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www.materialstoday.com
Materials for energyPowering the future
NOVEMBER 2011 | VOLUME 14 | NUMBER 11
MT1411pCovers.indd 1 17/10/2011 14:30:25
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MT1411pCovers.indd 2 17/10/2011 14:30:45
EDITORIAL
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 505
Caroline Baillie
Queens University, Canada
Zhenan Bao
Stanford University, USA
Alejandro Lopez Briseno
University of Massachusetts, USA
Chris Ewels
CNRS, France
Peter Goodhew
University of Liverpool, UK
Alan Heeger
University of California, USA
Suwan Jayasinghe
University College London, UK
Mark Johnson
Naval Research Laboratory, USA
David Kisailus
University of California
Steven Lenhert
Florida State University, USA
Tae Won Noh
Seoul National University, Korea
Aleksandr Noy
University of California, USA
Stephen Pearton
University of Florida, USA
David N. Seidman
Northwestern University
Helena Van Swygenhoven
Paul Scherrer Institute, Switzerland
George Whitesides
Harvard University, USA
Jackie Yi-Ru Ying
Institute of Bioengineering and
Nanotechnology, Singapore
Editorial Advisory PanelEditorial Advisory Panel
You cannot pick-up a popular scientific periodical these
days, such as Materials Today or the New Scientist, without
finding some news on energy related research. The
editorial team on Materials Today are seeing a significant
surge in interest and high quality research papers
appearing on energy related topics. Whether it be new
materials in the generation of energy, storage, improved
performance or policy, energy is definitely here to stay,
as the world’s energy needs continue to increase and our
conventional energy sources become even further strained.
To further facilitate research and discovery in the field
Deborah Logan our Publishing Director in Materials Science
is launching a new journal in 2012 with Prof Zhong Lin
Wang from the Georgia Institute of Technology on Nano
Energy. You can actually hear more from Professor Wang
and the journal by listening to the podcast between
Professor Wang and Dr Stewart Bland, Assistant Editor
on Materials Today. www.materialstoday.com/podcasts.
You can also find our podcasts through your iTunes
account, search for materials today. You can find
out more information about Nano Energy by visiting
www.materialstoday.com/view/21675/nano-energy/
Professor Wang will launch Nano Energy at the Fall MRS
meeting so please come along to meet him. We will host
a meet the Editor-in-Chief event at our stand on Tuesday
29th December from 3pm where you can come along
and chat with Professor Wang and enjoy some cheese
and wine. The same evening we’ll have a more formal
reception to launch the journal at the Sheraton hotel from
6.30 – 8.30pm. To attend just pick up an invitation from
our stand during the day.
Turning to this month’s issue our lead authors Jerry D.
Murphy and Thanasit Thamsiriroj look for a single solution
in the replacement of petroleum products with renewable
transport fuels. Their lively paper entitled “what will fuel
transport systems of the future” makes fascinating reading.
Our second paper by Aaron D. LaLonde et al. looks at
thermoelectric power generation. The growth in interest
in this amazing material has been phenomenal and the
authors look at its application in waste heat recovery.
Meilin Liu et al. bring us up to date with new advances
and tools in solid oxide fuel cells.
Whilst Sergei Kalinin et al. look at the optimization of
energy storage and conversion materials by understanding
better their ionic and electrochemical functionality.
Kalinin et al. use electrochemical strain microscopy in the
studies of Li-ion cathode and anode materials.
I hope you enjoy this issue of Materials Today and if you
are travelling to the Fall MRS meeting in Boston do drop
by our stand and say hello, we look forward to welcoming
you!
Published byElsevier Ltd.The Boulevard, Langford Lane,Kidlington, OX5 1GB, UK
EditorialCommercial Editor Jonathan AgbenyegaE-mail: [email protected] Assistant Editor Stewart BlandE-mail: [email protected] Support Manager Lin LucasE-mail: [email protected]
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Jonathan Agbenyega | Editor, Materials Today | [email protected]
Power to the future
Energy generation, storage and utilization
MT1411p505.indd 505 14/11/2011 11:08:16
MT1411p506_509.indd 506 01/11/2011 14:53:19
Contents
www.materialstoday.com CONTENTS
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 507
Regulars Editorial 505
Power to the futureEnergy generation, storage and utilization
Comment | Peter Thrower 510 Awards and prizesResearch isn’t about collecting awards, but are scientists being overlooked when it comes to recognizing and rewarding service to society?
Research News 512Memristors are made of this | Hubble bubble | Copying nature | Thermoelectrics, to go | Lights up time for graphene devices | Algae, for cheaper batteries | Synthesizing chromosomes offers DNA breakthrough | Smooth operator | Is it or isn’t it? | New photovoltaic mechanism | Belting up ultraviolet visibility | Mechanical chemistry
Updates Feature Comment 560
Scientific coopertition: can it scale and work?In materials science, collaborations tend to be limited to the cooperation of several small research groups. Meanwhile, in the world of particle physics, it’s an entirely different story. Markus Nordberg and Fabiola Gianotti from the LHC’s ATLAS experiment discuss the rewards and difficulties of large scale collaboration.
Markus Nordberg and Fabiola Gianotti
Books & Media 565
Uncovered | Benjamin J. Jones 567Nano fingerprints This month’s cover shows a back scattered electron micrograph of a fingermark developed using novel techniques. Benjamin Jones discusses the science of fingerprint detection and speculates on future techniques.
Diary 568
512 Hubble bubble.
515 Smooth operator.
517 Belting up ultraviolet visibility.
MT1411p506_509.indd 507 01/11/2011 14:53:39
CONTENTS www.materialstoday.com
NOVEMBER 2011 | VOLUME 14 | NUMBER 11508
www.materialstoday.com
Materials for energyPowering the future
NOVEMBER 2011 | VOLUME 14 | NUMBER 11
MT1411pCovers.indd 1 17/10/2011 14:30:25
Materials for energy Review 526
Lead telluride alloy thermoelectricsThe possibility of using solid-state thermoelectrics for waste heat recovery has reinvigorated the field of thermoelectrics. In this review, Snyder et al. examine the past and present successes of PbTe as a thermoelectric material, as well as identifying the issues related to maximizing performance in other thermoelectric materials.
Aaron D. LaLonde, Yanzhong Pei, Heng Wang, and G. Jeffrey Snyder
Review 534Rational SOFC material design: new advances and toolsSolid oxide fuel cells offer great prospects for the most efficient and cost-effective utilization of a wide variety of fuels. Liu et al. highlight some of the recent progress that has been made in the tools used to study electrode reactions.
Meilin Liu, Matthew E. Lynch, Kevin Blinn, Faisal Alamgir, and
YongMan Choi
Review 548Li-ion dynamics and reactivity on the nanoscaleProgress in the development and optimization of energy storage and conversion materials necessitates understanding their ionic and electrochemical functionality on the nanometer scale of single grain clusters, grains, or extended defects. Kalinin and colleagues review electrochemical strain microscopy, focussing on applications for Li-ion cathode and anode materials.
Sergei Kalinin, Nina Balke, Stephen Jesse, Alexander Tselev,
Amit Kumar, Thomas M. Arruda, Senli Guo, and Roger Proksch
Lead story 518What will fuel transport systems of the future?Can there be a “silver bullet” to solve our energy needs for transport? In this paper, Murphy and Thamsiriroj question the notion of whether there could ever be a single solution to the replacement of petroleum products with renewable transport fuels. Their paper examines available renewable transport fuels through an analytical review of a number of technologies.
Jerry D Murphy and Thanasit Thamsiriroj
Next issueMaterials Today looks at memory and more
Organic ferroelectric opto-electronic memoriesOrganic non-volatile memory
devices based on ferroelectricity
are a promising approach
towards the development of a
low-cost memory technology
based on a simple cross-bar
array. In this review article, the
latest developments in this
area are discussed, with a focus
on the most promising opto-
electronic device concepts.
Phase change memories: properties and applicationsAfter revolutionizing the
technology of optical data
storage, phase change materials
have been successfully adopted
in nonvolatile semiconductor
memories. The paper reviews the
key physical properties making
phase change materials so special,
the quantitative framework
describing cell performance and
the future perspectives of phase-
change memory devices.
Antibody-sensed protein surface conformationUsing an antibody-modified
atomic force microscope tip,
Bhushan et al. examine protein
surface conformation. Their
finding demonstrate that block
copolymer nanomorphology
can be used to regulate protein
conformation, and potentially
cellular response.
MT1411p506_509.indd 508 01/11/2011 14:53:58
COMMENT
NOVEMBER 2011 | VOLUME 14 | NUMBER 11510
Awards and prizes
Just over 50 years ago I reported for duty at the
Atomic Energy Research Establishment at Harwell, UK.
After signing various documents I was unexpectedly
told that I was to work in the Carbon and Graphite
Group where I had the task of trying to understand
some of the problems of radiation damage in
graphite, a major component in current UK nuclear
power reactors. I first had to read a number of reports
and then learn to prepare few-layer graphene. Of
course, we did not use the word “graphene” because
its use was not recommended by IUPAC until 35
years later. The technique of thinning natural graphite
crystals by repeated cleavage using adhesive tape was
being explored by several researchers at that time and
some perfectly transparent samples were occasionally
produced. The resulting material could be examined
in the transmission electron microscope, and at least
two studies of dislocations in graphite prepared using
this method were published in 1960.
Readers will know that 50 years later the Nobel Prize
for Physics was awarded to Geim and Novoselov for
“producing, isolating, identifying and characterizing
graphene”. As is often the case, there were arguments
over whether other scientists should have been
recognized by the award. When I raised the subject
with a senior colleague, I received a reply that
contained the comment “science is not about awards
and prizes”.
Many countries have a system of awards, and in the
UK we have a system of honors and awards that
recognizes “outstanding achievement and service
across the whole of the United Kingdom”. The usual
awards range from the title of Sir (Knight) or Dame
to MBE (Member of the Order of the British Empire).
Many people who win gold medals in the Olympic
Games, Nobel Prize winners and people retiring
from high government office are often given the
title Sir or Dame, while an MBE may be awarded for
“achievement or service in and to the community
of a responsible kind which is outstanding in its
field”. An Honours List is published twice a year. The
awards have no monetary value. While some are for
achievement (winning an Olympic gold medal or a
Nobel Prize), most are simply a public recognition of
service to a community. As illustrated by the recent
failed bid to host the 2018 Football (Soccer) World
Cup, success is not a criterion: the people involved in
the bid were recognized for their service.
When I retired from university life almost thirteen
years ago I returned to the village in England where
I was born and raised. The village is situated in the
middle of the Norfolk Broads: a picturesque area of
shallow lakes (broads) and rivers that is renowned
for its wildlife. I know many people in the village,
some of whom I went to elementary school with. A
few years ago, one of the villagers was awarded an
MBE for “services to the Broads”. He had worked as
a reed-cutter on the Broads for around 35 years and
had given outstanding local service. [Norfolk reed
is recognized as a premium thatching material for
the roofs of houses.] This prompted a good friend to
comment: “if (he) can get an award for cutting reed,
why can’t you get one for what you do?” I must say
that the friend did not really have any knowledge of
what I do, except that I had long been involved with
editing a major scientific journal.
The answer to this question is probably quite
simple. I have never been nominated. Most people
who are nominated for awards in the UK come
from government departments, from national
to local, and quangos. The Broads Authority was
undoubtedly responsible for the nomination referred
to above. The latest Honours List includes three
school crossing wardens awarded MBEs for guiding
children across the street outside a school for many
years. They were certainly nominated by the local
council. But any organization or person is allowed to
nominate, and one wonders how often it is done?
When it was suggested that I write this Comment
I was reluctant to do so because I did not want
readers to get the impression that I was “fishing” for
a nomination. I also certainly do not want people
to think that I am of the opinion that the people
mentioned above did not deserve their awards. They
did! They all devoted a major part of their lives to
a task that was not highly rewarded financially, and
they were reliable and diligent in what they did as
a service to their local community. But such is true
of many people who do not work in a government
organization. How often do such employees get
nominated for awards? Who was the last person
recognized for “services to scientific publishing”?
Is a publisher going to take time to prepare a
nomination? Do they even think about doing so?
In the last thirteen years I have nominated several
people for different awards and I am proud to say
that they have all been successful. Why? I think there
are two reasons. First, the nominee has to be truly
worthy of the award. Second, the nomination has
to be carefully prepared, presented and supported. It
takes a lot of effort and time, but it is satisfying to
see people receive the recognition they are due.
The latest issue of my Cambridge University Alumni
Magazine reports that six people from the university
received awards in the latest Honours List. Five of
these were professors, as one might expect, but
one was presented to a department administrator,
for services to “Higher Education”. Two names
further down the list one sees a person recognized
for services to “Ploughing in Wales”. The range of
activities recognized is enormous!
Winning a Nobel Prize may guarantee consideration
for a national award, but there are those who work
“behind the scenes” that are also deserving of
recognition. It is our responsibility to see that steps
are taken to ensure that this is done.
Peter A. Thrower | Editor-in-Chief, Carbon | [email protected]
Research isn’t about collecting awards, but are scientists being overlooked when it comes to recognizing and rewarding service to society?
MT1411p510_511.indd 510 01/11/2011 14:50:08
RESEARCH NEWS
NOVEMBER 2011 | VOLUME 14 | NUMBER 11512
Resistors, capacitors, and inductors have been the three
most fundamental passive components of electronic
circuitry for decades. But, there is a fourth two-terminal
element: the memory resistor, or memristor.
Resistance in a memristor increases as current flows
through it in one direction, but falls when the current
is flowing in the opposite direction. What makes
memristors potentially very useful in a wide range of
applications is that when the current is switched off
a memristor maintains the value of its resistance at
that point.
Although memristors were first theorized in the
early 1970s, it was not until 2008 that researchers
announced a thin film titanium dioxide memristor
device for applications in nanoelectronics, computer
logic components, and novel computer architectures
that mimic the plasticity of the human brain. Now,
researchers in Singapore have side-stepped metal
oxides, chalcogenides, amorphous silicon, or carbon,
and polymer-nanoparticle composites to address
the possibility of creating a flexible, memristor
using more life-like molecules instead, in the form
of proteins [Chen et al., Small (2011) doi:10.1002/
smll.201101494].
Other researchers have, of course, investigated biological
macromolecules, including peptides and proteins and
nucleic acids, as possible components of nano-electronic
devices. The Singapore team has embedded a bipolar
memristive structure composed of protein into a nanogap
using the chemistry based nanofabrication technique of
on-wire lithography (OWL). Using OWL they were able
to template gold nanowires in a silicon dioxide layer in
which a controllable gap can be etched. The primary
iron-storage protein, ferritin, was then used to fill the gap
to create a redox-active constituent of their memristor.
Covalent bonds between sulfur atoms in the ferritin
molecules and the gold atoms of the template electrode
surfaces ensure strong coupling, the team explains.
The team tested the protein-based memristor and
demonstrated that there are marked forward and
reverse current biases. Moreover, the signal traces for
forward and reverse cross from positive to negative
and vice versa at zero voltage, proving that the bipolar
behavior of the memristor is not simply a capacitance
effect. They suggest a mechanism taking place within
the ferritin in which iron(III) atoms within the protein
are reduced to iron(II). Iron(II) atoms move more
readily in ferritin and thus give rise to the bias in the
device, with current flow rising as more and more
iron(III) is reduced. The iron(II) atoms do not revert
to iron(III) when the current is switched off, so the
resistance level is maintained. A reverse current nudges
the process in the other direction.
“We are now working with bioengineers who can
control the number of iron ions in ferritin, which we
think will help to modulate the memristors,”
Xiaodong told Materials Today. “We are also working
with organic chemists to synthesize bio-inspired
polymers to have iron ions, which we hope will have
memristor behaviour similar to our results in the Small
paper.”
David Bradley
Memristors are made of thisELECTRONIC MATERIALS
There has recently been an explosion of interest in
developing devices based on graphene, which is not
surprising when you consider the range of remarkable
electronic, optical, and mechanical properties it
possesses. Now, the group of researchers from the
University of Manchester that won the 2010 Nobel
Prize in Physics for their groundbreaking work on
graphene has proposed a new use for the wonder
material: adaptive focal lenses [Georgiou et al., Appl
Phys Lett (2011) 99, 093103].
The lenses that Novoselov and colleagues are
proposing rely on the bubbles that can form at
the interface between graphene and a substrate.
Although the exact origin of these bubbles is still
not quite clear, the bubbles appear to arise from
small quantities of gas that are trapped below the
graphene.
The bubbles are evident using an optical microscope,
and come in a range of shapes and sizes; from tens
of nanometers to several microns, with circular,
square, and triangular shapes. The bubbles are
visible thanks to the phenomenon of Newton’s rings:
when illuminated with monochromatic light, the
interference between the light reflected from the
curved bubble and flat substrate surface causes a
series of concentric rings to appear. By measuring
these rings it is possible to extract the height and
shape of the bubbles.
The team have suggested that a lens could be formed
from either filling the bubble with a liquid with a high
refractive index, or even by surrounding the bubble
with a liquid from the outside.
But the really exciting aspect of the lenses is that their
shape can be changed by applying a small voltage. The
team estimate that if a lens was constructed from one
of the bubbles, they could produce a device with a
focal length tuning ratio of 15 %. The ability to focus
a lens that is potentially cheap and easy to produce
could find application in a wide range of devices.
But work is ongoing, and lenses only represent the tip
of the iceberg. According to Prof Kostya Novoselov,
“The importance of the paper goes far beyond
optics. We need to learn how to control the strain in
graphene, which would allow dynamic change of the
band-structure.”
Stewart Bland
Hubble bubbleCARBON
Topography AFM scan of a bubble. Reprinted with
permission from Georgiou et al., Appl Phys Lett (2011)
99, 093103. © 2011, American Institute of Physics.
Ferritin molecule. Courtesy Xiaodong Chen.
MT1411p512_517.indd 512 31/10/2011 16:40:04
RESEARCH NEWS
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 513
Researchers have developed synthetic crystals
whose structures and properties copy those
of naturally occurring biological minerals, a
breakthrough that could help in the development
of high-performance materials made under
more environmentally friendly conditions than
conventional synthetic materials.
Biominerals, as composite materials created from
inorganic minerals such as calcium carbonate
that contain a small amount of an organic
material, are commonplace in the natural world
in seashells, bones, and teeth. Synthetic versions
of these tough structures are usually made under
high temperatures and pressures, a process
that provides a lack of full control over their
properties. However, a team of scientists from
the universities of Leeds, Sheffield, Manchester
and York in the UK, as well as the Israel Institute
of Technology, have now developed artificial
biominerals with similar properties to biominerals,
but which can be designed and produced in a
much easier and more eco-friendly way.
For the study, published in Nature Materials [Kim et al. Nature Mater (2011) doi:10.1038/
nmat3103], the researchers grew calcite
crystals in the presence of synthetic polymer
nanoparticles, which acted as artificial proteins,
with these nanoparticles being integrated into
the structure of the crystal as it grew to create
a composite material, while recording the
responses with a nanoindenter.
As leader of the study Fiona Meldrum points out,
“This method of creating synthetic biominerals
gives us a unique insight into the structure
of these incredible materials and the way the
organic molecules are incorporated into the
crystal structure at a microscopic level. We can
then relate this microscopic structure to the
mechanical properties of the material.”
Initially, they examined the properties of
biominerals, with the team keen to explore the
possibilities of using this property to develop
composite materials using a straightforward
one-pot method. The artificial biomineral
produced was much harder than the pure calcite
mineral because it is a composite material, and
allowed the addition of something soft to a hard
substance to create a material even harder than
its constituent parts. This is significant because
it offers a flexible, simple way of generating
composite materials, and an understanding of
how to control crystal properties through the
inclusion of additives in a crystal.
The study also allowed them to link the
microscopic properties of the crystals to the
macroscopic to better understand how some
of the properties of biominerals can be applied
synthetically. It is hoped the study will lead to
applying this process to the manufacture of a
range of crystals with enhanced mechanical
properties, and perhaps even for generating
many different composite systems. They will try
different additives to assess how well they can
control crystal properties, which would help in
designing new materials based on this technology.
Laurie Donaldson
Copying natureBIOMATERIALS
SEM image of a calcite crystal. Reprinted by
permission from Macmillan Publishers Ltd: Nature
Mater (2011) doi: 10.1038/nmat3103, © 2011.
Just a few months ago we reported on the use of
microwaves to quickly and safely lock away radioactive
by-products from nuclear reactors inside robust
compounds [Mater Today (2011) 14, 304]. Continuing
the theme of using domestic microwaves as a short
cut to materials synthesis, a group of researchers
from Oregon State University is now using a similar
technique to shave days off the production time of a
promising thermoelectric [Biswas et al., Mater Res Bull
(2011) doi:10.1016/j.materresbull.2011.08.058].
The potential of thermoelectric materials is obvious
when we consider how much energy is wasted as
heat. From conventional light bulbs to gasoline fueled
engines, the problem is clear. Thermoelectrics provide
the opportunity for some of that wasted heat energy
to be recovered, by converting it into electricity.
One promising group of materials for thermoelectric
applications are the filled skutterudites.
The team, led by Prof Mas Subramanian, have
managed to rapidly synthesize In0.2Co4Sb12: an indium
filled skutterrudite. Skutterudites are materials that
adopt a cubic structure, with large voids located at
their centers. By filling these voids with guest atoms,
so called rattlers, it is possible to simultaneously
increase the material’s thermoelectric power factor
while reducing the lattice component of the thermal
conductivity. The result being that filled skutterudites
make excellent thermoelectric materials.
Conventional synthesis methods for skutterrudites
can take several days, as the calcination process can
take up to 48 hours. However, by using a microwave,
Subramanian and colleagues have reduced the process
to just two minutes.
To produce the thermoelectric, the team mixed the
powdered elemental components together and placed
them within a tube inside a mass of CuO: a material
that strongly absorbs microwaves. After switching
the microwave on, the CuO rapidly increases in
temperature, reaching over 650 °C in two minutes.
The resulting materials were then ball milled in
acetone, and sintered for several hours under nitrogen
and oxygen.
The overall production time can be compressed to just
a few hours. But the new process does not just save
time; it saves energy.
Subsequent testing revealed that the samples
prepared using the new method were as good as those
produced using a conventional furnace. The team is
now hoping that it will be possible to produce different
types of skutterudites using the same method.
Stewart Bland
Thermoelectrics, to goENERGY
The unit cell of InxCo4Sb12, showing the indium
rattler at the center.
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11514
RESEARCH NEWS
A new study has shown that alginates taken from
fast-growing brown algae could offer extended energy
storage capabilities for a new generation of lithium ion
batteries using environmentally friendly manufacturing
technologies. Such cheap, lightweight, and improved
batteries could benefit a range of applications, such as
electrical cars, computers, and cell phones.
The researchers, from the Georgia Institute of Technology
and Clemson University in the United States, whose work
was published in Science Express [Kovalenko et al. Science
(2011) doi:10.1126/science.1209150], showed that the
algae, which are produced in stalks as long as 60 meters
in large oceanic clusters, can provide more energy storage
and output than the two standard types of commercial
electrodes; graphite-based and silicon-based.
They initially looked for a natural replacement
binder in aquatic plants that grow in salt water with
high concentration of ions. The binder is crucial in
suspending the silicon or graphite particles that
interact with the electrolyte and produce power.
Lithium ion batteries transfer lithium ions between
two electrodes through a liquid electrolyte, so the
easier the lithium ions can enter the electrodes during
charge and discharge, then the greater the capacity of
the battery.
The low-cost alginate-nanoSi-electrode can be
extracted from seaweed using a straightforward soda-
based process that produces a uniform material. From
there, the anodes can be developed using a water-
based slurry to suspend either the silicon or graphite
nanoparticles. Such electrodes have the advantage of
being compatible with existing methods of production
and can therefore be integrated into standard designs
for batteries.
The alginate needs to deal with decomposition
occurring when the lithium ion electrolyte forms
a solid electrolyte interface (SEI) on the surface of
the anode, hampering the potential of high-energy
silicon anodes. The SEI has to be stable to allow
the lithium ions to pass through it as well as to
restrict the flow of fresh electrolyte. For graphite
particles, the SEI remains stable as the volume
does not change; the alginate also manages to bind
silicon nanoparticles to each other as well as the
anode, and coat the silicon nanoparticles themselves
to offer a rigorous support for the SEI, therefore
stopping any degradation.
As researcher Gleb Yushin points out “The carboxylic
groups in alginate actively interact with ions from
water and the intracellular environment, protecting the
cell from an excess of toxic chemicals. We utilize these
uniformly distributed carboxylic groups to improve the
performance of battery electrodes.”
Algae can be produced on salt water or waste water
land and do not use valuable agricultural land, and
also need less area to produce the same amount of
biomass as regular crops. The team expects such use
of plant matter to increase and believe this should
be prioritized to help achieve greater sustainability,
especially as alginates are already extensively used in
the paper, pharmaceutical, biotechnological, dental,
and food industries.
Laurie Donaldson
Algae, for cheaper batteriesENERGY
SEM Aliginate-nanoSi-electrode. Courtesy of Gleb Yushin.
Lights up time for graphene devicesCARBON
Graphene has been touted as the natural
successor to silicon in microelectronics devices.
Indeed, experimental and theoretical results
suggest that it is has many properties that will
make it the perfect material for building a new
generation of transistors, chemical sensors,
composites, nanoeletromechanical (NEMS)
devices and optoelectronics components that
might operate at 10 gigabits per second.
However, there is an obstacle visible on the
roadmap: graphene-based photodetectors
demonstrate a poor response when compared to
conventional semiconductor devices.
Now, UK researchers have combined graphene
with plasmonic nanostructures to boost
their photodetector sensitivity twentyfold
[Novoselov et al., Nature Commun (2011)
doi:10.1038/ncomms1464] The research also
suggests that it is possible to achieve wavelength
and polarization selectivity by tweaking the
nanoscopic geometry of the materials.
Geim and Novoselov and colleagues at
Manchester and the University of Cambridge
explain that graphene-based photodetectors
ought to have excellent characteristics in terms
of quantum efficiency and reaction time and
indeed they do. But they absorb light only
inefficiently and it is difficult to extract electrons
from the critical p-n junctions in any such device.
In order to circumvent this latter obstacle to the
development of graphene-based photodetectors,
Geim and colleagues have focused on
incorporating plasmonic nanostructures close
to the junctions. The team explains that these
plasmonic structures can absorb photons,
producing plasmonic oscillations, which in
turn boosts the local electric field guiding the
electromagnetic energy to the p-n junction.
With this in mind, the team therefore prepared
graphene flakes using their Nobel-winning
micromechanical exfoliation technique to
peel off monoatomic layers of carbon from
an adhesive surface. Raman spectroscopy and
optical contrast techniques proved that the
necessary templated layouts were produced.
They then created various nanostructures at
the p-n terminals of these graphene layouts.
They tested the photo response of the devices
using several low-intensity lasers coupled to a
microscope to scan the points of illumination.
The researchers were able to measure the local
photovoltage and photocurrent response. Their
theoretical calculations agreed with the responses,
however, they were unable to make a direct
quantitative comparison between the theoretical
field enhancement and the photovoltaic signals
obtained. Nevertheless, they point out that the
qualitative correspondence between theory and
experiment is sufficient and it “proves the viability
of the concept of using field amplification by
plasmonic nanostructures for light harvesting in
graphene-based photonic devices,” they conclude.
David Bradley
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RESEARCH NEWS
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 515
In what could be a crucial breakthrough in synthetic
biology, researchers have synthesized one of the
largest DNA molecules ever by replacing the DNA in
the arm of a yeast chromosome with synthetically
produced DNA. This computer-designed DNA is
different from the original but can still produce
a viable yeast cell using the chromosome arm to
produce a new way of mixing up the genetic structure
of the organism.
The study, which was published in Nature
[Dymond et al., Nature (2011) doi:10.1038/nature10403],
showed that large chunks of DNA can be synthesized
and inserted into a chromosome, and also developed a
way to change the structure of the synthetic DNA (called
scrambling), a process that could be applied to a range
of other organisms. The synthesis and scrambling of the
DNA in the chromosome arm is another stage in the
synthesis of all DNA in a yeast cell, and the similarities
between yeast cells and human cells could provide
information about which structural arrangements are
possible and compatible with life.
Although there have been previous studies
that synthesized bacterial chromosomes, yeast
chromosomes are bigger and more complex, making
them harder to synthesize. The team first developed
semi-synthetic DNA using a computer-generated
blueprint for the sequence of nucleotides, the building
blocks of DNA, before the resulting semi-synthetic
DNA replaced a particular chromosome arm of a
yeast cell without impacting its health. Researcher
Jef D. Boeke, from Johns Hopkins University School
of Medicine, stated “The yeast that underwent
this process were indistinguishable in their growth
properties from the native yeast.”
The synthesized DNA in the arm was then scrambled,
with a chemical added to the yeast culture that caused
major changes to gene-sized blocks of nucleotides in
the synthesized DNA. The shuffling caused some of the
genes to be lost, while the order of other genes was
also changed. The process was repeated for a range of
yeast cultures to produce many modified arms. The
differences between the scrambled genetic codes of
the yeast cultures meant these cultures showed trait
differences.
As Boeke pointed out “We were able to track the
changes we made relative to the native yeast and
isolate scrambled derivatives from the semi-synthetic
yeast. We thereby generated a wide range of different
derivatives from the semi-synthetic strain.” With
yeast being involved in a wide variety of industrial
fermentation processes, being able to more effectively
confer desired traits on yeast DNA and make it
more flexible could lead to the development of new
medicines, vaccines, and more efficient biofuels.
It is also hoped the ability to trigger huge
combinatorial genome rearrangements will resolve
other issues, such as how much you can reduce the
genome and still grow yeast and whether yeast is
restricted to 16 chromosomes or can be made with
less or more.
Laurie Donaldson
Synthesizing chromosomes offers DNA breakthroughBIOMATERIALS
A population of synthetic or semi-synthetic yeast
(yellowish cells) is scrambled. Courtesy Jessica S.
Dymond.
There is nothing worse than a ragged ribbon, especially
if it’s a graphene nanoribbon destined for the future
world of spintronics and nanoelectronics. Thankfully,
a US team has developed an approach to nanoribbons
that smoothes the edges paving the way for the
exploitation of their quantum-confined bandgaps and
magnetic edge states.
Rough edges and defects are part and parcel of
making graphene nanoribbons using lithography. But,
a novel approach that involves opening and unrolling
single-walled carbon nanotubes can be used to create
nanoribbon quantum dots that have all the properties
desired of such structures. Specifically, the researchers
demonstrated how these materials have well defined
quantum transport phenomena [Dai et al., Nature
Nanotechnol (2011) doi:10.1038/nnano.2011.138].
The concept of graphene nanoribbons, also referred
to as nano-graphene ribbons, was first postulated in
the 1990s as a suggestion for investigating the edge
effects and effects of nanostructure on these intriguing
materials. While a graphene sheet might be thought
of as a single layer of the carbon allotrope graphite,
it is the edges that are thought to give rise to specific
functionality. Theoretically, there are two arrangements:
the zigzag and the armchair. In the former, the
carbons along the edge resemble the carbon backbone
of a simple, unbranched, and essentially unending,
hydrocarbon chain. In the armchair motif, one might
imagine the edge as resembling an alternating E, Z
carbon=carbon double bond chain. In reality, these neat
motifs are distorted by dangling groups, gaps and other
defects and deviations from the norm.
There is good reason to investigate graphene nanoribbons
as theory suggests that quantum effects will arise that
are not observed with graphene sheets and that zigzag
ribbons are “metallic” whereas armchair ribbons are either
semiconducting or metallic depending on their width.
Studying these effects requires smooth, ordered edges
otherwise the phenomena observed would not be
consistent between experiments.
Moreover, one might thus be forced to conclude
that it is the defects that are giving rise to specific
measurements. This is supported by the studies carried
out by Dai et al. from which they infer that, “the
quantum transport features of graphene nanoribbons
are highly reflective of ribbon quality.” Moreover,
their graphene nanoribbons have conductivities 700
to 800 times higher than previously reported devices,
presumably due to their smooth edges.
“The next step should be to further improve nanoribbon
quality and edge structures, exploring the signature
of the edge-induced quantum phenomena such as
magnetic edges states in transport measurements, and
harvesting these states for future spintronic and other
device applications,” Dai told Materials Today.
David Bradley
Smooth operatorCARBON
AFM of a smooth GNR device. Courtesy Hongjie Dai.
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11516
RESEARCH NEWS
Is it or isn’t it?BIOMATERIALS
A team of scientists from South Korea have
revealed that a developed self-assembled
phosphoryl cholin (PC) nanostructure is capable
of mimicking the natural cell membrane bilayer,
and significantly affect the gene delivery process
through favorable interactions with the cell
membrane. The team investigated this capability
with an experiment that utilized nanostructured
film-mediated gene delivery through an
examination of the interactions between the
cell membrane and an artificial self-assembled
nanostructure.
They used the nanostructure as a cell membrane
mimicking biointerface as it could interact with
the natural cell membrane, which has a strong
affinity toward naturally occurring PC groups
in cell membranes, and is structural similarity
with the phospholipid layer. The study, which
was published in Small [Son et al., Small (2011)
doi:10.1002/smll.201100232], produced two
key breakthroughs. The first was the effective
development of brush PC polymers that form
cell membranes that mimic multi-layer film and
elucidate the structural features of the multi-layer
structure by grazing incidence wide-angle x-ray
diffraction (GIWAXD). Also, the researchers looked
at the usefulness of brush polymers as artificial
cell membranes and the potential application for
tissue engineering and drug delivery substrates.
The other significant result was a greater
knowledge of the mechanism of the interaction
between the PC polymer surface and the natural
cell membrane by evaluating the gene delivery
efficiency. The team found that the polymer
surface interacted with the cell membrane
through the fusiogenic process, offering an
advantage for the delivery of payloads.
The research group had previously developed
polymer-based nanoparticles for the delivery of
therapeutic genes in the solution phase and, to
improve delivery, attempted to synthesize the
polymeric template, which can load therapeutic
genes and also interact with cell surfaces
efficiently. One researcher, Professor Moonhor
Ree, showed cell-mimicking synthetic polymers
that consisted of a polymeric backbone and PC
side chains. This polymer had similar structural
features to natural cell membranes, such as
aliphatic alkyl tails, hydrophilic PC head, and a
forming self-assembly structure.
The team therefore decided to use artificial
polymer films for efficient surface gene
delivery because of its close resemblance to
cell membrane. For this study, they revealed
that cells seeded on the PC polymer surfaces
could proliferate very efficiently, while the
initial cell attachment was inhibited by the
PC group. To fully investigate how useful
the PC polymer could be, it was necessary
to examine its structural features, and
ensure the polymerization conditions were
optimized. The team also stressed the
importance of understanding and mimicking
the cell membrane to unravel these biological
phenomena and the development of a
biointerface platform.
There are many potential applications for the
research. For instance, the PC polymer has good
biocompatibility with cells and can be easily
coated onto a range of substances, including
silicon wafer, glass and metals; as the polymer
scaffold also has useful biocompatibility without
any cytotoxicity, it could also help to create the
next generation of tissue engineering materials.
Laurie Donaldson
New photovoltaic mechanismELECTRONIC MATERIALS
Last year, a study in Nature Nanotechnology reported
on the discovery of a new mechanism for the
photovoltaic (PV) effect at domain walls in bismuth
ferrite, a ferroelectric that has recently been the focus
of many studies [Yang et al., Nature Nanotech (2010)
5, 143]. The researchers found that they could produce
voltages that were larger than the band gap of the
material: a value that limits the maximum voltage in
conventional semiconductor photovoltaics.
Now, the same team of researchers have succeeded
in unraveling this mystery and explaining the science
behind the phenomenon, which occurs in ferroelectrics
with periodic domain structures [Seidel et al., Phys Rev
Lett (2011) 107, 126805].
Ferroelectrics are materials that posses a net electric
polarization, such that one side is negatively charged
and one side is positive (just as a ferromagnet has a
north and south pole). However, regions with particular
polarizations can be separated within the bulk of a
material into individual domains, divided by domain walls.
The team, predominantly from Berkeley, studied thin
films of bismuth ferrite grown on insulating DyScO3
using chemical vapor deposition. By varying the
thickness of the film, the team was able to control the
domains, producing periodic structures that spanned
hundreds of microns. The domains were formed in
stripes, with widths between 50 to 300 microns, and
where each stripe has the opposite polarization to
that of its neighbour. The result is striped film, which
retains a net polarization across the film thanks to the
zig-zag ferroelectric pattern.
Just as in a regular PV, when exposed to light, free
electrons and holes are formed. But the team has
found that thanks to the presence of the domain walls,
electrons and holes collect on opposite sides of the
walls, delaying the electron/hole recombination. A
current then results that is perpendicular to the walls,
as the opposite charges attempt to recombine. But
it’s the repetitive nature of the domain structure that
is key to the large voltage, as the effect is additive.
According to co-author of the study Joel Ager “It’s like
a bucket brigade, with each bucket of electrons passed
from domain to domain. As the charge contributions
from each domain add up, the voltage increases
dramatically.”
Bismuth ferrite is not the ideal material for solar cells,
as it’s sensitive to blue and near UV light, but the
same mechanism should work in any ferroelectric
with a periodic domain structure, and the team is now
looking into new material possibilities.
Stewart Bland
Piezoresponse force microscopy image of the aligned domain walls. Reprinted with permission from Seidel at al., Phy Rev Lett (2011) 107, 126805. © 2011 by the American Physical Society.
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RESEARCH NEWS
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 517
A novel and facile approach to detecting ultraviolet
radiation in the hazardous 320 – 400 nm (UV-A) part
of the spectrum has been developed by researchers
in China (Fudan University) and Japan (NIMS). Their
high performance photodetector comprises niobium(V)
oxide nanobelts that are 100 – 500 nm wide and
2 – 10 micrometres long. The team synthesized these
quasi-aligned nanobelts using hydrothermal treatment
of niobium foil in potassium hydroxide solution
and subsequent proton exchange and calcinations
[Fang et al., Adv Funct Mater (2011) doi:10.1002/
adfm.201100743]. The same detectors might be useful
in optoelectronics circuitry operating in the UV-A
band.
Photodetectors are becoming increasingly important
in many applications. Fundamentally, they convert
an optical signal into an electrical one and so can
be used, as the name would suggest, simply as a
light detector, as well as acting as binary optical
switches for imaging, optical communications, and
optoelectronic circuits. Given the growing interest in
nanotechnology it requires no great stretch of the
imagination to see that integrating photodetectors at
the nanoscale is the inevitable next step. Indeed, such
nano devices are inherently more effective than their
“bulk” semiconductor counterparts because of their
higher surface to volume ratio, even in bridging the
gap between micro and nano.
Researchers across the globe have thus focused
on creating one-dimensional nanowire based
photodetectors and efforts have been made to nudge
the sensitivity of these devices into the ultraviolet
part of the electromagnetic spectrum. Unfortunately,
most efforts have led to only poorly efficient UV-A
detectors. Fang and colleagues hoped to fill the gap by
turning to niobium(V) oxide, a material transparent to
visible light that has a bandgap of 3.4 eV, which they
suggest, makes it an ideal candidate for a “visible-
blind” UV-A photodetector. The visible transparency
means that the detection process essentially ignores
incident visible light.
Tests on their nanobelt UV-A photodetector reveal
it to live up to expectations with high sensitivity
and high external quantum-efficiency of well over
6000 %. The prototype nanobelt detector also has a
photocurrent stability of more than 40 minutes. The
team suggests that optimization of the annealing
process used in the final stage of preparation of the
nanobelts, could be further optimized to improve the
active life time of the materials. There is also a need
to eliminate defects and so improve efficiency and
sensitivity still further.
David Bradley
Belting up ultraviolet visibilityNANOTECHNOLOGY
Nanobelt arrays. Courtesy Xiaosheng Fang.
Click chemistry describes the process of quickly and
easily joining smaller molecules together to form
larger ones. However, in spite of a name which implies
a kind if chemical Lego, while moving forward is easy,
reversing the reactions can be rather difficult. Such is
the case for the highly stable 1,2,3-triazole moiety,
which strongly resists being reverted into its azide
and alkyne precursors, rebuffing attempts to reverse
the reaction using simple chemical and thermal
techniques.
Tackling the problem will instead require a different
approach, and thanks to a team from the University
of Texas at Austin, it looks as though we may have a
solution to “unclick the click” [Brantley et al., Science
(2011) 333, 1606].
Prof Christopher Bielawski and colleagues implanted
the stable triazole inside a polymer chain, and then
managed to break the chain at the triazole site using
ultrasonic sound waves. Speaking to Materials Today,
Bielawski exaplained how such a mechanochemical
approach works: “The polymer chains function as
handles that respond to the forces generated
under ultrasonication. In an acoustic field, solvent
cavitation generates small bubbles that rapidly
expand and implode. Solvated polymer chains near
these growing cavities essentially are pulled toward
the void volume. If this happens to a polymer
chain attached to one side of the triazole, but not
to the polymer chain attached to the other side
of the triazole, tensile forces are generated in the
center of the chain, right where the triazole is
located. It is believed that this mechanical force
destabilizes the molecule through bond distortion,
which ultimately lowers the energy needed for the
cycloreversion to occur.”
As the length of the polymer chain is dependent
on sonochemical reactions, as well as the position
of the mechanically sensitive molecule within
the chain, the team was also able to demonstrate
that the reversion was purely the result of the
mechanical action, rather than the effect of
any induced heating. These control experiments
suggest that the triazole must be located near
the center of the chain in order to experience the
required force.
The researchers believe that such an ability to
“selectively deconstruct tiazoles with high fidelity”
could find use in mechanoresponsive materials.
Bielawski revealed, “An interesting application of
our work could be the development of systems
or sensors that use mechanical forces to
reversibly label biomolecules (e.g., proteins) with
a variety of small molecules.”
The team is “currently undertaking a theoretical
study to understand the role that mechanical forces
play in the reactivity we have observed. We are
also exploring new areas, such as the application of
mechanical forces in a biological context.”
Stewart Bland
Mechanical chemistryTOOLS AND TECHNIQUES
Bielawski and colleagues make reversing the
reaction look easy. Courtesy Christopher Bielawski.
MT1411p512_517.indd 517 31/10/2011 16:40:11
ISSN:1369 7021 © Elsevier Ltd 2011NOVEMBER 2011 | VOLUME 14 | NUMBER 11518
When discussing energy, it is important to understand the relevant
units. Box 1 outlines the relationship between the tonne of oil
equivalent (toe), the Joule, and the GWeh. In addition, primary and
final energy must also be differentiated, particularly for electricity.
Primary energy is the energy contained in primary resources prior
to conversion or transformation into a form that is used by the
final consumer. Energy used by the final consumer is final energy.
For example the energy content of the coal used in a power plant
is primary energy while final energy describes the electricity
produced by the power plant. The ratio of primary energy to final
energy is approximately 2.5 if electricity is produced at 40 %
electrical efficiency. This relationship is less clear for renewable
sources. For example, for wind power primary energy is similar to
the final energy.
Background and perspectiveThe world’s Total Primary Energy Supply (TPES) was 514 EJ
(12 267 Mtoe) in 2008, which is double the figure of 1973 (256 PJ)1. In
2008 renewable sources accounted for 12.9 % of the TPES. On a global
scale the ratio of primary energy to final energy was 1.471.
There tends (particularly in the media) to be a preoccupation
with renewable electricity rather than renewable energy. Electricity
consumption in 2008 equated to 60.6 EJ or 17.2 % of the Total Final
Consumption (TFC) while transport equated to 95 EJ or 27 % of the
TFC1. On a global scale 18.7 % of the electricity is produced from
renewable sources; the majority of which is from hydro-electricity
(15.9 %). Renewables play a smaller role in transport. In 2008 liquid
biofuels accounted for 1.92 EJ or about 2 % of the energy used in
transport2. This relatively small proportion has been controversial,
This paper seeks to decry the notion of a single solution or “silver bullet” to replace petroleum products with renewable transport fuel. At different times, different technological developments have been in vogue as the panacea for future transport needs: for quite some time hydrogen has been perceived as a transport fuel that would be all encompassing when the technology was mature. Liquid biofuels have gone from exalted to unsustainable in the last ten years. The present flavor of the month is the electric vehicle. This paper examines renewable transport fuels through a review of the literature and attempts to place an analytical perspective on a number of technologies.
Jerry D. Murphy and Thanasit Thamsiriroj
Environmental Research Institute and Department of Civil and Environmental Engineering, University College Cork, Ireland
E-mail: [email protected]; [email protected]
What will fuel transport systems of the future?
MT1411p518_525.indd 518 31/10/2011 14:24:38
What will fuel transport systems of the future? REVIEW
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 519
leading to a food versus fuel debate3,4. In 2007 – 2008 the prices
of wheat, rice, and maize increased by 130 %, 98 %, and 38 %
respectively; this was attributed to the maize ethanol market5. This may
indicate that we need to worry about transport fuels of the future.
Land, population, food, biomass, and biofuelsIf biofuels are a solution to renewable transport fuel we must consider
the available land. The land area on Earth is 149 × 106 km2; 55.7 % of
this is forest, 16.1 % (or 24 × 106 km2) is pastureland and 9.4 % (or
14 × 106 km2) is arable land6. The population of the world is growing.
In 1804 there were 1 billion people on the planet; in 2000 this number
had increased by a factor of 6. By 2013 another billion people will
occupy the planet7. Global arable land averages only 0.2 ha per person
and this number is decreasing. Life styles are such that more people
require meat. A meat diet requires more land than a vegetarian diet.
Thus our finite agricultural land is required to produce more food for
a growing population of humans and animals as well as renewable
thermal and transport energy. Is this possible?
Ireland: a case studyThe world is variable; bioenergy systems are geography specific.
Sugar cane grows in tropical climates not in temperate ones. Even in
particular climatic regions the yield of crops varies; maize for example
provides yields in the range of 9 to 30 tonnes of dry solids (tDS) per
hectare per annum8. This paper will use Ireland as an example where
necessary. The Republic of Ireland is part of the island of Ireland and
is situated at the western extreme of Europe. It has a population of
4.5 million and a land area of 6.8 million ha. The agricultural area is
of the order of 4.4 million hectares of which 4 million are deemed
pastureland (including rough grazing) and 400 000 hectares are arable9.
Energy forecasts for IrelandIreland is a member state of the EU. The EU has set targets for Ireland
of 16 % renewable energy supply (RES) in 2020. They have further
set a specific target of 10 % renewable energy supply in transport
(RES-T)10. Policy in Ireland has a particular focus on renewable
electricity (RES-E). Ireland has independently set a target of 40 %
RES-E; this will be met predominately through wind power. Ireland’s
forecast for total final energy in transport in 2020 (allowing for the
implementation of energy efficiency and renewable energy plans) is
188 PJ11 (Table 1). In 2008 Ireland had 2.497 million vehicles of which
1.92 million were private cars. The private car density thus equated to
ca. 430 per 1000 population, with an average annual distance travelled
of 16 708 kilometers12.
Contribution of electric vehicles to renewable transportThe role of electricity in renewable transportElectric vehicles (EVs) are expected to make a significant impact on
the international transport fleet with several manufacturers rolling out
EV models. The EV (Fig. 1) is recharged from the electricity grid and is
not only beneficial to the vehicle users, but also to electricity providers.
EVs can act as an energy storage system by recharging at night when
Table 1 Forecasted final energy consumption in Ireland
in 2020. Adapted from11.
PJ % total
Electricity 124 21.5
Thermal 223 38.9
Transport (road and rail) 188 32.8
Other transport (not covered by RES-T) 39 6.8
Total 574 100
Box 1 Energy units and prefixesAt a national or global scale, energy may be described in:
PJ (1015 J) or EJ (1018 J).
Alternatively, Million tonnes of oil equivalent (Mtoe) may be used.
1Mtoe = 41.9 PJ.
Electricity may be described by the TWeh (1 TWeh = 3.6 PJ).
1 kWeh = 3.6 MJ.
k = 103: M = 106: G = 109: T = 1012 :P = 1015; E = 1018.
Box 2 The role of EVs in renewable energyRenewable energy associated with EVs300 000 EVs in 2020, each travelling 16708 km/a.
5 billion km/a at a fuel efficiency of 6 km/kWeh = 835 GWeh/a or
3 PJ/a.
Final energy consumption for transport Ireland in 2020 is
projected to be 188 PJ.
300 000 EVs equates to 1.6 % of the energy in transport (2.4 %
of the energy in electricity).
The target for green electricity in 2020 is 40 %.
300 000 EVs equates to 0.64 % green energy in transport.
The relationship between EVs and 3 MWe turbinesAllowing for 8 % losses between the source and plug in point, and
a 12 % loss from plug to battery14, a 3 MWe wind turbine at a
capacity factor of 30 % generates:
3 MWe × 8760 h/a × 0.3 × 0.92 × 0.88 × 10-3 = 6.38 GWeh/a.
300 000 EVs require 3 PJ or 835 GWeh/a.
One hundred and thirty one 3 MWe wind turbines would be
required.
One 3 MWe turbine will fuel about 2300 EVs.
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11520
the electricity demand is normally low and electricity from wind may
otherwise be wasted. The primary disadvantage of the EV is the battery,
which has a relatively short lifetime, a long recharging time and results
in a short driving distance per charge.
The proposed role of EVs in IrelandThe Irish Government has set an ambitious target of 10 % of all
vehicles in the transport fleet to be powered by electricity by 2020.
This will require between 250 000 – 300 000 EVs12. With reference to
Box 2 it may be noted that this amounts to only 1.6 % of the energy
in transport and as only 40 % of electricity is proposed to be green,
accounts for only 0.64 % RES-T. The EU Renewable Energy Directive10
allows a weighting of 2.5 to green electricity, thus this again equates to
1.6 % RES-T. The rationale for this weighting is to incentivize EVs, but
there is some logic to this value as it is similar to the ratio of primary
energy to final energy. The value of 1.6 % RES-T is very similar to
values obtained by Foley and co-workers13.
EVs as a variable electricity storage mechanismOne issue with producing electricity from wind is its intermittency
and the inability to store it on a large scale. Much of the potential
electricity that could be produced by wind at night is lost to the
system. Electricity demand in Ireland is expected to be 124 PJ in 2020
(Table 1) and 40 % of this (ca. 50 PJ) is targeted to be renewable; as
wind is the dominant renewable energy source, the production will
be variable. On a very simplified basis it can be assumed that a third
of this electricity (ca. 17 PJ) will be produced by night when demand
is very low. The 300 000 EVs plugged in at night will require 3 PJ/a;
averaged over the year EVs could store on the order of 18 % of the
electricity produced from wind during the night.
On the most advantageous sites in Ireland a wind turbine will
generate electricity at a 40 % capacity factor; as more turbines are
built this has dropped to ca. 30 %. With reference to Box 2 it may be
noted that a 3 MWe turbine can provide electricity for 2300 EVs; one
hundred and thirty one 3 MWe turbines are required to fuel 300 000
EVs. It is obvious that although EVs have a significant role to play in
renewable energy, other sources are required.
Liquid biofuelsLiquid biofuels are dominated by bioethanol and biodiesel.
Approximately 67 billion liters of bioethanol were produced in
20089. This equates to 1.4 EJ or (1.47 % of the energy in transport).
Approximately 12 billion liters of biodiesel were produced in 20089
(400 PJ or 0.42 % of the energy in transport).
BioethanolBioethanol may be produced from sugars (sugar cane, sugar beet)
or starches (corn/maize, wheat, barley). Ethanol production requires
fermentation of six-carbon sugars with saccharomyces cerevisiae as the
prime yeast species15. Sucrose (C12H22O11) can be easily converted to
glucose and therefore juice or molasses from sugar cane and sugar beet
do not require hydrolytic pre-treatment16. Starch however is a complex
carbohydrate (C6H10O5)n which requires hydrolytic pre-treatment prior
to fermentation17. Starch is the most utilized feedstock for ethanol
production16; its conversion is energy intensive18. For example wheat
ethanol has a by-product known as wet distiller’s grain and solubles
(WDGS) which contains 33 % of the starting solids at ca. 12 % solid
content. It is used as cattle feed after drying to 9 % water content.
This drying process can account for up to 35 % of the total parasitic
thermal demand of the ethanol process18.
BiodieselBiodiesel is produced from rapeseed and sunflower in Europe,
soybean in Southern America, and palm oil in South East Asia. It is
produced through a transesterification process whereby oil (ca. 90 %)
and methanol (ca. 10 %) together with a catalyst are converted to
fatty acid methyl ester (FAME) or biodiesel (ca. 90 %) and glycerol
Fig. 1 (a) Wind turbine and (b) plug-in electric vehicle.
(a)
(b)
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11 521
(ca. 10 %). Physical properties of the specific oil depend on the portion
of triglycerides and free fatty acids (FFA). For example, fresh vegetable
oils comprise 90 – 98 % triglyceride with a small portion of FFA19,20
while used cooking oil is high in FFA content. Two production methods
are currently available at commercial scales21: (1) Alkaline catalyzed
transesterification; (2) Acid and alkaline catalyzed transesterification in
a two-stage process.
The first method is used to transesterify oil with low FFA content;
the process can be established on a small scale. The second method is
for oils high in FFA. The two-stage process begins with an esterification
reaction using an acid catalyst to convert FFA into biodiesel;
subsequently a transesterification reaction (method 1) is used to
convert the remaining triglyceride into biodiesel. Animal fat obtained
from the rendering process and used cooking oil from catering are
normally high in FFA content, and require the two-stage technology.
Sustainability of first generation biofuelsIssues related to liquid biofuels may be separated into energy balances
and green house gas analyses.
Energy balanceGross energy reflects the yield of biofuel per hectare. The net energy
deducts the energy input to the crop production and to the process. For
example, 18 GJ of direct and indirect energy is required per hectare to
produce wheat. Indirect sources include the energy required to produce
the fertilizers. Direct energy includes diesel to power agricultural
machinery22.
In wheat ethanol approximately 66 GJ of ethanol are produced
per hectare in Ireland (372 L ethanol / tonne of grain × 8.4 tonnes
of grain / hectare × 21.1 MJ / L of ethanol). However in a standard
ethanol system using electricity from the grid powered by fossil fuel
and natural gas for thermal energy about two thirds of the output
energy is used in the process18. Thus wheat ethanol has a gross energy
of 66 GJ/ha/a (3125 L of ethanol per hectare) while the net energy
can be as low as 4 GJ/ha/a (Fig. 2). This highlights one difference
between modern biofuel facilities in the developed world and those
in the developing world. Sugarcane ethanol facilities use the residue
of the cane (bagasse) as a source of combined heat and power to
fuel the system and as such the net energy is not much lower than
the gross energy (Fig. 2). Systems can always be improved. Murphy
and Power18 showed that by using stillage (WDGS) to produce
biomethane, and straw as a source of thermal energy, the gross
energy of the bioenergy system could be increased by 27 % and
the net energy from 4 to 43 GJ/ha/a. For an optimum sized ethanol
facility (150 million liters / annum) the land under grain (and straw)
is 48 000 ha. As straw is a bulky, voluminous biomass the developer
may find this logistically difficult and expensive. How is the developer
persuaded to be sustainable?
Greenhouse gas balanceA greenhouse gas balance outlines the sustainability of the biofuel
system. The EU Renewable Energy Directive10 states that to be deemed
sustainable the biofuel system must affect a 60 % saving in greenhouse
gas emissions compared to the displaced fossil fuel. Table 2 highlights
data from the Directive for various biofuel systems. A lot of negative
energy balances and life cycle analysis have been attributed to biofuel
systems as non-biofuel products are neglected in the analysis. Fig. 2 only
allows for energy in fuel. If, for example, the stillage from a grain ethanol
facility is fed to cattle and displaces grass silage, no credit is given to
the ethanol system. The present authors23 investigated biodiesel for use
in Ireland through comparison of indigenous Irish rape seed and palm
oil biodiesel produced in Thailand. The paper highlighted the benefits
of the palm oil system as the by-products provide the parasitic energy
demand of the palm oil biodiesel system. The paper also highlighted a
short fall in the analysis of biofuel systems in the developed world. Of
the 4 tonnes of rape seed produced, 1.2 tonnes is converted to biodiesel
while 2.8 tonnes is converted to rape cake23. A further paper by the same
Fig. 2 Gross and net energy balance of selected biofuel systems. Adapted from9.
Table 2 Typical values for greenhouse gas savings from
the EU renewable energy directive. Adapted from10.
Biofuel system % savings in greenhouse gas emissions as compared to fossil fuel replaced
Wheat ethanol 32
Rape seed biodiesel 45
Sunflower biodiesel 58
Sugarcane ethanol 61
Palm oil biodiesel 62
Biogas from MSW 80
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REVIEW What will fuel transport systems of the future?
NOVEMBER 2011 | VOLUME 14 | NUMBER 11522
authors24 found that allocation by energy content attributes almost half
the greenhouse gas emissions to rape cake (a co-product). Rape cake
substitutes for importation of soybean from South America and thus
saves on emissions through deforestation and/or ploughing of grass
lands. They found that the system could be sustainable when produced
glycerol is used as a source of heat, and rape straw pellets are used in lieu
of peat (an environmentally damaging indigenous fuel source in Ireland).
Bioresources of first generation biofuels in Ireland Murphy and Power25 highlighted the quantity of land required to meet
the 2010 5.75 % biofuel target for Ireland. The fuel required equated to
11.3 PJ/a or 538 million L/a of ethanol or 323 million L/a of biodiesel.
The land take is excessive. With reference to Table 3, rape seed, which
is a one in four year rotational crop, requires 280 % of the arable land
involved in a rape seed rotation to meet the 5.75 % biofuel target. This
is not possible.
Second generation biofuelsSecond generation biofuels are derived from lignocellulosic feedstocks.
These feedstocks do not (directly) compete with food production but may
compete for resources such as water and land. Thus indirectly there is
potential for conflict with food if lignocellulosic crops (such as Miscanthus)
are grown on arable land. The beneficial use of whole crop (straw and
cereal) for biofuel production has a drawback in that carbon that may
have been ploughed back in (in the form of straw) is now not available.
This can lead to carbon depletion of the soil. Care must be taken that
carbon is recycled to the soil where lignocellulosic biomass is produced. In
the long term it must also be noted that fertilizer is dependent on fossil
fuel, and as fossil fuels deplete, fertilizer will become very expensive.
Lignocellulosic biomass typically comprises 35 – 50 % cellulose,
15 – 25 % hemicellulose, 15 – 30 % lignin and small amounts of extractive
substances and ash26. Bio-refineries convert lignocellulosic biomass into
biofuels and smaller quantities of high value products (e.g., chemicals)27.
Two particular issues require caution with regard to assuming that second
generation biofuels are superior to first generation biofuels, namely: the
feedstock and the process. Second generation biofuels are not always
free or cheap. In 2006 – 2007 grain prices were of the order of �110/t in
Ireland28. Straw, for example, is a second generation feedstock with a yield
of ca. 5 t/ha/a (compared to ca. 8.5 t/ha/a of wheat grain)28. Straw requires
collection, baling, and transport and has a minimum cost of ca. �65/t29. In
Denmark straw is used in CHP facilities, which drives up the price further29.
Straw is voluminous and bulky and as such has high transport costs for the
high distances associated with a commercial ethanol facility. It produces
37 % of the ethanol produced by grain per unit mass (140 L/t versus
372 L/t)30 and as such should be at a maximum 37 % the price of grain.
The process for production of straw ethanol requires a pre-treatment step
before the first generation technology30. This is typically a steam explosion
step which drives up the capital and operating costs.
Hydrogen Hydrogen is seen as a clean, abundant energy source with water vapor
as the only emission in combustion. The merits of hydrogen are based
on the fact that its energy content per unit mass is very high. The
demerit of hydrogen is that its energy value per unit volume is low. If
we consider diesel has an energy value of ca. 37 MJ/L then 1 L of diesel
has an energy content similar to 1000 L of methane and 3000 L of
hydrogen (Table 4). Hydrogen tends to be bound in compounds such as
water or in hydrocarbons such as gas. To be used as an energy source
it has to be separated from carbon in gas or oxygen in water. Typically
hydrogen is produced using two methods.
Steam reforming of natural gasApproximately 95 % of hydrogen used in the United States is
generated from natural gas31. Steam is used to reduce methane to
hydrogen and carbon dioxide. The energy demand is of the order of
20 to 30 %32. Carbon dioxide may be removed through pressure swing
adsorption and ideally carbon should be captured and stored. Hydrogen
from steam reforming will always be more expensive than natural gas.
Water electrolysisFor renewable hydrogen, renewable electricity must be sourced. The energy
efficiency of commercial electrolyzers is ca. 74 %33. This value refers only
to the efficiency with which electrical energy is converted into the chemical
energy of hydrogen. Distribution losses of 4 – 8 % must be added 34.
Table 3 Land required to meet the 2010 biofuels target
in Ireland. Adapted from25.
Biofuel Land take (kha/a)
% of agricultural land
% of arable land (9 % of agricultural land is arable)
Biodiesel Rape seed 279.1 6.3 70
Ethanol Wheat 172.3 3.9 43
Ethanol Sugar beet 107.1 2.4 26
Table 4 Comparison of hydrogen and methane as
sources of transport fuel.
Hydrogen Methane
Energy value 142 MJ/kg 55.6 MJ/kg
Molecular weight 2.016 16.042
Density 0.085 kg/mn3 0.677 kg/mn
3
Energy value 12.1 MJ/mn3 37.6 MJ/mn
3
Compression 700 bar 220 bar
Energy per unit compressed storage 8.47 MJ/L 8.27 MJ/L
Energy to compress 13 % 3.3 %
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Hydrogen versus natural gasWhy convert methane (natural gas) to hydrogen to use as a transport
fuel? The natural gas system in Ireland is extensive, is interconnected
to the European gas network, and at least 40 % of the population have
access to natural gas in their homes. To construct a similar hydrogen
distribution system would entail a massive infrastructural project
over many years35. Similarly, conversion of natural gas to hydrogen
requires significant infrastructural investment and is energy intensive
and expensive. Compressed natural gas is a mature technology; there
are 12 million natural gas vehicles (NGVs) in the world. Methane
is an excellent fuel in terms of local air quality and greenhouse gas
emissions. Studies suggest a reduction of 18 – 38 % and 2 – 21 % for
petrol and diesel, respectively36-38.
Hydrogen must be compressed for transport fuel use. The
current standard is compression to 700 bar. This requires 13 % of
the energy content of the gas39. In comparison, compressed natural
gas (200 – 220 bar) requires of the order of 3.3 % of the energy of
the gas25. At 700 bar the volumetric energy content of compressed
hydrogen is of a similar order to CNG at 220 bar. Safety is a key
concern as 700 bar is a very high pressure.
Efficiency of hydrogen productionHydrogen produced at a power plant or wind farm must be compressed
and transported. Losses between production and application are in
the range of 39 to 49 % for steam reforming (20 – 30 % in steam
reforming, 6 % loss in pipelines, 13 % in compression) and 49 – 53 % for
electrolysis (26 % in electrolyzing, 4 – 8 % loss in grid transmission, 6 %
loss in pipelines, 13 % in compression)40. According to Bossel41 for each
100 kWeh of electricity, the net energy used by an EV will be 69 kWeh,
while that of a fuel cell vehicle operating on hydrogen will be 23 kWeh.
Biogas and biomethaneBiogas or biomethane can be produced from a range of feed stocks such
as organic waste materials (Fig. 3a) and crops including those not used
directly for human consumption. Marginal land and land unsuitable for
food production can be used. The feedstock is digested in a sealed vessel.
The produced biogas is scrubbed and upgraded to 97 % plus methane,
which may be discharged to the gas grid (Fig. 3b) or injected directly
into a NGV vehicle25 (Fig. 3c). Table 2 highlights the sustainability of
compressed biomethane from municipal solid waste (MSW). Singh and
co-workers42 highlighted that waste resources can typically allow for
2 % of the energy in transport through digestion of slaughter waste,
slurries, and MSW. These are all highly sustainable biofuel systems. To
achieve more than 2 % of the transport fuel market, biomethane must
Table 5 Energy production from crop digestion. Adapted
from6.
Maize Fodder beet Grass
Methane yield m3/ha 5748 6624 4303
GJ/ha 217 250 163
Process energy demand for
digestion GJ/ha
33 38 24
Energy requirement in cropping
GJ/ha
17 20 17
Total energy requirement GJ/ha 50 58 41
Net energy yield GJ/ha 167 192 122
Output (GJ/ha) Input (Total
Energy)
4.3 4.3 4.0
Fig. 3 (a) Facility producing biomethane from waste food in Austria. (b) Upgrading of biogas and injection of biomethane into the natural gas grid (yellow valve centre of picture) in Austria. (c) Injecting biomethane into a bus in Linkoping, Sweden.
(a)
(b)
(c)
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11524
be produced from agricultural crops. With reference to Table 5 (and
comparison to Fig. 2) it may be noted that the energy balances are far
superior to first generation liquid biofuels. A simple calculation highlights
the potential of this technology. Allowing for an average net energy yield
of 120 GJ per hectare per year produced on 20 % of all arable and pasture
land (7.6 × 106 km2 or 7.6 × 108 ha) the potential production is 91.2 EJ;
this is almost equivalent to the world’s TFC in transport (95 EJ) in 20086.
Algae are considered to be the holy grail of biofuels. The energy
balance (and associated cost) is significantly affected by the dilute
nature of micro algae and the requirement to dry the algae to allow
esterification of the lipids. It is suggested that biomethane is preferable
to liquid biofuel generated from micro algae as the process does not
require drying43,44. Marine algae (or macro algae) may be very suited
to multi-feedstock anaerobic digesters in coastal areas. Biomethane
may be the optimal vector for energy from algae.
ConclusionsThis paper, in its brevity, can not deal with every aspect of renewable
transport energy but has the ambition of assessing the big questions.
Electrification of all transport is unlikely due to the scale of energy in
transport but should be a big part of the solution, as it allows for storage
of variable night time electricity. First generation liquid biofuels do not
optimize energy return per unit of land in the form of transport biofuel, will
struggle with sustainability issues, and will only ever account for less than
10 % of the energy in transport. Second generation liquid biofuels need a
cheap abundant source of lignocellulosic feedstock. EVs are more efficient
than hydrogen fuelled vehicles when the hydrogen is sourced from
electrolysis. Methane is always cheaper than hydrogen and biomethane
has a superior energy balance to first generation liquid biofuels. Transport
fuels of the future will require numerous sources; there is no silver bullet.
Detailed research in biofuel materials and technologies are required to
optimize the resources of all renewable transport systems.
AcknowledgementsThe ideas in this broad paper were developed based on interaction
with good colleagues and by the funders of research. Particular thanks
on this paper are due to Brian O‘ Gallachoir and Clare Dunne. Thanks
to the funders, including: the Environmental Protection Agency; The
Department of Agriculture, Food and Fisheries; The Higher Education
Authority; Science Foundation Ireland; and Bord Gais Eireann.
REFERENCES
1. International Energy Agency (IEA), Key World Energy Statistics 2010, SORE GRAPH, France, 2010.
2. ENERS Energy Concept, Biofuels Platform 2009, Lausanne, Switzerland.
3. Koh, L. P., and Ghazoul J., Biological Conservation (2008) 141, 2450.
4. Rosegrant, M. W., Biofuels and Grain Prices: Impacts and policy Responses, Testimony for the U.S. Senate Committee on Homeland Security and Governmental Affairs, USA., 2008.
5. Asian Development Bank, Soaring Food Prices: Response to the Crisis, Publication Stock No. 041908, the Philippines, 2008.
6. Murphy, J., et al., Biogas from Crop Digestion, IEA Bioenergy Task 37 – Energy from Biogas, 2011
7. World Bank, World Development Indicators 2011, USA. 2011.
8. KTBL, Faustzahlen für die Landwirtschaft, Kuratorium für Technik und Bauwesen in der Landwirtschaft e.V. (KTBL), Germany, (2005), 1095 S.
9. Smyth, B., et al., J Clean Prod (2010) 18, 1671.
10 Official Journal of the European Union. Directive 2009/28/EC of the European Parliament and of the Council of 23 April on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC. 5 June 2009.
11. Clancy, M., et al., Energy Forecasts for Ireland to 2020, Sustainable Energy Association of Ireland, 2010.
12. Howley, M., et al., Energy in transport 2009 report, Sustainable Energy Association of Ireland, 2010.
13. Foley, A., et al., Electric vehicles and energy storage: a case study on Ireland. In IEEE International Vehicle Power and Propulsion Conference. Institute of Electrical and Electronics Engineers, Dearborn, Michigan, (2009).
14. Foley, A., et al., Electric vehicles and displaced gaseous emissions. In IEEE Vehicle
Power and Propulsion Conference pp.1-6, doi:10.1109/VPPC.2010.5729228 (2010)
15. Bai, F. W., et al., Biotechnology Advances (2008) 26, 89.
16. Cardona, C. A., and Sánchez, Ó. J., Bioresource Technol (2007) 98, 2415.
17. Verma, G., et al., Bioresource Technol (2000) 72, 261.
18. Murphy, J. D., and Power, N., Fuel (2008) 87, 1799.
19. Srivastava, A., and Prasad, R., Renew Sust Energ Rev (2000) 4, 111.
20. Sharma, Y. C., et al., Fuel (2008) 87, 2355.
21. Thamsiriroj, T., and Murphy, J. D., Fuel (2010) 89, 3579.
22. Nicholas, E. K., et al., Biofuel Bioprod Bior (2010) 4, 310.
23. Thamsiriroj, T., and Murphy J. D., Appl Energ (2009) 86, 595.
24. Thamsiriroj, T., and Murphy, J. D., Energy Fuels (2010) 24, 1720.
25. Murphy J. D., and Power, N., Biomass Bioenerg (2009) 33, 504.
26. Kamm, B., et al., Lignocellulose-Based Chemical Products and Product Family Trees, In Biorefineries - Industrial Processes and Products: Status Quo and future Directions, Kamm, B., et al., (eds.), Wiley-VCH Verlag GmbH & Co. KGaA, Germany (2006) 2, 97.
27. International Energy Agency, From 1st- to 2nd- Generation Biofuel Technologies:
An Overview of Current Industry and RD&D Activities, OECD/IEA, 2008.
28. Power, N., et al., Renew Energ (2008) 33, 1444.
29. Smyth, B. Optimal biomass technology for the production of heat and power, MEngSc Thesis, University College Cork, Ireland, 2007.
30. Ballesteros, I., et al., Appl Biochem Biotech (2006) 129-132, 496.
31. U.S. Department of Energy Hydrogen Program, Hydrogen Production, 2006.
32. Mueller-Langer F., et al., Int J Hydrogen Energ (2007) 32, 3797.
33. Hammerschlag, R., and Mazza, P., Energ Policy (2005) 33, 2039.
34. Belati, E. A., and da Costa, G. R. M., Int J Elec Power (2008) 30, 291.
35. Thamsiriroj, T., et al., Renew Sust Energ Rev (2011) doi:10.1016/j.rser.2011.07088.
36. Hekkert, M.P., et al., Energ Policy (2005) 33, 579.
37. Engerer, H., and Horn, M., Energ Policy (2010) 38, 1017.
38. EUCAR, CONCAWE, JRC. Well-to-wheels analysis of future automotive fuels and
powertrains in the European context, Well-to-Wheels Report Version 2b. May 2006.
39. Jensen, J. O., et al., J Alloy Compsd (2007) 446-447, 723.
40. Page S., and Krumdieck, S., Energ Policy (2009) 37, 3325.
41. Bossel, U., P IEEE (2006) 94, 1826.
42. Singh, A., et al., Renew Sust Energ Rev (2010) 14, 277.
43. Singh, A., et al., Bioresource Technol (2011) 102, 10.
44. Singh, A., et al., Bioresource Technol (2011) 102, 26.
MT1411p518_525.indd 524 31/10/2011 14:24:58
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MT1411p518_525.indd 525 31/10/2011 14:24:58
ISSN:1369 7021 © Elsevier Ltd 2011NOVEMBER 2011 | VOLUME 14 | NUMBER 11526
The basic concept of thermoelectric power generation is rather
simple; when a temperature difference exists across a material
a proportional voltage is generated between opposing ends
of the material, which can be connected to a load to provide
electrical power (Fig. 1a1). Because the charge carriers are directly
driven by the flow of heat through the material, thermoelectric
generators have a distinct advantage over other heat engines by
operating without moving parts, thus providing a device that is
robust and requires no maintenance. While the heat in a typical
generator is provided by burning a fuel, or through radioactive
decay, the possibility of renewable sources of heat such as energy
harvesting from body heat2 or waste heat recovery from industry
or automobiles has renewed interest in thermoelectrics to target
energy sustainability3.
A thermoelectric material’s potential to convert heat into electricity
is quantified by the thermoelectric figure of merit, zT. To date, the
widely believed peak zT for single phase PbTe-based material, which
has been successfully used for several NASA space missions since the
1960s, has been ~0.8. Recent studies including precise compositional
control and modern characterization have revealed that maximum zT
The opportunity to use solid-state thermoelectrics for waste heat recovery has reinvigorated the field of thermoelectrics in tackling the challenges of energy sustainability. While thermoelectric generators have decades of proven reliability in space, from the 1960s to the present, terrestrial uses have so far been limited to niche applications on Earth because of a relatively low material efficiency. Lead telluride alloys were some of the first materials investigated and commercialized for generators but their full potential for thermoelectrics has only recently been revealed to be far greater than commonly believed. By reviewing some of the past and present successes of PbTe as a thermoelectric material we identify the issues for achieving maximum performance and successful band structure engineering strategies for further improvements that can be applied to other thermoelectric materials systems.
Aaron D. LaLonde, Yanzhong Pei, Heng Wang, and G. Jeffrey Snyder*
California Institute of Technology, Materials Science, 1200 East California Boulevard, Pasadena CA 91125, USA
*E-mail: [email protected]
Lead telluride alloy thermoelectrics
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11 527
values of ~1.4 are, in fact, intrinsic to this material for both n- and
p-type materials. Further enhancement of the figure of merit has
been achieved in alloys where zT reaches a value approaching ~1.8
for homogeneous PbTe-PbSe materials. These recent findings from
PbTe-based alloys have shed new light on this classic thermoelectric
material and provide encouragement for the further development of
thermoelectric technologies on Earth.
Many types of PbTe and related compounds, alloys, and composites
have been studied as thermoelectrics for many years. There are several
comprehensive reviews of the older results that can be found in
reference 4, while recent developments in PbTe based nanostructured
composites are described in references 5 and 6. The focus of this review
is to highlight the potential of PbTe utilizing only small concentrations
of dopants (assuming the bands remain rigid) or alloying that produces
only minor perturbations to tune the band structure.
The history of PbTeDuring the Cold War and the Space Race of the middle part of the 20th
century it was with a sense of pride that President Eisenhower of the
United States was presented with the “world’s first atomic battery”
in the oval office of the White House7,8 (Fig. 1b9). This radioisotope
thermoelectric generator (RTG) contained simple alloys of PbTe for
both the n- and p-type elements. NASA used this design for its first
RTG powered spacecraft, the Transit 4A, and modified designs and
materials based on PbTe in the Apollo missions and the 1975 launch
of the Viking 2 mission to Mars10. With the advent of the Voyager
missions, NASA switched to Si-Ge alloys which were of more interest
to the scientific community following the 1960s. Although the
excitment of the Space Race has subsided, there is new enthusiasm in
NASA’s forthcoming mission; the Mars Science Laboratory (MSL), which
will probe the possibility of life on Mars. Although it has been nearly
35 years, NASA will return to PbTe based alloys as the thermoelectric
material of choice to provide power to the most sophisticated Mars
rover to date10 ( Fig. 1c11).
The early promotion of PbTe in thermoelectric generators was
made by Soviet physicist A. F. Ioffe, reportedly as early as 192812,
and numerous thorough investigations were performed at the
Semiconductor Institute in Leningrad. While the Soviets initiated the
Fig. 1 Thermoelectric materials and electric power generators. (a) Schematic of a thermoelectric device consisting of both n- and p-type thermoelectric materials1.
(b) President Eisenhower (far left) in the Oval Office of the White House being presented the “world’s first atomic battery” by officials from the Atomic Energy
Commission. The radioisotope powered thermoelectric generator (left most object on the desk) is powering a fan next to it9. Image courtesy of the US Department
of Energy. (c) A depiction of the Mars Science Lab rover Curiosity showing the thermoelectric power source (right end of the rover) containing PbTe-based
materials11. Image courtesy of NASA.
(a)(b)
(c)
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lead telluride research, the 3M Corporation was actively pursuing
similar work in the United States. Scientific reports on these materials
started appearing in the literature in the early 1950s and 1960s from
both the United States and the Soviet Union12-15, during which time
a vast amount of experimental data was gathered on PbTe and similar
alloys for thermoelectric applications.
Evolution of high temperature thermal conductivity measurementsThe experimental data that are most frequently acquired to
characterize the performance of a thermoelectric material are
the Seebeck coefficient (S), the electrical resistivity (ρ), and the
total thermal conductivity (κ). These three properties, along with
temperature (T), constitute the dimensionless thermoelectric figure
of merit, zT = (S2T)/(ρκ). The thermal conductivity of a material is
given by κ = κE + κL, where κE is the electronic component and κL
is the lattice component. The electronic component is related to the
electrical resistivity and is calculated by the Wiedemann-Franz law,
κE = LT/ρ, where L is the Lorenz number. The lattice component can be
estimated by subtracting the electronic component, calculated using
the measured electrical resistivity, from the measured total thermal
conductivity.
At the time of the initial interest in PbTe the measurement of the
electrical resistivity and the Seebeck coefficient could be performed
accurately in research laboratories throughout the world. On the
other hand, the measurement of the thermal conductivity at high
temperatures was known to be a very difficult measurement to
perform accurately16. It is likely that as a result of this difficulty,
Fritts at 3M used a more confidently measured room temperature
thermal conductivity value, in combination with the resistivity and
Seebeck measurements, to estimate κ and zT at the temperatures of
interest for PbTe thermoelectric materials15. The room temperature
κL value was treated as a constant (Fig. 2) and combined with the
reliable electrical resistivity measurements at various temperatures
to determine κE and thus determine a value of κ without actual
measurements of the high temperature thermal conductivity.
Fritts himself knew that this approach would result in a significant
overestimate of κ and therefore lead to underestimated values
of zT. By the time an acceptable high temperature measurement
method, the flash diffusivity technique, became available in the
USA in the early 1960s16, the focus of thermoelectric research in
the United States had shifted away from PbTe. The research in the
Soviet Union continued to pursue the understanding of the physics
of PbTe, however, the work did not utilize the newly available flash
diffusivity technique17-21. These initial results for the high temperature
thermal properties of PbTe, which were more recently used for
thermal conductivity calculations22, give values for κ that are up to
~30 % higher than those measured today using the flash diffusivity
technique. Despite extensive research comparing results to PbTe, the
underestimated figure of merit values by Fritts have subsisted within
the field until it was revealed very recently that optimally doped PbTe
is, in fact, almost two times better than commonly believed23,24.
By using the now commonplace flash thermal diffusivity
measurement technique the thermal conductivity of a material can
be accurately determined if the specific heat capacity and density are
known. Because the largest source of error with this method is the
measurement of specific heat capacity, published values measured
by drop-calorimetry are likely to be the most accurate. The recent
findings shown i n Fig. 3 reveal that, in fact, the zT value of the historic
n-type PbTe material is ~1.4 for several sample compositions over
a temperature range of 150 degrees (700 – 850 K)24. The electronic
transport properties (ρ and S) were found to be in excellent agreement
with numerous previously reported studies on the same compositions,
allowing the increase in zT to be almost entirely attributed to the
difference in thermal conductivity determined for the material. These
results have revealed the inherent properties of PbTe and provide
new motivation to continue research and development of this highly
functional thermoelectric material.
The low thermal conductivity intrinsic to PbTe may be due to
many factors. In the Umklapp dominated phonon scattering regime
above 300 K, this can be traced to the low speed of sound due to
Fig. 2 Comparison of the thermal conductivity recently reported for PbTe,
determined by laser flash diffusivity measurement, and thermal conductivity
data reported by Fritts in 1960. The Fritts data is from n-type 0.055 %
PbI2 and p-type 1 % Na15. The recently reported data is from n-type
PbTe1-xIx (x = 0.0012)24 and p-type NaxPb1-xTe (x=0.01)23. Also shown is
the constant κL for both n- and p-type PbTe at high temperatures; a value that
Fritts a ssumed.
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the harmonic lattice vibrations and the relatively high anharmonicity,
which is quantified by the Grüneisen parameter. Low speed of sound is
generally found in materials with soft bonds from large, heavy atoms,
which also correlates with a high coordination number and Grüneisen
parameter. The large Grüneisen parameter may also be due to unusual
structural features relating to the near ferroelectric instability in PbTe
and related materials25-28.
Maximizing PbTe performance via doping optimizationA similar oversight in thermal characterization also exists for
p-type PbTe doped with sodium, significantly contributing to an
underestimated zT value23. The value found recently is nearly double
the value of Fritts that is commonly reported, showing p-type
PbTe with a maximum zT value of 0.7 , (Fig. 3). However, in this
case, the accurately measured thermal conductivity is only partially
responsible for the large discrepancy in the figure of merit. Additionally
contributing to the larger zT value is an improvement of the electronic
transport properties, which are attributed to two-band conduction
behavior in heavily doped p-type PbTe.
The two-band model has been proposed because of significant
deviations of the transport data compared to that expected from
a single band model12,29-34. As schematically shown in (Fig. 4a), at
lower carrier concentrations the transport properties are dominated
by a light mass band (with extrema at the L point of Brillouin zone)
and as the carrier density increases a heavy effective mass band
(with extrema along the Σ line of the Brillouin zone) plays a more
significant role contributing to the carrier transport. Additionally, the
position of the bands is found to be temperature dependent so that
the importance of the Σ band to the transport properties increases
with temperature12,29,35-37. As the contribution from each band
is dependent on both carrier concentration and temperature, it is
intuitive that there will be optimized values that will result in the best
thermoelectric performance. Correspondingly, it is possible to “tune”
carrier concentration (and the band structure as described below) for
higher zT at the desired temperature.
The zT for each individual band (as if the other valence band did
not exist), as well as for the coexistence of both bands as a function
of carrier concentration is show n in Fig. 4b, where it is clear that the
performance of the individual bands is optimized in different regions.
The early material development per formed by Fritts for NASA is
now known to have been too lightly doped15,23,38. It can be seen
quantitative ly in Fig. 4b that at 750 K the carrier concentration in
Fritts’ (and similar material from the USA38,39) is only about half of
the optimally required amount for maximizing zT. It can also be s een
in Fig. 4b that if the transport properties were to be the result of the
Σ band alone (no L band) that the zT would have a maximum value of
only 1.1. However, when the Σ band is supplemented by the light mass
carriers of the L band, a maximum zT of ~1.4 can be realized at 750 K
(Fig. 3b) as long as it is properly doped (~2 × 1020 cm-3).
High performance PbTe analogsEarly studies, particularly from the 1950s to the 1970s have
surveyed many IV-VI compounds (such as PbTe, PbSe, GeTe, etc.) for
thermoelectric applications4, which have similar atomic and electronic
structures. PbTe was considered the best candidate and thus is the
Fig. 3 Overview of zT values for materials reviewed here and those in reference 1, including the n- and p-type values for PbTe reported by Fritts in 1960 for (a)
p-type and (b) n-type thermoelectric materials. The n-type PbTe1-xIx (x = 0.0012) and p-type NaxPb1-xTe (x = 0.01)23,24 are shown to have significantly higher zT
values than previously believed. Additionally, it can be seen that p-type PbSe (carrier concentration 3 × 1019 by Na doping43) is a very promising alternative to PbTe
and that Na doped alloys of PbTe1-xSex (x = 0.15)50 show extraordinary perfor mance.
(a) (b)
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11530
most studied of the lead salts4,12,13,41. As a result of the growing
expense of Te there is a renewed interest in materials that do not
contain Te. Among the alternate materials is PbSe, which had been
thought to have much lower performance compared with PbTe12,20
due to its lower band gap (actually, only at low temperatures) and
the general trend that isostructural compounds composed of lighter
elements have higher κL. However, recent theoretical work and
experimental results have reported that this material may in fact
outperform PbTe at high temperatures42, 43.
When PbSe is made p-type by doping with Na it is found that
the electronic transport properties can be explained by the same
type of complex band structure as that found in Na-doped PbTe43
and this leads to substan tial zT (Fig. 3). The maximum zT achieved
in samples with hole concentrations of 3 × 1019 cm-3 is near 1.
However, a significant contribution from the presence of the heavy
hole band and a resulting peak zT > 1.2 is realized in samples having
hole concentrations even greater than 1 × 1020 cm-3. In heavily doped
(1 × 1020 cm-3) PbSe samples the large band gap at high temperatures
inhibits thermally activated minority carriers that degrade charge
carrier transport, the bipolar effect, and consequently the zT of these
samples does not appear to be approaching a maximum value at
850 K.
While the increase in Seebeck accounts considerably for the zT
values observed, the thermal conductivity of PbSe is much lower than
expected as compared to PbTe, which has a larger molar mass, and is
partially responsible for the high zT observed. The low lattice thermal
conductivity can be attributed to the larger Grüneisen parameter12
corresponding to the stronger anharmonic nature of the lattice
vibrations25.
High zT is well known in p-type AgSbTe2 and TAGS (GeTe alloy
with 15 % AgSbTe2)25,40 and is likely due to the structure, which is
similar to PbTe, along with the same reasons that make PbTe and PbSe
good thermoelectric materials, although additional factors (e.g., due to
alloying) are also present.
Band structure engineering by alloyingIncorporating additional elements into PbTe opens new opportunities
for tuning the electronic transport properties through band
structure engineering as well as providing a route to κL reductions.
The reduction of lattice thermal conductivity by alloying (such as
PbTe1-xSex) due to the scattering of phonons by point defects is
well known and was promoted by A. F. Ioffe well before 195713.
The substitution of Te with Se reduces κL by ~35 % at 300 K for
x = 0.15, but greater amounts of Se present in the material result in
a detrimental net effect due to the simultaneous reduction of the
carrier mobility caused by scattering of carriers by the additional
Se atoms.
Concurrent to the rather simple effects on κL due to the addition
of Se in PbTe, a more complicated impact on the electronic band
structure is also observed, which provides an additional avenue for
Fig. 4 (a) Schematic of energy bands in PbTe and how the bands move as temperature increases. The shaded areas schematically represent the hole population of
each band. The orange line indicates the movement of the L band as temperature increases. (b) The calculated zT value as a function of carrier concentration for
p-PbTe comparing the result of transport from the L or Σ band alone (as if the other band did not exist). It is seen that when the interaction between the two bands
is modeled, "Σ + L", the result is a significant increase of peak zT, which can only be realized at carrier concentrations twice that reported by Fritts.
(a) (b)
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developing high performance thermoelectric materials based on band
structure engineering.
It is straightforward to show that if it can be assumed that carrier
concentration can be tuned to optimize zT, this optimized zT value
depends on the thermoelectric quality factor, B, determined by the
lattice thermal conductivity, κL, and the electronic band structure44-47.
B ∝ μ—N—v
κL—Mb—
*3/
–2
– (1)
Here, μ is the carrier mobility, κL is the lattice thermal conductivity,
and mb* is the density-of-states effective mass of a single carrier
pocket. While μ decreases rapidly with mb*, it is clear that large
valley degeneracy (NV) is good for thermoelectric materials. Valley
degeneracy arises when multiple bands have the same energy (are
degenerate) at the band extrema (orbital degeneracy), or when there
are multiple degenerate carrier pockets in the Brillouin zone due to the
symmetry of the crystal. High symmetry crystals can have very high
valley degeneracy when the band extrema are located at low symmetry
points in the Brillouin zone48. The known good thermoelectric materials
such as Si1-xGex, Bi2Te3, and CoSb3 have NV of 6 or less. In p-type
PbTe the valence band maxima occur at the L point in the Brillouin
zone where NV is 4, while the Σ band has an exceptionally large NV
of 1212,41,49,50, which certainly contributes to its good electronic
properties and high zT at high carrier concentration. One can think
of valley degeneracy as leading to multiple (NV) pathways for charge
carriers to participate in electronic transport without altering the
Seebeck coefficient (determined by the Fermi level).
One way to increase NV, and therefore zT, is to converge different
bands which are not required to be degenerate because of orbital
or Brillouin zone symmetry. Such bands are effectively converged
when their band extrema are within a few kBT of each other (kB is
the Boltzmann constant). This concept formed the basis of early
carrier pocket engineering attempts to increase zT in low dimensional
thermoelectric materials51.
Perhaps the most dramatic demonstration of band structure
engineering to increase NV, to date, has been in p-type PbTe1-xSex
alloys where the alloying allows small, controlled manipulation of the
band energies. The relative energies of the L and Σ bands in PbTe are
temperature dependent and as the temperature increases the band
energies converge to produce a combined valley degeneracy of 16. As
Se is added to PbTe the energy difference between the L and Σ bands
increases, raises the temperature at which the band convergence
occurs, and makes the convergence effect more noticeable as the peak
zT approaches ~1.8 in bulk PbTe1-xSex.
Alloying can also be used to introduce resonant electronic states,
such as Tl in PbTe52. Additional electronic states brought by the
incorporation of resonant impurities to increase NV will increase
thermoelectric properties in the same manner as discussed above.
Resonant states can also introduce resonant scattering, which is
proposed to improve thermoelectric performance at low temperature
by a different means53.
Band convergence by alloying has a distinct advantage over
nanowires or superlattices because the high symmetry (e.g., cubic in
PbTe), and therefore high inherent NV, for each band is maintained
while low dimensional structures break this symmetry. In addition,
small concentrations of alloying elements can be used to enable fine,
precise adjustments in the band energies.
Potential for future band structure engineeringThe effectiveness of alloying to induce band convergence has only
begun to be fully appreciated in PbTe and a few other systems54,55.
The quality factor can be further developed to encourage other
strategies for band structure engineering, or even the search for entirely
new thermoelectric materials. Since virtually all good thermoelectric
materials have carrier mobility limited by acoustic phonon scattering,
the deformation potential theory15,56 can be used to show that
B ∝ T m—
I*Nv—
Ξ2– Cκ—l
L— (2)
where Cl is a combination of elastic constants57 that will be correlated
with κL, mI* is the inertial effective mass along the conduction
direction, and Ξ is the deformation potential coefficient that describes
the change in energy of the electronic band with elastic deformation.
This formula suggests that low effective masses and low deformation
should be targeted in future band structure engineering58,59. In
addition, it is also desirable to engineer increases in band gap, as long
as it does not include detrimental effects to NV, mI*,or Ξ, as it enables
higher temperature operation without the influence of minority
carriers16,45.
Ultimately, however, band structure engineering will be used to
improve the average zT (or more precisely, device zT1) for higher
overall thermoelectric efficiency rather than peak zT, which will lead to
somewhat different goals60,61.
Summary and outlook Simple binary lead telluride alloy thermoelectric materials have
demonstrated exceptional thermoelectric performances, with an
optimized peak zT of ~1.4, far exceeding the values commonly reported
since 1960. Two key aspects for high performance n- and p-type PbTe
are the use of modern thermal diffusivity measurements for thermal
conductivity characterization and optimal dopant concentration.
The large Grüneisen parameter and high valley degeneracy in p-type
materials contribute to the exceptional zT in PbTe and related IV-VI
semiconductors. These trends, understandable from the perspective
of simple semiconductor physics, can help guide other thermoelectric
material systems as well as the search for new thermoelectric
materials.
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11532
Additionally, the concept of band structure engineering to achieve
band convergence is demonstrated with the exceptional peak zT
of ~1.8 in PbTe1-xSex alloys. Such a small modification of the band
structure through alloying promises to be a fruitful route to tune other
band parameters as well. Alternatively, the more dramatic changes
brought about by resonant states may provide another mechanism
for increasing the number of converged bands and would expand the
possibilities for further improvements. A summary of the materials
reviewed in this paper and the corresponding zT values is shown in
Fig. 3, as compared to other thermoelectric materials.
While alloying has proven successful in reducing lattice thermal
conductivity, other methods such as nanostructuring6,62 should lead
to further improvements, particularly at low temperatures where
boundary scattering of phonons is most effective, raising the average
zT60. However, with such strategies, it will be important to consider the
effect on other parameters that determine the thermoelectric quality
factor to ensure a net increase in the performance of the material.
Small improvements in sample homogeneity, fine tuning of the doping
and alloy concentrations, microstructure and composite control, as
well as functionally grading63 could all combine to produce a truly
optimized thermoelectric material.
Even though PbTe is one of the oldest and most studied
thermoelectric materials for power generation, recent work has
demonstrated several new possibilities that can be explored to ensure
a bright future for the further development and use of PbTe-based
thermoelectric ma terials.
REFERENCES
1. Snyder, G., et al., Nat Mater (2008) 7, 105.
2. Snyder, G., The Electrochemical Society, Interface (2008) Fall, 54.
3. Bell, L.E., Science (2008) 321, 1457.
4. Wood, C., Rep Prog Phys (1988) 51, 459.
5. Sootsman, J. R., et al., Angew Chem Int Ed (2009) 48, 8616.
6. Kanatzidis, M. G., Chem Mater (2010) 22, 648.
7. Gamarekian, E., In: Washington Post (Jan. 17th 1959.) A1.
8. US Dept of Energy, “Atomic power in space, a history,” Available at:
http://www.fas.org/nuke/space/index.html.
9. US Dept of Energy, “Image id: Snap-3 President Eisenhower”.
10. Abelson, R. In: Thermoelectrics Handbook: Macro to Nano (CRC/Taylor & Francis, Boca Raton, 2006) Chap. 56.
11. Image courtesy of NASA, Available at: http://marsprogram.jpl.nasa.gov/ .
12. Ravich, Y., et al., Semiconducting Lead Chalcogenides, edited by Stil’bans, L. (Plenum Press, 1970).
13. Ioffe, A., Semiconductor Thermoelements and Thermoelectric Cooling (Infosearch, London, 1957).
14. Heikes, R., et al., In: Thermoelectricity: Science and Engineering, edited by Heikes, R. and Ure, R. (Interscience Publishers, New York, 1961) p. 405-442.
15. Fritts, R. In: Thermoelectric Materials and Devices, edited by Cadoff, I. and Miller, E. (Reinhold Pub. Corp., New York, 1960) p. 143-162.
16. Parker, W., et al., J Appl Phys (1961) 32, 1679.
17. Petrov, A., In: Thermoelectric properties of Semiconductors, edited by Kutasov, V. (Consultants Bureau, New York, 1964) p. 17.
18. Devyatkova, E. and Saakyan, V., Izvestiia Akademii Nauk SSSR (1967) 2, 14.
19. Efimova, B., et al., Sov Phys Semicond (1971) 4, 1653.
20. Alekseeva, G., et al., Semiconductors (1996) 30, 1125.
21. Alekseeva, G., et al., Fiz Tverd Tela (1981) 23, 2888.
22. Gelbstein, Y., et al., Proc 21st Int Conf Thermoelectrics (2002) 21, 9.
23. Pei, Y., et al., Energy Environ Sci (2011) 4, 2085.
24. LaLonde, A., et al., Energy Environ Sci (2011) 4, 2090.
25. Morelli, D.T., et al., Phys Rev Lett (2008) 101, 035901.
26. Bozin, E., et al., Science (2010) 330, 1660.
27. An, J., et al., Solid State Commun (2008) 148, 417.
28. Delaire, O., et al., Nat Mater (2011) 10, 614.
29. Andreev, A. and Radinov, V., Sov Phys Semicond (1967) 1, 145.
30. Chernik, I., et al., Sov Phys Semicond (1968) 2, 645.
31. Crocker, A. and Rogers, L., J Phys Colloques (1968) C4, 129.
32. Crocker, A. and Rogers, L., Br J Appl Phys (1967) 18, 563.
33. Airapetyants, S., et al., Sov Phys-Sol State (1966) 8, 1069.
34. Allgaier, R., J Appl Phys (1961) 32, 2185.
35. Gibson, A., Proc Phys Soc B (1952) 65, 378.
36. Saakyan, V. and Devyatkova, E., Sov Phys-Sol State (1966) 7, 2541.
37. Tsang, Y. and Cohen, M., Phys Rev B (1971) 3, 1254.
38. Kudman, I., J Mater Sci (1972) 7, 1027.
39. Kudman, I., Metall Trans (1971) 2, 163.
40. Skrabek, E. and Trimmer, D., CRC Handbook of Thermoelectrics (CRC Press, New York, 1995) Chap. 22.
41. Nimtz, G. and Schlicht, B., Springer Tracts In Modern Physics (1983) 98, 1.
42. Parker, D. and Singh, D., Phys Rev B (2010) 82, 035204.
43. Wang, H., et al., Adv Mat (2011) 23, 1366.
44. Chasmar, R. and Stratton, R., J Electronics and Control (1959) 7, 52.
45. Goldsmid, H., Thermoelectric Refrigeration (Plenum, 1964).
46. Slack, G. CRC Handbook of Thermoelectrics (CRC Press, Boca Raton, 1995) Chap. 34.
47. Mahan, G. Solid State Physics (Academic Press, 1998) pp. 81–157.
48. DiSalvo, F., Science (1999) 285, 703.
49. Sitter, H., et al., Phys Rev B (1977) 16, 680.
50. Pei, Y., et al., Nature (2011) 473, 66.
51. Rabina, O., et al., Appl Phys Lett (2001) 79, 81.
52. Heremans, J.P., et al., Science (2008) 321, 554.
53. Ravich, Y.I., CRC Handbook of Thermoelectrics (CRC Press, Boca Raton, 1995) Chap. 7.
54. Fedorov, M., European Conference on Thermoelectrics (2007) Odessa, Ukraine.
55. Lenoir, B., et al., Proc 15th Int Conf Thermoelectrics (1996), 15.
56. Bardeen, J. and Shockley, W., Phys Rev (1950) 80, 72.
57. Herring, C. and Vogt, E., Phys Rev (156) 101, 944.
58. Pei, Y., et al., (2011) submitted.
59. Wang, H., et al., (2011) submitted.
60. Pei, Y., et al., Energy Environ Sci (2011) 4, 3640.
61. Pei, Y., et al., Adv Mater (2011) accepted.
62. Minnich, A.J., et al., Energy Environ Sci (2009) 2, 466.
63. Pei, Y., et al., Adv Energy Mater (2011) 1, 291.
MT1411p526_533.indd 532 31/10/2011 14:27:55
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MT1411p526_533.indd 533 31/10/2011 14:27:56
ISSN:1369 7021 © Elsevier Ltd 2011NOVEMBER 2011 | VOLUME 14 | NUMBER 11534
The demand for clean, secure, and sustainable energy sources
has stimulated great interest in fuel cells: devices that convert
a chemical fuel directly to electricity. Among all types of fuel
cell, solid oxide fuel cells (SOFCs) represent the cleanest, most
efficient, and versatile energy conversion system1, offering the
prospect of efficient and cost effective utilization of hydrocarbon
fuels, coal gas, biomass, and other renewable fuels2,3. However,
SOFCs must be economically competitive to be commercially
viable. An effective approach to cost reduction is to drastically
reduce the operating temperature to 400 – 700 ºC, thereby
allowing the use of much less expensive materials in the
components4. Unfortunately, lowering the operating temperature
also lowers the fuel cell performance, as the electrode and
electrolyte materials become less conductive and less catalytically
active. Long term performance of SOFCs also degrades due to
poisoning of the cathode by chromium from interconnect layers5,6,
deactivation of the conventional anode by carbon deposition4-7
and poisoning by contaminants (e.g., sulfur) in the fuel gas8-12.
One of the grand challenges facing the development of a new
generation of low cost SOFCs is the creation of novel materials with
unique compositions, structures, morphologies, and architectures that
promote the fast transport of ionic and electronic defects, facilitate
rapid surface electrochemical reactions, and enhance the tolerance to
contaminants at low temperatures. In this article, we will highlight some
Solid oxide fuel cells (SOFCs) offer great prospects for the most efficient and cost-effective utilization of a wide variety of fuels. However, their commercialization hinges on the rational design of low cost materials with exceptional functionalities. This article highlights some recent progress in probing and mapping surface species and incipient phases relevant to electrode reactions using in situ Raman spectroscopy, synchrotron based x-ray analysis, and multi-scale modeling of charge and mass transport. The combination of in situ characterization and multi-scale modeling is imperative to unraveling the mechanisms of chemical and energy transformation: a vital step for the rational design of next generation SOFC materials.
Meilin Liua,b*, Matthew E. Lyncha, Kevin Blinna, Faisal M. Alamgira, and YongMan Choia,c
a Center for Innovative Fuel Cell and Battery Technologies, School of Materials Science and Engineering, Georgia Institute of Technology,
771 Ferst Drive, Atlanta, GA 30332-0245, USAb World Class University (WCU), UNIST, South Koreac Current address: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA
* E-mail: [email protected]
Rational SOFC material design: new advances and tools
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11 535
recent progress, the remaining challenges, and future perspectives in the
modeling, simulation, and in situ characterization of SOFC materials, in
order to unravel the mechanism of electrode processes and ultimately
achieve rational design of new materials and structures using a multi-
scale computational framework rigorously validated by experiments at
each scale.
Recent progress in materials developmentThe electrolyteOxygen ion (or vacancy), proton, and mixed ion conductors have been
used for SOFCs (Fig. 1)13. As is well known, the advantages of SOFCs
based on oxygen ion conductors include the formation of H2O and
CO2 on the fuel side of the cell, which facilitates the use of carbon
containing fuels through steam (H2O) and dry (CO2) reforming.
However, the reaction products dilute the fuel. Although many
candidate oxygen ion conductors have been studied14,15, the materials
that attract the most attention include doped zirconia, ceria, and
LaGaO3. For SOFCs based on proton conductors14, the H2O will form
on the cathode side, diluting the air, not the fuel. Direct utilization of
carbon-containing fuels is no longer possible with proton-conducting
electrolytes. One prominent group of proton conductors is the
BaZr0.1Ce0.7Y0.2O3-δ (BZCY) system16, representing a good compromise
between ionic conductivity and stability. It is also reported as a mixed
ion conductor, allowing transport of both proton and oxygen ions.
In particular, the BaZr0.1Ce0.7Y0.2-xYbxO3-δ (BZCYYb) system offers
the highest ionic conductivity in the intermediate temperature range
and the ionic transference number may be tailored to some degree2.
An ideal situation is to tailor the proton and oxygen ion transference
number of the mixed ion conductor, allowing CO2 to form on the
fuel side while allowing most of the H2O to form on the air side.
Proton conductors have attracted considerable attention because of
the low activation energy for proton conduction and thus high ionic
conductivity at low temperatures. This class of mixed proton and
oxygen ion conductors holds great potential for a new generation of
low temperature SOFCs2.
The air electrode (or cathode)Cathode polarization still contributes considerably to energy loss
in SOFC operation, more so at lower operating temperatures. The
search for materials and architectures that are more active for
oxygen reduction reactions (ORR) at lower temperatures has led
to many candidate cathode materials17, including doped SrFeO318,
Ba0.5Sr0.5Co0.8Fe0.2O3-δ composites19, and Sr0.5Sm0.5CoO3-δ with
a cone-shaped microstructure20. In particular, mixed ionic and
electronic conductors (MIECs) have attracted attention because
a high ambipolar conductivity may extend the active sites
beyond the triple-phase boundaries (TPBs), thus offering better
performance than a predominantly electronic conductor such as
La1-xSrxMn O3−δ (LSM). To date, however, LSM-based composites
(> 800 °C) and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) (< 750 °C) still remain
the most widely used cathodes for SOFC development; the adoption
of alternative cathode materials is hindered by their unproven
long-term stability and limited compatibility with electrolyte/other
cell components, especially at high temperatures required for cell
fabrication.
To counter this problem, many different types of catalytically active
cathode materials have been infiltrated into the scaffolds of electrolytes
(to form composite cathodes) followed by firing at a much lower
temperature to avoid reactions between the electrolyte and electrode
materials21. While it appears possible to make use of more active
cathode materials and to show reasonable performance in small button
cells using heavy coatings of Pt paste or mesh as a current collector,
several critical issues still remain: the long-term stability of the cathodes
is yet to be demonstrated due to degradation issues21 and the poor
conductivity for current collection could be much more severe in real
cells or stacks where the use of Pt is no longer practical.
One reliable and effective approach is to modify the surface of the
state-of-the-art cathode with a thin-film coating of a catalyst with higher
stability and catalytic activity toward ORR. One example is a cathode
consisting of a porous LSCF backbone and a thin coating of LSM22. The
LSM-infiltrated LSCF allows the use of the best properties of two different
materials: the excellent ambipolar conductivity of LSCF and the high
stability and catalytic activity of LSM. The catalyst coating could be a
porous layer of discrete particles or a dense, continuous film, as shown in
Figs. 2a and 2b, respectively. Nanoparticles of ionic conductors such as
Fig. 1 (a) Schematic of an SOFC single cell based on proton and/or oxide ion conductors and (b) a cross-sectional view (SEM micrograph) of an SOFC single cell showing the typical microstructures of a dense electrolyte and porous electrodes.
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SDC have been successfully deposited on LSCF surfaces by the infiltration
of aqueous nitrate solutions23. In contrast, dense films of LSM have
been prepared using non-aqueous solutions24, as shown in Figs. 2c-e.
The challenges lie in how to achieve a rational design of the desired
architecture and microstructure for each component, and how to fabricate
a cathode with a reduced polarization and enhanced stability at low cost.
It is not always completely clear how to correlate the electrochemical
performance quantitatively with the local structure, composition, and
morphology of surfaces and interfaces of a cathode. Most approaches
to electrode materials development tend to be very empirical in nature:
a qualitative idea is developed, an electrode is fabricated, and a test is
performed. The idea is considered a success if the performance meets
or exceeds expectations. Part of the reason for the continued role of
empiricism is the inability to establish the scientific basis for the rational
design of better cathodes with enhanced stability and activity for
oxygen reduction, which is a very hard problem. Nevertheless, rational
design provides important insights into how to achieve higher cathode
efficiencies through new architectures and new materials. Recent efforts
in this direction are discussed later in this paper.
The fuel electrode (or anode)Ni-YSZ cermet anodes are known to exhibit excellent performance in
clean hydrogen or reformed fuels; however, Ni metal is susceptible to
re-oxidation, carbon buildup (coking) in carbon-containing fuels, and
deactivation by fuel contaminants (e.g., sulfur).
To overcome these critical technical barriers to fuel flexibility,
a large number of “Ni-free” alternative anode materials have been
developed; one prominent group of which is the conducting metal oxides,
including La0.75Sr0.25Cr0.5Mn0.5O3-δ, (with a Ce0.8Gd0.2O2-δ interlayer)25,
Sr2Mg1−xMnxMoO6−δ (0 ≤ x ≤ 1)26, and doped (La,Sr)(Ti)O327,28. Indeed,
many alternative anode materials have shown much-improved tolerance
to coking and contaminant poisoning; however, they have limited
Fig. 2 A highly efficient cathode consists of an LSCF backbone having high ionic and electronic conductivity and (a) a porous layer of catalyst particles or (b) a dense
thin film of catalyst having high stability and catalytic activity toward O2 reduction (e.g., LSM), making effective use of the desirable properties of two different
materials. (c) An LSM film (40 to 50 nm thick) on an LSCF substrate, (d) LSM infiltrated into a porous LSCF cathode (TEM image) after operation at 750 °C at 0.8 V
for 900 hours, and (e) a closer view of the LSM coated LSCF grain in the LSM infiltrated LSCF cathode shown in (d). Parts b-e reprinted from22 with permission of the
Royal Society of Chemistry.
(a) (b)
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physical, chemical, and thermal compatibility with the YSZ electrolyte
during fabrication at high temperatures, which severely hinders their
applicability to actual fuel cell systems.
Fabrication by infiltration of alternative electrode materials into a
scaffold of electrolyte is perceived to offer advantages for improved
structural stability and better thermal expansion matching. However,
these claims are yet to be proven by experimental results.
Recently, a very different approach was adopted to achieve better
sulfur tolerance: replacing the oxygen ion conductor YSZ in a Ni-YSZ
cermet anode with a mixed-ion conductor like BZCYYb2; the -OH
groups produced on anode surfaces by dissociative adsorption of water
greatly facilitate oxidation of H2S to SO2 (thus removing sulfur from
anode surfaces) and in situ reformation of carbon-containing fuels
(thus minimizing coking) under typical SOFC operating conditions. This
Ni-BZCYYb cermet anode showed superior sulfur tolerance at 750 °C for
up to ~20 ppm H2S using a cell based on a dense BZCYYb electrolyte
and up to ~50 ppm H2S using a cell based on Sm-doped ceria (SDC)
dense electrolyte, suggesting that the critical pH2S/pH2 values are two
to three orders of magnitude higher than that for a conventional Ni-YSZ
cermet anode under similar conditions10,29. The sulfur tolerance exhibited
is also significantly better than the Ni-YSZ anodes with YSZ replaced by
other oxygen ion conductors of higher conductivity such as Gd-doped
ceria (GDC) and Sc-stabilized zirconia (ScSZ)30.
Among all anode materials ever studied, a composite anode
consisting of Ni and the electrolyte still represents the state-of-the-art
for anode-supported SOFCs because of the excellent catalytic activity
for hydrogen oxidation, electrical conductivity for current collection,
and compatibility with the electrolyte. One effective approach to
making these composite anodes contaminant-tolerant is to modify
the Ni-electrolyte surface by particles of catalysts that may promote
the removal of contaminants (e.g., carbon or sulfur) while maintaining
the unique properties of Ni required for high performance. As reported
recently3, small amounts of BaO spread over the surface of Ni grains
in a Ni-BZCYYb cermet anode during processing at high temperatures
may play a vital role in achieving the observed sulfur tolerance. In
fact, when nano-sized BaO islands were created on the surface of the
Ni grains in a Ni-YSZ cermet anode using a vapor phase deposition
(Fig. 3), the resistance to coking was dramatically enhanced3. The
nanostructured BaO/Ni interfaces seem very efficient for a water-
mediated carbon removal. They also showed good sulfur tolerance
while maintaining high performance.
Challenges in the rational design of materialsTo date the development of new materials or structures has been
guided largely by experience and chemical/physical intuition rather
than by scientific theories or models, because the mechanisms of
many charge and mass transport processes associated with fuel
cell operation are still lacking. In this section we outline the main
challenges we are facing in unraveling the mechanisms of electrode
processes and the nature of the rate-limiting step, using both
computational and experimental approaches, in order to control the
rate of electrode processes or to achieve the rational design of better
materials by changing the composition, structure, and morphology of
materials.
Fig. 3 Microanalysis of a BaO-modified Ni-YSZ anode: (a) top view (SEM image) showing the nano-islands of BaO on a Ni grain, (b) a cross-sectional view (bright-field
TEM image) of a BaO/Ni interface, (c) HRTEM image of the BaO/Ni interface (the [112–
] zone axis of the Ni under the BaO island is perpendicular to the page), and
(d) Fourier-filtered [112–
] zone axis image of the Ni under the BaO island. Adapted from3 by permission from Macmillan Publishers Ltd: Nature Communications, ©2011.
(a)
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Well-designed cells for electrochemical measurementsThe rates of many chemical and energy transformation processes are
limited by the charge and mass transfer along surfaces and across
interfaces. A fundamental understanding of these processes, especially
the rate-limiting step, is vital to enhancing electrode performance.
Electrochemical measurement is an effective technique for
quantifying the performance of a cell or a cell component, offering
phenomenological parameters such as charge or mass transfer
resistance or area specific resistance (ASR), exchange current density,
and transference numbers. These parameters are helpful when
predicting performance using continuum models. However, they may
not represent the intrinsic catalytic properties of an electrode material
because they may be influenced by many factors which are difficult
to control experimentally, including the geometry, microstructure,
and transport properties of the electrode as well as its physical and
chemical compatibility with the electrolyte.
To preclude the effect of extrinsic factors (e.g., geometry and
microstructure), a test cell platform with well-controlled geometry
must be used31. One example is a dense or patterned film electrode
of the material to be examined, which functions as the working
electrode, as schematically shown in Figs. 4a and b, respectively22.
The secondary current collection layer, the MIEC layer in Fig. 4a, is
sufficiently thick to alleviate sheet resistance and therefore does not
require a densely packed metal current collector, allowing exposure of
the working electrode surface for other in situ characterization. This
cell is ideally suited for characterization of the intrinsic properties
of an electrode material (the top layer), which is also open to other
in situ characterization such as Raman spectroscopy and scanning
probe microscopy. When dense, the working electrode material must
have some degree of mixed conductivity (poor ionic conductivity
may be mitigated by reducing the thickness). To minimize the sheet
resistance effect of a thin-film electrode, an MIEC of high ambipolar
conductivity has to be used as the current collector, which can be
a homogenous (like LSCF) or a composite (like YSZ-LSM or Ni-YSZ,
Fig. 4c) MIEC. Another advantage of this cell design is that only the
top surface is active for electrochemical reactions, which is open to
other simultaneous in situ measurements such as Raman spectroscopy
or x-ray analysis due to the underlying current collection layer and,
Fig. 4 (a) Schematic of a test cell with well controlled geometry: an MIEC film (current collector, the 2nd layer) coated with a dense film of a second cathode material
(working electrode, the top layer), both deposited onto an electrolyte layer with a highly active counter electrode opposite. (b) A top and cross-sectional view of a
test cell with a patterned electrode. Reproduced with permission from33. © 2008, The Electrochemical Society. (c) A schematic and an SEM image of a composite MIEC
consisting of two phases (e.g., Ni-YSZ), which can be used as the current collector (as the 2nd layer) or be characterized (as the top layer) in the test cell.
(a)
(b)
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therefore, lack of interference by bulky metal mesh or even metal
paste.
To further isolate certain charge or mass transfer step(s) of the
electrode reactions, continuum models have been developed for
the careful design of cell geometry (e.g., the thickness of MIEC film)
to minimize the sheet-resistance effect in the thin-film working
electrodes32. Models have also been developed for the design of cells
with patterned electrodes for direct correlation between performance
and electrode geometry (e.g., thickness, TPB length) to minimize
experimental errors33. These cell designs are instrumental for the
reliable determination of the intrinsic properties of electrode materials.
The use of well-designed cells is vital for the collection of useful
information, since the interpretation of data becomes difficult for cells
with complicated porous electrodes. For example, while it is possible
to achieve satisfactory or even perfect curve fitting to impedance data
involving intricate charge and mass transfer processes with similar
relaxation time constants, there is no way of knowing if the assumed
number of processes adequately reflects what is occurring in the cell,
if the proposed equivalent circuits are meaningful, or if the assigned
values for circuit elements are valid, let alone the errors associated
with the de-convolution of the overlapping data. When complications
occur in data interpretation, the best solution is to simplify the cell
for electrochemical measurements by isolating the response from a
cell component or separating charge from mass transfer, as discussed
earlier. It is noted, however, that some difference may exist between
bulk properties of a porous material and those of the material as a
thin film34,35. Careful validation is necessary to make well designed
test cells a valuable tool for the evaluation of fundamental electrode
properties.
However, an electrochemical measurement cannot provide direct
information for identifying the chemical species involved in electrode
reactions or changes in electrode materials under the test conditions.
In situ characterization techniques can be performed alongside
electrochemical measurements, including Raman spectroscopy and
AFM (see Fig. 4a). Probing and mapping the evolution of surface
composition and structure or incipient new phase formation on
electrode surfaces relevant to electrode reactions under practical
fuel cell operating conditions may provide critical insights into the
Fig. 5 (a) Optical micrograph of a patterned Ni electrode (lighter area) on YSZ after exposure to CH4 at 625 °C for 12h. (b) Raman intensity map for a 1580 cm-1
Raman shift (carbon G-band) of the same area shown in (a). The detailed experimental arrangement for the Raman measurement is described elsewhere10.
(c) Comparison of coking behavior between bare a Ni mesh electrode and BaO-modified electrode using Raman spectromicroscopy.
(a)
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mechanisms of these reactions, which are vital to achieving rational
design of better electrodes, catalysts, and interfaces.
In situ Raman spectroscopy Since a number of chemical and electrochemical reactions limit SOFC
performance, a detailed knowledge of the surface species involved
in those processes is vital to the design of new SOFC materials and
microstructures. Among the relatively few in situ surface analysis
methods (FTIR, Raman, EXAFS, and small-angle x-ray scattering)36,
Raman spectroscopy is the most flexible in terms of having the largest
window of operating conditions and the greatest range of surface
species that can be probed and mapped, especially oxygen37,38,
sulfur10,39,40, carbon9,41,42, hydrocarbons43, and water2,3. Raman
spectroscopy can be used in situ (and ex situ) alongside electrochemical
measurements to probe and map surface species (e.g., reaction
intermediates) and chemical phases relevant to the electrode reactions
under practical fuel cell operating conditions, allowing the direct
correlation of electrochemical performance to surface chemistry and
structure in the electrochemical environment of an operating cell.
A typical arrangement for in situ Raman analysis of SOFC electrode
materials is described elsewhere10.
A multitude of research efforts in recent years have led to marked
progress in the development of Raman spectroscopy as an effective
tool for studying SOFC materials, including the investigation of sulfur
poisoning on Ni-based anodes39,40, chromium poisoning of LSM
cathodes in cells with metallic interconnects6, and the oxidation state
of GDC electrolytes under fuel cell atmospheres44. Studies of coking
on SOFC anodes operating on carbon-containing fuels have garnered
attention from researchers in this area due to the reasonable sensitivity
of Raman spectroscopy to carbon species9,41,42,45-47. Walker’s group
was the first to develop and demonstrate a Raman microscope system
for in situ studies of coking on Ni-YSZ anodes45, and this methodology
was adapted for similar investigations by others42,46,47.
More recently, efforts have been devoted towards vastly improving
spatial resolution in these types of studies41. For example, Fig. 5a
shows an optical image of a patterned Ni strip electrode deposited
by PVD on YSZ that was exposed to CH4 at 625 °C for 12 hours,
while Fig. 5b shows an intensity map of the same area for a 1580
cm-1 Raman shift, corresponding to the carbon G-band. The intensity
map very clearly resolves the Ni electrode, on which the coking
should preferentially occur. Additionally, Raman spectromicroscopy
has been used to characterize coking resistance conveyed by surface
modifications to Ni electrodes. Fig. 5c shows a comparison of coking
behavior between a plain Ni mesh electrode and one that has been
modified with BaO (see above), the latter of which shows no coking
even after 16 hours of exposure to C3H8 at 625 °C under OCV
conditions due to BaO islands on the surface, of which clusters are
visible in the micrograph. The spectra shown were collected in situ
under treatment conditions.
In the future, Raman spectroscopy will provide critical insights into
the pathway, sequence, and mechanism of reactions occurring on
SOFC electrode surfaces under electrochemically polarized conditions.
The presence of specific chemical species on electrode surfaces will be
correlated with impedance spectroscopy data under various operating
conditions to understand how different species impact fuel cell
performance. A profound understanding of the detailed electrode reaction
mechanism and knowing which step affects cell performance the most will
guide the development of new electrode materials and microstructures.
While Raman spectroscopy has been successfully used to probe
and map carbon, sulfur, Cr-containing phases, water, and oxygen
species, many challenges still remain in pin-pointing important reaction
mechanisms relevant to fuel cell operation. These obstacles include a
lack of sensitivity for probing and mapping the earliest stages of phase
formation related to coking and sulfur poisoning as well as the limited
amount of signal from oxygen species on cathode surfaces, due to the
small Raman cross-sections of those species. In particular, for common
cathode materials like LSM and LSCF, a high tendency for fluorescence
necessitates the use of a lower excitation power, diminishing the useful
Raman signal. Additionally, the high temperatures associated with
in situ testing further decrease the sensitivity and may lead to a shifting
of the characteristic Raman bands. To overcome these difficulties,
surface-enhanced Raman scattering (SERS) methods applicable to SOFC
materials are being developed to increase sensitivity48. In addition,
Raman spectroscopy integrated with scanning probe methods, which
can allow for tip-enhanced Raman spectroscopy (TERS) can potentially
increase both sensitivity and spatial resolution49.
In situ synchrotron-based XRD/XAS Synchrotron-based x-ray techniques provide yet another methodology
for in situ investigations of SOFC materials. When acting as in situ
Fig. 6 A schematic arrangement for an in situ synchrotron-based XAS study of SOFC materials under conditions similar to fuel cell operation alongside electrochemical measurements.
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structure probes for SOFC materials, x-ray absorption spectroscopy
(XAS) is excellent in probing the local atomistic and electronic
structure, as shown schematically in Fig. 6, while x-ray diffraction
(XRD) is a powerful tool for eliciting lattice or long-range order. Often,
the structures of SOFC materials may change due to a chemical,
electrochemical, and/or thermal effect under typical operating
conditions. The atom specificity of XAS makes the technique very
powerful for characterizing electrode reaction mechanisms. Since the
near-edge structure of XAS data (XANES) probes the chemical state
of a particular elemental component while the extended fine structure
(EXAFS) describes its local atomic environment, combining XANES and
EXAFS of multiple atomic species over an SOFC reaction can be used
to create a reaction roadmap. At the same time, XRD can identify the
crystalline phases involved in the reactions and draw out the phase
transformation windows in temperature, PO2, or other external stimuli.
XAS can be performed in multiple modes simultaneously, barring
constraints of sample geometry and sample chamber pressure and
temperature. Transmission mode XAS measurements at hard x-ray
energies are forgiving experiments to set up under in situ conditions
and provide access to the K-edges of 3d transition metals, Sr,
and Y, and the L3-edges of Ba, La, and Ta. Fluorescence yield (FY)
measurements here are particularly useful when the element being
probed is a dilute component of the material (e.g., the dopants).
Near-surface electronic structure measurements, using total or
partial electron yield (TEY/PEY), can be made simultaneously with
corresponding bulk measurements using fluorescence yield (FY) or
direct transmission, and provide contrast between surface and bulk
phenomena. As described earlier, XANES provides the chemical state
of a species, but to be more specific, it provides a spectral function
that is proportional to the unoccupied densities of state local to that
species. In the commonly found transition metals with octahedral
symmetry in SOFC materials, for example, the oxygen K-edge or the
metal L-edge will show the split t2g and eg LUMO orbitals (provided
they have empty states). Fig. 7 shows the deconvoluted oxygen
K-edge (transition between the oxygen 1s initial state and LUMO final
states with p-symmetry) in LSF with and without Ti and Ta doping50.
After deconvoluting the oxygen K-edge prepeaks into individual
t2g and eg spin up and spin down bands, Braun et al. discovered a
correlation between a ratio of peak heights, the conductivity, and
the Fe4+ /Fe3+ ratio. Also shown in Fig. 7 is the Mn L-edge data
(Mn 2p electrons excited to Mn 3d final states) in LSM, clearly showing
the contributions of three types of Mn ion51.
Fig. 7 (a-d) Deconvoluted oxygen 1s spectra with a pre-peak near EF for LSF and LSF doped with Ti (20 mol.%) and Ta (10 and 20 mol.%). Numerals indicate
peak position and spectral weight. Reproduced from50 with permission of the American Institute of Physics. (e) Mn L3-edge XAS of LSM as-processed (blue), SOFC
environment exposed (black), and activated (red) films. The Mn charge-state multiplet line shape peak energies are plotted as vertical lines. Reproduced with
permission from51. © 2011, The Electrochemical Society.
(a) (b)
(c) (d)
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Transmission in situ XAS can be carried out in a proper furnace at
reaction temperature and PO2 like the one at beamline X18B of the
National Synchrotron Light Source where multiple samples can be
placed in a carousel and exposed to the same reaction conditions.
An example of in situ XANES of SOFCs can be seen in the work of
Yildiz and co-workers who studied the activation of La0.8Sr0.2MnO3
and La0.8Ca0.2MnO3 on single crystal YSZ electrolytes using in situ
XANES (Mn K-edge and La L2-edge and x-ray reflectivity)52. For
enhanced surface sensitivity under in situ conditions, a grazing
incidence FY geometry can be used, such as the one demonstrated by
Shinoda et al.53.
There are currently a handful of examples for the use of in situ
XRD for structural characterization of SOFC materials. Liu et al.
studied phase and strain distributions associated with reactive
contaminants in SOFCs54. Using in situ measurements, Shultz et al.
characterized the dissociation and crystallization of the amorphous
precursor powders of lanthanum strontium gallates55. More recently,
Hashimoto et al. investigated the changes in the lattice parameters
of La0.6Sr0.4Co1−yFeyO3−δ (y = 0.2, 0.4, 0.6, and 0.8)56. Further, a
combination of both XAS and XRD would be a very powerful approach
to probing local and long-range atomistic structures of SOFC materials
under practical fuel cell operating conditions, offering structural
information that has never before been accessible.
Prediction of intrinsic properties of materials Proper modeling techniques and prediction tools are essential to a
comprehensive understanding of SOFC materials. For example, density
functional theory (DFT)57 is an effective computational framework for
prediction of electronic structures and other fundamental properties
of materials58-60. In particular, it has provided detailed, molecular-
level information that may not be readily obtained experimentally,
including probable electrode reaction sequence, mechanisms, and
stable intermediates, surface coverage of adsorption sites, mobility of
electro-active species along surfaces, and rate constants for adsorption/
dissociation, reduction/oxidation, and incorporation/release of species
at surface sites. For example, the significance of the MnO2-terminated
(001) surface of LaMnO3 was reported61 and, in conjunction with
molecular dynamics (MD) simulations62 and kinetic theory63, several
properties of LaMnO3-based materials were examined for SOFC
applications62-64 including reaction sequences and charge transfer
mechanism. Recently, an ab initio thermodynamic approach was used
to examine oxygen reduction on SOFC cathodes.65,66 Fig. 8 depicts a
molecular-level computational screening based on surface and bulk
properties to search for cathode materials of higher performance than
conventional ones67. DFT calculations were also used to design sulfur-
and carbon-tolerant anode materials59,68-70. The interactions of sulfur-
containing compounds (e.g., H2S) with anode materials under SOFC
operating conditions were predicted using ab initio thermodynamics12;
providing important insights into the mechanism of sulfur poisoning.
Recently, a water-mediated removal of carbon species from nano-
structured BaO/Ni interfaces was modeled successfully (Fig. 9)3.
Furthermore, the predicted vibrational frequencies of surface species and
new phases were confirmed using Raman spectroscopy71.
Fig. 9 DFT-predicted energy profile for removing chemisorbed carbon species
on BaO/Ni(111) interfaces through a water mediated process, where *
denotes an adsorbed species on the surface. Adapted from3 by permission from
Macmillan Publishers Ltd, © 2011.
Fig. 8 (a) Illustration of a bulk La0.5Sr0.5MnO2.75 (110) structure before and
after ionic conduction. V and ON (N = 1 or 2) are, respectively, an oxygen
vacancy and the nearest neighboring oxygen to V. (b) Trajectory of oxygen ion
conduction through La0.5Sr0.5MnO2.75 (110). O1 and O2 are the initial and
final states of the oxygen ion conduction. Reprinted from67 with permission
from Elsevier, © 2011.
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While DFT-based calculations have been instrumental in gaining
critical insights into intrinsic properties of many materials72, new
methodologies must be developed to bridge the gap between
theoretical predictions and experimental measurements, notably for the
materials with open shells of d or f electrons commonly encountered in
SOFC systems60,73. For example, the DFT+U theory has been proposed
to mitigate the limitations of conventional DFT for these materials.
Further, time-dependant DFT60 has also been applied to the analyses of
x-ray absorption spectra of SOFC materials.
On the other hand, DFT calculations become increasingly more
difficult for more complex material systems, such as the BZCYYb
electrolyte, the LSCF cathode, and LSTC anode materials, due
partially to the large number of atoms/ions (or potential reactive
sites) that must be considered in the calculations. It is still a grand
challenge to identify approximate descriptors for the computational
design of better SOFC materials, something similar to the d band
model applied to the characterization of the oxygen reduction reactions
(ORR) in low temperature fuel cells. Moreover, electrochemical hot
spots, such as the heterogeneous boundaries and junctures in a porous
electrode (e.g., TPB), are even more difficult to predict using DFT-based
calculations. The quantum mechanics/molecular mechanics (QM/MM)
methodology74 may help model reactions at or near the TPB region.
To link the intrinsic properties of a material with its performance
in a fuel cell, however, DFT/MD simulations must be combined with
continuum modeling that can predict the phenomenological behavior
of materials, which can be confirmed directly by experiments.
Continuum modeling Coupled continuum and phenomenological models can be used to
predict experimentally measurable parameters, such as exchange
current density and ASR of a cell, providing a means to evaluate
response over larger length scales than those available to DFT/MD
simulations. Phenomenological models provide a means to understand
the rates of charge and mass transfer processes in detail and predict
them in a variety of circumstances. The rate expressions serve as
boundary conditions for continuum models. Fig. 10 illustrates the
interdependence of models at different length and time scales (from
DFT to continuum) for the rational study of SOFC materials.
Once phenomenological parameters are known, these models in
turn can predict the behavior of materials or the performance of fuel
cells under various conditions. For example, continuum models have
been successfully applied to various aspects of electrode operation75,
from the global response of porous mixed conducting electrodes76,77,
to the performance of heterogeneous composite electrodes described
using particle/resistor networks78 and homogenized treatment79,
and to detailed reaction rates or intermediate steps of surface and
interfacial processes80.
Further, continuum models can link DFT/MD calculations indirectly
to experimental measurements, thus providing a means of verifying
their predictions. For example, continuum models can be used to
predict performance of fuel cells from materials properties derived from
DFT/MD simulations, including the molecular level reaction sequence,
rate-limiting steps, detailed defect structures, surface structures,
and phenomenological parameters (e.g., surface adsorbed oxygen
concentration73), some of which may not be readily accessible from
experiments. Phenomenological/continuum models guided by DFT/MD
can then be compared to experimental results to verify and/or refine
these calculations.
Moreover, phenomenological/continuum models conformal to
electrode geometry have been successfully used with cells consisting
of thin-film/patterned electrodes. On the simplest level, they help to
quantify the characteristic activity under the framework of linearized
parameters, such as the length-normalized resistance to oxygen
reduction at the TPB81 or the area-normalized equivalent circuit
parameters of the bulk pathway82. A model conformal to electrode
geometry is required to examine the effect of the microstructure on
the performance of a porous electrode. Conformal models have been
successfully coupled with phenomenological models to explain83 and
help mitigate32 sheet resistance observed in experimental patterned
electrode results, thus providing guidelines for the better design
of test cells and for the proper interpretation of electrochemical
measurements84. As described in detail elsewhere32,33, these models
have been used to predict potential and defect distribution in a thin-
film working electrode with current collectors of different geometries
(strips, grids, and circular pads), the critical spacing between current
collectors to minimize the effect of sheet resistance on performance,
and the relative contributions from competing bulk and TPB pathways
of a patterned electrode under cathodic polarization. Recently,
phenomenological modeling was used together with DFT simulation
predictions and TEM analysis to examine the trend of the ASR of
uncoated and LSM-coated LSCF electrodes under various cathodic
polarizations22.
The reactions on both cathode and anode are quite complex, often
involving adsorption, dissociation, and reduction/oxidation of gas
molecules, transport of adsorbed surface species, and participation of
point defects (e.g., oxygen vacancies and electrons/electron holes). An
electrochemical driving force may not only alter the concentrations
of surface species within a mixed conductor but also change their
energies85. Further, the composition and structure of an active
electrode surface may be different from those of the bulk phase due
to surface elemental segregation86. The linking of electrochemical
response with detailed surface properties and reaction mechanisms,
therefore, is quite a challenge and continues to be an important
research pursuit.
Quantification of the microstructure effectThe performance of an electrode is determined critically by the porous
3D microstructure. Important factors include the exposed catalyst
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11544
surface area, facility of gas transport through pores, resistance to ionic
and electronic transport through solid phase, and length of TPB lines.
The complexity of mass and charge transport in just the solid phase
is illustrated in Figs. 11a,b. Many strategies have been employed to
optimize the microstructure, including the formation of composite
electrodes, functionally graded microstructures, and infiltration of active
electrode phases on electrolyte scaffolds. Optimization of the electrodes
is a very difficult task because many of the important features compete
with one another; for example, surface area may increase at the expense
of gas-phase diffusion.
Modeling on an electrode level75,87 is useful for understanding
performance. In particular, equivalent circuit models78,88 describe the
performance based upon linearized parameters. Another model type
for mixed ionic-electronic conducting electrodes is based upon porous
electrode theory76 and uses homogenized microstructural parameters
and linear irreversible thermodynamics in reaction rates. Recent results
show that it is reasonable for many MIECs, but not for those where
diffusion length is on the order of particle size89, because they neglect
the fine details of the microstructure and can lack detailed predictive
capability.
3D reconstruction by FIB/SEM90,91 and phase-sensitive x-ray computed
tomography92 is a recent and promising development, providing high-
resolution microstructural details of porous electrodes. This information
has been used in homogenized models for performance prediction.
Recently, researchers also began to use the 3D reconstructions
as the domain for electrochemical simulations using the Lattice-
Boltzmann method93,94 or the finite element method95. The former
has been applied primarily in the anode using models developed for
nickel patterned electrodes, gas diffusion, and ionic transport. The latter
has been applied to an LSCF cathode using effective linear irreversible
thermodynamic parameters (not detailed reaction rates) based on
surface exchange and tracer diffusion coefficients. We have developed
FEM models for the simulation of the electrochemical response of
3D porous electrodes reconstructed by x-ray computed tomography92;
the preliminary results are shown in Fig. 11c.96
While simulations conformal to the reconstructed electrode
microstructures constitute a powerful computational framework,
some challenges still remain. First, the 3D reconstructions may require
extensive and skilled FIB/SEM or synchrotron work. Second, simulations
deployed on the actual porous structures require sophisticated
numerical methods and, depending on the complexity of phenomena
modeled, can require complicated constitutive equations and/or
parameter determination. All are the subject of current research within
the field.
Such simulations can corroborate the accuracy of homogenized
models and, when homogenized models break down, provide the
most accurate and detailed means of simulation. The detailed
microstructure may also be able to act as a sort of well-defined
electrode in and of itself: the a priori digital representation of explicit
microstructural geometry might allow fundamental study. The
ultimate goal is to use the 3D geometry for numerical simulation
of electrode performance in engineering design, in conjunction with
Fig. 10 Interdependence of models at different length and time scale and characterization techniques for the rational study and design of SOFC materials.
REVIEW Rational SOFC material design: new advances and tools
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Rational SOFC material design: new advances and tools REVIEW
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 545
detailed, mechanistic, nonlinear phenomenological rate expressions
serving as boundary conditions97 and informed by parameters derived
from patterned or porous electrodes.
New directions and future perspectives One important direction is to exploit nanostructures and nano-
architectures derived from a variety of templates in order to transcend
some of the difficulties facing materials development for energy
applications98. In particular, hierarchical 3D porous architectures99
may dramatically enhance the rates of change and mass transfer
processes while improving the mechanical integrity and robustness.
These nanostructured electrodes and interfaces are known for increased
numbers of active sites, reduced length of ion diffusion to active sites,
and greater flexibility in surface modification for catalysis and electro-
catalysis98,100.
Another important new direction is to develop a predictive
multi-scale (from DFT to continuum) computational framework,
through rigorous validation at each scale by carefully designed
experiments under in situ conditions, for the rational design of
better materials and structures for a new generation of SOFCs to
be powered by readily available fuels. While significant progress has
been made in developing SOFC materials in probing and mapping
electrode surface species relevant to electrode processes, and in
unraveling some of the mechanisms of the electrode processes,
many challenges still remain to bridge the gaps between models at
different scales or between theoretical predictions and experimental
observations10. Only when the detailed mechanisms of the rate-
limiting steps are clearly understood will it be possible to rationally
design better materials.
It is vital to perform well designed experiments at each scale under
in situ conditions in order to validate and perfect the predictability of
the individual models at different scales. It is still a grand challenge
to link the global performance or functionality of a 3D porous
electrode with the local structure, composition, and morphology of
nanostructured surfaces and interfaces. Validation and integration of
information collected from different scales are critical to developing a
computational framework across multiple scales for the rational design
of materials with exceptional functionality.
AcknowledgmentsThis material is based upon work supported as part of the HeteroFoaM
Center, an Energy Frontier Research Center funded by the U.S.
Department of Energy (DOE), Office of Science, Office of Basic
Energy Sciences (BES) under Award Number DE-SC0001061. Partial
support from the World Class University (WCU) program, UNIST, South
Korea, is also acknowledged. The authors wish to thank Prof. Wilson
Chiu, Dr. George Nelson, William Harris, and Jeffrey Lombardo at
the University of Connecticut for x-ray tomography data used for the
simulation shown in Fig. 11c.
Fig. 11 Schematic diagram of charge and mass transport within and on the
surface of (a) a single-phase mixed conducting porous electrode and (b) a
composite (mixed conductor + electrolyte) porous electrode. (c) Initial 3D
FEM simulation of adsorbed oxygen species on the surface and at the TPBs of a
porous LSM electrode96.
(a)
(b)
(c)
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11546
REFERENCES
1. Mi nh, N. Q., and Takahashi, T., Science and Technology of Ceramic Fuel Cells. Elsevier: Amsterdam, 1995.
2. Ya ng, L., et al., Science (2009) 326 (5949), 126.
3. Ya ng, L., et al., Nat Commu (2011) 2, 357.
4. Xi a, C. R., et al., Solid State Ionics (2002) 149(1-2), 11.
5. Ji ang, S. P., et al., J Eur Ceram Soc (2002) 22(3), 361.
6. Ab ernathy, H. W., et al., J Phys Chem C (2008) 112(34), 13299.
7. At kinson, A., et al., Nature Mater (2004) 3(1), 17.
8. Kr umpelt, M., et al., Catal Today (2002) 77(1-2), 3.
9. Zh a, S., et al., J Electrochem Soc (2004) 151(8), A1128.
10. C heng, Z., et al., Energy Environ Sci (2011) 4, 4380.
11. Z ha, S., et al., J Electrochem Soc (2007) 154(2), B201.
12. W ang, J. -H., and Liu, M., Electrochem Commun (2007) 9(9), 2212.
13. K reuer, K. D., Ann Rev Mater Res (2003) 33, 333.
14. M alavasi, L., et al., Chem Soc Rev (2010) 39(11), 4370.
15. K harton, V. V., et al., Solid State Ionics (2004) 174 (1-4), 135.
16. Z uo, C. D., et al., Adv Mater (2006) 18(24), 3318.
17. S un, C. W., et al., J Solid State Electrochem (2010) 14(7), 1125.
18. N iu, Y. J., et al., J Electrochem Soc (2011) 158(2), B132.
19. C hen, D. J., et al., Electrochem Commun (2011) 13(2), 197.
20. Ju, Y. W., et a l., J Electrochem Soc (2011) 158(7), B825.
21. Shah, M., et al ., Solid State Ionics (2011) 187 (1), 64.
22. Lynch, M. E., e t al., Energy Environ Sci (2011) 4, 2249.
23. Nie, L. F., et al., J Power Sources (2010) 195(15), 4704.
24. Choi, J., et al ., J Am Ceram Soc (2011), in press.
25. Tao, S., and Ir vine, J. T. S., Nature Mater (2003) 2(5), 320.
26. Huang, Y. H., e t al., Science (2006) 312(5771), 254.
27. Ruiz-Morales, J . C., et al., Nature (2006) 439(7076), 568.
28. Marina, O. A., et al., Solid State Ionics (2002) 149(1-2), 21.
29. Liu, M., et al. , J Power Sources (2011) 196(17), 7277.
30. Lussier, A., et al., Int J Hydrogen Energy (2008) 33(14), 3945.
31. Fleig, J., Soli d State Ionics (2003) 161(3-4), 279.
32. Lynch, M. E., a nd Liu, M. L., J Power Sources (2010) 195(16), 5155.
33. Lynch, M. E., e t al., J Electrochem Soc (2008) 155(6), B635.
34. Kawada, T., et al., J Electrochem Soc (2002) 149(7), E252.
35. la O’, G. J., e t al., Angew Chem Int Ed (2010) 49(31), 5344.
36. Stierle, A., an d Molenbroek, A. M., MRS Bull (2007) 32(12), 1001.
37. Choi, Y. M., et al., ChemPhysChem (2006) 7(9), 1957.
38. Pushkarev, V. V ., et al., J Phys Chem B (2004) 108(17), 5341.
39. Cheng, Z., et a l., J Phys Chem C (2007) 111(49), 17997.
40. Cheng, Z., and Liu, M., Solid State Ionics (2007) 178(13-14), 925.
41. Blinn, K., et a l., J Power Sources (2011), submitted.
42. Eigenbrodt, B. C., et al., J Phys Chem C (2011) 115(6), 2895.
43. Yang, H. Z., et al., J Phys Chem B (2006) 110(35), 17296.
44. Maher, R. C., a nd Cohen, L. F., J Phys Chem A (2008) 112(7), 1497.
45. Pomfret, M. B., et al., J Phys Chem C (2008) 112(13), 5232.
46. Su, C., et al., J Power Sources (2011) 196(4), 1967.
47. Yoshinaga, M., et al., J Ceram Soc Jpn (2011) 119(1388), 307.
48. Blinn, K. S., e t al., Advances in Solid Oxide Fuel Cells V (2010) 30(4), 65.
49. Kudelski, A., S urf Sci (2009) 603(10-12), 1328.
50. Braun, A., et a l., Appl Phys Lett (2009) 94(20), 202102.
51. Piper, L. F. J. , et al., J Electrochem Soc (2011) 158(2), B99.
52. Yildiz, B., et al., In Proceedings of the Lucerne Fuel Cell Forum, Argonne National Laboratory, (2006),
53. Shinoda, K., et al., Surf Interface Anal (2010) 42(10-11), 1650.
54. Liu, D. J., and Almer, J., Appl Phys Lett (2009) 94(22), 224106.
55. Schulz, O., and Martin, M., Solid State Ionics (2000) 135(1-4), 549.
56. Hashimoto, S., et al., Solid State Ionics (2011) 186(1), 37.
57. Kohn, W., and S ham, L. J., Phys Rev (1965) 140, A1133.
58. Choi, Y., et al ., Top Catal (2007) 46(3-4), 386.
59. Wang, J. H., et al., In Quantum Chemical Calculations of Surfaces and Interfaces
of Materials Basiuk, V. A., and Ugliengo, P., (eds.) American Scientific Publishers, Los Angeles, (2008).
60. Huang, P., and Carter, E. A., Annu Rev Phys Chem (2008) 59, 261.
61. Kotomin, E. A., et al., Phys Chem Chem Phys (2005) 7(11), 2346.
62. Choi, Y. M., et al., Angew Chem Int Ed (2007) 46(38), 7214.
63. Choi, Y. M., et al., J Phys Chem C (2009) 113(17), 7290.
64. Chen, H. T., et al., Langmuir (2011) 27(11), 6787.
65. Mastrikov, Y. A ., et al., J Phys Chem C (2010) 114(7), 3017.
66. Piskunov, S., e t al., Phys Rev B (2011) 83(7), 073402.
67. Choi, Y., et al ., J Power Sources (2010) 195(5), 1441.
68. Nikolla, E., et al., J Am Chem Soc (2006) 128(35), 11354.
69. Galea, N. M., e t al., J Phys Chem C (2007) 111(39), 14457.
70. Shishkin, M., a nd Ziegler, T., J Physl Chem C (2010) 114(49), 21411.
71. Wang, J. H., et al., J Chem Phys (2007) 127(21), 214705.
72. Crabtree, G., a nd Sarrao, J., Ann Rev Cond Matt Phys (2011) 2, 287.
73. Lee, Y. L., et al., Phys Rev B (2009) 80(22), 224101.
74. Lin, H., and Tr uhlar, D. G., Theor Chem Acc (2007) 117(2), 185.
75. Fleig, J., Annu Rev Mater Res (2003) 33, 361.
76. Adler, S. B., e t al., J Electrochem Soc (1996) 143(11), 3554.
77. Virkar, A. V., et al., Solid State Ionics (2000) 131(1-2), 189
78. Sunde, S., J El ectrochem Soc (1995) 142(4), L50.
79. Costamagna, P., et al., Electrochim Acta (1998) 43(3-4), 375.
80. Svensson, A. M. , et al., J Electrochem Soc (1997) 144(8), 2719.
81. Radhakrishnan, R., et al., J Electrochem Soc (2005) 152(1), A210.
82. Baumann, F. S., et al., Solid State Ionics (2006) 177, 1071.
83. Mebane, D. S., et al., J Electrochem Soc (2007) 154(5), A421.
84. Koep, E., et al ., Electrochem Solid State Lett (2005) 8(11), A592.
85. Lankhorst, M. H . R., et al., Solid State Ionics (1997) 96(1-2), 21.
86. Simner, S. P., et al., Electrochem Solid State Lett (2006) 9(10), A478.
87. Singhal, S. C., and Kendall, K., High Temperature Solid Oxide Fuel Cells:
Fundamentals, Design and Applications. Elsevier: Amsterdam, 2003.
88. Liu, M. L., J E lectrochem Soc (1998) 145(1), 142.
89. Lu, Y. X., et a l., J Electrochem Soc (2009) 156(4), B513.
90. Wilson, J. R., et al., Nature Mater (2006) 5 (7), 541.
91. Gostovic, D., e t al., Electrochem Solid State Lett (2007) 10 (12), B214.
92. Grew, K. N., et al., J Electrochem Soc (2010) 157 (6), B783.
93. Shikazono, N., et al., J Electrochem Soc (2010) 157 (5), B665.
94. Joshi, A. S., e t al., J Power Sources (2007) 164 (2), 631.
95. Joos, J., et al ., ECS Transactions (2011) 35, 2357.
96. Lynch, M. E., e t al., in preparation (2011).
97. Mebane, D. S., and Liu, M., J Solid State Electrochem (2006) 10, 575.
98. Song, M.-K., et al., Mater Sci Eng R (2011) 72, 203.
99. Kroger, N., and Sandhage, K. H., MRS Bull (2010) 35(2), 122.
100. Bao, Z., et al ., Nature (2007) 446(7132), 172.
MT1411p534_547.indd 546 01/11/2011 14:34:15
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ISSN:1369 7021 © Elsevier Ltd 2011NOVEMBER 2011 | VOLUME 14 | NUMBER 11548
Ionic transport and electrochemical transformations underpin a
broad variety of modern energy and information technologies.
The classical examples include primary and secondary batteries
ranging from centuries-old Volta piles1 and Leclanche elements2 to
modern Li-ion and flow batteries that hold the promise of viable
hybrid and electric vehicle technologies and grid-level storage3 ,4 .
Ionic phenomena are at the core of the operation of solid oxide
and polymer electrolyte fuel cells, which offer some of the
highest efficiencies of fuel-to-energy conversion5-7. Finally, the
development of secondary metal-air and metal-water batteries
will potentially open the pathway for energy storage at densities
comparable to fossil fuels8.
Equally important are ionic phenomena in many areas of condensed
matter physics. Recent examples include electroresistive and
memristive electronic devices as components of non-volatile storage
and neuromorphic logic9,10. Similarly, ionic effects can play a significant
and potentially definitive role in the functionality of molecular
electronic devices11, strongly affect the piezoresistance12, induce
ferroelectric-like dielectric behaviors13, contribute to ferroelectric
resistive switching14, and couple to other physical phenomena in
nanoscale oxides. Finally, ionic phenomena are an integral part of
the long-term degradation phenomena in ferroelectrics and dielectric
materials15-17, and hence are directly relevant to the optimization and
implementation of oxide electronic devices.
Progress in the development and optimization of energy storage and conversion materials necessitates understanding their ionic and electrochemical functionality on the nanometer scale of single grain clusters, grains, or extended defects. Classical electrochemical strategies based on Faradaic current detection are fundamentally limited on the nanoscale. Here, we review principles and recent applications of electrochemical strain microscopy (ESM), a scanning probe microscopy (SPM) technique utilizing intrinsic coupling between ionic phenomena and molar volumes. ESM imaging, as well as time and voltage spectroscopies, are illustrated for several Li-ion cathode and anode materials. Finally, perspectives for future ESM developments and applications to other ionic systems are discussed.
Sergei Kalinina,* Nina Balkea, Stephen Jessea, Alexander Tseleva, Amit Kumara, Thomas M. Arrudaa, Senli Guoa, and Roger Prokschb
aThe Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USAbAsylum Research Corporation, Santa Barbara, CA, USA
*E-mail: [email protected]
Li-ion dynamics and reactivity on the nanoscale
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Li-ion dynamics and reactivity on the nanoscale REVIEW
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 549
Progress in these applications requires probing electrochemical and
ionic phenomena on the nanometer scale, establishing the origins of
observed physical behaviors, and linking macroscopic device or material
functionality and advanced theoretical studies18. However, while
such an understanding was developed for, e.g., semiconductor and
structural materials leading to exponential technology development
(Moore’s law), an understanding of the electrochemical processes in
solids and at solid-gas/liquid interfaces remains elusive.
This dearth of information is related to two factors, namely
the complexity of ionic systems and the very small length scales
of relevant interactions. For example, operation of battery or fuel
cell devices will involve stages of simultaneous electronic and
ionic transport (possibly mediated by the presence of electrolytes,
percolating and non-percolating diffusion paths, second phase
inclusions, and conductive dopants) in cathodes and anodes, chemical
reactions at the interfaces with solid or liquid electrolytes, and
ionic transport in electrolytes3,4. The electrochemical reactions and
intercalation processes in solid components are typically associated
with significant volume change effects that can lead to large (and
poorly understood) strains and, potentially, failure of the material. Also
significant, and poorly understood, are irreversible processes associated
with the formation stages of the device (i.e., initial operational cycle),
including solid-electrolyte interphase (SEI) formation in batteries19-21
and electroforming in memristors and electroresistive materials22,23.
Complimentary to this extreme complexity are the small length
scales of relevant physical phenomena and materials morphologies.
For example, modern Li-ion battery cathodes are formed by multilevel
assemblies, with the characteristic sizes of particles of the order
of 50 – 200 nm24,25. Similarly, the active zone of the TiO2 based
memristor device is often a nanometer-scale filament formed by
conductive Magneli phases26. More generally, it is well recognized that
the functionality of solids is controlled by atomic- and nanometer scale
defects that act as nucleation centers for new phases, pinning centers
for moving transformation fronts, etc. Defects and defect-mediated
functionality thus play a universally important role in virtually all phase
and chemical transformations, including those in energy systems.
Characterization of these phenomena on the level of a single
morphological or structural element and eventually, single defects
(or defect-free segment of material), requires extending classical
electroche mi cal strategies27,28 to the nanometer scale. While clearly
a challenge, recent progress in several areas of nanoscale science
suggests that such developments are possible once the proper tools
are developed. As a comparative example, the thermodynamics and
kinetics of macromolecular reactions have become accessible on a
singl e molecule level29-31, as a result of development of molecular
unfolding spectroscopy. Similarly, the development of Piezoresponse
Force Microscopy (PFM) and associated spectroscopic techniques
have allowed the exploration and control of polarization switching in
ferroelectric and mu ltiferroic materials at a single defect level32-34.
These examples suggest that comparable progress can be achieved in
all areas related to ionic and electrochemical behavior in solids once
proper nanoscale characterization tools are developed.
However, classical electrochemical methods based on the detection
of Faradaic currents offer very stringent limitations on the minimum
amount of material that can be probed. This necessitates the
development of alternative strategies to probe local electrochemical
functionality. In this review, we summarize the principles and
applications of electrochemical strain microscopy (ESM), a novel
scanning probe microscopy (SPM) method specifically aimed at probing
electrochemical and ionic phenomena in solids on the nanometer scale.
Scanning probe microscopy in electrochemistry: current and strain detectionProbing electrochemical processes by scanning probe microscopy
brings the dual challenge of inducing electrochemical processes below
the tip and detecting the associated changes in materials, e.g., changes
in the ionic concentration, size of the nucleated second phase, etc.
Tip-induced electrochemical reactions have been reported since the
very dawn of SPM, usually in the context of (highly undesirable)
processes that interfere with SPM imaging35,36. In the 1990s, it was
recognized that local electrochemical reactions can be used as a
basis for nanofabrication in the nano-oxidation of semiconductors
and metals37,38, electromachining, or deposition of carbon39,
semiconductors40, or metals41. In all these cases, the presence and
extent of (thermo)electrochemical processes can be established from
the changes in surface topography readily accessible by SPM in situ or
post mortem, or using spatially-resolved chemical analysis (e.g., micro
Raman).
The measurements of reversible ionic and electrochemical processes
necessitate the detection of transient signals directly during SPM imaging
or spectrum acquisition. One such approach utilizes the local surface
potential with variants of kelvin probe force microscopy42 or electrostatic
force microscopy43 following the application of bias pulses to the probe.
The relaxation of induced surface charge directly coupled to the local
ion concentration yie lds information on ion dynamics44-46. Schirmeisen
and his colleagues47,48 explored the direct relaxation of the electrostatic
force microscopy (rEFM) signal, extending the detection limit to the
millisecond range. Similar time resolved spectroscopies are actively being
pursued by the Ginger group for mapping light-induced phenomena in
photovoltaic systems49. However, the primary limitation of this approach
is only an indirect link between the potential and ionic concentration (e.g.,
the uncompensated injected charges are expected to affect electrostatic
forces much stronger then weak changes in the work function induced by
changes of ionic concentration while maintaining electroneutrality).
The alternative strategy for detecting bias-induced transformations
under the tip is direct detection of the electronic current by an SPM probe.
For materials with electronic and mixed electronic-ionic conductivities
the detected current will be dominated by electronic currents and contain
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11550
only indirect information on the electrochemical transformations50-53.
For materials with purely ionic conductivity (transference number t = 1),
the electronic current flowing though the (metallic) tip is balanced by
the ionic current though the material, as illustrated, for example, for
proton-conducting Nafion membranes by Burrato54 and molten salts
by Haile55. However, the limitations of this approach stem directly from
the sensitivity limits of current AFM. For example, a ~10 pA current
for 1 second is equivalent to 6.2 × 107 electrons, or, e.g., complete
electrochemical transformation in a 1.43 × 105 nm3 volume of material
such as LiCoO2 (assuming a cathode with 1 g LiCoO2 and the theoretical
capacity of 275 mAh/g). This introduces stringent limits on Faradaic
current measurements in SPM. In comparison, static and dynamic strain
detection can probe volumes as small as several 10s of nm3 as discussed
below, suggesting that strain detection can be used to probe local
electrochemistry at much higher resolutions.
As an illustration, Fig. 1 shows the surface topography and I-V
curve on th surface of the Li-ion conductive electrolyte56. Currents
on the order of 10s of nA are observed for biases in the range of
8 to 13 V, corresponding to purely Faradaic currents in the reaction
of Li+ + 1 e- → Li(s). Note that the total number of metallic Li atoms
deposited (via integration of topography) on the surface is on the
order of ~1010 which scales linearly with the total transferred charge.
However, the fact that AFM can readily detect nanoparticles of the
~1 – 2 nm scale (depending on background surface roughness) suggests
that current measurement is indeed not the optimal measurement
strategy for local electrochemical probing. For example, a Li particle
with a 4 nm diameter and 4 nm height having the same shape as the
Fig. 2 Limitations of conductive SPM based on environment and material. (a) Schematic image of current detection in liquid showing that stray currents are dominant and lead to non-locality of current detection. (b) Surface topography and (c) current image at 0 V bias for a LiCoO2 surface, illustrating electrical inhomogeneity in the electrode material.
Fig. 1 Surface topography of Li-ion conductive glass ceramics (LiCGC) (a) after and (b) before application of a bias pulse, and (c) locally measured I-V curve with inset showing the linear correlation (fit: y = 0.9389x) between the total charge transferred and volume of Li deposited.
(a)
(b)
(c)
(a) (b)
(c)
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NOVEMBER 2011 | VOLUME 14 | NUMBER 11 551
particles in Fig. 1a, would require < 1000 electrons (corresponding
to sub ~fA currents for 1 s), which is well below the noise levels and
detection limit of typical current measuring amplifiers used in SPM.
Even given the limitations of direct Faradaic current detection,
local conductivity can offer valuable indirect information on local phase
composition (conductive/non-conductive phases) and bias-induced
transformations (e.g., electroforming57, electrochemical reactions at
the junction58, or polarization switching in ferroelectrics59). However,
the significant limitation of the current-based techniques follows from
the non-local nature of the detection mechanism, as illustrated in
Fig. 2. In conductive electrolytes, the current will be dominated by non-
local current flow to the tip and cantilever. Even with the introduction
of insulated and shielded probes60-62, the measured current will be
determined by all resistive elements between the tip-surface junction and
the current collector. While for (homogeneous) materials the spreading
resistance of the tip-surface-junction is the dominant resistive element,
this is not necessarily the case for complex architectures forming energy
materials with multiple interfaces and grain boundaries, thus complicating
the interpretation of conductive AFM (cAFM) data. Finally, the presence
of multiple electrochemically active pairs (e.g., cathode materials and
carbon particles) results in a large inhomogeneity of current responses
and presence of “nanobatteries”, as can be deduced from non-zero
currents at zero bias (e.g., shown in Figs. 2b,c). This inhomogeneity further
complicates interpretation of cAFM data in terms of battery functionality.
We further note that some information on the local chemical changes
induced by bias can be obtained from mechanical property changes,
e.g., in atomic force acoustic microscopy and related techniques or even
AFM phase imaging. Comparative analysis of SPM methods for probing
electrochemical processes in solids is given in reference63.
Imaging by electrochemical strain microscopyRecently, the electrochemical strain microscopy approach was
suggested for probing ionic dynamics based on local dynamic strain
detection64. While scanning tunneling microscopes (STMs) measure
electronic currents and atomic force microscopies (AFM) measures
forces, ESM measures the direct coupling of ionic currents to strain
(or position) measurements, providing a new tool for mapping
electrochemical phenomena on the nanoscale. The operation of ESM
is reminiscent of the dilatometric measurements broadly used for
the characterization of oxide conductors65-67 in that the deformation
of the material in response to electrochemical stimulus (tip bias) is
determined. Note that the direct (deformation induced by bias) and
reverse (bias induced by deformation) electromechanical coupling
effects have been reported for macroscopic geometries as well68,69.
Principles of ESMESM imaging (Fig. 3) is based on detecting the strain response of a
material to an applied electric field though a blocking or electrochemically
active SPM tip (functionalized directly or placed in an ion-containing
medium). The biased SPM tip concentrates an electric field in a
nanometer-scale volume of material, inducing interfacial electrochemical
processes at the tip-surface junction and ionic currents through the
solid71. The intrinsic link between concentration of ionic species and/or
oxidation states of the host cation and the molar volume of the material
Fig. 3 The principle of electrochemical strain microscopy. (a) In ESM, a periodic bias is applied to the SPM tip in contact with the sample surface. The applied bias
induces ionic motion in the sample and the resulting surface deformation is detected by the SPM probe and electronics, generating an image that maps the ionic
motion at the nanoscale. Reprinted from63 with permission from Wiley. (b) The dependence of the c-lattice parameter (perpendicular to the layers) on lithiation in
a prototypical LixCoO2 cathode material. Reproduced from70 by permission of The Electrochemical Society.
(a) (b)
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results in electrochemical strain and surface displacement. This is the case
for many ionic and mixed ionic-electronic conductors such as ceria72,
cobaltites73,74, nikelates74, and manganites75. Similarly, insertion and
extraction of Li-ions in a Li-battery electrodes produce changes of molar
volume75,76. For the example of LiCoO2, the lattice parameter changes
with the degree of lithiation, x, by 40 pm (Fig. 3b) in the operation region
of LixCoO2 from x = 1 to x = 0.5. Combined with the 3 – 4 pm sensitivity
limit of modern AFMs, this suggests that a lithiation state change of just
10 % can be measured through 1 unit cell of material.
The electrochemical interactions in the tip-surface junction are
governed by the nature of the tip and surface material and the
surrounding medium, much like reversible and polarizable electrodes in
classical electrochemical strategies29,30. For the blocking tip electrode,
the electron transfer between tip and surface and the non-uniform
electrostatic field result in mobile ion redistribution within the solid but
no electrochemical process at the interface. Note that for sufficiently
high bias, “non-standard” electrochemical processes become possible,
e.g., the formation of SiC on the SiO2 surface in hexane77, formation
of carbon from CO278, or electrochemical injection of dislocations in
oxides79. In principle, ESM can be performed in a liquid Li-containing
electrolyte, even for finite electronic conductivities the ac electric field
is concentrated in the tip-surface junction, as recently demonstrated in
ferroelectric materials80,81. However, corresponding image formation
mechanisms can be expected to differ significantly from the ambient
Fig. 4 Mapping the electrochemical strain response with an amorphous Si anode. (a) Contact resonance amplitude map of a 1 × 1 μm area with the AFM deflection (topography) signal (inset), and (b) resonance frequency map showing the heterogeneous strain response and a strong correlation between the resonance frequency and topography. (c) Single-point contact resonance peak with (d) the corresponding phase from the grain and grain boundary region. (e,f) Corresponding amplitude and phase profiles illustrating variations of signal strength (color) and resonant frequency (vertical axis) along the surface (horizontal axis).
(a) (b)
(c) (d)
(e)
(f)
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situation. Under ambient conditions, the formation of a liquid
droplet at the tip-surface junction82,83 provides a Li-ion reservoir,
rendering electrodes partially reversible. A similar effect can occur for
blocking electrodes at high biases (Li-extraction and tip plating) or for
Li-electrolyte-coated electrodes. Finally, ESM can be performed on
the surface of top-electrode devices, or on a uniformly biased mixed
electron-ionic conductor, similar to piezoresponse force microscopy of
capacitor structures36,84.
Dynamic modes in ESMIn contrast to the well-polished ceramics or smooth epitaxial films
typically studied in the context of condensed matter physics and
materials science, one of the significant difficulties in imaging
electrochemical materials is their high intrinsic surface roughness.
This topographic roughness can couple into the measured ESM signal
through direct effects, where the signal depends on contact radius
and local slope (e.g., contact stiffness), and indirect effects where the
frequency dependent transfer function of the cantilever depends on
the contact radius and local slope. This cross coupling is well known
in SPM (generally referred to as “topographic cross-talk”), and hinders
quantitative and even qualitative measurements.
Crosstalk with surface topography can be avoided or minimized using
multifrequency excitation schemes including band excitation (BE)85, dual
AC resonance tracking (DART)86, and other dynamic methods87-89. In BE,
Fig. 5 (a) Correlation between OP and IP ESM response and grain orientation. (b) Topography, (c) deflection, (d) OP ESM map, (e) IP ESM map for a 2 × 2 μm area
on a LiCoO2 thin film. Note the complementary character of information in the OP and IP maps in (d) and (e).
(a)
(e)
(b) (c)
(d)
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the excitation and detection are performed using a signal with defined
amplitude and phase content over a given frequency interval. In DART,
the amplitude-based feedback is used to track cantilever resonance and
its quality factor. These methods allow effective use of cantilever resonant
amplification and decouple the intrinsic surface response from variations in
resonant frequency induced by surface topography. This approach avoids
the indirect topographic cross-talk inevitable in single frequency SPMs90.
As an illustration of BE ESM imaging, Fig. 4 shows an example of
an amorphous Si anode. Figs. 4a,b illustrate the electrochemical strain
microscopy response amplitude and resonant frequency obtained
at each spatial point from resonant frequency curves as shown in
Figs. 4c,d. Note the strong variability of resonant frequency within
the grains by ~30 kHz, as compared to the resonant peak width of
3 – 5 kHz. In the constant frequency method, this will lead to very
strong cross-talk with topography, whereas feedback-based methods
will lose stability due to a lack of a well-defined response phase
(for phase locked-loop based methods) or zero amplitude at certain
locations (for DART). The typical evolution of the amplitude and phase
response signal across the line of the surface is illustrated in Figs. 4e,f.
Vector ESM imagingThe bias-induced surface displacement in ESM is generally a vector
having three non-zero components. The full surface displacement vector
in ESM can be characterized through cantilever deflection and torsion
measurements, similar to the approach used in vector piezoresponse force
microscopy of ferroelectric and piezoelectric materials91. The measurement
of the full displacement vector is especially important for materials with an
anisotropic ionic conduction and volume change, as schematically shown in
Fig. 5. For layered LiCoO2, chosen here as an example, the ionic transport
is fastest along the (001) Li-planes, whereas the strongest volume change
occurs along the [001] direction perpendicular to the planes. The vertical
(out-of-plane, OP) and lateral (in-plane, IP) component of the surface
displacements for differently oriented LiCoO2 is shown (Fig. 5a).
Figs. 5b-e display the correlation between topography and the
measured OP and IP ESM amplitude signals for LiCoO2 thin films. Here,
OP and IP ESM signals are recorded around the deflection and torsional
contact resonance frequencies of the cantilever. The topography and
deflection signals are shown in Figs. 5b and c, respectively. The maximum
OP and IP ESM amplitudes are displayed in Figs. 5d,e. Both images show
strong variations in the ESM response across the scanned area. In addition,
the OP and IP ESM amplitude maps do not show the same features,
demonstrating no or minimum cross-talk between the cantilever deflection
and torsion. When Figs. 5d,e are compared, grains with OP and IP response
(#1), no OP but IP response (#2), and OP but no IP response (#3) can be
identified. In the future, these local variations of IP and OP ESM can be
related to crystallographic orientation and electrochemical activity.
Spatially-resolved spectroscopies in ESMThe ESM signal can be used as a basis for a broad set of voltage and
time spectroscopies. The spectroscopic techniques in ESM have been
Fig. 6 Mapping of Li-ion relaxation in an amorphous thin film Si anode. (a) Scheme of voltage pulses applied to measure relaxation maps. (b) Single-point relaxation curves from two points in the map. (c) Deflection signal of a 1 × 1 μm area showing grain boundaries. (d) Map of maximum displacement measured after a −18 V voltage pulse of 30 ms duration.
(a) (b)
(c) (d)
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developed following the protocols of classical electrochemical methods
(e.g., potentiostatic intermittent titration, galvanostatic intermittent
titration, electrochemical impedance spectroscopy), in which the local
electrochemical strain signal substitutes for macroscopic Faradaic
currents. Once properly calibrated, these techniques offer the potential
for implementation of the rich panoply of electrochemical techniques
on the nanoscale level in a spatially-resolved fashion, some of which are
illustrated below.
Fig. 7 Mapping Li-ion diffusion. (a) Deflection signal of a 1 × 1 μm area showing a triple boundary junction of an amorphous Si anode. (b) Map of displacement
loop opening for a voltage sweep of 7 Hz and ±15 V. Reprinted with permission from98. ©2010 American Chemical Society. The loop opening is a direct measure of
Li-ion diffusivity. (c) Single-point displacement loops from three different areas as indicated in (b). Similarly, (d) deflection, (e) displacement loop opening map,
and (f) single-point displacement loops for a LiCoO2 cathode film.
Fig. 8 Separation of transport and electrochemical reaction within a Si anode. (a) Scheme of the expected displacement loop opening as a function of voltage used
to measure the displacement loops. The case of linear diffusion and diffusion after activating the electrochemical reaction with an onset voltage is shown.
(b) Displacement loops as a function of maximum bias voltage. (c) Map of electrochemical reaction onset voltage in a 500 × 500 nm area around a triple boundary
junction. (d) Single-point curves extracted from the boundary and grain region as indicated in (c).
(a) (b) (c)
(d) (e) (f)
(a) (b)
(c) (d)
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ESM time spectroscopyIn ESM time spectroscopy92,93, the signal is measured after the application
of a single voltage pulse to the probe, and the response is measured
over a long time (ideally, comparable to the diffusion time of Li-ions).
In the mapping mode, the relaxation curves are measured over a grid
of locations on the sample surface giving rise to a 3D data set, and the
characteristic parameters (relaxation times, relaxation amplitude) are then
extracted from each relaxation curve and plotted as 2D maps94,95. The
diffusion length can often be determined from spatially resolved images,
thus allowing quantitative determination of the diffusion coefficient.
This spectroscopic method is somewhat reminiscent of the well-known
potentiostatic and galvanostatic intermittent titration techniques96,97, but
is performed on the nanometer scale in a spatially-resolved fashion.
An example of relaxation mapping of the amorphous Si anode
is illustrated in Fig. 6. Here, the excitation waveform at each spatial
location is formed by a sequence of positive and negative bias pulses
to minimize the device charging. Following the bias pulse application,
the relaxation of the electromechanical signal is probed by a sequence
of BE pulses. The relaxation curves from two surface locations are
shown in Fig. 6b. The corresponding surface deflection map is shown in
Fig. 6c. The maximum relaxation amplitude shown in Fig. 6d illustrates
that strong relaxation of the ESM signal is observed only in the grain
boundary like regions and a number of “hot spots” within the material.
These regions thus correspond to the locations with maximal ionic
activity and can be mapped with ~10 nm spatial resolution.
Voltage spectroscopy As an alternative to time spectroscopy, the ESM measurements can be
performed in voltage spectroscopic mode. In this ca se, voltage pulses of
increasing and decreasing amplitude are applied to the probe, and the
electrochemical strain response is tested after each pulse. The voltage
sweep provides the advantage of faster measurements (compared to
time spectroscopy, where only one voltage is tested at a time) and
yields information about voltage-activated electrochemical processes
and transport in the probed volume.
Fig. 7 shows an example of ESM voltage spectroscopy of Si anode
and LiCoO2 cathode materials. The surface topography of the Si anode
illustrates the presence of grain-boundary-like features, likely induced
by the roughness of the alumina substrate and associated with the
disruption of short range order in the amorphous Si (as evidenced by
sharpness of the feature). The ESM hysteresis loops are measured and
the areas of the loops are plotted as a 2D spatial map in Fig. 7b. The
hysteresis loops extracted from several spatial locations are shown in
Fig. 7c. The open loops are highly localized at the grain-boundary-like
features, with effective resolution well below 10 nm. This behavior is
indicative of high localization of Li activity at the grain boundaries. Note
that this behavior cannot be ascribed to topographic crosstalk, since the
latter primarily affects conservative tip-surface interactions rather than
the hysteresis of bias response. Figs. 7d-f show the same measurement
performed on a thin film LiCoO2 cathode material. As for the Si anode,
areas of different Li-ion transport properties can be identified.
Fig. 9 Evolution of loop opening maps with battery cycling at 7 Hz with ±16 V. Loop opening map after (a) 103, (b) 104, (c) 3 × 104, and (d) 105 measurement cycles. (e) Strain hysteresis of the boundary region for increasing cycle number. (f) Charging curves of fresh and high-frequency, strongly cycled batteries. Reprinted with permission from98. ©2010 American Chemical Society.
(a) (b) (c) (d)
(e) (f)
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Reaction-transport separationElectrochemical processes are typically composed of several interfacial
reactions and diffusion steps. A reaction is typically exponentially
dependent on overpotential while transport (diffusion and/or
migration) changes linearly with driving force. Consequently, for low
potentials the process is limited by reaction, while for high potentials it
is limited by diffusion. Hence, measuring the ESM signal as a function
of bias pulse magnitude allows differentiation of the reaction and
diffusion, as illustrated in Fig. 8 for the case of the Si anode.
Evolution of Li concentration in Si anode on chargeESM allows one to systematically follow in detail the changes in Li-ion
diffusion on a local scale during battery charging/discharging or battery
fading. Fig. 9 illustrates the evolution of ESM activity on a silicon anode
surface at different stages of cycling. Note the gradual disappearance
of defect-related “hot spots” and the increase of contrast at grain
boundaries. This behavior is attributed to the accumulation of Li-ions in
the grain boundaries and exclusion of the dominant parts of the sample
from the electrochemical process, providing insight into the causes of
the loss of capacity on subsequent charging.
Future challengesAs reviewed above, ESM is a technique capable of probing
electrochemical reactivity and ionic flows in solids on the sub-10
nanometer level, made possible using the intrinsic link between
electrochemical processes and strains. ESM can be extended to a
broad spectrum of time and voltage spectroscopies; however, broad
acceptance and implementation of ESM hinges on a number of technical
breakthroughs. Some of these are standard targets of SPM development,
such as higher spatial resolution, sensitivity, energy and time resolutions.
At the same time, several development directions are specific to ESM,
including development of SPM platforms operating under realistic
electrochemical conditions (electrolytes, reactive gases, temperature
ranges), theoretical understanding of ESM signals, extending ESM
towards mapping irreversible electrochemical processes, and collecting
chemical information.
For in situ ESM, imaging in conductive liquids presents an obvious
problem. While ac electric fields can be localized even in conductive
liquids91, the dc tip-surface potential difference cannot be maintained92.
While multiple prototypes of insulated and shielded probes have been
developed68-70, such probes are not yet broadly available. Similarly, ESM
studies of fuel call materials necessitates development of appropriate
environmental cells maintaining temperatures, gas compositions and
pressures similar to operational fuel cells.
Quantitative interpretation of ESM requires both understanding
aspects of SPM operation (i.e., the relationship between the surface
deformation below the tip and signal detected by SPM electronics)
and materials behavior (relationship between electromechanical
response and local electrochemical functionality) which has to be
addressed by combining experiment and theory. For a simplified case
of purely diffusional coupling, the theoretical principles of ESM have
been considered in several recent publications99,100. However, the
future development of ESM requires the development of analytical
approximations and numerical models reproducing voltage-divider effects
in the tip-surface junction and bulk, strain evolution in the material,
and associated surface deformations. Recent analyses by Garcia101 and
Ciucci102,103 illustrated possible pathways toward addressing these
problems.
The primarily limitation of ESM is the lack of chemical information,
especially limiting for multi-component materials or in studying complex
sets of electrochemical and chemical transformations. The combination of
ESM and microRaman/near field scanning optical microscopy (NSOM) will
allow us to add local chemical sensitivity and systematically explore the
local electrochemistry at the tip-surface junction. Similar opportunities
can be provided by the combination of ESM with focused synchrotron
x-ray microdiffraction and microfluorescence which can be a powerful
tool to provide the needed phase composition, crystal orientation, and
micron-scale chemical and strain changes104.
OutlookThe capability for probing electrochemical processes and ionic transport
in solids is invaluable for the study and improvement of a broad range
of energy technologies and applications, including batteries and fuel
cells. However, the progress in this field has been limited by an almost
complete lack of tools capable of probing the local electrochemical
activity on the nanoscale.
ESM offers a universal method for probing ionic and electrochemical
processes in solids on the nanoscale. To date, ESM has been demonstrated
for a variety of lithium-ion materials (including layered transition metal
oxide cathodes79, silicon anodes105, and electrolytes such as LISICON),
oxygen electrolytes (including yttria-stabilized zirconia [YSZ] and
samarium-doped ceria106), mixed electronic-ionic conductors for fuel cell
cathodes, and some proton conductors. The data, as well as the near-
universal presence of chemical expansion in electrochemical systems,
suggest an extremely promising potential for future developments and
applications.
AcknowledgementsThe effort by SVK and NB was supported as a part of the Fluid
Interface Reactions, Structures and Transport (FIRST) Center at Oak Ridge
National Laboratory, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy Sciences
under Award Number ERKCC61. Parts of this research (SJ, TMA, AK, AT) were
performed at the Center for Nanophase Materials Science sponsored by the
Office of Science, Basic Energy Sciences Program, Division of User Facilities.
TMA was supported in part by DOE SISGR program. The authors are deeply
grateful to J. Budai for valuable advice regarding x-ray microprobe, and A.
Borisevich and R. Unocic for multiple discussion of STEM-SPM combinations.
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REFERENCES
1. http://en.wikipedia.org/wiki/Volta_pile
2. http://en.wikipedia.org/wiki/Leclanch%C3%A9_cell
3 Huggins, R. A., Advanced Batteries: Materials Science Aspects, Springer-Verlag, New York, (2008).
4. Nazri, G. A., and Pistoia, G., (eds.), Lithium Batteries: Science and Technology, Springer-Verlag, New York, (2009).
5. O’Hayre, R., et al., Fuel Cell Fundamentals, John Wiley & Sons, New York, (2009).
6. Bagotsky, V. S., Fuel Cells: Problems and Solutions, Wiley, New York, (2009).
7. O’Hayre, R., Probing Electrochemistry at the Micro Scale: Applications in Fuel Cells, Ionics, and Catalysis, VDM Verlag, Saarbruecken, (2008).
8. Girishkumar, G., et al., J Phys Chem Lett (2010) 1, 2193.
9. Sawa, A., Mater Today (2008) 11, 28.
10. Strukov, D. B., et al., Nature (2008) 453, 80.
11. Yao, J., et al., J Am Chem Soc (2011) 133, 94.
12. Milne, J. S., et al., Phys Rev Lett (2010) 105, 226802.
13. Soukiassian, A., et al., Appl Phys Lett (2010) 97, 192903.
14. Nonnenmann, S. S., et al., Appl Phys Lett (2010) 97, 102904.
15. Wang, J. L., and Trolier-McKinstry, S., Appl Phys Lett (2006) 89, 172906.
16. Meyer, R., et al., Appl Phys Lett (2005) 86, 112904.
17. Tagantsev, A. K., et al., J Appl Phys (2001) 90, 1387.
18. Basic research needs for electrical energy storage, Report of the DOE BES workshop on energy storage, (2007).
19. Sethuraman, V. A., et al., J Power Sources (2010) 195, 3655.
20. Vetter, J., et al., J Power Sources (2005) 147, 269.
21. Moss, P. L., et al., J Power Sources (2009) 189, 66.
22. Menke, T., et al., J Appl Phys (2009) 106, 114507.
23. Yang, J. J ., et al., Nanotechnology (2009) 20, 215201.
24. Magasinski, A., et al., Nature Mater (2010) 9, 353.
25. Winter, M., et al., Adv Mater (1998) 10, 725.
26. Kwon, D. H., et al., Nature Nano (2010) 5, 148.
27. Bard, A. J ., and Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications, John Wiley & Sons, New York, (2001).
28. Newman, J. S., Electrochemical Systems, Prentice Hall, New Jersey, (1980).
29. Rief, M., et al., Science (1997) 275, 1295.
30. Noy A., (Ed.), Handbook of Molecular Force Spectroscopy, Springer, New York, (2010).
31. Ritort, F., J Phys:Condens Matter (2006) 18, R531.
32. Gruverman, A., and Kholkin, A., Rep Prog Phys (2006) 69, 2443.
33. Kalinin, S. V., et al., Rep Prog Phys (2010) 73, 056502.
34. Kalinin, S. V., et al., Adv Mater (2010) 22, 314.
35. Fan, F. R., and Bard, A. J., Science (1995) 270, 1849.
36. Freund, J., et al., Micros Res and Tech (1999) 44, 327.
37. Martinez, R. V., and Garcia, R., Nano Lett (2005) 5, 1161.
38. Martinez, R. V., et al., Nanotechnology (2007) 18, 084021.
39. Garcia, R., et al., Appl Phys Lett (2010) 96, 143110.
40. Chien, F. S. S., et al., J Appl Phys (2002) 91, 10044.
41. Lee, M., et al., Appl Phys Lett (2004) 85, 3552.
42. Glatzel, T., Kelvin Probe Force Microscopy, Springer, New York, (2011) to be published
43. Kalinin, S. V., and Gruverman, A., (Eds.) Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale, Springer, New York, (2006).
44. Cunningham, S., et al., Appl Phys Lett (1998) 73, 123.
45. Kalinin, S. V., and Bonnell, D. A., Nano Lett (2004) 4, 555.
46. Lee, W., et al., Nanotechnology (2009) 20, 445706.
47. Schirmeisen, A., et al., Phys Rev Lett (2007) 98, 225901.
48. Schirmeisen, A., et al., Z Phys Chem (2010) 224, 1831.
49. Coffey, D. C., et al., Nano Lett (2007) 7, 690.
50. Park, M., et al., J Power Sources (2010) 195, 7904.
51. Kostecki, R., et al., Electrochim Acta (1999) 45, 225.
52. Matsuo, Y., et al., J Electrochem Soc (2001) 148, A687.
53. McEvoy, T. M., and Stevenson, K. J., Langmuir (2005) 21, 3529.
54. Bussian, D. A., et al., Nano Lett (2007) 7, 227.
55. Louie, M. W., et al., ACS Nano (2010) 4, 2811.
56. Arruda, T., et al., Nano Lett (2011) dx.doi.org/10.1021/nl202039v.
57. Szot, K., et al., Nature Mater (2006) 5, 312.
58. Kalinin, S. V., et al., ACS Nano (2011) 5, 5683.
59. Maksymovych, P., et al., Science (2009) 325, 1421.
60. Rosner, B. T., and van der Weide, D.W., Rev Sci Instrum (2000) 73, 2505.
61. Frederix, P. L. T. M., et al., Nanotechnology (2005) 16, 997.
62. Rodriguez, B. J., et al., Appl Phys Lett (2007) 91, 093130.
63. Kalinin, S. V., and Balke, N., Adv Mater (2010) 22, E193.
64. Balke, N., et al., “Real Space Mapping of Ionic Diffusion and Electrochemical Activity in Energy Storage Materials” (patent pending).
65. Adler, S. B., J Am Ceram Soc, (2001) 84, 2117.
66. Kharton, V. V., et al., Chem Mater (2007) 19, 2027.
67. Zuev, A. Y., and Tsvetkov, D. S., Solid State Ionics (2010) 181, 557.
68. Chin, T. E., et al., Electrochem. Solid State Lett (2006) 9, A134.
69. Pannikkat, A. K., and Raj, R., Acta Mater, (1999) 47, 3423.
70. Amatucci, G. G., et al., J Electrochem Soc (1996) 143, 1114.
71. Balke, N., et al., Nature Nano (2010) 5, 749.
72. Bishop, S. R., et al., Electrochimica Acta (2009) 54, 1436.
73. Zuev, A. Y., et al., Solid State Ionics (2008) 179, 1876.
74. Lein, H. L., et al., Solid State Ionics (2006) 177, 1795.
75. Cheng, Y. T., and Verbrugge, M.W., J Power Sources (2009) 190, 453.
76. Zhang, X., et al., J Electrochem Soc (2008) 155, A542.
77. Garcia, R., et al., J Chem Soc Rev (2006) 35, 29.
78. Garcia, R., et al., Appl Phys Lett (2010) 96, 143110.
79. Ueno, K., et al., Appl Phys Lett (2010) 96, 252107.
80. Rodriguez, B. J., et al., Phys Rev Lett (2006) 96, 237602.
81. Rodriguez, B. J., et al., Phys Rev Lett (2007) 98, 247603.
82. Weeks, B. L., et al., Langmuir (2005) 21, 8096.
83. Scovell, D. L., et al., Chem Phys Lett (1998) 294, 255.
84. Kalinin, S. V., et al., Appl Phys Lett (2008) 92, 152906.
85. Jesse, S. et al., Nanotechnology (2007) 18, 435503.
86. Rodriguez, B. J., et al., Nanotechnology (2007) 18, 475504.
87. Proksch, R., and Dahlberg, E. D., Rev Sci Instr (1993) 64, 912.
88. Kos, A. B., and Hurley, D. C., Meas Sci Technol (2008) 19, 015504.
89. Nath, R., et al., Appl Phys Lett (2008) 93, 072905.
90. Jesse, S., et al., Nanotechnology (2010) 21, 405703.
91. Kalinin, S. V., et al., Microscopy and Micoanalysis (2006) 12, 206.
92. Guo, S., et al., J Electrochem Soc (2011) 158, A982.
93. Jesse, S., et al., J Electrochem Soc (2011) submitted.
94. Kalinin, S. V., et al., Appl Phys Lett (2009) 95, 142902.
95. Kalinin, S. V., et al., Phys Rev B (2010) 81, 064107.
96. Wen, C. J., et al., J Electrochem Soc (1979) 126, 2258.
97. Weppner, W., and Huggins, R. A., Annu Rev Mat Sci (1978) 8, 269.
98. Balke, N., et al., Nano Lett (2010) 10, 3420.
99. Morozovska, A. N., et al., J Appl Phys (2010) 108, 053712.
100. Morozovska, A. N., et al., Phys Rev B (2011) 83, 195313.
101. Garcia, R. E., et al., J Electrochem Soc (2011) submitted.
102. Ciucci, F., et al., Phys Chem Chem Phys (2011) 13, 2121.
103. Lai, W. and Ciucci, F., Electrochem Acta (2010) 56, 531.
104. Ice, G. E., et al., Mat Sci Eng A (2009) 524, 3.
105. Balke, N., et al., ACS Nano (2010) 4, 7349.
106. Kumar, A., et al., Nature Chem (2011) 3, 707.
MT1411p548_559.indd 558 31/10/2011 14:40:54
Submit your paper todaywww.nanoenergyjournal.com
‘Research in energy will be at the core of science and technology for decades to come, and will affect the quality of life for every one of us. My intention is that the journal of Nano Energy will become a leading platform to communicate global research in green and sustainable energy using nanomaterials and nanotechnology.’ Z.L. Wang, Editor-in-ChiefSchool of Materials Science & Engineering, Georgia Institute of Technology, 500 10th Street NW, Atlanta, GA 30332, USA, Email: [email protected]
Nano Energypublishes original experimental and theoretical research on all aspects of energy-related research which utilizes nanomaterials and nanotechnology. Manuscripts of four types are considered: review articles which inform readers of the latest research and advances in energy science; rapid communications which feature exciting research breakthroughs in the fi eld; full-length articles which report comprehensive research developments; and news and opinions which comment on topical issues or express views on the developments in related fi elds.
The editors welcome contributions on a variety of topics such as: Batteries | Fuel Cells | Hydrogen generation and storage | Light emitting diodes | Optoelectronic devices for effi cient energy usage | Photovoltaics | Piezoelectric nanogenerators | Policy and perspectives in energy | Self-powered nanodevices/nanosystems | Supercapacitors | Thermoelectrics
MT1411p548_559.indd 559 31/10/2011 14:40:54
NOVEMBER 2011 | VOLUME 14 | NUMBER 11560
FEATURE COMMENT
Scientific coopertition: can it scale and work?
Collaboration and competition. The two words any ambitious scientist
knows too well and has learned to live with. On the one hand, addressing
cutting-edge scientific problems requires an increasing amount of knowledge
and expertise, as well as inter-disciplinary skills, demanding some form of
collaboration with colleagues and external experts. But on the other hand,
there are only limited number of good research and teaching positions, and
research funds are limited. Time is also of the essence. Direct competition is
therefore unavoidable.
In industry, this mix of collaboration and competition is better known as
“coopertition” and has been exercised for years. For example, in the automobile
In materials science, collaborations tend to be limited to the cooperation of several small research groups. Meanwhile, in the world of particle physics, it’s an entirely different story. Markus Nordberg and Fabiola Gianotti from the LHC’s ATLAS experiment, discuss the rewards and difficulties of large scale collaboration.
Markus Nordberg and Fabiola Gianotti | ATLAS Experiment, CERN | [email protected]
The ATLAS detector. Forty five meters long and seven
thousand tons in weight. ATLAS Experiment © 2011 CERN.
MT1411p560_563.indd 560 13/10/2011 12:49:44
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 561
and avionic industries, large manufacturers use the same few subcontractors and
occasionally form specific alliances among each other. The number of competing
partners in such alliances can run in the excess of tens of companies.
But how about in scientific research, say in physics in general, where the
specific research domains are rather well established, not expanding rapidly, and
where scientists (more or less) know each other? Can research undertakings
involving a lot more than say, twenty contributing scientists, work? And above
all, does it make any sense?
Come together, right nowThis question is surely not new. A healthy suspicion about scaling up scientific
collaborations has existed ever since the mid 1930s when the epoch of “Big
Science” started. This is when physicists in the US started to team up to design
and run particle physics accelerators and later on, to construct and operate large
physics experiments. The trend was later picked up in Europe and elsewhere.
And the last 60 years or so has shown that this approach works, and that highly
ambitious, complex scientific goals are accessible with enough people, cash, and
even more patience.
As an example, we can examine the ATLAS experiment at the Large Hadron
Collider (LHC) at CERN, of which we are members. ATLAS brings together more
than 3000 scientists, engineers, and PhD students from more than 170 scientific
institutions around the world. We study the conditions of our Universe just a
fraction after the Big Bang. We look at traces of particles we believe existed
then and try to understand the underlying laws behind their interactions. To
achieve our research goals, we needed a new tool: a large particle detector.
ATLAS was built by our own community and completed in 2008. It lies in a large
FEATURE COMMENT
The Large Hadron Collider is the world’s largest particle accelerator; measuring twenty seven kilometers in circumference, and buried beneath the Franco-Swiss border. © 2011 CERN.
“Can research undertakings involving a lot more than say, twenty contributing scientists, work?
And above all, does it make any sense?”
MT1411p560_563.indd 561 13/10/2011 12:49:50
NOVEMBER 2011 | VOLUME 14 | NUMBER 11562
FEATURE COMMENT
cavern some 100 meters below the ground. It is about 45 meters long, 23 wide,
and weights about 7000 tons: as much as the Eiffel Tower. It has over 10 million
functional elements, perhaps ten times more in terms of pieces touched by a
human hand at least once during the production phase that took over 10 years
to complete.
In order to bring this all together, a community had to be established and
it had to learn to work together. How did this happen? First, a small group of
scientists got together in a rather informal way to discuss how a detector could
be built to best explore the expected new physics. These coffee-table discussions
then started to include more colleagues, who performed computer simulations
of both the physical processes and behavior of the detector, explored the use of
suitable materials, and considered the related signal collection and processing
technologies. These interactions in turn led into larger, common meetings,
where scientists and engineers started to work towards splitting the detector
into meaningful sub-projects or sub-systems.
Second, communities started to emerge around the proposed sub-systems
and they in turn started to organize themselves, benefitting from general
guidance from the core group that had started the process. Moreover, technical
review panels of external experts were set up to evaluate the documented
technical solutions that the sub-systems were proposing. In parallel, CERN, in
its role of the Host Lab, put in place a peer review system to follow and finally
approve the progress being made on the sub-system level.
Third, an overall collaboration structure was put in place. This meant
establishing a Collaboration Board that every participating institute could join,
all with equal voting powers. The rules for such a Collaboration Board were
worked out, again based on the principle of minimum control. The rules laid out
procedures for joining the experiment, formalized the shared obligations, both in
terms of the nature of the scientific and financial input, and stated the common
scientific policies (e.g., concerning working together on individual topics and
signing off scientific papers). In addition to the Collaboration, an Executive Board
was established for the running of day-to-day operations, spearheaded by a small
Management Team. Each sub-system in turn set up their own internal structures
including their Institute Board and Project Leaders, somewhat in a symmetrical
way to the overall ATLAS organization. This is probably the key feature that allows
the collaboration to scale. Later on, as the experiment started to collect data, the
physics groups consolidated their structures around topics of major interest.
Fourth, the funding mechanisms and resources allocation processes were
established. This meant creating a Resources Review Board so that the ATLAS
Management could present, to all participating Funding Agencies, on a periodic
basis, the progress made and the resources consumed, and make resource
requests for the future. As a point of reference, the (direct) construction costs
of ATLAS amounted to some 540 million Swiss Francs and the annual operation
costs amount to about 4 % of that.
Finally, all of the above was recorded and agreed to in a nine page
Memorandum of Understanding (MoU) which all of the Funding Agencies
signed. This MoU was light in structure and mainly based on the principle of
deliverables, where participating institutes commit to building an agreed set of
hardware or software at a fixed cost. This meant that the bulk of the spending
took place at home, facilitating book keeping and financial reporting. The MoU
tried to give maximum flexibility to all partners to facilitate what was expected
from them and in fact, the MoU was declared as legally non-binding. We were
later told by lawyers that the notion of such an agreement is very strange, but
was undoubtedly effective!
Should you try this at home?Although putting all this together may sound straightforward, even if it appears
bulky, it took almost 10 years for the ATLAS community to establish itself
and commit on paper to build the detector described above. The way forward
was determined by history, a compelling vision, a common passion for new
discoveries, pragmatism, tolerance, and common sense; rather than by glossy
diagrams adapted from business management books. We are often asked
whether our structures or management approach was influenced by large,
A proton collision, as measured by ATLAS. ATLAS Experiment © 2011 CERN.
“A lot of diplomatic skills are required, and in a scientific community like ours it can
sometimes be difficult to find!”
“Highly ambitious, complex scientific goals are accessible
with enough people, cash, and even more patience.”
MT1411p560_563.indd 562 13/10/2011 12:49:57
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 563
high-tech corporations. It was not, we are embarrassed to admit; although
some management scholars think we should not be!
And such a collaboration had many advantages. First, the collaboration
mode offered full access to bright individuals and expertise in 38 different
countries. This allowed an effective way to expose, address, and solve many
scientific and technical challenges as they emerged that otherwise may have
jeopardized the success of ATLAS in the later years. Second, the collaboration
permitted us to make the best of available industrial technologies at the best
cost: components of ATLAS could be manufactured in the countries most
suitable for producing them, benefiting from a local ATLAS presence. Third, it
offered a new and exciting learning environment for young PhD students: today,
ATLAS has more than 1000 PhD students working on physics and detector
technologies. They get immediate exposure to an international community and
get to present their work in front of a large international audience of leading
experts. They are allowed to grow and blossom in a multi-cultural environment
and learn about tolerance and interacting within the community.
But it’s obvious also that there is a downside. Working together with 3000
scientists and engineers is not the same as doing “garage experiments”. One
needs structures, procedures, and patience. It can sometimes be heavy and
slow and the project reporting lines unclear. The construction phase certainly
was not without some very difficult moments. For example, in the early days
there were several parallel and promising R&D projects on-going to solve
serious technological challenges. Due to limited funding, it became obvious
that only one solution could be followed, and that the funding from the other
competing projects should be used to implement it. So which one to choose?
How to keep everyone on board? How to avoid creating an unhelpful class
of “winners” and “losers”? A lot of diplomatic skills are required, and in a
scientific community like ours it can sometimes be difficult to find!
Working in a large collaboration like ATLAS also has implications back
at the home institutions. Our colleagues do need help when defending their
applications for tenure when being asked by colleagues in other fields about
their individual contribution to a scientific paper carrying 3000 authors (even if
there will soon be over a hundred such papers). It is certainly a valid question
and answering it requires explaining the nature of large collaborations and the
structures within.
The question of competition between ATLAS collaborators and those
in other experiments is also an important factor. Luckily, it has so far not
jeopardized the success of ATLAS. We believe this is because there is a common,
shared motivation which overrides the desire to withhold knowledge from
colleagues in fear of compromising future tenure or senior research positions.
The individuals are usually known within the overall particle physics community
and their work can be assessed and ranked rather well, despite any boundary
conditions that may be implied from their association to a given experiment.
Competition is tough but fair.
Despite the above hardship, we believe large collaborations provide the
required academic, educational, and financial stability to encounter ambitious
projects in fundamental science which extend beyond a decade or two.
Youngsters need to enter the game early and learn the tricks of the trade.
Working closely with people from so many different countries and cultures gives
a totally new meaning to the concept of problem solving and innovation. Even
if one shares a common language, in ATLAS it is English, people see the same
thing in many different ways. It’s a bit like making a raw diamond shine: it’s
hard work, not always fun, but it’s worth it.
Follow the Yellow Brick Road?Although large scale collaboration is the norm in the modern world of particle
physics, it is not intended to imply that our colleagues in other fields of science
should necessarily follow suit. Although following the Yellow Brick Road may
offer plenty of excitement and magic along the way, size alone does not always
matter. What matters is the nature of the scientific goals, the anticipated means
to achieve these goals within a meaningful time frame, and concrete, available
funding. And this, of course, varies across different scientific disciplines.
As illustrated earlier, the strive for such large, global collaboration in
experimental particle physics has been determined by a pragmatic need to
pool resources together and share the work load. This is necessary because the
increased scientific and technological complexity and the way forward has been
part of a natural evolution over decades to accommodate the many boundary
conditions imposed by academic institutions and their funding agencies around
the world. There is some indication that our colleagues in the fields of nuclear
physics, bioinformatics, and computing are testing their toes in the strange
waters of truly global collaboration. Whether other fields find themselves facing
similar challenges remains to be seen.
And for ourselves, shall we continue growing bigger? We don’t know.
Much depends on what new exciting physics comes out of the LHC program
and what the next steps should be. Our expectations are high but in the end,
we can’t force Mother Nature’s hand, either. But irrespective of what there
is for us to discover, we do know that the next generation of collaborations
will look different from the present ones. Perhaps future efforts will be even
more concentrated geographically. Perhaps the community needs to shrink
in size. In any event, the passion to dream about new worlds will remain and
a set of minimal, simple rules will guide the crowd forward. And this is what
fundamental research has always been about.
FEATURE COMMENT
“Although large scale collaboration is the norm in the modern world of particle physics, it is not
intended to imply that our colleagues in other fields of science should necessarily follow suit.”
MT1411p560_563.indd 563 13/10/2011 12:50:01
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MT1411p564_565.indd 564 31/10/2011 14:42:19
BOOKS & MEDIA
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 565
Q & AKirsten BodleySTEMNET | www.stemnet.org.uk
Kirsten Bodley is the Chief Executive
of STEMNET, an organization
dedicated to inspiring young people
in science, technology, engineering,
and mathematics. Materials Today
caught up with Kirsten to find out
what inspires her.
MT: When did your interest in science begin?
KB: I became interested in science at school, thanks to encouraging chemistry and physics teachers who let us take part in engaging and exciting experiments; having a go ourselves. Seeing scientific theories come to life through practicals made learning much more interesting.
MT: What’s the most rewarding part of your job?
KB: One of the most rewarding aspects of my job is working with our nationwide network of volunteers: STEM Ambassadors. I enjoy meeting these role models who volunteer their time to inspire young people in STEM: they always have great stories to share from the school activities they have taken part in!
MT: What did you want to be when you were younger?
KB: Believe it or not, a ballerina! However, It was not to be. I found chemistry at school and always wanted to be in an industry related to it.
MT: What’s your biggest achievement?
KB: Completing and enjoying my NQT year, before moving on to teach Year 5/6. Although challenging (the children never miss a trick!), I found these experiences richly rewarding.
MT: Which scientists have inspired you?
KB: Scientists from the past who have inspired me include Rosalind Franklin, for her pioneering work on the DNA structure, and Marie Curie. In the present day, I find Maggie Aderin-Pocock hugely inspirational, for not only encouraging the next generation of scientists and engineers, but also for increasing public engagement with science. It was an honor to have her as a presenter at The Big Bang London and South East in July; a celebration of STEM for young people, coordinated by STEMNET.
To hear more from Kirsten, as well as a
plethora of speakers from academia and
industry, visit our podcast page at
www.materialstoday.com/podcasts
or by searching for Materials Today on
iTunes.
Industrial Biofouling
Reg Bott started to investigate industrial biofouling long before it acquired the attention that it actually deserves, and he was the first to state through straightforward, elegant experiments, published in the eighties, that nutrients are a key factor for the progress of biofouling, leading to the conclusion that nutrients are a major fouling factor. This is remarkable compared to the common anti-fouling approaches which simply focus on killing bacteria instead of limiting their food.
He now presents a brand-new book on “Industrial Biofouling”. He did his major work on the biofouling of heat exchangers, and, consequently, he begins with a chapter on fluid flow, mass, and heat transfer. Here, solid text book material is presented, helping readers to understand the influence of biofilms on all of these factors. A clear definition of biofouling would have been good, but is missing.
A general chapter on biofilms follows, which unfortunately is strangely outdated. In the introduction, he elaborates briefly on bacteria, but from a time before molecular biology took over and provided a much better understanding of bacterial species, physiology, interactions, and identification methods. The entire chapter is derived from work from the eighties, as is represented by the references: out of 66 citations, 33 are older than 20 years, another 28 are older than 10 years, leaving only 5 recent publications; two of them by the author. In the section about primary adhesion, he explains the physico-chemical approaches for understanding the mechanisms. However, it has long been known that none of these approaches really fit and allow for predictions. As a graphical description of biofilms, unfortunately, he chose an image published in New Scientist that is clearly very uninspired and schematic. In the meantime, there are many other, much better biofilm schemes available; and much deeper, detailed, and process-oriented descriptions of biofilms too.
The chapter on biofouling control reflects the ample experience of the author. Much of it is presented without reference but is very trustworthy. As the obvious, most common countermeasure, he presents the use of biocides. Implicitly, he distinguishes between the killing of bacteria (which will result in leaving biofilms in place) and of the removing of biofilms. But this aspect should have deserved much more attention as it is one of the main reasons for failures of biocide applications: these failures are based on the medical paradigm that killing the problem-causing microorganisms will cure the problem.
For living systems this applies because the immune system performs the cleaning. In technical environments, there are no such systems and dead biomass represents nutrients for subsequent living organisms. It would have been particularly interesting to have provided more information on biodispersants, because they hold the potential to remove biofilms. The methods of physical control concentrate mainly on mechanical principles and are very interesting. It could easily have been amalgamated with the subsequent, very brief, chapter on cleaning.
Then he addresses a very important aspect in anti-fouling strategies; that of monitoring. As we usually don´t have
“eyes in the system”, there is a poor capacity for early warning, and problems are recognized late and not really localized. The practice of water sampling is insufficient to locate or quantify biofilms. He presents a very interesting system, but many others are not mentioned, although in heat exchanger technology, a number of interesting and promising methods have been developed, e.g., based on ultrasound, heat transfer, friction resistance, or optical/spectroscopic principles. It remains surprising and disappointing that this entire branch of industry still operates without proper monitoring as long as they can get away with just
dumping biocides, saving the investment for developing
monitors for industrial application. Here, a considerable optimization potential is wasted. And the chapter is very scarce about these aspects.
Finally, a chapter on biofilms in industry follows. It begins with a superficial section on biofilm reactors, followed by brief sections on biofilms in water treatment and in the food industry. Again, two thirds of the references are more than 10 years old, and are mostly textbook material.
In the concluding remarks, Reg Bott states “As many of the references quoted in this publication illustrate, much research work has been done or is in progress to provide a better understanding of the fundamentals of biofouling and hence the development of improved methods of control.” He is absolutely right. And quite a bit has been achieved in the meantime. Why didn’t he say more about it?
Reg Bott is a long standing member of my hall of fame, and in this text he discusses the challenges and benefits of biofilms on industrial surfacesHans-Curt Flemming | [email protected]
T. Reg. Bott
Industrial Biofouling
Elsevier • 2011 • 220 pp
ISBN: 978-0-444-53224-4
$260.00
MT1411p564_565.indd 565 31/10/2011 14:42:21
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MT1411p566-567.indd 566 31/10/2011 14:51:01
UNCOVERED
NOVEMBER 2011 | VOLUME 14 | NUMBER 11 567
Nano fingerprints
The use of powders to develop latent fingerprints left after criminal activity has been established for many years. However, various types of substrate surfaces, such as rough materials, fabrics, and adhesives are not well suited to this type of technique. Other methods have been developed, including acid dyes, cyanoacrylate fuming (CA), and the evaporation of metals such as gold, zinc, and silver. Protocol tables have been established that apply broad classifications to surfaces and outline appropriate development techniques.
Fingerprints are biochemically complex, containing fatty acids, glycerides, amino acids, and metal ions in various proportions, excreted from eccrine and sebaceous glands. Print composition also varies from person to person, and is strongly affected by factors such emotional state, grooming regime, and intake of food and drugs. This inter- and intra- donor variability further complicates detection and interpretation.
A US report in February 2009 outlined the need for additional and rigorous research on forensic techniques, and this was backed up by practitioners such as the chief forensic pathologist of the New York State Police, who stated in a New Scientist interview, “so many innocent people get convicted … based on junk science”1. More recently, the UK Forensic Science Regulator has stressed the importance of ensuring the validity of methods, with peer-review and publication featuring as cornerstones of quality in forensic practice. This conclusion is reinforced by other studies and court actions that challenge the application and interpretation of fingerprint evidence. Therefore the development of a scientific understanding of forensic fingerprint evidence is both timely and critical in ensuring the continued trust in forensics and the validity of investigative methodology. Researchers across academia and the forensic provider sector have stepped up to the challenge, with recent advances in areas such as quantum dots for fingerprint development, detection of drug residues within fingerprint deposits, and statistical analysis and representation of uncertainties within the courtroom.
Research at Brunel University in London, in association with the UK Home Office, has been pioneering the use
of micro and nanotechnological analysis to improve understanding of the operation and interaction of fingerprint development techniques. A recent study investigated titanium dioxide powders in suspension for developing fingerprints on adhesive tapes, for example from drug packaging2. This work demonstrated a nanoscale variation in particulate coating in commercial formulations that is responsible for the significant differences in the effectiveness of different powder suspensions. This also highlights a problem in detecting fingerprints: no one formulation is effective across all fingerprint donors, or on every material. Research on the surface interaction of development agents can
therefore help to improve development agent selection and hence enhance the detection process3.
Multiple techniques can sometimes be utilized to aid development of fingermarks or obtain additional details, for example, when investigating fingermarks in blood4. However, the interaction of two techniques can sometimes be detrimental and obscure information from the fingerprint, therefore further elucidation of the operation of multiple techniques helps to ensure validity.
This month’s cover image shows a back scattered electron micrograph of a fingermark developed with two sequential techniques. Here, vacuum metal deposition of gold and zinc, following cyanoacrylate development of a latent print leads to zinc nanoparticulate decoration of the polycyanoacrylate deposits5. The image was captured using a field emission scanning electron microscope, the contrast
is dependent on atomic number. Operating in variable pressure mode enables imaging and analysis without the usual addition of a conducting coating.
There is more that a fingerprint could tell us. A wide consortium of research laboratories is investigating the potential to capitalize on the inter-donor variability of the biochemistry of fingerprints. Although a problem in developing prints and designing effective techniques, this variability may make it possible to gather extra intelligence about the victims or perpetrators of crime, such as age, gender, smoking, or drug habits, which could facilitate criminal investigations.
This work aims to improve the efficiency and performance of fingerprint detection, as well as aid the selection of the most appropriate development systems, and so facilitate enhanced and reliable collection of forensic information.
Gathering intelligence
Benjamin J. Jones
Experimental Techniques Centre, Brunel University, UK
E-mail: [email protected]
REFERENCES
1. Geddes, L., New Sci (2009) 201(2697), 6.
2. Jones, B. J., et al., Sci Justice (2010) 50, 150.
3. Jones, B. J., et al., Surf Interface Anal (2010) 42, 438.
4. Au, C., et al., Forensic Sci Int (2011) 204, 13.
5. Jones, B. J., et al., J Forensic Sci, doi: 10.1111/j.1556-4029.2011.01952.x.
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MT1411p566-567.indd 567 31/10/2011 14:51:02
EVENTS DIARY
NOVEMBER 2011 | VOLUME 14 | NUMBER 11568
28 November – 2 December 2011
2011 MRS Fall Meeting & Exhibit
Boston – MA – USA
MRS conducts two major meetings every year, the Spring Meeting in San Francisco and the Fall Meeting in Boston. Consisting of topical symposia, the meetings have become important and not-to-be-missed events for materials researchers around the world for presenting their work, for getting information on up-to-the minute developments in their field, and for networking.
www.mrs.org/fall2011/
8 – 10 December 2011
ICANN-2011 — 2nd International Conference on Advanced Nanomaterials and Nanotechnology
Guwahati – India
The 2nd International Conference on ‘Advanced Nanomaterials and Nanotechnology (ICANN-2011)’ is being organized jointly by the Department of Physics and Centre for Nanotechnology at the Indian Institute of Technology Guwahati (IITG), India. This is going to be another major international conference being held in the North-Eastern region of India, in the area of Nanoscience and Nanotechnolgy. The international conference intends to bring eminent scientists, technologists, and young researchers from several disciplines across the globe together to provide a common platform for discussing their achievements and new directions of research.
www.iitg.ernet.in/icann2011/
11 – 15 December 2011
2nd Nanotoday Conference
Marriott Resort Waikoloa – Hawaii – USA
This second international meeting on nanostructured materials and devices, organized by the journal NanoToday, will showcase the latest research and advances in this increasingly multidisciplinary field. This conference will present the latest research achievements and commercial applications of nanostructured materials and devices.
www.nanotoday-conference.com
11 – 14 December 2011
4th International Conference on the Mechanical Biomaterials and Tissues
Marriott Resort Waikoloa – Hawaii – USA
The conference provides a forum for the discussion of the modeling and measurement of deformation and fracture behavior in biological and replacement materials, and the role mechanical properties play in physiological and disease conditions.
www.mechanicsofbiomaterials.com
8 – 14 January 2012
2012 Winter Conference on Plasma Spectrochemistry
Tucson – Arizona – USA
More than 500 scientists are expected to participate, and over 300 papers on modern plasma spectrochemistry will be presented. A three day exhibition will feature new plasma instrumentation and many supporting products. Six plenary lectures and 24 invited speakers will highlight critical topics in 12 symposia. In addition, six Heritage Lectures will feature outstanding senior researchers.
http://icpinformation.org/2010_Winter_Conference.html
9 – 11 January 2012
BTS 2012 — Biotech Showcase 2012
San Francisco – United States
Biotech Showcase™ is a forum devoted to providing biotechnology and medtech companies, investors, and pharmaceutical executives an opportunity to meet in one place during the course of one of the largest annual healthcare conferences that attracts investors and biopharmaceutical executives from around the world.
www.ebdgroup.com
18 – 20 January 2012
Electronic Materials and Applications 2012
Orlando – Florida – USA
EMA 2012 focuses on electronic materials for energy generation, conversion, and storage applications, highlighting renewable energy, innovative hybrid and all-electric transportation development, electrical ceramics, and advanced microelectronics.
http://ceramics.org/meetings/electronic-materials-and-applications-2012
21 – 26 January 2012
SPIE Photonics West
San Francisco – California – USA
See over 1150 industry-leading companies at the industry’s largest photonics and laser event. If you’re in the photonics and laser industry, kick off the new year at SPIE Photonics West, the essential photonics and laser event.
http://spie.org/x2584.xml
22 – 27 January 2012
36th International Conference and Expo on Advanced Ceramics and Composites
Daytona Beach – Florida – USA
ICACC’12 showcases cutting-edge research and product developments in advanced ceramics, armor ceramics, solid oxide fuel cells, ceramic coatings, bioceramics and more.
http://ceramics.org/meetings/36th-international-conference-and-expo-on-advanced-ceramics-and-composites
15 – 17 February 2012
BioMed 2012 – The Ninth International Conference on Biomedical Engineering
Innsbruck – Austria
In recent years, with the aid of engineering and information technology, biomedical engineering has emerged as a high-tech field, generating innovation in such areas as medical imaging, bioinformatics, MEMS and nanotechnology, new biomaterials and sensors, medical robotics, and neurobiology. Scientists and engineers in this field have recently been working towards such advances as developing artificial organs that mimic natural human organs, conducting telemedicine, performing surgeries with robots, creating a laboratory-on-a-chip, and controlling robots through natural animal brain matter.
www.iasted.org/conferences/home-764.html
26 February – 1 March 2012
Materials Challenges in Alternative and Renewable Energy
Clearwater – Florida – USA
MCARE 2012 facilitates information sharing on the latest materials developments and innovations for solar, wind, hydro, geothermal, biomass, nuclear, hydrogen, electric grid, materials availability, and battery and energy storage.
http://ceramics.org/meetings/materials-challenges-in-alternative-rewable-energy-2012
27 February – 2 March 2012
APS March Meeting 2012
Boston – Massachusetts – USA
The American Physical Society will hold its 2012 March Meeting in Boston, Massachusetts. The conference is expected to play host to over 7000 top scientists involved in physics research and applied physics from around the world.
www.aps.org/meetings/march/
6 – 7 March 2012
Nanomaterials for Biomedical Technologies
Frankfurt am Main – Germany
Nanomaterials in biomedical applications either in vitro or in vivo have raised high expectations for new and ground breaking diagnostic and therapeutic solutions in health care and are already moving from the laboratory bench to clinical application. The success of nanomaterials in these fields is founded on our advanced understanding of molecular mechanisms in biology, the progress of nanostructure sciences in physics, chemistry, and engineering, and our quickly improving ability to mimic biological signals by increasingly complex synthetic structures and interaction functionalities.
www.processnet.org/nanoBiomed2012
FORTHCOMING EVENTS
diaryIf you are organizing a future conference or workshop and would like to have it listed in Materials Today please contact Jonathan Agbenyega – [email protected].
Events Materials Today has a contra deal with and that are relevant to the current issue of the magazine are listed below.
If, as an organizer, you would like to discuss a contra deal, please contact Lucy Rodzynska – [email protected]
For further information please visit www.materialstoday.com/events
28 November – 2 December 2011
2011 MRS Fall Meeting & Exhibit
Boston – MA – USA
MRS conducts two major meetings every year, the Spring meeting in San Francisco and the Fall Meeting in Boston. Consisting of topical symposia, the Meetings have become important and not-to-be-missed events for materials researchers around the world for presenting their work, for getting information on up-to-the minute developments in their field, and for networking.
www.mrs.org/fall2011/
11 – 15 December 2011
2nd Nanotoday Conference
Waikoloa – Hawaii – USA
This second international meeting on nanostructured materials and devices, organized by the journal NanoToday, will showcase the latest research and advances in this increasingly multidisciplinary field. This conference will present the latest research achievements and commercial applications of nanostructured materials and devices.
www.nanotoday-conference.com
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