HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS

Jasmina Grbović Novaković

…because

reserves of fossil fuels are limited???

European economy depends on petroleum exporting countries

we need to secure future individual mobility

we want to reduce greenhouse gases

we aspire to protect our environment by using clean forms of energy

(Why) Do we need alternative fuels and energy carriers?

Alternative fuel

Distributed Generation

TransportationBiomass

HydroWindSolar

Coal

Nuclear

Natural Gas

Oil

Wit

h C

arb

on

Seq

ues

trat

ion

HIGH EFFICIENCY & RELIABILITY

ZERO/NEAR ZEROEMISSIONS

Why Hydrogen?Why Hydrogen? It’s abundant, clean and can be derived from diverse resources.

Alternative fuel

Why not Hydrogen?Why not Hydrogen?

Problem is safe, efficient and cost-effective storage

Alternative fuel

Common requirements for hydrogen storage

• High gravimetric and volumetric storage capabilities

• Cost• Efficiency• Safety• Life cycle• Environmental impact

Hydrogen storage

Hydrogen Storage Options

Mobile applications Volume of 4 kg of hydrogen compacted in different ways, with size relative to the size of the car

Stationary applications

Schlapbach & Züttel, Nature, 15 Nov. 2001

• high pressure cylinders ( up to 35MPa consumes 20% of its total energy content

• cryogenic storage of liquid hydrogenat low temperature (consumes nearly 30% of total energy content

• metal hydride, where hydrogen is chemically bound to a metallic material

• complex hydride

• metal-organic framework materials

• zeolites

• carbon fibres and nanotubes

Solid-state Storage :safer and more efficient

Hydrogen storage

The first attractive interaction of hydrogen molecule approaching the metal surface is Van der Waals force, leading to a physisorbed state. The physisorption energy is typically of order.

molKJEphys

10

molKJEphys

50

In the next step the hydrogen has to overcome an activation barrier for dissociation and for the formation of the hydrogen metal bond. This process is called chemisorption. The chemisorption energy is typically of order . After dissociation on the metal surface, the H atoms generally diffuse rapidly through the bulk metal even at room T to form M-H solid solution

How does solid storage occur?

Solid state storage

Lennard-Jones potential of hydrogen approaching a metallic surface.

The reaction of hydrogen gas with metal can be described in terms of a simplified one-dimensional potential energy curve

Solid state storage

Solid state storage

fcc bcc hcp

In many cases H occupies interstitial sites tetrahedral and octahedral.

A medium value of electronegativity Indicates that H can form various kinds of chemical bonds with

various elements

a) I and II group of elements which has small electronegativity H forms ionic compounds called saline hydrides (M+H- and Mg2

+H2-)

b) Most of Group III–V non-metallic elements form covalently bonded crystals

c) But there is still a large number of elements having comparable electronegativites, namely, d-band metals, lanthanides and actinides, which form metallic hydrides.

Metallic hydrides, by nature of metallic bonding, commonly exist

over extended ranges of nonstoichiometric compositions. These hydrides can be called interstitial alloys, where interstitials sites of metal lattice are occupied by H atom, randomly at high temperatures and in some regular ways at lower T

Hydrides

The TaHx phase diagram according to Schober. α and α‘ are disordered BCC solutions of H in Ta. ε is a tetragonal phase and β, δ, ζ and γ are orthorhombic. The α‘-β is a disorder-order transformation for the Hatoms.

Details of the phase diagram of NbHx. [ Schober and Wenz ]. The full line is a calculation by Kuji and Oates.

Hydrides

T

R

Hydrides

NF 2F is the degree of freedom

is the number of phases

number of chemical species

R

S

RT

Hp

ln

H is enthalpy,

S is entropy

is gas constant

is temperature

N

We mast make a series of isothermal measurements of the equilibrium composition of a specimen as a function of the pressure of surrounding gas e.t PCI

Van’t Hoff plots of some technically important reversible metal hydrides

Hydrides

Hydride Comparison

Classical/ interstitial metal hydrides

No structure changes

Reversible @ ambient T

Tailorable thermodynamic properties

Chemical hydrides

Structure changes

Reversible @ ambient T or irreversible

No tailorability

Hydrogen storage system challenge:

Pack H as close as possible to reach high volumetric densities and use as little additional materials as possible

…we need materials satisfying simultaneously all these requirements?!

Hydrides

Complex light metal hydrides

Structure changes

non Reversible @ ambient T

tailorability

Hydrides

• Rutile-type structure (H/M=2)• Unit cell volume : 33% larger

than metallic Mg large nucleation energy

barrier high temperature and pressure for activation

• Mixture of covalent and ionic

bonds• Heat of formation(-75 kJ/mol H2)

•: high dissociation temperature

•Severe surface oxidation and pyrophoricity

MgH2

• High gravimetric (7.6 wt.%) and volumetric

(130 kg H2/m3) storage capabilities • Endothermic desorption reaction• Low cost

Nanostructuring and nano-scale catalysis through ball-milling

High density of extended defects acting as short circuit path for hydrogen atom diffusion

Can be used to introduce a small amount of catalyst able to support the molecule dissociation

Increase kinetics: diffusion time

Possibility of co-existence of chemi- and physi sorption

Possibility of changing thermodynamic properties

Long H-diffusion distances in bulk materials reduced H-exchange rate

Short H-diffusion distances in

nanoparticle: fast H-exchange rate

Ball milling and catalysis

high energy ball milling to achieve nanostructure- Spex mixer/mill 8000 with hardened steel vials and balls

ball-to-powder weight ratio: has great influence on morphology

time of milling

atmosphere: Ar or H2

Nanostructuring and nano-scale catalysis through ball-milling

Low energy ball milling to introduce catalyst

Ball milling and catalysis

Ball milling and catalysis

DSC trace of MgH2 before and after 20 h of milling.

J. Huot, G. Liang, S. Boily, A. V. Neste R. Schulz, J. Alloys Comp. 1999,293-295, p.495

X-ray powder diffraction of nanocrystalline MgH2

as a function of the milling time

Thermal desorption mass spectra (TDMS) of hydrogen for pure MgH2 milled for 2 h and catalyzed MgH2 with 1 mol % ,Cu, Fe, Co and Ni

Ball milling and catalysis

J. Phys. Chem. B 2005, 109, 7188-7194

N. Hanada, T.Ichikawa, H. Fujii

a) carbon and carbon containing liquid additives, b) catalytic metals c) intermetallic compounds

Different approaches set up in order to improve the hydruration/dehydruration

Improvement of hydrogen storage properties

Mg -C and MgH2- C composites

It has been shown that mechanical milling of magnesium and carbon, in the presence of organic additives (tetrahydrofuran, cyclohexan, benzene, etc), results in material, which has enhance absorption/desorption kinetics.

DSC traces for various (Mg/G)BN , (Mg/G)none and Mg samples.

The (Mg/G) composites were prepared by grinding with benzene (8.0 cm3 BN ) for (a) 4 h, (b) 10 h, (c) 20 h, (d) 30 h and (e) 40 h. (Mg/G) wasprepared by grinding without benzene for 15 h.

Imamura et al.

Improvement of hydrogen storage properties

By addition of C, the time of first hydrogen uptake can be significantly reduced. There is completely transformation of Mg to MgH2. Therefore, a minimal amount of graphite has to be added in order to have synergetic effect.

Improvement of hydrogen storage properties

Montone et al.

H-desorption: DSC scans

300 350 400 450 5000

5

10

15

20 (Mg70 C30) none

(Mg70 C30) 1/3

(Mg70 C30) 3/1

(Mg97 C7) 1/3

(Mg85 C15) 1/6

(Mg85 C15) 1/3

H

eat F

low

(W

/g)

Temperature (°C)

(Mg70 C30)3/1

(Mg85 C15) 1/6

en

do

150 200 250 300 350 400 450 500

0

2

4

6

8

10

12

CFe

=10 wt.%

Hea

t Flo

w (

W/g

)

T (°C)

BPR= 1:1 3:1 10:1 20:1

BPR:20:1BPR:10:1

BPR:3:1 BPR:1:1

CFe=10wt.%

MgH2-Fe

Improvement of hydrogen storage properties

MgH2-intermetallic compounds

450 500 550 600 650 700 750

0

2

4

6

8

10

12

14

BPR 20:1

Milling time: A 1h B 5h C 10h

Hea

t F

low

(W

/g)

Temperature (K)

DSC traces of MgH2 –35 wt.% Mg2NiH4 composite.

Improvement of hydrogen storage properties

Ball-milled mixtures of MgH2 and Mg2NiH4 exhibit a synergetic effect of hydrogen sorption that results in excellent kinetic properties of the composite material. Sample desorbs hydrogen quickly at temperatures around 220 -240C with hydrogen capacity exceeding 5 wt.%. This result is remarkable in that the dissociation of magnesium hydride does not normally occur at temperatures below at least 280C.

Cycling life

1)Hamiltonian of the many-electron system is unique functional of spin densities

NNxcNeeestotEEEETE

)()()()()(

)(

sT

)(

eeE

kinetic energy (of the non-interacting particles,

electron-electron repulsion,

nuclear-electron attraction,)(

NeE

exchange-correlation energy, we do not know this term !

)(

xcE

An efficient way for solving the many-electron problem of a crystal (with nuclei at fixed positions) are the calculations based on density functional theory. DFT is based on following assumptions:

Theoretical approach

the repulsive Coulomb energy of the fixed nuclei and the electronic contributionsNNE

Theoretical approach

2)Minimal energy obtained through variation principle corresponds to spin densities of basic state of system.

Everything works fine if one knows all terms of Hamiltonian. However this is not the case. We need the way to describe exchange-correlation part of interaction.

drExcxc

][*)(

xcii), the particular form chosen for

xcE

xc

Theoretical approach

Two approximations comprise the LSDA, i), the assumption that can be written in terms of a local exchange-correlation energy density times the total (spin-up plus spin-down) electron density as:

Theoretical approach

The most effective way known to minimize Etot by means of the variational principle is to introduce orbitals constrained to construct the spin densities and then solve Kohn -Sham equation

ik

rrVVVikikikxceeNe

2

So????????

Theoretical approach

Like most “energy-band methods“, the LAPW (linearized augmented plane waves ) method is a procedure for solving the Kohn-Shamequations for the ground state density, total energy, and (Kohn-Sham) eigenvalues (energy bands) of a crystal by introducing a basis set which is especially adapted to the problem.

We dividing the unit cell into:(I) non-overlapping atomic spheres (centeredat the atomic sites. The sphere could be described by linearization of radial function in order to exclude energy dependence) and (II) an interstitial region. The interstitial region could be described by plane waves

The density of states (DOS)

X-ray absorption and emission spectra

X-ray structure factors

Optical properties

An analysis of the electron density according to Bader’s “atoms in molecules” theory can be made

Theoretical approach

Theoretical approach

What we can obtain using WIEN 2k?

Theoretical approach

Charge densitiesDOS

Predicted values of the formation enthalpy of binary metal hydrides obtained from DFT-GGA calculations vs. experimental values

Thermodynamically Favorable Does Not Mean Kinetically Favorable

Theoretical approach

100 200 300 400 500 600-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

T/T

T

MgH2-Ti

MgH2-Co

Obtained Hf for Ti -60KJ/molH2

for Co -55KJ/molH2

Summary

• The challenge is clear and fascinating: supplying more and more abundant and clean energy, consuming less and less natural resources and finding the appropriate solutions for any corner of the planet.

• Fundamental theoretical and experimental research is needed to understand the interaction of hydrogen in solid-state materials in order to realize the potential of these materials for hydrogen storage.

The challenge still remains!!!

Public perceptions

Thank you and see you next year

Siemens Nixdorf Notebook powered by a PEM fuel cell /metal hydride tank

At the Hannover Fair 1998 a Siemens Nixdorf laptopcomputer was demonstrated , which was powered by a laboratory PEM fuel cell (FhG ISE Freiburg,Germany) and a commercial metal hydride tank SL002(GfE Metalle und Materialien GmbH, Germany),

A small atomic size of hydrogen:

One might consider intuitively that a hydrogen atom should be small in size because it has one e-. The situation in fact is not so

simple:

H+ has ionic radia from (0.18-0.38 Å) depending on the number of surrounding anions. So what that actually implies is that bare proton causes contractions of the neighboring bonds by the effect of hydrogen bonding

H- has ionic radius 2.1 Å ( halogens has 1.95-2.1 Å)

H has radius of 0.529 Å

Solid state storage

• Severe surface oxidation and pyrophoricity• Sluggish hydrogen diffusion kinetics• Metal-Hydride volume mismatch large nucleation

energy barrier high temperature and pressure for activation

• Large enthalpy of hydride formation

Problems

MgH2

Ball milling and catalysis

Varin et el.