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Construction and Characterization of Microbial
Fuel Cells Using a Defined Co-culture of
G. sulfurreducens and E. coli
by
Nicholas Bourdakos
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Nicholas Bourdakos (2012)
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Construction and Characterization of Microbial Fuel Cells Using a Defined
Co-culture of G. sulfurreducens and E. coli
Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2012
Abstract
An air cathode, membrane-less microbial fuel cell (MFC) containing a co-culture of Geobacter
sulfurreducens and Escherichia coli was constructed and compared to pure culture MFCs of
both organisms. The E. coli containing MFCs were unsparged and relied on E. coli for oxygen
removal. The pure G. sulfurreducens MFC had a power output of 128 mW/m2, compared to 63
mW/m2 for the co-culture at an early stage and 56 mW/m2 for the late stage co-culture. The
limiting current density is 404 mA/m2 for the pure G. sulfurreducens culture, 184 mA/m2 for
the early co-culture, and 282 mA/m2 for the late co-culture, despite an increase in internal
resistance between the early and late co-culture cells. Analysis of metabolites has shown that
succinate production is likely to have negatively affected current production by G.
sulfurreducens, and the removal of succinate is responsible for the increased current density in
the late co-culture cell.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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Acknowledgements
I would like to emphatically thank the following people for all their help with every stage of this
project, for donated equipment and for sharing their experience. Without them, this thesis
would probably not exceed 12 pages in length and most likely be ripe with errors and
omissions.
-Prof. Radhakrishnan Mahadevan, University of Toronto, Department of Chemical Engineering.
-Prof. Donald Kirk, University of Toronto, Department of Chemical Engineering.
-Prof. Elizabeth Edwards, University of Toronto, Department of Chemical Engineering.
-Dr. Enrico Marsili, Dublin City University, School of Biotechnology.
-Wallace Wee , University of Toronto, Department of Electrical Engineering.
-LMSE Group, University of Toronto, Department of Chemical Engineering.
-Prof. Christopher Yip, University of Toronto, Department of Chemical Engineering.
-Dr. Ashley Franks, University of Massachusetts Amherst.
-NSERC, OGS, ERA, Funding
Finally I would like to express my sincere thanks to my friends and family. They have made the
last few years, and this project, the largest and most difficult undertaking of my life thus far,
among the most enjoyable times I can remember. While I am glad to be completing my thesis, I
will be hard-pressed to find anything like the sense of community and genuine affection for my
surroundings that I have had at U of T in my work in the future. I thank you all immensely.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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Table of Contents
List of Tables .............................................................................................................. vi
List of Figures ............................................................................................................ vii
List of Appendices ...................................................................................................... ix
Nomenclature ............................................................................................................. x
1 – Introduction & Background .................................................................................... 1
1.1 – Introduction to Microbial Fuel Cells .................................................................................. 1
1.1.1 – Background, Fuel Cells ............................................................................................... 1
1.1.2 – Background, Microbial Fuel Cells ............................................................................... 4
1.1.3 – Background, Bacterial Metabolism ............................................................................ 6
1.1.4 – Fuel Cell Chemistry ..................................................................................................... 9
1.2 – MFC Limitations .............................................................................................................. 10
1.3 – Project Rationale ............................................................................................................. 12
1.4 – Objectives & Hypothesis .................................................................................................. 13
1.5 – Relevant Literature .......................................................................................................... 15
1.5.1 – Types of MFC ............................................................................................................ 15
1.5.2 – Anode electrode ....................................................................................................... 19
1.5.3 – Cathode electrode.................................................................................................... 20
1.5.4 – Setup and operating conditions ............................................................................... 21
1.5.5 – Geobacter sulfurreducens ........................................................................................ 23
1.5.6 – Community MFCs ..................................................................................................... 26
1.5.7 – Power and Efficiency ................................................................................................ 27
2 – Materials and Apparatus ..................................................................................... 29
2.1 – MFC Anode Chamber ...................................................................................................... 29
2.2 – Electrodes ........................................................................................................................ 30
2.3 – MFC Assembly ................................................................................................................. 32
2.4 – Temperature Control ....................................................................................................... 34
2.5 – MFC Monitoring .............................................................................................................. 35
2.6 – Gassing Station ............................................................................................................... 36
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2.7 – HPLC Analysis .................................................................................................................. 39
3 – Experiments and Methods ................................................................................... 41
3.1 – Experiments conducted ................................................................................................... 41
3.2 – Cultures and Media ......................................................................................................... 42
3.3 – E. coli Strain Selection ..................................................................................................... 43
3.4– MFC Setup and Inoculation .............................................................................................. 44
3.5 – MFC Operation ................................................................................................................ 49
3.6 – Sterilization of Components and Maintenance of Sterility ............................................. 50
3.7 – Operating curve and Power Density Curve Determination ............................................. 52
3.8 – Coulombic efficiency determination ............................................................................... 53
4 – Results ................................................................................................................ 55
4.1 – Pure culture Geobacter sulfurreducens MFCs ................................................................. 55
4.2 – Co-culture Geobacter sulfurreducens and E. coli MFCs .................................................. 57
4.3 – Pure culture E. coli MFCs ................................................................................................. 59
4.4 – Pure culture E. coli Bottles ............................................................................................... 62
4.5 – Pure culture G. sulfurreducens and co-culture MFC operating and power curves ......... 64
5 – Discussion ........................................................................................................... 66
5.1 – Comparison of MFC metabolite curves ........................................................................... 66
5.2 – Effect of succinate on MFC power production ................................................................ 69
5.3 – Control of pH in co-culture and pure culture cells ........................................................... 71
5.4 – Effect of E. coli on MFC performance .............................................................................. 72
6 – Conclusions ......................................................................................................... 74
7 – Recommendations ............................................................................................... 75
8 – References .......................................................................................................... 78
Appendix A – Anode Chamber Schematic .................................................................. 86
Appendix B – Replicate MFC Data .............................................................................. 87
Appendix C – Sample Calculations for Determining Coulombic Efficiency ................... 93
Appendix D – Proton Balance in Pure Culture G. sulfurreducens MFCs ....................... 97
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List of Tables
Table 3.3.1 - Phenotypic growth of 3 strains of E. coli in LB and NB media _______________ 44
Table 3.6.1- Sterilization of MFC components______________________________________ 51
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List of Figures
Fig. 1.1.1.1 – Typical fuel cell operation for a hydrogen fuel cell. ________________________ 2
Fig. 1.1.1.2 – Operating curve and power curve for a typical fuel cell. ____________________ 3
Fig. 1.1.2.1 – Typical fuel cell operation for a single-chamber air cathode microbial fuel cell,
with acetate as electron donor and G. sulfurreducens as anodic bacteria. _________________ 5
Fig. 1.1.3.1 – The electron transport chain, with O2 as terminal electron acceptor (blue circle).
ATP synthase (yellow) can be seen phosphorylating ADP into ATP. (figure modified from
Brownlee, 2001) [10]. __________________________________________________________ 7
Fig. 1.5.1.1 – Sediment MFCs from 6 different sediment sources. _______________________ 16
Fig. 1.5.1.2 – H-cell MFC design (Oh and Logan 2006) [32]. ___________________________ 17
Fig. 1.5.1.3 – A) 2-chambered MFC with aqueous cathode [7]. B) Typical schematic for a 2
chambered MFC [5]. __________________________________________________________ 17
Fig. 1.5.1.4 – Tubular upflow air cathode MFC. A) Section and top views. B) System view. C)
Porous monolithic carbon anode. (Kim et al., 2009 )[38]. _____________________________ 18
Fig. 1.5.2.1 – Some possible materials for MFC anodes. A) Graphite plate. B) Carbon paper. C)
Carbon cloth. D) Carbon felt. E) Graphite fiber “bottle brush” [48]. F) Carbon fiber tissue. ___ 20
Fig. 1.5.4.1 – G. sulfurreducens EET. a, b) Ferric oxide grown cells with OmcS (black dots)
aligned along pili [63]. c) Close up image of pilus with OmcS [63]. d) Schematic of EET to ferric
oxide [62]. e) Schematic of EET to anode, OmcZ accumulating on the anode surface [62]. ___ 26
Fig. 2.1.1 – MFC anode chamber. A) Inlet and B) outlet for continuous operation, C) Flange with
opening for ion exchange, D) Inlet and E) outlet for sparging, F) Sampling (bottom) and
reference electrode (top) ports. _________________________________________________ 29
Fig. 2.2.1 – Anode (left), with Ni wire inserted between fiber layers, and cathode (right). ____ 31
Fig. 2.3.1 – Order of components for MFC assembly. 1) Polycarbonate anode chamber; 2)
Carbon cloth anode (nickel wire current collector); 3) Plastic separator mesh; 4) Silicon sponge
gasket; 5) Carbon paper cathode (30% wet proof, 0.5 mg/cm2 Pt catalyst loading on Vulcan
XC72) , copper wire current collector; 6) Polycarbonate cell front cover. _________________ 33
Fig. 2.4.1 – Incubator containing 3 MFCs. _________________________________________ 35
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Fig. 2.5.1 – VMP 16 channel potentiostat for MFC monitoring _________________________ 36
Fig. 2.6.1 – Copper reduced (left) and oxidized (right) for oxygen removal in gassing station. _ 37
Fig. 2.6.2 – Gassing station, entire system view. ____________________________________ 38
Fig. 2.6.3 – Gas saturation bottle for MFCs. ________________________________________ 39
Fig. 2.7.1 – Dionex Ultimate 3000 HPLC system and Shodex RI-101 detector (left). _________ 40
Fig. 3.4.1 – Sterile filling system for MFCs. _________________________________________ 46
Fig. 3.4.2 – Final assembled 6 cell MFC system, with connections to potentiostat and gassing. 47
Fig. 3.4.2 – Final assembled 6 cell MFC system, incubator view. ________________________ 48
Fig. 3.4.3 – Final assembled MFCs (cell view). A) Cannula for gas inlet B) Stopper to seal gassing
effluent when sparging is stopped, C) Septum, D) Reference electrode, E) Cathode. ________ 48
Fig. 3.8.1 – Evaporation rate for co-culture MFC. ___________________________________ 54
Fig. 4.1.1 – Current evolution of pure culture G. sulfurreducens MFC. ___________________ 55
Fig. 4.1.2 – Metabolite concentration of key metabolites for pure culture G. sulfurreducens
MFCs. Arrows indicate acetate addition. __________________________________________ 56
Fig. 4.2.1 – Current evolution of co-culture G. sulfurreducens and E. coli MFC. ____________ 57
Fig. 4.2.2 – Metabolite concentration of key metabolites for co-culture G. sulfurreducens and E.
coli MFCs. __________________________________________________________________ 58
Fig. 4.3.1 – Current evolution of pure culture E. coli MFC. _____________________________ 59
Fig. 4.3.2 – Metabolite concentration of key metabolites for pure culture E. coli MFCs. _____ 60
Fig. 4.3.3 – Base addition for neutralization of pure culture E. coli MFCs. _________________ 61
Fig. 4.4.1 – Pure anaerobic bottle culture E. coli metabolite curve. ______________________ 62
Fig. 4.4.2 – Base addition for neutralization of pure culture E. coli bottles. _______________ 63
Fig. 4.5.1 – Operating curves for co-culture MFCs at various times. _____________________ 64
Fig. 4.5.2 – Power curves for co-culture MFCs at various times. ________________________ 65
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List of Appendices
Appendix A – Anode Chamber Schematic _________________________________________ 86
Appendix B – Replicate MFC Data _______________________________________________ 87
Appendix C – Sample Calculations for Determining Coulombic efficiency ________________ 93
Appendix D – Proton Balance in Pure Culture G. sulfurreducens MFC ___________________ 97
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Nomenclature
ΔGcell Gibbs’ Free Energy of net cell reaction
ADP Adenosine diphosphate
ATCC American Type Culture Collection
ATP Adenosine triphosphate
CGSC Coli Genetic Stock Center, Yale University
CMC Carboxymethyl cellulose
COD Chemical oxygen demand
Ecell Cell potential
EET Extracellular electron transfer
ETC Electron transport chain
F Faraday’s constant, 96485 C/mol e-
HPLC High-performance liquid chromatography
I Current
LB Lysogeny broth
MEA Membrane electrode assembly
MFC Microbial fuel cell
MN301 Macherey-Nagel 301 fibrous cellulose
n Number of moles of electrons
NADH Nicotinamide adenine dinucleotide
NB Nutrient broth medium
NBAF Nutrient broth medium with acetate/fumarate
Omc Outer membrane cytochrome
PEM Proton exchange membrane
PTFE Polytetrafluoroethylene
R Resistance
Rinternal Internal resistance
SMFC Sediment microbial fuel cell
V Voltage
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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1 – Introduction & Background
1.1 – Introduction to Microbial Fuel Cells
1.1.1 – Background, Fuel Cells
Microbial fuel cells (MFCs) are electrochemical conversion devices, similar to most fuel cells,
excepting that the power generated is derived from bacterial metabolism. Fuel cells are divided
into two chambers, each containing an electrode; an anode and a cathode. An electron donor
on the anode side, often hydrogen or methanol, is oxidized on the anode surface, leading to
the formation of electrons and cations. The electrons reduce the anode and create current in
the circuit. A voltage difference across the circuit is the driving force for the reaction. The
cations from the anode side then traverse a cation selective membrane to the cathode side of
the fuel cell, in order to equalize the charge transferred by the electrons. In some fuel cells,
anionic transfer from the cathode to the anode occurs in lieu of this process. The oxidation of
the cathode by the electrons created on the anode is the second step of the redox reactions
that create power in fuel cells, and requires some oxidized electron acceptor [1].
If a load is placed between the anode and cathode, then the energy of these electrons can be
harnessed. Catalysts are often required on either the anode or cathode surfaces, or both, in
order to allow the electron donor in the anode to be oxidized and the cathodic acceptor to be
reduced. A diagrammatic representation of this process for a hydrogen fuel cell with a proton
exchange membrane (PEM) can be seen in fig. 1.1.1 below. In this process, hydrogen gas is split
into protons and electrons on the anode surface. Electrons travel through the external circuit
and the load, R, providing power. Protons pass through a cation selective membrane to the
cathode, where they react with oxygen and the electrons that have passed through the
external circuit to form water. In most cases, both electrodes would be catalyzed with platinum
or palladium in order to increase the power from the cell.
In the fuel cell two half reactions have occurred; the oxidation of hydrogen gas into protons
and electrons on the anode, and the reduction of oxygen in the presence of protons to make
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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water on the cathode. The net reaction in the fuel cell is the same as the combustion of
hydrogen gas. The same process has occurred, releasing the same amount of energy, except
that electrical energy has been harvested instead of heat.
Anode Reaction: 2H2 → 4H+ + 4e-
Cathode Reaction: 4H+ + 4e- + O2 → 2H2O
Cell Reaction: 2H2 + O2 → 2H2O ΔGcell= -474.4 kJ/mol [1]
Fig. 1.1.1.1 – Typical fuel cell operation for a hydrogen fuel cell.
Fuel cells are usually characterized by two curves, the power and operating curves, which show
how the voltage and power vary with current. These curves allow for the determination of
important properties of the cells, such as the maximum power, optimal external resistance and
operating voltage, and can also provide a measure of the internal resistance in the cell. The
operating curve (example shown in fig. 1.1.1.2) has a characteristically sigmoid shape. At open
circuit, where no current is passing through the external circuit, the potential is at its maximum
level for the system. As the current increases, there are ohmic losses as well as activation
losses for the first non-linear portion of the operating curve. The activation losses occur at low
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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current densities and are due mostly to slow kinetics at the anode or more usually cathode
electrodes. These losses are very pronounced at low current densities due to activation energy
required to push the reaction forward, and increase logarithmically according to the Tafel
equation as current density increases. The linear portion of the curve consists primarily of
potential decreasing due to ohmic losses, resistive losses from the electrolyte and electrodes,
though activation losses are still present, but are far less drastic as current density increases.
This portion of the curve follows Ohm’s law (V = IR) and can be used to give a reasonable
approximation of the cell’s internal resistance.
The final portion of the operating curve, which shows an abrupt loss in potential with
increasing current, is the mass transfer limited region of the curve. The losses here are due to
the diffusive limit being reached in some electrically active species in the cell. Either the
electron donor or acceptor cannot diffuse to the electrode rapidly enough, or the charge
neutralizing ion cannot travel across the electrolyte rapidly enough. This mass transfer
limitation implies that there is a limiting current density possible for the cell, and any attempt
to lower the external resistance will lead to no additional current and give potential losses.
Fig. 1.1.1.2 – Operating curve ( ) and power curve ( ) for a typical fuel cell.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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1.1.2 – Background, Microbial Fuel Cells
In a microbial fuel cell, the same process occurs as in a typical fuel cell, however the anode
reduction is part of bacterial metabolism. One way to view MFCs is that the bacteria are the
anodic catalysts for the oxidation of the electron donor, which can be whichever substrate the
bacteria are consuming, often glucose or acetate. The cathode can use a variety of electron
acceptors; however the most widely used are ferricyanide and oxygen [2]. Ferricyanide is often
used in MFC research when cathode effects are not of interest to the researcher, since it has a
very high potential and makes for an excellent cathode reagent, with little or no limitation to
the system. This allows researchers to investigate the effects of only the anode compartment.
The problem with ferricyanide is that it is not a cost effective way to run an MFC and can
potentially leak across to the anodic side of the cell. It is a toxic compound, and can have a
detrimental impact on the anodic cultures. Furthermore, the spent ferrocyanide must be
disposed of, which leads to waste and a potential hazard for people coming in contact with it [2,
3].
For these reasons, most MFC systems use an oxygen based cathode, identical to that used in a
hydrogen fuel cell. In order to reduce the cost involved in purifying oxygen, ambient air is
typically used instead. A diagrammatic view of an air cathode microbial fuel cell can be seen in
fig. 1.1.2.1. The cathode is often catalyzed with platinum [4, 5].
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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Fig. 1.1.2.1 – Typical fuel cell operation for a single-chamber air cathode microbial fuel cell, with acetate as electron donor and G. sulfurreducens as anodic bacteria.
The system in air cathode MFCs does not always contain a PEM. In hydrogen fuel cells, since
both anodic and cathodic reagents are gaseous, a physical barrier is necessary to prevent
anodic and cathodic contents from mixing and create the potential difference across the cell.
The PEM is used as this separator, though any material that allowed for ionic conductivity for
the internal circuit can be used. For example, in alkaline fuel cells, a caustic solution is used as
the electrolyte between the two electrodes, and the mobile ion is hydroxide. The net reaction
is the same, but water is evolved on the anode side. In air cathode MFCs, the culture medium
between the anode and cathode can serve as an electrolyte and the PEM can be removed
entirely. The advantage of this is a pronounced reduction in internal cell resistance to ionic
transfer, as protons move far more readily through culture medium than through the PEM
(typically Nafion, a sulfonated tetrafluoroethylene based polymer); however this leads to
increased oxygen diffusion into the system [6, 7].
MFCs are characterized using operating and power curves in the same way as conventional fuel
cells. The curves have the same shape and give the same information, with one principal
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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difference. The mass transfer limiting region can now also be explained by a limitation in the
metabolic rate of respiration of the anodic bacteria, rather than a diffusional limitation. In
either case the effect is the same on power production. The other measure of fuel cell efficacy
that is frequently employed in MFCs is the coulombic efficiency. This figure is essentially a
measure of the yield on electrons, and is defined as the quotient of the number of electrons
harvested by the MFC and the number of available electrons when fully oxidizing the food
source to CO2 and water [8, 9].
1.1.3 – Background, Bacterial Metabolism
As previously mentioned, the bacteria in microbial fuel cells act as catalysts for anode
reduction by some bacterial substrate. The mechanism by which the electrons are produced in
the bacteria is the final step in energy production for oxidative phosphorylation, the electron
transport chain (ETC). The electrons from the electron donor have been stored in shuttle
molecules, such as NADH, during the course of metabolic oxidation of the food source. These
molecules then pass the electrons through a series of membrane bound proteins in the cell’s
periplasmic membrane. When a trans-membrane protein is reduced, it undergoes a
conformational change which causes protons to be pumped across the membrane. This creates
both charge and diffusional gradients across the membrane, which creates a proton motive
force. The protons have very few ways to pass back through the membrane, and these
chemical potential gradients are exploited by bacteria for a number of purposes. The principal
use of this gradient is to create ATP, by passing protons through the ATP synthase protein. As
the protons pass through this protein, it phosphorylates ADP into ATP, the cell’s main energy
molecule. A diagrammatic view is shown in figure 1.1.3.1.
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Fig. 1.1.3.1 – The electron transport chain, with O2 as terminal electron acceptor (blue circle). ATP synthase (yellow) can be seen phosphorylating ADP into ATP. (figure modified from Brownlee, 2001) [10].
In figure 1.1.3.1, oxygen is the terminal electron acceptor for the electrons, after they have
passed through the ETC, which is the process in aerobic organisms. This is not the case for
anaerobic organisms, many of which are incapable of oxidative phosphorylation and rely on
fermentation to create ATP in a much less efficient process. Other bacteria that are found in
anaerobic environments have evolved an ETC that makes use of other molecules in lieu of
oxygen as a terminal electron acceptor. Those that are of most interest in microbial fuel cells
are capable of reducing high oxidation state metals, such as Fe3+ and U6+, to a lower oxidation
state, by directly transferring electrons to them. These metals are usually present as solid
oxides in sediment, and the bacteria have evolved methods for reducing them extracellularly
[11, 12, 13, 14, 15]. This can be accomplished by the use of mediators which act as shuttles for
electrons that are secreted by the organism, or by another organism growing in consortium
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with them [9, 16, 17]. The other method is direct electron transfer, either by making contact
directly with outer membrane cytochromes, or by electron transfer through conductive pilin
proteins which can significantly extend the range of electron transfer [9, 11, 12, 14]. This last
method is predominantly seen in bacteria of the genus Geobacteraceae, such as Geobacter
metallireducens and Geobacter sulfurreducens. The mechanism for electron transfer to
external Fe3+ by G. sulfurreducens is discussed further in section 1.5.5.
In these organisms the critical consideration is adequate potential difference between the
bacteria and external terminal acceptor; thus many of these bacteria with mechanisms for
extracellular transfer have been shown to be capable of respiring on fuel cell anodes as well,
provided that the potential is high enough from the anode to the cathode and that the
resistance in the circuit is not too great.
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1.1.4 – Fuel Cell Chemistry
The most intensely studied organism in MFCs and the one that has shown the most promise in
terms of power density is Geobacter sulfurreducens. The preferred substrate for this organism
is acetate and this is therefore used as an example in this section. The half cell reactions for a
bacteria respiring with acetate as the electron donor and an air cathode system are as follows:
Anode Reaction: CH3COO- + H+ + 2H2O → 2CO2 + 8H+ + 8 e-
Cathode Reaction: 8H+ + 8e- + 2O2 → 4H2O
Cell Reaction: CH3COO- + H+ + 2O2 → 2CO2 + 2H2O ΔGcell= -842.2 kJ/mol [18]
Using the Nernst equation [1], this Gibbs free energy value corresponds to a theoretical cell
voltage of:
This is the maximum possible voltage that an MFC respiring on acetate could achieve with an
oxygen cathode system; however in practice it is typically much lower, in the range of 0.6 V to
0.8 V for an open circuit [5]. This discrepancy is due to losses during metabolism and energy
taken up by the bacteria, which cannot be avoided if the bacteria are to derive any gain from
respiring on the anode. Furthermore, this formula assumes a pure oxygen cathode in lieu of an
air cathode, and an activity of 1 for all species present (implying 1M acetate, pH 0). Even if
corrected to reflect expected acetate concentration and pH, it is difficult to make an accurate
prediction as intracellular acetate concentrations are more important than the extracellular
ones, and are difficult to measure.
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1.2 – MFC Limitations
In order to make use of and improve MFC technology, it is important to understand the
limitations that it faces. One of the principal limitations is its relatively low power density
compared to other fuel cell systems. The US Department of Energy (DOE) has the current
status of portable energy density in fuel cells at between 7 and 30 kW/m3 [19], with constantly
increasing future goals. The highest reported MFC power density is 2.15 kW/m3 and only
considers anode volume, not system volume [5]. Because of this, these systems tend to have
large footprints and are thus not usually implemented on a small scale or in households, as
other systems such as hydrogen, direct methanol, or phosphoric acid fuel cells, might be. Any
small scale application of MFCs is typically used to produce a very small amount of power, such
as marine sediment MFCs that are used to provide power for temperature reading probes.
Another major limitation, and in fact probably the most important one, is cost. MFCs on a
laboratory scale consist of expensive components, such as the graphite or carbon cloth anodes,
PEMs, and most notably in the case of air cathode systems, the Pt catalyzed cathodes. On their
own, these aren’t prohibitively expensive, however coupled with the relatively modest power
density obtained from MFC, and the number of stacks in series and parallel required to obtain
a reasonable power output, the stack costs can become far too great to be considered
favourable over traditional power generation methods, many of which could be constructed
using the same components. In order to effectively implement an MFC system, materials must
be selected for their ability to give the greatest possible power density for the cost. Thankfully
the typical feed into microbial fuel cells is far less expensive than in other fuel cell systems,
because there are little or no purity requirements on either anode or cathode sides of the cell,
and the materials oxidized by the system are very often wastes and in fact have revenue
associated with their removal. It is clear from these considerations that the most cost effective
MFC systems will deal with very large quantities of wastewater in order to take advantage of
any of the cost savings associated with scale-up, and the increased revenue from treating large
quantities of water.
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Another limitation in the case of air cathode MFCs, and that which will be looked at in this
study, is the effect of oxygen diffusion into the anode chamber. Oxygen is a high potential
molecule which would raise anode potential if it were present and reduce the cell potential
significantly. In addition to this, oxygen in large quantities is toxic to many species of
electrogens, including Geobacter sulfurreducens, the most frequently studied electrogen and
that which is used in this study [20, 21, 22]. Even in small quantities, oxygen can slow
metabolism or cause it to shift in a manner which bypasses anode reduction and thus
decreases MFC power density [20, 21]. In order to maintain an anaerobic condition in MFCs,
researchers generally sparge the anode with either N2 or a mixture of N2/CO2 (80% N2, 20% CO2
by volume), depending on the culture medium. While this eliminates the effect of oxygen for
laboratory scale experiments, in practice the process of creating and subsequently compressing
this gas likely consumes more energy than the MFCs produce. Furthermore, in order to reduce
internal cell resistance, a very important factor in for power production, the anode should be
placed as close to the cathode as possible [23, 24]. This means that the most sensitive area to
oxygen, the anodic culture, is also the most likely to be exposed to oxygen in the event of a
diffusive leak. Sparging right at the anode surface creates too much agitation to allow for a
thick biofilm to grow. Thicker biofilms lead to greater current production, and therefore it is
not practical or efficient to sparge near the anode, where it is most necessary to do so. It is
therefore evident that some other method is desirable to maintain an anaerobic condition for
practical application of this technology.
Finally, MFCs are limited by the substrates they can consume. Typical electrogenic bacteria
consume fermentation products such as acetate or lactate, and are rarely capable of
consuming a more complex substrate. These simpler substrates are available in from a variety
of sources, but a broader range of possible substrates, especially some which are not easily
metabolized, would be beneficial. Typically the more complex substrates are broken down
abiotically or by different bacteria, without contributing to power generation. These bacteria
can lead to competition in MFCs and lower the power production. It is therefore important to
understand the interaction between different bacteria in MFCs and how they affect each other.
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1.3 – Project Rationale
MFCs have applications in several fields, a number of which are key engineering concerns.
Globally, energy production is an issue that we are constantly aware of and alternative and
clean sources of energy are being researched and implemented extensively. One of the largest
consumers of electricity in North America is wastewater treatment, accounting for
approximately 4% of the power utilized in the United States [25, 26]. A great deal of the power
for these treatment processes is used to physically separate the waste from the water, and
subsequently treat and handle the residual waste, often using biological processes [25, 26].
Microbial fuel cells have the capability to remediate waste while simultaneously producing
small scale power. The MFC approach to wastewater treatment reduces the requirement for
separating the waste, since much of it is treated in the water itself, and allows for the recovery
of some of the power put into the process. Microbes are already being used to remediate
wastewater; the challenge is to incorporate power generation with microbial fuel cells into the
system. The environmental and financial impacts of microbial fuel cell wastewater treatment
systems could potentially be very significant, especially with a worsening energy crisis causing
us to look more and more to alternative sources of electricity. A better understanding of the
processes that occur within these MFCs is thus very important to the sustainable energy
movement.
In order to look at MFCs used in wastewater treatment, it’s important to understand the
interactions on a species to species level, and to see how the electrogenic bacteria are affected
by the other bacteria present. It is not economical or practical to have sterile wastewater feed
and so any large scale MFC would likely be a large community. This study seeks to establish the
relationship between two common types of microbe found in these communities, and to see if
there is any positive or negative interaction between them that could potentially be mitigated
in actual wastewater application. Furthermore, this study seeks to determine if one of the
most expensive stack components, the PEM, can be removed from the system to make it more
economically feasible with the addition of a facultative anaerobe.
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1.4 – Objectives & Hypothesis
The aim of this project is to attempt to create and understand a microbial community that
would allow for the elimination or reduction of some of the limitations mentioned in section
1.2. The principle hypothesis is that in creating a defined co-culture of an electrogen along with
another facultative anaerobic organism capable of degrading higher order substrates will be
beneficial to the system in a number of ways.
Firstly, a facultative anaerobic organism will eliminate the requirement for sparging MFCs for
oxygen removal. Any oxygen that makes its way into the cell will be consumed by the
secondary organism and will not be able to interfere with the electrogenic organism on the
anode. This also has the advantage of being a method of oxygen removal that does not disturb
the anode with sparging, or have the energy requirements of sparging. This has the added
advantage of allowing for the removal of the PEM from the system. As mentioned in section
1.1.2, in an air cathode system the PEM is not strictly necessary, and leads to a significant
increase in the internal resistance of the cell. The downside of its removal is increased oxygen
diffusion into the cell. It is hypothesized that the secondary bacteria will allow for MFCs to be
operated without sparging, at similar power densities or at least at power densities that
produce more net power from the system, when subtracting the power required for sparging
from the output of a pure culture sparged cell.
Secondly, the facultative anaerobe will enable the MFC to be fed a more complex substrate
than simply acetate, which will in turn be degraded into substrates that the electrogen is
capable of consuming on the anode and converting to electrical energy.
The objectives of this project are therefore as follows:
1) To construct several MFCs that can be run in a reproducible manner for comparison
between experiments.
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2) To establish a co-culture MFC comprised of an electrogen as well as a facultative anaerobe
which can be operated without sparging and in the absence of a PEM, and which can be fed a
more complex substrate than that simply required for the electrogen.
3) To compare this co-culture to a sparged pure culture electrogen MFC, without a PEM.
4) To characterize the co-culture and determine and explain any interactions, favourable or
unfavourable, between the two microbes present, and to recommend possible future steps for
the amelioration of the culture.
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1.5 – Relevant Literature
In preparation for this project, a great deal of literature has been reviewed for microbial fuel
cell design and construction, as well as MFC operating conditions, cultures, power production
and experimental design. The most relevant papers are mentioned in this section, with relation
to which aspects of design they pertain to. The papers omitted from this discussion but
referenced by this thesis are either redundant or less clear than the ones included; however
they have also helped influence the experimental design.
1.5.1 – Types of MFC
As mentioned in section 1.1.2, there are two main cathodic electron acceptors in MFCs; oxygen
and ferricyanide. Though other options have been explored, such as hydrogen peroxide [27],
the majority of studies deal with these two acceptors, and almost every cell that has practical
application as a consideration uses oxygen; it is generally considered impractical and expensive
to use ferricyanide [2, 3, 26]. For these reasons, this study uses air cathodes and ferricyanide
based studies are only considered when used to determine properties of relevant anodic
cultures.
There are a number of different designs for microbial fuel cells. Many of the designs have little
practical application outside of a laboratory setting, however are easy to work with and allow
for experiments to be conducted with relative ease. The simplest form of MFC is the sediment
MFC (SMFC), where an anode is buried sufficiently deep in sediments for there to be no
remaining oxygen present, as it is consumed by aerobes or facultative anaerobes in the
sediment above the anode [28, 29]. The cathode is suspended in the oxygenated water above
the anode [28, 29]. These MFCs generally provide very little power, however are very
inexpensive as they usually use no cathodic catalysts and require no ion exchange membrane,
ions seep through pores in the sediment. These cells have very little potential for wastewater
remediation but can provide power for some niche applications, such as small marine
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monitoring probes or bioremediation of contaminated sediments [30, 31]. These SMFCs can be
studied in a lab setting by collecting sediments and burying an anode in a jar or cylinder.
Fig. 1.5.1.1 – Sediment MFCs from 6 different sediment sources.
The simplest form of MFC after SMFCs is an H-type cell. This cell consists of two glass bottles
that have been attached to a tube at the bottom. These two tubes are clamped together with
some form of ion exchange membrane between them to connect the internal circuit of the cell.
Electrodes are inserted through septa or holes drilled in the lids of these bottles [32]. Typically
the cathodic chamber is sparged with oxygen or air, or else it is filled with a ferricyanide
solution [32,33]. The advantage of this type of design is its ease of construction, sterilization,
and it’s relatively low cost. The disadvantage is a very low area for ion transfer between the
anode and cathode, as well as the need to force oxygen into the cathode chamber in most
cases [32, 33, 34]. Furthermore, operating under continuous flow is very difficult in this type of
system, as the effluent cannot simply drain out by gravity once it reaches a certain level, it
must be pumped.
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Fig. 1.5.1.2 – H-cell MFC design (Oh and Logan 2006) [32].
A slightly more complex design involves a 2 chambered MFC, typically of rectangular
construction, with both chambers being pressed together by external bolts. Once again, the
cathode is aqueous in this system. The advantages of this design over an H-cell are the
increased area for ionic transfer, and the ease of adding ports for continuous flow. The
disadvantages are the need to sparge the cathode, as well as the difficulty involved in assembly
[26, 5].
Fig. 1.5.1.3 – A) 2-chambered MFC with aqueous cathode [7]. B) Typical schematic for a 2 chambered MFC [5].
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The most common and seemingly most practical design is the air cathode system described in
section 1.1.2. This system has an aqueous anode chamber and a gaseous cathode. Typically the
cathode makes use of ambient air rather than using pure oxygen, in order to keep costs down.
These air cathode MFCs are also very often rectangular in design (see fig. 1.1.2.1), though a
number of different systems and orientations have been attempted. For example, tubular
upflow air cathode MFCs are common for continuous application, with regularly spaced holes
in the tubular anode chamber, and the cathode wrapped around it [35, 36, 37, 38]. The
advantage of this type of system is the typically large area for ion transfer, as well as the
elimination of the need to sparge the cathode chamber, which reduces the energy requirement
for operating the cell. Furthermore, in this system, with an appropriate cathode to prevent
leakage, the ion exchange membrane can be removed [6, 36]. This membrane is often the most
expensive component of an MFC, and also leads to a large increase in internal cell resistance,
thus its removal can greatly improve cell performance in some cases[3, 6, 26]. The principal
disadvantage to this design is the increased possibility of oxygen diffusion into the anodic
chamber of the cell [6]. This can significantly decrease the efficiency of the cell and potentially
be toxic to anodic cultures [20, 21, 33].
Fig. 1.5.1.4 – Tubular upflow air cathode MFC. A) Section and top views. B) System view. C) Porous monolithic carbon anode. (Kim et al., 2009 )[38].
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1.5.2 – Anode electrode
A variety of different materials have been used for MFC anodes. More simple anodes are
typically made of a planar conductive material. G. sulfurreducens has been shown to be able to
respire on a graphite electrode, as well as a polished gold electrode and stainless steel plate
electrode [39, 40, 41]. Any conductive material that is non-toxic to the microbes can be used,
however certain materials (such as copper) do not allow for biofilm formation [42]. In MFCs
exclusively using mediators to transfer electrons, adhesion is not a significant concern. For
cultures that require biofilm formation, more rough or porous electrodes are generally
required to generate a higher power density [43, 44, 45].
These more porous cathodes are typically made from conductive carbon. Carbon fiber tissue,
carbon cloth, carbon paper and carbon felt are common MFC anode materials, because of their
high conductivity and large surface area compared to a polished plate of graphite [43, 45].
Granular graphite connected by a rod has also been used to enhance surface area for microbial
attachment [35, 36, 46, 47]. The “bottle brush” style electrode, developed by Logan et al. in
2007, has also shown promise as it allows for a very large surface area for bacterial adhesion
[48]. In addition to offering a large surface area, carbon based electrodes are typically resistant
to corrosion, unlike metal-based electrodes [42]. A concern with these electrodes is the
difficulty in diffusion of substrate into the electrodes, as well as the diffusion of protons out of
it, which can lead to anode acidification and an increased internal resistance, particularly in
carbon felt [43, 45, 47]. It is difficult to quantify the effective surface area of many of these
anode designs, and so projected surface areas are often reported.
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Fig. 1.5.2.1 – Some possible materials for MFC anodes. A) Graphite plate. B) Carbon paper. C) Carbon cloth. D) Carbon felt. E) Graphite fiber “bottle brush” [48]. F) Carbon fiber tissue. 1.5.3 – Cathode electrode
The cathode electrode in an air cathode MFC is typically a commercially available cathode for a
hydrogen fuel cell. Hydrogen fuel cells tend to have their electrodes all in one piece in a
membrane electrode assembly (MEA) [49]. This MEA consists of the anode and cathode
electrodes hot pressed onto either side of the PEM. Often, additional layers are added to these
electrodes, including gas diffusion layers, which allow for a more even diffusion of hydrogen
and oxygen to the electrode surface, and PTFE wet proofing, to allow for water elimination on
the cathode [49, 50]. The oxygen reaction is very slow without a catalyst; typical catalysts are
platinum or platinum and ruthenium [43, 49]. Due to the cost of these metals, it is desirable
that as much of the surface of the catalyst is exposed as possible, to minimize the amount
required. This catalyst is usually prepared by spattering platinum nanoparticles onto carbon
black particles. The nanoparticles ensure that the surface area to volume ratio of the platinum
is very high. The catalyst loading employed in most published MFC literature is 0.5 mg/cm2
based on platinum mass [38, 51, 52].
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The microbial fuel cell MEA is a half MEA, typically not hot pressing the anode to the PEM.
There are several intuitive reasons for this. Firstly, many anodes are made up of materials that
cannot be hot pressed to electrodes. Secondly, the act of hot pressing a planar material to the
electrode, such as carbon paper, effectively halves the surface area for bacterial adhesion and
for diffusive transfer of substrate into the biofilm or mediator to the electrode in a mediated
cell. Lastly, in an air cathode cell, oxygen diffuses in from the cathode through the PEM [23, 24].
Hot pressing the anode to the PEM would place the electrically active biofilm at the point of
highest oxygen concentration in the anode.
Several studies have shown that there are benefits to removing the ion exchange membrane
from an MFC in an air cathode system and allowing anodic medium to act as the electrolyte
between the two electrodes [6, 24, 36]. This would lead to a stand-alone cathode. As the
membrane is not very permeable to water, when it is removed, the cathode must not allow
water to pass through to prevent drying out the cell. This is typically accomplished by wet
proofing with PTFE [23, 26, 43, 50]. This wet proofing is usually used to rapidly cause water to
bead off of hydrogen fuel cell cathodes, in order to remove it as a limitation to oxygen diffusion
to the electrode.
1.5.4 – Setup and operating conditions
There are a number of considerations when setting up and operating MFCs that can have a
significant effect on reproducibility and effective power production. One of the critical factors
in cell design is the presence of a membrane. As previously mentioned membrane removal
reduces internal resistance and increases oxygen diffusion into the cell. Liu and Logan (2004)
compared a membrane-less system to one using a Nafion 117 PEM. The maximum power
density in the membrane-less cell increased to 494 mW/m2 compared with 262 mW/m2 in a
cell with the PEM [6]. Fan et al. (2007) showed that by placing two layers of J-cloth between
anode and cathode in lieu of a PEM, very little additional ionic resistance is observed, while
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oxygen diffusion into the MFC is significantly decreased, and a radically increased power
production is observed [53].
Another effect that is often seen in air cathode systems with a PEM is the diffusion of other
cations across the membrane and their accumulation on the cathode. Other ions, such as Na+
or Mg2+ are in much higher concentrations in MFCs than protons, and are often able to pass
through the PEM and balance the charge transfer caused by electron movement [54]. Since the
cathode reaction requires protons, and not enough protons are moving through the PEM, the
pH rises. The process of the pH rising naturally slows down the anodic reaction, which requires
protons. Precipitates of hydroxides and of these other cations can form on the cathode surface
[54]. These precipitates must be removed by rinsing the outer surface of the cathode with
deionized or slightly acidic water, which is impractical and adds operational complexity to the
system.
The distance between anode and cathode is another important concern in MFC configuration.
In a PEM cell, the internal resistance is the sum of the resistance of the electrolytic medium
and the resistance of the PEM added in series. In a membrane-less cell, the internal resistance
is proportional to the distance between electrodes. There is also the added resistance of
protons passing through the biofilm or through the electrode in a porous electrode. The only
controllable aspect of these considerations is minimizing the distance between the electrodes.
Liu et al. (2005) showed the trend of increasing MFC power with reduced electrode spacing. In
their study, electrodes spaced 4 cm apart produced 720 mW/m2 of power, as compared with
1210 mW/m2 for electrodes spaced 2 cm apart [23]. This is supported by findings by
Ghangrekar and Shinde (2007), which showed that for a spacing of 20cm, 24cm and 28cm, the
power densities obtained were 10.9, 8.6 and 7.4 mW/m2 respectively [24].
Medium ionic strength also affects the conductivity of the electrolyte. In many fuel cells or
batteries, a very high ionic strength is achieved by creating a very acidic or basic solution for
the electrolyte. In a microbial fuel cell this is not possible as these conditions would kill the
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bacteria, however increasing the ionic strength of the medium is possible using salts. Liu et al.
(2005) demonstrated that increasing the NaCl concentration and thus the solution ionic
strength from 100 mM to 400 mM nearly doubled the power output of the MFC [23].
1.5.5 – Geobacter sulfurreducens
Geobacter sulfurreducens is one of the principal organisms used in MFC experiments because
of its prevalence in mixed community MFCs on the anode, as well as its very high power
density in pure culture MFCs [5]. The ability of this organism to reduce the electrode comes
from its use of high oxidation state metals as electron acceptors, typically Fe3+existing as iron
oxide in sediments. G. sulfurreducens has also shown a very high propensity to form thick
biofilms on this iron oxide and on MFC anodes. G. sulfurreducens is typically fed acetate as a
substrate in bottle cultures and in MFCs, however it has been known to consume lactate,
hydrogen and pyruvate as well [55, 56, 57]. When culturing G. sulfurreducens, media are
supplemented with a form of ferric iron, typically ferric citrate or ferrihydrite (iron oxide),
however growth is much faster when using fumarate as electron acceptor, and having it
reduced to succinate [57, 58].
G. sulfurreducens was once considered a strict anaerobe, however it was shown (Lin et al. 2004)
that it is aerotolerant [20]. Oxygen is toxic to G. sulfurreducens in large quantities, greater than
10% in the gas phase, and can severely limit growth even in small quantities. G. sulfurreducens
exposed to atmospheric quantities of oxygen for different time periods and then re-inoculated
to an anaerobic medium, using fumarate as an electron acceptor, exhibited a longer than
normal lag phase in its growth. This lag phase increased in length with increased oxygen
exposure [20]. The medium used after the exposure to oxygen contained 1mM cysteine and
0.5% yeast extract in order to eliminate residual oxygen and promote growth [20]. When
anaerobic cultures of G. sulfurreducens were grown in a bottle with 5mM fumarate instead of
the usual 40mM fumarate, growth stopped completely once fumarate was depleted. At this
point, the addition of 5% oxygen and 10% oxygen to the headspace showed further growth and
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consumption of acetate [20]. Higher concentrations of oxygen (15% and 20%) showed slower
growth than the 5% oxygen case, and the growth stopped within a day. In order to examine the
possibility of sustained growth on oxygen, additions of 5% and 10% oxygen as well as fresh
acetate were added to the bottles regularly. Growth stopped after the second addition of 10%
oxygen, however continued for 10 days with 5% oxygen additions [20]. It was also found that
the growth of these cultures on fumarate prior to oxygen exposure was required. Inocula from
the fed batch 5% oxygen culture would not grow when inoculated into fresh 5% oxygen
medium in the absence of fumarate [20].
Of particular interest to this thesis was the stoichiometry of the oxygen reaction. When
growing with oxygen as electron acceptor and acetate as the donor, the stoichiometry of the
reaction calls for 2 oxygen molecules per acetate molecule. It is expected, as the culture is still
growing, that the observed ratio be somewhat lower, in order to account for carbon going
towards biomass formation, as is seen with the fumarate consumption earlier in this culture,
which has a stoichiometric ratio of 4 and had an observed ratio of 3.8 [20]. The observed ratio
for growth on oxygen however was 2.4, 20% higher than the stoichiometry [20]. This would
imply that oxygen is being consumed in another process, possibly the oxidation of residual
malate or succinate in the bottles, or the direct oxidation of fumarate. A C-13 flux analysis of G.
sulfurreducens (Yang et al., 2010) showed that when both fumarate and acetate are present, G.
sulfurreducens uses both as a carbon source, which may explain the discrepancy [57]. This
effect was more pronounced under acceptor limiting conditions than donor limiting conditions;
furthermore, the flux between succinate and fumarate in the TCA cycle, in both directions, was
more elevated under acceptor limited conditions [57].
A recent study (Nevin et al., 2011) showed that a strain isolated from G. sulfurreducens, known
as strain KN400, pregrown on an anode in an anaerobic MFC was capable of power production
under aerobic conditions [22]. The cells died or were severely compromised when external
acetate was provided in the medium, but the cells were healthy when a concentrated acetate
solution was placed inside the anode and allowed to diffuse through the electrode. The
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hypothesis given is that high acetate and oxygen concentrations at the same point in the
biofilm leads to rapid metabolism and free radical formation which negatively affects the cells.
With acetate diffusing from the inside of the electrode and oxygen from the outer edge of the
biofilm, the concentration of oxygen and acetate are never both elevated and thus the cells
continue to grow [22]. The cells showed significantly less power than anaerobic cultures of
KN400 growing under the same conditions, and even when the anode was potentiostatically
controlled no increase in current was observed, implying that some other factor was limiting in
the system [22]. This study is important as it demonstrates than G. sulfurreducens can grow on
an anode and produce current, and under conditions where a small amount of oxygen is
present (e.g., deep in the biofilm) can also grow aerobically at the same time.
The G. sulfurreducens mechanism for respiration on an electrode is not fully understood,
however in recent years advancements have been made. The process of reducing species
external to the cell is known as extracellular electron transfer (EET). The organism contains a
number of outer membrane cytochromes (Omc) that could be responsible for the final
reduction step, however it seems more likely that each has a different purpose and reduces a
different acceptor [59]. In addition to these proteins, G. sulfurreducens has a pilin type protein,
pilA, which has been recently conclusively shown to be conductive [11, 60, 61, 62]. The pili are
3-5 nm in diameter and 10-20 μm long [11, 62]. The present understanding of the EET for ferric
oxide reduction in G. sulfurreducens is that OmcS, a cytochrome necessary for the reduction of
iron oxides, aligns along the pilus filaments, in order to increase the surface area and number
of contact points to iron oxide [63]. The actual iron reduction takes place from OmcS to the
iron, and the pilus serves as a nanowire to conduct the electrons [62, 63]. In biofilms respiring
on an electrode, less OmcS is present in the biofilm; in electrode systems OmcZ is the most
abundant cytochrome in the extracellular biofilm matrix and accumulates near the electrode
surface. It appears to be responsible for electrode reduction in the same manner than OmcS is
responsible for ferric oxide reduction [62, 64]. It is not yet clear how electrons are transferred
from the cell into the biofilm [62]. The entire biofilm is conductive and has tunable metal like
properties, with a conductivity of approximately 5 mS/cm, and a range of electron transfer of
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about 1 cm [61]. This allows for a thick biofilm of G. sulfurreducens to respire, without the cell
wall making contact with the anode, increasing the total anodic biomass.
Fig. 1.5.4.1 – G. sulfurreducens EET. a,b) Ferric oxide grown cells with OmcS (black dots) aligned along pili [63]. c) Close up image of pilus with OmcS [63]. d) Schematic of EET to ferric oxide [62]. e) Schematic of EET to anode, OmcZ accumulating on the anode surface [62].
1.5.6 – Community MFCs
There have been many studies of MFCs inoculated with wastewater cultures and pure cultures,
however relatively few used defined consortia. The problem often seen in wastewater cultures
is organisms competing for the carbon source and a relatively low proportion of the available
energy going towards power production [5, 65, 66]. Most of the pure cultures have a relatively
high coulombic efficiency as compared with mixed culture MFCs [5, 67].
A study using a co-culture MFC of G. sulfurreducens and C. cellulolyticum (Ren et al., 2007)
showed that while maintaining similar overall COD removal to a wastewater sludge inoculate
MFC, the co-culture had a significantly higher coulombic efficiency [68]. The coulombic
efficiency was 47% as compared with 27% for the sludge when fed with sodium carboxymethyl
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cellulose (CMC), and 39% as compared with 22% for the sludge when fed with fibrous cellulose
(MN301) [68]. Furthermore, the cellulose degradation in the co-culture MFC was 64% for CMC
as compared with 42% for a pure culture of C. cellulolyticum and 41% for the sludge MFC [68].
This clearly demonstrates the potential advantages of a co-culture or defined culture system,
as the co-culture was not only able to degrade more cellulose than either the pure C.
cellulolyticum culture or the sludge, but was able to do so with a higher coulombic efficiency
than the sludge culture, which leads to a significantly greater current output given the same
feed.
The beneficial effect of a co-culture on G. sulfurreducens metabolism was also observed
(Straub and Schink, 2004) when a secondary bacterium was added to G. sulfurreducens
respiring on ferrihydrite [69]. G. sulfurreducens showed an increased rate of ferrihydrite
reduction when grown in co-culture with 3 different secondary bacteria; E. coli, P. stutzeri and
L. lactis [69]. Of the three co-cultures with secondary bacteria, the one with E. coli showed the
greatest increase in the rate of ferrihydrite reduction, and increased the rate even more than
the addition of L-cysteine, a common additive to anaerobic medium, used to reduce the
medium and lower the redox potential [69]. Furthermore, the addition of E. coli to the medium
lowered the redox potential of the culture medium, as determined with redox dyes, to -390 mV,
as compared with -110 mV for G. sulfurreducens alone [69]. This lowered potential is of great
consequence to an MFC system and could lead to greater overall cell potential.
1.5.7 – Power and Efficiency
MFC parameters are reported in a number of ways, which often makes comparison difficult.
Very frequently researchers will report power density normalized to anodic surface area,
however it is also very often reported normalized to membrane area, cathodic surface, or
anodic chamber volume. The issue with normalizing to volume is that frequently the same
anode could be placed in a much smaller volume and have the same current density, while
drastically increasing the observed power density.
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The highest reported volumetric power density to date, to the best of our knowledge, is 2.15
kW/m3 (Nevin et al. 2008) [5]. This study used a pure culture of G. sulfurreducens in a two
chamber MFC stack with ferricyanide as the acceptor and a Nafion 117 PEM. The anode was
successively shrunk to 1/8 the size of the cathode until cathodic limitations were removed;
potentiostatic control of the anode potential showed no increase in current, implying that the
microbes rather than the system were the limiting factor [5]. The volumetric density for the cell
under normal operation was 43.3 W/m3. The cell volume was then brought down to a
minimum in order to obtain the power density reported. Normalized to the anode surface, this
system had a power density of 1.9 W/m2 [5]. Changing to an air cathode had no effect on the
power densities as the limiting factor was not the cathode. The same system inoculated with
wastewater sludge had a power density of 1.6 W/m2 and only 1.4 W/m3 [5]. The coulombic
efficiency of the pure culture was 100% after the biofilm had been established, whereas the
sludge inoculated culture cells were between 40% and 45%. It is not uncommon for cultures of
G. sulfurreducens to achieve coulombic efficiencies higher than 95%, while sludge cultures can
range from almost 0% to about 90% [5, 35, 48, 50, 52, 66, 67, ].
Membrane removal in air cathode systems generally leads to a lower coulombic efficiency
because of oxygen being directly reduced by the anodic organisms, or because it disturbs the
metabolism of these organisms. An air cathode system (Liu and Logan, 2004) achieved a
coulombic efficiency of 40-55% with a PEM and 9-12% without [6]. In one air cathode
wastewater system (Fan et al., 2007) the addition of two layers of J-cloth led to an increase
from 35% to 71% in coulombic efficiency due to reduced oxygen diffusion through the cloth
layer [53].
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2 – Materials and Apparatus
2.1 – MFC Anode Chamber
Ten identical single chamber air cathode MFCs were used for these experiments. Previously
developed MFCs with a 1 L volume were too large to run several replicates concurrently and
led to difficulties in replenishing the large volume of medium that would evaporate from them.
Furthermore, the large cathodes were prone to rupture during sterilization and were very
expensive. The cells used for this study were therefore reduced in volume, to 400 mL.
Dimensions and drawings for these cells can be found in Appendix A. The cells are made of
polycarbonate, chosen for its ability to be autoclaved and bleached without losing structural
integrity. The chamber is a box with the front face leaving a 5cm x 5cm opening for ion
exchange with the cathode, and a flange on the front face for stack assembly, with 12 holes for
bolting the cover on. On top of the cell, two gassing ports, consisting of polycarbonate tubing,
are present at the back of the chamber, for gas inflow and outflow. Two holes in the middle of
the top of the cell are to allow for a reference electrode and sampling septum. In order to
allow for continuous flow, two more pieces of polycarbonate tubing feed into and out of the
cell, the inlet at the lower back of the cell, and the outlet at the upper front, on opposite sides.
The empty MFC chamber can be seen in fig. 2.1.1.
Fig. 2.1.1 – MFC anode chamber. A) Inlet and B) outlet for continuous operation, C) Flange with opening for ion exchange, D) Inlet and E) outlet for sparging, F) Sampling (bottom) and reference electrode (top) ports.
C
D
E
F A
B
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A schematic of the chamber with exact dimensions can be found in Appendix A.
2.2 – Electrodes
The anode electrode is a 4cm x 4cm piece of carbon fiber electrode tissue, with a pure nickel
wire (Alfa Aesar, 99.8% Ni, 0.38mm diameter) woven through as a current collector. The tissue
is rigid and is formed of two layers of fibers held together by some cross woven fibers. In order
to make electrical connection without creasing or severing fibers, and to ensure that the
electrode maintains a planar shape once wetted, the wire is slipped between the two layers in
two places. The layout is such that no part of the anode is more than 1 cm away from the
current collector, and the resistance in each anode is less than 2 Ω at any point on the surface.
In order to prevent slipping and secure the connection, the portion of the anode in contact
with the wire is coated with conductive carbon paint. The carbon fiber anode was chosen as it
is porous enough to allow for biofilm formation, however not too expansive which would
require a very large cathode to handle the produced current. Furthermore, the planar shape of
the electrode allows for a consistent distance between anode and cathode, and therefore a
consistent internal resistance between MFCS, leading to more reproducible results. This would
be much more difficult to accomplish with a thicker 3 dimensional carbon “bottle brush” *48]
or felt style electrode.
Different anode sizes were attempted, originally the anode was 5cm x 5cm, however it was
reduced in size after cyclic voltammetry of a pure culture biofilm showed a current greater
than 10 mA, leading to an overflow error in the potentiostat. Furthermore, the effective
cathode surface area is 5cm x 5cm, and it was desirable to have an oversized cathode in order
to ensure that any cell limitations occurred on the anode side, as the anodic culture is the
parameter of interest. A 3cm x 3cm anode gave very low current, however a 4cm x 4cm anode
did not lead to overflow and produced significantly better current than the 3cm x 3cm anode.
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Fig. 2.2.1 – Anode (left), with Ni wire inserted between fiber layers, and cathode (right).
The cathode was obtained from Fuel Cell Earth LLC. It is a 6cm x 6cm carbon paper gas
diffusion cathode, with 30% PTFE wet proofing and 0.5 mg/cm2 platinum on carbon black
catalyst. The gas diffusion layer is to ensure equal distribution of oxygen over the cathode
surface and avoid hot spots where the catalyst may degrade more rapidly. The wet proofing
serves two purposes, the primary purpose is to prevent water loss from leakage through the
cathode. The secondary purpose is to allow water formed on the cathode during the cathodic
reaction to rapidly bead off and thus not effect oxygen diffusion to the active sites. When
assembled into the stack, as previously mentioned, the effective surface area of the cathode is
5cm x 5cm. The current is collected using a copper wire current collector that is pressed against
the cathode in the outer 0.5 cm of inactive cathode. For some preliminary experiments,
identical cathodes to those described above were obtained that had been hot pressed to a
Nafion 115 PEM. These PEM containing electrode assemblies were later discarded in favor of
membrane-less MFCs.
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2.3 – MFC Assembly
MFC assembly’s main goal is to avoid leakage in the cell and assemble all cells in an identical
manner. The first step is to place the anode over the center of the opening on the MFC flange.
After this, an inert plastic mesh is placed over the anode. In subsequent steps, the anode and
cathode will be pushed together to minimize the distance between them and thus the internal
resistance. The approximate distance between anode and cathode is 5 mm. This mesh serves
as a spacer that will prevent a short circuit in the cell. On top of the mesh is placed a closed cell
sponge silicon gasket. This material was chosen because it is soft and compressible and thus
will form a good seal without damaging the cathode, for its thermal properties which allow it
to be autoclaved, and finally because its closed cell structure makes it gas-tight and will not
allow oxygen diffusion into the cell. The cathode electrode is placed on top of the gasket,
followed by the copper wire current collector. The copper wire is bent into a pleated shape in
order to increase the contact area and ensure good contact even in the event of a small shift in
either cathode or current collector position. A second gasket is placed over the current
collector, and finally the front cover.
The cover is bolted to the MFC chamber using 12 stainless steel machine screws and wide
washers to distribute the pressure. Each nut is tightened so that a standard distance is
between the nut and the end of the machine screw is maintained. Under tightening can lead to
leaks and the shifting of the components. Over tightening compresses the gaskets and causes
them to expand outwards. The lateral force caused by this expansion on the cathode can cause
rips in the carbon paper, so it is very important to have a standard tightness for each cell. Due
to the compressibility of the gaskets, the cell tightness will also have an effect on the distance
between anode and cathode, which is a further reason why a standard tightness is desirable.
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Fig. 2.3.1 – Order of components for MFC assembly. 1) Polycarbonate anode chamber; 2) Carbon cloth anode (nickel wire current collector); 3) Plastic separator mesh; 4) Silicon sponge gasket; 5) Carbon paper cathode (30% wet proof, 0.5 mg/cm2 Pt catalyst loading on Vulcan XC72) , copper wire current collector; 6) Polycarbonate cell front cover.
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After the cell is assembled, the inlet and outlet ports for continuous operation are clamped
shut with screw compressor clamps. The cannula is inserted into the gassing inlet port and
attached to the anaerobic tubing. The other end of the tubing is capped with a 25G needle and
wrapped in aluminum foil. The gassing outlet port is capped with a needle and wrapped in
aluminum foil. A rubber septum is inserted in the sampling port, and a second septum is placed
in the port for the reference electrode. This is a placeholder for the reference electrode, which
cannot be autoclaved. MFCs are filled with milli-Q water and allowed to sit for several hours in
order to ensure that there are no leaks present prior to sterilization.
2.4 – Temperature Control
Temperature control for all MFC experiments is accomplished with a VIP 420 CO2 incubator.
The incubator has been calibrated to a temperature of 30°C with an air flow rate of 5L/min. Air
to the incubator is building air passed through a microfilter to ensure sterility and reduce the
likelihood of contamination. 6 MFCs can be set up in the incubator at one time. Tubes for
sparging enter the incubator though small gaps cut in the door gasket, while potentiostat
connections and external resistors are simply compressed between door and gasket. Bottle
cultures used for comparison were also kept in this incubator in anaerobic serum bottles.
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Fig. 2.4.1 – Incubator containing 3 MFCs.
2.5 – MFC Monitoring
Fuel cells were monitored using a VMP 16 channel potentiostat, and EC-Lab software. Working
and counter electrode potential was recorded, as well as the cell potential. This was
accomplished by setting the VMP to monitor open circuit potential between working (anode)
and counter (cathode) electrodes. In order to determine an accurate measurement, both
electrode potentials are measured against a reference electrode in the anode chamber. The
reference was an Ag/AgCl reference electrode, (BASi model RE-5B), stored in 3M NaCl when
not being used in MFC experiments. A constant external resistance of 240 Ω was applied to the
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cells unless power curves were being taken or unless otherwise stated. Determination of
current and power was determined from the cell potential using Ohm’s law (V = IR).
Fig. 2.5.1 – VMP 16 channel potentiostat for MFC monitoring
2.6 – Gassing Station
Sparging of MFCs and of culture bottles in order to ensure an anaerobic condition was
accomplished using a gassing station. The gassing station can hold 3 cylinders, which are
hydrogen, nitrogen, and N2/CO2 (80:20, v/v). Each of these gases first passes through a
volumetric flow meter for controlling flow rate. The station essentially ensures that all gas
passing through is anaerobic by removing trace oxygen in the gas stream. This is accomplished
by passing the gas through an electrically heated glass column filled with copper gauze. Any
oxygen present in the column will oxidize the copper and thus be removed from the gas stream.
The copper column is reduced with hydrogen gas prior to use, and when running for extended
periods of time, is reduced as necessary (typically every 48 hours unless the copper has visibly
oxidized).
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Fig. 2.6.1 – Copper reduced (left) and oxidized (right) for oxygen removal in gassing station.
The gas can be directed either to a vent, to a series of 7 cannula ports, or to a syringe port. The
vent is used for purging the system and to send hydrogen gas used to reduce the copper
column to the fume hood. The cannulas are used for purging anaerobic bottles for bottle
cultures, and for purging large medium bottles for MFC filling. The syringe port is used to flush
syringes with anaerobic gas, for inoculation or substrate addition to bottles or MFCs, for
sampling syringes for bottles or MFCs, and in any other scenario where sterile gas is required.
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Fig. 2.6.2 – Gassing station, entire system view.
Sparging of the MFCs is accomplished by connecting a gas tight tube to the first cannula port
and running the gas to the incubator. Prior to passing into the cannula port, the gas is passed
through the copper column to remove trace oxygen. The gas then passes through a glass wool
column and a 0.22 micron filter to ensure sterility and the lack of any entrained solids. The gas
finally passes though a cannula into a sealed 200 mL anaerobic serum bottle filled with
sterilized and anaerobic milli-Q water, kept at 70°C on a hot-plate. The gas bubbles up through
the bottle and then passes into the tubes leading to the MFCs. This step is necessary to reduce
water loss, which was found during preliminary experiments to be a major issue. The heated
water essentially saturates the warm dry gas and thus reduces the amount of moisture lost in
the cells. The tubing for MFC sparging is connected to the serum bottle by inserting a 21 G
syringe though the septum. The gas passes through the syringe and into the tube, which is
attached to a cannula whose outlet is in the lower back corner of the MFC. Total flow into the
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system is determined by volumetric flow meters on the gassing station. Any inequality in
sparging between the 6 MFCs is adjusted by attaching a screw compressor clamp to the tubes
and loosening or tightening them to increase or reduce flow as necessary.
Fig. 2.6.3 – Gas saturation bottle for MFCs.
2.7 – HPLC Analysis
HPLC analysis was done with a Dionex Ultimate 3000 HPLC system with a BioRad HPX 87H
cation exchange column, using a Shodex RI-101 RI detector. The samples were eluted with
5mM H2SO4 in water, at 0.4mL/min and 42°C. Samples were measured for glucose, acetate,
lactate, fumarate, succinate and ethanol, which were determined to be the major fermentation
products for the system. Data was analyzed using Chromeleon software. A five point
calibration curve was determined for each metabolite.
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Fig.2.7.1 – Dionex Ultimate 3000 HPLC system and Shodex RI-101 detector (left).
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3 – Experiments and Methods
3.1 – Experiments conducted
A number of preliminary experiments were conducted with pure culture G. sulfurreducens
MFCs, in order to determine typical system parameters. Some preliminary experiments also
used cathodes with Nafion 115 PEMs hot pressed to them. These cathodes were discarded as it
was deemed more important to develop a culture that was capable of power generation
without them.
All experiments were conducted with a minimum of 3 replicate MFCs. This number was chosen
based on preliminary experiments, which showed that approximately one third of MFCs
inoculated would fail to produce power or develop a leak or contamination and have to be
discarded.
Originally, the intended order of inoculation was to first add E. coli to the MFCs, and later add
G. sulfurreducens once the anaerobic condition had been established. Two runs of 6 replicate
MFCs showed no current production using this method. Another two runs of 6 replicates
showed no current production when the cultures were added simultaneously. It was therefore
determined that MFCs should be inoculated initially with G. sulfurreducens and allowed to
form a power producing anodic biofilm, and that E. coli should then be subsequently added to
the cell once the G. sulfurreducens had reached a steady current density. A run of 6 replicate
MFCs was therefore inoculated in this manner.
In order to determine the effect of interaction between cultures, pure culture MFCs of both G.
sulfurreducens and E. coli were also run. The first phase of the co-culture cells mentioned
above, which contained a sparged pure culture of G. sulfurreducens, served as the pure culture
for that organism. A separate 3 MFCs were inoculated with E. coli alone. Finally, in order to
observe the effect of E. coli growing in MFCs as compared to bottle culture, a duplicate
anaerobic bottle culture of E. coli was also characterized.
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3.2 – Cultures and Media
All anaerobic media described in this section were sparged with N2/ CO2 (80:20, v/v) to
establish an anaerobic condition. 10 mL tubes were sparged for 15 minutes into the liquid
space and for an additional 5 minutes into the headspace. 30 mL bottles were sparged for 20
minutes into the liquid space and for an additional 5 minutes into the headspace. 100 mL
bottles were sparged for 30 minutes into the liquid space and for an additional 5 minutes into
the headspace. MFC medium was sparged in two separate 2L bottles. These bottles have a cap
with a gas-tight rubber septum, through which a 10 mL serum tube with a cut off end has been
inserted, effectively making them into 2L serum bottles. These were sparged for 2.5 hours in
the liquid phase and an additional ½ hour into the headspace.
Geobacter sulfurreducens strain DL1 was obtained from the University of Massachusetts in
Amherst and was used for all cultures of G. sulfurreducens in these experiments. G.
sulfurreducens was grown at 30°C in strictly anaerobic tubes and bottles in NBAF medium as
previously described by Coppi et al. (2001) [70], containing 20mM acetate as electron donor
and 40mM fumarate as electron acceptor. First generation cultures were inoculated from
freezer stocks into 10 mL tubes of NBAF, and contained 0.3 mL of 0.1M cysteine and 0.2 mL of
5% yeast extract. All subsequent cultures of G. sulfurreducens were grown in NBAF medium
without cysteine, yeast extract or rezazurin in order to avoid the addition of mediators to the
MFC broth. G. sulfurreducens was grown for 5 generations, using a 5% inoculum from the
previous generation, prior to inoculation into MFCs.
E. coli cultures were initially plated from freezer stocks onto LB plates and incubated overnight
at 37°C. Colonies were then transferred to 10mL aerobic tubes of LB medium, containing
100mM D-glucose as electron donor, and incubated at 37°C for 12 hours. Subsequent
generations were grown at 30°C in anaerobic NB medium sparged with N2/ CO2 (80:20, v/v) to
establish an anaerobic condition. NB medium for E. coli contained no acetate or fumarate but
instead contained 100mM D-glucose as substrate. Again, the NB medium contained no
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cysteine, yeast extract or rezazurin to avoid the presence of mediators. E. coli cultures were
grown for 5 generations in NB medium, using a 5% inoculum from the previous generation,
prior to inoculation into MFCs. The first generation in NB medium, with inoculum taken from
the LB medium, grew within 12 hours to a high density. The subsequent generation (second
generation NB) was the slow step, as there was very little yeast extract present from the LB
remaining and the E. coli had to adapt to a new medium. This generation took between 36 and
48 hours to reach a similar density to the first generation NB culture. All subsequent
generations in NB medium reached the appropriate density within 12 hours of inoculation.
MFCs were kept at 30°C; MFC medium was NB medium containing no acetate, fumarate,
cysteine or rezazurin, with an additional 10mM NaCl to increase conductivity. For co-culture
MFCs the medium initially contained 10mM acetate as the substrate for G. sulfurreducens, and
was supplemented with additional acetate as necessary. Prior to E. coli addition, 100mM D-
glucose was added to the MFCs. In pure culture E. coli MFCs, 100mM D-glucose was added
initially.
3.3 – E. coli Strain Selection
The goal of this project is to produce a co-culture that can degrade a variety of wastes, using a
facultative anaerobe. E. coli is the best understood of the facultative anaerobes and is a model
organism. In addition, there exist many strains of E. coli that have been genetically engineered
to consume a variety of substrates, which is desirable for a robust MFC system. Furthermore, E.
coli generates acetate as one of its primary fermentation products; reducing acetate
production is the focus of a number of E. coli genetic engineering studies. Furthermore, acetate
toxicity becomes an issue in E. coli culture when it is allowed to accumulate, and so the
removal of acetate by G. sulfurreducens is desirable. As acetate is the ideal substrate for G.
sulfurreducens, E. coli is a seemingly ideal choice in terms of understanding interactions and
working in syntrophic culture.
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It is desirable to keep G. sulfurreducens in its ideal medium in order to achieve the highest
possible power production. The strain of E. coli was therefore determined based on the ability
of the strain to grow anaerobically in NB medium. Wild type E. coli strain K-12 cultures showed
no growth in the second generation NB transfer. The only very significant difference between
NB medium and the M9 minimal medium in which E. coli K-12 grew well is the bicarbonate
buffer in the NB. Three strains of wild type E. coli were investigated based on their previously
documented ability to grow in bicarbonate buffered media. The strains were obtained from the
E. coli Genetic Stock Centre (CGSC) at Yale University. Selected strains were: W3110 (CGSC
4474), B (CGSC 5713), and C (CGSC 3121). All three strains were grown as described in section
3.2. Table 3.3.1 below summarizes the findings.
Table 3.3.1 – Phenotypic growth of 3 strains of E. coli in LB and NB media.
- Growth, - Slow or limited growth, - No growth.
Strain LB Medium 1st Generation NB
Medium
2nd Generation NB
Medium
W3110
B
C
Based on this analysis, E. coli strain C (CGSC 3121, ATCC 23461) was chosen to perform MFC
experiments. All subsequent references to E. coli refer to E. coli strain C.
3.4– MFC Setup and Inoculation
MFC medium is prepared and sterilized in the 2L serum bottles described in section 3.2.
Preliminary attempts to autoclave assembled MFCs filled with medium failed due to two
separate issues. Firstly, in previous cells with larger cathodes, increased pressure during
autoclaving caused some cathodes to burst, and cause leaks in others. Cathode assemblies
including a PEM saw a good deal of separation between the PEM and the cathode surface,
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diminishing the effective cathode surface area. In the cells used for these experiments, cathode
leaks were rare but were nevertheless observed, and led to poor reproducibility and an
elevated cost for cathode replacement. Secondly, NB medium forms precipitates when
autoclaved in the presence of oxygen, and the MFCs cannot maintain a perfect anaerobic
condition without sparging. For these reasons, MFCs are filled with only 10 mL of milliQ water
for autoclaving, in order to provide evaporative heating. The medium is autoclaved separately
and the cells filled after.
Prior to filling the MFCs, the electron donor (10mM acetate for co-culture cells or 100mM D-
glucose for pure culture E. coli cells) is added to the cell with a syringe and 0.22 micron filter,
using a 22G needle through the septum. The cells are filled using gravity flow and N2/CO2 gas
pressure by inverting the 2L serum bottles on a retort stand. A tube attached to a sterile 18G
needle on either end is inserted into the septum of the serum bottle. The other end is inserted
into the septum on the MFC acting as a placeholder for the reference electrode. This tube is
autoclaved with the MFCs to ensure sterility. Both septa are swabbed with 70% ethanol and
flamed prior to being pierced with the needle. In order to break the vacuum formed in the 2L
serum bottles and force the medium out, N2/CO2 is sparged into the bottle. This is
accomplished by attaching a 0.22 micron filter to the end of one of the cannula ports and
attaching a sterile 22G needle to the filter. This 22G needle is also passed through the 2L serum
bottle septum. The entire process is done next to a Bunsen burner flame. Once the level in the
MFC has reached 330 mL, the needle is removed from the cell, the tube is pinched to stop flow,
and a new 18G needle is attached and used to fill the next MFC. The filling system can be seen
in fig. 3.4.1.
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Fig. 3.4.1 – Sterile filling system for MFCs.
Once the MFC has been filled, the reference electrode, which has been sitting in a 5% bleach
solution for 2 hours, is removed from the bleach and thoroughly rinsed with 20 mL of sterile
milliQ water, using a 20 mL syringe and a 0.22 micron filter. The septum acting as a placeholder
is removed from the cell and the reference electrode quickly inserted. The seal is formed using
an O-ring and autoclaved silicone grease. Finally, the electron donor is added through the
remaining rubber septum using a sterile syringe and 22G needle.
Fully assembled MFCs are placed in the incubator and connected to the gassing station as
described in section 2.6, by inserting a 21G needle through the septum of the gas saturation
bottle. The 25G needle for gas outlet is replaced with a fresh sterile one to ensure that the
path is not blocked by condensation from the autoclave. MFCs are then sparged vigorously for
3 hours prior to inoculation. During this time, potentiostat is connected to anode current
collector, cathode current collector and reference electrode using crocodile clips. A 240Ω
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resistor is also connected between anode and cathode current collectors using crocodile clips.
The sparging rate was then decreased to approximately 0.8 mL/min.
MFCs are inoculated with 20mL of a mid log phase culture of either G. sulfurreducens in co-
culture cells, and subsequently with 20mL of E. coli once current had reached a steady level, or
solely with 20mL of E. coli in pure culture cells. This brings the cells to the operating volume of
350 mL.
Fig. 3.4.2 – Final assembled 6 cell MFC system, with connections to potentiostat and gassing.
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Fig. 3.4.2 – Final assembled 6 cell MFC system, incubator view.
Fig. 3.4.3 – Final assembled MFCs (cell view). A) Cannula for gas inlet B) Stopper to seal gassing effluent when sparging is stopped, C) Septum, D) Reference electrode, E) Cathode.
B
D C
E
A
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3.5 – MFC Operation
After inoculating the MFCs, the potentiostat is started and begins monitoring the cells and
measures the cell potential every 120 seconds. Sparging continues for G. sulfurreducens cells
until 12 hours after E. coli addition. E. coli cells are sparged only for the first 12 hours. When
sparging is stopped, the screw compressor clamps controlling flow to the cells are clamped
shut, to prevent any contact between cells via the gas saturation bottle once the positive
pressure is lost.
Immediately after inoculation, a 2mL sample was taken from each cell for HPLC analysis of
metabolites. Samples were for the most part taken every 24 hours. At the beginning of the
experiments when rapid changes were occurring, samples were taken with greater frequency,
and samples were taken when a significant change was seen in the potential evolution curve
being logged by the potentiostat. In the later stages of the experiment when very little was
changing, samples were taken every 48 to 72 hours as necessary. Samples were analyzed by
HPLC and measured for pH using litmus paper. Samples not immediately analyzed by HPLC
were filtered with a 0.22 micron filter to ensure no further microbial activity and kept
refrigerated at 4°C.
Preliminary experiments showed a tendency of pure culture E. coli MFCs to acidify as
fermentation progressed. In these cells, pH dropped to 4 and subsequently metabolic activity
slowed and eventually ceased entirely. In the experiments shown in this study, cell pH was not
allowed to fall below pH 6. Pure culture E. coli cells were neutralized with 1M anaerobic NaOH
as necessary. The amount of NaOH addition was determined by titration into a small 3mL
aliquot from the MFC that required it.
Due to sampling and evaporation, the cell volume was significantly reduced and had to be
regularly supplemented with fresh medium. Medium was taken from the 2L serum bottles
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using sterile 60 mL syringes, and vacuum in the serum bottles was broken as in the cell filling in
section 3.4. The fresh medium was added through the MFC sampling septum.
Operating curves were taken for pure culture G. sulfurreducens cells and co-culture cells
whenever a new steady voltage was observed. After determining internal resistance from
these curves (see section 3.7) the external resistance was varied to better match the internal
one if necessary.
3.6 – Sterilization of Components and Maintenance of Sterility
Considering that these experiments run for long periods of time with pure cultures, sterility is a
major concern in the MFCs. The system is too large to assemble all the components and
autoclave all at once, and so several of the components must be sterilized separately. Table
3.6.1 summarizes sterilization method for individual system components.
Electron donors (acetate and glucose) were autoclaved separately in serum bottles and their
septa were rubbed with 70% ethanol and flamed with a Bunsen burner prior to any syringe
punctures. All substrate or fresh medium addition is injected into MFCs through a 0.22 micron
syringe filter. The septa on the MFC anode chamber were also rubbed with 70% ethanol and
flamed with a handheld lighter prior to any syringe puncture.
In order to maintain sterility after cell assembly, building air passing into the incubator is first
passed through a sterile gas filter. The incubator is kept shut except for sampling and
medium/substrate addition.
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Table 3.6.1 – Sterilization of MFC components
Items Requiring Sterilization Method of Sterilization
Anode Chamber (PC box) Submerged in 5% bleach for 2 hours. Rinsed with
milli-Q water. Autoclaved after cell assembly.
Anode Electrode Autoclaved after cell assembly.
Cathode Electrode Autoclaved after cell assembly.
Reference Electrode Submerged in 5% bleach for 2 hours. Rinsed with
sterile milli-Q water next to a flame.
Electrode Separator Mesh Autoclaved after cell assembly.
Gaskets Autoclaved after cell assembly.
Sampling septum Autoclaved after cell assembly.
MFC medium Autoclaved in 2L medium bottles.
Tubing for filling MFC with medium Autoclaved with capped ends.
N2/CO2 gas Passed through 0.22 micron filter.
Cell gassing cannulas Autoclaved after cell assembly.
Gassing tubing Autoclaved after cell assembly, ends capped.
Serum bottle and cannula for gas saturation Autoclaved, cannula wrapped in foil.
Incubator interior surfaces 70% ethanol spray. Temperature raised to 80°C
(maximum level) for 2 hours.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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3.7 – Operating curve and Power Density Curve Determination
The fuel cells were allowed to reach a steady state, where the voltage remained more or less
constant for a number of days. At this point, the operating curves were determined. This was
done by varying the external resistance in the circuit in order to draw more or less current, and
the voltage was measured. Anodic cultures have been shown to adapt to changes in external
resistance given a long enough time frame [47]. In order to avoid this effect, each new
resistance was changed and allowed to steady for at most one hour. If the voltage reached a
steady value prior to this, the resistance was changed again. The cell was first allowed to reach
open circuit potential, and subsequently the following order of resistors were used: 10 kΩ, 3
kΩ, 1 kΩ, 750 Ω, 560 Ω, 240 Ω, 100 Ω, 51 Ω, 24 Ω. If the curve lacked resolution in a particular
region, additional resistors were used to provide more points. If the curve reached limiting
current density at a higher resistance, some of the lower resistances were omitted.
Internal resistance was approximated by taking the slope of the linear portion of the operating
curve, with non-normalized current density. The power curve power and current are
normalized to the projected anodic surface area of 32 cm2.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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3.8 – Coulombic efficiency determination
The coulombic efficiency was determined based on the total amount of substrate degraded.
Each substrate has an equivalent number of electrons that should be harvested as power if the
entirety of the electrons from that substrate g towards reducing the anode. The two substrates
of interest in this case are acetate and glucose. Their oxidation reactions are :
Acetate: CH3COO- + 2H2O → 2CO2 + 7H+ + 8e-
Glucose: C6H12O6 + 6H2O → 6CO2 + 24H+ + 24e-
Thus, for each mole of acetate or glucose oxidized, 8 moles or 24 moles of electrons
respectively can conceivably be collected. The coulombic efficiency is the percentage of the
theoretical electrons that can be harvested that are converted to current. In order to
determine the efficiency for each stage of MFC operation, the total amount of oxidized
substrate must be determined.
For acetate, this is determined by subtracting the final amount of acetate, based on the cell
volume at the final time point and the concentration at that point, from the initial amount of
acetate, based on the initial cell volume and initial measured concentration. It is important not
to assume a constant cell volume, as evaporation is constantly occurring and the fresh medium
addition is sporadic. From the rate of medium addition, however, the evaporation rate in the
cell can be determined and accurately predicted at any time. This was determined to be 6.09
mL/hr by plotting total medium addition against time, in fig. 3.8.1. It is evident that the
evaporation rate is linear and fairly constant. The initial volume is known to be 350 mL, the
final volume is determined from the evaporation curve. Once total acetate consumption is
determined, it must be converted to coulombs.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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y = 0.6093xR² = 0.9973
0
100
200
300
400
500
600
700
800
0 200 400 600 800 1000 1200
Vo
lum
e lo
st (
mL)
Time (h)
Fig. 3.8.1 – Evaporation rate for co-culture MFC. The conversion to coulombs is accomplished using the following equation:
The number of coulombs in collected on the anode is determined by integrating the current vs.
time curve for the time period in question to determine the area. This is accomplished using EC
Lab’s built in integration feature.
In order to determine the coulombic efficiency for co-culture MFCs, the same process is done
using glucose concentrations and stoichiometry (24 electrons vs. 8 for acetate). The only added
step is to consider residual acetate from the previous stage of the co-culture cell.
Sample calculations for the coulombic efficiency can be found in Appendix
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
55
4 – Results
4.1 – Pure culture Geobacter sulfurreducens MFCs
A total of 6 replicate cells were run, results shown are for one representative cell. Further
replicate data can be found in Appendix B.
The pure culture G. sulfurreducens MFC showed a typical growth curve, reaching a steady
current at 240 Ω of 1.3 mA. The current evolution of the cell can be seen in fig. 4.1.1. When a
sudden decrease in current was observed, additional acetate was added to the cell. Sudden
drops in the current curve represent fresh medium or acetate additions, which disturbed the
anode and cause a drop in current output.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 50 100 150 200 250 300 350 400 450 500
Cu
rre
nt
(mA
)
Time (hrs)
Fig. 4.1.1 – Current evolution of pure culture G. sulfurreducens MFC, under a constant external resistance of 240Ω. Sudden spikes in current are due to medium or acetate addition.
Metabolite concentrations for this MFC can be seen in fig. 4.1.2. Residual fumarate from the
inoculum is reduced to succinate within the first 40 hours, during which time the G.
sulfurreducens begins to grow on the anode and produce current. Succinate is completely
depleted by 65 hours into the batch. The coulombic efficiency based on acetate for this pure
culture region of the curve is 6.99% (see Appendix C for sample calculations). This value is
Power/Operating Curve Taken
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
56
lower than previously determined literature values in section 1.5.7, in all likelihood due to the
lack of a PEM which leads to some oxygen leaking into the cell and prevents the anodic culture
from respiring effectively using the anode. The remaining 93.01% of the electrons may have
gone towards biomass formation or the aerobic consumption of acetate. When dividing the
curve into the growth phase (0 to 142 hours) and the stationary phase (142 – 486 hours), the
coulombic efficiencies are 4.77% and 7.50% respectively. This is to be expected as more
acetate is being diverted to biomass in the growth phase, and less once the biofilm has
established in the stationary phase.
The pH of the MFCs was measured when samples were taken. The pH of the pure culture G.
sulfurreducens MFC rose steadily from 7 to 7.8 over the 486 hour period of measurement. As
the changes were not abrupt and did not become basic enough to hinder growth, no measures
were taken to control pH in these cells.
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300 350 400 450 500
Co
nce
ntr
atio
n (
mM
)
Time (hrs)
Acetate
Addition
Fig. 4.1.2 – Metabolite concentration of key metabolites for pure culture G. sulfurreducens MFCs. Arrows indicate acetate addition. ( - acetate, – succinate, - fumarate).
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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4.2 – Co-culture Geobacter sulfurreducens and E. coli MFCs
E. coli culture was inoculated after 487 hours of operation as a pure culture G. sulfurreducens
MFC, when a sustained current was observed. The current evolution for the co-culture MFC
can be seen in fig. 4.2.1. There was an immediate decrease in cell current after inoculation,
which dropped to 0.5 mA prior to power and operating curves being taken at 163 hours after E.
coli addition (650 hours after G. sulfurreducens addition). After evaluating the internal
resistance from this operating curve, the external resistance was increased to 560 Ω as it was
determined that 240 Ω was in the mass transfer limited region of the curve (see fig. 4.5.1).
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600 700 800
Cu
rre
nt
(mA
)
Time (hrs)
Fig. 4.2.1 – Current evolution of co-culture G. sulfurreducens and E. coli MFC. E. coli and glucose added at 0 hours, corresponding to 487 hours after G. sulfurreducens inoculation. The external resistance of 240Ω changed to 560Ω after first power curve. Sudden spikes in current are due to medium addition.
The metabolite curve for the co-culture MFC can be seen in fig. 4.2.2. The main fermentation
products of acetate and lactate began to accumulate in the cells as glucose was depleted,
eventually reaching 53mM and 56mM respectively. Notably, succinate also began to
accumulate, though at a significantly lower rate, reaching a peak of only 5 mM. Upon glucose
being completely consumed at approximately 315 hours, succinate, lactate and acetate
Power/Operating Curve Taken
R changed to 560 Ω
Power/Operating Curve Taken
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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concentrations all began to decrease as they were consumed, with succinate being completely
removed from the system after 550 hours. This is followed by a decline in cell current until 600
hours after E. coli addition (~1090 hours after G. sulfurreducens addition), when the cell
suddenly began to produce much more current again. Acetate and lactate concentrations
continued to decline during this resurgence in current production. A final power and operating
curve was taken for the cell at 755 hours (1246 hours after G. sulfurreducens addition). The
coulombic efficiency based on acetate and glucose present at E. coli inoculation was 1.97%.
Ethanol never reached any significant level in the MFC.
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600 700 800
Co
nce
ntr
atio
n (
mM
)
Time (hrs)
Fig. 4.2.2 – Metabolite concentration of key metabolites for co-culture G. sulfurreducens and E. coli MFCs. E. coli and glucose added at 0 hours, corresponding to 487 hours after G. sulfurreducens inoculation. ( - glucose, - acetate, x– lactate, – succinate, - ethanol).
Upon addition of E. coli to the MFCs, the pH gradually dropped to 6.8 due to the production of
organic acids in fermentation (first 300 hours), and then reverted back to pH 7.5 after these
acids were consumed by G. sulfurreducens. This system therefore required no pH adjustment
as the pH did not vary radically with time or reach a value that inhibited power production.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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4.3 – Pure culture E. coli MFCs
These MFCs were run in triplicate and agreed very well. Results shown are of one
representative MFC. The current evolution for the pure culture E. coli MFC is shown in fig 4.3.1.
No significant current evolution is seen, as the maximum current never exceeded 10% of the
current seen in the G. sulfurreducens or co-culture cells (note the difference in the scale of in
fig. 4.3.1 compared to figs. 4.1.1 and 4.2.1), and therefore power and operating curves were
not taken for these MFCs. The maximum current production occurs early on while E. coli is
consuming glucose, however the reading is noisy and no steady current is ever established for
this MFC.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 100 200 300 400 500 600 700 800
Cu
rre
nt
(mA
)
Time (hrs)
Fig. 4.3.1 – Current evolution of pure culture E. coli MFC. The external resistance is 240Ω. Sudden spikes in current are due to medium addition.
The metabolite curve for the pure culture MFC of E. coli is shown in fig. 4.3.2. Glucose is
completely consumed within 120 hours, with the main fermentation products again being
lactate, acetate and succinate. Lactate reached a maximum concentration of 57mM, acetate of
34mM, and succinate reached 19mM. After glucose was completely consumed, the
concentrations of these products began to steadily decline, though initially only lactate and
succinate are decreasing; a lag is observed for the consumption of acetate for nearly 250 hours
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
60
after glucose is depleted. Once again, ethanol production in the MFCs was negligible.
Measurements were taken for these MFCs for the same timescale as the co-culture MFCs in
the previous section.
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600 700 800
Co
nce
ntr
atio
n (
mM
)
Time (hrs)
Fig. 4.3.2 – Metabolite concentration of key metabolites for pure culture E. coli MFCs. ( - glucose, - acetate, x– lactate, – succinate, - ethanol).
The pure culture E. coli MFCs showed a rapid drop in pH that needed to be neutralized with
sterile 1M NaOH in order to keep the bacteria metabolically active and continue breaking down
glucose. The total base addition is shown in fig. 4.3.3. The pH of the MFC was never allowed to
drop below pH 6 at any time, and was neutralized to pH 7 at each time point in fig. 4.3.3.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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0
5
10
15
20
25
30
35
40
45
50
0 20 40 60 80 100 120
To
tal
Ad
de
d V
olu
me
1M
Na
OH
(m
L)
Time (hrs)
Fig. 4.3.3 – Base addition for neutralization of pure culture E. coli MFCs. 1M anaerobic and sterile NaOH was used for neutralization. After 110 hours, no further base addition was required.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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4.4 – Pure culture E. coli Bottles
In order to compare with MFC cultures, pure culture E. coli was also characterized in anaerobic
bottle culture. Bottle culture experiments were done in duplicates; results shown are the
average of two bottles. The metabolite curve is shown in fig. 4.4.1. Glucose is completely
consumed within 160 hours, after which point metabolite concentrations stop changing
significantly for the following 600 hours. Lactate, acetate and succinate are once again the
principal fermentation products, reaching concentrations of 68mM, 14mM and 15mM
respectively. The acetate concentration stops rising at approximately 50 hours, significantly
before glucose is completely consumed. Ethanol does reach approximately 2mM in these
bottles, however this is still a very low concentration compared to other metabolites.
0
10
20
30
40
50
60
70
80
90
0 100 200 300 400 500 600 700 800
Co
nce
ntr
atio
n (
mM
)
Time (hrs)
Fig. 4.4.1 – Pure anaerobic bottle culture E. coli metabolite curve. ( - glucose, - acetate, x– lactate, – succinate, - ethanol).
The pure culture E. coli bottles also exhibited a drop in pH that needed to be neutralized with
sterile 1M NaOH. The total base addition is shown in fig. 4.4.2. The pH of the MFC was never
allowed to drop below pH 6 at any time, and was neutralized to pH 7 at each time point in fig.
4.4.2.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120
Tota
l Ad
de
d V
olu
me
1M
NaO
H (
mL)
Time (hrs)
Fig. 4.4.2 – Base addition for neutralization of pure culture E. coli bottles. 1M anaerobic and sterile NaOH was used for neutralization. After 110 hours, no further base addition was required.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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4.5 – Pure culture G. sulfurreducens and co-culture MFC operating and power curves
Operating curves for the pure G. sulfurreducens culture, the early co-culture and the late co-
culture are shown on the same plot in fig 4.5.1. From the slope of the linear region of these
curves we can see that E. coli addition dramatically increased the internal resistance in the cell,
with a resistance of 244 Ω for the G. sulfurreducens culture increasing to 363 Ω 163 hours after
E. coli addition, and to 523 Ω 759 hours after E. coli addition. The open circuit potential for the
G. sulfurreducens culture was 750 mV, which decreased to 681 mV for the early co-culture and
rose back to 725 mV for the late co-culture.
y = -243.91x + 685.24R² = 0.9987
y = -363.76x + 578.2R² = 0.9997
y = -522.83x + 612.34R² = 0.9988
0
100
200
300
400
500
600
700
800
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Po
ten
tial
(m
V)
Current (mA)
Fig. 4.5.1 – Operating curves for co-culture MFCs at various times. – Pure G. sulfurreducens cells, 310 hours. – Co-culture cells, 650 hours (163 hours after E. coli addition). – Co-culture cells, 1246 hours (755 hours after E. coli addition).
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
65
The power curves for these same cultures, shown in fig. 4.5.2, quite clearly show that the most
power by far is achieved by the pure G. sulfurreducens culture, as would be expected from the
increased internal resistance in the co-culture cells. The pure culture reaches a peak power of
128 mW/m2, compared to 63 mW/m2 for the early co-culture and 56 mW/m2 for the late co-
culture. The limiting current density is different in each case; it is 404 mA/m2 for the pure G.
sulfurreducens culture, 184 mA/m2 for the early co-culture, and 282 mA/m2 for the late co-
culture.
0
20
40
60
80
100
120
140
0 50 100 150 200 250 300 350 400 450
Po
we
r (m
W/m
2)
Current (mA/m2)
Fig. 4.5.2 – Power curves for co-culture MFCs at various times. – Pure G. sulfurreducens cells, 310 hours. – Co-culture cells, 650 hours (163 hours after E. coli addition). – Co-culture cells, 1246 hours (755 hours after E. coli addition).
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
66
5 – Discussion
5.1 – Comparison of MFC metabolite curves
The pure culture G. sulfurreducens, co-culture and pure culture E. coli MFCs all differ
significantly from one another. The most obvious difference is the complete lack of current
generation in the E. coli pure culture MFCs as compare with the other two cultures. This is to
be expected, as E. coli in the absence of electron transfer mediators has not been seen to
produce significant current in the past, and great care was taken to remove any potential
external mediators from the MFC medium. The more significant differences are in the
metabolite curves.
The pure culture G. sulfurreducens MFCs behaved as expected, reducing any residual fumarate
from the inoculum to succinate and then growing on acetate with the anode as electron
acceptor. The only unexpected result is the removal of the residual succinate from the system
(fig. 4.1.2). From the co-culture and E. coli pure culture MFC metabolite curves in figs. 4.2.2 and
4.3.2, it is plain that the cultures follow a similar trend in metabolic activity, with some notable
discrepancies. Firstly, complete consumption of glucose occurs much faster in the pure culture
than in the co-culture, 120 hours compared with 315 hours. When looking at the bottle culture
E. coli metabolite curve (fig. 4.4.1), it completely consumes glucose within 160 hours, which
agrees better with the pure culture E. coli MFC. It would therefore appear that the E. coli in the
co-culture cell is growing more slowly than in the bottle or pure culture MFCs. One reason for
this could be the higher acetate concentration; there is less acetate in the pure culture E. coli
MFCs, due to residual acetate from the previous stage of pure culture G. sulfurreducens MFCs,
approximately 32 mM, present at E. coli inoculation. The bottle culture of E. coli having slower
growth is likely due to the lack of oxygen, the bottles were completely anaerobic, whereas the
MFCs allowed for a small amount of oxygen to leak into the cell and E. coli will break down
glucose more rapidly in the presence of oxygen. The presence of the anode as an electron sink
could also have contributed to the more rapid glucose consumption; however it is much more
likely that oxygen was the more important factor, due to the very low current observed on the
anode (fig. 4.3.1).
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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Lactate concentrations are virtually identical after fermentation of glucose stops in both E. coli
containing MFCs. In pure E. coli and co-culture cells, lactate concentration reaches a peak at 59
and 57 mM respectively, after which any remaining glucose seems to be converted to acetate.
This lactate peak occurs at approximately 110 hours for the pure culture and 255 hours for the
co-culture. Acetate levels are different, 54 mM in the co-culture and only 33 mM in the pure
culture, but once again this difference can be explained by residual acetate from the pure
culture G. sulfurreducens phase of the co-culture cells. The most notable difference in
metabolite concentrations between the two cultures is succinate. The peak succinate
concentration in the pure culture E. coli MFC is 19 mM, compared with only 5 mM in the co-
culture.
Once all the glucose is consumed, lactate, acetate and succinate begin to be depleted in both
cells. In the pure culture E. coli MFC, acetate is at first consumed at a much lower rate than
lactate (180-350 hours), and then later on more rapidly but still not as quickly as lactate.
Succinate is initially rapidly consumed, at a similar rate to lactate, however as the
concentration reaches a lower value (~8 mM) the succinate removal begins to slow. In the co-
culture MFC the rate of consumption is almost identical between acetate and lactate, and
succinate consumption is much slower. Despite the much faster fermentation of glucose in the
pure culture E. coli MFC, the consumption of fermentation products is significantly more rapid
in the co-culture. At 760 hours, 445 hours after glucose is completely consumed in the co-
culture MFC, succinate is completely consumed, and only 4 mM lactate and 10 mM acetate
remain in the system. In the pure culture E. coli MFC, while succinate is fully consumed, there
are still large amounts of lactate and acetate remaining in the system, 27 mM and 17mM
respectively, 680 hours after glucose is completely consumed.
The consumption of lactate, acetate and succinate in the pure culture E. coli cell can be seen as
a consequence of oxygen diffusion into the cell, as E. coli is incapable of oxidizing these
compounds without oxygen. This is evident from the anaerobic bottle culture metabolite curve
in fig. 4.4.1, where acetate, lactate and succinate concentrations all remain constant after
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
68
glucose is completely oxidized. The only changing variable between the bottle culture and the
pure culture MFC is the possibility of oxygen as an electron acceptor for the ETC, or the
possibility of the electrode as a terminal acceptor; however no significant current was ever
seen in the pure culture E. coli MFCs in fig. 4.3.1. Thus, any degradation seen after glucose is
consumed is a result of oxygen diffusion into the cell. Pure culture MFCs should have the same
oxygen diffusion rate as the co-culture cells, as they are made up of identical components and
operating under the same conditions. This would imply that the increased rate of fermentation
product degradation in the co-culture cells is due to G. sulfurreducens activity.
Comparison of the acetate consumption in figs. 4.2.2 and 4.3.2 supports this idea. As
mentioned above, in the co-culture MFC, after glucose is completely removed from the system,
acetate immediately begins being consumed, whereas in the pure culture E. coli MFC there is a
lag period where lactate and succinate are being consumed at a much higher rate than acetate
(180 hrs – 350 hrs, fig. 4.3.2). This difference can be explained by the consumption of acetate
by G. sulfurreducens, which is the preferred substrate for that organism. The current
production, while lower than in the pure G. sulfurreducens cells, continued after E. coli addition,
which is evidence that G. sulfurreducens was still respiring on the anode.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
69
5.2 – Effect of succinate on MFC power production
The least readily explained difference between pure and co-culture MFC metabolite curves
after glucose is completely consumed, is that there is significantly more succinate in the pure
culture fuel cell (19mM) than is present in the co-culture (5mM). The bottle culture of E. coli
also reaches a much higher succinate concentration of 15mM. This leads to the conclusion that
either succinate is being consumed in the co-culture MFC at the same time as it is being
generated, or that more succinate is being produced in the pure culture MFC and the bottle
culture than in the co-culture MFC.
Careful observation of the metabolite and current evolution curves make it appear most likely
that G. sulfurreducens is consuming the succinate as it is produced, without transferring
electrons to the anode, or with a lower rate of electron transfer. This is supported by observing
that once succinate is completely removed from the co-culture MFC in fig. 4.2.2, the cell
current in fig. 4.2.1 rapidly increases. It is also supported by the observation that in the pure
culture G. sulfurreducens MFCs, the residual fumarate from the NB medium is reduced to
succinate, and there is a visible stay in the exponential current evolution (fig. 4.1.1) until
succinate is fully removed from the system, at 65 hours (fig. 4.1.2). At this point, G.
sulfurreducens is the only microbe present, so it is clear that succinate is being depleted by this
organism, and this process appears to have a negative effect on current production on the
anode; it is either slower than acetate consumption, leading to the lower current output, or it
is not related to anode reduction. Immediately upon addition of E. coli to the co-culture MFCs,
and as the production of succinate begins and succinate levels begin to rise, the cell current
steadily drops, until succinate is completely removed from the cell (refer to figs. 4.2.1 and
4.2.2). From fig. 4.2.2 it is also clear that acetate is taken up by G. sulfurreducens at the same
time.
A possible mechanism for this reaction is that succinate is being consumed aerobically by G.
sulfurreducens present on the anode. In section 1.5.5, the ability of G. sulfurreducens to grow
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
70
under microaerobic conditions is mentioned [20]. It is also seen that it is likely that not only
acetate is being consumed while aerobic growth takes place, due to the observed
stoichiometry of the reaction [20]. Another study showed using C13 analysis that in the
presence of fumarate and acetate, G. sulfurreducens would uptake both and that the flux
between fumarate and succinate in both directions is elevated [57]. This suggests that it is
possible that G. sulfurreducens could potentially simultaneously uptake and utilize both acetate
and succinate. Finally, it has also been observed that a strain of G. sulfurreducens can sustain
growth on an electrode in the presence of oxygen, provided that the concentration is not too
elevated in the biofilm; however this leads to a decrease in power production [22].
It is therefore proposed that G. sulfurreducens on the anode surface is consuming succinate
aerobically in the microaerobic conditions present at the anode. In the absence of succinate,
while G. sulfurreducens is consuming only acetate, the organism favors electrode reduction
over oxygen reduction. This would imply a decrease in the amount of biomass respiring with
the anode as electron acceptor in the presence of succinate, and therefore a decrease in the
current. In this case, it would be expected that a lower limiting current density would be
observed in an MFC containing succinate than would be present in one where succinate had
been depleted. This is exactly what is seen in figs. 4.5.1 and 4.5.2. The early co-culture, whose
operating and power curves were taken at 163 hours after E. coli addition, in the presence of
succinate (see fig. 4.2.2), has a limiting current of 184 mA/m2. The late co-culture, whose
operating curve was taken at 759 hours after E. coli addition and long after all succinate had
been depleted in the cell (see fig 4.2.2) has a limiting current of 282 mA/m2. This occurs
despite an increased internal resistance in the late co-culture, which typically would be
expected to cause a decreased limiting current.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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5.3 – Control of pH in co-culture and pure culture cells
Another notable difference between pure and co-culture MFCs is the necessity for pH control.
The pH was measured when samples were taken for HPLC analysis. In the first stage of the co-
culture cells, pH rose from 7 to 7.8 due to G. sulfurreducens growth. This can be explained by
proton depletion at the cathode, as the cells were fed sodium acetate rather than acetic acid,
with the balanced redox equations being:
Anode: CH3COO- + 2H2O → 2CO2 + 8e- + 7H+ Cathode: 2O2 + 8H+ + 8e- → 4H2O
This system therefore depletes protons from the culture medium and would lead to a basic
condition, which was observed.
A pure culture E. coli MFC rapidly acidifies, due to the production of organic acids by
fermentation. In a preliminary experiment (data not shown), a cell left without neutralization
did not fully consume glucose and stopped fermenting after reaching a pH of 4. This led to the
decision to neutralize pure culture E. coli MFCs and co-culture cells if necessary.
The co-culture MFC did not become acidic or too basic; the process of acidification by E. coli
was cancelled out by acetate consumption and proton depletion by G. sulfurreducens. This
process allowed for the E. coli to consume more glucose than they would have been capable of
without neutralization. This self-regulation of pH appears to be one of the principal advantages
of the co-culture system, in addition to the oxygen removal by E. coli. The advantages of this
outcome to a scaled up system are significant in terms of operating costs both for mixing and
for the raw material cost of the acid or base; the system would also be capable of a greater
degree of degradation and lead to a cleaner effluent due to the enhanced capacity for glucose
utilization by E. coli.
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5.4 – Effect of E. coli on MFC performance
It is clear from figs. 4.5.1 and 4.5.2 that E. coli addition had a negative effect on power
production in the co-culture MFCs. The pure culture G. sulfurreducens MFC produced a
maximum power of 128 mW/m2, compared to 63 mW/m2 for the early co-culture and 56
mW/m2 for the late co-culture. However, in order to compare on an even scale, the power
required for the sparging of the anode chamber must be considered. As the co-culture cells
were not sparged and used E. coli to remove trace oxygen, they have no additional power
requirements. The power required to sparge the anode chamber of the pure G. sulfurreducens
MFCs is estimated to be roughly 2 W/m2. This value was obtained assuming the reversible
compression of an ideal gas from atmospheric pressure to the pressure in the gas cylinder. This
does not take into account the additional energy required to create the anaerobic N2/CO2 gas
in the first place. When considering the net power to the system, including the power required
to sparge the anode chamber, clearly a co-culture system is vastly superior to the pure culture
G. sulfurreducens cell. The relative power required for sparging would likely decrease
significantly with scale up of the anode for greater current production, however it is obviously
preferable to avoid sparging altogether.
From the coulombic efficiencies of 6.99% for the pure culture G. sulfurreducens MFC and 1.97%
for the co-culture MFC, we can see that much more of the electrons in the electron donor went
towards the anode in the pure G. sulfurreducens cell, which is the expected result, though the
very low numbers in both cases do not agree very well with literature (see section 1.5.7). Pure
culture G. sulfurreducens often reaches coulombic efficiencies greater than 90%. The most
likely explanation is the presence of oxygen. Even in the pure G. sulfurreducens MFC, the anode
is only 5 mm away from the cathode, which is the source of oxygen diffusion into the cell, and
the cannula for sparging is roughly 6 cm away in order to not disturb the biofilm. While
sparging can reduce the oxygen concentration to a very low level, it can never ensure truly
anaerobic conditions which are necessary for the high coulombic efficiencies seen in pure
culture G. sulfurreducens cells.
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A steady increase in internal cell resistance is evident in fig. 4.5.1, the resistance rises from 244
Ω for the G. sulfurreducens culture, to 363 Ω 163 hours after E. coli addition, and to 523 Ω 759
hours after E. coli addition. This increase is almost certainly due to biomass accumulation on
the cathode surface, which inhibited ion transfer to the cathode, thus increasing the internal
resistance. E. coli would naturally concentrate at the cathode, as this is the site where oxygen
leakage into the cell occurs and would thus have the highest oxygen concentration. Once
glucose was exhausted and fermentative growth no longer possible, aerobic growth is even
more favoured by the E. coli, and the cathode biofilms likely grew even thicker. This behavior is
desirable in E. coli, as the anode is immediately next to the cathode and thus oxygen removal
in this region is critical. Upon disassembly of the cells, very thick E. coli biofilms were observed
to have grown on the cathode surface. The sudden resurgence of power production in the late
stages of the co-culture MFC could be in part due to this film acting as an increased diffusive
barrier for oxygen transfer. The oxygen levels near the anode may have dropped significantly
and stopped limiting current production in G. sulfurreducens, though it seems that the
elimination of succinate is more likely the cause because of the stay observed in current
evolution for the pure G. sulfurreducens in fig. 4.1.1, where no E. coli biofilm is present.
Construction and Characterization of Microbial Fuel Cells Nicholas Bourdakos (2012) Using a Defined Co-culture of G. sulfurreducens and E. coli
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6 – Conclusions
A reproducible air cathode MFC system was constructed in which it was possible to operate in
the absence of a PEM. A method was also determined for creating a co-culture of the
electrogen Geobacter sulfurreducens and the facultative anaerobe Escherichia coli. This culture
was able to grow on glucose as the sole electron donor.
The use of a co-culture system has significant advantages over a pure culture MFC, both from
an operational and practical perspective. Firstly, the co-culture eliminates the need for anode
sparging, which in smaller MFC systems often consumes more power than is generated by the
cell and even in larger systems is difficult to implement and costly. Secondly, the tendency of G.
sulfurreducens MFCs to become basic very effectively counteracts the acidification of the MFC
by E. coli fermentation products, which in this case has eliminated the need for pH control, and
would certainly mitigate pH effects in a larger scale system. Finally and perhaps most
importantly, it is possible to feed a more complex feed substrate to G. sulfurreducens in a
consortium, rather than the usual substrates of acetate or lactate. Since MFCs are typically
used for wastewater treatment, this expands the utility of this culture and allows it to degrade
a wide array of wastes, essentially any wastes fermentable by E. coli, one of the most studied
organisms and one for which mutants exist with a wide range of possible substrates.
There are however disadvantages to the co-culture system. There is a significant reduction in
coulombic efficiency in the co-culture system. In addition, there is the issue of biomass
accumulation leading to an increased internal resistance on the cathode surface, reducing the
power and voltage obtained from this system. Finally, the production of succinate seems to
have an inhibitory effect on power production, and so strains must be chosen to minimize
succinate production in these systems, or a reduced power output must be tolerated. The
system does however show promise for a variety of applications and helps to elucidate some of
the interactions between anode respiring bacteria and the aerobic and facultative anaerobic
organisms in a wastewater treatment system.
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7 – Recommendations
There are several recommendations to be made in order to possibly improve the system
configuration and performance, and to better understand some of the observed trends in the
metabolism of the MFC cultures.
In terms of system configuration, it is recommended that the effect of the anode being moved
back further away from the cathode, or of some barrier to oxygen diffusion be added to the
system, such as the J-cloth suggested by Fan et al. (2007), be investigated [53]. Both of these
options would lead to an increased internal resistance; however the gain in current output and
increase in coulombic efficiency would likely have a greater positive effect on the performance
of the MFCs, and some optimum distance would be determined.
The inoculum of each organism is 20 mL into a 350 mL anode, and so residual products from
bottle culturing are introduced along with the bacteria. In order to reduce this effect, which
does slightly affect the metabolite curves and leads to a rapid decline in current generation
when E. coli is added, inocula could be spun down in a centrifuge and resuspended in fresh
medium. This way, less volume would need to be added, which disturbs the anodic biofilm, and
the effect of succinate would be less pronounced as none would be present until it began to be
produced by the E. coli.
A possible mechanism for aerobic succinate removal by G. sulfurreducens has been proposed,
however it is not a verified conclusion, nor has there been a specific instance of this behaviour
well documented in literature about the organism. It is recommended that this effect be
investigated further in pure culture MFCs with a ferricyanide cathode. This configuration would
allow for a cell to be operated in a completely anaerobic environment, such as a glove box. If
succinate removal is observed in the anaerobic system, then the proposed mechanism must be
revised or discounted. If not, a small amount of oxygen could be added to the system, in order
to observe if succinate was then removed from the system. Another possible method to
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determine the mechanism of succinate removal by G. sulfurreducens would be to add succinate
to an established culture and then once succinate removal is observed, open the circuit. At this
point, any anode reducing activities would cease, and if succinate removal continued then it
would clearly be due to aerobic consumption and not anode reduction. Together, these
experiments could show if one or the other or a combination of both mechanisms were
responsible for succinate removal.
Even if the above experiment shows aerobic succinate removal, the question remains as to
how this process affects the biofilm respiring on the anode. The cells consuming succinate
aerobically on the biofilm may simply be shifting their metabolism away from anode reduction,
and once succinate is exhausted they are resuming anode reduction, which causes the
resurgence in power in the late co-culture MFC. It is also possible that the cells that are
consuming succinate aerobically are dying due to free radial formation; similar to the KN400
strain cells in the study by Nevin et al., (2011) [22]. In this case the resurgence in power
observed in the late co-culture would likely be due to new cell growth and biofilm formation. In
order to determine which is occurring, some method would have to be developed for
determining the state of the anodic biofilm. The method used in other studies of Geobacter
biofilm is confocal laser scanning microscopy with a live/dead BacLight stain [22, 55, 61], which
would be able to determine which of the two processes are occurring (if not both).
Strains of E. coli with a diminished or even no capacity for succinate excretion exist [71], and if
succinate is in fact found to be the cause of the current inhibition, one of these strains could be
substituted for strain C. Attempts could also be made to improve the culture by genetically
engineering E. coli for improved cooperation with G. sulfurreducens. A mutant strain of E. coli
strain C with a lactate dehydrogenase knockout has been constructed for a separate project
(work not shown). It grows significantly slower in bottle culture than the wild-type, and the
principal fermentation product in this strain is acetate, which is more favourable for G.
sulfurreducens. Use of this strain and would likely lead to an increased coulombic efficiency for
the co-culture, as more of the carbon from glucose would be directed towards acetate, and the
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slower metabolism of the mutant E. coli would lead to less acetate and lactate consumption by
that organism, and more by G. sulfurreducens. The slower metabolism may also lead to less
oxygen removal from the cell and act as a detriment to the MFC performance; an in depth
study of this co-culture would be required.
Finally, a third organism could be added to the co-culture to create a consortium capable of
degrading even more complex substrates. C. cellulolyticum has been grown in co-culture with G.
sulfurreducens in an anaerobic system [68]. With the addition of C. cellulolyticum to the
consortium, or another organism capable of more complex waste degradation, a more
versatile culture and greater understanding of community interaction in MFCs could be
obtained. In order to do this, much greater barriers to oxygen diffusion into the anode or much
rapider oxygen removal by facultative anaerobes must be accomplished, as C. cellulolyticum is
an obligate anaerobe.
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Appendix A – Anode Chamber Schematic
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Appendix B – Replicate MFC Data
For pure culture G. sulfurreducens MFCs and co-culture MFCs, 6 replicate MFCs were run at
once. The results displayed in the body of the document are for one representative cell.
Averaging the runs would have led to a lack of definition in the metabolite and current
evolution curves, due to small differences in the time scale; for example, the initial lag in
exponential growth in the pure culture G. sulfurreducens cells is not well defined and this lag
occurred at slightly different times in different runs. This may have averaged to a smooth curve
or one that did not correlate with succinate removal from the system, leading to a very
different interpretation of the data. For this reason only one representative cell was shown,
and the replicates are shown in this appendix.
The reason 6 replicate MFCs were run was that the experiment was over a long time scale and
the chance of contamination or of cell malfunction is reasonably high. It is often the case that
some MFCs will never start generating power for any number of reasons, due to the anodic
culture not adhering to the anode, which may be caused by poor connection or by the culture
not being inoculated at the right time, or a slightly higher rate of oxygen diffusion into that
particular cell. In the case of these 6 replicates, one cell did not establish any power and was
discarded, another cell developed a visible mould contamination after 430 hours of operation
and the results had to be discarded, and a third cell leaked severely and had to be discarded.
Two nearly identical replicates were observed, as well as the one remaining cell, which
exhibited similar behaviour but at different times, due to a small leak in the cell. This leak
caused the metabolites to wash out slightly more rapidly, however led to no contamination
and the cell still has value for comparative purposes.
The replicate cell data that agrees well with that in the main document, designated MFC2, is
shown below in figs. B.1, B.2, B.3 and B.4. The data for the cell with a slight leak, designated
MFC3, is shown in figs. B.5, B.6, B.7 and B.8.
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Fig. B.1 – Current evolution of pure culture G. sulfurreducens MFC2, under a constant external resistance of 240Ω. Sudden spikes in current are due to medium or acetate addition.
Fig. B.2 – Metabolite concentration of key metabolites for pure culture G. sulfurreducens MFC2. Arrows indicate acetate addition. ( - acetate, – succinate, - fumarate).
Power/Operating Curve Taken
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Fig. B.3 – Current evolution of co-culture G. sulfurreducens and E. coli MFC2. E. coli and glucose added at 0 hours, corresponding to 487 hours after G. sulfurreducens inoculation. The external resistance of 240Ω changed to 560Ω after first power curve. Sudden spikes in current are due to medium addition.
Fig. B.4 – Metabolite concentration of key metabolites for co-culture G. sulfurreducens and E. coli MFC2. E. coli and glucose added at 0 hours, corresponding to 487 hours after G. sulfurreducens inoculation. ( - glucose, - acetate, x– lactate, – succinate, - ethanol).
Power/Operating Curve Taken
Power/Operating Curve Taken
R changed to 560 Ω
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The current evolution curve for the pure culture G. sulfurreducens portion of the operation in
MFC2 also shows a slowing down of exponential growth until succinate removal is observed in
the cell, and reaches a similar peak current (fig. B.1). The metabolite curve for this portion of
the operation (fig. B.2) shows fumarate and subsequently succinate removal, as well as regular
acetate additions and removal, with the final acetate concentration reaching 34 mM, nearly
the same as in fig. 4.1.2. The current evolution for MFC2 in the co-culture stage of the
experiment (fig. B.3) follows a similar trend to that in fig. 4.2.1, rapidly dropping subsequent to
E. coli addition and showing a resurgence at approximately 550 hours. By observation of fig.
B.4, we can see that the profile is very similar to that in fig. 4.2.2, and that once again, the
resurgence in current production in fig. B.3 seems to be correlated with succinate removal
from the system.
MFC3 had a small leak that could not be patched during operation, and therefore needed
fresh medium addition more frequently. This led to more disturbances in the current evolution
curves and to a washout of the metabolites. Thus, in figs. B.6 and B.8, metabolite
concentrations are slightly lower and are removed from the system slightly more quickly. Once
again, the slowdown of exponential growth is observed while succinate is being removed in the
pure G. sulfurreducens portion of the experiment is seen. The co-culture section of the curve
differs significantly from the other two replicates in the time-scale. The E. coli addition has a
pronounced effect on current production and this production begins to decrease rapidly, as in
the other two MFCs. Succinate is consumed and washed out due to leakage more rapidly than
in the other replicates. Once this is observed, at 400 hours in fig. B.8, the resurgence of current
occurs in fig. B.7. Eventually, the leak intensified, which led to a lower liquid level in the cell
and to the sudden cessation of current production at approximately 630 hours (see fig. B.7).
This also corresponded with lactate removal from the system, and this could also have caused
the decline, if E. coli consumed oxygen more slowly while consuming acetate or began to
compete with G. sulfurreducens for the remaining acetate. Nevertheless, this cell supports the
idea of succinate removal being responsible for the current resurgence, as it occurs at a
different time point and still shows the same effect on the system.
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Fig. B.5 – Current evolution of pure culture G. sulfurreducens MFC3, under a constant external resistance of 240Ω. Sudden spikes in current are due to medium or acetate addition.
Fig. B.6 – Metabolite concentration of key metabolites for pure culture G. sulfurreducens MFC3. Arrows indicate acetate addition. ( - acetate, – succinate, - fumarate).
Power/Operating
Curve Taken
Power/Operating
Curve Taken
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Fig. B.7 – Current evolution of co-culture G. sulfurreducens and E. coli MFC3. E. coli and glucose added at 0 hours, corresponding to 487 hours after G. sulfurreducens inoculation. The external resistance of 240Ω changed to 560Ω after first power curve. Sudden spikes in current are due to medium addition.
Fig. B.8 – Metabolite concentration of key metabolites for co-culture G. sulfurreducens and E. coli MFC3. E. coli and glucose added at 0 hours, corresponding to 487 hours after G. sulfurreducens inoculation. ( - glucose, - acetate, x– lactate, – succinate, - ethanol).
Power/Operating Curve Taken
R changed to 560 Ω
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Appendix C – Sample Calculations for Determining Coulombic Efficiency
Sample calculations in this section are for the pure culture G. sulfurreducens MFC shown in
section 4.1 and the co-culture MFC described in section 4.2.
In order to determine the coulombic efficiency, first we must determine the total number of
coulombs output by the cell. This is accomplished using the EC-Lab software that was used to
plot and monitor the MFCs during their operation. The plotting software allows for the data to
be transformed from cell potential (V) to current (A) by dividing the voltage data uniformly by
the external applied resistance of 240Ω. For co-culture cells, the data was divided by 560Ω
after the time point at which the external resistance was adjusted. The time axis was changed
to units of seconds. Therefore what remains is a plot of current (A) vs. time (s). The integral of
this curve gives the number of coulombs output during the selected time scale. Fig. C.1 below
shows the graphical interface from which the first portion (first 250 hours) of the current
production of the MFC described in section 4.1. The potentiostat had to be stopped
momentarily at 250 hours and thus the data for the pure culture G. sulfurreducens MFCs is in
two separate files that must be added together.
Fig. C.1 – EC-Lab plot of current (A) vs. time (s). Integrator program in window shows the integral for the shaded area is 878.986C.
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From this curve, it is determined that the charge transferred through the MFC is 878.986C. A
similar analysis of the second portion of the curve (250 – 486 hours) yields a result of 891.922C.
The total coulombs gathered are therefore 1770.908 C for the pure culture G. sulfurreducens
MFC.
Now that we know the charge output by the MFC, we must compare it to the known quantity
of acetate that has been consumed. From the half cell reaction for acetate, it is apparent that
theoretically, 8 moles of electrons are generated per mole of acetate consumed:
CH3COO- + H+ + 2H2O → 2CO2 + 8H+ + 8 e-
The total acetate consumed is the sum of the total amount of acetate added, less any residual
acetate in the cell at the end of the time period in question, and less any acetate removed from
sampling. For the 486 hour period of monitoring the G. sulfurreducens, a total of 44.0 mL of 1M
sodium acetate was added to the MFCs in 9 separate additions (1 at the time of inoculation
and 8 subsequently when acetate was fully consumed). Therefore, the total acetate into the
cell is 44 mmoles.
At 486 hours, the acetate concentration is 36.006 mM from the HPLC analysis. From the
evaporation rate plot (fig. 3.8.1), it is known that 0.6093 mL of water are lost due to
evaporation each hour. The cell was filled to the operating volume at 405 hours, thus the cell
volume has been decreasing for 81 hours at this rate. The total water loss is thus:
, and the cell volume is therefore
Thus, the remaining acetate at the end of the time period is:
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The acetate lost to sampling is determined by multiplying the concentration measured by HPLC
by the sample volume (2.0 mL). This data is shown in table C.1 below.
Table C.1 – Sampling loss of acetate for coulombic efficiency determination
Sample time (h) Acetate concentration (mM) mmoles lost due to sampling
0 10.201 0.0204
8 10.295 0.0206
20 10.471 0.0209
32 9.0978 0.0182
44 9.2241 0.0184
56 17.352 0.0347
71 4.4015 0.0088
80 20.992 0.0420
95 18.055 0.0361
117 12.493 0.0250
142 8.0119 0.0160
486 36.006 0.0720
The sum of the rightmost column in Table C.1 is 0.3332 mmoles, and represents the total
acetate lost to sampling. Therefore, the total acetate consumed is:
The number of coulombs associated with this is:
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Therefore, the coulombic efficiency is 6.99 % for the pure culture G. sulfurreducens portion of
the MFC curve.
Since it is expected that the coulombic efficiency would be lower during the period of biomass
growth (exponential portion of the growth curve), the pure G. sulfurreducens curve was further
split into the growth phase (0-142 hours) and the stationary phase (142-486 hours). During the
growth phase, 227.061 C were harvested based on the integral up to this point, and by
subtraction 1543.847 C were harvested in the stationary phase. Using the same analysis as
above, the coulombic efficiency was determined to be 4.77% for the growth phase, and 7.50%
for the stationary phase. This implies that the G. sulfurreducens is in fact diverting more of the
acetate to biomass during the early growth phase, and using acetate more efficiently for
electricity production in the stationary phase.
Coulombic efficiency calculations for the co-culture are determined in the same manner;
however the values are based on glucose concentrations, and take into account acetate that is
initially present and remaining at the end of the time period as well.
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Appendix D – Proton Balance in Pure Culture G. sulfurreducens MFCs
Since the MFCs were fed with sodium acetate rather than acetic acid, there is a proton
imbalance when acetate is consumed by G. sulfurreducens, according to the half reactions
below, discussed in section 5.3 :
Anode: CH3COO- + 2H2O → 2CO2 + 8e- + 7H+ Cathode: 2O2 + 8H+ + 8e- → 4H2O
For each acetate molecule consumed on the electrode, a proton is removed from the solution,
leading to the solution becoming more basic. Analogously, for each 8 electrons that have been
harvested, a proton is depleted from the anode chamber. From Appendix C, we know that for
the pure culture G. sulfurreducens MFC a total of 1770.908 C were harvested. Thus,
The pH rose from 7 to 7.8 in this MFC. The required amount of protons to accomplish this in a
350 mL MFC is:
Clearly far more protons were depleted than was required for the pH change, however it must
be noted that the medium contained a sodium bicarbonate buffer of 1.8 g/L. This corresponds
to 21.43 mM NaHCO3. There was also 4.03 mM sodium carbonate added in order to establish
the carbonate/bicarbonate equilibrium. Assuming that each mole of bicarbonate can
counteract proton removal by splitting into a mole of protons and a mole of carbonate ions,
the 350 mL MFC has a buffering capacity of:
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It must be noted that it is possible that some of the bicarbonate exists as carbonate once it is
first dissolved into the medium, and thus is not available as buffer, and so this value may in
actual fact be significantly lower. With the assumption that all the bicarbonate is available to
act as buffer, this buffer capacity is more than adequate to absorb the proton losses, which is
why no base addition was required for the system. The slight increase in pH would have been
far more dramatic in a system without a buffer.
The fact that the pH rose could be due to a shift in the bicarbonate/carbonate equilibrium. It is
also likely that some acetate is being consumed aerobically, which would also lead to proton
depletion if it was broken down to carbon dioxide and water (the net reaction for aerobic
acetate degradation has the same stoichiometry as the fuel cell reaction). The coulombs to the
electrode only account for 6.99% of the acetate removed, however it is not possible to
determine for this system how much of the residual acetate went to biomass or if it was
consumed aerobically.