The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
Bioengineered Fuel Cells: Optimization via Genetic Approaches
and Multi-Scale Modeling
O. Bretschger1, S. Finkel1, L. Iverson1, B. Kim1, F. Mansfeld1, K. Nealson, S. Prakash1,
P. Ronney1,*, H. Wang1, A. Lüttge2
1
University of Southern California, Los Angeles CA; 2Departments of Earth Sciences and Chemistry, Rice
University, Houston, TX
Abstract
While enormous effort has been spent on engineering of conventional fuel cells (CFCs) using hydrogen and methanol,
relatively little engineering has been devoted to Microbial Fuel Cells (MFCs). Engineering of any system requires sufficient
understanding of its characteristics to develop a predictive model. Development of a model for MFCs is not a simple
extension of models of CFCs because of the many differences between MFCs and CFCs. These differences include much
lower anode (fuel-side) activity (expressed, for example, in terms of current density); life-support and health requirements of
the anode "catalyst" (bacteria); variability and adaptation (both desirable and undesirable) of the anode; and fuel (nutrient)
complexity and flexibility. Despite these differences, we believe reaction-diffusion modeling, typically employed in chemical
systems such as CFCs and combustion, will be a valuable tool for MFC engineering, especially when combined with genetic
approaches, examples of which are described. Furthermore, because of the differences between MFCs and CFCs, it is
considered very likely that viable MFCs will require novel architectures not employed in any CFCs. Candidate architectures
for MFCs are discussed.
Keywords: Microbial Fuel Cells; Multiscale modeling; Reaction-diffusion systems; Shewanella Oneidensis
1 - INTRODUCTION
The development of microbial fuel cells (MFCs) represents
an area of great potential through the conversion of complex
and impure fuel sources into electrical energy. However, this
potential has remained largely unrealized because of the
historically low power densities produced by MFCs. The
problem of low power generation must be solved if MFCs of
any size are to be useful in portable devices. Improving
power density will require solving a number of fundamental
problems including:
(A) understanding the microbial
mechanisms whereby electron transport to solid electrode
surfaces occurs; (B) interfacing these microbial catalysts with
proper MFC design to optimize current production under
widely varying conditions; and (C) understanding,
manipulating, and improving the microbial communities to
obtain those communities most capable of converting
complex and variable organic carbon sources into electrical
energy.
The performance of MFCs has been studied by many
researchers, for example [1], but these results are difficult to
interrelate because of differences in bacteria, MFC design
and operating conditions. Our attention is focused primarily
on anode performance since existing air cathodes and Proton
Exchange Membranes (PEMs) are capable of orders-ofmagnitude larger current density (≈ 1 amp/cm2) than existing
MFC anodes. For the basic understanding of the mechanism
of current production, we use a model organism, Shewanella
oneidensis MR-1. Deletion mutants were used to identify
those components necessary for current production by MR-1.
Several key genes of MR-1 that are required for iron
reduction are also involved in current production. These
include the genes that code for the outer membrane decaheme
cytochromes (mtrA,B,C, and omcA. Under conditions of
electron limitation, MR-1 forms abundant thread-like
appendages called nanowires [2] (Figure 1) thought to be
involved with current production. Mutations in a number of
regulatory genes lead to phenotypes that are incapable of
current generation, as do mutants in the synthesis of certain
pili (Table 1) that are involved in the production of these
nanowires. Hence, the goal of this investigation is to
determine the roles of transport (of nutrients, waste, protons
and electrons) and genetic structure on the power production
capability of microbial fuel cells.
2 - APPARATUS AND PROCEDURES
A “standard” MFC (Figure 2) was used to measure current
produced by a number of different deletion mutants of MR-1.
Because it is made of glass, it can be autoclaved repeatedly.
without loss of structural integrity. The cathode (air) side
consists of a platinum electrode impregnated into a graphite
felt current collector. Air is bubbled through a water-filled
glass chamber, maintaining the water at oxygen-saturation
conditions. On the anode (fuel) side, microbes are grown on
a second graphite felt current collector.
A Nafion™
membrane is used to allow protons to pass from the anode to
*
Corresponding author; Department of Aerospace and Mechanical Engineering, OHE 430J, 3650 McClintock
Ave, University of Southern California, Los Angeles, CA 90089-1453; (213) 740-0490; ronney@usc.edu.
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The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
.
Figure 1.
Low-resolution (top) and high-resolution
(bottom) images of MR-1 cells forming a biofilm with
MFC1
“nanowire” appendages [2].
MR-1 Wildtype
Figure 2. Microbial Fuel Cell used to gather data shown in
Table 1. Top: schematic diagram; bottom: photograph.
0.3
3 - RESULTS
A typical example of the current density obtained from
Shewanella Oneidensis MR-1 are shown in Fig. 3. It can be
seen that each time 0.002 moles of nutrient (lactate) was
added to the cell, the current production increased
dramatically. The time constant for consumption of this
nutrient is about 15 hours, with a total current production of
about 8 coulombs or 0.00022 moles of electrons, thus the
ratio of nutrient (lactate) molecules to electrons produced is
roughly 10:1.
0.25
Current (mA)
0.2
0.15
0.1
0.05
0
0
10
20
30
40
Time (hrs)
50
60
70
80
Figure 3 - Example of current production from Shewanella
Oneidensis MR-1 microbial fuel cell. The three sharp
increases in current correspond to the introduction of 2 mM
lactate into the anode side of the MFC.
cathode. A Keithley source meter was used to determine
current production. Lactate was used as the nutrient in the
tests described here.
Preliminary results of the effect of deletion of specific
sequences in the genome are shown in Table 1. These
mutants targeted either regulatory genes or specific
cytochromes. Interestingly, at least one cytochrome mutant
was enhanced for current production suggesting that it might
be involved with a competing pathway of electron flow
within the cell.
Optimization of current production will require an
understanding of the physiology and energy flow within the
bacteria. A major effort will be made to understand the
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The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
factors involved in the regulation of energy flow and the
partitioning of energy between growth and maintenance. As
improvements are made in the electron transfer ability of
MR-1, these will be analyzed in terms of MFC power
production and system efficiency. For example, our work has
shown that various strains of Shewanella can oxidize (i.e.,
eat) a wide variety of organic carbon compounds. For
example, MR-1 can use at least 18 different compounds and
others strains can use more than 40. However, we have
found that many of these compounds that can be oxidized
will not support growth. These may be precisely the
compounds we would like to use in MFCs – is it possible to
feed the bacteria one compound to grow a biofilm on the
anode, then feed this biofilm different compounds to produce
a non-growing, current-producing system? Alternatively,
some Shewanella strains produce copious amounts of formate
when grown on more complex carbohydrates. This suggests
that a double-electrode fuel cell might be possible, harvesting
electrons directly from the bacteria and harvesting further
electrons via a formate fuel cell.
Mutant
Designation
MR-1
E. coli
mtrA
mtrB
mtrC
mtrD
mtrE
mtrF
omcA
gspD
tatC
crp
The phenomenological part of the model will characterize
anode performance on the basis of fuel cell responses (e.g.,
current density, products of metabolic activity) to test
parameters. Based on the dependence of power density as a
function of microbial cell density N and active electron
transfer sites per cell A postulated in Figure 3, the kinetic rate
expression for the power density Re may be written as
!
[
"#
+ ( Re,$ )
"# #
]
density, where Re is expected to be proportional to the cell
density and probably the active electron transfer sites per cell
A, i.e.,
Re,0 (A) = aAN
(2)
Re,∞ corresponds to the limit of large cell density, where the
microbial cells undergo excessive competition for nutrients
and electron transfer sites on the anode. Therefore an
increase the cell density can cause Re,∞ to decrease, i.e.,
Re," (A) = b /N
(1)
Here Re,0(A) is the power density at the limit of low cell
!
Figure 4 – Schematic of dependence of power density
on microbial cell density and electron transfer sites per
cell.
% Max.
Current
100 %
nd
nd
nd
≈ 30 %
100 %
≈ 160 %
≈ 80 %
nd
≈ 10 %
≈ 15 %
≈ 20 %
Wild Type
Wild Type
Fe reduction structural gene
Fe reduction structural gene
Fe reduction structural gene
Unknown structural gene
Unknown structural gene
Unknown structural gene
Fe reduction structural gene
Type II secretion system
Membrane protein insertion
Regulatory gene
(C-metabolism)
Fur
Regulatory gene
≈ 25 %
(Fe metabolism)
luxS
Regulatory gene
100 %
(quorum sensing)
Table 1 - Results of MFC testing of some mutants of MR-1.
The normal strain (wild type) produces ≈ 70 µA of current
(with 18 cm2 anode area), while a negative control (E. coli)
produces a baseline current of about 5 µA. Note that several
mutants (mtrABC, omcA) produced no measurable current
(nd), while others were impaired with regard to current
production, and one, mtrE produced more current than the
wild type. A number of regulatory mutants (tatC, fur, crp)
are depressed with regard to current production.
4 - DISCUSSION
A robust multi-scale, multi-phenomenon computer model of
an MFC system is being developed that subsequently can be
used for testing, predicting and optimizing fuel cell design
(e.g., miniaturization) and efficiency. The primary focus is
on the anode because of its low current density. Initially, the
PEM and cathode processes are not expected to be ratelimiting.
Re = ( Re,0 )
Mutant Class
!
(3)
The kinetic expression (1) intends to capture the transition
region, from the low-to-high cell density limits, where
cooperation within the microbial cell community is amplified.
Beyond the phenomenological model, a physical model will
be developed and critically validated within the active
parameter space.
The low currents obtained from MFCs may suggest a
paradigm shift in terms of the optimal operating conditions.
For example, we note that the O2 permeability of Nafion-117
at 25˚C is about 10-14 Mole/m-s-Pa [3]. Assuming an air
cathode (PO2 = 21,000 Pa) with a 100 µm membrane, this O 2
leakage results in a 0.82 A/m2 current loss – trivial for
hydrogen , methanol or formic acid fuel cells whose current
density may approach 10,000 A/m2, but substantial for MFCs
whose current density (in our baseline case) is
(70 µA)/(18 cm2) (10-4 m2/cm2) ≈ 0.04 A/m2. In contrast, for
high-current fuel cells using conventional fuels, the main loss
mechanism is not oxygen diffusion, but rather resistive losses
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The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
in the membrane. For example, the proton conductance of
Nafion-117 at 25˚C is about 0.08 S/cm [4], which results in a
12 µV drop for 1 A/m2 - trivial for MFCs with a typical open
circuit voltage of ≈ 0.3 volts, but results in a 0.12 V drop for
a current density of 1 A/cm2 - a dominant factor for
conventional fuel cells.
Since resistive losses in Nafion is not an issue for MFCs, but
O2 crossover is, one could use different material with lower
O2 permeability at the expense of lower proton conductivity,
or an entirely different type of fuel cell structure, an example
of which is shown in Fig. 5. In this design, “fins” are
employed using laminated Nafion. The optimal ratio of fin
width to length can be calculated for a give current density,
oxygen permeability and proton resistance. Note also that in
the design shown in Fig. 5, the inner PEM layers should be
thicker than the outer layers since the protons must be
conducted farther in the case of the inner layers. Also, as a
result of this structure, the anode area will be much greater
than cathode area, which is appropriate since the anode
surface reaction rate is much smaller than that on the cathode.
With this scheme, it is potentially possible to practically
eliminate O2 crossover without incurring significant voltage
loss due to proton resistance.
conventional and microbial fuel cells is required, for example
relative importance of oxygen crossover vs. proton resistance
and the opportunities to increase current production via
genetic approaches.
ACKNOWLEDGEMENTS
This work was supported by the AFOSR MURI program,
Award No. FA9550-06-1-0292.
REFERENCES
[1] Jang, J. K., T. H. Pham, I. S. Chang, T. H. Kang, H. S. Moon,
K. S. Cho, and B. H. Kim. Construction and operation of a novel
mediator- and membrane-less microbial fuel cell. Process
Biochem. 39:1007-1012, 2004.
[2] Gorby, Y., et al., Electrically conductive bacterial nanowires
produced by Shewanella oneidensis strain MR-1 and other
microorganisms. Proc. Nat. Acad. Sci. U.S.A., 103:11358 – 11363
(2005).
[3] Kocha, S. S. Yang, J. D., Yi, J. S., Characterization of Gas
Crossover and Its Implications in PEM Fuel Cells. AIChE J. 52,
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[4] Siu, A., Schmeisser, J., Holdcroft, S., Effect of Water on the
Low Temperature Conductivity of Polymer Electrolytes. J. Phys.
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Figure 5 – Possible configuration of microbial fuel cells for
optimization of power generation by minimizing oxygen
crossover without significantly increasing resistive losses
(only applicable for low-current devices such as MFCs).
5 - CONCLUSIONS
Experiments using Shewanella Oneidensis MR-1 in a
microbial fuel cell with a conventional Nafion membrane
demonstrate the possibility of generated electrical power
from common nutrient feedstocks. However, to exploit this
opportunity, consideration of the differences between
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