Download PDF w - American Chemical Society

ARTICLE
pubs.acs.org/est
Characterization of Microbial Fuel Cells at Microbially and
Electrochemically Meaningful Time scales
Zhiyong Ren,*,†,‡ Hengjing Yan,† Wei Wang,‡ Matthew M. Mench,§ and John M. Regan†
†
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802,
United States
‡
Department of Civil Engineering, University of Colorado Denver, Denver, Colorado 80204, United States
§
Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, Knoxville, Tennessee 37996,
United States
bS Supporting Information
ABSTRACT: The variable biocatalyst density in a microbial fuel cell (MFC) anode biofilm is a unique feature of MFCs relative to
other electrochemical systems, yet performance characterizations of MFCs typically involve analyses at electrochemically relevant
time scales that are insufficient to account for these variable biocatalyst effects. This study investigated the electrochemical
performance and the development of anode biofilm architecture under different external loadings, with duplicate acetate-fed singlechamber MFCs stabilized at each resistance for microbially relevant time scales. Power density curves from these steady-state reactors
generally showed comparable profiles despite the fact that anode biofilm architectures and communities varied considerably,
showing that steady-state biofilm differences had little influence on electrochemical performance until the steady-state external
loading was much larger than the reactor internal resistance. Filamentous bacteria were dominant on the anodes under high external
resistances (1000 and 5000 Ω), while more diverse rod-shaped cells formed dense biofilms under lower resistances (10, 50, and 265 Ω).
Anode charge transfer resistance decreased with decreasing fixed external resistances, but was consistently 2 orders of magnitude
higher than the resistance at the cathode. Cell counting showed an inverse exponential correlation between cell numbers and
external resistances. This direct link of MFC anode biofilm evolution with external resistance and electricity production offers several
operational strategies for system optimization.
’ INTRODUCTION
Microbial fuel cells (MFCs) or bioelectrochemical systems
present a promising technology for concurrent waste treatment
and electricity generation. Using microorganisms as biocatalysts,
the performance of MFC systems ultimately depends on the
effective biocatalyst availability and activity. Different approaches
have been used to improve MFC performance, including reducing internal resistance,1,2 optimizing operations by sequential
anode-cathode flow-through or electrolyte recirculation,3-5
and improving biocatalyst attachment on the electrodes.6,7
However, few studies have provided information on the relationship between biocatalyst density and diversity and the system
electrochemical performance, and how to regulate the growth of
the anode biofilm and therefore optimize the system for different
objectives.8,9 As MFC technology becomes more economically
and practically feasible for full-scale applications, such as in
wastewater treatment processes or remote power production
from benthic deployments, the understanding and utilization of
the biological and electrochemical relationship should become
very important.
The resistance of the external circuit in an MFC directly
influences the anode potential and the resultant bioavailability
of the anode for exoelectrogenic bacteria, offering an operational parameter to influence anode biofilm development and
performance.10 Low resistances lead to more positive potentials, which provide more free energy to the microorganisms
r 2011 American Chemical Society
and enable a higher flux of electrons through exoelectrogenic
metabolisms, imparting a selective advantage to exoelectrogens over competing functional groups.11,12 For example,
several studies have demonstrated that low external loadings
could limit methanogenesis in the MFC anode chamber.13,14 In
addition to influencing this competition with other groups, the
external resistance (anode potential) also exerts a selective
pressure on the exoelectrogenic community composition
due to their different attributes related to anode affinity and
maximum substrate utilization rate.10,15 This presents an opportunity to tailor the anode biofilm structure and composition,
and potentially MFC performance with respect to power output
and substrate utilization, through the adjustment of external
resistance.
The typical method of characterizing MFC power production involves operating the MFC at a fixed external resistance
or applied potential, and then transiently obtaining polarization data by applying a series of external resistances each for a
5-30 min interval.16 These measurements are based on the
observation that the voltage stabilizes within such short
periods. However, there is a significant difference in the time
Received: September 13, 2010
Accepted: January 14, 2011
Revised:
January 10, 2011
Published: February 17, 2011
2435
dx.doi.org/10.1021/es103115a | Environ. Sci. Technol. 2011, 45, 2435–2441
Environmental Science & Technology
scales required to achieve electrochemical versus microbial
steady states. When the external resistance is changed, the
electrochemical response of the biofilm established at the
antecedent resistance quickly stabilizes. However, the biofilm
takes much longer to stabilize, with changes in biofilm structure and community composition potentially leading to a longterm stable performance that differs from the short-term
electrochemical response. A full cycle at each external resistance, which is the approach used in some MFC studies, could
partially reduce this time-scale discrepancy, but may still be
insufficient for steady-state biofilm development at each resistance. Measuring a polarization curve generated with electrochemically steady-state data may misrepresent the true
biological steady state of the system.
In this study, we looked at the long-term operation of MFCs at
different fixed resistances and the performances, architectures,
and compositions of these different steady-state biofilms. Our
objective was to characterize the effects of external resistances on
both electrochemical and microbiological attributes, and to
propose operational strategies to adjust to different goals in
waste-to-energy processes.
’ MATERIALS AND METHODS
MFC Construction and Operation. Duplicate single-chamber air-cathode bottle MFCs were operated in fed-batch mode
under five different external loadings at an incremental increase of a factor of 5 (10, 50, 265, 1000, and 5000 Ω) for more
than 3 months. Reactor configurations and experimental conditions were identically maintained for all MFCs except the
difference of external loadings. The anodes were made of plain
carbon paper (BASF, NJ) and the air-cathodes with diffusion
layers were made according to Cheng et al.17 MFCs were
inoculated with secondary effluent from the Pennsylvania State
University Wastewater Treatment Plant. The reactors were fed
with 250 mL of medium containing 1.0 g/L sodium acetate,
0.31 g/L NH4Cl, 0.13 g/L KCl, 5.85 g/L NaH2PO4 3 H2O, 4.09
g/L Na2HPO4, 12.5 mL/L mineral solution, and 12.5 mL/L
vitamin solution.18 Except for batches in which Coulombic
efficiency was calculated, where medium was replaced after
whole-batch operation for individual reactors, medium was
replaced at the same time for all reactors when the voltage of
the 10 Ω reactors dropped to below ∼7 mV, which generally
took 7-8 days. Reference electrodes (Bioanalytical Systems, Inc., OH) were introduced into the anode chamber
for conducting electrochemical measurements on individual
electrodes.
Analyses. Cell voltage across the external resistor was continuously monitored using a data acquisition system (Keithley
Instruments, OH). Polarization data were collected during
the stable power production stage of each batch, and the same
resistance series was applied on each reactor using a variable
resistor box. To achieve steady electrochemical conditions,
the cells were held at one resistance for approximately 25 min
before switching to the next resistance. Both low-to-high and high-tolow measurement directions were conducted for comparison
purpose. The calculations of power density and Coulombic
efficiency (i.e., the fraction of electrons removed from the
electron donor that are recovered as current through the external
circuit) were performed as previously described.16,18 Data were
averaged over triplicate tests from two reactors running with the
same resistance (mean ( SD).
ARTICLE
Electrochemical impedance spectroscopy (EIS) and reactor
ohmic resistances were tested using a Zahner IM6ex potentiostat-AC frequency analyzer and the results were analyzed using
ZView software. Impedance measurements were taken on two
configurations designated anode (A) and cathode (C). Because
the reference electrode (RE) was installed adjacent to the
anode, the solution effects on the anode impedance were
minimized, while the cathode impedance taken with respect
to the RE also included the electrolyte resistances. To obtain
electrochemical charge transfer resistances for the electrodes,
two equivalent circuit models were adapted from our previous
EIS studies.19,20 Specifically, the cathode equivalent circuit
consisted of an ohmic resistance, followed by an electrochemical charge transfer resistance (Rct) and a Warburg’s diffusion
element (W) in parallel with a constant phase element (CPE)
(Figure S1A, Supporting Information). The anode data were
fitted to an equivalent circuit with two sets of parallel resistorcapacitor elements connected in series (Figure S1B). The
frequency of the AC signal was varied from 10 kHz to 100
mHz with an amplitude of (10 mV. Impedance experiments
were performed under galvanostatic closed-circuit conditions
at 50 μA. To ensure steady state during galvanostatic operation, the MFCs were allowed to equilibrate for 10 min before
applying the AC signal.
After over 3 months of operation, anode biofilm samples were
prepared and analyzed as described previously.21,22 Briefly, a
section of anode for each reactor was randomly selected and
mounted onto a glass slide. The anode biofilm was then stained
for 15 min using the LIVE/DEAD BacLight Bacterial Viability
Kit (Invitrogen, CA). The stained biofilm was observed and
analyzed by a confocal laser scanning microscope (CLSM;
Olympus America Inc., NY) equipped with 3 lasers (peaks at
488, 543, and 633 nm). The 3-dimensional biofilm architecture
(z-stack) was scanned and displayed as an ortho view. Images
were analyzed by Photoshop and FV10-ASW software. Biofilm
cell counts were performed by removing the biofilm from a 1 cm2
anode sample and washing and suspending the cells in 2 mL of 50
mM phosphate buffer. Suspended cells were serially diluted and
stained using the viability staining kit, filtered through a 0.45-μm
membrane filter, and counted using a Zeiss Axiophot epifluorescent microscope. The cell density per anode geometric area
was calculated using the dilution factor, the filtered volume, and
the ratio of total filtered area to image area.
Phylogenetic Analysis. The composition of anode biofilm
communities was determined by 16S rRNA gene-targeted PCR,
denaturing gradient gel electrophoresis (DGGE) screening of
PCR products, cloning and sequencing of prominent DGGE
bands, and phylogenetic analysis as described in detail elsewhere.22 Briefly, genomic DNA was extracted from anode
subsamples using the PowerSoil DNA Isolation Kit (MO BIO
Laboratories, Inc., Carlsbad, CA), a fragment of the 16S rRNA
gene was PCR amplified using primers 968F (50 -AACGCGAAGAACCTTAC-30 ) with a GC clamp and 1401R (50 -CGGTGTGTACAAGACCC-30 ),23 and DGGE was performed with a
denaturing gradient ranging from 30 to 60%. Prominent DGGE
bands were excised, eluted, and used as the template for a PCR
with the primers listed above except that the forward primer
lacked the GC clamp. PCR products were purified and cloned,
and the inserts from five randomly selected clones for each band
were sequenced to determine whether multiple 16S rRNA gene
fragments had comigrating on the DGGE band. The 16S rRNA
gene sequences were analyzed in the GenBank database and have
2436
dx.doi.org/10.1021/es103115a |Environ. Sci. Technol. 2011, 45, 2435–2441
Environmental Science & Technology
Figure 1. Profile of cell voltage production at different external
resistances during the acclimation period.
Figure 2. Power density curves derived from electrochemical steadystate reactors at different resistances. The same series of resistors was
used for polarization tests.
been deposited in the GenBank database under accession
numbers HQ157169-HQ157181.
’ RESULTS
Electricity Generation and Coulombic Efficiency As a
Function of External Resistance. Duplicate reactors at each
of the five resistor settings showed identical voltage profiles at
steady state. Reactors under higher resistance showed reduced
periods before reaching steady-state voltage, consistent with
another study.24 The lag time for 5000, 1000, 265, 50, and 10 Ω
reactors to reach 80% of their maximum voltage was 91, 104,
106, 108, and 120 h, respectively (Figure 1). Power density
curves generated from polarization data collected after the
3-month biofilm-development period generally showed similar
profiles that were also consistent with the steady-state powers
of systems at each fixed resistance (Figure 2). The highest power
density among the reactors ranged from 139 to 157 mW/m2, all
ARTICLE
achieved at the 265 Ω resistance. The maximum current
density of the 5000 Ω reactor was much lower than that of
the other reactors, suggesting that the restricted growth of
biofilm could not catalyze a high current in the short duration
of the polarization tests. The data from the other paired
reactors showed a pattern of slightly higher maximum current
density with decreasing steady-state resistance. Moreover,
bigger variations of power densities among the reactor pairs
were generated at lower resistances, as small voltage differences created much higher power density variations through
the square transform (P = V2/R). Higher power density values
were observed when external resistances were changed from
high to low (from 5000 to 10 Ω) compared to low to high
(from 10 to 5000 Ω) at 25 min intervals (data not shown), but
the variations were within standard deviation limits.
All reactors achieved more than 90% COD removal in each
batch, but the Coulombic efficiency showed an inverse correlation with the applied external loading. The average Coulombic
efficiency for the 10 Ω resistor MFCs was 45%, while the average
value for 5000 Ω resistor MFCs was only 6% (Table 1). The low
electron recovery at high resistances was mainly due to the long
batch duration, which resulted in more electron loss to nonelectricity related reactions such as aerobic respiration and perhaps methanogenesis, though the latter was not measured.
Electrochemical Impedance Variation As a Function of
External Resistance. The EIS data were fitted with a Randle’s
type equivalent electrical circuit to obtain values for the anode
and cathode charge transfer resistances.22 As shown in the
Figure 3 inset graph, an additional Nyquist arc in higher frequency became visible as the external resistance decreased.
With decreasing external loading, the anode charge transfer
resistance decreased consistently from 3.55 kΩ 3 cm2 under
5000 Ω resistance to 0.72 kΩ 3 cm2 under 10 Ω resistance
(Table 1). This is consistent with the accompanied development of thicker biofilm, which enables higher current density.
In contrast, the effects of external resistances on the MFC aircathode seemed to be minimal, as the cathode charge transfer
resistances at different external loadings were mostly about
two magnitudes lower than the anode resistances and stable
throughout the experiment. The cathode biofilm on one 10 Ω
reactor fell off before the EIS measurement and caused a reduced
cathode charge transfer resistance and increased measured
power density.22,25
Anode Biofilm Variation As a Function of External Resistance. Despite the similar electrochemical performance, the
biofilm structures and compositions at different resistances after
3-month continuous operation were quite different from each
other (Figure 4). Bacteria with a filamentous structure dominated and formed relatively thin and patchy biofilm (0-30 μm)
on the anodes under high external resistances (1000 and 5000 Ω),
while rod-shaped cells accumulated and formed dense biofilms
(more than 50 μm) that covered the anodes under lower
resistances (10, 50, and 265 Ω). As the external resistance
decreased, the biofilm cells tended to aggregate and finally
covered the whole anode with thick biofilm. This is consistent
with the total direct cell counts, which showed an inverse exponential correlation with external resistances. The total cell density
on the 10 Ω anodes was 1.49 109 cells/cm2 of projected anode
surface area, which was 13 times higher than the cell density on
the 5000 Ω anode (Table 1).
DGGE screening of amplified 16S rRNA gene fragments
showed two common sequences among all anode communities
2437
dx.doi.org/10.1021/es103115a |Environ. Sci. Technol. 2011, 45, 2435–2441
Environmental Science & Technology
ARTICLE
Table 1. Summary of the Coulombic Efficiency, Total Cell Counts, and Charge Transfer Resistance of the Reactors at Different
External Resistances
5000 Ω
reactor
265 Ω
50 Ω
10 Ω
Coulombic efficiency (%)
6.1 ( 1.4
10.8 ( 3.0
28.2 ( 6.2
42.5 ( 7.0
45.0 ( 5.6
total cell count ( 108/cm2 anode)
1.16 ( 0.20
1.52 ( 0.06
2.59 ( 0.23
7.00 ( 0.34
14.9 ( 1.2
Rcta (kΩ 3 cm2)
a
1000 Ω
anode
3.55 ( 0.07
1.79 ( 0.10
1.62 ( 0.08
1.04 ( 0.05
0.72 ( 0.12
cathode
0.047 ( 0.003
0.034 ( 0.002
0.041 ( 0.005
0.034 ( 0.003
0.005 ( 0.001
Rct: Charge transfer resistance
Figure 3. Nyquist plots showing ohmic resistances (X-intercept) and
anode polarization resistances (arc radius) for steady-state reactors
running at different external resistances.
(Table 2, Figure 5). These two sequences are most similar to
Clostridium sp. FF08 (band 1) and Bacillus sp. KX6 (band 14).
No other sequenced bands were found in more than 2 anode
samples, indicating a diverse community structure among the
reactors running at different loadings. The 10 Ω and 50 Ω
anodes shared several bands, such as band 2 (Lachnospiraceae
DJF_CP76), band 3 (Fusibacter sp. SA1), and band 6
(Pelobacter propionicus), but none of these sequences were
found in reactors operated at higher external resistances. On
the other hand, some sequences found in higher resistances,
such as band 8 (Clostridiales JN18_A56_K) and band 10
(Anaerovorax odorimutans strain NorPut) were not observed
in lower resistance reactors. Surprisingly for an acetate-fed
system, most of the retrieved fragment sequences were most
similar to members of Firmicutes, and none of the predominant
DGGE bands were associated with known exoelectrogenic
bacteria. This may partially be due to the sensitivity limitation
of the DGGE analysis, and the microbial community in the
original inoculum.
’ DISCUSSION
External resistances regulate the availability of the MFC anode
as the electron acceptor for microbial electron transfers, and
therefore affect the anode biocatalyst activity and electrochemical
performance. However, how this regulator affects the relationship between the electrochemical and biological mechanisms has
not yet been fully investigated. In this study, both electrochemical response and anode biocatalyst variation were investigated
and correlated during 3 months of continuous operation. Duplicate reactors running at different external resistors showed
similar power production profiles using power density curves
obtained at 25-min measurement intervals, but the anode biofilm
architecture and community varied significantly among the
reactors with different operating resistances. Although the results
in this study support the hypothesis that electrochemical steady
state can be achieved within .5 h and verify the traditional
polarization testing method, they also show that this short testing
period will not reveal the steady state anode biofilm.
The applied external resistance controls the anode potential
and the rate of electron flow. The energy that can be used by
microorganisms during substrate oxidation is proportional to the
difference between the substrate redox potential and the anode
potential, as well as the number of electrons transferred to the
electrode.26 As a result, at higher anode potential and current,
more exoelectrogens should be able to transfer electrons to the
anode and gain more energy, likely leading to a more diverse
and denser anode biofilm. This could explain why the 5000 Ω
reactors showed similar power output in high resistances but
failed in low resistances, because the thin biofilms could not
sustain higher current under low resistances. On the other hand,
higher external resistance could accelerate the biofilm acclimation process by providing a lower anode potential for a faster
reactor start-up. However, with the exception of the highest
resistance system (5000 Ω), all reactors under different external
loadings (10-1000 Ω) showed similar power density but
different biofilm morphologies and densities. This suggests that
though the biofilm developed differently under variable external
loadings (anode potentials) as expected, the corresponding
electrochemical output was not generally affected by the bacterial
density because of the physicochemical constraints of the
selected MFC system. The measured total internal resistance
of the bottle reactors was around 190 Ω, which explains why the
peak power from all reactors was achieved at the 265 Ω
resistance.16
The similar power density supported by very different anode
biofilms shows the limited influence of microbial reactions on
electrochemical performance in reactor-constrained systems.
Though the physical and chemical constraints in MFCs have
been relieved significantly in recent years, this study shows that
current popular MFC configurations are still not good enough to
reveal microbially constrained electrochemical performance.
More advances are needed to further reduce the reactor limitation on microbial electrochemical activity. In addition, this
finding showed that steady state external resistance that
bracketed internal resistance had no meaningful differences in
scanned electrochemical performance, even though the biofilm
developed at high external resistance (1000 Ω) could have been
microbially constrained at a lower applied resistance (e.g., 10 Ω)
during polarization measurements.
2438
dx.doi.org/10.1021/es103115a |Environ. Sci. Technol. 2011, 45, 2435–2441
Environmental Science & Technology
ARTICLE
Figure 4. Confocal images (composite plan and cross section) of anode biofilms at (A) 5000 Ω, (B) 1000 Ω, (C) 265 Ω, and (D) 10 Ω. Samples were
viability stained, resulting in green live cells and red dead cells. The 50 Ω anode biofilm had architecture similar to the 10 Ω anode (data not shown).
Table 2. Phylogenetic Identification of Predominant 16S
rRNA Gene Fragment DGGE Bands
identity
band
closest cultivated isolate
(%)
1 Clostridium sp. FF08 (AB276319)
98
2 Lachnospiraceae DJF_CP76 (EU728729)
94
3 Fusibacter sp. SA1 (AF491333)
92
5 Clostridium sp. PPf35E6 (AY548783)
6 Pelobacter propionicus (X70954)
98
97
7 Clostridiaceae FH042 (AB298771)
94
8 Clostridiales JN18_A56_K (DQ168652)
99
9 Pseudomonas aeruginosa ZQP5 (GU384228)
99
10 Anaerovorax odorimutans strain NorPut (NR_028911)
93
11 Eubacterium saphenum ATCC 49989 (NR_026031)
94
12 Clostridium indolis strain 7 (NR_026493)
99
13 Clostridium aminovalericum strain DSM 1283 (NR_029245)
14 Bacillus sp. KX6 (AB043862)
97
90
Despite the limitation of current MFC configurations, the
effects of external resistances on MFC performance and biofilm
architecture carry important practical significance for the application at wastewater treatment facilities. Multiple MFC reactors
may be needed in series or parallel and operated accordingly
based on different objectives, such as more efficient COD
removal, higher voltage, or higher current output (i.e., shorter
retention time). So the external load may be used as a tuning
knob to properly regulate the operation in different stages and
meet different objectives. For example, high resistance can be
applied to accelerate biofilm acclimation and power production
during MFC start up. When the voltage stabilizes, the resistance
can be reduced to increase biofilm growth and current density.
On the other hand, the loading could be switched from low to
high when the reactor needs to “hibernate” but the biofilm needs
to be kept for the next active operation. Lower resistance is
generally preferred in active wastewater treatment processes, as
the reactor will have higher efficiency in COD removal and
electron recovery, but the associated biosolid production will
increase as well due to accelerated biofilm growth. Moreover, the
resistance tuning knob can also be used to select an ideal bacterial
community and inhibit unnecessary or harmful biofilm growth.
As indicated in Figure 4, filamentous bacteria tend to dominate
when a high resistance is applied, while using lower resistance
could inhibit such bacteria from growing and thus prevent
clogging problems during operation or potential bulking in
subsequent clarifiers, if needed.
There are many factors that affect MFC performance in addition to external resistances, such as reactor configuration, substrate characteristics, planktonic microorganisms, and flow rate in
2439
dx.doi.org/10.1021/es103115a |Environ. Sci. Technol. 2011, 45, 2435–2441
Environmental Science & Technology
Figure 5. DGGE profile of the anode biofilm samples running at
different external resistances. Selected bands are labeled.
continuous operation. All these factors need to be considered
systematically for better system design and operation. This study
demonstrates the effects of external loadings on electrochemical
performance and biological variations.
’ ASSOCIATED CONTENT
bS
Supporting Information. One additional figure. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]; phone: (303) 556-5287;
fax: (303) 556-2368.
’ ACKNOWLEDGMENT
This work was supported by National Science Foundation
Grant CBET-0834033 and King Abdullah University of Science
and Technology (KAUST; Award KUS-I1-003-13).
’ REFERENCES
(1) Liu, H.; Logan, B. E. Electricity generation using an air-cathode
single chamber microbial fuel cell in the presence and absence of a
proton exchange membrane. Environ. Sci. Technol. 2004, 38, 4040–6.
(2) He, Z.; Wagner, N.; Minteer, S. D.; Angenent, L. T. An upflow
microbial fuel cell with an interior cathode: Assessment of the internal
resistance by impedance spectroscopy. Environ. Sci. Technol. 2006, 40,
5212–5217.
(3) Cheng, S.; Liu, H.; Logan, B. E. Increased power generation in a
continuous flow MFC with advective flow through the porous anode and
reduced electrode spacing. Environ. Sci. Technol. 2006, 40, 2426–32.
(4) Freguia, S.; Rabaey, K.; Yuan, Z. G.; Keller, J. Sequential anodecathode configuration improves cathodic oxygen reduction and effluent
quality of microbial fuel cells. Water Res. 2008, 42, 1387–1396.
ARTICLE
(5) Zhang, F.; Jacobson, K.; Torres, P.; He, Z. Effects of anolyte
recirculation rates and catholytes on electricity generation in a liter-scale
upflow microbial fuel cell. Energy Environ. Sci. 2010, 1347–1352.
(6) Wang, X.; Cheng, S. A.; Feng, Y. J.; Merrill, M. D.; Saito, T.;
Logan, B. E. Use of Carbon Mesh Anodes and the Effect of Different
Pretreatment Methods on Power Production in Microbial Fuel Cells.
Environ. Sci. Technol. 2009, 43, 6870–6874.
(7) Logan, B.; Cheng, S.; Watson, V.; Estadt, G. Graphite fiber brush
anodes for increased power production in air-cathode microbial fuel
cells. Environ. Sci. Technol. 2007, 41, 3341–3346.
(8) Reguera, G.; Nevin, K. P.; Nicoll, J. S.; Covalla, S. F.; Woodard,
T. L.; Lovley, D. R. Biofilm and nanowire production leads to increased
current in Geobacter sulfurreducens fuel cells. Appl. Environ. Microbiol.
2006, 72, 7345–7348.
(9) Rabaey, K.; Rodriguez, J.; Blackall, L. L.; Keller, J.; Gross, P.;
Batstone, D.; Verstraete, W.; Nealson, K. H. Microbial ecology meets
electrochemistry: Electricity-driven and driving communities. ISME J
2007, 1, 9–18.
(10) Torres, C. I.; Krajmalnik-Brown, R.; Parameswaran, P.; Marcus,
A. K.; Wanger, G.; Gorby, Y. A.; Rittmann, B. E. Selecting AnodeRespiring Bacteria Based on Anode Potential: Phylogenetic, Electrochemical, and Microscopic Characterization. Environ. Sci. Technol. 2009,
43, 9519–9524.
(11) Jung, S.; Regan, J. M. External Resistance Influences Electrogenesis, Methanogenesis, and Anode Prokaryotic Communities in
MFCs. Appl. Environ. Microbiol. 201010.1128/AEM.01392-10.
(12) Rismani-Yazdi, H.; Christy, A. D.; Carver, S. M.; Yu, Z.;
Dehority, B. A.; Tuovinen, O. H. Effect of external resistance on bacterial
diversity and metabolism in cellulose-fed microbial fuel cells. Bioresour.
Technol. 2011, 102, 278–83.
(13) Jung, S.; Regan, J. M. Comparison of anode bacterial communities and performance in microbial fuel cells with different electron
donors. Appl. Microbiol. Biotechnol. 2007, 77, 393–402.
(14) Lee, H. S.; Parameswaran, P.; Kato-Marcus, A.; Torres, C. I.;
Rittmann, B. E. Evaluation of energy-conversion efficiencies in microbial
fuel cells (MFCs) utilizing fermentable and non-fermentable substrates.
Water Res. 2008, 42, 1501–1510.
(15) Torres, C. I.; Marcus, A. K.; Lee, H. S.; Parameswaran, P.;
Krajmalnik-Brown, R.; Rittmann, B. E. A kinetic perspective on extracellular electron transfer by anode-respiring bacteria. FEMS Microbiol.
Rev. 2010, 34, 3–17.
(16) Logan, B.; Hamelers, B.; Rozendal, R.; Schr€oder, U.; Keller, J.;
Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. Microbial Fuel
Cells: Methodology and Technology. Environ. Sci. Technol. 2006, 40,
5181–5192.
(17) Cheng, S.; Liu, H.; Logan, B. E. Increased performance of
single-chamber microbial fuel cells using an improved cathode structure.
Electrochem. Commun. 2006, 8, 489–494.
(18) Ren, Z. Y.; Ward, T. E.; Regan, J. M. Electricity production from
cellulose in a microbial fuel cell using a defined binary culture. Environ.
Sci. Technol. 2007, 41, 4781–4786.
(19) Ramasamy, R. P.; Ren, Z.; Mench, M. M.; Regan, J. M. Impact
of initial biofilm growth on the anode impedance of microbial fuel cells.
Biotechnol. Bioeng. 2008, 101, 101–8.
(20) Ramasamy, R. P.; Gadhamshetty, V.; Nadeau, L. J.; Johnson,
G. R. Impedance Spectroscopy as a Tool for Non-Intrusive Detection of
Extracellular Mediators in Microbial Fuel Cells. Biotechnol. Bioeng. 2009,
104, 882–891.
(21) Ren, Z.; Steinberg, L. M.; Regan, J. M. Electricity production
and microbial biofilm characterization in cellulose-fed microbial fuel
cells. Water Sci. Technol. 2008, 58, 617–622.
(22) Ren, Z.; Ramasamy, R. P.; Cloud-Owen, S. R.; Yan, H.; Mench,
M. M.; Regan, J. M. Time-course correlation of biofilm properties and
electrochemical performance in single-chamber microbial fuel cells.
Bioresour. Technol. 2011, 102, 416–421.
(23) Ren, N.; Xing, D.; Rittmann, B. E.; Zhao, L.; Xie, T.; Zhao, X.
Microbial community structure of ethanol type fermentation in biohydrogen production. Environ. Microbiol. 2007, 9, 1112–1125.
2440
dx.doi.org/10.1021/es103115a |Environ. Sci. Technol. 2011, 45, 2435–2441
Environmental Science & Technology
ARTICLE
(24) Lyon, D.; Buret, F.; Vogel, T.; Monier, J. Is resistance futile?
Changing external resistance does not improve microbial fuel cell
performance. Bioelectrochemistry 2010, 78, 2–7.
(25) Kiely, P. D.; Radera, G.; Regan, J. M.; Logan, B. E. Long-term
cathode performance and the microbial communities that develop in
microbial fuel cells fed different fermentation endproducts. Bioresour.
Technol. 2011, 102, 361–366.
(26) Aelterman, P.; Versichele, M.; Marzorati, M.; Boon, N.; Verstraete,
W. Loading rate and external resistance control the electricity generation
of microbial fuel cells with different three-dimensional anodes. Bioresour.
Technol. 2008, 99, 8895–8902.
2441
dx.doi.org/10.1021/es103115a |Environ. Sci. Technol. 2011, 45, 2435–2441