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Quantification of the Internal Resistance
Distribution of Microbial Fuel Cells
Yanzhen Fan, Evan Sharbrough, and Hong Liu
Environ. Sci. Technol., 2008, 42 (21), 8101-8107 • DOI: 10.1021/es801229j • Publication Date (Web): 24 September 2008
Downloaded from http://pubs.acs.org on December 10, 2008
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Environ. Sci. Technol. 2008, 42, 8101–8107
Quantification of the Internal
Resistance Distribution of Microbial
Fuel Cells
YANZHEN FAN, EVAN SHARBROUGH, AND
HONG LIU*
Department of Biological and Ecological Engineering, Oregon
State University, 116 Gilmore Hall, Corvallis, Oregon 97331
Received August 11, 2007. Revised manuscript received
July 30, 2008. Accepted August 11, 2008.
Identifying the limiting factors in a microbial fuel cell (MFC)
system requires qualifying the contribution of each component
of an MFC to internal resistance. In this study, a new method
was developed to calculate the internal resistance distribution of
an MFC. Experiments were conducted to identify the limiting
factors in single-chamber MFCs by varying the anode surface
areas, cathode surface areas, and phosphate buffer concentrations. For the MFCs with equally sized electrodes (7 cm2) and
200 mM phosphate buffer, the anode contributed just 5.4% of
the internal resistance, while the cathode and the electrolyte
each contributed 47.3%, indicating that the anode was not
the limiting factor in power generation. The limitation of the
cathode was further revealed by the 780% higher area-specific
resistance (284.4 Ω cm2) than the 32.3 Ω cm2 of the anode.
The electrolyte limitation was also evidenced by the greatly
increased contribution of electrolyte in internal resistance from
47.3 to 78.2% when the concentration of phosphate buffer
was decreased from 200 to 50 mM. An anodic power density
of 6860 mW/m2 was achieved at a current density of 2.62 mA/
cm2 using the MFCs with an anode/cathode area ratio of 1/14
and 200 mM phosphate buffer. The method was also
successfully applied to analyze the internal resistance
distribution of the two chamber MFCs from a previously
reported study. The comparison of the internal resistances of
the two air cathode systems indicates that the much lower
resistances, including anode, cathode, and membrane resistances,
contributed to the much better performance of the singlechamber MFCs than the two-chamber system.
Introduction
Microbial fuel cells (MFCs), which can directly generate
electricity from biodegradable substances, have rapidly
gained increasing research attention. Although the power
density of an MFC is still low compared to a hydrogen fuel
cell, the renewable and widely available fuel sources and
moderate operational conditions make it very promising in
renewable energy generation, wastewater treatment, and as
potable power supplies or power sources for remote electronic devices. Significant increases in power density have
been achieved in recent studies (1-4). Power densities of
115 W/m3 (1) and 2400 mW/m2 (73 W/m3) (2) using air
cathode MFCs were reported. An even higher power density
of 500 W/m3 was achieved with a miniature MFC using
* Corresponding Author phone: 541-737-6309; fax: 541-737-2082;
e-mail: liuh@engr.orst.edu.
10.1021/es801229j CCC: $40.75
Published on Web 09/24/2008
 2008 American Chemical Society
ferricyanide as catholyte (3). More recently, Fan et al.
increased the power density to 1010 W/m3 using air cathode
MFCs with cloth electrode assemblies (CEAs) (4). However,
further amplifying the power density remains one of the
greatest challenges for realizing the practical applications of
MFCs.
Identifying the limiting factors in an MFC system is critical
for further enhancing the MFC performance. Internal
resistance, including anode resistance, cathode resistance,
electrolyte resistance, and membrane resistance (if any),
limits the power output of an MFC (5). The internal resistance
can be reduced by increasing the anode surface area (2, 6),
the cathode surface area (7), the surface area of the proton
exchange membrane (PEM) (6), the ionic strength of the
electrolyte (8), or the pH (1). Identification of the limiting
factor of an MFC requires quantification of the contribution
of each component of an MFC to internal resistance. Like
any electrochemical cell, the internal resistance of an MFC
can also be classified as ohmic resistance, charge-transfer
resistance, and diffusion resistance (12), which can be
analyzed using some alternating current (AC) methods such
as electrochemical impedance spectroscopy (EIS) (9). The
resistance of each fuel cell component may also be estimated
using EIS if an appropriate equivalent circuit can be
constructed (9). EIS has been utilized to separate the ohmic
resistance from the other resistances of MFCs (2, 4, 10).
However, the analysis of charge-transfer resistance and
diffusion resistance of an MFC remains a great challenge
(2, 4, 10, 11) probably due to the 3-dimensional nature of
the MFC anode, especially with a developed biofilm, and the
relative larger electrode spacing (9). The internal resistance
has to be measured over a wide frequency range in order to
cover the diffusion resistance, which is usually found in the
lowest frequency region, down to 10 mHz, or even lower for
chemical fuel cell (9). The relatively larger electrode spacing
in MFCs requires longer time constant thus even lower
frequency (9). However, the EIS of MFCs at low frequency
region is often unstable and inaccurate. In an up-flow two
chamber MFC system, the EIS data were inaccurate beyond
the range 9 Hz to 4.4 kHz (11). The lowest frequency (9 Hz)
might be too high to analyze the diffusion resistance, which
might be the reason for the inconsequential diffusion
resistance. Although much wider frequency range (5 mHz to
300 kHz) were employed in single-chamber MFCs (2, 4, 10),
the charge transfer and diffusion resistances haven′t been
separated. Direct current (DC) analytical methods such as
polarization curve can be used to measure the total internal
resistance (12, 13), but not the resistance of the individual
components. There is substantial need for the development
of a novel method to analyze the quantitative contribution
of anode, cathode, membrane, and electrolyte resistances to
the overall internal resistance. Such a quantitative method
can be used to identify the limiting factor of an MFC that
governs the internal resistance, and in turn the power output.
Recently, some computational models have been developed
(14-16), which provide comprehensive insights of the biofilm
development and kinetics of the anodic reaction. However,
it remains difficult to model the whole microbial fuel cell,
including anode, cathode, electrolyte, and membrane.
In this paper, a new method was developed to calculate
the contribution of the anode, cathode, electrolyte, and
membrane to internal resistance. Experiments were conducted to investigate the limiting factors in single-chamber
MFCs by varying the anode surface areas, cathode surface
areas, and phosphate buffer concentrations. The method
was also successfully implemented to calculate the distribuVOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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(ohmic) region of the curve (5). For the linear polarization
curve, the relationship between the external voltage (E) and
current (I) can be described as follows:
E ) Eb - RintI
FIGURE 1. The polarization curve of an MFC shows the three
regions of polarization overpotentials.
tion of internal resistance of two-chamber MFCs using the
data reported in a previous study (6).
Materials and Methods
MFC Construction and Operation. Single-chamber membrane-free MFCs (12 mL liquid volume, 1.7 cm electrode
spacing) were constructed as reported previously (17) and
were used to investigate the power density at various anode
areas (1-7 cm2; nonwet proofing, type A, E-Tek) using 7 cm2
carbon cloth cathodes. In addition, a double-cathode reactor
setup was arranged wherein the 1 cm2 anode was placed
equidistant between two 7 cm2 cathodes in order to further
investigate the maximum power density generated using this
carbon fiber cloth. The cathode has a platinum loading of
0.5 mg/cm2 Pt coated with a PTFE layer to prevent water
loss. Details of cathode fabrication can be found in a previous
study (18).
The MFCs were inoculated with a mixed bacterial culture
from the anode of a single-chamber MFC, which was
originally inoculated with domestic wastewater (Corvallis
Wastewater Treatment Plant, Corvallis, OR) and has been
operated for about 1 year using acetate. All MFCs were
operated in batch mode using nutrient medium (4) amended
with acetate (30 mM). The electrolyte solution was replaced
when the voltage decreased to less than 30 mV. The system
was considered to be operating under steady conditions when
voltage output was reproducible after refilling reactor with
medium at least twice. Repeatable power output can normally
be obtained in about 50 h (three batches) at a fixed external
resistance of 1000 Ω. Resistance was gradually decreased
from 1000 to 60 Ω for polarization curve generation. At each
resistance, MFCs were run for at least two batches for
reproducible voltage output. The 10 data points around the
maximum voltage at each batch were averaged and used to
make the polarization curve. 2-3 weeks (15-20 batches) of
operation was sufficient to finish one polarization curve.
Voltage (V) was measured using a multimeter with a data
acquisition system (2700, Keithley, U.S.) and used to calculate
the power (P) according to P ) IV. Power density was then
calculated using the power normalized by the cross sectional
area (projected) of the anode.
Calculation of Internal Resistance Distribution. A polarization curve is commonly used to study the performance
of a fuel cell and the various current related losses and
includes three regions (Figure 1). The cell voltage drops
rapidly and nonlinearly in (1) the activation region, followed
by a slow and near-linear drop in (2) the ohmic region. The
continuous current increase may reflect (3) the concentration
polarization region or a condition where mass transport of
reactants becomes the limiting factor. In this region, the
voltage drops rapidly and nonlinearly with the current
increase. In a MFC, operation often takes place in the linear
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 21, 2008
(1)
where Rint is the total internal resistance of an MFC, which
can be easily obtained from the slope of the linear curve. The
intercept of the linear curve with the voltage (E) axis, Eb, is
referred to as the linear extrapolation open circuit voltage
(LE-OCV) in this study, i.e., the voltage that is determined
by the extrapolation of the linear part of the polarization
curve. The value of Eb should be close to, but less than, the
measured open circuit voltage (OCV).
The calculated resistance using this method is referred to
as internal resistance rather than ohmic resistance because
activation loss and concentration polarization affect the slope
of the polarization curve, and the influence has been
considered in the calculation of the internal resistance using
this method.
The total internal resistance Rint of an MFC can be further
dissected into anodic resistance Ra, cathodic resistance Rc,
membrane resistance Rm (if it exists), and electrolyte resistance Re. Therefore, eq 1 can be rewritten as follows:
E ) Eb - (Ra + Rc + Rm + Re)I
(2)
The Ra, and Rc, refer to the resistances caused by bioelectrochemical reactions at anode and oxygen reduction at
cathode, respectively. The contact resistance is neglected
because it is normally less than 1% of the total internal
resistance in a well designed MFC. It can be simply included
in eq 2 as a constant if its value is too large to be neglected.
The anode resistance Ra depends on the size of the anode
and the activity of the electricity-generating bacteria. Both
the biofilm on the anode and planktonic bacteria in bulk
solution (through self-produced mediators) (19) may contribute to the power generation of MFCs when mixed bacterial
cultures are utilized. However, the planktonic bacteria had
a negligible contribution to the power generation in the tested
MFC system (see the Supporting Information). Therefore,
for a plate-type anode (e.g., carbon cloth), Ra can be assumed
inversely proportional to the projected area of the anode Sa,
i.e.,
Ra ) ra ⁄ Sa
(3)
where ra is the area-specific resistance (ASR) of the anode,
or the resistance normalized by anode area.
Similarly,
Rc ) rc ⁄ Sc
(4)
Rm ) rm ⁄ Sm
(5)
Re ) re ⁄ Sr
(6)
where rc, rm, and re are the ASRs of the cathode, membrane,
and electrolyte, respectively; Sc, Sm, and Sr are the projected
areas of the cathode, membrane, and electrolyte (reactor),
respectively.
It should be noted that the values of Ra, Rc, and Rm might
be different from the values measured with traditional
electrochemical methods, e.g., EIS using Luggin capillary
reference electrode. While it is impossible to totally separate
the contribution of electrodes or membranes from that of
electrolytes using the traditional methods, the method
developed here includes the contribution of electrolyte only
in electrolyte resistance Re, which represents all the resistances caused by electrolyte, including ohmic resistance,
charge transfer resistance, and diffusion resistance. Assuming
Re is proportional to the electrode spacing L, while inversely
proportional to the cross-sectional area of the reactor Sr and
the concentration of charge transfer electrolyte Ce (10), the
electrolyte resistance can be computed as follows:
Re ) aL ⁄ (Sr × Ce)
(7)
pmax ) 0 . 25Eb2 ⁄ Rint
where a is a constant.
Therefore, eq 2 can be rewritten as
E ) Eb - (ra ⁄ Sa + rc ⁄ Sc + rm ⁄ Sm + aL ⁄ (Sr × Ce))I
(8)
The total internal resistance, Rint, can be written as
Rint ) ra ⁄ Sa + rc ⁄ Sc + rm ⁄ Sm + aL ⁄ (Sr × Ce)
(9)
Equation 8 can be further transformed into the equation
below in order to calculate the power density directly.
p ) Ebi - Rinti2
From eq 10, we can easily derive that the power density
(p) of an MFC is the quadratic function of the current density
(i), while the maximum power density, or the maximum value
of the quadratic function, can be computed as follows:
(10)
where p is the power density and i the current density.
(11)
The values of Eb, ra, rc, rm, a, and pmax can be determined with
a best fit of the experimental data with eqs 8, 10 or 11 using
the SOLVER function in Microsoft Office Excel. In this study,
the sums of the squares of the differences in the observed
and calculated values were minimized by varying the initial
guess 1000 times over a wide range of possible parameter
values with a quasi-Newton search method.
Results and Discussion
Single-Chamber MFC System. Power Generation. To explore
the power density generated by the mixed culture using
carbon cloth electrodes and the relationship between the
FIGURE 2. Comparison of observed (A) voltage and (B) anodic power density (points) and the calculated results (lines) as a function
of current density in MFCs with different anode areas (1, 2, 3.6, and 7 cm2), cathode areas (7 and 14 cm2), and phosphate buffer
concentrations (50, 100, and 200 mM). Inserts show the correlations between the observed and calculated voltage/power density. The
format of the legend is anode area (cm2)scathode area (cm2)sbuffer concentration (mM).
VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Internal Resistance Distribution and Anodic Maximum Power Density of MFCs with Different Electrode Areas and
Phosphate Buffer Concentrations
MFC
b
1
2
3
b
4
b
5
b
6
7
7
1
1
1
1
b
b
a
Sa cm2
Sc cm2
Cb mM
7
7
7
7
14
∞
200
50
50
200
200
∞
Ra Ω (%)
Rc Ω (%)
Re Ω (%)
RintΩ
pmax,ra mW/m2
4.6 (5.4%)
4.6 (2.2%)
32.3(13.7%)
32.3(28.4%)
32.3(34.6%)
32.3(100%)
40.6(47.3%)
40.6(19.5%)
40.6(17.2%)
40.6(35.8%)
20.3(21.8%)
0
40.7(47.3%)
162.7(78.2%)
162.7(69.0%)
40.7(35.8%)
40.7(43.6%)
0
85.9
208.0
235.7
113.6
93.3
32.3
1040
430
379
787
958
-
Based on the cross-sectional area of the MFC reactor.
b
anode and cathode surface areas, the cross-sectional areas
were systematically decreased from 7 to 3.6, 2, and 1 cm2
using single or double cathode arrangements. Decreasing
the anode surface area relative to the cathode surface area
increased the power density (Figure 2). A maximum power
density of 1050 mW/m2 was obtained at a current density of
0.39 mA/cm2 in MFCs using equally sized electrodes (7 cm2)
(Figure 2). This result is comparable to a previous study using
the same electrode materials and similar solution chemistry
(8). Reducing the anode surface area from 7 to 1 cm2 resulted
in a 430% increase in anodic power density (5570 mW/m2).
A power density of 6860 mW/m2 was achieved when double
cathodes (14 cm2) were used, corresponding to an anodic
current density of 2.62 mA/cm2.
The power density of the reactor was also greatly affected
by the concentration of the phosphate buffer. As demonstrated in Figure 2, the measured maximum power density
was 2680 mW/m2 for MFCs with 50 mM phosphate buffer.
The maximum power density increased by 49% to 4000 mW/
m2 when the buffer concentration increased to 100 mM.
Further increasing the buffer concentration to 200 mM
resulted in an additional 39% increase in power density.
Internal Resistance Distribution. The increase in current
density resulted in a uniformly linear decrease in voltage,
especially in a voltage range of 0.2-0.45 V. For the air cathode
single-chamber MFCs used in this study, there was no
membrane and the dimensions of the MFCs (L ) 1.7 cm; Sr
) 7 cm2) were fixed. Therefore, eq 8 can be simplified to the
following:
E ) Eb - (ra ⁄ Sa + rc ⁄ Sc + ac ⁄ Cb)I
(12)
where E (V) is the external voltage, Eb (V) is the linear
extrapolation open circuit voltage, ra (Ω cm2) is the anodic
ASR, Sa (cm2) is the surface area of anode, rc (Ω cm2) is the
cathodic ASR, Sa (cm2) is the surface area of cathode, ac (Ω
mM) is a constant, Cb (mM) is the buffer concentration, and
I (A) is the current.
Similarly, eq 10 can be simplified as follows:
p ) Ebi - (ra ⁄ Sa + rc ⁄ Sc + ac ⁄ Cb)i2
(13)
Both eqs 12 and 13 were used to fit all the experimental data
except those with an actual voltage greater than 0.45V which
were not in the linear region of the polarization curve. Similar
results were obtained using eqs 12 and 13 and both equations
fit the experimental data very well (R2 > 0.99, n ) 47) (Figure
2). The calculated LE-OCV of the MFCs (0.5004 V) was much
lower than the OCV of the same MFCs, which was in the
range of 0.7-0.8 V. The ASR of the cathode (284.4 Ω cm2) was
780% higher than the 32.3 Ω cm2 of the anode, indicating
that the cathode rather than the anode was limiting the MFC
performance.
The distributions of internal resistances for MFCs with
different electrode areas and buffer concentrations are shown
in Table 1. For all tested MFCs, the electrolyte resistance
accounted for 36-78% of the total resistance, while the
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 21, 2008
pmaxb mW/m2
1040
430
2656
5509
6708
19369
Based on the projected area of the anode.
resistance of the anode ranged from 2 to 35%. For the MFCs
with equally sized anodes and cathodes (7 cm2) and 200 mM
phosphate buffer, the anode only accounted for 5.4% of the
internal resistance, while the cathode and the electrolyte
each contributed 47.3%. Increasing the phosphate buffer
concentration from 50 to 200 mM greatly decreased the total
internal resistance from 208 Ω (MFC no. 2) to 85.9 Ω (MFC
no. 1), whereas the contribution of electrolyte to the internal
resistance decreased from 78.2 to 47.3%. The decrease of
anode area from 7 cm2 to 1 cm2 only slightly increased the
internal resistance to 235.7 Ω (MFC no. 3) because the
electrolyte (50 mM phosphate buffer) was still limiting
the performance of the MFC. The MFC (no. 4) with a 1 cm2
anode, 7 cm2 cathode, and 200 mM phosphate buffer resulted
in a more balanced internal resistance distribution. Doubling
the surface area of the cathode resulted in an 18% decrease
in internal resistance from 113.6 Ω to 93.3 Ω (MFC no. 5).
The calculated anodic maximum power density was only
430 mW/m2 for MFC no. 2 with equally sized electrodes and
50 mM buffer. Increasing the cathode/anode area ratio and
the buffer concentration can greatly increase the anodic
power density. A power density of 6860 mW/m2 was achieved
with a cathode/anode area ratio of 14 and 200 mM buffer,
indicating that future studies on this kind of MFC system
should focus on the development of high performance
cathode materials, buffer solution, and reactor design to
minimize the internal resistances. Although platinum cathode
has achieved very high current density in chemical fuel cells,
the performance of the cathode in MFCs is greatly reduced
due to the extremely low availability of protons at the catalytic
sites at near neutral pH condition (10). New cathode design
should focus on the structure of cathode to improve the
proton availability. The reduction in electrolyte resistance
can be achieved through the increase in pH buffer concentration and the reduction in electrode spacing. In a recent
study, the electrolyte resistances of MFCs with double cloth
electrode assemblies (CEAs) (14 cm2, 100 mM PBS) were
reduced to about one tenth of that in MFC no. 1 (7 cm2, 200
mM PBS) due to the much smaller electrode spacing in CEA
MFCs (about 1 mm). A maximum anodic power density of
19.4 W/ m2 is predicted assuming the carbon cloth anode,
microbial consortia, and biofilm thickness and activity are
the same and assuming both cathode resistance and electrolyte resistance are neglectable. Such a power density is
about 1 order of magnitude higher than the maximum power
density achieved using the state-of-the-art MFCs.
Two-Chamber MFC System. Membrance Resistance. Oh
and Logan tested the power generation in two-chamber MFCs
with various electrodes and PEM areas (6). Equation 8 can
be modified to fit the data presented in that study. Figure 5
in Oh and Logan′s paper (6) illustrated the effect of the PEM
area on the power generation of MFCs using dissolved oxygen
or ferricyanide as electron acceptors. The anodic resistance,
cathodic resistance and electrolyte resistance were fixed in
this case since the electrode areas (22.5 cm2), reactor
FIGURE 3. Comparison of (A) the observed voltage and (B) anodic power density (points) and the calculated results (lines) of MFCs
with different PEM areas (3.5 cm2, 6.2 cm2, and 30.6 cm2). The open marks and dashed lines are the results for air cathode MFCs
(denoted air), and solid marks and solid lines are the results for MFCs using ferricyanide as the catholyte (denoted FeCN). Inserts
show the correlation between the observed and the calculated voltage/power density. Original data were from Figure 5 of ref (6).
configuration, and electrolyte were unchanged. Therefore,
eq 8 can be simplified as
E ) Eb - (Rs + rm ⁄ Sm)I
(14)
where E (V) is the external voltage; Eb (V) is the linear
extrapolation open circuit voltage; Rs (Ω) is the sum of anodic
resistance, cathodic resistance, and electrolyte resistance;
rm (Ω cm2) is the ASR of the PEM; Sm (cm2) is the surface area
of the PEM; and I (A) is the current.
Similarly, to calculate the power P (W), eq 10 can be
simplified as
P ) EbI - (Rs + rm ⁄ Sm)I2
(15)
Both eqs 14 and 15 fit well with the data from Figure 5 of ref
6 with a voltage greater than 0.1 V. Figure 3 illustrates the
fitting results using eq 14 for air cathode MFCs and eq 15 for
ferricyanide cathode MFCs. The calculated Eb, Rs, Rm, and
Pmax are listed in Table 2. The calculated voltages and powers
agree very well with the experimental results with correlation
coefficients of R2 ) 0.9924 (n ) 59) for power and R2 ) 0.9632
(n ) 59) for voltage.
The calculated Eb (0.739 V) of MFCs using ferricyanide
was higher than the 0.652 V of air cathode MFCs. The relatively
lower internal resistances of MFCs using ferricyanide also
contributed to the improved power generation. The major
TABLE 2. Calculated Internal Resistance Distribution and
Maximum Power of Two-Chamber MFCs with Different PEM
Areas
MFCa
Sm
cm2
Eb V
Rs Ω (%)
air-3.5
3.5 0.652 145 (16%)
air-6.2
6.2 0.652 145 (25%)
air-30.6
30.5 0.652 145 (62%)
FeCN-3.5
3.5 0.739 102 (14%)
FeCN-6.2
6.2 0.739 102 (22%)
FeCN-30.6 30.5 0.739 102 (58%)
Rm Ω (%)
Rint Ω
Pmax
mW
780 (84%)
440 (75%)
89 (38%)
633 (86%)
357 (78%)
72 (42%)
925
585
234
735
459
174
0.115
0.182
0.454
0.186
0.297
0.783
a
Air: MFCs using dissolved oxygen as electron acceptor;
FeCN: MFCs using ferricyanide as electron acceptor; The
number indicates the area of the PEM (cm2).
(38-86%) contribution of the PEM to the total internal
resistance indicates that the PEM may limit the performance
of two-chamber MFCs. Therefore, the role of the PEM needs
to be carefully evaluated and considered in the design and
operation of MFCs.
Internal Resistance Distribution. The data in Figure 4 of
ref 6 can be used to further calculate the distribution of the
Rs. In this figure, the areas of the anode, cathode and PEM
were varied, while the reactor dimension and electrolyte were
fixed, allowing eq 10 to be rewritten as
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P ) EbI - (ra ⁄ Sa + rc ⁄ Sc + rm ⁄ Sm + Re)I2
(16)
To reduce the number of variables, the values of Eb (0.652
V) and rm (2729 Ω cm2) obtained above were used directly
in fitting the experimental results using eq 16. All the data
from the figure were used except two points with measured
voltages of 0.079 V, which fell out of the linear region of the
polarization curve. Figure 4 shows that the calculated power
agrees well with the experimental data (R2 ) 0.9861, n ) 23).
The calculated internal resistance distribution of the twochamber MFCs is listed in Table 3.
Single-Chamber MFCs vs Two-Chamber MFCs. A comparison of the calculated parameters of single-chamber and
two-chamber MFCs is shown in Table 3. The calculated
electrolyte resistances for the two chamber MFCs (33.6 Ω)
were lower than that of single-chamber MFCs (40.7 Ω),
possibly due to differences in reactor dimensions and
electrolyte chemistry. The Eb value (0.5004 V) of the singlechamber air cathode MFCs was lower than the 0.652 V of
two-chamber MFCs which may have resulted from the higher
oxygen cross-over in the single-chamber system. However,
the much lower internal resistances of single-chamber MFCs
make it possible to produce more than a 10-fold increase in
power density.
The anodic ASR of two chamber MFCs with carbon paper
anodes were 24 times higher than the single-chamber MFCs
with carbon cloth anodes, especially considering similar
performances were seen in single-chamber MFCs using both
materials (8). The differences in microbial consortia may be
the major reason for such a difference. Besides the difference
in the inocula, the high resistances of the membrane,
electrolyte and cathode in the two-chamber MFCs, which
limited the anodic current density, may also affect the
development of biofilms and highly efficient microbial
consortia. Although both the single-chamber MFCs and the
two-chamber air cathode MFCs used platinum as the catalyst
and oxygen as the electron acceptor, the cathodic ASR of the
two-chamber system was 8 times higher than that of the
single-chamber system, indicating direct air is much better
than dissolved oxygen in supplying oxygen to the catalytic
sites. The difference in supplying oxygen might be the result
from the much higher oxygen concentration and mass
transfer rate in air than those in water. Although the
microorganisms and exopolymeric substance (EPS) on the
cathode of a single-chamber MFC might increase the resistances of the cathode, the cathode resistance of the singlechamber MFC was less than 1/8 of the cathode or membrane
resistance of the two-chamber MFC (Table 3), indicating the
resistance caused by microbes and EPS was not significant.
The high membrane resistance (2729 Ω cm2), which is not
present in membraneless single-chamber MFCs, is another
important factor that caused the poor performance of the
two chamber system. The inclusion of a PEM, such as Nafion
117, needs to be carefully considered due to its high area
specific resistance, which could contribute 38-86% of the
total internal resistances of two-chamber MFCs. The total
ASR of the single-chamber MFC with equal sized electrodes
(1.7 cm spacing, 200 mM PBS) was 601 Ω cm2. It is lower than
the 3220 Ω cm2 of an upflow MFC with interior cathode
(membrane surface area 188 cm2) (11), but higher than the
ohmic ASRs of 49-56 Ω cm2 of MFCs with cloth-electrode assemblies (CEAs) (4, 10) and 56 Ω cm2 (based on the
cathode projected area of 7 cm2) of a brush-anode MFC using
0.2 M phosphate buffer (2).
Implications. The method developed in this study
provides a versatile tool to evaluate the limiting factor(s) and
to simulate/predict the power generation of an MFC. In
addition to the tested single-chamber and two-chamber MFC
systems, the method can also be easily adapted to other MFC
systems. The calculated parameters, such as ra, rc, rm, and Re
8106
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 21, 2008
FIGURE 4. Comparison of observed (points) and calculated
(lines) power of MFCs with different electrode and PEM areas.
Original data were from Figure 4 of ref (6).
TABLE 3. Comparison of the Calculated Parameters of
Single-Chamber and Two-Chamber Air Cathode MFCs
MFC
singlechamber
twochamber
Eb
V
ra
Ω cm2
rc
Ω cm2
rm
Ω cm2
re
Ω cm2
Re
Ω
0.500
32.3
284
0
285a,b
40.7a
2566
2729
1028c
33.6
0.652
a
820
b
200 mM phosphate buffer. Based on cross-section
area of 7 cm2. c Based on cross-section area of 30.6 cm2.
are linked directly to the components of MFCs including the
anode, cathode, membrane, and electrolyte. These parameters can then be directly used to evaluate the performance
of the MFC components and the whole MFC system. A
comparison of these parameters can be utilized to determine
the limiting factors of MFCs. In addition, these parameters
can also be compared among different types of MFCs,
providing valuable information in reactor design, electrode/
membrane material selection and development, and electrolyte selection. It should be noted that this is a simplified
method and is only suitable for MFCs with near linear
polarization curves, which is the case for the majority of the
reported MFC systems (5). Further improvement can be made
to simulate the non-linear polarization curves through the
incorporation of the Butler-Volmer equation for electron
transfer rates, first-order mass transport rates, and ohmic
potential drop across the membrane and in solution (20).
The power generation of MFCs with different anode
surface areas, cathode surface areas, and phosphate buffer
concentrations indicated that carbon cloth anodes with well
developed biofilms did not limit the power generation of the
single-chamber air cathode MFCs. The electrolyte resistance,
membrane resistance, and cathode resistance contribute the
most to the internal resistance of most current MFC systems.
Significant enhancement in the performance of MFCs can
be achieved through the development of high efficiency
cathodes and the reduction of the internal resistance by
reducing the electrode spacing, increasing the concentration
of pH buffers, and selecting membranes with low resistance.
An anodic power density of 6860 mW/m2 was achieved using
single-chamber air cathode MFCs with an anode/cathode
area ratio of 1/14 and 200 mM phosphate buffer, indicating
the great potential of MFC technology.
Acknowledgments
This research was partially supported by Agricultural Research
Foundation and Oregon State University General Research
Fund. We acknowledge Sang-Eun Oh and Bruce Logan for
providing the data of Figures 4 and 5 in ref 6.
Supporting Information Available
The experimental results and discussion on the contribution
of biofilms and planktonic bacteria to power generation of
single-chamber MFCs. This material is available free of charge
via the Internet at http://pubs.acs.org.
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