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Environ. Sci. Technol. 2009, 43, 6870–6874
Use of Carbon Mesh Anodes and
the Effect of Different Pretreatment
Methods on Power Production in
Microbial Fuel Cells
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Publication Date (Web): July 17, 2009 | doi: 10.1021/es900997w
XIN WANG,† SHAOAN CHENG,‡
Y U J I E F E N G , * ,† M A T T H E W D . M E R R I L L , ‡
TOMONORI SAITO,‡ AND
B R U C E E . L O G A N * ,†,‡
State Key Laboratory of Urban Water Resource and
Environment, No 73 Huanghe Road, Nangang District,
Harbin 150090, China, and Department of Civil and
Environmental Engineering, Penn State University, 231Q
Sackett Building, University Park, Pennsylvania 16802
Received April 2, 2009. Revised manuscript received June
15, 2009. Accepted July 7, 2009.
Flat electrodes are useful in microbial fuel cells (MFCs) as
close electrode spacing improves power generation. Carbon
cloth and carbon paper materials typically used in hydrogen fuel
cells, however, are prohibitively expensive for use in MFCs.
An inexpensive carbon mesh material was examined here as
a substantially less expensive alternative to these materials
for the anode in an MFC. Pretreatment of the carbon mesh was
needed to ensure adequate MFC performance. Heating the
carbon mesh in a muffle furnace (450 °C for 30 min) resulted
in a maximum power density of 922 mW/m2 (46 W/m3) with this
heat-treated anode, which was 3% more power than that
produced using a mesh anode cleaned with acetone (893 mW/
m2; 45 W/m3). This power density with heating was only 7%
less than that achieved with carbon cloth treated by a high
temperature ammonia gas process (988 mW/m2; 49 W/m3). When
the carbon mesh was treated by the ammonia gas process,
power increased to 1015 mW/m2 (51 W/m3). Analysis of the cleaned
or heated surfaces showed these processes decreased
atomic O/C ratio, indicating removal of contaminants that
interfered with charge transfer. Ammonia gas treatment also
increased the atomic N/C ratio, suggesting that this process
producednitrogenrelatedfunctionalgroupsthatfacilitatedelectron
transfer. These results show that low cost heat-treated
carbon mesh materials can be used as the anode in an MFC,
providing good performance and even exceeding performance
of carbon cloth anodes.
Introduction
Microbial fuel cells (MFCs) are devices that produce electrical
energy from organic wastes using exoelectrogenic bacteria
on the anode (1-3). The application of MFCs for wastewater
treatment or bioenergy production requires the use of
inexpensive electrode materials that are electrochemically
and biologically stable, and that have a high specific surface
* Address correspondence to either author. B.E.L. e-mail: blogan@
psu.edu; phone: (1)814-863-7908; Y.F. e-mail: [email protected];
phone: (86)451-86283068.
†
State Key Laboratory of Urban Water Resource and Environment.
‡
Penn State University.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 17, 2009
area and electrical conductivity. Many carbon-based materials have been used as the anode, including carbon paper,
cloth, felt, or foam (4-6); reticulated vitreous carbon (RVC)
(7); graphite sheets, rods, and granules (8-10); and graphite
fiber brushes (5). The best performance of these materials
depends on many factors, including electrode spacing,
solution conductivity, and substrate (4, 11, 12).
Brush electrodes provide a high surface area for bacteria
and high electrode porosities. One limitation of the brush
architecture is that the minimum electrode spacing is
constrained by the brush size, which can lead to larger
electrode distances and thus higher ohmic resistances than
more closely spaced flat electrodes. With flat carbon cloth
electrodes, it has been shown that reducing electrode spacing
from 4 to 2 cm increased power from 720 to 1210 mW/m2
(18 to 60 W/m3) (11). Using brush electrodes treatment with
an ammonia gas treatment further increased power production to 2400 mW/m2 (normalized to the cathode projected
surface area) at a similar (4 cm) electrode spacing (brush
core to cathode) (5). Power production based on liquid
volume was 73 W/m3. By placing two flat electrodes on either
side of a cloth separator (to provide insulation between the
electrodes and to reduce oxygen transfer from the cathode
to the anode), the volumetric power density was increased
to 627 W/m3 by using a very small volume reactor made
possible by the flat electrode architecture (fed-batch mode)
(13).
While high volumetric power densities can be achieved
using flat carbon electrodes and close electrode spacing, the
cost of carbon cloth used in previous tests has been too high
for practical applications. For example, the cost of carbon
cloth commonly used in MFCs costs between $100 and $1000
per m2 (BASF, USA; depending on quantity ordered). Creating
a reactor with the specific surface area used by Fan et al. of
280 m2/m3 at $100/m2 would therefore cost ∼$28,000/m3
(anodes only). The use of brush electrodes for the anode on
the basis of 9600 m2/m3 (16 kg graphite fiber per m3) would
cost only ∼$270/m3, but a disadvantage would be lower power
production on a volumetric basis.
Carbon mesh is a possible alternative material for an MFC
anode. It has a more open structure than cloth electrodes
due to a more open weave, which could help with reducing
biofouling, and it has a low cost of ca. $25 per m2 as purchased
here (Gaojieshi Graphite Products Co. Ltd., Fujian, China;
Figure 1) although we estimate costs could be as low as $10
if bought in bulk from other vendors. We investigated the
performance of this carbon mesh in comparison to the best
performing carbon cloth material treated with a high
temperature ammonia gas process (14). We used membraneless (separatorless) MFCs so that the performance of
the MFC was not affected by other materials. To maintain
low treatment costs of materials, we explored alternative
methods to ammonia gas treatment to improve carbon mesh
performance, and explored reasons for changes in performance by examining the surfaces of these materials using
cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy (XPS).
Materials and Methods
Anode Materials. Carbon mesh was used as received (without
pretreatment; CM) or treated by cleaning (CM-C), heating
in air (CM-H), or heating with an ammonia gas atmosphere
(CM-A). All mesh were cut into circles 4-cm in cross section
area (Figure 1). CM-C mesh was cleaned using acetone
(soaking overnight), and then it was rinsed 5 times in
ultrapure water. CM-H meshes were prepared by heating in
10.1021/es900997w CCC: $40.75
 2009 American Chemical Society
Published on Web 07/17/2009
TABLE 1. Voltage Produced by MFCs with Different Anodes in
MFCs Inoculated with Domestic Wastewater: Ammonia
Treated Carbon Cloth (CC-A), Ammonia Treated Carbon Mesh
(CM-A), Heat-Treated Carbon Mesh (CM-H), Cleaned Carbon
Mesh (CM-C), and Original Carbon Mesh (CM) (1000 Ω
Resistance; Error Bars ± SD Based on the Voltages from
Duplicate Reactors)
Vmax (mV)
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Publication Date (Web): July 17, 2009 | doi: 10.1021/es900997w
FIGURE 1. Carbon mesh (A) and carbon cloth (B) without
bacteria and with bacteria (C, D).
a muffle furnace at 450 °C for 30 min (TGA results provided
in Supporting Information (SI)). CM-A and carbon cloth
(BASF Fuel Cell Inc., Somerset, NJ; CC-A) were treated using
a high-temperature ammonia gas process previously described (700 °C for 60 min in 5% ammonia gas) (14).
MFC Configuration and Operation. Air-cathode cubicshaped MFCs having a cylindrical chamber 7 cm2 in projected
area, with an electrode spacing of 2 cm, were constructed as
previously described (12). Cathodes were made of carbon
cloth (30% wet proofed, BASF, US) containing a Pt catalyst
(0.35 mg/cm2, BASF) on the water-facing side, with four PTFE
diffusion layers and one carbon base layer on the air-facing
side (15).
MFCs tests (duplicate reactors) were inoculated either
with domestic wastewater (Pennsylvania State University
Wastewater Treatment Plant; 50%, v/v) or a preacclimated
suspension of bacteria. Reactors were fed a 50 mM nutrient
buffer solution (NBS; Na2HPO4 4.09 g/L, NaH2PO4 · H2O 2.93
g/L, trace minerals 12.5 mL/L, vitamins 5 mL/L) (16) containing 1 g/L sodium acetate as substrate. The inoculum was
omitted from the solution when the maximum voltage output
was similar for at least two consecutive cycles (1000 Ω fixed
external resistance). The preacclimated inoculum was obtained by using a suspension of bacteria obtained from a
mixture of the reactor effluents that had been operated for
>30 days. All the tests were performed in a 30 °C constant
temperature room.
Electrochemical and Material Analysis. Electrochemical
active surface area of different anode material was estimated
by cyclic voltammetry (CV) using a ferrocyanide solution
(17, 18). A deoxygenated solution of 5 mM K4Fe(CN)6
containing 0.2 M Na2SO4 was placed in the MFC reactor
containing a new plain or treated anode (working electrode)
and a Pt/C cathode (counter electrode) that was sealed on
the air-facing side with a silicon gel (all preparations done
in an anaerobic glovebox). The reactors were then removed
from the glovebox, and CVs were conducted over the range
of -0.2 V to +1.0 V. The peak current, ip (A) and effective area
of the working electrode was obtained using Matsuda’s
equation:
ip ) 0.4464 × 10-3n3/2F3/2A(RT)-1/2D1/2C *Rv1/2
(1)
where n ) 1 is the number of electrons transferred, F )
96487 C/mol is Faraday’s constant, R ) 8.314 J/mol · K is the
gas constant, T ) 303 K is the temperature, and CR* (mol/L)
is the initial ferrocyanide concentration, and v ) 0.05 V/s is
the scan rate. The effective diffusion coefficient of K4Fe(CN)6
was calculated as DR ) 5.79 × 10-6 cm2/s from the value of
cycle
CC-A
CM-A
CM-H
CM-C
CM
3
4
5
6
7
8
9
10
15
20a
5(2
12 ( 3
87 ( 23
300 ( 26
479 ( 30
508 ( 8
553 ( 2
549 ( 3
551 ( 1
-
8(1
12 ( 3
138 ( 38
460 ( 32
487 ( 26
556 ( 6
560 ( 2
562 ( 1
564 ( 3
-
18 ( 2
77 ( 3
109 ( 7
183 ( 17
342 ( 56
495 ( 7
546 ( 5
548 ( 3
545 ( 6
-
7(3
41 ( 1
200 ( 8
457 ( 37
507 ( 2
514 ( 3
528 ( 4
528 ( 2
524 ( 4
-
0
0
0
0
0
1
1
1
245 ( 9
437 ( 3
a
Polarization curves were performed on MFCs with
CC-A, CM-A, CM-H, and CM-C anodes.
ip using eq 1 with a flat sheet of stainless steel as the working
electrode (A ) 7 cm2) (19), which is comparable with that
previously reported (∼8 × 10-6 cm2/s; 35 °C) (20). The
stainless steel was cleaned prior to tests using 0.5 M H2SO4.
Based on these values, the electrochemical active area (cm2)
is simplified to A ) 1.395 × 103 × ip.
The Butler-Volmer equation is:
i ) i0{exp[βnF∆V/(RT)] - exp[-(1 - β)nF∆V/(RT)]}
(2)
where i0 (A) is the exchange current, β is the transfer
coefficient, and ∆V (V) is the voltage change. The last term
in eq 2 describes the cathode, but because we were only
assessing the anode this term was excluded. For the given
conditions, the transfer coefficient of the anode can be
determined from the slope of a V-log i curve using logi )
logi0 + 16.63β∆V. For an example of this process, see Figure
S2B.
Maximum power densities were obtained from polarization curves using a single resistor (1000 Ω to 50 Ω) over a
complete fed batch cycle (∼1 day per cycle). Ohmic resistances were determined from Nyquist plots using electrochemical impedance spectroscopy (EIS) performed at the
open circuit voltage (OCV) over a frequency range of 105 to
0.1 Hz with a sinusoidal perturbation of 10 mV in amplitude
(5).
The composition of the anode materials was examined
using X-ray photoelectron spectroscopy (XPS; PHI model
5600 MultiTechnique) with a monochromated Al KR X-ray
source. Before each analysis, the carbon samples were dried
under vacuum at 80 °C. Spectra obtained over a scan range
of 1350-0 eV were recorded and stored using the PHI ACCESS
data system, and analyzed using CasaXPS software (Version
2.3.12Dev9). All peaks were identified except Auger peaks.
Results
Electricity Generation Using Different Anode Materials.
Approximately 200 h after inoculation with domestic wastewater, stable voltages were produced from MFCs with all
anodes except the untreated carbon mesh (CM). The cleaned
and heat-treated anodes produced voltages (CM-C, 528 ( 4
mV; CM-H, 546 ( 5 mV) only slightly less than those of the
ammonia treated electrode materials (CM-A: 560 ( 2 mV;
CC-A: 553 ( 2 mV) (Figure S3, Table 1). Voltage was eventually
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Publication Date (Web): July 17, 2009 | doi: 10.1021/es900997w
FIGURE 2. (A) Polarization and power density curves, and (B)
electrode potentials (vs Ag/AgC/l electrodes) as a function of
the different anodes for reactors inoculated with domestic
wastewater.
increased for the untreated carbon mesh, but after 400 h the
voltage was still much less than that of the other anodes.
The maximum power density of the MFCs with the
different types of anodes, obtained from polarization data,
show that the heated carbon mesh produced 720 mW/m2
(36 W/m3), which was 15% larger than that of the mesh
chemically cleaned with acetone (624 mW/m2; 31 W/m3)
(Figure 2A). Ammonia treatment of the carbon mesh
produced the highest power density of 801 mW/m2 (40 W/m3),
which is 7% higher than that obtained with the ammonia
treated carbon cloth (749 mW/m2; 37 W/m3). Measurement
of the electrode potentials shows that the differences in power
production were due to performance of the anodes and not
the cathode (Figure 2B). The open circuit potentials (OCP)
of both electrodes were the same independent of the type
of anode (anode: 503 ( 4 mV; cathode: 324 ( 3 mV; Ag/
AgCl).
MFC Performance with Pre-acclimated Inocula. The use
of preacclimated inocula for the MFCs with the different
anodes further increased power densities by 27-43% compared to the reactors inoculated with domestic wastewater.
The overall trend in power generation with material type
was the same as that previously obtained with the wastewater
inoculum, with the maximum power densities decreasing in
the following order: 1015 mW/m2 (51 W/m3; CM-A); 988 mW/
m2 (49 W/m3; CC-A); 922 mW/m2 (46 W/m3; CM-H); and 893
mW/m2 (45 W/m3; CM-C) (Figure 3A). This effect of increased
power through acclimation in MFCs was consistent with that
found by others using different types of MFCs (9, 21).
Coulombic efficiencies (CEs) ranged from 22% to 76%,
and increased in all cases with the current density (Figure
4). This increase in CE with current has been shown in
previous studies (11, 12) and is due in part to the reduction
in the loss of substrate to oxygen diffusing into the reactor
due to a decreased cycle time at higher current densities.
There was no observable trend in CE with the carbon mesh
treatment, consistent with our expectation that the CE would
primarily be a function of the cathode performance and cycle
time (1). These CEs were similar to those obtained in previous
studies using the same reactor configuration and medium
(CE of 30-60%) (14).
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 17, 2009
FIGURE 3. (A) Polarization and power density curves, and (B)
electrode potentials (vs Ag/AgC/l electrodes) as a function of
the different anodes for reactors with preacclimated inocula.
FIGURE 4. Coulombic efficiencies (CEs) of MFCs using difference anode materials (preacclimated inocula).
TABLE 2. Electrochemical Active Area and Transfer
Coefficients of Carbon Meshes Estimated by CV (1, 2 were
Two Paralleled Measurements)
CM-A1
CM-A2
CM-H1
CM-H2
CM-C1
CM-C2
CM-1
CM-2
ip (A)
A
(cm2)
0.0382
0.0391
0.0412
0.0423
0.0145
0.0141
0
0
53
55
57
59
20
20
0
0
average A
(cm2)
54
58
20
0
slope
β
13.14
14.37
8.16
7.13
5.17
5.34
-
0.79
0.86
0.49
0.43
0.31
0.32
-
average
(β)
0.83
0.46
0.32
0
Carbon Mesh Characteristics after Treatment. Several
different methods were used to examine the reasons for the
different characteristics of the carbon mesh. EIS was used to
measure the ohmic resistances of the different reactors, and
36 ( 1 Ω was obtained for all carbon mesh materials. This
shows that there was no difference in architecture or solution
chemistry that could have affected power production by the
different reactors through changing the ohmic resistance.
Thus, the observed differences in the power production were
due to the anode material.
The peak currents obtained from CV varied from 0 to 0.42
A with the K4Fe(CN)6 solution, with electrochemical active
areas that ranged from 0 to 59 cm2 (Figure S2A, Table 2). The
untreated carbon mesh did not have any current response
FIGURE 6. Correlation of the maximum power density with the
charge transfer coefficient for different anode materials as a
function of the two different inocula: green triangle, domestic
wastewater; red dot, preacclimated inocula.
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Discussion
FIGURE 5. Overall elemental analysis of XPS spectra from
different carbon mesh materials (auger peaks were not
marked).
TABLE 3. XPS Atomic Fractions, N/C Ratios, and O/C Ratios of
Carbon Mesh with Different Pretreatments
mesh
C (%)
O (%)
N (%)
N/C (%)
O/C (%)
CM
CM-C
CM-H
CM-A
80.8
84.7
90.7
92.3
17.6
13.2
6.7
3.5
1.7
2.2
2.6
4.2
2.0
2.6
2.9
4.6
21.8
15.5
7.4
3.8
in the CV tests showing that without treatment the surface of this material had little activity toward electron transfer
with the K4Fe(CN)6 solution. Following cleaning with acetone
the carbon mesh active surface area increased to 20 cm2,
and the charge transfer coefficient increased to 0.32 (Figure
S2B, Table 2). The active area and charge transfer coefficients
were further increased to 58 ( 1 cm2 and 0.46 ( 0.03 by
heating. The active surface area measured for the high
temperature ammonia gas treatment of the mesh (54 ( 1
cm2 for CM-A) was similar to the heat treated mesh despite
having a much larger transfer coefficient of 0.83 ( 0.03.
XPS analysis of the different carbon mesh showed the
presence of C (BE ≈ 284.5 eV), N (BE ≈ 400 eV), and O (BE
≈ 531.5 eV) atoms on the surface (Figure 5). A small amount
of Zn (<0.01%) was found on the ammonia treated mesh,
likely due to contamination by the container used for the
ammonia treatment process. The most noticeable change in
the surface composition was the relative oxygen to carbon
ratio that decreased with the different treatments, from 21.8
for the untreated mesh to 3.8 for the ammonia treated mesh
(Table 3), with maximum power densities increasing as a
result of these changes. This indicates that the different
treatment methods reduced the concentration of oxygen
material from the surface, with the ammonia treated materials
having the lowest O/C ratios. A substantial change was noted
in the nitrogen to carbon ratio for the ammonia treated mesh
(N/C ) 4.6%) compared to the other materials (2.0-2.9%).
The ammonia treatment therefore increased the nitrogen
content of the surface by ∼100% compared to the cleaned
electrode, and the maximum power densities increased by
∼28%.
Treated carbon mesh anodes had improved electrochemical
activities as determined by K4Fe(CN)6 oxidation, and produced power densities comparable to those of much more
expensive carbon cloth (Table 2). A simple heat treatment
and inoculation with a preacclimated inoculum produced
922 mW/m2, compared to 720 mW/m2 with a wastewater
inoculum. Previous tests with a plain carbon cloth anode
produced 811 mW/m2 using the same solution in a reactor
with the same configuration and a wastewater inoculum (12).
Ammonia treatment increased the power output, by 10% to
1015 mW/m2 for the carbon mesh, and to 988 mW/m2 for
the carbon cloth. This shows the ammonia gas treatment
works for a variety of materials (including brush anodes)
(5, 14), but it is not clear that the cost of this ammonia
treatment process would be warranted (based on costs and
complexity of the process) when building much larger
reactors.
The success of the different anode treatments was
observed to be correlated to a decrease in the O/C ratio for
the cleaning, heating, and ammonia gas treatments (Table
3). This suggests that there was material on the surface that
could be removed as a result of these different treatments.
The change in the electrochemically active surface area was
important, as the active area increased to 20 cm2 with
cleaning. The active area was further increased to 54-58
cm2 for ammonia treated or the heat treatments, with no
difference in power density that could be attributed to
differences in effective area. While the various treatments
did change the surface area as measured by CV, they had a
more apparent and consistent change in the charge transfer
coefficient as indicated by power production that followed
a saturation kinetics of the form
P ) Pmax β/(Ks + β)
(3)
where Pmax (mW/m2) is the maximum power output and Ks
is the half-saturation constant. This relationship varied for
the two different inocula, with the Pmax ) 967 mW/m2 for the
wastewater inoculum, and Pmax ) 1102 mW/m2 for the
acclimated inoculum (Figure 6). The correlation of power
with the O/C ratio and the charge transfer coefficient suggests
these are useful parameters for evaluation other carbon
electrode materials.
The increase in power generation achieved with ammonia
gas treatment, compared to heating, was primarily associated
with an increase in the N/C ratio as shown by XPS analysis
(Table 2). Previous tests with ammonia treated carbon cloth
showed that this process also increased surface charge (14),
making bacterial adhesion more favorable to the surface.
While adhesion could explain a more rapid acclimation time,
it does not directly explain the 10% increase in power
densities. Others have reported that amine groups, including
C-N and CdN, were detected after high-temperature amVOL. 43, NO. 17, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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monia treatment was performed on active carbon by Fourier
transform infrared (FTIR) spectroscopy (22, 23). FTIR analysis
of the carbon mesh and cloth used here, however, was
inconclusive relative to the presence of nitrogen bonds (data
not shown). Thus, it seems likely that amine groups were
produced on the surface that facilitated electron transfer
from bacteria to electrodes (24).
Future Use of Carbon Mesh Anodes in MFCs. The low
cost and good performance of the carbon mesh could allow
for close placement of the anodes next to the cathodes. Closely
placed carbon cloth electrode spacing, coupled with the use
of a cloth separator, has resulted in very high volumetric
power densities. While the carbon mesh could replace the
carbon cloth as the anode, it does not appear possible at this
time to use the carbon mesh as a cathode. Preliminary
experiments in our laboratory showed that the carbon mesh
did not perform well when coated with several layers of PTFE
(unpublished results). The use of a specialized membrane or
separator is needed to prevent water leakage from flat
electrodes, or alternatively cathode tubes can be used (25).
An optimized cathode structure remains a need for reducing
the costs of MFCs while maintaining or increasing power
production (26).
Acknowledgments
We thank Tad Daniel and Josh Stapleton from MRI for their
help on XPS and other surface measurements. This research
was supported by Award KUS-I1-003-13 from the King
Abdullah University of Science and Technology (KAUST),
the U.S. National Science Foundation (CBET-0730359),
National Science Foundation of China (50638020), the
National Creative Research Groups of China (50821002), and
a scholarship from the China Scholarship Council (CSC).
Supporting Information Available
Additional text and graphics. This material is available free
of charge via the Internet at http://pubs.acs.org.
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