Article
pubs.acs.org/est
Enhanced Activated Carbon Cathode Performance for Microbial Fuel
Cell by Blending Carbon Black
Xiaoyuan Zhang,† Xue Xia,‡ Ivan Ivanov,† Xia Huang,‡ and Bruce E. Logan*,†
†
Department of Civil & Environmental Engineering, Penn State University, 231Q Sackett Building, University Park,
Pennsylvania 16802, United States
‡
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University,
Beijing 100084, P. R. China
S Supporting Information
*
ABSTRACT: Activated carbon (AC) is a useful and environmentally sustainable
catalyst for oxygen reduction in air-cathode microbial fuel cells (MFCs), but there is
great interest in improving its performance and longevity. To enhance the performance of AC cathodes, carbon black (CB) was added into AC at CB:AC ratios of
0, 2, 5, 10, and 15 wt % to increase electrical conductivity and facilitate electron
transfer. AC cathodes were then evaluated in both MFCs and electrochemical cells
and compared to reactors with cathodes made with Pt. Maximum power densities of
MFCs were increased by 9−16% with CB compared to the plain AC in the first week.
The optimal CB:AC ratio was 10% based on both MFC polarization tests and three
electrode electrochemical tests. The maximum power density of the 10% CB cathode was initially 1560 ± 40 mW/m2 and
decreased by only 7% after 5 months of operation compared to a 61% decrease for the control (Pt catalyst, 570 ± 30 mW/m2
after 5 months). The catalytic activities of Pt and AC (plain or with 10% CB) were further examined in rotating disk electrode
(RDE) tests that minimized mass transfer limitations. The RDE tests showed that the limiting current of the AC with 10% CB
was improved by up to 21% primarily due to a decrease in charge transfer resistance (25%). These results show that blending CB
in AC is a simple and effective strategy to enhance AC cathode performance in MFCs and that further improvement in
performance could be obtained by reducing mass transfer limitations.
■
INTRODUCTION
Microbial fuel cells (MFCs) are being explored as a technology
for energy recovery and wastewater treatment based on
electricity generation from wastewater organics using exoelectrogenic bacteria.1−3 Air cathodes are used in MFCs to produce
high power from readily available oxygen in air, without the
need for wastewater aeration.4,5 Catalysts are needed to reduce
the overpotential for oxygen reduction, and Pt is commonly
used in lab-scale reactors. However, Pt is very expensive and a
precious metal, and its catalytic performance can significantly
decrease over time due to chemical and biological fouling.6,7
Various alternatives to Pt have been proposed that can
primarily be separated into three types.8 The first is transition
metal macrocyclic compounds, such as cobalt tetramethylphenylporphyrin (CoTMPP),9 Co-naphthalocyanine (CoNPc),10
and iron(II) phthalocyanine (FePc).11 The second is non-noble
metallic oxides, such as manganese dioxides (MnO2)12,13 and
lead dioxide (PbO2).14 However, these two types of alternative
catalysts have cost or performance limitations. The third and
most promising alternative is activated carbon (AC)-based
catalysts due to their low cost and good performance.8,15−20
AC cathodes are typically constructed using certain types
of carbon-based materials that have good catalytic activity,
but the electrical conductivity of the AC is poor compared to
that of carbon cloth. Therefore, the AC is usually pressed onto
a stainless steel mesh that provides structural support and
© 2014 American Chemical Society
functions as a current collector. Poly(tetrafluoroethylene)
(PTFE) is normally used as a binder, as opposed to Nafion
with Pt, which greatly reduces the cost of the cathode materials.
AC cathodes can be made with a PTFE binder by batch18,20 or
continuous (rolling) cold-pressing of the AC onto the stainless
steel mesh.16,17,21 The mass of AC used per area of cathode can
affect performance, with an optimal performance determined in
one study of 0.43 kg/m2 (tested range of 0.07 to 1.71 kg/m2).20
AC cathodes can produce power densities similar to those of
Pt in MFCs, but there is still considerable interest in further
improving cathode performance and longevity. Selection of
the material used to make the AC is important for power
production. Peat-based and coal-based carbons have been
shown to produce higher power densities than coconut shellbased, hardwood-based, and phenolic resin-based carbons.18
The AC can be chemically modified to improve performance as
well. Pyrolysis of AC with iron ethylenediaminetetraacetic acid
(FeEDTA) was shown to improve maximum power densities
in MFCs by 10% compared to plain AC.6 High temperature
ammonia gas treatment can also improve AC performance,
but the procedure is energy-intensive.22 Pt cathodes have been
Received:
Revised:
Accepted:
Published:
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November 12, 2013
January 12, 2014
January 15, 2014
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shown to be reduced in performance by ∼20% to >50% over
one year, depending on what substrate is used.23 In one study,
the performance of the AC was reduced by ∼20−40% after one
year (same time frame as Pt).24 MFC performance is mainly
limited by oxygen reduction kinetics at the cathode.25,26 While
it is important to improve cathode performance and longevity,
such improvements must be made using methods that do not substantially increase cathode materials and manufacturing costs.
In order to improve cathode performance we examined
the addition of carbon black (CB) particles with AC during
cathode construction. While CB has poor performance as an
oxygen reduction catalyst,15,27 it primarily is used in Pt cathodes
as a catalyst support due to its high electrical conductivity. CB
powder is relatively inexpensive and can easily be incorporated
into the AC cathode due to its much smaller particle size
compared to most AC particles. Catalyst materials for oxygen
reduction should have both high catalytic activity and conductivity. We hypothesized that adding CB into AC would
facilitate electron transfer and thus would improve overall
catalytic performance. To determine the impact of CB on
cathode performance, different ratios of CB were added to AC.
The performance of these cathodes was examined in both
electrochemical tests and in MFCs over time.
other chamber facing the air as a working electrode, and a
Ag/AgCl reference electrode (3 M KCl, +0.21 V versus a
standard hydrogen electrode; RE-5B; BASi, West Lafayette,
USA) was placed close to the cathode. A 50 mM phosphate
buffer solution (PBS) was used as medium, which contained
4.57 g/L Na2HPO4, 2.45 g/L NaH2PO4·H2O, 0.31 g/L NH4Cl,
and 0.13g/L KCl. Chronoamperometry tests were carried out
by setting a potential in a stepwise manner after the reactor
operated in open circuit condition for 3 h. Each potential
(0.2, 0.1, 0, −0.1, and −0.2 V versus Ag/AgCl) was applied for
2 h.
A rotating disc electrode (RDE) can be used to reduce mass
transfer limitations to catalysts, allowing measurement of the
limiting current due to reaction kinetics. RDE tests were
therefore conducted to evaluate the catalyst activity of the AC
materials compared to that of Pt. Catalyst ink was prepared
by adding 40 mg of the powdered sample (10% Pt on Vulcan
XC-72, plain AC powder, or AC powder with 10 wt % CB) to
0.8 mL of isopropyl alcohol, followed by ultrasonication for
15 min. Nafion (5 wt % solution, 0.2 mL) was added to the
suspensions, with ultrasonication for an additional 15 min.
The ink suspension (40 μL) was drop coated onto a 1.3-cm
diameter graphite carbon disk and allowed to dry in the air
overnight, leading to a sample loading of 1.2 mg/cm2. The
catalyst ink samples were used for kinetic studies and electrochemical impedance spectroscopy (EIS) analysis. To better
simulate catalyst conditions in MFCs, small cathodes (1.3-cm
diameter) were fabricated following the same procedure
described above, but without the diffusion layer, allowing the
catalyst layer to direct face the solution during RDE tests. The
effective projected area during the tests was 0.283 cm2 (0.6 cm
in diameter) for both two types of samples.
RDE tests were carried out using a modulated speed rotator
(MSR rotator, PINE Instruments, USA) and a medium of 50 mM
PBS sparged with pure oxygen. Before the tests, solutions were
sparged for at least 30 min with oxygen, and cyclic voltammetry
(CV) was run at 20 mV/s between 0.3 to −0.8 V vs Ag/AgCl
until the current response was the same from cycle to cycle. The
reproducibility demonstrated an absence of contaminants on
the electrode surface or excess oxygen trapped in the pores of the
sample.18 Chronoamperometry tests were then run by setting a
potential in a stepwise manner (from 0 to −0.6 V, with 30 min at
each potential), at rotation rates between 100 and 2500 rpm. The
average number of electrons transferred (n) and kinetic current
(iK) for the oxygen reduction reaction were calculated based on
the Koutecky−Levich analysis using29
■
MATERIALS AND METHODS
Air-Cathode Material and Fabrication. AC (Norit SX
plus, Norit Americas Inc., USA) was selected on the basis of its
better performance compared to other types of readily available
carbons18 and applied at a constant optimal loading of 0.43 kg/m2
(300 mg for each 7 cm2 cathode). CB powder (Vulcan XC-72,
Cabot Corporation, USA) was added into AC, and the content
was varied at weight ratios of CB:AC = 0% (pure AC control),
2%, 5%, 10%, and 15%. Cathodes were fabricated using a batch,
cold-press process.6,20 The AC (300 mg) and CB (0−45 mg)
powders were added into a vial and mixed by vortexing for
0.5 min. The binder solution was prepared by adding 37.8 μL of
60% PTFE and 700 μL of DI water into a beaker, followed by
ultrasonication for 1 min. Then the AC and CB powders were
transferred into the beaker with the binder and mixed in a blender
for 0.5 min to form a paste. The paste was then ultrasonicated
for 1 min and spread using a spoon onto one side of a stainless
steel mesh (50 × 50, type 304, McMaster-Carr, USA) that served
as the support material and current collector. Diffusion layers
were prepared by applying two layers of poly(dimethylsiloxane)
(PDMS) solution28 onto a textile material (Amplitude Prozorb,
Contec Inc., USA), with the textile placed onto the cathode side
facing the AC. Then, the cathode and the textile were pressed
together at 40 MPa for 20 min (Model 4386, Carver Inc., USA)
and dried at 80 °C in an oven overnight before use.6 Pt cathodes
were also prepared to benchmark performance compared to the
AC cathodes by applying a Pt catalyst layer (5 mg/cm2 10% Pt
on Vulcan XC-72 and Nafion binder) on one side of a stainless
steel mesh and two diffusion layers of PDMS on the other side of
the mesh.28
Electrochemical Analysis. Electrochemical tests were conducted using a potentiostat (VMP3Multichannel Workstation,
BioLogic Science Instruments, USA). Cathodes were examined
in an abiotic and electrochemical reactor that was cubic shaped,
with two 2-cm cylindrical chambers bolted together with an
anion exchange membrane (AEM; AMI-7001, Membrane
International Inc., USA) in the middle.5 A high purity platinum
plate (99.99%, 1 cm2) was placed in the middle of one chamber
as a counter electrode, a cathode was placed on one side of the
⎛
⎞ −1/2
1
1
1
⎟ω
=
+⎜
2/3
1/6
−
⎝ 0.620nFAD v
i
iK
C⎠
(1)
where i is the measured current, F Faraday’s constant, A the
effective projected area of the disk electrode (0.283 cm2), D
the diffusion coefficient of oxygen (2.7 × 10−5 cm2/s), v is the
kinematic viscosity (8.08 × 10−3 cm2/s), C is the concentration
of oxygen in the solution (2.3 × 10−7 mol/cm3), and ω is the
rotation rate of the electrode. The average number of electrons
transferred (n) for Pt, AC, and AC with 10% CB was calculated
using these constants (D, v, and C) based on typical values of
seawater with a solution conductivity of 7.5 mS/cm, which was
similar to that of 50 mM PBS.
EIS was carried out for the catalyst ink at the highest rotation
rate of 2500 rpm, when the mass transfer limitations were significantly reduced. Impedance measurements were conducted at
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−0.1 V vs Ag/AgCl over a frequency range of 10,000 to
0.002 Hz with a sinusoidal perturbation of 10 mV amplitude. EIS
data were analyzed by fitting the spectra to an equivalent circuit
model (Supplementary Figure S1).20
MFC Experiments. Cubic-shape single-chamber MFCs
were constructed using a 4-cm length Lexan block with a 3-cm
diameter inner cylindrical chamber, as previously described.30 The
anode was a graphite fiber brush (2.5 cm in both diameter and
length) with a core of two twisted titanium wires that functioned
as a current collector. The anode was heat treated in a muffle
furnace at 450 °C for 30 min and then placed horizontally in the
middle of the cylindrical chamber. The cathode was placed at the
other side of the reactor, with the diffusion layers facing the air.
All MFCs were inoculated with the effluent of MFCs that
were operated for over 1 year. The medium was 1 g/L sodium
acetate in 50 mM PBS amended with 12.5 mL/L minerals and
5 mL/L vitamins.31 All MFCs were operated in batch mode with
a 1000 Ω external resistor (except as noted) in a 30 ± 1 °C
room. Initially, a standard Pt/C carbon cloth cathode4 was
used for all the MFCs during start-up to ensure that the anodes
achieved the same performance during acclimation. After
1 week, a repeatable cycle of voltage was produced by the
MFCs. To ensure all the anode biofilms were fully acclimated,
the MFCs were operated for 1 month, and then the Pt/C
cathodes were removed and replaced with the new AC based
cathodes or new Pt/C stainless steel cathodes. All MFC tests
were conducted in duplicate.
Voltages (U) were recorded across an external resistance (R)
every 20 min using a multimeter with a computerized data
acquisition system (2700, Keithley Instrument, USA). Polarization curves were obtained using a multicycle method, by
applying different external resistors, with each resistance used
for a complete cycle.32 Polarization tests were conducted after
the MFCs had been operated after 1 week, 3.5 months, and
5 months, by varying the external resistances from 1000 to 20 Ω.
Current densities (J) and power densities (P) were normalized
by air-cathode projected area (A = 7 cm2), using J = U/RA and
P = JU.32 Anode potentials were reported versus an Ag/AgCl
reference electrode (+0.21 V versus a standard hydrogen
electrode). Coulombic efficiencies (CEs) were calculated at each
external resistance as previously described.32
Figure 1. Current−potential curves of different cathodes in electrochemical cells.
MFC Performance and Durability. Power generation was
enhanced in MFCs by adding CB into the AC, with the same
optimum ratio of 10% as that obtained in abiotic tests. After
1 week of operation, the MFCs with 10% ratio cathodes
produced the highest power density of 1560 ± 40 mW/m2,
followed by the 5% (1510 ± 10 mW/m2), 15% (1510 ±
10 mW/m2), and 2% ratio cathodes (1460 ± 10 mW/m2)
(Figure 2A). The power density of the MFCs with the 10%
ratio cathode was 16% higher than those with the plain AC
cathodes (1340 ± 120 mW/m2) and 7% higher than MFCs
with a Pt cathode (1460 ± 10 mW/m2). Anode potentials were
essentially the same at the same current densities in all MFCs,
indicating that the cathode potentials were responsible for the
differences in power generation (Figure 2B).
After 3.5 months of operation, the MFCs with 10% CB still
produced the highest maximum power density of 1500 ±
210 mW/m2, which was 29% higher than the plain AC
cathodes (1160 ± 120 mW/m2). The cathodes with the 5% and
15% ratios still produced similar maximum power densities of
1340 ± 40 mW/m2 (5% ratio) and 1330 ± 40 mW/m2 (15%
ratio). The MFCs with the Pt cathodes had greatly reduced
power production, with a 55% reduction in maximum power to
650 ± 10 mW/m2. This large decrease with Pt was consistent
with previous reports6 as further discussed below. The AC with
a 10% CB:AC ratio had the lowest decrease in performance
of 4% in the maximum power densities, compared to a 13%
decrease for plain AC. The CEs of MFCs with different cathodes
increased with the current densities (Figure 3), also consistent
with previous reports.33 The MFCs with 10% CB achieved the
highest CE of 74% at the current density of 9.9 A/m2 (Figure 3).
After 5 months, the MFCs with the 10% ratio continued to
produce the highest power density of 1450 ± 10 mW/m2,
which was now 150% higher than Pt (570 ± 30 mW/m2)
and 14% higher than plain AC (1270 ± 80 mW/m2).
The maximum power densities for this 10% ratio cathode
were decreased by only 7% compared to the first week,
demonstrating that AC based cathodes sustained high power
generation. The slight decrease of AC cathode performance
over time might be due to the clogging of micropores in
the AC.24 There could also be small amounts of loss of the
AC from the cathode support, but this might not affect the
overall performance since the loading of AC was relatively high
(86 times that of Pt).20 In contrast, maximum power of the
Pt cathode decreased by 61% compared to that after 1 week.
This decrease was likely due to biofouling and Pt catalyst
losses (Supplementary Figure S2). When carbon cloth (fuel cell
grade) was used as the current collector with the Pt catalyst, the
durability was similar to that of the AC cathodes,20 but the cost
■
RESULTS AND DISCUSSION
Electrochemical Performance of Different Cathodes.
Chronoamperometry tests on the abiotic cathodes in the
electrochemical reactor showed that the cathode performance
was enhanced by blending CB with AC, with an optimal ratio of
10% (Figure 1). This cathode had an open circuit potential
(OCP) of 0.25 V, which was higher than the one without CB
(0.20 V) or the other ratios (2%, 0.22 V; 5%, 0.23 V; 15%,
0.23 V). The Pt cathode produced the highest OCP of 0.32 V.
The cathode with the 10% CB ratio generated the highest
current density of 8.7 A/m2 at −0.2 V, which was 23% higher
than that produced without CB (7.1 A/m2) and 12% higher
than Pt (7.8 A/m2). All cathodes with CB showed improved
performance compared to plain AC. The current densities at
−0.2 V were 8.0−8.7 A/m2, increasing with the CB:AC ratio
from 2% to 10%, and were enhanced by 13% to 23% compared
to plain AC (7.1 A/m2). Adding more CB (15%) produced a
current density of 8.5 A/m2, which was 20% higher than that of
plain AC but was not more than that obtained with 10% CB
(Figure 1).
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Figure 2. (A−C) Power densities and (D−F) electrode potentials as a function of current density in MFCs using different cathodes after 1 week,
3.5 months, and 5 months of operation.
plain AC (0.28 mA). The Pt/C ink generated the highest
limiting current of 0.40 mA (Figure 4E).
The kinetic current of the AC with 10% CB, based on the
K-L analysis, was 2.39 mA compared to 1.07 mA for plain AC
and 8.39 mA for Pt/C (Figure 4F). The average number of
electron transferred was 3.2 for Pt and was 2.9 for the 10% CB,
compared to 2.8 for the plain AC. These numbers for Pt are
much lower than those previously reported for Pt-catalyzed
electrodes (n = 4),18 suggesting that the constants chosen for
eq 1 might not be adequate for the solution conditions used
here.
EIS tests were conducted for the catalyst inks at the highest
rotation rate of 2500 rpm. The use of 10% CB decreased charge
transfer resistances by 25% to 576 Ω compared to plain AC
(766 Ω) (Figure 5 and Supplementary Table S1). This reduction in charge transfer resistance therefore demonstrated our
hypothesis that adding CB would facilitate electron transfer and
enhance the overall catalytic performance of AC based catalysts.
The Pt/C had the highest performance, with 237 Ω of ohmic
resistance and 315 Ω of charge transfer resistance (Figure 5 and
Supplementary Table S1).
To simulate cathode conditions more similar to that in MFCs
with the same catalyst and binder loading and the same current
collector, but with controlled mass transfer, RDE samples were
also prepared following the above mentioned fabrication procedure
Figure 3. Coulombic efficiency as a function of current density in
MFCs using different cathodes after 3.5 months of operation.
of carbon cloth (fuel cell grade, $500−$600/m2, www.
fuelcellearth.com) is much higher than that of stainless steel
mesh ($10−$30/m2, www.alibaba.com).
Catalyst Activity and Kinetics. Catalyst performance
was further evaluated using RDE with two different methods
of sample preparation. In the RDE tests using catalyst ink, the
limiting current of AC was improved with 10% CB at all rotation
rates (Figure 4). At the highest rotation rate (2500 rpm) the
mixture with 10% CB was 0.34 mA, 21% higher than that of
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Figure 4. Current−potential curves of catalyst inks of Pt/C (A), plain AC (B), and AC with 10% CB (C) in the RDE tests. Current comparison of
different catalyst inks at rotation rates of 100 rpm (D) and 2500 rpm (E). Koutecky−Levich plots for oxygen reduction on different catalyst inks (F).
but without diffusion layers. Blending 10% CB into AC enhanced
the current generation among all rotation rates (Figure 6). At
the lowest rotation rate of 100 rpm, the Pt/C produced current
densities similar to those with plain AC and AC with 10% CB
(Figure 6). At the higher rotation rates, Pt/C showed higher
current production at potentials that were more negative than
−0.3 V. For example, at 900 rpm, the current was 1.42 mA for
Pt/C at −0.6 V, which was much higher than that of plain
AC (0.90 mA) or AC with CB (0.96 mA) (Figure 6). However,
in the MFCs, the maximum power densities were normally
obtained among the cathode potential range between −0.05 to
−0.15 V (around −0.1 V in most cases) (Figure 2). At
−0.1 V, at the highest rotation rate of 2500 rpm, the current
for Pt/C was 0.58 mA, compared to 0.57 mA for plain AC, and
0.63 mA with 10% CB. This trend showing slightly better
performance of the 10% CB loading was similar to results
obtained on the basis of polarization tests in MFCs. Taken
together, these results demonstrated that blending 10% CB
into AC was a simple and effective strategy to improve the AC
cathode performance.
Implications of Catalyst Activities for MFC Performance. By blending 10% CB into AC, the catalyst activity was
improved and the maximum power densities of MFCs with
10% CB cathodes were increased by 16% in the first week and
by 29% after 3.5 months, compared to those of the plain AC.
The cathodes with the 10% CB also showed much better
longevity compared to the Pt/C cathodes. The cost of CB is
Figure 5. (A) Nyquist plots using catalyst ink consisting of Pt/C, plain
AC, or AC with 10% CB at a rotation rate of 2500 rpm in RDE tests.
(B) Nyquist plots with Z′ axis shifted to omit ohmic resistance for
comparison of charge transfer resistances.
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Figure 6. Current−potential curves of the cathodes without diffusion layers using Pt/C (A), plain AC (B), and AC with 10% CB (C) as catalysts in
the RDE tests. Current comparison of different samples at rotation rates of 100 rpm (D), 900 rpm (E), and 2500 rpm (F).
■
only ∼$1 per kg (www.alibaba.com). The AC loading in this
study is 0.43 kg/m2, and therefore the cost for a 10% CB
loading (0.043 kg/m2) is negligible (∼$0.043/m2) compared to
other cathode costs. Recently, AC modified by pyrolyzed with
iron ethylenediaminetetraacetic acid (FeEDTA), at a weight
ratio of FeEDTA:AC = 0.2:1, also showed an enhanced catalyst
activity in MFC systems, producing 10% higher maximum
power than that of plain AC in the first week.6 The performance
and durability of these different approaches and the factors
determining the lifetime of the catalysts will therefore need to
be considered in future studies. Binders, diffusion layers, and
fabrication methods can also affect AC based cathode
performances, so there will need to additional considerations
given to these components. At this point, peat-based AC
(0.43 kg/m2) blended with 10% CB appears to be the most
cost-effective catalyst mixture for making MFC air cathodes
using cold-pressing procedures.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: (1) 814-863-7908. Fax: (1)814-863-7304. E-mail:
[email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
REFERENCES
The authors thank Valerie Watson and Marta Hatzell for the
help with RDE and EIS analyses and David Jones for laboratory
support. This research was supported by the Strategic Environmental Research and Development Program (SERDP) and
Award KUS-I1-003-13 from the King Abdullah University of
Science and Technology (KAUST). The authors thank the
anonymous reviewers for their instructive comments.
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