Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (nafion and PTFE) in single chamber microbial fuel cells.

Abstract

Cathode catalysts and binders were examined for their effect on power densities in single chamber, air-cathode, microbial fuel cells (MFCs). Chronopotentiometry tests indicated thatthe cathode potential was only slightly reduced (20-40 mV) when Pt loadings were decreased from 2 to 0.1 mg cm(-2), and that Nafion performed better as a Pt binder than poly(tetrafluoroethylene) (PTFE). Replacing the precious-metal Pt catalyst (0.5 mg cm(-2); Nafion binder) with a cobalt material (cobalt tetramethylphenylporphyrin, CoTMPP) produced slightly improved cathode performance above 0.6 mA cm(-2), but reduced performance (<40 mV) at lower current densities. MFC fed batch tests conducted for 35 cycles (31 days) using glucose showed that replacement of the Nafion binder used for the cathode catalyst (0.5 mg of Pt cm(-2)) with PTFE reduced the maximum power densities (from 400 +/- 10 to 480 +/- 20 mW m(-2) to 331 +/- 3 to 360 +/- 10 mW m(-2)). When the Pt loading on cathode was reduced to 0.1 mg cm(-2), the maximum power density of MFC was reduced on average by 19% (379 +/- 5 to 301 +/- 15 mW m(-2); Nafion binder). Power densities with CoTMPP were only 12% (369 +/- 8 mW m(-2)) lower over 25 cycles than those obtained with Pt (0.5 mg cm(-2); Nafion binder). Power densities obtained using with catalysts on the cathodes were approximately 4 times more than those obtained using a plain carbon electrode. These results demonstrate that cathodes used in MFCs can contain very little Pt, and that the Pt can even be replaced with a non-precious metal catalyst such as a CoTMPP with only slightly reduced performance.

Full text

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Power Densities Using Different
Cathode Catalysts (Pt and CoTMPP)
and Polymer Binders (Nafion and
PTFE) in Single Chamber Microbial
Fuel Cells
S H A O A N C H E N G ,†H O N G L I U ,†A N D
B R U C E E . L O G A N *, † , ‡
Department of Civil and Environmental Engineering and The
Penn State Hydrogen Energy (H2E) Center, The Pennsylvania
State University, University Park, Pennsylvania 16802
Cathode catalysts and binders were examined for their
effect on power densities in single chamber, air-cathode,
microbial fuel cells (MFCs). Chronopotentiometry tests
indicatedthatthecathodepotentialwasonlyslightlyreduced
(20-40 mV) when Pt loadings were decreased from 2 to
0.1 mg cm-2, and that Nafion performed better as a Pt binder
thanpoly(tetrafluoroethylene)(PTFE).Replacingtheprecious-
metal Pt catalyst (0.5 mg cm-2; Nafion binder) with a
cobaltmaterial(cobalttetramethylphenylporphyrin,CoTMPP)
produced slightly improved cathode performance above
0.6 mA cm-2, but reduced performance (<40 mV) at lower
current densities. MFC fed batch tests conducted for 35
cycles (31 days) using glucose showed that replacement of
the Nafion binder used for the cathode catalyst (0.5 mg
ofPtcm-2)withPTFEreducedthemaximumpowerdensities
(from 400 ( 10 to 480 ( 20 mW m-2to 331 ( 3 to 360
(10mWm-2).WhenthePtloadingoncathodewasreduced
to 0.1 mg cm-2, the maximum power density of MFC
was reduced on average by 19% (379 ( 5 to 301 ( 15
mWm-2;Nafionbinder).PowerdensitieswithCoTMPPwere
only 12% (369 ( 8 mW m-2) lower over 25 cycles than
those obtained with Pt (0.5 mg cm-2; Nafion binder). Power
densities obtained using with catalysts on the cathodes
were∼4timesmorethanthoseobtainedusingaplaincarbon
electrode. These results demonstrate that cathodes used
in MFCs can contain very little Pt, and that the Pt can even
be replaced with a non-precious metal catalyst such as
a CoTMPP with only slightly reduced performance.
Introduction
A microbial fuel cell (MFC) is a device that uses bacteria to
catalyzetheconversionoforganicmatterintoelectricity(1-
9).Substrateisoxidizedbybacteriageneratingelectronsand
protons at the anode. Electrons are transferred through an
externalcircuitwhiletheprotonsdiffusethroughthesolution
to the cathode, where electrons combine with protons and
oxygen to form water. It is now known that no exogenous
mediatorsneedtobeaddedintoanMFC(10,11).Ithasbeen
found that several microorganisms including Shewanella
putrefaciens (4, 12, 13), Geobacteraceae (5, 14-17) Clostrid-
ium butyricum (18), and Rhodoferax ferrireducens (19) can
produce electricity in the absence of exogenous mediators
from chemicals such as glucose, acetate, lactate, pyruvate,
and formate. Mixed cultures of bacteria have been also
reported to generate electricity from domestic wastewater
(6-8, 20) and marine sediments (4, 15).
The performance of an MFC is influenced by several
factors including the microbial activity, chemical substrate
(fuel), type of proton exchange material (or even absence of
thismaterial),resistanceofthecircuit,andanodeandcathode
materials (4-6, 8, 9, 19, 21-23). The cathode performance
is an important factor to the performance of an MFC due to
the poor kinetics of oxygen reduction in the medium (6, 24).
CathodeperformancecanbeimprovedifPtcatalystscanbe
made more effective at room temperature, if the internal
resistance of the reactor is reduced, or if more effective
oxidants than oxygen (such as ferricyanide) are used. For
example, power densities of MFCs have been increased by
replacing aqueous cathodes with either direct-air carbon
cathodes containing Pt (8, 24) or graphite electrodes con-
tainingFe3+(22).Ohetal.(9)foundthatthemaximumpower
achieved using ferricyanide ion as oxidant in the cathode
chamber was 50-80% greater than that obtained with
dissolved oxygen and Pt. Power densities as large as 6000
mW m-2have been reported for MFCs using ferricyanide
(23). However, ferrocyanide must be replaced after it is
reduced, while systems using oxygen can be continuously
operated and therefore self-sustaining.
MFCs produce lower power densities than other types of
fuel cells, but their most promising application in the near
future is likely to be as a process for wastewater treatment
(7,25).ReducingthecostofthematerialsusedtomakeMFCs
is essential for building an economical treatment system.
The typical manufactured components of an MFC are a
proton exchange membrane (PEM), Pt catalyst on the
cathode,andcarbonelectrodes.However,recentstudieshave
shown that the proton exchange membrane is not needed
fortheoperationofMFCs(8,14)andthatremovingitactually
increases maximum power densities (8). Pt is an effective
catalyst used for both electrodes in hydrogen fuel cells, but
it is an expensive component of the MFC cathode. While
alternatives to Pt have been sought, none have approached
the performance of Pt in hydrogen fuel cells (26). Pt is used
on the cathode in air-cathode MFCs, so minimizing or
eliminating the need for Pt can reduce the system capital
costs.InmostMFCstudiescommerciallyproducedcathode
electrodes are used that contain 0.5 mg cm-2Pt loading (8),
althoughgraphiteelectrodescontaining0.28mg/cm2Pthave
also been used (24).
TheeffectofdifferentPtloadingshasnotbeenpreviously
examined for MFCs, and few alternatives to Pt have been
exploredforuseinair-cathodeMFCs.TheeffectofPtloading
on power generation has been examined for other types of
fuel cells, but these systems operate under much different
conditions of pH (highly acid or alkaline conditions; refs
27-29) and temperature (50-1000 °C), and therefore the
results are not directly translatable to the performance of
MFCs.MaximumpowerdensitiesinaqueouscathodeMFCs
can be reduced by an order of magnitude when Pt is not
used on the cathode (30). When Pt is used as a catalyst on
a carbon electrode, it is usually bound to the electrode
substrate using a polymer. Perfluorosulfonic acid (Nafion)
andpoly(tetrafluoroethylene)(PTFE)aretwocommonlyused
binders for Pt in chemical fuel cells (31-33), but Nafion can
cost 500 times more than PTFE (mass basis). Nafion is a
* Corresponding author phone: (814) 863-7908; fax: (814) 863-
7304; e-mail: blogan@psu.edu.
†Department of Civil and Environmental Engineering.
‡The Penn State Hydrogen Energy (H2E) Center.
Environ. Sci. Technol. 2006, 40, 364-369
3649ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 200610.1021/es0512071 CCC: $33.50
 2006 American Chemical Society
Published on Web 11/23/2005

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proton conductive polymer that has hydrophilic ionic
clusters, a hydrophobic base, and a transition region that
allows for effective proton transfer to the catalyst. PTFE is
ahighlyhydrophobicmaterialthatisusefulinlimitingwater
leakagethroughthecathodeinanMFCanditiswell-known
to have good oxygen transfer properties, but unlike Nafion
it is not an electrolyte. In addition, because PTFE is highly
hydrophobic,itcancausetheenvironmentofthecatalystto
become too dry, limiting effective proton transfer to the
catalyst. The effect of different binders on the performance
of air-cathode MFCs has not been previously addressed.
In this study, we examined the effect of Pt loading (0.1-2
mgcm-2)usingtwodifferentbindersontheelectrochemical
performance of the cathodes in a bacteria-free electrochemi-
cal cell using chronopotentiometery. We then evaluated the
performance of cathodes having a fixed Pt content, and two
differentbinders,overmanyfeedingcyclesinfedbatchMFC
systems in terms of power densities and Coulombic ef-
ficiencies. We also compared the performance of the Pt-
containingcathodestothatofaninexpensivetransitionmetal
basedcatalyst(cobalttetramethylphenylporphyrin,CoTMPP)
using chronopotentiometery and in fed batch MFC tests.
Materials and Methods
Electrodes. The cathode electrodes used in electrochemical
celltestshadaprojectedsurfaceareaof0.64cm2,whileboth
electrodes used in MFC had a projected surface area of 7
cm2. Anodes were made of non-wet-proofed carbon cloth
(type A, E-TEK). For Pt-containing cathodes, a commercial
Pt catalyst (10 wt % Pt/C, E-TEK) was mixed with a chemical
binder (5% Nafion solution or 2% PTFE suspension) to form
a paste (7 µL of binder per mg of Pt/C catalyst). The paste
was applied to one side of the wet-proofed carbon cloth (30
wt %, type B, E-TEK), and dried at room temperature for 24
h (Nafion) or at 350 °C for 0.5 h (PTFE). The Pt content was
varied in the range of 0.1-2 mg cm-2by changing the mass
of Pt catalyst used in the paste. Pt loadings less than 0.1 mg
cm-2were not tested as it was not possible using our
application method to evenly apply the paste onto the
cathode at lower Pt loadings.
CoTMPP was also examined as an alternative to a Pt-
based catalyst as it has been demonstrated to have a high
activity for oxygen reduction in direct methanol fuel cells
(34).CoTMPP(Acros)wasdissolvedinchloroform(0.001M)
and mixed with Vulcan XC-72 carbon (320 mg per 100 mL)
as previously described (35). The mixture was air-dried and
thenheatedinaninert-gasatmosphereat800°Cfor1h.The
material (∼0.6 mg cm-2CoTMPP loading) was applied to
carbon cloth using a Nafion binder by the same method
described above for the Pt catalyst cathode. A cathode
containingonlycarbonpowder(VulcanXC-72)wasprepared
as a non-catalyst control.
MFC Tests. All MFC tests were conducted using single
chamberMFCsconstructedaspreviouslydescribed(8),and
operated at a fixed external circuit resistance (1000 Ω). The
coated side of the cathode was placed facing the solution,
with the uncoated side exposed directly to air. MFCs were
inoculatedusingdomesticwastewater(pH7.3-7.6,chemical
oxygen demand (COD) ) 200-300 mg L-1) collected from
the primary clarifier of the Pennsylvania State University
Wastewater Treatment Plant. The wastewater was replaced
threetofivetimesbeforethesolutionwasswitchedtosolution
containingglucose(1gL-1)andaphosphatebufferednutrient
medium(PBM)containingNH4Cl(0.31gL-1),NaH2PO4‚H2O
(4.97 g L-1), Na2HPO4‚H2O (2.75 g L-1), KCl (0.13 g L-1), and
a metal (12.5 mL) and vitamin (12.5 mL) solution (36). The
chamberwasrefilledeachtimethevoltagedecreasedtoless
than 50 mV, forming one complete cycle of operation. All
testswereconductedina30°Ctemperature-controlledroom.
Glucose utilization was calculated on the basis of change
in COD, which was measured using standard methods (37).
All samples were filtered through a 0.22 µm (pore diameter)
membrane filter prior to COD measurements.
Electrochemical Cell Tests. The cathode potential was
measured by applying a constant current in potentiometry
tests(chronopotentiometry;ref38)usingathree-chambered
electrochemicalcellcontainingaworkingelectrode(cathode
electrode with 0.64 cm2projected surface area), a counter
electrode (platinum plate with a projected surface area of 2
cm2), and an Ag/AgCl reference electrode (EE009 no-leak
electrode, Cypress Systems). The catalyst-coated side of the
cathode was placed facing the solution, with the uncoated
side exposed directly to air. Chronopotentiometry studies
were conducted using a PC4/750 potentiostat (Gamry
Instruments) by applying a constant current for 60 min and
recording the stable value of the response potential. A curve
of the cathode potentials against current densities was used
to evaluate the performance of cathodes. For a fixed current
density, the higher the potential the better the performance
of the cathode material.
Calculations. Cell voltage was recorded using a multi-
meter and a data acquisition system (Model 2700, Keithly).
Currentdensityinchronopotentiometrytestswascalculated
as i ) I/A, where I (mA) is the applied constant current and
A (cm2) is the projected surface area of the electrode. Power
density in MFC tests was calculated according to P (mW
m-2) ) 10V2/(RA), where V (mV) is the voltage and R (Ω) is
theexternalresistance.Coulombicefficiencywascalculated
as Ec) Cp/Cth× 100%, where Cp(C) is the total coulombs
calculatedbyintegratingthecurrentovertime,andCthisthe
theoreticalamountofcoulombsavailablebasedonmeasured
COD removal in the MFC.
Results
Performance of Pt Cathodes with Different Pt Loadings
and Binders. To assess the effect of the Pt loading on their
performance at the current densities typical of MFCs,
electrodes were tested using chronopotentiometry at Pt
loadings ranging from 0.1 to 2 mg cm-2with Nafion or PTFE
asthebinder.AtaPtloadingof0.5mgcm-2(Nafionbinder),
the cathode potential rapidly decreased from 307 to -239
mV for current densities up to 2 mA cm-2(Figure 1A). These
potentials are much lower than those theoretically possible
under these solution conditions (620 mV vs Ag/Ag/Cl for 25
°C at pH 7) or those theoretically possible in hydrogen fuel
cellsundermoreacidicconditionsandathighertemperatures
(1030 mV vs Ag/Ag/Cl, pH 0, 80 °C) where the cathode is in
contact with a PEM and not water.
There was only a small change in the potential when the
Pt loading was changed within a range of 0.1-2 mg cm-2.
The difference in the potential of the other cathodes relative
to that of the cathode with a Pt loading of 0.5 mg/cm2(E -
E0.5) was typically less than 10-20 mV (Figure 1A). Similar
results were obtained using PTFE as a binder for Pt loadings
of 0.25-1 mg cm-2(Figure 1B). The potential decreased
rapidly from 288 to -257 mV at current densities up to 2 mA
cm-2. The potential differences (E - E0.5) of the cathodes
with various Pt loadings relative to that obtained at 0.5 mg
cm-2Ptwasgenerallylessthan10-15mV,oronlyadifference
of 4-10% in potential at a current density of 1 mA cm-2
(Figure 1B).
A comparison of the two different bonding materials at
a fixed Pt loading of 0.5 mg cm-2showed that Nafion
produced slightly more positive potentials than PTFE when
comparedatthesamecurrentdensity(Figure2).Forexample,
at 1 mA cm-2, the measured potential with Nafion was 12%
more positive than that obtained using PTFE as the binder.
PerformanceoftheCoTMPPCathode.Theperformance
of a CoTMPP cathode (Nafion binder) was compared to Pt
VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9365

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catalyst cathodes (Nafion or PTFE binders) using chrono-
potentiometry. At current densities above 0.6 mA cm-2, the
potentials produced using CoTMPP were 3-15 mV more
positive than those using Pt and Nafion (Figure 2). At lower
current densities (<0.6 mA cm-2) the CoTMPP cathode
potential was up to 36 mV more negative than that obtained
with Pt (Nafion binder). Potentials produced with CoTMPP
wereconsistentlymorepositive(1-35mV)thanPtcathodes
using a PTFE binder (see inset in Figure 2). In all cases the
Pt and CoTMPP catalyst potentials were substantially more
positive than those obtained using a plain carbon cathode
(prepared in the same manner as the other cathodes, and
containing the carbon paste, but lacking a catalyst).
MFC Performance Using a Pt Catalyst with Different
Binders.TheperformanceofthePtcatalyst(0.5mgcm-2Pt)
withaNafionorPTFEbinderwasexaminedinMFCsoperated
in a fed batch mode over 31 days (35 cycles). Following
acclimation,glucosewasusedasasubstrateduringthetests
with each cycle requiring ∼24 h before the voltage was
reduced to below <50 mV. The maximum voltage produced
with the Nafion binder varied between 0.5 and 0.6 V each
time the MFC was refilled, with this voltage generally
decreasingovertime(Figure3A).Forexample,themaximum
power density achieved in the MFC decreased from an
averageof480(20mWm-2((SD(standarddeviation)based
on the averages of the maximum voltages for the first three
cycles; range of 464-508 mW m-2; Figure 3B) to 400 ( 10
mWm-2(lastthreecycles;rangeof390-414mWm-2;Figure
3C).Thecurrentdensityrangedfrom0.075to0.102mAcm-2.
Although the maximum power density decreased, the Cou-
lombicefficiencyincreasedduringthesameperiodfrom8.9
( 0.4% to 18.6 ( 0.5% (Figure 3D).
When PTFE was used as the Pt (0.5 mg cm-2) binder on
the cathode, the maximum cell voltage was slightly lower
(<0.5V)thanthatobtainedwithNafion(Figure4A).However,
themaximumvoltageforeachcycledidnotdecreaseasmuch
over the operation period using PTFE as the binder as it did
with Nafion. The maximum power density for the PTFE
cathode with Pt decreased only 9%, or from 360 ( 10 mW
m-2(cycles 2-4) to 331 ( 3 mW m-2(last three cycles)
compared to a 19% decrease using Nafion (Figure 4B). The
Coulombicefficiencyalsovariedoverasmallerrangeduring
the same period, or from 9.5 ( 1.5% to 13.1 ( 0.3%.
When the Pt loading was reduced to 0.1 mg cm-2(Nafion
binder), the maximum power density of MFC averaged 340
( 20 mW m-2(from 379 to 301 mW m-2) for 24 cycles of
operation(Figure5A).Thesemaximumpowerdensitieswere
19% lower on average than those obtained with 0.5 mg cm-2
Pt and the same binder. The Coulombic efficiency ranged
from 4.5% to 7.5%, which is about half that obtained at the
higher Pt loading with a Nafion binder (Figure 5B).
MFC Performance Using a CoTMPP Cathode or in the
AbsenceofaCatalyst.IntestsusingCoTMPPasthecathode
catalyst, the power density was 369 ( 8 mW m-2on average
(range from 397 to 361 mW m-2; 25 cycles), which is 12%
lowerthanthatobtainedwithPt(0.5mgcm-2;Nafionbinder)
(Figure 5A). The Coulombic efficiency ranged from 7.9% to
16.3% (Figure 5B), similar to that obtained with 0.5 mg cm-2
Pt and Nafion (Figure 3D).
In the absence of a cathode catalyst, the average power
density was reduced to 93 ( 13 mW m-2(range from 104 to
88 mW m-2). This power density is 73% lower than that
obtained with Pt (0.1 mg cm-2; Nafion binder). The Cou-
lombic efficiency, however, was 10-20% lower than that
obtained with Pt (0.1 mg cm-2; Nafion binder) (Figure 5B).
Discussion
MFC tests at a fixed Pt loading (0.5 mg cm-2) demonstrated
that Nafion was a better binder than PTFE on the basis of
maximumpowerdensitiesmeasuredin35repeatedfedbatch
cycles. The maximum power density with Nafion averaged
480 mW m-2, while that obtained with PTFE was 14% less
at the same Pt loading. Reducing the Pt loading from 0.5 to
0.1 mg cm-2(Nafion binder) produced a similar reduction
inpower,averaging19%lessthanthatobtainedatthehigher
Pt loading. CoTMPP proved to be a suitable replacement for
Pt on the MFC cathode. In fed batch tests over many cycles,
the maximum power density using CoTMPP was actually
8.5% greater than that obtained at the lower Pt loading (0.1
mg/cm2) using a Nafion binder, and 5.4% greater compared
tothatusingaPTFEbinderevenatthehigherPtloading(0.5
mg cm-2Pt). The maximum power with CoTMPP produced
only 12% less power than the Pt catalyst (Nafion binder) at
thehigherPtloading(0.5mgcm-2).TheCoulombicefficiency
of the CoTMPP was also similar to that obtained with the
higher Pt loading and Nafion binder, indicating that there
was greater overall conversion of substrate into electricity.
Chronopotentiometrictestsweresuccessfulinpredicting
the relational performance of the different catalysts and
binders, although the actual performance differences were
larger than the differences implied by the electrochemical
tests. On the basis of maximum power densities, the
performance of the different systems can be ordered ac-
cording to the following: 0.5 mg cm-2Pt and Nafion binder
>CoTMPP>0.5mgcm-2PtandPTFEbinder>0.1mgcm-2
Pt and Nafion binder > plain carbon electrode. This is the
sameorderexpectedfromthechronopotentiometrictestsat
current densities similar to that produced in the MFC tests
(<0.5mAcm-2).Thus,chronopotentiometrytestswereuseful
as a rapid method of evaluating and ranking the different
electrode materials and preparation techniques.
FIGURE1. CathodeperformanceasafunctionofPtloadingmeasured
in the electrochemical cell using (A) Nafion or (B) PTFE binders.
The insets show the potential difference taking 0.5 mg cm-2Pt
loading as reference.
3669ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006

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In general, there were larger differences in the maximum
powerdensitiesmeasuredinMFCteststhanthosefoundfor
the differences in potentials measured in choronopoten-
tiometric tests. In addition, there were changes in the
maximum power densities observed over time in MFC tests
thatcouldnotbeanticipatedfromthechronopotentiometric
tests. Thus, some of the differences in maximum power
densities observed for the different catalysts and binders
during the MFC tests must result from other factors, such as
biofilmformationonthecathodesandtheeffectsofchemicals
produced by the bacteria during glucose oxidation. Upon
completion of the MFC fed batch cycle tests, for example, it
was observed that the biofilm formed on PTFE cathode was
thin and loose compared to that formed on Nafion cathode.
Whilewecannotconclusivelylinkthisqualitativedifference
in the biofilm to the performance of the two systems, we do
notethatbiofilmformationisafunctionofthehydrophobicity
of the surface (39-41). It seems that biofilm formation on
thecathodesurfacecontributedtotheincreaseinCoulombic
efficiency with the operation time of MFC according to the
results using different binders.
The function of a catalyst binder should not be confused
withthefunctionofaPEM,althoughthesamematerialscan
be used for both. Ideally, the main function of the binder is
to hold the catalyst in contact with the conductive electrode
surface, providing the catalyst as a site for the reaction of
oxygen, protons, and electrons. The purpose of a PEM is to
conduct protons from the anode to cathode. In an MFC,
wateristheprotoncarrierbetweentheelectrodes.Removing
the PEM in an MFC can increase power (8), while the binder
may increase or decrease power depending on the nature of
the material. The lack of a PEM does reduce the Coulombic
efficiencycomparedtoMFCswithaPEM,aspreviouslynoted
(8).TheCoulombicefficienciesobtainedhere,whilelow,are
FIGURE 2. Comparison of cathode potentials using Pt loading of
0.5 mg cm-2and a chemical binder (Nafion or PTFE) compared to
a plain carbon cathode (no Pt, Nafion as a binder) and CoTMPP
cathode(Nafionasabinder).Theinsetshowsthepotentialdifference
taking 0.5 mg cm-2Pt Nafion cathode as reference (except carbon
cathode).
FIGURE 3. Performance of MFC with Nafion binder and Pt loading
of 0.5 mg cm-2: (A) cell voltage over whole operation period; (B)
expanded view of cell voltage during first four filling cycles; (C)
cell voltage during last three cycles; (D) power density and
Coulombic efficiency as a function of number of cycles.
FIGURE4. PerformanceoftheMFCwithPTFEbinderandPtloading
of 0.5 mg cm-2: (A) cell voltage as a function of time and (B) power
densityandCoulombicefficiencyasafunctionofnumberofcycles.
FIGURE 5. Comparison of power densities (A) and Coulombic
efficiencies (B) achieved in MFCs containing different types of
cathodes.NumbersinparenthesesinthelegendindicatePtloading
in mg cm-2.
VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY9367

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consistentwithvaluespreviouslyreportedforthissystem(8,
14).
The relatively small effect of different Pt loadings on the
performance of MFCs is different from what could be
expected from hydrogen fuel cells, where the performance
isgreatlyaffectedbyPtloading.Ithasbeenreportedthatthe
power density of a hydrogen fuel cell decreased by a factor
of2whenthePtloadingwasreducedfrom0.387to0.074mg
cm-2(also using E-TEK materials, but with 40% Pt/C) (42).
Thesedifferentresultslikelyarisefromthedifferentoperating
conditions of hydrogen fuel cells and MFCs, such as
electrolyte, temperature, and current density. For example,
the water content of the cathode in a hydrogen fuel cell is
controlled to prevent cathode flooding, while there is no
PEMintheMFCusedhereandthecathodeisplaceddirectly
in contact with water. Hydrogen fuel cells operate at higher
temperatures (50-80 °C) and at current densities of 100 mA
cm-2or more, while MFCs operate at much lower temper-
atures that bacteria can tolerate (30°C in the present study),
and produce current densities below 1 mA cm-2. Thus, it is
difficult to predict the magnitude of differences in these
systems produced solely by the materials used.
While it has been shown here that the performance of Pt
cathode with Nafion is better than that with PTFE, this
comparisondoesnotconsiderthecostofthesetwochemicals.
Lowering the Pt loading by a factor of 5 reduced overall
performance by a factor of only 1.2, a difference that might
be acceptable given the cost of the catalyst and the large
surface areas needed for MFC operation. The CoTMPP
catalyst showed excellent promise as an alternative to Pt for
MFCs, but additional studies on loadings and long-term
stability of this material compared to Pt are needed. Thus,
further comparisons of the different cathode materials and
catalystsbasedonconstructioncostsandlong-termstability
are needed to better compare all aspects of using materials
in MFCs.
Acknowledgments
We thank Dr. Tom Mallouk for suggesting the use of the
CoTMPP material for the cathode. This research was sup-
portedbyNationalScienceFoundationGrantsBES-0331824
and BES-0401885, a seed grant from The Huck Institutes of
theLifeSciencesatPennState,andtheStanandFloraKappe
Endowment.
Supporting Information Available
Electrochemical cell used to examine cathode performance
asafunctionofPtloadingandchemicalbinder.Thismaterial
is available free of charge via the Internet at http://
pubs.acs.org.
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