5897r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 5897–5902 : DOI:10.1021/ef100825hPublished on Web 10/06/2010

Carbon Nanotube/Platinum (Pt) Sheet as an Improved Cathode forMicrobial Fuel Cells

David V. P. Sanchez,† Phuong Huynh,‡ Mikhail E. Kozlov,‡ Ray H. Baughman,‡

Radisav D. Vidic,*,† and Minhee Yun*,§

†Department of Civil and Environmental Engineering, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, Pennsylvania15260, United States, ‡Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, 800 West Campbell Road,

Richardson, Texas 75080, United States, and §Department of Electrical and Computer Engineering,University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States

Received June 29, 2010. Revised Manuscript Received September 8, 2010

We report a characterization and evaluation of a single-wall carbon nanotube (SWNT) sheet electrode withinfused platinum nanoparticles (nPts) as a cathode in a microbial fuel cell. The design is intended to increaseelectrode efficiency by increasing the surface/volume ratio and the available surface area of a platinum catalyst.The electrode fabrication procedure is an extension of the conventional bucky-paper-like fabrication techniqueto a two-component system and incorporates nPts throughout the thickness of the sample. The electrodes werecharacterized via scanning electronmicroscopy (SEM),Raman spectroscopy, transmission electronmicroscopy(TEM), and cyclic voltammetry (CV). Our characterizations confirmed the architecture of the electrodes, andthe current density from our microbial fuel cell (MFC) increased significantly, approximately an order ofmagnitude, when an e-beam-evaporated platinum cathode was replaced with this SWNT/nPt sheet electrode.The enhancement of catalytic activity can be associatedwith the increase of the catalyst surface area in the activecathode layer. Finally, our data suggest that nanoparticles co-deposited into layers of nanotubes can moreefficiently catalyze the cathodic reaction than electrode architectures of conventional design.

Introduction

Microbial Fuel Cells (MFCs). MFCs and microbial elec-trolysis cells (MECs) are expected to become an importantsource of renewable energy in the near future.1 A typicalMFC consists of the anode and cathode compartmentsseparated by an ion-exchange membrane. A substrate(electron donor) is pumped into the anode compartment,where anode-respiring bacteria (ARB) or fermenters con-sume it. In the absence of other thermodynamically favor-able electron acceptors, ARB transfer the electrons to theanode.2,3 Electrons pass through an external circuit, gener-ating electricity and reducing oxygen to water at thecathode.4 A conceptual illustration can be seen in Figure 1.

To become a viable source of renewable energy, MFCsrequire an increase of current densities relative to their abioticcounterparts.5,6 The increase of the current density requires adecrease of the internal resistance (often characterized by ohmic

losses), an improvement of proton transport out of the biofilm,7

and a reduction of overpotentials for both anodic8 and cathodicreactions.9,10

Addressing these issues for both anode respiration and theoxygen reduction reactions significantly affect MFC perfor-mance. For example, cathodic performance was shown toimprove greatly by augmenting proton transport with buffers,using low pH catholyte solutions and incorporating platinum(Pt) or other catalysts into cathodes.11 However, to attain highcurrent densities platinizedMFC electrodes have typically usedhigh Pt surface loading (>0.4mg/cm2),6 expensive binders (i.e.,Nafion),12 and artificial mediators (i.e., FeCN). Increasingcurrent densities by loading Pt efficiently and eliminatingbinders and artificial mediators are the steps needed to makeMFCs an industrially viable technology.

Mass-Transfer Limitations and Oxygen Reduction Kinetics.

The high overpotentials of cathodic reactions in oxygen-basedmicrobial fuel cells6 originate from (i) the concentration over-potential caused by inadequate proton transport to the cathodeand (ii) the surface overpotential, a consequence of the four-electron transfer needed for oxygen reduction (see eq 1).13

O2 þ 4Hþ þ 4e- f 2H2O ð1ÞThe rate-determining reaction on a modified cathode was

shown to change from 4e-/4Hþ to 4e-/2Hþwith a variationof pH. As pH increased, the oxygen reduction potential

*To whom correspondence should be addressed. Telephone: (412)648-8989. Fax: (412) 648-8003. E-mail: vidic@pitt.edu (R.D.V.);yunmh@engr.pitt.edu (M.Y.).(1) Rittmann, B. E. Biotechnol. Bioeng. 2008, 100, 203.(2) Reguera, G.; Nevin, K. P.; Nicoll, J. S.; Covalla, S. F.; Woodard,

T. L.; Lovley, D. R. Appl. Environ. Microbiol. 2006, 72, 7345.(3) Marcus, A. K.; Torres, C. I.; Rittmann, B. E. Biotechnol. Bioeng.

2007, 98, 1171.(4) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schroder,U.; Keller, J.;

Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. Environ. Sci.Technol. 2006, 40, 5181.(5) Du, Z.; Li, H.; Gu, T. Biotechnol. Adv. 2007, 25, 464.(6) Ramani, V. Electrochem. Soc. Interface 2006, 15 (1), 41.(7) Torres, C. I.; Marcus, A. K.; Rittmann, B. E. Biotechnol. Bioeng.

2008, 100, 872.(8) Pham, T. H.; Aelterman, P.; Verstraete, W. Trends Biotechnol.

2009, 27, 168.(9) Rozendal, R. A.; Hamelers, H. V. M.; Rabaey, K.; Keller, J.;

Buisman, C. J. N. Trends Biotechnol. 2008, 26, 450.

(10) Oh, S.; Min, B.; Logan, B. E. Environ. Sci. Technol. 2004, 38,4900.

(11) Zhao, F.; Harnisch, F.; Schr€oder, U.; Scholz, F.; Bogdanoff, P.;Herrmann, I. Electrochem. Commun. 2005, 7, 1405.

(12) Cheng, S.; Liu, H.; Logan, B. E. Environ. Sci. Technol. 2006, 40,364.

(13) Zhao, F.; Harnisch, F.; Schroder, U.; Scholz, F.; Bogdanoff, P.;Herrmann, I. Environ. Sci. Technol. 2006, 40, 5193.

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decreased. Negative slopes of 59 and 29 mV/pH for pHbelow and above 7, respectively, were demonstrated forcathodes with surface catalysts.11 As a quick illustration,open-circuit potentials of ∼0.6, ∼0.33, and ∼0.27 V wereshown for a carbon foil electrode with a cobalt tetramethox-yphenylporphyrin (CoTMPP) catalyst at pH values of∼1,∼7, and ∼9.3, respectively.11

Additionally, the rate-determining reactions (i.e., four-and two-electron transfers for oxygen reduction) have beenstudied in alkaline media using bulk platinum and are shownin eqs 2 and 3, respectively.14

O2 þ 2H2Oþ 4e- f 4OH- ð2Þ

O2 þH2Oþ 2e- f HO-2 þOH- ð3Þ

Because the concentrationof protons at neutral pH is about 3orders of magnitude lower than the concentration of oxygen inair-saturated water (∼9 mg/L) and four protons per oxygenmolecule are needed for the cathodic reaction to occur (eq 1), aproton mass-transfer limitation can typically be expected inMFCs. Torres et al. and Zhao et al. defined proton mass-transfer limitations for both bio-anodic (proton-generating)and cathodic (proton-consuming) reactions.7,13 These mass-transfer limitations can be reduced by the addition of concen-trated phosphate buffer to the anolyte or acid to the catholyte.Only when proton mass-transfer limitations are removed areoxygen mass-transfer limitations realized. For example, underacidic conditions and a high catalyst loading of 1 mg cm-2,Zhao et al. demonstrated amass-transfer limitation for oxygen.Ultimately, the cathode performance primarily depends uponthe proton concentration, so that the oxygen reduction rateincreases with a decreasing pH and/or an increasing bufferconcentration.

Enhancing the reactivity at the cathode is importantbecause oxygen reduction requires a four-electron transfer.Fortunately, the reactivity can be improved via the additionof catalysts, such as Pt. For neutral pH solutions, which arepreferred forMFCs, a higher catalyst loading correlateswith anincreased rate of oxygen reduction and higher currentdensities.13 To demonstrate the trade-off between maintaininga neutral pH and adding a catalyst, it was shown that, as the Ptloading increased from0.5 to2mg cm-2, the current density in apH neutral solution with a phosphate buffer concentration of0.1 M increased from 45 to 74% of the value recorded for asolution at pH 3 with 0.5 M phosphate buffer.13

Nanostructured Electrodes.Nanostructured carbon-basedmaterials, in particular nanotubes, can have a high surfacearea/volume ratio, mechanical strength, high electrical con-ductivity, and catalytic properties. As a result, nanomaterialscan be beneficial to theMFCperformance if they are used forelectrodes instead of conventional bulk graphite.15,16 Con-sidering that carbon nanotubes can be assembled into highlyporousmacroscopic paper-like sheets similar to carbonmatsfrom Toray Industries,17 the scalability issues for sheetfabrication can be similar. In fact, the carbon nanotubesheets can be manufactured from liquid-dispersed carbonnanotubes, using a process similar to that used for makingordinary paper.

MFCs using nanostructured electrodes have yielded pro-mising results. Park et al. showed that electron-beam(e-beam) Pt deposition improves the performance ofplain carbon paper from Toray Industries.17 Sharma et al.reported the impact of noble nanoparticles and artificialmediators upon improving the current density,18 and Zouet al. improvedMFC performance using polypyrrole-coatednanocomposites.19 These and other examples demonstratethe ability of catalysts to decrease activation overpotentialsand the utility of high surface/volume ratio electrode struc-tures for improving MFC performance.

Ultimately the concept of an improved MFC cathodeusing a buffered supporting electrolyte is driven by the factthat the reduction of oxygen at the cathode in fuel cells is arelatively slow reaction that can be improved with catalysts.At the same time, it is important to consider that crossover ofthe substrate from the anode to cathode can cause sidereactions that decrease oxygen reduction efficiency on Ptelectrodes,20 illustrating yet again the importance of increas-ing the available Pt surface area on cathode electrodes.

In the present study, we inoculated aMFC using activatedsludge and a buffered electrolyte with an e-beam Pt-depos-ited cathode. We then substituted the cathode with a single-wall carbon nanotube (SWNT)/platinum nanoparticle (nPt)electrode and evaluated resulting energy generation. Here,we focused on the characterization of the cathode fabricatedfrom SWNTs embedded with nPt and the evaluation of itsperformance in a two-compartment MFC.

Figure 1. Schematic diagram of a MFC system. As bacteria (yellowrods) consume glucose, the produced free electrons flow from theanode to cathode via the electrical circuit, while protons aretransferred from the anode to cathode through a proton-exchangemembrane (Nafion).

(14) Genies, L.; Bultel, Y.; Faure, R.; Durand, R. Electrochim. Acta2003, 48, 3879.

(15) Zhang, M.; Fang, S.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.;Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309,1215.

(16) Oh, S.-E.; Logan,B.E.Appl.Microbiol. Biotechnol. 2006, 70, 162.(17) Park,H. I.;Mushtaq,U.; Perello,D.; Lee, I.; Cho, S.K.; Star, A.;

Yun, M. Energy Fuels 2007, 21, 2984.(18) Sharma, T.;MohanaReddy, A. L.; Chandra, T. S.; Ramaprabhu,

S. Int. J. Hydrogen Energy 2008, 33, 6749.(19) Zou, Y.; Xiang, C.; Yang, L.; Sun, L.-X.; Xu, F.; Cao, Z. Int. J.

Hydrogen Energy 2008, 33, 4856.(20) Harnisch, F.; Wirth, S.; Schr€oder, U. Electrochem. Commun.

2009, 11, 2253.

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Materials and Methods

Electrode Fabrication Procedure. SWNTs made by the high-pressure carbon monoxide (HiPco) process were purchasedfrom Unidym, Inc. (Sunnyvale, CA). About 15 mg of SWNTswas placed in an aqueous surfactant solution and subjected toprobe sonication (Fisher Scientific model 500) for about 25min,in 5 min cycles. The dispersion was kept in an ice bath to avoidoverheating.Two typesof surfactants,Triton-X-100andPluronicX(Aldrich), were evaluated in this study.

Approximately 0.1 g of Triton-X-100 in 50 mL of water wasadded to the SWNT dispersions. Sonication continued for∼5-10 min until a good dispersion was achieved, thus allowingfor the uniform addition of ∼15 mg of nPt (Aldrich PlatinumblackHiSPEC1000). The solutionwas dilutedwith 1Lofwater,and a decanting method was used to repeat the process. Thewhole solution was then filtered through a vacuum filter appa-ratus with a 47 mm diameter filter [Millipore, 10 μm MITEXpolytetrafluoroethylene (PTFE) membrane filters]. After filtra-tion, 1000 mL of water was passed through the filter until allfoamdisappeared.A second 1000mL solution of 30%methanolwas passed through the filter. The methanol solutions werediluted to ensure that methanol would not react directly withoxygen in the air using Pt in the carbon nanotube paper as acatalyst. The vacuum was released, and the vacuum filtrationapparatus was disassembled. Immediately, another 10 μm MI-TEX PTFE membrane filter was placed on top of the carbonnanotube sheet forming a “sandwich”. The whole vacuumfiltration apparatus was reassembled, and the vacuum wascontinually applied for 1 h. The purpose of enclosing carbonnanotube paper between two membranes was to maintain a flatand uncurled sheet. After air-drying for an additional 1 h, themembrane filter was peeled off. The process was also conductedusing PluronicX in place of Triton-X-100. The carbon nanotubepaper with nPt was then used as the anode and/or cathode of theMFC. It is important to note that the size and shape of theSWNT/nPt sheet prepared in this way is limited only by the sizeand shape of the membrane filter used.

Characterization of SWNT/nPt Electrode. In this study,SWNT/nPt electrodes were characterized using three differentmethods: scanning electron microscopy (SEM), Raman spec-troscopy, and transmission electron microscopy (TEM).

SEM: To ensure that the nPt had not completely agglomer-ated throughout the sample or on the surface of the SWNT/nPtsheet, SEM images were taken of the electrode surface and fromthe fractured sample using a LEO 1530VP field emissionmicroscope. Electrode samples were cut at random positionsand viewed in a scanning electron microscope at differentmagnifications for the surface analysis.

Raman Spectroscopy: Raman spectroscopy provides spectrabetween 100 and 3500 cm-1 for 633 nm laser excitation, whichwas used to characterize the SWNTs in the electrodes and todetect the presence of contaminants, such as surfactants ororganic residue from the production of carbon nanotubes, inthe electrodes. A Jobin Yvon LabRam HR800 Raman micro-scope was used.

TEM:Toconfirmthedistributionand sizesofnPt in theSWNT/nPt matrix, a small sample (∼0.25 cm2) was sonicated in isopropylalcohol for a total of 35 min. Water in the sonication bath wasexchanged every 7 min to avoid a temperature increase in thesolution. A sample of the isopropyl alcohol solution containingSWNT/nPTwas deposited on a lacey carbon TEMgrid and dried.Images were taken at several magnifications using a JEOL 2100FTEM/STEM machine at 200 kV. The images were used to deter-mine the size of the nPt and the relative dispersion.

MFC System. A dual-compartment MFC, separated by aNafion proton-exchange membrane (Nafion 112, DuPont,

Wilmington, DE)21 was used with each compartment holding a4� 1 cm electrode. An external resistor (10Ω) was used as a fixedload, and the potential across the resistor was measured with aKeithley meter (model 2701 DMM, Keithley Instruments, Inc.,Cleveland,OH) andcollected via apersonal computer.17 Influentwaspumped into the reactor using aperistaltic pump (Watson-Marlow323S Bredel, Watson-Marlow, Inc., Wilmington, MA) andattached cassette (Watson-Marlow 314MC cassette) at a rate of3 rpm using marprene tubing with a 2.79 mm bore. The anodecompartment was continuously sparged with nitrogen throughoutthe experiment.

Anode:Glucose (50 ppm), glutamic acid (50 ppm), and phos-phate buffer (100mM, pH 7.5) were added to amedia consistingof a trace element and salt solution described previously.22 Eachreactor was inoculated with activated sludge (biomass takenfrom the aeration tank in a wastewater treatment plant) fromAlleghenyCounty SanitaryAuthority (ALCOSAN)wastewatertreatment plant (Pittsburgh, PA) and was recycled through thereactors until current was generated. The substrate was then fedin continuously until a steady-state current evolved. The start-up procedure lasted about 1 week, after which cathode-testingcommenced. After testing in the MFC was complete, anodeswere extracted and imaged using SEM (SEM, e-LiNE, RaithGmbH, Germany) to ensure the accumulation of a biofilm.

Cathode: The cathode was fed with air-saturated water con-taining phosphate buffer and the same salt solution as in theanolyte. The influent was pumped using the same setupdescribed above. For the first part of the experiment, 1000 A Ptwas deposited on plain Toray carbon paper (TGPH-120, E-tek,Somerset, NJ) using an e-beam evaporator (VE-180, Thermio-nics Laboratory). This e-beam Pt-deposited electrode wasinserted as the cathode. The e-beam Pt-deposited electrodeswere used as a base for comparison because theywere previouslytested in MFCs.17 It is also important to note that e-beamevaporation does not deposit Pt throughout the thickness ofthe sample. The e-beam Pt-deposited electrodes (1000 A) in thecathode compartment were used in operation for 2 days andwere than replaced by a SWNT/nPt sheet electrode to evaluaterelative performance. The MFC was again operated for a dayafter the replacement. Samples of both SWNT/nPt and e-beamPt (1000 A) electrodes, 1 cm2 in size, were also placed in a 100mMphosphate buffer solution with an Ag/AgCl reference electrode,and a 2 cm2 e-beam Pt counter electrode. The solution was con-tinuously sparged with air. Cyclic voltammetry was performedon each cathode individually in the solution using a CH Instru-ments 1040A potentiostat at a scan rate of 2 mV s-1 in the rangefrom -0.2 to 1.2 V (versus Ag/AgCl).

Results and Discussion

A SEM image in Figure 2A shows the uniform architec-ture of the SWNT/nPtmatrix at themicro scale.Assemblies ofSWNT bundles were seen throughout the thickness of thesample. Figure 2B at higher magnification shows the uneventopography and high surface area that we expected.

The Raman spectra of SWNT samples with and withoutnPt showed identical Raman features (Figure 3). The G, G0,and D bands were at the usual positions for SWNT spectra.23

The spectra showed about the same intensity for SWNT andSWNT/nPt and revealed no significant shifting or broadeningof the peaks. The dominant peak near 1580 cm-1 is character-istic of SWNT and typically relates to the phonon modes.23

These spectra indicate a consistency in the process of electrodefabrication. Most importantly, the material did not changeduring the process, and no contaminant, such as the surfactant

(21) Gil, G.-C.; Chang, I.-S.; Kim, B. H.; Kim,M.; Jang, J.-K.; Park,H. S.; Kim, H. J. Biosens. Bioelectron. 2003, 18, 327.

(22) Park, H. I.; Sanchez, D.; Cho, S. K.; Yun, M. Environ. Sci.Technol. 2008, 42, 6243.

(23) Dresselhaus, M. S.; Eklund, P. C. Adv. Phys. 2000, 49, 705.

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used to disperse SWNTs or organic material from sourcecarbon nanotubes, was found.

The TEM images in Figure 4 illustrate the size and dis-tribution of nPt throughout the SWNT sheet. Relativelycomplete coverage of SWNT with nPt can be seen inFigure 4A. The distribution of these nPts throughout theSWNT matrix was such that nPts were clearly separated,evenly distributed, and had rare occurrences of agglomera-tion. Sizes of nPts were between 4 and 10 nm and wereconfirmed with images of increasing magnification shown inpanels B-D of Figure 4.

Figure 5A shows the difference in anodic performance forthe SWNT with embedded nPt made with different surfac-tants; e-beam Pt (1000 A) electrodes were used as cathodes. Itis important to note that e-beam Pt (1000 A) cathodes werepreviously shown to increase current density (∼0.2 A m-2)when substituted for plain carbon cathodes. This serves as ourperformance reference to improve the cathode electrode witha catalyst. The shapes of the current density profiles inFigure 5A were similar to each other, and the difference inperformance of the two electrodes was small. The shape of thecurrent densities/voltage discharge seen in panels A and B ofFigure 5 is similar to those reported previously.19 The initialdischarge is typically followed by a plateau of the currentdensity, which could be due to activation overpotentials of thebiofilmanode and/or slowdiffusionof reducedmediators thatmay be in the biofilm. To test the reactivity of the cathode, theinfluent pump for the cathode was shut off for 5 h to decreasethe oxygen reduction rate but allow charge (i.e., reducedmediators and/or proteins) to accumulate within the biofilmanode as bacteria continued to consume the substrate. Theshut-off period occurred from hours 22 to 27 in Figure 5A,which resulted in the increase in charge in the biofilm anode.

As the influent pump for the cathode was restarted at the 27thhour to increase the oxygen concentration in the cathode andthus the oxygen reduction rate, a second peak dischargeoccurred (Figure 5A). This peak discharge confirms theaccumulationof charge in the biofilm anode and characterizesthe reactivity of the cathode.

InFigure 5B, the e-beamPt cathode that is coupledwith thelower performing anode (SWNT/nPt/Triton-X) is swappedfor a SWNT/nPt/Triton-X cathode. The discharge peak andcurrent density plateau are then increased by almost an orderofmagnitude. This ismost interestingwhenwe examine the Ptloading for each electrode. The loading for an e-beam evapo-rated Pt electrode at 1000 A thickness is 0.215 mg cm-2, andthe Pt loading for the SWNT/nPt electrode is more than

Figure 2. SEM images of a fracture surface of SWNT/nPt matrix at(A) lowand (B) highmagnifications. The images indicate the fibrousnature of the electrode and the fact that the platinum nanoparticlesare not highly agglomerated.

Figure 3. Raman spectra of SWNT samples with and withoutplatinum nanoparticles. The samples were measured using 633 nmlaser excitation. This image shows that there is no notable shift in theG, G0, and D bands between SWNTs with and without platinumnanoparticles.

Figure 4. TEM images of SWNT/nPt samples at (A) lowmagnifica-tion, (B) medium magnification, (C) high magnification, and (D)magnification of the inset in B. The images show 4-10 nm platinumnanoparticles evenly dispersed in the SWNT matrix.

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double that amount (0.576 mg cm-2). However, when weconservatively compare the mass-specific current densities(current/Pt loading in milligrams), the SWNT/nPt/Triton-Xelectrode (0.1A m-2/0.576 mg) outperforms the e-beam Ptcathode (0.01 A m-2/0.215 mg) by a ratio of 4:1.

Cyclic voltammograms (CVs) for both SWNT/nPt ande-beam evaporated Pt (1000 A) electrodes are provided inFigure 6. The reaction rate for oxygen reduction on SWNT/nPt when compared to the e-beam Pt electrode is significantlyhigher than for the e-beam evaporated Pt (1000 A) electrode.The results are consistent over the 4 cycles.

The SEM image in Figure 7 confirms the accumulation of abiofilm on the anode. The bacteria aremostly rod-shaped andcan be seen throughout the sample. The biofilm is similar toother biofilms that have been imaged and dimensioned.24 Thetop left of the image shows a couple of putative bionanowires

that have also been shown in previous publications.24 Thesewere scattered throughout the biofilm.

Conclusions

In this paper, we report a technology for the preparation ofMFC cathodes using SWNT/nPt dispersions and their char-acterizations. This technology is an extension of the conven-tional bucky-paper fabrication technique for a multi-component system that combines nPt and SWNT into filtersheets. The electrodes were characterized with SEM, Ramanspectroscopy, and TEM, confirming nPt size, the even dis-persion of nPt, and the composition and structure of ourelectrode. These characterizations confirmed a successfulintegration of the two components: SWNT and nPt.

The novel electrode/catalyst architecture improved thecathodic reaction rate in MFCs, with the average currentdensity increasing approximately anorder ofmagnitudewhenused in lieu of an e-beam Pt (1000 A) cathode. These findingssuggest that, when the cathode is under kinetic control (i.e.,mass transfer is not limiting), the oxygen reduction reactioncan be effectively and more efficiently catalyzed by smaller Ptconstituents (nPt) that are deposited throughout the electrode

Figure 5. Current density profiles from a MFC employing (A)SWNT/nPt pluronic acid (blue 9) and SWNT/nPt/Triton-X (redb) anodes with e-beam Pt (1000 A) cathodes and (B) SWNT/nPtelectrodes loaded with Pt (0.5 mg/cm2) and treated with Triton-X(green 2) as the anode and cathode superimposed in panel A. Notethat changing the cathode from an e-beam Pt electode (1000 A) to aSWNT/nPt electrode improved the current density approximatelyan order of magnitude.

Figure 6. Cyclic scans of SWNT/nPt and e-beam Pt (1000 A)electrodes, illustrating the electrode effect on the oxygen reductionreaction.Ata scan rateof2mV/s in the range from-0.2 to1.2V (versusAg/AgCl), the SWNT/nPt demonstrated superior performance.

Figure 7. SEM image of the biofilm accumulated on the SWNT/nPtanode surface in aMFC.Most of the bacteria are rod-shaped, whichwas consistent throughout the sample.

(24) Torres, C. I.; Krajmalnik-Brown, R.; Parameswaran, P.; Marcus,A. K.; Wanger, G.; Gorby, Y. A.; Rittmann, B. E. Environ. Sci. Technol.2009, 43, 9519.

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as opposed to solely on the surface. Increasing the surfacearea/volume ratio of platinum using nanoparticles and main-taining an even distribution of those nanoparticles within aSWNT matrix increases the catalyst efficiency of Pt 4-fold.

Finally, the catalyst structure and deposition methoddetermine the catalyst surface area available for the oxygenreduction reaction and are paramount to improving theoxygen reduction rates in MFC cathodes. Nanoparticlesintermixed with SWNTs hold great potential to improve thecathodic reaction inMFCs.As a result, future research should

focus on further decreasing the nPt loading, using non-noblemetal nanoparticles, and long-term feasibility studies.

Acknowledgment. This work was supported in part by theMascaroCenter forSustainable Innovation(MCSI).D.V.P.S. thanksthe IntegratedEducationResearchTrainee (IGERT) andGraduateResearch Fellowship programs (National Science Foundation),the Alfred P. Sloan Foundation, ALCOSANWastewater Treat-ment Plant, Yushi Hu, Jiyong Huang, and Michael Nayhouse.The authors also appreciate theTEMsupport fromDr. SeongYongPark and Dr. Moon J. Kim.