A Highly Efficient Catalyst toward Oxygen Reduction Reaction in

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A Highly Efficient Catalyst toward Oxygen Reduction Reaction in Neutral Media for Microbial Fuel Cells Yunhe Su,† Yihua Zhu,*,† Xiaoling Yang,† Jianhua Shen,† Jindan Lu,† Xiaoyan Zhang,‡ Jianding Chen,† and Chunzhong Li† †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, and ‡School of Biotechnology, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: A nanocomposite of cobaltosic oxide and nitrogen-doped graphene (Co3O4/N-G) was prepared by the facile hydrothermal method. Morphology characterizations show that the Co3O4 nanoparticles with crystalline spinel structure are uniformly dispersed on the nitrogen-doped graphene nanosheets, and the graphene weight fraction in Co3O4/N-G composite is estimated to be ∼20%. Meanwhile, electrochemical measurements reveal that the as-prepared Co3O4/N-G nanocomposite exhibits a high catalytic activity and long-term stability in neutral electrolyte. Moreover, the use of Co3O4/N-G as cathode catalyst for oxygen reduction in microbial fuel cells (MFCs) to produce electricity was also investigated. The obtained maximum power density was 1340 ± 10 mW m−2, which was as high as almost four times that of the plain cathode (340 ± 10 mW m−2), and only slightly lower than that of a commercial Pt/C catalyst (1470 ± 10 mW m−2). All the results prove that a Co3O4/N-G hybrid can be a good alternative to platinum catalysts for practical MFC applications.

1. INTRODUCTION Microbial fuel cells (MFCs) are promising devices that use bacteria as catalysts to oxidize organic matter and generate current.1 Because of their eminent merits of universality in various organic fuels, capability in treating wastewater, mild reaction conditions, and low cost, MFCs have attracted considerable interest in scientific research recently.2 In a MFC device, oxygen has been used as the most sustainable electron acceptor in the cathode, because of its easy availability in the environment and capacity to produce a high power output.3 However, the poor kinetics of oxygen reduction reaction (ORR) at neutral pH and low temperature hinder improvement in the performance of MFCs.4,5 To improve the slow ORR rate, platinum and platinum-based catalysts have been commonly used as MFCs cathode catalyst, because of their effectiveness in lowering the activation energy of cathodic reactions. Unfortunately, their high cost, limited supply, and weak durability severely restrict their applicability to broad commercialization.6,7 Hence, researchers are faced with the challenge of developing efficient, durable, and inexpensive non-platinum ORR catalysts for MFCs.8 In the search for ideal catalysts, iron(II) phthalocyanine (Pc) and cobalt tetramethoxyphenyl-porphyrin (CoTMPP)-based oxygen reduction cathode catalysts have never been proposed as platinum-free alternatives to catalyze ORR in MFCs.9 Nevertheless, the sophisticated preparation procedure and long-term instability of the transition-metal macrocycles and phthalocyanines make these alternatives impractical.10 Moreover, transition-metal oxides have been widely used as catalysts for oxygen reduction in fuel cells. Morris et al.11 compared PbO2 to platinum as a cathode catalyst in a double-chamber MFC utilizing glucose as the substrate. The results indicated that the PbO2 cathode produced 2−4 times more power than the platinum cathode. However, because of lead leaching from the © 2013 American Chemical Society

PbO2 cathodes, the lead compound may dissolve into cathode chamber solution, although various methods and binders have been attempted to stabilize the material on the cathode. Recently, nitrogen-doped carbon-based nanomaterials, such as nitrogen-doped carbon nanotubes and nitrogen-doped graphene, have been demonstrated to be high-performance electrocatalysts that show high electrocatalytic activity for ORR under alkaline conditions.12−17 Qu et al.13 applied nitrogendoped graphene as the catalyst for the ORR process in alkaline fuel cells and obtained power performances comparable to those of conventional platinum catalysts. Nevertheless, the nitrogen-doped carbon nanomaterials are usually competitive with platinum-based catalysts in an alkaline medium, which greatly limits their applications in proton exchange membrane fuel cells (PEMFCs) running in acid media and in MFCs operating in neutral solution. Lately, Dai et al.18 reported that the hybrid material consisting of Co3O4 nanocrystals grown on nitrogen-doped reduced graphene oxide had a high bifunctional electrocatalytic activity for ORR and oxygen evolution reaction (OER), because of the unusual catalytic activity that arises from synergetic chemical coupling effects between Co3O4 and nitrogen-doped graphene, which exhibits similar catalytic activity but superior stability to platinum in alkaline solutions. However, to the best of our knowledge, only a few studies have reported on the use of ORR catalysts in neutral solution and the practical applications in biological systems. In addition, the use of Co3O4/N-graphene hybrid as the catalyst for cathodic ORR in MFCs has never been reported. Received: Revised: Accepted: Published: 6076

February 1, 2013 March 24, 2013 April 16, 2013 April 16, 2013 dx.doi.org/10.1021/ie4003766 | Ind. Eng. Chem. Res. 2013, 52, 6076−6082

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Figure 1. (a) Scanning electron microscopy (SEM) and the corresponding particle size distribution (PSD) histogram (inset), (b) transmission electron microscopy (TEM) and the corresponding selected-area electron diffraction (SAED) pattern (inset), (c) X-ray diffraction (XRD), and (d) thermogravimetric (TG) images of Co3O4/N-G.

supernatant GO aqueous solution was redispersed in anhydrous ethanol (EtOH). The concentration of the final GO EtOH suspension was ∼0.5 mg mL−1 by measuring the mass of the GO lyophilized from a certain volume of the suspension. To synthesize Co3O4/N-G, 1.5 mL of 0.2 M Co(Ac)2 aqueous solution was added to 25 mL of GO EtOH suspension, followed by the addition of 0.50 mL of NH4OH (30% solution) and 0.50 mL of water at room tempreture. The reaction was kept at 80 °C with stirring for 12 h. After that, the reaction mixture was transferred to a 40 mL autoclave for hydrothermal reaction at 150 °C for 3 h, which reduced GO to graphene. The resulted product was collected by centrifugation and washed with ethanol and water. The resulting Co3O4/N-G was ∼25 mg after lyophilization. For comparison, nitrogen-doped graphene (N-graphene) or free Co3O4 nanoparticles were made through the same steps as those used to make Co3O4/N-G without adding any cobalt salt or GO in the stirring step. 2.3. Ink and Electrode Preparation for Co3O4/N-G Catalysts. The catalyst ink formulation used in electrochemical measurements and MFC tests was the same: 1 mg of catalyst was dispersed in 1 mL of a solvent mixture of Nafion (5 wt %) and anhydrous ethanol (1:9, v:v) for 0.5−1 h under sonication.26,27 For the electrochemical tests, a 10-μL portion of catalyst suspension was lightly deposited dropwise onto a prepolished glass carbon electrode 3 mm in diameter (catalyst loading ≈

Thus, the main purpose of this study was to examine the electrochemical properties of Co3O4/N-graphene hybrid under neutral conditions and the feasibility of Co3O4/N-graphene hybrid as cathode catalyst for oxygen reduction in MFCs. To achieve this aim, the nanocomposite of Co3O4 and nitrogendoped graphene (Co3O4/N-G) was synthesized in solution by a general two-step method,19,20 and the role of Co3O4/N-G on cathodic ORR in neutral solution was elucidated by electrochemical measurements. Moreover, we have set up homemade, pure-bactierium-inoculated MFCs to evaluate the CO3O4/N-G in cathodic catalysis, and the performance of pure-bacteriuminoculated MFCs with Co3O4/N-G cathode catalyst for oxygen reduction was compared with the commercially available Pt/C catalyst.

2. EXPERIMENTAL SECTION 2.1. Materials. All the chemicals were of analytical grade and used without any further purification. Cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O) was purchased from Sigma−Aldrich Chemicals Co. In addition, all other chemicals were purchased from Shanghai Chemical Reagent Co. Ultrapure water (18 MΩ cm) was used for all experiments. 2.2. Synthesis of Co3O4/N-G Catalysts. The preparation of graphene oxide (GO) was carried out using a modified Hummers method21−25 and then centrifugated at a low speed of 5000 rpm to obtain a homogeneous supernatant. The 6077

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Figure 2. Cyclic voltammograms (CVs) for (a) Co3O4/N-G, (b) N-graphene, (c) free Co3O4 nanoparticles, and (d) comparison for the samples in O2-saturated (solid line) or N2-saturated neutral PBS (dashed line). (Catalyst loading: 0.14 mg cm−2, normalized to the surface area of a 3-mmdiameter GC electrode.)

0.14 mg cm−2). In contrast, a commercially available catalyst of 20 wt % Pt/C powder was used and a 1 mg mL−1 Pt/C suspension was prepared by following the same procedure as that detailed above. The electrodes were then dried overnight at room temperature before measurement. For MFC cathode fabrication, the catalyst inks were spun onto the 4 cm2 indium tin oxide (ITO) substrate with a rotational speed of 1000 rpm, and the catalyst loading on the ITO substrate was 1 mg cm−2. 2.4. MFC Construction and Operation. For the MFC experiments, we used Shewanella oneidensis MR-1 (ATCC 70050) as the biocatalyst. Shewanella oneidensis MR-1 was grown aerobically in 100 mL of Tripticase Soy Broth (TSB) under 30 °C for 36 h, then centrifuged and resuspended in 1 L of a nutrient medium28,29 previously purged with nitrogen gas to eliminate oxygen. Lactate (0.02 M) and fumarate (0.1 M) were added to the medium as an electron donor and acceptor, respectively. After anaerobic incubation under 30 °C for 48 h, the cells were harvested again by centrifugation, resuspended in 100 mL of anolyte medium, and transferred to the anode chamber under anoxic conditions.30 The anode chamber was filled with enriched Shewanella oneidensis MR-1 cells, carbon granules (1−3 mm in diameter), and a nutrient medium containing lactate (0.02 M). The catholyte in the cathode chamber was a phosphate buffer solution (PBS) (100 mM, pH ∼7.0). Oxygen was purged into the cathode compartment in order to supply the oxygen needed for the electrochemical reaction.

The MFC experiments were carried out in batch mode, using a self-made aqueous cathode dual-chambered microbial fuel cell model, which was composed of two 250-mL bottles pressed together at laterally inserted windows. A Nafion 117 membrane 3 cm in diameter was clamped between the windows separating the anode and cathode compartments. Stainless steel wire and some copper wires were used to connect the circuit. All experiments were performed at room temperature. 2.5. Characterizations and Electrochemical Analyses. To demonstrate the overall morphology and structure, the samples were examined by scanning electron microscopy (SEM), using a JEOL Model SM-6360LV microscope (JEOL, Japan), and transmission electron microscopy (TEM), with a JEOL Model 2011 microscope (JEOL, Japan) operated at 200 kV. The crystalline structure was investigated by powder X-ray diffraction (XRD) (Rigaku, Model D/MAX 2550 VB/PC, Japan) and the thermogravimetric (TG) measurement was carried out with a Mettler STARe thermal analyzer. Electrochemical measurements of cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and current−time chronoamperometric response (i−t curve) were carried out with a Model CHI 660C electrochemical workstation (CH Instruments, Chenhua Instrument Co., China) connected to a personal computer. A threeelectrode configuration was employed, consisting of glassy carbon electrode (3 mm in diameter) or glassy carbon rotating disk electrode (5 mm in diameter) serving as the working electrode, Ag/AgCl (3 M KCl) reference electrode and platinum wire counter electrode,31 respectively. All electro6078

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To gain insight into the ORR activity of Co3O4/N-G, we first examined the cyclic voltammogram (CV) of Co3O4/N-G in N2 and O2-saturated 0.1 M PBS at a scan rate of 50 mV s−1 (Figure 2a). For comparison, we also tested the N-graphene (Figure 2b) and free Co3O4 nanocrystals (Figure 2c). In the case of N2saturated solution, CVs within the potential range of −1.2 V to +0.2 V did not show any significant peak for all three samples. In contrast, when the electrolyte was saturated with O2, the three electrodes showed a substantial reduction process. Apparently, as shown in Figure 2d, Co3O4/N-G exhibited a pronounced electrocatalytic ORR activity associated with a more positive ORR potential peak and higher current density value than those of N-graphene and free Co3O4 nanocrystals.39 Subsequently, LSV was used to further investigate the ORR catalytic activity of Co3O4/N-G in O2-saturated 0.1 M PBS at a scan rate of 10 mV s−1 from 0.4 V to −0.6 V vs Ag/AgCl reference electrode. The bare glass carbon rotating-disk electrode (bare RDE), free Co3O4 nanocrystals, N-graphene and Pt/C were also measured for comparison. As shown in Figure 3, the current density and the onset ORR potential of

chemical experiments were carried out at room temperature. The power density (P = IV/A) for the MFCs was measured by a Model 2400 SourceMeter (Keithley, Cleveland, OH), according to the measured voltage (V), current (I = V/R), and surface area of the cathode electrode (A). The maximum power output was determined by the polarization curves.

3. RESULTS AND DISCUSSION Figure 1a shows the SEM image of Co3O4/N-G. As clearly illustrated, the Co3O4 nanoparticles were dispersed uniformly on the entire surface of graphene nanosheets. The inset image of Figure 1a represents the corresponding particle size distribution histogram and confirms that the average diameter of the majority of the nanoparticles is ∼10 nm, which is consistent with the results of TEM observation shown in Figure 1b. The selected area electron diffraction (SAED) pattern from the inset of Figure 1b indicated a polycrystalline crystal structure of Co3O4 nanoparticles. Moreover, the diffraction rings in the SAED pattern corresponded to the (220), (311), (400), (511), and (440) planes of Co3O4 nanoparticles from inside to outside, respectively, which could be indexed to the XRD pattern. Figure 1c shows the XRD pattern of the prepared Co3O4/N-G hybrid. The well-defined diffraction peaks at ∼19°, 31°, 37°, 45°, 59°, and 65° are indicative to crystalline spinel structure nanosized Co3O4, which is in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS File Card No. 42-1467).32 The results of XRD analysis reveal broad diffraction peaks, for example, with the full width at halfmaximum (fwhm) of the (311) peak being 0.948°. The size (S) of the nanocrystals is calculated to be ca. 9.8 nm, using the Scherrer formula: S=

Kλ B cos θ

where K is a constant (K = 0.9), λ the wavelength of the X-ray (1.54056 Å), B the fwhm (in radians), and θ the Bragg angle of the (311) peak. The calculated value agrees well with the microscopy data. Graphene nanosheets displayed a relatively low diffraction peak for graphitic (002), which is indicating that significant face-to-face stacking is broken, because of the introduction of Co3O4 nanoparticles on both sides of graphene sheets.33,34 To quantify the amount of graphene in the Co3O4/ N-G hybrid, TG analysis was carried out at a heating rate of 10 °C min−1 in an air atmosphere. As shown in Figure S1 in the Supporting Information, the TG curve of N-graphene reveals an completely thermal decomposition in an air atmosphere, while for the Co3O4/N-G hybrid, as presented in Figure 1d, a weight loss (∼2.0 wt %) below 110 °C is attributed to the evaporation of moisture and residual solvents. The gradual weight loss (∼3.0 wt %) beginning from 110 °C can be assigned to the decomposition of residual oxygen groups in graphene. An abrupt weight loss (∼18 wt %) occurs between 200 °C and 400 °C, indicating the oxidation and decomposition of graphene in air. In addition, to further confirm the composition of the residual sample after TG analysis of Co3O4/ N-G hybrid, XRD analysis has been carried out. As shown in Figure S2 in the Supporting Information, the XRD pattern of the residual sample after TG analysis of Co3O4/N-G hybrid is indicative of crystalline spinel structured Co3O4, which is in good agreement with the JCPDS data (File Card No. 42-1467). Therefore, it can be calculated that the mass fraction of graphene in the Co3O4/N-G hybrid is ∼20 wt %.35−38

Figure 3. Linear sweep voltammetry (LSV) for different samples in O2-saturated neutral PBS at a rotating speed of 1600 rpm. (Catalyst loading: 0.051 mg cm−2, normalized to the surface area of a 5-mmdiameter GC electrode.)

Co3O4/N-G electrode are significantly improved than those of the free Co3O4 nanocrystals and N-graphene, but not as good as the Pt/C catalyst. The results suggest that the Co3O4/N-G hybrid exhibits a high catalytic activity when free Co3O4 nanocrystals loaded on nitrogen-doped graphene, which can be therefore inferred that the high ORR activity results from the synergetic chemical coupling effect between Co3O4 and nitrogen-doped graphene. The long-term stability of catalysts is one of the major concerns in microbial fuel cell technology. The stability of Co3O4/N-G and Pt/C catalysts was tested at a constant voltage of −0.4 V (vs Ag/AgCl) for 20 000 s in O2-saturated 0.1 M PBS (Figure 4). The chronoamperometric response of Co3O4/N-G, under neutral conditions, retained a higher relative current of ∼83%, compared to Pt/C, which showed a relative current of ∼66% after 20 000 s of operation. This result indicates that the long-term stability of Co3O4/N-G in neutral PBS is better than that of the Pt/C catalyst. The electrochemical impedance spectra (EIS) carried out at potential of −0.3 V and frequency range of 0.1 Hz to 10000 Hz were used to investigate the remarkable rate performances of 6079

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Figure 4. Chronoamperometric responses of Co3O4/N-G and Pt/Cmodified GC electrodes at −0.4 V in O2-saturated 0.1 M PBS.

Co3O4/N-G and Pt/C. As demonstrated in Figure 5, the Nyquist plots show the diameter of the semicircle for Pt/C in

Figure 6. (a) Photograph of the H-shaped aqueous cathode MFC and (b) the corresponding schematic of MFC to generate electricity.

Figure 5. Electrochemical impedance spectra of Co3O4/N-G and Pt/ C-modified GC electrodes; inset shows the equivalent circuit of the electrochemical interface.

diagram of the H-shaped aqueous cathode MFC to generate electricity. As shown in Figure 7, the open-circuit voltage for Co3O4/NG hybrid was 0.68 ± 0.05 V and displayed a power density of 1340 ± 10 mW m−2, which is as high as nearly four times of plain cathode (340 ± 10 mW m−2, 0.36 ± 0.05 V), and only slightly lower than commercial Pt/C catalyst (1470 ± 10 mW

the high−medium frequency region is smaller than that of Co3O4/N-G, which suggests that Pt/C electrodes possess lower contact and charge-transfer resistances. The kinetic differences of Pt/C and Co3O4/N-G electrodes were further investigated by fitting Nyquist data to a hypothetical equivalent circuit40−42 (inset of Figure 5). In the equivalent circuit, Re is the electrolyte resistance, and C and Rct are the double-layer capacitance and charge-transfer resistance, respectively. Zw is the Warburg impedance related to the diffusion of oxygen into the bulk electrodes. The value of charge-transfer resistance (Rct) for Co3O4/N-G was 101.2, while the Pt/C electrode had an Rct value of 67.06, which could be inferred as the cause of superior electrocatalytic activity of Pt/C than Co3O4/N-G. To study the possibility of using Co3O4/N-G as an alternative cathode catalyst to Pt/C, we set up a homemade, pure-bactierium-inoculated H-shaped aqueous cathode MFC (Figure 6a) and examined the MFC performances with (Co3O4/N-G and Pt/C) and without cathode catalyst (plain ITO). Figure 6b demonstrated the corresponding schematic

Figure 7. Polarization curve (thin line) and power density curve (bold line) of MFC with cathode catalysts of Co3O4/N-G and Pt/C and without catalyst. (Catalyst loading: 1 mg cm−2, normalized to the cathode surface area.) 6080

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m−2, 0.83 ± 0.05 V). These results suggest that, using Co3O4/ N-G hybrid as the cathode catalyst in pure-bactieriuminoculated MFCs, the oxygen reduction reaction can be efficient and comparable to commercial platinum catalyst. Therefore, extending to the more representative and practical anodic systems (for example, a mixed-bacteria-inoculated MFCs system), Co3O4/N-G can also be expected to replace the expensive Pt/C as a promising cathode catalyst.

(4) HaoYu, E.; Cheng, S.; Scott, K.; Logan, B. Microbial fuel cell performance with non-Pt cathode catalysts. J. Power Sources 2007, 171, 275. (5) Lu, M.; Guo, L.; Kharkwal, S.; Wu, H.; Ng, H. Y.; Li, S. F. Y. Manganese−polypyrrole−carbon nanotube, a new oxygen reduction catalyst for air-cathode microbial fuel cells. J. Power Sources 2013, 221, 381. (6) Dong, G.; Huang, M.; Guan, L. Iron phthalocyanine coated on single-walled carbon nanotubes composite for the oxygen reduction reaction in alkaline media. Phys. Chem. Chem. Phys. 2012, 14, 2557. (7) Li, S.; Hu, Y.; Xu, Q.; Sun, J.; Hou, B.; Zhang, Y. Iron- and nitrogen-functionalized graphene as a non-precious metal catalyst for enhanced oxygen reduction in an air-cathode microbial fuel cell. J. Power Sources 2012, 213, 265. (8) Yuan, Y.; Ahmed, J.; Kim, S. Polyaniline/carbon black compositesupported iron phthalocyanine as an oxygen reduction catalyst for microbial fuel cells. J. Power Sources 2011, 196, 1103. (9) Zhao, F.; Harnisch, F.; Schröder, U.; Scholz, F.; Bogdanoff, P.; Herrmann, I. Application of pyrolysed iron(II) phthalocyanine and CoTMPP based oxygen reduction catalysts as cathode materials in microbial fuel cells. Electrochem. Commun. 2005, 7, 1405. (10) Liu, X.; Sun, X.; Huang, X.; Sheng, G.; Zhou, K.; Zeng, R. J.; Dong, F.; Wang, S.; Xu, A.; Tong, Z.; Yu, H. Nano-structured manganese oxide as a cathodic catalyst for enhanced oxygen reduction in a microbial fuel cell fed with a synthetic wastewater. Water Res. 2010, 44, 5298. (11) Morris, J. M.; Jin, S.; Wang, J.; Zhu, C.; Urynowicz, M. A. Lead dioxide as an alternative catalyst to platinum in microbial fuel cells. Electrochem. Commun. 2007, 9, 1730. (12) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 2009, 323, 760. (13) Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells. ACS Nano 2010, 4, 1321. (14) Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; Shi, G.; Dai, L.; Qu, L. Nitrogen-doped graphene quantum dots with oxygen-rich functional groups. J. Am. Chem. Soc. 2012, 134, 15. (15) Geng, D.; Chen, Y.; Chen, Y.; Li, Y.; Li, R.; Sun, X.; Ye, S.; Knights, S. High oxygen-reduction activity and durability of nitrogendoped graphene. Energy Environ. Sci. 2011, 4, 760. (16) Zhao, Y.; Hu, C.; Hu, Y.; Cheng, H.; Shi, G.; Qu, L. A versatile, ultralight, nitrogen-doped graphene framework. Angew. Chem., Int. Ed. 2012, 51, 11371. (17) Lin, Z.; Waller, G.; Liu, Y.; Liu, M.; Wong, C.-P. Facile synthesis of nitrogen-doped graphene via pyrolysis of graphene oxide and urea, and its electrocatalytic activity toward the oxygen-reduction reaction. Adv. Energy Mater. 2012, 2, 884. (18) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780. (19) Wang, H.; Robinson, J. T.; Diankov, G.; Dai, H. Nanocrystal growth on graphene with various degrees of oxidation. J. Am. Chem. Soc. 2010, 132, 3270. (20) Wang, H.; Liang, Y.; Mirfakhrai, T.; Chen, Z.; Casalongue, H.; Dai, H. Advanced asymmetrical supercapacitors based on graphene hybrid materials. Nano Res. 2011, 4, 729−736. (21) Shen, J.; Zhu, Y.; Yang, X.; Zong, J.; Zhang, J.; Li, C. One-pot hydrothermal synthesis of graphene quantum dots surface-passivated by polyethylene glycol and their photoelectric conversion under nearinfrared light. New J. Chem. 2012, 36, 97. (22) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets. J. Am. Chem. Soc. 2008, 130, 5856. (23) Zhao, J.; Pei, S.; Ren, W.; Gao, L.; Cheng, H.-M. Efficient preparation of large-area graphene oxide sheets for transparent conductive films. ACS Nano 2010, 4, 5245.

4. CONCLUSIONS In summary, a hybrid of nitrogen-doped graphene-loaded Co3O4 was synthesized via a facile hydrothermal method. Electrochemical measurements under neutral condition and application in an aqueous cathode microbial fuel cell (MFC) as cathode catalyst for Co3O4/N-G were examined. The results demonstrated that the Co3O4/N-G nanocomposite exhibited high electrocatalytic activity for oxygen reduction reaction (ORR) under neutral conditions and MFC performance comparable to that of a commercial Pt/C catalyst, which suggested that Co3O4/N-G could be a promising alternative to platinum for MFCs in neutral solution. With further optimization, the relative low-cost Co3O4/N-G catalysts could be improved to achieve higher power densities and promote the practical application and commercialization of MFCs.

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ASSOCIATED CONTENT

S Supporting Information *

TGA result of N-graphene in air atmosphere and the XRD pattern of the residual sample after TG analysis of Co3O4/N-G hybrid (Figures S1 and S2, respectively). Details for calculating the number of electrons transferred of Co3O4/N-G and Pt/C. Koutecky−Levich (K-L) plots for Co3O4/N-G and Pt/C electrodes (Figures S3a and S3b, respectively). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64250624. Fax: +86 21 64250624. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 21236003, 21206042, 20925621, 20976054, and 21176083), the Fundamental Research Funds for the Central Universities, the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0825), and the Shanghai Leading Academic Discipline Project (Project No. B502) for financial support.

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REFERENCES

(1) Schroder, U. Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency. Phys. Chem. Chem. Phys. 2007, 9, 2619. (2) Wen, Z.; Ci, S.; Zhang, F.; Feng, X.; Cui, S.; Mao, S.; Luo, S.; He, Z.; Chen, J. Nitrogen-enriched core-shell structured Fe/Fe3C-C nanorods as advanced electrocatalysts for oxygen reduction reaction. Adv. Mater. 2012, 24, 1399. (3) Feng, L. Y.; Chen, Y. G.; Chen, L. Easy-to-operate and lowtemperature synthesis of gram-scale nitrogen-doped graphene and its application as cathode catalyst in microbial fuel cells. ACS Nano 2011, 5, 9611. 6081

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dx.doi.org/10.1021/ie4003766 | Ind. Eng. Chem. Res. 2013, 52, 6076−6082