nanocarbon hybrid materials as alternative cathode catalyst for oxygen reduction in microbial fuel cell

nanocarbon hybrid materials as alternative cathode catalyst for oxygen reduction in microbial fuel cell

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 8 6 8 e3 8 7 4

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Cobalt oxide/nanocarbon hybrid materials as alternative cathode catalyst for oxygen reduction in microbial fuel cell Tian-Shun Song a,b, De-Bin Wang a,b, Haoqi Wang b, Xiaoxiao Li c, Yongye Liang c,**, Jingjing Xie a,b,* a

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China b College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 210009, PR China c Department of Materials Science & Engineering, South University of Science & Technology of China, Shenzhen 518055, China

article info

abstract

Article history:

Cobalt oxide/nanocarbon hybrid materials (graphene and carbon nanotube) are used as

Received 12 November 2014

alternative cathode catalysts for oxygen reduction reaction in air-cathode microbial fuel

Accepted 19 January 2015

cell (MFC) for the first time. Electrochemical results reveal that these hybrid materials

Available online 12 February 2015

exhibit high catalytic performance. In MFCs, the maximum power density of 469 ± 17 mW m2 is achieved from the Co3O4/NCNT cathode, which is 5.3 times larger than

Keywords:

that of the NCNT cathode. This value is competitive with those obtained using Pt/C

Microbial fuel cell

(603 ± 23 mW m2). Therefore, Co3O4/NCNT nanocomposite is an efficient and cost-

Oxygen reduction reaction

effective cathode catalyst for practical MFC applications.

Cobalt oxide

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Graphene

reserved.

Carbon nanotube

Introduction Microbial fuel cells (MFCs) utilize microorganisms as catalysts, which can harvest electricity from oxidation of organic or inorganic matters [1]. Since they represent a novel technology for electricity generation and waste treatment, MFCs gets more and more research attentions recently [2,3]. Although

being a promising biotechnology, the practical application of MFCs faces many challenges. Oxygen has been considered as one of the most suitable electron acceptors in MFCs due to its high redox potential, and easy accessibility in environment compared to other cathodic electron acceptors. However, the poor kinetics of oxygen reduction reaction (ORR) at neutral pH seriously limits the power density of MFCs. Although platinum (Pt) and its alloys have been widely used to enhance the

* Corresponding author. Present address: 30 South Puzhu Road, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, PR China. Tel.: þ86 25 58139939. ** Corresponding author. Present address: 1088 Xueyuan Blvd., Xili Nanshan District, Department of Materials Science & Engineering, South University of Science & Technology of China, Shen Zhen 518055, PR China. Tel.: þ86 755 88018306. E-mail addresses: [email protected] (Y. Liang), [email protected] (J. Xie). http://dx.doi.org/10.1016/j.ijhydene.2015.01.119 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 8 6 8 e3 8 7 4

ORR in MFCs, Pt remains impractical due to its high cost and sensitivity to poisoning [4,5]. Therefore, developing costeffective and durable non-platinum catalysts for ORR has aroused extensive research interests. Several metal based catalyst, i.e. cobalt [6], manganese [7], iron [8], copper [9], nickel [10] have been found to catalyze the ORR and used as new cathode catalysts in MFCs. Among these metals, cobalt is appealing because of its high catalytic ORR activities. Several cobalt-based catalysts such as cobalt oxide [6,11,12], cobalt tetramethoxyphenylporphyrin [13,14], Co-naphthalocyanine [15] have been explored as cathodic catalysts for MFC applications. Furthermore, the ORR activity of metal based catalyst is influenced by the hybrid method [16], and the structure of supporting material for the metal active site can significantly affect the kinetics of the ORR [17]. Because of the high electrical conductivity, large surface area and high mechanical properties, nanocarbon materials (such as graphene and carbon nanotube) could be used as substrate for producing enhanced hybrid and composite materials for various electrical applications [18e20]. Indeed, nanocarbon materials have been made as Pt catalyst supports [21,22] to improve ORR activity and stability as compared to other carbon-based supports. However, the use of cobalt oxide/nanocarbon hybrid materials as cathodic catalyst in MFC remains little explored. In this study, we employed a new type of nanocarbon hybrid materials with cobalt oxide nanocrystals grown on nitrogen doped carbon nanotube or nitrogen doped graphene oxide as catalysts for MFCs. Five cathode catalysts (nitrogendoped reduced graphene oxide (NGO), Co3O4 on nitrogendoped reduced graphene oxide (Co3O4/NGO), nitrogen-doped CNT (NCNT), Co3O4 on nitrogen-doped CNT (Co3O4/NCNT)) and Pt on carbon black (Pt/C) as control were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD) and cyclic voltammetry (CV). Further electrochemical performance examinations were evaluated in MFCs with the catalysts as air-cathode. Co3O4/NCNT demonstrated comparable performance to Pt/C and outperformed other catalysts, which exhibited good cycling stability. This is expected to offer a new strategy to develop efficient cathode catalyst for improving ORR process in MFC.

Materials and methods Synthesis of oxidized carbon nanotubes (oxCNT) and GO The oxCNT and GO was prepared by a modified Hummers method from the reaction using a low concentration of oxidizing agent [23,24]. Multi-walled CNT (MWCNT) (Shenzhen Nanotech Port Co., Ltd.) was first purified by calcinations for 2 h at 500  C and washed with 10% hydrochloric acid. Graphite (1 g, Adamas 99%) was grounded with NaCl for 20 min in mortar. Afterwards, the NaCl was removed by rinsing with ultrapure water with a vacuum filtration apparatus. The purified graphite was dried in an oven at 80  C for 30 min. The purified MWCNT (1 g) (or graphite) was transferred to a 250 mL round-bottom flask. 23 mL of concentrated sulfuric acid was added, and the mixture was stirred at room

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temperature overnight. Then, the flask was moving to a 40  C oil bath and 200 mg of NaNO3 was added to the suspension and continued stirring for 5 min. This step was followed by the slow additive of 1 g of KMnO4, keeping the temperature of oil bath below 45  C. The solution was continued to stir for 30 min. Afterward 3 mL of water was added to the flask, followed by another 3 mL after 5 min. After another 5 min, 40 mL of water was added. 15 min later, the flask was removed from the oil bath, and 140 mL of water and 10 mL of 30% H2O2 were added to end the reaction. This suspension was stirred for 5 min at room temperature. It was then centrifuged and washed with 5% HCl solution twice and copious amounts of water repeatedly. The final precipitate was dispersed in 10 mL of water and lyophilized. Finally, the dry product was collected and designated as oxCNT. Graphite: The final precipitate was dispersed in 100 mL of water and bath sonicated for 2 h. Any indispensable solid was crushed down by a centrifugation at 5000 rpm 5 min. A brown homogeneous supernatant was collected and designated as GO.

Synthesis of Co3O4/NCNT, NCNT, Co3O4/NGO and NGO The preparation of Co3O4/NCNT and Co3O4/NGO was followed the method reported in the literatures [23,24], and the detailed procedures were given below: Co3O4/NCNT: 270 mg of oxCNT was dispersed in 240 mL of ethanol (EtOH) and 5 mL of water. Next, 6 mL of 0.6 M Co(OAc)2 aqueous solution was added to the suspension, followed by the addition of 5 mL of NH4OH at room temperature. The reaction was kept at 80  C with stirring for 20 h. After that, the suspension was centrifuged and washed with ethanol and water. The precipitate was calcined under N2 at 500  C for 2 h in tube furnace after lyophilization. This sample was flagged for Co3O4/NCNT. NCNT: The method for the preparation of NCNT was similar to Co3O4/NCNT without adding any Co salt in the first step. Co3O4/NGO: 90 mg of GO was dispersed in 80 mL of ethanol (EtOH) and 1.67 mL of water. Next, 2 mL of 0.6 M Co(OAc)2 aqueous solution was added to the suspension, followed by the addition of 1.67 mL of NH4OH at room temperature. The reaction was kept at 90  C with stirring for 20 h. After that, the suspension was centrifuged and washed with ethanol and water. The precipitate was calcined under N2 at 500  C for 2 h in tube furnace after lyophilization. This sample was flagged for Co3O4/NGO. NGO: In addition to the first step of water instead of Co(OAc)2 solution, NGO was completed through the same method as prepared Co3O4/NGO.

Electrode preparation The air-cathode was prepared according to a previously reported procedure [25]. Cathode electrode for MFC studies were non wet-proofed carbon cloth. On the air facing side, four poly-tetrafluoroethylene (PTFE) diffusion layers were applied. For catalyst layer was prepared by mixing catalyst and Nafion solution to form catalyst ink which painted on the side of the carbon cloth exposed to solution. The catalyst ink formulation was: 3.5 mg of catalyst, 11.6 ml of isopropyl

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alcohol, 23.3 ml of 5% Nafion solution and 2.9 ml double distilled water. The mixture was ultrasonicated for 30 min. The catalyst loaded on the air cathode was 0.5 mg cm2. Commercial Pt/C catalyst (40%, Hesen, China) was also loaded into the air cathode at 0.5 mg cm2 by using the same procedure as described above.

MFC construction and operation All MFC tests were carried out using a single-chamber air cathode MFC with empty bed volume of 25 mL. The singlechamber MFC with a diameter of 30 mm and a length of 40 mm was made of plexiglas. The anode was made of non wet-proofed carbon cloth (effective area of 7 cm2) and was fixed on one side of the MFC. A carbon cloth containing catalysts placed on the other side served as the air-cathode. The distance between anode and cathode was 40 mm. The MFCs were inoculated by mixing 5 mL anaerobic sludge with 20 mL culture media. The culture media was prepared containing the following compounds (per liter of deionized water): Glucose, 1 g; NH4Cl, 0.31 g; KCl, 0.13 g; Na2HPO4$12H2O, 11.53 g, NaH2PO4$2H2O, 2.77 g; and the trace mineral element solution, 12.5 mL [26]. For comparing the performance of MFCs with different cathode catalysts, five air-cathode single chamber MFC reactors with Co3O4/NCNT, NCNT, Co3O4/NGO, NGO and Pt/C were set up for the experiments. MFCs were operated in batch mode with 1000 U external resistance and maintained at 25  C.

Analytical methods The morphologies of the catalyst were studied by using the Field Emission-Scanning Electron Microscope (FE-SEM) (Hitachi-S-4700; Japan). The X-ray diffraction (XRD, Shimadzu) equipped with Cu Ka radiation (l ¼ 0.1541 nm) over the 2q rang of 20e70 was used to characterize the structure of the catalysts. Thermal properties of prepared materials were examined by Perkin Elmer TGA instrument under a nitrogen atmosphere with a heating rate of 20  C min1. CV was performed on a potentiostat (CHI660E Chenhua Instrument Co., China) in a three-electrode configuration. A three electrode cell assembly consisting of a working electrode, an Ag/AgCl reference electrode and a Pt sheet counter electrode was placed in 50 mM phosphate buffer (pH 7.0) and aerated by oxygen. The working electrodes were prepared as follows: 2 mg catalyst was added into 0.4 mL ethanol and 0.1 mL of 5% Nafion solution, and then subjected to ultrasonic 30 min. Subsequently, the prepared catalyst ink (2 ml) was dropped on the GC electrode. The electrode was dried at 25  C for 2 h. The potentials were shifted from 600 to 600 mV at a scan rate of 5 mV s1. The MFCs voltages were continuously monitored using a precision multimeter and a data acquisition system (Keithley Instruments 2700, USA). The polarization curve was then constructed by discharging the cell with external resistor of various resistance values from 50 to 2000 U. Internal resistance was calculated by the polarization slope method [27]. The performances of the electrode potential were measured and the coulombic efficiencies were calculated as previously described [28].

Results and discussion Characterization of Co3O4/NCNT and Co3O4/NGO To determine the mass ratio of GO or CNT in the hybrids, a thermal-gravimetric analysis method was used. The hybrids were heated in air at 500  C for 2 h to remove the carbon materials (GO or CNT). Weight losses of ~45% for GO hybrid and ~51% for CNT hybrid were measured. Thus, Co3O4 was about 55 wt% in Co3O4/NGO hybrid and 49 wt% in Co3O4/NCNT hybrid. Both NGO and NCNT exhibited a basal reflection peak at 2q ¼ 26 (Fig. 1), which correspond to the stacking of the graphitic planes. The Co3O4/NGO and Co3O4/NCNT hybrids exhibited the characteristic peaks of Co3O4 (JCPDS 01-0741656). However, the characteristic peak of NGO and NCNT disappeared, possibly due to the isolation of graphitic planes by the Co3O4 nanoparticles. Furthermore, the morphologies of nanoparticles Co3O4 on nanocarbon materials were presented in Fig. 2. The SEM image clearly reveals the nanoscale Co3O4 particles with the size of about 5e10 nm were coated uniformly on the surface of nanocarbon materials.

Electrochemical characterization of Co3O4/NCNT and Co3O4/ NGO The ORR activity of catalysts was analyzed by CV. As shown in Fig. 3, a new reduction peak was observed when the CV was conducted in O2-saturated solution with various electrodes. A reduction peak at 0.02 V and 0.06 V appeared at the Co3O4/NCNT catalyst and Co3O4/NGO catalyst, which was more positive than those at the NCNT catalyst (0.309 V) and

Fig. 1 e X-ray diffraction patterns of (a) Co3O4/NGO (b) NGO (c) Co3O4/NCNT (d)NCNT.

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Fig. 2 e SEM image of Co3O4/NCNT (A, B) and Co3O4/NGO (C, D).

MFCs performance

the NGO catalyst (0.273 V). These were close to the reduction peak of Pt/C at 0.1 V (All the potential values were versus Ag/AgCl electrode). Co3O4/NCNT exhibited maximum current (0.09 mA) at reduction peak, which was close to that of Pt/C (0.11 mA), followed by Co3O4/NGO (0.07 mA), relatively lower current output (0.04 mA and 0.02 mA) were recorded in NGO and NCNT. Compared to bare NGO and NCNT, the electrodes with Co3O4/NCNT and Co3O4/NGO showed the positive shifts of the electroreduction peak as well as the higher peak current density. This phenomenon confirmed that electrodes with nanocarbon materials supported Co3O4 have higher catalytic activity toward oxygen reduction.

The voltages produced in MFCs equipped with different cathodes were showed in Fig. 4. Reproducible cycles of electricity generation were obtained in all MFCs after inoculation. The voltage of MFC rapidly increases upon the replacement of the fresh culture media, maintains its steady value for a period time, and gradually decreases due to depletion of the substrate. The steady voltage of MFC was greatly affected by the cathode materials. Stable maximum voltage of 460 ± 1 mV and 424 ± 2 mV are obtained for the MFC with Co3O4/NCNT cathode and the MFC with Co3O4/NGO cathode respectively, while the stable maximum voltage were only 296 ± 4 mV and

Fig. 3 e Cyclic voltammogram curves of the Pt/C, Co3O4/ NGO, NGO, Co3O4/NCNT, NCNT in oxygen saturated buffer solution.

Fig. 4 e Voltage generation of MFCs produced with various cathodes.

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Fig. 6 e Voltage versus current densities curves with various cathodes.

Fig. 5 e Power densities versus current densities curves with various cathodes.

234 ± 3 mV for MFC with NGO cathode and MFC with NCNT cathode, respectively. The maximum power density (Pmax) and polarization curves as a function of current density of the different MFCs were determined at the steady value (Fig. 5). MFC with Co3O4/ NCNT cathode generated a higher power density of 469 ± 17 mW m2, followed by that of MFC with Co3O4/NGO cathode (312 ± 15 mW m2). MFC with NGO cathode (135 ± 3 mW m2) and MFC with NCNT cathode (74 ± 8 mW m2) generated the lower Pmax. The Pmax of SMFC with Co3O4/NCNT cathode was 5.3 times larger than that of the SMFC with NCNT cathode, while The Pmax of SMFC with Co3O4/NGO cathode was 1.3 times larger than that of the SMFC with NGO cathode. All the MFC with Co3O4 nanocrystal on nanocarbon materials can obtain the higher Pmax than the bare nanocarbon materials, demonstrating that both Co3O4 and nanocarbon materials for cathode catalyst have the synergistic effects to improve the MFC performance. Once the stabilized performance of MFC was determined, polarization curves (Fig. 6) were conducted by changing the external circuit load, internal resistances were also estimated from the slope of the plot of voltage versus current (Table 1). The lowest internal resistance was 270 ± 1 U in MFC with Co3O4/NCNT cathode, while the MFC with NCNT cathode had the highest internal resistance (481 ± 20 U). The result demonstrated that a combination of Co3O4 and NCNT modified cathode can generate the highest Pmax and the lowest internal resistance.

The curves of individual electrode potentials versus current densities were showed in Fig. 7. It was observed that potential variations for cathode were much more distinctive as compared with the anode potential variation for MFCs with different cathode. Anode potentials were showed almost no differences among all MFCs whereas cathode potentials varied in a wide difference. The variation in cathode potentials was mainly due to the efficiency of the different catalyst towards oxygen reduction. Other parameters of the MFCs with various cathodes were also studied as depicted in Table 1, the open circuit voltage (OCV) of the MFC with Co3O4/NCNT cathode was 629 ± 1 mV, followed by MFC with Co3O4/NGO cathode (606 ± 2 mV), which was close to that of MFC with Pt/C cathode (699 ± 3 mV), and much higher than that of NGO cathode (383 ± 7 mV) and NCNT cathode (310 ± 5 mV). The MFCs with different cathodic oxygen reduction catalysts showed the similar CE value, which could attribute to the same structure of reactors and electrode.

Table 1 e Comparison of different MFC parameters with various cathode materials. Cathode

Rin (U)

Pt/c NGO NCNT Co3O4/NGO Co3O4/NCNT

242 361 481 337 270

± 13 ± 14 ± 20 ± 26 ±1

OCV(mV) 699 383 310 606 629

± ± ± ± ±

3 7 5 2 1

Pmax (mW m2) 603 135 74 312 469

± 23 ±3 ±8 ± 15 ± 17

CE (%) 30 29 28 26 28

± ± ± ± ±

1 2 2 3 2

Fig. 7 e Electrode potentials (vs. Ag/AgCl) as a function of different cathodes.

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Fig. 8 e Performance of MFC with Co3O4/NCNT cathode during a long-term operation.

Long term stability test was studied to confirm the durability of the catalyst (Fig. 8). For each cycle, when the fresh culture media was added, electricity started to generate and increased rapidly until it reached a certain level and then started to decline. More than 16 cycles were tested at the MFC with Co3O4/NCNT cathode during over one month. It was found that power density of MFC with Co3O4/NCNT cathode kept at the almost same level. These results showed that Co3O4/NCNT has stable performance as a catalyst for ORR in a MFC. The development of oxygen reduction catalysts instead of Pt in MFCs is one of the major challenges for field scale application [29]. Under alkaline conditions, Co3O4/Nanocarbon hybrids have the high activity for ORR as previously reported [23,24]. For the first time, the performance of MFC with Co3O4/Nanocarbon hybrids as cathode catalysts was studied. The Co3O4/Nanocarbon hybrids exhibited high catalytic activity for ORR under neutral conditions in MFC. As electrode materials, the relatively new classes of carbonbased nanomaterials, graphene and carbon nanotube have their own advantages, such as high electrical conductivity, large surface area, high mechanical strength, and structural flexibility [24]. In contrast to nanocarbon materials (graphene and carbon nanotube), Co3O4/Nanocarbon hybrids showed even superior performance (Fig. 8). The excellent performance of Co3O4/Nanocarbon hybrids cathode can be attributed to the large catalyst surface area, which is provided by the nanoscale size of the Co3O4 catalyst particles (5e10 nm) and the high degree of homogeneous dispersion of Co3O4 on the surface of the nanocarbon materials (Fig. 2). The Co3O4 and nanocarbon materials also showed very good stability. As the cathodic materials, the hybrids can maintain high catalytic activity in the over one month run. Comparing the performances of the nanocarbon materials and the hybrids as the cathodic materials in MFC, it showed that Co3O4/NCNT > Co3O4/NGO > NGO > NCNT. It has been proposed that the active sites on NGO or NCNT are originated from the heteroatoms on the carbon matrix [30]. As NGO is a two-dimensional material with higher surface area than

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NCNT [31], the performance of MFC with NGO cathode was better than that of the MFC with NCNT cathode. However, the performance of MFC with Co3O4/NCNT was better than that of the MFC with Co3O4/NGO. Different from NCNT and NGO, the ORR active sites in these hybrid materials are originated from the Co3O4 nanoparticles on the carbon substrates, which are highly related to the substrate conductivity. GO needed a suitable degree of oxidation functional groups for nucleating and anchoring nanocrystals on GO sheet, however, the existence of defects and functional groups on graphene sheets would lower its electro-conductivity and limit the catalytic performance of the hybrid material. Owing to the inner graphitic walls of highly conducting network, the functionalized NCNT has better electrical conductivity than NGO. As a result, Co3O4/NCNT is an excellent catalyst for the ORR in MFC. The results showed that Co3O4/NCNT was still lower than that of the Pt. The performance of Co3O4/NCNT catalyst can be improved by catalyst loading increase and catalyst preparation process optimization.

Conclusion A new type of carbon hybrid material with Co3O4 nanoparticles grown on nanocarbon substrates was studied as cathode materials for MFC applications. The electrode modified by Co3O4/NCNT and Co3O4/NGO hybrids outperformed the NCNT and NGO in terms of catalytic activity for ORR. Furthermore, the maximum power density of 469 ± 17 mW m2 obtained with Co3O4/NCNT catalyst was close to that of the Pt/C catalyst. Good stability had been demonstrated in MFC with Co3O4/NCNT cathode with little decay for 16 cycles. The results demonstrate that these nanocarbon hybrid materials could be a class of promising alternative cathode catalyst to Pt for ORR in MFCs.

Acknowledgments This work was supported by the National Basic Research Program of China (973) (Grant No.: 2012CB721100, 2011CBA00806); the National Science Fund of China (Grant No.: 51209116, 21306083, 21390201); Fund from the State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201312); Program for New Century Excellent Talents at the Ministry of Education of China (Grant No.: NCET-11-0987); the Research Fund for the Doctoral Program of Higher Education of China (RFDP) (Grant No.: 20113221120007); the Shenzhen Fundamental Research Programs (Grant No.: JCYJ20130401144532128) and the Priority Academic Program from Development of Jiangsu Higher Education Institutions.

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