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 x x x ( 2 0 1 2 ) 1 e8 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials Yaping Zhang a,b,1, Jian Sun a,b,2, Yongyou Hu a,b,*, Sizhe Li a,b,3, Qian Xu a,b,4 a Ministry of Education Key Laboratory of Pollution Control and Ecosystem Restoration for Industrial Agglomeration Area, College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China b State Key Lab of Pulp and Paper Engineering, College of Light Industry and Food Science, South China University of Technology, Guangzhou 510640, China article info abstract Article history: The choice of the cathode material is crucial for every bio-cathode microbial fuel cell (MFC) Received 9 June 2012 setup. The commonly used biocathode materials, Graphite felt (GF), carbon paper (CP) and Received in revised form stainless steel mesh (SSM) were compared and evaluated in terms of current density, 8 August 2012 power density, and polarization. The maximum current density and power density of the Accepted 13 August 2012 MFC with GF-biocathode achieved 350 mA m2 and 109.5 mW m2, which were higher than Available online xxx that of the MFC with CP-biocathode (210 mA m2 and 32.7 mW m2) and the MFC with SSMbiocathode (18 mA m2 and 3.1 mW m2). The polarization indicated that the biocathode Keywords: was the limiting factor for the three MFC reactors. Moreover, cyclic voltammetry (CV) Microbial fuel cell showed that the microorganisms on the biocathode played a major role in oxygen reduc- Biocathode tion reaction (ORR) for GF- and CP-biocathode but SSM-biocathode. Electrochemical Graphite felt impedance spectroscopy suggested that GF biocathode performed better catalytic activity Carbon paper toward ORR than that of CP- and SSM-biocathode, also supported by CV test. Additionally, Stainless steel mesh the MFC with GF-biocathode had the highest Coulombic Efficiency. The results of this study demonstrated GF was the most suitable biocathode for MFCs application among the three types of materials when using anaerobic sludge as inoculums. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Microbial fuel cell (MFC) is a bio-electrochemical device that utilizes microorganisms as catalysts to decompose organic or inorganic matter and simultaneously harvest electricity [1,2]. The conventional MFC consists of an electronegative bioanode and an electro positive abiotic cathode that are separated by a proton exchange membrane [3]. Cathode plays the role of an * Corresponding author. College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China. Tel.: þ86 20 39380506; fax: þ86 20 39380508. E-mail addresses: [email protected] (Y. Zhang), [email protected] (J. Sun), [email protected] (Y. Hu), scutlsz@gmail. com (S. Li), [email protected] (Q. Xu). 1 Tel.: þ86 15920480187. 2 Tel.: þ86 20 39383779. 3 Tel.: þ86 15989036737. 4 Tel.: þ86 15013030126. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.064 Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064 2 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 x x x ( 2 0 1 2 ) 1 e8 electron sink to accept electrons and protons that are produced from the anodic oxidation of substrates. Generally, oxygen is the preferred electron acceptor due to its limitless availability, high redox potential, and the lack of chemical waste product [4]. However, the sluggish kinetics of the oxygen reduction reaction (ORR) in a medium at a pH near neutral diminishes the oxygen cathode performance [5,6]. Thus, several types of catalysts, such as platinum [5], manganese oxides [7,8], iron complexes [4,9], and cobalt complexes [10], have been investigated as cathodic catalysts to enhance the ORR (decrease the overpotential) in MFCs. Unfortunately, these catalysts are often expensive, unsustainable or time-consuming in preparation, and might be subject to poisoning or secondary pollution, which is detrimental to further scale up in waste treatment scenarios. Recently, microbial bio-cathodes in which microorganisms are the electrocatalytic agents of the desired oxygen reduction reaction have received considerable attention [1,2]. He and Angenent [11] reviewed several possible biological cathodic processes, which included the reduction of oxygen. Clauwaert et al. [12] firstly demonstrated bio-cathode MFC using microorganisms as catalyst and achieved the reduction of oxygen and nitrite. Compared with an abiotic cathode MFC, microbial bio-cathode MFC has the advantage of self-regeneration of the catalyst, low cost, sustainability, and greatest activity at neutral pH. The microbial metabolism in bio-cathodes may also be utilized to produce valuable products or to remove unwanted compounds, for example, hydrogen [13] or methane [14] bioproduction, nitrate [12] or perchlorate [15] reduction. Electrode material is known to play an important role in MFCs. The characteristics and configuration of biocathode materials are the major factors affecting the performance of biocathode [16]. A wide variety of materials have been examined as bio-cathodes in MFCs, including carbon paper [17], graphite fiber brush [18], graphite felt [12], and stainless steel mesh [19]. De Schamphelaire et al. [20] reported that carbon felt was more suitable than stainless mesh for biocathodes. In another study under a polarized potential of 0.6 V vs Ag/AgCl, stainless steel performed better than graphite in supporting biocathode when the reactor was inoculated with Geobater sulfurreducens [21]. The interaction between the microbial biofilms and electrode surface has to be considered to be decisive for the overall cathode performance, such as microbial attachment, electron transfer, electrode resistance and the rate of electrode surface reaction [16,22]. Thus, the choice of the cathode material is crucial for every bio-cathode MFC setup. Nevertheless, to our knowledge, no systematically comparative study on the bioelectrocatalytic activity of mixed culture microbial biofilms on different biocathode materials using an identical source of bacteria as inoculums has been conducted so far. In this study, we present the investigation of MFCs with three different materials used as bio-cathodes for oxygen reduction based on aerobic microorganisms as catalysts. Performance of these bio-cathode MFCs were compared and evaluated via current density, power density, and polarization curves. Moreover, the electrochemical capability of cathodic biofilms and the electrochemical impedance behavior of the three different bio-cathodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. This work aimed to study and compare the commonly used biocathode materials, Graphite felt (GF), carbon paper (CP) and stainless steel mesh (SSM) in MFCs, and could help to advance the knowledge base needed for the biocathode MFC designs and applications. 2. Materials and methods 2.1. MFC construction and operation The anodic chamber was 2 cm long and 5 cm in diameter and its volume was 40 mL with a net volume (liquid volume) of 30 mL. For the MFC start-up, the carbon paper coated with Pt of 0.5 mg cm2 (Shanghai Hesen Co., Ltd.) was used as the cathode. The coated side of the cathode was placed facing the cation exchange membrane (CEM, Zhejiang Qianqiu Group Co., Ltd.), with the uncoated side directly exposed to air. Graphite felt with a projected surface area of 7 cm2 was used as an anode without further treatment and was positioned in the chamber at a distance of 0.5 cm from the CEM. The singlechamber MFC reactors were inoculated using anaerobic sludge collected from the Liede municipal wastewater treatment plant, Guangzhou, China. All reactors were fed with a medium containing sodium acetate (1000 mg L1), a phosphate buffer solution (PBS, 100 mM), minerals (12.5 mL L1), and vitamin solution (12.5 mL L1) [23]. After the output voltage was stabilized at 350 mV (with an external resistance of 1000 U), the MFC was operated in a dualchamber model. The cathodic chamber was of the same shape and size to the anodic chamber. Graphite felt (GF, thickness of 5 mm, diameter of 15 mm, Beijing Sanye Co., Ltd.), carbon paper (CP, thickness of 0.35 mm, diameter of 10 mm, Shanghai Hesen Co., Ltd.) and stainless steel mesh (SSM, thickness of 100 mm, diameter of 50 mm, Anping Count Resen Screen Co., Ltd. China), with a projected surface area of 7 cm2, were used as the cathode, respectively. Prior to use, these electrodes were soaked in the mixture of H2SO4eHNO3 (volume ratio: 3:1) for 1 h and subsequently in de-ionized water for 24 h. The biocathodic chamber inoculation was the same as the anodic chamber and was fed with a similar medium to the anode without the sodium acetate. The cathodic chamber was continuously aerated with air as a cathodic electron acceptor by using a fish pump. All MFCs were operated in fed-batch mode and conducted in a temperature-controlled room at 30 1 C. The anode solution was refreshed when the voltage decreased to below 20 mV. 2.2. Analysis and calculation The voltage of the MFC across a 1000 U external resistor was recorded every 5 min with a multimeter and a data acquisition system (Model 2700, Keithly Instruments, USA). Polarization curves were obtained by varying the external resistance (Rex) over a range from 50 to 8000 U when the voltage output approached a steady and repeatable state. Current density (A m2) was calculated as I ¼ U/(RexS ), and power density (mW m2) was calculated according to P ¼ 1000 UI/A, where I (A) is the current, U (V) is the voltage, and A (m2) is the projected surface area of the cathode. Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064 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 x x x ( 2 0 1 2 ) 1 e8 3 The soluble chemical oxygen demands (COD) were measured according to standard methods [24] and the COD removal efficiency (dCOD) was calculated as dCOD ¼ (CODint CODout)/CODint 100%, where CODint represents the initial COD concentration (mg L1) in the feed and CODout denotes COD concentration at the end of the batch test. The Coulombic Efficiency (CE) was obtained as CE (%) ¼ CP/CT 100%, where CP is the total Coulombs calculated by integrating the current over time, and CT is the theoretical amount of coulombs based on COD removal by assuming 4 mol of electrons/mol of COD. Electrochemical analysis of cathodic biofilms and the electrochemical impedance behavior of the three different bio-cathodes in MFCs were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), which were performed using an electrochemical workstation (Model 2273, Princeton Applied Research) with a three-electrode consisting of a working electrode (the three different bio-cathodes), a saturated calomel electrode (SCE, þ0.242 V vs standard hydrogen electrode, SHE) reference electrode, and a platinum foil counter electrode. CV tests were performed under open circuit voltage (OCV) conditions and the voltage was changed from 0.6 V to 0.3 V in forward and reverse scans at a scan rate of 25 mV s1. EIS tests were conducted under OCV conditions over a frequency range of 10 kHze5 mHz with sinusoidal perturbation of 10 mV amplitude and the obtained data were fitted and simulated to predetermined equivalent electrical circuit and were then analyzed using ZSimpWin 3.10 software (Echem, US). The surface morphologies of bio-cathodes were observed by an environmental scanning electron microscope (SEM) (XL30, Philips Holland). Before observation, the bio-cathodes sample with bacterial was collected and fixed overnight with paraformaldehyde and glutaraldehyde in a buffer solution (0.1 M cacodylate, pH ¼ 7.5, 4 C), followed by washing and dehydration in water/ethanol. Samples were then coated with Pt before SEM observation. 3. Results and discussion 3.1. Power output and polarization Stable voltage (350 mV, 1 KU) produced from the MFCs with Pt cathodic catalyst was observed in several consecutive cycles, showing that exoelectrogenic biofilm had been acclimated on anode. The MFC was then installed with a two-chamber mode and the anaerobic sludge was inoculated into the cathode chamber. To our knowledge, GF and CP are commonly used as anode materials for MFCs [12,17], while SSM used for marine MFCs [21], and they performed differently. Their performance as biocathode was also different, as shown in Fig. 1A and B. In the MFC with GF, the maximum current density and maximum power density reached 350 mA m2 and 109.5 mW m2, respectively. By comparison, those were 210 mA m2 and 32.7 mW m2 for the MFC with CP, while only 18 mA m2 and 3.1 mW m2 for the MFC with SSM, respectively. During the 50 days operation, the current density of all the three MFCs kept at a stable level. Fig. 1 e Performance of MFC equipped with graphite felt (GF), carbon paper (CP) and stainless steel mesh (SSM) as biocathode: (A) current density as a function of time, (B) power density and cell polarization curves, and (C) anode and cathode polarization curves. The polarization curves in Fig. 1C showed the anode potential was not appreciably affected by cathode, and the difference in cathode potential (the open circuit potential and slop of the cathode polarization curve) was consistent with the orders in power density for MFCs with different cathode materials. For instance, the open circuit potential of SSM biocathode (5.9 mV vs SCE) was lower than that of GF (54.6 mV vs SCE) and CP biocathode (76.1 mV vs SCE). Also, the Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064 4 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 x x x ( 2 0 1 2 ) 1 e8 slop of SSM biocathode polarization curve showed a more rapid decrease as compared to that of GF and CP biocathode. As a result, the difference in performance of the MFCs can be attributed to the difference in biocathode performance. The results obtained in this section showed that GF used as biocathode performed better than CP and SSM, which was probably due to better growth of microorganisms with electrocatalytic oxygen reducing activity on GF-biocathode. It seems most likely that the rough surface of GF is responsible for the better development of electrochemically active microbial biofilm onto the electrode. However, the current and power density of the MFC with GF, CP and SSM were lower than those reported previously [25,26]. In addition, it was evident from Fig. 1C that the cathode was the limiting factor in these MFCs. For example, in the MFC with GF, with increased current densities of 0e1.33 A m2, the anode potential increased insignificantly from 548 to 500 mV, whereas the biocathode potential dropped from 55 to 454 mV. The larger driving force with an overpotential of 510 mV required for the biocathode compared to the value of 48 mV required for the anode indicated that power generation from the MFC was dominated by cathode polarization. Therefore, biologicallycatalyzed reactions toward oxygen reduction reaction (ORR) of the biocathode could be improved to enhance the MFC performance. Potentially, modification of anode electrode surface to enhance the electron transfer between electrode and bacteria might be employed to strengthen the electron transfer rate to oxygen in ORR of biocathode in MFCs [17] and this experiment is conducting in our laboratory. Moreover, CV and EIS tests were carried out to determine the electrocatalytic behavior of different biocathode materials in MFCs in the following part. 3.2. Fig. 2 e Cyclic voltammetry curves for (A) raw cathode electrodes and (B) cathode electrodes with biofilm attached. Electrochemical characterization of the biocathode CV was performed to examine the catalytic behavior of the biocathodes. As shown in Fig. 2A, no distinct redox peak was observed from all three abiotic cathode either in air- or nitrogen-saturation solution, but all the currents were higher in air-saturation solution than that in nitrogen-saturation solution because of the oxygen reduction on the abiotic electrode. By comparison, the abiotic GF exhibited higher catalytic activity than the abiotic CP and SSM electrodes. In order to further ascertain the biofilm-associated ORR, the three biocathodes were tested under the same conditions. As shown in Fig. 2B, for the GF biocathodes, the maximum current of 9 mA was reached at a cathode potential of 0.6 V vs SCE in airsaturation solution. However, the current was considerably decreased to 0.8 mA when the solution was purged with nitrogen, indicating that indeed oxygen reduction was the reaction that was catalyzed. When comparing these results with the CV of abiotic cathode in air-saturation solution (Fig. 2A), the maximum current of GF biocathodes was much higher than that of the abiotic GF cathodes, suggesting that indeed the ORR was catalyzed by the microorganisms. The CP biocathode showed similar catalytic behavior to the GF biocathode, the main difference being a lower maximum current compared to the GF biocathode. In addition, one pair of redox wave was observed for the GF and CP biocathodes either in airor nitrogen-saturation solution (Fig. 2B). It proved that some components in the culture could be oxidized or reduced during the potential sweep. One possibility is that they were redox active compounds excreted by microorganism, some of which have the ability to react with oxygen and give an electrocatalytic effect on the CV [27]. In contrast, the CV of SSM Fig. 3 e EIS (Nyquist plots) of the biocathode in MFCs. The inset shows the details at high frequency. Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064 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 x x x ( 2 0 1 2 ) 1 e8 Fig. 4 e COD removal and coulombic efficiency (CE) of biocathodic materials. biocathode in Fig. 2B showed similar behavior and no redox wave in air-saturation solution as compared with that in nitrogen-saturation solution, implying that the microorganism developed poorly on the SSM electrode. The results demonstrated that GF biocathode performed better catalytic activity toward ORR than that of CP and SSM biocathodes, which was consistent with the power generation. Clearly, further studies are warranted to better understand the mechanism of ORR in biocathodes. After 50 days operation from initiation, the charge transfer resistance (Rct) and diffusion resistance (Rdif) were 5 estimated from Nyquist plots (Fig. 3) to investigate the biocathode performance of MFCs. The inset in Fig. 3 illustrated the high-frequency part of the result. All plots consisted of semicircles at high-frequencies and straight-line features in a low-frequency region, corresponding with the Rct at the cathode interface and the Rdif (diffusion process of oxygen at the electrode/electrolyte interface), respectively. As seen from the inset in Fig. 3, plots of GF-biocathode and CP-biocathode revealed a smaller depressed semi-circle than that of SSM-biocathode. Here, the value of Rct is indicated by the diameter of the first semicircle in the Nyquist curve [28]. As shown in Fig. 3, the Rct of GF-biocathode was approximately 11 U, whereas that of CP-biocathode was approximately 23 U, and both were much lower than that of SSM-biocathode (w820 U). It is well known that the electrochemical reaction rate is inversely proportional to the electron transfer resistance (Rct) [29]. Therefore, it can be concluded that GF-biocathode exhibited the best catalytic behavior toward ORR. This result also indicated that the SSM-biocathode required a greater overpotential than that of GF-biocathode and CP-biocathode. This could be one of reasons why the MFCs with GF-biocathode and CPbiocathode showed a better performance than the MFC with SSM-biocathode. The straight line region over low-frequency of SSMbiocathode was significantly smaller than that of GFbiocathode and CP-biocathode as shown by the linear portion. This revealed that Rdif was dominant for GFbiocathode and CP-biocathode but SSM-biocathode. The high values of Rdif for GF-biocathode and CP-biocathode showed that the diffusion term significantly contributed to the oxygen reduction process, which was partly a result of the Fig. 5 e SEM images of biocathodes from (A) graphite felt, (B) carbon paper, (C) stainless steel mesh and (D) amplification of stainless steel mesh (D). Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064 6 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 x x x ( 2 0 1 2 ) 1 e8 poor solubility, the shortage and limited diffusivity of oxygen in water [30]. This meant that the mass transfer (diffusion process) was the main factors limiting the performance of GFbiocathode and CP-biocathode. It was reported that increasing aeration fluxes or pumping pure oxygen instead of air could lead to a decrease in diffusion resistance [31]. Therefore, these methods could be employed and helpful for improving the performance of GF- and CP-biocathode. When compared with GF- and CP-biocathode, the SSM-biocathode had a smaller Rdif and a larger Rct, revealing that the ORR rate at SSM-biocathode was not limited by diffusion but by charge transfer, which was likely due to the less developed bacteria on SSM-biocathode. Overall, the EIS tests demonstrated that GF was the most suitable biocathode for MFCs application among the three materials, which was in good agreement with the result of the current generation and CV test. 3.3. COD removal and CE COD removal and CE for different biocathode materials were calculated upon a batch-operated basis, as shown in Fig. 4. The acetate removal efficiency remained at a high level for all MFCs with the three biocathodes. 93.8% and 91.9% of COD was removed for MFC with GF-biocathode and CP-biocathode, respectively. These values were a little higher than that of MFC with SSM-biocathode (86.3%). The MFC with GF- Table 1 e Cathode materials, configuration and performance in MFCs. Cathode materials Electrode Cathode configuration size/chamber volume Catalyst MFC configuration Maximum power density (W m3) Reference Graphite felt Plane 7 cm2; 30 mL Anaerobic sludge Dual chamber 2.6 Graphite felt Plane 110 mL Carbon paper Plane 7 cm2; 30 mL Aerobic/anaerobic sludge Anaerobic sludge Dual-chamber flat plate MFC Dual chamber 14.1 (cathode volume) 0.8 Carbon paper Carbon paper Plane Plane Stainless steel mesh Plane 9 cm2; 30 mL 25 cm2; 125 mL 7 cm2; 30 mL Anaerobic sludge Dual chamber Mixed culture of Dual chamber denitrify bacteria Anaerobic sludge Dual chamber 2.5 0.19 (anodic volume) 0.07 Stainless steel mesh Plane 368 cm2 Earthen pot Carbon nanotubes/chitosan-coated carbon paper Graphite fiber brush Carbon cloth Graphene-coated carbon cloth Graphite granules/graphite fiber brush Plane 9 cm2; 30 mL Oxidation pond sewage Anaerobic sludge Dual chamber 1.7 (anodic volume) 5.7 [17] Brush Plan Plan Packed Aerobic sludge Anaerobic sludge Anaerobic sludge Topsoil from turf Dual Dual Dual Dual chamber chamber chamber chamber 68.4 (anodic volume) 8.1 (cathode volume) 16.2 (cathode volume) 99.8 (cathode volume) [18] [40] [40] [25] Topsoil from turf Dual chamber 72.8 (cathode volume) [25] Topsoil from turf Dual-chamber flat plate MFC Dual-chamber flat plate MFC Dual-chamber flat plate MFC Dual-chamber flat plate MFC Dual chamber 72.4 (cathode volume) [25] 20.1 (cathode volume) [26] 24.3 (cathode volume) [26] 14.7 (cathode volume) [26] 32 (total MFC volume) [41] Dual chamber 2.55 (total volume) [42] Dual chamber 15 (total volume) [35] Cylindrical dual 83 (total volume) chamber with a internal anode chamber [12] Graphite granules Packed Graphite fiber brush Brush 56 mL 5 cm2; 10 mL 5 cm2; 10 mL Diameter: 1e5 mm; 51 mL Diameter: 1e5 mm; 51 mL 51 mL Semicoke Packed 110 mL Activated carbon Packed 110 mL Carbon felt Plane 110 mL Manganese/iron solution-modified Granular carbon Graphite granules Packed Packed Diameter: 2.5e4 mm; 75 mL 319 mL Carbon fiber Packed 12 14 cm; Manganese oxide-coated graphite felt Tubular 40 mL Aerobic/anaerobic sludge Aerobic/anaerobic sludge Aerobic/anaerobic sludge Digested sludge Aerobic/anaerobic sludge Effluent of anode chamber Mixture of sediment, aerobic and anaerobic sludge This study [16] This study [17] [34] This study [19] Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064 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 x x x ( 2 0 1 2 ) 1 e8 biocathode had the highest CE as compared to that of CP- and SSM-biocathode, and this agreed with the results of polarization and power measurements. This likely resulted from faster oxygen reduction kinetics of GF-biocathode with more developed microorganisms as catalysts, promoting quantitative conversion of organic substrates to Coulombs. However, the CE in this study was only in the range of 1.5e11.7% at the external resistance of 1000 U, indicating that a substantial number of electrons were lost. The factors impact electron recovery can be complicated and mainly appeared to result from the diffusion of molecular oxygen from the cathode to the anode [32,33] and the loss of substrate due to alternative microbial (such as acetoclastic methanogenesis) processes in the anode chamber [34]. 3.4. Morphological characterization of biocathodes SEM images of biocathodes showed sparse coverage of bacteria (Fig. 5). Specially, Fig. 5A and B suggested biomass attached to the surface of GF- and CP-biocathode, which was in contrast to Fig. 5C and D, where significantly less biomass attachment was visible on the SSM-biocathode. Compared with GF and CP, the surface of SSM is more smooth, probably less suitable for efficient microbial biofilm formation. The low power production from MFC with SSM-biocathode seemed due to increased charge transfer resistance with less developed bacteria on the cathode. The amounts of bacteria observed on SEM images were consistent with the results of power generation, EIS test and polarization behavior as discussed above. Therefore, it is assumed here that amounts of bacteria on the biocathode should be one of the factors to determine the charge transfer resistance and power generation. Overall, the bacterial densities on all the three biocathodes were exceptionally low, which was similar to previous studies that also showed low microbial densities on the biocathode for ORR [35]. This could be one of reasons why the cathode was the limiting factor in biocathode MFCs. It is more difficult to obtain electron-accepting biofilms on cathodes under anoxic conditions than it is to produce exoelectrogenic biofilms on anodes [36]. Thus, surface modifications such as carboxylation, nanostructured materials modification and immobilization of electron redox mediators [37e39] likewise to the anode modification should be employed and could increase the catalytic activity and improve the colonization on the biocathode. 3.5. Comparison to previous biocathode studies Biocathode alleviates the need to use noble metal or nonnoble metal catalysts for ORR, which increases the viability and sustainability of MFCs. As shown in Table 1, biocathode electrodes are mainly composed of graphite felt, carbon paper, graphite granules, graphite fiber brush, carbon fiber, carbon felt and activated carbon, as well as stainless steel mesh. The power densities obtained in our test were lower than those of previously reported in Table 1. Except of biocathode material, the different reactor configuration, inoculation, operation parameters and anodic performance might also be responsible for the different performance of MFCs. So this comparison between different studies did not appear to be proper. However, GF, CP and SSM biocathode should be optimized to 7 improve its performance, thus enhancing the power generation of MFCs. Recently, Liu et al. [17] fabricated a biocathode by electrodepositing carbon nanotubes and chitosan onto a carbon paper, and the use of this nanocomposite increased the maximum power density of MFC by 130%. Zhuang et al. [40] reported that the maximum power density achieved by using graphene-modified carbon cloth could be 2 times higher than that achieved with bare carbon cloth. Exploration of appropriate nanomaterials to modify biocathode is a new topic of interest and is required to enhance the performance of MFCs. 4. Conclusion The commonly used biocathode materials, GF, CP, and SSM were tested and compared to evaluate their behaviors and suitability in biocathode MFCs when using anaerobic sludge as inoculums. The MFC with GF-biocathode exhibited the best electrochemical performance (power generation, polarization, CV and EIS performance) and the highest utilization for electricity generation (Coulombic Efficiency), followed by CPbiocathode MFC and SSM-biocathode MFC. We demonstrated that GF was the most suitable biocathode for MFCs application among the three types of materials. The results obtained in this work could help advance the knowledge base needed for the biocathode MFC designs and applications. However, to make the biocathode MFC more effective and applicable, more studies are required to further elucidate the biofilm-driven catalysis mechanisms and to optimize the electrode materials and MFC construction. 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Please cite this article in press as: Zhang Y, et al., Bio-cathode materials evaluation in microbial fuel cells: A comparison of graphite felt, carbon paper and stainless steel mesh materials, International Journal of Hydrogen Energy (2012), http:// dx.doi.org/10.1016/j.ijhydene.2012.08.064
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