Power Densities Using Different Cathode Catalysts (Pt and CoTMPP) and Polymer Binders (Nafion and PTFE) in Single Chamber Microbial Fuel Cells SHAOAN CHENG, † HONG LIU, † AND BRUCE E. LOGAN* ,†,‡ Department of Civil and Environmental Engineering and The Penn State Hydrogen Energy (H2E) Center, The Pennsylvania State University, University Park, Pennsylvania 16802 Cathode catalysts and binders were examined for their effect on power densities in single chamber, air-cathode, microbial fuel cells (MFCs). Chronopotentiometry tests indicated that the cathode potential was only slightly reduced (20-40 mV) when Pt loadings were decreased from 2 to 0.1 mg cm -2 , and that Nafion performed better as a Pt binder than poly(tetrafluoroethylene) (PTFE). Replacing the precious- metal Pt catalyst (0.5 mg cm -2 ; Nafion binder) with a cobalt material (cobalt tetramethylphenylporphyrin, CoTMPP) produced slightly improved cathode performance above 0.6 mA cm -2 , but reduced performance (<40 mV) at lower current densities. MFC fed batch tests conducted for 35 cycles (31 days) using glucose showed that replacement of the Nafion binder used for the cathode catalyst (0.5 mg of Pt cm -2 ) with PTFE reduced the maximum power densities (from 400 ( 10 to 480 ( 20 mW m -2 to 331 ( 3 to 360 ( 10 mW m -2 ). When the Pt loading on cathode was reduced to 0.1 mg cm -2 , the maximum power density of MFC was reduced on average by 19% (379 ( 5 to 301 ( 15 mW m -2 ; Nafion binder). Power densities with CoTMPP were only 12% (369 ( 8 mW m -2 ) lower over 25 cycles than those obtained with Pt (0.5 mg cm -2 ; Nafion binder). Power densities obtained using with catalysts on the cathodes were ∼4 times more than those obtained using a plain carbon electrode. These results demonstrate that cathodes used in MFCs can contain very little Pt, and that the Pt can even be replaced with a non-precious metal catalyst such as a CoTMPP with only slightly reduced performance. Introduction A microbial fuel cell (MFC) is a device that uses bacteria to catalyze the conversion of organic matter into electricity (1- 9). Substrate is oxidized by bacteria generating electrons and protons at the anode. Electrons are transferred through an external circuit while the protons diffuse through the solution to the cathode, where electrons combine with protons and oxygen to form water. It is now known that no exogenous mediators need to be added into an MFC (10, 11). It has been found that several microorganisms including Shewanella putrefaciens (4, 12, 13), Geobacteraceae (5, 14-17) Clostrid- ium butyricum (18), and Rhodoferax ferrireducens (19) can produce electricity in the absence of exogenous mediators from chemicals such as glucose, acetate, lactate, pyruvate, and formate. Mixed cultures of bacteria have been also reported to generate electricity from domestic wastewater (6-8, 20) and marine sediments (4, 15). The performance of an MFC is influenced by several factors including the microbial activity, chemical substrate (fuel), type of proton exchange material (or even absence of this material), resistance of the circuit, and anode and cathode materials (4-6, 8, 9, 19, 21-23). The cathode performance is an important factor to the performance of an MFC due to the poor kinetics of oxygen reduction in the medium (6, 24). Cathode performance can be improved if Pt catalysts can be made more effective at room temperature, if the internal resistance of the reactor is reduced, or if more effective oxidants than oxygen (such as ferricyanide) are used. For example, power densities of MFCs have been increased by replacing aqueous cathodes with either direct-air carbon cathodes containing Pt (8, 24) or graphite electrodes con- taining Fe 3+ (22). Oh et al. (9) found that the maximum power achieved using ferricyanide ion as oxidant in the cathode chamber was 50-80% greater than that obtained with dissolved oxygen and Pt. Power densities as large as 6000 mW m -2 have been reported for MFCs using ferricyanide (23). However, ferrocyanide must be replaced after it is reduced, while systems using oxygen can be continuously operated and therefore self-sustaining. MFCs produce lower power densities than other types of fuel cells, but their most promising application in the near future is likely to be as a process for wastewater treatment (7, 25). Reducing the cost of the materials used to make MFCs is essential for building an economical treatment system. The typical manufactured components of an MFC are a proton exchange membrane (PEM), Pt catalyst on the cathode, and carbon electrodes. However, recent studies have shown that the proton exchange membrane is not needed for the operation of MFCs (8, 14) and that removing it actually increases maximum power densities (8). Pt is an effective catalyst used for both electrodes in hydrogen fuel cells, but it is an expensive component of the MFC cathode. While alternatives to Pt have been sought, none have approached the performance of Pt in hydrogen fuel cells (26). Pt is used on the cathode in air-cathode MFCs, so minimizing or eliminating the need for Pt can reduce the system capital costs. In most MFC studies commercially produced cathode electrodes are used that contain 0.5 mg cm -2 Pt loading (8), although graphite electrodes containing 0.28 mg/cm 2 Pt have also been used (24). The effect of different Pt loadings has not been previously examined for MFCs, and few alternatives to Pt have been explored for use in air-cathode MFCs. The effect of Pt loading on power generation has been examined for other types of fuel cells, but these systems operate under much different conditions of pH (highly acid or alkaline conditions; refs 27-29) and temperature (50-1000 °C), and therefore the results are not directly translatable to the performance of MFCs. Maximum power densities in aqueous cathode MFCs can be reduced by an order of magnitude when Pt is not used on the cathode (30). When Pt is used as a catalyst on a carbon electrode, it is usually bound to the electrode substrate using a polymer. Perfluorosulfonic acid (Nafion) and poly(tetrafluoroethylene) (PTFE) are two commonly used binders for Pt in chemical fuel cells (31-33), but Nafion can cost 500 times more than PTFE (mass basis). Nafion is a * Corresponding author phone: (814) 863-7908; fax: (814) 863- 7304; e-mail: blogan@psu.edu. † Department of Civil and Environmental Engineering. ‡ The Penn State Hydrogen Energy (H2E) Center. Environ. Sci. Technol. 2006, 40, 364-369 364 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006 10.1021/es0512071 CCC: $33.50  2006 American Chemical Society Published on Web 11/23/2005