Microbial Fuel Cell Cathodes With Poly(dimethylsiloxane) Diffusion Layers Constructed around Stainless Steel Mesh Current Collectors FANG ZHANG, † TOMONORI SAITO, †,‡ SHAOAN CHENG, † MICHAEL A. HICKNER, ‡ AND BRUCE E. LOGAN* ,† Department of Civil and Environmental Engineering, Penn State University, 212 Sackett Building, University Park, Pennsylvania 16802, and Department of Materials Science and Engineering, Penn State University, Steidle Building, University Park, Pennsylvania 16802 Received July 3, 2009. Revised manuscript received December 17, 2009. Accepted January 8, 2010. A new and simplified approach for making cathodes for microbial fuel cells (MFCs) was developed by using metal mesh current collectors and inexpensive polymer/carbon diffusion layers (DLs). Rather than adding a current collector to a cathode material such as carbon cloth, we constructed the cathode around the metal mesh itself, thereby avoiding the need for the carbon cloth or other supporting material. A base layer of poly(dimethylsiloxane) (PDMS) and carbon black was applied to the air-side of a stainless steel mesh, and Pt on carbon black with Nafion binder was applied to the solution- side as catalyst for oxygen reduction. The PDMS prevented water leakage and functioned as a DL by limiting oxygen transfer through the cathode and improving coulombic efficiency. PDMS is hydrophobic, stable, and less expensive than other DL materials, such as PTFE, that are commonly applied to air cathodes. Multiple PDMS/carbon layers were applied in order to optimize the performance of the cathode. Two PDMS/ carbon layers achieved the highest maximum power density of 1610 ( 56 mW/m 2 (normalized to cathode projected surface area; 47.0 ( 1.6 W/m 3 based on liquid volume). This power output was comparable to the best result of 1635 ( 62 mW/ m 2 obtained using carbon cloth with three PDMS/carbon layers and a Pt catalyst. The coulombic efficiency of the mesh cathodes reached more than 80%, and was much higher than the maximum of 57% obtained with carbon cloth. These findings demonstrate that cathodes can be constructed around metal mesh materials such as stainless steel, and that an inexpensive coating of PDMS can prevent water leakage and lead to improved coulombic efficiencies. Introduction A microbial fuel cell (MFC) is a novel technology that can be used for direct bioelectricity generation (1-6). One of the most promising applications for MFCs is wastewater treat- ment, as organic matter can be removed while at the same time producing power (1-3). Various oxidants have been used as the electron acceptor at the cathode (1, 3, 7-11), but oxygen is the most promising electron acceptor for MFC applications because it is freely available, sustainable, and oxygen reduction is a well studied and widely applied reaction. Air cathode MFCs, which have cathodes exposed to air on one side and water on the other, are the most practical approach for designing MFC cathodes due to not having to aerate the water, and their ability to generate high power densities (7-9, 12). Scale-up is an important issue for practical applications of MFCs for wastewater treatment and bioenergy production. The main challenges for commercializing scalable MFCs are developing materials that are cost-effective, efficient in power generation, and identifying architectures that can be used at larger scales. Maximum power densities in most high power MFCs are largely limited by cathode surface area and performance (9, 12, 13), and the price of cathode materials can account for the greatest percentage (47%) of the MFC capital costs (14). Most laboratory MFCs use small electrodes made of carbon cloth or paper or gas-permeable membranes coated with conductive paints. Carbon cloth purchased for fuel cell applications can be expensive (ca. $1000/m 2 ). Recent tests have shown very promising results for overcoming the high costs of the anode by using a very inexpensive carbon mesh ($10-40/m 2 )(15). The weave of the carbon mesh is very loose, and thus we found it could not be water sealed when used as a cathode (unpublished data). Tubular cathodes can be designed to provide high surface areas needed for cathodes (13, 16-19), and tubes made of carbon cloth can provide power output similar to that of flat carbon cloth on the basis of equivalent projected surface area (13), but the carbon cloth can be expensive. To reduce the cost of a tubular cathode, one approach has been to use a membrane as the supporting material for the application of a conductive coating and catalyst (16, 20). Power densities have so far been low with these alternative materials, for example 403 mW/m 2 using two ultrafiltration membrane tube cathodes and a flat carbon paper anode. Power was increased (450 mW/m 2 ) using a less expensive and flat anion exchange membrane (AEM) and a graphite fiber brush anode. Scaling up MFCs with carbon cloth or coated membranes alone is likely not practical as the resistance of these materials becomes large as the reactor size increases. The electrical resistivity of carbon materials is relatively high which can produce high electrode ohmic losses in large-scale systems. For example the electrical resistivity of graphite is 1375 μΩ cm, compared to only 42 μΩ cm for titanium (14). To avoid having large resistances, metal current collectors are often used with carbon materials to reduce the overall resistance of the cathode. In one recent approach for the MFC anode, a graphite fiber brush electrode was developed that used a twisted core titanium wire as the current collector (12). The graphite fibers provide a high surface area for bacteria, and as they are only 1.3-2.5 cm long in these brushes, voltage losses across the length of the fibers are small. Electrons can flow along the titanium wire which has a much lower electrical resistivity. Brushes have also been used as cathodes for sediment MFCs in natural systems, where dissolved oxygen is available in the water, but the cost for aeration in an engineered system and the low power densities typically produced by the sediment MFC architecture would likely prohibit the use of brush cathodes in wastewater treatment systems. Current collectors have been used to improve performance of MFC cathodes. For example, power was increased from 450 to 575 mW/m 2 by adding a piece of * Corresponding author phone: (1)814-863-7908; e-mail: blogan@ psu.edu. † Department of Civil and Environmental Engineering. ‡ Department of Materials Science and Engineering. Environ. Sci. Technol. 2010, 44, 1490–1495 1490 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 4, 2010 10.1021/es903009d  2010 American Chemical Society Published on Web 01/25/2010