Microfabrication of anodes for use in microbial fuel cells

The recent discovery that metal-reducing bacteria are also capable of reducing solid electrodes has enabled the study of microbial fuel cells or sensors utilizing bacteria as catalysts. However, use of large (~50 cm²) electrodes, cut from variable surface area graphite materials, restricts the throughput, precision, and resolution of microbial fuel cell research. Therefore, we are investigating technology for miniaturization and better characterization of the microbial fuel cell. First, a scalable technique for constructing membrane-electrode assemblies (MEAs) was developed, via the hot-pressing of carbon-based anodes to proton exchange membranes and Pt/C-based cathodes. Anaerobic glass and polycarbonate chambers able to accept these MEAs were tested with Geobacter metallireducens as the microbial catalyst at the anode and a solution of potassium ferricyanide at the cathode. These cells were found to completely oxidize substrates, converting organic compounds into electricity. To replace the anodes in these chambers, we combined a series of designs enabling silicon to act as both a porous membrane and substrate for microscale conductive parts, for current collection (W. Smyrl et. al, AlChE J. 48, 1071) as well as for high-resolution voltammetric characterization (E.Yoon et. al, Jpn J. Appl. Phys. 39, 7159). Nitride-coated silicon wafers ([100] P-type) were photolithographically patterned with a series of windows ranging from 2 mum² to 800 mum² in size, and etched completely through using both wet and dry etching techniques. Using metal evaporation, one side of this porous silicon was patterned with a 1 nm thick Ti/Au layer, providing conductive sites for microbial reduction that could be controlled for active surface area. Silicon-based anodes were tested as components in MEA-based fuel cells. Depending on the pattern of Au deposition and pore profile, anodes had characterized active surface areas ranging from a few square microns to- three orders of magnitude higher. Based on attachment and current production characteristics of G. metallireducens with these microfabricated anodes, designs were selected for refinement and miniaturization, with the goal of producing arrays of porous anodes, (approximately 0.25 cm²), for growth and testing of multiple electrode-reducing cultures, as well as flat anodes (on the order of 5 mum²) suitable for parallel, high-resolution electrochemical characterization of respiration at the microscale