Characterization and performance of activated carbon catalysts and polymer membrane layers for microbial fuel cell cathodes and an analysis of power overshoot

Microbial fuel cells (MFCs) are a promising technology for treatment of wastewater streams in combination with electricity production. Linear sweep voltammetry (LSV, 1 mV s-1) and variable external resistances (at fixed intervals of 20 min) over a single fed-batch cycle in an MFC both resulted in power overshoot in power density curves due to anode potentials. Increasing the anode enrichment time from 30 days to 100 days did not eliminate overshoot, suggesting that insufficient enrichment of the anode biofilm was not the primary cause. Running the reactor at a fixed resistance for a full fed-batch cycle (∼1 to 2 days), however, completely eliminated the overshoot. These results show that acclimation at low fixed resistances are needed to stabilize current generation by bacteria in MFCs, and that even relatively slow LSV scan rates and long times between switching circuit loads during a fed-batch cycle may produce inaccurate polarization and power density results for these biological systems.;Membrane separators reduce oxygen flux from the cathode into the anolyte in MFCs, but water accumulation and pH gradients between the separator and cathode reduces performance. To avoid these problems, air cathodes were spray-coated (water-facing side) with anion exchange, cation exchange, and neutral polymer coatings of different thicknesses to incorporate the separator into the cathode structure. The anion exchange polymer coating resulted in greater power density (1167 ± 135 mW m-2) than a cation exchange coating (439 ± 2 mW m-2). This power output was similar to that produced by a Nafion-coated cathode (1114 ± 174 mW m-2), and slightly lower than the uncoated cathode (1384 ± 82 mW m-2). Thicker coatings reduced oxygen diffusion into the electrolyte and increased coulombic efficiency (CE = 56 – 64%) relative to an uncoated cathode (29 ± 8%), but decreased power production (255–574 mW m -2). Electrochemical characterization of the cathodes using abiotic anodes in separate reactors showed that the cathodes with the lowest charge transfer resistance and the highest oxygen reduction activity produced the most power in MFC tests. The results using hydrophilic cathode separator layers revealed a tradeoff between power and CE. Cathodes coated with a thin coating of anion exchange polymer showed the most promise for controlling oxygen transfer while minimally affecting power production.;Platinum is commonly used as the catalyst in MFC cathodes, but platinum is an expensive and limited resource. Activated carbon (AC) is a promising material for the replacement of platinum catalysts because it is inexpensive and can be made from renewable waste sources, but its catalytic performance in neutral solutions used in MFCs in not well understood. Commercially available AC powders made from different precursor materials (coal, peat, coconut shell, hardwood, and phenolic resin) were evaluated as oxygen reduction catalysts, and tested as cathode catalysts in MFCs. Carbons were characterized in terms of surface chemistry, specific surface area, and pore volume distribution, and kinetic activities were compared to carbon black and platinum catalysts using a rotating disk electrode (RDE). There was a strong inverse relationship between onset potential and the quantity of strong acid (pKa < 8) functional groups, and a larger fraction of microporosity was negatively correlated with power production in MFCs. These results showed that surface area alone was a poor predictor of catalyst performance, and that a high quantity of acidic surface functional groups was detrimental to oxygen reduction and cathode performance.;Four of the commercially available AC powders (peat, coconut shell, coal, and hardwood) were treated with ammonia gas at 700 °C in order to improve their performance as oxygen reduction catalysts. Ammonia treatment resulted in a decrease in oxygen (by 29 – 58%) and an increase in nitrogen content (total abundance up to 1.8 atomic %) on the carbon surfaces, which also resulted in an increase in the basicity of the bituminous, peat, and hardwood ACs. The kinetic activity and selectivity of ammonia–treated carbons were evaluated using a rotating ring disk electrode (RRDE), and compared to untreated ACs and platinum. All of the ammonia– treated ACs exhibited better catalytic performance than their untreated precursors, with the bituminous and hardwood based samples showing the most improvement. (Abstract shortened by UMI.).