Development of an Aerobic Biocathode for Microbial Fuel

Abstract

Microbial fuel cells (MFCs), which convert organic waste to electricity using microbes, could be used to make the wastewater infrastructure more energy efficient and sustainable. However, the chemical catalysts which catalyse the oxygen reduction reaction (ORR) at the cathode of MFCs are expensive and unsustainable. Mixed community aerobic biocathode biofilms are an alternative to chemical catalysts. However, little is known about the bacteria, their metabolism, and their mechanisms of electron transfer with the electrode. A novel 4-electrode method was used to determine the minimum potential for production of peroxide on a porous carbon felt biocathode support. Biocathodes with a high onset potential for the ORR of +0.4 V vs Ag/AgCl were then cultivated in poised-potential half-cells at working electrode potentials of -0.1 and +0.2 V vs Ag/AgCl. These biofilms show what may be an electrode potential-dependent switch in an electron transfer mechanism from -0.1 to +0.2 V vs Ag/AgCl. The biofilms were dominated by unidentified Gammaproteobacteria, not present in unpolarised controls, which were most likely responsible for the ORR catalysis. This is the first time that a link has been made between a high onset potential for ORR catalysis of +0.4 V vs Ag/AgCl, and the bacteria responsible for this catalysis. Using half-cells, the aerobic biocathodes were enriched and used to replace existing abiotic Pt cathodes in operational MFCs. MFC performance was found to be limited by high external resistance and oxygen mass transfer. The MFC with a biocathode achieved a 9-fold increase in peak power from 7 to 62 W/cm2 using a carbon electrode with a biocathode compared to a plain carbon electrode. A simple battery separator was shown to be as effective as an ion exchange membrane through novel abiotic analysis of this membrane, and the MFC with a battery separator was found to give similar performance to the MFC with an ion exchange membrane.