Improvement of the oxygen reduction cathodes of microbial fuel cells designed to treat municipal wastewater

Abstract

The Microbial Fuel Cell is a technology for self-powered pollution remediation, receiving widespread academic interest only since the turn of the 21st century. The device centres on immobilised anaerobic microbes that oxidise organic pollutants in industrial or domestic wastewater and generate an electrical charge. To generate useful energy from this charge, oxygen is commonly used as an electron acceptor at a cathode to complete the cell. This oxygen reduction reaction (ORR) requires catalysis and is thought to produce H2O at pH7. High cost materials such as Platinum and energy in-efficient materials such as activated carbon are typically used to catalyse this reaction in MFC’s. The majority of ORR catalysis research is based in acid or alkali media. To facilitate MFC commercialisation the project aim was to enhance cathode performance by developing an active, selective, stable and low cost oxygen reduction catalyst. Presented within this thesis is a fundamental study of the enzyme mimic catalyst Iron Phthalocyanine (FePc). With the addition of a novel anion selective binder and membranes, the low cost cathodes are applied to laboratory scale single chamber MFC’s fed with primary clarifier influent wastewater. With use of a rotating ring disc electrode, the O2 reduction mechanism was found to produce OH- and the O2 adsorption step was not rate limiting. The mechanism with the lowest overpotential proceeds through an intermediary of strongly adsorbed peroxide. Unfortunately, partial release of this H2O2, ranging from 0.5 to 7%, caused catalyst de-stabilisation. The traditional approach of catalyst pyrolysis was found to be ineffective remedy, reducing the number of viable sites (by 96%) and overall activity. It was hypothesised that pH splitting from OH- production could be reduced with anion selective materials. A Quaternary-1,4-diazabicyclo-[2.2.2]-octane Polysulfone (QDPSU) anion exchange ionomer utilising a Dabco anion exchange group was implemented in thin films and MFC cathodes as a substitute for Nafion. A facile tafel slope of 25.4 mV per decade of current implied a decrease in the overall activation energy for ORR. Oxygen diffusivity was comparable with Nafion and in real wastewater the air cathodes producing an average of 34% more power in MFC’s. An impedance spectroscopy study identified a numerical way of quantifying the poisoning of anion exchange groups. The addition of ion selective membranes increased the resistance showing this process to be related to ion diffusion, thin membranes with quaternary ammonium produced the best results.