Comprehensive study of biocathode in Bio-electrochemical system (BES) for energy harvesting and CO2 conversion

Abstract

The focus of this thesis is studying the mixed community biocathode to solve the bottlenecks for the scale-up and commercialisation of Bio-electrochemical Systems (BESs). First, the utilisation of aerobic iron-oxidising bacteria enriched from natural environment and Fe2+ in Microbial fuel cell (MFC) equipped with gas diffusion electrodes was studied as an efficient alternative to Pt-based catalysts for MFCs with unlimited oxygen diffusion. Controlling the system pH during continuous operational mode, MFCs produced maximum power density of 0.251 mW cm-2, compared to 0.134 mW cm-2 for MFCs equipped with Pt catalyst (0.5 mg cm-2 Pt on carbon paper) indicating advantages of fully microorganism catalysed system, and importance of operational parameters in such systems. In microbial electrosynthesis (MES) converting CO2 to various organic compounds, optimisation of operational parameters, such as applied potential on the cathode, pH, and operational mode were investigated. The community analysis revealed different dominant communities on the electrode and in solution suggesting an electro-active biofilm was formed on the cathode. It also suggested the importance of cathodic potentials corresponding to sufficient energy provided for MES and inorganic carbon sources (CO2 and HCO3-) on biofilm formation and composition of bacterial community in biofilm responsible for MES, respectively. It was found that -1.0 V (Ag/AgCl) was required for MES, and CO2 was preferred inorganic carbon source for microorganisms for MES. Mechanisms of MES and chain elongation were studied. The hypothesis on hydrogen mediated electron transfer mechanism was confirmed from cyclic voltammetry showing a positive shift on hydrogen evolution onset potential with optimum conditions. The role of cathode on providing electrons was explored comparing the systems operated at open circuit potential (OCP) and applied potential. In addition, two different strategies of 1) impact of continuous operational mode and, 2) addition of external electron donors were investigated to promote chain elongation through acetate, triggering crucial parameters affecting production through MES. Acetate concentration reached the maximum of 6.8 g L-1 over fed-batch mode with a production rate of 660 ppm day-1 (maximum columbic efficiency~69%), while the highest acetate production rate in this study was reached over continuous operational mode (803 ppm day-1, maximum columbic efficiency~90%). Over longer operational time and providing extra electron donors, longer chain organic acids (Butyrate) and alcohols (Butanol and Hexanol) were produced, concluding MES is a promising technology alternative to petrochemical production.