Carbon oxidation with sacrificial anodes to inhibit O2 evolution in membrane-less bioelectrochemical systems for microbial electrosynthesis

Abstract

Microbial electrosynthesis is an emerging biosynthesis technology that produces value-added chemicals and fuels and, at the same time, reduces the environmental carbon footprint. However, constraints, such as low current densities and high inner resistance, disfavor this technology for industrial-scale purposes. The cathode performance has been strongly improved in recent years, while the anode performance has not been given enough attention despite its importance in closing the electric circuit. For traditional water electrolysis, O₂ is produced at the anode, which is toxic to the anaerobic autotrophs that engage in microbial electrosynthesis. To overcome O₂ toxicity in conventional microbial electrosynthesis, the anode and the cathode chamber have been separated by an ion-exchange membrane to avoid contact between the microbes and O₂. However, ion-exchange membranes increase the maintenance costs and compromise the production efficiency by introducing an additional internal resistance. Furthermore, O₂ is inevitably transferred to the catholyte due to diffusion and electro-osmotic fluxes that occur within the membrane. Here, we proved the concept of integrating carbon oxidation with sacrificial anodes and microbes to simultaneously inhibit the O₂ evolution reaction (OER) and circumvent membrane application, which allows microbial electrosynthesis to proceed in a single chamber. The carbon-based anodes performed carbon oxidation as the alternative reaction to the OER. This enables microbial electrosynthesis to be performed with cell voltages as low as 1.8–2.1 V at 10 A m⁻². We utilized Methanothermobacter thermautotrophicus ΔH in a single-chamber Bioelectrochemical system (BES) with the best performing carbon-based anode (i.e., activated-carbon anode with soluble iron) to achieve a maximum cathode-geometric CH₄ production rate of 27.3 L m⁻² d⁻¹, which is equal to a volumetric methane production rate of 0.11 L L⁻¹ d⁻¹ in our BES, at a coulombic efficiency of 99.4%. In this study, M. thermautotrophicus ΔH was majorly limited by the sulfur source that inhibited electromethanogenesis. However, this proof-of-concept study allows microbial electrosynthesis to be performed more energy-efficiently and can be immediately utilized for research purposes in microbial electrosynthesis.