Startup cathode potentials determine electron transfer behaviours of biocathodes catalysing CO2 reduction to CH4 in microbial electrosynthesis

Microbial electrosynthesis is an innovative technology capable of producing biofuels (particularly CH4) via CO2 reduction. Electron-uptaking from biocathode by microorganisms is the key step in the process of microbial electrosynthesis. However, the lack of understanding of electron transfer behaviors and their affecting factors largely limit the development of microbial electrosynthesis. Herein, we investigated the effects of initial startup cathode potentials (-0.7, -0.8, -0.9, -1.0 and -1.1 V vs. Ag/AgCl) on the electron transfer behaviors of biocathodes. The cyclic voltammetry (CV) showed typical sigmoidal catalytic currents corresponding to the CO2 reduction on the biocathodes started up at -0.7 and -0.8 V, but not on the other biocathodes. The CH4 production rate, Faradaic efficiency, scanning electron microscopy (SEM), and microbial community were also investigated. All the results indicated that the startup cathode potentials determine the electron transfer behaviors of biocathode: the biocathodes started up at -0.7 and -0.8 V mainly possessed a direct electron transfer manner, while the biocathodes started up at -0.9, -1.0 and -1.1 V mainly possessed an indirect electron transfer pathway. This study provides a guidance for the development of CH4-producing biocathodes with a desired electron transfer behavior.