Alternating direction of catholyte forced flow-through 3D-electrodes improves start-up time in microbial electrosynthesis at applied high current density

Microbial electrosynthesis is an uprising concept for the combined carbon dioxide reduction and electricity storage in the form of green chemical compounds. Although several proof of principle studies show great promise, mass-transfer limitations of substrates, protons and products remains one of the issues that needs to be addressed to bring the systems towards greater scale applications. A previously tested solution formed force flow-through catholyte recirculation, but this set-up encountered difficulties with gas accumulation during start-up at higher current densities (∼ −10 kA/m3), creating the need for a bypass to release gas. In this study, start-up at high current density was achieved without a bypass by using an alternating flow-through regime. This regime decreased the operating energy input from 221 to 136 kWh per kg of produced hydrogen and reached acetate production within 10 days after start-up at high current density and elongation to n-caproate after 45 days. Mass-transfer studies were included by microsensor measurements of local conditions (hydrogen concentration, pH) combined with thermodynamic calculations at the start and end of 60-days biotic experiments. The microorganisms on the cathode decreased pH gradients and consumed the formed hydrogen. The presence of Clostridium sensu stricto 12 and Peptococcaceae species were related to chain elongation activity, and the presence of Methanobrevibacter was linked to methanogenesis activity. By identifying the effects of different flow-through strategies on local concentrations and functional microbial groups, this work provides insights on the optimal conditions for microbial CO2 conversion and highlight the application potential of microbial electrosynthesis.