Development of biocathode during repeated cycles of bioelectrochemical conversion of carbon dioxide to methane

Functioning biocathodes are essential for electromethanogenesis. This study investigated the development of a biocathode from non-acclimated anaerobic sludge in an electromethanogenesis cell at a cathode potential of −0.7 V (vs. standard hydrogen electrode) over four cycles of repeated batch operations. The CO₂-to-CH₄ conversion rate increased (to 97.7%) while the length of the lag phase decreased as the number of cycles increased, suggesting that a functioning biocathode developed during the repeated subculturing cycles. CO₂-resupply test results suggested that the biocathode catalyzed the formation of CH₄ via both direct and indirect (H₂-mediated) electron transfer mechanisms. The biocathode archaeal community was dominated by the genus Methanobacterium, and most archaeal sequences (>89%) were affiliated with Methanobacterium palustre. The bacterial community was dominated by putative electroactive bacteria, with Arcobacter, which is rarely observed in biocathodes, forming the largest population. These electroactive bacteria were likely involved in electron transfer between the cathode and the methanogens.