Enhanced CO₂ bioelectromethanogenesis by Fe(OH)_x nanosheets arrays decorated graphite felt via synergy of promoting biofilm formation and the extracellular electron transfer

Electroactive biofilm development and extracellular electron transfer at the cathode represent a major bottleneck in CO₂ bioelectromethanogenesis. To address this limitation, Fe(OH)_x nanosheet arrays were electrodeposited onto graphite felt (GF) to construct a Fe(OH)_x@GF biocathode. The Fe(OH)_x@GF biocathode achieved rapid start-up within 10 days and achieved (methane) CH₄ production rate of 204.6 ± 28.0 mL/L/d at −1.0 V vs. Ag/AgCl, which was 2.3-fold higher than that of the raw GF biocathode. The highest CH₄ coulombic efficiency reached 88.4 ± 13.3%, a 1.3-fold increase over GF. Fe(OH)_x@GF enriched Methanothrix and acetate-producing bacteria, indicating acetate-mediated methanogenesis as the primary pathway in the first 20 days. The community shifted toward Methanobacterium dominance after the 20th day, suggesting enhanced autotrophic CO₂ methanogenesis via direct extracellular electron transfer (EET). Functional gene enrichment of CO₂ methanogenesis pathways further supported the accelerated electron flux into the methane production. The Fe(OH)_x@GF biocathode provides an effective strategy for regulating biofilm development and EET, thereby significantly enhancing CO₂ bioelectromethanogenesis.