Alternating polarization steered CO2 bio-electroreduction selectivity with stepped extracellular electron transfer

Convoluted selectivity trend under mismatched electron transfer speed and a missing link between product distribution and redox microenvironment on electrode surface could hinder the applicability of bio-electrochemical CO2 reduction reaction for renewable fuels products. Herein, asymmetrically alternating po-larization was employed to establish stepped electron transfer pathways in CO2-fixation biohybrids to enhance the selectivity of CO2 to CH4 at high overpotentials. An optimized dynamic balance between oxidized and reduced humic acid improved microbes' coverage ratio and bioactivity for a maximum of 1.8-fold methane production rate (1131.0 -2954.8 mmol/m2/day). Promoting interspecies electron transfer for methane pro-duction was based on electron relay from humic acid in Clostriadiales and Bacterodiale to coenzyme F420 in Methanobacterium. Unblocking clogged electrons in humic acid with only reduced status boosted extracellular electron transfer for 1.31 -2.47-fold CH4 selectivity (57.11 -87.36 %). Alternating polarization optimized the redox microenvironment around the electrode and facilitated the electron transfer process to promote meth-anogenesis selectivity. Therefore, the appropriate polarization method provided a unique control strategy for establishing high-performance CO2-fixation systems.