Electrocatalytic CO2 Reduction Coupled to Formate Fermentation: An Electro-Bio Cascade Approach in Biocompatible Electrolytes

Integration of electrochemical CO2 reduction with microbial fermentation enables conversion of CO2 into valuable chemicals but poses challenges at the electrolysis-fermentation interface. The electrolyte must ensure efficient CO2 reduction while remaining compatible with microbial growth. We investigated various electrolytes for coupling CO2 electroreduction to formate with formate fermentation by Methylorubrum extorquens TK 0001. Electrolyte performance was evaluated by formate production and microbial growth. A phosphate-based buffer demonstrated the best overall compatibility. Optimal microbial growth occurred at 0.1 mol L-1 KPi, with tolerance of up to 111 mmol L-1 formate. Continuous CO2 electrolysis in 1.0 mol L-1 KPi produced 2.0 mol -1 formate in 48 h. Formate fermentation with M. extorquens showed biomass yield of 107 mg CDW gformate-1 and a growth rate of 0.10 h-1. These results highlight the crucial role of buffer composition and concentration in balancing efficient CO2 electroreduction with stable fermentation. Optimizing this electrochemical–biological interface enables direct utilization of CO2-derived formate as a substrate for sustainable microbial production, offering a promising scalable route for industrial biotechnology.