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 L⁻¹ formate in 48 h. Formate fermentation with M. extorquens showed biomass yield of 107 mg CDW g_formate⁻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.