Electrochemical CO2 reduction to formate at indium electrodes with high efficiency and selectivity in pH neutral electrolytes

Abstract Electrochemical reduction of CO2 is promising for a bio-based economy as it combines utilization of CO2 as feedstock and provides a pathway for the utilization and (temporary) storage of electric energy. Among different products formate (HCOO-) can be produced with high rates and selectivity using indium as electrocatalyst. This can be achieved at mild biocompatible reaction conditions, e.g. ambient temperature, ambient pressure and neutral pH. Formate can serve as a source of carbon and energy for the biosynthesis of energy carriers or chemicals. However, the in situ interfacing of electrochemical CO2 reduction and biosynthesis creates challenges for electrochemical engineering. It is demonstrated that the electrode potential is the main steering parameter affecting the columbic efficiency, selectivity and rate of formate production in NaHCO3 electrolyte solution at biocompatible conditions. Coulombic efficiencies and formate production rates of 94.5 ± 2% and 0.136 ± 0.016 mmol h-1 cm-2 (at -2.2 vs. Ag/AgCl and ĸ = 10 mS cm-1), respectively, were achieved. Further, increasing the conductivity using inert electrolytes can enhance formate space-time yields up to 0.254 ± 0.031 mmol h-1 cm-2. Surprisingly, high NaHCO3 concentrations do not further increase formate production which supports that HCO3- is not electrochemically converted but only acting as CO2/H+ reservoir. Based on kinetic modeling insight on the inter-conversion of the carbonaceous species by CO2 sparging of the electrolyte solution is provided. Importantly, the influence of O2 on the electrochemical CO2 reduction was revealed to be marginal. This study, providing principles on the engineering of electrochemical CO2 reduction to formate for future interfacing to biosynthesis, demonstrates its feasibility to become technologically relevant.