Highly selective formate formation via bicarbonate conversions†

Electrocatalytic conversion of liquid bicarbonate feedstock to formate is a promising reactive CO₂ capture technology. However, bicarbonate-fed electrolyzers have shown insufficient faradaic efficiencies (FEs) for formate production due to competing hydrogen evolution reactions. In this study, we developed a bicarbonate electrolyzer incorporating a porous membrane between a proton exchange membrane (PEM) and a hydrophilic bismuth cathode. By employing the intermediate membrane to enhance in situ CO₂ generation from 3.0 M KHCO₃, we achieved a formate FE of 84.6% even at a high current density of 300 mA cm⁻². This electrolyzer also achieved high CO₂ utilization efficiency (89%) and low full-cell voltage (3.1 V) at 100 mA cm⁻² owing to the rational designs of membrane electrode assemblies. Bicarbonate conversion to formate is accelerated through in situ CO₂ generation and selective CO₂ reduction reaction at a gas–liquid–catalyst triple-phase boundary. Additionally, the bicarbonate electrolyzer demonstrates high CO₂ utilization efficiency, long-term stability, and production of pure formate salt.