Controlled Acidity Gradients Enable CO2 Reduction to Formic Acid (Not Formate) by Molecular Electrocatalysts.

Neutral or basic conditions are commonly required for the selective electrochemical reduction of CO2, leading to the accumulation of carbonate salts and the generation of formate rather than formic acid. A generalizable strategy for obtaining formic acid (not formate) in the electroreduction of CO2 with molecular catalysts is introduced, based on controlling acidity gradients using a dual-electrolyte cell with a proton-exchange membrane. This approach uses anodic water oxidation as the source of protons and electrons for CO2 reduction to formic acid, while mitigating H2 evolution near the cathode and avoiding carbonate formation. Mechanistic studies, including systems modeling, provide insight into the origin of the formic acid selectivity and guide the broader implementation of this strategy in molecular electrocatalysis for CO2 utilization.