Proton Activity as a Promoter for the CO₂ Electroreduction to Formic Acid in Acidic Media

The electrochemical carbon dioxide reduction reaction (CO₂RR) in acidic electrolytes addresses the critical issue of carbonate formation but is intrinsically limited by the parasitic hydrogen evolution reaction (HER). While prevailing strategies aim at suppressing the HER by depleting interfacial proton concentration, this approach is detrimental for products whose formation kinetics are proton-dependent. We demonstrate that for the electrosynthesis of formic acid (FA), the maintenance of high interfacial proton activity is not merely a challenge but a requirement for higher FA formation rates. We show that, rather than arising from suppression of the competing HER, FA synthesis is directly coupled to proton availability and is strongly enhanced at higher proton activities. Our findings establish that an efficient acidic CO₂RR to FA requires a distinct design paradigm, which balances the kinetic competition between HER and CO₂RR while ensuring sufficient proton flux to drive the kinetics of FA formation.