Boosting Formic Acid Production in Mildly Acidic Media─The Role of Native Surface Oxide on the CO ₂ Reduction Performance

Bismuth catalysts hold significant promise for the electroreduction of CO₂ to formic acid (FA), yet their performance in acidic electrolytes remains largely unexplored despite the clear advantages for product separation and direct industrial integration. Here, we systematically investigate the catalytic activity and long-term stability of bismuth catalysts under acidic conditions (pH 3). At this pH, the hydrogen evolution reaction bifurcates into distinct proton and water reduction pathways with the CO₂ reduction reaction to FA taking place exactly between them. We demonstrate that bismuth catalysts efficiently suppress the proton reduction, leading to over 95% selectivity toward FA in acidic media. Furthermore, long-term measurements reveal the governing catalytic role of an inherently formed surface oxide, which develops upon catalyst ink preparation in ambient air, even if metallic bismuth is used as catalyst. This is evidenced by cyclic voltammetry, X-ray photoelectron spectroscopy, and potentiostatic product detection via online gas chromatography. Our work reports on successful electrocatalytic formic acid production from CO₂ at pH 3, which opens viable pathways for the implementation of this reaction through proton-exchange membrane technologies.