Ion-Pair-Driven Electrolyte Engineering for Optimized KHCO 3 Reduction to Formate

Electrochemical CO 2 conversion to formate offers a promising route for environmentally sustainable production of carbon-based fuels and chemicals, yet the mechanistic role of electrolytes in CO 2 electroreduction remains debated. Here, we reveal that electrochemical formate production on Sn/SnO 2 electrodes in KHCO 3 electrolytes is strongly influenced by bicarbonate-based speciation and is consistent with the potential electroreduction of neutral KHCO 3 ion pairs. Electrolyte engineering with KHCO 3 and KCl identifies 1.75 M KHCO 3 /2 M KCl as the optimal composition, yielding a peak formate partial current density of ∼20 mA·cm –2 on planar Sn/SnO 2 electrodes at reduced overpotentials. Tafel analysis shows that KCl addition to 1.75 M KHCO 3 electrolytes enhances charge-transfer kinetics for formate electrosynthesis. Higher limiting currents in the polarization curves reflect improved mass transport of the proposed electroactive KHCO 3 species. The results highlight the significance of tuning the balance between KHCO 3 ion pairs and free bicarbonate for scalable formate electrosynthesis. These insights establish electrolyte design principles for advancing the selectivity and efficiency of CO 2 conversion processes with broad implications for other electrochemical technologies.