Electrolyte Effects\non Electrochemical CO<sub>2</sub> Reduction Reaction at Sn Metallic\nElectrode

Understanding the electrolyte factors governing the electrochemical\nCO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR) is fundamental\nfor selecting the optimized electrolyte conditions for practical applications.\nWhile noble metals are frequently studied, the electrolyte effects\non the CO<sub>2</sub>RR on Sn catalysts are not well explored. Here,\nwe studied the electrolyte effect on Sn metallic electrodes, investigating\nthe impact of electrolyte concentration, cation identity, and anion\nproperties, and how it shapes the CO<sub>2</sub>RR activity and selectivity.\nThe activity for formic acid and carbon monoxide increases with the\ncation concentration and size at mild acid conditions. In contrast,\nhydrogen production is not strongly influenced by the cathodic potential,\nelectrolyte concentration, and cation size. Furthermore, we have compared\nthe CO<sub>2</sub>RR performance at a constant cation concentration\nin K<sub>2</sub>SO<sub>4</sub> (pH 4) and KHCO<sub>3</sub> (pH 7),\nwhere we show that the reaction rate toward HCOOH and CO are minimally\nimpacted by the anion identity on the SHE scale, while being affected\nby the cations in solution, which we attribute to the reaction being\nlimited by cation-coupled electron transfer steps rather than by a\nproton-coupled electron transfer step. We propose that the HCOOH forms\nvia adsorbed hydrides leading to *OCHO intermediate, while CO forms\nthrough an electron transfer step, producing *CO<sub>2</sub><sup>δ−</sup>. Cations facilitate both processes by stabilizing the negatively\ncharged intermediates, and the difference in the extent of the promotion\nof HCOOH over CO formation would stem from the stronger cation effects\non *H compared with *CO<sub>2</sub><sup>δ−</sup> species.\nAdditionally, the presence of HCO<sub>3</sub><sup>–</sup> at\nhigh concentrations (1.0 mol L<sup>–1</sup>) is shown to significantly\nenhance the production of H<sub>2</sub> at high overpotentials (>-1.0\nV vs RHE) due to bicarbonate ions acting as protons donors, outcompeting\nwater reduction. These findings underscore the significance of electrolyte\nengineering for enhanced formic acid synthesis, offering valuable\ninsights for optimizing the CO<sub>2</sub>RR processes on Sn electrocatalysts.