Electrolyte Effects on Electrochemical CO₂ Reduction Reaction at Sn Metallic Electrode

Understanding the electrolyte factors governing the electrochemical CO₂ reduction reaction (CO₂RR) is fundamental for selecting the optimized electrolyte conditions for practical applications. While noble metals are frequently studied, the electrolyte effects on the CO₂RR on Sn catalysts are not well explored. Here, we studied the electrolyte effect on Sn metallic electrodes, investigating the impact of electrolyte concentration, cation identity, and anion properties, and how it shapes the CO₂RR activity and selectivity. The activity for formic acid and carbon monoxide increases with the cation concentration and size at mild acid conditions. In contrast, hydrogen production is not strongly influenced by the cathodic potential, electrolyte concentration, and cation size. Furthermore, we have compared the CO₂RR performance at a constant cation concentration in K₂SO₄ (pH 4) and KHCO₃ (pH 7), where we show that the reaction rate toward HCOOH and CO are minimally impacted by the anion identity on the SHE scale, while being affected by the cations in solution, which we attribute to the reaction being limited by cation-coupled electron transfer steps rather than by a proton-coupled electron transfer step. We propose that the HCOOH forms via adsorbed hydrides leading to *OCHO intermediate, while CO forms through an electron transfer step, producing *CO₂ ^δ-. Cations facilitate both processes by stabilizing the negatively charged intermediates, and the difference in the extent of the promotion of HCOOH over CO formation would stem from the stronger cation effects on *H compared with *CO₂ ^δ- species. Additionally, the presence of HCO₃ ^- at high concentrations (1.0 mol L^-1) is shown to significantly enhance the production of H₂ at high overpotentials (>-1.0 V vs RHE) due to bicarbonate ions acting as protons donors, outcompeting water reduction. These findings underscore the significance of electrolyte engineering for enhanced formic acid synthesis, offering valuable insights for optimizing the CO₂RR processes on Sn electrocatalysts.