Anion\nEffect in Electrochemical CO<sub>2</sub> Reduction:\nFrom Spectators to Orchestrators

The electrochemical\nCO<sub>2</sub> reduction reaction (eCO<sub>2</sub>RR) offers a pathway\nto produce valuable chemical fuels from\nCO<sub>2</sub>. However, its efficiency in aqueous electrolytes is\nhindered by the concurrent H<sub>2</sub> evolution reaction (HER),\nwhich takes place at similar potentials. While the influence of cations\non this process has been extensively studied, the influence of anions\nremains largely unexplored. In this work, we study how eCO<sub>2</sub>RR selectivity and activity on a gold catalyst are affected by a\nwide range of inorganic and carboxylate anions. We utilize <i>in situ</i> differential electrochemical mass spectrometry (DEMS)\nfor real-time product monitoring coupled with molecular dynamics (MD)\nsimulations. We show that anions significantly impact eCO<sub>2</sub>RR kinetics and eCO<sub>2</sub>RR selectivity. MD simulations reveal\na new descriptorfree energy of anion physisorptionwhere\nweakly adsorbing anions enable favorable CO<sub>2</sub> reduction\nkinetics, despite the negative charge carried by the electrode surface.\nBy leveraging these fundamental insights, we identify propionate as\nthe most promising anion, achieving nearly 100% Faradaic efficiency\nwhile showing high CO production rates that are comparable to those\nin bicarbonate. These insights underscore the vital role of anion\nselection in achieving a highly efficient eCO<sub>2</sub>RR in aqueous\nelectrolytes.