Anion Effect in Electrochemical CO₂ Reduction: From Spectators to Orchestrators

The electrochemical CO₂ reduction reaction (eCO₂RR) offers a pathway to produce valuable chemical fuels from CO₂. However, its efficiency in aqueous electrolytes is hindered by the concurrent H₂ evolution reaction (HER), which takes place at similar potentials. While the influence of cations on this process has been extensively studied, the influence of anions remains largely unexplored. In this work, we study how eCO₂RR selectivity and activity on a gold catalyst are affected by a wide range of inorganic and carboxylate anions. We utilize in situ differential electrochemical mass spectrometry (DEMS) for real-time product monitoring coupled with molecular dynamics (MD) simulations. We show that anions significantly impact eCO₂RR kinetics and eCO₂RR selectivity. MD simulations reveal a new descriptor─free energy of anion physisorption─where weakly adsorbing anions enable favorable CO₂ reduction kinetics, despite the negative charge carried by the electrode surface. By leveraging these fundamental insights, we identify propionate as the most promising anion, achieving nearly 100% Faradaic efficiency while showing high CO production rates that are comparable to those in bicarbonate. These insights underscore the vital role of anion selection in achieving a highly efficient eCO₂RR in aqueous electrolytes.