Anion effect in electrochemical CO2 reduction: from spectators to orchestrators

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. 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 highly efficient eCO2RR in aqueous electrolytes.