Anion Effect in Electrochemical CO2 Reduction: Enhanced Selectivity Enabled Through Anion Modulation

Electrochemical CO2 reduction reaction (eCO2RR) offers a pathway to produce valuable chemical fuels from CO2 molecules. However, its efficiency in aqueous electrolytes is hindered by the concurrent H2 evolution reaction (HER), which takes place at similar potentials. While recent studies have highlighted critical role of cation selection to improve efficiency of eCO2RR, targeted studies elucidating anion effect on eCO2RR activity and selectivity are lacking. Herein, we present a systematic investigation of the anion-dependent eCO2RR selectivity and activity on gold catalyst using in situ Differential Electrochemical Mass Spectrometry (DEMS) among wide range of anions. Our results reveal that by replacing bicarbonate with carboxylate anions, HER is largely suppressed. Furthermore, we show that propionate and acetate can promote eCO2RR on par with bicarbonate, unlike other studied anions that display largely increased eCO2RR overpotential. Notably, propionate benefits from both suppressed HER and eCO2RR kinetics comparable to bicarbonate: reaching impressive Faradaic Efficiency close to 100% while displaying high CO production rates that are comparable to bicarbonate. These insights underscore the vital role of anion selection in achieving highly efficient eCO2RR in aqueous electrolytes.