Ammonium Cation-Promoted CO2 Electroreduction on Au in Acidic Media

The electrochemical reduction of CO2 in acidic media has received considerable attention, as it can mitigate the carbonate formation issue. However, the available protons in acidic media can boost the hydrogen evolution reaction (HER), so alkali metal cations are generally employed to promote the CO2 reduction reaction (CO2RR) while suppressing the HER. Here we report that NH4+ cations are more effective promoters of CO2 electrolysis on Au in acidic media than Na+ and K+, achieving a 3-fold improvement in the CO2RR activity. The promotional effect of NH4+ cations is mainly attributed to their enhanced electrostatic stabilization of the CO2 adsorption, which is the rate-limiting step for the CO2RR on Au. An estimation of the local pH under relevant conditions indicates that NH4+ can mitigate the interfacial pH swing during the CO2RR compared to Na+ and K+. Further studies on Au nanocatalysts of varying sizes indicate that the cation effect is independent of the catalytic sites. Our work advances the understanding of the cation effect on CO2RR using nonmetal cations and demonstrates a viable cation strategy to enhance CO2 electrolysis.