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.