Theoretical insights into the promotion effect of alkali metal cations on the electroreduction mechanism of CO₂ into C1 products at the Cu(111)/H₂O interface

A mechanistic study of the promotion effect of alkali metals on CO₂ electroreduction into C1 products was carried out at Cu(111)/H₂O electrochemical interfaces, for which Na and Cs with different atomic sizes were selected. The results show that the presence of alkali metal cations can notably change CO₂ electroreduction reaction pathways, activity, and product selectivity. Alkali metal cations with different atomic sizes can lead to differences in the reduction pathway of CO₂ into CH₄. Barriers to the CO formation pathway as the rate-determining step in CO₂ electroreduction to C1 products on clean Cu(111) surfaces are significantly decreased in the presence of alkali metal cations, thus enhancing the CO₂ electroreduction activity. The Na cation with a smaller atomic size is more favorable for CO formation, which is in agreement with experimental observations. The enhanced CO₂ electroreduction activity on alkali metal cation-promoted Cu electrodes may be attributed to the decreased limiting potential of the potential-limiting step. The origin of alkali metal promotion effects on CO₂ electroreduction has been revealed. The difference in the geometric configuration may be the physical origin of the promoter effect. The electron structure analyses showed that significantly more electrons are transferred to adsorbed CO₂ molecules on alkali metal cation-promoted Cu(111) surfaces, resulting in the formation of the anion radical ^.CO₂^- and explaining why CO formation is more facile. More distorted CO₂ and more electron transfer to CO₂ on the Na cation-promoted Cu(111) surface explain why CO formation is more favorable than that in the presence of the Cs cation. The difference in the promotion mechanism of alkali metals may lead to different initial CO₂ electroreduction activities where Na is regarded as an electron donor, whereas Cs with a larger atomic size is an effective electron acceptor. (authors)