Alkali Cations Promote CO₂ Electroreduction on Cu(100) Surfaces under Acidic Conditions by Suppressing Surface...

The promotional effect of cations on the CO₂ reduction reaction (CO₂RR) on Cu is well-established experimentally, yet the underlying mechanism remains debated. This is further complicated by an underexplored factor: in acidic solution, the Cu surface may be covered by a H adlayer rather than being pristine. We employ a multiscale modeling approach to investigate how the H adlayer and electric double layer environment affect CO₂RR activity on Cu(100). GC-DFT calculations demonstrate that the H adlayer on Cu lowers the d-band center of Cu, attenuating CO₂RR activity and correctly identifying CO₂ chemisorption as the rate-limiting step. This H-passivated surface provides a novel lens through which to view the cation effects. Molecular dynamics simulations reveal that the direct Cs⁺ stabilization of intermediates is a minor effect. More importantly, Cs⁺ thermodynamically favors the transition to lower H coverage and suppresses H₃O⁺ adsorption on the Cu surface. We thus propose a dual-role mechanism: alkali cations promote CO₂RR also by acting as depassivants that reduce H coverage, restoring the catalytic activity of the Cu surface for the CO₂RR.