The Role of Cation Acidity on the Competition between Hydrogen Evolution and CO 2 Reduction on Gold Electrodes.

CO 2 electroreduction (CO 2 RR) is a sustainable alternative for producing fuels and chemicals. Metal cations in the electrolyte have a strong impact on the reaction, but mainly alkali species have been studied in detail. In this work, we elucidate how multivalent cations (Li + , Cs + , Be 2+ , Mg 2+ , Ca 2+ , Ba 2+ , Al 3+ , Nd 3+ , and Ce 3+ ) affect CO 2 RR and the competing hydrogen evolution by studying these reactions on polycrystalline gold at pH = 3. We observe that cations have no effect on proton reduction at low overpotentials, but at alkaline surface pH acidic cations undergo hydrolysis, generating a second proton reduction regime. The activity and onset for the water reduction reaction correlate with cation acidity, with weakly hydrated trivalent species leading to the highest activity. Acidic cations only favor CO 2 RR at low overpotentials and in acidic media. At high overpotentials, the activity for CO increases in the order Ca 2+ < Li + < Ba 2+ < Cs + . To favor this reaction there must be an interplay between cation stabilization of the *CO 2 - intermediate, cation accumulation at the outer Helmholtz plane (OHP), and activity for water reduction. Ab initio molecular dynamics simulations with explicit electric field show that nonacidic cations show lower repulsion at the interface, accumulating more at the OHP, thus triggering local promoting effects. Water dissociation kinetics is increasingly promoted by strongly acidic cations (Nd 3+ , Al 3+ ), in agreement with experimental evidence. Cs + , Ba 2+ , and Nd 3+ coordinate to adsorbed CO 2 steadily; thus they enable *CO 2 - stabilization and barrierless protonation to COOH and further reduction products.