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. ■ INTRODUCTIONThe electrochemical reduction of carbon dioxide (CO 2 RR) offers a sustainable pathway to produce fuels or base chemicals without relying on finite and nonrenewable resources such as oil and gas.[1][2][3] During CO 2 RR in aqueous electrolyte, the hydrogen evolution reaction (HER) occurs concomitantly, which may significantly lower the faradaic efficiency of CO 2 RR.[4][5][6] Depending on the reaction conditions, specifically the electrolyte pH, the production of hydrogen can happen through either the direct reduction of protons (2H + + e -→ H 2 ) or the reduction of the solvent itself (2H 2 O + 2e -→ H 2 + 2OH -).Different strategies have been adopted to favor CO 2 RR over HER, such as enhancing mass transport, 6 using porous catalysts, 7,8 or modifying the catalyst surface.9 An alternative approach is to change the electrolyte composition, since the cation identity, for instance, is known to highly influence both the CO 2 RR 10-12 reaction and HER 13,14 rates.Up to now, mainly alkali cations have been employed for CO 2 RR, but recent theoretical work suggests that multivalent cations may lead to an even larger enhancement of the reaction activity.11 Still, in order to use that in favor of the CO 2 RR in aqueous media, one also has to understand how cation properties affect the competing HER.Since the work of Hori and co-workers, it is known that the species in the electrolyte can influence the product selectivity and faradaic efficiency of the CO 2 RR.[15][16][17] Despite the strong effect that cations have on CO 2 RR, mainly alkali metals in neutral or alkaline media have been investigated using copper, 11 silver, 18 or gold 19,20 electrodes.It has been established that the CO 2 RR activity increases in the order Li + < Na + < K + < Cs + .Three main theories have been suggested to explain this trend, namely, local buffering at the interface, 21 changes in the (local) electric field, 11,22,23 and electrostatic interactions with reaction intermediates.22,24 In a recent work, we elucidated that the key specific role of cations on CO 2 RR to CO is to stabilize the negatively charged reaction intermediate, *CO 2 -. 25 We observe that without a metal cation in the electrolyte, CO is not produced on gold, silver, and copper electrodes.Through ab initio molecular dynamics