Electrochemical CO₂ reduction (eCO₂R) is a promising technology to store renewable energy into chemical bonds and close the carbon cycle. However, to date the industrial exploitation of this process is limited to CO and HCOO^–, whilst production of desirable C₂₊ chemicals is possible only at laboratory scale and on copper-based catalysts. The main obstacles to eCO₂R toward multicarbon molecules are (i) poor selectivity, due to the wide spectrum of intermediates and reaction products and (ii) limited long-term stability. The role of the electrolyte, and more specifically of the cations, is well-known to affect CO₂R selectivity since the pioneering work from Hori. [1] Recently, periodic cathodic reactivation through alkali cations enabled eCO₂R to CO on Ag at high partial current densities and outstanding stability (400 mA cm^–2 for > 200 h). [2] Albeit these experimental observations, the mechanism underlying the so-called “cation effect” is still under debate. Different hypotheses have been proposed, as buffering of interfacial pH, mean electric field effects, and explicit electrostatic interactions with key adsorbates. [3] Here, by means of ab initio molecular dynamics (AIMD) simulations, we rationalize experimental observations on the triggering role of metal cations in eCO₂R on gold, silver, and copper, [4] and propose cation acidity and cation-CO₂ coordination as descriptors for the competition between CO₂ reduction and hydrogen evolution reaction (HER) on gold.

In the first study, [4] we rationalized sound experimental evidence on the absence of eCO₂R on gold, silver, and copper without a metal cation in solution. By applying AIMD on a large Au supercell with 72 explicit water molecules and 1 cation, we demonstrated that cations steadily coordinate with adsorbed CO₂, with a coordination rate which increases with the ionic cation radius. Overall, such coordination accounts for a short-range electrostatic interaction which enables eCO₂R by stabilizing CO₂ activation by 0.5 eV and assisting the first electron transfer from the surface. Specific eCO₂R activity trends are solely due to larger accumulation at the Outer Helmholtz plane (OHP) for weakly solvated cations.

In the second study, we investigated the role of alkali, divalent, trivalent cations in the competition between hydrogen evolution and CO₂ reduction. By extending the previous model including electric field and higher cation concentration at the surface (~1.6 M), we demonstrated that cation acidity rules cation accumulation at the OHP, motivating the enhanced reduction performance for less acidic cations among each valency group. Besides, the thermodynamics and kinetics for H₃O⁺ and H₂O, respectively proton sources at acidic and neutral-to-alkaline surface pH, correlate with cation acidity, whilst *H binding is independent from cationic species. This explains the absence of “cation effect” on proton reduction on gold and the enhanced water reduction (H₂OR) performance of acidic cations (e.g., Nd³⁺, Al³⁺). Finally, CO₂R activity correlates with cation-CO₂ coordination for all the considered species, with Cs⁺, Ba²⁺, and Nd³⁺ showing the best performance, thus extending the validity of this descriptor to multivalent cations.

Both studies provide a rigorous protocol to assess explicit cation effects on electrocatalytic reactions such as eCO₂R, HER, and H₂OR, thus building a bridge between theory and experiments. Additionally, by defining a specific set of conditions (pH, applied potential, cationic species) to maximize CO₂R selectivity, this work provides clear guidelines for electrolyte optimization toward high performance devices.