Cation-induced changes in the inner- and outer-sphere mechanisms of electrocatalytic CO2 reduction

Abstract The electrocatalytic CO2 reduction reaction (CO2RR) is an attractive approach to produce fuels and chemicals through CO2 recycling and renewable energy. While it is well-known that metal cations greatly affect the CO2RR activity and selectivity, the underlying mechanism remains debated. Herein, we study cation effects on the CO2RR kinetics and thermodynamics over Au by simulating both outer-sphere electron transfer (OS-ET) and inner-sphere electron transfer (IS-ET) pathways during the first CO2 reduction step via advanced constrained density functional theory molecular dynamics (cDFT-MD) simulations and DFT-MD with slow-growth sampling (SG-DFT-MD) technique, respectively. Our computational analysis shows that without any cations only the OS-ET is feasible with a barrier of 1.21 eV. In the presence of K+, OS-ET shows a very high barrier of 2.93 eV and is prohibited by cations. In stark contrast, cations promote the CO2 activation through IS-ET and the barrier for generating adsorbed CO2δ−, the key CO2RR intermediate, is only 0.61 eV. Without cations, CO2-to- CO2δ−(ads) conversion cannot proceed. We also find that the cation effects arise from the short-range Coulomb, ionic-like interactions between cations and reaction intermediates rather than the long-range electrostatic interactions. Overall, our results disclose that cations modulate the inner- and outer-sphere pathways of CO2RR at Au-water interfaces through short-range interactions. These findings provide substantial new insights on the cation specificity in the initial CO2RR steps.