Determining the Effect of Ion Clustering on Electrochemical CO2 Reduction

The interfacial region between the electrode and electrolyte, called the electric double layer, contains a high density of ions recruited from the bulk electrolyte to screen the surface potential. While it is accepted that the electric double layer has a defining impact on the rate and selectivity of electrochemical reactions, general understanding of double layers remains limited under conditions of high ion concentrations and large surface potentials common to many electrochemical systems. Under these conditions, the formation of multi-ion clusters and correlated ion networks impact reactivity in ways unaccounted for by classical electrolyte theory, which assumes noninteracting ions. Here we study electrochemical CO2 reduction to CO and use complementary molecular dynamics simulations to extract the effects of ion clustering on charge transfer kinetics and electrolyte properties. We explore clustering behavior using ionic liquids dissolved in acetonitrile, dimethyl sulfoxide, and propylene carbonate to generalize the effects of clustering across solvents. We show that peak CO2 electroreduction rates correspond to conditions where the number of mobile charge carriers, or the sum of isolated ions and charged ion clusters in solution, are also maximized in all solvent tested. We attribute this to the enhanced screening ability of the electrolyte and further demonstrate how ion clustering can be used to explain changes in both bulk and interfacial properties, such as conductivity and capacitance. Overall, we highlight the critical impact of ion correlations in electrochemical systems and show that modulating ion clustering can be an effective method to accelerate electrochemical reactions.