(6h) Alkali Cation Spatial Distribution at Electrocatalytic Interfaces By Molecular Simulations

Alkali cations at an aqueous electrocatalytic interface can affect the kinetics of crucial reactions such as CO₂ reduction (CO₂RR) or hydrogen evolution. While there is a wealth of mechanistic studies on how cations specifically interact with surface reaction intermediates, studies aimed at understanding the spatial distribution of alkali cations at the electrochemical double layer (EDL) are currently lacking.

In this study, we use classical molecular dynamics to compute the potential of mean force (PMF) of cations approaching a polarizable metal electrode. The PMFs are decomposed into contributions from the solvent and the metal surface to physically explain the laterally averaged concentration profiles of cations with respect to electrode distance. Near a negatively charged electrode, we found bigger (in ionic radii) cations exhibit more pronounced clustering (Cs⁺ > K⁺ > Na⁺ > Li⁺), consistent with interpretation by past experimental work on CO₂RR. At close distances to the surface, bigger cations electrostatically interact with the surface more strongly because of weaker short-range screening by looser hydration shells, as well as less deformation of the hydration shells.

Furthermore, we interrogate two physical quantities that encode some information about cations distribution in the EDL: 1) the non-Faradaic electrosorption valency—first derivative of cation adsorption free energy versus electrode potential; 2) the interfacial capacitance—first derivative of surface charge density versus electrode potential. Bigger cations yield larger electrosorption valency and capacitance, attributed to a larger amount of screening counter-charges on the electrode surface. Nevertheless, both quantities fail to infer the preferred cation-electrode distance, even with invoking the Gouy-Chapman-Stern theory to explain the cation-dependent capacitance.