Interfacial Electrolyte Solution Reorganization as a Rate-Limiting Step for CO Adsorption on Pt in Electrochemical Environments

Electrolyte solutions play a central role in determining the thermodynamics and kinetics of electrochemical reactions. Recent experimental and theoretical studies have proposed that reorganization of interfacial solvent structures can be correlated with electrocatalytic reaction kinetics on surfaces. However, the influence of electrolyte-solution reorganization on the kinetics of molecular adsorption processes, a ubiquitous elementary step in electrocatalysis, remains largely unexplored, limiting quantitative connections between electrolyte composition and catalytic kinetics. In this study, we measure the rate of CO adsorption on Pt nanoparticles under electrochemical conditions via rotating disk electrode (RDE) voltammetry as a function of electrolyte composition, H2O-D2O switching, temperature, and CO partial pressure. An apparent activation Gibbs energy of 63 ± 1 kJ mol-1 at 295 K for CO adsorption on Pt is observed across various electrolyte compositions. Electrolyte compositions influenced the temperature-dependent CO adsorption rate in a cation-specific manner, with the apparent activation Gibbs energy distinctly partitioned between enthalpic and entropic contributions across different cations. Microkinetic analysis identifies interfacial electrolyte solution reorganization (IESR) as the primary rate-limiting step for CO adsorption. This microkinetic lens provides insight into how electrolytes control CO adsorption kinetics. Notably, Li+ and Cs+, which are distinctly structure-making and -breaking ions in water, produce larger enthalpic and less unfavorable entropic contributions to the adsorption free energy barrier than Na+ and K+, highlighting a nonmonotonic dependence of barrier partitioning on cation identity. This work establishes IESR as a distinct kinetic contribution to adsorption at electrochemical interfaces and demonstrates that collective water-ion restructuring can govern adsorption kinetics, thereby controlling surface coverage and potentially the overall rate of electrocatalytic reactions.