(523d) Interactions between Solvent Molecules and Reactive Intermediates within the Electrochemical Double Layer Dictate Epoxidation and O2 Evolution Kinetics

The formation of reactive intermediates and transition states during electrocatalysis induces changes to the solvent network within the electrochemical double layer (EDL), which in turn manipulates the stability of surface species and affects reaction rates and selectivities. Here, we probe interactions between solvent molecules within the EDL and the respective transition states for alkene epoxidation and O₂ evolution over an Au anode in an aqueous-CH₃CN electrolyte through comparison of reaction kinetics at different potentials.

Epoxidation rates and Faradaic efficiencies increase by an order of magnitude and decrease by a factor of two, respectively, as applied potential increases (1.15-1.29 V_Fc/Fc+). The rate differences do not result from changes in reaction mechanism or mass transfer limitations, but rather changes in transition state stability as quantified, through combination of transition state theory and Butler-Volmer kinetics, by apparent activation enthalpies (ΔH^‡) and entropies (ΔS^‡). A hypothetical Born-Haber thermochemical cycle deconvolutes the activation barriers to four separate components (Figure 1a). Both the epoxidation and O₂ evolution enthalpic barriers decrease with potential (Figure 1b), likely due to increased transition state stabilization from the stronger electric field (step 3, ΔG_TS-elec). Both reactions undergo higher entropic penalties at higher potentials (Figure 1c) because the more compact solvent structures within the EDL at higher potentials results in larger entropy losses for the reactants in the bulk electrolyte and requires more energy to displace solvents for the respective transition states (step 1, ΔG_ΔSolv). We investigate effects of alkene chain length (C₃-C₁₂) and electrolyte composition (8-18 M H₂O, CH₃CN) on apparent reaction barriers to decouple the molecular origins of differences in epoxidation and O₂ evolution reaction barriers. The quantification of differences in apparent activation energy barriers induced by EDL solvent structure changes enables informed process and electrolyte engineering for the promotion of epoxidation over O₂ evolution.