Scanning Electrochemical Microscopy Study of Proton-Coupled Electron Transfer Processes in Aqueous and Concentrated Hydrogen-Bonded Electrolytes: Reorganization Energies and Kinetic Correlations with Medium Acidity

Electrochemical proton-coupled electron transfer (PCET) reactions are ubiquitous, yet quantitative kinetic analysis of soluble redox species is often hindered by practical experimental limitations, such as mass transport, and mechanistic complexity. Here, we employed scanning electrochemical microscopy (SECM) in the feedback mode under stationary, time-independent conditions to investigate the electrochemical PCET kinetics of freely diffusing quinone molecules in aqueous and concentrated hydrogen-bonded electrolytes (CoHBEs) media by constructing rate constant (log k) vs. overpotential (η) plots (k-plots). This approach enabled the direct determination of the apparent electron transfer rate constant at each applied potential, bypassing the reliance on a specific kinetic model. It also captured potential-dependent changes in slope and curvature features that could be described by Butler-Volmer (BV) and Marcus-Hush-Chidsey (MHC) models to obtain standard rate constants (k0) and the reorganization energies (λ) under different reaction conditions. Using anthraquinone-2,7-disulfonic acid (AQDS) as a redox probe, we examined the proton-to-electron transfer ratios during PCET processes across acidic to basic pH conditions through Pourbaix analysis, in situ spectroelectrochemical methods including shifted excitation Raman difference spectroscopy (SERDS), and electronic absorption spectroscopy. We observed that, in both aqueous and CoHBE solvents with varying acidity, more basic environments exhibit larger k0 and smaller λ values as acidity decreases. Density functional theory (DFT) calculations further revealed that multiple PCET pathways coexist, with their relative contributions depending on pH and applied potential. The greater contribution of sequential proton-electron transfer (SPET) pathways and lower activation barriers under more basic conditions provide a mechanistic understanding for the experimentally observed 2 / 41 faster kinetics because concerted PCET steps have larger reorganization energies. Furthermore, extending our approach to the benzoquinone (BQ)/hydroquinone (H2BQ) redox couple enabled the construction of two-tailed k-plots containing both anodic and cathodic branches, revealing redox asymmetry in PCET reactions. Theoretical modeling suggested that different operative reaction mechanisms between oxidation and reduction reactions as a potential origin of apparent redox asymmetry. This work lays the foundations for extracting electrochemical PCET kinetics for a variety of redox systems, electrode materials, and solution compositions and conditions of broad interest for energy storage and beyond.