A Molecular-Level Mechanistic Framework for Interfacial Proton Coupled Electron Transfer Kinetics

Electrochemical proton coupled electron transfer (PCET) reactions are critical to energy conversion and catalysis. These reactions can be driven by outer-sphere electron transfer to a soluble molecule (OS-PCET) or through an inner-sphere mechanism by interfacial polarization of a surface-bound active site (I-PCET). The pH-dependent kinetics of OS-PCET have been extensively studied at the molecular level, but the inherent heterogeneity of most surfaces has impeded molecular-level understanding of I-PCET. Herein, we employ graphite conjugated carboxylic acids (GC COOH) as molecularly well-defined hosts of I-PCET to isolate the intrinsic pH dependent kinetics of this reaction. Using variable scan rate voltammetry, we measure the rates of I-PCET across the entire pH range and find a pronounced “V”-shaped dependence on pH with a rate minimum at pH 10 that rises log-linearly to pH 0 and pH 14. This kinetic profile spans three orders of magnitude in rate and lacks the pH-independent regions characteristic of electrochemical OS-PCET reactions. To explain these trends, we develop a mechanistic model for I PCET that invokes CPET involving hydronium/water or water/hydroxide donor-acceptor pairs. This relatively simple model captures the entire data set with only four adjustable parameters corresponding to the standard rate constants and charge transfer coefficients of the two donor/acceptor couples. From this analysis, we find that I PCET with water/hydroxide is four-fold more sluggish than with hydronium/water, but both reactions display similar charge transfer coefficients near 0.7, indicating a late transition state. These studies highlight the key mechanistic and kinetic distinctions between OS PCET and I-PCET and provide a baseline framework for understanding and modeling more complex I PCET reactions critical to energy conversion and catalysis.