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.
A Molecular-Level Mechanistic Framework for Interfacial Proton Coupled Electron Transfer Kinetics -- Supporting Information
Methods, Supplementary Discussion, and Representative data for "A Molecular-Level Mechanistic Framework for Interfacial Proton Coupled Electron Transfer Kinetics"