(190f) Mechanisms of Proton-Coupled Electron Transfer at Electrochemical Interfaces

The applied potential at an electrode plays a significant role in electrochemical proton-coupled electron transfer (PCET) by modifying interfacial electrostatics and the driving forces of redox reactions. Here, periodic density functional theory (DFT) calculations are used to investigate the interplay between surface charging, electrostatic potentials, interfacial electric fields, and PCET reaction mechanisms. To gain mechanistic insights into interfacial PCET, we investigate proton discharge from a nitrile-tagged proton donor to a silver electrode. The vibrational Stark shift of the nitrile probe beyond and within the electrical double layer is determined using a grid-based method that directly accounts for anharmonicity. These computational results are compared to operando surface-enhanced Raman spectroscopy data to assign the infrared peaks as a function of applied potential, determine the reactant desolvation potential, and explain the delayed onset of steady-state current.¹ In another study, DFT is used to probe the spatial inhomogeneity of interfacial electrostatic potentials and electric fields across graphite-conjugated (GCC) organic acids to demonstrate the importance of continuous conjugation between electrode surfaces and their active sites during heterogeneous PCET. Proton-coupled redox potentials are computed for a series of GCC organic acids, and upon unit cell size extrapolation, exhibit good agreement with experimental measurements.² Beyond this approach, we show that variable charge calculations combined with a grand canonical thermodynamic analysis can be used to evaluate constant potential PCET thermochemistry at fixed unit cell size by deconvoluting capacitive contributions from charged adsorbates and electrode surfaces. These two studies provide fundamental insights into PCET mechanisms and the corresponding role of electrostatics at charged interfaces.

References
(1) Sarkar, S.; Maitra, S.; Lake, W. R.; Warburton, R. E.; Hammes-Schiffer, S.; Dawlaty, J. M. submitted.
(2) Warburton, R. E.; Hutchison, P.; Jackson, M. N.; Pegis, M. L.; Surendranath, Y.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2020, 142, 20855.