Mechanistic Insights about Electrochemical Proton-Coupled Electron Transfer Derived from a Vibrational Probe

Abstract

Proton-coupled electron transfer (PCET) is a fundamental step in a wide range of electrochemical processes, including those of interest in energy conversion and storage. Despite its importance, several mechanistic details of such reactions remain unclear. Here, we have combined a proton donor (tertiary ammonium) with a vibrational Stark-shift probe (benzonitrile), to track the process from the entry of the reactants into the electrical double layer (EDL), to the PCET reaction associated with proton donation to the electrode, and the formation of products. We have used operando vibrational spectroscopy and periodic density functional theory under electrochemical bias to assign the reactant and product peaks and their Stark shifts. We have identified three main stages for the progress of the PCET reaction as a function of applied potential. First, we have determined the potential necessary for desolvation of the reactants and their entry into the polarizing environment of the EDL. Second, we have observed the appearance of product peaks prior to the onset of steady state electrochemical current, indicating formation of a stationary population of products that does not turn over. Finally, more negative of the onset potential, the electrode attracts additional reactants, displacing the stationary products and enabling steady state current. This work shows that the integration of a vibrational Stark-shift probe with a proton donor provides critical insight into the interplay between interfacial electrostatics and heterogeneous chemical reactions. Such insights cannot be obtained from electrochemical measurements alone.

Anodized Steel: The Most Promising Bifunctional Electrocatalyst for Alkaline Water Electrolysis in Industry

“…[ 38 ] Most importantly, we see that the main peaks of oxides and oxyhydroxides show a red‐shift as the current density decreases (Figure 5b), which demonstrates the interaction among the absorbed protons and electrons on the electrode surface, indicating the fast desorption of hydrogen molecule (H 2 ). [ 39 ]…”

“…[38] Most importantly, we see that the main peaks of oxides and oxyhydroxides show a red-shift as the current density decreases (Figure 5b), which demonstrates the interaction among the absorbed protons and electrons on the electrode surface, indicating the fast desorption of hydrogen molecule (H 2 ). [39] After OER, there is also no obvious new phase (Figure S37, Supporting Information), and the α′-Fe and γ-Fe peaks shift to the right as the lattice distortion is eased due to the dissolution (Figure S37b, Supporting Information). Raman spectra show that the oxides transform to oxyhydroxides, which are the dominant phases after OER.…”

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer

“…Periodic DFT calculations using the grid-based approach depicted in panel A were used to interpret these spectra in terms of different stages of the Volmer reaction. Panel reproduced with permission from ref . Copyright 2021 American Chemical Society.…”

Probing Hydrogen-Bonding Interactions within Phenol-Benzimidazole Proton-Coupled Electron Transfer Model Complexes with Cryogenic Ion Vibrational Spectroscopy

“…Further, solution-phase spectroscopic experiments to date have exclusively focused on the electron transfer half of PCET by monitoring, for example, the recovery of the electronic ground-state transition of the appended photoexcitable group (e.g., Ru­(bpy) 3 2+ ). The proton-transfer component has only been inferred through kinetic isotope effects, the appearance of fingerprint vibrations associated with transfer to the base, or frequency shifts of a vibrational probe group . Proton stretching vibrations have yet to be monitored and, thus, direct interrogation of the proton-transfer reaction coordinate remains unexplored.…”

“…The proton-transfer component has only been inferred through kinetic isotope effects, the appearance of fingerprint vibrations associated with transfer to the base, 16 or frequency shifts of a vibrational probe group. 25 Proton stretching vibrations have yet to be monitored and, thus, direct interrogation of the proton-transfer reaction coordinate remains unexplored.…”