Proton-coupled electron transfer (PCET) is a fundamental step in a wide range of electrochemical reactions, including those of interest in energy conversion and storage. Despite its importance, several mechanistic details of such reactions remain unclear, partly due to the difficulty of following them with well-defined spectroscopic handles. Here, we have combined a proton donor (tertiary ammonium) with a Stark-shift probe (benzonitrile), to track the progress of the reaction from the entry of the reactants into the electric double layer (EDL), proton donation to the electrodes, and formation of products. We have used <i>operando</i> vibrational spectroscopy and density functional theory for assigning the reactant and product peaks and their Stark shifts. We have reported three major findings. First, we have found the potential at which the reactants desolvate and enter the EDL. Second, we have observed the appearance of product peaks prior to the onset of steady-state electrochemical current. This indicates the formation of an initial stationary population of products that do not turn over continuously at a threshold far below the onset of steady-state current. Finally, more negative of the onset potential, the electrode attracts additional reactants, displacing the stationary products and enabling steady-state current. We have also observed that the generated products continue to Stark shift indicating that at least some of the products adsorb on the surface or remain within the double layer. 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. Electrochemical measurements alone cannot provide such insights.

Mechanistic insights about electrochemical proton-coupled electron transfer derived from a vibrational probe

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