Kinetic Competence of Surface-Bound Species at Electrodes During Decarboxylation and Hydrogen Evolution

The development of sustainable electrochemical processes relies heavily on understanding the mechanisms occurring at the electrode-electrolyte interface. Establishing the role of surface-bound species remains a critical challenge at both oxidative and reductive potentials. This work addresses this challenge by combining electrochemical, computational, and advanced in-situ spectroscopic investigations to decipher the kinetic competence and roles of surface intermediates in model electrochemical reactions.

First, the oxidative (non-)Kolbe oxidation of biomass-derived valeric acid on RuO₂ anodes was investigated to understand the relative competition between the oxygen evolution reaction (OER) and decarboxylation. In-situ spectroscopy reveals that carboxylates bind to the RuO₂ surface at potentials corresponding to product formation. However, these bound species are long-lived at the surface, and their primary role is to suppress the competitive and parasitic OER. Electrokinetic studies in the absence of water suggest decarboxylation proceeds via an outer-sphere electron transfer mechanism. Hence, the role of the RuO₂ anode is to favor carboxylate binding to suppress OER, rather than directly participating in the electrooxidation of the acid.

Conversely, in reductive electrocatalysis, the detection and kinetic competence of surface-bound hydrogen intermediates during the hydrogen evolution reaction (HER) were explored on silver cathodes. Using surface-enhanced infrared absorption spectroscopy (SEIRAS) in an aprotic solvent with select Brønsted acids enabled the controlled generation and characterization of intermediate surface-bound species. Weak acids accumulate stable, linearly bound surface hydrogen, whereas strong acids do not give rise to detectable surface-bound hydrogen, although electrokinetic data are consistent with hydrogen evolution proceeding via the rate-limiting formation of the same bound hydrogen. By subsequently reacting the isolated surface bound hydrogen species with a stronger proton donor, we both elucidate the kinetic competence of the surface-bound species and determine that the HER proceeds via the Volmer-Heyrovský pathway on silver.

Finally, additional work extends the utility of SEIRAS to coordination polymers for the nonaqueous carbon dioxide reduction reaction and to cobalt for aqueous acetone hydrogenation. These studies highlight the power of in-situ spectroscopy in characterizing complex interfacial phenomena.