Dynamic Probing of Intermediates in (Photo)electrocatalytic Reactions Using Frequency-Resolved Excitation-Modulated Absorption Spectroscopy.

Operando examination of reaction intermediates during (photo)-electrocatalytic processes, particularly with respect to their kinetics, is a critical but challenging task. Spectroelectrochemical (SEC) methods based on UV-vis spectroscopy offer potential insights into intermediate species, although they are often applied under steady-state conditions. In this work, we employ excitation-modulated absorption spectroscopy (EMAS), which utilizes periodic modulation of either potential or light intensity, to dynamically probe the SEC responses of a hematite (FeO) photoanode, NiFeO -type electrocatalysts, and their composites. Comparison of spectral profiles in the frequency domain under electrocatalytic (EC) and photoelectrocatalytic (PEC) conditions allows for the identification of key reaction intermediates and their corresponding redox rate constants, which further facilitates the revelation of key steps of charge dynamics. Specifically, EC and PEC water oxidation on bare FeO photoanodes proceed via the same reaction intermediate Fe-(IV) with similar kinetics. For composite hematite photoanodes, a faster redox rate of the oxidized catalytic reaction intermediates of the electrocatalyst upon accepting holes from the semiconductor is crucial for achieving a high photocurrent density. These results highlight EMAS as a powerful and versatile tool to understand (photo)-electrocatalytic systems.