Understanding the Activation Mechanism of RhCrO Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes

Mixed oxides of Rh-Cr (RhCrO), containing Rh³⁺ and Cr³⁺ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrO, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrO produces reduced Rh (Rh⁰) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (V_RHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr³⁺ (CrO) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 V_RHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 V_RHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.