Characterization of Rh:SrTiO3 photoelectrodes surface-modified with a cobalt clathrochelate and their application to the hydrogen evolution reaction

Water is a promising source of clean hydrogen. Besides water electrolysis, the direct photoelectrochemical dissociation of water that combines light absorption and electrochemical water splitting into a single device is actively investigated by the scientific community. We report results on the p-type rhodium-doped strontium titanate (Rh:SrTiO₃) surface-modified by adsorption of a 3d-transition metal complex which is known to be a good catalyst of the hydrogen evolution reaction (HER) from water. The tris-dioximate cobalt hexachloroclathrochelate with an encapsulated cobalt(II) ion was selected as a surface HER co-catalyst for this study. Samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The kinetics of the photoelectrochemical water dissociation was studied using the open circuit photovoltage decay (OCPD) and photoelectrochemical impedance spectroscopy (PEIS) methods. Under open circuit conditions, the cobalt cage complex did not substantially affect the HER kinetics compared to the bare doped Rh:SrTiO₃ semiconductor. But when a reverse bias was applied, a significant difference caused by presence of the clathrochelate was observed. The rate constants of the charge transfer and the recombination processes were both affected, leading to the conclusion that the macrobicyclic cobalt-encapsulated species not only increased the rate of charge transfer, but also played a role as recombination centers for photoexcited minority carriers. The space charge capacitance was determined under the inversion conditions. Only under strong reverse bias it was found that, whereas bulk Rh:SrTiO₃ has a p-type conductivity, the surface of both bare Rh:SrTiO₃ and the cobalt clathrochelate-covered Rh:SrTiO₃ show the properties of a n-type semiconductor.