Abstract
Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a spectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide-based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to compare directly the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen.
Highlights
Multi-electron heterogeneous catalysis is a pivotal element in theelectrochemical generation of solar fuels
Precious metals that exhibit near ideal catalytic behaviour such as platinum have been employed as electrocatalysts for the hydrogen evolution reaction (HER)[4,9]
These systems provide an attractive model for the study of the HER catalysis, as well as enabling a direct comparison of the catalytic function under conditions of dark electrochemical and irradiated photoelectrochemical proton reduction
Summary
Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. This electrocatalyst is based upon a nanostructured, amorphous, highly porous RuOx that is deposited onto FTO (fluorine-doped tin oxide) and onto a multilayer Cu2O-based photoelectrode (Fig. 1) Such photocathodes, where the Cu2O is protected against photocorrosion by thin Al:ZnO (AZO) and TiO2 overlayers, have achieved remarkable solar-to-hydrogen yields and large (near 100%) faradaic efficiencies for solar-driven water splitting[19,20,21]. This Cu2O/AZO/TiO2/RuOx (referred as [Cu2O]/RuOx) assembly can be considered an example of a ‘buried junction’ photoelectrode, in which the generation and separation of photogenerated charges in the Cu2O/AZO/TiO2 layers is at least partially decoupled from the catalytic function by the catalyst overlayer As such, these systems provide an attractive model for the study of the HER catalysis, as well as enabling a direct comparison of the catalytic function under conditions of dark electrochemical and irradiated photoelectrochemical proton reduction. For many (photo)electrocatalytic systems, determination of rate laws for multi-redox reactions is a non-trivial challenge, with only limited studies reported in the literature to date
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