Ruthenium Oxide Hydrogen Evolution Catalysis on Composite Cuprous Oxide Water‐Splitting Photocathodes

Photocathodes based on cuprous oxide (Cu 2 O) are promising materials for large scale and widespread solar fuel generation due to the abundance of copper, suitable bandgap, and favorable band alignments for reducing water and carbon dioxide. A protective overlayer is required to stabilize the Cu 2 O in aqueous media under illumination, and the interface between this overlayer and the catalyst nanoparticles was previously identified as a key source of instability. Here, the properties of the protective titanium dioxide overlayer of composite cuprous oxide photocathodes are further investigated, as well as an oxide‐based hydrogen evolution catalyst, ruthenium oxide (RuO 2 ). The RuO 2 ‐catalyzed photoelectrodes exhibit much improved stability versus platinum nanoparticles, with 94% stability after 8 h of light‐chopping chronoamperometry. Faradaic efficiencies of ∼100% are obtained as determined by measurement of the evolved hydrogen gas. The sustained photocurrents of close to 5 mA cm −2 obtained with this electrode during the chronoamperometry measurement (at 0 V vs. the reversible hydrogen electrode, pH 5, and simulated 1 sun illumination) would correspond to greater than 6% solar‐to‐hydrogen conversion efficiency in a tandem photoelectrochemical cell, where the bias is provided by a photovoltaic device such as a dye‐sensitized solar cell.