On the Substantially Improved Photoelectrochemical Properties of Nanoporous WO3 Through Surface Decoration with RuO2

Solar fuel generation, either through photochemical or photoelectrochemical (PEC) routes, is one of the most prominent pathways to harvest sunlight in order to contribute to a diversified and sustainable energy supply. Oxygen evolution reaction (OER) from aqueous solutions is the half reaction of different energy related reaction schemes, most importantly stoichiometric water splitting (H-2 generation) or CO2 reduction. In this paper, we present PEC water oxidation (OER) on nanoporous WO3 films, decorated with RuO2 nanoparticles. The morphology and the composition of these nanostructured assemblies were characterized by high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry while their photo-electrochemical behavior was evaluated by photovoltammetry and incident photon-to-current conversion efficiency (IPCE) measurements. The O-2 evolution capability was directly assessed by determining the amount of evolved O-2 gas during PEC oxidation of water. The RuO2 electrocatalyst substantially increased photoanodic current flow through facile transfer of photogenerated holes from WO3 to the solution, thereby improving interfacial charge transfer kinetics. Both absolute and relative enhancement of photocurrents was analyzed as a function of the applied external bias potential giving important insight into the mechanistic details. Furthermore, the decisive roles of the amount of RuO2 cocatalyst and thermal pretreatment were established by synthesizing hybrid assemblies with different RuO2 coverage, and applying subsequent thermal treatment, respectively. Through careful optimization of the composition of the hybrid material, the IPCE value doubled at lower bias potentials and was increased close to the theoretical 100 % limit at higher positive potential values. These results demonstrate the synergy gained by combining the excellent photoanodic properties of nanoporous WO3 with the robust O-2 evolution capabilities of RuO2.