Assessment of a W:BiVO4-CuBi2O4Tandem Photoelectrochemical Cell for Overall Solar Water Splitting.

We assess a tandem photoelectrochemical cell consisting of a W:BiVO4 photoanode top absorber and a CuBi2O4 photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO4 photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi2O4 photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuOx is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi2O4 rather than proton reduction. Protecting the CuBi2O4 photocathode with a CdS/TiO2 heterojunction and adding RuOx as a cocatalyst prevents photocorrosion and allows for photoelectrochemical production of hydrogen at potentials ≤0.3 V vs RHE. A tandem photoelectrochemical cell composed of a W:BiVO4/CoPi photoanode and a CuBi2O4/CdS/TiO2/RuOx photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO4 and conduction band of CuBi2O4 are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes, that is, the relatively low quasi-Fermi level splitting within BiVO4 and CuBi2O4. This work is the first experimental demonstration of hydrogen production from a BiVO4-CuBi2O4-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi2O4 photocathodes.