Assessment of a W:BiVO-CuBiOTandem Photoelectrochemical Cell for Overall Solar Water Splitting.

We assess a tandem photoelectrochemical cell consisting of a W:BiVO photoanode top absorber and a CuBiO photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBiO photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuO is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBiO rather than proton reduction. Protecting the CuBiO photocathode with a CdS/TiO heterojunction and adding RuO 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:BiVO/CoPi photoanode and a CuBiO/CdS/TiO/RuO photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO and conduction band of CuBiO 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 BiVO and CuBiO. This work is the first experimental demonstration of hydrogen production from a BiVO-CuBiO-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBiO photocathodes.