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

We assess a tandem photoelectrochemical cell consisting of a W:BiVO 4 photoanode top absorber and a CuBi 2 O 4 photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO 4 photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi 2 O 4 photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuO x is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi 2 O 4 rather than proton reduction. Protecting the CuBi 2 O 4 photocathode with a CdS/TiO 2 heterojunction and adding RuO x 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 4 /CoPi photoanode and a CuBi 2 O 4 /CdS/TiO 2 /RuO x photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO 4 and conduction band of CuBi 2 O 4 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 4 and CuBi 2 O 4 . This work is the first experimental demonstration of hydrogen production from a BiVO 4 -CuBi 2 O 4 -based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi 2 O 4 photocathodes.