Assessmentof a W:BiVO4–CuBi2O4TandemPhotoelectrochemical Cell for OverallSolar Water Splitting
2020
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.
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