Abstract

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.

Highlights

  • Solar photoelectrochemical (PEC) water splitting with the employment of photoelectrodes in a PEC cell has been demonstrated to be a promising approach for converting abundant solar energy into chemical bonds in the form of hydrogen fuel, which is transportable and does not emit greenhouse gases.[1−4] During the past few decades, extensive research efforts have been focused on exploring metal oxidebased semiconductors as photoelectrodes for photoelectrochemical water splitting

  • Among the n-type semiconductors under investigation, bismuth vanadate (BiVO4) with a bandgap of 2.4−2.5 eV has recently received a great deal of attention as a photoanode material, and it has been optimized to be one of the highest performing metal oxide photoanode materials for water oxidation.[15−17] For p-type semiconductors, CuBi2O4 has recently been discovered as a possible photocathode material due to its optimal optical bandgap in the range 1.5−1.8 eV, conduction band location that is more negative than the thermodynamic potential for proton reduction, and positive

  • We reveal the upper limit in stable photocurrent density that can be achieved from our W:BiVO4− CuBi2O4 tandem cell by utilizing H2O2 as an electron and hole scavenger, and we measure the actual performance of the tandem cell for overall water splitting with the addition of cobalt phosphate (CoPi) on W:BiVO4 (W:BiVO4/CoPi) and Pt or ruthenium oxide (RuOx) on CuBi2O4 as cocatalysts

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Summary

■ INTRODUCTION

Solar photoelectrochemical (PEC) water splitting with the employment of photoelectrodes in a PEC cell has been demonstrated to be a promising approach for converting abundant solar energy into chemical bonds in the form of hydrogen fuel, which is transportable and does not emit greenhouse gases.[1−4] During the past few decades, extensive research efforts have been focused on exploring metal oxidebased semiconductors as photoelectrodes for photoelectrochemical water splitting. Experiments are typically performed in a PEC cell in three-electrode configuration with the semiconductor photoelectrode as the working electrode, a metal such as Pt as the counter electrode, and a reference electrode such as Ag/AgCl.[5−7] In a three-electrode configuration the working electrode only drives one of the two halfreactions for water splitting while an additional bias is usually applied to the counter electrode to drive the other halfreaction.[8] In practical water splitting applications, a PEC cell would be used in two-electrode configuration, preferably without the application of an external bias This can be achieved by using a single photoelectrode with band positions that straddle the redox potentials for both of the water splitting half-reactions or by combining a photoanode and photocathode in a tandem PEC device.[9−14]. We demonstrate hydrogen production from a W:BiVO4/CoPi-CuBi2O4/CdS/ TiO2/RuOx tandem device, and we discuss the limitations of using CuBi2O4 as the photocathode material in terms of photocorrosion and photovoltage

■ RESULTS AND DISCUSSION
■ CONCLUSION
■ ACKNOWLEDGMENTS
■ REFERENCES
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