Understanding Energy Conversion and Loss Mechanisms in Ternary Metal Oxide Photoelectrodes: The Case of Copper Vanadate

Essential photoelectrochemical (PEC) functionalities are systematically analyzed on a series of copper vanadate photoanodes with different Cu:V elemental ratios. Homogeneous, highly continuous, and phase-pure thin films of <i>β</i>-Cu<sub>2</sub>V<sub>2</sub>O<sub>7</sub>, <i>γ</i>-Cu<sub>3</sub>V<sub>2</sub>O<sub>8</sub>, Cu<sub>11</sub>V<sub>6</sub>O<sub>26</sub> and Cu<sub>5</sub>V<sub>2</sub>O<sub>10</sub> are grown via reactive co-sputtering deposition and then evaluated for their performances in light-driven oxygen evolution reaction (OER). Despite all four compounds have similar 1.8 – 2.0 eV bandgaps, Cu-rich phases are found to exhibit shorted absorption length in addition to higher charge separation efficiencies at the semiconductor/electrolyte junction. In the presence of sacrificial hole acceptor, the superior bulk properties of Cu<sub>5</sub>V<sub>2</sub>O<sub>10</sub> photoanode translate to the most cathodic (0.67 V vs. RHE) onset potential and a 206 μA/cm<sup>2</sup> photocurrent density that is four times higher than <i>β</i>-Cu<sub>2</sub>V<sub>2</sub>O<sub>7</sub> at 1.23 V. vs. RHE. Nevertheless, the sluggish OER kinetics competes with carrier recombination through Cu-associated surface states, and transient photocurrent spectroscopy quantitatively reveals the deterioration of surface catalytic activity with increasing Cu:V elemental ratio. This comprehensive analysis of PEC characteristics – light absorption, carrier separation, and heterogeneous charge transfer – not only gives insights into functional roles of individual elements in ternary metal oxide photoanodes, but also provides strategies for rational discovery, design, and engineering of new photoelectrode materials for solar fuel production.