"Double-Use" Strategy for Improving the Photoelectrochemical Performance of BiVO Photoanodes Using a Cobalt-Functionalized Polyoxotungstate.

Doping and surface-modification are well-established strategies for the performance enhancement of bismuth vanadate (BiVO) photoanodes in photoelectrochemical (PEC) water splitting devices. Herein, a "double-use" strategy for the development of high-performance BiVO photoanodes for solar water splitting is reported, where a molecular cobalt-phosphotungstate (CoPOM = Na[Co(HO)(PWO)]) is used both as a bulk doping agent as well as a surface-deposited water oxidation cocatalyst. The use of CoPOM for bulk doping of BiVO is shown to enhance the electrical conductivity and improve the charge separation efficiency, resulting in the enhancement of the maximum applied-bias photoconversion efficiency (ABPE) by a factor of ∼18 to 0.54% at 0.87 V vs. RHE, as compared to pristine BiVO (0.03% at 1.04 V vs. RHE). The ratio of W/Co on the surface of the photoanode is related to the activity and stability. In addition, modification of CoPOM-doped BiVO with CoPOM as a surface cocatalyst enhances the hole extraction and improves the water oxidation kinetics, resulting in the overall enhancement of the ABPE to 0.79% (at 0.82 V vs. RHE), i.e., by a factor of ∼26 with respect to pristine BiVO. This study establishes the "double-use" strategy involving CoPOMs as an effective, straightforward, and easily scalable approach for the development of high-quality photoanodes for solar water splitting and highlights the future potential of utilizing well-designed polyoxometalates as precursors for the synthesis of energy materials.