“Double-Use” Strategy for Improving the Photoelectrochemical Performance of BiVO ₄ Photoanodes Using a Cobalt-Functionalized Polyoxotungstate

Doping and surface-modification are well-established strategies for the performance enhancement of bismuth vanadate (BiVO₄) photoanodes in photoelectrochemical (PEC) water splitting devices. Herein, a "double-use" strategy for the development of high-performance BiVO₄ photoanodes for solar water splitting is reported, where a molecular cobalt-phosphotungstate (CoPOM = Na₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂]) 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 BiVO₄ 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 BiVO₄ (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 BiVO₄ 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 BiVO₄. 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.