A Tandem Water Splitting Cell Based on Nanoporous BiVO4 Photoanode Cocatalyzed by Ultrasmall Cobalt Borate Sandwiched with Conformal TiO2 Layers

The BiVO4 photoanode with 2.4 eV band gap and a-Si photocathode with 1.7 eV band gap are ideal to form a self-driven tandem photoelectrochemical (PEC) water-splitting cell. The main obstacles for such tandem cells are the relatively low activity and stability of BiVO4 photoanode. Herein, we demonstrate a highly active undoped BiVO4 cocatalyzed with ultrasmall cobalt borate (CoB,) via photoassisted electrodeposition. The conformal TiO2 layer deposited on BiVO4 surface helps the formation of ultrasmall CoB, cocatalysts with excellent charge transfer properties. The resulting BiVO4 photoanode exhibited a remarkably stable photocurrent of similar to 2.5 mA/cm(2) at 1.23 V vs RHE. By combining the highly active and stable BiVO4 photoanode with a-Si photocathode, a solar-driven tandem water-splitting cell is successfully fabricated with up to 3% solar-to-hydrogen (STH) conversion efficiency, a fairly high efficiency yet reported for undoped BiVO4 photoanode.