Cobalt-Modified Amorphous Nickel–Iron–Molybdate Cocatalysts to Enhance the Photoelectrochemical Water Splitting Performance of BiVO₄ Photoanodes

Abstract

Incorporation of a low content of Co into the structure of the amorphous nickel–iron–molybdate cocatalyst is found to dramatically enhance the photoelectrochemical water splitting performance of BiVO 4 at low bias potentials. After being activated via a cyclic voltammetry-related in situ activation process, the photocurrent of the amorphous cocatalyst-modified BiVO 4 at 1.23 V RHE was further improved to 5.47 mA/cm 2 under AM 1.5G (100 mW/cm 2 ) illumination. Compared to pure BiVO 4, the activated composite BiVO 4 photoanode exhibits a greatly enhanced interfacial charge injection efficiency in the whole potential range and displays a low onset potential of 0.28 V RHE, which delivers the photoanode an applied bias photon-to-current efficiency (ABPE) of 1.78% at 0.65 V RHE . During the in situ activation process, a significant loss of Mo cations accompanied by an incorporation of phosphate ions was revealed, which suggests that a rapid surface reconstruction process was triggered in the surface-loaded amorphous cocatalyst. In addition to the surface reconstruction, the oxidation state of Ni ions and the density of oxygen vacancies in the amorphous cocatalyst were also found to be increased. The combined effect of incorporation of Co and surface reconstruction is believed to induce a rational adjustment of the electronic structure of the amorphous cocatalyst, which forms dense active sites and ultimately enhances the photoelectrochemical water splitting performance of BiVO 4 in the whole potential range.