Amorphous NiFeMo oxides (a-NiFeMo) synthesized via a simple supersaturated coprecipitation method are explored as a cocatalyst to improve the photoelectrochemical water splitting performance of BiVO 4 . The performance of the composite photoelectrode is found to be further enhanced through a cyclic-voltammetry-related in situ activation process, and a photocurrent of 5.0 mA/cm 2 is achieved at 1.23 V RHE with improved photoelectrochemical stability. X-ray photoelectron spectroscopy (XPS), Raman, and electron spin resonance (ESR) characterization results indicate that the surface-loaded a-NiFeMo cocatalyst undergoes a rapid surface reconstruction during the in situ activation process, which results in an incorporation of phosphate ions in the surface-loaded a-NiFeMo, along with an increased oxidation state of Ni ions and enriched oxygen vacancies. These combined effects lead to an improved oxygen evolution reaction performance and finally result in a reduced charge-transfer resistance at the solid/liquid interface, causing the interfacial charge-injection efficiency to be enormously enhanced from the original 25.1% in pure BiVO 4 to 83.3% in BiVO 4 /a-NiFeMo after being in situ activated. Our result indicates that the rapid surface-reconstruction phenomenon shown in the amorphous materials may provide a promising strategy for designing a highly efficient cocatalyst for photoelectrodes.

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.

The bare surface of BiVO4 photoanode usually suffers from extremely low interfacial charge transfer efficiency which leads to a significantly suppressed photoelectrochemical water splitting performance. Various strategies, including surface modification and the loading of co-catalysts, facilitate the interface charge transfer process in BiVO4. In this study, we demonstrate that CoS2 synthesized from the hydrothermal method can be used as a high-efficient co-catalyst to sufficiently improve the interface charge transfer efficiency in BiVO4. The photoelectrochemical water splitting performance of BiVO4 was significantly improved after CoS2 surface modification. The BiVO4/CoS2 photoanode achieved an excellent photocurrent density of 5.2 mA/cm2 at 1.23 V versus RHE under AM 1.5 G illumination, corresponding to a 3.7 times enhancement in photocurrent compared with bare BiVO4. The onset potential of the BiVO4/CoS2 photoanode was also negatively shifted by 210 mV. The followed systematic combined optical and electrochemical characterization results reveal that the interfacial charge transfer efficiency of BiVO4 was largely improved from less than 20% to more than 70% due tor CoS2 surface modification. The further surface carrier dynamics study performed using an intensity modulated photocurrent spectroscopy displayed a 6–10 times suppression in surface recombination rate constants for CoS2 modified BiVO4, which suggests that the key reason for the improved interfacial charge transfer efficiency possibly originates from the passivated surface states due to the coating of CoS2.

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