Bismuth vanadate (BiVO4) is one of the most efficient metal oxides for solar water splitting applications. It is an n-type photoanode material, with a bandgap energy of 2.4 eV, a theoretical efficiency of ~9% STH, and is made of cheap, earth abundant, non-toxic elements. However, it suffers from substantial recombination losses that limit its performance to well below its theoretical maximum. While deposition of a co-catalyst, doping, nanostructuring and passivation have been reported to successfully improve the overall PEC activity of BiVO4, they add extra steps to the material processing and introduce significant complexity in optimizing a practical PEC water splitting device.

In our recent work we have reported for the first time on a photoelectrochemical procedure called photocharging (PC), that enables to successfully address limitations of BiVO4 photoanodes. We have demonstrated that BiVO4 photoanodes immersed in an aqueous electrolyte under open circuit conditions and exposed to simulated solar irradiation for prolonged time can greatly increase its solar water oxidation efficiency via a reduced onset potential and an increased maximum photocurrent density. According to our results, the presence of a liquid electrolyte is essential to the PC effect, and no PC can take place without it. These findings indicate that the PC effect should be studied in the context of the semiconductor-liquid junction (SLJ), the key interface in any PEC system

Herein we provide new insights on the possible mechanisms of photocharging in BiVO4 photoanodes, and how it directly leads to improved PEC performance. We show that alkaline conditions favor the PC effect, specifically BiVO4 photoanodes subjected to PC treatment in pH 10 achieve a record high photocurrent for undoped and uncatalyzed BiVO4 of 4.3 mAcm-2 @ 1.23 VRHE, an outstandingly low onset potential of 0.25 VRHE, and a very steep photocurrent onset. Alkaline conditions also facilitate excellent external and internal quantum efficiencies of 75 and 95 % respectively (average in the 440 nm > λ > 330 nm range). Moreover, impedance spectroscopy and in-situ X-ray absorption study suggests that electronic, structural and chemical properties of the bulk of these films remain fairly unchanged during the PC treatment, however, appreciable changes in the surface-related properties take place. Ultimately, our results indicate that the improved activity of PC-BiVO4 is enhanced by surface reaction pathways of the SLJ, which is directly correlated to the electrochemical environment it is modified in.