Effect of pH on the stability and charge carrier dynamics in BiVO4 photoanode for solar water oxidation

Abstract

Providing clean energy is one of the goals of green chemistry and sustainability. In photoelectrochemical systems for solar fuel production, the long-term stability of semiconductors at the electrode/electrolyte interfaces is a fundamental challenge for production efficiency. In this study, we investigate the photodegradation of bismuth vanadate, a state-of-the-art photoanode, in potassium phosphate buffer in acidic (pH= 5.8), neutral (pH= 7.2), and alkaline (pH= 12.6). For the first time, in addition to investigating the changes in physical and photoelectrochemical characterisation, operando photoinduced absorption spectroscopy is used to monitor the dynamics of BiVO4 photocarriers that begin to degrade under photoelectrochemical water oxidation at different pHs. The analysis of the physicochemical properties shows severe corrosion of the photoanode at high pH under PEC operating conditions, i.e., under illumination at an applied bias of 1.4 VRHE. Surprisingly, on the other hand, the photoelectrochemical performance shows slower photocurrent degradation for the photoanode in alkaline than for the photoanodes in acidic and neutral media. This is attributed to the preferential water oxidation in alkaline media, although most of the BiVO4 film is corroded. As for the decay of steady-state PIAS signals, it shows additional oxidation processes than the water oxidation of the degraded BiVO4 under all electrolyte pH conditions. The decays are faster for the degraded BiVO4 under both acidic and alkaline conditions. At a pH of 5.8, the oxidation of water is more hindered, and the accumulated holes are consumed in other oxidation processes nearby, which we have not probed. In the neutral electrolyte, the decay behaviour before and after the photoelectrochemical experiments was almost the same and the film showed no obvious corrosion. However, its lower performance in water oxidation could indicate the occurrence of self-passivation, which is not detected by the characterisations. In the case of pH 12.6, we can say that although accumulated holes are consumed in the water oxidation and/or photocorrosion of the photoanode itself, the alkaline medium enhances the water oxidation. These investigations are of interest to gain more insight into the photodegradation mechanisms of photoelectrodes to improve their stability in sustainable energy production systems through water-splitting.