The development of photoelectrochemical (PEC) systems for solar fuel production has been focused mainly on the fabrication and study of highly efficient materials. However, when the main target is to increase performance in terms of photocurrent densities, the selection of operating conditions becomes inconsistent due to ad hoc approaches preferred by researchers to maximise photocurrent densities at the expense of stability, scalability and practical application. The operating conditions for increased stability and for improved solar-to-fuel efficiency are usually at odds. For example, BiVO₄ photoelectrodes report higher stability when used in near-neutral electrolytes, although photocurrent densities are higher when alkaline electrolytes are used [1]. Similarly, chronoamperometries performed at low electrode potentials usually register more stable, although lower, photocurrent densities compared to those at high electrode potentials. For BiVO₄ photoelectrodes, the effect of different operating condition has been studied separately, e.g. irradiance [2], temperature [3] and pH [4], although with a focus on performance instead of stability. More recently, the authors decoupled the effect of increased irradiance and temperature on the dissolution of BiVO₄ by examining the competing behaviour between these two factors [5]. However, the effect of other relevant operating conditions such as electrolyte concentration, pH, electrode potential and electrolyte flow, and their interactions, has yet to be reported.

In this study, we realised a systematic experimental study of different factors, including irradiance, temperature and electrolyte nature (pH, concentration and flow) and its effect on the degradation of spray pyrolyzed BiVO₄ photoelectrodes. For this, a 4-cell array was developed to perform simultaneous tests under different conditions. Fresnel lenses were used to achieve irradiances between 1 and 110 suns, and an external water bath to control the electrolyte temperature (25 to 50 °C). KPi buffer solution was used as electrolyte at different pHs (5 to 11) and concentrations (0.1 to 1.0 M). Chronoamperometries were performed for long term tests, while electrochemical impedance spectroscopy was used to assess in-operando the charge transfer processes at different stages during the degradation and to quantify the effect of operating conditions on the properties of the semiconducting material, i.e. flat band potential and donor density. The array allowed to investigate a wide range of operating conditions, and its combinatorial effect on the dissolution of BiVO₄. Additionally, seldom reported reproducibility and sensitivity studies were also performed.

It was found that increased temperature impacts negatively the stability of the photoelectrodes, while increasing the irradiance limits the amount of charge towards the dissolution process. However, increased irradiance also induces concomitant effects, that is, increased temperature and lowered pH at the surface of the oxygen-evolving photoanode. These effects were successfully decoupled by using the above-described experimental design. It was also found that the electrolyte concentration, and respective buffer capacity, have a significant effect on the stability of the photoelectrode, although to a lesser extent compared to pH, while the electrolyte flow has a minor but noticeable effect on the of the photoelectrochemical performance.

This study contributes to the broader field of solar fuel production by highlighting the importance of stability alongside efficiency in the development of new photoelectrode materials. Understanding the complex interaction between operating conditions and degradation mechanisms can prove useful in future research to tailor these conditions to optimize the longevity and efficiency of PEC systems. Additionally, the methodology discussed here can serve as a framework for upcoming systematic studies on photoelectrode materials.