Photocorrosion of CuBi₂O₄ Monitored by Operando Surface-Sensitive X-Ray Scattering

The low stability of most semiconducting materials is one of the key factors that hinders the development of efficient and durable PEC water splitting cells. Monitoring the semiconductor-electrolyte interface during operation is crucial to understand all the underlying photocorrosion processes and in turn establish appropriate mitigation strategies.<br/>In this work, we show a novel method to study operando the semiconductor-electrolyte interface during PEC operation. In particular, we developed a custom-built PEC cell that allows to assess operando the crystalline and morphological evolution of the semiconductor surface by grazing-incidence X-ray scattering, making use of two detectors to collect simultaneously the total scattering (TS) and the small-angle X-ray scattering (SAXS) signals. We applied the technique to monitor the evolution of CuBi₂O₄ films, a promising p-type semiconductor to be used as a light harvesting material in the cathodic compartment of a PEC cell.<br/>Our operando approach, together with complementary X-ray absorption near edge spectroscopy (XANES) and inductively coupled plasma mass spectroscopy (ICP-MS) measurements, allows us to uncover the multiple degradation pathways affecting CuBi₂O₄ films performance during PEC operation. We find that CuBi₂O₄ reduces to metallic Bi and Cu, with the first one being the fastest process. We also find that Cu ions are released in the electrolyte during long-term stability tests, while at the same time BiPO₄ is formed at the surface of the CuBI₂O₄ film, due to the presence of PO₄^3- ions in the electrolyte. Our work provides a detailed picture of the degradation mechanisms occurring at the surface of CuBi₂O₄ electrodes under operation and poses the methodological basis to study the photocorrosion processes of a wide range of PEC materials.