pH-Dependent Stability of α‑SnWO₄ Photoelectrodes

Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is a promising approach in the quest for “green” hydrogen production. One of the most important aspects in the search for suitable photoelectrode materials is the resistance against corrosion under PEC operating conditions. Among the various photoelectrode materials that have been investigated, α-SnWO₄ is particularly promising due to the favorable charge carrier transport properties, similar to those of BiVO₄, but with a lower band gap of ∼1.9 eV. However, potential challenges regarding stability were indicated in previous studies, and the corrosion resistance has not been explored in detail. In this study, the stability of α-SnWO₄ photoanodes prepared by pulsed laser deposition is thoroughly investigated in a broad range of applied potentials and pH values, using a combination of inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and in situ spectro­(photo)­electrochemistry measurements. The experimentally observed pH and potential dependences are also compared with a calculated Pourbaix diagram. Based on this complete analysis, a stability window is defined and an optimal operating window for α-SnWO₄ is proposed. Finally, the formation of a self-passivating oxide layer on the surface of α-SnWO₄ and the self-terminating nature of the reaction are experimentally confirmed and shown to protect against photocorrosion, even after the extended operation of up to 24 h. Such a passivation layer is an important property of a photoelectrode, as it fully protects the photoelectrode from further degradation even in the presence of pinholes.