Abstract
Photoelectrochemical (PEC) water electrolysis is an important energy conversion (power-to-chemical) method, providing a solution to the intermittent nature of solar energy. However, as PEC systems usually suffer from low operational stability, they are seriously lagging in up-scaled demonstrations and viability. PEC systems are based on semiconductor/liquid interfaces, which have been extensively studied by experiments and theory, but there is a significant knowledge gap in the energetics of such interfaces during operation. In this work, operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been used to characterize the electrical and spectroscopic properties of a pristine Ta3N5 photoelectrode and a Ta3N5/NiOx protection/passivation layer system, which stabilizes an otherwise quickly corroding pristine photoelectrode. We directly observed Fermi-level pinning of Ta3N5 within the applied potential window under both dark and illumination conditions, detrimental to the performance and stability of the photoelectrode. Interestingly, in the Ta3N5/NiOx protection/passivation layer system, the Fermi level gets unpinned under illumination, allowing quasi-Fermi-level splitting and sustaining a significant PEC performance as well as high stability.