Interrogation of the Interfacial Energetics at a Tantalum Nitride/Electrolyte Heterojunction during Photoelectrochemical Water Splitting by Operando Ambient Pressure X-ray Photoelectron Spectroscopy

Photoelectrochemical (PEC) water electrolysis is an importantenergyconversion (power-to-chemical) method, providing a solution to theintermittent nature of solar energy. However, as PEC systems usuallysuffer from low operational stability, they are seriously laggingin up-scaled demonstrations and viability. PEC systems are based onsemiconductor/liquid interfaces, which have been extensively studiedby experiments and theory, but there is a significant knowledge gapin 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 spectroscopicproperties of a pristine Ta3N5 photoelectrodeand a Ta3N5/NiO x protection/passivation layer system, which stabilizes an otherwisequickly corroding pristine photoelectrode. We directly observed Fermi-levelpinning of Ta3N5 within the applied potentialwindow under both dark and illumination conditions, detrimental tothe performance and stability of the photoelectrode. Interestingly,in the Ta3N5/NiO x protection/passivation layer system, the Fermi level gets unpinnedunder illumination, allowing quasi-Fermi-level splitting and sustaininga significant PEC performance as well as high stability.