Maximizing Oxygen Evolution Performance on a Transparent NiFeO_x/Ta₃N₅ Photoelectrode Fabricated on an Insulator

A transparent Ta₃N₅ photoanode is a promising candidate for the front-side photoelectrode in a photoelectrochemical (PEC) cell with tandem configuration (tandem cell), which can potentially provide high solar-to-hydrogen (STH) energy conversion efficiency. This study focuses in particular on the semiconductor properties and interfacial design of transparent Ta₃N₅ photoanodes fabricated on insulating quartz substrates (Ta₃N₅/SiO₂), typically the geometric area of 1 × 1 cm² in contact with indium on its edge. This material utilizes the self-conductivity of Ta₃N₅ to make the PEC system operational, and the electrode would strongly reflect the intrinsic nature of Ta₃N₅ without a back contact that is commonly introduced. First, PEC measurements using acetonitrile (ACN)/H₂O mixed solution were made to elucidate the intrinsic photoresponse in the presence of tris(2,2'-bipyridine)ruthenium(II) bis(hexafluorophosphate) (Ru(bpy)₃(PF₆)₂) without water contact which avoids a multielectron-transfer oxygen evolution reaction (OER) and photoinduced self-oxidation. The potential difference between the onset potential of Ru²⁺ PEC oxidation by Ta₃N₅/SiO₂ and the redox potential of Ru^2+/3+ in the nonaqueous environment was about 0.7 V. While a stable photoanodic response was observed for Ta₃N₅/SiO₂ in the nonaqueous phase, the addition of a small quantity of water into this nonaqueous system led to the immediate deactivation of Ta₃N₅/SiO₂ photoanode under illumination by self-photooxidation to form TaO_x at the solid/water interface. In aqueous phase, flatband potentials estimated from Mott-Schottky analysis varied with solution pH (constant potential against reversible hydrogen electrode (RHE)). Photoelectrode modification by a transparent NiFeO_x layer was attempted. The complete coverage of the Ta₃N₅ surface with transparent NiFeO_x electrocatalysts, achieved by an optimized spin-coating protocol with controlled Ni-Fe precursors, allowed for the successful protection of Ta₃N₅ and demonstrated an extremely stable photocurrent for hours without any additional protective layers. The stability of the resultant NiFeO_x/Ta₃N₅/SiO₂ was limited not by Ta₃N₅ but mainly by a NiFeO_x electrocatalyst due to Fe dissolution with time.