Maximizing Oxygen Evolution Performance on a Transparent NiFeOx/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 x 1 cm(2) 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 Ru2+/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 TaOx 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 NiFeOx layer was attempted. The complete coverage of the Ta3N5 surface with transparent NiFeOx 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 NiFeOx/Ta3N5/SiO2 was limited not by Ta3N5 but mainly by a NiFeOx electrocatalyst due to Fe dissolution with time.