Maximizing Oxygen Evolution Performance on a Transparent NiFeO/TaN Photoelectrode Fabricated on an Insulator.

A transparent TaN 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 TaN photoanodes fabricated on insulating quartz substrates (TaN/SiO), typically the geometric area of 1 × 1 cm in contact with indium on its edge. This material utilizes the self-conductivity of TaN to make the PEC system operational, and the electrode would strongly reflect the intrinsic nature of TaN without a back contact that is commonly introduced. First, PEC measurements using acetonitrile (ACN)/HO mixed solution were made to elucidate the intrinsic photoresponse in the presence of tris(2,2'-bipyridine)ruthenium(II) bis(hexafluorophosphate) (Ru(bpy)(PF)) without water contact which avoids a multielectron-transfer oxygen evolution reaction (OER) and photoinduced self-oxidation. The potential difference between the onset potential of Ru PEC oxidation by TaN/SiO and the redox potential of Ru in the nonaqueous environment was about 0.7 V. While a stable photoanodic response was observed for TaN/SiO in the nonaqueous phase, the addition of a small quantity of water into this nonaqueous system led to the immediate deactivation of TaN/SiO photoanode under illumination by self-photooxidation to form TaO 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 layer was attempted. The complete coverage of the TaN surface with transparent NiFeO electrocatalysts, achieved by an optimized spin-coating protocol with controlled Ni-Fe precursors, allowed for the successful protection of TaN and demonstrated an extremely stable photocurrent for hours without any additional protective layers. The stability of the resultant NiFeO/TaN/SiO was limited not by TaN but mainly by a NiFeO electrocatalyst due to Fe dissolution with time.