Design of semitransparent tantalum nitride photoanode for efficient and durable solar water splitting

Abstract

Unbiased solar water splitting via a photoelectrochemical–photovoltaic (PEC–PV) tandem device is a promising strategy for efficient, low-cost, and sustainable hydrogen production to address growing energy demands. The bandgap of Ta₃N₅ is 2.1 eV for a theoretical limit of solar-to-hydrogen (STH) energy conversion efficiency of 15.3%, but the inefficient utilization of photogenerated holes limits the STH efficiency to 7% when Ta₃N₅ is used as a single photoanode. In addition, the formation of a TaO_x insulating layer on the bare Ta₃N₅ surface caused by the self-photooxidation of the Ta₃N₅ surfaces leads to the poor stability of the water oxidation reaction. In this study, we fabricated a void-free Ta₃N₅ planar thin film, originating from metallic Ta deposition by high-power sputtering followed by nitridation in ammonia treatment at high temperature, grown on a transparent GaN/Al₂O₃ substrate. With the uniform decoration of the Ta₃N₅ surface with an ultrathin NiFeO_x electrocatalyst layer, the semitransparent Ta₃N₅ photoanode drastically improved the stability and generated a photocurrent of 7.4 mA cm⁻² at 1.23 V vs. a reversible hydrogen electrode under simulated AM1.5G solar illumination. Unassisted water splitting by a transparent Ta₃N₅ photoanode coupled with CuInSe₂ PV was demonstrated with an initial STH efficiency of 9%, which is the highest efficiency ever reported among metal oxide/nitride-based PEC–PV tandem cells. With the homogeneous electrocatalyst, the tandem cell achieved the stabilized STH efficiency of 4% up to 2 h of device operation. Using measurements and theoretical modeling, the charge carrier kinetics and transport were determined to identify the most crucial Ta₃N₅-thin-film parameters for further performance enhancement.