Enabling an integrated tantalum nitride photoanode to approach the theoretical photocurrent limit for solar water splitting

Abstract

The feasibility of photoelectrochemical (PEC) water-splitting cells relies on the development of high-performance photoanodes. Significant progress has been made in the discovery of narrow bandgap semiconductors as promising photoanodes. However, the rational design of photoanode architecture that brings the potentials of narrow bandgap semiconductors into fruition for efficient PEC water oxidation still remains a key challenge. Herein, we show a highly efficient photoanode system consisting of a tantalum nitride (Ta₃N₅) semiconductor for light harvesting, hole-storage layers (Ni(OH)_x/ferrhydrite) that mediate interfacial charge transfer from Ta₃N₅ to coupled molecular catalysts (Co cubane and Ir complex) for water oxidation and a TiO_x blocking layer that reduces the surface electron–hole recombination. The integrated Ta₃N₅ photoanode exhibits a record photocurrent of 12.1 mA cm⁻² at 1.23 V vs. the reversible hydrogen electrode (RHE), which is nearly its theoretical photocurrent limit under sunlight (12.9 mA cm⁻²), suggesting that almost each pair of photogenerated charge carriers in Ta₃N₅ has been efficiently extracted and collected for solar water splitting.