Unlocking the potential of semi-transparent Ta3N5 photoelectrodes for high-performance, reproducible solar redox flow cells†

A solar redox flow cell (SRFC) converts solar energy into storable electrochemical energy and heat; when connected to a redox flow battery, it can produce dispatchable electricity. Despite its versatility, a SRFC is still considered to be at a low technology readiness level (TRL), mainly due to the absence of abundant, efficient, and stable semiconductors. Tantalum nitride (Ta₃N₅) photoelectrodes have garnered special interest for photoelectrochemical water-splitting applications, particularly those using opaque Ta substrates. However, for SRFCs, which are normally based on coloured electrolytes, Ta₃N₅ needs to be semi-transparent to allow backside sunlight illumination. Herein, for the first time, the electrophoretic deposition technique was optimized for synthesizing semi-transparent Ta₃N₅. The best-performing bare photoelectrodes were prepared over a 30 nm Ta-doped TiO₂ (TTO) underlayer, and with an electrophoretic time of 7 min and an annealing temperature of 425 °C in an NH₃ atmosphere, displaying an unprecedented photocurrent density of ca. 4.0 mA cm⁻², and a maximum power density of ca. 1.1 mW cm⁻², using a ferrocyanide-based electrolyte. These conditions allowed improving the charge-transfer kinetics and reducing the recombination rates, as observed by electrochemical impedance spectroscopy analysis. The optimized Ta₃N₅ photoelectrode was paired with a perovskite solar cell, demonstrating ca. 100 h of operation in an aqueous alkaline electrolyte, based on ferrocyanide (K₄Fe(CN)₆) and anthraquinone-2,7-disulphonate (2,7-AQDS) redox pairs.