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

Abstract

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 cm2, and a maximum power density of ca. 1.1 mW cm2, 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. (c) 2025 The Royal Society of Chemistry.