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

Abstract

A solar redox flow cell (SRFC) is singular in its ability to use a photoelectrochemical (PEC) cell for efficiently converting solar energy into electrochemical storable energy and heat. The generated solar fuels can be easily converted into electrical power at a redox flow battery (RFB). However, the SRFCs remain at a low Technology Readiness Level (TRL) mainly due to the low semiconductor efficiency, system durability, and lack of validated demonstrators. By leveraging the unique properties of tantalum nitride (Ta3N5) photoelectrodes, this work studies the role of this semiconductor material for the development of high performing SRFC devices. Opaque Ta3N5 photoelectrodes reached special interest for PEC water splitting due to the impressive photocurrent density achieved so far. However, for SRFC applications based on coloured electrolytes, Ta3N5 needs to be semi-transparent to allow backside sunlight illumination. Electrophoretic deposition (EPD) stands out as a suitable method for preparing semi-transparent Ta3N5 films. For the first time, the synthesis conditions were optimized, focusing on the EPD cycle time and annealing temperature, in an NH3 atmosphere, as well as employing a Ta‑doped TiO2 (TTO) underlayer for addressing simultaneously reproducibility and high efficiency. The best-performing bare Ta3N5 photoelectrodes were prepared with an electrophoretic deposition time of 7 min and annealed at 425 ºC, 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 charge transfer kinetics and reducing recombination rates, as observed by electrochemical impedance spectroscopy. The optimized Ta3N5 photoelectrode was then paired with a perovskite solar cell, in a PEC-PV arrangement, to demonstrate 100 h operation of an aqueous alkaline SRFC, based on ferrocyanide (K4Fe(CN)6) and anthraquinone-2,7-disulphonate (2,7-AQDS) redox pairs both dissolved in KOH solutions.