Doped <scp> Ta ₃ N ₅ </scp> Semi‐Transparent Photoanode Enabling Stable Solar Redox Flow Cell Operation

Solar redox flow cells (SRFCs) are photoelectrochemical devices that convert sunlight into electrochemical fuels by photocharging electrolytes, which can later discharge in redox flow cells to produce electricity. Advancing SRFCs requires semiconductors that are efficient, stable, abundant, and affordable. Tantalum nitride (Ta 3 N 5 ), with a bandgap of ~2.1 eV, offers high photocurrents but suffers from limited stability. This study presents semitransparent Ta 3 N 5 photoelectrodes enhanced by an aluminim‐doped titanium dioxide (Al:TiO 2 ) underlayer and doped with tungsten (W) and lanthanum (La). Their performance was evaluated using J – E curves, stability tests, TRPL, EIS, XPS, XRD, and SEM/EDS. The optimal configuration—Ta 3 N 5 with 2.8% La on a 20 nm Al:TiO 2 layer—achieved a photocurrent density of ~ 4.0 mA cm −2 at 1.3 V RHE , a peak power density of ~1.8 mW cm −2 , and stability exceeding 90 h in potassium ferrocyanide. When integrated with ferrocyanide and anthraquinone redox chemistry, the system completed 12 photocharge–discharge cycles, demonstrating promising scalability and durability.