High-Performance Hematite Photoanodes for Unassisted Recharging of Solar Redox Flow Battery

Solar redox flow batteries (SRFB) have attracted increasing interest for simultaneous capture and storage of solar energy by integrating a photoelectrochemical cell with a redox flow battery. Herein, a scalable, nanostructured α-Fe₂O₃ photoanode exhibiting a high photovoltage of 0.68 V in a fully integrated Na₄Fe(CN)₆/AQDS SRFB is demonstrated. Thanks to its optimal band alignment, it uniquely enables stable, unassisted photocharging of the SRFB up to a state-of-charge (SOC) higher than 50%. Concurrently, its improved charge transfer results in a record unbiased photocurrent density of 0.22 mA cm⁻², with a sixfold increase at zero SOC compared to α-Fe₂O₃ film. Through an in-depth optical and photoelectrochemical characterization of different α-Fe₂O₃ morphologies, the impact of nanostructuring on charge transfer is quantified. Most interestingly, an increase in unbiased photocurrent is observed at 10% SOC (0.31 mA cm⁻²) and attributed to adsorption of ferricyanide, which enhances charge transfer. Importantly, it is demonstrated that the superior performance is retained after device scale-up to 5.72 cm². Overall, the demonstrated unassisted device is on par with previously reported dye-sensitized solar cell-assisted hematite-based SRFBs. More broadly, this work contributes to the real-world deployment of cost-effective SRFBs based on Earth-abundant materials.