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

Abstract

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.