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

Solar redox flow batteries (SRFB) have attracted increasing interest for in situ capture and storage of intermittent solar energy by integrating a photoelectrochemical cell with a redox flow battery. Device performance has significantly advanced via heterostructure engineering or dual-photoelectrode designs. Yet, cost-effective single-photoelectrodes, like hematite, featuring a suitable bandgap for solar absorption, have not achieved unassisted photocharging up to high state-of-charge (SOC). Here, we demonstrate a scalable, nanostructured α-Fe2O3 photoanode exhibiting a high photovoltage of 0.68 V in an integrated type-III Na4Fe(CN)6/AQDS SRFB. Thanks to its optimal band alignment, it uniquely enables stable, unassisted photocharging of the SRFB up to a SOC higher than 50%. Concurrently, its improved charge transfer results in a record unbiased photocurrent density of 0.22 mA/cm2 and a sixfold increase in solar-to-chemical efficiency (STC) at zero SOC compared to bulk α-Fe2O3 film. Through an in-depth optical and photoelectrochemical characterization of four different α-Fe2O3 morphologies, we quantify the impact of nanostructuring on charge transfer. Most interestingly, we consistently observe an increase in unbiased photocurrent from 0% SOC to 10% SOC, reaching 0.31 mA/cm2 in the best device. We attribute this to the adsorption of ferricyanide ligands on the photoanode surface, which enhances charge collection and catalytic performance while suppressing back electron recombination. Importantly, we demonstrate 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 can pave the way to the realization and real-world deployment of cost-effective SRFBs based on earth-abundant materials