A 25 cm2 Solar Redox Flow Cell: Facing the Engineering Challenges of Upscaling

The solar redox flow cell (SRFC) is an emerging technology that uses semiconductors to photocharge redox pairs, storing solar energy in electrochemical fuels and heat. Despite being in its infancy, significant efforts have been made in the development of high‐efficient materials and in understanding the fundamental processes. However, little attention has been given to device architecture and scalability, which may prove equally important in bringing this technology closer to commercialization. This work is the first attempt at upscaling SRFCs, proposing an innovative 25 cm2 photoactive‐area device: the SolarFlow25 cell. A computational fluid dynamics model is developed to implement design key features aiming at: maximizing light absorption by the semiconductor, ensuring effective diffusion and convection of redox species, and guaranteeing minimal electronic and ionic transport resistances. After being connected to a redox flow cell (RFC), the combined SRFC/RFC device is used to photocharge–discharge a ferrocyanide/anthraquinone (2,7‐AQDS) chemistry continuously. A nanostructured hematite (α‐Fe2O3) photoelectrode combined in series with a dye‐sensitized solar cell (DSSC) produces an unbiased photocurrent of ≈40 mA. The solar‐to‐output‐electricity‐efficiency remains stable at ≈0.44%, fourfold higher than any other reported hematite‐driven SRFC. The guidelines provided here are expected to help design and upscale future SRFC devices, making solar energy accessible in decentralized locations. The highest photoactive‐area solar redox flow cell (SRFC) is proposed—the SolarFlow25 cell. Computational fluid dynamics (CFD) simulations are used to optimize the electrolyte flow path inside the cell and its applicability is demonstrated using hematite photoelectrodes to photocharge a ferrocyanide/anthraquinone flow cell. Record solar‐to‐output‐electricity‐efficiency is obtained for hematite‐based SRFCs. Moreover, device design guidelines are provided to help develop future upscaled photoelectrodes and devices.