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Mitigation of ohmic losses and mass transport limitations enables a large area BiVO4-based water splitting device with a solar-to-hydrogen efficiency of 2.1%.

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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.

Photoelectrochemical (PEC) cells for water splitting generally have a transparent front window for the sunlight to reach the surface of the photoelectrode or tandem photoelectrode. The overall efficiency of a PEC system for solar hydrogen production is strongly affected by the evolved gases that get trapped in the front window. This negative effect is clearly observed when the PEC cells are placed in tilted positions to maximize light harvesting. Titanium dioxide coatings become superhydrophilic when exposed to UV light, facilitating the gas bubbles to slip up. The present study focuses on the development of a thin TiO 2 coating to minimize the adhesion of the evolved bubbles in the front window of PEC cells, thereby maximizing its transparency. Highly transparent, crack-free, and stable thin films of TiO 2 were prepared by spin coating followed by sintering at 465 C for 45 min. A water contact angle (WCA) of 0 was obtained after irradiating the surface of the sample for 30 min with UV-light (365 nm, 2 W m À2), confirming the superhydrophilic behaviour. The irradiance loss during the evolution of H 2 /O 2 was assessed using a silicon PV cell; the cell, tilted at 45 and equipped with a TiO 2 -coated glass window, showed ca. 10% higher irradiance as compared to the uncoated glass window cell for both hydrogen and oxygen evolutions, whereas no significant differences were observed when the cells were vertically placed.

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