Ferrites: emerging light absorbers for solar water splitting

Solar water splitting in a photoelectrochemical cell is a highly promising technology to produce clean and storable hydrogen energy. However, there are still a large number of challenges to overcome in order for solar water splitting to become a practically viable technology that can replace current hydrogen production from fossil fuels. The most critical barrier is the light absorber, which can convert solar photons to charge carriers to conduct the water splitting reaction. Iron-containing metal oxides, commonly called "ferrites", are a relatively recently explored class of materials owing to their unadulterated merits of earth abundancy, nontoxicity, and intrinsic stability in aqueous solutions. They generally have small band gaps, which are suitable to harvest sufficient amounts of photon flux from the solar spectrum, and suitable band positions to drive the redox reactions of water splitting. Unfortunately, their performance is fairly poor at the moment in comparison to that of other well-established materials, primarily because of their indirect bandgap characteristics and poor charge carrier dynamics. Another reason is that these materials have been studied as photoelectrodes only recently, and the time spent on their development is not as long as that of well-developed materials. Hence, intensive research should be continued to obtain a fundamental understanding of ferrite materials, develop ingenious synthetic methods, and improve their performance by various modification strategies to transform them into viable photoelectrodes. In this review, we summarise the current state of the progress made in specific ferrite-based photoelectrodes for overall water splitting and provide a perspective of the major challenges and opportunities they face