Photocatalytic water splitting is an active area of research in materials chemistry. Photo-irradiated semiconductor materials provide electrons or holes to water to split oxygen and hydrogen. They can be used as photoanode (oxygen evolution) and photocathode (hydrogen evolution), (1) and sometimes both are combined or even connected to each other to directly produce oxygen and hydrogen at the same time (Z-scheme material). (2) The semiconductor materials are made from inexpensive natural resources without using much energy, and the processing cost is expected to be much lower than today’s technology.

       Although this technology is promising, it is still in the development stage and needs to be greatly improved in terms of efficiency, because the efficiency is usually measured in STH (solar-to-hydrogen efficiency), which indicates how much light energy is converted into a photocurrent (used to produce hydrogen). The STH of photocatalytic water splitting is about 1% for the best case scinario.  (3) However, this value is too low compared to the combination of a solar cell and an electrolyzer. A general silicon solar cell has a conversion efficiency of about 20%, and the efficiency of a polymer electrolyte membrane (PEM) electrolyzer is 70-80%, and the overall efficiency can go >10%, which is 10 times higher than the best photocatalytic water splitting system.

        However, in a recent paper published in Nature, the authors increased the efficiency by more than 9%, almost an order of magnitude increase in efficiency using the photocatalytic water splitting.(4) They used a sophisticated material (InGaN/GaN nanowire) with cocatalysts (Rh/Cr2O3/Co3O4) for efficient oxygen and hydrogen evolution, of course, but the key improvement is the use of concentrated solar light through a Fresnel lens and the reaction temperature control at 70 degC. This research possibly suggests that the photocatalytic reactions at the interface has not been optimized in most of cases in terms of the reaction energetics and kinetics and mass transfer processes.

(1) J. Josny, et al, Int. J. Hyd. Ener., 2018, 43, 4804.
(2) Wang, Zheng, et al, Chemical Society Reviews 2019, 48, 2109–25. https://doi.org/10.1039/C8CS00542G.
(3) T. Takata,et al,. Nature 581, no. 7809: 411–14. https://doi.org/10.1038/s41586-020-2278-9.
(4) P. Zhou, et al., Nature 613, no. 7942 66–70. https://doi.org/10.1038/s41586-022-05399-1.