An updated review and perspective on efficient hydrogen generation via solar thermal water splitting

Solar thermal water splitting (STWS) produces renewable (or green) hydrogen from water using concentrated sunlight. Because STWS utilizes energy from the entire solar spectrum to drive the reduction–oxidation (redox) reactions that split water, it can achieve high theoretical solar‐to‐hydrogen efficiencies. In a two‐step STWS process, a metal oxide that serves as a redox mediator is first heated with concentrated sunlight to high temperatures (T >1000°C) to reduce it and evolve oxygen. In the second step, the reduced material is exposed to steam to reoxidize it to its original oxidation state and produce hydrogen. Various aspects of this process are comprehensively reviewed in this work, including the reduction and oxidation chemistries of active materials considered to date, the solar reactors developed to facilitate the STWS reactions, and the effects of operating conditions—including the recent innovation of elevated oxidant pressure—on efficiency. To conclude the review, a perspective on the future optimization of STWS is provided. This article is categorized under: Sustainable Energy > Solar Energy Emerging Technologies > Hydrogen and Fuel Cells Emerging Technologies > New Fuels