Solar thermal water splitting (STWS) produces renewable hydrogen from water using concentrated sunlight. Because it utilizes energy from the entire solar spectrum to directly drive the redox reactions that split water, it can achieve high theoretical solar-to-hydrogen efficiencies. In two-step STWS, a metal oxide is first heated by concentrated sunlight to high temperatures to reduce it and produce O 2 . In the second step, the reduced material is exposed to H 2 O to reoxidize it to its original oxidation state and produce H 2 . Various aspects of this process are reviewed in this work, including the reduction and oxidation chemistries of the active redox materials, the effects of operating conditions, and the solar thermal reactors in which the STWS reactions occur, and a perspective is given on the future optimization of STWS.

A comprehensive solar-to-hydrogen (STH) efficiency model, which includes the effect of oxidation kinetics, is developed for two-step solar thermochemical redox water splitting processing. Active materials flow through separate reduction and oxidation reactors. Two active redox materials are considered and compared in order to assess the impact of the rate of redox and the hydrogen productivity per cycle on STH efficiency. Reported oxidation rates for reduced cerium oxide (fast kinetics/lower H2 productivity/cycle) and a ferrite/zirconia composite (slow kinetics/higher H2 productivity/cycle) are used in the model in order to make a realistic comparison. Generally, the efficiency at thermodynamic equilibrium is higher for the ferrite/zirconia composite than ceria. Interactions between material specific parameters are compared, such as the combination of heat capacity and flow rate on sensible heating loads.Additionally, the sensitivity of oxidation kinetics on the overall cycle efficiency is illustrated.Model results show that kinetics can have a drastic effect on STH efficiency. Near-isothermal redox processing is more optimal for materials with slower kinetics, and especially with moderate to high gas heat recuperation. The kinetic effects are negligible for those active materials having fast oxidation rates, i.e. ceria, which benefit from a larger temperature difference (thermodynamic driving force) between the reduction and oxidation steps. This leads to different optimal operating conditions when oxidation kinetics are included in the analysis as compared to prior models when only thermodynamic equilibrium is considered.