One of the most eco-friendly methods for replacing fossil fuels involves producing H2 through water splitting. Thermochemical cycles that use chemical reagents offer scalability advantages compared to other methods of H2 production from water splitting. In this chapter, various hydrogen production technologies are discussed based on pure and hybrid thermochemical cycles, involving two-, three-, and four-steps. In this regard, zinc oxide (ZnO/Zn), hybrid sulfur (HyS), sulfur-iodine (S-I), and metallic chlorides groups including magnesium-chlorine (Mg-Cl), copper-chlorine (Cu-Cl), vanadium-chlorine (V-Cl), and iron-chlorine (Fe-Cl) are evaluated in detail. Moreover, the main challenges, recent progress, and future directions of these cycles are introduced for analysis and design purposes. Finally, a comparative assessment of the thermochemical cycles is carried out, focusing on the global warming potential (GWP), the cost of H2 production, as well as exergy and energy efficiencies. It is found that in terms of GWP, the HyS and S-I cycles show the most promise, with GWP values of 0.5 and 0.48 kg CO2-eq/kg H2, respectively, while the V-Cl cycle exhibits the highest exergy efficiency (i.e., 77%). © 2025 selection and editorial matter, Mohammad Reza Rahimpour, Mohammad Amin Makarem, and Parvin Kiani.
