A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production

Thermochemical cycles are considered as promising technologies for long-term, large-scale hydrogen production in a clean and sustainable manner. In this study, various hydrogen production methods through thermochemical and hybrid cycles, including two step (Zinc oxide), Three step (Sulfur-Iodine), four-step (Iron-Chlorine, Magnesium-Chlorine and Copper-Chlorine) and hybrid types (Hybrid Sulfur) are discussed and comparatively evaluated. In this regard, major challenges, recent advancements and future directions of these methods are presented and discussed for design, analysis and assessment purposes. Moreover, a comparative evaluation of the selected thermochemical cycles is extensively performed based on the cycle's energy and exergy efficiencies, hydrogen production cost and global warming potential (GWP). A comparative study shows that vanadium-chlorine offers the highest exergy efficiency of 77% while in terms of GWP, Sulfur-Iodine and hybrid sulfur cycles become the most promising with GWP of 0.48 and 0.50 kg CO2-eq/kg H-2, respectively. The hybrid Cu-Cl cycle offers a great potential in terms of integration with nuclear heat or industrial process heat or renewable or waste heats.