Water Splitting: Thermochemical

Abstract

AbstractHydrogen can be produced by direct or single‐step splitting of water. The Gibbs free energy change for direct thermochemical water splitting is zero at about 4300 K at 1 bar. However, in practice, direct thermochemical splitting of water poses challenging high‐temperature materials and product separation issues, among others. One way to lower the extremely high temperatures required for direct splitting of water and mitigate gas separation issues is the use of multistep cycles. To date, more than 350 thermochemical cycles for splitting water has been conceived. Of these, fewer than two dozen cycles still remain subject to further research and development. Among them are several “sulfur family” cycles as well as a number of volatile and nonvolatile metal oxide cycles. All of these cycles consist of at least two process steps, and some are hybrid cycles requiring both heat and work (e.g., electricity) input. This article presents a brief review of the status of multistep thermochemical water‐splitting cycles (TCWSCs) for the production of hydrogen.