ABSTRACT
Thermochemical water-splitting cycles transform nuclear or solar heat energy into hydrogen energy that works as an energy carrier. Reserves of fossil resources in the world are limited and will be depleted in future. In addition, it is concerned that carbon dioxide produced by use of fossil fuel is a major causative factor of global warming [1]. Therefore, carbon-free energy system is attracting lots of interest. Hydrogen energy system is one of such future energy systems if hydrogen is produced carbon dioxide free. Among technical challenges for realizing the hydrogen energy system, technologies for the production and the storage of hydrogen represent the most important ones. As for the former, concept of thermochemical water-splitting cycle offers an attractive candidate for the large-scale and economical hydrogen production. The direct thermal decomposition of water requires high temperature heat of exceeding a few thousand Kelvin. On the contrary, thermochemical water-splitting cycles make it possible to decompose water at lower temperatures by combining high-temperature endothermic chemical reactions and low-temperature exothermic ones, where the net chemical change is water splitting [2].
