oxygen. All other chemicals are recycled. The iron oxide process requires an efficient source of heat. The sulfur-iodine cycle (S-I cycle) is a series of thermochemical processes used to produce hydrogen. The S-I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled. The S-I process requires an efficient source of heat. More than 352 thermochemical cycles have been described for water splitting or thermolysis., These cycles promise to produce hydrogen oxygen from water and heat without using electricity. Since all the input energy for such processes is heat, they can be more efficient than high-temperature electrolysis. This is because the efficiency of electricity production is inherently limited. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient. For all the thermochemical processes, the summary reaction is that of the decomposition of water: 2H2O <=>[\ce{Heat}] 2H2{} + O2 All other reagents are recycled. None of the thermochemical hydrogen production processes have been demonstrated at production levels, although several have been demonstrated in laboratories. There is also research into the viability of nanoparticles and catalysts to lower the temperature at which water splits. Recently Metal-Organic Framework (MOF)-based materials have been shown to be a highly promising candidate for water splitting with cheap, first row transition metals. Research is concentrated on the following cycles: See also Water gas shift reaction References External links JEAC Environmental chemistry Fuels Hydrogen production Industrial gases