Thermochemical Water Decomposition
A thermochemical hydrogen-production process is one which requires only water as a material input and mainly thermal energy, or heat, as an energy input. The output of the process is hydrogen and oxygen and perhaps some waste heat. The process itself comprises a series of chemical reactions which sum to water decomposition. In any direct decomposition process there will be severe practical problems related to the very high temperatures required. Materials of construction, heat exchangers, separation membranes, and reaction vessels will present considerable difficulties, but the real and unavoidable drawback arises from the small entropy increase for the water decomposition reaction. Thermodynamic property changes for each reaction and heating and cooling step, and a summation which must equal those values for water decomposition.