Thermodynamics of hydrogen production from water

The possibility of hydrogen Production from water using only heat submitted and extracted at different temperatures is analysed. One step direct water decomposition as well as thermo-chemical cycles are considered. It is emphasised the importance for this analysis of the so called ''zero work'' temperature, at which the Gibbs energy of a particular reaction becomes zero. It is demonstrated that in any case when work is substituted by heat the exergy of this amount of heat has to be equal to the substituted work.