Features of solar thermochemical redox cycles for hydrogen production from water as a function of reactants’ main characteristics

Promising applications of concentrated solar energy are thermochemical cycles based on metal–metal oxide redox reactions for hydrogen production. These cycles usually consist of two steps: metal hydrolysis followed by solar reduction or thermal decomposition of the metal oxide. Thermodynamic analysis sustained by experimental results obtained for different reactants such as boron, zinc, tin and cadmium indicates that the cycle efficiency essentially depends on molar weight, valence, vapor pressure of reduced metals at reaction temperature, and strength of metal–oxygen (Me–O) bonds. Metals with lower molecular weight-to-valence ratio and stronger Me–O bonds demonstrate higher hydrogen productivity, better conversion and larger amount of heat released during the hydrolysis reaction. However, they require higher temperatures or multiple steps for the reduction of their oxides. This paper compares previously published results about these two-step processes complemented by more recent ones and describes the main aspects of selecting solid reactants to enable effective, reliable and safe operation of both the hydrolysis and the reduction steps.