Thermochemical water splitting cycles for hydrogen production: Perspectives for thermodynamic approaches

Water splitting through thermochemical cycles for hydrogen production is environmentally benign and potentially efficient. Improving the thermodynamic analysis is critical to close the gaps between the theoretical and the practical thermochemical cycle efficiencies. Although the theoretical efficiency can be up to 60-70 %, most of the experimental efficiencies obtained were lower than 8 %. The current state-of-the-art review on the thermodynamic analysis of the thermochemical cycles is presented in this work. Prior to review the thermodynamic analysis, the reaction routes of different thermochemical cycles are introduced and discussed, which is helpful to understand the application scope of the thermodynamic analysis. Much research has been conducted to investigate the energy or exergy loss of the thermochemical cycle. As the loss distribution varies with either system setup or reactant properties, it is difficult to predict the experimental efficiency with theoretical models. The Gibbs function analysis and the T-S diagram have been widely used. But both of them rely on ideal assumptions including the chemical equilibrium. Recently, a novel diagram-based method without too much hypotheses has been proposed and validated by comparing theoretical and experimental results. Further considering the loss through the thermal conduction, convection and radiation of the reactors in the diagram-based method would be helpful to minimize the gap between the theoretical and experimental efficiencies.