Solar Hybrid Photo-Thermochemical Sulfur-Ammonia Water-Splitting Cycle: Photocatalyst, Thermodynamics and Plant Analysis

The solar driven hybrid sulfur-ammonia water splitting cycle (HySA) is a promising technology for the environmentally sustainable production of hydrogen fuel. It is a state-of-the-art process that integrates a solar-photocatalytic hydrogen production step (driven by the photonic portion of solar irradiance) with a high-temperature solar thermochemical oxygen evolution step (driven by the thermal portion) and efficient thermal energy storage as an integral part of the cycle. This work summarizes the recent findings on the two subcycles and the overall process design and optimization, following detailed experimental and numerical calculations. First, multiple photocatalysts, for the hydrogen production step, were assessed and characterized, finally, a binary band gap photocatalyst based on ZnS-CdS core shell particles doped with Pt was selected. Second, the materials and their properties for the oxygen evolution step were verified using thermogravimetric experiments, that supported a detailed thermodynamic analysis for the selection and design of the overall process. Finally, all information was assembled in one process that takes advantage of the hybrid solar photo-thermo character of the cycle and the incorporation of a portion of the process-fluids to design an integrated thermal-storage cycle. The solar to H₂ efficiency of the process was estimated up to 25% depending on reactor configurations.