Advancements in metal oxide redox cycles for solar-driven chemical processes: oxygen separation, fuel synthesis, ammonia generation, and thermochemical energy storage

This review article presents an in-depth exploration of metal oxide redox cycles applied to solar chemical processing, encompassing thermochemical oxygen separation, fuel production (H2, CO), ammonia synthesis, and thermochemical energy storage. The focus is on solar-driven thermochemical H2O and CO2 splitting cycles, utilizing monolithic solar reactors with a porous reactive structure for efficient two-step redox reactions. Isothermal plots highlight the relationship between δ and pO2 in CeO2/CeO2−δ systems with temperature and pressure variations. The study showcases hierarchically ordered porous structures achieved through additive manufacturing, enhancing solar radiation absorption. Customized ceria materials, demonstrated in packed-bed cavity-type solar reactors, exhibit notable O2, CO, and H2 production rates during consecutive thermochemical splitting cycles. The review further discusses the synthesis of La/Sr/Mn perovskites via solution combustion, presenting CO production yield patterns. Ammonia synthesis reactions are assessed through Gibbs free energy variation with temperature, while AlN hydrolysis extent is evaluated at different H2O concentrations. In addition, the concept of thermochemical energy storage is outlined, elucidating its potential in supplying high-temperature process heat for electricity or fuel generation. This comprehensive review contributes to the understanding and advancement of solar-driven chemical processes and their pivotal role in sustainable energy technologies.