Unlocking thermochemical CO2/H2O 2 /H 2 O splitting by understanding the solid-state enthalpy and entropy of material reduction process

Two-step redox thermochemical cycles, capable of directly converting CO2 2 and H2O 2 O respectively into CO and H2, 2 , offer a promising synthesis route towards green carbon-neutral fuels. The performance of such two-step cycles depends highly on the thermodynamic properties of splitting materials, particularly the solid-state enthalpy ( Delta h solid ) and entropy ( Delta s solid ) changes during the reduction process. Here, we report an investigation into the roles of the Delta h solid and Delta s solid . We shall show that a high Delta s solid relaxes both reduction temperature and oxygen pressure, but increases oxidant consumption. Conversely, an increase in Delta h solid enhances reduction resistance while promotes oxidation reactions. There are therefore no perfect materials, and a trade-off is needed for an optimal solution. We also defined a thermodynamic region based on Delta h solid and Delta s solid and typical operating conditions, and showed that higher values of both Delta h solid and Delta s solid provided a larger reaction space. While lower Delta h solid and negative Delta s solid may be more suitable for isothermal cycles. Our analyses also suggest future efforts in searching for splitting materials with a high Delta s solid within an appropriate range of Delta h solid (280-460 - 460 kJ/ mol).