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

Abstract

Two-step redox thermochemical cycles, capable of directly converting CO2 and H2O respectively into CO and H2, 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 (Δhsolid) and entropy (Δssolid) changes during the reduction process. Here, we report an investigation into the roles of the Δhsolid and Δssolid. We shall show that a high Δssolid relaxes both reduction temperature and oxygen pressure, but increases oxidant consumption. Conversely, an increase in Δhsolid 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 Δhsolid and Δssolid and typical operating conditions, and showed that higher values of both Δhsolid and Δssolid provided a larger reaction space. While lower Δhsolid and negative Δssolid may be more suitable for isothermal cycles. Our analyses also suggest future efforts in searching for splitting materials with a high Δssolid within an appropriate range of Δhsolid (280–460 kJ/mol).