CeTiO-A Promising Oxide for Solar Thermochemical Hydrogen Production.

A large entropy of reduction is crucial in achieving materials capable of high-efficiency Solar Thermochemical Hydrogen (STCH) production through the two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic configurational entropy of reduction attains an extreme value of 4.26 k at 1500 K in Ce→ Ce redox reactions, which explains the high performance and uniqueness of CeO as an archetypal STCH material. However, ceria requires high temperatures (T > 1500 °C) to achieve a reasonable reduction extent due to its large enthalpy of reduction, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and low reduction enthalpy are required. Here, we perform a systematic screening to search for Ce-based oxides which possess thermodynamics superior to CeO for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce-based oxides and subsequently use Density Functional Theory (DFT) to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). We find CeTiO with the brannerite structure is the most promising candidate for STCH  since it processes three essential characteristics of a STCH material: (i) a smaller reduction enthalpy compared to ceria yet large enough to split water, (ii) a high thermal stability, as reported experimentally, and iii) a large entropy of reduction associated with Ce→ Ce redox. Our proposed design strategy suggests that further exploration of Ce oxides for STCH production is warranted.