CeTi₂O₆ - 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 two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic entropy of reduction attains an extreme value of 4.26 k_B at 1500 K in Ce⁴⁺ → Ce³⁺ redox reactions, which explains the high performance and uniqueness of CeO₂ as an archetypal STCH material. However, ceria requires high temperatures (T > 1500 °C) to achieve a reasonable reduction extent because of its large reduction enthalpy, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and lower reduction enthalpy are required. Here, we perform a systematic screening to search for Ce⁴⁺-based oxides which possess thermodynamics superior to CeO₂ for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce⁴⁺-based oxides and subsequently use density functional theory to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). We find that CeTi₂O₆ with the brannerite structure is the most promising candidate for STCH because it possesses three essential characteristics of an 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 Ce⁴⁺ → Ce³⁺ redox. Our proposed design strategy suggests that further exploration of Ce⁴⁺ oxides for STCH production is warranted.