A new highly oxygen-deficient and cubic Pr<sub>3</sub>ZrO<sub>8-δ</sub> for intermediate-temperature thermochemical production of oxygen and hydrogen.

Abstract

Two-step thermochemical cycles offer a clean route for hydrogen and oxygen production but are typically limited to high temperatures exceeding 1500 °C. Lowering operating temperatures would enable the use of alternative heat sources such as industrial waste heat. Here, we report Pr₃ZrO₈ as a new enabling material for efficient intermediate-temperature redox cycling, with thermal reduction at 900 °C in argon and steam oxidation at 400 °C. Pr₃ZrO₈ adopts a face-centered cubic structure similar to CeO₂ but exhibits significantly greater oxygen deficiency, achieving average oxygen and hydrogen fluxes of 331.7 and 70.3 µmol·g^-1, respectively, over ten cycles at 20%H₂O. These values surpass those of leading CeO_2₋δ and perovskite oxides under comparable or more severe conditions. In-situ neutron and X-ray diffraction determine the phase stability boundaries of Pr₃ZrO₈, while density functional theory identifies O-H bond cleavage as the rate-limiting step. These results establish Pr₃ZrO₈ as a promising material for intermediate-temperature thermochemical oxygen and hydrogen production.