Highly Oxygen Deficient and Face Centered Cubic Pr3ZrO8-δ as a New Redox Material for Intermediate Temperature Thermochemical Productions of Oxygen and Hydrogen

The “two-step” thermochemical processes are a promising clean oxygen and hydrogen production technology. It is traditionally driven by solar energy due to the high operating temperatures (>1500oC) required for redox-active oxides (RAOs) to achieve reasonable O2 and H2 yields. Lowering this temperature threshold would enable the use of alternative heat sources, such as waste heat from power and chemical plants, thereby broadening the impact of the technology. Here, we report Pr3ZrO8 (PZO) as a new class of redox materials capable of operating redox cycles at an intermediate temperature window within 900°C (thermal reduction in argon) and 400°C (chemical oxidation in H2O). We show that PZO crystallizes in a face-centered cubic structure like CeO2 but exhibits significantly higher oxygen off-stoichiometry than CeO2- δ . Over ten consecutive redox cycles, PZO demonstrates an average oxygen flux of 331.7 µmol/g at 900°C/Ar and an average hydrogen flux of 70.3 µmol/g at 400°C/20%H2O – significantly outperforming the benchmark CeO2- δ as well as perovskite oxides under comparable or even higher-temperature conditions. We also examine PZO’s structure, oxygen off-stoichiometry, and phase stability as a function of temperature and oxygen partial pressure to identify the operation envelope for stable redox cycles. The kinetics of the slow water splitting reaction have been investigated through density functional theory (DFT) calculations to understand the rate limiting step (RLS). The results indicate that the cleavage of the O-H bond on the PZO (111) surface is the RLS. Applying surface catalysts to promote the cleavage of O-H bond could be a strategy to enhance H2 production. Overall, this study demonstrates PZO as a promising RAO for reduced-temperature thermochemical processes, significantly expanding the technology’s applicability to utilize lower-temperature heat sources.