Reactive stability of promising scalable doped ceria materials for thermochemical two-step CO2 dissociation

Abstract

Metal-doped ceria (Ce_1−xM_xO_2−δ) is an attractive redox-active material for thermo/electrochemical synthesis of renewable fuels due to its high mixed ionic/electronic conductivity and variable valence (Ce⁴⁺/Ce³⁺) and oxygen nonstoichiometry (δ) at high temperatures. Previously, we have investigated all 26 potentially tetravalent dopants for efficient thermochemical splitting of CO₂. Here, we fine-tune the dopant activity (x = 0.10 Zr⁴⁺, 0.10 Hf⁴⁺, 0.07 Ta⁵⁺, and 0.05 Nb⁵⁺) of all thermally stable ceria materials with an oxygen exchange capacity (OEC) surpassing that of pristine ceria (CeO_2−δ), and we employ thermogravimetric analysis to evaluate long-term stability of their OEC over 50 consecutive redox cycles. Each cycle swings between 40 min ceria oxidation with approximately 500 mbar CO₂ at 1000 °C and 90 min ceria reduction in about 0.01 mbar O₂ at 1500 °C. Along with analyses of phase purity and stability (PXRD), of composition and dopant concentration (EDX and ICP-MS), and of sintering via SEM, the cycling results show long-term stable OEC and kinetics of the oxygen exchange for Zr-, Hf-, and Nb-doped ceria, despite their distinctly sintered particle surfaces. This attractive performance is rationalized by characterizing oxidation states and oxygen vacancies and by excluding surface carbonation through Raman and FT-IR spectroscopy. Furthermore, we find that introducing stable oxygen vacancies in Ce_0.95Hf_0.05O_2−δ by doping with additional 5% lower-valent Li⁺, Mg²⁺, Ca²⁺, Y³⁺, and Er³⁺ does not significantly accelerate the oxygen exchange kinetics. From this first comprehensive long-term stability study of systematically optimized ceria, we propose ceria co-doped with permutations of Hf⁴⁺, Zr⁴⁺, and Nb⁵⁺, yielding an optimal average dopant radius of 0.8 Å, as the benchmark redox material for thermochemical production of solar fuels.