Thermodynamic stability of doped ceria for solar reactors: Sublimation and surface segregation

The stability of ceria‑based redox oxides under high‑temperature solar‑thermal operation is crucial for fuel‑producing reactors. This study demonstrates that the gas pressure governs CeO₂ sublimation from Gd_0.1Ce_0.9O_1.95 (GDC-10) under solar thermochemical conditions. Annealing GDC-10 at 1400 °C in vacuum (≈10^-4 bar) produces a porous, sponge like surface layer enriched in Gd, confirming selective CeO₂ sublimation; identical treatments in argon at the same temperature and duration do not generate comparable porosity. Time resolved electron microscopy reveal a diffusion controlled, linear growth of the cerium depleted zone, while dilatometry records irreversible chemical expansion, indicating structural damage. Co-doping with 5 mol % Zr (Ce_0.9Gd_0.05Zr_0.05O_1.975) reduces CeO₂ loss and yields a saturated surface. Knudsen effusion mass spectrometry shows reduced CeO₂ volatility for the co-doped material after aging, linking thermodynamic stability to improved microstructural integrity. These results establish Zr co-doping as a potential strategy to stabilize GDC type redox oxides for durable solar thermochemical fuel production.