Multiple and nonlocal cation redox in Ca-Ce-Ti-Mn oxide perovskites for solar thermochemical applications

Modeling-driven design of redox-active off-stoichiometric oxides for solar thermochemical H₂ production (STCH) seldom has resulted in empirical demonstration of competitive materials. We report the theoretical prediction and experimental evidence that the perovskite Ca_2/3Ce_1/3Ti_1/3Mn_2/3O₃ is synthesizable with high phase purity, stable, and has desirable redox thermodynamics for STCH, with a predicted average neutral oxygen vacancy (V_O) formation energy, E_v = 3.30 eV. Flow reactor experiments suggest potentially comparable or greater H₂ production capacity than recent promising Sr-La-Mn-Al and Ba-Ce-Mn metal oxide perovskites. Utilizing quantum-based modeling of a solid solution on both A and B sub-lattices, we predict the impact of nearest-neighbor composition on E_v and determine that A-site Ce⁴⁺ reduction dominates the redox-activity of Ca_2/3Ce_1/3Ti_1/3Mn_2/3O₃. X-ray absorption spectroscopy measurements provide evidence that supports these predictions and reversible Ce⁴⁺-to-Ce³⁺ reduction. Our models predict that Ce⁴⁺ reduces even when it is not nearest-neighbor to the V_O, suggesting that refinement of Ce stoichiometry has the possibility of further enhancing performance.