Quantum Chemical Design of Doped Ca₂MnAlO_5+δ as Oxygen Storage Media

Abstract

Brownmillerite Ca2MnAlO5 has an exceptional capability to robustly adsorb half-molecules of oxygen and form Ca2MnAlO5.5. To utilize this unique property to regulate oxygen-involved reactions, it is crucial to match the oxygen release-intake equilibrium with targeted reaction conditions. Here we perform a comprehensive investigation of the strategy of tuning the oxygen storage property of Ca2MnAlO5 through chemical doping. For undoped Ca2MnAlO5+δ, our first-principles calculation predicts that the equilibrium temperature at a pressure of 1 atm of O2 is 848 K, which is in excellent agreement with experimental results. Furthermore, the doping of alkaline earth ions at the Ca site, trivalent ions at the Al site, and 3d transition metal ions at the Mn site is analyzed. By the doping of 12.5% of Ga, V, and Ti, the equilibrium temperature shifts to high values by approximately 110-270 K, while by the doping of 12.5% of Fe, Sr, and Ba, the equilibrium temperature is lowered by approximately 20-210 K. The doping of these elements is thermodynamically stable, and doping other elements including Mg, Sc, Y, Cr, Co, and Ni generates metastable compounds. The doping of a higher content of Fe, however, lowers the oxygen storage capacity. Finally, on the basis of our calculated data, we prove that the formation energetics of nondilute interacting oxygen vacancy in doped Ca2MnAlO5.5 scale linearly with a simple descriptor, the oxygen p-band position relative to the Fermi level. The higher-oxygen p-band position leads to a lower vacancy formation energy and thus a lower oxygen release temperature. Understanding such a relationship between fundamental quantum chemical properties and macroscopic properties paves the road to the design and optimization of novel functional oxides.

Sr1-xCaxFe1-yCoyO3-δ as facile and tunable oxygen sorbents for chemical looping air separation

“…Perovskite based oxygen sorbents have the advantage of reversibly absorb and desorb oxygen at significantly lower temperatures (e.g. ≤ 600 • C), owing to their structural flexibility to accommodate significant amount of oxygen vacancies [30][31][32][33][34][35][36]. For example, La 0.1 Sr 0.9 Co 0.9 Fe 0.1 O 3-δ has a wide range of oxygen vacancy depending on temperature and oxygen partial pressure [37,38].…”

A high-performance oxygen evolution catalyst in neutral-pH for sunlight-driven CO2 reduction

“…As an oxygen-deficient derivative of perovskite, the crystalline structures of Brownmillerite oxides include several variations of oxygen vacancy ordering 17,18 . The compositional degree of freedom, as well as the structural polymorphism of Brownmillerite oxides, offers a great platform to tune the functionality in various oxygen-related applications, such as oxygen storage materials 19 , heterogeneous catalysts 20,21 and oxygen-ion conductors; 22,23 but the usage of these compounds as OER catalyst has seldom been explored 24,25 . In the current study, we found that SGC had the specific activity about one order of magnitude higher than that of IrO 2 and showed superior durability without observable performance degradation in neutral pH.…”

Sr substitution effects on atomic and local electronic structure of Ca2AlMnO5+δ

“…In the case of application for OSM, the reversible topotactic transformation between AB O 3 and AB O 3‐δ occurs in the oxygen intake and release process. Recently, it has been reported that brownmillerite oxides, which are the vacancy‐ordered perovskite oxides, show high oxygen storage capability and cycle prperties . Especially, Ca 2 AlMnO 5+δ , which shows the ready availability of the constituent elements and 3 wt% oxygen storage, is one of the promising OSM.…”

“…Recently, it has been reported that brownmillerite oxides, which are the vacancy-ordered perovskite oxides, show high oxygen storage capability and cycle prperties. [1][2][3] Especially, Ca 2 AlMnO 5+δ , which shows the ready availability of the constituent elements and 3 wt% oxygen storage, 3 is one of the promising OSM.…”

“…It has also been reported that we can control the oxygen storage properties of Ca 2 AlMnO 5+δ , for instance, equilibrium temperature and oxygen partial pressure for the oxygen storage, through the substitution of constituent elements. 2 For further material design using heteroatom substitution, we have to understand the substitution effect on the atomic and local electronic structure. Because of its vacancy-ordered structure, brownmillerite oxide Ca 2 AlMnO 5.5 contains complex chemical bonding, ie, five nonequivalent oxygen sites.…”