First Principles Study of Oxygen Incorporation Reactions in Oxides

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The oxygen incorporation reaction, and its opposite, the oxygen deletion reaction, may be rate limiting reactions in solid oxide fuel cells. This study examines the insertion and deletion reactions in the popular solid oxide fuel cell electrolyte and electrode materials (Y2O3)x(ZrO2)1−x (YSZ), (Y2O3)x(CeO2)1−x (YDC), (Gd2O3)x(CeO2)1−x (GDC), LaMnO3 (LMO), (LaxSr1−x)MnO3 (LSM), and (LaxSr1−x)(CoxFe1−y)O3 (LSCF). A Kröger-Vink diagram is constructed for SOFC anode and cathode operating conditions, and defect equilibrium considerations are used to explain the experimental result that oxygen incorporation in bilayer electrolytes shows higher surface exchange rates. Quantum simulations of the incorporation reaction and vacancy formation energetics support this conclusion. Simulations of oxygen incorporation in perovskites agree with experimental trends in reactivity LSCF > LSM > LMO, and suggest the reason to be due to structural rather than electronic properties. A new model of oxygen deletion at the anode is proposed and shown to have lower energy barriers than sequential deletion and reaction with hydrogen to form the hydroxide ion.
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