Mixed ionic and electronic conducting (MIEC) oxides are multifunctional materials used as catalysts, [1] superconductors, [2] fuel-cell and battery electrodes, [3] as well as membranes for gas separation. [4] Among the applications, MIEC membranes with fast transport of oxygen ions at elevated temperature have received increasing attention because the membranes can potentially be used for highly pure oxygen production with low costs; they are highly efficient as membrane reactors for the selective oxidation of C 1 and C 2 molecules, [4g,h] water splitting for hydrogen production reactions, [5] and CO 2 capture integrated with oxy-fuel combustion technology. [6] Despite their widespread use, the hightemperature (usually 800-1000 8C) operation of the membrane modules requires special sealants sustaining highpressure gradients and special stainless steel resisting high temperatures and oxidation, which have become the bottleneck technology of the application of MIEC membrane technology. [4c] Great energy consumption and high investment on high-temperature membrane modules count against the reduction of the costs for oxygen production. The bottleneck technology is easy to be overcome, as well as the energy consumption and the investment can be significantly reduced at low-temperature (LT; 350-650 8C) operation. Furthermore, many catalytic oxidation reactions of C 3 -C 6 organic molecules can be performed in the MIEC membrane reactors with high efficiency at LT. [7] However, most MIEC materials suffer from large and irreversible performance losses during long-term operation at LT that limit their commercial application. [8] This problem is particularly acute for many of the recent higher-performance perovskite-based MIEC materials such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd (BSCF) [4a, 9] and La 1Àx Sr x Co 1Ày Fe y O 3Àd (LSCF, 0 x 1, 0 y 1) [10] that have been designed for low-temperature fuel cells, gas separations, and catalytic processes. Up to now, it is still difficult to stabilize oxygen permeation fluxes of MIEC membranes at LT. In general, degradation processes of materials become more severe with increasing temperature. Thus, the fact that MIEC membrane degradation becomes more severe at lower temperatures stands in stark contrast to the conventional wisdom. Two main mechanisms have been proposed to account for the LT degradation in perovskite MIEC membranes. The first is that oxygen vacancy ordering occurs in these materials during sustained operation at LT, which severely decreases their ionic conductivity; [10] the second is that LT operation introduces thermodynamic driving forces that favor kinetic demixing in these materials under oxygen permeation conditions, producing new phases with low ionic conductivity or undesirable changes to the surface elemental composition and morphology. [11] However, these two proposed mechanisms cannot explain why degradation is observed even in many otherwise structure-stable membranes, for example BaZr x Co y Fe 1ÀxÀy O 3Àd . [4b, 12] Here, we identify a third imp...

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