Functional Materials for Energy

Epitaxial oxygen sponges as low temperature catalysts

Crystal structure of SrCoO2.5 superimposed on a scanning transmission electron microscopy image of an epitaxially stabilized oxygen sponge.

Fast and reversible redox reactions at considerably reduced temperatures are achieved by epitaxial stabilization of multivalent transition metal oxides. This illustrates the unprecedented potential of complex oxides for oxide-ionics, where oxidation state changes are used for energy generation, storage and electrochemical sensing.

Thermomechanical degradation reduces the overall performance and lifetime of many perovskite oxides undergoing reversible redox reactions, such as those found in solid oxide fuel cells, rechargeable batteries, electrochemical sensors, oxygen membranes, and catalytic converters. To mitigate this degradation, these reactions must occur at lower temperatures. However, high temperatures (>700 oC) are often required in conventional perovskites for fast catalysis and bulk diffusion. The epitaxial stabilization of strontium cobaltite (SrCoOx) is here shown to lower the redox temperature to ~ 200°C. Switching between two distinct topotactic crystalline phases, either the perovskite SrCoO3-δ or the brownmillerite SrCoO2.5, occurs rapidly and reversibly. Theory and optical spectroscopy indicate that this transformation corresponds to a metal-insulator transition. Quite interestingly, the SrCoO2.5 film also exhibited significant activity for carbon monoxide oxidation, which suggests its potential as a heterogeneous catalyst.

H. Jeen, W. S. Choi, M. D. Biegalski, C. M. Folkman, I. C. Tung, D. D. Fong, J. W. Freeland, D. Shin, H. Ohta, M. F. Chisholm,, and H. N. Lee, “Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge” Nature Materials, DOI: 10.1038/NMAT3736 (2013).

W. S. Choi, H. Jeen, J. H. Lee, S. S. A. Seo, V. R. Cooper, K. M. Rabe, and H. N. Lee, “Reversal of the lattice structure in SrCoOx epitaxial thin films studied by real-time optical spectroscopy and first-principles calculations,” Phys. Rev. Lett. 111, 097401 (2013).

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