Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge

Journal name:
Nature Materials
Year published:
Published online


Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cation’s oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoOx) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO3−δ or the brownmillerite SrCoO2.5. Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200–300 °C) in a considerably short time (< 1 min) without destroying the parent framework. The fast, low-temperature redox activity in SrCoO3−δ is attributed to a small Gibbs free-energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low-temperature cathode materials.

At a glance


  1. Epitaxial synthesis of two topotatic SrCoOx phases.
    Figure 1: Epitaxial synthesis of two topotatic SrCoOx phases.

    a,b, XRD θ–2θ scans of a brownmillerite SrCoO2.5 film on (001) STO (a) and a perovskite SrCoO3−δ film on (001) LSAT (b). Insets are schematics of SrCoO2.5 and SrCoO3. c, Cross-sectional Z-contrast STEM image of a SrCoO2.5 film on STO along the [110] STO direction. The arrow indicates the interface between the film and substrate. A magnified image of SrCoO2.5 on the right-hand side is shown to clearly visualize the one-dimensional oxygen-vacancy channels and subsequent local structural distortions.

  2. Comparison of oxidation states and magnetism.
    Figure 2: Comparison of oxidation states and magnetism.

    a, XAS O K-edge spectra of SrCoO3−δ and SrCoO2.5 films on LSAT and STO, respectively. A clear pre-peak at around 527 eV clearly indicates different oxygen contents in SrCoO3−δ (solid line) and SrCoO2.5(dashed line) films. b, XAS Co L2,3-edge spectra. The shift of the L3-edge towards the higher energy (> 0.7 eV) in SrCoO3−δ indicates that the Co ions in SrCoO3−δ are in a higher valence state. c, XMCD spectra of the two phases at 5 T.

  3. Magnetic and d.c. transport properties.
    Figure 3: Magnetic and d.c. transport properties.

    a,b, Temperature-dependent magnetization of SrCoO2.5 and SrCoO3−δ thin films at 1,000 Oe (a) and magnetic hysteresis loops at 10 K (b). c, Resistivity of SrCoO2.5 and SrCoO3−δ thin films as a function of temperature, clearly showing insulating and metallic states, respectively. The ρ(T) curve marked with T-SCO is obtained from a perovskite film, topotactically oxidized from a brownmillerite film by annealing at 300 °C in 0.67 bar of O2 for 5 min, confirming the successful phase conversion based on the clear metallic behaviour similarly seen from the in situ grown perovskite film. d, Thermoelectromotive force (ΔV) at 300 K with a strong contrast in the thermopower (S) between the two phases.

  4. Direct probing of reversible redox activity.
    Figure 4: Direct probing of reversible redox activity.

    a,b, Real-time temperature-dependent XRD θ–2θ scans around the 002 LSAT reflection, clearly revealing the SrCoO3−δ–SrCoO2.5 transition (reduction) in vacuum (a) and the SrCoO2.5–SrCoO3−δ transition (oxidation) in oxygen (b). The SrCoO3−δ–SrCoO2.5 transition is at ~210 °C, and the reversal is at ~350 °C. Note that, owing to the large difference in the thermal expansion coefficient between the film and substrate, the 002 SrCoO3−δ film peak overlaps with the substrate one at high temperatures as marked with the dashed lines.

  5. Thermodynamic competition.
    Figure 5: Thermodynamic competition.

    Gibbs energy differences between SrCoOx and SrMnOx with different oxygen contents shown as a function of temperature. Note that SrCoO2.5 and SrMnO3−δ phases are energetically more favourable than the other phases. When the temperature decreases, the energy difference is reduced significantly in the case of SrCoOx, facilitating the topotatic phase conversion. The opposite trend is observed in SrMnOx.

  6. Gas phase catalysis.
    Figure 6: Gas phase catalysis.

    Temperature-programmed CO oxidation reaction over a brownmillerite film on a LSAT substrate. a,b, The CO conversion (a) and the CO2 production (b) both show catalytic activity above ~320 °C.


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Author information


  1. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Hyoungjeen Jeen,
    • Woo Seok Choi,
    • Dongwon Shin,
    • Matthew F. Chisholm &
    • Ho Nyung Lee
  2. Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Michael D. Biegalski
  3. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

    • Chad M. Folkman &
    • Dillon D. Fong
  4. Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA

    • I-Cheng Tung &
    • John W. Freeland
  5. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA

    • I-Cheng Tung
  6. Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan

    • Hiromichi Ohta


H.J. conducted sample synthesis, XRD, d.c. transport and SQUID measurements with help from W.S.C., and H.J. and M.D.B. performed the high-temperature environmental XRD, under the direction of H.N.L. M.F.C. performed STEM measurements. I-C.T. and J.W.F. measured XAS and XMCD, and H.O. worked on thermopower measurements. D.S. performed the thermodynamic modelling. C.M.F. and D.D.F. worked on the catalysis measurement. H.N.L. initiated the research and supervised the work. All authors participated in writing the manuscript.

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