Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface

Journal name:
Nature Materials
Volume:
15,
Pages:
1010–1016
Year published:
DOI:
doi:10.1038/nmat4659
Received
Accepted
Published online

Abstract

Segregation and phase separation of aliovalent dopants on perovskite oxide (ABO3) surfaces are detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H2O and CO2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants (for example, ) by the positively charged oxygen vacancies ( ) enriched at the surface. Here we show that reducing the surface concentration improves the oxygen surface exchange kinetics and stability significantly, albeit contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O2 molecules. We take La0.8Sr0.2CoO3 (LSC) as a model perovskite oxide, and modify its surface with additive cations that are more and less reducible than Co on the B-site of LSC. By using ambient-pressure X-ray absorption and photoelectron spectroscopy, we proved that the dominant role of the less reducible cations is to suppress the enrichment and phase separation of Sr while reducing the concentration of and making the LSC more oxidized at its surface. Consequently, we found that these less reducible cations significantly improve stability, with up to 30 times faster oxygen exchange kinetics after 54h in air at 530°C achieved by Hf addition onto LSC. Finally, the results revealed a ‘volcano’ relation between the oxygen exchange kinetics and the oxygen vacancy formation enthalpy of the binary oxides of the additive cations. This volcano relation highlights the existence of an optimum surface oxygen vacancy concentration that balances the gain in oxygen exchange kinetics and the chemical stability loss.

At a glance

Figures

  1. Surface oxygen exchange kinetics and stability on LSC dense thin film cathodes.
    Figure 1: Surface oxygen exchange kinetics and stability on LSC dense thin film cathodes.

    a, Oxygen surface exchange coefficient, kq, quantified from electrochemical impedance spectroscopy measurements over time at 530°C in air, for the LSC and LSC-Me films. b, Atomic force microscopy images of the LSC, LSC-V12, LSC-Nb19, LSC-Ti15, LSC-Hf16 and LSC-Al15 films for which the test results are shown in a. Scale bar, 1μm.

  2. Surface chemical stability on LSC dense thin films.
    Figure 2: Surface chemical stability on LSC dense thin films.

    ac, Concentration ratios [Sr]Total/([La] + [Sr]) (a), [Sr]Non-lattice/([La] + [Sr]) (b) and [Sr]Lattice/[Co] (c) at the surface of the LSC and LSC-Me thin films measured in situ at different temperatures and oxygen partial pressures by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). d, Ex situ atomic force microscopy images of the LSC and LSC-Me films following the AP-XPS measurements in a. Scale bar, 400nm.

  3. Oxidation state of Co based on Co L2,3-edge XAS on LSC dense thin films.
    Figure 3: Oxidation state of Co based on Co L2,3-edge XAS on LSC dense thin films.

    a, Co L2,3-edge X-ray absorption spectra on LSC-Hf16 at different temperatures and oxygen partial pressures. The dashed line marks the Co L3-edge main peak at 300°C and 0.76torr as a reference, to monitor the relative changes in Co oxidation state. b, Co L3-edge peak positions at 300°C, 0.76torr for LSC, LSC-Ti3, LSC-Ti15 and LSC-Hf16 are shown by the solid symbols. From left to right the x-axis shows the direction of decreasing Co oxidation state, that is, increasing oxygen vacancy concentration. The open symbols represent the oxygen vacancy formation enthalpy for the binary oxides HfO2 (ref. 29) and TiO2 (ref. 26), and also for LSC (ref. 30). The dashed arrows in b are guides to the eye and do not imply a quantitative linearity.

  4. Oxidation state on LSC based on the valence band and O K-edge.
    Figure 4: Oxidation state on LSC based on the valence band and O K-edge.

    a,b, Evolution of the valence band structure from X-ray photoelectron spectra measured in situ on LSC (a) and LSC-Hf16 (b). The arrow indicates the low-energy peak, which reflects the hybridization of Co t2g states with the O 2p orbital. The greater the intensity of this peak, the more electrons in the t2g states of Co. c,d, O K-edge spectra of LSC (c) and LSC-Hf16 (d) films at different temperatures and oxygen partial pressures. The dashed lines in each plot mark the position of the O 2p ligand hole peak. The presence of this peak indicates p-type doping, and therefore an increased Co oxidation state, as seen on LSC-Ti15 and LSC-Hf16.

  5. Coordination environment of Ti on LSC-Ti15.
    Figure 5: Coordination environment of Ti on LSC-Ti15.

    Ti L2,3-edge X-ray absorption spectra under different measurement conditions. The dashed lines mark the separation of t2g and eg peaks in both the L2 and the L3 edges. On the right is a schematic representation of the evolution of the Ti coordination at the LSC surface, from disordered at 300°C to perovskite coordination of Ti atoms at the B-site of LSC at 450–550°C (visualized using the VESTA software47).

  6. Dependence of oxygen surface exchange kinetics on the reducibility of the LSC surface.
    Figure 6: Dependence of oxygen surface exchange kinetics on the reducibility of the LSC surface.

    The oxygen surface exchange kinetics of LSC-Me, represented by the kinetic coefficient kq, exhibit a volcano-like dependence on the enthalpy of oxygen vacancy formation (ΔHfV) in the binary oxides, MeOx. The x-axis is the difference between the ΔHfV of the binary oxides (that is, V2O5 (α-phase, orthorhombic)25, Nb2O5 (α-phase, orthorhombic)28, TiO2 (rutile phase)25, 27, ZrO2 (monoclinic phase)29, HfO2 (monoclinic phase)29 and Al2O3 (α-phase, hexagonal)26) and that of LSC30. The y-axis shows kq on LSC-Me, where the surface Me concentrations are within 12–19%, measured after 27h of testing at 550°C in air. The dashed line is a guide for the eye.

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Author information

  1. These authors contributed equally to this work.

    • Nikolai Tsvetkov &
    • Qiyang Lu

Affiliations

  1. Laboratory for Electrochemical Interfaces, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Nikolai Tsvetkov,
    • Qiyang Lu,
    • Lixin Sun &
    • Bilge Yildiz
  2. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Nikolai Tsvetkov,
    • Lixin Sun &
    • Bilge Yildiz
  3. Department of Material Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Qiyang Lu &
    • Bilge Yildiz
  4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Ethan J. Crumlin

Contributions

N.T. and Q.L. prepared the samples. N.T. performed electrochemical measurements. Q.L., N.T., B.Y. and E.J.C. performed XPS and XAS measurements. All authors analysed and discussed the results and wrote the paper. B.Y. designed and supervised the research.

Competing financial interests

The authors declare no competing financial interests.

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