Phase Composition, Cation Distribution and Proton Uptake of Self-Generated Ba(Ce,Fe,In)O3-δ Composites

Werner Sitte^a, Christina Nader^a, Andreas Egger^a, Edith Bucher^a, Judith Lammer^b, Werner Grogger^b, Rotraut Merkle^c, Joachim Maier^c

^aChair of Physical Chemistry, Montanuniversitaet Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria

^bInstitute of Electron Microscopy and Nanoanalysis (FELMI), Graz University of Technology & Graz Centre for Electron Microscopy (ZFE), Steyrergasse 17, A-8010 Graz, Austria

^cMax Planck Institute for Solid State Research, Heisenbergstrasse 1, DE-70569 Stuttgart, Germany

Proceedings of 24th International Conference on Solid State Ionics (SSI24)

Emerging Materials for High-Performance Devices

London, United Kingdom, 2024 July 14th - 19th

Organizers: John Kilner and Stephen Skinner

Invited Speaker, Werner Sitte, presentation 071
Publication date: 10th April 2024

Triple-conducting oxides, which conduct oxygen ions and electrons or holes as well as protons, are a promising option for electrodes of protonic ceramic fuel and electrolyser cells (PCFCs, PCECs), as the reaction zone extends beyond the three-phase boundary of the air electrode, increasing the rate of oxygen reduction. However, for single-phase triple-conducting perovskites, a conflict between electronic conductivity and proton uptake has been observed [1].

The present study focuses on self-generated Ba(Ce,Fe,In)O_3-δ composites of Ce-rich and Fe-rich perovskites that collectively satisfy the desired properties for proton uptake and transport properties of air electrodes in PCFCs and PCECs. In the system BaCe_0.5Fe_0.5O_3-δ [2], In³⁺ is substituted on the B-site to induce oxygen vacancies and increase the hydration capacity. Crystal structures, lattice parameters and the relative amounts of the formed phases were determined by X-ray diffraction. The local chemical composition (cation distribution) within the individual phases, which is decisive for the transport properties of the composite, was determined by scanning transmission electron microscopy including energy-dispersive X-ray spectroscopy. Proton uptake was analysed by thermogravimetry.

Annealing experiments showed that the miscibility gap of the BaCe_0.8-xFe_xIn_0.2O_3-δ system ranges from [Ce]/([Ce]+[Fe]) ratios of approximately 0.2 to 0.9 and narrows with increasing dopant concentration. The acceptor In³⁺ tends to accumulate in the Fe-rich phase, similar to Y³⁺ in the Ba(Ce,Fe,Y)O_3-δ system [3]. The ratio of In(Ce-rich phase) / In(Fe-rich phase) ranges from 0.3 to 0.7.

The proton concentrations of the Ba(Ce,Fe,In)O_3-δ composites are within the range of 1-4 mol% at 400°C. The proton uptake increases with higher amounts of In and lower amounts of Fe in the precursor in the composite samples as well as in Ce- or Fe-rich Ba(Ce,Fe,In)O_3-δ single phases. Measurements on BaCe_0.4Fe_0.4Acc_0.2O_3‑δ (Acc = Y, Yb, Gd, Sm, Sc) composites show a general increase in proton uptake compared to the undoped system BaCe_0.5Fe_0.5O_3-δ [4]. The variation of the acceptor ion has only a minor effect on the proton uptake. The driving forces for the dopant accumulation in the Fe-rich phase such as size mismatch and acid/base properties will be discussed.

References:

[1] Zohourian R.; Merkle R.; Raimondi G.; Maier J. Mixed-Conducting Perovskites as Cathode Materials for Protonic Ceramic Fuel Cells: Understanding the Trends in Proton Uptake. Adv. Funct. Mater.2018, 28, 1801241.

[2] Cheng S.; Wang Y.; Zhuang L.; Xue J., Wei Y.; Feldhoff A.; Caro J.; Wang H. A Dual-Phase Ceramic Membrane with Extremely High H2 Permeation Flux Prepared by Autoseparation of a Ceramic Precursor, Angew. Chem. Int. Ed. 2016, 55, 10895–10898.

[3] Berger C.; Bucher E.; Merkle R.; Nader C.; Lammer J.; Grogger W.; Maier J.; Sitte W. Influence of Y-substitution on phase composition and proton uptake of self-generated Ba(Ce,Fe)O3−δ–Ba(Fe,Ce)O3−δ composites, J. Mater. Chem. A, 2022, 10, 2474-2482.

[4] C. Nader, J. Lammer, Andreas Egger, C. Berger, W. Sitte, W. Grogger, R. Merkle, J. Maier, E. Bucher, Phase composition and proton uptake of acceptor-doped self-generated Ba(Ce,Fe)O3−δ–Ba(Fe,Ce)O3−δ composites, Solid State Ionics, 2024, accepted for publication.

Acknowledgements:

The authors thank the Austrian Research Promotion Agency FFG (grant no. 871659) for funding.

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