High-temperature devices such as solid oxide fuel cells (SOFCs) and oxygen transport membranes (OTMs) require materials with high chemical, microstructural stability as well as high catalytic activity towards the oxygen reduction reaction. In its cubic phase the mixed ionic-electronic conductor Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) presents outstanding oxygen transport properties and exceedingly low cathode resistance [1]. However, the application in OTMs has been hampered by its degrading oxygen permeability in the relevant temperature range (600 – 800 °C). It was shown by our group [2], that the multivalent Co-cation prefers within this temperature range a higher valence which destabilizes the cubic structure favoring the hexagonal phase formation.

Various doping strategies with transition metals like, e.g., Y have shown beneficial effects with respect to stabilizing the cubic BSCF phase and preservation of the oxygen conductivity [3,4]. In this direction the effect of B-site partial substitution with Y, Ti or Nb has been studied thoroughly by our group using scanning electron microscopy (SEM) and analytical (scanning) transmission electron microscopy ((S)TEM) combined with energy dispersive X-ray spectroscopy (EDXS).

In addition, the oxygen reduction reaction of pure and doped BSCF electrodes is investigated in oxygen-nitrogen mixtures (pO₂ = 0.02 to 0.4 atm) as well as in CO₂-atmospheres (0.1…1 vol. % CO₂) using symmetrical SOFC cells with Ce_0.9Gd_0.1O_2-δ (CGO) as electrolyte from 600 to 800 °C. Characterization by electrochemical impedance spectroscopy (EIS) combined with the distribution of relaxation times (DRT) reveals an enhanced tolerance of doped BSCF cathodes, in particular Y-doped, towards the adsorption-desorption of CO₂ molecules. Channeling enhanced microanalysis (ALCHEMI) investigations on Y-doped BSCF demonstrate the coexistence of Y ions on both cation lattice sites with up to 55 % of the Y-ions located on the unintended A-lattice site [5,6].

Overall this study shows that enhancing the phase stability and the CO₂ tolerance of BSCF by a rational substitution of the B-site is plausible while the high performance and its excellent oxygen transport properties are kept.

References:

[1] Z. Shao, S.M. Haile. Nature, 431 (2004) 170-173.

[2] P. Müller, M. Meffert, H. Störmer, D. Gerthsen. Microsc. Microanal., 19 (2013) 1595-1605.

[3] P. Haworth, S. Smart, J. Glasscock, J.C. Diniz da Costa. Sep. Purif. Technol., 94 (2012) 16-22.

[4] M. Meffert, L-S Unger, L. Grünewald, H. Störmer, S. F. Wagner, E. Ivers-Tiffée, D. Gerthsen. J. Mater. Sci., 52 (2017) 2705-2719.

[5] J. C. H. Spence and J. Taftø. J. Microsc., 130 (1983) 147-154.

[6] M. Meffert, H. Störmer, D. Gerthsen. Microsc. Microanal., 22 (2016) 113-121.