Modification of Oxygen/(Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ� Interfaces Derived by Metal-Organic Deposition

Mixed ionic-electronic conducting (MIEC) perovskite oxides ABO_3-δ have attracted great interest as materials for a variety of high-temperature applications, e.g., ceramic oxygen transport membranes (OTM) for oxy­­gen separation with a high permea­bility and selectivity for O₂/N₂ separation.

Several MIEC materials, namely members of the com­position A_xSr_1-xCo_yFe_1-yO_3-δ (A = La, Ba), are known to exhibit ex­cellent oxygen-ionic and electronic trans­port pro­per­ties at tem­peratures of 700…1000 °C [1]. Among these Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), in par­ticular, has been reported to exhibit unmatched permeation properties to-date [2,3].

Permeation mainly depends on both the chemical oxygen bulk diffusion and the surface transfer of oxygen between gas phase and oxide. The former can be accelerated by limiting the diffusion path length, i.e., thinning the OTM. With the help of thin-film technology, microscale thicknesses are readily achievable.

However, the oxygen flux cannot be significantly increased by fabricating membranes with a thickness below a certain material-specific value (“characteristic length”) because surface transfer limitations dominate. In order to further increase the oxygen permeation one must therefore improve the oxygen surface exchange.

Recently, an enhancement of O₂ flux through a BSCF membrane was achieved with a microscale functional layer [4]. Furthermore, it was reported that metal-organic deposition (MOD)-derived nanoscale thin-film SOFC cathodes made from La_0.6Sr_0.4CoO_3-δ exhibit ex­tre­mely low polarization losses [5]. These results suggest that the oxygen flux through a thin BSCF OTM can be significantly enhanced by nanoscale surface mo­di­fications leading to a geometric in­crease of the gas/ solid interface. Recent MIEC performance-modelling ac­ti­vities [6] also addressed the influence of microstructure on oxygen-transport kinetics.

In this work, nanoscaled BSCF thin films (thicknesses below 100 nm) have been successfully deposited onto NdGaO₃ single crystal substrates from a homogeneous BSCF starting solution using an MOD method. The films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis.

In contrast to physical vapor deposition techniques, e.g. PLD or magnetron sputtering, BSCF thin films formed by an MOD method provide the possibility to custom-tailor the surface morphology by subsequent annealing treatments. Fig. 1 shows the surface morpho­logy variation of BSCF. Depending on the annealing process, different surface morphologies could be observed, e.g., oriented grain growth with a textured surface as shown in Fig. 1(i) or a smooth, stepped surface as shown in Fig. 1(ii).

Changes of surface area or phase composition (cu­bic, hexagonal BSCF, others) are expected to affect the catalytic activity of OTM membranes. A systematic study of surface morphologies achie­ved by a parameter variation in the annealing process is presented.

The samples were characterized electrically and elec­tro­chemically (conductivity σ(T,pO₂), chemical oxy­­gen sur­face exchange coefficient k^δ(T)) and results compared to those obtained on epitaxial BSCF thin films and other MIEC compositions.

References

[1] Y. Teraoka et al., Chem. Lett. (1988), 503.

[2] Z. P. Shao et al., J. Membrane Sci. 172 (2000), 177.

[3] J. Sunarso et al., J. Membrane Sci. 320 (2008), 13.

[4] S. Baumann et al., J. Membrane Sci. 377 (2011), 198.

[5] J. Hayd et al., J. Electrochem. Soc. 160 (2013), F351. 

[6] A. Häffelin et al., J. Electrochem. Soc. 161 (2014), F1409.