(Invited) MIEC Materials for Membrane Applications: Enhancing the Oxygen Transport

Mixed ionic-electronic conducting (MIEC) materials are of great interest for a variety of high-temperature applications, such as solid oxide fuel cell (SOFC) cathodes or dense ceramic membranes for gas separation. Several MIEC perovskite oxides, namely of the com­position A_xSr_1-xCo_yFe_1-yO_3-d (A = La, Ba), exhibit ex­cellent oxygen-ionic and electronic transport properties. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), La_0.58Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF), or La_0.6Sr_0.4CoO_3-δ (LSC) are amongst these state-of-the-art high-flux materials and, hence, very promising candidates for a high-permea­tion oxygen transport membrane (OTM).

In order to assess their applicability for OTMs under operating conditions, it is essential, though, to determine their electrochemical transport properties not only under oxidizing conditions (pure oxygen, air), but also as a function of oxygen partial pressure pO₂ down to low values of, e.g., 10^-6 bar. This can be achieved in a closed tubular zirconia “oxygen pump” setup [1,2] facilitating precise and continuous pO₂ con­trol in the entire range between pO₂ = 10^-20...1 bar above 700 °C. By performing fast pO₂ step changes and applying the electrical conductivity relaxation (ECR) method to selected MIEC compositions, the oxygen equilibration kinetics of dense ceramic bulk samples of BSCF, LSCF, or LSC have been studied, yielding chemi­cal dif­fusion coefficients, D^δ, and surface exchange co­efficients, k^δ, as f(T,pO₂) for 700 ≤ T / °C ≤ 900 and 10^-5 ≤ pO₂ / bar ≤ 0.21. Furthermore, chemical stability issues of MIEC oxides are also addressed over a wide pO₂ range using this “oxygen pump”.

By reducing the thickness of an OTM, the oxygen flux will be increased. However, upon reducing the thickness below a “characteristic” membrane thickness [3], given by the ratio of D^δ and k^δ, surface exchange becomes rate-determining for thin membranes. No enhancement of the oxygen flux is achieved by thinning the membrane any further if one cannot enhance the surface oxygen exchange. This, however, is possible by modifying the membrane surfaces with a porous functional/catalytic layer that provides more surface area for the exchange of oxygen between the gas phase and the membrane material. This becomes especially important for the permeate-side OTM surface where k^δ values are reduced as a result of the low pO₂. 

This concept has already been successfully applied to OTMs [4], facilitating unprecedented oxygen fluxes in the case of a dense BSCF membrane covered with a microporous BSCF functional layer [5], but also to SOFCs where nanoscaled LSC thin-film cathodes with a nano­porous micro­struc­ture [6] led to a large enhancement of oxygen surface reduction and, in com­bi­nation with the occur­rence of chemical hetero-interfaces [7], thus resulted in the best performance of SOFC cathodes reported so far in literature.

With the help of a 3D FEM OTM transport model [8] the interplay of transport parameters (D^δ and k^δ) and functional-layer micro­structure (thickness, porosity, particle sizes) can be readily assessed. In our present study it is shown that the choice of materials in an OTM depends not only on their pO₂- and temperature-dependent transport parameters, but also on phase stability, chemical compatibility and the optimum functional-layer microstructure. To this end high-resolution structural analyses (TEM) are also required. 

References

[1] C. Niedrig et al., J. Electrochem. Soc. 160 (2013), F135.

[2] C. Niedrig et al., manuscript in preparation (2013).

[3] H. J. M. Bouwmeester and A. J. Burggraaf, in P. J. Gellings and H. J. M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton FL (1997), p. 481.

[4] S. Baumann et al., J. Eur. Ceram. Soc. 33 (2013), 1251.

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

[6] J. Hayd et al., J. Power Sources 196  (2011), 7263.

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

[8] A. Häffelin et al., ECS Trans. 57 (2013), 2543.