The performance of electroceramic high temperature oxygen-transport membranes (OTM) is deter­mined by ionic and electronic transport processes. Oxygen transport kinetics is determined by the transport parameters, namely the surface exchange coefficient k^δ and the bulk diffusion coefficient D^δ. Mixed-conducting perovskite oxides, e.g., A_1-xSr_xCo_1-yFe_yO_3-δ (A = Ba, La, Pr…) [1], are materials with excellent oxygen transport properties. Especially Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) shows an excellent OTM performance [2] in its cubic phase [3] at high temperatures (T > 850 °C). For an energy-efficient application it makes sense to use these oxygen separation membranes at lower temperatures (T< 700 °C). However, the occurrence of secondary phases in this temperature range [4] leads to a decreased oxygen transport through the BSCF membrane. Secondary phase formation can, though, be suppressed by suitable doping strategies [5,6].

OTM permeation occurs due to an oxygen partial pressure (pO₂) gradient applied across the dense membrane. Reducing the thickness of the membrane increases the oxygen flux. However, below a certain thickness, the surface transfer of oxygen becomes rate-limiting; an improvement in oxygen flux can only be realized by surface activation, e.g., by a nanostructured layer. A promising candidate for such a layer is La_0.6Sr_0.4CoO_3-δ (LSC) [7]. On the one hand, nanoporous LSC can enlarge the geo­metrical surface area and consequently increase the effective k^δ value which is important especially at the low-pO₂ side of the membrane because oxygen transport parameters strongly decrease with lower pO₂ [8]. On the other hand, the occurrence of chemical “hetero-interfaces” as previously reported in LSC [9,10] may also improve oxygen surface exchange.

The oxygen transport parameters (k^δ, D^δ) of uncoated and LSC-coated dense BSCF ceramic bulk samples are determined by electrical conductivity relaxation (ECR) measurements. Combining the results from these ECR measurements under varying partial pressures (pO₂ = 10^-2…1 bar) and transmission electron microscopy (TEM) analysis the influence of the nanostructured functional LSC layer on the dense BSCF ceramic membrane is investigated and discussed. Elemental distributions before and after aging obtained by analytical scanning TEM are presented to address cation interdiffusion between LSC and BSCF.

References:

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

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  [5] P. Haworth et al., Sep. Purif. Technol. 81, 88 (2011).

  [6] M. Meffert, L.-S. Unger et al., manuscript in preparation.

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

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