The rate of the oxygen exchange at the gas solid interface is of key importance for the performance of solid oxide fuel cells (SOFCs), especially at low temperature, as well as for the overall durability of solid oxide electrolysis cells (SOECs), when operated at high current density. Hence, details of this process are of great technological importance. Whereas simple guidelines like; i) Combine an electrolyte material and an electro-catalyst in the electrode, ii) maximize triple phase boundary length per unit area, iii) maximize surface area of the electro catalyst, iv) use preferably a mixed conducting electro-catalyst, v) maximize ionic conductivity in both phases,... are well established in the field, and have lead to development of well performing electrodes, the rate limiting steps of the exchange process itself are not well established. It is very challenging to elucidate these on the “real” geometrically very complex composite systems. This has spurred interest in studying simpler model electrode systems [1,2,3], for example in the form of ultra-thin dense films (with a well-defined surface area) and also in trying to model electrode performance from a detailed geometrical description of the electrode structure and the material properties deduced from studies of the materials in bulk form. The study of thin film model electrode systems certainly lends insight into the details of the exchange process, but the approach is not without problems, as the model system may differ from the “real” nanostructured composite electrodes in a number of ways, e.g. with respect to surface structure/termination, chemical composition [4], purity and possibly strain, which may all affect the exchange process.

In this paper we discuss challenges in predicting performance of real electrodes from studies on model systems. We shall compare trends observed on dense thin films of LSC (La_0.5Sr_0.5CoO₃), LSCF (La_0.58Sr_0.4Co_0.2Fe_0.8O_3-δ) and LSF (La_0.6Sr_0.4FeO₃) with trends observed when these materials are used in porous composites. Also a model study of a multi-phase nano-structured electrode, prepared by infiltrating a porous CGO backbone structure with “LSC” will be presented, and it is shown that performance can be well accounted for when assuming exchange properties as deduced from LSC in bulk form. 

Finally, recent results obtained on two other types of simplified model electrode systems are presented. The first one is based on a dense composite of two phases (Fig. 1, A and B) and in the second type a mixed conducting material is studied with and without second phase surface decorations (Fig. 1, D). In both cases, the oxygen exchange is studied using conductivity relaxation (Fig. 1, C). The advantage of these geometries are that they are simpler than the real electrodes, e.g. the TPP can be accurately assessed, and the manufacturing route in many respects lies closer to the one applied on technological electrodes than does the ones typically applied when making ultra-thin films.

On a LSF/CGO (La_0.6Sr_0.4)_0.98FeO₃-Ce_0.9Gd_0.1O_1.95) composite system with different volume ratios between the two phases (0%, 30 %, 50 %, 70 % CGO) we find that the relaxation time for re-equilibrating the oxygen stoichiometry in the sample after a sudden change in oxygen activity reduces strongly the more CGO is added. This is not only an effect of the increased ionic conductivity in the composite (CGO has a higher ionic conductivity than LSF) but also the surface exchange rate is increased (by up to a factor of 25) relative to the one of the pure LSF sample. Effects of adding small amounts of second phase decorations on the surface on both the composites and the LSF end-member were studied. Results are compared to recent findings in literature [5] and the underlying mechanisms for the increased surface exchange rates are discussed.

References

[1] S. Baumann, J. Fleig Solid State Ionics 177(2006).

[2] P. Plonczak, A. Bieberle-Hütter, M. Søgaard, T. Ryll, J. Martynczuk, P.V. Hendriksen, L. J. Gauckler, Advanced Mat.  21 (2011) 2764.

[3] P. Plonczak, M. Sogaard, A. Bieberle-Hutter, P. V. Hendriksen and L. J. Gauckler Journal of the Electrochemical Society 159, (2012) B471-B482.

[4] M. Kubicekz, A. Limbeck, T. Frömling, H. Hutter, and J. Fleig Journal of The Electrochemical Society, 158B727-B734 (2011).

[5] Tao Hong, Lei Zhang, Fanglin Chen, Changrong Xia, Journal of Power Sources 218 (2012) 254