Determination of relevant factors affecting the surface oxygen exchange coefficient of solid oxide fuel cell cathode with ionic conducting oxide coating

In order to investigate the relevant factors affecting the transport properties of solid oxide fuel cell cathodes upon coating with Ce0.9Gd0.1O1.95 (GDC), the surface oxygen exchange coefficient, k*, and oxide ion diffusivity, D*, of La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were measured by a combination of isotope exchange technique and secondary ion mass spectrometry (SIMS). The k* of LSCF/GDC enhanced compared to a bare LSCF for all measured temperatures. In contrast, the k* of LSC/GDC only showed an enhancement compared to the bare LSC at relatively low temperatures (below 873 K) but not at higher temperatures. From these results, GDC has a less significant effect on the k* of LSC compared to LSCF. This suggests that the enhancement of k* is possibly induced only when the oxygen vacancy concentration, δ, of the cathode is lower than that of GDC, although another factor may affect the enhancement. The enhancement of k* is attributed to the formation of triple-phase boundary where a spill-over mechanism controlled the oxygen reduction reaction. The change of rate-determining step of LSCF due to the GDC coating suggests that the enhancement of the k* is not only with respect to the incorporation process, but it is possibly enhancing the reaction steps that involve δ as well.