MICROSTRUCTURAL AND PHASE EVOLUTION OF ATMOSPHERIC PLASMA SPRAYED MANGANESE COBALT IRON OXIDE PROTECTION LAYERS IN SOLID OXIDE FUEL CELLS

Chromium containing steels are widely used as interconnects in solid oxide fuel cell stacks (SOFCs) because of their advantageous mechanical and electrical properties. The major drawback of this material is the evaporation of chromium containing species at the high SOFC operation temperatures, which lead to a strong degradation of commonly used cathode materials. Covering the interconnect’s surface with a protection layer enables a strong reduction of the chromium related degradation. The use of dense layers of atmospherically plasma sprayed (APS) Mn1,0Co1,9Fe0,1O4 (MCF) showed remarkably low degradation rates within operated stacks in Jülich. Although the stacks show good performance, the APS-MCF layers undergo strong changes that are just partially described in literature [1,2], but not fully understood yet. This study analyses the microstructural evolution and phase changes of APS-MCF layers within samples tested for annealing times of up to 10.000 hours and also within components of test stacks operated at JÜLICH. A crack healing effect is observed by annealing in air at low temperature of 500°C. Measurements with X-ray diffraction, wet chemical analyses and thermo gravimetry support a theory describing this effect by a volume expansion that is induced by an oxidation process of the plasma sprayed MCF. Air leakage measurements reveal a strong increase of gas-tightness providing increased chromium retention. Annealing at higher temperatures of 700°C to 850°C leads to a segregation and a phase separation observed in electron microscopic images. Oxidation driven diffusion of Cobalt ions to the layer’s surface build up a dense Co3O4 layer in a spinel crystal structure. This layer decelerates further oxidation of the coatings bulk. Combining these results with the performed long-term annealing tests can provide lifetime predictions of APS-MCF protective layers. Thereby a reduction of the cathode related degradation can be achieved.References[1] R. Vaßen, N. Grünwald, D. Marcano, N.H. Menzler, R. Mücke, D. Sebold, Y.J. Sohn, O. Guillon, Aging of atmospherically plasma sprayed chromium evaporation barriers, Surface and Coatings Technology 291 (2016) 115–122.[2] J. Puranen, M. Pihlatie, J. Lagerbom, G. Bolelli, J. Laakso, L. Hyvärinen, M. Kylmälahti, O. Himanen, J. Kiviaho, L. Lusvarghi, P. Vuoristo, Post-mortem evaluation of oxidized atmospheric plasma sprayed Mn–Co–Fe oxide spinel coatings on SOFC interconnectors, International Journal of Hydrogen Energy 39 (2014) 17284–17294