Analysis of Structural Degradation Effects in a Solid Oxide Fuel Cell after Long-Term Operation

Abstract

The sustainable production of hydrogen as well as its re-conversion into electricity will be a key aspect in a future green energy environment that is depending on regenerative resources. In order to ensure the eminent position of solid oxide cells (SOC) in this future hydrogen economy their reliability and long-term performance still needs to be improved. Having this in mind, we performed detailed investigations on samples from cells that were operated for 100,000h to get a profound understanding of structural degradation effects after long-term operation.The investigated samples originate from a two layer stack of fuel electrode supported solid oxide cells that were operated at 700°C and 0.5 A/cm2 in fuel cell mode. The cells consist of a Ni/Y-stabilized ZrO2 (YSZ) fuel electrode, a dense YSZ electrolyte layer, a Gd doped CeO2 (GDC) barrier layer and a La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF) air electrode. Furthermore, an additional contact layer of La0.97Mn0.4Co0.3Cu0.3O3-d was applied on the air electrode surface and, in order to inhibit Cr-poisoning the interconnector material was coated with a manganese oxide layer by wet powder spraying. The stack represents the usual material status from 2009 when the stack operation has been starting.In the post-test analysis the microstructure and chemical composition was investigated by scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDS). The EDS analysis revealed localized, correlated enrichments of Sr and Cr in the bulk of the LSCF air electrode as well as at the LSCF / GDC interface. Moreover, non-correlated, localized Co enrichments could also be detected at the LSCF / GDC interface. In order to get an insight into localized structural changes and to identify impurity phases after long-term operation Raman spectroscopy measurements were performed. Hereby, it was for example possible to visualize, in accordance with the EDS analysis, the formation of a SrCrO4 impurity phase on the LSCF air electrode surface as well as inside the bulk of the LSCF by running Raman measurements on the air electrode surface as well as on embedded and polished cross-section samples. To study the structural degradation in detail with high spatial resolution, additional µ-beam Laue diffraction measurements were performed at the CRG-IF BM32 beamline at European Synchrotron Radiation Facility (ESRF).