Microstructural Degradation of (La,Sr)MnO[sub 3]∕YSZ Cathodes in Solid Oxide Fuel Cells with Uncoated E-Brite Interconnects

Abstract

Transmission electron microscopy was used to investigate the local chemistry and microstructure of the active cathodes in solid oxide fuel cells operated for 500 h ͑at 700 and 800°C͒ under different electrochemical conditions while exposed to a chromiaforming stainless steel. Several distinct microstructural changes were observed owing to chromium interactions with the ͑La,Sr͒MnO 3 ͑LSM͒-yttria stabilized zirconia ͑YSZ͒ cathode; the nature and magnitude of which depended on the temperature, electrochemical load, and physical location. Nanosized particles of ͑Cr,Mn͒ 3 O 4 and Cr 2 O 3 were observed on the surface of the YSZ, regardless of the overall extent of degradation; the quantity increased with decreasing temperature and increasing current.The results indicate that Mn species facilitate the formation of a stable Cr-Mn-O nuclei on the YSZ, on which further growth occurs, including growth of Cr 2 O 3 . In cases of severe performance degradation, LSM decomposes completely, which does not appear to be strongly correlated with the nanoparticles on the YSZ, but results from a more destructive mechanism. Following this decomposition, severe pore filling of Cr-containing species occurs. The amount of microstructural degradation was largest near the cathode/electrolyte interface directly beneath the interconnect-cathode contact channels. These results indicate that two distinct mechanisms of degradation occur, with the electrochemical decomposition of ͑La,Sr͒MnO 3 as the primary cause for severe performance degradation.Solid oxide fuel cells ͑SOFCs͒ that operate between 600 and 800°C commonly employ chromia-forming ferritic stainless steels as interconnect materials. Unfortunately, cell performance is known to degrade over time when using such materials uncoated for interconnects. 1-23 Cell performance degradation is evinced as a decrease in the cell potential with time at a given current density. Microstructural changes in the active cathode are also known to be correlated with Cr-containing species owing to the volatilization of the steel's oxide layer in the wet cathode environment. 1,2,15,[24][25][26][27][28][29][30][31][32][33][34] Though several mechanisms have been proposed, discussed briefly below, to correlate the microstructural degradation products with the performance degradation, it is still unclear as to which proposed mechanism ultimately leads to the cell failure. In this work, we investigate these correlations in more detail, as a follow up to a recent paper describing the spatially averaged observations. 23 Microstructural changes associated with the performance degradation have been investigated previously, primarily using scanning electron microscopy ͑SEM͒ methods, 8,10,11,16,22,23,32 with a scant few detailed transmission electron microscopy ͑TEM͒ investigations. 15,35-37 Based on these observations, several mechanisms for Cr deposition have been put forward to explain the cell degradation behavior. Hilpert et al. 27 suggested that the cathode degradation should be associated with Cr transport in...

The Orientation Distributions of Lines, Surfaces, and Interfaces around Three‐Phase Boundaries in Solid Oxide Fuel Cell Cathodes

“…Their results imply that the local performance will vary with phase fractions, tortuosity, and connectivity, which would also impact the local electrochemical driving forces for microstructural evolution considered in this work. Clearly, much more research is required to understand how the local electrochemical properties vary within cathodes, especially because degradation events, such as Cr‐poisoning, are known to be correlated with local properties, not average properties …”

“…Clearly, much more research is required to understand how the local electrochemical properties vary within cathodes, especially because degradation events, such as Crpoisoning, are known to be correlated with local properties, not average properties. 57,58 The last point of interest here is to discuss the impact of these observations on SOFC activity. While we do observe weak texture in the cathodes, one could reasonably approximate the distributions as isotropic.…”

Quantitative Analysis of Multi-Scale Heterogeneities in Complex Electrode Microstructures

“…Such variations not only impact local cell performance and cell reliability, but can impact negatively the long-term performance if they accelerate degradation phenomenon related to the overpotential. 47,52,54,55,59,60 In other words, heterogeneous cells may also degrade faster than homogeneous cells. To investigate this, an in-house multiphysics model called DREAM-SOFC, 68,69 was used to model the effect that microstructural heterogeneity will have on performance.…”

“…Summary.-Prior microstructural research with nanoscale resolution of phases in electrodes has successfully characterized and correlated microstructural properties to local electrochemical performance, 47,54,55,59,60 but could not assess variability in microscale properties across devices. This work combined analysis on multiple PFIB-SEM scanned electrode microstructure reconstructions from the same cells, as well as different cells of the same manufacturer, which allowed quantifying heterogeneities over length scale that has not been studied before.…”

Effect of Strontium Content on Chromium Deposition and Poisoning in Ba1−xSrxCo0.8Fe0.2O3−δ(0.3 ≤x ≤0.7) Cathodes of Solid Oxide Fuel Cells

“…Using high resolution TEM, Krumpelt reported that the observed microstructrual degradation and partial decomposition of LSM cathode in the presence of un-coated E-Brite interconnect is most likely related to the high cell overpotential and the presence of Cr species. 20,21 Very different from that observed on the LSM electrode, the chromium deposition on (La,Sr)(Co,Fe)O 3 (LSCF) electrodes occurs predominantly on the surface of the LSCF cathode, forming SrCrO 4 . 8,22 Matsuzaki et al studied the performance of an LSCF electrode/samaria-doped ceria (SDC) electrolyte in the presence of a chromia-forming alloy and found no preferential precipitation of Cr species near the electrode/electrolyte interface.…”