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Fuel Cell Microstructure from FIB Tomography
Overview
The lifetime and degradation of solid oxide fuel cells (SOFCs) are affected by stresses generated within different layers of a device. While macro-scale modelling of stress in fuel cells can be used for homogeneous studies, more accurate stress analysis can be made of heterogeneous multiphase porous layers within electrodes on the micron and sub-micron scale using Focused Ion Beam (FIB) tomography and Finite Element (FE) modelling. By combining microstructural characterisation of a porous solid oxide fuel cell with volume meshing in Simpleware software, researchers were able to perform a 3D FE stress analysis of the individual phases and boundaries of a fuel cell electrode. The results indicate the potential for characterising principal stresses across phases and interfaces in SOFCs, and for developing a better understanding of fuel cell failure.
Thanks to
Imperial College and University College London:
R. Clague • P.R. Shearing • P.D. Lee • Z. Zhang • A.J. Marquis • N.P. Brandon • D.J.L. Brett
Financial support provided by the EPSRC Supergen Fuel Cells programme; FIB-SEM work carried out at the ion beam facility at the London Centre for Nanotechnology
FIB Tomography Scanning
Electrolyte supported nickel and yttria-stabilised zirconia (Ni-YSZ) symmetrical cells were manufactured using screen printing, with half cells then chemically reduced and characterised using electrochemical impedance spectroscopy. After electrochemical testing, the samples were scanned using FIB tomography (Zeiss xB-1540 FIB-FEG SEM, London Centre for Nanotechnology) to characterise the Ni-YSZ electrode microstructure. 170 images with pixel dimensions of 20 nm were created with an inter-slice thickness of 15 nm. 100 images were selected and segmented for further analysis.
FE Model Generation
Simpleware +FE was used to handle the volume meshing of the segmented tomography data. The software's automatic handling of multi-parts enabled a multiphase mesh to be created with conforming interfaces and shared nodes. The FE model consisted of 285,000 four-node tetrahedral elements (Abaqus type C3D4), with the number of elements determined from a mesh independence test. During the meshing process, the Ni and YSZ phases were assigned to one of two material sections to allow allocation of appropriate material properties; node and element phases were automatically generated on the model cut planes and at the interface of the two phases.
Simulation and Results
The mesh was exported to Abaqus/CAE where boundary conditions and load-cases were applied. Stress analysis was run using an implicit solution technique, with the analysis assuming that there was no separation of the Ni and YSZ phases, and that the material behaviour was linear and elastic. The simulation allowed researchers to use a simple elastic analysis to approximate peak maximum principal stresses in response to thermal expansion within Ni and YSZ phases and their boundaries. The analysis found that the yield strength of nickel is exceeded at the interface of the Ni and YSZ phases, suggesting its importance for stress relief in the electrodes as they are heated or cooled.