Elsevier

Journal of Power Sources

Volume 273, 1 January 2015, Pages 486-494
Journal of Power Sources

Analyses of microstructural and elastic properties of porous SOFC cathodes based on focused ion beam tomography

https://doi.org/10.1016/j.jpowsour.2014.09.131Get rights and content

Highlights

  • Microstructures of porous LSCF cathodes were studied based on FIB/SEM tomography.

  • Important microstructural and elastic parameters were quantified and correlated.

  • Their sensitivities to the image segmentation threshold value (TV) were studied.

  • Results showed that these parameters had very different sensitivities to TV.

  • Porosity played an important role in elastic modulus determination.

Abstract

Mechanical properties of porous SOFC electrodes are largely determined by their microstructures. Measurements of the elastic properties and microstructural parameters can be achieved by modelling of the digitally reconstructed 3D volumes based on the real electrode microstructures. However, the reliability of such measurements is greatly dependent on the processing of raw images acquired for reconstruction. In this work, the actual microstructures of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathodes sintered at an elevated temperature were reconstructed based on dual-beam FIB/SEM tomography. Key microstructural and elastic parameters were estimated and correlated. Analyses of their sensitivity to the grayscale threshold value applied in the image segmentation were performed. The important microstructural parameters included porosity, tortuosity, specific surface area, particle and pore size distributions, and inter-particle neck size distribution, which may have varying extent of effect on the elastic properties simulated from the microstructures using FEM. Results showed that different threshold value range would result in different degree of sensitivity for a specific parameter. The estimated porosity and tortuosity were more sensitive than surface area to volume ratio. Pore and neck size were found to be less sensitive than particle size. Results also showed that the modulus was essentially sensitive to the porosity which was largely controlled by the threshold value.

Introduction

Solid oxide fuel cells (SOFCs) are promising energy conversion devices which directly generate electricity from the electrochemical reactions of fuels and air (oxygen) with high efficiency and low environmental impact [1]. A SOFC commonly consists of a dense ceramic electrolyte with high oxygen ionic conductivity, which is supported on either side by a porous cathode with mixed ionic-electronic conductivity (MIEC) and a porous anode, respectively. Perovskite-structure materials such as La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) have been widely applied as SOFC cathodes due to their good MIEC, particularly at intermediate operating temperatures [2], [3], [4], [5].

The electrochemical performance and mechanical properties of an operating LSCF cathode are strongly dependent on their three-dimensional (3D) microstructures [6], [7], which include the porosity, distributions of particle and pore sizes, surface area and phase tortuosity. Often larger porosities are favourable for enhancing the cathode's electrochemical performance while its mechanical strength increases when the opposite is the case. Therefore, efforts must be made to trade off the electrochemical performance and the ability to withstand mechanical constraints.

The development of advanced 3D tomographic techniques has made it possible to analyse real spatial microstructures of SOFC electrodes by 3D reconstruction [8]. The reconstructed actual 3D microstructures allow mechanical and electrochemical simulations which can help improve the mechanical and electrochemical properties. One of the most widely used techniques is the focused ion beam/scanning electron microscope (FIB/SEM) tomography, which was first applied to the characterisation of SOFC materials by Wilson et al. [9] and has been implemented to reconstruct 3D microstructures of SOFC anodes thanks to its advanced slicing and imaging capability, and high spatial resolution. With the aid of the FIB/SEM technique, studies have been reported of the 3D reconstruction of LSCF cathode microstructures and the relationship to their electrochemical performance [10], [11], [12]. However, none of these 3D reconstructions of porous thin LSCF cathodes was correlated to their mechanical properties, which on the other hand has been rarely studied [7], as most mechanical studies in the literature were based on nominally dense LSCF bulk samples [13], [14], [15]. However, porous films may behave mechanically very differently from bulk samples of the same materials/compositions.

On the other hand, reliable quantification of the microstructural parameters and mechanical/electrochemical simulations require accurate 3D microstructure datasets, which hinge largely on the quality of the binarised 2D sequential images of the sample volume. The segmentation process, which generally involves grayscale thresholding to identify the two phases (i.e. pore and solid phase), is crucial to the resulting binary image quality and thus the 3D microstructures reconstructed.

The current study aims to investigate the sensitivity of key microstructural and elastic parameters to the grayscale threshold value applied in the image segmentation process. The important microstructural parameters involved porosity, tortuosity, specific surface area, particle and pore size distributions, and inter-particle neck size distribution, all of which may, to different extents, influence the elastic properties (i.e. elastic modulus and Poisson's ratio) calculated by finite element modelling based on the reconstructed 3D microstructures. The input Poisson's ratio of dense LSCF material was also examined to check its influence on the elastic parameters of the porous microstructures.

Section snippets

Sample preparation

LSCF cathode films were fabricated by tape casting of an ink slurry on CGO (Ce0.9Gd0.1O2−δ) pellet substrates, followed by sintering at 900 °C in air for 4 h. The resulting films had smooth and crack-free surface, without interfacial delamination from the substrate. The detailed sample fabrication processes can be found in Refs. [7], [16].

FIB/SEM slicing and viewing for image acquisition

The as-sintered porous films were coated with a thin gold layer on the top and subjected to slicing and viewing with a combined focused ion beam and scanning

3D microstructure reconstruction

In the current study, three volumes of interest were sampled in different locations of the films sintered at 900 °C. Fig. 3(a) shows an image stack consisting of 250 slices obtained by FIB/SEM slicing and viewing with image size of 250 × 250 pixel2. Fig. 3(b) is the corresponding histogram which plots the number of voxels of each grayscale value for the above image stack, with 0 corresponding to black (i.e. pore phase in Fig. 3(a)). It shows a reasonably good bimodal feature which was

Conclusions

The mechanical properties of porous thin SOFC electrode films strongly rely on some of their key microstructural parameters, which can be made available by quantification based on 3D reconstructed microstructures using FIB/SEM tomography. Reliable microstructural parameter quantification and mechanical simulation require accurate processing of the as-collected sequential 2D image stacks. The segmentation process, which generally involves grayscale thresholding to identify the two phases (i.e.

Acknowledgements

This research was carried out as part of the UK Supergen consortium project on “Fuel Cells: Powering a Greener Future”. The Energy Programme is an RCUK cross-council initiative led by EPSRC and contributed to by ESRC, NERC, BBSRC and STFC. Z. Chen is especially grateful to the Chinese Government and Imperial College for financial support in the form of scholarships.

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