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Materials 2015, 8(9), 5554-5585; doi:10.3390/ma8095265

3D Microstructure Effects in Ni-YSZ Anodes: Prediction of Effective Transport Properties and Optimization of Redox Stability

1
Institute of Computational Physics, Zurich University of Applied Sciences, Winterthur 8400, Switzerland
2
Institute for Building Materials, ETH Zurich, Zurich 8093, Switzerland
3
Hexis SA, Winterthur 8404, Switzerland
4
Scientific Center for Optical and Electron Microcopy (ScopeM), ETH Zurich, Zurich 8093, Switzerland
5
Institute of Stochastics, Ulm University, Ulm 89069, Germany
*
Author to whom correspondence should be addressed.
Academic Editor: Federico Bella
Received: 13 July 2015 / Revised: 4 August 2015 / Accepted: 13 August 2015 / Published: 26 August 2015
(This article belongs to the Special Issue Electrode Materials)
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Abstract

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 °C. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φeff), constriction factor (β), and tortuosity (τ). The effective conductivity (σeff) is described as the product of intrinsic conductivity (σ0) and the so-called microstructure-factor (M): σeff = σ0*M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, Mpred = εβ0.365.17, and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (Msim). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers. View Full-Text
Keywords: cermet; degradation; microstructure; tomography; conductivity; solid oxide fuel cells; Ni-YSZ; redox cycling cermet; degradation; microstructure; tomography; conductivity; solid oxide fuel cells; Ni-YSZ; redox cycling
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Pecho, O.M.; Stenzel, O.; Iwanschitz, B.; Gasser, P.; Neumann, M.; Schmidt, V.; Prestat, M.; Hocker, T.; Flatt, R.J.; Holzer, L. 3D Microstructure Effects in Ni-YSZ Anodes: Prediction of Effective Transport Properties and Optimization of Redox Stability. Materials 2015, 8, 5554-5585.

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