Characterization of Cracks and their Effects on the Effective Transport Pathways in Ni-YSZ Anodes after Reoxidation Using X-Ray Nanotomography

Abstract

Reduction-oxidation (redox) cycling of Ni-based electrodes for solid oxide fuel/electrolysis cell irreversibly alters their microstructure and can cause the fracture of the electrolyte. Non-destructive 3-D imaging techniques enable tracking the microstructural changes that occur during the cycling [1]. Despite the recent advances, the understanding of how local 3-D geometrical features in the heterogeneous electrode material contribute to the material degradation remains incomplete. Absorption contrast X-ray nanotomography (XNT) of a same Ni(O)-yttria-stabilized zirconia (YSZ) sample was performed at the NiO K-edge white-line peak (8348 eV), before and after exposure to air at 800°C during 45 minutes. A complimentary XNT at 8376 eV confirmed a degree of oxidation in the range of 98%. The morphology of the Ni(O) phase was as expected completely different after re-oxidation. The spatial resolution in the range of 20 nm [2] further enabled the detection of cracks in the brittle YSZ phase. 3-D local curvature measurements were first performed to identify and characterize the crack initiation sites. Then, the capabilities of simple fracture mechanics models combined with curvature measurements were tested in the view of rapidly pinpointing the locations vulnerable to redox cycling. Finally, the detrimental effects of the cracks on the effective 3-D transport pathways in the Ni-YSZ anode under polarization was investigated using a skeleton-based discrete representation of the imaged volume and an analytical electrochemical fin model [3]. Topological properties, effective ionic conductivity and polarization resistance were calculated before and after oxidation. For the latter estimate, the effect of cracked YSZ network was considered first alone; that of the spatial re-distribution of triple-phase boundaries induced by re-oxidation will be included in the future. [1] A. M. Kiss, W. M. Harris, S. Wang, J. Vila-Comamala, A. Deriy and W. K. S. Chiu, Applied Physics Letters 102 (2013) 053902. [2] J. Vila-Comamala, Y. Pan, J. J. Lombardo, W. M. Harris, W. K. S. Chiu, C. David, Y. Wang, J. Synchrotron Rad. 19 (2012), 705–709. [3] G. J. Nelson, A. Nakajo, B. N. Cassenti, M. B. DeGostin, K. R. Bagshaw, A A. Peracchio, G. Xiao, S. Wang, F. Chen, W. K. S. Chiu, Journal of Power Sources 246 (2014) 322-334.