High operating temperature affords solid oxide fuel cells (SOFCs) fuel flexibility, favorable kinetics, enhanced transport properties and high quality heat for secondary thermodynamic cycles.¹ Elevated temperatures, however, present challenges to ensuring cell durability.² Coarsening of nickel in the Ni-YSZ anode is one such challenge that can lead to significant performance degradation.

The fundamental driving force Δμ for coarsening phenomena is given by Eq. 1.³ The expression, which assumes isotropic interfacial energy, indicates that the chemical potential μ of a curved interface differs from that of a flat interface μ₀ by the factor V_mγH, where H is mean curvature, γ interfacial energy and V_m molar volume. 

      Δμ = μ - μ₀ = V_mγH                         (1)

Eq. 1 makes evident that variations in interface curvature, i.e., interface shape, directly correspond to variations in chemical potential. Furthermore, it follows from Eq. 1 that microstructural coarsening is fundamentally related to variations in interface shape.

The link between interface shape and coarsening propensity is well known and methods have been developed to directly measure interfacial curvature distributions for complex morphologies in 3-D.^4,5 In this work, these techniques are applied to 3-D images obtained by x-ray nanotomography to study coarsening in microstructures obtained from Ni-YSZ solid oxide fuel cell anodes aged for varying lengths of time. This work builds on a previous characterization of the same samples.⁶

The curvature measurements collected suggest that a combination of coarsening and morphological instability, which leads to channel pinch-off, severely diminish the connectivity of the anode’s Ni network and, in addition, lead to a significant loss of both nominal and effective triple-phase boundary (TPB) density. Both the loss of connectivity and TPB density coincide with cell performance degradation that was observed in the aging experiment⁷ from which the anode samples were taken.

References

1. Singhal, S. C.; Kendall, K. High Temperature Solid Oxide Fuel Cells, Elsevier, 2003.
2. Yokokawa, H.; Tu, H.; Iwanschitz, B.; Mai, A. Fundamental mechanisms limiting solid oxide fuel cell durability. J. Power Sources 2008, 400-412.
3. Voorhees, P. W. The Theory of Ostwald Ripening. J. Stat. Phys. 1985, 231-252.
4. Glicksmann, M. E.; Voorhees, P. W. Ostwald Ripening and Relaxation in Dendritic Structures. Metall. Trans. A, 1984, 995-1001.
5. Mendoza, R.; Savin, I.; Thornton, K.; Voorhees, P. W. Topological complexity and the dynamics of coarsening. Nat. Mater., 2004 385-388.
6. Nelson, G. J.; Grew, K. N.; Izzo, J. R. Jr.; Lombardo, J. J.; Harris, W. M.; Faes, A.; Hessler-Wyser, A.; Van herle, J.; Wang, S.; Chu, Y. S.; Virkar, A. V.; Chiu, W. K. S. Three-dimensional microstructural changes in the Ni-YSZ solid oxide fuel cell anode during operation. Acta Mater., 2012 3491-3500.
7. 7.     Faes, A.; Hessler-Wyser, A.; Presvytes, D.; Vayenas, C. G.; Van herle, J. Nickel-Zirconia Anode Degradation and Triple Phase Boundary Quantification from Microstructural Analysis. Fuel Cells, 2009 841-851.

 Acknowledgements

Financial support from the National Science Foundation (Award CBET-1134052) and an Energy Frontier Research Center on Science Based Nano-Structure Design and Synthesis of Heterogeneous Functional Materials for Energy Systems (HeteroFoaM Center) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Award DE-SC0001061) is gratefully acknowledged. X-ray nanotomography was performed with Dr. Steve Wang at the Advanced Photon Source supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357. Ni-YSZ samples were provided by MER Dr. J. Van herle (Ecole Polytechnique Federale de Lausanne, Switzerland).