Advanced Impedance Spectroscopy Study of the Influence of the Crystalline Structure on the Ionic Conduction of Thin Solid-State Electrolytes

Solid electrolytes exhibit electrical conductivity due to the presence of mobile ions which act as charge carriers through the crystalline structure. For their use in solid oxide fuel cells (SOFCs), a sufficient electrical conductivity is achieved by operating at high temperatures. To reduce the application temperature, research is dedicated to increase the electrical conductivity of the ceramic material otherwise. One way is decreasing the film thickness. Yet, the relationship between the material crystallinity and the ion conductivity may be reinterpreted when reducing the thickness to the micrometer and nanometer scales.

Another way is by reducing the grain size. At present it is not clear how the microstructure of solid state electrolytes affect the electrical conduction. In literature, the experiments on nanocrystalline solid oxides yield contradictory observations [1-6]. A good understanding of the relationship between microstructure and ion conductivity would open the possibility to engineer advanced thin materials for solid oxide fuel cells.

We propose an experimental and modeling approach that aims at unraveling the electrical behavior of thin film solid state electrolytes, by considering the crystalline nature of the material. Through careful comparison of simulations and measurements we want to explain the observed electrical properties of these materials to predict how their conductivity can be enhanced.

In this research, the complex electrical properties at different temperatures of yttrium-doped ceria electrolytes, with different thicknesses and grain sizes, are determined by electrochemical impedance spectroscopy (EIS). The solid oxide is deposited by reactive magnetron sputtering, which allows us to modify the morphology and composition of the thin film in a controlled and flexible way [7].

In the EIS characterization, improved electrode geometry is also searched. Experiments with platinum co-planar electrodes are performed and compared with those with an interdigital electrode structure. The specific electrode geometry is designed by 2D simulations for a solid oxide film in contact with the substrate.

A finite element model of the electrical conductivity was recently developed to simulate impedance spectra and electrical conductivities as a function of grain size and temperature [8]. More rigorous than the dilute solution models, it is based on the application of the linear phenomenological relations to a crystal lattice.

The impedance modeling provides a description of the electrical behavior of the material. The EIS results are then compared with the ones generated by the finite element model. The combined analysis aims to elucidate the experimental evidence that is not explained by the available physical models. Therefore, it represents a strong approach to understand the electrical properties and ion conduction of solid electrolytes.

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[2] J. Maier “Nano-sized mixed conductors (Aspects of nano-ionics. Part III)”, Solid State Ionics 148 (2002) 367–374.

[3] X. Guo, R. Waser “Electrical properties of the grain boundaries of oxygen ion conductors: Acceptor-doped zirconia and ceria”, Progress in Materials Science 51 (2006) 151–210.

[4] J. Maier “Nanoionics: ionic charge carriers in small systems”, Physical Chemistry Chemical Physics 11 (2009) 3011–3022.

[5] C. Peters, A. Weber, B. Butz, D. Gerthsen, E. Ivers-Tiffée “Grain-Size Effects in YSZ Thin-Film Electrolytes”, Journal of the American Ceramic Society 92 (9) (2009) 2017–2024.

[6] X. Guo “Can we achieve significantly higher ionic conductivity in nanostructured zirconia?”, Scripta Materialia 65 (2011) 96–101.

[7] S. Mahieu, P. Ghekiere, G. De Winter, S. Heirwegh, D. Depla, R. De Gryse., O.I. Lebedev, G. Van Tendeloo “Mechanism of preferential orientation in sputter deposited titanium nitride and yttria-stabilized zirconia layers”, Journal Of Crystal Growth 279 (1-2) (2005) 100-109.

[8] D. Van Laethem, A. Hubin, J. Deconinck “Finite element modelling of the electrical conductivity of acceptor doped ceria”, 6^th International Conference on Fundamentals and Development of Fuel Cells, 3^rd- 5^th February, Toulouse (France).