Impedance Spectroscopy Analysis of Ni/YSZ Interfaces Prepared by Liquid Precursor Deposition

The state-of-the-art material for SOFC anodes is a Ni-yttria-stabilized zirconia (Ni-YSZ) based cermet, due primarily to the excellent electrocatalytic activity of Ni towards H₂oxidation. To design and construct Ni-YSZ anodes with maximum performance, it is important to understand the reaction mechanism at the Ni/YSZ interface. Therefore, model anode designs, in which dense Ni films were deposited on dense YSZ electrolytes in the form of regularly spaced strips, have been developed by multiple groups [1, 2] to control the Ni/YSZ interfacial length, with impedance spectroscopy (EIS) used to evaluate performance. However, a consensus has not been reached yet on the exact reaction steps and how they change with temperature. Charge transfer at the Ni/YSZ interface, hydrogen diffusion on Ni surface and hydrogen adsorption/desorption reaction, have all been suggested as the rate-limiting steps in the literature [1, 2]. Thus, a clear identification of the rate limiting step at the Ni/YSZ interface and its correlation with the Ni-YSZ composite anodes is still required.

In this work, solution precursor deposited Ni thin films were deposited on dense Zr oxide discs for comparison with Ni-YSZ composite anodes that were prepared by the infiltration of the Ni solution precursor into porous YSZ scaffolds. EIS analysis was carried out, and a transmission line model, which allows the separation of the porous YSZ scaffold ionic resistance and the Ni/YSZ interfacial impedance, was used to fit the impedance data. The dominant impedance arcs in the thin film Ni anodes and the Ni/YSZ interfacial impedance, extracted from the transmission line modelling of the Ni-YSZ composites, both exhibited an activation energy of ~1.3 eV. The same activation energy obtained for the Ni/YSZ interfacial impedance obtained from both Ni thin film anodes and infiltrated Ni-YSZ anodes validates the transmission line fitting approach while the obtained activation energy value of ~1.3 eV indicates that the charge transfer process occurring at the triple phase boundary is the rate determining reaction step in Ni-YSZ anodes.

Acknowledgements:

The authors gratefully acknowledge the Eyes High PDF Program at the University of Calgary and Alberta Innovates – Technology Futures (AITF) for the support of AB, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC) for the overall financial support of this work.

References:

1. A. Bieberle, L. P. Meier and L. J. Gauckler,  J. Electrochem. Soc., 148 (2001) A646.

2. A. Utz, H. Stormer, D. Gerthsen, A. Weber, E and Ivers-Tiffee, Solid State Ionics, 192 (2011) 565.