Physically based impedance modeling of Ni/YSZ cermet anodes

The electrochemical impedance of Ni/yttria-stabilized zirconia (YSZ) cermet anodes is generally observed to consist of multiple features that can span the entire frequency range from hertz to megahertz. In order to assign the origin of these features, we apply a two-dimensional multiscale modeling approach that includes elementary kinetic electrochemistry, electrical double-layer formation, diffusive transport in the porous cermet and the current collector mesh, gas-phase transport in the supply channel, and cermet microstructure description based on percolation theory. The model parameters are chosen to represent the electrolyte-supported symmetrical cell setup of Sonn [J. Electrochem. Soc., 155, B675 (2008)]. The experimental data show three distinct impedance features. Using sensitivity analysis and model reductions, the physical model allows the assignment of the origin of these impedance features: (i) distributed charge transfer and electrical double layer cause an asymmetrical and depressed semicircle at high frequencies (similar to 5 kHz); (ii) gas diffusion in the cermet and current collector mesh causes a small feature at intermediate frequencies (similar to 1 kHz); and (iii) gas diffusion in the supply channels causes a large, sometimes dominant feature at low frequencies (similar to 6 Hz). Two-dimensional spatial and temporal evolution of electrical current, potential, and gas composition during the impedance measurement are discussed. (C) 2008 The Electrochemical Society.