Impedance spectra of mixed conductors: a 2D study of ceria

In this paper we develop an analytical framework for the study of electrochemical impedance of mixed ionic and electronic conductors (MIEC). The framework is based on non-equilibrium thermodynamics and it features the coupling of electrochemical reactions, surface transport and bulk transport processes. We utilize this work to analyze two-dimensional systems relevant for fuel cell science via the finite element method (FEM). Alternate current impedance spectroscopy (AC-IS or IS) of a ceria symmetric cell is simulated near equilibrium conditions (zero bias) for a wide array of working conditions including variations of temperature and H-2 partial pressure on a two-dimensional doped ceria sample with patterned metal electrodes. The model shows agreement between computed IS curves and the experimental literature where the relative error on the impedance is consistently below 2%. Important two-dimensional effects such as the impact of thickness decrease and the influence of variable electronic and ionic diffusivities on the impedance spectra are also explored.