Physically Based Impedance Modelling of Ni/8YSZ-Cermet Anodes

Abstract

Electrochemical impedance spectroscopy (EIS) is an indispensable tool to investigate the polarization processes in complex electrochemical systems such as Solid Oxide Cells (SOC), which can be operated in both fuel cell and electrolysis mode. We have introduced a methodology to analyze the measured impedance spectra by the distribution of relaxation times [1] and a subsequent Complex Non-linear Square (CNLS) approximation to an equivalent circuit model (ECM) [2] in order to identify and quantify the individual contribution of different loss mechanisms. It has been demonstrated, that electrochemical properties of SOCs operated in fuel cell mode and electrolysis mode can be described by the same ECM [3]. The complex anode electrochemistry is one major loss mechanism that highly affects the performance and durability of SOCs. Porous Ni/8YSZ anodes provide a large number of electrochemically active triple phase boundary (TPB) points, where the electrooxidation of hydrogen couples the electronic, the ionic and the gaseous transport paths. A basic equivalent circuit element designed to describe this configuration is the transmission line model (TLM) [4]. The corresponding TLM deconvolutes the contribution of (i) the ionic transport through the 8YSZ-matrix , (ii) the electronic transport through the Ni-matrix and (iii) the charge transfer reaction at the TPB z (fig. 1). In this study the established ECM [2] for anode supported cells (ASC) is extended by a physically meaningful TLM in order to describe the anode electrochemistry. The required model parameters are obtained from EIS measurements on patterned model anodes [5], 4-point DC conductivity measurements on electrolyte bulks and microstructure properties extracted from FIB-tomography [6] (fig. 1). Using this approach the number of free fitting parameters can be reduced to one. The model has been validated by EIS measurements for a broad range of operation conditions such as temperature and gas composition. We will present a detailed study on the development and parameterization of the TLM model, as well as the temporal evolution of the abovementioned processes (i) to (iii) during durability tests.