Advanced impedance modelling of Ni/8YSZ cermet anodes

Abstract

The efficiency and long-term stability of Solid Oxide Fuel Cells (SOFC) are both highly reliant on the performance of the porous fuel electrode. The electro-oxidation reactions and transport processes that take place in the nickel/yttria stabilized zirconia cermet anode can be described by a transmission line model (TLM). Herein, the ionic conduction pathway in the electrolyte matrix is coupled to the electronic conduction pathway in the nickel matrix at every individual triple phase boundary point, where the gaseous species H2 and H2O react. With a well-parametrized TLM, the spatial expansion of the electro-oxidation reaction into the anode volume becomes predictable as a function of temperature and gas composition.In this work, a physicochemically meaningful TLM was set up using parameters acquired by (i) EIS measurements on patterned model anodes, (ii) 4-point DC conductivity measurements on porous YSZ-matrix samples and (iii) microstructure characteristics gained from FIB/SEM-tomography. The application of these parameters in the TLM provided a good agreement with measured impedance spectra over a wide temperature and fuel composition range. Further, the representation of the impedance spectra became even more precise when they were used as starting values in a CNLS-fit, with all parameters kept within physicochemically meaningful ranges.As such, the TLM can predict the impact of microstructural and material properties on the anode performance. Simulation studies were performed and we evaluated the impact of tortuosity, volume specific triple phase boundary length, double layer capacity and ionic conductivity on the impedance spectrum.