How Sulfur Tolerance of Two-Layered Ni/YSZ Anodes is Governed by Variations in Microstructure and Thickness

Abstract

In most anode-supported SOFCs the fuel electrode is realized by two porous Ni/YSZ layers, the supporting substrate and the anode functional layer (AFL) with specified microstructure properties. These electrodes, especially the AFL, provide a large number of electrochemically active triple phase boundaries, where (i) the electrochemical oxidation of hydrogen couples (ii) the electronic conduction in the nickel matrix, (iii) the ionic conduction in the YSZ-matrix and (iv) the gas diffusion in the pores. In this study a physical meaningful modelling approach based on a three channel transmission line model was developed in order to describe the coupling of all abovementioned processes. The extension of the electrochemical active zone into the substrate is considered for the first time by investigating AFLs varying from 3 to 22 µm. The model is capable of correlating the microstructure and thickness of the AFL with the performance of Ni/YSZ cermet anodes. Anodes with thicker functional layer revealed a poorer initial performance due to enhanced gas diffusion, whereas the tolerance towards sulfur is increased.