Thermal analysis of the cyclic reduction and oxidation behaviour of SOFC anodes

Abstract

The reduction and oxidation (redox) kinetics of a Ni/YSZ cermet were studied at temperatures between 400 and 850 °C by thermogravimetric analysis (TGA) and reduction and oxidation activation energies were determined. In addition, the volume change after redox cycling was examined by thermomechanical analysis (TMA). Samples with fine microstructure similar to a typical SOFC anode functional layer (AFL) and samples with coarse microstructure similar to a typical anode substrate were characterized. Coarse structured samples experienced no volume change or cracking upon redox cycling. Fine structured anode samples did not change in volume after reduction, but expanded between 0.9% and 2.5% after oxidation. The samples were significantly cracked after oxidation. The amount of expansion and cracking was reduced by lowering the Ni content of the anode and by reducing the oxidation temperature. The reduction of the NiO/YSZ samples followed linear kinetics with an activation energy of 78 kJ/mol. Ni/YSZ oxidation followed parabolic kinetics at temperatures lower than 700 °C. A divergence from parabolic kinetics was seen at higher temperatures (700–850 °C). This divergence has been seen in many other kinetic studies and is usually attributed to short-circuit diffusion mechanisms. An activation energy of 87 kJ/mol was calculated for oxidation.