Post-treatment of Plasma-Sprayed Zirconia Based Electrolytes

Abstract

Yttria-stabilized zirconia (YSZ) is conventionally used as electrolyte material in solid oxide fuel cells (SOFCs). The ionic conductivity of YSZ shows a strong dependency on thickness of the electrolyte layer and on operating temperature of the cell. Due to temperature dependency of the ionic conductivity of YSZ, SOFCs with YSZ electrolyte operate in a temperature range of 800 to 1000 °C to reduce ohmic losses. An important research and development goal for SOFCs is to reduce the operating temperature below 800 °C. Though, the reduced operating temperature promotes durability of cells and decreases stringent demands on peripheral components, the ionic conductivity of electrolytes decreases following Arrhenius law. Reducing the thickness of conventionally used YSZ electrolyte by using nanostructured particles as feedstock or by using an electrolyte with improved ionic conductivity for intermediate temperature (IT)-SOFCs, e.g. scandia-stabilized zirconia (ScSZ), can solve this problem. ScSZ, owing to its high ionic conductivity, is of great interest as a potential low temperature electrolyte. Within this work layers of YSZ and of nanostructured YSZ were deposited on metal substrates using plasma spraying. As all thermal sprayed coatings contain some porosity, which influences the cell performance, the sprayed electrolyte layers were sintered in a second step. Conventional sintering of electrolytes is performed over several hours at temperatures above 1400°C. Nanostructured material assisted in enhancing the kinetics for sintering and grain growth. Plasma-sprayed coatings were under compressive stresses. In order to achieve a better understanding of differences in the sintering behaviour, plasma sprayed conventional YSZ and nanostructured YSZ layers were sintered under constrained and non-constrained conditions in the temperature range of 800 to 1520 °C and characterised. Sintering properties, microstructure, and conductivity of sprayed and sintered YSZ electrolyte layers were investigated by SEM, 4-point dc method, and mercury intrusion porosimetry.