The Reduction of NiO-YSZ Anode Precursor and Its Effect on the Microstructure and Elastic Properties at Ambient and Elevated Temperatures

Abstract

NiO-8YSZ anode supported half cells were reduced for different periods of time at 800°C in 5% H2 environment. The reduction of NiO is associated with significant changes in the crystalline phases, porosity and microstructure and hence in elastic properties. The Young’s modulus was determined after reduction at room temperature and at elevated temperatures using the Impulse Excitation Technique. The Young’s modulus was found to be decreasing with increasing porosity. The decrease in the Young’s modulus was about 50% and can be attributed mainly to the changes in the microstructure particularly increasing porosity from ≈12% to 37% after 8 hrs of reduction. The elastic properties of the reduced anodes were estimated at elevated temperatures in air. INTRODUCTION Recently, there have been tremendous research interests on the development of solid oxide fuel cells (SOFCs) due to its potential as a highly efficient source of environmentally clean energy. SOFCs, which convert the chemical energy of a gaseous fuel into electricity, have a basic structure of a highly dense ion-conducting electrolyte layer, sandwiched between electron conducting anode and cathode layers. Since the electrolyte is kept as thin as possible to decrease ohmic losses through it and to lower the operating temperature of the cell and the cathode is a weak and compliant material, 12 often in certain designs the anode becomes exclusively responsible for the structural integrity and reliability of the cell. In general, Ni-Y2O3 stabilized ZrO2 (Ni-YSZ) based porous cermet is the most favored anode material in SOFCs, due to its enviable properties of excellent electro-catalytic activity for the H2 oxidation reaction at the triple phase boundary (TPB) between Ni, YSZ and gaseous H2, high electronic conductivity, high mechanical stability and compatibility with YSZ electrolyte, and structural stability at high operating temperatures (700 to 1000°C) in H2 atmosphere. The stability and performance of the Ni-YSZ anode structure is crucially dependent on the phase distribution and microstructure in the cermets. Usually, the Ni-YSZ anode structure is fabricated by reducing a NiOYSZ anode precursor structure which readily generates the desired porosity, microstructure, and mechanical strength in the anode. It is observed that the microstructure in the Ni-YSZ cermets is considerably dependent on the composition, morphology and process of synthesis of the precursor, resulting in significant changes in the overall performance of the SOFC. For the optimum performance of the anode, at least 40 vol% of Ni 21,22 is required along with a homogeneous distribution of Ni and YSZ phase. The distribution of the metal ions controls the coefficient of thermal expansion (CTE) of the anode and determines the structural stability of the SOFC during operation. Although, small particle size is advantageous for obtaining high length of triple phase boundary (TPB), however, enlarging the YSZ particle size helps to diminish the CTE effects and often results in a well connected YSZ structure, thereby improving the thermo/electro-mechanical performance of the anode. However, the themomechanical properties, particularly the elastic properties are sensitive to porosity and the decrease of Young’s and shear moduli of NiO-YSZ with porosity have been reported earlier by Seluk and Atkinson and Radovic and Lara-Curzio. However, the reported data on the elastic properties are determined mostly at the ambient temperature.