Densification, microstructure and electrical properties of auto-combustion derived nanostructured NiO/Ni-8YSZ composites for IT-SOFC application

Abstract

The development of nanostructured NiO/Ni-8YSZ powders has been a major area of research to bring down the operating temperature of SOFC into intermediate range due to the better percolation of fine NiO/Ni particles in 8YSZ matrix and enhancement of the triple phase boundary. In this work, NiO-8YSZ nanopowders were successfully synthesized by a low cost combustion method using metal nitrates and glycine as oxidizer and fuel respectively, at a calcination temperature of 650°C. The crystallite sizes for NiO and 8YSZ were calculated to be 15.2nm and 19.4nm respectively. With increase in calcination temperature, the NiO crystallites exihibited higher tendency to grow as compared to 8YSZ. The powders showed a very good sinterability at a temperature as low as 1200°C and a density of 87% was achieved by manipulating the binder concentration and compaction pressure. The presence of homogeneously distributed NiO and 8YSZ grains were observed from the back scattered SEM micrographs. The average grain size of the NiO and 8YSZ increased with increase in sintering temperature. The complete reduction of NiO into Ni was confirmed from the XRD, after heating the samples in hydrogen atmosphere. The hydrogen fired porosity was found to vary linearly with air fired porosity. The thermal expansion coefficient (TEC) was found to be 12.5×10-6/°K. The presence of well-connected grains with sufficient porosity was observed from the SEM micrographs of Ni-8YSZ. The measurement of electrical conductivity in addition to TEC and microstructure showed the effectiveness of this method and material for use as anode in IT-SOFC.