Manufacturing of pyrochlore (A2B2O7) nanoparticles using sol gel method

Pyrochlores have been considered as an important class of electrolyte materials for solid oxide fuel cell (SOFC), gas sensors and oxygen separation membranes. In this research, ionic conductivities of A2B2O7 type materials have been investigated to check their suitability as an electrolyte materials of SOFC. Nanopowders of four different pyrochlore materials have been synthesised using sodium alginate mediated sol-gel process, and their properties have been evaluated for use in the intermediate-temperature Solid Oxide Fuel Cell (IT-SOFC) applications. Sodium alginate based ion exchange process was used to synthesise high purity nanopowders of holmium hafnate (Ho2Hf2O7), holmium zirconate (Ho2Zr2O7), lanthanum hafnate (La2Hf2O7) and lanthanum zirconate (La2Zr2O7) after calcining the dried gel. Dried gel was analysed using simultaneous thermogravimetric analysis and differential thermal scanning calorimetry (TGA/DSC) and high temperature X-ray diffraction (HT-XRD) to determine the evolution of crystallite structure as a function of calcination temperature. Nano-crystalline powders of high purity, single phase holmium hafnate and holmium zirconate were obtained by calcining the dried gel at 700°C and lanthanum hafnate and lanthanum zirconate at 900°C. Rietveld refinement of X-ray diffraction (XRD) data confirmed the formation of single phase defect fluorite structure of Ho2Hf2O7 and Ho2Zr2O7 and evidence of presence of oxygen ion vacancy was supported by Raman spectroscopy. La2Hf2O7 and La2Zr2O7 has the cubic pyrochlore structure as confirmed by XRD and transmission electron microscopy (TEM).
Microstructural and ac-impedance analyses of sintered pellets were carried out to confirm their suitability for fuel cell applications. Holmium based materials can be considered as a promising new oxide ion conducting solid electrolyte for IT-SOFC. Lanthanum based materials showed lower ionic conductivities than holmium based materials.
Process description of the sol gel method along with theoretical detail on kinetic study of diffusion have also been provided. Evolved gas analysis (EGA) from TGA were also carried out for holmium hafnate to estimate from the gaseous emission for mass and energy balance of the process.
Sodium alginate based sol-gel method, also named as Leeds alginate Process (LAP), has the capability of consistently yielding the single phase nanoparticles of electrolyte materials for SOFC in environmentally sustainable, cost effective and energy efficiently manner.