Synthesis and property study of Ni-based cathod for IT-SOFCs

Abstract

This thesis focuses on improvements of cathode materials for IT-SOFCs through an investigation of new synthesis route and new materials with A and B site substitution on Ni-based perovskite structure. The work was divided into three parts. The first part focuses on an investigation of a new synthesis route to obtain IT-SOFCs cathode La₂_ₓSrₓNiO₄ with improved properties. In this work, La₂_ₓSrₓNiO₄ (κ ≤0.8) were, for the first time, successfully synthesized via a simple room-temperature sol-gel process using a water-based solvent. The La₂_ₓSrₓNiO₄ compounds were found to be more stable than the non-substituted La₂NiO₄ with more superior properties in term of thermal expansion coefficient matching to that of electrolyte and in term of electrical conductivity. The second part involved a more in depth investigation on the effect of Sr substitution on oxygen content and structure in relation to transport properties of the materials synthesized in the first part. Oxygen tracer diffusivity was found to follow an increasing trend with increasing oxygen content. Consequently, the highest diffusion coefficient was found for the minimum amount of Sr substitution and continuously decreasing with x until x = 0.6. An unusual increase in D* was observed when the Sr content increased up to x = 0.8. The last part of the thesis focuses on the development of two new cathodes. La₃Ni₂NbO₉ and La₃Ni₂TaO₉ double perovskite materials were successfully synthesized in this work. However, low conductivities of the two new compositions were found and thus, further improvements are needed.