Stability and cation diffusion in BaZrO3-based proton conducting solid state electrolytes

Abstract

Fuel cells stands as an attractive technology to efficiently convert chemical energy (fuels) to electricity. Protonic ceramic fuel cells have gained considerable interest in recent years due to higher fuel utilization at lower operation temperatures compared to conventional solid oxide fuel cells. It is the electrolyte, which determines the operation and properties of a fuel cell. Proton conducting yttrium-doped barium zirconate (BZY) is a promising solid electrolyte for fuel cells and electrolyzers with high proton conductivity. However, BZY-materials have not been 0characterized sufficiently with respect to chemical and mechanical stability.

The initial part of the thesis was dedicated to the processing of dense and phase pure BZY-ceramics from spray pyrolyzed powders. In order to evaluate the stability of BZY-materials, reaction of BZY materials with CO2 and steam and cation diffusion in BZ and related ceramics were investigated. The Vickers indentation method was introduced to monitor the changes in mechanical properties and chemical stability of BZY materials exposed to CO2 or steam. It was demonstrated that BZY-materials exposed to CO2 exhibited chemical and mechanical degradation due to the formation of BaCO3 and Ba-deficiency in BZY lattice. In contrast to the effect of reaction with CO2, hydration of BZY resulted in toughening of the ceramics despite the transient stress associated with hydration/dehydration. Hydration caused the grain boundaries of BZY ceramics to become robust compared to the grains evidenced by a change in fracture mode from intergranular to transgranular. The cation transport study in pure barium zirconate (BZ) highlighted apparent differences between barium and zirconium cations, the latter being much slower, what also reflects the high sintering temperature required for BZ-materials. Diffusion of both cations occurred through barium vacancies. Systematic studies of the cation mobility and activation energy in BZ and similar AIIBIVO3 perovskites demonstrated the importance of crystal symmetry and cation size.