Gallium-doped lanthanum germanates as electrolyte material of solid oxide fuel cells

Abstract

In this study, the effects of various amounts of Ga³⁺ dopants on the densification, phase structure, microstructure and electrical conductivity of La_9.5Ge_6.0O_26.25 ceramics are examined. The incorporation of Ga³⁺ ions into the La_9.5Ge_6.0O_26.25 lattices leads to the retardation of the densification and an increase in the triclinic structure, caused by the evaporation of GeO₂ and the largeness of the size difference between the Ga³⁺ ions and Ge⁴⁺ ions. Of the compositions studied, the La_9.5Ge_5.7Ga_0.3O_26.1 ceramic sintered at 1450°C shows an electrical conductivity of 4.02 × 10⁻² Scm⁻¹ at 800°C, which is higher than that of an 8YSZ ceramic. The calculated thermal expansion coefficient (CTE) of this ceramic of 10.4 × 10⁻⁶ K⁻¹ appears to be compatible with those of the common adjacent materials used in solid oxide fuel cells (SOFCs). A single cell with a La_9.5Ge_5.7Ga_0.3O_26.1 electrolyte 0.51 mm in thickness is built and evaluated. The cell has R₀ and R_P values of 0.80 and 0.28 Ω·cm², respectively, at 850°C. The open circuit voltage (OCV) and maximum power density (MPD) of the single cell are recorded as 1.002 V and 0.24 Wcm⁻² at the measurement temperature of 850°C. Through the incorporating of a thick La_9.5Ge_5.7Ga_0.3O_26.1 layer, the cell performance of the single cell with the La_9.5Ge_5.7Ga_0.3O_26.1 electrolyte is superior to that of SOFC cells with a similar electrolyte thickness reported in the literature, thus qualifying the La_9.5Ge_5.7Ga_0.3O_26.1 ceramic as a potential electrolyte material for SOFCs.