Development of Gadolinium Doped Ceria (GDC) Electrolytes and Effects of Lutetium (Lu) Addition for Intermediate Ttemperature Solid Oxide Ffuel Cell (IT-SOFC) Applications

Abstract

Solid oxide fuel cells (SOFCs) have attracted a great attention due to its higher conversion efficiency, high waste heat utilization, greater fuel flexibility and the ability of environmental friendship. Yttria stabilized zirconia (YSZ) is the common material used as an electrolyte for high temperature solid oxide fuel cells (SOFC). Due to high operating temperature, various design issues arise. Therefore, low to intermediate temperature solid oxide fuel cells (SOFCs) are actively being developed. Ceria based materials are among the best options for intermediate temperature SOFC (500-800˚C). In the present study, successful synthesis of gadolinium doped ceria (GDC) electrolyte material with a formula Ce1-xGdxO2-x/2 (x=0.15) was carried out by two different techniques namely sol-gel and co-precipitation. Both techniques have the advantages of high phase purity, homogeneity and low processing temperatures along with strong control on crystallite size. The synthesized materials were characterized by X-ray Diffraction, Scanning Electron Microscopy with EDX etc. We have detected a cubic fluorite structure for both synthesized compounds. The co-precipitation synthesis resulted in better control on crystallite size in comparison to that by sol-gel route. The average particle size was found to be in the range of 40-70 nm. The electrical conductivity of the GDC was measured in the temperature range of 220-660 ˚C in air by using two probe LCR meter. GDC synthesized by sol-gel method was found to have highest conductivity value of 2.45 10-4 S/cm at 600 ˚C with activation energy of 0.31 keV. Moreover, the effects of lutetium (Lu) addition on structure, morphology and electrical properties of gadolinium doped ceria (GDC) electrolyte were investigated. Gd and Lu co-doped ceria Ce0.85Gd0.15-xLuxO2-δ (at x = 0.00, 0.05, 0.075, 0.10 and 0.15) electrolytes were synthesized by solid state reaction method. All the synthesized materials were found to be single phase with cubic fluorite structure. The most intense peak crystallite size was calculated by Scherrer equation for all samples. The elastic strain present in the Ce0.85Gd0.10Lu0.05O2-δ sample is found to be negligible as compared to singly doped Gd and Lu samples. The grain sizes were examined by SEM images and it is found to be in the range of 30-60 nm. The electrical conductivity as a function of temperature and compositions were measured in air in the temperature range of 200 to 600°C and found to be linearly rising with the ii increase in temperature. The electrical conductivity of samples varies with the increase in dopant (Lu) content. The maximum conductivity was observed to be 0.0217 S/cm at 600°C for Ce0.85Gd0.10Lu0.05O2-δ sample, which is slightly higher than that for pure gadolinium doped ceria Ce0.85Gd0.15O2-δ and quite higher than for pure lutetium doped ceria Ce0.85Lu0.15O2-δ.