| When energy source and environment are in contradiction, solid oxide fuel cell(SOFC), as an all solid-state device, causes more and more attention due to its high comprehensive energy utilization rate, wide range of fuel source and low-cost.Therefore, research on SOFC has also been booming.La Ga O3-based solid electrolyte(LSGM) has a high ionic conductivity, good chemical stability in oxidation and reducing atmosphere at high-temperature, hence it has been considered as an electrolyte with great potential for development. However,the conductive activation energy of this material is high, and not suitable for media- and low-temperature use. Furthermore, its pure phase is difficult to be synthesized, which reduces the overall performance. Previous reports show that doping at A sites with small rare earth ions such as Pr and Sm can increase the orthorhombic-rhombohedral phase transition temperature; the structure of La Ga O3 becomes close to cubic when doping with 30% Sm, and such a high symmetry structure facilitates the ion transport; the slight decrease of the radius of A site ions in La Ga O3 can decrease the conductivity activation energy. As a result, doping Sm or Pr for La in La Ga O3 can improve the performance at media- and low-temperatures.Sm doped LSGM samples have been synthesized by traditional solid-state reaction.When Sm content increases, peaks in XRD patterns gradually move to higher angle, and no raw material peaks were found, indicating that Sm3+ ions entered the lattice of La Ga O3. Different sintering conditions are suitable for different systems.La0.85Sm0.05Sr0.1Ga0.85Mg0.15O3 sintered at 1400 °C for 12 h possesses the lowest activation energy of 78.80 k J/mol, and the energy barrier for oxygen ions to overcome during migration is 60.67 k J/mol. Its conductivity is 3.2×10-2 S/cm at 800 °C, and 1.5× 10-2S/cm at 700 °C. La0.8Sm0.1Sr0.1Ga0.83Mg0.17O3 synthesezed at 1400 °C for 4 h possesses the highest conductivity, i.e., 3.8 × 10-2 S/cm at 800 °C, 1.6×10-2 S/cm at700 °C, and corresponding activation energy is 83.17 k J/mol.High temperature and high pressure have been used to avoid impurities created in solid state technique. The impurity content is much lower in samples obtained under high temperature and high pressure compared with solid state reaction. Furthermore,synthesizing samples under high temperature and high pressure condition needs shorter time and lower temperature, indicating its high efficiency. Due to theorthorhombic-rhombohedral transition experienced at high pressure and the residual strss in LSGM, the conductivity is lower for high pressure samples than for solid state technique ones. Sm doping can reduce the activation energy of LSGM, e.g., the activation energy of La0.75Sm0.15Sr0.1Ga0.8Mg0.2O3 is 51.43 k J /mol, much lower than undoped sample La0.9Sr0.1Ga0.8Mg0.2O3(105.45 k J/mol). Dielectric relaxation peaks of loss are related to the migration process of oxygen ions in lattice, and different process corresponds to different peak. The intensity of loss of high pressure samples is significantly lower than samples obtained by solid state technique, indicating the crack of grains during preparation hinders the transport of oxygen ions, hence the conductivity is lower.Pr-doped La Ga0.9Mg0.1O3 electrolytes were successfully synthesized under high temperature and high pressure conditions. All the samples are orthorhombic,corresponding to Pbnm space group. Retieveld refinement results show that the larger the bottle neck of transport passage for oxygen ions and the smaller the thickness of energy barrier are, the lower the conducting activation energy is. The relationship between activation energy and the thickness of energy barrier is linear. Sample La Ga0.9Mg0.1O3 possesses the lowest activation energy of 45.73 k J/mol, which is more suitable for application at lower temperatures. |
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