Synthesis And Properties Of B Site Doped LnGaO₃ -Based Solid Electrolytes (ln=la,pr,Nd)

As a new clean energy generator, Solid Oxide Fuel Cell (SOFC) converts the chemical energy of fuel directly to electrical energy. SOFC attracts much attention in many countries because of its advantages such as wide selection of fuels, low pollution, high efficiency, no noise, no leakage, long lifetime, and so on.SOFC is composed of a dense electrolyte sandwiched between anode and cathode. As an important component, electrolyte determines the operating temperature of the SOFC, materials for other components, as well as the performance of the cell. Rare earth gallates possess high ionic conductivity and low electron or hole conductivity which are stable in a wide range of oxygen partial pressure, indicating its great potential for the electrolyte of SOFCs operated at mid- and low-temperatures.In this paper, LnGaO₃ (Ln=La,Pr,Nd) are selected for investigation. LnGa1-xMgxO₃ (Ln=La,Pr,Nd; x = 0,0.05,0.1,0.15,0.2,0.25) electrolytes were synthesized successfully by traditional solid state reaction. Ga doping at Mg sites introduces oxygen vacancies in electrolytes, distorts the lattice, and changes the volume of unit cell and activation energy, which exerts influences on conductivity. The solution limits of Mg in LaGaO₃, PrGaO₃ and NdGaO₃ are 0.2, 0.2 and 0.15, respectively. LaGa_0.8Mg_0.2O₃ possesses the highest conductivity of 10.14×10-2S/cm at 1073K, slightly lower than the optimal one, i.e., La_0.8Sr_0.2Ga_0.83Mg_0.17O_3-δ with the conductivity of 0.17S/cm at 1073K. The ion transport number of LaGa_0.8Mg_0.2O₃ increases with increasing temperature and close to 1 in the range of 973K¹173K, e.g., 0.98 at 1173K. The highest conductivity in PrGa_1-xMg_xO₃ series lies at x = 0.1, which is 8.14×10-2 S/cm at 1073K. As for NdGa_1-xMg_xO₃, the conductivity attains the maximum of 1.96×10-2 S/cm for x = 0.15 at 1073K. The activation energy of PrGa_1-xMg_xO₃ is the lowest among the three series, for instance, 0.41eV for PrGa0.9Mg0.1O₃. At temperature lower than 1033K, PrGa0.9Mg0.1O₃ exhibits higher conductivity than LaGa_0.8Mg_0.2O₃, which is the highest in this temperature range.Transition metal Co doping increases further the conductivity of LaGa_0.8Mg_0.2O₃ at mid- and low-temperature, while at high temperature, the conductivity decreases first, then increases with Co content. Moreover, Co doping lowers the activation energy.Order-disorder transition of oxygen vacancies happens at 873K in some electrolytes, resulting in the change of thermal expansion coefficient. This is in consistence of the conductivity results, which signals the transition between high conducting and low conducting phases. The open circuit voltage of H₂-O₂ SOFC based on LaGa_0.8Mg_0.2O₃ electrolyte is above 1 V, indicating the dominant role of ion conducting. However, the power density is rather too low for the large resistance of Ag electrodes.Above all, the conductivity of Mg-doped rare earth gallates is higher than YSZ and is potential candidates for electrolyte in mid- and low-temperature SOFCs.