Investigation On The Properties Of (La, Pr) SrMnO Perovskite Cathode Materials Of Solid Oxide Fuel Cell

As the important part of solid oxide fuel cell (SOFC), the properties of the cathode will directly affect the performance of solid oxide fuel cells. The resistance losses in cathode are main part of the overall resistance losses of the cell. For example, more than half part of losses of cell is due to the polarization resistance on the cathode. So investigating and improving the properties of the cathode materials are the important task in developing the solid oxide fuel cell.Currently the most investigated and widely used cathode materials is the ABO₃-type perovskite structure oxides, which possesses the mixed ionic-electronic conducting properties, better thermal stability in high temperature, and perfect accordance with the electrolyte materials. These cathode materials can be applied in the high temperature solid oxide fuel. High temperature electrical conductivity is the very important properties of the cathode materials. Generally, electronic conductivity of the ABO₃-type perovskite structure cathode materials is due to the variable valences of the ions on the B-site, which is followed by the small polaron hopping mechanism. However, whether the ions on the A-site contributing to the electronic conductivity of the materials are not reported completely. We prepared three series of cathode materials: La_0.7-xPr_xSr_0.3MnO₃, La_0.6-yPr_ySr_0.4MnO₃ and La_1-zPr_zMnO₃. By gradually replacing La with Pr, we study the ions'contribution to the conductivity of cathode materials on the A-site.La_0.7-xPr_xSr_0.3MnO₃, La_0.6-yPr_ySr_0.4MnO₃ and La_1-zPr_zMnO₃ are prepared by sol-gel process. Except for La0.1Pr0.6Sr0.3MnO₃, Pr_0.7Sr_0.3MnO₃, La_0.3Pr_0.7MnO₃, La_0.2Pr_0.8MnO₃, La0.1Pr0.9MnO₃, and PrMnO₃, which have little Pr6O11, all the other samples are found to be single phase. The result demonstrated lattice volumes have some contraction when the Pr doping increase, which is because the radius of Pr3+ is smaller than La3+. All the samples have the measured densities well above 90% of the theoretical values. The value of the TEC is smaller with the doping of Pr in the series of La_0.7-xPr_xSr_0.3MnO₃, but the TEC of La_0.6-yPr_ySr_0.4MnO₃ have some increase. When La and Pr are both in the A-site, high temperature electrical of the cathode materials prepared is larger than La or Pr. In the series of La_0.7-xPr_xSr_0.3MnO₃, LPS3M5 has the largest electrical conductivity, up to 183 S/cm in the 850℃. LPS4M5 has the highest electrical conductivity in the series of La_0.6-yPr_ySr_0.4MO₃, 145 S/cm. Electrical conductivity of LPM4, LPM5 and LPM6 in the series of La_1-zPr_zMnO₃ cathode materials are larger, 109, 111 and 110 S/cm respectively. Activation energy of the two series of materials, La_0.7-xPr_xSr_0.3MnO₃ and La_0.6-yPr_ySr_0.4MO₃, both has a trend that first increases and then decreases. Activation energy of the La_1-zPr_zMnO₃ changes in a decreasing trend.Investigation on the electrochemical properties of La_0.7-xPr_xSr_0.3MnO₃ and La_0.6-yPr_ySr_0.4MnO₃ is started that: in the intermediate temperature range (600℃). The impedance of cathode materials doping Pr is larger in the La_0.7-xPr_xSr_0.3MnO₃. But in the high temperature range (800℃), LPS3M3 has lower impedance, about 2.31Ω·cm², larger polarization current density, 2.52 A/cm². In the measured temperature range, the total impedance of LPS4M0 is lowest, 1.24Ω·cm² (800℃), highest polarization current density, 4.69 A/cm². With the Pr doping increasing, the value of the total impedance of the both series of cathode materials has a trend of first increases and decreases.