Preparation And Performance Of The Nanometer La_1-xSr_xMnO₃

The oxygen reduction catalyst is the key material of air electrode used in the metal-air battery and the electrochemical oxygen generator. In this paper, nanometer La1-xSrxMnO3 catalyst of perovskite type with high property was prepared by sol-gel method, and the preparation and properties of the material were studied as well.In this paper, the optimum conditions of catalyst preparation were found by studying the influences of the sol-gel process parameters on the crystal structure of the nanometer La1-xSrxMnO3, such as the value of pH, the temperature of water bath, the drying method of gel and the sintering mode of sol. The catalyst with good performance were prepared as follows: mixed the metal ion and citric acid together by molar ratio of 1:2, then the mixture was dissolved in 150ml water with original solution pH value of 0.5 to prepare the sol; then the water in the sol was vaporized for 5.5h in water bath at temperature of 80 ℃, and the gel is dried in vacuum for 10h at the temperature of 100 ℃. At last, the catalyst is prepared after roasting the gel for 3h at 700℃. The average diameter of the catalysts, observed by the Transmission Electron Microscope (TEM),is 20~50nm.The softly coacervated nanometer La1-xSrxMnO3 was separated by ultrasonic separating and supported on active carbon uniformly thus the usage ratio of the material was increased. The voidage of the electrode was improved and the three phases boundary was ameliorated by the thermal annealing process of air electrode, and the performance of catalyst was improved as a result.The influence of the doping Sr on the catalytic performance was studied by measuring the steady-state polarization curve of the air cathode. According to the results, the main factors affecting the catalyst's performance are the deformation of the La1-xSrxMnO3 structure by the doping of Sr, the concentration of oxygen vacancies in the crystal, the ratio of Mn4+/Mn3+ and the change of La1-xSrxMnO3 paramagnetism. The catalytic performance of La1-xSrxMnO3increased with the amount of doping Sr at first, and the performance was best when x=0.4, then it decreased. The current density reached 324mA/cm2 at -200mV vs Hg/HgO while electrolyte is alkaline solution at the normal tempreture.The study shows that there are two reaction processes in the air electrode: First, the oxygen vacancies in La1-xSrxMnO3 deacidize oxygen into HO2-, and the Mn4 +/Mn3+deacidizes HO2- into OH-. Second, active carbon with high surface area deacidizes oxygen into HO2- , and the La1-xSxMnO3 deacidizes HO2- into OH- With the associated catalyzing of active carbon and La1-xSrxMnO3, the oxygen is deacidized into OH" by two electrons reaction process and the transformation of four electrons is realized. The catalyzing of the highly performanced catalysts made reaction a quasi four electrons process.The catalyst was applied in the electrochemical oxygen generator and the effect was very good.