Preparation And Characterization Of Perovskite-type Oxide Catalyst

Because the high cost of the platinum-based catalysts limits the commercializa-tion of the Polymer Electrolyte Membrane Fuel Cell (PEMFC) applications, to find a non-precious catalyst becomes one of the major objectives for the PEMFC research fields. This paper chose perovskite-type oxides as the possible non-precious metal catalysts because of their low price and simple synthesis techniques. A powder of La0.65Sr0.3MnO3 (LSMO) with the diameter of 50 nm has been synthesized by sol-gel method. It's electrocatalytic activity for oxygen reduction of the resulting compound were characterized, the results indicated that:(1) The optimum conditions of catalyst preparation were found by studying the influences of the sol-gel process parameters such as pH values, amount of citric acid. The catalyst with good performance were prepared as follows:the molar ratio of the mixed citric acid and metal ion is 2:1, pH value is 3. The appropriate calcination process is 25℃-400℃,holding for 1h, then up to 700℃, holding for 1h. After calcinations, pure perovskite-type oxides with diameter of 50nm are obtained.(2) The result of the linear sweep indicates that La0.65Sr0.3MnO3 exhibit catalytic activity for Oxygen Reduction Reaction in 0.1 M H2SO4, the onset potential is 0.5 V (vs SCE). Adding carbon powder with the catalyst could enhancing the catalytic activity, and with 20 wt% carbon powder BP2000, the catalyst shows better activity(0.670mA/cm2@-0.4V (vs.SCE)) than that of carbon powder XC-72(0.507mA/cm2@-0.4V (vs.SCE)), because BP2000 has larger specific surface area.(3) The oxygen reduction rate on La0.65Sr0.3MnO3 catalyst is determined by the diffusion of oxygen in the electrolyte.(4) The solubility of La0.65Sr0.3MnO3 in the 25℃,0.1mol L-1H2SO4 is 6×10-5mol/L. Comparing to commercial Pt/C catalyst, the solubility of La0.65Sr0.3MnO3 is larger by an order of magnitude. Thus the stability of La0.65Sr0.3Mn03 needs further improvement. (5) site cation segregation occurs on the surface of La0.65Sr0.3MnO3 powder, and Sr is preferentially segregated, at the same time there would produce cation vacancies in the bulk phase. Under cathodic polarization, the oxygen vacancies would form in the oxides, enhancing the oxygen dissociative adsorption and diffusion on the Lao.65Sr0.3MnO3 powder, thus accelerate the oxygen reduction reaction.