Studies On Oxygen Reduction Reaction Of Pt/C Electro-catalyst Modified By Fe₃O₄

Electro-catalyst is one of the key materials in direct methanol fuel cells and strongly affect the performance,stability and durability of cells. Exploring cathodic electro-catalysts with low cost and methanol tolerant has important value both in theory and application for alkaline batteries. The study applies magenatic material Fe₃O₄, which has a cubic inverse spinel structure and ferro-magnetism, to reform the traditional cathodic catalyst (Pt/C) to enrich oxygen and acquire high oxygen reduction activity.Firstly, nano Fe₃O₄ and carbon coating (Fe₃O₄/C) is prepared by co-precipitation method and ethyl acetate treatment. The samples were characterized by XRD (x-ray diffraction) and TEM (Transmission Electronic microscope). The results show that all the samples are in the range of nano meters and uniform in morphology. The size bout is 11.2 nm and 14.3 nm, respectively.Modified Pt/C electro-catalysts with different amount of Fe₃O₄ were prepared by ultrasonic blending method. XRD and TEM results show additive of Fe₃O₄ can improve the dispersion of Pt. Better dispersion of Pt appears for the Fe₃O₄/C-Pt/C catalyst then Fe₃O₄-Pt/C because of the compatibility of carbon film.The cyclic voltammetry under nitrogen and the linear voltammogram under air and oxygen conditions for two samples were used to test the oxygen reduction activity. The results show that adding appropriate amount of Fe₃O₄ to Pt/C can improve the activity sites of the catalysts, the starting oxygen potential and the maximum current density. The best oxygen reduction activity of the modified catalyst appears when the content of Fe₃O₄ is 7wt% for both catalysts. Under the air and oxygen condition, the oxygen-reduction potential of Fe₃O₄/C-Pt/C improves about 41mv and 30 mv and the maximum current density is about 13.0 mA cm-2 and 12.45 mAcm-2. Pt/C catalyst modified by Fe₃O₄/C shows better ORR activity the by Fe₃O₄.Tafel curve results of Pt/C and Fe₃O₄/C-Pt/C catalysts show that the mix potential are 0.473 V and 0.525 V (vs.Hg/HgO), respectively. The two peaks are ascribed to two-electron and four-electron reduction of O2. The adding of Fe₃O₄ and its promotion in 4-electron direct oxygen reduction reaction (ORR) is confirmed. AC impedance tests show 38.086 ? and 17.768 ? for the two catalysts, which in turn support that the additive of Fe₃O₄ accelerates oxygen reduction reaction.Curie temperature is the critical temperature from ferro-magnetism to paramagnetism. Curie temperature for Fe₃O₄ in 10-40 nm is about 120 - 140oC. The Pt/C catalysts modified by Fe₃O₄ under 3 h heat treatment of 80-220 oC illustrate different ORR activity. ORR activity gets better from 80-140 oC and then gets worse to 220 oC. The starting oxygen reduction potential is 0.608 V, Taffel mix potential is 0.55 V (vs.Hg/HgO),AC impedance is 15.447 ?. The results suggest that 140 oC is possiblely the maximum temperature for Fe₃O₄ to keep ferro-magnetism. The ferro-magnetism of Fe₃O₄ leads to the enrichment of oxygen and therefore, improve the ORR activity.