Synthesis, Characterization And Electrocatalytic Performance Of Anode Catalysts For Direct Methanol Fuel Cells

Direct methanol fuel cell(DMFC)is believed to be one of the ideal small or mobile energy sources due to its high energy conversion efficiency.However,there are two urgent problems faced in DMFC:one of which is that the electrocatalytic activity of methanol at anode is low at low termperature;the other is that the penetration of methanol from anode to cathode is quite severe.Therefore it is a urgent need to prepare a kind of anode catalyst with high electrocatalytic activity for methanol oxidation.Transition metal Pt is the first candidate for anode catalysts of DMFC due to their excellent catalytic activity.As is well known,catalytic activity depends on the size,particle distribution,surface area and the shape of the metal particles,and therefore the synthesis of well-controlled shapes and sizes of Pt nanocatalysts could be critical for improving catalytic activity.In this article,research has proceeded mainly with preparation methods of carbon supported Pt catalysts.Highly dispersed carbon supported Pt nanoparticle catalysts have been prepared by microwave polyol process and the effect of the synthesis solution pH and the addition of a small amount of sodium acetate on the Pt particle size and size distribution have been investigated;Pt-CeO₂/C and Pt-ZrO₂/C catalysts have been one-step synthesized by microwave polyol process;carbon supported hollow Pt nanosphere catalyst has been prepared by a metal replacement reaction at room temperature.The structrue and composition of catalysts have been characterized by energy-dispersive X-ray spectroscopy(EDX),transmission electron microscopy(TEM)and X-ray diffraction(XRD).And catalytic activity of methanol oxidation has been investigated by electrochemical analysis.The original results obtained are as follows: 1)Pt/XC-72 and Pt/CNTs nanoparticle catalysts have been successfully prepared by a microwave polyol process.The results demonstrate that the as-synthesized Pt nanoparticles are homogeneous and highly dispersed on surface of XC-72 carbon and carbon nanotubes due to rapidness and uniformity of microwave heating.TEM results show that the average Pt particle size of microwave-synthesized Pt/XC-72 and Pt/CNTs catalysts is 3.2 and 2.7 nm respectively.Electrochemical measurements show that microwave polyol synthesized Pt/XC-72 and Pt/CNTs catalysts have higher electrochemical active area and better electrocatalytic activity for methanol oxidation compared with KBH4-reduced Pt/XC-72 catalyst.2)XC-72 carbon supported size-controlled Pt have been synthesized by microwave polyol process through adjusting the synthesis solution pH or adding a small amount of sodium acetate as stabilizer.The effects of pH and the content of sodium acetate on the mean particle size,size uniformty and size ditribution of microwave synthesized Pt nanoparticles have been investigated.The results demonstrate that the mean size of microwave synthesized Pt nanoparticles can be controlled by adjusting pH value or the amount of acetate added in the synthesis solution.When the synthesis solution pH is 9.5 or 0.5 mL 1 M sodium acetate is added into 50 mL synthesis solution,microwave synthesized Pt particles have uniform particle size with the mean particle size of 2.7 nm and 2.6 nm respectively and highly disperse on the carbon support.Mcrowave polyol synthesized Pt/XC-72 catalysts have higher electrochemical active area and better electrocatalytic activity for methanol oxidation.3)Pt nanoparticles supported on carbon nanotubes(CNTs)have been synthesized by microwave polyol process through adjusting the synthesis solution pH. The research results demonstrate that the mean size of microwave synthesized Pt particles gradually decreases and the size uniformty and dispersion uniformty on CNTs surface get improved with increasing the synthesis solution pH. Electrochemical measurements demonstrate that the elctrochemical surface area and electrocatalytic acitivity for methanol oxidation of Pt/CNTs increases when the synthesis solution pH increases from 3.6 to 7.4.The electrocatalytic acitivity for methanol oxidation decreases,though the elctrochemical surface area and electrocatalytic acitivity for methanol oxidation of Pt/CNTs increases when pH increases to 9.2.The preliminary analysis propose that Pt particles with the right size can be synthesized under the right synthesis solution pH value,which can make Pt surface reach a right co-adsorption ratio of methanol and water to optimize the electrocatalytic activity for methanol oxidation.4)Hollow Pt/C catalyst has been prepared by a metal replacement reaction at room temperature.TEM results show that Pt nanospheres are hollow nanostructure, which are composed of some smaller discrete Pt nanoparticles.The average diameter of hollow Pt nanospheres is statistically calculated to be 24.5 nm and the thickess of shell is about 2.3 nm.XRD and EDX results show that hollow Pt nanospheres are faced-centered cubic(fcc)with the Pt loading of 20%.The electrochemical active surface area of hollow Pt nanospheres is 138.7 m²/g,which is much higher than that of solid Pt/C catalyst(73 m²/g).Based on the study of methanol oxidation,it is found that the hollow Pt/C catalysts display much higher electrocatalytic activity for methanol electrooxidation than the E-TEK Pt/C and SN-Pt/C electrocatalysts.5)Pt-CeO₂/C and Pt-ZrO₂/C catalysts have been one-step prepared by microwave heating of polyol solutions of H₂PtCl₆,(NH₄)₂Ce(NO₃)₆ or Zr(NO₃)₄ and sodium acetate as stabilizer.Characterization results show that Pt nanoparticles of microwave polyol one-step synthesized Pt-CeO₂/C and Pt-ZrO₂/C catalysts are ultrafine and highly dispersed on carbon support.Compared with Pt/C(E-TEK) catalyst,the onset potentials of methanol oxidation of Pt-CeO₂/C and Pt-ZrO₂/C catalysts shift to the negative potential and the peak currents of Pt-CeO₂/C and Pt-ZrO₂/C catalysts for methanol oxidation become much higher.The further studies show that Pt-CeO₂/C catalyst has higher electrochemical surface area and better electrocatalytic activity for methanol oxidation.This is due to that the interaction of Pt and CeO₂ or ZrO₂ lead to form a large quantity of oxygen vacancies on the surface of oxide,which can oxidized CO and release active sites on Pt surface to enhance the ability to resist CO posioning and improve the electrocatalytic performance of Pt/C catalyst.