Studies On Cathode Catalysts And Kinetics Of Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell(PEMFC) has attracted significant attention of many developed nations at both research and development for its special property of high energy density, low operation temperature, no corrosion problem and its prospective application to serve as clean power generation, transportation, space navigation, china has also attached great importance to this domain. The catalysts for proton exchange membrane fuel cell were prepared by using XC-72 carbon as supporter, the catalyst performance were enhanced by the way of supporter modification and platinum alloying, and then, the hydrogen-oxygen fuel cell were fabricated with Nafion117 as electrolyte, the effect of electrode structure and operation condition on the fuel cell performance were studied in this paper. The electrochemical dynamics of oxygen reduction at pt/Nafion interface were studied by using solid electrolyte electrochemical cell. And a model for mass transportation and electrochemical dynamics in catalytic layer was established. The transportation and reduction dynamics of oxygen in catalytic layer were simulated by the model.The preparation technology of electrode&membrane assemblies for PEMFC were studied, the effect of electrode composition on the fuel cell performance were investigated, the optimum composition were obtained with 40%PTFE in diffusion layer, Nafion 0.8mg/cm~2, platinum lmg/cm~2,PTFE 0.6mg/cm~2 in catalytic layer. The steady operation condition proton exchange membrane fuel cell studied, the results show that it improve the cell performance by increasing cell temperature, humidifying reaction gas and increasing operation pressure.The carbon supported catalysts were prepared by immersion-reduction method, the particle size of supported platinum were 3-6nm, the performance of prepared catalysts closed to the simple from oversea. The effect of different surface functional on the catalytic performance of supported platinum were studied. The results show that the electrocatalytic activity of oxygen reduction on the electrode were relevant to the property of surface functional. The C=O-functionalized electrodes play better electro-catalytic performance on oxygen reduction than the unfunctionalized. On the functionalized Pt-GC electrode, the C-N functionalized Pt-GC electrode show superior electro-catalytic activity to the nonfunctionalized, and the C=O-functionalized electrodes go the opposite. The reasons of functionalities effect on the electro-catalytic performances of Pt-GC electrodes were discussed from the point of view of electronegative and electrocatalysis of functionalities in this paper.The platinum based alloy catalysts were prepared by aquous aggradation and high tempreture alloying method, and the performances of alloy catalysts for PEMFC were tested. The results shows that the activity of catalysts were enhance by base metal. The activity of catalysts were relevant to the content of base metal. Both too higher and too lower content of base metal could not enhance the catalytic activity. The best content was atom ratio of Pt/M=3:1. XRD patterns for prepared catalysts shift positively compared to pure platinum. The structure determination show the shortening of Pt-Pt atom distance. Chemical shift of XPS pattern for Pt 4f electron were abserved, which indicated the transfer of electron cloud from platinum to base metal, as a result, it increased the 3d-vacancy of Pt, simultaneously quickened the adsorption and bond incision of dioxygen on platinum. The mechanism for base metal enhancing the catalytic activity for oxygen reduction on platinum were discussed. The electrocatalysis procedure for Griffith and Pauling adsorption of oxygen on platinum were proposed. That the improvement of catalyst performance were result from the change of structure and eletronic property of platinum by introducing basemetal were concluded.Carbon nanotubes supported catalyst were prepared by the way of aqueous reduction and support platinum onto carbon nanotubes. The catalysts were used to prepare gas diffusion electrode. Its electrocatalytic performance as a cathode for PEMFC were tested. The results show its catalytic activity were improved by the using of carbon nanotubes as supporter. The cyclic voltammograms of the catalysts were determined in H2SO4 solutions. The results show the stability of prepared catalysts in acid. After modification, the surface functional on nanotubes changed. Oxidation treatment richened oxygenic groups on carbon surface. However, after treated by ammonia, oxygenic groups decreased. And as a contrary the nitrogenous groups increased, which benefit to the dispersing of platinum. It was proposed that the special structure and surface property improved the activity of nanotubes supported platinum catalysts.The electrochemical dynamics of oxygen reduction at Pt/Nafion interface were studied by using solid electrochemical cell. At higher potential, the exchange current density was low. at lower potential, the exchange current density was high. That indicated different mechanism for oxygen reduction at different potential. Furthermore, the dynamic parameters were relevant to water content of Nafion. both solubility and diffusion coefficient of oxygen in Nafion increase with water content in Nafion. Impedance of oxygen reduction at Pt/Nafion interface and fuel cell cathode were determined. There were two relaxation in the impedance of oxygen reduction at Pt/Nafion interface. Membrane relaxation were relevant to water content of Nafion. When water content of Nafion increased, the grain boundary resistance minished. The membrane resistance mainly origin from bulk resistance. The fuel cell reaction were limited by electrochemistry at lower overpotential and were limited by diffusion at higher overpotential. The equivalent current for oxygen reduction at Pt/Nafion interface and fuel cell cathode were proposed.By using Stefan-Maxwell equation to describe molecular diffusion in multi-composition gas mixture, Butler-Volmer equation to describe oxygen electrochemical reduction in catalytic layer, Nernst-Plank equation to describe proton transfer in membrane, a steady state mathematic model was established. Membrane phase current gradually conversed to electron current in cathode catalytic layer and finally reached 0 at the boundary of catalytic layer and diffusion layer. At lower current density, the change of proton current were nearly consistent in catalytic layer. As current density increased, the current mainly generated at the region near diffusion layer/catalyst layer. At dimensionless thickness 0.5 it reached limited current of 1.1 A/cm2 and keep this current to catalyst layer/membrane interface. The oxygen fraction grads increased with the current density rised. At higher current density, most oxygen were consumed in the region close to diffusion layer and go down to 0 in the region close to membrane which formed a inactive domain. Electrolyte phase potential goes up from catalyst layer/diffusion layer to membrane. The overpotential show the same trend and reached the maximum at catalyst layer/membrane interface. The minimum appear at the catalyst layer/diffusion layer. The fuel cell performance were simulated by the established model, the experimental results were better described by the simulation output.