Study Of Novel Anode Catalysts For Direct Ethanol Fuel Cells

Direct ethanol fuel cells (DEFCs) have attracted much attention for their potential applications as clean and mobile power sources due to their high energy density, low toxicity, abundant availability, portable and easy storage. However, the complete electro-oxidation of ethanol involves a 12-electron process, the cleavage of C-C bond and muche more adsorbed intermediates, which set a practical challenge for the effectiveness of catalysts. So electrocatalysts with high catalytic activity and low cost are needed for commercialization of DEFCs. This work has developed a series of catalysts in order to solve those problems. The innovative achievements in this work are as follows:1. Size-controllable tin oxide nanoparticles were prepared by microwave heating ethylene glycol solutions containing SnCl2 at atmospheric pressure. The particles were characterized by means of transmission electron microscopic (TEM), X-ray diffraction (XRD) studies. TEM micrographs show that the obtained material are spherical nanoparticles, the size and size distribution of which depends on the initial experimental conditions of reaction time, water concentration, and tin precursor concentration. The XRD pattern result shows that the obtained powder is SnO2 with tetragonal crystalline structure. On the basis of UV/vis, the formation mechanism of SnO2 nanoparticles is deduced. Moreover, the SnO2 nanoparticles were employed to synthesize Pt-SnO2/C catalyst. Cyclic voltammetry and chronoamperometry showed that Pt-SnO2/C have much higher catalytic activity and CO tolerance for ethanol electroxidation than that of commercial Pt/C(JM).2. SnO2/C composites were prepared by an intermittent microwave heating method and used as the support to prepare Pt-Rh-SnO2/C with different Pt:Rh atomic ratio. The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show that the as-prepared catalysts were composed of well-dispersed Pt-Rh-SnO2 nanoparticles with an average particle size less than 4nm. All the as-prepared catalysts have face-centered cubic (fcc) crystalline structure. With an increase of Pt:Rh molar ratio, the particle sizes are increasing. Electrochemical studies are carried out using cyclic voltammetry and chronoamperometry. The results show that the highest electrocatalytic activity and best stability for ethanol electrooxidation were obtained at Pt:Rh molar ratio with 3:1.3. A novel electrocatalyst structure of carbon nanotubes (CNT) coated with thin SnO2 and Pt (Pt/(CNT@SnO2)) has been successfully designed and prepared. Careful structural design allowed the Pt nanoparticles to homogeneously disperse on a SnO2 layer, which means that all of the Pt nanoparticles could be in direct contact with SnO2. Oxygen containing species (OHads), which can be easily produced on SnO2 by the decomposition of water, will conveniently react with the CO-like species produced from ethanol oxidation to free Pt active sites according to the bifunctional mechanism. We find that the amount of HCl added plays a key role in the preparation of coated layer. Without adding HCl, SnO but not SnO2 coated MWNTs is obtained due to the hydrolysis of the SnCl2 in water. In the case of adding the 0.15ml or 0.7 mL of 38% HCl, which can completely suppress the hydrolysis, a continuous SnO2 coating is formed on MWNTs. The CNT@SnO2 composites with a thin shell (about 2nm) are prepared by adding 0.7mL 38%HCl.4. The Pt/(CNT@SnO2) catalyst was prepared by first microwave heating H2PtCl6 in NaOH ethylene glycol solution and then depositing Pt nanoparticles onto CNT@SnO2 composites. High resolution transmission electron microscopy and X-ray diffraction show that crystalline SnO2 about 2 nm thick is coated uniformly on the surface of the CNTs. Pt nanoparticles of about 3.5 nm in diameter are homogenously dispersed on the SnO2 surface. Electrochemical studies were carried out using cyclic voltammetry and chronoamperometry. The results showed that Pt/(CNT@SnO2) catalysts have much higher catalytic activity and CO tolerance for ethanol electro-oxidation than that of Pt/CNT.5. Sulfated SnO2 modified multi-walled carbon nanotubes (MWCNTs) composites were successfully prepared by washing with the acetate solution, sulfating with H2SO4 solution and calcination without destroy the core-shell structure of SnO2/MWCNTs composites. The Pt-S-SnO2/MWCNTs catalyst is synthesized by pulse-microwave assisted polyol methods. X-ray diffraction and transmission electron microscopy show that sulfated SnO2 is coated uniformly on the surface of the CNTs. Pt nanoparticles of about 3.0 nm in diameter are homogenously dispersed on the S-SnO2 surface. The electrocatalytic properties of the Pt-S-SnO2/MWCNTs catalyst for ethanol oxidation reactions are investigated by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The results show that Pt-S-SnO2/MWCNTs catalyst exhibits higher catalytic activity for ethanol oxidation than Pt supported on non-sulfated SnO2/MWCNTs composites. The increase the catalytic activity and the utilization of Pt in Pt-S-SnO2/MWCNTs is due to the high proton conductivity of sulfated SnO2, which can decrease the charge transfer resistance and increase the rate and current density of ethanol oxidation.6. Pt-Rh-SnO2/CNTs catalysts with different microstructures were prepared by pulse-microwave assisted polyol methods with the precursors of H2PtCl6 and RhCl3 adjusting the catalyst preparation process. Pt-Rh-SnO2/CNTs catalyst with Pt (or Rh) rich on the nanoparticle surface was obtained by adopting the successive reduction method. Pt-Rh-SnO2/CNTs-Pt(f) catalyst with Rh rich on the nanoparticles surface was prepared by reducing Pt precursor in a polyol process at first, and then Rh was reduced successively on Pt nanoparticles. Pt-Rh-SnO2/CNTs-Rh(f) catalyst with Pt rich on the nanoparticles surface was prepared by reducing Rh precursor in a polyol process at first, and then Pt was reduced successively on Rh nanoparticles. Pt-Rh-SnO2/CNTs-(co) catalyst with PtRh alloy rich on the nanoparticles surface were prepared by co-reducing the Pt and Rh precursors together. X-ray diffraction and transmission electron microscopy show the metal particles in all the three catalyst were homogenously dispersed on the SnO2 surface with an average particle size less than 3.5 nm. The electrocatalytic properties of the as-prepared catalysts for ethanol oxidation reactions are investigated by cyclic voltammetry, chronoamperometry. The catalytic activity and CO tolerance for ethanol electro-oxidation on the three catalysts in the following order: Pt-Rh-SnO2/CNTs-Pt(f)>Pt-Rh-SnO2/CNTs-(co)>Pt-Rh-SnO2/CNTs-Rh(f).The mechanism of ethanol oxidation on Pt-Rh-SnO2/CNTs-Pt(f) catalyst was also proposed. There are 57 pictures,5 tables and 194 references in this thesis.