WC Enhanced Catalysis Of Noble Metallic Catalysts For Fuel Cells

Direct alcohol fuel cells(DAFCs) have some advantages such as good theoretical energy density and energy efficiency. DAFCs are of extensive applying prospect for electric vehicles power and portable electrical source. Comparing with PEMFC, DAFCs have been studied intensively for the reason that they own obvious volume ratio energy advantage. However, one of the main obstacles for the development of DAFCs is a high anodic overpotential and serious activating polarization when alcohol is oxided. The improvement of the activation for the anodic electro catalysts is needed. DAFCs are far from producing cosmically in commerce, which is caused by the reason that the kinetics of alcohol oxidation is slow and the catalysts are easy to be poisoned. In addition, the crossover of liquid alcohol through the polymer electrolyte membrane into the cathodic compartment results in mixed potential in the cathode side.This study focuses on the study of cathode catalysts for direct alcohol fuel cells. At the same time, we also primarily study the anode catalysts for direct alcohol fuel cells. Currently the most widely used cathode catalysts are Pt-based catalysts, but they have many shortcomings such as expensive, easy to be poisoned and with high overpotential for ORR. Therefore, the new kind of cathode catalysts which have low overpotential of ORR, good selectivity towards the ORR and low price must be studied for a broad commercial application. Based on the above shortcomings of Pt-based catalysts, nano-tungsten carbide enhanced Pt-WC /C and AuPd-WC/C noble metallic composite catalysts were studied.Nano-tungsten carbide was prepared by intermittent microwave heating method (IMH) and pure nano-tungsten carbide was prepared by using improved microwave oven. IMH can reach a high temperature at a short time, and the reaction rate is very fast. Furthermore, the prepared catalysts are of small particles. By controlling the concentration of precursor we can prepare two kinds of tungsten carbide: WC/C and W2C/WC/C. The results of XRD for the catalysts show the forming of crystal tungsten carbide. The results of TEM for the catalysts demonstrate that tungsten carbide was uniformly dispersed on the carrier. The particle size is less than 10 nm. By controlling the mass ratio in the precursor to 30%, pure tungsten carbide was acquired. The results of HRTEM for the catalysts demonstrate that the distance between crystal lattices (d value) is 2.53? which shows pure tungsten carbide grows along the (100) wafer. Tungsten carbide was uniformly dispersed on the carrier. The particle size is less than 10 nm. The as-prepared tungsten carbide had catalytic activity towards ORR in alkaline and it had good selectivity towards ORR.Nano-tungsten carbide promoted composite catalyst Pt-WC /C was prepared by IMH and on-site reducion method. The composite catalysts showed the synergistic effect between Pt and tungsten carbide. The composite catalyst showed higher activity than that of commercial Pt/C catalyst with more positive onset potential and smaller overpotential in acidic solution. The onset potential of ORR on the composite catalyst shifted positively more than 150 mV than Pt/C catalyst. The exchange current density of ORR on the composite catalyst rised by three quantitatively rates. The composite catalyst had good selectivity towards ORR.Nano-tungsten carbide promoted composite catalyst AuPd-WC/C was prepared. In acid and alkaline solutions, the composite catalyst showed higher activity than that of commercial Pt/C catalyst with more positive onset potential and smaller overpotential. The onset potential of ORR on the composite catalyst shifted positively more than 70 mV than Pt/C catalyst. The exchange current density of ORR on the composite catalyst rised by a quantitatively rate. Influence of different concentration of ethanol on the ORR was studied and it turned out that in alkaline the ORR on the composite catalyst was seriously affected by ethanol. However, influence of different concentration of ethanol on the ORR for the composite catalyst was hardly affected by ethanol both for onset potential and current density in acidic solution. This proved that the ORR on the composite catalyst was hardly affected by ethanol. AuPd-WC/C composite catalyst had good selectivity for ORR in acidic solution.In this thesis the methanol and ethanol oxidation reaction (EOR) on AuPd-WC/C composite catalyst was studied in alkaline. The results show that the performance of EOR on AuPd-WC/C composite catalyst is better than that of methanol oxidation reaction. EOR on AuPd-WC/C composite catalyst results in high current density peak and the onset potential was -600mV vs Hg/HgO which was lower 500mV than that of commercial Pt/C catalyst. When the mass ratio Au:Pd:WC was 1:2:1, composite catalyst owns best catalysis performance. EOR on composite catalyst was more seriously affected by concentration of alcohol and temperature in alkaline than that of Pt/C catalyst. Chronopotentiometry on the performance of the catalyst shows potential oscillation appeared at low current density for Pt/C catalyst while there was no oscillation even at high current density for AuPd-WC/C composite catalyst during polarization.In this research novel composite catalysts were prepared and the performance was studied. Then pure nano-tungsten carbide was also prepared. A series of composite catalysts were prepared based on nano-tungsten carbide. The nano-tungsten carbide enhanced Pt-WC /C and AuPd-WC/C noble metallic composite catalysts had good catalytic activity towards ORR and AuPd-WC/C composite catalysts had good catalytic activity towards EOR. These composite catalysts can be used in low temperature fuel cells not only for cathode catalyst but also for anode catalyst.