Preparation Of Novel Metallo-phthalocyanine Catalysts Used In DMFC Cathode

Direct methanol fuel cells (DMFCs) are attractive as promising power source for portable applications due to several advantages: high power density, low or zero emissions, quick start capability ,easy fuel carriage.and instantaneous refueling. However, two major technical gaps limit their development: one problem is the slower kinetics of the methanol oxidation reaction; and the other is crossover of methanol through. Two solutions are mainly envisaged for the second problem: the use of polymer membranes less permeable to methanol between both electrodes and/or the use of methanol tolerant (inactive) cathode electrocatalysts. But the best solution is to develop efficient methanol-tolerant cathode catalyst. As one of nonprecious macrocycle catalysts, metallophthalocyanine catalysts are resistant totally to methanol. But so far, the major drawback of this kind of catalys is its low electrocatalytic activity and stabilities.In this paper, four kinds of single-ring metallophthalocyanines with different strong electron-donating groups and four kinds of double-ring metallophthalocyanines were designed and synthesized by DBU method and a method in kerosene raffinate solvent in the presence of ammonium molybdate at about 180～200℃. This molecular configurations can strengthen the electron-donating capability of central metal ions to oxygen with a view to improving the catalytic performance and the selectivity for oxygen reduction reaction (ORR). The metallophthalocyanines were characterized by elemental analysis, UV-Vis, IR and ¹HNMR. The heat stability of the metallophthalocyanines were tested by TG-DTA in N2. The thermal decomposition was about 220℃, which showed remarkable heat-stability.The metallophthalocyanines were then laded on activated carbon (Vulcan XC-72) with different M-N₄ (M=metal) content (1%, 2%, 4%, 6%, 10%) and heat-treated under the protection of Ar at different temperature (400℃, 600℃, 800℃or 850℃and 1000℃or 1050℃) in order to find out suitable conditions of immboliztion and pyrogenation. SEM,TEM and XRD were used to characterize the micromechanism of the catalyst, EDS was used to characterize the elements on the surface of HNODPcFe/C, and rotating disk electrode (RDE) method was used to inspecte the catalystic activity of the metallophthalocyanine catalysts for ORR affected by M-N₄ loading and pyrogenation temperature in different electrolytes. A broad maximum of activity for ORR was observed in these pyrolyzed catalysts at temperature ranging between 600℃and 850℃. The best M-N₄ loading was 6wt% or 4% for Fe-N₄ and Co-N₄. The results showed the metallophthalocyanines were absorbed on the surface of carbon black uniformly at medium temperature, metal-cluster appeared when pyrogenation temperature was higher than 850℃. Compare two central metal catalyst in the same amount of M-N4 loading, the result showed that iron catalyst has a better catalytic performangce than cobalt catalyst in (weak) alkaline solution, and more inclined to deoxidize oxygen to water by 4e route. The electrocataiytic activity of single-ring metallophthalocyanines was comparative to that of the double-ring ones. Compare the metallophthalocyanine catalysts with the 30wt%Pt/C produced by E-Tek, the result is that HNODPcFe/C, TFEPPcCo/C and TQOPcCo/C catalysts showed analogical electrocatalystic activity with 30wt%Pt/C in 0.5M NaHCO₃.With the presence of 1M methanol, all the macrocycle catalysts showed totally methanol tolerance.