Catalysts. — 20% Pt/C and 20% Pt - 10% Ru/C catalysts were purchased from Electrochem. Inc. (Woburn. MA) and used for the oxygen reduction and methanol oxidation reaction in DMFC. respectively.

Fabrication of the membrane/electrode assembly.— The M&E assemblies were prepared according to a "paste process" procedure already developed for SPE fuel cclls [11. Catalysts were directly mixed with a colloidal suspension of Nafion® in butyl acctatc solution in an ultrasonic bath. The obtained paste was spread on carbon cloth backings. Pt loadings were 0.8 mg cm'2 in anode and 0.5 mg cm'2 in cathodc. Nafion® loading was 1 mg cm"2 in both electrodes. The clcctrodcs were pressed onto a purified Nafion® 117 membrane at 150 °C and 90 atm for 1.5 min.

Single cell. —Electrode/membrane assemblies having 50 cm2 geometrical electrode area were loaded into a single cell test fixture. The cell was operated at 95 °C. Gaseous CH3OH solutions of various concentrations were supplied to the anode (1 atm). Humidified oxygen (100° C) was fed to the cathodc at 3.5 atm.

A" - ray diffraction analysis. — Fig. 1 shows the X-ray diffraction patterns of 20% Pt - 10% Ru/C and 20% Pt/C catalysts. Both catalysts exhibit the characteristic diffraction peaks of the Pt fee structure. The diffraction peaks in the Pl-Ru catalyst slightly shift to higher two-thcta values with respect to the same reflections of Pt in the JCPDS card. Such cvidcncc accounts for the presence of a Pt-Ru alloy in the catalyst. alcc values of 3.921 and 3.905 A for Pt/C and Pt-Ru/C catalysts, respectively, were obtained from the 220 peak profile fitting. According to the variation of fee latticc parameter with composition for Pt-Ru bulk alloys an atomic fraction of about 20% Ru should be present in the carbon supported alloy. The average particle size in the two catalysts was determined from the broadening of the (220) reflection of the fee lattice by using the Debye-Sherrer equation. The average particle size in Pt/C catalyst was 23 A A larger particle size was determined for the Pt-Ru catalyst i.e. 38 Â as clearly evidenced by the increase in sharpness of the diffraction peaks in this sample (Fig. 1).

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