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In this work, carbon-supported Pt 70 Co 30 nanoparticles were prepared by a polyol process using a long-chain diol as reducer (hexadecanediol) and oleic acid and oleylamine as stabilizers. Depending on the synthesis conditions, Pt 70 Co 30 /C nanocatalysts with very small particle size (1.9 ( 0.2 nm) and narrow distribution homogeneously dispersed on the carbon support and having a high degree of alloying without the need of thermal treatments were obtained. The as-prepared catalyst presents an excellent performance as proton exchange membrane fuel cells (PEMFC) cathode material. In terms of mass activity (MA), the Pt 70 Co 30 /C electrocatalysts produced single fuel cell polarization response superior to that of commercial catalyst. To analyze alloying effects, the result of thermal treatment at low temperatures (200-400 °C) was also evaluated and an increase of average crystallite size and a lower degree of alloying, probably associated to cobalt oxidation, were evidenced.

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In this work, the mechanism of CO tolerance for the hydrogen oxidation reaction (HOR) was investigated on Mo-based carbon dispersed electrocatalysts (Mo/C and PtMo/C). Gas-diffusion electrodes with a diffusion layer formed by Mo/C and a catalyst layer composed by Pt/C, PtRu/C, or PtMo/C, showed enhanced electrocatalytic activity for the HOR in the presence of CO. This phenomenon was associated to a lowering of the CO concentration in the gas channel of the electrodes promoted by a water gas shift (WGS) reaction taking place in the Mo/C layer. The occurrence of this reaction was confirmed by gas chromatographic analyses of the CO content, conducted at the outlet of a Mo/C reactor operating at 100°C and fed with different H 2 O / H 2 / CO / CO 2 mixtures. Experiments made in the presence of CO 2 indicated an additional CO poisoning effect due to the reversing of the WGS reaction, but the Mo-based electrodes still showed a performance superior to that of the standard state-of-the-art PtRu/C. © 2004 The Electrochemical Society. All rights reserved.

LiCoO 2 powders were prepared by combustion synthesis, using metallic nitrates as the oxidant and metal sources and urea as fuel. A small amount of the LiCoO 2 phase was obtained directly from the combustion reaction, however, a heat treatment was necessary for the phase crystallization. The heat treatment was performed at the temperature range from 400 up to 700 jC for 12 h. The powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and specific surface area values were obtained by BET isotherms. Composite electrodes were prepared using a mixture of LiCoO 2 , carbon black and poly(vinylidene fluoride) (PVDF) in the 85:10:5% w/w ratio. The electrochemical behavior of these composites was evaluated in ethylene carbonate/dimethylcarbonate solution, using lithium perchlorate as supporting electrolyte. Cyclic voltammograms showed one reversible redox process at 4.0/3.85 V and one irreversible redox process at 3.3 V for the LiCoO 2 obtained after a post-heat treatment at 400 and 500 jC.Raman spectroscopy showed the possible presence of LiCoO 2 with cubic structure for the material obtained at 400 and 500 jC. This result is in agreement with X-ray data with structural refinement for the LiCoO 2 powders obtained at different temperatures using the Rietveld method. Data from this method showed the coexistence of cubic LiCoO 2 (spinel) and rhombohedral (layered) structures when LiCoO 2 was obtained at lower temperatures (400 and 500 jC). The single rhombohedral structure for LiCoO 2 was obtained after post-heat treatment at 600 jC. The maximum energy capacity in the first discharge was 136 mA g À 1 for the composite electrode based on LiCoO 2 obtained after heat treatment at 700 jC. D

The proton transport properties of Nafion membranes were studied in a wide range of temperature by using an air-tight sample holder able to maintain the sample hydrated at high relative humidity. The proton conductivity of hydrated Nafion membranes continuously increased in the temperature range of 40–180 °C with relative humidity kept at RH = 100%. In the temperature range of 40–90 °C, the proton conductivity followed the Arrhenius-like thermal dependence. The calculated apparent activation energy Ea values are in good agreement with proton transport via the structural diffusion in absorbed water. However, at higher measuring temperatures an upturn of the electrical conductivity was observed to be dependent on the thermal history of the sample.

Our experiments disentangle the low and high frequency dispersions in perfluorosulfonate ionomer solutions and membranes, providing a reasonable model for understanding these relaxations. Dielectric spectroscopy (DS) and small angle x-ray scattering (SAXS) measurements revealed that the dielectric relaxations observed at low (α relaxation) and high (β relaxation) frequencies show typical features of the longitudinal and radial polarization, respectively, of rodlike polymeric aggregates. Such relaxations were attributed to counterion fluctuations in the vicinity of sulfonic acid groups, in resemblance with polyelectrolytes. Characteristic correlation lengths calculated from both DS and SAXS data are in good agreement adding further evidence to the proposed model. Such description provides insights for the understanding of the crossover from polyelectrolytes, dominated by charge repulsion, to ionomers, dominated by dipolar attraction.