Showing papers on "Direct methanol fuel cell published in 2003"

Preparation and Characterization of Multiwalled Carbon Nanotube-Supported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells

Abstract: Multiwalled carbon nanotube-supported Pt (Pt/MWNT) nanocomposites were prepared by both the aqueous solution reduction of a Pt salt (HCHO reduction) and the reduction of a Pt ion salt in ethylene glycol solution. For comparison, a Pt/XC-72 nanocomposite was also prepared by the EG method. The Pt/MWNT catalyst prepared by the EG method has a high and homogeneous dispersion of spherical Pt metal particles with a narrow particle-size distribution. TEM images show that the Pt particle size is in the range of 2-5 nm with a peak at 2.6 nm, which is consistent with 2.5 nm obtained from the XRD broadening calculation. Surface chemical modifications of MWNTs and water content in EG solvent are found to be the key factors in depositing Pt particles on MWNTs. In the case of the direct methanol fuel cell (DMFC) test, the Pt/MWNT catalyst prepared by EG reduction is slightly superior to the catalyst prepared by aqueous reduction and displays significantly higher performance than the Pt/XC-72 catalyst. These differences in catalytic performance between the MWNT-supported or the carbon black XC-72-supported catalysts are attributed to a greater dispersion of the supported Pt particles when the EG method is used, in contrast to aqueous HCHO reduction and to possible unique structural and higher electrical properties when contrasting MWNTs to carbon black XC-72 as a support.

Preparation of PtNi nanoparticles for the electrocatalytic oxidation of methanol

Abstract: Carbon supported PtNi nanoparticles were prepared by hydrazine reduction of Pt and Ni precursor salts under different conditions, namely by conventional heating (PtNi-1), by prolonged reaction at room temperature (PtNi-2) and by microwave assisted reduction (PtNi-3). The nanocomposites were characterized by XRD, EDX, XPS and TEM and used as electrocatalysts in direct methanol fuel cell (DMFC) reactions. Investigations into the mechanism of PtNi nanoparticle formation revealed that platinum nanoparticle seeding was essential for the formation of the bimetallic nanoparticles. The average particle size of PtNi prepared by microwave irradiation was the lowest, in the range of 2.9–5.8 nm. The relative rates of electrooxidation of methanol at room temperature as measured by cyclic voltammetry showed an inverse relationship between catalytic activity and particle size in the following order PtNi-1 < PtNi-2 < PtNi-3.

Ruthenium Crossover in the Direct Methanol Fuel Cell with a Pt-Ru Black Anode

Abstract: In this contribution, we present the problem of rut henium crossing from the DMFC anode catalyst, typically a Pt-Ru alloy black, through the polymer electrolyte membra ne to the cathode, typically a Pt black. We focus on the nature of the process, the impact it has on the cathode ac tivity, and the ways of avoiding it. Ru contamination of the DMFC cathode has been studied in this work with the electrochemical CO stripping technique. 7 In this method, CO is first chemisorbed on the electrode from the gas phase and then stripped off the surface in a single positive-going voltammetric sca n. The position, shape, and charge of the CO stripping pea k are all indicative of the surface area and composition. Example CO stripping voltammograms for one of the cathodes in a six-cell DMFC stack running for 850 h ours are shown in Figure 1 . The stack was operated on 0.3 M methanol at 75oC, current density of 80 mA cm -2 and

A micro methanol fuel cell operating at near room temperature

Abstract: We present a bipolar micro direct methanol fuel cell (μDMFC) with high-power density and simple device structure. A proton exchange membrane-electrode assembly was integrated in a Si-based μDMFC with micro channels 750 μm wide and 400 μm deep, fabricated using silicon micromachining. The μDMFC has been characterized at near room temperature, showing a maximum power density of 47.2 mW/cm2 when 1 M methanol was fed at 60 °C. The cell voltage dependence on the current density agrees well with the modified Tafel model, in which regimes of kinetic polarization and ohmic polarization are observed without significant presence of the concentration polarization.

Structural, Chemical, and Electronic Properties of Pt/Ni Thin Film Electrodes for Methanol Electrooxidation

Abstract: Pt/Ni thin-film electrodes were fabricated by e-beam evaporation of metal layers and rapid thermal annealing (RTA), to achieve alloy formation between the Pt and Ni layers. The structural, chemical, and electronic properties of thin-film electrodes annealed at 200, 300, and 500 °C were classified as follows: Pt-dominant (as-Pt/Ni or 200 °C Pt/Ni), Pt-based (300 °C Pt/Ni), and Ni-dominant (500 °C Pt/Ni). These Pt/Ni thin-film electrodes were matched well with Pt/Ni(3:1), -(1:1), and -(1:3) nanoparticles synthesized by borohydride reduction, for use in methanol electrooxidation in a direct methanol fuel cell. The characteristics of the thin-films and nanoparticles were correlated using X-ray diffraction analysis, Auger electron spectroscopy, X-ray photoelectron spectroscopy, and electrochemical measurements. The modified electronic properties of platinum in Pt/Ni alloy electrodes as well as a higher catalytic activity for methanol electrooxidation could be attributed to the surface and bulk structure of Pt...

An In Situ Method for Determination of Current Distribution in PEM Fuel Cells Applied to a Direct Methanol Fuel Cell

Abstract: This paper describes and demonstrates a new method for determination of current density distribution in an operating polymer electrolyte membrane ~PEM! fuel cell. The technique is a modification of the current mapping technique that relies on an array of shunt resistors embedded within a current collecting plate. Standard, nonaltered membrane electrode assemblies are utilized with gas diffusion layers in direct contact with an electrically segmented current collector/flow field. Multiple current measurements are taken simultaneously, allowing transient distribution detection with a multichannel potentiostat. Both steady state and transient data are presented for an operating liquid fed direct methanol fuel cell. Cathode flooding is predicted, and shown to occur at relatively high cathode flow rates. This technique can contribute to knowledge and understanding of key phenomena including water management and species distribution in PEM fuel cells.

Performance Modeling of a Direct Methanol Fuel Cell

Abstract: A new two-dimensional model of a direct methanol fuel cell (DMFC) has been developed and numerically tested. In complement to the existing developments, this model regards the diffusion layers as water-gas systems in the pore space with saturation and permeability varying according to capillary effects. The presence of hydrophilic and hydrophobic pores has been taken into account by introducing a new parametrization of the relationship between capillary pressure and saturation. Thus, mass transport occurs in parallel in the two phases, gas and liquid. The exchange between these phases is due to condensation and evaporation with rates given by the available exchange surface and the temperature. The gas transport is governed by the Stefan-Maxwell equations incorporated into the two-phase flow modeling approach. Instead of the often used Tafel and Butler-Volmer equations which are insufficient in the case of catalytic methanol oxidation and oxygen reduction, according to recent investigations, the electrochemical reactions are split up into reaction chains involving the covering of the catalysts with the various intermediate species. The main advantage of this approach is that it incorporates the effects of the limitation of the reaction rates due to the limited number of catalyst sites in a natural manner. The resulting system of transport and reaction equations is discretized in time by the backward Euler method and in space by a finite volume technique with proper upwinding.