Showing papers on "Direct methanol fuel cell published in 2001"

A Pt−Ru/Graphitic Carbon Nanofiber Nanocomposite Exhibiting High Relative Performance as a Direct-Methanol Fuel Cell Anode Catalyst

Abstract: Multistep deposition and reactive decomposition of a precursor molecule containing one Pt and one Ru atom on herringbone graphitic carbon nanofibers (GCNFs) affords a Pt−Ru/GCNF nanocomposite containing Pt−Ru alloy nanoclusters widely dispersed on the GCNF support. The nanocomposite has a total metal content of 42 wt % with a bulk Pt/Ru atomic ratio of ca. 1:1, and metal alloy nanoclusters having average particle sizes of 6 nm as calculated from XRD peak widths or 7 nm as measured directly from TEM images. XRD and electrochemical analysis of the nanocomposite as-prepared and stored under ambient conditions reveals the presence of small amounts of Ru metal and oxidized metal species. Comparative testing of this nanocomposite and an unsupported Pt−Ru colloid of similar surface area and catalyst particle size as anode catalysts in a working direct-methanol fuel cell (DMFC) reveals a 50% increase in performance for the Pt−Ru/GCNF nanocomposite. More detailed study of the catalytic performance of metal alloy/G...

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Polymer Electrolyte Membrane Fuel Cell Applications: II. Methanol Uptake and Methanol Permeability

Abstract: Sol-gel derived Nafion/silica hybrid membranes were investigated as a potential polymer electrolyte for direct methanol fuel cell applications. Methanol uptake and methanol permeability were measured in liquid and vapor phase as a function of temperature, methanol vapor activity, and silica content. Decreased methanol uptake from liquid methanol was observed in the hybrid membranes with silica contents of 10 and 21 wt %. The hybrid membrane with silica content of ≈20 wt % showed a significant lower methanol permeation rate when immersed in a liquid methanol-water mixture at 25 and 80°C. Methanol uptake from the vapor phase by the hybrid membranes appears similar to that of unmodified Nafion. Methanol diffusion coefficients, as determined from sorption experiments, were slightly lower in the hybrid membranes than in unmodified Nafion. However, in direct permeation experiments, significantly lower methanol vapor permeability was seen only in the hybrid membrane with silica content of ≈20 wt %. Based on these results, Nafion/silica hybrid membranes with high silica content have potential as electrolytes for direct methanol fuel cells operating either on liquid or vapor-feed fuels. © 2001 The Electrochemical Society. All rights reserved.

Investigation of Ternary Catalysts for Methanol Electrooxidation

Abstract: The electrochemical oxidation of methanol was investigated on Pt–Ru–X ternary metallic catalysts (with X=Au, Co, Cu, Fe, Mo, Ni, Sn or W). The catalysts were prepared by electrochemical deposition and dispersed in a conductive three-dimensional matrix, an electronic conducting polymer, polyaniline (PAni). A comparative study of the behaviour of several ternary catalysts towards the electro-oxidation of methanol shows that PAni/Pt–Ru–Mo is the most efficient anode at potentials up to 500mV vs RHE. This latter ternary electrocatalyst leads to current densities up to 10 times higher than those measured with PAni/Pt–Ru in this potential range. Moreover, the catalyst appears to be stable for potentials lower than 550mV vs RHE. According to EDX analysis, the good behaviour of the Pt–Ru–Mo ternary catalyst seems to result from the presence of a small amount of the third metal, at an atomic ratio close to 5%. This set of encouraging results has also been confirmed by preliminary measurements in a single cell direct methanol fuel cell (DMFC) containing a home made PAni/Pt–Ru–Mo anode. The ternary catalyst leads to higher power densities than the PAni/Pt–Ru binary catalyst under the same experimental conditions.

Current Status of and Recent Developments in the Direct Methanol Fuel Cell

Abstract: The direct methanol fuel cell (DMFC) has been discussed recently as an interesting option for a fuel-cell-based mobile power supply system in the power range from a few watts to several hundred kilowatts. In contrast to the favoured hydrogen-fed fuel cell systems (e.g. the polymer electrolyte membrane fuel cell, PEMFC), the DMFC has some significant advantages. It uses a fuel which is, compared to hydrogen, easy to handle and to distribute. It also comprises a fairly simple system design compared to systems utilising liquid fuels (like methanol) to produce hydrogen from them by steam reforming or partial oxidation to finally feed a standard PEMFC. Nevertheless, many severe problems still exist for the DMFC, hindering its competitiveness as an option to hydrogen-fed fuel cells. This work reviews the major research activities concerned with the DMFC by highlighting the problems (slow kinetics of the anodic methanol oxidation, methanol permeation through the membrane, carbon dioxide evolution at the anode) and their possible solutions. Special attention is devoted to the steady state and dynamic simulation of these fuel cell systems.

Mechanistic aspects of methanol oxidation on platinum-based electrocatalysts

Abstract: An understanding of the overall mechanism of the electrooxidation of methanol is of considerable interest in relation to the optimization of the direct methanol fuel cell. This paper describes in detail the different steps in the oxidation of methanol on platinum-based electrocatalysts with the identification of the key adsorption steps and of the different intermediates involved. From these fundamental studies, it is shown how it is possible to design multimetallic electrocatalysts for the electrooxidation of methanol under experimental conditions suitable for fuel cell application.

Rapid Synthesis of a Pt1Ru1/Carbon Nanocomposite Using Microwave Irradiation: A DMFC Anode Catalyst of High Relative Performance

Abstract: Thermal treatment of (η-C2H4)(Cl)Pt(μ-Cl)2Ru(Cl)(η3:η3-2,7-dimethyloctadienediyl) (1)/Vulcan carbon composites under appropriate oxidizing and reducing conditions using microwave dielectric loss heating affords PtRu/Vulcan carbon nanocomposites consisting of PtRu alloy nanoparticles highly dispersed on a powdered carbon support. Two such nanocomposites containing 16 or 50 wt % total metal and alloy nanoclusters of 3.4- or 5.4-nm average diameter are formed within only 100 or 300 s of total microwave heating. XRD and on-particle EDS analyses reveal that complex 1 serves as a reliable single-source molecular precursor for the formation of PtRu nanoparticles having a nearly 1:1 metal alloy stoichiometry. Preliminary measurements of the catalytic activity of these nanocomposites as supported direct methanol fuel cell (DMFC) anode catalysts indicate that the 50 wt % nanocomposite has a performance superior to that of a 60 wt % commercial catalyst and a normalized performance equivalent to that of a proprietary...

Electrochemical and gas evolution characteristics of direct methanol fuel cells with stainless steel mesh flow beds

Abstract: Carbon dioxide gas management and bipolar plate/(flow bed) design are important to the development of direct methanol fuel cell (DMFC) systems. If the gas produced at the anode is not removed rapidly and efficiently a gradual deterioration in electrical performance can occur. This paper examines the feasibility of using stainless steel mesh materials as flow beds for the DMFC. A flow visualization study, using a high-speed video camera and appropriate computer software, of the anode side, carbon dioxide gas evolution and flow behaviour with flow beds based on stainless steel mesh is reported. The electrochemical behaviour of the direct methanol fuel cell with stainless steel flow beds is also reported. A number of the flow bed designs, based on stainless steel mesh, showed promising behaviour in terms of gas removal characteristics and electrical performance.

Direct methanol fuel cell including a water recovery and recirculation system and method of fabrication

Abstract: A fuel cell device and method of forming the fuel cell device including a base portion, formed of a singular body, and having a major surface At least one fuel cell membrane electrode assembly is formed on the major surface of the base portion and includes an electrically conductive hydrophilic material for the wicking of reaction water and providing for electrical conduction to a current collector A fluid supply channel including a mixing chamber is defined in the base portion and communicating with the fuel cell membrane electrode assembly for supplying a fuel-bearing fluid to the membrane electrode assembly An exhaust channel including a water recovery and recirculation channel is defined in the base portion and communicating with the membrane electrode assembly and the electrically conductive hydrophilic material The membrane electrode assembly and the cooperating fluid supply channel and cooperating exhaust channel forming a single fuel cell assembly

Measurement of methanol crossover in direct methanol fuel cell

Abstract: In a direct methanol fuel cell (DMFC) with a Nafion membrane, a methanol solution fed to the anode permeates to the cathode. This phenomenon causes the depolarization loss at the cathode and the DMFC performance grows worse. In this research, by continuously measuring the concentration of CH3OH, CO and CO2 in the exhaust gas of the cathode, the amount of methanol crossover was determined. Also, the effect of the thickness of polymer electrolyte membrane and the concentration of methanol solution on the amount of methanol crossover were investigated.

Electrodes modified by electrodeposition of CoTAA complexes as selective oxygen cathodes in a direct methanol fuel cell

Abstract: The oxygen reduction reaction (ORR) at cobalt tetraazaanulene (CoTAA) modified electrodes was investigated. As a first approach, modified electrodes were prepared by electrodeposition of CoTAA on glassy carbon (GC). The modification of the GC surface was monitored by u.v.–vis. differential reflectance spectroscopy (UVDRS). The recorded spectra (i.e., absorbance as a function of wavelength and time) showed that the electrodeposition of CoTAA at 0.8 V vs Ag|AgCl, that is, at a potential where the TAA ligand is oxidized to TAA+•, seems to produce a thin polymer film. Starting from these preliminary results, porous rotating disc electrodes (RDEs) were prepared by electrodeposition of CoTAA (0.8 V vs Ag|AgCl, 1 min) on graphite powder embedded in a recast Nafion® film. The use of a porous RDE allowed comparison of the activity and selectivity of Pt nanoparticles and CoTAA for the ORR under experimental conditions close to those of a fuel cell cathode, that is, at the catalyst|Nafion® interface. The activity towards the ORR of a porous electrode modified by electrodeposition of CoTAA is not affected when methanol is present in the electrolyte phase, whereas a noticeable decrease in the activity of Pt-based oxygen cathodes was observed under the same conditions. Half-cell life tests showed that CoTAA-modified electrodes and Pt-based electrodes have a comparable stability over a period of 90 min.

Apparatus and methods for sensor-less optimization of methanol concentration in a direct methanol fuel cell system

Abstract: Apparatus and methods for regulating methanol concentration in a direct methanol fuel cell system without the need for a methanol concentration sensor. One or more operating characteristics of the fuel cell, such as the potential across the load, open circuit potential, potential at the anode proximate to the end of the fuel flow path or short circuit current of the fuel cell, are used to actively control the methanol concentration.

Analysis of the nonlinear dynamics of a direct methanol fuel cell

Abstract: This paper is related to the analysis of the dynamic behaviour of a liquid-feed direct methanol fuel cell (DMFC) under different operating conditions, based on an isothermal model accounting for the mass balances, the charge balances, the reaction micro-kinetics and the mass transport phenomena. Conceptually, the fuel cell system is decomposed into its subsystems (anode and cathode compartments, diffusion layers, catalyst layers on both electrodes, proton exchange membrane (PEM)). The models of the subsystems are coupled to a DMFC model which is represented by a set of differential-algebraic equations of index one. Dynamic simulations with this model show that the undesired cross-over of the reactant methanol through the PEM can be reduced by periodically pulsed methanol feeding to the anode compartment. The simulated results are in good agreement with experimental cell voltage data obtained from a laboratory-scale DMFC which was operated with different dynamic feeding strategies.

Synthesis and characterization of catalyst layers for direct methanol fuel cell applications

Abstract: A preparation method for catalyst layers of platinum nanoparticles, as used in direct methanol fuel cells, based on pulsed electrochemical deposition within the three-phase boundary regions responsible for electrochemical reactions, was developed. A platinum precursor was brought into a carbon–Nafion layer and platinum was deposited in situ on the carbon surface. The size, size distribution and crystallinity of the deposited catalyst particles were determined by means of high-resolution transmission electron microscopy and X-ray diffraction measurements. The electrochemically active surface area of the deposited catalyst material was investigated by cyclic voltammetry, analysing the Pt–H oxidation peak.

Catalytic processes in solid polymer electrolyte fuel cell systems

Abstract: Industrial application of fuel cell technology requires suitable electrocatalysts. This is true for all different types of fuel cells. These catalysts are responsible for the oxidation of the fuel (i.e. hydrogen, hydrogen rich gases or methanol) as well as for the oxygen reduction. This paper focuses on solid polymer electrolyte fuel cell systems for mobile applications. Here the demands on the catalyst are most severe due to the low-temperature operating regime. Two system configurations are possible: either the carbon containing fuel is processed by a fuel converter to a hydrogen rich gas mixture and this in turn is fed into the fuel cell. Alternatively fuels such as methanol can be supplied directly into the fuel cell. In the first case, contaminations of CO in the feed gas have to be taken into account. These strongly absorbs on the surface of the catalysts (carbon-supported Pt or Pt-alloys) thereby inhibiting the hydrogen oxidation reaction. Electro-oxidation mechanisms of adsorbed CO as well as methanol—as an example for a direct fuel cell system—is discussed on Pt-Ru and other catalysts. Finally catalysts for methanol and hydrocarbon reforming reactions as well as for the shift reaction are reviewed.

Development and operation of a 150 W air-feed direct methanol fuel cell stack

Abstract: A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm2 geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm−2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm2) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm−2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.

Monopolar cell pack of proton exchange membrane fuel cell and direct methanol fuel cell

Abstract: A proton exchange membrane fuel cell and a direct methanol fuel cell pack using a monopolar electrode are provided. The fuel cell pack includes a plurality of cells each having a membrane in its middle and a cathode and an anode at both sides of the membrane, collector plates contacting the cathode and the anode, respectively, in each cell, and an electrical connection member for electrically connecting adjacent cells. The cells are evenly disposed on an arbitrary plane with a hollow interposed between two adjacent cells. The electrical connection member is positioned in the hollow. The fuel cell pack also includes a porous fuel diffusion member contacting the anode of each cell; a porous air contact member contacting the cathode of each cell; an anode end plate and a cathode end plate disposed at the side of the anodes of the cells and at the side of the cathodes of the cells, respectively, for protecting the cells; a fuel supply and discharge unit for supplying fuel toward the anodes in the hollow and discharging the fuel; a fuel flow stopper disposed at a portion at the part of the cathodes in the hollow, for preventing fuel flowing at a portion at the part of the anodes in the hollow from flowing toward the portion at the part of the cathodes in the hollow; and a sealing member for sealing the anodes of the cells and the portion of the hollow corresponding to the anodes. Accordingly, circulation of fuel for the plurality of cells is performed through a single inlet and a single outlet so that a fuel supply line is very simple compared to a structure of a fuel supply line for each cell in a conventional cell pack having a structural limitation. In addition, the cell pack generates current of a high density without a separate cooling device.

Catalyst inks and method of application for direct methanol fuel cells

Abstract: Inks are formulated for forming anode and cathode catalyst layers and applied to anode and cathode sides of a membrane for a direct methanol fuel cell. The inks comprise a Pt catalyst for the cathode and a Pt—Ru catalyst for the anode, purified water in an amount 4 to 20 times that of the catalyst by weight, and a perfluorosulfonic acid ionomer in an amount effective to provide an ionomer content in the anode and cathode surfaces of 20% to 80% by volume. The inks are prepared in a two-step process while cooling and agitating the solutions. The final solution is placed in a cooler and continuously agitated while spraying the solution over the anode or cathode surface of the membrane as determined by the catalyst content.

Monopolar cell pack of direct methanol fuel cells

Abstract: A monopolar cell pack for a direct methanol fuel cell is provided. In the monopolar cell pack, series-connection of electrodes makes it unnecessary for connections to pass through an electrolyte membrane and allows single cells to be electrically connected on the first and second surfaces of the ion exchange membrane (101), respectively, thereby preventing leakage of fuel. As a result, the internal electric circuit according to the present invention can be simplified. Also, since current collectors (150,160) contact the anodes (121) and cathodes (131) entirely rather than partially, contact resistance can be considerably reduced, thereby greatly reducing a loss due to resistance. Carbon dioxide, which is a byproduct of the reaction, can be easily exhausted through an exhaust path installed in each current collector, thereby improving performance of a cell pack.