Patent Application
20060228604
1. Field of the Invention
The invention relates generally to membranes for fuel cells and, more particularly, to a membrane for a fuel cell, for example, a direct methanol fuel cell (DMFC), a method for making the same and a membrane-electrode assembly having the same.
2. Discussion of Related Art
Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy with no intermediary stage, and thus do so with high efficiency. Fuel cells function on the principle of reverse electrolysis, that is, fuel (such as hydrogen) and oxidant (such as oxygen) are respectively oxidized and reduced at an anode electrode and a cathode electrode, yielding only water and heat as byproducts while converting chemical energy into electricity. According to the electrolyte used, fuel cells can be classified into general groups including phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, proton exchange membrane fuel cells (PEMFCs), and alkaline fuel cells. The PEMFCs perform well at low working temperatures (down to about 80 degrees Celsius) have high efficiency, long operation life, high specific power density and high specific energy density.
Theoretically, hydrogen gas and oxygen gas are ideal fuel and oxidant for PEMFCs. However, pure hydrogen gas is unsafe because of its high chemical reactivity and difficulty in storage. Therefore, direct methanol fuel cells (DMFCs) were developed to meet these problems. DMFCs utilize methanol as fuel, thus rendering the fuel cell safely as methanol does not spontaneously react with oxygen at room temperature.
Conventional DMFCs suffer from a problem which will be familiar to those skilled in the art: cross-over of methanol from the anode to the cathode through electrolyte membranes, causing significant loss in efficiency. Cross-over occurs because of the high solubility of methanol in the membrane. In order to minimize cross-over, and thereby minimize the loss of efficiency, the concentration of methanol in the fuel feed stream should be kept low by dilution with water. However, dilution of the methanol introduces other disadvantages: (1) fuel cell construction becomes more complicated and costly because of additional structures and processes needed to store and manage the water; and (2) the energy per unit volume of the fuel cell, which is a critical factor in terms of the fuel cell's potential commercial applications, is reduced.
What is needed, therefore, is a electrolyte membrane which can lower or eliminate cross-over of methanol from an anode to a cathode, while allowing protons to pass through.
In one aspect, of the present invention, a membrane for fuel cells is provided. The membrane includes a polymer base and a metal layer formed on the polymer base. The polymer base is comprised of a material selected from the group consisting of polyethylene, polypropylene, polysulfone, polyimide, polyvinylidenefluoride, polyurethane, polystyrene, polyvinylchloride, cellulose, nylon, a copolymer of vinylidenefluoride and hexafluoropropylene, a copolymer of vinylidenefluoride and trifluoroethylene, a copolymer of vinylidenefluoride and tetrafluoroethylene, acrylate based polymer, polyacrylonitrile, polyvinylacetate, polyethyleneoxide, and polypropyleneoxide. The metal layer is comprised of a material selected from the group consisting of nickel, palladium, platinum and gold.
In another aspect of the present invention, a method for making the above-described membrane is provided. The method includes the steps of providing a polymer base, forming a first metallic layer on the polymer base, and forming a second metallic layer on the first metallic layer. The first metallic layer is made by a sputtering method. The second metallic layer is made by an electroless plating method.
In still another aspect of the present invention, a membrane-electrode assembly is provided. The membrane electrode assembly includes an anode, a cathode and the above-described membrane.
Advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings.
Many aspects of the present membrane, membrane electrode assembly, and method for making the membrane can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present membrane, method for making the same and membrane electrode assembly.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the present invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Reference will now be made to the drawings to describe embodiments of the present membrane, membrane electrode assembly having the same, and method for making the membrane, in detail.
Referring to
The metal layer 120 is made of a metal selected from the group consisting of nickel, palladium, platinum and gold. The relatively un-reactive material, platinum, is also used as catalyst particles for catalyzing electrochemical reaction. The metal layer 120 can prevent fuels, such as methanol, from crossing-over the polymer base 110. Thickness of the metal layer 120 can be adjusted according to the kind of material being blocked.
A method for making the above-described membrane 100 includes the steps of providing a polymer base, forming a first metallic layer on the polymer base, and forming a second metallic layer on the first metallic layer.
Referring to
On the first metallic layer 121, a second metallic layer 122 is formed by an electroless plating method for enhancing the ability of preventing fuels, such as methanol, crossing over the membrane 110. The second metallic layer 122 is formed in a plating solution containing metal ions and a reducing agent. A catalyst is utilized to catalyze the redox reaction, thereby the metal ions are reduced from their ionic state into a solid metal state. The reducing agent is generally sodium borohydride or hypophosphite. The plating solution is an acid solution with a PH value in the range from about 4.2 to about 4.8. The plating solution may be an alkaline solution. The thickness of the second metallic layer 122 can be adjusted by controlling the time period of the electroless plating process.
Referring to
The anode 200 and the cathode 300 are electrically conductive. Materials of the anode 200 and the cathode 300 are selected from graphite, carbon composites and conductive metals. Catalysts, such as platinum and ruthenium, are coated on the anode 200 and cathode 300 uniformly for inducing electrochemical reaction. The catalytic anode 200 and cathode 300 are intimately bonded to each side of the above-described membrane 100 to form the membrane-electrode assembly 1000.
The membrane-electrode assembly 1000 can be used for direct methanol fuel cells (DMFC), polymer electrolyte fuel cells (PEFC) or proton exchange membrane fuel cells (PEMFC). The fuel can be a liquid or a vapor methanol/water mixture and the oxidant can be air or oxygen. Protons are produced by oxidation of methanol at the anode 200 and pass through the membrane 100 from the anode 200 to the cathode 300. Electrons produced at the anode 200 in the oxidation reaction flow in the external circuit to the cathode 300, driven by the potential difference between the anode 200 and the cathode 300,thus producing a current capable of performing useful work.
The electrochemical reactions occurring in a direct methanol fuel cell which contains the membrane electrode assembly 1000 are:
Anode CH.sub.3OH+H.sub.2O.fwdarw.CO.sub.2+6H.sup.++6e.sup.− (1)
Cathode3/2O.sub.2+6H.sup.++6e.sup.−.fwdarw.3H.sub.2O (2)
Overall CH.sub.3OH+3/2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O (3)
Compared with conventional systems, the present invention uses a metal layer to prevent fuels such as methanol from crossing over an electrolyte membrane, thus minimizing or eliminating energy efficiency loss.
It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200510034108.3 | Apr 2005 | CN | national |
