Patent Application
20080248359
The application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. P2007-080317, filed on Mar. 26, 2007; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel cell, more particularly, to a direct methanol fuel cell.
2. Description of the Related Art
In a direct methanol fuel cell, there is a known method to provide a gas/liquid separation structure on an anode side of the cell, and to separate gas (CO2 gas) generated in a reaction in the anode side from liquid fuel and water. In the conventional gas/liquid separation structure on the anode side, a lyophobic or lyophilic gas/liquid separation layer is provided between an anode passage plate and an anode electrode, and the gas/liquid separation is performed by the gas/liquid separation layer. In such a way, an anode circulation system in the fuel cell is unnecessary or can be miniaturized, so as to contribute to miniaturization of the entire system of the fuel cell.
However, in the conventional gas/liquid separation structure on the anode side, a complicated structure includes a plural of parts, and accordingly, it is difficult to integrally mold the gas/liquid separation structure. Moreover, it is impossible to use a material containing a solvent, such as an adhesive, that adversely affects the electrode, and a material from which metal ions are eluted. Therefore, the individual parts are positioned, stacked on one another, and pressed after being prepared. However, there is a case where a gap occurs between the anode passage plate and the lyophobic or lyophilic gas/liquid separation layer. In this case, the fuel will leak to a gas passage side, and there is a possibility that the function of the gas/liquid separation is not performed.
An object of the present invention is to provide a fuel cell with improve reliability for gas/liquid separation provided on an anode side in a direct methanol fuel cell.
An aspect of the present invention inheres in a fuel cell including: a membrane electrode assembly including an electrolyte membrane, and anode and cathode electrodes sandwiching the electrolyte membrane there between; a gas/liquid separation layer provided at an opposite side of the anode electrode with the electrolyte membrane, and configured to separate fluid generated by a reaction in the anode electrode into gas and liquid; an auxiliary porous layer provided on the gas/liquid separation layer; and an anode passage plate provided on the auxiliary porous layer, including a fuel passage supplying a fuel to the anode electrode and a gas passage discharging the gas, wherein the auxiliary porous layer is softer than the gas/liquid separation layer and the anode passage plate, and includes lyophobic, electric conductive and gas permeability properties.
Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified.
Generally and as it is conventional in the representation of devices, it will be appreciated that the various drawings are not drawn to scale from one figure to another nor inside a given figure, and in particular that the layer thicknesses are arbitrarily drawn for facilitating the reading of the drawings.
A direct methanol fuel cell (DMFC) using methanol as a fuel will be described as a first embodiment of the present invention.
As shown in
In the membrane electrode assembly 1, the anode electrode 101 is composed of an anode catalyst layer 12, a carbon micro porous layer 14, and an anode gas diffusion layer 16. Moreover, the cathode electrode 102 is composed of a cathode catalyst layer 13, a carbon micro porous layer 15, and a cathode gas diffusion layer 17.
The electrolyte membrane 11 has a proton (H+)-conductive polymer electrolyte membrane, such as a Nafion membrane (registered trademark). For the anode catalyst layer 12, for example, platinum ruthenium (PtRu) and the like can be used. For the cathode catalyst layer 13, for example, platinum (Pt) and the like can be used. For the anode gas diffusion layer 16, for example, a water repellent treatment is implemented by polytetrafluoroethylene (PTFE) for commercially available carbon paper. As the cathode gas diffusion layer 17, for example, commercially available carbon cloth attached to the carbon micro porous layer is usable. The anode gas diffusion layer 16 smoothly supplies fuel to the anode catalyst layer 12, discharges a product generated by an anode reaction, and collects current. The cathode gas diffusion layer 17 smoothly supplies air to the cathode catalyst layer 13, discharges a product generated by a cathode reaction, and collects current.
The gas/liquid separation layer 2 provides the properties of electric conductivity, lyophobic (water repellency), and gas permeability. As the gas/liquid separation layer 2, a porous layer such as carbon paper, carbon cloth and carbon nonwoven fabric is usable.
The auxiliary porous layer 3 has higher degree of softness than the gas/liquid separation layer 2 and the anode passage plate 4, and is lyophobic (water repellency), electric conductive, and gas permeability. As the auxiliary porous layer 3, a micro porous layer (MPL) is usable. It is possible to manufacture the MPL in such a manner that carbon powder and the PTFE are mixed as slurry by use of a solvent, followed by baking at about 380° C.
In the anode passage plate 4, the fuel passage 5 and the gas passage 6 are formed. The fuel passage 5 supplies the fuel or a fuel solution from a fuel supply port 50 to the anode electrode 101, and discharges the fuel solution that is not reacted in the anode electrode 101, and the like from a fuel discharge port 51. The gas passage 6 discharges the gas (CO2 gas) generated by the anode reaction from a gas discharge port 60. Anode-side openings of the fuel passage 5 are positionally aligned with openings 31 of the auxiliary porous layer 3 and openings 21 of the gas/liquid separation layer 2. Anode-side openings of the gas passage 6 are in contact with the auxiliary porous layer 3.
A cathode collector (cathode passage plate) 7 is disposed on an outside of the cathode gas diffusion layer 17. The cathode collector 7 supplies the air from openings 8 to the cathode electrode 102, and collects current. An anode gasket 9 and a cathode gasket 10 prevent leakage of the fuel and the air to the outside.
In the fuel cell according to the first embodiment of the present invention, as shown in
CH3OH+H2O→6H++6e−+CO2 (1)
6H++6e−+3/2O2→3H2O (2)
Protons (H+) generated in the anode reaction flow into the cathode electrode 102 through the electrolyte membrane 11. Electrons (e−) generated in the anode reaction are carried to the cathode electrode 102 via the anode passage plate 4, an external circuit (not shown), and the cathode collector 7. CO2 generated in the anode reaction is more likely to pass through the lyophobic gas/liquid separation layer 2 than to form air bubbles in the liquid in the fuel passage 5, and accordingly, permeates the lyophobic gas/liquid separation layer 2 and the auxiliary porous layer 3, and is discharged from the gas passage 6. A part of the water that is not reacted in the anode electrode 101 is mixed with the methanol solution in the fuel passage 5, and the rest thereof permeates the electrolyte membrane 11, and is discharged from the cathode electrode 102 to the outside. A part of the water generated in the cathode reaction is reversely diffused to the anode catalyst layer 12 side through the electrolyte membrane 11, and the rest thereof is discharged from the openings 8 of the cathode collector 7 to the outside.
At this time, since the auxiliary porous layer 3 having higher degree of softness than the gas/liquid separation layer 2 and the anode passage plate 4 is disposed between the gas/liquid separation layer 2 and the anode passage plate 4, the liquid can be prevented from leaking to the gas passage 6 without forming any gap between the gas/liquid separation layer 2 and the anode passage plate 4, and reliability of such a gas/liquid separation structure can be improved. Moreover, no matter which direction the membrane electrode assembly 1 may be directed, CO2 can be separated from the methanol solution, and can be discharged.
Furthermore, the auxiliary porous layer 3 has not only a packing effect but also functions as a fluid element for improving the reliability of the gas/liquid separation. As shown in
PMeOH-PCO2≧ΔP1+ΔP2 (3)
When the pressure loss (ΔP1+ΔP2) when CO2 passes through the gas/liquid separation layer 2 and the auxiliary porous layer 3 is larger than the pressure difference (PMeCH-PCO2) between the fuel passage 5 and the gas passage 6, there is a possibility that CO2 generated by the anode reaction may be emitted into the fuel passage 5. If the CO2 is emitted into the fuel passage 5, this could result in breakage of the gas/liquid separation. Accordingly, the pressure loss (ΔP1+ΔP2) is designed by the auxiliary porous layer 3 to control porosity thereof so that the pressure loss (ΔP1+ΔP2) can be smaller than the pressure difference (PMeCH-PCO2) between the fuel passage 5 and the gas passage 6, whereby the reliability on the gas/liquid separation can be improved.
Moreover, as shown in
ΔPc>PMeOH-PCO2 (4)
When the surface tension ΔPc is smaller than the pressure difference (PMeCH-PCO2) between the fuel passage 5 and the gas passage 6, there is a possibility that the gas/liquid separation is disrupted. A magnitude of the surface tension ΔPc is controlled by using the auxiliary porous layer 3 to control the pore diameter thereof, whereby the reliability of the gas/liquid separation structure can be improved.
As described above, in accordance with the fuel cell according to the first embodiment of the present invention, the auxiliary porous layer 3 is disposed between the lyophobic gas/liquid separation layer 2 and the anode passage plate 4, whereby liquid leakage from the fuel passage 5 to the gas passage 6 can be prevented without forming any gap between the lyophobic gas/liquid separation layer 2 and the anode passage plate 4, and the reliability on the gas/liquid separation can be improved.
In an example of the fuel cell according to the first embodiment of the present invention, the anode passage plate 4 was fabricated, in which a width of the fuel passage 5 is 1 mm, a width of the gas passage 6 is 1 mm, and a land width is 0.8 mm. The anode passage plate 4 was pressed at a pressure of approximately 3.9 MPa while using carbon paper as the lyophobic gas/liquid separation layer 2 and an MPL with a thickness of 50 μm as the auxiliary porous layer 3. Then, the anode passage plate 4 was able to endure an inner pressure of the fuel passage 5 that was approximately 3 kPa. As a comparative example, when the auxiliary porous layer 3 was not provided, the gas/liquid separation was disrupted under an inner pressure of several ten Pa. As a result, it is understood that, thanks to the auxiliary porous layer 3, a resistance pressure of the gas/liquid separation structure is improved by approximately double digits.
As shown in
The second embodiment of the present invention is different from the first embodiment of the present invention in that a lyophilic porous layer is used as the gas/liquid separation layer 2. The carbon paper, the carbon cloth, the carbon nonwoven fabric, and the like are usable as the gas/liquid separation layer 2.
As shown in
The anode-side openings of the fuel passage 5 are positionally aligned with the second openings 32 of the auxiliary porous layer 3. Therefore, the auxiliary porous layer 3 does not inhibit the fuel supply from the fuel passage 5 to the gas/liquid separation layer 2. The anode-side openings of the gas passage 6 are positionally aligned with the openings 21 of the lyophilic gas/liquid separation layer 2 and the first openings 31 of the auxiliary porous layer 3. Other structures in the fuel cell shown in
In the fuel cell according to the second embodiment of the present invention, as shown in
CO2 generated by the anode reaction is discharged from the gas passage 6 through the openings 21 of the lyophilic gas/liquid separation layer 2 and the openings 31 of the auxiliary porous layer 3.
Here, as shown in
ΔP′c>P′MeOH-P′CO2 (5)
When the surface tension ΔP′c is smaller than the pressure difference (P′MeCH-P′CO2) between the fuel passage 5 and the gas passage 6, it is possible that the gas/liquid separation maybe broken. The magnitude of the surface tension ΔP′c is controlled by using the auxiliary porous layer 3 that can control the pore diameter thereof, whereby the liquid leakage from the fuel passage 5 to the gas passage 6 can be prevented, and the reliability of the gas/liquid separation structure can be improved.
As described above, in accordance with the fuel cell according to the second embodiment of the present invention, fuel leakage from the fuel passage 5 to the gas passage 6 can be prevented without forming any gap between the anode passage plate 4 and the lyophilic gas/liquid separation layer 2, and the reliability of the gas/liquid separation structure can improved.
Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.
DMFC has been explained as a fuel cell system in the first and second embodiment of the present invention. However, the present invention may be applied to various fuel cell systems. Various alcohols, ethers or the like instead of methanol may be used as the fuel.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-080317 | Mar 2007 | JP | national |
