Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-06-07
2002-11-05
Ryan, Patrick (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C429S047000, C429S047000
Reexamination Certificate
active
06475249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a method for manufacturing a fuel cell, and more particularly to a method for manufacturing a membrane electrode assembly of the fuel cell with solvent pretreatment to the electrolyte membrane of the membrane electrode assembly.
2. Description of the Prior Art
A fuel cell converts chemical energy into electrical energy and thermal energy by means of chemical reaction between hydrogen-containing fuel and oxygen. Benefits of the fuel cell include low pollution, high efficiency, high energy density and simple fuel recharge. Applications of the fuel cells include electrochemical engines, portable power supplies, standby power supply facilities, power generating systems, and so on.
The chemical reaction of a fuel cell requires the presence of an electrolyte, electrodes and catalysts. Based on the electrolyte, the fuel cell is classified as AFC, PAFC, MCFC, SOFC, and proton exchange membrane. During recent years, the proton exchange membrane type fuel cell is one of the most intensely-researched fuel cell. The proton exchange membrane may be classified as PEMFC and DMFC. The difference between PEMFC and DMFC is the fuel that they take. A PEMFC uses hydrogen or reformed gases containing rich hydrogen while a DMFC uses methanol solution.
A typical proton exchange membrane type fuel cell comprises a seven-layered structure, including a central electrolyte membrane for the transmission of protons, two catalyst layers on opposite sides of the electrolyte membrane in which the chemical reactions occur, two gas diffusion electrodes stacked on the catalyst layers comprising low porosity carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell, and two flow field plates stacked on the gas diffusion electrodes. The flow field plates are made of carbon plates, metal plates or composite graphite fiber plates. Gas guide channels are defined on the gas diffusion electrode facing sides of the flow field plates. Reactants and reaction products are guided into/out of the cell through the flow field plates. The structure mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units arranged to form a stack and is serially connected together. If desired, cooling plates and humidifying plates may be added to ensure the operation and performance of the fuel cell stack.
Examples of fuel cells and the manufacturing techniques thereof were disclosed in U.S, Pat. Nos. 5,252,410, 5,399,184, 5,523,177, 5,683,828, 5,723,173, 5,723,288, 5,869,201, and 6,010,606.
Porous material is used as the electrode of the fuel cell for the reaction gas coming in and the product gas going out, so called Gas Diffusion Electrode.
FIG. 1
shows a conventional basic fuel cell unit, comprising a central electrolyte membrane
10
. One side of the electrolyte membrane
10
is coated with a cathode catalyst layer
21
, and the other side of the electrolyte membrane
10
is coated with an anode catalyst layer
22
. Two gas diffusion electrodes
31
and
32
, which are usually made of carbon cloth or carbon paper, are formed on the outer side of the catalyst layers
21
and
22
respectively. The Conventional process for manufacturing the fuel cell unit is coating the catalyst slurry on the inner side of the gas diffusion electrodes
31
and
32
to form catalyst layers
21
and
22
respectively. Then, the electrolyte membrane
10
is interposed between the cathode gas diffusion electrode
31
and the anode gas diffusion electrode
32
coated with catalyst layer to form a basic fuel cell unit
1
. It is found that the conventional method has a serious disadvantage that the catalyst slurry is easily permeating into the carbon cloth or carbon paper of the gas diffusion electrode, and the thickness of the catalyst layer coated is not easy to be controlled.
In another manufacturing method in prior art, the catalyst slurry is coated on both sides of the electrolyte membrane
10
, as shown in FIG.
2
. The structure fabricated by this approach is so called Membrane Electrode Assembly (MEA). In this process, the surfaces of both sides of the electrolyte membrane
10
are first coated with a cathode catalyst layer
21
and an anode catalyst layer
22
respectively. Then, the electrolyte membrane
10
is interposed between the gas diffusion electrodes
31
and
32
to form a basic fuel cell unit
1
.
FIG. 3
is a left side elevational view showing the electrolyte membrane
10
and the cathode catalyst layer
21
shown in FIG.
2
.
FIG. 4
is a right side elevational view showing the electrolyte membrane
10
and the anode catalyst layer
22
shown in FIG.
2
.
FIG. 5
is a perspective view showing a fuel cell stack comprising a basic fuel cell unit
1
, an anode gas distribution plate
4
for transporting hydrogen, a cathode gas distribution plate
6
for transporting oxygen, and a cooling plate/humidifying plate
5
. The anode gas distribution plate
4
and cathode gas distribution plate
6
may be combined to be a bi-polar plate
7
.
Coating catalyst layer on the electrolyte membrane directly makes the catalyst slurry and the electrolyte membrane contact better, thereby decreasing reaction resistance and increasing activity. It is easier to control the thickness and quantity of the catalyst layer, thereby decreasing the quantity used and cost of the catalyst slurry.
However, the polymeric material of the electrolyte membrane available in the market, such as Nafion produced by DuPont or the products produced by Dow, Asahi Chemical, or Asahi Glass, has feature of good water adsorption. It is found that the polymeric material of the electrolyte membrane will absorb the solvent of the catalyst slurry when coating the slurry on the electrolyte membrane, thereby causing the electrolyte membrane expanding and deforming. With this character, it becomes a problem to coat the catalyst slurry on both sides of the electrolyte membrane evenly for making MEA.
For overcoming the problem of the electrolyte membrane deformation discussed above, a plurality of new manufacturing processes are developed. One of the new processes is coating the catalyst slurry on a transfer paper, then transferring the catalyst layer to the electrolyte membrane by hot pressing.
FIG. 6
shows a conventional manufacturing process for manufacturing a fuel cell unit. The method comprises the following steps: preparing an electrolyte membrane
100
, changing the property of the electrolyte membrane
101
, coating catalyst layer on one side of the electrolyte membrane
102
, pre-drying
103
, coating catalyst layer on the other side of the electrolyte membrane
104
, drying
105
, changing the property of the electrolyte membrane again
106
, finishing MEA
107
, interposing the MEA between two gas diffusion electrodes and hot pressing
108
, and finished the basic fuel cell unit
109
.
In the prior art manufacturing method mentioned above, the property of the electrolyte membrane needs to be changed from H-form to Na-form by ion exchanging in step
101
. Then, the Na-form membrane must be changed back to H-form in step
106
for purpose of proton transmission. In addition, the processes of this prior art must include coating a first catalyst layer on one side of the electrolyte membrane at first, pre-drying the electrolyte membrane coated with catalyst layer, coating a second catalyst layer on the other side of the electrolyte membrane, and drying the electrolyte membrane again. It is noted that the processes are so complex.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a method for manufacturing membrane electrode assembly of a fuel cell for overcoming the problem of deforming when coating the catalyst slurry on the electrolyte membrane.
Another object of the present invention is to provide a simple method for manufacturing membrane electrode assembly of a fuel cell. The property of the electrolyte membrane doesn't need to be changed w
Cheung Yaw Chung
Hsu Cheng Hsien
Li Yingjeng James
Wan Chi Chao
Industrial Technology Research Institute
Ruthkosky Mark
Ryan Patrick
Troxell Law Office PLLC
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