JP2006147339A - Membrane-electrode assembly for solid polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid polymer electrolyte fuel cell that is less likely to cause voltage drop in a low-current region and is capable of suppressing dissolution of acid ions, by improving a bonding state of catalyst layers 2a and 2b to a polymer ion exchange membrane 3. <P>SOLUTION: In this membrane-electrode assembly, a hydrogen electrode-side catalyst layer 2a and an oxidation electrode-side catalyst 2b are pressed into a polymer ion exchange membrane 3 and brought into partial contact with each other in the polymer ion exchange membrane 3. By pressing-in and partial contact of the catalyst layers 2a and 2b, the catalyst layers 2a and 2b are firmly bonded to the polymer ion exchange membrane 3, and own effective resistance of the polymer ion exchange membrane 3 is also reduced. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a membrane-electrode assembly for a polymer electrolyte fuel cell that suppresses acidic ion elution from an electrolyte and prolongs the life of a fuel cell.

Solid polymer fuel cells have little impact on the environment, and are expected to be a movable or stationary electric energy source in various fields including the power source of automobiles because of their advantages of starting and generating power even at low temperatures of about room temperature. Yes. In the polymer electrolyte fuel cell, a hydrogen electrode 12 and an oxidation electrode 13 are arranged on both surfaces of a polymer ion exchange membrane 11, and a cell structure 10 (FIG. 1) sandwiched between separators 14 and 15 is used as one unit. A plurality of grooves 14 a for feeding the hydrogen-containing fuel are formed in the hydrogen electrode side separator 14. The oxidation electrode-side separator 15 is formed with a plurality of grooves 15a for sending oxygen or air and a groove 15b for sending water generated by the battery reaction. It is also known to form a flow path by laminating carbon paper or expansive graphite on a flat metal plate as a separator on the hydrogen electrode side and the oxidation electrode side (Patent Document 1).
JP 2000-123850 A

When the hydrogen-containing fuel is sent to the hydrogen electrode 12 (fuel electrode) side through the hydrogen supply groove 14a, H ₂ becomes proton H ⁺ on the fuel electrode 12. Proton H ⁺ permeates the polymer ion exchange membrane 11 in the presence of water, moves to the catalyst electrode layer 13 (oxidation electrode), and passes through the oxygen supply groove 15a to the oxidation electrode 13 side. It reacts with O ₂ and the electron e ⁻ flowing from the external circuit 16 and is discharged out of the system as water (reaction product) through the drainage groove 15b.
The flow of electrons e ⁻ along the external circuit 16 is extracted as electric energy, but the amount of electricity extracted from the single membrane-electrode assembly 10 is very small. Therefore, power that can be put to practical use is obtained by stacking a large number of cell structures 10.

The water produced during the battery reaction is theoretically pure water, and should not adversely affect the environment even if discharged. However, as a result of the power generation test by the present inventors, when a fluorine-based resin is used for the polymer ion exchange membrane 11, fuel is used in a low current density operation region where the cell voltage is high, including no-load operation (open circuit). It shows that acidic ions are contained in the water discharged from the battery (FIG. 2).
Although the outflow source of acidic ions is not clear, battery characteristics are significantly deteriorated if the operating conditions in which acidic ions are eluted are continued for a long time. When acidic ions are dissolved in the humidified water wetH ₂ supplied to the fuel cell or the generated water (reaction product), the metal separators 14 and 15, the metal drain pipe, and the like are easily corroded.

  In particular, the polymer electrolyte fuel cell is expected to be frequently started and stopped by taking advantage of the low temperature operation. Since it becomes a no-load state in which acidic ions are likely to elute at the time of starting and stopping, the life of the fuel cell is shortened in applications where the starting and stopping are frequently repeated. The concentration of acidic ions in the waste water is also high, which is not preferable from the environmental viewpoint. In view of the influence caused by the elution of acidic ions, it is desired to avoid as much as possible no-load operation that does not enter the operation region for power generation and operation in a low current density region, and the restrictions imposed on the operation conditions increase.

  The present invention has been devised to solve such problems, and by reducing the cell voltage during no-load operation or operation in a low current density region not practically used, elution of acidic ions and metal An object of the present invention is to provide a polymer electrolyte fuel cell that suppresses corrosion of components, maintains high power generation efficiency over a long period of time, and has no adverse effect on the environment.

  In order to achieve the object of the membrane-electrode assembly for a polymer electrolyte fuel cell of the present invention, the gas diffusion electrodes joined to the hydrogen electrode side catalyst layer and the oxidation electrode side catalyst layer are provided on both surfaces of the polymer ion exchange membrane. The hydrogen electrode side catalyst layer and the oxidation electrode side catalyst layer are pushed into the polymer ion exchange membrane and are in partial contact with each other inside the polymer ion exchange membrane.

Embodiment

The polymer electrolyte fuel cell according to the present invention includes a membrane-electrode assembly produced in the following process.
The catalyst layers 2a and 2b are formed by applying an ink containing a carbon carrier carrying a platinum-based catalyst, an electrolyte, polytetrafluoroethylene, etc. on one side of the gas diffusion electrodes 1a and 1b such as carbon paper and carbon cloth. . The ink is applied by an appropriate method, but if the spray coating method is adopted, the surfaces of the catalyst layers 2a and 2b are uneven, and the hydrogen electrode side catalyst layer 2a and the oxidation electrode side catalyst layer 2b are gas diffused during hot press molding. It is easily coupled to the electrodes 1a and 1b.

  The polymer ion exchange membrane 3 is sandwiched between the gas diffusion electrodes 1a and 1b combined with the catalyst layers 2a and 2b, and the polymer ion exchange membrane 3 is pressurized while heating the gas diffusion electrodes 1a and 1b by hot pressing or the like. . The polymer ion exchange membrane 3 softens during the pressure heating, and the catalyst layers 2a and 2b are partially pushed into the polymer ion exchange membrane 3. The indentation of the catalyst layers 2a and 2b becomes more prominent as the catalyst layers 2a and 2b are made uneven by spray coating or the like.

  The pressure heating condition is set so that the hydrogen electrode side catalyst layer 2a and the oxidation electrode side catalyst layer 2b are in partial contact. Specifically, the electrical resistance between the hydrogen electrode side catalyst layer 2a / oxidation electrode side catalyst layer 2b is monitored during hot pressing, and the hydrogen electrode side catalyst layer is selected by selecting a pressurizing condition that provides a specified resistance value. 2a and partial contact of the oxidation electrode side catalyst layer 2b become possible. (Fig. 3a)

  When the hydrogen electrode side catalyst layer 2a and the oxidation electrode side catalyst layer 2b are partially brought into contact with each other, a short circuit occurs and the effective resistance of the polymer ion exchange membrane 3 itself decreases. As the junction area between the catalyst layers 2a and 2b and the polymer ion exchange membrane 3 increases, the ion conductivity also increases. Further, since the catalyst layers 2a and 2b are pushed into the polymer ion exchange membrane 3, the catalyst layers 2a and 2b are firmly joined to the polymer ion exchange membrane 3, thereby improving the ion conductivity and consequently the battery performance.

  In this respect, the conventional membrane-electrode assembly has a structure (FIG. 3b) in which the gas diffusion electrodes 1a, 1b combined with the catalyst layers 2a, 2b are simply laminated on the polymer ion exchange membrane 3 (FIG. 3b). The electric resistance increases according to the film thickness of the ion exchange membrane 3. Further, the bonding strength of the catalyst layers 2a and 2b to the polymer ion exchange membrane 3 tends to be insufficient, and the ion conductivity is low, and the battery performance of the obtained fuel cell is not sufficient.

A catalyst ink was prepared by mixing 10 g of 5% Nafion (manufactured by DuPont) and 10 ml of water with 1 g of platinum-supported carbon black. After spray coating the catalyst ink on carbon paper (TGP-H-120: manufactured by Toray Industries, Inc.), the catalyst layer was fixed by heating to 80 ° C.
After 5% Nafion solution was applied to the carbon paper on which the catalyst layer was formed, the polymer ion exchange membrane was sandwiched therebetween, and the carbon paper was bonded to both sides of the polymer ion exchange membrane by hot pressing. The heating temperature during hot pressing was set to three conditions of 120 ° C., 150 ° C., and 175 ° C., and the influence of the heating temperature on the polymer ion exchange membrane / carbon paper bonding was investigated.

  In the membrane-electrode assembly hot-pressed at 120 ° C where the polymer ion exchange membrane does not substantially soften, the catalyst layer is not pushed into the polymer ion exchange membrane, and the interface between the polymer ion exchange membrane and the catalyst layer is It was flat (Fig. 3b). When the heating temperature is 150 ° C., the polymer ion exchange membrane partially softens during hot pressing, but the catalyst layer is not sufficiently pressed, and the contact between the hydrogen electrode side catalyst layer / oxidation electrode side catalyst layer does not reach. It was. In contrast, when hot pressing was performed at a heating temperature of 175 ° C., the catalyst layer was pushed in as the polymer ion exchange membrane softened, and the hydrogen electrode side catalyst layer partially contacted the oxidation electrode side catalyst layer.

Each produced membrane-electrode assembly is incorporated into a fuel cell, the cell temperature is maintained at 80 ° C., and hydrogen and non-humidified oxygen at 90 ° C. are supplied as reaction gases at a flow rate of 0.5 liter / min. Under operating conditions, the current-voltage characteristics and the pH value of the waste water were investigated.
The fuel cell using the membrane-electrode assembly produced at a hot press temperature of 120 ° C. and 150 ° C. had an open circuit voltage of 1.0 to 1.05 V, and the pH of the waste water was 4 or less. When the current was taken out at an open circuit voltage of 1 V, the cell voltage decreased, and a large voltage drop occurred particularly in the low current density region.

  On the other hand, the fuel cell using the membrane-electrode assembly produced at a hot press temperature of 175 ° C. has an open circuit voltage of 0.7 V, the pH of the waste water is 5 or more, and acidic ions are hardly eluted. (FIG. 4). Further, in the example of the present invention, although the open circuit voltage was low, the cell voltage only dropped slightly when a minute current was taken out (FIG. 5). The cell voltage drop is suppressed because the hot press temperature is high, the catalyst layer is firmly bonded to the polymer ion exchange membrane, and a short circuit occurs due to partial contact of the hydrogen electrode side catalyst layer / oxidation electrode side catalyst layer. This is presumably due to a decrease in the effective resistance of the ion exchange membrane.

  As a result of observing the cross section of the membrane-electrode assembly taken out from the cell after the battery test, in the membrane-electrode assembly hot-pressed at 120 ° C. and 150 ° C., the hydrogen electrode side catalyst layer and the oxidation electrode side are separated by the polymer ion exchange membrane. The catalyst layer was completely separated (Fig. 3a). In the membrane-electrode assembly produced by hot pressing at 175 ° C., the partial contact (FIG. 3b) of the hydrogen electrode side catalyst layer / oxidation electrode side catalyst layer is still maintained, and the catalyst layer is bonded to the polymer ion exchange membrane. The condition was good.

A fuel cell unit incorporating a membrane-electrode assembly produced at a hot press temperature of 120 ° C. was continuously operated with no load, and changes with time in cell voltage and drainage pH were investigated. As the battery operation time became longer, the pH value of the wastewater decreased, and reached pH 3.3 on the oxidation electrode side after 40 hours of continuous operation. Further, when the operation was continued, the pH of the drainage was increased, but it did not reach pH 5 or higher.
A fuel cell unit incorporating a membrane-electrode assembly produced by hot pressing at 175 ° C. for 30 minutes was examined for changes over time in cell voltage and drainage pH under the same conditions. Although the cell voltage was slightly decreased, the pH of the drainage was slightly decreased on both the hydrogen electrode side and the oxidation electrode side, but remained at a high pH value.

  As explained above, by bringing the hydrogen electrode side catalyst layer and the oxidation electrode side catalyst layer sandwiching the polymer ion exchange membrane into partial contact with each other in the polymer ion exchange membrane, the open circuit voltage is reduced, and the acidic ions are reduced. Elution is also suppressed. Therefore, the burden on the cell is reduced even in an open circuit state, and acidic ions are not eluted. A fuel cell incorporating this membrane-electrode assembly has high durability under actual use and is environmentally superior because it does not elute acidic ions.

Schematic showing the laminated structure of a polymer electrolyte fuel cell Graph showing drainage pH according to current density The figure which compared the membrane-electrode assembly (a) of this invention with the conventional membrane-electrode assembly (b) Graph showing the relationship between hot press temperature of membrane-electrode assembly, cathode drainage pH, and open circuit voltage Graph showing current-voltage characteristics of a cell equipped with a membrane-electrode assembly according to the present invention

Explanation of symbols

1a, 1b: Gas diffusion electrode 2a, 2b: Catalyst layer 3: Polymer ion exchange membrane 11: Polymer ion exchange membrane 12, 13: Catalyst electrode layer 14, 15: Separator

Claims (1)

  A gas diffusion electrode joined with a hydrogen electrode side catalyst layer and an oxidation electrode side catalyst layer is laminated on each side of the polymer ion exchange membrane, and the hydrogen electrode side catalyst layer and the oxidation electrode side catalyst layer are pushed into the polymer ion exchange membrane. A membrane-electrode assembly for a polymer electrolyte fuel cell, wherein the membrane-electrode assembly is in partial contact with each other inside the polymer ion exchange membrane.

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