JPH0766810B2 - Fuel cell - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a fuel cell, and more particularly to a fuel cell capable of enhancing the hydrophobicity of an electrode catalyst layer.

[Technical Background of the Invention] A fuel cell is a device that directly oxidizes the energy released along with the oxidation reaction by oxidizing the chemical energy of the fuel in an electrochemical process. It is expected that the power plant using this fuel cell will have a thermal efficiency of 40 to 50% even on a relatively small scale, and that it will far surpass new thermal power. In addition, the emission of SOx and NOx, which are pollution factors that have become a big social problem in recent years, is extremely small, a large amount of cooling water is not required because the combustion cycle is not included in the power generator, and the vibration noise is small. In addition to being expected to have high energy conversion efficiency, there are few environmental problems such as noise and exhaust gas, and moreover, it has characteristics such as good responsiveness to load changes.
Expectations and interests in research on their development and practical application are high.

FIG. 3 is an exploded perspective view showing a structural example of a unit cell in an electrode type fuel cell with ribs, which uses phosphoric acid as an electrolyte based on the above principle. In the figure, 1 is an electrolyte layer in which phosphoric acid as an electrolyte is impregnated in a matrix, and 3a and 3b are an anode electrode and a cathode electrode made of a porous carbon material arranged with the electrolyte layer 1 in between. Catalysts 2a and 2b are applied on the side in contact with the layer 1, and ribs 4a and 4b and grooves 5a and 5b through which fuel gas and oxidant gas flow are provided on the back side, respectively. Here, the groove 5a through which the fuel gas flows and the groove through which the oxidant gas flows
A plurality of 5b are regularly formed in parallel in a direction orthogonal to each other. A unit cell is formed as described above, and a plurality of such unit cells are sandwiched between separators 6 to form a unit cell stack.

By the way, in the fuel cell using phosphoric acid as an electrolyte as described above, there is no problem in performance even if carbon dioxide is mixed, but on the other hand, the polarization in the electrode reaction, especially in the positive electrode reaction is large. The use of active electrocatalysts to accelerate the reaction is necessary. That is, the porous electrode is one in which a supported catalyst in which a catalyst is supported on carbon particles is adhered and formed in layers on a gas-permeable conductive porous substrate made of a carbon material.

By the way, in the phosphoric acid fuel cell using phosphoric acid as an electrolyte among the above-mentioned fuel cells, the oxidizer electrode: 1 / 2O ₂ + 2H ⁺ + 2e → H ₂ O (1) Fuel electrode: H ₂ → 2H ⁺ + 2e (2) The reaction proceeds. Such a reaction proceeds in the catalyst layer formed on the electrolyte side of the electrode. The catalyst is in contact with the electrolyte on one side and with the gas phase on the other side, and in order for the above reaction to proceed rapidly, the exchange of atoms between the gas phase and the liquid phase is on the solid phase, that is, on the catalyst. Needs to be done efficiently. That is, an effective three-phase interface of gas / liquid / solid phase is required. In order to maintain this three-phase interface for a long period of time, a technique has been developed in which a hydrophobic portion (for example, polytetrafluoroethylene) and a hydrophilic portion (for example, platinum) are microscopically mixed in the catalyst layer to form an electrode. ing. In this case, the hydrophilic portion serves as a passage for communicating the liquid phase to the gas phase side, and the hydrophobic portion serves as a passage for the gas phase to the liquid phase side.
Ideally, the three-phase interface is three-dimensionally formed uniformly.

[Problems of Background Art] In the fuel cell described above, if the hydrophobicity of the electrode catalyst layer is not sufficiently ensured, the water generated by the reaction formula (I) may be aged during the operation of the cell. As a result, the electrode catalyst layer is wetted, the above-mentioned passage of the gas phase to the liquid phase side is not secured, and the generated water covers the electrode catalyst layer and the three-phase interface is lowered. Therefore, as described above, conventionally, a fluorine resin, for example, polytetrafluoroethylene is mixed in order to make the electrode catalyst layer hydrophobic. However, although this fluororesin is excellent in hydrophobicity, on the contrary, it is an electrical insulator that does not have the electrical conductivity essential for electrodes, so it is added to the electrode catalyst layer so that sufficient hydrophobicity is obtained. The fluorine-based resin produced causes a decrease in electrical conductivity of the electrode, resulting in a decrease in battery characteristics.

On the other hand, as another method of providing the electrode catalyst layer with hydrophobicity, there is a method of improving the hydrophobicity of the carbon carrier carrying the catalyst. Supporting the catalyst on the carbon support is generally carried out by a method of depositing the catalyst on the carbon support in a solution. At this time, if the carbon carrier has a strong hydrophobicity, the carbon carrier is poorly compatible with the carbon carrier and the catalyst cannot be uniformly dispersed on the carbon carrier. As a result, the catalyst is supported in an agglomerated state, leading to a decrease in the three-phase interface, which causes a problem that good battery characteristics cannot be obtained.

Furthermore, when the fuel cell was operated and the fluid discharged from the cell was inspected, fluorine ions were detected in the condensed water condensed from the discharged fluid. This is probably because the fluororesin in the electrode catalyst layer was decomposed and carried out of the fuel cell. That is, the fluororesin is excellent in electrolyte resistance and heat resistance, but in a fuel cell, since it is heated to a high temperature by reaction heat in a state of being immersed in the electrolyte, decomposition of the fluororesin occurs. It is believed that there is.

[Object of the invention] The present invention has been made to solve the above problems, and an object thereof is to provide a sufficient and stable hydrophobic property for a long period of time without deteriorating the electrical conductivity of the electrode. (EN) Provided is a fuel cell excellent in heat resistance and electrolyte resistance that can be maintained at

[Summary of the Invention] In order to achieve the above object, in the present invention, an electrolyte is impregnated between a pair of electrodes formed by coating a catalyst layer on one surface of a conductive porous substrate having a groove for gas flow. In a fuel cell constituted by sandwiching the electrolyte layer, the supported catalyst having a catalytic function that is an activation function for promoting the reaction at the electrode, a binding function for maintaining the catalyst layer, and an excess electrolyte Fluorine-based resin having a hydrophobic function to prevent penetration,
The catalyst layer is formed by mixing with a highly hydrophobic carbon having a hydrophobic function for preventing the permeation of excess electrolyte.

Here, fine graphite powder is used as the carbon having a particularly high hydrophobicity.

Further, as the highly hydrophobic carbon, carbon having a high degree of crystallinity and no functional group or carbon having a high degree of crystallinity and significantly less functional group is used.

[Examples of the Invention] First, the present invention relates to a pair of electrodes formed by coating a catalyst layer on one surface of a conductive porous substrate having a groove for gas flow,
In the above-mentioned fuel cell constituted by sandwiching an electrolyte layer impregnated with an electrolyte, it is possible to form the above catalyst layer by mixing a supported catalyst, a fluorine-based resin as a binder, and carbon having strong hydrophobicity. It is what

More specifically, the present invention relates to a catalyst layer of an electrode of a fuel cell, and particularly to realize long-term stability of the catalyst layer. That is, the constitution of the catalyst layer of the electrode of the fuel cell of the present invention is as follows: (a) supported catalyst (catalyst supported on carbon carrier) (b) fluororesin (mainly polytetrafluoroethylene) (c) hydrophobic It is a catalyst layer made of strong carbon (eg graphite).

Here, the supported catalyst has an activation function for promoting the reaction at the electrode, that is, a so-called catalytic function.

Further, the fluorine-based resin has a binding function for maintaining the catalyst layer and a hydrophobic function for preventing excessive permeation of the electrolyte.

Further, the highly hydrophobic carbon has a hydrophobic function for preventing permeation of an excessive electrolyte.

Now, the catalyst layer of the electrode of the conventional fuel cell is composed of the above-mentioned (a) supported catalyst and (b) fluororesin. However, as explained in the background art problem, there is a problem in the physical properties and electrolyte resistance of the fluororesin, and in the constitution consisting only of (a) the supported catalyst and (b) the fluororesin, sufficient battery characteristics are not obtained. Stability over time cannot be obtained.

Therefore, in the present invention, in order to eliminate the problem with the conventional catalyst layer, that is, the problem that the fluororesin has, that is, in order to eliminate the poor stability over time due to the poor corrosion resistance of the fluororesin, c) Stabilization of battery characteristics is attempted by adding a highly hydrophobic carbon component to form a catalyst layer.

Hereinafter, a specific embodiment of the present invention based on the above concept will be described with reference to the drawings. That is,
As an example, as shown in FIG. 1, first, the supported catalyst 7 in which white metal particles are supported on carbon particles is 30 to 60 wt%.
Then, 30 to 50 wt% of a fluorine-based resin 8 such as polytetrafluoroethylene as a binder and 10 to 20 wt% of carbon 9 having a strong hydrophobic property such as fine graphite powder are uniformly mixed in water. Next, this is sprayed onto one surface of the conductive porous substrate using air spray or the like, pressurized at a predetermined pressure, and then heated and baked at about 330 ° C. to form an electrode catalyst layer. In this case, as the highly hydrophobic carbon 9, carbon having a high degree of crystallinity and no functional group or carbon having a high degree of crystallinity and significantly less functional group is used. Further, as the polytetrafluoroethylene 8 which is a binder, one that can sufficiently bind the supported catalyst 7 when the electrode catalyst layer is heated and baked at 330 ° C. is used, and it does not affect the electrical conductivity. I keep it to.

In the fuel cell having the electrode catalyst layer formed as described above, highly hydrophobic carbon 9 having high crystallinity and no functional group, or high crystallinity and significantly less functional group is used as the electrode catalyst layer. Since it is mixed, the hydrophobicity of the electrode catalyst layer can be enhanced without using a large amount of polytetrafluoroethylene 8 which is a binder, and a decrease in electric conductivity of the electrode can be prevented. .
Further, since the highly hydrophobic carbon 9 having excellent heat resistance and electrolyte resistance is used, it becomes possible to maintain sufficient and stable hydrophobicity in the electrode catalyst layer for a long period of time.

FIG. 2 shows current-voltage characteristics of the fuel cell using the electrodes on which the electrode catalyst layers were formed by the present example and the conventional method, respectively. In the figure, A shows the characteristic according to this embodiment, and B shows the characteristic according to the related art. As shown in FIG. 2, the battery in which the hydrophobicity of the electrode catalyst layer was reinforced by the method of the present embodiment is higher in battery than the conventional one in which the hydrophobicity of the electrode catalyst layer is reinforced by polytetrafluoroethylene 8. It was found that the characteristics were exhibited, and the decrease with time was better than that of the one in which the hydrophobicity of the electrode catalyst layer was enhanced by polytetrafluoroethylene 8.

Although the supported catalyst 7, the fluorine-based resin 8 and the highly hydrophobic carbon 9 are mixed in a liquid such as water in the above-mentioned embodiment, the present invention is not limited to this, and they may be mixed in a dry state. However, the same effect can be obtained.

Further, although polytetrafluoroethylene is used as the fluorine-based resin 8, the same effect can be obtained by using other fluorine-based resin.

In addition, the present invention can be variously modified and implemented within the scope of the invention.

[Effects of the Invention] As described above, according to the present invention, an electrolyte is impregnated between a pair of electrodes formed by coating a catalyst layer on one surface of a conductive porous substrate having a groove for gas flow. In a fuel cell constituted by sandwiching an electrolyte layer, a supported catalyst having a catalytic function which is an activation function for promoting a reaction at an electrode,
Fluorine resin, which has a binding function to maintain the catalyst layer and a hydrophobic function to prevent the penetration of excess electrolyte, and a highly hydrophobic carbon, which has a hydrophobic function to prevent the penetration of excess electrolyte, are mixed. Since the catalyst layer is formed, heat resistance and electrolyte resistance that allow the electrode catalyst layer to maintain sufficient and stable hydrophobicity for a long period of time without deteriorating the electrical conductivity of the electrode. A superior and highly reliable fuel cell can be provided.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an electrode catalyst layer showing one embodiment of the present invention,
FIG. 2 is a diagram showing current-voltage characteristics of a battery according to the present invention and a conventional one, and FIG. 3 is an exploded perspective view showing a structure of a unit cell of a fuel cell. 1 ... Electrolyte layer, 2a, 2b ... Catalyst, 3a ... Anode electrode, 3b ...
Cathode electrodes, 4a, 4b ... Ribs, 5a, 5b ... Grooves, 6 ... Separator, 7 ... Supported catalyst, 8 ... Fluorine-based resin, 9 ... Strongly hydrophobic carbon.

Claims (3)

[Claims] 1. A fuel cell in which an electrolyte layer impregnated with an electrolyte is sandwiched between a pair of electrodes formed by coating a catalyst layer on one surface of a conductive porous substrate having a groove for gas flow. A supported catalyst having a catalytic function which is an activation function for promoting the reaction at the electrode, and a fluorine-based catalyst having a binding function for maintaining a catalyst layer and a hydrophobic function for preventing the permeation of an excessive electrolyte. A fuel cell, characterized in that the catalyst layer is formed by mixing a resin and a highly hydrophobic carbon having a hydrophobic function for preventing permeation of an excessive electrolyte. 2. The fuel cell according to claim 1, wherein fine graphite powder is used as the highly hydrophobic carbon. 3. A carbon having a high degree of crystallinity and no functional group, or a carbon having a high degree of crystallinity and a remarkably small number of functional groups is used as the highly hydrophobic carbon. A fuel cell according to item (1).