JP3565077B2 - Electrode for fuel cell and method for producing the same - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a polymer electrolyte fuel cell, and more particularly, to an electrode that is a constituent element thereof.
[0002]
[Prior art]
BACKGROUND ART In recent years, polymer electrolyte fuel cells have attracted attention as power supplies for electric vehicles and distributed power supplies. At present, the required ionic conductivity of a polymer electrolyte used in a polymer electrolyte fuel cell is maintained when the polymer electrolyte is sufficiently wetted with water. On the other hand, the electrode reaction as a battery is a reaction for generating water generated at a three-phase interface between a catalyst, a polymer electrolyte, and a reaction gas, and water vapor generated in a supplied gas and water generated by the electrode reaction are not quickly discharged. If it stays in the electrode or the diffusion layer, the gas diffusion becomes worse, and the battery characteristics are adversely degraded.
[0003]
From such a viewpoint, for the electrodes used in the polymer electrolyte fuel cell, measures are taken to promote the moisturization of the polymer electrolyte and the discharge of water. As a general electrode, an electrode obtained by forming a carbon powder supporting a noble metal serving as a catalyst layer on a porous conductive electrode substrate serving as a gas diffusion layer is used. As the porous conductive substrate, carbon paper or carbon cloth made of carbon fiber is used. These porous conductive substrates are subjected to a water-repellent treatment using a dispersion of a polytetrafluoroethylene-based material in advance, so that the water generated by the electrode reaction can be quickly discharged, and In general, the electrolyte membrane and the polymer electrolyte in the electrode are brought into an appropriate wet state. In addition, as another method, a countermeasure for mixing water-repellent carbon particles into the electrode catalyst layer to discharge excess water generated in the electrode catalyst layer has been taken.
[0004]
[Problems to be solved by the invention]
As described above, an electrode used in a conventional polymer electrolyte fuel cell is obtained by subjecting a porous conductive substrate serving as a gas diffusion layer to a water-repellent treatment. For this reason, although the water discharge property of the gas diffusion layer is improved, the water discharge property in the catalyst layer and the gas diffusion property to the catalyst layer are deteriorated. There was a problem that the battery characteristics deteriorated.
[0005]
When carbon that has been subjected to water-repellent treatment using submicron-order polytetrafluoroethylene-dispersed particles is introduced into the electrode catalyst layer, the polymer electrolyte in the catalyst layer is often present in the water-repellent treated carbon particles. It is adsorbed, and the degree of contact between the polymer electrolyte and the catalyst particles is insufficient and the catalyst particles become uneven, or the catalyst particles are covered with PTFE, so that a sufficient three-phase interface cannot be secured. there were. Furthermore, if the carbon particles supporting the catalyst fine particles serving as a catalyst exhibit water repellency, the wet state of the polymer electrolyte in the polymer electrolyte membrane or the electrode catalyst layer shifts to the drying direction, and the battery characteristics deteriorate. There was a problem to do.
[0006]
As described above, there is a demand for a high-performance electrode designed so that water does not stay in the electrode catalyst and the polymer electrolyte is kept in an appropriate wet state.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the fuel cell electrode of the present invention has a hydrogen ion conductive solid polymer electrolyte membrane, a pair of electrodes sandwiching the hydrogen ion conductive solid polymer electrolyte membrane, and the electrode In a fuel cell including an electrode-electrolyte assembly in which a pair of diffusion layers are stacked, the electrode includes a catalyst in which catalyst particles are supported on a hydrophilic carbon material, a hydrogen ion conductive polymer electrolyte, and a water-repellent carbon material. And a hydrophilic layer is chemically bonded to at least a part of the surface of the catalyst particles .
[0009]
Further, it is preferable that a catalyst body in which catalyst particles are supported on a hydrophilic carbon material is selectively disposed on the hydrogen ion conductive polymer electrolyte membrane side, and a water-repellent carbon material is selectively disposed on the diffusion layer side.
[0010]
At this time, it is effective that the water-repellent carbon material has a monomolecular layer in which a part or the entire surface of the carbon material is chemically bonded to a silane coupling agent having a hydrophobic part.
[0011]
Further, it is effective that the hydrophilic carbon material has a monomolecular layer in which a part or the entire surface of the carbon material is chemically bonded to a silane coupling agent having a hydrophilic part.
[0012]
In the above, it is effective that the carbon material and the silane coupling agent are chemically bonded via at least one functional group selected from a phenolic hydroxyl group, a carboxyl group, a lactone group, a carbonyl group, a quinone group and a carboxylic anhydride. is there.
[0013]
Further, in the production method, at least one of the catalyst particles or the carbon material is immersed in a solvent containing a silane coupling agent, so that the silane coupling agent is applied to at least a part of the catalyst particle surface or the carbon material surface. After the chemical adsorption, the surface of the catalyst particles or the surface of the carbon material is chemically bonded to a silicon atom in the molecule of the silane coupling agent to form a layer having hydrophilicity or water repellency. Features.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
As described above, in the fuel cell electrode according to the present invention, the catalyst layer is composed of the polymer electrolyte, the hydrophilic carbon material, and the water-repellent carbon material. In addition, water is appropriately held by the hydrophilic carbon material, and excess water generated is quickly discharged by the adjacent water-repellent carbon material.
[0015]
As a result, even when the fuel cell is operated at a relatively low current density, it is expected that the electrode maintains a constant water holding power by the hydrophilic carbon material, and high characteristics can be expected. In addition, when operated at a relatively high current density, excess water is quickly discharged by the water-repellent carbon material located very close to the hydrophilic carbon material, and the flooding phenomenon is less likely to occur, improving battery performance. I do.
[0016]
When the hydrophilic carbon material is disposed on the polymer electrolyte membrane side and the water-repellent carbon material is disposed on the gas diffusion layer side, the polymer electrolyte membrane side becomes a highly humidified atmosphere, and the ionic conductivity of the polymer electrolyte membrane becomes higher. And the battery characteristics are improved.
[0017]
Further, in the fuel cell electrode of the present invention, on the surface of the carbon particles, the hydrolyzable group of the silane coupling agent having a hydrophobic site is hydrolyzed by water in solution or air, and water adsorbed on the carbon surface. Then, it is converted into an active silanol group (基 SiOH) and reacts with a functional group on the carbon surface to form a strong bond. Thereby, a very microscopic monomolecular water-repellent layer having a thickness of several nm to several tens nm is formed on the surface of the carbon particles. By using the water-repellent carbon particles, even if the electrode is formed by mixing with the hydrophilic catalyst-carrying carbon particles, the catalyst particles in the electrode are coated as in the case of using the submicron-order PTFE dispersion particles. And does not hinder the supply of the reaction gas.
[0018]
Furthermore, in the fuel cell of the present invention, the hydrolyzable group of the silane coupling agent is formed on the surface of the catalyst particles or on the surface of the carbon particles carrying the catalyst by the same method as described above. Is converted into active silanol groups (水分 SiOH) and reacts with the functional groups on the carbon surface to form strong bonds. By providing the silane coupling agent with a hydrophilic group such as a sulfone group or a carboxyl group, the catalyst surface becomes hydrophilic, and the wet state near the three-phase interface is maintained.
[0019]
As described above, when the electrode of the present invention is used, in the vicinity of the three-phase interface where the electrode reaction occurs, the wet state is appropriately maintained by the hydrophilic catalyst-carrying carbon particles, and excess water is generated by the adjacent water-repellent carbon. As a result, a polymer electrolyte fuel cell having higher performance than before can be constructed.
[0020]
Hereinafter, the fuel cell of the present invention will be described with reference to the drawings.
[0021]
【Example】
( Reference Example 1 )
First, a method for producing a water-repellent carbon material will be described. The entire surface of the carbon powder was directly adsorbed with a silane coupling agent by a chemical adsorption method in a nitrogen gas atmosphere to form a monomolecular film composed of the silane coupling agent. As a silane coupling agent, CH ₃ — (CH ₂ ) n —SiCl ₃ (n is an integer of 10 or more and 25 or less) having a linear hydrocarbon chain is used, and hexane dissolved at a concentration of 1% by weight is used. The solution was prepared and the carbon particles were immersed. The carbon particles used at this time were made of graphitizable carbon having a phenolic hydroxyl group and a carboxyl group left on the surface, and a dehydrochlorination reaction of this functional group and the above-mentioned silane coupling agent -SiCl ₃ was carried out. A monomolecular water-repellent film was formed using a ring agent. This is shown in FIG.
[0022]
In FIG. 1, 1 is a carbon particle, and 2 is a monomolecular water-repellent film. The thickness of the monomolecular water-repellent film 2 was about 2 to 10 nm. Here, by changing the molecular weight of the single molecule, this film thickness could be made 1 to 100 nm. Further, the material for chemical adsorption is not limited to the silane-based surfactant used in this embodiment as long as it contains a group having a binding property to an -OH group, for example, a ≡SiCl group.
[0023]
Next, a hydrophilic carbon powder with platinum as an electrode catalyst supported 25 wt%, was mixed with the water-repellent carbon material, this was dispersed -SO ₃ H groups pendent polyfluorocarbon polymer electrolyte solution (FSS-1, manufactured by Asahi Glass) and butanol. The ink carbon paper as a gas diffusion layer (manufactured by Toray Industries, TGP-H-120, thickness 360 .mu.m) on was coated by screen printing, to remove Butanoru by heating and drying, the electrode of the present embodiment A.
[0024]
In the above process, a carbon powder carrying platinum (Vulcan XC72R, manufactured by Cabot Corporation) having many functional groups on the surface and having hydrophilicity was used. The amount of platinum per unit area was 0.5 mg / cm ² . Furthermore, the mixed weight ratio of the platinum-supported hydrophilic carbon powder, the water-repellent carbon material, and the polyfluorocarbon-based polymer electrolyte was 100: 20: 3 after finishing.
[0025]
Next, a comparative electrode B was prepared. The comparative electrode B has a conventionally proposed configuration in which a carbon powder supporting a noble metal serving as a catalyst and a water repellent are formed on a porous conductive electrode base material serving as a gas diffusion layer. Was. As the porous conductive substrate, the same carbon paper (TGP-H-120, Toray Co., Ltd., film thickness: 360 μm) as that used in the above-mentioned electrode A was used, and this was a polyfluorocarbon-based material which was previously pendant with an —SO ₃ H group. Water repellency treatment was performed using a solution (FSS-1, manufactured by Asahi Glass) in which a polymer electrolyte was dispersed. In the above configuration, a carbon powder having a small surface functional group and exhibiting water repellency (Denka Black, manufactured by Denki Kagaku Kogyo) was used. Further, the water-repellent agent was used a solution prepared by dispersing the -SO 3 _H groups pendent polyfluorocarbon polymer electrolyte (FSS-1, manufactured by Asahi Glass). The other configuration was the same as the electrode A described above.
[0026]
The electrode A of the present reference example and the electrode B of the comparative example thus produced were arranged on both sides of a polymer electrolyte membrane (manufactured by Dupont, Nafion 112) and hot pressed to produce an electrode-electrolyte assembly. This was set in the unit for measuring single cells shown in FIG. 2 to form a single cell. In FIG. 2, reference numerals 3, 4, and 5 indicate the electrode-electrolyte junctions.
[0027]
In these cells, hydrogen gas is supplied to the fuel electrode and air is supplied to the air electrode. The cell temperature is 75 ° C., the fuel utilization is 80%, the air utilization is 30%. The dew point was adjusted to 65 ° C. FIG. 3 shows the current-voltage characteristics of the battery at this time.
[0028]
In FIG. 3, it was confirmed that the electrode using the electrode A of the present reference example exhibited superior characteristics as compared with the electrode B having a conventionally proposed configuration. The reason for this is that when water-repellent carbon powder treated with a silane coupling agent is used, the catalyst fine particles in the electrode are coated as in the case of PTFE-supported carbon powder using PTFE dispersion particles on the order of submicrons. It is considered that this does not hinder the supply of the reaction gas.
[0029]
( Reference Example 2 )
In this reference example , an electrode was prepared in which the electrode catalyst layer was disposed on the polymer electrolyte membrane side and the water-repellent carbon particles were disposed on the diffusion layer side, and the characteristics were evaluated. First, the catalyst-supporting carbon powder shown in Reference Example 1 and the water-repellent carbon powder treated with a silane coupling agent were applied as separate inks to form electrodes. This is shown in FIG. First, water-repellent carbon powder (DENKA BLACK, manufactured by Denki Kagaku Kogyo) was made into an ink using butanol , and screen-printed on carbon paper (manufactured by Toray, TGP-H-120, film thickness: 360 μm). After drying, the catalyst-supporting carbon powder 6 was made into an ink using a solution (FSS-1, manufactured by Asahi Glass) in which a polyelectrolyte solution of a polymer electrolyte solution in which a -SO ₃ H group-pendant polyfluorocarbon polymer electrolyte was dispersed, and butanol. An electrode was prepared by coating the carbon paper 8 coated with the water-repellent carbon powder 7 by a screen printing method.
[0030]
Using thus prepared electrode, the electrode - to prepare an electrolyte assembly were constructed similarly unit cell shown in FIG. 2 as in Reference Example 1. Table 1 shows the battery voltage when a current of 700 mA / cm ² was passed through the cell. Table 1 also shows the characteristics of the battery using the electrodes A and B prepared in Reference Example 1 . In Table 1, the electrode catalyst layer employed in this Example 2 the polymer electrolyte membrane side, by using an electrode arranged on the diffusion layer side water repellency charcoal particles were treated with a silane coupling agent of Reference Example 1 It was found that the same performance as that obtained by mixing and using the carbon powder was shown. From this, it has been found that a battery having more excellent characteristics can be formed by using an electrode in which the electrode catalyst layer is disposed on the polymer electrolyte membrane side and the water-repellent carbon particles are disposed on the diffusion layer side.
[0031]
[Table 1]
┌─────┬─────────┐
│ │ Battery voltage (mV) │
├─────┼─────────┤
│Reference Example 2 │710 │
├─────┼─────────┤
│electrode A │690 │
├─────┼─────────┤
│electrode B │620 │
└─────┴─────────┘
[0032]
( Example 1 )
Next, a case where the catalyst-supporting carbon powder is treated with a silane coupling agent will be described with reference to FIG. Using platinum-carrying carbon powder used in Reference Example 1, in place of the silane coupling agent used in Reference Example 1, having a hydrocarbon chain and a fluorocarbon chain in the main chain, having a sulfonic group at the end, ClSO ₂ - _{(CH 2) n- (CF 2} ) m-SiCl 3 (n, m is 25 an integer of 10 or more) with, by reaction with steam, and the surface of the platinum particles 9, the surface of the carbon particles 1 Thus, a monomolecular film 10 having a sulfone group was formed. This monomolecular film has hydrophilicity due to a terminal sulfone group. Further, the silane coupling agent is not limited to the silane-based surfactant shown in the examples as long as it contains a site exhibiting hydrophilicity.
[0033]
The platinum-supported carbon powder subjected to the hydrophilic treatment and the water-repellent carbon powder treated with the silane coupling agent used in Reference Example 1 were mixed to prepare an electrode in the same manner as in Reference Example 1, and this was used in FIG. The indicated cell was prepared.
[0034]
When the characteristics of this battery were evaluated under the same conditions as in Reference Example 1 , the voltage at a current density of 700 mA / cm ² was 720 mV. This is a characteristic higher than that of the battery manufactured in Reference Example 1. This is because the wettability near the three-phase interface is improved by performing a hydrophilic treatment on the carbon powder supporting the catalyst fine particles using a silane coupling agent. I think it is due to improvement.
[0035]
In the present invention, a chlorosilane-based surfactant having a sulfone group is used as a silane coupling agent. However, any silane coupling agent having a hydrophilic site may be used as long as it has a site such as a carboxyl group. But it doesn't matter. Also, in this case, both the catalyst fine particles and the carbon particles are treated, but if the present invention can be applied, only one of them can be treated. Furthermore, the present invention is not limited to the carbon powder used and the catalyst-supporting carbon powder as long as the present invention can be applied.
[0036]
【The invention's effect】
As described above, as apparent from the description of the examples, in the fuel cell according to the present invention, the catalyst layer is composed of the polymer electrolyte, the hydrophilic catalyst-carrying carbon particles, and the water-repellent carbon particles. In the vicinity of the generated three-phase interface, the wet state is appropriately maintained by the hydrophilic catalyst-supported carbon particles, and excess water is quickly discharged by the adjacent water-repellent carbon.
[0037]
When the hydrophilic catalyst-carrying carbon particles are arranged on the polymer electrolyte membrane side and the water-repellent carbon particles are arranged on the gas diffusion layer side, the polymer electrolyte membrane side becomes a highly humidified atmosphere, and the ion of the polymer electrolyte membrane becomes higher. The conductivity is improved, and the battery characteristics are improved.
[0038]
Further, in the fuel cell of the present invention, on the surface of the carbon particles, the hydrolyzable group of the silane coupling agent having a hydrophobic site is hydrolyzed by water in solution or air, and water adsorbed on the carbon surface, It changes to active silanol groups (≡SiOH) and reacts with functional groups on the carbon surface to form strong bonds. Thereby, a very microscopic monomolecular water-repellent layer having a thickness of several nm to several tens nm is formed on the surface of the carbon particles. By using the water-repellent carbon particles, even if the electrode is formed by mixing with the hydrophilic catalyst-carrying carbon particles, the catalyst fine particles in the electrode are coated as in the case of using submicron-order PTFE dispersion particles. And does not hinder the supply of the reaction gas.
[0039]
Furthermore, in the fuel cell of the present invention, the hydrolyzable group of the silane coupling agent is formed on the surface of the catalyst particles or on the surface of the carbon particles carrying the catalyst by the same method as described above. Is converted into active silanol groups (水分 SiOH) and reacts with the functional groups on the carbon surface to form strong bonds. By providing the silane coupling agent with a hydrophilic group such as a sulfone group or a carboxyl group, the catalyst surface becomes hydrophilic, and the wet state near the three-phase interface is maintained.
[0040]
As described above, when the electrode of the present invention is used, in the vicinity of the three-phase interface where the electrode reaction occurs, the wet state is appropriately maintained by the hydrophilic catalyst-supporting carbon particles, and excess water is generated by the adjacent water-repellent carbon. As a result, a polymer electrolyte fuel cell having higher performance than before can be constructed.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a configuration of a carbon particle surface used in a first reference example of the present invention. FIG. 2 is a graph showing a relationship between current and voltage of a single cell of a fuel cell according to a first reference example of the present invention. indicated Figure 3 is a conceptual diagram showing a configuration of a second reference example is that the electrode catalyst supporting conceptual diagram showing the configuration of the carbon particles [4] the present invention is a second exemplary embodiment of the present invention FIG. 5 is a conceptual diagram showing the structure of a catalyst-carrying carbon particle according to a first embodiment of the present invention.
REFERENCE SIGNS LIST 1 carbon particles 2 monomolecular water-repellent film 3 solid polymer electrolyte membrane 4 negative electrode 5 positive electrode 6 catalyst-supporting carbon powder 7 water-repellent carbon powder 8 carbon paper

Claims (6)

A fuel comprising an electrode electrolyte assembly in which a hydrogen ion conductive solid polymer electrolyte membrane, a pair of electrodes sandwiching the hydrogen ion conductive solid polymer electrolyte membrane, and a pair of diffusion layers sandwiching the electrode are stacked. In the battery, the electrode has at least a catalyst body in which catalyst particles are supported on a hydrophilic carbon material, a hydrogen ion conductive polymer electrolyte, and a water-repellent carbon material, and further, at least a part of the surface of the catalyst particles. An electrode for a fuel cell, characterized in that a hydrophilic layer is chemically bonded to the electrode. Claims a catalyst body carrying a catalyst particle in the hydrophilic carbon material, and selectively positioned proton conductive polymer electrolyte membrane side, characterized in that selectively place the water repellent carbon material on the diffusion layer side Item 2. An electrode for a fuel cell according to Item 1 . 3. The fuel cell according to claim 1 , wherein the water-repellent carbon material has a monomolecular layer in which a part or the entire surface of the carbon material is chemically bonded to a silane coupling agent having a hydrophobic site. 4. electrode. 4. The electrode for a fuel cell according to claim 1 , wherein the hydrophilic carbon material has a layer in which a part or the entire surface of the carbon material is chemically bonded to a silane coupling agent having a hydrophilic part. . A phenolic hydroxyl group, a carboxyl group, claims lactone group, via at least one functional group selected from a carbonyl group, quinone group or carboxylic acid anhydride, and wherein the chemically bonded to the carbon material and the silane coupling agent 5. The electrode for a fuel cell according to 3 or 4 . By immersing at least one of the catalyst particles or the carbon material in a solvent containing a silane coupling agent, the silane coupling agent is chemically adsorbed on at least a portion of the catalyst particle surface or the carbon material surface, A layer having hydrophilicity or water repellency is formed by performing a chemical bond between a catalyst particle surface or the carbon material surface and a silicon atom in a molecule of the silane coupling agent . 6. The method for producing a fuel cell electrode according to 2, 3, 4 or 5 .

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