EP1540753A2

EP1540753A2 - Conductive carbon, electrode catalyst for fuel cell using the same and fuel cell

Info

Publication number: EP1540753A2
Application number: EP03792667A
Authority: EP
Inventors: Motokazu Kobayashi; Teigo Sakakibara; Masayuki Yamada; Shinji Eritate; Zuyi Zhang
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2002-08-21
Filing date: 2003-08-11
Publication date: 2005-06-15
Also published as: TW200405608A; AU2003253438A1; WO2004019435A2; WO2004019435A3; TWI226140B; JP2004079420A

Abstract

To provide conductive carbon which carries at least platinum as a catalyst and has an organic functional group capable of causing hydrogen ion dissociation. The use of such conductive carbon as an electrode catalyst layer of a fuel cell enables the effective conduction of hydrogen ions and electrons generated on the catalyst, thereby providing a fuel cell highly efficient in electricity generation.

Description

DESCRIPTION

CONDUCTIVE CARBON, ELECTRODE CATALYST FOR FUEL CELL USING THE SAME AND FUEL CELL

TECHNICAL FIELD

The present invention relates to conductive carbon for use in a fuel cell, an electrode catalyst using the same, a fuel cell and a solid polymer type fuel cell.

BACKGROUND ART

A solid polymer type fuel cell has a layered structure where a solid polymer type electrolyte membrane is held between a fuel electrode (anode) and an oxidizer electrode (cathode) . In general, the fuel electrode ' and the oxidizer electrode are made up of a mixture of: a catalyst which contains conductive carbon carrying noble metals, such as platinum, or organic metal complexes; a polymer electrolyte; and a binder. The fuel supplied to the fuel electrode passes through the fine pores in the electrode and reaches the catalyst, by the action of which it releases electrons and becomes hydrogen ions. The hydrogen ions pass through the polymer electrolyte membrane between the two electrodes and reach the oxidizer electrode, and the hydrogen ions react with oxygen supplied to the oxidizer electrode and electrons flowing into the oxidizer from an external circuit to form water. The electrons' released from the fuel are conducted out through the catalyst and catalyst-carrying conductive carbon in the electrode to the external circuit and flow into the oxidizer electrode from the external circuit. As a result, the electrons flow from the fuel electrode toward the oxidizer electrode in the external circuit, and electric power is taken out from the fuel cell by this mechanism.

For example, when using hydrogen as the fuel, the following reaction occurs in the fuel electrode.

H₂ → 2H⁺ + 2e^~ (1) And in the oxidizer electrode the following reaction occurs .

-0₂ + 2H⁺ + 2e^~ → H₂0 (2) 2

The conductive carbon, which is a carrier for catalysts such as platinum, functions as a conductor for conducting the electrons generated in the above reaction, whereas the polymer electrolyte contained in the mixture functions as a conductor for conducting the hydrogen ions. Therefore, in the interface of each electrode and the polymer electrolyte membrane, the conductive carbon and polymer electrolyte included in the mixture are required to form networks, respectively, so that the conduction of the electrons and hydrogen ions are carried out smoothly.

In order to smoothly conduct the electrons and hydrogen ions to improve the characteristics of the fuel cell, the form and dispersion state of the catalyst carried by the conductive carbon have undergone several refinements. For example, Japanese Patent Application Laid-Open No. 63-319050 discloses that the three-dimensional structure of carbon fine powder, as a carrier for catalysts, is broken and thereby the adsorption sites for adsorbing noble metal particles are increased so that the noble metal particles are carried by the carbon fine powder in a highly dispersed state.

Further, Japanese Patent Application Laid-Open No. 63-97232 discloses that noble metal colloidal particles of 2 to 4 run in size are prepared and attached on the surface of commercially available carbon powder with a specific surface of 50 to 300 m²/g. Still further, Japanese Patent Application Laid-Open No. 6-19671 discloses that requirements are established for the pore diameter and specific surface of carbon fine powder so that catalyst particles are carried by the catalyst carrier in a more highly dispersed state.

Since the polymer electrolyte included in the mixture and the polymer electrolyte membrane provided between two electrodes are conductors for conducting hydrogen ions, they conduct the hydrogen ions generated in accordance with the above reaction formula (1) from the fuel electrode side to the oxidizer electrode side. And the electrons generated at the same time of generating as the hydrogen ions are conducted through the catalyst or from the conductive carbon, which carries the catalyst, to the adjacent conductive carbon, brought into a collector, and allowed to flow into the external circuit. Accordingly, to take out electric power from the fuel cell, it is necessary to take out the hydrogen ions and electrons generated by bringing the fuel into contact with the catalyst, which means that the catalyst is required to be in contact with both the polymer electrolyte and the conductive carbon. In other words, the catalyst not in contact with both of the polymer electrolyte and the conductive carbon does not contribute to the above reaction.

When using carbon powder having so fine pores that polymer electrolyte cannot exist therein, the methods described in the above patent publications cannot effectively use an expensive noble metal catalyst, since it is difficult to bring the catalyst into contact with the polymer electrolyte. Moreover, when a thick polymer electrolyte exists around a catalyst, since it is difficult that the fuel reaches the catalyst, there exists some part of catalyst which does not contribute to the reaction.

DISCLOSURE OF THE INVENTION

The present invention solves these problems separately or together. Accordingly, the present invention provides conductive carbon which permits the separation and effective conduction of hydrogen ions and electrons generated on a catalyst; an electrode catalyst using the same; and a polymer electrolyte fuel cell having high discharge properties .

Specifically, a first aspect of the present invention is conductive carbon which carries at least platinum, as a catalyst, and has an organic functional group capable of causing hydrogen ion dissociation.

A second aspect of the present invention is an electrode catalyst for use in a fuel cell which includes the above conductive carbon.

A third aspect of the present invention is a fuel cell including an electrode catalyst layer on the fuel electrode side, an electrode catalyst layer on the oxidizer electrode side, and a polymer electrolyte membrane provided between the above electrode catalyst layers, characterized in that at least one of the electrode catalyst layer on the fuel electrode side and the electrode catalyst layer on the oxidizer electrode side contains the above electrode catalyst. A fourth aspect of the present invention is a fuel cell apparatus, characterized in that it includes the above fuel cell, means for supplying a fuel to the fuel electrode side of the above fuel cell, and means for supplying an oxidizer to the oxidizer electrode side of the above fuel cell.

Such constitution permits effective conduction of hydrogen ions and electrons generated on a catalyst, since the constitution includes conductive carbon having an organic functional group capable of causing hydrogen ion dissociation and carrying a platinum catalyst.

The use of such conductive carbon as an electrode catalyst provides a solid polymer type fuel cell having high discharge characteristics. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a partial schematic illustration of a fuel cell of the present invention; Fig. 2 is a schematic view illustrating the structure of an electrode catalyst of the present invention; and

Fig. 3 is a graph showing the relation between the current and the voltage of the fuel cell in Evaluation Examples 1 to 7.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to accomplish the above object, the present invention is characterized by an electrode catalyst including conductive carbon which carries at least a platinum catalyst and has an organic functional group capable of causing hydrogen ion dissociation. Further, the present invention is characterized by an electrode catalyst including conductive carbon which carries platinum and ruthenium catalysts and has an organic functional group capable of causing hydrogen ion dissociation. Further, the present invention is characterized by the above electrode catalyst, wherein the organic functional group capable of causing hydrogen ion dissociation is at least one selected from the functional group consisting of sulfonic acid functional group, sulfinic acid functional group, carboxylic acid functional group, phosphonic acid functional group, phosphinic acid functional group, phosphoric acid functional group and hydroxyl functional group.

Still further, the present invention is characterized by a solid polymer type fuel cell including: electrode catalyst layers provided on both the main surfaces of a solid polymer type electrolyte membrane in such a manner as to hold the electrolyte between the electrode catalyst layers; porous diffusion layers provided on both the outsides of the above electrode catalyst layers; and means for supplying a fuel to one of the above diffusion layers and an oxidizer to the other diffusion layer, wherein at least a surface of the above electrode catalyst layer to which the fuel is supplied contains the above electrode catalyst.

As described above, directly modifying the catalyst-carrying conductive carbon permits electrons generated on the catalyst to be conducted through the conductive carbon, as a path, and hydrogen ions generated on the catalyst to be conducted through the organic functional group capable of causing hydrogen ion dissociation, as a path.

Since both the paths for conducting the hydrogen ions and for conducting the electrons can be ensured, the hydrogen ions and electrons generated on the catalyst can be separated effectively. This results in improvement of the discharge characteristics of a solid polymer type fuel cell.

In the following the present invention will be described in detail with reference to the drawings.

A partial schematic illustration of one example of the fuel cell according to the present invention is shown in Fig. 1.

Referring to Fig. 1, in the fuel cell of the present invention, there are provided electrode catalyst layers 2a and 2b on both sides of a polymer electrolyte membrane 1, diffusion layers 3a and 3b on the outsides of the electrode catalyst layers, and electrodes 4a (fuel electrode) , 4b (oxidizer electrode) as collectors on the outsides of diffusion layers . As the polymer electrolyte membrane 1, a perfluorosulfonic acid polymer membrane, represented by Nafion™ membrane manufactured by Du Pont, and a hydrocarbon-based membrane manufactured by Hoechst are preferably used but the membrane is not limited to them. A wide variety of polymer membranes can be used as long as they have hydrogen-ion-conductive functional functional groups, such as sulfonic acid functional group, sulfinic acid functional group, carboxylic acid functional group, phosphonic acid functional group, phosphinic acid functional group, phosphoric acid functional group and hydroxyl functional group. A hybrid electrolyte membrane consisting of an inorganic electrolyte and a polymer membrane, which is prepared by the sol-gel processing, can also be used. To prevent the fuel crossover, a coating can be applied to the surface of the electrolyte membrane.

Either or both of the electrode catalyst layers 2a and 2b contain an electrode catalyst which includes conductive carbon. The schematic illustration of the electrode catalyst is shown in Fig. 2. In the same figure, reference numeral 5 denotes conductive carbon, 6 a catalyst carried on the surface of the conductive carbon, and 7 an organic functional group capable of causing hydrogen ion dissociation which is directly bound to the conductive carbon.

For example, the electrode catalyst layer 2a on the fuel electrode side is made up of an electrode catalyst including the conductive carbon 5 which carries at least the platinum catalyst 6 and has the organic functional group 7 capable of causing hydrogen ion dissociation.

Preferably the platinum catalyst is carried on the surface of the conductive carbon. And preferably the particle diameter of the catalyst carried by the conductive carbon is small. In particular, the particle diameter is in the range of 0.5 nm to 20 nm, preferably in the range of 1 nm to 10 nm. If the particle diameter is less than 0.5 nm, the single catalyst particle is too highly active to handle, whereas if the particle diameter is more than 20 nm, the surface area of the catalyst is decreased, and hence the reaction site of the catalyst reduces, resulting in the risk of lowered activity of the catalyst.

Instead of the platinum catalyst, metals of platinum group, such as rhodium, ruthenium, iridium, palladium and osmium, and the platinum alloys thereof can also be used. Particularly when using methanol as a fuel, preferably a platinum-ruthenium alloy is used for the electrode catalyst on the fuel electrode side.

The conductive carbon can be selected from the group consisting of carbon black, carbon fiber, graphite, carbon nanotube and the like.

Preferably the average particle diameter of conductive carbon is in the range of 5 nm to 1000 nm, more preferably in the range of 10 nm to 100 n . In order to allow the conductive carbon to carry the above-described catalyst, the BET specific surface area should be large to some extent. The BET specific surface area is preferably 50 m²/g to 3000 m²/g, more preferably 100 m²/g to 2000 rα²/g.

As the method of allowing the conductive carbon to carry the catalyst, a wide variety of known methods can be used. For example, a method has been known in which conductive carbon is allowed to carry a catalyst on its surface by impregnating the conductive carbon with a solution of platinum or other metal and then reducing the noble metal ions . This method is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2-111440 and 2000- 003712. Alternatively, a vacuum film-forming method such as a sputtering method can also be used to allow conductive carbon to carry an intended noble metal by sputtering using a target of the noble metal.

The amount of the catalyst carried on the surface of the conductive carbon is in the range of 5 to 80% by weight of the total amount of the electrode catalyst, preferably in the range of 10 to 70% by weight. If the amount is less than 5% by weight, the catalyst might not fully exhibit its performance, whereas if the amount is more than 80% by weight, not only the catalyst production cost is increased, but also handling the catalyst in the production process becomes very difficult because it becomes very likely to ignite. Thus the amount outside the above range is not preferable. An organic functional group capable of causing hydrogen ion dissociation is directly bound to or physically adsorbed on the conductive carbon of the present invention. The organic functional groups capable of causing hydrogen ion dissociation include, for example, sulfonic acid functional group, sulfinic acid functional group, carboxylic acid functional group, phosphonic acid functional group, phosphinic acid functional group, phosphoric acid functional group and hydroxyl functional group. Of the above functional groups, sulfonic acid functional group is particularly preferable from the property and production point of view. For example, directly binding of sulfonic acid functional group to conductive carbon can be easily achieved by heat treatment with fuming sulfuric acid or the like. Further, conductive carbon on which sulfonic acid functional group has been physically adsorbed can be obtained by treating conductive carbon with sulfuric acid. Whether an organic functional group is directly bound to or physically adsorbed on the conductive carbon can be selected appropriately depending on the type of conductive carbon and that of organic functional group. However, it is preferable from the viewpoint of durability that an organic functional group is directly bound to conductive carbon. From among the above organic functional groups capable of causing hydrogen ion dissociation, more than one organic functional group can also be selected and used. The amount of the organic functional group capable of causing hydrogen ion dissociation which is bound to the conductive carbon is in the range of 0.0001 mmol/g to 100 mmol/g based on the weight of the conductive carbon, preferably in the range of 0.001 mmol/g to 10 mmol/g. If the amount is less than 0.0001 mmol/g, the proton conductivity of the organic functional group is not developed, whereas if the amount is more than 100 mmol/g, the conductivity of the conductive carbon is inhibited. Thus the amount outside the above range is not preferable.

The order of the step of directly binding or physically adsorbing an organic functional group capable of causing hydrogen ion dissociation to or on the conductive carbon and the step of allowing the conductive carbon to carry the catalyst is not limited.

By the direct binding or physically adsorbing of an organic functional group capable of causing hydrogen ion dissociation to or on the conductive carbon, the electrode catalyst of the present invention permits electrons generated on the catalyst on the fuel electrode side to be effectively transported to the electrode through the conductive carbon, as a path, and hydrogen ions generated on the catalyst to be effectively . transported to the electrolyte through the .organic functional group capable of causing hydrogen ion dissociation, as a path.

The electrode catalyst of the present invention may be used on the oxidizer electrode side. The use of the electrode catalyst enables the effective transportation of hydrogen ions received from the electrolyte membrane provided between the two electrodes as well as electrons flowing in from the external circuit and moreover, improves the reactivity with oxygen, as an oxidizer.

Particularly in the present invention, since the organic functional group capable of causing hydrogen ion dissociation is directly bound to or physically adsorbed on the conductive carbon, the localization of the organic functional group due to the passage of a fuel or oxygen can be prevented. In addition, since the conductive carbon, organic functional group capable of hydrogen ion dissociation and catalyst are directly in contact with the reaction site near the surface, the reaction efficiency can be improved.

The electrode catalysts thus produced are provided alone in such a manner as to be contact with the polymer electrolyte membrane. Or the electrolyte catalysts are provided in the form of a mixture with a binder, a polymer electrolyte, a water-repellent agent, conductive carbon and a solvent in such a manner as to be in contact with the polymer electrolyte membrane. When the fuel cell includes diffusion layers, the electrolyte catalysts are provided in such a manner as to be in the contact with the diffusion layers, too.

The diffusion layers 3a and 3b serve to effectively and uniformly introduce hydrogen, reformed hydrogen, methanol or dimethyl ether, as a fuel, and air or oxygen, as a oxidizer, into the electrode catalyst layers, and in addition, to deliver electrons in such a state that it is in contact with the electrodes. In general, conductive and porous membranes are preferable as the diffusion layers, and carbon paper, carbon cloth or a composite sheet of carbon and polytetrafluoroethylene are used for the diffusion layers.

The diffusion layers may be subjected to water repellent treatment, before using, by coating the surface and the inside thereof with a fluorine-based coating.

As the electrodes 4a and 4b, those conventionally used can be used without any limitation, as long as they can supply a fuel and an oxidizer effectively to the diffusion layers and can transfer electrons from or to the diffusion layers. The fuel cell of the present invention is produced in such a manner as to stack the polymer electrolyte membrane, the electrode catalyst layers, the diffusion layers and the electrodes, as shown in Fig. 1. The shape is arbitrary. And the production method is not limited to any specific one, and those commonly in us.e can be employed.

In the following the present invention will be described in further detail by giving examples . However, it should be understood that these examples are not intended to limit the present invention. (Production Example of Electrode Catalyst) Examples 1 to 3

As conductive carbon, VULCAN XC 72-R (manufactured by Cabot Corporation) was used. And a platinum (30% by weight) -ruthenium (15% by weight) alloy was carried on the surface of the conductive carbon in such a manner as to allow a platinum compound and a ruthenium compound to be carried on the surface of the conductive carbon using an aqueous solution of platinum chloride acid and that of ruthenium chloride, as raw materials, and reducing the conductive carbon carrying the compounds .

The catalyst-carrying conductive carbon was dried, heat treated with fuming sulfuric acid, and washed in ion-exchanged water so that sulfonic acid was directly bound to the catalyst-carrying conductive carbon. The conductive carbon as an electrode catalyst of the present invention was obtained after fully drying the above conductive carbon. In the treatment with fuming sulfuric acid, the treatment time was changed variously to change the amount of sulfonic acid bound to the conductive carbon.

The amount of sulfonic acid functional group was quantitatively determined by the elemental analysis, and three types of electrode catalysts shown in Table 1 below were obtained. Table 1

Example 4

As conductive carbon, KETJENBLACK EC (manufactured by Lion Corporation) was used. And the conductive carbon was treated with chloroalkylphosphonic acid to bind phosphonic acid functional group directly to the surface of the conductive carbon. Then a platinum (30% by weight) - ruthenium (15% by weight) alloy was carried on the surface of the conductive carbon in the same manner as in Examples 1 to 3. The conductive carbon as an electrode catalyst of the present invention was obtained after fully drying the above conductive carbon.

The amount of phosphonic acid functional group was quantitatively determined by the elemental analysis, and an electrode catalyst shown in Table 2 below was obtained.

Table 2

Examples 5 to 7 As conductive carbon, VULCAN XC 72-R

(manufactured by Cabot Corporation) was used. And platinum (50% by weight) was carried on the surface of the conductive carbon in such a manner as to allow a platinum compound to be carried on the conductive carbon using an aqueous solution of platinum chloride acid, as a raw material, and reducing the conductive carbon carrying the compound.

The catalyst-carrying conductive carbon was dried, treated with fuming sulfuric acid, and washed in ion-exchanged water so that sulfonic acid was directly bound to the catalyst-carrying conductive carbon. The conductive carbon as an electrode catalyst of the present invention was obtained after fully drying the above conductive carbon. In the treatment with fuming sulfuric acid, the treatment time was changed variously to change the amount of sulfonic acid bound to the conductive carbon.

The amount of sulfonic acid functional group was quantitatively determined by the elemental analysis, and three types of electrode catalysts shown in Table 3 below were obtained.

Table 3

Comparative Example 1

Platinum (30% by weight) -ruthenium (15% by weight) -carrying VULCAN XC 72-R used in Example 1 was used without binding a sulfonic acid functional group thereto. Comparative Example 2

Platinum (50% by weight) -carrying VULCAN XC 72- R used in Example 5 was used without binding a sulfonic acid functional group thereto. Evaluation 4 g of each of the conductive carbon produced in Examples 1 to 7 and Comparative Examples 1 to 2, which carried their respective catalysts and had their respective organic functional groups, was mixed with 10 g of water and 8 g of 5% Nafion™ solution (manufactured by Wako Pure Chemical) to form into apaste.

This paste was applied to 0.1 mm-thick carbon paper (TGP-H-30, Toray) which had been subjected to water repellent treatment, dried at room temperature, and dried at 50°C. In these examples, the amount of the platinum or platinum-ruthenium alloy applied was about 4 mg/cm² for each. Electrode catalysts were prepared for the fuel electrode side and the oxidizer electrode side in combinations shown in Table 4 below, and Nafion™ 112 (co-polymer of tetrafluoroethylene and perfluorovinylether sulfonic acid) (manufactured by Du Pont) as an electrolyte membrane was held between each combination of the electrode catalysts. The electrolyte membrane and the electrode catalysts were press treated at 100°C and 4.9 MPa (50 kgf/cm²) with a hot press to produce an MEA (Membrane Electrode Assembly) .

Table 4

Each MEA prepared as above was incorporated as a single cell of a fuel cell to produce a fuel cell. The cell area was 25 cm².

For each cell, 5% by weight methanol aqueous solution was supplied to the fuel electrode side at a rate of 10 ml/min and air at atmospheric pressure was supplied to the oxidizer electrode side at a rate of 200 ml/min, and electricity generation was carried out while keeping the whole cell at 75°C.

The relations between the current and the voltage in Evaluation Examples 1 to 7 are shown in Fig. 3. Fig. 3 shows that in the fuel cells of the Evaluation Examples 1 to 6 according to the present invention, output up to 0.5 A/cm² can be produced stably, whereas in the fuel cell of the Evaluation Example 7, output of only 0.2 A/cm² or less is produced. It is apparent that this is because sulfonic acid functional group or phosphonic acid functional group bound to conductive carbon serves as an effective path for hydrogen ions . When using hydrogen, reformed hydrogen, methanol or dimethyl ether as a fuel, the same results were obtained.

INDUSTRIAL APPLICABILITY As described so far, according to the present invention, since the conductive carbon is used which has an organic functional group capable of causing hydrogen ion dissociation and carries a platinum catalyst, hydrogen ions and electrons generated on the catalyst can be effectively conducted.

The use of such conductive carbon as an electrode catalyst makes it possible to provide a solid polymer type fuel cell having high discharge characteristics.

Claims

1. Conductive carbon carrying at least platinum as a catalyst and having an organic functional group capable of causing hydrogen ion dissociation.

2. The conductive carbon according to claim 1, further carrying ruthenium as a catalyst.

3. The conductive carbon according to claim 1, wherein the organic functional group is at least one selected from the functional group consisting of sulfonic acid functional group, sulfinic acid functional group, carboxylic acid functional group, phosphonic acid functional group, phosphinic acid functional group, phosphoric acid functional group and hydroxyl functional group.

4. An electrode catalyst for use in a fuel cell, comprising conductive carbon which carries at least platinum as a catalyst and has an organic functional group capable of causing hydrogen ion dissociation.

5. A fuel cell comprising: an electrode catalyst layer on a fuel electrode side; an electrode catalyst layer on an oxidizer electrode side; and a polymer electrolyte membrane provided between the electrode catalyst layers, wherein at least one of the electrode catalyst layer on the fuel electrode side and the electrode catalyst layer on the oxidizer electrode side contains an electrode catalyst which includes conductive carbon carrying platinum, as a catalyst, and having an organic functional group capable of causing hydrogen ion dissociation.

6. The fuel cell according to claim 5, further comprising one collector for collecting electric power provided on an outside of the electrode catalyst layer on the fuel electrode side, and the other collector for collecting electric power on an outside of the electrode catalyst layer provided on the oxidizer electrode side.

7. The fuel cell according to claim 6, further comprising a porous diffusion layer between the electrode catalyst layer on the fuel electrode side and the one collector, and a porous diffusion layer between the electrode catalyst layer on the oxidizer electrode side and the other collector.

8. A fuel cell apparatus comprising: a fuel cell comprising an electrode catalyst layer on a fuel electrode side, an electrode catalyst layer on an oxidizer electrode side, and a polymer electrolyte membrane provided between the electrode catalyst layers, wherein at least one of the electrode catalyst layer on the fuel electrode side and the electrode catalyst layer on the oxidizer electrode side containing an electrode catalyst which includes conductive carbon carrying at least platinum as a catalyst and having an organic functional group capable of causing hydrogen ion dissociation; means for supplying a fuel to the fuel electrode side of the fuel cell; and means for supplying an oxidizer to the oxidizer electrode side of the fuel cell.