MEMBRANE ELECTRODE ASSEMBLY FOR FUEL CELL, AND FUEL CELL

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to a membrane electrode assembly for a fuel cell, and a fuel cell including the membrane electrode assembly.

2. Description of the Related Art

Fuel cells directly transform chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes, and electrochemically oxidizing the fuel. The fuel cells are not limited by the Carnot cycle, unlike thermal power generation, and thus exhibit high energy-conversion efficiency. A fuel cell is generally made up of a plurality of laminated unit cells. Each unit cell has, as its basic structural element, a membrane electrode assembly in which an electrolyte membrane is interposed between a pair of electrodes. In particular, polymer electrolyte fuel cells using a solid polymer electrolyte membrane as the electrolyte membrane are drawing attention, especially as power sources for mobile equipment and mobile bodies, because of their advantages of ease of downsizing and operability at low temperatures.

At an anode (fuel electrode) of a polymer electrolyte fuel cell, a reaction represented by the following formula (1) proceeds: E ₂ → 2if + 2e - (l).

Electrons generated by the reaction of the formula (1) pass through an external circuit, work at an external load, and reach a cathode (oxidizing electrode). Protons generated by the reaction of the formula (1) move from the anode to the cathode through the solid polymer electrolyte membrane by electroendosmosis, as they are hydrated with water. At the cathode, a reaction represented by the following formula (2) proceeds:

4H ⁺ + O ₂ + 4e ^' → 2H ₂ O - (2).

The reaction at each electrode proceeds at a three-phase boundary where a reaction gas (H ₂ or O ₂ ), the protons (H ⁺ ), and the electrons (o ^~ ) can be exchanged, as represented by the above formulas (1) and (2). Each electrode provided on both surfaces of the

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electrolyte membrane in the polymer electrolyte fuel cell typically has a structure in which a catalyst layer and a gas diffusion layer are laminated in this order over the electrolyte membrane. The catalyst layer is where the reaction at each electrode occurs, and generally includes a conductive material carrying a catalyst component and a polymer electrolyte resin (proton-conducting material). The catalyst layer typically has a porous structure in which the polymer electrolyte resin is suitably permeated through voids in the conductive material carrying the catalyst component, with some voids remaining unpermeated. In such a catalyst layer, the polymer electrolyte resin forms a proton-conducting pathway, the conductive material forms an electron-conducting pathway, and the voids in the porous structure form a passage for the reaction gas.

In order to obtain a high power fuel cell, it is important to improve the proton conductivity between the. electrolyte membrane and the catalyst layer and the proton conductivity in the catalyst layer. In order to control the proton conductivity in the catalyst layer, for example, the quantity of the polymer electrolyte resin in the catalyst layer is controlled in general.

In the case where the quantity of the polymer electrolyte resin in the catalyst layer is increased and thus the quantity of proton-conducting groups (proton dissociating polar groups) present in the catalyst layer is increased, the proton conductivity in the catalyst layer is improved. However, as the polymer electrolyte resin increases, the voids in the porous structure are filled and the quantity of water retained in the polymer electrolyte resin which is hydrophilic is increased, which unfavorably reduces the reduction gas permeability and the water drainage properties of the catalyst layer. This makes flooding more likely to occur and thus reduces the power generation performance at high current range. On the other hand, in the case where the quantity of the polymer electrolyte resin in the catalyst layer is reduced, the proton conductivity in the catalyst layer is reduced, which reduces the area of the three-phase boundary and thus reduces the power generation performance at low current range. As described above, it is considered to be difficult to achieve high power generation performance over a wide current range from the low current range to the high current range by merely controlling the proton conductivity by increasing

and reducing the quantity of the polymer electrolyte resin in the catalyst layer.

In recent years, fullerene derivatives obtainable by introducing a proton dissociating polar group such as a sulfonic acid group into a fullerene have been drawing attention as solid electrolyte materials having proton conductivity. For example, the Japanese patent application publication No.JP-A-2005-251505 describes a proton conductor constituted of a fullerene derivative obtainable by coupling at least one organic group having a specific dissociating acidic group to a fullerene molecule via a hetero atom selected from an oxygen atom and a sulfur atom, and proposes its usage as a solid electrolyte in a fuel cell. The Japanese patent application publication No.JP-A-2005-527957 describes a surface modified fullerene containing a plurality of sulfonate substituents of the general formula-SO3M (where M represents hydrogen or a cationic species) surface bonded thereto in a specific quantity.

The international patent application publication No.WO02/013295 describes a proton-conducting electrode constituted of a mixture containing a fullerene derivative obtainable by introducing a proton dissociating group into a carbόm atom constituting a fullerene molecule, and an electron conductive catalyst. Such a proton-conducting electrode uses only the fullerene derivative obtainable by introducing a proton dissociating group, and not a polymer electrolyte resin such as NAFION (trade name), as a proton conductor. The fullerene derivatives having a proton dissociating polar group such as described above can have a large number of proton dissociating polar groups introduced thereinto, and thus can express high proton conductivity. However, it is difficult to achieve satisfactory battery performance in the case where only the fullerene derivative is used as an electrolyte in the catalyst layer. Fuel cells using a fullerene derivative as well as a polymer electrolyte resin are also proposed. For example, the Japanese patent application publication No.JP-A-2005-527957 proposes the use of the above surface modified fullerene along with a conductive polymer in devices such as fuel cells. The Japanese patent application publication NoJP-A-2004-185863 describes an electrode for a fuel cell including a catalyst particle, a carrier for carrying the catalyst particle, a catalyst layer

containing an ion exchange resin, and a conductive porous base for supporting the catalyst layer, in which the catalyst layer includes a proton-conducting substance. A fullerene derivative containing a proton dissociating polar group is indicated as an example of the proton-conducting substance. The Japanese patent application publications No.JP-A-2005-527957 and

JP-A-2004-185863 mentioned above, however, do riot sufficiently discuss the ratio between the ion exchange resin and the fullerene derivative, in spite of the fact that the ratio between an ion exchange resin and a fullerene derivative is important to sufficiently improve the power generation performance of a fuel cell.

SUMMARY OFTHE INVENTION

The present invention provides a membrane electrode assembly for a fuel cell that exhibits excellent power generation performance (I-V characteristics) by the use of a polymer electrolyte resin and a fullerene derivative as solid electrolytes, and a fuel cell that includes the membrane electrode assembly.

A first aspect of the present invention is directed to a membrane electrode assembly for a fuel cell that includes an electrolyte membrane, which contains a polymer electrolyte resin having a proton dissociating polar group, and catalyst layers, which contain a polymer electrolyte resin having a proton dissociating polar group and a catalyst component. The catalyst layers are provided on both surfaces of the electrolyte membrane. In the membrane electrode assembly for a fuel cell, at least one of the catalyst layers provided on both surfaces of the electrolyte membrane contains a fullerene derivative having a proton dissociating polar group, and assuming that the total quantity of the polymer electrolyte resin and the fullerene derivative contained in the catalyst layer is 100 wt%, the content of the fullerene derivative is 35 wt% to 75 wt%.

In the catalyst layer containing the polymer electrolyte resin and the fullerene derivative at a specific ratio such as described above, both the proton conductivity and the gas diffusion and water drainage properties are ensured from the high current range to the low current range. Thus, the membrane electrode assembly for a fuel cell according to

the above aspect expresses excellent power generation performance over a wide current range from the high current range to the low current range.

In the above aspect, the fullerene derivative may be a sulfonated fullerene having a sulfonic acid group. With this composition, there can be obtained a fuel cell with high output .over a wide current range.

A second aspect of the present invention is directed to a fuel cell. The fuel cell is characterized by including the membrane electrode assembly for a fuel cell according to the first aspect described above.

According to either of the first and second aspects, there can be obtained a membrane electrode assembly for a fuel cell, and a fuel cell, which exhibits high proton conductivity even at low current range. and high gas diffusion properties and water drainage properties even at high current range and which expresses excellent power generation performance, by the use of the polymer electrolyte resin and the fullerene derivative mixed at a specific ratio as solid electrolytes contained in the catalyst layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a cross sectional view of a unit cell including a membrane electrode assembly according to an embodiment of the present invention;

FIG 2 shows graphs showing the results of I- V tests on an example and comparative examples; FIG 3 shows graphs showing the results of impedance measurements on the example and the comparative examples; and

FIG 4 shows graphs showing the results of CV on the example and the comparative examples.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

A membrane electrode assembly for a fuel cell according to the present invention includes an electrolyte membrane containing a polymer electrolyte resin having a proton dissociating polar group, and catalyst layers, containing a polymer electrolyte resin having a proton dissociating polar group and a catalyst component, provided on both surfaces of the electrolyte membrane, in which at least one of the catalyst layers provided on both surfaces of the electrolyte membrane contains a fullerene derivative having a proton dissociating polar group, and assuming that the total quantity of the polymer electrolyte resin and the fullerene derivative contained in the catalyst layer is 100 wt%, the content of the fullerene derivative is 35 wt% to 75 wt%.

The proton dissociating polar group refers to a group (functional group) from which a hydrogen atom may be dissociated and released as a proton (H ⁺ ). The proton dissociating polar group may be present at the terminal, or at the middle, of a molecule chain (main chain or side chain). To be specific, the proton dissociating polar group introduced at the terminal may be, but is not limited to, -OH, -OSO ₃ H ₃ -COOH, -SO ₃ H and -OPO(OH) ₂ . Among others, a sulfonic acid group (-SO ₃ H) or a sulfonic acid ether group (-OSO ₃ H) is preferable for their high proton conductivity and for being easily introducible into resins, fullerene molecules, etc.

A membrane electrode assembly for a fuel cell (hereinafter occasionally referred to simply as "membrane electrode assembly") according to an embodiment of the present invention will now be described with reference to FIG. 1. FIG. 1 shows a cross sectional view of a unit cell that includes a membrane electrode assembly according to an embodiment of the present invention. As shown in FIG 1, a cathode (oxidizing electrode) 4a and an anode (fuel electrode) 4b are provided on one and the other sides, respectively, of an electrolyte membrane 1 to form a membrane electrode assembly 5. In this embodiment, the cathode 4a and the anode 4b have a structure in which a cathode side catalyst layer 2a and a cathode side gas diffusion layer 3a, or an ^' anode side catalyst layer 2b and an anode side gas diffusion layer 3b, respectively, are laminated in this order over the electrolyte membrane 1. In the membrane electrode assembly 5 according to the

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present invention, the structure of the cathode and the anode is not limited to that in this embodiment, and may be a single layer structure having only a catalyst layer or may have additional layers other than a catalyst layer and a gas diffusion layer, for example.

A unit cell 100 is made up of the membrane electrode assembly 5, and a cathode side separator 6a and an anode side separator 6b and the assembly 5 is held between the cathode side separator 6a and the anode side separator 6b. The cathode side separator 6a and. the anode side separator 6b define flow paths 7a and 7b for supplying a reaction gas (fuel gas or oxidizing gas) to the cathode 4a and the anode 4b, respectively. The separators 6a and 6b also provide a gas seal between the unit cells and function as a current collector. The cathode 4a is supplied with an oxidizing gas (a gas containing oxygen, generally air) from the flow path 7a. The anode 4b is supplied with a fuel. gas (a gas containing hydrogen or a gas that produces hydrogen, generally a hydrogen gas) from the flow path 7b.

In the present invention, the electrolyte membrane contains a polymer electrolyte resin having a proton dissociating polar group (hereinafter occasionally referred to simply as "polymer electrolyte resin"). The polymer electrolyte resin having a proton dissociating polar group is a resin having a proton dissociating polar group such as described above, and may be fluorine-based polymer electrolyte resins, typically a perfluorocarbon sulfonic acid resin membrane such as NAFION (trade name, manufactured by Dupont), hydrocarbon-based polymer electrolyte resins obtainable by introducing a proton dissociating polar group such as described above into aromatic plastics such as polyether ether ketone, polyether ketone, polyether sulfone, polyphenylene sulfide and polyphenylene ether, and general purpose resins such as polystyrenes, ABS resins and AS resins, etc. The hydrocarbon-based polymer electrolyte resins typically contain no fluorine, but may be partially fluorine-substituted or may contain a heteroatom other than fluorine.

The electrolyte membrane may contain one or more kinds of polymer electrolyte resins such, as described above. Each polymer electrolyte resin may have one or more kinds of proton dissociating polar groups. The electrolyte membrane may appropriately contain components other than polymer electrolyte resins. In general, the thickness of the

electrolyte membrane is preferably, but not limited to, about 10 to 100 μm.

The catalyst layer provided on the surface of the electrolyte membrane contains a polymer electrolyte resin having a proton dissociating polar group, and a catalyst component. The polymer electrolyte resin may be, but is not limited to, those mentioned as polymer electrolyte resins constituting the electrolyte membrane. The catalyst component is not specifically limited as long as it catalyzes an oxidation reaction of hydrogen at the fuel electrode and a reduction reaction of oxygen at the oxidizing electrode. For example, the catalyst component may be platinum, or an alloy of platinum and a metal such as ruthenium, iron, nickel, manganese, cobalt, gold, and irϊdium. The catalyst component is contained in the catalyst layer as it is carried on a conductive material, in general. The conductive material may be carbonaceous materials such as carbonaceous particles and carbonaceous fibers, metal particles, and metal fibers, etc.

The membrane electrode assembly according to the present invention is characterized in that the catalyst layers contain a fullerene derivative having a proton dissociating polar group, in addition to the polymer electrolyte resin described above, as a solid electrolyte, and that the quantity of the fullerene derivative to the total quantity of the polymer electrolyte resin and the .fullerene derivative contained in the catalyst layer [fullerene derivative / (fullerene derivative + polymer electrolyte resin) * 100wt%] is 35 wt% to 75 wt%. The fullerene derivative having a proton dissociating polar group is obtainable by introducing a proton dissociating polar group such as described above into a carbon atom constituting a fullerene molecule directly or via a linking group. The fullerene molecule of the fullerene derivative may be a fullerene core of Ceo, but is not specifically limited thereto and may alternatively be a fullerene core of C36, C ₇ O, C ₇ g, C ₇ g, Cgo, Cg ₂ , Cg ₄ , C ₉₀ , C _% , etc. A plurality of fullerene molecules such as described above may be used as they are linked to each other. .

One or more kinds of proton dissociating polar groups may be introduced into the fullerene molecule. The linking group linking the carbon atom and the proton dissociating polar group of the fullerene derivative is not specifically limited, and may be

-(CH ₂ )n-, -(CFa)m- and -0-(CH ₂ )Ii-, for example. Preferably, n = about 1 to 6 and m = about 1 to 8. The fullerene derivative is preferably a sulfonated fullerene, which is obtainable by introducing a sulfonic acid group into a fullerene molecule, for its high proton conductivity. The power generation performance at low and high current ranges is improved by mixing a fullerene derivative such as described above with the polymer electrolyte resin at a specific ratio, and having the mixture contained in the catalyst layer. The proton conductivity of the catalyst layer is higher as the catalyst layer contains a larger number of proton dissociating polar groups, which add proton conductivity to the catalyst layer. An attempt to increase the quantity of the proton dissociating polar groups in the catalyst layer by increasing the quantity of the polymer electrolyte resin, however, unfavorably reduces the gas diffusion properties and the water drainage properties of the catalyst layer, and significantly reduces the power generation performance especially at high current range where flooding tends to occur, as described above. In view of the above, in the present invention, the quantity of the proton dissociating polar groups in the catalyst layer is increased, while the gas diffusion properties and the water drainage properties of the catalyst layer are secured, through the combined use of the polymer electrolyte resin and the fullerene derivative at a specific ratio as a solid electrolyte to be contained in the catalyst layer. A large number of proton dissociating polar groups may be introduced into the fullerene derivative, thereby increasing the quantity of proton dissociating polar groups per unit area of the catalyst layer. In addition, gaps are formed in the fullerene derivative even if it is aggregated. Therefore, the fullerene derivative does not fill up voids in the catalyst layer, unlike the polymer electrolyte resin, and thus does not greatly reduce the gas diffusion properties and the water drainage properties of the catalyst layer. Consequently, the use of the fullerene derivative can increase the quantity of the proton dissociating polar groups present in the catalyst layer without impeding the gas diffusion and water drainage properties of the catalyst layer.

Thus, in the catalyst layer of the membrane electrode assembly, flooding is not likely

to occur and a sufficient quantity of reaction gas is supplied to the catalyst component, even under conditions where the water content in the electrode is relatively large, such as during operation at high current or at high humidity.

In an electrode containing a polymer electrolyte resin, as known, the polymer electrolyte resin forms a cluster as a proton conductor. By the combined use of a fullerene derivative and a polymer electrolyte resin as components of an electrode, however, it is presumed that the fullerene derivative enters into and thus expands such a cluster, thereby improving the proton conductivity of the electrode. A cluster formed of only a polymer electrolyte resin easily changes in size due to expansion and contraction of the polymer electrolyte resin along with changes in humidity, etc., in the electrode. By the combined use, however, it is considered that the fullerene derivative enters into the cluster and thus the size of the cluster is fixed, thereby obtaining an electrode expressing stable proton conductivity.

Unlike the polymer electrolyte resin, the fullerene derivative does not excessively cover the surface of the catalyst component and the conductive material carrying the catalyst component. Thus, it is easy to secure the electron conductivity by the conductive material. In addition, the presence of the fullerene derivative at the interface between the electrolyte membrane and the electrode advantageously improves the joint between the electrolyte membrane and the electrode. An improvement in the joint between the electrolyte membrane and the electrode improves the proton conductivity between the electrolyte membrane and the electrode and the durability of the membrane electrode assembly.

The polymer electrolyte resin tends to interact with the conductive material carrying the catalyst component, and thus forms a three-phase boundary more easily than the fullerene derivative does. Therefore, a sufficient three-phase boundary can be formed by the combined use of the fullerene derivative and the polymer electrolyte resin, in spite of the fact that in a catalyst layer containing only a fullerene derivative as an electrolyte, a three-phase boundary having an area matching an increased quantity of proton dissociating polar groups cannot be formed. Thus, in the catalyst layers in the membrane electrode

assembly according to the present invention, a proton is supplied to the catalyst component (at the cathode) and moved from the catalyst component to the electrolyte membrane (at the anode) efficiently even at low current range, thereby maintaining high electrode reactivity. As described above, a membrane electrode assembly exhibiting excellent power generation performance over a wide current range from the high current range to the low current range can be obtained by the combined use of the polymer electrolyte resin and the fullerene derivative as a solid electrolyte in the catalyst layer.

In order to obtain the above effect by using the fullerene derivative and the polymer electrolyte resin in the catalyst layer, the ratio between the fullerene derivative and the polymer electrolyte resin to be contained in the catalyst layer is important. Assuming that the total quantity of the fullerene derivative and the polymer electrolyte resin in the catalyst layer is 100 wt%, the quantity of the fullerene derivative should be 35 wt% to 75 wt% [fullerene derivative / (fullerene derivative + polymer electrolyte resin) * 100 wt% = 35 to 75 wt%] to obtain a fuel cell exhibiting high power generation performance over a wide current range from the low current range to the high current range.

The ratio of the fullerene derivative should be not less than 35 wt% to prevent voltage reduction at low current range, and should be not more than 75 wt% to prevent flooding of generated water at high current range. The fullerene derivative content is preferably not less than 40 wt%, more preferably not less than 45 wt%, and preferably not more than 70 wt%, and more preferably not less than 65 wt%.

The catalyst layer can be formed using, for example, catalyst ink obtainable by mixing and dispersing in a solvent the fullerene derivative and the polymer electrolyte resin in such, quantities that [fullerene derivative / (fullerene derivative + polymer electrolyte resin) * 100 wt%] is 35 to 75 wt%, and the conductive material carrying the catalyst component in an arbitraty quantity. The solvent for the catalyst ink may be, but is not limited to, alcohols such as ethanol, methanol, propanol, and propylene glycol, organic solvents such as dimethyl sulfoxide, N-methyl-2-pyrrolidone, dimethylacetamide, and tetrahydrofuran, water, and a combination thereof, for example. The thickness of the

catalyst layer is not specifically limited, and may be about 3 to 50 μm. In addition to the polymer electrolyte resin, the fullerene derivative, the catalyst, component and the conductive material, the catalyst. layer may appropriately contain other materials such as water repellent polymers, binders, etc. In the membrane electrode assembly according to the present invention, the catalyst layer containing the fullerene derivative and the polymer electrolyte resin at a ratio such as described above may be provided on only either one of the cathode side and the anode side. However, the catalyst layer containing the fullerene derivative and the polymer electrolyte resin is preferably provided on both the cathode side and the anode side to more effectively increase the power generation performance.

A description will now be made of a manufacturing method of the membrane electrode assembly according to the present invention. The method of forming a membrane electrode assembly in which a pair of electrodes are provided on both surfaces of an electrolyte membrane is not specifically limited, and may be a common method. For example, one method is to 1) apply catalyst ink to both surfaces of an electrolyte membrane and dry the ink to form a catalyst layer, on the surfaces of the electrolyte membrane, and then bond a gas diffusion layer sheet constituting a gas diffusion layer onto the catalyst layer. Another method is to 2) apply catalyst ink to a surface of a gas diffusion layer sheet on a catalyst layer side and dry the ink to form a catalyst layer, and bond the sheet to an electrolyte membrane such that the catalyst layer is interposed between the electrolyte membrane and the gas diffusion layer. Still another method is to 3) apply catalyst ink to a surface of a base made of polytetrafluoroethylene or the like, dry the ink to obtain a catalyst layer sheet, bond the sheet to an electrolyte membrane or a gas diffusion layer sheet, peel the base thereof, and bond it to a gas diffusion layer sheet or an electrolyte membrane such that the catalyst layer is interposed between the electrolyte membrane and the gas diffusion layer.

The gas diffusion layer should be formed using a gas diffusion layer sheet having gas diffusion properties allowing efficient gas supply to the catalyst layer, conductivity, and strength required as a material for the gas diffusion layer. The gas diffusion layer sheet

may be constituted of, for example, carbonaceous porous bodies such as carbon paper, carbon cloth and carbon felt, and conductive porous bodies such as metal meshes and metal porous bodies made of metals such as titanium, aluminum, copper, nickel, nickel chrome alloys, copper, copper alloys, silver, aluminum alloys, zinc alloys, lead alloys, niobium, tantalum, iron, stainless steels, gold, and platinum. The thickness of the conductive porous body is preferably about 15 to 100 μm.

The gas diffusion layer may have a single layer of a conductive porous body such as described above, but may be provided with a water repellent layer on the side facing the catalyst layer. The water repellent layer generally has a porous structure containing conductive powder such as carbon particles and carbon fibers, water repellent resins such as polytetrafiuoroethylene, etc. Although not absolutely necessary, the water repellent layer advantageously can increase the water drainage properties of the gas diffusion layer while suitably keeping the water content in the catalyst layer and the electrolyte membrane, and can improve the electrical contact between the catalyst layer and the gas diffusion layer.

The conductive porous body may be processed by impregnation-applying a water repellent resin such as polytetrafiuoroethylene to the surface facing the catalyst layer using a bar coater or the like, so that water in the catalyst layer is efficiently drained out of the gas diffusion layer. In the above methods, the method of applying the catalyst ink to the surface of the electrolyte membrane, the gas diffusion layer and the base is not specifically limited, and may be spraying, screen printing, doctor blading, gravure printing, and die coating, for example. The electrolyte membrane and the layers may be bonded by heat pressing, for example by hot pressing. The membrane electrode assembly formed by bonding a pair of electrodes to an electrolyte membrane as described above is further interposed between separators to form a unit cell. The separator may be a carbon separator containing a high concentration of carbon fiber and made of a composite material with a resin, a metal separator made of a metal material, etc. The metal separator may be made of a metal material with excellent

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corrosion resistance, an element with its surface coated with carbon, a metal material with excellent corrosion resistance or the like to increase its corrosion resistance, etc. In general, a plurality of such unit cells are laminated together and assembled into a fuel cell. As described above, there can be obtained a high-output fuel cell expressing high power generation performance over a wide current range by using the membrane electrode assembly for a fuel cell according to the present invention with excellent proton conductivity and high electrode reactivity even at low current range, and with excellent gas diffusion properties and water drainage properties even at high current range. [Example] [Synthesis of Sulfonated Fullerene (Fullerene Derivative)]

Fifteen ml of fuming sulfuric acid was added to 1 g of a fullerene (Ceo), and the mixture was subjected to a heated reflux at 60 ⁰ C for three days under a nitrogen atmosphere. Two hundred ml of diethyl ether was added to the obtained suspension, and the mixture was stirred in an ice bath. The deposited precipitate was extracted using a centrifuge, and dried under a reduced pressure at 40 ⁰ C for 24 hours, to obtain Qo(SO ₄ ),!- Forty ml of purified water was added to 2 g of the obtained Qo(SO ₄ ),, _: , and the mixture was heat refluxed at 85°C for 10 to 15 hours. A small quantity of purified water was added to the obtained dark brown precipitate, and the mixture was centrifuged and dried under a reduced pressure at 40 ⁰ C for 24 hours, to obtain Cg _O (OH) _2n . Then, 12 ml of fuming sulfuric acid was added to 200 mg of Ce _O (OH) _2n , and the mixture was stirred in an ice bath for 3 days. One hundred and twenty ml of diethyl ether was added to the obtained orange color solution, and the mixture was stirred in an ice bath. The deposited precipitate was extracted using a centrifuge, and dried under a reduced pressure at 40 ⁰ C for 24 hours, to obtain a sulfonated fullerene Cg _O (OSOaH) _2n . [Fabrication of Membrane Electrode Assembly] (Example 1)

Ten g of a 60 wt% polytetrafluoroethylene (PTFE) aqueous solution was added to 110 g of water to prepare a 5wt% PTFE aqueous solution. Carbon paper was immersed in the PTFE aqueous solution for 3 minutes, and dried for 10 minutes. After being dried by

winds, the carbon paper was placed on a hot plate preheated to 35O ⁰ C, and heat-treated for 30 minutes.

Then, 10 g of carbon particles carrying 50% by weight of platinum, 2.5 g of a perfluorocarbon sulfonic acid resin (trade name: NAFION, manufactured by Dupont), and 2.5 g of the fullerene derivative synthesized as described above were dispersed in 100 ml of a mixed solution of water and ethanol (water: ethanol = 1: 1) to prepare catalyst ink with the ratio [fullerene derivative / (fullerene derivative + perfluorocarbon sulfonic acid resin)

* 100 wt%] being 50 wt%.

The obtained catalyst ink was sprayed onto both surfaces of a perfluorocarbon sulfonic acid resin membrane (trade name: NAFION, manufactured by Dupont, 10 cm by 10 cm) and dried to form catalyst layers (3.6 cm by 3.6 cm). The obtained electrolyte membrane with catalyst layers was interposed between two sheets of the carbon paper after the water repellent treatment described above, and hot pressed,- to fabricate a membrane electrode assembly. The membrane electrode assembly was interposed between separators to thereby obtain a unit cell of Example 1. (Comparative Example 1)

A unit cell of Comparative Example 1 differs from the unit cell of Example 1, in that 2.5 g of a perfluorocarbon sulfonic acid resin and 2.5 g of the fullerene derivative were replaced with 5 g of a perfluorocarbon sulfonic acid resin (trade name: NAFION, manufactured by Dupont) to prepare catalyst ink with the ratio [fullerene derivative / (fullerene derivative + perfluorocarbon sulfonic acid resin) * 100 wt%] being 0 wt%. (Comparative Example 2)

A unit cell of Comparative Example 2 differs from the unit cell of Example 1 in that 2.5 g of a perfluorocarbon sulfonic acid resin and 2.5 g of the fullerene derivative were replaced with 4 g of a perfluorocarbon sulfonic acid resin (trade name: NAFION ₅ manufactured by Dupont) and 1 g of the fullerene derivative to prepare catalyst ink with the ratio [fullerene derivative / (fullerene derivative + perfluorocarbon sulfonic acid resin)

* 100 wt%] being 20 wt%. (Comparative Example 3)

A unit cell of Comparative Example 3 differs from the unit cell of Example 1 in that 2.5 g of a perfluorocarbon sulfonic acid resin and 2.5 g of the fullerene derivative were replaced with 1 g of a perfluorocarbon sulfonic acid resin (trade name: NAFION, manufactured by Dupont) and 4 g of the fullerene derivative to prepare catalyst ink with the ratio [fullerene derivative / (fullerene derivative + perfluorocarbon sulfonic acid resin) * 100 wt%] being 80 wt%. [Power Generation Test]

I-V tests and impedance measurements were conducted on the unit cells of Example 1 and Comparative Examples 1 to 3, obtained as described above, under the conditions (1) to (3) below. The results are shown in FIG 2 (I-V tests) and FIG 3 (impedance measurements). <Power Generation Evaluation Conditions>

• Fuel (hydrogen gas): 300 ml/min

• Oxidant (air): 1000 ml/min (1) Condition 1

• Cell temperature: 80 ⁰ C

• Bubbler temperature: 80 ⁰ C

(2) Condition 2

^■ Cell temperature: 80 ⁰ C • Bubbler temperature: 60 ⁰ C

(3) Condition 3

• Cell temperature: 90 ⁰ C

• Bubbler temperature: 80 ⁰ C <Impedance Conditions> • Frequency range: 1 kHz to 100 kHz

• DC bias voltage: 0.5 V <I-V Characteristics>

As shown in FIG 2, the unit cell of Example 1, which contained 50 wt% of the Mlerene derivative and 50 wt% of the perfluorocarbon sulfonic acid resin (polymer

electrolyte resin), generated high voltages over the entire range from the low current range to the high current range under all of the conditions (1) to (3), compared to those of Comparative Examples 1 to 3. Also, the unit cell of Example 1 has higher in boundary current value than those of Comparative Examples 1 to 3 under the conditions (1) to (3). The unit cell of Comparative Example 2, which contained 20 wt% of the fullerene derivative and 80 wt% of the perfluorocarbon sulfonic acid resin, was low in power generation performance compared to that of Comparative Example 1, which contained no fullerene derivative. Thus, it is understood that containing 20 wt% of the fullerene derivative does not provide the effect with the fullerene derivative but rather is . counterproductive. This is considered to be because the quantity of sulfonic acid groups in the catalyst layers is short. The unit cell of Comparative Example 3, which contained 80 wt% of the fullerene derivative and 20 wt% of the perfluorocarbon sulfonic acid resin, was able to operate only up to about 400 mA/cm ² . This is considered to be because the area of the three-phase boundary is reduced. <Impedance Measurement

From the intersection point between the left end of the curve obtained from the impedance measurements and the axis and the diameter of the arc shown in FIG. 3, it is understood that the unit cell of Example I ₃ which contained 50 wt% of the fullerene derivative and 50 wt% of the perfluorocarbon sulfonic acid resin (polymer electrolyte resin), was low in IR resistance and low in concentration overvoltage under any of the conditions (1) to (3), compared to those of Comparative Examples 1 to 3. The results, support the results of the I-V tests described above, that the unit cell of Example 1 exhibited excellent I-V performance over the entire current range. [Effective Area of Platinum] The effective area S of platinum (Pt) contained in the catalyst layers was calculated by cyclic voltammetry (CV) on the unit cells of Example 1 and Comparative Examples 1 to 3. In order to calculate the effective area S of platinum by CV, the area S' of oxidation current at 0.05 to 0.4 V (quantity of electricity: shaded area) was calculated in the CV curve shown in FIG. 4, and the obtained area S' of oxidation current was divided by the quantity of

electricity E due to hydrogen desorption from Pt at 0.05 to 0.4 V (210 μC/crn ² ), to obtain the value of the effective area of Pt (SVE). That is, the size of the shaded area in FIG. 4 indicates the size of the effective area S of Pt.

The results of CV measurements are shown in FIG. 4. From FIG. 4, it is understood that the unit cell of Example 1 had a larger effective area of platinum than any of those of Comparative Examples 1, to 3. That is, the three-phase boundary was large and the reaction at the electrode proceeded actively in the unit cell of Example 1. The results support the results of the I-V tests described above, that the. unit cell of Example 1 exhibited excellent I-V performance over the entire current range.