Method for operating a fuel cell

Abstract

Provided is a method for operating a fuel cell that can sufficiently humidify a polymer electrolyte membrane even under high-temperature conditions and that can achieve excellent power generation performance. The present invention is a method for operating a fuel cell that includes a membrane electrode composite having an electrolyte membrane, a catalyst layer, and a gas diffusion layer, the method being characterized by including a step of setting the operating temperature of the fuel cell to 100°C or higher, in which step the relative humidity of a supply gas supplied to the fuel cell is 70% or higher, and the back pressure of the supply gas is 330 kPa or higher.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for operating a fuel cell having a membrane electrode assembly including an electrolyte membrane, a catalyst layer, and a gas diffusion layer, which is capable of obtaining excellent power generation performance even under high-temperature conditions by increasing the humidity and back pressure of a supplied gas during high-temperature operation. BACKGROUND

[0002] A fuel cell is a power generation device that extracts electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and in recent years, it has attracted attention as a clean energy supply source. Among them, a solid polymer fuel cell has a standard operating temperature as low as about 100°C and a high energy density, and thus is expected to be widely used as a power generation device for small-scale decentralized power generation facilities, vehicles, ships, and the like. In addition, it has attracted attention as a power source for small mobile devices and portable devices, and is expected to replace secondary batteries such as nickel-hydrogen batteries and lithium-ion batteries, and to be mounted on mobile phones, computers, and the like.

[0003] A solid polymer fuel cell is generally configured by a membrane electrode assembly (hereinafter sometimes referred to as MEA) in which a gas diffusion layer that supplies a fuel gas and an oxidizing gas to a catalyst layer, the catalyst layer in which an anode and a cathode that undergo a reaction for power generation are provided, and a polymer electrolyte membrane that is a proton conductor between the anode catalyst layer and the cathode catalyst layer are combined, and a single cell in which the MEA is sandwiched by separators.

[0004] As a required characteristic of the above-mentioned polymer electrolyte membrane, first, a high proton conductivity can be mentioned, and in particular, a high proton conductivity under high-temperature low-humidification conditions is also required. In the past, Nafion (registered trademark) (manufactured by DuPont) that is a perfluorosulfonic acid-based polymer has been widely used for a polymer electrolyte membrane. Nafion (registered trademark) exhibits a high proton conductivity through a proton conduction path generated by a cluster structure, and on the other hand, there is a problem in the proton conductivity under low-humidification conditions.

[0005] On the other hand, in recent years, development of a hydrocarbon-based polymer electrolyte membrane that can replace Nafion (registered trademark) has become active, and in particular, in order to improve the proton conductivity, a block copolymer composed of a hydrophobic segment and a hydrophilic segment is used, and several attempts to form a microphase separation structure have been made, but the proton conductivity under low-humidification conditions is still a problem. In view of this situation, in a fuel cell, it has become important to manage water in a membrane electrode assembly (particularly, the water content of an electrolyte membrane).

[0006] On the other hand, to further improve the performance of solid polymer fuel cells, higher operating temperatures exceeding 100°C are required. Increasing the operating temperature enhances catalyst activity, improves power generation performance, and increases heat dissipation efficiency through the radiator, enabling miniaturization of the fuel cell system. Furthermore, it reduces catalyst poisoning caused by poisoning sources such as carbon monoxide in the fuel gas and suppresses performance degradation due to impurities. However, increasing the operating temperature causes dehydration of the membrane electrode complex, particularly the electrolyte membrane, reducing proton conductivity and thus hindering the achievement of optimal performance. Therefore, the development of electrolyte membrane materials usable at high temperatures, especially exceeding 100°C, and operational fuel cell systems has been undertaken.

[0007] Patent Document 1 describes a battery structure for a solid polymer fuel cell operating at temperatures above 100°C, which includes a separator structure to suppress gas pressure loss and increase the pressure difference between the anode and cathode. The document discloses a structure that reduces pressure loss in the cathode separator and improves energy efficiency by making the cross-sectional area of ​​the flow path downstream of the gas flow direction of the cathode separator larger than the cross-sectional area upstream.

[0008] Furthermore, Patent Document 2 discloses a solid polymer membrane-electrode structure for fuel cells. This solid polymer membrane-electrode structure has a proton-conducting membrane with high proton conductivity and is difficult to swell under high temperature and high humidity conditions, exhibiting excellent dimensional stability. It discloses a structure that provides a membrane-electrode structure with small dimensional changes even under high sulfonic acid equivalents by making the electrolyte membrane a branched polyarylene copolymer with specific structural units.

[0009] Furthermore, Patent Document 3 describes a high-temperature polymeric electrolyte membrane fuel cell and its operation method that operate substantially independently of moisture within the fuel cell. It discloses a method for mitigating the effects of CO concentration in the process gas and moisture content within the cell in order to operate in a substantially moisture-free state. This method utilizes an electrolyte containing self-dissociating compounds such as phosphoric acid within the membrane, and maintains an operating temperature of 80–300°C and an operating pressure of 0.3–5 bar.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2007-115413

[0013] Patent Document 2: Japanese Patent Application Publication No. 2009-238468

[0014] Patent Document 3: Japanese Patent Publication No. 2003-504805 Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] However, the inventors have discovered that for membrane electrode composites using the solid polymer electrolyte membranes described in Patent Documents 1 and 2, if the humidification amount is increased to maintain proton conductivity at high temperatures, the increased water content in the supply gas leads to a decrease in the concentration of the reactant gas, particularly the oxidizing gas, resulting in increased mass diffusion resistance and a decline in performance. This point has not been mentioned in any of the literature.

[0017] Furthermore, the electrolyte described in Patent Document 3 has a high acidity, which raises concerns about strong catalyst poisoning and reduced power generation performance in high-temperature regions. Additionally, the decrease in proton conductivity with use is also a concern. Therefore, to maintain high power generation performance in high-temperature regions, it is necessary to appropriately humidify the electrolyte, which uses a solid polymer that does not contain self-dissociating compounds such as phosphoric acid in the membrane, while simultaneously suppressing the decrease in the concentration of reactive gases near the electrodes.

[0018] In view of the background of the prior art, the present invention provides a method for operating a fuel cell that can be fully humidified even under high temperature conditions, thereby achieving excellent power generation performance.

[0019] Problem-solving methods

[0020] To address this issue, the present invention employs the following method.

[0021] That is, the fuel cell operation method of the present invention is an operation method of a fuel cell having a membrane electrode assembly (MEA) having an electrolyte membrane, a catalyst layer and a gas diffusion layer, characterized in that it includes a step of setting the operating temperature of the fuel cell to 100°C or higher, setting the relative humidity of the supply gas supplied to the fuel cell in the step to 70% or higher and setting the back pressure of the supply gas to 330 kPa or higher.

[0022] Furthermore, the fuel cell system of the present invention is a fuel cell system used in the above-described operation method of the fuel cell of the present invention, characterized in that it comprises a fuel cell having a membrane electrode complex, a humidifier for humidifying the supply gas supplied to the fuel cell, and a compressor for increasing the back pressure of the supply gas, wherein the membrane electrode complex comprises an electrolyte membrane, a catalyst layer and a gas diffusion layer.

[0023] Invention Effects

[0024] According to the present invention, a method for operating a fuel cell with high power generation performance under high temperature conditions can be provided. Attached Figure Description

[0025] Figure 1 This is a schematic cross-sectional view illustrating the fabrication method of the membrane electrode composite produced in Embodiment 1 of the present invention.

[0026] Figure 2 This is a schematic cross-sectional view illustrating the method for fabricating the membrane electrode composite produced in Embodiment 2 of the present invention.

[0027] Figure 3 This is a perspective view illustrating the structure of the fuel cell unit of the present invention.

[0028] Figure 4 This is a schematic diagram illustrating the fuel cell system of the present invention. Detailed Implementation

[0029] The present invention will now be described in detail.

[0030] [Membrane electrode complex]

[0031] The membrane electrode composite (MEA) of the present invention has an electrolyte membrane, a catalyst layer disposed on both sides of the electrolyte membrane, and a gas diffusion layer disposed in contact with the opposite side of the electrolyte membrane to the catalyst layer.

[0032] (electrolyte membrane)

[0033] The electrolyte membrane contained in the membrane electrode composite of the present invention is not particularly limited, but is preferably an electrolyte membrane containing a solid polymer electrolyte. As a solid polymer electrolyte, it is preferably an electrolyte containing a proton-conducting polymer.

[0034] In this invention, perfluorosulfonic acid polymers, which have been widely used as polymeric electrolyte membranes in the past, can be used as proton-conducting polymers. However, polymeric electrolyte membranes containing hydrocarbon polymers, which have been actively developed in recent years, are preferred. Considering their low cost, ability to inhibit fuel cross-over, excellent mechanical strength, high softening temperature, and high-temperature resistance, polymeric electrolyte membranes containing hydrocarbon polymers can replace perfluorosulfonic acid polymers.

[0035] In particular, to improve proton conductivity in low-humidity environments, several attempts have been made to use block copolymers composed of hydrophobic and hydrophilic segments to form microphase-separated structures. By using polymers with this structure, mechanical strength is improved through hydrophobic interactions or aggregation between hydrophobic segments, while clustering occurs through electrostatic interactions between the ionic groups of hydrophilic segments, thus enhancing proton conductivity by forming ion conduction channels.

[0036] As a mechanism for proton movement in these electrolyte membranes, a vehicle-mounted mechanism (beagle mechanism) for the self-movement of hydrogen ions after proton hydration and a grotthus mechanism for protons bound to substrates jumping to other substrates were proposed. Under low humidification conditions with few water molecules, the jumping movement of sulfonic acid groups based on the vehicle-mounted hydration mechanism predominated.

[0037] In this context, with fluorinated electrolyte membranes, the sulfonic acid groups in the molecular structure have a small acid dissociation constant, making proton dissociation easier and thus facilitating hopping-based proton conduction. Conversely, in polymeric electrolyte membranes containing hydrocarbon polymers, the sulfonic acid groups have a larger acid dissociation constant than in fluorinated electrolyte membranes, making proton dissociation less likely. Therefore, the decrease in proton conductivity under low humidification conditions is greater in fluorinated electrolyte membranes than in fluorinated electrolyte membranes. The acid dissociation constant, as referred to here, is one of the indicators used to express the acid strength of a substance, and is represented by the negative logarithm pKa of the equilibrium constant in the dissociation reaction that releases protons from the acid.

[0038] In this invention, aromatic hydrocarbon polymers are preferred as hydrocarbon polymers. Specific examples of aromatic hydrocarbon polymers include polysulfones, polyethersulfones, polyphenylene ethers, polyarylene ether polymers, polyphenylene sulfide, polyphenylene sulfide sulfone, poly(p-phenylene), polyarylene polymers, polyarylene ketones, polyether ketones, polyarylene phosphine oxides, polyether phosphine oxides, and polybenzo[a]benzene, all of which have aromatic rings on their main chains. Polymers such as azoles, polybenzothiazoles, polybenzimidazoles, aromatic polyamides, polyimides, polyetherimides, and polyimide sulfones.

[0039] In addition, "polyethersulfone" is a general term for polymers that have ether bonds and sulfone bonds in their molecular chains. Similarly, "polyetherketone" is a general term for polymers that have ether bonds and ketone bonds in their molecular chains, including polyetherketoneketone, polyetheretherketone, polyetheretherketoneketone, polyetherketoneetherketone, polyetherketoneetherketone, polyetherketoneetherketone, etc., without limiting any specific polymer structure.

[0040] Among these aromatic hydrocarbon polymers, considering mechanical strength, physical durability, processability, and hydrolysis resistance, polymers such as polysulfone, polyethersulfone, polyphenylene ether, polyarylene ether polymers, polyphenylene sulfide, polyphenylene sulfide sulfone, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, and polyether phosphine oxide are preferred, with polyether ketone being more preferred. As a polyether ketone, a block copolymer composed of segments having a benzophenone structure with ionic groups and segments having a dioxolane ring structure is more preferred.

[0041] There are no particular limitations on the synthesis methods of aromatic hydrocarbon polymers, as long as the above-mentioned characteristics or requirements are met. For example, methods described in the Journal of Membrane Science, 197, 2002, pp. 231-242 can be used.

[0042] As an example, the preferred polymerization conditions for synthesizing aromatic hydrocarbon polymers via condensation polymerization are shown below. Polymerization can be carried out in a temperature range of 0–350°C, but is preferably in the range of 50–250°C. Below 0°C, the reaction tends to be incomplete, and above 350°C, polymer decomposition tends to occur. The reaction is preferably carried out in a solvent. Examples of solvents that can be used include, but are not limited to, aprotic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, sulfolane, 1,3-dimethyl-2-imidazolinone, and hexamethylphosphoric triamine, as long as they are stable solvents in aromatic nucleophilic substitution reactions. These organic solvents can be used alone or as a mixture of two or more.

[0043] When carrying out a condensation reaction in a solvent, it is preferable to combine the monomers in such a way that the resulting polymer concentration is 5 to 50% by weight. When the polymer concentration is less than 5% by weight, there is a tendency to make it difficult to increase the degree of polymerization. On the other hand, when the polymer concentration is greater than 50% by weight, the viscosity of the reaction system becomes too high, and the post-processing of the reactants tends to become difficult.

[0044] In this invention, aromatic hydrocarbon polymers may also possess ionic groups. Methods for introducing ionic groups into aromatic hydrocarbon polymers include polymerization using monomers possessing ionic groups and introducing ionic groups during the polymer reaction. As a method of polymerization using monomers possessing ionic groups, any monomer possessing ionic groups in the repeating unit can be used; if necessary, appropriate protecting groups can be introduced, followed by deprotection after polymerization.

[0045] For example, methods for introducing ionic groups, such as methods for sulfonating aromatic rings, i.e., introducing sulfonic acid groups, are described in Japanese Patent Application Publication No. 2-16126 or Japanese Patent Application Publication No. 2-208322.

[0046] Specifically, sulfonation can be achieved, for example, by reacting the aromatic ring with a sulfonating agent such as chlorosulfonic acid in a solvent such as chloroform, or by reacting it in concentrated sulfuric acid or fuming sulfuric acid. There are no particular limitations on the sulfonating agent, as long as it can sulfonate the aromatic ring; sulfur trioxide can also be used in addition to the methods mentioned above. When sulfonating the aromatic ring using this method, the degree of sulfonation can be easily controlled by adjusting the amount of sulfonating agent used, the reaction temperature, and the reaction time. Introducing sulfonylimide groups into aromatic polymers can be achieved, for example, by reacting a sulfonic acid group with a sulfonamide group.

[0047] The ionic group preferably has a negatively charged functional group, and particularly preferably has a proton exchange capability. As such functional groups, sulfonic acid group, sulfonylimide group, sulfate group, phosphonic acid group, phosphate group, and carboxylic acid group are preferred. Here, the sulfonic acid group is represented by the following general formula (f1), the sulfonylimide group is represented by the following general formula (f2) [in general formula (f2), R represents any organic group], the sulfate group is represented by the following general formula (f3), the phosphonic acid group is represented by the following general formula (f4), the phosphate group is represented by the following general formula (f5) or (f6), and the carboxylic acid group is represented by the following general formula (f7).

[0048]

[0049] The ionic group includes the case where the aforementioned functional groups (f1) to (f7) are salts. Examples of cations forming the salt include any metal cation, NR4+ (where R is any organic group). The valence of the metal cation is not particularly limited. Specific examples of preferred metal ions include Li, Na, K, Rh, Mg, Ca, Sr, Ti, Al, Fe, Pt, Rh, Ru, Ir, and Pd ions. Among these, Na, K, and Li ions, which are inexpensive and readily proton-substituted, are preferably used in the block copolymers used in this invention.

[0050] These ionic groups can be present in the polymer in two or more forms, and the combination can be appropriately determined according to the polymer structure. From the viewpoint of high proton conductivity, sulfonic acid groups, sulfonylimide groups, or sulfate groups are more preferred, and from the viewpoint of raw material cost, sulfonic acid groups are the most preferred.

[0051] As the electrolyte membrane of the present invention, its softening temperature is preferably 120°C or higher. If the softening temperature is lower than 120°C, the mechanical strength of the electrolyte membrane decreases at operating temperatures exceeding 100°C, which may sometimes cause deterioration such as creep or membrane rupture. To maintain durability under high-temperature conditions, it is preferable to use an electrolyte membrane with a softening temperature of 120°C or higher. In the present invention, the softening temperature is the temperature at which the slope of the stored elastic modulus in the dynamic viscoelasticity measurement of the electrolyte membrane shows an inflection point.

[0052] As a polymeric electrolyte membrane with such a high softening temperature, a polymeric electrolyte membrane containing the aforementioned hydrocarbon-based polymer is preferred. Typical perfluorosulfonic acid polymers have a softening temperature around 80°C, and sometimes lack sufficient mechanical strength at operating temperatures exceeding 100°C. On the other hand, hydrocarbon-based polymers have a higher softening temperature, making it easier to produce electrolyte membranes with softening temperatures of 120°C or higher. Therefore, as an electrolyte membrane contained in a fuel cell operating under high-temperature conditions, a polymeric electrolyte membrane formed from a hydrocarbon-based polymer can be more preferably used.

[0053] As the electrolyte membrane of the present invention, its oxygen permeability coefficient at 90°C and 80% RH is preferably 1.0 × 10⁻⁶. -9 cm 3 ·cm / cm 2 Less than ·sec·cmHg, more preferably 5.0×10 -10 cm 3 ·cm / cm 2 Less than 1.0 × 10⁻⁶ cmHg, and more preferably 1.0 × 10⁻⁶ cmHg. -10 cm 3 ·cm / cm 2 Below ·sec·cmHg. Due to the high oxygen permeability of the electrolyte membrane, the amount of hydrogen peroxide generated, which causes chemical degradation of the membrane, increases. This hydrogen peroxide is produced by the chemical reaction between oxygen permeating the membrane and hydrogen supplied to the counter electrode. Especially under high temperature conditions, the saturated solubility of gases in the electrolyte membrane usually tends to decrease, but due to the significantly increased diffusion rate of gases in the electrolyte membrane, the gas permeability coefficient often increases. To maintain sufficient chemical durability at operating temperatures exceeding 100°C, if the oxygen permeability coefficient at 90°C and 80% RH is 1.0 × 10⁻⁶, the chemical permeability coefficient is required to achieve this. - 9 cm 3 ·cm / cm 2 Below ·sec·cmHg, it can suppress the decrease in chemical durability that accompanies the generation of hydrogen peroxide.

[0054] As the electrolyte membrane of the present invention, its hydrogen permeation coefficient at 90°C and 80% RH is preferably 5.0 × 10⁻⁶. -9 cm 3 ·cm / cm 2 Less than 1.0 × 10⁻⁶ sec·cmHg, more preferably 1.0 × 10⁻⁶ -9 cm 3 ·cm / cm 2Below ·sec·cmHg. Due to the high hydrogen permeability of the electrolyte membrane, the amount of hydrogen peroxide generated, which causes chemical degradation of the membrane, increases due to the chemical reaction between hydrogen permeating the membrane and oxygen supplied to the counter electrode. Especially under high temperature conditions, the saturated solubility of gases in the electrolyte membrane usually tends to decrease, but the gas permeability coefficient often increases due to the significantly increased diffusion rate of gases in the electrolyte membrane. To maintain sufficient chemical durability at operating temperatures exceeding 100°C, a hydrogen permeability coefficient of 5.0 × 10⁻⁶ at 90°C and 80% RH is required. -9 cm 3 ·cm / cm 2 Below ·sec·cmHg, it can suppress the decrease in chemical durability that accompanies the generation of hydrogen peroxide.

[0055] In this invention, the gas permeability coefficients of oxygen and hydrogen in an electrolyte membrane at 90°C and 80% RH were determined under the following conditions. The average value of three tests was calculated as the gas permeability coefficient.

[0056] Apparatus: Differential pressure gas permeability measurement system GTR-30AX (manufactured by GTR Tech Co., Ltd.)

[0057] Temperature × Relative Humidity: 90℃ × 80% RH

[0058] Test gases: oxygen, hydrogen

[0059] Test gas pressure: The total pressure including water vapor is 101.3 kPa (atmospheres).

[0060] At 90℃ and 80% RH, the partial pressure of each gas measured was 45.2 kPa.

[0061] Gas permeability area: 3.14 cm² 2 (Circular sample with a diameter of 2.0 cm) * Masking is implemented.

[0062] Number of measurements (n): 3 (using the same sample)

[0063] Because it easily reduces the gas permeability coefficient of the electrolyte membrane, the polymer electrolyte used in this invention is preferably a hydrocarbon-based polymer. Furthermore, to obtain sufficient mechanical strength and gas barrier properties, the polymer electrolyte is preferably a crystalline aromatic hydrocarbon-based polymer. Here, "crystalline" means having the property of crystallization upon heating, or already being crystallized.

[0064] The presence or absence of crystallinity is confirmed by differential scanning calorimetry (DSC) or wide-angle X-ray diffraction. In this invention, it is preferable that the heat of crystallization measured by differential scanning calorimetry after film formation is 0.1 J / g or more, or the degree of crystallinity measured by wide-angle X-ray diffraction is 0.5% or more. That is, if no crystallization peak is found in differential scanning calorimetry, it can be considered that the film has already crystallized or that the polymer electrolyte is non-crystalline; however, if the film has already crystallized, the degree of crystallinity measured by wide-angle X-ray diffraction is 0.5% or more.

[0065] There is no particular limitation on the thickness of the electrolyte membrane. However, if it is thicker than 20 μm, there is a tendency for reduced power generation performance. If it is thinner than 5 μm, there is a tendency for reduced durability and operability. The preferred thickness is between 5 μm and 20 μm. When the electrolyte membrane thickness is less than 5 μm, the amount of water retained in the membrane is low, and the membrane dries prematurely under high temperature conditions, which may sometimes lead to a decrease in power generation performance.

[0066] (Catalyst layer)

[0067] The catalyst layer of this invention comprises an ionic conductor and catalyst-supported particles on a support. The catalyst is preferably a noble metal such as platinum, gold, ruthenium, or iridium, which exhibits high activity in oxidation and reduction reactions, but is not limited thereto. As the support, carbon particles or oxide particles with high conductivity, chemical stability, and high surface area are preferred, with metal oxide particles being particularly preferred. Examples of carbon particles include acetylene black, Ketjen black, and Vulcan carbon; examples of metal oxide particles include tin oxide and titanium oxide.

[0068] In this invention, a chemically stable metal oxide support is particularly preferred, even under oxidizing atmospheres above 100°C. Carbon particles are accelerated to oxidize in oxidizing atmospheres above 100°C, sometimes accelerating the detachment of catalyst particles supported on the carbon particles or deterioration caused by sintering. By using a metal oxide support, the deterioration of the catalyst support under high-temperature operating conditions can be suppressed, maintaining high power generation performance.

[0069] (Gas diffusion layer)

[0070] The gas diffusion layer of the present invention comprises a carbon sheet and a microporous layer. That is, it can be fabricated by forming a microporous layer on the carbon sheet.

[0071] The microporous layer is composed of hydrophobic resins such as PTFE and conductive fillers. Carbon powder is preferred as the conductive filler. Examples of carbon powder include furnace black, acetylene black, lampblack, and pyrolysis carbon black; flake graphite, scaly graphite, amorphous graphite, artificial graphite, expanded graphite, and thin-film graphite; carbon nanotubes; and finely ground carbon fibers. Among these, carbon black is more preferably used as the filler, and acetylene black is preferred due to its low impurity content.

[0072] In this invention, from the viewpoint of improving water retention, it is preferable to reduce the amount of hydrophobic resin used in the microporous layer. Furthermore, by using a hydrophilic resin with adhesive properties instead of a hydrophobic resin, the water retention of the membrane electrode composite can be further improved.

[0073] The carbon sheet is important due to its high gas diffusivity for diffusing gas supplied from the separator to the catalyst layer and its high drainage capacity for discharging water generated during the electrochemical reaction to the separator. Furthermore, the carbon sheet of the present invention preferably has high electrical conductivity in order to extract the generated current. Therefore, to obtain the carbon sheet, a porous body with electrical conductivity is preferred. More specifically, the porous body used to obtain the carbon sheet is preferably a porous body containing carbon fibers, such as carbon fiber fabric, carbon paper, and carbon fiber nonwoven fabric, or a carbonaceous foam porous body containing carbon fibers.

[0074] Among them, due to its excellent corrosion resistance, a porous body containing carbon fibers is preferred in order to obtain carbon sheets. Furthermore, due to its excellent "springiness" in absorbing dimensional changes in the direction perpendicular to the surface of the electrolyte membrane (thickness direction), carbon paper made by bonding carbon fiber paper bodies with carbides (binder materials) is preferred.

[0075] (Method for fabricating membrane electrode composites)

[0076] The fabrication methods of the above-mentioned membrane electrode complex (MEA) having an electrolyte membrane, a catalyst layer and a gas diffusion layer are roughly divided into: (I) fabricating a gas diffusion electrode (GDE) with a catalyst layer formed on one side of the gas diffusion layer, and stacking the fabricated gas diffusion electrode (GDE) with the electrolyte membrane; and (II) preparing an electrolyte membrane (CCM) with a catalyst layer, and stacking the fabricated electrolyte membrane (CCM) with the gas diffusion layer.

[0077] Figure 2 This is a schematic cross-sectional view used to illustrate the method described in (I) above (the method of Embodiment 2 described later).

[0078] In the case of method (I), firstly, two gas diffusion electrodes (GDEs) are formed on the microporous layer forming surfaces of the anode gas diffusion layer 1a and the cathode gas diffusion layer 1b, respectively, to form an anode catalyst layer 2a and a cathode catalyst layer 2b. Then, an electrolyte membrane is configured to directly contact and bond with the catalyst layer forming surfaces of the anode and cathode gas diffusion electrodes.

[0079] Figure 1 This is a schematic cross-sectional view used to illustrate the method described in (II) above (the method of Example 1 described later).

[0080] In the case of method (II), firstly, an electrolyte membrane (CCM) with catalyst layers is fabricated, wherein an anode catalyst layer 2a and a cathode catalyst layer 2b are stacked on both sides of the electrolyte membrane 3. Then, the anode and cathode electrode substrates (anode gas diffusion layer 1a and cathode gas diffusion layer 1b) are configured to be in direct contact with and bonded to the catalyst layer forming surface of the CCM.

[0081] There are no particular limitations on the bonding method for the electrolyte membrane, catalyst layer, and gas diffusion layer, and well-known methods can be used (e.g., the chemical plating method described in Electrochemistry, 1985, 53, p. 269, and the heating and pressurizing bonding method for the gas diffusion electrode described in Electrochemical Society, J. Electrochem. Soc., and Electrochemical Science and Technology, 1988, 135, 9, p. 2209).

[0082] When integrating the electrolyte membrane, catalyst layer, and gas diffusion layer through pressing, the temperature and pressure should be appropriately selected based on the thickness of the electrolyte membrane, its moisture content, and the catalyst layer or electrode substrate. Specific pressing methods include roller pressing with a specified pressure or gap, and flatbed pressing with a specified pressure. From the viewpoint of industrial productivity or suppressing the thermal decomposition of polymers with ionic groups, these methods are preferably performed within the range of 0°C to 250°C. From the viewpoint of protecting the electrolyte membrane or electrodes, the pressure should be as low as possible; in the case of a flatbed press, a pressure of 10 MPa or less is preferred. From the viewpoint of preventing short circuits between the anode and cathode electrodes, it is also a preferred option to modularize the fuel cell by overlapping the electrodes and electrolyte membrane without implementing composite processing based on the pressing process. In this method, when the fuel cell is repeatedly generating electricity, there is a tendency to suppress electrolyte membrane degradation suspected to be caused by short circuits, resulting in good fuel cell durability.

[0083] Specifically, it is preferable to arrange the electrolyte membrane, gas diffusion layer, and catalyst layer as described above. Figure 1 and Figure 2The layers are stacked as shown and pressed under certain temperature and pressure conditions to manufacture MEAs. This stacking and pressing can be performed simultaneously on both sides or sequentially, one side at a time.

[0084] One method for continuously manufacturing membrane electrode composites includes stacking a roll-shaped electrolyte membrane with a catalyst layer and / or a gas diffusion layer, followed by pressing under specific temperature and pressure. When stacking membrane components such as a substrate, electrolyte membrane, or electrolyte membrane with a substrate, it is preferable to apply tension to each membrane component simultaneously. This can be varied by installing tension cutters between each step. Tension cutters can be devices such as those equipped with a motor, clutch, and brake on the roller, and preferably include a detection mechanism for detecting the tension applied to the film. Rollers used in tension cutters can include clamping rollers, suction rollers, or combinations of multiple rollers. Clamping rollers hold the film, and the feed speed of the film is controlled by the frictional force generated by the clamping pressure. As a result, the pressure applied to the film can vary before and after the roller. A suction roller attracts a film-like component by creating a negative pressure inside a roller with multiple holes on its surface, or a roller with threads wound around its surface to form a mesh or curtain. The feed speed of the film-like component is controlled by the frictional force generated by this attraction, which results in the pressure applied to the film-like component varying back and forth on the roller.

[0085] [Fuel Cell Unit]

[0086] Figure 3 This is a perspective view illustrating the structure of the fuel cell unit 10 of the present invention.

[0087] The membrane electrode assembly 4, manufactured as described above, is joined with the anode separator 5a and the cathode separator 5b to form a fuel cell unit 10. Multiple grooves are formed on the surface of the anode separator 5a that is joined with the anode gas diffusion layer 1a, serving as flow paths for hydrogen gas 6. Hydrogen gas 6 supplied to the grooves in the anode separator 5a passes through the anode gas diffusion layer 1a and reaches the anode catalyst layer 2a for oxidation. Similarly, multiple grooves are formed on the surface of the cathode separator 5b that is joined with the cathode gas diffusion layer 1b, serving as flow paths for air or oxygen 7. Air or oxygen 7 supplied to the grooves in the anode separator 5b passes through the cathode gas diffusion layer 1b and reaches the cathode catalyst layer 2b for reduction.

[0088] [Fuel Cell System]

[0089] Figure 4 This is a schematic diagram illustrating the fuel cell system 20 of the present invention.

[0090] The fuel cell system 20 mainly consists of a fuel cell stack 11, humidifiers 12a and 12b, compressors 13a and 13b, back pressure valves 14a and 14b, piping connecting them, and various sensors. The fuel cell unit 10 is manufactured as described above. Figure 3 The fuel cell stack 11 is formed by alternating connections and integration of the fuel cell stack 11 with the cooling plate (not shown).

[0091] (humidifier)

[0092] A humidifier is used to humidify the gas supplied to the fuel cell unit. The humidifier is positioned upstream of the gas supply port to the fuel cell stack 11. By controlling the amount of water supplied from the humidifier according to the operating temperature of the fuel cell, the electrolyte membrane can be appropriately humidified regardless of the operating temperature. Humidification methods include passing the supply gas through a layer of water containing heated water (bubbling method) or directly adding water vapor to the supply gas to mix it in (water vapor addition method). In the operating method of the present invention, since the operating temperature of the fuel cell exceeds 100°C, the gas supplied to the humidifier also reaches a similarly high temperature. Therefore, a humidifier that can adequately humidify the supply gas within a temperature range above 100°C and has high-temperature durability is preferred.

[0093] (compressor)

[0094] The compressor is used to pressurize the supply gas to the fuel cell unit. The compressor compresses the supply gas, particularly the cathode gas (air or oxygen), supplying pressurized gas to the fuel cell. The compressor, which compresses the cathode gas, uses air as the cathode gas and is positioned between the air intake and the gas supply port of the fuel cell stack, provided that air is always drawn in from outside the fuel cell system. In a structure that circulates the supply gas within the fuel cell system, it is positioned along the path from the gas outlet to the gas supply port.

[0095] (coolant)

[0096] Coolant is used to control the operating temperature of the fuel cell unit. The coolant is supplied to the cooling plate via a coolant circulation pump, absorbing the heat generated during power generation by the fuel cell stack and dissipating it in a radiator (not shown). In the operating method of this invention, since the operating temperature of the fuel cell exceeds 100°C, the coolant also becomes at a similarly high temperature. Therefore, a coolant with low vapor pressure that can adequately cool the fuel cell stack within a temperature range above 100°C is preferred.

[0097] [How a fuel cell operates]

[0098] The method for operating the fuel cell of the present invention includes a step of using the above-described fuel cell system to set the operating temperature of the fuel cell to 100°C or higher. In this step, the relative humidity of the supply gas supplied to the fuel cell is set to 70% or higher, and the back pressure of the supply gas is set to 330 kPa or higher.

[0099] In this invention, relative humidity (%RH) refers to the water vapor pressure relative to the saturated water vapor pressure at a certain temperature, and back pressure refers to the pressure of the supply gas at the outlet of the fuel cell stack. Furthermore, pressure in this invention means absolute pressure.

[0100] The following uses Figure 4 (Fuel Cell System 20) describes a specific method for operating the fuel cell of the present invention.

[0101] In this invention, each fuel cell unit 10 within the fuel cell stack 11 ( Figure 3 The operating temperature of the fuel cell unit 10 can be heated to 100°C or higher, for example, by using a heater or similar external heating source to heat the fuel cell stack 11. The temperature of the fuel cell unit 10 can be set to 100°C or higher by simultaneously adjusting the heating temperature of the heater or similar source while measuring the temperature using a thermocouple embedded within the unit or by using thermal imaging (infrared temperature imaging device). A temperature distribution can also be generated within the fuel cell stack 11, but it is necessary to set all units 10 within the fuel cell stack 11 to 100°C or higher. In this invention, the operating temperature of the fuel cell unit 10 only needs to be 100°C or higher, preferably 105°C or higher, more preferably 110°C or higher, and even more preferably 115°C or higher. Furthermore, the upper limit of the operating temperature of the fuel cell unit 10 is generally set to 150°C or lower, preferably 140°C or lower, and more preferably 130°C or lower. Any combination of the above-mentioned upper and lower limits of the operating temperature can be used.

[0102] Hydrogen, used as fuel gas, is stored in hydrogen tank 18. Hydrogen is supplied from hydrogen tank 18 to compressor 13a via hydrogen supply pipe 6c. In compressor 13a, the hydrogen is compressed and pressurized. The pressurized hydrogen is supplied to humidifier 12a. At this time, the hydrogen is humidified to a relative humidity of 70% or higher relative to the temperature of fuel cell unit 10, and the temperature of the hydrogen is set to the same dew point as the humidification amount or heated to a temperature higher than that dew point. Furthermore, the humidity of the hydrogen is confirmed by humidity sensor 16 set to a predetermined humidification amount. Next, the humidified hydrogen is supplied from the hydrogen supply port of fuel cell stack 11 into the interior of fuel cell stack 11, and supplied to the anode separator 5a of each fuel cell unit. Figure 3 ).

[0103] Hydrogen gas 6 not used in the fuel cell unit is discharged from the hydrogen outlet of the fuel cell stack 11 via the hydrogen discharge pipe 6d. At this time, the back pressure of the hydrogen measured at the outlet of the fuel cell unit stack is set to 330 kPa or higher. The back pressure of the hydrogen can be measured by a pressure sensor 17 installed on the hydrogen discharge pipe 6d, and can be adjusted to a specified back pressure by a compressor 13a and a back pressure valve 14a.

[0104] On the other hand, air, as an oxidizing gas, is introduced from the air intake and supplied to the compressor 13b via the air supply pipe 7c. In the compressor 13b, the air is compressed and pressurized. The pressurized air is supplied to the humidifier 12b. At this time, the air is humidified to a relative humidity of 70% or higher relative to the temperature of the fuel cell unit 10, and the air temperature is set to the same dew point as the humidification amount or heated to a temperature higher than that dew point. Furthermore, the air humidity is confirmed by the humidity sensor 16, which sets a predetermined humidification amount. Next, the humidified air is supplied from the air supply port of the fuel cell stack 11 into the interior of the fuel cell stack 11, and supplied to the cathode separator 5b of each fuel cell unit. Figure 3 ).

[0105] Air 7 not used in the fuel cell unit is discharged from the air outlet of the fuel cell stack 11 via the air exhaust pipe 7d. At this time, the back pressure of the air measured at the outlet of the fuel cell unit stack is set to 330 kPa or higher. The back pressure of the air can be measured by the pressure sensor 17 installed on the air exhaust pipe 7d, and can be adjusted to a specified back pressure by the compressor 13b and the back pressure valve 14b.

[0106] Within the fuel cell stack 20, heat is generated through the power generation of the fuel cells. To recover this heat, coolant is supplied to the fuel cell stack 20 via a circulation pump 19. The cooling water supplied to the fuel cell stack 20 recovers heat via cooling plates (not shown) disposed between each fuel cell unit 10 and is discharged as warm water to the outside of the fuel cell stack 20. The recovered waste heat can be further utilized effectively.

[0107] That is, in this invention, when the operating fuel cell reaches a high temperature of 100°C or higher, by setting the relative humidity of the supply gas to 70% or higher, an amount of water vapor sufficient to appropriately humidify the membrane electrode complex, particularly the electrolyte membrane, can be supplied. Furthermore, even when the water vapor pressure rises, by increasing the back pressure of the supply gas to a specific value or higher, a sufficient amount of reactant gas can be supplied to the electrodes of the membrane electrode complex. Thus, while improving catalyst activity and heat dissipation efficiency, the increase in proton conduction resistance of the electrolyte membrane and mass diffusion resistance in the electrode reaction can be suppressed, thereby achieving high performance in the fuel cell.

[0108] When the fuel cell is operating, in fuel cell unit 10 (Figure 3 In this process, hydrogen gas (6) is supplied to the anode side, and air or oxygen gas (7) is supplied to the cathode side. At the anode electrode, hydrogen gas is reduced to generate protons and electrons. The protons conducted in the electrolyte membrane and the electrons conducted in the external circuit react with oxygen at the cathode electrode to produce water. The consumption of hydrogen and oxygen at the anode and cathode electrodes is directly proportional to the current flowing through the external circuit. When the supply of hydrogen and oxygen near the electrodes is insufficient, it becomes a major cause of performance degradation due to resistance to mass diffusion.

[0109] As mentioned above, water in the membrane contributes to proton conduction in the solid polymer electrolyte, and its conductivity depends on the membrane's water content. When the water content in the membrane decreases due to a decrease in humidity in the feed gas or an increase in operating temperature, the proton conduction resistance increases, leading to a performance degradation. Therefore, to obtain a high-performance fuel cell, it is necessary to properly manage the hydrogen, oxygen, and water content in the feed gas.

[0110] In high-temperature regions above 100°C, the dehydration rate from the electrolyte membrane is high, requiring a high-humidity gas supply to maintain the water content in the membrane. Specifically, in this invention, by ensuring that the relative humidity of at least one of the air or oxygen supplied to the cathode side and the hydrogen supplied to the anode side of the fuel cell is 70% or more, preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more, the increase in proton conduction resistance can be suppressed.

[0111] On the other hand, in the high-temperature region above 100°C, the saturated water vapor pressure becomes very high compared to the temperature region below 100°C. Therefore, in order to maintain the humidity in the gas at the same level as in the temperature region below 100°C, the water vapor partial pressure becomes very high. Even under such conditions, in order to supply the hydrogen and oxygen required for the electrode reaction, the back pressure of the supply gas needs to be set to a predetermined value or higher. In this invention, specifically, by setting a back pressure of 330 kPa or higher, preferably 350 kPa or higher, more preferably 370 kPa or higher, and even more preferably 390 kPa or higher, the increase in mass diffusion resistance of the anode and cathode electrodes can be suppressed even in the high current density region.

[0112] In particular, the operating method of the present invention is effective when the gas supplied to the cathode side is air. When the gas supplied to the cathode side is air, the amount of oxygen near the electrode tends to decrease because the oxygen concentration in the supply gas drops to about 1 / 5. Even under such conditions, by increasing the gas supply pressure to 330 kPa or higher, a sufficient amount of oxygen can be supplied to the cathode electrode, resulting in a high-performance fuel cell.

[0113] Example

[0114] The present invention will be described in more detail below through embodiments, but the present invention is not limited thereto.

[0115] Synthesis of electrolyte membranes

[0116] [Synthesis example 1]

[0117] Synthesis of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (K-DHBP) represented by the following general formula (G1)

[0118]

[0119] 49.5 g of 4,4'-dihydroxybenzophenone, 134 g of ethylene glycol, 96.9 g of trimethyl orthoformate, and 0.50 g of p-toluenesulfonic acid monohydrate were dissolved in a 500 ml flask equipped with a stirrer, thermometer, and distillation tube. The resulting solution was then stirred at 78–82 °C for 2 hours. Subsequently, the internal temperature was gradually increased to 120 °C until the distillation of methyl formate, methanol, and trimethyl orthoformate completely stopped. After cooling the reaction solution to room temperature, it was diluted with ethyl acetate, and the organic layer was washed with 100 ml of 5% potassium carbonate aqueous solution. After separation, the solvent was removed by distillation. 80 ml of dichloromethane was added to the residue, crystals precipitated, filtered, and dried to obtain 52.0 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane. GC analysis of the crystal revealed 99.8% 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane and 0.2% 4,4'-dihydroxybenzophenone.

[0120] [Synthesis example 2]

[0121] The synthesis of sodium 3,3'-disulfonate-4,4'-difluorobenzophenone represented by the following general formula (G2)

[0122]

[0123] 109.1 g of 4,4'-difluorobenzophenone (Aldrich reagent) was reacted in 150 mL of fuming sulfuric acid (50% SO3) (Wako Purified Pharmaceutical Reagent) at 100 °C for 10 hours. Then, the reactants were added little by little to a large volume of water, neutralized with NaOH, and precipitated by adding 200 g of sodium chloride. The precipitate was filtered and recrystallized from the ethanol solution to obtain 3,3'-disulfonate sodium-4,4'-difluorobenzophenone as shown in the above general formula (G2). The purity was 99.3%. The structure was confirmed by 1H-NMR. Impurities were quantitatively analyzed by capillary electrophoresis (organic matter) and ion chromatography (inorganic matter).

[0124] [Synthesis example 3]

[0125] The following general formula (G5) represents the synthesis of polyetherketone-based polymer electrolyte membranes.

[0126]

[0127] Polymerization was carried out in N-methylpyrrolidone (NMP) at 210°C using 6.91 g of potassium carbonate, 7.30 g of sodium 3,3'-disulfonate-4,4'-difluorobenzophenone (G2) with ionic groups obtained in Synthesis Example 2 above, 10.3 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (G1) with hydrolyzable groups obtained in Synthesis Example 1 above, and 5.24 g of 4,4'-difluorobenzophenone.

[0128] A 25 wt% N-methylpyrrolidone (NMP) solution containing the obtained block copolymer was pressure filtered through a glass fiber filter, then cast onto a glass substrate. After drying at 100°C for 4 hours, it was heat-treated at 150°C for 10 minutes under nitrogen to obtain a polyketide membrane. The polymer exhibited excellent solubility. After immersion in a 10 wt% sulfuric acid aqueous solution at 95°C for 24 hours to undergo proton exchange and deprotection reactions, the membrane was further immersed in a large excess of pure water for 24 hours and thoroughly washed to obtain a polymeric electrolyte membrane. The softening temperature of the obtained polymeric electrolyte membrane was determined by dynamic viscoelasticity measurement, and the result was 160°C. The oxygen permeability coefficient of the obtained polymeric electrolyte membrane at 90°C and 80% RH was 4.5 × 10⁻⁶. -11 cm 3 ·cm / cm 2 •sec·cmHg, hydrogen permeability coefficient is 5.6×10 -10 cm 3 ·cm / cm 2 ·sec·cmHg.

[0129] [Fabrication of Membrane Electrode Complex]

[0130] [Example 1]

[0131] In the polyetherketone polymer electrolyte membrane (film thickness: 10 μm, size: 70 mm × 70 mm) prepared in Synthesis Example 3 above, transfer sheets with anolyte catalysts (size: 50 × 50 mm) and transfer sheets with cathode catalysts (size: 50 × 50 mm) were disposed on both sides. The membrane was then heated and pressed at 160 °C, 4.5 MPa, and 5 min to form a catalyst-coated electrolyte membrane (CCM). Platinum-based catalysts supported on a carbon support were used as both the anolyte and cathode catalysts.

[0132] like Figure 1As shown in the cross-sectional view, an anode gas diffusion layer 1a (size: 50mm × 50mm) and a cathode gas diffusion layer 1b (size: 50mm × 50mm) are disposed on both sides of the CCM fabricated above. The anode gas diffusion layer 1a and the cathode gas diffusion layer 1b are formed on a porous carbon sheet (Toray Industries, Inc. "TGP-H-060"), creating a microporous layer containing PTFE and carbon black. The membrane electrode composite is fabricated by hot pressing at 160°C for 5 minutes and 4.5 MPa.

[0133] [Example 2]

[0134] An anode catalyst layer 2a is formed on the surface of the anode gas diffusion layer 1a where a microporous layer is formed, thus fabricating an anode electrode for a gas diffusion electrode (GDE). Similarly, a cathode catalyst layer 2b is formed on the surface of the cathode gas diffusion layer 1b where a microporous layer is formed, thus fabricating a cathode electrode for a gas diffusion electrode (GDE). The anode gas diffusion layer 1a, cathode gas diffusion layer 1b, anode catalyst, and cathode catalyst are the same as those used in Example 1. Figure 2 As shown in the cross-sectional view, the above-mentioned anode electrode (size: 50mm × 50mm) and cathode electrode (size: 50mm × 50mm) are disposed on both sides of the polyetherketone polymer electrolyte membrane (membrane thickness: 10μm, size: 70mm × 70mm) prepared in Synthesis Example 3 above. The membrane electrode composite is prepared by hot pressing at 160°C for 5 minutes and 4.5MPa.

[0135] [High-Temperature Power Generation Evaluation (Power Generation Performance)]

[0136] The membrane electrode composite fabricated using the methods described in Examples 1 and 2 was placed in a JARI standard unit "Ex-1" (electrode area 25 cm²) manufactured by Eiwa Co., Ltd., as a module for power generation evaluation. Hydrogen was supplied as fuel gas to one anode electrode, and air was supplied as oxidizing gas to the other cathode electrode. Power generation evaluation was performed under the following conditions, with the current scanned from 0 A / cm² to 1.2 A / cm² until the voltage was below 0.2 V. In this invention, the voltage at a current density of 1 A / cm² was compared. Furthermore, a pressure of 0.7 GPa was applied when the membrane electrode composite was placed in the aforementioned unit.

[0137] Electronic load device: PLZ664WA electronic load device manufactured by Kikusui Electronics Industry Co., Ltd.

[0138] Unit temperature: 65℃, 120℃

[0139] Gas humidification conditions (hydrogen and air): 60% RH, 90% RH

[0140] Gas back pressure (hydrogen and air): 200 kPa, 330 kPa

[0141] Gas utilization rate: 70% for anode and 40% for cathode.

[0142] The measurement results are shown in Table 1 below.

[0143]

[0144] As shown in the table, the membrane electrode composites of Examples 1 and 2 exhibit voltage drops when the operating temperature increases from 65°C to 120°C, at a relative humidity of 60%, and at gas back pressures of 200 kPa and 330 kPa, respectively. On the other hand, at a relative humidity of 90%, no voltage drop occurs even when the operating temperature increases from 65°C to 120°C, and good power generation performance exceeding 65°C is achieved at a back pressure of 330 kPa.

[0145] [Evaluation of High-Temperature Power Generation (Humidity Dependence)]

[0146] The membrane electrode composite fabricated using the method described in Example 1 was placed in a JARI standard unit "Ex-1" (electrode area 25 cm²) manufactured by Eiwa Co., Ltd., as a module for power generation evaluation. Hydrogen was supplied as fuel gas to one anode electrode, and air was supplied as oxidizing gas to the other cathode electrode. Power generation evaluation was conducted under the following conditions: while maintaining a current density of 1 A / cm², the voltage was compared as the humidity changed from 30% RH to 95% RH. Furthermore, a pressure of 0.7 GPa was applied when the membrane electrode composite was placed in the aforementioned unit.

[0147] Electronic load device: PLZ664WA electronic load device manufactured by Kikusui Electronics Industry Co., Ltd.

[0148] Unit temperature: 120℃

[0149] Gas humidification conditions (hydrogen and air): 30%RH~95%RH

[0150] Gas back pressure (hydrogen and air): 330 kPa

[0151] Gas utilization rate: 70% for anode and 40% for cathode.

[0152] The measurement results are shown in Table 2 below.

[0153]

[0154] As shown in the table, the membrane electrode composite of Example 1, under operating conditions of 120°C and 330 kPa back pressure, showed a voltage increase of 1 A / cm² with increasing relative humidity. The voltage exhibited a high humidity dependence between 30% and 60% RH, while the dependence decreased at humidity levels above 70% RH. Therefore, by setting the humidity to above 70% RH, stable and high power generation performance could be achieved.

[0155] Explanation of symbols in attached drawings

[0156] 1a: Anode gas diffusion layer

[0157] 1b: Cathode gas diffusion layer

[0158] 2a: Anode catalyst layer

[0159] 2b: Cathode catalyst layer

[0160] 3: Electrolyte membrane

[0161] 4: Membrane electrode complex (MEA)

[0162] 5a: Anode separator

[0163] 5b: Cathode separator

[0164] 6: Hydrogen

[0165] 6c: Hydrogen supply pipe

[0166] 6d: Hydrogen exhaust pipe

[0167] 7: Air

[0168] 7c: Air supply pipe

[0169] 7d: Air exhaust pipe

[0170] 10: Fuel Cell Unit

[0171] 11: Fuel Cell Stack

[0172] 12a, 12b: Humidifier

[0173] 13a, 13b: Compressors

[0174] 14a, 14b: Back pressure valve

[0175] 15: Temperature sensor

[0176] 16: Humidity sensor

[0177] 17: Pressure sensor

[0178] 18: Hydrogen Tank

[0179] 19: Coolant circulation pump

[0180] 20: Fuel Cell System

Claims

1. A method for operating a fuel cell, the fuel cell comprising a membrane electrode assembly having an electrolyte membrane, a catalyst layer and a gas diffusion layer, the method being characterized in that it includes a step of setting the operating temperature of the fuel cell to 100°C or higher, wherein the relative humidity of the supply gas supplied to the fuel cell is 70% or higher, and the back pressure of the supply gas is 330 kPa or higher.

2. The method of operating a fuel cell according to claim 1, wherein the supply gas is air or oxygen supplied to the cathode side of the fuel cell, and / or hydrogen supplied to the anode side of the fuel cell.

3. The method of operating a fuel cell according to claim 2, wherein the supply gas is air supplied to the cathode side of the fuel cell.

4. The method of operating a fuel cell according to any one of claims 1 to 3, wherein the electrolyte membrane contains a solid polymeric electrolyte.

5. The method for operating a fuel cell according to claim 4, wherein the solid polymeric electrolyte contains a proton-conducting polymer.

6. The method for operating a fuel cell according to claim 5, wherein the proton-conducting polymer is a hydrocarbon polymer.

7. The method of operating a fuel cell according to any one of claims 1 to 3, wherein the softening temperature of the electrolyte membrane is 120°C or higher.

8. The method of operating a fuel cell according to any one of claims 1 to 3, wherein the oxygen permeability coefficient of the electrolyte membrane at 90°C and 80% RH is 1.0 × 10⁻⁶. -9 cm 3 ·cm / cm 2 Below ·sec·cmHg.

9. The method of operating a fuel cell according to any one of claims 1 to 3, wherein the hydrogen permeability coefficient of the electrolyte membrane at 90°C and 80% RH is 5.0 × 10⁻⁶. -9 cm 3 ·cm / cm 2 Below ·sec·cmHg.

10. The method of operating a fuel cell according to any one of claims 1 to 3, wherein the catalyst layer contains an oxide support.

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