JP2008204664A - Membrane-electrode assembly for fuel cell, and fuel cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means providing high power generation performance even when structure or physical properties of materials constituting each element or operation conditions are varied in a PEFC. <P>SOLUTION: An MEA has a solid polymer electrolyte membrane and a pair of catalyst layers comprising a cathode catalyst layer and an anode catalyst layer between which the solid polymer electrolyte membrane is placed. In at least one of the cathode catalyst layer and the anode catalyst layer, the surface resistance of the catalyst layer is varied along at least one direction of the stacking direction and the surface direction of the catalyst layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a membrane electrode assembly for a fuel cell and a fuel cell using the same.

  A polymer electrolyte fuel cell (PEFC) generally has a structure in which a membrane electrode assembly (MEA) is sandwiched between a pair of gas diffusion layers and a pair of separators. The MEA has a configuration in which a solid polymer electrolyte membrane is sandwiched between a pair of catalyst layers (an anode side catalyst layer and a cathode side catalyst layer). The catalyst layer includes an electrode catalyst and a proton conductive solid polymer electrolyte, and has a porous structure in order to diffuse reaction gas supplied from the outside. The electrode catalyst generally has a structure in which a catalyst component is supported on a conductive carrier.

  In the PEFC MEA, the following electrochemical reaction proceeds. First, hydrogen contained in the fuel gas supplied to the anode side is oxidized by the catalyst component in the anode side catalyst layer to become protons and electrons. The generated protons then pass through the solid polymer electrolyte contained in the anode side catalyst layer, and further through the solid polymer electrolyte membrane in contact with the anode side catalyst layer, and reach the cathode side catalyst layer. On the other hand, the electrons generated in the anode side catalyst layer are the conductive carrier constituting the anode side catalyst layer, and further the gas diffusion layer in contact with the side of the anode side catalyst layer facing the solid polymer electrolyte membrane, The cathode side catalyst layer is reached through the gas separator and the external circuit. The protons and electrons that have reached the cathode side catalyst layer react with oxygen contained in the oxidant gas supplied to the cathode side catalyst layer to generate water. In PEFC, electricity can be taken out through the above-described electrochemical reaction.

Conventionally, various attempts have been made to improve the output performance of fuel cells. For example, Patent Document 1 discloses ion exchange of an ionomer (fluorocarbon sulfonic acid type ion exchange resin) contained in a fuel electrode (anode) for the purpose of providing a PEFC having a high output current density and little deterioration of battery characteristics over time. A technique for making the capacity larger than that of the air electrode (cathode) is disclosed. According to such a technique, in the anode, hydrogen permeability is improved, and hydrogen supply from the gas phase to the catalyst component can be ensured. On the other hand, in the cathode, the swelling of the ionomer covering the catalyst is suppressed, and a gas diffusion path can be secured.
JP-A-9-213350

  In general, during the operation of PEFC, a temperature gradient, a humidity gradient, a gas concentration gradient, and the like are generated along the flow direction of fuel gas, oxidant gas, cooling water, and the like. As a result, a power generation distribution is generated due to these gradients, and there may be a region where power generation is concentrated locally.

  Moreover, the structure of the material which comprises PEFC is various. For example, PEFC is composed of an electrolyte membrane made of ionomer, an electrode catalyst and a catalyst layer made of ionomer, a gas diffusion layer made of porous conductive carbon fiber, etc., and the structure and physical properties of each material constituting each layer (proton of ionomer) The number of combinations of conductivity, water transfer characteristics, gas diffusion layer thermal conductivity, gas diffusion characteristics, etc. is extremely large. Furthermore, during the operation of PEFC, the operating conditions (temperature conditions and humidity conditions) generally vary. Therefore, even if the structure, physical properties, or operating conditions of the materials that make up each element of the PEFC vary, the power generation performance does not become unstable, and the PEFC exhibits stable and excellent power generation performance. It has been demanded.

  However, in the conventionally known PEFC as described in Patent Document 1, for example, if the structure, physical properties, operating conditions, etc. of the material constituting the PEFC fluctuate, the generation of power generation is still unavoidable. There was a problem that the power generation performance was also lowered.

  Therefore, an object of the present invention is to provide means capable of exhibiting excellent power generation performance even in the case where the structure, physical properties, or operating conditions of materials constituting each element in PEFC are changed.

  The present inventors have intensively studied to solve the above problems. In the process, an attempt was made to control the sheet resistance of the PEFC catalyst layer. As a result, it has been found that the above problems can be solved by changing the surface resistance of the catalyst layer along the stacking direction and / or the surface direction of the catalyst layer, and the present invention has been completed.

  That is, a first aspect of the present invention is an MEA for a fuel cell having a solid polymer electrolyte membrane and a pair of catalyst layers comprising a cathode catalyst layer and an anode catalyst layer sandwiching the solid polymer electrolyte membrane. In addition, in at least one of the cathode catalyst layer and the anode catalyst layer, the surface resistance of the catalyst layer changes along at least one of the stacking direction and the surface direction of the catalyst layer. It is MEA for batteries.

  The inventors have also found a production method capable of producing the first MEA of the present invention.

  That is, the second of the present invention is the first MEA production method of the present invention, wherein a catalyst ink containing an electrode catalyst, a solid polymer electrolyte, and a solvent for forming a catalyst layer is prepared, Forming a coating film from the catalyst ink and drying the coating film to form the catalyst layer, the type or composition of the catalyst ink, the coating film forming conditions, or the coating film drying conditions Is changed along the stacking direction or the surface direction of the catalyst layer to change the sheet resistance in the catalyst layer.

  According to the MEA for a fuel cell of the present invention, it is possible to exhibit excellent power generation performance even in the case where the structure, physical properties, or operating conditions of the material constituting each element changes in PEFC.

  A first aspect of the present invention is a fuel cell MEA having a solid polymer electrolyte membrane and a pair of catalyst layers comprising a cathode catalyst layer and an anode catalyst layer sandwiching the solid polymer electrolyte membrane, In at least one of the cathode catalyst layer and the anode catalyst layer, the surface resistance of the catalyst layer varies along at least one of the stacking direction and the surface direction of the catalyst layer. MEA.

  Hereinafter, several preferred embodiments of the first MEA of the present invention will be described in detail with reference to the drawings. Each drawing is exaggerated for convenience of explanation, and the dimensional ratio of each component in each drawing may be different from the actual one.

(First embodiment)
First, the first embodiment will be described.

  FIG. 1 is a schematic cross-sectional view of a polymer electrolyte fuel cell (PEFC) of a first embodiment. FIG. 1 shows a single cell of PEFC.

  The PEFC 100 shown in FIG. 1 has an MEA 130 in which a solid polymer electrolyte membrane 110 is sandwiched between a pair of catalyst layers including a cathode catalyst layer 120c and an anode catalyst layer 120a. In the PEFC 100, the MEA 130 is sandwiched between a pair of gas diffusion layers including a cathode side gas diffusion layer 140c and an anode side gas diffusion layer 140a. Further, the MEA 130 and the gas diffusion layer (140c, 140a) are sandwiched by a pair of separators including a cathode side separator 150c and an anode side separator 150a. Here, the cathode side gas diffusion layer 140c side surface of the cathode side separator 150c is provided with an oxidant gas flow path 152c through which the oxidant gas flows during operation, and the opposite surface has a coolant during operation. A cooling flow path (not shown) through which is circulated is provided. On the other hand, the anode side gas diffusion layer 140a side surface of the anode side separator 150a is provided with a fuel gas flow path 152a through which fuel gas flows during operation, and the coolant flows through the opposite surface during operation. A cooling channel (not shown) is provided. Around the PEFC 100, a gasket 160 is disposed so as to surround the pair of catalyst layers (120c, 120a) and the pair of gas diffusion layers (140c, 140a). In the present application, the “membrane electrode assembly” means an assembly having a solid polymer electrolyte membrane 110 and a pair of catalyst layers (120c, 120a) sandwiching the solid polymer electrolyte membrane 110. .

The MEA 130 included in the PEFC 100 according to the present embodiment is characterized in that the sheet resistance in the cathode catalyst layer 120c changes stepwise along the surface direction of the catalyst layer 120c. FIG. 2 is a schematic plan view of the cathode catalyst layer in the polymer electrolyte fuel cell (PEFC) of the first embodiment as viewed from the X direction shown in FIG. Specifically, as shown in FIG. 2, the sheet resistance in the cathode catalyst layer 120c is relatively small in the upstream half in the oxidant gas flow direction (for example, 0.15 Ωcm ² ), and is downstream in the oxidant gas flow direction. The side half is relatively large (eg, 0.30 Ωcm ² ). Accordingly, roughly speaking, current flows more easily in the upstream half of the cathode catalyst layer 120c than in the downstream half. The sheet resistance of the anode catalyst layer 120a is smaller than the sheet resistance of any one half of the cathode catalyst layer 120c (for example, 0.05 Ωcm ² ). Accordingly, roughly speaking, the anode catalyst layer 120a is more likely to cause a current to flow than the cathode catalyst layer 120c. Here, “surface resistance” is represented by the reciprocal of conductivity per unit area of a thin film substance, and is used as an index representing the difficulty of current flow in a direction perpendicular to the surface direction of the substance. That is, the smaller the surface resistance of a substance, the better the conductivity in the direction perpendicular to the surface of the substance. The unit of sheet resistance is [Ωcm ² ].

  In the MEA 130 of the present embodiment and the PEFC 100 having the same, the cathode catalyst layer 120c has a surface resistance that gradually increases from upstream to downstream in the oxidant gas flow direction (a current hardly flows from upstream to downstream). Is configured. For this reason, evaporation of liquid water is promoted due to relatively large Joule heat generation on the downstream side. On the other hand, the evaporation of liquid water is not promoted so much on the upstream side due to the relatively small Joule heat generation. As a result, on the upstream side, the dry-out of the electrolyte membrane 110, which is likely to occur due to large heat generation, is suppressed, and as a result, it can contribute effectively to the improvement of the power generation performance of the PEFC. Further, on the downstream side, the flooding phenomenon that is likely to occur due to the relatively large amount of water generated as a result of the power generation reaction is suppressed, and the power generation performance of PEFC is also improved. Note that the above-described mechanism is merely based on speculation, and the technical scope of the present invention is not affected at all even if the operational effects of the present invention are obtained by different mechanisms. This is also valid for each of the other preferred forms of mechanisms described below.

  As described above, the MEA and PEFC of the present invention are characterized by the structure of the cathode catalyst layer. Therefore, the MEA and PEFC configurations other than the characteristic configuration may be appropriately changed or modified based on conventionally known knowledge.

  The MEA and PEFC components of the present invention will be described below with reference to the drawings. The technical scope of the present invention should be determined based on the description of the claims.

[Solid polymer electrolyte membrane]
The solid polymer electrolyte membrane 110 is composed of a proton-conducting solid polymer electrolyte (hereinafter also simply referred to as “ionomer”), and protons generated in the anode catalyst layer 120a during the operation of the PEFC 100 are cathoded along the film thickness direction. It has a function of selectively permeating the catalyst layer 120c. The solid polymer electrolyte membrane 110 also has a function as a partition for preventing the fuel gas supplied to the anode side and the oxidant gas supplied to the cathode side from being mixed.

  The specific configuration of the solid polymer electrolyte membrane 110 is not particularly limited, and a conventionally known ionomer membrane can be appropriately employed in the technical field of fuel cells. The solid polymer electrolyte membrane 110 is roughly classified into a fluorine-based solid polymer electrolyte membrane and a hydrocarbon-based solid polymer electrolyte membrane according to the type of ionomer that is a constituent material.

  Examples of the fluorine-based solid polymer electrolyte include perfluorocarbon sulfonic acids such as Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like. Polymer, perfluorocarbon phosphonic acid polymer, trifluorostyrene sulfonic acid polymer, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride-perfluorocarbon sulfonic acid polymer Etc. From the viewpoint of power generation performance such as heat resistance and chemical stability, these fluorine-based solid polymer electrolyte membranes are preferably used, and particularly preferably a fluorine-based solid polymer electrolyte membrane composed of a perfluorocarbon sulfonic acid polymer. Is used.

  Examples of the hydrocarbon solid polymer electrolyte include sulfonated polyethersulfone (S-PES), sulfonated polyaryletherketone, sulfonated polybenzimidazole alkyl, phosphonated polybenzimidazole alkyl, sulfonated polystyrene, and sulfonated. Examples include polyetheretherketone (S-PEEK) and sulfonated polyphenylene (S-PPP). These hydrocarbon-based solid polymer electrolyte membranes are preferably used from the viewpoint of production such that the raw materials are inexpensive, the production process is simple, and the material selectivity is high. In addition, as for the ion exchange resin mentioned above, only 1 type may be used independently and 2 or more types may be used together.

  Materials other than the ionomer constituting the solid polymer electrolyte membrane described above may be used as the ionomer. As such materials, for example, liquids, solids, and gel materials having high proton conductivity can be used, and solid acids such as phosphoric acid, sulfuric acid, antimonic acid, stannic acid, and heteropoly acid, phosphoric acid, and the like. Hydrocarbon polymer compound doped with a non-organic acid, organic / inorganic hybrid polymer partially substituted with proton conductive functional group, gel impregnated with phosphoric acid solution or sulfuric acid solution in polymer matrix An example is a proton conductive material. A mixed conductor having both proton conductivity and electron conductivity can also be used as an ionomer.

  The thickness of the solid polymer electrolyte membrane 110 is appropriately determined in consideration of the characteristics of the MEA 130 and the PEFC 100, and is not particularly limited. However, the thickness of the solid polymer electrolyte membrane 110 is preferably 5 to 300 μm, more preferably 10 to 200 μm, still more preferably 15 to 150 μm, and particularly preferably 25 to 50 μm. When the thickness is within such a range, the balance between strength during film formation, durability during use, and output characteristics during use can be appropriately controlled.

[Cathode catalyst layer]
The cathode catalyst layer 120c is a layer containing an electrode catalyst on which a catalyst component is supported and an ionomer.

In the cathode catalyst layer 120c, the electrode catalyst is composed mainly of protons (H ⁺ ) via the solid polymer electrolyte membrane 110, electrons (e ⁻ ) via the external circuit, and oxygen (O 2) derived from the oxidant gas. It has a function of promoting a reaction for generating (that is, a reduction reaction of oxygen).

  Simply speaking, the electrode catalyst has a structure in which a catalyst component such as platinum is supported on the surface of a conductive carrier made of carbon or the like.

  The electroconductive carrier constituting the electrode catalyst may be any one having a specific surface area sufficient to support the catalyst component in a desired dispersed state and sufficient electron conductivity. In the composition of the conductive carrier, the main component is preferably carbon. Specific examples of the material for the conductive carrier include carbon black, activated carbon, coke, natural graphite, and artificial graphite. “The main component is carbon” means that the main component contains carbon atoms, and is a concept that includes both carbon atoms and substantially carbon atoms. In some cases, elements other than carbon atoms may be included in order to improve the characteristics of the fuel cell. Note that “substantially consisting of carbon atoms” means that contamination of about 2 to 3 mass% or less of impurities can be allowed.

The BET specific surface area of the conductive carrier is not particularly limited as long as it is a specific surface area sufficient to carry the catalyst component in a highly dispersed manner, but is preferably 100 to 1500 m ² / g, more preferably 600 to 1000 m. ² / g. When the specific surface area of the conductive support is in such a range, the balance between the dispersibility of the catalyst component on the conductive support and the effective utilization rate of the catalyst component can be appropriately controlled.

  Although there is no restriction | limiting in particular also about the average particle diameter of an electroconductive support | carrier, Usually, it is 5-200 nm, Preferably it is about 10-100 nm. In addition, as a value of “average particle diameter of the conductive carrier”, a value calculated by a primary particle diameter measurement method using a transmission electron microscope (TEM) is adopted.

  The catalyst component supported on the conductive carrier is not particularly limited as long as it has a function of catalytically promoting the oxygen reduction reaction described above, and a conventionally known catalyst component can be appropriately used. Specific examples of the catalyst component include platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and alloys thereof. Etc. Among these, the catalyst component preferably contains at least platinum from the viewpoint of excellent catalytic activity, poisoning resistance to carbon monoxide and the like, and heat resistance. The composition of the alloy in the case of using an alloy as the catalyst component in the cathode catalyst layer 120c varies depending on the type of metal to be alloyed and the like, and can be selected as appropriate by those skilled in the art, but is preferably about 30 to 90 atomic% platinum. The other metal to be converted is about 10 to 70 atomic%. Note that an “alloy” is a general term for a metal element having one or more metal elements or non-metal elements added and having metallic properties. The alloy structure consists of a eutectic alloy, which is a mixture of the component elements as separate crystals, a component element completely melted into a solid solution, and a component element composed of an intermetallic compound or a compound of a metal and a nonmetal. There is what is formed, and any may be used in the present application. Here, the alloy composition can be specified by using an ICP emission analysis method.

  The shape and size of the catalyst component are not particularly limited, and the same shape and size as those of conventionally known catalyst components can be adopted as appropriate, but the shape of the catalyst component is preferably granular. And the average particle diameter of a catalyst particle becomes like this. Preferably it is 1-30 nm, More preferably, it is 1.5-20 nm. When the average particle diameter of the catalyst particles is within such a range, the balance between the catalyst utilization rate related to the effective electrode area where the electrochemical reaction proceeds and the ease of loading can be appropriately controlled. In the present invention, the value of the “average particle diameter of the catalyst particles” is the particle diameter of the catalyst component determined from the crystallite diameter determined from the half-value width of the diffraction peak of the catalyst component in X-ray diffraction or the transmission electron microscope image. It can be calculated as an average value of diameters.

  The ratio of the content of the conductive support and the catalyst component in the electrode catalyst is not particularly limited. However, the content (supported amount) of the catalyst component is preferably 110 to 300% by mass, more preferably 130 to 250% by mass, and further preferably 150 to 200% based on 100% by mass of the conductive carrier. % By mass. When the content of the catalyst component is 100% by mass or more, the catalyst-producing ability of the electrode catalyst is sufficiently exerted, which can contribute to the improvement of the power generation performance of PEFC100. On the other hand, the catalyst component content of 300% by mass or less is preferable because aggregation of the catalyst components on the surface of the conductive support is suppressed and the catalyst components are highly dispersed and supported. In addition, as a value of the ratio of content mentioned above, the value measured by ICP emission spectrometry shall be employ | adopted.

  The cathode catalyst layer 120c further includes an ionomer in addition to the electrode catalyst described above. The specific form of the ionomer contained in the cathode catalyst layer 120c is not particularly limited, and conventionally known knowledge can be appropriately referred to in the technical field of fuel cells. For example, as the ionomer included in the cathode catalyst layer 120c, the ionomer that constitutes the above-described solid polymer electrolyte membrane 110 can be similarly used. Therefore, details of the specific form of the ionomer are omitted here. In addition, the ionomer contained in the cathode catalyst layer 120c may be one kind alone, or two or more kinds.

  The ion exchange capacity of the ionomer contained in the cathode catalyst layer 120c is preferably 0.9 to 1.5 mmol / g, and preferably 1.1 to 1.5 mmol / g, from the viewpoint of excellent ion conductivity. Is more preferable.

  The “ion exchange capacity” of an ionomer means the number of moles of sulfonic acid groups per unit dry mass of the ionomer. The value of “ion exchange capacity” can be calculated by removing the dispersion medium of the ionomer dispersion by heat drying or the like to obtain a solid polymer electrolyte, and performing neutralization titration.

  There is no particular limitation on the ionomer content in the cathode catalyst layer 120c. However, the ratio of the ionomer content to the conductive carrier content in the cathode catalyst layer 120c (ionomer / conductive carrier mass ratio) is preferably 0.5 to 2.0, more preferably 0.7. It is -1.5, More preferably, it is 0.9-1.3. The mass ratio of ionomer / conductive carrier is preferably 0.9 or more from the viewpoint of suppressing the internal resistance of the membrane / electrode assembly. On the other hand, an ionomer / conductive carrier mass ratio of 1.3 or less is preferable from the viewpoint of suppressing flooding.

  As described above, the MEA 130 included in the PEFC 100 of the present embodiment is characterized in that the sheet resistance in the cathode catalyst layer 120c changes stepwise along the surface direction of the catalyst layer 120c.

In the present embodiment, the sheet resistance in the cathode catalyst layer 120c is relatively small in the upstream half in the oxidant gas flow direction and relatively large in the downstream half in the oxidant gas flow direction (see FIG. 2). Here, there is no restriction | limiting in particular about the specific value of the surface resistance in the cathode catalyst layer 120c, It can adjust suitably with reference to a conventionally well-known knowledge. As an example, the sheet resistance in the cathode catalyst layer 120c is preferably 0.01～1.00Omucm ² mm, more preferably 0.02～0.50Omucm ^2, more preferably 0.05 to 0 .30 Ωcm ² . If the surface resistance in the cathode catalyst layer 120c is 0.01 Ωcm ² or more, it is preferable in that Joule heat is supplied to the liquid water to change the state to a gas. On the other hand, if the surface resistance in the cathode catalyst layer 120c is 1.00 Ωcm ² or less, the resistance overvoltage in the cathode catalyst layer 120c is prevented from becoming too large, and the obtained PEFC can exhibit sufficient power generation performance. It becomes. In the present application, the value of the “surface resistance” in the cathode catalyst layer 120c is the electrochemical impedance described in a conventionally known document (Electrochemical and Solid-State Letters, 2 (6) 259-261 (1999), Experimental). The value calculated according to the law shall be adopted. Specifically, an anode catalyst layer is disposed on one surface of the solid polymer electrolyte membrane, and a cathode catalyst layer for measuring surface resistance is disposed on the other surface to form an MEA. Thereafter, the MEA is sandwiched between a pair of gas diffusion layers to form a single cell. The sheet resistance was measured by setting the cell temperature to 25 ° C., supplying saturated humidified hydrogen (temperature: 25 ° C.) to the anode side, and supplying saturated humidified nitrogen gas (temperature: 25 ° C.) to the cathode side. This is performed by controlling the potential and current with a potentio galvanostat connected to the response analyzer. At this time, the cathode potential is fixed in the range of 0.4 V to 0.6 V with respect to the anode potential (for example, 0.45 V), the frequency is automatically scanned, and the impedance at each frequency is calculated from the obtained response. The value of the surface resistance of the cathode catalyst layer can be obtained by analyzing by a known method.

  In this embodiment, the sheet resistance in the cathode catalyst layer 120c is relatively small on the upstream side in the oxidant gas flow direction and relatively large on the downstream side, as shown in FIG. 2, but the technical scope of the present invention is such It is not limited only to the form, and the form opposite to that shown in FIG. 2, that is, the upstream side resistance in the oxidant gas flow direction may be relatively large and the downstream side resistance may be relatively small.

Further, in the present embodiment, the surface resistance of the cathode catalyst layer 120c changes (increases) in a “stepwise” manner from the upstream to the downstream in the oxidant gas flow direction. However, the present invention is not limited to the form that changes “stepwise” in this way. For example, a form that changes (increases or decreases) “inclined” can also be adopted. Here, “the surface resistance of the cathode catalyst layer 120c changes in an inclined manner” means that the value of the surface resistance gradually increases or decreases continuously along the direction of change. According to the form in which the surface resistance changes “inclined”, an excellent effect of uniform current distribution can be obtained. When the sheet resistance changes stepwise, the difference in sheet resistance between adjacent catalyst layers in the surface direction is preferably about 0.01 to 0.3 Ωcm ² . According to such a form, it is preferable from a viewpoint that the electric current in a catalyst layer surface is suppressed.

  Furthermore, in the present embodiment, the surface resistance of the cathode catalyst layer 120c changes “only once” stepwise from upstream to downstream in the oxidant gas flow direction. However, it is not limited only to such a form, You may change in step shape over multiple times. Note that a step-like change in surface resistance and a slope-like change may be combined.

[Anode catalyst layer]
The anode catalyst layer 120a is a layer containing an electrode catalyst on which a catalyst component is supported and an ionomer.

In the anode catalyst layer 120a, the electrode catalyst has a function of promoting a reaction in which hydrogen (H2) derived from fuel gas is converted to protons (H ⁺ ) and electrons (e ⁻ ) (that is, an oxidation reaction of hydrogen).

  As for specific forms of the electrode catalyst and ionomer constituting the anode catalyst layer 120a, the forms described above in the column of the cathode catalyst layer 120c can be similarly adopted. Therefore, detailed description is omitted here. The cathode catalyst layer 120c and the anode catalyst layer 120a may have the same or different specific forms of electrode catalyst and ionomer constituting the catalyst layer.

  In the first fuel cell MEA of the present invention, the average thickness (Ya) of the anode catalyst layer 120a is preferably smaller than the average thickness (Yc) of the cathode catalyst layer 120c. When Ya is smaller than Yc, proton conduction to the cathode catalyst layer 120c is improved. Specifically, Ya / Yc is preferably 0.1 to 0.9, and more preferably 0.2 to 0.5. In addition, the average thickness of the anode catalyst layer 120a is preferably 1.0 to 20.0 μm, and more preferably 2.0 to 5.0 μm. In addition, as a value of the thickness of the catalyst layer, a value obtained by observing a cross section of the catalyst layer with a scanning electron microscope (SEM) is adopted.

  In the first fuel cell MEA of the present invention, the anode catalyst layer 120a preferably has a single layer structure. This is because the anode catalyst layer 120a has a smaller difference in reactivity between the electrolyte membrane side and the gas diffusion layer side than the cathode catalyst layer 120c. This is because an increase in the layer average thickness due to the use of a plurality of layers can be avoided. However, embodiments in which the anode catalyst layer 120a has a laminated structure including two or more catalyst layers are also included in the technical scope of the present invention.

  In the fuel cell MEA 130 and PEFC 100 of the present embodiment, the sheet resistance of the anode catalyst layer 120a is smaller than the sheet resistance of the cathode catalyst layer 120c. According to such a configuration, it is possible to reduce the resistance overvoltage in the anode catalyst layer 120a while arbitrarily maintaining the resistance overvoltage (heat generation) of the cathode catalyst layer 120c during operation of the PEFC 100. As a result, dry-out due to drying of the electrolyte membrane is suppressed, and output can be stably obtained from the PEFC 100.

  As described above, the MEA of the present invention is characterized by the catalyst layer. Therefore, as for other members constituting the MEA, a conventionally known configuration in the field of the fuel cell can be employed as it is or after being appropriately improved. Hereinafter, typical forms of members other than the catalyst layer will be described for reference, but the technical scope of the present invention is not limited to the following forms.

[Gas diffusion layer]
The pair of gas diffusion layers (140c, 140a) are arranged so as to sandwich the MEA 130 composed of the above-described solid polymer electrolyte membrane 110 and the catalyst layers (120a, 120c). The gas diffusion layers (140c, 140a) diffuse gas (anode side: fuel gas, cathode side: oxidant gas) supplied through a gas flow path of the separator described later to the catalyst layers (120c, 120a). And a function as an electron conduction path.

  The material which comprises the base material of a gas diffusion layer is not specifically limited, A conventionally well-known knowledge can be referred suitably. For example, a sheet-like material having conductivity and porosity such as a carbon woven fabric, a paper-like paper body, a felt, and a non-woven fabric can be used. The thickness of the substrate may be appropriately determined in consideration of the characteristics of the obtained gas diffusion layer, but may be about 30 to 500 μm. If the thickness of the substrate is within such a range, the balance between mechanical strength and diffusibility such as gas and water can be appropriately controlled.

  The gas diffusion layers (140c, 140a) are preferably subjected to a hydrophilic treatment. Since the gas diffusion layer has been subjected to the hydrophilic treatment, discharge of excess moisture present in (or into) the catalyst layers (120c, 120a) is promoted, and the occurrence of the flooding phenomenon can be effectively suppressed. Here, as a specific form of the hydrophilic treatment applied to the gas diffusion layer, for example, a treatment such as a coating of titanium oxide on the carbon substrate surface or a treatment of modifying the carbon substrate surface with an acidic functional group Is mentioned. However, it is not limited only to these forms, and other hydrophilic treatments may be employed depending on circumstances.

  Further, in order to promote the discharge of excess moisture present in the catalyst layer and suppress the occurrence of flooding phenomenon, the gas diffusion layer has a carbon particle layer containing carbon particles on the catalyst layer side of the substrate. May be.

  The carbon particles contained in the carbon particle layer are not particularly limited, and conventionally known materials such as carbon black, graphite, and expanded graphite can be appropriately employed. Among them, carbon black such as oil furnace black, channel black, lamp black, thermal black, acetylene black and the like can be preferably used because of excellent electron conductivity and a large specific surface area. The average particle diameter of the carbon particles is preferably about 10 to 100 nm. Thereby, while being able to obtain the high drainage property by capillary force, it becomes possible to improve contact property with a catalyst layer.

  The carbon particle layer may contain a water repellent. Examples of the water repellent include fluorine-based polymer materials such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Examples include polypropylene and polyethylene. Among these, fluorine-based polymer materials can be preferably used because of excellent water repellency, corrosion resistance during electrode reaction, and the like.

[Separator]
The separators (150c, 150a) have a function of electrically connecting the cells in series when a plurality of single cells of a fuel cell such as the PEFC 100 are connected in series to form a fuel cell stack. The separator also functions as a partition that separates the fuel gas, the oxidant gas, and the coolant from each other. Therefore, each separator is provided with a gas channel and a cooling channel. Specifically, as described above, the cathode side gas diffusion layer 140c side surface of the cathode side separator 150c is provided with the oxidant gas flow path 152c through which the oxidant gas flows during operation. Is provided with a cooling channel (not shown) through which a coolant flows during operation. On the other hand, the anode side gas diffusion layer 140a side surface of the anode side separator 150a is provided with a fuel gas flow path 152a through which fuel gas flows during operation, and the coolant flows through the opposite surface during operation. A cooling channel (not shown) is provided.

  As a material constituting the separator, conventionally known materials such as dense carbon graphite, carbon such as a carbon plate, and metal such as stainless steel can be appropriately used without limitation.

  The thickness and size of the separator and the shape and size of each channel provided are not particularly limited, and can be appropriately determined in consideration of the desired output characteristics of the obtained PEFC.

[gasket]
The gasket 160 is arranged around the PEFC 100 so as to surround the pair of catalyst layers (120c, 120a) and the pair of gas diffusion layers (140c, 140a), and the gas supplied to the catalyst layers leaks to the outside. It has a function to prevent this.

  The material constituting the gasket 160 is not particularly limited, but rubber materials such as fluorine rubber, silicon rubber, ethylene propylene rubber (EPDM), polyisobutylene rubber, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) Fluorine polymer materials such as polyhexafluoropropylene and tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and thermoplastic resins such as polyolefin and polyester. Moreover, there is no restriction | limiting in particular also in the thickness of a gasket, Preferably it is 50 micrometers-2 mm, More preferably, what is necessary is just to be about 100 micrometers-1 mm.

(Second Embodiment)
Next, the second embodiment will be described.

  FIG. 3 is a schematic cross-sectional view of the PEFC of the second embodiment. FIG. 3 also shows a single PEFC cell, and the same components as those in the first embodiment described above are denoted by the same reference numerals.

  The PEFC 100 of the present embodiment shown in FIG. 3 differs from the first embodiment described with reference to FIGS. 1 and 2 only in the arrangement form of the cathode catalyst layer 120c, and other specific examples. The configuration is the same as in the first embodiment. Therefore, the description of components and the like that can be implemented by referring to the first embodiment will be omitted as appropriate.

The MEA 130 included in the PEFC 100 of the present embodiment is characterized in that the sheet resistance in the cathode catalyst layer 120c changes stepwise along the stacking direction of the catalyst layer 120c. Specifically, the cathode catalyst layer 120c is composed of _two catalyst layers (120c ₁ and 120c ₂ ) stacked in the film thickness direction, and is a layer 120c ₁ located on the side in contact with the solid polymer electrolyte membrane 110. surface resistance is relatively small (0.03Ωcm ^2), the surface resistance of the layer 120c ₂ located on the side in contact with the cathode-side gas diffusion layer 140c is relatively large (0.15Ωcm ^2). Accordingly, roughly speaking, current flows more easily on the electrolyte membrane side of the cathode catalyst layer 120c than on the gas diffusion layer side.

    In the MEA 130 of this embodiment and the PEFC 100 having the same, the two catalyst layers having different sheet resistances have a small sheet resistance disposed on the electrolyte membrane side, and the sheet resistance disposed on the gas diffusion layer side. The cathode catalyst layer 120c is configured by laminating in the film thickness direction so as to increase. In this way, by providing a distribution so that the surface resistance increases along the heat transfer direction (catalyst layer → gas diffusion layer → separator → coolant) in the PEFC 100, the catalyst layer on the gas diffusion layer side during the power generation operation of the PEFC The temperature can be made relatively high with respect to the catalyst layer temperature on the electrolyte membrane side. As a result, an excellent effect of suppressing flooding can be exhibited.

  In the present embodiment, the sheet resistance of the cathode catalyst layer 120c is relatively small on the electrolyte membrane side and relatively large on the gas diffusion layer side as shown in FIG. 3, but the technical scope of the present invention is limited to such a form. There is no limitation, and the configuration opposite to that in FIG. 3, that is, the electrolyte membrane side may be relatively large and the gas diffusion layer side may be relatively small.

  Also in the present embodiment, as in the first embodiment, the surface resistance of the cathode catalyst layer 120c changes (increases) “stepwise” from the electrolyte membrane side toward the gas diffusion layer side. However, the present invention is not limited to the form that changes “stepwise” in this way. For example, a form that changes (increases or decreases) “inclined” can also be adopted.

  Further, in the present embodiment, the sheet resistance of the cathode catalyst layer 120c changes “only once” stepwise from the electrolyte membrane side toward the gas diffusion layer side. However, it is not limited only to such a form, You may change in step shape over multiple times. Note that a step-like change in surface resistance and a slope-like change may be combined.

  In the present embodiment, as a more preferable form, the relationship between various physical properties of the cathode side gas diffusion layer 140c and the surface resistance of the layer in contact with the cathode side gas diffusion layer 140c among the layers constituting the cathode catalyst layer 120c. Is defined. In the following description, the “layer in contact with the cathode side gas diffusion layer” is not limited to the layer in direct contact, but other layers are interposed between the cathode catalyst layer 120c and the cathode side gas diffusion layer 140c. Even in this case, the layer closest to the cathode-side gas diffusion layer 140c is meant.

For example, in this embodiment, when the thermal conductivity of the cathode side gas diffusion layer 140c is 0.5 [W / mK] or more, the cathode side gas diffusion layer among the layers constituting the cathode catalyst layer 120c. The surface resistance of the layer in contact with 140c is preferably 0.10 Ωcm ² or more, more preferably 0.12 Ωcm ² or more, and further preferably 0.15 Ωcm ² or more. At this time, the upper limit value of the surface resistance is not limited, but from the viewpoint of reducing resistance overvoltage, it is preferably 0.5 Ωcm ² or less, and more preferably 0.3 Ωcm ² or less.

  Here, when the thermal conductivity of the cathode side gas diffusion layer 140c is high to some extent, the gas diffusion layer is excellent in heat conductivity, and therefore, the temperature decrease of the cathode catalyst layer 120c can be promoted. As a result, there is a problem in that evaporation of liquid water generated in the cathode catalyst layer is suppressed and a flooding phenomenon is likely to occur.

  On the other hand, according to the above-described form, the catalyst layer having a large surface resistance is arranged so as to be in contact with the cathode-side gas diffusion layer 140c, so that the temperature of the catalyst layer is reduced during the operation of the PEFC by the generated Joule heat. Since it becomes possible to increase the evaporation of the produced water in the cathode catalyst layer, it is possible to prevent the performance of the fuel cell from being deteriorated due to the flooding phenomenon.

In the present embodiment, when the water vapor diffusion coefficient of the cathode side gas diffusion layer 140c is 1.0 × 10 ⁻⁵ [m ² / s] or more, among the layers constituting the cathode catalyst layer 120c, the cathode side The surface resistance of the layer in contact with the gas diffusion layer 140c is preferably less than 0.10 Ωcm ² , more preferably less than 0.07 Ωcm ² , and even more preferably less than 0.05 Ωcm ² . At this time, there is no limit to the lower limit of the surface resistance, from the viewpoint of flooding suppression is preferably at 0.01? Cm ² or more, more preferably 0.02 .OMEGA.cm ² or more.

  Here, when the water vapor diffusion coefficient of the cathode side gas diffusion layer 140c is high to some extent, evaporation of liquid water generated in the cathode catalyst layer is promoted, and there is a problem that the electrolyte membrane is likely to dry out.

  On the other hand, according to the above-described embodiment, the catalyst layer having a small surface resistance is disposed so as to be in contact with the cathode-side gas diffusion layer 140c, thereby suppressing the generation of Joule heat in the catalyst layer during the PEFC operation. As a result, it is possible to suppress the evaporation of the produced water in the cathode catalyst layer, so that the performance degradation of the fuel cell due to the occurrence of dryout of the electrolyte membrane can be prevented.

Furthermore, in the present embodiment, the cathode catalyst layer 120c is configured when the overall heat transfer coefficient between the cathode catalyst layer 120c and the cooling flow path of the cathode separator 150c is 200 [W / m ² K] or more. Of the layers, the surface resistance of the layer in contact with the cathode-side gas diffusion layer 140c is preferably 0.1 Ωcm ² or more, more preferably 0.15 Ωcm ² or more, and further preferably 0.20 Ωcm ² or more. In this case, the upper limit value of the sheet resistance is not limited, but from the viewpoint of suppressing resistance overvoltage, it is preferably 0.50 Ωcm ² or less, and more preferably 0.30 Ωcm ² or less.

  Here, when the overall heat transfer coefficient between the cathode catalyst layer 120c and the cooling flow path of the cathode side gas diffusion layer 140c is high to some extent, the gas diffusion layer is excellent in heat transfer, so the temperature of the cathode catalyst layer 120c. Decrease can be promoted. As a result, there is a problem in that evaporation of liquid water generated in the cathode catalyst layer is suppressed and a flooding phenomenon is likely to occur.

  On the other hand, according to the above-described form, the catalyst layer having a large surface resistance is arranged so as to be in contact with the cathode-side gas diffusion layer 140c, so that the temperature of the catalyst layer is reduced during the operation of the PEFC by the generated Joule heat. Since it becomes possible to increase the evaporation of the produced water in the cathode catalyst layer, it is possible to prevent the performance of the fuel cell from being deteriorated due to the flooding phenomenon.

(Production method)
The method for producing MEA or PEFC of the present invention is not particularly limited, and conventionally known knowledge can be appropriately referred to in the field of fuel cell production. Hereinafter, although the preferable one form of the manufacturing method of MEA and PEFC of this invention is demonstrated, the technical scope of this invention is not limited only to the specific form mentioned later.

  First, an electrode catalyst is produced by supporting a catalyst component on a conductive carrier. In order to support the catalyst component particles on the conductive support, a known method such as impregnation method, liquid phase reduction support method, evaporation to dryness method, colloid adsorption method, spray pyrolysis method, reverse micelle (microemulsion method) is used. Just do it. Moreover, you may use the electrode catalyst marketed.

  Thereafter, the electrode catalyst and ionomer are mixed with water or an alcohol solvent to prepare a catalyst ink.

  When preparing the catalyst ink, the dispersibility of the conductive carrier may be adjusted by adjusting the stirring time and the like. Thereby, the porosity, surface resistance, etc. in the obtained catalyst layer can be adjusted. Examples of the stirring means include known means such as a homogenizer, an ultrasonic dispersion device, a jet mill, and a bead mill.

  Since the specific configurations of the electrode catalyst and ionomer included in the catalyst ink are as described above, detailed description thereof is omitted here. The solvent used in the catalyst ink is not particularly limited, but examples include water, propylene glycol, normal propyl alcohol, isopropyl alcohol, butanol, methanol, ethanol, benzene, toluene, xylene, and mixed solvents thereof. Can be mentioned. Besides these, butyl acetate alcohol, dimethyl ether, ethylene glycol, and the like may be used as a solvent. Preferably, normal propyl alcohol or propylene glycol is used as the solvent.

  The content of each component in the catalyst ink is not particularly limited. For example, the electrode catalyst is preferably 2 to 20% by mass, the ionomer is 1 to 15% by mass, and the solvent based on the total amount of the catalyst ink. Is 70 to 95% by mass, more preferably 4 to 10% by mass of the electrode catalyst, 4 to 10% by mass of the ionomer, and 80 to 95% by mass of the solvent. When the composition of the catalyst ink is within such a range, it is preferable from the viewpoint of the uniformity of the thickness of the coating film.

  Next, the previously prepared catalyst ink is applied onto a substrate (for example, a PTFE sheet) to form a coating film, and the coating film is further dried.

  As a base material to which the catalyst ink is applied, a solid polymer electrolyte membrane or a gas diffusion layer used for MEA may be used as the base material in addition to the PTFE sheet. Moreover, using the solid polymer electrolyte membrane and the gas diffusion layer as a base material, an electrode catalyst layer (1) is created on the solid polymer electrolyte membrane, an electrode catalyst layer (2) is created on the gas diffusion layer, The catalyst layer can also be formed by a method of joining them by hot pressing or the like. In order to apply the catalyst ink on the substrate, a known method such as a die coater method, a screen printing method, a doctor blade method, or a spray method may be used.

  At this time, the type of the catalyst ink (for example, the type of ionomer or solvent), the composition of the catalyst ink (the mass ratio of the electrode catalyst to the ionomer), the conditions for forming the coating film (for example, the catalyst ink applied by one application) Value of the catalyst layer obtained by controlling the amount of coating, the number of times of coating-drying), the drying conditions of the coating film (for example, drying temperature, drying time, pressure during drying, atmosphere during drying), etc. Can be controlled.

  Specifically, assuming that other conditions are the same, reducing the ion exchange capacity of the ionomer increases the surface resistance of the resulting catalyst layer. In addition, when the value of the mass ratio of the electrode catalyst to the ionomer (electrode catalyst / ionomer) is decreased, the surface resistance value of the obtained catalyst layer is increased. Furthermore, if the number of times of coating and drying is increased during the formation of the coating film, the surface resistance of the resulting catalyst layer increases. Moreover, when the drying temperature of a coating film is made high, the surface resistance of the catalyst layer obtained will become large.

In addition to the various conditions described above, the sheet resistance in the catalyst layer can be controlled by adjusting the ion exchange capacity of the ionomer contained in the catalyst layer. That is, as the ion exchange capacity of the ionomer increases, proton conductivity improves. As a result, the surface resistance of such a catalyst layer containing a large amount of ionomer is reduced. On the other hand, as the ion exchange capacity of the ionomer decreases, proton conductivity decreases. As a result, the sheet resistance of such a catalyst layer containing a large amount of ionomer increases. Table 1 below shows the results of measuring the surface resistance of each catalyst layer prepared using two ionomers having different ion exchange capacities. In addition, the kind of catalyst layer raw material used in the experiment shown in Table 1 and the specifications of the catalyst layer are as follows:
<Catalyst layer material>
Electrocatalyst: 50% by mass platinum-supported carbon (Ketjen black carrier, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.);
Ionoma A = Nafion solution (manufactured by DuPont, containing 10% by mass Nafion, EW1000);
Ionoma B = Nafion solution (manufactured by DuPont, containing 10% by mass Nafion, EW1100);
<Catalyst layer specifications>
Platinum mass: 0.4 mg / cm ² ;
Ionomer mass / carbon mass ratio: 1.0

  The results shown in Table 1 indicate that the sheet resistance in the catalyst layer can be controlled by adjusting the ion exchange capacity of the ionomer contained in the catalyst layer.

Further, the sheet resistance in the catalyst layer can be controlled by adjusting the ratio of the content of ionomer to the content of conductive carrier in the catalyst layer (the mass ratio of ionomer / conductive carrier). Specifically, when the amount of ionomer is increased and the mass ratio is increased, proton conductivity is improved. As a result, the surface resistance of such a catalyst layer containing a large amount of ionomer is reduced. Table 2 below shows the results of measuring the sheet resistance of the catalyst layers prepared using three types of catalyst inks having different ionomer / conductive carrier mass ratios. The types of catalyst layer raw materials used in the experiments shown in Table 2 and the specifications of the catalyst layers are as follows:
<Catalyst layer material>
Electrocatalyst: 50% by mass platinum-supported carbon (Ketjen black carrier, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.);
Ionoma = Nafion solution (manufactured by DuPont, containing 20% Nafion)
<Catalyst layer specifications>
Platinum mass: 0.4 mg / cm ² ;

  The results shown in Table 2 indicate that the sheet resistance in the catalyst layer can be controlled by adjusting the mass ratio of ionomer / conductive carrier in the catalyst layer.

  In addition to these, it is possible to control the surface resistance value of the resulting catalyst layer by adjusting various conditions. For example, the present inventors have confirmed that the surface resistance of the catalyst layer can be controlled even if the type and composition of the solvent used for the preparation of the catalyst ink are changed (see Reference Example 1 described later).

  The ink application-drying step for forming a coating film from the catalyst ink is preferably performed twice or more, more preferably 2 to 10 times, when forming one catalyst layer. Such a form is preferable from the viewpoint of forming a catalyst layer having a uniform thickness. Moreover, there is no restriction | limiting in particular about the coating method of a coating film, For example, a coater method, a spray method, a screen printing method etc. are mentioned. Of these, a coater method and a screen printing method can be preferably employed. Although there is no restriction | limiting in particular also about the drying conditions of a coating film, if an example is given, drying temperature becomes like this. Preferably it is 25-150 degreeC, More preferably, it is 60-130 degreeC, More preferably, it is 80-120 degreeC. . Moreover, drying time becomes like this. Preferably it is 5 to 60 minutes, More preferably, it is 10 to 30 minutes. Furthermore, the pressure at the time of drying becomes like this. Preferably it is 0.01-1.0 MPa, More preferably, it is 0.05-0.2 MPa. The drying treatment is preferably performed in a nitrogen atmosphere or an air atmosphere.

  In the production method of the present invention, the above-described various conditions are performed so as to change along the stacking direction or the surface direction of the catalyst layer desired to be produced. As a result, a catalyst layer having a characteristic configuration of the first fuel cell MEA of the present invention in which the sheet resistance is changed along the stacking direction or the surface direction of the catalyst layer can be manufactured.

  By the method as described above, the cathode catalyst layer is formed on one surface of the solid polymer electrolyte membrane, and the anode catalyst layer is formed on the other surface, whereby the first MEA for fuel cell of the present invention is completed.

  Then, PEFC is produced using obtained MEA.

  Specifically, for example, there is a method of hot pressing in a state where the MEA obtained above is sandwiched between a pair of gas diffusion layers. However, it is not limited only to such a method, A conventionally well-known method can be referred suitably.

  At this time, the hot pressing is preferably performed at 110 to 170 ° C. and a pressing pressure of 1 to 5 MPa with respect to the electrode surface. Thereby, the bondability between the solid polymer electrolyte membrane and the catalyst layer can be enhanced.

  When a water repellent is contained in the gas diffusion layer, a general water repellent treatment method may be used. For example, after immersing the base material used for a gas diffusion layer in the dispersion liquid of a water repellent agent, the method of heat-drying with oven etc. is mentioned.

  When forming a carbon particle layer on a substrate in a gas diffusion layer, the carbon particles, water repellent, etc. are in a solvent such as water, alcohol solvents such as perfluorobenzene, dichloropentafluoropropane, methanol, ethanol, etc. A slurry is prepared by dispersing in a slurry, and the slurry is applied on a substrate and dried, or the slurry is dried and pulverized to form a powder, and this is applied to the gas diffusion layer. Use it. Then, it is preferable to heat-process at about 250-400 degreeC using a muffle furnace or a baking furnace.

Furthermore, a fuel cell stack in which a plurality of MEAs are stacked via a separator and connected in series may be formed so that the desired voltage or the like of the fuel cell can be obtained. The shape of the fuel cell is not particularly limited, and may be determined as appropriate so that desired battery characteristics such as voltage can be obtained.

  For reference, FIG. 4 shows a schematic cross-sectional view of a fuel cell stack 200 in which a plurality of PEFCs 100 of the first embodiment are connected in series.

  In the fuel cell stack 200, a plurality of PEFCs 100 of the first embodiment are connected in series via separators (150c, 150a), and both ends are fixed by current collector plates 210 and end plates 220. Here, in the center of the two gas flow paths (fuel gas flow path: 152a, oxidant gas flow path: 152c) of the separator, a cooling flow path 230 for circulating a coolant (refrigerant) is provided. .

  Further, a vehicle equipped with the above-described PEFC or fuel cell stack of the present invention is also included in the technical scope of the present invention. Since the PEFC and fuel cell stack of the present invention are very excellent in output performance, they are suitable for vehicle applications that require high output.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the technical scope of the present invention is not limited to the following examples.

  In some examples described later, GDL having physical properties (thermal conductivity and water vapor diffusion coefficient) shown in Table 3 below was used.

<Reference Example 1>
(Preparation of electrode catalyst)
An electrode catalyst (platinum-supported carbon black) was prepared by the following method.

  First, carbon black powder (Cabot Vulcan XC-72) as a conductive carrier was sufficiently dispersed in a 1% by mass Pt-containing chloroplatinic acid aqueous solution using a homogenizer. Next, sodium citrate was added and dissolved sufficiently to prepare a reaction solution. Thereafter, using a reflux reactor, the reaction solution was heated and refluxed at 85 ° C. for 5 hours with stirring, and platinum was reduced and supported on the surface of the carbon black powder.

  After completion of the reaction, the sample solution was allowed to cool to room temperature, and the carbon black powder carrying platinum was filtered off with a suction filtration device and washed thoroughly with water. And the powder of the electrode catalyst (platinum carrying | support carbon black) was obtained by drying the powder washed with water under reduced pressure at 80 degreeC for 6 hours.

  The obtained electrode catalyst powder was quantitatively analyzed by atomic absorption spectrophotometry. As a result, the amount of platinum supported on the electrode catalyst was 50.0% by mass. From the observation with a transmission electron microscope (TEM), the average particle diameter of the supported platinum was estimated to be 2.0 nm.

(Preparation of catalyst ink)
A catalyst ink was prepared by the following method.

  First, to the electrode catalyst prepared above, based on the amount of carbon in the electrode catalyst (1 part by mass), after adding 10 parts by mass of ion exchange water, 10 parts by mass of normal propyl alcohol is added, Further, a Nafion solution (containing 20% by mass Nafion manufactured by DuPont) was added so that the mass of the electrolyte was 1 part by mass. The slurry thus obtained was sufficiently dispersed using an ultrasonic homogenizer and then subjected to vacuum defoaming treatment to prepare catalyst ink A.

  On the other hand, after adding 10 parts by mass of ion-exchanged water to the electrode catalyst prepared as described above based on the amount of carbon in the electrode catalyst (1 part by mass), 10 parts by mass of propylene glycol was added. Added a Nafion solution (containing 20% by mass Nafion manufactured by DuPont) so that the weight of the electrolyte was 1 part by mass. The slurry thus obtained was sufficiently dispersed using an ultrasonic homogenizer, and then subjected to vacuum defoaming treatment to prepare catalyst ink B.

(Production of GDL sandwiching MEA using catalyst ink A: application twice)
In the examples of the present application, the catalyst layer was produced by a screen printing method, and the MEA was produced by a transfer method. According to the screen printing method, the number of coatings and the coating amount can be easily controlled by changing the roughness of the screen mesh. That is, when the mesh eyes are large, the application amount per time increases, and when the mesh eyes are small, the application amount per time decreases.

The catalyst ink A prepared above was applied onto a PTFE sheet by a screen printing method using an appropriate screen mesh. Next, the film was dried at 25 ° C. for 15 minutes in an air atmosphere, and then dried in an oven at 60 ° C. for 30 minutes. Then, the basis weight of platinum contained in the coating film was 0.2 mg / cm ² . This coating-drying operation was further repeated once to prepare a catalyst layer A having a platinum basis weight of 0.4 mg / cm ² on the PTFE sheet.

Using the catalyst layer A created above as a cathode catalyst layer, bonded by a transfer method to a solid polymer electrolyte membrane (NRE211 made by DuPont) with an anode catalyst layer at 120 ° C. for 20 minutes at a pressure of 1.0 MPa Then, the PTFE sheet was peeled off to prepare an MEA. The MEA obtained in this manner was sandwiched between a pair of GDLs (both manufactured by SGL Carbon Co., 25BC) to produce a GDL sandwiching MEA. The anode catalyst layer is formed on the solid polymer electrolyte membrane by applying the catalyst ink A prepared above on a PTFE sheet by a screen printing method using an appropriate screen mesh, and then in an air atmosphere, 25 The drying was performed at 15 ° C. for 15 minutes. The basis weight of platinum contained in the coating film was 0.05 mg / cm ² .

When the sheet resistance of the cathode catalyst layer in the GDL sandwiching MEA obtained above was measured, it was 0.05 Ωcm ² .

<Reference Example 2>
(Preparation of GDL sandwiching MEA using catalyst ink B: application twice)
The catalyst ink B prepared above was applied onto a PTFE sheet by a screen printing method using an appropriate screen mesh. Next, the film was dried at 25 ° C. for 15 minutes in an air atmosphere, and then dried in an oven at 60 ° C. for 30 minutes. Then, the basis weight of platinum contained in the coating film was 0.2 mg / cm ² . This coating-drying operation was further repeated once to prepare a catalyst layer B having a platinum basis weight of 0.4 mg / cm ² on the PTFE sheet.

  Using the catalyst layer B created above as a cathode catalyst layer, a GDL sandwiching MEA was produced in the same manner as in Reference Example 1 described above.

When the sheet resistance of the cathode catalyst layer in the GDL sandwiching MEA obtained above was measured, it was 0.15 Ωcm ² .

<Reference Example 3>
(Preparation of GDL sandwiching MEA using catalyst ink B: 4 times application-low temperature drying)
The catalyst ink B prepared above was applied onto a PTFE sheet by a screen printing method using an appropriate screen mesh. Next, the film was dried at 25 ° C. for 15 minutes in an air atmosphere, and then dried in an oven at 60 ° C. for 30 minutes. Then, the basis weight of platinum contained in the coating film was 0.1 mg / cm ² . This coating-drying operation was further repeated three times to produce a catalyst layer C having a platinum basis weight of 0.4 mg / cm ² on the PTFE sheet.

  Using the catalyst layer C created above as a cathode catalyst layer, a GDL sandwiching MEA was produced by the same method as in Reference Example 1 described above.

When the sheet resistance of the cathode catalyst layer in the GDL sandwiching MEA obtained above was measured, it was 0.30 Ωcm ² .

<Reference Example 4>
(Preparation of GDL sandwiching MEA using catalyst ink B: 4 times application-high temperature drying)
The catalyst ink B prepared above was applied onto a PTFE sheet by a screen printing method using an appropriate screen mesh. Next, after drying at 25 ° C. for 15 minutes in an air atmosphere, drying was performed at 130 ° C. for 30 minutes in an oven. Then, the basis weight of platinum contained in the coating film was 0.1 mg / cm ² . This coating-drying operation was further repeated three times to produce a catalyst layer D having a platinum basis weight of 0.4 mg / cm ² on the PTFE sheet.

  Using the catalyst layer D created above as a cathode catalyst layer, a GDL sandwiching MEA was produced in the same manner as in Reference Example 1 described above.

When the sheet resistance of the cathode catalyst layer in the GDL sandwiching MEA obtained above was measured, it was 0.60 Ωcm ² .

  According to the above Reference Examples 1 to 4, the surface resistance of the catalyst layer to be formed is changed by changing the kind of the solvent added at the time of catalyst ink preparation, the number of times of applying the catalyst ink at the time of forming the catalyst layer, the drying temperature, and the like. Is shown to be controllable.

<Example 1>
The PEFC 100 having the configuration shown in FIGS. 1 and 2 was manufactured by the following method.

  First, using the catalyst ink A prepared above, a rectangular catalyst layer A was produced on a PTFE sheet by the same method as in Reference Example 1 described above, and used as an anode catalyst layer 120a.

  On the other hand, using the catalyst ink B prepared above, the rectangular shape of the catalyst layer A is divided into two equal parts in the longitudinal direction (“oxidant gas flow upstream side” shown in FIG. 2). The catalyst layer 1 was produced on the PTFE sheet by the same method as that for forming the catalyst layer B in Reference Example 2 described above.

  Further, using the catalyst ink B prepared above, the rectangular shape of the catalyst layer A is divided into two equal parts in the longitudinal direction (“oxidant gas flow downstream side” shown in FIG. 2). The catalyst layer 2 was produced on the PTFE sheet by the same method as that for forming the catalyst layer C in Reference Example 3 described above. And the said catalyst layer 2 was united with the catalyst layer 1 mentioned above, and it was set as the cathode catalyst layer 120c.

  The anode catalyst layer 120a and the cathode catalyst layer 120c obtained above are respectively transferred onto both surfaces of the solid polymer electrolyte membrane 110 (NRE211 manufactured by DuPont) by the same transfer method as in Reference Example 1 described above to produce an MEA. did. The MEA thus obtained is sandwiched between a pair of GDLs (anode side GDL 140a and cathode side GDL 140c) (both 25BC manufactured by SGL Carbon Co.), and a pair of carbon resin separators (anode side separator 150a and cathode). Side separator 150c; each was sandwiched between straight gas flow paths (152a, 152c) and a manifold (not shown) for supplying gas, thereby producing PEFC100. At this time, the catalyst layer 1 is arranged on the upstream side in the oxidant gas flow direction (X direction in FIG. 1) in the oxidant gas flow path 152c of the cathode side separator 150c, and the catalyst layer 2 is arranged on the downstream side. The cathode catalyst layer 120c was disposed.

<Example 2>
The cathode catalyst is arranged such that the catalyst layer 2 is disposed upstream of the oxidant gas flow direction (X direction shown in FIG. 1) in the oxidant gas flow path 152c of the cathode side separator 150c, and the catalyst layer 1 is disposed downstream. A PEFC 100 having the form shown in FIGS. 1 and 2 was produced by the same method as in Example 1 described above except that the layer 120c was disposed.

<Example 3>
The PEFC 100 having the form shown in FIG. 3 was produced by the following method.

  First, using the catalyst ink A prepared above, a rectangular catalyst layer A was produced on a PTFE sheet by the same method as in Reference Example 1 described above, and used as an anode catalyst layer 120a.

On the other hand, the catalyst ink A prepared above was applied onto a PTFE sheet by a screen printing method using an appropriate screen mesh. Next, after drying for 15 minutes at 25 ° C. in an air atmosphere, the catalyst layer 3 was dried on an PTFE sheet for 30 minutes at 60 ° C., and the catalyst weight of platinum was 0.2 mg / cm ² on the PTFE sheet. Was made.

Further, the catalyst ink B prepared above was applied on a PTFE sheet by a screen printing method using an appropriate screen mesh. Next, after drying at 25 ° C. for 15 minutes in an air atmosphere, the coating was dried in an oven at 60 ° C. for 30 minutes to form a coating film having a platinum basis weight of 0.05 mg / cm ² on the PTFE sheet. Produced. This coating-drying operation was further repeated three times to produce a catalyst layer 4 having a platinum basis weight of 0.4 mg / cm ² on the PTFE sheet.

  The anode catalyst layer 120a and the catalyst layer 3 obtained above were transferred to both surfaces of the solid polymer electrolyte membrane 110 (NRE211 manufactured by DuPont) by the same transfer method as in Example 1 described above. Furthermore, the catalyst layer 4 obtained above was transferred onto the catalyst layer 3 by the same transfer method to produce an MEA. The MEA thus obtained was sandwiched between a pair of GDLs composed of an anode side GDL 140a (25BC manufactured by SGL Carbon Co.) and a cathode side GDL 140c (GDL (A) shown in Table 3 above). PEFC100 was produced by sandwiching with a pair of carbon resin separators in the same manner as in Example 1.

<Example 4>
The same procedure as in Example 3 described above was used, except that the screen mesh used for producing the catalyst layer 4 was changed and the catalyst layer 5 having a platinum basis weight of 0.2 g / cm ² was produced by a single application. A PEFC 100 having the form shown in FIG. 3 was produced.

<Example 5>
A PEFC 100 having the form shown in FIG. 3 was produced in the same manner as in Example 3 described above, except that GDL (B) shown in Table 3 was used instead of GDL (A).

<Example 6>
A PEFC 100 having the form shown in FIG. 3 was produced in the same manner as in Example 4 described above, except that GDL (C) shown in Table 3 was used instead of GDL (A).

<Example 7>
A PEFC 100 having the form shown in FIG. 3 was produced in the same manner as in Example 6 except that GDL (D) shown in Table 3 was used instead of GDL (C).

<Comparative example>
Except that the cathode catalyst layer was formed by the same method as the anode catalyst layer (that is, the anode catalyst layer and the cathode catalyst layer have the same composition), the fuel cell unit was formed by the same method as in Example 1 described above. A cell was produced.

<Evaluation example>
About the fuel cell single cell produced by said Examples 1-7 and a comparative example, the electric power generation test was done and the cell voltage at the time of an operation was measured. Specifically, pure hydrogen (saturated humidification; humidifier temperature = 60 ° C.) is used as the fuel gas on the anode side of the single fuel cell, and air (saturated humidification; humidifier temperature = 70) is used as the oxidant gas on the cathode side. ° C) was fed in parallel flow. At this time, the supply pressure of both gases was atmospheric pressure. The temperature of the single fuel cell was set to 80 ° C., and the fuel gas utilization rate was 70% and the oxidant gas utilization rate was 67%. The fuel cell was operated at a current density of 1.0 A / cm ² for 30 minutes.

  The results obtained by this power generation test are shown in Table 4 below. In addition, it means that the output characteristic of a single cell is excellent, so that a cell voltage is high.

  Referring to Table 4, it can be seen from the comparison between each example and the comparative example that the MEA for fuel cell of the present invention and the PEFC single cell using the same have excellent power generation performance.

  Further, from comparison between Example 1 and Example 2, when the sheet resistance in the cathode catalyst layer changes along the surface direction of the catalyst layer, the sheet resistance decreases from upstream to downstream in the oxidant gas flow direction. It can be seen that the power generation performance can be further improved by increasing gradually toward the.

Further, from the comparison between Example 3 and Examples 4 and 5, when the cathode catalyst layer is composed of two layers having different surface resistances, the thermal conductivity of the cathode side gas diffusion layer is 0.5 [W / mK] or more, and among the layers constituting the cathode catalyst layer, when the surface resistance of the layer in contact with the cathode gas diffusion layer is 0.1 Ωcm ² or more, it can be seen that the power generation performance can be further improved.

Further, from comparison between Example 6 and Example 7, when the cathode catalyst layer is composed of two layers having different surface resistances, the water vapor diffusion coefficient of the cathode side gas diffusion layer is 1.0 × 10 ⁻⁵ [ m ² / s] or more, and among the layers constituting the cathode catalyst layer, when the surface resistance of the layer in contact with the cathode-side gas diffusion layer is less than 0.1 Ωcm ² , the power generation performance can be further improved. Recognize.

<Example 8>
A fuel cell stack was prepared by the following method, a power generation test was performed, and the stack voltage during operation was measured.

Specifically, the fuel cell single cell manufactured in Example 3 described above was subjected to gold plating after press-molding a separator (aluminum plate (JISA1050 material)) provided with a cooling channel for circulating cooling water. 20 cells were stacked through a stack, and the stack was further sandwiched between a pair of end plates to produce a fuel cell stack. At this time, the overall heat transfer coefficient (per fuel cell single cell) between the cathode catalyst layer and the cooling channel of the cathode side separator was 200 [W / m ² K].

Next, pure hydrogen (saturated humidification; humidifier temperature = 55 ° C.) is used as the fuel gas on the anode side of the fuel cell stack produced above, and air (saturated humidification; humidifier temperature = 70) is used as the oxidant gas on the cathode side. ° C) was fed in countercurrent. At this time, the supply pressure of both gases was atmospheric pressure. Moreover, 70 degreeC cooling water was distribute | circulated to the cooling flow path of the separator as a parallel flow with oxidizing agent gas (air). The fuel gas utilization rate was 70% and the oxidant gas utilization rate was 67%, and the operation was performed for 30 minutes at a current density of 1.0 A / cm ² .

  The results obtained by this power generation test are shown in Table 5 below. Note that the higher the stack voltage, the better the output characteristics of the stack.

<Example 9>
A fuel cell stack was produced and a power generation test was performed in the same manner as in Example 8 except that the separator was changed to a mixture of expanded graphite and phenol resin. At this time, the overall heat transfer coefficient (per fuel cell single cell) between the cathode catalyst layer and the cooling channel of the cathode separator was 25 [W / m ² K]. The results obtained by the power generation test are shown in Table 5 below.

From the results shown in Table 5, when the cathode catalyst layer is composed of two layers having different surface resistances in the fuel cell single cell constituting the fuel cell stack, the cathode catalyst layer and the cooling flow path of the cathode side separator When the overall heat transfer coefficient is 200 [W / m ² K] or more and the surface resistance of the layer in contact with the cathode gas diffusion layer among the layers constituting the cathode catalyst layer is 0.1 Ωcm ² or more, It can be seen that the power generation performance of the fuel cell stack can be further improved.

It is a schematic cross section of the polymer electrolyte fuel cell (PEFC) of 1st Embodiment. FIG. 2 is a schematic plan view of a cathode catalyst layer in the polymer electrolyte fuel cell (PEFC) of the first embodiment viewed from the X direction shown in FIG. 1. It is a schematic cross section of PEFC of a 2nd embodiment. It is a schematic cross section of a fuel cell stack in which a plurality of PEFCs of the first embodiment are connected in series.

Explanation of symbols

100 polymer electrolyte fuel cell (PEFC),
110 solid polymer electrolyte membrane,
120a anode catalyst layer,
120c cathode catalyst layer,
120c ₁ cathode catalyst layer located on the side in contact with the solid polymer electrolyte membrane,
120c ₂ cathode catalyst layer located on the side in contact with the cathode side gas diffusion layer,
130 Membrane electrode assembly (MEA),
140a anode side gas diffusion layer,
140c cathode side gas diffusion layer,
150a anode side separator,
150c cathode side separator,
152a fuel gas flow path,
152c oxidant gas flow path,
160 gasket,
200 Fuel cell stack,
210 current collector,
220 end plate,
230 Cooling flow path.

Claims (18)

A solid polymer electrolyte membrane;
A pair of catalyst layers comprising a cathode catalyst layer and an anode catalyst layer sandwiching the solid polymer electrolyte membrane;
A fuel cell membrane electrode assembly comprising:
In at least one of the cathode catalyst layer and the anode catalyst layer, the surface resistance of the catalyst layer varies along at least one of the stacking direction and the surface direction of the catalyst layer. Membrane electrode assembly.   The membrane electrode assembly for a fuel cell according to claim 1, wherein the surface resistance of the anode catalyst layer is smaller than the surface resistance of the cathode catalyst layer.   The membrane electrode assembly for a fuel cell according to claim 1 or 2, wherein the surface resistance in the catalyst layer is changed in at least one of a stepped shape and an inclined shape.   The membrane electrode assembly for a fuel cell according to any one of claims 1 to 3, wherein a sheet resistance in the catalyst layer changes along a surface direction of the catalyst layer.   The membrane electrode assembly for a fuel cell according to claim 4, wherein the sheet resistance in the catalyst layer gradually increases from upstream to downstream in the oxidant gas flow direction.   The membrane electrode assembly for a fuel cell according to any one of claims 1 to 5, wherein a surface resistance of the catalyst layer changes along a stacking direction of the catalyst layer.   The cathode catalyst layer is composed of two or more layers having different surface resistances, and the surface resistance of the layer located on the side in contact with the solid polymer electrolyte membrane is the smallest among the layers constituting the cathode catalyst layer; The fuel cell membrane electrode assembly according to claim 6. A fuel cell membrane electrode assembly according to any one of claims 1 to 7,
A pair of gas diffusion layers comprising a cathode side gas diffusion layer and an anode side gas diffusion layer sandwiching the membrane electrode assembly;
A cathode-side separator having an oxidant gas flow path through which an oxidant gas flows, a cooling flow path through which a coolant flows, and a fuel gas flow path through which fuel gas flows, sandwiching the membrane electrode assembly and the gas diffusion layer A pair of separators, and an anode side separator having a cooling channel through which a coolant flows,
A fuel cell. When the cathode catalyst layer is composed of two or more layers having different surface resistances,
The thermal conductivity of the cathode side gas diffusion layer is 0.5 [W / mK] or more,
9. The fuel cell according to claim 8, wherein a surface resistance of a layer in contact with the cathode-side gas diffusion layer among the layers constituting the cathode catalyst layer is 0.1 Ωcm ² or more. When the cathode catalyst layer is composed of two or more layers having different surface resistances,
The cathode gas diffusion layer has a water vapor diffusion coefficient of 1.0 × 10 ⁻⁵ [m ² / s] or more,
9. The fuel cell according to claim 8, wherein a surface resistance of a layer in contact with the cathode-side gas diffusion layer among layers constituting the cathode catalyst layer is less than 0.1 Ωcm ² . When the cathode catalyst layer is composed of two or more layers having different surface resistances,
The overall heat transfer coefficient between the cathode catalyst layer and the cooling flow path of the cathode side separator is 200 [W / m ² K] or more;
The fuel cell according to claim 8 or 9, wherein a surface resistance of a layer in contact with the cathode-side gas diffusion layer among the layers constituting the cathode catalyst layer is 0.1 Ωcm ² or more.   A fuel cell stack comprising the fuel cell according to any one of claims 8 to 11.   A vehicle equipped with the fuel cell according to any one of claims 8 to 11 or the fuel cell stack according to claim 12. It is a manufacturing method of the membrane electrode assembly according to any one of claims 1 to 7,
Preparing a catalyst ink containing an electrode catalyst, a solid polymer electrolyte, and a solvent for forming a catalyst layer, forming a coating film from the catalyst ink, and drying the coating film to form the catalyst layer; Have
The surface resistance in the catalyst layer is changed by changing the type or composition of the catalyst ink, the formation conditions of the coating film, or the drying conditions of the coating film along the stacking direction or the surface direction of the catalyst layer. The manufacturing method characterized by the above-mentioned.   The production method according to claim 14, wherein the solvent is selected from the group consisting of propylene glycol, normal propyl alcohol, isopropyl alcohol, butanol, methanol, ethanol, benzene, toluene, xylene, and a mixed solvent thereof.   The manufacturing method of Claim 14 or 15 which forms the said coating film by apply | coating the said catalyst ink twice or more.   The manufacturing method of any one of Claims 14-16 whose drying temperature of the said coating film is 25 degreeC or more.   The production method according to any one of claims 14 to 17, wherein the coating film is formed from the catalyst ink by a transfer method, a coater method, or a spray method.

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