JP2009152128A - Membrane / electrode assembly - Google Patents

Abstract

A gas diffusion layer that has elasticity capable of absorbing load changes and dimensional changes in the stacking direction of a single cell and that can reduce damage to an electrolyte membrane due to conductive carbonaceous fibers that are constituent materials, A membrane / electrode assembly having excellent durability is provided.
At least one of the electrodes has a laminated structure in which a catalyst layer and a gas diffusion layer are laminated in order from the electrolyte membrane side, and the gas diffusion layer has a fiber length direction on the surface of the gas diffusion layer. And having a structure in which a plurality of conductive carbonaceous fibers oriented in the direction are bound with a binder resin, and in the stacking direction of the stacked structure, (A) the fiber diameter of the conductive carbonaceous fibers is on the electrolyte membrane side And (B) in the fiber length direction of the conductive carbonaceous fiber, among the binding points at which the conductive carbonaceous fiber and other conductive carbonaceous fibers bind, the fiber length direction The membrane-electrode assembly has a distribution in which the interval between adjacent binding points decreases toward the electrolyte membrane side.
[Selection figure] None

Description

  The present invention relates to a membrane / electrode assembly that can be suitably used in a fuel cell.

  A fuel cell directly converts chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes and causing the fuel to be oxidized electrochemically. Unlike thermal power generation, fuel cells are not subject to the Carnot cycle, and thus exhibit high energy conversion efficiency. A fuel cell is usually formed by laminating a plurality of single cells having a basic structure of a membrane / electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes. Among them, a solid polymer electrolyte fuel cell using a solid polymer electrolyte membrane as an electrolyte membrane has advantages such as easy miniaturization and operation at a low temperature. It is attracting attention as a power source for the body.

FIG. 7 is a cross-sectional view showing an example of a single cell in a conventional general solid polymer electrolyte fuel cell. The unit cell 200 is provided with a fuel electrode (anode) 11 on one side of a solid polymer electrolyte membrane for fuel cell (hereinafter, simply referred to as an electrolyte membrane) 10 and an oxidant electrode (cathode) 12 on the other side. The membrane / electrode assembly 15 is provided. The fuel electrode 11 has a structure in which a fuel electrode side catalyst layer 13a and a fuel electrode side gas diffusion layer 14a are laminated in order from the electrolyte membrane 10 side. Similarly, the oxidant electrode 12 has a structure in which an oxidant electrode side catalyst layer 13b and an oxidant electrode side gas diffusion layer 14b are laminated in order from the electrolyte membrane 10 side.
The membrane / electrode assembly 15 is sandwiched between two separators 16 a and 16 b, thereby forming a single cell 200. On one side of each of the separators 16a and 16b, grooves for forming a flow path for the reaction gas (fuel gas, oxidant gas) are provided. The fuel is formed by these grooves and the outer surfaces of the fuel electrode 11 and the oxidant electrode 12. A gas flow path 17a and an oxidant gas flow path 17b are defined. In order to obtain a desired output, a fuel cell is usually equipped with a cell stack in which a plurality of single cells are stacked.

  In a fuel cell having a single cell as described above, since the power generation efficiency of the fuel cell decreases as the contact resistance between each layer in the single cell and between the single cells increases, a predetermined load is applied in the stacking direction of the single cells. Single cells and cell stacks are fastened in this state. This load is desirably kept uniform and constant in the surface direction of the single cell from the viewpoint of reducing contact resistance and preventing damage to the constituent members of the single cell.

However, with changes in the operating environment of the fuel cell, each component of the single cell undergoes dimensional changes such as thermal expansion, swelling due to water absorption, and shrinkage due to drying. Therefore, when the fastening mode of the single cell cannot absorb the dimensional change in the stacking direction of the single cell, the contact resistance increases because the load applied to the single cell is reduced when the constituent members of the single cell contract in the stacking direction. However, when the constituent members of the single cells expand in the stacking direction, the load applied to the single cells increases, and the constituent members of the single cells may be damaged. Moreover, the load in the surface direction of a single cell may become non-uniform | heterogenous by the dimensional change in the lamination direction of a structural member.
Therefore, a fastening method that can absorb the dimensional change in the stacking direction of the single cells has also been proposed. Specifically, there is a method in which a dimensional change such as expansion or contraction of the single cell can be absorbed by using an elastic material as the fastening member of the single cell.

  On the other hand, it has also been proposed to keep the load constant by using a member that can absorb dimensional changes of other constituent members as a constituent member of a single cell. Specifically, a membrane / electrode assembly configured using an elastically deformable gas diffusion layer formed of a fibrous carbonaceous material (carbonaceous fiber) is exemplified (for example, Patent Document 1). However, the carbonaceous fibers constituting the gas diffusion layer may protrude from the surface of the gas diffusion layer and penetrate through the catalyst layer and the electrolyte membrane. As a result, a short circuit occurs, the output of the fuel cell decreases, and a through hole that connects the fuel electrode and the oxidant electrode is formed, so that a so-called cross leak occurs in which the reaction gas leaks to the counter electrode side. As a result, there arises a problem that the power generation efficiency and durability of the fuel cell are lowered.

Various techniques have been proposed in order to solve the problem caused by such penetration of carbonaceous fibers (Patent Documents 1 and 2, etc.). For example, Patent Document 1 discloses a gas diffusion layer having a first bonding surface to be bonded to a catalyst layer, the first gas diffusion layer including first fibers therein, and the first A gas diffusion layer having a second bonding surface bonded to the gas diffusion layer, the gas diffusion layer including a second gas diffusion layer including a second fiber therein, wherein the first gas diffusion layer includes the second gas diffusion layer. The number of the first fibers per unit area that is larger than a predetermined value at an angle with respect to the first bonding surface at one bonding surface is equal to the angle with respect to the second bonding surface at the second bonding surface. A fuel cell electrode is described that is configured to be less than the number of second fibers per unit area that is greater than the value of.
Further, Patent Document 2 has a fiber layer containing a carbon fiber having a coil shape on the side where the gas diffusion layer is in contact with the membrane / electrode assembly, and the carbon fiber having the coil shape has a fiber average diameter of 1 nm to A fuel cell is described in which 1 μm, average coil length or twist average length is 5 nm to 10 μm, and average coil pitch or twist average pitch is 1 nm to 1 μm.

JP 2005-1000066 A JP 2006-85930 A JP 2005-1000074 A

On the other hand, Patent Document 3 includes conductive carbon and a hydrophobic polymer for the purpose of improving the mechanical strength of the gas diffusion layer and the diffusibility of the reactants and products, and has flexibility. An electrolyte membrane for a polymer electrolyte fuel cell, comprising: a diffusion layer having a structure in which at least one unit layer comprising sheets is laminated; a fuel electrode and an oxygen electrode including a catalyst layer in contact with the diffusion layer; and a solid electrolyte membrane In the electrode assembly, the conductive carbon contained in the unit layer on the side close to the catalyst layer is granular conductive carbon, and the conductive carbon contained in the unit layer on the side away from the catalyst layer is The fibrous conductive carbon and the unit layer disposed at a position farther from the catalyst layer has a larger average fiber length and average fiber diameter of the fibrous conductive carbon contained therein. Electrolyte membrane electrode assembly It has been described.
Since the gas diffusion layer provided in the membrane / electrode assembly described in Patent Document 3 has a small fiber diameter and fiber length on the electrolyte membrane side, sufficient elasticity may not be obtained in the thickness direction. There is.

  The present invention has been achieved in view of the above circumstances, and has elasticity capable of absorbing load changes and dimensional changes in the stacking direction of the single cells, and damage to the electrolyte membrane due to the conductive carbonaceous fibers that are constituent materials. An object of the present invention is to provide a membrane / electrode assembly including a gas diffusion layer that can be reduced and having excellent power generation performance and durability.

The membrane / electrode assembly of the present invention is a membrane / electrode assembly comprising an electrolyte membrane and a pair of electrodes sandwiching the electrolyte membrane, wherein at least one of the electrodes is in order from the electrolyte membrane side. It has a laminated structure in which a catalyst layer and a gas diffusion layer are laminated,
The gas diffusion layer has a structure in which a plurality of conductive carbonaceous fibers having a fiber length direction oriented in a plane direction of the gas diffusion layer are bound with a binder resin,
In the stacking direction of the stacked structure,
(A) a distribution in which the fiber diameter of the conductive carbonaceous fiber increases toward the electrolyte membrane;
(B) Among the binding points at which the conductive carbonaceous fiber and other conductive carbonaceous fibers bind in the fiber length direction of the conductive carbonaceous fiber, the adjacent binding point intervals in the fiber length direction. Is a distribution that decreases toward the electrolyte membrane side,
It is characterized by having.

  In the membrane / electrode assembly of the present invention, the gas diffusion layer has the fiber diameter distribution shown in (A) and the binding point interval distribution shown in (B), so that It has elasticity that can absorb load changes and dimensional changes, and can suppress the sticking of the conductive carbonaceous fibers constituting the gas diffusion layer into the electrolyte membrane.

The form of the specific gas diffusion layer having the distributions (A) and (B) is not particularly limited. For example, the form of the gas diffusion layer having the distribution (A) may be the electrolyte membrane side. In order, the first gas diffusion layer having an average fiber diameter of the conductive carbonaceous fiber of 7 μm or more and the second gas diffusion layer having an average fiber diameter of the conductive carbonaceous fiber of 5 μm or less The form which has the laminated | stacked two-layer structure is mentioned. In addition, as a form of the gas diffusion layer having the distribution of (B), for example, in order from the electrolyte membrane side, the first gas diffusion layer having an average binding point interval of less than 60 μm, and the binding Examples include a two-layer structure in which a second gas diffusion layer having an average point interval of 100 μm or more is laminated.
From the viewpoint of ensuring the elasticity of the gas diffusion layer, the thickness of the second gas diffusion layer is preferably thicker than the thickness of the first gas diffusion layer.

  The membrane / electrode assembly of the present invention is capable of maintaining a constant load of a single cell even when a load change or dimensional change in the stacking direction of the single cell occurs, and is a conductive carbonaceous material that is a constituent material. It has a gas diffusion layer that can reduce damage to the electrolyte membrane caused by the fibers. Therefore, according to the present invention, it is possible to provide a membrane / electrode assembly excellent in power generation performance and durability.

The membrane / electrode assembly of the present invention is a membrane / electrode assembly comprising an electrolyte membrane and a pair of electrodes sandwiching the electrolyte membrane,
At least one of the electrodes has a laminated structure in which a catalyst layer and a gas diffusion layer are laminated in order from the electrolyte membrane side,
The gas diffusion layer is
While having a structure in which a plurality of conductive carbonaceous fibers oriented in the fiber length direction in the plane direction of the gas diffusion layer are bound with a binder resin,
In the stacking direction of the stacked structure,
(A) a distribution in which the fiber diameter of the conductive carbonaceous fiber increases toward the electrolyte membrane;
(B) Among the binding points at which the conductive carbonaceous fiber and other conductive carbonaceous fibers bind in the fiber length direction of the conductive carbonaceous fiber, the adjacent binding point intervals in the fiber length direction. Is a distribution that decreases toward the electrolyte membrane side,
It is characterized by having.

  Hereinafter, the membrane / electrode assembly provided by the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional view showing an embodiment of a unit cell including the membrane / electrode assembly of the present invention, FIG. 2 is a partially enlarged view of FIG. 1, and FIG. 3 is a schematic enlarged view of a gas diffusion layer. .

  In FIG. 1, a single cell 100 is a membrane in which a fuel electrode (anode) 2 and an oxidant electrode (cathode) 3 are provided on one side of a solid polymer electrolyte membrane (hereinafter sometimes simply referred to as an electrolyte membrane) 1. -It has the electrode assembly 6. The fuel electrode 2 has a structure in which a fuel electrode side catalyst layer 4a and a fuel electrode side gas diffusion layer 5a are laminated in order from the electrolyte membrane 1 side. Similarly, the oxidant electrode 3 has a configuration in which an oxidant electrode side catalyst layer 4b and an oxidant electrode side gas diffusion layer 5b are laminated in order from the electrolyte membrane 1 side.

  The membrane / electrode assembly 6 is sandwiched between two separators 7 a and 7 b to constitute a single cell 100. Each separator 7a, 7b is formed with a groove that forms a flow path for each reaction gas (fuel gas, oxidant gas), and a flow for supplying and discharging the reaction gas and the outermost surface of each electrode 2, 3. Paths 8a and 8b are defined. The flow path 8a on the fuel electrode side is a flow path for supplying fuel gas (a gas containing hydrogen or generating hydrogen) to the fuel electrode 2 and discharging unreacted fuel gas, and on the oxidant electrode side. The flow path 8b is a flow path for supplying an oxidant gas (a gas containing oxygen or generating oxygen) to the oxidant electrode 3 and discharging an unreacted portion of the oxidant gas.

As shown in FIG. 3, the gas diffusion layer 5 has a porous structure in which a plurality of conductive carbonaceous fibers 9 are arranged in a mesh shape. The conductive carbonaceous fibers are oriented with the fiber length direction being the plane direction of the gas diffusion layer, and are bound to other conductive carbonaceous fibers by a binder resin. Here, the fiber length direction of the conductive carbonaceous fiber refers to the longitudinal direction of the fiber (see FIG. 3 (3A)). Further, the surface direction of the gas diffusion layer is the direction in which the surface of the gas diffusion layer expands, that is, the direction in which the surface orthogonal to the stacking direction of the catalyst layer and the gas diffusion layer expands. It should be noted that all the conductive carbonaceous fibers forming the gas diffusion layer may not be oriented with the fiber length direction being the plane direction of the gas diffusion layer.
As described above, elasticity in the thickness direction of the gas diffusion layer 5 is ensured by overlapping the plurality of conductive carbonaceous fibers in the thickness direction of the gas diffusion layer.

Further, the gas diffusion layer 5 depends on the fiber diameter of the conductive carbonaceous fibers 9 forming the gas diffusion layer 5 and the binder resin in the stacking direction with the catalyst layer 4, that is, in the thickness direction of the gas diffusion layer. The binding point interval has the following distributions (A) and (B).
(A) Distribution in which the fiber diameter of the conductive carbonaceous fiber increases toward the electrolyte membrane (B) In the fiber length direction of the conductive carbonaceous fiber, the conductive carbonaceous fiber and the other conductive carbonaceous fiber are Among the binding points to be bound, a distribution in which the spacing between adjacent binding points in the fiber length direction decreases toward the electrolyte membrane side.

First, the distribution (A) will be described. The conductive carbonaceous fiber becomes easier to bend as the fiber diameter (diameter) is smaller. Therefore, in the gas diffusion layer in which the conductive carbonaceous fiber is oriented in the plane direction in the fiber length direction, the elastic force in the thickness direction decreases as the fiber diameter of the conductive carbonaceous fiber forming the gas diffusion layer decreases. (See FIGS. 4 and 5). On the other hand, the conductive carbonaceous fiber has a larger force per unit area at its tip as the fiber diameter is smaller, so it penetrates the adjacent catalyst layer and pierces the electrolyte membrane, and also strikes the electrolyte membrane. It becomes easy to form a through hole in the electrolyte membrane.
Therefore, it is possible to suppress damage to the electrolyte membrane due to the piercing of the conductive carbonaceous fiber by configuring the electrolyte membrane side of the gas diffusion layer with the conductive carbonaceous fiber having a large fiber diameter that is difficult to pierce the electrolyte membrane. It is. And, it is possible to ensure elasticity in the thickness direction of the gas diffusion layer by configuring the separator side of the gas diffusion layer, which is unlikely to cause damage to the electrolyte membrane, with conductive carbonaceous fibers having a small fiber diameter. it can.

Next, the distribution (B) will be described. In the gas diffusion layer, the conductive carbonaceous fibers are bound by a binder resin and fixed to each other. At this time, in the fiber length direction of the conductive carbonaceous fiber in which the fiber length direction is oriented in the plane direction of the gas diffusion layer, the binding point where the conductive carbonaceous fiber and the other conductive carbonaceous fiber bind to each other Among them, the longer the distance L between adjacent binding points in the fiber length direction (see FIG. 3 (3A)), that is, the smaller the number of places fixed to other conductive carbonaceous fibers in the fiber length direction, Carbonaceous fibers are more flexible. That is, the gas diffusion layer is more easily bent in the thickness direction and has a higher elastic force in the thickness direction as the distance between the binding points at which the conductive carbonaceous fibers are fixed is smaller (FIGS. 4 and 6). reference). On the other hand, the larger the binding point interval between the conductive carbonaceous fibers, the easier the conductive carbonaceous fibers protrude from the surface of the gas diffusion layer due to the smaller fixing force in the gas diffusion layer, and the easier it is to pierce the electrolyte membrane.
Therefore, by reducing the binding point interval between the conductive carbonaceous fibers on the electrolyte membrane side of the gas diffusion layer, it becomes difficult for the conductive carbonaceous fibers to penetrate the electrolyte membrane, and the conductive side on the separator side of the gas diffusion layer. The elasticity in the thickness direction of the gas diffusion layer can be ensured by increasing the distance between the binding points of the carbonaceous fibers.

Here, the binding point interval of the conductive carbonaceous fiber in (B) will be described in detail.
The interval (binding point interval) at which the conductive carbonaceous fibers constituting the gas diffusion layer are bonded to other conductive carbonaceous fibers by the binder resin in the fiber length direction is determined by various factors. .
For example, when the gas diffusion layer is formed using the same amount (weight) of conductive carbonaceous fibers, if conductive carbonaceous fibers having a long fiber length are used, the number of conductive carbonaceous fibers used is reduced. The conductive carbonaceous fibers overlap each other, and the number of places bound by the binder resin is also reduced. As a result, the binding point interval at which the conductive carbonaceous fibers are bound to each other is also increased. On the other hand, if conductive carbonaceous fibers having a short fiber length are used, the number of conductive carbonaceous fibers to be used increases, so that the conductive carbonaceous fibers overlap and the number of places bound by the binder resin also increases. As a result, the binding point interval at which the conductive carbonaceous fibers are bound to each other is also reduced.

  In addition, even when conductive carbonaceous fibers having the same fiber length are used, if the amount of binder resin used is large, the distance between the binding points of the conductive carbonaceous fibers becomes small. The interval between the binding points of the carbonaceous fibers becomes large.

  The binding point interval of the conductive carbonaceous fiber in the gas diffusion layer can be calculated as follows. First, a load is applied in the thickness direction of the gas diffusion layer, the thickness of the gas diffusion layer at that time is measured, a thickness-load curve indicating the relationship between the load and the thickness of the gas diffusion layer is obtained, and the thickness direction of the gas diffusion layer is determined. Calculate the Young's modulus at. Next, since there is a correlation between the Young's modulus in the thickness direction of the gas diffusion layer, the fiber diameter of the conductive carbonaceous fiber, and the interval between the binding points, the fiber diameter of the used conductive carbonaceous fiber and the above are obtained. Further, the binding point interval can be calculated from the value of the Young's modulus of the gas diffusion layer.

The gas diffusion layer having both the distributions of (A) and (B) includes the elastic force in the thickness direction due to the fiber diameter distribution of the conductive carbonaceous fibers and the difficulty of sticking the conductive carbonaceous fibers, and the conductive carbonaceous fibers. Conductivity to form the gas diffusion layer while ensuring excellent elasticity in the thickness direction by a synergistic effect with the elastic force in the thickness direction due to the spacing between the binding points and the stiffness of the conductive carbonaceous fiber It is possible to suppress damage to the electrolyte membrane due to the carbonaceous fibers sticking into the electrolyte membrane.
That is, the membrane / electrode assembly according to the present invention can be used even when a dimensional change in the thickness direction of a member constituting a single cell such as an electrolyte membrane occurs or when a load applied in the stacking direction of the single cell changes. The elastic force of the diffusion layer suppresses contact resistance reduction and damage to each member, and exhibits stable power generation performance. Furthermore, since the short circuit and the cross leak due to the conductive carbonaceous fiber constituting the gas diffusion layer passing through the catalyst layer and the electrolyte membrane are prevented, the power generation performance is excellent and the durability is high.

The gas diffusion layer having the above distributions (A) and (B) may be provided in at least one of the oxidant electrode and the fuel electrode. The present invention includes the durability of the membrane / electrode assembly, the power generation performance, and the like. In order to sufficiently enhance the effect obtained by the above, it is preferable that both the oxidant electrode and the fuel electrode include gas diffusion layers having the distributions (A) and (B).
In the present invention, the distributions (A) and (B) described above may change stepwise or in an inclined manner in the thickness direction of the gas diffusion layer. In the case of having a stepwise distribution, the number of steps is not particularly limited, but from the viewpoint of securing elastic force in the thickness direction and preventing piercing of the conductive carbonaceous fibers into the electrolyte membrane, it is about 2 to 3 steps. In particular, two stages are particularly preferable. Note that the forms of changes in the distribution of (A) and (B) may not be the same as each other. For example, one of them may change in an inclined shape, the other may change in a stepped shape, Even in the case of a change, the number of stages may be different, or the number of stages may be the same or the boundary position of the distribution may be different.

  Here, the gas diffusion layer having the distributions (A) and (B) that change stepwise will be described. In FIG. 2, the gas diffusion layer 5 has a two-layer structure in which a first gas diffusion layer 5 ′ and a second gas diffusion layer 5 ″ are laminated in order from the electrolyte membrane 1 side.

The first gas diffusion layer disposed on the electrolyte membrane side is composed of conductive carbonaceous fibers having a larger average fiber diameter than the second gas diffusion layer (distribution (A)). If the average fiber diameter of the conductive carbonaceous fibers constituting the first gas diffusion layer is larger than that of the second gas diffusion layer, the conductivity constituting the first gas diffusion layer and the second gas diffusion layer The specific fiber diameter of the carbonaceous fiber is not particularly limited.
From the viewpoint of preventing the conductive carbonaceous fibers from sticking into the electrolyte membrane, the average fiber diameter of the conductive carbonaceous fibers constituting the first gas diffusion layer is preferably 7 μm or more. Further, from the viewpoint of elasticity in the thickness direction of the gas diffusion layer, the average fiber diameter of the conductive carbonaceous fibers constituting the second gas diffusion layer is preferably 5 μm or less.

In addition, the first gas diffusion layer disposed on the electrolyte membrane side has a smaller average value of the binding point intervals of the conductive carbonaceous fibers than the second gas diffusion layer (distribution (B)). If the average binding point interval of the conductive carbonaceous fibers in the first gas diffusion layer is smaller than that in the second gas diffusion layer, the specific average in the first gas diffusion layer and the second gas diffusion layer The interval between the binding points is not particularly limited.
From the viewpoint of preventing the conductive carbonaceous fibers from sticking to the electrolyte membrane, the average binding point interval of the conductive carbonaceous fibers in the first gas diffusion layer is preferably less than 60 μm. In addition, from the viewpoint of elasticity in the thickness direction of the gas diffusion layer, the average binding point interval of the conductive carbonaceous fibers in the second gas diffusion layer is preferably 100 μm or more.

  In the gas diffusion layer having the above two-layer structure, the thickness of the first gas diffusion layer and the second gas diffusion layer is not particularly limited, but from the viewpoint of ensuring elasticity in the thickness direction of the gas diffusion layer. The second gas diffusion layer is preferably thicker than the first gas diffusion layer. The first gas diffusion layer is preferably as thin as possible if the conductive carbonaceous fibers constituting the gas diffusion layer can be prevented from sticking into the electrolyte membrane.

  The conductive carbonaceous fiber can be used without particular limitation as long as it can be used as fibrous carbon forming the gas diffusion layer. The binder resin is not particularly limited as long as it is a resin capable of binding conductive carbonaceous fibers. For example, polytetrafluoroethylene or the like can be used.

The structure of the membrane / electrode assembly of the present invention having the gas diffusion layer as described above is not particularly limited. Here, FIG. 1 will be described as an example.
Examples of the electrolyte membrane 1 include, for example, a fluorine-based electrolyte membrane such as a perfluorocarbon sulfonic acid resin membrane represented by Nafion (trade name), and a hydrocarbon resin such as polyether ketone, polyether ether ketone, and polyether sulfone. Commonly used ones such as hydrocarbon electrolyte membranes into which ion exchange groups such as sulfonic acid groups, phosphoric acid groups, and carboxyl groups are introduced can be used.

The catalyst layer 4 is usually formed using a catalyst ink containing an electrode catalyst having catalytic activity for an electrode reaction in each electrode and an electrolyte material.
As the electrode catalyst, one in which a catalyst component is supported on conductive particles is usually used. The catalyst component is not particularly limited as long as it has catalytic activity for the oxidation reaction of the fuel at the fuel electrode or the reduction reaction of the oxidant at the oxidant electrode, and is generally used for polymer fuel cells. What is used for can be used. For example, platinum or an alloy of platinum and a metal such as ruthenium, iron, nickel, manganese, cobalt, copper, and the like can be given. As the conductive particles as the catalyst carrier, carbon particles such as carbon black, conductive carbon materials such as carbon fibers, and metal materials such as metal particles and metal fibers can also be used.
As the electrolyte material, the electrolyte resin exemplified as the electrolyte resin constituting the electrolyte membrane can be used.

  The catalyst ink is obtained by dissolving or dispersing the above catalyst and an electrolyte material in a solvent. The solvent of the catalyst ink may be appropriately selected. For example, alcohols such as methanol, ethanol and propanol, organic solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), or organic solvents such as these Mixtures and mixtures of these organic solvents and water can be used. In addition to the electrode catalyst and the electrolyte material, the catalyst ink may contain other components such as a binder and a water repellent resin as necessary.

  The method for forming the catalyst layer is not particularly limited. For example, the catalyst layer may be formed on the surface of the gas diffusion layer sheet to be the gas diffusion layer by applying and drying the catalyst ink on the surface of the gas diffusion layer, Alternatively, the catalyst layer may be formed on the electrolyte membrane surface by applying and drying the catalyst ink on the electrolyte membrane surface. Alternatively, a transfer sheet is prepared by applying and drying a catalyst ink on the surface of the transfer substrate, and the transfer sheet is joined to the electrolyte membrane or gas diffusion layer sheet by thermocompression bonding or the like, and the catalyst is formed on the electrolyte membrane surface. A layer may be formed, or a catalyst layer may be formed on the surface of the gas diffusion layer sheet.

The method for applying the catalyst ink, the drying method, and the like can be selected as appropriate. For example, examples of the coating method include a spray method, a screen printing method, a doctor blade method, a gravure printing method, and a die coating method. Examples of the drying method include reduced-pressure drying, heat drying, and reduced-pressure heat drying. There is no restriction | limiting in the specific conditions in reduced pressure drying and heat drying, What is necessary is just to set suitably.
The amount of catalyst ink applied varies depending on the composition of the catalyst ink and the catalyst performance of the catalyst metal used in the electrode catalyst, but the amount of catalyst component per unit area is about 0.1 to 2.0 mg / cm ^2. What should I do? The thickness of the catalyst layer is not particularly limited, but may be about 1 to 50 μm.

  A water-repellent layer can be provided between the gas diffusion layer and the catalyst layer as necessary. The water-repellent layer usually has a porous structure containing conductive particles such as carbon particles and carbon fibers, water-repellent resin such as polytetrafluoroethylene (PTFE), and the like. The water-repellent layer is not always necessary, but it can improve the drainage of the gas diffusion layer while maintaining an appropriate amount of water in the catalyst layer and the electrolyte membrane. There is an advantage that electrical contact can be improved.

  The method for forming the water repellent layer between the gas diffusion layer and the catalyst layer is not particularly limited. For example, water repellent obtained by mixing conductive particles such as carbon particles, water repellent resin, and other components as necessary with an organic solvent such as ethanol, propanol, propylene glycol, water or a mixture thereof. The layer ink may be applied to the side of the gas diffusion layer sheet facing the catalyst layer, and then dried and / or fired. The thickness of the water repellent layer may usually be about 1 to 150 μm. Examples of the method of applying the water repellent layer ink to the gas diffusion layer sheet include a screen printing method, a spray method, a doctor blade method, a gravure printing method, and a die coating method.

  The electrolyte membrane and gas diffusion layer sheet on which the catalyst layer has been formed by the above-described method are appropriately overlapped and subjected to thermocompression bonding or the like, and joined together to obtain a membrane / electrode assembly.

  The produced membrane / electrode assembly is further sandwiched between separators to form a single cell. The separator has conductivity and gas sealing properties, and can function as a current collector and gas sealing body, for example, a carbon separator containing a high concentration of carbon fiber and made of a composite material with resin, metal A metal separator using a material can be used. Examples of the metal separator include those made of a metal material excellent in corrosion resistance, and those coated with a coating that enhances the corrosion resistance by coating the surface with carbon or a metal material excellent in corrosion resistance. .

[Reference Experiment 1: Relationship between Fiber Diameter and Bonding Point Spacing of Conductive Carbonaceous Fiber and Elasticity of Gas Diffusion Layer]
Using conductive carbonaceous fibers having an average fiber diameter of 5 μm, 6 μm, 7 μm, and 8 μm, the average binding point interval Lave is different (10 μm, 20 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 60 μm, 70 μm, 80 μm). A load was applied to the gas diffusion layer (binder resin: PTFE) in the thickness direction, the amount of change in thickness (μm) was measured, and the elasticity in the film thickness direction was examined. The results are shown in FIG.
As shown in FIG. 4, the elasticity in the thickness direction of the gas diffusion layer increased as the fiber diameter of the conductive carbonaceous fiber decreased and as the binding point interval increased.

[Reference Experiment 2: Relationship between Fiber Diameter of Conductive Carbonaceous Fiber, Elasticity of Gas Diffusion Layer, and Leakage Current]
Using conductive carbonaceous fibers having an average fiber diameter of 5 μm, 6 μm, 7 μm, 8 μm, and 10 μm, for the gas diffusion layer (the same amount of binder and conductive carbonaceous fibers are used) as in Reference Experiment 2, The elasticity in the film thickness direction was examined. The results are shown in FIG.
Further, using each gas diffusion layer, a membrane / electrode assembly was produced as follows. Specifically, catalyst ink (platinum-supported carbon and perfluorocarbon sulfonic acid resin dispersed and dissolved in a solvent) was applied to both surfaces of the electrolyte membrane, dried, and then sandwiched between the two gas diffusion layers. With respect to the obtained membrane / electrode assembly, a current value when a constant voltage was applied was measured, and a leak current value was calculated. The results are shown in FIG.

As shown in FIG. 5, it can be seen that the smaller the fiber diameter of the conductive carbonaceous fiber, the larger the leak current value, that is, the short circuit is likely to occur. Moreover, it turns out that the elasticity of a gas diffusion layer becomes so high that the fiber diameter of electroconductive carbonaceous fiber is small.
The smaller the leakage current value, the better. However, since it is sufficient if it is 30 mA or less, it can be said that the electrolyte membrane side of the gas diffusion layer is preferably composed of conductive carbonaceous fibers having a fiber diameter of 7 μm or more. On the other hand, since the elasticity of the gas diffusion layer is preferably 12 μm or more, it can be said that the separator side of the gas diffusion layer is preferably composed of conductive carbonaceous fibers having a fiber diameter of 5 μm or less.

[Reference Experiment 3: Relationship between Bonding Point Spacing of Conductive Carbonaceous Fiber, Elasticity of Gas Diffusion Layer, and Leakage Current]
Using conductive carbonaceous fibers and a binder resin, gas diffusion layers having binding point intervals of 20 μm, 40 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, and 120 μm were prepared, and gas diffusion was performed in the same manner as in Reference Experiment 2. The elasticity of the layer in the film thickness direction and the leakage current value of the membrane / electrode assembly were calculated. The results are shown in FIG.

As shown in FIG. 6, it can be seen that the larger the binding point interval of the conductive carbonaceous fiber is, the larger the leak current value is, that is, a short circuit is likely to occur. Moreover, it turns out that the elasticity of a gas diffusion layer becomes so high that the binding point space | interval of electroconductive carbonaceous fiber is large.
The smaller the leak current value, the better. However, it is sufficient that the leakage current value is 30 mA or less. Therefore, it can be said that the electrolyte membrane side of the gas diffusion layer preferably has a binding point interval of conductive carbonaceous fibers of 60 μm or less. On the other hand, since the elasticity of the gas diffusion layer is preferably 12 μm or more, it can be said that the separator side of the gas diffusion layer preferably has an interval between the binding points of the conductive carbonaceous fibers of 100 μm or more.

It is sectional drawing which shows one example of a single cell provided with the membrane electrode assembly of this invention. It is a partial expanded sectional view of the membrane electrode assembly of the present invention. It is a schematic diagram explaining the gas diffusion layer of the membrane electrode assembly of this invention. 6 is a graph showing the results of Reference Experiment 1. 10 is a graph showing the results of Reference Experiment 2. 10 is a graph showing the results of Reference Experiment 3. It is sectional drawing which shows the example of a form of the conventional common cell for fuel cells.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electrolyte membrane 2 ... Fuel electrode (anode)
3 ... Oxidant electrode (cathode)
4a ... Anode side catalyst layer 4b ... Cathode side catalyst layer 5a ... Anode side gas diffusion layer 5b ... Cathode side gas diffusion layer 5 '... First gas diffusion layer 5 "... Second gas diffusion layer 6 ... Membrane / electrode junction Body 7a ... Anode-side separator 7b ... Cathode-side separator 8a ... Anode-side gas flow path 8b ... Cathode-side gas flow path 9 ... Conductive carbonaceous fiber 10 ... Electrolyte membrane 11 ... Fuel electrode (anode)
12 ... Oxidant electrode (cathode)
13a ... Anode side catalyst layer 13b ... Cathode side catalyst layer 14a ... Anode side gas diffusion layer 14b ... Cathode side gas diffusion layer 15 ... Membrane / electrode assembly 16a ... Anode side separator 16b ... Cathode side separator 17a ... Anode side gas flow path 17b ... Cathode side gas flow path 100 ... Single cell 200 ... Single cell

Claims (4)

A membrane-electrode assembly comprising an electrolyte membrane and a pair of electrodes that sandwich the electrolyte membrane,
At least one of the electrodes has a laminated structure in which a catalyst layer and a gas diffusion layer are laminated in order from the electrolyte membrane side,
The gas diffusion layer is
While having a structure in which a plurality of conductive carbonaceous fibers oriented in the fiber length direction in the plane direction of the gas diffusion layer are bound with a binder resin,
In the stacking direction of the stacked structure,
(A) a distribution in which the fiber diameter of the conductive carbonaceous fiber increases toward the electrolyte membrane;
(B) Among the binding points at which the conductive carbonaceous fiber and other conductive carbonaceous fibers bind in the fiber length direction of the conductive carbonaceous fiber, the adjacent binding point intervals in the fiber length direction. Is a distribution that decreases toward the electrolyte membrane side,
A membrane / electrode assembly comprising:   The gas diffusion layer includes, in order from the electrolyte membrane side, a first gas diffusion layer having an average fiber diameter of the conductive carbonaceous fiber of 7 μm or more and an average fiber diameter of the conductive carbonaceous fiber of 5 μm or less. The membrane / electrode assembly according to claim 1, which has a two-layer structure in which a second gas diffusion layer is laminated.   The gas diffusion layer includes, in order from the electrolyte membrane side, a first gas diffusion layer having an average binding point interval of less than 60 μm and a second gas diffusion having an average binding point interval of 100 μm or more. The membrane / electrode assembly according to claim 1, which has a two-layer structure in which layers are laminated.   4. The membrane-electrode assembly according to claim 2, wherein a film thickness of the second gas diffusion layer is larger than a film thickness of the first gas diffusion layer.