JP7383463B2 - Fuel cell - Google Patents

Description

The present invention relates to fuel cells.

Polymer electrolyte fuel cells generate electricity by causing a chemical reaction between hydrogen gas supplied as fuel gas and oxygen gas in the air. Generally, a polymer electrolyte fuel cell has a structure in which a membrane electrode assembly that performs the above chemical reaction is sandwiched between a pair of separators.

The membrane electrode assembly has an electrolyte membrane and a pair of electrodes on both sides of the electrolyte membrane. In order to reduce the resistance of membrane electrode assemblies and increase power generation efficiency, thinning the electrolyte membrane is being considered, but thinning tends to reduce the strength of the electrolyte membrane. Therefore, an electrolyte membrane having a porous membrane inside as a reinforcing layer has been proposed (see, for example, Patent Document 1).

International Publication No. 2016-056430

A flow path passing through the entire surface of the membrane electrode assembly is provided between the membrane electrode assembly and the separator as a fuel gas supply path. In order to uniformly supply fuel gas in the in-plane direction of the membrane electrode assembly, various flow path patterns such as straight, wavy, serpentine, etc. have been proposed.

Within the flow path, non-uniform distribution of fuel gas concentration, heat, pressure, etc. exists. For example, the concentration of fuel gas is highest at the entrance of the flow path, and gradually decreases as it is consumed downstream. At the cathode to which air is supplied as fuel gas, oxygen gas in the air is consumed, and the concentration of oxygen gas decreases from upstream to downstream in the flow path.

As this oxygen gas is consumed, the concentration of nitrogen gas in the air on the cathode side increases from upstream to downstream of the flow path. On the downstream side where the nitrogen gas concentration is high, nitrogen gas tends to leak from the cathode to the anode via the electrolyte membrane. Making the electrolyte membrane thinner may facilitate leakage of nitrogen gas.

The present invention aims to reduce leakage of fuel gas from one electrode to the other.

According to one aspect of the present invention, there is provided a fuel cell (10) that generates electricity through a chemical reaction of fuel gas, which includes an electrolyte membrane (1) and a pair of electrodes (2) disposed on both sides of the electrolyte membrane (1). ), a membrane electrode assembly (3) for chemically reacting the fuel gas, and a pair of separators arranged on both sides of the membrane electrode assembly (3) and sandwiching the membrane electrode assembly (3). , the fuel gas flow path (20) is provided between the separator (4) and the membrane electrode assembly (3), and the electrolyte membrane (1) is provided with the flow path (20) A fuel cell (10) is provided which is divided into a plurality of areas in the flow direction of the fuel gas in the fuel cell and has different gas shielding rates depending on the areas (E1 to E3).

According to the present invention, leakage of fuel gas from one electrode to the other electrode can be reduced.

FIG. 1 is an exploded perspective view schematically showing the configuration of a fuel cell according to the present embodiment. FIG. 3 is a plan view showing the surface of a separator. FIG. 3 is a plan view showing an example of an area of an electrolyte membrane. FIG. 7 is a plan view showing another example of the area of the electrolyte membrane. FIG. 2 is a cross-sectional view showing an example of a porous base material of an electrolyte membrane. It is a graph showing the shielding rate of gas in each area. It is a graph showing the leakage amount of nitrogen gas in each area. FIG. 3 is a cross-sectional view showing another example of a porous base material of an electrolyte membrane. FIG. 3 is a cross-sectional view showing another example of a porous base material of an electrolyte membrane. FIG. 3 is a cross-sectional view showing another example of a porous base material of an electrolyte membrane.

Hereinafter, one embodiment of the fuel cell of the present invention will be described with reference to the drawings. The configuration described below is an example (representative example) of the present invention, and is not limited thereto.

The fuel cell of this embodiment is mounted on a moving body such as a vehicle, generates electricity by chemically reacting fuel gas, and supplies driving power to the moving body; however, the present invention is not limited thereto; for example, The present invention can also be applied to fuel cells such as stationary power generation systems.

(Fuel cell)
FIG. 1 schematically shows the configuration of a fuel cell 10 of this embodiment.
As shown in FIG. 1, the fuel cell 10 includes a membrane electrode assembly (MEA) 3, a pair of separators 4 on both sides of the MEA 3, and a subgasket 5 surrounding the outer periphery of the MEA 3. .

The MEA 3 includes an electrolyte membrane 1 and a pair of electrodes 2, and causes fuel gas to undergo a chemical reaction. A pair of electrodes 2 are arranged on both sides of the electrolyte membrane 1.

(electrolyte membrane)
The electrolyte membrane 1 is an ion-conductive polymer electrolyte membrane. As the electrolyte membrane 1, for example, perfluorosulfonic acid polymers such as Nafion (registered trademark) and Aquivion (registered trademark); aromatic polymers such as sulfonated polyetheretherketone (SPEEK) and sulfonated polyimide; polyvinyl sulfonic acid; Examples include aliphatic polymers such as polyvinyl phosphoric acid.

The electrolyte membrane 1 of this embodiment is a composite membrane in which a porous base material 111 contains a polymer electrolyte. The porous base material 111 can improve the mechanical strength of the electrolyte membrane 1 and improve the durability of the fuel cell 10, and can also make the electrolyte membrane 1 thinner. By making the membrane thinner, the amount of water backdiffused from the cathode to the anode increases, so that the humidity dependence of the resistance and proton conductivity of the electrolyte membrane 1 can be reduced and power generation efficiency can be improved.

The porous base material 111 is not particularly limited as long as it can support a polymer electrolyte, and porous, woven, nonwoven, fibrillar, or other membranes can be used. Although the material of the porous base material 111 is not particularly limited, a polymer electrolyte as described above can be used from the viewpoint of improving ionic conductivity. Among them, fluororesins such as polytetrafluoroethylene, polytetrafluoroethylene-chlorotrifluoroethylene copolymer, and polychlorotrifluoroethylene have excellent strength and shape stability.

The porous base material 111 is obtained by forming the polymer electrolyte into a film using a known method such as extrusion molding or rolling. Further, the porous base material 111 can also be obtained by forming a nonwoven fabric-like membrane using an electrospray method. Among these, stretching after extrusion molding is preferable since the porosity of the porous base material 111 can be easily adjusted.

The porosity of the porous base material 111 is usually 35 to 90%. From the viewpoint of sufficiently impregnating the polymer electrolyte, the porosity is preferably 40% or more, more preferably 50% or more. From the viewpoint of obtaining sufficient strength, the porosity is preferably 80% or less, more preferably 70% or less.
The porosity can be determined as the area ratio (%) occupied by pores within a certain area when a cross section is observed using a transmission or scanning electron microscope, for example.

(electrode)
One of the pair of electrodes 2 is an anode, also called a fuel electrode. The other electrode 2 is a cathode, also called an air electrode. As fuel gas, hydrogen gas is supplied to the anode, and air containing oxygen gas is supplied to the cathode. In FIG. 1, black arrows represent air, and white arrows represent hydrogen gas.

At the anode, a reaction occurs that generates electrons (e ⁻ ) and protons (H ⁺ ) from hydrogen gas (H ₂ ). Electrons move to the cathode via an external circuit (not shown). This movement of electrons generates a current in the external circuit. Protons move to the cathode via the electrolyte membrane 1.

At the cathode, oxygen ions (O ₂ ⁻ ) are generated from oxygen gas (O ₂ ) by electrons transferred from an external circuit. The oxygen ions combine with protons (2H ⁺ ) transferred from the electrolyte membrane 1 to become water (H ₂ O).

The electrode 2 includes a catalyst layer 21 . The electrode 2 of this embodiment further includes a gas diffusion layer 22 to improve the diffusibility of fuel gas.

The catalyst layer 21 promotes the reaction of hydrogen gas and oxygen gas using a catalyst. The catalyst layer 21 includes a catalyst, a carrier supporting the catalyst, and an ionomer covering these.
Examples of the catalyst include metals such as platinum (Pt), ruthenium (Ru), iridium (Ir), rhodium (Rh), palladium (Pd), and tungsten (W), and mixtures and alloys of these metals. Among these, platinum, platinum-containing mixtures, alloys, and the like are preferred from the viewpoints of catalytic activity, carbon monoxide poisoning resistance, heat resistance, and the like.

Examples of the carrier include conductive porous metal compounds having pores such as mesoporous carbon and Pt black. Mesoporous carbon is preferred from the viewpoints of good dispersibility, large surface area, and little particle growth at high temperatures even when a large amount of catalyst is supported.
As the ionomer, an ion-conductive polymer electrolyte similar to that of the electrolyte membrane 1 can be used.

The gas diffusion layer 22 can uniformly diffuse the fuel gas supplied to the fuel cell 10 over the entire surface of the catalyst layer 21 .
The gas diffusion layer 22 can be formed by arranging a gas diffusion layer sheet as the outermost layer of the MEA 3. Examples of the gas diffusion layer sheet include porous fiber sheets such as carbon fibers having electrical conductivity, gas permeability, and gas diffusivity, as well as metal sheet materials such as foamed metal and expanded metal.

The subgasket 5 is a film or plate that surrounds the outer periphery of the MEA 3 and functions as a support for the MEA 3. As the material of the subgasket 5, a resin with low conductivity can be used. The resin material is not particularly limited, and examples thereof include polyphenylene sulfide (PPS), glass-filled polypropylene (PP-G), polystyrene (PS), silicone resin, and fluororesin.

(Separator)
The separator 4 is also called a bipolar plate. As the material of the separator 4, for example, a conductive material such as carbon or stainless steel is used.

FIG. 2 shows the surface of the separator 4.
As shown in FIG. 2, four through holes P1 to P4 are provided at the four corners of the separator 4. The through hole P1 is an air supply port, and the through hole P2 is an air discharge port. The through hole P3 is a hydrogen gas supply port, and the through hole P4 is a hydrogen gas discharge port.

The separator 4 of this embodiment has a surface provided with a recess 4a. Specifically, the separator 4 of this embodiment includes a plurality of ribs 4b on its surface, and a recess 4a is provided between each rib 4b. Note that as long as the recess 4a can be provided on the surface of the separator 4, a groove may be provided on the surface of the separator 4, or both the rib 4b and the groove may be provided.

When the surface of the separator 4 provided with the recess 4a faces the MEA 3, a flow path 20 is provided between the separator 4 and the MEA 3. The flow path 20 is provided over the entire surface of the electrolyte membrane 1 . The flow path 20 is not only a supply path for fuel gas but also a discharge path for water generated in the electrolyte membrane 1 during power generation.

The recess 4a of the separator 4 on the cathode side forms an air flow path 20 communicating from the through hole P1 to the through hole P2. The recess 4a of the separator 4 on the anode side forms a hydrogen gas flow path 20 communicating from the through hole P3 to the through hole P4. The flow path 20 has an area larger than the electrolyte membrane 1.

(Gas shielding rate of electrolyte membrane)
The electrolyte membrane 1 of this embodiment is divided into a plurality of areas in the air flow direction of the channel 20 on the cathode side. The gas shielding rate of the porous base material 111 in the downstream area in the flow direction is higher than that in the upstream area.

FIG. 3 shows an example of the area of the electrolyte membrane 1.
As shown in FIG. 3, the electrolyte membrane 1 is divided into three parts: an area E1 that is the most upstream in the direction of air flow from the through hole P1 to the through hole P2, and an area that is downstream of the area E1 and upstream of the area E3. area E2 and the most downstream area E3.

The example of dividing the area of the electrolyte membrane 1 is not limited to this, as long as it is divided into a plurality of areas from upstream to downstream in the flow direction. For example, the number of divisions is not limited to three, and may be two or four or more. Further, since the flow direction differs depending on the pattern of the flow path 20, the area of the electrolyte membrane 1 may be divided according to the pattern.

FIG. 4 shows another example of the area of the electrolyte membrane 1. Areas E1 to E6 shown in FIG. 4 can be used, for example, as a flow path 20 in which air meanders obliquely from the through hole P1 to reach the through hole P2.

FIG. 5 is a sectional view taken along line II-II in FIG. 3.
As shown in FIG. 5, the porous base material 111 in the electrolyte membrane 1 is composed of a part a1 in an area E1, a part a2 in an area E2, and a part a3 in an area E3. The gas shielding rate of each part a1 to a3 is highest in part a3 in area E3 on the most downstream side, next highest in part a2 in area E2, and lowest in part a1 in area E1 on the most upstream side.

Oxygen gas in the air flowing through the channel 20 on the cathode side undergoes a chemical reaction and is consumed from the upstream side. The oxygen gas concentration decreases as it goes downstream, and the nitrogen gas concentration in the air flowing through the flow path 20 relatively increases as it goes downstream. That is, the nitrogen gas concentration is lowest in area E1, higher in area E2 than area E1, and highest in area E3.

In this way, as the nitrogen gas concentration increases toward the downstream side, the gas shielding rate of the porous base material 111 also increases. Since the higher the nitrogen gas concentration in the area, the higher the nitrogen gas shielding rate, leakage of nitrogen gas from the cathode to the anode can be effectively suppressed. Nitrogen gas leaking from the cathode side to the anode side via the electrolyte membrane 1 can prevent the hydrogen gas concentration on the anode side from decreasing and the hydrogen utilization rate from decreasing.

Note that the gas shielding rate can be determined as the rate at which the fuel gas is shielded by the porous base material 111. For example, the gas shielding rate can be determined by determining the difference between the gas concentration (V1) on one surface side of the porous base material 111 to which fuel gas is supplied and the gas concentration (V2) on the other surface side. It can be expressed as a 100th fraction {(V1-V2)/V1×100} (%) when divided by the gas concentration (V1) on one surface side. The larger the value of this gas shielding rate, the more easily the gas is shielded.

FIG. 6 shows the gas shielding rate L (%) in each area E1 to E3. FIG. 7 shows the leakage amount M (vol%) of nitrogen gas from the cathode to the anode.
As shown in FIG. 6, a general porous base material has a constant gas shielding rate L0 regardless of the areas E1 to E3. As shown in FIG. 7, the leakage amount M0 of nitrogen gas when the gas shielding rate L0 increases toward the downstream side.

On the other hand, as shown in FIG. 6, the gas shielding rate L1 of the porous base material 111 of this embodiment becomes higher in the downstream area. Therefore, as shown in FIG. 7, the leakage amount M1 of nitrogen gas when the gas shielding rate L1 is compared with the leakage amount M0 when the gas shielding rate L0 is constant, the nitrogen gas leakage amount M1 in the case of the gas shielding rate L1 is higher than that in the downstream areas E2 and E3. is suppressed in

The parts a1 to a3 having different gas shielding rates may be formed as one film, or may be formed as separate films and bonded together with an adhesive. The adhesive is not particularly limited, and any known adhesive used for bonding resins can be used.

Self-healing polymers can also be used as adhesives. As the self-healing polymer, known materials can be used. As known materials, see, for example, Japanese Patent Application Publication No. 2018-39876, International Publication No. 2017/159346, and the document "Synthesis of Self-Healing Polymers by Scandium-Catalyzed Copolymerization of Ethylene and Anisylpropylenes" (Haobing Wang and five others, J. Am. Chem. Soc., American Chemical Society, 2019, 141, p. 3249-3257), but are not limited thereto.

The gas shielding rate of each part a1 to a3 can be adjusted by changing the porosity of the porous base material 111. The smaller the porosity, the higher the gas shielding rate.
For example, in the case of the porous base material 111 obtained by stretching a PTFE resin after extrusion molding, the porosity also decreases if the stretching ratio is decreased. When each part a1 to a3 is one PTFE membrane, the stretching ratio when stretching the PTFE membrane in the transverse direction (TD) with a tenter may be increased stepwise in the machine direction (MD). In the case of the porous base material 111 obtained by extruding a PTFE resin into a nonwoven fabric, the porosity is reduced by increasing the density at which the resin is extruded or by extruding it in multiple layers.

Further, the gas shielding rate of each part a1 to a3 can be adjusted by changing the thickness of the porous base material 111. The thicker the porous base material 111, the higher the gas shielding rate.
For example, the thickness of the porous base material 111 can be adjusted by adjusting the thickness, density, stretching ratio, etc. when extruding the PTFE resin, similar to the porosity.

FIG. 8 shows an example of a porous base material 112 having different thicknesses in each area E1 to E3.
As shown in FIG. 8, the porous base material 112 is composed of part b1 in area E1, part b2 in area E2, and part b3 in area E3. The porosity of each part b1 to b3 is the same, but the part closer to the most downstream area E3 is thicker and has a higher gas shielding rate.

The gas shielding rate may be adjusted by both the porosity and thickness of the porous base material 111.
9 and 10 show an example in which the gas shielding rate is adjusted by the porosity and thickness.

The porous base material 113 shown in FIG. 9 is composed of a part c1 in an area E1, a part c2 in an area E2, and a part c3 in an area E3. The part c1 on the most upstream side is composed only of the first layer 71 with a low porosity, but the part c2 has a second layer 72 with a higher porosity than the first layer 71, which is twice that of the part c1. It has a thickness of The part c3 on the most downstream side has a third layer 73 having the highest porosity layered on the first layer 71 and the second layer 72, and has a thickness three times that of the part c1. Since the downstream area has higher thickness and porosity, the gas shielding rate is adjusted to be higher.

In the porous base material 114 shown in FIG. 10, the thickness of the part c1 in the area E1 of the porous base material 113 is tripled, and the thickness of the second layer 72 of the part c2 in the area E2 is doubled. There is. The porous base material 114 is adjusted to have a higher gas shielding rate in areas E2 and E3 than the porous base material 113, and has higher strength.

The porous base materials 113 and 114 can be formed by, for example, forming the first layer 71, the second layer 72, and the third layer 73 individually and bonding them together, or by forming each part c1 to c3 individually and bonding them together. can get.

Note that the gas shielding rate may have a gradient within each of the areas E1 to E3. For example, the gas shielding rate may gradually increase from the upstream side to the downstream side within each of the areas E1 to E3.

When electrolyte membranes 1 having the same equivalent mass (EW) have different gas shielding rates, the resistance value of the electrolyte membrane 1 tends to gradually increase. In order to suppress such an increase in resistance, the EW of the electrolyte membrane 1 may be configured to be higher in areas where the gas shielding rate is higher. If the EW is high, even if the gas shielding rate is high, the reactivity of the fuel gas will be good, and a decrease in power generation performance can be suppressed. EW is the reciprocal of ion exchange capacity (IEC), which represents the number of ion exchange groups per unit mass of the resin, and can usually be determined by acid-base titration, quantification of sulfur atoms, FT-IR, etc. .

The EW of the electrolyte membrane 1 can be made different by, for example, impregnating each area of the porous substrates 111 to 114 with a polymer electrolyte having a different EW.

As described above, according to the fuel cell 10 of the present embodiment, the electrolyte membrane 1 is divided into a plurality of areas in the air flow direction of the flow path 20, and the gas shielding rate of the downstream area is lower than that of the upstream area. higher than Although the nitrogen gas concentration increases as the air moves downstream in the flow direction, the gas shielding rate of the electrolyte membrane 1 also increases, so leakage of nitrogen gas from the cathode to the anode can be suppressed. It is possible to prevent the hydrogen gas concentration on the anode side from decreasing due to the leaked nitrogen gas and the hydrogen utilization rate from decreasing.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the gist.

In the embodiment described above, the electrolyte membrane 1 is divided into a plurality of areas in the air flow direction in order to suppress leakage of nitrogen gas from the cathode to the anode. However, the present invention is not limited to this, as long as the electrolyte membrane 1 is divided into a plurality of areas in the flow direction of the fuel gas in the flow path 20 and has a different gas shielding rate depending on the area.

For example, in order to suppress leakage of hydrogen gas from the anode to the cathode, the electrolyte membrane 1 is divided into multiple areas in the direction of flow of hydrogen gas, and the gas shielding rate of the upstream area where the hydrogen gas concentration is higher than that of the downstream side. can also be made higher.

DESCRIPTION OF SYMBOLS 10... Fuel cell, 3... MEA, 1... Electrolyte membrane, 111-114... Porous base material, E1-E3... Area, 2... Electrode, 20... Flow 4... Separator

Claims (6)

A fuel cell (10) that generates electricity by a chemical reaction of fuel gas,
a membrane electrode assembly (3) comprising an electrolyte membrane (1) and a pair of electrodes (2) disposed on both sides of the electrolyte membrane (1), and causing a chemical reaction in the fuel gas;
A pair of separators (4) arranged on both sides of the membrane electrode assembly (3) and sandwiching the membrane electrode assembly (3),
The fuel gas flow path (20) is provided between the separator (4) and the membrane electrode assembly (3),
The electrolyte membrane (1) is divided into a plurality of areas (E1 to E3) in the flow direction of the fuel gas in the flow path (20), and has a different gas shielding rate depending on the area (E1 to E3). ,
The electrolyte membrane (1) includes a porous base material (111 to 114) containing a polymer electrolyte,
The larger the gas shielding rate in the area (E1 to E3), the smaller the porosity of the porous base material (111 to 114).
Fuel cell (10).
The flow direction of the fuel gas is the flow direction of air supplied to the cathode side of the pair of electrodes (2),
The shielding rate of the gas in the downstream area (E1 to E3) in the air flow direction is higher than the upstream area (E1 to E3),
A fuel cell (10) according to claim 1. The electrolyte membrane (1) includes a porous base material (111 to 114) containing a polymer electrolyte,
The larger the gas shielding rate is in the area (E1 to E3), the thicker the porous base material (111 to 114) is.
The fuel cell (10) according to claim 1 or claim 2 . A fuel cell (10) that generates electricity by a chemical reaction of fuel gas,
a membrane electrode assembly (3) comprising an electrolyte membrane (1) and a pair of electrodes (2) disposed on both sides of the electrolyte membrane (1), and causing a chemical reaction in the fuel gas;
A pair of separators (4) arranged on both sides of the membrane electrode assembly (3) and sandwiching the membrane electrode assembly (3),
The fuel gas flow path (20) is provided between the separator (4) and the membrane electrode assembly (3),
The electrolyte membrane (1) is divided into a plurality of areas (E1 to E3) in the flow direction of the fuel gas in the flow path (20), and has a different gas shielding rate depending on the area (E1 to E3). ,
The electrolyte membrane (1) includes a porous base material (111 to 114) containing a polymer electrolyte,
The larger the gas shielding rate is in the area (E1 to E3), the thicker the porous base material (111 to 114) is.
Fuel cell (10). The equivalent mass of the electrolyte membrane (1) is higher in areas (E1 to E3) where the gas shielding rate is higher;
A fuel cell (10) according to any one of claims 1 to 4. A fuel cell (10) that generates electricity by a chemical reaction of fuel gas,
a membrane electrode assembly (3) comprising an electrolyte membrane (1) and a pair of electrodes (2) disposed on both sides of the electrolyte membrane (1), and causing a chemical reaction in the fuel gas;
A pair of separators (4) arranged on both sides of the membrane electrode assembly (3) and sandwiching the membrane electrode assembly (3),
The fuel gas flow path (20) is provided between the separator (4) and the membrane electrode assembly (3),
The electrolyte membrane (1) is divided into a plurality of areas (E1 to E3) in the flow direction of the fuel gas in the flow path (20), and has a different gas shielding rate depending on the area (E1 to E3). ,
The equivalent mass of the electrolyte membrane (1) is higher in areas (E1 to E3) where the gas shielding rate is higher;
Fuel cell (10).

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