JP2020155386A - Fuel battery cell - Google Patents

Abstract

To provide a fuel cell for achieving both flooding resistance and reduction of contact resistance between a separator and a gas diffusion layer.SOLUTION: The fuel battery cell is composed of a proton conductive polymer electrolyte membrane, a pair of electrodes sandwiching the polymer electrolyte membrane, and a pair of separators sandwiching the electrodes. The electrodes include a catalyst layer on a side where the electrodes are in contact with the polymer electrolyte membrane and a gas diffusion layer on a side where the electrodes are in contact with the separators. A ratio of an amount of fluorine on a surface in contact with the catalyst layer to an amount of fluorine on a surface of the gas diffusion layer on the side in contact with the separators is 1.5 or more and 11 or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a fuel cell.

A fuel cell supplies chemical energy directly to electricity by supplying fuel gas (hydrogen gas) and oxidant gas (oxygen gas) to two electrically connected electrodes and electrochemically causing oxidation of the fuel. Convert to energy. This fuel cell is usually configured by stacking a plurality of fuel cell cells (single cells) having a membrane electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes. Among them, the polymer electrolyte type fuel cell using the proton conductive polymer electrolyte membrane as the electrolyte membrane has advantages such as easy miniaturization and operation at a low temperature, and is particularly portable. It is attracting attention as a power source for mobile bodies.

In the polymer electrolyte fuel cell, the reaction of the following equation (1) proceeds at the anode (fuel electrode) to which hydrogen is supplied.
_{^{H 2 → 2H + + 2e -}} ··· (1)

The electrons generated by the above equation (1) reach the cathode (oxidizing agent electrode) after working under an external load via an external circuit. On the other hand, the protons generated by the above equation (1) move from the anode side to the cathode side in the polymer electrolyte membrane by electroosmosis in a state of being hydrated with water.

On the other hand, at the cathode, the reaction of the following equation (2) proceeds.
2H ^＋ ＋ 1 / 2O ₂ ＋ 2e ⁻ → H ₂ O ・ ・ ・ (2)

Therefore, the chemical reaction shown in (3) below proceeds in the entire battery, electromotive force is generated, and electrical work is performed on the external load.
H ₂ + 1 / 2O ₂ → H ₂ O ・ ・ ・ (3)

Each electrode of the anode and the cathode generally has a structure in which a catalyst layer and a gas diffusion layer are laminated in this order from the electrolyte membrane side. The catalyst layer usually contains an electrode catalyst such as platinum or a platinum alloy for promoting the electrode reaction, a polymer electrolyte for ensuring proton conductivity, and a conductive material for ensuring electron conductivity. ing. Further, the gas diffusion layer is usually formed by using a conductive porous body capable of supplying the reaction gas to the catalyst layer, discharging excess water in the electrode, and the like.

In a fuel cell provided with a polymer electrolyte membrane represented by a perfluorocarbon sulfonic acid resin membrane, it is important to maintain a wet state of the electrolyte membrane and the catalyst layer in order to ensure ionic conductivity. Therefore, in general, the reaction gas is supplied to the electrode in a pre-humidified state.

On the other hand, in a fuel cell, as described above, water is generated along with power generation. Most of the generated water is discharged to the outside of the cell together with the unreacted gas (reaction gas that did not contribute to the electrode reaction) discharged from the electrode and the humidified water supplied as water vapor. However, a part of the generated water and the humidified water supplied as water vapor to the electrode together with the reaction gas is condensed depending on the environment in the fuel cell and exists in the electrode in a liquid state. At this time, if the condensed water (liquid water) stays in the electrode, the liquid water is clogged, that is, so-called flooding occurs, the supply of the reaction gas is hindered, and the power generation performance deteriorates. In particular, when the fuel cell is operated under high humidification conditions, a large amount of liquid water is generated, so it is necessary to ensure the discharge of liquid water from the electrodes.

Therefore, it is desired that the liquid water in the electrode is quickly discharged to the outside of the cell to prevent the liquid water from staying in the electrode.

As a gas diffusion layer for suppressing this flooding, a structure in which a microporous layer (hereinafter referred to as MPL) having a pore diameter smaller than that of the porous base material layer is laminated on a porous base material layer having a porous structure. Those with are known. It is said that by providing this MPL, the ultrafine porous structure of the MPL inhibits the formation of a liquid water film and improves the drainage property of the gas diffusion layer. Further, before forming the MPL, a water-repellent substance may be impregnated into the porous base material layer to perform a water-repellent treatment (see Patent Documents 1 to 3).

Japanese Unexamined Patent Publication No. 2001-085019 International Publication No. 2015/12575 International Publication No. 03/034519

Such MPL is formed by applying an MPL forming coating solution containing a carbon-based filler and a water-repellent material such as fluororesin to a porous base material layer and firing the MPL. The water-repellent material in the coating liquid for forming may permeate into the porous base material layer, and a phenomenon of strike-through that penetrates the porous base material layer may occur. When the water-repellent material penetrates into the porous base material layer and strike-through occurs, the surface of the gas diffusion layer opposite to the surface on which the MPL is formed also becomes water-repellent, but the fuel cell was assembled. At that time, the electric resistance and the thermal resistance at the interface with the separator increase, and as a result, the voltage of the fuel cell decreases and the output decreases. Further, the same problem occurs when a water-repellent substance is impregnated into the porous base material layer before forming the MPL.

The present invention has been made in view of the above circumstances, and reduces the contact resistance of the gas diffusion layer having MPL formed at the interface with the separator to achieve both flood resistance and reduction of contact resistance between the separator / gas diffusion layer. It is an object of the present invention to provide a fuel cell.

The present invention achieves the above object by the following means.

A fuel cell composed of a proton-conducting polymer electrolyte membrane, a pair of electrodes sandwiching the polymer electrolyte membrane, and a pair of separators sandwiching the electrodes, wherein the electrodes are the polymer electrolyte membrane. A catalyst layer is provided on the contacting side, and a gas diffusion layer is provided on the side in contact with the separator. A fuel cell that is 1.5 or more and 11 or less.

According to the fuel cell of the present invention, the ratio of the amount of fluorine on the surface of the gas diffusion layer on the surface in contact with the separator to the amount of fluorine on the surface in contact with the separator is 1.5 or more and 11 or less. The surface in contact with the layer has a high proportion of fluorine and thus has high water repellency, while the surface in contact with the separator has a low proportion of fluorine, that is, a high proportion of carbon and thus has a low electron resistance. Therefore, in the fuel cell using this gas diffusion layer, both flooding resistance and reduction of contact resistance between the separator / gas diffusion layer can be achieved.

It is sectional drawing which shows the structure of a fuel cell. It is sectional drawing which shows the structure of a gas diffusion layer. It is a graph which shows the measurement result of the amount of fluorine (%) in each part of the gas diffusion layer in an Example and a comparative example. It is a graph which shows the measurement result of the power generation performance of the fuel cell which incorporated the gas diffusion layer manufactured in the Example and the comparative example.

The fuel cell of the present invention is composed of a proton conductive polymer electrolyte membrane, a pair of electrodes sandwiching the polymer electrolyte membrane, and a pair of separators sandwiching the electrodes, and the electrodes are the polymer electrolyte membrane. A catalyst layer is provided on the side in contact with the separator, and a gas diffusion layer is provided on the side in contact with the separator. , 1.5 or more and 11 or less.

As described above, in the conventional gas diffusion layer provided with MPL, the water-repellent material in the MPL forming coating liquid permeates into the porous base material layer. Fluororesin, which is generally used as this water-repellent material, has an insulating property, and when the water-repellent material permeates, the surface of the gas diffusion layer in contact with the separator becomes insulating, and electrical conduction at the interface with the separator. The property and thermal conductivity are reduced.

On the other hand, according to the present invention, the ratio of the amount of fluorine on the surface of the gas diffusion layer forming the MPL to the amount of fluorine on the surface in contact with the separator is 1.5 or more and 11 or less. While establishing flood resistance by increasing the amount of fluorine on the surface of the gas diffusion layer in contact with the catalyst layer, by decreasing the amount of fluorine on the surface of the gas diffusion layer in contact with the separator, the gas diffusion layer and the gas diffusion layer The contact resistance at the interface of the separator can be reduced, and the decrease in electrical conductivity can be suppressed.

Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist of the present invention.

<Fuel cell configuration>
FIG. 1 is an explanatory diagram showing a schematic configuration of a fuel cell as an embodiment of the present invention. In FIG. 1, the fuel cell 10 is composed of a proton conductive polymer electrolyte membrane 1, a pair of electrodes 3 sandwiching the polymer electrolyte membrane 1, and a pair of separators 7 sandwiching the electrodes 3. The electrode 3 is provided with a catalyst layer 2 on the side in contact with the polymer electrolyte membrane 1 and a gas diffusion layer 6 on the side in contact with the separator 7.

The polymer electrolyte membrane 1 is a solid polymer thin film that exhibits good proton conductivity in a wet state, and is composed of, for example, an ion exchange membrane of a fluororesin such as Nafion (registered trademark). Each of the catalyst layers 2 has a catalyst (not shown) that promotes a chemical reaction between hydrogen and oxygen. Each of the catalyst layers 2 is formed as a dry coating film of a dispersion solution of conductive particles (for example, platinum-supported carbon) carrying a catalyst and an electrolyte resin of the same type or similar to that of the polymer electrolyte membrane 1.

Each of the gas diffusion layers 6 includes an MPL 4 and a porous base material layer 5, the MPL 4 is in contact with the catalyst layer 2, and the porous base material layer 5 is in contact with the separator 7. By compressing the fuel cell 10 in the stacking direction, each of the gas diffusion layers 6 is pressure-bonded to each catalyst layer 2 and integrated as an electrode 3.

Each of the gas diffusion layers 6 distributes the reaction gas supplied through the gas flow path groove 8 formed in the separator 7 to the entire catalyst layer 2. Further, each of the gas diffusion layers 6 functions as a conductive path between the electrode 3 and the separator 7. During the operation of the fuel cell, the reaction gas supplied to the gas diffusion layer 6 flows through the through pores inside the gas diffusion layer 6 along the plane direction orthogonal to the stacking direction of the gas diffusion layer 6. It reaches the catalyst layer 2 while being diffused.

In the MPL4 provided on the surface of the gas diffusion layer 6 in contact with the catalyst layer 2, the ultrafine porous structure inhibits the formation of a liquid-water film, so that the water generated by the chemical reaction of the reaction gas is the reaction gas. As a result, the gas diffusivity and drainage property on the surface in contact with the catalyst layer 2 are ensured. The porous base material layer 5 provided on the surface in contact with the separator 7 is composed of a conductive porous body, disperses the reaction gas supplied through the gas flow path groove 8, and uniformly supplies the reaction gas to the catalyst layer 2. Then, it has a role of discharging the generated water generated by the oxidation reaction in the catalyst layer 2 to the outside of the single cell.

The pair of separators 7 are plate-like members having conductivity and gas impermeableness, and are composed of, for example, a metal plate. Each of the separators 7 is laminated and arranged on the respective surfaces of the gas diffusion layer 6. Each of the separators 7 is formed with a gas flow path groove 8 through which hydrogen or oxygen as a reaction gas flows.

Although illustration and detailed description are omitted, an insulating seal portion is provided on the outside of each fuel cell 10 to prevent leakage of a fluid such as a reaction gas and short circuit between separators 7. Further, on the outside of the fuel cell 10, a manifold for supplying the reaction gas to the fuel cell 10 is provided.

<Structure of gas diffusion layer>
FIG. 2 shows the configuration of the gas diffusion layer 6 in the fuel cell of the present invention. In the gas diffusion layer 6, MPL4 composed of conductive particles is formed on the surface of the porous base material layer 5, penetrates into the porous base material layer 5, and is inside from the surface of the porous base material layer 5. There is a water repellent material 9 distributed in.

As the conductive particles constituting MPL4, carbon particles, particularly carbon particles having an average particle size of 20 to 150 nm, for example, carbon black having excellent conductivity and a large specific surface area can be used, and particularly, the conductivity is high. Acetylene black is preferred.

The porous base material layer 5 includes a sheet-like material having conductivity and porosity generally used as a base material of a gas diffusion layer of a fuel cell, for example, carbon paper, carbon cloth, carbon non-woven fabric, or the like. A porous sheet material made of carbon fiber can be used.

As the water repellent material 9, fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-pa-fluoroalkyl vinyl ether copolymer (PFA) are used. It is possible.

In the present invention, with respect to the amount of fluorine on the surface of the gas diffusion layer on the side in contact with the separator, that is, the side opposite to the side on which the MPL is formed, the side in contact with the catalyst layer, that is, the side on which the MPL4 is formed. The ratio of the amount of fluorine on the surface is 1.5 or more and 11 or less.

In order to use such a ratio of the amount of fluorine, for example, the amount of fluorine on the surface of the gas diffusion layer in contact with the separator is 2% or less as the amount of fluorine per unit area of the gas diffusion layer on the surface of the gas diffusion layer. The amount of fluorine on the surface of the gas diffusion layer on the side in contact with the catalyst layer is 2 to 13%. Here, the surface of the gas diffusion layer means a layer in the range from the outermost surface to a depth of 20 μm. Further, the amount of fluorine per unit area can be measured by using, for example, an energy dispersive X-ray analysis (EDX) apparatus.

<Manufacturing method of fuel cell>
The fuel cell of the present invention can be manufactured, for example, by the following method. First, the MPL forming coating liquid is applied to one surface of the porous base material layer by a known method such as a die-type coater. Before applying this MPL forming coating liquid, a slurry containing a water repellent material may be applied to the porous base material layer in advance.

Here, the slurry containing the water-repellent material is obtained by dispersing the above-mentioned fluororesin in a solvent. The solvent is not particularly limited, and various liquid agents such as water, methanol, and ethanol can be used.

The coating liquid for MPL formation may be in the form of a paste or a slurry in which conductive particles, a water repellent material and a solvent are mainly mixed and dispersed. Additives such as a dispersant can be added to the MPL forming coating liquid as needed, but it is preferable that the coating liquid does not contain metal in order to avoid contamination.

As the conductive particles, carbon particles, particularly carbon particles having an average particle diameter of 20 to 150 nm, for example, carbon black having excellent conductivity and a large specific surface area can be used, and acetylene black having high conductivity is particularly preferable. .. As the water repellent material, the above-mentioned fluororesin can be used. The solvent is not particularly limited, and various liquids such as methanol and ethanol can be used. The surfactant as a dispersant is also not particularly limited, and various surfactants such as various nonionic surfactants such as ester type, ether type and ester ether type can be used.

The composition of the coating liquid for forming MPL is, for example, 70 to 90% by mass of conductive particles, 15 to 25% by mass of a binder, and a dispersant, assuming that the total solid content of the conductive particles, the binder, and the dispersant is 100% by mass. Is adjusted to be 5 to 15% by mass. The physical properties of the coating liquid for MPL formation are as follows: a solid content ratio of 15 to 25% by mass, a viscosity of 500 to 2500 mPa · s (50 / s) at a shear rate of 50 s ^-1 , and a storage elastic modulus of 500 to 500. It may be set to 5500 Pa.

After applying the MPL forming coating liquid to the porous substrate, it is dried and then fired using, for example, a heating device. This firing may be performed in a general heating furnace. By this firing, the coating film of the coating liquid for forming the MPL layer is fixed on the porous base material layer as a fine porous layer, and a gas diffusion layer in which the porous base material layer and the MPL are laminated is formed.

In the present invention, as described above, the ratio of the amount of fluorine on the surface of the gas diffusion layer in contact with the catalyst layer to the amount of fluorine on the surface in contact with the separator is 1.5 or more and 11 or less. When applying the forming coating liquid, the infiltration of the MPL forming coating liquid into the porous base material layer is suppressed, and the surface of the porous base material layer opposite to the surface on which the MPL is formed is formed. By suppressing the infiltration of the water-repellent material into the porous base material layer in the coating liquid for MPL formation, the amount of fluorine on the surface of the gas diffusion layer in contact with the separator is reduced, and this requirement is met. Can be achieved. Alternatively, the coating liquid for forming MPL is applied, and after firing, the fluororesin is removed from the surface of the porous base material layer opposite to the surface on which MPL is formed, or this surface is polished to form a porous group. By crushing the carbon constituting the material layer and increasing the ratio of carbon to the fluororesin, the fluororesin grain / base material grain on the surface of the gas diffusion layer can be lowered, and this requirement can be achieved.

A fuel cell can be obtained by joining the gas diffusion layer thus obtained to a catalyst layer bonded to a polymer electrolyte membrane and further arranging a separator.

(Example 1)
A carbon paper (TGP-H060 manufactured by Toray Industries, Inc.) was cut into a size of 34 cm × 50 cm, and the carbon paper was impregnated in a slurry containing PTFE as a fluororesin. Here, the fluororesin basis weight / carbon paper basis weight was set to 7%. The impregnated carbon paper was dried in a drying oven at 330 ° C. for 15 minutes.

An MPL forming coating solution having the following composition was applied to one side of the dried carbon paper using a die coating machine (MPL basis weight: 2.0 mg / cm ² ).

A carbon paper coated with a coating liquid for MPL formation was put into a furnace, and while flowing argon gas into the furnace at 0.2 L / min, the temperature was raised from room temperature to 350 ° C. and kept at 350 ° C. for 15 minutes. After that, the heat history of furnace cooling was added. After the temperature inside the furnace reached room temperature, the carbon paper was taken out of the furnace. Next, using a circular water resistant paper (# 1500), the surface of the carbon paper opposite to the surface provided with the MPL was polished for 20 seconds to obtain a gas diffusion layer.

(Example 2)
A gas diffusion layer was produced in the same manner as in Example 1 except that the fluororesin basis weight / carbon paper basis weight was set to 3%.

(Comparative Example 1)
A gas diffusion layer was produced in the same manner as in Example 1 except that it was not polished with water resistant paper.

<Evaluation>
(Measurement of fluorine content at each site by SEM-EDX)
The obtained gas diffusion layer was cut diagonally in the thickness direction with respect to the surface using a design cutter, and the catalyst layer side (the side where the MPL was formed), the intermediate point, and the thickness direction of the cross section. Three points on the separator side were observed by SEM, and the amount of carbon and fluorine at the intersection of carbon fibers was quantified by EDX.

The results are shown in Table 2 and FIG. 3 below.
Further, in the obtained gas diffusion layer, the amount of F (wt%) on the catalyst layer side / the amount of F (wt%) on the separator side is shown in Table 2 below.

The outermost surface (separator side) of the gas diffusion layers of Examples 1 and 2 had a small amount of fluorine of 1 wt% and 2 wt%, while it was 9 wt% in Comparative Example 1. Considering that the surface roughness of carbon paper is about 20 μm, in Examples 1 and 2, the amount of fluorine is 2% or less at a depth of 20 μm from the outermost surface, and 3 to 11 other than that. It is considered to be about%.

(Evaluation of power generation performance)
Using the gas diffusion layers produced in Examples 1 and 2 and Comparative Example 1, a 1 cm ² size fuel cell was produced, and the power generation performance was evaluated under the conditions shown in Table 3 below. The gas at each of the cathode and the anode was decided to flow to the cell after passing through the bubbler whose dew point was set in Table 3.

The results are shown in FIG. In the fuel cell using the gas diffusion layer of the present invention (Examples 1 and 2), the voltage is higher than that of the fuel cell using the gas diffusion layer of the prior art (Comparative Example 1) at an arbitrary current value. It is considered that the same output can be obtained even if the power generation area is reduced. This is generated by power generation because the contact resistance between the gas diffusion layer and the separator is small and the water repellent material is present inside by reducing the water repellent material in the surface layer on the opposite side of the MPL. This is because the water could be drained.

1 Polymer electrolyte membrane 2 Catalyst layer 3 Electrode 4 Microporous layer (MPL)
5 Porous base material layer 6 Gas diffusion layer 7 Separator 8 Gas flow path groove 9 Water repellent material 10 Fuel cell

Claims (1)

A fuel cell composed of a proton-conducting polymer electrolyte membrane, a pair of electrodes sandwiching the polymer electrolyte membrane, and a pair of separators sandwiching the electrodes, wherein the electrodes are the polymer electrolyte membrane. A catalyst layer is provided on the contacting side, and a gas diffusion layer is provided on the side in contact with the separator. A fuel cell that is 1.5 or more and 11 or less.