JP2013182740A - Method of observing reaction layer of fuel battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To grasp a distribution state of hydrophilic group in a reaction layer of a fuel battery.SOLUTION: Sulfonic acid group bonded to hydrophilic group in a reaction layer is subjected to dying by allowing the sulfonic acid group to be bonded to the hydrophilic group (sulfone group) contained in the reaction layer of a fuel battery, and by loading into the reaction layer metal ions, such as ruthenium ions, capable of forming a nitrosyl complex together with the sulfonic acid group. When the hydrophilic group is aggregated, the sulfonic acid group bonded thereto is also aggregated. By subjecting the sulfonic acid group to dying with ruthenium, the ruthenium is also aggregated. This is observable with an electron microscope. The sulfonic acid group has an affinity for the hydrophilic group by bringing NOx gas into contact with the reaction layer.

Description

  The present invention relates to a method for observing a reaction layer of a fuel cell.

A membrane electrode assembly used in a fuel cell has a structure in which a solid polymer electrolyte membrane is sandwiched between a hydrogen electrode and an air electrode. The hydrogen electrode and the air electrode are each provided with a reaction layer and a diffusion layer from the solid polymer electrolyte membrane side. It is laminated sequentially.
The reaction layer is composed of a mixture of a catalyst and an electrolyte, and is required to have electron and proton conductivity and air permeability. Here, since protons move in the form of H ₃ O ⁺ with water, it is necessary to maintain the reaction layer in a wet state. Of course, excessive moisture in the reaction layer inhibits air permeability (so-called flooding phenomenon), so the moisture in the reaction layer must always be maintained at an appropriate amount.
In order to satisfy this requirement, the present applicant has proposed a catalyst paste including a thin water film between the catalyst and the electrolyte in Patent Document 1 and Patent Document 2. Such a catalyst paste can be obtained by preparing a pre-paste in which a catalyst and water are mixed in advance, mixing the pre-paste and the electrolyte solution, and employing an appropriate stirring method. In the catalyst paste formed in this way, the hydrophilic group of the electrolyte is attracted to and opposed to the water film covering the catalyst, and a hydrophilic region is formed between the electrolyte and the catalyst. Become. Hereinafter, a structure having a hydrophilic region between the catalyst and the electrolyte in the mixture of the catalyst and the electrolyte may be referred to as a “PFF structure”.
By making the hydrophilic region between the electrolyte and the catalyst continuous (that is, by making the hydrophilic region not patchy), uneven distribution of water in the reaction layer is prevented. Further, even when the fuel cell is operated in a low humidified environment, water is present in this hydrophilic region, so that overdrying can be prevented. Further, in operation in a highly humidified environment, excess water is discharged to the outside (diffusion layer side) through this hydrophilic region, so that flatting can be prevented.
In addition, please refer to patent document 3 and nonpatent literatures 1-3 as literature which discloses the technique relevant to this invention.

JP 2006-140061 A JP 2006-140062 JP JP 2009-104905 A

Journal of Electrochemical Society 2005, vol.152, No.5, PP.A970-A977 MAKHARIA Rohit; MATHIAS Mark F.; BAKER Daniel R. “Measurement of catalyst layer electrolyte resistance in PEFCs using electrochemicalimpedance spectroscopy” Journal of Electroanalytical Chemistry 475, 107-123 (1999) M.Eikerling and A.A.kornyshev “electrochemical impedance of Cathode Catalyst Layer of Polymer Electrolyte Fuel Cells” "Electrochemical Impedance Method" (Maruzen, Masayuki Itagaki) 8 Electrochemical Impedance Analysis Using Distributed Constant Type Equivalent Circuit (pp133-146) J. Power Sources, Vol.138, No 1/2 216 (2004)

Not only the PFF structure but also a water path must be formed to ensure proton conductivity. It is the hydrophilic group (for example, sulfone group) of the electrolyte that is essentially hydrophilic in the material constituting the reaction layer, and this hydrophilic group aggregates and forms the aggregate in order to form a water path in the reaction layer. The collection must be continuous.
The PFF structure described above is one in which aggregates of hydrophilic groups are collected on the surface of the catalyst and a water path is formed on the surface of the catalyst.
For example, when hydrophilic groups are dispersed, water adsorbed on the hydrophilic groups is separated from each other, and a water path is not formed. Therefore, protons supplied from outside cannot be delivered. Moreover, it is not preferable also from a viewpoint of draining excess generated water.
Thus, in order to know the characteristics of the reaction layer of the fuel cell, it is important to grasp the distribution state of the hydrophilic group of the electrolyte. However, the conventional technology does not provide any method for grasping this.

The present inventor has repeatedly investigated the distribution state of hydrophilic groups of the electrolyte in the reaction layer, and made the functional group affinity to the hydrophilic group of the electrolyte, and stained the functional group with a metal ion to observe the distribution state of the hydrophilic group with a microscope. It was noticed that it was observable, and the invention described in Japanese Patent Application No. 2010-195090 was completed.
That is, the step of making the functional group affinitize with the hydrophilic group of the electrolyte constituting the reaction layer of the fuel cell;
Introducing a metal ion capable of forming a complex with the functional group into the reaction layer, and staining the functional group with the metal ion.
According to such an observation method, since the hydrophilic group of the electrolyte is dyed with the metal ion indirectly through the functional group, the distribution state of the hydrophilic group can be visually observed with a microscope.

The electrolyte used for the reaction layer of the fuel cell is made of a polymer compound, and for example, a fluorine-based polymer such as Nafion (trade name of DuPont, the same applies hereinafter) is generally used. As shown in FIG. 1, the polymer compound has a hydrophobic main chain 100 and a side chain 101 having a hydrophilic ion exchange group. The hydrophilic ion exchange group is composed of, for example, a sulfone group (SO ₃ ⁻ ).
Such an electrolyte is dissolved in a solvent. This solvent consists of a mixture of water and an organic solvent. The electrolyte solution is mixed with a catalyst to form a catalyst paste.
There is the following relationship between the electrolyte and the amount of water in the electrolyte solution.
When the concentration of water in the electrolyte solution is reduced, the viscosity of the electrolyte solution increases even when the concentration of the electrolyte in the electrolyte solution is the same. Conversely, when the concentration of water is increased, the viscosity of the electrolyte solution decreases. The reason for this is estimated as follows.
That is, when the concentration of water in the electrolyte solution is high, as shown in FIG. 1A, water is adsorbed on the side chain 101 of the electrolyte 82 and the solid content of the electrolyte 82 is aggregated in the electrolyte solution. Viscosity decreases. Further, when the water concentration of the electrolyte solution is slightly lowered, the solid content of the electrolyte 82 is opened in the electrolyte solution by the action of the organic solvent contained in the electrolyte solution, as shown in FIG. As a result, the viscosity of the electrolyte solution increases.

When the reaction solution is formed by mixing the electrolyte solution containing the solid content of the electrolyte 82 (FIG. 1A) in which the aggregation has progressed, it is considered that the reaction layer is in a state as shown in FIG. That is, since the solid content of the electrolyte 82 is aggregated, the side chain 101 extends in multiple directions. Then, the side chains 101 and the water in the reaction layer are adsorbed, so that hydrophilic regions 83 are dispersed and formed in the reaction layer. For this reason, in the reaction layer where the solid content of the electrolyte 82 is aggregated, protons and water hardly move in the reaction layer due to ionic resistance in the reaction layer. For this reason, in a low humidification state, the performance degradation by drying of the electrolyte membrane and the electrolyte 82 in each catalyst layer is caused, and in the excessive humidification state, a performance degradation by flooding is caused.
In other words, in order to make the hydrophilic region (side chain 101) of the electrolyte face the catalyst and to reliably form a hydrophilic region between the two, the electrolyte is in the state of FIG. It is preferable to do. For this purpose, as described above, the amount of water contained in the electrolyte solution is set to 10% by weight or less of the electrolyte solution.

The cathode catalyst layer when the electrolyte in the state of FIG. 1B is used is considered to be in the state of FIG.
The side chain 101 of the electrolyte 82 extends in one direction. Therefore, in the catalyst paste, that is, the fuel cell reaction layer, hydrophilic ion exchange groups (sulfone groups) adsorb water in the pre-paste. It will be. For this reason, as shown in FIG. 3, in this reaction layer, the hydrophilic group 101 of the electrolyte 82 faces the surface of the catalyst 81, and a hydrophilic region 83 is formed between the electrolyte 82 and the catalyst 81. . Then, it is considered that the hydrophilic region 83 is continuously formed around the catalyst 81 and is in a state of communicating with each other as the sulfone group adsorbs with the water in the pre-paste as described above. For this reason, in the reaction layer using this catalyst paste, as shown in FIG. 3, protons and water easily move, and the electrochemical reaction proceeds smoothly. A fuel cell having such a reaction layer can have a high power generation capacity regardless of whether it is in a low humidified state or an excessively humidified state.
For details, refer to Japanese Patent Application No. 2010-002362.

As the functional group having an affinity for the hydrophilic group of the electrolyte, the previous application (Japanese Patent Application No. 2010-195090) exemplifies at least one selected from a nitrate group, an amino group, a sulfonic acid group, a hydroxyl group and a halogen group. When the hydrophilic group of the electrolyte is a sulfone group, the preferred functional group is a nitrate group, an amino group or a sulfonic acid group, and more preferably a nitrate group.
The purpose of observing the state of the hydrophilic group of the electrolyte is to grasp and improve the characteristics of the fuel cell. Therefore, it is preferable that the characteristics of the fuel cell are not affected as much as possible when the functional group is made to affinity for the hydrophilic group of the electrolyte. In other words, when observing the reaction layer of the fuel cell, it is preferable not to give a large environmental change from the usage state.

As a result of intensive studies to solve the above problems, the present inventor does not give a large stress to the reaction layer if the functional group can be compatible with the electrolyte of the reaction layer in a gas phase environment, that is, when used in a fuel cell. We thought that the reaction layer could be observed while maintaining the state. Then, when NOx gas is supplied to the reaction layer in a water-containing state, the present invention is conceived by focusing on the fact that this NOx gas dissolves in water and becomes nitrate ions.
That is, the first aspect of the present invention is defined as follows.
A step of making a nitrate group affinity with a hydrophilic group of an electrolyte constituting a reaction layer of a fuel cell;
Introducing a metal ion capable of forming a complex with the functional group into the reaction layer, and dyeing the functional group with the metal ion. Is a method for observing a fuel cell reaction layer, characterized in that NOx gas is brought into contact with the reaction layer in a water-containing state.

According to the observation method of the first aspect thus defined, nitrate groups as functional groups can be made to affinity for the electrolyte of the reaction layer in a gas phase environment in which NOx gas is brought into contact. Therefore, it is possible to dye the electrolyte in the reaction layer while maintaining the state at the time of use or reducing the environmental change from the time of use.
The method for supplying NOx gas to the reaction layer is not particularly limited, but it is preferable to use an inert gas such as air or nitrogen gas as the carrier gas. The amount of NOx gas mixed with the carrier gas can be arbitrarily set according to the wet conditions of the reaction layer, the environmental temperature, the flow rate of the carrier gas, and the like. For example, when air is used as the carrier gas, the amount of NOx gas mixed is preferably 5 to 100 ppm.

After making the nitrate group affinity with the hydrophilic group of the electrolyte, a metal ion capable of forming a complex with the nitrate group is introduced into the reaction layer. Specifically, ruthenium ions and osmium ions can be used, each forming a nitrosyl complex.
The method for introducing metal ions is not particularly limited, but a method of immersing the reaction layer in an aqueous solution of the metal ions can be employed.
In the case of other functional groups, an appropriate metal ion is adopted.
The amount of metal ions introduced is equal to or greater than the equivalent of the nitrate group of the electrolyte contained in the reaction layer.

FIG. 1 is a schematic diagram showing the form of the electrolyte in the electrolyte solution. FIG. 2 is a schematic diagram for explaining a PFF structure corresponding to FIG. FIG. 3 is a schematic diagram for explaining the PFF structure corresponding to FIG. FIG. 4 is a schematic diagram of the MEA evaluation apparatus. FIG. 5 is a schematic diagram showing a distribution model of nitrate groups in the reaction layer and staining of the nitrate groups with ruthenium ions.

1 g of carbon supported catalyst is prepared as a raw material catalyst. This raw material catalyst has carbon black particles as a carrier and catalyst platinum particles supported thereon by a well-known method (platinum supported amount: 50%).
Next, the catalyst is pulverized. The pulverized catalyst is put into a container together with 100 mL of water, and defoamed using a hybrid mixer (manufactured by Keyence Corporation, model number HM-500). The defoaming time is 4 minutes.
After the defoaming treatment, the mixture is allowed to stand overnight, the supernatant is discarded, 10 g of electrolyte (5% aqueous solution of Nafion) is added, and the mixture is stirred (centrifugation with a hybrid mixer (4 minutes)).

The paste thus obtained is applied to each diffusion layer on the hydrogen electrode side and oxygen electrode side, and dried to form a reaction layer. This reaction layer is bonded to the solid polymer electrolyte membrane by hot pressing.
The reaction layer thus obtained has a PFF structure.
The hydrophilic region formed between the electrolyte layer and the catalyst is the main body of the PFF structure, but the sulfone groups that could not be fully oriented to the catalyst side in the electrolyte layer are solidified to form clusters. It is done.

The membrane electrode assembly (MEA) thus obtained is incorporated into a standard MEA evaluation apparatus shown in FIG. 4, and a fuel cell reaction is performed.
In FIG. 4, reference numeral 201 denotes an MEA, 203 denotes an air electrode side diffusion layer, 205 denotes a hydrogen electrode side diffusion layer, and 211 and 213 denote support plates.
The fuel cell reaction is performed under high humidification conditions in which the produced water is sufficiently contained in the electrolyte of the MEA reaction layer.
Thereafter, nitrogen dioxide (NO2) is introduced into the air supplied to the air electrode side at a concentration of 5 to 100 ppm for 1 to 24 hours while maintaining the execution of the fuel cell reaction under high humidification conditions.
Thereby, the nitrate group has an affinity for the hydrophilic group of the electrolyte in the reaction layer. Refer to Non-Patent Document 4 for this phenomenon.
By performing the fuel cell reaction, the generated water is generated and moved, so that nitrate groups diffuse throughout the reaction layer.

In the above embodiment, unused MEA is incorporated into the fuel cell evaluation apparatus, and after the fuel cell reaction is performed, NOx gas is mixed into the air supplied at that time while maintaining the fuel cell reaction, and thus NOx Gas is brought into contact with the reaction layer.
Further, NOx gas can be brought into contact with the reaction layer while the fuel cell reaction is stopped.

Next, the reaction layer is cut out from the membrane electrode assembly and immersed in a ruthenium oxide solution or brought into contact with a ruthenium tetroxide gas, as shown in FIG. 5, nitrate groups (nitrate ions) having affinity for hydrophilic groups. Is dyed with ruthenium.
At this time, if the sulfone group which is a hydrophilic group is aggregated (PFF structure, cluster, etc.), the nitrate group is also aggregated. Accordingly, the nitrate group stained with ruthenium ions can be observed with a microscope (electron microscope).

  The present invention is not limited to the description of the embodiments and examples of the invention described above. Various modifications are also included in the present invention as long as those skilled in the art can easily conceive without departing from the scope of the claims.

81 catalyst 81a carrier 81b platinum catalyst fine particle 82 electrolyte 83 hydrophilic region

Claims (4)

A step of making the functional group affinity with the hydrophilic group of the electrolyte constituting the reaction layer of the fuel cell;
Introducing a metal ion capable of forming a complex with the functional group into the reaction layer, and staining the functional group with the metal ion;
In the method of observing the fuel cell reaction layer containing
The method for observing a fuel cell reaction layer is characterized in that the step of making the functional group affinity to the hydrophilic group comprises adding water to the reaction layer and adding NOx to the water to make the nitrate group affinity.   The method for observing a fuel cell reaction layer is characterized in that the step of making the nitric acid group compatible is performed by mixing NOx gas into the air while supplying air to the air supply system of the fuel cell.   The observation method according to claim 1, wherein the metal ion is a metal ion capable of forming a nitrosyl complex.   The observation method according to claim 3, wherein the metal ions are ruthenium ions and / or osmium ions.