JP5644277B2 - Method for observing the reaction layer of a fuel cell - Google Patents

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)

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 present invention was completed.
That is, the first aspect of the present invention is defined as follows.
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.
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 shall apply 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, at least one selected from a nitric acid group, an amino group, a sulfonic acid group, a hydroxyl group, and a halogen group can be selected. 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.
A method for making the functional group affinity to the hydrophilic group of the electrolyte is not particularly limited, but it is preferable to give the functional group to the catalyst side in advance. Here, affinity means a state in which a functional group is chemically adsorbed to a hydrophilic group.

When introducing the functional group into the catalyst, it is preferable to concentrate the functional group on the catalyst metal particles of the catalyst. This is because, when the electrolyte solution and the catalyst are mixed, if the functional group is dispersed, the electrolyte may interfere with the hydrophilic group of the electrolyte, and the electrolyte may not be kept open (see FIG. 1B).
In order to concentrate the functional groups on the catalyst metal particles, the same or the same kind of metal complex as the catalyst metal particles and containing the functional group is bonded to the catalyst metal particles.
When platinum particles or platinum alloy particles are employed as the catalyst metal particles, the catalyst metal (Pt) particles can be modified with functional groups by using the following solution.
(1) the functional groups: Examples of a nitrate group Jinitoroji ammine platinum (II) nitrate solution (cis- [Pt (NH3) 2 (NO2) 2] / HNO3 sln.)
Aged product (cis- [Pt (NO2) 4] / HNO3 sln.)
Hexahydroxoplatinum (IV) acid nitric acid solution ((H2Pt (OH) 6) / HNO3 sol.)
(2) Functional groups: Examples of sulfonic acid groups Hexahydroxoplatinum (IV) acid sulfuric acid solution ((H2Pt (OH) 6) / H2SO4 sol.)
(3) Functional group: Example of amino group Tetraammineplatinum (II) hydroxide aqueous solution ([Pt (NH3) 4 (OH) 2] / H2O sln.)

As the fuel cell reaction proceeds in the reaction layer, the functional groups concentrated on the catalyst metal particles are dispersed in the reaction layer away from the catalyst metal particles as the produced water is generated and moved. Thereby, a functional group couple | bonds with the hydrophilic group of the electrolyte in a reaction layer.
The functional group may be introduced later to the catalyst paste or the reaction layer without previously introducing the functional group to the catalyst side.
In order to spread the functional group throughout the reaction layer, it is preferable to introduce the functional group into the reaction layer and then perform a fuel cell reaction to generate generated water and move it. At this time, it is preferable that the fuel cell reaction is performed under high humidification conditions and is rich in generated water.
The amount of the functional group to be introduced should be equal to or more than the equivalent of the hydrophilic group, but from the viewpoint of executing the fuel cell reaction, the amount of the functional group introduced is preferably 0.05 to 0.6 equivalent. .
Of course, by infiltrating a large amount of functional groups into the reaction layer, the sensitive layer can be made to have a uniform affinity to the hydrophilic groups of the electrolyte in the entire reaction layer.

After the functional group is made to affinity with the hydrophilic group of the electrolyte, a metal ion capable of forming a complex with the functional group is introduced into the reaction layer.
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.
When a nitrate group is employed as a functional group, ruthenium ions and osmium ions can be used, and each form a nitrosyl complex.
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 hydrophilic group of the electrolyte contained in the reaction layer.
The reaction layer is washed in order to remove the functional group having an affinity for the hydrophilic group of the electrolyte and unreacted metal ions. If the fuel cell reaction is possible even after the introduction of metal ions, excess metal ions can be removed with the produced water.

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 showing the structure of the reaction layer immediately after production in the embodiment. FIG. 5 is a schematic diagram showing the structure of the reaction layer after the fuel cell reaction is performed. FIG. 6 is a schematic diagram showing staining with ruthenium ions. FIG. 7 is a 3D-TEM photograph of the reaction layer stained with ruthenium.

A carbon supported catalyst was 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 (supported amount: 50%).
Furuya aqueous nitric acid Jinitoroji ammine platinum to provide a metal (strain) (Pt0.05 / 150ml), a raw material catalyst 1g was added to a nitric acid concentration 0.07% (0.01 M), for 5 hours stirrer stirring at room temperature. Then, it is filtered and dried in an air atmosphere at 60 ° C. for 2 hours. Further, heat treatment is performed at 150 ° C. for 2 hours in a nitrogen atmosphere. The final weight of the obtained sample was 1.012 g, and the Pt yield determined from the residual amount of Pt in the filtrate was 84.3%.
As a result, the complex platinum is adsorbed on the catalyst platinum particles of the raw material catalyst, so that nitrate groups exist around the catalyst platinum particles.

Next, the obtained catalyst was pulverized. The pulverized catalyst was 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 was 4 minutes.
After carrying out the defoaming treatment, the mixture was left overnight, the supernatant was discarded, 10 g of electrolyte (5% aqueous solution of Nafion) was added, and the mixture was stirred (centrifugated with a hybrid mixer (4 minutes)).

The paste thus obtained was applied to each diffusion layer on the hydrogen electrode side and oxygen electrode side, and dried to obtain a reaction layer. This reaction layer was bonded to the solid polymer electrolyte membrane by hot pressing.
The reaction layer thus obtained has a PFF structure as shown in FIG. 4, and nitrate groups exist around the catalyst platinum particles.
In FIG. 5, 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 thought that.

A fuel cell reaction was performed using the membrane electrode assembly thus obtained.
When the fuel cell reaction is performed, the surface of the catalyst platinum particles is activated and the nitrate groups are easily released, and the nitrate groups diffuse throughout the reaction layer as the produced water is generated and moved.
The fuel cell reaction is preferably performed for 1 to 4 hours (highly humidified condition).
Next, the reaction layer is cut out from the membrane electrode assembly, immersed in a ruthenium oxide solution, and nitrate groups (nitrate ions) are stained with ruthenium as shown in FIG.
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).
A 3D-TEM observation image is shown in FIG. The photograph shown in FIG. 7 is a tomographic image. Large black circles in the figure indicate platinum particles, and small black dots are ruthenium. The solid portion indicates a catalyst or an electrolyte in which a hydrophilic group does not exist or a hydrophilic group is highly dispersed, and any of them does not substantially contribute to proton transfer .

  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.   The observation method according to claim 1, wherein the functional group is a nitrate group, and the metal ion is a metal ion capable of forming a nitrosyl complex.   The observation method according to claim 2, wherein the metal ions are ruthenium ions and / or osmium ions. A method for observing a reaction layer of a fuel cell comprising a mixture of a catalyst having platinum-based catalytic metal particles and an electrolyte,
Stirring the catalyst in an aqueous solution of a platinum complex having a nitrate group, modifying the catalyst metal particles of the catalyst with the nitrate group, and hydrophilizing the catalyst;
Mixing the hydrophilized catalyst and the electrolyte solution to form a catalyst paste;
Forming a reaction layer of the fuel cell using the catalyst paste;
Affinity of the nitrate group to the hydrophilic group of the electrolyte of the reaction layer;
Staining the nitrate group with ruthenium ions;
Method for observing a fuel cell reaction layer containing