JPH0782865B2 - Matrix holding electrolyte for phosphoric acid fuel cell - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrolyte retention matrix of a fuel cell using phosphoric acid as an electrolyte.

[Conventional technology]

The basic structure of a unit cell of a fuel cell using phosphoric acid as an electrolyte consists of a fuel electrode and an air electrode separated by an electrolyte holding matrix (hereinafter referred to as matrix). The H ⁺ generated at the fuel electrode diffuses in the phosphoric acid retained in the matrix and reaches the air electrode, where it is consumed as a depolarizer. Matrix, as its basic property,
In addition to being required to have no electronic conductivity and not to be attacked by phosphoric acid at the battery operating temperature, it is also necessary to have good phosphoric acid retention. That is, it is necessary that the matrix contain as much phosphoric acid as possible and that the phosphoric acid does not flow out to the electrode side during long-term battery operation.

Japanese Patent Publication No. 46-2304, Japanese Patent Laid-Open No. 59-117075, and Japanese Patent Laid-Open No. 60-30060 disclose that the difference in pore size distribution between the electrode matrix and the electrode matrix is a driving force for flowing out phosphoric acid from the fuel cell matrix. It is stated that there are forces such as capillary phenomenon based on the above, expansion and contraction of phosphoric acid due to temperature change, generation of water at the air electrode and dilution of phosphoric acid due to it, air pressure, differential pressure of fuel pressure, etc. There is. These forces cause a slow flow of phosphoric acid over a long period of time even if it is weak, and make the distribution of phosphoric acid between the electrode and the matrix uniform, resulting in an excess of phosphoric acid on the electrode and phosphoric acid on the matrix. Fall short. As a method for preventing the flow of phosphoric acid, for example, the following method has been conventionally known.

(1) Phosphates such as Zr, Si, and Al, which are unrelated to the redox reaction at the electrode, alone or by mixing these phosphates with a phosphoric acid-resistant substance to form a matrix. A method based on the thickening effect of dissolving in phosphoric acid.

(2) Oxides such as Zr, Si, Al, hydroxides, chlorides, etc. are mixed with phosphoric acid-resistant substances to form a matrix, and these oxides, etc. react with phosphoric acid to form phosphates. To produce a thickening effect similar to (1).

The mechanism of action of these methods is said to be that "phosphate" causes "gelation" by dissolving in phosphate.
However, the solubility of phosphate is said to be 1 to 5%,
The flow characteristics of phosphoric acid heated in the coexistence with a large excess of these phosphates were close to those of concentrated phosphoric acid, and were greatly separated from agar, which is a typical gel.

Bubble pressure is used as a measure of the phosphoric acid retention of the matrix. This is because the matrix is impregnated with phosphoric acid and sandwiched between the fuel electrode and the air electrode, and after being maintained for a certain period of time under the cell operating conditions, the pressure difference between the fuel at the fuel electrode and the air at the air electrode is gradually increased. It is the pressure difference when the gas on the high pressure side passes through and goes out on the low pressure side. Conventional Zr, S
The phosphate matrix containing i, Al, etc. did not show stable high foam pressure. Further, there is a drawback that the creep phenomenon that the thickness of the matrix gradually decreases when the laminated battery is tightened becomes larger as the phosphate content ratio becomes higher.

In this way, the cause of the case where the bubble pressure is low is
When oxides such as Zr, Si, Al, etc. react with phosphoric acid to form phosphate, the phosphate separates the phosphoric acid resistant substance particles and precipitates as coarse particles, after which comparison is still made. This is because the large pores remain, and the flow of phosphoric acid passing therethrough cannot be stopped by the thickening effect of the phosphate dissolved in phosphoric acid. The above-mentioned creep phenomenon occurs because the phosphate particles are destroyed when the surface pressure when the fuel cell is made into a laminated cell is applied to the mechanically weak phosphate particles. As mentioned above, the precipitated particles are Zr, A
In the case of l, it is a phosphate crystal, and in the case of Si, it is an amorphous brittle glass flake, but the situation of precipitation is the same.

FIG. 2 is a schematic view of a conventional phosphoric acid retaining matrix disclosed in, for example, Japanese Patent Laid-Open No. 59-117075. (1) shows a matrix skeleton material made of phosphoric acid resistant substance powder, for example, a particle size of 2 μm. SiC and / or ZrSiO with a particle size of 1 μm
₄ , (2) is a phosphate forming pores with the matrix skeleton (1), and (3) is pores formed by the matrix skeleton (1) and the phosphate (2). . The pores (3) communicate with each other to form meandering capillaries (not shown), and the capillaries serve as a flow path for phosphoric acid, which causes outflow of phosphoric acid.

In addition, in general, when the liquid permeates the fine particle packed layer, the Kozeny-Carm (Kozeny-Carm) shown in the equation (1) is used.
The expression of an) is established.

u = [ε ³ / 5Sv ² (1-ε) ² ] · [ΔP / μL] (1) where u is the apparent flow velocity, ε is the porosity of the fine particle packed layer,
Sv is the specific surface area per unit volume of the particles only, μ is the viscosity coefficient of the liquid, L is the thickness of the particle layer, and ΔP is the differential pressure.
As long as equation (1) holds, u is not 0 no matter how small ΔP is, and the liquid flows for a long time. In order to prevent the flow of phosphoric acid, it is necessary to create a system in which the flow of phosphoric acid does not start at ΔP below a certain limit. The flow does not follow equation (1), It must be expressed in any format.

In this case, the bubble pressure is a measure of α.

[Problems to be Solved by the Invention]

In the above matrix, since relatively large pores remain in the matrix, there are problems that the retention of phosphoric acid is poor and the creep phenomenon becomes large. Furthermore, the pores formed in the matrix become the flow path of phosphoric acid,
There was a problem that caused the outflow of phosphoric acid.

The present invention has been made to solve the above problems, in which phosphoric acid-resistant substance particles are in contact with each other, and amorphous phosphate forms a matrix structure that precipitates so as to fill the gap, It is an object of the present invention to obtain a phosphoric acid fuel cell matrix capable of achieving a stable bubble pressure.

[Means for Solving the Problems]

The phosphoric acid fuel cell matrix according to the present invention has a mean particle size of 20 nm or less, silica powder, which is a substance that reacts with phosphoric acid to produce amorphous phosphate, and a particle size of 1 μm.
It is made from a mixture with a phosphoric acid resistant powder which is ˜5 μm.

[Work]

In the present invention, the phosphoric acid-resistant substance particles are in contact with each other or the distance between the particles is at most 20 nm, while the amorphous phosphate particles precipitated by the reaction with phosphoric acid are phosphoric acid-resistant. The gap between the substance particles is filled up, and the phosphoric acid flow path is blocked by a part thereof, or the flow path is remarkably narrowed. Therefore, it is possible to more closely fill the spaces between the phosphoric acid resistant substance particles than before the reaction with phosphoric acid,
It is possible to prevent the flow of phosphoric acid [Example] In order to stably obtain the matrix according to the present invention,
The particle size of the phosphoric acid resistant substance particles is selected to be in the range of 1 μm to 5 μm, that is, 90% by weight or more. At the same time, the average particle size (hereinafter simply referred to as particle size) of the substance that reacts with phosphoric acid to form phosphate is 20 nm or less (preferably 20 nm).
The following is selected to be ~ 5 nm) and uniformly mixed with the phosphoric acid resistant substance particles. The mixing ratio of these is 1 wt% to 50 wt% of the substance that forms an amorphous phosphate, relative to 99 wt% to 50 wt% of the phosphoric acid resistant substance powder. The content ratio as amorphous phosphate is 60% by weight to 50% by weight, preferably 50% by weight to
10% by weight.

As phosphorous acid resistant substance powder, α type SiC, β type SiC, SiC
Powders such as fiber, α-type Si ₃ N ₄ , β-type Si ₃ N ₄ , WC, Ta ₂ O ₅ , Nb ₂ O ₅ , diamond, Ta ₄ C ₄ and B ₄ C ₃ can be used, α-type S
Use of iC powder is preferred. It is important that the metal contained in the substance that forms the amorphous phosphate is not related to the redox reaction at the electrode and that it does not inhibit the catalytic action at the electrode. Silica powder is preferable as the substance that produces an amorphous phosphate. Of these, for example, fumed silica is excellent as a substance that reacts with phosphoric acid to form an amorphous phosphate. These substances are commercially available under the trade name of "Aerosil" (manufactured by Aerosil Co., Ltd.) and have an average particle size of 10
It is a fine powder of high-purity silica with a particle size of ~ 20 nm. Since these fine particles are ultrafine particles that do not form secondary particles that strongly adhere to each other, they are easily mixed uniformly with the phosphoric acid resistant powder particles, have a rapid reaction with phosphoric acid, and are molded as a matrix. The paste for this purpose has the advantages that it is homogeneous, does not contain bubbles easily, and has a high viscosity and is easy to mold. By utilizing these, a matrix having excellent phosphoric acid retention can be easily manufactured industrially.

The substance that produces amorphous phosphate is mixed with powder of phosphoric acid-resistant substance, and water or an organic solvent is added to form a paste.
Apply on one electrode. Appropriate organic substances may be added to adjust the paste properties required for coating. However, it is necessary that all the organic substances including the organic solvent are evaporated or decomposed and removed in the subsequent process.

Polyethylene glycol, polyethylene oxide, etc. can be used as an additive for adjusting the properties of the paste containing water.When using these, heat treatment in an inert atmosphere at a temperature of 300 ° C or higher after coating and drying the paste. However, these additives must be decomposed and removed. After the paste is applied and dried, it is impregnated with a calculated amount (described later) of phosphoric acid and heated and maintained at a temperature of 150 ° C or higher and 200 ° C or lower, that is, a substance that produces phosphoric acid and an amorphous phosphate. Complete the reaction with the reactive material particles. Since phosphoric acid is consumed during this reaction, the amount of phosphoric acid to be impregnated depends on the amount of phosphoric acid consumed by the reaction, the amount of phosphoric acid required to fill the pore volume of the matrix, and the underlying electrode. Calculated as the sum of power phosphoric acid amounts. When the substance that produces a phosphate is silica, the produced phosphate is an amorphous gel that does not have a stoichiometric composition.
Therefore, the amount of phosphoric acid consumed by the production of phosphate cannot be calculated. In this case, the calculation standard of the amount of phosphoric acid is the ratio of the amount of phosphoric acid excluding the amount of phosphoric acid to be impregnated to the underlying electrode and the amount of silica, 20: 1 to 10: 1 by weight,
The ratio is preferably in the range of 15: 1 to 10: 1. The amount of phosphoric acid is 100% phosphoric acid conversion number. Since the reaction between the reactive substance and phosphoric acid is rapid, the reaction is completed before phosphoric acid has sufficiently penetrated into the underlying electrode, phosphoric acid is fixed in the matrix, and the electrode becomes phosphoric acid deficient. There is. In order to prevent this, after the impregnation with phosphoric acid, 150 ℃ or 1 ℃ for the second time.
It is desirable to keep at 70 ° C. It is necessary to hold for 12 hours or more to complete the reaction.

A method of using phosphoric acid as a solvent for the matrix paste in order to uniformly distribute phosphoric acid in the matrix is already known, and it is possible to use phosphoric acid as a solvent for coating the matrix. In this case, since the reaction between the reactive substance particles and phosphoric acid proceeds slowly even at room temperature, there is a difficulty in that the viscosity of the paste gradually increases with time, but the penetration of phosphoric acid into the underlying electrode is impaired. A sufficient phenomenon can occur at the same time. To avoid this, the underlying electrode is impregnated with the required amount of phosphoric acid in advance, and the phosphoric acid-containing paste is applied on top of it, and the temperature is maintained at 170 ° C or higher.
The matrix can be formed by heating and holding at a temperature of 00 ° C. or lower. In this case, after the required amount of phosphoric acid is contained, the absorption rate of phosphoric acid from the matrix paste of the electrode is slow, so that the so-called flooding phenomenon in which the electrode is in excess of phosphoric acid does not occur. Forming the matrix in this way has the effect of reducing the internal resistance of the battery. This is presumably because the interface between the matrix and the electrode was better adapted to facilitate the transfer of H ⁺ across the interface.

FIG. 1 is a schematic diagram showing an embodiment of the present invention.
A) and (2A) correspond to (1) and (2), respectively, and (3) is exactly the same as the above-mentioned conventional one. The phosphoric acid-resistant substance particles are in contact with each other or the distance between the particles is 20 nm at maximum. On the other hand, the amorphous phosphate particles precipitated by the reaction with phosphoric acid fill the pores (3) which are the gaps of the matrix skeleton material (1A) made of the phosphoric acid resistant substance particles, and the phosphoric acid flow path is formed. It is blocked at a part of the pores (3) or the phosphoric acid channel is made extremely narrow. The formation reaction of the phosphate (2A) is an expansion reaction when only particles of the substance that forms an amorphous phosphate are taken. Therefore, the pores (3) can be filled more closely than before the reaction with phosphoric acid. In the case of silica, the reaction product is amorphous, and glass flakes fill the pores (3). This amorphous material is a solid that is not plastic and brittle, and can hold 15 or less molecules of phosphoric acid per Si atom, but has H ⁺ conductivity and can block the flow of phosphoric acid. In this way, the phosphate is generated, the electrode coated with a matrix containing amorphous phosphoric acid, moisture absorption from the air, dew condensation on both the matrix side and the electrode side, in terms of battery performance, It may adversely affect handling. Therefore, this electrode should be used in a low humidity environment or 150
It should be stored in an air bath maintained at temperatures between ℃ and 170 ℃.

Example 1 4 parts of fumed silica having a particle size of 20 nm and 53 parts of 100% phosphoric acid were added to 46 parts of α-type SiC having a particle size of 2 μm to prepare a matrix paste. The viscosity was adjusted by adding water. This matrix paste was immediately applied onto the electrode with a squeegee, and the temperature was maintained at 150 ° C. for 2 hours and then maintained at 190 ° C. by heating overnight. The thickness of the matrix was 300 μm. The bubble pressure and electric resistance of this matrix were measured and found to be 0.4 kg / cm ² and 5.2 mΩ, respectively.
The measurement area was 100 cm ² . This bubble pressure is larger than the value in Comparative Example 1 described later, and this is because the difference in particle size of silica is reflected. The X-ray diffraction pattern of the matrix after the measurement was only α-type SiC, and the reaction product of fumed silica and phosphoric acid was amorphous.

Comparative Example 1. 53 parts of 100% phosphoric acid was added to 46 parts of α-type SiC having a particle size of 2 μm and 4 parts of amorphous fused silica powder having a particle size of 5 μm to prepare a matrix paste. The viscosity was adjusted by adding water. Apply this matrix paste on the electrode with a squeegee and hold at 150 ° C for 2 hours, then at 190 ° C
It was kept heated overnight. The thickness of the matrix was 250 μm. The bubble pressure and electric resistance of this matrix were measured and found to be 0.5 kg / cm ² and 5 mΩ, respectively. The measurement area was 100 cm ² . The measurement result by the X-ray diffraction of the matrix after the measurement is the same as in Example 1, and α-type SiC
Was only. The difference in bubble pressure from Example 1 is based on the difference in particle size of silica used.

Example 2 A matrix paste was prepared by adding 46 parts of α-type SiC having a particle diameter of 2 μm and 57 parts of 100% phosphoric acid to 4 parts of fumed silica having a particle diameter of 20 nm. The viscosity was adjusted by adding water. This matrix paste was applied onto the electrode with a squeegee, and heated and held at 150 ° C. for 2 hours and then at 200 ° C. overnight. The coating thickness of the matrix was 190 μm. The foam pressure and electric resistance of this matrix are 2.0k each.
It was g / cm ² and 3.0 mΩ.

Comparative Example 2. On the other hand, a matrix paste was prepared by adding 51 parts of 100% phosphoric acid to the same mixture of 46 parts of α-type SiC and 4 parts of fumed silica. This matrix paste was applied on the electrode previously impregnated with phosphoric acid with a squeegee and
The temperature was kept at 2 ° C for 2 hours and then at 200 ° C overnight. The coating thickness of the matrix was 190 μm. When the bubble pressure and electric resistance of this matrix were measured,
It was 2.0 kg / cm ² and 2.4 mΩ. The difference in electrical resistance value is based on the difference in whether or not phosphoric acid is previously contained in the electrodes when the matrix is applied.

〔The invention's effect〕

As described above, the phosphoric acid fuel cell holding matrix according to the present invention comprises a phosphoric acid-resistant substance powder having a particle size of 1 μm to 5 μm, and a silica powder having a particle size of 20 nm or less, which is extremely small and has a small weight. Since it was made by reacting a mixture with 10 to 15 times the weight of this silica powder with phosphoric acid, the phosphate formed is amorphous and liquid phosphoric acid is almost nonexistent.

The phosphoric acid-resistant substance particles are in contact with each other or exist very close to each other, and the precipitated amorphous phosphate has a structure of filling the gap.

This makes it possible to obtain a stable high foam pressure against external pressure, good retention of phosphoric acid in the matrix, and low outflow of phosphoric acid, resulting in a retention matrix for phosphoric acid fuel cells. .

[Brief description of drawings]

FIG. 1 is a schematic diagram according to an embodiment of the present invention, and FIG. 2 is a schematic diagram of a conventional matrix. In the figure, (1) and (1A) are matrix skeleton materials,
(2) and (2A) are amorphous phosphates, and (3) are pores. In each drawing, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A silica powder having an average particle size of 20 nm or less and reacting with phosphoric acid to form a phosphate, and a particle size of 1 μm to 5 μm.
In a matrix holding an electrolyte for a phosphoric acid fuel cell, which is produced by reacting a mixture of a phosphoric acid-resistant substance powder having a particle size of μm and phosphoric acid, a mixing ratio of the silica powder and the phosphoric acid-resistant substance powder is , Silica powder 1% by weight to 50
The phosphoric acid-resistant substance powder is 99 wt% to 50 wt% with respect to wt%, and the mixing weight ratio of the added phosphoric acid and the silica powder is 10 to 15 to 1, A matrix holding an electrolyte for a phosphoric acid fuel cell, wherein the phosphate is an amorphous phosphate.