JPH01204366A - Manufacture of electrolyte retaining matrix for fuel cell - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a fuel cell using phosphoric acid as an electrolyte, and particularly to a method for manufacturing an electrolyte retention matrix that retains the electrolyte.

[Conventional technology]

A fuel cell using phosphoric acid as an electrolyte usually has platinum supported on electrode base materials (la) and (lb) made of conductive carbon vapor, etc., as shown in the exploded perspective view of Figure 1. Water-repellent polytetrafluoroethylene (P)
Catalyst jliJ (2a
) * An electrolyte holding matrix (3) holding phosphoric acid as an electrolyte is interposed between a pair of gas diffusion electrodes consisting of (2b) to form a unit cell (4), and gas is further supplied. A separator plate (5) with a groove (5) for
6) is constructed by stacking a plurality of the above-mentioned unit batteries with each other interposed therebetween.

The phosphoric acid fuel cell described above generates electricity through an electrochemical reaction process by supplying pure hydrogen or hydrogen mixed gas as a fuel gas to one of the gas diffusion electrodes and supplying pure oxygen or air as an oxidant gas to the other. It's something you get.

In the above-mentioned phosphoric acid fuel cell, the matrix that holds the phosphoric acid electrolyte plays a very important role in improving the cell characteristics and ensuring its long-term stability, so it must have the following characteristics. required.

(1) It should be thermally and chemically stable to phosphoric acid with a concentration of 95% or more at a temperature of 180 to 230°C, which is the operating condition of a fuel cell.

(2) It has a high affinity with the electrolyte phosphoric acid, allows phosphoric acid to penetrate well, and has a high retention power.

(3) It is a good conductor of hydrogen ions and at the same time an insulator of electrons.

(4) Fuel gas and oxidant gas permeate the matrix,
Must have sufficient bubbling pressure to prevent direct contact and reaction.

(5) In order to minimize the internal resistance of the battery, the thickness of the matrix should be as thin as possible within the range allowed by mechanical strength.

Conventional electrolyte retention matrices for phosphoric acid fuel cells generally use silicon carbide (SiG), which has excellent heat resistance to phosphoric acid, as an electrolyte retention agent and fluorine resin as a binder, and are manufactured through a firing process. was.

[Problem to be solved by the invention]

However, conventional electrolyte retention matrices for phosphoric acid fuel cells have high water resistance because they are manufactured through a calcination process, and the problem is that they are still not fully satisfactory in terms of permeability and retention of phosphoric acid. There was a point.

In addition, Japanese Patent Application Laid-open No. 59-171472, 59-17147
Publication No. 3 proposes and discloses an electrolyte retention matrix using a composite oxide whose main components are silicon carbide and a phosphoric acid resistant substance as an electrolyte retention agent in order to increase phosphoric acid resistance and improve battery life characteristics. However, this electrolyte retention matrix could not solve the above problems.

This invention was made to solve the above-mentioned problems, and was carried out with the aim of improving the permeability and retention of phosphoric acid in the electrolyte retention matrix, improving battery characteristics and long-term stability. It is something that

[Means to solve the problem]

The method for producing an electrolyte retention matrix for fuel cells of the present invention involves kneading and molding an electrolyte retention agent, a binder, and a plasticizer made of boron carbide and silicon carbide, and removing the plasticizer by solvent extraction.

A method for producing an electrolyte retention matrix for fuel cells according to another aspect of the present invention includes kneading and molding an electrolyte retention agent, a binder, and a plasticizer made of carbon powder and silicon carbide, and removing the plasticizer by solvent extraction. It is something.

[Effect]

The electrolyte retention matrix of the present invention uses mixed grains containing SiC and boron carbide or carbon powder that has excellent affinity with phosphoric acid as an electrolyte retention agent, and also has no sintering process. The fibrillated binder in the process only fixes the electrolyte retaining agent with entangled fibrils, and unlike conventional methods, the molten binder does not cover the surface of the electrolyte retaining agent, which reduces the permeability of phosphoric acid and Improves retention. In addition, since no firing process is performed, the electrolyte retention matrix is extremely flexible, and as shown in Figure 1, an electrolyte retention matrix containing phosphoric acid is laminated between a pair of gas diffusion electrodes. When pressure is applied, the adhesion between the electrode and matrix is high, and ohmic loss due to contact resistance is significantly reduced.

Furthermore, when carbon powder is included, the carbon powder acts as a lubricant when the composition is roll-molded into a sheet, making it easier to form a thin film.

〔Example〕

The boron carbide (B4C) according to this invention is silicon carbide (S
Like iC), it is a covalent carbide and has the characteristics of being extremely hard and chemically extremely stable.

The density is 2.5 f/crn'', the melting point is 2450°C, and the specific electrical resistance is 0.9%.Since the density is close to 3.2 of SiC, it is easy to mix particles, and the specific electrical resistance is also io of SiC.

~200Ω・The specific electrical resistance is lower than that of the box, but compared to 10−5 to 10−4Ω・α of interstitial carbides, which are carbides of transition metal elements, the specific electrical resistance is sufficiently high, and by mixing SiC grains,
It is expected that it will exhibit electronic insulating properties to a level that does not pose any practical problems. The mixing ratio of boron carbide to the electrolyte holding agent is preferably 10% by weight or more. 10% by weight
Below, the effect of improving the affinity with phosphoric acid is small.

As the carbon powder according to the present invention, for example, at least one type of amorphous carbon crystalline graphite is used, and graphite having an average particle size of 0.1 to 10 μm is particularly preferably used. Since graphite has electronic conductivity7, it is judged to be unsuitable as a matrix material from the viewpoint of electrical insulation as shown in the required properties of the matrix above, but graphite has a specific electrical resistance of 100 to 200 Ω. By mixing grains with SiC, it is expected that electronic insulation properties to a level that does not pose a problem in practical use will be exhibited. The mixing ratio of carbon powder to the electrolyte holding agent is preferably 10 to 35% by weight. If it is less than 10% by weight, the effect of improving the affinity with phosphoric acid will be small, and if it is more than 35% by weight, the resistance will be low.

The binder used in this invention is, for example, polytetrafluoroethylene (PTFE), which has excellent heat resistance and chemical resistance.
, tetrafluoroethylene hexafluoropropylene copolymer (FE
P), polyperfluoroalkoxyethylene (PFA)
, polyfluorinated ethylene propylene ether (PFEPE)
) and the like are preferably used. If the mixing ratio of the binder to the electrolyte holding agent is less than 3% by weight, the mechanical strength will be weak and it will be difficult to form a sheet, and if it is more than 15% by weight, the water resistance will become too high, so a ratio of 3 to 15% by weight is suitable.

As the plasticizer according to the present invention, dialkylphthalates such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dipropylphthalate, diacyl phthalate, and dioctyl phthalate are preferably used.

The content of plasticizer in the freeze-dried powder consisting of an electrolyte holding agent and a binder is preferably 30 to 50% by weight.

If it is less than 30% by weight, it will not become clay-like and will fall apart, and if it is more than 50% by weight, it will become too soft and the moldability will be poor in both cases, so the above range is preferable.

Alcohols such as methyl alcohol, ethyl alcohol and propyl alcohol are preferably used as the solvent in this invention.

A method for producing an electrolyte retention matrix for a fuel cell according to an embodiment of the present invention will be described below.

Aqueous dispersion of PTFH using as a binder an electrolyte retaining agent of mixed particles containing silicon carbide (SiC) and boron carbide (B4C) or carbon powder (graphite) in an amount of 10% by weight based on the binder. Disperse it in an aqueous solution diluted with a predetermined amount of pure water, transfer it to a shallow vat, and freeze it in a freezer. The frozen state was suctioned and dried using a vacuum pump equipped with a trap to prepare a freeze-dried powder in which the electrolyte holding agent and binder were uniformly dispersed.

Next, add 4 ml of dimethyl phthalate as a plasticizer to the freeze-dried powder.
0% by weight was added and kneaded using a pressure kneader and a mixing roll. Although the degree of fibrillation of PTFE varies depending on the crosstalk conditions, a sticky clay-like composition can be obtained.

Form the above composition into a sheet with a thickness of 1 to 2 mm using a mixing roll, and then use a calender roll to form a sheet with a thickness of 10 mm.
It was molded into a sheet having a size of 0 to 300 μm.

Finally, plasticity, which is essential for crosstalk and forming into a sheet, but unnecessary when used as an electrolyte retention matrix, is extracted and removed using methyl alcohol as a solvent and dried. I got the matrix.

The electrolyte retention matrix must be a good conductor of hydrogen ions and an electronic insulator as described above. That is, it is necessary to be an electronic insulator with an electrical resistance of 10 6 Ω· or more in the absence of phosphoric acid, which is an electrolyte.

Electronic insulation evaluation test (1) SiC mixed particle ratio is 0.20, 40, 60.80
Weight%, that is, the mixed grain ratio of B and C is 100.80, 60.40
An electrolyte retention matrix was prepared using an electrolyte retention agent containing 20% by weight, and for comparison, a sheet-like electrolyte retention matrix was also prepared using an electrolyte retention agent with SiC of 10096 in the same manner as above. Retention agent Si
The relationship between C mixed particle ratio and specific electrical resistance was investigated. The above electrolyte retention matrix was sandwiched between electrodes without phosphoric acid, and 4 kf
The specific electrical resistance was measured with a surface pressure of 7 cm2 applied. The results are shown in (A1) in the characteristic diagram of FIG. The vertical axis represents the specific electrical resistance (Ω·cIrL), and the horizontal axis represents the SiC mixed particle ratio (wt%). As is clear from Figure 2, Si
It was confirmed that even when the mixed particle ratio of C was 0% by weight, electronic insulation of 10 6 Ω·α or more was exhibited, which fell into the insulator region.

When phosphoric acid is added to the electrolyte retention matrix, the electrical resistance is 10-2 to 10-1 as shown in the curve (13+) in Figure 2 because hydrogen ions act as charge carriers.
Ω·α, and exhibits electronic insulating properties to the extent that it does not pose a practical problem even when the SiC mixed particle ratio is 0% by weight.

In addition, when the mixed grain ratio of SiC is high, the effect of improving the affinity with phosphoric acid is small, so considering the above results, B4C
The mixed grain ratio is preferably in the range of 10% by weight or more.

(2) Graphite with an average particle size of 1 μm and an average particle size of 2 μm
using SiC with a graphite mixed grain ratio of 20 and 40
.

Electrolyte retention matrices were prepared using 60 and 80 wt% electrolyte retention agents, and for comparison, graphite or SiC was used in the same manner as above.

% electrolyte retention agent was used to prepare a sheet-like electrolyte retention matrix, and the relationship between graphite mixed particle ratio of the electrolyte retention agent and specific electrical resistance was investigated. The above electrolyte holding matrix was sandwiched between electrodes without phosphoric acid, and 4 ky/crn2
The specific electrical resistance was measured with a surface pressure of . The results are shown in curve (A2) in FIG. The vertical axis is the specific electrical resistance (
Ω·C7rL), and the horizontal axis represents the graphite mixed particle ratio (weight 96). As is clear from Figure 3, the specific electrical resistance is 10 when the graphite mixed particle ratio is up to 35% by weight.
It exhibits an electronic insulation property of more than 6 Ω·α, but when it exceeds 35% by weight, it becomes less than 10 6 Ω·α, which falls into the twisted conductor region. The specific electrical resistance when phosphoric acid is added to the electrolyte holding matrix is 10-2 to 10-1 Ω, as shown in curve (B2) in Figure 3, because it acts as a hydrogen ion or charge carrier.
If the proportion of mixed graphite particles is 35% by weight or less, it exhibits electronic insulation to the extent that it does not pose a practical problem.

If the mixed grain ratio of graphite is high, electronic insulation properties cannot be maintained, and if the mixed grain ratio of SiC is high, the effect of improving the affinity with phosphoric acid is small. Considering the above results, the mixed grain ratio should be 10~ Preferably, it is in the range of 35% by weight.

Next, graphite or B4 is added to the SiC of the embodiment of this invention.
In order to confirm the effect of improving the phosphoric acid affinity of the electrolyte retention matrix mixed with C, the improvement of battery characteristics, and the improvement of its long-term stability, graphite or B was added to SiC.
A sheet-like electrolyte retention matrix containing 4C particles was prepared. The manufacturing method will be specifically described below by giving examples.

Example 1 11! 60% by weight aqueous dispersion of PTFE (manufactured by Mitsui Fluorochemicals) 16tyy (PTF
The amount of E is 100 F), and the mixture is heated to 50 Or
B, C90J with an average particle size of 1.5 μm while stirring at pm
After adding little by little and mixing uniformly, the mixture was transferred to a stainless steel vat and frozen in an ultra-low temperature freezer at -40°C.

The frozen state is suctioned and dried using a vacuum pump with a trap, and the electrolyte retaining agent and binder are uniformly dispersed.
A freeze-dried powder of f was prepared. The composition of the above freeze-dried powder is B
100% by weight of electrolyte retention agent consisting of 4C, 1% of binder
It is 0% by weight.

A mixture of 1 ky of the freeze-dried powder and 400 f of dibutyl phthalate was kneaded using a pressure kneader.
A sticky viscous composition was obtained. The above composition was formed into a sheet with a thickness of about 2711 ml using a mixing roll, and then formed into a sheet with a thickness of about 200 μl using a calendar roll.
It was molded into a sheet shape of m.

Next, the matrix formed into a sheet is immersed in a stainless steel vat containing ethyl alcohol for 2 hours to completely extract and remove dibutyl phthalate, and then dried. A spore retention matrix was obtained.

Example 2 180g of B4C and 5iC720F with an average particle size of 2fim were used as the electrolyte retention agent to be added, and B4C was 20% by weight.
A sheet-shaped electrolyte retention matrix for a fuel cell according to another example of the present invention having a film thickness of 220 μm was obtained in the same manner as in Example 1 except that the SiC content was 80% by weight.

Example 3 40% by weight of B4C as an electrolyte retention agent, average particle size 2μ
Example 1 except that the mixed grain ratio of m SiC was 60% by weight.
In the same manner as above, a sheet-like electrolyte retention matrix for a fuel cell according to another example of the present invention having a film thickness of 200 μm was obtained.

Example 4 60% by weight of B4C as an electrolyte retention agent, average particle size 2μ
Example 1 except that the mixed grain ratio of m SiC was 40% by weight.
In the same manner as above, a sheet-like electrolyte retention matrix for a fuel cell according to another example of the present invention having a film thickness of 210 μm was obtained.

Example 5 80% by weight of B4C as electrolyte retention agent, average particle size 2μ
Example 1 except that the mixed grain ratio of m SiC was 20% by weight.
In the same manner as above, a 210 .mu.m thick sheet-like 7 solyte retaining matrix for fuel cells according to another embodiment of the present invention was obtained.

Example 6 Same as Example 1 except that graphite of 90F with an average particle size of 1 μm and SiC of 81of with an average particle size of 2 μm were used as electrolyte retention agents, and the composition of the freeze-dried powder was 10% by weight of graphite and 90% by weight of 5iC. The film thickness is 220μ
An electrolytic matrix for a fuel cell according to another example of the present invention was obtained in the form of a sheet of m.

Example 7 20% by weight of graphite and SiC7 as electrolyte retention agent
A sheet-shaped electrolyte retention matrix for a fuel cell according to another example of the present invention having a film thickness of 210 μm was obtained in the same manner as in Example 6 except that the mixed grain ratio was 0% by weight.

Example 3 Graphite 30% by weight, SiC7 as electrolyte retention agent
A sheet-shaped electrolyte retention matrix for a fuel cell according to another example of the present invention having a film thickness of 200 μm was obtained in the same manner as in Example 6 except that the mixed grain ratio was 0% by weight.

Comparative example 5iC1 with 90 of SiC added as an electrolyte holding agent
The film thickness was 23% by weight in the same manner as in Example 1 except that the
A sheet-like conventional electrolyte retention matrix for fuel cells with a thickness of 0 μm was obtained.

In the above Examples and Comparative Examples, the mixed particle ratio of B and C used as electrolyte holding agents and the prepared electrolyte Table 1 Table 2 According to it, since the average particle diameters of B, C and graphite and SiC are close to each other, the pore size, Both porosity are 10
It can be said that it is almost the same as the electrolyte retention matrix of the comparative example using 0% SiC.

Phosphoric Acid Affinity Test The contact angle of phosphoric acid of the electrolytic retention matrices for fuel cells (Examples 1 to 8) of the above embodiments of the present invention is measured. That is, the electrolyte matrices (Examples 1 to 8) according to the above embodiments of the present invention were placed horizontally on a sample heating table maintained at a predetermined temperature in the temperature range of room temperature to 200°C, and
The change in contact angle over time after dropping phosphoric acid was observed.

The results are shown in Figure 4 (Examples 1 to 5) of the characteristic diagram showing the temperature change in contact angle 5 minutes after dropping and Figure 5 (Examples 6 to 8) of the characteristic diagram showing the change in contact angle over time. show.

In the figure, the vertical axis represents the contact angle (C degrees), and the horizontal axis represents the sample temperature (0
The characteristic curve (B) of Example 1 is the characteristic curve (B) of Example 5, the characteristic curve (Q is of the comparative example), and the characteristic curve (■) is the characteristic curve (T) of Example 6. ) shows the characteristic curve of Example 8 (F'l shows the characteristic of Example 8).

Example 2 overlapped with the characteristic curve, and Examples 3 and 4 were between the characteristic curve (and characteristic curve (B)).

As is clear from FIGS. 4 and 5 above, 100
Compared to a conventional electrolyte retention matrix using 96 SiC, it was confirmed that the sample containing graphite or B4C mixed with SiC had an improved affinity with phosphoric acid, resulting in a lower contact angle with phosphoric acid. . In particular, graphite and S
It can be seen that in the sample mixed with iC particles, the contact angle decreases more quickly over time because the iC particles penetrate more easily into the interior than the surface.

Phosphoric Acid Retention Test Next, as a measure of the retention of phosphoric acid, which is an electrolyte, the amount of phosphoric acid transferred was measured based on the change over time in the amount of phosphoric acid in the matrix constituting a unit cell as shown in FIG. The results are shown in FIGS. 6 and 7, which are characteristic diagrams showing changes over time in the amount of phosphoric acid transferred. In the figure, the vertical axis is the amount of phosphoric acid transferred (m17cm”)
, the horizontal axis represents the elapsed time (h), the characteristic curve ■ is for Example 1, the characteristic curve (L) is for Example 3, and the characteristic curve (goods) is
is the characteristic curve of Example 5, and the characteristic curve is of Comparative Example.
Characteristic curve (0) shows the change over time in the amount of phosphoric acid transferred in Example 6, characteristic curve (P) in Example 7, and characteristic curve (P) in Example 8. As is clear from FIGS. 6 and 7, the electrolyte retention matrix in which B4C or graphite is mixed with SiC particles has a small amount of phosphoric acid transfer, and it can be seen that the phosphoric acid retention property is improved. Furthermore, the foaming pressure was also improved due to the improved phosphoric acid retention of the matrix.

Furthermore, the electrolyte retention matrix according to the present invention is not fired and is therefore extremely flexible, so it has excellent adhesion to the electrodes, and the internal resistance of the battery is reduced by the thickness of the matrix.
The contact resistance extrapolated to the conventional (100% SiC,
1/3 to 1/3 of the contact resistance of PTFE 3 to 10%, fired product)
Improved to 1/2.

Battery Characteristics and Lifetime Test Next, in order to confirm the improvement effect on battery characteristics and lifespan, a unit battery as shown in FIG. 1 was constructed, and changes over time in ohmic loss based on cell voltage and internal resistance were measured. The results are shown in the 8th section.
9 and 9.

In the figure, the vertical axis is the cell voltage (mV) and the ohmic loss (
mV), and the horizontal axis represents operating time (h).

Also, the solid line plot (qr) (yl) (R1) is the cell voltage, and the broken line plot (Q2) (Y2) (R2
) corresponds to ohmic loss. Characteristic curve (Ql) (Q2)
is Example 3, characteristic curve (R1) (R2) is Example 8,
Characteristic curve (Yl) (Y2) shows the case where the electrolyte retention matrix of the comparative example was used. As is clear from Figures 8 and 9, the electrolyte matrix containing B, C, or graphite mixed with SiC has improved phosphoric acid retention, and as a result, the ohmic loss does not change even after 1000 hours, and the cell voltage Also, the decrease was less than 10 mV after 1000 hours.

On the other hand, in the comparative example, the ohmic loss gradually started to increase after 600 hours, and the cell voltage also increased by 2 after 1000 hours.
'It showed a decrease of more than OmV. Therefore, by using a matrix in which B4C or graphite is mixed with SiC, the battery characteristics were improved and long-term stability could be achieved.

〔Effect of the invention〕

As explained above, the present invention improves phosphoric acid permeability and retention by kneading and molding an electrolyte retention agent, a binder, and a plasticizer made of boron carbide and silicon carbide, and removing the plasticizer by solvent extraction. It is possible to obtain a method for producing an electrolytic retention matrix for a fuel cell, which has improved properties, improved cell properties, and excellent long-term stability.

Another invention of the present invention is to knead and mold an electrolyte holding agent, a binder, and a plasticizer made of carbon powder and silicon carbide,
By removing the plasticizer by solvent extraction, it is possible to obtain a method for producing an electrolyte retention matrix for a fuel cell that not only provides the above effects but also allows easy formation of a thin film.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view of a general fuel cell, and FIG. 2 is a comparison of the SiC mixed particle ratio (wt% ) A characteristic diagram showing the change in specific electrical resistance (Ω・crrL), and FIG. 3 shows the graphite mixed particle ratio (weight 96 ) by specific electrical resistance (Ω
・It) A characteristic diagram showing changes in the phosphoric acid contact angle (0) comparing an electrolyte retention matrix mixed with B4C according to an embodiment of the present invention with a conventional one, and FIG. Figure 5 is a characteristic diagram showing the change in phosphoric acid contact angle over time, comparing the graphite-mixed electrolyte retention matrix according to an embodiment of the present invention with a conventional matrix. A characteristic diagram showing the change in phosphoric acid transfer B1 (my/Crf) over time (hours) comparing an electrolyte retention matrix mixed with B4C and a conventional one. Elapsed time C of phosphoric acid transfer amount (my/crIL2) comparing the electrolyte retention matrix and the conventional one
Fig. 8 is a battery characteristic diagram comparing the electrolyte retention matrix mixed with B4C according to the embodiment of this invention and a conventional one, and Fig. 9 is a characteristic diagram showing the change in graphite according to the embodiment of this invention. FIG. 3 is a battery characteristic diagram comparing a mixed electrolyte retention matrix and a conventional one. In the figure, (3) is an electrolyte retention matrix.

Claims (2)

[Claims] (1) A method for producing an electrolyte retention matrix for a fuel cell, which comprises kneading and molding an electrolyte retention agent, a binder, and a plasticizer made of boron carbide and silicon carbide, and removing the plasticizer by solvent extraction. (2) A method for producing an electrolyte retention matrix for a fuel cell, which comprises kneading and molding an electrolyte retention agent, a binder, and a plasticizer made of carbon powder and silicon carbide, and removing the plasticizer by solvent extraction.