JP2005174587A - Gel electrolyte and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gel electrolyte capable of stably exerting excellent proton conductivity over a long time in a work condition without humidification or at relative humidity below 50% at working temperature around 100-300°C. <P>SOLUTION: This gel electrolyte is prepared by mixing an acid with a polymer compound swollen by the acid. The gel electrolyte wherein the polymer compound is partially-methylated polybenzimidazole prepared by using a methyl group in at least a part of a substituted group R of a polybenzimidazole structure is employed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gel electrolyte exhibiting good proton conductivity even at an operating temperature of 100 ° C. or higher and 300 ° C. or lower and no humidification or a relative humidity of 50% or less, and a fuel cell using this gel electrolyte.

  In fuel cells, from the viewpoint of power generation efficiency, system efficiency, and long-term durability of components, good protons can be obtained at operating temperatures of about 100 ° C to 300 ° C under non-humidified or low humidity operating conditions of 50% or less. There is a demand for an electrolyte membrane that exhibits stable long-term conductivity. In the development of a conventional solid polymer electrolyte fuel cell, it has been studied in view of the above requirements, but perfluorocarbon sulfonic acid membranes have sufficient protons at an operating temperature of 100 ° C. to 300 ° C. and a relative humidity of 50% or less. There was a drawback that conductivity and output could not be obtained.

The following patent document discloses a solid electrolyte membrane made of polybenzimidazole doped with a strong acid such as phosphoric acid. According to this type of solid electrolyte membrane, it has excellent oxidation resistance and heat resistance, and can be operated even at a high temperature up to 200 ° C.
Japanese National Patent Publication No. 11-503262

  However, even the solid electrolyte membrane described in Patent Document 1 has a problem that sufficient proton conductivity cannot be obtained in an environment where the relative humidity is low.

  The present invention has been made in view of the above circumstances, and, at an operating temperature of about 100 ° C. to 300 ° C., stably exhibits long-term proton conductivity under an operating condition of no humidification or a relative humidity of 50% or less. It is an object of the present invention to provide a gel electrolyte that can be used and a fuel cell using the gel electrolyte.

In order to achieve the above object, the present invention employs the following configuration.
The gel electrolyte of the present invention is a gel electrolyte formed by mixing an acid and a polymer compound that swells with respect to the acid, and the polymer compound has a polybenzimidazole structure substitution represented by the following [Chemical Formula 1]. It is a partially methylated polybenzimidazole in which at least a part of the group R is a methyl group. However, in the following [Chemical Formula 1], R is CH ₃ or H, and n is 10 to 100,000.
Examples of the acid include phosphoric acid. The phosphoric acid includes both orthophosphoric acid and condensed phosphoric acid.

  According to the above configuration, partially methylated polybenzimidazole is used as the polymer compound, and the higher the degree of methylation, the more acid is taken in and swelling (gelation). Compared with, proton conductivity of the gel electrolyte can be improved.

  The gel electrolyte of the present invention is the gel electrolyte described above, wherein the partially methylated polybenzimidazole is polymethylated benzimidazole having a methylation rate of less than 80 mol%.

  According to said structure, since a methylation rate is less than 80 mol%, a partially methylated polybenzimidazole is not dissolved with an acid.

  The gel electrolyte of the present invention is the gel electrolyte described above, wherein the partially methylated polybenzimidazole is a poly-N methylbenzimidazole having a methylation rate of 100 mol% and a polybenzimidazole having a methylation rate of 0 mol%. And the poly N methylbenzimidazole content is less than 80 mol%.

  According to the above configuration, as the partially methylated polybenzimidazole, a mixture of polymethylbenzimidazole and polybenzimidazole having a methylation rate of 0 mol% is used, and the partial methylation can be achieved by changing the blending of the mixture. The methylation rate of polybenzimidazole can be easily changed, and the properties of the gel electrolyte can be easily optimized.

The gel electrolyte of the present invention is the gel electrolyte described above, wherein the partially methylated polybenzimidazole is a polymethylated benzimidazole having a methylation rate of X mol%, and a polybenzimidazole having a methylation rate of 0 mol%. When the weight of the polymethylated benzimidazole is A and the weight of the polybenzimidazole is B, f obtained by f (mol%) = AX / (A + B) is less than 80 mol%. It is characterized by that.
However, said X is 80 mol% or more and less than 100 mol%.

  According to the above configuration, as the partially methylated polybenzimidazole, a mixture of polymethylated benzimidazole with a methylation rate of X% and polybenzimidazole with a methylation rate of 0 mol% is used, and the blending of the mixture is changed. By doing so, the methylation rate of the partially methylated polybenzimidazole can be easily changed, and the characteristics of the gel electrolyte can be easily optimized.

  Next, a fuel cell according to the present invention includes a pair of electrodes and an electrolyte membrane disposed between the electrodes, and a part or all of the electrolyte membrane is the gel electrolyte described above. In addition, the gel electrolyte is contained in a part of the electrode.

  According to the above configuration, the gel electrolyte excellent in proton conductivity is provided as an electrolyte membrane, and further this gel electrolyte is provided in a part of the electrode, so that the internal impedance of the fuel cell can be reduced, The current density can be increased. In particular, when the gel electrolyte is contained in a part of the electrode, protons are easily conducted to the inside of the electrode, and the internal resistance of the electrode itself can be reduced.

  The fuel cell of the present invention preferably has an operating temperature in the range of 100 ° C. or higher and 300 ° C. or lower.

  As described above, according to the gel electrolyte of the present invention, the proton conductivity can be increased, and the current density of the fuel cell can be increased by using this gel electrolyte in the fuel cell, and the high output A simple fuel cell can be configured.

Hereinafter, embodiments of the present invention will be described in detail.
The fuel cell according to the present invention includes a hydrogen electrode (electrode), an oxygen electrode (electrode), and a gel electrolyte disposed between the hydrogen electrode and the oxygen electrode, and operates at a temperature of 100 ° C to 300 ° C. To do. The gel electrolyte according to the present invention has proton conductivity, and conducts protons (hydrogen ions) generated on the hydrogen electrode side to the oxygen electrode side. Protons conducted by the gel electrolyte electrochemically react with oxygen ions at the oxygen electrode to generate water and generate electrical energy.

  In the fuel cell according to the present invention, the hydrogen electrolyte and the oxygen electrode also contain a gel electrolyte. That is, the hydrogen electrode and the oxygen electrode contain an electrode material such as activated carbon and a binder for solidifying and forming the electrode material, and a part or all of this binder is the gel electrolyte according to the present invention. Yes. With this configuration, protons are easily conducted between the inside and outside of the electrode, and the internal resistance of the electrode is reduced.

Next, the gel electrolyte according to the present invention is configured by mixing phosphoric acid and a polymer compound that swells with respect to phosphoric acid. Examples of phosphoric acid include orthophosphoric acid and condensed phosphoric acid. Moreover, as a high molecular compound, the partially methylated polybenzimidazole which made the methyl group the at least one part of the substituent R of the polybenzimidazole structure shown in said [Chemical Formula 1] can be illustrated. In the following [Chemical Formula 1], R is CH ₃ or H, and n is 10 to 100,000. When n is less than 10, the mechanical strength is usually inferior, which is not preferable. When n exceeds 100,000, the solubility in a solvent or the like is significantly reduced.

  Partially methylated polybenzimidazole incorporates phosphoric acid and gels, and in particular, the higher the degree of methylation, the more phosphoric acid is incorporated and swells (gels). Thus, since the gel electrolyte which concerns on this invention can contain many phosphoric acids, it can improve proton conductivity.

As the partially methylated polybenzimidazole according to the present invention, any of the following (1) to (3) can be used.
(1) A polymethylated benzimidazole having a methylation rate adjusted to 5 mol% or more and less than 80 mol%, preferably 20 mol% or more and less than 80 mol%.
(2) A mixture of poly N methylbenzimidazole with a methylation rate of 100 mol% and polybenzimidazole with a methylation rate of 0 mol%, and the content of poly N methylbenzimidazole is 5 mol% or more and less than 80 mol% , Preferably 20 mol% or more and less than 80 mol%.
(3) A mixture of a polymethylated benzimidazole with a methylation rate of X mol% and a polybenzimidazole with a methylation rate of 0 mol%, where the weight of the polymethylated benzimidazole is A and the weight of the polybenzimidazole is B , F obtained by f (mol%) = AX / (A + B) is 5 mol% or more and less than 80 mol%, preferably 20 mol% or more and less than 80 mol%. However, said X is 80 mol% or more and less than 100 mol%.

According to the constitutions (1) to (3), the methylation rate of the partially methylated polybenzimidazole is 5 mol% or more and less than 80 mol%, preferably 20 mol% or more is less than 80 mol%. Can do. That is, in (1), the methylation rate can be adjusted by adjusting the degree of reaction for methylation. In (2), the methylation rate can be substantially adjusted by adjusting the content of poly-N methylbenzimidazole. Furthermore, in (3), the methylation rate can be substantially adjusted by adjusting the value of f.
When the methylation rate of the partially methylated polybenzimidazole is less than 5 mol%, phosphoric acid can be sufficiently taken in and proton conductivity becomes insufficient (because it decreases). When the conversion ratio is 80 mol% or more, phosphoric acid is excessively incorporated and the partially methylated polybenzimidazole is dissolved, which is not preferable.

  In the present specification, a part of the substituent R of the polybenzimidazole structure shown in the above [Chemical Formula 1] is a methyl group and the remainder is hydrogen is called polymethylated benzimidazole. Moreover, what made all the substituents R of the polybenzimidazole structure shown in the above [Chemical Formula 1] a methyl group is called poly N methylbenzimidazole. Furthermore, what made all the substituent R of the polybenzimidazole structure shown in the above [Chemical Formula 1] hydrogen is called polybenzimidazole.

  As described above, according to the gel electrolyte according to the present invention, the proton conductivity can be increased, and the current density of the fuel cell can be increased by using this gel electrolyte in the fuel cell. An output fuel cell can be configured.

  Next, the gel electrolytes of Examples 1 to 4 and Comparative Examples 1 to 2 and fuel cells using the gel electrolytes were manufactured, and various characteristics were evaluated.

[Example 1]
(Partial methylation of polybenzimidazole)
A dimethylacetamide solution prepared so that the concentration of polybenzimidazole was 10% by mass was prepared, and 30.08 g thereof was weighed into a Schlenk tube. Under an argon gas flow, an excess amount of lithium hydride (0.24 g) was added little by little at room temperature, and then refluxed at 80 ° C. Once the temperature was lowered to room temperature, the reaction solution was cooled to 0 ° C. using an ice bath, 1.56 g of methyl iodide was weighed and slowly added dropwise to the reaction solution. After dropping, the mixture was gradually returned to room temperature while stirring and refluxed again at 60 ° C. The resulting reaction solution was cooled to room temperature and then developed into tetrahydrofuran to obtain a powdery solid. Further, the solid was washed with water until the pH became 7, and then vacuum-dried to obtain the desired partially methylated polybenzimidazole. The methylation rate was about 20 mol% when confirmed by the NMR integration ratio.

(Production of gel electrolyte)
The synthesized partially methylated polybenzimidazole is again dissolved in dimethylacetamide so as to be a 10% by mass solution, and this solution is coated on a glass plate using a doctor blade, and when the surface becomes opaque. It dried once at 50 degreeC and also dried at 150 degreeC. Then, the whole glass plate was immersed in the water tank, and the swollen film was peeled off. Thereafter, vacuum drying was performed at 60 ° C. and 0.1 torr to obtain a polymer film. The film thickness was about 30 μm.
Next, the obtained polymer film was directly immersed in 85% phosphoric acid at room temperature. After 2 hours, the membrane was pulled up and the surface phosphoric acid was wiped off with a Kimwipe. Thus, the gel electrolyte of Example 1 was manufactured.

[Examples 2-3]
The gel electrolyte of Example 2 was produced in the same manner as in Example 1 except that the methyl iodide charge ratio was changed to 40 mol%. Similarly, the gel electrolyte of Example 3 having a methylation rate of 60 mol% was produced.

[Example 4]
A dimethylacetamide solution containing 10% by mass of all-methylated poly-N methylbenzimidazole and a dimethylacetamide solution containing 10% by mass of polybenzimidazole not methylated at all are mixed at a mass ratio of 1: 1. did. Using this mixed solution, a polymer film was obtained by coating on a glass plate in the same manner as in Example 1, drying and immersing in a water bath. This polymer membrane was immersed in phosphoric acid in the same manner as in Example 1 to produce a gel electrolyte of Example 4 having a substantial methylation rate of 50 mol%.

  Further, a 10-fold diluted solution of the above-described polymer film was thinly applied to carbon paper coated with platinum-supported carbon manufactured by EI Electrochem, and then vacuum-dried to obtain an electrode. A fuel cell of Example 4 was manufactured by preparing two of the electrodes and sandwiching the gel electrolyte of Example 4 between the electrodes.

[Comparative Example 1]
A polymer film of partially methylated polybenzimidazole was obtained in the same manner as in Example 1 except that the methyl iodide charge ratio was changed to 80 mol%. This was used as a sample of Comparative Example 1.

[Comparative Example 2]
A gel electrolyte and a fuel cell of Comparative Example 4 made of polybenzimidazole having a methylation rate of 0% were prepared.

[Evaluation]
(Swelling rate and proton conductivity of gel electrolyte)
For the gel electrolytes of Examples 1 to 3 and Comparative Example 1, the swelling rate was calculated from the mass (M1) of the polymer membrane before soaking in phosphoric acid and the mass (M2) of the gel electrolyte after soaking in phosphoric acid. Calculated. The swelling rate (mass%) was obtained by the swelling ratio (mass%) = M2 / M1 × 100. The relationship between the immersion time in phosphoric acid and the swelling rate is shown in FIG.

  In addition, Table 1 shows the proton conductivity of the gel electrolytes of Examples 1 to 3 and Comparative Example 1. In order to measure proton conductivity under conditions close to non-humidification, the proton conductivity is punched into a circular shape having a diameter of 13 mm, sandwiched between platinum blocking electrodes, and left at 70 ° C. for 1 hour. The resistance between the electrodes was measured by the AC impedance method.

  As shown in FIG. 1, the swelling rate reached an equilibrium state after about 30 minutes. The equilibrium swelling ratio measured at the time when 70 minutes had elapsed was 450% in Example 1, 530% in Example 2, and 660% in Example 3. It can be seen that the higher the methylation rate, the higher the swelling rate of the gel electrolyte. On the other hand, in Comparative Example 1 in which the methylation rate was 80 mol%, the gel electrolyte itself was dissolved after 70 minutes from the immersion in phosphoric acid.

  Next, as shown in Table 1, it can be seen that in Examples 1 to 3, the proton conductivity improves as the methylation rate increases. On the other hand, since the comparative example 1 was not obtained as a self-supporting film as described above, the ionic conductivity could not be measured.

Similarly, when the swelling rate and proton conductivity of the gel electrolyte of Example 4 were measured, the equilibrium swelling rate after 70 minutes was about 600%, and the proton conductivity at 70 ° C. was 4.00 mS · cm ^{−. 1}
Furthermore, when the swelling rate and proton conductivity of Comparative Example 2 were measured, the equilibrium swelling rate was about 400%, and the proton conductivity at 70 ° C. was 1.90 mS · cm ⁻¹ .

(Fuel cell performance)
For the fuel cells of Example 4 and Comparative Example 2, a power generation test was conducted using a laminate of an electrode and a gel electrolyte sandwiched between carbon separators, using hydrogen as the anode gas and oxygen as the cathode gas. The battery temperature was 130 ° C., the supply amounts of hydrogen and oxygen were 100 ml / min, and the supply gas was not particularly humidified. Moreover, the electrode area of Example 4 was 7.84 cm < ² >, and the electrode area of Comparative Example 2 was 10.24 cm < ² >. FIG. 2 shows the relationship between the voltage and current density of the fuel cell.

As shown in FIG. 2, in Example 4, power generation was possible until the current density reached 1.6 A / cm ² , whereas in Comparative Example 2, power generation was possible only up to 0.8 A / cm ^2. It was. In the fuel cell of Example 4, since the proton conductivity of the gel electrolyte is high, it is considered that the internal resistance of the fuel cell is kept low, thereby obtaining a high output.

FIG. 1 is a graph showing the relationship between the swelling ratios of gel electrolytes of Examples 1 to 3 and Comparative Example 1 and the immersion time of phosphoric acid. FIG. 2 is a graph showing the relationship between the voltage and current density of the fuel cells of Example 4 and Comparative Example 2.

Claims (6)

A gel electrolyte obtained by mixing an acid and a polymer compound that swells with respect to the acid, wherein the polymer compound has at least a part of the substituent R of the polybenzimidazole structure represented by the following [Chemical Formula 1]. A gel electrolyte characterized by being a partially methylated polybenzimidazole having a methyl group.

In the above [Chemical Formula 1], R is CH ₃ or H, and n is 10 to 100,000.   The gel electrolyte according to claim 1, wherein the partially methylated polybenzimidazole is a polymethylated benzimidazole having a methylation rate of less than 80 mol%.   The partially methylated polybenzimidazole is a mixture of polyN methylbenzimidazole having a methylation rate of 100 mol% and polybenzimidazole having a methylation rate of 0 mol%, and the content of the poly N methylbenzimidazole is 80%. The gel electrolyte according to claim 1, wherein the gel electrolyte is less than mol%. The partially methylated polybenzimidazole comprises a mixture of a polymethylated benzimidazole having a methylation rate of X mol% and a polybenzimidazole having a methylation rate of 0 mol%, and the weight of the polymethylated benzimidazole is A, The gel electrolyte according to claim 1, wherein f obtained by f (mol%) = AX / (A + B) is less than 80 mol%, when the weight of polybenzimidazole is B.
However, said X is 80 mol% or more and less than 100 mol%.   It is comprised from a pair of electrode and the electrolyte membrane arrange | positioned between each electrode, A part or all of the said electrolyte membrane is made into the gel electrolyte in any one of Claims 1 thru | or 4, and A fuel cell, wherein the gel electrolyte is contained in a part of the electrode. The fuel cell according to claim 5, wherein the operating temperature is in a range of 100 ° C. or more and 300 ° C. or less.

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