JP2008153114A - Solid polymer electrolyte for fuel cell, manufacturing method of solid polymer electrolyte for fuel cell, and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid polymer electrolyte for a fuel cell having high heat resistance, high proton conductivity, and capable of manufacturing at a low price, and to provide its manufacturing method and a fuel cell equipped with the electrolyte. <P>SOLUTION: The solid polymer electrolyte for the fuel cell is prepared by adding a basic compound to a fluorocarbon polymer having at least (-CF<SB>2</SB>-CF(M)CH<SB>2</SB>-CF<SB>2</SB>-) structure (wherein M is either one of CX<SB>3</SB>, CX<SB>2</SB>H, and CXH<SB>2</SB>; and X is F), and impregnating at least one kind of acid selected from the group comprising orthophosphoric acid, condensed phosphoric acid, alkyl phosphoric acid and phosphonic acid. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid polymer electrolyte for a fuel cell, a method for producing a solid polymer electrolyte for a fuel cell, and a fuel cell, and in particular, under an operating temperature condition of 100 ° C. or higher, no humidification or a relative humidity of 50% or less. However, the present invention relates to a polymer electrolyte fuel cell having excellent proton conductivity and good power generation performance.

Regarding polymer electrolyte membranes for fuel cells, many researches on hydrocarbon polymer membranes were conducted in the early stages of the 1960s. In 1968, EIDu Pont de Nemours, Inc. As a result of the development of perfluorinated sulfonic acid (Nafion) membranes made of perfluoronate sulfonic acid, research has begun in earnest, and in particular, Nafion membranes have been mainly used as electrolyte membranes for stationary fuel cells and portable fuel cells. Applied and developing.
However, perfluorosulfonic acid membranes such as Nafion have the disadvantage that the manufacturing process is complicated and very expensive, despite the high chemical stability. In the case of a purple sulfonic acid membrane, a hydrophobic main chain and a hydrophilic side chain are phase-separated to form a so-called ion cluster structure. As a proton transport form, it is understood that many water molecules are taken into this cluster structure, and the dissociation of the sulfonic acid group is promoted, and at the same time, the high mobility of the water molecules is utilized to express high proton conductivity. ing. However, under a high temperature condition of 100 ° C. or higher, there is a problem that the operating temperature is limited to 70 ° C. to 80 ° C. because proton conductivity is reduced due to a decrease in water molecules and the membrane itself cannot maintain its shape. was there.

  For the purpose of improving proton conductivity of polymer electrolyte membrane under high temperature conditions, US Pat. No. 5,525,436 discloses an electrolyte membrane in which polybenzimidazole membrane (PBI), which is a basic polymer, is doped with phosphoric acid. ing. The membrane formed with the complex of the basic polymer and phosphoric acid exhibits proton conductivity even in the absence of water due to proton hopping by the basic site and phosphoric acid. However, the hydrocarbon-based electrolyte membrane does not have sufficient oxidation resistance performance, and has a drawback in that it exhibits sufficient power generation performance for a long time as a fuel cell. Further, PBI has a problem that it has high crystallinity because it has a benzene ring in the polymer main chain and low molecular mobility because the molecular chain is rigid.

Japanese Patent Application Laid-Open No. 2001-213987 discloses a high-temperature proton conductive electrolyte in which polyvinyl pyridine is graft-polymerized on an ethylene-tetrafluoroethylene copolymer film substrate (ETFE) using an electron beam irradiation method, and doped with phosphoric acid. Is disclosed. However, in general, when a film having high ion conductivity is produced using a fluororesin by a radiation graft reaction such as an electron beam or γ-ray, the film is very brittle and has a problem in durability. In addition, when polyvinylpyridine is graft-polymerized and a pyridine polymer is bonded as a side chain to the main chain of ETFE, the crystallinity of ETFE itself is lowered, thereby reducing the heat resistance of ETFE, and the fuel cell. When used as an electrolyte, there is also a problem that durability is lowered.
Furthermore, in order to carry out a radiation graft reaction such as an electron beam or γ-ray, an electron beam or γ-ray irradiation device is required, which increases the manufacturing cost.
US Pat. No. 5,525,436 JP 2001-213987 A

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a solid polymer electrolyte for a fuel cell excellent in heat resistance and proton conductivity, and a fuel cell provided with the electrolyte. In addition, the present invention can easily add a monomer having a basic functional group to a polymer of the main chain without using a radiation graft reaction, so that the solid state for fuel cells having excellent heat resistance and proton conductivity can be obtained. It aims at providing the method of manufacturing a molecular electrolyte.

In order to solve the above problems, superimposed present inventors have conducted extensive studies, soluble _{(-CF 2 -CF (CF 3)} CH 2 -CF 2 -) in an organic solvent to a fluorine-based polymer containing a structural As a result of research on introducing various basic compounds by reacting them in an organic solvent, the basic compound can be easily added to the fluorine-based polymer by selecting a basic compound containing an NH group or the like. It has been found that an added polymer can be synthesized. Further, the synthesized polymer has heat resistance and acid resistance at high temperature (150 ° C.) and has been found to exhibit high proton conductivity by doping the polymer with phosphoric acid or the like.
That is, the present invention employs the following configuration.

更に、本発明の燃料電池用高分子電解質においては、前記塩基性化合物が付加されたフッ素系ポリマーが、下記式（３）〜（５）に示す構造を有するものであることが好ましい。但し、式（３）または（４）において、ＭはＣＸ_３、ＣＸ_２Ｈ、ＣＸＨ_２の何れかであり、ＸはＦであり、ｍは１〜３の整数であり、Ｒ^２はＨまたはＣ_ｎＨ_２ｎ＋１（ｎは１〜３の整数）である。

The polymer electrolyte for fuel cells of the present invention (hereinafter sometimes referred to as electrolyte) has at least a (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure (where M is CX ₃ , CX _2). H, CXH ₂ and X is F), a basic compound is added to the fluoropolymer, and from among orthophosphoric acid, condensed phosphoric acid, alkylphosphoric acid, and phosphonic acid It is characterized by being impregnated with at least one selected acid.
As the basic compound, a compound having an NH group is preferable.
In the polymer electrolyte for a fuel cell of the present invention, it is preferable that a part of the basic compound crosslinks the fluoropolymer.
In the polymer electrolyte for a fuel cell of the present invention, the fluorine-based polymer is selected from vinylidene fluoride-hexafluoropropylene copolymer and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer. It is preferably at least one selected.
Furthermore, in the polymer electrolyte for a fuel cell of the present invention, the basic compound is preferably a heterocyclic compound containing an endocyclic nitrogen atom.
Furthermore, in the polymer electrolyte for fuel cells of the present invention, the basic compound is preferably an imidazole compound.
In the polymer electrolyte for fuel cells of the present invention, the basic compound is preferably an imidazole compound represented by the following formula (1) or (2). However, in the formula (1), ^{R 1} is H or _C m _H m _NH 2 is (m is an integer of 1 to 3), ^{R 2} is H or _C n _{H 2n +} 1 (n is an integer of 1 to 3) is there.

Furthermore, in the polymer electrolyte for a fuel cell of the present invention, it is preferable that the fluorine-based polymer to which the basic compound is added has a structure represented by the following formulas (3) to (5). However, in Formula (3) or (4), M is any one of CX ₃ , CX ₂ H, and CXH ₂ , X is F, m is an integer of 1 to 3, and R ² is H or C _n H _{2n + 1} (n is an integer of 1 to 3).

Next, the method for producing a polymer electrolyte for a fuel cell according to the present invention has at least a (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure (where M is CX ₃ , CX ₂ H, CXH ₂ ). And X is F) and a basic compound is dissolved in an organic solvent to form a mixed solution, and the above-mentioned (—CF ₂ —CF (M) CH ₂ —CF ₂ —) A step of reacting the structure with the basic compound, a step of heat-treating the precipitate obtained by removing the organic solvent from the mixed solution after the reaction, and orthophosphoric acid, condensation on the precipitate after the heat treatment And impregnating with at least one acid selected from phosphoric acid, alkylphosphoric acid and phosphonic acid.
In the method for producing a polymer electrolyte for a fuel cell according to the present invention, the fluorine-based polymer may be a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer. It is preferable that it is at least 1 type chosen from these.
Furthermore, in the method for producing a polymer electrolyte for a fuel cell according to the present invention, the basic compound is preferably a heterocyclic compound containing an endocyclic nitrogen atom.
Furthermore, in the method for producing a polymer electrolyte for a fuel cell of the present invention, the basic compound is preferably an imidazole compound.
Moreover, in the manufacturing method of the polymer electrolyte for fuel cells of this invention, it is preferable that the said basic compound is an imidazole type compound shown to following formula (6) or (7). However, in the formula (6), ^{R 1} is H or _C m _H m _NH 2 is (m is an integer of 1 to 3), ^{R 2} is H or _C n _{H 2n +} 1 (n is an integer of 1 to 3) is there.

Next, the fuel cell of the present invention comprises a pair of electrodes and an electrolyte membrane disposed between the electrodes, and the electrolyte membrane is the polymer electrolyte for a fuel cell according to any one of the above. It is characterized by becoming.
In the fuel cell of the present invention, it is preferable that a part of the electrode contains the polymer electrolyte for a fuel cell described above.

According to the polymer electrolyte for a fuel cell of the present invention, by adding a basic compound to the fluorine-based polymer, an acid such as phosphoric acid is easily doped at the position where the basic compound is added. Proton conductivity can be expressed. In addition, since the basic compound added to the fluorine-based polymer is a monomer, the crystallinity of the fluorine-based polymer is unlikely to decrease, thereby preventing the heat resistance of the fluorine-based polymer from decreasing and the operating temperature from being reduced. It can be suitably used as a polymer electrolyte of a fuel cell at about 100 ° C to 200 ° C.
In addition, since a part of the basic compound crosslinks the fluorine-based polymer, the stability of the electrolyte itself is improved, so that the temperature is about 100 ° C. to 200 ° C. in the state impregnated with an acid such as phosphoric acid. Even when placed in a high temperature atmosphere, there is no risk of the electrolyte dissolving.
In addition, since an imidazole compound having relatively excellent heat resistance is used as the basic compound, the heat resistance of the fluoropolymer to which the basic compound is added does not decrease, and the operating temperature is 100 ° C. to It can be suitably used as a polymer electrolyte of a fuel cell at about 200 ° C.

  According to the method for producing a polymer electrolyte for a fuel cell of the present invention, a basic compound can be easily added to a fluoropolymer by reacting a fluoropolymer having a specific structure with a basic compound in an organic solvent. Can be made. Moreover, since a part of basic compound bridge | crosslinks by heat-processing to the fluorine-type polymer after adding a basic compound, the electrolyte excellent in chemical stability can be manufactured.

According to the fuel cell of the present invention, since the above-mentioned electrolyte excellent in heat resistance and proton conductivity is provided as an electrolyte membrane, a condition of no humidification or a relative humidity of 50% or less at an operating temperature of 100 ° C. to 200 ° C. Even when it is operated with a good power generation performance can be shown.
In addition, since the electrolyte is contained in the electrode, proton conductivity inside the electrode can be increased.

  Hereinafter, the polymer electrolyte for fuel cells of the present invention, a method for producing the same, and a fuel cell will be described in detail.

[Polymer electrolyte for fuel cells]
The polymer electrolyte for a fuel cell according to the present invention (hereinafter sometimes referred to as an electrolyte) includes a basic compound added to a fluorine-based polymer having a specific structure, orthophosphoric acid, condensed phosphoric acid, It is constituted by impregnating at least one acid selected from alkylphosphoric acid and phosphonic acid.

(Fluoropolymer)
The fluorine-based polymer constituting the electrolyte according to the present invention has at least a (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure (where M is any one of CX ₃ , CX ₂ H, and CXH ₂ ). , X is F). An example of such a fluorine-based polymer is a polymer having a structure in which vinylidene fluoride and hexafluoropropylene are bonded. From the viewpoint of producing an electrolyte, a fluorine polymer that dissolves in an organic solvent such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), N, N-dimethylformamide (DMF), or ethyl acetate. preferable. Examples of such a fluoropolymer include vinylidene fluoride-hexafluoropropylene copolymer or vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer. All of these polymers are CF ₃ groups in which M is an electron withdrawing group. Then, the presence of the CF ₃ group, the carbon atom to which a CF ₃ group is attached, it occurs deviation in the electron cloud between the carbon atom bonded two hydrogen atoms as well as adjacent to the carbon atom, a CF ₃ group The carbon atom to which is bonded is σ−, and the carbon atom to which a hydrogen atom is bonded is σ +. As will be described later, a basic compound is added to a carbon atom to which a hydrogen atom is bonded to constitute an electrolyte according to the present invention.

(Basic compound)
The basic compound constituting the electrolyte according to the present invention is preferably a monomer of a heterocyclic compound having an NH group and containing an endocyclic nitrogen atom. As such a basic compound, for example, an imidazole compound represented by the following formula (8) or (9) is preferably used. Here, R ¹ in Formula (8) is H or C _m H _m NH ₂ (m is an integer of 1 to 3), and R ² is H or C _n H _{2n + 1} (n is an integer of 1 to 3). is there. The imidazole compound represented by the formula (8) or (9) is provided with an NH group, and nitrogen of the NH group is bonded to a carbon atom to which a hydrogen atom in the fluorine-based polymer is bonded. Further, an acid such as phosphoric acid is doped at the site of the nitrogen atom in the ring contained in the imidazole compound.

  Among the compounds represented by the general formulas (8) and (9), basic compounds represented by the following (10) to (14) are more preferable.

(acid)
The acid constituting the electrolyte according to the present invention is preferably at least one acid selected from orthophosphoric acid, condensed phosphoric acid, alkylphosphoric acid, and phosphonic acid. The acid content (doping amount) in the electrolyte is preferably in the range of 10% by mass to 500% by mass, and more preferably in the range of 50% by mass to 400% by mass. If the acid content is less than this range, the proton conductivity decreases, which is not preferable. On the other hand, if the acid content exceeds this range, the chemical stability in the temperature range of 100 ° C. to 200 ° C., which is the operating temperature of the fuel cell, is not preferable.

(Electrolyte structure)
As described above, the electrolyte according to the present invention is configured by adding a basic compound to a fluoropolymer and further impregnating with an acid. The fluoropolymer to which a basic compound is added is considered to have a structure represented by the following formula (15) to the following formula (17). In the formula (15) or (16), M is any one of CX ₃ , CX ₂ H, and CXH ₂ , X is F, m is an integer of 1 to 3, and R ² is H or C _n H _{2n + 1} (n is an integer of 1 to 3).
The following formula (15) is an example in which an imidazole compound in which R ¹ in formula (8) is H (hydrogen) is added. Further, the following formula (16) is an example in which an imidazole compound in which R ¹ in the formula (8) is a C _m H _2m NH ₂ group is added. Furthermore, the following formula (17) is an example in which a benzimidazole compound represented by the formula (9) is added.

In any case, it is considered that the imidazole compound is added to the fluoropolymer by bonding the nitrogen atom of the NH group of the imidazole compound to the carbon atom to which the hydrogen atom in the fluoropolymer is bonded. Thereby, it is considered that the ring nitrogen atom contained in the imidazole compound is left as it is, and the acid is doped at the position of the ring nitrogen atom.
Moreover, it is thought that another imidazole compound does not add with respect to the added imidazole compound. That is, the imidazole compound monomer is added to the fluoropolymer, and the imidazole compound polymer is not added. For this reason, the electrolyte according to the present invention has a structure in which a monomer composed of a basic compound is added to a main chain polymer composed of a fluorine-based polymer, which greatly reduces the crystallinity of the fluorine-based polymer. It is considered that the heat resistance is improved.

Moreover, in the electrolyte which concerns on this invention, it is preferable that a part of added basic compound bridge | crosslinks the main chains of a fluorine-type polymer. Since the chemical stability of the electrolyte is improved by crosslinking a part of the basic compound, even when it is placed in an environment where the operating temperature of the fuel cell is 100 ° C. to 200 ° C. in the state of being impregnated with an acid. The electrolyte does not dissolve.
However, if all of the basic compound is crosslinked, the acid is not impregnated, which is not preferable. The degree of crosslinking can be adjusted by controlling the heat treatment conditions after adding the basic compound to the fluoropolymer.

[Method for producing polymer electrolyte for fuel cell]
Next, the manufacturing method of the electrolyte which concerns on this invention is demonstrated. This manufacturing method is roughly composed of a reaction process, a heat treatment process, and an impregnation process.
(Reaction process)
First, a fluorine-based polymer having a (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure and a basic compound are prepared, and these are dissolved in an organic solvent to obtain a mixed solution.
Each of the fluorine-based polymer and the basic compound may be dissolved in the same type of organic solvent, or may be dissolved in different types of organic solvents and then mixed with each other. When different types of organic solvents are used, compatible solvents are preferable. As the organic solvent for the fluorine polymer, for example, N-methylpyrrolidone (NMP), N, N-dimethylformamide (DMF), ester solvents such as ethyl acetate, ketone solvents, and the like are preferable.
When the fluorine-based polymer is VdF-HFP, NMP, DMF, or the like is preferable. When VdF-HFP-TFE is used, ethyl acetate (ester solvent), acetone, methyl ethyl ketone (ketone solvent), or the like is preferable.
Moreover, as a solvent for a basic compound, NMP, dimethylacetamide (DMAc), N, N-dimethylformamide (DMF), water and the like are preferable.
Among the solvents for dissolving the fluoropolymer, NMP, DMF, DMAc, and ethyl acetate are compatible with each other. Therefore, a compound soluble in these may be selected as the solvent for the basic compound. Further, since ethyl acetate is compatible with water, water can be used as a solvent for basic compounds.
The mixing ratio of the fluorine-based polymer and the basic compound is preferably in the range of fluorine-based polymer: basic compound = 9: 1 to 1: 1 by mass ratio, and more preferably in the range of 8: 1 to 2: 1. If the amount of the basic compound is small, the impregnation rate of phosphoric acid and the like is not preferable. An excess of the basic compound is not preferable because the chemical stability of the electrolyte is lowered.

  Next, the basic solution is added to the fluoropolymer by heating the mixed solution at a temperature equal to or lower than the boiling point of the organic solvent and the basic compound. Note that this reaction can be performed at room temperature, but it is preferable to heat the reaction in order to accelerate the reaction and shorten the reaction time. The heating temperature varies depending on the boiling points of the solvent and the basic compound, but is preferably set to be equal to or lower than the boiling points of the solvent and the basic compound. However, heating may be performed at a temperature equal to or higher than the boiling point as long as an apparatus including a reflux mechanism can be used.

(Heat treatment process)
Next, the organic solvent is removed from the mixed solution after the reaction step to obtain a precipitate. Specifically, for example, after the reaction mixture is cast on a glass plate, the solvent is removed by heating to obtain a film-like precipitate. The film thickness of the precipitate can be controlled by adjusting the casting amount of the mixed solution. By this casting method, it is possible to simultaneously remove the solvent and form the electrolyte.
Next, the deposited precipitate is heat treated. By this heat treatment, a part of the basic compound in the precipitate is crosslinked. The heat treatment temperature and time may be set as appropriate, but as a guide, the heat treatment temperature and time may be set to such an extent that the acid impregnation rate in the impregnation step described later does not decrease.

(Impregnation process)
Next, the precipitate after the heat treatment is impregnated with an acid such as phosphoric acid. At this time, the time and temperature at which the impregnation with phosphoric acid or the like is performed can be appropriately selected depending on the type of precipitates and the like. In order to shorten the time, it is preferable to heat phosphoric acid or the like. However, if phosphoric acid or the like is impregnated at an excessively high temperature, the physical properties of the precipitate may change, so an appropriate temperature needs to be set.
Thus, the electrolyte according to the present invention is manufactured.

[Fuel cell]
FIG. 1 is an exploded perspective view showing an example of the fuel cell of the present embodiment, and FIG. 2 is a schematic sectional view of a membrane-electrode assembly constituting the fuel cell of FIG.
The fuel cell 1 shown in FIG. 1 has a schematic configuration in which two single cells 11 are sandwiched between a pair of holders 12 and 12. The single cell 11 includes a membrane-electrode assembly 10 and bipolar plates 20 and 20 disposed on both sides in the thickness direction of the membrane-electrode assembly 10, and has an operating temperature of 100 ° C. to 200 ° C. and humidity is not humidified or It operates under conditions of relative humidity of 50% or less. The bipolar plates 20 and 20 are made of conductive metal, carbon, or the like, and function as current collectors by being joined to the membrane-electrode assembly 10 respectively, and the catalyst of the membrane-electrode assembly 10. Supply oxygen and fuel to the bed.
The fuel cell 1 shown in FIG. 1 has two single cells 11, but the number is not limited to two, and may be increased to several tens to several hundreds depending on the characteristics required for the fuel cell. .

As shown in FIG. 2, the membrane-electrode assembly 10 was laminated on the electrolyte membrane 100, the catalyst layers 110 and 110 ′ disposed on both sides in the thickness direction of the electrolyte membrane 100, and the catalyst layers 110 and 110 ′, respectively. The first gas diffusion layers 121 and 121 ′ and the second gas diffusion layers 120 and 120 ′ stacked on the first gas diffusion layers 121 and 121 ′, respectively. The catalyst layers 110 and 110 ′, the first gas diffusion layers 121 and 121 ′, and the second gas diffusion layers 120 and 120 ′ constitute a pair of electrodes.
The electrolyte membrane 100 is made of the above-described polymer electrolyte for fuel cells, and is configured by adding a basic compound to a fluoropolymer and impregnating an acid such as phosphoric acid. The thickness of the electrolyte membrane 100 is preferably in the range of about 20 μm to 200 μm.

The catalyst layers 110 and 110 ′ function as a fuel electrode and an oxygen electrode, and each of the catalyst layers 110 and 110 ′ includes a catalyst material mainly composed of carbon black or the like and a binder that solidifies and forms the catalyst material. It is a sexual body. The catalyst material is configured such that a catalyst substance is supported on carbon black or the like. The catalyst material is not particularly limited as long as it is a metal that promotes the oxidation reaction of hydrogen and the reduction reaction of oxygen. For example, lead, iron, manganese, cobalt, chromium, gallium, vanadium, tungsten, ruthenium, iridium, palladium, platinum , Rhodium or alloys thereof. A catalyst material can be constituted by supporting such a metal or alloy on carbon black or the like.
As the binder, a fluororesin excellent in heat resistance may be used, or the polymer electrolyte for fuel cells according to the present invention may be used. By using the polymer electrolyte for the fuel cell as the binder, proton diffusion inside the catalyst layers 110 and 110 ′ can be efficiently performed, and the impedance of the catalyst layers 110 and 110 ′ is lowered to improve the output of the fuel cell.
When a fluororesin is used as the binder, a fluororesin having a melting point of 400 ° C. or lower is preferable. As such a fluororesin, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkylvinylether copolymer, polyvinylidene fluoride, Resins excellent in hydrophobicity and heat resistance such as tetrafluoroethylene / hexafluoroethylene copolymer and perfluoroethylene can be used. By adding the hydrophobic binder, it is possible to prevent the catalyst layers 110 and 110 ′ from being wetted excessively by the water generated during the power generation reaction, and the fuel gas and oxygen inside the fuel electrode and the oxygen electrode can be prevented. Diffusion inhibition can be prevented.

  Furthermore, a conductive material may be added to the catalyst layers 110 and 110 '. As the conductive material, any conductive material may be used, and various metals and carbon materials may be used. Examples thereof include carbon black such as acetylene black, activated carbon and graphite, and these are used alone or in combination.

  The first gas diffusion layers 121 and 121 ′ and the second gas diffusion layers 120 and 120 ′ are each formed of, for example, a carbon sheet, and catalyze oxygen and fuel supplied via the bipolar plates 20 and 20. Diffusion over the entire surface of the layers 110, 110 ′.

The fuel cell 1 including the membrane-electrode assembly 10 operates at a temperature of 100 ° C. to 200 ° C., for example, hydrogen is supplied as fuel to the one catalyst layer side via the bipolar plate 20, and the other catalyst layer side For example, oxygen is supplied as an oxidizing agent through the bipolar plate 20. Then, hydrogen is oxidized in one catalyst layer to generate protons, and the protons pass through the electrolyte membrane 4 to reach the other catalyst layer, and the proton and oxygen react electrochemically in the other catalyst layer. In addition to producing water, it generates electrical energy.
The hydrogen supplied as the fuel may be hydrogen generated by reforming hydrocarbons or alcohols, and the oxygen supplied as the oxidant may be supplied in a state of being included in the air.

According to the above polymer electrolyte for fuel cells, by adding a basic compound to the fluorine-based polymer, an acid such as phosphoric acid is easily doped at the position where the basic compound is added, thereby Conductivity can be expressed. In addition, since the basic compound added to the fluorine-based polymer is a monomer, the crystallinity of the fluorine-based polymer is unlikely to decrease, thereby preventing the heat resistance of the fluorine-based polymer from decreasing and the operating temperature from being reduced. It can be suitably used as a polymer electrolyte of a fuel cell at about 100 ° C to 200 ° C.
In addition, since a part of the basic compound crosslinks the fluorine-based polymer, the stability of the electrolyte itself is improved, so that the temperature is about 100 ° C. to 200 ° C. in the state impregnated with an acid such as phosphoric acid. Even when placed in a high temperature atmosphere, there is no risk of the electrolyte dissolving.
In addition, since an imidazole compound having relatively excellent heat resistance is used as the basic compound, the heat resistance of the fluoropolymer to which the basic compound is added does not decrease, and the operating temperature is 100 ° C. to It can be suitably used as a polymer electrolyte of a fuel cell at about 200 ° C.

  Moreover, according to said manufacturing method, a basic compound can be easily added to a fluorine-type polymer by making the fluorine-type polymer which has a specific structure, and a basic compound react in an organic solvent. Moreover, since a part of basic compound bridge | crosslinks by heat-processing to the fluorine-type polymer after adding a basic compound, the electrolyte excellent in chemical stability can be manufactured cheaply.

Furthermore, according to the fuel cell, since the electrolyte membrane is provided with the electrolyte having excellent heat resistance and proton conductivity and excellent water repellency, it is not humidified at an operating temperature of 100 ° C. to 200 ° C. Or even if it is a case where it operate | moves on the conditions of relative humidity 50% or less, favorable electric power generation performance can be shown.
In addition, since the electrolyte is contained in the electrode, proton conductivity inside the electrode can be increased.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in more detail, this invention is not limited by the following example.

"Example 1: Production example of electrolyte membrane"
(Experimental example 1)
Polyvinylidene fluoride (manufactured by Kureha Chemical Industry Co., Ltd., KF Polymer W # 1100 (hereinafter referred to as PVdF)) is prepared as a fluorine-based polymer, and this PVdF is dissolved in N-methylpyrrolidone (hereinafter referred to as NMP). A mass% solution was obtained. Further, imidazole (hereinafter referred to as Im) as a basic compound was dissolved in NMP to obtain a 20% by mass solution.
Then, a 20% by mass solution of PVdF and a 20% by mass solution of Im are mixed at a weight ratio of PVdF: Im = 4: 1, and stirred and refluxed at a temperature below the boiling point of NMP (80 ° C.). I let you. The reaction time was 60 hours.
The reaction solution after the reaction is cast on a glass substrate, and then the solvent is removed under the conditions of a heating temperature of 150 ° C. and a heating time of 30 minutes, and finally heat treatment is performed at 180 ° C. for 2 hours, did. The film thickness of the polymer film was 25 μm.
Then, the obtained polymer film was impregnated in an 85% phosphoric acid solution at 60 ° C. for 4 hours to dope phosphoric acid. In this way, the electrolyte membrane of Experimental Example 1 was manufactured.
The phosphoric acid dope rate was measured about the obtained electrolyte membrane. The doping rate is defined as W ₁ is the mass (g) of the polymer membrane, W ₂ is the mass (g) of the electrolyte membrane after phosphoric acid doping, and the doping rate (%) = 100 × (W ₂ −W ₁ ) / It was determined by the formula W _1. The results are shown in Table 1.

(Experimental example 2)
Using a vinylidene fluoride-hexafluoropropylene copolymer (ARKEMA, KYNAR2821 (hereinafter referred to as VdF-HFP)) as a fluorine-based polymer, 20% by mass solution of VdF-HFP dissolved in NMP and Im were added to NMP. In the same manner as in Experimental Example 1 except that the mixed solution with the 20% by mass solution dissolved in was stirred and reacted at a temperature lower than the boiling point of NMP (60 ° C.) at 60 ° C. for 60 hours. The electrolyte membrane of Experimental Example 2 was manufactured. The film thickness of the polymer film was 25 μm.
The phosphoric acid dope rate was measured about the obtained electrolyte membrane. The results are shown in Table 1.

(Experimental example 3)
Vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer as a fluoropolymer (manufactured by 3M, Dyneon THV 220A (VdF: 40 mol%, HFP: 20 mol%, TFE: 40 mol%) (hereinafter, VdF -HFP-TFE)) is dissolved in ethyl acetate to form a 20 mass% solution, and a mixed solution of a 20 mass% solution of VdF-HFP-TFE and a 20 mass% solution in which Im is dissolved in NMP is heated to a temperature. An electrolyte membrane of Experimental Example 3 was produced in the same manner as in Experimental Example 1 except that the reaction was performed while stirring under reflux at 60 ° C. for 60 hours. The film thickness of the polymer film was 25 μm.
The phosphoric acid dope rate was measured about the obtained electrolyte membrane. The results are shown in Table 1.

As shown in Table 1, it was found that the electrolyte membrane of Experimental Example 1 had a phosphoric acid doping rate of 0% and was hardly doped. In Experimental Example 1, when PVdF and Im were mixed and reacted, the reaction hardly proceeded and it was almost impossible to add PVdF to Im. This is considered to be because the addition reaction of Im did not proceed because the (—CF ₂ —CF (CF ₃ ) CH ₂ —CF ₂ —) structure does not exist in PVdF.
On the other hand, as shown in Table 1, the electrolyte membranes of Experimental Examples 2 and 3 had phosphoric acid doping rates of 10% and 43%, respectively, and it was found that phosphoric acid could be impregnated into the polymer membrane. This is probably because Im was added to VdF-HFP and VdF-HFP-TFE, respectively. In VdF-HFP and VdF-HFP-TFE, there is a (—CF ₂ —CF (CF ₃ ) CH ₂ —CF ₂ —) structure, which is considered to be due to the progress of the addition reaction of Im. .

In FIG. 3, the measurement result of the spectrum of FT-IR of the polymer film before impregnating the phosphoric acid in Experimental example 3 is shown. FIG. 3 shows the spectrum of VdF-HFP-TFE alone and the spectrum of Im simultaneously.
As shown in FIG. 3, several absorption peaks not observed in the spectrum of VdF-HFP-TFE alone are observed between 700 and 1500 cm ⁻¹ of the spectrum of Experimental Example 3. Some of the absorption peaks detected in Experimental Example 3 also have an absorption peak that overlaps with the absorption peak of the Im spectrum.
Thus, the spectrum of Experimental Example 3 is clearly different from the spectrum of VdF-HFP-TFE alone, which is thought to be due to the addition of Im.

FIG. 4 shows the TG (heating loss) measurement results of the polymer film before impregnating with phosphoric acid of Experimental Example 3. FIG. 4 also shows a TG curve of VdF-HFP-TFE alone and an Im TG curve.
As shown in FIG. 4, in the TG curve of Experimental Example 3, it is understood that a gradual weight reduction starts from around 200 ° C., and no weight reduction occurs at around 150 ° C., which is the Im weight reduction start temperature. . This indicates that Im is not impregnated in the fluorine-based polymer but Im is chemically bonded to the fluorine-based polymer.
In addition, it can be seen that the polymer membrane of Experimental Example 3 has almost no weight loss between 100 and 200 ° C., which is the operating temperature of the fuel cell, and is excellent in thermal stability.

“Example 2: Production example of electrolyte”
(Experimental Examples 4 to 8)
The above-mentioned VdF-HFP-TFE was prepared as a fluorine-based polymer, and this VdF-HFP-TFE was dissolved in ethyl acetate to obtain a 20% by mass solution. Further, imidazole, 1-methylimidazole, 2-methylimidazole, 2-ethylimidazole and benzimidazole were dissolved in NMP as basic compounds to form a 20% by mass solution.
And the 20 mass% solution of VdF-HFP-TFE and the 20 mass% solution of various basic compounds were mixed by the ratio of 4: 1 by weight ratio, and it stirred and made it react, making it recirculate | reflux at the temperature of 60 degreeC. The reaction time was 60 hours.
The reaction solution after the reaction is cast on a glass substrate, and then the solvent is removed under the conditions of a heating temperature of 150 ° C. and a heating time of 30 minutes. A membrane was obtained. The film thickness of each polymer film was 25 μm.
Each polymer film was impregnated with phosphoric acid by impregnating the polymer film in an 85% phosphoric acid solution at 60 ° C. for 4 hours. In this way, electrolyte membranes of Experimental Examples 4 to 8 were manufactured.
About the obtained electrolyte membrane, it carried out similarly to Example 1, and measured the phosphoric acid dope rate. The results are shown in Table 2.

As shown in Table 2, it was found that the electrolyte membrane of Experimental Example 5 had a phosphoric acid doping rate of 0% and was hardly doped. In Experimental Example 5, 1-methylimidazole in which H of the NH group of imidazole was substituted with CH ₃ was used, but this 1-methylimidazole does not have an NH group reactive to a fluorine-based polymer. Therefore, there is no reaction with respect to the fluorine-based polymer, and it is considered that phosphoric acid was hardly doped for this reason.
Further, it was found that the electrolyte of Experimental Example 8 had a low phosphoric acid doping rate of 1%. The benzimidazole used in Experimental Example 8 was expected to have a reaction activity because it had an NH group, but it is considered that the addition reaction did not proceed due to some other factor.

"Example 3: Production example of electrolyte membrane"
The above-mentioned VdF-HFP-TFE was prepared as a fluorine-based polymer, and this VdF-HFP-TFE was dissolved in ethyl acetate to obtain a 20% by mass solution. Further, imidazole as a basic compound was dissolved in NMP to obtain a 20% by mass solution.
Then, a 20 mass% solution of VdF-HFP-TFE and a 20 mass% solution of various basic compounds were mixed at a weight ratio of 9: 1, 4: 1, 3: 1 and 2: 1, respectively. The mixture was stirred and reacted while refluxing at a temperature of 60 ° C. The reaction time was 60 hours. The ratio of imidazole to the total amount of fluoropolymer and imidazole is 10% by mass (mixing ratio 9: 1), 20% by mass (mixing ratio 4: 1), 25% by mass (mixing ratio 3: 1) and It was set to 33.3 mass% (mixing ratio 2: 1).

Each reaction solution after the reaction is cast on a glass substrate, and then the solvent is removed under the conditions of a heating temperature of 150 ° C. and a heating time of 30 minutes, and finally heat treatment is performed at 180 ° C. for 2 hours. A molecular film was obtained. The film thickness of the polymer film was 25 μm.
Each polymer film was impregnated with phosphoric acid by impregnating the polymer film in an 85% phosphoric acid solution at 60 ° C. for 4 hours. In this way, various electrolyte membranes were produced.
About the obtained electrolyte membrane, the dope rate of phosphoric acid was measured. FIG. 5 shows the relationship between the doping rate and the ratio of imidazole to the total amount of fluoropolymer and imidazole.

  As shown in FIG. 5, it can be seen that as the ratio of imidazole increases, the doping rate also improves. However, for the electrolyte membrane having an imidazole ratio of 33.3% by mass, the addition reaction proceeded smoothly and a homogeneous polymer membrane was obtained. However, when the polymer membrane was impregnated with phosphoric acid, the strength was It was insufficient to maintain the shape of the film, and the dope rate could not be measured.

"Example 4: Production example of electrolyte membrane"
VdF-HFP-TFE was prepared as a fluorine-based polymer, and this VdF-HFP-TFE was dissolved in ethyl acetate to obtain a 20% by mass solution. Further, imidazole as a basic compound was dissolved in NMP to obtain a 20% by mass solution.
Then, a 20% by mass solution of VdF-HFP-TFE and a 20% by mass solution of various basic compounds were mixed at a weight ratio of 3: 1 and stirred and reacted while refluxing at a temperature of 60 ° C. . The reaction time was 60 hours. The ratio of imidazole to the total amount of fluoropolymer and imidazole was 25% by mass.

Each reaction solution after the reaction is cast on a glass substrate, then the solvent is removed under the conditions of a heating temperature of 150 ° C. and a heating time of 30 minutes, and finally heat treatment is performed under the conditions of 180 ° C. and 2 to 4 hours. The polymer film was made. The film thickness of the polymer film was 25 μm.
Each polymer film was impregnated with phosphoric acid by impregnating the polymer film in an 85% phosphoric acid solution at 60 ° C. for 4 hours. In this way, various electrolyte membranes were produced.
About the obtained electrolyte membrane, the dope rate of phosphoric acid was measured. Table 3 shows the relationship between the doping rate and the heat treatment time.

  As shown in Table 3, it can be seen that when the heat treatment time is changed from 2 hours to 3 hours, the doping rate is significantly reduced. This is presumably because the cross-linking reaction with imidazole progressed as the heat treatment time increased, and the phosphoric acid dope sites decreased. In the case of an electrolyte membrane having a heat treatment time of 4 hours, when the polymer membrane was impregnated with phosphoric acid, the membrane became brittle and torn, making it impossible to measure the doping rate.

“Example 5: Proton conductivity of electrolyte membrane”
Next, proton conductivity of the electrolyte membrane having a heat treatment time of 2 hours in Example 4 was measured. The proton conductivity was measured by sandwiching the electrolyte membrane between two platinum electrodes (diameter 13 mm), obtaining the proton conductivity from the resistance value obtained from the complex impedance measurement, and measuring the temperature dependence of the proton conductivity. The results are shown in FIG.

The electrolyte membrane with a heat treatment time of 2 hours had a proton conductivity at 150 ° C. of 5.8 × 10 ⁻³ (S / cm). Moreover, as shown in FIG. 6, it turns out that the proton conductivity stable in the measurement temperature range (60 degreeC-150 degreeC) is shown.

“Example 6: Power generation characteristics”
Using the electrolyte membrane with a heat treatment time of 2 hours in Example 4, the characteristics when a fuel cell was produced were evaluated.
First, a catalyst layer containing carbon powder carrying 50% by mass of platinum was formed, and this catalyst layer was laminated on a carbon porous body to obtain a porous electrode for a fuel cell.
Then, an electrolyte membrane having a heat treatment time of 2 hours in Example 4 was sandwiched between the pair of porous electrodes to form a single cell. Hydrogen was supplied as a fuel at a flow rate of 100 cc / min and air as an oxidant was supplied at a flow rate of 200 cc / min, and a power generation test was conducted at 150 ° C. and without humidification. The results are shown in FIG.
As shown in FIG. 7, a voltage of about 0.9 to 0.5 V was obtained at a current density of an open circuit voltage of 0 to 0.6 A / cm ² . As described above, the electrolyte membrane according to the present invention can be operated under a high temperature non-humidified condition. Moreover, although the doping rate of phosphoric acid was relatively low, excellent power generation performance was exhibited.

FIG. 1 is a perspective exploded view showing a main part of a fuel cell according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing the membrane-electrode assembly provided in the fuel cell of FIG. FIG. 3 is a graph showing the FT-IR spectrum of the electrolyte membrane before doping with phosphate, imidazole, and VdF-HFP-TFE in Experimental Example 3. FIG. 4 is a graph showing a heating loss curve of the electrolyte membrane before the phosphoric acid doping of Experimental Example 3, imidazole, and VdF-HFP-TFE. FIG. 5 is a graph showing the relationship between the doping rate of phosphoric acid and the ratio of imidazole to the total amount of fluoropolymer and imidazole in Example 3. FIG. 6 is a graph showing temperature dependence of proton conductivity in Example 5. FIG. 7 is a graph showing the relationship between battery voltage and current density in Example 6.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Fuel cell, 10 ... Membrane-electrode assembly, 20 ... Bipolar plate, 100 ... Electrolyte membrane (polymer electrolyte for fuel cells), 110, 110 '... Catalyst layer, 120, 120', 121, 121 '... Gas Diffusion layer

Claims (14)

A fluorine-based polymer having at least a (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure (where M is any one of CX ₃ , CX ₂ H, and CXH ₂ , and X is F) On the other hand, a basic compound is added and at least one acid selected from orthophosphoric acid, condensed phosphoric acid, alkylphosphoric acid and phosphonic acid is impregnated. Molecular electrolyte.   2. The polymer electrolyte for fuel cells according to claim 1, wherein a part of the basic compound crosslinks the fluoropolymer.   The fluoropolymer is at least one selected from vinylidene fluoride-hexafluoropropylene copolymer and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer. The polymer electrolyte for fuel cells according to claim 1 or 2.   The polymer electrolyte for fuel cells according to any one of claims 1 to 3, wherein the basic compound is a heterocyclic compound containing an endocyclic nitrogen atom.   The polymer electrolyte for a fuel cell according to any one of claims 1 to 4, wherein the basic compound is an imidazole compound. The polymer electrolyte for a fuel cell according to any one of claims 1 to 5, wherein the basic compound is an imidazole compound represented by the following formula (1) or (2).

However, in the formula (1), ^{R 1} is H or _C m _H m _NH 2 is (m is an integer of 1 to 3), ^{R 2} is H or _C n _{H 2n +} 1 (n is an integer of 1 to 3) is there. The fuel cell according to any one of claims 1 to 6, wherein the fluorine-based polymer to which the basic compound has been added has a structure represented by the following formulas (3) to (5). For polymer electrolytes.

However, in Formula (3) or (4), M is any one of CX ₃ , CX ₂ H, and CXH ₂ , X is F, m is an integer of 1 to 3, and R ² is H or C _n H _{2n + 1} (n is an integer of 1 to 3). A fluorine-based polymer having at least a (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure (where M is any of CX ₃ , CX ₂ H, CXH ₂ , and X is F); A step of dissolving a basic compound in an organic solvent to form a mixed solution, and reacting the (—CF ₂ —CF (M) CH ₂ —CF ₂ —) structure with the basic compound;
Performing a heat treatment of the precipitate obtained by removing the organic solvent from the mixed solution after the reaction;
A step of impregnating the precipitate after the heat treatment with at least one acid selected from orthophosphoric acid, condensed phosphoric acid, alkylphosphoric acid, and phosphonic acid. A method for producing a polymer electrolyte.   The fluoropolymer is at least one selected from vinylidene fluoride-hexafluoropropylene copolymer and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer. 9. A method for producing a polymer electrolyte for a fuel cell according to 8.   The method for producing a polymer electrolyte for a fuel cell according to claim 8 or 9, wherein the basic compound is a heterocyclic compound containing an endocyclic nitrogen atom.   The method for producing a polymer electrolyte for a fuel cell according to any one of claims 8 to 10, wherein the basic compound is an imidazole compound. The method for producing a polymer electrolyte for a fuel cell according to any one of claims 8 to 11, wherein the basic compound is an imidazole compound represented by the following formula (6) or (7).

However, in the formula (6), ^{R 1} is H or _C m _H m _NH 2 is (m is an integer of 1 to 3), ^{R 2} is H or _C n _{H 2n +} 1 (n is an integer of 1 to 3) is there.   A fuel cell polymer electrolyte according to any one of claims 1 to 7, wherein the electrolyte membrane comprises a pair of electrodes and an electrolyte membrane disposed between the electrodes. A fuel cell.   14. The fuel cell according to claim 13, wherein a part of the electrode contains the polymer electrolyte for a fuel cell according to claim 1.

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