JP2010103079A - Polymer electrolyte, membrane electrode assembly, and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily provide a polymer electrolyte exhibiting high proton conductivity in low humidity conditions and having improved durability and mechanical strength. <P>SOLUTION: The polymer electrolyte is obtained by mixing proton conductive sulfonated polyether sulfone (C1) having sulfonic acid groups (A) as proton acid groups, or sulfonated polyphenylene sulfide with 1,4-benzene dimethanol (B) as cross-linking agent having methylol groups, and giving heating treatment thereto to promote the reaction. Other aromatic ring portions than the sulfonic acid groups (A) of the proton conductive sulfonated ether sulfone (C1) having a plurality of molecules are chemically bonded to one another via residues (B') of the 1,4-benzene dimethanol. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polymer electrolyte, a membrane electrode assembly, and a fuel cell used for a solid polymer fuel cell.

In recent years, fuel cells have attracted attention as effective solutions for environmental problems and energy problems. A fuel cell oxidizes a fuel such as hydrogen by using an oxidant such as oxygen and converts chemical energy associated therewith into electric energy.
Fuel cells are classified into alkali type, phosphoric acid type, solid polymer type, molten carbonate type, solid oxide type, etc., depending on the type of electrolyte. The polymer electrolyte fuel cell (PEFC) is operated at a low temperature, has a high output density, and can be reduced in size and weight. Therefore, it is expected to be applied as a portable power source, a household power source, and an in-vehicle power source. ing.

  As electrolytes for polymer electrolyte fuel cells (PEFCs), there are perfluoro-type electrolytes represented by Nafion (registered trademark of Nafion / DuPont), and various hydrocarbon-type electrolytes represented by practical stability. It is used. However, these electrolytes exhibit high proton conductivity, but have a problem of high cost.

  In order to solve the above problems, inexpensive hydrocarbon electrolytes have been studied. For example, Patent Documents 1 and 2 report sulfonated engineering plastics. However, these electrolytes exhibit high proton conductivity under high humidity conditions, but the proton conductivity decreases under low humidity conditions. As a method of showing high proton conductivity even under low humidity conditions, attempts have been made to increase the concentration of protonic acid groups. However, increasing the concentration of protonic acid groups reduces the water resistance of the membrane, making it practical for use. There is a problem that it is impossible to obtain the mechanical strength.

  On the other hand, in order to improve the durability, water resistance, and mechanical strength of the hydrocarbon electrolyte membrane, a method of crosslinking the electrolyte has been proposed (Patent Document 3). However, in this crosslinked electrolyte, since the crosslinking reaction proceeds via a protonic acid group bonded to the proton conducting polymer, the hydrogen ion exchange capacity of the crosslinked electrolyte decreases when the crosslinking density is improved. For this reason, there exists a problem that proton conductivity falls. Moreover, the crosslinked electrolyte proposed in Patent Document 4 has a problem that it is necessary to introduce a crosslinking group into the electrolyte, and the crosslinked electrolyte cannot be easily obtained.

Japanese Patent Laid-Open No. 11-116679 JP 2007-329120 A JP 2007-70563 A Japanese Patent Application Laid-Open No. 2004-10777

An object of the present invention is to crosslink an electrolyte without reducing ion exchange capacity in polyethersulfone and polyphenylene sulfide electrolytes, which are general-purpose engineering plastics, exhibit high proton conductivity, durability, mechanical It is easy to provide a polymer electrolyte excellent in strength.
Another object of the present invention is to provide a membrane electrode assembly and a fuel cell using the polymer electrolyte.

  In order to solve the above problems, the present inventors have made extensive studies and as a result, in the polyethersulfone-based and polyphenylenesulfide-based electrolytes that are general-purpose engineering plastics, the electrolyte and the crosslinking agent are separated from the proton acid group. As a result, the inventors have obtained the knowledge that an electrolyte having excellent durability and mechanical strength can be easily provided at low cost while maintaining high proton conductivity.

  If the electrolyte and the cross-linking agent are chemically bonded via a portion other than the proton acid group, the electrolyte can be cross-linked without reducing the hydrogen ion exchange capacity, and the proton conductivity is prevented from being reduced.

  In addition, since the electrolyte and the crosslinking agent are chemically bonded by heating, an electrolyte having excellent durability can be easily provided.

  The invention according to claim 1 is a cross-linked polymer having a proton acid group in a molecular chain and obtained by cross-linking a proton conductive polymer and a cross-linking agent through a portion other than the proton acid group. In the polymer electrolyte formed as a main component, the proton conductive polymer has a structural unit represented by the following general formula (1a) or (1b).

The invention according to claim 2 is the polymer electrolyte according to claim 1, wherein the proton-conducting polymer has a hydrogen ion exchange capacity of 0.5 meq / g or more and 5 meq / g or less. .
The invention according to claim 3 is the polymer electrolyte according to claim 1 or 2, wherein the crosslinking agent has at least one methylol group in the molecule.
The invention according to claim 4 is the polymer electrolyte according to any one of claims 1 to 3, wherein the proton conducting polymer and the cross-linking agent are chemically bonded by heating. is there.
The invention according to claim 5 is the polymer electrolyte according to claim 4, wherein the heating temperature is 60 ° C. or higher and 250 ° C. or lower.
The invention according to claim 6 is a membrane electrode assembly using the polymer electrolyte according to any one of claims 1 to 5.
The invention described in claim 7 is a fuel cell using the polymer electrolyte according to any one of claims 1 to 5.

  The polymer electrolyte according to claim 1 has a protonic acid group in a molecular chain, and is a crosslinked polymer obtained by a crosslinking reaction between a proton conductive polymer and a crosslinking agent via a portion other than the protonic acid group. In the cross-linked polymer electrolyte membrane formed by using as a main component, the proton conductive polymer has a structural unit represented by the following general formula (1a) or (1b), and has a high proton conductivity. Since the molecules are bonded to each other except for the protonic acid group, they can be crosslinked without lowering the ion exchange capacity and proton conductivity, and a polymer electrolyte having excellent durability can be obtained at a low cost. There is a remarkable effect of being able to.

  The polymer electrolyte according to claim 2, wherein the proton-conducting polymer has a hydrogen ion exchange capacity of 0.5 meq / g or more and 5 meq / g or less, and exhibits high proton conductivity. Further, it is possible to obtain a further remarkable effect that the internal resistance of the fuel cell can be reduced to obtain a high output density.

  The polymer electrolyte according to claim 3 is characterized in that the cross-linking agent has at least one methylol group in the molecule, and reacts without passing through a protonic acid group contained in the proton conductive polymer. Therefore, there is a further remarkable effect that the proton conductivity is not impaired by the reaction and the reaction easily proceeds by heating.

  The polymer electrolyte according to claim 4 is characterized in that the proton conductive polymer and the cross-linking agent are chemically bonded by heating, and the reaction can be easily advanced by heating. Therefore, it is possible to simplify the apparatus and the process, and to produce a further remarkable effect that the reaction can be performed easily and economically.

  The polymer electrolyte according to claim 5 is characterized in that the heating temperature is 60 ° C. or more and 250 ° C. or less, and it suppresses the separation or decomposition reaction of proton acid groups contained in the proton conductive polymer. , There is a further remarkable effect that the reaction can easily proceed.

  Invention of Claim 6 is a membrane electrode assembly using the polymer electrolyte in any one of Claims 1-5, and can suppress degradation of electrolyte, and is a fuel cell In the long-time operation, high power generation characteristics and power generation stability can be ensured, and the reliability is improved.

  Invention of Claim 7 is a fuel cell using the polymer electrolyte in any one of Claims 1-5, It can suppress degradation of electrolyte, Furthermore, the length of a fuel cell It has a remarkable effect that high power generation characteristics and power generation stability can be ensured in time operation and reliability is improved.

FIG. 1 is an explanatory view schematically showing the molecular structure of the polymer electrolyte of the present invention. FIG. 2 is an explanatory view schematically showing the molecular structure of the polymer electrolyte of the present invention. FIG. 3 is a cross-sectional explanatory view of one embodiment of the membrane electrode assembly of the present invention. FIG. 4 is an exploded sectional view showing the configuration of an embodiment of a single cell of a polymer electrolyte fuel cell equipped with a membrane electrode assembly.

Hereinafter, the present invention will be described in detail.
The present invention relates to a polymer electrolyte used in a polymer electrolyte fuel cell, which has a proton acid group in a molecular chain, and combines a proton conductive polymer and a crosslinking agent with a portion other than the proton acid group. In the cross-linked polymer electrolyte formed using a cross-linked polymer obtained by cross-linking reaction as a main component, the proton-conductive polymer has a structural unit represented by the following general formula (1a) or (1b) It relates to a polymer electrolyte.

  FIG. 1 is an explanatory view schematically showing the molecular structure of the polymer electrolyte of the present invention. The polymer electrolyte shown in FIG. 1 includes, for example, a proton conductive sulfonated polyethersulfone (C1) having a sulfonic acid group (I) as a protonic acid group and 1,4-benzenedisilane as a crosslinking agent having a methylol group. A polyelectrolyte obtained by mixing methanol (b) with heat treatment and proceeding with the reaction, wherein the aromatic ring other than the sulfonic acid group (i) of the proton-conductive sulfonated ether sulfone (c) of a plurality of molecules Are respectively chemically bonded via a residue (b) of 1,4-benzenedimethanol.

  FIG. 2 is an explanatory view schematically showing the molecular structure of the polymer electrolyte of the present invention. The polymer electrolyte shown in FIG. 2 includes, for example, proton conductive sulfonated polyphenylene sulfide (C2) having a sulfonic acid group (I) as a protonic acid group and 1,4-benzenedimethanol as a cross-linking agent having a methylol group. A polyelectrolyte obtained by mixing (b) with heat treatment and proceeding with the reaction, wherein the aromatic ring other than the sulfonic acid group (ii) of the proton conductive sulfonated polyphenylene sulfide (C2) of a plurality of molecules Each part is constituted by chemical bonding through a residue (b) of 1,4-benzenedimethanol.

The hydrogen ion exchange capacity of the proton conducting polymer having proton conductivity used in the present invention is preferably 0.5 meq / g to 5 meq / g in view of ion conductivity.
When the hydrogen ion exchange capacity is less than 0.5 meq / g, the proton conductivity is inferior, and when used in a fuel cell, the internal resistance may be large and the output density may be lowered. It can be difficult.

  As the polymer used as the base of the proton conductive polymer used in the present invention, a polymer having a structural unit represented by the following general formula (1a) or (1b) is preferable. Copolymers and derivatives of these polymers may be used, and these may be used alone or in combination of two or more. In addition, general formula (1a) or (1b) is a general formula for demonstrating the base | substrate of a proton conductive polymer, and the proton acid group is abbreviate | omitted.

In the proton conductive polymer used in the present invention, a proton acid group is introduced into the general formula (1a) or (1b). As the proton acid group, a sulfonic acid group (—SO ₃ H), a phosphoric acid group (—PO ₃ H), a carboxylic acid group (—COOH), or the like can be used. Among these, a sulfonic acid group (—SO ₃ H) and a phosphoric acid group (—PO ₃ H) can be preferably used.

  Moreover, in order to improve mechanical strength and water resistance, it can be used in combination with other polymers. Specifically, aromatic polyether, aromatic polyether ketone, aromatic polyether ether ketone, aromatic polyether sulfone, aromatic polysulfone, aromatic polyether nitrile, aromatic polyether pyridine, aromatic polyimide, aromatic An aromatic polyamide, an aromatic polyamideimide, an aromatic polyazole, an aromatic polyester, an aromatic polycarbonate and the like can be mentioned. These polymers may be sulfonated or not sulfonated.

The cross-linking agent used in the present invention allows the reaction to proceed at a portion other than the proton acid group without going through the proton acid group of the proton conductive polymer, and changes to the proton conductivity of the proton conductive material before and after the reaction. There is no particular limitation as long as it is a cross-linking agent that does not affect the surface. Among these, a crosslinking agent having a structure in which a methylol group represented by —CH ₂ OH is bonded to an aromatic ring is preferable because the reaction easily proceeds with the proton conductive material by heating and chemically bonds.

  As a cross-linking agent having a methylol group, a compound having at least one aromatic ring having a methylol group in the molecule proceeds without passing through a proton acid group contained in the proton conductive polymer. Since proton conductivity is not impaired and the reaction proceeds easily by heating, it can be used more preferably in the present invention.

  The reaction between the proton conductive polymer of the present invention and the crosslinking agent is carried out by heating, and the heating temperature is preferably 60 ° C. or higher and 250 ° C. or lower. In addition, the presence of an acid catalyst during the reaction is preferable because the reactivity is improved.

The proton conductive polymer and the crosslinking agent are preferably used in a proportion of 0.5 to 50% by weight with respect to the proton conductive polymer.
The weight average molecular weight of the polymer electrolyte of the present invention is not particularly limited, but is preferably 1000 to 1000000.
The polymer electrolyte of the present invention may be a copolymer using other structural units other than the structural unit represented by the general formula (1).

FIG. 3 is a cross-sectional explanatory view of one embodiment of the membrane electrode assembly of the present invention.
The membrane electrode assembly of the present invention has a laminated structure as shown in FIG.
The membrane electrode assembly 12 is formed by joining and laminating the air electrode side electrode catalyst layer 2 and the fuel electrode side catalyst layer 3 on both surfaces of the polymer electrolyte 1 of the present invention produced as described above by a conventional method. The electrode catalyst layers 2 and 3 each have a conductive agent and a reaction catalyst.

The reaction catalyst used in the present invention includes platinum, palladium, ruthenium, iridium, rhodium, osmium, platinum group elements, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and the like. A metal or an alloy thereof, or an oxide or a double oxide can be used. Moreover, since the activity of a catalyst will fall when the particle size of these catalysts is too large, and stability of a catalyst will fall when too small, 0.5-20 nm is preferable.
More preferably, 1-5 nm is good.

  Generally, carbon particles are used for the electron conductive conductive agent used in the present invention carrying these reaction catalysts. Any carbon particles can be used as long as they are in the form of fine particles, have conductivity and are not affected by the catalyst, but carbon black, graphite, graphite, activated carbon, carbon fiber, carbon nanotube, and fullerene are used. it can. If the particle size of the carbon particles is too small, it becomes difficult to form an electron conduction path. If the particle size is too large, the gas diffusibility of the electrode catalyst layer is reduced or the utilization rate of the catalyst is reduced. Is preferred. More preferably, 10-100 nm is good.

  FIG. 4 is an exploded cross-sectional view showing a configuration of an embodiment of a single cell of a polymer electrolyte fuel cell equipped with the membrane electrode assembly 12. An air electrode having a structure in which a mixture of carbon black and polytetrafluoroethylene (PTFE) is applied to carbon paper facing the air electrode side electrode catalyst layer 2 and the fuel electrode side electrode catalyst layer 3 of the membrane electrode assembly 12. A side gas diffusion layer 4 and a fuel electrode side gas diffusion layer 5 are disposed. Thereby, the air electrode 6 and the fuel electrode 7 are comprised, respectively. Then, a gas flow path 8 for reaction gas flow is provided facing the air electrode side gas diffusion layer 4 and the fuel electrode side gas diffusion layer 5, and a conductive water flow path 9 for cooling water flow is provided on the opposite main surface. A single cell 11 is formed by being sandwiched by a pair of separators 10 made of a gas-impermeable material. Then, an oxidant such as air or oxygen is supplied to the air electrode 6, and a fuel gas containing hydrogen or an organic fuel is supplied to the fuel electrode 7 to generate electric power.

  An example is further demonstrated to the manufacturing method of the membrane electrode assembly (MEA) of this invention. An ink containing a reaction catalyst and a conductive agent is applied on the gas diffusion layers 4 and 5 made of the conductive porous body for uniformly supplying the fuel gas into the electrode catalyst layers 2 and 3, and then dried. The membrane electrode assembly (MEA) 12 is manufactured by laminating the catalyst electrode layers 2 and 3 and then sandwiching the polymer electrolyte 1 between the catalyst electrode layers 2 and 3 and joining them by thermocompression bonding. Can be used. A doctor blade method, a screen printing method, a spray method, or the like can be used as a method for applying the ink to form the catalyst electrode layers 2 and 3 on the gas diffusion layers 4 and 5.

  As a method for manufacturing the MEA 12, a method in which the catalyst electrode layers 2 and 3 are formed on both surfaces of the polymer electrolyte 1 by transfer or spray spraying and then sandwiched between the gas diffusion layers 4 and 5 may be used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, the scope of the present invention is not limited by these examples.

Example 1
1 g of sulfonated polyethersulfone and 0.15 g of 1,4-benzenedimethanol (Tokyo Kasei Kogyo Co., Ltd.) as a crosslinking agent were mixed in an organic solvent.

Next, the prepared solution is formed on a polyimide substrate by a casting method, and after the solvent is dried, the obtained film is heated and pressed to advance the reaction to obtain the polymer electrolyte of the present invention. It was.
The hot pressing conditions were a press temperature of 120 ° C., a press time of 3 hours, and a press pressure of 60 kgf / cm ² .
When the obtained polymer electrolyte was immersed in dimethylformamide (DMF), it was confirmed that the polymer electrolyte was crosslinked because it did not dissolve. The obtained polymer electrolyte was mixed in a 2N aqueous sodium nitrate solution for 2 days, filtered through a filter to remove the polymer electrolyte, and the acidic solution from which the polymer electrolyte had been removed was titrated with an aqueous sodium hydroxide solution. The ion exchange capacity of the polymer electrolyte was calculated from the neutralization point. The obtained polymer electrolyte had a hydrogen ion exchange capacity of 0.7 meq / g.

(Comparative Example 1)
A proton conductive polymer electrolyte was obtained in the same manner as in Example 1 except that no crosslinking agent was added. When the obtained polymer electrolyte was immersed in dimethylformamide (DMF), it was dissolved. Since it was dissolved in DMF, it was confirmed that the sulfonated polyethersulfone was not crosslinked. Further, the hydrogen ion exchange capacity of the obtained polymer electrolyte was determined by the same method as in Example 1, and found to be 0.8 meq / g.

(Example 2)
1 g of sulfonated polyphenylene sulfide and 0.15 g of 1,4-benzenedimethanol (Tokyo Kasei Kogyo Co., Ltd.) as a crosslinking agent were mixed in an organic solvent.

Next, the prepared solution is formed on a polyimide substrate by a casting method, and after the solvent is dried, the obtained film is heated and pressed to advance the reaction to obtain the polymer electrolyte of the present invention. It was.
The hot pressing conditions were a press temperature of 120 ° C., a press time of 3 hours, and a press pressure of 60 kgf / cm ² .
When the obtained polymer electrolyte was immersed in dimethylformamide (DMF), it was confirmed that the polymer electrolyte was crosslinked because it did not dissolve. The obtained polymer electrolyte was mixed in a 2N aqueous sodium nitrate solution for 2 days, filtered through a filter to remove the polymer electrolyte, and the acidic solution from which the polymer electrolyte had been removed was titrated with an aqueous sodium hydroxide solution. The ion exchange capacity of the polymer electrolyte was calculated from the neutralization point. The obtained polymer electrolyte had a hydrogen ion exchange capacity of 2.0 meq / g.

(Comparative Example 2)
A proton conductive polymer electrolyte was obtained in the same manner as in Example 1 except that no crosslinking agent was added. When the obtained polymer electrolyte was immersed in dimethylformamide (DMF), it was dissolved. Since it was dissolved in DMF, it was confirmed that the sulfonated polyphenylene sulfide was not crosslinked. Further, the hydrogen ion exchange capacity of the obtained polymer electrolyte was determined by the same method as in Example 1, and found to be 1.8 meq / g.

Sulfonated poly Ether sulfone, in the sulfonated polyphenylene sulfide, -SO 2 is an electron withdrawing group in the backbone is a substrate _- comprises a group. In the present invention, even if a skeleton that is a substrate is provided with a —SO ₂ — group that is an electron-withdrawing group, by using a cross-linking agent having a methylol group in the molecule, parts other than the protonic acid group are solved. Thus, the polymer electrolyte of the present invention could be obtained.

DESCRIPTION OF SYMBOLS 1 Polymer electrolyte 2 Air electrode side electrode catalyst layer 3 Fuel electrode side electrode catalyst layer 4 Air electrode side gas diffusion layer 5 Fuel electrode side gas diffusion layer 6 Air electrode 7 Fuel electrode 8 Gas flow path 9 Cooling water flow path 10 Separator 11 Single Cell 12 Membrane electrode assembly

Claims (7)

A polymer electrolyte having a protonic acid group in the molecular chain and a main component of a crosslinked polymer obtained by crosslinking a proton conducting polymer and a crosslinking agent via a portion other than the protonic acid group Wherein the proton conductive polymer has a structural unit represented by the following general formula (1a) or (1b).

  2. The polymer electrolyte according to claim 1, wherein the proton-conducting polymer has a hydrogen ion exchange capacity of 0.5 meq / g or more and 5 meq / g or less.   The polymer electrolyte according to claim 1 or 2, wherein the crosslinking agent has at least one methylol group in the molecule.   The polymer electrolyte according to any one of claims 1 to 3, wherein the proton conductive polymer and the cross-linking agent are chemically bonded by heating.   The polymer electrolyte according to claim 4, wherein the heating temperature is 60 ° C. or higher and 250 ° C. or lower.   A membrane / electrode assembly using the polymer electrolyte according to claim 1.   A fuel cell comprising the polymer electrolyte according to claim 1.

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