JP2008166199A - POLYMER ELECTROLYTE, POLYMER ELECTROLYTE MEMBRANE, AND FUEL CELL INCLUDING THE SAME - Google Patents

Abstract

[PROBLEMS] To provide a polymer electrolyte and a polymer electrolyte membrane that are heat resistant and have excellent proton conductivity even in a low-humidity state, and can improve productivity by a simple chemical synthesis method using inexpensive raw materials. And a fuel cell including the same.
The polymer electrolyte of the present invention is a polymer electrolyte mainly composed of a hyperbranched polymer having an oligoethylene oxide structure in the main chain, and at least acidic as a terminal functional group of the side chain of the hyperbranched polymer. It has a functional group and a crosslinkable functional group. Examples of the hyperbranched polymer include poly [bis (oligoethylene glycol) benzoate].
[Selection figure] None

Description

  The present invention relates to a polymer electrolyte for a fuel cell having proton conductivity, a polymer electrolyte membrane, and a fuel cell including the same.

  Proton-conducting polymer electrolytes can be applied to electrochemical devices including solid polymer fuel cells, and in particular, fuel cells that are attracting attention as next-generation clean energy have recently been used at high temperatures of 100 ° C. or higher. Attention has been focused on the development of electrolyte membranes capable of conducting protons under non-humidification or low humidification that enables operation at low temperatures. If a membrane capable of proton conduction at a high temperature that does not depend on water is provided, the fuel cell system can be simplified. For example, it can be expected to be widely used as a household cogeneration application or an automobile application.

A polymer electrolyte fuel cell has a structure in which a polymer electrolyte membrane having proton conductivity is sandwiched between a fuel electrode and an oxygen electrode composed of a catalyst, a gas diffusion layer, and the like. By supplying air or oxygen to the electrode, an electrochemical reaction of the following formula occurs and an electromotive force is obtained.
(Fuel electrode) H ₂ → 2H ⁺ + 2e ⁻
(Oxygen electrode) 2H ⁺ + (1/2) O ₂ + 2e ⁻ → H ₂ O

  By the way, examples of the polymer electrolyte membrane having proton conductivity described above include perfluorocarbon sulfonic acid membranes (for example, Nafion (registered trademark) membrane manufactured by DuPont, USA), sulfonated polyether ether ketone, fluorocarbon sulfonic acid and polyvinylidene. Conventionally known are fluoride mixed membranes, those obtained by grafting trifluoroethylene on a fluorocarbon matrix, and those obtained by using a cation exchange membrane made of a polystyrene-based cation exchange membrane having a sulfonic acid group. The proton conduction of these polymer electrolytes is generally applied to fuel cells in a certain wet state by humidification or the like because water molecules around the sulfonic acid are involved in conduction. Therefore, the operating temperature is generally set to 80 ° C. or lower. However, this temperature restriction causes problems such as carbon monoxide poisoning of the catalyst. Therefore, selective carbon monoxide removal is necessary, and the system is more complicated and The situation is inevitably high.

  Therefore, the solid polymer electrolyte membrane has characteristics such as (1) excellent proton conductivity, (2) easy water management in the electrolyte, and (3) excellent heat resistance. Is required.

  In order to satisfy these required characteristics, for example, in Patent Document 1, polyvinyl pyridine is graft-polymerized on a base polymer that can be graft-polymerized, and phosphoric acid is doped on the graft base material, whereby a high-temperature condition of 100 ° C. or higher is achieved. It is said that an electrolyte membrane excellent in proton conductivity can be supplied by crystallization.

  In Patent Document 2, a basic polymer (for example, propylene glycol) is introduced into an acidic polymer (for example, perfluorosulfonic acid) by impregnation, and good proton conductivity is obtained even at a high temperature and low humidity of 150 ° C. It is said that you can get.

JP 2001-213987 A JP 2001-236773 A International Publication No. 2005/090910 Pamphlet Takahito Itoh, et al, “Ionic conductance of the hyperbranched polymer-lithium metal salt systems”, J. Am. of Power Sources, 81-82 (1999), p. 824-829

  However, the inventions described in Patent Documents 1 and 2 have a problem that productivity is low because expensive raw materials and complicated processes are required and production costs are high.

  Therefore, the present invention has been made in view of such problems, and its purpose is to provide a simple chemical synthesis using inexpensive raw materials because it has heat resistance and excellent proton conductivity even in a low humidity state. It is an object of the present invention to provide a new and improved polymer electrolyte, a polymer electrolyte membrane, and a fuel cell including the same, which can improve productivity by the method.

  As a result of intensive studies to solve the above problems, the present inventors have developed a hyperbranched polymer having a tree-like structure developed by the present inventors as an electrolyte material for lithium batteries (see Non-Patent Document 1). We have intensively studied the technology for making the basic polymer skeleton into a fuel cell electrolyte membrane. Here, the hyperbranched polymer having a dendritic structure (also referred to as a dendritic polymer) is a polymer in which branch molecules are three-dimensionally extended in a dendritic shape from a central core molecule. This dendritic hyperbranched polymer is sterically bulky, highly soluble, amorphous and excellent in processability compared to ordinary branched polymers and linear polymers, and has many functional groups at the molecular ends. Can be introduced.

  As a result of the above studies, the present inventors manufactured a solid polymer electrolyte membrane for fuel cells excellent in proton conductivity by replacing the functional group at the end of the side chain from an acetyl group to an acidic functional group such as phenylsulfonic acid. It was found that this can be done (see Patent Document 3). Furthermore, the present inventors have changed the functional group at the end of the side chain from an acetyl group to a crosslinkable functional group such as a vinyl group, so that the solid polymer for fuel cells has excellent proton conductivity and has self-supporting properties. It has been found that an electrolyte membrane can be produced, and the present invention has been completed based on these findings.

  That is, according to one aspect of the present invention, there is provided a polymer electrolyte mainly comprising a hyperbranched polymer having an oligoethylene oxide structure in the main chain, and at least an acidic functional group as a terminal functional group of a side chain of the hyperbranched polymer. And a polyelectrolyte having a crosslinkable functional group.

  Since the polymer electrolyte of the present invention has such a configuration, the hyperbranched polymer functions as a charge transfer medium using a polyethylene oxide chain as a main chain, and an acidic functional group functions as a proton-donating carrier. Even without a charge transfer medium such as water, self-dissociation is possible and protons can be easily conducted. Moreover, it is also possible to perform a crosslinking reaction as required by the crosslinkable terminal functional group arranged in the molecule of the hyperbranched polymer, and thus it is possible to provide an electrolyte membrane having self-supporting properties.

  Here, part or all of the acidic functional group is preferably a phosphonic acid group. Thereby, it can be expected that the heat resistance of the polymer electrolyte is remarkably improved. For example, Japanese Patent Application Laid-Open No. 2006-265497 discloses a background art that “the use of high temperatures causes the elimination of sulfo groups over time, resulting in a decrease in ion exchange capacity and a decrease in performance. These phenomena are particularly noticeable at high temperatures. ”As can be seen from the above, not only polyimide-based polymer electrolytes, but also polymer compounds having sulfonic acid groups as ionic functional groups can be used at high temperatures. However, the phosphoric acid group and the phosphonic acid group have not been reported in the same way, and as seen in phosphoric acid fuel cells, phosphoric acid groups are expected to lose their properties. Alternatively, the phosphoric acid compound is generally stable at high temperatures. Therefore, for example, when used in phosphoric acid fuel cells, solid oxide fuel cells, molten carbonate fuel cells, etc. that need to be operated at high temperatures, while maintaining ionic conductivity at a certain level, When importance is placed on the property, it is preferable to have a phosphonic acid group as part of the acidic functional group.

  Moreover, it is preferable that a part or all of the crosslinkable functional group is an acryloyl group. Thereby, the synthesis process of the polymer electrolyte can be simplified.

Examples of the hyperbranched polymer include poly [bis (oligoethylene glycol) benzoate] represented by the following general formula 1, and among these, (CH ₂ CH ₂ O) _m (m = 1) and oligoethylene oxide chain represented by 6) is preferably a polymer having a tree-like structure obtained by polymerizing an a _{2 B} type monomers synthesized from di oxybenzoate. Whereas the end of the linear polymer is two, in this way, by making the hyperbranched polymer have a tree-like structure, many ends of the polymer are formed. It is possible to introduce a terminal functional group. In the present invention, if a large number of acidic functional groups are introduced as terminal functional groups, proton conductivity can be improved, and if a large number of crosslinkable functional groups are introduced as terminal functional groups, mechanical cross-linking between molecules can be achieved. Strength can be improved.

  Moreover, it is preferable that the said crosslinkable functional group is contained 10 mol% or more and less than 60 mol% with respect to the whole terminal functional group in a hyperbranched polymer.

  Moreover, according to another viewpoint of this invention, the polymer electrolyte membrane formed by bridge | crosslinking the hyperbranched polymer which is a main component of the polymer electrolyte mentioned above is provided.

  Moreover, according to another viewpoint of this invention, a fuel cell provided with the said polymer electrolyte membrane is provided. In this way, the fuel cell system can be simplified by using the polymer electrolyte membrane that exhibits proton conductivity even under the above-described conditions of high temperature and low humidity and has self-supporting property as a charge transfer medium. it can.

  According to the present invention, it has heat resistance and excellent proton conductivity even in a low-humidity state, so that productivity can be improved by a simple chemical synthesis method using inexpensive raw materials. It is possible to provide a polymer electrolyte having a property, a polymer electrolyte membrane, and a fuel cell including the same.

  Hereinafter, preferred embodiments of the present invention will be described in detail.

  As described above, the inventors have heat resistance and excellent proton conductivity even in a non-humidified state, so that productivity can be improved by a simple chemical synthesis method using inexpensive raw materials. In order to obtain a simple polymer electrolyte, a highly branched polymer having a tree-like structure developed by the present inventors as an electrolyte material for lithium batteries is used as a basic polymer skeleton, and the functional group at the side chain terminal is changed from an acetyl group to an acidic functional group. It has been found that a polymer electrolyte membrane for a fuel cell excellent in proton conductivity can be produced by substituting for.

  However, the above-mentioned tree-like hyperbranched polymer having an acidic functional group is a sticky liquid alone, and a cross-linking agent has to be used as the second component for its immobilization (electrolyte film formation). The dendritic hyperbranched polymer forms a semi-IPN (Interpenetrating Polymer Network) type polymer together with the crosslinkable polymer, so that a part of the polymer may be dissolved in water generated on the oxygen electrode side. Furthermore, since the cross-linking reaction in this polymer is forced, the cross-linking itself is likely to be non-uniform, and a non-uniform electrolyte membrane (a part of the membrane has pores) due to elution as part of a paste. Etc.).

  Therefore, as a result of repeated studies by the present inventors to solve the above problems, the present inventors have added in addition to acidic functional groups as terminal functional groups of the side chains of the above-described highly branched polymer having a tree-like structure. It has been found that by introducing a crosslinkable functional group, it becomes possible to completely immobilize the whole molecule with a hyperbranched polymer having a tree-like structure alone, and a so-called self-supporting solid polymer electrolyte membrane can be obtained. It was. Thereby, the solubility with respect to the water generated on the oxygen electrode side is eliminated, and a uniform electrolyte membrane can be formed.

  Here, in general, when an acidic functional group such as a sulfonic acid group and a crosslinkable functional group such as an acryloyl group are introduced into a single molecule, the reactivity of the crosslinkable functional group is high, so the acidic functional group and the crosslinkable functional group It is considered that it is not technically easy to introduce both of them. That is, the present inventors have also found that it is impossible to introduce both an acidic functional group and a crosslinkable functional group in a one-step reaction, and that an acidic functional group cannot be introduced first. It is obtained by experiment. In contrast, the present inventors have found that it is only necessary to introduce a crosslinkable functional group first, isolate an intermediate product once obtained, and then introduce an acidic functional group again. Was completed.

  In addition, when using a phosphonic acid group as an acidic functional group, since a phosphonic acid group will contribute to reaction, a phosphonic acid group cannot be directly introduce | transduced into the said dendritic hyperbranched polymer. Therefore, after introducing a phosphonic acid ester, hydrolysis can be performed to obtain a phosphonic acid, whereby a target compound (a dendritic hyperbranched polymer having an acidic functional group introduced at the end of a side chain) can be obtained.

(Structure of polymer electrolyte according to the present invention)
Hereinafter, the structure of the polymer electrolyte according to the present invention thus completed will be described in detail.

  The polyelectrolyte according to the present invention is mainly composed of a hyperbranched polymer having an oligoethylene oxide structure in the main chain, and has at least an acidic functional group and a crosslinkable as a terminal functional group of a side chain of the hyperbranched polymer. Has a functional group.

  According to such a configuration, the hyperbranched polymer of the present invention has a function as an oligoethylene oxide main chain as a charge transfer medium and an acidic functional group as a proton-donating carrier, and therefore there is a charge transfer medium such as water. Even if not, a polymer electrolyte capable of self-dissociation and capable of conducting protons can be provided. Moreover, it is also possible to perform a crosslinking reaction as required by the crosslinkable terminal functional group arranged in the molecule of the hyperbranched polymer, and thus it is possible to provide an electrolyte membrane having self-supporting properties.

  Here, examples of the highly branched polymer having an oligoethylene oxide structure in the main chain include poly [bis (oligoethylene glycol) benzoate] represented by the following general formula 1.

As such poly [bis (oligoethylene glycol) benzoate], in particular, A ₂ B synthesized from an oligoethylene oxide chain represented by (CH ₂ CH ₂ O) _m (m = 1 to 6) and dioxybenzoate. A polymer having a tree-like structure obtained by polymerizing a type monomer is preferred.

  According to such a configuration, the end of the linear polymer is two, but in this way, by forming the hyperbranched polymer to have a tree-like structure, many ends of the polymer are formed. This makes it possible to introduce many terminal functional groups. In the present invention, if a large number of acidic functional groups are introduced as terminal functional groups, proton conductivity can be improved, and if a large number of crosslinkable functional groups are introduced as terminal functional groups, mechanical cross-linking between molecules can be achieved. Strength can be improved.

  Examples of the acidic functional groups include sulfonic acid groups, sulfinic acid groups, phosphoric acid groups, phosphonic acid groups, alkylsulfonic acid groups such as methanesulfonic acid groups, fluoroalkylsulfonic acid groups such as fluoromethanesulfonic acid groups, Examples thereof include functional groups capable of releasing protons such as acid groups and fluoroboric acid groups. Such acidic functional groups can be introduced singly or in combination of two or more, and particularly preferably contain a phosphonic acid group. By arranging phosphonic acid as a part of the acidic functional group or as all the terminal functional groups excluding the crosslinkable functional group, the heat resistance of the polymer electrolyte containing the highly branched polymer can be remarkably improved.

  Examples of the crosslinkable functional group include a functional group having a double bond such as a vinyl group, an allyl group, a 1-propenyl group, an acryloyl group, a glycidyl ether group, an isocyanate group, and a thioisocyanate group. . Such a crosslinkable functional group can be introduced singly or in combination of two or more, and particularly preferably contains an acryloyl group. By arranging an acryloyl group as a part of the crosslinkable functional group or as all the terminal functional groups excluding the acidic functional group, the synthesis process of the polymer electrolyte can be simplified.

  In the polymer electrolyte of the present invention, it is preferable that the crosslinkable functional group is contained at an introduction rate of 10 mol% or more and less than 60 mol% with respect to the entire terminal functional group in the hyperbranched polymer described above. More preferably, it is contained at an introduction rate of 30 mol% or more and less than 60 mol%.

  If the introduction ratio of the crosslinkable functional group is less than 10 mol%, the intermolecular crosslinking effect is insufficient, so that the mechanical strength is low and an electrolyte membrane having self-supporting properties may not be obtained. The crosslinkable functional group is preferably contained at an introduction rate of 10 mol% or more. On the other hand, when the introduction ratio of the crosslinkable functional group is 60 mol% or more, the crosslink density is too high, the electrolyte membrane becomes brittle, and the proton conductivity is lowered. It is preferable that it is contained at a rate.

  The introduction rate of the crosslinkable functional group (the ratio of the acryloyl group to the entire terminal functional group) is the reaction time of the reaction for introducing the crosslinkable functional group, the amount of N, N-dicyclohexylcarbodiimide (DCC) used as a catalyst, and It is determined by the amount of a crosslinking agent such as acrylic acid introduced as a crosslinkable functional group. Specifically, in order to make the introduction rate of the crosslinkable functional group within the above range, the reaction time is 24 to 48 hours, the amount of DCC is 1 to 4 times the number of moles of the repeating unit of the hyperbranched polymer, The amount of the agent is preferably 0.5 to 3 times the number of moles of the repeating unit of the hyperbranched polymer.

  As described above, the polyelectrolyte of the present invention has a crosslinkable functional group arranged at an appropriate introduction rate at the end of the highly branched polymer, and this crosslinkable functional group forms an intermolecular crosslink by a crosslink reaction. An electrolyte membrane having supportability can be obtained, and by extension, it can be applied to an electrochemical device that requires proton conductivity, such as a fuel cell having such a self-supporting electrolyte membrane.

(Method for producing polymer electrolyte membrane according to the present invention)
The structure of the polymer electrolyte according to the present invention has been described above. Next, a method for producing such a polymer electrolyte membrane will be described. In the following description, a hyperbranched polymer having an oligoethylene oxide structure in the main chain is synthesized from an oligoethylene oxide chain represented by (CH ₂ CH ₂ O) _m (m = 1 to 6) and dioxybenzoate. The case of poly [bis (oligoethylene glycol) benzoate] having a dendritic structure obtained by polymerizing the prepared A ₂ B type monomer will be described as an example.

First, an oligoethylene oxide chain represented by (CH ₂ CH ₂ O) _m (m = 1 to 6) and an A ₂ B type monomer synthesized from dioxybenzoate as a raw material are polymerized to produce poly [bis (oligoethylene glycol). ) Benzoate]. Next, a crosslinkable functional group such as an acryloyl group is introduced at the end of the side chain of poly [bis (oligoethylene glycol) benzoate].

  As a method for introducing a crosslinkable functional group into the side chain terminal, for example, a substance having one or more polymerizable functional groups at the molecular terminal, such as acrylic acid, which can be cross-linked is converted to N, N-dicyclohexylcarbodiimide ( DCC) and 4- (N, N-dimethylamino) pyridine (DMAP) together with the above poly [bis (oligoethylene glycol) benzoate] introduced a crosslinkable functional group at the molecular end. Poly [bis (oligoethylene glycol) benzoate] can be synthesized.

  Furthermore, after isolating poly [bis (oligoethylene glycol) benzoate] into which a crosslinkable functional group has been introduced, an acidic functional group such as a sulfonic acid group or a phosphonic acid group is introduced at the end of the side chain of this molecule.

  Examples of the method for introducing an acidic functional group into the side chain terminal include esterification of the side chain terminal with an alkali metal salt of o-sulfobenzoic acid, m-sulfobenzoic acid or p-benzoic acid, or disulfobenzoic acid. Thereafter, poly [bis (oligoethylene glycol) benzoate] in which a crosslinkable functional group and an acidic functional group (in this example, a sulfonic acid group) are introduced at the molecular end can be synthesized by conversion to a sulfonic acid group. .

  Further, as another method for introducing the acidic functional group to the side chain terminal, for example, after esterifying the side chain terminal with a benzoic acid compound having phosphoric acid or phosphonic acid ester as a functional group, A crosslinkable functional group and an acidic functional group (in this example, a phosphoric acid group or a phosphonic acid group) are also introduced into the molecular terminal by converting the side chain terminal to an acidic functional group by hydrolysis of an acid or phosphonic acid ester. Poly [bis (oligoethylene glycol) benzoate] can be synthesized.

  Thus, the polymer electrolyte membrane according to the present invention can be produced by a simple chemical synthesis method using inexpensive raw materials such as oligoethylene oxide chains and dioxybenzoate. Therefore, the productivity of the polymer electrolyte membrane can be improved. In addition, the polymer electrolyte membrane produced as described above has an acidic functional group in the molecule, and thus has heat resistance and excellent proton conductivity even in a low humidity state. Furthermore, since the polymer electrolyte membrane of the present invention has not only acidic functional groups but also crosslinkable functional groups in the molecule, it is possible to produce a membrane having high mechanical strength and self-supporting properties.

  In the production of the polymer electrolyte according to the present invention, as described above, after the crosslinkable functional group is first introduced, the acidic functional group must be introduced. This is because when an acidic functional group and a crosslinkable functional group are introduced into a single molecule, it is generally impossible to introduce them simultaneously in one step because the reactivity of the crosslinkable reactive group is high. Furthermore, it is because it became clear from an experiment by the present inventors that an acidic functional group cannot be introduced first. Therefore, it is necessary to introduce the crosslinkable functional group first, isolate the intermediate product once obtained, and then introduce the acidic functional group again.

  Moreover, electrochemical devices, such as a fuel cell, can be manufactured by a well-known method using the polymer electrolyte membrane manufactured as mentioned above.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

<Example>
(Synthesis of A ₂ B type monomer)
First, as an example of an A ₂ B type monomer synthesized from an oligoethylene oxide chain and dioxybenzoate, Methyl 3,5-bis [(8′-hydroxy-3 ′, 6′-dioxotactic) oxy] benzoate (3) is used. Synthesized. That is, as shown in the following reaction formula 1, in a 300 ml eggplant flask equipped with a magnetic stirrer and a Dimroth, Methyl 3,5-dihydroxybenzoate (1) (8.41 g, 50.0 mmol), Triethylene monochloromonohydrin (2) (18. 5 g, 110 mmol), K ₂ CO ₃ (49.8 g, 361 mmol), 18-Crown-6 (0.30 g, 1.15 mmol), and 200 ml of acetonitrile were weighed, and the flask was refluxed for 50 hours under a nitrogen stream. . The precipitated white solid was removed by suction filtration, and the solvent was distilled off from the filtrate by an evaporator to obtain an oily product. The obtained oil was passed through a silica gel column packed with dichloromethane, the first and second bands containing unreacted substances were removed with ethyl acetate, the eluent was changed to methanol, the third band was collected, and the solvent was distilled off under reduced pressure. As a result, Methyl 3,5-bis [(8′-hydroxy-3 ′, 6′-dioxoctyl) oxy] benzoate (3) was obtained as a pale yellow transparent oily substance. The yield was 15.3 g, and the yield was 71.0%.

(Synthesis of hyperbranched polymer with dendritic structure)
Next, as an example of a hyperbranched polymer having a tree-like structure obtained by polymerizing the A ₂ B type monomer synthesized as described above, Poly [bis (triethylene glycol) benzoate] (4) was synthesized. That is, as shown in the following reaction formula 2, Methyl 3,5-bis [(8′-hydroxy-3 ′, 6′-dioxoctyl) oxy] benzoate (3) (4) was added to a 30 ml eggplant flask equipped with a magnetic stirrer. 01 g, 9.28 mmol) was measured, and Tributyltin (IV) Chloride (0.05 g, 0.15 mmol) was added as a catalyst, and then the flask was placed in a nitrogen stream and heated to 210 ° C. for polymerization for 2 hours. . The obtained rubber-like solid was dissolved in a small amount of THF, precipitated in hexane, and dried under reduced pressure, whereby poly [bis (triethylene glycol) benzoate] (4) (molecular weight: Mn = 4,000) was made viscous. Obtained as a solid. The yield was 1.77 g, and the yield was 44.1%.

(Synthesis of a highly branched polymer having a phosphonic acid group and a crosslinkable functional group introduced at the terminal (1))
Next, in order to introduce a crosslinkable functional group and a phosphonic acid group at the end of the side chain of Poly [bis (triethylene glycol) benzoate] (4) synthesized as described above, a terminal hyperbranched polymer (HBP- PE-Ac) (8) was synthesized. That is, as shown in the following reaction formula 3, Poly [bis (triethylene glycol) benzoate] (4) 3.64 g (9.10 mmol) (molecular weight: M _n = 4,100), purified acrylic acid (5 ) 0.66 g (9.10 mmol), N, N-dicyclohexylcarbodiimide (DCC) 1.88 g (9.10 mmol), 4- (N, N-dimethylamino) pyridine (DMAP) 1.11 g (9.10 mmol) Then, 60 ml of methylene chloride was weighed and stirred at room temperature for 48 hours in a nitrogen atmosphere, and a crosslinkable functional group was introduced into the end of the side chain of Poly [bis (triethylene) benzoate] (4). In a solution containing Poly [bis (triethylene glycol) benzoate] (4) having a crosslinkable functional group introduced, 4- (diethylphosphorylmethyl) benzoic acid (7) 2.48 g (9.10 mmol), N, N-dicyclohexyldiimide (DCC) 1.88 g (9.10 mmol) and 4- (N, N-dimethylamino) pyridine (DMAP) 1.10 g (9.03 mmol) were added, and the mixture was stirred for 48 hours in a nitrogen atmosphere. After suction filtration, the solvent was distilled off from the filtrate under reduced pressure. The resulting viscous solid was purified by a dissolution-reprecipitation method using a mixed solvent of methylene chloride, ethanol and isopropyl ether to obtain HBP-PE-Ac (8) as a pale yellow viscous solid. The yield was 3.64 g, and the acryloyl content was 33% as calculated by ¹ H-NMR.

(Synthesis of a highly branched polymer having a phosphonic acid group and a crosslinkable functional group introduced at the end (2))
Finally, the phosphonic acid ester introduced at the end of the side chain of HBP-PE-Ac (8) obtained as described above is hydrolyzed to introduce a phosphonic acid group and a crosslinkable functional group at the end. As an example of the resulting highly branched polymer, HBP-PA-Ac (9) was synthesized. That is, as shown in the following reaction formula 4, 3.64 g of HBP-PE-Ac (8) was weighed into an eggplant flask and dissolved in 30 ml of methylene chloride. Next, 2.2 ml (17.0 mmol) of bromotrimethylsilane (TMSBr) was added, and the mixture was stirred at room temperature for 17 hours under a nitrogen atmosphere. After the solvent was distilled off under reduced pressure, 40 ml of methanol was added and the mixture was further stirred for 12 hours. The residue obtained by distilling off the solvent under reduced pressure was purified by the dissolution-reprecipitation method using N, N-dimethylformamide-isopropyl ether, and finally washed with isopropyl alcohol to give a highly branched polymer as a pale yellow solid. HBP-PA-Ac (9) was obtained. The yield was 3.59g. The FT-IR spectrum of HBP-PA-Ac (9) is shown in FIG.

(Synthesis of hyperbranched polymers with sulfonic acid groups and crosslinkable functional groups introduced at the ends)
Moreover, in order to introduce | transduce a crosslinkable functional group and a sulfonic acid group into the terminal of the side chain of Poly [bis (triethyleneglycol) benzoate] (4) synthesize | combined as mentioned above, a terminal hyperbranched polymer (HBP-Ac -OH) (10) was synthesized. That is, as shown in the following reaction formula 5, Poly [bis (triethylene glycol) benzoate] (4) 1.99 g (5.0 mmol) (molecular weight: M _n = 3,000), purified acrylic acid (5 ) 0.72 g (9.9 mmol), N, N-dicyclohexylcarbodiimide (DCC) 1.03 g (5.0 mmol), 4- (N, N-dimethylamino) pyridine (DMAP) 0.61 g (5.0 mmol) Then, 30 ml of methylene chloride was weighed and stirred at room temperature for 60 hours in a nitrogen atmosphere. After suction filtration, the solvent was distilled off from the filtrate under reduced pressure. The resulting viscous solid was purified by a dissolution-reprecipitation method using a mixed solvent of methylene chloride, ethanol and isopropyl ether to obtain HBP-Ac-OH (6) as a brown viscous solid. The yield was 1.92 g, and the acryloyl content was 46% as calculated by ¹ H-NMR.

The flask was charged with HBP-Ac-OH (6) (1.92 g), 4-Sulfobenzoic Acid Monopotassium Salt (10) (2.06 g, 8.6 mmol), DMAP (0.52 g, 4.3 mmol), DCC (1. 77 g, 8.6 mmol) and 30 mL of DMF were weighed and stirred for 26 hours under a nitrogen atmosphere. After suction filtration, the solvent was distilled off from the filtrate under reduced pressure. The obtained viscous solid was purified by a dissolution-reprecipitation method using DMF and isopropyl ether to obtain HBP-SO ₃ K-Ac (11) as a yellow viscous solid. The yield was 1.95 g, and the acryloyl content and SO ₃ K content calculated by ¹ H-NMR were 44% and 56%, respectively. The FT-IR spectrum of HBP-SO ₃ K-Ac (11) is shown in FIG.

(Manufacture of polymer electrolyte membrane)
A terminal hyperbranched polymer synthesized as described above HBP-PA-Ac (9) and CH ₂ Cl ₂ were prepared at a mass ratio of 50:50, cast onto a polyethylene terephthalate film (PET film), A cross-linking reaction was performed. The crosslinking reaction was performed in the order of 24 hours at room temperature, 24 hours at room temperature in a vacuum, and 24 hours by heating to 130 ° C. in a vacuum. The polymerized sample was immersed in CH ₂ Cl ₂ , H ₂ O, MeOH, DMSO (dimethyl sulfoxide), but swelled without dissolving, suggesting the progress of the crosslinking reaction by heating.

Further, a synthesized terminal hyperbranched polymer HBP-SO ₃ K-Ac (11) and DMAc were prepared at a mass ratio of 50:50, cast onto a PET film, and subjected to a crosslinking reaction. The crosslinking reaction was carried out in the order of 48 hours by heating on a hot plate at 80 ° C. for 24 hours and in vacuum at 120 ° C. Although the polymerized sample was immersed in H ₂ O, MeOH, and DMF, it swelled without dissolving, suggesting the progress of the crosslinking reaction by heating. Furthermore, when the polymerized sample film is immersed in 1N hydrochloric acid for 24 hours, it is converted from sulfonic acid potassium salt to sulfonic acid (crosslinked HBP-SO ₃ H-Ac). The FT-IR spectrum of the crosslinked HBP-SO ₃ H-Ac is shown in FIG.

(Production and evaluation of solid polymer fuel cell)
Solid polymer fuel using a proton conductive solid polymer electrolyte membrane (hereinafter referred to as “crosslinked HBP-PA-Ac membrane”) obtained by crosslinking the terminal hyperbranched polymer HBP-PA-Ac (9). In order to produce a battery, two commercially available gas diffusion electrodes with catalyst (electroplated by 1 mg / cm ^{2 of} carbon paper) manufactured by Electrochem were cut into 3 cm squares, which were used as a positive electrode and a negative electrode, respectively. A proton conductive solid polymer electrolyte membrane obtained by crosslinking HBP-PA-Ac (9) was cut into 5 cm square and sandwiched so as to be arranged in the center as an electrolyte layer to form a membrane electrode assembly. At this time, no hot pressing or the like was performed. In addition, in order to avoid gas leakage from the electrolyte part of the fabricated membrane electrode assembly, a fluororubber gasket is provided, sandwiched between two carbon separators having grooves to form gas flow paths, and further, the carbon separator ends from above. The plate was placed and tightened at 5 kgf / cm ² using a torque wrench to produce a polymer electrolyte fuel cell. Using this polymer electrolyte fuel cell, a power generation test was conducted using hydrogen as the anode gas and oxygen as the cathode gas. The battery temperature (cell temperature) was 130 ° C. (the test temperature was maintained at 130 ° C. for 2 hours), the supply amounts of hydrogen and oxygen were 100 ml / min and 100 ml / min, respectively, and the supply gas was not humidified. The polymer electrolyte fuel cell was evaluated by examining the voltage (DC polarization characteristics) that changed by performing current scanning using an electrochemical measuring device HZ-3000 manufactured by Hokuto Denko Corporation. In FIG. 4, the graph which showed the direct current | flow polarization characteristic of the fuel battery cell of a present Example is shown. In FIG. 4, the vertical axis represents voltage [V], and the horizontal axis represents current [mA].

  As shown in FIG. 4, the fuel cell was able to generate power up to a current of about 15 mA. The fuel cell was able to generate power even at a high temperature of 130 ° C. even though the supply gas was not humidified. Therefore, a humidifier is generally required when operating the fuel cell at a high temperature. According to the present invention, a fuel cell capable of operating at a high temperature without supplying a humidifier is provided. It was suggested that it would be possible.

<Ion conductivity test>
The ion conductivity of the polymer electrolyte membrane film (crosslinked HBP-PA-Ac film) obtained by the above method was measured. In this preliminary film, it was confirmed that the shape change caused by softening did not occur even at high temperatures, and the mechanical strength was sufficient, so that it was not necessary to use a support such as an O-ring. A cell was prepared. The sample was left in a 60 ° C. atmosphere for half a day, then heated to 150 ° C. and left for half a day. Then, resistance was measured with the amplitude of 10 mmV of AC 2 terminal methods (1 MHz-1 Hz) with the blocking electrode with the complex alternating current impedance measuring apparatus on the temperature fall conditions for every 10 degreeC. And the ionic conductivity in each temperature was calculated | required from the measured resistance value.

The measurement result of the ionic conductivity is shown in FIG. FIG. 5 is an Arrhenius plot showing the ionic conductivity of the polymer electrolyte membrane (crosslinked HBP-PA-Ac film) according to the present example. The ordinate represents ionic conductivity [Scm ⁻¹ ] and the abscissa represents the ionic conductivity. (1000 / T) [K ⁻¹ ] is shown as a temperature parameter. As can be seen from FIG. 5, the polymer electrolyte membrane according to the present example is practically sufficient at 10 ⁻⁵ S / cm ² to 10 ⁻⁶ S / cm ² even under conditions of a high temperature of 150 ° C. and no humidification. Showed good ionic conductivity. On the other hand, Nafion membranes used industrially are insulators under non-humidified conditions and cannot be used as electrolyte membranes.

<Heat resistance test>
Moreover, the result of measuring the hyperbranched polymer (9) having an acidic functional group (here, phosphonic acid group) and a crosslinkable functional group synthesized as described above by thermogravimetric analysis (TG / DTA) is shown in FIG. It was shown to. FIG. 6 is a graph showing the results of thermogravimetric analysis of the polymer electrolyte according to the example of the present invention. As a result, with respect to the thermal properties of the hyperbranched polymer, there was almost no weight loss at 200 ° C. or less, and the weight loss at 202.2 ° C. was 0.5%, confirming heat resistance.

<Mechanical strength test>
Furthermore, a mechanical strength test was performed on the polymer electrolyte membrane (crosslinked HBP-PA-Ac film) according to the example of the present invention. Specifically, five test pieces of the polymer electrolyte membrane obtained as described above were stacked, and thermomechanical measurement (TMA: Thermal Mechanical Analysis) was performed in the needle penetration mode. In the TMA measurement, the temperature was increased from room temperature to 150 ° C. at a rate of 5 ° C./min. During that time, the needle penetration pressure was set to a constant value of 50 mN, and the displacement of the polymer electrolyte membrane was investigated.

  The result of the TMA measurement is shown in FIG. FIG. 7 is a graph showing displacement due to needle penetration of the polymer electrolyte membrane with respect to temperature change. The vertical axis shows the membrane displacement [μm], and the horizontal axis shows the temperature [° C.]. As can be seen from FIG. 7, the polymer electrolyte membrane according to the embodiment of the present invention expands with heating, and the displacement of the membrane increases linearly, so that there is no penetration of the needle into the membrane. The film was not melted by heat. In other words, it is clear from FIG. 7 that the polymer electrolyte membrane according to the embodiment of the present invention linearly expands with respect to the heat-weighted response. In the case of this test, it turns out that it has a film | membrane shape, without melt | dissolving to 140 degreeC. In addition, about the polymer electrolyte which concerns on the comparative example 1 (one which does not have a crosslinkable functional group in a polymer terminal), since the film | membrane which has a self-supporting property cannot be formed, this test was impossible.

<Comparative Example 1>
As Comparative Example 1, Poly [bis (triethylene glycol) benzoate] (4) as an example of a hyperbranched polymer (4) has only an acidic functional group at the end of the side chain, and does not have a crosslinkable functional group HBP-PA ( 11) was synthesized.

(Synthesis of HBP-PE)
Specifically, first, as shown in the following reaction formula 6, Poly [bis (triethylene glycol) benzoate] (4) 3.62 g (9.03 mmol) (molecular weight: M _n = 4,000) in an eggplant flask. 4- (diethylphosphomethyl) benzoic acid (7) 2.50 g (9.03 mmol), N, N-dicyclohexylcarbodiimide (DCC) 1.86 g (9.03 mmol), 4- (N, N-dimethylamino) pyridine (DMAP) ) 1.10 g (9.03 mmol) was added and stirred for 48 hours under a nitrogen atmosphere. After suction filtration, the solvent was distilled off from the filtrate under reduced pressure. The obtained viscous solid was purified by a dissolution-reprecipitation method using a mixed solvent of methylene chloride, ethanol and hexane to obtain HBP-PE (10) (molecular weight M _n = 4,300) as a pale yellow viscous solid. . The yield was 5.56g.

(Synthesis of HBP-PA)
Next, 5.56 g (8.59 mmol, M _n = 4,300) of HBP-PE (10) obtained as described above was weighed into an eggplant flask and dissolved in 70 ml of methylene chloride. Then, as shown in the following reaction formula 7, 3.0 ml (23.3 mmol) of bromotrimethylsilane (TMSBr) was added and refluxed for 14 hours in a nitrogen atmosphere. To the residue obtained by distilling off the solvent under reduced pressure, 70 ml of methanol was added and stirred for 10 hours. The reaction product was purified by a dissolution-reprecipitation method using N, N-dimethylformamide-isopropyl ether and then dried by heating under reduced pressure to obtain HBP-PA (11) as a pale yellow solid. The yield was 3.13g.

  Although the obtained polymer HBP-PA (11) was a paste-like semi-solid, it was impossible to form a self-supporting film with this polymer alone. This suggests that a self-supporting membrane can be obtained by introducing a crosslinkable functional group into the polymer terminal.

<Comparative example 2>
On the other hand, as Comparative Example 2, a highly branched polymer Acrylate which has only a crosslinkable functional group at the end of the side chain of Poly [bis (triethylene glycol) benzoate] (4) as an example of a hyperbranched polymer and does not have an acidic functional group. Poly [bis (triethylene glycol) benzoate] (1d) (introduction rate of crosslinkable functional group = 100%) was synthesized.

Specifically, as shown in the following reaction formula 8, Poly [bis (triethylene glycol) benzoate] (4) (4.05 g, 9.35 mmol), Acryloyl Chloride (2. 30 g, 28.0 mmol) was measured, and while stirring, Trithylamine (4.0 g, 28.0 mmol) dissolved in 25 ml of methylene chloride was added dropwise, and the mixture was stirred at room temperature for 24 hours. The reaction solution was placed in a separatory funnel, methylene chloride and saturated brine were added for liquid separation, and the extracted methylene chloride layer was dried over anhydrous magnesium sulfate. The solvent was removed under reduced pressure by filtration. A small amount of methylene chloride was added to the obtained oily product, and the solution was precipitated and purified in isopropyl ether, and then dried under reduced pressure to give an acrylated poly [bis (triethyleneglycol) benzoate] (1d) (M _n = 3,200) was obtained as a yellow viscous oil. The yield was 3.23 g and the yield was 76%.

  Next, 10% by mass of benzoyl peroxide (BPO) was added to the yellow viscous oil (polymer) obtained as described above, and polymerization was performed for 48 hours. As a result, the obtained product was a brown hard film and insoluble in water. This suggested that a crosslink was formed by the acryloyl group introduced at the end of the highly branched polymer.

Further, as expected from the fact that the polymer film according to Comparative Example 2 does not have a component capable of forming an ion in the molecule, the complex alternating current as performed for the polymer electrolyte according to the example of the present invention is used. Impedance measurement was not possible. From the performance of the measuring device, it was confirmed that it was a non-conductor of 10 ⁷ Ω or more.

  From the above results, it was found that when the molecular terminal is only a crosslinkable functional group and no acidic functional group is present, it does not have ionic conductivity. Further, as suggested from the results of the ionic conductivity test performed on the polymer electrolytes according to the examples of the present invention described above, the increase in crosslinkable functional groups (in other words, It is clear that the decrease in ionic conduction is proportional to the decrease in ion carriers in the polymer (see Table 1 below). Therefore, in order to maintain good ion conductivity, it is preferable that the carrier density is high, that is, there are many acidic functional groups and few crosslinkable functional groups. On the other hand, as described above, as can be seen from the fact that the polymer of Comparative Example 1 does not form a self-supporting film, it is necessary that at least a part of the polymer electrolyte has a crosslinkable functional group. When the introduction ratio of the crosslinkable functional group is 10% or more and less than 60%, a film having self-supporting property can be formed while maintaining good ion conductivity.

<Ion conductivity thermal stability test>
Next, the polymer electrolyte (crosslinked HBP-PA-Ac film) according to the examples of the present invention, and an electrolyte membrane (hereinafter, referred to as Nafion) using commercially available Nafion as a general perfluorosulfonic acid type polymer electrolyte. A test of stability of ionic conductivity against heat was conducted on “Nafion membrane”. A sample of Nafion membrane was prepared as follows. That is, a polymer electrolyte membrane was obtained from a commercially available 20% solution of Nafion (registered trademark) (52,712-2 manufactured by Aldrich) by a casting method using a doctor blade. The drying temperature was set to 40 ° C., and the obtained membranes (the polymer electrolyte membrane and the Nafion membrane according to the examples of the present invention) were peeled off from the glass substrate in water, and a wet membrane was used. Then, a comparative evaluation of the thermal stability of the inverse of the ionic conductivity multiplied by a constant was performed.

  The result of the thermal stability test of the ionic conductivity is shown in FIG. FIG. 8 is a graph showing changes in membrane resistance with respect to elapsed time of a polymer electrolyte according to an example of the present invention and a Nafion membrane which is a perfluorosulfonic acid type polymer electrolyte membrane as a comparative example. The vertical axis shows the rate of increase in membrane resistance [%], and the horizontal axis shows the elapsed time [hour]. As shown in FIG. 8, the polymer electrolyte membranes according to the examples of the present invention show stable ion conductivity with little increase in membrane resistance even under high temperature conditions (150 ° C. in this test). I understood. On the other hand, it was found that with the Nafion membrane, with the passage of time (in the present experiment, after about 8 hours), the membrane resistance suddenly increased and the ionic conductivity significantly decreased.

  Perfluorosulfonic acid type polymer solid electrolytes such as Nafion (registered trademark) have excellent ionic conductivity in a state where moisture is retained. Therefore, these membranes are suitable for fuel cell applications using polymer solid electrolyte membranes. is there. However, since it is an ionic conduction mechanism using water as a medium, the ionic conductivity is remarkably lowered in a temperature range of 100 ° C. or higher, as is apparent from the above experiment. However, since ionic conduction in the electrolyte of the present invention does not use water as a medium, stable membrane resistance, that is, stable ionic conductivity can be maintained even at 150 ° C. Therefore, the above experiments and studies suggest that the polymer electrolyte of the present invention can achieve stable ionic conduction even at high temperatures.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

It is a FT-IR spectrum of HBP-PA-Ac (9) concerning one example of the present invention. It is an FT-IR spectrum of _HBP-SO 3 K-Ac according to an embodiment of the present invention (11). It is an FT-IR spectrum of crosslinking _HBP-SO 3 _H-Ac according to an embodiment of the present invention. 3 is a graph showing a direct current polarization characteristic of a fuel cell according to an embodiment of the present invention. It is an Arrhenius plot which shows the ionic conductivity of the polymer electrolyte membrane which concerns on one Example of this invention. It is a graph which shows the result of the thermogravimetric analysis of the polymer electrolyte which concerns on the Example of this invention. It is a graph which shows the displacement by the needle | hook penetration of the polymer electrolyte membrane with respect to the change of temperature. It is the graph which showed the change of the membrane resistance with respect to elapsed time of the polymer electrolyte which concerns on the Example of this invention, and the Nafion membrane which is a perfluorosulfonic acid type polymer electrolyte membrane as a comparative example.

Claims (8)

A polymer electrolyte mainly composed of a highly branched polymer having an oligoethylene oxide structure in the main chain,
A polymer electrolyte comprising at least an acidic functional group and a crosslinkable functional group as a terminal functional group of a side chain of the hyperbranched polymer.   The polymer electrolyte according to claim 1, wherein a part or all of the acidic functional group is a phosphonic acid group.   The polymer electrolyte according to claim 1 or 2, wherein a part or all of the crosslinkable functional group is an acryloyl group. The polymer electrolyte according to claim 1, wherein the highly branched polymer is poly [bis (oligoethylene glycol) benzoate] represented by the following general formula 1.

The poly [bis (oligoethylene glycol) benzoate] is an A ₂ B type monomer synthesized from an oligoethylene oxide chain represented by (CH ₂ CH ₂ O) _m (m = 1 to 6) and dioxybenzoate. The polymer electrolyte according to claim 4, which is a polymer having a tree-like structure obtained by polymerization.   6. The polymer electrolyte according to claim 1, wherein the crosslinkable functional group is contained in an amount of 10 mol% or more and less than 60 mol% with respect to the entire terminal functional group in the hyperbranched polymer.   A polymer electrolyte membrane formed by crosslinking the hyperbranched polymer that is a main component of the polymer electrolyte according to claim 1.   A fuel cell comprising the polymer electrolyte membrane according to claim 7.

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