JP2014044875A - Polymer electrolyte, membrane electrode assembly using the same, fuel cell, and method for producing polymer electrolyte - Google Patents

Abstract

The present invention provides a polymer electrolyte membrane that can be used in a polymer electrolyte fuel cell and exhibits excellent proton conductivity at low humidity.
A cross-linked site is formed on a cross-linked polymer electrolyte formed mainly of a cross-linked polymer obtained by cross-linking a proton conducting polymer and a cross-linking agent via a portion other than a proton acid group. A polymer electrolyte membrane was obtained by sulfonation.
[Selection] Figure 1

Description

  The present invention relates to a polymer electrolyte for a polymer electrolyte fuel cell, a membrane electrode assembly using the polymer electrolyte, a fuel cell, and a method for producing the polymer electrolyte.

In recent years, fuel cells have attracted attention as effective solutions for environmental problems and energy problems. A fuel cell is a system that oxidizes a fuel such as hydrogen using an oxidant such as oxygen and converts chemical energy associated therewith into electrical energy.
Fuel cells are classified into alkaline, phosphoric acid, solid polymer, molten carbonate, solid oxide, etc., depending on the type of electrolyte. The polymer electrolyte fuel cell (PEFC) is operated at low temperature, has high output density, and can be reduced in size and weight. ing.

As an electrolyte of PEFC, a perfluoro electrolyte represented by Nafion (registered trademark of Nafion, DuPont) having practical stability 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 hydrocarbon-based electrolytes exhibit high proton conductivity under high humidity conditions, but the proton conductivity decreases under low humidity conditions.

As a method of exhibiting high proton conductivity even under low humidity conditions, attempts have been made to increase the concentration of proton acid groups. However, when the concentration of the proton acid group is increased, there is a problem that the water resistance of the membrane is lowered, and the mechanical strength that can withstand practical use cannot be obtained.
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 (for example, 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 will fall rather than the original electrolyte.

  Moreover, in order to suppress the reduction | decrease of the above hydrogen ion exchange capacities, the method of bridge | crosslinking not via a proton acid group is also known (for example, patent document 4). However, this method also has a problem that the proton conductivity under low humidity conditions is low because the entire hydrogen ion exchange capacity is lowered.

Japanese Patent Laid-Open No. 11-116679 JP 2007-329120 A JP 2007-70563 A JP 2009-59522 A

As described above, a number of methods have been proposed as methods for maintaining high proton conductivity, but none of these methods is sufficient, and it is possible to easily obtain an electrolyte with better proton conductivity under low humidity conditions. A new method was desired.
An object of the present invention is to provide a polymer electrolyte excellent in proton conductivity under low humidity conditions, a membrane electrode assembly using the polymer electrolyte, a fuel cell, and a method for producing the polymer electrolyte.

In order to solve the above problems, the present inventors have made extensive studies, and as a result, by sulfonated the cross-linked site of the cross-linked polymer, an electrolyte having excellent proton conductivity at low humidity can be easily produced at low cost. The knowledge that it can be provided has been obtained, and the present invention has been achieved.
In one embodiment of the present invention, there is provided a cross-linked site of a cross-linked polymer electrolyte formed mainly of a cross-linked polymer obtained by a cross-linking reaction between a proton conductive polymer and a cross-linking agent via a portion other than a proton acid group. A polymer electrolyte characterized by being sulfonated.
The proton exchange capacity of the polymer electrolyte of the above embodiment may be 4.0 meq / g or more and 8.0 meq / g or less.

Another aspect of the present invention is a membrane electrode assembly including an electrolyte membrane, wherein the polymer electrolyte according to the above aspect is used as the electrolyte membrane.
Another aspect of the present invention is a fuel cell including an electrolyte membrane, wherein the polymer electrolyte described in the above aspect is used as the electrolyte membrane.
Another aspect of the present invention includes a step of cross-linking a proton conductive polymer and a cross-linking agent through a portion other than a proton acid group to form a cross-linked polymer, and sulfonating a cross-linked site of the cross-linked polymer. And a step of forming a polymer electrolyte containing the cross-linked polymer as a main component in this order.

  According to the present invention, a cross-linked site of a cross-linked polymer electrolyte formed mainly of a cross-linked polymer obtained by a cross-linking reaction between a proton conductive polymer and a cross-linking agent via a moiety other than a proton acid group, Since the polymer electrolyte is obtained by sulfonation, the polymer electrolyte having excellent proton conductivity at low humidity can be obtained at a low cost.

Moreover, since the proton exchange capacity of the polymer electrolyte is 4.0 milliequivalent / g or more and 8.0 or less milliequivalent / g, it exhibits high proton conductivity, and when applied as an electrolyte constituting a fuel cell, There is a further remarkable effect that a high output density can be obtained by reducing the internal resistance of the fuel cell.
Further, when such a polymer electrolyte is applied to a membrane electrode assembly constituting a fuel cell, there is a further remarkable effect that a high output density can be obtained by reducing the internal resistance of the fuel cell. .
Further, by using such a polymer electrolyte as an electrolyte membrane of a fuel cell, there is a further remarkable effect that the internal resistance of the fuel cell can be reduced and a high output density can be obtained.

It is sectional drawing which shows an example of the membrane electrode assembly formed by forming an electrode catalyst layer on both surfaces of the polymer electrolyte membrane of this invention. FIG. 2 is an exploded sectional view showing a configuration of a single cell of a fuel cell equipped with the membrane electrode assembly shown in FIG. It is a graph which shows transition of proton conductivity to change of humidity.

Hereinafter, the present invention will be described in detail.
The present invention relates to a polymer electrolyte for a solid polymer fuel cell, and mainly comprises a crosslinked polymer obtained by chemically bonding a proton conductive polymer and a crosslinking agent via a moiety other than a protonic acid group. In the crosslinked polymer electrolyte formed as above, the crosslinked site is sulfonated to obtain a polymer electrolyte.
The proton conductive polymer is preferably an aromatic polymer from the viewpoint of heat resistance, and specifically, polyether ketone, polyether ether ketone, polyether sulfone, or polyphenylene is preferable.
The type of protonic acid group in the proton conductive polymer is not particularly limited, but sulfonic acid group, phosphonic acid group, hydroxyl group, carboxyl group, etc. can be used, among others, dissociation constant, stability to water, etc. In view of this, a sulfonic acid group is preferable.

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. As long as it has a structure in which a sulfone group can be introduced later in the molecule. 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 the crosslinking agent having a methylol group, a compound having at least two aromatic rings having a methylol group in the molecule is preferable. That is, a compound having at least two aromatic rings having a methylol group in the molecule undergoes a reaction without passing through a protonic acid group contained in the proton-conducting polymer, so that the proton conductivity is not impaired by the reaction, Moreover, since the reaction easily proceeds by heating, it is preferable as a crosslinking agent in the present invention.

As the aromatic ring, those having a benzene ring as a basic skeleton are preferable. Specific examples include a benzene ring, a naphthalene ring, and an anthracene ring. In addition, a benzene ring having a plurality of bonds such as biphenyl and terphenyl may be used.
As a crosslinking method, a light irradiation method, a heating method, a pH adjustment method, or the like can be used. However, since the solution of the proton conductive polymer used in the present invention is acidic, a pH adjustment method that reacts under acidic conditions is preferable. .

As a sulfonating agent used for sulfonation, concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid and the like can be used. Sulfonation is carried out by reacting the crosslinked electrolyte with these reagents as they are or diluted with a solvent or the like.
The reaction temperature in the sulfonation is preferably room temperature, and in some cases, it may be heated.

The reaction time in sulfonation is preferably 1 hour or more and less than 48 hours. If it is less than 1 hour, sulfonation may be insufficient, and if it is more than 48 hours, decomposition of the electrolyte may occur.
The proton exchange capacity of the sulfonated crosslinked polymer is preferably 4.0 meq / g or more and 8.0 meq / g or less. If the proton exchange capacity is less than 4.0 milliequivalent / g, sufficient proton conductivity may not be obtained at low humidity. If the proton exchange capacity is 8.0 milliequivalent / g or more, the water resistance and durability of the electrolyte membrane may be insufficient. May be damaged.

  The sulfonated crosslinked polymer can be used in combination with other polymers in order to improve mechanical strength and water resistance. 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.

Next, a membrane electrode assembly (MEA) to which the present invention is applied will be described.
FIG. 1 is a cross-sectional view showing an embodiment of a membrane electrode assembly using a polymer electrolyte according to the present invention. The membrane electrode assembly of the present invention has a laminated structure as shown in FIG.
That is, the membrane electrode assembly 12 is formed by bonding and laminating the air electrode side electrode catalyst layer 2 and the fuel electrode side catalyst layer 3 to both surfaces of the polymer electrolyte 1 of the present invention produced as described above by a conventional method. It becomes. The electrode catalyst layers 2 and 3 are formed by reacting carbon black particles as a conductive agent with a reaction catalyst, the proton conductive polymer, or the proton conductive polymer and a crosslinking agent, respectively. It is composed of a polymer electrolyte obtained by chemical bonding through a portion other than the group.

  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 nm or more and 20 nm or less are preferable. More preferably, it is 1 nm or more and 5 nm or less.

  In order to support these reaction catalysts, carbon particles are generally used as the electron conductive conductive agent used in the present invention. 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 decreases or the utilization factor of the catalyst decreases. The degree is preferred. More preferably, it is 10 nm or more and 100 nm or less.

FIG. 2 is an exploded cross-sectional view showing the configuration of an embodiment of the unit cell 11 of the 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.

As a result, the air electrode 6 is formed by the air electrode side electrode catalyst layer 2 and the air electrode side gas diffusion layer 4, and the fuel electrode 7 is formed by the fuel electrode side electrode catalyst layer 3 and the fuel electrode side gas diffusion layer 5. The A single cell 11 is formed by facing each of the air electrode side gas diffusion layer 4 and the fuel electrode side gas diffusion layer 5 by sandwiching each of these layers by a set of separators 10. The separator 10 has a gas flow path 8 for reaction gas flow and has a cooling water flow path 9 for flow of cooling water on the opposing main surface, and is made of a conductive and gas-impermeable material. .
Electric power is generated by supplying an oxidant such as air or oxygen to the air electrode 6 and supplying a fuel gas containing hydrogen or an organic fuel to the fuel electrode 7.

The manufacturing method of the membrane electrode assembly (MEA) of the present invention will be further described.
Proton conductivity having a reaction catalyst, the conductive agent and a proton acid group 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 The electrode catalyst layers 2 and 3 are laminated | stacked by apply | coating the ink which consists of polymers, and making it dry after that. Thereafter, a method of manufacturing a membrane electrode assembly (MEA) 12 by sandwiching a polymer electrolyte membrane (proton conductive crosslinked polymer electrolyte membrane) 1 between the electrode catalyst layers 2 and 3 and joining them by thermocompression bonding is used. Can do. As a method for applying the ink for forming the electrode catalyst layers 2 and 3 on the gas diffusion layers 4 and 5, a doctor blade method, a screen printing method, a spray method, or the like can be used.
The membrane electrode assembly (MEA) 12 can be produced by producing electrode catalyst layers 2 and 3 on both surfaces of a polymer electrolyte membrane (proton conductive cross-linked polymer electrolyte membrane) 1 by transfer or spray spraying. Alternatively, a method of sandwiching 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.
In this example, the proton exchange capacity and proton conductivity were determined as follows.
(Measurement of proton exchange capacity)
The crosslinked electrolyte membrane was stirred overnight in a 2 mol / L sodium chloride aqueous solution, the filtrate was titrated with a 0.1 mol / L sodium hydroxide solution, and the proton exchange capacity was calculated from the neutralization point.

(Measurement of proton conductivity)
The AC resistance was obtained from the measurement of AC impedance between platinum wires by pressing a platinum wire (φ = 0.5 mm) against the surface of the strip-shaped polymer electrolyte membrane and holding the sample in a constant temperature and humidity device. That is, the impedance at alternating current was measured at 80 ° C. under a predetermined humidity environment (relative humidity of 30% to 90%). Four platinum wires were pressed against each other at intervals of 5 mm, and the distance between the wires was changed in the range of 5 mm to 15 mm, and the AC resistance was measured. Membrane resistance was calculated from the distance between lines and the resistance value, and proton conductivity was calculated from this value.

Example 1
First, 1 g of sulfonated polyetheretherketone (trade name: 450PF, manufactured by Victrex, hereinafter referred to as sulfonated PEEK) and 0.15 g of p-xylylene glycol as a crosslinking agent in γ-butyrolactone. Mixed.
Next, the prepared solution was formed into a film on a polyimide substrate by a casting method, and the solvent was dried. Then, the obtained film was heated and pressed to advance a crosslinking reaction, thereby obtaining a crosslinked electrolyte film. The hot pressing conditions were a press temperature of 120 ° C., a press time of 1 hour, and a press pressure of 60 kgf / cm 2. The proton exchange capacity of the obtained crosslinked electrolyte membrane was 2.3 meq / g.
Subsequently, the crosslinked electrolyte membrane was immersed in fuming sulfuric acid for 30 minutes for sulfonation. After the reaction, the membrane was washed with ultrapure water and then dried. And the proton exchange capacity and proton conductivity of the membrane were measured.

(Example 2)
A membrane was produced in the same manner as in Example 1 except that the reaction time with fuming sulfuric acid was changed from 30 minutes to 24 hours, and the proton exchange capacity and proton conductivity were measured.
(Comparative Example 1)
The crosslinked electrolyte membrane prepared in Example 1 was measured for proton exchange capacity and proton conductivity before sulfonation.
The measurement results of the proton exchange capacities of Example 1, Example 2, and Comparative Example 1 were 4.8 meq / g, 6.0 meq / g, and 2.3 meq / g, respectively.

FIG. 3 shows the transition of proton conductivity at each humidity for each of the crosslinked electrolyte membranes of Example 1, Example 2, and Comparative Example 1. In FIG. 3, the horizontal axis represents humidity [%], and the vertical axis represents proton conductivity [S / cm].
From FIG. 3, it was confirmed that Example 1 and Example 2 in which the crosslinking site was sulfonated showed a large improvement in proton conductivity in the pre-humidity region as compared with Comparative Example 1 in which the crosslinking site was not sulfonated. .

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 (5)

  The cross-linked site of a cross-linked polyelectrolyte formed using a cross-linked polymer obtained by cross-linking a proton conducting polymer and a cross-linking agent through a portion other than a proton acid group as a main component is sulfonated. A characteristic polymer electrolyte.   2. The polymer electrolyte according to claim 1, wherein the proton exchange capacity is 4.0 milliequivalent / g or more and 8.0 or less milliequivalent / g.   A membrane / electrode assembly comprising an electrolyte membrane, wherein the polymer electrolyte according to claim 1 or 2 is used as the electrolyte membrane.   A fuel cell comprising an electrolyte membrane, wherein the polymer electrolyte according to claim 1 or 2 is used as the electrolyte membrane. A step of cross-linking the proton conductive polymer and the cross-linking agent through a portion other than the proton acid group to form a cross-linked polymer;
Sulfonating the cross-linked site of the cross-linked polymer;
In this order. A method for producing a polymer electrolyte.

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