JP4814860B2 - Method for producing electrolyte membrane for fuel cell comprising cross-linked fluororesin substrate - Google Patents

Description

  The present invention relates not only to a polymer electrolyte membrane suitable for a polymer electrolyte fuel cell, but also to a fluorine-based polymer ion exchange membrane having a wide range of performance as an electrolytic membrane and excellent in dimensional stability.

Conventional technology

  The polymer electrolyte fuel cell is expected as a power source for electric vehicles, household stationary devices and electronic devices because of its high energy density. In this fuel cell, an ion exchange membrane as an electrolyte is one of the most important members. In the polymer electrolyte fuel cell, gas diffusion electrodes are joined to both surfaces of the electrolyte membrane, and the membrane and the electrode are substantially integrated. For this reason, the electrolyte membrane acts as an electrolyte for conducting protons, and also has a role as a diaphragm for preventing hydrogen or methanol as a fuel from directly contacting oxygen even under pressure. Such an electrolyte membrane has a high proton transfer rate and a high ion exchange capacity, and since a large current flows for a long period of time, the water has a constant and high water retention in order to keep the membrane chemical stability and electrical resistance low. Is required. On the other hand, due to its role as a diaphragm, the mechanical strength of the membrane is strong and its dimensional stability is excellent, and it does not have excessive gas permeability with respect to hydrogen gas or oxygen gas as a fuel. Long-term durability is required.

  As a membrane suitable for this application, a fluoropolymer perfluorosulfonic acid membrane “Nafion (registered trademark of DuPont)” developed by DuPont and the like has been generally used.

  However, fluorine-based polymer ion exchange membranes such as “Nafion” are excellent in chemical durability and stability, but their ion exchange capacity is as small as about 1 meq / g, so that sufficient electric output can be obtained. In addition, the water retention is insufficient and the ion exchange membrane is dried, resulting in a decrease in proton conductivity, or the reaction of the fuel gas or the oxidant gas in the electrode catalyst may be hindered. In addition, fluorine-based polymer ion exchange membranes such as Nafion are difficult and complicated to synthesize monomers, and the process for polymerizing them to produce polymer membranes is also very expensive. Efforts are being made to develop alternative low cost, high performance electrolyte membranes.

  In addition to the above, the conventional fluorine-based polymer ion exchange membrane has not been able to increase the ion exchange capacity because a crosslinked structure cannot be introduced. That is, if a large number of sulfonic acid groups are to be introduced in order to increase the ion exchange capacity, the membrane strength is remarkably lowered due to the absence of a crosslinked structure in the polymer chain, and the membrane is easily damaged. Therefore, in the conventional ion exchange membranes of fluoropolymers, it is necessary to suppress the amount of sulfonic acid groups to such an extent that the membrane strength is maintained, so that only a relatively small ion exchange capacity can be achieved. This was not provided with the performance required for an ion exchange membrane for passing a large current for a fuel cell or the like.

  In the radiation graft polymerization method closely related to the present invention, an attempt has been made to produce a solid polymer electrolyte membrane by grafting a monomer capable of introducing a sulfonic acid group into a fluorine-based polymer membrane. However, when a fluorine-based polymer film is irradiated with radiation such as an electron beam or γ-ray to perform a graft reaction, a significant decrease in film strength is observed due to deterioration due to irradiation, and the graft rate is also extremely low. You can only get things. For this reason, when a fluorine-based ion exchange membrane is produced by the radiation graft method, the membrane is very fragile and only a membrane having an extremely low ion exchange capacity can be produced. there were.

  For example, in polytetrafluoroethylene (PTFE), PTFE-hexafluoropropylene copolymer (FEP), PTFE-perfluoroalkyl vinyl ether copolymer (PFA), etc., the main chain of the polymer is marked when irradiated with radiation. It is known that cutting occurs. In a battery produced by using a solid polymer electrolyte membrane in which styrene is radiation graft-polymerized on a FEP membrane and sulfonic acid groups are introduced into the FEP membrane, sulfonic acid groups are removed or the membrane Swelling occurs, and as a result, the internal resistance of the battery is increased, and it is reported that the battery performance is significantly deteriorated even in a short operation of several tens of hours (Electrochimica Acta 40, 345 (1995)). On the other hand, in the case of a fluorinated polymer partially containing an olefinic hydrocarbon structure in the polymer main chain, the cleavage of the main chain by irradiation is greatly reduced. For example, an ion exchange membrane synthesized by introducing a styrene monomer into an ethylene-tetrafluoroethylene copolymer membrane containing a hydrocarbon structure by a radiation graft reaction and then sulfonating functions as an ion exchange membrane for a fuel cell (special feature). Kaihei 9-102322). However, as a disadvantage, since there is no or little cross-linked structure, the degree of swelling of the membrane is large, the dimensions are different during assembly and operation, and membrane breakage and sealing are insufficient. Furthermore, in a direct methanol fuel cell using methanol as a direct fuel, the electrolyte membrane swells and methanol diffuses to the positive electrode, so that power generation efficiency is significantly reduced. In JP-A-9-102322, it is presumed that a crosslinked structure is formed by irradiation with radiation for grafting. However, the radiation dose is as low as 1 to 100 kGy, and it cannot be said that the structure is sufficiently crosslinked. Further, the operating temperature of the fuel cell is currently around 70 ° C., but in order to use the carbon monoxide poisoning of the platinum catalyst and the obtained hot water for cooling, in the future, a high temperature such as 100 ° C. to 150 ° C. Driving is desired.

  The present invention suppresses the degree of swelling of the electrolyte membrane of the fuel cell, reduces the dimensional change during long-term operation, and prevents the membrane from being damaged or causing a seal failure, and also uses methanol as a direct fuel. In the direct methanol fuel cell, methanol is prevented from diffusing to the positive electrode, thereby reducing power generation efficiency. It is another object of the present invention to provide an electrolyte membrane suitable for high-temperature operation such as 100 ° C. to 150 ° C. in the future.

  The fluoropolymer substrate that can be used in the present invention is an ethylene-tetrafluoroethylene copolymer (hereinafter abbreviated as ETFE) and polyvinylidene fluoride (hereinafter abbreviated as PVDF). The present invention is achieved by positively cross-linking ETFE or PVDF base materials and then radiation-grafting styrenic monomers on the cross-linked base materials.

  By irradiating the fluoropolymer substrate with a radiation dose of 100 kGy to 500 kGy and highly crosslinking, the cell characteristics of the fuel cell having an electrolyte membrane made of the crosslinked substrate are remarkably improved. When the radiation dose is 100 kGy or less, the effect of crosslinking hardly appears. Moreover, if it is 500 kGy or more, the decomposition of the base material cannot be ignored and there is no merit.

  When the process from crosslinking to grafting is carried out in a short period of time, the radicals generated by irradiation for crosslinking can be used as they are for graft polymerization, but when graft polymerization is not performed immediately after crosslinking, irradiation is performed again before graft polymerization. It is necessary to create a radical.

  The cross-linked base material has many amorphous portions in view of its molecular structure, and can solve the disadvantage that the graft ratio is low in the fluoropolymer film. For example, when styrene is used as the graft monomer, the cross-linked base material can increase the graft ratio compared to an uncross-linked base material. Acid groups can be introduced into the crosslinked substrate.

BEST MODE FOR CARRYING OUT THE INVENTION

  As the graft polymerization method in the present invention, a pre-irradiation method in which the crosslinked fluoropolymer base material is irradiated with radiation and then brought into contact with a monomer to perform graft polymerization can be employed. Then, both of a polymer radical method in which irradiation is performed in the absence of oxygen and then graft polymerization without contact with oxygen and a peroxide method in which graft polymerization is performed after contact with oxygen can be employed. As the monomer used in the present invention, a styrene monomer can be used. Specifically, styrene and trifluorostyrene.

  Graft polymerization is carried out in an inert gas, usually in the temperature range of 30 ° C. to 150 ° C., in the monomer alone or in a solution obtained by diluting the monomer with a solvent. Since the presence of oxygen inhibits the grafting reaction, these series of operations are performed in an inert gas such as argon gas or nitrogen gas, and the monomer or a solution in which the monomer is dissolved in a solvent is treated in a conventional manner (freeze degassing or Use with oxygen removed by bubbling.

  The radiation dose for grafting is almost proportional to the graft rate, and the higher the dose, the higher the graft rate, but it gradually becomes saturated when the graft rate reaches 100% by weight or more. The graft ratio is 5 to 200 wt%, more preferably 15 to 150 wt%, based on the crosslinked base material. When the graft ratio is 150% or more, the mechanical strength of the membrane when it is hydrated decreases. Here, the “graft ratio” refers to the weight ratio (%) of the styrene monomer grafted onto the fluoropolymer substrate.

  In the graft chain, a fluorine-based polymer ion exchange membrane having a crosslinked structure can be produced by copolymerization with divinylbenzene. This can be obtained by adding 1 to 10 wt% of divinylbenzene, which is a crosslinking aid, to the above styrene monomer amount when performing radiation grafting on the above-mentioned crosslinked base material. By introducing a crosslinked structure into the graft chain of the crosslinked substrate, the oxidation resistance of the present fluoropolymer ion exchange membrane can be improved. When this membrane is used as an ion exchange membrane for fuel cells, if the water content is too low, the output voltage decreases when the pressure of oxygen or hydrogen is low or when air is used as the oxygen source, resulting in high current density and high output. Cannot be maintained. In addition, slight changes in operating conditions change the electrical conductivity and gas permeability coefficient, which is not preferable. Therefore, it is necessary that the ion exchange membrane is not easily dried and the change in gas permeability coefficient and electrical conductivity is relatively small.

  The water content of the ion exchange membrane of the present invention can be controlled in the range of 10 to 80 wt%. In general, the water content increases as the ion exchange capacity increases. However, since the water content of the ion exchange membrane of the present invention can be changed, the water content of the membrane is 10 to 100 wt%, preferably 10 to 80 wt%. It is. In the fluoropolymer according to the present invention, despite the high ion exchange capacity, an increase in the moisture content due to membrane swelling due to the entanglement of the crosslinked base material is suppressed, and an appropriate membrane strength can be maintained. Here, “the water content of the membrane” means a state where the ion exchange membrane has been stored for 24 hours or more in water at room temperature, and “the water content” means the weight of the ion exchange membrane stored in water. And the weight percentage of the film when the film was vacuum-dried at 60 ° C. for 16 hours.

  Only an ion exchange membrane having an ion exchange capacity of around 1 meq / g could be put to practical use from the viewpoint of the mechanical strength and dimensional stability of the membrane. In the uncrosslinked base material, the film strength is mainly maintained by the crystal part of the base material. For this reason, when a large amount of graft chains and sulfonic acid groups are introduced, the strength of the base material is drastically lowered and it cannot be used.

  On the other hand, the fluoropolymer having a cross-linked substrate structure of the present invention can maintain the mechanical properties and dimensional stability of the membrane even when a large amount of graft chains and sulfonic acid groups are introduced up to an ion exchange capacity of about 2.0 meq / g. Since the property is maintained, it can be put to practical use. Although a membrane having an ion exchange capacity of 2.0 meq / g or more can be produced, the mechanical properties of the membrane are lowered and the dimensional stability of the membrane is lowered. From these, the fluorine-based polymer ion exchange membrane in the present invention has an ion exchange capacity of 0.5 to 2.0 meq / g, and the tensile breaking strength of the membrane material in a water-containing state is 3 to 25 MPa, More preferably, it is 5-25 MPa. At this time, the tensile elongation of the film material is 15% or more, more preferably 30% or more. Here, “ion exchange capacity (meq / g)” is an index representing the ease of ion flow in the electrolyte membrane, and is the number of milliequivalents of sulfone groups per 1 g of electrolyte membrane.

  Membranes with high ion exchange capacity and excellent mechanical properties are extremely important for practical use. From the mechanical properties of the membrane, the graft ratio is 5 to 200 wt%, more preferably 15 to 150 wt%.

The higher the electric conductivity of the polymer ion exchange membrane for fuel cells, the lower the electric resistance, and the higher the performance as an electrolyte membrane. When the electric conductivity of the ion exchange membrane at 25 ° C. is 0.05Ω ⁻¹ · cm ⁻¹ or less, the output performance as a fuel cell often decreases significantly. 0.05Ω ⁻¹ · cm ⁻¹ or higher, and higher performance ion exchange membranes require 0.10 Ω ⁻¹ · cm ⁻¹ or higher. On the other hand, it is known that the strength of the ion exchange membrane is lowered when the electrical conductivity of the ion exchange membrane at 25 ° C. is 0.12 Ω ⁻¹ · cm ⁻¹ or more in a normal fluorine ion exchange membrane. That is, if the exchange capacity of the ion exchange membrane is increased and the electric conductivity is increased too much, there is a disadvantage that the strength of the membrane is lowered. However, it has been revealed that the ion exchange membrane according to the present invention maintains a high membrane strength even when the electric conductivity of the ion exchange membrane at 25 ° C. is 0.11 Ω ⁻¹ · cm ⁻¹ . From these facts, the fluoropolymer ion exchange membrane of the present invention has an electric conductivity at 25 ° C. of 0.03 to 0.25Ω ⁻¹ · cm ⁻¹ , preferably 0.05 to 0.25 Ω ⁻¹ · cm. ^-1 .

  In order to improve the characteristics of the fuel cell, it is conceivable to reduce the thickness of the ion exchange membrane. However, at present, a thin ion exchange membrane is easily damaged, and since the absolute amount of water contained in the ion exchange membrane is small, the ion exchange membrane is easy to dry and cannot maintain high performance for a long time. There is a case. Therefore, an ion exchange membrane having a thickness of 30 to 200 μm is usually used. In the present invention, the film thickness is not particularly limited, but those in the range of 15 μm to 200 μm are effective.

As described above, the fluoropolymer ion exchange membrane of the present invention has important characteristics as a membrane, that is, the ion exchange capacity is in a wide range of 0.5 to 2.0 meq / g, and the membrane strength is 5 to 25 MPa. The water content is 10 to 80 wt%, and the electrical conductivity at 25 ° C. can be controlled within the respective numerical ranges of 0.05 to 0.25Ω ⁻¹ · cm ⁻¹ . It is also a feature of the present invention that the characteristics can be controlled within these limited ranges.

  Hereinafter, although an example and a comparative example explain the present invention, the present invention is not limited to this.

In addition, each measured value was calculated | required by the following measurements.
(1) Graft ratio When the cross-linked fluoropolymer is the main chain portion and the graft polymerized portion of the styrene monomer is the graft chain portion, the weight ratio of the graft chain portion to the main chain portion is the graft ratio (X _dg ( wt%)).

_{X dg = 100 (W 2 -W} 1) / W 1 (1)
W ₁ : Weight of crosslinked fluoropolymer film before grafting (g)
W ₂ : Weight of the graft copolymer film (dry state) after grafting (g)
(2) The ion exchange capacity (I _ex (meq / g)) of the ion exchange capacity membrane is expressed by the following equation.

I _ex = n (acid group) _obs / W _d (2)
n (acid group) _obs : _Acid group concentration of the ion exchange membrane (mM / g)
W _d : dry weight of ion exchange membrane (g)
For the measurement of n (acid group) _obs , the membrane was again immersed in a 1 M (1 mol) sulfuric acid solution at 50 ° C. for 4 hours to completely complete the acid type (H type). Then, it was immersed in 3 M NaCl aqueous solution at 50 ° C. for 4 hours to form —SO ₃ Na type, and the substituted proton (H ⁺ ) was neutralized and titrated with 0.2 N NaOH to determine the acid group concentration.

(3) Electrical conductivity The electrical conductivity of the ion exchange membrane was measured by an alternating current method (New Experimental Chemistry Course 19, Polymer Chemistry <II>, p. 992, Maruzen). made in LCR meter, using the E-4925A was measured film resistance _{(R m).} The cell was filled with a 1M aqueous sulfuric acid solution, and the resistance between platinum electrodes (distance 5 mm) depending on the presence or absence of the film was measured. The electric conductivity (specific conductivity) of the film was calculated using the following equation.

κ = 1 / R _m · d / S (Ω ⁻¹ cm ⁻¹ ) (3)
κ: electrical conductivity of the film (Ω ⁻¹ cm ⁻¹ )
d: thickness of ion exchange membrane (cm)
S: Current-carrying area of the ion exchange membrane (cm ² )
For comparison of electrical conductivity measurements, Mark W. Verbrugge, Robert F. et al. Measurement was performed using a cell, a potentiostat, and a function generator similar to those of Hil 1 et al. (J. Electrochem. Soc., 137, 3770-3777 (1990)). There was a good correlation between the measured values of the AC and DC methods. The values in Table 1 below are measured values by the AC method.

(4) Oxidation resistance (weight residual rate%)
The weight after vacuum drying at 60 ° C. for 16 hours is defined as W _3, and the weight after drying of a film treated with a 3% hydrogen peroxide solution at 80 ° C. for 24 hours is defined as W ₄ .

Oxidation resistance = 100 (W ₄ / W ₃ )
(5) Measurement was performed at a tensile rate of 200 mm / min with an all-purpose tensile tester in an atmosphere having a strength temperature of 70 ° C. and a relative humidity of 95% in a high temperature and high humidity atmosphere.

Example 1
First, a 50 μm-thick ETFE film was crosslinked by irradiating it with 300 kGy of an electron beam in air. After leaving at 30 ° C. for 1 week, an electron beam was irradiated again in air for 30 kGy to generate radicals. Immediately, it was immersed in styrene from which oxygen had been removed in advance by a flask, and graft polymerization was carried out by heating at 70 ° C. for 90 minutes while flowing 500 cc of nitrogen per minute. The graft rate was 32%. Next, in order to introduce a sulfone group, it was immersed in a dichloroethane solution containing 35% by weight of chlorosulfonic acid at 30 ° C. for 1 hour.

(Example 2)
A PVDF film having a thickness of 50 μm was irradiated with 300 kGy of an electron beam in the air to crosslink. After leaving at 30 ° C. for 1 week, radicals were generated again by irradiation with 50 kGy of electron beams in air. Immediately, the flask was immersed in trifluorostyrene from which oxygen had been removed by pre-aeration in a flask, and was graft-polymerized by heating at 60 ° C. for 90 minutes while flowing 500 cc of nitrogen per minute. The graft rate was 29%. Next, in order to introduce a sulfone group, it was immersed in a dichloroethane solution containing 35% by weight of chlorosulfonic acid at 30 ° C. for 1 hour.

(Example 3)
In Example 1, the graft monomer was changed to 98% by weight of styrene and 2% by weight of divinylbenzene, and the other operations were performed in the same manner. The graft rate was 33%.

Example 4
The same operation was performed except that the crosslinking dose of Example 1 was 150 kGy. The graft rate was 32%.

(Comparative Example 1)
In Example 1, an uncrosslinked ETFE film having a thickness of 50 μm was used, and the others were operated in the same manner. The graft rate was 21%.

(Comparative Example 2)
In Example 1, the same operation was performed except that an uncrosslinked ETFE film with a thickness of 50 μm was used and the electron beam irradiation in air was changed to 60 kGy. The graft rate was 31%.

The fluororesin ion exchange membrane of the present invention has an ion exchange capacity of 1.5 to 2.0 meq / g, a membrane material tensile strength at 9 to 5 MPa at 70 ° C. and 95% RH, and an electrical conductivity at 25 ° C. of 0. 0.09 to 0.11 Ω ⁻¹ · cm ⁻¹ and very high oxidation resistance. It is a low-cost fluorine-based polymer ion exchange membrane with a wide range of ion exchange capacities, high oxidation resistance and membrane strength. The ion exchange membrane of the present invention is particularly suitable for a fuel cell membrane. Moreover, it is useful as an inexpensive and durable electrolytic membrane or ion exchange membrane.

Claims (2)

  A direct methanol fuel cell electrolyte in which a styrene monomer is radiation graft copolymerized with an ethylene-tetrafluoroethylene copolymer film or polyvinylidene fluoride film previously cross-linked by radiation irradiation, and a sulfonic acid group is introduced into the graft chain. A method for producing a membrane.   The method for producing an electrolyte membrane for a direct methanol fuel cell according to claim 1, wherein the crosslinking dose is 100 kGy to 500 kGy.