JP2013229325A - Polymer electrolyte membrane, membrane electrode assembly and fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a polymer electrolyte membrane excellent in dimensional stability in a wet state, improved in mechanical strength, and low in environmental loads in disposal; a membrane electrode assembly including the polymer electrolyte membrane; and a fuel cell including the membrane electrode assembly.SOLUTION: A polymer electrolyte membrane is a membrane containing sulfoalkyl cellulose and cellulose nanofibers and including a crosslinked structure formed by a crosslinking agent therein. A membrane electrode assembly includes the polymer electrolyte membrane. A fuel cell includes the membrane electrode assembly.

Description

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

Solid polymer fuel cells that use proton-conducting polymers as electrolytes are attracting attention from the viewpoints of energy saving and environmental problems because they are clean power generators with high energy conversion efficiency and no generation of pollutants. . Furthermore, the polymer electrolyte fuel cell has features such that it operates at a relatively low temperature, can be miniaturized, and has a high output density, and is expected as a power source for electric vehicles and households. In addition, a direct methanol fuel cell using methanol as a kind of solid polymer fuel cell is expected to be a portable power source for small electronic devices because of its high energy density.

Conventionally, perfluorosulfonic acid polymer membranes such as Nafion (registered trademark) have been used as electrolyte membranes for polymer electrolyte fuel cells. However, the perfluorosulfonic acid polymer membrane has a problem that the manufacturing process is complicated and very expensive, and since it contains fluorine,
It is also necessary to consider the environmental impact at the time of disposal. In addition, when used as an electrolyte membrane in a direct methanol fuel cell, the fuel methanol easily permeates (crosses over) the electrolyte membrane, so the methanol that reaches the air electrode directly reacts with oxygen, reducing the fuel utilization efficiency. In addition, there is a problem that the overvoltage increases and the potential is lowered.

Therefore, Patent Document 1 discloses a crosslinked sulfoalkyl cellulose electrolyte membrane as an electrolyte membrane that has low methanol permeability and does not contain fluorine.

JP 2010-218742 A

However, the polymer electrolyte membrane described in Patent Document 1 has insufficient dimensional stability and mechanical strength when it contains water, and has practical problems as an electrolyte membrane for fuel cells. In the membrane electrode assembly using and the fuel cell using this membrane electrode assembly, there was a problem in resistance to the aqueous methanol solution.

Therefore, the present invention has been made to solve the above-described problems, and the electrolyte membrane does not swell or break with the aqueous methanol solution, and has excellent dimensional stability when containing water,
It is an object of the present invention to provide a polymer electrolyte membrane with improved mechanical strength, a membrane electrode assembly using the polymer electrolyte membrane, and a fuel cell using the membrane electrode assembly.

In order to achieve the above object, a first aspect of the present invention is a polymer electrolyte membrane comprising a sulfoalkyl cellulose and cellulose nanofiber, wherein the membrane is provided with a crosslinked structure by a crosslinking agent. This is the gist.

The second aspect of the present invention is a membrane electrode assembly comprising the polymer electrolyte membrane described in the first aspect of the present invention and an electrode provided on at least one surface of the polymer electrolyte membrane. And

The third aspect of the present invention includes (a) a methanol aqueous solution fuel tank, (b) the membrane electrode assembly described in the second aspect of the present invention, and (c) sandwiching the membrane electrode assembly from both sides. A fuel cell comprising: a methanol aqueous solution supplied from an oxygen and methanol aqueous solution fuel tank; and a pair of conductive separators having a groove or an opening through which oxygen supplied from an air supply device can pass. .

According to the present invention, the electrolyte membrane does not swell or break due to the aqueous methanol solution, has excellent dimensional stability when containing water, improved mechanical strength, and has a low environmental impact when discarded. A membrane electrode assembly using a polymer electrolyte membrane and a fuel cell using the membrane electrode assembly can be provided.

It is a figure which shows typically an example of the cross-section of the membrane electrode assembly of this invention. It is a figure which shows typically an example of the cross-section of the fuel cell of this invention. 6 is a graph showing the relationship between the current density and output characteristics of a fuel cell in Example 4. 6 is a graph showing the relationship between the current density and voltage of a fuel cell with respect to each methanol concentration in Example 5. 10 is a graph showing the relationship between the current density and output density of a fuel cell with respect to each methanol concentration in Example 5.

The polymer electrolyte membrane of the present invention is obtained by providing a crosslinked structure between sulfoalkylcellulose and cellulose nanofiber molecules by performing a crosslinking reaction on a composite membrane in which sulfoalkylcellulose is combined with cellulose nanofiber. It is done.

The cellulose nanofiber used by this invention should just be a cellulose fiber with an average fiber diameter of 200 nm or less, and a well-known thing can be utilized. Cellulose nanofibers that can be used include bacterial cellulose produced by bacteria such as acetic acid bacteria, and fibrillated cellulose derived from plants.

Sulfoalkyl cellulose is obtained by sulfoalkylating cellulose.
The production method is not particularly limited. For example, a method in which cellulose is reacted with a sulfoalkylating agent such as halogenated alkanesulfonic acid, halogenated alkanesulfonic acid salt or sultone in a solvent can be used.

A composite membrane in which cellulose nanofibers are combined with sulfoalkyl cellulose can be produced as follows. By dissolving sulfoalkyl cellulose in a soluble solvent such as water and mixing with cellulose nanofibers using a mixer or homogenizer,
A dispersion solution in which cellulose nanofibers are dispersed in a sulfoalkylcellulose solution is obtained. Next, this dispersion solution is cast on a glass or resin support substrate, and the solvent is volatilized and dried to obtain a composite membrane.

The composite membrane can be produced by impregnating a cellulose or non-woven cellulose nanofiber with a sulfoalkyl cellulose solution and then evaporating and drying the solvent. Direct sulfoalkylation is also possible, and a method of converting a part of cellulose nanofibers to sulfoalkylcellulose is also possible.

Moreover, the composite film containing a crosslinking agent can also be obtained by mixing the sulfoalkyl cellulose solution with a crosslinking agent described later and then producing the composite film.

The thickness of the membrane is preferably 10 to 200 μm. If the thickness is less than 10 μm, the strength is lowered, and the permeation of hydrogen or methanol as a fuel may not be sufficiently prevented.
Exceeding this is undesirable because the film resistance increases.

The cellulose nanofiber content in the composite membrane is preferably 5% by mass or more and 80% by mass or less. When the content of the cellulose nanofiber is less than 5% by mass, the dimensional stability and strength of the film are lowered, which is not preferable. If it exceeds 80% by mass, the sulfonic acid group concentration of the electrolyte membrane will be low, and good proton conductivity will not be obtained.

The method for imparting a crosslinked structure to the composite membrane is not particularly limited. For example, by immersing the composite membrane in a reaction solution containing a crosslinking agent to carry out a crosslinking reaction, or by heating the composite membrane containing a crosslinking agent. A method of allowing the crosslinking reaction to proceed can be used.

Examples of the crosslinking agent include aldehydes such as formaldehyde, acetaldehyde, and glutaraldehyde, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycidyl ethers such as bisphenol A diglycidyl ether, succinic acid, oxalic acid, Examples thereof include polyvalent carboxylic acids such as maleic acid, epihalohydrin compounds such as epichlorohydrin, metal alkoxides such as tetramethoxysilane and tetraethoxysilane.

The polymer electrolyte membrane of the present invention preferably has an ion exchange capacity of 0.5 to 3.0 meq / g. If it is less than 0.5 meq / g, the proton conductivity becomes too low, which is not preferable.
If it exceeds 3.0 meq / g, swelling is increased and methanol permeability is increased, which is not preferable.

When the polymer electrolyte membrane of the present invention is used in a fuel cell, it is used after the electrolyte membrane is immersed in an acid aqueous solution such as a hydrochloric acid aqueous solution or a sulfuric acid aqueous solution, and the sulfonate is ion-exchanged to sulfonic acid as necessary.

FIG. 1 schematically shows an example of a cross-sectional structure of the membrane electrode assembly of the present invention. The membrane / electrode assembly 10 of the present invention includes the electrode 4 on at least one surface of the polymer electrolyte membrane 1 and can be manufactured by a known method. For example, it can be obtained by bonding the electrode 4 to the surface of the polymer electrolyte membrane 1 by hot pressing or the like. The electrode 4 is the catalyst layer 2 only, or the catalyst layer 2
And the diffusion layer 3. The catalyst layer 2 is a layer formed from a catalyst component and an electrolyte, and the catalyst component can be a fine particle such as platinum or platinum-ruthenium alloy, or a material in which these fine particles are supported on a carrier such as carbon. A sulfoalkyl cellulose, a perfluorosulfonic acid resin, or the like can be used. As the diffusion layer 3, carbon paper, carbon cloth, or the like can be used. It is also possible to manufacture the membrane electrode assembly 10 by forming the catalyst layer 2 directly on the surface of the polymer electrolyte membrane 1 by a coating method or the like.

A method for constructing a fuel cell using the polymer electrolyte membrane of the present invention is not particularly limited, and various known structures can be employed. FIG. 2 schematically shows an example of a cross-sectional structure of the fuel cell of the present invention.
The fuel cell 11 includes a part of the structure in which the membrane electrode assembly 10 is sandwiched from both sides by a pair of conductive separators 5 provided with grooves or openings for supplying a reaction gas and an aqueous methanol solution. The fuel cell 11 can also be obtained by sandwiching both surfaces of the polymer electrolyte membrane 1 with a pair of the electrodes 4 and sandwiching the pair of electrodes 4 with a pair of separators 5.
A structure in which many membrane electrode assemblies 10 are sandwiched between a pair of separators 5 may be stacked in series.

Embodiments of the present invention will be described in more detail below, but the technical scope of the present invention is not limited thereto.

(Synthesis Example 1: Synthesis of sulfoethyl cellulose)
1.6 g of cellulose and 4.0 mL of a 30% by mass sodium hydroxide aqueous solution were added to 25 mL of 2-propanol and stirred for 1 hour. Sodium 2-bromoethanesulfonate 2
. 1 g was added, and the sulfoethylation reaction was carried out at 70 ° C. with stirring for 5 hours. The reaction product was filtered, washed with a 70 mass% aqueous methanol solution, further washed with methanol, and dried to obtain sulfoethylcellulose.

Example 1
About 2% by mass by dissolving the sulfoethylcellulose prepared in Synthesis Example 1 in distilled water
An aqueous solution was obtained. This aqueous solution was mixed with bacterial cellulose by a homogenizer to obtain a dispersion solution. The mass ratio of sulfoethyl cellulose to bacterial cellulose was adjusted to 1: 1. This dispersion solution was cast on a plastic petri dish and dried at room temperature to prepare a composite membrane.
40 parts of ethylene glycol diglycidyl ether and 20 parts of 0.1N sodium hydroxide aqueous solution were mixed at a ratio of 20 parts to 40 parts of 2-propanol to prepare a crosslinking reaction solution.
The composite film was immersed in this reaction solution, and a crosslinking reaction was performed at 60 ° C. for 5 hours. This membrane is 0
. After immersing in a 5N hydrochloric acid solution for 2 hours, sodium ions were exchanged for hydrogen ions, and then sufficiently washed with distilled water to obtain a polymer electrolyte membrane.

(Example 2)
A polymer electrolyte membrane was obtained in the same manner as in Example 1 except that the mass ratio of sulfoethyl cellulose to bacterial cellulose was 3: 1.

(Example 3)
A polymer electrolyte membrane was obtained in the same manner as in Example 1 except that the mass ratio of sulfoethyl cellulose to bacterial cellulose was 1: 3.

(Comparative Example 1)
About 1.5 mass% aqueous solution was obtained by melt | dissolving the sulfoethyl cellulose produced by the synthesis example 1 in distilled water. This aqueous solution was cast on a plastic petri dish and dried at room temperature to form a film.
40 parts of ethylene glycol diglycidyl ether and 20 parts of 0.1N sodium hydroxide aqueous solution were mixed at a ratio of 20 parts to 40 parts of 2-propanol to prepare a crosslinking reaction solution.
The film was immersed in this reaction solution, and a crosslinking reaction was performed at 60 ° C. for 5 hours. This membrane is 0.5
After ion exchange of sodium ions for hydrogen ions by immersion in N hydrochloric acid solution for 2 hours,
The polymer electrolyte membrane was obtained by thoroughly washing with distilled water.

With respect to the polymer electrolyte membranes of Examples 1 to 3 and Comparative Example 1, the dimensional change rate, tensile strength and elongation at break, and proton conductivity were measured by the following methods, and the results are shown in Table 1.

(Measurement of dimensional change rate during swelling)
The size (Lw) in the surface direction of the electrolyte membrane immersed in distilled water or an aqueous methanol solution having a concentration of 5 M for 1 day and the size (Ld) in the surface direction of the electrolyte membrane vacuum-dried for 1 day were measured. Thus, the dimensional change rate was obtained.
Dimensional change rate (%) = (Lw−Ld) / Ld × 100

(Measurement of tensile strength and elongation at break)
Tensile speed of 10% for dry and water-containing electrolyte membrane cut into 5.0mm width rectangle
A tensile test was performed at mm / min to determine tensile strength and elongation at break.

(Measurement of proton conductivity)
The electrolyte membrane is sandwiched between fluororesin cells equipped with platinum-plated electrodes plated with platinum black, and S
The resistance of the solid polymer electrolyte membrane was measured by an alternating current impedance method in the frequency range of 0.1 Hz to 100 kHz at room temperature using an impedance analyzer (SI-1260) manufactured by polartron to determine proton conductivity. During the measurement, the electrolyte membrane was kept in a wet state.

Example 4
Platinum ruthenium-supported carbon for the anode and platinum-supported carbon for the cathode were mixed with a Nafion (registered trademark) solution to prepare a catalyst ink. The catalyst ink was applied onto carbon paper serving as a gas diffusion layer so that the amount of the catalyst supported was 2 mg / cm ² , respectively, to produce an electrode. A membrane / electrode assembly having an electrode area of 9 cm ² was produced by sandwiching both sides of the polymer electrolyte membrane produced in Example 1 with anode and cathode electrodes and hot pressing.
The membrane electrode assembly was sandwiched between a pair of graphite separators from both sides and incorporated into a cell having a methanol aqueous solution fuel tank and an air supply opening to produce a passive direct methanol fuel cell.
Using the above fuel cell, the fuel cell characteristics were evaluated using a methanol aqueous solution having a concentration of 5M at room temperature as a fuel. As a result, the maximum power density was 4.3 mW / cm ^{2 as} shown in FIG.

(Example 5)
A membrane / electrode assembly was produced in the same manner as in Example 4 except that the electrode area was 4 cm ² .
The membrane electrode assembly is sandwiched from both sides by a pair of graphite separators provided with a groove for supplying methanol aqueous solution or oxygen, and the outside is sandwiched by two current collector plates and two clamping plates. Thus, a direct methanol fuel cell was produced.
Using the above fuel cell, the fuel cell characteristics were evaluated at a temperature of 25 ° C., a methanol aqueous solution flow rate of 3 mL / cm ² , and an oxygen flow rate of 50 mL / cm ² . The concentration of aqueous methanol solution is 1-1.
Evaluated at 8M. As a result, as shown in FIGS. 4 and 5, power generation is possible even when a high concentration methanol aqueous solution having a concentration of 18M is used, and when a methanol aqueous solution having a concentration of 5M is used, the maximum output density is 11 mW / cm. ² .

(Comparative Example 2)
Using the polymer electrolyte membrane produced in Comparative Example 1, a membrane electrode assembly and a fuel cell were produced in the same manner as in Example 4, and the fuel cell characteristics were evaluated. As a result, almost no power was generated. When the state of taking out the membrane electrode assembly from the fuel cell was confirmed, the electrolyte membrane was damaged. It is considered that the electrolyte membrane was swollen and damaged by the methanol aqueous solution inside the fuel cell.

As described above, in the present invention, by combining with cellulose nanofibers, the electrolyte membrane does not swell or break with the aqueous methanol solution, and has excellent dimensional stability.
A polymer electrolyte membrane with improved mechanical strength and a membrane electrode assembly using the polymer electrolyte membrane can be obtained. The polymer electrolyte membrane of the present invention and the membrane electrode assembly using this polymer electrolyte membrane can be suitably used for direct methanol fuel cells.

DESCRIPTION OF SYMBOLS 1 Polymer electrolyte membrane 2 Catalyst layer 3 Diffusion layer 4 Electrode 5 Separator 10 Membrane electrode assembly 11 Fuel cell

Claims (10)

A polymer electrolyte membrane comprising a sulfoalkyl cellulose and cellulose nanofibers, wherein a crosslinked structure is imparted to the membrane by a crosslinking agent. The polymer electrolyte membrane according to claim 1, wherein the sulfoalkyl cellulose is sulfoethyl cellulose. The polymer electrolyte membrane according to claim 1 or 2, wherein the crosslinking agent is a crosslinking agent having an epoxy group. The polymer electrolyte membrane according to claim 3, wherein the crosslinking agent having an epoxy group is ethylene glycol diglycidyl ether. A membrane comprising sulfoalkyl cellulose and cellulose nanofibers, wherein the membrane is provided with a crosslinked structure by a crosslinking agent; and
A membrane / electrode assembly comprising: an electrode provided on at least one surface of the polymer electrolyte membrane. The membrane electrode assembly according to claim 5, wherein the sulfoalkyl cellulose is sulfoethyl cellulose. The membrane / electrode assembly according to claim 5 or 6, wherein the crosslinking agent is a crosslinking agent having an epoxy group. A methanol aqueous fuel tank,
A membrane electrode assembly;
A pair of conductive separators sandwiching the membrane electrode assembly from both sides and having a groove or an opening through which oxygen and a methanol aqueous solution supplied from the methanol aqueous solution fuel tank can pass, the membrane electrode assembly comprising: A polymer electrolyte membrane comprising a sulfoalkyl cellulose and cellulose nanofibers, wherein a polymer electrolyte membrane is provided with a crosslinking structure by a crosslinking agent, and an electrode provided on at least one surface of the polymer electrolyte membrane. A fuel cell comprising the fuel cell. The fuel cell according to claim 8, wherein the sulfoalkyl cellulose is sulfoethyl cellulose. The fuel cell according to claim 8 or 9, wherein the cross-linking agent is a cross-linking agent having an epoxy group.

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