TECHNICAL FIELD

The present invention relates to a method for producing a liquid composition, and methods for producing a polymer electrolyte membrane, a catalyst layer and a membrane/electrode assembly by using the liquid composition.

BACKGROUND ART

A polymer electrolyte fuel cells is, for example, a stack of a plurality of cells, each comprising a membrane/electrode assembly sandwiched between two separators. The membrane/electrode assembly comprises an anode and a cathode each having a catalyst layer, and a polymer electrolyte membrane disposed between the anode and the cathode. The polymer electrolyte membrane and the catalyst layers contain a fluoropolymer having sulfonic acid groups, as an ion exchange resin.

As oxygen is reduced at the cathode of a polymer electrolyte fuel cell via hydrogen peroxide, hydrogen peroxide or peroxide radicals form at the cathode. Furthermore, as oxygen molecules can pass through the polymer electrolyte membrane from the cathode to the anode, hydrogen peroxide or peroxide radicals can form at the anode as well. Hydrogen peroxide or peroxide radicals formed at the cathode or the anode are known to deteriorate the fluoropolymer having sulfonic acid groups contained in the polymer electrolyte membrane and the catalyst layers.

A proposal to use the following liquid composition was made for formation of polymer electrolyte membranes and catalyst layers resistant to hydrogen peroxide or peroxide radicals.

A liquid composition comprising water, a fluoropolymer having sulfonic acid groups and trivalent or tetravalent cerium ions (Patent Document 1).

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: WO 2005/124912

DISCLOSURE OF INVENTION

Technical Problem

However, the liquid composition contains tetravalent cerium ions as cerium (IV) hydroxide (Ce(OH)4). Because cerium (IV) hydroxide has a low solubility in water, a liquid composition containing cerium (IV) hydroxide is whitish and clouded with cerium (IV) hydroxide particles. A liquid composition containing cerium (IV) hydroxide particles form polymer electrolyte membranes and catalyst layers with poor resistance.

Trivalent cerium ions could be used to avoid a liquid composition clouded with cerium (IV) hydroxide particles. However, even in the case of cerium (III) carbonate hydrate, for example, it partly contains tetravalent cerium resulting from oxidization by atmospheric oxygen. Therefore, after addition of cerium (III) carbonate hydrate to a solution containing water and a fluoropolymer having sulfonic acid groups, at least 100 days of stirring is needed to completely dissolve cerium (IV) hydroxide. Patent Document 1 discloses in paragraph [0055] that addition of 1 g of cerium (III) carbonate hydrate to a solution containing ethanol (42 mass %), water (28 mass %) and a fluoropolymer having sulfonic acid groups (30 mass %) followed by 8 hours of stirring gave a clear liquid composition. However, in reality, the liquid composition was almost clear but contained a trace of undissolved cerium (IV) hydroxide.

A recent study by the present inventors revealed that when a solution containing water and a fluoropolymer having sulfonic acid groups was shielded from light as in a mixing tank on an actual production line, even a month of stirring was not enough to dissolve cerium (IV) hydroxide after addition of cerium (III) carbonate hydrate to the solution.

The present invention provides a method for producing a liquid composition which is capable of eliminating clouding of a liquid with cerium (IV) hydroxide particles in a relatively short time, and methods for producing a polymer electrolyte membrane, a catalyst layer and a membrane/electrode assembly, each having excellent durability, in a relatively short time.

Solution to Problem

The present invention provides the following embodiments.

[1] A method for producing a liquid composition containing a fluoropolymer having sulfonic acid groups, trivalent cerium ions and water, which comprises irradiating a solution containing at least one cerium compound selected from the group consisting of cerium carbonate, cerium hydroxide and cerium oxide, the fluoropolymer and the water, with light at least partially in a wavelength region from 300 to 400 nm so that the ultraviolet irradiance on the surface of the solution is at least 0.1 mW/cm2.
[2] The method for producing a liquid composition according to [1], wherein the light comes from a light source having a dominant wavelength within a range of from 300 to 400 nm.
[3] A method for producing a liquid composition containing a fluoropolymer having sulfonic acid groups, trivalent cerium ions and water, which comprises adding a reducing agent to a solution containing at least one cerium compound selected from the group consisting of cerium carbonate, cerium hydroxide and cerium oxide, the fluoropolymer and the water.
[4] The method for producing a liquid composition according to [3], wherein the reducing agent is at least one member selected from the group consisting of hydrogen peroxide, acetaldehyde, formic acid and oxalic acid.
[5] The method for producing a liquid composition according to [3] or [4], wherein the reducing agent is added in amount of from 0.47 to 90 mol % relative to elemental cerium (100 mol %) in the cerium compound.
[6] The method for producing a liquid composition according to any one of [1] to [5], wherein the content of the fluoropolymer having sulfonic acid groups in the liquid composition (100 mass %) is from 1 to 50 mass %.
[7] The method for producing a liquid composition according to any one of [1] to [6], wherein the content of the water in the liquid composition (100 mass %) is from 20 to 60 mass %.
[8] The method for producing a liquid composition according to any one of [1] to [7], wherein the content of trivalent cerium ions is from 1.6 to 23.3 equivalents relative to the sulfonic acid groups in the fluoropolymer (100 equivalents).
[9] The method for producing a liquid composition according to any one of [1] to [8], wherein the liquid composition further contains an organic solvent having a hydroxyl group.
[10] The method for producing a liquid composition according to any one of [1] to [9], wherein the proportion of the organic solvent having a hydroxyl group to the sum of the water and the organic solvent having a hydroxyl group (100 mass %) is from 1 to 90 mass %.
[11] A method for producing a polymer electrolyte membrane for a polymer electrolyte fuel cell, which comprises obtaining a liquid composition by the method as defined in any one of [1] to [10], and then forming a polymer electrolyte membrane for a polymer electrolyte fuel cell by using the liquid composition.
[12] A method for producing a catalyst layer for at least one of the cathode and the anode of a polymer electrolyte fuel cell, which comprises obtaining a liquid composition by the method as defined in any one of [1] to [10], mixing the liquid composition and a catalyst to obtain a catalyst layer forming coating solution and forming the catalyst layer by using the liquid composition.
[13] A method for producing a membrane/electrode assembly comprising an anode having a catalyst layer, a cathode having a catalyst layer and a polymer electrolyte membrane disposed between the anode and the cathode, which comprises at least one of the following steps (α) and (β):

step (α): producing a liquid composition by the method as defined in any one of [1] to [10] and producing the polymer electrolyte membrane by using the liquid composition;

step (β): producing a liquid composition by the method as defined in any one of [1] to [10], preparing a catalyst layer forming coating solution by mixing the liquid composition and a catalyst, and producing the catalyst layer of at least one of the cathode and the anode by using the catalyst layer forming coating solution.

Advantageous Effects of Invention

The method for producing a liquid composition of the present invention is capable of eliminating clouding of a liquid with cerium (IV) hydroxide particles in a relatively short time.

The methods for producing a polymer electrolyte membrane, a catalyst layer and a membrane/electrode assembly of the present invention can produce electrolyte membranes, catalyst layers and membranes and membrane/electrode assemblies, each having excellent durability, in a relatively short time.

