The present invention provides a catalyst layer, a catalyst layer ink and a membrane-electrode assembly which enable provision of fuel cells with high power efficiency. The catalyst layer of the present invention comprises a carbon alloy catalyst and an ion exchange polymer which comprises at least one species of units having a cyclic ether structure selected from the group consisting of units represented by the formula (u11), units represented by the formula (u12), units represented by the formula (u21), units represented by the formula (u22) and units represented by the formula (u24).

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Provided are an ion exchange membrane that has excellent mechanical strength as well as can exhibit an excellent proton conductivity over a long period, a membrane electrode assembly, a fuel cell, a redox flow secondary battery, a water electrolyzer, and an electrolyzer for organic hydride synthesis.

An ion exchange membrane containing:

an electrolyte containing a perfluorocarbon sulfonic acid polymer; and

glass fiber having a SiO.sub.2 content of 99.9% by mass or more.

Provided are an ion exchange membrane that has excellent mechanical strength as well as can exhibit an excellent proton conductivity over a long period, a membrane electrode assembly, a fuel cell, a redox flow secondary battery, a water electrolyzer, and an electrolyzer for organic hydride synthesis.

An ion exchange membrane containing:

an electrolyte containing a perfluorocarbon sulfonic acid polymer; and

glass fiber having a SiO.sub.2 content of 99.9% by mass or more.

A method for producing a liquid composition containing a fluoropolymer having sulfonic acid groups, trivalent cerium ions and water, by (1) irradiating a solution containing at least one cerium compound selected from 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/cm.sup.2 or (2) adding a reducing agent to a solution containing at least one cerium compound selected from cerium carbonate, cerium hydroxide and cerium oxide, the fluoropolymer and the water.

A method for producing a liquid composition containing a fluoropolymer having sulfonic acid groups, trivalent cerium ions and water, by (1) irradiating a solution containing at least one cerium compound selected from 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/cm.sup.2 or (2) adding a reducing agent to a solution containing at least one cerium compound selected from cerium carbonate, cerium hydroxide and cerium oxide, the fluoropolymer and the water.

An electrolyte membrane of a membrane-electrode assembly is formed by a manufacturing method yielding a membrane with improved chemical durability. The manufacturing method includes preparing an antioxidant solution, mixing the antioxidant solution and a first ionomer dispersion solution, drying the mixture to produce a composite having an antioxidant and a first ionomer surrounding the antioxidant, introducing and mixing the composite with a second ionomer dispersion solution, and applying that mixture to a substrate and drying the mixture to manufacture an electrolyte membrane. The resulting electrolyte membrane includes the composite having an antioxidant in an ionic state and a first ionomer surrounding the antioxidant.

To provide a liquid composition with which a catalyst layer and a polymer electrolyte membrane will hardly be broken at the time of their formation and a method for producing the liquid composition; and a method for producing a membrane/electrode assembly by which a catalyst layer and a polymer electrolyte membrane will hardly be broken at the time of their formation. A liquid composition comprising a polymer having ion exchange groups, water and an organic solvent, wherein the average secondary particle size of the polymer having ion exchange groups is from 100 to 3,000 nm, and the primary particle size parameter represented by the product of the average primary particle size (nm) and the ion exchange capacity (meq/g dry resin) of the polymer having ion exchange groups, is from 12 to 20.

To provide a liquid composition with which a catalyst layer and a polymer electrolyte membrane will hardly be broken at the time of their formation and a method for producing the liquid composition; and a method for producing a membrane/electrode assembly by which a catalyst layer and a polymer electrolyte membrane will hardly be broken at the time of their formation. A liquid composition comprising a polymer having ion exchange groups, water and an organic solvent, wherein the average secondary particle size of the polymer having ion exchange groups is from 100 to 3,000 nm, and the primary particle size parameter represented by the product of the average primary particle size (nm) and the ion exchange capacity (meq/g dry resin) of the polymer having ion exchange groups, is from 12 to 20.

A composite includes a fluorinated polymer and nanoparticles of a metal salt. The metal salt has a solubility product of not more than 1×10.sup.−4. The fluorinated polymer includes a fluorinated polymer backbone chain and a plurality of groups represented by formula —SO.sub.2X, in which each X is independently —NZH, —NZSO.sub.2(CF.sub.2).sub.1-6SO.sub.2X′, —NZ[SO.sub.2(CF.sub.2).sub.dSO.sub.2NZ].sub.1-10SO.sub.2(CF.sub.2).sub.dSO.sub.2X′ or —OZ, and Z is independently a hydrogen, an alkali-metal cation, or a quaternary ammonium cation, X′ is independently —NZH or —OZ, and each d is independently 1 to 6. A polymer electrolyte membrane, an electrode, and a membrane electrode assembly including the composite are also provided.

A composite includes a fluorinated polymer and nanoparticles of a metal salt. The metal salt has a solubility product of not more than 1×10.sup.−4. The fluorinated polymer includes a fluorinated polymer backbone chain and a plurality of groups represented by formula —SO.sub.2X, in which each X is independently —NZH, —NZSO.sub.2(CF.sub.2).sub.1-6SO.sub.2X′, —NZ[SO.sub.2(CF.sub.2).sub.dSO.sub.2NZ].sub.1-10SO.sub.2(CF.sub.2).sub.dSO.sub.2X′ or —OZ, and Z is independently a hydrogen, an alkali-metal cation, or a quaternary ammonium cation, X′ is independently —NZH or —OZ, and each d is independently 1 to 6. A polymer electrolyte membrane, an electrode, and a membrane electrode assembly including the composite are also provided.