A method is provided to fabricate an electrolyte membrane. The membrane has an asymmetric structure and is a polybenzimidazole membrane doped with phosphoric acid. The asymmetric structure comprises a dense layer and a porous layer. The content of phosphoric acid introduced into the polybenzimidazole membrane reaches 20 phosphoric acid molecules per polymer repeating unit. The proton conductivity of the polybenzimidazole membrane reaches 510.sup.2 siemens per centimeter (S/cm). An electrode made with the polybenzimidazole membrane can be smoothly operated in a proton exchange membrane fuel cell.

Provided is a cation exchange membrane having excellent mechanical strength against folding and the like and capable of delivering stable electrolytic performance for a long time, an electrolysis vessel using the cation exchange membrane and a method for producing the cation exchange membrane. A cation exchange membrane 1 at least includes: a membrane body containing a fluorine-based polymer having an ion-exchange group; and two or more reinforcing core materials arranged approximately in parallel within the membrane body. The membrane body is provided with two or more elution holes 12 formed between the reinforcing core materials 10 adjacent to each other. In addition, assuming that a distance between the reinforcing core materials 10 adjacent to each other is represented by a, a distance between the reinforcing core materials 10 and the elution holes 12 adjacent to each other is represented by b, a distance between the elution holes 12 adjacent to each other is represented by c, and the number of the elution holes 12 formed between the reinforcing core materials 10 adjacent to each other is represented by n, then a, b, c, and n satisfying the relationship represented by the following expression (1) or expression (2) are at least present.
b>a/(n+1)(1)
c>a/(n+1)(2)

The present invention aims to provide a biaxially stretched porous membrane having high strength, a small pore size, and excellent homogeneity. The biaxially stretched porous membrane of the present invention includes polytetrafluoroethylene obtained by copolymerizing tetrafluoroethylene and perfluoro(methyl vinyl ether).

One aspect of the present invention provides a polymer electrolyte membrane including: a porous substrate including a first part including a first polyolefin modified with a halogen-based compound, and a second part including a second polyolefin; and a halogen-based polymer electrolyte impregnated in pores of the porous substrate, wherein in the porous substrate, the first part and the second part constitute a discontinuous phase and a continuous phase, respectively, and a content of the halogen-based compound in the porous substrate is 0.5 to 10 wt %, and a method of manufacturing the same.