Composite ionomeric membrane

A composite ionomeric membrane comprising a layer or film of a porous inert material on which a sulphonic fluorinated ionomer is deposited, the sulphonic fluorinated ionomer comprising

• side chains of formula:
• —O—CF2—CF2—SO3—M+, wherein M is hydrogen or an alkaline metal,
• wherein the sulphonic fluorinated ionomer has an equivalent weight from 380 to 1,700 g/eq,
• wherein said composite ionomeric membrane, after drying at 105° C. under vacuum for one hour and subsequent dipping in demineralized water at 100° C. for 30 minutes, has size variations lower than 25% in both the orthogonal directions of the xy plane, and
• wherein the composite ionomeric membrane is prepared by a process comprising the following steps:
• 1) preparing a liquid dispersion comprising a fluorinated ionomer in acid or salified form;
• 2) depositing the dispersion on a surface of a porous inert material and removing excess dispersion to obtain a film or layer of impregnated porous inert material;
• 3) applying a non-porous support that is inert under the process conditions to one side of the film or layer of impregnated porous inert material;
• 4) annealing the film or layer of impregnated porous inert material and applied to the non-porous support at temperatures from 130° to 280° C.;
• and
• 5) detaching the film or layer of impregnated porous inert material from the non-porous support.