TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compound: The initiator is a photo-initiator from benzophenone, benzoin, and/or 1-chloroanthracene. It is a thermal-initiator from benzoylperoxide, and/or 2,2'-azobisisobutyronitrile.    TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The crosslinked hydrocarbon-based ionomer is obtained by crosslinking a monomer containing ionic groups; a crosslinking agent; a monomer for controlling mechanical properties; and an initiator. The monomer-containing ionic group is a sulfonic or carboxyl group. The initiator is a photopolymerization or a thermal polymerization initiator. Preferred Composition: The polymer electrolyte composition comprises (wt.%): crosslinked hydrocarbon-based ionomer (0.01-99.99); perfluorinated ionomer (0.01-99.99). The crosslinked hydrocarbon-based ionomer comprises (wt.%): monomer-containing ionic group (0.1-80); and crosslinking agent (0.1-50). Preferred Compound: The monomer-containing ionic group is acrylamidomethylpropane-sulfonic acid, styrenesulfonic acid, methacryloxyethane-sulfonic acid, methylpropanesulfonic acid, and/or hydroxypropane-sulfonic acid. It is methylmethacrylic acid, ethylacrylic acid, acrylic acid, and/or their derivatives. The crosslinking agent is hexanediolethoxylate diacrylate, hexanediolpropoxylate diacrylate, dimethylacrylate, polyethyleneglycol dimethacrylate, polyethyleneglycol diacrylate, trimethylolpropane, and/or trimethacrylate. The monomer for controlling mechanical properties is vinyl-based monomers, acrylate-based monomers, and/or methacrylate-based monomers. It is ethylhexylacrylate, ethylhexylmethacrylate, ethyl-methacrylate, n-butylacrylamide, vinylacetate, and/or alpha-olefin-based monomers. The perfluorinated ionomer is a perfluorosulfonic acid ionomer. EXAMPLE - Nafion 112 membrane was pretreated in hydrogen peroxide for 2 hours, in 1 M sulfuric acid for 2 hours, and in water for 2 hours. The pretreatment was carried out at 80degreesC. The pretreated membrane was impregnated in a solution containing 2-acrylamido-2-methyl-1-propanesulfonic acid (0.6 g), 1,6-hexanediol ethoxylate diacrylate (0.2 g), and 2-ethylhexyl acrylate (0.4 g) dissolved in dimethylformamide (40 ml), and benzophenone (0.002 g) was added. 2-Ethylhexyl acrylate was added. The mixture was subjected to a photocrosslinking reaction at room temperature for 10 minutes, to fabricate a membrane. The experimental results demonstrated that methanol crossover was suppressed, while the proton conductivity was maintained at a level similar to that of Nafion membrane.