TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Electrolyte: The polymer electrolyte further comprises an acid (3 - 12 mol.% based on the sulfonated polyether ketone ketone). Preferred Components: In the fuel cell, at least one of the cathode and the anode comprises (I) and the cross-linking agent.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The acid is at least one of phosphoric acid and a solid inorganic proton conductor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The polymerization initiator is at least one of benzoin ethyl ether, diethoxyacetophenone, benzyl dimethyl ketal and 2,2'-azobisisobutylnitrile.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: (I) Is prepared through sulfonication of polyether ketone ketone. The degree of sulfonication of (I) is 30 - 40 wt.%; and the weight average molecular weight of the sulfonated polyether ketone ketone is 60000 - 200000. The cross-linking agent comprises at least one of polyethyleneglycol diacrylate, polyethyleneglycol dimethacrylate, polyethyleneglycol dimethyl ether, and polyethyleneglycol methacrylate. Preferred Method: The method further involves adding the acid to the solution. The method involves: dissolving (I) in the solvent (preferably dichloroacetic acid) (400 - 1000 parts by weight based on 100 parts by weight of (I)); adding the cross-linking agent (80 - 250 parts by weight based on 100 parts by weight of (I)) and the polymerization initiator (1 - 30 parts by weight based on 100 parts by weight of (I)) to the solution; and curing the resulting mixture using at least one of heat, light, and electron beam irradiation (preferably by heating at 40 - 90degreesC and/or ultraviolet rays). EXAMPLE - A polyether ketone ketone (PEKK) (50 g) was added to 950 g of a mixture of 57:43 (vol./vol.) sulfuric acid and fuming sulfuric acid (30% free SO3); and reacted at room temperature for 72 hours. Then the reaction mixture was added dropwise into excessive volumes of rapidly stirred deionized ice water to form a precipitate. The precipitate was filtered and washed with deionized water to remove excessive sulfuric acid. The resultant residue was dried at 110degreesC for 3 days to obtain sulfonated polyether ketone ketone of formula (I) (SPEKK) (test) (having weight average molecular weight of 109000, n = 265, and degree of sulfonication of 32.6%). The SPEKK (degree of sulfonication=32.6%) (test) (10 g) was dissolved in dichloroacetic acid (90 g). Polyethyleneglycol dimethacrylate (PEGDMA) (1 g) was added dropwise to the SPEKK solution (10 g) and stirred. Benzoin ethyl ether (0.05 g) and phosphoric acid (2 g) were sequentially added to the mixture. The mixture was then stirred for 2 hours to obtain a polymer electrolyte forming composition. The polymer electrolyte forming composition was cast on a glass plate and a film was formed. The resulting film was dried at 60degreesC for 5 hours; and then cured by exposing it to UV rays for 20 minutes to obtain a polymer electrolyte (P1). A fuel cell was fabricated using the polymer electrolyte membrane and an electrode of EFCG-S type (containing 10 wt % of Pt impregnated on carbon paper, at loading amount of 0.6 mg/cm2). In comparative polymer electrolyte, a solution (5 g) of polybenzoimidazole (PBI) in dimethylacetimide was cast on a glass plate, dried at room temperature for 30 minutes, and then dried at 120degreesC for 1 hour. The glass plate was immersed in water, and the PBI film was removed and then dried in a vacuum oven at 60degreesC for 8 hours. The PBI film (P2) was impregnated in 85% phosphoric acid in a drying oven at 60degreesC for 2 hours, and its ionic conductivity was measured. At room temperature, (P1) was found to have a higher ionic conductivity than that of (P2). The ionic conductivity of (P1) was found to decrease at less rate at 130degreesC, whereas that of (P2) decreased significantly at 110degreesC.