TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Conditions: The water content of the membrane varies continuously across the membrane from the negative electrode side to the positive electrode side, preferably within a range of 30-200 wt.%, and is preferably at least 5 wt.% higher on the negative side. The water content is varied by regulating the ion exchange capacity of the membrane and/or its degree of crosslinking.    TECHNOLOGY FOCUS - POLYMERS - Preferred Material: The membrane may consist of a copolymer with a main chain formed by copolymerization of vinylic fluoro- or perfluoro-carbon monomers and vinylic hydrocarbon monomers and side chains of a crosslinked polymer derived from an olefinic hydrocarbon with sulfonic acid groups and a diolefin. The membrane may also be in the form of a laminate comprising at least two layers with different water contents, consisting of a fluorocarbon/hydrocarbon copolymer with sulfonated hydrocarbon side chains.This copolymer comprises main and side chains of formula (I) and (II) respectively:R1 = F, 1-3 C fluoroalkyl;R2-R5 = H or 1-3C alkyl;m, n, s = 1 or more;t = 1 or more.Preferred copolymers comprise ethylene/tetrafluoroethylene copolymer main chains with sulfonated polystyrene side chains. The degree of grafting on the main chain, and hence the water content of the membrane, may be changed by irradiation with a controlled dose of gamma rays or electrons and/or by changing the grafting temperature and time. EXAMPLE - A 50 microns ethylene/tetrafluoroethylene copolymer film was irradiated with gamma rays (10 kGy) in air at room temperature and then grafted with styrene for 2 hours at 60degreesC by contacting one surface of the film with a 100:20 (by volume) mixture of styrene and xylene and the other surface with xylene only.    The dried film was then sulfonated for 1 hour at room temperature with a 5:60 mixture of chlorosulfonic acid and 1,2-dichloroethane, hydrolyzed in 1M potassium hydroxide solution, dipped in 1M hydrochloric acid and washed for 1 hour with water at 90degreesC. The membrane obtained showed an ion exchange capacity of 1.69 (1.80) meq/g, a water content of 71 (86) wt% at 80degreesC and contact angles (with water) of 30degrees on the side treated with styrene and 72degrees on the side treated with xylene only. (Values in brackets are for 25-micron copolymer film modified by the same process.)    Gas diffusion electrodes (positive and negative) were made from commercial carbon paper by coating with a Teflon(TM) (polytetrafluoroethylene) dispersion, calcining and coating on one side with a mixture of platinum/carbon (40 wt.% Pt), Nafion(TM) solution and isopropanol to form a layer of 0.35 mg/cm2 based on Pt. The membrane was placed between the two electrodes and the combination was compressed with heating to form a fuel cell. When tested at 80degreesC with hydrogen at 2.5 atmospheres (80% utilization) and air at 2.5 atmospheres (40%), this cell showed an output voltage of 0.52 V at a current density of 1 A/cm2, compared with 0.42 V for a cell in which the membrane was made by treating both sides of the film with a 100:30 mixture of styrene and xylene.