the fluorine - containing solid polymer ion exchange membrane for use in the ion exchange layer can be selected from among many perfluoro ion exchange membranes known in the art and commercially available . for example , perfluoro ion exchange membranes sold under the trademark nafion ® manufactured by the dupont co . can be used . selection of the perfluoro ion exchange membrane is made according to its desired function in an electrochemical cell . polytetrafluoroethylene is the preferred material for the porous film of the seal layer . polytetrafluoroethylene is well known as a chemically inert polymer which will not react with ion exchange materials , electrode or catalyst materials , liquid and solid electrolytes , and the like , encountered in electrochemical apparatus environments , nor will it contribute contaminants which can adversely affect such materials . polytetrafluoroethylene has a very broad service temperature range , from cryogenic temperatures to over 250 ° c ., and , particularly in its preferred porous form , good strength , compressibility , and sealing characteristics . it is , furthermore , hydrophobic and , ever in porous form , resists wetting by water or aqueous solutions . porous polytetrafluoroethylene sheet or film suitable for use in the seal layer of the assembly can be made by processes known in the art , for example , by stretching or drawing processes , by papermaking processes , by processes in which filler materials are incorporated with the ptfe resin and which are subsequently removed to leave a porous structure , or by powder sintering processes . preferably the porous polytetrafluoroethylene film is porous expanded polytetrafluoroethylene film having a structure of interconnected nodes and fibrils , as described in u . s . pat . nos . 3 , 953 , 566 and 4 , 187 , 390 . the porous polytetrafluoroethylene film should be at least 5 micrometers thick , preferably in the range of 10 to 500 micrometers thick . the film should have a pore volume in the range of 20 to 95 percent , preferably in the range 50 to 85 percent ; and a nominal pore size in the range 0 . 03 to 3 micrometers , preferably in the range 0 . 1 to 1 micrometer . due to the well known release characteristics of polytetrafluoroethylene , if the pore volume of the porous polytetrafluoroethylene film is less than about 20 percent , or if the nominal pore size is less than 0 . 03 micrometers , it will be difficult to coat the film and penetrate the pores sufficiently with a solid polymer electrolyte so that the solid polymer electrolyte forms interlocking bonds within the porous structure which are required for securely bonding the film to the solid polymer electrolyte membrane of the ion exchange layer . consequently , adhesion between the seal layer and ion exchange layer will be poor , and separation of the layers may occur in handling or in service . on the other hand , if the pore volume exceeds about 95 percent , or if the nominal pore size exceeds about 3 micrometers , the solid polymer electrolyte may penetrate the film excessively , fully occupying the pore volume , and permeating to the opposite side of the porous polytetrafluoroethylene film . this is undesirable in that one surface of the film must be free of solid polymer electrolyte to preserve desired surface and electrical properties , and sufficient pore volume devoid of solid polymer electrolyte must be retained in the film to ensure its sealing function . the solid polymer electrolyte applied to the porous polytetrafluoroethylene film of the seal layer is preferably the same material as forms the solid polymer electrolyte membrane of the ion exchange layer . the solid polymer electrolyte should be applied to the film surface so that it is less than 100 micrometers thick , preferably 10 or less micrometers thick . it may be possible to apply the solid polymer electrolyte to the porous film solely by hot pressing . however , if only hot pressing is used , very high pressure and heat are required to obtain good joining strength ; and , as this can cause the polar groups of the solid polymer electrolyte ( such as sulfonic acid groups or carboxylic acid groups ) to decompose or undergo other undesirable modification , it is not the method of choice . a preferred method is to apply the solid polymer electrolyte in solution form , for example , dissolved in an alcohol , to one side of the porous polytetrafluoroethylene film . care must be taken to avoid excessive penetration of the solution into the porous film which can result in the solid polymer electrolyte being carried through the porous film to the opposite side . the amount of solution penetrated into the film and the depth of penetration can be controlled by adjusting the viscosity of the solution , for example , by raising or lowering the concentration of solid polymer electrolyte in the solution . alternatively , penetration of the solution can be controlled by including water or other surface tension raising solvent in the solution . the solution can be applied by conventional coating methods , such as roll - coating , spraying , or the like , so as to form a continuous non - porous film of solid polymer electrolyte on one surface of the porous polytetrafluoroethylene film . it can also be applied to the surface of the porous film in discrete separated spots or lines ( including crisscrossing lines ) by conventional means such as gravure printing , screen printing , or the like . in either case , a sufficient amount of solution must penetrate into the film to a sufficient depth so that the solid polymer electrolyte , after drying to remove the solvent , forms an interlocking bond with the porous structure of the film . also , in either case , the amount of solid polymer electrolyte on the surface in the form of a continuous non - porous film , or discrete spots or lines , should be 100 micrometers thick or less , preferably 10 micrometers thick or less . as noted above , none of the solid polymer electrolyte should penetrate through the film to the surface opposite the coated side to ensure that the uncoated surface characteristics remain those of polytetrafluoroethylene only , and sufficient porosity should remain to ensure good sealability . the material thus formed for the sealing layer is cut to the desired size and shape , placed in the seal areas on one or both sides of the solid polymer electrolyte membrane of the ion exchange layer , with the solid polymer electrolyte surfaces facing each other , and securely bonded to the solid polymer electrolyte membrane by hot pressing at 130 ° c . and about 3 to 50 kg / cm 2 pressure to form the assembly of the invention . the unitary assembly thus formed can be used in an electrochemical apparatus as is , or can be further processed before assembly and use in an electrochemical apparatus . an assembly consisting of a sealing layer and a solid polymer ion exchange membrane was made as follows : a porous expanded polytetrafluoroethylene film ( gore - tex ® expanded ptfe , manufactured by japan gore - tex , inc .) was used in the sealing layer . the porous ptfe film was about 50 micrometers thick and had a pore volume of about 75 %, a nominal pore size of 0 . 3 micrometers , and a tensile strength of about 0 . 7 kg / cm . the porous ptfe film was coated on one side with a solution having a 5 % concentration of a fluorine - containing ion exchange polymer ( obtained by hydrolyzing a copolymer of c 2 f 4 and cf 2 ═ cfo ( cf 2 ) n so 2 ) in a 1 : 1 mixture of alcohol and water , after which the coated film was heated and dried to remove the solvent . the dried coating , in the amount of about 0 . 3 mg / cm 2 , formed on the porous polytetrafluoroethylene film completed the sealing layer material . the sealing layer material was placed on a fluorine - containing ion exchange membrane ( nafion ® 117 perfluoro ion exchange membrane , manufactured by the dupont co .) so as to cover the full surface of the membrane . the sealing layer material was arranged so that the coated surface was in contact with the ion exchange membrane , and the two layers were securely bonded by hot pressing at a temperature of about 130 ° c . and 3 kg / cm 2 pressure to complete the assembly . the assembly was tested by immersing it in water . the assembly underwent severe curling , with the sealing layer on the inside . when the assembly was forcibly straightened out , the sealing layer did not peel or separate from the ion exchange membrane , indicating that a strong bond between the layers had been obtained . furthermore , the uncoated side of the sealing layer material resisted wetting by water , forming drops on the surface , indicating that the ion exchange polymer coated on the surface of the porous polytetrafluoroethylene film had not penetrated through the polytetrafluoroethylene film . a comparative assembly was prepared as described in example 1 , except that a solid non - porous polytetrafluoroethylene film , 50 micrometers thick , was substituted for the porous polytetrafluoroethylene film of the sealing layer material . the comparative assembly was tested by immersing it in water . the assembly underwent severe curling , with the sealing layer on the inside . when the assembly was forcibly straightened out , the sealing layer peeled and separated from the surface of the ion exchange membrane . an assembly consisting of sealing layers adhered to both sides of a composite ion exchange layer was made as follows : two washer - shaped rings , 60 mm inside diameter and 120 mm outside diameter , were cut from the sealing layer material of example 1 . a flat disc , 120 mm in diameter , of a fluorine - containing ion exchange membrane ( nafion ® 117 perfluoro ion exchange membrane , manufactured by the dupont co .) was placed between the rings . the rings of sealing layer material were arranged so that the coated surfaces were in contact with the ion exchange membrane , and securely bonded to the ion exchange membrane by hot pressing at a temperature of about 130 ° c . and 3 kg / cm pressure to form an assembly . the assembly was subjected to chemical plating according to the method disclosed in japanese laid - open patent application no . 57 - 134586 . in accordance with the above method , the assembly was treated with hydrochloric acid and completely converted into an h + type , after which a platinum amine complex was adsorbed on to the surface . the platinum amine complex was reduced with sodium borohydride to precipitate the platinum . when additional platinum was grown , using the precipitated platinum as nuclei , the surface of the ion exchange membrane not covered by the sealing layer was uniformly plated with platinum to complete formation of the composite ion exchange layer . the uncoated surfaces of the sealing layers were not plated at all . the assembly thus obtained was used as the solid polymer electrolyte in a hydroelectrolysis apparatus . when hydroelectrolysis was conducted , with pure water fed to both electrodes , the apparatus operated stably at a current density of 40 amps / dm 2 and a voltage of 2 volts . this example demonstrates that the sealing layers of the assembly of the invention provided support during formation of the composite ion exchange layer ; functioned well as a mask to prevent deposition of platinum on covered areas of the ion exchange membrane ; were unaffected by the harsh chemical environments associated with the plating and hydroelectrolysis steps , preserving the electrical insulating properties of the uncoated surfaces ; reinforced the ion exchange layer against the compressive force needed for sealing , and provided an effective gas and liquid seal .