DETAILED DESCRIPTION OF THE INVENTION

[0019] FIG. 1 illustrates a cross-sectional view of an ion-exchange membrane 10 at various stages of being coated with a catalyst composition. The thickness of membrane 10 is commonly 25 to 175 microns, and typically 25 to 125 microns. A representative commercial sulfonated perfluorocarbon membrane is sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®.

[0020] In a first step A, a surface 12 of membrane 10 is softened by heating. For example, softening of surface 12 may be achieved by heating to a temperature between 20° C. and 50° C. above the glass transition temperature (Tg) of the ion-exchange membrane. In one embodiment, surface 12 is heated to a temperature between 30° C. and 40° C. above Tg. For example, if a NAFION® membrane is used, surface 12 may be heated to a temperature between 130° C. and 150° C. (Tg for NAFION® is around 100° C.). By softening surface 12 , better contact and improved adhesion is observed when membrane 10 is subsequently coated with catalyst layer 14 as in step B.

[0021] The electrocatalyst of catalyst layer 14 may be a metal black, an alloy or a supported metal-based catalyst, for example, platinum supported on carbon particles may be used. Other catalysts include other noble or transition metal catalysts. The catalyst layer may also include organic binder such as PTFE, polymer electrolyte powder and fillers. Ionically conductive ionomer particles, if present, improve fuel cell performance by providing ion-conducting paths between the electrocatalyst surface and the ion-exchange membrane.

[0022] Once catalyst layer 14 is deposited onto heated surface 12 of membrane 10 in step B, the final step C may be performed by compacting catalyst layer 14 into membrane 10 to yield catalyst-coated membrane (CCM) 20 . As shown in FIG. 2 , the method as described above in reference to FIG. 1 may readily be adapted to the continuous production of CCM 20 .

[0023] Membrane 10 starts in roll 30 and is then fed past infrared lamps 32 which are used to soften membrane by heating as in step A of FIG. 1 . Infrared lamps 32 are particularly well suited for continuous processing though other methods of heating may be used.

[0024] A fluidized bed reactor 34 is used in the embodiment illustrated in FIG. 2 for the deposition of catalyst layer 14 on membrane 10 as in step B of FIG. 1 . In general, a fluidized bed is produced by passing a stream of gas upward through a bed of particles at sufficient velocity to suspend the particles. In this state, the mixture of particles behaves like a liquid having density equal to the bulk density of the particles. In this manner, the particles are “fluidized”.

[0025] A polymer electrolyte, if desired, may be added to fluidized bed reactor 34 as a powder or, after the dry components have been fluidized, as a fine aerosol of ionomer droplets. Once the system is homogeneous, a second stream of gas 36 directs the catalyst composition to the heated surface 12 (not shown in FIG. 2 ) of membrane 10 . Gas 36 may be, for example, compressed air or an inert gas such as nitrogen.

[0026] Compaction rolls 44 compact the catalyst/membrane system to provide the CCM 20 as in step C in FIG. 1 . Compaction rolls 44 are particularly well suited for continuous processing, although other methods of compaction may also be used. For example, pressure may be carried out by manual presses, flat plate presses, a roller or rollers pressing against a flat plate backup member or a roller or any other suitable means of applying pressure, manually or automatically. Compaction rollers 44 , or any other suitable compaction device, would typically have a release surface, such as a coating of PTFE, fluorocarbon or other suitable release material thereon.

[0027] Adhesion of the catalyst layer to membrane 10 may be further improved by, for example, applying a slight vacuum 38 opposite fluidized bed reactor 34 . A “slight” vacuum may be between 10 and 50 mbar and may result from, for example, a draft fan effect or by charging fluidized bed reactor 34 and ion exchange membrane 10 with opposite charge. Also, an aerosol coater 40 may be used to coat the catalyst layer with an aerosol spray comprising ionomer droplets. Such an aerosol spray may be beneficial regardless of whether polymer electrolyte has been added to fluidized bed reactor 34 . Further improvement in catalyst adhesion may be observed by placing additional infrared lamps 42 to soften the catalyst/membrane system immediately prior to compaction. If necessary, infrared lamps 42 may also be used to dry the catalyst layer. For example, in one embodiment, infrared lamps 42 heat the membrane/catalyst system to a temperature between 60° C. and 80° C.

[0028] The catalyst loading on CCM 20 will vary depending on several factors, including concentration of catalyst in fluidized bed reactor 34 , the linear speed of membrane 10 , and/or the pressure of gases that deposit catalyst particles onto membrane 10 . In addition to catalyst loading, different catalyst layer structures can be designed by varying the particle size of the catalyst components, the type and amount of any binder used and/or the temperature of surface 12 .

[0029] The catalyst loading can be monitored in a number of ways. For example, X-ray fluorescence (XRF) offers elemental analysis of a wide variety of materials in a highly precise and generally non-destructive way. XRF spectrometers operate by irradiating a sample with a beam of high energy X-rays and exciting characteristic X-rays from those elements present in the sample. The individual X-ray wavelengths are sorted via a system of crystals and detectors, and specific intensities are accumulated for each element. Chemical concentrations of individual elements can then be established by reference to stored calibration data. Alternatively, catalyst loadings can be determined from the concentration of catalyst in the fluidized bed and by measuring the thickness of the deposited catalyst layer.

[0030] After the first side of membrane 10 has been coated with a catalyst layer, the same process can be repeated for the other side. Alternatively, both sides of membrane 10 may be coated simultaneously or steps A and B may be performed on one side, then the other prior to compaction of both sides of membrane 10 simultaneously or any other combination or permutation. Electrodes may then be hot-bonded or laminated on the coated membrane to produce a continuous MEA that can be cut to the desired shape and size for use in electrochemical cells. Alternatively, the electrodes may be bonded simultaneous with step C.

[0031] While particular steps, elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by persons skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications as incorporate those steps or elements that come within the spirit and scope of the invention. Lastly, all of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.