Abstract:
A method for fabricating a fuel cell includes fixedly attaching a reinforcement to a proton-exchange membrane and to an electrode placed against a first face of the proton-exchange membrane. The reinforcement has a median aperture through which an interior portion of the electrode is exposed. Fixedly attaching the reinforcement includes superimposing an inner edge of the reinforcement over a periphery of the electrode, and causing a projecting portion of the reinforcement to project the proton-exchange membrane so as to limit gas permeation into the proton-exchange membrane, and forming filigrees by a wet process in a gas diffusion layer, thereby forming a recess therein, and placing the gas diffusion layer so that the inner edge of the reinforcement extends into the recess in the gas diffusion layer.

Description:
RELATED APPLICATIONS 
       [0001]    Under 35 USC 119, this application claims the benefit of the priority date of French application FR 1258197 filed on Sep. 3, 2012, the content of which is herein incorporated by reference. 
       FIELD OF THE DISCLOSURE 
       [0002]    The invention pertains to proton-exchange membrane fuel cells, and in particular, to methods for fabricating fuel cells. 
       BACKGROUND 
       [0003]    Fuel cells are envisaged as an electric power supply system for future mass-produced motor vehicles as well as for a large number of applications. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. Hydrogen (H2) or molecular hydrogen is used as a fuel for the fuel cell. The hydrogen gas is oxidized and ionized on an electrode of the cell and oxygen (O2) or molecular oxygen from the air is reduced on another electrode of the cell. The chemical reaction produces water at the cathode, oxygen being reduced and reacting with the protons. The great advantage of the fuel cell is that it averts rejection of atmospheric pollutant compounds at the place where electricity is generated. 
         [0004]    Proton exchange membrane (PEM) fuel cells have particularly interesting properties of compactness. Each cell has an electrolytic membrane enabling only the passage of protons and not the passage of electrons. The membrane comprises an anode on a first face and a cathode on a second face to form a membrane-electrode assembly known as an MEA. 
         [0005]    At the anode, the hydrogen (H2) is ionized to produce protons passing through the membrane. The electrons produced by this reaction migrate to a flow plate and then pass through an electrical circuit external to the cell to form an electrical current. 
         [0006]    The fuel cell can comprise several flow plates, for example made of metal, stacked on one another. The membrane is positioned between two flow plates. The flow plates can comprise channels and holes to guide the reactants and products to and from the membrane. The plates are also electrically conductive so as to form collectors for the electrons generated at the anode. 
         [0007]    Gas diffusion layers are interposed between the electrodes and the flow plates and are in contact with the flow plates. 
         [0008]    The methods for assembling the fuel cell and especially the methods for fabricating the MEA are of decisive importance for the performance characteristics of the fuel cell and its service life. 
         [0009]    A known method for fabricating membrane electrode assemblies is currently being favored in order to obtain an optimal compromise between the performance of the MEA and its service life. This method comprises a preliminary step for printing a layer of electrocatalyst ink on a smooth and hydrophobic support, insensitive to the solvents present in the ink. The printing support especially has a very small surface energy and very low roughness. After the formation of an electrode by the drying of the electrocatalyst ink, the electrode is joined with the membrane by hot-pressing. Owing to the low adhesion of the electrode to the printing support, this hot-pressing can be done under reduced temperature and pressure. The deterioration of the membrane during the hot-pressing step is thus reduced. Moreover, the electrode formed by printing on a smooth support has homogenous thickness and composition, thus also limiting the deterioration of the membrane during the hot-pressing. Moreover, since the electrode is joined with the membrane after drying, the membrane is not placed in contact with the solvents of the ink and does not undergo any corresponding deterioration. 
         [0010]    The document US2008/0105354 describes a method of this kind for assembling membranes/electrodes on a fuel cell. The membrane/electrode assembly formed comprises reinforcements or subgaskets. Each reinforcement surrounds the electrodes. The reinforcements are formed out of polymer films and reinforce the membrane/electrode assembly at the gas and cooling liquid inlets. The reinforcements facilitate the handling of the membrane/electrode assembly to prevent its deterioration. The reinforcements also limit the dimensional variations of the membrane according to temperature and humidity. In practice, the reinforcements are superimposed on the periphery of the electrodes in order to limit the phenomenon of gas permeation which is the source of deterioration of the membrane/electrode assembly. 
         [0011]    According to this method, a reinforcement is made by forming an aperture in the median part of a polymer film. The reinforcement comprises a pressure-sensitive adhesive on one face. A membrane/electrode assembly is recovered and the aperture of the reinforcement is positioned so as to be plumb with an electrode. The reinforcement covers the periphery of this electrode. A pressing is then done to fixedly attach the reinforcement with the membrane and the edge of the electrode by means of the adhesive. Cut-outs are then made in the reinforcement to form the inlets of gas and liquid. 
         [0012]    Gas diffusion layers are then placed in contact with the uncovered part of the electrodes. A hot-pressing operation is frequently performed to favor contact between a gas diffusion layer and its electrode. The periphery of each gas diffusion layer covers at least a part of a respective reinforcement in order to limit direct shear forces on the membrane. Locally, the superimposition of an electrode, a reinforcement and a gas diffusion layer induces excess thickness. During a hot-pressing step, this zone undergoes higher local pressure, potentially the source of deterioration, especially on the membrane, leading to a diminishing of the service life of the fuel cell. The non-homogeneity of the pressure during the hot-pressing can additionally cause gaps between the reinforcement, the gas diffusion layer and the electrode, and cause deterioration in the performance of the fuel cell. 
         [0013]    When designing a fuel cell, an increase of its power is generally obtained either by increasing the number of stacked electrochemical cells, or by increasing the surface of the membrane/electrodes assemblies and of the bipolar plates. Such a design increases in the same proportion the weight and the dimensions of the fuel cell, as well as the volume and the cost of the gas diffusion layer. In numerous applications, the dimensions and the weight of a fuel cell are strongly limited. 
       SUMMARY 
       [0014]    The invention seeks to resolve one or more of the foregoing drawbacks. 
         [0015]    In one aspect, the invention features a method for fabricating a fuel cell. Such a method includes fixedly attaching a reinforcement to a proton-exchange membrane and to an electrode placed against a first face of the proton-exchange membrane. The reinforcement has a median aperture through which an interior portion of the electrode is exposed. Fixedly attaching the reinforcement includes superimposing an inner edge of the reinforcement over a periphery of the electrode, and causing a projecting portion of the reinforcement to project the proton-exchange membrane so as to limit gas permeation into the proton-exchange membrane, and forming filigrees by a wet process in a gas diffusion layer, thereby forming a recess therein, and placing the gas diffusion layer so that the inner edge of the reinforcement extends into the recess in the gas diffusion layer. 
         [0016]    In some practices of the invention, forming filigrees includes applying layer of an aqueous solution including carbon fibers and a binding material, and solidifying components of the aqueous solution to form the gas diffusion layer. Among these practices are those in which the gas diffusion layer formed includes a recess having a depth between 25 μm and 75 μm, and those in which the gas diffusion layer formed includes a recess having a width between 500 μm and 3000 μm. 
         [0017]    In other practices, the gas diffusion layer placed has a substantially homogenous composition. 
         [0018]    In yet other practices, the gas diffusion layer includes a first face, in which the recess is formed, and a second face, wherein a part of the second face that is disposed over the recess is in alignment with a median part of the second face. 
         [0019]    Another aspect of the invention features a method for fabricating a fuel cell. Such a method includes fixedly attaching a reinforcement having an aperture in a median part thereof, to a proton-exchange membrane and to an electrode placed against one face of the proton-exchange membrane, so that an inner edge of the fixedly attached reinforcement covers a periphery of the electrode, with a projection onto the proton-exchange membrane, forming a recess on a periphery of a gas diffusion layer by forming filigrees by a wet process in the gas diffusion layer, and placing the gas diffusion layer so that the recess is positioned plumb with the inner edge of the reinforcement. 
         [0020]    In some practices, forming filigrees includes applying layer of an aqueous solution including carbon fibers and a binding material, and solidifying components of the aqueous solution to form the gas diffusion layer. Among these practices are those in which the gas diffusion layer formed includes a recess having a depth between 25 μm and 75 μm and also those in which the gas diffusion layer formed includes a recess having a width between 500 μm and 3000 μm. 
         [0021]    In some practices, the gas diffusion layer placed has a substantially homogenous composition. 
         [0022]    In other practices, the gas diffusion layer includes a first face, in which the recess is formed, and a second face, wherein a part of the second face that is plumb with the recess is in the alignment with a median part of the second face. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Other features and advantages of the invention shall appear more clearly from the following description, given by way of an indication that is in no way exhaustive, with reference to the appended drawings, of which: 
           [0024]      FIG. 1  is a schematic view in cross-section of an example of a fuel cell; 
           [0025]      FIG. 2  is a view in cross-section of the periphery of a membrane/electrode assembly devoid of the gas diffusion layer; 
           [0026]      FIG. 3  is a view in section of the periphery of a gas diffusion layer which can be integrated into a fuel cell; 
           [0027]      FIGS. 4 to 10  illustrate different steps of an example of a method for fabricating a fuel cell according to the invention; 
           [0028]      FIG. 11  is a schematic view in cross-section of an example of a fuel cell focusing on its membrane/electrodes assembly; 
           [0029]      FIG. 12  is a view in cross-section of an interface between several electrochemical cells; 
           [0030]      FIG. 13  is a view in cross-section of a gas diffusion layer during another method for fabricating a fuel cell; 
           [0031]      FIG. 14  is a top view of the method step of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  is a view in section of an example of a fuel cell  1  including a membrane/electrode assembly fabricated according to an example of a method according to the invention. The fuel cell  1  is of the proton exchange membrane or polymer electrolyte membrane type. Although this is not illustrated, the fuel cell can comprise several superimposed electrochemical cells. The fuel cell  1  comprises a motor fuel source feeding an inlet of each cell with hydrogen (H2) from the air. The fuel cell  1  also has an air source feeding an inlet of each cell with air, containing oxygen used as an oxidant. Each cell also comprises exhaust channels. Each cell can also have a cooling circuit known per se. 
         [0033]    Each cell comprises a membrane/electrode assembly or MEA. Each membrane/electrode assembly comprises a layer of electrolyte formed for example by a polymer membrane  100 . 
         [0034]    The membrane/electrode assembly also comprises a cathode  111  and an anode  112  placed on either side of the membrane  100 . The cathode  111  and the anode  112  are advantageously fixed to this membrane  100  by any appropriate means (for example hot-pressing). 
         [0035]    The electrolyte layer forms a semi-permeable membrane  100  enabling proton conduction while at the same time being impermeable to the gases present in the cell. The membrane  100  also prevents a passage of electrons between the anode  112  and the cathode  111 . 
         [0036]    The fuel cell  1  further comprises reinforcements or subgaskets  131  and  132  positioned on the periphery respectively of the cathode  111  and the anode  112 . The reinforcements  131  and  132  are superimposed on the periphery of the electrodes with a projection over the membrane  100  in order to limit the phenomenon of gas permeation which causes deterioration in the membrane/electrode assembly. The reinforcements  131  and  132  are typically formed by polymer films and reinforce the membrane/electrode assembly at the gas and cooling liquid inlets. The reinforcements  131  and  132  also facilitate the handling of the membrane/electrode assembly to prevent its deterioration. The reinforcements  131  and  132  also limit dimensional variations in the membrane  100  as a function of temperature and humidity. 
         [0037]    Each cell has flow-guiding plates  101  and  102 , positioned so as to respectively face the cathode  111  and the anode  112 . Each cell has a gas diffusion layer  21  positioned between the cathode  111  and the guiding plate  101 . Each cell furthermore has a gas diffusion layer  22  positioned between the anode  112  and the guiding plate  102 . Two guiding plates for adjacent cells can form one bipolar plate in a manner known per se. The guiding plates can be formed by metal sheets comprising a surface in relief defining flow channels. 
         [0038]    Flow channels  103  and  104  are distributed along the z direction and extend according to the x direction, as illustrated at  FIG. 12 . The stacked electrochemical cells are compressed (as known per se) to make the periphery of the electrochemical cells waterproof, and to press the gas diffusion layers on their respective electrodes and guiding plates. 
         [0039]    In a manner known per se, during the operation of the fuel cell  1 , air flows between the MEA and the guiding plate  101 , and hydrogen (H2) flows between the MEA and the guiding plate  102 . At the anode  112 , hydrogen (H2) is ionized to produce protons which pass through the MEA. The electrons produced by this reaction are collected at the guiding plate  101 . The electrons produced are then applied to an electrical load connected to the fuel cell  1  to form an electric current. At the cathode  111 , oxygen is reduced and reacts with the protons to form water. The reactions at the anode and the cathode are controlled as follows: 
         [0000]      H 2 →2H + +2 e   −  at the anode;
 
         [0000]      4H + +4 e   − +O 2 →2H 2 O at the cathode.
 
         [0040]    When it is in operation, a cell of the fuel cell usually generates a DC voltage of the order of 1V between the anode and the cathode. 
         [0041]      FIG. 2  is a view in section of the periphery of the membrane/electrode assembly of the fuel cell of  FIG. 1 . For reasons of readability, gas diffusion layers are not illustrated in this view. 
         [0042]    The reinforcement  131  shall be described in detail here below. The reinforcement  132  can have a substantially identical structure. The reinforcement  131  has an internal border  134  which covers the periphery of the cathode  111 . The covering of the periphery of the cathode  111  by the internal border  134  advantageously extends over a width ranging from 500 to 3000 μm. The internal border  134  is fixedly joined to the cathode  111 . The reinforcement  131  extends beyond the periphery of the cathode  111  and forms a projection onto the membrane  100 . The reinforcement  131  is fixedly attached to the membrane  100 . The fixed attachment of the reinforcement  131  to the cathode  111  and to the membrane  100  can be set up by any appropriate means, for example by hot-pressing or by printing the cathode  111  on the reinforcement  131 . The reinforcement  131  has an aperture  133  in its median part. The aperture  133  thus uncovers the median part of the cathode  111 . 
         [0043]    The reinforcements  131 ,  132  and the electrodes  111 ,  112  generally have homogenous thicknesses. Consequently, the overlap between the internal border of a reinforcement and the periphery of an electrode can create a slight local excess thickness. This excess thickness can correspond appreciably to the thickness of the electrode. The electrodes  111  and  112  generally have a thickness ranging from 5 μm to 25 μm. The reinforcements  131  and  132  generally have a thickness ranging from 25 μm to 75 μm. 
         [0044]      FIG. 3  is a magnified view in section of the periphery of an example of a gas diffusion layer which can be used on the anode side or the cathode side of a fuel cell  1  according to the invention. 
         [0045]    The gas diffusion layer  21  illustrated has two faces  214  and  215 . The face  214  is intended for coming into contact with a guiding plate  101 . The face  215  is intended for coming into contact with an electrode (the cathode  111  in this case), through the aperture  133  of the reinforcement  131 . The gas diffusion layer  21  comprises a recess  211  on its periphery, this recess  211  being prepared in the face  215 . The median part of the face  215  thus forms a bulge (relative to the recess  211 ), passing through the aperture  133  of the reinforcement  131  in order to come into contact with the cathode  111 . 
         [0046]    The recess  211  is intended to be plumb with the internal border  134  of the reinforcement  131 . Thus, a superimposition is created between the internal border  134 , in limiting or eliminating the thickness locally formed by this superimposition. To this end, the recess  211  advantageously has a depth Pr ranging from 0.8*Epr to 1.1*Epr, with Epr being the thickness of the reinforcement  131  at its internal border  134 . The depth Pr advantageously ranges from 25 μm to 75 μm. The gas diffusion layer  21  advantageously has a thickness Ep ranging from 200 μm to 400 μm at its median part. The thickness of the recess  211  advantageously ranges from 500 μm to 3000 μm. The sizing of the recess  211  is advantageously made so that the internal border  134  does not extend up to the median part of the face  215  or so that the bulge formed gets housed within the aperture  133 . 
         [0047]    The junction between the recess  211  and the median part of the face  215  can advantageously present a chamfer or a connection radius. 
         [0048]    To limit local excess pressure during a hot-pressing step if any, the gas diffusion layer  211  advantageously has a face  214  in which a portion  212  is made to align with a portion  213 . The portion  212  corresponds to that part of the face  214  which is plumb with the recess  211 . The part  213  corresponds to the median part of the face  214 . Thus, the level of the face  214  is appreciably homogenous during a hot-pressing step or during the joining of the plates  101  and  102 . Advantageously, the face  214  is substantially plane. Advantageously, the gas diffusion layer  211  has a substantially homogenous composition throughout its surface. 
         [0049]      FIGS. 4 to 10  illustrate different steps of the fabrication of a fuel cell  1  according to one example of the method of the invention. The method described with reference to  FIGS. 4 to 10  can be implemented on the cathode side  111  and/or the anode side  112 . 
         [0050]      FIG. 4  is a top view of a supplied support  130 . The support  130  is an advantageously plane support. A pre-cut contour  135  can be made in the support  130 . The pre-cut contour  135  thus divides the support  130  between a peripheral part and a median part. 
         [0051]      FIG. 5  is a top view of the support  130  after the deposition of an electrocatalyst ink in liquid phase, designed to form an electrode  110  after drying. The electrode  110  can be solidified by any appropriate means. The electrode  110  formed extends beyond the pre-cut contour  135 . Thus, a superimposition is created between an internal border of the peripheral part and the periphery of the electrode  110 . 
         [0052]    The electrocatalyst material has catalytic properties suited to the catalytic reaction to be obtained. The electrocatalyst material can take the form of particles or nano-particles including metal atoms. The catalyst material can especially include metal oxides. The electrocatalyst material can be a metal such as platinum, gold, silver, cobalt, ruthenium. 
         [0053]      FIG. 6  is a view in section of a cell of a fuel cell  1  formed by using a support (such as the reinforcement  131 ) and a cathode  111 , as well as a support (such as a reinforcement  132 ) and an anode  112 , obtained according to the steps illustrated in  FIGS. 4 and 5 . At this step of the fabricating process, a membrane/electrode assembly is obtained by fixedly attaching, firstly, the support/reinforcement  131  and the cathode  111  to a face of the membrane  100 , and, secondly, the support/reinforcement  132  and the anode  112  to another face of the membrane  100 . A reinforcement and an electrode can thus be fixedly attached to the membrane  100  during a same hot-pressing step. 
         [0054]    To favor the adhesion of an electrode  110  to the membrane  100  during a hot-pressing step, the membrane  100  and the electrode  110  advantageously comprise a same polymer material. This polymer material advantageously has a glass transition temperature below the hot-pressing temperature. The polymerizable material used to form this polymer material could be the ionomer commercially distributed under the commercial reference Nafion DE2020. 
         [0055]    For adhesion by hot-pressing, the hot-pressing temperature advantageously ranges from 100° C. to 130° C., and preferably from 110° C. to 125° C. 
         [0056]      FIG. 7  is a view in section of the cell of a fuel cell  1  after the withdrawal (cutting out along the contours  135 ) of the median part of the reinforcements  131  and  132  respectively so as to prepare their aperture  133 . The apertures  133  respectively uncover the median part of the cathode  111  and the median part of the anode  112 . Thus, reinforcements can be formed from supports made by deposition of an electrocatalyst ink. 
         [0057]    Advantageously, at the end of this withdrawal step, the reinforcements  131  and  132  can be subjected to operations for cutting out through-holes at their periphery, for example to make passages for the flow of gas or cooling liquid. 
         [0058]      FIG. 8  is a view in section of the cell of a fuel cell  1  after the positioning of the gas diffusion layers  21  and  22 . The gas diffusion layer  21  is thus placed in contact with the uncovered part of the cathode  111  through the aperture  133 . The periphery of the gas diffusion layer  21  covers the internal edge  134  of the reinforcement  131 . The internal edge  134  of the reinforcement  131  therefore gets housed in the recess  211  of the gas diffusion layer  21 . The gas diffusion layer  22  is placed in contact with the uncovered part of the anode  112  through the aperture  133 . The periphery of the gas diffusion layer  22  covers the internal border  134  of the reinforcement  132 . The internal border  134  of the reinforcement  132  therefore gets housed in the recess  211  of the gas diffusion layer  22 . 
         [0059]    To obtain the cell of a fuel cell  1  illustrated in  FIG. 1 , the membrane/electrode assembly provided with gas diffusion layers  21  and  22  can then be included between two flow-guiding metal plates  101  and  102 . 
         [0060]    A recess  211  can be formed by using known methods for forming filigree patterns in paper pulp. A gas diffusion layer comprising a recess  211  in filigree form can especially be obtained by wet process.  FIG. 9  is a schematic view in section illustrating a step of an example of a method for the fabrication, by wet process, of a gas diffusion layer comprising a recess  211 . 
         [0061]    According to such a method using a wet process, an aqueous solution  12  is applied to a porous support  31  having a structure known per se. This support  31  is surmounted by an added-on relief feature  32  (sometimes called a galvano relief or galvano), defining a shape for the recess  211 . The combination of a support  31  and an added-on relief  32  for the formation of a gas diffusion layer with recess is illustrated in a top view in  FIG. 10 . A device  34  for retrieving excess water is positioned beneath the support  31  and includes for example a vacuum suction device. The support  31  is designed to allow the filtering of water included in the aqueous solution  12  to preserve the remainder of the constituents of the solution above this support  31 . 
         [0062]    The aqueous solution includes carbon fibers (known per se in the formation of gas diffusion layers) and a binder material (for example polyvinyl alcohol). The aqueous solution  12  can take the form of a dispersion including the different elements. 
         [0063]    As illustrated in the example, the aqueous solution  12  can for example be applied by means of a spraying nozzle  33  that is mobile relatively to the support  31 . In preparation for such an application of the aqueous solution  12 , this solution can have a proportion by mass in carbon fibers smaller than or equal to 0.02% (for example equal to 0.01%) during the spraying. The binder material can for example constitute 5 to 10% of the proportion by mass of the gas diffusion layer formed. 
         [0064]    Once the aqueous solution  12  is applied to the support  31 , the major part of the water from this solution is allowed to get discharged through the support  31  until a material is obtained that is solid enough to enable it to be handled. The solidified element comprises the recess  211  defined by the shape of the relief  32 . The solidified element can then undergo other processing operations such as oven drying, pressing, impregnation or graphitization, until a gas diffusion layer  21  that must be assembled inside the fuel cell  1  is obtained. 
         [0065]    The solidified element can have a recess depth greater than that of the formed gas diffusion layer, especially when the solidified element undergoes a pressing step. The thickness of the relief feature  32  will advantageously be defined to take account of these subsequent steps of the process. The width of the relief will advantageously range from 500 μm to 3000 μm in order to define the width of the recess  211  to be formed. 
         [0066]      FIG. 11  is a schematic view (in cross section) of another embodiment of fuel cell  1 , focusing on the membrane/electrode assembly. The contact interfaces between the gas diffusion layers  21  and  22  and the cathode  111  and the anode  112  respectively are wave-shaped. 
         [0067]    For that purpose, the face of the gas diffusion layer  21  in contact with the cathode  111  is wave-shaped. Similarly, the face of contact of the gas diffusion layer  22  with the anode  112  is wave-shaped. The membrane/electrode assembly is flexible and is wave-shaped by the gas diffusion layers  21  and  22 . 
         [0068]    Thus, with a slightly increased thickness of the electrochemical cell and with a same area of the guiding plates  101  and  102 , the exchange area between the gas diffusion layers  21 ,  22  and the electrodes  111 ,  112  is increased. Such a fuel cell  1  has an increased power with an almost unchanged volume. An increase power is obtained with an unchanged volume of the gas diffusion layers. The cost of the gas diffusion layers (generally the most expensive parts of the fuel cell) is almost unchanged. 
         [0069]    In the example of  FIG. 11 , the wave shape has a period P along the x direction. Period P is advantageously comprised between 50 and 250 μm, and preferably between 75 and 150 μm (for instance 100 μm). Period P is low enough to obtain a contact area increase without increasing the thickness of the membrane/electrode assembly. Period P is high enough to avoid excessive deformations of the membrane/electrode assembly. The fastening between electrodes  111 ,  112  and the membrane  100  is thereby not altered. The contact between the electrodes and their respective gas diffusion layers is also maintained. 
         [0070]    The wave shape has advantageously a homogeneous height A. This height is advantageously comprised between 15 and 50 μm, and preferably between 20 and 45 μm. This height is preferably comprised between 5 and 20% of the thickness of the gas diffusion layer, and preferably comprised between 5 and 15%. Height A is the depth between the top and the bottom of the wave shape. Height A is high enough to significantly increase the exchange area between a gas diffusion layer and its respective electrode. Height A is low enough to avoid excessive deformations of the membrane/electrode assembly. The ration between period P and height A is preferably comprised between 2 and 5. Gas diffusion layers  21  and  22  have preferably a thickness comprised between 150 μm and 500 μm, and preferably comprised between 200 and 300 μm. 
         [0071]    With such parameters, the exchange area between a gas diffusion layer and its respective electrode can be increased (between 10% and 25%). 
         [0072]    The wave shape has preferably no sharp edge and has preferably a high radius of curvature. The membrane/electrode assembly is thereby not altered. A homogenous contact between an electrode and its gas diffusion layer is maintained as well. The contact face between an electrode and its gas diffusion layer has preferably an extrusion shape. 
         [0073]    The membrane/electrode assembly can be easily shaped without being altered, when its thickness is comprised between 35 and 130 μm. The thickness of electrodes  111  and  112  is preferably comprised between 5 and 15 μm. The thickness of the membrane  100  is preferably comprised between 20 and 100 μm. 
         [0074]      FIGS. 13 and 14  illustrate another embodiment of the fabrication step illustrated at  FIGS. 9 and 10 . During this step, a gas diffusion layer having a wave-shaped contact surface is obtained by wet process. The support  31  is additionally surmounted by an added-on relief feature  35 , defining the wave shape in the middle portion of the gas diffusion layer. The relief feature  35  defines recesses in the middle portion of the gas diffusion layer with an appropriate wave shape. 
         [0075]    The gas diffusion layer obtained by such a wet process step has a flat upper surface.