Abstract:
The present invention relates to en electrochemical cell, a gas diffusion layer of an electrochemical cell, and a method of using the gas diffusion layer. The electrochemical cell includes (a) a membrane electrode assembly; (b) a first reactant flow field plate for providing a first reactant flow field disposed on one side of the membrane electrode assembly; (c) a first seal disposed between the first reactant flow field plate and the membrane electrode assembly for impeding leakage of process fluids of the electrochemical cell; (d) a first gas diffusion layer disposed between the first reactant flow field plate and the membrane electrode assembly for diffusing reactant from the first reactant flow field to the membrane electrode assembly; and (e) a second reactant flow field plate for providing a second reactant flow field disposed on the other side of the membrane electrode assembly. The gas diffusion layer provides a peripheral support structure for supporting the membrane electrode assembly at a periphery between the first reactant flow field and the first seal to impede substantial distortion of the membrane electrode assembly between the first reactant flow field and the first seal.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to an electrochemical cell. More particularly, this invention relates to a gas diffusion layer for an electrochemical cell and a method of making such a gas diffusion layer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrochemical cells are energy conversion devices and usually are used to collectively indicate fuel cells and electrolyzer cells. Fuel cells have been proposed as a clean, efficient and environmentally friendly power source that has various applications. A conventional proton exchange membrane (PEM) fuel cell includes an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. A fuel cell generates electricity by bringing a fuel gas (typically hydrogen) and an oxidant gas (typically oxygen) respectively to the anode and the cathode. In reaction, a fuel such as hydrogen is oxidized at the anode to form cations (protons) and electrons by the reaction H 2 =2H + +2e−. The proton exchange membrane facilitates the migration of protons from the anode to the cathode while preventing the electrons from passing through the membrane. As a result, the electrons are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts with electrons returned from the electrical circuit to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction by-product following ½O 2 +2H − +2e−=H 2 O.  
           [0003]    On the other hand, an electrolyzer uses electricity to electrolyze water to generate oxygen from its anode and hydrogen from its cathode. Similar to a fuel cell, a typical solid polymer water electrolyzer (SPWE) or proton exchange membrane (PEM) electrolyzer includes an anode, a cathode and a proton exchange membrane disposed between the two electrodes. Water is introduced to, for example, the anode of the electrolyzer which is connected to the positive pole of a suitable direct current voltage. Oxygen is produced at the anode by the reaction H 2 O=½O 2 +2H − +2e−. The protons then migrate from the anode to the cathode through the membrane. On the cathode which is connected to the negative pole of the direct current voltage, the protons conducted through the membrane are reduced to hydrogen according to the equation 2H++2e−=H 2 .  
           [0004]    A typical electrochemical cell employing PEM comprises an anode flow field plate, a cathode flow field plate, and a membrane electrode assembly (MEA) disposed between the anode and cathode flow field plates. Each reactant flow field plate has an inlet region, an outlet region, and open-faced channels to fluidly connect the inlet to the outlet, and provide a way for distributing the reactant gases to the outer surfaces of the MEA. The MEA comprises a PEM disposed between an anode catalyst layer and a cathode catalyst layer A first gas diffusion layer (GDL) is disposed between the anode catalyst layer and the anode flow field plate, and a second GDL is disposed between the cathode catalyst layer and the cathode flow field plate. The GDLs facilitate the diffusion of the reactant gas, either the fuel or oxidant, to the catalyst surfaces of the MEA. Furthermore, the GDLs enhance the electrical conductivity between each of the anode and cathode flow field plates and the electrodes.  
           [0005]    One of the questions that have to be answered in the design of such electrochemical cells is how to ensure the MEA works properly while preventing leakage of process fluids. This almost always involves proper support of the MEA when the electrochemical cell stack is assembled Extensive efforts have been made in this regard. However, these efforts have been focused on design changes of reactant flow field plates, which are complicated and expensive Compromise has to be made in these designs and hence this often results in the substitution of one problem for another.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with a first aspect of the present invention, there is provided an electrochemical cell comprising: (a) a membrane electrode assembly; (b) a first reactant flow field plate for providing a first reactant flow field disposed on one side of the membrane electrode assembly; (c) a first seal disposed between the first reactant flow field plate and the membrane electrode assembly for impeding leakage of process fluids of the electrochemical cell; (d) a first gas diffusion layer disposed between the first reactant flow field plate and the membrane electrode assembly for diffusing reactant from the first reactant flow field to the membrane electrode assembly; (e) a second reactant flow field plate for providing a second reactant flow field disposed on the other side of the membrane electrode assembly; and, (f) a peripheral support structure for supporting the membrane electrode assembly at a periphery between the first reactant flow field and the first seal to impede substantial distortion of the membrane electrode assembly between the first reactant flow field and the first seal.  
           [0007]    In accordance with a second aspect of the present invention, there is provided a method of impeding leakage of process fluids from an electrochemical cell having a membrane electrode assembly, a first reactant flow field plate for providing a first reactant flow field disposed on one side of the membrane electrode assembly, a seal disposed between the first reactant flow field plate and the membrane electrode assembly for impeding leakage of process fluids of the electrochemical cell, and a second reactant flow field plate for providing a second reactant flow field disposed on the other side of the membrane electrode assembly. The method comprises (a) providing a gas diffusion layer disposed between the first reactant flow field and the membrane electrode assembly; and, (b) providing the gas diffusion layer with an edge portion for supporting the membrane electrode assembly at a periphery between the reactant flow field and the seal to impede substantial distortion of the membrane electrode assembly between the reactant flow field and the seal.  
           [0008]    In accordance with a third aspect of the present invention, there is provided a gas diffusion layer for an electrochemical cell. The gas diffusion layer comprises a first side for receiving process fluids of the electrochemical cell; a second side opposite to the first side; a porous body for diffusing the process fluids from the first side to the second side, the porous body being electrically conductive, and an edge portion surrounding the porous body, wherein the edge portion is substantially impermeable to the process fluids.  
           [0009]    In accordance with a fourth aspect of the present invention, there is provided a gas diffusion layer for an electrochemical cell. The gas diffusion layer comprising a first side for receiving process fluids of the electrochemical cell; a second side opposite to the first side; a porous body for diffusing the process fluids from the first side to the second side, the porous body being electrically conductive, and, an edge portion surrounding the porous body, wherein a thickness of the edge portion is reduced from one side of the porous body to provide a step between the porous body and the edge portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made to the accompanying drawings which show, by way of example, preferred embodiments of the present invention, and in which;  
         [0011]    [0011]FIG. 1 shows an exploded perspective view of an electrochemical cell unit located within an electrochemical cell stack;  
         [0012]    [0012]FIG. 2 shows a schematic view of the front face of a flow field plate of an electrochemical cell;  
         [0013]    [0013]FIG. 3 shows a schematic sectional view through part of an electrochemical cell stack incorporating a prior art gas diffusion layer;  
         [0014]    [0014]FIG. 4 shows a front elevational view of a gas diffusion layer according to a first embodiment of the present invention;  
         [0015]    [0015]FIG. 5 shows a side elevational view of the gas diffusion layer of FIG. 4;  
         [0016]    [0016]FIG. 6 shows a schematic sectional view through part of an electro chemical cell stack incorporating the gas diffusion layer of a first embodiment of the present invention;  
         [0017]    [0017]FIG. 7 shows a schematic view of an electrochemical cell stack in accordance with a second embodiment of the present invention;  
         [0018]    [0018]FIG. 8 shows a schematic view of an electrochemical cell stack in accordance with a third embodiment of the present invention;  
         [0019]    [0019]FIG. 9 shows a schematic view of an electrochemical cell stack in accordance with a fourth embodiment of the present invention;  
         [0020]    [0020]FIG. 10 shows a schematic view of an electrochemical cell stack in accordance with a fifth embodiment of the present invention;  
         [0021]    [0021]FIG. 11 shows a schematic view of an electrochemical cell stack in accordance with a sixth embodiment of the present invention;  
         [0022]    [0022]FIG. 12 shows a schematic view of an electrochemical cell stack in accordance with a seventh embodiment of the present invention;  
         [0023]    [0023]FIG. 13 shows a schematic view of an electrochemical cell stack in accordance with an eighth embodiment of the present invention; and,  
         [0024]    [0024]FIG. 14 shows a schematic view of an electrochemical cell stack in accordance with a ninth embodiment of the present invention 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The present invention relates to gas diffusion layers for electrochemical cells. Hereinafter, the present invention will be described in detail by taking a PEM fuel cell as an example. It Is to be understood that the present invention has applications not limited to PEM fuel cells, but rather any type of electrochemical cells, such as electrolyzers  
         [0026]    Referring first to FIG. 1, this shows an exploded perspective view of a single fuel cell unit  100  located within a fuel cell stack according to the present invention. It is to be understood that while a single fuel cell unit  100  is detailed below, in known manner the fuel cell stack will usually comprise a plurality of fuel cells stacked together. Each fuel cell of the fuel cell unit  100  comprises an anode flow field plate  120 , a cathode flow field plate  130 , and a membrane electrode assembly (MEA)  124  disposed between the anode and cathode flow field plates  120 ,  130 . Each reactant flow field plate has an inlet region, an outlet region, and open-faced channels to fluidly connect the inlet to the outlet, and provide a way for distributing the reactant gases to the outer surfaces of the MEA  124 . The MEA  124  comprises a solid electrolyte (i.e. a proton exchange membrane)  125  disposed between an anode catalyst layer (not shown) and a cathode catalyst layer (not shown). A first gas diffusion layer (GDL)  122  is disposed between the anode catalyst layer and the anode flow field plate  120 , and a second GDL  126  is disposed between the cathode catalyst layer and the cathode flow field plate  130 . The GDLs  122 ,  126  facilitate the diffusion of the reactant gas, either the fuel or oxidant, to the catalyst surfaces of the MEA  124 . Furthermore, the GDLs are electrically conductive and thereby enhance the electrical conductivity between each of the anode and cathode flow field plates  120 ,  130  and the MEA  124 .  
         [0027]    Still referring to FIG. 1, hereinafter the designations “front” and “rear” with respect to the anode and cathode flow field plates  120 ,  130  indicate their orientation with respect to the MEA  124 . Thus, the “fronts” face indicates the side facing towards the MEA  124 , while the “rear” face indicates the side facing away from the MEA  124 . A first current collector plate  116  abuts against the rear face of the anode flow field plate  120 . Similarly, a second current collector plate  118  abuts against the rear face of the cathode flow field plate  130 . The current collector plates  116 ,  118  collect the current from the flow field plates  120 ,  130 , and are connected to an external electrical circuit (not shown). First and second insulator plates  112 ,  114  are located immediately adjacent the first and second current collector plates  116 ,  118 , respectively. First and second end plates  102 ,  104  are located immediately adjacent the first and second insulator plates  112 ,  114 , respectively. Pressure may be applied on the end plates  102 ,  104  to press the unit  100  together. Moreover, sealing means are usually provided between each pair of adjacent plates. Preferably, a plurality of tie rods  131  may also b provided. The tie rods  131  are screwed into threaded bores in the anode endplate  102 , and pass through corresponding plain bores in the cathode endplate  104 . In known manner, fastening means, such as nuts, bolts, washers and the like are provided for clamping together the fuel cell unit  100  and the entire fuel cell stack  
         [0028]    Still referring to FIG. 1, the endplate  104  is provided with a plurality of connection ports for the supply of various fluids. Specifically, the second endplate  104  has first and a second air connection ports  106 ,  107 , first and second coolant connection ports  108 ,  109 , and first and second hydrogen connection ports  110 ,  111 . As will be understood by those skilled in the art, the MEA  124 , the anode and cathode flow field plates  120 ,  130 , the first and second current collector plates  116 ,  118 , the first and second insulator plates  112 ,  114 , and the first and/or second end plates  102 ,  104  have three inlets near one end and three outlets near the opposite end thereof, which are in alignment to form fluid ducts for air as an oxidant, a coolant, and hydrogen as a fuel. Also, it is not essential that all the outlets be located at one end, i.e., pairs of flows could be counter current as opposed to flowing in the same direction. Although not shown, it will be understood that the various ports  106 - 111  are fluidly connected to ducts that extend along the length of the fuel cell unit  100 .  
         [0029]    Referring now to FIG. 2, there is illustrated in a schematic view, the front face of a flow field plate  150 . It is to be understood that this only illustrates an example of a flow field plate which forms no part of the present invention. The flow field plate  150  as shown can be either an anode flow field plate or a cathode flow field plate. The flow field plate  150  has three apertures near one end thereof, namely an air inlet aperture  136 , a coolant inlet aperture  138 , and a hydrogen inlet aperture  140 , in fluid communication with the first air connection port  106 , the first coolant connection port  108 , and the first hydrogen connection port  110 , respectively. The flow field plate  150  has three apertures near the opposite end, namely an air outlet aperture  137 , a coolant outlet aperture  139  and a hydrogen outlet aperture  141 , in fluid communication with the second air connection port  107 , the second coolant connection port  109 , and the second hydrogen connection port  111 , respectively.  
         [0030]    In FIG. 2, the front face of the flow field plate  150  is provided with a reactant flow field  132  comprising a plurality of open-faced channels. For illustration only, this reactant flow field  132  fluidly connects the hydrogen inlet aperture  140  to the hydrogen outlet aperture  141  and hence can be referred to as hydrogen flow field. It can be understood that the reactant flow field  132  may alternatively be an air flow field that fluidly connects the air inlet aperture  136  to the air outlet aperture  137 . As is known to those skilled in the art, when reactant flows along the channels in the flow field  132 , at least a portion of the reactant diffuses across the first or second GDL  122  or  126  and reacts at the respective catalyst layer of the MEA. The excess reactant continues to flow along the reactant flow field  132 , and ultimately exits the flow field plate  150  via the associated reactant outlet aperture, in this example, hydrogen outlet aperture  141 .  
         [0031]    Still referring to FIG. 2, a seal  200  is provided around the reactant flow field  132  and the various inlets and outlets to prevent leaking or mixing of reactant gases and coolant. The seal, usually a gasket made of resilient materials compatible with a fuel cell environment, is seated in a seal groove (not shown) provided on the front face of the reactant flow field plate  150 ; The seal  200  is formed such that it completely encloses the flow field  132  and the inlet and outlet apertures, permitting a reactant to only flow from associated inlet aperture to outlet aperture. The gasket seals between adjacent plates when the fuel cell stack is assembled  
         [0032]    Although not shown, it is known to those skilled in the art that the rear face of the reactant flow field plate  150  is usually provided with coolant flow field that fluidly connects the coolant inlet aperture  138  to the coolant outlet aperture  139 . Similarly, seal gasket can be provided on the rear face of the reactant flow field plate  150 . The first and second GDLs  122  and  126  typically cover the flow field, i.e. the active area of respective anode and cathode flow field plates  120  and  130 , as shown in FIG. 1, to facilitate the diffusion of reactants.  
         [0033]    Similarly, PEM electrolyzers may employ similar design to that described above. Specifically, PEM electrolyzers also have flow field plates, MEA, GDLs, end plates and terminal plates corresponding to current collector plate of fuel cells that conduct current between the electrochemical cell and external circuit. In electrolyzers, the reactant is water and the products are hydrogen on cathode and oxygen on anode. On the anode, the gas diffusion layer enables the diffusion of the process water to the surface of the MEA, the diffusion of the product gas (oxygen) away from the surface of the MEA and provides conduction of electricity between the anode flow field plate and the MEA. On the cathode, the gas diffusion layer enables the diffusion of the product gas (hydrogen) away from the surface of the MEA and provides conduction of electricity between the cathode flow field plate  32  and the MEA. In electrolyzer application, the configuration may be slightly different. Electrolyzer flow fields may not have a cathode inlet and only a cathode outlet, as hydrogen emerges from cathode without introducing any fluid directly into the cathode side thereof.  
         [0034]    Now, reference will be made to FIG. 3, which shows a schematic sectional view through part of an electrochemical cell stack incorporating a prior art gas diffusion layer. FIG. 3 shows the anode flow field plate  120 , the first gas diffusion layer  122 , the MEA  124 , the second gas diffusion layer  126  and the cathode flow field plate  130 , stacked one on top of another. The anode flow field plate  120  is provided with a first seal gasket  200   a  enclosing the anode flow field and the cathode flow field plate  130  is provided with a second seal gasket  200   b  enclosing the cathode flow field. As aforementioned, apertures on the anode flow field plate  120 , the cathode flow field plate  130  and the MEA align to form a reactant duct throughout the electrochemical cell stack. In existing electrochemical cell stacks, the first and second gas diffusion layers  122  and  126  substantially cover the active areas, i.e. open-channels of the associated flow field plates, and hence do not extend to the duct formed by these apertures. Since the seal gaskets  200   a  and  200   b  usually protrude from the front face of anode and cathode flow field plates  120  and  130  respectively, a gap  162  is formed between the end of the first gas diffusion layer  122  and the first seal gasket  200   a . Similarly, a gap  164  is formed between the end of the second gas diffusion layer  126  and the second seal gasket  200   b . During operation of the electrochemical cell, pressure of the reactant gases or product gases on one side of the MEA  124  may not be the same as that on the other side Therefore, the MEA  124  will be forced to distort into one of the gaps  162  and  164 . In FIG. 3, for illustration only, a distorted portion  170  of MEA  124  is formed in the gap  162 . This reduces the lifespan of MEA and leads to lower efficiency and eventually leakage of reactants or products.  
         [0035]    [0035]FIGS. 4 and 5 show a gas diffusion layer  300  according to the first embodiment of the present invention. As is known to those skilled in the art, the gas diffusion layer  300  is made of porous materials, for example, Carbon Felts, paper and cloth, allowing fluids to pass therethrough. When used in electrolyzer cells, a typical material is sintered metal. The present gas diffusion layer  300  is provided with a central portion  310  and an edge portion  320  having reduced thickness. One side  340  of the gas diffusion layer  300  is flat and the other side  330  has a step  325  around the central portion  310 .  
         [0036]    Referring to FIG. 6, there is illustrated in a schematic sectional view an electrochemical cell stack incorporating first and second gas diffusion layers  300  and  300 ′. For clarity, analogous elements between the electrochemical cell stacks of FIGS. 3 and 6 are denoted by the same numerals. As can be seen in FIG. 6, the first and second gas diffusion layers  300  and  300 ′ are respectively disposed between anode flow field plate  120  and MEA  124 , and the cathode flow field plate  130  and the MEA  124 . Similar to the designation used for flow field plates, the “front” and “rear” faces of the first and second gas diffusion layers  300  and  300 ′ are respectively used to indicate faces of the gas diffusion layers facing towards and away from the MEA  124 . The front faces  340  and  340 ′ are the flat faces of the associated gas diffusion layers  300  and  300 ′ respectively. Each of the edge portions  320  and  320 ′ of the gas diffusion layers  300  and  300 ′ overlaps corresponding seal gasket  300   a  and  300   b  such that each of the steps  325  and  325 ′ abuts against the seal gasket  300   a  and  300   b , respectively. Therefore, when the electrochemical cell stack is assembled and compressed, the MEA  124  is properly supported on both sides in the active area, i.e. the area corresponding to the reactant flow field of each reactant flow field plate  120  or  130 .  
         [0037]    As can be appreciated by those skilled in the art, the edge portions  320  and  320 ′ have to be impermeable to reactant or product fluids to prevent leakage of these fluids. Further, a seal must be formed between the top surface of the edge portions  320  and  320 ′ and the MEA  124  In contrast, the area of the gas diffusion layer  300  corresponding to the active area of flow field plates has to be permeable to process fluids for electrochemical cells. This can be achieved by compressing the edge of the porous material to form physically compressed and hence impermeable portion  320 . Commonly used gas diffusion layers comprise sintered metal, carbon cloth, and other similar materials. Only the porous portions of the gas diffusion layers need be electrically conductive. These materials usually can be physically compressed to obtain impermeable, reduced thickness portions. Other methods of providing non-porous portion may also be used, such as joining non-porous materials to the edge of porous material. However, compressing is a simple and inexpensive technique.  
         [0038]    Referring to FIG. 7, there is illustrated, in a schematic sectional view, an electrochemical cell stack  1000  comprising a gas diffusion layer  1122  in accordance with a second embodiment of the invention. For clarity, analogous elements of the electrochemical cell stack of FIG. 3 and that of FIG. 7 are denoted by the same numerals, but with 1000 added to the reference numerals of the electrochemical cell stack  1000  of FIG. 7. As shown in FIG. 7, in accordance with the second embodiment of the present invention, the gas diffusion layer  1122  extends beyond the seal gasket  1200   a . Although the edge portion  1350  surrounding the central portion has the same thickness as the central portion, as in the first embodiment, the edge portion  1350  is impermeable to process fluids for electrochemical cells. Again, this can be done by compressing the porous material used to manufacture the gas diffusion layer to eliminate the pores. Since the front face of the gas diffusion layer  1122  of this embodiment is flat, the MEA  1124  is properly supported. The impermeable edge portion  1350  together with the seal gasket  1200   a  seal the process fluids from leaking  
         [0039]    Referring to FIG. 8, there is illustrated in a schematic sectional view, an electrochemical cell stack  2 ( 000  comprising a gas diffusion layer  2122  in accordance with a third embodiment of the invention. For clarity, analogous elements of the electrochemical cell stack  1000  of FIG. 7 and the electrochemical cell stack  2000  of FIG. 8 are denoted by the same numerals, but with 1000 added to the reference numerals of the electrochemical cell stack  2000  of FIG. 8. The structure and operation of the electrochemical cell stack  2000  of FIG. 8 is analogous to that of FIG. 7 except as described below. For brevity, the description of FIG. 7 is not repeated with respect to FIG. 8. In the cell stack  2000  of FIG. 8, a silk screened gasket (or other thin gasket material)  2400  is added on the edge portion  2350  of the gas diffusion layer  2122  on the front face thereof. This further improves the sealing between the MEA  2124  and the front face of the edge portion  2350 . In other respects, the gas diffusion layer  2122  of FIG. 8 resembles the gas diffusion layer  1122  of FIG. 7.  
         [0040]    Referring to FIG. 9, there is illustrated in a schematic sectional view, an electrochemical cell stack  3000  comprising a gas diffusion layer  3122  in accordance with a fourth embodiment of the invention. For clarity, analogous elements of the electrochemical cell stack  1000  of FIG. 7 and the electrochemical cell stack  3000  of FIG. 9 are denoted by the same reference numerals, but with 2000 added to the reference numerals of the electrochemical cell stack  3000  of FIG. 9. The structure and op ration of the electrochemical cell stack  3000  of FIG. 9 is analogous to that of FIG. 7 except as described below. For brevity, the description of FIG. 7 is not repeated with respect to FIG. 9.  
         [0041]    In the cell stack  3000  of FIG. 9, the edge portion  3320  is compressed with a silk screened gasket  3400  added on the front face thereof. As with the edge portion  2350  of the gas diffusion layer  2122  of the electrochemical cell stack  2000  of FIG. 8, the addition of the silk screened gasket  3400  further improves the sealing between an MEA  3124  and the front face of the edge portion  3320 . However, unlike the edge portions  1350  and  2350  of the gas diffusion layers  1122  and  2122  of FIGS. 7 and 8 respectively, the edge portion  3320  of the electrochemical cell stack  3000  is compressed, similar to the edge portions  320  and  320 ′ of FIG. 6.  
         [0042]    Referring to FIG. 10, there is illustrated in a schematic sectional view, an electrochemical cell stack  4000  comprising a gas diffusion layer  4122  in accordance with a fifth embodiment of the invention. For clarity, analogous elements between the electrochemical cell stack  1000  of FIG. 7 and the electrochemical cell stack  4000  of FIG. 10 are denoted by the same numerals, but with  3000  added to the reference numerals of the electrochemical cell stack  4000  of FIG. 10. The structure and operation of the electrochemical cell stack  4000  of FIG. 10 is analogous to that of FIG. 7 except as described below. For brevity, the description of FIG. 7 is not repeated with respect to FIG. 10.  
         [0043]    The cell stack  4000  of FIG. 10 includes a gas diffusion layer  4122  that is flat on both the front and rear faces. The edge portion  4350  and central portion are all porous. Liquid silicone material is injected onto the edge portion  4350 , to form an integral silicone gasket  4500 . The integral silicone gasket  4500  protrudes from the front and rear faces of the gas diffusion layer  4122  and forms a seal between a MEA  4124  and the gas diffusion layer  4122 , and between a reactant flow field plate  4120  and the gas diffusion layer  4122 .  
         [0044]    Referring to FIG. 11, there is illustrated in a schematic sectional view, an electrochemical cell stack  5000  comprising a gas diffusion layer  5122  and a sealing insert  5600  in accordance with a sixth embodiment of the invention. For clarity, analogous elements between the electrochemical cell stack  1000  of FIG. 7 and the electrochemical cell stack  5000  of FIG. 11 are denoted by the same numerals, but with 4000 added to the reference numerals of the electrochemical cell stack  5000  of FIG. 11. The structure and operation of the electrochemical cell stack  5000  of FIG. 11 is analogous to that of FIG. 7 except as described below. For brevity, the description of FIG. 7 is not repeated with respect to FIG. 11.  
         [0045]    The gas diffusion layer  5122  of the cell stack  5000  of FIG. 11 comprises an edge portion  5320  with reduced thickness, similar to the first and fourth embodiments shown in FIGS. 6 and 9 respectively. A step  5325  of the gas diffusion layer  5122  faces towards a MEA  5124  when the gas diffusion layer  5122  is disposed within the electrochemical cell  5000 . The sealing insert  5600  is attached onto the edge portion  5320  and abuts against the step  5325 . The sealing insert  5600  extends beyond the end face of the gas diffusion layer  5122  and has a thickness substantially the same as the height of the step  5325  to provide a substantially flat surface to support MEA  5124 . The sealing insert  5600  can be a metal ring bonded onto the edge portion  5320 . In this case, the sealing insert  5600  is configured such that the end face of the metal ring substantially aligns with the end face of the gas diffusion layer  5122  so as to properly support the MEA  5124 . That is, the sealing insert  5600  is configured such that the MEA  5124  is fully supported and does not collapse into a gap formed around the edge seal as shown in FIG. 3. In the embodiment of FIG. 11, the sealing insert  5600  need not seal at all, as the seal itself is outboard of the sealing insert  5600 .  
         [0046]    Referring to FIG. 12, there is illustrated in a schematic sectional view, an electrochemical cell stack  6000  in accordance with a seventh embodiment of the invention. For clarity, analogous elements of the electrochemical cell stack  5000  of FIG. 11 and the electrochemical cell stack  6000  of FIG. 12 are denoted by the same numerals, but with 1000 added to the reference numerals of the electrochemical cell stack  6000  of FIG. 12. Th structure and operation of the electrochemical cell stack  6000  of FIG. 12 is analogous to that of FIG. 11 except as described below.  
         [0047]    As with the electrochemical cell stack  5000 , the electrochemical cell stack  6000  of FIG. 12 comprises a sealing insert  6600  provided on an edge portion  6320  of a gas diffusion layer  6122 . The edge portion  6320  has a reduced thickness similar to the edge portion  5320  of FIG. 11 such that the gas diffusion layer  6122  with the sealing insert  6600  provides a substantially flat surface for supporting the MEA  6124 . In addition, a silk screened gasket  6400  is provided on the sealing insert  6600  to improve the sealing between a MEA  6124  and the sealing insert  6600 . In this embodiment, and in contrast to the cell stack  5000  of FIG. 11, the sealing insert  6600  is used to provide a seal as well as to support the MEA  6124 .  
         [0048]    Referring to FIG. 13, there is illustrated in a schematic sectional view, an electrochemical cell stack  7000  in accordance with an eighth embodiment of the present invention. For clarity, analogous elements between the electrochemical cell stack  6000  of FIG. 12 and the electrochemical cell stack  7000  of FIG. 13 are denoted by the same numerals, but with 1000 added to the reference numerals of the electrochemical cell stack  7000  of FIG. 13. The structure and operation of the electrochemical cell stack  7000  of FIG. 13 is analogous to that of FIG. 12 except as described below. For brevity, the description of FIG. 12 is not repeated with respect to FIG. 13.  
         [0049]    The electrochemical cell stack  7000  of FIG. 13 comprises a sealing insert  7700  provided adjoining an edge portion  7320  of a gas diffusion layer  7122 . A conventional sealing gasket  7200   a  can be disposed in a slot  7750  and provides a seal between a flow field plate  7120  and a MEA  7124 . Although not shown, it can be appreciated that the sealing insert  7700  surrounds a step  7325  where the thickness of the gas diffusion layer  7122  is reduced at the edge portion  7320 . Hence, the sealing insert  7700  is a loop.  
         [0050]    Depending on the configuration of the flow field plate, the edge portion of the gas diffusion layer may or may not be impermeable to process fluids for electrochemical cells. In the embodiments shown in FIGS.  11 - 13 , the end face of the edge portion engages the flow field plate and then an additional sealing insert is provided that overlaps this juncture of the end face of the edge portion with the flow field plate. Hence, it is not necessary for the edge portion to be impermeable. However, when used together with flow field plates having other configurations, it may be necessary for the edge portion to be impermeable.  
         [0051]    Referring to FIG. 14, there is illustrated in a schematic sectional view, an electrochemical cell stack  8000  in accordance with a ninth embodiment of the present invention. For clarity, analogous elements between the electrochemical cell stack  5000  of FIG. 11 and the electrochemical cell stack  8000  of FIG. 14 are denoted by the same numerals, but with  3000  added to the reference numerals of the electrochemical cell stack  8000  of FIG. 14. The structure and operation of the electrochemical cell stack  8000  of FIG. 14 is analogous to that of FIG. 11 except as described below. For brevity, the description of FIG. 11 is not repeated with respect to FIG. 14.  
         [0052]    The electrochemical cell stack  8000  comprises a sealing insert  8800 . This sealing insert  8800  includes a step  8825 . Gas diffusion layer  8122  includes an edge  8320  of reduced thickness, which edge  8320  defines a step  8325  at which the thickness of the gas diffusion layer  8122  is reduced. The steps  8325  and  8825  are configured to engage one another such that the sealing insert  8800  aligns with the step  8325  and edge  8320  of the gas diffusion layer  8122 . Again, as with the embodiments of FIGS.  11  to  13 , as the sealing member  8800  provides a complete seal between flow field plate  8120  extending from a flow field plate  8120  to a MEA  8124 , the edge portion  8320  of gas diffusion layer  8122  need not be impermeable. Given the thickness of the sealing insert  8800 , the flow field plate  8120  need not have a raised containment edge around the perimeter of the active area. The sealing insert  8800  includes a sealing material between its bottom surface and the flow field plate  8120  (not shown) and optionally may include a sealing material between its top surface and the MEA  8124  (as in FIGS. 8, 9 and  12 )  
         [0053]    It should be appreciated that the spirit of the present invention is concerned with the novel structure of the gas diffusion layer and its application in electrochemical cells. The design of flow field on the reactant flow field plate, and structure of end plates, current collector plates do not form part of the present invention Moreover, the present invention is applicable to various types of fuel cells and electrolyzers. For example, in case of fuel cell, the fuel may not be pure hydrogen and the oxidant may not be air. In case of electrolyzer, the reactant may not be pure water. But rather it can be tap water or water solution of other substance.  
         [0054]    It is to be anticipated that those having ordinary skill in this art can make various modification to the embodiments disclosed herein. However, these modifications should be considered to fall under the protection scope of the invention as defined in the following claims.