Patent Publication Number: US-2023155152-A1

Title: Fuel cell device comprising a membrane electrode assembly

Description:
RELATED APPLICATION 
     This application claims the benefit of priority of Europe Patent Application No. 21207900.8 filed on Nov. 12, 2021, the contents of which are incorporated by reference as if fully set forth herein in their entirety. 
     FIELD AND BACKGROUND OF THE INVENTION 
     The present application relates to a fuel cell device having a stack arrangement configured with a membrane-electrode assembly having a difference in a first and a second gas diffusion layer. 
     The invention is suitable to be integrated in the stack design of a compact fuel cell system, in particular a compact fuel cell for transportation applications having a proton exchange membrane (PEM-FC), or any other type of electrochemical device comprising a stack of cells each including a membrane-electrode assembly to be alternatingly stacked between bipolar plates and accommodating channels or other kinds of pathways for conducting fluids for electrochemical reaction in the cells. 
     For an economical manufacturing and assembling, the design of a fuel cell stack strives for stackable parts identically formed as possible for a repeatedly stacking of so-called unit cells building up the fuel cell stack. Also in terms of an economical manufacturing and assembling, the design strives for a small number of components. Further, in purpose of a compact design each component needs to integrate more than one of all required functionalities. 
     Several compact fuel cell stack designs integrate channels or ports and manifolds for supply of hydrogen and air as well as discharge of exhaust gas and water or wet gas fluid within the stack. It is known to provide so-called transition regions to e.g. two opposite sides of an active region of the cells. Such transition regions provide a fluid communicating interface to the cells, i.e. the membrane-electrode assemblies. The sections of the cell to be addressed for fluid supply or discharge exhibit a flat flow cross-section, and a port design within such transition regions aims to achieve a pre-ferrable uniform mass flow distribution within the flat flow cross-section of the interface for an as possible even fluid distribution into each of channels across a gas diffusion layer (GDL) of the cell (not illustrated). 
     The common stack design is constituted by the membrane-electrode assemblies of each cell to be stacked between bipolar plates providing electrical conductivity between the cells, wherein the bipolar plates have a longer dimension than the active part of the membrane-electrode assemblies (MEA) so as to enclose and protect the latter. In a common compact stack design, such transition regions are provided within outer sections of the bipolar plates extending beyond a dimension of the membrane-electrode assemblies. In this compact stack design, the supply and discharge of fluid in communication with the cells is accommodated in the form of internal channels or pathways of different flow designs integrated in the bipolar plates by pathway cavities divided by channeling plate features, wherein such channels are built by open half cross-sections recessed in surfaces of adjacently stacked bipolar plates or any stacking elements. 
     In a common prior art design, a frame member supporting the membrane-electrode assembly and arranged to mount the latter between the bipolar plates was covered by a pair of the gas diffusion layers of the respective membrane-electrode assembly on both sides for cushioning said frame member against plate features. However, in this case, a problem became apparent that the flow of fluid in the pathway cavities of the bipolar plates was obstructed or at least reduced by the soft open porous material of the gas diffusion layers. The soft material is compressed between the frame member and the plate features so as to deflect or pillow while constituting a restraining intrusion in the open half cross-sections of the pathway cavities between the plate features. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to improve the flow efficiency of the transition regions while not adding additional durability risks associated with impingement damage or leakage or severe flow restriction at interfacing surfaces between stacking elements of a fuel cell stack having integrated internal fluid pathways. It is a further object of the invention to provide a design for a fuel cell stack without raising complexity or manufacturing costs whilst enabling an enhanced durability and operation safety. 
     The object is solved by an electrochemical fuel cell device having the features of claim 1. Further features and details of the invention result from the dependent claims, the specification, and the drawings. 
     A basic core idea underlying the invention relates to a membrane-electrode assembly for an electrochemical fuel cell stack having stacked cells, wherein the membrane-electrode assembly being provided for each cell and comprises: an ion-conducting membrane for conducting ions between two sides of the membrane, catalyst layers or a catalyst coating for electrochemical conversion reactions across each side of the membrane, and a first gas diffusion layer arranged on one side of the membrane and a second gas diffusion layer arranged on the opposite side of the membrane for distributing fluids across each side of the membrane. The first gas diffusion layer is formed to extend across and beyond planar dimensions of the membrane into at least one region adjacently arranged to the planar dimensions of the membrane for covering an interfacing surface in the fuel cell stack. The second gas diffusion layer is formed to extend across the planar dimensions of the membrane without extending beyond, or is formed to extend across and beyond the planar dimensions of the membrane into another one of the at least one region adjacently arranged to the planar dimensions of the membrane for covering an interfacing surface of the fuel cell stack. 
     Realizing the latter membrane-electrode assembly within a stack design results in the invention that proposes: an electrochemical fuel cell device having an electrochemical fuel cell device having a stack arrangement of stacked cells, comprising bipolar plates stacked between each of the cells for providing electrical conduction between the cells. A cross-section of the stack arrangement is divided into an active region for accommodating an electrochemical reaction of the cells, and at least one transition region arranged adjacent to the active region for accommodating pathways for supplying and returning fluids reacting in the cells. Each cell includes a membrane-electrode assembly comprising: an ion-conducting membrane for conducting of ions between two sides of the membrane, catalyst layers or a catalyst coating for electrochemical conversion reactions across each side of the membrane, and a first gas diffusion layer arranged on one side of the membrane and a second gas diffusion layer arranged on the opposite side of the membrane for distributing fluids across each side of the membrane. With respect to the cross-section of the stack arrangement of the fuel cell device, the first gas diffusion layer is formed to extend across the active region and across or into the at least one transition region for covering an interfacing surface of a bipolar plate. The second gas diffusion layer is formed to extend across the active region without extending beyond the active region, or is formed to extend across the active region and across or into another one of the at least one transition region for covering an interfacing surface of a bipolar plate. 
     Hence, for the first time, the invention provides a fuel cell stack design and a single assembly component design, where gas diffusion layers on each side of the membrane differ from each other in their dimensions or extension outside active elements of the cell, and where an extended section of a single gas diffusion layer serves for cushioning a contact in interfacing areas between stacked elements within a respective tran-sition region. 
     As mentioned before, according to this disclosure, the term “transition region” defines areas of the cross-section of the stack provided as a fluid communicating interface to the cells, i.e. the membrane-electrode assemblies, for fluid supply or discharge in a flat flow cross-section. A design within such transition regions aims to achieve a preferra-ble uniform mass flow distribution within a flat flow cross-section for an as possible even fluid distribution between a fluid port and a plurality of small channels across a gas diffusion layer (GDL) of the cell (not illustrated), i.e. within the active region of the stack’s cross section. The design of a transition region for achieving an even distribution across the cell might include an array of pillars resulting in a multidirectional pressure drop, or an aligned bunch of small channels expanding between a fluid port and an interface of channels across resulting in a multidirectional guidance to the flat flow cross-section of the the gas diffusion layer within the cell. 
     As a first advantage, impingement damages or leakages due to contact of sharp edges of the bipolar plates in surface contacts of stacking that should constitute sealing interfaces within the above-described transition regions of the stack arrangement can be avoided by means of the comparably softer material of the gas diffusion layer arranged in between. This holds true also for an arrangement where the frame member still gets in direct contact on one side with one of two facing bipolar plates, since the achieved dampening is sufficient for effectively suppressing peaks of detrimental impingement for both sides of the frame member. 
     As a second advantage, due to the use of a single gas diffusion layer arranged on one side of the cell, a respective fluid flow might be affected merely on one side of the frame member supporting the membrane-electrode assembly, or more particular, open cross-sections of fluid pathways in only one of two bipolar plates might be diminished by material of a gas diffusion layer within the transition region of the stack arrangement. On the other side of the cell, the full flow cross-section of pathways recessed in the respective bipolar plate remains available to an entire depth of profile. The design according to the invention thus provides a reasonable relation between the avoidance of impingement damages or leakage while maintaining decent flow capacities by leaving more pathways unaffected, more particular, leaving on at least one side of the plate all flow cross-sections without any intrusions of compressed material of a gas diffusion layer. Such ratio is achieved while keeping thickness dimensions of the cell or the bipolar plates unamended, i.e. improving compactness. 
     Furthermore, a third advantage is to be seen in the integration of these beneficial properties without the need of manufacturing any further element or substantial modification of a present component. As a result, the beneficial effects according to the invention can be achieved by solely amending a positioning or cutting of two gas diffusion layers with respect to the cell, i.e. the planar dimensions of the membrane or the active region in terms of the fuel cell stack. Hence, the invention might be realized without any substantial expenses on efforts or costs but an implementation of amended sizes or positions of the gas diffusion layers in an existing manufacturing or design of a sandwich structure of a PEM fuel cell. 
     Summerizing the advantages, the invention provides a design having positive effect-son a region where the reactant gas flows are distributed from the fluid ports to the channels of a flow field in the cells. The ability of this region to evenly distribute gases is dependent on its volume. With a membrane electrode assembly (MEA) that has merely one gas diffusion layer instead of two gas diffusion layers in these transition regions, more of the volume in that transition region can be used for distributing gases, since the space otherwise taken up by one of the gas diffusion layers is thus available for distributing gas to channels in the flow field of a cell. This allows for plates to be designed thinner or smaller which improves the power density of the fuel cell stack. On the other hand, since the invention still retains one of the gas diffusion layers in the transition region, and because the gas diffusion layers have a compressible material or at least a porous structure, the one remaining gas diffusion layer allows to still support and protect the membrane electrode assembly against damages by edges of plate features in the case of an over compression in the transition region. 
     According to a preferred embodiment, the first gas diffusion layer and the second gas diffusion layer of the membrane-electrode assembly have different planar dimensions. 
     According to a preferred alternative embodiment, the first gas diffusion layer and the second gas diffusion layer of the membrane-electrode assembly have basically the same planar dimensions, wherein the first gas diffusion layer and the second gas diffusion layer of the membrane-electrode assembly are arranged in shifted positions with respect to the planar dimensions of the membrane. In this way, the same gas diffusion layers having the same cutting dimensions can still be used by only altering a position of same during the assembly. 
     According to an advantageous aspect of the invention, wherein a respective gas diffusion layer section formed to extend beyond the planar dimensions for covering an interfacing surface of the fuel cell stack, includes impregnation or inclusion of a further material, including an elastic material for sealing and/or compensating of tolerances. 
     According to an alternative advantageous aspect of the invention, a respective gas diffusion layer section formed to extend beyond the planar dimensions of the membrane for covering an interfacing surface of the fuel cell stack, is built by another material than a respective gas diffusion layer section extending within planar dimensions of the membrane only, for distributing fluids. 
     In an elaborated embodiment, a respective gas diffusion layer section formed to extend beyond the planar dimensions of the membrane for covering an interfacing surface of the fuel cell stack has another thickness of the same material, particularly is thinner than a respective gas diffusion layer section extending within planar dimensions of the membrane only, for distributing fluids. 
     In an elaborated alternative embodiment, the first gas diffusion layer has another material and/or thickness than the second gas diffusion layer. 
     According to an aspect of the invention relating to the fuel cell device with a stack arrangement divided into an active region and at least one transition region, as well as including ports and pathways for supplying and returning fluids; the fuel cell device preferably also comprises a frame member. Such frame member is overlapping with the membrane-electrode assembly in a peripheral section for supporting the membrane-electrode assembly, and extending outside the peripheral section for fixing the frame member between interfacing surfaces of two bipolar plates; wherein a respective gas diffusion layer section in a respective transition region covers interfacing surfaces between the frame member and one of the two bipolar plates. 
     According to a preferred embodiment, the stack arrangement of the fuel cell comprises at least two transition regions diametrically arranged on opposite sides with respect to the active region; and the first gas diffusion layer extends across the at least two transition regions, and the second gas diffusion layer has smaller planar dimensions and extends into none of the at least two transition regions. 
     According to a preferred alternative embodiment, again, the stack arrangement comprises at least two transition regions diametrically arranged on opposite sides with respect to the active region; and, however, the first gas diffusion layer extends across one of the at least two transition regions, and the second gas diffusion layer is shifted in position so as to extend across another of the at least two transition regions. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further objectives, advantages, features and applications of the present invention arise from the following description of the exemplary embodiments with reference to the drawings. In the drawing: 
         FIG.  1 A  shows a schematic view on an upper side of a membrane-electrode assembly supported in a frame member according to a first embodiment of the present invention; 
         FIG.  1 B  shows a schematic view on a lower side of the membrane-electrode assembly according to the embodiment of first the present invention; 
         FIG.  2 A  shows a schematic view on an upper side of the membrane-electrode assembly supported the frame member according to a second embodiment of the present invention; 
         FIG.  2 B  shows a schematic view on a lower side of the membrane-electrode assembly according to the second embodiment of the present invention; 
         FIG.  3    shows a cross-section of a stacked cell in a stack arrangement of a fuel cell device according to an embodiment of the present invention; and 
         FIG.  4    shows corresponding cross-sections of a stacked cell in prior art designs of a fuel cell stack arrangement. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
       FIGS.  1 A and  1 B  show an upper side and a lower side of a cell comprising a membrane-electrode assembly  20  framed by a frame member  10 , wherein the shape and the perspective on the frame member  10  also represent an overall cross-section of a stack arrangement which is perpendicular to a stacking direction in a fuel cell device  100 . Certain regions marked by dashed lines define for the whole stack arrangement a centrally arranged active region  120  and two transition regions  130  diametrically arranged on opposite sides of the active region  120 . The active region accommodates the cells, or more particularly is occupied by a membrane-electrode assembly (MEA) including all electrochemically active elements of the cells. The transition regions  130  accommodate fluid pathways having a flat shaped flow cross-section for fitting into or between the stacked elements, particularly bipolar plates  30 . Furthermore the transition regions  130  constitute a fluid interface enclosing internal pathways forming a fluid communication between the cells and outer arranged ports  110  being part of an internal manifold or other structures for a supply of air and a hydrogen based fuel gas or for a discharge of exhaust gas and water. One port  110  per side of a cell may also deliver and return a coolant for cooling the cell, however such port is not in fluid communication with the cell  20  via internal pathways enclosed with in one of the transition regions  130 . 
     The frame member  10  illustrated in  FIG.  1 A  and  FIG.  1 B , comprise basically the same outer contours shared with bipolar plates  30  and is mostly formed flat with planar interface surfaces to be stacked properly when facing each other with two facing bipolar plates, as to be seen in  FIG.  3   . The frame member  10  serves for mechanical supporting and mounting the electrochemically active membrane-electrode assembly  20  in the stack arrangement. 
     Within the transition regions  130  of the stack arrangement marked by dashed lines on the bipolar plate in  FIG.  3   , the bipolar plates  30  provide dedicated plate features  32  and fluid pathways  31  recessed therebetween in a surface of the bipolar plates  30 . The plate features  32  and fluid pathways  31  have distinctive shapes for promoting, in a flat flow cross-section, a flow distribution of a fluid to be supplied or discharged throughout a transverse dimension of the membrane-electrode assembly  20  while achieving a preferable even pressure distribution across said flat flow cross-section. Such distinctive shapes for promoting a diffusion of fluid pressure might represent an array of round pillars form arranged in equal distances. However, alternative embodiments might provide another suitable design of plate features  32  for effecting the desired flow dynamics including, for example, plate features  32  in the form of channel walls for a splitting-up channel arrangement or the like. 
       FIG.  3    shows a focussed part of a cross-section of one stacked cell in the stack arrangement of the fuel cell device  100 . This cross-section is parallel to a stacking direction of the stack arrangement and reveals the sandwich structure of the membrane-electrode assembly  20 . The membrane-electrode assembly  20  includes an ion-conductive membrane  23  or proton-exchange membrane (PEM) having a catalytic surface coating on each side. The membrane  23  is embedded between a first gas diffusion layer  21 , also shown from an upper side plan view in  FIG.  1 A , and a second gas diffusion layer  22 , also shown from a lower side plan view in  FIG.  1 B . The two gas diffusion layers  21 ,  22  serve for receiving and distributing or dissipating and leading away a gas or fluid in communication between a transition region  130  and the surface areas of the membrane  23 . 
       FIGS.  1 A and  1 B  show a first embodiment of the invention. It becomes apparent in a comparison of  FIGS.  1 A and  1 B  that the upper arranged first gas diffusion layer  21  has a larger planar dimension than the lower arranged second gas diffusion layer  22 . The first gas diffusion layer  21  covers the active area  120  or the membrane-electrode assembly  20  and reaches out across the transition region  130  to cover an interfacing between the frame member  10  and the upper bipolar plate  30 . On the other side, the second gas diffusion layer  22  covers the active area  120  only. This arrangement corresponds to an axis symmetrical configuration as focussed by  FIG.  3    on both sides of the stack arrangement in the fuel cell device  100 . 
     The illustrated cross-section of  FIG.  3    also shows the before mentioned surfaces and recesses in the profile of the plate features  32  and fluid pathways  31  of an embodiment of the bipolar plates  30  within the transition region  130 . The frame member  10  is fixed to the stack arrangement of the fuel cell device  100  by being clamped between the two adjacently facing bipolar plates  30 , also and particularly within the transition region  130  of the stack arrangement. Areas of extensive partial contact pressure and sharp edges exhibited by the plate features  32  in the profile of the bipolar plates  30  directly cushioned on the upper side of the frame member  10  by means of the first diffusion layer  21  and are at least indirectly dampened to a substantially reduced impact on the lower side of the frame member  10 . 
     Furthermore, flow cross-sections of the fluid pathways  31  recessed in the lower bipolar plate  30  remain unobstructed to a full depth of profile and providing a maximum flow capacity with respect to the thickness dimension of the bipolar plates  30  in the respective design of stack arrangement. With the first gas diffusion layer reaching into the transition region  130  on the upper side only, a small portion of the height of the flow cross-sections of the fluid pathways  31  recessed in the upper bipolar plate  30  (a much smaller portion of height than depicted in the drawings for illustrative reasons) is subjected to intrusion of pillowing material of the gas diffusion layer. Without any elaborated modifications hereto, a slightly reduced flow capacity with respect to the plate thickness is to be expected due to material of the first gas diffusion layer intruding within a potential full depth of profile. The latter issue of a reduced flow capacity might however be already addressed at the stage of plate designing by taking such intrusion of pillowing material into account. 
       FIGS.  2 A and  2 B  show a second embodiment of the invention. It becomes apparent in a comparison of  FIGS.  2 A and  2 B  that the upper arranged first gas diffusion layer  21  and the lower arranged second gas diffusion layer  22  have the same planar dimensions, but are positioned differently with respect to the cell. The first gas diffusion layer  21  covers the active area  120  or the membrane-electrode assembly  20  and reaches out across the transition region  130  illustrated on the left-hand side to cover an interfacing between the frame member  10  and the upper bipolar plate  30 . The second gas diffusion layer  22  covers the active area  120  or the membrane-electrode assembly  20  and reaches out across the transition region  130  illustrated on the right-hand side to cover an interfacing between the frame member  10  and the upper bipolar plate  30 . This arrangement corresponds to point symmetrical configuration as focussed by  FIG.  3    with respect to both sides of the stack arrangement in the fuel cell device  100 . 
       FIG.  4    shows a prior art design where the gas diffusion layers on both sides of the membrane are extended to reach into the transition region. However, in this way, flow cross-sections of all pathways in all bipolar plates are reduced by intrusion of gas diffusion layer material with respect to the potential full depth of plate feature profile on both sides of the plate (please note: realistic depth of intrusion is smaller than depicted in the drawings for illustrative reasons). 
     Furthermore,  FIG.  4    also shows a prior art design where the profile features for diffusion of a fluid are arranged in direct contact on both sides of a MEA frame. Due to a hardness of materials commonly used for the bipolar plates and sharp edges of such profiles, this structure is prone to problems like impingement leaks and excessive wear on interface surfaces, as mentioned in the introductory part of the present disclosure. 
     
       
         
           
               
               
             
               
                 List of Reference Characters 
               
             
            
               
                 
                   10 
                 
                 frame member 
               
               
                 
                   20 
                 
                 membrane-electrode assembly ( 20 ) 
               
               
                 
                   21 
                 
                 first gas diffusion layer 
               
               
                 
                   22 
                 
                 second gas diffusion layer 
               
               
                 
                   23 
                 
                 membrane 
               
               
                 
                   30 
                 
                 bipolar plate 
               
               
                 
                   31 
                 
                 fluid pathway 
               
               
                 
                   32 
                 
                 plate feature 
               
               
                 
                   100 
                 
                 fuel cell device 
               
               
                 
                   110 
                 
                 fluid port 
               
               
                 
                   120 
                 
                 active region 
               
               
                 
                   130 
                 
                 transition region