Abstract:
A process for fabricating a unitized electrode assembly for a polyelectrolyte membrane is disclosed. The process includes providing a gas diffusion medium and a membrane electrode assembly, printing an adhesive on the gas diffusion medium, locating the gas diffusion medium relative to the membrane electrode assembly and pressing the gas diffusion medium and the adhesive against the membrane electrode assembly.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to fuel cells which generate electricity to power vehicles or other machinery. More particularly, the present invention relates to a novel fabrication process for a unitized electrode assembly (UEA) to improve tolerances and performance of the UEA in a fuel cell.  
       BACKGROUND OF THE INVENTION  
       [0002]     A membrane electrode assembly (MEA) for a fuel cell includes an ionomer membrane which is coated with catalyst layers on opposite sides. Gas diffusion medium (GDM) layers are attached to or abut against the respective catalyst layers. In the fuel cell, a bipolar plate having flow field channels abuts against each GDM layer for the flow of reaction and by-product gases.  
         [0003]     For multiple reasons, it is preferred to integrate the catalyst layers and the ionomer membrane of the MEA with the gas diffusion medium (GDM) layers into a single component which is known as a unitized electrode assembly (UEA). As the GDM layers are attached to the MEA, several criteria must be met. First, the attachment process must not alter the properties of the ionomer membrane or catalyst. This requires precise control of the bonding temperature and pressure.  
         [0004]     Second, the attachment process must not alter the properties of the GDM layers. This requires avoidance of gross mistakes such as denting, scratching and tearing of the GDM, as well as precise control of attachment process parameters such as the magnitude of pressure applied to the GDM. Third, the interface between the GDM and each catalyst layer must remain unaffected by the attachment process.  
         [0005]     Fourth, if adhesive is used to attach the GDM to the MEA, the adhesive must not adversely affect GDM performance. Fifth, the GDM must be precisely positioned with respect to the MEA during attachment. If the attachment process is capable of only marginal tolerances, the UEA will cause an overall fuel cell size increase. Additionally, there are performance and stability issues associated with large tolerances in GDM placement.  
         [0006]     There are several important considerations which relate to the application of adhesive to the GDM. One of these considerations is the need to facilitate precise positioning of the adhesive on the GDM, since such positioning can affect the performance, size and durability of the fuel cell. Another important consideration involves the need to apply a layer of adhesive having minimal thickness such that compression of or local stress concentration in the active area of the MEA is avoided. Furthermore, the adhesive must be compatible with a fuel cell environment and must not introduce any contaminants to the MEA or GDM.  
         [0007]     Therefore, a fabrication process for a UEA is needed which satisfies the above-mentioned criteria and results in a UEA that is characterized by enhanced tolerances and performance in a fuel cell.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is generally directed to a novel fabrication process for a unitized electrode assembly (UEA) to improve tolerances and performance of the UEA in a fuel cell. The fabrication process includes precisely applying an adhesive to a gas diffusion medium (GDM) using a silkscreen or other printing method, precisely locating the GDM relative to a membrane electrode assembly (MEA), and lightly pressing the GDM onto the MEA without the use of excessive temperature or pressure.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0010]      FIG. 1  is a flow diagram which illustrates sequential process steps carried out according to a typical embodiment of the UEA fabrication process of the present invention;  
         [0011]      FIG. 2A  is a side view of a gas diffusion medium (GDM), illustrating the printing of an adhesive onto the GDM;  
         [0012]      FIG. 2B  is a side view of a MEA and a GDM, both of which are mounted in an assembly fixture (shown in phantom), and illustrating alignment of the GDM with the MEA prior to applying the GDM to the MEA; and  
         [0013]      FIG. 2C  is a side view of a UEA after the GDM is pressed onto the MEA. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     The present invention contemplates a novel unitized electrode assembly (UEA) fabrication process which results in a UEA that is characterized by improved tolerances and performance in a fuel cell. According to the UEA fabrication process of the invention, an adhesive is printed on a gas diffusion medium (GDM) using a silkscreen or other printing method. The GDM is then precisely located relative to a membrane electrode assembly (MEA), and then lightly pressed onto the MEA without the use of excessive temperature or pressure. The UEA fabrication process can be carried out as a batch process, although the process is amenable to high volume, continuous process production.  
         [0015]     The UEA fabrication process is carried out in a clean room environment. Dust, dirt, metal filings, fibers and the like must be prevented from contacting the MEA or GDM during the entire process. Any debris present on the MEA or GDM during the process must be removed prior to UEA assembly.  
         [0016]     Referring to the flow diagram of  FIG. 1 , in conjunction with  FIGS. 2A-2C , the UEA fabrication process is begun by precisely cutting the GDM to the desired dimensions, as indicated in step  1  of  FIG. 1 . The cutting step will facilitate precise location of an adhesive on the GDM in a subsequent processing step which will be hereinafter described. Preferably, the cutting step is implemented outside a clean room environment, since the cutting process introduces particulate debris from the GDM which is being cut, into the environment. The cut GDM is typically then subjected to flowing air to remove extraneous debris therefrom.  
         [0017]     As indicated in step  2  of  FIG. 1 , and as shown in  FIG. 2A , the GDM  18  is next placed in a fixture  26  of a silkscreen printer, for example. The fixture  26  properly locates the GDM  18  in the printer for precise application of adhesive  19  to the GDM  18 . Next, the silkscreen printer applies a thin layer of the adhesive  19  to the GDM  18  (step  3 ) in a precisely-controlled pattern, typically along the edges of the GDM  18 . The screen mesh size is a design parameter that is used to control the thickness and distribution of the adhesive  19  on the GDM  18 . Preferably, the adhesive  19  is uniformly distributed in the desired area of the GDM  18 , without voids or excessive thickness. The adhesive  19  should not be more than 25 um thick.  
         [0018]     The adhesive application step may be carried out as a batch printing process using the silkscreen printer, as described above. However, it is to be understood that the adhesive application step is not limited to silkscreen printing or batch printing. The adhesive  19  can be applied to the GDM  18  in a precisely-controlled pattern using a variety of techniques known to those skilled in the art, including but not limited to continuous process printing and decal transfer processes, for example.  
         [0019]     After the adhesive  19  is applied to the GDM  18 , the GDM  18  is removed from the fixture  26  of the silkscreen printer and placed in an assembly fixture  20 , as shown in  FIG. 2B  and indicated in step  4  of  FIG. 1 . An MEA  12 , having an ionomer membrane  14  which is coated on both sides with a catalyst layer  16 , is also placed in the assembly fixture  20 . The GDM  18  is located on a vacuum support  22 , and the MEA  12  is located on a vacuum support  24  which is spaced-apart from the vacuum support  22 , of the assembly fixture  20  using precise datums (step  5  in  FIG. 1 ). Once the GDM  18  is precisely located on the vacuum support  22  and the MEA  12  is precisely located on the vacuum support  24 , vacuum pressure is applied by the vacuum supports  22 ,  24  to hold the GDM  18  and MEA  12 , respectively, in place. In the assembly fixture  20 , the adhesive  19 , which was previously applied to the GDM  18  in step  3 , faces the catalyst layer  16  on the MEA  12  to which the GDM  18  is to be attached.  
         [0020]     Precise location of the GDM  18  and MEA  12  in the assembly fixture  20  requires control of the temperature and humidity of the clean room. The size of the MEA  12  will change as a function of the ambient humidity and temperature. Thus, excessive levels of humidity or temperature in the clean room environment will render impossible alignment of the GDM  18  with respect to the MEA  12  to within narrow tolerances. The temperature and humidity ranges which are permissible for the clean room environment during this step of the UEA fabrication process will depend on the particular type of GDM and MEA used. Ambient humidity and temperature levels which are optimum for the UEA fabrication process are about 42±7% relative humidity and about 21±2 degrees C. Moreover, the magnitude of vacuum pressure applied to the GDM  18  by the assembly fixture  20  must be adjusted according to the properties of the GDM  18 , since various types of GDM  18  exhibit variations in gas permeation rates, and therefore, require various magnitudes of vacuum pressure to hold them in place.  
         [0021]     Due to the fragility of the ionomer membrane  14  in the MEA  12 , vacuum pressure should not be applied to active areas of the ionomer membrane  14  during positioning of the MEA  12  in the assembly fixture  20 . Otherwise, the membrane  14  has a tendency to wrinkle or crease the catalyst layers  16 . Therefore, it is necessary to adjust both the position of vacuum openings (not shown) in the vacuum support  24  with respect to the MEA  12 , as well as the magnitude of vacuum pressure applied by the vacuum support  24  against the MEA  12 , based on the architecture of the MEA  12 .  
         [0022]     After the GDM  18  and MEA  12  have been precisely aligned in the assembly fixture  20 , a UEA  10  is formed by pressing the GDM  18  onto the MEA  12  by operation of the assembly fixture  20 , as shown in  FIG. 2C  and indicated in step  6  of  FIG. 1 . Depending on the tooling, this step can be carried out either in a continuous process or on a part-by-part basis. Three key controls are necessary for proper execution of this press-attachment step. First, the magnitude of vacuum pressure exerted on the GDM  18  and MEA  12 , respectively, by the assembly fixture  20  must be timed appropriately with the pressing step. Preferably, automated control is used to achieve greater process control on both vacuum timing and press dwell times. Automated control of the vacuum timing and press dwell time will ensure that UEA and its components are transferred at the appropriate time from one process step to the next. If manual control is used, one could envision a scenario where human error could cause the vacuum pressure to shut off prematurely which may cause the UEA to have GDM location issues.  
         [0023]     Second, the magnitude of pressure used to press the adhesive-coated GDM  18  to the MEA  12  must be tightly controlled. If too little pressure is used, the attachment bond will not be sufficiently robust to survive handling of the assembled UEA  10  during subsequent fabrication of a fuel cell. If excessive pressure is used, on the other hand, a permanent compression set will be induced in the GDM  18 . This will compromise the functional capacity of the GDM  18  in the assembled fuel cell. The target pressure used to press the adhesive-coated GDM  18  to the MEA  12  is dependent on the type of GDM used. Third, the temperature and humidity of the clean room in which the press-attachment step is carried out must be controlled. Preferably, the relative humidity of the clean room is maintained at about 42±7% and the temperature is maintained at about 21±2 degrees C.  
         [0024]     After the GDM  18  is pressed onto the MEA  12 , the fabricated UEA  10  is removed from the assembly fixture  20 . As indicated in step  7  of  FIG. 1 , the assembled UEAs  10  are placed in airtight bags (not shown) to prevent contamination and retain dimensional stability of the UEAs  10  pending assembly of the UEAs  10  into fuel cells.  
         [0025]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.