Manufacture of unitized electrode assembly for PEM fuel cells

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.

FIELD OF THE INVENTION

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

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.

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.

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.

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.

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.

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

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.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

Referring to the flow diagram ofFIG. 1, in conjunction withFIGS. 2A-2C, the UEA fabrication process is begun by precisely cutting the GDM to the desired dimensions, as indicated in step1ofFIG. 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.

As indicated in step2ofFIG. 1, and as shown inFIG. 2A, the GDM18is next placed in a fixture26of a silkscreen printer, for example. The fixture26properly locates the GDM18in the printer for precise application of adhesive19to the GDM18. Next, the silkscreen printer applies a thin layer of the adhesive19to the GDM18(step3) in a precisely-controlled pattern, typically along the edges of the GDM18. The screen mesh size is a design parameter that is used to control the thickness and distribution of the adhesive19on the GDM18, Preferably, the adhesive19is uniformly distributed in the desired area of the GDM18, without voids or excessive thickness. The adhesive19should not be more than 25μm thick.

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 adhesive19can be applied to the GDM18in 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.

After the adhesive19is applied to the GDM18, the GDM18is removed from the fixture26of the silkscreen printer and placed in an assembly fixture20, as shown inFIG. 2Band indicated in step4ofFIG. 1. An MEA12, having an ionomer membrane14which is coated on both sides with a catalyst layer16, is also placed in the assembly fixture20. The GDM18is located on a vacuum support22, and the MEA12is located on a vacuum support24which is spaced-apart from the vacuum support22, of the assembly fixture20using precise datums (step5inFIG. 1). Once the GDM18is precisely located on the vacuum support22and the MEA12is precisely located on the vacuum support24, vacuum pressure is applied by the vacuum supports22,24to hold the GDM18and MEA12, respectively, in place. In the assembly fixture20, the adhesive19, which was previously applied to the GDM18in step3, faces the catalyst layer16on the MEA12to which the GDM18is to be attached.

Precise location of the GDM18and MEA12in the assembly fixture20requires control of the temperature and humidity of the clean room. The size of the MEA12will 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 GDM18with respect to the MEA12to 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 GDM18by the assembly fixture20must be adjusted according to the properties of the GDM18, since various types of GDM18exhibit variations in gas permeation rates, and therefore, require various magnitudes of vacuum pressure to hold them in place.

Due to the fragility of the ionomer membrane14in the MEA12, vacuum pressure should not be applied to active areas of the ionomer membrane14during positioning of the MEA12in the assembly fixture20. Otherwise, the membrane14has a tendency to wrinkle or crease the catalyst layers16. Therefore, it is necessary to adjust both the position of vacuum openings (not shown) in the vacuum support24with respect to the MEA12, as well as the magnitude of vacuum pressure applied by the vacuum support24against the MEA12, based on the architecture of the MEA12.

After the GDM18and MEA12have been precisely aligned in the assembly fixture20, a UEA10is formed by pressing the GDM18onto the MEA12by operation of the assembly fixture20, as shown inFIG. 2Cand indicated in step6ofFIG. 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 GDM18and MEA12, respectively, by the assembly fixture20must 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.

Second, the magnitude of pressure used to press the adhesive-coated GDM18to the MEA12must be tightly controlled. If too little pressure is used, the attachment bond will not be sufficiently robust to survive handling of the assembled UEA10during subsequent fabrication of a fuel cell. If excessive pressure is used, on the other hand, a permanent compression set will be induced in the GDM18. This will compromise the functional capacity of the GDM18in the assembled fuel cell. The target pressure used to press the adhesive-coated GDM18to the MEA12is 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.

After the GDM18is pressed onto the MEA12, the fabricated UEA10is removed from the assembly fixture20. As indicated in step7ofFIG. 1, the assembled UEAs10are placed in airtight bags (not shown) to prevent contamination and retain dimensional stability of the UEAs10pending assembly of the UEAs10into fuel cells.