Patent Publication Number: US-9847535-B2

Title: Fuel cell stack with enhanced freeze-thaw durability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/078,168 filed in the United States Patent and Trademark Office on Apr. 1, 2011, which claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2010-0122439 filed Dec. 3, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     (a) Technical Field 
     The present invention relates to a fuel cell stack with enhanced freeze-thaw durability. More particularly, it relates to a fuel cell stack that is designed to reduce contact resistance in a fuel cell to prevent water that is a by-product of an electrochemical reaction from being frozen under a sub-zero temperature condition when reactant gases such as hydrogen and oxygen gases are supplied to the fuel cell stack. 
     (b) Background Art 
     Polymer electrolyte membrane fuel cells (PEMFCs) have been widely used as a fuel cell for a vehicle. For a fuel cell stack, manufactured by stacking hundreds of unit cells of the polymer electrolyte membrane fuel cell, to be properly mounted in a vehicle, it is essential that it shows high power performance of at least tens of kilowatts (kW), and thus requires stable operation in a wide range of current density. 
     In a reaction for generating electricity in a fuel cell, after hydrogen supplied to the anode at which oxidation occurs in a membrane-electrode assembly (MEA) of the fuel cell is divided into hydrogen ions (protons) and electrons, hydrogen ions move to the cathode at which reduction occurs through a polymer electrolyte membrane, and electrons move to the cathode through an external circuit. Also, in the cathode, oxygen molecules, hydrogen ions, and electrons react with each other to generate electricity and heat and water as a by-product. 
     If a suitable amount of water is generated from the electrochemical reaction in the fuel cell, the generated water may serve to maintain suitable humidity conditions for the membrane-electrode assembly. However, if the amount of water generated is excessive, the excessive water may not be removed at a high current density, thus causing flooding of water throughout the cell. The flooding may prohibit reactant gases from being efficiently supplied to the fuel cell, thus deepening a voltage loss. 
     Water generates from the reaction between hydrogen and oxygen in the air in the polymer electrolyte membrane fuel cell. If the freeze-thaw cycle is repetitively changed from a sub-zero temperature to an ordinary temperature, components of the fuel cell and interfaces between the components such as an MEA and a gas diffusion layer (GDL) may be physically damaged thereby reducing its electrochemical performance and durability. Therefore, for the stable operation of a hydrogen fuel cell vehicle, it is crucial to increase the durability of a fuel cell stack under such a freeze-thaw cycle condition. 
     Various attempts have been conducted to increase the freeze-thaw durability of a typical fuel cell. For example, Korean Pat. No. 10-0802749, registered in 2008, discloses a technology of increasing the durability by optimizing a fuel cell cooling line structure to reduce the freeze-thaw cycle. U.S. Pat. Application Publication Nos. 2010/0143813 and 2008/0102326 disclose technologies of increasing freeze start capability by optimizing a method for controlling operation of a fuel cell. Also, U.S. Pat. Application Publication No. 2008/0241608 discloses a method of operating a fuel cell by removing ice generated at a sub-zero temperature by heat. However, these methods are too complex to apply in reality, and their effects are also limited. Accordingly, for a mass production of hydrogen fuel cell vehicles, it is necessary to develop a new technology to improve the freeze-thaw durability while at the same time making the implementation process as simple as possible. 
     As commercialization of fuel cells progresses, much research and development is being conducted on a gas diffusion layer (GDL) that is an essential component for managing water in a fuel cell. A GDL is attached to the outer surface of anode and cathode catalyst layers in an MEA of a fuel cell to perform various functions such as supply of reactant gases (hydrogen and oxygen gases in the air), transport of electrons generated from an electrochemical reaction, and minimize flooding in the fuel cell by discharging water generated from the reaction. 
     A GDL has been currently commercialized has a dual layer structure of a microporous layer (MPL) and a macro-porous substrate (or backing). The MPL has a pore size of less than about 1 μm when measured by a mercury intrusion method. The macro-porous substrate, on the other hand, has a pore size of about 1 μm to about 300 μm. 
     The MPL of the GDL may be manufactured by mixing carbon power such as acetylene black carbon and black pearls carbon with hydrophobic agent based on polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (PEP), and then may be coated on one or both surfaces of the macro-porous substrate according to applications. On the other hand, the macro-porous substrate of the GDL may be typically formed of carbon fiber and hydrophobic agents based on PTFE or PEP, and may include carbon fiber cloth, carbon fiber felt, and carbon fiber paper. 
     Since the GDL for the fuel cell has to be designed to have appropriate performance according to operation conditions and specific application fields of the fuel cell for, e.g., transportation, portable, and residential power generation, the GDL based on either carbon fiber felt or carbon fiber paper (in which overall characteristics such as supply of reactant gas, discharge of generated water, and compressibility/handling property for stack assembly are excellent), is more widely used for a fuel cell vehicles than carbon fiber cloth. 
     Also, a GDL has a significant influence on performance of a fuel cell according to various characteristics such as gas permeability, compressibility, degree of hydrophobicity of MPL and macro-porous substrate, structure of carbon fiber, porosity/pore distribution, tortuosity of pore, electrical resistance, and bending stiffness. Particularly, the GDL has a significant influence on the performance in the mass transport zone. 
     The gas diffusion layer needs to show excellent performance in a fuel cell, and have appropriate stiffness for excellent handling properties when hundreds of unit cells are assembled into a fuel cell stack. On the other hand, when the stiffness of the gas diffusion layer is too high in a direction of a roll, the gas diffusion layer is difficult to store in a roll form, thus reducing its mass-productivity capabilities. 
     Alternatively as noted above, when the stiffness of a gas diffusion layer  106  is deficient in a fuel cell, as shown in  FIG. 1 , the GDL  106  may intrude into a flow field channel  202  of a bipolar plate (also called as a separator)  200  (thus causing GDL intrusion). Thus, when the GDL  106  intrudes into the flow field channel  202  of the bipolar plate  200 , a channel space for transferring materials such as reactant gases and generated water may not have enough room. Also, since the contact resistance between the GDL  106  and the rib (or land)  204  of the bipolar plate  200  and between the GDL  106  and an MEA  100  increases, the performance of the fuel cell may be considerably reduced. 
     Particularly, when the contact resistance in a cell increases, an interface between the GDL and the MEA or between the GDL and the bipolar plate may not be suitably maintained to generate an unnecessary gap. In this case, water generated in the fuel cell may be frozen to ice in the unnecessary gap under a freeze-thaw condition. 
     Thus, when there is ice generation, repetitive freeze-thaw cycles may damage the interface between components in the fuel cell. Accordingly, in order to increase the durability of a fuel cell, it is important to reduce the contact resistance so as not to generate a gap at the interface among the components of the fuel cell. 
     Generally, a bipolar plate for a fuel cell includes a major flow field and a minor flow field. Here, it is necessary for a GDL not to intrude into a channel of the major flow field. Therefore, it is important to increase the stiffness of the GDL oriented in the width (W) direction which is perpendicular to the major flow field direction of the bipolar plate than that oriented in the length (L) direction which is parallel with the major flow field direction of the bipolar plate (see  FIGS. 2 and 3 ). Otherwise, as shown in  FIG. 1 , when a GDL having a low stiffness is arranged in the width direction of the major flow field of the bipolar plate, the GDL may further intrude into the major flow field channel of the bipolar plate. Accordingly, since a space in which ice may be generated at a sub-zero temperature (due to increase of damage or deformation of the interface in the fuel cell) increases, the freeze-thaw durability of the fuel cell may be reduced. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE DISCLOSURE 
     The present invention provides a fuel cell stack that can reduce contact resistance in a fuel cell and improve freeze-thaw durability, using a GDL that is manufactured by using a typical art stack fabrication process without an additional modification of the process and optimizing a method of cutting the GDL in a sheet size appropriate to the fuel cell, i.e., a GDL in which its stiffness in a width direction perpendicular to a major flow field direction of a bipolar plate is increased by cutting a rolled GDL material at a certain angle such that a machine direction (MD) or high stiffness direction of GDL roll is not in parallel with the major flow field direction of the bipolar plate in order to minimize the intrusion of the GDL into a gas channel of the bipolar plate. 
     In one aspect, the present invention provides a fuel cell stack with enhanced freeze-thaw durability, the fuel cell stack including a GDL between an MEA and a bipolar plate, wherein the GDL has a structure that reduces contact resistance in a fuel cell and the GDL has a stiffness in a width direction perpendicular to a major flow field direction of a bipolar plate that is increased by cutting a rolled GDL material at a certain angle such that the MD (high stiffness direction) of GDL roll is not in parallel with the major flow field direction of the bipolar plate. 
     In some embodiments of the present invention, the GDL may be cut such that an angle between the MD (high stiffness direction) of the GDL roll and the major flow field direction of the bipolar plate is greater than about 0 degree, and equal to or smaller than about 90 degrees. 
     In another embodiment, the GDL may be cut such that an angle between the MD (high stiffness direction) of the GDL roll and the major flow field direction of the bipolar plate is greater than about 25 degrees, and equal to or smaller than about 90 degrees. In still another embodiment, the GDL may have a Taber bending stiffness of the MD (high stiffness direction) of the GDL roll that ranges from about 20 g f ·cm to about 150 g f ·cm. 
     In yet another embodiment, the GDL may have a Taber bending stiffness of the MD (high stiffness direction) of the GDL roll that ranges from about 50 g f ·cm to about 100 g f ·cm. 
     In still yet another embodiment, the GDL may include a MPL contacting an outer surface of each electrode of an MEA, and a macro-porous substrate contacting a flow field of the bipolar plate, and the macro-porous substrate may be formed of one of carbon fiber felt and carbon fiber paper or a combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a diagram illustrating intrusion of a GDL into a main flow field of a bipolar plate by bipolar plate land compression when fuel cells are assembled to each other in a typical stack; 
         FIG. 2  is a diagram illustrating a method (90° GDL) of cutting the sheet of a GDL according to an exemplary embodiment of the present invention, compared to a method (0° GDL) of cutting the sheet of a GDL according to a related art; 
         FIG. 3  is a diagram illustrating (a) an arrangement of a GDL&#39;s MD (high stiffness direction) and a major flow field direction of the bipolar plate in a 0° GDL-applied stack according to related art and (b) an arrangement of a GDL&#39;s MD (high stiffness direction) and a major flow field direction of the bipolar plate in a 90° GDL-applied stack according to an exemplary embodiment of the present invention; 
         FIG. 4  is a scanning electron microscope (SEM) photograph (500×) illustrating a macro-porous substrate of the GDL used in an exemplary embodiment of the present invention and in the related art; 
         FIG. 5  is a graph illustrating the electrochemical performances before and after 1000 freeze-thaw cycles of the 0° GDL-applied stack according to a related art and the 90° GDL-applied stack according to an exemplary embodiment of the present invention; 
         FIG. 6  is graphs illustrating the cell voltage decay as a function of number of freeze-thaw cycles of the 0° GDL-applied stack according to a related art and the 90° GDL-applied stack according to an exemplary embodiment of the present invention, in which graph (a) shows a result at a current density of 800 mA/cm 2  and graph (b) shows a result at a current density of 1,400 mA/cm 2 ; 
         FIG. 7  is a graph illustrating the high frequency resistances before and after 1000 freeze-thaw cycles of the 0° GDL-applied stack according to a related art and the 90° GDL-applied stack according to an exemplary embodiment of the present invention; 
         FIG. 8  is graphs illustrating the increase of high frequency resistance as a function of number of freeze-thaw cycles of the 0° GDL-applied stack according to a related art and the 90° GDL-applied stack according to an exemplary embodiment of the present invention, in which graph (a) shows a result at a current density of 800 mA/cm 2  and graph (b) shows a result at a current density of 1,400 mA/cm 2 . 
     
    
    
     Reference numerals set forth in the Drawings includes reference to the following elements as further discussed below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 100: MEA 
                 106: GDL 
               
               
                   
                 200: bipolar plate 
                 202: bipolar plate channel 
               
               
                   
                 204: bipolar plate land 
               
               
                   
                   
               
            
           
         
       
     
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION 
     Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
     Hereinafter, exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     The present invention provides a fuel cell stack with enhanced freeze-thaw durability by reducing generation of ice in a fuel cell through application of a GDL that can minimize contact resistance in the cell under a fuel cell freeze-thaw cycle condition. 
     The inherent anisotropic characteristics of a gas diffusion layer may be used to reduce the contact resistance in a fuel cell. 
     In a conventional manufacturing process, in a GDL including carbon fiber felt or carbon fiber paper that is widely used for a fuel cell vehicle as a macro-porous substrate, the carbon fiber may be preferentially oriented in one direction (i.e., MD) to have mechanical properties such as bending stiffness and tensile stress greater than those in the other direction (i.e., cross-machine direction (CMD), also called as transverse direction (TD)). Accordingly, the MD of fabrics of a GDL roll may usually be a high stiffness direction, and the CMD may be a low stiffness direction. 
     In the fuel cell stack according to the exemplary embodiment of the present invention, an angular range between the MD (high stiffness direction) of the GDL roll and the major flow field direction of the bipolar plate may be greater than about 0 degrees, and equal to or smaller than about 90 degrees. More preferably, the GDL roll may be cut such that the angular range becomes greater than about 25 degrees, and equal to or smaller than about 90 degrees. 
     More specifically, as shown in a related art of  FIG. 3A , when a GDL roll is cut, it may be cut such that the MD of the GDL runs parallel to the major flow field direction of the bipolar plate. However, as shown in an embodiment of  FIG. 3B , a GDL may be cut such that the MD of the GDL does not run parallel to the major flow field direction of the bipolar plate. Accordingly, the stiffness of the GDL may increase in the width (W) direction that crosses the major flow field direction of the bipolar plate. 
     It has been described as an example that the cutting of the GDL roll is performed such that the MD of GDL roll and the major flow field direction of the bipolar plate cross each other at an angle equal to or smaller than about 90 degrees. For example, the GDL roll may be cut such that the two directions cross each other at an angle of about 30 degrees, 45 degrees, and 60 degrees in order to increase the freeze-thaw durability of the fuel cell stack. 
     Thus, the GDL intrusion into gas channels of the bipolar plate can be reduced and an unnecessary gap in which water is frozen into ice can be reduced at an interface between the GDL and the MEA or the GDL and the bipolar plate. Accordingly, the freeze-thaw durability of the fuel cell stack can be improved. 
     The Taber bending stiffness of the MD (high stiffness direction) of the GDL roll may range from about 20 g f ·cm to about 150 g f ·cm, and in some instances from about 50 g f ·cm to about 100 g f ·cm. If the Taber bending stiffness is smaller than about 20 g f ·cm, the stiffness is too small for the GDL to be used for a fuel cell vehicle. If the Taber bending stiffness is greater than 150 g f ·cm, the GDL becomes so stiff that the GDL cannot be stored in a roll form, thus causing a reduction in the mass-productivity of GDL. 
     Also, the macro-porous substrate of the GDL mounted in the stack according to an exemplary embodiment of the present invention may be configured with carbon fiber felt, carbon fiber paper, or a combination thereof. Thus, since the MD (high stiffness direction) of the GDL and the major flow field direction of the bipolar plate are not parallel to each other, and carbon fiber felt or carbon fiber paper is used as a macro-porous substrate of the GDL, the contact resistance of the fuel cell can be reduced, and the interface between components of the fuel cell can be suitably maintained, thereby minimizing ice generation. 
     That is, the decrease in performance of the fuel cell due to increase in the contact resistance can be reduced by decreasing the contact resistance between the GDL and the bipolar plate land, or between the GDL and the MEA. Also, since the interface between the GDL and MEA, or between the GDL and the bipolar plate can be suitably maintained, a gap in which generated water is frozen into ice may be significantly reduced, thereby improving the freeze-thaw durability. 
     The fundamental characteristics of the carbon fiber felt-type GDL used in the present embodiment will be described in Table 1 below. The macro-porous substrate may be configured with typical carbon fiber felt as shown in  FIG. 4  that is a magnified view of about 500× by a scanning electron microscope. It can be seen that the carbon fibers are irregularly entangled. 
     As described in Table 1 below, the bending stiffness of the GDL has been measured with respect to MD and CMD at a bending angle of 15 degrees using a Taber Industries Stiffness Tester. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Type of Macro- 
                   
                 Weight  
                 Bending Stiffness 
               
               
                 Porous 
                 Thickness 
                 per Unit 
                 [Taber Stiffness Unit, g f  · cm] 
               
            
           
           
               
               
               
               
               
            
               
                 Substrate 
                 [μm] 
                 Area [gm −2 ] 
                 MD 
                 CMD 
               
               
                   
               
               
                 Carbon Fiber 
                 426 ± 10 
                 135 ± 2 
                 64.87 ± 6.97 
                 12.62 ± 0.38 
               
               
                 Felt 
               
               
                   
               
            
           
         
       
     
     Hereinafter, an embodiment and a test example of the present invention will be described in further detail. 
     Embodiment 
     As an embodiment of the present, a GDL roll was cut such that the MD (high stiffness direction) of the GDL roll is perpendicular to the major flow field direction of the bipolar plate (cutting angle of about 90 degrees). The GDLs were assembled into a 5-cell stack together with overall components such as MEAs, metallic bipolar plates, end plates and other assembly members. 
     Comparative Example 
     As a comparative example, a GDL roll was cut such that the MD (high stiffness direction) of the roll runs parallel to the major flow field direction of the bipolar plate (cutting angle of about 0 degree). The GDLs were assembled into a 5-cell stack together with overall components such as MEAs, metallic bipolar plates, end plates and other assembly members. 
     Test Example 
     Electrochemical performance of the GDLs according to the embodiment and the comparative example was tested. That is, the electrochemical performance of fuel cell stacks including the GDLs according to the exemplary embodiment and the comparative example was compared by measuring the current-voltage (I-V) polarization characteristics based on a 5-cell stack. A typical commercialized tester was used as a tester for measuring electrochemical cell performance. 
     In this case, the test of the electrochemical performance of the fuel cell stacks having the GDLs according to the illustrative embodiment of the present invention and the comparative example were performed under the following conditions.
         Temperature at the inlet of the fuel cell=65° C.,   Hydrogen anode/air cathode relative humidity (RH)=50%/50%,   Hydrogen anode/air cathode stoichiometric ratio (S.R.)=1.5/2.0       

     A freeze-thaw cycle condition applied to the exemplary embodiment of the present invention and the comparative example includes putting a five-cell stack into an environmental chamber in which the temperature is adjustable, repeating 1,000 cycles at chamber temperatures between −25° C. and 15° C., and measuring and comparing the electrochemical performance and high frequency resistance (HFR) of a stack for every 50 cycles. Here, the HFR that was measured may be a factor representing contact resistance in the cell. If the HFR increases, an interface between components is damaged or deformed, and contact becomes poor. In this instance, the HFR was measured by a typical commercialized tester under a condition of amplitude of about 5 A and frequency of about 1 kHz. 
     Test results of the electrochemical performance of the fuel cell stacks including the GDLs according to the illustrative embodiment of the present invention and the comparative example are shown in  FIGS. 5 through 8 . 
     The electrochemical performance of the stacks having the GDLs according to the present embodiment (90° GDL) and the related art (0° GDL) was compared with each other after completion of 1,000 freeze-thaw cycles. As shown in  FIG. 5 , the electrochemical performance of the stacks of the embodiment and the comparative example were both reduced. On the contrary, the stack including the GDL according to the present embodiment showed a greater increase in electrochemical performance than the stack including the GDL according to the related art after both 0 freeze-thaw cycles and 1,000 freeze-thaw cycles. Further, the same was true for the performance decay rate. 
     For the quantitative evaluation of the electrochemical performance decay rate of the stacks as a function of the number of the freeze-thaw cycles, a medium current density of about 800 mA/cm 2  and a high current density of about 1,400 mA/cm 2  were selected as representative fuel cell operation conditions. Cell voltage drops at the above current densities were compared to each other. As shown in  FIG. 6A , when the current density was 800 mA/cm 2 , the cell voltage of the stack including the GDL according to the related art (0° GDL) decreased at a rate of about −38 μV/cycle, but the cell voltage of the stack including the GDL according to the present embodiment (90° GDL) decreased at a rate of about −27 μV/cycle. Accordingly, it can be seen that the performance of the cell stack according to the present invention was more gradually decreased than that of the cell stack according to the related art. 
     As shown in  FIG. 6B , when the current density was 1,400 mA/cm 2 , the cell performance decay rate increased. The cell voltage of the stack including the GDL according to the related art decreased at a rate of about −109 μV/cycle, but the cell voltage of the stack including the GDL according to the present embodiment decreased at a rate of about −66 μV/cycle. Accordingly, it can be seen that the performance of the cell stack according to the present invention was more gradually decreased at the high current density as well. 
     When changes in the contact resistance in the cell of the fuel cell stack were compared to each other, the HFRs with respect to the stack according to the comparative example of the related art and the stack according to the present embodiment were compared to one another after 1,000 freeze-thaw cycles were completed. As shown in  FIG. 7 , the HFRs increased in both the stack according to the present invention and the stack according to the related art after 1,000 freeze-thaw cycles. 
     However, the HFR with respect to the stack according to the present embodiment was smaller than the stack according to the stack according to the related art after both 0 cycles and 1,000 cycles. This means that the contact state between components in the stack cell of the present embodiment is greater than that in the stack cell according to the related art. Accordingly, it is very unlikely that ice may be generated at interfaces between components in the stack according to the present embodiment at a sub-zero temperature, and thus the cells may be less damaged under freezing conditions. 
     Additionally, the HFR increase rates as a function of the number of the freeze-thaw cycles were quantitatively measured. As shown in  FIG. 8A , when the current density was 800 mA/cm 2 , the HFR of the stack according to the related art increased at a rate of about 43 μΩcm 2 /cycle, but the HFR of the stack according to the present embodiment increased at a rate of 34 μΩcm 2 /cycle. Accordingly, it can be seen that the HFR of the stack according to the present embodiment increased more gradually than that of the stack according to the related art. 
     As shown in  FIG. 8B , when the current density is about 1,400 mA/cm 2 , the HFR increase rate increases in both stacks. The HFR of the stack according to the related art increased at a rate of about 50 μΩcm 2 /cycle, but the HFR of the stack according to the present invention increased at a rate of about 38 μΩcm 2 /cycle. Accordingly, it can be seen that the HFR of the stack according to the present embodiment increased more gradually at the high current density. 
     For reference, the decay rate of the cell voltage and the increase rate of the HFR after 1,000 freeze-thaw cycles are summarized as shown in Table 2 below. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Cell Voltage Decay Rate 
                 HFR Increase Rate  
               
               
                   
                 [μV/cycle] 
                 [μΩ cm 2 /cycle] 
               
            
           
           
               
               
               
               
               
            
               
                   
                 800 
                 1,400 
                 800 
                 1,400 
               
               
                 Type of Stack 
                 mAcm −2   
                 mAcm −2   
                 mAcm −2   
                 mAcm −2   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0° GDL Stack 
                 −38 
                 −109 
                 43 
                 50 
               
               
                 according to 
                   
                   
                   
                   
               
               
                 Related Art 
                   
                   
                   
                   
               
               
                 90° GDL Stack 
                 −27 
                 −66 
                 34 
                 38 
               
               
                 according to 
                   
                   
                   
                   
               
               
                 Present 
                   
                   
                   
                   
               
               
                 Invention 
               
               
                   
               
            
           
         
       
     
     As described above, compared to the stack including the GDL (the MD of the GDL roll is parallel to the major flow field direction of the bipolar plate (cutting angle is 0 degree)) according to the related art, the stack including the GDL (the MD of the GDL roll is perpendicular to the major flow field direction of the bipolar plate (cutting angle is 90 degrees) according to the present embodiment has higher electrochemical performance, slower performance decay rate, smaller contact resistance in the cell, and a slower contact resistance increase rate during the freeze-thaw cycle. Accordingly, since the probability that ice is generated at interfaces in the cell is low, the freeze-thaw durability can be improved. 
     According to an embodiment of the present invention, the fuel cell stack with the GDL in which its stiffness in a width direction perpendicular to a major flow field direction of a bipolar plate is increased by cutting a rolled GDL material at a certain angle such that a MD (high stiffness direction) of GDL roll is not in parallel with the major flow field direction of the bipolar plate can have enhanced durability under freeze/thaw cycling conditions. Also, the freeze-thaw durability may be improved by reducing generation of ice at interfaces in the fuel cell. 
     The invention has been described in detail with reference to embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.