Abstract:
A fuel cell that includes a blocking agent for preventing hydrogen and air from contacting bare membrane. This in turn prevents the reaction of air and hydrogen gases at outside edges of the catalyst layers. The blocking agent is deposited within diffusion media layers on one or both of the anode and cathode sides of the fuel cell. The blocking agent extends into the diffusion media layers far enough so that it is within outside edges of the catalyst layers. In one embodiment, the blocking agent is a thermoplastic polymer, such as PVDF, that flows into the diffusion media layers in a melted format, where it hardens.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to a fuel cell and, more particularly, to a fuel cell including diffusion media layers having selectively positioned blocking agents that prevent hydrogen gas and oxygen gas from combining and reacting at outside edges of the catalyst layers that might otherwise cause membrane failure.  
         [0003]     2. Discussion of the Related Art  
         [0004]     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today&#39;s vehicles employing internal combustion engines.  
         [0005]     A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode, with the aid of a catalyst, to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons at the cathode, with the aid of a catalyst, to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before arriving at the cathode. The work acts to operate the vehicle.  
         [0006]     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorinated acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).  
         [0007]     Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas, such as air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.  
         [0008]      FIG. 1  is a cross-sectional view of a fuel cell  10  of the type discussed above. The fuel cell  10  includes a cathode side  12  and an anode side  14  separated by an electrolyte membrane  16 . A cathode diffusion media layer  20  is provided at the cathode side  12 , and a cathode catalyst layer  22  is provided between membrane  16  and the diffusion media layer  20 . Likewise, an anode diffusion media layer  24  is provided at the anode side  14 , and an anode catalyst layer  26  is provided between the diffusion media layer  24  and the membrane  16 . The catalysts layers  22  and  26  and the membrane  16  define an MEA. The diffusion media layers  20  and  24  are porous layers that provide for input gas transport to and water transport from the MEA. Various techniques are known in the art for depositing the catalyst layers  22  and  26  on the diffusion media layers  22  and  24 , respectively, or on the proper side of the membrane  16 .  
         [0009]     A bipolar plate  18  including flow fields provides an airflow  36  for the cathode side  12  and an opposing bipolar plate  30  including flow fields provides a hydrogen gas flow  28  for the anode side  14 . The bipolar plates  18  and  30  separate the fuel cells in a fuel cell stack, as is well known in the art. As discussed above, the hydrogen gas flow  28  reacts with the catalyst in the catalyst layer  26  to dissociate the hydrogen ions and the electrons. The hydrogen ions are able to propagate through the membrane  16  where they electrochemically react with the airflow  36  and the return electrons in the catalyst layer  22  to generate water.  
         [0010]     Fuel cells must have a certain durability to be viable in an automotive application or otherwise. It has been observed that the membrane  16  sometimes prematurely fails adjacent to an outside edge  32  and  34  of the catalyst layers  22  and  26 , respectively, thus reducing the fuel cell&#39;s durability and longevity. It is known that the membrane  16  does not possess infinite resistance to gas permeation. It is believed that one or both of the hydrogen gas flow  28  and/or the airflow  36  crosses the membrane  16  outside of the catalyst layer edges  32  and  34 , and reacts with the other hydrogen gas flow  28  or airflow  36  at the catalyst layer edges  32  and  34 . The outside edges  32  and  34  of the catalyst layers  22  and  26  are the first location that the mixture of gases encounters the catalyst.  
         [0011]     This overall reaction is the same reaction as the sum of the half-reactions that occur at the catalyst layers  22  and  26 ; however, none of the energy of this reaction from the gas crossover operates to move electrons through an external circuit. This excess energy manifests itself in the form of heat production. In other words, because it is the air and hydrogen gases that react spontaneously at the outside edges  32  and  34  of the catalyst layers  22  and  26  instead of the oxygen and hydrogen ions, none of the energy produced by the reaction is captured by the external circuit. All the energy that is generated by the gas reaction is converted to heat, which causes a premature failure of the membrane  16  adjacent to the catalyst layer edges  32  and  34 . If the airflow  36  crosses the membrane  16 , H 2 O 2  is formed, which can also chemically degrade the membrane  16 .  
       SUMMARY OF THE INVENTION  
       [0012]     In accordance with the teachings of the present invention, a fuel cell is disclosed that includes a blocking agent for preventing hydrogen and air from contacting bare membrane. This in turn prevents the reaction of air and hydrogen gases at outside edges of the catalyst layers. The blocking agent is deposited within diffusion media layers on one or both of the anode and cathode sides of the fuel cell. The blocking agent extends into the diffusion media layers far enough so that it is within outside edges of the catalyst layers. In one embodiment, the blocking agent is a thermoplastic polymer, such as PVDF, that flows into the diffusion media layers in a melted format, where it hardens.  
         [0013]     Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a cross-sectional view of a known fuel cell;  
         [0015]      FIG. 2  is a cross-sectional view of a fuel cell employing blocking agents in diffusion media layers, according to an embodiment of the present invention; and  
         [0016]      FIG. 3  is a graph with run time on the horizontal axis and cell voltage on the vertical axis showing the relationship between cell voltage and run time for a known fuel cell and a fuel cell employing a blocking agent of the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0017]     The following description of the embodiments of the invention directed to a fuel cell employing hydrogen and air blocking agents is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.  
         [0018]      FIG. 2  is a cross-sectional view of a fuel cell  40  similar to the fuel cell  10 , where like reference numerals identify like elements. According to an embodiment of the present invention, the diffusion media layers  20  and  24  include a blocking agent  42  that extends from the ends of the diffusion media layers  20  and  24  to a location some suitable distance within the edges  32  and  34  of the catalyst layers  22  and  26 , respectively. The blocking agent  42  can be any suitable material formed within the diffusion media layers  20  and  24  that acts to block or restrict one or both of the airflow  36  and the hydrogen gas flow  28  from propagating through the membrane  16  outside of the catalyst layers  22  and  26 . In other words, the blocking agent  42  forces the airflow  36  and the hydrogen gas flow  28  to enter the catalyst layers  22  and  26 , respectively, before the membrane  16 . Therefore, the blocking agent  42  prevents the airflow  36  and the hydrogen gas flow  28  from passing to the membrane  16  without first passing through the catalyst layers  22  and  26 .  
         [0019]     Since gas that reaches the catalyst layers  22  and  26  react, no gas reaches the membrane  16 , and no gas passes through the membrane  16 . This prevents uncontrolled reaction of hydrogen and oxygen gas at the outside edges  32  and  34 , which in turn prevents the membrane  16  from failing adjacent to the outside edges  32  and  34  of the catalyst layers  22  and  26 .  
         [0020]     In this embodiment, the blocking agent  42  is provided through the entire thickness of the diffusion media layers  20  and  24 . This is by way of a non-limiting example in that the blocking agent  42  can be selectively formed within the diffusion media layers  20  and  24  so that it only goes through a portion of the thickness of the diffusion media layers  20  and  24 , preferably nearest to the membrane  16 .  
         [0021]     Also, in this embodiment, the blocking agent  42  is provided in both of the diffusion media layers  20  and  24 . It is not particularly clear if premature failure is caused by one or both of the airflow  36  or the hydrogen gas flow  28  that propagates through the membrane  16 . Therefore, the blocking agent  42  may only be necessary in one of the diffusion media layers  20  and  24 , such as the anode diffusion media layer  24 .  
         [0022]     The blocking agent  42  does not necessarily have to be resistant to diffusion of the flows  36  and  28 . Even if the gas diffusion of the blocking agent  42  is not negligible, the thickness of the diffusion media layers  20  and  24  should be sufficient to force the flows  36  and  28  towards the region of the diffusion media layers  20  and  24  adjacent to the catalyst layers  22  and  26 , respectively. This is because the blocking agent  42  need only fill the pores of the diffusion media layers  20  and  24  to increase the gas diffusion length of the flows  36  and  28 .  
         [0023]     The blocking agent  42  can be any blocking agent suitable for the purposes described herein. For example, the blocking agent  42  can be a thermoplastic polymer, such as polyaryl (ether ketone) or polyethylene. In one embodiment, the blocking agent  42  is polyvinylidene fluoride (PVDF). PVDF provides a good blocking agent because its melting temperature is approximately 170° C., which is above the operating temperature of the fuel cell  40 , yet it is not so hot to be difficult to be melted by standard processes and forced into the diffusion media layers  20  and  24 . PVDF is also chemically stable in acidic environments, such as in fuel cells.  
         [0024]     The following description provides one technique for introducing the PVDF into the diffusion media layers  20  and  24 . In one embodiment, a standard Toray  060  diffusion media (7% PTFE added) with dimensions of 73 mm 2  was used. Two pieces of 0.003 inch thick Kynar® PVDF were cut into frames having outer dimensions of 74 mm 2  and inner dimensions of 66 mm by 67 mm. The frames were centered on both sides of the diffusion media layers  20  and  24  so that there was equal overlap of the PVDF frames on all sides. The PVDF-DM-PVDF sandwich was placed between two pieces of Kapton® brand polyimide film and two pieces of Gylon® brand PTFE. The entire layer structure was placed between two aluminum plates and hot pressed at 0.1 tons for ten minutes at 350° F. and then at 0.5 tons for ten minutes at 350° F. After hot pressing, the material was removed and investigated. The PVDF was fully imbibed, or consistent throughout the diffusion media layer.  
         [0025]     The modified diffusion media layers  20  and  24  with the blocking agent  42  were then placed in a 50 cm 2  fuel cell to test their effectiveness.  FIG. 3  is a graph with run time on the horizontal axis and fuel cell voltage on the vertical axis showing the test results. The fuel cell containing the modified diffusion media layers  20  and  24  including the blocking agent  42  are represented by graph lines  50  and  52 . A base line fuel cell containing the same type of MEA and non-modified standard diffusion media layers are represented by graph lines  54  and  56 . The graph lines  50  and  54  represent data taken with no current drawn from the fuel cell, and the graph lines  52  and  56  represent data taken with a normalized current of 0.8 A/cm 2  drawn from the fuel cell. Both fuel cells were run at 95° C. and 200 kPa pressure with a relative humidity of 75% at the anode inlet and 50% at the cathode inlet. The graph lines  54  and  56  indicate that the voltage decreased rapidly and failure occurred in the baseline fuel cell after approximately 100 hours. When the fuel cell was disassembled, significant edge failure was observed.  
         [0026]     The fuel cell using the PVDF imbibed diffusion media layer was run out to approximately 175 hours with a much less dramatic cell voltage loss. Additionally, ex-situ shorting current and gas crossover currents were measured and are presented in Table I below. The crossover current did not increase significantly during the run time of the fuel cell. If the MEA was seriously degraded, an increase in the crossover current would be expected. When the fuel cell was disassembled, there was no indication of catalyst layer edge failure. The majority of the failures occurred in the active region of the MEA.  
                                             TABLE I                       Run   Shorting   Crossover   Crossover current with ΔP       Time (hr)   current (A)   current (A)   (anode-cathode) of 3 psi (A)                                0   0.011   0.018   0.024       70   0.026   0.036   0.044       112   0.022   0.031   0.046       173   0.028   0.030   0.062                  
 
         [0027]     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.