Abstract:
A flow field plate for a fuel cell that includes an outer layer of a metal oxide or other material that makes the plate hydrophilic. The particular metal oxide and the thickness of the metal oxide layer are selected so that hydrofluoric acid generated by the fuel cell continuously etches away the layer at a predetermined rate so that a surface of the layer is free of contaminants over the entire life of the fuel cell. If the fuel cell does not employ a perfluorosulfonic acid membrane, then a separate hydrofluoric acid source can be provided that injects a low level solution of hydrofluoric acid into one or both of the reactant gas streams.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to bipolar plates for fuel cells and, more particularly, to a bipolar plate for a fuel cell that includes an outer coating that makes the plate hydrophilic, and degrades in the presence of hydrofluoric acid to continuously expose a clean hydrophilic surface during operation of the fuel cell.  
         [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 in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to 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 perfluorosulfonic 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 catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the 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]     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred bipolar plates. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen 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 reactant gas that flows into the anode side of the stack.  
         [0008]     The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.  
         [0009]     The bipolar plates are typically made of a conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, etc., so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. Metal bipolar plates typically produce a natural oxide on their outer surface that makes them resistant to corrosion. However, the oxide layer is not conductive, and thus increases the internal resistance of the fuel cell, reducing its electrical performance. Also, the oxide layer makes the plate more hydrophobic.  
         [0010]     US Patent Application Publication No. 2003/0228512, assigned to the assignee of this application and herein incorporated by reference, discloses a process for depositing a conductive outer layer on a flow field plate that prevents the plate from oxidizing and increasing its ohmic contact. U.S. Pat. No. 6,372,376, also assigned to the assignee of this application, discloses depositing an electrically conductive, oxidation resistant and acid resistant coating on a flow field plate. US Patent Application Publication No. 2004/0091768, also assigned to the assignee of this application, discloses depositing a graphite and carbon black coating on a flow field plate for making the flow field plate corrosion resistant, electrically conductive and thermally conductive.  
         [0011]     As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm 2 , the water accumulates within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the hydrophobic nature of the plate material. The contact angle of the water droplets is generally about 90° in that the droplets form in the flow channels substantially perpendicular to the flow of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels flow in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.  
         [0012]     It is usually possible to purge the accumulated water in the flow channels by periodically forcing the reactant gas through the flow channels at a higher flow rate. However, on the anode side, this increases the parasitic power applied to the air compressor, thereby reducing overall system efficiency. Moreover, there are many reasons not to use the hydrogen fuel as a purge gas, including reduced economy, reduced system efficiency and increased system complexity for treating elevated concentrations of hydrogen in the exhaust gas stream.  
         [0013]     Reducing accumulated water in the channels can also be accomplished by reducing inlet humidification. However, it is desirable to provide some relative humidity in the anode and cathode reactant gases so that the membrane in the fuel cells remains hydrated. A dry inlet gas has a drying effect on the membrane that could increase the cell&#39;s ionic resistance, and limit the membrane&#39;s long-term durability.  
         [0014]     It has been proposed by the present inventors to make bipolar plates for a fuel cell hydrophilic to improve channel water transport. A hydrophilic plate causes water in the channels to form a thin film that has less of a tendency to alter the flow distribution along the array of channels connected to the common inlet and outlet headers. If the plate material is sufficiently wettable, water transport through the diffusion media will contact the channel walls and then, by capillary force, be transported into the bottom corners of the channel along its length. The physical requirements to support spontaneous wetting in the corners of a flow channel are described by the Concus-Finn condition, β+α/2&lt;90°, where β is the static contact angle and α is the channel corner angle. For a rectangular channel α/2=45°, which dictates that spontaneous wetting will occur when the static contact angle is less than 45°. For the roughly rectangular channels use in current fuel cell stack designs with composite bipolar plates, this sets an approximate upper limit on the contact angle needed to realize the beneficial effects of hydrophilic plate surfaces on channel water transport and low load stability.  
         [0015]     A design concern needs to be addressed when providing a hydrophilic coating on bipolar plates in fuel cells. Because hydrophilic coatings have a high surface energy, they will attract particles and other contaminants entering the fuel cell from the gaseous fuel and/or oxygen streams, from humidifiers and upstream piping, or generated internally by other components, such as the MEA, diffusion media, seals, composite plate materials, etc. Accumulation of these contaminants on the coating will, over time, significantly reduce the hydrophilicity of the coating. Even if provisions are made to control contamination through the use of gas filtering and ultra-clean components, it is unlikely that degradation of a hydrophilic coating or other surface treatment would not occur during the desired 6,000 hour lifetime of a fuel cell.  
       SUMMARY OF THE INVENTION  
       [0016]     In accordance with the teachings of the present invention, a flow field plate or bipolar plate for a fuel cell is disclosed that includes an outer layer of a metal oxide, or other material, that makes the plate hydrophilic. Suitable metal oxides include at least one of SiO 2 , HfO 2 , ZrO 2 , Al 2 O 3 , SnO 2 , Ta 2 O 5 , Nb 2 O 5 , MoO 2 , IrO 2 , RuO 2 , metastable oxynitrides, nonstoichiometric metal oxides, oxynitrides and mixtures thereof. The particular metal oxide and the thickness of the metal oxide layer are selected so that hydrofluoric acid generated by the perfluorosulfonic acid membrane in the fuel cell etches away the layer at a desired rate so that a clean surface of the layer is continuously exposed that is free of contaminants over the entire life of the fuel cell. If the fuel cell does not employ a perfluorosulfonic acid membrane, then a separate hydrofluoric acid source can be provided that injects a low level solution of hydrofluoric acid into one or both of the reactant gas streams.  
         [0017]     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  
       [0018]      FIG. 1  is a cross-sectional view of a fuel cell in a fuel cell stack that includes bipolar plates having an outer layer that makes the plate hydrophilic, according to an embodiment of the present invention; and  
         [0019]      FIG. 2  is a plan view of a fuel cell system including a fuel cell stack and a source of hydrofluoric acid for emitting hydrofluoric acid into a reactant stream of the fuel cell stack. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0020]     The following discussion of the embodiments of the invention directed to a bipolar plate for a fuel cell that includes a coating that makes the bipolar plate hydrophilic and is etched away at a predetermined rate in the hydrofluoric acid environment of the fuel cell.  
         [0021]      FIG. 1  is a cross-sectional view of a fuel cell  10  that is part of a fuel stack of the type discussed above. The fuel cell  10  includes a cathode side  12  and an anode side  14  separated by a perfluorosulfonic acid membrane  16 . A cathode side diffusion media layer  20  is provided on the cathode side  12 , and a cathode side catalyst layer  22  is provided between the membrane  16  and the diffusion media layer  20 . Likewise, an anode side diffusion media layer  24  is provided on the anode side  14 , and an anode side catalyst layer  26  is provided between the membrane  16  and the diffusion media layer  24 . The catalyst 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  20  and  24 , respectively, or on the membrane  16 .  
         [0022]     A cathode side flow field plate or bipolar plate  18  is provided on the cathode side  12  and an anode side flow field plate or bipolar plate  30  is provided on the anode side  14 . The bipolar plates  18  and  30  are provided between the fuel cells in the fuel cell stack. A hydrogen reactant gas flow from flow channels  28  in the bipolar plate  30  reacts with the catalyst layer  26  to dissociate the hydrogen ions and the electrons. Airflow from flow channels  32  in the bipolar plate  18  reacts with the catalyst layer  22 . The hydrogen ions are able to propagate through the membrane  16  where they electro-chemically react with the airflow and the return electrons in the catalyst layer  22  to generate water as a by-product.  
         [0023]     In this non-limiting embodiment, the bipolar plate  18  includes two sheets  34  and  36  that are stamped and welded together. The sheet  36  defines the flow channels  32  and the sheet  34  defines flow channels  38  for the anode side of an adjacent fuel cell to the fuel cell  10 . Cooling fluid flow channels  40  are provided between the sheets  34  and  36 , as shown. Likewise, the bipolar plate  30  includes a sheet  42  defining the flow channels  28 , a sheet  44  defining flow channels  46  for the cathode side of an adjacent fuel cell, and cooling fluid flow channels  48 . In the embodiments discussed herein, the sheets  34 ,  36 ,  42  and  44  are made of an electrically conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, etc.  
         [0024]     According to one embodiment of the invention, the bipolar plates  18  and  30  are coated with a metal oxide layer  50  and  52 , respectively, that make the plates  18  and  30  hydrophilic. The layers  50  and  52  can also be made of materials other than metal oxide that make plates  18  and  30  hydrophilic within the scope of the present invention. The hydrophilicity of the layers  50  and  52  causes the water within the flow channels  28  and  32  to form a film instead of water droplets so that the water does not significantly block the flow channel. Particularly, the hydrophilicity of the layers  50  and  52  decreases the contact angle of water accumulating within the flow channels  32 ,  38 ,  28  and  46 , preferably below 40°, so that the reactant gas is still able to flow through the channels at low loads.  
         [0025]     Suitable metal oxides for the layers  50  and  52  include, but care not limited, to silicon dioxide (SiO 2 ), hafnium dioxide (HfO 2 ), zirconium dioxide (ZrO 2 ), aluminum oxide (Al 2 O 3 ), stannic oxide (SnO 2 ), tantalum pent-oxide (Ta 2 O 5 ), niobium pent-oxide (Nb 2 O 5 ), molybdenum dioxide (MoO 2 ), iridium dioxide (IrO 2 ), ruthenium dioxide (RuO 2 ), metastable oxynitrides, nonstoichiometric metal oxides, oxynitrides and mixtures thereof.  
         [0026]     Before the layers  50  and  52  are deposited on the bipolar plates  18  and  30 , the bipolar plates  18  and  30  are cleaned by a suitable process, such as ion beam sputtering, to remove the resistive oxide film on the outside of the plates  18  and  30  that may have formed. The metal oxide material can be deposited on the bipolar plates  18  and  30  by any suitable technique including, but not limited to, physical vapor deposition processes, chemical vapor deposition (CVD) processes, thermal spraying processes and sol-gel. Suitable examples of physical vapor deposition processes include electron beam evaporation, magnetron sputtering and pulsed plasma processes. Suitable chemical vapor deposition processes include plasma enhanced CVD and atomic layer deposition processes.  
         [0027]     As is understood in the art, hydrofluoric acid (HF) is generated as a result of degradation of the perfluorosulfonic ionomer in the membrane  16  during operation of the fuel cell. The hydrofluoric acid has a corrosive effect on the various coating materials discussed herein because it etches away the metal oxide layers  50  and  52 . The etching of the layers  50  and  52  is desirable because a clean surface of the layers  50  and  52  that is free of contaminants is continuously exposed during operation of the fuel cell  10 . Therefore, the desired hydrophilicity of the layers  50  and  52  is maintained.  
         [0028]     The thickness of the layers  50  and  52  needs to be sufficient to handle the degradation caused by the fluoride ions in the hydrofluoric acid over the desired lifetime of the fuel cell  10  without being completely etched away. In one embodiment, the desired lifetime of the fuel cell  10  is about 6000 hours. The necessary thickness of the layers  50  and  52  is dependent on the layer material. In other words, the layers  50  and  52  need to be thicker for materials that are quickly etched away by the hydrofluoric acid and the layers  50  and  52  can be thinner for materials that are slowly etched away by the hydrofluoric acid. In one non-limiting embodiment, the layers  50  and  52  are 80-100 nm thick. Certain of the suitable metal oxide materials, such as ZrO 2 , are more resistant to the fluoride ions, and still provide the desired hydrophilicity, which could be more desirable in certain fuel cell stacks. Moreover, ZrO 2  acts as a scavenger of fluoride ions, further enhancing its durability in applications involving stainless steel.  
         [0029]      FIG. 2  is block diagram of a fuel cell system  54  including a fuel cell stack  56 . A hydrogen source  58  provides a hydrogen reactant gas input on an anode input line  60  that is sent to the anode side of the fuel cells within the fuel cell stack  56 . A compressor  62  provides compressed air on a cathode side input line  64  that is sent to the cathode side of the fuel cells in the fuel cell stack  56 . A humidifier  66  humidifies the air before it is input into the fuel cell stack  56  to provide increased cell membrane humidity. In this embodiment, the fuel cells in the fuel cell stack  56  do not have a perfluorosulfonic acid membrane, but use other types of membranes known in the art, such as the hydrocarbon based membrane. Therefore, the membranes in the fuel cell stack  56  do not generate hydrofluoric acid to etch away the layers  50  and  52  to maintain the hydrophilicity of the layers  50  and  52 , as discussed above. According to this embodiment of the invention, a hydrofluoric acid source  68  is provided that provides a controlled amount of low level hydrofluoric acid to one or both of the reactant gas input lines  60  and  64 . The concentration of the hydrofluoric acid is determined for the desired etch rate of the metal oxide layers, which is based on the metal oxide material and the thickness of the layers, as discussed above. Additionally, the hydrofluoric acid from the source  68  can be applied to the humidifier  66 .  
         [0030]     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.