Abstract:
A method for depositing a hydrophilic coating on flow field plates or bipolar plates and manifolds in a fuel cell stack after the stack is assembled. The method includes preparing a solution that contains hydrophilic nano-particles suspended in a suitable solvent. The cathode and anode inlet and outlet manifolds and the cathode and anode flow channels are filled with the solution. The solution is then pumped out of the stack using, for example, a stream of nitrogen. The stack is allowed to dry, using heat if desirable, to provide a film of the nano-particles formed on the anode and cathode flow channels and manifolds within the stack.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for depositing a hydrophilic coating on the bipolar plates of a fuel cell and, more particularly, to a method for depositing a hydrophilic coating on the bipolar plates of the fuel cells in a fuel cell stack that includes running a solution including the hydrophilic material through the reactant gas flow channels in the bipolar plates after the stack is assembled. 
     2. Discussion of the Related Art 
     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. 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. 
     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). 
     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 two hundred or more fuel cells. 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. 
     The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. 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 reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows. 
     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. It is known in the art to deposit a thin layer of a conductive material, such as gold, on the bipolar plates to reduce the contact resistance between the plate and diffusion media in the fuel cells. 
     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 may accumulate 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 relatively hydrophobic nature of the plate material. 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 are 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. 
     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 cathode 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. 
     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. 
     It has been proposed in the art to deposit a hydrophilic layer on the bipolar plates to improve channel water transport. The hydrophilic layer 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 hydrophilic 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 used 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.
 
     Another potential water-related problem in fuel cell stacks is electrode carbon corrosion as a result of anode hydrogen starvation. Further, it has been demonstrated that excessive channel water can lead to freeze damage, as well as increasing freeze start-up time. 
     Removing liquid water from the flow channels through the manifold to the outside of the stack enclosure requires wetting surfaces not only in the channels of the bipolar plates, but also in the transition between the end of a channel, around the gaskets and into the manifold. The most effective system will have a hydrophilic coating throughout the entire gas volume of the stack. This will effectively reduce the resistance of water removal associated with the changes in surface energy. 
     Known hydrophilic bipolar plate treatments are typically very expensive relative to the projected plate cost targets. Adding a hydrophilic layer to the bipolar plates requires both process and material optimization for cost reduction. Further, the hydrophilic coatings have been shown to degrade after many hours of run-time, which may impact the fuel cell stability at mid-life of the fuel cell stack. Thus, a low cost repair for this failure mechanism may be essential for long fuel cell stack life. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method for depositing a hydrophilic coating on flow field plates or bipolar plates in a fuel cell stack after the stack is assembled is disclosed. The method includes preparing a solution that contains hydrophilic nano-particles suspended in a suitable solvent. The cathode and anode inlet and outlet manifolds and the cathode and anode flow channels are filled with the solution. The solution is then forced out of the stack using, for example, a stream of nitrogen. The stack is allowed to dry, using heat if desirable, to provide a film of the nano-particles formed on the anode and cathode flow channels and manifolds within the stack. 
     Additional 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 
         FIG. 1  is a cross-sectional view of a fuel cell in a fuel cell stack that includes bipolar plates having a hydrophilic coating; and 
         FIG. 2  is a plan view of a system for depositing a hydrophilic coating on anode and cathode flow channels in a fuel cell stack, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a process for depositing a hydrophilic coating on the flow channels of the bipolar plates in a fuel cell stack after the stack is assembled is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a cross-sectional view of a fuel cell  10  that is part of a fuel cell 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 . 
     A cathode side flow field plate or bipolar plate  28  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  28  and  30  are provided between the fuel cells in the fuel cell stack. A hydrogen reactant gas flow from flow channels  32  in the bipolar plate  30  reacts with the catalyst layer  26  to dissociate the hydrogen ions and the electrons. Airflow from flow channels  34  in the bipolar plate  28  reacts with the catalyst layer  22 . The hydrogen ions are able to propagate through the membrane  16  where they carry the ionic current through the membrane  16 . The end product is water, which does not have any negative impact on the environment. 
     In this non-limiting embodiment, the bipolar plate  28  includes two stamped sheets  36  and  38  that are welded together. The sheet  36  defines the flow channels  34  and the sheet  38  defines flow channels  40  for the anode side of an adjacent fuel cell to the fuel cell  10 . Cooling fluid flow channels  42  are provided between the sheets  36  and  38 , as shown. Likewise, the bipolar plate  30  includes a sheet  44  defining the flow channels  32 , and a sheet  46  defining flow channels  48  for the cathode side of an adjacent fuel cell. Cooling fluid flow channels  50  are provided between the sheets  44  and  46 , as shown. The bipolar plates  28  and  30  can be made of any suitable conductive material that can be stamped, such as stainless steel, titanium, aluminum, etc. 
     The bipolar plate  28  includes a coating  52  and the bipolar plate  30  includes a coating  54  that makes the plates conductive, corrosion resistant, hydrophilic and/or stable in a fuel cell environment. As will be discussed in more detail below, the present invention proposes a process for depositing the coatings  52  and  54  on the bipolar plates  28  and  30  after the fuel cell stack has been assembled. Thus, the lands  56  and  58  between the flow channels  34  and  32 , respectively, are not coated with the hydrophilic material, and thus the electrical properties of the plates  28  and  30  for conducting electricity through the fuel cell  10  is not affected. However, the part of the diffusion media layers  20  and  24  facing the flow channels  34  and  32  is coated with the hydrophilic material. It is possible that the hydrophilic solution can be optimized for poor adhesion to the diffusion media relative to the channels. 
     The process of the invention has particular application for depositing a coating of silicon dioxide (SiO 2 ) nano-particles on the bipolar plates  28  and  30 . However, other metal oxides can be used for the hydrophilic coatings including, but not limited to, hafnium dioxide (HfO 2 ), zirconium dioxide (ZrO 2 ), aluminum oxide (Al 2 O 3 ), tin 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 ) and mixtures thereof. 
     The metal oxides can be doped to make them electrically conductive. Suitable dopants can be selected from materials that can create suitable point defects, such as N, C, Li, Ba, Pb, Mo, Ag, Au, Ru, Re, Nd, Y, Mn, V, Cr, Sb, Ni, W, Zr, Hf, etc. and mixtures thereof. In one particular embodiment, the doped metal oxide is niobium (Nb) and tantalum (Ta) doped titanium oxide (TiO 2 ) and fluorine (F) doped tin oxide (SnO 2 ). The amount of dopant in the coatings can be in the range of 0-10% of the composition of the coatings. 
     In another embodiment, the hydrophilic coatings  52  and  54  are carbides that are conductive, corrosion resistant, hydrophilic and stable in the fuel cell environment. Suitable carbides may include, but are not limited to, chromium carbide, titanium carbide, tantalum carbide, niobium carbide and zirconium carbide. 
       FIG. 2  is a plan view of a system  60  for depositing the hydrophilic coatings  52  and  54  on the bipolar plates  28  and  30 . The system  60  includes a fuel cell stack  62  shown in cross-section. The fuel cell stack  62  includes an active region  64 , an anode inlet manifold  66 , a cathode inlet manifold  68 , an anode outlet manifold  70  and a cathode outlet manifold  72 . The anode and cathode flow channels shown in  FIG. 1  would extend from the inlet manifolds  66  and  68  through the active region  64  to the outlet manifolds  70  and  72 . 
     According to the invention, the entire anode and cathode volume of the fuel cell stack  62 , including the flow channels and the inlet and outlet manifolds, is filled with a solution including suspended hydrophilic nano-particles. In one non-limiting embodiment, the solution is SiO 2  nano-particles suspended in a solvent, such as ethanol. A solution of SiO 2  nano-particles suspended in ethanol is available as a commercial product, referred to as nano-X, from nano-XGmbh of Saarbrucken, Germany. A source  76  of the solution can be pumped by a pump  78  into the anode and cathode inlet manifolds  66  and  68 . A pressurized inert gas, such as nitrogen, from a source  80  is then allowed to flow into the anode and cathode inlet manifolds  66  and  68  through a three-way valve  86  that forces the solution out of the stack  62  through the outlet manifolds  70  and  72 . A three-way valve  82  can be switched between the source  76  and the source  80 . Nitrogen may continue to flow after all the hydrophilic solution is removed to aid evaporation and remove solvent vapor from the system. A thin film of the solution is left on the flow channels, stack manifolds and inlet header plumbing. The stack  62  is allowed to dry so that the solvent in the wet film evaporates, leaving a thin film of the hydrophilic nano-particles. Any suitable technique can be used to dry the solvent, such as by heating the stack or flowing a dry inert gas through the stack  12 . In one embodiment, the film has a thickness on the order of 100 nm. The solution from the stack  62  can be collected in a container  84  to be used for another fuel cell stack. 
     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, etc. Accumulation of these contaminants on the hydrophilic coatings will, over time, significantly reduce the hydrophilicity of the coating. Thus, the hydrophilic coatings will degrade after many hours of run time, which may impact the fuel cell stability. Therefore, the present invention also proposes using the process of depositing the hydrophilic coatings on the flow channels as a service of the vehicle. Particularly, at some point in the life of a fuel cell vehicle, it may have stability problems. The fuel cell stack in the vehicle can be connected to a suitable fixture at a service center that fills the stack with the hydrophilic solution, as discussed above, and then pumps the solution out of the stack using the inert gas to recoat the flow channels with the hydrophilic material. 
     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.