FUEL CELL HAVING A HYDROPHILIC NANOPOROUS REGION Technical Field

[0001] The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell having at least one hydrophilic nanoporous region to minimize problems associated with freezing of fuel cell water .

Background Art

[0002] Fuel cells are well known and are commonly used to produce electrical power from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as generators and transportation vehicles. In fuel cells of the prior art, it is known to utilize a proton exchange membrane ("PEM") as the electrolyte. As is well known, protons formed at an anode catalyst move through the electrolyte to a cathode catalyst while electrons move through a circuit to power a load. Fuel cell product water is formed at the cathode catalyst as the electrons complete the circuit back to the fuel cell and as an oxidant passes adjacent the cathode catalyst.

[0003] Use of such fuel cells to power transportation vehicles necessarily involves many start- stop cycles, some of which will occur in sub-freezing ambient temperatures. Many efforts have been undertaken to minimize problems associated with freezing of fuel cell water. Typically, fuel cell product water in a PEM fuel cell is at least partially recycled or utilized to hydrate membranes, to humidify reactant streams, to remove heat from a membrane electrode assembly ("MEA") , to support fuel reformers, and for other well-known purposes. In sub-freezing ambient conditions, freezing of fuel cell water may block flow paths that direct reactant streams through the fuel cell, thereby disrupting fuel cell performance. Start-up of a fuel cell having frozen fuel cell water within reactant flow paths may be so substantially impeded that auxiliary heating systems may be required that utilize parasitic power, that are time consuming and costly to install and maintain .

[0004] An example of an effort to minimize fuel cell water freezing problems is shown in U.S. Patent No. 6,794,076 to Busenbender which discloses use of a coating of fuel cell flow paths with a material defining pores having sizes in the nanometer range, such as between about 2 to 10 nanometers ("nm") . Busenbender states that liquid water within such nanoporous coatings will not freeze at temperatures as low as minus 40 degrees Celsius ("-40°C) . However, additional fuel cell product water within the catalyst and related layers may nonetheless freeze in such conditions, even though water in the coating may remain in liquid form. Consequently, there is a need for a fuel cell that minimizes problems associated with freezing of fuel cell product water. Summary

[0005] The disclosure is a fuel cell having at least one hydrophilic nanoporous region, wherein the fuel cell generates electrical energy from hydrogen-rich reducing fluid fuel and oxygen-containing oxidant reactant streams. The fuel cell includes an anode gas diffusion layer for convective and/or diffusive flow of the fuel reactant stream, and an anode catalyst layer having a first planar surface secured in fluid communication with the anode gas diffusion layer. A planar electrolyte membrane is secured adjacent an opposed second planar layer of the anode catalyst. A cathode catalyst layer having a first planar surface is also secured adjacent and in fluid communication with an opposed side of the planar electrolyte membrane. A cathode gas diffusion layer for convective and/or diffusive flow of the oxidant reactant stream is secured in fluid communication with an opposed second planar surface of the cathode catalyst layer. A hydrophilic nanoporous region is disposed within and in fluid communication with at least one of the anode gas diffusion layer and the cathode gas diffusion layer. (For purposes herein, fuel cell structures, such as the anode and/or gas diffusion layers, flow fields, pores of catalyst layers, etc., that serve to permit convective and/or diffusive flow of fluids, or that serve to direct flow of flowing fluids, will be collectively referred to for convenience as permitting flow of fluids by the phrase "for diffusing flow of".)

[0006] The hydrophilic nanoporous region is integral with the anode gas diffusion layer and/or the cathode gas diffusion layer. Preferably the hydrophilic nanoporous region is coextensive with, and completely overlies the planar surface of the electrolyte membrane. However, for certain operating conditions, it may be desirable to have an additional hydrophilic nanoporous region, a thicker hydrophilic nanoporous region, or an isolated, non-coextensive hydrophilic nanoporous region adjacent an area of proportionately higher fuel cell liquid water concentration, such as adjacent a cathode flow field exhaust.

[0007] For the hydrophilic nanoporous region to be integral with either or both of the anode and cathode gas diffusion layers, the gas diffusion layers may include a mixture of macroporous and hydrophilic nanoporous materials. (For purposes herein the term "macroporous" or "macropores" is to mean defined pores ranging in diameter from between about 10 microns ("μτη") and about 60 pm, and the term "hydrophilic nanoporous" or "hydrophilic nanopores" is to mean defined pores ranging in diameter from between about 3 nanometers ("nm") and about 100 nm with a contact angle of less than 90 degrees. (It is to be understood that 1 pm equals 1,000 nm) . (It is also to be understood that the hydrophilic nanopores and macropores referred to herein are open pores permitting flow through the pores and the dimensions of the pores refers to average diameters of the pores.) Additionally, the word "about" is to mean plus or minus twenty percent ("20%") . In the gas diffusion layers, the total porosity includes between about 90% and about 99% macropores, and between about 10% and about 1% hydrophilic nanopores.

[0008] Exemplary materials that would define the macropores include carbon fibers commonly used in fuel cell gas diffusion layers. Exemplary materials that would define the hydrophilic nanopores include materials commonly used in fuel cell technology to define open nanopores . Such a material is an ionomer component that is a hydrated nanoporous ionomer consisting of any suitable resin that is compatible with an operating environment of an electrochemical cell. An exemplary material for constituting a nanoporous ionomer region is a perflourosulfonic acid ionomer sold under the brand name "NAFION" by the E.I. DuPont company of Wilmington, Delaware, U.S.A. that has open pores having a diameter of about 4 nm when the ionomer is hydrated. Additionally, carbon fibers coated with nanoporous carbon black may be utilized to define both macroporous and hydrophilic nanoporous regions. An exemplary carbon black is that sold under the brand name Black Pearls by the Cabot company of Boston, MA, U.S.A.

[0009] In yet another embodiment, a "microporous layer" may be secured adjacent the cathode gas diffusion layer. The microporous layer typically consists of a polytetrafluoroethylene resin and carbon particles. The microporous layer is secured on a surface of the gas diffusion layer adjacent the catalyst layer, and serves during fuel cell start-up to thermally insulate the electrolyte membrane and adjacent catalyst layers. (The electrolyte membrane and adjacent catalyst layers are referred to frequently as a "membrane electrode assembly" or "MEA".) However, the microporous layer also reduces movement of fuel cell product water out of the MEA and in particular the cathode catalyst layer. Securing a hydrophilic nanoporous region within the gas diffusion layer leads to wicking of residual fuel cell product water away from the catalyst towards the nanoporous region during the freezing process. The mechanism by which water moves under a temperature gradient during the freezing of water is the same as that seen commonly in soil science and is heretofore referred to as the "frost- heave" mechanism. The movement of water away from the catalyst layer minimizes problems associated with freezing of that liquid water within and adjacent the MEA. Similarly, if no microporous layer is utilized, the nanopores of the hydrophilic nanoporous region act as a water sink, drawing liquid water out of the catalyst layers during the freezing process. Again, residual fuel cell product water is wicked away from the catalyst layer towards the hydrophilic nanopores contained within the macroporous layer by the well known "frost-heave" mechanism, i.e. capillary pressure gradient caused by the freezing of water in small pores under a temperature gradient.

[0010] Accordingly, it is a general purpose of the present invention to provide a fuel cell having a hydrophilic nanoporous region that overcomes deficiencies of the prior art.

[0011] It is a more specific purpose to provide a fuel cell having a hydrophilic nanoporous region that minimizes problems associated with freezing of fuel cell product water.

[0012] These and other purposes and advantages of the present fuel cell having a hydrophilic nanoporous region will become more readily apparent when the following description is read in conjunction with the accompanying drawing. Brief Description of the Drawing

[0013] Figure 1 is a simplified schematic representation of a fuel cell having a hydrophilic nanoporous region constructed in accordance with the present disclosure.

Description of the Preferred Embodiments

[0014] Referring to the drawing in detail, a simplified schematic representation of a fuel cell having a hydrophilic nanoporous region is shown in FIG. 1, and is generally designated by the reference numeral 10. The fuel cell 10 includes an anode flow field 12 that receives a hydrogen rich fuel reactant stream from a fuel source 14 and fuel inlet 16 and directs the fuel stream through the anode flow field 12 and out of the fuel cell 10 through an anode exhaust 18. The anode flow field 12 also distributes the fuel to pass into an anode gas diffusion layer 20 for diffusing flow of the fuel reactant stream. An anode catalyst layer 22 is secured in coextensive relationship with and in fluid communication with the anode gas diffusion layer 20. (By the phrase "coextensive relationship with" or the word "coextensive", it is meant that a permeable surface of a first layer completely overlies a permeable surface of a second layer.) The anode catalyst layer 22 has a first planar surface 24 secured in coextensive relationship with and in fluid communication with the anode gas diffusion layer 20. A planar electrolyte membrane 26 is secured adjacent an opposed second planar surface 28 of the anode catalyst 22.

[0015] A cathode catalyst layer 30 having a first planar surface 32 is also secured adjacent and in fluid communication with an opposed side of the planar electrolyte membrane 26. A cathode gas diffusion layer 34 for diffusing flow of an oxidant reactant stream is secured coextensively with and in fluid communication with an opposed second planar surface 36 of the cathode catalyst layer 30. The oxidant reactant stream is directed from an oxidant source 38 through an oxidant inlet 40 into an oxidant flow field 42 secured in fluid communication with the cathode gas diffusion layer 34. The oxidant stream then passes out of the fuel cell 10 through a cathode exhaust 44. The anode and cathode flow fields 12, 42 may be configured in a manner disclosed in U . S . Patent 7, 201, 992 that issued on April 10, 2007, which patent is owned by the owner of all rights in the present disclosure.

[0016] A first hydrophilic nanoporous region 46 may be formed in integral association with the cathode gas diffusion layer 34 (as represented in FIG. 1 by the dots in the cathode gas diffusion layer 34). Additionally, a second hydrophilic nanoporous region 48 may be formed in integral association with the anode gas diffusion layer 20. The first and second hydrophilic nanoporous regions 46, 48 may be configured to only overlie particular areas of the electrolyte membrane 26, or may define a greater proportion of a hydrophilic nanoporous region adjacent high liquid water concentration areas of the fuel cell 10, such as adjacent the oxidant exhaust 44.

[0017] The first and second hydrophilic nanoporous regions 46, 48 include a mixture of macroporous and hydrophilic nanoporous materials. (For purposes herein the term "macroporous" or "macropores" is to mean defined pores ranging in diameter from between about 10 microns ("μιη") and about 60 pm, and the term "hydrophilic nanoporous" or "hydrophilic nanopores" is to mean defined pores ranging in diameter from between about 3 nanometers ("nm") and about 100 nm with a contact angle of less than ninety degrees ("90°") . (It is to be understood that 1 m equals 1,000 nm) . (It is also to be understood that the nanopores and macropores referred to herein are open pores permitting flow through the pores and the dimensions of the pores refers to average diameters of the pores.) In the anode and cathode gas diffusion layers 20, 34, the total porosity includes between about 90% and about 97% macropores, and between about 10% and about 3% nanopores. As recited above, exemplary materials that would define the macropores include carbon fibers (not shown) commonly used in fuel cell gas diffusion layers. Exemplary materials that would define the hydrophilic nanopores include materials commonly used in fuel cell technology to define open hydrophilic nanopores such as hydrated hydrophilic nanoporous ionomers.

[0018] In another embodiment of the fuel cell 10, a "microporous layer" 50, such as a

"polytetrafluoroethylene layer", may be secured adjacent and coextensive with the cathode gas diffusion layer 34. The microporous layer 50 typically consists of a mixture of polytetrafluoroethylene resin and carbon particles. Micropores are understood to mean defined pores ranging in diameter from between about 0.5 urn to 10 urn and having a majority of pores being hydrophobic, having a contact angle of greater than 90 degrees. The microporous layer 50 is secured on a surface of the cathode gas diffusion layer 34 adjacent the cathode catalyst layer 30. The microporous layer 50 serves during fuel cell 10 start-up to thermally insulate the electrolyte membrane 26 and adjacent anode and cathode catalyst layers 22, 30 (referred to as a "membrane electrode assembly" or an "MEA") . However, the microporous layer 50 also reduces movement of fuel cell 10 product water out of the MEA and in particular the cathode catalyst layer 30. The microporous layer 50 may be formed of a mixture of polytetrafluoroethylene resin and carbon particles, such as carbon particles available under a brand name "VULCAN XC72", available from the CABOT company of Boston, Massachusetts, U.S.A. The mixture may be coated on a surface of the cathode gas diffusion layer 34 facing the second surface 36 of the cathode catalyst layer 30 or may be coated on the second surface 36 of the cathode catalyst layer 30 facing the cathode gas diffusion layer 34. The mixture may be coated using a dry powder process or a liguid coating process to secure the microporous layer 50 to the cathode gas diffusion layer 34.

[0019] Securing the first hydrophilic nanoporous region 46 within the cathode gas diffusion layer 34 leads to a wicking of residual fuel cell product water during the freezing process into and through the microporous layer 50 to thereby minimize liguid water in or adjacent the MEA. This would also minimize problems associated with freezing of that liguid water within and adjacent the cathode catalyst layer 30. If no microporous layer 50 is utilized, the hydrophilic nanopores of the first and/or second hydrophilic nanoporous regions 46, 48 act as a water sink within or adjacent the anode and cathode gas diffusion layers 20, 34, drawing liguid water out of the anode and cathode catalyst layers 22, 30 and electrolyte membrane 26 as the freezing process occurs in the hydrophilic nanoporous regions 46, 48. [0020] The present invention also includes a method of minimizing ice blocked reactant stream diffusion pathways in and adjacent the cathode catalyst layer 30 and/or the anode catalyst layer 22 of the fuel cell 10. The method includes diffusing a fuel reactant stream through an anode flow field 12 and anode gas diffusion layer 20 of the fuel cell 10 so that the fuel reactant stream passes adjacent an anode catalyst layer 22; diffusing an oxidant reactant stream through an oxidant flow field 42 and oxidant gas diffusion layer 34 of the fuel cell 10 so that the oxidant reactant stream passes adjacent a cathode catalyst layer 30 to operate the fuel cell 10; flowing fuel cell product water generated during operation of the fuel cell 10 into at least one hydrophilic nanoporous region 46, 48 disposed within and in fluid communication with at least one of the anode gas diffusion layer 20 and the cathode gas diffusion layer 34.

[0021] By having a substantial portion of fuel cell 10 residual product water preferentially flowing into and held within the hydrophilic nanopores of the regions 46, 48 by "frost heave" action, the flow or diffusion paths or any open micro and macro pores within and adjacent the cathode catalyst layer 30 will retain an absolute minimum of liquid water. Therefore, upon shutting down of the fuel cell 30 in subfreezing ambient conditions, frozen water will remain within the hydrophilic nanopores of the nanoporous regions 46, 48 to minimize ice blockage of macro and micro pores in and adjacent the anode and/or cathode catalyst layers 22, 30.

[0022] While the present invention has been disclosed with respect to the described and illustrated fuel cell having a hydrophilic nanoporous region 10, it is to be understood that the invention is not to be limited to those embodiments. For example, while the fuel cell 10 is shown for purposes of explanation as a single cell 10, it is to be understood that the use of the fuel cell 10 is more likely to be within a variety of adjacent fuel cells (not shown) arranged with cooperative manifolds, etc., in a well know fuel cell stack assembly. Accordingly, reference should be made primarily to the following claims rather than the foregoing description to determine the scope of the invention.