Abstract:
This disclosure is about invention of fuel cells comprising vertically free-standing graphene and carbon nanosheets in the components, and methods of making thereof. Performance enhancement effect of the fuel cell is achieved by using: an electrically conducting component whose surface is covered by vertically free-standing graphene and carbon nanosheets; and/or a catalyst supporting structure made of vertically free-standing graphene and carbon nanosheets; and/or a catalyst based on vertically free-standing graphene and carbon nanosheets. Layers of fuel cells embedded with vertically free-standing graphene and carbon nanosheets can be mechanically strengthened. Vertically free-standing graphene and carbon nanosheets embedded in the electrodes can also enhance the electrical conductivity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is the Nonprovisional application for a former provisional application with the same title, submitted on Dec. 11, 2015. EFS ID: 24333936, Application No.: 62,266,091. 
     
    
     BACKGROUND 
       [0002]    This invention is in the technical field of fuel cell. More particularly, it is in the technical field of manufacturing fuel cell components, and fabrication of fuel cell components incorporating vertically free-standing graphene and carbon nanosheets. 
         [0003]    A fuel cell is a device that electrochemically converts energy from a fuel into electricity either through reducing positively charged ions (e.g. protons, H + ) or via oxidizing a fuel (e.g. hydrogen gas, H 2 ) agent. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by their operating temperature and the type of electrolyte they use, e.g. proton exchange membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC), alkaline fuel cell (AFC), direct-methanol fuel cell (DMFC), molten-carbonate fuel cell (MCFC), and phosphoric-acid fuel cell (PAFC). 
         [0004]    A simplified diagram of a PEMFC is shown in  FIG. 1 , whose components consists of, but not limited to, an anode  110 , a cathode  130 , and an electrolyte  120  that allows charged ions to move between cathode and anode. In a PEMFC, the anode and cathode contain catalyst  114  and  134 , where the reactions occur. Reactant channel plate  111  conducts fuels (e.g. H 2 ) into the anode. Reactant channel plate  131  conducts oxidizing agent (e.g. air or O 2 ) into the cathode. 
         [0005]    In most types of PEMFC, current collectors  112  and  132  are attached to the inner side or outer side of reactant channel plate to collect electrical current generated by the fuel cell. 
         [0006]    In some types of fuel cells, e.g. PEMFC, gas diffusion layers (GDL)  113  and  133  are inserted between current collector and catalyst layer to electrically connect the catalyst and current collector. 
         [0007]    As one kind of thin film material, a carbon nanosheet is a novel carbon nanomaterial with a graphene and graphitic structure developed by Dr. J. J. Wang et al. at the College of William and Mary. As used herein, a “carbon nanosheet” refers to a carbon nanomaterial with a thickness of two nanometers or less. A carbon nanosheet is a two-dimensional graphitic sheet made up of a single to several layers of graphene. Thus, thickness of a carbon nanosheet can vary from a single graphene layer to multiple layers, such as one to seven layers of graphene. For example, a carbon nanosheet may comprise one to three graphene layers and has thickness of one nanometer or less. Edges of a carbon nanosheet usually terminate by a single layer of graphene. The specific surface area of a carbon nanosheet is between 1000 m 2 /g to 2600 m 2 /g. The height of a carbon nanosheet varies from 100 nm to 8 μm, depending on fabrication conditions. The width of a carbon nanosheet also varies from hundreds of nanometers to a few microns. 
         [0008]    A plurality of carbon nanosheets, each of which comprises at least one layer of graphene, are disposed orthogonally to a coated surface of a substrate. Essentially, the plurality of vertically free-standing carbon nanosheets are functioning as space-organizers at nanoscale. By partitioning the space above the surface of the substrate, these vertically free-standing carbon nanosheets can greatly enlarge the surface area of the substrate. 
         [0009]    Hereby the term “free-standing” or the term “vertically free-standing” refers to attaching carbon nanostructures to a surface orthogonally, or at various angles from  0  to 180 degree with respect to the surface. Furthermore, carbon nanostructures stretch out not only in a straight way, but also can have a crumpling, tilting, folding, sloping, or “origami”-like structure. 
         [0010]    By virtue of their graphene and graphitic structure, carbon nanosheets have very high electrical conductivity. Graphene is known as one of the strongest materials, and it has a breaking strength over  100  times greater than that of a hypothetical steel film of the same thickness. Morphology of carbon nanosheets can remain stable at temperatures up to 1000° C. A carbon nanosheet has a large specific surface area because of its sub-nanometer thickness. Referring to FIG. 4 , it shows an exemplary carbon nanosheet consisting of one layer of graphene. With only 1 to 7 layers of graphene, the carbon nanosheet is about 1 nm thick. Its height and length is about 1 micrometer respectively. The structure and fabrication method of carbon nanosheets have been published in several peer-reviewed journals such as: Wang, J. J. et al., “ Free - standing Subnanometer Graphite Sheets ”, Applied Physics Letters 85, 1265-1267 (2004); Wang, J. et al., “ Synthesis of Carbon Nanosheets by Inductively Coupled Radio - frequency Plasma Enhanced Chemical Vapor Deposition”,  Carbon 42, 2867-72 (2004); Wang, J. et al., “ Synthesis and Field - emission Testing of Carbon Nanoflake Edge Emitters”,  Journal of Vacuum Science &amp; Technology B 22, 1269-72 (2004); French, B. L., Wang, J. J., Zhu, M. Y. &amp; Holloway, B. C., “ Structural Characterization of Carbon Nanosheets via X - ray Scattering ”, Journal of Applied Physics 97, 114317-1-8 (2005); Zhu, M. Y. et al., “ A mechanism for carbon nanosheet formation”,  Carbon, 2007.06.017; Zhao, X. et al., “ Thermal Desorption of Hydrogen from Carbon Nanosheets”,  Journal of Chemical Physics 124, 194704 (2006), as well as described by Zhao, X. in U.S. Patent “Supercapacitor using carbon nanosheets as electrode” (U.S. Pat. No. 7,852,612 B2); and Wang, J. et al., in U.S. Patent “Carbon nanostructures and methods of making and using the same” (U.S. Pat. No. 8,153,240 B2), which are incorporated herein by reference in their entirety. 
       SUMMARY OF THE INVENTION 
       [0011]    This invention is a fuel cell, whose components incorporate vertically free-standing graphene and carbon nanosheets. 
         [0012]    Performance enhancement mechanism of vertically free-standing graphene and carbon nanosheets for a fuel cell is based on unique properties of the graphene material: high electrical conductivity, large specific surface, high structural strength, high chemical stability, and large amount of active sites. 
         [0013]    The current collectors of a fuel cell, which incorporate vertically free-standing graphene and carbon nanosheets on their surface, can reduce inner resistance, hence increase power output and enhance the total efficiency of the fuel cell. In the same way, other components in a fuel cell that need conduct electrical current can also benefit from coating of vertically free-standing graphene and carbon nanosheets on the surface. 
         [0014]    Vertically free-standing graphene and carbon nanosheets can work as a supporting structure for catalyst in a fuel cell. The very large active surface area of vertically free-standing graphene and carbon nanosheets can enhance load of mass and efficiency of the catalyst. 
         [0015]    Dopants of other atomic elements on vertically free-standing graphene and carbon nanosheets can chemically bond to carbon atoms. The doped graphene materials form a low-cost carbon-based catalyst, compared to expensive noble-metal-based catalysts. Meanwhile, the strengthened mechanical structure and effort of high mass loading due to large active surface area of the graphene materials can remain. 
         [0016]    Further more, any layer-shaped component of in a fuel cell (e.g. polymer or ceramic films) can be structurally strengthened by integration of vertically free-standing graphene and carbon nanosheets. The electrical conductivity of any layers can also be enhanced by embedding vertically free-standing graphene and carbon nanosheets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view of a simplified proton exchange membrane fuel cell (PEMFC) in a cross-sectional view; 
           [0018]      FIG. 2  is a schematic diagram of an exemplary vertically free-standing carbon nanosheet in a cross-sectional view; 
           [0019]      FIG. 3  is a schematic diagram of exemplary vertically free-standing graphene and carbon nanosheets directly grown on surface of a solid-state substrate, which could be any layer-shaped components in any types of fuel cells. Surface of the layers/components can achieve excellent electrical conductivity via the graphene materials; 
           [0020]      FIG. 4  is a schematic diagram of exemplary catalyst particles disposed on vertically free-standing graphene and carbon nanosheets; 
           [0021]      FIG. 5  is a schematic diagram of an exemplary vertically free-standing graphene and carbon nanosheets being doped with other elemental atoms in order to form a catalyst; 
           [0022]      FIG. 6  is a schematic diagram of an exemplary layer in a fuel cell (e.g. PEMFC or SOFC) strengthened by a plurality of embedded vertically free-standing graphene and carbon nanosheets, in a cross-sectional view. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In accordance with techniques of certain exemplary embodiments, a fuel cell adopting vertically free-standing graphene and carbon nanosheets, in the cell&#39;s components of cathode, anode and electrolyte, is described herein. In the following description, for purpose of explanation, numerous specific details are set forth to provide a thorough understanding of the exemplary embodiments. It will be evident, however, to person skilled in the art that the exemplary embodiments may be practiced without these specific details. 
         [0024]    Referring now to the invention in more details, in  FIG. 3 , it shows a plurality of vertically free-standing graphene and carbon nanosheets  320  coated on surface of a solid state substrate  310 . The substrate  310  is made of an electrically conductive material such as copper, nickel, stainless steel, and various metals or alloys. The substrate  310  can be prepared into various morphologies, such as a foil, a thin film coated on other substrate, a plane structure with holes or a mesh. The surface can be roughened, trenched, etched, foamed or “corrugated” in order to enlarge the active surface area. The substrate  310  can also be a graphite, carbon paper, and carbon cloth. 
         [0025]    For the detailed structure of vertically free-standing graphene and carbon nanosheets  320 , refer to  FIG. 2 . 
         [0026]    A plurality of carbon nanosheets  320  can be incorporated to or grow up in-situ on the substrate  310  through various methods known in prior art such as a thermal chemical vapor deposition method or a Microwave/RF plasma-enhanced chemical vapor deposition method. Surface of the carbon nanosheets  320  can be activated by various methods. Likewise, the density (e.g. spatial density and width/height) of the carbon nanosheets  320  and the attachment geometry between the carbon nanosheets  320  and the substrate  310  may vary. The carbon nanosheets  320  can grow orthogonally on the substrate  310  (e.g. vertically free-standing from surface of the substrate  310 ). By varying the spatial density of the carbon nanosheets  320 , active surface area of the substrate  310  can be modulated. The carbon nanosheets  320  can also be of various sizes, thicknesses, and shapes (width and height). For instance, the carbon nanosheets  320  can have a single layer or multiple layers of graphene. 
         [0027]    The first exemplary embodiment is to directly grow up vertically free-standing graphene and carbon nanosheets on surface of a component  111   112   113   114   120   134   133   132   131  in a fuel cell for the purpose of enhancing their electrical current conductivity in general. Referring to  FIG. 1 , based on different functions, the components are defined as: Current Collectors  112  and  132  (CC) which are usually made of metal film and metal mesh; Gas Diffusion Layers  113  and  133  (GDL) which are usually made of a carbon cloth and/or carbon paper; reactant channel plates  111  and  131  (RCP) which is made of graphite and/or metal; Catalyst Layers  114  and  134  (CL), or Proton Exchange Membrane (PEM) layer  120 . 
         [0028]    In the first exemplary embodiment, the plurality of vertically free-standing graphene and carbon nanosheets  320  enhance surface electrical conductivity of a component in general. Especially, on the interface of a component and a fuel (gas or liquid), due to large contact surface, good electrical conductivity of the graphene materials, and more accessibility to catalysts&#39; surface, the structure of vertically free-standing graphene and carbon nanosheets  320  can dramatically enhance transport of electrons from the fuel to the cell&#39;s external electrical circuit, decrease the inner resistance, hence can enhance power, thus, increase totally efficiency of the fuel cell. 
         [0029]    The second exemplary embodiment is to directly grow up vertically free-standing graphene and carbon nanosheets  420  on a component for the purpose of enhancing performance of catalysts. 
         [0030]    Referring to  FIG. 4 , it shows a schematic diagram of a fuel cell component  400 , which comprises of a supporting substrate  410 , a plurality of vertically free-standing graphene and carbon nanosheets  420 , and catalyst particles  430 . 
         [0031]    Referring to  FIG. 1 , the anode catalyst  114  breaks down the fuel into electrons and protons, and is usually made of platinum particles. The particles generally have diameters in a few nanometers. The cathode catalyst  134  turns the protons and oxygen into water. The cathode catalyst is often made up of platinum, nickel or other nanomaterial-based catalysts. Contact of catalyst to the reactant (fuel and oxygen) is very important to the reaction, so that space distribution and total surface area of the catalyst particles is more critical than total mass of the catalyst. A good electrical conductivity of catalyst to the supporting structure/component is also important to improve electrical transport of the fuel cell. 
         [0032]    In the second exemplary embodiment, the plurality of vertically free-standing graphen and carbon nanosheets  420  provides a very large surface area. Further more, due to the properties of vertically free-standing graphene and carbon nanosheets  420 , they provide a strong mechanical support to the catalyst with high electrical conductivity. The substrate  410  is usually made of but not confined to carbon papers or carbon clothes. It is known in the prior arts that other materials (e.g. metal mesh) can also be used as substrate. The catalyst particle  430  can be metal (e.g. platinum and nickel) particles, metal oxide (e.g. CoO2) particles, or other materials in prior arts. The catalyst particles  430  can be loaded by various methods like vapor deposition, sputtering deposition, electroplating, electrodeposition, printing, paste coating and chemical deposition. 
         [0033]    The third exemplary embodiment is to incorporate vertically free-standing graphene and carbon nanosheets  420  into a component not suitable for in-situ growth. In the third exemplary embodiment, the vertically free-standing graphene and carbon nanosheets  420  with catalyst particles  430  can be peeled of via a film transfer technique, then can be mixed with other material to form an ink-like catalyst paste. Such a composite can be directly applied to surface of a gas diffusion layer or surface of an electrolyte membrane layer to form a catalyst embedded component. Noticeably, in the second embodiment, the vertically free-standing graphene and carbon nanosheets are grown up in-situ on a fuel cell component. Contrast to the second embodiment, the vertically free-standing graphene and carbon nanosheets in the third embodiment are grown up ex-situ of a fuel cell component, then they were incorporated into the fuel component. 
         [0034]    The fourth exemplary embodiment is to dope vertically free-standing graphene and carbon nanosheets with active atoms for the purpose of creating low-cost novel catalyst. Referring to  FIG. 5 , it shows a schematic diagram that a fuel cell catalyst  500  made of vertically free-standing graphene and carbon nanosheets  510  doped with active atoms  520 . 
         [0035]    Among substitutes for precious metals (e.g. Pt) as a fuel cell catalyst, carbon-based catalysts have a promising future. The dopants of other elements like nitrogen, iodine, sulphur, iron, etc. are bonded with carbon atoms. In the fourth exemplary embodiment, the dopant atoms  520  are bonded with the carbon atoms of the vertically free-standing graphene and carbon nanosheets  510 . The dopant atoms  520  can be bonded in graphene plane of vertically free-standing graphene and carbon nanosheets  510  structure as a substitute for the carbon atom. The dopant atoms  520  can also be bonded out of the graphene plane of vertically free-standing carbon nanosheets  510  structure. Further more, the dopant atoms  520  can be bonded to the edge of vertically free-standing carbon nanosheets  510 . 
         [0036]    To prepare the catalyst of the fourth embodiment, the ionized atoms of dopant need to be presented during the plasma enhanced chemical vapor deposition process of vertically free-standing graphene and carbon nanosheets growth. A high temperature (normally higher than 800° C.) chemical vapor deposition or physical vapor deposition can also bond the dopant atoms to the already formed vertically free-standing graphene and carbon nanosheets, especially on the edge. 
         [0037]    The fifth exemplary embodiment is to use vertically free-standing graphene and carbon nanosheets to strengthen brittle layered-components in a fuel cell, e.g. an electrolyte layer in a fuel cell. The electrolyte layer/component is a proton exchange membrane (PEM) for PEMFC or a Yttria-stabilized Zirconia ceramic layer for solid oxide fuel cell (SOFC). The brittle layers also include the electrode layers in a fuel cell, e.g. anode and cathode layers for a SOFC, which is normally made by ceramic materials. Referring to  FIG. 6 , it shows a schematic diagram of the fuel cell electrolyte supporting structure  600  comprising of a membrane  610  strengthened by a plurality of vertically free-standing graphene and carbon nanosheets  620 , in a cross-sectional view. 
         [0038]    Benefiting from the high strength and flexibility of the carbon nanosheets, a layer-shaped component embedded with a plurality of vertically free-standing graphene and carbon nanosheets  620  becomes much stronger in structure, which makes the layer durable and be able to endure larger temperature shock. Applied to initially brittle materials, such structure is also favorable for the roll-to-roll manufacturing method. For application of the electrolyte layers, the carbon nanosheets embedded membrane is very favorable as a substitute to the expensive Nafion membrane and membranes used in high temperature fuel cell. For application of the electrodes, besides strengthening their mechanical structure, the embedded vertically free-standing graphene and carbon nanosheets  620  can also enhance the electrical conductivity for the bulk. 
         [0039]    To make the structure described in the fifth exemplary embodiment, the polymer or ceramic material is impregnated into the free-standing graphene and carbon carbon nanosheets being grown up on a substrate, by various methods like vapor deposition, sputtering deposition, electroplating, electrodeposition, printing, spraying, paste coating and chemical deposition. After a membrane formation process (e.g. melting and concreting for polymer, annealing for ceramic) is applied, the membrane can be peeled off integrally with assistance of other techniques, like thermal release and ultrasonic release.