Abstract:
An anode structure comprises an array of carbon nanotubes having a diffusion side and a membrane side, and catalyst particles interspersed on inner surfaces of the membrane side of the carbon nanotubes. The carbon nanotubes have an average diameter greater than the size of the hydrogen molecule but smaller than the size of the carbon monoxide molecule. Thus, hydrogen flowing toward the catalyst particles interspersed inside the carbon nanotubes are able to go through, while the flow of trace amounts of carbon monoxide contained in the hydrogen is blocked, preventing the poisoning of the catalyst particles by the carbon monoxide. A fuel cell utilizing the anode structure and a method for manufacturing the anode structure are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to an apparatus for protecting the catalyst particles of proton exchange membrane (PEM) fuel cells from poisoning by carbon monoxide, a fuel cell utilizing the apparatus, and a method for manufacturing the apparatus. 
     Fuel cells use the chemical energy produced in the reaction between hydrogen and oxygen to generate electricity that does not require combustion of fossil fuels and is clean. In a basic configuration of a fuel cell, hydrogen gas flows to the anode on one side of the fuel cell, while air flows to the cathode on the other side of the fuel cell. A platinum catalyst deposited on the anode splits the hydrogen molecule into two positive hydrogen ions (protons) and two negatively charged electrons. A proton exchange membrane (PEM) positioned between the anode and the cathode allows only the positively charged hydrogen protons to pass through it and reach the cathode. The negatively charged electrons travel along an external electrical circuit to the cathode, creating an electrical current. At the cathode, the negatively charged electrons and positively charged hydrogen ions combine with the oxygen typically from an air feed to form water which flows out of the cell. 
     The platinum catalyst of a fuel cell is susceptible to trace amounts of poisons, most notably carbon monoxide, which are generally present in hydrogen gas manufactured from the reforming of hydrocarbons. Catalyst poisons, such as carbon monoxide bind with the platinum thereby poisoning the electrode and decreasing fuel cell performance over time. Even trace amounts of carbon monoxide as low as 10 ppm can have a detrimental effect on the shelf life of the expensive platinum catalyst. 
     As can be seen, there is a need for a solution to reduce or eliminate the poisoning of platinum catalysts used for splitting the hydrogen molecule by carbon monoxide. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, an anode structure comprises an array of carbon nanotubes having a diffusion side and a membrane side, the carbon nanotubes having an average nanotube diameter and average nanotube length; and catalyst particles interspersed on inner surfaces of the membrane side of the carbon nanotubes. 
     In another aspect of the present invention, a fuel cell comprises a proton exchange membrane having an anode side and a cathode side; an anode structure having a diffusion side and a membrane side, the anode structure containing an array of carbon nanotubes and catalyst particles interspersed on inner surfaces of the membrane side of the array of carbon nanotubes, the membrane side of the anode structure being disposed in direct contact with the anode side of the proton exchange membrane; a cathode having a membrane side and a diffusion side, the membrane side of the cathode being disposed in direct contact with the cathode side of the proton exchange membrane; a hydrogen-side gas diffusion layer having an anode side and a diffusion side, the anode side of the hydrogen-side gas diffusion layer being disposed in direct contact with the diffusion side of the anode structure; an oxygen-side gas diffusion layer having a cathode side and a diffusion side, the cathode side of the oxygen-side gas diffusion layer being disposed in direct contact with the diffusion side of the cathode; a hydrogen-side bipolar plate having a channel side and a flat side, the channel side of the hydrogen-side bipolar plate being disposed in direct contact with the diffusion side of the hydrogen-side gas diffusion layer; an oxygen-side bipolar plate having a channel side and a flat side, the channel side of the oxygen bipolar plate being disposed in direct contact with the diffusion side of the oxygen-side gas diffusion layer; an external electrical circuit containing an electrical load, the electrical circuit connecting a collecting point on the hydrogen-side bipolar plate to a discharge point on the oxygen-side bipolar plate; a hydrogen-side end plate disposed in contact with the flat side of said hydrogen-side bipolar plate; and an oxygen-side end plate disposed in contact with the flat side of the oxygen-side bipolar plate. 
     In yet another aspect of the present invention, a method for manufacturing an array of carbon nanotubes for an anode structure comprises growing an array of carbon nanotubes on a silica crystal, the array of carbon nanotubes having a diffusion side, a membrane side, the membrane side having inner surfaces, the carbon nanotubes being characterized by an average nanotube diameter and average nanotube length; dipping the membrane side of the array of nanotubes in a hexachloroplatinum solution; heat treating the array of carbon nanotubes to decompose the hexachloroplatinum into platinum particles and chlorine and to remove the chlorine from the platinum particles in a manner that leaves the platinum particles interspersed on the inner surfaces of the membrane side of the array of nanotubes; exposing the platinum particles to hydrogen to effect a reduction of the platinum particles; and mechanically removing the array of carbon nanotubes from the crystal. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptualized illustration of filtering out the carbon monoxide molecules according to an embodiment of the present invention; 
         FIG. 2  is a magnified perspective front view of a multi-walled array of carbon nanotubes according to an embodiment of the present invention; 
         FIG. 3  is a perspective view of a fuel cell according to an embodiment of the present invention; 
         FIG. 4  is an exploded view of a fuel cell according to an embodiment of the present invention; 
         FIG. 5A  is a magnified perspective view of the hydrogen-side diffusion layer, the anode structure and the proton exchange membrane of  FIG. 3  according to an embodiment of the present invention; 
         FIG. 5B  is magnified perspective view of the anode structure of  FIG. 5  according to an embodiment of the present invention; and 
         FIG. 6  is a flow chart showing a method according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can be each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems above may not be fully addressed by any of the features described below. 
     Broadly, embodiments of the present invention provide for an anode structure for a fuel cell configured to block catalyst poisons, such as carbon monoxide or sulfur dioxide from reaching and contaminating the fuel cell catalyst, a fuel cell containing the anode structure, and a method of manufacturing the anode structure. The anode structure of the present invention contains an array of carbon nanotubes arranged to have a diffusion side and a membrane side and catalyst particles interspersed on the nanotubes&#39; membrane side inner surfaces. The catalyst may be platinum or an alloy of platinum family metals including but not limited to ruthenium and palladium. The average inner diameter of the nanotubes of the present invention must be greater than the size of a hydrogen molecule, which is about 0.29 nm, but smaller than the size of a carbon monoxide molecule, which is about 0.37 nm. A hydrogen gas, having molecules smaller than the nanotubes&#39; average diameter, may flow through the nanotubes, make contact with the catalyst particles and dissociate into hydrogen ions and electrons used to generate electrical power. The carbon monoxide present in the hydrogen, having a larger size than the nanotubes&#39; average diameter, may be prevented from entry into the nanotubes. In one embodiment, the average diameter of the carbon nanotube of the present invention may range from about 0.31 nm to about 0.36 nm. In another embodiment, the average diameter of the carbon nanotube of the present invention may range from about 0.33 nm to about 0.34 nm. The carbon nanotubes may have a single wall structure, a multiple wall structure or be vertically aligned. Carbon nanotubes having these characteristics have been disclosed in U.S. Pat. Nos. 6,835,330 and 6,916,434 that disclose a nested multi-walled carbon tube having mean spacing between hexagonal carbon layers, also referred to as d 002  spacing, of 0.34 nm or less. U.S. Pat. No. 6,630,772 discloses a nanotube formed by a laser ablation process that was measured having an inter-shell spacing d 002  of 3.4 Angstrom (equal to 0.34 nm). U.S. Pat. No. 6,869,583 suggests internal pores in a real material have gateways smaller that the kinetic diameter of about 0.36 nm. US pre-grant publication 20070195488 discloses interlayer distances of carbon materials in the range of 0.33-0.36 nm. It is desirable to manufacture the array of nanotubes with an average length that is as low as possible in order to minimize the length the hydrogen molecules need to travel, yet practical to manufacture. The preferred average length of the nanotubes of the present invention may range from about 100 nm to about 100,000 nm. 
     Referring to  FIG. 1 , there is shown a conceptualized illustration of a nanotube  51  showing hydrogen molecules going through the nanotubes which have an average diameter larger than the size of the hydrogen molecules. However, a carbon monoxide molecule may be blocked from entering the nanotube as its size is larger than the average nanotube diameter. 
     Referring to  FIG. 2 , there is shown a generalized illustration of a multi-walled array of nanotubes  70  showing rolled up carbon layers  72  having a nanotube  71  in the center. 
     The fuel cell containing the anode structure of the present invention may be assembled in the form that combines material layers and plates into a vertical assembly. The layers may be stacked and compressed against each other or against the plates to insure close contact between the surfaces. 
     Referring to  FIG. 3 , there is shown a front view of one embodiment of a fuel cell  10  according to one embodiment of the present invention including an anode structure  11 , a cathode  13 , a proton exchange membrane (PEM)  14  disposed between the anode structure  11  and the cathode  13 , a hydrogen-side gas diffusion layer  15  placed next to the anode structure  11 , an oxygen-side gas diffusion layer  12  positioned against the cathode  13 , a hydrogen-side bipolar plate  16  positioned against the hydrogen-side gas diffusion layer  15 , and an oxygen-side bipolar plate  18  disposed in contact with the cathode side gas diffusion layer  12 . External electrical circuit  17  may connect the hydrogen-side bipolar plate  16  with the oxygen-side bipolar plate  18 . The external electrical circuit  17  may contain an electrical load powered by the flow of the electrons such as an electrical appliance or an electric car. The hydrogen-side bipolar plate  16  and oxygen-side bipolar plate  18  may contain channels for distributing the hydrogen and air across the entire surface of the anode structure  11  and cathode  13  respectively. 
     In a typical operation, hydrogen flows through the hydrogen-side bipolar plate  16  and the hydrogen-side gas diffusion layer  15  which is in contact with the surface of the anode structure containing the catalyst. The hydrogen-side bipolar plate  16  and the hydrogen-side gas diffusion layer  15  help distribute the hydrogen flow over the entire area of the anode structure  11  to fully utilize the catalyst particles. The catalyst particles may react with a hydrogen molecule splitting it into two protons and two electrons:
 
H 2             2H + +2 e   − 

     The hydrogen protons may travel across the proton exchange membrane  14  designed to allow passage only to the positively charged protons and only in one direction toward the cathode  13 . The electrons may be generated around the catalyst inside the anode structure  11  and may travel across the array of carbon nanotubes, across the hydrogen-side gas diffusion layer  15  and across the hydrogen-side bipolar plate  16  to reach a collecting point on the hydrogen-side bipolar plate  16  from where they may enter the external electrical circuit  17 . Thus, the array of carbon nanotubes, the hydrogen-side gas diffusion layer  15  and hydrogen-side bipolar plate  16  should exhibit sufficient conductivity to allow the electrons to flow freely through these layers. Likewise, the oxygen-side bipolar plate  18  and gas diffusion layer  12  should be sufficiently conductive to enable the electrons to reach the cathode and combine with the oxygen from the air and hydrogen protons that reach the cathode through the proton exchange membrane to form water which flows out of the fuel cell as follows:
 
4H + +4 e   − +O 2             2H 2 O

     To perform their functions properly, the gas diffusion layers may be made of a porous material composed of a dense array of carbon fibers, which provides an electrically conductive pathway for current collection and helps provide a pathway for excess water removal. The materials of construction for bipolar plates may include graphite, graphite composites, and stainless steel and other metals. The proton exchange membrane may be made of a thin fluoropolymer, such as DuPont&#39;s Nafion® that will allow hydrogen protons to pass through. The end plates may be made from aluminum, titanium, or stainless steel alloys, while the cathode&#39;s material of construction may be graphite. 
     Referring to  FIG. 4 , there is shown an exploded view of the components of the fuel cell  20  according to an embodiment of the present invention including a cathode-side end plate  21 , a cathode-side bipolar plate  22 , a cathode-side gas diffusion layer  23 , a cathode  24 , a proton exchange membrane  25 , an anode structure  26 , a hydrogen-side gas diffusion layer  27 , an anode-side bipolar plate  28 , and an anode-side end plate  29 . The end plates  21  and  29 , also referred to as separator plates, may provide mechanical structure support for the fuel cell and act as a barrier to contain the hydrogen and oxygen. The end plates  21  and  29  may also separate multiple cells connected in series. 
     The fuel cell according to an embodiment of the present invention may optionally include a cooling system, such as a cooling plate, to maintain component temperatures in the proper range of about 80° C., and a moisture control system for the proton exchange membrane. The cooling system of the present invention may be manufactured according to the embodiment of a cooling system disclosed in US Pre-grant publication number 20050221149. The moisture control system of the fuel cell of the present invention may be manufactured according to the embodiment of a moisture control device disclosed in US Pre-grant publication number 20080292927. 
     Referring to  FIG. 5 , there is shown an illustration of an embodiment of the anode structure  32  according to an embodiment of the present invention disposed between a hydrogen-side gas diffusion layer  31  and a proton exchange membrane  33 . The hydrogen-side gas diffusion layer  31  may be porous and configured to contain channels for distributing the hydrogen across the entire area of the anode structure. The catalyst particles  34  are shown positioned at the end closest to the membrane  33 . In one embodiment of the present invention, the anode structure may be disposed next to and in intimate contact with the proton exchange membrane  33 . In another embodiment of the present invention, the membrane side of the anode structure containing the catalyst may be bonded to the membrane  33  by a process of pressing. The pressing procedure, as known in the art, typically requires pressing with a platen heated to about 120° C. at pressures ranging from about 80 lbs to about 200 lbs. 
     Referring to  FIG. 5A , there is shown a magnified perspective view of the anode structure of  FIG. 5  shown containing catalyst particles  42  interspersed on inner surfaces of the nanotube membrane side  41 . 
     Referring to  FIG. 6 , there is shown a flow chart depicting a method  60  according to the present invention. Step  62  may include growing an array of nanotubes on a silica crystal, the array of nanotubes having a diffusion side and a membrane side, the membrane side having inner surfaces. Step  64  may include dipping the membrane side of the array of nanotubes in a hexachloroplatinum solution. Dipping the membrane side in the hexachloroplatinum may have the effect of concentrating the catalyst particles on the membrane side of the array of nanotubes. Step  66  may include heat treating the array of carbon nanotubes to decompose the hexachloroplatinum into platinum particles and chlorine, in the form of hydrochloric acid, and to remove the chlorine from the platinum particles in a manner that leaves the platinum particles interspersed on the inner surfaces of the membrane side of the array of nanotubes. The heat treatment of the array of carbon nanotubes may be carried out at about 400° C. Step  67  may include exposing the platinum particles to hydrogen to effect a reduction of the platinum particles and step  68  may include mechanically removing the array of carbon nanotubes from the crystal. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.