Patent Publication Number: US-2009235651-A1

Title: Device and method for recovering catalyst for fuel cell

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2008-0024666 filed Mar. 18, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (a) Technical Field 
     The present invention relates to a device for recovering a catalyst for a fuel cell. More particularly, the present invention relates to a device and method for recovering a catalyst for a fuel cell, in which a carbon nanotube filter is provided in an air circulation loop of the fuel cell to recover platinum catalyst particles washed away from a catalyst layer of an air electrode during operation of the fuel cell. 
     (b) Background Art 
     Generally, a fuel cell is a device that generates electrical energy through an electrochemical reaction between hydrogen (H 2 ) and oxygen (O 2 ) and includes a membrane electrode assembly (MEA). The MEA includes a fuel electrode (anode) as an electrode catalyst layer to which hydrogen is supplied, and an air electrode (cathode) as an electrode catalyst layer to which air is supplied, with an electrolyte membrane, where hydrogen ions (H + ) are transmitted, interposed therebetween. The MEA further includes a gas diffusion layer (GDL) disposed on the outside of the anode and the cathode, respectively. 
     Accordingly, electrical energy is generated by an electrochemical reaction occurring when hydrogen as a fuel or a mixed gas containing a large amount of hydrogen is supplied to one of the electrode catalyst layers (anode, fuel electrode, or hydrogen electrode), and oxygen or air containing oxygen is supplied to the other electrode catalyst layer (cathode, air electrode, or oxygen electrode). 
     That is, the hydrogen supplied to the fuel electrode is dissociated into hydrogen ions (H + ) and electrons (e − ). The dissociated hydrogen ions move to the air electrode through the electrolyte membrane and, at the air electrode, the hydrogen ions (H + ) transferred from the fuel electrode combine with the electrons (e − ) transferred through an external conducting wire and oxygen supplied to the air electrode to produce water and heat at the same time, thus generating electrical energy. 
     A fuel cell system based on the above-described principle of electricity generation includes a fuel cell stack for generating electrical energy, a fuel supply system (hydrogen tank, hydrogen recirculation line, etc.) for supplying fuel (hydrogen) to the fuel cell stack, an air supply system (air supplier, membrane humidifier, etc.) for providing oxygen in the air, which is an oxidizer required for the electrochemical reaction, to the fuel cell stack, and a thermal management system (coolant pump, radiator, etc.) for removing the reaction heat of the fuel cell stack to the outside of the system and controlling the operation temperature of the fuel cell stack. 
     In particular, the air supply system includes an air blower for introducing fresh air from outside, an air supply line connected between an outlet of the air blower and an inlet of the air electrode of the fuel cell stack, a humidifier provided on the air supply line to humidify the fresh air (dry air), and an air discharge line through which the air after the reaction is discharged from the air electrode of the fuel cell stack to the humidifier. 
     Accordingly, the fresh air (dry air) introduced into the air supply system by the air blower is humidified by the humidifier, and then the humidified air is supplied to the air electrode of the fuel cell stack through the air supply line. 
     As a result, the oxygen supplied to the air electrode combines with the hydrogen ions (H + ) transferred from the fuel electrode and the electrons (e − ) transferred through an external conducting wire to produce water and heat at the same time, thus generating electrical energy. 
     However, there is a problem in that catalyst particles (e.g., particles containing platinum (Pt)) may be washed away from the air electrode, i.e., the electrode catalyst layer, during a long-term operation of the fuel cell stack. 
     That is, carbon corrosion occurs during the operation of the fuel cell stack and, especially, the catalyst is washed away from the catalyst layer of the air electrode during the long-term operation of the fuel cell stack. 
       FIG. 1  are microphotographs of a fresh MEA containing platinum as a catalyst and the MEA after being used for about 1500 hours. The fresh MEA shows no catalyst loss. In contrast, the used MEA shows a catalyst loss; that is the thickness of the electrode catalyst layer is reduced due to the catalyst loss from the catalyst layer of the air electrode after the long-term operation of the fuel cell stack. 
     Since the platinum of the catalyst layer of the air electrode is washed away in the form of ions, the platinum is drained away to the outside and thus cannot be recovered. As a result, it is not possible to recycle the platinum after mass production of the fuel cell vehicle. 
     The reason that the catalyst is washed away during the operation of the fuel cell will be described in detail below. 
     When the start-up and shut-down of the fuel cell are repeated for a long time, a carbon carrier in the catalyst layer of the air electrode, as shown in the dotted line box of  FIG. 2 , is oxidized and, at the same time, the platinum (Pt) is oxidized in the form of PtO X . At this time, since there are plentiful humidification water and product water in the corresponding air electrode as a result of the reaction, the platinum (Pt) may be eluted in an aqueous solution. 
     Like this, when the platinum of the electrode catalyst layer is washed away due to the repetition of the start-up and shut-down for a long time, the MEA itself is damaged, and accordingly the cell voltage is continuously lowered as shown in the graph of  FIG. 3 , thus deteriorating the performance of the fuel cell. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, the present invention has been made in an effort to solve the above-described drawbacks. 
     In one aspect, the present invention provides a device for recovering a catalyst for a fuel cell, the device comprising at least one filter means provided at a predetermined position or positions of an air circulation loop of a fuel cell stack to recover catalyst particles washed away from a catalyst layer of an air electrode of the fuel cell stack. 
     In another aspect, the present invention provides a method for recovering a catalyst for a fuel cell, the method comprising: passing air discharged from an air electrode of a fuel cell stack through a carbon nanotube filter; performing, at the carbon nanotube filter, a spontaneous reduction of catalyst ions and, at the same time, adsorbing reduced catalyst nanoparticles to carbon nanotubes to be recovered; introducing the air passing through the carbon nanotube filter and containing a small amount of catalyst ions into a humidifier; and mixing, at the humidifier, the small amount of catalyst ions with water and supplying the mixture to the fuel cell stack together with fresh air. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like. 
     The above and other features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinafter by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  are microphotographs showing that a catalyst layer of an air electrode is washed away from an MEA after a long-term operation in comparison with a fresh MEA; 
         FIG. 2  is a schematic diagram of a fuel cell stack; 
         FIG. 3  is a graph illustrating that a cell voltage is continuously lowered after a long-term operation of the MEA; 
         FIG. 4  is a schematic diagram showing a device for recovering a catalyst for a fuel cell in accordance with the present invention; and 
         FIG. 5  is a flowchart illustrating a method for recovering a catalyst for a fuel cell in accordance with the present invention. 
     
    
    
     Reference numerals set forth in the Drawings includes reference to the following elements as further discussed below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 10: fuel cell stack 
                 12: air blower 
               
               
                   
                 14: air electrode (cathode) 
                 16: air supply line 
               
               
                   
                 18: humidifier 
                 20: air discharge line 
               
               
                   
                 22: carbon nanotube filter 
                 24: exhaust line 
               
               
                   
                   
               
            
           
         
       
     
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the drawings attached hereinafter, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures. 
       FIG. 5  is a flowchart illustrating a method for recovering a catalyst for a fuel cell in accordance with the present invention. In the illustrated fuel cell, platinum is used as a catalyst. It should be noted that the method for recovering a catalyst can be applied to the fuel cell systems containing other catalysts than platinum. 
     An air supply system of a fuel cell includes an air blower  12  for introducing fresh air from outside, an air supply line  16  connected between an outlet of the air blower  12  and an inlet of an air electrode  14  of a fuel cell stack  10 , a humidifier  18  disposed on the air supply line  16  and humidifying the fresh air (dry air), and an air discharge line  20  through which the air after the reaction is discharged from the air electrode  14  of the fuel cell stack  10  to the humidifier  18 . 
     Accordingly, the fresh air (dry air) introduced into the air supply system by the air blower  12  is humidified by the humidifier  18 , and then the humidified air is supplied to the air electrode  14  of the fuel cell stack  10  through the air supply line  16 . Subsequently, the air after the reaction at the air electrode  14  of the fuel cell stack  10  is discharged to the humidifier  18  along the air discharge line  20  together with product water. 
     A filter means, which can recover the platinum catalyst particles washed away from the catalyst layer of the air electrode  14  of the fuel cell stack  10 , is provided at a position in an air circulation loop of the fuel cell stack  10 . 
     The filter means is a carbon nanotube filter  22 , in which single-walled carbon nanotubes or multi-walled carbon nanotubes are filled. The carbon nanotube filter  22  may be provided on either or both the air supply line  16  and the air discharge line  20 . 
     For reference, the carbon nanotube has a graphite structure consisting of carbon atoms and has electrical conductivity since p-orbital electrons around the carbon atoms are arranged in the shape of a plate like free electrons of a metal. On the other hand, since all bonding electrons are bonded in a diamond structure, differently from the graphite structure, the carbon nanotube has no electrical conductivity. As an atomic orbital of the carbon atom forming the graphite structure, p-orbitals shown in red are arranged in a line between atoms and form Tr bonding orbitals, thus having electrical conductivity. In the carbon nanotubes having the above-described atom and electron arrangements, a nanotube having a graphite plane is called a single-walled carbon nanotube (SWNT) and a nanotube having at least two graphite planes is called a multi-walled carbon nanotube (MWNT). 
     The operation of recovering the washed platinum catalyst particles by the carbon nanotube filter will be described in detail below. 
     The oxygen supplied to the air electrode  14  of the fuel cell stack  10  combines with the hydrogen ions (H + ) transferred from the fuel electrode and the electrons (e − ) transferred through an external conducting wire to produce water and heat at the same time, thus generating electrical energy. 
     During the long-term operation of the fuel cell stack  10 , the platinum (Pt) catalyst is washed away from the air electrode  14 , i.e., the electrode catalyst layer, and the Pt catalyst dissolved in water is mixed with the air discharged from the air electrode  14  after the reaction. 
     Accordingly, when the air including excessive air, water, and dissolved Pt ions and discharged from the air electrode  14  of the fuel cell stack  10  is supplied through the air discharge line  20 , it passes through the carbon nanotube filter  22  provided on the air discharge line  20 . 
     At this time, the Pt ions are reduced to the carbon nanotubes by a spontaneous reduction in the carbon nanotube filter  22 , and thus the reduced Pt nanoparticles are adsorbed to the carbon nanotubes and recovered. 
     That is, the Pt nanoparticles are adsorbed to the surface of the carbon nanotubes and recovered. 
     Subsequently, the exhaust air passing through the carbon nanotube filter  22  and containing a small amount of Pt ions is introduced into the humidifier  18  along the air discharge line  20 . 
     The small amount of Pt ions mixed with water is supplied to the air electrode  14  of the fuel cell stack  10  together with fresh air (humidification air) supplied to the inside of the humidifier  18  by the air blower  12 . 
     In this case, when the exhaust air passing through the carbon nanotube filter  22  is introduced into the humidifier  18  having the function of a gas-liquid separator along the air discharge line  20 , an excessive amount of air and a portion of water vapor are discharged to the outside through an exhaust line  24  of the humidifier  18 . However, since the Pt ions have a low vapor pressure, they are mixed with water in the humidifier  18  and returned to the air electrode  14  of the fuel cell stack  10 . 
     Thus, since the Pt ions are repeatedly circulated between the air electrode  14  of the fuel cell stack  10  and the humidifier  18 , the Pt ions may be easily adsorbed to the carbon nanotube filter  22 . 
     That is, since the elution rate of the Pt during the operation of the fuel cell stack  10  is not so much high, a small amount of Pt ions may be continuously detected, and thus the Pt ions can be recovered by the repetitive circulation between the air electrode  14  of the fuel cell stack  10  and the humidifier  18 . 
     As a result, it is possible to readily recover the platinum particles washed away from the air electrode of the fuel cell stack using the carbon nanotubes of the carbon nanotube filter. That is, assuming that the efficiency of the filter is 100%, if the amount of platinum used at the air electrode is about 56 g and half of the platinum is washed away after a long-term operation, it is possible to recover the platinum in an amount of 28 g per unit. 
     As described above, according to the present invention, it is possible to recover and reuse the catalyst in a simple and cost-effective manner and without the use of a peripheral power source. 
     The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.