Abstract:
A liquid-gas separator for a direct liquid feed fuel cell includes a tube having an opening portion at a sidewall thereof; liquid extracting members that selectively transmit the liquid in the tube and located at both ends of the tube; a gas extracting membrane that selectively transmits the gas and covers the opening portion; an inlet that guides the liquid and the gas into the tube; chambers that surround an outer side of the liquid extracting member; and outlets that guide the liquid in the chambers to the outside by being connected to the chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2005-55115, filed Jun. 24, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     An aspect of the present invention relates to a liquid-gas separator for segregating carbon dioxide and an unreacted liquid fuel discharged from an anode electrode of a direct liquid feed fuel cell.  
         [0004]     2. Description of the Related Art  
         [0005]     A direct liquid feed fuel cell is an apparatus that generates electricity by electrochemical reactions between an organic fuel, such as methanol or ethanol, and an oxidant, i.e., oxygen. The electricity generated by the direct liquid feed fuel cell has a high specific energy density and a high power density. Also, since liquid fuel, i.e., methanol, is fed directly to the cell, the direct feed fuel cell does not require a peripheral device, such as a fuel reformer, and storing and supplying the liquid fuel are easy.  
         [0006]     As depicted in  FIG. 1 , the direct feed fuel cell has a structure including an anode electrode  2 , a cathode electrode  3 , and an electrolyte membrane  1  interposed between the two electrodes  2  and  3 . The anode electrode  2  includes a diffusion layer  22  for supplying and diffusing fuel, a catalyst layer  21  at which oxidation reaction of the fuel occurs, and an electrode supporting layer  23 . The cathode electrode  3  also includes a diffusion layer  32  for supplying and diffusing the fuel, a catalyst layer  31  at which reduction reaction occurs, and an electrode supporting layer  33 . The catalyst for generating the electrode reaction is formed of a precious metal, such as platinum, having superior catalytic characteristics at low temperature. Alternately, to avoid catalyst poisoning by CO, which is a by-product of the electrode reaction, a transition metal alloy catalyst comprising ruthenium, rhodium, osmium, or nickel can be used. The electrode supporting layers  23  and  33  can be made of waterproofed carbon paper or waterproofed carbon fiber for easily supplying fuel and discharging reaction products. The electrolyte membrane  1  is a hydrogen ion exchange membrane having ion conductivity and containing moisture, and is formed of a polymer membrane having a thickness of 50-200 μm.  
         [0007]     An electrode reaction of a direct methanol fuel cell (DMFC), which is a type of direct liquid feed fuel cell, includes an anode reaction where fuel is oxidized and a cathode reaction where hydrogen and oxygen are reduced, as described below. 
 
CH 3 OH+H 2 O→CO 2 +6H + +6e −  (Anode reaction)  [Reaction 1]
 
3/2 O 2 +6H + +6e − →3H 2 O (Cathode reaction)  [Reaction 2]
 
CH 3 OH+3/2 O 2 →2H 2 O+CO 2  (Overall reaction)  [Reaction 3]
 
         [0008]     Carbon dioxide, hydrogen ions, and electrons are produced at the anode electrode  2  where the fuel is oxidized (reaction 1). The hydrogen ions migrate to the cathode electrode  3  through a hydrogen ion exchange membrane  1 . Water is produced by the reduction reaction between hydrogen ions, electrons transferred from an external circuit, and oxygen at the cathode electrode  3  (reaction 2). Accordingly, water and carbon dioxide are produced as the result of an overall electrochemical reaction (reaction 3) between methanol and oxygen. Two moles of water are produced when one mole of methanol reacts with oxygen.  
         [0009]     The liquid fuel used in the fuel cell may not be pure methanol, but may be a mixture with water produced in the system or already stored in the fuel cell system. When a fuel of high concentration is used, the performance of the fuel cell is greatly reduced due to crossover of the fuel through the electrolyte membrane (hydrogen ion exchange membrane). Therefore, methanol diluted to a low concentration, such as 0.5 to 2 M (2 to 8 volume %), is generally used.  
         [0010]      FIGS. 2A and 2B  are cross-sectional views of a liquid-gas separator used for a fuel cell. The orientation of the liquid-gas separator  10  used for a mobile fuel cell is not fixed at one orientation. At a normal orientation (refer to  FIG. 2A ), unreacted fuel and carbon dioxide enter the liquid-gas separator  10  through an inlet  11 . Carbon dioxide is exhausted into the air through a hole  12  formed on a ceiling of the liquid-gas separator body, and the unreacted fuel is recovered to the fuel cell through an outlet  13  formed on a lower part of the liquid-gas separator body.  
         [0011]     However, at a reversed orientation (refer to  FIG. 2B ) of the liquid-gas separator  10 , the outlets  12  and  13  of the unreacted fuel and carbon dioxide are changed. Accordingly, the carbon dioxide may pass into the anode electrode, and the unreacted fuel can be discharged to the outside.  
       SUMMARY OF THE INVENTION  
       [0012]     An aspect of the present invention provides a liquid-gas separator that performs liquid-gas separation regardless of its orientation, and a direct liquid feed fuel cell having the liquid-gas separator.  
         [0013]     According to an aspect of the present invention, there is provided a liquid-gas separator of a direct liquid feed fuel cell, which receives a gas and a liquid from the direct liquid feed fuel cell and separates the liquid and the gas, the liquid-gas separator including a tube having an opening portion at a sidewall thereof; liquid extracting members that selectively transmit the liquid in the tube to an outside area and are located at both ends of the tube; a gas extracting membrane that selectively extracts the gas and covers the opening portion; an inlet that guides the liquid and the gas into the tube; chambers that surround an outer side of the liquid extracting member; and outlets that guide the liquid in the chambers to the outside by being connected to the chamber.  
         [0014]     The liquid extracting member may be a first member having pores of 100 μm or less.  
         [0015]     The liquid extracting member may further include a second member having pores of a greater diameter than the first member, the second member located on the first member opposite to the liquid in the tube with respect to the first member.  
         [0016]     The second member may line the chamber.  
         [0017]     The gas extracting membrane may be formed of polytetrafluoro ethylene (PTFE).  
         [0018]     The gas extracting membrane may be formed by pressing the PTFE with a porous reinforcing member.  
         [0019]     The tube may further include a liquid absorbing member inside the tube.  
         [0020]     The liquid absorbing member may be beads having a predetermined diameter.  
         [0021]     The liquid absorbing member may have a specific gravity of 0.95 or less.  
         [0022]     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0024]      FIG. 1  is a cross-sectional view of the basic configuration of a direct liquid feed fuel cell;  
         [0025]      FIGS. 2A and 2B  are cross-sectional views of a liquid-gas separator used for a fuel cell;  
         [0026]      FIG. 3  is a schematic drawing of the conceptual configuration of a direct liquid feed fuel cell system having a liquid-gas separator according to an embodiment of the present invention;  
         [0027]      FIG. 4  is a cross-sectional view of a liquid-gas separator according to an embodiment of the present invention;  
         [0028]      FIG. 5  is a cross-sectional view of the liquid-gas separator of  FIG. 4  when liquid extraction units are positioned up and down;  
         [0029]      FIG. 6  is a graph showing the variation of internal pressure of a cylindrical tube of a liquid-gas separator according to an embodiment of the present invention; and  
         [0030]      FIG. 7  is a cross-sectional view of a liquid-gas separator according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0031]     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
         [0032]      FIG. 3  is a schematic drawing of the conceptual configuration of a direct liquid feed fuel cell system having a liquid-gas separator according to an embodiment of the present invention.  
         [0033]     Referring to  FIG. 3 , a direct liquid feed fuel cell system includes a direct liquid feed fuel cell, such as a fuel cell stack  190 , a liquid-gas separator  100  that exhausts carbon dioxide into the atmosphere and delivers liquid fuel to an anode using a water pump  191  after receiving diluted unreacted liquid fuel and carbon dioxide which is a product from an electrochemical reaction, a water pump  192  that conveys the diluted liquid fuel (methanol) from a fuel tank  195  to the fuel cell stack  190 , and a blower  193  that supplies air to the fuel cell stack  190 . Water produced at a cathode electrode is discharged or can be circulated to the liquid-gas separator  100  or the fuel tank  195 .  
         [0034]      FIG. 4  is a cross-sectional view of a liquid-gas separator  100  according to an embodiment of the present invention.  
         [0035]     Referring to  FIG. 4 , liquid extracting members  120 , which are separated a predetermined distance and face each other, are formed at ends of a cylindrical tube  110  having a predetermined diameter. An opening portion  111  is formed at a side wall of the cylindrical tube  110 , and a gas extracting membrane  115  is formed to cover the opening portion  111 . Chambers  130  are formed on the liquid extracting member  120 . Each chamber  130  includes an outlet  132 , and unreacted fuel in the chamber  130  is conveyed to the anode electrode of fuel cell stack  190  (at this time, the water pump P can be used) through the outlet  132 . An inlet  140  that guides the liquid fuel and carbon dioxide into the cylindrical tube  110  from the anode electrode of the fuel cell stack  190  is formed at an outer circumference of the cylindrical tube  110 .  
         [0036]     Each of the liquid extracting members  120  includes a first member  121  that contacts a material in the cylindrical tube  110  and a second member  122  that faces the chamber  130 . The first member  121  can be a foam member having pores of 100 μm or less, and the second member  122  can be a foam member having pores of 100 μm to 1 mm. The first member  121  selectively discharges the liquid fuel in the cylindrical tube  110  to the chamber  130 , and the second member  122  facilitates the discharge action of the first member  121 . The second member  122  may be installed to line the chamber  130 .  
         [0037]     The gas extracting membrane  115  can be formed of polytetrafluoro ethylene (PTFE) which is porous and has a hydrophobic property. The gas extracting membrane  115  may be shaped by pressing the PTFE with a porous reinforcing member (not shown) such as a porous cloth. The gas extracting membrane  115  blocks the outflow of the liquid fuel from the cylindrical tube  110  and allows the exhaustion of gas, i.e., carbon dioxide.  
         [0038]     The first member  121  of the liquid extracting members  120  includes a first surface  121   a  which faces the liquid fuel in the cylindrical tube  110  and a second surface  121   b  that contacts the second member  122 . When the liquid fuel in the tube  110  contacts a part of the liquid extracting member  120 , the liquid extracting member  120  gets soaked due to its hydrophilic properties under normal operating condition of the fuel cell system. Therefore, the gas in the tube  110  which is facing the first surface  121   a  must overcome the first capillary force of the first surface  121   a  of the first member  121  to infiltrate into the first member  121  through the first surface  121   a , and must overcome the second capillary force of the of the second surface  121   b  to infiltrate into the second member  122 . When the gas pressure P 1  of a gas entering through the inlet  140  is greater than the second capillary force, the liquid fuel infiltrates into the second member  122  through the first member  121  since the first capillary force is not generated at a region of the first surface  121   a  that contacts the liquid fuel. The second member  122  easily discharges the liquid fuel received from the first membrane  121  into the chamber  130 . The gas in the cylindrical tube  110  is exhausted to the atmosphere through the gas extracting membranes  115  by the internal pressure of the cylindrical tube  110 .  
         [0039]     The chamber  130  can be formed of metal, plastic, or flexible vinyl.  
         [0040]     The liquid-gas separator  100  can reliably separate the gas and liquid fuel when the liquid-gas separator  100  rotates around an axis that connects the two liquid extracting members  120 .  
         [0041]      FIG. 5  is a cross-sectional view of the liquid-gas separator of  FIG. 4  when liquid extraction members are positioned up and down. The liquid fuel moves to the lower chamber  130  through the first member  121  and the second member  122 , and the gas is exhausted to the atmosphere through the gas extracting membranes  115 .  
         [0042]     As described above, the liquid-gas separator  100  according to an embodiment of the present invention performs separation of the liquid-gas regardless of the position of the liquid-gas separator  100 .  
         [0043]      FIG. 6  is a graph showing a variation of internal pressure of a cylindrical tube of a liquid-gas separator according to an embodiment of the present invention.  
         [0044]     To measure the internal pressure of the cylindrical tube  110 , the inlet  140  of the cylindrical tube  110  is connected to an outlet of the water pump P, and an inlet of the water pump is connected to the outlet  132  of the cylindrical tube. As air is injected into the cylindrical tube  110  at a flow rate of 150 ml/min (stage 1), water is circulated at a flow rate of 40 ml/min (stage 2). That is, the liquid fuel from the water pump P and the gas are injected into the cylindrical tube  110 , and the liquid from the outlet  132  of the chamber  130  is circulated back to the water pump P. Next, the liquid-gas separator  100  is positioned as depicted in  FIG. 5  (stage 3). Referring to  FIG. 6 , the pressure in the cylindrical tube  110  is maintained constant, and accordingly, stable liquid-gas separation is observed.  
         [0045]      FIG. 7  is a cross-sectional view of a liquid-gas separator  200  according to another embodiment of the present invention. Reference numerals common to  FIG. 7  and  FIG. 4  denote like elements, and thus the descriptions thereof will be omitted.  
         [0046]     Referring to  FIG. 7 , liquid extracting members  120 , which are separated a predetermined distance and face each other, are formed at ends of a cylindrical tube  110  having a predetermined diameter. An opening portion  111  is formed at a sidewall of the cylindrical tube  110 , and a gas extracting membrane  115  is formed to cover the opening portion  111 . Chambers  130  are formed on the liquid extracting member  120 . Each chamber  130  includes an outlet  132 , and unreacted fuel in the chamber  130  is conveyed to the anode electrode of fuel cell stack  190  in  FIG. 3  through the outlet  132 . An inlet  140  that guides the liquid fuel and carbon dioxide into the cylindrical tube  110  from the anode electrode of the fuel cell stack  190  is formed at an outer circumference of the cylindrical tube  110 .  
         [0047]     The cylindrical tube  110  is filled with a liquid absorbing member that absorbs supersaturated liquid, such as a plurality of beads  210 . The beads  210  reduce the discharging of liquid from the cylindrical tube  110  by adsorbing the vapor state of water or the liquid fuel that passes through the gas extracting membranes  115  on their surfaces. The beads  210  preferably float in the cylindrical tube  110  by being formed of plastic having a lower specific gravity than the liquid entering the cylindrical tube  110  to contact the gas. Most of the liquid entering the cylindrical tube  110  is water, and the specific gravity of the liquid is close to 1 since the specific gravity of methanol is 0.79. Most plastics can be used for forming the beads  210  since they have a specific gravity lower than water. The beads  210  are preferably formed of a plastic having a specific gravity of 0.95 or less.  
         [0048]     A plastic having a honeycomb structure (not shown) can be used as the liquid absorbing member.  
         [0049]     The operation of the liquid-gas separator  200  according to another embodiment of the present invention is substantially the same as the liquid-gas separator  100  of  FIG. 4 , except for the fact that the beads  210  reduce the amount of the vapor state of water or liquid fuel that passes through the gas extracting membranes  115 .  
         [0050]     As described above, the liquid-gas separator according to an aspect of the present invention separates liquid and gas regardless of the position of the liquid-gas separator, when the liquid-gas separator is applied to a mobile direct liquid feed fuel cell in which the position of the liquid-gas separator can change at any time. Accordingly, a direct liquid feed fuel cell having the liquid-gas separator performs the liquid-gas separation function regardless of the position of the liquid-gas separator.  
         [0051]     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.