Patent Publication Number: US-2011065016-A1

Title: Fuel cell and fuel cell layer

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel cell and a fuel cell layer. 
     BACKGROUND ART 
     For the power source of portable electronic devices and the like that support the information society, the expectation for a fuel cell is increasing in recent years in view of the high power generation efficiency and high energy density as a unitary power generation device. A fuel cell is based on electrochemical reaction including oxidation of a reductant (for example, methane gas, hydrogen, methanol, ethanol, hydrazine, formalin, formic acid, or the like) at an anode electrode, and reduction of an oxidant (for example, the oxygen in the air, hydrogen peroxide, or the like) at a cathode electrode, generating electrical energy through the reaction. 
     Particularly, a direct methanol fuel cell (DMFC) utilizing methanol as the reductant does not require a reformer, and uses liquid fuel having a higher volume energy density than gaseous fuel. This provides the advantage that the fuel container can be reduced in size as compared to the case where a high-pressure gas cylinder typical of hydrogen is used. Therefore, a DMFC is suitably applicable in the usage of replacing a power source directed to small equipment, particularly a secondary battery for portable equipment. 
     Further, a DMFC allows the narrow and curved space that is dead space in a conventional fuel cell system to be used as a fuel storage space by virtue of the fuel being a liquid, providing the advantage that the design is not readily susceptible to restriction. This advantage facilitates the preferable application of the DMFC to portable small electronic equipment and the like. 
     Generally in a DMFC, a reaction set forth below occurs at the anode electrode and cathode electrode. At the anode electrode side, methanol and water react to generate carbon dioxide gas, protons, and electrons. At the cathode electrode side, the oxygen in the air, protons and electrons react to generate water. 
       CH 3 OH+H 2 O→CO 2 +6H + +6 e   −   Anode electrode
 
       O 2 +4H + +4 e   − →2H 2 O  Cathode electrode
 
     However, a DMFC conventionally has a low output per volume. It is desirable to improve the output per volume in view of reducing the size and weight of a fuel cell. 
     In general, a conventional fuel cell such as a polymer electrolyte fuel cell, a solid oxide fuel cell, a direct methanol fuel cell (DMFC), and an alkaline fuel cell is configured of the stacked layers including an anode separator having a fuel flow channel to supply a reductant; an anode collector and an anode gas diffusion layer for collecting electrons from an anode catalyst layer; the anode catalyst layer for promoting a reduction reaction; an electrolyte membrane for maintaining electrical insulation and for transmitting ions in precedence; a cathode catalyst layer for promoting an oxidation reaction; a cathode collector for supplying electrons to a cathode gas diffusion layer and the cathode catalyst layer; and a cathode separator having an air flow channel to supply an oxidant, in this order. 
     The anode separator and cathode separator generally serve to supply a reductant and an oxidant individually to the anode catalyst layer and the cathode catalyst layer, respectively, and also function as an anode collector and a cathode collector, respectively, using electrically conductive material. Based on the fact that the voltage of each unit fuel cell is low, a fuel cell is typically configured as a fuel cell stack capable of high voltage output, having stacked unit fuel cells such that an anode electrode and a cathode electrode of each unit fuel cell are brought into contact alternately. 
     In such a layered fuel cell stack, close electrical contact between respective layers must be maintained. If the contact resistance therebetween is increased, the internal resistance of the fuel cell will become higher to reduce the overall power generation efficiency. Further, a fuel cell stack generally has a sealing member in each fuel cell to prevent leakage of the reductant and oxidant. In order to ensure sufficient sealing and electrical conductance, each layer conventionally has to be constricted by a strong force. This induces the need of a fastening member such as a pressing plate, bolt, nut or the like to constrict each layer, leading to the problem that the fuel cell stack is increased in size and weight, and reduced in output density. 
     For example, Japanese Patent Laying-Open No. 2006-216449 (Patent Document 1) discloses a fuel cell including an anode catalyst layer and a cathode catalyst layer, and an anode diffusion layer and a cathode diffusion layer, stacked at either side of a solid electrolyte membrane, and an anode hydrophobic insulation layer and a cathode hydrophobic insulation layer, formed around the catalyst layers and diffusion layers, wherein the thicknesses of the anode hydrophobic insulation layer and the cathode hydrophobic insulation layer are less than or equal to the total thickness of the anode catalyst layer and the anode diffusion layer, and the total thickness of the cathode catalyst layer and the cathode diffusion layer, respectively. 
     Further, a general fuel cell has sealing members sandwiching a membrane electrode assembly formed of an anode, a solid electrolyte membrane, and a cathode, and the stacked body is further subject to pressure by means of a fastening member to improve the adherence between the layered members (for example, refer to Japanese Patent Laying-Open No. 2006-269126 (Patent Document 2)). 
     Moreover, as a fuel cell directed to reducing the size and weight, there is proposed a configuration that does not use a fastening member and that does not sandwich the solid electrolyte membrane with a sealing member such as a hydrophobic insulation layer while the membrane electrode assembly as well as a fuel supplying part and a cathode side separator constitute the same cross section at the side face of the fuel cell, which is sealed by a sealing member in order to prevent fuel leakage and oxidant leakage from each contacting face. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Laying-Open No. 2006-216449 
     Patent Document 2: Japanese Patent Laying-Open No. 2006-269126 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The fuel cell of Patent Document 1 does not have a fastening member constricting the fuel cell from the anode side and cathode side. Therefore, although the fuel cell stack is reduced in size and weight, the adherence at the contacting face between a fuel supplying part and the anode hydrophobic insulation layer, between the anode hydrophobic insulation layer and the solid electrolyte membrane, between the solid electrolyte membrane and the cathode hydrophobic insulation layer, and between the cathode hydrophobic insulation layer and a cathode side separator is insufficient. Thus, there was a problem that a gap is generated at these contacting faces, leading to the leakage of fuel and oxidant from the contacting faces. 
     Further, a fuel cell employing a fastening member may have the solid electrolyte membrane damaged and fractured by the contact with the sealing member caused by the intense constriction due to its thin thickness, leading to the problem of difficulty in supplying power stably to portable electronic equipment and the like. 
     Moreover, in the case where a fuel cell layer is configured having a plurality of fuel cells disposed apart, the gap region between adjacent fuel cells will be partially occupied by the sealing member. Therefore, there is a problem that it is difficult to form a sealing layer of high dimension accuracy. Thus, it is difficult to ensure a gap region of high dimension accuracy, leading to the problem of reduction in the diffusion region of the oxidant. 
     The present invention is directed to solving the problem set forth above. An object of the present invention is to provide a fuel cell and a fuel cell layer allowing fuel leakage and oxidant leakage to be suppressed without using a fastening member. 
     Means for Solving the Problems 
     The present invention provides a fuel cell including a membrane electrode assembly having a cathode electrode, an electrolyte membrane and an anode electrode in this order, and an anode collector layer. The anode collector layer includes a pair of first walls provided along two opposite sides. The membrane electrode assembly is fitted between the paired first walls such that the anode electrode faces the anode collector layer. 
     Preferably, the fuel cell of the present invention further includes a pair of second walls formed on the pair of first walls. Preferably, there is a gap space between the membrane electrode assembly and the second walls. Preferably, the gap space is filled with an insulative sealant to form an insulative sealant layer. 
     A side face of the membrane electrode assembly and a side face of the second wall facing the membrane electrode assembly may be substantially parallel. Further, the side face of the second wall facing the membrane electrode assembly may be inclined relative to the side face of the membrane electrode assembly. Moreover, the side face of the second wall facing the membrane electrode assembly may have a recess and a projection. The second wall is preferably formed of an electrically insulative material. 
     In the present invention, the second wall may be a layer formed of a porous material including an insulative sealant, arranged to form contact with the side face of the membrane electrode assembly. The second wall is preferably formed integrally with the anode collector layer. 
     The present invention also provides a fuel cell layer having a plurality of the fuel cells set forth above disposed with a gap region. 
     EFFECTS OF THE INVENTION 
     According to the present invention, there can be provided a fuel cell and a fuel cell layer absent of fuel leakage and oxidant leakage, without using a fastening member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically representing a preferable example of a fuel cell of the present invention. 
         FIG. 2  is a sectional view schematically representing another preferable example of a fuel cell of the present invention. 
         FIG. 3  is a sectional view schematically representing a further preferable example of a fuel cell of the present invention. 
         FIG. 4  is a sectional view schematically representing a further preferable example of a fuel cell of the present invention. 
         FIG. 5  is a sectional view of a fuel cell produced in Example 1. 
         FIG. 6  is a sectional view of a fuel cell produced in Comparative Example 1. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of a fuel cell and a fuel cell layer of the present invention will be described in detail hereinafter. The embodiments set forth below are all directed to a direct methanol fuel cell (DMFC) generating power by extracting protons directly from methanol. A methanol solution is used as a fuel, whereas air (specifically, the oxygen in the air) is used as an oxidant. 
     First Embodiment 
       FIG. 1  is a sectional view schematically representing a preferable example of a fuel cell of the present invention. A fuel cell  101  shown in  FIG. 1  includes a membrane electrode assembly  107  consisting of an electrolyte membrane  102 , an anode catalyst layer  103  arranged at one surface of electrolyte membrane  102 , a cathode catalyst layer  104  arranged at the other surface of electrolyte membrane  102 , an anode gas diffusion layer  105  arranged in contact with a surface of anode catalyst layer  103  opposite to the surface meeting electrolyte membrane  102 , and a cathode gas diffusion layer  106  arranged in contact with a surface of cathode catalyst layer  104  opposite to the surface meeting electrolyte membrane  102 . Cathode catalyst layer  104  and cathode gas diffusion layer  106  constitute a cathode electrode. Anode catalyst layer  103  and anode gas diffusion layer  105  constitute an anode electrode. An anode collector layer  108  is provided in contact with a surface of anode gas diffusion layer  105  opposite to the surface meeting anode catalyst layer  103 . Anode collector layer  108  has a fuel flow channel  109  that is the space for fuel transportation. Further, a cathode collector layer  113  is stacked in contact with a surface of cathode gas diffusion layer  106  opposite to the surface meeting cathode catalyst layer  104 . Cathode collector layer  113  has a through hole  112  to introduce air to the cathode electrode. 
     The fuel cell of the present embodiment includes the anode gas diffusion layer and the cathode gas diffusion layer. In the case where oxygen in the air is supplied uniformly to the cathode catalyst layer, and fuel is supplied uniformly to the anode catalyst layer, the anode gas diffusion layer and the cathode gas diffusion layer are dispensable. One or both of the anode gas diffusion layer and the cathode gas diffusion layer may be omitted. 
     Fuel cell  101  also includes an insulative sealing layer  114  formed at the side face of membrane electrode assembly  107 , and a second wall  116  provided on anode collector layer  108  to cover membrane electrode assembly  107  and insulative sealing layer  114 . 
     &lt;Electrolyte Membrane&gt; 
     The material for electrolyte membrane  102  is not particularly limited as long as it has proton conductivity and is electrically insulative. Preferably, the conventionally well-known appropriate polymer membrane, inorganic membrane, or composite membrane is employed. Examples of the polymer membrane include, for example, perfluorosulfonic acid based electrolyte membrane (NAFION (registered trademark) from E.I. du Pont de Nemours &amp; Co.), a Dow membrane (registered trademark, from Dow Chemical Company), ACIPLEX (registered trademark, from Asahi Kasei Corporation), Flemion (registered trademark, from Asahi Glass Co., Ltd.), as well as a hydrocarbon based electrolyte membrane such as of polystyrene sulfonic acid, sulfonated polyether ether ketone, and the like. Examples of the inorganic membrane include, for example, membranes of phosphate glass, cesium hydrogen sulfate, polytungstophosphoric acid, ammonium polyphosphate, and the like. Examples of the composite membrane include, a GORE-SELECT membrane (GORE-SELECT (registered trademark): by W.L. Gore &amp; Associates Inc.). 
     In the case where the fuel cell attains a temperature in the vicinity of or above 100° C., the electrolyte membrane is preferably composed of a material having high ion conductivity even in a low moisture content such as sulfonated polyimide, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), sulfonated polybenzimidazole, phosphonated polybenzimidazole, cesium hydrogen sulfate, ammonium polyphosphate, ionic liquid (ambient temperature molten salt) or the like. 
     The proton conductivity of the electrolyte membrane is preferably greater than or equal to 10 −5  S/cm. More preferably, a polymer electrolyte membrane having a proton conductivity greater than or equal to 10 −3  S/cm such as of perfluorosulfonic acid polymer, a hydrocarbon based polymer or the like is used. 
     &lt;Anode Catalyst Layer and Cathode Catalyst Layer&gt; 
     Anode catalyst layer  103  includes a catalyst promoting oxidation of the fuel. By causing oxidation reaction of the fuel on the catalyst, protons and electrons are generated. Cathode catalyst layer  104  includes a catalyst promoting reduction of the oxidant. The oxidant combines with the protons and electrons on the catalyst to cause reduction reaction. 
     For the aforementioned anode catalyst layer  103  and cathode catalyst layer  104 , a layer including a catalyst-supported carrier and an electrolyte, for example, may be employed. In this case, the anode catalyst in anode catalyst layer  103  functions to promote the reaction rate of generating protons and electrons from, for example, methanol and water. The electrolyte functions to transport the generated protons to the electrolyte membrane. The anode carrier functions to conduct the generated electrons to the anode gas diffusion layer. In cathode catalyst layer  104 , the cathode catalyst functions to promote the reaction rate of generating water from oxygen, protons, and electrons. The electrolyte functions to transport protons from the electrolyte membrane to the proximity of the cathode catalyst. The cathode carrier functions to conduct electrons to the cathode catalyst from cathode gas diffusion layer  106 . 
     The anode carrier and the cathode carrier are capable of conducting electrons and the catalyst also has electron conductivity. Therefore, anode catalyst layer  103  and cathode catalyst layer  104  do not necessarily have to include a carrier. In this case, supply or reception of electrons to/from anode gas diffusion layer  105  or cathode gas diffusion layer  106  is effected by the anode catalyst or cathode catalyst, respectively. 
     Examples of the anode catalyst and the cathode catalyst include a noble metal such as Pt, Ru, Au, Ag, Rh, Pd, Os and Ir; a base metal such as Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W and Zr; an oxide, a carbide, and a carbonitride of the noble metal or the base metal; and carbon. The material set forth above may be employed singularly or in combination of two or more types as the catalyst. The anode catalyst and the cathode catalyst may be of the same or different type of catalyst. 
     For the carrier employed in anode catalyst layer  103  and cathode catalyst layer  104 , a carbon based material having high electrical conductivity is preferable. Such carbon based material includes, for example, acetylene black, Ketchen black (registered trademark), amorphous carbon, carbon nanotube, carbon nanohorn and the like. In addition to such carbon based materials, a noble metal such as Pt, Ru, Au, Ag, Rh, Pd, Os and Ir; a base metal such as Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W and Zr; an oxide, a carbide, a nitride, and a carbonitride of the noble metal or the base metal can be enumerated. The material set forth above may be employed singularly or in combination of two or more types as the carrier. Further, a material having proton conductivity, specifically sulfated zirconia, zirconium phosphate, and the like may be employed for the carrier. 
     Although the material of the electrolyte employed in anode catalyst layer  103  and cathode catalyst layer  104  is not particularly limited as long as it has proton conductivity and electrically insulative, a solid or gel not dissolved by methanol is preferable. Specifically, for the material of the electrolyte, organic polymer having a strong acid group such as sulfonic acid group and phosphoric acid group or a weak acid group such as carboxyl group is preferable. Examples of such organic polymer include sulfonic acid group containing perfluorocarbon (NAFION (registered trademark), from E.I. du Pont de Nemours &amp; Co.), carboxyl group containing perfluorocarbon (Flemion (registered trademark): from Asahi Kasei Corporation), polystyrene sulfonic acid copolymer, polyvinyl sulfonic acid copolymer, ionic liquid (ambient temperature molten salt), sulfonated imide, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and the like. In the case where the aforementioned carrier provided with proton conductivity is used, anode catalyst layer  103  and cathode catalyst layer do not necessarily have to include the electrolyte since the carrier has proton conductivity. 
     A thickness of anode catalyst layer  103  and cathode catalyst layer  104  is preferably set less than or equal to 0.5 mm in order to reduce the resistance in proton conduction and electron conduction, as well as to reduce diffusion resistance in the fuel (for example, methanol) or the oxidant (for example, oxygen). Further, the thickness of anode catalyst layer  103  and cathode catalyst layer  104  is preferably at least 0.1 μm since sufficient amount of catalyst must be carried to improve the output as a cell. 
     &lt;Anode Gas Diffusion Layer and Cathode Gas Diffusion Layer&gt; 
     Anode gas diffusion layer  105  and cathode gas diffusion layer  106  are preferably formed of an electrically conductive porous body. For example, carbon paper, carbon cloth, metallic foam, sintered metal, nonwoven fabric of metal fiber, and the like can be employed. 
     A porosity of cathode gas diffusion layer  106  is preferably greater than or equal to 30% in order to reduce oxygen diffusion resistance, and preferably less than or equal to 95% in order to reduce the electrical resistance. More preferably, the porosity is 50 to 85%. A thickness of cathode gas diffusion layer  106  is preferably greater than or equal to 10 μm in order to reduce oxygen diffusion resistance in a direction perpendicular to the stacked direction of cathode gas diffusion layer  106 , and preferably less than or equal to 1 mm in order to reduce oxygen diffusion resistance in the stacked direction of cathode gas diffusion layer  106 . More preferably, the thickness is 100 to 500 μm. 
     &lt;Anode Collector Layer&gt; 
     Anode collector layer  108  is provided adjacent to anode gas diffusion layer  105 , and functions to transmit/receive electrons to/from anode gas diffusion layer  105 . In the present invention, one or more fuel flow channels  109  are formed at the anode collector layer. Examples of a suitable material employed for anode collector layer  108  include a carbon material; an electrically conductive polymer; a noble metal such as Au, Pt and Pd; a metal other than the noble metal such as Ti, Ta, W, Nb, Ni, Al, Cr, Ag, Cu, Zn and Su; Si; a nitride, a carbide, and a carbonitride of these metals; an alloy such as stainless steel, Cu—Cr, Ni—Cr, Ti—Pt and the like. More preferably, the material constituting the anode collector layer includes at least one element selected from the group consisting of Pt, Ti, Au, Ag, Cu, Ni and W. The inclusion of such elements reduces the specific resistance of the anode collector layer, which in turn alleviates reduction in the voltage caused by the resistance of the anode collector layer. Thus, a higher power generation property can be achieved. In the case where a metal having poor corrosion resistance under an acidic atmosphere such as Cu, Ag, or Zn is used, a coat of a noble metal having corrosion resistance such as Au, Pt, Pd, another metal having corrosion resistance, an electrically conductive polymer, an electrically conductive nitride, an electrically conductive carbide, an electrically conductive carbonitride, an electrically conductive oxide or the like may be applied to the surface. Accordingly, the lifetime of the fuel cell can be lengthened. 
     Fuel flow channel  109  is a flow passage for supplying fuel to anode catalyst layer  103 . The shape of the fuel flow channel is not particularly limited. For example, the cross section thereof may take a rectangular shape, as shown in  FIG. 1 . Fuel flow channel  109  can be provided by forming one or more grooves at the surface of anode collector layer  108  facing anode gas diffusion layer  105 . The fuel flow channel has a width of preferably 0.1 to 1 mm, and a cross sectional area of preferably 0.01 to 1 mm 2 . The width and the cross sectional area of the fuel flow channel are preferably determined taking into account the electrical resistance of anode collector layer  108  and the contacting area between anode collector layer  108  and anode gas diffusion layer  105 . 
     In the present embodiment, anode collector layer  108  has a pair of linear first walls  120  provided along two opposite sides. A recess is formed at the surface of anode collector layer  108  by the pair of first walls  120 . Fuel flow channel  109  is located at the bottom plane of the recess. Membrane electrode assembly  107  is fitted into the recess, so that a portion of the side face of anode gas diffusion layer  105  forms contact with the inner sidewall face of first wall  120  of anode collector layer  108 . The fitting of membrane electrode assembly  107  into the recess of anode collector layer  108  facilitates alignment between membrane electrode assembly  107  and anode collector layer  108  in the fabrication process. Thus, the fabrication cost can be reduced by simplifying the fabrication process of the fuel cell. In the case where second wall  116  is provided on first wall  120 , as will be described later, second wall  116  can be disposed with a predetermined distance from membrane electrode assembly  107  in high accuracy. Therefore, a space between membrane electrode assembly  107  and second wall  116  can be uniformly filled with an insulative sealing layer  114 . Accordingly, fuel leakage and oxidant leakage can be further suppressed. 
     A thickness of the portion of anode collector layer  108  in contact with the side face of membrane electrode assembly  107  (that is, a height of first wall  120  or a depth of the recess) is preferably set less than or equal to the total thickness of electrolyte membrane  102 , anode catalyst layer  103 , and anode gas diffusion layer  105 . Accordingly, contact between second wall  116  and the cathode electrode can be avoided suitably to prevent electrical shorting. 
     &lt;Second Wall&gt; 
     On the pair of linear first walls  120  of anode collector layer  108 , linear second wall  116  is preferably provided. Second wall  116  is arranged on first wall  120  so that a gap space is formed between a side face of membrane electrode assembly  107  and a side face of second wall  116  facing the side face of membrane electrode assembly  107 . Insulative sealing layer  114  that will be described afterwards is preferably formed in this gap space. 
     For the material of second wall  116 , an electron conductive material can be used. The usage of the electron conductive material allows second wall  116 , in addition to anode collector layer  108 , to function as an anode collector layer, thus suppressing reduction in power generation caused by voltage reduction resulting from lower resistance value. For the electron conductive material, a material similar to that of anode collector layer  108  can be preferably used. Examples of the electron conductive material include a carbon material; an electrically conductive polymer; a noble metal such as Au, Pt and Pd; a metal other than the noble metal such as Ti, Ta, W, Nb, Ni, Al, Cr, Ag, Cu, Zn and Su; Si; a nitride, a carbide, and a carbonitride of these metals; an alloy such as stainless steel, Cu—Cr, Ni—Cr, Ti—Pt and the like. More preferably, the material constituting the second wall includes at least one element selected from the group consisting of Pt, Ti, Au, Ag, Cu, Ni and W. In the case where a metal having poor corrosion resistance under an acidic atmosphere such as Cu, Ag, or Zn is used, a coat of a noble metal having corrosion resistance such as Au, Pt, Pd, another metal having corrosion resistance, an electrically conductive polymer, an electrically conductive nitride, an electrically conductive carbide, an electrically conductive carbonitride, an electrically conductive oxide or the like may be applied to the surface. 
     For the material employed for second wall  116 , it is more preferable to use an electron insulative material. Accordingly, electrical shorting can be prevented even if both the anode electrode and cathode electrode of membrane electrode assembly  107  form contact with second wall  116 . Examples of the insulative material preferably employed include an organic polymer material such as acrylic resin, ABS resin, polyimide resin, Teflon (registered trademark) resin, silicone resin and the like. More preferably, acrylic resin or ABS resin having favorable adherence with insulative sealing layer  114  that will be described afterwards is used. By increasing the binding force with the insulative sealing layer, the possibility of detachment between second wall  116  and the insulative sealing layer is eliminated. Thus, leakage of fuel and introduction of the oxidant to the anode electrode can be suppressed more effectively, and the reliability of the fuel cell can be increased. 
     Second wall  116  is formed so as to provide a predetermined gap space between second wall  116  and membrane electrode assembly  107  for introducing insulative sealing layer  114 . A width of second wall  116  is not particularly limited as long as a gap space for introducing insulative sealing layer  114  is formed between second wall  116  and membrane electrode assembly  107 . Although a thickness of second wall  116  is not particularly limited as long as a space for introducing insulative sealing layer  114  can be provided between second wall  116  and cathode collector layer  113 , durability against vibration in a direction perpendicular to a direction of the layer thickness can be increased by minimizing the space between second wall  116  and cathode collector layer  113  where insulative sealing layer  114  is to be introduced. Accordingly, the structure of the fuel cell and fuel cell layer can be enforced. 
     Although the configuration of second wall  116  is not particularly limited as long as the space for introducing insulative sealing layer  114  can be provided between second wall  116  and membrane electrode assembly  107 , the cross sectional shape of second wall  116  is preferably a rectangle, as shown in  FIG. 1 . In this case, the side face of membrane electrode assembly  107  and the side face of second wall  116  facing membrane electrode assembly  107  is parallel, or approximately parallel. 
     The cross sectional shape of the second wall is more preferably a triangle, or a pentagon, or a trapezoid like a second wall  216  shown in  FIG. 2 . In this case, the side face of the second wall facing the membrane electrode assembly is inclined with respect to the side face of membrane electrode assembly  107 , or has an inclined face with respect to the same. Such a configuration causes increase in the contacting area between the second wall and the insulative sealing layer, allowing the binding force to be increased. Therefore, fuel leakage and introduction of an oxidant to the anode electrode caused by detachment at the joining region can be suppressed further effectively. 
     Referring to  FIG. 3 , the side face of a second wall  316  facing membrane electrode assembly  307  (the side face in contact with insulative sealing layer  314 ) may have a recess and a projection. Thus, the contacting area between second wall  316  and insulative sealing layer  314  is increased to further secure the adherence between the two layers. Therefore, deviation of the arrangement in the stacked direction of membrane electrode composite  307  and insulative sealing layer  314  can be avoided even when a fuel cell does not have a cathode collector layer like a fuel cell  301 , allowing electric power to be supplied stably. Moreover, the number of components for the fuel cell can be reduced to lower the fabrication steps and fabrication cost. In addition, leakage of fuel and introduction of the oxidant to the anode electrode can be suppressed further effectively. 
     The second wall may be formed integrally with the anode collector layer by processing the base material constituting the anode collector layer through etching, cutting, or the like, likewise with the first wall. Alternatively, the second wall formed as a distinct member from the anode controller layer having the first wall may be coupled to the first wall of the anode collector layer. In the former case, durability against the force in a direction perpendicular to the stacked direction is improved. In addition, durability against towards bending stress is also improved. Accordingly, the structure of the fuel cell and fuel cell layer can be enforced. In the latter case, the material for the second wall can be selected without being influenced by the material for the anode collector layer. Accordingly, the cost for manufacturing a fuel cell can be reduced by selecting an economic material. Further, the adherence to the insulative sealing layer can be improved. 
     &lt;Cathode Collector Layer&gt; 
     Cathode collector layer  113  functions to transmit/receive electrons to/from cathode gas diffusion layer  106 , and includes a through hole  112  for communication between the outside of the fuel cell and cathode gas diffusion layer  106 . Since the cathode collector layer is generally maintained at a potential higher than that of the anode collector layer during power generation of the fuel cell, the material for the cathode collector layer preferably should have a corrosion resistance of a level equal to or greater than that of the anode collector layer. 
     The material for cathode collector layer  113  may be identical to that of anode collector layer  108 . In particular, it is preferable to use a carbon material; an electrically conductive polymer; a noble metal such as Au, Pt, Pd, a metal other than the noble metal such as Ti, Ta, W, Nb, Cr; a nitride and a carbide of these metals; an alloy such as stainless steel, Cu—Cr, Ni—Cr, Ti—Pt, or the like. In the case where a metal having poor corrosion resistance under an acidic atmosphere such as Cu, Ag, Zn, Ni is used, a coat of a noble metal having corrosion resistance, another metal having corrosion resistance, an electrically conductive polymer, an electrically conductive oxide, an electrically conductive nitride, an electrically conductive carbide, an electrically conductive carbonitride or the like may be applied to the surface. 
     A shape of cathode collector layer  113  is not particularly limited as long as oxygen in the air can be introduced into cathode gas diffusion layer  106 . In the case where cathode collector layer  113  of fuel cell  101  is greatly exposed to the atmosphere, and the concentration of the oxygen around cathode collector layer  113  does not decrease significantly even during operation of fuel cell  101 , cathode collector layer  113  preferably includes a plurality of through holes  112  extending in the direction of the layer thickness. Accordingly, the oxygen can be introduced efficiently from the air through the least number of through holes  112 , and reduction in the volume of cathode collector layer  113 , i.e. increase in the electric resistance, can be suppressed. This leads to suppressing reduction in the potential at cathode collector layer  113 , allowing electric power to be supplied stably. 
     In the case where a plurality of fuel cells  101  constitute a stacked structure, layered in the thickness direction, cathode collector layer  113  preferably includes a plurality of through holes extending in the direction of the plane, in addition to the plurality of through holes extending in the layer thickness direction. Accordingly, in a stacked structure where an anode collector layer of a second fuel cell is stacked close to a cathode collector layer of a first fuel cell, oxygen in air can be introduced into a cathode gas diffusion layer of the first fuel cell through the through holes extending in the plane direction, provided at a side face of the cathode collector layer. 
     Examples of cathode collector layer  113  of the above-described shape include foam metal, metal fabric, sintered metal, carbon paper, carbon cloth and the like. In a fuel cell  101  of the present invention, cathode collector layer  113  may be omitted. 
     &lt;Insulative Sealing Layer&gt; 
     Insulative sealing layer  114  is formed by filling the gap space located between membrane electrode assembly  107 , cathode collector layer  113 , and second wall  116  with an insulative sealant. By forming insulative sealing layer  114  at the gap space provided between membrane electrode assembly  107 , cathode collector layer  113  and second wall  116 , the adherence between members constituting the fuel cell is improved to prevent fuel leakage from the side face of membrane electrode assembly  107  and introduction of an oxidant from the side face of membrane electrode assembly  107  to the anode electrode. Further, by forming insulative sealing layer  114  to fill the gap space provided between membrane electrode assembly  107 , cathode collector layer  113  and second wall  116  in a fuel cell layer having a plurality of fuel cells arranged apart, or having a plurality of fuel cells so that a gap region is formed between fuel cells, the running of the insulative sealant from the side face of fuel cell  101  can be prevented in the filling step of the insulative sealant. As such, a region for diffusing an oxidant provided between adjacent fuel cells (the gap region provided between fuel cells) can be ensured in high accuracy. Thus, there can be provided a fuel cell and fuel cell layer allowing stable supply of electric power. 
     The insulative sealant employed for insulative sealing layer  114  prefereably contains a hydrophobic polymer material. The usage of an insulative sealant of such a material can prevent fuel leakage over a long period of time since swelling, hydrolysis, or the like by methanol solution fuel does not readily occur. The insulative sealant preferably consists of a material having high adherence with respect to membrane electrode assembly  107 , cathode collector layer  113 , and second wall  116 . 
     Examples of a specific material employed for the insulative sealant include fluorine-containing resin, fluorine-containing rubber, fluorine based surface finishing agent, silicon-containing resin, silicon-containing rubber, epoxy based resin, olefin based resin, polyamide based resin, and the like. 
     By providing insulative sealing layer  114  between second wall  116  and membrane electrode assembly  107  that allows adherence between each of the constituent members in a fuel cell of the above-described configuration, durability against vibration is increased so that electric power can be supplied stably. 
     Second Embodiment 
       FIG. 4  is a sectional view schematically representing another preferable example of a fuel cell of the present invention. A fuel cell  401  of  FIG. 4  includes a second wall  416 , between an anode collector layer  408  and a cathode collector layer  413 , and in contact with a membrane electrode assembly  407 . Second wall  416  is a layer formed of a porous material in which micropores are filled with an insulative sealant. In other words, second wall  416  is coupled to the side face of the membrane electrode assembly without the provision of a gap space between the second wall and the membrane electrode assembly, differing from the first embodiment set forth above. In the present embodiment, second wall  416  also functions as the aforementioned insulative sealing layer. The remaining configuration is similar to that of the first embodiment. 
     By employing a second wall of the above-described configuration, advantages similar to those of the first embodiment can be achieved. Further, since most of the side face of membrane electrode assembly  407  is arranged in contact with second wall  416 , the alignment between the membrane electrode assembly and the anode collector layer is facilitated in the fabrication process, allowing the fabrication cost to be reduced by simplifying the fabrication steps of the fuel cell. 
     EXAMPLES 
     The present invention will be described in further detail based on examples. It is to be understood that the present invention is not limited to these examples. 
     Example 1 
     A fuel cell  501  having the structure shown in  FIG. 5  was fabricated as set forth below. For an electrolyte membrane  502 , Nafion (registered trademark)  117  (from E.I. du Pont de Nemours &amp; Co.) of 40×40 mm and having a thickness of approximately 175 μm was employed. 
     Catalyst pastes were prepared by the procedures set forth below. Catalyst-supported carbon particles formed of Pt particles, Ru particles and carbon particles, having a Pt content of 32.5 wt % and a Ru content of 16.9 wt % (TEC66E50, from TANAKA KIKINZOKU KOGYO K.K.), an alcohol solution of 20 wt % Nafion (from Aldrich), ion-exchanged water, isopropanol, and zirconia beads were placed in a PTFE vessel at a predetermined ratio. These ingredients were mixed for 50 minutes at 500 rpm using a stirrer, followed by removing the zirconia beads to prepare a catalyst paste for an anode. In addition, a catalyst paste for a cathode was prepared under conditions similar to those of preparing the catalyst paste for an anode, using catalyst-supported carbon particles formed of Pt particles and carbon particles, having a Pt content of 46.8 wt % (TEC10E50E, from TANAKA KIKINZOKU KOGYO K.K.). 
     The anode catalyst paste was applied to the center on one surface of Nafion  117  that is the electrolyte membrane using a screen-printing plate having a window of 23×23 mm such that the catalyst content is 2 mg/cm 2 . Then, drying was performed at room temperature to form an anode catalyst layer  503  having a thickness of approximately 30 μm. Similarly, the catalyst paste for a cathode was applied to the center on the other surface of the Nafion  117  at a position corresponding to anode catalyst layer  503  to perform screen-printing in a manner similar to that described above such that the catalyst content is 3 mg/cm 2 . Then, drying was performed at room temperature to form a cathode catalyst layer  504  having a thickness of approximately 20 μm. Hereinafter, Nafion  117  having anode catalyst layer  503  and cathode catalyst layer  504  formed is referred to as CCM (Catalyst Coated Membrane). 
     For an anode gas diffusion layer  505  and a cathode gas diffusion layer  506 , two sheets of carbon paper GDL25BC (from SGL CARBON JAPAN Co., Ltd) having a water-repellant layer at one surface were cut to a size of 23×23 mm. 
     The CCM was superimposed on the water-repellant layer of the carbon paper such that the anode catalyst layer of the CCM is consistent with the carbon paper. Then, the other carbon paper qualified as cathode gas diffusion layer  506  was superimposed thereon such that the cathode catalyst layer of the CCM is consistent with the carbon paper. A stainless steel spacer of 600 μm in thickness was arranged along the perimeter of the CCM with respective members still superimposed. A hot press treatment was performed for two minutes at 130° C. and 10 kN to integrate each of the members to form a membrane electrode assembly. 
     The obtained membrane electrode assembly was sandwiched with polyethylene films and was cut to the size of 11 mm×21 mm by pressing a trimming knife perpendicularly while being held down by means of a plastic plate to obtain a membrane electrode assembly  507 . Each of the constituent layer formed the same cross section at all the four sides of membrane electrode assembly  507 . 
     An anode collector layer  508  was produced as set forth below. A flat plate of acid-resistant stainless steel having an outer shape of 14 mm×30 mm and a thickness of 500 μm was etched to have a groove of 300 μm in depth and 13 mm in width dug in the longitudinal direction. Thus, a linear second wall  513  of 500 μm in width was formed at both sides in the longitudinal direction of the anode collector layer. Then, a groove (recess) of 100 μm in depth and 11 mm in width was dug in the longitudinal direction, resulting in an anode collector layer having first wall  520  and second wall  530  formed in the longitudinal direction. First wall  520  had a width of 1.5 mm, on which second wall  513  having a width of 500 μm was formed. Further by etching, grooves of 100 μm in depth and 2 mm in width were formed in the longitudinal direction at the pitch of 1 mm, identified as fuel flow channels  509 . Thus, anode collector layer  508  was obtained. 
     The obtained membrane electrode assembly  507  was fitted in the recess of anode collector layer  508 . Epoxy resin was applied and spread into the gap space between the side face of membrane electrode assembly  507  and second wall  513  to obtain insulative sealing layer  511 . 
     Then, a silicon tube having an outer diameter of 2.5 mmφ (inner diameter 1.5 mmφ) (product of Tech-Jam Co., Ltd. ST1.5-2.5) identified as a fuel supply tube had a cut of 15 mm length formed in the longitudinal direction. The fuel cell was inserted in the cut so that the side face of the fuel cell where the end of the anode collector layer is open was inserted as far as the central region of the tube. The gap was filled with a sealant of silicon resin, followed by drying to form a connection portion of fuel supply. Thus, fuel cell  501  was obtained. 
     3M methanol aqueous solution was supplied at the rate of 0.5 ml/min. using a diaphragm pump to the obtained fuel cell  501 . It was confirmed that the fuel was not leaking during the supply of the fuel. 
     Comparative Example 1 
     A fuel cell  601  having the structure shown in  FIG. 6  was fabricated as set forth below. Membrane electrode assembly  607  was fabricated in a manner similar to that of Example 1. An anode collector layer  608  was produced as set forth below. A flat plate of acid-resistant stainless steel having an outer shape of 11 mm×30 mm and a thickness of 200 μm was etched to dig grooves of 100 μm in depth and 2 mm in width at the pitch of 1 mm, resulting in fuel flow channels  609 . Thus, anode collector layer  608  was obtained. 
     The obtained membrane electrode assembly  607  was arranged on anode collector layer  608 . Epoxy resin was applied and spread as thin as possible to both side faces formed by membrane electrode assembly  607  and anode collector layer  608  to form an insulative sealing layer  611 . A fuel supply tube was attached in a manner similar to that of Example 1 to obtain fuel cell  601 . 
     3M methanol aqueous solution was supplied at the rate of 0.5 ml/min. using a diaphragm pump to the obtained fuel cell  601 . During the supply of the fuel, fuel leakage was identified visually. 
     It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modification within the scope and meaning equivalent to the terms of the claims. 
     DESCRIPTION OF THE REFERENCE SIGNS 
       101 ,  201 ,  301 ,  401 ,  501 ,  601  fuel cell;  102 ,  202 ,  302 ,  402 ,  502 ,  602  electrolyte membrane;  103 ,  203 ,  303 ,  403 ,  503 ,  603  anode catalyst layer;  104 ,  204 ,  304 ,  404 ,  504 ,  604  cathode catalyst layer;  105 ,  205 ,  305 ,  405 ,  505 ,  605  anode gas diffusion layer;  106 ,  206 ,  306 ,  406 ,  506 ,  606  cathode gas diffusion layer;  107 ,  207 ,  307 ,  407 ,  507 ,  607  membrane electrode assembly;  108 ,  208 ,  308 ,  408 ,  508 ,  608  anode collector layer;  109 ,  209 ,  309 ,  409 ,  509 ,  609  fuel flow channel;  112 ,  212 ,  412  through hole;  113 ,  213 ,  413  cathode collector layer;  114 ,  214 ,  314 ,  511 ,  611  insulative sealing layer;  116 ,  216 ,  316 ,  416 ,  513  second wall;  120 ,  520  first wall.