Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/8605—Porous electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/921—Alloys or mixtures with metallic elements
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
  • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
  • H01M2004/8684—Negative electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M2008/1095—Fuel cells with polymeric electrolytes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1009—Fuel cells with solid electrolytes with one of the reactants being liquid, solid or liquid-charged
  • H01M8/1011—Direct alcohol fuel cells [DAFC], e.g. direct methanol fuel cells [DMFC]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the present invention relates to a fuel cell anode, a membrane electrode assembly comprising the same, and a fuel cell using them. Further, the present invention also relates to a process for production of said fuel cell anode or membrane electrode.
• Polymer electrolyte fuel cells in particular, using aqueous solutions of methanol as fuels have been vigorously studied to use as electric power supplies for mobile devices because they can work even at a low temperature and can be downsized and lightened.
• conventional fuel cells still have insufficient performance to be used widely.
• a fuel cell is a device that converts chemical energy into electric energy by a catalytic reaction on the electrode.
• protons are generated on the anode catalyst and then transfer to the cathode through a proton-conductive polymer membrane. Therefore, for improving the performance of fuel cell, it is indispensable to develop a material having excellent proton-conductivity and to produce an electrode using that material.
• Proton-conductive polymers containing perfluorosulfonic acid are well-known proton-conductive materials.
• a tetrafluoroethylene/perfluorovinyl ether copolymer containing sulfonic acid groups as ion-exchange groups (e.g., Nafion [trademark], available from DuPont) is widely adopted as the proton-conductive material of the electrode catalyst layer.
• Nafion [trademark] available from DuPont
• a sulfuric acid-supported metal oxide e.g., disclosed in JP-A 2004-158261 (KOKAI)
• the sulfuric acid-supported metal oxide is a proton-conductive inorganic oxide having solid super-strong acidity, and is prepared by the steps of: loading sulfate ion on the surface of oxide containing at least one element selected from the group consisting of zirconium, titanium, iron, tin, silicon, aluminum, molybdenum, and tungsten; and then heating to fix the sulfate ion on the surface of the oxide.
• the sulfuric acid-supported metal oxide can have proton-conductivity.
• the sulfuric acid-supported metal oxide is an unstable proton-conductive material when used in a fuel cell, in particular, using a liquid fuel because the fuel cell generates water together with electric power. It is, therefore, presumed that there is much room for improvement in the sulfuric acid-supported metal oxide in consideration of ensuring stable electric power for a long time.
• JP-A 2006-32287 the present inventors have already proposed a fuel cell electrode comprising oxide carriers such as oxide of Ti, oxide particles such as oxide of W, and a binder for uniting them.
• oxide carriers such as oxide of Ti
• oxide particles such as oxide of W
• binder for uniting them.
• the binder is presumed to inhibit the correlations among the oxide super-strong acids or those among the PtRu catalyst carriers.
• a fuel cell anode comprises an electrode catalyst layer comprising : a supported catalyst comprises electrically conductive carriers and fine catalytic particles supported on said electrically conductive carriers, a proton-conductive inorganic oxide supporting SiO 2 on its surface, and a proton-conductive organic polymer binder; wherein the weight ratio (WS A +SJO 2 /W C ) of the SiO 2 -containing proton-conductive inorganic oxide (SA+SiU2) and the supported catalyst (C) is in the range of 0.037 to 0.25, the weight ratio (W P /WsA + si02) of the proton-conductive organic polymer binder (P) and the SiO2-containing proton-conductive inorganic oxide (SA-I-SiO 2 ) is in the range of 3.5 to 12, and the weight ratio (WSJO2/W S A) of the SiO 2 and the proton-conductive inorganic oxide (SA) is in the range of 0.1 to 0.5.
• a supported catalyst comprises electrically conductive carriers and fine catalytic particles supported
• a membrane electrode assembly comprises an anode, a cathode, and an electrolyte membrane placed between said anode and said cathode; wherein said anode has a catalyst layer which comprises an electrode catalyst layer comprising a supported catalyst comprises electrically conductive carriers and fine catalytic particles supported on said electrically conductive carriers, a proton-conductive inorganic oxide supporting SiO 2 on its surface, and a proton-conductive organic polymer binder, provided that the weight ratio (WSA+SJO 2 /W C ) of the SiO 2 -containing proton-conductive inorganic oxide (SA+SiO 2 ) and the supported catalyst (C) is in the range of 0.037 to 0.25, the weight ratio (W P /WSA+SJO2) of the proton-conductive organic polymer binder (P) and the SiO 2 -containing proton-conductive inorganic oxide (SA+SiO 2 ) is in the range of 3.5 to 12, and the weight ratio (WSIO 2 /W
• a fuel cell comprises an anode, a cathode, and an electrolyte membrane placed between said anode and said cathode; wherein said anode has a catalyst layer which comprises an electrode catalyst layer comprising a supported catalyst comprises electrically conductive carriers and fine catalytic particles supported on said electrically conductive carriers, a proton-conductive inorganic oxide supporting S1O 2 on its surface, and a proton-conductive organic polymer binder, provided that the weight ratio (WS A +S I O 2 /WC) of the Si ⁇ 2 ⁇ containing proton-conductive inorganic oxide (SA+Si ⁇ 2) and the supported catalyst (C) is in the range of 0.037 to 0.25, the weight ratio (W P /WSA+SIO2) of the proton-conductive organic polymer binder (P) and the Si ⁇ 2 -containing proton-conductive inorganic oxide (SA+Si ⁇ 2) is in the range of 3.5 to 12, and the weight ratio (WSJO2/WSA) of the Si ⁇ 2 and
• a process according to one embodiment of the present invention for preparation of a proton-conductive inorganic oxide comprises the steps of: dispersing SiU2 particles and oxide carriers containing at least one element Y selected from the group consisting of Ti, Zr and Sn 7 in a solution containing at least one element X selected from the group consisting of W, Mo and V; removing the solvent; and subjecting the residue to heat treatment.
• the present invention provides an inorganic oxide having high proton-conductivity. Particles of that inorganic oxide are prevented from growing, which lowers the power of fuel cell, and accordingly the power of fuel cell can be improved by the present invention.
• Figure 1 is a schematic cross-sectional view showing a fuel cell according to one embodiment of the present invention.
• the present inventors have studied about the relation between the composition of electrode and the synthesizing process of proton-conductive inorganic oxide. As a result, it is found that, if Si ⁇ 2 particles are used to synthesize the inorganic oxide, particle growth of the oxide can be inhibited to improve the proton-conductivity, and thus a proton-conductive inorganic oxide having high proton-conductivity can be obtained.
• the proton-conductive inorganic oxide can be freely selected from oxides known to those skilled in the art.
• it may be a proton-conductive inorganic oxide comprising oxide carriers containing at least one element Y selected from the group consisting of Ti, Zr and Sn, and thereon-supported oxide particles containing at least one element X selected from the group consisting of W, Mo and V.
• ⁇ X and Y-containing proton-conductive inorganic oxide As ⁇ X and Y-containing proton-conductive inorganic oxide.
• each of the X and Y may be a mixture of two or more elements, but even in that case, the mixture of elements selected from the group of X or Y is simply referred to as X or Y, respectively, just for the sake of convenience.
• the proton-conduction mechanism in the X and Y-containing proton-conductive inorganic oxide is not yet revealed in detail, but is presumed as follows.
• the oxide particles containing the element X (hereinafter, often referred to as "oxide particles A"), such as WO3, MOO3 or V2O5, are supported on the surfaces of the oxide carriers containing the element Y (hereinafter, often referred to as ⁇ oxide carriers B"), such as Ti ⁇ 2, Zr ⁇ 2 or SnCh, and thereby Lewis acid points are formed in the oxide carriers B.
• the Lewis acid points are then hydrated to be br ⁇ nsted acid points, which form a proton-conduction field.
• the formation of Lewis acid points is supposed to be further promoted.
• the number of accompanying water molecules, which are necessary for the proton-conduction can be reduced and, in addition, protons are generated by the reaction at the Lewis acid points.
• the proton conduction can be provided by only a small amount of water existing on the surface of the proton-conductive inorganic oxide. This means that large electric power can be generated without strictly controlling water.
• the proton-conductive inorganic oxide is incorporated in at least one of the anode, the cathode and the electrolyte membrane, and thereby the cell resistance can be reduced to increase the maximum power generated by the fuel cell.
• the oxide particles A are often water-soluble although the solubility thereof depends upon the elements or pH condition. Even so, however, since the oxide particles A are supported on the oxide carriers B having a low solubility in water, they are prevented from dissolving in water and hence the proton-conductive inorganic oxide is stable against water and liquid fuels. At the same time, other parts of the fuel cell and peripheral devices are also protected from contamination caused by ions released from the dissolved oxide particles A. As a result, according to the present invention, the fuel cell can keep high reliability for a long time. Further, if an inexpensive base material is used as the oxide carriers B, the production coat of the fuel cell can be reduced.
• the proton-conductive inorganic oxide further comprises Si ⁇ 2 supported on the surface.
• the supported Si ⁇ 2 may be in the form of fine particles or an accumulated layer.
• the Si ⁇ 2 supported on the surface can prevent the proton-conductive inorganic oxide particles from aggregating to grow (as described later in detail).
• TEM transmission electron microscopy
• XRD X-ray diffraction
• XPS X-ray photoelectron spectroscopy
• the oxide carriers B can be synthesized by the gas-phase method in which a gas containing the element Y is decomposed and converted into the oxide, or by the sol-gel method in which the oxide is obtained from a solution containing the element Y or from a metal alkoxide of the element Y.
• the synthesizing method is not particularly restricted.
• the oxide carriers B may be a compound oxide containing two or more metal elements. In order to obtain enough high proton-conductivity, SnCh is preferred. On the other hand, however, in order to obtain high proton-conductivity without increasing the production cost, Ti ⁇ 2 is preferred.
• the oxide carriers B which may be in the shape of, for example, particles, fibers, tabular grains, laminate flakes or porous grains.
• the surfaces of the oxide carriers B serve as proton-conductive sites, and for increasing the proton-conductive sites the oxide carriers B are preferably in the form of nanoparticles.
• the oxide particles A are preferably loaded on the surfaces of the oxide carriers B by the steps of: dispersing the oxide carriers B in a solution in which a substance containing the element X is dissolved, for example, dispersing the oxide carriers B in an aqueous solution of chloride, nitrate, hydroacid salt or oxo-acid salt of the element X or in an alcohol solution of the element X alkoxide; removing the solvent to load the substance of the element X on the oxide carriers B; and subjecting the substance to heat treatment so as to convert the substance into the oxide particles A.
• the process for loading the oxide particles A on the oxide carriers B is not restricted to the above.
• the oxide particles A may be a compound oxide containing two or more metal elements.
• the solvent is normally heated to remove. However, the solvent may be removed without heating under reduced pressure, or may be heated under reduced pressure.
• the oxide particles A may be supported on only a part of the surfaces of the oxide carriers B.
• the surfaces of the oxide carriers B may be dotted with the oxide particles A, or otherwise the oxide carriers B may be covered with a layer of the oxide particles A.
• the crystallinities of the oxide particles A and the oxide carriers B there is no particular restriction on the crystallinities of the oxide particles A and the oxide carriers B, as long as the oxide particles A are supported on the surfaces of the oxide carriers B.
• both of the oxide particles A and the oxide carriers B are preferably amorphous in consideration of promoting the formation of Lewis acid points, of improving the acidity, of reducing the production cost, and of simplifying the production process. More preferably, the oxide particles A are amorphous while the oxide carriers B are crystalline. However, both of the oxide particles A and the oxide carriers B may be crystalline, or otherwise the oxide particles A and the oxide carriers B may be crystalline and amorphous, respectively.
• the solvent is removed to load the material of the oxide particles A on the oxide carriers B.
• precursor particles of the proton-conductive inorganic oxide are formed.
• the precursor particles, in which the material of the oxide particles A is supported on the oxide carriers B are liable to aggregate firmly. It is not easy to break the aggregated particles, and hence the production cost may be increased.
• the oxide carriers B are sintered to grow.
• the particles are liable to grow even at a temperature much lower than the melting point of the oxide carriers B. If the particles grow in such manner, the specific surface area of the particles is lowered to decrease the proton-conductive sites. Consequently, the cell resistance increases and hence the resultant fuel cell as a whole cannot provide high electric power.
• the precursor particles of the proton-conductive inorganic oxide can be easily prevented from aggregation if SiCh particles are added when the oxide carriers B are dispersed in the solution in which a substance containing the element X is dissolved.
• the solvent is removed to obtain the oxide carriers B on which the SiO 2 particles are supported.
• the SiO 2 particles may be directly attached on the surfaces of the oxide carriers B, or they may be dissolved in the solution and then deposited on the oxide carriers B. It was confirmed that the added SiO 2 can prevent the oxide carriers B from sintering to grow in the heat treatment.
• the proton-conductive inorganic oxide synthesized in the above manner comprises not only the oxide particles A supported on the oxide carriers B but also the oxide particles A supported on the SiO 2 particles. It is difficult to isolate and remove the oxide particles A supported on the SiO 2 particles. Since SiO 2 is hardly sintered and the oxide particles A are not well-fixed thereon, the oxide particles A supported on the SiO 2 may be released and components of the released oxide particles A may contaminate parts of the fuel cell while the cell is working.
• all the oxide particles A supported on the SiO 2 particles are preferably washed away with cool or hot water.
• the unrefined proton-conductive inorganic oxide which is a mixture of the oxide particles A supported on the oxide carriers B and the oxide particles A supported on the SiO 2 particles, is washed with water to be a mixture of the oxide particles A supported on the oxide carriers B and the SiO 2 particles.
• the SiO 2 particles can prevent the particle growth, it is necessary to control the weight ratio (WSJO2/WSA) of the SiO 2 particles and the proton-conductive inorganic oxide (SA) in the range of 0.1 to 0.5. If the ratio is less than 0.1, the particle growth is insufficiently prevented. On the other hand, if it is more than 0.5, the proton- and electric conductivities of the electrode catalyst layer are reduced to lower the output power.
• the size of the SiO2 particles is preferably not larger than that of the oxide carriers B. In consideration of treatability and prevention of the particle growth, it is preferably in the range of 1 to 15 nm. Further, in view of availability, it is more preferably in the range of 3 to 10 nm. There is no particular restriction on the shape and crystallinity of the SiO2 particles, and hence favorable SiO ⁇ particles can be selected from the viewpoint of, for example, the production cost.
• the proton-conductive inorganic oxide preferably has a specific surface area as large as possible since the surface of the oxide serves as a proton-conductive site.
• the specific surface area is larger than 2000 m 2 /g, it is difficult to treat and to synthesize homogeneously the inorganic oxide.
• the specific surface area is preferably in the range of 10 to 2000 m 2 /g.
• the ratio (X/Y) of the elements X and Y contained in the particles A and the carriers B, respectively is preferably in the range of 0.0001 to 5, more preferably in the range of 0.01 to 1. If the element ratio (X/Y) is less than 0.0001, the supported particles A are often in such a small amount that the proton conduction field is insufficiently formed to lower the proton-conductivity. However, if the element ratio (X/Y) is more than 5, the supported particles A are often in such a large amount that the proton conduction field is covered with the particles A containing the element X and, consequently, that the proton-conductivity is impaired.
• the proton-conductive inorganic oxide used in the present invention is, for example, prepared by the steps of: loading the precursor of the oxide particles A on the oxide carriers B, and subjecting the obtained material to the heat treatment under oxidative atmosphere such as in air. If the heat treatment is carried out at a temperature below 200 0 C, it is difficult to convert the precursor into the oxide particles A and hence it is difficult to form sufficient chemical bonds between the oxide particles A and the oxide carriers B. As a result, the resultant oxide is liable to have low proton-conductivity. On the other hand, if the temperature is higher than 1000 0 C, the particles are often sintered and grow to decrease the specific surface area, so that high proton-conductivity cannot be obtained.
• the temperature of the heat treatment is preferably in the range of 200 to 1000 0 C, more preferably in the range of 400 to 800 0 C.
• the temperature is 200 0 C
• the temperature is nearly 1000 0 C
• the proton-conductive inorganic oxide can be synthesized by the heat treatment for a short time because the bonds are easy to form.
• the proton-conductive inorganic oxide preferably has solid super-strong acidity.
• the degree of proton dissociation can be represented by the acidity, and the acidity of the solid acid can be indicated by Hammett acidity function Ho.
• the Hammett acidity function Ho of sulfuric acid is -11.93.
• the acidity of the solid super-strong acid can be estimated by color reactions of indicators.
• the acid indicator changes the color when adsorbed on the surface of the solid super-strong acid.
• the acidity can be estimated by observing the change of the color.
• the solid acidity of colored oxide super-strong acid such as Sn ⁇ 2 by the color reactions of the indicators.
• the solid super-strong acidity can be estimated by the temperature-programmed desorption (TPD) of ammonia.
• TPD temperature-programmed desorption
• the process of the TPD comprises the steps of: adsorbing ammonia gas on the sample of the solid acid, heating the sample to release the ammonia, measuring and analyzing the amount of the released ammonia and the temperature.
• the fuel cell anode catalyst according to the present invention contains a supported catalyst comprising electrically conductive carriers and fine catalytic particles supported thereon.
• the fine catalytic particles can be freely selected from known fuel cell anode catalysts, and normally contains Pt and Ru as the main catalytic elements. Further, auxiliary catalytic elements can be incorporated so as to enhance the catalytic activity. In many cases, an alloy of Pt and Ru is used because Pt is effective both in oxidation of hydrogen and in dehydrogenation of the organic fuel and because Ru is very effective in prevention of poisoning by CO. It should be noted that the catalytic activity is lowered if the content of Ru is too small.
• the electrically conductive carriers are, for example, carbon black particles.
• the conductive carriers there is no particular restriction on the conductive carriers as long as they have enough electric conductivity and stability to serve as the carriers.
• nano-carbon materials developed in these days such as carbon nanoparticles, carbon nanofiber and carbon nanocoils, can be employed.
• electrically conductive ceramic materials are also employable as the carriers. The process for preparation of the supported catalyst used in the present invention is then explained below.
• the fine catalytic particles can be loaded on the electrically conductive carriers by known methods such as the impregnated method, the precipitation method and the colloidal method. However, those methods are often unsuitable for preparation of a metal catalyst oxidizable enough to react with water. In that case, the catalytic particles are generally loaded on the conductive carriers directly by the sputtering method or by the vapor-deposition method. According to those methods, a catalyst having metallic bonds in a particular mixed state is easily obtained. However, there is no particular restriction on the method, and the catalytic particles may be loaded on the electrically conductive carriers by any method.
• the mean particle size of the catalytic particles is preferably 10 nm or less. If it is larger than 10 nm, the catalytic activity is liable to deteriorate considerably.
• the mean particle size is more preferably in the range of 0.5 to 10 nm. If the mean size is smaller than 0.5 nm, it is difficult to control the synthesizing process of the catalyst and hence it costs a lot to synthesize the catalyst.
• the catalytic particles having a mean size of 10 nm or less (primary particles) can be singly used, but aggregates of them (secondary particles) may be used.
• the mean particle size is calculated from the half-width of the peak obtained by the XRD measurement.
• the proton-conductive organic polymer binder is preferably an organic polymer having sulfonic acid groups since it has high proton-conductivity.
• Examples of the proton-conductive polymer include sulfonic acid group-containing fluororesins such as Nafion ([trademark], available from DuPont, FLEMION ([trademark], available from Asahi Glass Co., Ltd.) and Ashiplex ([trademark], available from Asahi Kasei Corporation).
• Nafion [trademark] available from DuPont
• FLEMION [trademark], available from Asahi Glass Co., Ltd.
• Ashiplex [trademark], available from Asahi Kasei Corporation
• a fuel cell anode comprises an electrode catalyst layer.
• the electrode catalyst layer comprises: the supported catalyst comprises electrically conductive carriers and fine catalytic particles supported on said electrically conductive carriers; the proton-conductive inorganic oxide supporting SiO2 on its surface; and the proton-conductive organic polymer binder.
• the weight ratio (Ws A+ si02/W c ) of the Si ⁇ 2-containing proton-conductive inorganic oxide (SA-I-SiO 2 ) and the supported catalyst (C) be in the range of 0.037 to 0.25; that the weight ratio (Wp/W SA +sio2) of the proton-conductive organic polymer binder (P) and the SiO 2 -containing proton-conductive inorganic oxide (SA-I-SiO 2 ) be in the range of 3.5 to 12; and that the weight ratio (W S JO 2 /WSA) of the SiO 2 and the proton-conductive inorganic oxide (SA) be in the range of 0.1 to 0.5.
• the ratio of WSA+SIO2/W C is smaller than 0.037, the proton-conductivity is lowered to impair the output power.
• the WsA + sio 2 /W c is larger than 0.25, the electric conductivity is lowered to impair the output power.
• the ratio of WP/WS A +SIO 2 is smaller than 3.5, the proton-conductivity is lowered to impair the output power.
• the W P /WS A +SIO2 is larger than 12, the electric conductivity is lowered to impair the output power. It has been hitherto unknown that the performance of fuel cell can be remarkably improved by optimizing the weight ratios among the supported catalyst, the proton-conductive inorganic oxide, and the proton-conductive organic polymer binder.
• a membrane electrode assembly comprises an anode (fuel electrode), a cathode (oxidant electrode), and a proton-conductive membrane placed between the anode and the cathode. Further, a fuel cell according to one embodiment of the present invention comprises that membrane electrode assembly.
• FIG. 1 is a schematic cross-sectional view showing a fuel cell according to one embodiment of the present invention.
• a membrane electrode assembly (hereinafter, often referred to as "MEA") 12 comprises an anode 2, a cathode 3 and a proton-conductive membrane 1.
• the anode 2 comprises a diffusion layer 4 and an anode catalyst layer 5 provided thereon.
• the cathode 3 comprises another diffusion layer 6 and a cathode catalyst layer 7 provided thereon.
• the anode 2 and the cathode 3 are laminated on the opposite sides of the proton-conductive membrane 1 so that the anode catalyst layer 5 and the cathode catalyst layer 7 may face to each other.
• the MEA is, for example, placed between a cathode holder 9 equipped with an oxidant gas-supplying path 8 and an anode holder 11 equipped with a fuel-supplying path 10, to form a single cell as shown in Figure 1.
• the anode catalyst layer 5 contains the supported catalyst described above, the proton-conductive inorganic oxide, and the proton-conductive organic polymer binder.
• the supported catalyst is comprises the fine catalytic particles and the electrically conductive carriers.
• the anode catalyst layer 5 contains the supported catalyst, the proton-conductive inorganic oxide, and the proton-conductive organic polymer binder for uniting them together.
• the contents thereof preferably satisfy the above conditions to ensure optimal proton-conduction.
• a concentrated fuel for example, 100% methanol is used for a long time
• the proton-conductive organic polymer binder is often so volume-expanded that the supported catalyst and the proton-conductive inorganic oxide cannot be united enough to keep the catalyst layer. Therefore, a non-proton-conductive binder may be incorporated so as to keep the catalyst layer.
• the non-proton-conductive binder since the non-proton-conductive binder has neither electric conductivity nor proton conductivity, it is liable to increase the cell resistance or to prevent the reactant from diffusing, and as a result, it often impairs the output power.
• the content of the non-proton-conductive binder is in the range of preferably 0 to 10%, more preferably 0 to 5% per the total weight of the catalyst layer.
• non-proton-conductive binder examples include polytetrafluoroethylene (PTFE) resin, tetrafluoroethylene-hexafluoropropyrene copolymer (FEP) resin, and tetrafluoroethylene-fluoroalkylvinyl- ether copolymer (PFA) resin.
• PTFE polytetrafluoroethylene
• FEP tetrafluoroethylene-hexafluoropropyrene copolymer
• PFA tetrafluoroethylene-fluoroalkylvinyl- ether copolymer
• the cathode catalyst layer 7 contains a cathode catalyst and a proton-conductive binder.
• a cathode catalyst for example, Pt can be used.
• the catalyst may be loaded on carriers, but does not need to be supported.
• the diffusion layers 4 and 6 may be electrically conductive porous sheets, which are made of gas- or liquid-penetrating materials such as carbon cloth or carbon paper.
• the anode and the cathode can be produced by known methods.
• the materials described above are mixed and dispersed in water or an organic solvent such as an alcohol to prepare catalytic slurry, which is then coated on the aforementioned electrically conductive porous sheet, dried and, if needed, fired to form a catalyst layer.
• the dispersing method is not particularly restricted, and, for example, a dissolver or a ball-mill can be used.
• the electrolyte membrane and the electrodes are unified by means of an apparatus capable of heating and/or pressing, normally, by means of a hot-press.
• the pressing temperature is higher than the glass transition temperature of hydrophilic polymer adopted as an adhesive between the electrolyte membrane and the electrodes, and is generally in the range of 100 to 400 0 C.
• the pressing pressure depends upon the hardness of the electrodes, but is normally in the range of 5 to 200 kg/cm 2 (19.6MPa).
• a fuel cell according to one embodiment of the present invention comprises the aforementioned MEA, a means for supplying a fuel to the anode, and a means for supplying an oxidant to the cathode.
• the fuel cell may comprise only one MEA or plural MEAs.
• the plural MEAs enable the fuel cell to output high electric power.
• As the fuel methanol, ethanol, formic acid and an aqueous solution containing at least one of them are employable, for example. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
• a proton-conductive inorganic oxide was synthesized.
• aqueous ammonia 1.5 g of vanadium oxide V2O5 was dissolved.
• 9 g of titanium oxide ⁇ O2 was dispersed in 500 ml of distilled water.
• the V2O5 solution and the TiCh dispersion liquid were mixed, and 3.0 g of silicon oxide Si ⁇ 2 was dispersed therein.
• the obtained mixture was then heated with stirring to 8O 0 C, and was evaporated at a water-vaporizing speed of 100 ml/hour. The residue was dried at 100 0 C in an oven for 12 hours to obtain a powder.
• the powder was placed in an alumina crucible, heated to 500 0 C at an elevation rate of 100 0 C /hour and left at 500 0 C for 4 hours to prepare a mixture of vanadium oxide-supported titanium oxide and vanadium oxide-supported silicon oxide. Successively, the mixture was washed with hot water to obtain another mixture of silicon oxide and vanadium oxide-supported titanium oxide in which the element ratio X/Y between vanadium (X) in the vanadium oxide and titanium (Y) in the titanium oxide was 0.1. The mixture of silicon oxide and vanadium oxide-supported titanium oxide was subjected to X-ray diffraction analysis.
• the element ratio (X/Y), the solid acidity and the specific surface area of the obtained powdery proton-conductive inorganic material were measured by the inductively coupled plasma-atomic emission spectroscopy (ICP-AES), by the acid indicators or the temperature-programmed desorption (TPD) of ammonia and by the BET method, respectively. The results were as set forth in Table 1.
• anode was produced from the obtained proton-conductive inorganic oxide.
• a mixture of 2 g of PtRu-supported catalyst, 0.24 g of the proton-conductive inorganic oxide, 7.47 g of 20 % Nafion ([trademark], available from DuPont) solution, 3 g of water and 3 g of ethanol was stirred and dispersed well to prepare a slurry.
• the obtained slurry was coated by means of a control coater onto a carbon paper (350 ⁇ m, available from Toray Industries, Inc.) previously subjected to water-repelling treatment, and dried to produce an anode containing the noble metal catalyst in a loading density of 1 mg/cm 2 .
• the weight ratio (W S A + si02/W c ) of the Si ⁇ 2 ⁇ containing proton-conductive inorganic oxide (SA+Si ⁇ 2) and the supported catalyst (C), the weight ratio (W P /WSA+SJO2) of the proton-conductive organic polymer binder (P) and the Si ⁇ 2-containing proton-conductive inorganic oxide (SA+Si ⁇ 2), and the weight ratio (WSJO2/W S A) of the S1O2 and the proton-conductive inorganic oxide (SA) were as set forth in Table 1.
• Example 1 The procedure of Example 1 was repeated, except that 1.5 g of vanadium oxide V 2 Os was replaced with 2.4 g of molybdenum oxide MoO3, the amount of silicon oxide SiO 2 was changed from 3.0 g to 3.2 g, and the temperature of heating was changed from 500 0 C to 600 0 C, to obtain a mixture of silicon oxide and molybdenum oxide-supported titanium oxide as a proton-conductive inorganic oxide.
• the element ratio X/Y between molybdenum (X) in the molybdenum oxide and titanium (Y) in the titanium oxide was 0.1. Thereafter, an anode was produced from the obtained proton-conductive inorganic oxide in the same manner as in Example 1.
• Example 1 The procedure of Example 1 was repeated, except that 1.5 g of vanadium oxide V2O5 was replaced with 4.0 g of tungsten oxide WO3, the amount of silicon oxide SiO 2 was changed from 3.0 g to 3.5 g, and the temperature of heating was changed from 500 0 C to 700 0 C, to obtain a mixture of silicon oxide and tungsten oxide-supported titanium oxide as a proton-conductive inorganic oxide.
• the element ratio X/Y between tungsten (X) in the tungsten oxide and titanium (Y) in the titanium oxide was 0.1. Thereafter, an anode was produced from the obtained proton-conductive inorganic oxide in the same manner as in Example 1.
• Example 5 The procedure of Example 1 was repeated, except that 9.0 g of titanium oxide TiO 2 was replaced with 13 g of zirconium oxide ZrO 2 , and the amount of silicon oxide SiO 2 was changed from 3.0 g to 4.2 g, to obtain a mixture of silicon oxide and vanadium oxide-supported zirconium oxide as a proton-conductive inorganic oxide.
• the element ratio X/Y between vanadium (X) in the vanadium oxide and zirconium (Y) in the zirconium oxide was 0.1. Thereafter, an anode was produced from the obtained proton-conductive inorganic oxide in the same manner as in Example 1.
• Example 5 Example 5
• Example 3 The procedure of Example 3 was repeated, except that 9.0 g of titanium oxide TiO 2 was replaced with 13 g of zirconium oxide ZrU2, and the amount of silicon oxide SiO 2 was changed from 3.5 g to 4.7 g, to obtain a mixture of silicon oxide and tungsten oxide-supported zirconium oxide as a proton-conductive inorganic oxide.
• the element ratio X/Y between tungsten (X) in the tungsten oxide and zirconium (Y) in the zirconium oxide was 0.1. Thereafter, an anode was produced from the obtained proton-conductive inorganic oxide in the same manner as in Example 1.
• Example 8 The procedure of Example 1 was repeated, except that 9.0 g of titanium oxide TiO 2 was replaced with 17 g of tin oxide SnO 2 , and the amount of silicon oxide SiO 2 was changed from 3.0 g to 5.4 g, to obtain a mixture of silicon oxide and vanadium oxide-supported tin oxide as a proton-conductive inorganic oxide.
• the element ratio X/Y between vanadium (X) in the vanadium oxide and tin (Y) in the tin oxide was 0.1. Thereafter, an anode was produced from the obtained proton-conductive inorganic oxide in the same manner as in Example 1.
• Example 8 Example 8
• Example 3 The procedure of Example 3 was repeated, except that 9.0 g of titanium oxide TiCh was replaced with 17 g of tin oxide Sn ⁇ 2, and the amount of silicon oxide Si ⁇ 2 was changed from 3.5 g to 5.9 g, to obtain a mixture of silicon oxide and tungsten oxide-supported tin oxide as a proton-conductive inorganic oxide.
• the element ratio X/Y between tungsten (X) in the tungsten oxide and tin (Y) in the tin oxide was 0.1. Thereafter, an anode was produced from the obtained proton-conductive inorganic oxide in the same manner as in Example 1.
• Comparative Example 1 The procedure of Example 1 was repeated, except that the amounts of the proton-conductive inorganic oxide and 20 % Nafion ([trademark], available from DuPont) solution were changed from 0.24 g to 0.05 g and from 7.47 g to 1.56 g, respectively, to produce an anode.
• the amounts of the proton-conductive inorganic oxide and 20 % Nafion ([trademark], available from DuPont) solution were changed from 0.24 g to 0.05 g and from 7.47 g to 1.56 g, respectively, to produce an anode.
• Example 3 The procedure of Example 1 was repeated, except that the amounts of the proton-conductive inorganic oxide and 20 % Nafion ([trademark], available from DuPont) solution were changed from 0.24 g to 0.55 g and from 7.47 g to 17.1 g, respectively, to produce an anode. Comparative Example 3
• Example 1 The procedure of Example 1 was repeated, except that the amount of 20 % Nafion ([trademark], available from DuPont) solution was changed from 7.47 g to 3.73 g, to produce an anode.
• Example 1 The procedure of Example 1 was repeated, except that the amount of 20 % Nafion ([trademark], available from DuPont) solution was changed from 7.47 g to 16.8 g, to produce an anode.
• Comparative Example 5 The procedure of Example 1 was repeated, except that the amount of silicon oxide Si ⁇ 2 was changed from 3.0 g to 0.1 g, to produce an anode.
• Comparative Example 6 The procedure of Example 1 was repeated, except that the amount of silicon oxide SiCb was changed from 3.0 g to 7.0 g, to produce an anode.
• Comparative Example 7 A mixture of 3 g of PtRu-supported catalyst, 8 g of pure water, 15 g of 20 % Nafion ([trademark], available from DuPont) solution and 10 g of ethanol was stirred and dispersed well to prepare a slurry. The obtained slurry was coated by means of a control coater onto a carbon paper (350 ⁇ m, available from Toray Industries, Inc.) previously subjected to water-repelling treatment, and dried to produce an anode containing the noble metal catalyst in a loading density of 1 mg/cm 2 .
• a carbon paper 350 ⁇ m, available from Toray Industries, Inc.
• Cathode A mixture of 2 g of Pt-supported catalyst, 5 g of pure water, 5 g of 20 % Nafion ([trademark], available from DuPont) solution and 20 g of 2-ethoxyethanol was stirred and dispersed well to prepare a slurry. The obtained slurry was coated by means of a control coater onto a carbon paper (350 ⁇ m, available from Toray Industries, Inc.) previously subjected to water-repelling treatment, and dried to produce a cathode containing the noble metal catalyst in a loading density of 2 mg/cm 2 .
• a carbon paper 350 ⁇ m, available from Toray Industries, Inc.
• the anode and the cathode were individually cut into square pieces of 3.2 cm x 3.2 cm so that the electrode area might be 10 cm 2 .
• a film of Nafion 117 ([trademark], available from DuPont) as the proton-conductive polymer membrane was placed between the anode and the cathode, and hot-pressed at 125°C and 30 kg/cm 2 for 10 minutes to obtain a membrane electrode assembly.
• the membrane electrode assembly and delivery plates were combined to assemble a single cell of direct feed solid polymer electrolyte fuel cell. While the single cell was kept at 65°C, air and IM methanol aqueous solution as a fuel were supplied to the cathode and the anode in amounts of 200 ml/minute and 0.6 ml/minute, respectively, to generate electric power in a current density of 150 mA/cm 2 for 30 minutes. Thereafter, the voltage of the cell was measured. The results were as set forth in Table 1.
• Example 7 which comprises an anode containing the supported catalyst and the proton-conductive organic polymer binder
• Examples 1 to 9 each of which comprises an anode further containing the proton-conductive inorganic oxide.
• Example 1 in which the weight ratio (WS A+SI O 2 /WC) of the Si ⁇ 2 ⁇ containing proton-conductive inorganic oxide (SA+Si ⁇ 2) and the supported catalyst (C) is out of the range of 0.037 to 0.25. As a result, it was found that Example 1 is superior to Comparative Examples 1 and 2 in the cell voltage. Further, Example 1 was also compared with Comparative Examples 3 and 4, in which the weight ratio (WP/WSA+SIO2) of the proton-conductive organic polymer binder (P) and the Si ⁇ 2-containing proton-conductive inorganic oxide (SA+SiU2) is out of the range of 3.5 to 12. As a result, it was found that Example 1 is also superior to Comparative Examples 3 and 4 in the cell voltage.
• Example 1 was still also compared with Comparative Examples 5 and 6, in which the weight ratio (WSJO2/W S A) of the SiOz and the proton-conductive inorganic oxide (SA) is out of the range of 0.1 to 0.5. As a result, it was found that Example 1 is still also superior to Comparative Examples 5 and 6 in the cell voltage.
• the present invention can provide a fuel cell capable of giving high output power.

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