Patent Application
20060177708
The present invention relates to a fuel cell power system equipped with a plurality of electrically connected single cells, each having an anode and having a cathode with an electrolyte membrane between the anode and the cathode, to generate a power with a liquid fuel, e.g., methanol, dimethyl ether or ethylene glycol, and a method for operating the same. The present invention is suitable as a compact, portable fuel cell system.
Portable electronic devices, e.g., cellular phones, laptop computers, audio-visual devices or mobile information terminals, have been rapidly spreading in tandem with the electronic technology advances, which make them compacter. These portable electronic devices have been driven by a secondary battery, which must be charged after discharging a certain amount of power and hence involves problems of needing a charging device and relatively long charging time.
More recently, use of a fuel cell as a power source for portable electronic devices has been considered. A fuel cell electrochemically converts chemical energy of a fuel directly into electrical energy. It needs no motive energy section, and is highly realizable as a compact power-generating device. Moreover, it can continuously generate power without stopping the service by only exchanging or replenishing a fuel, unlike a secondary battery, which is stopped temporarily while being charged.
Polymer electrolyte fuel cells (PEFCs) of high output density have been in development as fuel cells for portable electronic devices. A PEFC uses an electrolyte membrane of perfluorocarbon sulfonic acid resin to oxidize hydrogen gas at the anode and reduce oxygen at the cathode. However, it is unsuitable for compact devices, because it uses hydrogen gas of low volumetric energy density as a fuel, which increases size of the fuel tank. By contrast, a liquid fuel is more advantageous than a gas fuel for fuel cells for compact devices, because of its higher density. Therefore, methanol, ethanol, propanol, dimethyl ether, ethylene glycol or the like is a promising liquid fuel for a fuel cell as a power source for compact electronic devices serviceable for extended periods. One of the known methanol-firing fuel cells uses an electrolyte membrane of sulfonated polyether sulfone resin and supplies a fuel of 20% by weight methanol solution by a siphon effect (see, e.g., JP-A-2003-323904).
A liquid-firing fuel cell power source is described by taking, as an example, a standard direct methanol fuel cell (DMFC). A DMFC dissociates an aqueous methanol solution, supplied to an anode catalyst layer, into carbon dioxide, hydrogen ion and electron (Formula (1)).
CH2OH+H2O→CO2+6H++6e− (1)
The hydrogen ion thus produced moves from the anode to the cathode side, to react on a cathode catalyst layer with oxygen gas diffusing from ambient air and also with the electron on the cathode catalyst layer to produce water (Formula (2)).
6H++3/2O2+6e−→3H2O (2)
As a net, the overall chemical reaction is reduced to oxidation of methanol with oxygen to produce carbon dioxide gas and water (Formula (3)), and the chemical reaction is the same in form as that involved in flaming combustion of methanol.
CH3OH+3/2O2→CO2+3H2O (3)
A fuel cell with an aqueous methanol solution as a fuel generates power by converting chemical energy of methanol directly into electrical energy by the electrochemical reaction described above. However, few DMFCs totally consume an aqueous methanol solution supplied on the anode, part of the methanol solution being discharged to the outside. Therefore, utilization factor of the solution is generally low. Returning the solution discharged from the cell back to a solution tank is considered. This, however, gradually dilutes the solution in the tank, because methanol is consumed with equimolar water in the anode catalyst layer, causing problems of methanol shortage within the cell and sharply decreased electromotive force.
One approach for solving these problems is controlling an aqueous methanol solution concentration at a given level by sensing the concentration.
It is an object of the present invention to provide a power system for liquid-firing fuel cells, which can stably produce power for extended periods by keeping the liquid fuel composition at a constant level.
The inventors of the present invention have studied in detail causes for changed concentration of liquid fuel for fuel cells to simplify mechanisms for controlling liquid fuel concentration, achieving the present invention. As a result, the four major causes are identified; consumption of fuel and water on the anode for power generation according to Formula (1), water associated with movement of H+ from the anode to the cathode, fuel cross-over and water cross-over.
Consumption of fuel and water on the anode for power generation according to Formula (1), occurring only in the power generation process of a fuel cell, is an essential phenomenon for power generation, and the consumption can be estimated from power output. Production of water associated with movement of H+ from the anode to the cathode, occurring only in the power generation process of a fuel cell, is also an essential phenomenon for power generation. The associated water production can be also estimated from power output, when dependences of moved hydrated proton (associated water) on temperature and fuel concentration are predetermined. Fuel cross-over occurs when a liquid fuel is in contact with a polymer electrolyte membrane and is, as it were, a phenomenon equivalent to self-discharge of a cell. Amount of cross-over liquid is difficult to estimate simply by calculation, because it depends on various factors, e.g., electrolyte membrane material and thickness, fuel concentration and temperature, and liquid fuel and air (oxygen) supplies. Water cross-over, on the other hand, is negligibly small.
The inventors of the present invention have found, after having studied these causes in detail, that a liquid fuel can have an essentially constant concentration of 15 to 20% by weight after a lapse of certain time to produce a stable output without a mechanism of estimating and controlling liquid fuel concentration in a fuel cell, when an electrolyte membrane having a make-up liquid fuel permeability of 70 mA/cm2 or less, preferably 0.02 to 70 mA/cm2, is used and make-up liquid fuel is kept at a concentration of 15% by weight or more. A liquid fuel having a concentration above 60% by weight is not desirable, because of accelerated cross-over to make the system inefficient.
It is also found that use of an electrolyte membrane having a make-up liquid fuel permeability of 70 mA/cm2 or less, preferably 0.02 to 70 mA/cm2, brings other favorable effects of reduced amount of steam/water production on the cathode and prevention of interrupted flow of oxygen only by natural aspiration without need of forced removal, recovery or the like of steam/water, to achieve stable operation. As a result, it removes adverse effects of steam/water on devices in which it is used, e.g., PDAs, personal computers and cellular phones.
As described above, the present invention provides a fuel cell power system equipped with a plurality of electrically connected single cells, each having an anode for oxidizing a liquid fuel supplied to the cells and cathode for reducing oxygen with an electrolyte membrane in-between, wherein the electrolyte membrane has a liquid fuel permeability of 0.02 to 70 mA/cm2 and the liquid fuel having a concentration of 15% by weight or more is supplied to generate power.
The present invention also provides a method for operating a fuel cell power system equipped with a plurality of electrically connected single cells, each having an anode for oxidizing a liquid fuel supplied to the cells and cathode for reducing oxygen with an electrolyte membrane in-between to generate power, wherein the electrolyte membrane has a liquid fuel permeability of 0.02 to 70 mA/cm2 and the liquid fuel having a concentration of 15 to 60% by weight is supplied.
The present invention also provides a fuel cell power system equipped with an anode and cathode with an electrolyte membrane in-between, holding a liquid fuel having a concentration of 90% by weight or more, mixing water produced on the cathode with the liquid fuel and having a liquid fuel supply system which continuously supplies the liquid fuel to the anode, wherein the electrolyte membrane has a liquid fuel permeability of 0.02 to 70 mA/cm2.
The present invention also provides an electronic device having a fuel cell power system equipped with a plurality of electrically connected single cells, each running on a liquid fuel and having an anode for oxidizing the liquid fuel and cathode for reducing oxygen with an electrolyte membrane in-between, wherein the electrolyte membrane has a liquid fuel permeability of 0.02 to 70 mA/cm2 and the liquid fuel having a concentration of 15% by weight or more is supplied to generate power.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
1: DMFC, 2: Polymer electrolyte membrane, 3: Anode catalyst layer, 4: Cathode catalyst layer, 5: Anode collector, 6: Cathode collector, 7: Air passage plate, 8: Air supply port, 9: Air exhaust, 10: Air passage, 11: Oxidant, 13: Fuel passage plate, 14: Fuel supply port, 15: Fuel exhaust, 16: Fuel passage, 17: Aqueous methanol solution container, 21: Water container, 23: Methanol solution container, 27: External circuit, 101: Fuel cell, 102: Fuel cartridge tank, 103: Output terminal, 104: Off-gas port, 105: DC/DC converter, 106: Controller, 107: Gasket, 111: MEA equipped with a diffusion layer, 112: Fuel chamber, 113a: Anode terminal plate, 113c: Cathode terminal plate, 114: Fuel cartridge holder, 115: Screw, 116: Connecting terminal, 201: Display, 202: Main board, 203: Antenna, 204: Hinge equipped with a cartridge holder, 205: Diaphragm, 206: Lithium ion secondary battery, 210: Box
The present invention secures an essentially constant liquid fuel concentration in a fuel cell after a lapse of certain time to produce a stable output without a mechanism of sensing or estimating and controlling liquid fuel concentration.
The present invention reduces amount of steam/water production on the cathode and prevents interrupted flow of oxygen only by natural aspiration without need of forced removal, recovery or the like of steam/water by use of an electrolyte membrane having a liquid fuel permeability of 0.02 to 70 mA/cm2. As a result, it removes adverse effects of steam/water on devices in which it is used, e.g., PDAs, personal computers and cellular phones.
Electronic devices driven by a secondary battery, e.g., cellular phones, portable personal computers, portable audio and/or visual equipment, and other portable information terminals, can be serviceable for extended periods, when the fuel cell power system is incorporated as a battery charger. Moreover, it can work as a directly built-in power source for a device. Such a device is serviceable continuously by supplying a fuel to the cell.
The embodiments of the present invention are described in detail. The essential points of the present invention are use of an electrolyte membrane having a liquid fuel permeability of 70 mA/cm2 or less, preferably 0.02 to 70 mA/cm2, and keeping a liquid fuel concentration at 15% by weight or less. The electrolyte membrane is not limited for the present invention, so long as it has a liquid fuel permeability of 70 mA/cm2 or less, preferably 0.02 to 70 mA/cm2. The materials suitable for the electrolyte membrane include sulfonated engineering plastics, e.g., sulfonated polyetheretherketone, sulfonated polyether sulfone, sulfonated acrylonitrile-butadiene-styrene polymer, sulfonated polysulfide and sulfonated polyphenylene; sulfoalkylated engineering plastics, e.g., sulfoalkylated polyetheretherketone, sulfoalkylated polyether sulfone, sulfoalkylated polyetherether sulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide and sulfoalkylated polyphenylene; and hydrocarbon-based ones, e.g., sulfoalkyletherified polyphenylene. Of these, sulfoalkylated hydrocarbon-based ones and sulfoalkyletherified hydrocarbon-based ones are more preferable viewed from their liquid fuel permeability, ion conductivity, swelling characteristics, among others. The fuel cell is serviceable at a higher temperature range by incorporating a composite electrolyte membrane or the like with a proton-conductive inorganic material dispersed microscopically in a heat-resistant resin. These inorganic materials include tungsten oxide hydrate, zirconium oxide hydrate, tin oxide hydrate, silicotungsten acid, silicomolybdenum acid, tungstophosphoric acid and molybdenum acid. An acidic electrolyte membrane of the above-described hydrate is generally swollen and deformed, and may have an insufficient mechanical strength when it is highly ion-conductive. In such a case, the membrane is preferably reinforced with a non-woven or woven fabric of fibers of high mechanical strength, durability and heat resistance as a core, or as a filler to be incorporated in the membrane production step. The membrane can have a reduced liquid fuel permeability, when it is made of a polybenzimidazole doped with sulfuric, phosphoric, sulfonic or phosphinic acid. The polymer electrolyte membrane for the present invention may be incorporated with an additive for common polymers, e.g., plasticizer, stabilizer or releasing agent, within limits not harmful to the object of the present invention.
The polymer electrolyte membrane contains sulfonic acid at 0.5 to 2.0 milliequivalents (meq)/g-dry resin, preferably 0.8 to 1.5 milliequivalents/g-dry resin. The content outside of the above range is not desirable. At lower than 0.5 milliequivalents/g-dry resin, the membrane will have an excessive ion-conducting resistance. At higher than 2.0 milliequivalents/g-dry resin, on the other hand, the membrane tends to be dissolved in an aqueous fuel solution, e.g., methanol solution. Thickness of the polymer electrolyte membrane is not limited, but preferably in a range from 10 to 300 μm, particularly preferably 15 to 200 μm. It is preferably thicker than 10 μm for securing a sufficiently high practical strength, and thinner than 200 μm for reducing membrane resistance, i.e., for improving power-generating capacity. When the membrane is produced by solution casting, its thickness can be controlled by solution concentration or thickness of the solution film formed on a base film. When it is formed from a molten state, its thickness can be controlled by drawing the film of a given thickness formed by pressing or extrusion at a given drawing ratio.
An electrode for an MEA to be assembled in a DMFC is composed of an electroconductive material impregnated with fine catalyst metal particles. It may be incorporated with a water repellent agent or binder, as required. Moreover, a layer of electroconductive material, free of catalyst and incorporated with a water repellent agent or binder as required, may be formed on a catalyst layer. Any catalyst metal may be used for the electrode, so long as it can promote oxidation of a liquid fuel and reduction of oxygen. The catalyst metal useful for the present invention include platinum, gold, silver, palladium, iridium, rhodium, ruthenium, iron, cobalt, nickel, chromium, tungsten, manganese, vanadium, titanium and an alloy thereof, of which platinum in particular is used in many cases. The catalyst metal has a particle size normally in a range from 10 to 30 Å. It is advantageous costwise to deposit the metal on a carrier, e.g., carbon, preferably to 0.01 to 10 mg/cm2 in a condition after the electrode is formed, because of reduced catalyst requirement.
Any method can be used for joining a fuel cell electrolyte membrane to an electrode, and may be selected from known ones. An MEA can be produced by assembling an electrolyte membrane by hot pressing with a catalyst layer which is formed by spreading and heat-treating a suspension of carbon-supported platinum catalyst particles and polytetrafluoroethylene on paper-shaped carbon, after the catalyst layer is coated with a binder of electrolyte solution of the same material as the electrolyte membrane or fluorine-based electrolyte. The other useful methods include coating platinum catalyst particles with an electrolyte solution of the same material as the electrolyte membrane, printing a catalyst paste, spreading a catalyst on an electrolyte membrane by spraying or ink jetting, electrolessly plating an electrode on an electrolyte membrane, and adsorbing and later reducing a complex ion of platinum group metal on an electrolyte membrane. Of these, spreading a catalyst paste by ink jetting on an electrolyte membrane is more advantageous, because of reduced catalyst loss.
Any electroconductive material can be used for the present invention, so long as it is electron-conductive. These materials include various metals, and carbonaceous materials, e.g., carbon black (e.g., furnace black, channel black and acetylene black), activated coal and graphite. They may be used either individually or in combination. Water repellent agents useful for the present invention include fluorinated carbon. The binder for the present invention is preferably in the form of solution of hydrocarbon-based electrolyte similar to that for the electrolyte membrane, viewed from adhesiveness. However, it may be selected from various resins. It may be incorporated with a fluorine-containing resin having water repellent capacity, e.g., polytetrafluoroethylene, tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer or tetrafluoroethylene/hexafluoropropylene copolymer.
A DMFC is composed of a plurality of single cells stacked via a cooling plate or the like, where each single cell comprises a grooved fuel and oxidant distributor plates formed on the MEA assembled by the above-described procedure, each plate working as a fuel or oxidant passage and current collector. However, the connecting method is not limited. For example, the single cells may be connected to each other on a two-dimensional plane, instead of being stacked.
Working temperature of fuel cell is not limited, although it is preferably higher for enhanced electrode catalyst activity and reduced electrode over-voltage. The cell can operate after evaporating the liquid fuel.
A plurality of single cells, each comprising an anode, electrolyte membrane and cathode, when connected to each other in series by electroconductive interconnections, can produce a higher voltage. The present invention can realize a compact power source serviceable for extended periods to continuously generate power by use of a liquid fuel, e.g., aqueous solution of methanol, of high volumetric energy density without needing an auxiliary device, e.g., that for forced supply of fuel or oxidant, or for forced cell cooling. It is built in various devices, e.g., cellular phones, laptop personal computers and portable video cameras to drive them continuously for extended periods, when supplied with a given fuel as required. It is an effective approach to greatly reduce frequency of fuel supply in the above case by combining the compact power source with a charger for a cellular phone, laptop personal computer or portable video camera driven by a secondary battery. In this case, it is built in a device-containing case to work as a battery charger. The portable electronic device, when it is to be used, is driven by the secondary battery after being taken out of the case and, when it is not used, contained in the case, where the built-in compact power source charges the secondary battery to which it is connected via the charger. This allows to increase fuel tank capacity thereby greatly reducing frequency of fuel supply.
The present invention is described in more detail by EXAMPLES. It is however to be understood that the present invention is not limited to .these examples.
(1) Synthesis of Chloromethylated Polyether Sulfone
A 500 mL four-mouthed, round-bottomed flask, equipped with a stirrer, thermometer, calcium chloride tube and reflux condenser, was charged with 30 g of polyether sulfone (PES), 250 mL of tetrachloroethane and then 40 mL of chloromethylmethyl ether, after it was purged with nitrogen, to which a mixed solution of 1 mL of anhydrous tin (IV) chloride and 20 mL of tetrachloroethane was added dropwise, and the mixture was heated at 80° C. for 90 minutes with stirring. Then, the resulting reaction solution was thrown into 1 L of methanol, to separate out the polymer produced. The separated precipitate was crushed by a mixer and washed with methanol to prepare chloromethylated polyether sulfone. It had a chloromethyl group introduction rate (ratio of structural units with introduced chloromethyl group to the total structural units in the compound of Chemical Formula (1), i.e., sum of “x” and “y”) of 36%, as confirmed by a nuclear magnetic resonance spectral analysis.
(2) Synthesis of Acetylthio(Polyether Sulfone)
A 1000 mL four-mouthed, round-bottomed flask, equipped with a stirrer, thermometer, calcium chloride tube and reflux condenser, was charged with the chloromethylated polyether sulfone prepared above, to which 600 mL of N-methylpyrrolidone (NMP) was added. Then, a solution of 9 g of potassium thioacetate dissolved in 50 mL of NMP was added, and the mixture was heated at 80° C. for 3 hours with stirring. The resulting reaction solution was thrown into 1 L of water, to separate out the polymer produced. The separated precipitate was crushed by a mixer and washed with water and dried under heating to prepare 32 g of acetylthio(polyether sulfone).
(3) Synthesis of Sulfomethylated Polyether Sulfone
A 500 mL four-mouthed, round-bottomed flask, equipped with a stirrer, thermometer, calcium chloride tube and reflux condenser, was charged with 20 g of the acetylthio(polyether sulfone) prepared above, and further with 300 mL of acetic acid and 20 mL of hydrogen peroxide solution. The mixture was heated at 45° C. for 4 hours with stirring. Then, the resulting reaction solution was put in 1 L of 6N aqueous solution of sodium hydroxide while it was being cooled, and stirred for a while. The resulting polymer was separated by filtration, and washed with water until the alkaline component was removed. Then, the polymer was added to 300 mL of 1N hydrochloric acid, and the mixture was stirred for a while. The polymer was separated by filtration, washed with water until the acidic component was removed and dried under a vacuum to quantitatively prepare 20 g of sulfomethylated polyether sulfone. Presence of sulfomethyl group was confirmed, because the chemical shift of NMR-determined methylene proton was shifted to 3.78 ppm. The product had a sulfomethyl group introduction rate (ratio of structural units with introduced sulfomethyl group to the total structural units in the compound of Chemical Formula (2), i.e., sum of “x” and “y”) of 36%, based on chloromethyl introduction rate of 36%.
(4) Preparation of Electrolyte Membrane
The sulfomethylated polyether sulfone prepared in (3) above was dissolved in a (50/50) mixed solvent of dimethylacetoamide and methoxy ethanol to 5% by weight. The solution was developed on glass by spin coating, dried in air and then dried at 80° C. under a vacuum, to form a 42 μm thick electrolyte membrane of sulfomethylated polyether sulfone. It had a methanol permeability of 12 mA/cm2 and ion conductivity of 0.053 S/cm at room temperature.
(5) Preparation of Membrane Electrode Assemblies (MEAs)
Preparation of MEA (1)
A carbon carrier impregnated with fine catalyst particles of platinum/ruthenium alloy (50/50 by atom) at 50% by weight was slurried with a 30% by weight solution of the sulfomethylated polyether sulfone prepared in (3) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol. The slurry was spread on a polyimide film to form an about 30 μm thick catalyst layer by screen printing, to prepare a porous anode film. Next, a carbon carrier impregnated with fine catalyst particles of platinum at 30% by weight was slurried with a mixed water/alcohol solvent containing a binder composed of the sulfomethylated polyether sulfone prepared in (3) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol. The slurry was spread on a polyimide film to form an about 10 μm thick catalyst layer by screen printing, to prepare a porous cathode film. The porous anode and cathode films were cut into 10 mm wide, 20 mm long shapes, to prepare the anode and cathode. About 0.5 mL of a 5% by weight solution of the sulfomethylated polyether sulfone prepared in (3) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol was penetrated into the anode surface. Then, the surface-treated anode was bonded to the power-generation section of the electrolyte membrane of sulfomethylated polyether sulfone prepared in (4) above, cut into a 16 mm wide, 33 mm long shape, and dried at 80° C. for 3 hours under a load of about 1 kg. Next, about 0.5 mL of a 5% by weight solution of the sulfomethylated polyether sulfone prepared in (3) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol was penetrated into the cathode surface. Then, the surface-treated cathode was bonded to the electrolyte membrane in such a way to lap over the anode layer prepared above, and dried at 80° C. for 3 hours under a load of about 1 kg, to prepare an MEA (1).
Preparation of MEA (2)
A porous anode film was prepared in the same manner as that for the MEA (1). For preparation of a porous cathode layer, a carbon carrier impregnated with fine catalyst particles of platinum at 30% by weight was slurried with a mixed water/alcohol solvent containing an electrolytic binder composed of 30% by weight perfluorocarbon sulfonic acid (Nafion 117, trade name, DuPont), and the slurry was spread on a polyimide film to form an about 10 μm thick catalyst layer by screen printing. The porous anode and cathode films were cut into 10 mm wide, 20 mm long shapes, to prepare the anode and cathode. About 0.5 mL of a 5% by weight solution of the sulfomethylated polyether sulfone prepared in (3) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol was penetrated into the anode surface. Then, the surface-treated anode was bonded to the power-generation section of the electrolyte membrane of sulfomethylated polyether sulfone prepared in (4) above, cut into a 16 mm wide, 33 mm long shape, and dried at 80° C. for 3 hours under a load of about 1 kg. Next, about 0.5 mL of a 5% by weight solution of Nafion 117 dissolved in a mixed solvent of water, isopropanol and normal propanol (20/40/40 by weight, Fluka Chemika) was penetrated into the cathode surface. Then, the surface-treated cathode was bonded to the electrolyte membrane in such a way to lap over the anode layer prepared above, and dried at 80° C. for 3 hours under a load of about 1 kg, to prepare an MEA (2).
Preparation of MEA (3)
A carbon carrier impregnated with fine catalyst particles of platinum/ruthenium alloy (50/50 by atom) at 50% by weight was slurried with a mixed water/alcohol solvent (water, isopropanol and normal propanol, 20/40/40 by weight) containing an electrolytic binder composed of 30% by weight perfluorocarbon sulfonic acid (Nafion 117, trade name, DuPont). The slurry was spread on a polyimide film to form an about 30 μm thick catalyst layer by screen printing, to prepare a porous anode film. Next, a carbon carrier impregnated with fine catalyst particles of platinum at 30% by weight was slurried with a mixed water/alcohol solvent containing an electrolytic binder composed of 30% by weight perfluorocarbon sulfonic acid (Nafion 117, trade name, DuPont). The slurry was spread on a polyimide film to form an about 10 μm thick catalyst layer by screen printing, to prepare a porous cathode film. The porous anode and cathode films were cut into 10 mm wide, 20 mm long shapes, to prepare the anode and cathode. About 0.5 mL of a 5% by weight solution of Nafion 117 dissolved in a mixed solvent of water, isopropanol and normal propanol (20/40/40 by weight, Fluka Chemika) was penetrated into the anode surface. Then, the surface-treated anode was bonded to the power-generation section of the electrolyte membrane of sulfomethylated polyether sulfone prepared in (4) above, cut into a 16 mm wide, 33 mm long shape, and dried at 80° C. for 3 hours under a load of about 1 kg. Next, about 0.5 mL of a 5% by weight solution of Nafion 117 dissolved in a mixed solvent of water, isopropanol and normal propanol (20/40/40 by weight, Fluka Chemika) was penetrated into the cathode surface. Then, the surface-treated cathode was bonded to the electrolyte membrane in such a way to lap over the anode layer prepared above, and dried at 80° C. for 3 hours under a load of about 1 kg, to prepare an MEA (3).
(6) Preparation of Fuel Cell (DMFC) Power Systems
The DMFC power system, illustrated in
(1) Preparation of Solution for Coating Electrode Catalyst and Electrolyte Electrode Assembly (MEA)
A carbon carrier impregnated with fine catalyst particles of platinum/ruthenium alloy (50/50 by atom) at 50% by weight was slurried with a mixed water/alcohol solvent (water, isopropanol and normal propanol, 20/40/40 by weight) containing an electrolytic binder composed of 30% by weight perfluorocarbon sulfonic acid (Nafion 117, trade name, DuPont). The slurry was spread on a polyimide film to form an about 30 μm thick catalyst layer by screen printing, to prepare a porous anode film. Next, a carbon carrier impregnated with fine catalyst particles of platinum at 30% by weight was slurried with a mixed water/alcohol solvent containing an electrolytic binder composed of 30% by weight perfluorocarbon sulfonic acid (Nafion 117, trade name, DuPont). The slurry was spread on a polyimide film to form an about 10 μm thick catalyst layer by screen printing, to prepare a porous cathode film. The porous anode and cathode films were cut into 10 mm wide, 20 mm long shapes, to prepare the anode and cathode. About 0.5 mL of a 5% by weight solution of Nafion 117 dissolved in a mixed solvent of water, isopropanol and normal propanol (20/40/40 by weight, Fluka Chemika) was penetrated into the anode surface. Then, the surface-treated anode was bonded to the power-generation section of the electrolyte membrane of Nafion 115 (thickness: 125 μm), cut into a 16 mm wide, 33 mm long shape, and dried at 80° C. for 3 hours under a load of about 1 kg. Next, about 0.5 mL of a 5% by weight solution of Nafion 115 dissolved in a mixed solvent of water, isopropanol and normal propanol (20/40/40 by weight, Fluka Chemika) was penetrated into the cathode surface. Then, the surface-treated cathode was bonded to the electrolyte membrane in such a way to lap over the anode layer prepared above, and dried at 80° C. for 3 hours under a load of about 1 kg, to prepare an MEA (4).
(2) Preparation of Fuel Cell (DMFC) Power System
The DMFC power system, illustrated in
As discussed above, a fuel cell power system can produce a stable output without a mechanism of sensing and controlling liquid fuel concentration in a fuel cell, when an electrolyte membrane having a liquid fuel permeability of 12 mA/cm2 is used and a make-up liquid fuel is kept at a concentration of 20% by weight or more, because the liquid fuel can have an essentially constant concentration after a lapse of certain time, unlike a system which uses an electrolyte membrane having a liquid fuel permeability of 100 mA/cm2 and runs on a methanol fuel having a concentration of 20% by weight.
(1) Synthesis of Sulfopropylated Polyether Sulfone
A 500 mL four-mouthed, round-bottomed flask, equipped with a stirrer, thermometer, calcium chloride tube and reflux condenser, was charged with 21.6 g of polyether sulfone (PES), 12.2 g (0.1 mols) of propane sultone and 50 mL of dried nitrobenzene, after it was purged with nitrogen, to which a 14.7 g (0.11 mols) of anhydrous aluminum chloride was added with stirring in about 30 minutes. On completion of addition of anhydrous aluminum chloride, the mixture was treated under reflux for 8 hours. Then, the reaction solution was poured into 500 mL of ice water incorporated with 25 mL of concentrated sulfuric acid to terminate the reaction, to which 1 L of deionized water was added dropwise to separate out sulfopropylated polyether sulfone. It was recovered by filtration. The separated precipitate was washed with deionized water in a mixer and filtered under a vacuum for recovery repeatedly until the filtrate became neutral, and then dried at 120° C. for a night under a vacuum. The sulfopropylated polyether sulfone produced contained the ion-exchanged group at 1.1 meq/g.
(2) Preparation of Electrolyte Membrane
The product prepared in (1) above was dissolved in a mixed solvent of N,N′-dimethylformamide, cyclohexanone and methylethylketone (20/80/25 by volume) to a varying content. The solution was developed on glass by spin coating, dried in air and then dried at 80° C. under a vacuum, to form an electrolyte membrane of varying thickness (14, 21, 31, 42, 63, 103 or 202 μm). The characteristics of the electrolyte membranes are given in Table 1.
(3) Preparation of Membrane Electrode Assemblies (MEAs)
A carbon carrier impregnated with fine catalyst particles of platinum/ruthenium alloy (50/50 by atom) at 50% by weight was slurried with a 30% by weight solution of the sulfopropylated polyether sulfone prepared in (1) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol. The slurry was spread on a polyimide film to form an about 30 μm thick catalyst layer by screen printing, to prepare a porous anode film. Next, a carbon carrier impregnated with fine catalyst particles of platinum at 30% by weight was slurried with a mixed water/alcohol solvent containing a binder composed of the sulfopropylated polyether sulfone prepared in (1) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol. The slurry was spread on a polyimide film to form an about 10 μm thick catalyst layer by screen printing, to prepare a porous cathode film. The porous anode and cathode films were cut into 10 mm wide, 20 mm long shapes, to prepare the anode and cathode. About 0.5 mL of a 5% by weight solution of the sulfopropylated polyether sulfone prepared in (1) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol was penetrated into the anode surface. Then, the surface-treated anode was bonded to the power-generation section of the electrolyte membrane of sulfopropylated polyether sulfone prepared in (2) above, cut into a 16 mm wide, 33 mm long shape, and dried at 80° C. for 3 hours under a load of about 1 kg. Next, about 0.5 mL of a 5% by weight solution of the sulfopropylated polyether sulfone prepared in (1) above dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol was penetrated into the cathode surface. Then, the surface-treated cathode was bonded to the electrolyte membrane in such a way to lap over the anode layer prepared above, and dried at 80° C. for 3 hours under a load of about 1 kg, to prepare an MEA. In this manner, a total of 12 MEAs (MEA (5) to (11) of varying electrolyte membrane thickness were prepared.
(4) Preparation of Fuel Cell (DMFC) Power Systems
The DMFC power system, illustrated in
(1) Preparation of Electrolyte Membrane of Sulfomethylated Polyether Sulfone
A total of 5 types of chloromethylated polyether sulfone of different chlotromethyl group introduced were prepared in the same manner as in EXAMPLE 1 (1) for synthesis of chloromethylated polyether sulfone, except that a composition of polyether sulfone, chloromethyl ether and anhydrous tin chloride. The same procedures as EXAMPLE 1 (2) for synthesis of acetylthio(polyether sulfone) and (3) for synthesis of sulfomethylated polyether sulfone were adopted to substitute the chloromethyl group in the chloromethylated polyether sulfone by acetylthio group and then to synthesize sulfomethylated polyether sulfone. The same film-making procedure as EXAMPLE 1 (4) preparation of electrolyte membrane was adopted to prepare an about 40 μm thick electrolyte membrane of each sulfomethylated polyether sulfone type. The characteristics of the electrolyte membranes are given in Table 2.
(2) Preparation of Membrane Electrode Assemblies (MEAs)
A carbon carrier impregnated with platinum/ruthenium at 40% by weight was uniformly dispersed in a solution of the sulfomethylated polyether sulfone prepared in EXAMPLE 1 (3) dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol to have a platinum/ruthenium catalyst/high-molecular-weight electrolyte ratio of 2/1 by weight. The resulting paste was used as a solution for coating the electrode catalyst. It was spread on one side of the electrolyte membrane prepared in EXAMPLE 1 (1) and dried to prepare an anode impregnated with platinum/ruthenium at 6 mg/cm2. Next, a carbon carrier impregnated with platinum at 40% by weight was uniformly dispersed in a solution of the sulfomethylated polyether sulfone prepared in EXAMPLE 1 (3) dissolved in a mixed solvent of 1-propanol, 2-propanol and methoxy ethanol to have a platinum catalyst/high-molecular-weight electrolyte ratio of 2/1 by weight. The resulting paste was used as a solution for coating the electrode catalyst. It was spread on the other side of the electrolyte membrane and dried to prepare a cathode impregnated with platinum at 2 mg/cm2. These anode and cathode were used to prepare the MEAs (12) to (18).
(3) Preparation of Fuel Cell (DMFC) Power Systems
The DMFC power system, illustrated in
The DMFC power system, illustrated in
(1) Preparation of Fuel Cell Power System
The slits 122a for distributing fluids, e.g., fuel and oxidant gas, have apertures running in parallel to each other in the structure illustrated in
The current collectors 142 are preferably fit into, and joined by an adhesive agent to, the respective spot-face sections in such a way that they are arranged to form the same plane as far as possible. The adhesive agent is not limited, so long as it is not dissolved in or swollen by an aqueous methanol solution, and electrochemically more stable than methanol. An epoxy resin agent is one of the preferable examples. The current collector 142 is not necessarily fixed by an adhesive agent. For example, it may be fixed by providing projections on the base board 181 and fitting them into part of the slits 122b on each of the current collectors 42 or specially provided holes. Moreover, it is not essential that each side of the current collectors 142 and base board 181 form the same plane. In the case of the plane of a stepped structure, for example, it is possible to join the current collectors 142 without the spot-faced sections 182 on the base board 181 by altering structure and thickness of the sealing gasket.
Materials for the current collector are not limited. They include carbon, metals (e.g., stainless steel, titanium and tantalum), composites including these metals (e.g., carbon steel, stainless steel, copper and nickel clads). When a metallic current collector is used, plating the fabricated current-carrying member with a corrosion-resistant noble metal (e.g., gold) or coating the member with an electroconductive carbon paint or the like to reduce contact resistance in the assembly is effective to increase cell output density and secure functional stability for extended periods.
FIGS. 25 (b) and (c) illustrate the cathode diffusion layer 170c and anode diffusion layer 170a structures for the present invention, respectively. The cathode diffusion layer 170c comprises the water-repellent layer 172 and porous carbon base board 171c, the former increasing water repellency of the layer to increase steam pressure around the cathode, thereby promoting diffusion/discharge of steam produced and also preventing condensation of steam. The water-repellent layer 172 is stacked in such a way to come into contact with the electrode 162c. Plane contact between the anode diffusion layer and anode is not limited. In EXAMPLE 5, the porous carbon base board 171a was used for the plane contact. The porous carbon base board 171c for the cathode diffusion layer 170c is made of an electroconductive, porous material. It is generally in the form of woven or non-woven fabric of carbon fibers.
The cathode diffusion layer 170c is described in detail. Carbon paper (TGP-H-060, Toray) is cut into a shape of given dimensions. The carbon paper of predetermined water absorption rate is dipped in a polytetrafluorocarbon/water dispersion (D-1, Daikin Industries) diluted in such a way to keep the carbon paper content at 20 to 60% by weight after baking, and dried at 120° C. for 1 hour. It is then baked in air at 270 to 360° C. for 0.5 to 1 hour. The powdered carbon (XC72R, Cabot) is kneaded with the polytetrafluorocarbon/water dispersion to have a content of 20 to 60% by weight. The resulting pasty mixture is spread on one side of the carbon paper, treated to be water-repellent as described above, to a thickness of 10 to 30 μm. It is then dried at 120° C. for about 1 hour, and fired in air at 270 to 360° C. for 0.5 to 1 hour to prepare the cathode diffusion layer 170c. Air and moisture permeability, i.e., supplied oxygen and produced water diffusion capacities, of the cathode diffusion layer 170c greatly depend on addition rate and dispersibility of polytetrafluoroethylene and baking temperature. Therefore, the adequate conditions are selected in consideration of design performance and service environments of the fuel cell.
The anode diffusion layer 170a can be made of a woven or non-woven fabric of carbon fibers which can satisfy the conditions related to electroconductivity and porosity. The suitable woven fabrics of carbon fibers include carbon cloth (TORAYCA cloth, Toray) and carbon paper (TGP-H-060, Toray). The anode diffusion layer 170a works to accelerate supply of an aqueous solution fuel and discharge of carbon dioxide produced. Therefore, the porous carbon base board 171a is preferably treated to have the hydrophilic surface by mild oxidation or UV irradiation, or treated with a hydrophilic resin or strongly hydrophilic material (represented by titanium oxide)dispersed in the porous carbon base board 171a. These methods bring effects of preventing growth of bubbles of carbon dioxide gas, produced on the anode, within the porous carbon base board 171a and thereby enhancing fuel cell output density. Materials for the anode diffusion layer 170a are not limited to those described above. The other suitable materials include substantially electrochemically inactive metals, e.g., non-woven fabric of stainless steel fibers, and a porous material such as porous titanium and tantalum.
The above-described catalyst materials are described more specifically by citing EXAMPLES and COMPARATIVE EXAMPLE. In this example, a platinum/ruthenium alloy is described as the catalyst metal. The catalyst, however, is not limited to the above. For example, a platinum-containing catalyst metal can be used for a DMFC cathode.
Here, one embodiment of DMFC for a portable information terminal.
The current collector is made of a 0.3 mm thick titanium plate, and coated with an about 0.1 μm thick gold layer on the portion coming into contact with the electrode, which is deposited by vacuum evaporation after the collector surface is cleaned.
The power source thus prepared is 115 by 90 by 9 mm in size. The MEA which constitutes the power generation section of the DMFC assembled in the power source allows the DMFC to produce a higher output than a conventional DMFC by use of the catalyst described in EXAMPLE 1.
On the other hand, a power system in which the MEA (4) prepared in COMPARATIVE EXAMPLE is used in place of the MEA (13) cannot stably produce an output, because of immediately decreased liquid fuel concentration. Moreover, steam is condensed by natural aspiration on the anode to cause oxygen shortage and unstable output. Therefore, removal of condensate is prerequisite for the power system.
The MEA which constitutes the power generation section of the DMFC assembled in the portable information terminal allows the DMFC to produce a higher output than a conventional DMFC by use of the catalyst described in EXAMPLE 1, and hence can increase a maximum output which the terminal requires.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
The compact fuel cell power system of the present invention contributes to spread of fuel cells.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-029154 | Feb 2005 | JP | national |
