JP2012212570A - Fuel permeation amount adjusting membrane for liquid fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high-output fuel cell in which methanol crossover is suppressed and an MEA is moisturized.SOLUTION: The fuel cell comprises a gas-liquid separating membrane for supplying at least part of a liquid fuel as a gas fuel, a fuel permeation amount adjusting membrane for adjusting a permeation amount of the gas fuel, an anode-side collector layer, an anode for oxidizing the gas fuel, a solid electrolyte membrane, a cathode for reducing oxygen, and a cathode-side collector layer in this order. The fuel cell is characterized in that the fuel permeation amount adjusting membrane contains polytetrafluoroethylene (PTFE) and inorganic particles.

Description

  The present invention relates to a fuel cell that vaporizes liquid fuel and generates power using the vaporized fuel gas.

  In recent years, a battery having a high energy capacity has been demanded due to the development of small portable electronic devices. Lithium ion secondary batteries are mainly developed, but fuel cells that can continuously generate power by replacing fuel cartridges are promising next-generation power sources with high energy density. Examples of the fuel include hydrogen-containing substances such as hydrogen and sodium borohydride, alcohols such as methanol and ethanol, and other organic fuels. Among these, methanol has a high volumetric energy density, is a liquid, and is easy to carry, so it is suitable for small portable devices.

A direct methanol fuel cell (DMFC) generates electricity by an oxidation reaction of methanol (1 type) at the anode and by an air reduction reaction (2 type) by supplying air to the cathode.
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1)
3 / 2O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O (2)

  Usually, perfluorosulfonic acid represented by Nafion (registered trademark) is used for the electrolyte membrane through which protons pass. The anode is mainly made of platinum and ruthenium catalysts that oxidize methanol. An electrolyte necessary for fixing the catalyst and carrying protons generated by the oxidation reaction is used as a binder to form a catalyst layer. Similarly, the cathode is composed of a catalyst represented by platinum, which is an oxygen reduction catalyst, and an electrolyte. For this electrolyte, a perfluorosulfonic acid resin such as Nafion similar to the electrolyte membrane is usually used.

  DMFC using conventional methanol aqueous solution has sufficient water content of MEA, and the generated water and permeated water condenses on the cathode side and flooding impedes air diffusion. Further, methanol permeates to the cathode side and the battery performance decreases. There was a problem of methanol crossover. In order to suppress the methanol crossover, it is necessary to reduce the concentration of methanol supplied to, for example, about 1M. The fuel diluted to a low concentration has a reduced volumetric energy density and is not suitable for miniaturization. In addition, in order to suppress methanol crossover and water permeation, it has been attempted to apply a hydrocarbon ionomer such as aromatic instead of perfluorosulfonic acid to the electrolyte membrane. However, the proton conductivity of hydrocarbon ionomers tends to be lower than that of perfluorosulfonic acid ionomers. Therefore, it is very difficult to conduct protons well from the anode to the cathode while suppressing methanol crossover.

  On the other hand, a gas-phase DMFC that supplies methanol by vapor has also been proposed. For example, according to WO 2005/112172 (Patent Document 1), high-concentration methanol (liquid) is supplied to the fuel chamber as fuel, and the vaporized methanol passes through a gas-liquid separation membrane disposed between the fuel chamber and the anode. The gas is once stored in a vaporized fuel storage chamber (so-called vapor reservoir) disposed between the liquid separation membrane and the anode, and gradually diffuses in the gas diffusion layer from there to be supplied to the anode. This states that a large amount of vaporized fuel can be prevented from being supplied to the anode at one time, and methanol crossover can be suppressed. Further, in Patent Document 1, a moisture retention layer is provided between the gas-liquid separation membrane and the anode, water that is reversely diffused from the cathode to the anode is retained in the moisture retention layer, and is diffused into the vaporized fuel storage chamber to dilute the fuel. To prevent.

  In Patent Document 1, the fuel concentration is adjusted in the vaporized fuel storage chamber to prevent excessive methanol from being supplied, but that much space is required, and it is unclear how effective it is in a narrow space. There may be local fuel concentration non-uniformity.

International Publication Number WO 2005/112172

  The present invention has been made in view of the above problems in the conventional fuel cell described above, and an object of the present invention is to obtain a high-power fuel cell that suppresses methanol crossover, moisturizes MEA, and high output.

According to the present invention, the following is provided.
(1) A gas-liquid separation membrane that supplies at least a part of liquid fuel as gaseous fuel, a fuel permeation adjustment membrane that adjusts the permeation amount of the gaseous fuel, a negative current collector layer, and the gaseous fuel is oxidized A fuel cell comprising a negative electrode, a solid electrolyte membrane, a positive electrode for reducing oxygen, and a positive-electrode current collecting layer in this order, wherein the fuel permeation adjustment membrane is polytetrafluoroethylene (PTFE) and A fuel cell comprising inorganic particles.
(2) The fuel cell according to (1), wherein the inorganic particles are silica porous fine particles.
(3) The fuel cell according to (2), wherein the inorganic particles further contain activated carbon.
(4) The fuel cell according to (2) or (3), wherein the weight composition ratio of the PTFE and the porous silica fine particles is in the range of 1: 3 to 1: 1.
(5) The fuel cell according to any one of (1) to (4), wherein the liquid fuel is at least one selected from the group consisting of methanol, ethanol, dimethyl ether and formic acid.
(6) The fuel cell according to any one of (1) to (5), wherein the concentration of the liquid fuel is 70 wt% or more.
(7) The fuel cell according to any one of (1) to (6), wherein the gas permeability (Gurley number) of the fuel permeation amount adjusting membrane is in a range of 2300 seconds or more.
(8) The fuel cell according to any one of (1) to (7), wherein the fuel adjustment film has a thickness of 100 μm or more and 1 mm or less.
(9) The fuel cell according to any one of (1) to (8), wherein the gas-liquid separation membrane uses expanded porous PTFE subjected to an oil repellent treatment.
(10) The fuel cell according to any one of (1) to (9), wherein the gas-liquid separation membrane and the fuel permeation amount adjusting membrane are laminated and used.
(11) The fuel cell according to any one of (1) to (10), wherein the fuel permeation amount adjusting membrane includes an expanded porous PTFE layer on a negative electrode side current collecting layer side.
(12) The fuel cell according to any one of (1) to (11), wherein a particle size distribution of the inorganic particles is 0.01 μm or more and 500 μm or less.

  According to the fuel gas permeation adjustment membrane of the present invention, methanol can be supplied to the anode as a gas, and methanol crossover can be suppressed by supplying an amount of methanol that is necessary for the reaction but not excessive. Further, since the hydrophilic inorganic fine particles constituting the fuel gas permeation amount adjusting membrane retain the generated water that has been reversely diffused from the cathode, the methanol can be humidified and supplied to the anode, so that the MEA can be moisturized. Therefore, a high output fuel cell can be obtained by the present invention.

1 shows a schematic cross-sectional view of a liquid fuel supply type fuel cell according to an embodiment of the present invention. The outline of a methanol gas penetration amount evaluation apparatus is shown. The electric power generation test result of the Example and comparative example of this invention is shown.

  FIG. 1 shows a schematic cross-sectional view of a liquid fuel supply type fuel cell according to an embodiment of the present invention. The fuel cell according to the present invention will be described with reference to FIG. A liquid fuel supply type fuel cell according to the present invention includes a gas-liquid separation membrane that supplies at least a part of liquid fuel as gaseous fuel, a fuel permeation adjustment membrane that adjusts the permeation amount of the gaseous fuel, and a negative current collector layer And a negative electrode that oxidizes the gaseous fuel, a solid electrolyte membrane, a positive electrode that reduces oxygen, and a positive-electrode current collecting layer in this order, and the fuel permeation adjustment membrane is polytetrafluoroethylene It comprises (PTFE) and inorganic particles.

  As shown in FIG. 1, the liquid fuel supply type fuel cell according to the present invention is supplied with a liquid fuel, for example, an aqueous methanol solution (MeOHaq), from the side opposite to the negative electrode of the gas-liquid separation membrane. At least a part of the supplied liquid fuel evaporates to become gaseous fuel. The gas-liquid separation membrane allows only gas to permeate without permeating the liquid. Therefore, the fuel gas evaporated at the interface between the gas-liquid separation membrane and the liquid fuel permeates the gas-liquid separation membrane and becomes the fuel permeation adjustment membrane. To reach.

The gas-liquid separation membrane is required to have heat resistance and corrosion resistance under fuel cell operation conditions in addition to good air permeability, water repellency and oil repellency. For example, expanded porous polytetrafluoroethylene (PTFE) can be used as the gas-liquid separation membrane. As this expanded porous PTFE, it is preferable to use one having a porosity of 10% or more, preferably 10 to 90%. If this porosity is less than 10%, the amount of fuel gas evaporated at the interface with the liquid fuel is reduced, the release of CO ₂ gas generated at the negative electrode is inhibited, and so on. Performance is insufficient. On the other hand, if the porosity exceeds 90%, the liquid fuel can be permeated and a crossover phenomenon may occur. The average pore diameter (diameter) of the expanded porous PTFE is generally in the range of 0.01 to 50 μm, preferably 0.05 to 15 μm, more preferably 0.1 to 3 μm. When the average pore diameter is less than 0.01 μm, the permeation amount of the evaporated fuel gas is reduced. On the other hand, when the average pore diameter exceeds 50 μm, a crossover phenomenon due to the permeation of liquid fuel tends to occur. The thickness of the gas-liquid separation membrane containing expanded porous PTFE is generally in the range of 10 to 500 μm, preferably 50 to 300 μm. If the thickness of the gas-liquid separation membrane is less than 10 μm, the liquid fuel can permeate and a crossover phenomenon may occur. On the other hand, if it exceeds 500 μm, the permeation amount of the fuel gas evaporated at the interface with the liquid fuel is reduced, the release of CO ₂ gas generated at the negative electrode is inhibited, and the power generation performance of the fuel cell is insufficient. It becomes. Particularly preferred expanded porous PTFE for use as a gas-liquid separation membrane according to the present invention is commercially available from Japan Gore-Tex Corporation.

  In order to reliably prevent liquid fuel from permeating the gas-liquid separation membrane and further promote vaporization of the liquid fuel, an oil repellent treatment that reduces the wettability of the expanded porous PTFE to the liquid fuel is applied to the expanded porous PTFE. It is preferable to give it. As a method for subjecting stretched porous PTFE to an oil repellency treatment, for example, as described in JP-A-7-126428, a fluororesin having a fluorinated aliphatic ring structure in the main chain and a polyfluoroalkyl group are used. A method of treating expanded porous PTFE with a composition containing a fluorine-containing polymer to be contained, as described in JP-A-8-511040, fluorination having an average particle size of 0.01 to 0.5 μm And a method of treating expanded porous PTFE with an aqueous latex containing organic polymer particles having organic side chains. By performing such an oil repellency treatment, the wettability of the stretched porous PTFE to the liquid fuel (methanol) can be reduced while maintaining the breathability and water repellency inherent to the stretched porous PTFE. For details of the oil-repellent treatment of stretched porous PTFE, refer to the above-mentioned JP-A-7-126428 and JP-A-8-511040.

  Only the gaseous fuel reaches the fuel permeation amount adjusting membrane through the gas-liquid separation membrane, where the fuel permeation amount adjusting membrane comprises polytetrafluoroethylene (PTFE) and inorganic particles. PTFE is a fluororesin excellent in heat resistance, corrosion resistance and workability. PTFE can be formed into a film by various known methods such as extrusion and bead rolling. By appropriately combining rolling and stretching, the PTFE membrane has extremely fine holes, and gas fuel can be transmitted through the fine holes. At least a part of the fine holes are stretched in the fuel permeation amount adjusting film and communicated with each other, whereby the vaporized fuel is uniformly diffused through the fine holes and supplied to the negative electrode. When processing the PTFE to form a membrane, inorganic particles can be mixed to obtain a fuel permeation adjustment membrane containing PTFE and inorganic particles. The inorganic particles can at least partially block the fine pores. Further, when the mixing ratio of the inorganic particles is increased, voids between the inorganic particles are generated. If the gap is not sufficiently filled with PTFE, the gap may remain as an airway in the fuel permeation amount adjusting membrane. Furthermore, when the fuel can permeate the inorganic particles themselves, the fuel can permeate through the inorganic particles. Thus, various effects are produced by combining PTFE and inorganic particles. Therefore, these effects can be combined and adjusted so as to obtain a desired fuel permeation amount.

  As PTFE contained in the fuel permeation amount adjusting membrane, expanded PTFE can be used as in the gas-liquid separation membrane. By appropriately combining rolling and stretching, it is possible to appropriately adjust the porosity, average pore diameter, and thickness of the PTFE membrane included in the fuel permeation amount adjusting membrane to obtain a desired fuel permeation amount.

  The inorganic particles contained in the fuel permeation amount adjusting membrane are preferably those having high resistance to the fuel that permeates the fuel permeation amount adjusting membrane. In addition, the inorganic particles are desirably hydrophilic. The inorganic particles may be porous. This is because hydrophilic inorganic particles can retain moisture, and porous inorganic particles can retain moisture in the porous portion. By holding this moisture, it is possible to prevent the drying of the negative electrode, particularly the negative electrode, in the vicinity of the fuel permeation adjustment film, and to smoothly proceed with the negative electrode power generation reaction that requires water. The retained moisture may be the moisture supplied through the gas-liquid separation membrane together with the vaporized fuel for the negative electrode reaction, and the moisture generated in the positive electrode power generation reaction at the positive electrode passes through the electrolyte membrane to adjust the fuel permeation amount. It may be one that reaches the membrane. Moisture generated in the positive electrode power generation reaction is retained by the fuel permeation amount adjustment film, so that flooding can be prevented from occurring in the positive electrode and the vicinity thereof, and oxygen diffusion necessary for the negative electrode power generation reaction proceeds smoothly.

Representative examples of inorganic particles include, but are not limited to, silica, alumina, zirconia, titania, activated carbon, glass and the like. Among these, silica and activated carbon are preferable because they are hydrophilic and can easily obtain a large pore volume. These inorganic particles may be used individually by 1 type, and 2 or more types may be mixed and used for them. Examples of the porosity of the inorganic particles include those in which the cumulative pore volume of pores having a pore radius of 3 to 10 nm (hereinafter referred to as “V _3-10 ”) is at least 0.2 ml / g. If V _3-10 is less than 0.2 ml / g, the hygroscopicity may not be sufficient. However, it should be noted that the effect of the present invention can be produced by adjusting the diameter and porosity of the fine pores of PTFE, the particle size distribution of the silica porous fine particles, and the like outside this range. . Note that the “pore radius” and “pore volume” are the known nitrogen adsorption methods (measurement of the amount of nitrogen adsorbed on the sample and the pressure at that time to obtain the adsorption isotherm, and the Kelvin equation is used to determine the pore size. The method of calculating the radius and the pore volume is measured by using a gas adsorption measuring device “ASAP2010” manufactured by Shimadzu Corporation) or by a known water vapor adsorption method (the method described in Japanese Patent No. 3122205). In this method, the relationship between the amount of water adsorbed on the sample and the water vapor pressure at that time was measured to obtain the adsorption isotherm, and the pore radius and pore volume were calculated using the Kelvin equation).

  Depending on the operating conditions, the gas-liquid separation membrane may get wet and liquid fuel may be supplied to the fuel permeation adjustment membrane. For example, since the amount of vaporized fuel required according to the amount of power generation varies, the supplied vaporized fuel may transiently become excessive or insufficient. When the vaporized fuel is excessively present, the excessive amount aggregates into liquid fuel, which may wet the gas-liquid separation membrane or the fuel permeation adjustment membrane. However, according to the present invention, the liquid fuel may exist in the fine pores of PTFE included in the fuel permeation amount adjusting membrane, may exist in the porous portion of the inorganic particles, or may penetrate into the inorganic particles themselves. it can. These phenomena have an effect of suppressing excessive fuel supply. Further, the liquid fuel present in the PTFE and the inorganic particles has an effect of vaporizing again to compensate for the shortage of fuel when the fuel is short. That is, the fuel permeation amount adjusting film can be a buffer for supplying fuel. With the fuel permeation amount adjusting membrane of the present invention, it is possible to supply a fuel that is not excessive or insufficient in accordance with fluctuations in the amount of power generation, which leads to efficient fuel cell operation. Since excessive fuel supply is suppressed, the crossover phenomenon in which the fuel passes through the negative electrode current collecting layer, the negative electrode, and the electrolyte membrane and reaches the positive electrode can be remarkably reduced.

  The mass composition ratio of PTFE and silica porous fine particles contained in the fuel permeation amount adjusting membrane is preferably in the range of 1: 3 to 1: 1. When PTFE is larger than this range, that is, when the porous silica fine particles are smaller than this range, the effect of including inorganic particles may not be sufficiently exhibited. The effect is, for example, that the porous pores of the PTFE are closed by the porous silica fine particles, the airways based on the voids between the porous silica fine particles are formed, or the fuel penetrates into the inorganic particles themselves. On the other hand, if the PTFE is less than this range, that is, if the porous silica fine particles are more than this range, the fuel permeation adjustment membrane is likely to get wet and the possibility of fuel crossover increases, or the fuel permeation adjustment membrane itself. There is a concern that the strength of the resin may decrease (due to insufficient PTFE as a binder). Therefore, the mass composition ratio between PTFE and silica porous fine particles is preferably in the range of 1: 3 to 1: 1. However, it should be noted that the effects of the present invention can be produced by adjusting the fine pore diameter and porosity of PTFE, the particle size distribution of the silica porous fine particles, and the like, outside the above-described composition ratio range. Should.

  By appropriately adjusting the particle size distribution of the inorganic particles according to the diameter of the PTFE micropores, the PTFE micropores can be appropriately blocked. Therefore, the lower limit of the particle size of the inorganic particles may be approximately the same as the diameter of the PTFE micropores. In that case, the lower limit of the particle size of the inorganic particles is 0.01, preferably 0.05, and more preferably 0.1 μm. The particle size of the inorganic particles affects the size of the airway based on the voids between the inorganic particles. If the airway is too large, there is a concern such as crossover, so the upper limit of the particle size of the inorganic particles is 500, preferably 300, more preferably 100 μm. In general, the broader particle size distribution of the inorganic particles is preferably within the above range, and more preferably, for example, 0.01 μm to 500 μm. This is not bound by a specific theory, but if the particle size distribution is wide, small to large particles are mixed, airways based on the voids created between the particles increase, the air permeability increases, That is, it is considered that the fuel permeability increases and the power generation performance increases.

  The Gurley number of the fuel permeation amount adjusting membrane is preferably 2300 seconds or more. This is because if it is less than 2300 seconds, liquid fuel can permeate and a crossover phenomenon may occur.

The thickness of the fuel permeation adjustment membrane is generally in the range of 10 μm to 3 mm, preferably 100 μm to 1 mm, and more preferably 200 to 600 μm. If the thickness of the fuel permeation adjustment film is less than 10 μm, liquid fuel can be permeated and a crossover phenomenon may occur. On the other hand, if it exceeds 3 mm, the power generation performance of the fuel cell becomes insufficient due to a decrease in the amount of permeated fuel gas and the inhibition of the release of CO ₂ gas generated at the negative electrode.

  A fuel permeation adjustment membrane and a gas-liquid separation membrane may be laminated. That is, it is not necessary to provide the vaporized fuel storage chamber (so-called vapor reservoir) of Patent Document 1 between the fuel permeation adjustment membrane and the gas-liquid separation membrane. Therefore, it is possible to save the space of the fuel cell, and there is no concern about uneven fuel concentration in the fuel reservoir. As a lamination method, any conventionally known method can be adopted as long as it can be laminated accurately without damaging these films. For example, the laminated film may be formed by thermocompression bonding or electrostatic adsorption of the fuel permeation amount adjusting film and the gas-liquid separation film.

  An expanded porous PTFE layer can also be provided between the fuel permeation amount adjusting membrane and the negative electrode side current collector (not shown). This stretched porous PTFE layer can exhibit the same function as the gas-liquid separation membrane described above. In the unlikely event that the fuel permeation amount adjusting membrane gets wet, this expanded porous PTFE layer (between the fuel permeation amount adjusting membrane and the negative electrode side current collector) further supplies only vaporized fuel to the negative electrode. Can be certain.

  As shown in FIG. 1, an electromotive part of a fuel cell is composed of a negative electrode, a positive electrode, and a proton (hydrogen ion) conductive solid electrolyte membrane sandwiched between the negative electrode, a membrane electrode assembly (MEA). : MebraneElectrodeAssembly).

The solid electrolyte membrane in the present invention is not particularly limited as long as it has high proton (H ⁺ ) conductivity, electronic insulation, and gas impermeability, and is a known solid electrolyte membrane. I just need it. A typical example is a resin having a fluorine-containing polymer as a skeleton and a group such as a sulfonic acid group, a carboxyl group, a phosphoric acid group, or a phosphonic group. Since the thickness of the solid electrolyte membrane has a great influence on the resistance, a thinner one is required as long as the electronic insulation and gas impermeability are not impaired. Specifically, the thickness is 1 to 100 μm, preferably 5 to 50 μm. Is set within the range. The material of the solid electrolyte membrane in the present invention is not limited to a perfluorinated polymer compound, and is a mixture with a hydrocarbon polymer compound or an inorganic polymer compound, or a C—H bond and C— in the polymer chain. It may be a partially fluorinated polymer compound containing both F bonds. Specific examples of hydrocarbon polymer electrolytes include polyamides, polyacetals, polyethylenes, polypropylenes, acrylic resins, polyesters, polysulfones, polyethers, etc., and derivatives thereof (aliphatic carbonization) into which electrolyte groups such as sulfonic acid groups have been introduced. Hydrogen-based polymer electrolytes), polystyrene having electrolyte groups such as sulfonic acid groups introduced therein, polyamides having aromatic rings, polyamideimides, polyimides, polyesters, polysulfones, polyetherimides, polyethersulfones, polycarbonates, and derivatives thereof (Partial aromatic hydrocarbon polymer electrolyte), polyetheretherketone, polyetherketone, polyethersulfone, polycarbonate, polyamide, polyamideimide, polyester into which electrolyte groups such as sulfonic acid groups are introduced Polyphenylene sulfide and the like, and derivatives thereof (fully aromatic hydrocarbon-based polymer electrolyte), and the like. Specific examples of the partially fluorinated polymer electrolyte include polystyrene-graft-ethylenetetrafluoroethylene copolymer, polystyrene-graft-polytetrafluoroethylene, etc., into which electrolyte groups such as sulfonic acid groups have been introduced, and derivatives thereof. It is done. Specific examples of the perfluorinated solid electrolyte membrane include Nafion (registered trademark) membrane (manufactured by DuPont) which is a perfluoropolymer having a sulfonic acid group in the side chain, Aciplex (registered trademark) membrane (manufactured by Asahi Kasei Corporation), and Examples include Flemion (registered trademark) membrane (manufactured by Asahi Glass Co., Ltd.). As the inorganic polymer compound, a siloxane-based or silane-based, particularly alkylsiloxane-based organosilicon polymer compound is suitable, and specific examples include polydimethylsiloxane, γ-glycidoxypropyltrimethoxysilane, and the like. It is done.

The negative electrode and the positive electrode sandwiching the solid electrolyte membrane are each composed of a catalyst layer and a gas diffusion layer. The catalyst layer of the negative electrode and the positive electrode is not particularly limited as long as it contains catalyst particles and an ion exchange resin, and conventionally known ones can be used. The catalyst particles are usually carried on a conductive material. The catalyst particles may be any catalyst particles that have a catalytic action in the oxidation reaction of hydrogen or the reduction reaction of oxygen. In addition to platinum (Pt) and other noble metals, iron, chromium, nickel, and alloys thereof may be used. it can. As the conductive material, carbon-based particles such as carbon black, activated carbon, graphite and the like are suitable, and fine powder particles are particularly preferably used. A typical catalyst includes a carbon black particle having a surface area of 20 m ² / g or more loaded with noble metal particles such as Pt particles or alloy particles of Pt and other metals. In particular, for catalyst particles for negative electrodes, Pt is vulnerable to carbon monoxide (CO) poisoning, so when using a fuel that produces CO by side reaction, such as methanol, or a gas that is modified methane or the like. Is preferably alloy particles of Pt and ruthenium (Ru). The ion exchange resin in the catalyst layer is a material that supports the catalyst and serves as a binder for forming the catalyst layer, and has a role of forming a passage for ions and the like generated by the catalyst to move. As such an ion exchange resin, the thing similar to what was demonstrated in relation to the solid electrolyte membrane previously can be used. The catalyst layer is preferably porous so that vapor of liquid fuel such as methanol can be brought into contact with the catalyst as much as possible on the negative electrode side and oxygen gas such as oxygen and air can be brought into contact with the catalyst as much as possible on the positive electrode side. Further, the amount of catalyst contained in the catalyst layer is in general but may be in the range of 0.1 to 10 mg / cm ^2, in particular in the range of 1.0~7.0mg / cm ² for a negative electrode, also The positive electrode is preferably in the range of 0.3 to 4.0 mg / cm ² . The thickness of the catalyst layer may generally be in the range of 1 to 200 μm, but it is particularly preferable that the thickness of the catalyst layer is in the range of 50 to 150 μm, and that of the positive electrode is in the range of 10 to 100 μm.

  The gas diffusion layer constitutes an electrode (negative electrode or positive electrode) in combination with the catalyst layer. The gas diffusion layer serves to uniformly supply a fuel such as methanol or an oxidant such as oxygen to the catalyst layer, and also has a function of conducting electrons between the current collecting layer and the catalyst layer. . Therefore, the gas diffusion layer has air permeability and conductivity, and is generally a sheet material. Representative examples include those obtained by subjecting a breathable conductive base material such as carbon paper, carbon woven fabric, carbon nonwoven fabric, carbon felt or the like to a water repellent treatment. A porous sheet obtained from carbon-based particles and fluorine-based resin can also be used. For example, a porous sheet obtained by forming carbon black into a sheet using polytetrafluoroethylene as a binder can be used. The thickness of the gas diffusion layer is generally 50 to 500 μm, preferably 100 to 350 μm.

  A membrane electrode assembly (MEA) is produced by joining a catalyst layer, a gas diffusion layer, and a solid electrolyte membrane. As a joining method, any conventionally known method can be adopted as long as a precise joining with low contact resistance is achieved without damaging the solid electrolyte membrane. At the time of joining, after first forming a negative electrode or a positive electrode by combining a catalyst layer and a gas diffusion layer, these can be joined to a solid electrolyte membrane. For example, a negative electrode or a positive electrode is formed by preparing a coating solution for forming a catalyst layer containing catalyst particles and an ion exchange resin using an appropriate solvent, and applying it to a sheet material for a gas diffusion layer. Can be joined by hot pressing. Further, after combining the catalyst layer with the solid electrolyte membrane, a gas diffusion layer may be combined on the catalyst layer side. When combining the catalyst layer and the solid electrolyte membrane, a conventionally known method such as a screen printing method, a spray coating method, or a decal method may be employed.

The amount of the fuel that has permeated the gas-liquid separation membrane is adjusted by the fuel permeation amount adjustment membrane, passes through the negative electrode side current collecting layer, is further diffused by the negative electrode gas diffusion layer, and is supplied to the negative electrode catalyst layer. The fuel supplied to the negative electrode catalyst layer undergoes an internal reforming reaction of methanol represented by the following formula (1).
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ Formula (1)
On the other hand, the air taken in from the moisturizing membrane / external filter diffuses through the positive electrode current collecting layer and the positive electrode gas diffusion layer and is supplied to the positive electrode catalyst layer. The air supplied to the positive electrode catalyst layer causes a reaction represented by the following formula (2), which is a reduction reaction, together with protons that have diffused through the solid electrolyte membrane and electrons that have flowed through the external circuit. By this reaction, water is generated and a power generation reaction occurs.
(3/2) O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O Formula (2)

When the reactions of the above formulas (1) and (2) occur simultaneously, the power generation reaction as a fuel cell is completed. When the power generation reaction proceeds, water (H ₂ O) generated in the positive electrode catalyst layer by the reaction of the above-described formula (2) increases the amount of water in the positive electrode catalyst layer. As a result, due to the osmotic pressure phenomenon, water generated in the positive electrode catalyst layer passes through the solid electrolyte membrane and moves to the negative electrode catalyst layer, and is used for the methanol oxidation reaction represented by the above-described formula (1). In this way, the methanol oxidation reaction can be continued without supplying water from the outside. In addition, the water which moved to the negative electrode catalyst layer stays in the fuel permeation amount adjusting film, and can also provide moisture retention in the vicinity thereof, particularly in the negative electrode.

  The negative current collecting layer is formed of a conductive material in order to fulfill the function of conducting electrons generated in the negative electrode. The negative electrode side current collecting layer is formed of a porous layer such as a mesh in order to fulfill the function of supplying fuel and water to the negative electrode. As the material for the negative electrode side current collecting layer, any conventionally known material can be employed. Specific examples of the material for the negative electrode current collecting layer include a titanium (Ti) porous plate, a SUS porous plate, and a carbon porous plate. The thickness of the negative electrode side current collecting layer is generally in the range of 10 μm to 5 mm.

  The positive electrode side current collecting layer can be made of the same material as the negative electrode side current collecting layer. Specifically, the positive current collecting layer is formed of a conductive material in order to fulfill the function of conducting electrons used in the positive electrode. Further, the positive electrode side current collecting layer is formed of a porous layer such as a mesh in order to supply oxygen (air) to the positive electrode and to discharge the generated surplus water to the outside. Any conventionally known material can be adopted as the material for the positive electrode current collecting layer. Specific examples of the material for the positive current collecting layer include a titanium (Ti) porous plate, a SUS porous plate, a carbon porous plate, and the like. The thickness of the positive electrode side current collecting layer is generally in the range of 10 μm to 5 mm.

  In the liquid fuel supply type fuel cell according to the present invention, the liquid fuel is supplied to the negative electrode side. Examples of the liquid fuel include methanol, ethanol, dimethyl ether, formic acid, 1-propanol, 2-propanol, sodium borohydride, potassium borohydride, lithium borohydride and the like. The lower limit of the solute concentration in these aqueous solutions is generally 2% by mass or more, preferably 20% by mass or more, more preferably 50% by mass or more, still more preferably 70% by mass or more, and most preferably 90% by mass or more. When the solute concentration is less than 2% by mass, the energy density of the fuel cell decreases. In the fuel cell of the present invention, since fuel crossover is suppressed, high-concentration fuel can be used. The upper limit of the solute concentration in these aqueous solutions is not particularly limited. That is, the liquid fuel supply type fuel cell according to the present invention can use a fuel having a solute concentration of 100% by mass, and can generate power with high efficiency. Needless to say, from the viewpoints of availability and manageability, it is possible to optionally use a fuel having a solute concentration of less than 100% by mass, for example, 99% by mass or less, 90% by mass or less, and further 80% by mass or less. is there.

  Hereinafter, the present invention will be specifically described by way of examples.

  Fabrication of membrane electrode assembly (MEA)

(Production of diffusion layer)
Prepare carbon paper (TGP-H-060 made by Toray Industries, Inc.) and immerse it in PTFE (Daikin, D1E, PTFE content 60 mass%) diluted to 20% (diluted 5 times with water). After drying in an oven, the mixture was baked at 350 ° C. for 2 hours to produce a water-repellent diffusion layer.

(Preparation of negative electrode)
Carbon-supported platinum ruthenium catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC61E54DM: platinum / ruthenium (PtRu) supported amount of 50% by mass) 5 g, 17 mass of perfluorosulfonic acid electrolyte (EW = 700) represented by the following formula (1) A mixed ink was prepared by mixing with 17.6 g of% ethanol solution. The mixed ink was sprayed on the solid electrolyte membrane side of the negative electrode diffusion layer to deposit a catalyst layer, thereby forming a negative electrode having a catalyst noble metal amount of 3 mg / cm ² .

(Preparation of positive electrode)
5 g of a carbon-supported platinum catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E70TPM: 70 mass% platinum supported) in the perfluorosulfonic acid electrolyte (EW = 700) solution, and the mass ratio of the carbon content of the catalyst to the electrolyte is 1.0. A mixed ink was prepared by mixing so that The mixed ink was sprayed on the solid electrolyte membrane side of the diffusion layer to deposit a catalyst layer, and a positive electrode having a catalyst noble metal amount of 1.3 mg / cm ² was formed.

(MEA)
An ion exchange membrane GORE-SELECT (registered trademark) (manufactured by Japan Gore-Tex Co., Ltd.) having a size of 5 × 5 cm and a thickness of 30 μm is prepared as a polymer electrolyte membrane. Each of the positive electrodes (catalyst layer side) was placed on the surface, and hot pressure (160 ° C., 1 MPa, 5 minutes) was applied by a hot press for lamination.

Assembly of liquid fuel supply type fuel cell (current collection layer)
A titanium (Ti) porous plate (size: 5 × 5 cm, thickness: 2 mm, opening ratio: 50%) provided with holes having a diameter of 2 mm in a grid pattern as a current collecting layer was prepared. It arrange | positioned so that the diffusion layer side might be touched.

(Fuel permeation adjustment membrane)
A porous membrane (size: 5 × 5 cm, manufactured by Japan Gore-Tex) containing inorganic fine particles made of silica and PTFE was prepared and used as a fuel permeation adjustment membrane. In each example, films having different compositions and thicknesses were produced. This fuel permeation amount adjusting membrane was disposed so as to be in contact with the side opposite to the solid electrolyte membrane of the negative electrode side current collecting layer.

(Gas-liquid separation membrane)
Oil-repellent stretched porous PTFE (size 5 x 5 cm, thickness 160 μm, porosity 80%, manufactured by Japan Gore-Tex) is prepared as a gas-liquid separation membrane, which is opposite to the solid electrolyte membrane of the fuel permeation adjustment membrane It was arranged so that it touches the side.

<Power generation test>
A fuel container equipped with a thermocouple for measuring the fuel temperature was attached to the single fuel cell produced by the above procedure, and a potentiostat was further connected. The fuel container was filled with 5 ml of pure methanol, held at OCV (open voltage) for 5 minutes, and then generated at a constant voltage of 0.3 V for 15 minutes. Next, a resistance value of 5 kHz was obtained by an alternating current impedance (EIS) method. Thereafter, the voltage was swept from OCV to 0.1 V at 1 mV / sec to obtain an IV curve. As an evaluation environment, an environment left at room temperature for several hours was used. The fuel temperature was measured over time after the current started to flow, and the maximum temperature was recorded as the fuel temperature.

<Gurley number evaluation method>
In order to evaluate the gas air permeability of the permeation amount adjusting membrane, the Gurley number was evaluated based on JIS P 8117: 1998.

<Methanol gas penetration amount evaluation method>
FIG. 2 shows an outline of the evaluation apparatus. A gas-liquid separation membrane expanded PTFE and a fuel permeation amount adjustment membrane are laminated on an acrylic cell constituting the fuel chamber, sandwiched between Ti plates with an aperture ratio of 50% used in the power generation test, and a ventilation chamber is formed outside the laminate. Acrylic cells were placed and tightened with bolts. The fuel chamber was filled with methanol, an N ₂ gas line was connected to the vent chamber, and N ₂ was supplied at 20 ml / min. The N ₂ gas line from the vent chamber outlet was connected to a mass spectrometer, and the strength of methanol in the vent was measured. At the same time, N ₂ gas from the vent chamber outlet was also supplied to the gas chromatogram, and the amount of methanol was quantified. The methanol permeability was determined from the slope of the increase in methanol intensity per unit time when methanol was detected.

Comparative Example 1
In Comparative Example 1, no fuel permeation adjustment membrane was used, and only a fuel cell with an oil-repellent stretched porous PTFE (size 5 × 5 cm, thickness 160 μm, porosity 80%, manufactured by Japan Gore-Tex) as a gas-liquid separation membrane. Evaluation, Gurley number and methanol permeability were evaluated.

Example 1
Silica fine particles (Silysia 780, manufactured by Fuji Serial Chemical Co., Ltd., particle diameter 0.4 μm to 70 μm, mode diameter 20 μm): PTFE = 3: 1 (weight ratio) porous membrane (0.6 mm) was prepared, Laminated with a liquid separation membrane, the fuel cell was evaluated and the methanol permeability was evaluated. The Gurley number was evaluated using only the porous membrane before lamination with the gas-liquid separation membrane.

Example 2
A porous membrane (0.6 mm) of silica fine particles (Silicia 780): PTFE = 1: 1 (weight ratio) is prepared, and is laminated with a gas-liquid separation membrane, and evaluation of methanol permeability and a fuel cell Evaluated. The Gurley number was evaluated using only the porous membrane before lamination with the gas-liquid separation membrane.

Example 3
The porous film having a thickness of 0.6 mm used in Example 2 was rolled to produce a porous film (0.2 mm) of silica fine particles (Silicia 780): PTFE = 1: 1 (weight ratio). Laminated with a liquid separation membrane, and evaluated methanol permeability and fuel cell. The Gurley number was evaluated using only the porous membrane before lamination with the gas-liquid separation membrane.

Example 4
Silica fine particles (RA, particle diameter: 0.3 μm to 300 μm, mode diameter: 30 μm): PTFE = 1: 1 (weight ratio) porous membrane (0.2 mm) is prepared and laminated with a gas-liquid separation membrane And methanol permeability and fuel cell evaluation. The Gurley number was evaluated using only the porous membrane before lamination with the gas-liquid separation membrane.
Here, silica fine particles having a wide particle size distribution were used.

表１において、実施例３のガーレー数の測定不能とは１４、０００秒以上で測定限界を超えたことを示している。 Table 1 shows the evaluation results of the Gurley number and methanol gas penetration amount.

In Table 1, the inability to measure the Gurley number of Example 3 indicates that the measurement limit was exceeded in 14,000 seconds or more.

Comparative Example 1 does not contain silica and Example 3 contains silica. From this result, it was found that the inclusion of silica increases the Gurley number (time required for aeration) and decreases the methanol permeability. This is presumably due to the fact that the fine pores of PTFE were blocked by silica. However, the effect was greatly different depending on the composition and film thickness, as can be seen from the following example data.
When the silica content of Example 1 was increased from that of Example 2, Example 1 with a large amount of silica had a smaller Gurley number (time required for aeration) and increased methanol permeability. This is because silica particles increase, voids are generated between the silica particles, and the voids are not sufficiently filled with PTFE and remain in the membrane as airways, resulting in a reduction in the Gurley number (time required for ventilation). It is thought that the methanol permeability increased. The rate of change in methanol permeability was greater than the rate of change in Gurley number. This is because, in addition to methanol passing through the airways in the membrane, methanol can penetrate into the silica particles, so methanol passes through the increased silica particles, resulting in an increase in total methanol permeability. It is done.
Using Example 2 and Example 3, the influence of the film thickness on methanol permeability was examined. The porous membrane of Example 2 was 0.6 mm thick, and the porous membrane of Example 3 was 0.2 mm thick. Since the membrane of Example 3 is thinner, it is considered that the methanol permeability increased accordingly.
Using Example 3 and Example 4, the influence of the particle size distribution of silica particles on methanol permeability was examined. The silica particles (SY780) of Example 3 had a particle size of 0.4 μm to 70 μm and a mode diameter of 20 μm, and the silica particles (RA) of Example 4 had a particle size of 0.3 μm to 300 μm and a mode diameter of 30 μm. That is, the particle size distribution of Example 4 was wider and the mode diameter was larger. The methanol permeability was higher in Example 4. This is probably because in Example 4 where the particle size distribution is wide and particles having a large particle size are included, more airways remain based on voids generated between the silica particles than in Example 3. This is supported by the fact that the Gurley value of Example 4 is lower than that of Example 3, that is, the air permeability is high.
In addition, the particle size distribution measurement of the silica particle was performed in the following procedures. A sample of a small spatula was prepared for each silica particle, mixed with 20 ml of pure water 0.4 wt% Triton aqueous solution, and this mixture was placed in an ultrasonic cleaner for 2 minutes to prepare a silica particle dispersion. . Using a Shimadzu SALD-2000 (absorbance 0.1 to 0.2, refractive index 1.45 to 0.05i), the particle size distribution of each silica particle dispersion was measured.
As described above, it has been found that the fuel permeation amount adjustment film can adjust the fuel permeation amount by including inorganic particles including PTFE and silica particles.

FIG. 3 shows the power generation test results of Examples 1 to 4 and Comparative Example 1 described above.
In the case of only the gas-liquid separation membrane of Comparative Example 1, the methanol crossover was increased, and heat was generated to reduce the output. As can be seen from Table 1, in the case of only the gas-liquid separation membrane of Comparative Example 1, since the methanol permeability was remarkably high, the fuel supply was excessive and the methanol crossover was considered to be large.
In the case of the film of Example 1, a high output could be obtained. This is because the methanol permeation amount was appropriately adjusted.
The membrane of Example 2 has a relatively low methanol permeability and a high OCV, so that methanol crossover is suppressed to a very low level. However, the methanol concentration is too low and the fuel concentration is limited, so that no output can be obtained. .
The film of Example 3 has the same composition as Example 2 and a film thickness of 0.2 mm. In Example 3, since the methanol permeability was improved as compared with Example 2, the output was improved.
In the membrane of Example 4, the particle size distribution of silica was broadened, and the amount of methanol permeation increased compared to Example 3, thereby improving the output of DMFC. The methanol permeation amount was higher in Example 4 than in Example 1, and the Gurley number was lower in Example 4 than in Example 1. That is, in the case of Example 1, it is considered that the methanol permeation amount was adjusted more appropriately and the sweeping was possible up to a high current density.

In the above examples and comparative examples, pure methanol was used for the test. When a methanol aqueous solution is used as the fuel, the optimum membrane characteristics vary depending on the methanol concentration. Therefore, it is desired to optimize the physical properties of the fuel permeation amount adjusting film depending on the fuel concentration to be used.
From the above results, the fuel permeation amount can be appropriately controlled and the power generation performance of the fuel cell can be improved by using a porous membrane containing inorganic fine particles such as silica and PTFE as a fuel permeation amount adjustment membrane. It was shown that

Claims (12)

  A gas-liquid separation membrane that supplies at least a part of the liquid fuel as a gaseous fuel, a fuel permeation adjustment membrane that adjusts the permeation amount of the gaseous fuel, a negative current collecting layer, a negative electrode that oxidizes the gaseous fuel, A fuel cell comprising a solid electrolyte membrane, a positive electrode for reducing oxygen, and a positive current collecting layer in this order, wherein the fuel permeation adjustment membrane comprises polytetrafluoroethylene (PTFE) and inorganic particles. A fuel cell comprising the fuel cell.   The fuel cell according to claim 1, wherein the inorganic particles are silica porous fine particles.   The fuel cell according to claim 2, wherein the inorganic particles further comprise activated carbon.   4. The fuel cell according to claim 2, wherein the weight composition ratio of the PTFE and the silica porous fine particles is in the range of 1: 3 to 1: 1. 5.   5. The fuel cell according to claim 1, wherein the liquid fuel is at least one selected from the group consisting of methanol, ethanol, dimethyl ether and formic acid.   The fuel cell according to claim 1, wherein the concentration of the liquid fuel is 70 wt% or more.   The fuel cell according to any one of claims 1 to 6, wherein the gas permeability (Gurley number) of the fuel permeation amount adjusting membrane is in a range of 2300 seconds or more.   The fuel cell according to any one of claims 1 to 7, wherein the thickness of the fuel adjustment film is 100 µm or more and 1 mm or less.   The fuel cell according to any one of claims 1 to 8, wherein the gas-liquid separation membrane uses expanded porous PTFE subjected to an oil repellent treatment.   The fuel cell according to any one of claims 1 to 9, wherein the gas-liquid separation membrane and the fuel permeation amount adjusting membrane are stacked and used.   The fuel cell according to any one of claims 1 to 10, wherein the fuel permeation amount adjusting membrane includes an expanded porous PTFE layer on a negative electrode side current collecting layer side.   The fuel cell according to any one of claims 1 to 11, wherein a particle size distribution of the inorganic particles is 0.01 µm or more and 500 µm or less.

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