JP2011210503A - Membrane electrode conjugant and direct alcohol fuel cell using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a membrane electrode conjugant by which a crossover phenomenon of liquid fuel and the poisoning of a catalyst can be suppressed and a high concentration alcohol is used as fuel, and to provide a direct alcohol fuel cell using the membrane electrode conjugant.SOLUTION: The membrane electrode conjugant A includes: an electrolyte membrane 1 having inorganic ion exchanging bodies formed in and on a surface of a substrate formed by forming a timber into the shape of a thin sheet of which the thickness is 10 mm or smaller; and a pair of electrode bodies 2 and 3 which are joined to both sides of the electrolyte membrane 1, respectively and carry platinum-ruthenium catalyst layers on an electrode base member. The direct alcohol fuel cell is constituted of the membrane electrode conjugant A which is pinched between an air electrode separator 4 and a fuel electrode separator 5, and then crimped and held by an air electrode side plate 6 and a fuel electrode side plate 7.

Description

  The present invention relates to a direct alcohol fuel cell that directly supplies a liquid fuel mainly composed of alcohol to a fuel electrode.

  A polymer electrolyte fuel cell (hereinafter referred to as PEFC) using hydrogen gas as a fuel is being developed as a driving power source or a distributed power source for an electric vehicle or the like. The PEFC has a stack structure in which a plurality of cell units including a fuel electrode (anode), an oxidant electrode (cathode), and a polymer electrolyte membrane sandwiched between these electrodes are stacked. Hydrogen can be supplied to the fuel electrode of the cell unit and oxygen or air can be supplied to the oxidant electrode, and an electromotive force can be obtained by oxidation or reduction reaction occurring at each electrode. The product in the reaction is only water. There is a clean power source. However, PEFC is difficult to reduce in size of a fuel cell, and is considered difficult to apply to portable electronic devices.

  Technologies for facilitating the miniaturization of fuel cells include a technology that converts liquid fuel, such as alcohol, into hydrogen with a reformer, supplies hydrogen to the fuel electrode, and generates electricity by supplying alcohol directly to the fuel electrode. Technology has been proposed. When alcohol is supplied directly, there is an advantage that it is easy to miniaturize since a reformer is unnecessary. Alcohol has the advantage that it has a higher energy density than other fuels such as liquid hydrogen and high-pressure hydrogen, and is easy to handle in terms of safety. Among alcohols, methanol is mainly being developed, but alcohols such as ethanol and ethylene glycol, which have high energy density, are also being investigated as fuels.

  Regarding such a fuel cell using alcohol as a fuel, for example, in Patent Document 1, a performance test as a fuel cell using methanol is performed using an electrocatalyst assembly obtained by platinum-plating a thin wood piece impregnated with an inorganic ion exchanger. The point is described. Further, in Patent Document 2, a plate-shaped liquid fuel cell single cell forming member is overlapped to form a single cell of a liquid fuel cell, and the formed single cell is an air electrode side plate made of an insulator, and a fuel electrode. It is sandwiched between the side plate and the gasket for preventing liquid fuel leakage, and is assembled so as to be disassembled by a screw fastening assembly member, so that air can be supplied from the air electrode side plate and liquid fuel can be supplied from the fuel electrode side plate to the single cell. A liquid fuel cell is described. Patent Document 3 describes a polymer electrolyte fuel cell that uses, as a negative electrode, an electrode catalyst assembly in which a platinum-ruthenium catalyst is deposited on the surface of an ion exchange membrane to form a layer.

JP 2006-313650 A JP 2007-12304 A JP 2002-75384 A

  The direct methanol fuel cell developed in recent years has a problem of methanol crossover (fuel crossover). Methanol crossover refers to the phenomenon in which methanol diffuses and moves due to the difference in methanol concentration between the fuel electrode side and oxygen agent electrode side, and the hydrated methanol is transported by water movement caused by proton movement. In combination with the electroosmosis phenomenon, methanol permeates the electrolyte membrane from the fuel electrode side and leaks out to the oxygen agent electrode side.

  When the methanol crossover occurs, the leaked methanol is oxidized in the catalyst layer of the oxygen agent electrode, reducing the output voltage of the fuel cell and wasting fuel. Further, since a platinum (Pt) catalyst is used as the catalyst layer of the oxygen agent electrode instead of a platinum (Pt) -ruthenium (Ru) alloy catalyst, carbon monoxide (CO) is easily adsorbed on the catalyst surface. There is also a problem that poisoning of the catalyst occurs.

  Accordingly, the present invention provides a membrane electrode assembly capable of suppressing the liquid fuel crossover phenomenon and catalyst poisoning and using high-concentration alcohol as fuel, and a direct alcohol fuel cell using the membrane electrode assembly. It is intended.

  The membrane electrode assembly according to the present invention is joined to an electrolyte membrane in which an inorganic ion exchanger is generated on the inside and surface of a substrate formed of a thin plate having a thickness of 10 mm or less, and both sides of the electrolyte membrane. The electrode base material is provided with a pair of electrode bodies carrying a platinum-ruthenium catalyst layer. Further, the inorganic ion exchanger is zirconyl phosphate produced by treating a hydrochloric acid solution of zirconium oxychloride impregnated in the substrate or a nitric acid solution of zirconyl oxynitrate with a phosphoric acid solution. . Furthermore, the electrode base material is a nonwoven fabric made of carbon fiber.

  A direct alcohol fuel cell according to the present invention includes the membrane electrode assembly described above, and is characterized in that liquid fuel containing alcohol as a main component is directly supplied to the electrode body on the fuel electrode side of the membrane electrode assembly. To do. Furthermore, the liquid fuel is characterized in that one of methanol, ethanol, isopropyl alcohol, and ethylene glycol is a main component. Furthermore, the electrode body on the air electrode side is provided with paper made of bamboo fibers that absorbs water generated by the reaction.

  The present invention relates to an electrolyte membrane in which an inorganic ion exchanger is generated on the inside and surface of a substrate formed of a thin plate having a thickness of 10 mm or less, and bonded to both sides of the electrolyte body, and platinum- Since the pair of electrode bodies carrying the ruthenium catalyst layer are provided, it is possible to suppress the crossover phenomenon of the liquid fuel and the poisoning of the catalyst and to use high concentration alcohol as the fuel.

It is a schematic sectional drawing regarding the direct type | mold alcohol fuel cell which concerns on this invention.

  The present invention will be described in detail below. FIG. 1 is a schematic cross-sectional view of a direct alcohol fuel cell according to the present invention. The thin plate-like electrolyte membrane 1 is sandwiched from both sides by the sheet-like electrode body 2 carrying the catalyst layer held by the frame 2a and the sheet-like electrode body 3 carrying the catalyst layer held by the frame 3a. To form a membrane electrode assembly A.

  An air electrode separator 4 and an air electrode side plate 6 are disposed in pressure contact with the electrode body 2 side of the membrane electrode assembly A, and a fuel electrode separator 5 and a fuel electrode side plate 7 are disposed in pressure contact with the electrode body 3 side. Has been.

  The electrolyte membrane 1 is made of thin wood pieces having a thickness of 10 mm or less formed by cutting and drying plants belonging to trees or herbs, and using a thin plate-like piece of wood as a base, and an inorganic ion exchanger on and inside the base. The produced inorganic ion exchanger includes zirconyl phosphate and zirconyl tungstate.

  The type of wood used for the substrate is not particularly limited as long as it can be formed into a thin plate shape having a thickness of 10 mm or less and can be sufficiently impregnated with the liquid described later. Further, since the thickness of the base becomes easy to pass liquid if it becomes too thin, it is desirable to set it to a thickness that does not allow liquid to permeate and is preferably set to 100 μm or more, for example, depending on the type of wood. .

  When producing zirconyl phosphate as an inorganic ion exchanger on a substrate, first, pretreatment such as washing treatment and drying treatment of the piece of wood to be the substrate is performed, and the piece of wood is added to a hydrochloric acid solution of zirconium oxychloride or a nitric acid solution of zirconium oxynitrate. The pressure is reduced in the soaked state so that the wood pieces are sufficiently impregnated with the solution, and the impregnated wood pieces are immersed in the phosphoric acid solution and sufficiently reacted to form zirconyl phosphate on and inside the wood pieces. By producing zirconyl phosphate on the inside and on the surface of the wood piece, it has a liquid shielding property that makes it difficult for alcohol as a liquid fuel to pass through.

  When producing zirconyl tungstate as an inorganic ion exchanger on a substrate, first, pretreatment such as washing treatment and drying treatment of the piece of wood to be the substrate is performed, and the piece of wood is added to a hydrochloric acid solution of zirconium oxychloride or a nitric acid solution of zirconyl oxynitrate. The wood piece is sufficiently impregnated with the solution in a state of being immersed, and the impregnated wood piece is immersed in a sodium tungstate solution and sufficiently reacted to generate zirconyl tungstate in and on the wood piece. By producing zirconyl tungstate on the inside and on the surface of the piece of wood, it has liquid shielding properties like zirconyl phosphate.

  The electrode bodies 2 and 3 are composed of an electrode base material carrying a platinum-ruthenium catalyst layer. The electrode base material is preferably a nonwoven fabric or a woven fabric made of inorganic conductive fibers, and more preferably a carbon fiber nonwoven fabric. An electrode body carrying the catalyst tank is prepared by applying and fixing a coating liquid in which the catalyst is uniformly dispersed to the electrode substrate.

  Since the electrode bodies 2 and 3 arranged on both sides of the electrolyte membrane 1 of the membrane electrode assembly A both carry a platinum-ruthenium catalyst layer, any of them may be used as a fuel electrode or an air electrode. When it is assembled, it is not necessary to worry about the arrangement relationship between the membrane electrode assembly A and the separator, and it can be assembled easily. Further, the electrode bodies 2 and 3 may be integrated with the electrolyte membrane 1 in advance by a pressure press, and in that case, it is possible to further improve the assembly work.

  The air electrode separator 4 has a plurality of air holes 4a formed in a region in close contact with the electrode body 2, and the air electrode side plate 6 has an opening 6a corresponding to the region including the air holes 4a. Yes. Therefore, oxygen in the air is supplied to the electrode body 2 through the opening 6a and the air hole 4a. The portion between the air holes 4 a is in close contact with the electrode body 2.

  In the fuel electrode separator 5, a plurality of slits 5 a are formed in a region in close contact with the electrode body 3 along the vertical direction, and the slits 5 a are arranged at equal intervals in the horizontal direction. Therefore, the portion between the slits 5 a is in close contact with the electrode body 3. The fuel electrode side plate 7 is hollowed out in a container shape corresponding to a region including the slit 5a to form a storage portion 7a. A supply hole 7b penetrating to the outside is formed in the upper portion of the storage portion 7a. Has been.

  The air electrode separator 4 and the fuel electrode separator 5 are formed of a conductive material that does not transmit liquid. Examples of the conductive material include carbon black, metal, expanded graphite, conductive ceramic, and conductive polymer. Since the air electrode separator 4 and the fuel electrode separator 5 are made of a conductive material, the air electrode separator 4 is set in a conductive state in contact with the electrode body 2, and the fuel electrode separator 5 is connected to the electrode body 3. The contact state is set to the conductive state.

  The air electrode side plate 6 and the fuel electrode side plate 7 may be formed of an insulating synthetic resin material or glass that is durable against alcohol used as a liquid fuel. Since the air electrode side plate 6 and the fuel electrode side plate 7 are made of an insulating material, current does not leak even if they come into contact with the air electrode separator 4 and the fuel electrode separator 5, respectively.

  The liquid fuel used in the fuel cell is mainly composed of alcohol, for example, one mainly composed of methanol, ethanol, isopropyl alcohol, or diethylene glycol. A liquid fuel obtained by mixing a plurality of these alcohols is used. It can also be used.

The liquid fuel is supplied and filled into the reservoir 7a from the supply hole 7b. The filled liquid fuel is brought into contact with the electrode body 3 of the membrane electrode assembly A through the slit 5 a of the fuel electrode separator 5. In this case, the electrode body 3 becomes a fuel electrode, and the electrode body 2 becomes an air electrode. When the liquid fuel is methanol, the following reaction occurs on the fuel electrode side.
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻
Protons generated by the reaction on the fuel electrode side pass through the membrane electrode assembly A and reach the air electrode side. Further, if the air electrode separator 4 and the fuel electrode separator 5 are electrically connected, the electrode bodies 2 and 3 are set in a conductive state via the air electrode separator 4 and the fuel electrode separator 5, and the reaction on the fuel electrode side is performed. The electrons generated in this step reach the electrode body 2 on the air electrode side from the electrode body 3 on the fuel electrode side via the fuel electrode separator 5 and the air electrode separator 4. Then, oxygen in the air is supplied on the air electrode side and the following reaction occurs.
3/2 · O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O
Therefore, on the air electrode side, protons that have reached the air electrode side are oxidized and water is generated, and an electromotive force is generated between the electrode bodies 2 and 3.

When the liquid fuel is isopropyl alcohol, the following reaction occurs on the fuel electrode side.
(CH ₃ ) ₂ CHOH + 3H ₂ O → 3CO ₂ + 14H ⁺ + 14e ⁻
Further, the following reaction occurs on the air electrode side.
7/2 · O ₂ + 14H ⁺ + 14e ⁻ → 7H ₂ O
Therefore, as in the case of methanol, water is generated on the air electrode side, and an electromotive force is generated between the electrode bodies 2 and 3.

  When the water generated on the air electrode side adheres to the electrode body in the form of water droplets, the supply of oxygen is restricted and the output is reduced. Therefore, it is necessary to quickly discharge the water. Therefore, it is preferable to provide paper made of bamboo fibers in the air electrode separator 4. Paper made of bamboo fibers can absorb water efficiently and can be quickly discharged because the bamboo fibers are arranged substantially along one direction.

<Preparation of electrolyte membrane>
A warp prepared by cutting linden to a thickness of 0.3 mm was cut into a size of 30 mm × 40 mm, degreased with acetone, and the elution was removed with warm water to obtain a substrate. The prepared substrate was sufficiently impregnated by immersing it in a solution (concentration: 19% by weight) in which zirconyl nitrate was dissolved in dilute nitric acid. The substrate is taken out of the solution, the excess liquid is wiped off, and the substrate is immersed in a solution (concentration: 20% by weight) in which phosphoric acid is dissolved in water. Then, the solution is heated and reacted at 100 ° C. for 1 hour. After the reaction, the substrate is taken out from the solution and washed with water. When the pH of the washed solution reaches 5-6, the reaction is terminated and stored in water. The prepared electrolyte membrane is used by simply wiping water when assembling the fuel cell.

<Fabrication of fuel cell>
A fuel cell assembly kit (PEM-004DM) manufactured by Chemix Co., Ltd. was used as the fuel cell. Two electrode bodies (size: 2 cm x 2 cm) coated with a water-repellent carbon paper (carbon fiber nonwoven fabric) used in the assembly kit and coated with a platinum-ruthenium catalyst (size: 2 cm x 2 cm) were prepared from both sides of the prepared electrolyte membrane. The body was arranged so as to be sandwiched, and held in pressure contact with the assembly kit as shown in FIG. As shown in FIG. 1, a 20Ω fixed resistor R1 and a 0.2Ω shunt resistor R2 are connected in series between the fuel electrode separator and the air electrode separator of the fuel cell, and a voltage V1 of the fixed resistor R1 is obtained by a voltmeter 10. The voltage V2 of the shunt resistor R2 was measured with the voltmeter 11.

The voltage V and current I between both separators are obtained as follows based on the measured voltages V1 and V2. V = V1
I = V2 / 0.2
Therefore, the electromotive force W between both separators is obtained as follows based on the measured voltages V1 and V2.
W = V · I = V1 · V2 / 0.2

  In addition, as the air electrode separator, a bamboo paper manufactured by Bamboo Paper Studio-style bamboo paper cut into strips was arranged to absorb water generated by the reaction.

<Measurement of electromotive force>
(1) In the case of methanol fuel The liquid fuel prepared so that the concentration of methanol was 25% by weight was supplied to distilled water, and the electromotive force was measured. The measurement results were a voltage of 200 mV, a current of 10 mA, and an electromotive force of 2 mW. The power generation state could be maintained for 1000 days.

  When the membrane electrode assembly according to the present invention was used, even when a high-concentration methanol liquid fuel was supplied, it was possible to generate power normally without causing a crossover phenomenon.

(2) In the case of ethanol fuel The liquid electromotive force prepared by supplying ethanol to distilled water to a concentration of 10% by weight was measured. As a result of the measurement, the voltage was 51 mV, the current was 2 mA, and the electromotive force was 0.102 mW.

  In the case of high-concentration ethanol, an odor was generated during the reaction, but it was confirmed that power generation was successful without causing a crossover phenomenon.

(3) In the case of ethylene glycol fuel The liquid fuel prepared so that the density | concentration of ethylene glycol might be 10 weight% was supplied to distilled water, and the electromotive force was measured. The measurement results were a voltage of 55 mV, a current of 2.5 mA, and an electromotive force of 0.138 mW. And the power generation state could be maintained for 600 days or more. When the temperature was raised to 50 ° C. during the measurement test, the electromotive force could be raised to 2 to 3 mW. In the case of liquid fuel having an ethylene glycol concentration of 3% by weight, the voltage was 86 mV, the current was 3.5 mA, and the electromotive force was 0.301 mW.

  In the case of ethylene glycol, the electromotive force was low due to the high viscosity at low temperatures, but the electromotive force could be improved as the temperature increased.

(4) In the case of isopropyl alcohol fuel The liquid fuel prepared so that the concentration of isopropyl alcohol was 30% by weight was supplied to distilled water, and the electromotive force was measured. The measurement result was a current of 10 mA at a voltage of 208 mV, and an electromotive force of 2.08 mW. In the case of liquid fuel having an isopropyl alcohol concentration of 20% by weight, the voltage was 311 mV, the current was 15.5 mA, and the electromotive force was 4.82 mW. In the case of liquid fuel having an isopropyl alcohol concentration of 10% by weight, the voltage was 245 mV, the current was 12 mA, and the electromotive force was 2.94 mW.

  In the case of isopropyl alcohol, an electromotive force comparable to that of methanol was obtained without causing a crossover phenomenon. Although it is easily affected by the viscosity at low temperatures, as the temperature increases, the viscosity decreases and the reaction proceeds.

(5) In the case of a mixed fuel Next, in the case of a liquid fuel in which an equal amount of liquid fuel of 10% by weight of isopropyl alcohol and liquid fuel of 10% by weight of methanol are mixed in equal amounts, a current of 5.5 mA is obtained at a voltage of 114 mV. Was 0.627 mW. In the case of a liquid fuel in which an equal amount of liquid fuel of 10% by weight of isopropyl alcohol and liquid fuel of 10% by weight of ethylene glycol were mixed in equal amounts, the current was 1.5 mA at a voltage of 49 mV, and the electromotive force was 0.073 mW.

  It can be seen that when mixed fuel is used, a higher electromotive force can be obtained in the case of liquid fuel containing only isopropyl alcohol, although power is generated without causing a crossover phenomenon.

[Comparative example]
Using an assembly kit similar to the example, a carbon fiber non-woven fabric carrying a platinum-ruthenium catalyst on the fuel electrode side and a platinum catalyst on the air electrode side against Nafion (registered trademark) manufactured by DuPont, which is an electrolyte membrane. A membrane electrode assembly that was sandwiched and bonded from both sides by a supported carbon fiber non-woven fabric was prepared by pressing. The liquid fuel prepared so that the methanol concentration was 3% by weight was supplied to distilled water, and the electromotive force was measured. The measurement result was a voltage of 450 mV, a current of 4 mA, and an electromotive force of 1.8 mW.

  From the above measurement results, the fuel cell using the membrane electrode assembly according to the present invention can generate power stably using a wide variety of high-concentration alcohol fuel without causing a crossover phenomenon. In the case of isopropyl alcohol fuel, a higher electromotive force can be obtained than conventional fuel cells, and isopropyl alcohol, which is widely used for disinfection instead of toxic methanol, is a liquid fuel. Therefore, handling becomes simple and a highly practical fuel cell can be obtained.

A Membrane electrode assembly 1 Electrolyte membrane 2 Electrode body 3 carrying a catalyst layer 3 Electrode body carrying a catalyst layer 4 Air electrode separator 5 Fuel electrode separator 6 Air electrode side plate 7 Fuel electrode side plate

Claims (6)

  An electrolyte membrane in which an inorganic ion exchanger is formed on the inside and surface of a substrate formed of a thin plate having a thickness of 10 mm or less, and a platinum-ruthenium catalyst layer on the electrode substrate, and bonded to both sides of the electrolyte membrane. A membrane electrode assembly comprising a pair of supported electrode bodies.   The inorganic ion exchanger is zirconyl phosphate produced by treating a hydrochloric acid solution of zirconium oxychloride impregnated in the substrate or a nitric acid solution of zirconium oxynitrate with a phosphoric acid solution. 2. The membrane electrode assembly according to 1.   The membrane electrode assembly according to claim 1 or 2, wherein the electrode base material is a nonwoven fabric made of carbon fiber.   A membrane electrode assembly according to any one of claims 1 to 3, wherein a liquid fuel mainly composed of alcohol is directly supplied to the electrode assembly on the fuel electrode side of the membrane electrode assembly. Type alcohol fuel cell.   5. The direct alcohol fuel cell according to claim 4, wherein the liquid fuel is mainly composed of any one of methanol, ethanol, isopropyl alcohol, and ethylene glycol.   6. The direct alcohol fuel cell according to claim 4, wherein the electrode body on the air electrode side is provided with paper made of bamboo fiber that absorbs water generated by the reaction.

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