JP2006004695A - Membrane-electrode assembly of direct fuel cell, system and how to use the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide improvement of utilization factor by suppressing fuel transmission in a direct type fuel cell system. <P>SOLUTION: A membrane-electrode assembly 10 for a direct methanol fuel cell comprises an electrolyte layer 11, a cathode 13 joined one side of the electrolyte layer 11 to which air is supplied, and an anode 12 joined the other side of the electrolyte layer 11 by supporting a catalyst to which a methanol solution is supplied. The anode 12 is composed of a first catalyst layer 12a where the catalyst is supported, a magnetic layer 12b adjacent to the first catalyst layer 12a at the side of the electrolyte layer 11 where a magnetic body with an magnetic action is carried, and a second catalyst layer 12c adjacent to the side of the electrolyte layer 11 of the magnetic layer 12b. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a membrane electrode assembly, a system, and a method of using the system of a direct fuel cell.

  Conventionally, Patent Document 1 discloses a direct methanol fuel cell (DMFC) system, which is a type of direct fuel cell. This DMFC system includes a DMFC membrane electrode assembly (MEA). This membrane electrode assembly has an electrolyte layer, a cathode electrode joined to one surface of the electrolyte layer and supplied with air, and an anode electrode joined to the other surface of the electrolyte layer and supplied with an aqueous methanol solution. . A catalyst such as platinum (Pt) or platinum-ruthenium (Pt-Ru) having a catalytic action is supported on the cathode and anode.

  The membrane electrode assembly is sandwiched between separators (not shown) to form a fuel cell, which is the minimum power generation unit, and a number of these cells are stacked to form a fuel cell stack. Methanol is supplied to the anode electrode together with water by a methanol aqueous solution supply means, and air is supplied to the cathode electrode by air supply means. Thus, the DMFC system is configured.

  In this membrane / electrode assembly, when the catalyst supported on the anode electrode is Pt, Pt generates hydrogen ions from methanol by the reactions shown in the following chemical formulas 1 to 5.

(Chemical formula 1)
Pt + CH ₃ OH → Pt—CH ₂ OH + H ⁺ + e ⁻

(Chemical formula 2)
Pt—CH ₂ OH → Pt—CHOH + H ⁺ + e ⁻

(Chemical formula 3)
Pt—CHOH → Pt—COH + H ⁺ + e ⁻

(Chemical formula 4)
Pt—COH → Pt—CO + H ⁺ + e ⁻

(Chemical formula 5)
Pt—CO → Pt + CO ₂ + 2H ⁺ + 2e ⁻

  Thus, hydrogen ions, electrons and carbon dioxide are generated from the fuel methanol by the reaction shown in Chemical Formula 6 at the anode electrode.

(Chemical formula 6)
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻

The hydrogen ions move in the form of protons (H ₃ O ⁺ ) in the electrolyte layer toward the cathode electrode. Further, the electrons flow through the load connected to the DMFC system and flow to the cathode electrode.

  On the other hand, at the cathode electrode, water is generated from oxygen, hydrogen ions and electrons contained in the air by the reaction shown in Chemical formula 7.

(Chemical formula 7)
3 / 2O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O

  The DMFC system can continuously generate an electromotive force by continuously performing such a reaction. In addition, this DMFC system is a polymer electrolyte fuel cell (PEFC) system that uses hydrogen as a fuel, and methanol is used as fuel to reform methanol in advance and convert it to hydrogen to PEFC. Unlike the system of the reforming PEFC to be supplied, the fuel handling is easy, and there is an advantage that it is possible to reduce the overall size and weight by eliminating the reformer.

JP 2003-346836 A

  However, in the conventional DMFC system, methanol that has not reacted on the catalyst on the anode electrode passes through the electrolyte layer to reach the cathode electrode (crossover phenomenon), and the methanol is thrown into the air. There is a problem that the amount increases.

  The present invention has been made in view of the above-described conventional situation, and in the direct fuel cell system, it is an object to be solved to realize improvement in fuel utilization by suppressing fuel permeation.

The membrane electrode assembly for a direct fuel cell according to the present invention includes an electrolyte layer, a cathode electrode joined to one surface of the electrolyte layer and supplied with air, a catalyst supported and joined to the other surface of the electrolyte layer. In a membrane electrode assembly for a direct fuel cell having an anode electrode supplied with an aqueous solution,
A magnetic body having a magnetic action is supported on the anode pole.

  According to the knowledge of the inventors, the effects of the membrane electrode assembly of the present invention are as follows. That is, in this membrane electrode assembly, the unreacted fuel receives a large rejection force as a diamagnetic material by the magnetic action of the magnetic material supported on the anode electrode. For this reason, the unreacted fuel causes the reactions shown in the above chemical formulas 1 to 5 more reliably by the anode electrode catalyst, and is reliably decomposed by hydrogen ions, electrons, and the like. In other words, the crossover phenomenon of unreacted fuel is suppressed.

  Therefore, according to the membrane electrode assembly of the present invention, in the direct fuel cell system, it is possible to improve the fuel utilization rate by suppressing the permeation of fuel.

  The direct fuel cell according to the present invention can be a direct alcohol fuel cell using dimethyl ether (DME) as a fuel, in addition to a direct alcohol fuel cell using an alcohol such as methanol, ethanol, propanol or the like as a fuel.

  The membrane electrode assembly of the present invention has an electrolyte layer, a cathode electrode, and an anode electrode. As the electrolyte, an ion exchange resin such as Nafion (registered trademark) can be employed. The cathode electrode can be composed of a conductive base material such as carbon cloth, carbon paper, carbon felt, etc., and at least a catalyst and an electrolyte fixed to the base material. The substrate can have water repellency and gas permeability. In order to have water repellency, a water repellent material can be applied to a substrate such as carbon cloth. The anode electrode can be composed of a substrate and at least a catalyst and an electrolyte fixed to the substrate.

  The magnetic body has a magnetic action. As this magnetic body, platinum-iron (Pt-Fe), platinum-cobalt (Pt-Co), platinum-iron-rhodium (Pt-Fe-Lo) is preferable, and further, neodymium having a coated surface. Permanent magnets such as iron-boron magnets, samarium-cobalt magnets, and ferrite magnets can be employed. Further, the magnetic body may be a magnetic carrier for electrodes made of a magnetic body, and the permanent magnet described above is supported on the outer peripheral surface after the magnetic carrier for electrodes is coated with a conductive magnetic body. Also good. As the magnetic carrier for electrodes, a single metal or a multi-component alloy of binary or higher such as iron, cobalt, nickel, iron-cobalt alloy, iron-nickel alloy, alnico, etc. can be employed. Moreover, carbon etc. can be employ | adopted as an electroconductive magnetic body.

  A membrane electrode assembly in which a magnetic body is supported on the anode pole can be manufactured as follows. First, a conductive magnetic body, a magnetic body, etc. are prepared. Then, a conductive magnetic body, a magnetic body, and an electrolyte solution are mixed to produce a paste, and this paste is applied to a substrate and then dried to form an anode electrode. Further, after applying a paste in which at least the catalyst and the electrolyte solution are mixed to the base material, the paste is dried to manufacture the cathode electrode. The obtained cathode electrode, anode electrode and electrolyte layer are joined. In this way, a membrane electrode assembly is obtained.

  In addition, as described above, after the anode electrode is configured, the cathode electrode, the anode electrode, and the electrolyte layer are bonded to obtain a membrane electrode assembly. By placing this membrane electrode assembly in a magnetic field, the magnetic body can be magnetized to obtain a membrane electrode assembly.

  In the membrane electrode assembly of the present invention, the anode electrode preferably has a catalyst layer on which a catalyst is supported, and a magnetic layer on which the catalyst layer and the electrolyte layer are adjacent and on which a magnetic body is supported. In this case, the unreacted fuel is returned to the catalyst layer as a diamagnetic material due to the magnetic action of the magnetic material carried on the magnetic layer, as a diamagnetic material. For this reason, the unreacted fuel causes the reactions shown in the above chemical formulas 1 to 5 more reliably by the catalyst in the catalyst layer, and is reliably decomposed by hydrogen ions, electrons, and the like. In this case, the anode electrode may have a two-layer structure of a catalyst layer and a magnetic layer.

  In the membrane electrode assembly of the present invention, when the anode has a catalyst layer and a magnetic layer, it is more preferable that the anode has a second catalyst layer adjacent to the electrolyte layer side of the magnetic layer. In this case, even if the unreacted fuel passes through the magnetic layer without returning to the first catalyst layer on the non-electrolyte layer side, the unreacted fuel is converted into the above-mentioned chemical formula by the second catalyst layer on the electrolyte layer side. The reaction shown in .about.5 occurs more reliably and is reliably decomposed by hydrogen ions, electrons and the like. In this case, the anode electrode may have a three-layer structure of the first catalyst layer, the magnetic layer, and the second catalyst layer, and 4 of the first catalyst layer, the first magnetic layer, the second catalyst layer, and the second magnetic layer. The layer structure may be a five-layer structure including a first catalyst layer, a first magnetic layer, a second catalyst layer, a second magnetic layer, and a third catalyst layer.

  The magnetic body can also be configured to exert a strong magnetic force on the electrolyte layer side. In this case, the unreacted fuel is less likely to approach the electrolyte side, and the effects of the present invention are more likely to occur.

  By assembling the membrane electrode assembly of the present invention together with the aqueous fuel solution supplying means, the air supplying means, etc., the direct fuel cell system of the present invention is obtained.

  That is, the direct fuel cell system of the present invention includes the membrane electrode assembly, a fuel aqueous solution supply means for supplying a fuel aqueous solution to the anode electrode, and an air supply means for supplying air to the cathode electrode. It is characterized by.

  The aqueous fuel solution supply means supplies fuel to the anode electrode together with water. The fuel aqueous solution supply means may be a fuel cartridge, a fuel aqueous solution chamber of the separator, or the like. The air supply means supplies air to the cathode electrode. A blower, an air chamber of a separator, or the like can be used as an air supply means.

In addition, the method of using the direct fuel cell system of the present invention includes an electrolyte layer, a cathode electrode joined to one surface of the electrolyte layer and supplied with air, and an aqueous fuel solution joined to the other surface of the electrolyte layer. A membrane electrode assembly having an anode electrode,
A fuel aqueous solution supply means for supplying the fuel aqueous solution to the anode electrode;
A method of using a direct fuel cell system comprising air supply means for supplying air to the cathode electrode,
By carrying a magnetic body having a magnetic action on the anode pole, the magnetic body prevents diamagnetic fuel from moving to the electrolyte layer.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings. In the examples, as a direct alcohol fuel cell, a direct methanol fuel cell (DMFC) system using methanol as alcohol as fuel was formed.

  In this DMFC system, a plurality of cells 1 shown in FIG. 1 are used. Each cell 1 includes a membrane electrode assembly (MEA) 10 and a pair of separators 20.

  The membrane electrode assembly 10 includes an electrolyte layer 11 made of an ion exchange membrane of Nafion 117, an anode electrode 12 integrally bonded to one surface of the electrolyte layer 11, and a cathode integrally bonded to the other surface of the electrolyte layer 11. And poles 13.

  Each separator 20 has a methanol aqueous solution chamber 21 for supplying a methanol aqueous solution to the anode electrode 12 on one side, and an air chamber 22 for supplying air to the cathode 13 on the other side. is there.

  Each cell 1 is formed by laminating a membrane electrode assembly 10 and a pair of separators 20 such that a methanol aqueous solution chamber 21 faces the anode electrode 12 side and an air chamber 22 faces the cathode electrode 13 side. A stack is configured by sequentially laminating the membrane electrode assembly 10 and the separator 20. Further, the separator 20 common to the anode electrode 12 side and the cathode electrode 13 side is employed. Note that only the methanol aqueous solution chamber 21 or the air chamber 22 is formed in the separators 20 at both ends of the stack.

  Connected to the stack are a methanol cartridge 2 that communicates with a methanol aqueous solution chamber 21 of each cell 1 via a valve (not shown), and a blower 3 that communicates with an air chamber 22 of each cell 1. The methanol aqueous solution chamber 21 of the methanol cartridge 2 and the separator 20 is a methanol aqueous solution supply means for supplying methanol to the anode electrode 12. The air chamber 22 of the blower 3 and the separator 20 is air supply means for supplying air to the cathode electrode 13.

  As shown in FIG. 2, the anode electrode 12 includes a first catalyst layer 12a on which a catalyst made of a Pt—Ru alloy is supported, a first catalyst layer 12a, and an electrolyte layer. 11 includes a magnetic layer 12b that is adjacent to the magnetic layer 11 and carries a magnetic body, and a second catalyst layer 12c that is further adjacent to the electrolyte layer 11 side of the magnetic layer 12b.

  The magnetic body is a Pt—Fe alloy having a main phase of an fct structure in which a Pt—Fe alloy having an average particle diameter of 50 nm and Pt / Fe = 50/50 (at%) is heat-treated and rapidly cooled with water.

(Evaluation 1)
In this DMFC system, the magnetic force at room temperature near the anode 12 is simulated. The magnetic force is calculated by F = X / 2μ ₀ × ∇ (B) ² . μ ₀ is 4π × 10 ⁻⁷ (H / m), and X is a volume magnetic susceptibility (methanol is −0.528 × 10 ⁻⁶ ). Therefore, X / 2μ _{0 of} methanol is −2.10. The results are shown in FIG.

From FIG. 3, it can be seen that a magnetic force for removing methanol from the magnetic layer 12b acts on the anode pole 12 of the membrane electrode assembly 10. The repulsive force that methanol, which is a diamagnetic material, receives from the magnetic layer 12b is 10 times greater than 10 times the approximately 8000 N that 1 m ^{3 of} methanol receives from gravity even at the weakest position between the magnetic material and the magnetic material. ⁵ N / m ³ of magnetic repulsive force, which prevents methanol from entering the electrolyte layer 11.

That is, in this membrane electrode assembly 10, as shown in FIG. 2, methanol (MeOH) surely causes the reactions shown in Chemical Formulas 1 to 5 above by the catalyst of the first catalyst layer 12 a, and carbon dioxide, This produces hydrogen ions and electrons. However, unreacted methanol may remain in the first catalyst layer 12a. This unreacted methanol receives a large displacement force as a diamagnetic material by the magnetic action of the magnetic material carried on the magnetic layer 12b, and is returned to the first catalyst layer 12a. For this reason, unreacted methanol is reliably decomposed by carbon dioxide, hydrogen ions and electrons by the catalyst of the first catalyst layer 12a. Even if the unreacted methanol passes through the magnetic layer 12b without returning to the first catalyst layer 12a, the unreacted methanol is surely separated into carbon dioxide, hydrogen ions and electrons by the catalyst of the second catalyst layer 12c. Is broken down into
Thus, in this membrane electrode assembly 10, the crossover phenomenon of unreacted methanol is suppressed.

  Therefore, according to this membrane electrode assembly 10, in the DMFC system, it is possible to improve the utilization rate of methanol by suppressing the permeation of methanol.

(Evaluation 2)
The membrane electrode assembly 10 of the example in which the magnetic layer 12b is formed on the anode 12 and the membrane electrode assembly 10 of the comparative example in which the magnetic layer 12b is not formed on the anode 12 are prepared. Other configurations of the membrane electrode assemblies 10 of these examples and comparative examples are the same as those of the membrane electrode assembly 10 described above.

About the membrane electrode assembly 10 of an Example and a comparative example, the relationship between the density | concentration (Mol) of methanol in room temperature and the permeation | transmission rate (mMol / cm < ² > h) of methanol was calculated | required. The results when the current density is 0 mA / cm ² are shown in FIG.

  FIG. 4 shows that the membrane electrode assembly 10 of the example suppresses the permeation amount of methanol by the magnetic force even when the concentration of methanol is higher than that of the membrane electrode assembly of the comparative example.

(Evaluation 3)
About the membrane electrode assembly 10 of an Example and a comparative example, the relationship between the density | concentration (Mol) of methanol in room temperature and a fuel utilization factor (%) was calculated | required. The current density is 100 mA / cm ² . The results are shown in FIG.

  FIG. 5 shows that the membrane electrode assembly 10 of the example is superior in fuel utilization rate to the membrane electrode assembly 10 of the comparative example.

  While the present invention has been described with reference to the embodiments, it is needless to say that the present invention is not limited to the above-described embodiments and can be appropriately modified and applied without departing from the spirit thereof.

  The present invention can be used for a mobile power source for an electric vehicle or the like, or a stationary power source or a portable power source.

It is a DMFC system of an example, and is a principal section schematic cross section of a cell. It is a principal part expansion schematic diagram of the membrane electrode assembly of an Example. It is a graph which shows the magnetic force of an anode pole vicinity. It is a graph which shows the relationship between the density | concentration of methanol and the permeation | transmission rate of methanol in the membrane electrode assembly of an Example and a comparative example. It is a graph which shows the relationship between the density | concentration of methanol and a fuel utilization factor in the membrane electrode assembly of an Example and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Electrolyte layer 13 ... Cathode electrode 12 ... Anode electrode 10 ... Membrane electrode assembly 12a ... 1st catalyst layer 12b ... Magnetic layer 12c ... 2nd catalyst layer 2, 21 ... Methanol aqueous solution supply means (2 ... Methanol cartridge, 21 ... (Methanol aqueous solution chamber)
3, 22 ... Air supply means (3 ... Blower, 21 ... Air chamber)

Claims (5)

A direct fuel cell having an electrolyte layer, a cathode electrode joined to one surface of the electrolyte layer and supplied with air, and an anode electrode carrying a catalyst and joined to the other surface of the electrolyte layer and supplied with an aqueous fuel solution In membrane electrode assemblies for
A membrane electrode assembly for a direct fuel cell, wherein a magnetic body having a magnetic action is carried on the anode electrode.   2. The direct electrode according to claim 1, wherein the anode electrode has a catalyst layer on which the catalyst is supported, and a magnetic layer that is adjacent to the catalyst layer on the electrolyte layer side and on which the magnetic body is supported. Type membrane electrode assembly for fuel cell.   3. The membrane electrode assembly for a direct fuel cell according to claim 2, wherein the anode electrode has the second catalyst layer adjacent to the electrolyte layer side of the magnetic layer.   The membrane electrode assembly according to any one of claims 1 to 3, a fuel aqueous solution supply means for supplying the fuel aqueous solution to the anode electrode, and an air supply means for supplying air to the cathode electrode. A direct fuel cell system characterized by A membrane electrode assembly having an electrolyte layer, a cathode electrode joined to one surface of the electrolyte layer and supplied with air, and an anode electrode joined to the other surface of the electrolyte layer and supplied with an aqueous fuel solution;
A fuel aqueous solution supply means for supplying the fuel aqueous solution to the anode electrode;
A method of using a direct fuel cell system comprising air supply means for supplying air to the cathode electrode,
A method of using a direct fuel cell system, wherein a magnetic body having a magnetic action is supported on the anode pole to prevent diamagnetic fuel from moving to the electrolyte layer.

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