JP2007179859A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light compact fuel cell improved in output properties. <P>SOLUTION: The fuel cell comprises a fuel electrode, an air electrode and an electrolyte placed between the fuel electrode and the air electrode. The electrolyte has a feature of including a rigid substrate 1 with through-holes 2 running through in the thickness direction, electrolytes 3 held in at least part of the through-holes, catalysts 4 having methanol decomposition activity held in at least part of through-holes. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell suitable as a small fuel cell.

  In recent years, advances in electronic technology have led to downsizing, higher performance, and portability of electronic devices. In portable electronic devices, there is an increasing demand for higher energy density of batteries used. For this reason, there is a demand for a secondary battery having a high capacity while being lightweight and small.

  In response to the demand for such secondary batteries, for example, lithium ion secondary batteries have been developed. In addition, the operation time of portable electronic devices tends to increase further, and in lithium ion secondary batteries, the improvement in energy density is almost limited from the viewpoints of materials and structure, which is further demanded. It is becoming impossible to respond.

  Under such circumstances, small fuel cells are attracting attention instead of lithium ion secondary batteries. In particular, direct methanol fuel cells (DMFC) using methanol as fuel require more difficult handling of hydrogen gas and devices that produce hydrogen by reforming organic fuel, compared to fuel cells using hydrogen gas. It is thought that it is excellent in miniaturization.

  In DMFC, methanol is oxidatively decomposed at the fuel electrode to generate carbon dioxide, protons and electrons. On the other hand, in the air electrode, water is generated by oxygen obtained from air, protons supplied from the fuel electrode through the electrolyte membrane, and electrons supplied from the fuel electrode through an external circuit. In addition, power is supplied by electrons passing through the external circuit.

However, in the fuel cell having such a configuration, methanol permeates from the fuel electrode to the air electrode through the electrolyte membrane, and as a result, the power generation potential is lowered. In order to solve this phenomenon (methanol crossover), an attempt has been made to suppress a phenomenon in which an electrolyte is filled in a porous body and the electrolyte swells to promote methanol crossover (for example, Patent Document 1, Non-Patent Document 1). Patent Document 1).
JP 2002-83612 A Dongguan Synthetic Research Annual Report TREND 2004, pp. 34-36 Development of Fuel Cell Electrolyte Membrane by Pore Filling Polymerization New Product Development Laboratory Hideki Hiraoka Kozo Kubota

  The electrolyte membrane structures described in Patent Document 1 and Non-Patent Document 1 described above seem to have an effect of reducing methanol crossover, but basically they do not treat permeated methanol, so they have passed. Methanol is thought to cause a reaction and reduce the electromotive potential. Therefore, it is effective to dispose the catalyst in the pores in order to decompose the passing methanol in the membrane. For this reason, a catalyst layer is inserted between electrolyte membranes, or a mixture of an electrolyte and a catalyst is formed into a membrane shape. However, according to these methods, since the electrolyte membrane is deformed by carbon dioxide generated when the catalyst decomposes methanol, the contact characteristics with the electrodes are caused, and the output characteristics of the fuel cell are deteriorated.

  Accordingly, in view of this point, the present inventors provide a through-hole penetrating in the thickness direction in a rigid base material, and holds the electrolyte in at least a part of the through-holes, or the through-hole in which the electrolyte is held or By holding a catalyst having a methanol decomposing action in a different through hole, the gas generated in the methanol decomposing reaction can be quickly diffused from the surface of the electrolyte membrane, and at the same time, the methanol passes through the electrolyte membrane to the air electrode. This suppresses the transition, minimizes the formation of the mixed potential at the air electrode, and improves the output characteristics. In addition, since the position of the hole can be designed with high accuracy by using a rigid substrate, the catalyst can be disposed at a desired position of the electrolyte membrane. For the substrate having rigidity, a Si substrate porous body manufactured by MEMS (micro electro mechanical system) technology is preferable.

  The present invention seeks to provide a fuel cell with improved output characteristics.

The fuel cell according to the present invention is a fuel cell comprising a fuel electrode, an air electrode, and an electrolyte membrane disposed between the fuel electrode and the air electrode.
The electrolyte membrane includes a rigid substrate having a through-hole penetrating in the thickness direction, an electrolyte held in at least a part of the through-hole, and methanol decomposition held in at least a part of the through-hole. And a catalyst having an action.

  According to this fuel cell, methanol diffused from the fuel electrode to the electrolyte membrane can be decomposed in the electrolyte membrane, so that the amount of methanol reaching the air electrode can be significantly reduced. At the same time, carbon dioxide generated by the methanol decomposition reaction can be released quickly. In addition, it is possible to specify the location where the catalyst exists, so it is possible to select the location of the catalyst in the electrolyte membrane according to the electrode arrangement, and it is possible to effectively suppress methanol crossover, resulting in excellent output. A fuel cell can be provided.

  According to the present invention, it is possible to provide a fuel cell with improved output characteristics.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 schematically shows a perspective view of an electrolyte membrane for a direct methanol fuel cell according to an embodiment of the present invention. The porous film 1 is a porous substrate having a number of precision-processed through holes, and desirably has rigidity. The through hole penetrates in the thickness direction t of the porous membrane 1. The porous film 1 can be obtained, for example, by precisely designing the location of holes in a metal silicon substrate by lithography and then forming precise and fine through holes by etching. The diameter of the hole is in the range of 0.1 μm to 1000 μm, preferably 1 μm to 100 μm, and more preferably 2 μm to 10 μm. As the substrate material, any metal, ceramic, glass, or resin can be used as long as it is a rigid material that can be precisely processed. As the metal material, for example, silicon, stainless steel, titanium and the like can be considered, and aluminum may be used as long as the inserted electrolyte is glass. However, when using a metal, it is desirable to perform a surface treatment such as an oxidation treatment in order to suppress electron conduction on the surface. Examples of the ceramic include oxide ceramics such as alumina, silica, and zirconia, nitride ceramics such as silicon nitride, and carbide ceramics such as silicon carbide. Generally speaking, ceramics are polycrystalline, but single crystals may also be used. For example, a sapphire substrate can be considered. In addition, when glass is used, its fracture toughness becomes a problem, so the size may be limited. For example, use of quartz glass or the like is conceivable. As the resin material, a highly rigid engineering plastic material is desirable. For example, polyether ether ketone (PEEK) can be considered. However, it is not limited to these.

  The thermal conductivity of the porous substrate is desirably 1 W / mk or more. Thereby, the heat generated by the oxidation reaction at the air electrode can be quickly transmitted to the fuel electrode side to promote the fuel cell reaction, and the output characteristics can be further improved.

  As the electrolyte 3 inserted into all or some of the through holes of such a porous substrate, an organic polymer electrolyte, an inorganic glass electrolyte, or the like can be considered. In the organic polymer electrolyte, for example, a fluororesin having a sulfonic acid group, such as a perfluorosulfonic acid polymer (Nafion (registered trademark) {product name, manufactured by DuPont)}, Flemion (trade name, manufactured by Asahi Glass Co., Ltd.), etc. ), Hydrocarbon resins having sulfonic acid groups (polystyrene sulfonic acid, polyether ketone sulfonic acid, etc.) are conceivable. Examples of inorganic electrolytes include tungstic acid and phosphotungstic acid. Further, as an inorganic glass-based electrolyte, phosphate glass or the like can be considered. However, it is not limited to these.

  Further, as the methanol decomposition catalyst 4 disposed in all or some of the through holes of the porous substrate, a noble metal (for example, platinum, palladium, ruthenium, rubidium, etc.) supported on a carbon support, an oxide support (for example, A noble metal (such as platinum, palladium, ruthenium, or rubidium) supported on titanium oxide, zeolite, or aluminum oxide) or an unsupported noble metal that does not have a support (such as platinum, palladium, ruthenium, or rubidium) is conceivable. However, of course, it is not limited to these. The methanol decomposition catalyst 4 may be filled in a through hole in which the electrolyte 3 is held, or a through hole in which only the methanol decomposition catalyst 4 is filled may exist.

  An organic polymer electrolyte layer can be formed on one side or both sides of the porous membrane 1. It is desirable that at least a part of the organic polymer electrolyte layer is in contact with the electrolyte 3 or the methanol decomposition catalyst 4. Examples of the organic polymer electrolyte forming the organic polymer electrolyte layer include the same types as described in the electrolyte 3.

  FIG. 2 shows a cross-sectional view of an MEA in which a catalyst layer and a gas diffusion layer are adhered to such an electrolyte membrane. An air electrode and a fuel electrode are disposed across the electrolyte membrane 5. The fuel electrode includes a fuel electrode catalyst layer 6 stacked on one surface of the electrolyte membrane 5 and a fuel electrode gas diffusion layer 7 stacked on the fuel electrode catalyst layer 6. The fuel electrode gas diffusion layer 7 serves to uniformly supply fuel to the fuel electrode catalyst layer 6 and also serves as a current collector for the fuel electrode catalyst layer 6. On the other hand, the air electrode includes an air electrode catalyst layer 8 stacked on the opposite surface of the electrolyte membrane 5 and an air electrode gas diffusion layer 9 stacked on the air electrode catalyst layer 8. The air electrode gas diffusion layer 9 serves to uniformly supply the oxidant to the air electrode catalyst layer 8 and also serves as a current collector for the air electrode catalyst layer 8. A fuel electrode conductive layer (not shown) is laminated on the fuel electrode gas diffusion layer 7, and an air electrode conductive layer (not shown) is laminated on the air electrode gas diffusion layer 9. The fuel electrode conductive layer and the air electrode conductive layer are formed of a porous layer such as a mesh made of a conductive metal material such as gold.

  Such an MEA is installed in a fuel cell and generates electric power by supplying fuel and air. The fuel cell can be roughly divided from its form, and the fuel made of methanol aqueous solution is supplied to the fuel electrode of the MEA while adjusting the amount of the fuel with a pump so that the amount is constant, while the air is also pumped to the air electrode of the MEA. Active type fuel cell consisting of a system that supplies gas and methanol that is vaporized into the MEA by natural supply, while external air is also naturally supplied to the air electrode so that no extra equipment such as a pump is installed. Type fuel cell. The electrolyte membrane according to the present invention can be used for any of them, and the use thereof is not limited.

  Next, it will be described in the following examples that excellent output characteristics can be obtained with the fuel cell provided with the electrolyte membrane of the present invention thus provided.

Example 1
The direct methanol fuel cell shown in FIG. 3 was produced as follows.

  First, an electrolyte membrane was formed. In order to produce a precisely processed rigid porous film, the following processing was performed using a silicon substrate as a material.

A mask designed using a silicon substrate having a diameter of 6 inches and lithography of the surface of the silicon substrate so that 32 perforated plates having a 10 mm square and 5 μm diameter holes with an opening area ratio of 55% can be taken within a 20 mm square area. Was made. Using this mask, a photosensitive resist was applied to the surface of the silicon substrate by spin coating. OFPR5000 was used as a resist and applied twice to ensure a resist thickness of 7 μm. Each application was heated at 85 ° C. for 6 minutes to adhere the resist. After the second heating, side rinsing treatment was performed after the second heating, and then heating was again performed at 85 ° C. for 6 minutes in order to remove this rinsing agent. The resist-coated silicon substrate was exposed by lithography and developed to obtain a silicon substrate having a pattern transferred as designed. This silicon substrate was dry etched by the Bosch etching method using SF ₆ and C ₄ F ₈ gases as decomposition gases. Silicon was etched with SF ₆ , and polymer treatment was performed on the side surfaces of the etching holes with C ₄ F ₈ . The etching rate was about 2 μm per minute, and etching holes were formed to a depth of 100 μm or more over 1 hour. The obtained silicon substrate was subjected to ashing treatment to remove residual resist on the surface, and further polished from the back surface by CMP (Chemical Mechanical Polishing) to obtain a porous body having a thickness of 50 μm. This porous membrane was cut along a 20 mm region to obtain a porous body for an electrolyte membrane.

  Thereafter, the porous body was heat-treated in air at 800 ° C. to form an oxide film on the surface as an electrical insulating layer.

  Next, this porous body was immersed in a DE2020: Nafion (registered trademark) solution manufactured by Dupont, and the pores were impregnated to impregnate Nafion molecules. Thereafter, it was sufficiently dried at room temperature. When this process was repeated about 5 times, a 4-mm square area in the center of the substrate was screened with a slurry in which catalyst particles made of Tanaka Kikinzoku (TEC10E70TPM) with 70% platinum supported on carbon were dispersed in a NAFION solution. Printing was performed, and catalyst particles were filled in the holes in the center. Then, after fully drying at room temperature, it was further submerged in Dupont's DE2020: Nafion solution and decompressed, and Nafion molecules were impregnated in the pores and taken out, and sufficiently dried at room temperature. This was repeated twice.

  In this way, an electrolyte membrane 5 was obtained in which the catalyst was inserted into the holes in the center of the porous body, and all the holes of the porous body including these holes were filled with Nafion molecules.

  On the other hand, platinum-supported graphite particles were mixed with DE2020 manufactured by Dupont using a homogenizer to prepare a slurry, which was applied to carbon paper which is an air electrode gas diffusion layer. And this was dried at normal temperature and the air electrode was produced. Furthermore, carbon particles carrying platinum ruthenium alloy fine particles were mixed with DE2020 made by Wako Pure Chemical Industries, Ltd. and a homogenizer to produce a slurry, which was applied to carbon paper as a fuel electrode gas diffusion layer. And this was dried at normal temperature and the fuel electrode was produced.

  On the other hand, on the surface of the electrolyte membrane, DE2020 manufactured by Dupont was applied over the entire screen by screen printing so that the thickness before drying was about 10 μm, and then sufficiently dried at room temperature. After drying, the other side was similarly coated with DE2020 manufactured by Dupont and sufficiently dried at room temperature. In this way, a thin layer of organic electrolyte (Nafion) was formed on the surface of the electrolyte membrane, and this layer was in contact with the catalyst and electrolyte disposed in the pores.

The electrolyte membrane is interposed between the air electrode and the fuel electrode so that the screen-printed surface for placing the catalyst in the hole is in contact with the fuel electrode, the temperature is 120 ° C., and the pressure is 10 kgf / cm ^2. The membrane electrode assembly (MEA) 11 was produced by pressing under the above conditions.

  Subsequently, the membrane electrode assembly was sandwiched between gold foils having a plurality of holes for taking in air and vaporized methanol to form the air electrode conductive layer 11 and the fuel electrode conductive layer 12.

  A laminate in which the membrane electrode assembly (MEA) 11, the fuel electrode conductive layer 12, and the air electrode conductive layer 11 described above were laminated was sandwiched between two frames 13a and 13b made of resin. A rubber O-ring 14 is sandwiched between the air electrode side of the membrane electrode assembly 11 and the one frame 13a, and between the fuel electrode side of the membrane electrode assembly 11 and the other frame 13b. And sealed.

  Further, the fuel electrode side frame 13b was fixed to the liquid fuel tank 16 via a gas-liquid separation membrane 15 with screws. A 0.1 mm thick silicone sheet was used for the gas-liquid separation membrane. On the other hand, a porous plate was disposed on the air electrode side frame 13a to form a moisture retention layer 17. On this moisturizing layer 17, a cover 19 is formed by disposing a stainless steel plate (SUS304) having a thickness of 2 mm in which air inlets 18 (diameter 2.5 mm, number of ports 8) for taking in air are formed. And fixed by screwing.

  Into the liquid fuel tank 16 of the fuel cell formed as described above, 5 mL of pure methanol is injected as the liquid fuel 20, and the voltage in the open circuit state and the maximum output are obtained in an environment of a temperature of 25 ° C. and a relative humidity of 50%. The value was measured from the current value and the voltage value. Further, the maximum value of the surface temperature of the fuel cell was measured by a thermocouple attached to the surface of the cover 19.

  The measurement results are shown in Table 1 below.

(Example 2)
The configuration of the electrolyte membrane used in Example 2 is the same as that of the fuel cell of Example 1 except that the position of the hole into which the catalyst is inserted is located within a region having a width of 1 mm from the outermost periphery of the opening region of the porous membrane. Is the same. The measurement method and measurement conditions for the voltage in the open circuit state, the maximum output value, and the maximum value of the surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Example 3)
The configuration of the electrolyte membrane used in Example 3 is that the position of the hole into which the catalyst is inserted is the outermost circumference and the middle area from the center of the opening area of the porous membrane, 2 mm from the outer circumference, 2 mm from the middle, and 1 mm in width. The configuration is the same as that of the fuel cell of Example 1 except that it is located within the region. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

Example 4
The configuration of the electrolyte membrane used in Example 4 is the same as that of the fuel cell of Example 1 except that the holes into which the catalyst is inserted are all the holes of the porous membrane. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Example 5)
The electrolyte membrane used in Example 5 is different in the electrolyte filled in the porous portion. That is, instead of the Nafion solution used in Example 1, tetramethoxysilane and tetramethoxyphosphoric acid were used as main raw materials and the SiO ₂ —P ₂ O ₅ composition ratio was adjusted to 0.9: 0.1. The configuration of the fuel cell of Example 1 is the same as that of the fuel cell of Example 1 except that the sol is repeatedly filled by dipping and drying by the dipping method. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Example 6)
The electrolyte membrane used in Example 6 is different in the electrolyte filled in the porous portion. That is, the Nafion solution is impregnated in the same manner as in Example 1, but the impregnation and drying steps are repeated up to three times, and then SiO ₂ —P ₂ O ₅ using tetramethoxysilane and tetramethoxyphosphoric acid as main raw materials. The configuration of the fuel cell of Example 1 is the same as that of Example 1 except that the sol adjusted to a composition ratio of 0.9: 0.1 is filled by repeatedly dipping and drying by the dipping method. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Comparative Example 1)
The configuration of the electrolyte membrane used in Comparative Example 1 is the same as that of the fuel cell of Example 1 except that no catalyst is inserted into the holes. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Comparative Example 2)
The configuration of the electrolyte membrane used in Comparative Example 2 is the same as that of the fuel cell of Example 5 except that no catalyst is inserted into the holes. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Comparative Example 3)
The configuration of the electrolyte membrane used in Comparative Example 3 is the same as that of the fuel cell of Example 6 except that no catalyst is inserted into the holes. The measurement method and measurement conditions for the open circuit voltage, the maximum output value, and the maximum surface temperature of the fuel cell are the same as those in Example 1. The measurement results are shown in Table 1 below.

(Examination of measurement results of Examples and Comparative Examples)
From the measurement results shown in Table 1, the maximum value of output in Examples 1 to 4 was higher than that in Comparative Example 1. In addition, Example 5 showed a higher value than Comparative Example 2, and Example 6 showed a higher value than Comparative Example 3. As a result, it has been clarified that, in the electrolyte membrane in which the electrolyte is inserted into the porous membrane, the catalyst for decomposing methanol is disposed in the pores, thereby increasing the output.

  Moreover, when the maximum value of the output in Examples 1-3 and the maximum value of the output in Example 4 were compared, Example 4 showed a higher value. In Examples 1 to 3 and Example 4, the amount of catalyst in the electrolyte membrane is clearly different. For this reason, in Example 4, since methanol crossover decreased considerably, it is thought that the output fall was able to be suppressed. However, the methanol decomposition reaction in the electrolyte membrane was activated and considerable heat generation was observed. As a fuel cell used for portable equipment, it is necessary to control the temperature rise. In that sense, it is also important to control the temperature rise while increasing the output. Therefore, it can be considered from the results of Examples 1 to 3 that the amount of generated heat and the output can be controlled depending on where and how much the methanol decomposition catalyst is disposed at the pore position of the electrolyte membrane.

  Further, carbon dioxide generated by the methanol decomposition reaction is exhausted through the electrolyte in the porous body. When the amount of generated carbon dioxide is large, the electrolyte may be altered by long-time operation. Therefore, it has been clarified that the amount and arrangement of the catalyst arranged in the electrolyte membrane affect the characteristics of the fuel cell.

  As described above, in an electrolyte membrane in which an electrolyte is inserted into a porous membrane, a fuel that can control methanol crossover, high output, and temperature rise can be controlled by arranging a catalyst that decomposes methanol in the pores. It became clear that batteries could be provided.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the components without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The perspective view of the electrolyte membrane for direct methanol type fuel cells of one embodiment. 1 is a cross-sectional view of a power generation element including an electrolyte membrane for a direct methanol fuel cell, a fuel electrode, and an air electrode according to an embodiment. 1 is a schematic cross-sectional view showing a direct methanol fuel cell of Example 1. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Porous membrane, 2 ... Through-hole, 3 ... Electrolyte, 4 ... Methanol decomposition catalyst, 5 ... Electrolyte membrane, 6 ... Fuel electrode catalyst layer, 7 ... Fuel electrode gas diffusion layer, 8 ... Air electrode catalyst layer, 9 ... Air Polar gas diffusion layer, 10 ... Membrane electrode assembly, 11 ... Cathode conductive layer, 12 ... Anode conductive layer, 13a, 13b ... Frame, 14 ... Sealing material, 15 ... Fuel vaporization part, 16 ... Liquid fuel tank, 17 ... Moisturizing Layer, 18 ... oxidant inlet, 19 ... cover, 20 ... liquid fuel.

Claims (8)

In a fuel cell comprising a fuel electrode, an air electrode, and an electrolyte membrane disposed between the fuel electrode and the air electrode,
The electrolyte membrane includes a rigid substrate having a through-hole penetrating in the thickness direction, an electrolyte held in at least a part of the through-hole, and methanol decomposition held in at least a part of the through-hole. A fuel cell comprising a catalyst having an action.   2. The fuel cell according to claim 1, wherein the base material is made of an insulating material or has a surface covered with an insulating layer.   3. The fuel cell according to claim 1, wherein the electrolyte is composed of an organic polymer electrolyte, an inorganic electrolyte, or a mixture of both.   The fuel cell according to claim 3, wherein the inorganic electrolyte is a material containing phosphorus.   The fuel cell according to any one of claims 1 to 4, wherein the catalyst having a methanol decomposition action is supported on a carbon support.   The fuel cell according to any one of claims 1 to 4, wherein the catalyst having a methanol decomposition action is supported on an oxide carrier.   The fuel according to any one of claims 1 to 6, wherein an organic polymer electrolyte layer is formed on a surface of the substrate, and at least a part of the organic polymer electrolyte layer is in contact with the electrolyte. battery.   The organic polymer electrolyte layer is formed on the surface of the base material, and at least a part of the organic polymer electrolyte layer is in contact with the catalyst having the methanol decomposing action. The fuel cell according to 1.

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