JP2008198516A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell increasing power generation performance by giving uniform and high conductivity and water repellency to a gas diffusion layer. <P>SOLUTION: The fuel cell comprises cells each formed by stacking a conductive water repellent layer, the gas diffusion layer, and a separator on both of a pair of catalyst layers facing through an electrolyte layer in this order. At least one side of the gas diffusion layer is made of a substrate of carbon fibers having carbon-fluorine bond on the surface. By covering the surface of carbon fiber having high conductivity with carbon-fluorine bonds having high water repellency, uniform and high conductivity and water repellency can be kept, and high power generation performance is provided for a long time. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell, and more particularly to improvement of a gas diffusion layer.

  Fuel cells are classified into phosphoric acid type, alkaline type, molten carbonate type, solid oxide type, solid polymer type, etc., depending on the type of electrolyte used. Among these, polymer electrolyte fuel cells that can operate at a low temperature and have a high output density are being put into practical use in in-vehicle power supplies, household cogeneration systems, and the like.

  On the other hand, in recent years, in order to improve the convenience of portable devices such as notebook computers, mobile phones, and PDAs, a fuel cell that does not require charging unlike a secondary battery and can generate electricity only by replenishing fuel. However, it is expected as a future power source. As a fuel cell power source for such portable devices, a polymer electrolyte fuel cell (hereinafter referred to as PEFC) having a low operating temperature is attracting attention. Among them, a liquid fuel at room temperature is not reformed to hydrogen, A direct fuel oxidation type fuel cell that can directly oxidize at an electrode and extract electric energy is most expected because it does not require a reformer and can easily reduce the size of a power source.

  Low-molecular-weight alcohols and ethers have been studied as fuels for direct fuel oxidation fuel cells. Among them, direct methanol fuel cells (hereinafter referred to as DMFC) using methanol as a fuel, which can improve energy efficiency and output, are used. Most promising.

The reaction at the anode of the DMFC is represented by CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (hereinafter, reaction formula (1)), and the reaction at the cathode is 3 / 2O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O ( Hereinafter, it is represented by the reaction formula (2)). The oxygen introduced into the cathode is generally taken from air.

  A PEFC including a DMFC is called a cell by facing a pair of catalyst layers through an electrolyte layer and laminating a conductive water repellent layer, a gas diffusion layer, and a separator in this order on the other side of both catalyst layers. It forms the basic configuration. A configuration including an electrolyte membrane and a pair of catalyst layers is referred to as CCM (Catalyst Coated Membrane), and a configuration including a conductive water repellent layer and a gas diffusion layer is referred to as MEA (Membrane Electrode Assembly). The separator is provided with a flow path for circulating fuel and air to the MEA. The fuel is introduced into the flow path as an anode, and the air is introduced as a cathode. Since the power generation voltage of the fuel cell is 1 V or less and it is difficult to drive the device with a voltage generated from a single cell, it is possible to obtain a high voltage by stacking a plurality of the cells described above in series. It is common. Such a cell stack is called a stack.

  The gas diffusion layer is required to have high gas diffusibility and electronic conductivity. However, on the cathode side, as shown in the reaction formula (2), there is a reaction to generate water, so that the water is quickly removed from the outside so that the pores in this layer are saturated with water and air diffusion is not insufficient. Need to be discharged. Accordingly, at least the gas diffusion layer on the cathode side is required to have high water repellency. Therefore, a so-called carbon cloth, carbon non-woven fabric, carbon paper, etc., formed from a polyacrylonitrile-based carbon fiber into a woven fabric, non-woven fabric, or paper, is formed into a sheet shape with a porosity of 80% or more, and a gas diffusion layer It is common to use as

In order to impart water repellency to the gas diffusion layer, a sheet made of carbon fiber is immersed in a dispersion of a fluororesin such as polytetrafluoroethylene (PTFE), which is a highly water-repellent material, and dried to remove moisture. In general, a process of baking to remove the surfactant contained in the dispersion and coating the carbon fiber with a fluororesin is common. However, in this process, since the fluororesin concentrates on the portion where the dispersion liquid tends to stay, such as a gap of several μm formed by the fibers and around the intersection, it is difficult to cover the sheet with a structure having a uniform water repellent effect. Therefore, as in Patent Document 1, it has been proposed to use a nonwoven fabric in which a fluororesin fiber and a conductive fiber are woven.
JP-A-11-204114

  However, the fluororesin fiber does not have electronic conductivity, and it is difficult to uniformly weave two different fibers. Therefore, when viewed microscopically, the configuration of Patent Document 1 has a form in which a portion having poor conductivity mainly composed of fluororesin fibers and a portion not subjected to water repellent treatment mainly composed of carbon fibers are mixed. Especially, when driving for a long period of time, the problems of each part become conspicuous and it becomes difficult to obtain a desired effect.

  An object of the present invention is to solve the above problems and to provide a fuel cell with high power generation performance by imparting uniform and high conductivity and water repellency to a gas diffusion layer.

  In order to solve the above problems, the fuel cell of the present invention comprises a cell in which a conductive water-repellent layer, a gas diffusion layer, and a separator are laminated in this order on both of a pair of catalyst layers opposed via an electrolyte layer, At least one of the gas diffusion layers is characterized by using a carbon fiber having a carbon-fluorine bond on the surface as a base material.

  The base material itself of the gas diffusion layer is made of carbon fiber having high conductivity, and the surface thereof is covered with a carbon-fluorine bond having high water repellency, thereby mixing two kinds of fibers having different properties as in Patent Document 1. Compared to the case, both conductivity and water repellency can be maintained at a uniform and high level, so that excellent power generation performance can be exhibited over a long period of time.

  As described above, according to the present invention, since fluorine atoms are directly bonded to carbon fibers, the function does not become uneven as in the gas diffusion layer of Patent Document 1, and long-term storage stability and long-term continuous operation performance are achieved. It is possible to provide an excellent fuel cell.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  1st invention consists of the cell which laminated | stacked the electroconductive water-repellent layer, the gas diffusion layer, and the separator in order on both of a pair of catalyst layers opposed through the electrolyte layer, and at least one of the gas diffusion layers is on the surface. The present invention relates to a fuel cell comprising a carbon fiber having a carbon-fluorine bond as a base material. According to a second invention, in the first invention, the fuel is supplied to one of the gas diffusion layers and the gas containing oxygen is supplied to the other, and at least the gas containing oxygen is supplied. The gas diffusion layer is characterized by using a carbon fiber having a carbon-fluorine bond on the surface as a base material.

FIG. 1 is a schematic sectional view showing a fuel cell as a single cell of the present invention. A pair of catalyst layers 2 and 3 are opposed to each other through the electrolyte layer 1, and the conductive water repellent layer 4, the gas diffusion layer 6, and the separator 8 are laminated in this order on the other surface of the catalyst layer 2. By laminating the conductive water repellent layer 5, the gas diffusion layer 7, and the separator 8 in this order, a basic structure called a cell is formed. The structure composed of the electrolyte membrane 1 and the catalyst layers 2 and 3 is CCM, and the structure composed of the conductive water repellent layers 4 and 5 and the gas diffusion layers 6 and 7 is MEA. The separator 8 is provided with a flow path 9 for circulating fuel and air (as a gas containing oxygen) to the MEA. The fuel is introduced into the channel 9 as an anode, and the air is introduced as a cathode. While the CCM is responsible for power generation, it is responsible for the uniform dispersion of the fuel and air supplied by the MEA and the smooth discharge of the product water and carbon dioxide.

  Carbon fibers are generally used for the gas diffusion layers 6 and 7, but when carbon is exposed to a high potential and high humidity environment like a cathode for a long time, carbon atoms on the surface are oxidized and carbon- By forming an oxygen bond and increasing affinity with water, the pores are saturated with water and air diffusion becomes insufficient. In the first invention, at least one of the gas diffusion layers 6 and 7 is made of a carbon fiber having a high conductivity as a base material itself, and the surface thereof is covered with a carbon-fluorine bond having a high water repellency. In addition to solving this problem, both conductivity and water repellency can be maintained at a uniform and high level as compared with the case where two kinds of fibers having different properties are mixed as in Patent Document 1, so that it can be stored for a long time and used for a long time. Even in this case, it will be possible to demonstrate excellent power generation performance.

  In many direct oxidation fuel cells such as DMFC, since the fuel is water-soluble, a crossover in which the fuel passes through the electrolyte membrane 1 occurs. Since the crossover fuel lowers the cathode potential, in anticipation of this concern, the concentration of the aqueous solution of the fuel supplied to the anode is equal to the concentration of methanol and water shown in the reaction formula (1) (approximately 64). Will be set much lower than (weight percent). As a result, a difference in water activity occurs between the anode and the cathode, and a large amount of water moves from the anode to the cathode. In the second aspect of the present invention, at least one of the gas diffusion layers 6 and 7 that supplies air to the flow path 9 has the structure of the first aspect of the invention, so that the water that has moved from the anode to the cathode Or since it can discharge | emit efficiently from 3 to the direction of the flow path 9, the malfunction by the accumulation | storage of water (air diffusion is inhibited) is eliminated, and a high output is obtained.

  The third invention is characterized in that, in the first invention, the fuel is directly supplied. Specifically, it refers to a configuration in which fuel is directly sent without providing a reformer or the like until reaching the flow path 9. Since the water repellency of at least one of the gas diffusion layers 6 and 7 is increased in the first invention, the water that has moved from the anode to the cathode is quickly discharged even when the configuration of the third invention is adopted. Since the diffusibility of air can be maintained in the gas diffusion layer 6 or 7, a high output can be obtained.

  Below, the structure of this invention is demonstrated in detail.

  The electrolyte membrane 1 may be a membrane made of a proton-conducting solid electrolyte, such as a perfluorosulfonic acid polymer represented by Dupont's Nafion (trade name), an inorganic / organic composite membrane, or a composite membrane of organic materials. It is preferable to use a material having an effect of reducing the crossover of methanol as a fuel, such as a hydrocarbon-based polymer not containing fluorine.

The catalyst layers 2 and 3 are composed of a mixture of catalyst powder and a proton conductive solid electrolyte. As the catalyst powder, a noble metal catalyst powder typified by platinum is used, and there are a case where a powder consisting only of a noble metal called black is used and a case where a noble metal powder is supported on carbon particles. In order to reduce the poisoning of the active sites due to carbon monoxide, which is an intermediate product, the catalyst layers 2 and 3 generally use platinum and ruthenium alloy fine particles as catalyst powder. is there. On the other hand, platinum fine particles are used on the cathode side of the catalyst layers 2 and 3. As the proton-conducting solid electrolyte, like the electrolyte membrane 1, a perfluorosulfonic acid polymer or the like, or a material that reduces the crossover of methanol can be used.

The paste which is a precursor of the catalyst layers 2 and 3 is obtained by mixing the above-described catalyst powder and a dispersion liquid in which a solid electrolyte is dispersed in a solvent. There is a method of obtaining CCM by directly applying and drying this on the surface of the electrolyte membrane 1 by spraying. Alternatively, a paste is applied with a doctor blade on a sheet of polytetrafluoroethylene (PTFE) or the like, which is a support, and then dried to form catalyst layers 2 and 3 on the support. There is also a method in which a CCM is obtained by arranging on both surfaces of 1 and bonding by hot pressing or the like.
The conductive water-repellent layers 4 and 5 are made of carbon black (furnace black, acetylene black, etc.), graphite powder, porous metal powder, and other materials capable of forming conductive micropores, and water-repellent materials (PTFE, etc.). ) In a solvent to prepare a paste, which is applied on a smooth surface (such as PTFE) with a doctor blade and then dried.

  PTFE is a preferred water-repellent material, but is often delivered in the form of a dispersion. When the surfactant contained in the PTFE dispersion remains in the conductive water-repellent layers 2 and 3, the water repellency is lowered. Therefore, the surfactant is generally decomposed and removed by baking at a high temperature of 250 ° C. or higher. . Therefore, the sheet used as the support is preferably made of a resin or metal having high heat resistance that does not melt or significantly deform at high temperatures. Further, in the subsequent step, when a pressure process such as hot pressing is not performed, glass, ceramics, or the like may be used as the support. It is preferable to enhance the smoothness and heat resistance of the surface of any support so that a part of the conductive water-repellent layer is not peeled off when the support is removed.

It is important that the gas diffusion layers 6 and 7 on the cathode side impart high water repellency as described above. On the other hand, among the gas diffusion layers 6 and 7, the anode-side one is often used by mixing water with fuel as shown in the reaction formula (1). There may be no cases. That is, in DMFC, the material diffusion in the anode GDL is complicated, and the methanol aqueous solution as fuel diffuses from the separator 8 to the catalyst layer 6 or 7 on the anode side in a liquid or gas state, and the product carbon dioxide. Diffuses opposite it. Normally, the faster the diffusibility of methanol or water, the better the power generation performance. However, if the supply amount is excessive in the balance between the power generation consumption and supply amount of methanol, the anode catalyst layer 6 or 7 and the electrolyte Since the methanol concentration increases at the interface of the membrane 1, the amount of crossover increases, and the power generation performance decreases. Therefore, the water repellency of the gas diffusion layer 6 or 7 on the anode side needs to consider the balance with the water repellency of other materials. For the gas diffusion layers 6 and 7 based on carbon fibers having carbon-fluorine bonds on the surface, for example, a sheet made of carbon fibers is prepared and mixed with fluorine gas and inert gas (such as argon or helium) at room temperature. It is obtained by exposure to gas or low molecular weight fluorocarbon gas. However, in either case, it should be noted that the value of x represented by the chemical formula (CF _x ) _n becomes large and the electron conductivity becomes remarkable when the ratio of fluorine atoms becomes high.

  The carbon fibers used as the base material for the gas diffusion layers 6 and 7 are considered to be composed of carbon fibers partially graphitized. In accordance with the spirit of the present invention, in order to obtain high surface water repellency without losing the electronic conductivity inherent to carbon fibers, fluorine does not form a graphite intercalation compound, and only surface carbon bonds with fluorine. It is preferable to do.

  The separator 8 may be made of a material containing a carbon material such as graphite, and the flow path 9 may be formed by cutting or the like, or may be die-molded by injection molding, compression molding, or the like.

Since FIG. 1 shows the structure of a single cell, an end plate 10 is arranged on the outside of both separators 8 and pressure fastening using bolts and springs (not shown) is shown. However, the same applies to a stack in which a plurality of cells are directly stacked and connected. Since the interface between the MEA and the separator 8 bonded by hot pressing has poor adhesion, the aim is to reduce the contact resistance by pressure-fastening the cell or stack in the direction perpendicular to the component surface.

  Examples of the present invention are shown below.

(Example 1)
A dispersion liquid in which Nafion (trade name), which is a solid electrolyte, is dispersed as a catalyst powder obtained by supporting 50% by weight of a platinum-ruthenium alloy having an atomic ratio of 1: 1 on conductive carbon particles having an average primary particle diameter of 30 nm; Were mixed in water and defoamed to obtain a paste. This paste was applied onto a polypropylene sheet having a thickness of 50 μm using a bar coater, and allowed to stand for 1 day at room temperature and dried to obtain an anode-side catalyst layer.

  Mixing a dispersion liquid in which Nafion (trade name), which is a solid electrolyte, is dispersed in water using 50% by weight of platinum supported on conductive carbon particles having an average primary particle diameter of 30 nm as a catalyst powder. A paste was obtained by foaming. This paste was applied onto a polypropylene sheet having a thickness of 50 μm using a bar coater, and allowed to stand for 1 day at room temperature and dried to obtain a catalyst layer on the cathode side.

Nafion 117 (trade name, thickness 178 μm) is prepared as an electrolyte membrane, sandwiched between the above two catalyst layers, and pressurized with a hot press device at a setting of 125 ° C., 10 MPa for 3 minutes, the catalyst layer is electrolyte membrane And the polypropylene sheet was removed to obtain CCM (25 cm ² square).

  Using carbon paper TGP-H-090 (trade name, manufactured by Toray Industries, Inc.) as a base material, it is immersed in PTFE dispersion D-1 (trade name, manufactured by Daikin Industries, Ltd.) diluted to a desired concentration for 1 minute. Then, it is dried in a hot air dryer at 100 ° C., and baked in an electric furnace at 270 ° C. for 2 hours to produce a gas diffusion layer having a PTFE content of 10% by weight. It was.

  Using Avcarb 1071HCB (trade name, manufactured by Ballard Material Products) as a base material, it is allowed to stand for 10 minutes in a mixed gas at room temperature in which 0.1 mol% of fluorine gas is mixed with helium gas. A gas diffusion layer provided with a fluorine bond was prepared and used on the cathode side.

  As the conductive water repellent layer, on both the anode side and the cathode side, PTFE dispersion D-1 was stirred and mixed so that the solid content ratio of PTFE was 10% by weight, and the paste was prepared as described above. It apply | coated by the doctor blade method on the gas diffusion layer, and after drying with a 100 degreeC constant temperature layer, it baked in the electric furnace of 270 degreeC for 2 hours, the surfactant was removed, and MEA was obtained.

  The obtained MEA conductive water-repellent layer was arranged so as to face the catalyst layers on both sides of the CCM, and was joined by pressurizing at a pressure of 5 MPa for 1 minute with a hot press apparatus set at a temperature of 125 ° C. Furthermore, it arrange | positioned so that the gas diffusion layer of both MEA may oppose a separator. A 2 mm thick graphite plate was used as the separator, and a flow path for supplying fuel and oxygen was formed by cutting so that the cross section became 1 mm square, and this flow path was disposed on the gas diffusion layer side. In addition, the flow path was made into the serpentine type which meanders uniformly on the surface of MEA.

  These separators sandwiched between CCM and MEA were further sandwiched between end plates made of a stainless steel plate having a thickness of 1 cm. In addition, between the end plate and the separator, a 2 mm-thick copper plate with a gold plating on the surface was arranged, and connected to an electronic load device so that the power generation performance could be measured. This end plate was pressure-fastened using bolts, nuts, and disc springs to produce a DMFC single cell using methanol as fuel. This is cell A.

(Comparative Example 1)
Using Avcarb1071HCB as a base material, it was immersed in PTFE dispersion D-1 diluted to a desired concentration for 1 minute, pulled up, dried in a hot air dryer at 100 ° C., and baked in an electric furnace at 270 ° C. for 2 hours. Then, a gas diffusion layer having a PTFE content of 10% by weight was produced, and a DMFC single cell produced in the same manner as in Example 1 using the gas diffusion layer on the cathode side was designated as cell R.

The above Examples and Comparative Examples, while supplying the anode with a tube pump aqueous methanol solution 2 mol / L at a flow rate of 1 cm ³ / min as a fuel, two of 200 cm ³ / min and 1000 cm ³ / min The following evaluation was performed by supplying unhumidified air at a flow rate to the cathode while being controlled by a mass flow controller. The results are shown in (Table 1).

(Initial power generation characteristics)
After each cell is controlled to be in a 60 ° C. environment using a heating wire heater and a temperature controller, it is connected to an electronic load device PLZ164WA (manufactured by Kikusui Electronics Co., Ltd.), and the current density is 200 mA / by constant current control. The voltage was set to be cm ² and the voltage one minute after the start of power generation was recorded.

(Power generation characteristics after long-time operation)
The voltage when 1000 hours of continuous power generation was performed under the same conditions as the initial power generation characteristics described above was recorded.

（表１）からわかるように、本発明のセルＡは、比較例であるセルＲに比べて空気量の違いによる電圧差異が小さく、空気流量が小さくても高い発電電圧が得られることがわかる。この傾向は初期のみでなく、長時間運転後であっても同様であった。特に長時間運転においては、セルＲは空気の拡散が阻害されて発電性能が低下していると考えられるので、本発明のセルＡとの差が顕著であった。以上のように、本発明の燃料電池は従来のものに比べて、高い発電性能と高い連続発電性能とを得ることができることがわかった。

As can be seen from Table 1, the cell A of the present invention has a smaller voltage difference due to the difference in the air amount than the cell R as the comparative example, and a high power generation voltage can be obtained even when the air flow rate is small. . This tendency was the same not only in the initial stage but also after long-time operation. In particular, in the long-time operation, since the cell R is considered to have a reduced power generation performance due to inhibition of air diffusion, the difference from the cell A of the present invention was remarkable. As described above, it was found that the fuel cell of the present invention can obtain higher power generation performance and higher continuous power generation performance than those of the conventional one.

  The fuel cell of the present invention is useful as a power source for portable small electronic devices such as mobile phones, personal digital assistants (PDAs), notebook PCs, and video cameras. Further, it can be applied to a power source for an electric scooter.

Schematic cross-sectional view of a fuel cell according to an embodiment of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrolyte membrane 2, 3 Catalyst layer 4, 5 Conductive water-repellent layer 6, 7 Gas diffusion layer 8 Separator 9 Flow path 10 End plate

Claims (3)

A fuel cell comprising a cell in which a conductive water repellent layer, a gas diffusion layer, and a separator are laminated in this order on both of a pair of catalyst layers facing each other through an electrolyte layer,
A fuel cell in which at least one of the gas diffusion layers is based on a carbon fiber having a carbon-fluorine bond on its surface. The gas diffusion layer is configured to supply fuel to one of the gas diffusion layers and supply a gas containing oxygen to the other, and the gas diffusion layer supplying at least a gas containing oxygen is based on the carbon fiber. The fuel cell according to claim 1, wherein The fuel cell according to claim 1, wherein the fuel is directly supplied.

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