JP2007165204A - Porous base material, membrane-electrode junction, and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous base material for forming a durable and proton conductive fine pore filled electrolyte film, a durable and proton conductive film-electrode connected member, and a long life fuel cell having high power generation ability. <P>SOLUTION: The porous base material 22 has fine holes 34 and 36 with a diameter from 0.03 to 0.3μm, and a fine hole 32 with a diameter form 0.5 to 15μm formed separated in the thickness direction thereof. The holes having different diameters are connected with the two kinds of fine holes 32 to 36 above to form a continuous hole as the pathway thereof. The relatively small size fine hole 34 and 36 contributes to suppress the swelling of a proton conductive polymer 24, and the relatively large fine hole 32 contributes on the improvement of proton conductivity. The communication fine holes having the two kinds of fine holes 32 to 36 made to communicate with each other have both properties thereof. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a porous substrate used to form a membrane-electrode assembly (MEA) by filling a pore with a proton conductive material, an MEA using the porous substrate, and the MEA. Relates to the fuel cell.

A fuel cell uses a reducing agent such as hydrogen, methanol, or reformed hydrogen from fossil fuels as a fuel, and electrochemically oxidizes the fuel in the cell using air or oxygen as an oxidant, thereby chemical energy of the fuel. Is directly converted into electrical energy and extracted. As a result, it has higher efficiency and quietness compared to internal combustion engines, and has low emissions of NO _x , SO _x , particulate matter (PM), etc. that cause air pollution. It is attracting attention as an energy supply source. For example, it is expected to be used as a distributed power source or a thermoelectric supply system for automobile engines, residential use, etc.

  Such fuel cells are classified into alkali type, phosphoric acid type, molten carbonate type, solid oxide type, solid polymer type, and the like depending on the type of electrolyte used. Among these, the phosphoric acid form and the solid polymer form using a proton-conducting electrolyte can be operated with high efficiency without being restricted by the Carnot cycle in thermodynamics, and the theoretical efficiency is 25 (° C). It reaches 83 (%). In particular, solid polymer fuel cells have been remarkably improved in performance in recent years due to the development of electrolyte membranes and catalyst technology, and are attracting attention as a low-pollution automobile power source and a high-efficiency power generation method.

  Conventionally, as an electrolyte membrane for constituting a membrane-electrode assembly used in the above polymer electrolyte fuel cell, a perfluorosulfonic acid membrane (for example, Nafion (registered trademark) manufactured by DuPont or Dow Chemical manufactured by Dow Chemical Co., Ltd.) is used. Membranes and the like) are commonly used. However, since such a polymer electrolyte exhibits good proton conductivity in an appropriate water-containing state, membrane humidification is essential for moisture management. On the other hand, polymer electrolytes swell when contacted with water or methanol, and repeated swelling and drying cause deterioration. Therefore, there is a problem that the membrane-electrode assembly is easily deteriorated and the life of the fuel cell cannot be obtained. . Moreover, when the electrolyte membrane deteriorates, fuel such as hydrogen or methanol permeates from the fuel electrode side to the air electrode side (that is, crossover) and is directly oxidized at the air electrode, so that the electromotive force of the battery is also reduced.

On the other hand, it has been proposed to form a pore-filled polymer electrolyte membrane by filling the pores of a porous substrate with a polymer electrolyte material (see, for example, Patent Document 1). According to this, even when the polymer electrolyte material is swollen, its deformation and expansion are suppressed by the porous base material, and further, deformation of the electrolyte membrane is suppressed.
JP 2003-263998 A

  However, since the pore-filled electrolyte membrane as described above has a lower proton conductivity than the non-filled membrane in which the entire membrane is composed of an electrolyte, the proton conduction of the membrane-electrode assembly using this membrane Therefore, improvement was desired because the power generation capacity of the fuel cell remained low. In general, when the electrolyte filling amount is constant in the pore-filled electrolyte membrane, the larger the average pore diameter of the porous substrate, the larger the contact area between the electrolytes in the pores, resulting in higher proton conductivity. can get. On the other hand, as the average pore size increases, the effect of suppressing the swelling of the electrolyte is reduced, and when the filled electrolyte is dried and contracted, a gap is generated between the porous substrate and the electrolyte, and hydrogen leakage occurs. There's a problem. Therefore, in the past, for the purpose of sufficiently suppressing the swelling and increasing the durability, a porous substrate having a small average pore diameter of 0.1 (μm) or less, that is, a submicron level was used. Therefore, high proton conductivity could not be obtained.

  The electrolyte constituting the pore-filled electrolyte membrane is not limited to the polymer electrolyte as described above, but is a hydrocarbon-based electrolyte (for example, styrene-based, vinyl-based, aromatic polymer-based) or an inorganic phosphate-based electrolyte (for example, Cesium dihydrogen phosphate) may also be used. Although these electrolytes swell and do not deteriorate as a result of their swelling, since the mechanical strength is low, the pore-filled type has an advantage that the mechanical strength can be supplemented. In these, it is necessary to use a porous base material having a small average pore diameter in order to obtain a high reinforcing effect, suppress damage and extend durability. For this reason, it has been difficult to obtain high proton conductivity as in the case of a pore-filled electrolyte membrane using a polymer electrolyte.

  The present invention has been made against the background of the above circumstances, and its object is to provide a porous substrate that can constitute a pore-filled electrolyte membrane having high durability and high proton conductivity, Accordingly, it is an object of the present invention to provide a membrane-electrode assembly that has high proton conductivity and hardly deteriorates, and a fuel cell that has high power generation capability and a long life.

  In order to achieve such an object, the gist of the porous substrate of the first invention is that a solid substrate is provided with a large number of communicating pores penetrating in the thickness direction and filled with a proton conductive material in the pores. A porous base material used for constituting an electrolyte membrane, wherein a curve representing a log differential pore volume has a first convex curve portion over a predetermined first pore diameter range, and the first convex curve portion. A pore diameter distribution including a second convex curve portion over a predetermined second pore diameter range on the large diameter side, and at least some of the plurality of communicating pores have a pore diameter within the first pore diameter range. And having a portion having a pore diameter within the second pore diameter range on the communication path.

  Further, the gist of the membrane-electrode assembly of the second invention for achieving the above object is one side of a pore-filled electrolyte membrane in which the porous substrate of the first invention is filled with a proton conductive material. The air electrode and the fuel electrode on the other surface.

  Further, the gist of the polymer electrolyte fuel cell of the third invention for achieving the above object is that the membrane-electrode assembly of the second invention is provided.

  According to the first invention, the porous base material is provided with both a portion having a pore diameter in the first pore diameter range and a portion having a pore diameter in the second pore diameter range on the communication path in the thickness direction. Pores are provided. As described above, in the porous base material in which the proton conductive material is filled in the pores, the smaller the pore diameter, the higher the swelling suppression effect of the proton conductive material in the polymer electrolyte material. In the material, the reinforcing effect of the mechanical strength is increased, while the proton conductivity increases as the pore diameter increases. Therefore, pores in the first pore diameter range having a relatively small diameter contribute to swelling suppression and reinforcement, and pores in the relatively large second pore diameter range contribute to improvement in proton conductivity. Therefore, the communicating pores having both of these two types of pore diameter portions have both characteristics. Therefore, the size and ratio of the two kinds of pore diameters (that is, the ratio of the position and height of the first convex curve part and the second convex curve part), and the ratio of the pores having both of them on the communication path By appropriately determining the above according to the desired characteristics, a porous substrate capable of forming an electrolyte membrane having high proton conductivity and hardly causing swelling or damage of the proton conductive material is obtained.

  In addition, according to the second invention, since the porous base material having high proton conductivity and not easily deteriorated as described above is used, the membrane-electrode assembly having high proton conductivity and hardly causing deterioration or damage. Is obtained.

  Further, according to the third invention, since the membrane-electrode assembly using the porous base material having high proton conductivity and hardly deteriorated as described above is used, the power generation capacity is high and the life is long. A solid polymer fuel cell can be obtained.

  In the present application, the curve representing the log differential pore volume refers to a curve in which the horizontal axis represents the pore diameter D and the vertical axis represents dV / d (log D). However, dV is an increase in pore volume between measurement points, and d (logD) is a logarithmic difference value of the pore diameter D. Further, the convex curve portion means a portion that is convex upward in the pore distribution curve. The first convex curve portion and the second convex curve portion are partially stuck even if the peaks are completely separated (that is, there is a pore diameter of substantially zero pore volume between them) (That is, there is no pore diameter of substantially zero pore volume between them).

  Preferably, the porous base material includes a first layer having a pore size distribution corresponding to the first convex curve portion, a pore size distribution corresponding to the second convex curve portion, and the And a second layer stacked on the first layer. In this way, the porous substrate as a whole has a pore size distribution having the first convex curve portion and the second convex curve portion as described above, so that the first layer and the second layer are protons. By using the layers laminated in the permeation direction, a porous substrate capable of forming an electrolyte membrane having high proton conductivity and hardly causing swelling or damage of the proton conductive material is obtained.

  That is, in the present invention, a portion having a pore diameter in the first pore diameter range and a portion having a pore diameter in the second pore diameter range are both provided in the porous substrate, and both of them are on the communication path. If the pore which has is provided, the effect of invention can be enjoyed and the distribution form of these 2 types of pore diameter parts is not specifically limited. For example, these two types of pore diameter portions may be distributed throughout the porous substrate, but the respective distribution ranges may be separated into layers. The latter distribution mode is preferable because it can be easily obtained by laminating two layers having different average pore diameters. Note that the number of stacked layers is not limited to two, and may be three or more.

  Preferably, the first convex curve portion has a peak within a range of 0.01 to 0.3 (μm), and the second convex curve portion has a peak within a range of 0.5 to 15 (μm). is there. On the small-diameter side, a pore diameter of 0.3 (μm) or less is preferable for suppressing swelling of the proton conductive material and reinforcing mechanical strength, and from the viewpoint of obtaining sufficiently high proton conductivity, a pore diameter of 0.01 (μm) or more Is preferred. On the large diameter side, a pore diameter of 0.5 (μm) or more is preferable from the viewpoint of obtaining sufficiently high proton conductivity, and a pore diameter of 15 (μm) or less from the viewpoint of sufficiently suppressing hydrogen gas leakage. Is preferred.

  More preferably, the first convex curve portion has a peak in the range of 0.05 to 0.3 (μm), and more preferably has a peak in the range of 0.10 to 0.3 (μm). . That is, in order to obtain sufficiently high proton conductivity, the smaller diameter side is more preferably 0.05 (μm) or more, and further preferably 0.10 (μm) or more. More preferably, the second convex curve portion has a peak in the range of 0.5 to 10 (μm), and more preferably has a peak in the range of 0.5 to 5 (μm). It is. That is, in order to further suppress the leakage of hydrogen gas, the larger diameter side is more preferably 10 (μm) or less, and further preferably 5 (μm) or less.

  Preferably, the first convex curve portion is located within a range of 0.005 to 3.0 (μm), and the second convex curve portion is located within a range of 0.1 to 50 (μm). is there. On the small-diameter side, all pore diameters are preferably 3.0 (μm) or less in order to suppress swelling of the proton conductive material or to reinforce mechanical strength, and from the viewpoint of obtaining sufficiently high proton conductivity, The pore diameter is preferably 0.005 (μm) or more. On the large diameter side, from the viewpoint of obtaining sufficiently high proton conductivity, all pore diameters are preferably 0.1 (μm) or more, and from the viewpoint of sufficiently suppressing hydrogen gas leakage, all fine diameters are preferred. The pore diameter is preferably 50 (μm) or less. In the present application, the range of the first convex curve portion and the second convex curve portion is defined as the upper limit and lower limit when the pore volume distribution curve is a completely separated pore distribution curve. If there is no point indicating approximately zero between them, the minimum point existing between them is the upper limit of the first convex curve portion and the lower limit of the second convex curve portion. Judge that there is.

  More preferably, the first convex curve portion is located within a range of 0.010 to 1.0 (μm), and the second convex curve portion is located within a range of 0.1 to 20 (μm). That is, for the small diameter side, in order to obtain sufficiently high proton conductivity, it is more preferable that all the pore diameters on the small diameter side are 0.010 (μm) or more, from the viewpoint of suppression of swelling and reinforcement of mechanical strength. Is preferably 1.0 (μm) or less. On the large diameter side, in order to further suppress the leakage of hydrogen gas, it is more preferable that all pores on the large diameter side are 20 (μm) or less, and from the viewpoint of proton conductivity, 0.1 ( μm) or more is preferable.

  Preferably, the ratio of the total volume of the pores in the first pore diameter range to the total volume of the pores in the second pore diameter range is in the range of 20:80 to 80:20. If both of these two types of pores are provided, the effects of the present invention can be enjoyed. However, as the proportion of pores in the first pore size range having a relatively small pore size increases, the proton conductivity decreases. The fuel gas is less likely to leak from the fuel electrode side to the air electrode side. On the contrary, the smaller the number of pores in the first pore diameter range, the higher the proton conductivity, but the more likely the fuel gas leaks. Therefore, the ratio of the volumes of these two kinds of pores can be determined as appropriate according to the desired characteristics, but it is preferable to determine them in consideration of these.

  Further, as described above, in the aspect in which the porous substrate includes the first layer and the second layer, the total volume of the pores in the first pore diameter range and the entire pores in the second pore diameter range The ratio to the volume is determined by the ratio of these thickness dimensions. Therefore, the ratio can be appropriately determined according to the desired characteristics. For example, the thickness of the first layer: the thickness of the second layer = 20: 80 to 80:20. When the porous substrate is composed of three or more layers, the total thickness of the layers having pores in the first pore diameter range and the total thickness of the layers having pores in the second pore diameter range The ratio is preferably within the above range.

  Preferably, the porous base material has a pore diameter distribution in which a central portion in the proton permeation direction corresponds to one of the first convex curve portion and the second convex curve portion, and the remainder (that is, the proton permeation direction). (The front surface side and the back surface side of the porous substrate) have a pore diameter distribution corresponding to the other of the first convex curve portion and the second convex curve portion. In this way, since the porous base material has symmetrical physical properties in the thickness direction, the damage due to the difference in the amount of thermal expansion when the temperature rises during the manufacturing process or use is suitable. It is suppressed.

  Preferably, the porous substrate has a pore diameter distribution in which a portion including a surface located on the fuel electrode side in use corresponds to the first convex curve portion. In this way, since the pore diameter on the fuel electrode side exposed to hydrogen, methanol or the like is reduced, their penetration into the porous substrate is suitably suppressed, and higher durability can be obtained.

  Preferably, the porous substrate is made of ceramics. The porous substrate preferably does not have electronic conductivity, but the constituent material is not particularly limited as long as it can withstand the use temperature of the polymer fuel cell and hardly deteriorate due to fuel gas or water vapor. For example, porous polymers such as polyethylene and polyimide can be suitably used, but porous ceramics are particularly preferable from the viewpoints of price, strength, durability, controllability of pores, and the like. Examples of the ceramic material suitably used in the present invention include alumina, zirconia, silicon nitride, and silica.

  In addition, when the porous substrate as described above is made of ceramics, for example, it is prepared by preparing two types of raw material powders having different average particle diameters (however, more than two types may be used). can do. The ratio of the average particle size and size of the two types of raw material powders is appropriately determined according to the size of the pores to be formed. For example, the average particle size in the range of 0.02 to 12 (μm) The average particle diameter within the range of 0.4 to 200 (μm) is preferably determined so that the ratio of the average particle diameter of the former to the latter is within the range of 1: 2 to 1: 1000. Further, it is more preferable to use a raw material powder having an average particle size in the range of 0.04 to 1.2 (μm) and an average particle size in the range of 2.0 to 60 (μm), those average particle sizes The ratio is more preferably in the range of 1: 2 to 1: 100.

  In addition to or in addition to using raw material powders having different average particle diameters, two types of pores with different pore sizes (however, more than two types may be used) are formed. The porous substrate as described above can also be produced by mixing the agent with the raw material powder. As the pore forming agent, an organic substance that is burned off by the baking treatment is preferable.

  Further, when the porous substrate is manufactured using the raw material powder as described above, both the pores in the first pore diameter range and the pores in the second pore diameter range are included in the entire porous substrate. In the distributed mode, for example, the raw material powders may be mixed at an appropriate ratio, formed by an appropriate method such as press molding, and fired. In such an embodiment, when two kinds of raw material powders are used, the quantitative ratio thereof is appropriately determined according to the desired ratio of the pore volume, but for example within the range of 20:80 to 80:20 (For example, 80:20, 50:50, 20:80, etc.) are preferable. On the other hand, in the aspect provided with the first layer and the second layer having different pore size distributions, if one and the other of the first layer and the second layer are formed of raw material powders having different average particle sizes, Good. In this case, for example, a method of performing baking treatment by superimposing each after molding by press molding or the like, or performing firing treatment by dipping a base made of one raw material powder into a slurry containing the other raw material powder, etc. The method etc. can be taken.

  When a porous substrate is composed of a porous polymer, two kinds of pore forming agents capable of forming pores of different sizes are used as raw materials in addition to laminating and fixing two pieces having different pore distributions. There is a method of mixing and polymerizing.

Preferably, the proton conductive material is a polymer organic material. As a proton conductive material (that is, a polymer electrolyte) made of a polymer organic material, those conventionally used in solid polymer fuel cells can be used. For example, a homopolymer or copolymer of a monomer having an ion exchange group (such as —SO ₃ H group), a copolymer of a monomer having an ion exchange group and another monomer copolymerizable with the monomer, hydrolysis Similar to the homopolymer or copolymer (proton conductive polymer precursor) of a monomer having a functional group (that is, a precursor functional group of an ion exchange group) that can be converted into an ion exchange group by a post-treatment such as The thing etc. which processed are mentioned.

  Specific examples of the polymer electrolyte include, for example, perfluoro proton conductive polymer such as perfluorocarbon sulfonic acid resin, perfluorocarbon carboxylic acid resin film, sulfonic acid type polystyrene-graft-ethylenetetrafluoroethylene (ETFE). Copolymer membrane, sulfonic acid type poly (trifluorostyrene) -graft-ETFE copolymer membrane, polyetheretherketone (PEEK) sulfonic acid membrane, 2-acrylamido-2-methylpropanesulfonic acid (ATBS) membrane, carbonized Examples include hydrogen-based films.

  The method for filling the polymer electrolyte into the pores of the porous substrate is not particularly limited, and any known appropriate method can be used. For example, it can be filled by vacuum impregnation or dip coating.

  In addition, as the proton conductive material, for example, an inorganic electrolyte membrane such as a phosphate glass material is also preferably used.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are appropriately simplified or modified, and the dimensional ratios, shapes, and the like of the respective parts are not necessarily drawn accurately.

  FIG. 1 is a diagram schematically showing a cross-sectional structure of a membrane-electrode assembly 10 according to an embodiment of the present invention. The membrane-electrode assembly 10 is used in, for example, a polymer fuel cell (PTFE), and includes a solid electrolyte 12, catalyst layers 14 and 16 and electrodes 18 and 20 fixed to both surfaces thereof, respectively. Has been.

  The above-mentioned solid electrolyte 12 has a thickness dimension of, for example, about 200 (μm). For example, the solid electrolyte 12 has a rectangular shape with a planar dimension of about 5 (cm) × 5 (cm) and a flat plate type. It is to be made. The solid electrolyte 12 is a so-called pore-filled electrolyte membrane configured by filling the pores of a porous base material 22 as shown in FIG. 2 with a proton conductive polymer 24 as shown in FIG. is there.

  The porous substrate 22 is made of, for example, porous alumina, and has a base portion 26 having a relatively large pore diameter, and surface layer portions 28 and 30 having relatively small pore diameters provided on both surfaces thereof. It is composed of In this embodiment, the base portion 26 corresponds to the second layer, and the surface layer portions 28 and 30 correspond to the first layer. The thickness dimension of the base portion 26 is about 160 (μm), for example, and the thickness dimensions of the surface layer portions 28 and 30 are both about 20 (μm), for example.

Further, the base portion 26 is exclusively provided with pores 32 having a pore diameter in the range of 0.5 to 15 (μm), for example, and the average pore diameter is, for example, about 5 (μm). The peak of the pore distribution is located at a pore diameter of about 5 (μm), and the peak height is about dV / d (log D) = 4.80 × 10 ⁻¹ (ml / g). On the other hand, the surface layer portions 28 and 30 are exclusively provided with pores 34 and 36 having a pore diameter in the range of 0.03 to 0.3 (μm), for example, and the average pore diameter is, for example, about 0.1 (μm). The peak of the pore distribution is located at a pore diameter of about 0.1 (μm), and the peak height is dV / d (log D) = 2.30 × 10 ⁻¹ (ml / g). FIG. 4 shows an example of the pore size distribution of the entire porous substrate 22. The porous base material 22 has a bifurcated pore size distribution having two peaks as a whole, and each convex curve portion including both peaks draws a steep curve. Are separated from each other. Most of the pores 32 to 36 provided in the three layers 26 to 30 are communicated with each other, and the pores 34 and 36 of the surface layer portions 28 and 30 are open to the front surface 38 and the back surface 40. Therefore, the porous base material 22 is provided with a large number of communicating pores that communicate from the front surface 38 to the back surface 40. In this example, 0.5 to 15 (μm) corresponds to the second pore diameter range, and 0.03 to 0.3 (μm) corresponds to the first pore diameter range, and portions located between these in the pore distribution curve. Corresponds to the second convex curve portion and the first convex curve portion, respectively.

  The proton conductive polymer 24 filled in the pores 32 to 36 of the porous base material 22 is made of, for example, 2-acrylamido-2-methylpropanesulfonic acid resin or perfluoroalkylsulfonic acid resin. For example, it is filled at a ratio of about 80% of the total pore volume. In FIG. 3, the pores 32 to 36 are drawn in a completely filled state. However, for example, when the membrane-electrode assembly 10 is used, the proton conductive polymer 24 is swollen in this manner. It becomes a state.

  3 schematically shows the configuration of the solid electrolyte 12, and the pores 32 to 36 are actually bent in a complicated manner, but each has a shape that extends straight in the film thickness direction. It is drawn in. As shown in FIG. 3, relatively large diameter pores 32 are formed in the relatively thick base portion 26, while relatively small diameter pores 34 are formed in the relatively thin surface layer portions 28 and 30. , 36 are connected to each other to form a communication hole penetrating the solid electrolyte 12 in the film thickness direction.

  The catalyst layers 14 and 16 are made of, for example, Pt-supported carbon black. For example, those commercially available from Tanaka Kikinzoku Kogyo Co., Ltd. can be used. The electrodes 18 and 20 are made of, for example, carbon paper. The carbon paper is, for example, one having a thickness of 380 (μm) commercially available for fuel cells from Toray Industries, Inc. The outer peripheral edge of the membrane-electrode assembly 10 is sealed with a sealing material 42 made of polytetrafluoroethylene (PTFE) or the like and having a thickness of about 300 (μm).

  The membrane-electrode assembly 10 configured as described above is manufactured, for example, as follows. That is, first, for example, alumina powder having an average particle diameter of about 20 (μm) is prepared and press-molded by, for example, uniaxial pressing. Next, this is held at a firing temperature corresponding to the raw material powder, for example, a temperature of about 1200 (° C.) for about 2 hours, and a firing treatment is performed. Thereby, an alumina porous sintered body is obtained. Next, a slurry is prepared in which alumina powder having an average particle diameter of about 0.3 (μm) is dispersed in a suitable vehicle, and the slurry is applied to the sintered body obtained by the above-described firing treatment, for example, by dipping. The porous base material 22 is obtained by holding this at about 1200 (° C.) for about 2 hours after drying. When the average particle size of the two kinds of raw materials is determined so that the base 26 and the surface layer portions 28 and 30 having the thickness dimensions as described above can be obtained, the molding thickness of the press-formed body and the coating thickness of the slurry are determined as described above. The pore distribution curve as shown in FIG. That is, it is determined to have two peaks that are separated from each other and have a high height on the large diameter side.

  Next, the proton conductive polymer 24 is filled into the pores 32 to 36 of the porous substrate 22. This filling process is performed as follows, for example. That is, the porous substrate 22 is dipped in an electrolyte monomer solution prepared separately and evacuated to sufficiently impregnate the pores 32 to 36. Next, the monomer in the solution is polymerized by, for example, standing in a dryer at 80 (° C.) for about 2 hours. Next, this is washed with running water to remove excess polymer adhering to both surfaces of the porous substrate 22, thereby obtaining a pore-filled electrolyte. This pore-filled electrolyte is subjected to ion exchange, for example, by being immersed in 1M HCl.

  The electrolyte monomer solution includes, for example, an electrolyte monomer for synthesizing the proton conductive polymer 24, a crosslinking agent, a polymerization initiator, a surfactant, and water. For example, N, N-methylenebisacrylamide is used as the crosslinking agent. As the polymerization initiator, for example, V-50 manufactured by Wako Pure Chemical Industries, Ltd. can be used. Further, as the surfactant, an appropriate one can be used as long as it does not interfere with the polymerization. For example, sodium dodecylbenzenesulfonate is preferable.

  Next, the catalyst layers 14 and 16 and the electrodes 18 and 20 are provided on the pore-filled electrolyte, and the outer peripheral edge is sealed with a sealing material 42. The catalyst layers 14 and 16 are formed, for example, by dispersing the catalyst powder in an appropriate electrolyte polymer solution and applying it to the carbon paper constituting the electrodes 18 and 20 using a bar coater or the like. That is, after the catalyst layers 14 and 16 are formed on the carbon paper, the catalyst layers 14 and 16 and the electrodes 18 and 20 are provided by sandwiching the pore-filled electrolyte therebetween. And the said membrane-electrode assembly 10 is obtained by pressurizing from both surfaces, heating in the state which enclosed the outer periphery with the sealing material 42. FIG.

  FIG. 5 is a diagram schematically showing a configuration example of the fuel cell 44 using the membrane-electrode assembly 10 described above. When the fuel cell 44 is operated, first, the membrane-electrode assembly 10 is humidified, so that the proton conductive polymer 24 in the solid electrolyte 12 is hydrated to develop ionic conductivity.

Next, for example, hydrogen is supplied from the fuel supply source 46 to the anode side electrode 18 via the humidification tank 48 as fuel. An oxidation reaction of the following formula (1) occurs on the electrode 18 to which hydrogen and water are supplied, and protons H ⁺ and electrons e ⁻ are generated. Protons flow toward the cathode electrode 20 through the solid electrolyte 12 (more precisely, within the proton conductive polymer 24 filled in the pores 32 to 36 of the solid electrolyte 12), and the electrons are connected to the electrode 18. Is taken out from the terminal (not shown) and flows to the load 50 via an external circuit. The electrons supplied to the load 50 further travel to the cathode side electrode 20 via an external circuit. On the electrode 20, protons and electrons cause a reduction reaction of the following formula (2) between oxygen and water supplied from the oxygen supply source 52 via the humidification tank 54. Note that methanol may be supplied instead of hydrogen, and the oxidation reaction in this case is as shown in the equation (3).
_{^{3H 2 → 6H + + 6e -}} ··· (1)
^{_{3 / 2O 2 + 6H + +}} 6e - → 3H 2 O ··· (2)
_{CH 3 OH + H 2 O →} 6H + + CO 2 + 6e - ··· (3)

FIG. 6 and FIG. 7 are diagrams showing the results of measuring the output characteristics of the single cell fuel cell together with other examples and comparative examples. FIG. 6 shows the relationship between current density and voltage, and FIG. 7 shows the relationship between the current density and output density obtained from the current density and voltage. For the measurement, for example, a fuel cell measurement system manufactured by Toyo Technica is used, and the temperature of the membrane-electrode assembly 10, the piping temperature, and the temperatures of the humidifying tanks 48 and 52 are all maintained at 80 (° C.), and the H ₂ flow rate is set to 100. (ml / min), the O ₂ flow rate was 500 (ml / min), and the terminal voltage was measured while changing the current by adjusting the load. 6 and 7, sample 4 is the membrane-electrode assembly 10, and the open circuit voltage is less than 1 (V), 200 (mA / cm ² ) is 0.7 (V) or more, and 300 (mA / cm ² ). However, it has a high voltage characteristic of 0.5 (V) or higher, and a high output density as high as about 170 (mW / cm ² ) can be obtained.

Note that Samples 1 and 2 shown in FIGS. 6 and 7 are comparative examples outside the scope of the present invention, and a porous substrate having a pore distribution having a single convex curve portion was used. It is a membrane-electrode assembly. The porous base material used for sample 1 has a pore diameter in the range of 0.03 to 0.3 (μm) exclusively with an average pore diameter of about 0.10 (μm), for example, as shown in FIG. It has pores and has a pore distribution in which a peak of height dV / d (logD) = 2.30 × 10 ⁻¹ (ml / g) is located at 0.10 (μm). In addition, the porous substrate used in Sample 2 has an average pore diameter of about 5 (μm) and a pore diameter in the range of 0.5 to 15 (μm) as shown in FIG. 9 for example. And has a pore distribution in which a peak of height dV / d (logD) = 4.80 × 10 ⁻¹ (ml / g) is located at 5 (μm). That is, the porous substrate constituting Sample 1 has the same structure as the surface layer portions 28 and 30 of the porous substrate 22, and the porous substrate constituting Sample 2 is the base portion of the porous substrate 22. The same organizational structure as that of No. 26 is provided. These have different peak positions, but each has a pore size distribution with only one convex curve portion.

On the other hand, Sample 3 is another embodiment of the present invention, and is a membrane-electrode assembly using another porous substrate having a different configuration from the porous substrate 22. The porous substrate used in Sample 3 has a pore distribution having two convex curve portions as shown in FIG. 10 as an example of the pore diameter distribution. Has a peak of height dV / d (log D) = 2.30 × 10 ⁻¹ (ml / g) at 0.10 (μm) and covers a pore diameter range of 0.03 to 0.3 (μm). The other has a peak of height dV / d (log D) = 4.80 × 10 ⁻¹ (ml / g) at 5 (μm) and covers a pore diameter range of 0.5 to 15 (μm). That is, the porous substrate constituting the sample 3 has the same pore distribution as the porous substrate 22 as a whole, although the peak position, height, and shape of the distribution curve are slightly different.

  In each of Samples 1 to 3, a porous base material was produced by subjecting alumina powder to press molding and baking treatment in the same manner as the base portion 26, and the pores were the same as in the case of the porous base material 22 In this way, the electrolyte material is filled to produce a pore-filled electrolyte, which is further sandwiched between the catalyst layers 14 and 16 and the electrodes 18 and 20 to form a membrane-electrode assembly. However, when producing a porous base material in these, raw material powder different from the case of the base part 26 was used, and the thickness dimension after baking was set to about 200 (μm). That is, the thickness dimension of the porous substrate constituting Samples 1 to 3 is the same as the thickness dimension of the entire porous substrate 22 constituting the membrane-electrode assembly 10. In addition, the raw material powders used for each sample are those used for sample 1, for example, having an average particle diameter of about 0.3 (μm), those used for sample 2, for example, having an average particle diameter of about 20 (μm), The sample 3 used is, for example, a mixture of alumina powder having an average particle size of about 0.3 (μm) and alumina powder having an average particle size of about 20 (μm) in a ratio of 20:80, for example.

As shown in the above measurement results, sample 1 using a porous base material having a small average pore diameter of about 0.1 (μm) shows a characteristic of about 1 (V) at the open circuit voltage, that is, the same level as sample 4. However, the degree of decrease in the terminal voltage, which decreases as the current density is increased, is larger than that of sample 4, being less than 0.7 (V) at 200 (mA / cm ² ) and 0.5 (V) at 300 (mA / cm ² ). ). As a result, the power density remains below 140 (mW / cm ² ). If the pore volume is approximately the same, the resistance to proton conduction increases as the pore diameter decreases, and the resistance to proton conduction increases as the current density increases. Therefore, in Sample 1 in which the pore diameter of the porous substrate is small, a high terminal voltage can be obtained when the current density is small, but the voltage drop accompanying the increase in the current density is large, and the output density is also relatively low. It stays in value.

Moreover, in the sample 2 using the porous base material having a large average pore diameter of about 5 (μm), it remains at about 0.6 (V) at the open circuit voltage, and is below 0.4 (V) at 200 (mA / cm ² ). As a result, the power density remains below 80 (mW / cm ² ). As the pore diameter increases, the swelling suppression effect is reduced, and the filling property of the proton-conductive polymer 24 is reduced, so that a gap penetrating in the thickness direction of the porous substrate is easily formed in the pore. The amount of hydrogen gas that leaks in the form of gas molecules from the fuel electrode side to the air electrode side through the gap increases. For this reason, although the pore diameter is large and the resistance to proton conduction is small, the terminal voltage is lowered and the output density cannot be obtained.

  On the other hand, in sample 4, high proton conductivity is obtained in the base portion 26 having a large pore diameter, while swelling of the proton conductive polymer 24 is suitably performed in the surface layer portions 28 and 30 having a small pore diameter. In addition to being suppressed, the gap between the inner wall surface of the pores and the proton conductive polymer 24 is absent or extremely small, so that leakage of hydrogen gas from the fuel electrode side to the air electrode side is suppressed. Further, since the surface layer portions 28 and 30 having a small pore diameter are limited to a part of the total thickness of the porous base material 22, the proton conductivity is reduced as compared with the case where the whole is made of a material having a small pore diameter. Is suppressed. As a result, high proton conductivity can be obtained while suppressing leakage of hydrogen gas, so that a high terminal voltage and a high output density can be obtained as shown in FIGS.

Sample 3 was prepared by preparing a porous substrate from a powder obtained by mixing alumina powders having two types of average particle diameters as described above, so that pores having a small pore diameter of about 0.1 (μm) and 5 As a result, most of the pores penetrating in the thickness direction are small and small pores penetrating in the thickness direction. Both are provided with a hole diameter portion. Further, since the large pore diameter peak is higher than the small pore diameter peak as described above (that is, the volume occupied by the large pore diameter is relatively large), sufficiently high proton conductivity can be obtained. Thus, although slightly lower than the sample 4 in the open circuit voltage, draws a similar curve and this, 200 (mA / cm ²⁾ at 0.6 (V) or more, 300 (mA / cm ²⁾ even 0.5 (V) or The maximum output density is about 150 (mW / cm ² ).

  In addition, the mixing ratio of the two kinds of powders for constituting the porous base material of the sample 3 is a ratio of 80:20, which is larger in the case of the large diameter as described above. This is equal to the ratio of the thickness dimension of the base portion 26 and the surface layer portions 28 and 30 of the porous base material 22 constituting the sample 4. For this reason, when viewed as the whole porous substrate, these pore distributions substantially coincide with each other, so that similar characteristics can be obtained. However, since the porous base material of Sample 3 made of the mixed powder is difficult to avoid large-diameter pores penetrating in the thickness direction, leakage of hydrogen gas from the fuel electrode side to the air electrode side is a membrane-electrode. Compared to the joined body 10, the number is increased. Therefore, the open circuit voltage of the sample 3 is lower than that of the sample 4, and the output density is also lowered.

  Incidentally, in the fuel cell 44 of the present embodiment, the theoretical value of the open circuit voltage is about 1 (V), and the degree of decrease with respect to this corresponds to the degree of leakage of hydrogen gas from the fuel electrode side to the air electrode side. As described above, when there are large pores having an average pore diameter of about 5 (μm), hydrogen gas may leak, but the larger the proportion of large pores, the larger the amount of leakage. For this reason, in sample 2, the leakage of hydrogen gas is remarkably increased, so that the open circuit voltage is remarkably lowered, and the output density is also low.

  As shown in FIG. 6, the membrane-electrode assembly 10 (sample 4) has a slightly lower open circuit voltage than sample 1. This is because the porous substrate of sample 1 is composed exclusively of pores having a pore diameter of about 0.3 (μm) and the average pore diameter is about 0.1 (μm), whereas the membrane-electrode assembly 10 This is because the porous base material 22 has the same structure as the porous base material of the sample 1 in the surface layer portions 28 and 30, but the base portion 26 has a pore diameter of about 0.5 to 15 (μm). That is, from the viewpoint of suppressing hydrogen gas leakage, it is desirable that the pore diameter is 0.1 (μm) or less, and Sample 1 or a membrane-electrode assembly having pores with a maximum pore diameter of about 0.3 (μm) exists. Hydrogen gas can enter the 10 surface layer portions 28 and 30 as gas molecules. At this time, since the membrane-electrode assembly 10 has the base portion 26 having a large pore diameter, the invading hydrogen gas easily permeates to the air electrode side as compared with the sample 1 having no portion having such a large pore diameter. . For this reason, although the amount of leakage of hydrogen gas is relatively large, the open circuit voltage is lowered. However, since the amount of hydrogen gas that can permeate the surface layer portions 28 and 30 is very small, the difference between these is only about 0.01 (V), for example.

  In short, according to this embodiment, the porous substrate 22 has pores 34 and 36 having a pore diameter of 0.03 to 0.3 (μm) and pores 32 having a pore diameter of 0.5 to 15 (μm) in the thickness direction. Therefore, the pores connected to each other are provided with the two kinds of pores 32 to 36 on the communication path. Therefore, the relatively small diameter pores 34 and 36 contribute to suppression of swelling of the proton conducting polymer 24, and the relatively large diameter pore 32 contributes to improvement of proton conductivity. The communicating pores in which the pores 32 to 36 are communicated with each other have both characteristics. Therefore, the porous base material 22 that can form the solid electrolyte 12 that has high proton conductivity and hardly swells the proton conductive polymer 24 is obtained.

  Further, according to this example, since such a porous substrate 22 is used, the membrane-electrode assembly 10 having high proton conductivity and hardly deteriorated can be obtained, and the power generation capacity is high. A polymer electrolyte fuel cell 44 having a long life is obtained.

  As mentioned above, although this invention was demonstrated in detail with reference to drawings, this invention can be implemented also in another aspect, A various change can be added in the range which does not deviate from the main point.

It is a figure which shows typically the structure of the membrane-electrode assembly of one Example of this invention. It is a figure which shows typically the cross-sectional structure of the porous base material for comprising the membrane-electrode assembly of FIG. It is a figure which shows typically the structure where the proton-conductive polymer was filled in the porous base material of FIG. It is a figure which shows an example of the pore diameter distribution of the porous base material of FIG. 2, taking the pore diameter on the horizontal axis and the log differential pore volume on the vertical axis. It is a figure explaining the structure of the polymer fuel cell using the membrane-electrode assembly of FIG. It is a figure which shows the measurement result of the electric power generation characteristic in the fuel cell shown in FIG. 5 regarding the relationship between a current density and a voltage. It is a figure which shows the measurement result of the electric power generation characteristic in the fuel cell shown in FIG. 5 about the relationship between an electric current density and an output density. It is a figure which shows an example of the pore diameter distribution of the porous base material of a comparative example (sample 1), taking the pore diameter on the horizontal axis and the log differential pore volume on the vertical axis. Comparative Example (Sample 2) FIG. 3 is a diagram showing an example of the pore diameter distribution of the porous substrate of FIG. 2 with the horizontal axis representing the pore diameter and the vertical axis representing the log differential pore volume. It is a figure which shows an example of the pore diameter distribution of the porous base material of the other Example of this invention, taking a pore diameter on a horizontal axis and taking log differential pore volume on a vertical axis | shaft.

Explanation of symbols

10: membrane-electrode assembly, 12: solid electrolyte, 22: porous substrate, 24: proton conductive polymer, 26: base, 28, 30: surface layer, 44: fuel cell

Claims (6)

A porous substrate having a large number of communicating pores penetrating in the thickness direction and filling a proton conductive material in the pores to form a solid electrolyte membrane,
A first convex curve portion in which a curve representing a log differential pore volume extends over a predetermined first pore diameter range, and a second convex curve over a predetermined second pore diameter range on a larger diameter side than the first convex curve portion A portion having a pore diameter within the first pore diameter range and a portion having a pore diameter within the second pore diameter range. A porous base material characterized in that both are provided on the communication path.   The porous substrate has a first layer having a pore size distribution corresponding to the first convex curve portion, and a pore size distribution corresponding to the second convex curve portion, and is laminated on the first layer. The porous substrate according to claim 1, comprising a second layer.   The first convex curve portion has a peak in a range of 0.01 to 0.3 (µm), and the second convex curve portion has a peak in a range of 0.5 to 15 (µm). Item 3. A porous substrate according to Item 2.   The first convex curve portion is located within a range of 0.005 to 3.0 (µm), and the second convex curve portion is located within a range of 0.1 to 50 (µm). Item 4. The porous substrate according to any one of items 3 to 4.   An air electrode is provided on one surface of a pore-filled electrolyte membrane in which the porous substrate according to any one of claims 1 to 4 is filled with a proton conductive material, and a fuel electrode is provided on the other surface. Membrane-electrode assembly. A fuel cell comprising the membrane-electrode assembly according to claim 5.

Images