CATALYST FOR ELECTRODES, COMPOSITION FOR FORMING GAS DIFFUSION ELECTRODE, GAS DIFFUSION ELECTRODE, MEMBRANE ELECTRODE ASSEMBLY, AND FUEL CELL STACK
20230369608 · 2023-11-16
Assignee
Inventors
- Yasuhiro Seki (Tokyo, JP)
- Kiyotaka Nagamori (Tokyo, JP)
- Tomoteru Mizusaki (Tokyo, JP)
- Satoshi Aoki (Tokyo, JP)
- Yoko NAKAMURA (Tokyo, JP)
- Makoto Nishibetto (Tokyo, JP)
- Yuki Takii (Tokyo, JP)
Cpc classification
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
H01M4/86
ELECTRICITY
International classification
H01M4/86
ELECTRICITY
Abstract
The catalyst for electrodes comprises: a porous support which has nanopores having a pore diameter of from 1 nm to 20 nm and micropores having a pore diameter of less than 1 nm; and a plurality of catalyst particles which are supported by the support. The catalyst particles are supported by both inner portions and outer portions of mesopores of the support, and contain Pt (zerovalent). If an analysis of the particle size distribution of the catalyst particles is performed using three-dimensional reconstructed images obtained through a STEM-based electron tomography measurement, the condition of formula (S1), namely (100×(N10/N20)≤8.0) is satisfied, where N10 represents the number of noble metal particles that are not in contact with pores having a pore diameter of 1 nm or more; and N20 represents the number of catalyst particles that are supported by the inner portions of the nanopores of the support.
Claims
1. An electrode catalyst which includes a conductive porous carbon support having a nanopore of a pore size of 1 to 20 nm, and a plurality of catalyst particles supported on the support, wherein a region made of Pt (0 valence) is formed on at least a part of the surface of the catalyst particle, the catalyst particle is supported on both of inside of the nanopore and outside the nanopore of the support, and when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image obtained by an electron beam tomography measurement using an STEM (scanning transmission electron microscopy), the condition of the following equation (S1) is satisfied:
100×(N10/N20)≤8.0 (S1) in the above equation (S1), N10 is the number of non-contact particles (n101+n102), which is the sum of the number of particles (n101) of noble metal particles that are not in contact with pores having a pore diameter of 1 nm or more that can be confirmed by the electron tomography measurement and the number of noble metal particles (n102) that are not in contact with the porous carbon support itself and exist outside the porous carbon support, N20 is the number of particles of the catalyst particles supported inside the nanopores of the support.
2. The catalyst for electrode according to claim 1, wherein, when focusing on a catalyst aggregate composed of the catalyst particle and the support, which has a size that can be accommodated in a rectangular space with one side of 60 to 300 nm in the three-dimensional reconstructed image of the STEM, and looking at six square cross sections of a stereoscopic image with one side of 20 to 50 nm extracted from the inside region of the catalyst aggregate, at least one nanopore is formed in at least one cross section, and the nanopore formed in at least one of the six square cross sections has at least one opening in contact with a first side of four sides of the square cross section, and at least one opening in contact with a second side of the cross section of the square which is parallel to the first side, and has a shape of an intercommunicating pore which extends continuously from the opening on the first side to the opening on the second side without blocking.
3. The catalyst for electrode according to claim 2, wherein the intercommunicating pore has a shape having a plural of branches.
4. The catalyst for electrode according to claim 3, wherein the intercommunicating pore has two or more openings on the first side.
5. The catalyst for electrode according to claim 3, wherein the intercommunicating pore has two or more openings on the second side.
6. The catalyst for electrode according to claim 3, wherein the intercommunicating pore has at least one opening on the third side perpendicular to the first side.
7. The catalyst for electrode according to claim 3, wherein the intercommunicating pore has at least one opening on the fourth side perpendicular to the first side.
8. The catalyst for electrode according to claim 1, wherein a porosity measured by using the three-dimensional reconstructed image of STEM is 35% or more.
9. The catalyst for electrode according to claim 1, wherein a pore size of the nanopore is 1 to 10 nm.
10. The catalyst for electrode according to claim 1, wherein the porous carbon support further has a micropore having a pore size of less than 1 nm.
11. The catalyst for electrode according to claim 1, wherein the catalyst particle is made of Pt (0 valence).
12. The catalyst for electrode according to claim 1, wherein the catalyst particle is made of a Pt alloy.
13. The catalyst for electrode according to claim 1, wherein the catalyst particle is a core-shell catalyst particle, and the core-shell catalyst particle has a core particle, and a Pt shell layer corresponding to a region composed of Pt (0 valence) formed on at least a part of the surface of the core particle.
14. The electrode catalyst according to claim 1, wherein, when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image of the STEM, the condition of the following equation (S1) is satisfied:
100×{N10/(N20+N30)}≤5.0 (S2) in the formula (S2), N10 is synonymous with N10 in the formula (S1), N20 is synonymous with N20 in the formula (S1), N30 is the number of particles of the catalyst particles supported on the outside of the nanopores of the support.
15. The electrode catalyst according to claim 1, wherein, when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image of the STEM, with respect to the catalyst particles supported inside the nanopore of the support, an average distance from the inlet of the nanopores to the supported position of the catalyst particles is of 5.0 nm or more.
16. The electrode catalyst according to claim 1, wherein, when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image of the STEM, the catalyst particles supported inside the nanopore of the support exist within a range that a distance from the inlet of the nanopore to the supported position of the catalyst particles of 0 to 27 nm.
17. The electrode catalyst according to claim 1, wherein, when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image of the STEM, a particle size of the catalyst particles supported inside the nanopore of the support is more than 0 nm and 7 nm or less.
18. The electrode catalyst according to claim 1, wherein, when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image of the STEM, a ratio of the catalyst particles supported inside the nanopore is 50% or more.
19. The electrode catalyst according to claim 18, wherein, when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image of the STEM, a ratio of the catalyst particles supported inside the nanopore is 70% or more.
20. The catalyst for electrode according to claim 1, wherein at least a part of the region composed of the Pt (0 valence) of the surface of the catalyst particles is covered with a Pt oxide film.
21. The catalyst for electrode according to claim 1, wherein a BET specific surface area (nitrogen adsorption specific surface area) of the porous carbon support is 200 to 1500 m.sup.2 g.
22. A powder of a catalyst for electrode, which contains 10 wt % or more of the catalyst for electrode according to claim 1.
23. A composition for forming gas diffusion electrode, which comprises the catalyst for electrode according to claim 1.
24. A gas diffusion electrode, which comprises the catalyst for electrode according to claim 1.
25. A membrane-electrode assembly (MEA), which comprises the gas diffusion electrode according to claim 24.
26. A fuel cell stack, which comprises the membrane-electrode assembly (MEA) of claim 25.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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MODE FOR CARRYING OUT THE INVENTION
[0181] Hereinafter, with reference to figures as appropriate, a suitable embodiment of the present invention is explained in detail.
[0182] <Membrane-Electrode Assembly (MEA)>
[0183]
[0184] The MEA10 shown in
[0185] In this MEA10, at least one of the cathode 1 and the anode 2 has a configuration in which a electrode catalyst 20 (Pt catalyst 20) to be described later is contained.
[0186] The MEA10 can be produced by laminating the cathode 1, the anode 2, and the PEM 3 as shown in
[0187] <Gas Diffusion Electrode (GDE)>
[0188] The cathode 1 as a gas diffusion electrode has a configuration including a gas diffusion layer 1gd and a catalyst layer 1e, which is formed on the PEM 3 side surface of the gas diffusion layer 1gd. Further, the cathode 1 has a water repellent layer (Micro Porous Layer, hereinafter, referred to as “MPL” as needed) 1m arranged between the gas diffusion layer 1gd and the catalyst layer 1c.
[0189] Similarly to the cathode 1, the anode 2, which is a gas diffusion electrode, has a configuration including a gas diffusion layer 2gd and a catalyst layer 2c, which is formed on the PEM 3 side surface of the gas diffusion layer 2gd, and a MPL 2m, which is arranged between the gas diffusion layer 2gd and the catalyst layer 2c.
[0190] (Catalyst Layer (CL))
[0191] In the cathode 1, the catalyst layer 1c is a layer in which a reaction proceeds such that water is generated from air (oxygen gas) sent from the gas diffusion layer 1gd and hydrogen ions moving through the PEM 3 from the anode 2.
[0192] In addition, in the anode 2, the catalyst layer 2c is a layer in which a reaction in which hydrogen ions and electrons are generated from hydrogen gas sent from the gas diffusion layer 2gd proceeds.
[0193] At least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2 includes the electrode catalyst 20 of the present invention.
[0194] (Preferred Embodiment of the Catalyst for Electrode of the Present Invention)
[0195] Hereinafter, the preferred embodiment of the catalyst for electrode of the present invention will be described with reference to
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[0197] Further,
[0198] As shown in
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[0201] Further, from the viewpoint of obtaining the effect of the present invention more reliably, the catalyst for electrode 20 shown in
[0202] That is, as described above, when the microstructure is observed by using the three-dimensional reconstructed image of STEM as described above, the catalyst for electrode 20 has the structure which satisfies the condition (a) of the equation (S1), and the condition (ß) the nanopore is formed so as to have the aforementioned shape of the intercommunicating pore.
[0203] To explain the condition (a) in detail, when observing the fine structure of the catalyst for electrode 20 according to the information of the three-dimensional reconstructed image of STEM obtained in the above-described steps (A) to (C), the value of the equation (S1) [100×(N10/N20)] is preferably 8.0 or less.
[0204] Furthermore, from the viewpoint of obtaining the effects of the present invention more reliably, in the catalyst for electrode 20, the value of the equation (81) [100×(N10/N20)] is preferably 6.5 or less, more preferably 1.0 or less.
[0205] To explain the condition (B) in detail, when observing according to the information of the three-dimensional reconstructed image of STEM obtained in the above-described steps (D) to (G), the catalyst for electrode 20 has the following structure.
[0206] More specifically explaining, when looking at six square cross sections of “the stereoscopic image (one side of 20 to 50 nm) extracted from “the catalyst aggregate composed of the catalyst particle 20 (catalyst aggregate composed of the catalyst particle 23 which has a size that can be accommodated in a rectangular space with one side of 60 to 300 nm and the support 22)” which can be observed by the 3D-STEM image (three-dimensional reconstructed image), in at least one cross section, the catalyst for electrode 20 has the structure where at least one “intercommunication pore P1 in which a plurality of nanopores P22 are connected” having the following shape is formed.
[0207] That is, in the catalyst for electrode 20, when the fine structure of the catalyst for electrode is observed with the 3D-STEM image (three-dimensional reconstructed image), at least, the nanopore P22 formed in at least one of the six square cross sections of the stereoscopic image obtained by cutting the inside of the catalyst aggregate has at least one opening in contact with the first side of four sides of the square cross section, and at least one opening in contact with the second side of the cross section of the square which is parallel to the first side. In addition, the nanopore P22 has the shape of an intercommunicating pore P1 which extends continuously from the opening on the first side to the opening on the second side without blocking.
[0208] In the following, more specific description will be made by referring the example of the catalyst for electrode of Example 1 shown in
[0209] (D) First, with respect to the catalyst for electrode of Example 1 to be measured, a three-dimensional reconstructed image of STEM is obtained. From among the catalyst aggregates in the three-dimensional reconstructed image, a catalyst aggregate which has a size that can be accommodated in a rectangular space with one side of 60 to 300 nm (region of interest) is selected (
[0210] (E) Next, a stereoscopic image (20 to 50 nm on one side) is extracted from the inner region of the catalyst aggregate of the catalyst for electrode of Example 1 selected in the step (D) (
[0211] (F) Next, the stereoscopic image (three-dimensional reconstructed image of STEM) inside the catalyst aggregate of the catalyst for electrode of Example 1 obtained in the step (E) is observed, and by utilizing the difference in luminance, the part of the void (part of pore such as nanopore) and the part of the porous carbon support are segmented (
[0212] (G) Next, when observing the six square cross-sections of the stereoscopic image inside the catalyst aggregate of the catalyst for electrode of Example 1 after performing segmentation in the step (F), as shown in
[0213] From the viewpoint of obtaining the effects of the present invention more reliably, as shown in the example of Example 1 of
[0214] Furthermore, from the viewpoint of obtaining the effects of the present invention more reliably, as shown in the example of Example 1 of
[0215] Further, from the viewpoint of obtaining the effects of the present invention more reliably, as shown in the example of Example 1 of
[0216] Furthermore, from the viewpoint of obtaining the effects of the present invention more reliably, as shown in the example of Example 1 of
[0217] Furthermore, from the viewpoint of obtaining the effects of the present invention more reliably, as shown in the example of Example 1 of
[0218] Furthermore, from the viewpoint of obtaining the effects of the present invention more reliably, in the catalyst for electrode 20, it is preferable that a porosity measured by using the three-dimensional reconstructed image of STEM (stereoscopic image of interest) is 35% or more, more preferably 40% or more, more preferably 45% or more, furthermore preferably 50% or more, furthermore preferably 55% or more, furthermore preferably 60% or more, furthermore preferably 65% or more. On the other hand, from the viewpoint of durability, in the catalyst for electrode 20, it is preferable that a porosity measured by using the three-dimensional reconstructed image of STEM (stereoscopic image of interest) is 80% or less, more preferably 75% or less.
[0219] Here, a region made of Pt (0 valence) is formed on at least a part of the surface of the catalyst particle 23. Provided that, within the range where the effects of the present invention can be obtained, a Pt oxide layer may be formed on the region of the surface made of Pt (0 valence) of the catalyst particle 23.
[0220] As a more specific structure of the catalyst particle 23, when the catalyst particle 23 is made of Pt (0 valence), not particularly limited, when the catalyst particle 23 is made of a Pt alloy, it is preferable that the catalyst particle 23 is a core-shell catalyst particle.
[0221] When the catalyst particle 23 is made of a Pt alloy, the metal specie other than Pt, which is an element of the alloy is not particularly limited. From the viewpoint of obtaining excellent catalytic activity, it is preferable that the metal species other than Pt, which is an element of the alloy, is at least one metal of Co and Ni.
[0222] When the catalyst particle 23 is the core-shell catalyst particle, from the viewpoint of obtaining excellent catalytic activity, it is preferable that the core-shell catalyst particle is composed of a core particle and a Pt shell layer (a region composed of Pt (0 valence)) formed on at least a part of the surface of the core particle. The metal specie constituting the core particles is not particularly limited, but at least one of Pd, Ni and Co is preferable from the viewpoint of obtaining excellent catalytic activity. Further, the core particle may be an alloy of at least one of Pd, Ni and Co and other metal. From the viewpoint of reducing the amount of precious metal used, the core particle may contain at least one of a base metal other than precious metal and a base metal oxide, a base metal nitride, and a base metal carbide.
[0223] The catalyst for electrode 20 preferably has an average value of crystallite size of 3 to 16.0 nm as measured by powder X-ray diffraction (XRD).
[0224] Further, a Pt supporting ratio of the catalyst for electrode 20 is preferably 5.6 to 66.5 wt %.
[0225] The support 22 preferably satisfies the above conditions (α) and (ß) when used as an catalyst for electrode. From this point of view, as the support 22, a carbon selected among CNovel (manufactured by Toyo Tanso Co., Ltd., product name, registered trademark) which can satisfy the above conditions (α) and (ß) when used as an catalyst for electrode is preferable.
[0226] As shown in
[0227] Usually, this layered structure is produced by heat-treating the carbon materials at a certain temperature or higher. However, in general, since carbon materials shrink during the heat treatment, the pores tend to collapse and the specific surface area tends to decrease, and thus, when crystallinity develops, it is difficult to obtain a porous carbon having a high specific surface area.
[0228] On the other hand, since the support 22 has the nanopore P22 and the carbonaceous wall forming the outlines of the nanopores P22, it can withstand the shrinkage during the heat treatment, and the layered structure is sufficiently formed in the carbonaceous walls, and the specific surface area is sufficiently secured.
[0229] Further, since the carbonaceous wall has a three-dimensional network structure, the support 22 can carry catalyst particles as small as several nanometers in a highly dispersed state, and is suitable as a support for the catalyst layer of the fuel cell. It should be noted that the support 22 does not need to have the layered structure in the entire carbonaceous wall, and may partially have an amorphous portion.
[0230] Further, it is preferable that the support 22 has a specific surface area of 200 m.sup.2/g to 1500 m.sup.2/g. When the specific surface area is 200 m.sup.2/g or more, it becomes easier to form the three-dimensional network structure more reliably. Thereby, the pores can be sufficiently formed, and it becomes easier to have sufficient gas adsorption ability. On the other hand, when the specific surface area is 1500 m.sup.2/g or less, it becomes easier to form the carbonaceous wall more reliably. This makes it easier to sufficiently form the nanopores P22.
[0231] Here, as shown in
[0232] From the viewpoint of having sufficient electrical conductivity, the support 22 preferably has a specific resistance of 10.0×10.sup.2 Ω.Math.cm or less, more preferably 5.0×10.sup.2 Ω.Math.cm or less, and further preferably 1.0×10.sup.2 Ω.Math.cm or less.
[0233] Further, the support 22 may contain micropores having a pore diameter of less than 1 nm (relatively small pores among pores classified as so-called micropores), and micropores having a pore diameter of more than 20 nm to not more than 50 nm (relatively large pores among pores classified as so-called mesopores), within the range where the effects of the present invention can be obtained.
[0234] Furthermore, it is preferable that the support 22 is a porous carbon support having good dispersibility in the gas diffusion electrode forming composition including the catalyst for electrode 20 and excellent electrical conductivity.
[0235] Here, as shown in
[0236] The electrode catalyst 20 satisfies the condition of the following equation (S1) when the electron tomography is measured by 3D-STEM.
100×(N10/N20)≤8.0 (S1)
[0237] Here, in the above equation (S1), N10 is the number of non-contact particles 25 (n101+n102), which is the sum of (I) the number of particles (n101) of noble metal particles that are not in contact with pores having a pore diameter of 1 nm or more that can be confirmed by the electron tomography measurement and (II) the number of noble metal particles (n102) that are not in contact with the porous carbon support 22 itself and exist outside the porous carbon support.
[0238] N20 is the number of particles of the catalyst particles 23 supported inside the nanopores P22 of the support 22.
[0239] The electrode catalyst 20 which satisfies the condition of the equation (S1) has few the non-contact particles 25 (catalyst particle being hard to contribute to the electrode reaction) embedded in the micropores P24 of the support 22 as compared with the conventional electrode catalysts 200 (see
[0240] The catalyst particles 23 supported inside the nanopores P22 of such a support 22 are supported on the support 22 in a state in which these catalyst particles are hardly in direct contact with the polymer electrolyte present in the catalyst layer (catalyst layer 2c or catalyst layer 1c in
[0241] Furthermore, from the viewpoint of obtaining the effects of the present invention more reliably, the value of the equation (S1) [100×(N10/N20)] is preferably 6.5 or less, more preferably 1.0 or less.
[0242] It is preferable that the electrode catalyst 20 further satisfies the condition of the following equation (S2) when the electron tomography is measured by 3D-STEM.
100×{N10/(N20+N30)}≤5.0 (82)
[0243] Here, in the equation (82), N10 is synonymous with N10 in the formula (S1),
[0244] Further, in the equation (S2), N20 is synonymous with N20 in the formula (S1),
[0245] Furthermore, in the equation (S2), N30 is the number of particles of the catalyst particles 23 supported on the outside of the nanopores P22 of the support 22.
[0246] By supporting the catalyst particles on the support 22 so as to simultaneously satisfy the conditions of the above equation (S2), the electrode catalyst 20 of the present invention has few the non-contact particles 25 (catalyst particles) embedded in the micropores P24 of the support 22 as compared with the conventional electrode catalysts 200, and thus there are a relatively large number of highly active catalyst particles 23 are present inside the nanopores P22 of the support 22. And then, the electrode catalyst 20 of the present invention exhibits more reliably excellent catalytic activity. to contribute to the cost reduction of PEFC.
[0247] Here, from the viewpoint of obtaining the effects of the present invention more reliably, the value of the equation (S2) [100×{N0/(N20+N30)}] is preferably 3.0 or less, more preferably 1.0 or less.
[0248] Furthermore, in the electrode catalyst 20, when an analysis of a particle size distribution of the catalyst particles 23 is performed by using a three-dimensional reconstructed image obtained by an electron beam tomography measurement using an STEM (scanning transmission electron microscopy), it is preferable that with respect to the catalyst particles 23 supported inside the nanopore P22 of the support 22, an average distance from the inlet of the nanopores P22 to the supported position of the catalyst particles 23 is of 5.0 nm or more.
[0249] The present inventors consider that the catalyst particles 23 satisfying this condition are supported inside the nanopores P22 of the support 22 and can sufficiently prevent the contact with the polyelectrolyte. Further, it is considered that the catalyst particles 23 satisfying this condition are supported inside the nanopores P22 of the support 22, but exist at an appropriate depth from the inlet of the nanopores, so that the supply of protons, oxygen gas, and hydrogen gas can be relatively obtained.
[0250] From the viewpoint of obtaining the effect of the present invention more reliably, with respect to the catalyst particle 23 supported inside the nanopore P22 of the support 22, the average distance from the inlet of the nanopores P22 to the supported position of the catalyst particles 23 is preferably 5.0 to 8.5 nm, more preferably 5.0 to 5.5 nm. When the supported position of the catalyst particles 23 is set to the range of preferably 8.5 nm or less, more preferably 5.5 nm or less, even in the case that the MEA 10 is able to generate electricity under higher temperature conditions and lower humidification conditions than usual, there is a greater tendency to keep the supply of protons to the catalyst particles 23 inside the nanopores P22.
[0251] Furthermore, in the electrode catalyst 20, when an analysis of a particle size distribution of the catalyst particles 23 is performed by using a three-dimensional reconstructed image obtained by an electron beam tomography measurement using an STEM (scanning transmission electron microscopy), it is more preferable that the catalyst particles 23 supported inside the nanopore P22 of the support 22 exist within a range that a distance from the inlet of the nanopore P22 to the supported position of the catalyst particles 23 of 0 to 27 nm.
[0252] It is considered that the catalyst particles 23 satisfying this condition are supported inside the nanopores P22 of the support 22, but exist at an appropriate depth from the inlet of the nanopores P22, and the effect of obtaining a sufficient supply of reaction gases and protons can be obtained more reliably, while avoiding contact with the above-mentioned polymer electrolyte.
[0253] From the same point of view, it is further preferable that the catalyst particles 23 supported inside the nanopore P22 of the support 22 exist within a range that a distance from the inlet of the nanopore P22 to the supported position of the catalyst particles 23 of 0 to 18 nm.
[0254] Further, in the electrode catalysts 20, when an analysis of a particle size distribution of the catalyst particles 23 is performed by using a three-dimensional reconstructed image obtained by an electron beam tomography measurement using an STEM (scanning transmission electron microscopy), it is preferable that a particle size of the catalyst particles 23 supported inside the nanopore P22 of the support 22 is more than 0 nm and 7 nm or less.
[0255] It is considered that, since the catalyst particles 23 satisfying this condition have an appropriate particle size to give a sufficient reaction surface area, the electrode reaction can be sufficiently promoted even if the particles are supported inside the nanopore P22 of the support 22.
[0256] Further, in the electrode catalysts 20, when an analysis of a particle size distribution of the catalyst particles 23 is performed by using a three-dimensional reconstructed image obtained by an electron beam tomography measurement using an STEM (scanning transmission electron microscopy), it is preferable that a ratio of the catalyst particles 23 supported inside the nanopore P22 is 50% or more, more preferably 70% or more.
[0257] By supporting the catalyst particles 23 on the support 22 so as to satisfy the above conditions, a large number of the highly active catalyst particles 23 having a relatively small particle size exist inside the nanopores P22 of the support 22 as compared with the conventional electrode catalysts.
[0258] The catalyst particles 23 supported inside the nanopores P22 of the support 22 are supported on the support in a state in which these catalyst particles are hardly in direct contact with the polymer electrolyte present in the catalyst layer (catalyst layer 1c or catalyst layer 2c). Therefore, the electrode catalyst 20 reduces the decrease in catalytic activity due to poisoning of the Pt component and can exhibit an excellent catalytic activity when made into an electrode as compared with a conventional electrode catalyst 200. In addition, the electrode catalyst 20 also reduces the dissolution of the Pt component from the catalyst particles 23.
[0259] The method for producing the catalyst for electrode 20 is not particularly limited and can be produced by a known method, except that it includes a “support pretreatment step”, a “Pt addition step”, and a “reduction step” for satisfying the conditions of the equation (1), the equation (2) and the aforementioned other conditions.
[0260] In the support pretreatment step, the support 22 is put into an ultrapure water, and a pH adjuster is further added to prepare a dispersion whose pH is adjusted to 2 to 5. Furthermore, the temperature of the dispersion is kept at 80 to 99° C., preferably 90 to 99° C. for a predetermined time while stirring (however, the state of not boiling is maintained). Then, the temperature of the dispersion is lowered to room temperature.
[0261] Thus, the gas inside the nanopore P22 of the support 22 is removed, so that ultrapure water can sufficiently enter into inside the nanopore P22. Then, in the subsequent “Pt addition step”, the Pt raw materials are sufficiently held inside the nanopore P22 of the support 22. Thus, a large number of precursors of the Pt catalyst particle are supported inside the nanopore P22 of the support 22.
[0262] Note that “ultrapure water” used as the preparation of the aqueous solution in this support pretreatment step is water in which the specific resistance R (reciprocal of the electric conductivity measured by the JIS standard test method (JIS K0552)) represented by the following the equation (4) is 3.0 M Ω.Math.cm or more. In addition, it is preferable that “ultrapure water” has a quality equivalent to “A3” defined in JISK0557 “Water used for testing of water and waste” or higher than that.
[0263] This ultrapure water is not particularly limited as long as it has an electric conductivity satisfying the relation represented by the following equation (4). For example, ultrapure water produced using an ultrapure water producing apparatus “Milli-Q Series” (manufactured by Merck Co., Ltd.) and “Elix UV Series” (manufactured by Nippon Millipore Co., Ltd.) can be mentioned as the above ultrapure water.
R=1/ρ (4)
[0264] In the above equation (4), R represents a specific resistance, and p represents an electric conductivity measured by a JIS standard test method (JIS K0552).
[0265] The next step of the “support pretreatment step” is the “Pt addition step”. In this “Pt addition step”, an aqueous solution of a water-soluble Pt salt (N.E.CHEMCAT, trade name “A-salt” (concentration of Fe component: 8 ppm or less)) dissolved in ultrapure water is added to the dispersion liquid of the support 22 obtained through the “support pretreatment step” at room temperature.
[0266] The next step of the “Pt addition step” is the “reduction step”. In this “reduction step”, the temperature of the liquid obtained through the “Pt addition step” is raised to 50° C. or higher, and an aqueous solution in which a water-soluble reducing agent (preferably an acidic water-soluble reducing agent) is dissolved is added. After the addition of the reducing agent, the liquid temperature is maintained at 50° C. or higher for a predetermined period of time to allow the reduction reaction to proceed, and then the temperature of the liquid is lowered to room temperature.
[0267] The next step of the “reduction step” is the “washing step”. In this “washing step”, the solid component and the liquid component in the liquid obtained through the “reduction step” are separated, and the solid content (a mixture of a Pt/C catalyst and other impurities) is washed. For example, the solid component in the liquid obtained through the “reduction step” may be separated from the liquid component by using a filtering means such as filter paper or a filter cloth. The solid content may be washed with the above-mentioned ultrapure water, a pure water (specific resistance R represented by the above equation (4) is 0.1 MΩ.Math.cm or more and less than 3.0 MΩ.Math.cm), or a pure warm water (temperature of pure water being 40 to 80° C.) may be used. For example, when the pure warm water is used, the filtrate is washed repeatedly until the electrical conductivity after washing becomes less than 10 μS/cm.
[0268] The next step after the “washing step” is the “drying step”. In this “drying step”, water is separated from the solid component (mixture of Pt/C catalyst and water) obtained through the “washing step”. First, the solid component is air-dried, and then dried in a dryer at a predetermined temperature for a predetermined time.
[0269] The next step after the “drying step” is the “crushing step”. In this “crushing step”, the solid component (Pt/C catalyst) obtained from the “drying step” is crushed to the catalyst powder with a crushing means such as a mixer.
[0270] The polymer electrolyte contained in the catalyst layer 1c and the catalyst layer 2c is not particularly limited as long as it has hydrogen ion conductivity, and known ones can be used. For example, the polymer electrolyte can exemplify a known perfluorocarbon resin having a sulfonic acid group and a carboxylic acid group. Examples of easily available polymer electrolytes having hydrogen ion conductivity include Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), and Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.).
[0271] Then, at least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2 shown in
[0272] (Gas Diffusion Layer (GDL))
[0273] The gas diffusion layer 1gd provided in the cathode 1 shown in
[0274] In addition, the gas diffusion layer 2gd provided in the anode 2 is a layer provided for supplying a reducing agent gas (e.g., hydrogen gas) to the catalyst layer 2c. And, the gas diffusion layer 2gd serves to support the catalyst layer 2c.
[0275] The gas diffusion layer (1gd) shown in
[0276] The member which can be used for the gas diffusion layer (1gd) is not particularly limited, and a known member can be used. For example, preferably, there are exemplified carbon paper and other material, in which carbon paper is used as a main material and auxiliary materials including carbon powder, ion exchange water, and a polyethylene terephthalate dispersion as a binder is applied on the carbon paper.
[0277] (Water Repellent Layer (MPL))
[0278] As shown in
[0279] (Polymer Electrolyte Membrane (PEM))
[0280] The polymer electrolyte membrane (PEM) 3 shown in
[0281] <Modified Embodiment of MEA>
[0282] While a preferred embodiment of the MEA of the present invention (and the catalyst layer of the present invention, the gas diffusion electrode of the present invention) is described above, the MEA of the present invention is not limited to the configuration of the MEA 10 shown in
[0283] For example, the MEA of the present invention may have the configuration of the MEA 11 shown in
[0284]
[0285] <Membrane-Electrode Assembly (CCM)>
[0286] Next, a preferred embodiment of the membrane-electrode assembly (CCM: Catalyst Coated Membrane) of the present invention will be described.
[0287]
[0288] <Modified Embodiment of Membrane-Electrode Assembly (CCM)>
[0289] While a preferred embodiment of the CCM of the present invention has been described above, the CCM of the present invention is not limited to the configuration of the CCM 12 shown in
[0290] For example, the CCM of the present invention may have a configuration of the CCM 13 shown in
[0291]
[0292] <Gas Diffusion Electrode (GDE)>
[0293] Next, a preferred embodiment of the gas diffusion electrode (GDE) of the present invention will be described.
[0294]
[0295] <Modified Embodiment of Gas Diffusion Electrode (GDE)>
[0296] While a preferred embodiment of the GDE of the present invention has been described above, the GDE of the present invention is not limited to the configuration of the GDE 1B shown in
[0297] For example, the GDE of the present invention may have the composition of GDE 1C shown in
[0298]
[0299] <Composition for Forming Catalyst Layer>
[0300] Next, a preferred embodiment of the composition for forming catalyst layer of the present invention will be described.
[0301] A composition for forming catalyst layer of the present embodiment includes the catalyst for electrode 20, a polymer electrolyte, and a main component, and has a mass ratio N/C of mass N of polymer electrolyte to mass C of support 22 of the catalyst for electrode 20 of 0.5 to 1.2, more preferably 0.7 to 1.0.
[0302] Here, the composition of the liquid including the polymer electrolyte is not particularly limited. For example, a liquid including a polymer electrolyte may contain a polymer electrolyte having hydrogen ion conductivity described above, water, and an alcohol.
[0303] The composition ratio of the catalyst for electrode 20, the polymer electrolyte, and other components (water, alcohol, and the like) included in the composition for forming catalyst layer is appropriately set so that the dispersion state of the catalyst for electrode 20 in the obtained catalyst layer becomes good and the power generation performance of the MEA 10 including the catalyst layer can be improved.
[0304] The composition for forming catalyst layer can be prepared by mixing a liquid including the catalyst for electrode 20 and the polymer electrolyte and stirring the mixture. From the viewpoint of adjusting applicability, a polyhydric alcohol such as glycerin and/or water may be contained. When the liquid including the catalyst for electrode 20, the polymer electrolyte is mixed, a pulverizing and mixing machine such as a ball mill, an ultrasonic disperser and the like can be used.
[0305] At least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2 shown in
[0306] (Method for Producing Gas Diffusion Electrode)
[0307] Next, an example of a method of producing gas diffusion electrode of the present invention will be described. It is sufficient that the gas diffusion electrode is formed so as to include the catalyst layer of the present invention, and a known method can be employed for the producing method. It can be more reliably produced by using the composition for forming catalyst layer of the present invention.
[0308] For example, it may be produced by coating a composition for forming catalyst layer on a gas diffusion layer (or a water repellent layer of a laminate in which a water repellent layer is formed on a gas diffusion layer) and drying the composition.
[0309] <Fuel Cell Stack>
[0310]
[0311] The fuel cell stack 30 illustrated in
EXAMPLE
[0312] The present invention is further illustrated by the following examples, which are not intended to limit the present invention.
[0313] (I) Preparation of the catalyst for electrode to be used for the catalyst layer of the cathode of MEA
[0314] (1) Production of Pt/C Catalyst to be Used for the Cathode of MEA of Example 1
[0315] [Pt Catalyst Particle-Supported Carbon Catalyst “Pt/C Catalyst” Powder]
[0316] Powder of Pt/C catalyst powder in which catalyst particles made of Pt are supported on the following support {Pt loading ratio 48.0 wt %, trade name “SA50BM-A207”, manufactured by N.E. CHEMCAT)} was prepared.
[0317] The powder of this Pt/C catalyst (hereinafter, referred to as “Pt/C catalyst A” if necessary) was prepared in the following procedures.
[0318] (First Step (Support Pretreatment Step))
[0319] A dispersion liquid, in which a trial sample product “CNovel A” manufactured by Toyo Tanso Co., Ltd. as the porous carbon support (BET specific surface area: 1200 m.sup.2/g) was dispersed in the aqueous solution adjusted to pH=2 to 5 (prepared by adding a pH adjuster to ultrapure water), was held at 90 to 99° C. for about 0.5 hours while stirring (although a not boiled state was retained).
[0320] Note that “ultrapure water” used in this first step (support pretreatment step) was a water having a specific resistance R (reciprocal of electric conductivity measured by a JIS standard test method (JIS K0552)) represented by the following equation (4) of 3.0 MΩ.Math.cm or more. In addition, ultrapure water had a water quality equivalent to or higher than that of A3 specified in JISK0557 Water for Testing Water and Wastewater.
[0321] This ultrapure water was produced using an ultrapure water producing apparatus “Milli-Q Series” (manufactured by Merck Co., Ltd.) and “Elix UV Series” (manufactured by Nippon Millipore Co., Ltd.).
R=1/ρ (4)
[0322] In the above general equation (4), R represents a specific resistance, and ρ represents an electric conductivity measured by a JIS standard test method (JIS K0552).
[0323] (Second Step (Pt Addition Step))
[0324] After preparing a mixed solution by adding an aqueous solution of a water-soluble Pt salt (N.E.CHEMCAT, trade name “A-salt” (concentration of Fe component: 8 ppm or less)) in ultrapure water to the dispersion obtained through the first step, the pH was adjusted to 7 to 12, and the mixture was stirred while maintaining a predetermined temperature of 50° C. or higher for a predetermined time.
[0325] (Third Step (Reduction Step))
[0326] By adding an aqueous solution in which an acidic water-soluble reducing agent is dissolved to the liquid obtained through the second step, the Pt ions in the mixed liquid were reduced to obtain the Pt catalyst particle-supporting carbon “Pt/C” powder.
[0327] (Fourth Step (Washing Step))
[0328] By using a filter paper, the solid component and the liquid component in the liquid obtained through the “third step” were separated. Next, the solid content (a mixture of the Pt/C catalyst and other impurities) remaining on the filter paper was washed with the above-mentioned pure water and pure warm water. First, washing with pure water was performed. This washing was repeated until the electric conductivity of the filtrate after washing became less than 20 μS/cm. Next, washing with pure warm water was performed. This washing was repeated until the electric conductivity of the filtrate after washing became less than 10 μS/cm.
[0329] (Fifth Step (Drying Step))
[0330] The solid component (mixture of Pt/C catalyst and water) on the filter paper obtained through the “fourth step” was air-dried in the air in this state. After this air drying, the solid component on the filter paper was transferred to a magnetic dish and dried in an electric dryer at a predetermined temperature of 60° C. or higher for a predetermined time.
[0331] (Sixth Step (Crushing Step))
[0332] The solid component (Pt/C catalyst) obtained in the “fifth step” was crushed by using a mixer to obtain a powder of Pt/C catalyst A.
[0333] <Measurement of Loadingsupporting Ratio (ICP Analysis)>
[0334] For this Pt/C catalyst A, the Pt loading supporting ratio (wt %) was determined by the following methods.
[0335] The Pt/C catalyst A was immersed in aqua regia to dissolve the metal. The carbon as the insoluble component was then removed from the aqua regia. Next, the aqua regia from which the carbon was removed was analyzed by ICP.
[0336] As a result of the ICP analysis, this Pt/C catalyst A had the Pt loading ratio of 48.0 wt %.
[0337] <Surface Observation/Structure Observation of Catalyst for Electrode>
[0338] In order to observe the three-dimensional structure of this Pt/C catalyst A of Example 1, the electron tomography measurement was carried out by using the “USAL-KM3D analysis method” with the STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center Co., Ltd.
[0339] The electron tomography measurement with the STEM (scanning transmission electron microscope) was carried out according to the sample to be measured preparation method, the conditions and their analytical procedures, the conditions of (A) to (C) and (D) to (G), described above. More detailed information is described hereinbelow. [0340] STEM apparatus: JEM-ARM200F Atomic Resolution Analytical Electron Microscopy Made by JEOL [0341] Data analysis software: 3D reconfiguration software Composer, 3D data visualization software Visualizer-kai by System Infrontia, image analysis software Colorist
[0342] Measurement Conditions [0343] Acceleration voltage: 60 kV [0344] Observation magnification 800,000 to 1,000,000 times [0345] Tilt angle of the sample to be measured: −80° to +80° [0346] Tilt step angle of the sample to be measured: 2° [0347] Pixel Count 512×512 pixels 512×512 pixels [0348] Pixel size: 0.350 to 0.500 nm/pixel [0349] Volume Size: as shown in
[0350] With respect to the Pt/C catalyst A, by image analysis of a three-dimensional reconstructed image (3D-STEM image) obtained by electron beam tomography (electron tomography) measurement using an STEM (scanning transmission electron microscopy), the Pt catalyst particles (hereinafter, inner particles) present inside the carbon support and the Pt catalyst particles (hereinafter, outer particles) present on the surface portion of the carbon support were separated, and the particle size distribution of the Pt catalyst in each region was calculated.
[0351] A three-dimensional reconstructed image (3D-STEM image) of the Pt/C catalyst A is shown in
[0352] A graph showing the distribution state of Pt catalyst particle located outside and inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst of Example 1 shown in
[0353] A graph showing the distribution state of Pt catalyst particle located inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst of Example 1 shown in
[0354] A graph showing the distribution state of Pt catalyst particle located inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst of Example 1 shown in
[0355] The 3D-STEM image was obtained by reconstructing a plurality of two-dimensional STEM images obtained by stepwise tilting the sample stage under the above measuring conditions.
[0356] Further, the image analysis (particle size analysis) of three-dimensional reconstructed image (3D-STEM image) was carried out by the following procedures. The regions of the catalytic particles were first selected from the three-dimensional reconstructed images, and the respective catalytic particles were labeled (not shown). Next, the volume of the labeled Pt catalyst particles was obtained, the diameter of a sphere having the same volume as this volume (the equivalent diameter of the sphere) was calculated, and the particle size distribution (
[0357] Here, the sphere equivalent diameter was calculated by rounding up the value below the decimal point (value below 1 nm) using the unit of nm.
[0358] With respect to this Pt/C catalyst A, the ratio of the catalyst particles supported inside the nanopores of the support and the ratio of the catalyst particles supported outside the nanopores of the support were determined. The values of N10, N20, N30 were also obtained. The results are shown in Table 1 and Table 2.
[0359] In the electron tomography measurement of the electrode catalyst of Example 1, the presence of the noble metal particles of (II) described above was not confirmed. That is, there was no noble metal particle that was not in contact with the porous carbon support itself and existed outside the support (n102=0).
[0360] Furthermore, the mean value of the particle size of the catalyst particles of the Pt/C catalyst A measured from the STEM image was 3.1 nm (the average value of the particle size of the catalyst particles inside the nanopores: 3.1 nm, the average value of the particle size of the catalyst particles outside the nanopores: 3.2 nm).
[0361] (2) Production of the Pt/C Catalyst to be Used for the Cathode of the MEA of Example 2
[0362] [Pt Catalyst Particle-Supported Carbon Catalyst “Pt/C Catalyst” Powder]
[0363] The powder {Pt loading ratio: 48.0 wt %, trade name “SA50BM-B237”, manufactured by N.E.CHEMCAT} of the Pt/C catalyst used for the cathode of the MEA of Example 2 (hereinafter, referred to as “Pt/C catalyst B” if necessary) was prepared under the same conditions and procedures as the Pt/C catalyst A used for the cathode of the MEA of Example 1, except that a trial sample product “CNovel B” manufactured by Toyo Tanso Co., Ltd. as the porous carbon support (BET specific surface area: 800 m.sup.2/g)” was used as a porous carbon support.
[0364] <Surface Observation/Structure Observation of Catalyst for Electrode>
[0365] In order to observe the three-dimensional structure of this Pt/C catalyst B of Example 2, in the same method and conditions as the Pt/C catalyst of Example 1, the electron tomography measurement was carried out by using the “USAL-KM3D analysis method” with the STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center Co., Ltd.
[0366] An STEM image showing 3D-electron beam tomography measurement conditions (volume size) using an STEM of the catalyst for electrode of Example 2 is shown in
[0367] A 3D-STEM image (three-dimensional reconstructed image) of the catalyst for electrode of Example 2 is shown in
[0368] A graph showing the distribution state of Pt catalyst particle located outside and inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst for electrode of Example 2 shown in
[0369] A graph showing the distribution state of Pt catalyst particle located inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst for electrode of Example 2 in
[0370] A graph showing the distribution state of Pt catalyst particle located inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst for electrode of Example 2 shown in
[0371] For this catalyst for electrode (Pt/C catalyst B), the ratio of the catalyst particles supported inside the nanopores of the support and the ratio of the catalyst particles supported outside the nanopores of the support were determined. The values of N10, N20, N30 were also obtained. The results are shown in Table 1 and Table 2.
[0372] In the electron tomography measurement of the electrode catalyst of Example 2, the presence of the noble metal particles of (II) described above was not confirmed. That is, there was no noble metal particle that was not in contact with the porous carbon support itself and existed outside the support (n102=0).
[0373] Furthermore, the mean value of the particle size of the catalyst particle of the catalyst for electrode (Pt/C catalyst B) measured from the STEM image was 3.3 nm (the average value of the particle size of the catalyst particles inside the nanopores: 3.2 nm, the average value of the particle size of the catalyst particles outside the nanopores: 3.7 nm).
[0374] (3) Preparation of Pt/C Catalyst Powder Used for the Cathode of MEA of Comparative Example 1
[0375] As a Pt/C catalyst, a Pt/C catalyst manufactured by N. E. CHEMCAT with a Pt loading ratio of 50 wt % (trade name: “SA50BK”) was prepared. As the support of this Pt/C catalyst, a commercially available porous carbon support {manufactured by Lion Co., Ltd., trade name “Carbon ECP” (registered trademark) (Ketchen Black EC300J), specific surface area of 750 to 800 m.sup.2/g} was used.
[0376] <Surface Observation/Structure Observation of Catalyst for Electrode>
[0377] In order to observe the three-dimensional structure of this Pt/C catalyst of Comparative Example 1, in the same method and conditions as the Pt/C catalyst of Example 1.
[0378] The electron tomography measurement was carried out by using the “USAL-KM3D analysis method” with the STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center Co., Ltd.
[0379] An STEM image showing 3D-electron beam tomography (electron tomography) measurement conditions (volume size) using an STEM of the Pt/C catalyst of Comparative Example 1 is shown in
[0380] A 3D-STEM image (three-dimensional reconstructed image) of the Pt/C catalyst of Comparative Example 1 is shown in
[0381] A graph showing the distribution state of Pt catalyst particle located outside and inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst of Comparative Example 1 shown in
[0382] A graph showing the distribution state of Pt catalyst particle located inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst of Comparative Example 1 shown in
[0383] A graph showing the distribution state of Pt catalyst particle located inside nanopore of carbon support in the pore depth direction catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst of Comparative Example 1 shown in
[0384] With respect to this Pt/C catalyst, the ratio of the catalyst particles supported inside the nanopores of the support and the ratio of the catalyst particles supported outside the nanopores of the support were determined. The values of N10, N20, N30 were also obtained. The results are shown in Table 1 and Table 2.
[0385] In the electron tomography measurement of the electrode catalyst of Example 1, the presence of the noble metal particles of (II) described above was not confirmed. That is, there was no noble metal particle that was not in contact with the porous carbon support itself and existed outside the support (n102=0).
[0386] Furthermore, the mean value of the particle size of the catalyst particles of the Pt/C catalyst measured from the STEM image was 3.1 nm (the average value of the particle size of the catalyst particles inside the nanopores 3.1 nm, the average value of the particle size of the catalyst particles outside the nanopores: 3.2 nm).
[0387] (4) Production of the Pt/C Catalyst to be Used for the Cathode of the MEA of Comparative Example 2
[0388] [Pt catalyst particle-supported carbon catalyst “Pt/C catalyst” powder]
[0389] The powder {Pt loading ratio: 48.0 wt %, trade name “SA50BM-C207”, manufactured by N.E.CHEMCAT} of the Pt/C catalyst used for the cathode of the MEA of Example 2 (hereinafter, referred to as “Pt/C catalyst C”, if necessary) was prepared under the same conditions and procedures as the Pt/C catalyst A used for the cathode of the MEA of Example 1, except that a trial sample product “CNovel C” manufactured by Toyo Tanso Co., Ltd. as the porous carbon support (BET specific surface area: 800 m.sup.2/g)” was used as a porous carbon support.
[0390] <Surface Observation/Structure Observation of Catalyst for Electrode>
[0391] In order to observe the three-dimensional structure of this Pt/C catalyst C of Comparative Example 2, in the same method and conditions as the Pt/C catalyst of Example 1, the electron tomography measurement was carried out by using the “USAL-KM3D analysis method” with the STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center Co., Ltd.
[0392] Though with respect to this catalyst for electrode (Pt/C catalyst C) of Comparative Example 2, the information of the measurement of electron beam tomography by using the STEM (scanning transmission electron microscope) and the analysis results thereof were obtained as in Example 1 and Example 2 and Comparative Example 1, but the illustration is omitted.
[0393]
[0394] For this catalyst for electrode (Pt/C catalyst C) of Comparative Example 2, the ratio of the catalyst particles supported inside the nanopores of the support and the ratio of the catalyst particles supported outside the nanopores of the support were determined. The values of D10, D20, D1, D2, N1, and N2 were also obtained. The results are shown in Table 2 and Table 3.
[0395] Furthermore, the mean value of the particle size of the catalyst particles of the catalyst for electrode (Pt/C catalyst C) measured from the STEM image was 3.2 nm (the average value of the particle size of the catalyst particles inside the nanopores: 2.9 nm, the average value of the particle size of the catalyst particles outside the nanopores 3.5 nm).
[0396] (5) Confirmation of the Microstructure of the Catalyst for Electrodes of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 by Using the Three-Dimensional Reconstructed Image of STEM
[0397] With respect to the catalyst for electrodes of Example 1, Example 2 and Comparative Example 2, in order to confirm “whether or not the nanopore is formed so as to have the shape of the intercommunicating pore according to the present invention”, which is the aforementioned condition (ß), the study by using the three-dimensional reconstructed image of STEM was performed. Further, the porosity of each catalyst was determined by using the three-dimensional reconstructed image of STEM.
[0398] The results are shown in
[0399] As shown in Fid. 27(c) and
[0400] Furthermore, as shown in
[0401] Furthermore, as shown in
[0402] Further, as shown in
[0403] Furthermore, as shown in
[0404] Further, as shown in
[0405] As shown in
[0406] Furthermore, as shown in
[0407] On the other hand, as shown in
[0408] (II) Preparation of P/C Catalysts Used for the Anodes of the MEA of Example 1, Example 2, Comparative Example 1 and Comparative Example 2
[0409] The same Pt/C catalyst as the Pt/C catalyst used for the cathode of the MEA of Comparative Example 1 was used as the P/C catalyst used in the anodes of the MEA of Example 1 and Example 2, Comparative Example 1 and Comparative Example 2.
Example 1
[0410] In the following procedures, an MEA with the same configuration as the MEA 10 shown in
[0411] (1) Production of the Cathode
[0412] Cathode GDL
[0413] Carbon paper (trade name “TGP-H-60” manufactured by Toray Co., Ltd) was prepared as the GDL.
[0414] Ink for Forming Cathode MPL
[0415] Into a ball mill container made of Teflon (registered trademark) in which balls made of Teflon (registered trademark) were added, 1.5 g of carbon powder (trade name “Denkablack” manufactured by Electrochemical Industry Co., Ltd.), 1.1 g of ion-exchanged water, and 6.0 g of a surfactant (trade name “Triton” (35 wt % water solution) manufactured by Dow chemical Co., Ltd.) were charged and mixed.
[0416] Next, 1.75 g of polytetrafluoroethylene (PTFE) dispersion (trade name “31-JR” manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) was put into the ball mill container and mixed. Thus, an ink for forming cathode MPL was produced.
[0417] Cathode MPL
[0418] On one side of the GDL, an ink for forming cathode MPL was applied using a barcoater to form a coating film. Thereafter, the coating film was sufficiently dried in a dryer, and further subjected to a heat and pressure bonding treatment to prepare a laminate in which the MPL was formed on the GDL.
[0419] Ink for Forming Cathode Catalyst Layer
[0420] Into a ball mill container made of Teflon (registered trademark) containing a ball made of Teflon (registered trademark), the above-mentioned Pt/C catalyst A, ion-exchanged water, a 10 wt % Nafion aqueous dispersion (trade name “DE1021CS” manufactured by DuPont Co., Ltd.) and glycerin were charged and mixed to prepare an ink for forming cathode catalyst layer. Note that this ink was adjusted to have a N/C=0.7. Further, the core-shell catalyst A was adjusted to have carbon:ion-exchanged water:glycerin=1:10:0.8 (mass ratio).
[0421] Cathode Catalyst Layer (CL)
[0422] An ink for forming cathode catalyst layer described above was applied to the surface of the MPL of a laminate in which MPL was formed on MPL on the GDL described above by a bar coating method to form a coating film. This coating film was dried at room temperature for 30 minutes, and then dried at 60° C. for 1.0 hours to obtain a catalyst layer. In this way, a cathode which is a gas diffusion electrode was prepared. Note that the supporting amount of Pt supported on the catalyst layer of the cathode was set to be a numerical value shown in Table 1.
[0423] (2) Production of Anode
[0424] Anode GDL
[0425] As the GDL, carbon paper identical to that of the cathode was prepared.
[0426] Ink for Forming Cathode MPL
[0427] Into a ball mill container made of Teflon (registered trademark) in which balls made of Teflon (registered trademark) were added, 1.5 g of carbon powder (trade name “Denka black” manufactured by Electrochemical Industry Co., Ltd.), 1.0 g of ion-exchanged water, and 6.0 g of a surfactant (trade name “Triton” (35 wt % water solution) manufactured by Dow chemical Co., Ltd.) were charged and mixed.
[0428] Next, 2.5 g of a polytetrafluoroethylene (PTFE) dispersion (trade name “31-JR” manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) was charged into the ball mill container and mixed. Thus, an ink for forming anode MPL was produced.
[0429] Anode MPL
[0430] The ink for forming anode MPL was applied to one side of the GDL using a barcoater to form a coating film. Thereafter, the coating film was sufficiently dried in a dryer, and further subjected to a heat and pressure bonding treatment to produce a laminate in which MPL was formed on the GDL.
[0431] Ink for Forming Anode Catalyst Layer
[0432] Into a ball mill container made of Teflon (registered trademark) in which balls made of Teflon (registered trademark) were added, SA50BK (Pt loading ratio 50 wt %), ion-exchange water, 5 wt % Nafion alcohol dispersion (trade name “Nafion” 5 wt % dispersion, product number 274704, manufactured by SIGMA-ALDRICH's) and glycerin were charged and mixed to prepare an ink for forming anode catalyst layer. Note that this ink was adjusted to have N/C=1.2. Further, SA50BK was adjusted to have carbon:ion-exchanged water:glycerin=1:6:4 (mass ratio).
[0433] Anode Catalyst Layer (CL)
[0434] An ink for forming anode catalyst layer described above was applied to the surface of an MPL of a laminate in which MPL was formed on MPL on the GDL described above by a bar coating method to form a coating film. This coating film was dried at room temperature for 30 minutes, and then dried at 60° C. for 1.0 hours to obtain a catalyst layer. In this way, an anode which is a gas diffusion electrode was produced. Note that the Pt supporting amount of the catalyst layer of the anode was set as a 0.3 mg/cm.sup.2.
[0435] (3) Production of MEA
[0436] A polymer electrolyte membrane (trade name “Nafion NR212” manufactured by DuPont Co., Ltd.) was prepared. Alaminate in which this polymer electrolyte membrane was arranged between the cathode and the anode was produced, and heated and pressed by a hot pressing machine to produce an MEA. Incidentally, the hot pressing was carried out with the conditions of 140° C. at 5 KN for 5 minutes and, further, 140° C. at 25 KN for 3 minutes.
Example 2
[0437] Each MEA was prepared under the same conditions and procedures as in Example 1, except that the following conditions were changed for the catalyst layer of the cathode.
[0438] That is, in preparing the ink for forming the catalyst layer of the cathode, [0439] the aforementioned Pt/C catalyst B was used instead of the Pt/C catalyst A.
Comparative Example 1
[0440] Each MEA was produced under the same conditions and procedures as in Example 1, except that the following conditions were changed with respect to the cathode catalyst layer.
[0441] That is, in the preparation of the ink for forming cathode catalyst layer, [0442] the previously described P/C catalyst (trade name: “SA-50BK”) was used instead of the Pt/C catalyst A, [0443] a 5 wt % Nafion alcohol dispersion (trade name “DE520CS”; containing 48 wt % of 1-propanol manufactured by DuPont Co., Ltd.) was used instead of 10 wt % Nafion aqueous dispersion, [0444] the composition of the ink for forming cathode catalyst layer and the applying conditions of the ink were adjusted so that the Pt supported amount and the N/C had the numerical values shown in Table 1. [0445] carbon:ion-exchanged water:glycerin=1:10:1 (mass ratio) in the P/C catalyst (trade name: “SA50BH”).
Comparative Example 2
[0446] Each MEA was prepared under the same conditions and procedures as in Example 1, except that the following conditions were changed for the catalyst layer of the cathode.
[0447] That is, in preparing the ink for forming the catalyst layer of the cathode, [0448] the aforementioned Pt/C catalyst C was used instead of the Pt/C catalyst A.
[0449] <Cell Performance Evaluation>
[0450] The cell performance of the MEA of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 was carried out by the following cell performance evaluation method.
[0451] The MEA of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 were installed in a fuel cell unit cell evaluation device.
[0452] Next, the power generation reaction in the MEA was allowed to proceed under the following conditions.
[0453] The temperature of the unit cell (MEA) was set to 80° C. The anode was supplied with pure hydrogen humidified with saturated water vapor of 1.0 atm by adjusting the flow rate so that the utilization rate was 70%. Further, the cathode was supplied with pure oxygen humidified with saturated water vapor of 1.0 atm at 80° C. by adjusting the flow rate so that the utilization rate was 50%.
[0454] Evaluation of the unit cells (MEA) was performed by controlling the current by an electronic supporting device attached to the fuel cell unit cell evaluation device, and the current-voltage curves obtained by scanning the current values from 0 to 1.0 A/cm.sup.2 were acquired as data.
[0455] The X-axis (current density) from the data of the current-voltage curves was plotted as a logarithmic scale to obtain a graph (not shown), and a current density value at a voltage 850 mV (current value per unit area of the electrode) was obtained.
[0456] By dividing the current density value thus obtained by the platinum weight per unit area of the cathode, it was calculated as the activity per unit weight (Mass.Math.Act.) for platinum contained in the cathode, and was used as an indicator of the oxygen reduction ability of the catalyst contained in the cathode. The results are shown in Table 1.
[0457] In Table 1, a result of comparing Mass.Math.Act. obtained in the other examples as a relative value (relative ratio) using Mass.Math.Act. obtained in Comparative Example 1 as a reference (1.0) is shown.
TABLE-US-00001 Ratio of catalyst Structure of Carrier of particles supported BET specific catalyst for catalyst for inside nanopore of Relative Pt supporting Pt supporting surface area electrode of electrode of carrier of catalyst for value of Mass. amount of amount of of carrier cathode cathode electrode of cathode/% Act. @850 mV cathode g/cm2 anode g/cm2 m2/g Porosity % Ex. 1 Pt/C CNovel A 52 1.8 0.10 0.30 1300 68 Ex. 1 Pt/C CNovel B 80 1.5 0.10 0.30 800 68 Com. Pt/C Carbon ECP 43 1.0 0.10 0.30 800 Not Ex.1 measured Com. Pt/C CNovel C 46 1.1 0.10 0.30 1200 33 Ex.2
TABLE-US-00002 TABLE 2 Ratio of [non-contact particle] to Average distance from inlet of catalyst particle supported inside Ratio of [non-contact particle] to all nanopore of electrode catalyst Nanopore of electrode catalyst catalyst particles supported on carrier to sopporting position of carrier electrode catalyst carrier catalyst particle (N10/N20)/% [N10/(N20 + N30)]/% /nm Ex. 1 0.7 0.4 5.4 Ex. 2 6.1 4.9 8.3 Com. Ex. 1 8.3 3.6 5.6 Com. Ex. 2 59.0 17.0 8.1
[0458] From the results shown in Table 1 and Table 2, it was clarified that the MEA of Example and Example 2 has a high Pt mass activity compared with the MEA of Comparative Example 1 and Comparative Example 2.
[0459] In the above, in the present examples and comparative examples, there have been studied the embodiments where the catalyst particle is the catalyst particle of the most simple Pt. However, the characteristics of the catalyst for electrode of the present invention is that, when the microstructure is observed by using a three-dimensional reconstructed image of STEM, the microstructure has the stereoscopic structure which satisfies the aforementioned condition (α) of the equation (S1) (relating the conditions of the supporting position of the catalyst particle which constitutes the catalyst for electrode, and the number of the catalyst particle at the supporting position), and the condition (ß) that the nanopores are formed to have the shape of the intercommunicating pores described above (condition of microstructure of the nanopore of the support which composes the catalyst for electrode). Therefore, it is clear that similar results can be obtained by changing the chemical constituents of the catalyst particles. That is, as long as the stereoscopic structure has the characteristics of the present invention, when a Pt alloy particle containing Pt or core-shell particle having a Pt shell layer is used as the catalyst particle in the same manner as the particle made of Pt, it is clear that the excellent Pt mass activity similar to that of the aforementioned examples can be realized.
INDUSTRIAL APPLICABILITY
[0460] The catalyst for electrode of the present invention exhibits excellent catalytic activity. In addition, the GDE, CCM, MEA, and fuel cell stack including the catalyst layer of the present invention exhibit excellent cell properties that can contribute to cost reduction of PEFC.
[0461] Therefore, the present invention can be applied not only to the electrical equipment industry such as a fuel cell, a fuel cell vehicle and a portable mobile but also to ENE-FARM, a cogeneration system and the like and, therefore, contributes to the development of energy industry and environmental technology.
EXPLANATION OF NUMERALS
[0462] 1 . . . Cathode, [0463] 1A, 1B, 1C . . . Gas diffusion electrode (GDE) [0464] 1c . . . Catalytic layer (CL), [0465] 1m . . . Water repellent layer (MPL), [0466] 1gd . . . Gas diffusion layer (GDL), [0467] 2 . . . Anode, [0468] 2c . . . Catalytic layer (CL), [0469] 2m . . . Water repellent layer (MPL), [0470] 2gd . . . Gas diffusion layer (GDL), [0471] 3 . . . Polymer electrolyte membrane (PEM), [0472] 4, 5 . . . Separator [0473] 10, 11 . . . Membrane-electrode assembly (MEA). [0474] 12, 13 . . . Membrane catalyst layer assembly (CCM) [0475] 20 . . . Pt/C catalyst, [0476] 22 . . . Support, [0477] 23 . . . Catalyst particle, [0478] 25 . . . Non-contact particle, [0479] 30 . . . Fuel cell stack, [0480] P1 . . . Intercommunicating pore where a plural of nanopores P22 is connected [0481] P22 . . . Nanopores of the support [0482] P24 . . . Micropores of the support