WO2014175099A1

WO2014175099A1 - Catalyst, electrode catalyst layer using same, membrane electrode assembly and fuel cell

Info

Publication number: WO2014175099A1
Application number: PCT/JP2014/060637
Authority: WO
Inventors: 高橋　真一; 大間　敦史; 健秋月; 徹也眞塩; 義隆小野
Original assignee: 日産自動車株式会社
Priority date: 2013-04-25
Filing date: 2014-04-14
Publication date: 2014-10-30
Also published as: JP6113836B2; JPWO2014175099A1

Abstract

[Problem] To provide an electrode catalyst for fuel cells, which is increased in the rate of catalyst metal utilization and is capable of reducing the production cost of a fuel cell. [Solution] A catalyst which is composed of a catalyst carrier and a catalyst metal supported by the catalyst carrier, and which is characterized in that the BET specific surface area per carrier weight is more than 1200 m² per gram of the carrier and the amount of acidic groups per carrier weight is not less than 0.7 mmol per gram of the carrier.

Description

Catalyst, electrode catalyst layer using the catalyst, membrane electrode assembly, and fuel cell

The present invention relates to a catalyst, in particular, an electrode catalyst used in a fuel cell (PEFC), an electrode catalyst layer using the catalyst, a membrane electrode assembly, and a fuel cell.

A solid polymer fuel cell using a proton conductive solid polymer membrane operates at a lower temperature than other types of fuel cells such as a solid oxide fuel cell and a molten carbonate fuel cell. For this reason, the polymer electrolyte fuel cell is expected as a stationary power source or a power source for a moving body such as an automobile, and its practical use has been started.

In such a polymer electrolyte fuel cell, generally, an expensive metal catalyst represented by platinum (Pt) or a Pt alloy is used, which is a high cost factor of such a fuel cell. . For this reason, development of a technique capable of reducing the cost of fuel cells by reducing the amount of noble metal catalyst used is required.

For example, Patent Document 1 discloses a polymer electrolyte fuel cell catalyst in which platinum catalyst particles are supported on a carbon powder carrier having a specific surface area of 250 to 1200 m ² / g. The carbon powder carrier is bound with 0.7 to 3.0 mmol / g (based on the weight of the carrier) of hydrophilic groups, and the platinum particles have an average particle size of 3.5 to 8.0 nm. The platinum specific surface area (COMSA) is 40 to 100 m ² / g. This Patent Document 1 describes that the initial activity (initial power generation characteristics) can be ensured by introducing a hydrophilic group into a platinum catalyst that has been annealed and has lost its functional group on the surface of the carrier and has deteriorated wettability. Yes.

JP2012-124001A

However, the catalyst described in Patent Document 1 is such that when the platinum particles, which are catalytic metals, and the polymer electrolyte come into contact with each other, the polymer electrolyte is easily adsorbed on the surface of the platinum particles, resulting in a decrease in catalytic activity and power generation performance. It was found that would decrease. In order to obtain sufficient power generation performance, it is necessary to use a large amount of expensive metal such as platinum, which leads to an increase in the cost of the fuel cell.

Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fuel cell electrode catalyst capable of increasing the utilization rate of the catalyst metal and reducing the manufacturing cost of the fuel cell.

As a result of intensive studies to solve the above problems, the present inventors have found that a catalyst having a predetermined BET specific surface area and a predetermined amount of acidic groups solves the above problems. The invention has been completed.

That is, the present invention is a catalyst comprising a catalyst carrier and a catalyst metal supported on the catalyst carrier, wherein the BET specific surface area per carrier weight exceeds 1200 m ² / g carrier, and the amount of acidic groups per carrier weight Is a catalyst characterized by having a support of 0.7 mmol / g or more.

It is a schematic sectional drawing which shows the basic composition of the polymer electrolyte fuel cell which concerns on one Embodiment of this invention. It is a schematic sectional explanatory drawing which shows the shape and structure of the catalyst of this invention.

The catalyst of the present invention (also referred to herein as “electrode catalyst”) comprises a catalyst carrier and a catalyst metal supported on the catalyst carrier. Here, the catalyst satisfies the following configurations (a) to (b):
(A) the BET specific surface area per carrier weight is more than 1200 m ² / g carrier;
(B) The amount of acidic groups per carrier weight is 0.7 mmol / g or more.

According to the present invention, by controlling the BET specific surface area of the catalyst, it is possible to suppress a decrease in the catalytic activity due to the electrolyte adsorbed on the catalytic metal surface. Further, by controlling the amount of acidic groups in the catalyst, proton transport inside the pores of the carrier can be ensured, so that the utilization rate of the catalyst metal can be increased. As a result, the amount of catalyst metal used can be reduced, and the manufacturing cost of the fuel cell can be reduced.

When the present inventors prepared a catalyst layer using the catalyst described in Patent Document 1 and a polymer electrolyte, the electrolyte is more easily adsorbed on the surface of the catalyst metal as compared with a gas such as oxygen. It has been found that the reaction active area on the surface of the catalytic metal decreases when is contacted with the electrolyte. As a result, the catalytic activity is lowered and the power generation performance is lowered. Therefore, in order to obtain sufficient power generation performance, it is necessary to use a lot of expensive metals such as platinum, which increases the cost of the fuel cell. Will be invited. On the other hand, even when the catalyst metal does not come into contact with the electrolyte, it has been found that the reaction activity area of the catalyst metal can be secured by forming a three-phase interface with water, and the catalyst metal can be used effectively. Here, in a catalyst using a porous carrier such as carbon, sufficient mesopores can be secured by setting the BET specific surface area of the catalyst to more than 1200 m ² / g carrier. Therefore, by setting the BET specific surface area of the catalyst to more than 1200 m ² / g support, the catalyst metal can be supported inside the mesopores into which the electrolyte cannot enter, and the catalyst is formed by the electrolyte adsorbed on the surface of the catalyst metal. The decrease in activity can be suppressed. In the present specification, pores having a radius of less than 1 nm are also referred to as “micropores”. Further, in the present specification, holes having a radius of 1 to 5 nm are also referred to as “meso holes”.

On the other hand, in the electrode catalyst layer of the polymer electrolyte fuel cell, protons are conducted through the electrolyte and the water, which is a liquid proton conducting material, so that an electrochemical reaction proceeds and power generation occurs. When the catalyst metal is mainly supported inside the mesopores into which the electrolyte cannot enter, the electrolyte cannot enter the pores, so that water is responsible for proton transport around the catalyst metal in the pores. However, it has been found that if there is not enough water in the pores, the proton transportability is lowered and the power generation performance is lowered. Therefore, in such a case, it has become clear that the amount of catalyst metal used cannot be reduced for cost reduction.

On the other hand, according to the present invention, the hydrophilicity of the inner surface of the pores of the catalyst can be enhanced by controlling the amount of acidic groups present in the catalyst to a value above a certain value. Therefore, water can be adsorbed and held inside the pores. Since water is easily introduced into the pores in this way, proton transport is also promoted around the catalyst metal supported inside the pores of the catalyst, and the electrochemical reaction can proceed efficiently. Metal utilization can be improved. Therefore, the usage amount of the catalyst metal can be reduced, which can contribute to the reduction of the manufacturing cost of the fuel cell. In addition, the catalyst of the present invention can achieve a higher effect, particularly when used under conditions where the relative humidity is low. When the relative humidity is high, the catalyst pores are relatively easily filled with water. However, when the relative humidity is low, the conventional catalyst is not sufficiently filled with water inside the catalyst pores, and the proton transport resistance is low. The power generation performance is greatly reduced. However, according to the catalyst of the present invention, even when the relative humidity is low, water can be retained inside the pores, so that high proton transportability can be obtained. Therefore, the catalytic metal can be used effectively, and the effects of the present invention can be obtained more remarkably.

Hereinafter, an embodiment of the catalyst of the present invention and an embodiment of a catalyst layer, a membrane electrode assembly (MEA) and a fuel cell using the same will be described in detail with reference to the drawings as appropriate. However, the present invention is not limited only to the following embodiments. Each drawing is exaggerated for convenience of explanation, and the dimensional ratio of each component in each drawing may be different from the actual one. Further, in the case where the embodiment of the present invention is described with reference to the drawings, the same reference numerals are given to the same elements in the description of the drawings, and a duplicate description is omitted.

In the present specification, “X to Y” indicating a range means “X or more and Y or less”, “weight” and “mass”, “weight%” and “mass%”, “part by weight” and “weight part”. “Part by mass” is treated as a synonym. Unless otherwise specified, measurement of operation and physical properties is performed under conditions of room temperature (20 to 25 ° C.) / Relative humidity 40 to 50%.

[Fuel cell]
A fuel cell includes a membrane electrode assembly (MEA), a pair of separators including an anode side separator having a fuel gas flow path through which fuel gas flows and a cathode side separator having an oxidant gas flow path through which oxidant gas flows. Have. The fuel cell of this embodiment is excellent in durability and can exhibit high power generation performance.

FIG. 1 is a schematic diagram showing a basic configuration of a polymer electrolyte fuel cell (PEFC) 1 according to an embodiment of the present invention. The PEFC 1 first includes a solid polymer electrolyte membrane 2 and a pair of catalyst layers (an anode catalyst layer 3a and a cathode catalyst layer 3c) that sandwich the membrane. The laminate of the solid polymer electrolyte membrane 2 and the catalyst layers (3a, 3c) is further sandwiched between a pair of gas diffusion layers (GDL) (anode gas diffusion layer 4a and cathode gas diffusion layer 4c). Thus, the polymer electrolyte membrane 2, the pair of catalyst layers (3a, 3c), and the pair of gas diffusion layers (4a, 4c) constitute a membrane electrode assembly (MEA) 10 in a stacked state.

In PEFC1, the MEA 10 is further sandwiched between a pair of separators (anode separator 5a and cathode separator 5c). In FIG. 1, the separators (5 a, 5 c) are illustrated so as to be positioned at both ends of the illustrated MEA 10. However, in a fuel cell stack in which a plurality of MEAs are stacked, the separator is generally used as a separator for an adjacent PEFC (not shown). In other words, in the fuel cell stack, the MEAs are sequentially stacked via the separator to form a stack. In an actual fuel cell stack, a gas seal portion is disposed between the separator (5a, 5c) and the solid polymer electrolyte membrane 2, or between the PEFC 1 and another adjacent PEFC. These descriptions are omitted in FIG.

The separators (5a, 5c) are obtained, for example, by forming a concavo-convex shape as shown in FIG. 1 by subjecting a thin plate having a thickness of 0.5 mm or less to a press treatment. The convex part seen from the MEA side of the separator (5a, 5c) is in contact with the MEA 10. Thereby, the electrical connection with MEA10 is ensured. Further, a recess (space between the separator and the MEA generated due to the concavo-convex shape of the separator) viewed from the MEA side of the separator (5a, 5c) is a gas for circulating gas during operation of the PEFC 1 Functions as a flow path. Specifically, a fuel gas (for example, hydrogen) is circulated through the gas flow path 6a of the anode separator 5a, and an oxidant gas (for example, air) is circulated through the gas flow path 6c of the cathode separator 5c.

On the other hand, the recess viewed from the side opposite to the MEA side of the separator (5a, 5c) serves as a refrigerant flow path 7 for circulating a refrigerant (for example, water) for cooling the PEFC during operation of the PEFC 1. . Further, the separator is usually provided with a manifold (not shown). This manifold functions as a connection means for connecting cells when a stack is formed. With such a configuration, the mechanical strength of the fuel cell stack can be ensured.

In the embodiment shown in FIG. 1, the separators (5a, 5c) are formed in an uneven shape. However, the separator is not limited to such a concavo-convex shape, and may be any form such as a flat plate shape and a partially concavo-convex shape as long as the functions of the gas flow path and the refrigerant flow path can be exhibited. Also good.

The fuel cell having the MEA of the present invention as described above exhibits excellent power generation performance. Here, the type of the fuel cell is not particularly limited. In the above description, the polymer electrolyte fuel cell has been described as an example. However, in addition to the above, an alkaline fuel cell and a direct methanol fuel cell are used. And a micro fuel cell. Among them, a polymer electrolyte fuel cell (PEFC) is preferable because it is small and can achieve high density and high output. The fuel cell is useful as a stationary power source in addition to a power source for a moving body such as a vehicle in which a mounting space is limited. Among them, it is particularly preferable to use as a power source for a mobile body such as an automobile that requires a high output voltage after a relatively long time of operation stop.

The fuel used when operating the fuel cell is not particularly limited. For example, hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, secondary butanol, tertiary butanol, dimethyl ether, diethyl ether, ethylene glycol, diethylene glycol and the like can be used. Of these, hydrogen and methanol are preferably used in that high output is possible.

Further, the application application of the fuel cell is not particularly limited, but it is preferably applied to a vehicle. The electrolyte membrane-electrode assembly of the present invention is excellent in power generation performance and durability, and can be downsized. For this reason, the fuel cell of this invention is especially advantageous when this fuel cell is applied to a vehicle from the point of in-vehicle property.

Hereinafter, although the members constituting the fuel cell of the present embodiment will be briefly described, the technical scope of the present invention is not limited only to the following embodiments.

[Catalyst (Electrocatalyst)]
FIG. 2 is a schematic sectional explanatory view showing the shape and structure of a catalyst according to an embodiment of the present invention. As shown in FIG. 2, the catalyst 20 of the present invention includes a catalytic metal 22 and a support 23. Further, the catalyst 20 has pores (mesopores) 24. Further, the catalyst 20 has an acidic group 25. Here, the catalyst metal 22 is supported inside the pores (mesopores) 24. Further, it is sufficient that at least a part of the catalyst metal 22 is supported inside the pores (mesopores) 24, and a part thereof may be supported on the surface of the carrier 23. However, from the viewpoint of preventing contact between the electrolyte and the catalyst metal in the catalyst layer, it is preferable that substantially all of the catalyst metal 22 is supported inside the mesopores 24. Here, “substantially all catalytic metals” is not particularly limited as long as it is an amount capable of improving sufficient catalytic activity. “Substantially all catalyst metals” are present in an amount of preferably 50 wt% or more (upper limit: 100 wt%), more preferably 80 wt% or more (upper limit: 100 wt%) in all catalyst metals.

The BET specific surface area of the catalyst of the present invention (after supporting the catalytic metal) [the BET specific surface area of the catalyst per 1 g of support (m ² / g)] is more than 1200 m ² / g support. When the BET specific surface area of the catalyst is 1200 m ² / g or less, sufficient pores (mesopores) cannot be secured, and it is difficult to store (support) more catalyst metal inside the pores (mesopores). The catalytic metal supported on the surface of the support becomes relatively large. Therefore, the catalyst metal and the electrolyte are easily brought into contact with each other in the catalyst layer, and the ratio of the electrolyte covering the catalyst metal is increased. Therefore, the activity of the catalytic metal cannot be effectively used, and it becomes difficult to promote the catalytic reaction more effectively. Further, when the BET specific surface area of the catalyst is 1200 m ² / g or less, it is not easy to disperse the catalyst metal particles in a high state to sufficiently increase the effective surface area. The BET specific surface area of the catalyst is preferably 1500 m ² / g support or more, more preferably 1700 m ² / g support or more. The upper limit of the specific surface area is not particularly limited, but is preferably 3000 m ² / g or less.

In the present specification, the “BET specific surface area (m ² / g)” of the catalyst is measured by a nitrogen adsorption method. Specifically, about 0.04 to 0.07 g of catalyst powder is precisely weighed and sealed in a sample tube. This sample tube is preliminarily dried at 90 ° C. for several hours in a vacuum dryer to obtain a measurement sample. For weighing, an electronic balance (AW220) manufactured by Shimadzu Corporation is used. In the case of a coated sheet, a net weight of about 0.03 to 0.04 g of the coated layer obtained by subtracting the weight of Teflon (registered trademark) (base material) of the same area from the total weight is used as the sample weight. . Next, the BET specific surface area is measured under the following measurement conditions. On the adsorption side of the adsorption / desorption isotherm, a BET specific surface area is calculated from the slope and intercept by creating a BET plot from a relative pressure (P / P ₀ ) range of about 0.00 to 0.45.

The production method of the catalyst having the specific surface area as described above is not particularly limited, but usually, the methods described in JP 2010-208887 A, International Publication No. 2009/0775264, etc. are preferably used.

The material of the carrier is not particularly limited as long as it has mesopores and has a specific surface area sufficient to support the catalyst component in a dispersed state inside the mesopores and sufficient electron conductivity. Preferably, the carrier contains carbon, more preferably the main component is carbon. Specific examples include porous carbon particles made of carbon black (Ketjen black, oil furnace black, channel black, lamp black, thermal black, acetylene black, etc.), activated carbon, and the like. “The main component is carbon” means that the main component contains carbon atoms, and is a concept that includes both carbon atoms and substantially carbon atoms. It may be included. “Substantially consists of carbon atoms” means that contamination of impurities of about 2 to 3% by weight or less can be allowed. By using carbon as a carrier, electron conductivity is improved and electron conduction resistance is reduced, so that power generation performance can be improved.

More preferably, carbon black is preferably used, and Black Pearls (registered trademark) is particularly preferably used because a desired pore region is easily formed inside the carrier.

Furthermore, it is preferable to control the crystallinity of the carbon support for the purpose of improving the corrosion resistance of the catalyst layer. For the crystallinity and crystalline composition of the carbon material, for example, G band peak intensity and D band peak intensity calculated by Raman scattering spectroscopic analysis can be used.

When the carbon material is analyzed by Raman spectroscopy, peaks are usually generated around 1340 cm ⁻¹ and 1580 cm ⁻¹ . These peaks are usually referred to as “D band” and “G band”. Strictly speaking, the peak of diamond is 1333 cm ⁻¹ and is distinguished from the D band.

In one embodiment of the present invention, said carrier is carbon black half width of D band appearing at 1340 cm ^-1 in the Raman spectrum is 100 cm ^-1 or less. Further, in an embodiment of the present invention, the carrier, the half width of G band appearing at 1580 cm ^-1 in the Raman spectrum is 60cm ^-1 or less. In these cases, the corrosion resistance of the catalyst layer is improved by graphitization of the carbon support, whereby a catalyst layer having high initial performance and capable of maintaining the performance over a long period of time can be provided.

The lower limit value of the half width of the D band and the half width of the G band is not particularly limited. However, since the primary vacancies are closed simultaneously with the progress of graphitization of the support, the half-value width of the D band is 50 cm ⁻ in order to achieve both the graphitization of the support and securing the desired primary vacancy region. ^It is preferably ¹ or more, and the half width of the G band is preferably 40 cm ⁻¹ or more.

Here, the Raman spectrum is a spectrum indicating which light of which wavelength is scattered with what intensity with respect to the light scattered by the Raman effect. In the present invention, the half widths of the D band and the G band can be calculated using a Raman spectrum in which the wave number (cm ⁻¹ ) is represented on one axis and the intensity is represented on the other axis. Further, the “half-value width” is a value used for determining the distribution state of a predetermined absorption band, and refers to the spread width of the absorption band at a half height of the peak height of the absorption band. These Raman spectra may be measured for the carrier before supporting the catalyst, but it is preferable to measure for the carrier after supporting the catalyst. This is because the presence or absence of the catalyst in the carrier itself does not affect the Raman spectrum, but the surface of the carrier may be altered by the catalyst loading treatment.

When there is another absorption band in the vicinity of the D band or G band and the half width cannot be determined from the spectrum at first glance because it is bonded to the D band or G band, it is usually attached to the Raman spectrometer. The full width at half maximum can be determined by the analysis program. For example, the half-value width is determined by the process of drawing a straight baseline in the region containing the D-band and G-band peaks, performing curve fitting of the Lorentz waveform, and separating the peaks of the D-band and G-band. The

In addition to the above carbon, porous metals such as Sn (tin) and Ti (titanium), as well as conductive metal oxides such as RuO ₂ and TiO ₂ can be preferably used as the carrier. By using such a metal oxide, the corrosion of the support is reduced and the durability of the catalyst is further improved.

The BET specific surface area of the support may be a specific surface area sufficient to support the catalyst component in a highly dispersed state. The BET specific surface area of the carrier can be determined by the same method as the BET specific surface area of the catalyst described above. The BET specific surface area of the support is substantially equivalent to the BET specific surface area of the catalyst determined on the basis of the weight of the support. The BET specific surface area of the support is preferably more than 1200 m ² / g, more preferably 1500 m ² / g or more, and even more preferably 1700 m ² / g or more. If the specific surface area is as described above, sufficient mesopores can be secured, so that more catalyst metal can be stored (supported) in the mesopores. Therefore, it is possible to suppress the coating of the electrolyte on the catalyst metal in the catalyst layer (the contact between the catalyst metal and the electrolyte can be more effectively suppressed / prevented). Therefore, the activity of the catalytic metal can be utilized more effectively, and the catalytic reaction can be promoted more effectively. In addition, since the area to which the catalyst metal adheres can be increased, the catalyst metal particles having a small particle diameter can be distributed with high dispersion, and the effective surface area can be increased. As a result, the power generation performance when used as a fuel cell electrode catalyst can be improved. The upper limit value of the BET specific surface area of the support is not particularly limited, but is, for example, 3000 m ² / g or less.

The pore diameter of the carrier is not particularly limited as long as the carrier has mesopores, but preferably mesopores (radius 1 to 5 nm) and micropores (radius less than 1 nm, size lower limit is 0.3 nm). Have.

The pore volume of pores (micropores) having a radius of less than 1 nm of the carrier is not particularly limited, but is preferably 0.1 cc / g or more. More preferably, the pore volume of the micropores is 0.3 to 3 cc / g carrier, and particularly preferably 0.4 to 2 cc / g carrier. Such a void volume can more effectively suppress / prevent desorption of the catalytic metal under mechanical stress. In addition, sufficient micropores for gas transportation can be secured, and the gas transportation resistance is small. For this reason, since a sufficient amount of gas can be transported to the surface of the catalytic metal existing in the mesopores through the micropore (pass), the catalyst of the present invention can exhibit high catalytic activity, that is, promote the catalytic reaction. it can. In this specification, the pore volume of pores having a radius of less than 1 nm is also simply referred to as “micropore pore volume”.

The pore volume of the pores (mesopores) having a radius of 1 nm or more of the carrier is not particularly limited, but is 0.4 cc / g carrier or more, more preferably 0.4 to 3 cc / g carrier, particularly preferably. Is preferably 0.4 to 2 cc / g carrier. If the pore volume is in the range as described above, desorption of the catalytic metal under mechanical stress can be more effectively suppressed / prevented. In addition, a large amount of catalyst metal can be stored (supported) in the mesopores, and the electrolyte and catalyst metal in the catalyst layer are physically separated (contact between the catalyst metal and the electrolyte can be more effectively suppressed / prevented). Therefore, the activity of the catalytic metal can be utilized more effectively. In addition, the presence of many mesopores can more effectively promote the catalytic reaction by exerting the effects and advantages of the present invention more remarkably. In addition, the micropores act as a gas transport path, and a three-phase interface is formed more remarkably with water, so that the catalytic activity can be further improved. In the present specification, the void volume of holes having a radius of 1 nm or more is also simply referred to as “mesopore void volume”.

“The pore volume of micropores” means the total volume of micropores with a radius of less than 1 nm present in the carrier, and is expressed as the volume per gram of carrier (cc / g carrier). The “micropore pore volume (cc / g carrier)” is calculated as the area (integrated value) below the differential pore distribution curve obtained by the nitrogen adsorption method (MP method). Similarly, “pore volume of mesopores” means the total volume of mesopores having a radius of 1 nm or more present in the carrier, and is represented by the volume per gram of carrier (cc / g carrier). The “mesopore pore volume (cc / g carrier)” is calculated as the area (integrated value) below the differential pore distribution curve obtained by the nitrogen adsorption method (DH method).

The average particle size of the carrier is preferably 20 to 2000 nm. Within such a range, the mechanical strength can be maintained and the thickness of the catalyst layer can be controlled within an appropriate range even when the support is provided with the above-described pore structure. The value of the “average particle diameter of the carrier” is observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM) unless otherwise specified. The value calculated as the average value of the particle diameter of the particles shall be adopted. The “particle diameter” means the maximum distance among the distances between any two points on the particle outline.

In the present invention, it is not always necessary to use the granular porous carrier as described above as long as it has the specific surface area as described above.

That is, examples of the carrier include a non-porous conductive carrier, a non-woven fabric made of carbon fibers constituting a gas diffusion layer, carbon paper, and carbon cloth. At this time, the catalyst can be supported on these non-porous conductive carriers, or directly attached to a non-woven fabric made of carbon fibers, carbon paper, carbon cloth, etc. constituting the gas diffusion layer of the membrane electrode assembly. It is.

The catalytic metal that can be used in the present invention has a function of catalyzing an electrochemical reaction. The catalyst metal used in the anode catalyst layer is not particularly limited as long as it has a catalytic action in the oxidation reaction of hydrogen, and a known catalyst can be used in the same manner. The catalyst metal used in the cathode catalyst layer is not particularly limited as long as it has a catalytic action for the oxygen reduction reaction, and a known catalyst can be used in the same manner. Specifically, metals such as platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and alloys thereof Can be selected.

Among these, those containing at least platinum are preferably used in order to improve catalytic activity, poisoning resistance to carbon monoxide, heat resistance, and the like. That is, the catalyst metal is preferably platinum or contains a metal component other than platinum and platinum, and more preferably platinum or a platinum-containing alloy. Such a catalytic metal can exhibit high activity. Therefore, when used as an electrode catalyst for a fuel cell, high power generation performance can be obtained. Although the composition of the alloy depends on the type of metal to be alloyed, the content of platinum is preferably 30 to 90 atomic%, and the content of the metal to be alloyed with platinum is preferably 10 to 70 atomic%. In general, an alloy is a generic term for a metal element having one or more metal elements or non-metal elements added and having metallic properties. The alloy structure consists of a eutectic alloy, which is a mixture of the component elements as separate crystals, a component element completely melted into a solid solution, and a component element composed of an intermetallic compound or a compound of a metal and a nonmetal. There is what is formed, and any may be used in the present application. At this time, the catalyst metal used for the anode catalyst layer and the catalyst metal used for the cathode catalyst layer can be appropriately selected from the above. In the present specification, unless otherwise specified, the description of the catalyst metal for the anode catalyst layer and the cathode catalyst layer has the same definition for both. However, the catalyst metals of the anode catalyst layer and the cathode catalyst layer do not have to be the same, and can be appropriately selected so as to exhibit the desired action as described above.

The shape and size of the catalyst metal (catalyst component) are not particularly limited, and the same shape and size as known catalyst components can be adopted. As the shape, for example, a granular shape, a scale shape, a layered shape, and the like can be used, but a granular shape is preferable. At this time, the average particle diameter of the catalyst metal (catalyst metal particles) is not particularly limited, but is preferably 3 nm or more, more preferably more than 3 nm and not more than 30 nm, particularly preferably more than 3 nm and not more than 10 nm. If the average particle diameter of the catalyst metal is 3 nm or more, the catalyst metal is supported relatively firmly in the mesopores, and the contact with the electrolyte in the catalyst layer is more effectively suppressed / prevented. Further, the micropores remain without being clogged with the catalyst metal, and the gas transport path can be secured better, and the gas transport resistance can be further reduced. In addition, elution due to potential change can be prevented, and deterioration in performance over time can be suppressed. For this reason, the catalytic activity can be further improved, that is, the catalytic reaction can be promoted more efficiently. On the other hand, if the average particle diameter of the catalyst metal particles is 30 nm or less, the catalyst metal can be supported inside the mesopores of the support by a simple method, and the electrolyte coverage of the catalyst metal can be reduced. The “average particle diameter of the catalytic metal particles” in the present invention is the crystallite diameter determined from the half-value width of the diffraction peak of the catalytic metal component in X-ray diffraction, or the catalytic metal particles examined by a transmission electron microscope (TEM). It can be measured as the average value of the particle diameters.

When the catalyst of this embodiment is used as an electrode catalyst for a fuel cell, the catalyst content per unit catalyst coating area (mg / cm ² ) is particularly limited as long as sufficient degree of dispersion of the catalyst on the carrier and power generation performance can be obtained. For example, it is 0.01 to 1 mg / cm ² . However, when the catalyst contains platinum or a platinum-containing alloy, the platinum content per unit catalyst coating area is preferably 0.5 mg / cm ² or less. The use of expensive noble metal catalysts typified by platinum (Pt) and platinum alloys has become a high cost factor for fuel cells. Therefore, it is preferable to reduce the amount of expensive platinum used (platinum content) to the above range and reduce the cost. The lower limit is not particularly limited as long as power generation performance is obtained, and is, for example, 0.01 mg / cm ² or more. More preferably, the platinum content is 0.02 to 0.4 mg / cm ² . In this embodiment, since the activity per catalyst weight can be improved by controlling the pore structure of the carrier, the amount of expensive catalyst used can be reduced.

In this specification, inductively coupled plasma emission spectroscopy (ICP) is used for measurement (confirmation) of “catalyst (platinum) content per unit catalyst application area (mg / cm ² )”. A person skilled in the art can easily carry out a method of making the desired “catalyst (platinum) content per unit catalyst coating area (mg / cm ² )”, and control the slurry composition (catalyst concentration) and coating amount. You can adjust the amount.

In the catalyst of the present invention, the ratio of the catalyst metal contained in the catalyst (sometimes referred to as the amount of catalyst metal supported or the rate of support) is preferably relative to the total amount of the catalyst support (that is, the support and the catalyst). Is 50% by weight or less, more preferably 30% by weight or less. If the supported amount is within the above range, the catalyst metal having a small particle diameter is dispersed on the surface of the support, so that the surface area of the catalyst metal per weight is maintained even if the amount of catalyst metal used is reduced. That is, it is preferable because a sufficient degree of dispersion of the catalyst components on the carrier, improvement in power generation performance, economic advantages, and high catalyst activity per unit weight of the catalyst can be achieved. In particular, when the catalyst metal is platinum, or when platinum and a metal other than platinum are included, the amount of platinum used can be reduced by reducing to the above range, and it is preferable to reduce the cost. In particular, if the amount of catalyst metal supported is 30% by weight or less, the raw material cost of the catalyst metal can be reduced, and the weight fraction of the support with respect to the catalyst increases. Can be suppressed and durability can be improved. In addition, the lower limit value of the loading amount is not particularly limited, but is preferably 5% by weight or more from the viewpoint of obtaining high power generation performance.

The catalyst of the present invention has acidic groups on the surface of catalyst particles or the surface of pores, and the amount of acidic groups per carrier weight is 0.7 mmol / g or more.

The acidic group of the catalyst of the present invention is not particularly limited as long as it is a functional group that can be ionized to release protons, but contains at least one selected from the group consisting of a hydroxyl group, a lactone group, and a carboxyl group. Is preferred. When the support includes carbon, the acidic group preferably includes a hydroxyl group, a lactone group, or a carboxyl group, and when the support includes a metal oxide, the acidic group preferably includes a hydroxyl group. Such an acidic group is a hydrophilic group and can increase the amount of water adsorbed on the surface of the carrier, so that the proton transportability in the catalyst layer can be improved. Further, the durability of the catalyst can be improved.

The amount of acidic groups possessed by the catalyst is 0.7 mmol / g or more. When the amount of acidic groups in the catalyst is less than 0.7 mmol / g support, the hydrophilicity of the catalyst cannot be secured and sufficient proton transportability cannot be exhibited. For this reason, the utilization rate of the catalyst metal cannot be sufficiently increased, and in order to obtain sufficient power generation performance, a large amount of the catalyst metal must be used, which may increase the cost of the fuel cell. The amount of the acidic group is preferably more than 0.75 mmol / g carrier, more preferably 1.2 mmol / g carrier or more, and still more preferably 1.8 mmol / g carrier or more. The upper limit of the amount of acidic groups is not particularly limited, but is preferably 3.0 mmol / g carrier or less, more preferably 2.5 mmol / g carrier or less from the viewpoint of carbon durability.

The amount of the acidic group can be measured by a titration method using an alkali compound, and specifically, can be measured by the method described in Examples.

The method for adding an acidic group to the catalyst so that the amount of the acidic group is within the above range is not particularly limited. For example, a support (catalyst support) supporting a catalyst metal in an oxidizing solution containing an oxidizing agent is used. A dipping wet method may be employed. Details of this method will be described later.

[Catalyst layer]
As described above, the catalyst of the present invention can exhibit high proton transportability, that is, can promote an electrochemical reaction. Therefore, the catalyst of the present invention can be suitably used for an electrode catalyst layer for a fuel cell. That is, this invention also provides the electrode catalyst layer for fuel cells containing the catalyst and electrolyte layer of this invention.

As shown in FIG. 2, in the catalyst layer, the catalyst 20 is covered with the electrolyte 26. However, since the electrolyte 26 has a smaller surface opening diameter of the pores (mesopores) 24 than its molecular size, It does not enter the pores (mesopores) 24 of the (carrier 23). For this reason, the catalyst metal 22 on the surface of the carrier 23 is in contact with the electrolyte 26, but the catalyst metal 22 supported in the pores 24 is not in contact with the electrolyte 26. The catalytic metal in the pores forms a three-phase interface between oxygen gas and water in a non-contact state with the electrolyte, thereby ensuring a reaction active area of the catalytic metal.

The catalyst of the present invention may be present in either the cathode catalyst layer or the anode catalyst layer, but is preferably used in the cathode catalyst layer. As described above, the catalyst of the present invention can effectively use the catalyst by forming a three-phase interface with water without contacting the electrolyte, but water is formed in the cathode catalyst layer. .

The electrolyte is not particularly limited, but is preferably an ion conductive polymer electrolyte. Since the polymer electrolyte plays a role of transmitting protons generated around the catalyst active material on the fuel electrode side, it is also called a proton conductive polymer.

The polymer electrolyte is not particularly limited, and conventionally known knowledge can be appropriately referred to. Polymer electrolytes are roughly classified into fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes depending on the type of ion exchange resin that is a constituent material.

Examples of ion exchange resins constituting the fluorine-based polymer electrolyte include Nafion (registered trademark, manufactured by Dupont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like. Perfluorocarbon sulfonic acid polymer, perfluorocarbon phosphonic acid polymer, trifluorostyrene sulfonic acid polymer, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride-per Examples thereof include fluorocarbon sulfonic acid polymers. From the viewpoint of excellent heat resistance, chemical stability, durability, and mechanical strength, these fluorine-based polymer electrolytes are preferably used, and particularly preferably fluorine-based polymer electrolytes composed of perfluorocarbon sulfonic acid polymers. Is used.

Specific examples of the hydrocarbon electrolyte include sulfonated polyethersulfone (S-PES), sulfonated polyaryletherketone, sulfonated polybenzimidazole alkyl, phosphonated polybenzimidazole alkyl, sulfonated polystyrene, sulfonated poly Examples include ether ether ketone (S-PEEK) and sulfonated polyphenylene (S-PPP). These hydrocarbon polymer electrolytes are preferably used from the viewpoint of production such that the raw material is inexpensive, the production process is simple, and the selectivity of the material is high. In addition, as for the ion exchange resin mentioned above, only 1 type may be used independently and 2 or more types may be used together. Moreover, it is not restricted only to the material mentioned above, Other materials may be used.

Proton conductivity is important in polymer electrolytes responsible for proton transmission. Here, when the EW of the polymer electrolyte is too large, the ionic conductivity in the entire catalyst layer is lowered. Therefore, it is preferable that the catalyst layer of this embodiment contains a polymer electrolyte having a small EW. Specifically, the catalyst layer of this embodiment preferably has an EW of 1500 g / eq. The following polymer electrolyte is contained, More preferably, it is 1200 g / eq. The following polymer electrolyte is included, and particularly preferably 1000 g / eq. The following polymer electrolytes are included.

On the other hand, if the EW is too small, the hydrophilicity is too high and it becomes difficult to smoothly move water. From such a viewpoint, the EW of the polymer electrolyte is preferably 500 or more. Note that EW (Equivalent Weight) represents an equivalent weight of an exchange group having proton conductivity. The equivalent weight is the dry weight of the ion exchange membrane per equivalent of ion exchange group, and is expressed in units of “g / eq”.

Further, the catalyst layer includes two or more types of polymer electrolytes having different EWs in the power generation surface. At this time, the polymer electrolyte having the lowest EW among the polymer electrolytes has a relative humidity of 90% or less of the gas in the flow path. It is preferable to use in the region. By adopting such a material arrangement, the resistance value becomes small regardless of the current density region, and the battery performance can be improved. The EW of the polymer electrolyte used in the region where the relative humidity of the gas in the flow channel is 90% or less, that is, the polymer electrolyte having the lowest EW is 900 g / eq. The following is desirable. Thereby, the above-mentioned effect becomes more reliable and remarkable.

Furthermore, it is desirable to use a polymer electrolyte with the lowest EW in a region higher than the average temperature of the cooling water inlet and outlet. As a result, the resistance value is reduced regardless of the current density region, and the battery performance can be further improved.

Furthermore, from the viewpoint of reducing the resistance value of the fuel cell system, the polymer electrolyte having the lowest EW is within 3/5 from the gas supply port of at least one of the fuel gas and the oxidant gas with respect to the channel length. It is desirable to use it in the range area.

The catalyst layer of this embodiment may have a liquid proton conductive material such as water that can connect the catalyst and the polymer electrolyte in a proton conductive state between the catalyst and the polymer electrolyte. By introducing a liquid proton conductive material, a proton transport path through the liquid proton conductive material is secured between the catalyst and the polymer electrolyte, and protons necessary for power generation are efficiently transported to the catalyst surface. Is possible. Thereby, since the utilization efficiency of a catalyst improves, it becomes possible to reduce the usage-amount of a catalyst, maintaining electric power generation performance. The liquid proton conductive material only needs to be interposed between the catalyst and the polymer electrolyte, and the pores (secondary pores) between the porous carriers in the catalyst layer and the pores (micropores) in the porous carrier. Or mesopores: primary vacancies).

The liquid proton conductive material is not particularly limited as long as it has ion conductivity and can exhibit a function of forming a proton transport path between the catalyst and the polymer electrolyte. Specific examples include water, aqueous perchloric acid solution, aqueous nitric acid solution, aqueous formic acid solution, and aqueous acetic acid solution. In this embodiment, it is preferable to contain water.

When water is used as the liquid proton conductive material, water as the liquid proton conductive material is introduced into the catalyst layer by moistening the catalyst layer with a small amount of liquid water or humidified gas before starting power generation. Can do. Moreover, the water produced by the electrochemical reaction during the operation of the fuel cell can be used as the liquid proton conductive material. Therefore, it is not always necessary to hold the liquid proton conductive material when the fuel cell is in operation. For example, the surface distance between the catalyst and the electrolyte is preferably 0.28 nm or more, which is the diameter of oxygen ions constituting water molecules. By maintaining such a distance, water (liquid proton conductive material) intervenes between the catalyst and polymer electrolyte (liquid conductive material holding part) while maintaining a non-contact state between the catalyst and the polymer electrolyte. And a proton transport route by water between the two can be secured.

In the catalyst layer of the present embodiment, the total area of the catalyst (catalyst metal) in contact with the polymer electrolyte is smaller than the total area of the catalyst (catalyst metal) exposed at the liquid conductive material holding part. It is preferable that

These areas are compared, for example, with the capacity of the electric double layer formed at the catalyst-polymer electrolyte interface and the catalyst-liquid proton conducting material interface in a state where the liquid conducting material holding portion is filled with the liquid proton conducting material. This can be done by seeking a relationship. That is, the electric double layer capacity is proportional to the area of the electrochemically effective interface. Therefore, if the electric double layer capacity formed at the catalyst-electrolyte interface is smaller than the electric double layer capacity formed at the catalyst-liquid proton conductive material interface, the contact area of the catalyst with the electrolyte is exposed to the liquid conductive material holding part. It will be smaller than the area.

Here, a method for measuring the electric double layer capacity formed at the catalyst-electrolyte interface and the catalyst-liquid proton conducting material interface will be described. In other words, this is a method for determining the relationship between the contact area between the catalyst and the electrolyte and between the catalyst and the liquid proton conductive material (the relationship between the contact area of the catalyst with the electrolyte and the exposed area of the liquid conductive material holding part). is there.

That is, in the catalyst layer of this embodiment,
(1) Catalyst-Polymer electrolyte (CS)
(2) Catalyst-Liquid proton conductive material (CL)
(3) Porous carrier-polymer electrolyte (Cr-S)
(4) Porous carrier-liquid proton conducting material (Cr-L)
These four types of interfaces can contribute as electric double layer capacitance (Cdl).

Electric double layer capacitor, as described above, since that is directly proportional to the area of the electrochemically active surface, Cdl C-S _(catalytic - electric double layer capacity of the polymer electrolyte interface) and Cdl _{C-L (catalytic} - What is necessary is just to obtain | require the electrical double layer capacity | capacitance of a liquid proton conductive material interface. The contribution of the four types of interfaces to the electric double layer capacity (Cdl) can be separated as follows.

First, for example, the electric double layer capacity is measured under a high humidification condition such as 100% RH and a low humidification condition such as 10% RH or less. In addition, examples of the measurement method of the electric double layer capacitance include cyclic voltammetry and electrochemical impedance spectroscopy. From these comparisons, the contribution of the liquid proton conducting material (in this case “water”), that is, the above (2) and (4) can be separated.

Further, when the catalyst is deactivated, for example, when Pt is used as the catalyst, the catalyst is deactivated by supplying CO gas to the electrode to be measured and adsorbing CO on the Pt surface. The contribution to the multilayer capacity can be separated. In such a state, as described above, the electric double layer capacity under high and low humidification conditions is measured by the same method, and the contribution of the catalyst, that is, the above (1) and (2) is separated from these comparisons. be able to.

As described above, all the contributions (1) to (4) can be separated, and the electric double layer capacity formed at the interfaces of the catalyst, the polymer electrolyte, and the liquid proton conducting material can be obtained.

That is, the measured value (A) in the highly humidified state is the electric double layer capacity formed at all the interfaces (1) to (4), and the measured value (B) in the lowly humidified state is the above (1) and (3). The electric double layer capacity formed at the interface. Further, the measured value (C) in the catalyst deactivation / highly humidified state is the electric double layer capacity formed at the interface of the above (3) and (4), and the measured value (D) in the catalyst deactivated / lowly humidified state is the above It becomes an electric double layer capacity formed at the interface of (3).

Therefore, the difference between A and C is the electric double layer capacity formed at the interface of (1) and (2), and the difference between B and D is the electric double layer capacity formed at the interface of (1). Then, by calculating the difference between these values, (AC)-(BD), the electric double layer capacity formed at the interface of (2) can be obtained. In addition to the above, the contact area of the catalyst with the polymer electrolyte and the exposed area of the conductive material holding part can be obtained by, for example, TEM (transmission electron microscope) tomography.

In the catalyst layer of the present embodiment, the electrolyte coverage of the catalyst metal calculated from the ratio of the area where the catalyst metal surface is covered with the electrolyte to the surface area of the catalyst metal is preferably 0.3 or less. The electrolyte coverage of the catalyst metal is more preferably 0.25 or less, and more preferably 0.2 or less (lower limit: 0). If the electrolyte coverage is 0.3 or less, the catalytic activity (particularly the oxygen reduction reaction activity) is improved, so that the power generation performance can be improved.

The electrolyte coverage can be calculated from the electric double layer capacity, and specifically can be calculated by the method described in Examples.

For the catalyst layer, a water repellent such as polytetrafluoroethylene, polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer, a dispersing agent such as a surfactant, glycerin, ethylene glycol (EG), as necessary. ), A thickener such as polyvinyl alcohol (PVA) and propylene glycol (PG), and an additive such as a pore-forming agent may be contained.

The thickness (dry film thickness) of the catalyst layer is preferably 0.05 to 30 μm, more preferably 1 to 20 μm, still more preferably 2 to 15 μm. In addition, the said thickness is applied to both a cathode catalyst layer and an anode catalyst layer. However, the thickness of the cathode catalyst layer and the anode catalyst layer may be the same or different.

(Method for producing catalyst layer)
Hereinafter, although preferable embodiment for manufacturing a catalyst layer is described, the technical scope of this invention is not limited only to the following form. Further, since various conditions such as the material of each component of the catalyst layer are as described above, description thereof is omitted here.

First, a carrier (also referred to as “porous carrier” or “conductive porous carrier” in the present specification) is prepared, and the pore structure is controlled by heat-treating the carrier. Specifically, it may be produced as described in the method for producing the carrier. Thereby, the support | carrier which has a specific specific surface area is obtained.

The conditions for the heat treatment vary depending on the material and are appropriately determined so that a desired specific surface area can be obtained. Such heat treatment conditions may be determined according to the material while confirming the pore structure, and can be easily determined by those skilled in the art.

Next, the catalyst is supported on the porous carrier to obtain catalyst powder. The catalyst can be supported on the porous carrier by a known method. For example, known methods such as impregnation method, liquid phase reduction support method, evaporation to dryness method, colloid adsorption method, spray pyrolysis method, reverse micelle (microemulsion method) can be used.

Next, the obtained catalyst powder may be annealed as necessary. The particle diameter of the catalytic metal particles can be adjusted to a desired particle diameter by annealing treatment. The annealing treatment is not particularly limited, but can be performed by heat treatment in hydrogen gas. The temperature and time of the heat treatment are not particularly limited. For example, the heat treatment is performed at 600 to 1180 ° C., preferably 800 to 1000 ° C., and preferably 0.5 to 2 hours. If the temperature of heat processing is 600 degreeC or more, a particle diameter will not become small too much and activity can continue for a long time. If the temperature of heat processing is 1180 degrees C or less, a particle diameter will not become large too much and high mass activity can be obtained.

Next, the obtained catalyst is treated with an oxidizing solution to add an acidic group. A carrier such as carbon has a certain amount of a functional group such as a hydrogen atom or an acidic group as a terminal group. However, an acidic group is further added by treatment with an oxidizing solution to make the carrier 0.7 mmol / g or more. The oxidizing solution used is preferably an aqueous solution of sulfuric acid, nitric acid, phosphorous acid, potassium permanganate, hydrogen peroxide, hydrochloric acid, chloric acid, hypochlorous acid, chromic acid, or the like. This oxidizing solution treatment is performed by bringing the catalyst into contact with the oxidizing solution at least once. When performing acid treatment several times, you may change the kind of solution for every process. As conditions for the oxidizing solution treatment, the concentration of the solution is preferably 0.1 to 10.0 mol / L, and the catalyst is preferably immersed in the solution. The immersion time is preferably 0.5 to 3 hours, and the treatment temperature is preferably 50 to 90 ° C. The amount of acidic groups can be controlled by adjusting the BET specific surface area of the catalyst, the type of oxidizing solution, the concentration, the treatment time, and the treatment temperature.

Subsequently, a catalyst ink containing an acid group-added catalyst powder, a polymer electrolyte, and a solvent is prepared. The solvent is not particularly limited, and ordinary solvents used for forming the catalyst layer can be used in the same manner. Specifically, water such as tap water, pure water, ion exchange water, distilled water, cyclohexanol, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, isobutanol, tert-butanol, etc. And lower alcohols having 1 to 4 carbon atoms, propylene glycol, benzene, toluene, xylene and the like. Besides these, butyl acetate alcohol, dimethyl ether, ethylene glycol, and the like may be used as a solvent. These solvents may be used alone or in the form of a mixture of two or more.

The amount of the solvent constituting the catalyst ink is not particularly limited as long as it is an amount capable of completely dissolving the electrolyte. Specifically, the solid content concentration of the catalyst powder and the polymer electrolyte is preferably 1 to 50% by weight, more preferably about 5 to 30% by weight in the electrode catalyst ink.

In addition, when additives such as a water repellent, a dispersant, a thickener, and a pore-forming agent are used, these additives may be added to the catalyst ink. At this time, the amount of the additive added is not particularly limited as long as it is an amount that does not interfere with the effects of the present invention. For example, the amount of the additive added is preferably 5 to 20% by weight with respect to the total weight of the electrode catalyst ink.

Next, a catalyst ink is applied to the surface of the substrate. The application method to the substrate is not particularly limited, and a known method can be used. Specifically, it can be performed using a known method such as a spray (spray coating) method, a gulliver printing method, a die coater method, a screen printing method, or a doctor blade method.

At this time, a solid polymer electrolyte membrane (electrolyte layer) or a gas diffusion substrate (gas diffusion layer) can be used as the substrate on which the catalyst ink is applied. In such a case, after forming the catalyst layer on the surface of the solid polymer electrolyte membrane (electrolyte layer) or the gas diffusion base material (gas diffusion layer), the obtained laminate can be used for the production of the membrane electrode assembly as it is. Can be used. Alternatively, a peelable substrate such as a polytetrafluoroethylene (PTFE) [Teflon (registered trademark)] sheet is used as the substrate, and after the catalyst layer is formed on the substrate, the catalyst layer portion is peeled from the substrate. Thus, a catalyst layer may be obtained.

Finally, the coating layer (film) of the catalyst ink is dried at room temperature to 150 ° C. for 1 to 60 minutes in an air atmosphere or an inert gas atmosphere. Thereby, a catalyst layer is formed.

(Membrane electrode assembly)
According to a further embodiment of the present invention, the solid polymer electrolyte membrane 2, a cathode catalyst layer disposed on one side of the electrolyte membrane, an anode catalyst layer disposed on the other side of the electrolyte membrane, There is provided a membrane electrode assembly for a fuel cell having an electrolyte membrane 2 and a pair of gas diffusion layers (4a, 4c) sandwiching the anode catalyst layer 3a and the cathode catalyst layer 3c. In this membrane electrode assembly, at least one of the cathode catalyst layer and the anode catalyst layer is the catalyst layer of the embodiment described above.

However, in consideration of the necessity for improvement of proton conductivity and improvement of transport characteristics (gas diffusibility) of the reaction gas (especially O ₂ ), at least the cathode catalyst layer may be the catalyst layer of the embodiment described above. preferable. However, the catalyst layer according to the above embodiment may be used as an anode catalyst layer, or may be used as both a cathode catalyst layer and an anode catalyst layer, and is not particularly limited.

According to a further embodiment of the present invention, there is provided a fuel cell having the membrane electrode assembly of the above form. That is, one embodiment of the present invention is a fuel cell having a pair of anode separator and cathode separator that sandwich the membrane electrode assembly of the above-described embodiment.

Hereinafter, the components of the PEFC 1 using the catalyst layer of the above embodiment will be described with reference to FIG. However, the present invention is characterized by the catalyst and the catalyst layer. Therefore, the specific form of the members other than the catalyst layer constituting the fuel cell can be appropriately modified with reference to conventionally known knowledge.

(Electrolyte membrane)
The electrolyte membrane is composed of a solid polymer electrolyte membrane 2 as shown in FIG. The solid polymer electrolyte membrane 2 has a function of selectively permeating protons generated in the anode catalyst layer 3a during operation of the PEFC 1 to the cathode catalyst layer 3c along the film thickness direction. The solid polymer electrolyte membrane 2 also has a function as a partition wall for preventing the fuel gas supplied to the anode side and the oxidant gas supplied to the cathode side from being mixed.

The electrolyte material constituting the solid polymer electrolyte membrane 2 is not particularly limited, and conventionally known knowledge can be appropriately referred to. For example, the fluorine-based polymer electrolyte or hydrocarbon-based polymer electrolyte described above as the polymer electrolyte can be used. At this time, it is not always necessary to use the same polymer electrolyte used for the catalyst layer.

The thickness of the electrolyte layer may be appropriately determined in consideration of the characteristics of the obtained fuel cell, and is not particularly limited. The thickness of the electrolyte layer is usually about 5 to 300 μm. When the thickness of the electrolyte layer is within such a range, the balance of strength during film formation, durability during use, and output characteristics during use can be appropriately controlled.

(Gas diffusion layer)
The gas diffusion layers (anode gas diffusion layer 4a, cathode gas diffusion layer 4c) are catalyst layers (3a, 3c) of gas (fuel gas or oxidant gas) supplied via the gas flow paths (6a, 6c) of the separator. ) And a function as an electron conduction path.

The material which comprises the base material of a gas diffusion layer (4a, 4c) is not specifically limited, A conventionally well-known knowledge can be referred suitably. For example, a sheet-like material having conductivity and porosity such as a carbon woven fabric, a paper-like paper body, a felt, and a non-woven fabric can be used. The thickness of the substrate may be appropriately determined in consideration of the characteristics of the obtained gas diffusion layer, but may be about 30 to 500 μm. If the thickness of the substrate is within such a range, the balance between mechanical strength and diffusibility such as gas and water can be appropriately controlled.

The gas diffusion layer preferably contains a water repellent for the purpose of further improving water repellency and preventing flooding. The water repellent is not particularly limited, but fluorine-based high repellents such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Examples thereof include molecular materials, polypropylene, and polyethylene.

In order to further improve the water repellency, the gas diffusion layer has a carbon particle layer (microporous layer; MPL, not shown) made of an aggregate of carbon particles containing a water repellent agent on the catalyst layer side of the substrate. You may have.

The carbon particles contained in the carbon particle layer are not particularly limited, and conventionally known materials such as carbon black, graphite, and expanded graphite can be appropriately employed. Among them, carbon black such as oil furnace black, channel black, lamp black, thermal black, acetylene black and the like can be preferably used because of excellent electron conductivity and a large specific surface area. The average particle size of the carbon particles is preferably about 10 to 100 nm. Thereby, while being able to obtain the high drainage property by capillary force, it becomes possible to improve contact property with a catalyst layer.

Examples of the water repellent used for the carbon particle layer include the same water repellents as described above. Among these, fluorine-based polymer materials can be preferably used because of excellent water repellency, corrosion resistance during electrode reaction, and the like.

The mixing ratio of the carbon particles to the water repellent in the carbon particle layer is about 90:10 to 40:60 (carbon particles: water repellent) by weight in consideration of the balance between water repellency and electronic conductivity. It is good. In addition, there is no restriction | limiting in particular also about the thickness of a carbon particle layer, What is necessary is just to determine suitably in consideration of the water repellency of the gas diffusion layer obtained.

(Method for producing membrane electrode assembly)
A method for producing the membrane electrode assembly is not particularly limited, and a conventionally known method can be used. For example, it is possible to use a method in which a catalyst layer is transferred or applied to a solid polymer electrolyte membrane by hot pressing and a gas diffusion layer is bonded to a dried product. Alternatively, two gas diffusion electrodes (GDE) are prepared by applying a catalyst layer on one side of the base material layer in advance and drying the microporous layer side of the gas diffusion layer (if the microporous layer is not included) The gas diffusion electrode can be bonded to both sides of the solid polymer electrolyte membrane by hot pressing, and the coating and bonding conditions such as hot pressing can be performed in the solid polymer electrolyte membrane or the polymer in the catalyst layer. What is necessary is just to adjust suitably according to the kind (perfluorosulfonic acid type | system | group or hydrocarbon type) of electrolyte.

(Separator)
The separator has a function of electrically connecting each cell in series when a plurality of single cells of a fuel cell such as a polymer electrolyte fuel cell are connected in series to form a fuel cell stack. The separator also functions as a partition that separates the fuel gas, the oxidant gas, and the coolant from each other. In order to secure these flow paths, as described above, each of the separators is preferably provided with a gas flow path and a cooling flow path. As a material constituting the separator, conventionally known materials such as dense carbon graphite, carbon such as a carbon plate, and metal such as stainless steel can be appropriately employed without limitation. The thickness and size of the separator and the shape and size of each flow path provided are not particularly limited, and can be appropriately determined in consideration of the desired output characteristics of the obtained fuel cell.

The manufacturing method of the fuel cell is not particularly limited, and conventionally known knowledge can be appropriately referred to in the field of the fuel cell.

Furthermore, a fuel cell stack having a structure in which a plurality of membrane electrode assemblies are stacked and connected in series via a separator may be formed so that the fuel cell can exhibit a desired voltage. The shape of the fuel cell is not particularly limited, and may be determined as appropriate so that desired battery characteristics such as voltage can be obtained.

The relative humidity of the fuel gas or the oxidant gas supplied to the fuel cell of the present embodiment is not particularly limited. For example, when the relative humidity is 60% or less, particularly 40% or less (lower limit value: 0% or more), the present invention. The effect of can be obtained more remarkably. That is, the catalyst of the present embodiment can be suitably used as a fuel cell electrode catalyst under conditions where the relative humidity is 60% or less, particularly 40% or less. The fuel gas or oxidant gas supplied to the fuel cell can be used after adjusting the humidity to a desired humidity by a known method.

The PEFC and membrane electrode assembly described above use a catalyst layer that has a high utilization rate of catalyst metal and is excellent in power generation performance and durability. Therefore, the PEFC and membrane electrode assembly are excellent in power generation performance and durability, and the amount of catalyst metal used can be reduced, so that the manufacturing cost is reduced.

The PEFC of this embodiment and the fuel cell stack using the same can be mounted on a vehicle as a driving power source, for example.

The effect of the present invention will be described using the following examples and comparative examples. However, the technical scope of the present invention is not limited only to the following examples.

(Example 1)
A carrier A having a BET specific surface area of 1750 m ² / g was prepared. Specifically, carrier A was prepared by the method described in International Publication No. 2009/75264.

Using the carrier A, platinum (Pt) having an average particle diameter of 4 nm as a catalyst metal was supported so as to have a supporting rate of 50% by weight to obtain catalyst powder A. That is, 46 g of carrier A was immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid solution having a platinum concentration of 4.6% by mass, and 100 ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours, and platinum was supported on the carrier A. And it filtered and dried and obtained catalyst powder A with a loading rate of 50 weight%.

The annealing treatment was performed by holding the catalyst powder produced by the above process in 900% hydrogen gas at 900 ° C. for 1 hour.

Catalyst powder A was subjected to an oxidizing solution treatment for adding an acidic group. The catalyst powder A was immersed in a 3.0 mol / L nitric acid aqueous solution at 80 ° C. for 2 hours, and then filtered and dried to obtain a catalyst powder A having an acidic group.

The catalyst powder A thus obtained was measured for BET specific surface area and found to be 1750 m ² / g carrier.

A catalyst powder A having an acidic group and an ionomer dispersion (Nafion (registered trademark) D2020, EW = 1100 g / mol, manufactured by DuPont) as a polymer electrolyte, and a weight ratio of the carbon support to the ionomer of 0.9 It mixed so that it might become. Further, an n-propyl alcohol solution (50%) was added as a solvent so that the solid content (Pt + carbon carrier + ionomer) was 7% by weight to prepare a cathode catalyst ink.

Ketjen black (particle size: 30 to 60 nm) is used as a carrier, and platinum (Pt) having an average particle size of 2.5 nm is supported on the catalyst metal so that the supported amount is 50% by weight. Got. This catalyst powder was mixed with an ionomer dispersion (Nafion (registered trademark) D2020, EW = 1100 g / mol, manufactured by DuPont) as a polymer electrolyte so that the weight ratio of the carbon support to the ionomer was 0.9. Further, an anode catalyst ink was prepared by adding an n-propyl alcohol solution (50%) as a solvent so that the solid content (Pt + carbon carrier + ionomer) was 7% by weight.

Next, a gasket (manufactured by Teijin DuPont Films, Teonex (registered trademark), thickness: 25 μm (adhesive layer: 25 μm) around both sides of a polymer electrolyte membrane (Dupont, Nafion (registered trademark) NR211; thickness: 25 μm). 10 μm)). Next, the catalyst ink was applied to a size of 5 cm × 2 cm by spray coating on the exposed portion of one side of the polymer electrolyte membrane. The catalyst ink was dried by maintaining the stage for spray coating at 60 ° C. to obtain a cathode catalyst layer. The amount of platinum supported at this time is 0.15 mg / cm ² . Next, similarly to the cathode catalyst layer, spray coating and heat treatment were performed on the electrolyte membrane to form an anode catalyst layer, thereby obtaining a membrane electrode assembly (1) (MEA (1)) of this example.

(Example 2)
Black Pearls (registered trademark) (carrier B) having a BET specific surface area of 1440 m ² / g was prepared.

Using the carrier B, platinum (Pt) having an average particle size of 4 nm was supported on the catalyst metal so that the supporting rate was 30% by weight to obtain catalyst powder B. That is, 107 g of carrier B was immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid solution having a platinum concentration of 4.6% by mass and stirred, and then 100 ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours, and platinum was supported on the carrier B. And it filtered and dried and obtained catalyst powder B with the load of 30 weight%.

Catalyst powder B was treated with an oxidizing solution for addition of acidic groups. The catalyst powder B was immersed in a 3.0 mol / L nitric acid aqueous solution at 80 ° C. for 1 hour, then filtered and dried to obtain a catalyst powder B having an acidic group.

The catalyst powder B thus obtained was measured for BET specific surface area to be 1291 m ² / g carrier.

Membrane electrode assembly (2) (MEA (2)) was obtained in the same manner as in Example 1 except that the catalyst powder B thus obtained was used instead of the catalyst powder A.

(Example 3)
As the carrier C, the same carrier as in Example 1 was prepared.

Using the carrier C, platinum (Pt) having an average particle diameter of 4 nm was supported as a catalyst metal so that the supporting rate was 30% by weight, and catalyst powder C was obtained. That is, 107 g of carrier C was immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid solution having a platinum concentration of 4.6 mass%, and 100 ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours, and platinum was supported on the carrier C. And it filtered and dried and obtained catalyst powder C with the load of 30 weight%.

Catalyst powder C was treated with an oxidizing solution for adding an acidic group. The catalyst powder C was immersed in an aqueous 3.0 mol / L nitric acid solution at 80 ° C. for 1 hour, then filtered and dried to obtain catalyst powder C having an acidic group.

The catalyst powder C thus obtained was measured for BET specific surface area and found to be 1750 m ² / g carrier.

Membrane electrode assembly (3) (MEA (3)) was obtained in the same manner as in Example 1 except that the catalyst powder C thus obtained was used instead of the catalyst powder A.

(Comparative Example 1)
Ketjen Black EC300J (Ketjen Black International Co., Ltd.) (carrier D) having a BET specific surface area of 720 m ² / g was prepared.

Using the carrier D, platinum (Pt) having an average particle diameter of 5 nm was supported on the carrier metal so as to have a supporting rate of 50% by weight to obtain catalyst powder D. That is, 46 g of carrier D was immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid solution having a platinum concentration of 4.6% by mass and stirred, and 100 ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours, and platinum was supported on the carrier D. And it filtered and dried and obtained catalyst powder D with the load of 50 weight%.

Thereafter, in a hydrogen atmosphere, the temperature was maintained at 900 ° C. for 1 hour to obtain catalyst powder D. The catalyst powder D thus obtained was measured for BET specific surface area and found to be 705 m ² / g carrier.

A comparative membrane electrode assembly (1) (comparative MEA (1)) was obtained in the same manner as in Example 1 except that the catalyst powder D thus obtained was used instead of the catalyst powder A. .

(Comparative Example 2)
As the carrier E, the same carrier as in Example 2 was prepared.

Using the carrier E, platinum (Pt) having an average particle diameter of 4 nm was supported on the carrier metal so that the supporting rate was 50% by weight to obtain catalyst powder E. That is, 46 g of carrier E was immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid solution having a platinum concentration of 4.6% by mass and stirred, and then 100 ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours, and platinum was supported on the carrier E. And it filtered and dried and obtained catalyst powder E with the load of 50 weight%.

Thereafter, in a hydrogen atmosphere, the temperature was maintained at 900 ° C. for 1 hour to obtain catalyst powder E. The catalyst powder E thus obtained was measured for BET specific surface area to be 1291 m ² / g carrier.

A comparative membrane electrode assembly (2) (comparative MEA (2)) was obtained in the same manner as in Example 1 except that the catalyst powder E thus obtained was used instead of the catalyst powder A. .

(Measurement of acid group content)
The amount of acidic groups was measured by the following titration method. That is, first, 2.5 g of the catalyst powder having an acidic group was washed with 1 L of warm pure water and dried. After drying, the amount of carbon contained in the catalyst having an acidic group was measured to be 0.25 g, stirred with 55 ml of water for 10 minutes, and then subjected to ultrasonic dispersion for 2 minutes. Next, this catalyst dispersion was moved to a glove box purged with nitrogen gas, and nitrogen gas was bubbled for 10 minutes. Then, an excessive amount of a 0.1 M aqueous base solution was added to the catalyst dispersion, and the basic solution was subjected to neutralization titration with 0.1 M hydrochloric acid, and the amount of functional groups was determined from the neutralization point. Here, three types of NaOH, Na ₂ CO ₃ and NaHCO ₃ are used as the aqueous base solution, and neutralization titration work is performed for each. This is because the type of functional group to be neutralized differs for each base used. In the case of NaOH, the carboxyl group, lactone group, and hydroxyl group, and in the case of Na ₂ CO ₃ , the carboxyl group, lactone group, and NaHCO 3 are used. _This is because the case ₃ is neutralized with a carboxyl group. Then, the amount of acidic groups was calculated from the results of the types and amounts of the three types of bases added in these titrations and the amount of hydrochloric acid consumed. For confirmation of the neutralization point, a pH meter was used. The pH was 7.0 for NaOH, pH 8.5 for Na ₂ CO ₃ , and pH 4.5 for NaHCO ₃ . Thereby, the total amount of carboxyl groups, lactone groups, and hydroxyl groups added to the catalyst was determined.

[Electrolyte coverage]
The coverage of the electrolyte with respect to the catalyst metal was calculated by measuring the electric double layer capacity formed at the interface of the catalyst with the solid proton conductive material and the liquid proton conductive material. . In calculating the coverage, it was calculated from the ratio of the electric double layer capacity in the low humidified state to the high humidified state, and the measured values under the conditions of 5% RH and 100% RH were used as representatives of the humidity state. .

<Measurement of electric double layer capacity>
For the MEAs obtained in Example 3 and Comparative Example 1, the electric double layer capacities in the highly humidified state, the low humidified state, the catalyst deactivation and the highly humidified state, and the low humidified state were measured by electrochemical impedance spectroscopy. The contact area of the catalyst with both proton conducting materials in the electrode catalyst of both batteries was compared.

As the equipment used, the measurement conditions shown in Table 1 were employed using an electrochemical measurement system HZ-3000 manufactured by Hokuto Denko Corporation and a frequency response analyzer FRA5020 manufactured by NF Circuit Design Block.

First, each battery was heated to 30 ° C. with a heater, and the electric double layer capacity was measured in a state where nitrogen gas and hydrogen gas adjusted to the humidified state shown in Table 1 were supplied to the working electrode and the counter electrode, respectively.

When measuring the electric double layer capacity, as shown in Table 1, it was held at 0.45 V, and the working electrode potential was oscillated with an amplitude of ± 10 mV and a frequency range of 20 kHz to 10 mHz.

That is, the real part and imaginary part of the impedance at each frequency are obtained from the response when the working electrode potential vibrates. Since the relationship between the imaginary part (Z ″) and the angular velocity ω (converted from the frequency) is expressed by the following equation, the reciprocal of the imaginary part is arranged with respect to −2 to the angular velocity, and when the −2 to the angular velocity is 0 The electric double layer capacitance C _dl is obtained by extrapolating the value.

Such measurement was sequentially performed in a low humidified state and a high humidified state (5% RH → 10% RH → 90% RH → 100% RH condition).

Furthermore, after deactivating the Pt catalyst by flowing nitrogen gas containing CO at a concentration of 1% (volume ratio) to the working electrode at a rate of 1 NL / min for 15 minutes or more, the high humidification and low humidification conditions as described above The electric double layer capacity was measured in the same manner. These results are shown in Table 2. In addition, the obtained electric double layer capacity was shown in terms of a value per area of the catalyst layer.

Based on the measured values, the electric double layer capacity formed at the catalyst-solid proton conductive material (CS) interface and the catalyst-liquid proton conductive material (CL) interface was calculated.

In the calculation, the measured values under the conditions of 5% RH and 100% RH were used as representatives of the electric double layer capacity in the low and high humidification states.

Experiment 1: Evaluation of oxygen reduction reaction activity Membrane electrode assemblies (1) to (3) prepared in Examples 1 to 3 and Comparative membrane electrode assemblies (1) and (1) prepared in Comparative Examples 1 and 2 For 2), the current density (A / m ² -Pt) of the generated current per platinum surface area at 0.9 V was measured under the following evaluation conditions. Thereby, the oxygen reduction reaction (ORR) activity was evaluated.

The results are shown in Table 2 below.

From the results of Table 2 above, the catalysts of Examples 1 to 3 in which the BET specific surface area of the catalyst is more than 1200 m ² / g support and the amount of acidic groups is 0.7 mmol / g support or more are Comparative Examples 1, 2 It can be seen that the oxygen reduction reaction activity per platinum surface area is excellent as compared with the above catalyst. Further, from comparison between Example 2 and Example 3, the larger the BET specific surface area of the catalyst, the higher the oxygen reduction reaction activity per platinum surface area. Further, from comparison between Example 1 and Example 3, it was found that the oxygen reduction reaction activity could be improved when the platinum loading was 30% by weight or less. Moreover, if the electrolyte coverage is 0.3 or less, the oxygen reduction reaction activity can be improved. From these results, it can be seen that according to the catalyst of the present invention, the utilization rate of the catalyst metal can be increased, and the usage amount of the catalyst metal can be reduced to contribute to the reduction of the production cost of the catalyst.

This application is based on Japanese Patent Application No. 2013-092918 filed on April 25, 2013, the disclosure of which is incorporated by reference in its entirety.

1 Polymer electrolyte fuel cell (PEFC),
2 solid polymer electrolyte membrane,
3a anode catalyst layer,
3c cathode catalyst layer,
4a Anode gas diffusion layer,
4c cathode gas diffusion layer,
5a anode separator,
5c cathode separator,
6a Anode gas flow path,
6c cathode gas flow path,
7 Refrigerant flow path,
10 Membrane electrode assembly (MEA),
20 catalyst,
22 catalytic metal,
23 carrier,
24 pores (mesopores),
25 acidic groups,
26 Electrolyte.

Claims

A catalyst comprising a catalyst carrier and a catalyst metal supported on the catalyst carrier,
A catalyst characterized in that the BET specific surface area per weight of the support is more than 1200 m 2 / g support, and the amount of acidic groups per weight of the support is 0.7 mmol / g or more.
The catalyst according to claim 1, wherein the acidic group is at least one selected from the group consisting of a hydroxyl group, a lactone group, and a carboxyl group.
The catalyst according to claim 1 or 2, wherein the support has a BET specific surface area of 1500 m 2 / g support or more.
The catalyst according to any one of claims 1 to 3, wherein the carrier contains carbon.
The catalyst metal according to any one of claims 1 to 4, wherein the catalyst metal is platinum or contains a metal component other than platinum and platinum, and the amount of the catalyst metal supported on the catalyst is 50% by weight or less. catalyst.
The catalyst according to claim 5, wherein the amount of the catalyst metal supported on the catalyst is 30% by weight or less.
An electrode catalyst layer for a fuel cell, comprising the catalyst according to any one of claims 1 to 6 and an electrolyte.
8. The fuel cell according to claim 7, wherein in the catalyst, an electrolyte coverage of the catalyst metal calculated from a ratio of an area where the catalyst metal surface is covered with an electrolyte to a surface area of the catalyst metal is 0.3 or less. Electrocatalyst layer.
A fuel cell membrane electrode assembly comprising the fuel cell electrode catalyst layer according to claim 7 or 8.
A fuel cell comprising the fuel cell membrane electrode assembly according to claim 9.