JP2005190887A - ELECTRODE CATALYST HAVING SURFACE STRUCTURE FOR CONSTRUCTING CATALYST LAYER HAVING HIGH PERFORMANCE AND DUTY AND PROCESS FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell electrode catalyst having a surface structure suitable for constructing a catalyst layer having high performance and durability.
In an electrode catalyst in which a catalyst metal is supported on a support, (CO peak height) / (C obtained by peak splitting of a C1s peak obtained by X-ray photoelectron spectroscopy of the electrode catalyst. -C peak height) ratio is 0.2 or more, The electrode catalyst for fuel cells characterized by the above-mentioned.
[Selection] Figure 2

Description

  The present invention relates to a fuel cell electrode having a surface structure for constructing a catalyst layer of a fuel cell electrode having high performance and durability, and preferably an electrode catalyst that can be used for a polymer electrolyte fuel cell electrode and It relates to the manufacturing method.

  In recent years, in response to social demands and trends against the background of energy and environmental issues, polymer electrolyte fuel cells that can operate at room temperature and obtain high output density have attracted attention as power sources for electric vehicles and stationary power sources. .

  The main components of such a polymer electrolyte fuel cell are an anode and a cathode, a separator plate that forms a gas flow path, and a solid polymer electrolyte membrane that separates the electrodes. Protons generated on the anode catalyst move through the solid polymer electrolyte membrane, reach the cathode catalyst, and react with oxygen. Therefore, the ion conduction resistance between the two electrodes greatly affects the battery performance.

  In order to form a fuel cell, it is necessary to join the catalyst of both electrodes and the solid polymer electrolyte membrane through an ion conduction path. For this purpose, a catalyst layer in which an electrolyte polymer solution and catalyst particles are mixed, applied and dried to bond them together is used as an electrode, or the electrode catalyst and a solid polymer electrolyte membrane are pressed under heating. The technique was commonly used.

  As the electrolyte polymer responsible for ionic conduction, a polymer in which a sulfonic acid group is introduced into a perfluoro main chain is generally used. As specific products, Nafion manufactured by DuPont, Flemion manufactured by Asahi Glass Co., Ltd., Aciplex manufactured by Asahi Kasei Co., Ltd., and the like are used. A perfluoro-based electrolyte polymer is composed of a perfluoro main chain and a side chain having a sulfonic acid group, and is microphase-separated into a sulfonic acid group phase and a perfluoro main chain phase to form a sulfonic acid group. The phases are said to form clusters. What contributes to ionic conduction is the portion where sulfonic acid groups are gathered to form clusters.

  In the catalytic reaction, it is necessary that ions are efficiently exchanged on the catalyst surface of the electrode. Therefore, it is desirable that the sulfonic acid group contributing to ionic conduction is oriented on the catalyst surface. In order to obtain this state, the catalyst surface needs to be hydrophilic.

  However, the electrode catalyst is one in which a metal having catalytic activity such as platinum is supported on the surface of carbon black particles. The surface of the carbon black particles is originally hydrophobic, and the surface of the metal having catalytic activity is not in a hydrophilic state due to impurities or the like. That is, in such a carbon carrier, the graphitization treatment is performed to improve the corrosion resistance, and the carbon surface becomes more hydrophobic, so that the entanglement state between the polymer electrolyte and the electrode catalyst tends to decrease. It was. Therefore, if the interface between the electrode catalyst particles and the polymer electrolyte is formed without any special treatment, the sulfonic acid group cannot be oriented on the electrode catalyst surface. As a result, in the conventional method, it has been difficult to reduce the ion conduction resistance at the interface.

As a means to solve these problems, in a polymer electrolyte fuel cell, carbon that forms an interface with the electrolyte polymer in order to reduce the ionic conduction resistance between the electrolyte polymer and the catalyst that forms an interface with the electrolyte polymer and to obtain high power generation efficiency. The surface of the black particle carrying a metal having catalytic activity is hydrophilized by plasma irradiation or immersion in an aqueous solution containing hydrogen peroxide or an inorganic acid, and the like, for example, a sulfonic acid group (See, for example, Patent Document 1).
JP 2002-324557 A

  The above-mentioned Patent Document 1 describes a method for controlling the state of the catalyst surface and the interface between the electrolyte polymer (that is, a point for controlling the catalyst surface to be hydrophilic and to orient the sulfonic acid group phase of the electrolyte polymer). However, there has been no description of what catalyst surface state is more durable and suitable for a high-performance catalyst layer by its treatment. In addition, there is no description regarding the processing temperature, processing adjustment conditions, concentration, and the like necessary for it. Therefore, in reality, a catalyst having a surface structure suitable for constructing a catalyst layer having high performance and durability and a polymer electrolyte fuel cell using the same cannot be provided.

  Accordingly, an object of the present invention is to provide a fuel cell electrode catalyst having a catalyst surface structure optimized for constructing a catalyst layer having high performance and durability, a method for producing the same, and the use of the electrode catalyst. An electrode for a fuel cell and a method for manufacturing the same are provided.

  As a result of intensive studies by the present inventor to solve the above-mentioned problems, it is obtained by peak splitting of the C1s peak obtained by X-ray photoelectron spectroscopy (hereinafter, also simply referred to as XPS) of the catalyst ( It has been found that the above problem can be solved by adjusting the surface state so that the ratio of (C—O peak height) / (C—C peak height) is in a certain range.

  That is, the present invention provides an electrode catalyst in which a catalytic metal is supported on a support, and is obtained by peak splitting of the C1s peak obtained by XPS of the electrode catalyst (CO peak height) / (CC The fuel cell electrode catalyst is characterized by having a peak height ratio of 0.2 or more.

  In the electrode catalyst of the present invention, by adjusting the surface state so that the (CO peak height) / (CC peak height) ratio obtained by XPS of the catalyst is 0.2 or more, so to speak, The coverage of the hydrophilic group represented by the C—O peak height can be adjusted. That is, a hydrophilic group having high affinity with the electrolyte polymer can be provided on the surface. Thereby, the coverage of the electrolyte polymer can be controlled. As a result, the gas diffusion region and the proton conduction path can be controlled, and the catalyst layer can be arbitrarily designed. This contributes to the moisture management of the fuel cell, thereby improving the cell performance capability.

  Further, in the electrode catalyst of the present invention, by adjusting the surface state so that the (CO peak height) / (CC peak height) ratio obtained by XPS of the catalyst is 0.2 or more, A support surface having a surface covered with a hydrophilic group can be obtained. Thereby, the affinity with the hydrophilic part of the electrolyte polymer is improved, and the dispersibility of the catalyst in the catalyst layer and the affinity with the electrolyte polymer are also improved. Therefore, the proton conduction path is maintained, and the effects of improving proton conductivity and cell performance are obtained. As a result, it is possible to construct a high durability and high performance catalyst layer.

  The present invention provides (C—O peak height) / (C—C peak height) obtained by peak splitting of the C1s peak obtained by XPS of an electrode catalyst having a catalyst metal supported on a support. The fuel cell electrode catalyst is characterized in that the ratio is 0.2 or more, preferably 0.25 to 0.5.

  Hereinafter, the electrode catalyst of the present invention (hereinafter also simply referred to as catalyst) will be described in more detail.

  The basic structure of the electrode catalyst of the present invention is that a catalytic metal is supported on a support.

  The carrier is not particularly limited as long as it has sufficient electronic conductivity as a current collector as well as a substrate having a specific surface area sufficient to carry the catalyst in a highly dispersed manner. Conventionally known ones can be used. Among these, those whose main component is carbon are preferable. When the electric resistance of the carrier is high, the internal resistance of the electrode catalyst, and hence the electrode for the fuel cell, becomes high, resulting in a decrease in battery performance. The carrier can have a sufficiently high electronic conductivity by using carbon as a main component, and can reduce the electrical resistance. More specifically, as the support, carbon black such as channel black, furnace black, and thermal black; activated carbon obtained by carbonizing and activating a material containing various carbon atoms; and carbon such as graphitized carbon as a main component. Things.

  The catalyst metal has a function of promoting an electrode reaction. As the catalyst metal, a conventionally known metal can be used, but preferably contains at least one selected from Pt, Ir, Au, Ag, and Pd. Among them, Pt exhibits high oxygen reduction activity and hydrogen reduction activity, so that platinum is used alone as one of the preferred embodiments of the catalyst metal. Further, in order to enhance the stability and activity of the catalyst metal, it is another preferred embodiment of the catalyst metal to be used as a noble metal alloy based on Pt or a noble metal-base metal mixture based on Pt. . Here, the base metal is not particularly limited, and includes at least one base metal selected from the group consisting of chromium, manganese, iron, cobalt, and nickel.

  The smaller the average particle diameter of the catalyst metal, the higher the oxygen reduction activity because the effective electrode area where the electrochemical reaction proceeds increases. Actually, when the particle diameter of the catalyst metal becomes too small, a phenomenon is observed in which the activity is rather lowered. Therefore, the average particle diameter of the catalyst metal is preferably 1 to 10 nm, more preferably 2 to 5 nm.

  The average particle diameter of the catalyst metal can also be defined from the relationship with the support. That is, in conductive carbon black usually used as a carrier, primary carbon particles having a diameter of about 10 to 50 nm form an aggregate (aggregate) structure, which is further agglomerated. It forms an agglomerate (agglomerate) structure. Carbon having a large specific surface area has many fine pores in the order of nm in such a higher-order structure. Since it is desirable that the carrier that also functions as a current collector and the catalyst metal are in contact with each other more, the catalyst metal can penetrate into the microstructure of the carrier and is fine enough to achieve more uniform and highly dispersed support. It is required to be a particle. From the above, the average particle diameter of the catalyst metal that can be used in the present invention is 0.1 to 1 times, preferably 0.4 to 0.8 times the primary particle diameter of the support. desirable. This is because, if the average particle diameter of the catalyst metal is within such a range, the catalyst metal can penetrate into the fine structure of the support and can be uniformly dispersed and supported on the support.

  Here, the “average particle diameter of the catalyst metal” is measured by the average value of the crystallite diameter obtained from the half width of the diffraction peak of the catalyst metal in X-ray diffraction or the particle diameter of the catalyst metal determined from the transmission electron microscope image. be able to.

  The amount of the catalyst metal supported on the support is preferably 10 to 70% by mass, more preferably 20 to 60% by mass, and particularly preferably 30 to 40% by mass with respect to the total amount of the electrode catalyst. Good. If the supported amount of the catalyst metal is less than 10% by mass, there is a possibility that sufficient catalyst activity by the catalyst metal cannot be obtained. For this reason, in order to obtain high activity, the electrode must be thickened, which is undesirable because the battery performance may be deteriorated due to an increase in the internal resistance of the electrode. Further, when the amount of the catalyst metal supported exceeds 80% by mass, the overlap of the catalyst metals supported on the support increases, so that a sufficient catalytic metal reaction area cannot be obtained and the catalytic activity is lowered. There is a fear.

  In the electrode catalyst of the present invention, the (CO peak height) / (CC peak height) ratio (hereinafter simply referred to as the CO / CC ratio) obtained by the peak splitting of the C1s peak obtained by XPS of the catalyst. (Abbreviated) is 0.2 or more, preferably 0.25 to 0.5. In other words, the requirement is that the hydrophilic functional group (simply referred to as a hydrophilic group) on the catalyst surface is quantitatively defined, and the coverage by the hydrophilic group is defined. That is, when the CO / CC ratio is 0.2 or more, preferably 0.25 to 0.5, it can be said that the surface coverage by hydrophilic groups is 20% or more, preferably 25 to 50%. For example, when the hydroxyl group is coated on the surface, as shown in FIG. 2, if the CO peak height is 1 and the CC peak height is 4, the CO / CC ratio is 1 / 4 = 0.25. In this case, the coverage by the hydrophilic group (hydroxyl group) is defined as 25%. When the CO / CC ratio is less than 0.2 (less than 20% coverage), in addition to improving affinity with ionomers, it has a surface structure suitable for building a catalyst layer having high performance and durability. It becomes difficult to provide a new catalyst. The upper limit of the CO / CC ratio is not particularly limited, but when it exceeds 0.5, the hydrophobic functional group dominates the surface. This is disadvantageous in improving the affinity with the ionomer (forming a three-phase zone interface).

  Thus, by satisfying the above requirements of the present invention, the surface state of the catalyst is more durable and suitable for a high-performance catalyst layer, that is, the catalyst surface (mainly the support surface) is covered with hydrophilic groups. It can be in a broken state. As a result, an electrolyte polymer (ionomer; also referred to as an ionic polymer material) responsible for ion conduction can uniformly cover the catalyst.

  FIG. 1 shows an explanatory schematic surface schematically showing the surface state of these catalysts, the connection between the catalyst and the electrolyte polymer, and the reaction route (process).

As shown in FIG. 1, the catalyst 15 formed by supporting a catalytic metal 13 on carrier 11, a hydroxyl group (OH group) as a hydrophilic group, a carbonyl group (CO ^- group), a carboxyl group (COOH group) and the like catalysts The above requirements of the present invention can be satisfied by existing so as to cover the surface of 15. As a result, when the perfluoro type is taken as an example of the electrolyte polymer 17, the hydrophilic part 17 b of the sulfonic acid group (SO ₃ ^- group) introduced into the hydrophobic part 17 a of the main chain of the perfluoro type, and the hydrophilicity of the catalyst 15. Affinity between groups can be improved.

Therefore, in the catalyst layer using the catalyst, the dispersibility of the catalyst particles in the catalyst layer and the affinity with the electrolyte polymer can be greatly improved. Therefore, the proton conduction path (H ⁺ introduction path in FIG. 1) is maintained, and the effect of improving proton conductivity and cell performance is obtained (high performance catalyst layer can be constructed).

In addition, the surface state that satisfies the above-described structural requirements is a state in which the mixing of impurities on the surface is suppressed, and it can be said that there are few factors that inhibit the proton conduction path. Therefore, the active site that has been disturbed by the impurities can be recovered. That is, the surface of the catalytic metal is clean and there are no active sites due to contaminants, whereby the utilization rate of the catalytic active sites can be 100% and the catalytic activity is improved. Further, there are few impurities that are obstructive factors for the oxygen reduction 4-electron path (O ₂ introduction path in FIG. 1), and the effect of significantly suppressing or preventing H ₂ O ₂ generated when impurities are present (FIG. 1, there is an effect of preventing generation of H ₂ O ₂ on the catalyst metal 13. Further, poisoning of the catalyst metal (Pt or the like) and further deterioration of the electrolyte polymer or electrolyte membrane in the catalyst layer can be prevented. As a result, it is possible to contribute to improving the durability of the catalyst layer and, consequently, the battery.

  In the electrode catalyst of the present invention, it is preferable that at least one hydrophilic group among a hydroxyl group, a carbonyl group, and a carboxyl group exists on the support. These hydrophilic groups can be measured as the CO peak height, and are effective in adjusting and controlling the surface state of the support (hydrophilic group coverage) using the requirements of the CO / CC ratio. is there.

  Here, the hydrophilic group may be at least one of a hydroxyl group, a carbonyl group, and a carboxyl group, and two or more types may exist as shown in FIG. However, the hydrophilic group of the present invention is not limited to these.

  Next, the method for producing the electrode catalyst of the present invention as described above will be described.

  The method for producing an electrode catalyst of the present invention is characterized in that a catalytic metal is supported on a support and then treated (pretreated) with an aqueous solution containing active oxygen. Preferably, the pretreatment includes a step of treating with an aqueous solution containing active oxygen after supporting the catalyst metal on the support (oxidation step), and a step of cleaning with ultrapure water (cleaning step). It is a feature. More preferably, the oxidation step includes a step of dispersing an electrocatalyst carrying a catalytic metal on a carrier in an aqueous solution containing active oxygen (dispersion step), and oxygen generation in the dispersion obtained in the dispersion step. The cleaning step includes a step of performing filtration and catalyst cleaning with ultrapure water after the oxygen generation step is completed.

By the above production method, the catalyst surface (mainly the support surface) can be in a state of being covered with a hydrophilic group. Further, impurities are removed, the utilization rate of the catalytic activity site can be improved (a utilization rate of 100% is expected), and the catalytic activity is improved. Further, removal of impurities has an effect of preventing H ₂ O ₂ generated when contaminants are present during cell power generation, and can prevent deterioration of the electrolyte polymer and the electrolyte membrane in the catalyst layer due to generation of H ₂ O _2. . As a result, it is possible to improve the performance and durability of the catalyst layer, and consequently the battery performance.

  FIG. 8 is an explanatory schematic view schematically showing the surface state of the electrode catalyst before and after the pretreatment and the connection with the electrolyte polymer in the production method of the present invention. Among these, Fig.8 (a) is the explanatory schematic surface which represented typically the surface state in the electrode catalyst single before a pre-processing. FIG. 8B is an explanatory schematic view schematically showing the surface state of the electrode catalyst after the pretreatment and the connection with the electrolyte polymer. As shown in FIG. 8 (a), in the electrode catalyst 81 before the pretreatment, there exists a second component 87 to be dissolved in / on the catalyst metal 85 such as platinum alloy supported on a carrier 83 such as carbon. It is in the state. This can be eluted in advance by pretreatment. As a result, as shown in FIG. 8B, the second component 87 is not present in / on the catalyst metal 85 in the electrode catalyst 81 ′ after the pretreatment. Therefore, it is possible to prevent performance degradation and deterioration due to in-battery contamination of the electrolyte polymer 89 such as Nafion or the electrolyte membrane (not shown) due to elution from the catalyst metal 85 during battery operation. In addition, the said 2nd component to melt | dissolve means elements other than Pt, for example, Al, Si, Ti, V, Cr, Fe, Ni, Co, Cu, W, Ag, V etc. correspond.

  Hereinafter, the manufacturing method of the present invention will be described along with the steps.

  In the treatment step (oxidation step) with the above aqueous solution containing active oxygen, oxidation treatment (hydrophilic treatment) is carried out in an aqueous solution containing active oxygen, and the CO / CC ratio of the catalyst is 0.2 or more (coating with hydrophilic groups) Adjustment and control may be performed so that the rate is 20% or more. In other words, it can be said that the surface of the support may be oxidized so that a surface structure can be constructed in which the electrolyte polymer can uniformly cover the catalyst.

  Therefore, the step of treating with the aqueous solution containing active oxygen includes, for example, a method of performing a hydrophilic treatment by immersing an electrode catalyst carrying a catalyst metal on a carrier in an aqueous solution containing active oxygen. . More specifically, a process (dispersion process) in which an electrode catalyst having a catalyst metal supported on a support is dispersed in an aqueous solution containing active oxygen, and oxygen is generated in the dispersion solution obtained in the dispersion process. It is desirable to include a process (oxygen generation process). Hereinafter, the process of treating with an aqueous solution containing active oxygen will be described separately for the dispersion process and the oxygen generation process, but the present invention is not limited to these.

  In the dispersion process, an appropriate stirring / dispersing device such as a homogenizer, an ultrasonic dispersing device, a planetary ball mill, etc. is used so that the catalyst particles originally having a hydrophobic surface do not aggregate or agglomerate in an aqueous solution containing active oxygen. What is necessary is just to enable it to disperse | distribute and mix using an appropriate means.

  Here, the electrode catalyst in which the catalyst metal is supported on the support is as already described in the section “Basic structure of the electrode catalyst of the present invention”, and the description thereof is omitted here.

  Moreover, as a method of carrying | supporting a catalyst metal on a support body, a well-known method is mentioned, It does not specifically limit. For example, an electrode catalyst having a catalyst metal supported on the support can be obtained by dispersing the support in a catalyst metal compound solution and adding a reducing agent thereto.

  The catalyst metal compound solution is a solution containing a catalyst metal compound such as chloroplatinic acid, chloroplatinum platinum, or dinitrodiammine platinum when Pt is used as the catalyst metal. In order to obtain a noble metal-base metal mixture catalyst or the like, a base metal compound such as a desired base metal nitrate, chloride or sulfate may be dispersed in the solution in addition to platinum. As the solvent to which the catalytic metal compound is added, water, methanol, ethanol, the mixed solvent described on the left, and the like can be used.

  In order to disperse the carrier in the catalyst metal compound solution, an appropriate dispersing means such as a homogenizer or an ultrasonic dispersing device may be used. What is necessary is just to determine suitably the catalyst metal density | concentration in a catalyst metal compound solution, the compounding ratio of a catalyst metal compound, and a support body etc. so that the obtained electrode catalyst may have a desired characteristic.

  The reducing agent is not particularly limited as long as it can reduce the catalytic metal compound. Sodium thiosulfate, citric acid, sodium citrate, L-ascorbic acid, sodium borohydride, hydrazine, formaldehyde, methanol, ethanol, Hydrogen, ethylene, carbon monoxide, or the like can be used. By adding the reducing agent, the catalyst metal compound can be highly dispersed and supported as catalyst metal particles on the support.

  An appropriate amount of the above reducing agent is added to the above dispersion of the catalytic metal compound and the support, heated to 60 to 100 ° C., preferably 60 to 80 ° C. using a reflux reactor, and then allowed to cool to room temperature. Thus, reduction loading of the catalyst metal can be performed on the surface of the support.

  In order to alloy the catalyst particles, it is preferable to further perform firing. The calcination method varies depending on the dispersed state of catalyst particles on the support, the particle diameter, and the like, but preferably in an inert atmosphere such as nitrogen, helium or argon, calcination temperature is 300 to 1000 ° C., preferably 200 to 400. What is necessary is just to carry out at 1 degreeC for about 1 to 6 hours.

  Although the reducing agent is used in the above-described method for supporting the catalytic metal on the support, it is not particularly limited to this. For example, various known techniques such as an impregnation method, a coprecipitation method, and a competitive adsorption method can be used. Alternatively, an electrode catalyst may be produced using a microemulsion method described in JP-A-7-246343, JP-A-10-216517, and the like. Also by the microemulsion method, the catalyst metal can be highly dispersed and supported on the conductive support.

Next, the aqueous solution containing the active oxygen is not particularly limited, but as a solution condition for constructing a good catalyst surface, one containing hydrogen peroxide (H ₂ O ₂ ) is desirable. However, in the present invention, as an aqueous solution containing active oxygen, hydrogen peroxide (H ₂ O ₂ ), sodium hydroxide (NaOH), sodium hydrogen carbonate (NaHCO ₃ ), sodium hypochlorite, chlorine, dioxide monoxide Examples include, but are not limited to, an aqueous solution containing one or more of nitrogen, potassium permanganate, potassium dichromate, and inorganic acids.

  The inorganic acid here includes sulfuric acid, nitric acid and the like, but is not limited thereto.

  In the present invention, two or more kinds of aqueous solutions containing different active oxygens may be prepared, and the catalyst may be sequentially oxidized in these aqueous solutions (specifically, dispersed to generate oxygen). The aqueous solutions containing different active oxygens are not particularly limited, for example, those having different types of aqueous solutions, those having different concentrations, and those having different types and concentrations of the aqueous solutions.

The H ₂ O ₂ concentration of the solution containing hydrogen peroxide (H ₂ O ₂ ) is preferably 0.001 to 0.1% by mass as one of the solution conditions for constructing a good catalyst surface. When the H ₂ O ₂ concentration is less than 0.001% by mass, it becomes difficult to satisfy the requirement that the CO / CC ratio is 0.2 or more due to insufficient oxidizing power. When the H ₂ O ₂ concentration exceeds 0.1% by mass, the carbon support may be damaged by the oxidizing power. The upper limit of the hydrogen peroxide concentration is more preferably 1% by mass, and the lower limit is more preferably 0.001% by mass.

Similarly, the NaOH concentration of the solution containing sodium hydroxide (NaOH) is preferably 0.001 to 0.1% by mass as one of the solution conditions for constructing a good catalyst surface. When outside these ranges, it is the same as in the case of the H ₂ O ₂ concentration.

The NaHCO ₃ concentration of the solution containing sodium hydrogen carbonate (NaHCO ₃ ) is preferably 0.001 to 0.1% by mass as one of the solution conditions for constructing a good catalyst surface. When outside these ranges, it is the same as in the case of the H ₂ O ₂ concentration.

  The concentration of the inorganic acid in the solution containing the inorganic acid is preferably 0.01 to 0.1N. When the concentration of the inorganic acid is less than 0.01 N, it is difficult to satisfy the requirement that the CO / CC ratio is 0.2 or more. On the other hand, if it exceeds 1N, there is a risk of problems in handling such as heat generation during dilution. The concentration of the inorganic acid is more preferably 0.01 to 0.05N.

Next, in the oxygen generation process, oxidation treatment (hydrophilic treatment) may be performed until the CO / CC ratio is 0.2 or more = the coverage with hydrophilic groups is 20% or more. Thereby, a surface structure in which the electrolyte polymer can uniformly cover the catalyst can be constructed. Regarding the treatment temperature, treatment adjustment conditions, concentration, etc. at this time, these treatment conditions should be appropriately set so that impurities are removed (eluted) from the catalyst surface and the CO / CC ratio is 0.2 or more. That's fine. In addition, until the oxygen generation process is completed, a more uniform process is possible by stirring the dispersion solution using the apparatus used in the dispersion process.
In the present invention, it has been found that adjusting the conditions for the above-mentioned CO / CC ratio as the treatment conditions is more suitable for a durable and high-performance catalyst layer. Conventionally, it has not been described what kind of surface state is more durable and suitable for a high-performance catalyst layer by such treatment, and it has not been possible to find out how to set treatment conditions. .

  Next, in the step of cleaning with ultrapure water, the contaminants can be completely removed from the catalyst surface by performing cleaning with ultrapure water following the above-described treatment step with the active oxygen-containing aqueous solution. As a result, a good catalyst surface can be maintained.

  More specifically, the step of cleaning with ultrapure water preferably includes a step of performing filtration and catalyst cleaning with ultrapure water after the oxygen generation step is completed.

  Here, the completion of the oxygen generation process is a time when the requirement of the CO / CC ratio is satisfied. That is, the present invention has an advantage that it can be carried out while confirming in the production process whether the surface state of the catalyst is more durable and suitable for a high-performance catalyst layer. This can improve the productivity of the catalyst as compared with the conventional case where the catalyst performance cannot be judged without producing the fuel cell and evaluating the battery characteristics. That is, in the present invention, if the surface state of the catalyst is not sufficient during the production process, the oxygen generation process can be continued. Therefore, the yield of the product can be greatly improved, and a catalyst having a desired surface state can be obtained with a yield of almost 100%. For this reason, if the catalyst performance cannot be judged unless the fuel cell is manufactured, all battery products (parts) using such a catalyst are wasted. However, the present invention is advantageous in that it does not cause such a problem. .

  In the said filtration, what is necessary is just to be able to distinguish a catalyst particle and the aqueous solution containing used active oxygen. The aqueous solution containing used active oxygen on the filtrate side is desirably reused as an aqueous solution containing active oxygen after adjusting the concentration of active oxygen. A filtration apparatus for performing such filtration is not particularly limited as long as the above-described object can be achieved. For example, a filter using a millipore membrane filter can be used.

  As ultrapure water that can be used for the catalyst washing after the filtration, those having an electrical specific resistance of 18.3 MΩ · cm or less and a TOC (organic concentration) of 5 ppb or less are desirable. With respect to the electrical resistivity and TOC of these ultrapure waters, many measuring instruments are already available on the market and can be measured using them. Many ultrapure water production apparatuses that can satisfy these requirements are already on the market, and it is also possible to obtain desired ultrapure water using them.

  In addition, after washing the catalyst, the electrode catalyst may be dried while paying sufficient attention so that the hydrophilic group is not inactivated or impurities are not reattached to the catalyst surface. You may utilize for a slurry preparation process.

  The conditions for drying are not particularly limited, but it is desirable to heat and dry at about 60 to 80 ° C. in order to increase productivity. The drying method is not particularly limited as long as a known method such as vacuum drying, natural drying, rotary evaporator, and drying by a blower dryer is used. What is necessary is just to determine drying time etc. suitably according to the drying method.

  Moreover, in the manufacturing method of this invention, as shown to Fig.9 (a), when performing the said washing | cleaning process, the dispersibility to an ionomer solution cannot be maintained but it precipitates. On the other hand, as shown in FIG. 9B, dispersibility in the ionomer solution can be maintained when the washing step is not performed. This is because the catalyst is cleaned using ultrapure water, so that contaminants are completely removed and the surface is clean and hydrophilic, so that the catalyst particles easily aggregate in the solution. Conceivable. However, as will be described later, the inventors of the present invention can provide an electrode catalyst and an electrolyte polymer within a range of 10: 2 to 10: 6 by weight ratio. It has been found that it can be suitably dispersed in the catalyst slurry. Therefore, in the subsequent manufacturing process, the catalyst layer can be formed using conventional coating and printing techniques without problems due to aggregation of the catalyst particles.

In the production method of the present invention, the reduction treatment may be performed at an appropriate time in order to increase the catalytic activity of the electrode catalyst particles supporting the catalyst metal on the support. The reduction treatment may be performed by a known method such as a gas phase method using a reducing gas such as hydrogen or carbon monoxide; a liquid phase method (wet) using NaBH ₄ , formaldehyde, glucose, hydrazine, or the like. The reduction treatment is particularly limited after a catalyst metal is supported on a support, a desired electrode catalyst is produced by the pretreatment of the present invention, a catalyst layer is produced, or an electrode is produced. Not.

  According to the production method of the present invention, a hydrophilic group that improves the affinity with the electrolyte polymer can be introduced on the surface while maintaining a high specific surface area of the carrier. Moreover, since the introduced hydrophilic group can improve the affinity with the conductive electrolyte polymer, it is also possible to prevent a decrease in conductivity at these interfaces.

  In the method for producing an electrode catalyst of the present invention, an electrode having excellent characteristics as shown in FIGS. 3 to 6 is prepared by performing the pretreatment as described above and adjusting so as to satisfy the requirements of the CO / CC ratio. A catalyst can be obtained.

FIG. 3 shows a rotating electrode that is effective in evaluating the activity of the catalyst layer itself without the influence of the electrolyte membrane included in the power generation test in an actual fuel cell and the gas diffusion layer incidental to the catalyst layer. The catalyst layer is held on the electrode and the activity for the oxygen reduction reaction which is the positive electrode reaction is examined. Specifically, when rotating the rotating electrode at 1000 rpm, pretreated _(H 2 _{O 2} treatment) is prepared by using the electrode catalyst of the present invention in which a catalyst layer, and pretreatment _(H 2 _{O 2} treatment) 2 is a diagram showing a relationship between an oxygen reduction current I and an electrode potential E in a catalyst layer produced using a conventional electrode catalyst that is not used.

As can be seen from FIG. 3, in the case of using a conventional electrode catalyst that has not been pretreated (H ₂ O ₂ treatment), the limit current required for strict catalytic performance analysis does not appear due to impurities (dashed line in the figure). reference). On the other hand, in the case of using the electrode catalyst of the present invention that has been pretreated (H ₂ O ₂ treatment) so that the catalyst surface state is more durable and suitable for a high performance catalyst layer, strict catalytic ability analysis The critical current required for the is clearly shown (see the solid line in the figure). This shows that the impurities are completely removed by the pretreatment (H ₂ O ₂ treatment).

  FIG. 4 is a view showing a cyclic voltammogram in a sulfuric acid acidic solution of a catalyst layer (electrode) produced using the same electrode catalyst as in FIG.

Also from FIG. 4, in the case of using a conventional electrode catalyst that has not been pretreated (H ₂ O ₂ treatment), the number of catalytically active sites is reduced due to impurities. Therefore, a sufficient potential cannot be obtained (see the broken line in the figure). On the other hand, in the case of using the electrode catalyst of the present invention that is pretreated (H ₂ O ₂ treatment) so that the catalyst surface state is more durable and suitable for a high performance catalyst layer, the pretreatment (H ₂ O ₂ treatment) has an effect of cleaning the surface of Pt / C, and impurities are sufficiently removed. Therefore, the active site that has been disturbed by the impurities is recovered. As a result, a utilization rate of catalytic activity sites of 100% can be expected, and the catalytic activity is improved. Therefore, a high potential can be maintained (see the solid line in the figure). Further, the pretreatment (H ₂ O ₂ treatment) has an effect of preventing H ₂ O ₂ generated when impurities are present, and contributes to improvement of the durability of the catalyst layer and the battery. As a result, as shown in FIG. 4, the solid line currents (oxidation current and reduction current) are larger in a predetermined potential region, particularly in the low potential region than in the broken line. In such a region, the current value becomes an index of the activity of the individual catalyst particles, so that it can be confirmed that the electrode catalyst of the present invention is excellent. This is considered that the entire catalyst layer contributes to the reaction in the high potential region, whereas only the vicinity of the surface facing the aqueous solution contributes to the reaction in the low potential region. Therefore, it is considered that the activity of the catalyst of the present invention can be enhanced even in a low potential region by coating the surface with a hydrophilic group at a predetermined ratio. As a result, as shown in FIG. 4, the catalyst of the present invention can increase the oxygen reduction current over almost the entire potential region as compared with the conventional catalyst.

  FIG. 5 is a graph showing the results of XPS analysis of the electrode catalyst of the present invention produced in FIG. Specifically, it is a graph showing 4f5 / 2 peak height (75 ± 0.3 eV) and 4f7 / 2 peak height (72 ± 0.3 eV) obtained by peak splitting of the Pt4f peak obtained by XPS.

From FIG. 5, in the electrode catalyst of the present invention in which Pt / C is treated with H ₂ O ₂ , impurities other than impurities and second component Pt are present on the Pt surface. For example, Al, Si, Ti, V, Cr , Fe, Ni, Co, Cu, W, Ag, and V are not on the catalyst surface (see FIG. 8), and therefore, approximately equal Pt4f5 / 2 and Pt4f7 / 2 peaks are observed.

  FIG. 6 shows the power generation durability of a battery manufactured using the same electrode catalyst as in FIG. That is, it is a diagram showing the relationship between power generation time (time) and electrode potential (E).

As can be seen from FIG. 6, the conventional electrode catalyst that has not been treated with H ₂ O ₂ has a high voltage at the initial stage of power generation, but has insufficient affinity with the electrolyte polymer, and impurities (contamination). As a result, the number of catalytically active sites has decreased. In addition, deterioration of the electrolyte polymer and the like rapidly proceeds due to H ₂ O ₂ generated by impurities. As a result, the battery voltage rapidly decreases (see the broken line in the figure). On the other hand, in the case of using the electrode catalyst of the present invention, the pretreatment (H ₂ O ₂ ) treatment improves the affinity with the electrolyte polymer and can prevent H ₂ O ₂ generated when impurities are present. Also, the catalytic activity is improved. As a result, it can be seen that the durability of the catalyst layer and the battery is greatly improved, and the decrease in battery voltage can be suppressed over a long period of time (see the solid line in the figure).

As mentioned above, although it demonstrated per suitable embodiment of the manufacturing method of the electrode catalyst of this invention, the electrode catalyst of this invention is not limited to these manufacturing methods at all. For example, as a method of covering the support surface with a hydrophilic group by pretreatment, one kind of hydrogen peroxide (H ₂ O ₂ ), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO ₃ ), inorganic acid, and the like is used. In addition to the above-described method of immersing (oxidizing) in an aqueous solution containing active oxygen, a method of performing plasma treatment or the like can be used. A catalyst that satisfies the requirement that the CO / CC ratio is 0.2 or more can also be produced by the plasma treatment method.

  As a method using such a plasma treatment, a plasma irradiation method in which plasma is irradiated to an electrode catalyst having a catalyst metal supported on a carrier as a method that is easy to process and difficult to mix impurities that adversely affect battery performance. Can be mentioned. On the surface of the catalyst irradiated with plasma, impurities such as organic substances are removed, and hydrophilic groups such as hydroxyl groups are introduced.

  Also in the plasma irradiation method, plasma irradiation conditions (irradiation distance, irradiation level, irradiation time, number of irradiations, etc.) may be adjusted and controlled as appropriate so that the CO / CC ratio is 0.2 or more.

  There are many types of apparatuses for obtaining the above-described catalyst surface by plasma irradiation, but a so-called corona discharge treatment apparatus that discharges in air at normal temperature and pressure can be used. Depending on the properties of the catalyst particles, the catalyst particles may be easily scattered and irradiation treatment may be difficult. In such a case, the plasma irradiation treatment can be performed after the catalyst particles are slightly wetted in advance with a small amount of solvent. When the catalyst particles are agitated during irradiation, a more uniform treatment is possible.

  Next, the fuel cell electrode of the present invention comprises at least one electrode catalyst of the above-described electrode catalyst of the present invention or the electrode catalyst obtained by the manufacturing method of the present invention as a catalyst layer of the fuel cell electrode. And an electrolyte polymer (ionomer).

  The configuration of the electrode of the present invention is not particularly limited as long as the above requirements are satisfied. Hereinafter, an embodiment in which the catalyst layer is formed on the gas diffusion layer will be described as an example, and the catalyst layer and the gas diffusion layer will be described separately.

(1) Catalyst layer Since the electrode catalyst contained in the catalyst layer of the present invention is as already described, the description thereof is omitted here.

  The electrolyte polymer contained in the catalyst layer is not particularly limited as long as it can improve the affinity with the hydrophilic group introduced on the surface of the electrode catalyst. A conventionally well-known thing can be used. Specifically, an electrolyte polymer having a functional group capable of improving the affinity with the hydrophilic group introduced on the surface of the electrode catalyst can be used.

  As such an electrolyte polymer, a liquid, solid, gel-like material having at least high proton conductivity can be used. Specifically, solid acids such as phosphoric acid, sulfuric acid, antimonic acid, stannic acid, and heteropolyacid, Fluorosulfonic acid ionomer, inorganic acid such as phosphoric acid doped with hydrocarbon polymer compound, organic / inorganic hybrid polymer partially substituted with proton conductive functional group, phosphoric acid solution in polymer matrix And a gelled proton conductive electrolyte polymer impregnated with a sulfuric acid solution.

  In addition, as the electrolyte polymer, a mixed conductor having electronic conductivity such as a lanthanum gallate mixed conductor, an oxygen ion-electron mixed conductor, and a hydrogen ion-electron mixed conductor can be used.

Preferably, is one sulfonic acid functional groups capable of improving affinity between the hydrophilic groups of the carrier surface ^- with a (SO ₃ groups), styrene - divinylbenzene sulfonic acid resin, a perfluorocarbon sulfonic acid Of these, various ion exchange resins are used, but perfluorocarbon sulfonic acid resins are preferably used. In particular, when a perfluorocarbon sulfonic acid resin film is used as the electrolyte membrane, it is preferable to use a perfluorocarbon sulfonic acid resin of the same type.

  Specifically, Amberlite IR-120B (T-3) [Styrene-divinylbenzene sulfonic acid resin, Na type, 30 μm particle diameter, manufactured by Organo Co., Ltd., trade name) and NAFION (perfluorocarbon) are commercially available. Examples thereof include, but are not limited to, sulfonic acid resin, H type, 5% solution in a mixed solvent of aliphatic alcohol and water, Aldrich Chemical, trade name), and the like.

  However, the functional group that the electrolyte polymer of the present invention can improve the affinity with the hydrophilic group on the surface of the support is not limited to the sulfonic acid group, and examples thereof include a carboxyl group. It may have a functional group.

  By using the electrolyte polymer having such a functional group, the affinity with the hydrophilic group on the surface of the support is improved, the electrolyte polymer can uniformly cover the catalyst particles, and the dispersibility in the catalyst layer is also improved. . In particular, in the present invention, it is possible to control the coverage of the electrolyte polymer by controlling the coverage of the hydrophilic groups of the electrode catalyst (= control of the CO / CC ratio). Thereby, the gas diffusion region and the proton conduction path can be controlled, and the catalyst layer can be arbitrarily designed. This contributes to the water management of the battery, thereby improving the cell performance.

  The electrolyte polymer preferably covers the electrode catalyst from the viewpoint of a binder polymer. As a result, the structure of the catalyst layer can be stably maintained, and the reaction site where the electrode reaction proceeds (three-phase interface; the catalyst particles coexist with the electrolyte and the gas phase, and more of this three-phase interface is secured. Thus, the battery performance can be improved sufficiently, and high catalytic activity can be obtained.

  The content ratio of the electrode catalyst and the electrolyte polymer contained in the catalyst layer is not particularly limited, but the electrode catalyst and the electrolyte polymer are contained in a weight ratio of 10: 2 to 10: 6. desirable. By adjusting within this range, the catalyst particles can be added while maintaining the dispersibility of the catalyst particles in the electrolyte polymer during the production process. As a result, the surface of each catalyst particle highly dispersed in the catalyst layer can be in a state where the electrolyte polymer uniformly covers it. Therefore, the proton conduction path is maintained, and the effects of improving proton conductivity and cell performance are obtained.

  In addition, among the electrodes of the present invention, a large amount of water is generated by an oxygen reduction reaction at the cathode or the like. Since this generated water may close the pores for the reaction gas to diffuse, it is also important to ensure the water repellency of the electrode. Therefore, the catalyst layer may contain a water repellent such as polytetrafluoroethylene (hereinafter referred to as PTFE) as necessary. However, since the water repellent is an insulator, its amount is preferably as small as possible from the viewpoint of the output of the electrode, and its content is preferably 20 to 40% by mass with respect to the total amount of the catalyst layer.

(2) Gas diffusion layer In the electrode of this invention, it does not specifically limit as a gas diffusion layer which supports the said catalyst layer, A conventionally well-known thing can be used suitably. For example, porous carbon paper or a porous carbon substrate such as a carbon cloth can be used.

  In the electrode of the present invention, the catalyst layer and the gas diffusion layer are desirably thin in order to improve the reaction gas diffusibility. However, if the thickness is too thin, sufficient electrode output cannot be obtained. Therefore, it may be determined as appropriate so as to obtain an electrode having desired characteristics.

  The electrode of the present invention can be applied to both the anode and the cathode. Therefore, it may be used only for the anode, may be used only for the cathode, or may be used for both.

  Next, as described above, as a method for producing the electrode of the present invention, at least one electrode catalyst of the electrode catalyst of the present invention or the electrode catalyst obtained by the production method of the present invention and the electrolyte polymer are in a weight ratio. What is characterized by forming a catalyst layer using the material mixed so that it may become 10: 2-10: 6 is desirable. By adjusting the relationship between the amount of the electrode catalyst and the electrolyte polymer within the above range, it can be added while maintaining dispersibility.

  As a method for producing the electrode of the present invention, in addition to the material obtained by mixing the electrode catalyst and the electrolyte polymer in the above ratio, materials such as a water repellent, a pore-forming agent, a thickener, and a diluting solvent are used as necessary. Add and mix to form paste or slurry. A known method of forming a catalyst layer by applying this to a desired thickness by a screen printing method, a deposition method, a spray method or the like on a gas diffusion layer or an electrolyte layer described later can be used.

  FIG. 7 is a diagram showing the relationship between the battery potential (E) and the battery current (I) as a power generation test in an actual fuel cell. In FIG. 7, using the same electrode catalyst as in FIG. 3, the electrode catalyst and the electrolyte polymer were prepared such that the weight ratio was in the range of 10: 2 to 10: 6, and the electrode was further used. The power generation characteristics of the fuel cell fabricated in this way were investigated.

As can be seen from FIG. 7, the present invention is pre-treated (H ₂ O ₂ treatment) as compared with an electrode catalyst that has not been pre-treated (H ₂ O ₂ treatment) (broken line in the figure). It can be seen that the one using the electrode catalyst (solid line in the figure) shows better power generation characteristics (IV characteristics) from beginning to end. In this case, the coverage of the electrolyte polymer can be controlled by controlling the coverage of hydrophilic groups (= control of the CO / CC ratio) by pretreatment (H ₂ O ₂ treatment). Thereby, the gas diffusion region and the proton conduction path can be controlled, and the catalyst layer can be arbitrarily designed. In addition, cell performance is improved by contributing to moisture management of the battery. As a result, as shown in FIG. 7, the one using the electrode catalyst of the present invention (solid line in the figure) can exhibit excellent power generation characteristics from beginning to end. It can also be seen that, when the catalyst layer is produced, the dispersibility of the catalyst particles in the catalyst slurry can be maintained, so that there is no influence on the power generation characteristics due to the aggregation of the catalyst particles.

  Next, the polymer electrolyte fuel cell of the present invention has the above-described electrode of the present invention. An electrode using the electrode catalyst of the present invention can maintain high electrode performance over a long period of time. Therefore, if such an electrode is used as an electrode for a fuel cell, it may be possible to provide a fuel cell that can exhibit high performance over a long period of time. The type of fuel cell is not particularly limited as long as desired cell characteristics can be obtained, but it is used as a polymer electrolyte fuel cell (hereinafter also referred to as “PEFC”) from the viewpoints of practicality and safety. Is preferred. Hereinafter, the polymer electrolyte fuel cell will be described, but is not limited thereto.

  The PEFC generally has a structure in which a membrane-electrode assembly (hereinafter also referred to as “MEA”) is laminated (sandwiched) with a separator such as a carbon plate provided with a gas flow channel groove. The MEA is formed by sandwiching an electrolyte membrane (electrolyte layer) between electrodes. Therefore, at least one surface of the catalyst layer in the electrode is in contact with the electrolyte membrane.

  Since the electrodes in PEFC are as described above, the description thereof is omitted here. When the above-mentioned electrode is used as a cathode, the anode may be the same as the cathode, or a conventionally known anode electrode described in JP-A No. 2002-151089 or the like may be used. Not.

  The electrode used for the anode is not particularly limited as long as it has a high catalytic activity for the hydrogen oxidation reaction, and the same electrode as that used for the cathode may be used.

  The electrolyte membrane is a film-like solid polymer membrane made of a proton conductive member. The proton conductive member used for the electrolyte membrane is not particularly limited, and examples thereof include the same electrolyte polymers listed in the catalyst layer described above. Further, the electrolyte polymer and proton conductive member used for the catalyst layer and the electrolyte membrane may be the same or different.

  A known separator such as carbon paper or carbon cloth may be used as a separator for sandwiching the MEA. The separator has a function of separating air and fuel gas, and a channel groove may be formed in order to secure the channel. The groove width and pitch of the flow path groove of the separator are not particularly limited, but the gas diffusibility to the electrode is improved as the thickness is reduced, and a groove width of about 0.5 to 1.0 mm is usually used. In addition, in order to prevent water generated at the cathode from staying in the separator flow path, the flow path can be lengthened to increase the flow rate, or the separator can be set up so that the generated water can easily flow from top to bottom. Good.

  The MEA manufacturing method includes a method of directly forming an electrode on an electrolyte membrane, a method of forming an electrode on a separator and bonding it to the electrolyte membrane, a method of forming an electrode on a flat plate and transferring it to the electrolyte membrane, etc. The various methods are mentioned. When the electrode is formed on the separator separately from the electrolyte membrane, the electrode and the electrolyte membrane are preferably joined by a hot press method, an adhesive method (see Japanese Patent Laid-Open No. 7-220741) or the like.

  Furthermore, a stack in which a plurality of MEAs are stacked via a separator and connected in series may be formed so that a desired voltage or the like can be obtained by the PEFC. The shape of the PEFC is not particularly limited, and may be determined as appropriate so that desired battery characteristics such as voltage can be obtained.

  The fuel cell using the electrode of the present invention can maintain the cell performance for a long time as compared with the conventional one. Therefore, it is possible to provide a highly reliable fuel cell as a power source for a moving body such as a vehicle or a stationary power source. In addition, regarding the polymer electrolyte fuel cell described above, only one embodiment of the present invention is shown, and the present invention is not limited to this. Therefore, an electrode fuel cell using the electrode of the present invention is included in the scope of the present invention.

  Hereinafter, the present invention will be specifically described based on examples.

<Preparation of Pt / C>
1 g of conductive carbon black (Vulcan XC-72: manufactured by Cabot) powder was sufficiently dispersed in 250 g of a chloroplatinic acid aqueous solution containing 0.4% by mass of platinum using a homogenizer, and then sodium citrate was added thereto. 3 g was added, and the mixture was heated to 80 ° C. using a reflux reactor, and platinum was supported for reduction. And after standing to cool to room temperature, Pt carrying | support carbon black powder (Pt / C) was obtained by filtering carbon carrying | supporting platinum. The average particle diameter of platinum in Pt / C was about 2.9 nm from the result of observation with a transmission electron microscope. Further, as a result of examining the amount of Pt supported by inductively coupled plasma emission spectroscopy, it was confirmed that 40.6% by mass of Pt was supported.

<Preparation of electrode catalyst by H ₂ O ₂ treatment>
The Pt / C0.05g obtained above was dispersed using an ultrasonic homogenizer diffuser in aqueous solution of _H 2 _{O 2} of 0.04 wt% concentration. Stirring was continued for 0.5 hours at 25 ° C. in the obtained Pt / C-dispersed H ₂ O ₂ solution to generate oxygen (oxidation treatment).

  After the oxygen generation process is completed, filtration is performed using a Millipore filtration filter, and the catalyst cleaning is repeated twice using ultrapure water having an electrical resistivity of 18.3 MΩ · cm or less and a TOC (organic concentration) of 5 ppb or less. In this way, an electrode catalyst was produced.

<Analysis and evaluation of C1s peak of XPS of electrode catalyst>
The obtained electrode catalyst is measured by X-ray photoelectron spectroscopy (XPS), and a graph obtained by peak division of the C1s peak is shown in FIG. From FIG. 2, the ratio of (CO peak height) / (CC peak height) was determined.

From FIG. 2, in the electrode catalyst of this example, the C—O peak height obtained by the peak splitting of the C1s peak obtained by XPS was adjusted to 1 by adjusting the H ₂ O ₂ treatment condition. The -C peak height was 4. From this, it was confirmed that the CO / CC ratio = 1/4 = 0.25. That is, the coverage with hydroxyl groups (OH groups) was 25%.

<Activity analysis and evaluation for oxygen reduction reaction with rotating electrode using electrode catalyst>
Using the electrode catalyst that had been treated with H ₂ O ₂ obtained in this example and Pt / C that had not been treated with H ₂ O _{2, the} activity was compared with respect to the oxygen reduction reaction by the rotating electrode.

  The rotating electrode and the catalyst layer were produced by the following method.

A 5 wt% Nafion solution was diluted with ultrapure water at a mixing ratio of H ₂ O: Nafion = 100: 1 (weight ratio) to obtain an aqueous Nafion solution. Next, 20 mg of Pt / C was dispersed in 20 mL of H ₂ O using an ultrasonic homogenizer to obtain a catalyst suspension.

  10 μL of the above catalyst suspension was dropped onto the electrode substrate (on the Glassy Carbon) with a micropipette, and the water in the suspension was blown away with a vacuum dryer. After drying, 10 μL of the above Nafion aqueous solution was dropped on Pt / C using a micropipette. This was used for electrochemical measurements.

  Using the rotating electrode obtained above, as shown in FIG. 10, the catalyst layer 103 was held on the rotating electrode 101, and the activity for the oxygen reduction reaction as the positive electrode reaction was examined. Specifically, the oxygen reduction current I and the electrode potential E in the catalyst layer produced using the electrode catalyst of the present invention and the catalyst layer produced using the conventional electrode catalyst when the rotating electrode 101 is rotated at 1000 rpm. I investigated the relationship. The obtained results are shown in FIG.

From FIG. 3, it was confirmed that the limit current required for the strict catalytic analysis does not appear due to impurities (contamination) in the case of using a conventional electrode catalyst not treated with H ₂ O ₂ . On the other hand, in the case of using the electrode catalyst of the present invention that has been treated with H ₂ O ₂ so that the catalyst surface state is more durable and suitable for a high-performance catalyst layer, the limit necessary for strict catalytic performance analysis It was confirmed that the current clearly appeared. From this, it was confirmed that impurities on the catalyst surface were sufficiently removed in the electrode catalyst of the present invention.

<Polarization curve analysis and evaluation of oxygen reduction by rotating electrode using electrode catalyst>
Using the electrode catalyst that had been treated with H ₂ O ₂ obtained in this example and Pt / C that had not been treated with H ₂ O ₂ , the polarization curve of oxygen reduction by a rotating electrode was measured. The obtained results are shown in FIG. FIG. 4 is a view showing a cyclic voltammogram of the electrode in a sulfuric acid aqueous solution.

From FIG. 4, it was confirmed that in the case of using the conventional electrode catalyst not treated with H ₂ O ₂ (broken line in the figure), the potential appears to be low because the catalytic active site is reduced due to impurities. . On the other hand, in the case of using the electrode catalyst of the present invention treated with H ₂ O ₂ so that the catalyst surface state is more durable and suitable for a high performance catalyst layer (solid line in the figure), H ₂ O It was confirmed that the _two treatments had the effect of cleaning the surface of Pt / C and the impurities were sufficiently removed, so that the potential appeared high (solid line in the figure).

<Analytical evaluation of XPS Pt4f peak of electrode catalyst>
The H ₂ O ₂ -treated electrode catalyst obtained in this example was measured by X-ray photoelectron spectroscopy (XPS), and the 4f5 / 2 peak height (75 ± 0.00) obtained by peak splitting of the Pt4f peak. 3eV) and a graph representing the 4f7 / 2 peak height (72 ± 0.3eV) are shown in FIG.

From FIG. 5, in the electrode catalyst treated with H ₂ O ₂ obtained in this example, the Pt surface refers to elements other than impurities and Pt as the second component. For example, Al, Si, Ti, V, There is no Cr, Fe, Ni, Co, Cu, W, Ag, or V (see FIG. 8), and therefore, approximately equal Pt4f5 / 2 peak and Pt4f 57/2 peak are observed.

<Power generation durability test of battery using electrode catalyst>
As the power generation durability test in an actual fuel cell, and H ₂ O ₂ treated electrode catalyst obtained in the present _embodiment, the fuel cell by the following method using a Pt / C without _H 2 O ₂ treatment A cell was prepared, and a power generation durability test shown below was performed using the cell. FIG. 6 shows the relationship between the power generation time (number of cycles) obtained by the power generation durability test and the battery potential (E).

(Fabrication of fuel cell)
After adding 5 times the amount of purified water with respect to the weight of the electrode catalyst, 0.5 times the amount of isopropyl alcohol is added, and further, the Nafion solution (Aldrich 5 wt% so that the weight of Nafion is 1 time). Nafion containing) was added to form a mixed slurry. That is, in this example, a material in which the electrode catalyst and the electrolyte polymer Nafion were mixed at a weight ratio of 10:10 was used.

  The obtained mixed slurry was well dispersed with an ultrasonic homogenizer, followed by adding a vacuum degassing operation to prepare a catalyst slurry. A predetermined amount of catalyst slurry is printed on one side of a carbon paper (TGP-H-120 manufactured by Toray Industries, Inc.), which is a gas diffusion layer (GDL), by a screen printing method according to a desired thickness. After drying for a period of time, the surface coated with the catalyst layer was aligned with the electrolyte membrane and hot pressed at 120 ° C. and 0.1 MPa for 10 minutes to bond the cathode and the electrolyte membrane. Nafion 112 (thickness: about 50 μm) was used for the electrolyte membrane.

  Next, the anode was prepared in the same manner as the cathode, and the catalyst layer was bonded to the surface opposite to the cathode bonding surface of the electrolyte membrane to form an electrode (membrane-electrode assembly; MEA). The thickness of the anode and cathode catalyst layers was in the range of 8 to 12 μm for both cells.

In the obtained MEA, the amount of Pt used for both the anode and the cathode was 0.5 mg per 1 cm ^{2 of the} apparent electrode area, and the electrode area was 300 cm ² .

  The obtained MEA was sandwiched between carbon plates provided with gas flow channel grooves to form fuel cells.

(Battery power generation durability test)
The power generation durability test was performed by the following method.

When the fuel cell was operated for power generation, hydrogen was supplied as fuel to the anode side, and air was supplied to the cathode side. The supply pressure for both gases is atmospheric pressure, hydrogen is 80 ° C, air is saturated and humidified at 50 ° C, the temperature of the fuel cell body is set to 60 ° C, the hydrogen utilization rate is 70%, and the air utilization rate is 40%. Then, the operation was continued for 3 days at a current density of 0.7 A / cm ² to evaluate the durability of the fuel cell.

FIG. 6 shows that in the case of using an electrode catalyst not treated with H ₂ O ₂ (broken line in the figure), although the initial power generation has a high potential, the catalytic active site is reduced due to impurities, so the potential rapidly A significant decline was seen. On the other hand, in the case of using the electrode catalyst of the present invention that is treated with H ₂ O ₂ so that the catalyst surface state is more durable and suitable for a high-performance catalyst layer (solid line in the figure), the electrode potential is It was confirmed that the decrease was mild and the potential decrease was suppressed for a long time. From this, the H ₂ O ₂ treatment improves the affinity with the electrolyte polymer, prevents H ₂ O ₂ generated when impurities are present, suppresses catalyst deterioration, and improves catalyst activity. As a result, it can be seen that the durability of the catalyst layer and the battery is greatly improved, and that the battery voltage drop can be suppressed over a long period of time. <Evaluation of power generation characteristics of fuel cell using electrode catalyst>
As power generation test in an actual fuel cell, and H ₂ O ₂ treated electrode catalyst obtained in the present _embodiment, the fuel cell by the methods described above using Pt / C with no _H 2 O ₂ treatment It produced and the power generation test shown below was done using this. FIG. 7 shows the relationship between the battery potential E and the battery current I obtained by the power generation test.

(Power generation test with fuel cells)
The power generation test was performed by the same method as the power generation durability test of the battery.

From FIG. 7, the one using a conventional electrode catalyst not treated with H ₂ O ₂ (broken line in the figure) has a high voltage at the initial stage of power generation, but the catalytic active site is reduced due to impurities. A rapid drop in battery voltage was observed. On the other hand, in the case of using the electrode catalyst of the present invention treated with H ₂ O ₂ so that the catalyst surface state is more durable and suitable for a high performance catalyst layer (solid line in the figure), H ₂ O ₂ treatment prevents H ₂ O ₂ generated when impurities are present. As a result, as shown in FIG. 7, the durability of the catalyst layer and the battery is greatly improved, and the battery voltage is lowered for a long time. It can be seen that it can be suppressed.

Further, in the fuel cell manufactured using the electrode catalyst treated with H ₂ O ₂ obtained in the present embodiment of the present invention, hydrogen gas is supplied to the anode side so that the gas utilization rate is 70%. At normal pressure, air was supplied by adjusting the flow rate so that the utilization rate was 40%. Both gases were humidified and supplied so that the dew point was 60 ° C. The temperature of the fuel cell was adjusted to 70 ° C. This cell was operated at a current density of 400 mA / cm ² , and the output voltage after 24 hours from the start of measurement was 700 mV. Moreover, when the resistance between both electrodes was measured, it was 60 mΩ · cm ² . As a result of conducting a similar experiment with a fuel cell manufactured using a conventional electrode catalyst not treated with H ₂ O ₂ , the output voltage after 24 hours from the start was 650 mV, and the resistance between both electrodes was 68 mΩ · cm. ² . From the above, in this example, the surface of the electrode catalyst is oxidized in both the anode and the cathode, and the hydrophilic group coverage is controlled so as to exhibit a suitable catalyst surface state. It was confirmed that the coverage ratio was also controlled and the gas diffusion region and the proton conduction path could be controlled, so that the proton conductivity was improved and a higher voltage was obtained as the cell performance.

<Dispersibility test based on presence or absence of cleaning process in H ₂ O ₂ treatment>
In the above-mentioned “Preparation of Electrocatalyst by H ₂ O ₂ Treatment” in the present example, after the oxygen generation process is completed, it is filtered using a Millipore filter and then dried without washing the catalyst using ultrapure water. Thus, an electrode catalyst without a washing process was obtained.

An electrode catalyst with a cleaning process and an electrode catalyst without a cleaning process obtained as in the preparation of the electrode catalyst by the above H ₂ O ₂ treatment were respectively mixed in an ionomer solution (the electrode catalyst and the electrolyte polymer in weight ratio). 10:10) and stirred for 15 minutes using an ultrasonic homogenizer and then left for 1 hour. FIG. 9 is a drawing schematically showing the state of the electrode catalyst in the solution after being left standing.

  In the electrocatalyst with the washing process, as shown in FIG. 9A, the dispersibility in the ionomer solution was low, and it was confirmed that the electrode catalyst settled. On the other hand, as shown in FIG. 9B, the electrode catalyst without the washing process has high dispersibility in the ionomer solution and was confirmed to be uniformly dispersed. From the above, it was confirmed that it is difficult to ensure dispersibility in the ionomer solution because the contaminants are completely removed by performing catalyst cleaning using ultrapure water.

  However, when the same experiment was conducted so that the weight ratio of the electrode catalyst and the electrolyte polymer was within the range of 10: 2 to 10: 6, the dispersibility was improved even with the electrode catalyst having a cleaning process. It was confirmed that the good dispersion state shown in FIG. 9B could be maintained.

It is the explanatory schematic surface which represented typically the surface state of the electrode catalyst of this invention, the connection between electrolyte polymer, and the reaction path | route (process). A graph obtained by performing peak measurement of the C1s peak by measuring the electrode catalyst of the present invention and a conventional electrode catalyst by X-ray photoelectron spectroscopy (XPS) is shown. It is a graph showing the relationship between the oxygen reduction electric current I and the electrode potential E in the catalyst layer produced using the electrode catalyst of this invention, and the catalyst layer produced using the conventional electrode catalyst. The graph showing the cyclic voltammogram in the sulfuric acid aqueous solution of the electrode catalyst of this invention and the electrode produced using the conventional electrode catalyst is shown. About the electrode catalyst of this invention, the measurement by a X ray photoelectron spectroscopy (XPS) is performed, and the graph obtained by the peak division of Pt4f peak is shown. The graph which compared the change of the electric power generation endurance performance at the time of repeating an operation-stop cycle is shown for the fuel cell using the electrode catalyst of the present invention and the fuel cell using the conventional electrode catalyst. The graph which compared the performance change of the electric power generation characteristic (IV characteristic) at the time of operating continuously the fuel cell using the electrode catalyst of this invention and the fuel cell using the conventional electrode catalyst is shown. FIG. 8 (a) is a schematic diagram illustrating the surface state of the electrode catalyst before and after the pretreatment, and further schematically illustrating the surface state of the electrode catalyst alone before the pretreatment. FIG. 8B is an explanatory schematic surface schematically showing the surface state of the electrode catalyst after the pretreatment and the connection with the electrolyte polymer. FIG. 9A is a diagram schematically showing the state of an electrode catalyst in a solution after the electrode catalyst with a cleaning process of the present invention and an electrode catalyst without a cleaning process are added to and mixed with an active oxygen aqueous solution. Is the case where the electrode catalyst is used with the cleaning process, and FIG. 9B is the case where the electrode catalyst is used without the cleaning process. The catalyst layer fixed on the rotating electrode and the solution (electrolyte solution; 0.1M HClO saturated with oxygen) at the rotating electrode used for the comparative activity test for the oxygen reduction reaction by the rotating electrode of the example. ₄ is a drawing schematically showing the flow of ( ₄ aqueous solution).

Explanation of symbols

11 Carrier,
13 catalytic metal,
15 electrode catalyst,
17 Electrolyte polymer (ionomer),
17a hydrophobic part of the main chain of the electrolyte polymer,
17b hydrophilic part such as a side chain of the electrolyte polymer,
81 electrocatalyst,
83 carrier,
85 catalytic metal 87 second component to dissolve,
89 electrolyte polymer,
101 rotating electrode,
103 catalyst layer.

Claims (10)

In an electrode catalyst having a catalyst metal supported on a support,
The (C—O peak height) / (C—C peak height) ratio obtained by peak splitting of the C1s peak obtained by X-ray photoelectron spectroscopy (XPS) of the electrode catalyst is 0. 2. An electrode catalyst for a fuel cell, characterized by being 2 or more.   2. The fuel cell electrode catalyst according to claim 1, wherein at least one hydrophilic functional group among a hydroxyl group, a carbonyl group, and a carboxyl group is present on the support.   The ratio (CO peak height) / (CC peak height) obtained by XPS of the electrocatalyst is 0.25 to 0.5. The fuel cell electrode catalyst according to Item. It is a manufacturing method of the electrode catalyst according to claims 1-3,
A process for producing an electrode catalyst for a fuel cell, characterized in that a catalyst metal is supported on a support and then treated with an aqueous solution containing active oxygen. Treatment with an aqueous solution containing active oxygen after supporting the catalyst metal on the support;
The method for producing an electrode catalyst according to claim 4, comprising a step of washing with ultrapure water. A process of dispersing an electrocatalyst carrying a catalytic metal on a carrier in an aqueous solution containing active oxygen;
A process of generating oxygen in the dispersion obtained in the dispersion process;
After the oxygen generation process is completed, the process of filtering, cleaning the catalyst with ultrapure water,
The method for producing an electrocatalyst according to claim 4 or 5, characterized by comprising:   The method for producing an electrode catalyst according to any one of claims 4 to 6, wherein the aqueous solution containing active oxygen contains hydrogen peroxide.   The electrode layer for a fuel cell contains at least one electrode catalyst of the electrode catalyst according to claims 1 to 3 or the electrode catalyst obtained by the production method according to claims 4 to 7 and an electrolyte polymer. A fuel cell electrode characterized by comprising:   The electrode catalyst according to any one of claims 1 to 3 or the electrode catalyst obtained by the production method according to claims 4 to 7 and an electrolyte polymer in a weight ratio of 10: 2 to 10: 6. A method for producing an electrode for a fuel cell, wherein a catalyst layer is formed using a material containing a material mixed so that   A polymer electrolyte fuel cell comprising the electrode according to claim 8.