JP2017206568A - Carbon black and fuel cell catalyst prepared therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide carbon black having high dispersibility and suitable for a fuel cell catalyst carrier.SOLUTION: Carbon black has a DBP absorption of 200 ml/100g or more; a ratio of the DBP absorption to a specific surface of 0.60 or less (DBP absorption (ml/100 g)/specific surface (m/g)); a sulfur content of 50 ppm or less; and a bulk density of 0.050 g/cmor less. A fuel cell catalyst is obtained by carrying platinum particles and/or platinum alloy particles on the carbon black.SELECTED DRAWING: None

Description

The present invention relates to carbon black.

The cell structure of the polymer electrolyte fuel cell has a structure in which a gas diffusion layer, a catalyst layer, and an electrolyte membrane are sandwiched between separators provided with gas flow paths. This catalyst layer is composed of carbon black carrying platinum particles and / or platinum alloy particles (hereinafter referred to as “platinum particles”), and the platinum particles are carried on the surface of the carbon black in a highly dispersed state. Yes. Here, the carrying is a state in which particles such as platinum are attached to the surface of carbon black by chemical bonds or physical bonds. Since source gases such as hydrogen and oxygen are activated by contact with particles such as platinum to generate water, the reaction efficiency is higher when the particles such as platinum in the catalyst layer are supported in a highly dispersed state. On the other hand, when particles such as platinum are supported on carbon black in a low dispersion state, that is, when they are supported in an agglomerated state, the contact area between the source gas and the particles such as platinum is reduced, and the reaction efficiency is lowered. .

In order to increase the reaction efficiency of particles such as platinum, it has been proposed to use carbon black having a high specific surface area. However, in carbon black, a combination of primary particles called a structure develops in a branch shape, which may reduce the surface area effective for supporting the catalyst. In addition, the aggregated particles of carbon black generated by the entanglement of the structure cause the generation of protrusions in the catalyst layer which is becoming thinner, and the electrolyte membrane-electrode assembly (MEA) obtained by sandwiching the catalyst layer and the electrolyte membrane is formed. Possible damage.

As a method of reducing aggregated particles of carbon black, an external shearing machine (bead mill, ball mill, three rolls, etc.) and an internal shearing machine (ultrasonic homogenizer) are used for a slurry in which catalyst-supported carbon black, ion exchange resin and solvent are mixed , Jet mill, etc.) have been proposed (Patent Document 1). In addition, a method has been proposed in which carbon black itself is pulverized with various pulverization apparatuses (jet mill, planetary mill, etc.) in the form of powder (Patent Documents 2 and 3). However, the crushing treatment has a limit in crushing ability, and there are problems such as contamination of foreign substances and reduction in productivity.

Furthermore, carbon black often contains impurities such as sulfur, chlorine, potassium, lead, sodium, calcium and the like in the order of several tens of ppm to several percent, which has hindered long-term stability.

JP 2005-216661 A JP-A-2005-41967 JP 2006-274189 A

An object of the present invention is to provide a carbon black that is highly dispersible and suitable for a fuel cell catalyst carrier.

The present invention employs the following means in order to solve the above problems.
(1) DBP absorption is 200 ml / 100 g or more, and the ratio of DBP absorption to specific surface area is 0.60 or less in terms of DBP absorption (ml / 100 g) / specific surface area (m ² / g). Is a carbon black characterized by having a bulk density of not more than 50 ppm and a bulk density of not more than 0.050 g / cm ³ .
(2) A catalyst for a fuel cell, wherein platinum particles and / or platinum alloy particles are supported on the carbon black described in (1).

The carbon black of the present invention does not have an excessively developed structure and has few aggregated particles. Therefore, when used for a fuel cell catalyst carrier, particles such as platinum can be more uniformly supported in a highly dispersed state, and the performance of the fuel cell can be improved. Moreover, since it can disperse | distribute highly when mixed with a solvent, when coating a slurry and forming the catalyst layer of a fuel cell, the catalyst layer excellent in surface smoothness can be obtained.

The carbon black of the present invention preferably has a high specific surface area. Here, the specific surface area can be measured according to JIS K6217-2. When the specific surface area is low, particles such as platinum cannot be supported in a highly dispersed state, and the reaction efficiency of particles such as platinum cannot be increased. The reaction efficiency is the catalytic activity per unit mass of particles such as platinum, and when the supported amount is constant, the current potential curve in the fuel cell evaluation is compared, and it can be judged by the voltage level at the same current value. . The higher the voltage, the higher the reaction efficiency. The specific surface area is preferably 350 m ² / g or more, and more preferably 500 m ² / g or more. The specific surface area is preferably less than 1000 m ² / g. There exists a possibility that the characteristic of a fuel cell may fall that a specific surface area is 1000 m < ² > / g or more.

The DBP absorption amount of the carbon black of the present invention is 200 ml / 100 g or more. Here, the DBP absorption is an amount by which dibutyl phthalate is absorbed in the voids formed by the surfaces of the carbon black particles and the aggregated particles, that is, an index for evaluating liquid absorbency, and can be measured according to JIS K6217-4. When the DBP absorption amount is less than 200 ml / 100 g, the viscosity of the carbon black slurry produced in the step of supporting particles such as platinum is lowered, and it becomes difficult to apply a shearing force due to mixing, so that good dispersibility cannot be obtained. . The DBP absorption is preferably less than 400 ml / 100 g. When the DBP absorption amount is 400 ml / 100 g or more, the viscosity of the carbon black slurry becomes high, and the production of the catalyst layer becomes difficult.

The ratio of the DBP absorption amount to the specific surface area of the carbon black of the present invention is 0.60 or less as DBP absorption amount (ml / 100 g) / specific surface area (m ² / g). In the carbon black having a developed structure, the voids formed by the aggregated particles increase, and the DBP absorption amount increases. However, since the DBP absorption varies depending on the surface area of the carbon black, it is difficult to evaluate the degree of development of the structure only by the DBP absorption. Therefore, by taking the ratio between the DBP absorption amount and the specific surface area, it was used as an index of the degree of structure development. As a result of intensive studies to increase the reaction efficiency of particles such as platinum in the fuel cell, the present inventor has determined that the ratio of the DBP absorption amount to the specific surface area of the carbon black as the catalyst support is 0.60 (ml / 100 g) / ( It was found that m ² / g) or less is effective. When the ratio between the DBP absorption amount and the specific surface area exceeds 0.60 (ml / 100 g) / (m ² / g), the surface area effective for supporting the catalyst due to the excessively developed structure decreases. Further, the structures are easily entangled with each other, and aggregated particles are generated.

The sulfur content contained in the carbon black of the present invention is 50 ppm or less. The sulfur content contained in the carbon black exists as acidic functional groups such as sulfate groups on the surface of the carbon black. When the sulfur content exceeds 50 ppm, gas such as SOx is generated by an electrochemical reaction inside the battery. May occur and degrade battery performance. The sulfur content contained in the carbon black can be calculated by absorbing the combustion gas generated by burning the carbon black in an oxygen stream into hydrogen peroxide water and measuring it by ion chromatography.

The bulk density of the carbon black of the present invention is 0.050 g / cm ³ or less. When the bulk density exceeds 0.050 g / cm ³ , the structures are easily entangled with each other and aggregated particles are generated.

The carbon black of the present invention preferably has 10 ppm or less of aggregated particles of 20 μm or more. Aggregated particles of 20 μm or more can be measured according to JIS K6218-3 using a 635 mesh (20 μm mesh) sieve. Moreover, it is preferable that dispersion degree is 40 micrometers or less. Here, the degree of dispersion is an index for evaluating the size of the aggregated particles, and can be evaluated by measuring the particle size of the aggregated particles with a grind gauge (100 μm and 50 μm grooves) according to JIS K 5600-2-5. If aggregated particles of 20 μm or more are present in excess of 10 ppm or the degree of dispersion exceeds 40 μm, the surface smoothness cannot be maintained when applied as a catalyst layer, and when the MEA is assembled, it penetrates to the counter electrode side and is internally short-circuited. May cause.

In the process of producing carbon black, a raw material gas such as hydrocarbon or natural gas is supplied from a nozzle installed at the top of a vertical reactor, and carbon black is produced by a pyrolysis reaction and / or a combustion reaction. Collect from the bug filter directly connected to the bottom. The raw material gas to be used is not particularly limited, but it is preferable to use an acetylene gas having few impurities such as a sulfur content. Moreover, when removing sulfur content from the obtained carbon black, for example, a baking furnace such as a muffle furnace is used and heated at 1000 to 1500 ° C. for 1 hour or more in an inert atmosphere or in an inert air current. Thus, the sulfur content can be removed.

The specific surface area and DBP absorption amount of carbon black can be adjusted by controlling the shape of the reaction furnace and the temperature distribution in the furnace, but in order to obtain carbon black having a high specific surface area and a low structure as in the present invention, It is preferable to adjust by supplying oxygen gas and water vapor into the furnace. In particular, when acetylene gas is used as a raw material gas, when oxygen gas and water vapor are supplied, incomplete combustion of the acetylene gas occurs and the specific surface area increases. Further, since carbon black nuclei generated by the decomposition of the acetylene gas are diluted, it is possible to suppress fusion between the grain-grown carbon blacks, that is, formation of a structure. Although the reason is not clear, it is particularly preferable that the amount of water vapor supplied relative to the amount of oxygen gas supplied is 0.8 to 1.2 times in volume ratio.

In the present invention, in addition to oxygen gas and water vapor, for example, hydrocarbon gas, hydrogen gas, carbon dioxide gas and the like can be added. Examples of hydrocarbon gas are gasified gases such as methane, ethane, propane, ethylene, propylene, butadiene, etc., and oily hydrocarbons such as benzene, toluene, xylene, gasoline, kerosene, light oil, heavy oil, etc. . When these gases are added, the reaction temperature changes, so that the specific surface area of the resulting carbon black can be easily increased or decreased.

The carbon black of the present invention may have a high specific surface area by oxidation treatment. The oxidation treatment can be performed by, for example, a dry method using an oxidizing gas such as air or ozone, or a wet method using an aqueous solution containing an oxidizing agent. The dry method is preferred from the viewpoint of more uniform and large-scale treatment. In the dry method, a horizontal furnace maintained at a temperature of 500 to 800 ° C. is used, and the carbon black and the oxidizing gas are preferably brought into contact with each other preferably while stirring the carbon black. It can be performed by a method of spraying carbon black from the top of the dredged vertical furnace. The wet method can be performed by adding carbon black to an aqueous solution containing an oxidizing agent, treating at 50 to 120 ° C. for 5 to 30 hours, and then washing and drying. As the oxidizing agent, for example, inorganic acids such as aqueous hydrogen peroxide, hydrochloric acid, sulfuric acid and nitric acid, for example, salts such as sodium hypochlorite and potassium dichromate can be used.

In the carbon black of the present invention, the aggregated particles are reduced, but the remaining aggregated particles themselves are also easily loosened, so it is possible to further reduce the aggregated particles using a ball mill, a vibration mill, a jet mill, etc. It is. In particular, the use of a jet mill can reduce aggregated particles more effectively.

The catalyst for a fuel cell of the present invention is obtained by precipitating (supporting) particles such as platinum with high dispersion on the surface of the carbon black of the present invention. From the viewpoint of long-term stability of fuel cell performance, it is preferable that particles such as platinum are strongly supported on the surface of carbon black, and a method for supporting the particles will be described later. The size of particles such as platinum is preferably 10 to 50 mm.

As a material such as platinum, platinum alloy is used in addition to platinum. Examples of the platinum alloy-forming metal include palladium, rhodium, iridium, ruthenium, iron, titanium, nickel, cobalt, gold, silver, copper, chromium, manganese, molybdenum, tungsten, aluminum, silicon, rhenium, zinc, and tin. Among these, in the case of a direct methanol fuel cell, a platinum-ruthenium alloy is preferable because it is effective in preventing carbon monoxide poisoning. If an example of a platinum alloy composition is shown, platinum will be 30-90 mass%, and the metal to alloy will be 10-70 mass%.

There are no particular restrictions on the method for supporting particles such as platinum on carbon black, but the following method is preferred, for example. Carbon black is suspended in water to form a slurry, and an aqueous solution of hexachloroplatinic acid (IV) containing particles such as platinum is added thereto to obtain a mixed solution A. To this, 10 times equivalent sodium borohydride is added to the particles such as platinum. After adding (reducing treatment) and precipitating platinum particles or the like on the surface of carbon black, a fuel cell catalyst is produced by filtration, washing and drying.

In order to use platinum or the like as a platinum alloy, hexachloroplatinic acid (IV) containing platinum and an alloy-forming metal is used. For example, when ruthenium is used as the alloy-forming metal, a mixed solution B is prepared by adding a ruthenium (III) trichloride aqueous solution containing a predetermined amount of ruthenium to the mixed solution A. The blending amount of ruthenium is preferably 10 to 70% by mass with respect to platinum. Next, sodium borohydride equivalent to 10 times the platinum particles is added to the mixed solution B (reduction treatment), and the platinum particles are precipitated on the surface of the carbon black in the mixed solution B, followed by filtration, washing, A fuel cell catalyst is produced by drying.

Regardless of whether the particles such as platinum are platinum particles or platinum alloy particles, a pH adjuster such as an aqueous solution of sodium hydroxide can be added as appropriate when the particles are deposited on the surface of carbon black. An example of the supported amount of particles such as platinum is 10 to 80 parts by mass with respect to 100 parts by mass of carbon black.

The evaluation of the fuel cell catalyst of the present invention can be performed as follows, for example, in the case of a polymer electrolyte fuel cell. A fuel cell catalyst is mixed with tetrafluoride resin powder, and a paste obtained by adding alcohol is applied to one side of carbon paper to form a catalyst layer. And a Nafion solution is uniformly apply | coated to the surface of a catalyst layer, and it is set as an electrode. An electrode is placed on both sides of a Nafion membrane (perfluorosulfonic acid electrolyte membrane) so that the electrodes are in contact with each other and thermocompression bonded by a hot press to obtain an electrolyte membrane-electrode assembly (MEA). A single cell is completed if the MEA is sandwiched between a separator and a current collector, and a fuel cell can be evaluated by connecting an electronic load device and a gas supply device. Moreover, if the commercially available fuel cell single cell evaluation apparatus is used, the said evaluation can be performed more simply.

Hereinafter, the production method of the carbon black and the fuel cell catalyst according to the present invention will be described in detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following examples unless it exceeds the gist.

Example 1
Acetylene gas, oxygen gas and water vapor are mixed under the conditions shown in Table 1 and sprayed from a nozzle installed at the top of a carbon black production furnace (furnace length: 5 m, furnace diameter: 0.5 m) to thermally decompose acetylene gas and / or Carbon black was produced using a combustion reaction, and carbon black was collected from a bag filter directly connected to the lower part of the furnace. 500 g of the obtained carbon black was sampled and put into an electric furnace heated to 720 ° C., and then air was introduced at 30 L / hour for 1.5 hours while maintaining the pressure in the furnace at 0.1 kPa. Oxidation treatment was performed.

The obtained carbon black was measured for the following physical properties. The evaluation results are shown in Table 1.
(1) Specific surface area: Measured according to JIS K 6217-2.
(2) DBP absorption: measured according to JIS K 6217-4.
(3) Sulfur content: 1 g of a sample was accurately weighed on a magnetic boat and inserted into a reaction tube of a combustion absorber heated to 1300 ° C. An absorption bottle containing an absorption liquid (3.5 ml of hydrogen peroxide solution diluted with pure water to 1 L) was connected, oxygen gas was allowed to flow, and combustion gas was passed through the absorption bottle. The obtained absorbing solution was introduced into an ion chromatography analyzer, the peak area of sulfate ion was measured, and the sulfur content in the sample was calculated based on a calibration curve prepared in advance from a sulfate ion standard solution.
(4) Bulk density: Measured according to JIS K 5101-12-2.
(5) Sieve residue: Measured according to JIS K 6218-3 except that the sieve was 635 mesh (aperture 20 μm).
(6) Dispersity: As a pretreatment, 0.1 g of carbon black and 30 g of dibutyl phthalate are put into a centrifugal sedimentation tube, and mixed using a homogenizer (Biomixer BM-2 manufactured by Nippon Seiki Co., Ltd.) at a rotational speed of 2000 rpm for 1 minute. The particle size of the aggregated particles was measured with a grind gauge (100 μm and 50 μm grooves) according to JIS K 5600-2-5.

In order to evaluate carbon black as a fuel cell catalyst, a platinum-ruthenium alloy was supported by the following method. That is, carbon black was mixed with chloroplatinic acid and a ruthenium chloride aqueous solution. The mixing ratio was carbon black / platinum / ruthenium = 40/40/20 by mass ratio. The mixture was stirred at 80 ° C. for 30 minutes and then cooled to room temperature. 0.5M sodium borohydride was added in 5 portions to precipitate platinum and ruthenium as an alloy, filtered, washed and dried to obtain a fuel cell catalyst. About the obtained catalyst for fuel cells, the particle size of the platinum-ruthenium alloy was measured about 1000 particles by TEM observation (magnification of 100,000 times), and the average value was obtained. The evaluation results are shown in Table 2.

2500 g of Nafion was mixed with 1 g of the obtained fuel cell catalyst to form a paste, applied to carbon paper, and then dried at 80 ° C. to obtain a fuel electrode. For the surface smoothness of the fuel electrode, 10 fields of view were observed using SEM (magnification 1000 times). Evaluation was based on the number of aggregated particles of 10 μm or more present in a region of 100 μm in length × 100 μm in width. The evaluation results are shown in Table 2.

Next, Pt-black was used for the oxygen electrode, and the fuel electrode was overlapped with the Nafion membrane interposed between them and pressed at 135 ° C. and 9.8 MPa for 10 minutes to obtain an MEA. The fuel cell was configured by being sandwiched between a separator and a current collector and integrated. This fuel cell was operated continuously at a constant current of 500 mA / cm ² by introducing methanol at 4 ml / min and air at 60 ml / min at 90 ° C., and the initial output voltage and the output voltage after 1000 hours. Was measured. The evaluation results are shown in Table 2.

Examples 2-4, Comparative Examples 1-3
Carbon black was produced in the same manner as in Example 1 except that the type of raw material gas, the amount of raw material gas supplied, and the treatment time of the oxidation treatment were changed as shown in Table 1, and the catalyst for the fuel cell was produced. As evaluated. The evaluation results are shown in Tables 1 and 2.

From the evaluation results of Tables 1 and 2, the carbon black of the present invention is platinum-free when used in a fuel cell catalyst carrier having an excessively developed structure, few aggregated particles, and excellent dispersibility. The ruthenium alloy particles can be more uniformly supported in a highly dispersed state, and the performance of the fuel cell can be improved. Further, since it can be highly dispersed when mixed with a solvent, a catalyst layer having excellent surface smoothness can be obtained when a slurry is applied to form a catalyst layer of a fuel cell.

The carbon black of the present invention can be used as a catalyst for various fuel cells such as solid polymer fuel cells and phosphoric acid fuel cells.

Claims (2)

The DBP absorption is 200 ml / 100 g or more, the ratio of the DBP absorption to the specific surface area is 0.60 or less in terms of DBP absorption (ml / 100 g) / specific surface area (m ² / g), and the sulfur content is 50 ppm or less. A carbon black having a bulk density of 0.050 g / cm ³ or less. A catalyst for a fuel cell, comprising platinum particles and / or platinum alloy particles supported on the carbon black according to claim 1.