JP4508571B2 - Electrode catalyst and method for producing the same - Google Patents

Description

  The present invention relates to an electrode catalyst, and more specifically, electrodes of various electrochemical devices such as a fuel cell, a water electrolysis apparatus, a hydrohalic acid electrolysis apparatus, a salt electrolysis apparatus, an oxygen and / or hydrogen concentrator, a humidity sensor, and a gas sensor. The present invention relates to an electrode catalyst used in the above.

  In a polymer electrolyte fuel cell, an electrode generally has a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon fiber, carbon paper, or the like is used. The catalyst layer is a part that becomes a reaction field for electrode reaction, and is generally composed of a composite of an electrode catalyst and a solid polymer electrolyte.

  As an electrode catalyst used for such various electrochemical devices, generally, a catalyst in which fine particles of a catalyst component containing a noble metal such as Pt is supported on a carbonaceous support is used. The carbonaceous carrier used for the electrode catalyst has (1) a large surface area to support the catalyst component in a higher dispersion, (2) supply of oxygen and hydrogen as reaction gas components, and water as a product. In order to promote the discharge of particles, it is necessary to develop a chain-branched structure (structure) of aggregates of primary particles, to have an appropriate void volume, and (3) to have a necessary degree of conductivity. It is known.

  In addition, certain types of carbon black (for example, VULCAN XC-72 manufactured by Cabot) are known to provide good electrochemical characteristics and are used as a carbonaceous support for fuel cell electrode catalysts. ing.

Furthermore, in order to impart various functions to the carbon material, the surface of the carbon material is modified or modified. For example, in Non-Patent Document 1, in order to control the hydrophilicity of carbon black, the surface of carbon black is oxidized by treatment with nitric acid, a mixed gas of NO ₂ and air, ozone, etc. The point which gives sex is indicated.

Carbon Black Handbook, Carbon Black Association (ed.) Pp. 287-289

  In electrochemical devices used under harsh conditions such as polymer electrolyte fuel cells, hydrogen peroxide is generated as a result of side reactions of electrode reactions, and one of the causative substances that hydrogen peroxide degrades electrolyte membranes. It is known that In order to solve this problem, conventionally, it is common to use a perfluoro-based electrolyte having excellent oxidation resistance as the electrolyte membrane.

  However, with respect to the carbonaceous support used for the electrode catalyst, as described in Non-Patent Document 1 described above, only an improvement for controlling the hydrophilicity of the surface has been made. In order to solve the problem of deterioration due to the above, there has been no example in which the improvement was attempted by paying attention to the carbonaceous support used in the electrode catalyst.

  The problem to be solved by the present invention is to provide an electrode catalyst capable of suppressing the deterioration of the electrolyte membrane due to hydrogen peroxide and improving the durability of various electrochemical devices such as solid polymer fuel cells. It is in.

In order to solve the above problems, an electrode catalyst according to the present invention comprises a carbonaceous support , noble metal fine particles supported on the surface of the carbonaceous support, and the carbonaceous support chemically bonded to the surface of the carbonaceous support. The gist of the invention is that it comprises a surface layer (excluding a fluoride coating) that renders the surface electrochemically inactive.

In various electrochemical devices, hydrogen peroxide is generated mainly on the surface of the carbonaceous support in the surface of the electrode catalyst. Therefore, if a surface layer (excluding fluoride coating) that is chemically bonded to the surface of the carbonaceous support and is electrochemically inactive is formed on the surface of the carbonaceous support, hydrogen peroxide is generated. And the deterioration of the electrolyte membrane due to hydrogen peroxide can be suppressed.

  Hereinafter, an embodiment of the present invention will be described in detail. The electrode catalyst according to the present invention includes a carbonaceous support, noble metal fine particles supported on the surface of the carbonaceous support, and a surface layer that makes the surface of the carbonaceous support electrochemically inactive.

  In the present invention, the material of the carbonaceous support is not particularly limited, but in order to obtain good electrochemical characteristics, as described above, (1) a large surface area and (2) an appropriate void It is preferable to use a material having a volume and (3) a necessary degree of conductivity.

  Specific examples of the carbonaceous carrier material include certain types of carbon black (for example, VULCAN XC-72 manufactured by Cabot, Kechen Black EC manufactured by Kechen Black International, PRINTEX manufactured by Degussa, etc.), activated carbon, Suitable examples include graphite powder, glassy carbon, carbon fiber and the like.

  As the noble metal fine particles, those having a function as a catalyst, that is, those having activity for hydrogen oxidation and / or oxygen reduction are used. Specifically, Pt, Pd, Ir, Ru, and the like and alloys thereof, or alloys of these with other metal components are listed as preferable examples.

  The loading amount of the noble metal fine particles on the carbonaceous support is not particularly limited, and an optimum loading amount may be selected according to the use of the electrode catalyst, the characteristics required for the electrode catalyst, and the like. In addition, these carbonaceous support and noble metal catalyst may be used alone, or two or more kinds of materials may be used in combination for at least one of the carbonaceous support and the noble metal catalyst.

  The surface layer is used to electrochemically inactivate the surface of the carbonaceous support and suppress the hydrogen peroxide production reaction that occurs on the surface of the carbonaceous support. In order to make the surface of the carbonaceous support electrochemically inactive, a high resistance material having a higher specific resistance at room temperature than that of the carbonaceous support is used as the surface layer, or it reacts with carbon on the surface of the carbonaceous support. It is preferable to form a surface layer having a specific resistance higher than that of the carbonaceous support.

  When a high resistance material is used as the surface layer or a high resistance surface layer is formed on the surface, generation of hydrogen peroxide on the surface of the carbonaceous support can be efficiently suppressed. In order to efficiently suppress the generation of hydrogen peroxide, the specific resistance of the surface layer is preferably 10 Ωcm or more, and more preferably 100 Ωcm or more.

Specific examples of the material for the surface layer include vinyl polymers such as polystyrene, polyacrylonitrile, and polyvinyl acetate, polymer compounds having relatively high specific resistance such as polyethylene and polypropylene, silicon carbide (SiC), and silicon nitride. A suitable example is a carbon compound having a relatively high specific resistance such as (Si ₃ N ₄ ).

  The thickness of the surface layer formed on the surface of the carbonaceous support is not particularly limited, and an optimum thickness is selected according to the material of the carbonaceous support, the material of the surface layer, the characteristics required for the electrode catalyst, and the like. Just choose. In general, as the thickness of the surface layer increases, the probability that hydrogen peroxide is generated can be reduced.

  In addition, the area of the surface layer formed on the surface of the carbonaceous support is not particularly limited, and the optimum area is determined according to the material of the carbonaceous support, the material of the surface layer, the characteristics required for the electrode catalyst, etc. Should be selected. Generally, the probability that hydrogen peroxide is generated can be reduced as the ratio of the area of the surface layer to the entire surface of the carbonaceous support increases. On the other hand, if the area ratio of the surface layer becomes too large, the electron conduction between the carbonaceous supports is interrupted and the electrode reaction is inhibited, which is not preferable.

  Next, the method for producing the electrode catalyst according to the present invention will be described. The electrode catalyst according to the present invention can be produced by supporting noble metal fine particles on the surface of a carbonaceous support and then forming a surface layer on the surface of the carbonaceous support.

  In the present invention, the method for supporting the noble metal fine particles on the surface of the carbonaceous support is not particularly limited, and a well-known method can be used. Specifically, (1) a compound containing a noble metal which can be dissolved in an appropriate solvent (hereinafter referred to as “noble metal source”) is dissolved in the solvent, and (2) By immersing or dispersing the carbonaceous support in this solution, it is possible to support the noble metal on the surface of the carbonaceous support. In this case, if necessary, a reagent for precipitating noble metal atoms from the noble metal source may be added to the solution, or the carbonaceous carrier separated from the solution may be subjected to a reduction treatment under predetermined conditions. .

For example, when the noble metal is Pt, hexahydroxyplatinic acid (H ₂ Pt (OH) ₆ ) is used as a noble metal source, and this is dissolved in a sulfurous acid (H ₂ SO ₃ ) aqueous solution. Next, after dispersing the carbonaceous support in this solution, hydrogen peroxide is added thereto and heated at 80 ° C. to 100 ° C. for 0.1 to 3 hours to precipitate Pt on the surface of the carbonaceous support. Furthermore, a carbonaceous carrier carrying Pt can be obtained by reducing for 0.1 to 3 hours at 50 to 300 ° C. in a hydrogen stream.

  Next, a surface layer is formed on the surface of the carbonaceous support on which the noble metal fine particles are supported. Specific methods for forming the surface layer include the following methods. In the first method, a functional group capable of reacting with a monomer for synthesizing a polymer compound is introduced on the surface of a carbonaceous support, and then the monomer is added under conditions where the carbonaceous support into which the functional group is introduced coexists. In this method, a surface layer made of a polymer compound is formed on the surface of a carbonaceous carrier by polymerization.

Specifically, a —COCl group is preferred as the functional group to be introduced on the surface of the carbonaceous support. Further, the —COCl group specifically introduces an oxygen-containing functional group such as —OH group, —COOH group, and —CO group on the surface of the carbonaceous carrier by treating the carbonaceous carrier with an aqueous nitric acid solution, This oxygen-containing functional group can be introduced on the surface of the carbonaceous support by further treatment with thionyl chloride (SOCl ₂ ) or the like.

Moreover, the optimal monomer is used according to the material of the surface layer to be manufactured. Specific examples of the monomer include styrene (C ₆ H ₅ CH═CH ₂ ), acrylonitrile (CH ₂ ═CHCN), vinyl acetate (CH ₂ ═CHOCOCH ₃ ), ethylene (CH ₂ ═CH ₂ ), propylene ( CH ₃ CH═CH ₂ ) and the like are preferable examples.

  Furthermore, the monomer polymerization method is not particularly limited, and a known method such as a radical polymerization method, a thermal polymerization method, a photopolymerization method, or a radiation polymerization method can be used. Further, when the polymerization conditions are optimized in accordance with the polymerization method, the monomer and the type of functional group introduced on the surface of the carbonaceous support, the surface layer made of a polymer compound having a predetermined thickness and area on the surface of the carbonaceous support Can be formed.

  The second method is a method of forming a surface layer made of a carbon compound on the surface of the carbonaceous carrier by reacting a carbonaceous carrier carrying noble metal fine particles with a gas containing a compound capable of forming a carbon compound. .

For example, when the surface layer is silicon carbide (SiC), the reaction gas includes
(1) methyltrichlorosilane (CH ₃ SiCl ₃ ), methyldichlorosilane ((CH ₃ ) HSiCl ₂ ), dimethyldichlorosilane ((CH ₃ ) ₂ SiCl ₂ ), trimethylchlorosilane ((CH ₃ ) ₃ SiCl), etc. A mixed gas of organosilicon compound gas and hydrogen gas,
(2) It is preferable to use a mixed gas of silicon tetrachloride (SiCl ₄ ), hydrocarbons (methane, ethane, propane, etc.) and hydrogen.

  When the carbonaceous carrier and the reaction gas are heated at a predetermined temperature, carbon compounds having various compositions can be formed on the surface of the carbonaceous carrier according to the composition of the reaction gas. At this time, if the reaction conditions are optimized, the thickness and / or area of the surface layer can be controlled. In general, the higher the reaction temperature and / or the longer the reaction time, the thicker the surface layer formed on the surface of the carbonaceous carrier and / or the wider the area.

  For example, when a mixed gas of an organosilicon compound gas and hydrogen gas is used as the reaction gas, the reaction temperature is preferably 800 ° C. to 1300 ° C. The reaction time is selected according to the reaction temperature so that a surface layer having a predetermined thickness and / or area can be obtained.

  Next, the operation of the electrode catalyst according to the present invention will be described. It is known that when a hydrocarbon-based electrolyte is used as an electrolyte membrane of a polymer electrolyte fuel cell, the electrolyte membrane deteriorates in a relatively short time. It is also known that such deterioration of the electrolyte membrane is caused by hydrogen peroxide generated by a side reaction of the electrode reaction. For this reason, it is common to use a perfluoro-based electrolyte such as Nafion (registered trademark, manufactured by DuPont) for the electrolyte membrane for a polymer electrolyte fuel cell.

  However, even when a perfluoro-based electrolyte was used as the electrolyte membrane, a phenomenon that the perfluoro-based electrolyte membrane deteriorates was observed when the polymer electrolyte fuel cell was operated for a long time. As a result of examining the deterioration phenomenon in detail, hydrogen peroxide, which is a by-product of the electrode reaction, has a great influence on the deterioration, and this hydrogen peroxide is mainly generated on the surface of the carbonaceous support used as a catalyst support. I found out.

  That is, the reaction at the air electrode of the polymer electrolyte fuel cell is an electrochemical reduction reaction of oxygen, and as shown in the following equation (1), oxygen supplied to the air electrode is reduced to water. This is a 4-electron reduction reaction. Such a four-electron reduction reaction mainly occurs on the surface of noble metal fine particles such as Pt supported on a catalyst carrier.

O ₂ + 4H ⁺ + 4e ⁻ → H ₂ O (1)

  On the other hand, the production reaction of hydrogen peroxide as a by-product is a two-electron reduction reaction as shown in the following equation (2). Such a two-electron reduction reaction mainly occurs on the surface of a carbonaceous support used as a catalyst support. Therefore, although a carbonaceous carrier such as a certain type of carbon black has suitable conditions as a catalyst carrier, it must be said that it is disadvantageous in terms of the production of hydrogen peroxide. This was a cause of shortening the battery life.

O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O ₂ (2)

  On the other hand, when a part of the surface of the carbonaceous support on which the noble metal catalyst is supported is coated with a surface layer made of a high resistance material or the like, the two-electron reduction reaction represented by the formula (2) is suppressed (ie, the carbonaceous The surface of the carrier becomes electrochemically inactive), and the production of hydrogen peroxide can be suppressed. Therefore, the deterioration of the electrolyte membrane due to hydrogen peroxide can be suppressed.

  In the case where a perfluoro-based electrolyte is used as the electrolyte membrane, when the electrolyte membrane is deteriorated, the conductivity may be reduced, or harmful fluorine ions may be contained in the exhaust gas. On the other hand, if the electrode catalyst according to the present invention is used, it is possible to suppress such a decrease in conductivity and the discharge of fluorine ions.

3 g of hexahydroxyplatinic acid (H ₂ Pt (OH) ₆ ) was dispersed in 300 ml of pure water, and 100 ml of 6% aqueous sulfurous acid (H ₂ SO ₃ ) solution was added thereto and stirred for 1 hour. Then, it heated at 120 degreeC, the remaining sulfurous acid was removed, and pure water was added to make 500 ml, and it was set as the Pt chemical | medical solution (4g-Pt / L equivalent).

Next, 0.5 g of carbon black (manufactured by Cabot, VULCAN XC-72) is weighed and dispersed in water, and the Pt chemical solution has a ratio of Pt weight to carbon black weight of 3: 2. I put it in. Next, 50 ml of a 20% H ₂ O ₂ aqueous solution was added to this solution and heated at 120 ° C. for 1 hour to support Pt on carbon black. Further, the carbon black carrying Pt was filtered, washed with pure water, vacuum dried at room temperature, and pulverized. Finally, it was reduced in a hydrogen stream at 200 ° C. for 2 hours to obtain a Pt-supported carbon catalyst (hereinafter referred to as “Pt / C catalyst”).

Next, this Pt / C catalyst is inserted into an electric furnace and heated to 1000 ° C., and then a mixed gas of methyltrichlorosilane (CH ₃ SiCl ₃ ) and hydrogen (methyltrichlorosilane concentration: about 6%) is supplied. By reacting silicon with the carbon black surface, a Pt / C catalyst (hereinafter referred to as “Pt / C—SiC catalyst”) having a silicon carbide (SiC) film formed on the carbon black surface was obtained. .

  In this embodiment, the SiC coating is formed by intermittently supplying a reactive gas into the electric furnace, reacting for 1 second, and then exhausting residual unreacted gas and reaction product gas by 500 pulses. A chemical vapor infiltration reactive deposition process was used.

A Pt / C catalyst was prepared according to the same procedure as in Example 1. Next, this Pt / C was dispersed in a nitric acid aqueous solution and heated at 80 ° C. for one day and night while refluxing. After cooling, filtration and washing with pure water were repeated, followed by vacuum drying and pulverization. This nitrate-treated Pt / C was dispersed in benzene, an excess amount of thionyl chloride (SOCl ₂ ) was added and refluxed for 50 hours, and the oxygen-containing functional group introduced by the nitric acid treatment was converted to a —COCl group. .

Next, Pt / C into which —COCl group was introduced was washed with benzene and dried, and then dispersed in nitrobenzene. Silver perchlorate (AgClO ₄ ) was added as a polymerization catalyst, and the mixture was stirred for 48 hours. Next, styrene (C ₆ H ₅ CH═CH ₂ ) was added to this solution, and the polymerization reaction was allowed to proceed at 40 ° C. for 6 hours while stirring. After the completion of the reaction, it was washed with methanol and dried to obtain a Pt / C catalyst having polystyrene grafted on the carbon surface (hereinafter referred to as “Pt / C-PS catalyst”).

(Comparative Example 1)
The Pt / C catalyst obtained in Example 1 was used for the test as it was.

  Using the catalysts obtained in Examples 1 and 2 and Comparative Example 1 as an electrode catalyst, a small test fuel cell was produced according to the following procedure. That is, first, a diffusion layer was prepared by applying carbon black water-repellent with polytetrafluoroethylene (PTFE) dispersion to the surface of a carbon cloth and performing water-repellent treatment. Next, a catalyst layer is formed on the surface of the diffusion layer by applying and drying a mixture of a catalyst and a Nafion solution (polymer content 5%, manufactured by Aldrich) on the surface of the diffusion layer subjected to water repellency treatment. A diffusion electrode was obtained.

Next, with the catalyst layer inside, gas diffusion electrodes were thermocompression bonded from both surfaces of the electrolyte membrane (Nafion (registered trademark) membrane, manufactured by DuPont) having a thickness of about 50 μm to obtain a membrane electrode assembly (MEA). . The thermocompression bonding was performed under the conditions of a pressure bonding temperature: 130 ° C. and a pressure: 20 kg / cm ² (1.96 MPa). Further, the MEA was sandwiched between current collectors provided with a gas flow path on a graphite plate to obtain a test battery. The electrode area was 25 cm ² .

Next, the durability test was done using the obtained test battery. The durability test was performed by repeating a continuous 12-hour discharge at a current density of 0.1 A / cm ² and a 12-hour pause. The fuel electrode is supplied with hydrogen gas at a pressure of 1.5 at, temperature: 80 ° C. and dew point: 80 ° C., and the air electrode is at pressure: 1.5 at, temperature: 80 ° C., dew point: 70 ° C. Air was supplied. In addition, the test was conducted while supplying hydrogen and air during the rest.

  FIG. 1 is a plot of terminal voltages after 2 hours have elapsed since each discharge resumed against the total test elapsed time. In the case of the test battery using the Pt / C catalyst obtained in Comparative Example 1, the terminal voltage decreased when the total test elapsed time exceeded 400 hours. This is presumably because the electrolyte membrane is deteriorated by hydrogen peroxide generated on the carbon black surface.

  On the other hand, in the case of the test battery using the Pt / C-SiC catalyst obtained in Example 1 and the Pt / C-PS catalyst obtained in Example 2, the total test elapsed time exceeded 400 hours. The terminal voltage maintained almost the initial value. This is presumably because the carbon black surface became electrochemically inactive by forming a SiC film or polystyrene film on the carbo black surface.

  Furthermore, the water | moisture content discharged | emitted from the test battery was collected during the durability test, and the elution rate of the fluorine ion was computed from the fluorine ion concentration contained in the water | moisture content. As a result, the elution rates of fluorine ions from the test battery using the Pt / C-SiC catalyst of Example 1 and the test battery using the Pt / C-PS catalyst of Example 2 were respectively Pt of Comparative Example 1. It decreased to 1/20 times and 1/30 times the elution rate of the fluorine ion from the test battery using the / C catalyst, and it was confirmed that elution of the fluorine ion was suppressed.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The electrode catalyst according to the present invention is particularly suitable as an electrode catalyst for a polymer electrolyte fuel cell. However, the application of the present invention is not limited to this, and it can be used as an electrode catalyst used in various electrochemical devices. Can be used.

It is a figure which shows the result of the endurance test of the fuel cell using the electrode catalyst obtained in Example 1 (Pt / C-SiC), Example 2 (Pt / C-PS), and Comparative Example 1 (Pt / C). is there.

Claims (3)

A carbonaceous carrier;
Noble metal fine particles supported on the surface of the carbonaceous support;
An electrode catalyst comprising a surface layer (excluding a fluoride coating) that is chemically bonded to the surface of the carbonaceous support and renders the surface of the carbonaceous support electrochemically inactive.   The electrode catalyst according to claim 1, wherein the surface layer is an organic molecule that covers the surface of the carbonaceous support.   The electrode catalyst according to claim 1, wherein the surface layer is a carbon compound that coats a surface of the carbonaceous support.

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