JP2010092814A - Electrode catalyst for polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode catalyst for a polymer electrolyte fuel cell in which dispersibility of metal particulates is further improved, and which carries out superior catalyst performance. <P>SOLUTION: A conductive carbon carrier is used, of which the ratio (S1/S2) of BET specific surface area (S1) obtained by having a water molecule as adsorbed species, and BET specific surface area (S2) obtained by having nitrogen molecule as the adsorbed species is within a range of 0.4/1 to 0.8/1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrode catalyst for a polymer electrolyte fuel cell.

As an electrode catalyst for a polymer electrolyte fuel cell, for example, Patent Document 1 describes an electrode catalyst in which a catalytic metal is supported on a conductive carbon carrier provided with a silane coupling agent.
JP 2006-344553 A

  The electrode catalyst for a polymer electrolyte fuel cell described in Patent Document 1 is said to have improved dispersibility of metal fine particles and excellent electrode catalyst performance, but is sufficiently satisfactory for practical use. is not.

  An object of the present invention is to provide an electrode catalyst for a polymer electrolyte fuel cell that further improves the dispersibility of metal fine particles and exhibits more excellent catalytic performance.

According to the studies by the present inventors, it has been found that the above object can be achieved by the following invention.
(1) An electrocatalyst for a polymer electrolyte fuel cell in which a catalytic metal is supported on a conductive carbon support, wherein the conductive carbon support has a BET specific surface area (S1) and nitrogen determined using water molecules as adsorbed species. A solid polymer fuel characterized by using a fuel having a ratio (S1 / S2) to a BET specific surface area (S2) obtained by using molecules as adsorbed species in the range of 0.4 / 1 to 0.8 / 1 Battery electrode catalyst.
(2) The electrode catalyst for a polymer electrolyte fuel cell according to the above (1), wherein the conductive carbon carrier has a surface modified with SiO ₂ .
(3) the amount of SiO ₂ is, relative to the total mass of the conductive carbon carrier and SiO _2, 1 to 40 wt% at which solid polymer fuel cell electrode catalyst of the above (2).
(4) The solid polymer fuel according to (2) or (3) above, wherein the amount of the catalyst metal is 0.1 to 20% by mass with respect to the total mass of the conductive carbon carrier, SiO ₂ and the catalyst metal. Battery electrode catalyst.
(5) The electrode catalyst for a polymer electrolyte fuel cell according to any one of the above (2) to (4), wherein the specific surface area of the conductive carbon support modified with SiO ₂ is 100 to 800 m ² / g.

  The ratio (S1 / S2) is an index indicating the hydrophilicity of the surface of the conductive carbon carrier, as will be described later.

  In the electrode catalyst of the present invention, since the conductive carbon carrier has high hydrophilicity, the catalyst metal is supported with high dispersibility. Therefore, the electrode catalyst of the present invention exhibits excellent catalytic performance.

In the anode of a direct methanol fuel cell, a Pt—Ru alloy catalyst is generally used. However, the catalytic reaction mechanism is said to proceed as shown in the following formulas (1) to (3). . As shown in the formula (1), Pt oxidizes MeOH, and CO as an intermediate product is adsorbed on Pt. As in formula (2), Ru reacts with water to produce Ru-OH. The generated Ru—OH oxidizes CO adsorbed on Pt to CO ₂ (formula (3)).

Pt + MeOH → Pt—CO + 4H ⁺ + 4e ⁻ (1)
Ru + H ₂ O → Ru—OH + H ⁺ + e ⁻ (2)
Pt—CO + Ru—OH → CO ₂ + H ⁺ + e ⁻ (3)
In the electrode catalyst of the present invention, by using a conductive carbon support to SiO ₂ modification, the reaction for the silanol groups of the SiO ₂ surface is the same action as Ru formula (3) is promoted, MeOH oxidation performance of Is thought to improve.

  The electrode catalyst of the present invention has a ratio (S1 / S2) of the BET specific surface area (S1) obtained using water molecules as an adsorbing species and the BET specific surface area (S2) obtained using nitrogen molecules as an adsorbing species is 0.4 / 1. A catalyst carbon is supported on a conductive carbon support in a range of ˜0.8 / 1.

The BET specific surface area (S1) is calculated from the water vapor adsorption isotherm (298K), and the BET specific surface area (S2) is calculated from the nitrogen adsorption isotherm (77k). The water vapor adsorption isotherm and the nitrogen adsorption isotherm were measured using BELSORP18 Plus-T manufactured by Nippon Bell Co., Ltd. The measurement conditions are as follows.
(Measuring principle) Dissolution method (pretreatment conditions)
Water vapor adsorption: Vacuum heating was performed at 150 ° C. for 5 hours to remove water in the sample.

Nitrogen adsorption: Vacuum heating was performed at 200 ° C. for 2 hours to remove water in the sample.
(Sample amount) 50 mg to 100 mg
The ratio (S1 / S2) is an index for evaluating whether the surface of the conductive carbon carrier is hydrophilic or hydrophobic. The larger the ratio is, the more hydrophilic the surface is. It can be evaluated that it is hydrophobic.

  There is no restriction | limiting in particular as electroconductive carbon used for manufacture of the electrode catalyst of this invention, What is generally used for manufacture of this kind of electrode catalyst can be used. For example, carbon black, carbon nanophone, activated carbon, carbon nanotube, fullerene and the like are used, and among these, carbon black is preferably used.

The conductive carbon carrier having the above ratio (S1 / S2) in the range of 0.4 / 1 to 0.8 / 1 used in the present invention can be easily obtained by modifying the surface of general conductive carbon with SiO _2. can get.

In order to modify the surface of the conductive carbon with SiO ₂ , for example, the conductive carbon may be treated with a silane compound and / or a silane coupling agent by a conventional method. Specifically, a method of heat condensing conductive carbon and a silane coupling agent, a hydrothermal reaction method, a plasma reaction method, or the like is used.

  Examples of the silane compound include chlorosilanes such as methyltrichlorosilane, methyldichlorosilane, ethyltrichlorosilane, phenyltrichlorosilane, and diphenyldichlorosilane; alkoxysilanes such as tetramethoxysilane, methyltrimethoxysilane, and phenyltrimethoxysilane; tetraethylorthosilicate Etc.

  Examples of the silane coupling agent include vinyltriethoxysilane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ -Methacryloxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-chloropropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane and the like. Of these, ethyltrimethoxysilane and vinyltriethoxysilane are preferably used.

In SiO ₂ modified conductive carbon support, the amount of SiO _2, relative to the total weight of the conductive carbon carrier and SiO _2, 1 to 40 wt%, preferably from 5 to 30 mass%. When the amount of SiO ₂ is out of the above range, a SiO ₂ -modified conductive carbon support having a ratio (S1 / S2) in the range of 0.4 / 1 to 0.8 / 1 cannot be obtained.

The electrode catalyst of the present invention can be obtained by supporting a catalyst metal on the SiO ₂ modified conductive carbon support. Platinum, ruthenium, palladium, rhodium, nickel, cobalt, iron, gold or the like is used as the catalyst metal. These can be used alone or in combination of two or more. The catalyst metal can be supported according to a generally known method such as an impregnation method or an ion implantation method.

The amount of the catalyst metal is 0.1 to 20% by mass, preferably 1 to 10% by mass, based on the total mass of the conductive carbon carrier, SiO ₂ and the catalyst metal.

Hereinafter, the present invention will be specifically described with reference to examples.
(Example of carrier preparation)
To 250 g of ethanol, 5 g of 3-aminopropyltriethoxysilane and 10 g of carbon black (Cabot, Vulcan XC72) were added and stirred at room temperature for 30 minutes. Next, after filtration and washing with water, it was dried at 110 ° C. in a nitrogen atmosphere to obtain a SiO ₂ -modified conductive carbon carrier. Next, this SiO ₂ -modified conductive carbon carrier was added to 100 g of 1N aqueous nitric acid solution, stirred at room temperature for 2 hours, filtered, washed with water, and dried at 110 ° C. in a nitrogen atmosphere. Next, the nitric acid-treated SiO ₂ -modified conductive carbon carrier was added to a solution of 21 g of tetraethylorthosilicate and 230 g of ethanol, stirred for 15 minutes at room temperature, and then added with 6.8 g of 25% ammonia and 11.2 g of water. The mixture was stirred at room temperature for about 10 hours. Thereafter, filtration, washing with water were performed, and drying was performed at 110 ° C. in a nitrogen atmosphere, whereby Carrier A was obtained. The water vapor adsorption isotherm of the obtained carrier was measured. The results are shown in Table 1 and FIG.

上記表から、カーボンブラックにＳｉＯ_２ の層を形成させることにより、比（Ｓ１／Ｓ２）の値が大きく、より親水性が高まっていることがわかる。

From the above table, it can be seen that by forming a SiO ₂ layer on carbon black, the value of the ratio (S1 / S2) is large and the hydrophilicity is increased.

In FIG. 1, the horizontal axis represents the water vapor relative pressure, the vertical axis represents the water vapor adsorption amount, and the untreated carbon black and the carbon black formed with a SiO ₂ layer with a silane coupling agent and a silane compound. It shows a graph of investigation steam adsorption at 25 ° C. of; (SiO 2 -VulcanXC72 carrier _a). By treating with a silane coupling agent and a silane compound, the amount of water vapor adsorption is improved over that of the untreated.
Example 1
2 g of NaOH (granular) was added to 100 mL of ethylene glycol and dissolved at 70 ° C. in a nitrogen atmosphere. Next, 4.79 g of dinitrodiammine platinum nitrate aqueous solution (Pt: 0.386 g) and 5.93 g of ruthenium nitrate aqueous solution (Ru: 0.270 g) were added to 100 mL of ethylene glycol. To this ethylene glycol solution, an ethylene glycol solution in which NaOH was dissolved was added and stirred at room temperature for 1 hour under a nitrogen atmosphere (degassing). Next, this solution was refluxed at 90 ° C. (liquid temperature) for 3 hours under a nitrogen atmosphere. After cooling, 0.386 g of carrier A was added to this solution, stirred for 1 hour at room temperature in a nitrogen atmosphere (degassing), and then refluxed at 160 ° C. (liquid temperature) in a nitrogen atmosphere for 3 hours. After cooling, 1N nitric acid aqueous solution was gradually added dropwise with stirring to adjust the pH to 1. The solid was filtered, thoroughly washed with ion-exchanged water, dried at 110 ° C. under a nitrogen atmosphere, and then reduced with hydrogen at 300 ° C. for 2 hours to prepare Catalyst A. When the obtained catalyst A was analyzed, the composition was Pt: Ru: SiO ₂ : carbon black = 29: 30: 10: 31 (mass%).
(Comparative Example 1)
In Example 1, Catalyst B was prepared in the same manner as in Example 1 except that untreated carbon black (Cabot, Vulcan XC72) was used instead of the support A. When the obtained catalyst B was analyzed, the composition was Pt: Ru: carbon black = 29: 30: 41 (mass%).
(Comparative Example 2)
A catalyst C was prepared in the same manner as in Example 1 except that untreated carbon black (Johnson Matthey, HiSPEC 10100) was used in place of the support A. When the obtained catalyst C was analyzed, the composition was Pt: Ru: carbon black = 29: 30: 41 (mass%).
(Performance evaluation)
10 mg of Catalyst A was added to 1 mL of 5% Nafion solution (manufactured by Aldrich) and sufficiently stirred by ultrasonic waves to prepare a catalyst paste. Next, 5 μL of this catalyst paste was applied onto a glassy carbon electrode and dried. This catalyst layer was fixed on a glassy carbon electrode to obtain a test electrode. For the evaluation of the catalyst performance, methanol was added to a 1N aqueous sulfuric acid solution at 1 mol / L. The test electrode is immersed in this solution held at 25 ° C. to make a working electrode, a platinum wire is used as a counter electrode, a reversible hydrogen electrode (RHE) is used as a reference electrode, and a methanol oxidation current and an electrode by a potential regulating method. The potential relationship was measured and 0.6 V vs. A value obtained by dividing the oxidation current value in RHE by the mass of platinum contained in the catalyst applied on the glassy carbon electrode (oxidation current value per mass of platinum). The higher the current value, the better the catalyst performance. Table 2 shows the evaluation results when the catalysts A and B were used.

上記結果から、シラン化合物および／またはシランカップリング剤を用いて、導電性カーボンの表面をＳｉＯ_２ 修飾して親水性を高めた導電性カーボン担体を用いることにより、未処理の導電性カーボン担体を用いる場合に比べて、より高いメタノール酸化性能が得られることがわかる。

From the above results, an untreated conductive carbon carrier was obtained by using a conductive carbon carrier whose surface was modified with SiO ₂ to improve hydrophilicity using a silane compound and / or a silane coupling agent. It can be seen that higher methanol oxidation performance can be obtained compared to the case of using.

Carrier _A (SiO 2 -VulcanXC72) and untreated carbon black (Cabot Corporation, Vulcan XC72) is a graph showing the water vapor adsorption amount of.

Claims (5)

  An electrocatalyst for a polymer electrolyte fuel cell in which a catalytic metal is supported on a conductive carbon support, which adsorbs a BET specific surface area (S1) and nitrogen molecules determined by using water molecules as adsorbed species as the conductive carbon support. Electrode for polymer electrolyte fuel cell, characterized in that the ratio (S1 / S2) to the BET specific surface area (S2) obtained as a seed is in the range of 0.4 / 1 to 0.8 / 1 catalyst. _{2. The} electrode catalyst for a polymer electrolyte fuel cell according to claim 1, wherein the surface of the conductive carbon carrier is modified with SiO2. The electrode catalyst for a polymer electrolyte fuel cell according to claim _{2, wherein} the amount of SiO ₂ is 1 to 40% by mass with respect to the total mass of the conductive carbon support and SiO ₂ . The electrode catalyst for a polymer electrolyte fuel cell according to claim 2 or 3, wherein the amount of the catalyst metal is 0.1 to 20% by mass with respect to the total mass of the conductive carbon support, SiO ₂ and the catalyst metal. . 5. The electrode catalyst for a polymer electrolyte fuel cell according to claim 2, wherein the SiO ₂ -modified conductive carbon support has a specific surface area of 100 to 800 m ² / g.

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