CN1404178A - Electric Pt-C catalyst containing cocatalytic element and its prepn - Google Patents

Abstract

The present invention relates to electric Pt/C catalyst containing two or more cocatalyst elements and its preparation. The electric catalyst is prepared with active carbon obtained via cracking phonelic resin as material and through carrier surface treatment, liquid phase codeposition, heat treatment and other steps. The electric catalyst has a Pt content of 10-30 wt% and cocatalyst element content of 0.1-5 wt% with the cocatalyst elements being two or more of Fe, Co, Ni, Cr, Mn, and Ti. The Pt grains are distributed homogeneously and have average size of 3.5-4.5 nm. Electrochemical test shows the electric catalyst has high catalytic activity, and compared with E-TEK electric catalyst, it has one 40 mV raised oxygen electrode potential in work current density of 200 mA/sq cm.

Description

Platinum/carbon electrocatalyst containing catalytic promoter element and preparation method thereof

technical field

The invention relates to a platinum/carbon (Pt/C) electrocatalyst applied to a proton exchange membrane fuel cell (PEMFC) containing two or more catalytic promoter elements and a preparation method thereof, belonging to the technical field of electrocatalysis and energy.

Background technique

A fuel cell is a device that directly converts the chemical energy of fuel (such as hydrogen, methanol) and oxidant (such as oxygen, air) into electrical energy. Because energy conversion is not limited by the Carnot cycle and is environmentally friendly, fuel cells are gradually becoming mainstream products in the energy field in the new century. The concentrated advantages of PEMFC are high energy conversion efficiency, clean and pollution-free, and can be quickly started at room temperature. It is a promising category of fuel cells. Because of these advantages, PEMFC is particularly suitable as a power source for various mobile power sources, especially electric vehicles. At present, the research on the application of PEMFC to electric vehicles has received extensive attention from governments, major automobile companies and scientific research institutions all over the world, and they have invested a lot of manpower and material resources in related research and development. Some of these companies, such as Canada's Ballard Power Company, have already begun to prepare to push PEMFC to the commercial market due to their leadership in certain technologies. At the same time, many research institutes have continuously launched various laboratory prototypes, and the research on PEMFC for vehicles has reached a new high point. However, despite this, there is still a long way to go before the commercialization of PEMFC in automobiles. One of the reasons is related to the excessive price caused by electrocatalysts. This is because: (1) the Pt/C electrocatalyst used in PEMFC is expensive; (2) the PEMFC positive electrode electrocatalyst is used in a large amount, the utilization rate is low (<30%), and the total amount of Pt is high. The Pt/C electrocatalyst produced by E-TEK is currently the best combination of electrocatalytic activity and precious metal dosage, but there is still a certain gap between the dosage and utilization of Pt compared with the expected value. Although there are some literature reports [Taylor E J, Anderson E B and Vilamibi N R K.J.Electrochem.Soc., 1992, 139: L45～L46] about the new electrocatalyst Pt synthesized in the laboratory with a small amount and high utilization rate [Taylor E J, Anderson E B and Vilamibi N R K.J. The stability and lifetime of electrocatalysts have become new issues. These have prompted people to actively carry out research on PEMFC. One of them is the preparation of Pt/C electrocatalysts containing various catalytic promoters in order to obtain better electrocatalysts. It has been found through research that Pt is prone to migration and agglomeration on the surface of the carrier carbon, which will cause the Pt particles to become thicker and the activity will decrease; and the introduction of transition metals, rare earths and other catalytic promoter elements into the Pt/C system can effectively inhibit the Pt on the carbon surface. migration on the carbon surface, thereby enhancing the stability and lifetime of the electrocatalyst. Regarding the mechanism of action of catalytic promoter elements, researchers generally believe that the "anchor effect" produced by catalytic promoter elements can effectively inhibit the movement of Pt on the carbon surface [Wei Zidong, Guo Hetong, Tang Zhiyuan. Acta Catalytica Sinica, 1995, V16(2), pp141~144]. However, the researchers also found that different catalytic promoters have different catalytic effects. Some catalytic promoters can improve the electrocatalytic activity, but the stability has decreased; others have the opposite effect. Therefore, researchers often introduce two or more catalytic promoter elements into the electrocatalytic system at the same time in order to obtain the best effect. In recent years, people have begun to introduce co-catalysts into PEMFC, but there are not many specific reports.

From the existing methods, the liquid-phase deposition method (also known as "liquid-phase impregnation method") has become the most common method for preparing this type of electrocatalyst due to its simple operation. For example, USP 6,165,635 and EP0,665,985B1 disclose a method for preparing a Pt/Rh/Fe electrocatalyst synthesized by a liquid phase impregnation method (one pot process). The product remains in the liquid phase without having to be separated multiple times. The average size of the synthesized electrocatalyst particles is about 10nm. Francis J. Luczak introduced a liquid-phase method for the preparation of Pt/C electrocatalysts containing other metal elements such as Co, Cr, and Ir through a series of US patents (such as USP 4,447,506 and USP 5,013,618, etc.). The activity of Pt-Cr/C in the electrocatalyst synthesized by this method is more than 2.5 times greater than that of pure Pt/C electrocatalyst, but its stability is poor.

The choice of carbon support is an important step in the preparation of PEMFC electrocatalysts, and the performance of electrocatalysts obtained from different carbon supports varies greatly. This is caused by the different surface properties of different carrier carbons; at the same time, the different surface treatment methods of the same species are also the reasons for the differences in the performance of electrocatalysts. For example, for the widely used carrier carbon produced by Cabot Company at present, most researchers use it directly, and some only undergo simple drying treatment in an inert atmosphere. It is well known that the nature of the oxygen-containing groups on the surface of the carrier carbon, the characteristics of the pore structure, and the size of the specific surface area have an important impact on the adsorption behavior and action characteristics of the catalyst on the carrier, which is directly related to the performance of the synthesized catalyst. However, commercialized carrier carbon (such as Vulcan XC-72) often has a part of active sites lost during packaging, storage, and transportation, and needs to be reactivated during use [Shao Qinghui, Xie Fangyan, Tian Zhiqun, etc. Battery, 2002, Vol32( 3), 153-155]. In the existing technology, carbon dioxide surface treatment is relatively common. For example, CN 1267922A discloses a Pt/C electrocatalyst preparation method for PEMFC obtained by treating carrier carbon with carbon dioxide. This catalyst showed higher electrocatalytic activity compared with that without activation treatment.

Contents of the invention

The object of the present invention is to provide a Pt/C electrocatalyst containing two or more catalytic promoter elements applied to PEMFC and a preparation method thereof. Its preparation process is characterized in that:

(1) Carbon carrier surface treatment. The powdered activated carbon (over 1000 mesh sieve) prepared by phenolic resin cracking was added to the mixed system of concentrated nitric acid (60wt%) and concentrated phosphoric acid (75wt%) and soaked for 10 hours under stirring. The volume ratio of phosphoric acid is controlled between 1:1 and 1:4. The treated supported carbon was then filtered, washed with water, and dried in a vacuum oven.

(2) Preparation of Pt/C electrocatalysts containing cocatalyst elements by liquid phase co-deposition method. Under ultrasonic stirring, the carbon carrier obtained in (1) is dispersed into a suspension with distilled water, and a small amount of surfactant (such as oleic acid, silicone oil, sodium dodecylsulfonate, etc.) is added; then the temperature of the suspension is raised As high as 60 ° C, the pH of the solution is between 7 and 9.

Prepare a certain concentration of chloroplatinic acid (H ₂ PtCl ₆ 6H ₂ O) solution and a nitrate or halide solution containing catalytic element metal ions, mix these solutions evenly under ultrasonic stirring conditions, and then add them dropwise to In the solution containing carrier carbon obtained above; use NaHCO ₃ solution to adjust the pH value of the solution to 7-9. Stirring of the above solution was continued for 2 hours and then allowed to stand at room temperature for 1 hour.

Prepare a certain concentration of formaldehyde solution and other reducing solutions (such as sodium hyposulfite solution, potassium borohydride solution and hydrazine hydrate solution, etc.). Add the formaldehyde solution dropwise to the above-mentioned solution after standing under stirring condition, and keep the pH value of the solution between 7-9 with NaHCO ₃ solution. The reaction temperature is controlled at 60° C., and when the reaction time exceeds 3 hours, one or several other reducing solutions are added dropwise to continue the reaction. The reaction temperature is controlled to be 60° C., and the reaction time is 6 to 8 hours.

The reacted solution is filtered, washed with ammonium bicarbonate solution, and dried at a temperature of 120-150°C to obtain an electrocatalyst without heat treatment.

(3) The electrocatalyst obtained in (2) is heat-treated at 500 to 700 C for 0.5 to 2 hours under an Ar atmosphere to obtain the Pt/ C electrocatalyst.

The catalyst Pt content obtained through the above steps is between 10wt% and 30wt%, and the content of the catalytic promoter element is between 0.1 and 5wt%, and the catalytic promoter element is any of Fe, Cr, Co, Ni, Mn, Ti Two or more types. The distribution of Pt particles is uniform, with an average particle size of 3.5-4.5nm.

The particle size and distribution of the electrocatalyst obtained in the present invention are characterized by a high-resolution transmission electron microscope (TEM), and the electrochemical performance is obtained by a polarization experiment of an oxygen electrode. The preparation method of the oxygen electrode is: take a certain proportion of electrocatalyst, Nafion solution, PTFE emulsion and dispersant (a mixture of absolute ethanol and water, the volume ratio is 1:1), etc., and mix them under ultrasonic vibration to prepare ink-like slurry materials, and then evenly transfer them to the water-repellent treated carbon paper for drying. Under the pressure of 6-9MPa, the dried carbon paper containing electrocatalyst was hot-pressed onto the Nafion117 proton membrane at about 135°C to obtain the test oxygen electrode, in which the loading amount of Pt was 0.2mg/cm ² , The content of Nafion is 1.0 mg/cm ² . The test is carried out in a classic three-electrode two-loop system. The oxygen electrode and the Pt auxiliary electrode constitute a constant current loop. The reference electrode is a mercury/mercurous sulfate electrode, which forms a potential test loop through a 5mol/L sulfuric acid solution on one side of the proton membrane. The effective area of the oxygen electrode was 4 cm ² . The oxygen pressure is 0.2MPa.

Proved by electrochemical tests, the electrocatalyst provided by the present invention shows higher catalytic activity, suitable for use as the positive electrode electrocatalyst of the fuel cell, especially the positive electrode electrocatalyst of the PEMFC cell, compared with the E-TEK electrocatalyst, at the operating current When the density is 200mA/cm ² , the potential of the oxygen electrode can be increased by more than 40mV (see the examples for details).

Description of drawings

Fig. 1 is the polarization curve diagram of the electrocatalyst oxygen electrode described in embodiment 1 and comparative examples 1-3.

Detailed ways

The present invention is described in detail below by way of examples.

Example 1 Get 10g of activated carbon (over a 1000 mesh sieve) obtained by pyrolysis of phenolic resin and add it to the mixed system of 250ml of concentrated nitric acid and concentrated phosphoric acid under ultrasonic stirring conditions (the volume ratio of concentrated nitric acid and concentrated phosphoric acid is 1:2 ) soaking treatment for 10 hours. The treated carrier carbon was filtered, washed with water, and then dried in a vacuum oven at 120°C.

Weigh 0.60 g of the treated activated carbon. Under ultrasonic stirring, the support carbon was dispersed with 50 ml of distilled water, and 0.008 g of sodium dodecylsulfonate was added to the suspension. The temperature of the solution was then raised to 60°C and the pH of the solution was 7.5.

Prepare 25ml of 0.014mol/L chloroplatinic acid (H ₂ PtCl ₆ ·6H ₂ O) solution. Dissolve 0.0212g of anhydrous cobalt nitrate and 0.030g of anhydrous ferric nitrate in distilled water and mix evenly with chloroplatinic acid solution; add the obtained solution dropwise to the aqueous solution containing carrier carbon under ultrasonic stirring conditions. The temperature of the solution was maintained at 60 °C. The pH of the solution was adjusted to 8 with NaHCO ₃ solution.

Stirring of the above solution was continued for 2 hours and then allowed to stand at room temperature for 1 hour.

Under the condition of stirring, 10 ml of formaldehyde solution was added dropwise to the solution after standing, and the pH value of the solution was maintained at 8 with NaHCO ₃ solution. Control the reaction temperature to 60°C. When the reaction time exceeds 3 hours, add 25 ml of 0.5 mol/L sodium hyposulfite solution dropwise to continue the reaction. The reaction temperature was controlled to be 60° C., and the reaction time was 8 hours.

The reacted solution was filtered, washed with ammonium bicarbonate solution, and dried at 120° C. to obtain an electrocatalyst without heat treatment.

The electrocatalyst obtained above was heat-treated at 500° C. for 0.5 hour under an Ar atmosphere to prepare a supported electrocatalyst containing two catalytic promoters, Fe and Co. Wherein, the content of Pt is 10wt%, and the contents of Fe and Co are respectively 1wt%. It can be seen from the TEM photos that the Pt particles of the electrocatalyst are uniformly distributed, and the average particle size is 4.0nm. Compared with the E-TEK electrocatalyst, the oxygen electrode potential was increased by 30mV when the working current density was I=200mA/ ^cm2 .

In Example 2, only chromium is used to replace iron, that is, a supported electrocatalyst containing two promoter elements, Cr and Co, is prepared. Wherein, the content of Pt is 10wt%, and the contents of Cr and Co are respectively 1.5wt%. It can be seen from the TEM photos that the Pt particles of the electrocatalyst are uniformly distributed, and the average particle size is 4.0nm. Compared with the E-TEK electrocatalyst, when the working current density was I=200mA/cm ² , the potential of the oxygen electrode was increased by 22mV, and other conditions were the same as in Example 1.

In Example 3, only nickel was used instead of cobalt, that is, a supported electrocatalyst containing two catalytic promoters, Fe and Ni, was prepared. Wherein, the content of Pt is 20wt%, and the contents of Fe and Ni are respectively 5wt%. It can be seen from the TEM photos that the Pt particles of the electrocatalyst are distributed evenly, and the average particle size is 4.2nm. Compared with the E-TEK electrocatalyst, when the working current density was I=200mA/cm ² , the potential of the oxygen electrode was increased by 10mV, and other conditions were the same as in Example 1.

Embodiment 4 only introduces manganese element at the same time, and controls the content of each component according to the consumption of medicine. That is, a supported electrocatalyst containing three promoter elements of Cr, Co and Mn is obtained. Wherein, the content of Pt is 20wt%, and the contents of Fe, Co and Mn are respectively 0.5wt%. It can be seen from the TEM photos that the Pt particles of the electrocatalyst are uniformly distributed, and the average particle size is 3.9 nm. Compared with the E-TEK electrocatalyst, when the working current density was I=200mA/cm ² , the potential of the oxygen electrode was increased by 36mV, and other conditions were the same as in Example 2.

Embodiment 5 only introduces titanium element at the same time, and controls the content of each component according to the consumption of medicine. That is, a supported electrocatalyst containing four catalytic promoter elements of Co, Mn, Cr and Ti is prepared. Wherein, the content of Pt is 20wt%, and the contents of Fe, Co, Mn and Cr are respectively 0.5wt%. It can be seen from the TEM photos that the Pt particles of the electrocatalyst are uniformly distributed, and the average particle size is 3.8nm. Compared with the E-TEK electrocatalyst, when the working current density was I=200mA/cm ² , the potential of the oxygen electrode was increased by 41mV, and other conditions were the same as in Example 4.

The other conditions of Comparative Example 1 were the same as those of Example 1, except that the pretreatment conditions of the carrier carbon were changed. The carrier carbon was placed in a tube furnace, and CO ₂ was passed through, and activated at 450° C. for 3 hours. The electrochemical performance characterized by the oxygen electrode using the obtained electrocatalyst is shown in Fig. 1 .

Other conditions of Comparative Example 2 are the same as in Example 1, except that the pH condition of the reaction solution is changed to 10, and the electrochemical performance of the obtained electrocatalyst characterized by its oxygen electrode is shown in FIG. 1 .

In Comparative Example 3, the carrier carbon was not subjected to surface activation treatment, and other conditions were the same as in Example 1. The electrochemical performance of the obtained electrocatalyst characterized by its oxygen electrode is shown in FIG. 1 .

It should be noted that the polarization performance of the oxygen electrode is not only related to the quality of the catalyst itself, but also related to other factors (such as the preparation process of the oxygen electrode). Therefore, the advantages and disadvantages of catalysts given in Examples and Comparative Examples only have relative significance. The specific values may vary due to different conditions, but this change will not change the order of the catalysts.

Claims (9)

1. a platinum/carbon electrocatalyst containing promoter elements and preparation method thereof, is characterized in that: (1) The content of the catalyst Pt is between 10wt% and 30wt%, and the content of the promoter element is between 0.1 and 5wt%; (2) The catalytic promoter element is any two or more of Fe, Cr, Co, Ni, Mn, Ti; (3) Platinum particles are evenly distributed, with an average particle size of 3.5-4.5nm. 2. by the described platinum/carbon electrocatalyst of claim 1 containing promoter element and preparation method thereof, it is characterized in that catalyst Pt content is 10wt%, and the content of promoter element Fe and Co is respectively 1wt%; Pt particle uniform distribution , the average particle size is 4.0nm. 3. by the described platinum/carbon electrocatalyst of claim 1 containing promoter element and preparation method thereof, it is characterized in that catalyst Pt content is 10wt%, and the content of promoter element Fe and Co is respectively 1.5wt%; Pt particle is uniform Distribution, the average particle size is 4.0nm. 4. by the described platinum/carbon electrocatalyst of claim 1 containing promoter element and preparation method thereof, it is characterized in that catalyst Pt content is 20wt%, and the content of promoter element Fe and Ni is respectively 0.5wt%; Pt particle is uniform Distribution, the average particle size is 4.2nm. 5. by the described platinum/carbon electrocatalyst of claim 1 containing promoter element and preparation method thereof, it is characterized in that catalyst Pt content is 20wt%, and the content of promoter element Fe, Co and Mn is respectively 0.5wt%; Pt The particles are evenly distributed, with an average particle size of 3.9nm. 6. by the described platinum/carbon electrocatalyst of claim 1 containing promoter element and preparation method thereof, it is characterized in that catalyst Pt content is 20wt%, and the content of promoter element Fe, Co, Cr and Mn is respectively 0.5wt% ; Pt particles are evenly distributed, with an average particle size of 3.8nm. 7. by the described platinum/carbon electrocatalyst of claim 1 containing promoter element and preparation method thereof, it is characterized in that it comprises three technological processes: (1) Carbon carrier surface treatment: Add the powdery activated carbon obtained by phenolic resin cracking into the mixed system of concentrated nitric acid and concentrated phosphoric acid and soak for 10 hours under the condition of stirring. The volume ratio of concentrated nitric acid and concentrated phosphoric acid is controlled at Between 1:1～1:4, then the treated carrier carbon is filtered, washed with water, and then dried in a vacuum oven; (2) Preparation of Pt/C electrocatalyst containing cocatalyst elements without heat treatment by liquid phase co-deposition method: a. Under ultrasonic stirring, disperse the carbon carrier obtained in the above (1) into a suspension with distilled water, add a small amount of surfactant, including oleic acid, silicone oil or sodium dodecylsulfonate; then the temperature of the suspension Raise to 60 ° C, the pH of the solution is between 7 and 9; B. prepare chloroplatinic acid solution and nitrate or halide solution containing catalytic promoter element metal ion respectively, after these solutions are uniformly mixed under ultrasonic stirring condition, be added dropwise in the solution containing carrier carbon of above gained; NaHCO ₃ solution to adjust the pH value of the solution to 7-9; continue to stir the above solution for 2 hours, and then let it stand at room temperature for 1 hour; c. Add the formaldehyde solution drop by drop to the above-mentioned solution after stirring under the condition of stirring, and keep the pH value of the solution between 7 and 9 with NaHCO ₃ solution; Hours later, add one or several other reducing solutions dropwise to continue the reaction; control the reaction temperature to 60°C, and the reaction time is 6 to 8 hours; other reducing solutions are sodium hyposulfite solution or hydroboration Potassium solution or hydrazine hydrate solution; d. Filter the reacted solution, wash with ammonium bicarbonate solution, and dry at 120-150°C to obtain an electrocatalyst without heat treatment; (3) Heat-treat the unheated electrocatalyst obtained in the above (2) at 500-700° C. for 0.5-2 hours under an Ar atmosphere to obtain the PEMFC prepared by the present invention containing two or more than two kinds of co-catalysts. Pt/C electrocatalysts of catalytic elements. 8. The platinum/carbon electrocatalyst containing catalytic promoter elements and its preparation method according to claim 7, characterized in that the powdery activated carbon passes through a 1000 mesh sieve. 9. The platinum/carbon electrocatalyst containing catalytic promoter elements and the preparation method thereof according to claim 7, characterized in that the concentrations of the concentrated nitric acid and concentrated phosphoric acid are respectively 60wt% and 75wt%.