KR20140126209A - Method for preparing Pt catalyst, the Pt catalyst for oxygen reduction reaction prepared therefrom, and PEMFC including the Pt catalyst - Google Patents

Abstract

The present invention relates to a method for preparing a platinum catalyst, a platinum catalyst for oxygen reduction reaction prepared using the same, and a proton exchange membrane fuel cell including a platinum catalyst. More specifically, the method includes the steps of: dispersing a carbon carrier in element aqueous solution to prepare a carbon carrier dispersing solution; injecting platinum salts into the carbon carrier dispersing solution to form a platinum hydration complex compound and carrying the platinum hydration complex compound by the carbon carrier; and irradiating gamma rays to the platinum hydration complex compound to reduce platinum cation in the platinum hydration complex compound and forming a platinum cluster. When compared to the method for synthesizing a platinum catalyst of conventional technology, the method according to the present invention is capable of synthesizing smaller and more uniform nanoparticles and achieving more excellent dispersion, thereby providing a platinum catalyst having more excellent activity and stability for the oxygen reduction reaction. In particular, the platinum catalyst can be prepared in bulk enough to be commercially used by simple and eco-friendly processes.

Description

TECHNICAL FIELD The present invention relates to a platinum catalyst for oxygen reduction and a proton exchange membrane fuel cell comprising the same, and a proton exchange membrane fuel cell comprising the same,

The present invention relates to a method for producing a platinum catalyst for an oxygen reduction reaction used in a proton exchange membrane fuel cell.

Proton exchange membrane fuel cells (PEMFCs) are accepted as the most advanced fuel cell technology, and due to their high power density, relatively fast drive, fast response to various loads, and relatively low operating temperatures, low-pollution / pollution-free electric vehicles, , Power for small portable electronic devices, and the like. However, PEMFCs are constrained in large-scale commercialization due to the high cost of noble metal (platinum) catalysts and the occurrence of overvoltage at low temperatures due to slow oxygen reduction (ORR) rates. The electrode structure of the PEMFC is very important. Research has been carried out to improve the efficiency of the catalyst utilization by lowering the use of noble metals or reducing the particle size.

It has been found that the use of a carbon support to increase the catalyst utilization efficiency is effective in increasing the catalytic activity by increasing the active surface area while reducing the amount of noble metal in the catalyst. The carbon support includes carbon nanotubes , Carbon nanofiber, graphene, and mesoporous carbon are used. In addition, hollow mesoporous shell carbon (HCMSC) with a mesoporous outer structure with a hierarchical multi-structure carbon structure exhibits a large surface area, wide mesopore volume, and high electrical conductivity, It is expected that the catalytic activity can be greatly improved by widening the area and minimizing the resistance in the electrode. On the other hand, a method of reducing the polarization resistance of a fuel cell by reducing the thickness of a catalyst and a diffusion layer using a platinum supported catalyst having a large metal loading is widely proposed. However, when the amount of metal supported is high, platinum nanoparticles (NP) may aggregate during the synthesis process, so that it is important to uniformly disperse small-sized platinum nanoparticles on a carbon support. Therefore, in order to increase the electrocatalytic activity and utilization efficiency, researches for producing a platinum supported catalyst having a large active surface area have been actively conducted.

As a part of such studies, a wide variety of chemical methods for synthesizing platinum-supported catalysts such as NaBH ₄ reduction by wet impregnation, protective colloid method, microemulsion method, and ethylene glycol method using microwave have been reported. These methods are successful in synthesizing a platinum supported catalyst having a narrow particle size range of 1 to 5 nm in the case of low metal loading (for example, 20 wt% platinum), but in the case of high platinum loading (for example, 40 Weight percent platinum), it is difficult to control the particle size and the synthesis process is very complicated and time consuming. Moreover, these methods often result in lower activity of the catalyst due to the presence of surface protective agents or the formation of larger particles with a nonuniform particle distribution. Therefore, as a method for producing platinum nanoparticles in which platinum nanoparticles having a small diameter and a narrow size distribution are supported on a carbon support with a uniform dispersion, a reliable, easy and efficient synthesis route is required, It is very important for you.

Korean Patent Laid-Open Publication No. 2009-0073164 discloses a method for producing a platinum / carbon catalyst, a catalyst prepared using the same, and a proton exchange membrane fuel cell. In this method, the in-situ hydrolysis of urea Discloses a technique in which platinum hydroxide complex is homogeneously supported (Homogeneous Deposition (HD)) and uniformly reduced by ethylene glycol (EG) as the pH is gradually increased. It has an excellent effect in terms of size and dispersion. In this method, small platinum nanoparticles of about 2.8 nm in size are uniformly dispersed in a platinum loading amount of 60% by weight, and elements play an important role in this uniform dispersion. However, there is a problem that the above method is not suitable for mass production on a commercial scale.

Meanwhile, during the last several decades, a method of synthesizing various metals, metal alloys and metal oxide nanoparticles using gamma irradiation has been developed ((a) Fang, B .; Chaudhari, NK; Kim, M. -S .; Kim , JH; Yu, J.-S. J. Am . Chem . Soc . 2009 , 131 , 15330-15338 .; (b) Lee, K.-P .; Gopalan, AI; Santhosh, ;... Nho, YC Compos Sci Tech 2007, 67, 811-816 .; (c) Choi, S.-H .; Lee, S.-H .; Hwang, Y.-M .; Lee, K. -P .; Kang, H.-D. Rad. Phys . Chem. 2003, 67, 517-521 .; (d) Belloni, J .; Mostafavi, M .; Remita, H .; Marignier, J.-L .;. Delcourt, M.-O. New J. Chem 1998, 22, 1239-1255 .; (e) Sarkany, A .; Papp, Z .; Sajo, I .; Schay, Z. Solid State Ionics 2005 , 176 , 209-215 .; (f) Kageyama, S .; Seino, S .; Nakagawa, T .; Nitani, H .; Ueno, K .; Daimon, H .; Yamamoto, T. J. Nanopart . Res. 2011 , 13 , 5275-5287 .; (g) Ni, Y .; Ge, X .; Zhang, Z .; Ye, Q. Chem . Mater . 2002 , 14 , 1048-1052). In these methods, hydrated electrons, or active reduction species produced by radial decomposition of water, such as hydrogen atoms, are uniformly dispersed in the reaction solution, and the rate of formation of the active reducing species can be easily controlled by controlling the dose . It is an advantage of the gamma irradiation method that it is possible to synthesize environmentally friendly catalyst nanoparticles having a short reaction time, mild reaction conditions and a uniform size distribution. Despite these advantages, there has been no report on the use of gamma irradiation in the field of conventional electrocatalyst production.

Therefore, in the present invention, it is possible to synthesize smaller and more uniform nanoparticles than in the conventional method of synthesizing a platinum catalyst, and it is possible to attain excellent dispersion and to exhibit more excellent activity and stability in an oxygen reduction reaction, And a platinum catalyst for oxygen reduction reaction produced by the method, and a fuel cell comprising the same.

In order to achieve the first object of the present invention,

Dispersing a carbon carrier in an aqueous urea solution to prepare a carbon carrier dispersion;

Introducing a platinum salt into the carbon carrier dispersion to form a platinum anhydride complex and supporting the platinum anhydride complex on a carbon support; And

And irradiating the platinum oxide complex with gamma rays to reduce platinum cations in the platinum anhydride complex to form platinum clusters.

According to an embodiment of the present invention, the platinum oxide complex may be formed by coordinating a hydroxide ion formed by the following Reaction Scheme 1 with a platinum cation of a platinum salt:

[Reaction Scheme 1]

CO (NH ₂ ) ₂ + 3H ₂ O? 2NH ^{4 +} + CO ₂ + 2OH ^-

According to another embodiment of the present invention, the platinum salt is a chloroplatinic acid salt, and the coordination bond between the hydroxide ion and the platinum cation may be formed by the following reaction formula 2:

[Reaction Scheme 2]

_{[PtCl 6 -x (H 2 O} ) x] (2-x) - + yOH - → [PtCl 6 -x (OH) y (H 2 O) xy] (2-x + y) - + yH 2 O

Wherein x is 0, 1 or 2 and y is an integer less than or equal to x.

According to another embodiment of the present invention, the carbon carrier may be a carbon black, a carbon nanotube, a carbon nanofiber, a graphene, a mesoporous carbon, or a hierarchical multi-structure carbon structure.

According to another embodiment of the present invention, the formation of the platinum oxide complex and the step of supporting the platinum oxide complex on the carbon carrier may be performed at a temperature of 70 to 110 ° C and an acidity condition of pH 7 to pH 10.

According to another embodiment of the present invention, the supporting ratio of the platinum catalyst to the carbon carrier may be 10 to 80% by weight.

According to another embodiment of the present invention, hydrated electrons and radicals are generated by the irradiation of gamma rays, and the hydrated electrons and radicals lower the oxidation number by reducing the platinum cation in the platinum anhydride complex, The platinum nanoparticles can be formed and grown.

According to another embodiment of the present invention, the gamma ray irradiation condition may be performed at an irradiation speed of 0.5 x 10 ³ Gy / h to 4.0 x 10 ³ Gy / h for 0.5 to 6 hours.

According to another embodiment of the present invention, the particle size of the platinum nanoparticles may be 1.8 to 3.7 nm.

In order to achieve the second object, the present invention provides a platinum catalyst for an oxygen reduction reaction produced by the above method.

The present invention also provides a proton exchange membrane fuel cell comprising the platinum catalyst for oxygen reduction reaction to achieve the third object.

According to the present invention, it is possible to provide a platinum catalyst exhibiting more excellent activity and stability in an oxygen reduction reaction because it can synthesize smaller and more uniform nanoparticles than in the conventional method of synthesizing a platinum catalyst And in particular, these platinum catalysts can be produced in large quantities such that they can be commercialized by a simple and environmentally friendly process.

FIGS. 1A-1H are HR-SEM images (a, c, e, g) and HR-TEM images of catalysts prepared by the method according to Example 1, Comparative Example 1 and Comparative Example 2, and commercialized Premetek platinum catalysts b, d, f, h).
Figures 2A-2C illustrate the preparation of a catalyst comprising 60 wt% (Example 4, Figure 2a) of a catalyst prepared using hollow core-mesoporous shell carbon (HCMSC) as a carrier by the method according to Examples 4-6. , 40 wt% (Example 5, Fig. 2B) and 20 wt% (Example 6, Fig. 2C) platinum / HCMSC.
3A and 3B are graphs showing the XRD pattern (FIG. 3A) and the TGA graph (FIG. 3B) of the catalyst prepared by the method according to Example 1, Comparative Example 1 and Comparative Example 2, and the commercialized Premetek platinum catalyst .
FIGS. 4A to 4D show the results of XPS analysis of the degree of reduction and the oxidation state of platinum nanoparticles in a catalyst prepared by the method according to Example 1, Comparative Example 1, and Comparative Example 2, and a commercially available Premetek platinum catalyst FIG.
5a and 5b are graphs showing the electroadhesion curves at 50 mV s < ^-1 > in 0.5 MH ₂ SO ₄ of the catalyst prepared by the method according to Example 1, Comparative Example 1 and Comparative Example 2, and the commercialized Premetek platinum catalyst, respectively FIG. 5C is a graph showing the polarization performance of a PEMFC at 60 ° C. when the catalyst according to Examples 1 to 3 is used as a cathode catalyst. FIG.
FIG. 6 is a graph showing changes in current of a single cell over time for a period of 10 hours under a constant voltage of 0.75 V for a catalyst prepared by the method according to Example 1, Comparative Example 1 and Comparative Example 2, and a commercially available Premetek platinum catalyst Graph.

In the present invention, a platinum supported catalyst supported on a carbon support such as carbon black in a high metal deposition amount was synthesized by combining homogeneous deposition (HD) using urea and a synthesis method using gamma irradiation.

Therefore, in the first step of the present invention, the hydroxide ion is gradually and uniformly produced in the acidic platinum salt solution by using the urea, and the generated hydroxide ion reacts with the platinum ^{4 +} To form a complex and to be uniformly supported on the carbon carrier. Uniform supporting technique using these elements has been disclosed in another patent application of Korean Patent Application Publication No. 2009-0073164 by the present inventor. According to the above method, unlike the case where pH is controlled by adding KOH from the outside, there is an advantage that the platinum nanoparticles having a fine particle size can be uniformly supported on the carbon carrier since the problem of a rapid increase in pH locally or a rapid precipitation of platinum oxide complex can be avoided.

As a second step, the method of preparing a platinum catalyst according to the present invention comprises irradiating an ionized gamma ray to the supported platinum oxide complex to generate hydrated electrons and other radicals, and the resulting hydrated electrons and radicals are first reduced Lowering the oxidation number and subsequently forming a metallic platinum cluster. Finally, metallic platinum nanoparticles are formed by agglomeration and grow.

In particular, the method according to the present invention using a combination of HD and gamma ray irradiation is very efficient, environmentally friendly, easy to expand the production scale without using harmful reducing agents, and can save time and energy consumption. Further, the platinum catalyst prepared by the method according to the present invention is superior in terms of utilization efficiency, ORR catalyst activity, and fuel cell polarization performance as compared with the platinum supported catalyst or the commercialized catalyst synthesized by the commonly used impregnation-NaBH ₄ reduction method And it is easy to expand to commercial production scale.

Specifically, the process for producing a platinum catalyst according to the present invention comprises the steps of: preparing a carbon carrier dispersion by dispersing a carbon carrier in an aqueous urea solution; Introducing a platinum salt into the carbon carrier dispersion to form a platinum anhydride complex and supporting the platinum anhydride complex on a carbon support; And irradiating the platinum oxide complex with a gamma ray to reduce the platinum cation in the platinum oxide complex to form a platinum cluster.

In the process according to the present invention, the particle size of the platinum nanoparticles adsorbed on the carbon carrier depends on the acidity of the urea aqueous solution, and the acidity of the aqueous urea solution is controlled by the amount of the element dissolved in the water. The element helps to produce hydroxide ions in-situ, gradual, homogeneous, and prevents the formation of local supersaturation or precipitation. As a result, the active phase or its precursor is slowly and uniformly adsorbed to the carrier, and the chemical reaction between the reactant and the carrier causes the precipitation to occur only on the surface of the carrier and nucleation in the solution can be prevented from occurring . That is, the platinum oxide complex may be formed by coordination bonding of a hydroxide ion formed by the following Reaction Scheme 1 and a platinum cation of a platinum salt:

[Reaction Scheme 1]

CO (NH ₂ ) ₂ + 3H ₂ O? 2NH ^{4 +} + CO ₂ + 2OH ^-

For example, when a chloroplatinic acid salt is used as the platinum salt, the coordination bond between the hydroxide ion and the platinum cation may be formed by the following reaction formula 2:

[Reaction Scheme 2]

_{[PtCl 6 -x (H 2 O} ) x] (2-x) - + yOH - → [PtCl 6 -x (OH) y (H 2 O) xy] (2-x + y) - + yH 2 O

Wherein x is 0, 1 or 2 and y is an integer less than or equal to x.

On the other hand, the carbon carrier used in the present invention means a material to which platinum particles can be adsorbed. The carbon carrier may be a carbon black, a carbon nanotube, a carbon nanofiber, a graphene, a mesoporous carbon, Type can be used.

At this time, it is preferable that the formation of hydroxide ion, the reaction of formation of platinum oxide complex by hydrolysis of the urea, and the adsorption reaction on the carbon carrier proceed at a temperature of 70 to 110 캜 and an acidity condition of pH 7 to pH 10. When the temperature of the solution is less than 70 ° C, it is difficult to cause a smooth reaction. When the temperature exceeds 110 ° C, coagulation may occur. When the acidity of the solution is in the range of pH 7 to pH 10, . By the above process, the platinum catalyst is supported on the carbon carrier, and more specifically, it can be supported in a loading amount of about 10 to 80% by weight.

After the platinum anhydride complex is supported on the carbon support, the platinum anhydride complex is irradiated with gamma rays. Gamma irradiation generates hydrated electrons and radicals as shown in Scheme 3 below:

[Reaction Scheme 3]

H ₂ O → ne _eq ^- , H ^· , OH ^· , H ⁺

These hydrides and radicals first reduce platinum cations in the platinum anhydride complex to lower the oxidation number, followed by the formation of metallic platinum clusters, and finally, platinum nanoparticles are formed by aggregation of the clusters. Harmful oxidative OH ^· radicals can be disabled using, for example, 2-propanol:

[Reaction Scheme 4]

_{[PtCl 6 -x (OH) y} (H 2 O) xy] (2-x + y) - + (ne eq - or ^{H ·) → Pt 0 + H} + + (x + y) H 2 O

It is preferable that the gamma ray irradiation condition is performed for 0.5 to 6 hours at an irradiation speed of 0.5 x 10 ³ Gy / h to 4.0 x 10 ³ Gy / h, because the irradiation speed of the gamma ray is less than 0.5 x 10 ³ Gy / h There is a problem in that the reduction of the platinum oxide complex supported on the carbon carrier is not easy, and when it exceeds 4.0 × 10 ³ Gy / h, the platinum nanoparticles increase in size and aggregate, which is not preferable . When the irradiation time of the gamma ray is less than 0.5 hour, there is a problem that sufficient time can not be secured for reducing the platinum oxide complex supported on the carbon carrier. When the irradiation time exceeds 6 hours, excessive irradiation of the gamma- There arises a problem of causing increase in size and agglomeration.

As can be seen from the following examples, the platinum nanoparticles prepared using the method according to the present invention had a particle size of about 1.8 to 3.7 nm, and were prepared by the conventional impregnation-NaBH ₄ reduction method, HD-EG method, or commercial Premetek platinum %) / VC catalyst, and the efficiency of ECSA and platinum utilization is higher as the particles are uniformly dispersed on the carbon carrier.

The present invention also provides a platinum catalyst for oxygen reduction reaction produced by the above method and a proton exchange membrane fuel cell comprising the same. The platinum catalyst for the oxygen reduction reaction according to the present invention does not deteriorate the ORR performance even when the platinum loading is increased, since the size and coagulation phenomenon of the platinum nanoparticles are minimized. In addition, since the content of platinum (0) species in the final supported platinum nanoparticles is high and small and uniform particles are supported, dissolution and aggregation of the platinum nanoparticles are suppressed during the operation, so that compared with conventional platinum catalysts High initial current response and final current response.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are intended to assist the understanding of the present invention and should not be construed as limiting the scope of the present invention.

VC  Synthesis of supported platinum catalyst

20 wt%, 40 VC (Vulcan XC- 72) a supported platinum catalyst having a platinum loading of various synthetic methods% by weight and 60% by weight, that is impregnated -NaBH ₄ reduction (Steigerwalt, ES; Deluga, GA; Lukehart, CM J .. Phys Chem B 2002, 106 , 760-766;. Chai, GS;.. Yu, J.-S. J. Mater Chem 2009, 19, 6842-6848), EG using the elements (HD-EG) method (Oh, H.-S .; Oh , J.-G .; Hong, Y.-G .; Sharma, R .; Shul, Y.-G .; Kim, H. Res. Chem. Intermed. 2008, 34, 853-861; Fang, B .; Chaudhari, NK; Kim, M.-S .; Kim, JH;... Yu, J.-S. J. Am Chem Soc 2009, 131, 15330-15338) And the method according to the present invention. All chemicals were used as purchased without purification. Unless otherwise noted, the amount of deionized water (DI) and metal precursor in each synthesis method was fixed at 15 mL per 200 mL and 0.05 MH ₂ PtCl ₆ · 6H ₂ O aqueous solution, respectively. The platinum loading rate (%) on the VC was controlled by controlling the amount of VC.

Example  1 to 3. Synthesis of platinum (60 wt%, 40 wt% and 20 wt%) / VC catalyst according to the invention

4.62 g of element (urea / platinum mole ratio = 20) was dissolved in 1.0 L of DI, 0.5 g of carbon was added, and sonicated for 30 minutes. Then, 75 mL of 0.05 MH ₂ PtCl ₆ .6H ₂ O aqueous solution was added. After stirring for 3 hours, the mixture was heated to 90 < 0 > C and stirred for an additional 7 hours. Next, 50 mL of 2-propanol was added as a radical scavenger, and nitrogen was bubbled through the mixture to remove dissolved oxygen. Then, 40 kGy gamma rays were irradiated with a Co-60 gamma ray source at a surrounding temperature in a nitrogen atmosphere (Irradiation speed = 2.0 x 10 ³ Gy / h, exposure time = 2 hours). During the irradiation of the gamma rays, the solution was continuously stirred to uniformly disperse the platinum nanoparticles on the VC, thereby preparing a catalyst according to Example 1, which was a platinum (60 wt%) / VC catalyst. (40% by weight) / VC catalyst (Example 2) and platinum (20% by weight) / VC catalyst (40% by weight) were prepared by the same procedure as in Example 1, Example 3) was prepared.

Example  4 to 6. Synthesis of platinum (60 wt%, 40 wt% and 20 wt%) / HCMSC catalyst according to the invention

Unlike Examples 1 to 3, a supported catalyst was synthesized by using hollow core-mesoporous shell carbon (HCMSC) as a carrier of platinum catalyst. Urea (urea / platinum molar ratio = 20) of 0.925 g (60 wt% platinum / HCMSC), 0.411 g (40 wt% platinum / HCMSC) and 0.154 g (20 wt% platinum / HCMSC) 0.1 g HCMSC was added and sonicated for 30 min. Then, 15 mL (60 wt% platinum / HCMSC), 7 mL (40 wt% platinum / HCMSC) and 3 mL (20 wt% platinum / HCMSC) 0.05 MH ₂ PtCl ₆ .6H ₂ O aqueous solutions were added, respectively. After stirring for 3 hours, the mixture was heated to 90 < 0 > C and stirred for an additional 7 hours. Next, 50 mL of 2-propanol was added as a radical scavenger, and nitrogen was bubbled through the mixture to remove dissolved oxygen. Then, 40 kGy gamma rays were irradiated with a Co-60 gamma ray source at a surrounding temperature in a nitrogen atmosphere (Irradiation speed = 2.0 x 10 ³ Gy / h, exposure time = 2 hours). While the gamma ray was irradiated, the solution was continuously stirred to uniformly disperse the platinum nanoparticles on the VC.

Comparative Example  One. Impregnation - NaBH ₄  Platinum (60% by weight) / VC  Synthesis of Catalyst

0.1 g of VC was added to 200 mL of DI, sonicated for 30 minutes, and then 15 mL of 0.05 MH ₂ PtCl ₆ .6H ₂ O aqueous solution was added. After stirring for 3 hours, the pH of the mixed solution was adjusted to about 8 by adding 0.4 M KOH. Thereafter, an NaBH ₄ aqueous solution was added dropwise at a molar ratio (NaBH ₄ : Pt) of 10: 1, and the mixture was continuously stirred overnight. Then, the mixed solution was filtered and washed with a large amount of water to remove impurities. The residue was dried overnight at 80 < 0 > C to obtain a platinum / VC catalyst.

Comparative Example  2. HD - EG  Platinum (60% by weight) / VC  Synthesis of Catalyst

0.925 g of element (urea / platinum mole ratio = 20) was first dissolved in 200 mL of DI, 0.1 g of carbon was added, and the mixture was ultrasonicated for 30 minutes. Next, 15 mL of 0.05 MH ₂ PtCl ₆ .6H ₂ O aqueous solution was added and stirred for 3 hours. The temperature was then raised to 90 DEG C, allowing the urea to hydrolyze and stirring for 7 hours. The pH of the solution was around 7-10. The mixture was cooled to room temperature and 200 mL of EG was added. After stirring for an additional 3 hours, the solution was added to 120 < 0 > C and stirred overnight. The formed catalyst slurry was then filtered and washed with excess DI to remove impurities. The obtained platinum catalyst was dried at 80 占 폚 overnight.

Test Example  1. Surface Characterization

Platinum nanoparticles formed on a VC support were identified by X-ray diffraction (XRD) analysis at 40 kV, 20 mA using a Rigaku 1200 and Cu-K? Light source, Ni? -Filter. The mean particle size of the platinum nanoparticles synthesized by each method was determined from the 2? Reflection peak for the platinum (220) lattice surface using the Debye-Scherer equation. The platinum nanoparticles dispersed on the VC support were further analyzed by high-resolution scanning electron microscopy (HR-SEM) analysis and high-resolution transmission electron microscopy (HR-TEM) analysis. HR-SEM images were obtained using a Hitachi S-5500 microscope operating at 30 kV. HR-TEM images were obtained using a JEOL FE-2010 microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the oxidation state of the platinum nanoparticles formed. XPS spectra were obtained using AXIS-NOVA (Kratos) X-ray photoelectron spectroscopy and monochromatic Al Kα (150 W) light source under a base pressure of 2.6 x 10 ^-9 Torr. The metal loading of all samples was determined by thermogravimetric analysis (TGA) using a Bruker TG-DTA2000SA analyzer.

Test Example  2. Measurement of electrochemical performance

The electrochemical surface area (ECSA) of platinum of platinum (60 wt%) / VC catalyst synthesized by each method was measured by cyclic voltammetry (CV) using a three electrode (half-cell) configuration. CV measurements were performed in a 0.5 MH ₂ SO ₄ electrolyte at a voltage scan rate of 50 mV s ^-1 at room temperature. Plasma wire and Ag / AgCl (in 3.0 M NaCl solution) were used as a counter electrode and a reference electrode, respectively, as a working electrode, a Nafion-impregnated catalyst in the form of a thin film cast on a 3 mm diameter glassy carbon disk impregnated with a Teflon cylinder . The working electrode was prepared by the method described in literature (Fang, B .; Chaudhari, NK ; Kim, M.-S .; Kim, JH;... Yu, J.-S. J. Am Chem Soc 2009, 131 , 15330-15338). The amount of catalyst supported on the working electrode was 0.5 mg Pt / cm ^-2 . After the electrolytic solution was deaerated with high purity nitrogen for 1 hour, a cyclic voltammetric curve stabilized after 10 cycles was recorded by cyclic voltammetry.

The polarization performance of the fuel cell using each platinum (60 wt%) / VC cathode catalyst was evaluated by fabricating a unit cell. A cell electrode assembly (MEA) was fabricated using a membrane electrode assembly (MEA) having a surface area of 10 cm ² produced by pressing a pre-treated Nafion 115 membrane (DuPont) between the anode and the cathode at 130 ° C. The amount of the catalyst supported was 0.2 mg platinum / cm ^-2 for the anode and 0.4 mg platinum / cm ^-2 for the cathode. The test was carried out using platinum (60% by weight) / VC (Premetek) as the anode catalyst. Each platinum / VC catalyst was dispersed in a mixed solution of an appropriate amount of DI and a required amount of a 5 wt% Nafion ionomer solution (Aldrich) to prepare a catalyst ink. The contents of the Nafion ionomers in the anode catalyst layer and the cathode catalyst layer were 20 wt% and 25 wt%, respectively. An appropriate amount of the catalyst ink was coated on Teflon coated carbon paper (TGPH-090) and dried at 80 캜 overnight. The fabricated fuel cell was humidified at 75 ℃ and the polarization performance was measured by a WFCTS fuel cell tester at 60 ℃ under constant current and voltage. The H ₂ and O _2, respectively 200 mL min ^- at a rate of ¹ mL min ^-1 and 500 were supplied to the anode and the cathode.

Results analysis

(Comparative Example 1) using NaBH ₄ as a reducing agent (Comparative Example 1), HD-EG method using an element using EG as a reducing agent (Comparative Example 2), and three methods of the method according to the present invention ) Were compared with those of platinum nanoparticles synthesized on VC. In the case of Example 1, in the first step, the urea was hydrolyzed at 90 占 폚 so that the hydroxide ion was gradually and uniformly produced in the acidic platinum salt solution by the following mechanism.

CO (NH ₂ ) ₂ + 3H ₂ O? 2NH ^{4 +} + CO ₂ + 2OH ^-

The pH of the solution gradually rises to around 8 due to the OH ^- generated in situ and OH ^- reacts with the platinum ^{4 +} cation to form a platinum anhydride complex, which is supported uniformly on the carbon support. Unlike the case where the pH is controlled by adding KOH from the outside, it is possible to avoid the problem that the pH of the solution increases rapidly locally or that the platinum hydroxide precipitates rapidly.

_{[PtCl 6 -x (H 2 O} ) x] (2-x) - + xCl - + yOH - → [PtCl 6 -x (OH) y (H 2 O) xy] (2-x + y) - + yH ₂ O + xCl ^-

In the second step, the supported platinum complex is irradiated with ionized gamma rays to generate primary and secondary radiolytic species, hydrated electrons and other radicals.

H ₂ O → ne _eq ^- , H ^· , OH ^· , H ⁺

These hydrated electrons and other radicals first reduce the platinum cation to lower the oxidation number and then form a metallic platinum cluster. Finally, metallic platinum nanoparticles are formed by agglomeration and grow. Harmful oxidative ^· OH radical was deactivated by using a 2-propanol.

_{[PtCl 6 -x (OH) y} (H 2 O) xy] (2-x + y) - + (ne eq - or ^{H ·) → Pt 0 + H} + + (x + y) H 2 O

The size and distribution of the platinum nanoparticles produced during the reduction by gamma irradiation is clearly dependent on the state of the platinum complex supported on the carbon support in the homogeneous loading process using the elements. It has been confirmed that the addition of an element plays a key role in controlling the size and dispersion of platinum nanoparticles.

(A, c, e, g) and HR-TEM images (b, d, f, h) of platinum (60 wt%) / VC catalyst synthesized by each method are shown in Figs. Respectively. The platinum nanoparticles (g and h) synthesized according to Comparative Examples 2 (e and f) and Example 1 were added to the catalysts (a and b) prepared by Comparative Example 1 or the commercially available Premetek platinum catalysts (c and d) It can be seen that it is superior in dispersion and size. When the reducing agent NaBH ₄ was added ex situ, the platinum (IV) complex could not be uniformly reduced on the VC, and some platinum nanoparticles aggregated in the reducing process to exhibit a size distribution of about 3.5 to 4.8 nm 1b), which is thought to be due to the strong reducing power of NaBH ₄ at room temperature. In addition, the pH of the solution increases instantaneously and locally in the process of adjusting the pH by adding 0.4 M KOH, so that the dispersion of the platinum nanoparticles becomes uneven. In the case of commercialized Premetek catalysts, smaller size of platinum nanoparticles were observed in comparison with the case of NaBH ₄ reduction method, and it is easy to control the particle size in the catalyst synthesis process. However, the size distribution of platinum nanoparticles is in the range of 3.2 to 4.3 nm Respectively.

In contrast, the platinum / VC nanoparticles prepared by the HD-EG method (Comparative Example 2) exhibited a homogeneous distribution and a rough distribution of about 2.3 to 3.4 nm as shown in the HR-SEM and HR-TEM images of FIGS. It shows small particle size. For the method according to the invention, the particle size calculated from the HR-SEM image and the HR-TEM image was about 2.2 to 3.1 nm. The advantages of Comparative Example 2 and Example 1 in terms of uniformity and size control are that OH ^- ions released when the element is hydrolyzed are exchanged with the chloride ion of the metal salt while increasing the pH of the aqueous solution so that uniformity of the metal complex This leads to sedimentation. In Comparative Example 2 and Example 1, aggregation of platinum nanoparticles was not observed. As a result of repeated synthesis and characterization, the platinum nanoparticles formed in Example 1 were generally smaller in size and superior in dispersibility than the platinum nanoparticles formed in Comparative Example 2. Thus, the decrease in particle size is due to the rapid karyotype and growth in the gamma irradiation process.

2A to 2C show the results of a comparison of a 60 wt% (Example 4, Fig. 2A), 40 wt% (Example 5, Fig. 2B) synthesized using a hollow core-mesoporous shell carbon (HCMSC) ) And a HR-TEM image of a platinum / HCMSC catalyst of 20 wt% (Example 6, Fig. 2C). The platinum nanoparticles synthesized by Examples 4 to 6 (FIGS. 2A to 2C) are superior in dispersion and size to the catalyst prepared by Comparative Example 1 or the commercially available Premetek platinum catalyst. Examples 4 to 6, which induced uniform precipitation of metal complexes on HCMSC through hydrolysis of urea, exhibited large surface area, wide mesopore volume, and high electrical conductivity, thus widening the reaction area of the metal catalyst particles, It is expected that the activity of the catalyst can be greatly improved.

FIG. 3A shows XRD patterns for the four catalysts. From the XRD results of FIG. 3A, it can be seen that the platinum nanoparticles produced by each of the synthesis methods form a disordered face-centered cubic (fcc) structure. The mean particle size of platinum (60% by weight) / VC calculated using the Debye-Scherer equation from the widening of the platinum (220) reflection peak was 4.3 nm and 2.9 nm in Comparative Example 1, Comparative Example 2 and Example 1, respectively nm and 2.7 nm, respectively, and 4.0 nm for the Premetek commercial catalyst. The particle sizes calculated from the XRD data of the platinum nanoparticles were generally in good agreement with the particle sizes arbitrarily selected from the HR-SEM and HR-TEM images of FIG. The sizes of the platinum particles determined through the platinum (220) reflection peak measurement of platinum (60 wt.%) / VC prepared by each method are shown in Table 1 below.

3B shows the TGA curves of platinum (60 wt.%) / VC and Premetek platinum (60 wt.%) / VC commercialized according to Comparative Example 1, Comparative Example 2 and Example 1, 1, the TGA curves of the catalysts in which the platinum loading amount was changed to 40 wt% (Example 2) and 20 wt% (Example 3) were also shown. From the TGA data, it can be seen that the actual metal loading of all platinum / VC catalysts is very similar to the calculated value, which means that the platinum loading is effectively controlled in each of the synthesis methods.

Meanwhile, the degree of reduction and oxidation state of the platinum nanoparticles in the synthesized catalyst were analyzed by XPS, and the results are shown in FIGS. 4A to 4D. The platinum / VC catalysts according to Comparative Example 1 (4a), Comparative Example 2 (4c) and Example 1 (4d) were found to be 71.4 eV and 74.7 eV in the platinum 4f binding energy region, beam was a strong peak at a, which means that the 4f _7/2 and 4f _5/2 for the VC, and supported platinum (60% by weight) is mainly zero oxidation number of platinum in the catalyst of each of the metallic platinum. The relative content of platinum (0), platinum (II) and platinum (IV) in the platinum nanoparticles was calculated by Gaussian curve fit analysis of the platinum 4f peak. As a result, the platinum / And platinum (0) and platinum (IV) contents were lower than those of commercial platinum catalysts. This result means that the adsorbed platinum (IV) complex is almost completely reduced by gamma irradiation, which means that the process according to the present invention is highly effective in the preparation of a platinum catalyst supported on a VC support with a high supported ratio. The detailed composition ratios of the 60 wt% platinum / VC catalyst are shown in Table 1 below.

Electrochemically active surface area of platinum nanoparticles (ECSA) is a very important parameter that reflects the unique electrocatalytic activity of the platinum catalyst, typically a supporting electrolyte (0.5 MH ₂ SO ₄₎ hydrogen in a normal state CV in under a N ₂ atmosphere Is determined by the integrated charge amount (excluding the capacitance component) of the absorption region (-0.2 to +0.05 V).

FIG. 5A shows the electroadhesion curves of the four catalysts (Comparative Examples 1 and 2, Example 1, commercial Premetek platinum / VC) at 50 mV s ^-1 in 0.5 MH ₂ SO ₄ . The ECSA values calculated by the equation ECSA = Q _H / [L xq _H ] (equation (1)) are summarized in Table 1 below. Wherein, Q _H is hydrogen absorption amounts, L is a platinum mass (mg cm ^-2), _H q is a charge amount required to oxidize the H ₂ single layer on the Pt (0.21 mC cm ^-2) of the supported electrode. ECSA measure of the catalyst according to Example 1 Referring to Table 1, it is 83 m ² g ^{- 1,} in Comparative Example 1 (37 m ² g ^-1) or Comparative Example ² (71 m 2 g ^-1) (60 wt%) / VC (48 m < ² > g < ^-1 >). These results indicate that the method according to the present invention can uniformly control the particle size and dispersion even when the amount of supported platinum nanoparticles is large.

On the other hand, the catalyst utilization efficiency, which is an important parameter in evaluating the catalytic activity, can be generally obtained by dividing ECSA by the chemical surface area (CSA). CSA can be calculated from the formula CSA = 6000 / ρd (Equation (2)) where ρ is the density of platinum (21.4 g cm ^-3 ) and d is the mean diameter of the platinum nanoparticles obtained from the XRD analysis. The CSA calculated value of platinum (60 wt.%) / VC prepared according to the example was 104 m ² g ^-1 , the platinum (60 wt.%) / VC (65 m ² g ^-1 ) (60 wt%) / VC (70 m ² g ^-1 ), and platinum (60 wt%) / VC (96 m ² g ^-1 ) according to Comparative Example 2. The utilization efficiencies of platinum (60% by weight) / VC according to Comparative Example 1, commercial Premetek, Comparative Example 2 and Example 1 were about 57%, 69%, 74% and 80%, respectively. The reason why the utilization efficiency of platinum (60% by weight) / VC according to Example 1 is the highest is also the smallest size of the platinum nanoparticles and is well dispersed on the VC, resulting in a large surface area for the adsorption and catalysis of the O ₂ reduction reaction .

Figure 5b shows the polarization performance of a PEMFC using each platinum (60 wt%) / VC cathode catalyst at 60 < 0 > C. In principle, polarization of a fuel cell using H ₂ as a fuel, which occurs at a low current density, is mainly due to electrochemical activation by ORR which proceeds slowly at the cathode surface. Platinum (60 wt.%) / VC according to Example 1 had less voltage loss due to polarization than other synthesis methods, which means that the catalytic activity for ORR is high. The current density and the maximum power density at 0.6 V of platinum (60 wt.%) / VC according to Example 1 were 627 mA cm ^-2 and 451 mW cm ^-2 , respectively, and that of Comparative Example 1 (381 mA cm ^-2 and 307 mW cm ^-2 ), commercial Premetek (537 mA cm ^-2 and 377 mW cm ^-2 ), and Comparative Example 2 (588 mA cm ^-2 and 429 mW cm ^-2 ). Thus, as the size of the platinum nanoparticles is small and the particles are uniformly dispersed on the VC support, the efficiency of utilization of ECSA and platinum is higher, and thus the catalyst synthesized by Example 1 exhibits higher catalytic activity and fuel cell polarization performance will be.

In general, the PEMFC polarization performance tends to decrease as the platinum loading in the catalyst increases. Qi et al. Reported that platinum / VC catalysts with a large platinum loading amount had lower PEMFC performance than platinum supported catalysts because of the much smaller platinum ECSA (Qi, Z.-G .; Kaufman, A. J. Power Sources 2003 , 113 , 37-43). However, the catalyst prepared according to the present invention shows a tendency to be significantly different from this conventional theory (see FIG. 5C). The catalyst prepared according to the present invention has a higher current density and maximum power density at 0.6 V as the platinum loading increases from 20 wt% to 60 wt%, compared with the case where the loading amount is 20 wt% (Example 3) And about 11% and 13%, respectively, in the case of 40 wt% (Example 2), about 9% and 10%, and the loading amount of 60 wt% (Example 1). The improvement of PEMFC performance is considered to be due to the minimization of the increase in the size of the platinum nanoparticles due to the increase of the platinum loading in the method according to the present invention. As a result, even when the amount of supported platinum is 60% by weight, the catalyst prepared according to the present invention has a high ECSA and an improved ORR performance because the increase of the platinum particle size and the platinum nanoparticle aggregation are not remarkable. The ability to improve the fuel cell performance of a catalyst with a high platinum loading (60 wt.%) Compared to medium platinum loading (40 wt.%) Or low (20 wt.%) Is of great importance in practical applications. If the amount of supported platinum is large, the amount of the catalyst support can be reduced, thereby reducing the manufacturing cost of the fuel cell and reducing the weight and size of the fuel cell stack. This increases the fuel efficiency and increases the possibility of commercialization due to diversification of the battery design. do.

Durability against long-term use of catalysts is also an important factor in terms of commercial viability. The durability of the four catalysts (Comparative Examples 1 and 2, Example 1, commercial Premetek platinum / VC) was confirmed by measuring the change in current of the single cell over time at a constant voltage of 0.75 V for 10 hours Reference). As can be seen from FIG. 6, the current response of each catalyst continuously decreased with time because sintering or migration of platinum particles on a carbon support, dissociation of platinum and dissolution in the electrolyte, production of toxic substances, and carbon Undesirable phenomena such as corrosion of the support occur. Nevertheless, the catalyst synthesized by Example 1 showed a higher initial current response and a much higher final current response than the catalyst by other methods (Example 1, Comparative Example 2, commercial Premetek catalyst, and initial And the current response retention rates were 73%, 65%, 62% and 46%, respectively). This behavior is due to the fact that the content of platinum (0) species is higher than that of other methods in Example 1, the size of platinum nanoparticles is smaller and distribution is more uniform, and dissolution and aggregation of platinum are inhibited during operation. Table 1 summarizes the physical and electrochemical properties of the four catalysts.

^a ) The grain size (D) of the platinum nanoparticles was determined by the Debye-Scherrer equation from the broadening of the platinum (220) reflection peak as a result of XRD analysis.

^b The chemical surface area (CSA) of the catalyst was calculated using equation (2).

^{c The} electrochemically active surface area (ECSA) was determined by equation (1).

^d Platinum utilization efficiency (PU) was calculated by the equation PU = ECSA / CSA x 100.

^e The current density (I) and maximum power density (P _max ) at 0.6 V were measured by a single cell test.

^f relative content of platinum (0), Pt (II) and platinum (IV) species was determined by the inverse convolution platinum 4f XPS data.

Claims (11)

Dispersing a carbon carrier in an aqueous urea solution to prepare a carbon carrier dispersion;
Introducing a platinum salt into the carbon carrier dispersion to form a platinum anhydride complex and supporting the platinum anhydride complex on a carbon support; And
And irradiating the platinum oxide complex with gamma rays to reduce platinum cations in the platinum oxide complex to form platinum clusters. The method for preparing a platinum catalyst according to claim 1, wherein the platinum oxide complex is formed by coordinating a hydroxide ion formed by the following Reaction Scheme 1 with a platinum cation of a platinum salt:
[Reaction Scheme 1]
CO (NH ₂ ) ₂ + 3H ₂ O? 2NH ^{4 +} + CO ₂ + 2OH ^- 3. The process for producing a platinum catalyst according to claim 2, wherein the platinum salt is a chloroplatinic acid salt, and the coordination bond between the hydroxide ion and the platinum cation is formed by the following Reaction Scheme 2:
[Reaction Scheme 2]
_{[PtCl 6 -x (H 2 O} ) x] (2-x) - + yOH - → [PtCl 6 -x (OH) y (H 2 O) xy] (2-x + y) - + yH 2 O
Wherein x is 0, 1 or 2 and y is an integer less than or equal to x. 2. The method of claim 1, wherein the carbon carrier is carbon black, carbon nanotube, carbon nanofiber, graphene, mesoporous carbon, or a hierarchical multi-structure carbon structure. The method according to claim 1, wherein the step of forming and supporting the platinum oxide complex on the carbon support is carried out at a temperature of 70 to 110 ° C and an acidity of pH 7 to 10. The method for producing a platinum catalyst according to claim 1, wherein the support ratio of the platinum catalyst to the carbon support is 10 to 80 wt%. The method according to claim 1, wherein the gamma ray irradiation generates hydrated electrons and radicals, the hydrated electrons and radicals reduce the platinum cations in the platinum anhydride complex to lower the oxidation number, Wherein the particles are formed and allowed to grow. The method of claim 1, wherein the gamma ray irradiation condition is performed for 0.5 to 6 hours at an irradiation rate of 0.5 x 10 ³ Gy / h to 4.0 x 10 ³ Gy / h. The method of claim 7, wherein the platinum nanoparticles have a particle diameter of 1.8 to 3.7 nm. A platinum catalyst for oxygen reduction reaction produced by the process according to any one of claims 1 to 9. A proton exchange membrane fuel cell comprising the platinum catalyst for oxygen reduction reaction according to claim 10.

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