Catalyst and method of preparing same

An electrode catalyst is configured such that non-noble metal particles, noble metal particles or nitride-doped noble metal particles are supported on a carbon support, wherein the carbon support has a 2D planar crystal structure or a 3D polyhedral crystal structure and is doped with nitrogen, thereby exhibiting increased catalytic activity.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of Korean Patent Application No. 10-2020-0058597, filed on May 15, 2020, the entire contents of which are incorporated herein by reference.

FIELD

The present disclosure relates to a catalyst and a method of preparing the same.

BACKGROUND

In order to increase the electrode reaction rate, a polymer electrolyte membrane fuel cell has to use platinum, which is a noble metal, as a catalyst. Platinum is highly priced and is limited in reserves.

Research is ongoing into catalysts in which a non-platinum transition metal for platinum replacement is joined through coordination bonding to nitrogen, and in particular, a porous-metal/organic framework is a complex composed of transition metal, nitrogen and carbon, and has superior properties due to large specific surface area, micropores, and coordination bonding of transition metal and nitrogen, and use of this complex as a non-platinum fuel cell electrode catalyst has been reported in a number of studies. However, development of non-platinum catalysts exhibiting sufficient performance is seldom reported.

When a small amount of platinum is used, carbon, having a large specific surface area and excellent pore structure, is utilized as a platinum support in order to increase the activity relative to mass through support of small-sized particles. Also, there are reports in which nitrogen, sulfur, phosphorus, etc. are added to the existing carbon structure, thus improving performance based on changes in electronic structures.

In order to reduce the use of platinum and increase the low activity of non-platinum catalysts, we have discovered that a composite catalyst in which two systems are combined is regarded as an ideal catalyst in view of both activity and price. However, the use of iron shortens the lifetime of the fuel cell due to the generation of hydroxyl radicals in side reactions.

SUMMARY

The present disclosure provides a method of preparing an electrode catalyst for a fuel cell or a water electrolysis cell, in which noble metal and non-metal metal may be efficiently applied.

The present disclosure provides an electrode catalyst in which the amount of noble metal is decreased due to the use of non-noble metal other than iron (Fe) and which has high activity.

The present disclosure provides a catalyst, including a carbon support doped with nitrogen and solid particles supported on the surface of the carbon support, in which the solid particles include particles of any one selected from the group consisting of non-noble metal particles, noble metal particles, nitride-containing noble metal particles and combinations thereof.

The carbon support may have a 2D planar crystal structure or a 3D polyhedral crystal structure.

The carbon support may have a porosity of 10% to 85%.

The nitride may include cobalt nitride.

The solid particles may include particles of any one selected from the group consisting of noble metal particles, cobalt-nitride-containing noble metal particles and combinations thereof.

The non-noble metal particles may include cobalt, and the noble metal particles may include platinum.

The catalyst may be contained in the electrode of a fuel cell or a water electrolysis cell.

In addition, the present disclosure provides a method of preparing a catalyst, including preparing a support by synthesizing a metal precursor and alkyl imidazole, preparing a carbon support doped with nitrogen by subjecting the support to primary heat treatment, subjecting the carbon support to secondary heat treatment, purifying the carbon support by removing metal particles from the carbon support through acid treatment, and preparing a composite catalyst by subjecting the carbon support to tertiary heat treatment.

The support may include ZIF-67, which is a zeolitic imidazolate framework.

The primary heat treatment may be performed at a temperature of 25° C. to 1000° C. for 10 min to 300 min in an inert gas atmosphere, and the secondary heat treatment may be performed at a temperature of 60° C. to 100° C. for 2 hr to 10 hr at a proton concentration (pH) of 7 or less.

The carbon support prepared through the primary heat treatment may include cobalt particles on the surface thereof.

The carbon support may be added to a solvent along with platinum and subjected to secondary heat treatment.

The carbon support subjected to secondary heat treatment may include, on the surface thereof, particles of any one selected from the group consisting of cobalt, nickel, platinum and combinations thereof.

The acid treatment may be performed at a temperature of 90° C. to 120° C. for 1 to 3 hr in a hydrochloric acid aqueous solution.

The metal particles removed from the carbon support through acid treatment may include pure cobalt not bound to platinum.

The tertiary heat treatment may be performed at a temperature of 300° C. to 600° C. for 10 min to 60 min in an ammonia gas atmosphere.

In addition, the present disclosure provides a method of preparing a catalyst, including preparing a support by synthesizing a metal precursor and alkyl imidazole, preparing a carbon support doped with nitrogen by subjecting the support to primary heat treatment, removing metal particles from the surface of the carbon support by subjecting the carbon support to acid treatment, and preparing a composite catalyst by subjecting the carbon support to secondary heat treatment.

The carbon support prepared through the primary heat treatment may include cobalt particles on the surface thereof, and the cobalt particles may be completely removed from the surface of the carbon support through the acid treatment.

The carbon support may be added to a solvent along with platinum and subjected to secondary heat treatment.

The carbon support subjected to secondary heat treatment may include platinum particles on the surface thereof.

According to the present disclosure, there can be provided a method of preparing an electrode catalyst for a fuel cell or a water electrolysis cell, in which noble metal and non-metal metal can be efficiently applied.

According to the present disclosure, there can be provided an electrode catalyst, in which the amount of noble metal is decreased due to the use of non-noble metal other than iron (Fe) and which has high activity.

The effects of the present disclosure are not limited to the foregoing, and should be understood to include all effects that can be reasonably anticipated from the following description.

DETAILED DESCRIPTION

For the sake of clarity of the present disclosure, the dimensions of structures are depicted as being larger than the actual sizes thereof. It will be understood that, although terms such as “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another element. For instance, a “first” element discussed below could be termed a “second” element without departing from the scope of the present disclosure. Similarly, the “second” element could also be termed a “first” element. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.

It will be further understood that the terms “comprise”, “include”, “have”, etc., when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof. Also, it will be understood that when an element such as a layer, film, area, or sheet is referred to as being “on” another element, it can be directly on the other element, or intervening elements may be present therebetween. Similarly, when an element such as a layer, film, area, or sheet is referred to as being “under” another element, it can be directly under the other element, or intervening elements may be present therebetween.

Unless otherwise specified, all numbers, values, and/or representations that express the amounts of components, reaction conditions, polymer compositions, and mixtures used herein are to be taken as approximations including various uncertainties affecting measurement that inherently occur in obtaining these values, among others, and thus should be understood to be modified by the term “about” in all cases. Furthermore, when a numerical range is disclosed in this specification, the range is continuous, and includes all values from the minimum value of said range to the maximum value thereof, unless otherwise indicated. Moreover, when such a range pertains to integer values, all integers including the minimum value to the maximum value are included, unless otherwise indicated.

In the present specification, when a range is described for a variable, it will be understood that the variable includes all values including the end points described within the stated range. For example, the range of “5 to 10” will be understood to include any subranges, such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, and the like, as well as individual values of 5, 6, 7, 8, 9 and 10, and will also be understood to include any value between valid integers within the stated range, such as 5.5, 6.5, 7.5, 5.5 to 8.5, 6.5 to 9, and the like. Also, for example, the range of “10% to 30%” will be understood to include subranges, such as 10% to 15%, 12% to 18%, 20% to 30%, etc., as well as all integers including values of 10%, 11%, 12%, 13% and the like up to 30%, and will also be understood to include any value between valid integers within the stated range, such as 10.5%, 15.5%, 25.5%, and the like.

The present disclosure pertains to a catalyst and a method of preparing the same. Specifically, the present disclosure pertains to an electrode catalyst configured such that non-noble metal particles, noble metal particles or nitride-containing noble metal particles are supported on a carbon support having a 2D planar crystal structure or a 3D polyhedral crystal structure and doped with nitride, thereby exhibiting increased catalytic activity.

FIGS.1to4show the configurations of the catalyst of the present disclosure. With reference to these drawings, the catalyst of the present disclosure is described, and with reference toFIGS.5to10, the method of preparing the catalyst of the present disclosure is described stepwise.

Catalyst

The catalyst of the present disclosure includes a support and solid particles supported on the surface of the support. Here, the solid particles may include particles of any one selected from the group consisting of non-noble metal particles, noble metal particles, nitride-containing noble metal particles and combinations thereof.

In one form, the support of the present disclosure is a carbon support doped with nitrogen.

The support of the present disclosure may have a 2D planar crystal structure or a 3D polyhedral crystal structure, as desired.

FIGS.1and2illustrate a catalyst including a carbon support having a 3D polyhedral crystal structure and doped with nitrogen, andFIGS.3and4illustrate a catalyst including a carbon support having a 2D planar crystal structure and doped with nitrogen.

The catalyst of the present disclosure includes a support, and particles of any one selected from the group consisting of a non-noble metal, noble metal, nitride-containing noble metal and combinations thereof, which are attached to the surface of the support. The particles disposed on the support may be selected as desired, and include a nitride-containing noble metal due to the beneficial effect thereof. Here, the nitride may include cobalt nitride, the non-noble metal may include cobalt, and the noble metal may include platinum.

FIG.1shows the catalyst according to one form of the present disclosure (CoN-Pt/CoNC-bulk) including nitride-containing noble metal particles301and a carbon support101doped with nitrogen, andFIG.2shows the catalyst according to another form of the present disclosure (Pt/CoNC-bulk) including noble metal particles300and a carbon support101doped with nitrogen.

The carbon support ofFIGS.1and2has a 3D polyhedral crystal structure. In one form, the diameter of the carbon support is approximately 20 to 1000 nm. Also, the porosity of the carbon support is approximately 30 to 85%.

FIG.3shows the catalyst according to still another form of the present disclosure (CoN-Pt/CoNC-2D) including nitride-containing noble metal particles301and a carbon support111doped with nitrogen having a 2D planar crystal structure, andFIG.4shows the catalyst according to yet another form of the present disclosure (Pt/CoNC-2D) including noble metal particles300and a carbon support111doped with nitrogen having a 2D planar crystal structure.

The carbon support ofFIGS.3and4has a 2D planar crystal structure. Here, the thickness of the carbon support is 10 to 200 nm and the diameter thereof is 30 to 5000 nm. Also, the carbon support has a porosity of 10 to 60%.

The method of preparing the electrode catalyst of the present disclosure may be classified into two types depending on the kind of particles provided on the support. The method of preparing the catalyst of the present disclosure may be classified into a method of preparing an electrode catalyst including nitride-containing noble metal particles and a method of preparing an electrode catalyst including noble metal particles.

Method of Preparing Catalyst Including Nitride-Containing Noble Metal Particles

The method of preparing a catalyst including nitride-containing noble metal particles according to the present disclosure may include preparing a support by synthesizing a metal precursor and alkyl imidazole, preparing a carbon support having an exposed non-noble metal and doped with nitrogen by subjecting the support to primary heat treatment, subjecting the carbon support to secondary heat treatment, purifying the carbon support by removing metal particles from the surface of the carbon support having the exposed non-noble metal through acid treatment, and preparing a composite catalyst by subjecting the carbon support not containing the metal particles to tertiary heat treatment.

FIG.5is a flowchart showing the process of preparing the electrode catalyst of the present disclosure, andFIGS.6and7show the catalyst preparation process according to various forms. With reference toFIGS.5to7, the steps thereof are described below.

Preparation of Support (S1)

A support100is prepared by synthesizing a metal precursor and alkyl imidazole.

In one form of the present disclosure, the metal precursor may include cobalt nitrate hydrate (Co(NO3)2.6H2O), and the alkyl imidazole includes 2-methylimidazole.

The support100includes a zeolitic imidazolate framework (ZIF) having a 3D polyhedral crystal structure, and ZIF-67 (Co(C4N2H5)2).

The support100includes carbon, nitrogen, hydrogen and cobalt, and in another form, the support100is configured such that carbon, nitrogen and hydrogen constitute the basic backbone of the support100and cobalt is joined through coordination bonding to a portion of nitrogen contained in the basic backbone.

In the present disclosure, the support100may be imparted with a 2D planar crystal structure by converting the crystal structure, as desired.

The support110having a 2D planar crystal structure is prepared by adding the support100having a 3D polyhedral crystal structure and cobalt nitrate hydrate to a solvent and carrying out a hydrothermal reaction. Here, the solvent includes methanol. The weight ratio of the support100and the cobalt nitrate hydrate, which are added to the solvent, is 1:1 to 1:3.

The hydrothermal reaction may be performed at a temperature of 140° C. to 180° C. for 30 min to 5 hr, and 150° C. to 160° C. for 1 hr to 2 hr.

FIG.6shows the process of preparing the catalyst using the support110having the converted 2D planar crystal structure, andFIG.7shows the process of preparing the catalyst using the support100having a 3D polyhedral crystal structure without performing the hydrothermal reaction.

In the present disclosure, the shape of the support100determines the shape of the final catalyst, and the use of a support110having a 2D planar crystal structure may be desired for the purpose of the present disclosure.

For the sake of description, in the present disclosure, the process of preparing the catalyst as shown inFIG.6is described.

Primary Heat Treatment (S2)

The support110is subjected to primary heat treatment to afford a carbon support111that has an exposed non-noble metal and is doped with nitrogen. Specifically, the support110is heat-treated in an inert gas atmosphere, so the non-noble metal particles200are provided on the support110and the support110is converted into a carbon support111doped with nitrogen.

In the present disclosure, the non-noble metal particles200provided on the support110include cobalt. Here, the cobalt provided on the support110is derived from cobalt ions joined through coordination bonding to nitrogen in a conventional support. Specifically, during carbonization through primary heat treatment, the cobalt ions joined through coordination bonding are reduced, and thus cobalt is deposited on the surface of the carbon support111.

In the present disclosure, the nitrogen that is doped is derived from nitrogen that is bound to carbon in a conventional support to constitute the basic backbone. Specifically, during carbonization through primary heat treatment, the carbon lattice is doped with nitrogen. Here, cobalt is deposited on the surface of carbon doped with nitrogen.

In one form, the primary heat treatment may be performed at a temperature of 25° C. to 1000° C. for 10 min to 300 min, and the inert gas may include nitrogen (N2).

In another form, the primary heat treatment is performed at 800° C. for 30 min to 120 min under conditions such that the temperature is elevated from 25° C. to 800° C. at a rate of 5° C./min in a nitrogen gas atmosphere.

Secondary Heat Treatment (S3)

The carbon support111doped with nitrogen and the non-noble metal particles200on the carbon support111are heat-treated. Specifically, the carbon support111including the non-noble metal particles200on the surface thereof and doped with nitrogen is added together with the noble metal to a solvent to afford a mixture, which is then heat-treated.

In the present disclosure, the noble metal that is added is supported on the surface of the non-noble metal particles200supported on the carbon support111doped with nitrogen through a galvanic conversion reaction.

The noble metal that is added to the solvent is used in the form of an ionic solution, and the solution includes platinum ions. Here, platinum is provided in the form of chloroplatinic acid (H2PtCl6). Specifically, the platinum ions are reduced on the non-noble metal particles200and deposited in the form of particles, and some of the non-noble metal particles200are oxidized and dissolved in the form of ions in the solvent.

The solvent may include ethylene glycol (EG).

The proton concentration (pH) of the mixture is approximately 7 or less.

The heat treatment is performed at a temperature of 60° C. to 100° C. for 2 hr to 10 hr.

Due to the heat treatment of the present disclosure, the carbon support111doped with nitrogen includes, on the surface thereof, particles of any one selected from the group consisting of non-noble metal, noble metal and combinations thereof. Here, the non-noble metal particles may include cobalt and the noble metal particles may include platinum.

The carbon support111having the exposed non-noble metal and doped with nitrogen is subjected to acid treatment to thus remove metal particles from the surface thereof, so the carbon support is purified. Specifically, in the purification step, pure non-noble metal particles200are removed from the carbon support111doped with nitrogen through acid treatment, and only the non-noble metal particles201on the noble metal particles300remain. Here, the pure non-noble metal particles200are non-noble metal particles that are neither attached to nor in contact with any noble metal particles in the present disclosure.

The acid treatment is performed by dispersing the washed carbon support111in a solvent including an acid and conducting a hydrothermal reaction. Specifically, in the purification step, the carbon support111doped with nitrogen, obtained through secondary heat treatment, is washed with distilled water and ethanol, dried, and dispersed in the solvent, and a hydrothermal reaction is carried out. Here, the solvent that is used may include hydrochloric acid, and the hydrothermal reaction may be carried out at a temperature of 90° C. to 120° C. for 1 hr to 3 hr.

Through the hydrothermal reaction in the solvent including hydrochloric acid, the non-noble metal particles200are removed from the surface of the carbon support111, and some of the non-noble metal particles201present on the noble metal particles300are removed.

Tertiary Heat Treatment (S5)

The carbon support111including, on the surface thereof, the noble metal particles300including the non-noble metal particles201and doped with nitrogen is subjected to tertiary heat treatment, thus preparing a composite catalyst. Specifically, the carbon support111including, on the surface thereof, platinum particles, some of which include cobalt particles, and doped with nitrogen is subjected to tertiary heat treatment in an ammonia (NH3) gas atmosphere, thus obtaining a composite catalyst.

The ammonia is synthesized into nitride during the heat treatment along with the non-noble metal particles201formed on the surface of the noble metal particles300. The tertiary heat treatment in an ammonia gas atmosphere enables the formation of nitride on the surface of the noble metal particles300.

The nitride includes cobalt nitride.

The tertiary heat treatment may be performed at a temperature of 300° C. to 600° C. for 10 min to 60 min in an ammonia gas atmosphere.

Through the heat treatment, a composite catalyst including the nitride-containing noble metal particles301and the carbon support111doped with nitrogen including the nitride-containing noble metal particles301on the surface thereof may be prepared.

Method of Preparing Catalyst Including Noble Metal Particles

The method of preparing an electrode catalyst including noble metal particles according to the present disclosure may include preparing a support by synthesizing a metal precursor and alkyl imidazole, preparing a carbon support having an exposed non-noble metal and doped with nitrogen by subjecting the support to primary heat treatment, purifying the carbon support by removing metal particles from the surface of the carbon support through acid treatment, and preparing a composite catalyst by subjecting the carbon support to secondary heat treatment.

FIG.8is a flowchart showing the process of preparing the electrode catalyst according to the present disclosure, andFIGS.9and10show the catalyst preparation process according to various forms. With reference toFIGS.8to10, the steps thereof are described below.

Preparation of Support (S′1)

A support100is prepared by synthesizing a metal precursor and alkyl imidazole. The present step is the same as the step of preparing the support (S1) in the method of preparing the electrode catalyst including nitride-doped noble metal particles described above, and thus a description thereof is omitted. For the sake of description, the process of preparing the catalyst as shown inFIG.9is described.

Primary Heat Treatment (S′2)

A carbon support111including non-noble metal particles200on the surface thereof and doped with nitrogen is prepared by subjecting the support100to primary heat treatment. The present step is the same as the primary heat treatment step (S2) in the method of preparing the electrode catalyst including nitride-doped noble metal particles described above, and thus a description thereof is omitted.

The carbon support111doped with nitrogen is subjected to acid treatment, thus removing metal particles from the surface thereof. Specifically, the acid treatment enables the removal of the non-noble metal particles200formed on the carbon support111in the primary heat treatment step, thereby affording a carbon support111doped with nitrogen.

In the present disclosure, the non-noble metal particles200that are removed are cobalt particles.

In the present step, the acid treatment is performed in a manner in which the carbon support111, including the non-noble metal particles200on the surface thereof and doped with nitrogen, is dispersed in the solvent and a hydrothermal reaction is carried out. Here, the solvent includes hydrochloric acid, and the hydrothermal reaction is conducted at a temperature of 90° C. to 120° C. for 1 hr to 3 hr.

The hydrothermal reaction enables the formation of a carbon support111not including any metal particles on the surface thereof and doped with nitrogen.

Secondary Heat Treatment (S′4)

The carbon support111not including any metal particles on the surface thereof and doped with nitrogen is subjected to secondary heat treatment to afford a composite catalyst including the carbon support111including noble metal particles300on the surface thereof and doped with nitrogen. Specifically, the carbon support111is added to a solvent together with the noble metal to form a mixture, which is then heat-treated. Here, the solvent may include ethylene glycol (EG).

The proton concentration (pH) of the mixture is 7 or less.

The heat treatment is performed at a temperature of 60° C. to 150° C. for 2 hr to 10 hr.

Through the heat treatment of the present disclosure, the carbon support111doped with nitrogen may include only the noble metal particles300on the surface thereof. Here, the noble metal particles300include platinum.

Electrode

In the present disclosure, an electrode including the catalyst prepared by the catalyst preparation method described above is provided. More specifically, the catalyst of the present disclosure may be included in the electrode for a fuel cell or a water electrolysis cell.

The catalyst may include an electrode catalyst including a carbon support including nitride-containing noble metal particles on the surface thereof and doped with nitrogen, or an electrode catalyst including a carbon support including noble metal particles on the surface thereof and doped with nitrogen.

A better understanding of the present disclosure will be given through the following examples. However, these examples are merely set forth to illustrate the present disclosure, and are not to be construed as limiting the scope of the present disclosure.

PREPARATION EXAMPLE 1

An aqueous solution, obtained by dissolving a metal precursor Co(NO3)2.6H2O (3.1 mmol) in 6 ml of distilled water, was supplied at a rate of 5 ml/h and mixed with an aqueous solution of 2-methylimidazole (12.2 mmol) in 40 ml of distilled water with stirring, after which the resulting mixed solution was stirred for 6 hr, centrifuged and dried, thus synthesizing a support having a polyhedral crystal structure (ZIF-67).

The ZIF-67 (0.5 g) was uniformly mixed with a cobalt nitrate hydrate Co(NO3)2.6H2O (1.0 g) and methanol, followed by hydrothermal reaction at 160° C. for 2 hr, whereby the support was converted so as to have a 2D planar crystal structure.

The temperature of the support having the converted crystal structure (2.0 g) was elevated from 25° C. to 800° C. at a rate of 5° C./min in an inert gas (N2) atmosphere, followed by heat treatment at 800° C. for 1 hr, thus forming a carbon support (Co/CoNC-2D) including cobalt non-noble metal particles on the surface thereof and doped with nitrogen.

The carbon support (Co/CoNC-2D) (0.1 g) was mixed with a platinum solution (40 μl, 1.93 M) in an ethylene glycol solvent (EG, 40 ml), after which the pH thereof was adjusted to about 7 using a KOH aqueous solution, followed by stirring, heat treatment at 80° C. for 8 hr, washing with distilled water and ethanol, drying, dispersion in a 0.5 M hydrochloric acid aqueous solution, and hydrothermal reaction at 100° C. for 2 hr, thus removing pure cobalt non-noble metal particles from the surface of the carbon support, after which drying was performed, thereby preparing a carbon support configured such that cobalt non-noble metal particles including platinum particles attached thereto were attached to the surface thereof.

Thereafter, heat treatment was performed at 500° C. for 30 min in an ammonia atmosphere, thereby preparing an electrode catalyst (CoN-Pt/CoNC-2D) including a carbon support doped with nitrogen and cobalt-nitride-containing platinum particles attached to the surface of the carbon support.

FIG.11Ashows a scanning electron microscope (SEM) image of the electrode catalyst prepared above,FIG.11Bshows a transmission electron microscope (TEM) image thereof, andFIG.11Cshows a high-resolution TEM image thereof and a platinum particle size distribution.

With reference toFIGS.11A to11C, the carbon support included in the electrode catalyst has a 2D planar crystal structure, pores therein, and uniform distribution of platinum particles having a size of 2 to 4 nm.

FIGS.12A to12Fshow the results of qualitative and quantitative analysis of the surface of the electrode catalyst prepared above using energy-dispersive X-ray spectroscopy (EDS).

With reference toFIGS.12A to12F, it can be seen that platinum, cobalt and nitrogen were uniformly distributed on the carbon support and also that cobalt and nitrogen were mainly superimposed at the location at which the platinum particles were distributed, based on which the platinum can be inferred to include cobalt nitride on the surface thereof.

Based on the results of an EDS line-scanning profile limited to one particle (FIG.12F), it can be confirmed that the particles were composed mainly of platinum and included cobalt nitride on the surface thereof.

PREPARATION EXAMPLE 2

An aqueous solution, obtained by dissolving a metal precursor Co(NO3)2.6H2O (3.1 mmol) in 6 ml of distilled water, was supplied at a rate of 5 ml/h and mixed with an aqueous solution of 2-methylimidazole (12.2 mmol) in 40 ml of distilled water with stirring, after which the resulting mixed solution was stirred for 6 hr, centrifuged, and dried, thus synthesizing a support having a polyhedral crystal structure (ZIF-67).

The temperature of the support (2.0 g) was elevated from 25° C. to 800° C. at a rate of 5° C./min in an inert gas (N2) atmosphere, followed by heat treatment at 800° C. for 1 hr, thus forming a carbon support (Co/CoNC-bulk) including cobalt non-noble metal particles on the surface thereof and doped with nitrogen.

The carbon support (Co/CoNC-bulk) (0.1 g) was mixed with a platinum solution (40 μ1, 1.93 M) in an ethylene glycol solvent (EG, 40 ml), after which the pH thereof was adjusted to about 7 using a KOH aqueous solution, followed by stirring, heat treatment at 80° C. for 8 hr, washing with distilled water and ethanol, drying, dispersion in a 0.5 M hydrochloric acid aqueous solution, and hydrothermal reaction at 100° C. for 2 hr, thus removing pure cobalt non-noble metal particles from the surface of the carbon support, after which drying was performed, thereby preparing a carbon support configured such that platinum particles including cobalt non-noble metal particles were attached to the surface thereof.

Thereafter, heat treatment was performed at 500° C. for 30 min in an ammonia atmosphere, thereby preparing an electrode catalyst (CoN-Pt/CoNC-bulk) including a carbon support including cobalt-nitride-containing platinum particles attached to the surface thereof and doped with nitrogen.

FIG.13Ashows a TEM image of the electrode catalyst prepared above,FIG.13Bshows a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image thereof, andFIGS.13C to13Fshow the results of surface qualitative analysis using energy-dispersive X-ray spectroscopy (EDS).

With reference toFIGS.13A to13F, it can be confirmed that the electrode catalyst included the support having a 3D polyhedral crystal structure, and also that the platinum particles were distributed on the support, cobalt and nitrogen were uniformly distributed throughout the catalyst, and cobalt and nitrogen were superimposed on some of the platinum particles that were distributed, indicating that cobalt nitride was formed on the platinum particles.

PREPARATION EXAMPLE 3

An aqueous solution, obtained by dissolving a metal precursor Co(NO3)2.6H2O (3.1 mmol) in 6 ml of distilled water, was supplied at a rate of 5 ml/h and mixed with an aqueous solution of 2-methylimidazole (12.2 mmol) in 40 ml of distilled water with stirring, after which the resulting mixed solution was stirred for 6 hr, centrifuged, and dried, thus synthesizing a support having a polyhedral crystal structure (ZIF-67).

The ZIF-67 (0.5 g) was uniformly mixed with a cobalt nitrate hydrate Co(NO3)2.6H2O (1.0 g) and methanol, followed by hydrothermal reaction at 160° C. for 2 hr, whereby the support was converted so as to have a 2D planar crystal structure.

The temperature of the support having the converted crystal structure (2.0 g) was elevated from 25° C. to 800° C. at a rate of 5° C./min in an inert gas (N2) atmosphere, followed by heat treatment at 800° C. for 1 hr, thus forming a carbon support (Co/CoNC-2D) including cobalt non-noble metal particles on the surface thereof and doped with nitrogen.

The carbon support thus obtained was washed with distilled water and ethanol, dried, dispersed in a 0.5 M hydrochloric acid aqueous solution, and subjected to a hydrothermal reaction at 100° C. for 2 hr, thus removing pure cobalt non-noble metal particles from the surface of the carbon support, after which drying was performed, thereby obtaining a carbon support not including metal particles on the surface thereof.

The carbon support (CoNC-2D) (0.1 g) was mixed with a platinum solution (40 μl, 1.93 M) in an ethylene glycol solvent (EG, 40 ml), after which the pH thereof was adjusted to about 7 using a KOH aqueous solution, followed by stirring and heat treatment at 140° C. for 8 hr, thus preparing an electrode catalyst (Pt/CoNC-2D) including a carbon support doped with nitrogen and platinum particles attached to the surface of the carbon support.

FIG.14Ashows a TEM image of the electrode catalyst prepared above,FIG.14Bshows a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image thereof, andFIGS.14C to14Gshow the results of surface qualitative analysis using energy-dispersive X-ray spectroscopy (EDS).

With reference toFIGS.14A to14G, it can be confirmed that the platinum particles were distributed on the carbon support but that cobalt and nitrogen were uniformly distributed throughout the carbon support, regardless of the distribution of the platinum particles.

TEST EXAMPLE (1)

The electrode catalysts prepared in Preparation Example 1, Preparation Example 2 and Preparation Example 3 were subjected to X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results thereof are shown inFIG.15and in Table 1 below.

With reference toFIG.15, in Preparation Example 2, cobalt metal was provided on the carbon support despite the hydrothermal reaction in the hydrochloric acid aqueous solution, and the diffractive pattern of platinum was not different from the reference pattern, indicating that platinum and cobalt were not alloyed but were present individually. In Preparation Example 1 and Preparation Example 3, cobalt metal was not provided on the carbon support but the diffractive pattern of platinum was not different from the reference pattern, indicating that platinum was present alone. In particular, in Preparation Example 2, since platinum was not alloyed with cobalt metal but cobalt nitride was distributed on the surface of platinum, the diffractive pattern of platinum was not changed.

Based on the results of Table 1, in Preparation Example 1, in which platinum was present along with cobalt nitride, cobalt and nitrogen were included in large amounts compared to Preparation Example 3, in which the platinum particles were present alone. In Preparation Example 2, the reason why the cobalt content was very high was that cobalt metal accumulated on polyhedral carbon and thus cobalt metal particles did not react but remained on the carbon support in the platinum supporting process and the hydrothermal reaction in the hydrochloric acid aqueous solution.

TEST EXAMPLE (2)

The specific surface area (FIG.16A) of the electrode catalysts prepared in Preparation Example 1 and Preparation Example 2 and the pores therein (FIG.16B) were measured. The results thereof are shown inFIGS.16A and16B.

With reference toFIG.16A, in Preparation Example 1 and Preparation Example 2, both micropores and mesopores were present, and in Preparation Example 1, macropores were also present, based on the adsorption/desorption near relative pressure of 1.0. Thus, Preparation Example 1 had a specific surface area of 287.8 m2/g and Preparation Example 2 had a specific surface area of 202.3 m2/g. With reference toFIG.16B, Preparation Example 1 included micropores having an average size of 1.2 nm (54.2 m2/g), mesopores having a size of 4 to 50 nm, and macropores having a size of 50 to 128 nm, the ratio of micropores relative to the specific surface area being 19%. Preparation Example 2 included micropores having a size of 0.6 to 2 nm (163.7 m2/g) and limited mesopores, the ratio of micropores relative to the specific surface area being 81%.

TEST EXAMPLE (3)

Compared with a platinum catalyst (Pt/C) supported on a carbon support (hereinafter referred to as Comparative Example 1), which is usually applied to a fuel cell, the electrode catalyst of each of Preparation Example 1, Preparation Example 2 and Preparation Example 3 was measured for the activity of an oxygen reduction reaction (ORR), which is a half reaction of the fuel cell, in a 3-electrode system (reference electrode: platinum wire; counter electrode: KCl saturated Ag/AgCl; working electrode: Preparation Example 1, Preparation Example 2, Preparation Example 3 and Comparative Example 1) using 0.1 M HClO4electrolyte saturated with oxygen. The results thereof are shown inFIGS.17A to17F.

FIG.17Ais a linear sweep voltammetry (LSV) graph showing the current density of a rotating disk at 1600 rpm in the presence of the catalyst of each of Preparation Example 1 to Preparation Example and Comparative Example 1,FIG.17Bshows the Tafel plots of Preparation Example 1 and Preparation Example 3, andFIG.17Cis graphs showing the production of hydrogen peroxide (H2O2) and the number of electrons transferred per oxygen molecule for the oxygen reduction reaction, calculated based on the current density of the rotating ring disk at 1600 rpm.FIGS.17D,17E and17Fare LSV graphs showing changes in current density of the rotating ring disk at 1600 rpm before and after an accelerated durability test (ADT) for the electrode catalysts of Preparation Example 1, Preparation Example 3 and Comparative Example 1.

Based on the results ofFIG.17A, by comparing the starting voltage (Eonset), the half-wave voltage (E1/2) and the threshold current density, it can be seen that the ORR activity of Preparation Example 1 was excellent. Based on the results ofFIG.17B, in the kinetic region of the ORR catalyst reaction, Preparation Example 1 exhibited 49.5 mV/dec, which is a Tafel slope smaller than Preparation Example 3 (65.8 mV/dec), thus enabling faster electron transfer. Based on the results ofFIG.17C, the number of electrons transferred during the catalyst reaction at 0.15 to 0.8 V in Preparation Example 1 was 3.99, and the production of hydrogen peroxide, a product of the 2-electron ORR reaction, was 0.26% or less, so the ORR of the 4-electron reaction was the main catalyst reaction. The results ofFIGS.17D,17E and17Fconfirmed the change in LSV due to the deterioration in the performance of the electrode catalyst after ADT. Preparation Example 1 was the most stable because the change in half-wave voltage (E1/2) was 17 mV in Preparation Example 1, 30 mV in Preparation Example 3, and 36 mV in Comparative Example 1.

Thereby, it can be found that the cobalt nitride on the surface of platinum improved the ORR activity and stability.

TEST EXAMPLE (4)

FIGS.18A and18Bare graphs showing a current-voltage polarization curve and power density in the presence of hydrogen/oxygen fuel and current per unit mass of platinum at 0.8 V of the corresponding polarization curve in an oxygen reduction reaction (ORR) electrode, which is a cathode of a membrane-electrode assembly of a fuel cell, manufactured using the electrode catalyst of each of Preparation Example 1, Preparation Example 3 and Comparative Example 1.

In this experiment, the electrode catalyst of Comparative Example 1 was used for the anode, and in each evaluation, the platinum content in both the anode and the cathode was 0.1 mgpt/cm2, and NR211 was used as the electrolyte membrane. The current-voltage polarization curve and the power density graph were determined under conditions of 0.5 bar of hydrogen and oxygen.

Based on the results ofFIG.18A, the maximum power density was 1.33 W/cm2in Preparation Example 1, 1.24 W/cm2in Preparation Example 3, and 1.10 W/cm2in Comparative Example 1, and the current per unit electrode area at 0.8 V was 0.207 A/cm2in Preparation Example 1, 0.158 A/cm2in Preparation Example 3, and 0.130 A/cm2in Comparative Example 1, indicating that Preparation Example 1 exhibited excellent membrane-electrode assembly activity. Based on the results ofFIG.18B, Preparation Example 1 manifested excellent current per unit mass of platinum at 0.8 V. Thereby, it can be found that the cobalt nitride on the surface of platinum improved the ORR activity in the membrane-electrode assembly.

TEST EXAMPLE (5)

FIGS.19A,19B,19C and19Dare a current change graph, a current-voltage polarization curve at the initial stage and after 100 hr, a power density graph at the initial stage and after 100 hr, and a current graph per unit mass of platinum at 0.8 V at the initial stage and after 100 hr, respectively, when a current per unit electrode area of 1 Acm−2was maintained for 100 hr in the presence of hydrogen/oxygen fuel in an oxygen reduction reaction (ORR) electrode, which is a cathode of a membrane-electrode assembly of a fuel cell, manufactured using the electrode catalyst of each of Preparation Example 1 and Comparative Example 1.

In this experiment, the electrode catalyst of Comparative Example 1 was used for the anode, and in each evaluation, the platinum content in both the anode and the cathode was 0.1 mgpt/cm2, and NR211 was used as the electrolyte membrane. The current-voltage polarization curve and the power density graph were determined in the presence of 0.5 bar of hydrogen and oxygen.

As shown inFIG.19A, the voltage at 1 A/cm2for 100 hr was decreased by 1.0% in Preparation Example 1 and by 6.7% in Comparative Example 1, indicating that Preparation Example 1 was more stable. As shown inFIG.19B, the current per unit electrode area at 0.8 V was 0.182 A/cm2in Preparation Example 1 and 0.088 A/cm2in Comparative Example 1, and as shown inFIG.19C, the maximum power density was 1.31 W/cm2in Preparation Example 1 and 1.06 W/cm2in Comparative Example 1. As shown inFIG.19D, the current reduction rate per unit mass of platinum at 0.8 V after 100 hr for current per unit electrode area of 1 A/cm2was 11.9% in Preparation Example 1 and 32.7% in Comparative Example 1, based on which the cobalt nitride on the surface of platinum was determined to improve ORR stability in the membrane-electrode assembly.