HETEROELEMENT-DOPED HIGH-GRAPHITE POROUS CARBON BODY, CATALYST COMPRISING SAME, AND METHOD FOR PRODUCING SAME
20230082237 · 2023-03-16
Inventors
Cpc classification
C25B11/091
CHEMISTRY; METALLURGY
C01P2002/88
CHEMISTRY; METALLURGY
H01M4/8803
ELECTRICITY
Y02E60/36
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C01P2002/74
CHEMISTRY; METALLURGY
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C01P2002/72
CHEMISTRY; METALLURGY
International classification
C25B11/091
CHEMISTRY; METALLURGY
Abstract
The present invention provides a method for producing a nitrogen-doped highly graphitic porous carbon body, and a nitrogen-doped highly graphitic porous carbon body produced according to the same. Also, the present invention provides a method for producing a sulfur and nitrogen double-doped highly graphitic porous carbon body, a sulfur and nitrogen double-doped highly graphitic porous carbon body produced according to the same, and an electrode catalyst for a fuel cell and/or a water electrolysis reaction comprising the carbon body.
Claims
1-6. (canceled)
7. A nitrogen-doped highly graphitic porous carbon body which contains 0.5 to 6 atom % of N as a heteroelement, and has a peak intensity ratio I.sub.D/I.sub.G of less than 1.0 as measured with a Raman spectrum (I.sub.D is a peak intensity of a D band (at around 1,350 cm.sup.−1) , and I.sub.G is a peak intensity of a G band (at around 1,580 cm.sup.−1) in the Raman spectrum).
8. The nitrogen-doped highly graphitic porous carbon body of claim 7, wherein the peak intensity ratio I.sub.2D/I.sub.G is 0.5 or more as measured with the Raman spectrum of the nitrogen-doped highly graphitic porous carbon body (I.sub.2D is a peak intensity of a 2D band (at around 2,700 cm.sup.−1), and I.sub.G is a peak intensity of a G band (at around 1,580 cm.sup.−1) in the Raman spectrum).
9. The nitrogen-doped highly graphitic porous carbon body of claim 7, wherein the nitrogen-doped highly graphitic porous carbon body has a BET specific surface area of 200 to 1,000 m.sup.2/g.
10. The nitrogen-doped highly graphitic porous carbon body of claim 7, wherein the nitrogen-doped highly graphitic porous carbon body includes a binding state of pyrrolic N, graphitic N, pyridinic N, or a combination thereof.
11. A method for producing a sulfur and nitrogen double-doped highly graphitic porous carbon body, the method comprising: polymerizing a sulfur and nitrogen element-containing precursor to produce a polymer; mixing the polymer and metal powder in an inert gas atmosphere, and then heat-treating the mixture at a temperature equivalent to or higher than a melting point of the metal; and pickling a product from the heat treatment.
12. The method for producing a sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 11, wherein the sulfur and nitrogen element-containing precursor is at least one selected from the group consisting of thiourea, ammonium thiocyanate, and thioacetamide.
13. The method for producing a sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 11, wherein the sulfur and nitrogen element-containing precursor includes a sulfur-containing precursor and a nitrogen-containing precursor, the sulfur-containing precursor is an organic molecular precursor or a polymer precursor including at least one selected from the group consisting of benzyl disulfide (BDS), thiophene, 2,2′-dithiophene, p-toluenesulfonic acid, and 2-thiophenemethanol, and the nitrogen-containing precursor is an organic molecule or a polymer precursor including at least one selected from the group consisting of guanine, adenine, and purine, melamine, urea, pyridine, aniline, dicyandiamide, ethylenediamine, benzimidazole, and ethylene diamine tetraacetic acid (EDTA).
14. The method for producing a sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 11, wherein the metal powder is alkaline earth metal powder.
15. The method for producing a sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 11, wherein the polymer and the metal powder are mixed at an atomic molar ratio of N+S:M of 1:0.3 to 1:7.5 (N and S are nitrogen and sulfur contained in the polymer as a heteroelement, respectively, M is a metal element of the metal powder, and N+S is a total of nitrogen and sulfur elements).
16. A sulfur and nitrogen double-doped highly graphitic porous carbon body which has a peak intensity ratio measured with a Raman spectrum satisfying the following Relations 1 and 2:
0≤I.sub.D/I.sub.G≤0.95 [Relation 1]
0.3≤I.sub.2D/I.sub.G≤3.0 [Relation 2] wherein I.sub.D is a peak intensity of a D band (at around 1,350 cm.sup.−1), I.sub.G is a peak intensity of a G band (at around 1,580 cm.sup.−1), and I.sub.2D is a peak intensity of a 2D band (at around 2,700 cm.sup.−1) in the Raman spectrum.
17. The sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 16, wherein the carbon body satisfies the following Relation 3:
V.sub.1/V.sub.2≤1.0 [Relation 3] wherein V.sub.1 is a volume (cm.sup.3/g) of micropores of the carbon body, and V.sub.2 is a volume (cm.sup.3/g) of mesopores of the carbon body.
18. The sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 16, wherein the carbon body contains 0.3 to 6.0 atom % of N and 0.1 to 3.0 atom % of S as heteroelements.
19. The sulfur and nitrogen double-doped highly graphitic porous carbon body of claim 16, wherein the carbon body has a BET specific surface area of 80 to 1,000 m.sup.2/g.
20. An electrode catalyst for a fuel cell and/or a water electrolysis reaction, comprising the highly graphitic porous carbon body of claim 7.
21. An electrode catalyst for a fuel cell and/or a water electrolysis reaction, comprising the highly graphitic porous carbon body of claim 16.
22. A method for producing the nitrogen-doped highly graphitic porous carbon body of claim 7, the method comprising: (a) polymerizing a nitrogen element-containing precursor to produce a polymer; (b) mixing the polymer and metal powder in an inert atmosphere, and then heat-treating the mixture at a temperature equivalent to or higher than a melting point of the metal; and (c) adding a product from the heat treatment to an acid solution and performing stirring.
23. The method for producing a nitrogen-doped highly graphitic porous carbon body of claim 22, wherein the nitrogen element-containing precursor is urea, dicyandiamide, melamine, aniline, ethylenediamine, ethylene diamine tetraacetic acid (EDTA), or a combination thereof, and the metal powder is magnesium (Mg), calcium (Ca), aluminum (Al), lithium (Li), sodium (Na), potassium (K), or mixed powder thereof.
24. The method for producing a nitrogen-doped highly graphitic porous carbon body of claim 22, wherein the polymer contains 40 to 80 atom % of N as a heteroelement.
25. The method for producing a nitrogen-doped highly graphitic porous carbon body of claim 22, wherein in (b), the polymer formed and the metal powder are mixed at a weight ratio of 1:0.5 to 1:3.
26. The method for producing a nitrogen-doped highly graphitic porous carbon body of claim 22, wherein in (b), the polymer and the metal powder are mixed at an atomic molar ratio of N:M of 1:0.3 to 1:7.5 (N is nitrogen contained in the polymer as a heteroelement, and M is a metal element of the metal powder).
Description
DESCRIPTION OF DRAWINGS
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BEST MODE
[0072] Advantages and features of the present invention and methods to achieve them will be elucidated from exemplary embodiments described below in detail with reference to the accompanying drawings. However, the present invention is not limited to exemplary embodiments disclosed below, but will be implemented in various forms. The exemplary embodiments of the present invention make disclosure of the present invention thorough and are provided so that those skilled in the art can easily understand the scope of the present invention. Therefore, the present invention will be defined by the scope of the appended claims. Detailed description for carrying out the present invention will be provided with reference to the accompanying drawings below. Regardless of the drawings, the same reference number indicates the same constitutional element, and “and/or” includes each of and all combinations of one or more of mentioned items.
[0073] Unless otherwise defined herein, all terms used in the specification (including technical and scientific terms) may have the meaning that is commonly understood by those skilled in the art. Throughout the present specification, unless explicitly described to the contrary, “comprising” any elements will be understood to imply further inclusion of other elements rather than the exclusion of any other elements. In addition, unless explicitly described to the contrary, a singular form includes a plural form herein.
[0074] In the present specification, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being on or above another element, it can be directly on the other element or intervening elements may also be present.
[0075] An exemplary embodiment of the present invention provides a method for producing a nitrogen-doped highly graphitic porous carbon body. The method is characterized by including: (a) polymerizing a nitrogen element-containing precursor to produce a polymer; (b) mixing the polymer and metal powder in an inert atmosphere, and then heat-treating the mixture at a temperature equivalent to or higher than a melting point of the metal; and (c) adding a product from the heat treatment to an acid solution and performing stirring.
[0076] In step (a), the polymerizing of a nitrogen element-containing precursor to produce a polymer may be, as a non-limiting example, producing a graphitic-C.sub.3N.sub.4 (g-C.sub.3N.sub.4) polymer, and a commonly known method of producing a graphitic-C.sub.3N.sub.4 (g-C.sub.3N.sub.4) may be applied. As a non-limiting example, the nitrogen element-containing precursor is added to an airtight container, and a reaction may be performed at a heating rate of 1 to 5° C./min, at a reaction temperature of 300 to 1,000° C. for 1 to 6 hours, and preferably at a heating rate of 2 to 4.5° C./min at a reaction temperature of 300 to 1000° C. for 3 to 6 hours to produce a polymer.
[0077] The nitrogen element-containing precursor may be urea, dicyandiamide, melamine, or a combination thereof. Since the nitrogen element-containing precursor described above is an organic compound which has a higher content of nitrogen than carbon, and may be easily decomposed at a reaction temperature (300 to 1,000° C.) in step (a) and polymerized, a polymer having a high nitrogen doping content may be produced, which is preferred.
[0078] The polymer produced is a heteroelement and may content 40 to 80 atom % of N, and specifically, may contain 40 to 60 atom % of N as a heteroelement. The polymer may be, as an example, graphitic-C.sub.3N.sub.4 (g-C.sub.3N.sub.4), but the present invention is not limited thereto. Since nitrogen contained in a carbon structure has a higher electron affinity than carbon and has an unshared electron pair, it easily forms an electronic structure, and thus, it is easy to provide a site appropriate for an oxygen reduction reaction. However, when the content of N is more than 80 atom %, a carbon body is, if produced, easily decomposed so that it is difficult to form a carbon-to-carbon bond and the crystallinity is poor, and when the content of N is less than 40 atom %, the content of nitrogen is insufficient in the production of the carbon body, so that the activity of the oxygen reduction reaction is not sufficiently secured, which is thus disadvantageous.
[0079] In step (b), the polymer and the metal powder are mixed in an inert atmosphere, and then a heat treatment is performed at a temperature equivalent to or higher than the melting point of the metal.
[0080] Since the metal powder melted by the heat treatment is applied all over the surface of the polymer, a metal-nitride may be formed well by the high reducibility of the metal. In the conventional technology, when there is no formation of metal-nitride, the heat treatment may be performed at lower than 2,000° C. or lower than 1,000° C., preferably 600 to 950° C., more preferably 600 to 900° C., or 600 to 850° C., and most preferably 600° C. or higher and lower than 850° C. for increasing the crystallinity of the carbon body. Since nitrogen doped into the polymer is decomposed into gaseous cyano-fragments (C.sub.xN.sub.y), N.sub.2, NH.sub.3, and the like at a temperature of about 600° C. or higher, as the heat treatment temperature is higher, it is unfavorable in terms of a doped nitrogen loss, and in contrast, as the heat treatment temperature is lowered, the crystallinity (graphiticity) of the synthesized carbon body is lowered. In the present invention, since the polymer and the metal powder are mixed and heat-treated, the above problems may be prevented.
[0081] When the metal of the metal powder has a melting point of 1,000° C. or lower, a doping nitrogen loss may be effectively suppressed. Specifically, when the melting point of the metal is lower than 1,000° C., 950° C. or lower, 900° C. or lower, 800° C. or lower, or 750° C. or lower, a problem that the metal powder is only partly melted at a temperature at which a doped nitrogen loss occurs (600° C.) so that the entire surface of the polymer is not covered is prevented, thereby suppressing the doping nitrogen loss in the heat treatment may be suppressed. In addition, when the preferred lower limit of the melting point of the metal is higher than 50° C., 100° C. or higher, 150° C. or higher, 550° C. or higher, or 600° C. or higher, problems that the metal powder is all melted at too low temperature to lower the crystallinity of carbon in the production of the carbon body and handling is difficult may be prevented.
[0082] The metal powder may be magnesium (Mg), calcium (Ca), aluminum (Al), lithium (Li), sodium (Na), potassium (K), or mixed powder thereof, and preferably magnesium. As an example, magnesium melted and applied on the surface of the polymer reacts with a part of doped nitrogen (N) to form Mg.sub.3N.sub.2. Thus, a carbon atom remaining in the carbon body is a sp.sup.2 hybridized carbon, and rapidly forms a carbon-carbon bond in spite of a relatively low heat treatment temperature (lower than 1,000° C.) to contribute conversion of the polymer into graphene having high crystallinity. In the crystallization process of the carbon body, a part of nitrogen is not decomposed to gaseous cyano-fragments (C.sub.xN.sub.y) and remains in a doped state in the highly crystalline carbon body.
[0083] Meanwhile, a representative reducing agent metal applied to a reduction process of a metal and the like is calcium, aluminum, magnesium, and the like in order of reducing power; however calcium is somewhat unfavorable in that it has a high melting point of 850° C. or higher so that it is inevitably operated at a high temperature, has a risk of explosion, and is expensive, and aluminum has excellent stability, is easy to handle, and has good reducing power, but has a disadvantage in that it is difficult to remove AlN, Al.sub.2O.sub.3, and the like which are mixed after the reduction process. However, since magnesium has a relatively low melting point of 650° C., is easy to handle, and has excellent reducing power, it is the most preferred to be applied to the present invention.
[0084] In step (b), the polymer and the metal powder may be mixed at a weight ratio of 1:0.5 to 1:3, preferably 1:0.7 to 1:2, or 1:0.7 to 1:1.5, more preferably 1:0.9 to 1:1.1, and most preferably about 1:1. In addition, the polymer and the metal powder may be mixed at an atomic molar ratio of N:M of 1:0.3 to 1:7.5, preferably 1:0.3 to 1:5 or 1:0.5 to 1:3, more preferably 1:0.7 to 1:2, more preferably 1:0.9 to 1:1.2, and most preferably about 1:1 (N refers to nitrogen contained in the polymer as a heteroelement, M refers to a metal element of the metal powder). When the polymer and the metal powder are included at the above contents, formation of metal-nitride may be optimized as described later.
[0085] The heat treatment in step (b) may be performed at a temperature of 550° C. or higher and lower than 1,000° C., 550 to 950° C., preferably 550 to 900° C., 600 to 850° C., 600 to 800° C., 600 to 750° C., and more preferably 600 to 700° C. Carbonization is performed at a relatively low temperature, and it is analyzed that as the temperature is lower in the heat treatment temperature range, the content of doped nitrogen is increased, and the ratio of pyrrolic-N in the doped nitrogen may be increased. Meanwhile, the nitrogen-doped highly graphitic porous carbon body representatively includes binding states of pyrrolic N, graphitic N, pyridinic N, and pyridinic N-oxide, and since the electron pairs of the pyrrolic N is involved in aromatic stabilization, the pyrrolic N has higher binding energy than graphitic N, pyridinic N, and the like, and thus, may improve the crystallinity of the nitrogen-doped carbon body and increase oxygen reduction activity.
[0086] Meanwhile, the heat treatment may be performed in an inert gas flow atmosphere, for example, a nitrogen flow atmosphere, or an argon, or Ar/H.sub.2 flow atmosphere, but is not limited thereto.
[0087] In step (c), the heat-treated product in step (b) is added to an acid solution and stirring is performed.
[0088] The acid solution may be removed by etching residual metal powder remaining in the heat-treated product and a metal-nitride as a by-product by an acid. In the site of the carbon body from which the residual metal powder and the metal-nitride have been removed by the acid etching, a pore structure including mesopores may be formed. In particular, since the pore structure is formed by the acid etching after the heat treatment is performed at a relatively low temperature, the pore structure which is uniformly formed on the inside or the surface of the nitrogen-doped highly graphitic porous carbon body may contribute to BET specific surface area improvement. However, when the crystallinity of the carbon body is increased by a simple high-temperature heat treatment as a conventional technology, a decomposition problem arises due to small fragments such as N.sub.2, NH.sub.3, CH.sub.x, and gaseous C.sub.xN.sub.y, and simultaneously, since the pores formed at this time is formed mainly on an edge surface, the BET specific surface area is decreased.
[0089] In step (c), the heat-treated product of step (b) may be added to a 0.5-3.5 M acid solution, for example, a 1-3 M hydrochloric acid solution, stirring may be performed for 1-10 hours, preferably 2-8 hours to perform acid leaching, and then the recovered product may be filtered by deionized water or the like and dried, thereby obtaining the nitrogen-doped highly graphitic porous carbon body according to an exemplary embodiment, but the present invention is not limited thereto.
[0090] Another exemplary embodiment of the present invention provides a nitrogen-doped highly graphitic porous carbon body. The nitrogen-doped highly graphitic porous carbon body is characterized by containing 0.5 to 6 atom % of N as a heteroelement, and having a peak intensity ratio I.sub.D/I.sub.G of less than 1.0 as measured with a Raman spectrum.
[0091] Specifically, it is preferred that the nitrogen-doped highly graphitic porous carbon body may contain 0.6 to 6 atom %, preferably 0.6 to 4 atom % of N as a heteroelement, and have a peak intensity ratio I.sub.D/I.sub.G of 0.1 to 0.95, preferably 0.1 to 0.8, more preferably 0.1 to 0.7, or 0.2 to 0.6, as measured with the Raman spectrum. I.sub.D is a peak intensity of a D band (at around 1,350 cm.sup.−1) in the Raman spectrum, and is a peak due to a defect in a crystal and known to be observed near the edge of a sample or when there are many defects in the sample in the case of graphene. I.sub.G is a peak intensity of a G band (at around 1,580 cm.sup.−1) in the Raman spectrum, and is a peak commonly found in graphite-based materials. That is, the present invention corresponds to the content of the doped nitrogen and the I.sub.D/I.sub.G range, and since the high crystallinity of the carbon body is secured even at a low temperature, the loss of the nitrogen doping amount occurring at a high temperature may be suppressed.
[0092] In addition, the nitrogen-doped highly graphitic porous carbon body has a peak intensity ratio I.sub.2D/I.sub.G of 0.8 to 1.7, preferably 0.9 to 1.6, as measured with the Raman spectrum. I.sub.2D is a peak intensity of a 2D band (at around 2,700 cm.sup.−1) in the Raman spectrum, and shows a tendency to be inversely proportional to the number of graphene layers. Since the carbon body of the present invention corresponds to the range of I.sub.2D/I.sub.G, it may be graphene formed of 1 to 3 layers, preferably 1 or 2 layers.
[0093] The nitrogen-doped highly graphitic porous carbon body may have a BET specific surface area of 100 m.sup.2/g or more, preferably 200 to 500 m.sup.2/g, and more preferably 300 to 450 m.sup.2/g, a total pore volume of 0.3 cm.sup.3/g or more, preferably 0.4 to 2.0 cm.sup.3/g, and more preferably 0.4 to 1.5 cm.sup.3/g, a mesopore volume of 0.5 cm.sup.3/g or more, 0.6 cm.sup.3/g or more, 0.7 cm.sup.3/g or more, or 0.8 cm.sup.3/g or more and 2 cm.sup.3/g or less or 1.5 cm.sup.3/g or less, and an average pore size of 1 to 30 nm, preferably 3 to 20 nm, and more preferably 6 to 15 nm.
[0094] Meanwhile, the BET specific surface area is calculated using a Brunauer-Emmett-Teller method based on the results of nitrogen adsorption at a relative pressure in a range of 0.05 to 0.2, and the total pore volume is calculated from a gas adsorption amount at a relative pressure of 0.99. In addition, the mesopore size is measured by a Barrett-Joyner-Halenda method based on a Kelvin equation, and the mesopore volume is analyzed using the amount of adsorbed nitrogen at a relative pressure of 0.95 or less.
[0095] The nitrogen-doped highly graphitic porous carbon body may include binding states of pyrrolic N, graphitic N, pyridinic N, or a combination thereof; 20% or more, preferably 50% or more with respect to 100 atom % of the nitrogen (N) as a heteroelement is present as a binding state of pyrrolic N, as a non-limiting example, the upper limit may be 75% or less or 70% or less, and graphitic N may be 10% or less, pyridinic N may be 15% or more, preferably 20% or more, and more preferably 25% or more. In particular, it is known that since the electron pair of the pyrrolic N is involved in aromatic stabilization, it has higher binding energy than graphitic N, pyridinic N, and the like. Since the present invention has a higher ratio of pyrrolic N in the doped N, the crystallinity of the nitrogen doped carbon body may be improved and the activity of the oxygen reduction reaction may be improved.
[0096] Another exemplary embodiment provides an electrode catalyst for a fuel cell and/or a water electrolysis reaction including a highly graphitic porous carbon body doped with a heteroelement such as nitrogen. The nitrogen-doped highly graphitic porous carbon body may have platinum (Pt) supported on the surface and the inside.
[0097] Meanwhile, a water electrolysis reaction is a technology producing hydrogen and oxygen from pure water using electric energy, is largely classified into alkaline water electrolysis, solid polymer electrolyte membrane (PEM) water electrolysis, and high-temperature water vapor electrolysis using a solid oxide, and the nitrogen and heteroelement-doped highly graphitic porous carbon body of the present invention may be used as an electrode catalyst for a water electrolysis reaction as well as an electrode catalyst for a fuel cell.
[0098] Another exemplary embodiment of the present invention provides a method for producing a sulfur and nitrogen double doped highly graphitic porous carbon body. The method is characterized by including: polymerizing a sulfur and nitrogen element-containing precursor to produce a polymer; mixing the polymer and metal powder in an inert gas atmosphere, and then heat-treating the mixture at a temperature equivalent to or higher than a melting point of the metal; and pickling a product from the heat treatment.
[0099] In the polymerizing step, the hybrid orbital of carbon changes from sp.sup.3 to sp.sup.2, and thus, crystal structure arrangement in which it changes to crystalline carbon (sp.sup.2) later in the heat treatment step (carbonization step) is easy. However, when the carbonization is performed using a monomolecular precursor without the polymerizing step, the hybridization and the crystal structure arrangement of carbon should be performed at the same time in the carbonization process, and thus, a heteroelement content is greatly decreased and process efficiency is not good.
[0100] The step of polymerizing of a precursor may include adding the sulfur and nitrogen element-containing precursor to an airtight container and reacting at a heating rate of 1.0 to 5.0° C./min at a reaction temperature of 300 to 1,500° C. for 1 to 6 hours, and preferably at a heating rate of 2.0 to 4.5° C./min at a reaction temperature of 300 to 1,000° C. for 3 to 6 hours.
[0101] The sulfur and nitrogen element-containing precursor may be at least one selected from the group consisting of thiourea, ammonium thiocyanate, and thioacetamide. The precursor has a higher content of nitrogen than carbon and is easily decomposed at a low temperature, and thus, is an organic compound which is difficult to polymerize. Meanwhile, an oxygen element which may be contained in the carbon body produced as a heteroelement corresponds to a defect, and decreases the crystallinity of the carbon body and needs an additional heat treatment to remove it, and thus, is not preferred.
[0102] Meanwhile, in addition to the precursor containing both sulfur and nitrogen elements described above, a precursor in which a sulfur-containing precursor and a nitrogen-containing precursor are mixed may be individually used. The sulfur-containing precursor may be an organic molecular precursor or a polymer precursor including at least one selected from the group consisting of benzyl disulfide (BDS), thiophene, 2,2′-dithiophene, p-toluenesulfonic acid, and 2-thiophenemethanol, and the nitrogen-containing precursor may be an organic molecule or a polymer precursor including at least one selected from the group consisting of amino acids such as guanine, adenine, and purine, melamine, urea, pyridine, aniline, dicyandiamide, ethylenediamine, benzimidazole, and ethylene diamine tetraacetic acid (EDTA).
[0103] As a non-limiting example, a precursor in which the sulfur-containing precursor and the nitrogen-containing precursor are mixed at a weight ratio of 5:5 to 1:9 may be used, and specifically, a precursor mixed at a weight ratio of 5:5 to 4:6, 3:7 to 2:8, or 1.5:8.5 to 1:9 may be used. Otherwise, a precursor which is mixed so that a ratio between the number of moles of S element in the sulfur-containing precursor and the number of moles of N element in the nitrogen-containing precursor is 1:1 to 1:2.5 may be used, and specifically, a precursor which is mixed so that the ratio is 1:1 to 1:1.2, 1:1.5 to 1:1.7, or 1:2 to 1:2.5 may be used. In this case, in the carbonization process, it is favorable that the nitrogen and sulfur doping amounts are individually precisely adjusted considering a loss degree of each doping element.
[0104] The polymer may include sulfur-doped graphitic C.sub.3N.sub.4 at 30 to 90 wt %, preferably 50 to 70 wt %, with respect to the total weight. It is very favorable for forming a carbon hexagonal structure in the carbonization step and may increase the crystallinity of the carbon body, in that its atomic arrangement and hybrid orbital are similar to those of the graphene structure.
[0105] In addition, the polymer produced may contain 40 to 80 atom % or 40 to 60 atom % of N and 0.01 to 10 atom % or 0.01 to 5 atom % of S as a heteroelement. Sulfur and nitrogen doping may improve adsorption of a reaction chemical species in a hydrogen generation reaction using the carbon body. Specifically, since nitrogen and sulfur contained in a carbon structure have a higher electron affinity than carbon and have an unshared electron pair, they easily form an electronic structure favorable to the reaction, and thus, it is easy to provide a site appropriate for hydrogen generation reaction/oxygen reduction reaction. However, when the N content is more than 80 atom % or the S content is more than 10 atom %, the carbon body is easily decomposed during its production so that it is difficult to form a carbon-to-carbon bond, and its crystallinity is poor, and when the N content is less than 40 atom % and the S content is less than 0.01 atom %, the activity of hydrogen generation reaction/oxygen reduction reaction is insufficient, which is thus unfavorable.
[0106] Subsequently, the polymer produced above and the metal powder are mixed in an inert atmosphere, and then a heat treatment is performed at a temperature equivalent to or higher than the melting point of the metal.
[0107] In the heat treatment step (carbonization step), since molten metal powder is applied all over the surface of the polymer, metal-nitride or metal-sulfide may be formed well by the high reducibility of the metal. When there is no formation of metal-nitride or metal sulfide, doped nitrogen and sulfur are decomposed in the heat treatment at 600° C. or higher, and thus, when the heat treatment temperature is higher, it is unfavorable in terms of doped nitrogen and sulfur loss, and in contrast, when the heat treatment temperature is lower, the crystallinity (graphiticity) of the synthesized carbon body is lowered, but the problem may be prevented in the present invention.
[0108] The heat treatment temperature may be 1,500° C. or lower, preferably 650 to 1,000° C., more preferably 650 to 950° C., and most preferably 700 to 900° C. In addition, when the metal of the metal powder has a melting point of 1,000° C. or lower, a doping nitrogen and sulfur loss may be effectively suppressed. Specifically, when the melting point of the metal is lower than 1,000° C., 950° C. or lower, 900° C. or lower, 800° C. or lower, or 750° C. or lower, a problem that the metal powder is only partly melted at a temperature at which a doped nitrogen or sulfur loss occurs (600° C.) so that the entire surface of the polymer is not covered may be prevented. In addition, when the preferred lower limit of the melting point of the metal is higher than 50° C., 100° C. or higher, 150° C. or higher, 550° C. or higher, or 600° C. or higher, problems that the metal powder is all melted at too low temperature to lower the crystallinity in the production of the carbon body and handling is difficult may be prevented.
[0109] The metal powder may be alkaline earth metal powder, and for example, may be magnesium (Mg) or calcium (Ca), preferably magnesium. A representative reducing agent metal applied to a reduction process of a metal and the like is calcium, aluminum, magnesium, and the like in order of reducing power; however calcium is somewhat unfavorable in that it has a high melting point of 850° C. or higher so that it is inevitably operated at a high temperature, has a risk of explosion, and is expensive, and aluminum has excellent stability, is easy to handle, and has good reducing power, but has a disadvantage in that it is difficult to remove AlN, Al.sub.2O.sub.3, and the like which are mixed after the reduction process. However, since magnesium has a relatively low melting point of 650° C., is easy to handle, and has excellent reducing power, it is the most preferred to be applied to the present invention. In addition, the metal powder may have a metal particle diameter of 70 to 100 μm or 40 to 70 μm. When the particle diameter is more than 100 μm, uniform mixing with the polymer is difficult, and it is unfavorable for uniform mesopore formation. When the metal particle diameter is less than 40 μm, it reacts with oxygen or water in the air so that it is difficult to maintain a metallic surface, which is unfavorable in terms of reactivity and reducing power.
[0110] Meanwhile, the particle diameter of the metal particles may refer to D50, and D50 refers to a diameter of a particle with a cumulative volume of 50% when cumulated from the smallest particle in measurement of a particle size distribution by a laser scattering method. Here, for D50, the particle size distribution may be measured by collecting a sample for a metal particle material according to the standard of KS A ISO 13320-1, using Mastersizer 3000 from Malvern. Specifically, a volume density may be measured after particles are dispersed in ethanol as a solvent, using an ultrasonic disperser, if necessary, but the present invention is not limited thereto.
[0111] In the heat treatment step, the polymer and the metal powder may be mixed at a weight ratio of 1:0.5 to 1:3, preferably 1:0.7 to 1:1.5, more preferably 1:0.9 to 1:1.1, and most preferably about 1:1. In addition, the bulk graphite type polymer and the metal powder may be mixed at an atomic molar ratio of N+S:M of 1:0.3 to 1:7.5, preferably 1:0.5 to 1:4, more preferably 1:0.7 to 1:2.0, more preferably 1:0.9 to 1:1.3, and most preferably about 1:1. When the polymer and the metal powder are included at the above content, formation of metal-nitride and metal-sulfide may be optimized, as described later, which is thus preferred. Meanwhile, N and S are nitrogen and sulfur contained in the polymer as a heteroelement, respectively, M is a metal element of the metal powder, and N+S is a total of nitrogen and sulfur elements.
[0112] The heat treatment may be performed in an inert gas flow atmosphere, for example, a nitrogen flow atmosphere, or an argon, hydrogen/argon mixed flow atmosphere, but is not limited thereto.
[0113] Subsequently, by pickling the heat-treated product, residual metal powder, and metal-nitride and metal-sulfide as a by-product may be removed by etching. As the etching effect, a mesopore structure (formation of mesopores having a size of 2 to 50 nm) may be formed in the site of the carbon body from which residual metal powder, metal-nitride, and metal-sulfide are removed. The residual metal powder, the metal nitride, and metal sulfide may be, for example, residual magnesium powder (Mg), magnesium nitride (Mg.sub.2N.sub.3), and magnesium sulfide (MgS) having a uniform size of 20 to 50 nm, and by removal by the pickling, a carbon body having mesopores formed may be produced. In particular, since the mesopore structure has a tendency of having a larger pore structure with a higher carbonization heat treatment temperature, a low-temperature carbonization process is essentially required. In addition, it is analyzed that magnesium nitride has a greater effect on mesopore formation than residual magnesium, and this may be possible by using magnesium at tens of μm particle size level (70 to 100 μm or 40 to 70 μm) to form magnesium nitride which is distributed in a liquid phase in the reaction to have a uniform size (20 to 50 nm).
[0114] In particular, since the mesopore structure is formed by the acid etching after the heat treatment is performed at a relatively low temperature, the pore structure which is uniformly formed inside or on the surface of the carbon body may contribute to specific surface area improvement. However, when the crystallinity of the carbon body is increased by a simple high-temperature heat treatment as a conventional technology, not only are nitrogen and sulfur lost, but also micropores are closed, and thus, the specific surface area is rather decreased.
[0115] The pickling may be performed by adding the heat-treated product to a 0.5 to 3.5 M acid solution, for example, a 1 to 3 M inorganic acid (hydrochloric acid (HCl), phosphoric acid (H.sub.3PO.sub.4), or sulfuric acid (H.sub.2SO.sub.4)) and stirring the solution for 1 to 10 hours, preferably 2 to 8 hours, and then the recovered product may be filtered with deionized water and dried to obtain the carbon body, but the present invention is not limited thereto.
[0116] Another exemplary embodiment provides a sulfur and nitrogen double doped highly graphitic porous carbon body. The carbon body may have a peak intensity ratio satisfying the following Relations 1 and 2 as measured with a Raman spectrum:
0≤I.sub.D/I.sub.G≤0.95 [Relation 1]
0.3≤I.sub.2D/I.sub.G≤3.0 [Relation 2]
[0117] wherein I.sub.D is a peak intensity of a D band (at around 1,350 cm.sup.−1) in a Raman spectrum, I.sub.G is a peak intensity of a G band (at around 1,580 cm.sup.−1) in the Raman spectrum, and I.sub.2D is a peak intensity of a 2D band (at around 2, 700 cm.sup.−1) in the Raman spectrum.
[0118] In Relation 1, I.sub.D/I.sub.G may be preferably 0.8 or less, more preferably 0.7 or less, or 0.6 or less. I.sub.D is a peak representing a defect in the crystal of the carbon body, and graphene is known to be observed near the edge of the specimen or when there are many defects in the sample. I.sub.G is a peak commonly found in a graphite-based material, and is a measure of graphitization crystallinity. Since the carbon body of the present invention satisfies Relation 1, the high crystallinity of the carbon body in the low-temperature carbonization step is secured, and the loss of nitrogen and sulfur may be suppressed. Meanwhile, the lower limit of I.sub.D/I.sub.G may be, for example, 0.1 or more or 0.2 or more.
[0119] In relation 2, I.sub.2D/I.sub.G may be preferably 0.8 to 1.7, more preferably 0.9 to 1.6. I.sub.2D shows a tendency to be inversely proportional to the number of graphene layers, and ideal single-layer graphene which is practically close to the definition shows a peak intensity of I.sub.2D/I.sub.G close to 2. Since the present invention corresponds to the range of I.sub.2D/I.sub.G, the carbon body of the present invention may be graphene formed of less than 5 layers, preferably 1 to 3 layers, and more preferably 1 or 2 layers.
[0120] In addition, the carbon body may satisfy the following Relation 3:
V.sub.1/V.sub.2≤1.0 [Relation 3]
[0121] wherein V.sub.1 is a volume (cm.sup.3/g) of micropores of the carbon body, and V.sub.2 is a volume (cm.sup.3/g) of mesopores of the carbon body.
[0122] Specifically, V.sub.1/V.sub.2≤0.5 or V.sub.1/V.sub.2≤0.3, preferably V.sub.1/V.sub.2≤0.08 or V.sub.1/V.sub.2≤0.05, and more preferably V.sub.1/V.sub.2≤0.02 or V.sub.1/V.sub.2≤0.01.
[0123] Generally, the micropore refers to a pore having a size of 2 nm or less, and the mesopore refers to a pore having a size of 2 to 50 nm. The pores formed on the surface and the inside of the carbon body are a kind of defect and means low crystallinity. In addition, the specific surface area is increased by pore formation, and in particular, micropores having a smaller pore size has a greater effect than mesopores. In the case of the present invention, since fewer micropores are formed by increasing the crystallinity of the carbon body and more mesopores are formed by a pickling process, the specific surface area is somewhat small as compared with the conventional technology, but mesopores are increased, which is thus more favorable in terms of mass transfer in an application reaction and may secure the crystallinity of the carbon body. Specifically, it is favorable for movement and transfer of a reactant, an intermediate, and a product in an oxygen reduction reaction and a hydrogen generation reaction, and in particular, when a catalyst material such as Pt, Rh, Ru, Mo, Co, and Ni is supported, the access of a reactant (water, OH.sup.− ion in a water electrolysis reaction) is facilitated and the release of a product (hydrogen gas) is facilitated. The mesopores have a greater effect on the material transfer in the application reaction as such than the micropores.
[0124] Meanwhile, the carbon body may have a total pore volume of 0.3 to 2.0 cm.sup.3/g, a micropore volume of 0.001 to 0.06 cm.sup.3/g, a mesopore volume of 0.3 to 1.8 cm.sup.3/g, and an average pore size of 10 to 50 nm, and more preferably, a total pore volume of 0.5 to 2.0 cm.sup.3/g, a micropore volume of 0.005 to 0.050 cm.sup.3/g, a mesopore volume of 0.5 to 1.9 cm.sup.3/g, and average pore size of 15 to 40 nm. In addition, the carbon body may have a BET specific surface area of 80 to 1000 m.sup.2/g, preferably 100 to 700 m.sup.2/g, and more preferably 150 to 500 m.sup.2/g.
[0125] Meanwhile, the BET specific surface area is calculated using a Brunauer-Emmett-Teller method based on the results of nitrogen adsorption at a relative pressure in a range of 0.05 to 0.2, and the total pore volume is calculated from a gas adsorption amount at a relative pressure of 0.99. In addition, the mesopore size is measured by a Barrett-Joyner-Halenda method based on a Kelvin equation, and the mesopore volume is analyzed using the amount of adsorbed nitrogen at a relative pressure of 0.95 or less.
[0126] The carbon body may include 0.3 to 6.0 atom % of N and 0.1 to 3.0 atom % of S as heteroelements. Preferably, the carbon body may include 0.6 to 4.0 atom % of N and 0.1 to 1.0 atom % of S. Since nitrogen and sulfur doping is a kind of defect of the carbon body, when a doping amount is large, crystallinity is decreased to lower electrical conductivity. For electrochemical application, a balance between the doping degree of heteroelement and the crystallinity of carbon body is needed, and it is preferred that the heteroelements N and S are included in the above range. In addition, in the present invention, the kind and the content of metal powder are adjusted, thereby allowing qualitative improvement.
[0127] Meanwhile, the carbon body may include binding states of pyrrolic N, graphitic N, pyridinic N, or a combination thereof; 10 atom % or more, preferably 30 atom % or more, and more preferably 50 atom % or more with respect to 100 atom % of the nitrogen (N) as a heteroelement is present as a binding state of pyrrolic N, as a non-limiting example, the upper limit may be 75 atom % or less, 70 atom % or less, or 60 atom % or less, graphitic N may be 50 atom % or less, and pyridinic N may be 20 atom % or more, preferably 50 atom % or more, and more preferably 60 atom % or more. In particular, since the unshared electron pair of the pyrrolic N is not involved in aromatic stabilization, it has lower binding energy than graphitic N, pyridinic N, and the like. Since the present invention has a higher ratio of pyridinic N in the doped N, the crystallinity of the nitrogen doped carbon body may be improved and the activity of the oxygen reduction reaction and the hydrogen generation reaction may be improved. However, the relative ratio of N species may be varied with a carbonization temperature, a time, and used metal powder.
[0128] Another exemplary embodiment provides an electrode catalyst for a fuel cell and/or a water electrolysis reaction including the carbon body. The carbon body may be characterized by having platinum (Pt), rhodium (Rh), ruthenium (Ru), nickel (Ni), cobalt (Co), iron (Fe), palladium (Pd), copper (Cu), iridium (Ir), osmium (Os), molybdenum (Mo), vanadium (V), silver (Ag), gold (Au), and the like, or a combination thereof supported on the surface and the inside.
[0129] The water electrolysis reaction is a technology of producing hydrogen and oxygen from pure water using electric energy, and is largely classified into alkaline water electrolysis, solid polymer electrolyte membrane (PEM) water electrolysis, and high-temperature water vapor electrolysis using a solid oxide, and the sulfur and nitrogen double doped highly graphitic porous carbon body of the present invention may be used as an electrode catalyst for a water electrolysis reaction as well as an electrode catalyst for a fuel cell.
Hereinafter, the preferred examples and the comparative examples of the present invention will be described. However, the following examples are only a preferred exemplary embodiment of the present invention, and the present invention is not limited thereto.
Example 1
Examples 1-1 to 1-4
[0130] 30 g of urea was added to an alumina crucible, the crucible was sealed, and a heat treatment at a heating rate of 3.5° C./min at 550° C. for 4 hours under an air atmosphere was performed to obtain g-C.sub.3N.sub.4. The obtained g-C.sub.3N.sub.4 was mixed with an equal amount of Mg powder, the well-mixed powder was added to the alumina crucible, and a heat treatment was performed at X° C. (650° C., 750° C., 850° C., and 950° C.) according to each example for 5 hours in a furnace under an argon (Ar), nitrogen (N.sub.2), or H.sub.2/Ar mixed gas flow atmosphere. Subsequently, the heat-treated product was added to a 2 M HCl solution, and stirring was performed for 5 hours to perform acid leaching. The recovered product was filtered with deionized water and dried to obtain a nitrogen-doped highly graphitic porous carbon body.
Comparative Examples 1-1 and 1-2
[0131] The process was performed in the same manner as in Example 1-1, except that the obtained g-C.sub.3N.sub.4 was added to an alumina crucible and the heat treatment was performed in a furnace under the atmosphere according to each comparative example (H.sub.2/Ar 5/95 vol %, Ar 100 vol %).
Comparative Examples 1-3 and 1-4
[0132] The process was performed in the same manner as in Example 1-4, except that the obtained g-C.sub.3N.sub.4 was added to an alumina crucible and the heat treatment was performed in a furnace under the atmosphere according to each comparative example (H.sub.2/Ar 5/95 vol %, Ar 100 vol %).
[0133] Hereinafter, the nitrogen-doped highly graphitic porous carbon bodies of Examples 1-1 to 1-4 are indicated as CNMg-650, CNMg-750, CNMg-850, and CNMg-950, and the carbon bodies of Comparative Examples 1-1 to 1-4 are indicated as CNH-650, CNAr-650, CNAr-950, and CNH-950.
Evaluation Example
[0134] [Evaluation Method]
[0135] In order to analyze the produced porous carbon body, X-ray diffraction analysis was performed at 40 kV, 30 mA, and a scan rate of 4°/min, using a Cu-Kα (A=0.15406 nm) ray, using Rigaku Smartlab X-ray diffraction apparatus.
[0136] Fourier transform infrared (FT-IR) spectrum was obtained using NICOLET CONTINUUM.
[0137] Transmission electron microscope (TEM EM912 Omega) observation was performed at 120 kV, and a JEOL FE-2010 microscope operated at 200 kV was used to obtain high resolution TEM (HR-TEM) and a STEM image. Scanning electron microscope (SEM) observation was performed using a Hitachi S-4700 microscope operated at an acceleration voltage of 10 kV.
[0138] Nitrogen adsorption-desorption isotherms were measured at −196° C. by degassing a sample at 150° C. at 20 mTorr for 12 hours and then using a Micromeritics ASAP 2460 acceleration surface area and porosity analyzer.
[0139] A BET specific surface area was calculated using a Brunauer-Emmett-Teller (BET) method based on the nitrogen adsorption results at a relative pressure in a 0.05 to 0.2 range.
[0140] Raman analysis was performed using a Raman spectrometer (NICOLET ALMECA XR, available from Thermo Scientific). A 532 nm laser beam was used.
[0141] X-ray photoelectron spectroscopy (XPS) analysis was performed using an ESCALAB 250 XPS system using a monochromatic Al Kα (150 W).
[0142] Metal loading amounts in all samples and thermal reactions were measured by thermogravimetric analysis (TGA) using a Bruker TGDTA2000SA analyzer.
[0143] Electrical conductivity was measured using pressurized 4-probe measurement equipment using Keithley 6220 and 2182A with a DC current source and a voltmeter.
[0144] 2.5 mg of a Pt-supported porous carbon body produced by supporting Pt by various methods such as Homogeneous deposition and NaBH.sub.4 was added to 0.5 ml of a mixed solvent (solvent of 5 wt % of a Nafion aqueous solution: isopropanol:water mixed at a volume ratio of 0.8:1.0:4.0), and then ultrasound was applied for 15 minutes to produce a catalyst ink. Thereafter, 4 μL of the catalyst ink was applied on the surface of a working electrode and dried at 45° C. to produce a catalyst electrode having 0.283 mg/cm.sup.2 of a catalyst (porous carbon body) loaded.
[0145] For comparison, in the production of the catalyst ink, 2.5 mg of a commercially purchased Pt/C catalyst (20 wt % of Pt on ketjen black, Tanaka Kikinzoku Kogyo Co., hereinafter, referred to as commercial Pt/C catalyst), or Pt/CNH, Pt/CNAr, or Pt/g-C.sub.3N.sub.4 of Comparative Examples 1 and 2 was used instead of 2.5 mg of the Pt-supported porous carbon body to produce a commercial catalyst ink, which was applied on the working electrode as described above to produce a commercial catalyst electrode.
[0146] All electrochemical measurements were performed in a 3-electrode cell at room temperature. A KCl-saturated Ag/AgCl and a platinum wire were used as a reference electrode and a counter electrode. A rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE) on which a catalyst was loaded as a working electrode were measured in an acid electrolyte of a O.sub.2-saturated chloric acid (HClO.sub.4) aqueous solution.
[0147] All potentials were measured with a reversible hydrogen electrode (RHE), and for this, an Ag/AgCl reference electrode was calibrated for RHE.
[0148] A vitreous carbon RDE having a diameter of 3 mm coated with Pt/CNMg-X or commercial Pt/C (Tanaka, 20 wt % Pt on ketjen black) produced was used as the working electrode. As the catalyst ink, a dispersion of a catalyst corresponding to 5.0 mg of a mixture of 0.1 ml of a 5% Nafion solution (Sigma Aldrich) and 0.9 ml of deionized water was produced. The dispersed ink was cast on the vitreous carbon electrode, and dried in an oven at 60° C. The Pt loading amount of the electrode was about 50 μg/cm.sup.2.
[0149] Cyclic voltammetry (CV) was measured in a N.sub.2 saturated 0.1 M HClO.sub.4 solution for ORR under acidic conditions at room temperature, in a range of +1.1 to −0.2 V (vs. RHE) at a scan rate of 50 mV/s.
[0150] RDE and RRDE measurements were performed with linear sweep voltammetry (LSV) curves in an O.sub.2-saturated 0.1 M HClO.sub.4 solution. The LSV curve for ORR was measured at 1600 rpm, +1.0 to −0.2 V, and a scan rate of 10 mV/s under acidic conditions, and Pt ring potential was measured at +1.1 V set potential.
[0151] Membrane Electrode Assemblies (MEA) and Single Cell Test
[0152] A membrane electrode assembly was produced using a catalyst coated membrane (CCM) having an active area of 10 cm.sup.2. In the fuel cell test, commercial Pt/C (Tanaka, 20 wt % Pt on ketjen black) was used as an anode catalyst, and synthesized Pt/CNMg-X and a commercial Pt/C catalyst were used as an active cathode electrode. All catalyst inks were formed of catalyst powder, 2-propanol, deionized water, and 5 wt % of a Nafion solution (Sigma Aldrich). A well-dispersed catalyst slurry was directly sprayed on a Nafion N211 membrane to produce CCM, which was dried at 60° C. for several hours. A catalyst loading amount was 0.12 mgPt/cm.sup.2 in all electrodes. A membrane electrode assembly was assembled in a commercial gas diffusion layer (GDL, SGL 39 BC) without hot-pressing, and performance of a polymer electrolyte membrane fuel cell (PEMFC) was evaluated under the conditions of supplying fully humidified H.sub.2 and 02 at 80° C. to a positive electrode and a negative electrode at a flow rate of 500 and 1200 mL, respectively. A back pressure during evaluation was maintained at 0.5 bar.
[0153] A polarization curve for the membrane electrode assembly was performed by an electronic load (PLZ664WA, Kikusui) with PEMFC test station (Scitech Inc., Korea) under constant current.
[0154] Impedance spectroscopy (electrochemical impedance spectroscopy, EIS) was performed at 5 kHz˜ 100 mHz at a cell current of 100 mA/cm.sup.2. A back pressure during evaluation was maintained 0.5 bar.
[0155] In order to measure the electrochemical active surface area of the prepared cathode catalyst, CV was obtained at 0.1 to 1.2 V at a scan rate of 50 mV/s. Potentiostat (Bio-Logic, SP-150) coupled with a booster (VMP3B-20) was used, and at this time, fully humidified H.sub.2 (100 sccm) and N.sub.2 (50 sccm) at 80° C. under the atmosphere was injected to a negative electrode and a positive electrode, respectively.
[0156] For evaluation of electrode catalyst durability, an accelerated durability test (ADT) was performed in accordance with the standard of department of energy (DOE). According to the electrocatalyst cycle protocol, a quadrangle voltage cycling test was performed between 0.6 V and 0.95 V for a maintenance time of 3 seconds at potentials on both sides. After 10 k, 20 k, and 30 k cycles, a polarization curve, a cyclic voltammetry (CV), and impedance spectroscopy (EIS) were obtained.
[0157] For evaluation of catalyst support durability, an accelerated durability test (ADT) was performed in accordance with the standard of department of energy (DOE). According to the cycle protocol of the catalyst support, a triangular voltage cycling test was performed between 1.0 V and 1.5 V at a scan rate of 500 mV/s. After 5 k cycles, a polarization curve, a cyclic voltammetry (CV), and impedance spectroscopy (EIS) were obtained.
Evaluation Example 1
[Evaluation Example 1-1] Results of Analyzing Properties Such as High Crystallinity of Produced Carbon Body
[0158] In
[0159] In (b) of
[0160] From (c) of
[0161] To sum up, when the nitrogen-doped highly graphitic porous carbon body was produced by the production method of the present invention, as in the schematic diagram of the synthesis process of (a) of
[0162] In the following Table 1, the results (see (b) of
TABLE-US-00001 TABLE 1 The number of Sample I.sub.D/I.sub.G I.sub.2D/I.sub.G graphene layers CNMg-650 0.57 1~0.91 >2 CNMg-750 0.50 1.24 1-2 CNMg-850 0.26 1.50 1-2 CNMg-950 0.21 1.54 1-2
[0163] Meanwhile, in the case of the products produced in Comparative Examples 1-1 to 1-3, since CNH-650 and CNAr-650 maintained g-C.sub.3N.sub.4, I.sub.D and I.sub.G peaks were not able to be confirmed like g-C.sub.3N.sub.4. In addition, it was confirmed that CNAr-950 and CNH-950 were almost all decomposed, so that it was difficult to recover the samples (see
[Evaluation Example 1-2] Results of Analyzing Chemical Structure and the Like of Produced Nitrogen-Doped Carbon Body
[0164] In
[0165] In (a) of
[0166] In (b) of
TABLE-US-00002 TABLE 2 C 1s N 1s O 1s Cl 2p Sample (at %) (at %) (at %) (at %) g-C.sub.3N.sub.4 47.15 50.9 1.95 — CNH-650 46.05 52.41 1.41 — CNAr-650 49.41 48.49 2.1 — CNMg-650 93.3 2.67 4.03 0 CNMg-750 95.35 1.93 2.6 0.11 CNMg-850 96.05 1.29 2.48 0.18 CNMg-950 96.04 0.90 3 0.06
[0167] From Table 2 and (b) of
TABLE-US-00003 TABLE 3 Binding state of doped nitrogen in carbon body (ratio % relative to total doped N) Pyri- Pyr- Pyri- Graph- dinic rolic dinic itic N- sp.sup.2 Sample N N N oxide N—C C—NH.sub.2 N—H g-C.sub.3N.sub.4 0 0 0 0 66.02 22.14 11.84 CNAr-650 0 0 0 0 64.63 24.11 11.26 CNH-650 0 0 0 0 59.44 26.83 13.73 CNMg-650 58.69 25.53 9.09 6.69 0 0 0
[0168] From Table 3, and (c) and (d) of
[Evaluation Example 1-3] Results of Analyzing Porosity and BET Specific Surface Area Properties of Produced Carbon Body
[0169] (a) to (e) of
TABLE-US-00004 TABLE 4 BET Total Average surface pore Micropore Mesopore pore area volume volume volume diameter Sample (m.sup.2/g) (cm.sup.3/g) (cm.sup.3/g) (cm.sup.3/g) (nm) g-C.sub.3N.sub.4 64.08 0.2881 0.0035 0.2846 19.36 CNAr-650 81.83 0.2078 0.0189 0.1889 20.59 CNH-650 105.83 0.4310 0.0145 0.4165 23.43 CNMg-650 375.58 0.8317 0.0173 0.8144 8.17 CNMg-750 355.33 1.2542 0.0100 1.2442 11.75 CNMg-850 288.64 1.2968 0.0020 1.2948 12.67 CNMg-950 251.30 0.9902 0.0031 0.9871 12.71
[0170] From Table 4 and
[0171]
[Evaluation Example 1-4] Evaluation of Electrochemical Performance and PEMFC Durability
[0172] Evaluation of Electrical Conductivity of Porous Carbon Body
[0173] The electrical conductivities of the porous carbon bodies of Examples 1-1 to 1-4, Comparative Examples 1-1 and 1-2, a bulk graphite type polymer (g-C.sub.3N.sub.4), Reference 1 (KB-EC600JD, LION SPECIALTY CHEM CO., LTD), and Reference 2 (Vulcan carbon, CABOT) depending on the pressurized pressure were measured, and the results are shown in (a) of
[0174] Referring to (a) of
[0175] In addition, all of the porous carbon bodies of Examples 1-1 to 1-4 had significantly higher electrical conductivity than a commercial carbon black product (Reference 2, Vulcan carbon), and Examples 1-3 and 1-4 of these showed higher performance than ketjen black (Reference 1, KB-EC600JD) which is known as a carbon body having high electrical conductivity, which are thus meaningful.
[0176] ORR Performance Evaluation
[0177] From (b) to (d) of
[0178] Evaluation of Single Cell Performance and Durability (Evaluation of Durability of Supported Platinum)
[0179] An accelerated durability test (ADT) is to test the stability of supported platinum of an electrocatalyst, and according to Evaluation Example 1, the test was performed with the protocol suggested by U.S. Department of Energy (DOE). Referring to (a) and (c) of
[0180] These results may be also confirmed in CV and ECSA evaluations. In Example 1, the hydrogen absorption/desorption peak area (@CV) was smaller in the CV curve than commercial Pt/C ((b) and (d) of
TABLE-US-00005 TABLE 5 Mass V @ Rct @ Cycle activ- 0.8 ECSA 0.1 num- P.sub.max ity A/cm.sup.2 (m.sup.2/ A/cm.sup.2 (Ω Catalyst ber (mW/cm.sup.2) (A/mg.sub.Pt) (V) g.sub.Pt) cm.sup.2) 20% initial 1275 1.15 0.67 66.4 0.77 Pt/C 10 k 870 0.72 0.62 35.8 0.95 20 k 580 0.55 0.54 24.5 1.11 30 k 465 0.47 0.50 17.0 1.1 Pt/CNMg- initial 1325 1.07 0.69 42.0 0.73 650 10 k 1160 0.85 0.65 32.5 0.84 20 k 1080 0.75 0.64 27.3 0.87 30 k 970 0.69 0.63 26.5 0.88 Pt/CNMg- initial 1300 1.02 0.68 36.2 0.75 750 10 k 1070 0.76 0.65 24.1 0.86 20 k 965 0.66 0.64 19.1 0.93 30 k 895 0.57 0.63 17.5 0.94 Pt/CNMg- initial 1300 0.97 0.68 32.3 0.78 850 10 k 1055 0.68 0.65 20.2 0.89 20 k 955 0.59 0.63 14.4 0.94 30 k 895 0.52 0.62 12.8 1.02 Pt/CNMg- initial 1305 1.03 0.68 33.8 0.78 950 10 k 1065 0.75 0.65 21.2 0.88 20 k 950 0.65 0.63 20.1 0.93 30 k 894 0.56 0.63 17.1 1.01
[Evaluation Example 1-5] Evaluation of Durability of Carbon Support of PEMFC
[0181] With the evaluation of durability of the Pt catalyst, another accelerated durability test (ADT) suggested by DOE is to test the stability of a catalyst support, and was performed for new MEA with DOE protocol.
[0182] According to Evaluation Example 1, in the test, 20 wt % of Pt/CNMg-650 and a Pt/C carbon support were oxidized between 1.0 V and 1.5 V at a scan rate of 500 mV/s by high-pressure application. Oxidation (corrosion) of carbon agglomerated Pt loaded in the cathode and decreased the activity of MEA. It was confirmed that both commercial Pt/C and CNMg-650 had decreased current densities and hydrogen absorption-desorption areas from the CV results ((a) to (d) of
[0183] The characteristics as such may be also confirmed from the changed results of ECSA. In the ADT durability test of the support, it was observed that the size of Pt particles was increased with the loss of the support point of the support, which led to the decreased active surface area of Pt. Accordingly, increased EIS and increased Rct by cycling were shown. Referring to (e) to (f) of
TABLE-US-00006 TABLE 6 Mass V @ Rct @ Cycle activ- 1.5 ECSA 0.1 num- P.sub.max ity A/cm.sup.2 (m.sup.2/ A/cm.sup.2 (Ω Catalyst ber (mW/cm.sup.2) (A/mg.sub.Pt) (V) g.sub.Pt) cm.sup.2) 20% Pt/C initial 1275 1.15 0.60 66.4 0.77 5 k 510 0.61 0.33 23.2 1.31 Pt/CNMg- initial 1325 1.07 0.60 42.0 0.73 650 5 k 920 0.67 0.51 26.5 1.05 Pt/CNMg- initial 1300 1.02 0.60 36.2 0.75 750 5 k 875 0.63 0.50 21.6 1.12 Pt/CNMg- initial 1300 0.96 0.60 31.3 0.78 850 5 k 870 0.59 0.50 18.2 1.18 Pt/CNMg- initial 1310 0.97 0.60 32.0 0.76 950 5 k 870 0.60 0.49 18.2 1.17
[0184]
[0185] To sum up, in the present invention, the improved performance of ORR and PEMFC was achieved by the magnesiothermic reduction of g-C.sub.3N.sub.4. It was shown that the suggested synthesis method was effective in terms of costs and energy saving, and high crystallization (graphiticity) and electrical conductivity were obtained in a low-temperature heat treatment process of lower than 1,000° C. (preferably 650° C.) from inexpensive chemical materials such as metal powder like magnesium and urea, thereby greatly improving the catalytic activity to ORR and the durability. In addition, by using the N-rich precursor, nitrogen doping in the synthesized carbon body was easily functionalized in a graphitic carbon skeleton, and interestingly, was carbonized at a relatively low temperature, and thus, a large amount of pyrrolic-N sites was formed. The pyrrolic N was thermodynamically unstable, but stabilized Pt by a strong bond to Pt, so that the size or the distribution of particles was not greatly changed, and showed high activity and durability properties as compared with commercial Pt/C. Subsequently, in the carbon body structure, pores and a 3D porous structure were formed by the pickling of magnesium and Mg.sub.3N.sub.2 particles, and thus, the specific surface area of the carbon body was increased. The pore structure may decrease the mass diffusion resistance of the graphitized carbon body.
[0186] An exemplary embodiment of the present invention provides a PEMFC single cell including a nitrogen-doped highly graphitic porous carbon body and a Pt catalyst, having high activity and durability, resulting from the excellent chemical/physical properties. The maximum power (P.sub.max) of the present invention was decreased by 28%, but the maximum power of commercial Pt/C was decreased by 64%, after 30 k cycles. In addition, the results of analyzing the mass activity (@ 0.8V) and ECSA after 30 k cycles teach that the Pt utilization of the present invention was superior to commercial Pt/C. These results mean that the nitrogen-doped carbon body having a high pyrrolic-N ratio improved stability to Pt and resistance to carbon corrosion. In the Pt catalyst ADT test, the results of the mass activity and ECSA maintenance after 20 k cycles were that the catalyst of the present invention was more stable than the commercial catalyst, and it was analyzed that this was due to nitrogen doping with high crystallinity (graphiticity) and high pyrrolic N ratio.
Example 2
Examples 2-1 to 2-4
[0187] 1) Production of Polymer (Hereinafter, Referred to as SCN) Using Nitrogen Element and Sulfur Element-Containing Precursor
[0188] 10 g of thiourea (Sigma Aldrich, 99%) was added to an alumina boat, the alumina boat was covered, and a heat treatment was performed in a tube furnace at 500° C. at a heating rate of 5° C./min under a nitrogen atmosphere for 2 hours to obtain a polymer (hereinafter, referred to as SCN) yellow powder doped with nitrogen (N) and sulfur (S).
[0189] 2) Production of Sulfur and Nitrogen Double Doped Highly Graphitic Porous Carbon Body (Hereinafter, Referred to as SCNMg-X)
[0190] 3 g of SCN obtained was mixed with the same weight of Mg powder (Sigma Aldrich, 99%) using a ball mill for 20 minutes, and the well-mixed powder was added to an alumina crucible and heat-treated in a tube furnace at a temperature according to each example, X° C. (650° C., 750° C., 850° C., 950° C., Examples 2-1 to 2-4), at a heating rate of 5° C./min in an argon (Ar) flow atmosphere (flow rate of 200 mL/min) for 5 hours, thereby obtaining black powder (hereinafter, referred to as SCNMg-X-BE).
[0191] Subsequently, the heat-treated product was added to a 0.5M H.sub.2SO.sub.4 solution, and stirred at 80° C. for 12 hours to perform acid leaching. The recovered product was filtered with deionized water, and dried under vacuum at 70° C. to obtain a sulfur and nitrogen double doped highly graphitic porous carbon body (hereinafter, referred to as SCNMg-X).
Evaluation Example 2
[Evaluation Example 2-1] Results of Analyzing Properties Such as High Crystallinity of Carbon Body
[0192]
[0193] In (a) of
[0194] Meanwhile, referring to
[0195] In (b) of
TABLE-US-00007 TABLE 7 The number of Sample I.sub.D/I.sub.G I.sub.2D/ I.sub.G graphene layers SCNMg-650 0.95 0.45 <5 SCNMg-750 0.92 0.5 <5 SCNMg-850 0.69 0.9 2~3 SCNMg-950 0.66 1.5 1~2 SCN — — —
[0196] Meanwhile, it was confirmed that in SCN-850 which was synthesized under the same conditions without Mg, SCN was almost all decomposed in the high-temperature treatment, so that it was difficult to recover the sample (see
[0197] In (c) of
TABLE-US-00008 TABLE 8 BET Average surface Total pore Micropore Mesopore pore size area volume volume volume diameter Sample (m.sup.2/g) (cm.sup.3/g) (cm.sup.3/g) (cm.sup.3/g) (nm) SCN 9.46 0.033 0.004 0.029 13.753 SCNMg-650 157 0.652 0.006 0.646 15.02 SCNMg-750 224 0.961 0.007 0.954 16.729 SCNMg-850 254 1.069 0.004 1.065 16.291 SCNMg-950 160 0.825 0.016 0.809 20.258
[0198] From Table 8 and (c) of
[0199] In (d) of
[Evaluation Example 2-2] Analysis of Morphology of Carbon Body
[0200] From
[Evaluation Example 2-3] Analysis of Chemical Composition and the Like of Carbon Body
[0201] The results of analyzing the chemical composition of the carbon body using a Vairo MACRO cube elemental analyzer are summarized in the following Table 9.
TABLE-US-00009 TABLE 9 N C H S Sample (atom. %) (atom. %) (atom. %) (atom. %) SCN 59.27 32.16 2.42 1.66 SCNMg-850-H.sub.2SO.sub.4 1.60 88.60 0.52 0.55 SCNMg-850-HCl 1.66 87.24 0.53 0.41
[0202] From Table 9, it was found that the contents of N and S elements were greatly decreased in the SCN structure, by acid leaching (pickling) after magnesiothermic reduction. Meanwhile, in order to confirm the effect on the elemental composition of the carbon body of the acid solution in the acid leaching step, 2M HCl was used instead of 0.5M H.sub.2SO.sub.4 to perform acid leaching, thereby producing SCNMg-850-HCl, and as a result, the content of a S element was somewhat decreased. The results imply that a small amount of H.sub.2SO.sub.4 may be adsorbed in the form of —SO.sub.x on the surface of SCNMg-850. In
TABLE-US-00010 TABLE 10 C 1s N 1s S 2p O 1s Sample (at %) (at %) (at %) (at %) SCN 48.63 47.71 0.58 3.08 SCNMg-850 96.58 0.6 0.23 2.56
[0203] Referring to
Examples 2-5 to 2-7
[0204] (Production of Electrode Catalyst for Fuel Cell or Water Electrolysis Reaction, SCNMg-X-YRh-Z)
[0205] SCNMg-850-YRh-Z in which a Rh metal was reduced to be supported on SCNMg-850 (Example 2-3) having excellent surface area and crystallinity among the high crystalline carbon supports produced in Examples 2-1 to 2-4 was synthesized.
[0206] 100 mg of SCNMg-850 produced in Example 2-3 was ultrasonically dispersed in 20 mL of a mixture (mixture at a volume ratio of isopanol:water=1:1), and a RhCl.sub.3.nH.sub.2O aqueous solution (18 μM, 36 μM, and 73 μM RhCl.sub.3.nH.sub.2O, Examples 2-5 to 2-7) (RhCl.sub.3.nH.sub.2O (n=3.5) (Kojima chemicals co., ltd, 99.9%)) was added to the dispersion. Subsequently, a heat treatment at 120° C. was performed in an oil bath to evaporate all of the solvent. Then, the obtained SCNMg-850-YRh (YRh refers to the concentration of RhCl.sub.3.nH.sub.2O, Y μM) was cleaned with ethanol and water to remove unreacted metal ions, centrifuged at 14,000 rpm, and then dried under vacuum at 70° C. The obtained SCNMg-850-YRh powder was heat-treated under a gas atmosphere (flow rate of 100 mL/min) at Z° C. (500, 700, and 900° C., respectively) for 1 hour to produce an electrode catalyst (hereinafter, referred to as SCNMg-850-YRh-Z).
Comparative Examples 2-1 to 2-3
[0207] Reduced graphene oxide (hereinafter, referred to as rGO), commercial Vulcan carbon XC-72 (Sigma Aldrich, hereinafter, referred to as VC), and SCN were prepared.
[0208] rGO was produced by a thermic reduction treatment of the graphene oxide synthesized by a Hummer method (hereinafter, referred to as GO) at 850° C. for 2 hours in a NH.sub.3 atmosphere.
[0209] Rh-supported catalysts, rGO-36Rh-500, VC-36Rh-500, and SCN-36Rh-500 (Comparative Examples 2-1 to 2-3) were produced in the same manner as in the production of SCNMg-850-36Rh-500 of Example 2-6, except that rGO, VC, and SCN prepared were used as a support instead of SCNMg-850 of the examples.
[Evaluation Example 2-4] Synthesis of Rh-Supported N and S Double Doped Porous Carbon Body Catalyst
[0210] (a) of
[0211] From
[Evaluation Example 2-5] Electrochemical Properties of Rh-Supported N and S Double Doped Porous Carbon Body Catalyst
[0212] (a) of
[0213] Referring to (b) of
[0214] In addition, a loading amount and an annealing temperature in the electrocatalytic reaction are one of core elements. A SCNMg-850-36Rh sample produced for analyzing a degree to which an annealing temperature affects the HER performance of the electrocatalyst was annealed at 500, 700, and 900° C., respectively, under a NH.sub.3 gas flow. Referring to (c) of
[0215] In addition, as described above, the carrier (support) has a very important role in the interaction of metal ions/nanoparticles. In order to evaluate the role of the support, the HER performances of the example (SCNMg-850-36Rh-500) and Comparative Examples 2-1 to 2-3 (rGO-36Rh-500, VC-36Rh-500, and SCN-36Rh-500) were compared. Referring to (d) of
[0216] Therefore, referring to the Nyquist plot in (e) of
[0217] The turn of frequency (TOF) of the catalyst was measured and is shown in (f) of
[0218] Although the exemplary embodiments of the present invention have been described above, the present invention is not limited to the exemplary embodiments but may be made in various forms different from each other, and those skilled in the art will understand that the present invention may be implemented in other specific forms without departing from the spirit or essential feature of the present invention. Therefore, it should be understood that the exemplary embodiments described above are not restrictive, but illustrative in all aspects.