WO2012074160A1 - Carbon nanofiber catalysts using nanofiber including low cost transition metal for fuel cells and manufacturing method thereof - Google Patents

Carbon nanofiber catalysts using nanofiber including low cost transition metal for fuel cells and manufacturing method thereof Download PDF

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WO2012074160A1
WO2012074160A1 PCT/KR2010/008892 KR2010008892W WO2012074160A1 WO 2012074160 A1 WO2012074160 A1 WO 2012074160A1 KR 2010008892 W KR2010008892 W KR 2010008892W WO 2012074160 A1 WO2012074160 A1 WO 2012074160A1
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transition metal
catalyst
carbon nanofiber
fuel cell
hours
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PCT/KR2010/008892
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French (fr)
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Jae Young Lee
Sung-Hyun Uhm
Beom-Gyun Jeong
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Gwangju Institute Of Science And Technology
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/9041—Metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/9075—Catalytic material supported on carriers, e.g. powder carriers
    • H01M4/9083—Catalytic material supported on carriers, e.g. powder carriers on carbon or graphite
    • Y—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
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present disclosure relates to a transition metal-carbon nanofiber catalyst for a fuel cell using a nanofiber including a low cost transition metal and a method for preparing the same. More particularly, it relates to a transition metal-carbon nanofiber catalyst having electrode activity and durability comparable to those of a platinum catalyst for oxygen reduction reaction in an alkaline medium while having improved cost competitiveness by using a low cost transition metal, and a method for preparing the same.
  • a fuel cell is not only environment-friendly but also is expected to sufficiently replace the existing energy systems, including gasoline engines and secondary cells, under the current situations where the demand on high-output portable power source is increasing rapidly.
  • An alkaline fuel cell is favorable in that it is cost-competitive because expensive electrode catalysts and acid-resistant elements are not required and in that the output voltage at the oxygen electrode may be further improved by 100 mV.
  • the alkaline fuel cell can directly utilize liquid fuels with high energy density such as alcohols including methanol and ethanol, and hydrogenated materials such as sodium borohydride.
  • An electrode of the fuel cell has a metal catalyst layer for oxidizing fuel and reducing oxygen.
  • precious metals such as platinum and palladium or expensive functional precursors are commonly used. As such, there is a need of reducing the use of the expensive precious metals and improving the production process in order to lower production cost .
  • the present disclosure is directed to providing a nanofiber including a low cost transition metal and prepared by electrospinning .
  • the present disclosure is also directed to providing a carbon nanofiber catalyst using a nanofiber including a transition metal (hereinafter, also referred to as a transition metal-carbon nanofiber catalyst) , which is prepared by oxidatively stabilizing and carbonizing the transition metal- including nanofiber.
  • a transition metal-carbon nanofiber catalyst hereinafter, also referred to as a transition metal-carbon nanofiber catalyst
  • the present disclosure is also directed to providing a transition metal-carbon nanofiber catalyst for an alkaline fuel cell.
  • the present disclosure provides a method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell, including: (1) mixing a polymer precursor with an organic solvent to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition metal precursor to prepare a spinning solution; (3) electrospinning the spinning solution to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate to yield a transition metal-carbon nanofiber catalyst.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell may further include: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700-1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell may further include: (7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell may further include: (8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5), (6) or (7).
  • the present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell prepared by a method including: (1) mixing a polymer precursor with an organic solvent at 40-60 °C for 3-24 hours to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition metal precursor at 40-60 °C for 5-24 hours to prepare a spinning solution; (3) electrospinning the spinning solution at a voltage of 18-60 kV and a flow rate of 1- 10 mL/h to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber by heating the transition metal- nanofiber from room temperature at a rate of 1-5 °C/min and keeping at a final temperature of 250-350 °C for 1-2 hours to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1, 000 °
  • the present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell prepared by the afore-described method, which has a diameter of 50-200 nm and an average specific surface area of 500 m 2 .
  • the carbon nanofiber catalyst using a nanofiber including a transition metal and the method for preparing the same allow to improve catalyst performance through effective modification the functional groups on the nanofiber surface, to use the prepared catalyst as an electrode as it is through effective control of size and thickness, and to use the catalyst for other applications through fine grinding by ball milling, ultrasonic pulverization, etc .
  • the transition metal-carbon nanofiber catalyst according to the present disclosure may be produced in large scale through relatively simple electrospinning and heat treatment processes without using expensive precious metal catalyst precursors such as platinum or reducing agents.
  • precious metal catalyst precursors such as platinum or reducing agents.
  • electrode activity and durability comparable to those of a platinum catalyst for oxygen reduction reaction in an alkaline medium, it provides improved cost competitiveness.
  • transition metal-carbon nanofiber catalyst and the method for preparing the same allow increase of specific surface area through activation, further improvement of catalytic activity and stability through nitrogen doping using ammonia, and construction of a structure with several to tens of nanometers thick, highly electrically conductive carbon fibers including transition metals through chemical vapor deposition (CVD) using a hydrocarbon.
  • CVD chemical vapor deposition
  • transition metal-carbon nanofiber catalyst and the method for preparing the same according to the present disclosure allow preparation of the catalyst in the form of fine powder through fine grinding by ball milling, ultrasonic pulverization, etc.
  • the resulting catalyst may be coated and pressed on a gas diffusion layer to make a catalyst electrode through various coating methods such as spray coating, brush coating and slurry coating.
  • the employment of the relatively simple electrospinning and heat treatment processes allows mass production and cost reduction.
  • Fig. 1 (a) shows an iron-cobalt included carbon fiber matrix prepared according to the present disclosure
  • Fig. 1 (b) shows a scanning electron microscopic image thereof
  • Fig. 1 (c) shows a transmission electron microscopic image thereof;
  • Fig. 2 shows a result of an electrochemical activity test for oxygen reduction reaction in Examples 1-6 and Comparative Examples 1-3 (The carbon nanofiber catalysts of Examples 1-6 and Comparative Examples 1-3 were coated on a rotating electrode and catalytic activity was measured in a 0.1 KOH solution saturated with oxygen while varying voltage at a rate of 5 mV/s . ) ;
  • Fig. 3 shows a result of measuring current for Example 1 and Comparative Example 1 (Current was measured with time at a constant voltage of -0.6 V as a measure of long-term stability.);
  • Fig. 4 shows a result of an electrochemical activity test for oxygen reduction reaction for various combinations of three transition metals iron, cobalt and nickel (Current density measurement at -0.6 V was compared.);
  • Fig. 5 shows a result of an electrochemical activity test for oxygen reduction reaction for catalysts having the same iron-cobalt composition of 1:1 but different overall metal contents (Current density measurement at -0.6 V was compared.);
  • Fig. 6 shows a result of an electrochemical activity test for oxygen reduction reaction for catalysts having the same iron-cobalt composition and overall metal contents but carbonized at different temperatures (Current density measurement at -0.6 V was compared.) .
  • the present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell using a nanofiber comprising a low cost transition metal and a method for preparing the same.
  • a nanofiber is prepared by electrospxnning using a spinning solutio containing a transition metal and a polymer precursor, it is stabilized and carbonized to prepare a carbon nanofiber catalyst comprising the transition metal.
  • the resulting carbon nanofiber catalyst has improved catalyst performance through effective modification of functional groups on the nanofiber surface combined with the transition metal nanoparticles supported on the carbon nanofibers and may be used as an electrode as it is through effective control of size and thickness. Further, the catalyst may be used for other applications through fine grinding by ball milling, ultrasonic pulverization, etc.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell comprises: (1) mixing a polymer precursor with an organic solvent to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition- metal precursor to prepare a spinning solution; (3) electrospinning the spinning solution to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate to yield a transition metal-carbon nanofiber catalyst.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700- 1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5) , (6) or (7) .
  • the organic solvent may be one or more selected from water, ethanol, N, -dimethylformamide (DMF) , acetone, benzene, toluene, hexane, acetonitrile, tetrahydrofuran, or the like.
  • the mixing proportion is not particularly limited.
  • the mixing proportion may be from 1:9 to 9:1.
  • the polymer precursor may be one or more selected from a group consisting of polyacrylonitrile (PAN) , polyamide (PA) , polyacrylamide (PAA) , polyurethane (PU) , polyetherimide (PEI), polyvinylpyrrolidone (PVP) and polybenzimidazole (PBI) .
  • PAN polyacrylonitrile
  • PA polyamide
  • PAA polyacrylamide
  • PU polyurethane
  • PEI polyetherimide
  • PVP polyvinylpyrrolidone
  • PBI polybenzimidazole
  • the polymer precursor mixture may comprise 5-30 wt% of the polymer precursor and 70-95 wt% of the organic solvent. Outside this range, it may be difficult to maintain adequate viscosity and dispersity of the metal precursor.
  • the polymer precursor may be mixed with the organic solvent sufficiently at 40-60 °C for 3-24 hours. Outside this range, the polymer precursor may not be fully dissolved, the monomers dissolved in the solvent may not form the polymer material, or the polymer precursor may be oxidized.
  • the transition metal precursor may be an alkoxide-based precursor comprising a transition metal selected from iron (Fe), cobalt (Co), nickel (Ni) , copper (Cu) or a combination of two or more of them, without particular limitation.
  • one or more selected from a group consisting of iron (II) acetylacetonate , iron (III) acetylacetonate, cobalt (II) acetylacetonate, cobalt (III) acetylacetonate, nickel (II) acetylacetonate, nickel (III) acetylacetonate, copper (II) acetylacetonate, copper ( III ) acetylacetonate, a metal acetate, a metal sulfide and a metal chloride may be used.
  • a combination iron (II or III) acetylacetonate and cobalt (II or III) acetylacetonate may be used.
  • the mixing proportion of the combination is not particularly limited.
  • the mixing proportion may be from 1:9 to 9:1.
  • the spinning solution may comprise 0.1-2 wt% of the transition metal precursor and 98.0-99.9 wt% of the polymer precursor mixture, in order to optimize solubility and catalytic activity depending on the particular transition metal precursor.
  • catalytic activity may be insufficient. And, if it exceeds 2 wt%, there may be a negative effect on dispersity and active site formation of the catalyst although electrical conductivity is maintained. As a result, catalytic activity may be not good.
  • the polymer precursor mixture may be mixed with the transition metal precursor at 40-60 °C for 5-24 hours. Outside this range, the transition metal precursor may not be fully dissolved.
  • the spinning solution is electrospun to prepare the transition metal-nanofiber .
  • the electrospinning may be performed at a voltage of 18-60 kV and a flow rate of 1-10 mL/h. If the voltage is below 18 kV, electrospinning may not occur because of insufficient electrostatic attraction/repulsion. And, if it exceeds 60 kV, spark may occur or the electrospun fabric may have uneven thickness due to fluid mechanical instability between the fiber and the solution. If the flow rate is below 1 mL/h, preparation speed may be too low. And, if it exceeds 10 mL/h, economic loss may occur because the polymer solution is not fully used for the spinning or fiber may not be obtained because the solvent is not evaporated completely.
  • the electrospinning may be performed using a commonly used electrospinning apparatus.
  • the spinning may be performed under controlled temperature and relative humidity conditions.
  • the transition metal-nanofiber is stabilized to obtain the transition metal-nanofiber intermediate
  • the stabilization may be performed by heating the transition metal-nanofiber from room temperature at a rate of 1- 5 °C/min and keeping at a final temperature of 250-350 °C for 1- 2 hours. During this process, the linear-structure polymer is oxidized and converted into a cyclic structure consisting of carbon and nitrogen atoms .
  • the heating rate is below 1 °C/min, too much time may be spent for the stabilization, nitrogen included in the polymer precursor may be removed, and fiber yield may decrease. Further, if the polymer remains for a long time near the glass transition temperature, the nanofiber structure may be deformed. And, if the heating rate exceeds 5 °C/min, the nanofiber becomes unstable due to fast reaction, resulting in melting or glass transition of the nanofiber. As a result, the fiber structure may not be maintained.
  • the stabilization may be incomplete, thereby resulting in melting or glass transition of the nanofiber. As a result, the fiber structure may not be maintained. And, if the final temperature exceeds 350 °C, reaction may occur too fast. As a result, there is a high risk of combustion of the polymer fiber.
  • the stabilization may be incomplete, as in the case where the final heat treatment temperature is below 250 °C.
  • the nanofiber may be unstable due to fast reaction, resulting in melting or glass transition of the nanofiber.
  • the fiber structure may not be maintained.
  • the reaction time is too short, it is difficult to maintain the fiber structure stably during the carbonization due to incomplete cyclization.
  • fiber yield or strength, conductivity, etc. of the fiber may be unsatisfactory.
  • a stabilization time exceeding 2 hours is unnecessary since the fiber remains stable once the cyclization is completed unless it is further heated. On the contrary, too long a stabilization time may result in mass loss caused by oxidation of carbon or nitrogen as well as undesired reactions .
  • the transition metal-nanofiber intermediate is carbonized in a nitrogen atmosphere to yield the transition metal-carbon nanofiber catalyst.
  • the carbonization may be performed by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1,000 °C for 30 minutes to 3 hours.
  • step (5) if the heating rate is below 3 °C/min, side reactions may occur or a lot of time and energy may be spent to maintain the high temperature. And, if the heating rate exceeds 7 °C/min, the temperature distribution of the nanofiber matrix becomes non-uniform, resulting in irreproducible quality of final product.
  • the cyclized polymer may not be converted to carbon arranged with a graphite structure, resulting in amorphous carbon with unsatisfactory mechanical strength and electrical conductivity. And, if it exceeds 1,000 °C, the functional groups consisting of nitrogen and oxygen atoms or the doping structure that exhibit the catalytic performance for oxygen reduction on the surface of the carbon nanofiber may be destroyed. Moreover, there could be an undesirable mass loss of carbon nanofibers leading to a lower yield of the catalyst.
  • carbonization time is shorter than 30 minutes, carbonization may not occur sufficiently. And, if it exceeds 3 hours, undesired reactions may occur and productivity may decrease due to waste of energy and time.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700- 1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours. Outside this range, catalytic activity and stability may not be ⁇ improved because of insufficient modification with nitrogen atoms and fiber loss may occur .
  • the hydrocarbon is not particularly limited. For example, one or more selected from methane, acetylene, etc. may be used.
  • the content of the hydrocarbon may be determined by those skilled in the art based on the present disclosure.
  • step (6) nitrogen-containing functional groups are formed on the surface of the transition metal-carbon nanofiber catalyst.
  • the transition metals and nitrogen atoms form complexes, the resulting nitrogen-doped catalyst may have improved catalytic activity and stability for oxygen reduction reaction.
  • multi-walled nanotubes (MWNT) including transition metal nanoparticles as catalyst may be grown via chemical vapor deposition (CVD) to improve specific surface area and electrical conductivity.
  • the nitrogen modification in the step (6) results in improved catalytic activity and stability through two actions.
  • nitrogen is doped into the carbon graphite structure of the transition metal-carbon nanofiber, nonmetallic active site consisting of nitrogen and carbon atoms are formed.
  • the transition metals included in the transition metal-carbon nanofiber form complexes with the nitrogen atoms, thereby improving catalytic activity and stability .
  • the addition of the hydrocarbon followed by heating is to prepare transition metal-based pyrolytic carbon nanofibers with high electrical conductivity.
  • a branch-like structure may be formed, with fine fibers having a thickness of several to tens of nanometers extending from carbon fibers with a thickness of hundreds of nanometers.
  • electrical conductivity of the electrode may be improved.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours. Outside this range, specific surface area may not be increased sufficiently and the activation reaction itself may not occur.
  • the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5) , (6) or (7) .
  • the fine grinding may be performed by ball milling, ultrasonic pulverization, or the like.
  • the ball milling may be performed by pulverizing at a rate of 100-300 rpm for 30 minutes to 4 hours using a ball mill and drying with hot air of 100-150 °C in a nitrogen atmosphere or in vacuum. Outside this range, the fine grinding may not be performed sufficiently.
  • the ultrasonic pulverization may be performed by pulverizing using an ultrasonic pulverizer with a power of 10-100 W for 1-5 minutes. Outside this range, loss of the catalyst may occur.
  • the present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell prepared by a method comprising: (1) mixing a polymer precursor with an organic solvent at 40-60 °C for 3-24 hours to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition metal precursor at 40-60 °C for 5-24 hours to prepare a spinning solution; (3) electrospinning the spinning solution at a voltage of 18-60 kV and a flow rate of 1-10 mL/h to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber by heating the transition metal- nanofiber from room temperature at a rate of 1-5 °C/min and keeping at a final temperature of 250-350 °C for 1-2 hours to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1, 000 °C for 30
  • the transition metal-carbon nanofiber catalyst according to the present disclosure may be prepared by a method comprising: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700-1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
  • the transition metal-carbon nanofiber catalyst according to the present disclosure may be prepared by a method comprising:
  • the transition metal-carbon nanofiber catalyst according to the present disclosure may be prepared by a method comprising:
  • the organic solvent may be one or more selected from water, ethanol, DMF, acetone, benzene, toluene, hexane, acetonitrile, tetrahydrofuran, or the like.
  • the mixing proportion is not particularly limited.
  • the mixing proportion may be from 1:9 to 9:1.
  • the polymer precursor may be one or more selected from a group consisting of PAN, PA, PAA, PU, PEI, PVP and PBI .
  • the polymer precursor mixture may comprise 5-30 wt% of the polymer precursor and 70-95 wt% of the organic solvent.
  • the transition metal precursor may be an alkoxide-based precursor comprising a transition metal selected from iron, cobalt, nickel, copper or a combination of two or more of them, without particular limitation.
  • a transition metal selected from iron, cobalt, nickel, copper or a combination of two or more of them, without particular limitation.
  • a combination of iron (II or III) acetylacetonate and cobalt (II or III) acetylacetonate may be used.
  • the mixing proportion of the combination is not particularly limited.
  • the mixing proportion may be from 1:9 to 9:1.
  • the spinning solution may comprise 0.1-2 wt% of the transition metal precursor and 98.0-99.9 wt% of the polymer precursor mixture, in order to optimize solubility and catalytic activity depending on the particular transition metal precursor.
  • the transition metal-carbon nanofiber catalyst according to the present disclosure may have a diameter of 50-200 nm an average specific surface area of 500 m 2 .
  • the transition metal-carbon nanofiber catalyst finely ground according to the present disclosure may have a diameter of 50-200 nm and a length of 20 ⁇ or smaller.
  • the electrospinning condition was 25 °C and 30% relative humidity.
  • the electrospun nanofiber was heated at a rate of 1 °C/min and then heat-treated at 280 °C in an oxygen atmosphere for 1 hour.
  • the nanofiber was carbonized at 800 °C in a nitrogen atmosphere for 1 hour after heating at a rate of 5 °C/min to prepare a carbon nanofiber matrix (see Fig. 1) .
  • a current density measurement result is shown in Fig. 2.
  • the carbon nanofiber catalyst prepared in Example 1 was activated at 250 °C in a steam atmosphere for 2 hours to prepare an activated carbon nanofiber catalyst.
  • the catalyst had a specific surface area of 700 m 2 .
  • a current density measurement result of the carbon nanofiber catalyst is shown in Fig. 2.
  • the carbon nanofiber catalyst prepared in Example 1 was pulverized using a ball mill at 200 rpm for 2 hours, and dried with hot air of 120 °C. As a result, a carbon nanofiber catalyst with an average length of 15 ⁇ was prepared. A current density measurement result is shown in Fig. 2.
  • the carbon nanofiber catalyst prepared in Example 2 was pulverized using a ball mill at 200 rpm for 2 hours, and dried with hot air of 120 °C. As a result, an activated carbon nanofiber catalyst with an average length of 15 ⁇ was prepared. A current density measurement result is shown in Fig. 2.
  • the carbon nanofiber catalyst prepared in Example 1 was heated at 800 °C in an ammonia atmosphere for 2 hours, and then at 800 °C in a hydrocarbon atmosphere for 2 hours.
  • a current density measurement result of thus prepared carbon nanofiber catalyst is shown in Fig. 2.
  • the carbon nanofiber catalyst prepared in Example 5 was pulverized using an ultrasonic pulverizer with a power of 10 W for 4 minutes. A current density measurement result of thus prepared carbon nanofiber catalyst is shown in Fig. 2.
  • a commercially available 20 wt% Pt/C catalyst was prepared.
  • Carbon black (Vulcan Corporation) was prepared as a catalyst .
  • 40 wt% of polyacrylonitrile was mixed with arid dissolved in 60 wt% of dimethylformamide at 25 °C for 2 hours to prepare a polymer precursor solution.
  • a polymer precursor solution To 75 wt% of the polymer precursor solution, 2 wt% of iron (II) acetylacetonate and 2 wt% of cobalt (II) acetylacetonate were added and completely dissolved.
  • the resulting mixture was spun at a flow rate of 6 mL/h by applying a voltage of 15 kV between a syringe and a rotating drum spaced by 15 cm, so as to form a nanofiber mat on the drum surface.
  • the electrospun nanofiber was heated at a rate of 1 °C/min and then heat-treated at 250 °C in an oxygen atmosphere for 16 hours. Then, the nanofiber was carbonized at 1100 °C in a nitrogen atmosphere for 2 hours after heating at a rate of 5 °C/min to prepare an iron-cobalt carbon nanofiber catalyst.
  • the fiber thickness was in micrometer order due to the high content of the polymer precursor, and side reactions occurred since the metal precursor was not completely dissolved due to its high content.
  • loss of the carbon nanofiber catalyst occurred as the functional groups or the doping structure were destroyed. As a result, the current density was very low.
  • Fig. 2 shows a result of an electrochemical activity test for oxygen reduction reaction for Examples 1-6 and Comparative Examples 1-3.
  • the iron-cobalt carbon nanofiber catalysts of Examples 1-6 exhibited oxygen reduction performance comparable to that of the platinum catalyst of Comparative Example 1.
  • Fig. 3 shows a constant-voltage test result.
  • the catalyst of Example 1 exhibited long-term stability comparable to that of the platinum catalyst of Comparative Example 1.
  • Fig. 4 shows a result of an electrochemical activity test for oxygen reduction reaction for various combinations of transition metals.
  • the iron-cobalt combination exhibited better activity than other pure transition metals or alloys thereof. This suggests that different results may be derived depending on the transition metal content or heat treatment temperature.
  • Fig. 5 shows a result of an electrochemical activity test for oxygen reduction reaction for iron-cobalt catalysts depending on metal content. Superior catalytic activity was obtained at 5-30 wt%. When the metal content exceeded 30 wt%, the catalytic activity decreased since the dispersity and active site formation of the catalyst were negatively affected.
  • a carbonization temperature range suited for formation of the catalytic active sites was 800-1,000 °C. Above 1,000 °C, the catalytic activity decreased due to improper formation of functional groups and loss of the carbon nanofiber .

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Abstract

Provided is a transition metal-carbon nanofiber catalyst for a fuel cell using a nanofiber including only a low cost transition metal. More particularly, the transition metal-including carbon nanofiber catalyst is prepared by preparing a nanofiber from electrospinning of a solution containing a transition metal and a polymer precursor, followed by oxidative stabilization and carbonization. As a result, catalyst performance may be improved through effective modification of the functional groups on the nanofiber surface. The catalyst may be used as an electrode as it is since the size and thickness may be controlled effectively. Further, the catalyst may be subjected to fine grinding by ball milling, ultrasonic pulverization, etc. for use in other applications. The catalyst may be produced in large scale through relatively simple process of electrospinning and heat treatment processes without using expensive precious metal catalyst precursors such as platinum or reducing agents. In particular, with electrode activity and durability comparable to those of a platinum catalyst for oxygen reduction reaction in an alkaline medium, it is useful as an oxygen reduction catalyst for use in an alkaline fuel cell with improved cost competitiveness.

Description

[DESCRIPTION]
[invention Title]
CARBON NANOFIBER CATALYSTS USING NANOFIBER INCLUDING LOW COST TRANSITION METAL FOR FUEL CELLS AND MANUFACTURING METHOD' THEREOF
[Technical Field]
The present disclosure relates to a transition metal-carbon nanofiber catalyst for a fuel cell using a nanofiber including a low cost transition metal and a method for preparing the same. More particularly, it relates to a transition metal-carbon nanofiber catalyst having electrode activity and durability comparable to those of a platinum catalyst for oxygen reduction reaction in an alkaline medium while having improved cost competitiveness by using a low cost transition metal, and a method for preparing the same.
[Background Art]
A fuel cell is not only environment-friendly but also is expected to sufficiently replace the existing energy systems, including gasoline engines and secondary cells, under the current situations where the demand on high-output portable power source is increasing rapidly. An alkaline fuel cell is favorable in that it is cost-competitive because expensive electrode catalysts and acid-resistant elements are not required and in that the output voltage at the oxygen electrode may be further improved by 100 mV. In addition, the alkaline fuel cell can directly utilize liquid fuels with high energy density such as alcohols including methanol and ethanol, and hydrogenated materials such as sodium borohydride.
An electrode of the fuel cell has a metal catalyst layer for oxidizing fuel and reducing oxygen. For the metal catalyst, precious metals such as platinum and palladium or expensive functional precursors are commonly used. As such, there is a need of reducing the use of the expensive precious metals and improving the production process in order to lower production cost .
[Disclosure]
[Technical Problem]
The present disclosure is directed to providing a nanofiber including a low cost transition metal and prepared by electrospinning .
The present disclosure is also directed to providing a carbon nanofiber catalyst using a nanofiber including a transition metal (hereinafter, also referred to as a transition metal-carbon nanofiber catalyst) , which is prepared by oxidatively stabilizing and carbonizing the transition metal- including nanofiber.
The present disclosure is also directed to providing a transition metal-carbon nanofiber catalyst for an alkaline fuel cell.
[Technical Solution]
In one general aspect, the present disclosure provides a method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell, including: (1) mixing a polymer precursor with an organic solvent to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition metal precursor to prepare a spinning solution; (3) electrospinning the spinning solution to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate to yield a transition metal-carbon nanofiber catalyst.
In an embodiment, the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell may further include: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700-1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
In an embodiment, the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell may further include: (7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours.
In an embodiment, the method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell may further include: (8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5), (6) or (7).
In another general aspect, the present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell prepared by a method including: (1) mixing a polymer precursor with an organic solvent at 40-60 °C for 3-24 hours to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition metal precursor at 40-60 °C for 5-24 hours to prepare a spinning solution; (3) electrospinning the spinning solution at a voltage of 18-60 kV and a flow rate of 1- 10 mL/h to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber by heating the transition metal- nanofiber from room temperature at a rate of 1-5 °C/min and keeping at a final temperature of 250-350 °C for 1-2 hours to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1, 000 °C for 30 minutes to 3 hours to yield a transition metal-carbon nanofiber catalyst.
In another general aspect, the present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell prepared by the afore-described method, which has a diameter of 50-200 nm and an average specific surface area of 500 m2.
Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.
[Advantageous Effects]
The carbon nanofiber catalyst using a nanofiber including a transition metal and the method for preparing the same, according to the present disclosure, allow to improve catalyst performance through effective modification the functional groups on the nanofiber surface, to use the prepared catalyst as an electrode as it is through effective control of size and thickness, and to use the catalyst for other applications through fine grinding by ball milling, ultrasonic pulverization, etc .
The transition metal-carbon nanofiber catalyst according to the present disclosure may be produced in large scale through relatively simple electrospinning and heat treatment processes without using expensive precious metal catalyst precursors such as platinum or reducing agents. In particular, with electrode activity and durability comparable to those of a platinum catalyst for oxygen reduction reaction in an alkaline medium, it provides improved cost competitiveness.
Further, the transition metal-carbon nanofiber catalyst and the method for preparing the same according to the present disclosure allow increase of specific surface area through activation, further improvement of catalytic activity and stability through nitrogen doping using ammonia, and construction of a structure with several to tens of nanometers thick, highly electrically conductive carbon fibers including transition metals through chemical vapor deposition (CVD) using a hydrocarbon. Such a structure improves specific surface area, catalyst utilization and electrical conductivity of the catalyst electrode.
In addition, the transition metal-carbon nanofiber catalyst and the method for preparing the same according to the present disclosure allow preparation of the catalyst in the form of fine powder through fine grinding by ball milling, ultrasonic pulverization, etc. The resulting catalyst may be coated and pressed on a gas diffusion layer to make a catalyst electrode through various coating methods such as spray coating, brush coating and slurry coating. Besides, the employment of the relatively simple electrospinning and heat treatment processes allows mass production and cost reduction.
[Description of Drawings]
The above and other objects, features and advantages of the present disclosure will become apparent from the following description of certain exemplary embodiments given in conjunction with the accompanying drawings, in which:
Fig. 1 (a) shows an iron-cobalt included carbon fiber matrix prepared according to the present disclosure, Fig. 1 (b) shows a scanning electron microscopic image thereof, and Fig. 1 (c) shows a transmission electron microscopic image thereof;
Fig. 2 shows a result of an electrochemical activity test for oxygen reduction reaction in Examples 1-6 and Comparative Examples 1-3 (The carbon nanofiber catalysts of Examples 1-6 and Comparative Examples 1-3 were coated on a rotating electrode and catalytic activity was measured in a 0.1 KOH solution saturated with oxygen while varying voltage at a rate of 5 mV/s . ) ;
Fig. 3 shows a result of measuring current for Example 1 and Comparative Example 1 (Current was measured with time at a constant voltage of -0.6 V as a measure of long-term stability.);
Fig. 4 shows a result of an electrochemical activity test for oxygen reduction reaction for various combinations of three transition metals iron, cobalt and nickel (Current density measurement at -0.6 V was compared.);
Fig. 5 shows a result of an electrochemical activity test for oxygen reduction reaction for catalysts having the same iron-cobalt composition of 1:1 but different overall metal contents (Current density measurement at -0.6 V was compared.); and
Fig. 6 shows a result of an electrochemical activity test for oxygen reduction reaction for catalysts having the same iron-cobalt composition and overall metal contents but carbonized at different temperatures (Current density measurement at -0.6 V was compared.) .
[Best Mode]
Hereinafter, the embodiments of the present disclosure will be described in detail with reference to accompanying drawings.
The present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell using a nanofiber comprising a low cost transition metal and a method for preparing the same.
After a nanofiber is prepared by electrospxnning using a spinning solutio containing a transition metal and a polymer precursor, it is stabilized and carbonized to prepare a carbon nanofiber catalyst comprising the transition metal. The resulting carbon nanofiber catalyst has improved catalyst performance through effective modification of functional groups on the nanofiber surface combined with the transition metal nanoparticles supported on the carbon nanofibers and may be used as an electrode as it is through effective control of size and thickness. Further, the catalyst may be used for other applications through fine grinding by ball milling, ultrasonic pulverization, etc.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure comprises: (1) mixing a polymer precursor with an organic solvent to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition- metal precursor to prepare a spinning solution; (3) electrospinning the spinning solution to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate to yield a transition metal-carbon nanofiber catalyst.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700- 1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5) , (6) or (7) .
In the step (1), the organic solvent may be one or more selected from water, ethanol, N, -dimethylformamide (DMF) , acetone, benzene, toluene, hexane, acetonitrile, tetrahydrofuran, or the like. When two or more solvents are used, the mixing proportion is not particularly limited. For example, the mixing proportion may be from 1:9 to 9:1.
In the step (1), the polymer precursor may be one or more selected from a group consisting of polyacrylonitrile (PAN) , polyamide (PA) , polyacrylamide (PAA) , polyurethane (PU) , polyetherimide (PEI), polyvinylpyrrolidone (PVP) and polybenzimidazole (PBI) .
The polymer precursor mixture may comprise 5-30 wt% of the polymer precursor and 70-95 wt% of the organic solvent. Outside this range, it may be difficult to maintain adequate viscosity and dispersity of the metal precursor.
In the step (1), the polymer precursor may be mixed with the organic solvent sufficiently at 40-60 °C for 3-24 hours. Outside this range, the polymer precursor may not be fully dissolved, the monomers dissolved in the solvent may not form the polymer material, or the polymer precursor may be oxidized.
In the step (2), the transition metal precursor may be an alkoxide-based precursor comprising a transition metal selected from iron (Fe), cobalt (Co), nickel (Ni) , copper (Cu) or a combination of two or more of them, without particular limitation. For example, one or more selected from a group consisting of iron (II) acetylacetonate , iron (III) acetylacetonate, cobalt (II) acetylacetonate, cobalt (III) acetylacetonate, nickel (II) acetylacetonate, nickel (III) acetylacetonate, copper (II) acetylacetonate, copper ( III ) acetylacetonate, a metal acetate, a metal sulfide and a metal chloride may be used. More specifically, a combination iron (II or III) acetylacetonate and cobalt (II or III) acetylacetonate may be used. The mixing proportion of the combination is not particularly limited. For example, the mixing proportion may be from 1:9 to 9:1.
The spinning solution may comprise 0.1-2 wt% of the transition metal precursor and 98.0-99.9 wt% of the polymer precursor mixture, in order to optimize solubility and catalytic activity depending on the particular transition metal precursor.
If the content of the transition metal precursor is below 0.1 wt%, catalytic activity may be insufficient. And, if it exceeds 2 wt%, there may be a negative effect on dispersity and active site formation of the catalyst although electrical conductivity is maintained. As a result, catalytic activity may be not good.
In the step (2), the polymer precursor mixture may be mixed with the transition metal precursor at 40-60 °C for 5-24 hours. Outside this range, the transition metal precursor may not be fully dissolved.
In the step (3) , the spinning solution is electrospun to prepare the transition metal-nanofiber . In the step (3) , the electrospinning may be performed at a voltage of 18-60 kV and a flow rate of 1-10 mL/h. If the voltage is below 18 kV, electrospinning may not occur because of insufficient electrostatic attraction/repulsion. And, if it exceeds 60 kV, spark may occur or the electrospun fabric may have uneven thickness due to fluid mechanical instability between the fiber and the solution. If the flow rate is below 1 mL/h, preparation speed may be too low. And, if it exceeds 10 mL/h, economic loss may occur because the polymer solution is not fully used for the spinning or fiber may not be obtained because the solvent is not evaporated completely.
The electrospinning may be performed using a commonly used electrospinning apparatus. The spinning may be performed under controlled temperature and relative humidity conditions.
In the step (4), the transition metal-nanofiber is stabilized to obtain the transition metal-nanofiber intermediate
The stabilization may be performed by heating the transition metal-nanofiber from room temperature at a rate of 1- 5 °C/min and keeping at a final temperature of 250-350 °C for 1- 2 hours. During this process, the linear-structure polymer is oxidized and converted into a cyclic structure consisting of carbon and nitrogen atoms .
In the step (4), if the heating rate is below 1 °C/min, too much time may be spent for the stabilization, nitrogen included in the polymer precursor may be removed, and fiber yield may decrease. Further, if the polymer remains for a long time near the glass transition temperature, the nanofiber structure may be deformed. And, if the heating rate exceeds 5 °C/min, the nanofiber becomes unstable due to fast reaction, resulting in melting or glass transition of the nanofiber. As a result, the fiber structure may not be maintained.
If the final temperature is below 250 °C, the stabilization may be incomplete, thereby resulting in melting or glass transition of the nanofiber. As a result, the fiber structure may not be maintained. And, if the final temperature exceeds 350 °C, reaction may occur too fast. As a result, there is a high risk of combustion of the polymer fiber.
If the stabilization is performed for less than 1 hour, the stabilization may be incomplete, as in the case where the final heat treatment temperature is below 250 °C. Thus, the nanofiber may be unstable due to fast reaction, resulting in melting or glass transition of the nanofiber. As a result, the fiber structure may not be maintained. Further, if the reaction time is too short, it is difficult to maintain the fiber structure stably during the carbonization due to incomplete cyclization. As a result, fiber yield or strength, conductivity, etc. of the fiber may be unsatisfactory. And, a stabilization time exceeding 2 hours is unnecessary since the fiber remains stable once the cyclization is completed unless it is further heated. On the contrary, too long a stabilization time may result in mass loss caused by oxidation of carbon or nitrogen as well as undesired reactions .
In the step (5), the transition metal-nanofiber intermediate is carbonized in a nitrogen atmosphere to yield the transition metal-carbon nanofiber catalyst.
The carbonization may be performed by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1,000 °C for 30 minutes to 3 hours.
In the step (5) , if the heating rate is below 3 °C/min, side reactions may occur or a lot of time and energy may be spent to maintain the high temperature. And, if the heating rate exceeds 7 °C/min, the temperature distribution of the nanofiber matrix becomes non-uniform, resulting in irreproducible quality of final product.
If the final temperature is below 700 °C, the cyclized polymer may not be converted to carbon arranged with a graphite structure, resulting in amorphous carbon with unsatisfactory mechanical strength and electrical conductivity. And, if it exceeds 1,000 °C, the functional groups consisting of nitrogen and oxygen atoms or the doping structure that exhibit the catalytic performance for oxygen reduction on the surface of the carbon nanofiber may be destroyed. Moreover, there could be an undesirable mass loss of carbon nanofibers leading to a lower yield of the catalyst.
If the carbonization time is shorter than 30 minutes, carbonization may not occur sufficiently. And, if it exceeds 3 hours, undesired reactions may occur and productivity may decrease due to waste of energy and time.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700- 1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours. Outside this range, catalytic activity and stability may not be ■ improved because of insufficient modification with nitrogen atoms and fiber loss may occur .
The hydrocarbon is not particularly limited. For example, one or more selected from methane, acetylene, etc. may be used. The content of the hydrocarbon may be determined by those skilled in the art based on the present disclosure.
In the step (6), nitrogen-containing functional groups are formed on the surface of the transition metal-carbon nanofiber catalyst. As the transition metals and nitrogen atoms form complexes, the resulting nitrogen-doped catalyst may have improved catalytic activity and stability for oxygen reduction reaction. Further, multi-walled nanotubes (MWNT) including transition metal nanoparticles as catalyst may be grown via chemical vapor deposition (CVD) to improve specific surface area and electrical conductivity.
More specifically, the nitrogen modification in the step (6) results in improved catalytic activity and stability through two actions. For one thing, as nitrogen is doped into the carbon graphite structure of the transition metal-carbon nanofiber, nonmetallic active site consisting of nitrogen and carbon atoms are formed. For another, the transition metals included in the transition metal-carbon nanofiber form complexes with the nitrogen atoms, thereby improving catalytic activity and stability .
In the step (6), the addition of the hydrocarbon followed by heating is to prepare transition metal-based pyrolytic carbon nanofibers with high electrical conductivity. As a result, a branch-like structure may be formed, with fine fibers having a thickness of several to tens of nanometers extending from carbon fibers with a thickness of hundreds of nanometers. As these fibers form a new, highly conductive carbon network, electrical conductivity of the electrode may be improved.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours. Outside this range, specific surface area may not be increased sufficiently and the activation reaction itself may not occur.
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to the present disclosure may further comprise: (8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5) , (6) or (7) .
In the (8), the fine grinding may be performed by ball milling, ultrasonic pulverization, or the like. The ball milling may be performed by pulverizing at a rate of 100-300 rpm for 30 minutes to 4 hours using a ball mill and drying with hot air of 100-150 °C in a nitrogen atmosphere or in vacuum. Outside this range, the fine grinding may not be performed sufficiently. The ultrasonic pulverization may be performed by pulverizing using an ultrasonic pulverizer with a power of 10-100 W for 1-5 minutes. Outside this range, loss of the catalyst may occur.
The present disclosure provides a transition metal-carbon nanofiber catalyst for a fuel cell prepared by a method comprising: (1) mixing a polymer precursor with an organic solvent at 40-60 °C for 3-24 hours to prepare a polymer precursor mixture; (2) mixing the polymer precursor mixture with a transition metal precursor at 40-60 °C for 5-24 hours to prepare a spinning solution; (3) electrospinning the spinning solution at a voltage of 18-60 kV and a flow rate of 1-10 mL/h to prepare a transition metal-nanofiber; (4) stabilizing the transition metal-nanofiber by heating the transition metal- nanofiber from room temperature at a rate of 1-5 °C/min and keeping at a final temperature of 250-350 °C for 1-2 hours to obtain a transition metal-nanofiber intermediate; and (5) carbonizing the transition metal-nanofiber intermediate by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1, 000 °C for 30 minutes to 3 hours to yield a transition metal-carbon nanofiber catalyst.
The transition metal-carbon nanofiber catalyst according to the present disclosure may be prepared by a method comprising: (6) heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700-1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
The transition metal-carbon nanofiber catalyst according to the present disclosure may be prepared by a method comprising:
(7) activating the transition metal-carbon nanofiber catalyst yielded in the step (5) or (6) in a steam atmosphere at 200-300 °C for 1-3 hours.
The transition metal-carbon nanofiber catalyst according to the present disclosure may be prepared by a method comprising:
(8) finely grinding the transition metal-carbon nanofiber catalyst yielded in the step (5), (6) or (7).
The organic solvent may be one or more selected from water, ethanol, DMF, acetone, benzene, toluene, hexane, acetonitrile, tetrahydrofuran, or the like. When two or more solvents are used, the mixing proportion is not particularly limited. For example, the mixing proportion may be from 1:9 to 9:1.
The polymer precursor may be one or more selected from a group consisting of PAN, PA, PAA, PU, PEI, PVP and PBI .
The polymer precursor mixture may comprise 5-30 wt% of the polymer precursor and 70-95 wt% of the organic solvent.
The transition metal precursor may be an alkoxide-based precursor comprising a transition metal selected from iron, cobalt, nickel, copper or a combination of two or more of them, without particular limitation. For example, one or more selected from a group consisting of iron (II) acetylacetonate, iron (III) acetylacetonate, cobalt (II) acetylacetonate, cobalt (III) acetylacetonate, nickel (II) acetylacetonate, nickel (III) acetylacetonate, copper (II) acetylacetonate, copper (III) acetylacetonate, a metal acetate, a metal sulfide and a metal chloride may be used. More specifically, a combination of iron (II or III) acetylacetonate and cobalt (II or III) acetylacetonate may be used. The mixing proportion of the combination is not particularly limited. For example, the mixing proportion may be from 1:9 to 9:1.
The spinning solution may comprise 0.1-2 wt% of the transition metal precursor and 98.0-99.9 wt% of the polymer precursor mixture, in order to optimize solubility and catalytic activity depending on the particular transition metal precursor.
The transition metal-carbon nanofiber catalyst according to the present disclosure may have a diameter of 50-200 nm an average specific surface area of 500 m2.
The transition metal-carbon nanofiber catalyst finely ground according to the present disclosure may have a diameter of 50-200 nm and a length of 20 μιη or smaller. [Mode for Invention]
The examples and experiments will now be described. The following examples and experiments are for illustrative purposes only and not intended to limit the scope of the present disclosure.
Examples 1-4 and Comparative Examples 1-4
Example 1
10 wt% of polyacrylonitrile was mixed with and dissolved in 90 wt% of dimethylformamide at 40 °C for 4 hours to prepare a polymer solution. To 80 wt% of the polymer precursor solution, 0.2 wt% of iron (II) acetylacetonate and 0.2 wt% of cobalt (II) acetylacetonate were added. After completely dissolving at 50 °C for 6 hours, a spinning solution was prepared. The spinning solution was spun at a flow rate of 3 mL/h for 10 hours by applying a high voltage of 24 kV between a syringe and a rotating drum spaced by 15 cm, so as to form a nanofiber on the drum surface. The electrospinning condition was 25 °C and 30% relative humidity. For oxidative stabilization, the electrospun nanofiber was heated at a rate of 1 °C/min and then heat-treated at 280 °C in an oxygen atmosphere for 1 hour. Then, the nanofiber was carbonized at 800 °C in a nitrogen atmosphere for 1 hour after heating at a rate of 5 °C/min to prepare a carbon nanofiber matrix (see Fig. 1) . A current density measurement result is shown in Fig. 2.
Example 2
The carbon nanofiber catalyst prepared in Example 1 was activated at 250 °C in a steam atmosphere for 2 hours to prepare an activated carbon nanofiber catalyst. The catalyst had a specific surface area of 700 m2. A current density measurement result of the carbon nanofiber catalyst is shown in Fig. 2.
Example 3
The carbon nanofiber catalyst prepared in Example 1 was pulverized using a ball mill at 200 rpm for 2 hours, and dried with hot air of 120 °C. As a result, a carbon nanofiber catalyst with an average length of 15 μπι was prepared. A current density measurement result is shown in Fig. 2.
Example 4
The carbon nanofiber catalyst prepared in Example 2 was pulverized using a ball mill at 200 rpm for 2 hours, and dried with hot air of 120 °C. As a result, an activated carbon nanofiber catalyst with an average length of 15 μπι was prepared. A current density measurement result is shown in Fig. 2.
Example 5
The carbon nanofiber catalyst prepared in Example 1 was heated at 800 °C in an ammonia atmosphere for 2 hours, and then at 800 °C in a hydrocarbon atmosphere for 2 hours. A current density measurement result of thus prepared carbon nanofiber catalyst is shown in Fig. 2.
Example 6
The carbon nanofiber catalyst prepared in Example 5 was pulverized using an ultrasonic pulverizer with a power of 10 W for 4 minutes. A current density measurement result of thus prepared carbon nanofiber catalyst is shown in Fig. 2.
Comparative Example 1
A commercially available 20 wt% Pt/C catalyst was prepared.
Comparative Example 2
Carbon black (Vulcan Corporation) was prepared as a catalyst .
Comparative Example 3
10 wt% of polyacrylonitrile was mixed with and dissolved in 90 wt% of dimethylformamide at 25 °C for 2 hours to prepare polymer precursor solution. The precursor solution was spun at a flow rate of 3 mL/h by applying a high voltage of 24 kV between a syringe and a rotating drum spaced by 15 cm, so as to form a nanofiber on the drum surface. For stabilization, the electrospun nanofiber was heated at a rate of 1 °C/min and then heat-treated at 280 °C in an oxygen atmosphere for 1 hour. Then, the nanofiber was carbonized at 800 °C in a nitrogen atmosphere for 1 hour after heating at a rate of 5 °C/min to prepare a carbon nanofiber catalyst.
Comparative Example 4
40 wt% of polyacrylonitrile was mixed with arid dissolved in 60 wt% of dimethylformamide at 25 °C for 2 hours to prepare a polymer precursor solution. To 75 wt% of the polymer precursor solution, 2 wt% of iron (II) acetylacetonate and 2 wt% of cobalt (II) acetylacetonate were added and completely dissolved. The resulting mixture was spun at a flow rate of 6 mL/h by applying a voltage of 15 kV between a syringe and a rotating drum spaced by 15 cm, so as to form a nanofiber mat on the drum surface. For stabilization, the electrospun nanofiber was heated at a rate of 1 °C/min and then heat-treated at 250 °C in an oxygen atmosphere for 16 hours. Then, the nanofiber was carbonized at 1100 °C in a nitrogen atmosphere for 2 hours after heating at a rate of 5 °C/min to prepare an iron-cobalt carbon nanofiber catalyst. However, the fiber thickness was in micrometer order due to the high content of the polymer precursor, and side reactions occurred since the metal precursor was not completely dissolved due to its high content. In particular, upon the carbonization at 1100 °C, loss of the carbon nanofiber catalyst occurred as the functional groups or the doping structure were destroyed. As a result, the current density was very low. Fig. 2 shows a result of an electrochemical activity test for oxygen reduction reaction for Examples 1-6 and Comparative Examples 1-3. The iron-cobalt carbon nanofiber catalysts of Examples 1-6 exhibited oxygen reduction performance comparable to that of the platinum catalyst of Comparative Example 1.
Fig. 3 shows a constant-voltage test result. The catalyst of Example 1 exhibited long-term stability comparable to that of the platinum catalyst of Comparative Example 1.
Fig. 4 shows a result of an electrochemical activity test for oxygen reduction reaction for various combinations of transition metals. The iron-cobalt combination exhibited better activity than other pure transition metals or alloys thereof. This suggests that different results may be derived depending on the transition metal content or heat treatment temperature.
Fig. 5 shows a result of an electrochemical activity test for oxygen reduction reaction for iron-cobalt catalysts depending on metal content. Superior catalytic activity was obtained at 5-30 wt%. When the metal content exceeded 30 wt%, the catalytic activity decreased since the dispersity and active site formation of the catalyst were negatively affected.
Referring to Fig. 6, a carbonization temperature range suited for formation of the catalytic active sites was 800-1,000 °C. Above 1,000 °C, the catalytic activity decreased due to improper formation of functional groups and loss of the carbon nanofiber .
The present application contains subject matter related to Korean Patent Application No. 10-2010-0119866, filed in the Korean Intellectual Property Office on November 29, 2010, the entire contents of which is incorporated herein by reference.
Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present disclosure. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the disclosure as set forth in the appended claims.

Claims

[CLAIMS]
[Claim l]
A method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell, comprising:
mixing a polymer precursor with an organic solvent to prepare a polymer precursor mixture;
mixing the polymer precursor mixture with a transition metal precursor to prepare a spinning solution;
electrospinning the spinning solution to prepare a transition metal-nanofiber;
stabilizing the transition metal-nanofiber to obtain a transition metal-nanofiber intermediate; and
carbonizing the transition metal-nanofiber intermediate to yield a transition metal-carbon nanofiber catalyst.
[Claim 2]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, which further comprises: heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700-1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours. [Claim 3]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, which further comprises: activating the resulting transition metal- carbon nanofiber catalyst in a steam atmosphere at 200-300 °C for 1-3 hours.
[Claim 4]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 2, which further comprises: activating the resulting transition metal- carbon nanofiber catalyst in a steam atmosphere at 200-300 °C for 1-3 hours.
[Claim 5]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to any one of claims 1 to 4, which further comprises: finely grinding the resulting transition metal-carbon nanofiber catalyst
[Claim 6]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, wherein the organic solvent is one or more selected from water, ethanol, N, N-dimethylformamide (DMF) , acetone, benzene, toluene, hexane, acetonitrile and tetrahydrofuran, the polymer precursor is one or more selected from a group consisting of polyacrylonitrile (PAN), polyamide (PA), polyacrylamide (PAA) , polyurethane (PU), polyetherimide (PEI), polyvinylpyrrolidone (PVP) and polybenzimidazole (PBI) , and the transition metal precursor is an alkoxide-based precursor comprising a transition metal selected from iron (Fe), cobalt (Co), nickel (Ni) , copper (Cu) or a combination of two or more of them.
[Claim 7]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 6, wherein the transition metal precursor is one or more selected from a group consisting of iron (II) acetylacetonate, iron (III) acetylacetonate, cobalt (II) acetylacetonate, cobalt (III) acetylacetonate, nickel (II) acetylacetonate, nickel (III) acetylacetonate, copper (II) acetylacetonate, copper (III) acetylacetonate, a metal acetate, a metal sulfide and a metal chloride. [Claim 8l
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, wherein the polymer precursor mixture comprises 5-30 wt% of the polymer precursor and 70-95 wt% of the organic solvent, and the spinning solution comprises 0.1-2 wt% of the transition metal precursor and 98.0-99.9 wt% of the polymer precursor mixture.
[Claim 9]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, wherein said mixing of the polymer precursor with the organic solvent is performed at 40-60 °C for 3-24 hours, said mixing of the polymer precursor mixture with the transition metal precursor is performed at 40-60 °C for 5-24 hours, and said electrospinning of the spinning solution is performed at a voltage of 18-60 kV and a flow rate of 1-10 mL/h.
[Claim 10]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, wherei'n said stabilizing of the transition metal-nanofiber is performed by heating the transition metal-nanofiber from room temperature at a rate of 1-5 °C/min and keeping at a final temperature of 250-350 °C for 1-2 hours.
[Claim 11]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 1, wherein said carbonizing of the transition metal-nanofiber intermediate is performed by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1, 000 °C for 30 minutes to 3 hours.
[Claim 12]
The method for preparing a transition metal-carbon nanofiber catalyst for a fuel cell according to claim 5, wherein said fine grinding of the transition metal-carbon nanofiber catalyst is performed by pulverizing the transition metal-carbon nanofiber catalyst using a ball mill at a rate of 100-300 rpm for 30 minutes to 4 hours and drying with hot air of 100-150 °C or in vacuum, or by pulverizing using an ultrasonic pulverizer with a power of 10-100 W for 1-5 minutes.
[Claim 13] A transition metal-carbon nanofiber catalyst for a fuel cell prepared by a method comprising:
mixing a polymer precursor with an organic solvent at 40-60 °C for 3-24 hours to prepare a polymer precursor mixture;
mixing the polymer precursor mixture with a transition metal precursor at 40-60 °C for 5-24 hours to prepare a spinning solution;
electrospinning the spinning solution at a voltage of 18-60 kV and a flow rate of 1-10 mL/h to prepare a transition metal- nanofiber;
stabilizing the transition metal-nanofiber by heating the transition metal-nanofiber from room temperature at a rate of 1- 5 °C/min and keeping at a final temperature of 250-350 °C for 1- 2 hours to obtain a transition metal-nanofiber intermediate; and carbonizing the transition metal-nanofiber intermediate by heating the transition metal-nanofiber intermediate in a nitrogen atmosphere at a rate of 3-7 °C/min and keeping at a final temperature of 700-1, 000 °C for 30 minutes to 3 hours to yield a transition metal-carbon nanofiber catalyst.
[Claim 14]
The transition metal-carbon nanofiber catalyst for a fuel cell according to claim 13, which is prepared by a method further comprising: heating the transition metal-carbon nanofiber catalyst in an ammonia atmosphere at 700-1500 °C for 1-3 hours to modify with nitrogen atoms and then, after adding a hydrocarbon, heating in a hydrocarbon atmosphere at 700-1500 °C for 1-3 hours.
[Claim 15]
The transition metal-carbon nanofiber catalyst for cell according to claim 13, which is prepared by a further comprising: activating the resulting transition carbon nanofiber catalyst in a steam atmosphere at 200 for 1-3 hours.
[Claim 16]
The transition metal-carbon nanofiber catalyst for a fuel cell according to claim 14, which is prepared by a method further comprising: activating the resulting transition metal- carbon nanofiber catalyst in a steam atmosphere at 200-300 °C for 1-3 hours.
[Claim 17]
The transition metal-carbon nanofiber catalyst for a fuel cell according to any one of claims 13 to 16, which is prepared by a method further comprising: finely grinding the resulting transition metal-carbon nanofiber catalyst.
[Claim 18]
The transition metal-carbon nanofiber catalyst for a fuel cell according to claim 17, wherein said fine grinding of the transition metal-carbon nanofiber catalyst is performed by pulverizing the transition metal-carbon nanofiber catalyst using a ball mill at a rate of 100-300 rpm for 30 minutes to 4 hours and drying with hot air of 100-150 °C or in vacuum, or by pulverizing using an ultrasonic pulverizer with a power of 10- 100 W for 1-5 minutes.
[Claim 19]
The transition metal-carbon nanofiber catalyst for a fuel cell according to claim 13, wherein the organic solvent is one or more selected from water, ethanol, N, N-dimethylformamide (DMF) , acetone, benzene, toluene, hexane, acetonitrile and tetrahydrofuran, the polymer precursor is one or more selected from a group consisting of polyacrylonitrile (PAN), polyamide (PA), polyacrylamide (PAA) , polyurethane (PU), polyetherimide (PEI), polyvinylpyrrolidone (PVP) and polybenzimidazole (PBI), and the transition metal precursor is an alkoxide-based precursor comprising a transition metal selected from iron (Fe) , cobalt (Co) , nickel (Ni) , copper (Cu) or a combination of two or more of them, the transition metal precursor being one or more selected from a group consisting of iron (II) acetylacetonate, iron (III) acetylacetonate, cobalt (II) acetylacetonate, cobalt (III) acetylacetonate, nickel (II) acetylacetonate, nickel (III) acetylacetonate, copper (II) acetylacetonate, copper (III) acetylacetonate, a metal acetate, a metal sulfide and a metal chloride, the polymer precursor mixture comprising 5-30 wt% of the polymer precursor and 70-95 wt% of the organic solvent, and the spinning solution comprising 0.1-2 wt% of the transition metal precursor and 98.0^99.9 wt% of the polymer precursor mixture.
[CIaim 20]
The transition metal-carbon nanofiber catalyst for a fuel cell according to any one of claims 13 to 16, which has a diameter of 50-200 nm an average specific surface area of 500 m2. [Claim 2l]
The transition metal-carbon nanofiber catalyst for a fuel cell according to claim 17, which has a diameter of 50-200 nm and a length of 20 μπι or smaller.
PCT/KR2010/008892 2010-11-29 2010-12-13 Carbon nanofiber catalysts using nanofiber including low cost transition metal for fuel cells and manufacturing method thereof WO2012074160A1 (en)

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