JP2019081667A - Nitrogen-containing carbon material, production method thereof, and fuel cell electrode using the same - Google Patents

Abstract

To provide a nitrogen-containing carbon material that has a high oxygen reducing activity and is capable of maintaining the activity, i.e., has durability.SOLUTION: A nitrogen-containing carbon material is provided in which the coefficient of moisture absorption at 70°C at a relative humidity 90% is less than 20%.SELECTED DRAWING: None

Description

  The present invention relates to a nitrogen-containing carbon material, a method of producing the same, and a fuel cell electrode using the same.

The polymer electrolyte fuel cell has advantages such as high power generation efficiency, high power density, rapid start / stop capability, small size and light weight, portable power source, mobile power source, and small size. Application to stationary generators etc. is expected.
In a polymer electrolyte fuel cell, platinum or a platinum alloy is generally used as a catalyst to promote the oxygen reduction reaction that occurs at the positive electrode. However, the amount of platinum resources is very small and expensive, which is a major barrier to practical use. Therefore, a carbon material (hereinafter referred to as "nitrogen-containing carbon material") that has developed oxygen reduction activity by containing nitrogen as a fuel cell electrode catalyst that does not require a noble metal such as platinum has attracted attention.

  For example, in Patent Document 1, a nitrogen-containing carbon material is synthesized by carbonizing a mixture of a transition metal complex having a metal such as iron phthalocyanine, cobalt phthalocyanine, copper phthalocyanine and the like and a nitrogen-containing organic ligand, and a phenol resin, It has been proposed to use this as an electrode catalyst of a fuel cell.

It is known that in a fuel cell using a nitrogen-containing carbon material as an electrode catalyst, the power generation performance significantly decreases with time, that is, the durability decreases. Although the cause is not necessarily clear, it has been pointed out that water generated at the time of power generation may be accumulated near the active site and the supply of oxygen to the active site may be inhibited.
In order to improve the durability of a fuel cell, Non-Patent Document 1 discloses a method of reducing the nitrogen content and reducing the hydrophilicity by heat treating a nitrogen-containing carbon material at a high temperature.

JP 2012-101155 A

Electro-Kimika Actor, 2015, No. 159, pp. 184-197

In the method of improving the durability of a fuel cell of a nitrogen-containing carbon material in Non-Patent Document 1, there is a problem that the initial oxygen reduction activity is low because high temperature treatment is performed.
Therefore, the present invention has been made in view of the above circumstances, and has an object of providing a nitrogen-containing carbon material having high oxygen reduction activity and capable of maintaining the activity, that is, having durability. Do.

  As a result of intensive studies to solve the above problems, the present inventors have found that nitrogen-containing carbon materials having a specific range of moisture absorption at 70 ° C. and 90% relative humidity have high oxygen reduction activity, and And durability have been found, and the present invention has been completed.

That is, the present invention is as follows.
[1]
Nitrogen-containing carbon material whose moisture absorption rate at 70 ° C. and relative humidity 90% is less than 20%.
[2]
The nitrogen-containing carbon material according to [1], having a hydrophobic functional group on the surface.
[3]
The nitrogen-containing carbon material according to [2], wherein the hydrophobic functional group is an alkyl group and / or a fluoroalkyl group.
[4]
The method for producing a nitrogen-containing carbon material according to [2] or [3], including the step of introducing a hydrophobic functional group after reduction treatment of the nitrogen-containing carbon material of the raw material.
[5]
The fuel cell electrode using the nitrogen-containing carbon material in any one of [1]-[3].

  According to the present invention, it is possible to provide a nitrogen-containing carbon material in which high oxygen reduction activity and durability are compatible.

  Hereinafter, although a mode for carrying out the present invention (hereinafter, simply referred to as “the present embodiment”) will be described in detail, the present invention is not limited to this, and various modifications can be made within the scope of the present invention. Variations are possible.

[Nitrogen-containing carbon material]
The nitrogen-containing carbon material of the present embodiment has a moisture absorption rate of less than 20% at 70 ° C. and 90% relative humidity (hereinafter, the nitrogen-containing carbon material of the present embodiment is also referred to as “hydrophobic nitrogen-containing carbon material”) . The moisture absorption rate is preferably less than 18%, more preferably less than 15%.
The lower limit of the moisture absorption rate is not particularly limited, and 0% is ideal from the viewpoint of durability, but may be 5% or more.
When the moisture absorption rate is less than 20%, the durability tends to be improved when used as an electrode of a fuel cell. Although the details of the reason are not clear, it is considered that water accumulation near the active point of the nitrogen-containing carbon material is suppressed.

The method of making the moisture absorption rate less than 20% is not particularly limited. For example, from the viewpoint of achieving both high oxygen reduction activity and hydrophobicity, a relatively hydrophilic nitrogen-containing carbon material (hereinafter referred to as The method etc. which introduce | transduce a hydrophobic functional group are mentioned to "the nitrogen-containing carbon material of a raw material."
The moisture absorption rate at 70 ° C. and 90% relative humidity can be measured by the method described in the examples described later.

(Atomic ratio (N / C))
The atomic ratio (N / C) of nitrogen atom to carbon atom in the nitrogen-containing carbon material and the hydrophobic nitrogen-containing carbon material of the raw material is preferably 0.005 to 0.300, more preferably 0.010 It is -0.200, and more preferably 0.020 to 0.150.
When the atomic ratio (N / C) is in the above range, the oxygen reduction activity tends to be higher.
The atomic ratio (N / C) can be controlled by adjusting the ratio of the carbon source to the nitrogen source in the method for producing a nitrogen-containing carbon material serving as a source described later.
The atomic ratio (N / C) can be measured by the method described in the examples described later.

(Transition metal)
It is preferable that the raw material nitrogen-containing carbon material and the hydrophobic nitrogen-containing carbon material further contain a transition metal. The nitrogen-containing carbon material and the hydrophobic nitrogen-containing carbon material, which are raw materials, tend to exhibit high oxygen reduction activity by containing nitrogen and a transition metal. The transition metal atom is not particularly limited, and examples thereof include Fe, Co, Ni, Cu, Mn and Cr, preferably Fe, Co and Cu, more preferably Fe and Co. These transition metal atoms may be used alone or in combination of two or more.

The content of transition metal atoms in the nitrogen-containing carbon material and the hydrophobic nitrogen-containing carbon material of the raw material is preferably 0.1 to 20% by mass, more preferably 0.3 to 15% by mass, respectively. More preferably, it is 0.5 to 10% by mass, and still more preferably 0.5 to 3.0% by mass.
When the content of transition metal atoms is in the above range, the oxygen reduction activity tends to be higher.
The content rate of transition metal atoms can be controlled by adjusting the blending amount of transition metal raw materials.
The content of transition metal atoms can be measured by the method described in the examples below.

(Hydrophobic functional group)
The nitrogen-containing carbon material of the present embodiment is preferably a nitrogen-containing carbon material having a hydrophobic functional group on the surface, from the viewpoint of having high oxygen reduction activity and maintaining the activity.
The hydrophobic functional group is not particularly limited as long as it reduces the hydrophilicity of the nitrogen-containing carbon material of the raw material, but, for example, an alkyl group, a fluoroalkyl group and the like are preferable because of their high hydrophobicity. These hydrophobic functional groups may be used alone or in combination of two or more.

As an alkyl group, a C1-C20 alkyl group can be mentioned, for example, Specifically, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, tert -Butyl group, pentyl group, isopentyl group, neopentyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, nonadecyl Groups, eicosyl groups and the like.
Among these, preferred is an alkyl group having 4 to 20 carbon atoms, and more preferred is an alkyl group having 8 to 20 carbon atoms.

The fluoroalkyl group is a group in which at least one hydrogen atom of the alkyl group is substituted with a fluorine atom. As a fluoroalkyl group, for example, trifluoromethyl group, perfluoroethyl group, perfluoro n-propyl group, perfluoroisopropyl group, perfluoro n-butyl group, perfluoro t-butyl group, perfluoro n-hexyl group And perfluoroalkyl groups such as perfluoro n-octyl group, perfluoro n-decyl group, and perfluoro n-dodecyl group; and perfluoroethyl group, perfluoro n-propyl group, perfluoroisopropyl group, perfluoro A methyl group or an ethyl group having any one of n-butyl group, perfluoro t-butyl group, perfluoro n-hexyl group, perfluoro n-octyl group, perfluoro n-decyl group and perfluoro n-dodecyl group Or perfluoroalkylalkyl such as n-propyl group ; And the like.
Among these, an ethyl group having a perfluoro n-octyl group is preferable, that is, 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7, 8,8-heptadecafluorodecyl group.

The method for introducing the hydrophobic functional group is not particularly limited. For example, a method of reacting an oxygen functional group on the surface of the raw material nitrogen-containing carbon material with a reagent having a hydrophobic functional group is preferably used. it can.
As an oxygen functional group, a hydroxy group, a carboxy group, a carbonyl group etc. are mentioned, for example, From a high viewpoint of the reactivity with the reagent which has a hydrophobic functional group, it is preferably a hydroxy group.

[Method of producing nitrogen-containing carbon material]
It is preferable that the manufacturing method of the nitrogen-containing carbon material of this embodiment includes the process of introduce | transducing a hydrophobic functional group into the nitrogen-containing carbon material of the said raw material.
Further, from the viewpoint of increasing the amount of introduced hydrophobic group, it is preferable to reduce the carboxy group, carbonyl group, lactone structure and acid anhydride structure on the surface of the nitrogen-containing carbon material of the raw material into a hydroxy group.
That is, it is more preferable that the manufacturing method of the nitrogen-containing carbon material of this embodiment includes the process of introduce | transducing a hydrophobic functional group, after carrying out the reduction process of the nitrogen-containing carbon material which is a raw material.

The nitrogen-containing carbon material as the raw material can be produced from the carbon raw material, the nitrogen raw material, and, if necessary, the transition metal raw material.
The method for producing the nitrogen-containing carbon material of the raw material is, for example, a precursor preparation step of preparing a precursor from a composition containing a carbon raw material, a nitrogen raw material, and, if necessary, a transition metal raw material, It is preferable to have a heat treatment step of obtaining a nitrogen-containing carbon material, and a particle diameter adjustment step of grinding the carbonized material to adjust the particle diameter.

[Precursor preparation step]
The precursor preparation step is a step of preparing a precursor by combining a carbon source and a nitrogen source, or a carbon source, a nitrogen source and a transition metal source.

(precursor)
The precursor may be a composite of a carbon source and a nitrogen source, or may be a composite of a carbon source, a nitrogen source, and a transition metal source. The precursor can also contain other components as needed. The other component is not particularly limited, and examples thereof include compounds containing boron and / or phosphorus.

The carbon raw material, the nitrogen raw material, and the transition metal raw material are not particularly limited as long as they each contain a carbon atom, a nitrogen atom, and a transition metal atom, and one kind of compound may be used as a raw material of a plurality of atoms, A plurality of compounds may be used as a source of a certain atom. To use one kind of compound as a raw material of a plurality of atoms means, for example, to use only a metal phthalocyanine containing a carbon atom, a nitrogen atom and a transition metal atom as a precursor raw material. Moreover, using a plurality of compounds as a raw material of a certain atom means, for example, using carbon black containing carbon atoms and polyaniline containing carbon atoms and nitrogen atoms as raw materials of precursors. .
Here, “composite” may be a state in which the carbon raw material and the nitrogen raw material, or the carbon raw material, the nitrogen raw material, and the transition metal raw material are physically mixed, and the carbon raw material and the nitrogen raw material, or The carbon source, the nitrogen source and the transition metal source may form a chemical bond. It is preferable that each raw material in the precursor is uniformly dispersed.

(Carbon raw material)
Although it does not specifically limit as a carbon raw material, For example, the organic compound which has a carbon atom, and carbon material itself are mentioned. As the organic compound having a carbon atom, a compound having a yield of 1% by mass or more of a carbon material obtained by heat treatment at 1000 ° C. for 1 hour under nitrogen gas flow is preferable.

  The organic compound having a carbon atom is not particularly limited, and examples thereof include: phenol resin, polyfurfuryl alcohol, furan, furan resin, phenol formaldehyde resin, epoxy resin, polyvinylidene chloride, polythiophene, polysulfone, polyvinyl alcohol, polyvinyl butyral, Polyester, polylactic acid, polyether, polyetheretherketone, cellulose, carboxymethylcellulose, lignin, pitch, polycarbazole, polyacrylic acid, polyacrylic ester, polymethacrylic ester, polymethacrylic acid and the like can be mentioned. These may be used alone or in combinations of two or more.

  The carbon material is not particularly limited, and examples thereof include graphite, activated carbon, amorphous carbon, carbon black, coal, charcoal, coke, carbon nanotubes, fullerene, graphene and the like. These may be used alone or in combinations of two or more.

(Nitrogen source)
The nitrogen source is not particularly limited. For example, a low molecular weight organic compound having a nitrogen atom or a high molecular weight organic compound having a nitrogen atom may be used, and a mixture of two or more thereof may be used.

  Although it does not specifically limit as a low molecular weight organic compound which has a nitrogen atom, For example, the organic compound which has a number average molecular weight of less than 1000 which has a nitrogen atom is mentioned. Specific examples of the low molecular weight organic compound having a nitrogen atom include diaminomaleonitrile, phthalocyanine, porphyrin, phenanthroline, melamine, acrylonitrile, pyrrole, pyridine, vinylpyridine, aniline, imidazole, 1-methylimidazole, 2-methyl Imidazole, benzimidazole, pyridazine, pyrimidine, piperazine, quinoxaline, pyrazole, morpholine, hydrazine, hydrazide, urea, salen, triazine, cyanuric acid and the like. These may be used alone or in combinations of two or more.

  Although it does not specifically limit as a polymeric organic compound which has a nitrogen atom, For example, the organic compound which has a number average molecular weight 1000 or more which has a nitrogen atom is mentioned. Specific examples of the organic compound having a nitrogen atom include azulmic acid, diaminomaleonitrile polymer, melamine resin, polyamideimide resin, polyacrylonitrile, polyacrylonitrile-polymethacrylic acid copolymer, polypyrrole and polyvinylpyrrole. Polyvinylpyridine, polyaniline, polybenzimidazole, polyimide, polyamide, chitin, chitosan, polyamino acid, silk, hair, nucleic acid, DNA, RNA, polyurethane, polyamidoamine, polycarbodiimide, polybismaleimide, polyaminobismaleimide, etc. It can be mentioned. These may be used alone or in combinations of two or more.

  The atomic ratio (N / C) of the nitrogen-containing carbon material used in the present embodiment can be adjusted by using such a nitrogen source.

  Among these, as the nitrogen raw material and the carbon raw material, it is preferable to contain a phenol resin and azulmic acid and / or diaminomaleonitrile, and a phenol resin and diaminomaleonitrile are more preferable. By containing a phenolic resin and azulmic acid and / or diaminomaleonitrile, the oxygen reduction activity of the nitrogen-containing carbon material powder tends to be further improved. In addition, by containing diaminomaleonitrile having higher solubility in a solvent, complex formation between the transition metal and the nitrogen source and the carbon source preferably proceeds, and thus, the nitrogen-containing carbon material in which the transition metal is more uniformly dispersed. As a result, the oxygen reduction activity of the nitrogen-containing carbon material tends to be further improved.

(Transition metal raw materials)
The transition metal source is not particularly limited. For example, transition metal salts and transition metal complexes can be used, and a mixture of two or more of them may be used. The transition metal is not particularly limited, and examples thereof include Fe, Co, Ni, Cu, Mn, and Cr. Fe, Co, and Cu are preferable, and Fe and Co are more preferable. The transition metal salt is not particularly limited, and examples thereof include chloride, bromide, iodide, nitrate, sulfate, phosphorus oxide, acetate, cyanide and the like of the transition metal. Examples of transition metal complexes include acetylacetone complexes of transition metals, cyclopentadienyl complexes, phthalocyanine complexes, porphyrin complexes, phenanthroline complexes and the like. The nitrogen-containing carbon material of the present embodiment tends to further improve the oxygen reduction activity by including such a transition metal.

  The iron (Fe) salt is not particularly limited. For example, iron (II) chloride, iron (II) chloride tetrahydrate, iron (III) chloride, iron (III) chloride hexahydrate, iron bromide (II), iron (II) bromide hexahydrate, iron (III) bromide, iron (III) bromide hexahydrate, ammonium hexacyanoferrate (II) trihydrate, hexacyanoferrate (II) Potassium trihydrate hydrate, ammonium hexacyanoferrate (III), potassium hexacyanoferrate (III), sodium hexacyanoferrate (II) decahydrate, sodium hexacyanoferrate (III) monohydrate, iron nitrate (II ) Hexahydrate, iron (III) nitrate nonahydrate, iron (III) thiocyanate, iron (II) carbonate, iron (II) carbonate monohydrate, methyl ammonium hexachloroferrate (III), tetrachloro Iron (II) acid tetramethyl Ammonium, potassium pentacyanonitrosylferrate (III) dihydrate, potassium hexacyanoferrate (II) iron (III) hydrate, sodium pentacyanonitrosylferrate (III) dihydrate, ammine pentacyanoferrate ( II) Sodium acid trihydrate, sodium aquapentacyanoferrate (II) heptahydrate, iron thiocyanate iron (II) trihydrate, iron acetate, iron oxalate pentahydrate, oxalic acid Iron (II) dihydrate, iron (III) citrate trihydrate, iron (II) iodide, iron (II) iodide tetrahydrate, iron (III) sulfate, iron (III) sulfate Hydrate, ammonium tetrachloroferrate (II), iron (II) perchlorate hexahydrate, iron (III) perchlorate hexahydrate, potassium aquapentafluoroferrate (III), potassium iron sulfate (III) Twelve Water , Bis (sulfato) iron (II) diammonium hexahydrate, tris (sulfate) iron (III) trihydrate, iron (III) phosphate dihydrate, iron (II) phosphate Hydrates, iron (II) sulfate heptahydrate and the like can be mentioned. Among these, preferred are iron (II) chloride, iron (III) chloride, iron (II) bromide, iron (III) bromide, iron (III) nitrate nonahydrate. These may be used alone or in combinations of two or more.

  The cobalt (Co) salt is not particularly limited. For example, potassium hexacyanocobalt (III), cobalt (II) nitrate hexahydrate, cobalt (II) fluoride, cobalt (II) bromide, cobalt bromide (II) Hexahydrate, cobalt (II) carbonate, cobalt (II) thiocyanate trihydrate, cobalt (II) acetate tetrahydrate, cobalt (III) acetate, cobalt (II) chloride, cobalt chloride II) Hexahydrate, cesium tetrachlorocobalt (II), potassium hexafluorocobalt (III), cobalt (II) iodide, cobalt (II) iodide hexahydrate, hexanitrocobalt (III) Examples include potassium, cobalt (II) phosphate, cobalt (II) phosphate octahydrate, cobalt (II) sulfate, cobalt (II) sulfate heptahydrate, etc. It is. Among these, preferred are cobalt (II) chloride, cobalt (II) bromide and cobalt (II) nitrate hexahydrate. These may be used alone or in combinations of two or more.

The nickel (Ni) salt is not particularly limited, and examples thereof include nickel (II) chloride, nickel (II) chloride hexahydrate, nickel (II) bromide, nickel (II) nitrate hexahydrate and the like. Be
The copper (Cu) salt is not particularly limited, and examples thereof include copper (II) chloride, copper (II) chloride dihydrate, copper (II) bromide, copper (II) nitrate dihydrate and the like. Be
The manganese (Mn) salt is not particularly limited, and examples thereof include manganese (II) chloride, manganese (II) bromide, manganese (II) nitrate tetrahydrate and the like.
The chromium (Cr) salt is not particularly limited, and examples thereof include chromium (III) chloride, chromium (III) bromide, and chromium (III) nitrate nonahydrate.
These may be used alone or in combinations of two or more.

  The content of transition metal atoms in the precursor is preferably 0.01% by mass to 10% by mass, more preferably 0.03% by mass to 5% by mass, still more preferably 0.05% by mass It is 3% by mass. When the content of transition metal atoms in the precursor is in the above range, the oxygen reduction activity of the nitrogen-containing carbon material tends to be further improved.

In addition, since the nitrogen content in the nitrogen-containing carbon material obtained by the heat treatment step is largely different depending on the carbon raw material and the nitrogen raw material used, the atomic ratio (N / C) of nitrogen atom to carbon atom of nitrogen-containing carbon material is desired It is preferable to adjust the ratio of the carbon source and the nitrogen source so as to be in the range of
The atomic ratio (N / C) can be controlled to increase by using a nitrogen source having a high atomic ratio (N / C) and / or increasing the ratio of the nitrogen source in the precursor, the atomic ratio It can be controlled to be small by using a low (N / C) nitrogen source and / or reducing the ratio of the nitrogen source in the precursor.

The method of combining the nitrogen source, carbon source, and transition metal source is not particularly limited. For example, a method in which each source is used in advance as a precursor is used as a precursor, and after each source is dissolved in a solvent, it is evaporated to dryness. The method of hardening, the method of physically mixing each raw material with a ball mill etc., etc. are mentioned.
The method for forming a composite is not particularly limited. For example, all the raw materials may be dissolved in one solvent, or the respective raw materials may be mixed after being dissolved in different solvents. The raw material “each raw material is complexed in advance” means, for example, a precursor such as an iron phthalocyanine complex which is a single compound to be a nitrogen raw material, a carbon raw material, and a transition metal raw material.

  The solvent is not particularly limited, and a solvent having high solubility of each raw material may be appropriately selected. The solvents may be used alone or in combination of two or more.

[Heat treatment process]
The heat treatment step is a step of heat treating a precursor containing a carbon source and a nitrogen source to obtain a nitrogen-containing carbon material. The heat treatment process may be a one-step heat treatment, or two or more heat treatments.
In addition, when the heat treatment step is performed in two or more steps, another step may be incorporated therebetween.
The heat treatment step includes a first heat treatment step of heat treating the precursor in an inert gas atmosphere, and a second heat treatment step of heat treating the precursor treated in the first heat treatment step in an ammonia-containing gas atmosphere Preferably,.
Among them, in the heat treatment step, the nitrogen-containing carbon material obtained in the first heat treatment step of heat-treating the precursor in an inert gas atmosphere and the first heat treatment step is pulverized to adjust the average particle diameter. It is preferable that the particle diameter adjusting step and the second heat treatment step of heat treating the nitrogen-containing carbon material powder whose average particle diameter is adjusted in an ammonia-containing gas atmosphere be performed in this order. The first heat treatment step in an inert gas atmosphere mainly aims at carbonization, and the second heat treatment step in an ammonia-containing gas atmosphere mainly aims at activation, By performing the heat treatment step, a nitrogen-containing carbon material powder which is more excellent in oxygen reduction activity tends to be obtained.

(First heat treatment process)
In the first heat treatment step, the precursor is heat-treated under an inert gas atmosphere. The inert gas is not particularly limited, and, for example, nitrogen, a rare gas, vacuum or the like can be used. The heat treatment temperature under an inert gas atmosphere is preferably 400 to 1500 ° C., more preferably 500 to 1400 ° C., and still more preferably 550 to 1300 ° C. When the heat treatment temperature is 400 ° C. or more, carbonization of the precursor tends to proceed sufficiently. Further, when the heat treatment temperature is 1500 ° C. or less, a sufficient yield tends to be obtained.
The heat treatment time under an inert gas atmosphere is preferably 5 minutes to 50 hours, more preferably 10 minutes to 20 hours, and still more preferably 20 minutes to 10 hours. When the heat treatment time is 5 minutes or more, carbonization of the precursor tends to proceed sufficiently. In addition, when the heat treatment time is 50 hours or less, a sufficient yield tends to be obtained.

(Second heat treatment process)
In the second heat treatment step, the precursor treated in the first heat treatment step is heat-treated in an ammonia-containing gas atmosphere. The target of the heat treatment in the second heat treatment step may be the nitrogen-containing carbon material powder after performing the pore forming step as described above.
The ammonia-containing gas is not particularly limited. For example, it is preferable to use only ammonia or a gas obtained by diluting ammonia with nitrogen or a rare gas. The heat treatment temperature in the ammonia-containing gas atmosphere is preferably 600 to 1200 ° C., more preferably 700 to 1100 ° C., and still more preferably 800 to 1050 ° C. When the heat treatment temperature is 600 ° C. or more, activation of the nitrogen-containing carbon material powder in which the pores are formed is sufficiently advanced, and a nitrogen-containing carbon material powder having excellent oxygen reduction activity tends to be obtained. In addition, when the heat treatment temperature is 1200 ° C. or less, a sufficient yield tends to be obtained.

  The heat treatment time in the ammonia-containing gas atmosphere is preferably 5 minutes to 5 hours, more preferably 10 minutes to 3 hours, and still more preferably 15 minutes to 2 hours. When the heat treatment time is 5 minutes or more, activation of the nitrogen-containing carbon material powder in which the pores are formed is sufficiently advanced, and a nitrogen-containing carbon material powder which is excellent in oxygen reduction activity tends to be obtained. In addition, when the heat treatment time is 5 hours or less, a sufficient yield tends to be obtained.

(Transition metal removal process)
Before or after the first heat treatment step in an inert gas atmosphere and / or the second heat treatment step in an ammonia-containing gas atmosphere, part of the transition metal may be removed using hydrochloric acid, sulfuric acid, or the like. In particular, when transition metal particles are formed on the surface by heat treatment, it is preferable to remove the transition metal particles after the heat treatment step from the viewpoint of suppressing the increase in the degree of crystallinity. The ease of transition metal particles varies depending on the type, concentration, dispersibility, or heat treatment temperature of the transition metal. In order to increase the removal rate of transition metal particles, the first heat treatment step under an inert gas atmosphere and / or the second heat treatment step under an ammonia-containing gas atmosphere is divided into a plurality of sub-steps, and heat treatment is performed. It is also preferable to repeat the process and the transition metal removal process.

[Average particle diameter adjustment step]
The average particle size (volume average particle size) of the nitrogen-containing carbon material is preferably 1 nm or more and 100 μm or less, and preferably 5 nm or more and 10 μm or less, from the viewpoint of activity as an electrode catalyst and material transport in the electrode. The thickness is more preferably 10 nm or more and 1 μm or less. The method of adjusting the particle diameter of the nitrogen-containing carbon material is not particularly limited, and the average particle diameter of the precursor may be controlled in the precursor preparation step, and the nitrogen-containing carbon material after the heat treatment step is pulverized and adjusted. May be When controlling the average particle size of the precursor in the precursor preparation step, there is no particular limitation. For example, a method of granulating a solution containing a carbon raw material, a nitrogen raw material and a transition metal raw material by a spray dryer, Can be used. Further, the method for pulverization is not particularly limited, and examples thereof include a method of pulverizing the precursor or the nitrogen-containing carbon material by a dry ball mill, a dry jet mill, a wet bead mill, a wet jet mill or the like.

Hydrophobing step
As described above, the method for producing a nitrogen-containing carbon material of the present embodiment preferably includes the step of introducing a hydrophobic functional group into the nitrogen-containing carbon material of the above-mentioned raw material. By contacting the reagent having a group, the oxygen functional group on the surface of the nitrogen-containing carbon material of the raw material is reacted with the reagent.
Further, in the method for producing a nitrogen-containing carbon material of the present embodiment, as described above, it is preferable to subject the nitrogen-containing carbon material of the raw material to reduction treatment.

In the reduction treatment of the nitrogen-containing carbon material of the raw material, the nitrogen-containing carbon material of the raw material is reacted with a reducing agent in the presence or absence of a solvent.
The reducing agent is not particularly limited as long as it can be converted to a hydroxy group by reducing a functional group such as a carboxy group, a carbonyl group, a lactone structure or an acid anhydride structure present on the surface of the nitrogen-containing carbon material of the raw material. For example, sodium borohydride, borane-tetrahydrofuran complex, lithium aluminum hydride and the like can be mentioned. Among these, sodium borohydride and borane-tetrahydrofuran complex are preferable from the viewpoint of the ease of removal of the salt by-produced after the reaction.
The amount of the reducing agent is not particularly limited, but is usually 0.1 to 100 times by mass, preferably 0.5 to 10 times by mass, more preferably 0 based on the mass of the nitrogen-containing carbon material of the raw material. It is .5-5.0 mass times.

The temperature when reacting with the reducing agent may be appropriately adjusted according to the reducing agent used, but from the viewpoint of promoting the reduction reaction, it is usually -20 ° C to 100 ° C, preferably 0 ° C to 80 ° C. C., more preferably 0.degree. C. to 40.degree. In order to handle the reducing agent safely and to allow the reduction reaction to proceed mildly, the temperature may be gradually raised to 4 ° C. or less when adding the reducing agent to the nitrogen-containing carbon material of the raw material.
The reaction time with the reducing agent may be appropriately adjusted depending on the reducing agent used, but is usually 0.5 hours to 3 days, preferably 1 hour to 2 days, more preferably 5 hours to 24 It's time.

The process of introducing a hydrophobic functional group may be performed without stopping the reduction reaction, removing the reducing agent from the reaction mixture, adding an aqueous acid solution such as aqueous hydrochloric acid to deactivate the reducing agent, etc. The reaction may be terminated and then the subsequent step of introducing a hydrophobic functional group may be performed.
The nitrogen-containing carbon material of the raw material after the reduction treatment is preferably purified by filtration, washing with water, drying and the like, and then subjected to the step of introducing a hydrophobic functional group.

  In the step of introducing a hydrophobic functional group, the nitrogen-containing carbon material of the raw material after reduction treatment in the presence or absence of a solvent or in the presence or absence of a base, a reagent having a hydrophobic functional group, and Contact

As the reagent having a hydrophobic functional group, for example, a halide having high reactivity with a hydroxy group is preferable.
As a reagent which has a hydrophobic functional group, when introduce | transducing an alkyl group, alkyl iodides, such as methyl iodide, an ethyl iodide, 1-i-propyl iodide etc., are mentioned, for example.
When a fluoroalkyl group is introduced as a reagent having a hydrophobic functional group, for example, perfluoroalkyl iodide or 1,1,1,2,2,3,3,4,4,5,5,6 And perfluoroalkylalkyl iodides such as 6,7,7,8,8,8-heptadecafluoro-10-iododecane and the like.
The amount of the reagent having a hydrophobic functional group is not particularly limited, but is usually 0.1 to 100 times by mass, preferably 0.5 to 10 times by mass the mass of the nitrogen-containing carbon material of the raw material. And more preferably 0.5 to 5.0 times by mass.

The base is not particularly limited as long as it can extract hydrogen of the hydroxyl group contained in the nitrogen-containing carbon material of the raw material, and examples thereof include sodium hydride, lithium diisopropylamide (LDA) and sodium bis (trimethylsilyl) amide ( NaHMDS) and the like. Among these, preferred is sodium hydride.
The amount of the base is not particularly limited, but is usually 0.1 to 100 times by mass, preferably 0.5 to 10 times by mass, more preferably 0. 1 to 10 times by mass the mass of the nitrogen-containing carbon material of the raw material. It is 5 to 5.0 mass times.

The reaction temperature in the step of introducing a hydrophobic functional group may be appropriately adjusted according to the reagent or base having a hydrophobic functional group to be used, but from the viewpoint of advancing the reaction, it is usually -20 ° C to 100 ° C. Preferably it is 0 degreeC-80 degreeC, More preferably, it is 0 degreeC-40 degreeC. In addition, from the viewpoint of handling the base safely and allowing the reaction to proceed gently, the temperature should be 4 ° C or lower when adding a reagent or base with a hydrophobic functional group to the nitrogen-containing carbon material of the raw material. May warm.
The reaction time may be appropriately adjusted according to the reagent or base having a hydrophobic functional group to be used, but is usually 0.5 hour to 24 hours, preferably 1 hour to 12 hours, and more preferably 1 It is 10 hours.

After completion of the reaction, the reaction may be stopped by removing the base from the reaction mixture or by adding an aqueous acid solution such as aqueous hydrochloric acid to neutralize the base.
In addition, the nitrogen-containing carbon material obtained may be appropriately purified by filtration, washing with water, drying, and the like, and then made to be a nitrogen-containing carbon material that has been made hydrophobic.

〔Fuel cell〕
The hydrophobic nitrogen-containing carbon material can be used for the positive electrode of a polymer electrolyte fuel cell, an anion exchange membrane fuel cell, a direct methanol fuel cell or the like. Although the structure of the fuel cell is not particularly limited, it mainly comprises a membrane electrode assembly (MEA), a positive electrode separator and a negative electrode separator.

[Manufacturing method of MEA]
The method for producing MEA is not particularly limited, and, for example, both the negative electrode side and the positive electrode side of an ink in which the components of the catalyst layer (positive electrode catalyst layer and negative electrode catalyst layer are collectively referred to as catalyst layer) are dispersed in a solvent A method may be prepared, applied to each of the one main surface and the other main surface of the polymer electrolyte membrane and dried, and pressure-bonded to the gas diffusion layer with a microporous layer after drying. In addition, the ink is applied to a film such as polytetrafluoroethylene and dried, and the polymer electrolyte membrane is sandwiched between the negative electrode side and the positive electrode side by thermocompression bonding, and then the films on both sides are peeled off to form a gas diffusion layer with a microporous layer. The method of crimping is mentioned. In addition, a general method such as a method of applying and drying the above-mentioned ink on a gas diffusion layer with a microporous layer, and thermocompression bonding sandwiching a polymer electrolyte membrane on the negative electrode side and the positive electrode side can be used. As described in the above section, the microporous layer-provided gas diffusion layer can be a commercially available one.

[Manufacturing method of negative electrode]
The electrode catalyst of the negative electrode catalyst layer is not particularly limited, but, for example, a general negative electrode of a direct methanol fuel cell such as a catalyst having platinum fine particles supported on carbon black, a catalyst having fine particles of platinum ruthenium alloy supported on carbon black Electrode catalysts can be used. As the polymer electrolyte, Nafion dispersion can be preferably used. As the solvent, water, lower alcohols (methanol, ethanol, isopropyl alcohol, normal propyl alcohol and the like) and mixtures thereof can be preferably used. 0.1-2 are preferable, 0.2-1 are more preferable, and, as for the ratio of (polyelectrolyte weight) / (catalyst weight), 0.3-0.8 are still more preferable. The ratio of the solvent to the solid content is not particularly limited, and it is preferable to adjust to a viscosity suitable for the coating apparatus to be used. The coating apparatus is not particularly limited, and for example, general methods such as a bar coater, a spray coater, and a screen printer can be used.

[Manufacturing method of positive electrode]
Hydrophobic nitrogen-containing carbon materials can be used as positive electrode catalysts. As the polymer electrolyte, Nafion dispersion can be preferably used. As the solvent, water, lower alcohols (methanol, ethanol, isopropyl alcohol, normal propyl alcohol and the like) and mixtures thereof can be preferably used. 0.1-5 are preferable, 0.2-3 are more preferable, and, as for the ratio of (polymer electrolyte weight) / (catalyst weight), 0.3-2 are still more preferable. The ratio of the solvent to the solid content is not particularly limited, and it is preferable to adjust to a viscosity suitable for the coating apparatus to be used. The coating apparatus is not particularly limited, and for example, general methods such as a bar coater, a spray coater, and a screen printer can be used.

[Use]
The above-mentioned hydrophobic nitrogen-containing carbon material can be used as a positive electrode catalyst of a polymer electrolyte fuel cell.

  The present embodiment will be described in more detail by way of examples and the like, but these are illustrative, and the present embodiment is not limited to the following examples. Those skilled in the art can implement the present embodiment as various modifications to the examples shown below, and such modifications are included in the scope of the present invention.

  The analysis method was as follows.

(Hygroscopicity)
The nitrogen-containing carbon material obtained in Examples and Comparative Examples was introduced into TG-DTA / HUM-1 manufactured by Rigaku, and first, the sample was dried at 100 ° C. for 1 hour under nitrogen gas flow. Thereafter, the sample temperature was lowered to 70 ° C., nitrogen gas humidified to 70% relative humidity at 90 ° C. was circulated, and held for 2 hours, and the moisture absorption rate was measured according to the following formula (1).
Moisture absorption rate (%) = {(humidified sample mass)-(dry sample mass)} / (dry sample mass) x 100 ... Formula (1)

(N / C)
The mass concentrations of carbon and nitrogen in the nitrogen-containing carbon materials obtained in Examples and Comparative Examples were measured according to JIS M 8819 "Coals and cokes-Elemental analysis method by instrumental analyzer". The atomic ratio was converted to N / C.
As a measuring apparatus, CHN coder MT-6 manufactured by Yanako Technical Science was used.

(Content of transition metal)
The nitrogen-containing carbon material obtained in the examples and comparative examples is heat-treated at 900 ° C. for 1 hour in an air stream to burn off carbon and nitrogen, and the residue is dissolved in aqua regia and subjected to ICP emission spectral analysis. The content of transition metal in the carbon material was measured.
As a measuring apparatus, SPS3520UV-DD made by SII Nano Technology was used.

(Oxygen reduction activity measurement)
Measurement of linear sweep voltammetry by an electrode production method and a rotating electrode method (using a rotating ring disk electrode device "HR-301" and a potentiostat "HZ-5000" by Hokuto Denko) used in Examples and Comparative Examples is as follows. It went as follows.
First, 5 mg of the nitrogen-containing carbon material prepared in Example or Comparative Example is weighed into a vial, and a glass spatula is filled with glass beads, and 50 μL of 5 mass% Nafion dispersion (manufactured by Sigma Aldrich Japan) and ions 150 μL each of exchange water and ethanol were added, and the mixture was irradiated with ultrasonic waves for 20 minutes to prepare a slurry.
4 μL of this slurry was weighed, applied onto glassy carbon (0.2828 cm ² ) of a rotating electrode, and vacuum dried. The rotating electrode after drying was used as a working electrode, and the reversible hydrogen electrode (RHE) was used as a reference electrode, and the carbon electrode was used as a counter electrode. 0.5 M sulfuric acid was used as an electrolyte, and nitrogen was first bubbled in the electrolyte for 30 minutes, and then sweeping from 1.1 V to 0 V at a sweep rate of 5 mV / s and a rotational speed of 1500 rpm to obtain a voltammogram.
Subsequently, after bubbling oxygen for 30 minutes, a voltammogram was obtained under the same conditions as above. The difference between the voltammogram under oxygen and the voltammogram under nitrogen is the oxygen reduction current, and the potential at which the oxygen reduction current becomes −10 μA / cm ² is the initiation potential.

(Durability measurement)
<MEA production>
An MEA was produced by the following method from the nitrogen-containing carbon material produced in Examples or Comparative Examples. First, 50 mg of a nitrogen-containing carbon material, 200 mg of ethanol and 1.5 g of 1 mmφ zirconia beads were placed in a microcentrifuge tube and stirred for 10 minutes at 2850 rpm using a disrupter homogenizer (manufactured by Scientific Industries). Next, add a 20% Nafion solution (Wako Pure Chemical Industries, Ltd., DE 2021 CS) so that I / C (= mass of Nafion / mass of nitrogen-containing carbon material) becomes a predetermined value, and then add Disruptor Homogenizer The mixture was stirred for 15 minutes at 2850 rpm to prepare a positive electrode ink. In addition, I / C was adjusted so that the electric power generation amount of MEA became the maximum for every nitrogen containing carbon material to be used. This positive electrode ink was applied to a gas diffusion layer with a microporous layer (SGL, 29BC) with a screen printer (MEC-2400, manufactured by Mitani Micronics, Inc.), and dried at 120 ° C. for 30 minutes. The application amount of the nitrogen-containing carbon material was 2 mg / cm ² . On the positive electrode catalyst layer-positive electrode microporous layer-positive electrode gas diffusion layer thus prepared, Nafion membrane (manufactured by DuPont, NR211), platinum-ruthenium supported catalyst-attached gas diffusion layer (Kycoms Co., Ltd., platinum-ruthenium) 0.5 mg / cm ² loading was placed thereon, and thermocompression bonding was performed at 150 ° C. and 0.2 MPa to obtain MEA.
<Fuel cell operation>
The above-mentioned power generation evaluation of MEA was carried out with a potential galvanostat (manufactured by Solartron, 1285A). Hydrogen (0.2MPa, 80 ° C, relative humidity 100%, flow rate 300cc / min) is supplied to the negative electrode, and air (0.2MPa, 80 ° C, relative humidity 40%, flow rate 300cc / min) atmospheric pressure to the positive electrode The cell temperature was brought to 80 ° C., and continuous operation was performed at 0.4 V for 100 hours. The current holding ratio (%) = (current value at 0.4 V after continuous operation) / (current value at 0.4 V before continuous operation) × 100, which was used as an indicator of durability.

Comparative Example 1
<Precursor preparation process>
6 g of diaminomaleonitrile, 6 g of phenol resin (Gunei Chemical Industries, Ltd., Redtop PSK-2320), 0.082 g of iron (II) chloride and 400 cc of methanol are added to a 1 L eggplant type flask, and the components are dissolved at room temperature Stir until until. After that, the solvent was removed using a rotary evaporator in a water bath at 60 ° C., and dried at 80 ° C. for 2 hours in a vacuum dryer to obtain a precursor.
<First heat treatment process>
12 g of the prepared precursor was placed on an alumina boat, which was housed in a box furnace. The temperature in the furnace was raised from room temperature to 800 ° C. over 60 minutes under nitrogen flow at atmospheric pressure and 1 NL / min, and held at 800 ° C. for 1 hour to obtain 6.6 g of a sample.
<Average particle diameter adjustment process>
A sample in which the average particle diameter is adjusted to 980 nm by dry-grinding the first heat treatment step sample with a planetary ball mill (using Pulverisette-5 manufactured by Fritsch Japan Ltd.) using zirconia balls of 10 mm in diameter I got
<Second heat treatment process>
The above-mentioned 6.6 g of the sample adjusted to the average particle size was placed on a quartz boat, and it was housed in a quartz tubular furnace with an inner diameter of 36 mm. The temperature in the furnace was raised from room temperature to 975 ° C. over 60 minutes under ammonia gas flow at atmospheric pressure and 2 NL / min, and held at 975 ° C. for 1 hour to obtain 3.3 g of a nitrogen-containing carbon material.
<Physical evaluation>
The moisture absorption rate measurement, the N / C measurement, the transition metal content measurement, and the oxygen reduction activity measurement were performed by the above-mentioned method. The results are shown in Table 1.
<Durability evaluation>
MEA was produced (I / C = 1.25) by the above-mentioned method, and endurance measurement was performed. The current retention rate was 36%.

Example 1
<Reduction treatment process>
The nitrogen-containing carbon material obtained in Comparative Example 1 was used as a nitrogen-containing carbon material as a raw material.
First, 50 mg of a nitrogen-containing carbon material as a raw material and 20 mL of tetrahydrofuran were added to a 50 mL eggplant type flask and cooled to 0 ° C. After 56 mg of sodium borohydride was added little by little there, it returned to room temperature and stirred overnight. Thereafter, while the flask was ice-cooled, several drops of ion exchange water were added dropwise, 30 mL of a hydrochloric acid aqueous solution of pH = 3 was added, the solution was returned to room temperature, stirred for 3 hours, and filtration was performed with a membrane filter. The residue on the filter was washed with 50 mL of pH 3 aqueous hydrochloric acid solution and 50 mL of ion-exchanged water, and then dried under reduced pressure at room temperature to obtain 51 mg of a reduced sample.
Hydrophobing process
15 mg of the above-mentioned reduced sample and 10 mL of tetrahydrofuran were added to a 50 mL eggplant type flask, cooled to 0 ° C., 22 mg of sodium hydride and 27 μL of methyl iodide were added. After returning to room temperature and stirring for 3 hours, 10 mL of ion exchange water was dropped, and filtration was performed with a membrane filter. The residue on the filter was washed with 100 mL of hexane and 100 mL of ethanol, dried at room temperature under reduced pressure, and dried at room temperature under reduced pressure to obtain 15 mg of a hydrophobicized nitrogen-containing carbon material.
<Physical evaluation>
The moisture absorption rate measurement, the N / C measurement, the transition metal content measurement, and the oxygen reduction activity measurement were performed by the above-mentioned method. The results are shown in Table 1.
<Durability evaluation>
An MEA was produced (I / C = 0.73) by the above-mentioned method, and the durability measurement was performed. The current retention rate was 44%.

Comparative Example 2
Hydrophobing process
The nitrogen-containing carbon material obtained in Comparative Example 1 was used as a nitrogen-containing carbon material as a raw material.
First, 15 mg of a nitrogen-containing carbon material as raw material and 20 mL of acetone were added to a 50 mL eggplant type flask, cooled to 0 ° C., and 10 mg of sodium hydride and 2.7 μL of methyl iodide were added. After returning to room temperature and stirring for 3 hours, 10 mL of ion exchange water was dropped, and filtration was performed with a membrane filter. The residue on the filter was washed with 50 mL of ion exchanged water and then dried under reduced pressure at room temperature to obtain 15 mg of a sample.
<Physical evaluation>
The moisture absorption rate measurement, the N / C measurement, the transition metal content measurement, and the oxygen reduction activity measurement were performed by the above-mentioned method. The results are shown in Table 1.

Comparative Example 3
<Physical evaluation>
The reduction treatment sample obtained in the reduction treatment step of Example 1 was subjected to the measurement of moisture absorption, measurement of N / C, measurement of transition metal content, and measurement of oxygen reduction activity by the method described above. The results are shown in Table 1.

Example 2
<Reduction treatment process>
The nitrogen-containing carbon material obtained in Comparative Example 1 was used as a nitrogen-containing carbon material as a raw material.
First, 50 mg of a nitrogen-containing carbon material as a raw material and 20 mL of tetrahydrofuran were added to a 50 mL eggplant type flask and cooled to 0 ° C. To this, 0.31 mL of borane-tetrahydrofuran complex (8.5% tetrahydrofuran solution) was added little by little, and then returned to room temperature and stirred for 4 hours. Thereafter, while the flask was ice-cooled, 50 mL of ion exchanged water was dropped, and filtration was performed with a membrane filter. The residue on the filter was washed with 300 mL of ion exchanged water and then dried under reduced pressure at room temperature to obtain 51 mg of a sample.
Hydrophobing process
The hydrophobization was carried out in the same manner as the hydrophobization step of Example 1.
<Physical evaluation>
The moisture absorption rate measurement, the N / C measurement, the transition metal content measurement, and the oxygen reduction activity measurement were performed by the above-mentioned method. The results are shown in Table 1.

[Example 3]
A nitrogen-containing carbon material was synthesized in the same manner as in Example 1 except that methyl iodide in Example 1 was changed to t-butyl iodide, and the physical properties were evaluated. The results are shown in Table 1.

Example 4
A nitrogen-containing carbon material was synthesized by the same method as in Example 1 except that methyl iodide in Example 1 was changed to n-decyl iodide, and physical property evaluation was performed. The results are shown in Table 1.

[Example 5]
Example 1 methyl iodide to 1,1,1,2,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluoro-10-iododecane A nitrogen-containing carbon material was synthesized in the same manner as in Example 1 except for the change, and the physical properties were evaluated. The results are shown in Table 1.
Moreover, MEA was produced by the above-mentioned method (I / C = 0.53), and durability measurement was implemented. The current retention rate was 48%.

  The nitrogen-containing carbon material of the present invention has industrial applicability as a fuel cell electrode.

Claims (5)

  Nitrogen-containing carbon material whose moisture absorption rate at 70 ° C. and relative humidity 90% is less than 20%.   The nitrogen-containing carbon material according to claim 1, having a hydrophobic functional group on the surface.   The nitrogen-containing carbon material according to claim 2, wherein the hydrophobic functional group is an alkyl group and / or a fluoroalkyl group.   The method for producing a nitrogen-containing carbon material according to claim 2 or 3, further comprising the step of introducing a hydrophobic functional group after reduction treatment of the raw material nitrogen-containing carbon material.   A fuel cell electrode using the nitrogen-containing carbon material according to any one of claims 1 to 3.