JP6257910B2 - Nitrogen-containing carbon material, method for producing the same, and electrode for fuel cell - Google Patents

Description

  The present invention relates to a nitrogen-containing carbon material, a method for 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 output density, rapid start / stop, small size and light weight, portable power, mobile power, small stationary It is expected to be applied to power generators.

  In a polymer electrolyte fuel cell, platinum or a platinum alloy is generally used as a catalyst in order to promote the oxygen reduction reaction occurring at the positive electrode. However, since the amount of platinum resources is extremely small and expensive, it is put to practical use. It has become a big barrier. Accordingly, carbon materials (hereinafter also referred to as “carbon catalysts”) that have developed oxygen reduction activity by containing transition metals and nitrogen as electrode catalysts for fuel cells that do not require noble metals such as platinum are attracting attention. .

  For example, in Patent Document 1, a carbon material having a large shell-like structure of 10 to 20 nm is synthesized by carbonizing a resin containing a transition metal such as cobalt, and nitrogen or It has been proposed to use a material into which nitrogen and boron are introduced as an electrode catalyst. Here, the (002) plane diffraction line of carbon having a shell-like structure appears as a sharp component in the X-ray diffraction diagram.

  In Patent Document 2, a carbon material having a large amorphous structure is synthesized by carbonizing a mixture of a transition metal complex having a nitrogen-containing organic ligand such as iron phthalocyanine, cobalt phthalocyanine, and copper phthalocyanine and a phenol resin. It has been proposed to use as an electrode catalyst. Here, the (002) plane diffraction line of amorphous carbon appears as a flat component in the X-ray diffraction diagram.

JP 2007-207662 A JP2012-101155A

  However, the above-mentioned catalyst does not have sufficient oxygen reduction activity as an electrode catalyst for a polymer electrolyte fuel cell, and development of an electrode catalyst having superior oxygen reduction activity is required.

  The oxygen reduction active site of carbon catalyst containing transition metal and nitrogen is a structure in which nitrogen is doped at the end or defect part of graphene skeleton, or its transition metal complexed with nitrogen, although the details are not clear it is conceivable that. A carbon catalyst mainly comprises a shell-like structural component and an amorphous structural component. Since the shell-like structural component is graphitized more than the amorphous structural component, the terminal portion and the defect portion are few, the nitrogen content is small, and the crystallinity is high. Therefore, when the content of nitrogen or transition metal and the specific surface area are in appropriate ranges, it is considered that the one with lower crystallinity (one with less shell-like structural components) can be highly active.

  However, there is no known method for synthesizing an amorphous structure while keeping the nitrogen content, transition metal content, and specific surface area within appropriate ranges.

  Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a nitrogen-containing carbon material having high oxygen reduction activity, a method for producing the same, and a fuel cell electrode using the same.

  As a result of intensive studies in order to solve the above problems, the inventors of the present invention have a nitrogen content, transition metal content, specific surface area, and a nitrogen-containing carbon material whose crystallinity is adjusted within a predetermined range. The present inventors have found that the above problems can be solved and have come to the present invention.

That is, the present invention is as follows.
[1]
The atomic ratio (N / C) of nitrogen atoms and carbon atoms obtained by an electron beam microanalyzer is 0.005 to 0.3,
The atomic ratio (M / C) of transition metal atoms to carbon atoms obtained by an electron beam microanalyzer is 0.0001 to 0.05,
The specific surface area determined by the BET method is 400 m ² / g or more,
The degree of crystallinity obtained from an X-ray diffraction diagram is 0.1 or less,
Nitrogen-containing carbon material.
[2]
The nitrogen-containing carbon material according to [1] above, wherein the transition metal atom is iron and / or cobalt.
[3]
A method for producing a nitrogen-containing carbon material according to [1] or [2] above,
A method for producing a nitrogen-containing carbon material, comprising a heat treatment step of heat treating a precursor obtained by combining a carbon raw material, a nitrogen raw material, and a transition metal raw material.
[4]
The method for producing a nitrogen-containing carbon material according to [3], wherein the precursor contains at least one selected from the group consisting of a compound having a quinolizinium structure and azulmic acid.
[5]
The method for producing a nitrogen-containing carbon material as described in [3] or [4] above, wherein in the heat treatment step, a heat treatment in an inert gas atmosphere and a heat treatment in an ammonia-containing gas atmosphere are performed.
[6]
A fuel cell electrode comprising the nitrogen-containing carbon material according to [1] or [2].

  ADVANTAGE OF THE INVENTION According to this invention, the nitrogen-containing carbon material which has high oxygen reduction activity, its manufacturing method, and the electrode for fuel cells using the same can be provided.

It is a figure which shows an example of intensity | strength correction | amendment and background correction | amendment performed by the measurement of crystallinity. In FIG. 1, it is a figure which shows an example of isolation | separation of the peak of a sharp component, and the peak of a flat component. It is a figure which shows the result of the electrochemical measurement in an Example and a comparative example.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail with reference to the drawings as necessary. However, the present invention is not limited to this. Various modifications are possible without departing from the scope of the invention.

[Nitrogen-containing carbon material]
The nitrogen-containing carbon material according to this embodiment is
The atomic ratio (N / C) of nitrogen atoms (N) and carbon atoms (C) obtained by an electron beam microanalyzer is 0.005 to 0.3,
The atomic ratio (M / C) of transition metal atoms (M) and carbon atoms (C) obtained by an electron beam microanalyzer is 0.0001 to 0.05,
The specific surface area determined by the BET method is 400 m ² / g or more,
The degree of crystallinity obtained from the X-ray diffraction diagram is 0.1 or less.

[Atomic ratio (N / C)]
In the present embodiment, the atomic ratio (N / C) of nitrogen atoms and carbon atoms obtained by an electron beam microanalyzer is 0.005 to 0.3, preferably 0.01 to 0.2. It is more preferable that it is 0.02-0.15. When the atomic ratio (N / C) is within the above range, the oxygen reduction activity becomes higher.

[Atomic ratio (M / C)]
Moreover, in this embodiment, the atomic ratio (M / C) of the transition metal atom and carbon atom obtained by an electron beam microanalyzer is 0.0001 to 0.05, and is 0.001 to 0.04. Is more preferable, and 0.005-0.03 is more preferable. When the atomic ratio (M / C) is within the above range, the oxygen reduction activity becomes higher.

  The nitrogen-containing carbon material according to the present embodiment expresses oxygen reduction activity by containing nitrogen and a transition metal. However, if the content of nitrogen or transition metal is too much, the oxygen reduction activity is lowered, so that There is an appropriate range for the content.

  The atomic ratio of nitrogen atom to carbon atom (N / C) and the atomic ratio of transition metal atom to carbon atom (M / C) can be determined by an electron beam microanalyzer. An “electron beam microanalyzer” is an apparatus that separates characteristic X-rays generated when an electron beam is irradiated onto a sample with a wavelength dispersive X-ray spectrometer, and identifies and quantifies elements contained in the sample. . As a specific method for measuring the atomic ratio, the methods described in Examples can be used.

  Although it does not specifically limit as a transition metal atom, For example, it is preferable that they are Fe, Co, Ni, Cu, Mn, and / or Cr, It is more preferable that they are Fe, Co, and / or Cu, More preferably, and / or Co. By using such a transition metal, the oxygen reduction activity of the nitrogen-containing carbon material tends to be further improved.

[BET specific surface area]
In the present embodiment, the specific surface area determined by the BET method, is at 400 meters ² / g or more, preferably 500 meters ² / g or more, more preferably 600 meters ² / g or more. The upper limit of the BET specific surface area is not particularly limited, but is preferably 3000 m ² / g or less. When the BET specific surface area is within the above range, the oxygen reduction activity becomes higher. The specific surface area of the nitrogen-containing carbon material according to the present embodiment can be measured according to JIS Z8830 “Method for measuring specific surface area of powder (solid) by gas adsorption”.

[Crystallinity]
In the present embodiment, the crystallinity obtained from the X-ray diffraction diagram is 0.1 or less, preferably 0.095 or less, and more preferably 0.09 or less. Further, the lower limit of the crystallinity is not particularly limited, and the lower the better. When the degree of crystallinity is within the above range, the oxygen reduction activity becomes higher.

  The “crystallinity” is an index of the ratio of the shell-like structural component to the total of the shell-like structural component and the amorphous structural component, and can be obtained by X-ray diffraction by the method described in the examples. In the X-ray diffraction diagram, the (002) plane diffraction line of the shell-shaped carbon has a sharp component, and the (002) plane diffraction line of the amorphous carbon has a flat component. The degree of crystallinity is calculated from the ratio of the area of the sharp component to the sum of the area of the sharp component and the area of the flat component. Since the crystallinity may cause an error depending on measurement conditions and fitting conditions, it is preferable to evaluate according to the crystallinity analysis method described in the examples.

(Average particle size)
The average particle diameter of the nitrogen-containing carbon material according to the present embodiment is preferably 1 nm or more and 100 μm or less, more preferably 5 nm or more and 10 μm or less, and further preferably 10 nm or more and 1 μm or less. When a nitrogen-containing carbon material is used as an electrode, it is preferable to appropriately adjust the average particle diameter (volume-based median diameter: 50% D) in order to efficiently exhibit the performance as an electrode. When the average particle diameter is 100 μm or less, the specific activity of the electrode tends to be further improved. Further, when the average particle diameter is 1 nm or more, the particles tend to be aggregated densely and the substance transport is inhibited from being inhibited. The substance transport is inhibited when, for example, it is difficult to supply oxygen molecules to the active site when used as a positive electrode catalyst of a polymer electrolyte fuel cell. The average particle diameter can be measured by a known method such as a laser diffraction / scattering method, a dynamic light scattering method, an image imaging method, or a gravity sedimentation method.

  The method for adjusting the average particle size is not particularly limited, and the average particle size may be controlled in the precursor preparation step described later, or may be adjusted by pulverizing the nitrogen-containing carbon material after the heat treatment step described later. . When controlling the average particle size in the precursor preparation step, for example, a method of granulating a solution containing a carbon raw material, a nitrogen raw material and a transition metal raw material with a spray dryer, or a method of obtaining fine particles by polymerization can be used. . The pulverization method is not particularly limited, and examples thereof include a method of pulverizing a precursor or a nitrogen-containing carbon material with a ball mill, a bead mill, a jet mill or the like.

[Method for producing nitrogen-containing carbon material]
The method for producing a nitrogen-containing carbon material according to this embodiment includes a heat treatment step of heat-treating a precursor obtained by combining a carbon raw material, a nitrogen raw material, and a transition metal raw material.

(precursor)
First, a carbon raw material, a nitrogen raw material, and a transition metal raw material are combined to prepare a precursor (hereinafter also referred to as “precursor preparation step”). The precursor is a composite of a carbon raw material, a nitrogen raw material, and a transition metal raw material. The precursor can also contain other components as required. Although it does not specifically limit as another component, For example, the compound etc. which contain boron and / or phosphorus are mentioned.

  Here, “composite” may be a state in which a carbon raw material, a nitrogen raw material, and a transition metal raw material are physically mixed, or a carbon raw material, a nitrogen raw material, and a transition metal raw material form a chemical bond. However, it is preferable that each of them is uniformly dispersed. In particular, when transition metal single particles, carbides, oxides, nitrides, and other nanoparticles are generated in the heat treatment process, they become catalysts for forming a shell-like structure, so that the transition metal is uniformly dispersed in the composite material. The transition metal concentration is preferably within a certain range.

(Carbon raw material)
Although it does not specifically limit as a carbon raw material, For example, the compound which has a quinolizinium structure, azulmic acid, polyaniline, polyvinyl pyridine, a melamine resin, a hydrophilic polymer, etc. are mentioned. Although it does not specifically limit as a hydrophilic polymer, For example, a phenol resin, polyvinyl alcohol, polyethyleneglycol, polyacrylic acid, polyamide, polyvinylpyrrolidone etc. can be used preferably. Among these, a phenol resin is more preferable from the viewpoint of easy carbonization in the heat treatment step.

(Nitrogen raw material)
Although it does not specifically limit as a nitrogen raw material, For example, the compound which has a quinolizinium structure, azurmic acid, polyaniline, polyvinyl pyridine, a melamine resin etc. are mentioned. Among these, a compound having a quinolizinium structure and azulmic acid are preferable. By using such a nitrogen raw material, the atomic ratio (N / C) of the nitrogen-containing carbon material can be kept within an appropriate range.

(Transition metal raw material)
The transition metal raw material is not particularly limited, and examples thereof include transition metal chlorides, bromides, iodides, nitrates, sulfates, phosphates, acetates, and cyanides. Of these, those that are soluble in polar solvents such as water and lower alcohols are preferred. Further, the transition metal is not particularly limited, but is preferably, for example, Fe, Co, Ni, Cu, Mn, Cr, more preferably Fe, Co, Cu, and Fe, Co. Further preferred. By using such a transition metal, the oxygen reduction activity of the nitrogen-containing carbon material tends to be further improved.

  The method for combining the nitrogen raw material, the carbon raw material and the transition metal raw material is not particularly limited. For example, the nitrogen raw material, the carbon raw material and the transition metal raw material are dissolved in a polar solvent, preferably water and / or lower alcohol. And evaporating the solvent to dryness is preferred.

  Although it does not specifically limit as a dissolution method, For example, although all the raw materials may be dissolved in one solvent, you may mix, after dissolving a raw material in a respectively different solvent. For example, when using a compound (1) having a quinolizinium structure as a nitrogen raw material, a phenol resin as a carbon raw material, and iron (II) bromide as a transition metal raw material, the compound (1) and iron (II) are water. Since the phenol resin easily dissolves in ethanol, an aqueous solution of the compound (1) and iron (II) bromide and an ethanol solution of the phenol resin are separately prepared and then mixed. Moreover, it is preferable to heat a solution from a viewpoint of the solubility improvement of a nitrogen raw material, a carbon raw material, and a transition metal raw material.

  The method for evaporating and drying is not particularly limited. For example, the solvent may be removed under reduced pressure using a rotary evaporator or the like, or the solvent may be volatilized using a spray dryer or the like. Among these, a method using a spray dryer is preferable from the viewpoint of maintaining a uniform composite state and from the viewpoint of granulation.

  The concentration of the transition metal in the precursor is preferably 0.01 wt% to 10 wt%, more preferably 0.05 wt% to 5 wt%, and even more preferably 0.1 wt% to 3 wt%.

  In addition, since the nitrogen content in the nitrogen-containing carbon material obtained by the heat treatment process varies greatly depending on the carbon material and the nitrogen material, the carbon material and the nitrogen are adjusted so that the atomic ratio (N / C) of the nitrogen-containing carbon material is within the above range. It is preferable to adjust the ratio of the raw materials. The atomic ratio (N / C) of the nitrogen-containing carbon material can be largely controlled by using a nitrogen material having a high N / C and / or increasing the ratio of the nitrogen material in the precursor, and having a low N / C. It can be controlled to be small by using a nitrogen raw material and / or decreasing the ratio of the nitrogen raw material in the precursor.

  Further, the atomic ratio (M / C) of the nitrogen-containing carbon material can be largely controlled by increasing M / C in the precursor, and can be controlled small by decreasing M / C in the precursor. it can.

  The method for producing the nitrogen-containing carbon material according to the present embodiment is not limited to the above, and various other combinations of nitrogen materials, carbon materials, and transition metal materials can be used. Among them, it is preferable to select a combination in which each raw material (especially a transition metal raw material) is uniformly dispersed and the concentration can be controlled within the above-mentioned predetermined range. From such a viewpoint, the precursor preferably contains at least one selected from the group consisting of a compound having a quinolizinium structure and azulmic acid. Since these compounds all have nitrogen atoms and carbon atoms, they can be used as nitrogen raw materials and carbon raw materials, have high affinity with transition metals, and are suitable for preparing precursors in which each raw material is uniformly dispersed. .

  Next, details will be described separately for the case where a compound having a quinolinium structure is used as the precursor raw material and the case where azulmic acid is used.

[When a compound having a quinolizinium structure is used as a precursor]
Although the compound which has a quinolizinium structure is not specifically limited, For example, it represents with following formula (1).

In formula (1), R ^{1 to} R ⁸ are each independently a hydrogen atom, alkyl group, aryl group, alkoxy group, aryloxy group, hydroxyl group, sulfonyl group, halogen atom, nitro group, thioalkyl group, cyano group. , A carboxy group, an amino group, an amide group or the like, or two adjacent groups among R ^{1 to} R ⁸ may form a condensed ring.

In formula (1), B ⁻ is a counter anion of quinolizinium, such as a halide ion, a hydroxide ion, an organic acid ion, a carbonate ion, a nitrate ion, a sulfate ion, or a phosphate ion.

  Since the compound having a quinolizinium structure is an organic salt, it is easily dissolved in a polar solvent, and tends to be uniformly complexed with a carbon raw material such as a hydrophilic polymer and a transition metal complex. In general, when a nitrogen-containing organic material is heat-treated to form a nitrogen-containing carbon material, most of the nitrogen atoms in the organic material are eliminated as nitrogen molecules, ammonia, hydrocyanic acid, etc., but in a compound having a quinolizinium structure Nitrogen atoms tend to remain in nitrogen-containing carbon materials. Thereby, it exists in the tendency which can obtain the nitrogen-containing carbon material which has higher oxygen reduction activity.

  The molecular weight of the compound having a quinolizinium structure is preferably 200 to 2500, more preferably 250 to 2000, and further preferably 300 to 1500. When the molecular weight is within the above range, a compound having a quinolizinium structure tends to be easily carbonized. The proportion of the molecular weight of the quinolizinium structure in the compound having a quinolizinium structure is preferably 5 to 100%, more preferably 10 to 95%, and still more preferably 20 to 90%. When the ratio of the molecular weight is within the above range, the oxygen-reducing activity of the nitrogen-containing carbon material tends to be superior.

  Although it does not specifically limit as a compound which has the said quinolizinium structure, For example, the compound represented by following formula (2)-(4) is mentioned. Such a compound has excellent solubility in polar solvents, particularly water and lower alcohols, and the ratio of the molecular weight of the quinolizinium structure in the compound having a quinolizinium structure and the molecular weight of the compound having a quinolizinium structure are in an appropriate range.

When the transition metal raw material and the compound having a quinolizinium structure are combined, the transition metal raw material is preferably chloride, bromide, or iodide. Although details of this are unknown, for example, it is known that 1-butylpyridinium bromide and iron (III) chloride are mixed to form 1-butylpyridinium tetrachloroferrate (III) (J. Mol. Liq. 169). Similarly, the counter anion of a compound having a quinolidinium structure and a transition metal chloride, bromide, or iodide are combined to form a compound having a quinolidinium structure and a transition metal complex. It is expected that uniform complexing has been achieved.

[When azulmic acid is used as a precursor]
Azulmic acid is a general term for polymers obtained by polymerizing hydrocyanic acid, and is an organic polymer containing nitrogen. Although the detailed chemical structure of azulmic acid has not been identified, Angew. Chem. 72, p379-384 (1960), and vacuum science, 16, p64-72 (1969) etc., it is estimated that the structure represented by the formula (5) is a typical structure. Yes.

  The method of combining azulmic acid and the transition metal raw material is not particularly limited. For example, a method of adding a transition metal raw material by dissolving azulmic acid in a solvent, a method of coexisting a transition metal complex during polymerization of azulmic acid Is mentioned. Azulmic acid is a substance that is poorly soluble in various solvents. Therefore, it is preferable to coexist with a transition metal complex during the polymerization of azulmic acid from the viewpoint of more uniform complexing.

  The method for polymerizing azulmic acid is not particularly limited. For example, a method for heating liquefied hydrocyanic acid or an aqueous solution of hydrocyanic acid, a method for leaving liquefied hydrocyanic acid or an aqueous solution of hydrocyanic acid for a long time, a method for adding a base to liquefied hydrocyanic acid or an aqueous solution of hydrocyanic acid, or liquefaction. A method of irradiating light to hydrocyanic acid or aqueous solution of hydrocyanic acid, a method of emitting high energy to liquefied hydrocyanic acid or aqueous solution of hydrocyanic acid, a method of performing various discharges in the presence of liquefied hydrocyanic acid or aqueous solution of hydrocyanic acid, a method of electrolyzing an aqueous solution of potassium cyanide, etc. Can be mentioned. From the viewpoint of complexing azulmic acid and a transition metal complex, a method of polymerizing by adding a water-soluble transition metal complex and a base to an aqueous hydrocyanic acid solution is preferable.

  Transition metal complexes that coexist during the polymerization of azulmic acid are transition metal chlorides, bromides, iodides, nitrates, sulfates, phosphates, acetates, and cyanides from the standpoint of ease of dissolution in water. Etc. are preferred. The base to be added at the time of polymerization is not particularly limited. For example, ammonia, amine, hydroxide of alkali metal; hydroxide of alkaline earth metal; metal alkoxide can be used. Among these, it is preferable to use ammonia or amine in which unnecessary metals are not mixed.

  When a water-soluble transition metal complex and a base are added to a hydrocyanic acid aqueous solution and polymerized, azulmic acid containing a transition metal is precipitated as brown particles. Thereafter, the solvent is removed by evaporation to dryness or filtration to obtain azulmic acid in which the transition metal complex is uniformly dispersed.

[Heat treatment process]
In the heat treatment step, the precursor obtained in the precursor preparation step is heat treated to obtain a nitrogen-containing carbon material. The heat treatment process may be one stage, but may be two or more stages. Among these, it is preferable to perform a heat treatment in an inert gas atmosphere and a heat treatment in an ammonia-containing gas atmosphere in the heat treatment step. By performing such a heat treatment step, a nitrogen-containing carbon material excellent in oxygen reduction activity tends to be obtained.

  Although it does not specifically limit as said inert gas, For example, nitrogen, a noble gas, a vacuum etc. can be used. When carbonization is performed under an inert gas, the carbonization tends to proceed sufficiently by being at a predetermined temperature or higher. Moreover, it exists in the tendency for the production | generation of a shell-like structure to be suppressed by being below predetermined temperature. Therefore, there is an appropriate range for the heat treatment temperature. The appropriate heat treatment temperature varies depending on the precursor. For example, when a precursor composed of a compound represented by formulas (1) to (4), a phenol resin and iron (II) bromide is used, the temperature is 400 to 800 ° C. It is preferable that the temperature is 450 to 750 ° C, more preferably 500 to 700 ° C. The heat treatment time is preferably 5 minutes to 50 hours, more preferably 10 minutes to 20 hours, and further preferably 20 minutes to 10 hours. On the other hand, when using the precursor which consists of azurmic acid and iron nitrate (III), it is preferable that it is 600-1100 degreeC, It is more preferable that it is 700-1000 degreeC, It is more preferable that it is 800-950 degreeC. . The heat treatment time is preferably 5 minutes to 50 hours, more preferably 10 minutes to 20 hours, and further preferably 20 minutes to 10 hours. The reason why the appropriate heat treatment temperature is different is considered to be because the easiness of carbonization and the ease of producing transition metal nanoparticles differ depending on the raw materials.

  Although the crystallinity of the nitrogen-containing carbon material finally obtained is 0.1 or less, the shell-like structure is more stable than the amorphous structure, and once the crystallinity is increased, the crystallinity is decreased again. It is difficult. Therefore, it is preferable to always maintain the crystallinity at 0.1 or less. The crystallinity can be controlled to be small by lowering the heat treatment temperature and / or shortening the heat treatment time. Although the transition metal raw material is decomposed by the heat treatment, particles such as simple metals, metal oxides, metal carbides, metal nitrides (hereinafter also simply referred to as “transition metal particles”) may be generated. It tends to improve the crystallinity of the nitrogen-containing carbon catalyst. In this case, the degree of crystallinity can be controlled to be small by appropriately removing the transition metal particles. Further, the crystallinity can be largely controlled by increasing the heat treatment temperature and / or lengthening the heat treatment time and / or not removing the transition metal.

  When the heat treatment is performed under an ammonia-containing gas, 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. When the heat treatment is performed under an ammonia-containing gas, the heat treatment temperature is preferably 600 to 1200 ° C, more preferably 700 to 1100 ° C, and further preferably 800 to 1050 ° C. The heat treatment time is preferably 5 minutes to 5 hours, more preferably 10 minutes to 3 hours, and further preferably 15 minutes to 2 hours. When the heat treatment temperature and the heat treatment time are within the above ranges, a nitrogen-containing carbon material excellent in oxygen reduction activity tends to be obtained.

  The atomic ratios (N / C) and (M / C) of the nitrogen-containing carbon material also change depending on the conditions of the heat treatment step. By increasing the heat treatment temperature and / or lengthening the heat treatment time, it is possible to control N / C to be small and M / C to be large, and to lower the heat treatment temperature and / or to shorten the heat treatment time. , N / C can be increased and M / C can be decreased.

  By performing the heat treatment under an ammonia-containing gas, the BET specific surface area of the obtained nitrogen-containing carbon material is increased. The BET specific surface area can be largely controlled by increasing the ammonia concentration and / or increasing the heat treatment temperature under the ammonia-containing gas and / or increasing the heat treatment time under the ammonia-containing gas, thereby lowering the ammonia concentration. In addition, the BET specific surface area can be controlled to be small by lowering the heat treatment temperature under the ammonia-containing gas and / or shortening the heat treatment time under the ammonia-containing gas.

  Before and after the heat treatment under an inert gas and / or the heat treatment under an ammonia-containing gas, a part of the transition metal may be removed using hydrochloric acid, sulfuric acid or the like. In particular, when transition metal particles are generated by heat treatment, it is preferable to remove the transition metal particles from the viewpoint of suppressing increase in crystallinity in a subsequent heat treatment step. The ease with which transition metal particles can be produced varies depending on the type of raw material, metal concentration, metal dispersibility, heat treatment temperature, and the like. For example, when a compound having a quinolizinium structure is used, transition metal particles are likely to be generated. Therefore, it is preferable to perform acid cleaning to remove the transition metal. On the other hand, since it is difficult to produce transition metal particles with azulmic acid, acid cleaning is often unnecessary. However, even when a compound having a quinolizinium structure is used, if the type and concentration of the metal complex are changed, acid cleaning may not be necessary. Further, in order to increase the removal rate of the transition metal particles, it is preferable to divide the heat treatment step under an inert gas and / or the heat treatment step under an ammonia-containing gas into a plurality of steps and repeatedly remove the transition metal.

[Use]
The nitrogen-containing carbon material according to the present embodiment can be suitably used for a fuel cell electrode or the like. A fuel cell electrode containing a nitrogen-containing carbon material has high oxygen reducing ability. A method for obtaining an oxygen reduction electrode, a fuel cell, and the like from the oxygen reduction catalyst is not particularly limited, and a general method for producing a solid polymer fuel cell can be used. (For example, see Patent Document 1)

  Hereinafter, the present embodiment will be described in more detail with reference to examples and the like. However, these are illustrative, and the present embodiment is not limited to the following examples. Those skilled in the art can implement the present embodiment by making 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.
<Analysis method>
(Electron microanalyzer measurement)
The carbon materials obtained in the examples and comparative examples were pressed and compacted with a manual tablet molding machine, and the electron beam microanalyzer (manufactured by JEOL Ltd., “JXA-8200” was used) was measured as a 3 mmφ pellet. The acceleration voltage was set to 15 kV, the irradiation current was set to 20 nA, the probe diameter was set to 50 μm, C, N, O, and Fe were measured and corrected by the “ZAF Metal” program attached to the apparatus. As a result, atomic ratios N / C and M / C of C, N and Fe (hereinafter also referred to as “M”) were obtained.

(BET specific surface area measurement)
It was measured according to JIS Z8830 “Method for measuring specific surface area of powder (solid) by gas adsorption”.

(Measurement of crystallinity)
The carbon materials obtained in the examples and comparative examples are put into the recesses (18 mm × 20 mm × thickness 0.2 mm) of the silicon non-reflective sample plate so that the heights of the carbon material surface and the reference surface of the sample plate match. The sample was uniformly filled while being pressed with a slide glass, and X-ray diffraction measurement was performed with an X-ray diffractometer (Ultima IV, manufactured by Rigaku). The applied voltage and current to the X-ray tube were 40 kV and 40 mA, respectively. For incident X-rays, CuKα was used and monochromatized with a graphite monochromator. The sampling interval was 0.1 °, the scanning speed was 1 ° / min, and the measurement range (2θ) was 5 to 100 °.

  In the diffraction diagram obtained by the X-ray diffraction measurement, as shown in FIG. 1, intensity correction and background correction were performed in the range of 15 to 32 °. First, the intensity was corrected by dividing the diffraction intensity at each diffraction angle by the intensity correction coefficient FCT, and then the background was corrected by subtracting the straight line connecting the vicinity of 15-20 ° and the vicinity of 30-35 ° as the background. .

FCT is the Lorentz factor L, the polarization factor P, expressed by the following equation using the atomic scattering factor f _c of the absorption factor A and the carbon.
FCT = L × P × A × f c 2
(In the formula, if the angle of the goniometer of the X-ray diffractometer is θ,
L = 1 / (sin ² θ · cos θ),
P = (1 + cos ² (2θ) · cos ² (26.56 °)) / (1 + cos ² (26.56 °))
A = (1− (sin (2θ)) / (2 μb _r )) × (1−exp (−2 μt / sin θ)) + ((2t cos θ) / b _r ) × exp (−2 μt / sin θ)
Here, μ is a linear absorption coefficient of carbon of 0.4219 mm ⁻¹ ,
b _r = R sin β (R is a goniometer radius of 285 mm, β is a divergence slit angle of 2/3 °),
t is a sample thickness of 0.2 mm,
^{_{f c = 2.26069exp (-0.226907 × s}} 2) + 1.56165exp (-0.00656665 × s 2) + 1.05075exp (-0.0975618 × s 2) + 0.839259exp (0.555949 × s 2) +0.286997, and s = (sin θ) /0.15419.

  In the X-ray diffraction diagram subjected to intensity correction and background correction, a diffraction peak of the carbon (002) plane appeared near 26 ° at the diffraction angle (2θ). This peak has a sharp peak of 26.4 ° ± 0.4 ° derived from the shell-like structure and a full width at half maximum of 1 ° or less, and a broader peak at a lower angle than the shell-like structure derived from the amorphous structure. Two types were mixed.

  As shown in FIG. 2, the X-ray diffraction diagram subjected to background correction and intensity correction was fitted by optimizing each component as a Gaussian function with the peak intensity, peak position, and full width at half maximum as parameters. The peaks were separated. In the case of a carbon material containing a large amount of an amorphous structure, the peak derived from the shell structure and the peak derived from the amorphous structure are integrated and sometimes cannot be clearly distinguished. At this time, if the integrated peak top was in the range of 26.4 ° ± 0.4 °, it was regarded as the peak position of the shell-like structure. If the integral and the peak top were not in the range of 26.4 ° ± 0.4 °, 26.4 ° was taken as the peak position of the shell-like structure. Also, fitting is performed under the condition that the full width at half maximum of the shell-like structure is 1 ° or less, the peak position of the amorphous structure is on the lower angle side than the peak position of the shell-like structure, and the full width at half maximum of the peak is 3 ° or more. went.

  By the above operation, the peak area derived from the shell structure with respect to the sum of the peak area derived from the shell structure obtained by peak separation and the peak area derived from the amorphous structure was defined as the crystallinity.

(Electrochemical measurement)
The measurement method of linear sweep voltammetry by using the electrode manufacturing method and the rotating electrode method (using a rotating ring disk electrode device “RRDE-1” manufactured by Nisshi Kogyo) used in Examples and Comparative Examples is shown below. First, 5 mg of the carbon material prepared in the example or the comparative example was weighed into a vial bottle, and a glass bead was filled with a spatula and a 5 mass% Nafion (trade name , registered trademark ) dispersion (manufactured by Sigma Aldrich Japan). And 150 μL each of ion-exchanged 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 the rotating electrode, and dried under saturated steam. The dried rotating electrode was the working electrode, the reversible hydrogen electrode (RHE) was the reference electrode, and the carbon electrode was the counter electrode. 0.5M sulfuric acid was used as an electrolyte, and oxygen was bubbled through the electrolyte for 30 minutes, and then the electrochemical measurement was performed by sweeping from 1.1 V to 0 V at a sweep speed of 5 mV / s and a rotation speed of 1500 rpm.

[Example 1]
<Precursor preparation step>
2.0 g of the compound represented by the above formula (2) (synthesized by the method described in Journal of Organic Chemistry (1964) Vol. 29, No. 4, pp. 856-858) and iron (II) bromide (Sigma) Aldrich Japan) (0.20 g) was dissolved in 60 mL of pure water at 70 ° C. Next, 1.3 g of phenol resin (manufactured by Gunei Chemical Industry Co., Ltd., REGITOP (registered trademark) PSK-2320) was dissolved in 50 mL of ethanol, and this was added dropwise to the solution at 70 ° C. At this time, since a phenol resin was precipitated, 70 mL of ethanol was further added and refluxed for 1 hour for dissolution, and then ethanol was distilled off under reduced pressure, followed by vacuum drying to obtain 3.2 g of a precursor.

<Heat treatment process>
3.2 g of the precursor obtained in the precursor preparation step was placed on a quartz boat, accommodated in a quartz tube furnace having an inner diameter of 35 mm, and heat-treated at 600 ° C. for 5 hours under a nitrogen flow of 1.2 NL / min. After cooling, it was dry-ground for 90 minutes using a planetary ball mill (made by Fritsch, “Pulverisete-7”) containing carbon nitride balls having a diameter of 10 mmφ. The pulverized carbide is passed through a sieve having an opening of 106 μm, and then is mixed with a planetary ball mill containing a water / ethanol = 1/1 (volume ratio) mixture and zirconia balls having a diameter of 0.5 mmφ. The mixture was wet pulverized for a minute to obtain an average particle size of 0.35 μm. The ground carbide (1.5 g) was placed in 500 mL of 36 mass% concentrated hydrochloric acid and stirred at room temperature for 4 hours to dissolve and remove iron on the surface of the carbide. This was filtered with a membrane filter, washed with ion exchange water, and then vacuum dried at 80 ° C. to obtain a carbide.

  The dried carbide was put in the same heating furnace as described above, and heat-treated at 800 ° C. for 1 hour under a flow of 1.2 NL / min ammonia gas / nitrogen = 1/1 (volume ratio) mixed gas. After cooling, 1.0 g of the carbide was put into 500 mL of 36% by mass concentrated hydrochloric acid and stirred at room temperature for 4 hours to dissolve and remove iron on the surface of the carbide. This was filtered with a membrane filter, washed with ion-exchanged water, and then vacuum-dried at 80 ° C.

  The dried carbide is put in the same heating furnace as described above, and heat treated at 1000 ° C. for 1 hour under a flow of 1.2 NL / min ammonia gas / nitrogen = 1/1 (volume ratio) mixed gas. .30 g was obtained. N / C and M / C obtained by electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”), and BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area obtained in step 1 and the crystallinity obtained by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

[Comparative Example 1]
<Heat treatment process>
A nitrogen-containing carbon material was prepared under the same conditions as in Example 1 except that the iron on the carbide surface after pulverization was not dissolved and removed, and an excessive amount of iron was left. N / C and Fe / C obtained by an electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”) and the BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area obtained in step 1 and the crystallinity obtained by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

[Example 2]
<Precursor preparation step>
In a mixed solution of 81 g of cyanuric acid, 6.4 g of acetic acid and 124 g of pure water, 1.5 g of ammonium hexacyanoferrate (II) n-hydrate (manufactured by Wako Pure Chemical Industries) and 14 g of 25% aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd.) ) Was added. Thereafter, the mixture was gradually heated and finally stirred at 80 ° C. for 7 hours. The black solid content generated in the mixed solution was filtered with a membrane filter and vacuum-dried to obtain 57 g of azulmic acid containing iron as a precursor.

<Heat treatment process>
3.0 g of the precursor obtained in the precursor preparation step was placed on a quartz boat, accommodated in a quartz tube furnace having an inner diameter of 35 mm, and heat-treated at 900 ° C. for 1 hour under a nitrogen flow of 1.2 NL / min. . After cooling, it was dry-ground for 90 minutes using a planetary ball mill (made by Fritsch, “Pulverisete-7”) containing carbon nitride balls having a diameter of 10 mmφ. The pulverized carbide is passed through a sieve having an opening of 106 μm, and then is mixed with a planetary ball mill containing a water / ethanol = 1/1 (volume ratio) mixture and zirconia balls having a diameter of 0.5 mmφ. Wet pulverize for a minute to obtain a carbide having an average particle size of 0.35 μm.

  The pulverized carbide was put into the same heating furnace as described above, and heat-treated at 850 ° C. for 1 hour under a flow of 1.2 NL / min ammonia gas / nitrogen = 1/1 (volume ratio) mixed gas. This was filtered through a membrane filter, washed pure, and then vacuum dried at 80 ° C. to obtain 0.14 g of a nitrogen-containing carbon material. N / C and M / C obtained by electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”), and BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area obtained in step 1 and the crystallinity obtained by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

[Comparative Example 2]
Azurmic acid having an iron content different from that in Example 2 was prepared. Specifically, in a mixed solution of 80 g of cyanic acid, 8.6 g of acetic acid and 130 g of pure water, 14 g of ammonium hexacyanoferrate (II) n-hydrate (manufactured by Wako Pure Chemical Industries) and 12 g of 25% aqueous ammonia (Wako Pure) Yakuhin Kogyo) was added. Thereafter, the mixture was gradually heated and finally stirred at 80 ° C. for 7 hours. The black solid content generated in the mixed solution was filtered through a membrane filter and vacuum dried to obtain 49 g of azulmic acid containing iron. A heat treatment step was performed using 3.0 g of this azulmic acid under the same conditions as in Example 2 to obtain 0.24 g of a nitrogen-containing carbon material. N / C and M / C obtained with an electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”) and the BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area obtained in step 1 and the crystallinity obtained by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

[Comparative Example 3]
Nitrogen having an average particle diameter of 1.2 μm was obtained by performing the heat treatment step under the same conditions as in Example 2 except that 3.0 g of the precursor obtained in Example 2 was used and no ammonia gas was used in the heat treatment step. 0.68g of carbon materials were obtained. N / C and Fe / C obtained by an electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”) and the BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area obtained in step 1 and the crystallinity obtained by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

[Comparative Example 4]
<Precursor preparation step>
Azulmic acid containing no iron was prepared. Specifically, 12 g of 25% aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd.) was added to a mixed solution of 80 g of hydrocyanic acid, 8.6 g of acetic acid and 130 g of pure water. Thereafter, the mixture was gradually heated and finally stirred at 80 ° C. for 7 hours. The black solid content generated in the mixed solution was filtered through a membrane filter and vacuum-dried to obtain 40 g of azulmic acid containing no iron. A heat treatment step was performed using 3.0 g of this azulmic acid under the same conditions as in Example 2 to obtain 0.24 g of a nitrogen-containing carbon material. N / C and M / C obtained with an electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”) and the BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area obtained in step 1 and the crystallinity obtained by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

[Comparative Example 5]
<Precursor preparation step>
3.3 g of phenol resin (manufactured by Gunei Chemical Industry Co., Ltd., Resist Top PSK-2320) was dissolved in 300 mL of acetone to obtain a solution, and 3.0 g of phthalocyanine iron (II) (manufactured by Tokyo Chemical Industry Co., Ltd.) was added to this solution. Thereafter, acetone was distilled off under reduced pressure and vacuum-dried to obtain 6.2 g of a solid content.

<Heat treatment process>
Of the precursors obtained in the precursor preparation step, 3.0 g was placed on a quartz boat, accommodated in a quartz tube furnace having an inner diameter of 35 mm, and heat-treated at 600 ° C. for 5 hours under a nitrogen flow of 1.2 NL / min. After cooling, it was dry-ground for 90 minutes using a planetary ball mill (made by Fritsch, “Pulverisete-7”) containing carbon nitride balls having a diameter of 10 mmφ. The pulverized carbide is passed through a sieve having an opening of 106 μm, and then is mixed with a planetary ball mill containing a water / ethanol = 1/1 (volume ratio) mixture and zirconia balls having a diameter of 0.5 mmφ. Wet pulverize for a minute to obtain a carbide having an average particle size of 0.35 μm.

  The ground carbide (1.5 g) was placed in 500 mL of 36 mass% concentrated hydrochloric acid and stirred at room temperature for 4 hours to dissolve and remove iron on the surface of the carbide. This was filtered through a membrane filter, washed with pure water, and then vacuum-dried at 80 ° C. The dried carbide was put in the same heating furnace as described above, and heat-treated at 800 ° C. for 1 hour under a flow of 1.2 NL / min ammonia gas / nitrogen = 1/1 (volume ratio) mixed gas.

  After cooling, 1.0 g of the carbide was put into 500 mL of 36% by mass concentrated hydrochloric acid and stirred at room temperature for 4 hours to dissolve and remove iron on the surface of the carbide. This was filtered through a membrane filter, washed with pure water, and then vacuum-dried at 80 ° C. The dried carbide is put in the same heating furnace as described above, and heat treated at 1000 ° C. for 1 hour under a flow of 1.2 NL / min ammonia gas / nitrogen = 1/1 (volume ratio) mixed gas. .51 g was obtained. N / C and Fe / C obtained by an electron beam microanalyzer (manufactured by JEOL, using “JXA-8200”) and the BET method (manufactured by Nippon Bell, using “Belsorb mini-II”). Table 1 shows the specific surface area determined in step 1 and the crystallinity determined by X-ray diffraction. Moreover, the result of the electrochemical measurement is shown in FIG.

  From the results shown in FIG. 3, it was found that Examples 1 and 2 had high oxygen reduction activity, and Comparative Examples 1 to 4 had low oxygen reduction activity.

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

Claims (1)

The atomic ratio (N / C) of nitrogen atoms and carbon atoms obtained by an electron beam microanalyzer is 0.005 to 0.3,
The atomic ratio (M / C) of transition metal atoms to carbon atoms obtained by an electron beam microanalyzer is 0.0001 to 0.05,
The transition metal atom is Fe, Co, Ni, Cu, Mn, and / or Cr;
The specific surface area determined by the BET method is 400 m ² / g or more,
The degree of crystallinity obtained from an X-ray diffraction diagram is 0.1 or less,
A method for producing a nitrogen-containing carbon material,
A heat treatment step of heat-treating a precursor compounded by dissolving a carbon raw material, a nitrogen raw material and a transition metal raw material in a solvent;
When the precursor contains azulmic acid, it is complexed by coexisting with a transition metal complex during the polymerization of azulmic acid,
The method for producing a nitrogen-containing carbon material, wherein the precursor contains at least one selected from the group consisting of a compound having a quinolizinium structure and azulmic acid.