JP2012245456A - Carbon catalyst, method for producing the same, cathode catalyst layer of fuel cell, and fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a carbon catalyst which has the oxygen reduction activity equivalent to that of another carbon catalyst having a nanoshell structure, without using a metal.SOLUTION: The carbon catalyst is prepared, which has a defect in a carbon structure thereof and is characterized in that a parameter W of an X-band electron spin resonance spectrum appearing at 3,200-3,500 gauss is ≤11 gauss. A method for preparing the carbon catalyst includes the steps of: heat-treating nanodiamond to about 1,400-1,600°C with the nanodiamond as a start material; and treating the nanodiamond having a nano-onion structure with nitrogen at ≥600°C.

Description

  The present invention relates to a carbon catalyst produced on the basis of nanodiamond, a method for producing the same, a cathode catalyst layer of a fuel cell using the carbon catalyst, and a fuel cell using the same.

  Currently, in order to reduce carbon dioxide, which is a cause of global warming, conversion from fossil fuels to clean energy has been screamed. Among them, the development of a practical fuel cell that does not emit carbon dioxide is expected. However, in the fuel cell, platinum, which is an expensive noble metal, is used as a catalyst for causing the electrode reaction. Therefore, this platinum catalyst causes an increase in cost, and is currently an obstacle to development and its spread.

  Accordingly, the inventors have been working on the development of an electrode catalyst that does not require an expensive noble metal such as platinum. One of them is the development of a carbon catalyst having a nanoshell structure (see Patent Document 1). The nanoshell is a type of nanocarbon with a spherical shell-like graphite structure formed by the catalytic action of fine metal particles generated during thermal decomposition by adding a metal complex in advance when making a carbon material from an organic compound. One.

  In this development, carbon with a nanoshell structure exhibits oxygen reduction activity, and the structure, physical properties, and chemical reactivity of the carbon material depend on the preparation conditions such as the type of organic material that is the raw material for carbon, the temperature and time of carbonization, etc. Utilizes dependency. Therefore, a non-platinum cathode catalyst using a carbon material was proposed by forming a nanoshell structure for imparting oxygen reduction activity to carbon and controlling the carbon material by a carbon alloy method (see Patent Document 1).

  Another method that the inventors have tried is a method of doping (doping) nitrogen and boron into a carbon material to be a cathode catalyst. By this method, it has been confirmed that the oxygen reduction reaction is enhanced by simultaneously doping nitrogen and boron into a carbon material (see Non-Patent Document 1).

JP 2008-282725 A

Jun-ichiOzaki, Naofumi Kimura, Tomonori Anahara, Asao Oya, Carbon 45 (2007) 1847-1853

However, in preparing carbon having a nanoshell structure as shown in Patent Document 1, it is indispensable to use a metal for the production method. This is because the catalytic function of modifying the carbonization process of the metal is utilized. However, the metal required to prepare the carbon having a nanoshell structure causes a chemical reaction with hydrogen peroxide (H ₂ O ₂ ), leading to deterioration of the electrolyte membrane or separator as a fuel cell member. After preparation, there was a problem that the used metal had to be removed.
The removal of the metal is performed using a hydrochloric acid aqueous solution or the like, but even if it is attempted to remove the metal with the hydrochloric acid aqueous solution, it is difficult to completely remove the metal. For this reason, it has been desired to prepare a carbon material having a structure similar to the nanoshell structure without using a metal.

  For this reason, the inventors tried a method of increasing the oxygen reduction activity by doping nitrogen and boron into a carbon material as disclosed in Non-Patent Document 1, but the carbon catalyst prepared by this method is The oxygen reduction activity could not be increased as much as the nanoshell structure. Of course, although some oxygen reduction activity can be expected, it has reached the peak, and the oxygen reduction activity as high as the nanoshell structure could not be obtained.

In the course of such research, the inventors considered whether a catalyst having oxygen reduction activity comparable to that of the nanoshell structure could be prepared without using a metal. Then, what came to mind was the use of commercially available nano tiremond (ND) used as an industrial abrasive.
Nanodiamond has a particle size of about 5 nm, and it is known that a cross-section called a nano-onion structure has an onion-like carbon structure when quenched at high temperature. The carbon structure in which the cross-section has an onion shape indicates that the carbon is laminated in an onion-like circular shape, and this structure is similar to the nanoshell structure prepared using a metal. . A carbon catalyst having a nano-onion structure prepared by the inventors is shown in FIG.

  In general, it is said that a curved portion existing in a carbon material has a π-electron strain. The inventors assumed that this was an oxygen reduction active site, and thought that the nano-onion structure obtained by heat-treating nanodiamond (ND) exhibited the same oxygen reduction activity as the nanoshell structure.

  An object of the present invention is to prepare a carbon catalyst having a nano-onion structure close to the above-described nano-shell structure without using a metal, and as a result, having a carbon catalyst having oxygen reduction activity comparable to that of a nano-shell structure carbon catalyst prepared using a metal. Is to provide.

  When the electron spin resonance spectrum of the carbon catalyst of the present invention is measured using an X-band spectrometer, the parameter determined by the absorption differential curve appearing at 3200 to 3500 gauss is 11 gauss or less. The carbon catalyst of the present invention is considered to have defects in the carbon lattice, for example, heat treatment from about 1400 ° C. to 1600 ° C. using nano diamond as a starting material to form nano onion nano diamond at 600 ° C. or higher. It can be prepared by nitrogen treatment.

Moreover, when the nitrogen ratio in carbon in the carbon catalyst of the present invention is measured with an X-ray photoelectron spectrometer, the nitrogen ratio relative to carbon is 4% or less.
Furthermore, when the cyclic voltammogram measured in the sulfuric acid aqueous solution was seen about the carbon catalyst of this invention, it was recognized that the electric current value of the low electric potential side is increasing compared with the high electric potential side.

  The method for producing a carbon catalyst of the present invention includes a first heating step of heating to about 1000 ° C. using nano diamond as a starting material, and heating the heated nano diamond to a higher temperature to 1400 ° C. to 1600 ° C. A second heating step for obtaining nano-onion-structured nanodiamond, and a step of preparing a carbon catalyst by performing nitrogen treatment by ammoxidation at a temperature of 600 ° C. or higher after the second heating step. ,including.

  According to the carbon catalyst of the present invention, there is no need to remove the metal because no metal is used unlike the conventional carbon catalyst having a nanoshell structure. And the catalyst which has equivalent oxygen reduction activity compared with the conventional carbon catalyst prepared using the metal can be provided.

It is a flowchart for demonstrating the process of manufacturing the carbon catalyst of this invention. FIG. 2 is a schematic diagram of a rotating disk electrode device for measuring the potential and current density of the carbon catalyst manufactured in the manufacturing process of FIG. 1. It is a voltammogram which shows the relationship between the electric potential regarding the oxygen reduction calculated | required using the apparatus of FIG. 2, and current density. It is a figure which shows the oxygen reduction start potential with respect to the ammoxidation processing temperature (nitrogen processing temperature) calculated | required from the graph of the relationship between the electric potential of FIG. 3, and current density. It is a figure which shows the change of the nitrogen dope ratio (N / C atomic ratio) with respect to the process temperature (nitrogen process temperature) of the ammoxidation of FIG. FIG. 6 is a diagram showing the relationship between the nitrogen doping ratio and the oxygen reduction start potential in FIG. 5 in a comparison between the ND1400 series and the ND1600 series. It is a figure which shows the result of having measured the ND1400 series and ND1600 series of the carbon catalyst manufactured in FIG. 1 by cyclic voltammetry (CV). It is a figure which shows oxygen reduction start potential (V vs. NHE) with respect to ratio of electric current value I (0.05V) of electric potential 0.05V, and electric current value I (0.6V) of electric potential 0.6V. It is a figure for demonstrating the method of calculating | requiring the parameter W from an ESR (electron spin resonance) spectrum. It is a spectrum when measuring ND1400 series using an X-band ESR apparatus (electron spin resonance apparatus). It is a spectrum when measuring ND1600 series using an X-band ESR apparatus (electron spin resonance apparatus). It is a figure which shows the oxygen reduction active potential with respect to the parameter W obtained from the X-band electron spin resonance measurement result of FIG. 10 and FIG. It is the schematic for demonstrating the principle of a fuel cell. It is a photograph when carbon nano-onion is observed with a transmission electron microscope.

Hereinafter, the carbon catalyst of the embodiment of the present invention (hereinafter sometimes referred to as “this example”) and the preparation method thereof, and particularly the oxygen reduction activity of the carbon catalyst of this example will be described with reference to FIGS. To do. The description will be given in the following order.
1. 1. Method for preparing carbon catalyst of embodiment of the present invention Description of the oxygen reduction activity of the carbon catalyst according to the embodiment of the present invention
Experimental Example 1: Measurement with a rotating disk electrode device
Experimental example 2: Measurement by X-ray photoelectron spectrometer
Experimental Example 3: Measurement by cyclic voltammetry (CV)
Experimental Example 4: Measurement with X-band ESR apparatus (electron spin resonance apparatus) Application to fuel cells

<1. Preparation method of carbon catalyst of this example>
Initially, the preparation method of a carbon catalyst is demonstrated with reference to the flowchart (process table) of FIG. First, a commercially available (made by PROSONIC) nano diamond (ND) is heat-treated (step S1). This heat treatment was performed by using an infrared image furnace and maintaining for 1 hour in a state where the temperature was raised to 1000 ° C. at a temperature raising rate of 10 ° C./min under a nitrogen flow.

  Next, prepare nano-diamonds that have been heat-treated in this way, using an ultra-high temperature furnace, heated to 1400 ° C under vacuum and those heated to 1600 ° C, and hold them again for 1 hour. Then, a high temperature heat treatment was performed (step S2). Then, the obtained heat-treated product was pulverized to 106 μm or less using a ball mill (step S3). The samples thus obtained were designated as ND1400 and ND1600 (see FIG. 3).

  Here, the two carbon catalyst samples ND1400 and ND1600 are taken up for the following reason. At the stage where the temperature was raised to 1000 ° C. in Step S1, the nanodiamond structure remained and did not become a nano-onion structure. In fact, it was found that when the heating temperature was increased in step S2, the structure of nanodiamond remained at 1200 ° C, and changed to a nano-onion structure when the temperature was raised to 1400 ° C (see FIG. 14). A slight nanodiamond structure remained even at 1400 ° C. In addition, it was confirmed that when the temperature was raised to near 2000 ° C, the nano-onion structure grew into a graphite structure (a structure in which large crystals developed at a high temperature) and became inactive, so ND1400 and ND1600 were used as samples. I decided to adopt it.

However, even with the samples ND1400 and ND1600 prepared in this way, as described later, sufficient elongation of oxygen reduction activity was not obtained. That is, it was not possible to reach the oxygen reduction activity of a nanoshell structured catalyst prepared using a metal as originally assumed.
Therefore, the inventors tried to dope the samples ND1400 and ND1600 with nitrogen by the ammoxidation method as the next step (step S4). That is, the method of doping nitrogen with a carbon material proposed by the inventors in Non-Patent Document 1 is incorporated.

The ammoxidation method is a method in which nitrogen is doped by heat-treating a sample in a flow of ammonia and dry air. This method is a method in which a mixed gas of ammonia and air is brought into contact with nanodiamonds ND1400 and ND1600 that have been heat-treated, and the carbon surface is activated with oxygen contained in the air and simultaneously doped with nitrogen.
Specifically, using an infrared image furnace, a rate of temperature increase of 30 ° C / min up to a predetermined temperature (400, 600, 700, 800, 900, 1000, 1100 ° C) under a nitrogen stream (200 ml / min) The temperature was raised. Then, the mixture was changed from nitrogen to a mixed gas of ammonia and air of 200 ml / min (ammonia concentration ratio 70%), heated to react for 2 hours, and then kept under a nitrogen stream for 10 minutes. The carbon catalyst sample prepared in this way is expressed as, for example, ND1400N600 (nitrogen treatment temperature 600 ° C.), ND1600N900 (nitrogen treatment temperature 900 ° C.), etc. (see FIG. 3 described later).

<2. Explanation of oxygen reduction activity of carbon catalyst of this example>
FIG. 2 is a schematic configuration diagram of an apparatus for measuring the oxygen reduction activity (reactivity of the catalyst) of the carbon catalyst (sample) thus prepared. This device is a device that is conventionally used to apply a substance to be measured (here, a carbon catalyst) to a disk electrode as a working electrode and observe a cathode electrode reaction of a fuel cell. Called.

  First, an outline of the structure of this electrochemical cell will be described. The electrochemical cell includes three electrodes, that is, a normal working electrode 3, a reference electrode 5, and a counter electrode 6. The electric current flowing between the working electrode 3 and the counter electrode 6 is measured by changing the potential of the working electrode 3 with respect to the reference electrode 5.

  In the electrochemical cell used for the measurement of the present invention, the glass vessel 1 supported by the three legs 9 is filled with the electrolyte solution 2 composed of a 0.5 mol oxygen saturated sulfuric acid aqueous solution, A working electrode (disk electrode) 3 formed of a disk is disposed. The working electrode 3 is coated with a sample to be measured, here a carbon catalyst. A ring electrode 3 a made of platinum is provided around the disk electrode 3. The potential of the ring electrode 3a is set independently of the potential of the disk electrode 3. When the disk electrode 3 is rotated, the electrolyte 2 is sucked toward the center of the disk electrode 3, and the electrolyte 2 flows parallel to the surface of the disk electrode 3 due to the centrifugal force of the rotation of the disk electrode 3. The ring electrode 3a is disposed in the flow of the electrolytic solution 2, and plays a role of electrolyzing and detecting a product generated by the reaction on the surface of the disk electrode 3.

As shown in FIG. 2, a glass container 4 different from the glass container 1 is provided, and a reference electrode 5 is provided in the glass container 4. As the reference electrode 5, for example, a silver / silver chloride electrode (Ag / AgCl) is used.
The glass container 1 and the glass container 4 are in communication, and both are filled with an electrolytic solution (aqueous sulfuric acid solution) 2. From the gas inlet 7, oxygen gas is introduced to saturate the electrolytic solution 2 with oxygen, and this oxygen gas bubbles the electrolytic solution 2 to obtain the electrolytic solution 2 saturated with oxygen. The electrolyte solution 2 saturated with oxygen causes an oxygen reduction reaction, which is a cathode reaction of the fuel cell, at the working electrode (disk electrode) 3. Thereafter, the oxygen gas introduced from the gas inlet 7 is discharged from the gas outlet 8.

  The procedure of operation using this electrochemical cell is as follows. First, a disk electrode 3, a ring electrode 3 a, and a counter electrode 6 that are working electrodes are set in an electrolytic solution (aqueous sulfuric acid solution) 2. In this state, nitrogen is introduced from the gas inlet 7 to forcibly discharge oxygen dissolved in the sulfuric acid aqueous solution (oxygen purge by nitrogen bubbling).

  Next, LSV (Linear Sweep Voltammetry) measurement is performed in an electrolyte purged with oxygen, and the voltammogram obtained here is used as a baseline. That is, the baseline is a current obtained by sweeping the potential in a nitrogen purged electrolyte, and specifically, a current response when the potential of the disk electrode 3 is linearly swept as a function of time. .

  Subsequently, oxygen is introduced into the sulfuric acid aqueous solution from the gas inlet 7 and oxygen bubbling is performed. Then, LSV is measured in the same manner using a sulfuric acid aqueous solution saturated with oxygen. In this LSV measurement, two currents are superimposed. One is a reaction current (Faraday current) based on oxygen reduction in the disk electrode 3, and the other is a current (non-Faraday current) due to charging / discharging of the electric double layer. By subtracting the previously obtained baseline current from the current obtained in the oxygen saturated sulfuric acid aqueous solution superimposed, a reaction current (Faraday current) based purely on oxygen reduction is obtained.

There are two reaction modes in the oxygen reduction reaction occurring on the surface of the working electrode (disk electrode) 3.
One is a reaction that produces water from oxygen (four-electron reaction), and the other is a reaction that produces hydrogen peroxide from oxygen (two-electron reaction). The latter is an undesirable reaction because the current efficiency per mole of oxygen is poor and hydrogen peroxide, a substance that causes battery deterioration, is produced. In the case of a carbon-based catalyst, it is important how hydrogen peroxide is not emitted, and the ring electrode 3a is provided to measure the amount of hydrogen peroxide that is an undesirable side reaction product. Here, the potential of the ring electrode 3a is set to a potential at which hydrogen peroxide is oxidized, electrolysis is performed up to oxygen, and the current accompanying the oxidation is measured.

  The glass container 4 provided with the reference electrode 5 is provided separately from the glass container 1 to minimize the diffusion of a saturated solution of potassium chloride (KCl) used for the reference electrode 5 and the reference electrode 5 is a working electrode. This is to prevent the electrolyte 2 from diffusing due to the rotation of 3. Further, a counter electrode 6 made of platinum is disposed at a position facing the working electrode 3 of the glass container 1. The counter electrode 6 is an electrode provided to allow a current to flow between the working electrode 3 and the counter electrode 6.

  The working electrode (disk electrode) 3 is rotationally driven by a motor (not shown) at a speed of 1500 revolutions per minute. The reason why the working electrode 3 is rotated at a high speed is that a laminar flow is formed on the electrode surface and the diffusion supply of the reactant (sample) is made constant. Further, the scanning speed of the potential of the working electrode 3 is 1 mV / s, and the working electrode 3 has a scanning speed of 0.8 V to −0.2 V with respect to the reference electrode 5 (Ag / AgCl). Scanned. As a result, the current flowing between the working electrode 3 and the counter electrode 6 is measured, and the current density per unit area is also measured.

<Experimental Example 1: Measurement with a rotating disk electrode device>
Using the rotating disk electrode device shown in FIG. 2, the oxygen reduction starting potentials of the carbon catalyst ND1400 series and ND1600 series samples of this example prepared in FIG. 1 were measured.
3A and 3B are oxygen reduction voltammograms of the carbon catalysts ND1400 series and ND1600 series of this example prepared using this method. In this oxygen reduction voltammogram, the horizontal axis represents the potential of the working electrode 3 with respect to a normal hydrogen electrode (NHE), and the vertical axis represents the current density (mA / cm ² ) flowing between the counter electrode 6 and the working electrode 3. ing.

  3A and 3B are diagrams showing the oxygen reduction activity of the carbon catalyst. In this figure, the working electrode 3 is at a higher potential (right side) than the standard hydrogen electrode (NHE), and the oxygen reduction activity increases as the current density between the counter electrode 6 and the working electrode 3 increases (goes down). Is high. As can be seen from FIG. 3, both the ND1400 series and ND1600 series samples have higher activity as the treatment temperature (nitrogen treatment temperature) of the ammoxidation method is higher, and the activity is higher in nitrogen-undoped samples (ND1400, ND1600). Becomes lower.

FIG. 4 is a graph obtained by measuring the oxygen reduction initiation potential (V vs. NHE) of the carbon catalyst of the heat treatment temperature 1400 ° C. series (● mark) and the heat treatment temperature 1600 ° C. series (circle mark) of nanodiamond ND. The horizontal axis represents the ammoxidation treatment temperature (nitrogen treatment temperature).
The oxygen reduction start potential (V vs. NHE) shown in FIG. 4 is obtained by enlarging the graph of FIG. 3 and obtaining the potential when the current density is −10 μA / cm ² . The oxygen reduction start potential is the potential of the working electrode 3 measured without flowing current between the working electrode (disk electrode) 3 and the counter electrode 6 in FIG. However, since it is difficult to measure the potential in a state where no current flows, the working electrode when a weak current having a current density of −10 μA / cm ² flows between the working electrode (disk electrode) 3 and the counter electrode 6 is used here. The potential of 3 was defined as the oxygen reduction starting potential. The potential of the working electrode 3 becomes the oxygen reduction start potential of the carbon catalysts ND1400 series and ND1600 series prepared in the present invention.

  In FIG. 4, in order to compare with the oxygen reduction start potential obtained from the oxygen reduction voltammogram, the oxygen reduction start potential of a conventional carbon catalyst prepared using a metal is indicated by ▲, and a conventional carbon catalyst prepared without using a metal is shown. The oxygen reduction start potential of the carbon catalyst is indicated by ■. Here, the conventional carbon catalyst prepared using metal is a carbon catalyst having a nanoshell structure described in Patent Document 1. It can be seen from FIG. 4 that the oxygen reduction activity is extremely high as compared with the carbon catalyst prepared without using a metal.

Further, as can be seen from FIG. 4, there is a correlation between an increase in the treatment temperature (nitrogen treatment temperature) of ammoxidation and an increase in oxygen reduction activity. That is, when the nitrogen treatment temperature is increased, the oxygen reduction start potential is increased, so it was initially thought that the oxygen reduction activity might be increased by increasing the proportion of nitrogen in the carbon.
However, in the experimental results, even if the nitrogen treatment temperature is increased, the nitrogen ratio in the carbon increases up to a certain temperature (for example, 600 ° C.), but if the treatment temperature is increased further, the nitrogen ratio in the carbon is increased. Conversely, it turned out to decrease. FIG. 5 illustrates this result.

<Experimental example 2: Measurement by X-ray photoelectron spectrometer>
Next, using XPS (X-ray photoelectron spectrometer), the nitrogen ratio (N / C atomic ratio) in carbon in the ND1400 series and ND1600 series of the carbon catalyst of this example was measured. This measurement method using XPS is a method of irradiating a sample with X-rays and detecting photoelectrons emitted thereby. Thereby, the element atomic composition and chemical bonding state of the sample surface can be known. The measurement results are shown in FIGS.

FIG. 5 is a plot of the N / C atomic ratio (nitrogen ratio in carbon) of the ND1400 series (● mark) and ND1600 series (◯ mark) when the nitrogen treatment temperature in FIG. 4 is taken on the horizontal axis. is there.
As can be seen from FIG. 5, both the ND1400 series and ND1600 series recorded the maximum N / C atomic ratio when the nitrogen treatment temperature was 600 ° C. When the nitrogen treatment temperature reached 600 ° C. or higher, the nitrogen treatment Conversely, as the temperature increases, the N / C atomic ratio decreases. On the other hand, as shown in FIG. 4, when the nitrogen treatment temperature is 600 ° C. or higher, the oxygen reduction activity increases as the temperature rises.

  The inventors thought that the amount of nitrogen doping initially increased the oxygen reduction activity, but this idea does not hold as long as FIGS. 4 and 5 are viewed. That is, when the nitrogen treatment temperature is 600 ° C. or higher, the oxygen reduction activity increases (FIG. 4) despite the N / C atomic ratio decreasing (FIG. 5) as the nitrogen treatment temperature increases.

  6 (A) and 6 (B) show the oxygen reduction initiation potential (V vs. NHE) as the nitrogen ratio (N / C atomic ratio) in the carbon material measured by XPS on the horizontal axis of FIG. It is. 6A shows the oxygen reduction start potential (V vs. NHE) of the ND1400 series sample (● mark), and FIG. 6B shows the oxygen reduction start potential (V) of the ND1600 series sample (circle mark). vs. NHE). Again, what can be seen from FIGS. 5 and 6 is that there is not necessarily a correlation between a high nitrogen ratio in the carbon (high N / C atomic ratio) and a high oxygen reduction activity.

  This suggests that the nitrogen content in the carbon greatly affects the oxygen reduction activity until the nitrogen treatment temperature is up to 600 ° C, but if the doping temperature is 600 ° C or higher, some other mechanism that is not the nitrogen doping amount acts. Therefore, it is natural to think that the oxygen reduction activity is increased. In the ND1400 series of FIG. 6A, when the nitrogen treatment temperature is 600 ° C. or higher and the nitrogen doping ratio (N / C atomic ratio) is 4% or lower, the oxygen reduction start potential becomes 0.6 V or higher and becomes high potential. ing. In the ND1600 series of FIG. 6B, when the nitrogen treatment temperature is 600 ° C. or higher and the nitrogen doping ratio (N / C atomic ratio) is 2% or lower, the oxygen reduction starting potential is as high as 0.55 V or higher. It is at electric potential. That is, it can be seen that high oxygen reduction activity is exhibited.

The inventors interpret this phenomenon as follows. In other words, in both the ND1400 series and ND1600 series, even if the nitrogen treatment temperature is raised to 600 ° C or higher, nitrogen does not enter the carbon, and conversely, the phenomenon that the nitrogen contained in the carbon lattice escapes. Isn't it? Then, the defect (lattice defect) generated by the separation of nitrogen may become an active site and increase the oxygen reduction activity.
Here, the defect means that the bond is in an unsaturated state. Since a radical is formed at a place where there is a defect, the radical becomes a defect existence index. In other words, since there is a tendency to become pure carbon (carbon) at high temperatures, it is presumed that nitrogen is released at high temperatures, and the sites (defects) from which the nitrogen has been released lead to an increase in radicals. .

<Experimental Example 3: Measurement by cyclic voltammetry (CV)>
In order to track this phenomenon in more detail, the inventors have used cyclic voltammetry (CV), nitrogen undoped samples of ND1400 series and ND1600 series (without ammoxidation treatment), and nitrogen treatment temperature. A sample subjected to (ammoxidation treatment temperature, 400 ° C., 600 ° C., 700 ° C., 800 ° C., 900 ° C., 1000 ° C., 1100 ° C.) was immersed in an aqueous sulfuric acid solution, and the current change with respect to the voltage was measured. 7A to 7D are diagrams showing the results.

  Here, cyclic voltammetry (CV) is a method for examining the electrochemical behavior of the interface by applying a change in potential and measuring the change in current. That is, charging / discharging of the electrochemical double layer formed at the carbon / electrolyte solution interface can be measured by changing the potential and measuring the current. The horizontal axis in FIG. 7 represents the potential with respect to the standard hydrogen electrode (NHE), and the vertical axis represents the current.

  It can be seen from FIG. 7 that the current value on the low potential side increases as the nitrogen treatment temperature increases to 400 ° C. and 600 ° C. Such an increase in current on the low potential side is a phenomenon observed in carbon doped with nitrogen, and charging / discharging is not electrostatic and depends on an electrochemical reaction involving a functional group formed by the introduced nitrogen. it seems to do.

  That is, in the CV measurement result, the closer to the rectangle, the closer to the response of a pure electric double layer, that is, only the physical capacitance contributes to the current. However, in FIG. 7, the rectangle is deformed on the low potential side. This deformation on the low potential side is considered to have a contribution of pseudo capacitance due to the presence of nitrogen in addition to physical capacitance, and can be interpreted as an increase in current due to this pseudo capacitance.

  The expression of the pseudo capacity means that surface functional groups containing nitrogen are present. This surface functional group usually appears at an interface potential around 0.05 V with respect to a standard hydrogen electrode (NHE). On the other hand, the current at a potential of 0.6 V is a current that depends purely on the physical capacitance. Accordingly, FIG. 8 shows the oxygen reduction starting potential (V vs. NHE) with respect to the ratio between the current values I (0.05 V) and I (0.6 V) of the interface potential (0.05 V) and the potential 0.6 V, respectively.

  From FIG. 8, in the state where the surface functional groups are low (for example, the nitrogen treatment temperature is 400 ° C.), both I (0.05 V) and I (0.6 V) contribute only to physical capacitance. It turns out that it becomes equal value and oxygen reduction activity is not high. When surface functional groups increase by nitrogen treatment, I (0.05V) becomes larger than I (0.6V), and oxygen reduction activity temporarily increases. However, it was also found that the nitrogen treatment temperature was up to 600 ° C., and I (0.05 V) / I (0.6 V) decreased and the oxygen reduction activity increased at temperatures higher than that. That is, when the change in the nitrogen treatment temperature is followed, as shown by the dotted lines in FIGS. I understand that.

<Experimental Example 4: Measurement by X-band ESR (electron spin resonance) apparatus>
Next, referring to FIG. 9 to FIG. 12, the carbon catalyst ND1400 series and ND1600 series of this example prepared in FIG. 1 are measured with an X-band ESR (Electron Spin Resonance) apparatus appearing at 3200 to 3500 gauss. The results will be described.
This ESR intensity (ESR Intensity) represents the absorption intensity of microwaves, and an arbitrary unit (au) is used in FIGS.
FIG. 9 is a diagram for explaining a method for obtaining the parameter W from the ESR spectrum, and represents the ESR intensity (ESR Intensity) with respect to the magnetic field intensity (Gauss). Here, a differential waveform commonly used in this measurement is used. The positive or negative waveform of the differential waveform has the same shape (point symmetry) only if the polarity is reversed if the component originates from a single cause. Therefore, either the positive or negative waveform was examined, and the width of the magnetic field at the portion where the amplitude value was halved was measured. The width of the magnetic field is an extremely meaningful value in describing the carbon catalyst of the present invention. Therefore, the width of the magnetic field is defined as “parameter W” and used thereafter.

  The differential waveform measured by ESR is considered to be a waveform obtained as a result of absorption of microwaves by radicals remaining in a defect portion formed by carbon-nitrogen bond or carbon-carbon bond cleavage. . For example, in the graphite system, it is known that oxygen is adsorbed on the edge surface of graphite, and that part has the highest reaction activity, and the carbon catalyst of this example has the same structure. Presumed. In other words, in the carbon catalyst of this example, the differential waveform measured by ESR serves as an indicator indicating a bond defect. In other words, since radicals that do not exist in pairs of electron spins absorb microwaves, searching for this spin reveals that there is a defect in the carbon structure or on its surface. And it is thought that oxygen reduction activity becomes strong because oxygen adsorbs to this defect.

FIGS. 10A to 10F show the carbon catalyst sample ND1400 series of this example prepared in FIG. 1, each of nitrogen undoped, nitrogen treatment temperatures of 400 ° C., 600 ° C., 900 ° C., 1000 ° C., and 1100 ° C. It is the figure which showed the result of having measured the ESR of the sample.
The parameter W in the nitrogen-undoped sample ND1400 in FIG. 10 (A) and the nitrogen treatment temperature ND1400N400 in FIG. 10 (B) is relatively wide, whereas ND1400N600 to ND1400N1100 with the nitrogen treatment temperature increased to 600 ° C. or higher. It can be seen that the parameter W of these samples (FIGS. 10C to 10F) is narrower than the samples of ND1400 and ND1400N400.

  Similarly, FIGS. 11A to 11F show the results of measuring ESR for the sample ND1600 series of the carbon catalyst of this example prepared in FIG. FIGS. 11A to 11F also correspond to samples with nitrogen undoped and nitrogen treatment temperatures of 400 ° C., 600 ° C., 900 ° C., 1000 ° C., and 1100 ° C., respectively. The ND1600 series sample of FIG. 11 is not as obvious as the ND1400 series sample of FIG.

  12 (A) and 12 (B) show the parameter W (unit: Gauss) on the horizontal axis and the oxygen reduction starting potential (V vs. NHE) on the vertical axis for the ND1400 series and ND1600 series carbon catalysts, respectively. FIG. FIG. 12C is a diagram in which FIGS. 12A and 12B are synthesized. As can be seen from FIG. 12A, in the ND1400 series, the parameter W is about 17 to 18 gauss and the oxygen reduction starting potential is as low as about 0.4 V at a nitrogen undoped and nitrogen treatment temperature of 400 ° C. (N400). It has become. On the other hand, in the samples with nitrogen treatment temperatures of 600 ° C., 900 ° C., 1000 ° C., and 1100 ° C., the parameter W is as narrow as 8 to 11 gauss and the oxygen reduction starting voltage is also around 0.6 to 0.7 V. You can see that it is getting higher.

  In the sample of ND1600 shown in FIG. 12B, the parameter W is about 13 Gauss and the oxygen reduction starting potential is as low as about 0.4 V when nitrogen is not doped and the nitrogen treatment temperature is 400 ° C. (N400). On the other hand, in the samples with nitrogen treatment temperatures of 600 ° C., 900 ° C., 1000 ° C., and 1100 ° C., the parameter W is as narrow as 8 to 10 gauss and the oxygen reduction start potential is as high as 0.5 to 0.7 V. It has become. In particular, the nitrogen treatment temperatures of 900 ° C., 1000 ° C., and 1100 ° C. have an oxygen reduction starting potential of about 0.7 V, which is higher than that of the nitrogen-undoped sample.

  As already explained, the ESR parameter W is different for each sample in both the ND1400 series and ND1600 series, which means that the carbon structure of the carbon catalyst and the carbon structure containing nitrogen or the state of defects on its surface are different. It is thought that it depends. This means that the ND1400 series and ND1600 series carbon catalysts prepared by the inventors are novel carbon catalysts having structural defects different from those of the carbon catalysts prepared by conventional methods.

  That is, the samples of the carbon catalyst ND1400 series and ND1600 series produced by the preparation method of this example shown in FIG. 1 and FIG. 2 are new that did not exist so far as long as the measurement results of the respective experimental examples described above are seen. It can be said that it is a good carbon catalyst. In other words, as shown in FIG. 3, the carbon catalyst of this example whose oxygen reduction starting potential (oxygen reduction activity) is comparable to that of a conventional nanoshell structure prepared using a metal is shown in FIG. The ESR parameter W can be specified to be 11 gauss or less.

<3. Application of Fuel Cell to Electrocatalyst>
The carbon catalyst of this example is used as a cathode catalyst layer of a fuel cell. FIG. 13 shows a basic configuration of the fuel cell.
The fuel cell 10 includes a fuel electrode (anode) 20, an air electrode (cathode) 30, and an electrolyte material 40. The fuel electrode 20 is composed of an anode catalyst 21 usually made of platinum and a porous conductive support layer 22 for supporting the anode catalyst 21, and the air electrode 30 is composed of a cathode catalyst 31 and a porous conductive support layer 32. A load 50 is connected between the anode 20 and the cathode 30. A separator 60 is provided outside the fuel electrode (anode) 20 and the air electrode (cathode) 30.

As shown in the figure, hydrogen (H ₂ ) is supplied to the fuel electrode (anode) 20 of the fuel cell configured as described above, and in the anode catalyst 21, hydrogen releases electrons to become hydrogen ions. The electrons move to the air electrode (cathode) 30 via the load (electric circuit) 50. On the other hand, hydrogen ions move in the electrolyte material 40 toward the air electrode (cathode) 30, and combine with oxygen and electrons moving through the load at the air electrode (cathode) 30 to generate water.
The carbon catalyst of this example is preferably used mainly as a cathode catalyst on the air electrode (cathode) 30 side.

  As described above, it has been proved through several experiments that the carbon catalyst prepared by the inventors exhibits an oxygen reduction activity equivalent to that of a carbon catalyst having a nanoshell structure produced using a conventional metal. Although the structure of the carbon catalyst of the present invention is not a stage that can be specifically identified, it can be applied to many uses including fuel cells in that it can obtain the same activity as a conventional carbon catalyst in that no metal is used. It is considered possible.

DESCRIPTION OF SYMBOLS 1, 4 ... Glass container, 2 ... Electrolyte solution (sulfuric acid aqueous solution), 3 ... Working electrode (disk electrode), 3a ... Ring electrode, 5 ... Reference electrode (Ag / AgCl), 6 ... Counter electrode (platinum), 7 ... Gas inlet, 8 ... Gas outlet, 20 ... Fuel electrode (anode), 30 ... Air electrode (cathode), 40 ... Electrolyte material

Claims (8)

  A carbon catalyst having an X-band electron spin resonance spectrum parameter W appearing at 3200-3500 gauss of 11 gauss or less.   The carbon catalyst according to claim 1, which has a defect in the carbon structure.   The carbon catalyst according to claim 1 or 2, wherein a nitrogen ratio relative to carbon is 4% or less when a nitrogen ratio in carbon is measured by an X-ray photoelectron spectrometer.   The carbon catalyst according to any one of claims 1 to 3, wherein a ratio of a current value of 0.05 V to the standard hydrogen electrode and a current value of 0.6 V to the standard hydrogen electrode is 1.3 or less.   The carbon catalyst according to any one of claims 1 to 4, which was prepared by heat-treating nanodiamond as a starting material to about 1400 ° C to 1600 ° C and subjecting nanodiamond having a nano-onion structure to nitrogen treatment at 600 ° C or higher. A first heating step of heating to about 1000 ° C. using nano diamond as a starting material,
The heated nanodiamond is further heated to a high temperature and heated to 1400 ° C to 1600 ° C to obtain a nanoonion-structured nanodiamond,
After the second heating step, a step of preparing the nanodiamond by performing nitrogen treatment by ammoxidation at a temperature of 600 ° C. or higher.   The cathode catalyst layer of the fuel cell using the carbon catalyst in any one of Claims 1-5.   A fuel cell using the cathode catalyst layer according to claim 7 as a cathode.