Patent Description:
In Patent Literature <NUM>, there is described an oxygen reduction catalyst, including a stack of single-layer graphene and a phosphorous compound, and having such a graphene skeleton that carbon atoms of the graphene are partially replaced by nitrogen atoms, in which the phosphorus compound has a peak of a phosphorus 2p orbital of from <NUM> eV to <NUM> eV in an X-ray photoelectron spectrum, or a peak of a phosphorus 2p orbital of the phosphorus compound is shifted by from <NUM> eV to <NUM> eV toward a high-energy side from a peak of a phosphorus 2p orbital of tetra-n-butylphosphonium bromide in an X-ray photoelectron spectrum. It is also described that the oxygen reduction catalyst further contains iron or cobalt, and that the oxygen reduction catalyst further contains platinum. Non-Patent Literature <NUM> describes producing a carbon catalyst that is doped with nitrogen, phosphorus and iron by thermally decomposing polyacrylonitrile nanospheres in the presence of (NH<NUM>)<NUM>HPO<NUM> and FeCl<NUM> ·<NUM><NUM>O. Further, Non-Patent Literature <NUM> discloses carbon catalysts for fuel cells comprising iron and phosphorus atoms with a certain ratio of a concentration of phosphorus atoms with respect to a concentration of carbon atoms in X-ray photoelectron spectroscopic measurement.

Hitherto, however, it has not been clear in which state phosphorus atoms contribute to the improvement of catalytic activity in a carbon catalyst.

The present invention has been made in view of the above-mentioned problem, and one of the objects of the present invention is to provide a carbon catalyst having improved catalytic activity, and an electrode and a battery including the carbon catalyst.

According to one embodiment of the present invention for solving the above-mentioned problem, there is provided a carbon catalyst including a metal and phosphorus atoms, wherein the metal is one or more kinds selected from a group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, wherein the carbon catalyst has a ratio of a concentration (atomic%) of the phosphorus atoms with respect to a concentration (atomic%) of carbon atoms in X-ray photoelectron spectroscopic measurement of <NUM> or more, the phosphorus atoms exhibiting a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, which is obtained by peak separation of a phosphorus atom P2p peak.

According to the embodiment of the present invention, a carbon catalyst having improved catalytic activity is provided.

In addition, the carbon catalyst may have a ratio of an area of the following peak (<NUM>) with respect to a total area of the following five peaks (<NUM>) to (<NUM>) in the X-ray photoelectron spectroscopic measurement of <NUM> or more, the five peaks being obtained by the peak separation of the phosphorus atom P2p peak: (<NUM>) a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; (<NUM>) a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM>. <NUM> eV; (<NUM>) a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; (<NUM>) a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; and (<NUM>) a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV.

In the present invention, the carbon catalyst has a carbon structure in which area ratios (%) of the following two peaks fbroad and fnarrow obtained by separating a peak in a vicinity of a diffraction angle (2θ) of <NUM>° in an X-ray diffraction pattern in powder X-ray diffraction measurement satisfy the following conditions (a) and (b): (a) a peak fbroad having a peak top within a range of a diffraction angle (<NUM>) of <NUM>°±<NUM>° and having a full width at half maximum of <NUM>°±<NUM>°: <NUM>% or more and less than <NUM>%; and (b) a peak fnarrow having a peak top within a range of a diffraction angle (<NUM>) of <NUM>°±<NUM>° and having a full width at half maximum of <NUM>°±<NUM>°. more than <NUM>% and <NUM>% or less.

According to one embodiment of the present invention for solving the above-mentioned problem, there is provided an electrode, including any one of the above-mentioned carbon catalysts. According to the embodiment of the present invention, an electrode having improved performance is provided.

According to one embodiment of the present invention for solving the above-mentioned problem, there is provided a battery, including the above-mentioned electrode. According to the embodiment of the present invention, a battery having improved performance is provided.

According to the present invention, a carbon catalyst having improved catalytic activity, and an electrode and a battery including the carbon catalyst are provided.

Now, embodiments of the present invention will be described. The present invention is as defined in the appended claims, but not limited to examples shown in these embodiments.

A carbon catalyst according to one embodiment of the present invention (hereinafter referred to as "catalyst of the present invention") includes a metal and phosphorus atoms, where the carbon catalyst has a ratio of a concentration (atomic%) of the phosphorus atoms with respect to a concentration (atomic%) of carbon atoms in X-ray photoelectron spectroscopic measurement of <NUM> or more, where the phosphorus atoms exhibit a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, which is obtained by peak separation of a phosphorus atom P2p peak.

That is, the catalyst of the present invention is a carbon catalyst containing a metal and phosphorus atoms. More specifically, the catalyst of the present invention is a carbonized material containing a metal and phosphorus atoms, which is obtained by carbonizing a raw material containing an organic substance, the metal, and a phosphorus compound as will be described later.

The metal contained in the catalyst of the present invention is one or more kinds selected from a group consisting of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn). In this embodiment, the metals belong to the fourth period of Groups III to XII in the periodic table.

The metal may be preferably one or more kinds selected from a group consisting of Fe, Cu, and Zn.

One of the characteristic features of the catalyst of the present invention is that a P<NUM>/C ratio is a predetermined threshold value or more, where the P<NUM>/C ratio is a ratio, which is obtained by the XPS measurement, of a concentration (atomic%) of particular phosphorus atoms exhibiting the above-mentioned peak (peak P<NUM>) having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV with respect to a concentration (atomic%) of carbon atoms.

That is, the inventors of the present invention have repeatedly conducted extensive investigations on technical means for improving the catalytic activity of a carbon catalyst containing a metal and phosphorus atoms. As a result, the inventors of the present invention have uniquely found that particular phosphorus atoms exhibiting the above-mentioned peak P<NUM> obtained by peak separation of a phosphorus atom P2p peak in an XPS spectrum contributes to the improvement of the catalytic activity of the carbon catalyst, to thereby achieve the present invention.

In the XPS spectrum, the P2p peak appears within a range of <NUM>±<NUM> eV. The P2p peak includes peaks of five kinds of phosphorus atoms having different oxidation states due to different binding manners to other atoms.

That is, the P2p peak can be separated into the following five peaks (<NUM>) to (<NUM>) by peak separation described later in detail in Examples: (<NUM>) a peak (peak P<NUM>) having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; (<NUM>) a peak (peak P<NUM>) having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; (<NUM>) a peak (peak P<NUM>) having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; (<NUM>) a peak (peak P<NUM>) having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV; and (<NUM>) a peak (peak P<NUM>) having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV.

As a result, the inventors of the present invention have found that phosphorus atoms exhibiting a particular peak that is the above-mentioned peak P<NUM> among those five peaks contributes to the catalytic activity of the carbon catalyst containing a metal and phosphorus atoms.

As described above, the peak P<NUM> is specified as one of the above-mentioned five peaks obtained by peak separation of a phosphorus atom P2p peak in an XPS spectrum of the catalyst of the present invention.

The phosphorus atoms exhibiting the peak P<NUM> encompass, for example, phosphorus atoms bound to one or two oxygen atoms and also bound to one or two carbon atoms. More specifically, the phosphorus atoms exhibiting the peak P<NUM> encompass, for example, phosphorus atoms bound to two oxygen atoms and bound to two carbon atoms, and/or phosphorus atoms bound to two oxygen atoms and bound to one carbon atom.

On the other hand, the peak P<NUM> is a peak derived from elemental phosphorus atoms. The peak P<NUM> is a peak derived from phosphorus atoms of a phosphoric acid type. The peak P<NUM> is a peak derived from phosphorus atoms of the -O-PO<NUM> atomic group. The peak P<NUM> is a peak derived from phosphorus atoms of a diphosphorus pentoxide type.

In some cases, at least one of the four peaks other than the peak P<NUM> is not substantially detected as a result of performing peak separation for separating the P2p peak into the above-mentioned five peaks in the catalyst of the present invention.

There is no particular limitation on the P<NUM>/C ratio of the catalyst of the present invention as long as the P<NUM>/C ratio is <NUM> or more, but for example, the P<NUM>/C ratio is preferably <NUM> or more, more preferably <NUM> or more. Further, when the P<NUM>/C ratio of the catalyst of the present invention is <NUM> or more, the P<NUM>/C ratio is preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, particularly preferably <NUM> or more.

There is no particular limitation on the upper limit value of the P<NUM>/C ratio of the catalyst of the present invention, but for example, the P<NUM>/C ratio may be <NUM> or less. As long as the P<NUM>/C ratio falls within any one of the above-mentioned ranges, the catalyst of the present invention has excellent catalytic activity.

The catalyst of the present invention may have a ratio of an area of the above-mentioned peak (<NUM>) (peak P<NUM>) with respect to a total area of the above-mentioned five peaks (<NUM>) to (<NUM>) (peak P<NUM>, peak P<NUM>, peak P<NUM>, peak P<NUM>, and peak P<NUM>), which are obtained by peak separation of a phosphorus atom P2p peak in the X-ray photoelectron spectroscopic measurement, of <NUM> or more.

That is, in this case, the ratio (P<NUM>/Ptotal ratio) of the area of the peak P<NUM> (area P<NUM>) with respect to the total of the area of the peak P<NUM>, the area of the peak P<NUM>, the area of the peak <NUM>, the area of the peak P<NUM>, and the area of the peak P<NUM> (area Ptotal) obtained by the XPS of the catalyst of the present invention is <NUM> or more.

When at least one of the four peaks other than the peak P<NUM> is not substantially detected as a result of performing peak separation for separating the P2p peak into the above-mentioned five peaks, the P<NUM>/Ptotal ratio is calculated with the area of the peak that has not been detected being zero.

There is no particular limitation on the ratio of a concentration (atomic%) of phosphorus atoms with respect to a concentration (atomic%) of carbon atoms (hereinafter sometimes referred to as "P/C ratio") obtained by the XPS of the catalyst of the present invention. However, for example, the P/C ratio may be <NUM> or more, preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, particularly preferably <NUM> or more. There is no particular limitation on the upper limit value of the P/C value of the catalyst of the present invention, but for example, the P/C ratio may be <NUM> or less.

There is no particular limitation on the concentration (atomic%) of the phosphorus atoms obtained by the XPS of the catalyst of the present invention. However, for example, the concentration may be <NUM> atomic% or more, preferably <NUM> atomic% or more, more preferably <NUM> atomic% or more, particularly preferably <NUM> atomic% or more. There is no particular limitation on the upper limit value of the concentration (atomic%) of the phosphorus atoms of the catalyst of the present invention, but for example, the concentration (atomic%) of the phosphorus atom may be <NUM> (atomic%) or less.

The catalyst of the present invention has a carbon structure in which area ratios (%) of the following two peaks fbroad and fnarrow obtained by separating a peak in the vicinity of a diffraction angle (2θ) of <NUM>° in an X-ray diffraction pattern in powder X-ray diffraction measurement satisfy the following conditions (a) and (b): (a) a peak fbroad having a peak top within a range of a diffraction angle (<NUM>) of <NUM>°±<NUM>° and having a full width at half maximum of <NUM>°±<NUM>°: <NUM>% or more and less than <NUM>%; and (b) a peak fnarrow having a peak top within a range of a diffraction angle (2θ) of <NUM>°±<NUM>° and having a full width at half maximum of <NUM>°±<NUM>°: more than <NUM>% and <NUM>% or less.

When the carbon catalyst has a laminate structure formed of a curved net surface that contributes to the catalytic activity thereof, a diffraction peak of a carbon (<NUM>) plane appears in the vicinity of a diffraction angle (2θ) of <NUM>° (within a range of <NUM>° or more and <NUM>° or less) in an X-ray diffraction pattern. This peak includes the following two kinds of peaks: a graphite structure peak (fnarrow) derived from a (<NUM>) plane of a graphite structure that is a high-crystalline component, and a peak (fbroad) derived from a low-crystalline component.

In this respect, through peak separation of the X-ray diffraction pattern, the peak in the vicinity of <NUM>° can be separated into two peaks fbroad and fnarrow. Specifically, the peak separation is performed by the following procedures. First, an X-ray diffraction pattern obtained by powder X-ray diffraction measurement is subjected to intensity correction of a polarization factor, a Lorentz factor, and an atom scattering factor of carbon, and is also subjected to background correction in which a straight line connecting the vicinity of the diffraction angle of from <NUM>° to <NUM>° to the vicinity of the diffraction angle of from <NUM>° to <NUM>° is defined as a background, and the background is subtracted from each diffraction intensity after the intensity correction. Next, in the corrected X-ray diffraction pattern, the peak having a peak top in the vicinity of the diffraction angle 2θ of <NUM>° is superimposed onto a Gaussian basic waveform to be approximated, to thereby optimize a peak intensity, a peak full width at half maximum, and a peak position, and each of two superimposed peaks included in the above-mentioned peak is subjected to curve fitting, to thereby perform peak separation. The curve fitting is performed so that a residual sum of squares becomes smallest. The residual square refers to a square of a residual error at each measured diffraction angle, and the residual sum of squares refers to a sum of residual squares. In addition, the residual error refers to a difference between the intensity of the peak having a peak top in the vicinity of the diffraction angle 2θ of <NUM>° in the corrected X-ray diffraction pattern and the sum of intensities of the two separated peaks (fbroad and fnarrow).

Through such peak separation, two peaks, that is, the peak fbroad of a low-crystalline component and the peak fnarrow of a high-crystalline component, are obtained. The peak fdreae has a peak top within a range of a diffraction angle of <NUM>°±<NUM>° and has a full width at half maximum of <NUM>°±<NUM>°. The peak fnarrow has a peak top within a range of a diffraction angle of <NUM>°±<NUM>° and has a full width at half maximum of <NUM>°±<NUM>°. The diffraction angle of the peak fnarrow is larger than that of the peak fbroad.

The two peaks of the catalyst of the present invention may satisfy the following conditions (a) and (b): (a) fbroad: <NUM>% or more and <NUM>% or less; and (b) fnarrow: <NUM>% or more and <NUM>% or less.

The lower limit value within each range of the above-mentioned condition (b) is not limited to <NUM>%. For example, each range of the condition (b) may be <NUM>% or more, and each range of the condition (a) may be <NUM>% or less.

The fact that the two peaks of the catalyst of the present invention satisfy the conditions (a) and (b), that is, the fact that the carbon structure of the catalyst of the present invention contains the low-crystalline component satisfying the condition (a) and the high-crystalline component satisfying the condition (b), effectively contributes to the catalytic activity and/or durability of the catalyst of the present invention.

The catalyst of the present invention has catalytic activity. Specifically, the catalyst of the present invention has, for example, oxygen reduction activity. In this case, the catalyst of the present invention effectively catalyzes an oxygen reduction reaction, for example, in an electrode for a fuel cell or an electrode for an air cell.

The oxygen reduction activity of the catalyst of the present invention is evaluated, for example, based on a current density i<NUM> (mA/cm<NUM>) at the time of application of a voltage of <NUM> V (vs. RHE) in data (oxygen reduction voltammogram) representing a relationship between the voltage and the current density obtained by performing sweep application of a potential through use of a rotating ring disk electrode device including a working electrode having the catalyst of the present invention applied thereto.

In this case, the current density i<NUM> exhibited by the catalyst of the present invention may be, for example, -<NUM> (mA/cm<NUM>) or less (e.g., from -<NUM> (mA/cm<NUM>) to -<NUM> (mA/cm<NUM>)), preferably -<NUM> (mA/cm<NUM>) or less (e.g., from -<NUM> (mA/cm<NUM>) to -<NUM> (mA/cm<NUM>)), particularly preferably -<NUM> (mA/cm<NUM>) or less (e.g., from -<NUM> (mA/cm<NUM>) to -<NUM> (mA/cm<NUM>) ).

The catalyst of the present invention is obtained by carbonizing a raw material containing an organic substance, a metal, and a phosphorus compound. That is, the catalyst of the present invention is a carbonized product of the raw material containing the organic substance, the metal, and the phosphorus compound. The organic substance contained in the raw material is not particularly limited as long as the organic substance can be carbonized. Specifically, as the organic substance, for example, high-molecular-weight organic compounds (e.g., resins, such as a thermosetting resin and/or a thermoplastic resin), and/or low-molecular-weight organic compounds are used. In addition, a biomass may be used as the organic substance.

As the organic substance, a nitrogen-containing organic substance is preferably used. The nitrogen-containing organic substance is not particularly limited as long as the nitrogen-containing organic substance is an organic substance containing an organic compound that contains a nitrogen atom in a molecule thereof, and any one or more kinds thereof are used. The catalyst of the present invention obtained through use of the raw material containing the nitrogen-containing organic substance contains a nitrogen atom.

The content of the organic substance in the raw material is not particularly limited as long as the content falls within a range in which the catalyst of the present invention is obtained. The content may be, for example, <NUM> mass% or more and <NUM> mass% or less, preferably <NUM> mass% or more and <NUM> mass% or less.

The metal contained in the raw material (the metal contained in the catalyst of the present invention) is one or more kinds selected from a group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, or preferably one or more kinds selected from a group consisting of Fe, Cu, and Zn. In this embodiment, the metals belong to the fourth period of Groups III to XII in the periodic table.

As the metal contained in the raw material, a simple substance of the metal or a compound of the metal is used. As the metal compound, one or more kinds selected from a group consisting of, for example, a metal salt, a metal oxide, a metal hydroxide, a metal nitride, a metal sulfide, a metal carbide, and a metal complex may be used.

The content of the metal in the raw material is not particularly limited as long as the content falls within a range in which the catalyst of the present invention is obtained. The content may be, for example, <NUM> mass% or more and <NUM> mass% or less, preferably <NUM> mass% or more and <NUM> mass% or less.

Carbonization is performed by heating a raw material and keeping the raw material at a temperature at which the raw material is carbonized (hereinafter referred to as "carbonizing temperature"). The carbonizing temperature is not particularly limited as long as the raw material is carbonized. The carbonizing temperature may be, for example, <NUM> or more, <NUM> or more, or <NUM> or more. More specifically, the carbonizing temperature may be, for example, <NUM> or more and <NUM>,<NUM> or less, <NUM> or more and <NUM>,<NUM> or less, or <NUM> or more and <NUM>,<NUM> or less.

The temperature increase rate up to the carbonizing temperature is, for example, <NUM>/min or more and <NUM>/min or less. The period of time for keeping the raw material at the carbonizing temperature is, for example, 5minutes or more and <NUM> hours or less. It is preferred that the carbonization be performed under the circulation of inert gas, such as nitrogen.

The catalyst of the present invention is a carbonized material obtained by the above-mentioned carbonization of a raw material. Specifically, the carbonized material obtained by the above-mentioned carbonization may be used directly as the catalyst of the present invention. In addition, the catalyst of the present invention may be obtained by subjecting the carbonized material obtained by the above-mentioned carbonization to further treatment. Specifically, the catalyst of the present invention may be obtained by, for example, subjecting the carbonized material to metal removal treatment (e.g., acid washing treatment or electrolytic treatment).

An electrode according to one embodiment of the present invention (hereinafter referred to as "electrode of the present invention") includes the catalyst of the present invention. Specifically, the electrode of the present invention is, for example, an electrode carrying the catalyst of the present invention. Specifically, the electrode of the present invention is, for example, an electrode including an electrode base material and the catalyst of the present invention carried on the electrode base material.

The electrode of the present invention is, for example, an electrode for a battery. Specifically, the electrode of the present invention is, for example, an electrode for a fuel cell or an electrode for an air cell. In addition, the electrode of the present invention is, for example, a cathode electrode or an anode electrode, preferably a cathode electrode.

A battery according to one embodiment of the present invention (hereinafter referred to as "battery of the present invention") includes the electrode of the present invention. Specifically, the battery of the present invention is, for example, a fuel cell or an air cell including the electrode of the present invention. The battery of the present invention may include, for example, a membrane/electrode assembly including the electrode of the present invention. The battery of the present invention is a battery including the electrode of the present invention as one or both of the cathode electrode and the anode electrode, preferably a battery including the electrode of the present invention as the cathode electrode.

Next, specific Examples according to the embodiments of the present invention will be described.

<NUM> of folic acid, <NUM> of iron(III) chloride hexahydrate (FeCl<NUM>·<NUM><NUM>O), and <NUM> of phosphoric acid were mixed to prepare a raw material to be carbonized. Then, the raw material was placed in a quartz tube and heated at a temperature increase rate of <NUM>/min in a nitrogen atmosphere in an image furnace and kept at <NUM>,<NUM> for <NUM> hour to be carbonized.

A carbonized material obtained by the carbonization was pulverized with a planetary ball mill (P-<NUM>, manufactured by Fritsch Japan Co. ) in which silicon nitride balls each having a diameter of <NUM> were set, and the resultant was sieved with a sieve of <NUM>. Further, a1 M HCl aqueous solution was added to the resultant, and the mixture was stirred for <NUM> hour. After that, the carbonized material was collected by suction filtration and subjected to thermal vacuum drying at <NUM>. Thus, a powdery carbon catalyst was obtained.

A raw material to be carbonized was prepared through mixing in the same manner as in Example <NUM> except that <NUM> of polyacrylonitrile was used in place of folic acid and <NUM> of phosphoric acid was used. Then, the obtained mixture was heated in the atmosphere to be subjected to infusibilization. Specifically, the mixture was heated in the atmosphere so as to be increased in temperature from room temperature to <NUM> over <NUM> minutes and then increased in temperature from <NUM> to <NUM> over <NUM> hours. After that, the mixture was kept at <NUM> for <NUM> hours to be subjected to infusibilization. Thus, the raw material to be carbonized was prepared. Then, carbonization, pulverization, acid treatment, and drying were performed in the same manner as in Example <NUM>. Thus, a carbon catalyst was obtained.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of riboflavin was used in place of folic acid and <NUM> of phosphoric acid was used.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of copper (I) chloride (CuCl) was further used for a raw material to be carbonized.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of zinc chloride (ZnCl<NUM>) was further used for a raw material to be carbonized.

<NUM> of riboflavin, <NUM> of iron (III) chloride hexahydrate (FeCl<NUM> · <NUM><NUM>O), and <NUM> of phosphoric acid were mixed to prepare a raw material to be carbonized. Then, the raw material was placed in a quartz tube and heated at a temperature increase rate of <NUM>/min in a nitrogen atmosphere in an image furnace and kept at <NUM> for <NUM> hour to be carbonized.

Further, after pulverization and acid washing in the same manner as in Example <NUM>, a carbonized material obtained by the carbonization at <NUM> was subjected to additional carbonization by being heated at a temperature increase rate of <NUM>/min in a nitrogen atmosphere and kept at <NUM>,<NUM> for <NUM> hour. The carbonized material obtained by the carbonization at <NUM>,<NUM> was pulverized in the same manner as in Example <NUM>. Thus, a powdery carbon catalyst was obtained.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of phosphoric acid was used.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of folic acid was used in place of riboflavin.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of zinc chloride (ZnCl<NUM>) was further used.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of iron (II) sulfate heptahydrate (FeSO<NUM> · <NUM><NUM>O) was used in place of iron chloride, <NUM> of a melamine resin was used in place of folic acid, and <NUM> of triphenylphosphine was used in place of phosphoric acid.

A carbon catalyst was obtained in the same manner as in Example <NUM> except that <NUM> of iron(III) chloride hexahydrate (FeCl<NUM> · <NUM><NUM>O) was used, and carbonization was performed at <NUM> instead of <NUM>,<NUM>.

Each of the carbon catalysts obtained as described above was analyzed by X-ray photoelectron spectroscopy (XPS). That is, surface elements of the carbon catalyst were analyzed with an X-ray photoelectron spectrometer (Kratos AXISNOVA, manufactured by Shimadzu Corporation) (X-ray: AlKα X-ray, output: <NUM> mA×<NUM> kV). Specifically, each surface element concentration (atomic%) of carbon atoms, nitrogen atoms, oxygen atoms, phosphorus atoms, and metal atoms was determined based on an area of each peak of a spectrum obtained by XPS measurement and a detection sensitivity coefficient, and a ratio (N/C) of the concentration (atomic%) of the nitrogen atoms with respect to the concentration (atomic%) of the carbon atoms on the surface, and a ratio (P/C) of the concentration (atomic%) of the phosphorus atoms with respect to the concentration (atomic%) of the carbon atoms on the surface were calculated as a ratio in concentration between the elements. A background at the time of quantitative calculation was determined by a Shirley method.

Then, through peak separation of a P2p peak in the obtained XPS spectrum, the P2p peak was separated into a peak P<NUM> having a peak top within a range of <NUM>. <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, a peak P<NUM> having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, a peak P<NUM> having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, a peak P<NUM> having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, and a peak P<NUM> having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV.

The peak separation was performed by superimposing overlapping peaks onto a Gaussian-Lorentzian basic waveform to approximate the overlapping peaks. In the spectrum subjected to energy value correction and intensity correction described later, a peak intensity, a peak full width at half maximum, and a peak position of a Gaussian-Lorentzian function, serving as each component, were optimized as parameters, and each of five overlapping peaks included in the above-mentioned peak was subjected to curve fitting, to thereby perform peak separation. The curve fitting was performed so that a residual sum of squares became smallest. The residual square refers to a square of a residual error at each measured energy value, and the residual sum of squares refers to a sum of residual squares. In addition, the residual error refers to a difference between the intensity of the P2p peak in the corrected spectrum and the sum of intensities of the five separated peaks.

First, the XPS spectrum obtained by the XPS measurement was subjected to energy value correction. In the energy value correction, measurement results of carbon atoms derived from C <NUM> were used. When a normal value (<NUM> eV in this case) of a peak top of a C <NUM> peak is shifted from a peak top of a measured C <NUM> peak, the measured value of the C <NUM> peak is subtracted from the normal value, and the numerical value thus obtained is added to a binding energy value of the P2p peak. Then, intensity correction was performed. A background intensity was subtracted from an intensity of the spectrum obtained by the energy value correction, and a straight line connecting an intensity of a binding energy of <NUM> eV to an intensity of a binding energy of <NUM> eV was subtracted from each intensity, to thereby perform intensity correction. The peak separation was performed through use of the spectrum thus obtained.

A ratio of an area of the peak P<NUM> (peak having a peak top within a range of <NUM>±<NUM> eV) with respect to a total area of the five peaks obtained by the above-mentioned peak separation was calculated as a P<NUM>/Ptotal ratio.

Further, through multiplication of the ratio (P/C ratio) of the concentration (atomic%) of the phosphorus atoms with respect to the concentration (atomic%) of the carbon atoms by the above-mentioned P<NUM>/Ptotal ratio, the ratios being measured by the XPS, a ratio (P<NUM>/C ratio) of the concentration (atomic%) of the phosphorus atoms exhibiting a P<NUM> peak with respect to the concentration (atomic%) of the carbon atoms was calculated.

A sample of the powdery carbon catalyst obtained as described above was placed in a concave portion (<NUM>×<NUM>×<NUM> in thickness) of a glass sample plate and pressed with a slide glass so as to be uniformly filled into the concave portion so that the surface of the sample was matched with a reference surface. Then, the glass sample plate was fixed onto a wide-angle X-ray diffraction sample stage so that the filled sample was not deformed.

Then, X-ray diffraction (XRD) measurement was performed through use of an X-ray diffraction device (XRD-<NUM>, manufactured by Shimadzu Corporation). The voltage and current applied to an X-ray tube were <NUM> kV and <NUM> mA, respectively. The sampling interval was <NUM>°, the scanning speed was <NUM>°/min, and the measurement angle range (2θ) was from <NUM>° to <NUM>°. As an incident X-ray, CuKα was used. The sample thickness was set to <NUM>, and the divergence slit width β was set to <NUM>/<NUM>°.

Further, through the peak separation of the X-ray diffraction data obtained by the XRD measurement, the peak in the vicinity of the diffraction angle (2θ) of <NUM>° (within a range of from <NUM>° to <NUM>°) was separated into two peaks fbroad and fnarrow. The peak separation was performed by superimposing overlapping peaks onto a Gaussian basic waveform to approximate the overlapping peaks. In a diffraction pattern subjected to intensity correction and background correction described later, a peak intensity, a peak full width at half maximum, and a peak position of a Gaussian function, serving as each component, were optimized as parameters, and each of two overlapping peaks included in the above-mentioned peak was subjected to curve fitting, to thereby perform peak separation. The curve fitting was performed so that a residual sum of squares became smallest. The residual square refers to a square of a residual error at each measured diffraction angle, and the residual sum of squares refers to a sum of residual squares. In addition, the residual error refers to a difference between the intensity of the peak having a peak top in the vicinity of the diffraction angle (2θ) of <NUM>° in the corrected X-ray diffraction pattern and the sum of intensities of the two separated peaks (fbroad and fnarrow).

The intensity correction was performed by dividing the diffraction intensity at each diffraction angle by an intensity correction coefficient. The intensity correction coefficient is represented by a product of a polarization factor (P), a Lorentz factor (L), and a square of an atom scattering factor of carbon (fc) as represented by the following expression: intensity correction coefficient=L×P×fc<NUM>.

The polarization factor (P) is represented by the following expression: P=(<NUM>+cos<NUM>2θ·cos<NUM>2θ')/(<NUM>+cos<NUM>2θ'). Here, θ represents an angle of a goniometer. θ' varies depending on a monochromatic procedure. θ' represents a diffraction angle of a monochromator crystal at a time when a counter monochromator is used. θ' is <NUM> ° in the case where a graphite monochromator is used. θ' is <NUM>° at a time when the counter monochromator is not used (at a time when a Ni filter is used). In this Example, a graphite monochromator was used.

The Lorentz factor (L) is represented by the following expression: L=<NUM>/(sin<NUM>θ·cosθ). In addition, the atom scattering factor of carbon (fc) is represented by the following expression: fc=<NUM>. 26069exp(-<NUM><NUM>)+<NUM>. 56165exp(-<NUM><NUM>)+<NUM>. 05075exp (-<NUM><NUM>)+<NUM>. 839259exp(-<NUM><NUM>)+<NUM>. Here, s is represented by the following expression: s=sinθ/λ. λ represents a wavelength of a characteristic X-ray used for obtaining X-ray diffraction data, and in this case, is a wavelength A of <NUM> of CuKα. In addition, calculation was performed with units of θ in the above-mentioned calculation expression being radians.

The background correction was performed by defining a straight line connecting the vicinity of a diffraction angle (2θ) of from <NUM>° to <NUM>° to the vicinity of a diffraction angle (2θ) of from <NUM>° to <NUM>° as a background, and subtracting the background from each diffraction intensity after the intensity correction. A ratio of each component was calculated based on an area of each peak obtained by the above-mentioned peak separation.

The oxygen reduction activity of the carbon catalyst obtained as described above was evaluated. First, a catalyst slurry containing the carbon catalyst was prepared. Specifically, <NUM>µL of a <NUM> wt% commercially available Nafion (trademark) solution (produced by Sigma-Aldrich) , <NUM>µL of ethanol, and <NUM>µL of distilled water were added to <NUM> of the carbon catalyst, and glass beads were added thereto. Then, the resultant was subjected to ultrasonic treatment for <NUM> minutes, to thereby provide a homogeneous catalyst slurry.

Then, the catalyst slurry was pipetted, and <NUM>µL thereof was applied to a disk electrode (diameter: <NUM>) of a rotating ring disk electrode device (RRDE-3A, Ver. <NUM>, manufactured by BAS Inc. ), followed by drying, to thereby manufacture a working electrode. A platinum electrode was used as a counter electrode, and a reversible hydrogen electrode was used as a reference electrode. A <NUM> sulfuric acid aqueous solution saturated with oxygen was used as an electrolyte solution.

Then, a current density obtained by rotating the electrode at a rotation speed of <NUM>,<NUM> rpm and sweeping a potential at a sweep speed of <NUM> mV/sec was recorded as a function of a potential. From the oxygen reduction voltammogram thus obtained, a current density i<NUM> (mA/cm<NUM>) at a time when a voltage of <NUM> V (vs. RHE) was applied was recorded.

The retention ratio of the oxygen reduction activity (performance retention ratio) of the carbon catalyst obtained as described above was evaluated. That is, each of the carbon catalysts obtained in Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM> was subjected to a start-stop test involving repeatedly applying a voltage. First, a catalyst slurry containing the carbon catalyst was prepared. Specifically, <NUM>µL of a <NUM> wt% Nafion (trademark) solution (produced by Sigma-Aldrich), <NUM>µL of ethanol, and <NUM>µL of distilled water were added to <NUM> of the carbon catalyst, and glass beads were added thereto. Then, the resultant was subjected to ultrasonic treatment for <NUM> minutes, to thereby provide a homogeneous catalyst slurry.

Then, <NUM>µL of the catalyst slurry was pipetted and applied to a disk electrode (diameter: <NUM>) of a rotating ring disk electrode device (RRDE-3A, Ver. <NUM>, manufactured by BAS Inc. ), followed by drying, to thereby manufacture a working electrode. A <NUM> sulfuric acid aqueous solution saturated with oxygen at normal temperature was used as an electrolyte solution. In addition, a reversible hydrogen electrode was used as a reference electrode. Then, a current density obtained by rotating the electrode at a rotation speed of <NUM>,<NUM> rpm and sweeping a potential at a sweep speed of <NUM> mV/sec was recorded as a function of a potential. From the oxygen reduction voltammogram thus obtained, a potential at which a current having a half current density of that at a time of <NUM> V (vs. RHE) flowed was recorded. The potential thus obtained was defined as a potential before the start-stop test.

Next, the electrolyte solution was replaced by an electrolyte solution obtained by saturating a <NUM> sulfuric acid aqueous solution with nitrogen at normal temperature, and <NUM> cycles of the start-stop test were performed through use of a triangular wave at a voltage of from <NUM> V to <NUM> V and a sweep speed of <NUM> mV/sec. Then, a current density was measured under the same conditions as those before the start-stop test, and a ratio of a potential measured after the start-stop test with respect to the potential measured before the start-stop test was determined as a performance retention ratio (%). As the performance retention ratio is higher, the durability of the carbon catalyst is more excellent.

In <FIG>, the results obtained by performing the peak separation of the P2p peak in the XPS spectrum of the carbon catalyst obtained in Example <NUM> are shown. In <FIG>, "P1" represents the peak P<NUM>, "P2" represents the peak P<NUM>, "P3" represents the peak P<NUM>, "P4" represents the peak P<NUM>, and "P5" represents the peak P<NUM>. Since the peak intensity of the "P3" was small, an enlarged diagram of the "P3" is also shown in <FIG>. In addition, the peak intensity of the "P5" was so small that the peak was not visually recognized in <FIG>.

In <FIG>, the results obtained by performing the peak separation in the XRD diffraction pattern of the carbon catalyst obtained in Example <NUM> are shown. In <FIG>, the results obtained by performing the peak separation in the XRD diffraction pattern of the carbon catalyst obtained in Comparative Example <NUM> are shown. In <FIG>, the results of the XPS measurement, the results of the XRD measurement, and the results of the oxygen reduction activity measurement in each of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM> are shown.

As shown in <FIG>, the current density i<NUM> (mA/cm<NUM>) of the carbon catalyst obtained in each of Examples <NUM> to <NUM> was significantly larger than that of the carbon catalyst obtained in each of Comparative Examples <NUM> and <NUM>. That is, the oxygen reduction activity of the carbon catalyst obtained in each of Examples <NUM> to <NUM> was significantly higher than that of the carbon catalyst obtained in each of Comparative Examples <NUM> and <NUM>.

In addition, the performance retention ratio (%) of the carbon catalyst obtained in each of Examples <NUM> to <NUM> was significantly larger than that of the carbon catalyst obtained in each of Comparative Examples <NUM> and <NUM>. That is, the carbon catalyst obtained in each of Examples <NUM> to <NUM> was also more excellent from the viewpoint of durability than the carbon catalyst obtained in each of Comparative Examples <NUM> and <NUM>.

Further, as shown in <FIG>, the P<NUM>/C ratio of the carbon catalyst obtained in each of Examples <NUM> to <NUM> was significantly larger than that of the carbon catalyst obtained in each of Comparative Examples <NUM> and <NUM>. Thus, it was considered that the carbon catalyst obtained in each of Examples <NUM> to <NUM> exhibited excellent oxygen reduction activity and durability by containing a relatively large amount of the particular phosphorus atoms that exhibited the peak P<NUM> having a peak top within a range of <NUM>±<NUM> eV in the XPS measurement (that is, due to the relatively large P<NUM>/C ratio).

Further, in the carbon catalyst obtained in each of Examples <NUM> to <NUM>, the P<NUM>/Ptotal ratio was <NUM> or more, which was larger than that of the carbon catalyst obtained in each of Comparative Examples <NUM> and <NUM>. In addition, as shown in <FIG>, in the carbon catalyst obtained in each of Examples <NUM> to <NUM>, the ratio of the peak fbroad derived from a low-crystalline component was more than <NUM>% (more specifically, <NUM>% or more), and the ratio of the peak fnarrow derived from ahigh-crystalline component was less than <NUM>% (more specifically, <NUM>% or less) in the XRD measurement.

In addition, in the carbon catalyst obtained in each of Examples <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, the current density i<NUM> and the performance retention ratio were particularly excellent compared to those of the carbon catalyst obtained in each of Examples <NUM>, <NUM>, <NUM>, and <NUM>. In this respect, the P<NUM>/C ratio of the carbon catalyst obtained in each of Examples <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> was more than <NUM> (more specifically, <NUM> or more), which was larger than that of the carbon catalyst obtained in each of Examples <NUM>, <NUM>, <NUM>, and <NUM>.

Claim 1:
A carbon catalyst, comprising a metal and phosphorus atoms, wherein the metal is one or more kinds selected from a group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn,
wherein the carbon catalyst has a ratio of a concentration (atomic%) of the phosphorus atoms with respect to a concentration (atomic%) of carbon atoms in X-ray photoelectron spectroscopic measurement of <NUM> or more, the phosphorus atoms exhibiting a peak having a peak top within a range of <NUM>±<NUM> eV and having a full width at half maximum of <NUM>±<NUM> eV, which is obtained by peak separation of a phosphorus atom P2p peak, and
wherein the carbon catalyst has a carbon structure in which area ratios (%) of the following two peaks fbroad and fnarrow obtained by separating a peak in a vicinity of a diffraction angle (2θ) of <NUM>° in an X-ray diffraction pattern in powder X-ray diffraction measurement satisfy the following conditions (a) and (b):
(a) a peak fbroad having a peak top within a range of a diffraction angle (2θ) of <NUM>°±<NUM>° and having a full width at half maximum of <NUM>°<NUM>°: <NUM>% or more and less than <NUM>%; and
(b) a peak fnarrow having a peak top within a range of a diffraction angle (2θ) of <NUM>°±<NUM>° and having a full width at half maximum of <NUM>°±<NUM>°: more than <NUM>% and <NUM>% or less.