JP4448792B2 - Metal oxynitride oxygen reduction electrode catalyst and method for producing the same - Google Patents

Description

The present invention, water electrolysis, organic electrolysis, to the oxygen reduction electrode catalyst and its method of manufacturing for electrochemical systems used in an acidic electrolyte in the field, such as fuel cells.

Precious metals, particularly platinum, are stable at high potentials and have high catalytic ability for various reactions, and thus are used as electrode catalysts for various electrochemical systems. However, since the price of platinum is high, the amount of resources is limited, and a highly active electrode catalyst is required as an electrode catalyst for fuel cells, an alternative material for the platinum catalyst is desired.
For example, metal oxides such as WO _3, TiO _2, ZrO _2, PtO, Sb ₂ O _{4 and} Sb ₂ O ₃ (Patent Document 1), oxides such as ruthenium oxide and nitrides such as molybdenum nitride (Patent Document 2) The invention relating to the electrocatalyst using) has been filed.

Metal oxynitrides have been developed as photocatalysts that decompose water with visible light (eg,
Non-patent document 1 and patent documents 3 to 5) have not been tried to be used as electrode catalysts.

M. Hara et al., Catal. Today., 78, 555 (2003) JP 2004-95263 A JP 2005-63677 JP 2002-66333 A JP 2002-154823 A JP2003-321782A

In general, it has been reported that many non-platinum compounds such as carbides are actively dissolved and cannot exist stably when the electrode potential is higher than 0.4 V in an acidic electrolyte (Hiroshima Yoneyama et al. , Electrochemistry, 41,719 (1973)), the scope of application as an electrode catalyst is limited to the case where the electrode potential is low, and an electrode catalyst that maintains its catalytic ability and improves stability under such conditions is developed. There was a need.

Among oxides and nitrides of transition metals, oxides and nitrides other than molybdenum, iron, and nickel-based Group 4 or Group 5 elements are actively dissolved in an acidic electrolyte and cannot exist stably. In the noble metal oxide, the active point of the oxygen reduction reaction is drastically reduced by forming the oxide, so that although it is stable, the oxygen reduction catalytic ability is drastically lowered. A valve metal made of Ta, Nb, Ti, and Zr has a strong binding force with oxygen, and oxides can exist stably even in an acidic electrolyte.

However, as shown in FIG. 11 for example, the current-potential curve of zirconium oxide and the current-potential curve of titanium nitride in FIG. 12, the oxide and nitride of the valve metal have no oxygen reduction catalytic ability. 11 and 12 are obtained by measuring a current-potential curve after polarization at a potential scanning speed of 5 mV / s in a 0.1 mol / dm ³ sulfuric acid solution at 30 ° C. in a nitrogen atmosphere and an oxygen atmosphere. It is. A reversible hydrogen electrode in a sulfuric acid solution with the same concentration was used as a reference electrode.

It is known that some transition metal oxynitrides obtained by substituting part of oxygen in an oxide with nitrogen have a photocatalytic function, and can exist stably in an acidic electrolyte. However, these transition metal oxynitrides have low reduction currents and low catalytic activity, and transition metal oxynitrides that have been used in acidic electrolytes at a potential of 0.4 V or higher with respect to the reversible hydrogen electrode potential. Ride was not found.

The present invention provides an electrocatalyst comprising an oxynitride containing at least one metal selected from the group of valve metals comprising Ta, Nb, Ti, and Zr. When the present inventors deposit an oxynitride of at least one metal element selected from the group of valve metals consisting of Ta, Nb, Ti, and Zr on an electrode catalyst support material by a sputtering method, the support material is heated at a high temperature. The metal oxynitride thin film obtained by heating to a high temperature exhibits excellent catalytic activity and provides corrosion resistance that does not dissolve even when used at a potential of 0.4 V or higher relative to the reversible hydrogen electrode potential in an acidic electrolyte. I found.

That is, the present invention is (1) an oxygen reduction electrode catalyst used in an acidic electrolyte at a potential of 0.4 V or more with respect to the reversible hydrogen electrode potential, and the electrode catalyst is formed on the surface of the support material by a sputtering method. A thickness of 20n adhered to the carrier material while heated to 200 ° C to 600 ° C.
An oxygen reduction electrocatalyst having an oxynitride thin film of at least one transition metal element selected from the group of valve metals consisting of Ta, Nb, Ti, and Zr of m to 50 nm.
In the present invention, (2) the ratio of the transition metal element, oxygen, and nitrogen in the atomic ratio is 1 ± 0.1: 1 ±.
The oxygen-reducing electrocatalyst described above, wherein 0.1: 1 ± 0.1.

The present invention is also characterized in that (3) the electrode catalyst carrier is electron conductive fine particles.
1. The oxygen reduction electrode catalyst according to 1 .
Further, the present invention is (4) used as an electrode catalyst for a fuel cell using an acidic electrolyte.
The oxygen reduction electrode catalyst as described above.

In addition, the present invention provides (5) T in a state where the electrode catalyst support material is heated to 200 ° C. to 600 ° C.
at least one transition metal element selected from the group of valve metals consisting of a, Nb, Ti, Zr
A thin film of oxynitride of 20 nm to 50 nm in thickness is formed by sputtering.
A method for producing the oxygen reduction electrode catalyst as described above, wherein the oxygen reduction electrode catalyst is attached to a material surface .

In the conventionally known transition metal oxynitride powder samples, the electron conductivity was not ensured, and the catalytic activity could not be confirmed. The present inventor has found that by producing a thin film by sputtering, it is possible to reduce the effects of electrical resistance of the metal oxynitride, a result was Heading first superior catalyst <br/> medium activity. Known methods for synthesizing metal oxynitrides include a method of heating a mixture of a metal oxide and urea, and a method of treating the oxide in a high-temperature gaseous nitrogen or ammonia atmosphere. For example, film formation using a sputtering method capable of controlling oxygen and nitrogen atmospheres can control a wide range of metal oxidation states from a reduced state to an oxidized state, thereby realizing an oxidized state having catalytic activity. It was.

The oxynitride electrocatalyst of at least one metal element selected from the group of valve metals consisting of Ta, Nb, Ti and Zr obtained by the production method of the present invention has high corrosion resistance at high electrode potential in an acidic electrolyte. And has an excellent oxygen reduction catalytic ability.

The electrode catalyst obtained by the production method of the present invention comprises oxynitride of at least one metal element selected from the group of valve metals consisting of Ta, Nb, Ti, and Zr, and has a reversible hydrogen electrode potential in an acidic electrolyte. On the other hand, it is used at a potential of 0.4V or more.

The valve metals Ta, Nb, Ti, and Zr are all stable because the oxide does not corrode at high potential in the acidic electrolyte, and these oxides form the catalyst surface, so the catalyst itself is stable. Can exist.
However, these oxides have no oxygen reduction catalytic ability. However, it is considered that the nitriding is partially nitrided to form oxynitride by the production method of the present invention, whereby the electronic state is continuously changed to have catalytic ability at a specific degree of nitriding. In general, the catalytic ability is preferably such that the ratio of transition metal, oxygen, and nitrogen is about 1 ± 0.1: 1 ± 0.1: 1 ± 0.1 in terms of the number of atoms.

The oxynitride electrode catalyst of at least one metal element selected from the group of valve metals consisting of Ta, Nb, Ti, and Zr obtained by the production method of the present invention is a conductive oxide such as carbon, tungsten oxide, or iridium oxide. A thin film can be formed by sputtering on a catalyst carrier that is an electron conductive powder such as the like. The electrode catalyst carrier is not particularly limited as long as it has conductivity and moderate corrosion resistance, but a metal plate such as titanium having sufficient electronic conductivity as an electrode catalyst carrier, glassy carbon, carbon black, graphitized carbon, It is desirable to use a conductive carbon material such as activated carbon.

These electrocatalyst carriers are preferably carriers that can take a large area in contact with the catalyst layer in order to obtain a state in which sufficient conductivity is obtained. In the case of a fuel cell, it is preferable to make the catalyst finer and mix well with carbon powder so as to obtain good conductivity, thereby increasing the amount of catalyst per unit volume.

Practically, the thickness formed by sputtering is preferably about 50 nm or less. As the thickness of the thin film becomes larger than 50 nm, the electric resistance of the catalyst layer increases, and the current value decreases due to the ohmic drop due to the electric resistance of the catalyst itself, thereby reducing the catalytic performance. Further, as the thickness is reduced, the substrate is affected by the base material, and the amount of the catalyst is reduced and the catalytic ability becomes insufficient. Therefore, the thickness is preferably about 20 nm or more.

In water electrolysis / organic electrolysis, plate-like electrodes are often used. For example, a film having a thickness of 50 nm or less may be formed on a titanium plate substrate. In the case of a fuel cell electrode catalyst, it is preferably used by being highly dispersed in electron conductive fine particles such as carbon. For example, a base material layer made of carbon powder is formed, and a catalyst layer is formed thereon by sputtering thinly to a thickness of 50 nm or less.

Further, a fine powder of oxynitride can be produced by sputtering, mixed together with carbon powder, applied to carbon paper or the like and used as an electrode catalyst layer. The particle size of the catalyst powder in this case is preferably 50nm or less.

As a specific example of the sputtering method for forming the oxynitride of Ta, Nb, Zr, or Ti of the valve metal, the partial pressure of nitrogen is 0.1 to 0.5 Pa and the partial pressure of oxygen is 0.05 with Ta, Nb, Zr, or Ti as a target.
Reactive sputtering is performed in a mixed gas atmosphere of nitrogen and oxygen of about 0.2 Pa. As the catalyst carrier serving as a sputtering substrate, electron conductive fine particles that are stable in an oxygen atmosphere in an acidic electrolyte, such as glassy carbon, can be used. At this time, by heating the substrate to 200 ° C. or higher, an electrode catalyst having higher crystallinity can be obtained, and the catalytic ability can be improved.

FIG. 1 shows the relationship between ZrON and FIG. 2 shows the relationship between the catalyst support heating temperature and the oxygen reduction current during sputtering for TaON. A clear catalytic activity is observed when the heating temperature of the catalyst carrier is about 200 ° C., and the catalytic activity increases as the heating temperature rises, and a high activity is obtained up to a heating temperature exceeding 600 ° C. Practically, it is preferably about 400 to 600 ° C. The reason why such an effect can be obtained by heating the catalyst carrier is not clear, but the heating improves the crystallinity of the metal oxynitride, promotes the adsorption of oxygen molecules, and further changes the band structure. It is presumed that the activation energy of the electron transfer reaction to the adsorbed oxygen species decreases, and as a result, the oxygen reduction reaction is promoted.

Fig. 3 shows a sample of ZrON having no catalyst activity when the catalyst support is not heated, and the catalyst support at 500 ° C.
2 shows the XRD of a sample with catalytic activity when heated at. When the catalyst support is not heated, it is in an amorphous state and a sharp peak is not confirmed in XRD, but it can be seen that the crystallinity increases when heated.

As a test piece corresponding to the base material, cylindrical glassy carbon having a diameter of 5.2 mm and a length of 20 mm was used, and a titanium oxynitride thin film was adhered to the bottom surface portion thereof by a sputtering method. The argon partial pressure during sputtering was about ¹ × 10 ⁻¹ Pa, the nitrogen partial pressure was about 4 × 10 ⁻¹ Pa, and the oxygen partial pressure was about ¹ × 10 ⁻² Pa. Ti was used as a sputtering target. The substrate was heated at 500 ° C. during sputtering. The amount of spatter was measured using a quartz vibration type film thickness meter, and a TiON electrode catalyst having a thin film thickness of about 40 nm was produced. By SEM observation, the entire bottom surface of the cylindrical glassy carbon was covered with a thin film.

The catalytic ability of the electrode thus produced was evaluated for the oxygen reduction reaction. The prepared electrode was polarized in a 0.1 mol / dm ³ sulfuric acid solution at a potential scanning speed of 5 mV / s in a nitrogen atmosphere and an oxygen atmosphere at 30 ° C., and a current-potential curve was measured. A reversible hydrogen electrode in a sulfuric acid solution with the same concentration was used as a reference electrode. The difference between the reduction current in the oxygen atmosphere and the reduction current in the nitrogen atmosphere was defined as the oxygen reduction current, and the catalytic ability was evaluated. Therefore, the potential at which the difference starts to appear is high, and the larger the difference, the higher the catalytic ability.

FIG. 4 shows a current-potential curve in a nitrogen atmosphere at a temperature of 30 ° C. when the potential of the produced titanium oxynitride thin film was scanned at 50 mV / s between 0.05 V and 1.0 V. It was found that even when the potential scan was repeated, the shape of the curve did not change and existed stably in the sulfuric acid solution.

FIG. 5 shows a current-potential curve obtained by potential scanning when the scanning speed is slowed down to 5 mV / s in a nitrogen atmosphere and an oxygen atmosphere. Compared with nitrogen atmosphere in oxygen atmosphere, it is about 0.
A reduction current is observed from 5 V, indicating that it has catalytic activity for the oxygen reduction reaction.

Comparative Example 1
A 40 nm TiON electrode catalyst was produced under the same conditions as in Example 1 except that the substrate was not heated.

FIG. 6 shows a sample (Example 1) in which the substrate was heated at 500 ° C. and a sample prepared without heating (Comparative Example 1).
2 shows a current-potential curve with respect to oxygen reduction. The measurement conditions were the same as described above. In the case of no heating, the reduction current is small and the catalytic activity is low. On the other hand, when heated at 500 ° C., a reduction current is observed from a higher potential. This indicates that the oxygen reduction catalytic ability is improved by heating the substrate.

The film thickness is about 40 under the same conditions as in Example 1 except that Zr is used as the sputtering target.
A zirconium oxynitride electrocatalyst of nm was prepared. The catalytic ability of the electrode thus produced was evaluated for the oxygen reduction reaction. The measurement conditions are the same as in Example 1.

FIG. 7 shows that the prepared zirconium oxynitride thin film has a potential of 0.05 V to 1.0 V and 5
A current-potential curve in a nitrogen atmosphere at a temperature of 30 ° C. when scanned at 0 mV / s is shown. It was found that even when the potential scan was repeated, the shape of the curve did not change and existed stably in the sulfuric acid solution.

FIG. 8 shows a current-potential curve obtained by potential scanning when the scanning speed is slowed down to 5 mV / s in a nitrogen atmosphere and an oxygen atmosphere. Compared with nitrogen atmosphere in oxygen atmosphere, it is about 0.
A reduction current is observed from 7V, indicating that it has catalytic activity for the oxygen reduction reaction.

Comparative Example 2
A ZrON electrocatalyst of approximately 40 nm was produced under the same conditions as in Example 2 except that the substrate was not heated.

FIG. 9 shows current-potential curves for oxygen reduction of a sample (Example 2) in which the substrate was heated at 500 ° C. and a sample (Comparative Example 2) produced without heating. The measurement conditions were the same as described above. In the case of no heating, the reduction current is small and the catalytic activity is low. On the other hand, when heated at 500 ° C., a reduction current is observed from a higher potential. This indicates that the oxygen reduction catalytic ability is improved by heating the substrate.

The thickness of the coating is approximately the same as in Example 1 except that Ta is used as the sputtering target.
A 40 nm tantalum oxynitride electrode catalyst was prepared. The catalytic ability of the electrode thus produced was evaluated for the oxygen reduction reaction. The measurement conditions are the same as in Example 1.

Comparative Example 3
A TaON electrode catalyst of approximately 40 nm was produced under the same conditions as in Example 3 except that the substrate was not heated.

FIG. 10 shows a sample in which the substrate was heated at 500 ° C. (Example 3) and a sample prepared without heating (Comparative Example 3).
2 shows a current-potential curve with respect to oxygen reduction. The measurement conditions were the same as described above. In the case of no heating, the reduction current is small and the catalytic activity is low. On the other hand, when heated at 500 ° C., a reduction current is observed from a higher potential. This indicates that the oxygen reduction catalytic ability is improved by heating the substrate.

Electrode catalysts of the present invention, water electrolysis, inorganic-organic electrolyte is used as an electrode catalyst for electrochemical systems used in contact with an acidic electrolyte in the field, such as fuel cells, a sputtering method
Is a useful method for producing a thin film capable of reducing the influence of electrical resistance using an inexpensive material.

6 is a graph showing the relationship between the heating temperature of the catalyst carrier and the oxygen reduction current for ZrON. 6 is a graph showing the relationship between the heating temperature of the catalyst carrier and the oxygen reduction current for TaON. It is a graph which shows the XRD at the time of the sample at the time of non-heating with no catalyst activity regarding ZrON and catalytic activity. 2 is a graph showing a current-potential curve of the electrode catalyst of Example 1 in a nitrogen atmosphere. 2 is a graph evaluating the catalytic ability of the electrode catalyst of Example 1 for an oxygen reduction reaction. 5 is a graph showing the influence of substrate heating on the catalytic ability of the oxygen reduction reaction of the electrode catalysts of Example 1 and Comparative Example 1. 6 is a graph showing a current-potential curve of the electrode catalyst of Example 2 in a nitrogen atmosphere. 4 is a graph showing an evaluation of the catalytic ability of the electrode catalyst of Example 2 for an oxygen reduction reaction. 6 is a graph showing the influence of substrate heating on the catalytic ability of the oxygen reduction reaction of the electrode catalysts of Example 2 and Comparative Example 2. 6 is a graph showing the influence of substrate heating on the catalytic ability of the oxygen reduction reaction of the electrode catalysts of Example 3 and Comparative Example 3. The electric current-potential curve in the oxygen atmosphere and nitrogen atmosphere of a zirconium oxide. Current-potential curve of titanium nitride in oxygen atmosphere and nitrogen atmosphere.

Claims (5)

An oxygen reduction electrocatalyst used in an acidic electrolyte at a potential of 0.4 V or higher with respect to the reversible hydrogen electrode potential, the electrocatalyst being coated with a support material on the surface of the support material by sputtering
Ta, Nb, having a thickness of 20 nm to 50 nm deposited in a heated state at 200 ° C. to 600 ° C.
An oxygen reduction electrocatalyst comprising an oxynitride thin film of at least one transition metal element selected from the group of valve metals consisting of Ti and Zr.
The ratio of transition metal element to oxygen and nitrogen in terms of atomic ratio is 1 ± 0.1: 1 ± 0.1: 1 ± 0.1
The oxygen reduction electrocatalyst according to claim 1. 2. The oxygen-reducing electrode contact according to claim 1, wherein the electrode catalyst carrier is an electron conductive fine particle.
Medium. 2. The electrode catalyst for a fuel cell using an acidic electrolyte.
Oxygen reduction electrode catalyst. Thickness of oxynitride of at least one transition metal element selected from the group of valve metals consisting of Ta, Nb, Ti, and Zr when the electrode catalyst support material is heated to 200 ° C. to 600 ° C.
Method for producing an oxygen reduction electrode catalyst according to any one of claims 1 to 4, <br/> characterized that you adhere to the carrier material surface by sputtering a thin film of 20 nm to 50 nm.

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