WO2015076375A1 - Oxygen reduction electrode catalyst, electrode catalyst for hydrogen generation reaction, and electrode - Google Patents

Abstract

To further improve the activity of an oxygen reduction electrode catalyst which uses boron nitride (BN). Oxygen reduction reaction activity is improved, in comparison to an electrode wherein a BN thin film is supported, by having the surface of Au or the like support a BN nanosheet. The activity can be further enhanced by having the BN nanosheet support Au fine particles. Vitreous carbon can be used for a substrate by having the BN nanosheet support Au fine particles. The oxygen reduction reaction activity can be further improved by using a BNNS that is further reduced in size. Also provided are: an electrode catalyst having high hydrogen generation reaction activity; and an electrode catalyst having high hydrogen generation reaction activity but not having hydrogen oxidation reaction activity, said hydrogen oxidation reaction being the reverse reaction of hydrogen generation reaction. High hydrogen generation reaction activity can be achieved by having the surface of Au substrate or the like support a BN nanosheet. The activity can be further improved by having the BN nanosheet support fine particles of Au or the like. An electrode catalyst which uses Au as a substrate has high hydrogen reaction activity but does not have hydrogen oxidation reaction activity.

Description

Oxygen reduction electrode catalyst, electrode catalyst for hydrogen generation reaction, and electrode

The present invention relates to an oxygen reduction electrocatalyst, for example, an oxygen reduction electrocatalyst that can be used for an oxygen electrode of a fuel cell, and in particular, an oxygen reduction electrocatalyst using boron nitride as a catalyst material and the catalyst. It relates to the oxygen electrode used.

The present invention also relates to an electrode catalyst for hydrogen generation reaction, for example, an electrode catalyst for hydrogen generation reaction (HER (hydrogen evolution reaction)) that can be used to decompose water to produce hydrogen and the catalyst. It relates to the HER electrode used.

Fuel cells are attracting attention because of their high energy conversion efficiency and low environmental impact. One of the problems to be solved in the practical application of fuel cells is that the oxygen reduction reaction at the oxygen electrode is slow. In order to promote this reaction, platinum is currently used as a catalyst for the oxygen electrode. However, the use of a platinum catalyst is not only insufficient in efficiency, but also has a problem that the catalyst has a short life, is expensive and has a small amount of resources. Therefore, development of an oxygen reduction electrode catalyst that solves the above problem without using a noble metal such as platinum has been promoted all over the world, but a satisfactory catalyst has not yet been obtained. Accordingly, there is a need for a search for a completely new oxygen reduction electrocatalyst using materials that have not been studied.

In response to this problem, the present inventors have previously found that a boron nitride (BN) nanostructure can be used as a catalyst for an oxygen reduction reaction without using platinum, which is a rare resource, and has filed a patent application related thereto. (Patent Document 1). In the example of Patent Document 1, a sample in which a BN thin film is deposited by sputtering on a conductive substrate having gold deposited on the surface, and a BN nanotube (BNNT) dispersed in a solvent on the same conductive substrate are used. The catalytic activity for the oxygen reduction reaction of the BN nanostructure was verified with the coated and dried sample. In addition, the inventors of the present application made theoretical calculations for predicting the oxygen reduction reaction activity (ORR (oxygen reduction reaction) activity) of BN, and published the results as Non-Patent Documents 1 and 2.

In addition, due to problems such as resource depletion and environmental pollution, various research and development for obtaining energy using sunlight have been actively conducted. One such solar cell is being put into practical use. Solar cells output power, but since it is difficult to store power, if the generated power is not used immediately, the secondary battery is charged, or fuel such as hydrogen is generated with the generated power. Countermeasures are being considered. However, with such measures, conversion is performed twice, which not only increases the complexity of the system and increases the price, but also accumulates energy loss that occurs every time conversion occurs, resulting in efficient use of sunlight. There was a problem that decreased.

In order to solve this problem, methods and devices for generating hydrogen directly from sunlight have been studied. However, platinum is still the best material for the catalyst used in HER at this time, so there is a catalyst that does not use such expensive and limited resources or reduces the amount of use as much as possible. It has been demanded.

JP 2013-230415 A

Since the oxygen reduction reaction is generally a slow reaction, improving the catalytic activity of the oxygen reduction reaction directly leads to an improvement in the efficiency of the entire fuel cell system. Therefore, it is always required to improve the catalytic activity of the oxygen reduction reaction catalyst as much as possible.
An object of the present invention is to further improve the oxygen reduction reaction catalytic activity of the BN nanostructure first shown by Patent Document 1 described above, thereby contributing to the improvement of fuel cell efficiency.
In addition, in the oxygen reduction reaction at the electrode, oxygen is reduced to water when all are carried out by a four-electron reaction, but in reality, a hydrogen peroxide generation reaction by a two-electron reaction also occurs. This is a factor that hinders the catalytic activity of the oxygen reduction reaction. Hydrogen peroxide also has the effect of damaging the electrolyte membrane. Therefore, it is desired to suppress hydrogen peroxide generated by the oxygen reduction reaction at the electrode.
Accordingly, an object of the present invention is to further suppress the hydrogen peroxide generated by the oxygen reduction reaction at the electrode to further improve the oxygen reduction reaction catalytic activity, and to contribute to the improvement of the fuel cell efficiency.

Also, an object of the present invention is to solve the above-described problems of the prior art in the field of hydrogen generation and to provide an HER electrode having high catalytic activity.

According to one aspect of the present invention, an oxygen reduction electrocatalyst is provided in which a boron nitride nanosheet is supported on the surface of at least one metal selected from the group consisting of Au, Ni, Pd, and Pt.
Here, the boron nitride nanosheet may carry at least one fine metal particle selected from the group consisting of elements of groups 8 to 11 of the periodic table.
According to another aspect of the present invention, boron nitride nanosheets are supported on the surface of glassy carbon, and the boron nitride nanosheets are at least one selected from the group consisting of elements of groups 8 to 11 of the periodic table of elements. An oxygen reduction electrocatalyst carrying metal particulates is provided. In this case, the metal element of the at least one metal fine particle supported on the boron nitride nanosheet may be Au.
According to another aspect of the present invention, the size of the boron nitride nanosheet supported on any one of the oxygen reduction electrocatalysts is such that the selectivity for the production of hydrogen peroxide by the oxygen reduction reaction at the electrode is suppressed. An oxygen reduction electrocatalyst is provided.
According to another aspect of the present invention, an oxygen electrode using any one of the above oxygen reduction electrode catalysts is provided.
According to another aspect of the present invention, a fuel cell comprising any one of the above oxygen electrodes is provided.
According to another aspect of the present invention, by adjusting the size of the boron nitride nanosheet supported on the oxygen reduction electrode catalyst, the generation of hydrogen peroxide due to the oxygen reduction reaction at the electrode is suppressed to reduce the oxygen reduction reaction. A method for improving the catalytic activity is provided.

According to another aspect of the present invention, there is provided an electrode catalyst for a hydrogen generation reaction (HER) in which a boron nitride nanosheet is supported on a surface of at least one metal selected from the group consisting of Au, Ni, Pd and Pt. It is done.
Here, the boron nitride nanosheet may carry at least one fine metal particle selected from the group consisting of elements of groups 8 to 11 of the periodic table.
The boron nitride nanosheet may cover a part of the surface of the metal (at least one metal selected from the group consisting of Au, Ni, Pd, and Pt).
The metal carrying the boron nitride nanosheet on the surface may be Au.
According to another aspect of the present invention, a hydrogen generating electrode using any one of the above HER electrode catalysts is provided.
According to another aspect of the present invention, there is provided a hydrogen generator comprising the HER electrode.

In the electrode realized in Patent Document 1, a BN nanosheet (BNNS) different from the BN nanostructure (boron nitride nanotube, boron nitride thin film, boron nitride nanoparticle, boron nitride nanoribbon) disclosed in the patent document is used as the BN. According to the oxygen reduction electrode catalyst of the present invention, the ORR activity can be improved. Further, by supporting Au fine particles on BNNS, higher ORR activity can be obtained. In addition, if BNNS to be supported on the surface of the substrate is made by supporting Au fine particles on BNNS, high ORR activity can be obtained even if glassy carbon is used as the substrate. The amount of noble metals used can be reduced. Moreover, ORR activity can be further improved by using BNNS whose size (size) is adjusted to be smaller. The present invention further provides an oxygen electrode with improved ORR activity and an efficient fuel cell.

In addition, according to the present invention, an electrode catalyst for HER having catalytic activity equivalent to that of platinum and, in some cases, higher than that of platinum can be obtained. Further, according to the present invention, if necessary, an HER electrode catalyst that does not exhibit catalytic activity for a hydrogen oxidation reaction (HOR (hydrogen-oxidation reaction)) that is a reverse reaction of HER can be obtained. According to the present invention, a HER electrode having high catalytic activity and an efficient hydrogen generator can be obtained.

The figure which shows notionally the preparation procedure of a BNNS / Au electrode. (A), (b): An HRTEM image of BNNS used to fabricate a BNNS / Au electrode, (a) clearly showing a honeycomb BN lattice, and (b) a BN hexagon. HRTEM image showing the zigzag edge structure of the composition. Here, the scale bar in the lower left corner of each image indicates 1 nm. (C): BNNS EESL spectrum. (A): SEM image of the surface of the Au electrode before supporting BNNS. (B), (c): SEM images of the BNNS / Au electrode surface after supporting BNNS. The conceptual diagram of the apparatus used when performing an electrochemical measurement by the rotating electrode method. (A): Alignment of an Au electrode in oxygen-saturated H ₂ SO ₄ (solid line; in FIG. 5 and subsequent drawings and in Table 1 this electrode is referred to as Bare Au) and the BNNS / Au electrode (dashed line) The figure which shows a running voltammogram. (B): A diagram showing a Tafel plot based on the measurement results shown in (a). The inset is a table comparing the kinetic parameters (Tafel slope and exchange current density). The figure which compares the linear scanning voltammogram of Au electrode, BNNS / Au electrode, GC electrode, and BNNS / GC electrode. The figure which shows the XRD pattern of BNNS and Au microparticles / BNNS. (A): TEM image of BNNS. (B), (c): HRNS image of BNNS. (D): Au fine particle / BNNS TEM image. (E), (f): HRTEM image of Au fine particles / BNNS. The inset of (b) shows the FFT pattern of a single layer BNNS. (A): Au electrode, BNNS / Au electrode, BNNS / GC electrode, Au fine particle / BNNS / Au electrode, Au in oxygen-saturated H ₂ SO ₄ when the rotational speed is 1500 rpm and the potential sweep speed is 10 mV / sec. The figure which shows the linear running voltammogram of microparticles | fine-particles / BNNS / GC electrode and BNNT / Au electrode. (B): Au electrode, GC electrode, BNNS / Au electrode, BNNS / GC electrode, Au fine particle / BNNS / Au in oxygen-saturated H ₂ SO ₄ when the rotational speed is 1500 rpm and the potential sweep speed is 10 mV / sec. The figure which shows the linear running voltammogram of an electrode, Au microparticles / BNNS / GC electrode, and Au microparticles / BNNS / GC electrode after heating at 600 degreeC. Au / h-BNNS electrode prepared without using an Au electrode and a filter, and Au / h-BNNS manufactured using three types of filters (1.0 μm, 0.45 μm, 0.22 μm) Current response curve with oxygen reduction reaction measured by the rotating ring disk electrode (RRDE) method of the Pt electrode and the rotating ring disk electrode (RRDE) method (where the electrode potential is measured in an oxygen-saturated 0.5 M sulfuric acid aqueous solution). Value). Au / h-BNNS electrode produced without using a filter at 0.1 V and Au / h- produced using three types of filters (1.0 μm, 0.45 μm, 0.22 μm) diagram showing the relationship between the ratio of _H 2 _{O 2} generated by BNNS electrode (%) and particle size of the h-BNNS (μm). The figure which shows various particle size distributions of h-BNNS obtained without using a filter and h-BNNS obtained by using three types of filters (1.0 μm, 0.45 μm, 0.22 μm). Linear scanning voltammograms of these electrocatalysts showing the HER characteristics of the example electrocatalysts using Au as the substrate. Linear scanning voltammograms of these electrocatalysts showing the HER characteristics of the example electrocatalysts using Pt as the substrate. Linear scanning voltammograms of these electrocatalysts showing the HER characteristics of the example electrocatalysts using Ni as the substrate. The Tafel plot based on the measurement result of the HER characteristic at the time of using the BNNS / Au electrode, Au-BNNS / Au electrode, Ni-BNNS / Au electrode, BNNS / Pt electrode, and Au-BNNS / Pt electrode in an Example. The figure which shows the HOR characteristic of these electrodes measured by the linear scanning voltammogram of the BNNS / Au electrode, Au-BNNS / Au electrode, BNNS / Pt electrode, and Au-BNNS / Pt electrode in an Example.

The inventor of the present application researched to further improve the ORR activity of an electrode using BN nanostructures (boron nitride nanotubes, boron nitride thin films, boron nitride nanoparticles, boron nitride nanoribbons) disclosed in Patent Document 1 instead of platinum. As a result, it has been found for the first time that it is effective to use BNNS, which is different from the BN nanostructure disclosed in the patent document, instead of the BN film or BNNT by sputtering verified in the patent document 1. It was. This improvement in ORR activity is realized by using BNNS which is much thinner than the BN thin film verified in Patent Document 1 and has higher crystallinity than the BN nanostructure disclosed in the Patent Document. it is conceivable that.

Furthermore, it was found that the ORR activity was further improved by supporting Au fine particles on this BNNS. Furthermore, it was found that an Au substrate is effective as a conductive substrate for supporting these BN films, not just a conductive substrate. Although not shown as an example, it has been confirmed that a further improvement in ORR activity can be achieved in the Ni substrate as well, and an improvement in ORR activity can also be achieved in the Pd or Pt substrate.

As described above, the reason why the ORR activity is further improved by supporting Au fine particles is that the electronic exchange between BN and the metal substrate affects the O—O bond of oxygen adsorbed on BN ( This is thought to be due to working in the direction of loosening the bond). In addition to Au fine particles, Pt, Ni, and fine particles of other elements belonging to groups 8 to 11 of the periodic table can obtain the same effect.

In addition, in the oxygen reduction electrode catalyst, when BNNS supporting Au fine particles is used as BNNS to be supported on the substrate, good ORR activity can be achieved even if an inexpensive glassy carbon substrate is used instead of an expensive Au substrate. I understood. In addition to Au fine particles, the same effect can be obtained with fine particles of Pt, Ni, and other elements in groups 8 to 11 of the periodic table. Further, when a heat treatment by heating is applied to the oxygen reduction electrode catalyst, the oxygen reduction current can be increased.

Further, it has been found that when BNNS having a small size is used, the generation of hydrogen peroxide by the oxygen reduction reaction at the electrode is suppressed. Specifically, hydrogen peroxide generated by an oxygen reduction reaction at an electrode using BNNS whose size was not adjusted and an Au substrate using BNNS whose size was adjusted to a smaller size was used. By comparing the results and measuring the results, the hydrogen peroxide generation rate decreases as the size of the BNNS is reduced (ie, the selectivity for hydrogen peroxide generation by the oxygen reduction reaction at the electrode is reduced). It was found to be suppressed. It was also found that all oxygen can be reduced to water by a four-electron reaction by using BNNS with a small size. As a result, it has been found that as the size of BNNS is adjusted to be smaller, the ORR activity by the oxygen reduction reaction at the electrode can be further improved, and the performance for oxygen reduction can be made closer to that of platinum. As a result, it has been found that a catalyst that can replace the conventional platinum catalyst can be obtained in terms of performance and the like.
The size of the BNNS may be smaller than that of the BNNS produced without adjusting the size, but is preferably as small as possible.
In addition to Au, Ni, Pd, Pt substrates, etc. can also be used as the substrate. Also, as BNNS, Au fine particles, Pt, Ni, fine particles of other elements in groups 8 to 11 of the periodic table of elements are used. It can also be used by being carried.

As described above, the inventor of the present application developed an oxygen reduction electrocatalyst using boron nitride (BN) (Patent Document 1) and continued research to further improve its oxygen reduction reaction (ORR) activity. It was found that it was effective to use BN nanosheets (BNNS) in place of the BN film or BN nanotubes (BNNT) obtained by sputtering verified in 1 and the ORR activity was further improved by supporting Au fine particles on this BNNS. I found out. Furthermore, the present inventors have found that an Au substrate is particularly effective as a conductive substrate for supporting BNNS, not just a conductive substrate. As a result of further research by the present inventor, the present inventors have found that the oxygen reduction electrocatalyst thus obtained is useful as an electrode catalyst for HER, and has also completed the present invention relating to an electrode catalyst for HER.

In addition, although not shown as an Example, it is thought that the improvement of HER activity is achieved similarly also with a Pd board | substrate. The fine particles supported on BNNS are not limited to Au, and fine particles of Ni or other elements belonging to groups 8 to 11 of the periodic table can also be used.

In addition, since the existing catalyst used for HER has the property of activating the reverse reaction (here, HOR) found in many catalysts, hydrogen generated in HER is not immediately removed from the reaction system. Then, hydrogen returns to hydrogen ions. For this reason, if hydrogen generated in HER is not immediately taken out of the reaction system, there is a problem that the yield of hydrogen decreases. However, in this regard, the substrate using the Au substrate as the substrate for the HER electrode catalyst of the present invention has a very advantageous feature that it does not show activity against HOR, which is a reverse reaction.

Note that, unless otherwise specified in the present application, the size of the metal fine particles supported on the BNNS is 5 to 30 nm. However, the size is not limited as long as the object of the present invention can be achieved.

Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the present invention is not limited to the following examples, and is defined only by the claims. There must be.

[Example 1: Au electrode carrying BNNS]
Regarding the electrode using the oxygen reduction electrode catalyst, an electrode (BNNS / Au electrode) having BNNS supported on an Au substrate was prepared as follows.

First, an Au electrode was prepared according to the procedure shown in FIG. That is, it was prepared by supporting a Ti layer (15 nm thickness) as a binder on a Si substrate by sputtering and then supporting Au (150 nm thickness) by sputtering. Then, BNNS / Au electrodes modified with a thin film made of BN nanosheets were prepared by supporting BNNS on Au electrodes by spin coating as follows. For comparison experiments, an electrode having a glassy carbon (hereinafter abbreviated as GC) surface modified with a thin film made of BNNS was also produced.

First, BNNS was prepared. Therefore, 3 mg of 1 μm BN powder was dispersed in 1 ml of isopropanol. The dispersion was sonicated for 96 hours, then centrifuged at 1500 rpm for 1 hour, and the upper 3/4 of the supernatant obtained by centrifugation was taken out for use as a spin coating solution. In the supernatant liquid thus taken out, BNNS peeled from the BN powder is dispersed.

The BNNS thus prepared was evaluated by a transmission electron microscope (TEM) and EELS (electron energy-loss spectroscopy). The result is shown in FIG. From this EELS spectrum, it was confirmed that BN had sp ² hybrid orbitals, that is, had a hexagonal structure. It was also confirmed that there was no contamination such as carbon.

The supernatant liquid in which BNNS is dispersed was coated on the Au electrode prepared as described above and a substrate having a GC surface by spin coating. In spin coating, it was first rotated at 500 rpm for about 90 seconds, and then rotated at 2000 rpm for 3 minutes. After completion of the spin coating, these electrodes were dried at 100 ° C. for 30 minutes in a hot air oven.

SEM images of the Au electrode surface before and after spin coating are shown in FIG. FIG. 3 (a) is an SEM image of the Au background before spin coating and with no BNNS supported. FIG. 3B is an SEM image obtained by observing the electrode surface carrying BNNS at a low magnification. FIG. 3C is an SEM obtained by observing the same electrode surface at a magnification approximately twice that of FIG. It is a statue. By comparing these SEM images, the electrode after spin coating is an Au / BN electrode in which BNNS is placed on the Au background, and BNNS does not completely cover the Au background. I understood it.

The redox characteristics of the BNNS / Au electrode thus prepared and the Au electrode before supporting BNNS as a comparison object are shown in FIG. 4 using a rotating electrode electrochemical cell whose structure is conceptually shown in FIG. It was measured by. By using the rotating electrode method, mass transfer to the electrode surface can be controlled, and the charge transfer reaction can be quantitatively analyzed with good reproducibility. In FIG. 4, a BNNS / Au electrode to be measured or an Au electrode not carrying BNNS was attached as a working electrode (WE). Oxygen-saturated 0.5M sulfuric acid was used as the electrolyte. In addition, although the specific electrolyte solution was mentioned here, this is only an illustration, and if it has electrical conductivity, the kind and concentration of the electrolyte are not particularly limited. A saturated NaCl silver / silver chloride electrode was used as the reference electrode (RE). Platinum wire was used as the counter electrode (CE). The measurement was performed at an electrode rotation speed of 1500 rpm and a potential sweep speed of 10 mV / sec.

FIG. 5A shows the measurement results of the BNNS / Au electrode supporting BNNS and the Au electrode not supporting BNNS. FIG. 5B is a graph in which the X axis and the Y axis of the data in FIG. As can be seen from these graphs, the absolute value of the current when the BNNS / Au electrode has the same potential is increased as compared with the unsupported gold electrode. From this, it was confirmed that oxygen reduction activity was improved by supporting BNNS on the Au electrode.

Further, the same measurement as above was performed for the GC electrode and an electrode in which BNNS was supported on the GC electrode by the same method as the production of the BNNS / Au electrode (hereinafter abbreviated as BNNS / GC electrode), and the Au electrode, BNNS / Comparison was made with the redox characteristics of the GC electrode, GC electrode, and BNNS / GC electrode. The result is shown in FIG. As can be seen from FIG. 6, there is no effect when GC is used as the base of the electrode for supporting BNNS. That is, it was found that even when BNNS was supported on GC, the oxygen reduction reaction activity was not improved.

[Example 2: Au electrode carrying Au fine particle carrying BN nanosheet]
Regarding the electrode using the oxygen reduction electrocatalyst, the ORR activity in the case where an Au fine particle-supported BNNS was used was examined below.

First, BNNS was produced by the Hummer method as follows. 2 g of BN powder was dispersed in 9 ml of concentrated nitric acid (97%), maintained at 0 ° C. and sufficiently stirred. 18 ml of concentrated sulfuric acid was slowly mixed there and continuously stirred. Further, 10 g of KMnO ₄ was slowly added over 1 hour, and stirring was continued for 5 days. The reaction mixture was diluted with 100 ml of diluted water, filtered, washed several times with water, and dried at 100 ° C. for 24 hours. Thereafter, thermal exfoliation was performed by heating to 1050 ° C. in an Ar atmosphere.

The BNNS obtained by the above procedure was modified with Au as follows (hereinafter, this product is abbreviated as Au fine particles / BNNS or Au-BNNS). First, BNNS was dispersed in 60 ml of isopropanol (IPA) and subjected to ultrasonic treatment for 3 hours. Next, 4 ml of 10 mM HAUCl ₄ solution (IPA was used as a solvent) was added, and the mixture was sufficiently stirred at room temperature for about 30 minutes. Next, 2 ml of 50 mM NaBH ₄ solution (IPA: H ₂ O = 4: 1 solution was used as a solvent) was slowly added, and the mixture was sufficiently stirred at room temperature for 3 hours. The product was washed several times with water and ethanol and dried overnight in a vacuum chamber.

FIG. 7 shows the result of the structural evaluation of the Au fine particles / BNNS thus produced by XRD. From the results of FIG. 7, it was confirmed that Au that was not present on the original BNNS was supported after the above-described treatment.

Furthermore, the result of having observed BNNS and Au microparticles / BNNS with TEM and HRTEM is shown in FIG. 8 (d) and 8 (e) are the TEM images and HRTEM images of the Au fine particles / BNNS after the Au modification, in which the fine particles not seen in the BNNS in FIGS. 8 (a), (b) and (c) are shown. In (f), it can be confirmed that fine particles having a size of about 10 nm are supported on the BNNS surface.

An electrode (Au fine particle / BNNS / Au electrode or Au-BNNS / Au electrode) in which the Au fine particles / BNNS were supported on the same Au electrode as in Example 1 was produced. Furthermore, an electrode (BNNT / Au electrode) in which BNNT was supported on an Au electrode was also prepared for comparison. A method for manufacturing these electrodes is described below.

In preparation of Au fine particles / Au electrode, first, 3 mg of BNNT was dispersed in 1 ml of IPA and subjected to ultrasonic treatment for 48 hours. This Au fine particle / BNNS dispersion was spin-coated on the Au electrode. Specifically, it was first rotated at 500 rpm for 90 seconds and then at 2000 rpm for 3 minutes. This was dried in a hot air oven at 100 ° C. for about 30 minutes to obtain Au fine particles / BNNS / Au electrodes.

In preparation of the BNNT / Au electrode, first, 3 mg of Au fine particles / BNNS were dispersed in 1 ml of IPA, subjected to ultrasonic treatment for 5 minutes, and then centrifuged at 1500 rpm for 1 hour. The upper 3/4 of the supernatant from the centrifugation was removed for use in the spin coat solution. This spin coating solution was spin coated on an Au substrate. Specifically, it was first rotated at 500 rpm for 90 seconds and then at 2000 rpm for 3 minutes. This was dried in a hot air oven at 100 ° C. for about 30 minutes to obtain Au fine particles / BNNS / Au electrodes.

Furthermore, an electrode (Au fine particle / BNNS / Au electrode or Au-BNNS / Au electrode) in which the same Au fine particles / BNNS were supported on the GC electrode was also produced.

The three types of electrodes were used as working electrodes under the same rotating electrode electrochemical cell and conditions as in Example 1, and the redox characteristics were measured. The measurement results are shown in FIG. 9A together with the measurement results for the gold electrode, the BNNS / Au electrode, and the BNNS / GC electrode in Example 1. Of the measurement results, the ORR potential at a current density of −0.02 mAcm ⁻² was applied to the gold electrode, the BNNS / Au electrode, the Au fine particle / BNNS / Au electrode, the BNNT / Au electrode, and the Au fine particle / BNNS / GC electrode. The results are shown in Table 1 below.

As is clear from FIG. 9A and Table 1, the Au fine particle / BNNS / Au electrode of Example 2 has better ORR activity than the BNNS / Au electrode of Example 1. Furthermore, it was confirmed that both the Au fine particle / BNNS / Au electrode and the BNNS / Au electrode were better than the BNNT / Au electrode verified in Patent Document 1.
Further, as is clear from FIG. 9 (a) and Table 1, the Au fine particle / BNNS / GC electrode with the least amount of Au used exhibits the same ORR active potential as the Au fine particle / BNNS / Au electrode. It was confirmed that it functions as an active oxygen reduction catalyst. In other words, even when GC is used instead of Au as the substrate, if Au fine particles / BNNS (BNNS carrying Au fine particles) are supported on the surface, the substrate functions as a highly active oxygen reduction catalyst, which is expensive. It has been found that it is possible to suppress the use of a noble metal as a substrate.

Further, in FIG. 9B, in addition to the measurement result of the Au fine particle / BNNS / GC electrode (no heat treatment by heating) shown in FIG. 9A, the Au fine particle / BNNS / GC electrode is 600 times. The measurement result of the heat treatment by heating at ° C. is the measurement result of the GC electrode or the measurement result of the other electrode shown in FIG. 9A (however, the above-mentioned shown in FIG. 9A) Measurement results of BNNT / Au electrode are not included.) The newly added electrode measurement method and measurement conditions are the same as in FIG.
As apparent from FIG. 9 (b), when the above heat treatment is applied, the ORR potential of the Au fine particles / BNNS / GC electrodes (no heat treatment by heating) shown in FIG. 9 (a) is hardly changed. It was confirmed that the oxygen reduction current can be remarkably increased although the results of supporting Au fine particles / BNNS on the Au substrate were not achieved (while maintaining good ORR activity).
Therefore, it was found that the Au fine particle / BNNS / GC electrode is effective in reducing the use of noble metal, and this oxygen reduction current can be improved by heat treatment.

[Example 3: Au electrode carrying BNNS with adjusted size (hereinafter also referred to as h-BNNS)]
Using an electrode (Au / h-BNNS electrode) on which an unsized h-BNNS is supported on an Au substrate and three types of filters (1.0 μm, 0.45 μm, 0.22 μm) Further, three types of electrodes (Au / h-BNNS electrodes) in which h-BNNS (hexagonal BN nanosheets) whose size was adjusted were supported on an Au substrate were prepared. For comparison, an Au electrode not supporting h-BNNS and a Pt electrode not supporting h-BNNS were also produced.
The Au electrode and the Pt electrode were prepared by the procedure shown in FIG. Specifically, a Ti layer (15 nm thickness) as a binder was supported on a Si substrate by sputtering, and then Au or Pt (150 nm thickness) was supported by sputtering.
Next, BNNS was produced by the Hummer method as follows.
2 g of BN powder was dispersed in 9 ml of concentrated nitric acid (97%), maintained at 0 ° C. and sufficiently stirred. 18 ml of concentrated sulfuric acid was slowly mixed there and continuously stirred. Further, 10 g of KMnO ₄ was slowly added over 1 hour, and stirring was continued for 5 days. The reaction mixture was diluted with 100 ml of diluted water, filtered, washed several times with water, and dried at 100 ° C. for 24 hours. Thereafter, thermal exfoliation was performed by heating to 1050 ° C. in an Ar atmosphere. BNNS obtained by the above procedure was dispersed in isopropanol (IPA) and filtered using three types of filters (1.0 μm, 0.45 μm, 0.22 μm), and each filtrate was spin-coated. The coating was performed on the Au electrode substrate produced as described above. In spin coating, it was first rotated at 500 rpm for about 90 seconds, and then rotated at 2000 rpm for 3 minutes. After completion of the spin coating, these electrodes were dried at 100 ° C. for 30 minutes in a hot air oven.
With respect to the above six types of electrodes, the redox characteristics were measured using the same rotating electrode electrochemical cell and conditions as in Example 1 as the working electrode. A BNNS / Au electrode to be measured or an Au electrode not supporting BNNS and a Pt electrode were attached as working electrodes (WE). Oxygen-saturated 0.5M sulfuric acid was used as the electrolyte. The measurement results are shown in FIG.
From FIG. 10, the Au electrode carrying BNNS has higher ORR activity than the Au electrode not carrying BNNS, and three types of sizes (1.0 μm) than BNNS without adjusting the electrode size. , 0.45 μm, 0.22 μm), the three electrodes using BNNS that were further sized only with higher ORR activity and as the size of the filter decreased (ie, BNNS It was confirmed that the ORR activity was further improved and the performance approached the Pt electrode as the size of the Pt electrode decreased.

Furthermore, using the electrode prepared by the above method and carrying an unsized h-BNNS on an Au substrate, and three types of filters (1.0 μm, 0.45 μm, 0.22 μm). Further, by using three electrodes in which h-BNNS of which only the size was adjusted was supported on an Au substrate, the production rate of hydrogen peroxide generated by the oxygen reduction reaction at each electrode was determined by the rotating ring-disk electrode method. The measurement was performed under the same electrochemical cell and conditions as in Example 1, and the redox characteristics of each electrode were examined. As the disk electrode, a removable Au electrode (φ5 mm × thickness 5 mm), an electrode carrying BNNS by a spin coating method, and an Au electrode not carrying BNNS were used, and Pt was used as a ring electrode. . Oxygen-saturated 0.5M sulfuric acid was used as the electrolyte. The potential of the ring electrode was always kept at +0.9 V during the measurement, and the generation rate of hydrogen peroxide was measured by detecting the oxidation current of hydrogen peroxide generated by the oxygen reduction reaction at the disk electrode. The result is shown in FIG. Here, the particle diameter / μm of various electrodes shown in FIG. 11 is an average value of the size (particle diameter) of h-BNNS used in various electrodes, calculated based on the result of FIG. is there.
From FIG. 11, h-BNNS in which only the size was adjusted by using three types of filters (1.0 μm, 0.45 μm, 0.22 μm) rather than the electrode using h-BNNS whose size was not adjusted was used. It was confirmed that the generation rate of hydrogen peroxide was smaller in the three electrodes using the hydrogen peroxide, and the generation rate of hydrogen peroxide decreased as the size of h-BNNS decreased.
The inventors of the present application have also confirmed that when the size of BNNS is reduced, all oxygen is reduced to water by a four-electron reaction on the electrode (not shown).

In addition, an electrode prepared by the above method and carrying an unsized h-BNNS on an Au substrate and a filter of three sizes (1.0 μm, 0.45 μm, 0.22 μm) are further used. H-BNNS of which only the size was adjusted was dispersed in an isopropanol solvent, and the flow diameter distribution was measured using a flow rate distribution measuring device. The result is shown in FIG.
As is clear from FIG. 12, the average value of the size (particle diameter) of h-BNNS that did not use the filter and did not adjust the size was further adjusted only by using the three types of filters. It is larger than the average value of any size (particle diameter) of h-BNNS, and as the size of these filters is reduced, the average value of the size (particle diameter) also decreases (ie, the size of h-BNNS). Was reduced).

[Example 4: Production of Au electrode supporting BNNS]
Regarding the electrode using the HER electrode catalyst, an electrode (BNNS / Au electrode) having BNNS supported on an Au substrate was prepared. The fabrication and confirmation were performed in the same manner as described in the previous section [Example 1: Au electrode supporting BNNS]. Specifically, it is as follows.

First, an Au electrode was prepared according to the procedure shown in FIG. That is, it was prepared by supporting a Ti layer (15 nm thickness) as a binder on a Si substrate by sputtering and then supporting Au (150 nm thickness) by sputtering.

Next, in order to prepare BNNS, 3 mg of 1 μm BN powder was first dispersed in 1 ml of isopropanol. The dispersion was sonicated for 96 hours, then centrifuged at 1500 rpm for 1 hour, and the upper 3/4 of the supernatant obtained by centrifugation was taken out for use as a spin coating solution. In the supernatant liquid thus taken out, BNNS peeled from the BN powder was dispersed.

The BNNS thus prepared was evaluated by a transmission electron microscope (TEM) and EELS (electron energy-loss spectroscopy). The result was confirmed to be the same as FIG. 2 (therefore, the result is substituted in FIG. 2). From this EELS spectrum, it was confirmed that BN had sp ² hybrid orbitals, that is, had a hexagonal structure. It was also confirmed that there was no contamination such as carbon.

The supernatant liquid in which BNNS is dispersed was coated on the Au electrode produced as described above by spin coating. In spin coating, it was first rotated at 500 rpm for about 90 seconds, and then rotated at 2000 rpm for 3 minutes. After completion of the spin coating, these electrodes were dried at 100 ° C. for 30 minutes in a hot air oven.

It was confirmed that the SEM images of the Au electrode surface before and after spin coating were the same as in FIG. 3 (therefore, the result is substituted in FIG. 3). FIG. 3 (a) is an SEM image of the Au background before spin coating and with no BNNS supported. FIG. 3B is an SEM image obtained by observing the electrode surface carrying BNNS at a low magnification. FIG. 3C is an SEM obtained by observing the same electrode surface at a magnification approximately twice that of FIG. It is an image and it can be seen that a single BNNS is on Au. By comparing these SEM images, the electrode after spin coating is an Au / BN electrode in which BNNS is placed on the Au background, and BNNS does not completely cover the Au background. I understood it.

[Example 5: Production of Au electrode carrying Au fine particle-carrying BN nanosheet]
Further, with respect to the electrode using the electrode catalyst for hydrogen generation reaction, Au fine particles were supported on BNNS, and Au electrodes further supporting BNNS were prepared. The preparation and confirmation were performed in the same manner as described in the previous section [Example 2: Au electrode carrying Au fine particle-carrying BN nanosheet]. Specifically, it is as follows.

First, BNNS was produced by the Hummer method as follows. 2 g of BN powder was dispersed in 9 ml of concentrated nitric acid (97%), maintained at 0 ° C. and sufficiently stirred. 18 ml of concentrated sulfuric acid was slowly mixed there and continuously stirred. Further, 10 g of KMnO ₄ was slowly added over 1 hour, and stirring was continued for 5 days. The reaction mixture was diluted with 100 ml of diluted water, filtered, washed several times with water, and dried at 100 ° C. for 24 hours. Thereafter, thermal exfoliation was performed by heating to 1050 ° C. in an Ar atmosphere. The BNNS manufacturing method here is slightly different from the BNNS manufacturing method used in manufacturing the BNNS / Au electrode described in the section [Example 4: Preparation of Au electrode carrying BNNS]. It has been confirmed that there is no influence on the HER characteristics due to the difference.

The BNNS obtained by the above procedure was modified with Au as follows (hereinafter, this product is abbreviated as Au fine particles / BNNS or Au-BNNS). First, 20 mg of BNNS was dispersed in 60 ml of isopropanol (IPA) by sonication for 3 hours. Next, 4 ml of 10 mM HAUCl ₄ solution (IPA was used as a solvent) was added, and the mixture was sufficiently stirred at room temperature for about 30 minutes. Next, 2 ml of 50 mM NaBH ₄ solution (IPA: H ₂ O = 4: 1 solution was used as a solvent) was slowly added, and the mixture was sufficiently stirred at room temperature for about 4 hours. The product was washed several times with water and ethanol and dried overnight in a vacuum chamber.

The result of the structural evaluation of the Au fine particles / BNNS thus produced by XRD was confirmed to be the same as in FIG. 7 (therefore, the result is substituted in FIG. 7). From the results of FIG. 7, it was confirmed that Au that was not present on the original BNNS was supported after the above-described treatment.

Furthermore, it was confirmed that the results of observing BNNS and Au fine particles / BNNS with TEM and HRTEM were the same as those in FIG. 8 (therefore, the results are substituted in FIG. 8). 8 (d) and 8 (e) are the TEM images and HRTEM images of the Au fine particles / BNNS after the Au modification, in which the fine particles not seen in the BNNS in FIGS. 8 (a), (b) and (c) are shown. In (f), it can be confirmed that fine particles having a size of about 10 nm are supported on the BNNS surface.

An electrode (Au fine particle / BNNS / Au electrode or Au-BNNS / Au electrode) in which the Au fine particles / BNNS are supported on the same Au substrate as the BNNS / Au electrode described above was produced. The manufacturing method is shown below.

In preparing the Au fine particles / BNNS / Au electrode, first, 3 mg of Au fine particles / BNNS was dispersed in 1 ml of IPA, subjected to ultrasonic treatment for 5 minutes, and then centrifuged at 1500 rpm for 1 hour. The upper 3/4 of the supernatant from the centrifugation was removed for use in the spin coat solution. This spin coating solution was spin coated on an Au substrate. Specifically, it was first rotated at 500 rpm for 90 seconds and then at 2000 rpm for 3 minutes. This was dried in a hot air oven at 100 ° C. for about 30 minutes to obtain Au fine particles / BNNS / Au electrodes.

[Example 6: Production of other electrodes]
Furthermore, an electrode in which the same Au fine particles / BNNS are supported on a Pt electrode (Au fine particles / BNNS / Pt electrode or Au-BNNS / Pt electrode), Ni fine particles / BNNS / Au electrode using Ni fine particles instead of Au fine particles (Ni-BNNS / Au electrode) and Ni fine particle / BNNS / Pt electrode (Ni-BNNS / Pt electrode), Au fine particle / BNNS / Ni electrode (Au-BNNS / Ni electrode) with the substrate changed to Ni, this Au Ni fine particles / BNNS / Ni electrodes (Ni-BNNS / Ni electrodes) in which the fine particles were changed to Ni fine particles were also produced. In addition, an electrode with an Au background (Au electrode), an electrode with an Pt background (Pt electrode), and an electrode with an Ni background (Ni electrode) were also prepared for comparison.

Here, the Ni fine particles / BNNS were prepared as follows.

First, BNNS was prepared using the same method as that described in [Example 5: Production of Au electrode carrying Au fine particle-carrying BN nanosheet]. Next, 20 mg of BNNS was dispersed in 60 ml of IPA by sonication for 3 hours. Next, 4 ml of a 10 mM NiCl solution (IPA) was added, and the mixture was sufficiently stirred at room temperature for about 30 minutes. Next, 2 ml of 50 mM NaBH ₄ solution (IPA: H ₂ O = 4: 1 solution was used as a solvent) was slowly added, and the mixture was sufficiently stirred at room temperature for about 4 hours. The product was washed several times with water and ethanol and dried overnight in a vacuum chamber.

The Ni fine particles / BNNS thus produced are supported on the Au electrode, the Pt electrode, and the Ni electrode in the same manner as in [Example 5: Preparation of Au electrode supporting Au fine particle-supported BN nanosheet]. Thus, a Ni fine particle / BNNS / Au electrode, a Ni fine particle / BNNS / Pt electrode, and a Ni fine particle / BNNS / Ni electrode were prepared, respectively. The Au fine particles / BNNS / Ni electrode was also produced by supporting the Au fine particles / BNNS produced earlier on the Ni electrode by the same method as above.

[Measurement of HER characteristics]
BNNS / Au electrode, Au fine particle / BNNS / Au electrode, Ni fine particle / BNNS / Au electrode, BNNS / Pt electrode, Au fine particle / BNNS / Pt electrode, Ni fine particle / BNNS / Pt electrode, BNNS / Measure the HER characteristics of Ni electrode, Au fine particle / BNNS / Ni electrode, Ni fine particle / BNNS / Ni electrode and Au electrode, Pt electrode and Ni electrode for comparison by rotating electrode method using rotating electrode electrochemical cell did. The diagram conceptually showing the configuration of the used rotating electrode electrochemical cell is the same as FIG. 4 described above. By using the rotating electrode method, mass transfer to the electrode surface can be controlled, and the charge transfer reaction can be quantitatively analyzed with good reproducibility. In FIG. 4, one of the electrodes to be measured was attached as a working electrode (WE). 0.5 M sulfuric acid saturated with hydrogen was used as the electrolyte. In addition, although the specific electrolyte solution was mentioned here, this is only an illustration, and if it has electrical conductivity, the kind and concentration of the electrolyte are not particularly limited. A saturated NaCl silver / silver chloride electrode was used as the reference electrode (RE). Platinum wire was used as the counter electrode (CE). The potential sweep rate was 0.1 mV / sec, and the measurement was performed without rotating the electrode.

FIG. 13 shows the apparatus shown above for the electrode catalyst of the above example using Au as a substrate, that is, the BNNS / Au electrode, the Au—BNNS / Au electrode, and the Ni—BNNS / Au electrode. A linear scanning voltammogram obtained using the measurement conditions is shown. Furthermore, the result of having performed the same measurement about Au electrode and Pt electrode is also shown in FIG. 13 for comparison. From the results of FIG. 13, although the Pt substrate shows the highest HER activity, the example using the Au substrate shows considerably higher HER activity than the Au substrate as a comparative example, and among them, the Au-BNNS / Au electrode and the BNNS The / Au electrode was found to exhibit HER activity much closer to the Pt electrode.

FIG. 14 shows an apparatus using Pt as a substrate in the electrocatalyst of the above example, that is, the apparatus shown above for the BNNS / Pt electrode, the Au—BNNS / Pt electrode, and the Ni—BNNS / Pt electrode, and A linear scanning voltammogram obtained using the measurement conditions is shown. Furthermore, the result of having performed the same measurement about Au electrode, Pt electrode, and Ni electrode is also shown in FIG. 14 for comparison. FIG. 14 shows that the Au—BNNS / Pt electrode and the Ni—BNNS / Pt electrode, which are examples of the present invention, exhibit higher HER activity than the Pt electrode that is a comparative example. This means that if the electrodes shown in these examples are used, the amount of Pt used can be reduced compared to conventional Pt electrodes.

FIG. 15 shows the apparatus shown above for the electrode catalyst of the above example using Ni as a substrate, that is, a BNNS / Ni electrode, an Au—BNNS / Ni electrode, and a Ni—BNNS / Ni electrode. A linear scanning voltammogram obtained using the measurement conditions is shown. Furthermore, the result of having performed the same measurement about Pt electrode and Ni electrode is also shown in FIG. 15 for comparison. From FIG. 15, even when Ni is used as the substrate, the HER activity is improved by supporting BNNS on the substrate, and the effect is further enhanced particularly when BNNS supporting Au fine particles is used. understood.

FIG. 16 is based on the measurement results of the HER characteristics when the BNNS / Au electrode, the Au—BNNS / Au electrode, the Ni—BNNS / Au electrode, the BNNS / Pt electrode, and the Au—BNNS / Pt electrode in the example are used. A Tafel plot is shown. For further comparison, FIG. 16 also shows a Tafel plot in the case of only an Au substrate (Au electrode), a Pt substrate (Pt electrode), and an Ni substrate (Ni electrode). Here, the higher the plot is in the upper right, the better the HER specification is. From FIG. 16, it was found that HER activity is improved by supporting BNNS, and that the effect is particularly great when Au nano particles are supported as BNNS.

The HOR activity of the HER electrode produced in this manner was examined. FIG. 17 shows only the BNNS / Au electrode, the Au—BNNS / Au electrode, the BNNS / Pt electrode, the Au—BNNS / Pt electrode, and the Au substrate (Au electrode) and Pt substrate (Pt electrode) as comparison targets in the examples. Shows a linear scanning voltammogram. In FIG. 17, the positive current density indicates HOR, and the negative current indicates HER. Many catalysts exhibit catalytic activity in the reverse reaction as well if they have catalytic activity for a certain reaction. However, as can be seen from FIG. 17, it was found that the BNNS / Au electrode and Au-BNNS / Au electrode, which are electrocatalysts using an Au substrate, did not show HOR activity despite showing high HER activity. . In contrast, the Pt electrode, the BNNS / Pt electrode, and the Au-BNNS / Pt electrode had both high HER activity and HOR activity. Therefore, by using an electrode catalyst that exhibits only HER activity, it is possible to avoid the problem that the yield of hydrogen decreases due to consumption of hydrogen generated by HER in HOR, which is a reverse reaction.

According to the present invention, since an oxygen reduction electrode catalyst and an oxygen electrode having good ORR activity can be obtained without using platinum, it is expected to greatly contribute to the further spread of fuel cells.
Furthermore, it is possible to provide an HER electrode catalyst that can reduce the amount of platinum that is used or not used, and it is possible to prevent HOR activity by appropriately selecting a substrate material. The use in the field can be greatly expected.

Claims (15)

1. An oxygen reduction electrocatalyst in which a boron nitride nanosheet is supported on the surface of at least one metal selected from the group consisting of Au, Ni, Pd and Pt.
2. The oxygen reduction electrocatalyst according to claim 1, wherein the boron nitride nanosheet carries fine particles of at least one metal selected from the group consisting of elements of groups 8 to 11 of the periodic table.
3. The oxygen reduction electrocatalyst according to claim 1 or 2, wherein the boron nitride nanosheet covers a part of the surface of at least one metal selected from the group consisting of Au, Ni, Pd, and Pt.
4. The oxygen-reducing electrode catalyst according to any one of claims 1 to 3, wherein the boron nitride nanosheet supported on the oxygen-reducing electrode catalyst has a size that suppresses selectivity for the production of hydrogen peroxide by an oxygen reduction reaction at the electrode. Reduction electrode catalyst.
5. An oxygen reduction electrocatalyst in which a boron nitride nanosheet carrying fine particles of at least one metal selected from the group consisting of elements of groups 8 to 11 of the periodic table is supported on the surface of glassy carbon.
6. The oxygen reduction electrode catalyst according to claim 5, wherein the metal fine particles supported on the boron nitride nanosheet are Au.
7. An oxygen electrode using the oxygen reduction electrode catalyst according to any one of claims 1 to 6.
8. A fuel cell comprising the oxygen electrode according to claim 7.
9. An oxygen reduction reaction catalyst that suppresses the production of hydrogen peroxide by an oxygen reduction reaction at an electrode by adjusting the size of the boron nitride nanosheet supported on the oxygen reduction electrode catalyst according to any one of claims 1 to 3. A method to improve activity.
10. An electrode catalyst for hydrogen generation reaction in which boron nitride nanosheets are supported on the surface of at least one metal selected from the group consisting of Au, Ni, Pd and Pt.
11. The electrode catalyst for hydrogen generation reaction according to claim 10, wherein the boron nitride nanosheet carries at least one fine metal particle selected from the group consisting of elements of groups 8 to 11 of the periodic table.
12. The electrode catalyst for hydrogen generation reaction according to claim 10 or 11, wherein the boron nitride nanosheet covers a part of the surface of at least one metal selected from the group consisting of Au, Ni, Pd and Pt.
13. The electrode catalyst for hydrogen generation reaction according to any one of claims 10 to 12, wherein the metal supporting the boron nitride nanosheet on the surface is Au.
14. A hydrogen generating electrode using the electrode catalyst for hydrogen generating reaction according to any one of claims 10 to 13.
15. A hydrogen generator comprising the hydrogen generating electrode according to claim 14.

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