JP2019155355A - Oxygen evolution catalyst - Google Patents

Abstract

To provide an oxygen evolution catalyst that has initial activity and durability equal to or higher than those of iridium oxide and is less costly than iridium oxide.SOLUTION: In the first embodiment, an oxygen evolution catalyst is provided which comprises a core and a shell covering the core surface, and in which the core includes ruthenium oxide or metal ruthenium at least on the surface, and the shell includes titania or a compound oxide of titanium and ruthenium. In the second embodiment, an oxygen evolution catalyst is provided which is identical to the oxygen evolution catalyst in first embodiment except that the oxygen evolution catalyst has a titania coverage factor of 0.05-5.0 ML (the number of titania atomic layers defined assuming that all titanium included in the shell becomes titania and that the titania uniformly covers the core surface). In the third embodiment, the oxygen evolution catalyst is provided which is identical to the oxygen evolution catalyst in the first or the second embodiment except that the oxygen evolution catalyst is used for an oxygen electrode of a water electrolysis apparatus. In the fourth embodiment, an oxygen evolution catalyst is provided which is identical to the oxygen evolution catalyst in the first or the second embodiment except that oxygen evolution catalyst is added to an anode of a fuel cell.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an oxygen generation catalyst, and more particularly to an oxygen generation catalyst that can be used in an oxygen electrode of a water electrolysis apparatus or an anode of a fuel cell.

Ruthenium oxide, iridium oxide, and the like are known as materials showing oxygen generation (OER) activity. Materials that exhibit such OER activity are:
(A) a catalyst on the oxygen electrode side of the water electrolysis device,
(B) When power generation is continued in a state where the fuel supply to some of the single cells in the fuel cell stack is interrupted, oxidation of the carbon material generated at the anode of the single cell (fuel shortage cell) where the fuel supply is interrupted It is used as a catalyst for suppression.

Among these, iridium oxide has higher durability of OER activity than other materials. Therefore, iridium oxide is often used as the oxygen generation catalyst. However, iridium oxide has low initial activity and high cost.
On the other hand, ruthenium oxide is lower in cost and higher in initial activity than iridium oxide. However, ruthenium oxide has a problem that durability of OER activity is low.

In order to solve this problem, various proposals have heretofore been made.
For example, Patent Document 1 discloses that
(A) To a suspension of TiO ₂ (BET> 300 m ² / g) was added a hexachloroiridium acid (H ₂ IrCl ₆ ) solution, and the suspension was heated to 70 ° C.
A process for producing an IrO ₂ / TiO ₂ catalyst is disclosed in which (b) the product is isolated by filtration and (c) the product is calcined at 400 ° C.

In the same document,
(A) By such a method, a catalyst in which iridium oxide (IrO ₂ ) particles are finely deposited on an inorganic oxide material (TiO ₂ ) can be obtained,
(B) When iridium oxide is deposited on an inorganic oxide material having a high specific surface area, agglomeration of iridium oxide is suppressed even after heat treatment, and a high specific surface area is maintained, whereas particles are aggregated only with iridium oxide. Easy to do and
(C) As a result, the IrO ₂ / TiO ₂ catalyst has a lower starting potential for oxygen generation (ie, higher OER activity) than the IrO ₂ catalyst,
Is described.

Patent Document 1 describes that a decrease in OER activity (aggregation of IrO ₂ ) is suppressed by supporting fine IrO ₂ particles on the TiO ₂ surface.
However, the catalyst described in this document is not configured to protect the catalyst surface that is the starting point of catalyst deterioration, and does not solve the problem of durability. Further, since IrO ₂ is used as the catalyst, the cost is high.

Special table 2007-514520 gazette

  The problem to be solved by the present invention is to provide an oxygen-generating catalyst having an initial activity and durability equivalent to or higher than those of iridium oxide and lower in cost than iridium oxide.

In order to solve the above-described problems, an oxygen generation catalyst according to the present invention includes:
A core and a shell covering the surface of the core;
The core includes ruthenium oxide or metal ruthenium at least on the surface,
The gist is that the shell contains titania or a composite oxide of titanium and ruthenium.

  The oxygen generation catalyst according to the present invention exhibits an initial activity and durability equal to or higher than those of conventional catalysts using iridium oxide. This is because the surface of the core containing ruthenium oxide or metal ruthenium is covered with titania or a shell containing a composite oxide of titanium and ruthenium, thereby protecting the catalyst surface that is the starting point of catalyst deterioration. Conceivable. Furthermore, since the oxygen generation catalyst according to the present invention is mainly composed of ruthenium oxide or metal ruthenium, the cost is lower than that of conventional catalysts mainly composed of iridium oxide.

2 is a TEM image of the oxygen generation catalyst obtained in Example 1 and Comparative Example 1. FIG. 2 is an EDX mapping of the oxygen generation catalyst obtained in Example 1. It is the initial water electrolysis activity of the oxygen generation catalyst obtained in Example 1 and Comparative Examples 1-3. It is an activity change in the potential cycle (0.07V⇔1.8V) of the oxygen generation catalyst obtained in Example 1 and Comparative Examples 1 to 3. FIG. 5A shows changes in the IV characteristics of the oxygen generation catalyst obtained in Example 1. FIG. FIG. 5B shows changes in the IV characteristics of the oxygen generation catalyst obtained in Comparative Example 1.

It is an activity change in the potential cycle (0.07V⇔1.8V) of the oxygen generation catalyst obtained in Examples 1-5 and Comparative Examples 1-2. It is the titania coverage dependency of the initial activity of the oxygen generation catalyst obtained in Examples 1-5 and Comparative Examples 1-2 and the activity after the durability test. It is an increase in potential when a potential cycle (0.07 V⇔1.8 V) is added to the oxygen generation catalysts obtained in Examples 1 to 5 and Comparative Examples 1 and 2. It is the titania coverage dependency of the increase width of the potential when the potential cycle (0.07 VＶ1.8 V) is added to the oxygen generation catalysts obtained in Examples 1 to 5 and Comparative Example 1.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Oxygen generation catalyst]
The oxygen generation catalyst according to the present invention is:
A core and a shell covering the surface of the core;
The core includes ruthenium oxide or metal ruthenium at least on the surface,
The shell includes titania or a composite oxide of titanium and ruthenium.

[1.1. core]
The core includes ruthenium oxide (RuO ₂ ) or metal ruthenium (Ru) at least on the surface. Ruthenium oxide or metal ruthenium may be contained only on the surface of the core, or may be contained in the entire core. The surface of the core is preferably substantially composed of ruthenium oxide or metal ruthenium, but may contain inevitable impurities. Since the central part of the core does not contribute much to the oxygen generation reaction, it does not necessarily need to be made of ruthenium oxide or metal ruthenium, and may be made of other materials.
The particle size of the core is not particularly limited. In general, the smaller the core particle size, the higher the effect obtained with a small amount. For this purpose, the average particle diameter of the core is preferably 1 μm or less. The average particle diameter of the core is preferably 500 nm or less, and more preferably 200 nm or less.

[1.2. shell]
The shell includes titania (TiO ₂ ) or a composite oxide of titanium and ruthenium ((Ti, Ru) O ₂ ). As will be described later, the shell is formed by forming a titania precursor around the core and firing it. At this time, ruthenium oxide or metal ruthenium on the core surface may react with the titania precursor to form a composite oxide. The shell is preferably composed essentially of titania or a complex oxide, but may contain inevitable impurities.
The shell covering the surface of the core is not particularly limited as long as it does not inhibit the OER activity of ruthenium oxide or metal ruthenium and can improve the durability of ruthenium oxide or metal ruthenium.

[1.3. Titania coverage]
“Titania coverage” refers to the number of titania atomic layers when it is assumed that the titanium contained in the shell is all titania (TiO ₂ ) and the titania covers the surface of the core uniformly. Say. “1ML” represents one atomic layer of titania. “1ML” is specifically defined when the surface atom number density of TiO ₂ (the total number of Ti and O per unit area) is 1.5 × 10 ¹⁵ cm ⁻² .

As the titania coverage decreases, the durability of the oxygen generating catalyst decreases. In order to obtain durability equal to or higher than that of iridium oxide, the titania coverage is preferably 0.05 ML or higher. The titania coverage is preferably 0.1 ML or more.
On the other hand, titania itself has no OER activity. Therefore, when the titania coverage is too high, the resistance of the diffusion of water to the core surface and the diffusion of oxygen from the core surface increases, and thus the OER activity decreases. Therefore, the titania coverage is preferably 5.0 ML or less. The titania coverage is preferably 0.5 ML or less.

[1.4. Application]
The oxygen generation catalyst according to the present invention is:
(A) a catalyst used for an oxygen electrode of a water electrolysis device,
(B) a catalyst added to the anode of the fuel cell (a catalyst for suppressing the oxidation of the carbon material generated at the anode of the fuel-deficient cell),
Can be used.

[2. Method for producing oxygen-generating catalyst]
The oxygen generation catalyst according to the present invention can be produced by a so-called sol-gel method.

[2.1. Preparation of dispersion]
First, core particles containing at least ruthenium oxide or metal ruthenium on the surface are dispersed in a solvent to obtain a dispersion. The solvent is
(A) the core particles can be dispersed, and
(B) A material capable of performing hydrolysis and polycondensation of a Ti source (alkoxide) so that the surface of the core particle is coated with a titania precursor,
If it is.
Examples of the solvent include alcohol, water, and a mixed solvent thereof.
The concentration of the core particles in the dispersion is not particularly limited as long as the core particles can be uniformly dispersed.

[2.2. Addition of Ti source]
Next, a Ti source is added to the dispersion. When a Ti source is added to the dispersion, hydrolysis and condensation polymerization of the Ti source proceed in the dispersion. As a result, precursor particles in which the surfaces of the core particles are coated with a titania precursor are obtained.
Examples of the Ti source include tetraisopropyl orthotitanate and tetrabutyl orthotitanate.
The amount of Ti source to be added to the dispersion is selected in accordance with the target composition.

[2.3. Heat treatment]
After the precursor particles are recovered from the dispersion and dried, the precursor particles are heat-treated. Thereby, the oxygen production | generation catalyst by which the surface of the core particle was coat | covered with the shell which consists of titania or the composite oxide of titanium and ruthenium is obtained.
The heat treatment is performed to dehydrate and crystallize the titania precursor in which OH groups remain. The heat treatment conditions are not particularly limited as long as the titania precursor can be dehydrated and crystallized. Usually, it is preferably heat-treated at 300 to 800 ° C. for about 0.5 to 3 hours in the air.

[3. Action]
Ruthenium oxide and metal ruthenium are lower in cost and higher in initial activity than iridium oxide. However, ruthenium oxide and metal ruthenium have low durability of OER activity.
In contrast, the oxygen generation catalyst according to the present invention exhibits an initial activity and durability equal to or higher than those of conventional catalysts using iridium oxide. This is because the surface of the core containing ruthenium oxide or metal ruthenium is covered with titania or a shell containing a composite oxide of titanium and ruthenium, thereby protecting the catalyst surface that is the starting point of catalyst deterioration. Conceivable. Furthermore, since the oxygen generation catalyst according to the present invention is mainly composed of ruthenium oxide or metal ruthenium, the cost is lower than that of conventional catalysts mainly composed of iridium oxide.

(Example 1, Comparative Examples 1-3)
[1. Preparation of sample]
[1.1. Example 1]
A commercially available ruthenium oxide catalyst (0.3 g) was dispersed in 50 mL of a solvent (isopropanol 80%, water 20%). To this, 0.6 mL of tetraisopropyl orthotitanate (TTIP) was added and stirred at room temperature for 4 hours. After stirring, the dispersion was filtered, and the catalyst precursor was recovered and dried. Further, the catalyst precursor was heat-treated at 400 ° C. for 1 hour in an air atmosphere to obtain an oxygen generation catalyst.
The amount of TTIP added in Example 1 corresponds to the amount of five titania atomic layers formed on the surface of ruthenium oxide. Hereinafter, Example 1 is also expressed as “5ML” using a unit of ML (Mono Layer).

[1.2. Comparative Examples 1-3]
A commercially available ruthenium oxide catalyst was heat-treated at 400 ° C. for 1 hour in an air atmosphere without being modified with titania (Comparative Example 1).
Further, a commercially available ruthenium oxide catalyst that was not subjected to heat treatment (Comparative Example 2) and a commercially available iridium oxide catalyst (Comparative Example 3) were subjected to the test as they were.

[2. Test method]
[2.1. TEM observation and EDS mapping]
TEM observation and EDX mapping of the catalyst of Example 1 and Comparative Examples 1 to 3 were performed.

[2.2. Activity and durability evaluation]
The catalysts of Example 1 and Comparative Examples 1 to 3 were each applied to a gold disk and dried. Using this as a working electrode, electrochemical measurement was performed. The catalyst loading was unified to 15 μgcm ⁻² . For Example 1, “catalyst loading” represents the amount excluding titania. The reference electrode was a reversible hydrogen electrode, the counter electrode was platinum, and the electrolyte was perchloric acid (0.1 M).
The measurement procedure is as follows.
(A) First, a potential scan of 1.0 V to 1.6 V was performed for one cycle.
(B) Next, a potential scan of 1.0 V to 1.7 V was performed for one cycle.
(C) Further, a potential scan of 0.07 V to 1.8 V was performed for 20 to 50 cycles.

[3. result]
[3.1. TEM observation and EDS mapping]
In FIG. 1, the TEM image of the oxygen production | generation catalyst obtained in Example 1 and Comparative Example 1 is shown. Overall, the particles of Example 1 appear to be rounder than Comparative Example 1. Further, when the TEM image of Example 1 is observed in detail, the ruthenium oxide particles (the region surrounded by the broken line in FIG. 1) are covered with the amorphous substance (the region surrounded by the broken line in FIG. 1). appear.

  FIG. 2 shows an EDX mapping of the oxygen generation catalyst obtained in Example 1. The distributions of Ru and Ti are almost overlapped, and it is considered that titania covers the entire surface of the ruthenium oxide particles.

[3.2. Activity and durability evaluation]
[3.2.1. Initial activity]
In FIG. 3, the initial water electrolysis activity (water electrolysis activity obtained by the measurement procedure (a)) of the oxygen generation catalyst obtained in Example 1 and Comparative Examples 1 to 3 is shown. Initial activity is as follows: untreated ruthenium oxide (RuO ₂ , Comparative Example 2)> unmodified heat-treated ruthenium oxide (HT-RuO ₂ , Comparative Example 1)> titania modified ruthenium oxide (HT-TiO ₂ —RuO ₂ , Example 1) The order was> IrO ₂ (Comparative Example 3). FIG. 3 shows that HT—TiO ₂ —RuO ₂ is less active than RuO ₂ and HT—RuO _2, but more active than IrO ₂ .

[3.2.2. durability]
FIG. 4 shows the activity change during the potential cycle (0.07 V⇔1.8 V) of the oxygen generation catalysts obtained in Example 1 and Comparative Examples 1 to 3. In FIG. 4, “potential @ 0.5 mA μg ⁻¹ ” on the vertical axis represents the potential when the current density reaches 0.5 mA μg ⁻¹ when the potential is increased from 0.07V. It represents that OER activity is so high that "electric potential @ 0.5mAmicrogram < ^-1 >" becomes small. Further, the value of the first cycle in FIG. 4 indicates that the current density is 0.5 mA μg ⁻¹ in the first cycle when the measurement procedure (c) is performed after the measurement procedures (a) and (b). Represents the potential at which

In FIG. 3, the activity of HT-RuO ₂ (Comparative Example 1) was greater than that of HT-TiO ₂ —RuO ₂ (Example 1). However, the activity of Example 1 in the first cycle of FIG. 4 was almost equivalent to that of Comparative Example 1. This is because the deterioration has already progressed in the measurement procedure (a) + measurement procedure (b), and the catalytic activities of both were equal at the start of the measurement procedure (c). From FIG. 4, HT-RuO ₂ (Comparative Example 1) and RuO ₂ (Comparative Example 2) show a decrease in activity during the cycle (catalyst deterioration), while Example 1 shows a decrease in activity. I understand that there is no.

  FIG. 5 shows changes in the IV characteristics of the oxygen generation catalysts obtained in Example 1 (FIG. 5A) and Comparative Example 1 (FIG. 5B). FIG. 5 shows that when ruthenium oxide is modified with titania, the activity does not decrease even when a potential cycle is added. The initial activity is Comparative Example 1> Example 1, but both activities are reversed during the cycle. From the above, it was found that an OER catalyst having high activity and high durability can be obtained by modifying the surface of ruthenium oxide with titania.

(Examples 2 to 5)
[1. Preparation of sample]
The amount of TTIP added was 0.06 mL (corresponding to 0.5 ML, Example 2), 0.03 mL (corresponding to 0.25 ML, Example 3), 0.012 mL (corresponding to 0.1 ML, Example 4), or An oxygen generation catalyst was produced in the same manner as in Example 1, except that the amount was 006 mL (equivalent to 0.05 ML, Example 5).

[2. Test method]
The activity and durability were evaluated in the same manner as in Example 1. The obtained current value was normalized by the weight of Ru.

[3. result]
[3.1. Activity change]
FIG. 6 shows a change in activity during the potential cycle (0.07 V⇔1.8 V) of the oxygen generation catalyst obtained in Examples 2 to 5. In addition, in FIG. 6, the result of Example 1 and Comparative Examples 1-2 was also shown collectively. In FIG. 6, the vertical axis represents the potential reaching 0.5 mA μg ^−1, and the lower the vertical axis, the higher the OER activity.

In Comparative Example 1 (HT-RuO ₂ ) and Comparative Example 2 (commercially available RuO ₂ ), the activity decreased during the cycle, and catalyst deterioration was observed.
On the other hand, the activity of Example 1 (5ML) hardly decreased. Furthermore, in Examples 2 to 5 (0.5 to 0.05 ML) in which the coverage was reduced, deterioration was observed from 1 cycle to 20 cycles. However, there was almost no deterioration over 20 to 50 cycles, and the activity was higher than that of Comparative Example 1 even after 50 cycles.

[3.2. Coverage dependence of activity change]
FIG. 7 shows the titania coverage dependency of the initial activity of the oxygen generation catalyst obtained in Examples 2 to 5 and the activity after the durability test. In addition, in FIG. 7, the result of Example 1 and Comparative Examples 1-2 was also shown collectively. In FIG. 7, the vertical axis represents the potential reaching 0.5 mA μg ⁻¹ , and indicates that the OER activity is higher toward the lower side of the vertical axis. In FIG. 7, black circles represent the first cycle potential (initial activity), and white circles represent the 50th cycle potential (endurance test activity). Further, in FIG. 7, the solid line represents the first cycle potential (initial activity) of Comparative Example 1 or 2, and the broken line represents the 50th cycle potential (endurance test activity).

  From FIG. 7, it can be seen that the initial activities of Examples 1-5 are higher than those of Comparative Examples 1-2. The activity after the durability test was the same. Furthermore, regarding the coverage dependency, both the initial activity and the activity after the durability test were maximized at 0.25 ML.

[3.3. Potential rise]
FIG. 8 shows the increase in potential when a potential cycle (0.07 V⇔1.8 V) is applied to the oxygen generation catalysts obtained in Examples 2 to 5. In addition, in FIG. 8, the result of Example 1 and Comparative Examples 1-2 was also shown collectively.
Here, the “potential increase width” means a potential difference between the first cycle and the Nth cycle. FIG. 8 is a graph obtained by normalizing FIG. 6 as the potential increase from the first cycle, and shows the degree of progress of deterioration from the first cycle. FIG. 8 shows that the deterioration progresses toward the upper side of the vertical axis.

  FIG. 8 shows that the degree of deterioration in Examples 1 to 5 is smaller than that in Comparative Examples 1 and 2. Furthermore, it can be seen from FIGS. 7 and 8 that in Examples 1 to 5, the OER activity itself is higher than that of Comparative Examples 1 and 2, and deterioration is less likely to proceed.

[3.4. Dependence of potential rise on titania coverage]
FIG. 9 shows the titania coverage dependency of the potential increase width when a potential cycle (0.07 V⇔1.8 V) is applied to the oxygen generation catalysts obtained in Examples 2 to 5. In FIG. 9, the results of Example 1 and Comparative Example 1 are also shown. In FIG. 9, black circles represent the potential difference between the first cycle and the 20th cycle, and white circles represent the potential difference between the first cycle and the 50th cycle. Moreover, FIG. 9 shows that a deterioration inhibitory effect is so large that it goes to the downward side of a vertical axis | shaft.

  From FIG. 9, it can be seen that the greater the titania coverage, the greater the effect of suppressing deterioration. In 5ML, there is no difference between the potential difference at the 20th cycle and the potential difference at the 50th cycle.

[3.5. Summary]
From the above, the effects of coating the surface of the ruthenium-based catalyst with titania can be summarized as follows.
(1) The OER activity after the durability test of Examples 1 to 5 (after 50 cycles of 0.07V⇔1.8V cycle) is higher than that of Comparative Example 1 (see FIG. 6).
(2) In Examples 1 to 5, both the initial activity and the activity after the durability test are higher than those of Comparative Example 1. In addition, both the initial activity and the activity after the durability test reach a maximum at 0.25 ML (see FIG. 7).
(3) The degree of deterioration with respect to the number of cycles in Examples 1 to 5 is smaller than those in Comparative Examples 1 and 2 (see FIG. 8). Further, the degree of deterioration decreases as the coverage increases (see FIG. 9).

  From the above, it became possible to obtain an OER catalyst having high activity and durability by coating the surface of the ruthenium-based catalyst with 0.05 to 5 ML of titania as in the present invention. Durability increased as the titania coverage increased, but the absolute value of activity was highest near 0.25 ML. Durability and activity were higher than those of Comparative Example 1 at all the covered ratios tested (0.05 ML or more and 5 ML or less), but the optimum coverage is considered to be around 0.25 ML.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The oxygen generation catalyst according to the present invention can be used as a catalyst used for an oxygen electrode of a water electrolysis apparatus, a carbon deterioration suppressing catalyst added to an anode of a fuel cell, and the like.

Claims (4)

A core and a shell covering the surface of the core;
The core includes ruthenium oxide or metal ruthenium at least on the surface,
The shell is an oxygen generation catalyst containing titania or a composite oxide of titanium and ruthenium. The oxygen-producing catalyst according to claim 1, wherein the titania coverage is 0.05 ML or more and 5.0 ML or less.
However, “titania coverage” means that all of the titanium contained in the shell is the titania, and the titania atomic layer is assumed to be uniformly coated on the surface of the core. The number of layers.   The oxygen generation catalyst according to claim 1 or 2, which is used for an oxygen electrode of a water electrolysis apparatus.   The oxygen generation catalyst according to claim 1 or 2, which is added to an anode of a fuel cell.

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