JP7528812B2 - Electrode materials, electrodes and batteries - Google Patents

Description

The present invention relates to an electrode material, an electrode, and a battery.

In recent years, growing concern about environmental issues has led to a shift in energy sources from fossil fuels such as oil and coal to electricity, which places less of a burden on the environment. As a result, the use of batteries is expanding not only in the electrical field, but also in the mechanical field, such as automobiles.
2. Description of the Related Art As the use of batteries spreads to various fields, there is an increasing demand for higher performance batteries, and research into batteries and various components that constitute batteries is being actively conducted in order to realize batteries that meet such demands.

Among the components constituting a battery, electrodes are one of the components that are being actively researched. Conventionally, carbon has been widely used as an electrode material, but when carbon is used for the electrodes of a fuel cell, a problem occurs in that the carbon reacts with water and decomposes at high potential. As a material that does not have such a problem, research on metal oxides with excellent electrical conductivity is being actively conducted. Among them, molybdenum dioxide and tungsten dioxide are known as highly conductive oxides, and for example, a turbostratic structure material containing molybdenum oxide (see Patent Document 1), a lithium ion capacitor containing tungsten dioxide as a negative electrode active material (see Patent Document 2), etc. have been disclosed.
Furthermore, molybdenum and tungsten oxides have been used in fields other than electrodes, and it has been disclosed that composite oxides of molybdenum or tungsten and titanium are used in skin preparations and denitrification catalysts (see Patent Documents 3 to 5). Furthermore, with regard to production methods, a method for producing titanium oxide particles containing molybdenum, which have a predetermined average particle size and a uniform particle shape that is approximately spherical or polyhedral (see Patent Document 6), and a method for producing metal oxide fine particles in which part of the titanium in titanium oxide is replaced with a different metal such as molybdenum or tungsten (see Patent Document 7) have been disclosed.

JP 2015-187066 A JP 2019-21770 A Japanese Patent Application Publication No. 11-35326 JP 2013-193938 A JP 2014-62012 A JP 2016-13954 A JP 2015-196634 A

As described above, molybdenum dioxide and tungsten dioxide have been proposed as electrode materials for batteries. Although these are materials with excellent electrical conductivity, there is a problem that they are oxidized to trioxides (WoO ₃ , MoO ₃ ) in an oxidizing environment and lose their electrical conductivity. Since the electrodes of fuel cells are exposed to an oxidizing environment, these cannot be said to be fully suitable as electrode materials for fuel cells. In addition, when molybdenum dioxide and tungsten dioxide are manufactured by a general manufacturing method, the specific surface area is about 1 m ² /g, but when used as an electrode, it is preferable to support the precious metal serving as a catalyst in a highly dispersed manner, since this improves performance, and in this respect, molybdenum dioxide and tungsten dioxide have room for improvement as electrode materials.

In view of the above-mentioned current situation, the present invention aims to provide an electrode material containing molybdenum or tungsten that has excellent electrical conductivity, excellent oxidation resistance even in an oxidizing environment, and a large specific surface area.

The inventors have investigated an electrode material containing molybdenum or tungsten that can maintain conductivity even in an oxidizing environment and has a larger specific surface area than molybdenum dioxide or tungsten dioxide produced by a general manufacturing method, and have found that a composite oxide in which titanium is added as a metal element to molybdenum or tungsten and the ratio of molybdenum or tungsten to the total metal elements is adjusted to 20 to 60 mol % has excellent conductivity and exhibits excellent oxidation resistance (composition stability) in an oxidizing environment, making it suitable as an electrode material. Furthermore, the inventors have also found that this composite oxide can be obtained as a material with a larger specific surface area than molybdenum dioxide or tungsten dioxide produced by a general manufacturing method, and have completed the present invention.

That is, the present invention relates to a compound represented by the following formula (1);
Ti _(1-x) A _x O ₂ (1)
(wherein A represents molybdenum or tungsten, and x is a number from 0.2 to 0.6)

The present invention also relates to an electrode formed using the electrode material of the present invention.

The present invention is also a fuel cell that is constructed using the electrode of the present invention.

The electrode material of the present invention has excellent electrical conductivity and exhibits excellent oxidation resistance (compositional stability) in an oxidizing environment, and can be obtained as a material with a larger specific surface area than molybdenum dioxide or tungsten dioxide produced by general manufacturing methods, so it can be suitably used as a material for battery electrodes, especially electrodes for fuel cells used in a strongly oxidizing environment.

The following provides a detailed description of a preferred embodiment of the present invention, but the present invention is not limited to the following description, and can be modified as appropriate without departing from the spirit of the present invention.

1. Electrode Material The electrode material of the present invention is a composite oxide containing titanium and molybdenum or tungsten as metal elements, in which the ratio of molybdenum element or tungsten element to the total metal elements is 20 to 60 mol %, specifically, a composite oxide represented by the following formula (1);
Ti _(1-x) A _x O ₂ (1)
(wherein A represents molybdenum or tungsten, and x is a number from 0.2 to 0.6).
In the above formula (1), x may be from 0.2 to 0.6, preferably from 0.3 to 0.5, and more preferably from 0.4 to 0.5.
Such composite oxides are less susceptible to oxidation than molybdenum dioxide or tungsten dioxide, and are therefore capable of maintaining electrical conductivity even in an oxidizing environment.

The composite oxide is a conductive composite oxide, and the volume resistivity is preferably 10 ³ /Ωcm or less, more preferably 10 ² /Ωcm or less, and even more preferably 10 ¹ /Ωcm or less.
The volume resistivity of the composite oxide can be measured by the method described in the Examples below.

The composite oxide can be obtained as a material having a larger specific surface area than molybdenum dioxide or tungsten dioxide produced by a general production method, and the specific surface area is preferably 5 m ² /g or more, more preferably 10 m ² /g or more, and even more preferably 20 m ² /g or more.
The specific surface area of the composite oxide can be measured by the method described in the Examples below.

The above composite oxide has high resistance to oxidation even in low pH environments and oxidizing environments, and can maintain electrical conductivity. In addition, since the specific surface area is larger than that of molybdenum dioxide or tungsten dioxide produced by a general production method, it can highly disperse and support precious metals and the like that serve as catalysts for electrode reactions. For this reason, the above composite oxide is suitable as an electrode material, and can be suitably used as an electrode material for fuel cells, in which the electrodes are exposed to an extremely strong oxidizing environment with a pH value of 3 or less during operation. It can also be suitably used as an electrode material for electrolysis cells, which are electrochemical devices with the same mechanism as fuel cells.
The present invention also includes an electrode formed using the electrode material of the present invention containing the above-mentioned complex oxide, and an electrochemical device, such as a fuel cell or an electrolysis cell, that is configured using the electrode.

The electrode material of the present invention has a large specific surface area and can support a highly dispersed precious metal or the like that serves as a catalyst for the electrode reaction. Therefore, an electrode in which a catalyst is supported on the surface of the electrode material of the present invention is one of the preferred embodiments of the electrode of the present invention.
Furthermore, when the electrode of the present invention is used in combination with a solid polymer electrolyte membrane, a membrane electrode assembly may be formed in which a catalyst layer containing the electrode material of the present invention is provided on at least one side of the solid polymer electrolyte membrane, and such a membrane electrode assembly is also one of the preferred embodiments of the electrode of the present invention.

2. Method for Producing the Composite Oxide The method for producing the composite oxide contained in the electrode material of the present invention is not particularly limited, but it can be obtained by a method including, for example, a first step of adding an aqueous solution of a titanium source to an aqueous solution of an ammonium salt of molybdenum or tungsten, or a mixed aqueous solution of a molybdenum source or tungsten source and a basic aqueous solution to obtain a mixed aqueous solution, a second step of adjusting the pH of the mixed aqueous solution to 1 to 7 to obtain a hydroxide precipitate, a third step of separating the obtained precipitate, and a fourth step of firing the separated precipitate under a reducing atmosphere. By producing by such a production method, a composite oxide with a large specific surface area can be obtained.
In the second step, a precipitate can also be produced by preparing an aqueous solution of molybdenum or tungsten and an aqueous solution of titanium, mixing the two solutions, and directly adjusting the pH to 1 to 7.

In the above first step, when an ammonium salt of molybdenum or tungsten is not used, the molybdenum source or tungsten source may be a sodium salt, a potassium salt, a chloride, or molybdic acid, tungstic acid, etc.

The basic aqueous solution used in the first step is not particularly limited, but examples thereof include an aqueous _NH3 solution, an aqueous NaOH solution, and an aqueous sodium carbonate solution.

The titanium source used in the first step can be a chloride, sulfate, or the like.

The amount of titanium source used in the first step is preferably 0.6 to 4.0 moles of titanium element per mole of molybdenum element or tungsten element mixed in the first step. More preferably, it is 0.7 to 3.0 moles, and even more preferably, it is 1.0 to 3.0 moles.

The amount of the solvent used in the first step is not particularly limited, but for example, it is preferably 100 to 1,000,000 parts by weight per 100 parts by weight of the titanium source used in the first step. It is more preferably 500 to 100,000 parts by weight, and even more preferably 1,000 to 50,000 parts by weight.

The temperature of the mixed aqueous solution in the first step is preferably 0 to 100°C in terms of the solubility of the raw materials. More preferably, it is 20 to 100°C.

In the second step, the pH of the mixed aqueous solution may be adjusted to 1 to 7, but in order to generate a precipitate more sufficiently, the pH is preferably 2 to 6. More preferably, it is 4 to 6.
In the second step, the temperature of the mixed aqueous solution when adjusting the pH to 1 to 7 is preferably 2 to 100° C., more preferably 10 to 70° C., from the viewpoints of the rate of precipitation and workability.

In the above-mentioned method for producing a composite oxide, a step of holding the mixed solution as it is for a predetermined time after adjusting the pH to a predetermined range in the second step and before the third step may be provided. By providing this step, it is possible to generate a precipitate more sufficiently.
The time for carrying out this step is not particularly limited, but in consideration of sufficient generation of precipitate and the efficiency of composite oxide production, it is preferably 10 to 600 minutes, and more preferably 30 to 180 minutes.

In the third step, the method for separating the hydroxide precipitate produced in the second step is not particularly limited, and filtration, centrifugation, evaporation under heating, etc. can be used.

In the fourth step, the temperature for calcination in a reducing atmosphere is not particularly limited as long as the hydroxide obtained in the second step is sufficiently calcined, but is preferably 200 to 800° C., more preferably 400 to 600° C., and even more preferably 500 to 600° C.
The calcination time is not particularly limited as long as the calcination is sufficiently performed, but is preferably 1 to 72 hours, more preferably 1 to 24 hours, and even more preferably 2 to 12 hours.

The reducing atmosphere in the fourth step is not particularly limited, and examples thereof include a hydrogen (H ₂ ) atmosphere, a carbon monoxide (CO) atmosphere, and a mixed gas atmosphere of hydrogen and an inert gas. Among these, a hydrogen atmosphere is preferred because it allows efficient production of a composite oxide. The hydrogen concentration is preferably in the range of 5 to 100 vol%, more preferably 50 vol% or more, even more preferably 75 vol% or more, and particularly preferably 100 vol%. In addition, the reducing atmosphere is preferably in a state in which a reducing gas is continuously injected and flows into the reaction field (also referred to as the system) where the reduction is performed.

The method for producing the composite oxide may include other steps in addition to the above-mentioned first to third steps and the step of holding the mixed solution as is after the above-mentioned second step. The other steps include a step of washing the hydroxide precipitate with water after separating the hydroxide precipitate in the third step and a step of drying the precipitate, a step of pulverizing the precipitate, a step of classifying the precipitate, and the like before the calcination in the fourth step.
The drying temperature in the step of drying the precipitate separated in the third step is not particularly limited as long as the precipitate is dried, but in order to sufficiently dry the precipitate, it is preferably 40 to 200° C., more preferably 50 to 130° C., but in order to prevent oxidation of the composite oxide, drying at a low temperature is particularly preferred, and drying at 70° C. or lower is particularly preferred.
The drying time may be adjusted appropriately depending on the amount of precipitate, but is preferably 1 to 72 hours, and more preferably 2 to 24 hours.

Specific examples are given below to explain the present invention in detail, but the present invention is not limited to these examples. Unless otherwise specified, "%" and "wt%" mean "weight % (mass %)." The methods for measuring and evaluating each physical property are as follows.

＜比表面積＞
ＪＩＳ Ｚ８８３０（２０１３年）の規定に準じ、試料を窒素雰囲気中、２３０℃で３０分間熱処理した後、マイクロトラックベル社製 Ｂｅｌｓｏｒｐ ＩＩを使用し、定容量式ガス吸着法により測定した。
＜耐酸化評価＞
（１）作用極の作製
測定対象のサンプルに、５重量％パーフルオロスルホン酸樹脂溶液（アルドリッチ社製）、イソプロピルアルコール（和光純薬工業社製）及びイオン交換水を加え、超音波により分散させてペーストを調製した。ペーストを回転グラッシーカーボンディスク電極に塗布し、充分に乾燥した。乾燥後の回転電極を作用極とした。
（２）サイクリックボルタンメトリー測定
Automatic Polarization System（北斗電工社製、商品名「ＨＺ－７０００」）に、回転電極装置（北斗電工社製、商品名「ＨＲ－３０１」）を接続し、作用極に、上記で得た測定サンプル付き電極を用い、対極と参照極には、それぞれ白金電極と可逆水素電極（ＲＨＥ）電極を用いた。
測定サンプル付き電極のクリーニングのため、２５℃で、電解液（０．１ｍｏｌ／ｌの過塩素酸水溶液）にアルゴンガスをバブリングしながら０．０５Ｖから１．２Ｖまでサイクリックボルタンメトリーに供した。その後、２５℃で、アルゴンガスを飽和させた電解液（０．１ｍｏｌ／ｌ硫酸水溶液）で０．０５Ｖから１．２Ｖまで掃引速度５０ｍＶ／ｓｅｃで１０００サイクルの試験を行い、酸化由来の電流発生の有無を確認した。 <Composition ratio>
The measurement was carried out by the X-ray fluorescence method using a scanning X-ray fluorescence analyzer ZSX Primus II manufactured by Rigaku Corporation.
<Volume resistivity>
Measurements were performed using the four-probe method using a powder resistivity measurement system MCP-PD51 manufactured by Nitto Seiko Analytech Co., Ltd. The powder resistivity measurement system is composed of a hydraulic powder press unit, a four-probe probe, and a high-resistance measurement device (Loresta GX MCP-T700 manufactured by the same company).
The volume resistivity (Ω·cm) was determined according to the following procedure.
1) A sample powder is placed in a press tool (diameter 20 mm) equipped with a four-point probe on the bottom, and the tool is set in the pressurizing section of the powder resistivity measurement system. The probe is connected to the high-resistance measurement device with a cable.
2) Using a hand press, apply a pressure of 20 kN. Measure the powder thickness with a digital caliper and measure the resistance value with a high resistance measuring device.
3) The volume resistivity (Ω·cm) is calculated from the base area, thickness, and resistance value of the powder according to the following formula 1.

<Specific surface area>
In accordance with the provisions of JIS Z8830 (2013), the sample was heat-treated in a nitrogen atmosphere at 230° C. for 30 minutes, and then measured by a constant volume gas adsorption method using a Belsorp II manufactured by Microtrackbel.
<Oxidation resistance evaluation>
(1) Preparation of working electrode A 5% by weight perfluorosulfonic acid resin solution (manufactured by Aldrich Co.), isopropyl alcohol (manufactured by Wako Pure Chemical Industries, Ltd.) and ion-exchanged water were added to the sample to be measured, and the mixture was dispersed by ultrasonic waves to prepare a paste. The paste was applied to a rotating glassy carbon disk electrode and thoroughly dried. The rotating electrode after drying was used as the working electrode.
(2) Cyclic voltammetry measurement
A rotating electrode device (manufactured by Hokuto Denko Corporation, product name "HR-301") was connected to an Automatic Polarization System (manufactured by Hokuto Denko Corporation, product name "HZ-7000"), and the electrode with the measurement sample obtained above was used as the working electrode, and a platinum electrode and a reversible hydrogen electrode (RHE) electrode were used as the counter electrode and reference electrode, respectively.
To clean the electrode with the measurement sample, the electrode was subjected to cyclic voltammetry from 0.05 V to 1.2 V while bubbling argon gas in an electrolyte (0.1 mol/l aqueous perchloric acid solution) at 25° C. Then, a test was performed for 1000 cycles at 25° C. with an electrolyte (0.1 mol/l aqueous sulfuric acid solution) saturated with argon gas from 0.05 V to 1.2 V at a sweep rate of 50 mV/sec to confirm the presence or absence of a current due to oxidation.

Example 1
A tungsten solution was obtained by mixing and stirring 6.7 g of ammonium tungstate parapentahydrate (Kanto Chemical Co., Ltd.) and 300 g of ion-exchanged water heated to 80° C. The resulting tungsten solution was mixed with 47.3 g of titanium trichloride solution (Fujifilm Wako Pure Chemical Industries, Ltd., _TiCl3 content 22 wt%) and stirred to prepare a solution (this is referred to as a "mixed aqueous solution").
1.0N ammonia water was added to the mixed aqueous solution, and the mixture was mixed and stirred while being heated to 30°C. When the pH reached 5.5, the addition of ammonia water was stopped, and the mixture was heated and maintained at 30°C for 1 hour. The precipitate obtained was filtered and washed in a conventional manner, and then dried at 60°C for 18 hours. The obtained powder was heated to 550°C in a hydrogen atmosphere, and then kept at 550°C for 4 hours, and then cooled to room temperature to obtain black powder 1 of a composite oxide. The molar ratio of Ti and W was confirmed from the results of fluorescent X-ray analysis of black powder 1. The results are shown in Table 1.

Example 2
A black powder 2 of a composite oxide was obtained in the same manner as in Example 1, except that 13.1 g of ammonium tungstate parapentahydrate (manufactured by Kanto Chemical Co., Ltd.) and 31.6 g of titanium trichloride solution were weighed. From the results of fluorescent X-ray analysis of the black powder 2, the molar ratio of Ti and W was confirmed. The results are shown in Table 1.

Example 3
A black powder 3 of a composite oxide was obtained in the same manner as in Example 1, except that ammonium molybdate tetrahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used instead of ammonium tungstate parapentahydrate, and 4.01 g of ammonium molybdate tetrahydrate and 47.3 g of titanium trichloride solution were weighed. From the results of fluorescent X-ray analysis of the black powder 3, the molar ratio of Ti and Mo was confirmed. The results are shown in Table 1.

Example 4
A black powder 4 of a composite oxide was obtained in the same manner as in Example 3, except that 8.03 g of ammonium molybdate tetrahydrate and 31.6 g of titanium trichloride solution were weighed. The molar ratio of Ti and Mo was confirmed from the results of fluorescent X-ray analysis of the black powder 4. The results are shown in Table 1.

Comparative Example 1
A gray composite oxide powder 5 was obtained in the same manner as in Example 3, except that 0.803 g of ammonium molybdate tetrahydrate and 24.7 g of titanium trichloride solution were weighed. The molar ratio of Ti and Mo was confirmed from the results of fluorescent X-ray analysis of powder 5. The results are shown in Table 1.

Comparative Example 2
5.0 g of molybdenum trioxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was heated to 500° C. in a hydrogen atmosphere, held at 500° C. for 4 hours, and then cooled to room temperature to obtain black powder 6.

Comparative Example 3
5.0 g of tungsten trioxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was heated to 580° C. in a hydrogen atmosphere, held at 580° C. for 4 hours, and then cooled to room temperature to obtain black powder 7.

The composition ratio, volume resistivity, and specific surface area of the composite oxides obtained in Examples 1 to 4 and Comparative Example 1, and the volume resistivity and specific surface area of the oxides in Comparative Examples 2 and 3 were measured by the methods described above, and the oxidation resistance was evaluated by the method described above. The results are shown in Table 1.

It is desirable for the electrode material to have a volume resistivity (Ωcm) of the order of ¹⁰³ or less and a specific surface area of 5 ^m2 /g or more. All of the composite oxides of Examples 1 to 4 met these standards, and were confirmed to have excellent oxidation resistance. On the other hand, the material of Comparative Example 1 had a significantly high volume resistivity, and the materials of Comparative Examples 2 and 3, which used molybdenum oxide or tungsten oxide, had small specific surface areas.
From these, it was confirmed that the composite oxide of the present invention, which contains titanium and either molybdenum or tungsten and in which the ratio of molybdenum element or tungsten element to the total metal elements is within a predetermined range, is suitable as an electrode material.

Claims (3)

The following formula (1):
Ti _(1-x) A _x O ₂ (1)
(In the formula, A represents molybdenum or tungsten, and x is a number from 0.2 to 0.6.)
An electrode material comprising a composite oxide represented by the formula: An electrode formed using the electrode material described in claim 1. A fuel cell constructed using the electrode according to claim 2.