Oxygen-evolving catalysts and electrochemical devices

Abstract

To provide an oxygen evolution catalyst that exhibits low overpotential in the oxygen evolution reaction. [Solution] An oxygen evolution catalyst comprising (i) Ni, (ii) Ru, or Ru and Fe, and (iii) O, or O and H in the form of a compound, wherein in the X-ray diffraction profile measured by X-ray diffraction (XRD), (a) a first peak is observed at 34°≦2θ≦35°, and (b1) a second peak is observed at 11°≦2θ≦12°, and the ratio of the intensity P2 of the second peak to the intensity P1 of the first peak P2 / P1 is 2.2 or less, or (b2) no second peak is observed at 11°≦2θ≦12°.

Description

Technical Field

[0001] The present disclosure relates to an oxygen generation catalyst and an electrochemical device.

Background Art

[0002] As an efficient method for using surplus power derived from renewable energy with large output fluctuations such as sunlight and wind power, hydrogen production by electrolysis of water (hereinafter referred to as water electrolysis), batteries, especially air batteries with high energy density, etc. have attracted attention. A common point of these applications is the reaction of generating oxygen (O2) from hydroxide ions (OH - -). Therefore, the development of highly active oxygen generation catalysts has been carried out.

[0003] Layered double hydroxides (hereinafter referred to as LDH) can increase the specific surface area and can adopt various combinations of metal ions, so they have attracted attention as promising oxygen generation catalysts, and various reports such as NiFe-LDH have been made. For example, Patent Document 1 (Japanese Patent No. 7008201) discloses an LDH containing two or more transition metals (for example, Ni and Fe) and a chelating agent, and this LDH is said to be able to exhibit higher catalytic activity than conventional ones in the anodic reaction of water electrolysis. Also, Non-Patent Document 1 (Yang Yang et al., Journal of The Electrochemical Society, 2022 169 024503) reports that Ru-NiFe-LDH obtained by doping ruthenium ions into NiFe-LDH shows higher oxygen generation reaction (OER) activity than NiFe-LDH.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005] [Non-Patent Document 1] Yang Yang et al., "Ru-Doped NiFe Layered Double Hydroxide as a Highly Active Electrocatalyst for Oxygen Evolution Reaction", Journal of The Electrochemical Society, 2022 169 024503 [Overview of the project]

[0006] There is still much room for improvement in LDHs such as NiFe-LDH used as oxygen evolution catalysts. In particular, when achieving highly efficient oxygen evolution, the magnitude of the overpotential in oxygen evolution reactions, such as those occurring in the anode of water electrolysis and the charging reaction of air batteries, is a concern. Therefore, oxygen evolution catalysts with low overpotential are desired.

[0007] The present inventors have now found that an oxygen evolution catalyst containing (i) Ni, (ii) Ru and optionally Fe, and (iii) O and optionally H in the form of a compound, exhibiting a predetermined X-ray diffraction profile, reduces the overpotential in the oxygen evolution reaction.

[0008] Therefore, the object of the present invention is to provide an oxygen evolution catalyst that exhibits low overpotential in the oxygen evolution reaction.

[0009] The following aspects are provided according to this disclosure. [Aspect 1] An oxygen evolution catalyst comprising (i) Ni, (ii) Ru, or Ru and Fe, and (iii) O, or O and H in the form of a compound, In the X-ray diffraction profile measured by X-ray diffraction (XRD), (a) The first peak is observed at 34°≦2θ≦35°, and, (b1) A second peak is observed at 11°≦2θ≦12°, and the ratio of the intensity P2 of the second peak to the intensity P1 of the first peak P2 / P1 is 2.2 or less, or (b2) Oxygen evolution catalyst for which no second peak is observed in the range 11°≦2θ≦12°. [Aspect 2] The oxygen evolution catalyst according to embodiment 1, wherein the compound is at least one selected from the group consisting of layered double hydroxides (LDHs), complex metal hydroxides, and complex metal oxides. [Aspect 3] The oxygen evolution catalyst according to embodiment 1 or 2, wherein the atomic ratio of Ni / (Fe+Ru) in the compound is 1.8 to 3.1. [Aspect 4] The oxygen evolution catalyst according to any one of embodiments 1 to 3, wherein the atomic ratio of Ru / (Ni+Fe+Ru) in the compound is 0.15 to 0.33. [Aspect 5] The oxygen evolution catalyst according to any one of embodiments 1 to 4, wherein the atomic ratio of Ru / (Ni+Fe+Ru) in the compound is 0.15 to 0.25. [Aspect 6] The oxygen-evolving catalyst according to any one of embodiments 1 to 5, wherein the oxygen-evolving catalyst is in particulate form. [Aspect 7] The oxygen-evolving catalyst according to any one of embodiments 1 to 6, wherein the oxygen-evolving catalyst has a particle size of 2 to 100 nm. [Aspect 8] An electrochemical device comprising an electrode containing an oxygen-evolving catalyst according to any one of embodiments 1 to 7. [Aspect 9] The electrochemical device according to embodiment 8, wherein the electrochemical device is a water electrolysis device. [Aspect 10] The electrochemical device according to embodiment 8, wherein the electrochemical device is a metal-air battery. [Aspect 11] The electrochemical device according to embodiment 8, wherein the electrochemical device is an ammonia electrolytic synthesis apparatus. [Brief explanation of the drawing]

[0010] [Figure 1] These are the XRD profiles of the oxygen evolution catalysts prepared in Examples 1-8.

Best Mode for Carrying Out the Invention

[0011] Oxygen-evolving catalyst The oxygen generation catalyst of the present disclosure contains (i) Ni, (ii) Ru, or Ru and Fe, and (iii) O, or O and H in the form of a compound. That is, while Ni, Ru, and O are essential elements, Fe and H are optional elements. This oxygen generation catalyst has an X-ray diffraction profile measured by the X-ray diffraction method (XRD) that satisfies the following conditions: (a) A first peak is observed at 34° ≤ 2θ ≤ 35°, and (b1) A second peak is observed at 11° ≤ 2θ ≤ 12°, and the ratio P2 / P1 of the intensity P2 of the second peak to the intensity P1 of the first peak is 2.2 or less, or (b2) No second peak is observed at 11° ≤ 2θ ≤ 12° is satisfied. Thus, by configuring an oxygen generation catalyst containing (i) Ni, (ii) Ru and optionally Fe, and (iii) O and optionally H in the form of a compound such that its X-ray diffraction profile satisfies the above conditions, an oxygen generation catalyst with a small overvoltage in the oxygen generation reaction can be provided.

[0012] As described above, there is still much room for improvement in LDH such as NiFe-LDH as an oxygen generation catalyst. In particular, when realizing highly efficient oxygen generation, the magnitude of the overvoltage in the oxygen generation reaction that proceeds in the anode in water electrolysis, the charging reaction of an air battery, etc. has been regarded as a problem. In this regard, in the present invention, by utilizing the characteristics of LDH that can greatly change the particle properties such as specific surface area and catalytic activity with a combination of metal ions, and changing or controlling the composition and crystallinity of a binary or ternary compound (for example, LDH or other oxides and hydroxides) using Ni and Ru, or Ni, Fe, and Ru as metal ions, it has been found that the above problems can be solved (that is, the overvoltage can be reduced). By reducing the overvoltage at the start of oxygen generation in this way, oxygen generation can proceed efficiently, and as a result, hydrogen production in water electrolysis, charging in an air battery, etc. can be carried out easily and efficiently.

[0013] The oxygen evolution catalysts of this disclosure comprise (i) Ni, (ii) Ru and optionally Fe, and (iii) O and optionally H in the form of compounds. Therefore, the oxygen evolution catalysts of this disclosure may be compounds of Ni, Ru, and O (i.e., Ni, Ru-containing oxides), compounds of Ni, Fe, Ru, and O (i.e., Ni, Fe, Ru-containing oxides), compounds of Ni, Ru, O, and H (i.e., Ni, Ru-containing hydroxides), and compounds of Ni, Fe, Ru, O, and H (i.e., Ni, Fe, Ru-containing hydroxides), or mixtures thereof. As described above, although the oxygen evolution catalysts of this disclosure are based on improvements to LDH, particularly NiFe-LDH, compounds showing reduced overpotential may have small or no peaks characteristic of NiFe-LDH. For this reason, the oxygen evolution catalysts of this disclosure may include those that can be identified as LDH and those that cannot be identified as LDH but can be determined to be at least one of the hydroxides or oxides. For example, as illustrated in Figure 1, the samples of Examples 1-6 and 8, which correspond to the oxygen evolution catalyst of this disclosure, show a first peak in their X-ray diffraction profiles at a position of 34°≦2θ≦35°, which is the same as or similar to the (012) peak of NiFe-LDH. Another characteristic peak in NiFe-LDH is the (003) peak. In this regard, the oxygen evolution catalyst of this disclosure may show a second peak at a position of 11°≦2θ≦12°, which is the same as or similar to the (003) peak of NiFe-LDH, as in the samples of Examples 1-5 shown in Figure 1, or it may not show a second peak at 11°≦2θ≦12°, as in the samples of Examples 6 and 8. In the former case, the second peak is smaller compared to Example 7 (a comparative example without Ru), which does not correspond to the oxygen evolution catalyst of this disclosure. In this regard, the oxygen evolution catalyst of the present disclosure is characterized by the absence of a second peak, or, if a second peak is present, by a ratio P2 / P1 of the intensity of the second peak to the intensity P1 of the first peak being 2.2 or less, preferably 1.5 or less, and more preferably 1.0 or less.Thus, the compounds included in the oxygen evolution catalyst of this disclosure may include those that can be identified as LDH by observing the (012) and (003) peaks of NiFe-LDH, and others that can be determined to be either a composite metal hydroxide or a composite metal oxide, even if they cannot be identified as LDH. In other words, the compounds constituting the oxygen evolution catalyst of this disclosure may be at least one selected from the group consisting of layered double hydroxides (LDH), composite metal hydroxides, and composite metal oxides.

[0014] As described above, the oxygen evolution catalyst of this disclosure exhibits a first peak in the X-ray diffraction profile at 34°≦2θ≦35°. A second peak may also be observed at 11°≦2θ≦12°. The peak positions (2θ) represent the positions where the target peaks are maximized. The XRD conditions are as described in the examples below.

[0015] The Ni / (Fe+Ru) atomic ratio in the compounds of this disclosure is preferably 1.8 to 3.1, more preferably 1.8 to 3.0, and even more preferably 1.8 to 2.1. The Ru / (Ni+Fe+Ru) atomic ratio in the compounds of this disclosure is preferably 0.15 to 0.33, more preferably 0.15 to 0.30, even more preferably 0.15 to 0.25, and particularly preferably 0.15 to 0.22. These atomic ratios can be determined by energy-dispersive X-ray spectroscopy (EDS). The EDS analysis is preferably performed using an EDS analyzer (e.g., JEOL JED-2300) by 1) acquiring an image with an acceleration voltage of 15 kV and a magnification of 50,000x, 2) performing a 3-point analysis in point analysis mode, 3) repeating steps 1) and 2) once more, and 4) calculating the average value of the total 6 points.

[0016] The oxygen evolution catalyst of this disclosure is preferably in particulate form. In this case, the particle size of the oxygen evolution catalyst is preferably 2 to 100 nm, and more preferably 10 to 100 nm. The particle size can be measured using an FE-SEM (electrolytic emission scanning electron microscope) as described in the examples below.

[0017] The oxygen evolution catalyst according to this disclosure is preferably applied to electrochemical devices in order to take advantage of its excellent catalytic activity, which involves low overpotential in the oxygen evolution reaction. Therefore, an electrochemical device is provided that includes an electrode containing the oxygen evolution catalyst. Such an electrochemical device is not particularly limited as long as it is a device that uses an electrode reaction involving an oxygen evolution reaction, but preferred examples include a water electrolysis apparatus, a metal-air battery, and an ammonia electrosynthesis apparatus. In the case of a water electrolysis apparatus, hydrogen production in water electrolysis can be easily and efficiently performed by using the oxygen evolution catalyst according to this disclosure. In the case of a metal-air battery, charging of the metal-air battery can be easily and efficiently performed by using the oxygen evolution catalyst according to this disclosure. In the case of an ammonia electrosynthesis apparatus, ammonia production can be easily and efficiently performed by using the oxygen evolution catalyst according to this disclosure.

[0018] The oxygen evolution catalyst of this disclosure preferably does not contain a chelating agent. As mentioned above, Patent Document 1 (Japanese Patent No. 7008201) discloses an LDH containing two or more transition metals (e.g., Ni and Fe) and a chelating agent, but the oxygen evolution catalyst of this disclosure is advantageous in that it can reduce the overpotential in the oxygen evolution reaction without using a chelating agent.

[0019] Manufacturing method The oxygen-evolving catalyst of this disclosure can be synthesized by mixing a first solution containing a nickel salt, a ruthenium salt, and optionally an iron salt with a second solution containing sodium carbonate and / or sodium hydroxide at a predetermined pH. Specifically, the process is as follows:

[0020] (1) Preparation of the first solution A first solution (metal ion aqueous solution) is prepared by dissolving nickel salts, ruthenium salts, and optionally iron salts in water or an aqueous solvent in predetermined molar ratios. The molar ratios should be determined such that the atomic ratio of Ni / (Fe+Ru) is 1.8 to 3.1 and the atomic ratio of Ru / (Ni+Fe+Ru) is 0.15 to 0.30. Examples of nickel salts include nickel(II) nitrate, nickel(II) chloride, and mixtures thereof, and these salts may be used in hydrate form. An example of ruthenium salt is ruthenium(II) chloride, and this salt may be used in hydrate form. Examples of iron salts include iron(III) nitrate, iron(III) chloride, and mixtures thereof, and these salts may be used in hydrate form.

[0021] (2) Preparation of the second solution A second solution (alkaline aqueous solution) is prepared by dissolving at least one of sodium carbonate and sodium hydroxide in water or an aqueous solvent. It is preferable to use both sodium carbonate and sodium hydroxide, and the mixing ratio of sodium carbonate and sodium hydroxide is preferably such that the molar ratio of NaOH:Na2CO3 is 5:1 to 3:1.

[0022] (3) Mixing of the first solution and the second solution The first solution is added dropwise to the second solution, and the pH of the mixed solution is maintained at a predetermined value (preferably 9 to 11) while stirring. The oxygen-evolving catalyst is precipitated as a precipitate by applying heat treatment to this mixed solution while stirring (preferably at a temperature of 20 to 30°C). The recovered precipitate is preferably washed with water and dried in the air. In this way, the oxygen-evolving catalyst powder of the present disclosure can be obtained. [Examples]

[0023] The present invention will be further described in detail by the following examples. However, the present invention is not limited to the following examples.

[0024] Examples 1-6 (1) Preparation of oxygen evolution catalyst To 50 ml of pure water, 120 mmol of sodium hydroxide (NaOH, manufactured by Fujifilm Wako Pure Chemical Corporation, reagent special grade) and 30 mmol of sodium carbonate (Na2CO3, manufactured by Fujifilm Wako Pure Chemical Corporation, reagent special grade) were added, and the mixture was stirred to prepare an alkaline aqueous solution. Next, 50 ml of an aqueous metal ion solution containing nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, manufactured by Fujifilm Wako Pure Chemical Corporation), ruthenium(II) chloride n-hydrate (RuCl2·nH2O, manufactured by Fujifilm Wako Pure Chemical Corporation), and iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, manufactured by Fujifilm Wako Pure Chemical Corporation) in the mixing ratio (atomic ratio of Ni:Fe:Ru) shown in Table 1 was prepared. 40 ml of the obtained aqueous metal ion solution was added to 40 ml of the alkaline aqueous solution and stirred. The obtained raw material aqueous solution was maintained at pH 10 and heat-treated with stirring to precipitate a precipitate. The aqueous solution containing the precipitate thus obtained was suction-filtered to recover the precipitate. The recovered precipitate was washed with water and dried at 80 °C in the air to obtain an oxygen generation catalyst powder.

[0025] (2) Evaluation The following evaluations were performed on the obtained oxygen generation catalyst powder.

[0026] <XRD Measurement> Using an X-ray diffractometer (manufactured by Rigaku Corporation, RINT-TTR III), with X-ray source: Cu-Kα ray, voltage: 50 kV, current value: 300 mA, step angle: 0.020°, scan speed: 2.00° / min, diffraction angle 2θ: 5° to 70°, X-ray diffraction (XRD) measurement was performed on the oxygen generation catalyst powder, and an X-ray diffraction profile as shown in Figure 1 was obtained.

[0027] <Measurement of Particle Size> <No. After dropping and drying an aqueous dispersion of the oxygen generation catalyst powder on a sample stage, two fields of particle images were obtained at a magnification of 50,000 times using a FE-SEM (field emission scanning electron microscope) (manufactured by JEOL Ltd., JSM-7900F). For each of the two obtained fields of images, 10 particle sizes were randomly measured in length, and the maximum and minimum values in the two fields were adopted as the particle sizes. The results were as shown in Table 1.

[0028] <Catalytic Activity Evaluation> The catalytic performance of oxygen evolution catalyst powder for the oxygen evolution reaction (OER) was evaluated using the Rotating Ring Disk Electrodes (RRDE) measurement method. This measurement was performed using a rotating ring disk electrode apparatus manufactured by BAS, with a platinum ring glassy carbon (GC) disk electrode manufactured by BAS as the electrode, and a 0.1 M KOH aqueous solution as the electrolyte. Specifically, 2.8 mg of the obtained sample (oxygen evolution catalyst powder), 1630 μL of ethanol (manufactured by Kanto Chemical Co., Ltd., special grade), and Nafion (polymer content 5% by weight) were used. (TM) Dispersion (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 5% Nafion) (TM) A measurement solution was obtained by mixing 20 μL of the dispersion solution (DE520) with the solution using ultrasound for 1 hour. 6 μL of this measurement solution was cast onto a disk electrode and dried. Convection voltammetry measurements were performed at a rotation speed of 1600 rpm, a chiller temperature of 24°C, and an oxygen atmosphere. The relationship between the potential and current density relative to the reversible hydrogen electrode (RHE) was found to be 1.0 mA / cm². 2 The potential at [location] was determined. The results are shown in Table 1.

[0029] Example 7 (comparison) Except for the absence of ruthenium(II) chloride n-hydrate, an aqueous metal ion solution containing nickel(II) nitrate hexahydrate and iron(III) nitrate nonahydrate was prepared in the proportions (atomic ratio of Ni:Fe:Ru) shown in Table 1, and the oxygen-evolving catalyst powder and its evaluation were carried out in the same manner as in Example 1. The results are shown in Table 1 and Figure 1.

[0030] Example 8 Except for preparing an aqueous metal ion solution containing nickel(II) nitrate hexahydrate and ruthenium(II) chloride n hydrate in the proportions (atomic ratio of Ni:Fe:Ru) shown in Table 1, without adding iron(III) nitrate nonahydrate, the oxygen-evolving catalyst powder and its evaluation were carried out in the same manner as in Example 1. The results are shown in Table 1 and Figure 1.

[0031] [Table 1]

[0032] The results shown in Figure 1 indicate that the catalysts synthesized in Examples 1-8 are at least one of LDH, complex metal hydroxides, and complex metal oxides. In particular, when focusing on the (012) peak (corresponding to the first peak at 34°≦2θ≦35°) and the (003) peak (corresponding to the second peak at 11°≦2θ≦12°), which are characteristic peaks of NiFe-LDH, the catalysts in Examples 1 and 2 show these peaks strongly and can be identified as LDH. In contrast, the catalysts in Examples 3, 4, and 5 show a decreasing second peak at 11°≦2θ≦12° in the order of Examples 3, 4, and 5, and no peak at 11°≦2θ≦12° is observed in Examples 6 and 8. Therefore, the catalysts in Examples 6 and 8 are understood to be amorphous LDH and / or complex metal hydroxides and / or complex metal oxides, or mixtures thereof. Examples 3, 4, and 5 are understood to be LDH, complex metal hydroxides, and complex metal oxides, or mixtures thereof.

[0033] As shown in Table 1, the catalysts synthesized in Examples 1-6 and 8 (executive examples) had a current density of 1.0 mA / cm² higher than the catalyst synthesized in Comparative Example 7. 2The potential or onset potential at this point is low, and therefore it is closer to the redox potential of water (1.23V vs. RHE). In other words, the activity of an oxygen evolution catalyst is higher when the overpotential is lower, with the redox potential of water (1.23V vs. RHE) as the reference. Furthermore, since oxygen evolution catalysts generally exhibit a reaction initiation potential at a higher potential than 1.23V vs. RHE, the closer the onset potential is to the reference 1.23V vs. RHE (i.e., the lower the onset potential), the higher the oxygen evolution catalyst activity. Therefore, it can be seen that the catalysts in Examples 1-6 and 8 have higher oxygen evolution catalyst activity than the catalyst in Example 7 (Comparative Example).

Claims

1. An oxygen evolution catalyst comprising (i) Ni, (iii) Ru, or Ru and Fe, and (iii) O, or O and H in the form of a compound, In the X-ray diffraction profile measured by X-ray diffraction (XRD), (a) The first peak is observed at 34° ≤ 2θ ≤ 35°, and, (b1) A second peak is observed at 11° ≤ 2θ ≤ 12°, and the intensity P of the first peak is １ The intensity P of the second peak relative to ２ Ratio P ２ / P １ is 2.2 or less, (b2) An oxygen-evolving catalyst in which no second peak is observed between 11° and 2θ.

2. The oxygen evolution catalyst according to claim 1, wherein the compound is at least one selected from the group consisting of layered double hydroxides (LDHs), complex metal hydroxides, and complex metal oxides.

3. The oxygen evolution catalyst according to claim 1 or 2, wherein the atomic ratio of Ni / (Fe+Ru) in the compound is 1.8 to 3.

1.

4. The oxygen evolution catalyst according to claim 1 or 2, wherein the atomic ratio of Ru / (Ni+Fe+Ru) in the compound is 0.15 to 0.

33.

5. The oxygen evolution catalyst according to claim 1 or 2, wherein the atomic ratio of Ru / (Ni+Fe+Ru) in the compound is 0.15 to 0.

25.

6. The oxygen-evolving catalyst according to claim 1 or 2, wherein the oxygen-evolving catalyst is in particulate form.

7. The oxygen-evolving catalyst according to claim 1 or 2, wherein the oxygen-evolving catalyst has a particle size of 2 to 100 nm.

8. An electrochemical device comprising an electrode containing the oxygen-evolving catalyst described in claim 1 or 2.

9. The electrochemical device according to claim 8, wherein the electrochemical device is a water electrolysis device.

10. The electrochemical device according to claim 8, wherein the electrochemical device is a metal-air battery.

11. The electrochemical device according to claim 8, wherein the electrochemical device is an ammonia electrolytic synthesis apparatus.

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