Water oxidation catalyst

Abstract

To provide a water oxidation catalyst capable of showing stability and high oxidation catalytic activity in an aqueous solution under neutral conditions.SOLUTION: A water oxidation catalyst according to the present embodiment contains an Fe-Ni-based catalyst, the Fe-Ni-based catalyst including Ni(OH)2 as a principal component, Fe element, and at least one source of an alkali metal element or an alkaline earth metal element.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water oxidation catalyst. [Background technology]

[0002] The use of hydrogen energy and carbon dioxide fixation technology are attracting attention as solutions to global environmental problems and the depletion of fossil fuels. In particular, the water splitting reaction, which splits water into hydrogen and oxygen at room temperature and pressure, and the carbon dioxide reduction reaction, which uses water as an electron source, are expected to be clean energy generation methods. These reactions include the water oxidation reaction: 2H2O→O2+4H + +4e - , 1.23V (vs. RHE) However, because the reaction efficiency is low, there has been a demand for catalysts that can promote the water oxidation reaction.

[0003] As water oxidation catalysts, catalysts using noble metals such as Ru, Ir, Sr, and Ga, as well as catalysts using non-noble metals such as iron, nickel, and cobalt have been reported (Non-Patent Documents 1 to 9). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] PNAS 26(2019)23915 [Non-patent document 2] Nat. Chem. 10(2018)149 [Non-patent document 3] Nat. Comm. 10(2019)4081 [Non-patent document 4] Adv. Mater. 32(2020)20002297 [Non-Patent Document 5] Feng, C. et al. Fe-Based Electrocatalysts for Oxygen Evolution Reaction: Progress and Perspectives. ACS Catal. 10, 4019-4047(2020) [Non-patent document 6] Hunter, BM, Gray, HB & Muller, AM Earth-Abundant Heterogeneous Water Oxidation Catalysts. Chem. Rev. 116, 14120-14136(2016) [Non-Patent Document 7] Li, PP et al. Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH. Small 15, 27(2019) [Non-patent document 8] Wang, WY et al. High-Efficiency and Durable Water Oxidation under Mild pH Conditions: An Iron Phosphate-Borate Nanosheet Array as a Non Noble-Metal Catalyst Electrode. Inorg. Chem. 56 3131-3135(2017) [Non-Patent Document 9] Jiang, N., Zhu, ZW, Xue, WJ, Xia, BY & You, B. Emerging Electrocatalysts for Water Oxidation under Near Neutral CO2 Reduction Conditions. Adv. Mater. 34, 22(2022) Summary of the Invention [Problem to be solved by the invention]

[0005] In order to carry out a carbon dioxide reduction reaction using water as an electron source in artificial photosynthesis, etc., an oxidation reaction of water through which carbon dioxide has been passed may be carried out. For example, when carbon dioxide is passed through a strong alkaline aqueous solution, the pH decreases, and when saturated with carbon dioxide, the pH becomes neutral (e.g., pH 6 to 8). However, in an aqueous solution in the neutral range, the catalytic activity of the water oxidation catalyst decreases.

[0006] Therefore, an object of the present invention is to provide a water oxidation catalyst that is stable and exhibits high oxidation catalytic activity in aqueous solutions in the neutral range. [Means for solving the problem]

[0007] A water oxidation catalyst according to an embodiment of the present invention comprises an Fe-Ni-based catalyst, characterized in that the Fe-Ni-based catalyst contains Ni(OH)2 as a main component, Fe element, and a source of at least one of an alkali metal element and an alkaline earth metal element.

[0008] In the water oxidation catalyst, the Fe—Ni-based catalyst preferably contains at least the alkali metal element out of the alkali metal element and the alkaline earth metal element.

[0009] In the water oxidation catalyst, the content of the Fe element is preferably 5 atomic % or more and 40 atomic % or less with respect to the total of the Ni element and the Fe element in the Fe—Ni-based catalyst.

[0010] In the water oxidation catalyst, the content of the Fe element is preferably 5 atomic % or more and 30 atomic % or less with respect to the total of the Ni element and the Fe element in the Fe—Ni-based catalyst.

[0011] Furthermore, the water oxidation catalyst preferably has a carrier on which the Fe—Ni-based catalyst is supported, and the carrier is preferably an Ni—Fe alloy foam or a Cu foam. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a water oxidation catalyst that is stable in aqueous solutions in the neutral range and exhibits high oxidation catalytic activity. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the configuration of a three-electrode experimental device. [Figure 2] FIG. 1 is a graph showing overvoltage-time characteristics when constant current electrolysis was performed using electrodes A and B as working electrodes. [Figure 3] FIG. 1 is a graph showing overvoltage-time characteristics when constant current electrolysis was performed using electrodes A and C as working electrodes. [Figure 4] FIG. 1 is a graph showing current density-time characteristics when constant-potential electrolysis was performed using electrodes A and D as working electrodes. [Figure 5] FIG. 1 is a graph showing current density-time characteristics when constant-potential electrolysis was performed using electrodes A and E as working electrodes. [Figure 6] The graph shows the average current density when constant-potential electrolysis was carried out using electrode A and electrodes F-1 to F-5 as working electrodes. [Figure 7] FIG. 1 is a graph showing current density-time characteristics when constant-potential electrolysis was performed using electrodes A and G-1 to G-4 as working electrodes. [Figure 8] FIG. 1(a) is a graph showing the results of the faradaic efficiency of oxygen when constant-potential electrolysis was performed using electrode A as the working electrode, and FIG. 1(b) is a graph showing the results of the faradaic efficiency of oxygen when constant-current electrolysis was performed using electrode H as the working electrode. [Figure 9] 1 shows XRD spectra of electrode A and a sample of electrode A during preparation. [Figure 10] (a) X-ray absorption near edge structure (XANES) spectra obtained by X-ray absorption spectroscopy of the oxidation catalyst before and after potentiostatic electrolysis using electrodes A and E as the working electrodes. (b) Fourier transform spectra of extended X-ray absorption fine structure (EXAFS) spectra obtained by X-ray absorption spectroscopy of the oxidation catalyst before and after potentiostatic electrolysis using electrodes A and E as the working electrodes. [Figure 11](a) to (d) are X-ray absorption near edge structure (XANES) spectra obtained by X-ray absorption spectroscopy of Samples 1 to 6. [Figure 12] (a) to (c) are the Fourier transform spectra of the extended X-ray absorption fine structure (EXAFS) spectra obtained by X-ray absorption spectroscopy measurements of samples 1 to 6. [Figure 13] (a)-(b) are X-ray absorption near edge structure (XANES) spectra obtained by X-ray absorption spectroscopy of samples 5 and 7, and (c) is the Fourier transform spectrum of the extended X-ray absorption fine structure (EXAFS) spectrum obtained by X-ray absorption spectroscopy of samples 5 and 7. [Figure 14] (a)-(b) are X-ray absorption near edge structure (XANES) spectra obtained by X-ray absorption spectroscopy of samples 5 and 8 after potentiostatic electrolysis, and (c) is the Fourier transform spectrum of the extended X-ray absorption fine structure (EXAFS) spectrum obtained by X-ray absorption spectroscopy of samples 5 and 8 after potentiostatic electrolysis. [Figure 15] FIG. 1 is a graph showing average values ​​of overvoltages when constant-current electrolysis was performed using electrodes having oxidation catalysts with different Ni / (Fe+Ni) ratios as working electrodes. [Figure 16] FIG. 1 is a graph showing current density-time characteristics when constant-potential electrolysis was performed using electrodes A and C as working electrodes. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the embodiments described herein.

[0015] "Water oxidation catalyst" The water oxidation catalyst according to the present embodiment includes an Fe-Ni catalyst containing Ni(OH)2 as a main component, Fe, and at least one of an alkali metal element and an alkaline earth metal element, where the main component refers to the component with the highest content among the components constituting the catalyst.

[0016] The content of Ni(OH)2 in the Fe-Ni catalyst is, for example, preferably 50% by mass or more and 90% by mass or less, and more preferably 60% by mass or more and 80% by mass or less, from the viewpoint of stabilizing or highly activating the oxidation catalyst activity. The crystalline state of Ni(OH)2 in the Fe-Ni catalyst is not particularly limited, but is preferably amorphous from the viewpoint of stabilizing or highly activating the oxidation catalyst activity. The Fe-Ni catalyst may contain other Ni components in addition to Ni(OH)2, but preferably contains as few trivalent Ni components as possible, such as NiOOH, from the viewpoint of stabilizing or highly activating the oxidation catalyst activity. The content of trivalent Ni components, such as NiOOH, is, for example, preferably 5% by mass or less, and more preferably 1% by mass or less.

[0017] The Fe element in the Fe—Ni-based catalyst may be, for example, doped into a Ni(OH)2 phase (crystalline phase and / or amorphous phase) or may exist as a compound containing Fe element. When present as a compound containing Fe element, for example, the compound containing Fe element may be arranged so as to surround the Ni(OH)2 phase, or the Ni(OH)2 phase may be arranged so as to surround the compound containing Fe element. Examples of the compound containing Fe element include iron oxyhydroxides such as α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH, and ε-FeOOH; iron oxides such as ferrihydrite, FeO, Fe2O3, and Fe3O4; hydroxides such as Fe(OH)2 and Fe(OH)3; and amorphous components thereof.

[0018] The content of Fe element in the Fe—Ni-based catalyst is preferably in the range of more than 0 atomic % and 40 atomic % or less, more preferably in the range of 10 atomic % or more and 40 atomic % or less, and still more preferably in the range of 10 atomic % or more and 30 atomic % or less, from the viewpoint of stabilizing or highly activating the oxidation catalyst activity.

[0019] The alkali metal element or alkaline earth metal element in the Fe-Ni catalyst is preferably doped into the Ni(OH)2 phase together with the Fe element, or into the structure of a composite compound composed of the Ni(OH)2 phase and a compound containing the Fe element. However, they may be present undoped around the Ni(OH)2 phase or the composite compound. From the viewpoint of stability and high activity of the oxidation catalyst, the Fe-Ni catalyst preferably contains at least an alkali metal element among alkali metal elements and alkaline earth metal elements, and more preferably contains at least one of sodium and potassium. The content of the alkali metal element or alkaline earth metal element is preferably 10% by mass or more and 25% by mass or less, and more preferably 15% by mass or more and 20% by mass or less, based on the amount of Ni, from the viewpoint of stability and high activity of the oxidation catalyst.

[0020] The content of each component in the Fe-Ni catalyst can be calculated by elemental analysis such as ICP (inductively coupled plasma optical emission spectroscopy), SEM-EDX (scanning electron microscope-energy dispersive X-ray spectroscopy), XPS (X-ray photoelectron spectroscopy), and STEM-EDS (scanning transmission electron microscope-energy dispersive X-ray spectroscopy).

[0021] The water oxidation catalyst preferably has a carrier on which an Fe-Ni catalyst is supported. The carrier is preferably a conductive carrier made of a carbon material, a metal material, or the like, and examples thereof include carbon paper, carbon fiber, metal foam (i.e., a porous metal), and metal mesh. Among these, metal foam is preferred in terms of achieving stability or high activation of the oxidation catalyst activity, and Ni-Fe alloy foam and Cu foam are more preferred.

[0022] "Method for producing Fe-Ni catalysts" An example of a method for producing an Fe—Ni-based catalyst according to an embodiment will now be described. First, a catalyst solution containing predetermined amounts of Fe and Ni raw materials dissolved therein is irradiated with ultrasonic waves for a predetermined period of time. Examples of Fe raw materials include iron-containing compounds such as iron nitrate, iron sulfate, iron chloride, and iron hydroxide. Examples of Ni raw materials include nickel-containing compounds such as nickel nitrate, nickel sulfate, and nickel hydroxide. The ultrasonic irradiation time may be, for example, about 1 to 20 minutes.

[0023] Next, the catalyst solution is coated onto the support, dried, and then calcined at a predetermined temperature for a predetermined time. Methods for coating the catalyst solution include a coating method and an immersion method. From the viewpoint of catalytic activity, it is preferable to immerse the support in the catalyst solution while irradiating it with ultrasonic waves. The calcination temperature is preferably in the range of 120°C to 250°C, for example. If the calcination temperature is outside the above range, the formation of trivalent Ni such as NiOOH in the catalyst may be promoted, or the formation of amorphous Ni(OH)2 may become difficult. The calcination time is not particularly limited, but is preferably in the range of 1 hour to 6 hours, for example.

[0024] The sample, in which the Fe—Ni catalyst obtained by calcination is supported on a carrier, is electrolyzed in an alkaline solution. The alkaline solution contains an alkali metal or alkaline earth metal, such as a potassium hydroxide solution or a barium hydroxide solution. The pH of the alkaline solution is, for example, 9 or higher. For example, the electrolysis in the alkaline solution can be performed by immersing the sample as the working electrode, a platinum wire as the counter electrode, and an Hg / HgO electrode as the reference electrode in the alkaline solution, and applying a constant voltage between the working electrode and the counter electrode from a bias power supply or scanning a predetermined potential range. This electrolysis dopes the Fe—Ni catalyst with, for example, an alkali metal element or an alkaline earth metal element. The applied voltage is, for example, in the range of 1.2 V to 1.8 V (vs. the reference electrode). The electrolysis time is, for example, 1 minute to 1 hour.

[0025] The Ni-Fe catalyst according to the embodiment contains Ni(OH)2 as a main component, and therefore is believed to exhibit high oxidation catalytic activity as a water oxidation catalyst in a solution in the neutral range. Furthermore, the Ni-Fe catalyst according to the embodiment contains iron, an alkali metal element, and an alkaline earth metal element in a catalyst containing Ni(OH)2 as a main component, and therefore is believed to suppress the structural change from Ni(OH)2 to NiOOH during the water oxidation reaction, thereby suppressing a decrease in catalytic activity. As a result, it is believed that stable oxidation catalytic activity is exhibited in an aqueous solution in the neutral range.

[0026] An electrode using the Ni-Fe-based catalyst according to the embodiment can be used as an oxidation electrode that extracts electrons and protons from water molecules, for example, in artificial photosynthesis systems, i.e., hydrogen synthesis, organic synthesis by CO2 reduction reactions, and ammonia synthesis by N2 reduction reactions. [Example]

[0027] <Experimental Example 1> In a beaker, 2 mmol of Fe(NO3)3 and 18 mmol of Ni(NO3)2 were dissolved in 10 mL of ethanol to prepare a raw material solution. After irradiating the prepared raw material solution with ultrasound for 5 minutes, carbon paper was immersed in the raw material solution for 3 seconds while irradiating with ultrasound. The sample in which the raw material solution was coated on the carbon paper was naturally dried overnight in the air, and then baked in the air at 150°C for 2 hours. This is called the precursor electrode. The electrode area, including the front and back, was 2 cm. 2 is.

[0028] An alkaline treatment was performed on a precursor electrode using a three-electrode experimental apparatus as shown in Figure 1. Specifically, a precursor electrode was used as the working electrode 1, a platinum wire as the counter electrode 2, and an Hg / HgO electrode as the reference electrode 3. A quartz prismatic cell was used as the reaction cell 4, 1 M KOH was used as the electrolyte 5, and an electrochemical analyzer (ALS) with the three electrodes connected to an electrochemical measurement device 6 was used. Then, while Ar gas was flowing through the electrolyte 5, a voltage of 0.58 V (vs. Hg / HgO) was applied to perform the alkaline treatment for 1 hour. After this alkaline treatment, the working electrode was washed with ultrapure water and dried in nitrogen gas. This working electrode was designated Electrode A in Experimental Example 1.

[0029] In addition, electrode B was prepared in the same manner as electrode A, except that ultrasonic irradiation was not performed.

[0030] Using the three-electrode experimental apparatus shown in Figure 1, constant-current electrolysis was performed to evaluate the water oxidation catalytic activity at electrodes A and B. Specifically, electrode A or electrode B was used as working electrode 1, a platinum wire as counter electrode 2, an Ag / AgCl electrode as reference electrode 3, and a CO2-saturated solution prepared by passing CO2 gas through a 1 M KHCO3 solution for 1 hour as electrolyte 5. Then, while continuing to pass CO2 gas through electrolyte 5, cyclic voltammetry was performed in the potential range of 1.2 to 1.8 V (vs. Ag / AgCl) at a sweep rate of 5 mV / sec, and then at a current density of 1, 10, or 50 mA / cm. 2 Constant current electrolysis was carried out at 400°C for 60 minutes, and the overvoltage was measured during this period.

[0031] Figure 2 shows the overvoltage-time characteristics when constant-current electrolysis was performed using electrodes A and B as the working electrodes. As shown in Figure 2, the use of ultrasonically treated electrode A resulted in a lower overvoltage than that of electrode B, which was not ultrasonically treated. Therefore, the use of ultrasonically treated electrode A improved the catalytic activity for water oxidation.

[0032] <Experimental Example 2> Electrode C was prepared in the same manner as electrode A, except that the baking at 150°C for 2 hours was not performed. Electrode C was used as the working electrode and constant current electrolysis was performed under the same conditions as in Experimental Example 1.

[0033] Figure 3 shows the overvoltage-time characteristics when constant-current electrolysis was performed using electrodes A and C as the working electrodes. As shown in Figure 3, the use of calcined electrode A resulted in a lower overvoltage than that of uncalcined electrode C. Therefore, the use of calcined electrode A improved the catalytic activity for water oxidation.

[0034] <Experimental Example 3> Electrode D was prepared in the same manner as electrode A, except that the alkali treatment was omitted. Potential electrolysis was performed using the three-electrode experimental apparatus shown in Figure 1 to evaluate the water oxidation catalytic activity of electrodes A and D. Specifically, electrode A or electrode D served as working electrode 1, a platinum wire as counter electrode 2, an Ag / AgCl electrode as reference electrode 3, and a CO2-saturated solution prepared by passing CO2 gas through a 1 M KHCO3 solution for 1 hour as electrolyte 5. While continuing to pass CO2 gas through electrolyte 5, cyclic voltammetry was performed over a potential range of 1.2 to 1.8 V (vs. Ag / AgCl) at a sweep rate of 5 mV / sec, followed by 8 hours of potential electrolysis at 1.0 V (vs. Ag / AgCl).

[0035] Figure 4 shows the current density-time characteristics when constant-potential electrolysis was performed using electrodes A and D as the working electrodes. Taking the current density 2 hours after the start of constant-potential electrolysis as the reference, the current density 8 hours after the start of constant-potential electrolysis decreased by approximately 1.2% when alkaline-treated electrode A was used, but decreased by approximately 4.5% when non-alkaline-treated electrode D was used. Therefore, the use of alkaline-treated electrode A improved the stability of the water oxidation catalytic activity. Furthermore, when electrode A was used, the current density was higher than when electrode D was used, indicating higher water oxidation catalytic activity.

[0036] <Experimental Example 4> Electrode E was prepared in the same manner as electrode A, except that Fe(NO3)3 was not used. Electrode E was used as the working electrode, and constant-potential electrolysis was performed under the same conditions as in Experimental Example 3. The atomic ratio of Fe to Ni elements (Fe:Ni) in the catalyst constituting electrode A was 1:9, and the atomic ratio of Fe to Ni elements (Fe:Ni) in the catalyst constituting electrode D was 0:10.

[0037] Fig. 5 shows the current density-time characteristics when constant-potential electrolysis was performed using electrodes A and E as the working electrodes. As shown in Fig. 5, when electrode A, which has a catalyst containing Fe, was used, the current density was higher and higher water oxidation catalytic activity was demonstrated compared to when electrode D, which has a catalyst not containing Fe, was used.

[0038] <Experimental Example 5> Electrode F-1 was prepared in the same manner as electrode A, except that 1 M KOH was replaced with 1 M LiOH. Electrode F-2 was prepared in the same manner as electrode A, except that 1 M KOH was replaced with 1 M NaOH. Electrode F-3 was prepared in the same manner as electrode A, except that 1 M KOH was replaced with 1 M CsOH. Electrode F-4 was prepared in the same manner as electrode A, except that 1 M KOH was replaced with saturated Ba(OH)2. Electrode F-5 was prepared in the same manner as electrode A, except that 1 M KOH in the alkali treatment was replaced with 30% by mass of ammonia water.

[0039] STEM-EDS analysis of the catalyst constituting electrode A confirmed that K was distributed within the catalyst of electrode A. Although STEM-EDS analysis was not performed on electrodes F-1 to F-4, because they were treated in the same way as electrode A, it is inferred that Li is distributed within the catalyst of electrode F-1, Na is distributed within the catalyst of electrode F-2, Cs is distributed within the catalyst of electrode F-3, and Ba is distributed within the catalyst of electrode F-4.

[0040] Potential electrolysis was carried out under the same conditions as in Experimental Example 3, except that electrodes F-1 to F-5 were used as working electrodes and the electrolysis time was set to 1 hour, and the average current density was measured.

[0041] Fig. 6 shows the average current density when constant-potential electrolysis was performed using electrode A and electrodes F-1 to F-5 as the working electrodes. As shown in Fig. 6, when electrode A and electrodes F-1 to F-4, which are composed of catalysts containing alkali metal elements or alkaline earth metal elements, were used, the average current density was higher, indicating higher water oxidation catalytic activity, compared to when electrode F-5, which is composed of a catalyst not containing alkali metal elements or alkaline earth metal elements, was used.

[0042] <Experimental Example 6> Electrode G-1 was prepared in the same manner as electrode A, except that the carbon paper was replaced with Ni foam. Electrode G-2 was prepared in the same manner as electrode A, except that the carbon paper was replaced with FeNi alloy foam. Electrode G-3 was prepared in the same manner as electrode A, except that the carbon paper was replaced with Cu foam. Electrode G-4 was prepared in the same manner as electrode A, except that the carbon paper was replaced with Co foam. Using electrodes G-1 to G-4 as working electrodes, constant-potential electrolysis was performed under the same conditions as in Experimental Example 3.

[0043] Figure 7 shows the current density-time characteristics when constant-potential electrolysis was performed using electrodes A and G-1 to G-4 as the working electrodes. As shown in Figure 7, when electrodes G-1 to G-4, which have a metal foam support, were used, the current density was higher and a higher water oxidation catalytic activity was demonstrated compared to when electrode A, which has a carbon paper support, was used. Furthermore, among electrodes G-1 to G-4, the use of electrode G-2, which has an FeNi alloy foam support, and electrode G-3, which has a Cu foam support, suppressed the decrease in current density over time and demonstrated stable water oxidation catalytic activity.

[0044] <Experimental Example 7> An ion exchange membrane was installed in the reaction cell of the three-electrode experimental apparatus shown in Figure 1, dividing it into a first compartment containing the working electrode 1 and reference electrode, and a second compartment containing the counter electrode 2. The working electrode 1 was the aforementioned electrode A or electrode H (a sample of electrode A in the process of preparation, consisting of carbon paper coated with the raw material solution). A platinum wire was used as the counter electrode 2, an Ag / AgCl electrode as the reference electrode 3, and a CO2-saturated solution prepared by passing CO2 gas through a 1 M KHCO3 solution for 1 hour was used as the electrolyte 5 contained in the first and second compartments. When electrode A was used, constant potential electrolysis at 1.0 V (vs. Ag / AgCl) was performed for 5 hours while continuing to pass CO2 gas through the electrolyte 5. On the other hand, when electrode H was used, CO2 gas was continuously passed through the electrolyte 5, and a current density of 5 mA / cm was used. 2 Constant current electrolysis was carried out at 40°C for 5 hours.

[0045] Figure 8(a) shows the results of the faradaic efficiency of oxygen when constant-potential electrolysis was performed using electrode A as the working electrode, and (b) shows the results of the faradaic efficiency of oxygen when constant-current electrolysis was performed using electrode H as the working electrode. When electrode H, which was not calcined and was composed of a catalyst containing no alkali metal elements, was used, the faradaic efficiency of oxygen was 79%. On the other hand, when electrode A, which was calcined and composed of a catalyst containing potassium elements, was used, the faradaic efficiency of oxygen was 99%, indicating higher catalytic activity than electrode H.

[0046] XRD measurements were performed on electrode A and samples during the preparation of electrode A. The samples during the preparation of electrode A were: Sample I: carbon paper; Sample II: carbon paper coated with the raw material solution; Sample III: precursor electrode (sample II obtained by heat treatment); Sample IV: precursor electrode treated with alkali; and Sample V: electrode A (the alkali-treated sample was washed and dried with nitrogen gas). XRD measurements were performed using an Ultima IV powder X-ray diffractometer manufactured by Rigaku Corporation at a tube voltage of 40 kV and a tube current of 40 mA (Cu-Kα radiation).

[0047] Figure 9 shows the XRD spectra of electrode A and a sample in the process of fabricating electrode A. The XRD spectra on the left of Figure 9 have the same vertical scale, while the XRD spectra on the right of Figure 9 have their vertical scales adjusted individually. As shown in Figure 9, samples II and III show numerous peaks, indicating that the structures of the raw materials Ni(NO3)2·6H2O and Fe(NO3)3·9H2O are maintained. However, samples IV and V show only small, broad peaks, suggesting that the water oxidation catalyst in electrode A contains amorphous Ni(OH)2 and iron compounds. STEM-EDS analysis was also performed on sample V (electrode A), and the results indicated that the water oxidation catalyst in electrode A is primarily composed of Ni(OH)2. STEM-EDS analysis also confirmed that potassium is uniformly dispersed within the oxidation catalyst in sample V.

[0048] (X-ray absorption spectroscopy) The local structure of the oxidation catalyst was investigated by X-ray absorption spectroscopy (XAS) before and after potentiostatic electrolysis using the aforementioned electrode A as the working electrode. For comparison, the local structure of the oxidation catalyst was also evaluated before and after potentiostatic electrolysis using the aforementioned electrode E, i.e., an electrode containing an oxidation catalyst that does not contain Fe, as the working electrode. As mentioned above, the atomic ratio of Fe to Ni in the catalyst that constitutes electrode A (Fe:Ni) is 1:9, and the atomic ratio of Fe to Ni in the catalyst that constitutes electrode E (Fe:Ni) is 0:10.

[0049] The measurements were performed using the BL33XU beamline at the SPring-8 synchrotron radiation facility of the Japan Synchrotron Radiation Research Center, with the attached transmission measurement unit, to measure X-ray absorption fine structure (XAFS) spectra. The oxidation catalyst used in the measurements before potentiostatic electrolysis was a pressed pellet made by mixing finely ground powder of electrode A or electrode E with boron nitride. The oxidation catalyst used in the measurements after potentiostatic electrolysis was a pressed pellet made by mixing finely ground powder of electrode A or electrode E with boron nitride after potentiostatic electrolysis at 1.0 V (vs. Ag / AgCl) for 1 hour using the three-electrode experimental apparatus shown in Figure 1.

[0050] Figure 10(a) shows the X-ray absorption near-edge structure (XANES) spectra obtained by X-ray absorption spectroscopy of the oxidation catalyst before and after potentiostatic electrolysis using electrodes A and E as the working electrodes. Figure 10(b) shows the Fourier transform spectra of the extended X-ray absorption fine structure (EXAFS) spectra obtained by X-ray absorption spectroscopy of the oxidation catalyst before and after potentiostatic electrolysis using electrodes A and E as the working electrodes. As shown in Figure 10(a), the peaks of both electrodes A and E shifted to higher energy after potentiostatic electrolysis. However, the shift to higher energy was suppressed for electrode A, which used an oxidation catalyst containing Fe. In addition, in the spectrum shown in Figure 10(b), the peak around 2.3 Å is attributed to the Ni-Ni bond of NiOOH, and the peak around 2.9 Å is attributed to the Ni-Ni bond of Ni(OH)2. After potentiostatic electrolysis, the peak intensity around 2.3 Å increased and the peak around 2.9 Å decreased for both electrodes A and E, but the increase in the peak intensity around 2.3 Å was suppressed for electrode A, which was composed of an oxidation catalyst containing Fe. In other words, the water oxidation reaction causes the valence of Ni in the oxidation catalyst to change from divalent to trivalent (from Ni(OH)2 to NiOOH), and it is presumed that this change was suppressed by using an oxidation catalyst containing Fe.

[0051] (Verification of the effect of ultrasonic treatment) Samples 1 to 6 were prepared by the following method and subjected to X-ray absorption spectroscopy (XAS) measurements. Sample 1 was prepared by irradiating a raw material solution of 2 mmol of Fe(NO3)3 and 18 mmol of Ni(NO3)2 in 10 mL of ethanol in a beaker with ultrasonic waves for 5 minutes, and then immersing carbon paper in the raw material solution for 3 seconds while the solution was being irradiated. Sample 2 was prepared by coating the raw material solution on carbon paper without ultrasonic irradiation, as in the preparation of Sample 1. Sample 3 was prepared by baking Sample 1 in air at 150°C for 2 hours (i.e., precursor electrode), and Sample 4 was prepared by baking Sample 2 in air at 150°C for 2 hours. Sample 5 was prepared by subjecting Sample 3 to the aforementioned alkali treatment, and Sample 6 was prepared by subjecting Sample 4 to the aforementioned alkali treatment.

[0052] Figures 11(a) to 11(d) show the X-ray absorption near-edge structure (XANES) spectra obtained by X-ray absorption spectroscopy for samples 1 to 6. The spectrum in Figure 11(d) is an enlarged version of the spectrum in Figure 11(c). Figures 12(a) to 12(c) show the Fourier transform spectra of the extended X-ray absorption fine structure (EXAFS) spectra obtained by X-ray absorption spectroscopy for samples 1 to 6. In the spectra shown in Figure 11, samples 1 and 2, and samples 3 and 4 all exhibited similar waveforms. However, the peak of sample 5 (ultrasonicated, calcined, and alkali-treated) is shifted to lower energy than the peak of sample 6 (unultrasonicated, calcined, and alkali-treated). This suggests that while alkali treatment tends to shift the Ni valence toward higher valences, ultrasonic treatment suppresses the shift toward higher valences that occurs during the subsequent alkali treatment, maintaining the Ni valence at a lower level. 12(c), sample 5 had a smaller peak intensity around 2.3 Å and a larger peak intensity around 2.9 Å compared to sample 6. This suggests that ultrasonic treatment suppresses the structural change from Ni(OH)2 to NiOOH that occurs during the subsequent alkali treatment, maintaining Ni(OH)2.

[0053] (Verification of the effect of baking) Sample 7 was prepared in the same manner as Sample 5 except that the firing step was not carried out, and was subjected to X-ray absorption spectroscopy (XAS) measurement.

[0054] Figures 13(a) and 13(b) show the X-ray absorption near-edge structure (XANES) spectra obtained by X-ray absorption spectroscopy for Samples 5 and 7. The spectrum in Figure 13(b) is an enlarged version of the spectrum in Figure 13(a). Figure 13(c) shows the Fourier transform spectrum of the extended X-ray absorption fine structure (EXAFS) spectra obtained by X-ray absorption spectroscopy for Samples 5 and 7. In the spectra shown in Figures 13(a) and 13(b), the peaks of Sample 5 (ultrasonicated, calcined, and alkali-treated) are shifted to lower energy than those of Sample 7 (ultrasonicated, uncalcined, and alkali-treated). This suggests that calcination suppresses the shift to higher valence states of Ni that occurs during the subsequent alkali treatment, maintaining the lower valence state. Furthermore, in the spectrum shown in Figure 13(c), Sample 5 exhibits a smaller peak intensity around 2.3 Å and a larger peak intensity around 2.9 Å compared to Sample 7. In other words, it is presumed that calcination suppresses the structural change from Ni(OH)2 to NiOOH that occurs during the subsequent alkali treatment, and maintains Ni(OH)2.

[0055] (Verification of the effect of alkaline treatment) Sample 8 was obtained in the same manner as in the preparation of Sample 5, except that the alkali treatment was not performed. Then, using the three-electrode experimental apparatus shown in Figure 1, Sample 5 or Sample 8 was used as the working electrode and subjected to constant-potential electrolysis at 1.0 V (vs. Ag / AgCl) for 1 hour. X-ray absorption spectroscopy measurements were performed on Samples 5 and 8 after this constant-potential electrolysis.

[0056] Figures 14(a) and 14(b) show X-ray absorption near-edge structure (XANES) spectra obtained by X-ray absorption spectroscopy of Samples 5 and 8 after potentiostatic electrolysis. The spectrum in Figure 14(b) is an enlarged version of the spectrum in Figure 14(a). Figure 14(c) shows the Fourier transform spectrum of the extended X-ray absorption fine structure (EXAFS) spectra obtained by X-ray absorption spectroscopy of Samples 5 and 8 after potentiostatic electrolysis. In the spectra shown in Figures 14(a) and 14(b), the peaks of Sample 5 after potentiostatic electrolysis (which underwent ultrasonic treatment, calcination, and alkali treatment) are slightly shifted to lower energy than those of Sample 8 after potentiostatic electrolysis (which underwent ultrasonic treatment and calcination but not alkali treatment), indicating that the shift to higher Ni valences was suppressed. Furthermore, in the spectrum shown in Figure 14(c), the peak intensity around 2.3 Å of Sample 5 after potentiostatic electrolysis was lower than that of Sample 8 after potentiostatic electrolysis. In other words, it is presumed that the inclusion of an alkali metal in the catalyst suppresses the structural change from Ni(OH)2 to NiOOH that occurs during electrolysis.

[0057] (Effect of catalyst composition on oxidation catalytic activity) In the process of fabricating electrode A described above, the amounts of Fe(NO3)3 and Ni(NO3)2 added were varied to fabricate electrodes composed of oxidation catalysts in which the ratio of the amount of Ni element to the total amount of Ni and Fe element (Ni + Fe) in the oxidation catalyst (Ni / (Fe + Ni) × 100) was 0 atomic %, 10 atomic %, 20 atomic %, 30 atomic %, 40 atomic %, 50 atomic %, 60 atomic %, 70 atomic %, 80 atomic %, 90 atomic %, or 100 atomic %. Other than the amounts of Fe(NO3)3 and Ni(NO3)2 added, the fabrication conditions were the same as for electrode A.

[0058] Using the three-electrode experimental apparatus shown in Figure 1, constant-current electrolysis was performed to evaluate the water oxidation catalytic activity of each electrode with oxidation catalysts with different Ni / (Fe+Ni) ratios. Specifically, each electrode with different Ni / (Fe+Ni) ratios was used as working electrode 1, a platinum wire as counter electrode 2, an Ag / AgCl electrode as reference electrode 3, and a CO2-saturated solution prepared by passing CO2 gas through a 1 M KHCO3 solution for 1 hour as electrolyte 5. Then, while continuing to pass CO2 gas through electrolyte 5, cyclic voltammetry was performed in the potential range of 1.2 to 1.8 V (vs. Ag / AgCl) at a sweep rate of 5 mV / sec, and a current density of 10 mA / cm was used. 2 Constant current electrolysis was carried out at 4000 kJ / min for 60 minutes, and the average value of the overvoltage during that period was measured.

[0059] FIG. 15 shows the average overvoltage when constant-current electrolysis was performed using electrodes with oxidation catalysts having different Ni / (Fe+Ni) ratios as the working electrode. As shown in FIG. 15, the electrode with an oxidation catalyst having a Ni / (Fe+Ni) ratio of 0 atomic % exhibited the highest overvoltage, and the overvoltage decreased as Ni was added. The preferred range of Ni / (Fe+Ni) is 60 atomic % or more and less than 100 atomic %, more preferably 60 atomic % or more and 90 atomic % or less, and even more preferably 70 atomic % or more and 90 atomic % or less. That is, the preferred range of Fe / (Fe+Ni) is more preferably more than 0 atomic % and 40 atomic % or less, more preferably 10 atomic % or more and 40 atomic % or less, and even more preferably 10 atomic % or more and 30 atomic % or less.

[0060] (Effect of calcination on oxidation catalyst activity) Using the three-electrode experimental apparatus shown in Figure 1, constant-potential electrolysis was performed using the aforementioned electrode A (calcined) or the aforementioned electrode C (uncalcined) as the working electrode. Specifically, electrode A or C was used as working electrode 1, a platinum wire as counter electrode 2, an Ag / AgCl electrode as reference electrode 3, and a CO2-saturated solution prepared by passing CO2 gas through a 1 M KHCO3 solution for 1 hour as electrolyte 5. Then, while continuing to pass CO2 gas through electrolyte 5, cyclic voltammetry was performed in the potential range of 1.2 to 1.8 V (vs. Ag / AgCl) at a sweep rate of 5 mV / sec, followed by constant-potential electrolysis at 1.0 V (vs. Ag / AgCl) for 8 hours.

[0061] Fig. 16 shows the current density-time characteristics when constant-potential electrolysis was performed using electrodes A and C as the working electrodes. As shown in Fig. 16, neither electrode A nor C showed a decrease in current density over time, but electrode A showed a higher current density than electrode C, indicating improved catalytic activity.

[0062] From the results of previous studies, it was found that the production of an oxidation catalyst was achieved by mixing Ni and Fe raw materials, ultrasonic treatment, calcination, and alkali treatment to add alkali metals or alkaline earth metals, which suppressed the formation of NiOOH and resulted in a catalyst with Ni(OH)2 as the main component, containing iron, alkali metals, and alkaline earth metals. Furthermore, the use of a catalyst containing Ni(OH)2 as the main component demonstrated high oxidation catalytic activity in aqueous solutions in the neutral range. Furthermore, the addition of iron, alkali metals, and alkaline earth metals to a catalyst containing Ni(OH)2 as the main component suppressed the structural transformation from Ni(OH)2 to NiOOH during the water oxidation reaction, demonstrating stable oxidation catalytic activity in aqueous solutions in the neutral range.

[0063] [Note] (1) A water oxidation catalyst comprising an Fe-Ni based catalyst, The Fe-Ni catalyst is a water oxidation catalyst characterized by containing Ni(OH)2 as a main component, Fe element, and at least one element selected from the group consisting of an alkali metal element and an alkaline earth metal element. (2) The water oxidation catalyst according to (1) above, wherein the Fe—Ni-based catalyst contains at least the alkali metal element out of the alkali metal element and the alkaline earth metal element. (3) The water oxidation catalyst according to (1) or (2) above, characterized in that the content of the Fe element is 10 atomic % or more and 40 atomic % or less with respect to the total of the Ni element and the Fe element in the Fe-Ni-based catalyst. (4) The water oxidation catalyst according to (3) above, characterized in that the content of the Fe element is 10 atomic % or more and 30 atomic % or less with respect to the total of the Ni element and the Fe element in the Fe-Ni-based catalyst. (5) a carrier on which the Fe-Ni-based catalyst is supported, The water oxidation catalyst according to any one of the above (1) to (4), wherein the support is a metal foam. (6) The water oxidation catalyst according to (5) above, wherein the metal foam is a Ni-Fe alloy foam or a Cu foam. [Explanation of symbols]

[0064] 1 Working electrode, 2 Counter electrode, 3 Reference electrode, 4 Reaction cell, 5 Electrolyte, 6 Electrochemical measurement apparatus.

Claims

1. A water oxidation catalyst comprising an Fe—Ni-based catalyst, The Fe-Ni catalyst is mainly composed of Ni(OH) ２ and an Fe element and at least one element selected from the group consisting of an alkali metal element and an alkaline earth metal element.

2. 2. The water oxidation catalyst according to claim 1, wherein the Fe--Ni catalyst contains at least the alkali metal element out of the alkali metal element and the alkaline earth metal element.

3. 3. The water oxidation catalyst according to claim 1, wherein the content of the Fe element is 10 atomic % or more and 40 atomic % or less with respect to the total of the Ni element and the Fe element in the Fe—Ni-based catalyst.

4. 4. The water oxidation catalyst according to claim 3, wherein the content of the Fe element is 10 atomic % or more and 30 atomic % or less with respect to the total of the Ni element and the Fe element in the Fe—Ni-based catalyst.

5. a carrier on which the Fe—Ni-based catalyst is supported, 3. The water oxidation catalyst according to claim 1, wherein the support is a metal foam.

6. 6. The water oxidation catalyst according to claim 5, wherein the metal foam is a Ni-Fe alloy foam or a Cu foam.

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