Electrochemical reaction catalyst, electrode, and electrochemical device

Abstract

To improve the catalytic activity of an electrochemical reaction catalyst while reducing the amount of noble metal to be used.SOLUTION: An oxygen or hydrogen electrochemical reaction catalyst is constituted of a perovskite type oxide or a spinel type oxide and includes cobalt (Co), iron (Fe) or nickel (Ni) and at least three species pf transition metal elements, with the ratio of the total of iron (Fe) and nickel (Ni) relative to the whole of metal elements included in the perovskite type oxide or in the spinel type oxide being more than 20 mol, and the ratio of the total of iron (Fe), nickel (Ni) and cobalt (Co) relative to the whole of the metal elements being 40 mol% or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a catalyst for an electrochemical reaction, an electrode, and an electrochemical device. [Background technology]

[0002] Known electrochemical reactions that proceed in various electrochemical devices involving energy conversion include the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER), and various electrochemical reaction catalysts and electrodes have been proposed to promote these electrochemical reactions. For example, Patent Document 1 discloses a configuration in which a layered rock salt type platinum-containing composite oxide (MPtO2) containing a metal element M is used as an electrolysis anode that promotes the oxygen evolution reaction (OER). Patent Document 2 also discloses a configuration in which a LaNi 1-x-y Cu x Fe y O 3-δ (wherein x>0, y>0, x+y<1, 0≦δ≦0.4) is disclosed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-163508 [Patent Document 2] International Publication No. 2016 / 147720 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 studies a catalyst with activity exceeding that of iridium oxide (IrO2), a precious metal catalyst commonly used as a catalyst for the oxygen evolution reaction (OER). It also makes it possible to use a relatively smaller amount of Pt compared to platinum bronze. However, the electrode described in Patent Document 1 uses platinum, a precious metal, as an essential element, and further reductions in the use of rare and expensive precious metals are desired. Furthermore, the electrode described in Patent Document 2 uses a composite oxide that does not contain precious metals, but there is room for improvement in catalytic activity, and further improvements in catalytic activity in electrochemical devices are desired. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms. [1] According to one embodiment of the present disclosure, there is provided a catalyst for electrochemical reactions of oxygen or hydrogen. The catalyst for electrochemical reactions is composed of a perovskite-type oxide or a spinel-type oxide, and contains cobalt (Co), iron (Fe) or nickel (Ni), and three or more transition metal elements, in which the total proportion of iron (Fe) and nickel (Ni) to all metal elements contained in the perovskite-type oxide or spinel-type oxide is greater than 20 mol %, and the total proportion of iron (Fe), nickel (Ni), and cobalt (Co) to all metal elements is 40 mol % or more. According to this embodiment of the electrochemical reaction catalyst, in an electrochemical reaction catalyst constituted by a perovskite-type oxide or a spinel-type oxide, by adjusting the type and content ratio of the transition metal element contained therein as described above, the conductivity of the electrochemical reaction catalyst can be increased and the catalytic activity can be improved. As a result, it is possible to ensure high catalytic activity while reducing or eliminating the use of precious metals in the electrochemical reaction catalyst. [2] The electrochemical reaction catalyst of the above embodiment may not contain a precious metal. With such a configuration, it is possible to achieve high catalytic activity and reduce costs while eliminating the need for a precious metal in the electrochemical reaction catalyst. [3] In the electrochemical reaction catalyst of the above embodiment, the ratio of cobalt (Co) to all the metal elements may be 10 mol % or more. With this configuration, it becomes easy to increase the catalytic activity of the electrochemical reaction catalyst. [4] In the electrochemical reaction catalyst of the above embodiment, when manganese (Mn) is contained as the transition metal element, the ratio of manganese (Mn) to the total of iron (Fe) and nickel (Ni) may be 20 mol % or less. With this configuration, it becomes easy to ensure high catalytic activity in the electrochemical reaction catalyst. [5] According to another aspect of the present disclosure, there is provided an electrode, the electrode including the catalyst for electrochemical reactions according to any one of [1] to [4]. According to this type of electrode, the activity of the electrochemical reaction that proceeds in the electrode can be increased while using an electrode that uses less or no precious metals. [6] According to yet another aspect of the present disclosure, there is provided an electrochemical device including the electrode of the above aspect. According to this type of electrochemical device, the activity of the electrochemical reaction that proceeds in the electrodes can be increased while using electrodes that use less or no precious metals, thereby improving the performance of the electrochemical device. The present disclosure may be realized in various forms other than those described above, such as a catalyst for the oxygen reduction reaction (ORR), a catalyst for the oxygen reduction reaction (ORR), a catalyst for the hydrogen evolution reaction (HER), a method for producing a catalyst for an electrochemical reaction, an alkaline water electrolysis device equipped with an electrode containing the catalyst, a metal-air secondary battery, a photoelectrode system, and a solid oxide electrolysis cell. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a flowchart showing an example of a method for producing a catalyst for electrochemical reactions according to an embodiment. [Figure 2] FIG. 2 is an explanatory diagram showing the configuration and evaluation results of each sample of the examples. [Figure 3] FIG. 1 is an explanatory diagram showing the results of electrochemical measurements for evaluating catalytic activity regarding OER. [Figure 4] 1 is a description showing a method for evaluating the electrical conductivity of each sample powder compact of the examples. DETAILED DESCRIPTION OF THE INVENTION

[0007] A. Catalysts for electrochemical reactions: The electrochemical reaction catalyst of this embodiment is a catalyst that promotes the electrochemical reaction of oxygen or hydrogen. "Electrochemical reaction of oxygen or hydrogen" refers to "an electrochemical reaction in which at least one of oxygen and hydrogen is a reactant or product." Specifically, examples of "electrochemical reaction of oxygen or hydrogen" include the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER).

[0008] The oxygen evolution reaction (OER) is a reaction expressed by the following formula (1) or (2), which occurs during the oxidation process of water. The oxygen reduction reaction (ORR) is a reaction expressed by the following formula (3), in which oxygen molecules are reduced to water or hydrogen peroxide. The hydrogen evolution reaction (HER) is a reaction expressed by the following formula (4), in which protons are reduced to produce hydrogen.

[0009] 4OH - →O2+2H2O+4e - … (1) 2H2O → O2+4H + +4e - … (2) O2+4H + +4e - → 2H2O … (3) 2H + +2e - → H2… (4)

[0010] The electrochemical reaction catalyst of this embodiment is composed of a perovskite-type oxide or a spinel-type oxide.

[0011] Perovskite oxides are composite oxides represented by the general formula ABO3 (where A is an element selected from rare earth elements and alkaline earth metal elements, and B is a transition metal element). La-based perovskite oxides containing the rare earth element lanthanum (La) at the A site generally have catalytic activity for the oxygen evolution reaction (OER) and are suitable for use as catalysts for the oxygen evolution reaction. It is also preferable to use perovskite oxides containing the alkaline earth metal strontium (Sr) at the A site, and it is also preferable to use LaSr-based perovskite oxides containing both lanthanum (La) and strontium (Sr) at the A site. Alternatively, perovskite oxides containing the rare earth element praseodymium (Pr) or the alkaline earth metal element calcium (Ca) at the A site may be used. In this way, the A site of the perovskite oxide may contain an alkali metal or alkaline earth metal in addition to a rare earth element selected from lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm), neodymium (Nd), and gadolinium (Gd).

[0012] A spinel-type oxide is, for example, a composite oxide represented by AB2O4. In such a composite oxide, the A site can contain one or more divalent metal elements. The B site can contain one or more trivalent metal elements, and can also contain a tetravalent metal element. In this specification, the term "spinel-type oxide" includes both normal spinel compounds having a normal spinel crystal structure and inverse spinel compounds having an inverse spinel crystal structure.

[0013] The perovskite-type oxide or spinel-type oxide constituting the electrochemical reaction catalyst of this embodiment contains cobalt (Co) and iron (Fe) or nickel (Ni), as well as three or more transition metal elements. Because the cobalt (Co), iron (Fe), and nickel (Ni) are transition metals, the electrochemical reaction catalyst of this embodiment may contain all three elements, i.e., cobalt (Co), iron (Fe), and nickel (Ni). In this case, it may further contain one or more other transition metal elements. Furthermore, it may contain one or more transition metal elements other than nickel (Ni) together with cobalt (Co) and iron (Fe), or it may contain one or more transition metal elements other than iron (Fe) together with cobalt (Co) and nickel (Ni). Examples of transition metal elements other than cobalt (Co), iron (Fe), and nickel (Ni) contained in the electrochemical reaction catalyst of this embodiment include transition metal elements in the fourth period of the periodic table, such as chromium (Cr), manganese (Mn), and copper (Cu). When the electrochemical reaction catalyst of this embodiment is a perovskite oxide, the transition metal element is contained in the B site. When the electrochemical reaction catalyst of this embodiment is a spinel oxide, the transition metal element can be contained in both the A site and the B site. In addition to the transition metal element of Period 4 of the periodic table, the B site of the perovskite oxide may further contain a valve metal such as zirconium (Zr), tantalum (Ta), niobium (Nb), antimony (Sb), or tungsten (W). By containing a small amount (for example, 5 mol % or less) of these metals, it is possible to improve the durability of the electrochemical reaction catalyst.

[0014] Here, cobalt (Co), which is contained as an essential component in the electrochemical reaction catalyst of this embodiment, is thought to be a component that contributes to increasing the activity of the catalyst for electrochemical reactions, particularly for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Iron (Fe), nickel (Ni), and manganese (Mn) contribute relatively significantly to the activity of the catalyst for electrochemical reactions, including the oxygen evolution reaction (OER). Among these, nickel (Ni) is thought to be more suitable as a catalyst for the hydrogen evolution reaction (HER). Iron (Fe), nickel (Ni), and copper (Cu) have relatively high electronic conductivities, and therefore contribute significantly to the electronic conductivity of the entire electrochemical reaction catalyst.

[0015] The transition metal element contained in the electrochemical reaction catalyst of this embodiment is preferably capable of taking multiple ionic valences. Cobalt (Co), contained as an essential component in the electrochemical reaction catalyst of this embodiment, is an element capable of taking multiple ionic valences, and is known to be capable of taking divalent, trivalent, and tetravalent states. Iron (Fe) is also known to be capable of taking divalent, trivalent, and tetravalent states, and nickel (Ni) is also known to be capable of taking divalent and trivalent states. Chromium (Cr), exemplified as another transition metal element, is also known to be capable of taking divalent, trivalent, tetravalent, pentavalent, and hexavalent states, manganese (Mn) is also known to be capable of taking divalent, trivalent, tetravalent, pentavalent, hexavalent, and heptavalent states, and copper (Cu) is also known to be capable of taking monovalent, divalent, and trivalent states.

[0016] Furthermore, the total proportion of iron (Fe) and nickel (Ni) to all metal elements contained in the perovskite oxide or spinel oxide constituting the electrochemical reaction catalyst of this embodiment may be greater than 20 mol %. This makes it easier to increase the electronic conductivity of the entire electrochemical reaction catalyst. From the perspective of increasing the electronic conductivity of the entire electrochemical reaction catalyst, the total proportion of iron (Fe) and nickel (Ni) to all the metal elements is more preferably 25 mol % or more, and even more preferably 30 mol % or more.

[0017] Furthermore, in the electrochemical reaction catalyst of this embodiment, the total proportion of iron (Fe), nickel (Ni), and cobalt (Co) to the total metal elements contained in the perovskite oxide or spinel oxide may be 40 mol % or more, which makes it easier to ensure the electronic conductivity and catalytic activity of the electrochemical reaction catalyst.

[0018] In particular, the ratio of cobalt (Co) to all metal elements contained in the perovskite oxide or spinel oxide constituting the electrochemical reaction catalyst of this embodiment is preferably 10 mol % or more. Furthermore, when the perovskite oxide or spinel oxide constituting the electrochemical reaction catalyst of this embodiment contains manganese (Mn), which is a transition metal element, the ratio of manganese (Mn) to the total of iron (Fe) and nickel (Ni) is preferably 20 mol % or less.

[0019] The composition of the perovskite oxide or spinel oxide constituting the electrochemical reaction catalyst of this embodiment can be confirmed by atomic emission spectroscopy (ICP-AES) using an inductively coupled plasma (ICP) as a light source. The composition of the electrochemical reaction catalyst thus determined can then be converted into the molar ratio of each constituent element.

[0020] B. Catalyst manufacturing method: Fig. 1 is a flowchart showing an example of a method for producing a catalyst for electrochemical reactions according to this embodiment. Fig. 1 shows a solid-phase method. The following production method can be applied in the same way whether the oxide constituting the catalyst for electrochemical reactions is a perovskite-type oxide or a spinel-type oxide.

[0021] 1, when producing the electrochemical reaction catalyst of this embodiment, first, raw material powders are mixed (step T100). Specifically, oxides, carbonates, hydroxides, etc. of constituent elements such as metals contained in the electrochemical reaction catalyst to be produced are weighed and mixed so as to have a desired composition ratio in the electrochemical reaction catalyst. Mixing can be performed wet using a ball mill or the like, for example, by adding a solvent such as ethanol.

[0022] The mixture obtained in step T100 is then dried to remove the solvent, followed by pulverization (step T110). The pulverized mixed powder is then calcined (step T120). Calcination may be carried out, for example, in an air atmosphere, with the calcination temperature and time appropriately set depending on the type of raw material used. Calcination conditions may be, for example, 1000 to 1500°C for 1 to 10 hours. After calcination in step T120, the calcined powder obtained is pulverized (step T130) to obtain a catalyst powder as a catalyst for electrochemical reactions. The pulverization in step T130 may be carried out wet, for example, by adding a solvent such as ethanol to the calcined powder obtained in step T120 using a ball mill or the like. Note that the mixed powder obtained in step T110 may be pre-calcined to allow solid-phase synthesis to proceed in advance prior to the main calcination step in step T120. Furthermore, as a method for producing the electrochemical reaction catalyst of this embodiment, a method other than the solid phase method described above may be adopted. For example, various methods known as methods for producing composite oxides, such as a citric acid complex polymerization method, a coprecipitation method, or a sol-gel method, may be used.

[0023] According to the electrochemical reaction catalyst of this embodiment configured as described above, the perovskite-type oxide or spinel-type oxide constituting the electrochemical reaction catalyst contains cobalt (Co) and iron (Fe) or nickel (Ni) as essential elements, and also contains three or more transition metal elements, with the content ratios of cobalt (Co), iron (Fe), and nickel (Ni) adjusted. This increases the activity of the electrochemical reaction catalyst, thereby reducing or eliminating the use of precious metals. Furthermore, the electronic conductivity of the electrochemical reaction catalyst is increased, further enhancing the activity of the electrochemical reaction catalyst. This also reduces or eliminates the need for conductive additives such as conductive carbon to ensure electronic conductivity in the electrochemical reaction catalyst.

[0024] Reactions that occur on catalysts are generally understood as follows: Reactant molecules are adsorbed onto "active sites" composed of metals on the catalyst surface, and the adsorbed reactants are transformed into reaction intermediates. The reaction then proceeds from the reaction intermediates, ultimately producing products, which are then desorbed from the catalyst surface. In the electrochemical reaction catalyst of this embodiment, for example, by including different types (three or more) of transition metal elements whose ionic valences can be changed, electron transfer between the active sites composed of transition metal elements such as cobalt (Co), iron (Fe), and nickel (Ni) and the reaction intermediates without the aid of a conductive additive is promoted, thereby enhancing catalytic activity. As described above, the inclusion of different types of transition metal elements whose ionic valences can be considered to promote electron transfer between the active sites and the reaction intermediates. Therefore, as long as the content ratios of cobalt (Co), iron (Fe), and nickel (Ni) are maintained as described above, the additional transition metal elements may be transition metal elements that generally do not contribute much to catalytic activity (are unlikely to form active sites). That is, even when copper (Cu) or chromium (Cr) is used as the additional transition metal element, the effect of promoting electron transfer and enhancing catalytic activity can be obtained.

[0025] In particular, spinel-type oxides can contain transition metal elements in both the A and B sites, which shortens the distance between transition metal elements where the ionic valence can change, compared to perovskite-type oxides that contain transition metal elements only in the B site. Therefore, in spinel-type oxides, once the surface charge increases, surface electronic conduction due to electron transfer becomes dominant over electronic conduction within the crystal, and electron transfer occurs quickly between transition metal elements that are closer together. This is thought to more significantly enhance the effect of increasing the speed of electron transfer between active sites and reaction intermediates by including transition metal elements. In other words, even if the electronic conductivity of the spinel-type oxide as a whole is relatively low, the speed of electron transfer during catalytic reactions can be increased, thereby improving catalytic activity.

[0026] The electrochemical reaction catalyst of this embodiment, which is composed of a perovskite oxide or a spinel oxide, can enhance catalytic activity, thereby reducing or eliminating the use of precious metals, as described above. Specifically, it can be a precious-metal-free electrochemical reaction catalyst. Here, "precious-metal-free" means that the catalyst is substantially free of precious metals, and allows the inclusion of precious metals as unavoidable impurities in raw materials, for example. An electrochemical reaction catalyst that is substantially free of precious metals refers to a catalyst in which the content of precious metals relative to the total metal elements contained in the perovskite oxide or spinel oxide is less than 1 mol %, even taking into account the unavoidable impurities. Note that the electrochemical reaction catalyst of this embodiment may contain 1 mol % or more of precious metal relative to the total metal elements. That is, in the electrochemical reaction catalyst of this embodiment, the catalytic activity can be further enhanced by adding a small amount of precious metal while maintaining the effect of improving catalytic activity described above by adjusting the type and ratio of the transition metal elements contained (particularly the ratio of Co, Fe, and Ni).

[0027] As described above, the electrochemical reaction catalyst of this embodiment ensures the electronic conductivity of the electrochemical reaction catalyst as a whole by adjusting the ratio of transition metal elements (the ratio of Co, Fe, and Ni) and by ensuring the content ratio of iron (Fe) and nickel (Ni), in particular. When the electrochemical reaction catalyst of this embodiment is used as an electrode catalyst, for example, it is thought that the composite oxide that is the constituent material of the electrode will be used in a state in which it contains more grain boundary resistance than in a bulk state. Therefore, when evaluating the electronic conductivity of the electrochemical reaction catalyst in accordance with the actual usage mode, it is thought that it is desirable to measure and evaluate the electronic conductivity in the state of a compact, for example. The electronic conductivity of the compact of the electrochemical reaction catalyst of this embodiment is, for example, 10 when the electrochemical reaction catalyst is a perovskite-type oxide. -2 S / cm or more is desirable, and when the electrochemical reaction catalyst is a spinel-type oxide, it is 10 -7 It is desirable to set it to S / cm or more.

[0028] As described below, the electrochemical reaction catalyst of this embodiment can be applied to various electrochemical devices and can be used as a constituent material for electrodes that constitute various electrochemical devices. When fabricating an electrode, for example, the electrochemical reaction catalyst of this embodiment can be mixed with a solvent such as water or ethanol and a binder to prepare a catalyst slurry, which can then be applied to the surface of an electrode substrate, dried to remove the solvent, and heat-treated at a temperature that does not decompose the binder. Alternatively, the electrochemical reaction catalyst of this embodiment can be solidified and used as a dry film-forming material, such as by sputtering, to fabricate an electrode. Alternatively, the electrochemical reaction catalyst of this embodiment can be mixed with the above-described solvent and binder to prepare a slurry, which can then be applied to the surface of an electrode substrate, and heat-treated at a temperature at which the solvent and binder volatilize, to form a wet coating material, to fabricate an electrode.

[0029] C. Application examples of catalysts: The electrochemical reaction catalyst of this embodiment can be applied, for example, to construct electrodes for various electrochemical devices. Examples of electrochemical devices that include the electrochemical reaction catalyst of this embodiment include alkaline water electrolysis apparatuses, metal-air secondary batteries, photoelectrode systems, and solid oxide electrolysis cells. When the electrochemical reaction catalyst of this embodiment is used as a catalyst for the oxygen evolution reaction (OER), an electrode that includes the electrochemical reaction catalyst of this embodiment can be used, for example, as an anode for alkaline water electrolysis, an air electrode for a metal-air secondary battery, an anode for a photoelectrode system, or an anode for a solid oxide electrolysis cell.

[0030] An alkaline water electrolysis device is a device that performs water electrolysis by disposing alkaline water as an electrolyte between a pair of electrodes, where the reaction of the following formula (5) proceeds at the anode, and the reaction of the following formula (6) proceeds at the cathode. In such an alkaline water electrolysis device, by using the anode, where the oxygen generation reaction proceeds, as an electrode comprising the electrochemical reaction catalyst of the present embodiment, the oxygen generation reaction that proceeds at the anode can be promoted, and the device performance can be improved.

[0031] 2OH - → H2O + 1 / 2O2 + 2e - … (5) 2H2O + 2e - → H2+ 2OH - … (6)

[0032] A metal-air secondary battery is a secondary battery that uses oxygen in the air as the positive electrode active material and a metal as the negative electrode active material. For example, in a zinc-air battery, the reaction of the following formula (7) proceeds at the air electrode (positive electrode) during discharge, and the reaction of the following formula (8) proceeds at the negative electrode. During charge, the reaction proceeds in the opposite direction to that during discharge. In such a metal-air secondary battery, by using the air electrode as an electrode comprising the electrochemical reaction catalyst of this embodiment, the oxygen evolution reaction that proceeds at the air electrode during charge can be promoted, thereby improving the device performance.

[0033] O2+ 2H2O + 4e - → 4OH - … (7) Zn + 2OH - → ZnO + H2O + 2e - … (8)

[0034] A photoelectrode system is a device that splits water using light such as sunlight, in which the reaction of formula (5) described above proceeds at the anode electrode, which is a photoelectrode, and the reaction of formula (6) described above proceeds at the cathode. By using the anode electrode, where the oxygen evolution reaction proceeds, as an electrode comprising the electrochemical reaction catalyst of this embodiment in such a photoelectrode system, the oxygen evolution reaction that proceeds at the anode photoelectrode can be promoted, and the device performance can be improved.

[0035] A solid oxide electrolysis cell (SOEC) is a water electrolysis device that uses a high-temperature solid electrolyte, in which the reaction of the following formula (9) proceeds at the anode, and the reaction of the following formula (10) proceeds at the cathode. In such a solid oxide electrolysis cell, by using the anode electrode comprising the electrochemical reaction catalyst of this embodiment, the oxygen evolution reaction that proceeds at the anode electrode can be promoted, and the device performance can be improved.

[0036] 2O 2- → O2+ 4e - … (9) H2O + 2e - → H2+ O 2- … (10)

[0037] Furthermore, when the electrochemical reaction catalyst of this embodiment is used as a catalyst for the oxygen reduction reaction (ORR), an electrode including the electrochemical reaction catalyst of this embodiment can be used, for example, as the air electrode of the metal-air secondary battery described above. In this case, the oxygen reduction reaction that proceeds at the air electrode during discharge of the metal-air secondary battery can be promoted, thereby improving device performance. Alternatively, when the electrochemical reaction catalyst of this embodiment is used as a catalyst for the hydrogen evolution reaction (HER), the electrochemical reaction catalyst of this embodiment can be used, for example, as the cathode of the alkaline water electrolysis device described above. In this case, the hydrogen evolution reaction that proceeds at the cathode of the alkaline water electrolysis device can be promoted, thereby improving device performance. [Example]

[0038] Figure 2 is an explanatory diagram showing the results of examining the performance of 20 types of catalysts, samples S1 to S20. Figure 2 shows the specific configuration of each sample and the results of evaluation of electrode performance. Of the catalysts shown in Figure 2, samples S1 to S6, S8 to S10, S15, S16, and S18 to S20 are composed of perovskite-type oxides. Samples S7 and S11 are composed of spinel-type oxides. Samples S12, S13, and S17 are comparative samples composed of perovskite-type oxides. Samples S12 and S13 contain less than 40 mol% of iron (Fe), nickel (Ni), and cobalt (Co). Sample S17 contains only two transition metal elements. Sample S14 is composed of iridium oxide (IrO), a conventional electrochemical reaction catalyst, rather than a composite oxide. In FIG. 2, the sample numbers of the comparative samples are indicated by hatching.

[0039] In Figure 2, the ratio of iron (Fe) and nickel (Ni) to the total metal elements contained in each sample is shown as the "(Fe + Ni) ratio." The ratio of manganese (Mn) to the total iron (Fe) and nickel (Ni) in each sample is shown as "Mn / (Fe + Ni)." The electronic conductivity measured for a compact prepared using each sample by the method described below is shown as the "electronic conductivity of the compact." The ratio of iron (Fe), nickel (Ni), and cobalt (Co) to the total metal elements contained in each sample is shown as the "(Fe + Ni + Co) ratio." The OER catalytic activity of electrodes prepared using each sample by the method described below is also shown as the "OER activity."

[0040] <Preparation of each sample> The composite oxides constituting each sample except for sample S14 were prepared according to the manufacturing method shown in Figure 1. When mixing the raw material powders in step T100, La(OH)3, SrCO3, Pr6O 11 The necessary raw material powders were selected from Sm2O3, Cr2O3, MnCO3, Fe2O3, Co3O4, NiO, CuO, Nb2O5, and WO3, and weighed to achieve the desired composition ratio shown in Figure 2. In step T100, ethanol was added to the weighed raw material powders, and the mixture was wet-mixed in a ball mill for 15 hours to obtain a slurry.

[0041] The slurry was then dried and pulverized to obtain a mixed powder (step T110). The resulting mixed powder was then calcined in an air atmosphere at 1000-1500°C for 1-10 hours (step T120). Ethanol was added to the calcined powder, and the mixture was wet-pulverized in a ball mill for 48 hours (step T130), to obtain catalyst powders for each sample. In sample S14, iridium oxide (IrO2) powder was used as the catalyst powder.

[0042] <Catalyst characteristic evaluation> [Electrode preparation] A catalytic electrode was fabricated using the oxide (catalyst) of each sample. First, catalytic ink was prepared using the catalyst powder of each sample. The catalytic ink was prepared by adding 50 mg of the catalyst powder of each sample to K + The catalyst ink was thoroughly dispersed using an ultrasonic cleaner or homogenizer, and then applied to a rotating disk electrode (glassy carbon (GC) with a diameter of 4 mm) at a catalyst concentration of 0.25 mg / cm. 2 After that, the coating was vacuum dried to obtain a catalyst electrode.

[0043] [Evaluation of catalytic activity for the oxygen evolution reaction (OER)] The catalytic activity was evaluated by the rotating disk electrode (RDE) method using a rotating ring-disk apparatus (RRDE-3A, BAS) equipped with an electrochemical analyzer (ALS 701E, BAS). While rotating at 1600 rpm, the potential was swept to a specific potential at a specific potential sweep rate (described below), and then swept back to the initial potential at the same potential sweep rate, and the current density was measured during this period. Specifically, electrochemical measurements were performed using the resulting catalytic electrode in a 0.1 M KOH aq. electrolyte solution, with a Pt coil electrode as the counter electrode and a Hg / HgO electrode (internal solution: 0.1 M KOH aq.) as the reference electrode. All measurements were performed at room temperature under oxygen saturation.

[0044] Figure 3 shows the results of electrochemical measurements for evaluating catalytic activity in the OER. In Figure 3, the horizontal axis represents electrode potential, and the vertical axis represents current density. To evaluate the properties of the catalytic oxide in the OER, the potential of the catalytic electrode was controlled at 0.3 to 1.1 V vs. Hg / HgO (1.2 to 2.0 V vs. RHE) relative to the reference electrode Hg / HgO at a potential sweep rate of 10 mV / s. In Figure 3, the potential is shown as a RHE-based potential, corrected for the iR drop (the voltage drop caused by the current flowing between the working electrode and the counter electrode due to the solution resistance between the working electrode and the reference electrode) due to the resistance component of the electrolyte. Figure 3 shows the electrochemical measurement results for samples S2, S3, S7, S10, S12, S14, and S18, out of the results for samples S1 to S20. FIG. 2 also shows the current density at an electrode potential of 1.7 V vs. RHE based on the electrochemical measurement results for each sample as a result of evaluating the catalytic activity for OER.

[0045] [Evaluation of the electrical conductivity (electronic conductivity) of powder compacts] FIG. 4 is an explanatory diagram showing the method for evaluating the electrical conductivity of each sample's compact. FIG. 4(A) shows a jig 10 used to prepare the compact used for evaluation, and FIG. 4(B) shows an electrochemical evaluation device 20. As previously described, the evaluation of the electrical conductivity of the compact is intended to evaluate the electrical conductivity of each sample under conditions that closely resemble the actual use of the sample as a catalytic electrode. To evaluate the electrical conductivity of the compact, as shown in FIG. 4(A), 0.1 g of catalyst powder 30 was placed in a jig 10 consisting of a cylindrical insulating tube 12 with an inner diameter of 10 mm and a mold 14. A pressure of 100 MPa was applied to the mold 14 for uniaxial compacting to produce a compact 32. The direction of pressure application is indicated by an arrow in FIG. 4(A). Then, as shown in FIG. 4(B), a fixing jig 22 was attached to the jig 10 holding the compact 32, and terminal screws (not shown) were attached to the upper and lower molds 14 to collect current, and the electrochemical evaluation device 20 was assembled. The DC resistance of each sample's powder compact was measured using the electrochemical evaluation device 20, and the electronic conductivity was calculated from the cross-sectional area and thickness of the powder compact. The DC resistance was measured by chronoamperometry, where a voltage of 10 mV was applied between two terminals and the change in current over time was measured. Specifically, the current value after 60 seconds was read and the DC resistance was calculated.

[0046] As shown in Figures 2 and 3, it was confirmed that samples S1 to S6, S8 to S10, S15, S16, and S18 to S20, which are perovskite-type oxides, and samples S7 and S11, which are spinel-type oxides, satisfy the above-mentioned requirements regarding the type and content ratio of the transition metal elements contained, and thereby exhibit catalytic performance equivalent to or better than iridium oxide (IrO2), which is conventionally known as a precious metal-containing electrochemical reaction catalyst, i.e., sample S14, which is a comparative example.

[0047] It was also shown that in order to achieve high catalytic activity, it is necessary that the total ratio of iron (Fe), nickel (Ni), and cobalt (Co) relative to the total metal elements be 40 mol% or more (see comparative samples S12 and S13). It was also shown that in order to achieve high catalytic activity, it is important to contain three or more transition metal elements (see comparative sample S17). It was also shown that in order to achieve high catalytic activity, it is desirable that the electrochemical reaction catalyst contain 10 mol% or more of cobalt (Co) relative to the total metal elements (see sample S1). It was also shown that in order to achieve high catalytic activity, when manganese (Mn) is included as a transition metal element, the ratio of manganese (Mn) relative to the total of iron (Fe) and nickel (Ni) is desirably 20 mol% or less (see samples S9 and S11).

[0048] Furthermore, when electrochemical reaction catalysts are compared between perovskite-type oxides and spinel-type oxides, it was shown that spinel-type oxides can achieve relatively high catalytic activity even though the electronic conductivity of the compact is lower than that of perovskite-type oxides. That is, as shown in Figures 2 and 3, perovskite-type oxides have an electronic conductivity of 1×10 -3 Sample S12, which has a relatively low conductivity of 3×10 S / cm, exhibits insufficient current flow (Fig. 3) and insufficient catalytic activity (Fig. 2). In contrast, spinel oxides have a compacted electronic conductivity of 3×10 -7 Even sample S7, which had a lower conductivity of 10 S / cm, still had a larger current flow (Fig. 3) and showed relatively high catalytic activity (Fig. 2). As mentioned above, this is thought to be because in spinel-type oxides, surface electron conduction due to electron transfer is dominant over electron conduction within the crystal, and electron transfer occurs quickly between transition metal elements that are closer together, increasing the speed of electron transfer between active sites and reaction intermediates. From the results shown in Fig. 2, for example, in perovskite-type oxides, the electronic conductivity of the compact is 10 -2 S / cm or more is desirable, and in the case of spinel-type oxides, the electronic conductivity of the compact is 10-7 It is considered desirable that the resistance be S / cm or higher.

[0049] The present disclosure is not limited to the above-described embodiments, and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted.

[0050] The present disclosure can also be realized in the following forms. [Application example 1] A catalyst for electrochemical reactions of oxygen or hydrogen, It is composed of perovskite-type oxide or spinel-type oxide, Cobalt (Co), iron (Fe) or nickel (Ni), and three or more transition metal elements; the total ratio of iron (Fe) and nickel (Ni) to all metal elements contained in the perovskite-type oxide or spinel-type oxide is more than 20 mol %, The total ratio of iron (Fe), nickel (Ni), and cobalt (Co) to the total metal elements is 40 mol % or more. Catalyst for electrochemical reactions. [Application example 2] The catalyst for electrochemical reactions according to Application Example 1, Characterized by not containing precious metals Catalyst for electrochemical reactions. [Application example 3] The electrochemical reaction catalyst according to Application Example 1 or 2, The ratio of cobalt (Co) to the total metal elements is 10 mol % or more. Catalyst for electrochemical reactions. [Application example 4] The catalyst for electrochemical reactions according to any one of Application Examples 1 to 3, When manganese (Mn) is contained as the transition metal element, the ratio of manganese (Mn) to the total of iron (Fe) and nickel (Ni) is 20 mol % or less. Catalyst for electrochemical reactions. [Application example 5] An electrode comprising the catalyst for electrochemical reactions according to any one of Application Examples 1 to 4. [Application Example 6] An electrochemical device comprising the electrode according to Application Example 5. [Explanation of symbols]

[0051] 10...Jig 12...Cylindrical insulating tube 14...Mold 20...Electrochemical evaluation device 30...Catalyst powder

Claims

1. A catalyst for electrochemical reactions of oxygen or hydrogen, It is composed of perovskite-type oxide or spinel-type oxide, Cobalt (Co), iron (Fe) or nickel (Ni), and three or more transition metal elements; the total ratio of iron (Fe) and nickel (Ni) to all metal elements contained in the perovskite-type oxide or spinel-type oxide is more than 20 mol %, The total ratio of iron (Fe), nickel (Ni), and cobalt (Co) to the total metal elements is 40 mol % or more. Catalyst for electrochemical reactions.

2. The catalyst for electrochemical reactions according to claim 1, Characterized by not containing precious metals Catalyst for electrochemical reactions.

3. The catalyst for electrochemical reactions according to claim 1, The ratio of cobalt (Co) to the total metal elements is 10 mol % or more. Catalyst for electrochemical reactions.

4. The catalyst for electrochemical reactions according to claim 1, When manganese (Mn) is contained as the transition metal element, the ratio of manganese (Mn) to the total of iron (Fe) and nickel (Ni) is 20 mol % or less. Catalyst for electrochemical reactions.

5. An electrode comprising the catalyst for electrochemical reactions according to any one of claims 1 to 4.

6. An electrochemical device comprising the electrode according to claim 5 .

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