JP2021107307A - Manganese oxide and method for producing the same, and electrode using the manganese oxide - Google Patents

Abstract

To provide a manganese oxide useful as an electrode active material, and to provide a method for producing the same.SOLUTION: A manganese oxide is formed by heating a manganese oxide laminate, the manganese oxide laminate being configured such that: a plurality of layers of manganese oxide are laminated; and water molecules and cations other than manganese ion exist among the layers of manganese oxide. In the manganese oxide, water molecules are removed. Preferably, the cations exist in a use environment of the manganese oxide and, for example, may be sodium ion. A method of the present invention for producing the manganese oxide includes: a first step of electrochemically oxidizing a divalent manganese compound or electrochemically reducing permanganate ion, in the presence of cations other than the manganese ion, to obtain the manganese oxide laminate; and a second step of heating the manganese oxide laminate at 300°C or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to novel manganese oxides.

Manganese oxides such as manganese dioxide (MnO ₂ ) have various crystal structures and can take multiple oxidation states by injecting electrons and cations. Therefore, they are used as positive electrode materials for alkaline batteries and lithium ion secondary batteries. , Used for various purposes.

In recent years, attempts have been made to use manganese oxide as a catalyst for a metal anode for electrolysis. For example, as an electrode for electrolysis of a chlorine ion-containing aqueous solution typified by seawater or an insoluble electrode for electrocorrosion protection, the surface of a conductive base material having excellent corrosion resistance such as titanium has conventionally been formed with a catalyst layer made of a noble metal or an oxide thereof. Although the coated material is widely used, it has been proposed to use manganese oxide, which is cheaper than the noble metal, as the material (electrode active material) of the catalyst layer (Patent Documents 1 to 3).

In seawater electrolysis using conventional electrolysis electrodes containing noble metals, where hydrogen and sodium hydroxide are generated at the cathode and chlorine is generated at the anode, chlorine (from chlorine and sodium hydroxide) is used only for the purpose of producing hydrogen. If the purpose is not to produce the sodium hypochlorite produced), the generation of chlorine can be a problem. Since chlorine is toxic, there is a concern that it may have an adverse effect on the ecosystem or equipment including the electrode depending on the environment in which seawater electrolysis is performed and chlorine is generated. In addition, the same adverse effect is also a concern in the electric corrosion protection of the external power supply system. In this regard, the manganese oxide electrodes described in Patent Documents 1 to 3 are oxygen-evolving electrodes that generate oxygen without generating chlorine in seawater electrolysis, and such concerns can be dispelled. The manganese oxide electrodes described in Patent Documents 1 to 3 contain γ (gamma) type manganese dioxide (γ-MnO ₂ ) having a tunnel structure as an electrode active material, and are typically a titanium substrate or platinum. It has a structure in which an intermediate layer containing a group and a catalyst layer containing γ-type manganese dioxide are sequentially laminated.

Conventionally, as a method for producing a manganese oxide electrode, a conductive base material such as titanium is used as an anode, and an electrolytic solution (acidic aqueous solution) containing manganese ions is electrolyzed to precipitate manganese dioxide on the surface of the anode. There are known "analysis method" (electrolysis method) and "thermal decomposition method" in which a thin layer of manganese dioxide is formed by heat-decomposing an aqueous solution of manganese nitrate on a conductive substrate such as titanium. The crystal structure of manganese dioxide obtained by the electrodeposition method is γ type, and that obtained by the thermal decomposition method is β (beta) type. The manganese oxide electrodes described in Patent Documents 1 to 3 are manufactured by an electrolysis method. In this conventional electrolysis method, the electrolytic solution in which the conductive base material is immersed is strongly acidic and has a high temperature of 90 ° C. or higher. Since it is necessary to keep it, there are problems in terms of work safety, equipment maintainability, and so on. Further, the conventional electrodeposition method has a problem that the current efficiency is low due to the generation of oxygen at the time of precipitation of manganese dioxide, and there is a report that the current efficiency is less than 10%. Therefore, it is difficult to control the thickness of the manganese dioxide layer formed on the surface of the base material by the conventional electrodeposition method.

Patent Document 4 describes an electrodeposition method capable of precipitating manganese dioxide under milder conditions than the electrodeposition methods described in Patent Documents 1 to 3. Specifically, the electrodeposition method described in Patent Document 4 uses a divalent manganese salt aqueous solution in which organic fourth ammonium ions coexist as an electrolytic solution, and manganese dioxide is applied to the surface of the anode by a predetermined voltage. _{The manganese dioxide obtained by this is a δ (delta) type manganese dioxide (δ-MnO 2} ) having a structure in which an organic fourth ammonium ion is intercalated between thin layers of a manganese oxide. ). δ-type manganese dioxide has a layered structure, has a continuous oxide layer for electron transfer and a continuous space for ion transfer, and has specific ion exchange due to such a unique layered structure. Since it has properties and electrochemical properties, it is attracting attention in various fields.

Japanese Unexamined Patent Publication No. 9-256181 Japanese Unexamined Patent Publication No. 2003-129267 Japanese Unexamined Patent Publication No. 2010-59524 Japanese Unexamined Patent Publication No. 2006-76865

The electrodeposition method described in Patent Document 4 requires that the electrolytic solution in which the conductive base material as the anode is immersed is neutral and has a temperature similar to room temperature, and the current efficiency is relatively high. Therefore, it is a δ type. It is useful as an industrial method for producing manganese dioxide. However, the δ-type manganese dioxide obtained by this electrodeposition method has room for improvement in terms of performance as an electrode active material.

An object of the present invention is to provide a technique capable of eliminating the above-mentioned drawbacks of the prior art, and more specifically, to provide a manganese oxide useful as an electrode active material.

In the present invention, a plurality of layers of manganese oxide are laminated, and a layered manganese oxide in which cations other than manganese ions and water molecules are present between the layers of the manganese oxide is heated, and the water molecules are formed. The removed manganese oxide.

Further, the present invention is an electrode in which the above-mentioned layer containing the manganese oxide of the present invention is fixed on the surface of a conductive base material.

Further, in the present invention, a divalent manganese compound is electrochemically oxidized in the presence of a cation other than manganese ion, or a permanganate ion is electrochemically reduced in the presence of a cation other than manganese ion. A method for producing manganese oxide, which comprises a first step of obtaining a layered manganese oxide and a second step of heating the layered manganese oxide at 300 ° C. or higher.

According to the present invention, a manganese oxide useful as an electrode active material and a method capable of producing the manganese oxide are provided. Since the electrode of the present invention using the manganese oxide of the present invention functions as an oxygen-evolving electrode, the load on the environment is reduced as compared with the conventional chlorine-evolving electrode, and an environment in which the chlorine-generating electrode cannot be used. However, it can be used in a wide range of applications such as electrodes for anticorrosion.

1 (a) to 1 (f) are analysis patterns by X-ray diffraction (XRD) of the precipitate layer in the electrode with the precipitate layer produced in each of the production examples. 2 (a) to 2 (f) are Na 1s spectra obtained by X-ray photoelectron spectroscopy (XPS) of the precipitated layer in the electrode with a precipitated layer produced in each of the production examples. 3 (a) to 3 (f) are O 1s spectra of the electrode with a precipitation layer produced in the production example by X-ray photoelectron spectroscopy (XPS). 4 (a) to 4 (f) are scanning electron microscope images of the surface of the precipitate layer in the electrode with the precipitate layer produced in the production example, respectively. 5 (a) to 5 (f) are anodic polarization curves of the electrode with a precipitation layer produced in the production example in an aqueous solution of sodium chloride or an aqueous solution of sodium perchlorate, respectively.

The manganese oxide of the present invention is obtained by heating a layered manganese oxide, and is a heated product of the layered manganese oxide. Hereinafter, the layered manganese oxide according to the present invention, which should be said to be a precursor of the manganese oxide of the present invention, will be described first.

The layered manganese oxide has a layered structure in which a plurality of layers of manganese oxide are laminated. The layered manganese oxide has, as a basic structure, a layered structure in which gaps exist between layers of manganese oxide, that is, a structure in which solid phase (manganese oxide) and spaces (gap) are alternately arranged. However, for example, a burnesite-type layered manganese oxide can be exemplified. As will be described later, cations other than manganese ions are present in the space (gap between layers) in the layered structure of the layered manganese oxide. The burnesite-type layered manganese oxide is a layer of manganese oxide _{(MnO 2} sheet) in _{which an octahedral structure represented by MnO 6 in} which six oxygens are arranged at the apex centered on manganese spreads while sharing the apex and ridge with each other. _{A manganese oxide (MnO 2} ) having a mixed valence of ^{Mn 3+} / Mn ⁴⁺ , which is formed by stacking layers of the plurality of manganese oxides to form a layered structure.

"Thickness of one manganese oxide layer" constituting the layered structure of layered manganese oxide and "distance between the one manganese oxide layer and the other manganese oxide layer closest thereto" (Hereinafter, also referred to as “interlayer distance”) has a considerable influence on the physicochemical stability and mechanical stability of the layered manganese oxide. In consideration of this point, the interlayer distance of the layered manganese oxide is preferably 0.4 to 5 nm, more preferably 0.5 to 3 nm, and further preferably 0.7 to 1.5 nm. The interlayer distance can be measured by _{obtaining the lattice constant (d 001} ) of the 001 plane from the Bragg equation (2dsin θ = nλ).

In the layered manganese oxide, cations other than manganese ions (hereinafter, also referred to as “non-manganese cations”) and water molecules are present between the layers of the manganese oxide. Typically, hydrated non-manganese cations are intercalated between the layers of manganese oxide in the layered manganese oxide.

The layered manganese oxide may be obtained by electrochemically oxidizing a divalent manganese compound in the presence of a cation other than the manganese ion. Further, the layered manganese oxide may be obtained by electrochemically reducing permanganate ions in the presence of cations other than manganese ions.

The manganese oxide of the present invention is characterized in that water molecules are removed from the layered manganese oxide which is a precursor of the manganese oxide. As a result of various studies on the layered manganese oxide produced by the method described in Patent Document 4, the present inventors have found that water molecules are present between the layers of the manganese oxide in the layered manganese oxide. It has been found that the catalytic activity of the manganese oxide is significantly improved by removing the water molecules existing between the layers by heating the layered manganese oxide. The manganese oxide of the present invention has been made based on such findings, has higher catalytic activity than the layered manganese oxide produced by the method described in Patent Document 4, and has high performance as an electrode material. The absence of water molecules between the layers each other manganese oxide in manganese oxide, using X-ray photoelectron spectroscopy (XPS), depending on whether a peak derived from a water molecule (H 2 _O) is obtained You can check.

Compared to the precursor layered manganese oxide, the manganese oxide of the present invention not only has water molecules removed, but also typically has a changed molecular structure. That is, as one embodiment of the manganese oxide of the present invention, one having no water molecule and having an amorphous structure can be mentioned. The "amorphous structure" referred to here is 1) a crystal structure that does not have a complete crystal structure, 2) a cryptocrystalline substance that does not have a crystal structure optically but shows weak diffraction in X-ray analysis, 3 ) The layering of crystal layers includes irregular layered structures. A manganese oxide having an amorphous structure has an increased specific surface area as compared with a manganese oxide having a crystal structure such as a layered manganese oxide, and therefore has high performance as an electrode material.

The manganese oxides of the present invention typically contain non-manganese cations. This non-manganese cation was present in the layered manganese oxide which is the precursor of the manganese oxide at least immediately after the production of the manganese oxide and usually before the manganese oxide was used as an electrode catalyst. Although it is the same and not crystallized, when the manganese oxide is used as an electrode catalyst, it can be desorbed from the manganese oxide by ion exchange with other cations existing around the manganese oxide. .. According to the findings of the present inventors, the desorption of non-manganese cations by ion exchange in this way may change the structure (amorphous structure) of manganese oxide, leading to deterioration of manganese oxide performance. ..

Therefore, as the non-manganese cation, it is preferable to use a cation that exists in the environment in which the manganese oxide is used. By doing so, for example, when manganese oxide is used as an electrode catalyst, even if the non-manganese cation in the manganese oxide is desorbed by ion exchange, the desorbed non-manganese present around the manganese oxide is present. Since a cation of the same type as the cation is present in the manganese oxide in place of the desorbed non-manganese cation, the non-manganese cation is substantially always present in the manganese oxide. As a result, the stability of the structure (amorphous structure) of the manganese oxide is improved, and the predetermined performance of the manganese oxide can be stably exhibited for a long period of time.

As described above, as the non-manganese cation, a cation existing in the environment in which the manganese oxide is used is preferable, and it may be appropriately selected in consideration of the environment in which the manganese oxide is used. As a preferable specific example of the non-manganese cation, sodium ion (Na ⁺ ) can be exemplified. The main uses of the manganese oxide of the present invention include "electrolysis of reinforcing bars in reinforced concrete structures" and "electrolysis of chloride-containing aqueous solution such as seawater". When manganese oxide is used in these applications. Is usually rich in sodium ions around the manganese oxide. Other than sodium ions, for example, K ⁺ , Mg ²⁺ , Ca ²⁺ , Al ³⁺ , Fe ²⁺ , Cr ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ , Sn ²⁺ are mentioned, and in the present invention, non-manganese cations are mentioned. As a result, one of these types can be used alone or in combination of two or more types.

The manganese oxide of the present invention can be obtained by heating a layered manganese oxide. The heating conditions (heating temperature and heating time of the layered manganese oxide) are not particularly limited as long as the water molecules existing in the layered manganese oxide can be removed. A preferred example of the method for producing a manganese oxide of the present invention is the method for producing a manganese oxide of the present invention, which will be described later.

The present invention includes an electrode in which the above-mentioned layer containing the manganese oxide of the present invention (hereinafter, _{also referred to as “anhydrous MnO 2} layer”) is adhered to the surface of a conductive base material. The anhydrous MnO ₂ layer is typically composed of the manganese oxide of the present invention and may have an amorphous structure.

The conductive base material may be any as long as it has conductivity and a layer containing a manganese oxide can be fixed, and the shape and material of the conductive base material should be appropriately selected according to the use of the electrode and the like. Can be done. The shape of the conductive base material is a flat plate shape, a curved plate shape, a rod shape, a lath shape, or the like. Examples of the material of the conductive base material include valve metals such as titanium, zirconium, and tungsten; conductive glass (FTO glass); and carbon-based electrodes such as carbon fiber, graphite, and artificial graphite.

In the electrode of the present invention, the anhydrous MnO ₂ layer is fixed to the whole or a part of the surface of the conductive base material. The fixation pattern and basis weight (mass per unit area) of the anhydrous MnO ₂ layer are not particularly limited and can be appropriately set according to the application of the electrode and the like. For example, when used as a sacrificial electrode, the basis weight of anhydrous MnO ₂ layer is preferably 0.1 to 1000 mg / cm ^2, more preferably at 0.1 to 10 mg / cm ^2.

The electrode of the present invention can be applied to various uses, and specific uses include an electrode for electrocorrosion protection (anode of an electrocorrosion protection system) and an electrode for electrolysis of a chlorine ion-containing aqueous solution (electrolysis for the purpose of hydrogen generation). The anode of the system) can be exemplified. Since the electrode of the present invention functions as an oxygen generating electrode in an electrocorrosion protection system or an electrolytic system and does not generate chlorine, it can be used even in an environment where a conventional chlorine generating electrode cannot be used due to chlorine generation. , Environmentally friendly electro-corrosion system, electrolysis system can be provided.

The electrode of the present invention is suitable as, for example, an electrode for electrocorrosion protection in an external power supply type electrocorrosion protection. In the external power supply system, an electrolytic anticorrosion electrode is installed on the surface of a concrete structure, and a steel material (reinforcing bar, etc.) in the concrete structure is used as a cathode, and a DC power supply device is used to direct current from the electrocorrosion electrode to the cathode. By supplying a current, the potential of the cathode is changed in the base direction to prevent corrosion of the steel material (cathode).

Next, the method for producing a manganese oxide of the present invention will be described. In the method for producing a manganese oxide of the present invention, a divalent manganese compound is electrochemically oxidized in the presence of a non-manganese cation, or a permanganate ion is electrochemically reduced in the presence of a non-manganese cation. It has a first step of obtaining the manganese oxide and a second step of heating the manganese oxide at 300 ° C. or higher.
It should be noted that the above-mentioned description of the manganese oxide and the electrode of the present invention is appropriately applied to the point that the method for producing the manganese oxide of the present invention is not particularly described.

The first step is typically by immersing the conductive substrate in an electrolytic solution containing divalent manganese ions or permanganate ions and a non-manganese cation, and energizing the conductive substrate. This is an electrolysis step of forming a _{precipitation layer (δ-MnO 2} layer) containing a manganese oxide on the surface of the conductive substrate. The conductive base material (the base material on which the precipitation layer is formed) is as described above. The counter electrode is not particularly limited, and for example, an electrode made of iron, copper, nickel, platinum or the like can be used. The electrolytic cell in which the electrolytic cell is housed may be a batch type or a circulation type, but a circulating electrolytic cell is preferable from the viewpoint of continuous operation and keeping the liquid concentration constant during the electrolytic oxidation / reduction reaction.

The electrolytic solution is typically a weakly acidic to neutral to weakly alkaline aqueous solution containing divalent manganese ions or permanganate ions, non-manganese cations, and water. The pH of the electrolytic solution at a liquid temperature of 25 ° C. is preferably 3 to 10, more preferably 4 to 7. The electrolytic solution can be prepared by dissolving a source of divalent manganese ions or permanganate ions and a source of non-manganese cations in water.

The manganese compound that is a source of divalent manganese ions is not particularly limited as long as it is a divalent manganese compound that is soluble in water, and for example, inorganic acids such as manganese sulfate, manganese chloride, manganese nitrate, and manganese carbonate. Salts; Examples thereof include organic manganese compounds such as ammonium manganese oxalate and potassium manganese oxalate. _{Among these, manganese sulfate (MnSO 4} ) is preferable from the viewpoint of easy precipitation of layered manganese oxide by electrolysis, easy availability, and the like.

The manganese compound serving as a source of permanganate ions is not particularly limited as long as it is a water-soluble permanganate compound, and examples thereof include permanganate salts. For example, alkali metal salts of permanganate, alkaline earth metal salts and the like can be mentioned. Among these, sodium salt or potassium salt is preferable from the viewpoint of easy precipitation of layered manganese oxide by electrolysis, easy availability, and the like.

When sodium ion is used as the non-manganese cation, it is not particularly limited as long as it is a sodium compound that is a source of sodium ion, and for example, sodium chloride, sodium nitrate, sodium sulfate, sodium perchlorate and the like can be used.

The concentration of divalent manganese ions in the electrolytic solution may be appropriately set depending on the use of the manganese oxide and the like, and is not particularly limited, but is preferably 0.1 to 1000 mM, more preferably 1 to 500 mM. The concentration of permanganate ion in the electrolytic solution can be set to be about the same as the concentration of divalent manganese ion. If the ion concentration is too low, the electrical resistance of the electrolytic solution increases, which may cause an unfavorable phenomenon such as electrolysis of other components such as water. On the contrary, if the ion concentration is too high, the conductive substrate may occur. There is a risk that the precipitation of manganese oxide on the surface of the surface will lack uniformity. From the same viewpoint, the concentration of non-manganese cations such as sodium ion in the electrolytic solution is preferably 1 to 1000 mM, more preferably 1 to 100 mM.

The temperature of the electrolytic solution can be set to a high temperature close to 90 ° C., for example, about 60 to 80 ° C., but can also be set to a lower temperature, for example, about room temperature. As described above, the conventional electrolysis method as described in Patent Documents 1 to 3 requires that the electrolytic solution is strongly acidic and has a high temperature of 90 ° C. or higher. However, in the electrodeposition step according to the present invention, the electrolytic solution is neutral and the temperature of the electrolytic solution may be about room temperature, so that the layered manganese oxide can be produced safely and efficiently. .. The temperature of the electrolytic solution is preferably 20 to 40 ° C, more preferably 25 to 40 ° C. If the temperature of the electrolytic solution is too high, the precipitation rate will increase, but the precipitation surface of the precipitated manganese oxide will tend to become rough, which is not only in terms of work safety, but also in terms of the quality of the layered manganese oxide. It is also not preferable in terms of points.

In the electrodeposition step (first step), the voltage during electrolysis is preferably 0.8 to 1.2 volts with respect to the silver / silver chloride reference electrode from the viewpoint of obtaining uniform manganese oxide precipitation. More preferably, constant voltage control is performed so as to maintain a voltage of 0.8 to 1.05 volts.

Regarding the electrolysis time (energization time) in the electrodeposition step (first step), in general, the number of layers of manganese oxide precipitated on the conductive substrate is proportional to the electrolysis time. Although it is possible to increase the number, if the electrolysis time is too long, the thickness of the precipitated manganese oxide becomes too large, and the layered structure tends to be disturbed. Considering these points, the electrolysis time in the electrodeposition step is preferably 10 to 300 minutes, more preferably 20 to 120 minutes.

In the method for producing a manganese oxide of the present invention, after the first step (electrodeposition step) and before the second step, the layered manganese oxide (conductive substrate) obtained in the first step is used. The precipitation layer containing the manganese oxide precipitated on the surface) may be dried. The purpose of such a drying treatment is mainly to evaporate the electrolytic solution adhering to the surface of the manganese oxide, and the manganese oxide may be dried within the range in which the purpose is achieved. The drying method is not particularly limited, and known drying methods such as vacuum drying, hot air drying, and air drying can be adopted.

In the second step, the layered manganese oxide obtained in the first step is heated at 300 ° C. or higher. That is, the layered manganese oxide is heated under the condition that the temperature of the layered manganese oxide is 300 ° C. or higher. When the first step is the above-mentioned electrodeposition step, the precipitated layer containing the layered manganese oxide precipitated on the surface of the conductive substrate is heated together with the conductive substrate. By such heat treatment, water molecules existing between layers in the layered structure of the manganese oxide are removed, and the manganese oxide of the present invention, which is a production target, is obtained. As described above, the manganese oxide thus obtained has no water molecule, has a non-manganese cation, and more typically has an amorphous structure.

The heating temperature of the manganese oxide in the second step is preferably 300 to 600 ° C, more preferably 300 to 400 ° C. The heating time (time for maintaining the temperature of the manganese oxide at 300 ° C. or higher) is preferably 1 to 5 hours, more preferably 1 to 2 hours. If the heating temperature of the manganese oxide is too high or the heating time is too long, the adhesion between the manganese oxide and the conductive base material may decrease. The heat treatment of the manganese oxide can be carried out according to a conventional method using a known heating means such as an electric furnace.

[Manufacturing example]
50 mL of an aqueous solution prepared by dissolving 50 mM of sodium chloride (NaCl) and _{2 mM of manganese sulfate (MnSO 4} ) was placed in a beaker type electrolytic cell to prepare an electrolytic solution. The electrolytic solution was made into a nitrogen atmosphere by bubbling nitrogen gas. The electrolytic solution had a liquid temperature of 25 ° C. and a pH of 6. A rectangular FTO glass electrode (length 40 mm in the longitudinal direction, length 10 mm in the width direction, thickness 2 mm), a platinum-plated titanium counter electrode having the same shape and dimensions as the glass, and a saturated KCl silver-silver chloride matching electrode (SSE). And was immersed in the electrolytic solution. These electrodes were degreased before being immersed in the electrolyte. After allowing the electrode to stand in the electrolytic solution for 10 minutes, a potential of +1.0 V (potential with respect to SSE) is applied to the glass electrode to form a precipitation layer containing manganese oxide on the surface of the glass electrode. An electrode with a precipitated layer was obtained (first step: electrodeposition step). The potential was applied until the integrated electric energy reached 200 mC / cm ^2. Next, after vacuum-drying the plurality of electrodes with a precipitation layer thus obtained in a desiccator for 30 minutes or more, the temperature in the furnace of some of the plurality of electrodes is set to a predetermined temperature (100 ° C., 200 ° C., 300 ° C.). , 400 ° C. or 500 ° C.), and the glass electrode was placed in an electric furnace and heat-treated for 2 hours (second step). In this way, an electrode with a precipitation layer without heat treatment and five types of electrodes with a precipitation layer that were heat-treated at a predetermined temperature were manufactured.

The precipitation layer (manganese oxide) in the six types of electrodes with a precipitation layer produced in the above [Production Example] is analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and the precipitation layer is analyzed. Was observed with a scanning electron microscope (SEM). FIG. 1 is an XRD pattern of the precipitation layer, FIG. 2 is an Na 1s spectrum by XPS of the precipitation layer, FIG. 3 is an O 1s spectrum by XPS of the precipitation layer, and FIG. 4 is an SEM image of the surface of the precipitation layer. In FIGS. 1 to 4, (a) is no heat treatment, (b) is a heating temperature of 100 ° C., (c) is a heating temperature of 200 ° C., (d) is a heating temperature of 300 ° C., and (e) is a heating temperature of 400 ° C. , (F) have a heating temperature of 500 ° C.

With reference to FIG. 1 (a), diffraction peaks (equally spaced peaks) peculiar to the layered manganese oxide are observed at 12.1 ° and 24.4 °. From Bragg's equation, the interlayer distance was calculated to be 0.73 nm. Further, from FIG. 2 (a), _{it was confirmed that sodium ions were present between the manganese oxide layers (MnO 2} sheet) in the layered manganese oxide, and from FIG. 3 (a), water molecules were present between the layers. It is confirmed that it will be done. From the above, the layered manganese oxide in which sodium ions (non-manganese cations) and water molecules are incorporated between layers by the first step (electrodeposition step) in the above [Production Example] is used as a precipitation layer on a conductive substrate (FTO). It is clear that it was formed on the surface of the glass electrode). Further, from the characteristics of the interlayer distance, it is presumed that the layered manganese oxide forming the precipitation layer is a burnesite type layered manganese oxide.

When the precipitated layer (burnesite type layered manganese oxide) is heat-treated at 200 ° C. with reference to FIG. 1, the peak shifts to the high angle side and the interlayer distance decreases from 0.73 nm before the heat treatment to 0.67 nm. (See FIG. 1 (c)). It is presumed that this is because the water molecules between the layers have been removed. This peak was significantly reduced by heat treatment at 300 ° C. (see FIG. 1 (d)) and completely disappeared by heat treatment at 400 ° C. or higher (see FIGS. 1 (e) and 1 (f)). On the other hand, since the diffraction peak due to the other crystal phase of the manganese oxide does not appear, it is presumed that the layered structure of the manganese oxide was changed to the amorphous structure by the heat treatment at least 300 ° C. or higher.

With reference to FIG. 2, since the peak of Na 1s was observed in all of FIGS. 2 (a) to 2 (f), the second step (heat treatment of the precipitated layer) in the above [Production Example] was performed. Even after that, it is confirmed that sodium ions (non-manganese cations) are present in the manganese oxide.

With reference to FIG. 3, peaks derived from water molecules were observed before the heat treatment (FIG. 3 (a)) and after the heat treatment at 100 ° C. (FIG. 3 (b)), but at 200 ° C. or higher. After the heat treatment of (FIGS. 3 (c) to 3 (f)), the peaks derived from water molecules disappeared completely. It is confirmed that the molecule has been removed. In addition, in the heat treatment at 200 ° C. or higher, a new peak derived from the metal-hydroxide bond (M-OH) appears in parallel with the decrease / disappearance of the peak derived from water molecules. It is suggested that MnOOH was formed by such heat treatment.

With reference to FIG. 4, the surface state of the precipitated layer before the heat treatment (FIG. 4 (a)) is basically unchanged after the heat treatment (FIGS. 4 (b) to 4 (f)). Therefore, it is considered that the amorphous structure of the manganese oxide is substantially unchanged and maintained by the heat treatment up to at least 500 ° C.

The anodic polarization curve was measured for the six types of electrodes with a precipitation layer manufactured in the above [Production Example]. The anodic polarization curve was measured by an automatic polarization method of 1 mV / s. In addition, the oxygen evolution efficiency of the electrode with a precipitation layer that had been heat-treated at 300 ° C. or higher was measured. Specifically, the amount of chlorine generated was quantified by the iodine titration method, the amount of chlorine generated in the amount of electricity was quantified from the quantified value, and the oxygen generation efficiency was calculated.

FIG. 5 shows the anodic polarization curves of the six types of electrodes with a precipitation layer in an aqueous solution of sodium chloride (NaCl) or an aqueous solution of sodium perchlorate (NaClO _4). The sodium chloride concentration in the sodium chloride aqueous solution used here and the sodium perchlorate concentration in the sodium perchlorate aqueous solution were 0.5 M, respectively. Only the oxygen evolution current is observed in the sodium perchlorate aqueous solution, and both the oxygen evolution current and the chlorine generation current are observed in the sodium perchlorate aqueous solution. Therefore, by comparing the anodic polarization curves in both electrolytes, the characteristics as an oxygen-evolving electrode can be qualitatively evaluated.

Before the heat treatment, it was _{inactive against oxygen evolution (2H 2} O → O ₂ + 4H + 4e ⁻ ) and chlorine evolution (2Cl ⁻ → Cl ₂ + 2e ⁻ ), and no current flowed (Fig. 5 (a)). .. On the other hand, after the heat treatment at 100 ° C. or 200 ° C., the ^{current in the presence of Cl −} ions increased slightly (FIGS. 5 (b) and 5 (c)). After 300 ° C. or more heat treatment, Cl ^- and both significantly increase the current in the ion presence and absence, and the difference in current in both electrolytic solution was hardly observed (Fig. 5 (d) ~ FIG. 5 (f)). From this, it is understood that a manganese oxide capable of functioning as an oxygen-evolving electrode can be obtained by heat treatment at least at 300 ° C. or higher.

Table 1 below shows the chlorine generation efficiency and oxygen generation efficiency of the electrode with _{a precipitation layer (anhydrous MnO 2} layer) heat-treated at 300 ° C. or higher. Table 1 below also shows the chlorine generation efficiency and oxygen generation efficiency of the metal oxide-coated titanium (MMO) electrode, which is a chlorine generation type electrode, for comparison. The oxygen evolution efficiency of the MMO electrode is 17%, whereas the oxygen evolution efficiency of the electrode with a precipitation layer heat-treated at 300 ° C. or higher, that is, the electrode using the manganese oxide of the present invention is 80% or higher. It was confirmed that the properties were completely opposite to those of the MMO electrode. From this, it is clear that the electrode using the manganese oxide of the present invention functions as an oxygen-evolving electrode.

Claims (9)

A plurality of layers of manganese oxide were laminated, and layered manganese oxide in which cations other than manganese ions and water molecules were present between the layers of manganese oxide was heated to remove the water molecules. Manganese oxide. The layered manganese oxide is obtained by electrochemically oxidizing a divalent manganese compound in the presence of a cation other than manganese ion, or permanganic acid in the presence of a cation other than manganese ion. The manganese oxide according to claim 1, which is obtained by electrochemically reducing ions. The manganese oxide according to claim 1 or 2, wherein the cation is a cation present in the environment in which the manganese oxide is used. The manganese oxide according to any one of claims 1 to 3, wherein the cation is a sodium ion. An electrode in which a layer containing the manganese oxide according to any one of claims 1 to 4 is adhered to the surface of a conductive base material. The electrode according to claim 5, which is for anticorrosion. A divalent manganese compound is electrochemically oxidized in the presence of a cation other than manganese ion, or a permanganate ion is electrochemically reduced in the presence of a cation other than manganese ion to obtain a layered manganese oxide. The first step to obtain and
A method for producing a manganese oxide, which comprises a second step of heating the layered manganese oxide at 300 ° C. or higher. In the first step, the conductive base material is immersed in an electrolytic solution containing divalent manganese ion or permanganate ion and the cation, and the conductive base material is energized to carry the conductive base material. The method for producing manganese oxide according to claim 7, which is an electrolysis step of forming a precipitation layer containing the layered manganese oxide on the surface of the material. The method for producing a manganese oxide according to claim 7 or 8, wherein the cation is a sodium ion.

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