JP2016132813A - Insoluble electrode and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new insoluble electrode suitable for an electrode for electrolysis, an anode for electric anticorrosion, and the like.SOLUTION: An insoluble electrode of the present invention consists of a substrate having electrical conductivity and a coating layer formed on a coating surface which is at least a part of the surface of the substrate. The coating layer according to the present invention contains iron oxide particles made of FeO. The insoluble electrode is insoluble and exhibits an excellent oxygen generating activity and the like. Such coating layer can be efficiently obtained, for example, by heat-treating in the atmosphere an iron plating layer formed using a treatment liquid containing an organic acid. Fe remains in the coating layer at a stage after the heat treatment, but the remaining Fe can be removed by reverse electrodeposition to obtain a coating layer formed of substantially only the iron oxide particles.SELECTED DRAWING: Figure 3C

Description

  The present invention relates to an insoluble electrode that can be used as an anode or the like used for electrolysis of water or the like and cathodic protection, and a method for producing the same.

  Many substances are produced by electrolysis, such as gases such as oxygen, hydrogen or chlorine, metals such as sodium (Na), aluminum (Al) or copper (Cu). Electrolysis is a method in which a voltage is applied to a compound and the compound is decomposed by an electrochemical oxidation-reduction reaction, but depending on the type (material) of the electrode used, its durability, as well as the product and input energy. Yields etc. are also different. For this reason, selection of an electrode material is important when performing electrolysis.

  For example, carbon (C) electrodes that are widely used industrially are inexpensive, but are easily consumed and require periodic replacement. Platinum (Pt) electrodes and gold (Au) electrodes are excellent in durability, but are expensive and have limited use. Therefore, an electrode made of a material different from them is proposed in the following patent document.

JP 2014-80668 A JP 2012-94509 A

Patent Document 1 proposes an insoluble electrode made of Co—Ti—P (—Fe). Patent Document 2 proposes a composite electrode material that is not an insoluble electrode but is used for a negative electrode of a metal-air battery. The composite electrode material, Specifically, diameter: 150 nm, fiber length: the (hollow) surface of the fibrous carbon of 10 to 20 [mu] m, by a solution polymerization method using an organic solvent containing an iron complex _{compound, D 90} In which iron oxide particles (Fe ₃ O ₄ fine particles) having a particle size of 50 nm or less are supported. In addition, the negative electrode for metal-air batteries is obtained by sandwiching a pellet obtained by solidifying the composite electrode material with a binder, and using iron oxide (Fe ₃ O ₄ ) as a negative electrode active material.

In Patent Document 2, in order to support Fe ₃ O ₄ fine particles on the surface of fibrous carbon, complicated and complicated processes such as synthesis, solid-liquid separation, and solid-phase drying are required. Moreover, the composite electrode material obtained by such a manufacturing method is merely a thing in which Fe ₃ O ₄ fine particles are scattered on the surface of fibrous carbon, and the binding force of Fe ₃ O ₄ fine particles to the surface of fibrous carbon is also weak. .

  This invention is made | formed in view of such a situation, and it aims at providing the insoluble electrode which has a new structure different from the conventional electrode etc., and its manufacturing method.

As a result of intensive research and trial and error to solve this problem, the inventor of the present invention heat treated an iron plating film obtained by electrodeposition in an iron salt solution containing an organic acid (ion), thereby forming Fe ₃ It was newly found that a coating layer composed of O ₄ nanoparticles (and Fe) was formed, and this coating layer exhibited high oxygen generation activity. By developing this result, various inventions described below have been completed.

<Insoluble electrode>
(1) The insoluble electrode of the present invention is an insoluble electrode comprising a conductive substrate and a coating layer formed on a coating surface which is at least a part of the surface of the substrate, and the coating layer Includes iron oxide particles made of Fe ₃ O ₄ .

(2) The insoluble electrode of the present invention is excellent in conductivity (low resistivity), insolubility (durability or corrosion resistance), etc., and can exhibit a high catalytic function (function of reducing activation energy). The insoluble electrode of the present invention is inexpensive because it is made of iron oxide and / or iron, and can be used for various industrial products. For example, when an anode comprising an insoluble electrode of the present invention is used to electrolyze water or an aqueous solution, a large oxygen generation current (density) can be obtained, and oxygen can be produced at low cost and high efficiency.

The reason why the insoluble electrode of the present invention is highly conductive (highly active) and insoluble is not necessarily clear, but is considered as follows. Fe ₃ O ₄ comprises a Fe ²⁺ and Fe ^3+, between Fe ²⁺ and Fe ^3+, the charge between valence moves, exhibit high conductivity. The Fe ₃ O ₄ particles corrosion is high because of the close contact with the dense substrate.

<Method for producing insoluble electrode>
(1) There are various methods for producing an insoluble electrode. For example, the following production method of the present invention is suitable. That is, a base material having conductivity is immersed in a treatment solution containing iron ions and organic acid or organic acid ions, and the base material contains iron by energizing the base material. It is characterized by comprising an electrodeposition step for forming a deposited layer, and a heat treatment step for heating the substrate after the electrodeposition step to produce Fe ₃ O ₄ in the deposited layer, whereby the coating layer described above is obtained. This is a method for producing an insoluble electrode.

(2) According to the production method of the present invention, an insoluble electrode in which almost the entire surface of the substrate (coated surface) is coated with the coating layer can be efficiently obtained at low cost. The reason for this is not necessarily clear, but when the organic acid (ion) taken into the deposited layer during the electrodeposition step is decomposed or released in the heat treatment step, fine Fe ₃ O ₄ particles are generated. It is considered that the coating layer according to the present invention was formed.

<Others>
(1) “Insoluble” as used herein means that the coating layer does not substantially elute during use of the electrode. It is not easy to specifically indicate the degree, but when the insoluble electrode of the present invention is used for an electrolysis anode or an anticorrosion anode, it is sufficient that the dissolution rate is not more than a desired value. .

Also, it is difficult to specify “conductivity”, “oxygen generation activity”, etc. of the insoluble electrode of the present invention in general. For example, when 1.5 V is applied to the reference electrode (saturated silver chloride electrode: SSE) The oxygen generation current density is preferably 5 × 10 ⁻⁴ A / cm ² or more, more preferably 1 × 10 ⁻³ A / cm ² or more.

(2) The insoluble electrodes (including insoluble electrodes) of the present invention are inevitable difficult to remove due to modifying elements that can further improve electrode characteristics (insoluble, conductive, oxygen generation activity, etc.) and cost or technical reasons. Impurity elements may be included.

(3) The present invention can be grasped not only as an insoluble electrode having a coating layer formed on the surface of a substrate, but also as an insoluble electrode film or an insoluble catalyst comprising the coating layer itself.

(4) Unless otherwise specified, “x to y” in the present specification includes the lower limit value x and the upper limit value y. Furthermore, a new arbitrary numerical range “ab” can be configured by appropriately combining numerical values described in the present specification and arbitrary numerical values included in “x to y” thereof.

3 is a graph showing a polarization curve according to Sample 1. 5 is a graph showing a polarization curve according to Sample 2. 5 is a graph showing a polarization curve related to Sample 3. 6 is a graph showing a polarization curve according to Sample 4. 10 is a graph showing a polarization curve according to Sample 5. 5 is a graph showing a polarization curve related to Sample 6. 10 is a graph showing a polarization curve according to Sample 7. It is a graph which shows the polarization curve concerning sample C1. It is the XRD profile which observed the Ti substrate surface before a coating process. It is the XRD profile which observed the coating surface after the electrodeposition process which concerns on the sample 3. FIG. It is the XRD profile which observed the surface after the heat processing process. It is the XRD profile which observed the surface after the polarization test. It is the SEM image which observed the Ti substrate surface before a coating process. 3 is an SEM image obtained by observing a coating surface after an electrodeposition process according to Sample 3. It is the SEM image which observed the surface after the heat treatment process. It is the SEM image which observed the surface after the polarization test. 2 is a TEM image obtained by observing the coating surface after an electrodeposition process and a heat treatment process according to Sample 1. FIG. 4 is a TEM image obtained by observing a cross section of a coating layer after a polarization test according to Sample 3. FIG. It is SAED which observed a part of the cross section.

  The present invention will be described in more detail with reference to embodiments of the invention. The contents described in this specification can be applied not only to the insoluble electrode according to the present invention but also to the manufacturing method thereof. One or two or more components arbitrarily selected from the present specification can be added to the above-described components of the present invention. If understood as a product-by-process claim, the content relating to the manufacturing method and the like can be a constituent element relating to the insoluble electrode and the like. Which embodiment is the best depends on the target, required performance, and the like.

"Base material"
The base material of the insoluble electrode may be any shape or size. The material may be any material as long as it has conductivity and is suitable for an insoluble electrode. For example, it may be a metal, or a non-metal such as carbon, ceramics, or resin. However, the base material is preferably made of a valve metal that forms an oxide film on the surface by anodic oxidation (anodic oxidation) and allows an electric current (anode current) to flow only in one direction. Specifically, the substrate is preferably made of a pure metal or alloy such as titanium, aluminum, zinc, chromium (including stainless steel), zirconium, hafnium, niobium, tantalum, tungsten, antimony, bismuth, and the like. In particular, the base material is preferably a pure metal or alloy such as titanium or aluminum, stainless steel, or the like, which forms a stable passive film (metal oxide) on the surface and exhibits excellent corrosion resistance.

<Coating layer>
(1) The coating layer according to the present invention is a single layer substantially composed of iron oxide particles (Fe ₃ O ₄ particles) even in a composite layer in which iron oxide particles are dispersed in a matrix such as iron (α-Fe). But you can. Incidentally, even when the coating layer is a composite layer, α-Fe or the like constituting the matrix is eluted during use of the insoluble electrode, and a single layer in which only the skeleton mainly composed of Fe ₃ O ₄ particles remains can be obtained. If the coating layer is a single layer of substantially Fe ₃ O ₄ particles, it may be a porous layer having fine pores of the Fe ₃ O ₄ particles and skeleton. In addition to α-Fe and Fe ₃ O ₄ , the coating layer may contain various elements (C, H, O, S, etc.) alone or as a compound. These elements can be introduced into the coating layer via the treatment liquid used during production. In particular, as will be described later, when a treatment liquid containing an organic acid is used, C may remain as fine Fe ₃ C or the like in the coating layer.

(2) The coating layer preferably contains 50% by volume or more, 60% by volume or more, 70% by volume or more, and further 80% by volume or more of iron oxide particles, with the entire coating layer being 100% by volume. When the volume ratio of the iron oxide particles is too small, the catalyst activity (oxygen generation activity) and the like can be reduced. This point is the same whether it is composed of a composite layer in which iron oxide particles are dispersed in a matrix or when the coating layer is mainly composed of a single layer of iron oxide particles. In addition, when there exists a fine hole in a coating layer, the volume (bulk volume) of the whole coating layer including the hole shall be 100 volume%.

By the way, it is not easy to directly specify the volume ratio of fine iron oxide particles dispersed in the coating layer. Therefore, in this specification, the area ratio of the iron oxide particles obtained as follows is used as the volume ratio of the iron oxide particles in the coating layer for convenience. First, the cross section in the thickness direction of the coating layer is observed with a transmission electron microscope (TEM) with five visual fields having different positions in the thickness direction. The TEM image obtained for each field of view is subjected to image processing, and the ratio of the total area of Fe ₃ O ₄ particles to the total area of the observation region (cross-sectional area ratio) is determined for each field of view. And the arithmetic mean value of the cross-sectional area ratio obtained for every visual field is calculated. The average cross-sectional area ratio obtained in this way is used as the volume ratio of the iron oxide particles referred to in this specification.

(3) The coating layer covers 70% or more, 80% or more, 90% or more, or 95% or more of the entire surface (coating surface) of the substrate on which the coating layer is formed as 100%. It is preferable. If the area ratio of the iron oxide particles is too small, the surface area of the iron oxide particles and hence the electrode may be reduced, and the catalytic activity (oxygen generation activity) and the like may be reduced. This is the same whether the coating layer is made of a single layer of iron oxide particles or a composite layer in which iron oxide particles are dispersed in a matrix. The coating layer according to the present invention is not a state in which the iron oxide particles are individually bound and supported on the surface of the substrate, but a film shape (porous film shape) covering at least a part of the surface of the substrate. Therefore, the coverage is high.

  The area ratio here is specified as follows. First, the surface (electrode surface) of the coating layer is observed with a scanning electron microscope (SEM) in five different fields of view. The SEM image of each field of view magnified 7500 times is image-processed, and the ratio (area ratio) of the total area occupied by other than the base material (iron oxide particles, etc.) with respect to the total area of the observation region for each field of view Ask for. The arithmetic mean value of the area ratio obtained for each visual field is calculated. The average area ratio obtained in this way is referred to as the coverage in this specification.

(4) Each iron oxide particle constituting the coating layer may be single crystal or polycrystalline, but in any case, it is considered that it is made of Fe ₃ O ₄ crystalline. Therefore, the coating layer preferably has three or more peaks indicating Fe ₃ O ₄ when analyzed by X-ray diffraction (XRD). Furthermore, considering the case where the coating layer contains iron (α-Fe) other than iron oxide particles (that is, the coating layer is a composite layer), three or more, and even four or more Fe on the XRD profile. _It can be said that the peak indicating ₃ O ₄ is preferably larger than the strongest peak indicating α-Fe.

(5) The finer the iron oxide particles, the better. For example, it is preferable that the average particle diameter of the iron oxide particles is 300 nm or less, 200 nm or less, and further 100 nm or less. If the area ratio of the iron oxide particles is excessive, the surface area of the iron oxide particles and thus the electrode may decrease, and the catalytic activity (oxygen generation activity) and the like may decrease. In addition, the particle diameter of the iron oxide particle as used in this specification is substantially synonymous with the crystal particle diameter.

  The average particle diameter of the iron oxide particles is specified by the intercept method (cutting method). Specifically, first, five fields of view on the surface of the coating layer (electrode surface) are observed with a TEM. On the TEM image of each field of view, three straight lines (total length L = 1710 nm) with an interval of 100 nm are drawn for each of the vertical direction and the horizontal direction. The number of particles (n) crossing each straight line is obtained, and the average intercept length (l) for each straight line is obtained. An average value per one visual field (average value of six straight lines) of the average intercept length is obtained, and further, an average value per five visual fields is obtained. The arithmetic average value thus obtained (the average value of the average intercept length of 5 × 3 × 2 straight lines) is defined as the average particle diameter of the iron oxide particles referred to in this specification.

(6) Even if the coating layer is thin, it can exhibit excellent electrode characteristics. For example, a thickness of about 10 to 1000 nm or about 50 to 300 nm is sufficient. An underlayer (or intermediate layer) that improves adhesion may be provided between the coating layer and the substrate surface. Further, the coating layer may have an inclined structure in which the composition and the structure are continuously changed in the depth direction or the thickness direction, or may have a multilayer structure in which the composition is changed discontinuously.

"Production method"
(1) Electrodeposition step The electrodeposition step according to the present invention is a step of forming a deposited layer containing iron on the coated surface of the substrate in a treatment liquid containing iron ions and organic acids or organic acid ions. This deposited layer is a so-called iron plating layer, but is not a plating layer made of mere pure iron (α-Fe), and contains at least an organic acid or an organic acid ion.

  The temperature (bath temperature) of the treatment solution (plating solution) during the electrodeposition process may be room temperature or warm. When the temperature is warm, the electrodeposition process is promoted. However, when the temperature of the processing liquid becomes high, the organic acid is decomposed or volatilized, and thus the management of the processing liquid becomes difficult. Therefore, the temperature of the treatment liquid is preferably 100 ° C. or lower, 80 ° C. or lower, 60 ° C. or lower, and further normal temperature range (40 ° C. or lower).

The organic acid contained in the treatment liquid, an organic acid ion (R-COO ^-, etc.) or an organic acid salt (. Them together, as appropriate, simply referred to as "organic acid") is, Fe in deposit (iron plating layer) Any kind can be used as long as it contributes to the precipitation of ₃ O ₄ . The organic acid according to the present invention is usually a water-soluble carboxylic acid having one or more carboxy groups (such as R-COOH). As such an organic acid, for example, one or more of L-ascorbic acid, citric acid, and fumaric acid are preferably used.

The concentration of the organic acid is preferably 1 to 100 mmol / L, more preferably 10 to 50 mmol / L, for example. If the concentration of the organic acid is too low, the amount of Fe ₃ O ₄ deposited will be too low. If the concentration of the organic acid is too high, the pH of the treatment solution will be too low and a large amount of H ₂ will be generated, resulting in poor current efficiency. .

The iron ions in the treatment liquid can be supplied from an iron salt such as iron sulfate or iron nitrate. The concentration of iron ions (particularly Fe ²⁺ ) is preferably 0.1 to 5 mol / L, 0.3 to 1.5 mol / L, and more preferably 0.5 to 1 mol / L. In other words, iron ions are preferably 5 to 250 g / L, 15 to 75 g / L, and more preferably 25 to 50 g / L. If the iron ion concentration is too low, it is difficult to efficiently form a deposited layer, and a large amount of H ₂ adheres to the surface of the cathode during the electrodeposition process, resulting in a decrease in current efficiency. If the iron ion concentration is excessive, it is uneconomical, and the concentration of the organic acid is relatively lowered, and the amount of Fe ₃ O ₄ deposited tends to be excessive. The molar concentration ratio of iron ions to organic acids (ions) is preferably 5 to 100, more preferably 10 to 50.

(2) Heat treatment step The heat treatment step is a step of heating the precipitate layer to generate Fe ₃ O ₄ in the precipitate layer. For example, the heating temperature is preferably 250 to 500 ° C, more preferably 300 to 400 ° C. Precipitation of fine Fe ₃ O ₄ becomes difficult when the heating temperature is too low, and precipitation of other iron oxides (FeO, Fe ₂ O ₃ ) and the like occur without precipitation of Fe ₃ O ₄ when the heating temperature is too high. become.

The heating time is preferably 10 seconds to 10 hours, 10 minutes to 3 hours, and more preferably 30 minutes to 2 hours. If the heating time is too short, the precipitation of Fe ₃ O ₄ becomes insufficient, and if the heating time is too long, it is uneconomical. The heating atmosphere may be an inert gas (Ar gas, N ₂ gas, etc.) atmosphere, but it is sufficient in the air.

(3) Electrolytic process The deposited layer after the heat treatment process is a composite layer in which iron oxide particles are dispersed in a matrix mainly composed of α-Fe. The coating layer according to the present invention may be the composite layer, or may be a layer obtained by previously removing α-Fe constituting the matrix from the composite layer. When the substrate coated with the composite layer is energized as an anode, α-Fe can be eluted from the composite layer and the matrix can be easily lost.

  Therefore, in the method for producing an insoluble electrode of the present invention, the residual iron that has not become iron oxide particles is further removed from the deposited layer after the heat treatment step by immersing the base material after the heat treatment step in the electrolytic solution as an anode and energizing. It is preferable to provide an electrolysis step for discharging.

<Application>
The insoluble electrode of the present invention is not limited in its use, such as an electrode for electrolysis of a compound such as water (particularly an anode electrode), an electrode for a stack type battery, an anode electrode for cathodic protection of various infrastructures such as electric wires and railways, etc. Various uses are possible. For example, when water or an aqueous solution is electrolyzed using the insoluble electrode of the present invention, hydrogen and oxygen expected as clean energy can be efficiently produced at a lower cost than before. In addition, when the insoluble electrode of the present invention is used as a sacrificial electrode, the replacement period is remarkably extended, and the management cost of various infrastructures can be reduced. Specifically, the insoluble electrode of the present invention is suitable for an anode for cathodic protection such as a steel material in concrete, a cathode for a Li ion battery, and the like.

  A sample (electrode) having a coating layer formed on the surface of a substrate (base material) was manufactured, and its characteristics (oxygen generation activity) were clarified by a polarization test. Moreover, the structure of the coating layer was clarified by observing the electrode surface with XRD, SEM and TEM. The present invention will be described more specifically by showing these contents.

<Production of sample>
(1) Electrodeposition process A rectangular substrate (hereinafter referred to as “Ti substrate”) made of pure titanium (15 mm × 60 mm × 0.2 mm) was prepared. The surface of the Ti substrate was previously degreased with acetone and then roughened with an aqueous HF solution and concentrated sulfuric acid (roughening treatment step).

  For each treatment solution shown in Table 1, the Ti substrate was immersed as a cathode and a pure iron plate as a sacrificial anode, and a current was passed between the electrodes to form a deposited layer on the surface of the Ti substrate. The temperature of the treatment liquid (bath temperature), energization amount (current density), and energization time (deposition time) are also shown in Table 1. Sample C1 is a case where the treatment liquid does not contain an organic acid.

(2) Heat treatment step Each Ti substrate after the electrodeposition step that was naturally dried was heated in an air atmosphere at 350 ° C for 1 hour using an electric furnace.

(3) Electrolytic process Each Ti substrate after the heat treatment process was immersed in a NaCl aqueous solution (concentration: 3 mass%) using an anode and a platinum plate (50 mm x 50 mm) as a cathode, and electricity was passed between these electrodes. This energization was carried out for 10 minutes while maintaining the distance between the electrodes at 7 cm and setting the voltage difference between the electrodes to 2V. Thus, an electrode of each sample having a coating layer formed on the surface of the Ti substrate was obtained.

《Polarization test》
(1) The electrode of each sample subjected to the electrolysis process was immersed in an aqueous NaCl solution (concentration: 3 mass%) as an anode and a saturated silver chloride electrode as a counter electrode (cathode / SSE), and a polarization test was performed. At this time, no deaeration treatment was performed. The polarization test was performed at room temperature, and the potential sweep rate was 20 mV / min. The (anode) polarization curves for the samples thus obtained are shown in FIGS. 1A to 1H (these are simply referred to as “FIG. 1”). In addition, in each figure, the polarization curve concerning the Ti substrate which performed only the roughening process mentioned above is also shown for reference.

(2) As is apparent from FIG. 1, all of the electrodes (samples 1 to 7) having a coating layer formed using a treatment liquid containing an organic acid are electrodes made of a Ti substrate without a coating layer (this The electrode was appropriately referred to as sample C0), and the current density at a potential of 1.5 V was improved to about 100 times, and it was found that the electrode showed very excellent oxygen generation activity.

  Each electrode of Samples 1 to 7 has the same polarization curve every time even when the above polarization test is repeated, and does not elute even when a high potential is applied, and exhibits excellent corrosion resistance (insoluble). Was also confirmed. During the polarization test, oxygen was generated from the surface of each electrode of Samples 1-7. Therefore, it can be said that the electrodes such as Samples 1 to 7 are suitable for insoluble electrodes used for electrolysis and anticorrosion.

  On the other hand, the electrode of sample C1 in which the coating layer was formed using the treatment liquid not containing an organic acid had a polarization curve almost the same as the electrode of sample C0. In the electrode of the sample C1, the coating layer (deposited layer) formed on the surface of the Ti substrate was eluted in the above-described electrolysis process, and was almost completely lost.

<< X-ray diffraction >>
(1) For the electrode of Sample 3, the surface of the Ti substrate before treatment, the surface after the electrodeposition step, the surface after the heat treatment step, and the surface after the polarization test were analyzed by XRD. The results are shown in FIGS. 2A to 2D (they are simply referred to as “FIG. 2”).

(2) First, as is clear from FIG. 2B, it can be seen that the surface of the Ti substrate is mainly coated with α-Fe by the electrodeposition process. That is, it can be seen that the plating layer (deposition layer) formed on the surface of the Ti substrate after the electrodeposition step is made of α-Fe (crystal).

Next, as apparent from FIG. 2C, it was also found that the plating layer made of α-Fe becomes the first coating layer made of α-Fe (residual iron) and Fe ₃ O ₄ after the heat treatment step. Further, as is clear from FIG. 2D, it was also found that the first coating layer became a second coating layer composed of Fe ₃ O ₄ from which α-Fe had disappeared after the polarization test (same after the electrolysis step). .

<< SEM >>
(1) About the electrode of the sample 3, the surface of the Ti substrate before a process, the surface after an electrodeposition process, the surface after a heat treatment process, and the surface after a polarization test were each observed by SEM. The states are shown in FIGS. 3A to 3D (these are simply referred to as “FIG. 3”). In addition, the lower SEM image of each figure expands a part of upper SEM image.

(2) First, as is clear from FIG. 3A, the surface of the Ti substrate before the treatment is formed into a fine and complicated uneven shape by the above-described roughening treatment, and due to the increase in the surface area and the anchoring effect, the precipitation layer It can be seen that the adhesion can be improved.

Next, as apparent from FIG. 3B, the surface after the electrodeposition process is found to be in a state where the entire surface of the Ti substrate is uniformly coated with a plating layer of α-Fe (see FIG. 2B). . Further, as apparent from FIG. 3C, the surface after the electrolysis process was smooth, but almost the entire surface of the Ti substrate was covered with Fe ₃ O ₄ (see FIG. 2C) grains after the heat treatment process. I found out that it was in a state.

As apparent from FIG. 3D, the surface after the polarization test exists deeply so that Fe ₃ O ₄ (see FIG. 2D) grains overlap with each other, and the Ti substrate is formed by these Fe ₃ O ₄ grains. It can be seen more clearly than the surface after the heat treatment step that the entire surface is covered. This is probably because iron (matrix) remaining on the surface after the heat treatment step was removed by the polarization test.

  Note that the coverage of the sample 3 was calculated by the method described above from the SEM image shown in FIG. 3D. The results are shown in Table 1 together with the coverage of other samples.

<< TEM (surface) >>
About the electrode of the sample 1, the surface after an electrodeposition process and the surface after a heat treatment process were observed by TEM, respectively. This is shown in FIG. As is clear from FIG. 4, the surface after the electrodeposition process was in a substantially uniform state where the entire surface was made of α-Fe (see FIG. 2B), but the surface after the heat treatment process was about several nm. It was found that a large number of Fe ₃ O ₄ particles of about several to several tens of nanometers were dispersed in the granular α-Fe. From the TEM image shown in FIG. 4, the average particle diameter of Fe ₃ O _{4 according} to Sample 1 was calculated by the method described above. The results are shown in Table 1 together with the coverage of other samples.

<< TEM (cross section) >>
Regarding the electrode of Sample 3, the cross section of the coating layer after the polarization test was observed with TEM. This is shown in FIG. The lower TEM image in FIG. 5 is an enlarged part of the upper TEM image. As is apparent from FIG. 5, the coating layer was formed of a substantially porous Fe ₃ O ₄ layer, and no Fe remained. As shown in FIG. 6, this is also confirmed from the result of analyzing a limited field electron diffraction pattern (SAED) obtained by observing the region in the TEM image shown in the lower part of FIG. That is, the main diffraction spot of SAED was Fe ₃ O ₄ (cubic structure with lattice constant a = 0.84 nm). From the TEM image shown in FIG. 5, the volume ratio of Fe ₃ O _{4 according} to the sample 3 was calculated by the method described above. The results are shown in Table 1 together with the volume ratio of other samples.

Claims (9)

A conductive substrate;
A coating layer formed on a coating surface which is a surface of at least a part of the substrate;
An insoluble electrode consisting of
The insoluble electrode, wherein the coating layer includes iron oxide particles made of Fe ₃ O ₄ .   The insoluble electrode according to claim 1, wherein the coating layer covers 70% or more of the entire area of the coated surface.   The insoluble electrode according to claim 1, wherein the iron oxide particles have an average particle size of 300 nm or less.   The insoluble electrode according to any one of claims 1 to 3, wherein the coating layer contains 50% by volume or more of the iron oxide particles with the entire coating layer being 100% by volume.   The insoluble electrode according to claim 1, wherein the base material is made of metal.   The insoluble electrode according to claim 1, which is an anode used for electrolysis of water or an aqueous solution.   The insoluble electrode according to any one of claims 1 to 5, which is an anode for cathodic protection. A deposited layer containing iron on at least a part of the surface of the substrate by immersing a conductive substrate in a treatment solution containing iron ions and organic acid or organic acid ions and energizing the substrate. An electrodeposition process to form
A heat treatment step of heating the substrate after the electrodeposition step to generate Fe ₃ O ₄ in the deposited layer,
A method for producing an insoluble electrode, wherein the coating layer according to claim 1 is obtained.   The insoluble electrode according to claim 8, further comprising an electrolysis step of discharging iron remaining from the deposited layer by immersing the base material after the heat treatment step in an electrolytic solution and energizing the base material as an anode. Production method.