Chlorine generating electrode

Abstract

To provide an electrode for chlorine generation with high chlorine generation efficiency and longer life under conditions where diluted salt water such as tap water is used at relatively high current density and the polarity of which is switched every short time.SOLUTION: An electrode for chlorine generation includes a substrate made of titanium or titanium base alloy, an intermediate layer comprising a porous platinum coating and titanium oxide on the substrate, and a catalyst layer, in which in terms of metal, specific molar contents of platinum and iridium oxide and tantalum oxide are each in a specific molar ratio. The electrode for chlorine generation is used to electrolyze diluted salt water by switching anode and cathode electrodes under specific conditions.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a chlorine generating electrode useful for generating electrolyzed water with bactericidal properties, for example, when used as an anode in dilute saline water such as tap water. More specifically, the present invention relates to a chlorine generating electrode that has high chlorine generation efficiency and a longer lifespan under conditions of switching polarity at short intervals with a relatively high current density. [Background technology]

[0002] For devices that electrolyze tap water to produce sterilized water, especially those attached to home appliances, there is a strong demand for chlorine generating electrodes that have high chlorine generation efficiency and a longer lifespan, under conditions of relatively high current density and frequent polarity switching, in order to achieve a small size that can be attached to home appliances.

[0003] An electrode for seawater electrolysis has been proposed, comprising a titanium or titanium-based alloy electrode substrate having a titanium oxide layer, a porous platinum coating layer, and an electrode catalyst layer supported on the platinum coating layer, which is a composite of 30-65 mol% iridium oxide, 10-40 mol% tantalum oxide, and 25-60 mol% platinum (see Patent Document 1). While this proposed electrode has the advantages of high chlorine generation efficiency and stability even under low potential conditions during pickling, it has the problem of insufficient lifespan under conditions of relatively high current density and frequent polarity switching.

[0004] [Patent Document 1] Japanese Patent Application Publication No. 08-170187 [Disclosure of the Invention] [Problems that the invention aims to solve]

[0005] In devices that directly electrolyze tap water to produce sterilized water, the electrode distance is often set to 2 mm or less due to the high electrical resistance of tap water. Furthermore, when the electrode distance is 2 mm or less, it is preferable from a maintenance-free perspective to switch the polarity at short intervals to remove scale components generated on the cathode side. In addition, in order to make the above device smaller, there is a strong demand for chlorine generation electrodes that have higher chlorine generation efficiency and longer lifespan under high current density. In other words, the challenge is to develop a chlorine generating electrode that has high chlorine generation efficiency and a longer lifespan when using dilute saline water, such as tap water, with a relatively high current density and switching polarity frequently. [Means for solving the problem]

[0006] As a result of diligent research to achieve the above objectives, the inventors of the present invention have discovered a chlorine generating electrode that exhibits high chlorine generation efficiency and a longer lifespan when a catalyst layer made of platinum, iridium oxide, and tantalum oxide is formed on an intermediate layer using a predetermined mixing ratio (Pt:Ir:Ta = 2~24 mol%:41~49 mol%:35~49 mol%) under conditions of switching the polarity of dilute saline water such as tap water at a relatively high current density at short intervals. This led to the completion of the present invention.

[0007] Thus, according to the present invention, A chlorine generating electrode comprising, in order, a substrate made of titanium or a titanium-based alloy, an intermediate layer, and a catalyst layer, The intermediate layer consists of a porous platinum coating and titanium oxide provided on the substrate. The catalyst layer consists of, in terms of metal equivalent, 2 mol% to 24 mol% platinum, 41 mol% to 49 mol% iridium oxide, and 35 mol% to 49 mol% tantalum oxide. Current density 0.05~0.25A cm -2 The present invention provides a chlorine generating electrode used to generate chlorine by electrolyzing dilute saline solution while repeatedly switching the polarity of the anode and cathode every 5 to 60 seconds. [Effects of the Invention]

[0008] The electrode of the present invention can provide a chlorine generating electrode that has high chlorine generation efficiency and a longer lifespan under conditions of switching polarity at relatively high current densities and short intervals. [Modes for carrying out the invention]

[0009] The present invention relates to a chlorine generating electrode comprising, in order, a substrate made of titanium or a titanium-based alloy, an intermediate layer, and a catalyst layer, wherein the intermediate layer consists of a porous platinum coating and titanium oxide provided on the substrate, and the catalyst layer consists of, in terms of metal equivalent, 2 mol% to 24 mol% platinum, 41 mol% to 49 mol% iridium oxide, and 35 mol% to 49 mol% tantalum oxide, with a current density of 0.05 to 0.25 A·cm². -2 This is a chlorine generating electrode used to generate chlorine by electrolyzing dilute salt water while repeatedly switching the polarity of the anode and cathode every 5 to 60 seconds.

[0010] Polarity switching refers to reversing the polarity of the voltage applied between the two electrodes each time the electrolysis time reaches a predetermined period (e.g., 5 to 60 seconds). When electrolysis is performed intermittently, the polarity of the voltage applied between the two electrodes is reversed each time the cumulative electrolysis time (cumulative electrolysis time) reaches a predetermined period (e.g., 5 to 60 seconds).

[0011] The catalyst layer of the electrode of the present invention consists of 2 mol% to 24 mol% platinum, 41 mol% to 49 mol% iridium oxide, and 35 mol% to 49 mol% tantalum oxide, in terms of metal content.

[0012] Current density 0.05~0.25A cm -2 In an electrode that generates chlorine by electrolyzing dilute saline solution while repeatedly switching the polarity of the anode and cathode every 5 to 60 seconds, durability decreases if the concentrations of iridium oxide and tantalum oxide in the catalyst layer fall below the above range. Conversely, chlorine generation efficiency decreases if the concentrations of iridium oxide and tantalum oxide in the catalyst layer exceed the above range. Here, dilute saline solution refers to water containing a chloride ion concentration of 5 to 100 ppm.

[0013] The catalyst layer is preferably composed of 4 mol% to 23 mol% platinum, 42 mol% to 48 mol% iridium oxide, and 35 mol% to 48 mol% tantalum oxide, in terms of metal equivalent. More preferably, the catalyst layer is composed of 4 mol% to 21 mol% platinum, 42 mol% to 48 mol% iridium oxide, and 37 mol% to 48 mol% tantalum oxide, in terms of metal equivalent.

[0014] The electrodes and their manufacturing methods of the present invention will be described in more detail below, although this is not limited to these descriptions.

[0015] <Base> Examples of substrate materials used in the present invention include titanium or titanium-based alloys. As titanium-based alloys, corrosion-resistant conductive alloys mainly composed of titanium are used. Examples include Ti-based alloys commonly used as electrode materials, consisting of combinations such as Ti-Ta-Nb, Ti-Pd, Ti-Zr, and Ti-Al. These electrode materials can be processed into desired shapes such as plates, perforated plates, rods, or mesh plates and used as base materials.

[0016] <Substrate pretreatment> It is desirable to pre-treat the above-mentioned substrates, as is commonly done in the relevant technical field. Suitable examples of such pre-treatments are described below. First, , the surface of the substrate made of the titanium or titanium-based alloy described above is degreased in accordance with a conventional method, for example, by washing with alcohol, acetone, etc. and / or by electrolysis in an alkaline solution, and then acid-treated with hydrofluoric acid having a hydrogen fluoride concentration of 1 to 20% by weight or a mixed acid of hydrofluoric acid and other acids such as nitric acid and sulfuric acid. By this acid treatment, the oxide film on the substrate surface is removed. Also, by this acid treatment, the crystal grain boundaries on the substrate surface are particularly etched. Although it is preferable that the oxide film on the substrate surface is completely removed, it is not limited to the substrate with the oxide film completely removed as long as it conforms to the object of the present invention. The acid treatment can be carried out at a temperature of room temperature to about 40 °C for several minutes to ten and several minutes according to the surface state of the substrate. In addition, a blasting treatment may be used in combination to sufficiently roughen the surface.

[0017] 〈Titanium hydride treatment〉 Next, the substrate surface is brought into contact with concentrated sulfuric acid (sulfuric acid treatment) to particularly finely roughen the portion other than the crystal grain boundaries on the substrate surface (inside the crystal grains) in a protruding shape and form a thin layer of titanium hydride on the titanium substrate surface.

[0018] Generally, concentrated sulfuric acid having a concentration of 40 to 80% by weight, preferably 50 to 60% by weight, is suitable for use. If necessary, a small amount of sodium sulfate, other sulfates, etc. may be added to this concentrated sulfuric acid for the purpose of stabilizing the treatment. The contact with the concentrated sulfuric acid can usually be carried out by immersing the titanium substrate in a bath of concentrated sulfuric acid. At that time, the bath temperature can generally be set within a range of about 100 to about 150 °C, preferably about 110 to about 130 °C, and the immersion time is usually about 0.5 to about 10 minutes, preferably about 1 to about 3 minutes, which is sufficient.

[0019] By the above sulfuric acid treatment, the portion other than the crystal grain boundaries on the substrate surface (inside the crystal grains) can be finely roughened in a protruding shape, and a very thin film of titanium hydride can be formed on the surface of the substrate. The sulfuric acid-treated titanium substrate is taken out from the sulfuric acid bath and preferably quenched in an inert gas atmosphere such as nitrogen or argon to lower the surface temperature of the titanium substrate to about 60 °C or lower. It is appropriate to use a large amount of cold water for this quenching, which also serves as washing.

[0020] Therefore, the substrate surface of the present invention that is in contact with the intermediate layer may be finely roughened in a protruding manner.

[0021] The substrate, with a very thin titanium hydride coating formed on its surface in this manner, is immersed in dilute hydrofluoric acid or a dilute fluoride aqueous solution (for example, an aqueous solution of sodium fluoride or potassium fluoride) to allow the titanium hydride coating to grow, thereby ensuring uniformity and stabilization of the coating. The concentration of hydrogen fluoride in the dilute hydrofluoric acid or dilute fluoride aqueous solution that can be used here can generally be in the range of 0.05 to 3% by weight, preferably 0.3 to 1% by weight, and the temperature during the immersion treatment with these solutions can generally be in the range of 10 to 40°C, preferably 20 to 30°C. This treatment can be carried out until a uniform titanium hydride coating with a thickness of typically 0.5 to 10 microns, preferably 1 to 3 microns, is formed on the substrate surface. This titanium hydride (TiHy, where y is a number between 1.5 and 2) exhibits a grayish-brown to dark brown color depending on the degree of hydrogenation. Therefore, the formation of titanium hydride coatings with the above-mentioned thickness range can be empirically controlled by the change in the color tone of the substrate surface through lightness contrast with a standard color source.

[0022] <Formation of porous platinum-coated materials> The substrate surface, roughened in this manner and coated with titanium hydride, is then subjected to timely treatments such as washing with water, after which a porous platinum coating is formed on its surface. This porous platinum coating can usually be formed by electroplating. The composition of the plating bath that can be used in this electroplating method is, for example, a platinum compound such as H2PtCl6, (NH4)2PtCl6, K2PtCl6, or Pt(NH3)2(NO2)2, which is mixed in a sulfuric acid solution (pH 1-3) or an aqueous ammonia solution to a concentration of 2-20 g / l, particularly 5-10 g / l, in terms of platinum. Examples include acidic or alkaline plating baths, which are prepared by dissolving the substance and, if necessary, adding small amounts of sodium sulfate (in the case of an acidic bath), sodium sulfite, or sodium sulfate (in the case of an alkaline bath) to stabilize the bath.

[0023] Platinum electroplating using such a plating bath is preferably carried out at a relatively low temperature within a range of about 30 to about 60 °C using a high-speed plating method such as so-called strike plating in order to suppress the decomposition of the titanium hydride film formed on the substrate surface as much as possible. By this electroplating, a porous platinum coating excellent in physical adhesion strength can be formed on the titanium hydride film of the substrate.

[0024] Here, the scale of "porosity" is specified by the "apparent density of the platinum coating". The apparent density of the porous platinum coating is 8 to 19 g / cm 3 Preferably, it is appropriate to be within the range of 12 to 18 g / cm 3 If the apparent density of the porous platinum coating is less than 8 g / cm 3 the bonding strength of platinum decreases and it becomes easy to peel off. On the contrary, if it exceeds 19 g / cm 3 it becomes difficult to stably support platinum and iridium oxide obtained by thermal decomposition described later. The control of the apparent density of the porous platinum coating can be performed by empirically adjusting, for example, the pretreatment conditions of the substrate, the bath composition of the platinum plating bath, and / or the plating conditions (current density, current waveform, etc.). In addition, when it is desired to obtain a more porous platinum coating, after forming the porous platinum coating, the porosity can be further increased by a chemical or electrochemical method.

[0025] In addition, the above platinum electroplating continues until the coating amount of platinum on the above substrate is usually at least 0.2 mg / cm 2 or more. When the coating amount of platinum is less than 0.2 mg / cm 2 oxidation of the titanium hydride film portion tends to proceed too much during the firing treatment described later, and the conductivity tends to decrease. The upper limit of the coating amount of platinum is not particularly limited, but even if it is made more than necessary, no corresponding effect can be obtained, and it becomes uneconomical, so usually a coating amount of 5 mg / cm 2 or less is sufficient. A preferable coating amount of platinum is 1 to 3 mg / cm 2It is within the range. Here, the coating amount of platinum in the porous platinum coating is the amount obtained as follows using the fluorescent X-ray analysis method. That is, as described above, the amount of platinum plating with various thicknesses was quantified by wet analysis method and fluorescent X-ray analysis method on the substrate pretreated as described above, and the analytical values obtained by both methods were plotted on a graph to create a standard calibration curve. Then, the actual sample is subjected to fluorescent X-ray analysis, and the coating amount of platinum is determined from the analytical value and the standard calibration curve. Also, the density (δ (g / cm 3 )) of the platinum coating amount is obtained by δ = w / t from the platinum coating amount (w (g / cm 2 )) obtained as described above and the thickness (t (cm)) of the platinum coating determined by microscopic observation of the cross section of the sample.

[0026] 〈Formation of Titanium Oxide〉 The substrate provided with the porous platinum film is then fired in the air. By this firing, the layer of the titanium hydride film under the platinum coating is thermally decomposed, and the titanium hydride in the layer of the titanium hydride film is substantially returned to almost metal, and further, at least the titanium of the substrate corresponding to the porous part of the platinum coating that is not coated with platinum can be changed to titanium oxide in a low oxidation state.

[0027] The above firing can generally be carried out by heating at a temperature of about 300 to about 600 °C, preferably about 300 to about 400 °C for about 10 minutes to 4 hours. Thereby, a very thin conductive titanium oxide is formed on the substrate surface. The thickness of this titanium oxide is generally preferably in the range of 100 to 1,000 angstroms, preferably 200 to 600 angstroms, and the composition of the titanium oxide is TiOx, and x is generally in the range of 1 < x < 2, particularly 1.9 < x < 2. Alternatively, the substrate with dispersed platinum coating may be directly subjected to the next step without performing the above firing treatment. In this case, during the thermal decomposition treatment in the next step, the layer of the titanium hydride film on the substrate surface is converted into metal and titanium oxide in a low oxidation state. Converted. In this way, a high adhesion strength between the porous platinum coating and the substrate can be maintained, and furthermore, an electrically conductive titanium oxide (passivation film) can be formed to enhance the chemical strength.

[0028] Therefore, the intermediate layer of the present invention preferably consists of a porous platinum coating and titanium oxide provided on a substrate. However, insofar as it is in line with the objectives of the present invention, it is not limited to the complete metallization of titanium hydroxide on the substrate surface beneath the platinum coating, or to the complete conversion of titanium oxide to a low-oxidation state in the titanium of the substrate corresponding to the porous portion of the platinum coating that is not coated with platinum.

[0029] <Formation of the catalyst layer> Next, a solution containing a platinum compound, an iridium compound, and a tantalum compound is applied to a substrate with a porous platinum coating, dried, and then calcined to form a layer composed of platinum, iridium oxide, and tantalum oxide.

[0030] The platinum, iridium, and tantalum compounds used herein are compounds that can decompose under the conditions described below to convert into platinum, iridium oxide, and tantalum oxide, respectively. Examples of platinum compounds include dinitrodiammineplatinum, chloroplatinic acid, and platinum chloride, with chloroplatinic acid being particularly preferred. Examples of iridium compounds include iridium chloride, iridium chloride, and potassium iridium chloride, with iridium chloride being particularly preferred. Examples of tantalum compounds include tantalum chloride and tantalum ethoxide.

[0031] On the other hand, lower alcohols are preferred as solvents for dissolving these platinum compounds, iridium compounds, and tantalum compounds. For example, methanol, ethanol, propanol, butanol, or mixtures thereof are advantageously used. Since dinitrodiammineplatinum does not dissolve directly in lower alcohols, it is preferable to first dissolve it in an aqueous nitric acid solution, adjust the concentration to 250-450 g / l in terms of platinum metal, and then dissolve it in the lower alcohol.

[0032] The total metal concentration of platinum, iridium, and tantalum compounds in a lower alcohol solution can generally be within the range of 20 to 200 g / l, preferably 40 to 150 g / l. If the metal concentration is lower than 20 g / l, the catalyst loading efficiency will be poor, and if it exceeds 200 g / l, the catalyst will be prone to aggregation, leading to problems such as non-uniformity of catalyst activity, loading strength, and loading amount.

[0033] Furthermore, the relative usage ratios of platinum compounds, iridium compounds, and tantalum compounds shall be calculated as follows, converted to metallic Pt, metallic Ir, and metallic Ta, respectively: platinum compounds shall be 2 mol% or more and 24 mol% or less; iridium compounds shall be 41 mol% or more and 49 mol% or less; and tantalum compounds shall be 35 mol% or more and 49 mol% or less of tantalum oxide.

[0034] A solution containing a platinum compound, an iridium compound, and a tantalum compound is applied to a substrate on which a porous platinum coating has been made. The solution is then dried at a temperature in the range of approximately 20 to 150°C and fired in an oxygen-containing gas atmosphere, for example, in air. The firing can be carried out by heating in a suitable heating furnace, such as an electric furnace, gas furnace, or infrared furnace, to a temperature generally in the range of approximately 450 to 650°C, preferably approximately 500 to 600°C. The heating time can be approximately 3 to 30 minutes, depending on the size of the substrate to be fired. This firing process allows for the formation and support of a layer composed of platinum, iridium oxide, and tantalum oxide.

[0035] Then, in a single loading operation, a sufficient amount of a layer consisting of platinum-iridium oxide-tantalum oxide is created. If it is not possible to form and support the material, the above-described steps of coating, drying, and firing the solution can be repeated a desired number of times.

[0036] In the platinum-iridium oxide-tantalum oxide layer (electrode catalyst layer / composite), the proportions of each component, converted to metal Pt, metal Ir, and metal Ta, are 2 mol% to 24 mol% for platinum, 41 mol% to 49 mol% for iridium oxide, and 35 mol% to 49 mol% for tantalum oxide.

[0037] In this way, an electrode composed of a catalyst layer (outer layer), an intermediate layer (porous platinum-coated material - titanium oxide), and a substrate can be manufactured. Specifically, titanium oxide is formed on the surface of the substrate in the porous portion of the platinum-coated material that is not coated with platinum.

[0038] Next, the manufacturing method and characteristics of the electrode of the present invention will be described in more detail with reference to examples. [Examples]

[0039] Examples 1-3, Comparative Examples 1-3 As a substrate, a JIS Class 1 titanium plate material (t0.5mm x 100mm x 100mm) was immersed in acetone and ultrasonically cleaned for 10 minutes to degrease it. Then, it was treated in an 8% by weight hydrofluoric acid aqueous solution at 20°C for 2 minutes, and subsequently treated in a 60% by weight sulfuric acid aqueous solution at 120°C for 3 minutes. Next, the substrate was removed from the sulfuric acid solution and rapidly cooled by spraying it with cold water in a nitrogen atmosphere. It was then immersed in a 0.3 wt% hydrofluoric acid solution at 20°C for 2 minutes and then washed with water.

[0040] After washing with water, dinitrodiammineplatinum was dissolved in sulfuric acid solution to adjust the platinum content to 5 g / L, pH ≈ 2, and temperature to 50°C. In this platinum plating bath, the flow rate was 30 mA / cm². 2 The plating was performed for approximately 6 minutes, resulting in an apparent density of 16 g / cm³. 3 The electrodeposition amount was 1.7 mg / cm². 2 A porous platinum-coated material was formed on the substrate.

[0041] After drying, it was baked in 400°C air for 1 hour.

[0042] Next, a butanol solution of chloroplatinic acid adjusted to a platinum concentration of 70 g / L, a butanol solution of chloroiridium acid adjusted to an iridium concentration of 100 g / L, and a butanol solution of tantalum ethoxide adjusted to a tantalum concentration of 200 g / L were weighed so that the Pt-Ir-Ta composition ratio in terms of metal equivalents was as shown in Table 1. These were then diluted with butanol so that the total concentration obtained by adding the metal equivalent values ​​of each metal component was 75 g / L, and electrode catalyst layer coating solutions were prepared with the metal equivalent composition ratios shown in Table 1. 250 μl of the coating solution was weighed using a pipette and applied to a substrate with a porous platinum coating. The substrate was tilted using tweezers to spread the solution over the entire surface of the substrate, then dried at room temperature and further calcined in air at 530°C for 10 minutes. This coating, drying, and calcination process was repeated four times to prepare electrodes for Examples 1-3 and Comparative Examples 1 and 2.

[0043] The chlorine generation efficiency was evaluated using the electrodes prepared in Examples 1-3 and Comparative Examples 1 and 2, as follows. Tap water (Soka City water) was used as the electrolyte. Electrode distance: 2 mm, flow rate: 0.3 L / min, current density: 0.12 A / cm² 2 Electrolysis was performed using polarity switching control every 30 seconds, and 10 mL of the electrolyzed solution was collected. The free chlorine concentration was measured by the DPD method, and the chlorine generation efficiency was calculated.

[0044] The lifespan of the electrodes prepared in Examples 1-3 and Comparative Examples 1 and 2 was evaluated as follows. Tap water (Soka City water) was used as the electrolyte. Electrode distance: 2 mm, flow rate: 0.3 L / min, current density: 0.12 A / cm² 2 Electrolysis tests were conducted using polarity switching control every 30 seconds. The end of the lifespan was determined when the chlorine generation efficiency fell below 1%.

[0045] The data obtained for Examples 1-3 and Comparative Examples 1 and 2 are shown in Table 1. [Table 1]

[0046] The electrodes in Examples 1-3 had a higher chlorine generation efficiency of 2.8% compared to the electrodes in Comparative Examples 1 and 2, and also had a longer lifespan of 220 hours or more.

[0047] From the above results, it can be seen that an electrode having an electrode catalyst layer in which the intermediate layer consists of a porous platinum coating and titanium oxide provided on the substrate, and the catalyst layer consists of 2 mol% to 24 mol% platinum, 41 mol% to 49 mol% iridium oxide, and 35 mol% to 49 mol% tantalum oxide (in terms of metal equivalent) exhibits good properties.

Claims

[Claim 1] A chlorine generating electrode comprising, in order, a substrate made of titanium or a titanium-based alloy, an intermediate layer, and a catalyst layer, The intermediate layer consists of a porous platinum coating and titanium oxide provided on the substrate. The catalyst layer consists of, in terms of metal equivalent, 4 mol% to 21 mol% platinum, 42 mol% to 48 mol% iridium oxide, and 37 mol% to 48 mol% tantalum oxide. Current density 0.05-0.25A・cm －２ The chlorine generating electrode is used to generate chlorine by electrolyzing dilute saline solution with a chloride ion concentration of 5 to 100 ppm while repeatedly switching the polarity of the anode and cathode every 5 to 60 seconds.

Images