JP5309813B2 - Oxygen generating electrode - Google Patents

Description

The present invention relates to an oxygen generating electrode that is used as an anode for electrolysis of an aqueous solution containing chlorine ions including seawater and generates oxygen while suppressing chlorine generation.

The electrolysis of seawater is usually performed to generate hydrogen and sodium hydroxide at the cathode, generate chlorine at the anode, and generate sodium hypochlorite from the generated sodium hydroxide and chlorine. As an anode used for this electrolysis, an electrode obtained by coating titanium, which is a corrosion-resistant metal, with an oxide of a platinum group metal is used as a high-performance electrode.

On the other hand, as with normal water electrolysis, in seawater electrolysis, which aims to obtain hydrogen and oxygen from seawater, hydrogen is generated at the cathode and chlorine is not generated at the anode. Therefore, a special anode that meets the purpose is required.

The inventors of the present invention previously applied a solution in which a salt of a specific metal is dissolved in a solvent, applied a conductive electrode substrate, dried, and then heated in the atmosphere to decompose the salt and convert it into an oxide. After repeating, heat treatment is performed to produce an electrode having an oxide adhered to the substrate as an active material, and when this is used as an anode for electrolyzing saline, it exhibits high activity against oxygen generation. It has been found that it is inert to chlorine generation (Patent Document 1). The oxygen generating electrode disclosed in Patent Document 1 has the following two modes in the oxide which is an electrode active material.
(1) An oxide comprising 0.2 to 20 mol% of Mo and W as a cation, and 0.2% by mole of Mn. (2) As a cation, 1 or 2 of Mo and W is 0. An oxide having an effective surface area increased by forming an oxide comprising 2 to 20 mol% and 1 to 30% of Zn and the balance being Mn, and then leaching Zn in a concentrated alkaline solution at a high temperature to elute Zn

In the invention of the electrode for oxygen generation described above, in the baking subsequent to the application of the metal salt, Mn is oxidized to trivalent to Mn ₂ O ₃ , and if Mn ₂ O ₃ contains Mo or W, oxygen is further added. It is based on the knowledge that the generation efficiency is improved. In the production of the electrode by the firing method, the crystal does not grow sufficiently when the firing temperature is low, and therefore the stability of the electrode is inferior. On the other hand, when firing at a high temperature, higher-order oxides decompose, Mn cannot be oxidized more than trivalent.

However, since higher order Mn oxides could be expected to show higher activity, the inventors tried a method of anodic deposition of oxides on the electrode substrate from a metal salt solution instead of the firing method. It was. As a result, when the electrode active material contains tetravalent Mn and is applied, an electrode capable of generating oxygen without generating chlorine when an aqueous solution containing sodium chloride such as seawater is electrolyzed can be manufactured. This was also disclosed (Patent Document 2) because it was confirmed that this electrode showed higher activity. What is disclosed in Patent Document 2 is that "a conductive electrode substrate is coated by an anodic deposition method with an oxide composed of 0.2 to 20 mol% of Mo and W and the balance Mn. Electrode for generating oxygen for seawater electrolysis ”, which is characterized by the use of an anodic deposition method.

As a result of related research, an electrolysis apparatus using this electrode as an anode and an ion exchange membrane as an electrolyte, an electrode assembly combining this electrode and a diode, and a method for manufacturing an anode have been developed (Patent Document 3). Patent Document 4 and Patent Document 5). Further improvement of the electrode was attempted, and the electrode using the double oxide obtained by adding Fe to Mn-Mo, Mn-W or Mn-Mo-W contains chlorine ions over a wide temperature range including high temperature up to just below the boiling point. It was found useful as an electrode for oxygen generation in a solution (Patent Document 6). In addition, an improved electrode manufacturing method has been proposed, including a method for preparing a Ti substrate in which a double oxide is electrodeposited by anodic deposition (Patent Document 7).

The inventors who have further improved the electrode for oxygen generation, as a result of the double oxide of Sn added to Mn-Mo, Mn-W or Mn-Mo-W generated oxygen in the chloride solution. It was found that even a strongly acidic solution has excellent oxygen generation efficiency and durability, and subsequently applied for a patent (Patent Document 8). The electrode for oxygen generation in Patent Document 8 is “on an electroconductive electrode substrate, in atomic percent of cations, 0.1 to 3 mol% of Sn, and Mo and / or W in a total amount with Sn. An electrode that occupies 0.2 to 20 mol% and the balance of Mn is produced by anodic deposition.
JP-A-9-256181 JP-A-10-287991 JP-A-11-256383 JP-A-11-256384 JP-A-11-256385 JP 2003-129267 A JP2007-138254 JP2007-302925

In general, the rate of product formation by electrolytic reaction is proportional to the current density, so that a predetermined current density must be achieved in order to achieve the desired reaction rate. There is a possibility that the overvoltage rises and the voltage between the anode and the cathode is increased. Needless to say, the lower the voltage between the anode and cathode, the lower the energy consumption and the higher the performance of the electrolysis system. The same applies to the electrolysis of an aqueous chloride solution using an oxygen generating anode, and it is desirable that the anode overvoltage at a predetermined current density is low. In view of reducing the electrode overvoltage in the electrode using the double oxide disclosed in Patent Document 8 as an active material, it is necessary to improve the conductivity of the double oxide. As a result of efforts to improve the substance, it was found that the addition of Sb is effective in improving the conductivity.

The purpose of the present invention is to utilize the latest knowledge obtained by the above-mentioned inventors and to generate oxygen without generating chlorine by using it for electrolysis of an aqueous solution containing chlorine, such as electrolysis of seawater. In addition, an electrode for oxygen generation that can perform electrolysis at a predetermined current density while keeping the voltage between the anode and the cathode low, resulting in low energy consumption and high performance electrolysis system can be provided. There is to do.

In the oxygen generating electrode of the present invention, the surface of the conductive metal substrate is coated with a metal double oxide layer that becomes the inner electrode layer (A), and a metal double oxide layer that becomes the outer electrode layer (B) is formed thereon. A coated oxygen generating electrode,
(A) The metal constituting the metal double oxide serving as the inner layer of the electrode is one or two of Sn, Sb, Mo and W, and Mn, and the composition is atomic% relative to the total cation, [Sn + Sb + Mo And one or two of W]: 0.2 to 20%, of which Sn: 0.1 to 3%, Sb: 0.01 to 1.3%, and Mn: the balance,
(B) The metal constituting the metal double oxide serving as the outer layer of the electrode is Sn, Mo and / or W and Mn, and the composition is atomic% relative to the total cation, [1 type of Sn + Mo and W Or 2 types]: 0.2 to 20%, of which Sn: 0.1 to 3%, Mn: the balance,
An oxygen generating electrode for generating oxygen by electrolyzing an aqueous solution containing chlorine ions, wherein both the inner and outer metal double oxide layers are formed by anodic deposition.

Table 1 shows the elements constituting the electrode active material of the oxygen generating electrode of the present invention and the contents thereof.
Table 1

According to the present invention, an (Mo, W) -Sn-Sb-O-based electrode inner layer and a (Mo, W) -Sn-O-based electrode outer layer are both formed on the conductive electrode substrate by anodic deposition. The electrode for oxygen generation produced by providing it has low oxygen overvoltage in electrolysis of an aqueous solution containing acidic chlorine ions, and can achieve high oxygen generation efficiency. That is, the electrode of the present invention is an energy-saving high-performance electrode as an oxygen generation electrode.

A typical embodiment of the method for producing the electrode of the present invention will be described below. In electrolysis in a chlorine ion-containing aqueous solution such as seawater, Ti or an alloy thereof having high corrosion resistance is suitable as a material for an electrode conductor that generates oxygen without generating chlorine. However, in an electrode in which Ti is directly coated with an electrode active material, an insulating film made of TiO ₂ is formed between the electrode active material and titanium in the process of using the electrode, and the electrode becomes unusable in a short time. In order to avoid this, Ti is coated with, for example, IrO ₂ having the same crystal structure as that of TiO _2, and this is used as an electrode substrate which is a conductor.

The deposition of the double oxide constituting the electrode inner layer (A) is performed by adding sulfuric acid to a solution containing one or two of Na ₂ WO ₄ and Na ₂ MoO ₄ together with a predetermined amount of MnSO ₄ , SnCl ₄ and SbCl _5. In addition, the pH is adjusted to a predetermined value, and this is heated to obtain an electrolytic solution, and electrolysis is performed using the above electrode substrate as an anode. Thereby, (Mo, W) -Mn-Sn-Sb-O oxide is obtained. Subsequently, anodic deposition of the double oxide constituting the outer electrode layer (B) is performed by adding sulfuric acid to a solution containing one or two of Na ₂ WO ₄ and Na ₂ MoO ₄ together with a predetermined amount of MnSO ₄ and SnCl _4. Is added to adjust to a predetermined pH, and this is heated to obtain an electrolytic solution, and the intermediate product having the electrode inner layer obtained as described above is used as an anode for electrolysis. Accordingly, when the (Mn, Mo) -Sn-O oxide is obtained, the electrode of the present invention is completed.

Next, the action of each component and the reason for limitation of the composition will be described for constituting the oxygen generating electrode of the present invention.
Mn is a basic component of the electrode of the present invention, and forms MnO ₂ necessary for generating oxygen upon electrolysis of an aqueous solution containing chlorine ions such as seawater.

Mo and W do not themselves generate an oxide exhibiting sufficiently high oxygen generation activity, but coexist with MnO ₂ to suppress generation of chlorine and improve the efficiency of oxygen generation, MnO ₂ has a function of preventing the dissolving is oxidized to permanganate. However, if Mo and W are added excessively, the oxygen generation efficiency is rather lowered. Therefore, in the double oxide constituting the electrode inner layer (A), the addition of one or two of Mo and W is limited to 0.2 to 20 mol% in the cation in total with Sn and Sb. There is a need to. Similarly, in the double oxide constituting the outer electrode layer (B), the addition of one or two of Mo and W is within the range of 0.2 to 20 mol% in the cation in total with Sn. Should be.

Sn is an element that improves the activity and durability of oxygen generation by forming a double oxide with Mn, Mo, and W. However, if excessively added, the oxygen generation efficiency is lowered. Therefore, it is necessary to limit the addition of Sn to 0.1 to 3 mol% in the cation.

Sb is an element effective for enhancing the conductivity of the oxide by being contained in the oxide in the oxidized state. However, it is not effective for oxygen generation itself. Since the effect is saturated even if it is added excessively, the addition of Sb must be limited to 0.01 to 1.3 mol% in the cation. Since Sb may be dissolved when in direct contact with a solution containing chloride, in order to avoid this, it is present only in the inner electrode layer (A) and is not added to the outer electrode layer (B). For this purpose, anodic deposition is performed in two stages, and an oxide containing no Sb is formed in the outer layer.

A solution composed of 0.2M MnSO ₄ −0.003M Na ₂ MoO ₄ −0.006M SnCl ₄ −0.006M SbCl ₅ was adjusted to pH = 0.1 by adding sulfuric acid, and warmed to 90 ° C. An electrode inner layer (A) was prepared by performing electrodeposition for 20 minutes ^{twice at} a current density of 600 A / m ² using a Ti electrode substrate coated with IrO ₂ as an anode and renewing the solution twice. ) Was formed. As a result of EPMA analysis, the cation composition of the obtained inner layer of the electrode was 94.80 mol% Mn-4.73 mol% Mo-0.14 mol% Sn-0.33 mol% Sb. According to X-ray diffraction, the material of the inner layer of the electrode is a MnO ₂ type single-phase oxide in which Mo, Sn, and Sb are dissolved, that is, a single-phase double oxide composed of Mn—Mo—Sn—Sb—O. It was confirmed.

Subsequently, sulfuric acid is added to a solution containing no Sb component in the above electrolyte solution, that is, 0.2M MnSO ₄ -0.003M Na ₂ MoO ₄ -0.006M SnCl ₄ to obtain pH = 0.1. The electrode was heated to 90 ° C. and used as the electrolyte, and the electrode inner layer (A) formed on the electrode substrate was used as the anode, and the anode current was 20 minutes at a current density of 600 A / m ^2. By performing deposition, an electrode outer layer (B) was formed. As a result of EPMA analysis, the cation composition of the obtained electrode outer layer was 92.24 mol% Mn-7.12 mol% Mo-0.64 mol% Sn. According to X-ray diffraction, the material of the outer electrode layer is a MnO ₂ type single-phase oxide in which Mo and Sn are dissolved, that is, a single-phase double oxide composed of Mn—Mo—Sn—O. It was confirmed that the outer electrode layer having the same structure was continuously formed.

Using the anode of the present invention produced as described above, constant current electrolysis was carried out at various current densities in a 0.5 M NaCl solution at pH = 1, and the polarization curve of oxygen evolution was measured. In the polarization curve, the logarithm of the current density increased linearly with increasing potential, and the potential at a current density of 1000 A / m ² was 1.37 V with reference to the silver / silver chloride electrode.

Comparative example

For comparison, a sulfuric acid acidic solution composed of 0.2M MnSO ₄ -0.003M Na ₂ MoO ₄ -0.006M SnCl ₄ was used as an electrolyte solution and the Ti electrode substrate was coated with IrO ₂ for comparison. The anode active electrodeposition was repeated three times for 20 minutes while renewing the solution at a current density of 600 A / m ² at 90 ° C., and the entire electrode active material was a single phase double oxidation of Mn—Mo—Sn—O. The electrode which consists of a thing was manufactured. Using this electrode as an anode, the above NaCl solution was electrolyzed, and a polarization curve of oxygen generation was measured. Also in the obtained polarization curve, the logarithm of the current density increased linearly as the potential increased, and the potential at a current density of 1000 A / m ² was 1.42 V with reference to the silver / silver chloride electrode. Therefore, it can be said that the electrode containing Sb in the electrode active material inner layer according to the present invention is an energy-saving electrode having a potential at a current density of 1000 A / m ² lower by 50 mV than the electrode of the comparative example.

For reference, a 0.5M NaCl solution at pH = 1 was electrolyzed at 1000 A / m ^{2 at} a current density of 1000 coulomb, and the amount of dissolved hypochlorous acid was quantified by an iodometric titration method to determine whether chlorine was generated. Tried to confirm. In all of the examples and comparative examples, no chlorine was detected, and both showed oxygen generation efficiency of 100%. Therefore, the electrode of the present invention and the electrode of the comparative example are both highly active electrodes for oxygen generation in electrolysis of a low pH solution, while the oxygen generation electrode of the present invention is an energy saving electrode. It was concluded.

With SbCl ₅ in various proportions, using 90 ° C. of sulfuric acid solution containing _{0.2M MnSO 4 -0.003M MnSO 4 -0.006M Na} 2 MoO 4 -0.006M SnCl 4 as the electrolyte, Ti The electrode substrate obtained by coating with IrO ₂ was used as an anode, and anode electrodeposition for 20 minutes was repeated twice while renewing the solution at a current density of 600 A / m ² . Mn—Mo—Sn—Sb—O type electrode inner layers (A) having various compositions were obtained. The cation composition of the obtained electrode inner layer was analyzed by EPMA. The results are shown in Table 2.

Sb-free solution containing 0.2 M MnSO ₄ -0.003 M Na ₂ MoO ₄ -0.006 M SnCl ₄ was adjusted to pH = −0.1 by adding sulfuric acid and electrolyzed at 90 ° C. An electrode outer layer was formed by performing anode electrodeposition for 20 minutes at a current density of 600 A / m ² using the electrode on which the electrode inner layer (A) was formed as an anode as an anode. As a result of EPMA analysis, the cation composition of the obtained electrode outer layer was 92.23 mol% Mn-7.14 mol% Mo-0.63 mol% Sn.

Constant current electrolysis was carried out at various current densities in a 0.5 M NaCl solution at pH = 1, and the potential was measured with reference to a silver / silver chloride electrode to obtain a polarization curve for oxygen generation. In the polarization curve, the logarithm of the current density increased linearly with the potential. The potential at a current density of 1000 A / m ² is shown in Table 2. Result of confirming the presence or absence of generation of chlorine by quantifying the amount of dissolved hypochlorous acid by iodine titration method after electrolysis of 0.5M NaCl solution of pH = 1 at a current density of 1000 A / m ² at 1000 coulomb. No chlorine was detected when any electrode was used, and all showed oxygen generation efficiency of 100%. This shows that the electrode of the present invention is an energy-saving electrode having a low polarization potential as an electrode that generates only oxygen in electrolysis of an aqueous solution containing acidic chlorine ions.

Table 2 ^* See silver-silver chloride electrode

A sulfuric acid solution having pH = −0.1 containing various ratios of MnSO ₄ , Na ₂ WO ₄ , SnCl ₄ , SbCl ₅ was used as an electrolyte, and a Ti electrode substrate coated with IrO ₂ was used as an anode. At 90 ° C., anodic electrodeposition for 20 minutes at a current density of 600 A / m ² was performed twice by renewing the solution, whereby inner layers of Mn—W—Sn—Sb—O type electrodes having various compositions ( A) was obtained. The cation composition of the obtained electrode inner layer was determined by EPMA analysis. The results are shown in Table 3. Next, SbCl ₅ was removed from the solution used for the formation of the electrode inner layer (A) as an electrolytic solution, adjusted to pH = −0.1, heated to 90 ° C., and applied onto the electrode inner layer. Anodic deposition was performed to form Mn—W—Sn—O electrode outer layers (B) having various compositions.

Using the obtained electrode of the present invention, constant current electrolysis was carried out at various current densities in a 0.5 M NaCl solution at pH = 1, the potential was measured with reference to the silver / silver chloride electrode, and the polarization of oxygen generation A curve was obtained. The polarization curve was such that the logarithm of current density increased linearly with potential. After electrolysis of 1000 coulombs at a current density of 1000 A / m ^{2 in} 1 liter of 0.5 M NaCl solution at pH 1, the amount of dissolved hypochlorous acid was quantified by an iodometric titration method to determine chlorine generation efficiency, The generation efficiency was determined. The results are shown in Table 3. This result also shows that the electrode of the present invention is a high-performance electrode that exhibits low oxygen overvoltage and high oxygen generation efficiency in electrolysis of an aqueous solution containing acidic chlorine ions.

Table 3

Claims (2)

The surface of a conductive electrode substrate made of Ti or a Ti alloy is coated with a metal double oxide layer serving as an electrode inner layer (A), and further thereon, a metal double oxide layer serving as an electrode outer layer (B) An electrode for oxygen generation having a structure coated with
(A) The metal constituting the metal double oxide serving as the inner layer of the electrode is one or two of Sn, Sb, Mo and W, and Mn, and the composition is atomic% relative to the total cation, [Sn + Sb + Mo And one or two of W]: 0.2 to 20%, of which Sn: 0.1 to 3%, Sb: 0.01 to 1.3%, and Mn: the balance,
(B) The metal constituting the metal double oxide serving as the outer layer of the electrode is Sn, Mo and / or W and Mn, and the composition is atomic% relative to the total cation, [1 type of Sn + Mo and W Or 2 types]: 0.2 to 20%, of which Sn: 0.1 to 3%, Mn: the balance,
An electrode for oxygen generation for electrolyzing an aqueous solution containing chlorine ions to generate oxygen, characterized in that both metal inner and outer electrode layers are formed by anodic deposition. . _2. The oxygen generating electrode according to claim 1, wherein the conductive electrode substrate is obtained by applying an IrO ₂ coating to an electrode base made of Ti or a Ti alloy.