JP5105406B2 - Electrode for reverse electrolysis - Google Patents

Description

  The present invention relates to an electrode in which a catalyst layer made of a mixture of iridium oxide and platinum is formed on a conductive substrate, and more particularly to an electrode used for reverse electrolysis that reverses polarity in its use.

  Using tap water, low-concentration saline, or an aqueous solution with a pH adjuster or calcium lactate added to them, a direct current or a rectangular wave with periodic polarity inversion is applied between at least two electrodes. By the electrolytic treatment, an aqueous solution generally known as electrolyzed water is obtained. Electrolyzed water is classified as strongly acidic water, acidic electrolyzed water, slightly acidic electrolyzed water, or strongly alkaline electrolyzed water depending on its pH, and depending on the components, effects, and applications contained in the electrolyzed water, not only pH but also alkaline ions Classifications such as water, acidic ionized water, electrolytic hypochlorite (electrolytic sodium hypochlorite water or electrolytic hypochlorite water) are also made.

  In addition, as a method of using such electrolyzed water, Patent Document 1 includes a humidifier that humidifies the introduced air and a water supply pipe that supplies humidification water to the humidifier. Comprises at least a pair of electrodes for electrolyzing tap water to produce sterilizing water containing hypochlorous acid, and the humidifier temporarily supplies the sterilizing water to the humidifier when air is not introduced. A sterilization apparatus configured to hold is disclosed, and electrolysis water containing hypochlorous acid generated in the apparatus causes propagation of miscellaneous bacteria in the humidification element and the humidifier, resulting in bacteria, odors, molds, etc. Blowing out with the blown air is suppressed.

  As described above, there are various properties, effects, applications, etc. of the electrolyzed water. In any case, at least two or more electrodes are used for producing electrolyzed water. When three or more electrodes are used, a plurality of pairs of electrodes are connected in series or in parallel, or an electrode not connected to the power source is arranged between a pair of electrodes connected to the power source. May be used as a bipolar electrode. In addition, when separating the anode and cathode with a diaphragm and collecting the electrolyzed water generated near each electrode separately, when collecting separately after mixing, or without using such a diaphragm, near the anode and the cathode In some cases, the electrolyzed water produced in each of these is mixed and then collected. In addition, as disclosed in Patent Document 2, it is connected to a water supply facility such as a water supply, electrolyzed in a flowing water state, and in the case of a flowing water type that produces acid water, alkaline water, or the like, and a simple connection that does not connect to the water supply facility. There is also a distinction such as a batch type in which water is electrolyzed in a residence state with a low-cost structure. Thus, the structure of the apparatus which manufactures electrolyzed water changes according to the property of the target electrolyzed water, supply required amount, etc.

  However, most of the electrodes used for the production of electrolyzed water have a titanium material as a base and a catalyst layer that is active for an electrochemical reaction at the anode or cathode is formed thereon. There is an electroplated platinum on top. In addition, Patent Document 3 introduces a platinum iridium fired electrode, a ruthenium-based electrode, and the like together with a platinum plating electrode, and it is known that the consumption rate of the electrode varies depending on the temperature of the aqueous solution used.

  The reason why an electrode composed of a titanium material and a platinum group metal as described above is used is that titanium and platinum groups are used for oxygen generation, which is a main reaction of an anode, and hydrogen generation, which is a main reaction of a cathode, in electrolysis of an aqueous solution. This is because the metal has an insoluble property that hardly dissolves. In particular, in an apparatus for producing alkaline ionized water used for drinking, or electrolytic hyponitrous acid used for sterilization and washing of foods and kitchens, the electrode material is electrolyzed water. Such elec- trode is often used because it is a big problem to elute in the lysate.

In particular, among platinum group metals used in the catalyst layer, platinum is not only insoluble, but also has good catalytic activity for both oxygen generation and hydrogen generation. If only oxygen generation is taken up, iridium oxide (IrO ₂ ) has better catalytic activity than platinum, and in the production of electrolyzed water, the electrode used is always fixed to the anode or cathode, that is, with polarity reversal. In such a case, an electrode formed on a titanium plate with iridium oxide or a mixture of iridium oxide and tantalum oxide as a catalyst layer for the anode and platinum as a catalyst layer for the cathode is suitable.

  However, in the production of electrolyzed water, an electrolysis method with periodic or intermittent polarity reversal is often used. The reason is that when alkaline earth metals, especially calcium ions, contained in the aqueous solution to be electrolyzed generate hydrogen at the cathode by electrolysis and the pH rises, the electrode or the electrolysis apparatus has a diaphragm on the diaphragm. This is because it precipitates as carbonates or hydroxides. This makes it difficult to energize on the cathode, and it is necessary to replace the electrode in order to increase the electrolysis voltage, and it obstructs the electrolysis apparatus by blocking or stagnating the water flow in the vicinity of the cathode or in the diaphragm. Undesirable effects that require maintenance. In view of this, a method is adopted in which, by reversing the polarity, an electrode that has been a cathode until then is energized as an anode to promote a decrease in pH and to re-dissolve carbonates and hydroxides deposited on the electrode. In Patent Document 4, during such reverse electrolysis, the discharge direction of negative anodized water is reversed and the negative anodized water is constantly discharged from a predetermined water discharge pipe. An electrolyzed water generating device having a function of replacing is disclosed.

That is, in the case of producing electrolyzed water by an electrolysis method with polarity reversal, so-called reverse electrolysis, the electrode to be used is not fixed to either the anode or the cathode, but becomes both the anode and the cathode by polarity reversal. Yeah. Therefore, in the production of electrolyzed water with energization accompanied by reverse electrolysis, the electrode used has high catalytic activity for both oxygen generation, which is the main reaction of the anode reaction, and hydrogen generation, which is the main reaction of the cathode reaction, It is desired to be insoluble and have high durability.
JP 2007-10201 A JP 2003-205288 A JP 2006-15220 A JP 2006-212607 A

When electrolyzed water is produced by an energization method involving reverse electrolysis, it is desirable that the catalyst water, insolubility, and durability be excellent for both oxygen generation and hydrogen generation. As an electrode having relatively good characteristics under such conditions, an electrode (hereinafter referred to as a Pt—IrO ₂ / Ti electrode) in which a catalyst layer made of a mixture of platinum and iridium oxide is formed on a titanium substrate by a thermal decomposition method is known. ing. In this electrode, generally, the main component of the catalyst layer is platinum, and by mixing iridium oxide with this, it is intended that the catalytic property for oxygen generation is higher than that of platinum alone. On the other hand, iridium oxide has a low catalytic property for hydrogen generation, but by reducing the mixing ratio thereof compared to platinum, there is an object of suppressing a significant decrease in catalytic property for hydrogen generation as compared with platinum alone.

However, the high catalytic property of oxygen generation inherent in iridium oxide is not sufficiently exhibited by the Pt—IrO ₂ / Ti electrode, and eventually the platinum catalyst as the main component rather than the effect of mixing iridium oxide. The influence of the oxygen generation potential on the sex was great. Therefore, although the theoretical voltage for electrolysis of water is 1.23 V, since the overvoltage for oxygen generation is larger than that for hydrogen generation, the actual electrolysis voltage is larger than the theoretical voltage, and the electrolysis voltage cannot be lowered. There was a problem. In particular, if the current density is increased to meet the demand for efficient production of electrolyzed water with a small electrode area, the electrolysis voltage will increase significantly due to oxygen overvoltage, and the applied current density cannot be increased, or the electrode area must be reduced. There was a problem that it was difficult. Furthermore, iridium oxide alone has a higher consumption rate when the pH in the vicinity of the electrode is lowered due to hydrogen generation than platinum alone, so that when reverse electrolysis is performed, iridium oxide is consumed first and the entire catalyst layer is deteriorated. In order to promote the penetration of the aqueous solution to be electrolyzed into the catalyst layer and eventually cause the catalyst layer, which is a mixture of platinum and iridium oxide, to peel off or fall off, there is a problem that it is necessary to improve the durability against reverse electrolysis. .

  In view of the above problems, the present invention aims to produce electrolyzed water using tap water, low-concentration saline, or an aqueous solution in which a pH adjuster, calcium lactate, or the like is added to at least two sheets. For reverse electrolysis that can reduce the overvoltage against oxygen generation and lower the electrolysis voltage in the electrode for performing electrolysis of the aqueous solution using the electrode of the electrode and performing reverse electrolysis by switching the polarity of the electrode An object of the present invention is to provide an electrode for reverse electrolysis that is excellent in durability for both positive and negative polarities with respect to reverse electrolysis that is repeatedly performed.

  The present inventor has developed an electrode for reverse electrolysis composed of a conductive base and a catalyst layer made of a platinum group metal, etc., and studied the constituent material, manufacturing method, and manufacturing conditions of the catalyst layer. Various studies were conducted to evaluate the electrochemical properties of the obtained electrode for reverse electrolysis in an aqueous solution and to evaluate the durability of the electrode for reverse electrolysis in a continuous electrolysis test with periodic polarity reversal. Based on the knowledge obtained from the results, the present invention has been made.

  That is, the present invention is a reverse electrolysis electrode in which a catalyst layer made of a mixture of iridium oxide and platinum is formed on a conductive substrate, and the iridium oxide is amorphous. Here, the conductive substrate is a valve metal such as titanium, tantalum, zirconium, or niobium, or an alloy or conductive material mainly composed of a valve metal such as titanium-tantalum, titanium-niobium, titanium-palladium, or titanium-tantalum-niobium. Diamond (for example, diamond doped with boron) is preferable, and titanium is particularly preferable. The shape can take various shapes such as a plate shape, a net shape, a rod shape, and a porous plate shape. Further, the above metal, alloy, or conductive diamond may be coated on the surface of a metal other than valve metal such as iron or nickel, or a conductive ceramic.

  In order to form a catalyst layer made of a mixture of platinum and amorphous iridium oxide on a conductive substrate, for example, iridium acid chloride or chloroplatinic acid is dissolved in an organic solvent such as water or butanol to remove platinum ions. There is a so-called pyrolysis method in which a coating solution containing iridium ions and iridium ions is prepared, coated on a conductive substrate, and then thermally decomposed at a predetermined temperature. At this time, an acid such as hydrochloric acid may be added to water or an organic solvent in order to promote dissolution of a compound serving as a supply source of platinum ions and iridium ions. In addition to the iridium acid chloride and chloroplatinic acid, other compounds including iridium chloride and platinum chloride may be used as the source compound for the platinum ion and iridium ion, and the iridium ion supply source compound. The oxidation state of iridium therein may be not only tetravalent but also trivalent.

  In the thermal decomposition method, in order to obtain amorphous iridium oxide, it is necessary to control the temperature at the time of thermal decomposition according to the solvent used in the coating solution. For example, when applying and thermally decomposing a coating solution on a titanium plate with water as a solvent and using iridium chloroacid and chloroplatinic acid as a source of each metal ion, it is formed on the titanium plate at a thermal decomposition temperature of 500 ° C. In the catalyst layer, both platinum and iridium oxide are crystalline. When this is carried out at a thermal decomposition temperature of 380 ° C., platinum is crystalline, but iridium oxide becomes amorphous. Of course, the conditions for forming a mixture of platinum and amorphous iridium oxide as a catalyst layer are not limited to these. In other words, in the pyrolysis method, the type of solvent used in preparing the coating solution to be used, the type and amount of additives added to the solvent, the type of compound serving as a source of platinum ions and iridium ions in the coating solution, etc. Accordingly, the minimum temperature at which platinum and iridium oxide are formed is different, and the crystal structure of platinum and the crystal structure of iridium oxide change depending on the thermal decomposition temperature, but the temperature at which the crystal changes from amorphous to crystalline is different. Different for platinum and iridium oxide. Depending on the composition of the coating solution, platinum is often crystalline even at relatively low temperatures, and iridium oxide is crystalline at high temperatures, and becomes amorphous when the temperature is lowered above the minimum temperature. Cheap. By utilizing the difference in temperature that causes such a structural change, a catalyst layer composed of crystalline platinum and amorphous iridium oxide can be formed by thermal decomposition at a constant temperature. Whether or not iridium oxide is amorphous in the obtained mixture layer is determined by the fact that a diffraction peak corresponding to iridium oxide is not observed or broadened by a commonly used X-ray diffraction method. I can know.

  Furthermore, in addition to the above thermal decomposition methods, for example, various physical vapor deposition methods such as generally known sputtering methods and CVD methods, chemical vapor deposition methods are used, and platinum or iridium oxide is targeted, or It is also possible to form a mixture of platinum and amorphous iridium oxide as a catalyst layer on a conductive substrate by introducing and reacting a compound serving as a supply source of platinum and iridium oxide into a vapor deposition apparatus. . In this case, it is necessary to select a temperature, a growth rate, a supply source, and the like that suppress crystallization of iridium oxide.

  In the electrode for reverse electrolysis in which a catalyst layer made of a mixture of platinum and amorphous iridium oxide is formed on a conductive substrate by the above method, the amorphous iridium oxide in the catalyst layer is converted into crystalline iridium oxide. In comparison, since the oxygen generation catalytic ability is high in terms of the effective surface area and the exchange current density, the oxygen generation overvoltage can be reduced. Therefore, when the same amount of catalyst layer is formed, oxygen generation is further promoted even with the same geometric electrode area if the iridium oxide is amorphous compared to the crystalline case. Therefore, even if the mixing ratio of platinum and iridium oxide and the weight or geometric area of the formed catalyst layer are the same, if the reverse electrolysis electrode of the present invention is used, the polarity reversal can be performed. In the production of electrolyzed water, the electrolysis voltage can always be lowered regardless of the polarity.

  In the reverse electrolysis electrode of the present invention, the mixing ratio of platinum and iridium oxide in the catalyst layer is preferably 50 to 95 mol%, particularly 80 to 90 mol%. If platinum is less than 50 mol%, the overvoltage for hydrogen generation tends to increase rapidly, and if it exceeds 95 mol%, the catalytic effect on oxygen generation by iridium oxide hardly appears. Further, considering the durability against reverse electrolysis, 80 to 90 mol% is preferable.

  Further, the present invention is an electrode for reverse electrolysis characterized in that an intermediate layer that suppresses corrosion of the conductive substrate is formed between the catalyst layer and the conductive substrate, and the intermediate layer is made of platinum. This is an electrode for reverse electrolysis. Here, the corrosion-resistant intermediate layer means that the aqueous solution that has penetrated the catalyst layer reaches the conductive substrate when used for a long time, and the conductive substrate is oxidized and corroded by the generation of oxygen on the conductive substrate. It has the function to prevent. By preventing the oxidation and corrosion of the conductive substrate, the ohmic loss due to the corrosion product is increased, the electrolysis voltage is increased accordingly, the adhesion of the catalyst layer on and around the corrosion product is decreased, peeling and dropping off. It has the effect of preventing the above. As the intermediate layer, valve metals such as tantalum and alloys thereof are preferable as the intermediate layer. In particular, platinum is a component of the catalyst layer. If this is used as an intermediate layer, high adhesion to the catalyst layer is achieved. It is preferable in that it is obtained. As a method for forming such an intermediate layer, a method of thermally decomposing with a coating solution containing only platinum ions can be used in the same manner as the above-described formation of the catalyst layer. In addition, a sputtering method, an ion plating method, a CVD method, An electroplating method or the like can be used.

The present invention has the following effects.
1) In the production of electrolyzed water that uses an aqueous solution and requires reverse electrolysis, the power used can be reduced by lowering the electrolysis voltage regardless of the polarity reversal, and electrolysis at a low overvoltage becomes possible. As a result, the characteristics of the electrolyzed water can be controlled at a low output.
2) Moreover, it has the effect that it becomes possible to reduce electrolysis cost by being able to reduce electric power used.
3) Since these effects can be maintained for a long time, the stability of the electrolyzed water can be improved, and the maintenance of the electrolyzer can be reduced.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example and a comparative example, this invention is not limited to a following example.

An electrode (Pt—IrO ₂ / Ti electrode) formed on a titanium substrate using a mixture of platinum and iridium oxide as a catalyst layer was produced. The coating solution was prepared by adding 0.6 mL of concentrated hydrochloric acid to 9.4 mL of distilled water, adding chloroplatinic acid hexahydrate and chloroiridate hexahydrate in a molar ratio of 85:15, and metal. It was performed by completely dissolving so that the total concentration of Pt and Ir was 70 g / L in terms of conversion. Next, after ultrasonically washing the titanium plate with acetone, it was immersed in a 10 wt% oxalic acid solution prepared using oxalic acid dihydrate at 90 ° C. for 60 minutes, and then the surface was thoroughly washed with distilled water. Air dried. The coating solution was thinly and uniformly applied to the titanium substrate pretreated in this way. Thereafter, the coating solution was dried at room temperature for 10 minutes in an air atmosphere, then held in an electric furnace at 120 ° C. for 10 minutes, and further held in an electric furnace at 380 ° C. for 20 minutes to thermally decompose the coating solution. The above steps from application to thermal decomposition were repeated 5 times to produce a Pt—IrO ₂ / Ti electrode.

A Pt—IrO ₂ / Ti electrode was produced by the same method and conditions except that the thermal decomposition temperature in Example 1 was changed to 400 ° C.

(Comparative Example 1)
A Pt—IrO ₂ / Ti electrode was produced by the same method and conditions except that the thermal decomposition temperature in Example 1 was changed to 450 ° C.

(Comparative Example 2)
A Pt—IrO ₂ / Ti electrode was produced by the same method and conditions except that the thermal decomposition temperature in Example 1 was changed to 500 ° C.

The structure of the Pt—IrO ₂ / Ti electrode prepared in Example 1, Example 2, Comparative Example 1, and Comparative Example 2 was examined by X-ray diffraction. First, in Comparative Examples 1 and 2, clear diffraction patterns corresponding to crystalline platinum and crystalline iridium oxide were obtained. On the other hand, in Example 1 and Example 2, only a diffraction pattern indicating crystalline platinum was obtained, and no diffraction line corresponding to iridium oxide was observed. From these results, the catalyst layers of Examples 1 and 2 are a mixture of crystalline platinum and amorphous iridium oxide, and the catalyst layers of Comparative Examples 1 and 2 are crystalline platinum and crystalline. It was found to be a mixture of iridium oxide.

Next, the electrochemical characteristics of the Pt—IrO ₂ / Ti electrodes prepared in Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were evaluated by a general three-electrode measurement method. Each electrode was used in a state where the electrode area was regulated to 1 cm × 1 cm using a holder made of Teflon (registered trademark). Pt—IrO ₂ / Ti thus formed was used as a working electrode, a Pt plate was used as a counter electrode, a KCl saturated Ag / AgCl electrode was used as a reference electrode, and a 1 mol / L Na ₂ SO ₄ solution was used as an electrolyte. Using a potentiogalvanostat, cyclic voltammetry was performed at a scanning speed of 5 mV / s to measure the electric double layer capacity and to evaluate the catalytic property against oxygen generation.
A cyclic voltammogram was measured in the range of 0.4 V to 0.7 V with respect to the reference electrode, and the electric double layer capacity per electrode area based on the geometry was determined from the result. FIG. 1 shows the result of comparing the electric double layer capacity. From FIG. 1, compared with Comparative Example 1 and Comparative Example 2, Example 1 and Example 2 have large electric double layer capacity, and in particular, Comparative Example 1 and Example 2 in which iridium oxide changes from crystalline to amorphous. It can be seen that the increase in electric double layer capacity is large. The electric double layer capacity obtained by the electrochemical measurement corresponds to a substantial electrode surface area in which the electrode reaction occurs. Even if the geometric area is the same, the larger the electric double layer capacity, the larger the reaction area. From the results of FIG. 1, the reaction area of the electrode in Example 1 and Example 2 is increased compared to Comparative Example 1 and Comparative Example 2, and in particular, Example 1 has a reaction four times or more that of Comparative Example 2. There was an increase in area.
Next, for each electrode, a cyclic voltammogram from the immersion potential to the anode direction was measured, and the results are summarized in FIG. In FIG. 2, the current flowing at 1.1 V or more is due to oxygen generation, but the current rises from the lowest potential in Example 1, and then in Example 2, Comparative Example 1, and Comparative Example 2. It became order. In addition, when compared at the same potential, the current of Example 1 was the largest, followed by Example 2, Comparative Example 1, and Comparative Example 2, but compared to Comparative Example 1 and Comparative Example 2, Example 1 and The current of Example 2 was very large, and clearly there was a difference in catalytic properties for oxygen generation between the Example and the Comparative Example. Further, when compared at the same current density, it was found that the potential of Example 1 was 200 mV or more lower than that of Comparative Example 2, that is, it was possible to greatly reduce the oxygen generation overvoltage.

After forming an intermediate layer of platinum on the titanium substrate, an electrode (hereinafter referred to as a Pt—IrO ₂ / Pt / Ti electrode) in which a mixture of platinum and iridium oxide was formed as a catalyst layer on the intermediate layer was produced. The coating solution for forming the intermediate layer was prepared by completely dissolving chloroplatinic acid hexahydrate at 70 g / L in terms of metal in a solution obtained by adding 0.6 mL of concentrated hydrochloric acid to 9.4 mL of distilled water. Prepared. Next, the coating solution for the intermediate layer was thinly and uniformly applied onto the titanium plate that had been subjected to the same pretreatment as in Example 2. Thereafter, the coating liquid was dried at room temperature for 10 minutes in an air atmosphere, then held in an electric furnace at 120 ° C. for 10 minutes, and further held in an electric furnace at 400 ° C. for 20 minutes to thermally decompose the coating solution. The above steps from application to thermal decomposition were repeated twice. Next, using the same coating solution as in Example 2, the steps from coating to thermal decomposition were repeated 5 times under the same method and conditions to form a Pt—IrO ₂ catalyst layer on the intermediate layer.

A reverse electrolysis test was performed on each of Pt—IrO ₂ / Ti produced in Example 2 and Pt—IrO ₂ / Pt / Ti produced in Example 3. In this test, two Pt—IrO ₂ / Ti electrodes or two Pt—IrO ₂ / Pt / Ti were immersed in a 1 mol / L Na ₂ SO ₄ solution, and energized for 10 minutes using an electrolytic test apparatus. A voltage between two electrodes was measured by applying a constant-current pulse wave that alternately repeated energization for 10 minutes. In addition, the electrode used for this test was regulated to 1 cm × 1 cm using a Teflon (registered trademark) holder, and the current density was set to 0.1 A / cm ² . The temperature of the electrolytic solution was adjusted to 30 ° C. In any electrode, when the electrolysis was continued, a change in which the voltage suddenly increased was observed, and this was evaluated as the lifetime of the electrode. As a result, the lifetime of the electrode of Example 2 was 350 to 400 hours, whereas in Example 3, electrolysis for 550 hours or more was possible. That is, it has been found that electrolysis time in reverse electrolysis is increased by introducing platinum as an intermediate layer.

  The present invention includes strongly acidic electrolyzed water, strongly alkaline electrolyzed water, slightly acidic electrolyzed water, electrolytic hyponitrous water, alkaline ionic water, acidic ionic water, variable acidic electrolyzed water, variable alkaline electrolyzed water, acidic electrolyzed water, alkaline electrolyzed water, It can be used for the production of electrolyzed water such as oxygen-based electrolyzed water and strong alkaline circulating electrolyzed water.

The electric double layer capacity of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 is shown. The cyclic voltammograms of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 are shown.

Claims (4)

  An electrode for reverse electrolysis for producing electrolyzed water by applying an electric current with periodic polarity reversal between at least two electrodes using an aqueous solution comprising tap water or low-concentration saline, and comprising iridium oxide An electrode for reverse electrolysis, wherein a catalyst layer made of a mixture of platinum and platinum is formed on a conductive substrate, and the iridium oxide is amorphous. The electrode for reverse electrolysis according to claim 1, wherein a pH adjusting agent or calcium lactate is further added to the aqueous solution.   The electrode for reverse electrolysis according to claim 1 or 2, wherein an intermediate layer for suppressing corrosion of the conductive substrate is formed between the catalyst layer and the conductive substrate. The electrode for reverse electrolysis according to claim 3, wherein the intermediate layer is platinum.

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