JP2000248393A - Electrolytic ozone generating element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the corrosion reaction at an anode and to improve the life thereof by disposing a thin film of a solid polymer electrolyte having the acidity weaker than the acidity of a solid polymer electrolyte film between the anode and the electrolyte film. SOLUTION: The film 20 of the solid polymer electrolyte having the weak acidity is formed as the extremely thin layer on the surface layer of the solid polymer electrolyte film 1 and is interposed between an anode catalyst layer 4 and the solid polymer electrolyte film 1 to prevent the contact of both. More preferably the solid polymer electrolyte film 1 is a sulfonic acid type and the film 20 is carboxylic acid type. When the generation of ozone is ceased by stopping the impression of voltage, the potential of the anode falls and the lead tends to elute from the anode catalyst layer 4 containing lead oxide. The acidity of the film 20 in contact with the lead dioxide is, however, weak and there is no possibility that the anode potential enters a region of Pb2+ if the atmosphere is the air atmosphere. Namely, the anode is stably maintained as the lead dioxide even if voltage is not impressed at the time of stoppage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic ozone generating element in which an anode and a cathode are opposed to each other with a solid polymer electrolyte membrane interposed therebetween and a DC voltage is applied to generate ozone from the anode.

[0002]

2. Description of the Related Art As a conventional electrolytic ozone generating element, a water supply type is generally used. However, practical use of an air supply type electrolytic ozone generating element that supplies air instead of water is also being studied.

FIG. 5 is a diagram showing the operating principle of a conventional air-supply type electrolytic ozone generating device using a solid polymer electrolyte membrane, as described in Japanese Patent Application Laid-Open No. 5-255879. 1, 1 is a solid polymer film, 2 is an anode, 3 is a cathode, 4 is lead dioxide, 5 is an expanded metal made of titanium, 6 is a cathode catalyst layer, 7 is carbon paper, 8 is an external circuit, and 9 is an external power supply. . The expanded metal 5 made of titanium is plated with platinum, and β-type lead dioxide is electrodeposited as a catalyst layer. The solid polymer electrolyte membrane 1 is made of Nafion (DuPont), and the cathode 3 of the electrolytic ozone generating element is made of carbon paper 7 with carbon carrying platinum to form a cathode catalyst layer 6. Is used. In addition to β-type lead dioxide (black), α-type lead dioxide (brown) is also used.

[0004]

[Problems to be solved by the invention]

In the conventional air supply type electrolytic ozone generating element as described above, a DC voltage of about 3.5 V is applied, ozone and oxygen are generated on the anode side, water is generated on the cathode side, and superacid is generated. Lead dioxide is gradually eluted by the solid polymer membrane, and the eluted lead dioxide is taken up by the solid polymer membrane to increase the ionic conduction resistance of the solid polymer electrolyte membrane, or the lead oxide in the form of lead monoxide at the cathode There were problems such as precipitation and inhibition of the cathode reaction.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to suppress a corrosion reaction at an anode in an electrolytic ozone generating element and improve the life.

[0007]

The invention according to claim 1 is
A solid polymer electrolyte membrane, a cathode and an anode facing each other with the solid polymer electrolyte membrane interposed therebetween, disposed between the anode and the solid polymer electrolyte membrane, and having a lower acidity than the solid polymer electrolyte membrane. This is an electrolytic ozone generating element having a thin film of a solid polymer electrolyte.

According to a second aspect of the present invention, there is provided a solid polymer electrolyte membrane, a cathode having a cathode catalyst layer opposed to the solid polymer electrolyte membrane, and an anode having an anode catalyst layer. And an electrolytic ozone generating element provided with a solid polymer electrolyte having a lower acidity than the solid polymer electrolyte membrane.

The invention according to claim 3 is the invention according to claim 1 or 2
In the electrolytic ozone generating element described above, the solid polymer electrolyte membrane is a sulfonic acid membrane, and the solid polymer electrolyte having a lower acidity than the solid polymer electrolyte membrane is a carboxylic acid.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The construction and operation of an electrolytic ozone generating element according to the present invention will be described below with reference to FIGS.

FIG. 1 is a sectional view showing the structure of an electrolytic ozone generating element according to an embodiment of the present invention. In the figure, 1 is a solid polymer film, 2 is an anode, 3 is a cathode, 4 is lead dioxide, 5 is an expanded metal made of titanium, 6 is a cathode catalyst layer, 7 is carbon paper, 8 is an external circuit, and 9 is an external power supply. The difference from the conventional electrolytic ozone generating element shown in FIG. 5 is that a thin film 20 of a solid polymer electrolyte having a lower acidity than the solid polymer electrolyte membrane 1 is provided.

Titanium expanded metal 5 for anode 2
Is a thin plate of titanium having a thickness of 50 μm, which is cut and elongated and expanded platinum is slightly plated with platinum. The catalyst layer 4 of the anode 2 is obtained by electroplating β-type lead dioxide on an expanded metal 5 made of titanium. The carbon paper 7 of the cathode 3 has a thickness of 0.3 mm.
Fine particles of polytetrafluoroethylene (PTFE) are attached to carbon paper made of
A material subjected to a water-repellent treatment by heat treatment at 0 ° C is used. The cathode catalyst layer 6 of the cathode 3 has a catalyst in which platinum particles are supported on carbon powder fixed to a carbon paper 7 with a binder, and has a thickness of about 20 μm. The binder used was a liquefied sulfonic acid film manufactured by DuPont.
As the solid polymer electrolyte membrane 1, a sulfonic acid membrane (thickness: about 100 μm) manufactured by DuPont was used, and a composite membrane having a thin carboxylic acid film formed on the solid polymer electrolyte membrane 1 was prepared. The acid film side was polished by sandblasting to reduce the thickness of the carboxylic acid film to about 5 μm, and this was boiled in boiling dilute hydrochloric acid for 24 hours to protonate the solid polymer electrolyte membrane 1. On the solid polymer electrolyte membrane 1 on which the carboxylic acid film is formed, the anode 2 and the cathode 3 are overlapped with the carboxylic acid film facing the anode 2 and
And hot pressed to produce an ozone generating element.

FIG. 2 schematically shows an enlarged view of the vicinity of the thin film 20 of the solid polymer electrolyte having weak acidity in FIG. The thin film 20 of the solid polymer electrolyte having weak acidity is formed as a very thin layer on the surface layer of the polymer electrolyte membrane 1, and is interposed between the lead dioxide layer 4 and the polymer electrolyte membrane 1. , To prevent contact between them.

FIG. 3 shows a comparison between the life performance 22 of the ozone generating element in this embodiment and the life performance 21 of the conventional ozone generating element for more than 4000 hours. Although the conventional ozone generating element is higher than the conventional one, the performance of the conventional ozone generating element is greatly reduced due to the elution of lead dioxide every time the element is stopped, whereas the effect of the stop is not observed in this embodiment. It can be seen that the life performance was greatly improved.

The reason for the lower initial performance of this embodiment is that the ionic conduction resistance is higher. The reason for the higher ionic conduction resistance is that the ionic conduction resistance is higher than that of the solid polymer electrolyte membrane 1. This is because a weak solid polymer electrolyte membrane is interposed between the lead dioxide layer 4 and the solid polymer electrolyte membrane 1. After the life test, the lead was decomposed and the distribution of lead was examined. In the first embodiment, it was confirmed that the elution of lead dioxide was small.

Next, the operation will be described. FIG.
And potential indicating the action of the electrolytic ozone generating element of FIG.
It is a pH diagram. The vertical axis in FIG. 4 indicates the potential, but RHE
(Reversible hydrogen electrode). The horizontal axis of the figure is the pH, that is, the acidity, and the acidity is weaker toward the right. In the drawing, dotted lines indicate a hydrogen generation potential near 0 V, an oxygen generation potential near 1.2 V, and an ozone generation potential near 1.5 V, respectively. The acidity of solid polymer electrolytes is not precisely understood, but in general, sulfonic acid membranes have high acidity and low ionic conduction resistance, and carboxylic acid membranes have low acidity and high ionic conduction resistance . In FIG. 4, the acidity 23 of a typical sulfonic acid film and the acidity 24 of a carboxylic acid film are shown by dashed lines. Note that the acidity also changes depending on the ion exchange capacity. On the other hand, the equilibrium state of lead is determined by the potential and the pH, and the potential of the anode of the ozone generating element is in the state of PbO ₂ when ozone is being generated. However, when the application of the voltage is stopped and the generation of ozone is stopped, the potential of the anode decreases, Pb ²⁺ enters a stable region, and lead elution occurs. For this reason, in the conventional example, the performance was lowered every time the operation was stopped. In the conventional ozone generation element, a voltage of about 1.5 V was applied even during the stop to prevent the elution of lead during the stop. Had to be kept. However, in this embodiment, the acidity of the carboxylic acid film in contact with the lead dioxide is weak, and there is no possibility that the anode potential enters the Pb ²⁺ region in an air atmosphere. That is, the anode is stably maintained as lead dioxide even when no voltage is applied during the stop.

When only a solid polymer electrolyte membrane having a weak acidity is used, the ionic conduction resistance is too high, so that an element cannot be formed. However, in the case of this embodiment, since most of the solid polymer electrolyte membrane is constituted by a membrane having a high acidity, an increase in resistance is minimized.

In this embodiment, a thin film of a carboxylic acid type solid polymer electrolyte having a lower acidity than that of the solid polymer electrolyte membrane 1 is formed on the surface of the sulfonic acid type solid polymer electrolyte membrane 1. Although the case was shown, even if the sulfonic acid type is used, for example, if the number of sulfonic groups is small or the protonation is small and the exchange capacity is small, the acidity becomes weak, so it is used instead of the carboxylic acid membrane. And a similar effect can be obtained.

Further, in this embodiment, the case where a thin film of the solid polymer electrolyte having a lower acidity than the solid polymer electrolyte membrane 1 is formed on the surface of the solid polymer electrolyte membrane 1 has been described. The thin film of the weak solid polymer electrolyte may be formed inside the anode catalyst layer. In that case, for example,
A solid polymer electrolyte having weak acidity may be dissolved in a solvent to form a solution and impregnated inside the catalyst layer, or may be mixed with a catalyst and formed on the electrode substrate or the solid polymer electrolyte membrane 1. . The essence of the present invention is to reduce the acidity of the electrolyte in contact with the anode catalyst layer exposed to the harsh environment while minimizing the increase in ionic conduction resistance, to prevent elution, and physically, the acidity is low. If the electrolyte is interposed between the anode catalyst and the solid polymer electrolyte membrane 1, the object can be achieved.

When an electrolyte having low acidity is impregnated inside the anode catalyst layer, a thinner film can be formed, and an increase in ion conduction resistance can be suppressed.

[0021]

According to the first and third aspects of the present invention, there is provided an electrolytic ozone generating element in which an anode and a cathode are opposed to each other with a solid polymer electrolyte membrane interposed therebetween. Since a thin film of the solid polymer electrolyte having a lower acidity than the solid polymer electrolyte membrane is arranged, the elution of lead dioxide when the application of the voltage is stopped is suppressed,
It is not necessary to apply an external voltage at the time of rest, and the life is improved.

According to the second and third aspects of the invention, there is provided an electrolytic ozone generating element in which a cathode having a cathode catalyst layer and an anode having an anode catalyst layer are opposed to each other with a solid polymer electrolyte membrane interposed therebetween. In the anode catalyst layer of the anode,
Since it contains a solid polymer electrolyte whose acidity is weaker than that of the solid polymer electrolyte membrane, elution of lead dioxide when the application of voltage is stopped is suppressed, and the application of an external voltage at rest is not required, and the life is improved. Is done.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an electrolytic ozone generating element according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view near an anode of an electrolytic ozone generating element according to an embodiment of the present invention.

FIG. 3 is a view showing the life performance of an embodiment of the present invention and a conventional electrolytic ozone generating element.

FIG. 4 is a diagram showing a potential-pH diagram of lead and an acidity of a solid polymer electrolyte.

FIG. 5 is a schematic sectional view showing the operation principle of a conventional electrolytic ozone generating element.

[Explanation of symbols]

1 solid polymer membrane, 2 anode, 3 cathode, 4 anode catalyst layer (lead dioxide), 5 expanded titanium metal, 6
Cathode catalyst layer, 7 carbon paper, 8 external circuit, 9
External power supply, 20 Thin film of solid polymer electrolyte with low acidity, 21 Lifetime performance of conventional ozone generating element, 22 Lifetime performance of ozone generating element of Embodiment 1, 24 Acidity of sulfonic acid film, 23 Carboxylic acid Membrane acidity, 25
PbO ₂ stable region, 26 Pb ²⁺ ion stable region

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hisatoshi Fukumoto 2-3-2 Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 4K021 AA01 AB15 DB13 DB18 DB19 DB21 DB22 DB31 DB33 DB53

Claims (3)

[Claims] 1. A solid polymer electrolyte membrane, a cathode and an anode facing each other with the solid polymer electrolyte membrane interposed therebetween, and the solid polymer electrolyte membrane disposed between the anode and the solid polymer electrolyte membrane. An electrolytic ozone generating element characterized by having a thin film of a solid polymer electrolyte having low acidity. 2. A solid polymer electrolyte membrane, a cathode having a cathode catalyst layer opposed to the solid polymer electrolyte membrane and an anode having an anode catalyst layer, and the solid contained in the anode catalyst layer of the anode. An electrolytic ozone generating element comprising a solid polymer electrolyte having a lower acidity than a polymer electrolyte membrane. 3. The solid polymer electrolyte membrane according to claim 1, wherein the solid polymer electrolyte membrane is a sulfonic acid membrane, and the solid polymer electrolyte having a lower acidity than the solid polymer electrolyte membrane is a carboxylic acid.
Or the electrolytic ozone generating element according to 2.