JPH01275791A - Cathode having high durability and low hydrogen overvoltage and production thereof - Google Patents

Abstract

PURPOSE:To obtain a cathode having high durability and low hydrogen overvoltage by adhering particles of a specified hydrogen occluding alloy to the surface of an electrode as the cathode for electrolyzing an aq. alkali halide soln. so that the particles are exposed. CONSTITUTION:When an aq. soln. of an alkali halide such as NaCl is electrolyzed in an electrolytic cell to produce NaOH and gaseous H2 on the cathode and gaseous Cl on the anode, the cathode is obtd. by electroplating the surface of an electrode core 1 in an Ni plating bath contg. dispersed particles of a hydrogen occluding alloy represented by formula I (where 0.3<=n<=4) so as to form an Ni plating layer 2 contg. exposed particles 3 of the hydrogen occluding alloy. The exposed particles 3 may be adhered to the surface of the core 1 by baking or melt coating. By the presence of the particles 3, the cathode has superior durability and low hydrogen overvoltage at the time of electrolysis of the aq. NaCl soln.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a highly durable low hydrogen overvoltage cathode, and particularly to a low hydrogen overvoltage cathode whose characteristics are extremely minimally deteriorated even in an oxidizing environment, and a method for producing the same.

(Prior Art) Various types of low hydrogen overvoltage cathodes have been proposed, particularly as cathodes for aqueous halogenated alkali solution electrolysis. Among these, Japanese Patent Application Laid-Open No. 1127-1987, which the present applicant has already proposed,
Although the electrode disclosed in Publication No. 85 has greater effects in terms of lower hydrogen overvoltage and durability than previously known electrodes, the present inventors have conducted further studies. As a result, we discovered that some of the electrodes disclosed in the above publications do not always have sufficient durability, and as a result of our earnest efforts to solve this problem, we have arrived at the present invention.

Producing halogen gas from the anode chamber and caustic alkali aqueous solution and hydrogen gas from the cathode chamber by electrolysis in an alkali halide aqueous solution electrolytic tank is already a well-known industrial method for producing chlorine and caustic alkali. . As the cathode of this electrolytic cell, the above-mentioned cathode with a low hydrogen overvoltage is preferably used, but the electrolytic cell may be stopped for various reasons during operation, and in this case, when the operation is resumed, the hydrogen overvoltage is low. was observed to increase. The present inventors investigated this phenomenon in depth, and found that when the electrolytic cell is stopped by short-circuiting the anode and cathode with a bus bar, the cathode is oxidized by the reverse current generated during the short circuit, and nickel and In the case of a cathode containing cobalt as an active ingredient, it was found that the electrode activity decreased due to the deterioration of cobalt into hydroxides, and the electrode activity did not return to its original active state even after restarting operation (i.e., the hydrogen overvoltage increased). .

In addition, even in the stopping method of stopping current supply without shorting the anode and cathode, if the cathode is immersed in high-temperature, high-concentration N,011 for a long time, if the cathode active component is nickel or cobalt, it will corrode. It was discovered that the electrode activity decreases when exposed to electric potential (this reaction is also a type of electrochemical oxidation reaction) and changes into hydroxide.

(Problems to be Solved by the Invention) The present invention provides an electrode whose electrode activity does not decrease even under the above conditions of use, and a method for manufacturing the same.

(Means for solving the problem) As a result of intensive studies to prevent this phenomenon, we found that if a hydrogen storage metal that electrochemically absorbs and releases hydrogen and has a low hydrogen overvoltage is used as an electrode active component, When the battery case is stopped, a large amount of hydrogen stored in the hydrogen storage metal is electrochemically oxidized, which effectively prevents the oxidation of the electrode active component, that is, the activity can be maintained for a long period of time. The present invention is based on an electrode in which a part of the electrode active metal particles is exposed on the surface of a layer provided on an electrode core, in which the electrode active particles are electrochemically hydrogenated. The gist of the present invention is to provide a highly durable low hydrogen overvoltage cathode which is a hydrogen storage metal capable of absorbing and desorbing hydrogen, and a method for manufacturing the above-mentioned highly durable low hydrogen overvoltage cathode described below.

Here, the hydrogen storage metal that can electrochemically store and release hydrogen refers to a metal that undergoes the following electrode reaction in an alkaline aqueous solution. That is, in the reduction reaction, hydrogen atoms generated by reducing water are occluded in the metal, and in the oxidation reaction, the occluded hydrogen is reacted with hydroxide ions on the metal surface to form water. The reaction formula is shown below.

M is a hydrogen storage metal, and Mllx represents its hydride. When a cathode containing this hydrogen storage metal as electrode active particles is used, for example, for salt electrolysis using an ionic membrane, hydrogen is stored in the hydrogen storage metal due to the rightward reaction of reaction formula (1) at the initial stage of energization. When hydrogen absorption eventually reaches saturation, hydrogen is generated on the surface of the hydrogen storage metal by reaction (2) shown below, and the original electrode reaction on the cathode proceeds.

11zO+ e+坏+1. +011- (2) On the other hand, when the battery is stopped due to a short circuit in the battery case, a large amount of hydrogen stored in the hydrogen storage metal electrochemically releases hydrogen through the leftward reaction of reaction formula (1). By oxidizing hydrogen and bearing the oxidation current, it is possible to effectively prevent the electrode active particles themselves from being oxidized.

As described above, the vine hydrogen storage metal used in the present invention is capable of electrochemically storing and releasing hydrogen,
Specifically, 1. aNia. A12. II! It is an alloy made of three metals: lanthanum, nickel, and aluminum, represented by n (0.3≦n≦4).

When n<0.3, although the hydrogen overvoltage is initially low,
Hydrogen overvoltage increases over time and cannot withstand long-term use. Moreover, when n>4, the amount of hydrogen stored and retained in this hydrogen storage metal decreases, and the effect of the present invention becomes insufficient. Therefore, it is necessary that 0.3≦n≦4, and preferably 0.5≦n≦2.

In addition, these hydrogen storage metals are known to cause brittle fracture and become pulverized due to absorption and release of hydrogen, so in order to prevent them from falling off due to pulverization, it is necessary to crush them mechanically or to pulverize hydrogen in the gas phase in advance. A metal that has been pulverized by repeated occlusion and release of gas may be used, or metal particles such as nickel powder may be used as a matrix material to prevent this from falling off, and polymer powder or the like may be used as a binder.

The average particle size of the above-mentioned hydrogen storage metal particles is related to the porosity of the electrode surface and the dispersibility of the particles during electrode production, which will be described later, but a range of 0.1 to 100 μ is sufficient.

In the above range, from the viewpoint of porosity of the electrode surface, etc., it is preferably 0.9 μ to 50 μ, more preferably lu to 30 μ.

Furthermore, the particles used in the present invention are preferably single-surface porous in order to achieve a lower hydrogen overpotential of the electrode.

This surface porosity does not mean only that the entire surface of the particle is porous; it is sufficient that only the portion exposed from the metal layer described above is porous.

The degree of porosity is preferably as large as possible; however, if the degree of porosity is excessively large, the mechanical strength of the layer provided on the electrode core will decrease.
It is preferable to set it to 0-90%. Within the above range, it is more preferably 35 to 85%, particularly preferably 50 to 80%.

The above porosity is a value measured by a known mercury intrusion method or water displacement method.

The layer on which the electrode active metal particles are firmly provided on the metal substrate is preferably made of the same metal as part of the components constituting the particles.

Thus, a large number of the above particles are attached to the electrode surface of the cathode of the present invention, and macroscopically, the cathode surface is microporous.

As described above, in the cathode of the present invention, a large number of particles having a low hydrogen overvoltage are present on the electrode surface, and as mentioned above, the electrode surface is microporous, so the active surface of the electrode becomes larger. These synergistic effects make it possible to effectively reduce hydrogen overvoltage.

Moreover, since the particles used in the present invention are firmly attached to the electrode surface by the layer made of the metal, the durability of the low hydrogen overvoltage can be dramatically extended without deterioration.

The electrode core of the present invention may be made of any suitable conductive metal, such as Ti, Zr, Fe, Ni, V, or Mo.

A metal selected from Cu, Ag, Mn, platinum group metals, graphite, and Cr, or an alloy selected from these metals can be used. Among these, Fe, Fe alloy (Fe-Ni alloy).

Fe-Cr alloy, Fe-Ni-Cr alloy, etc.),
It is preferable to use Ni, Ni alloy (Ni-Cu alloy, Ni-Cr alloy, etc.), Cu, Cu alloy, etc. Particularly preferable materials for the electrode core are Fe, Cu,
These are Fe-Ni alloy and Fe-Ni-Cr alloy.

The structure of the electrode core can be made into any suitable shape and size depending on the structure of the electrode used. Its shape can be, for example, plate-like, porous, net-like (for example, expanded metal), or blind-like.
It may be made into a cylindrical shape.

The thickness of the layer of the present invention depends on the particle size of the particles employed, but
20μ to 2mm is sufficient, more preferably 25μ
~I mm. This is because in the present invention, some of the particles described above are buried in a layer made of metal on the electrode core.
This is because it causes the particles to adhere. In order to facilitate understanding of this state, a cross-sectional view of the electrode surface of the present invention is shown in FIG. As shown in the figure, a layer 2 made of metal is provided on an electrode core 1, and a portion of electrode active metal particles 3 are contained in the acid layer so as to be exposed from the surface of the layer. The proportion of particles in layer 2 is preferably 5 to 80 wt%, more preferably IO to 60 wt%. In addition to this state, between the electrode core and the layer containing the particles of the present invention, Ni, Co,
By providing an intermediate layer made of a metal selected from Ag and Cu, the durability of the electrode of the present invention can be further improved. Such an intermediate layer may be of the same type or a different type from the metal of the above-mentioned layer, but from the viewpoint of adhesion between the intermediate layer and the above-mentioned layer, these intermediate layers and the metals of the layer are of the same type. It is preferable. The thickness of the intermediate layer is preferably 5 to 100μ from the viewpoint of mechanical strength, more preferably 20 to 80μ, particularly preferably 3
It is 0 to 50μ.

To facilitate understanding of the electrode provided with such an intermediate layer, a cross-sectional view of the electrode is shown in FIG.

1 is an electrode core, 4 is an intermediate layer, 2 is a layer containing particles, and 3 is an electrode active particle.

Various methods are used to specifically attach the electrode surface layer, such as composite plating, melt coating, baking,
Pressure forming and sintering methods are used. Among these, the composite plating method is particularly preferable because it allows the electrode active metal particles to be attached well.

The composite plating method is a bath in which particles containing, for example, nickel as a part of the alloy component are dispersed in an aqueous solution containing metal ions forming a metal layer, and plating is performed using the electrode core as a cathode. The metal and particles are co-electrodeposited on the core. In more detail, the particles become bipolar under the influence of the electric field in the bath, and when they approach the cathode surface, they increase the local current density of the plating.
It is thought that when it comes into contact with the cathode, it is co-electrodeposited onto the core by metal plating caused by reduction of ordinary metal ions.

For example, when a nickel layer is used as the metal layer, various nickel plating baths can be used, such as a total nickel chloride bath, a high nickel chloride bath, a nickel chloride-nickel acetate bath, a Watts bath, and an old sulfamic acid bath.

The proportion of such particles in the bath is between 742 and 200 g of Ig.
/β is preferable from the viewpoint of improving the number of particles on the electrode surface. Also, the temperature conditions during dispersion plating work are 20 to 80℃, and the current density is IA/dm'' to 20℃.
A/dm" is preferable.

It goes without saying that additives for reducing strain, additives for promoting co-electrodeposition, etc. may be added to the plating bath as appropriate.

In addition, in order to further improve the adhesion strength of the particles, after completion of composite plating, ordinary plating or electroless plating may be performed to the extent that the particles are not completely covered, or heating and baking in an inert or reducing atmosphere may be carried out as appropriate. You may go.

In addition, as described above, when providing an intermediate layer between the electrode core and the metal layer containing particles, the electrode core is first plated with Ni, Co, or Cu, and then the dispersion plating method described above or the melt plating method is applied. A metal layer containing particles is formed thereon by means of a spraying method.

As the plating bath in such a case, the various plating baths mentioned above can be used, and also for Cu plating, a known plating bath can be used.

In this way, an electrode is obtained in which electrode active metal particles made of a hydrogen storage metal are attached to the electrode core via a metal layer.

Next, another method for manufacturing the cathode of the present invention will be explained.

The cathode of the present invention can also be manufactured by a melt coating method or a baking method. That is, hydrogen-absorbing metal powder is adjusted to a predetermined particle size and melted and sprayed using plasma, oxygen/acetylene flame, etc. to obtain a partially exposed coating layer of these particles on the electrode core, or by dispersing these particles. A liquid or slurry is applied onto the electrode core and baked to obtain a desired coating layer.

The cathode of the present invention can also be obtained by preparing an electrode sheet containing a hydrogen-absorbing metal in advance and attaching it to an electrode core. In this case, the sheet is preferably formed by mixing hydrogen-absorbing metal particles with organic polymer particles and molding the mixture, or by baking the mixture after molding. Of course, in this case, the electrode active particles are exposed from the surface of the sheet.

The sheet thus obtained is pressed onto the electrode core and fixed onto the electrode core by heating.

The electrode of the present invention can of course be used as an electrode for ion exchange removal alkaline chloride aqueous solution electrolysis, especially as a cathode, but it can also be used as a porous diaphragm (for example, an asbestos diaphragm).
It can also be used as an electrode for aqueous alkali chloride electrolysis using.

When used as a cathode for alkali chloride electrolysis, iron dissolved from the electrolytic cell material into the catholyte may be deposited on the cathode, reducing electrode activity. To prevent this,
It is an effective method to deposit a non-electronically conductive material as disclosed in JP-A-57-143482 on the cathode of the present invention.

Examples 1 to 5 and Comparative Example 1.2 The hydrogen storage alloys shown in Table 1 were ground to 25 μm or less, and the powder was placed in a nickel chloride bath (NiC42m・-5Hz03
00g/flood, 11.80.38g/J2 >3.
Add N at a rate of 5g/fl and stir well
Composite plating was performed using i-manufactured Exband Metal as a cathode and a Ni plate as an anode. The temperature is 40℃, p■ is 2.5,
The current density was 3 A/dm2. As a result, the amount of eutectoid of the hydrogen storage alloy is 3. A composite plating layer of Ig/dm" was obtained. The thickness of this plating layer was about 130 μm, and the porosity was about 70%.

These electrodes were constructed using Run□-Tins as the anode, and a fluorine-containing cation exchange membrane (CF2=CF
A copolymer of 2 and CF, CFOfOFF) acOOcHs, ion exchange membrane ffk 1.45 meq/g resin) was used as a cathode for a salt electrolytic cell with an ion exchange membrane.
A short circuit resistance test was conducted. The anolyte is 3N Na
The following short circuit test was conducted 200 days after the start of electrolysis using a CJ2 solution and 35% NaOH as the catholyte at 90° C. and a current density of 30 A/dm.

First, the anode and cathode during electrolysis were short-circuited with a copper wire to stop electrolysis, and the electrolysis was left as it was for about 5 hours.

During this time, the current flowing from the cathode to the anode was observed.

Note that the temperature of the catholyte was maintained at 90°C. Thereafter, this copper wire was removed and electrolysis was performed for one day.

This operation was repeated 5 times.

After the test was completed, electrolysis was continued for another 30 days, and then the electrodes were taken out and electrolyzed with 35% NaOH, 90°C, and a current density of 30A/dm.
The hydrogen overvoltage of each electrode was measured and shown in Table 1 along with the values before the test. In Examples 1 to 5, there was almost no change from before the test, and the change in hydrogen overvoltage was 1 OmV or less.

Similar tests were conducted for Comparative Examples 1 and 2 as well. In the comparative example, the overvoltage change before and after the short circuit test was lOmV, which was the same as in the example, but there was a change in hydrogen overvoltage over time, which was not seen in the example, and on the 10th day after the start of electrolysis, As in Example 1, the voltage was 90 mV, but it had increased to IlOmV on the 200th day. On the other hand, in Comparative Example 2, the hydrogen overvoltage increased by 30 mV in the short circuit test.

Example 6 LaNi11. used in Example 2. sAl+, os coarse powder 70% by weight, carbonyl nickel powder 20% by weight, P
A mixture having a composition of 10% by weight of TFE powder was thoroughly mixed with a pestle and formed into a sheet.

The thickness of this sheet was about 1 mm, and the porosity was about 50%. This was press-bonded to nickel expanded metal, and then fired at 350° C. for 1 hour in an argon atmosphere to form an electrode. As a result of testing this in the same manner as in Example 1, the hydrogen overvoltage was IoomV, which was almost the same as before the test.

Example 7 LaN1aAli, rs powder used in Example 3,
An aqueous methyl cellulose solution was added as a thickener and mixed well to prepare a paste. This was uniformly applied onto a nickel punched metal substrate by screen printing. Next, this was dried in air at 100°C for 1 hour, and then fired at a vacuum width of about 1000°C for 1 hour to form a LaNi5A
A sintered layer of 1□16 was formed. The thickness of this layer is approximately 1mm
The porosity was approximately 50%. Using this, a test similar to Example 1 was conducted, and the hydrogen overvoltage was 100 m
V, which was almost the same as before the test.

Table 1

[Brief explanation of the drawing]

FIG. 1 is a partial cross-sectional view of the surface of an example of the electrode of the present invention, and FIG.
The figures each show a partial cross-sectional view of the surface of another example of the electrode of the present invention.

Claims (8)

[Claims] (1) In an electrode in which electrode active metal particles are provided on an electrode core, the electrode active metal particles are a hydrogen storage metal that can electrochemically absorb and release hydrogen, and the hydrogen storage metal has the following formula: LaNi_5_+_nAl_2_ ．． 18_n (However,
0.3≦n≦4) A highly durable low hydrogen overvoltage cathode. (2) A highly durable and low hydrogen overvoltage cathode according to claim (1), wherein the electrode active metal particles are adhered to the electrode core by plating metal. (3) The highly durable and low hydrogen overvoltage cathode according to claim (2), wherein the plating metal is the same metal as a part of the components constituting the electrode active metal particles. (4) The compositional formula is LaNi_5_+_nAl_2_. 18_n (However,
0.3≦n≦4) A composite plating method is performed in which the electrode core is immersed in a plating bath in which hydrogen-absorbing metal particles that can electrochemically absorb and release hydrogen are dispersed as electrode-active metal particles. A method for producing a highly durable and low hydrogen overvoltage cathode, comprising co-electrodepositing the electrode active metal particles together with a plating metal on the electrode core. (5) High durability and low hydrogen according to claim (4), wherein the plated metal is formed in a layer on the electrode core, and a part of the electrode active metal particles are exposed on the surface of the layer. Method of manufacturing overvoltage cathode. (6) A layer containing hydrogen-absorbing metal particles that can electrochemically absorb and release hydrogen as electrode-active metal particles is baked or melt-coated, so that some of the electrode-active metal particles are applied to the surface of the layer. A method for producing a highly durable and low hydrogen overvoltage cathode, which is characterized in that it is provided on an electrode core so as to be exposed. (7) Produce a sheet containing electrode active metal particles made of a hydrogen storage metal capable of electrochemically absorbing and desorbing hydrogen so that a portion thereof is exposed from the surface of at least one side, and the sheet A method for producing a highly durable and low hydrogen overvoltage cathode, in which the surface opposite to the exposed surface of the particles is fixed to an electrode core. (8) A method for producing a highly durable and low hydrogen overvoltage cathode according to claim (7), wherein the sheet contains organic polymer particles as a sizing agent.