JPH02310388A - Low hydrogen overvoltage cathode with high durability and its production - Google Patents

Abstract

PURPOSE:To obtain a low hydrogen overvoltage cathode minimal in deterioration in electrode activity even under an oxidizing environment by providing, as a cathode in an electrolytic cell for use in the electrolysis of salt solution, a metallic layer in which a part of hydrogen storage alloy with a specific composition is allowed to appear at the surface to the surface of an electrode core body. CONSTITUTION:As a cathode for use in an electrolytic cell for an aqueous solution of alkali halide, such as salt solution, electrode activation grains 3 of 0.1-100mum average grain size composed of a hydrogen storage alloy represented by LaNiXAlY or MmNiXAlY (where Mm means misch metal and the values of X and Y are 2-8 and >0-6, respectively) are formed by means of composite plating, etc., on the surface of an electrode core body 1 consisting of Fe, Fe-Ni alloy, Ni, Fe-Ni-Cr alloy, Cu, etc., so that part of the grains 3 are allowed to appear at the surface of a metallic layer 2 composed of Ni, etc., or further, Raney Ni and Raney Co are allowed to coexist with the hydrogen storage alloy, or, a metallic intermedi ate layer 4 composed of Ni, Co, Ag, Cu, etc., is provided between the electrode core body 1 and the metallic layer 2. By this method, the low hydrogen overvoltage cathode minimal in deterioration in electrode activity even if used in an oxidizing environment can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a highly durable low hydrogen overvoltage cathode, particularly a low hydrogen overvoltage cathode whose properties are 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 a lower hydrogen overvoltage and greater durability than previously known electrodes, the present inventors have conducted further studies. As a result, the inventors discovered that some of the electrodes disclosed in the above-mentioned publications do not necessarily have sufficient durability, and as a result of their earnest efforts to solve this problem, they have arrived at the present invention.

It is already a well-known industrial method for producing chlorine and caustic alkali to produce halogen gas from the anode chamber and a caustic alkali aqueous solution and hydrogen gas from the cathode chamber by electrolysis in an alkali halide aqueous solution electrolytic cell. As the cathode of this electrolytic cell, a cathode as described above 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 will be 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 It was discovered that in the case of a cathode containing cobalt as an active ingredient, the electrode activity decreases due to the deterioration of cobalt into hydroxides, and the electrode activity does not return to its original active state even after restarting operation (that is, the hydrogen overvoltage increases).

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 NaOH for a long time, if the cathode active ingredients are nickel or cobalt, they will reach a corrosion potential. (This reaction is also a type of electrochemical oxidation reaction) and the electrode activity was found to decrease.

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

(Means for solving the problem) Therefore, as a result of intensive studies to prevent this phenomenon, we decided to use a hydrogen storage metal that electrochemically absorbs and releases hydrogen and has a low hydrogen overvoltage as part of the active component of the electrode. If used in all cases, when the battery case is stopped as in the previous stage, a large amount of hydrogen stored in the hydrogen storage metal will be electrochemically oxidized, thereby effectively preventing the oxidation of the electrode active component.
That is, the present invention was completed by discovering that the activity can be maintained for a long period of time, and the present invention provides an electrode in which a part of the electrode active metal particles is exposed on the surface of a layer provided on the electrode core. The gist is a highly durable low hydrogen overvoltage cathode in which some of the metal particles are hydrogen storage metals that can electrochemically absorb and release hydrogen, and a method for producing the above highly durable low hydrogen overvoltage cathode described below. be.

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

Storage XH2O+xe+M # MHx4rxOH-(1
) Release M is a hydrogen storage metal and MHx represents its hydride. When a cathode in which this hydrogen storage metal is used as part or all of the electrode active metal is used, for example, for salt electrolysis using an ionic membrane, the hydrogen storage metal is absorbed by the rightward reaction of reaction formula (1) at the initial stage of energization. When the hydrogen storage 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.

H20+e++H2+OH- (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 reacts with the reaction formula (1).
Oxidation of the electrode-active metal particles themselves can be effectively prevented by releasing hydrogen through a leftward reaction, that is, by electrochemically oxidizing hydrogen and bearing an oxidation current.

As described above, the hydrogen storage metal that can be used in the present invention is one that can electrochemically store and release hydrogen, and specifically, LaN1xAly (where 2≦X
≦8.0×Y≦6. ) and/or MmNixAl
v (However, Mm is Mitsushmetal, 2≦X≦
8, O<Y≦6. ) Mitsushmetal −
It is a nickel-aluminum alloy. When X<2, the nickel content in the alloy is low, causing problems in corrosion resistance in a caustic alkaline solution, making it difficult to withstand long-term use, and hydrogen overvoltage cannot be made so low. When X>8, the amount of hydrogen absorbed by the hydrogen storage metal decreases, and the equilibrium pressure of absorption and desorption increases, so that the effect of the present invention becomes insufficient. When Y=O, the hydrogen overvoltage becomes high when X<5, and the equilibrium pressure of occlusion and desorption becomes high when X≧5, making the effect of the present invention insufficient. When V>6, there is a problem in corrosion resistance in a caustic alkaline solution, and the material cannot withstand long-term use. From the above, hydrogen storage metal L
aN! In xAIy and/or MmNiXAlY, it is necessary that 2≦(X≦8 and O×Y≦6, preferably 4≦X≦6 and 0<Y≦3.

In the present invention, the above-mentioned hydrogen storage metal can be used alone as the electrode active metal particles, or the above-mentioned hydrogen storage metal and Raney nickel and/or Raney cobalt can be used together. I'll come. When a hydrogen storage metal and Raney nickel and/or Raney cobalt are simultaneously used as electrode active metals, the hydrogen storage metal is used in an amount of 5 wt$ or more in the electrode active metals, particularly,
It is preferable that 10wt2 or more be present. This is because if the proportion of hydrogen storage metal is less than 5 wtx, the amount of hydrogen released upon short circuit is small, and the short circuit oxidizes the electrode active metal, reducing electrode activity and increasing hydrogen overvoltage.

In addition, these hydrogen-absorbing metals are known to undergo brittle fracture and become pulverized due to absorption and release of hydrogen, so in order to prevent decomposition due to pulverization, they must be mechanically pulverized or placed in a gas phase in advance. Metals that have been pulverized by repeated occlusion and release of hydrogen gas are used, and in order to prevent this from falling off, metal particles such as nickel powder and polymer powders are used as binders in addition to the Raney nickel and Raney cobalt as matrix materials. Good too.

The average particle diameter 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 μm to 100 μm is sufficient.

Within the above range, from the viewpoint of porosity of the electrode surface, preferably 0.9 μm to 50 μm, more preferably 1 μm to 30 μm.
It is μm.

Furthermore, the particles used in the present invention are preferably superficially 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, excessive porosity reduces the mechanical strength of the layer provided on the electrode core.
It is preferable that y) is 20 to 90χ. The above range Kai is preferably 30-851, particularly preferably 50-8
It is 0%.

Note that the above-mentioned 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.

In this way, in the cathode of the present invention, a large number of particles that themselves have 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 can 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 above-mentioned metal, they are difficult to deteriorate and can dramatically extend the sustainability of the above-mentioned low hydrogen overvoltage.

The electrode core of the present invention may be made of any suitable conductive metal, such as Ti5Zr, Fe, Ni, V, Mo, C.
A metal selected from u, Ag, metal, platinum group metal, graphite, and Cr, or an alloy selected from these metals can be used.

Among these, Fe, Fe alloy (Fe-Ni alloy, Fe-C
r alloy Je-N+-Cr alloy, etc.) Ni, Ni alloy (
(Ni-Cu alloy, Ni-Cr alloy, etc.) It is preferable to employ Cu, Cu alloy, etc. Particularly preferable materials for the electrode core are Fe, Cu, Ni, Fe-Ni alloy, Fe-
It is a 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
A thickness of 20 μm to 2 mm is sufficient, and more preferably 25 μm to 2 mm.
It is μm to 1 mm. This is because, in the present invention, some of the particles described above are attached to the electrode core while being buried in the metal layer. 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 this layer so as to be exposed from the surface of the layer. The proportion of particles in layer 2 is preferably 5 to 80%, more preferably 10 to 60%. In addition to this state, there are Ni, Co, Ag between the electrode core and the layer containing the particles of the present invention.
By providing an intermediate layer made of a metal selected from , 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 must be of the same type. is preferred. It is sufficient that the thickness of the intermediate layer is 5 to 100 μm from the viewpoint of mechanical strength, etc.
More preferably 20 to 80 μm 1 Especially preferably 30
~50 μm.

To make it easier to understand electrodes 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 can be used to specifically attach the electrode surface layer, such as a composite plating method, a melt coating method, a baking method, a pressure molding sintering method, and the like. Among these, the composite plating method is particularly preferable because the electrode active metal particles can 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 in the bath due to the influence of the electric field, increasing the local current density of plating when they come close to the cathode surface, and when they come into contact with the cathode, the particles become bipolar due to the reduction of normal metal ions. It is thought that it is co-electrodeposited on the core by plating.

For example, when using a nickel layer as a metal layer, do you choose a total nickel chloride bath, a high nickel chloride bath, a nickel chloride-nickel acetate bath, a Watts bath, or a nickel sulfamate bath? valley,
Various nickel plating baths such as nickel plating baths can be adopted.

The proportion of such particles in the bath ranges from 1 g/l to 2008
It is preferable to set the value to /1 from the viewpoint of improving the adhesion state of particles to the electrode surface. In addition, the temperature conditions during dispersion plating work are 20 to 80℃, and the current density is IA/dm2 to 2OA.
/dm2 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 mentioned above, when providing an intermediate layer between the electrode core and the metal layer containing particles, the electrode core is first plated with nickel, cobalt, or copper, and then the dispersion plating method described above or melt plating is applied. A metal layer containing particles is formed thereon by means of atomizing force.

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

In this way, an electrode is obtained in which electrode active metal particles containing 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 described.

The cathode of the present invention can also be manufactured by a melt coating method or a baking method. That is, a mixed powder of a hydrogen-absorbing metal powder and another low hydrogen overvoltage metal powder (obtained, for example, by a melt-grinding method) is adjusted to a predetermined particle size, and melted and blown using plasma, oxygen/acetylene flame, etc., to form an electrode core. A partially exposed coating layer of these particles is obtained on the electrode core, or a dispersion or slurry of these particles 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 storage metal in advance and attaching this to an electrode core. In this case, the sheet is formed by mixing hydrogen-absorbing metal particles and other metal particles (for example, Raney alloy showing low hydrogen overvoltage characteristics) with organic polymer particles, or by sintering the mixture after forming. The method is preferred.

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 membrane-based alkaline chloride aqueous solution electrolysis, especially as a cathode, but 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.

In the present invention, hydrogen storage metal LaN1xAIv and/or
Or MmN! xAIy (however, 2≦X≦8.0
Y≦6. ) The reason why the characteristics of the electrode are significantly improved by using the hydrogen storage metal of the present invention is that compared to the conventionally known hydrogen storage metals LaNi5 or MmN i s, the hydrogen storage metal of the present invention has a lower equilibrium pressure, which makes it easier to conduct electricity. This is thought to be based on the fact that hydrogen is easily absorbed chemically, the hydrogen stored in the hydrogen storage metal can act more effectively when the battery is stopped due to a short circuit, and the hydrogen overvoltage itself is low. It will be done.

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 6 and Comparative Examples 1 to 5 The lanthanum-nickel-aluminum hydrogen storage alloy shown in Table 1 (Comparative Example 1 was a lanthanum-nickel hydrogen storage alloy) was ground to 25 μm or less, and this powder was mixed with nickel chloride. Bath (NiCI2*6) 120300g/l, HaBOs
38g/I) at a ratio of 18/1, and while stirring well, composite plating was performed using a nickel expanded metal as a cathode and a nickel plate as an anode.

Temperature is 40℃, pH is 2.5, current density is 3A/di2
And so.

As a result, the amount of eutectoid hydrogen storage alloy is approximately 38/d.
A composite plating layer of m2 was obtained. The thickness of this plating layer was about 120 μm, and the porosity was about 651.

These electrodes have a Ru0a-Ti02 anode and a fluorine-containing cation exchange membrane (CFa=CF2 and CF2:CF
The following two types of tests were conducted using a hydrolyzate of a copolymer with O(CF2)3COOCH3 (resin with an ion exchange capacity of 1.45 meq/g) as an ion exchange membrane for salt electrolysis.

[Test ■] Resistance test against short circuit A zero-order short circuit test was carried out on the 200th day after the start of electrolysis using a 3N NaCl solution as the anolyte and 35X NaOH as the catholyte at 90° C. and a current density of 30 A/dl112.

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, the copper wire was removed and electrolysis was carried out for one day. This operation was repeated 5 times.

After the test was completed, electrolysis was continued for another 30 days, and then the electrode was taken out and treated with 35INaOH, 90°C, and a current density of 30A/dn.
The hydrogen overvoltage of each electrode was measured at +2.

[Test ■] Resistance test against minute reverse current Electrolysis was performed in the same manner as Test ■, and the following operation was performed 50 days after the start of electrolysis.

First, the ohmic loss occurring at both ends of the anode and cathode during electrolysis is 1
．． Short-circuit with a 2V copper wire to stop electrolysis, and then continue with 4V.
It was left for 8 hours. Furthermore, the short-circuited copper wire was replaced with a copper wire with an ohmic loss of O, SV at both ends, and the short-circuit continued.
It was left for 0 hours. During this time, the current flowing from the cathode to the anode was observed. The electrolytic cell was allowed to cool naturally at the same time as the short-circuiting operation was started.

Thereafter, the temperature of the electrolytic cell was raised to 90° C., the copper wire was removed, and electrolysis was carried out for one week. This operation was repeated four times.

After the test was completed, electrolysis was continued for another 30 days, and then the electrode was taken out and treated with 35X NaOH at 190°C and a current density of 30A/dm.
” was used to measure the hydrogen overvoltage of each electrode.

The results are shown in Table 1 along with the hydrogen overvoltage before the test.

Table 1 Example 6 Lanthanum-nickel-aluminum hydrogen storage alloy a
NiaAI2. This powder was added to nickel chloride at a ratio of 0.63/l, and then commercially available Raney nickel alloy powder (manufactured by Oko Rika, Ni 50
wt$, Al 50wt$, 500m2) was added at a rate of 2.7g/l at night, and composite plating was performed in the same manner as in Example 1. As a result, La
Ni5Al2. The eutectoid amount of s is 1.7 g/di2, the eutectoid amount of Raney nickel alloy is 1.7 g/dm2, that is, L
The ratio of aNiaAIg,s is 50! , LaN1eA12. with a Raney nickel alloy proportion of 501. A composite plating layer in which S and Raney nickel alloy coexisted was obtained. The thickness of this plating layer was about 150 μm, and the porosity was about 7 oz.

This sample was immersed in a 25.chi.NaOH solution at 90.degree. C. for 2 hours to develop AI of the Raney nickel alloy, and then the same test as in Example 1 was conducted using this sample as a cathode. As a result of measuring the hydrogen overvoltage after the test, it was 80 mV and hardly changed in both Test (■) and Test (■).

Example 7 Composite plating was performed in the same manner as in Example 1 except that the lanthanum-nickel-aluminum hydrogen storage alloy LaNi5Al in Example 1 was replaced with the Mitshu metal-nickel-aluminum alloy MmNi5Al+,s. As a result, MmN
A composite plating layer with an eutectoid amount of i5Al+;s of 3.1 g/drn2 was obtained, and the thickness of this plating layer was approximately 130 μm.
Porosity was approximately 7 oz.

A test similar to Example 1 was conducted using this sample as a cathode.

After the test, the hydrogen overvoltage was measured and the results were Test ■, Test ■
There was almost no change at 85 mV in either case.

[Brief explanation of drawings]

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. 1: Electrode core 2: Layer made of metal 3: Electrode active metal particles Figure 1 Figure 2

Claims (10)

[Claims] (1) In an electrode in which electrode active metal particles are provided on an electrode core, a part of the electrode active metal particles is a hydrogen storage metal that can electrochemically absorb and release hydrogen; A highly durable and low hydrogen overvoltage cathode represented by the following formula: LaNi_XAl_Y and/or MmNi_XAl_Y (where Mm is a misch metal, and 2≦X≦8, 0<Y≦6). (2) The highly durable and low hydrogen overvoltage cathode according to claim (1), wherein some of the electrode active metal particles are particles made of Raney nickel and/or Raney cobalt. (3) The proportion of hydrogen storage metal in the electrode active metal particles is 5%
The highly durable low hydrogen overvoltage cathode according to claim (1) above. (4) 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. (5) The highly durable and low hydrogen overvoltage cathode according to claim (4), wherein the plating metal is the same metal as a part of the components constituting the electrode active metal particles. (6) The compositional formula is LaNi_XAl_Y and/or MmNi_XAl_
Y (where Mm is a misch metal and 2≦X≦8, 0<Y≦6). The electrode core is immersed in a partially dispersed plating bath, and the electrode active metal particles are co-electrodeposited with the plating metal onto the electrode core by a composite plating method. Method for manufacturing hydrogen overvoltage cathode. (7) High durability and low hydrogen according to claim (6), 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. (8) A layer containing a hydrogen storage metal capable of electrochemically absorbing and desorbing hydrogen as at least a part of the electrode active metal particles is formed by a baking method or a melt coating method so that a part of the electrode active metal particles becomes part of the layer. A method for producing a highly durable and low hydrogen overvoltage cathode, the cathode being provided on an electrode core so as to be exposed on the surface of the cathode. (9) Contains electrode-active metal particles consisting of a hydrogen-absorbing metal that can electrochemically absorb and release hydrogen, or this metal and another low hydrogen overvoltage metal, so that a part of the particles is exposed from the surface of at least one side. Create a required sheet,
A method for producing a highly durable and low hydrogen overvoltage cathode, in which a surface of the sheet opposite to the exposed surface of the particles is fixed to an electrode core. (10) The method for producing a highly durable and low hydrogen overvoltage cathode according to claim (9), wherein the sheet contains organic polymer particles as a sizing agent.