KR101986631B1 - Method and apparatus for mass production of non-oxidative exfoliated graphite by electrochemical treatment - Google Patents

Abstract

The present invention relates to a method for mass-producing non-oxidized peeled graphite by electrochemical treatment and an apparatus therefor, and more particularly, to an electrolyte preparation method for preparing an electrolyte in which a sulfate is dissolved; An operation electrode disposing step of disposing N (N is an integer of 1 or more) graphite electrodes as working electrodes at predetermined intervals in the electrolyte; Arranging N + 1 metal electrodes at predetermined intervals in the counter electrode of the working electrode so that graphite electrodes are positioned between adjacent two metal electrodes; And a graphite peeling step of peeling the graphite from the graphite electrode by applying a voltage to the working electrode and the counter electrode, and a method for mass production of the electrochemical non-oxidized peeling graphite.

Description

TECHNICAL FIELD The present invention relates to a non-oxidative exfoliated graphite by electrochemical treatment,

The present invention relates to a method for mass production of non-oxidative exfoliated graphite by electrochemical treatment and an apparatus therefor.

More specifically, an electrolyte preparation step of preparing an electrolyte in which a sulfate is dissolved; An operation electrode disposing step of disposing N (N is an integer of 1 or more) graphite electrodes as working electrodes at predetermined intervals in the electrolyte; Arranging N + 1 metal electrodes at predetermined intervals in the counter electrode of the working electrode so that graphite electrodes are positioned between adjacent two metal electrodes; And a graphite peeling step of peeling the graphite from the graphite electrode by applying a voltage to the working electrode and the counter electrode, and a method for mass production of the electrochemical non-oxidized peeling graphite.

A graphene is a two-dimensional sheet consisting of sp2 carbons of a honeycomb structure and having a thickness of atoms. Since the first isolation of graphene by the Geim professor group in 2004, research in related fields has increased rapidly. However, much of this graphene-related technology is not directed to actual single-layer graphene but to multi-layer graphene having a thickness of 2 to 10 graphene layers, or to a large amount in the exfoliation process used to make graphene (GO) which is oxidized to the graphene oxide (GO).

In order to solve the problem of manufacturing nanometer-sized carbon materials, exfoliated graphite having micrometer scale characteristics similar to graphene by directly separating low-cost natural graphite through physical and chemical methods, And the like.

Recently, Professor Mullen, a professor at Max flanck in Germany, has reported that a simple electrochemical process allows a large amount of high-quality exfoliated graphite in a short period of time without using special physical pressure or toxic strong acids such as sulfuric acid and nitric acid. (Mullen et al., J. Am. Chem. Sos. 2014, 135, 6083.). Korean Patent Laid-Open Publication No. 2013-0087018 discloses a technique relating to graphite peeling using an organic solvent such as propylene carbonate or dimethylformamide, a lithium salt and ultrasonic waves, and Korean Patent Publication No. 2016-0079115 Discloses a technique of using sodium citrate as an electrolyte as a technique capable of producing graphene oxide with an appropriate yield and thickness while maintaining a desired level of oxidation by a stripping method through an electrochemical reaction.

However, platinum (Pt) electrodes used in most electrochemical reactions to date have high electrochemical activity and high corrosion resistance, although they are advantageous against corrosion. In particular, in order to improve the economical efficiency in producing high-quality exfoliated graphite, it is necessary to produce a large amount in a short period of time. In this case, in the configuration of the electrochemical reaction device and the like, the cost, shape and size of the platinum Difficulties have become a major obstacle to mass production and commercialization through the process.

Korean Patent Publication No. 2013-0087018 Korean Patent Publication No. 2016-0079115

Mullen et al., J. Am. Chem. Soc. 2014, 135, 6083.

The present invention has been developed in order to solve the above-mentioned problems. It is an object of the present invention to provide a non-oxidative exfoliated graphite which can replace platinum (Pt) electrodes for mass production of non-oxidative exfoliated graphite, To provide a metal electrode for electrochemical non-oxidative exfoliated graphite easy to be controlled in shape and size.

In addition, in the present invention, it is possible to efficiently manufacture an electrode, which can mass-produce electrochemically non-oxidative exfoliated graphite using a metal electrode which can replace a platinum (Pt) electrode, and a method of producing a large amount of exfoliated graphite To provide an apparatus for mass production of electrochemical non-oxidized peeled graphite.

In order to solve the above problems, in the present invention, working electrodes for arranging N graphite electrodes (N is an integer of 1 or more) as working electrodes are arranged at predetermined intervals in the electrolyte, The present invention provides a method for mass production of electrochemical non-oxidized peeled graphite composed of stainless steel or nickel (Ni) and having N + 1 metal electrodes arranged at predetermined intervals to peel off graphite from graphite electrodes.

Further, in the present invention, the slot-like electrode fixing portion provided in a pair of 2N + 1 (N is an integer of 1 or more) with symmetrical shapes on opposite sides facing each other in the reaction cell at predetermined intervals, A cartridge type in which a working electrode or counter electrode in the form of sheet, foil or plate is fixed to a central region of the frame, the plate-shaped frame being divided into three regions to be inserted into the fixing portion, The present invention provides an apparatus for mass production of electrochemical non-oxidized peeled graphite comprising an electrode.

According to the method of the present invention, it is possible to replace the existing expensive platinum (Pt) electrode and to use the stainless steel or nickel (Ni), which is free to adjust the shape and size of the electrode, There is an advantage that a large amount of exfoliated graphite can be produced.

In particular, the method of the present invention is advantageous in that the production efficiency is increased by peeling the graphite electrode from both sides by N + 1 metal electrodes disposed between the N graphite electrodes with a predetermined gap therebetween.

In addition, the apparatus for mass-production of electrochemical non-oxidized peeled graphite according to the present invention has a constant distance between electrodes through a multi-electrode system in which a plurality of electrodes are disposed, so that the fluctuation of the production efficiency of peeled graphite is small, And the distance between the graphite electrode and the metal electrode is kept constant, so that the quality of the peeled graphite can be kept constant and the risk of fire due to the contact between the electrodes can be reduced.

Particularly, the apparatus according to the present invention is capable of mass production of exfoliated graphite by supplying a large amount of electrolytic solution easily, and stable discharging of the electrolyte containing the exfoliated graphite produced, and continuous production is possible.

FIG. 1 schematically shows a method for producing electrochemical non-oxidative peeled graphite using a single electrode according to an embodiment of the present invention. Referring to FIG.
FIG. 2 shows LSV measurement results of various metal electrodes and corrosion phenomena of Cu and Fe electrodes according to an embodiment of the present invention.
FIG. 3 shows a method for producing electrochemical non-oxidized peeled graphite using stainless steel (a) and nickel (b) as metal electrodes according to an embodiment of the present invention.
4 shows a method (a) for producing peeled graphite using a multi-electrode according to an embodiment of the present invention and a large amount of peeled graphite (b) produced therefrom.
5 schematically shows a front view of an apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention.
6 schematically shows aspects of an apparatus for mass production of electrochemical non-oxidized peeled graphite according to an embodiment of the present invention.
7 is a perspective view of a reaction cell in an apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention.
FIG. 8 is an enlarged view of an electrode fixing portion A of an apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention.
9 shows a cartridge-like electrode according to an embodiment of the present invention.
10 is a photograph showing an electrode actually connected to a cartridge-type electrode in a reaction cell according to an embodiment of the present invention to produce peeled graphite.
11 is a photograph showing the entire apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. In particular, the technical idea of the present invention and its core structure and action are not limited by this. In addition, the content of the present invention can be implemented by various other types of equipment, and is not limited to the embodiments and examples described herein.

According to an aspect of the present invention, there is provided an electrolyte preparation method for preparing an electrolyte in which a sulfate is dissolved; An operation electrode disposing step of disposing N (N is an integer of 1 or more) graphite electrodes as working electrodes at predetermined intervals in the electrolyte; Arranging N + 1 metal electrodes at predetermined intervals in the counter electrode of the working electrode so that graphite electrodes are positioned between adjacent two metal electrodes; And a graphite peeling step of peeling the graphite from the graphite electrode by applying a voltage to the working electrode and the counter electrode, in a mass production method of non-oxidized peeled graphite.

Referring to the method of Figure 1 for a electrochemical non manufacturing method of oxide exfoliation of graphite with a single electrode in accordance with one embodiment of the invention, ammonium sulfate (NH _{4) 2} SO ₄ metal electrode and the graphite in the same electrolyte solution When the electrode is inserted and electricity is applied, SO ₄ ^2- ions and water molecules contained between the graphite layers are oxidized to generate SO ₂ and O ₂ gas, and the graphite layer is peeled off by the gasification reaction, The peeled graphite can be produced in a very short time of 5 to 30 minutes.

In this case, if ultrasonic treatment is performed in aqueous solution and organic solvent for a short time, separation of graphite composed of grains of about 3 to 5 layers can be more effectively performed, and a chemical agent or physical Since there are few defects, there is an advantage that a post-treatment step such as reduction is not required and electrical characteristics are excellent. Because non-oxidative exfoliated graphite uses low cost graphite as a raw material, it can be mass-produced at low cost and does not use acid or organic solvent. Therefore, it does not need special purification and post-treatment process. It can be advantageously applied to various fields such as an anode material and a conductive material of a secondary battery, a transparent electrode for a display, a heat radiation material, a gas and an optical sensor.

According to one embodiment of the present invention, the sulfates are ammonium sulfate _{((NH 4) 2 SO 4} ), potassium sulfate (K ₂ SO _4), and sodium sulfate (Na ₂ SO ₄₎ 1 jong or more selected from the group consisting of . And more preferably from an ammonium sulfate _{((NH 4) 2 SO 4} ).

According to an embodiment of the present invention, the electrolyte may be an aqueous solution in which the sulfate is dissolved at a concentration of 0.1 to 1.0 M. The higher the concentration of the electrolyte, the higher the amount of graphite to be peeled off compared to the same time. That is, the yield is increased. However, when the test is conducted at 1.0 M or more, graphite is not stably peeled off and the problem of graphite falling into a lump form frequently occurs Occurs. That is, when the sulfate was an aqueous solution dissolved at a concentration of 0.1 to 1.0 M, the graphite was stably peeled off and the yield was also good.

According to one embodiment of the present invention, the graphite electrode is made of a material selected from the group consisting of hard carbon, soft carbon, artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbead, petroleum coke, Lt; / RTI >

According to an embodiment of the present invention, the metal electrode is preferably stainless steel or nickel (Ni). The platinum (Pt) electrode used in most of the electrochemical reactions has excellent electrochemical activity and strong resistance to corrosion, but it is very expensive, and it is difficult to control the shape and size of the electrode, which is a hindrance to mass production and commercialization. Accordingly, in the present invention, for the low-cost production of peeled graphite, electrochemical peeling of graphite was tested for various electrodes of various metals which can replace the platinum (Pt) electrode.

FIG. 2 shows (a) LSV measurement results of various metal electrodes and corrosion phenomenon (b) of Cu and Fe electrodes according to an embodiment of the present invention. As can be seen from FIG. 2, most of the metal electrodes were found to have significantly lower electrophoretic activity than platinum (Pt) in the result of LSV (linear sweep voltammetry) test of the metal electrode. Especially, in the copper (Cu) A phenomenon was observed. Therefore, considering such electrochemical activity and corrosiveness, stainless steel (SUS) and (b) nickel (Ni) electrodes of FIG. 3 (a) can be effectively used, and the test results are shown in FIG.

According to an embodiment of the present invention, the working electrode or the counter electrode is preferably in the form of a sheet, a foil, or a plate.

Generally, the amount of exfoliated graphite produced through the electrochemical stripping process of a metal anode and a pair of graphite cathodes is inevitably limited, and it is also difficult to constantly control the quality of exfoliated graphite produced in various batches The production of large quantities of exfoliated graphite in a single electrochemical batch is a very important factor in low cost mass production processes. Therefore, in order to increase the single batch production amount of the peeled graphite, it is necessary to enlarge the area of the metal electrode and the graphite electrode. Therefore, the working electrode or the counter electrode is preferably in the form of sheet, foil or plate . However, as the size of the graphite electrode increases, the time required for the stripping process becomes longer and the possibility of electrooxidation due to electrochemical reaction for a long time increases, resulting in poor processability and poor quality of the produced peeled graphite.

The present invention provides a multi-electrode system in which the number of graphite electrodes and metal electrodes is increased to solve such a problem. In other words, it was confirmed that when the metal electrodes were disposed on both sides of the graphite electrode and the electricity was applied, the peeling phenomenon occurred on both surfaces of the graphite electrode and the process efficiency was increased. , When N + 1 metal electrodes are arranged at regular intervals and then electricity is applied, N graphite electrodes are peeled off at the same time, and a large amount of peeled graphite can be produced in a short time. At this time, peeling the N graphite electrodes at the same time not only increases the efficiency of the peeling process N times, but also has an advantage that the quality of the peeled graphite produced in the same batch can be controlled.

FIG. 4 shows a method for producing peeled graphite using a multi-electrode (a) according to an embodiment of the present invention and a large amount of peeled graphite (b) produced therefrom.

According to one embodiment of the present invention, the graphite peeling step is preferably performed at a voltage of 10 V for 20 to 60 minutes (min). Particularly, when the apparatus for mass production of electrochemical non-oxidative peeled graphite having a multi-electrode according to the present invention is used, productivity of peelable graphite of ~ 30 g can be produced within 1 hour.

Further, in the present invention, a reaction cell having a predetermined volume capable of containing an electrolyte in which a sulfate is dissolved;

(2N + 1) pairs (where N is an integer of 1 or more) at predetermined intervals in a symmetrical manner on opposite sides of the reaction cell,

Wherein the electrode fixing unit includes: a working electrode fixing unit capable of fixing N (N is an integer of 1 or more) graphite electrodes as working electrodes; And

And N + 1 counter electrode fixing portions for allowing graphite electrodes to be positioned between two metal electrodes adjacent to the counter electrode of the working electrode with a predetermined gap therebetween. The electrochemical non-oxidized peel graphite mass production Device.

FIG. 5 schematically shows a front view of a mass-production apparatus for electrochemical non-oxidative peeled graphite according to an embodiment of the present invention, FIG. 6 schematically shows a side view

According to an embodiment of the present invention, the apparatus for mass-producing electrochemical non-oxidized peeled graphite includes a reaction cell 20 disposed on an upper portion of the entire apparatus frame 10, And the separator 50 may be provided at a lower portion of the electrolyte outlet.

At this time, the opening and closing means 30 of the electrolyte outlet 40 may be embodied in the form of a cock or a valve as a representative example, and representative examples of the separating means 50 include a filter or a vacuum filtration . ≪ / RTI > According to one embodiment of the present invention configured as described above, the graphite peeled off at the electrolyte outlet of the lower portion immediately after the peeled graphite is produced in the upper reaction cell is discharged and simultaneously separated by a filter or the like to effectively produce peeled graphite .

FIG. 7 is a perspective view of a reaction cell in an apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention, and FIG. 8 is an enlarged view of the electrode fixing section A. FIG.

According to an embodiment of the present invention, an apparatus for mass-producing electrochemical non-oxidative exfoliated graphite includes a reaction cell 20 having a predetermined volume capable of accommodating electrolyte in which sulfate is dissolved;

Shaped electrode fixing parts A provided in pairs of 2N + 1 (N is an integer of 1 or more) with symmetrical shapes on opposite sides of the reaction cell at predetermined intervals,

Wherein the electrode fixing unit includes: a working electrode fixing unit capable of fixing N (N is an integer of 1 or more) graphite electrodes as working electrodes; And

And an N + 1 counter electrode fixing unit for allowing the graphite electrode to be positioned between two adjacent metal electrodes at a predetermined interval with the counter electrode of the working electrode.

At this time, the reaction cell having the predetermined volume may be manufactured in various shapes and sizes as required. Preferably, the reaction cell may be in the form of a hexahedron having an open top as shown in FIG. 7, , The material may be a nonconductive transparent material, and preferably a material such as acrylic.

The cartridge type electrode 60 may be inserted into the working electrode fixing portion and the counter electrode fixing portion of the reaction cell 20 according to the embodiment of the present invention.

FIG. 9 illustrates a cartridge-type electrode according to an embodiment of the present invention, wherein the cartridge-type electrode comprises a plate-shaped frame divided into three zones, and a sheet, a foil, A groove may be formed so that a working electrode or a counter electrode in the form of a plate or a plate can be fixed and the left and right portions of the three regions defined by the frame are open structures in which the electrolyte can freely move, And can be closed by the open structure or the fixing of the working electrode or the counter electrode. At this time, the central region of the cartridge-type electrode may be a sheet, a foil or a plate working electrode or a counter electrode which is inserted and fixed in a slot of two overlapping rectangular frames having a through hole formed therein It is more preferable in terms of preventing contact between adjacent electrodes and ensuring safety. In addition, there is an advantage that only a specific electrode can be easily replaced when necessary in the production of peeled graphite.

This type of cartridge-like electrode is easy to insert and fix in the slot-shaped electrode fixing portion A and can be easily replaced according to the use of the electrode, and N (where N is an integer of 1 or more) The graphite electrode as the working electrode and the metal electrode as the N + 1 counter electrode can be stably fixed, thereby ensuring productivity and safety.

FIG. 10 is a photograph showing a state in which an electrode is actually connected to a cartridge-type electrode in a reaction cell according to an embodiment of the present invention to produce peeled graphite. In FIG. 10, five graphite electrodes and six metal electrodes After the exfoliated graphite is produced, it is discharged through the outlet of the electrolyte to the electrolyte after the reaction in the lower part. Thus, the process of manufacturing the exfoliated graphite can be terminated. Thereafter, a large amount of exfoliated graphite can be produced repeatedly using the same apparatus, When replacement of the electrode is required as required, it is possible to continuously produce peeled graphite while replacing only the electrode portion constituted by the individual square frames.

In this case, the apparatus may further include voltage applying means for applying a voltage to the working electrode fixing portion and the counter electrode fixing portion of the reaction cell, or may further include voltage applying means for applying a voltage to the cartridge type electrode . FIG. 10 shows an electrode, a fixed portion, and a voltage applying means of an apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention, in which a voltage applying means is connected to a central portion of a cartridge- It is also possible to arrange the voltage application means of the cartridge-type electrode alternately to the left and right sides of the central portion of the cartridge-like electrode.

FIG. 11 is a photograph showing the entire apparatus for mass-producing electrochemical non-oxidized peeled graphite according to an embodiment of the present invention. The solution, which is an electrolyte before reacting, is supplied to a reaction cell, and a large amount of non- . The valve is opened to move the electrolyte solution and the peeled graphite to the bottom, and the solution and the peeled graphite are separated using a vacuum filtration apparatus. After such a series of processes, the production of the exfoliated graphite can be continuously repeated, and it is possible to manufacture the exfoliated graphite while replacing some or all of the cartridge type graphite electrode and the metal electrode as necessary. Particularly, in general, the metal electrode is not required to be replaced frequently, but in the case of the graphite electrode, since the thickness or the shape of the electrode is deformed due to the production of the peeled graphite, the electrode is required to be replaced at an appropriate time. It is advantageous to continuously produce peeled graphite by easily replacing some of the electrodes required.

Test Example  1: Metal electrode LSV  exam

The LSV test was performed under the following conditions using various metals as electrodes to replace platinum (Pt) electrodes. One working electrode is a graphite electrode, the counter electrode is Pt, SUS (stainless steel), Al. Brass, Cu, Fe, graphite foil, Ni, and Pb were used, and the area of the working electrode and the counter electrode was 1 cm ² . Ag / AgCl was used as a reference electrode. The electrolyte is 0.5M (NH ₄₎ was used for ₂ SO ₄ 3 ml. The measurement voltage range is 0 V to -2.0 V and the scan rate is 20 mV / s. The results are shown in Fig.

FIG. 3 shows a state where SUS (stainless steel) and Ni are used as an electrode during the above test to produce peeled graphite under the same conditions.

Example 1: Preparation of peeled graphite using a multi-electrode

The electrodes were arranged as shown in FIG. 4 using two graphite electrodes as a working electrode and three stainless steel electrodes as a counter electrode. Then, the electrolyte was 0.5 M (NH ₄ ) ₂ SO ₄ Respectively. Each electrode was immersed in an electrolyte, and a DC voltage of + 10V was applied to the graphite electrode for 20 minutes while maintaining the distance between the electrodes at 2 cm. The graphite was peeled off and then filtered and cleaned using a vacuum filtration apparatus. A large amount of peeled graphite could be obtained as shown in Fig. 4 (b).

Example 2 Mass Production of Peeled Graphite Using Multi-Electrode

The electrodes were arranged as shown in FIG. 10 using five graphite electrodes each having a size of 10 cm x 15 cm as a working electrode and six stainless steel electrodes each having a size of 10 cm x 15 cm with a counter electrode. 0.5M (NH ₄₎ was used for ₂ SO _4. Each electrode was immersed in an electrolyte, and a DC voltage of + 10V was applied to the graphite electrode for 20 minutes while maintaining the distance between the electrodes at 2 cm. The graphite was peeled off and then filtered and cleaned using a vacuum filtration apparatus. When using multi-electrodes, ~ 30 g of peeled graphite can be obtained in one experiment.

10: frame
20: reaction cell
30: opening and closing means
40: electrolyte outlet
50: separation means
60: Cartridge type electrode
A:

Claims (10)

An electrolyte preparation step of preparing an electrolyte in which a sulfate is dissolved;
An operation electrode disposing step of disposing N (N is an integer of 1 or more) graphite electrodes as working electrodes at predetermined intervals in the electrolyte;
Arranging N + 1 metal electrodes at predetermined intervals in the counter electrode of the working electrode so that graphite electrodes are positioned between adjacent two metal electrodes; And
And a graphite peeling step of peeling the graphite from the graphite electrode by applying a voltage to the working electrode and the counter electrode,
The working electrode and the counter electrode are in the form of sheet, foil or plate,
Wherein the metal electrode is made of stainless steel or nickel (Ni). The method according to claim 1,
The sulfate is ammonium sulfate _{((NH 4) 2 SO 4} ), potassium sulfate (K ₂ SO ₄₎ and sodium sulfate (Na ₂ SO ₄₎ electricity, characterized in that at least one member selected from the group consisting of chemically non-oxidizing peeling graphite Mass production method. The method according to claim 1,
Wherein the electrolyte is an aqueous solution in which the sulfate is dissolved at a concentration of 0.1 to 1.0 M. The method according to claim 1,
Characterized in that the graphite electrode is derived from the group consisting of hard carbon, soft carbon, artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesocarbon microbeads, petroleum cokes, resin composites, carbon fibers and pyrolytic carbon Method for mass production of non - oxidative exfoliated graphite. delete delete The method according to claim 1,
Wherein the graphite peeling step is performed at a voltage of 10 V for 20 to 60 minutes (min). A reaction cell having a predetermined volume capable of accommodating the electrolyte in which the sulfate is dissolved;
(2N + 1) pairs (where N is an integer of 1 or more) at predetermined intervals in a symmetrical manner on opposite sides of the reaction cell,
Wherein the electrode fixing unit includes: a working electrode fixing unit capable of fixing N (N is an integer of 1 or more) graphite electrodes as working electrodes; And
And an N + 1 counter electrode fixing portion for allowing a graphite electrode to be positioned between two adjacent metal electrodes with a predetermined gap therebetween as a counter electrode of the working electrode,
A cartridge-type electrode is inserted into the working electrode fixing portion and the counter electrode fixing portion,
The cartridge-type electrode comprises a plate-shaped frame divided into three zones, and a working electrode or counter electrode in the form of sheet, foil or plate is fixed to the central region of the frame Characterized in that the electrochemical non-oxidative peeled graphite mass production apparatus is characterized by: The method of claim 8,
Wherein an electrolyte outlet including an opening / closing means is provided at a lower portion of the reaction cell. The method of claim 8,
Wherein the metal electrode is made of stainless steel or nickel (Ni).

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