JPS6134516B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-purity halogens by electrolysis of hydrogen halide solutions, and in particular for producing chlorine by electrolyzing hydrochloric acid in a bath using a solid polymer electrolyte membrane and a catalytic anode and cathode bonded to at least one surface of the membrane. Regarding how to. The production of halogens by electrolysis of aqueous hydrogen halides, such as hydrochloric acid, is of great industrial interest because of the large amounts of hydrochloric acid left over as a by-product of organochlorination and other industrial processes throughout the world. On the other hand, the demand for hydrochloric acid itself has not kept pace with the supply of lower hydrochloric acids. Disposal of such surplus HC by-products poses a vexing environmental problem to the chemical industry. This is because such large quantities of hydrochloric acid must be disposed of without polluting the environment. Therefore, direct electrolysis of hydrochloric acid in an aqueous solution is very advantageous for the chemical industry.
This is because it provides a way to dispose of large amounts of surplus hydrochloric acid and at the same time produce chlorine, which is in great and increasing industrial demand. Conventionally, in order to produce chlorine from hydrochloric acid, an aqueous solution of hydrochloric acid is electrolyzed in an electrolytic cell having a diaphragm. Typically, in an electrolytic cell, solid graphite electrodes are separated by a suitable gasket, and the space between the electrodes is filled with a hydrochloric acid solution and separated by a perforated diaphragm. During electrolysis, chlorine is generated at the anode and hydrogen is generated at the cathode. but,
The working bath voltage in such industrially used electrolytic cells is significantly in excess of the theoretical voltage at which chlorine is generated at the anode and hydrogen is generated at the cathode. This excess voltage in industrially used HC electrolysers is in principle due to the chlorine overpotential at the graphite anode and the hydrogen overpotential at the graphite cathode, as well as the IR drop in the diaphragm and electrolyte. Since electrolyser operating costs are directly related to bath voltage, this excess voltage naturally affects the economics of the process. Reducing the overvoltage to reasonable values requires operation at low current densities, which has a negative impact on equipment costs. To reduce bath voltage and operating costs while operating at reasonable current densities, i.e., 300 amperes per square foot (ASF) or higher, operation at elevated temperatures, i.e., 80°C (176〓) or higher, is a challenge. Needed. Such high temperature operation, in turn, poses a number of additional problems. Furthermore, these systems use a perforated membrane to separate the anolyte and catholyte compartments, so that gaseous hydrogen generated at the cathode passes through the membrane and returns to the anode. As a result, the chlorine produced contains significant amounts of hydrogen, and gas separation equipment is required to obtain the desired high grade chlorine. There is also the problem of rapid depletion of the anolyte, resulting in a decrease in HC concentration and an increase in the rate of water electrolysis and therefore oxygen evolution. The evolution of oxygen is very troublesome because, as is well known, oxygen attacks graphite and rapidly deteriorates the electrode. The present invention seeks to overcome these conventional drawbacks, and its advantages will become clearer as the description proceeds. According to the present invention, a halogen such as chlorine is generated by electrolyzing hydrochloric acid in a tank separated into a catholyte compartment and an anolyte compartment by a solid polymer electrolyte in the form of a cation exchange membrane. bonding a catalytic electrode to at least one side, preferably both sides of the membrane;
The catalytic anode and cathode electrodes have extremely low halogen and hydrogen overvoltages. The HC aqueous solution is brought into continuous contact with the anode. Chlorine is generated at the anode,
H ⁺ ions are transferred to the cathode, where they are released. The catalytic electrode takes the form of a bonded mass of carbon fluoride (polytetrafluoroethylene) and graphite particles. Graphite-containing catalytic electrodes are other examples of useful platinum group element catalysts that include a catalytic material that has been thermally stabilized by heating a reduced oxide of at least one platinum group element in the presence of oxygen. These include platinum, palladium, iridium, rhodium, ruthenium and osmium. Suitable reduced metal oxides for the production of chlorine are reduced oxides of ruthenium or iridium. The electrocatalyst can be a single reduced platinum group element oxide, such as ruthenium oxide, iridium oxide, platinum oxide, and the like. However, mixtures or alloys of reduced platinum group element oxides have been found to be more stable. Electrodes of reduced ruthenium oxide containing up to 25% by weight of reduced oxide of iridium, preferably from 5 to 25% by weight of iridium oxide, have been found to be very stable. Graphite is present in an amount up to 50% by weight, preferably from 10 to 30%. Graphite has excellent conductivity with low halogen overpotentials and is much cheaper than platinum group elements, making it possible to create very inexpensive and highly effective halogen generating electrodes. One or more reduced oxides of valve metals such as titanium, tantalum, niobium, zircon, hafnium, vanadium or tungsten are added to the above components to make the electrodes resistant to oxygen, chlorine and generally harsh electrolytic conditions. It can be stabilized. Valve metal is effective in an amount of 50% by weight or less, preferably in an amount of 25 to 50% by weight. The novel aspects that characterize the invention are set forth in the claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to further clarify the configuration, embodiments, objects and effects of the present invention, the present invention will be described below in conjunction with the drawings. FIG. 1 is an exploded perspective view showing an electrolytic cell using a solid polymer electrolyte membrane suitable for the present invention. 1
The electrolytic cells collectively referred to as 0 have a cathode chamber 11 and an anode chamber 1.
2, and a solid polymer electrolyte membrane 13 separating these two chambers, the membrane 13 preferably being a hydration cation selective membrane. Bonded to both surfaces of membrane 13 are catalytic carbon fluoride bonded graphite electrodes. Catalytic carbon fluoride-bonded graphite electrodes can be used alone or with thermally stabilized reduced oxides of platinum group elements, such as ruthenium oxide RuOx, or iridium, ruthenium-iridium, ruthenium-titanium, ruthenium-tantalum or ruthenium-titanium. A mixture with a stabilized reduced oxide of iridium is used. A cathode, designated 14, is attached to one side of the membrane 13, and a catalytic anode (not shown) is attached to the opposite side of the membrane. The cathode is a Teflon bonded mass of catalyst particles which may be identical to the anode catalyst, ie, graphite alone or a mixture of graphite and thermally stabilized particles of reduced oxides of platinum group elements with or without the addition of transition valve metals. Alternatively, platinum black and Pt, Pt―Ir, Pt―
Mixtures and alloys of thermally stabilized reduced oxides of Ru, Pt-Ni, Pt-Pd, Pt-Au can be used.
This is based on the fact that the acid concentration on the cathode side due to transport with H ⁺ ions through the HC membrane is extremely low, less than 10% of the anolyte concentration. Current collectors in the form of metal screens or porous sheets, namely graphite 15 and 16, are pressed against the electrodes. The entire membrane/electrode assembly is sealed between housing members 11 and 12 with gaskets 17 and 18, such as Irving Moore Co., Ltd.
A filled rubber gasket sold under the trade name EPDM by Mocre Company, Massachusetts, USA, provides a secure hold. The electrolyte inlet 19 communicates with the inside of the anode chamber 20, and an aqueous hydrochloric acid solution is introduced through the inlet. The spent electrolyte and chlorine gas are removed from outlet conduit 21. A cathode outlet conduit 22 communicates with the cathode chamber 11 to remove the hydrogen produced at the cathode together with water or hydrochloric acid which is forced through the membrane 13 by the proton effect. A power cable 23 is inserted into the cathode chamber and a corresponding cable (not shown) is inserted into the anode chamber. Cables connect the current conducting screens 15 and 16 to a power source. Figure 2 is a cross-sectional view of the membrane, plotting the reactions that occur in various parts of the cell during the electrolysis of HC, and is useful for understanding the electrolysis process and the operation of the cell. An aqueous hydrochloric acid solution is introduced into the anode chamber separated from the cathode chamber by a cation membrane 13. stabilized by reduced oxides of iridium or titanium
A bonded graphite electrode containing a reduced oxide of Ru is pressed into the surface of membrane 13 as shown.
Current collectors 15 and 16 are pressed onto the surface of the catalyst electrode and are connected to the positive and negative terminals of a power source, respectively, thereby applying an electrolytic voltage between the electrodes. Hydrochloric acid introduced into the anode chamber is electrolyzed at the anode 24 to generate gaseous chlorine and hydrogen ions (H ⁺ ). This H ⁺ ion is transported across membrane 13 to cathode 14 along with some water and hydrochloric acid. Hydrogen ions are bonded to the surface of the membrane and ejected at a cathode electrode embedded within the surface. The cathode 14 is also composed of, for example, platinum group elements and valve metals 4 such as Ru, Ir,
It can be composed of fluorocarbon-bonded graphite containing thermally stabilized reduced oxides of Ti, Ta, etc.). The reactions in the various parts of the tank are as follows. Anodic reaction 2C ⁻ →C ₂ ↑＋2e ⁻ (1) Membrane transport 2H ⁺ (H ₂ O, HC) (2) Cathode reaction 2H ⁺ +2e ⁻ →H ₂ ↑ (3) Overall reaction 2HC→H ₂ +C ₂ ( 4) In this structure, the catalytic site of the electrode is connected to the cationic membrane and thus to the ion-exchange acid groups (SO ₃ H×H ₂ O sulfonic acid groups or COOH×
in direct contact with H ₂ O carboxylic acid groups). As a result, there is no IR drop in the anolyte or catholyte compartment (commonly referred to as "electrolyte voltage drop"), which is one of the major advantages of the present invention. Furthermore, since chlorine and hydrogen are generated at the interface between the electrode and the membrane, there is no voltage drop due to the so-called "bubble effect", which is gas mass transfer loss.
That is, in conventional systems, gas production occurs between the membrane and a catalytic electrode spatially separated from the membrane. This gas layer or film at least partially blocks ion transport between the catalytic electrode and the membrane and also results in a voltage drop. Electrodes Perfluorocarbon-polytetrafluoroethylene (known under the trade name "Teflon" from Dupont) bonded graphite electrodes with platinum group elements, such as ruthenium, to minimize chlorine overpotentials as anodes. , iridium, ruthenium-iridium, and other reduced oxides.
Effective long-life anodes obtained by stabilizing reduced ruthenium oxide are stable in acids and have extremely low chlorine overpotentials. To perform stabilization, temperature (thermal) stabilization is first performed. That is, the reduced ruthenium oxide is heated below the temperature at which the reduced oxide begins to decompose into pure metal. Therefore, the reduced oxide
Heating to 350-750°C for 30 minutes to 6 hours is preferred, and the thermal stabilization step can be suitably achieved by heating the reduced oxide at a temperature in the range of 550-600°C for 1 hour. . Teflon-bonded graphite electrodes containing reduced ruthenium oxide combine ruthenium with other platinum group elements, such as iridium (IrOx5~25
% iridium, 25% preferred), heat stabilized reduced oxides such as palladium, rhodium, or reduced titanium oxides (TiOx, 25-50% TiOx preferred) or reduced tantalum oxides (25% or more ) can be further stabilized by alloying or mixing with We also found that a ternary alloy of titanium, ruthenium and the reduced oxides of iridium (or tantalum) (Ru, Ir, Ti)Ox is very effective in producing stable, long-life anodes. In the case of a ternary alloy, the composition is preferably 5% by weight of reduced iridium oxide and equal proportions (47.5% by weight) of reduced oxides of ruthenium and the transition metal titanium. For reduced oxides of ruthenium and titanium, a preferred range is 50% ruthenium and 50% titanium by weight. Of course, titanium
It has the advantage of being much cheaper than either ruthenium or iridium. In the electrode structure
Other transition metals such as Nb, Zr or Hf can easily be used in place of Ti or Ta. An alloy of reduced noble metal oxides of ruthenium and iridium is mixed with Teflon along with a reduced oxide of titanium to form a homogeneous mixture. This is further blended with a graphite-Teflon mixture to form a noble metal activated graphite structure. A typical noble metal content for the anode is 0.6 mg/ ^cm2 of electrode surface, with a preferred range of 1-2.
mg/ ^cm2 . The cathode can likewise be a mixture of Teflon-bonded graphite with an alloy or mixture of reduced oxides of ruthenium, iridium and titanium, or with ruthenium itself. Or other precious metals,
For example, reduced oxides of platinum, Pt--Ir or Pt--Ru can also be used. The reason is that the cathode is not exposed to the high hydrochloric acid concentrations of the anode, which attack and rapidly dissolve platinum. HC at cathode
The concentration is typically more than 10 times more dilute than the anolyte. Like the anode, the cathode electrode is bonded to and embedded within the surface of the cationic membrane. Reduced ruthenium oxide lowers the overpotential for hydrogen evolution, and iridium and titanium stabilize the ruthenium. The anode current collector that engages the bonded anode layer has a higher chlorine overpotential than the catalytic anode. This reduces the possibility of electrochemical reactions such as chlorine generation occurring at the current collector surface. Suitable materials are Ta, Nb screens or porous graphite sheets. Since the bond electrode surface has a low chlorine overvoltage and a large voltage drop to the collector surface, the chlorine generation reaction is much more likely to occur at the feed electrode surface. Similarly, the cathode current collector is fabricated from a material that has a higher hydrogen overpotential than the cathode. As a result, hydrogen evolution at the current collector is less likely to occur because the overvoltage is low and the current collector shields or shields the electrode to some extent. By maintaining the bath voltage at a minimum level at which chlorine and hydrogen are generated at the electrodes, no gas generation occurs in the current collector since the gas generation overvoltage of the current collector is higher. Membrane Membrane 13 is preferably a stable hydrated cationic film characterized by ion transport selectivity. Cation exchange membranes allow the passage of positively charged cations, but minimize the passage of negatively charged anions. Various ion exchange resins can be formed into membranes that selectively transport cations. Two of these types are the so-called sulfonic acid cation exchange resins and carboxylic acid cation exchange resins. Sulfonic acid cation exchange resins are preferred, the ion exchange groups of which are hydrated sulfonic acid groups SO ₃ H
×H ₂ O, which is bonded to the polymer main chain through sulfonation. The ion-exchange acid groups are not mobile within the membrane and are fixedly bound to the polymer backbone, ensuring that the electrolyte concentration remains unchanged. As mentioned above, perfluorocarbon sulfonic acid cation membranes are preferred because they have excellent cation transport, are extremely stable, are not affected by acids and strong oxidants, and have excellent thermal stability. This is because it has the same characteristics and remains virtually unchanged over time. A suitable specific cationic polymer membrane is sold by DuPont under the trade name "Nafion" and is a hydrated copolymer of polytetrafluoroethylene (PTFE) and polysulfonyl fluoride vinyl ether containing terminal sulfonic acid groups. It is a membrane that is These membranes are used in hydrogen form, which is the usual form obtained from the manufacturer. The ion exchange capacity (IEC) of a particular sulfonic acid cation exchange membrane depends on the milliequivalents (MEW) of SO ₂ groups per gram of dry polymer. The greater the concentration of sulfonic acid groups, the greater the ion exchange capacity,
Therefore, the ability of the hydrated membrane to transport cations is increased. However, as the ion exchange capacity of the membrane increases, the water content also increases, reducing the membrane's ability to repel salts. In the case of hydrochloric acid electrolysis, a suitable ion exchange membrane is that sold by DuPont under the trade name "Nafion 120". The ion exchange membrane used is one in which the membrane's water content and transport properties are fixed by hydrating it in boiling water for one hour. Electrode production To produce reduced oxides of platinum group elements, such as ruthenium, iridium, and transition metals, such as titanium, tantalum, in combination with Teflon-bonded graphite, mixed metal salts can be added directly or with excess sodium salts, such as nitrates. , decomposes thermally in the presence of carbonates, etc. The actual production method modifies Adams' platinum catalyst production method by adding a thermally decomposable halide of iridium, titanium, tantalum or ruthenium, i.e., a salt such as iridium chloride, tantalum chloride, ruthenium chloride or titanium chloride. It is something that As an example (Ru, Ir,
In the case of a Ti)Ox ternary alloy, finely ground halogenated salts of ruthenium, iridium and titanium are combined with the same ruthenium,
Mix iridium and titanium in weight proportions. Excess sodium is added and the mixture is melted in a silica dish at 500-550°C for 3 hours. Wash the residue thoroughly to remove any remaining nitrates and halides. The suspension of mixed oxides thus obtained is reduced at room temperature using electrochemical reduction techniques or by bubbling hydrogen into the mixture. Dry the product completely, grind it,
Sieve through a mesh nylon sieve. The reduced oxide is thermally stabilized by heating a thin layer of catalyst to 550-600° C. for 1 hour before blending with Teflon. An alloy of heat stabilized reduced oxides of ruthenium, iridium and titanium is combined with Teflon and then mixed with a graphite-Teflon mixture. It is clear that when only binary alloy reduced oxides need to be produced, the appropriate combination of noble metal halides can be mixed in the weight ratio desired for the final alloy and then processed as described above. be. To manufacture the electrode, powdered graphite is first mixed with polytetrafluoroethylene particles. An example of a commercially available form of graphite is "Pocographite" manufactured by Union Oil Company.
1748” (trade name). Polytetrafluoroethylene particles are manufactured by DuPont under the trade name "Teflon T".
-30" can be obtained. The amount of Teflon is 15-35
% by weight. A preferred amount is 20% by weight. Combine the reduced oxide with the graphite-Teflon mixture. A mixture of graphite, Teflon and reduced oxide is placed in a mold and heated to sinter the composition into the desired patterned shape, which is then bonded to the surface of the membrane by applying pressure and heat, and the surface bury it inside. Various methods can be used, one example being U.S. Pat. No. 3,134,697 (May 1964) to L.W.
(Mon. 26) Details are described in "Fuel Cells".
In the method described in this patent, an electrode assembly is pressed onto the surface of a partially polymerized ion exchange membrane, thereby integrally bonding the gas-absorbing hydrophobic particle mixture to the membrane and depositing it into the surface of the membrane. Bury it. Operating conditions: In case of chlorine generation, aqueous hydrochloric acid solution is introduced into the anolyte chamber. Preferably, the feed rate is in the range 1-4/min-ft ² . With such feed rates and high acid concentrations, oxygen evolution at the anode can be minimized, resulting in oxygen concentrations below 0.02%.
If both the feed concentration and the flow rate are too low, the relative amount of water present at the anode that competes with the HC for catalytic reaction sites increases. As a result, water is electrolyzed to produce oxygen at the anode. Oxygen will attack graphite, so oxygen evolution should be minimized.
Hydrochloric acid concentration is 7N (equivalent/) or more, preferably 9
It is preferable to set it as the range of 12N. 1.8~, depending on electrode composition and HC concentration
A working voltage of 2.2 volts is applied to the vessel at 400 amps/square foot, and the vessel and feed solution are maintained at 30°C, or room temperature. Increasing the temperature at which the electrolyzer is operated to or above 80°C (176°C) increases the efficiency of the system and reduces the bath voltage required for chlorine generation. The effect of temperature on performance in a typical hydrated SPE electrolyzer operating at 9-12N HC is shown in the table. (ASF = Ampere/Square Feet)

Table: The best currently available industrial HC electrolysis systems can achieve a bath voltage of 2.1 volts at a current density of 370 amperes per square foot (ASF) only when operating at a temperature of 80°C (176°C). Only. As is evident from the table, the present invention allows operation at lower voltages, lower temperatures and higher current densities compared to such industrial systems. According to the present invention, the electrolytic process can be carried out at room temperature levels (around 30° C.), and the cell can be operated at a lower voltage than current higher temperature cells. Voltage savings (efficiency) can be improved by increasing the bath operating temperature, ie, voltage savings of 0.6-0.7 V at 80° C. for a given current density. As clearly shown in the Examples below, the chlorine electrolysis process of the present invention provides bath voltages equal to or lower than currently achievable values (1.80-2.2 volts), high current densities (400 ASF) and significantly lower Chlorine can be generated efficiently at a temperature (around 30℃). The economic advantages of operating at higher current densities, lower bath voltages and significantly lower temperatures are obvious and of great importance. When hydrochloric acid is electrolyzed, chlorine gas and hydrogen ions are generated at the anode. H ⁺ ions are transported across the membrane and expelled as hydrogen gas at the cathode. The chlorine gas and the spent hydrochloric acid feed are removed from the tank and fresh feed is introduced at a flow rate within the ranges described above. It is also desirable to conduct the electrolysis under superpressure to facilitate removal of gaseous electrolysis products. By pressurizing the anolyte and catholyte chambers above atmospheric pressure, the size of the bubbles formed in the electrodes is reduced. Small air bubbles detach much more easily from the electrode and electrode surface, thus facilitating the removal of gaseous electrolysis products from the cell. Furthermore, small bubbles also have an additional advantage in that they tend to eliminate or minimize the formation of gas films on the electrode surface that would impede access of the anolyte and catholyte to the electrode. . In an eclectic cell configuration in which only one electrode is bonded to the membrane, the reduced bubble size results in fewer interruptions in electrolyte passage, thereby reducing gas mixing and mass transfer losses in the space between the membrane and the non-bonded electrode (" voltage drop due to "bubble effect") is minimized. The cation exchange membrane can be about 4 to 12 mils thick. The material constituting the bath can be a material that is resistant to hydrochloric acid and chlorine in the case of the anolyte compartment and is not subject to hydrogen embrittlement in the case of the catholyte compartment. Thus, the anode housing can be made of tantalum, niobium and graphite, using a tantalum or niobium screen and a filled rubber gasket such as EPDM. Graphite is suitable as a material for forming the cathode. Alternatively, the entire vessel housing and end plates can be made from pure graphite or other organic materials that are not susceptible to attack by the fluids and gases present in the housing. EXAMPLE An electrolytic cell in which an electrode containing thermally stabilized reduced oxides of platinum group elements and transition metals was bonded to an ion exchange membrane was manufactured and tested under various operating conditions. The impact on the effectiveness of the system was specifically shown. The table shows the effect of various combinations of thermally stabilized reduced oxides of platinum group elements on bath voltage. The vessels were constructed by bonding Teflon-bonded graphite electrodes containing various combinations of reduced oxides to a 12 mil thick hydrated cation membrane. Current density 400 amperes/ft2, 30°C, feed flow rate 70 c.c./min (effective tank area 0.05 ft2), feed normality 9~
The tank was operated at 11N. Tables 1 and 2 show the effect of time on bath voltage under the same bath and under the same conditions. The table shows the oxygen evolved at the anode at various feed rates and various HC concentrations. The table shows the effect of feed acid concentration varying from 7.5 to 10.5N. A tank similar to Tank No. 5 in Table 1 was constructed with a heat stabilized reduced oxide of a platinum group element (Ru, 25% Ir) added to Teflon bonded graphite. The tank has a fixed flow rate of 150c.c./min (effective tank area)
0.05 sq ft), operated at 30°C and 400 ASF. Note that ASF in the table indicates ampere/square foot.

【table】

[Table] *See notes to table

【table】

【table】

[Table] As is clear from the above examples, chlorine containing substantially no oxygen is produced by electrolysis of hydrogen halide such as HC. Catalysts used in electrolyzers with coupled electrode structures are characterized by low overpotentials. Electrolysis can be carried out at low temperatures (around 30°C), resulting in economical operation of the electrolyzer. Furthermore, the data show that high current densities, especially 300~
This shows that it is an extremely efficient process with excellent performance at 400ASF. It goes without saying that this high current density has a positive effect on the equipment cost of the chlorine electrolyzer which is a specific example of the present invention.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view showing an electrolytic cell having a solid polymer electrolyte membrane used in the method of the present invention, and FIG. 2 is a cross-sectional view of the membrane with notes of reactions occurring in various parts of the electrolytic cell. 10... Electrolytic cell, 11... Cathode chamber, 12... Anode chamber,
13...Solid polymer electrolyte membrane, 14...Cathode, 1
5, 16... Graphite current collector, 17, 18... Gasket, 19... Electrolyte inlet, 20... Anode chamber interior, 21... Outlet conduit, 22... Cathode outlet conduit, 23
...cable, 24...anode.

Claims (1)

[Scope of Claims] 1. In a method of generating halogen by electrolyzing an aqueous solution of hydrogen halide between an anode and a cathode separated by an ion exchange membrane, at least one of the electrodes is gas permeable, It contains a conductive platinum group metal catalyst, and is in direct contact with the membrane to form an integrated membrane-electrode structure, in which gaseous products are generated by electrolysis. How it occurs. 2. The method of claim 1, wherein the at least one electrode is in the form of particles embedded in one or each surface of the membrane, forming an electrode permeable to gas and electrolyte. 3. A potential is applied to said at least one electrode by a conductive current collector with one surface in contact with said at least one electrode, said surface of the current collector being more hydrogen or halogen than the electrode with which said surface is in contact. 3. A method according to claim 1, wherein the method has a higher overvoltage. 4. The method according to any one of claims 1 to 3, wherein the hydrogen halide concentration is higher than 7N in order to maintain the oxygen concentration lower than 5% by volume. 5. The method according to any one of claims 1 to 4, wherein the oxygen content in the halogen generating electrode is lower than 2% by volume. 6. The method of claim 5, wherein the concentration of hydrogen halide is maintained in the range 7 to 12N to maintain the concentration of oxygen below 2% by volume. 7. Any one of claims 1 to 6, wherein the ion exchange membrane is impermeable to the electrolyte and the conductive catalyst is present as a plurality of conductive particles bound by a fluorocarbon polymer. 1
The method described in section. 8. The method of any one of claims 1 to 7, wherein both electrodes are respectively attached to each side of the membrane, and the current collector is in contact with each of the two electrodes. 9. The method of any one of claims 1 to 8, wherein the conductive catalyst comprises thermally stabilized reduced oxide particles of a platinum group metal. 10. The reduced oxide particles of ruthenium are activated by containing at least one thermally stabilized reduced oxide of another platinum group metal or a valve metal. Method. 11 Claim 9, in which reduced oxide particles of platinum group metal are present in the electrode layer together with conductive graphite particles
The method described in section. 12. The method according to claim 11, wherein the conductive graphite particles present in the electrode layers of the anode and cathode are activated with reduced oxide particles of ruthenium. 13 The conductive particles present in each electrode layer contain particles of at least two substances including a platinum group metal and a reduced oxide of a valve metal, and at least one of the at least two substances 10. The method according to claim 9, wherein one of the oxidants is activated by a reduced oxide of a platinum group metal. 14. The method of claim 13, wherein the reduced oxide of a platinum group metal is a thermally stabilized reduced oxide of ruthenium. 15. The anode electrode according to claim 14, wherein the anode electrode is further stabilized by a reduced metal oxide selected from reduced oxides of iridium, tantalum, titanium or niobium to form a binary system. Method. 16. The method of claim 15, wherein the anode electrode is further stabilized by containing a reduced oxide of iridium. 17. The method according to claim 16, wherein the anode electrode contains reduced oxides of ruthenium and iridium, and further contains reduced oxides of titanium, niobium, or tantalum to form a ternary system. 18. The method of claim 17, wherein the anode electrode comprises a reduced oxide of titanium to form a ternary system with reduced oxides of ruthenium and iridium.