FI67729B - KATOD FOER FRAMSTAELLNING AV ELEKTROLYTISKT VAETE - Google Patents

Description

1 67729

The present invention relates to a cathode for the electrolytic production of hydrogen.

Electrolysis methods generally aim to minimize the potential of electrochemical reactions at the electrodes.

This is especially the case in electrolysis processes which produce gaseous hydrogen at the cathode, such as in the electrolysis of water, aqueous hydrochloric acid solutions and aqueous solutions of sodium and potassium chloride.

The cathodes most commonly used in the electrolysis of aqueous solutions of sodium and potassium chloride to date generally consist of a sheet or mesh made of mild carbon steel. These known cathodes have the advantage of being easy to use and inexpensive.

However, the hydrogen overvoltage is relatively high with these known steel cathodes, which increases the cost of electrolysis processes in an undesirable manner.

To remedy the disadvantages of these known steel cathodes, German Patent Application No. 2620589 filed May 10, 1976 in the name of Hodogaya Chemical Co Ltd discloses cathodes comprising a substrate made of titanium, tantalum, zirconium, niobium or mixtures thereof, rhodium, ruthenium, an active coating made of iridium or platinum oxides. This active coating, which is intended to reduce the overvoltage of hydrogen release, can be prepared by adding a solution of a salt of ruthenium, rhodium, palladium, osmium or iridium to the cathode substrate, and then heating the solution so that the salt is converted to an oxide. Alternatively, the active coating of this known cathode can be further coated with a layer formed of an oxide of a metal of groups II and VI of the Periodic Table of the Elements, said layer being intended to prevent reduction of hypochlorite-10 ions when using the cathode in electrolysis of alkali metal chloride solutions.

U.S. Patent 4,100,049, issued July 11, 1978 to Diamond Shamrock Corporation, discloses cathodes for use in electrolytic cells comprising a metal substrate and an active coating thereon formed from a mixture of palladium oxide and zirconia.

To reduce the cost of the cathode, it is proposed to alternatively replace 50% by weight of palladium oxide with cobalt or nickel oxide.

Although the hydrogen release overvoltage is somewhat lower, these known cathodes still have the disadvantage of a high cost due to the precious metal necessary for their structure.

25 March 16, 1978 Tokuyama Soda K.K. German patent application 2811472, filed in the name of the companies, discloses, inter alia, cathodes for the electrolytic production of hydrogen, in which a support made of iron or nickel or an alloy of these metals has an active coating of Group VIII metal of the Periodic Table of the Elements obtained by electrolytic precipitation or or a thermally decomposable compound of said 3 67729 metal in a reducing atmosphere.

These known cathodes have the disadvantage of being cumbersome and expensive to manufacture, especially in the case where the active coating is formed by a thermal path. In fact, the thermal decomposition of a thermally decomposable compound requires very high temperatures, usually above 800 ° C, for the active metal to be released into the metal space. This results in high energy consumption and the risk that the cathode will be deformed by thermal stresses.

The object of the present invention is to obviate the above-mentioned drawbacks of the known cathodes by providing a cathode which has no noble metals at all, which is easy to manufacture and affordable, and which nevertheless has a low hydrogen release overvoltage.

The invention therefore relates to a cathode for the electrolytic production of hydrogen, which comprises an active region consisting of a thermally decomposable metal oxide obtained by thermal decomposition of a compound? according to the invention, the active region consists essentially of a metal oxide obtained by thermal decomposition of a thermally decomposable compound selected from cobalt, iron manganese and nickel.

By definition, a metal oxide in the active region of a cathode means at the same time a monometallic oxide of cobalt, iron, manganese or nickel, a solid solution of these metal oxides or a defined oxide compound.

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Defined compounds within the scope of the invention are, for example, spinel-structured compounds of the general formula M 0 ^, in which M denotes divalent iron, and / or cobalt and / or 5 magnesium and / or nickel, and ivr denotes one or more of these as trivalent (Structural Inorganic Chemistry - AF Wells - Oxford University Press - 1962 pages 487-490; Crystal Structure of Minerals - Bragg & Claringbull-Bell and Sons Ltd - 1965 - pages 102-106).

10 Magnetite of the general formula Fe 3 O 3 (FeIIFe 2 ΙΙ 3: 04>) gives particularly interesting results.

According to a preferred embodiment of the invention, the metal oxide of the active region of the cathode consists of hematite, which is a trivalent iron oxide and has the general formula Fe 2 O 2. This embodiment of the invention is particularly well suited to cases where the cathode is used by electrolysis of aqueous alkali metal hydroxide solutions such as sodium chloride solutions, sodium hydroxide solutions prepared in a cell with a permeable selective membrane 20 and basic sodium products prepared in cells with a permeable septum.

By selective permeable membrane is meant a separator made of a thin, non-porous, ion-exchanging material separating the anodes from the cathodes. Selective permeable membranes suitable for saline electrolysis cells include, for example, cationic membranes containing SO 2 groups obtained by copolymerization of tetrafluoroethylene. and sulfonated perfluorovinyl ether and these are, for example, the known films NAFION sold by E.I. du Pont de Nemours & Co.

By partition is meant an electrolyte-permeable wall of an anode cathode 35 made of an inert material. Such known partitions 5 67729 include, for example, asbestos walls as described in Stuert's U.S. Patent 1,855,497 of 7 May 1928 and Belgian Patent 773,918 of 14 October 1971 in the applicant's name, 5 and polytetrafluoroethylene porous sheets such as those described by Imperial Chemical Industries Belgian Patents 794,889, issued February 2, 1973, 817,675, 817,676 and 817,677, July 15, 1974.

In the cathode of the invention, the thermally decomposable compounds may be any compounds which, when heated in a controlled atmosphere, release iron, cobalt, manganese or nickel oxide or a mixture of at least two of said oxides, a solid solution or a defined compound.

They may be, for example, nitrates, sulfates, phosphates, salts of carboxylic acids such as formates, acetates, propionates, oxalates, etc. Thermally decomposable compounds may be used in solid form, for example as powders, or in liquid form, for example as molten salts, suspensions or solutions.

Thermal treatment, by definition, involves heating a thermally decomposable compound to a temperature high enough in a controlled atmosphere for the compound to decompose and the metal oxide to crystallize on the surface of the active region of the cathode.

The temperature of the heat treatment depends on several parameters, including the nature of the metal oxide to be formed, the nature of the thermally decomposable compound, its state (solid or liquid), the nature of the atmosphere used in the treatment and the pressure therein. A suitable temperature is generally 50-700 ° C, preferably less than 400 ° C. Temperatures between 100 ° C and 300 ° C have been found to be particularly suitable.

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Although thermal decomposition can be performed in an inert atmosphere (especially nitrogen or argon) under certain special conditions (especially when the selected thermally decomposable compound is a nitrate or oxalate), it is generally preferred to perform thermal decomposition in an oxidizing atmosphere, for example in the presence of air.

Alternatively, in addition to the cobalt, iron, magnesium and / or nickel metal oxide as defined above, the active region 10 of the cathode according to the invention may contain very small amounts of foreign matter, as long as these do not affect the metal oxide properties specifically with respect to hydrogen release voltage.

In the cathode according to the invention, the active region can be cathode-wide.

According to a preferred embodiment of the invention, the cathode consists of several parts and comprises a substrate made of 2q of electrically conductive material, the surface of which has an active region.

In this particular embodiment of the invention, the substrate material is generally selected to withstand the electrochemical conditions in which the cathode is intended to be used.

In the particular case that the cathode is intended for use in electrolytic cells of aqueous alkali metal chloride solutions, it is most preferable to select cobalt, chromium, iron, nickel, manganese or a mixture of these metals as the substrate material.

Iron and steel platforms are particularly well suited.

The metal oxide layer in the active surface of the substrate must be thick enough to withstand the abrasion of hydrogen gas and the wear caused by electrolytes in contact with the surface during electrolysis. In general, it is desirable that the thickness of the metal oxide in the active region on the surface of the substrate be at least 5 to 0.5 microns, preferably at least 5 microns. Excellent results are generally obtained with thicknesses of at least 10 microns and especially when said thickness is 50-250 microns.

To make the cathode of this preferred embodiment 10 of the invention, it is only necessary to add a thermally decomposable compound to the surface of the substrate and then heat it in a controlled atmosphere to decompose and crystallize the metal oxide in situ on the surface of the substrate.

For this purpose, it is desirable that the term-15 Sesto chipping compound be added in liquid form, preferably as a solution to the surface of the substrate. For this purpose, any suitable coating method can be used, such as immersing the substrate in a bath containing a thermally decomposable substance, brushing the substrate 20 with said liquid compound, or spraying this compound onto the substrate.

According to a preferred embodiment of the invention, the metal oxide is obtained by thermally decomposing the thermally decomposable compound so that it is first dissolved or suspended in a bath containing a soluble alkali metal salt.

In fact, it has been found that while all other factors are similar, the presence of a soluble alkali metal salt in a bath containing a thermally decomposable compound improves the performance of the cathode 30 of the invention, particularly by favorably reducing the hydrogen release overvoltage and increasing its effective operating time.

In this preferred embodiment of the invention, the choice of soluble alkali metal salt depends on the nature of the bath. In general, if a bath containing water is used, the best results are obtained with 8 67729 alkali metal halides or nitrates, of which sodium chloride is in any case the most preferred.

The minimum amount of soluble alkali metal salt to be used depends on the nature of the salt, the nature of the bath and the nature of the decomposing compound te ^ mi = es + -i.

It can be determined experimentally in the laboratory in each specific case. When sodium chloride is used in a bath containing water, the best results are obtained at concentrations of at least 0.2 moles per liter per bath, with concentrations of 0.5 moles / liter and above being more preferred.

The maximum amount of soluble alkali metal salt available is equal to its saturation concentration in the bath.

The shape of the cathode according to the invention is not critical. It can be, for example, a planar, curved or corrugated plate, possibly a perforated, 20 or straight or helical wire, or a routed network.

The code according to the invention is particularly well suited for use in cells for the electrolysis of sodium chloride solutions with a permeable septum and a selective permeable membrane, as described, for example, in French Patents 2,164,623, December 12, 1972, 2,223,083, 28 March 1973, 2,231 March 27, 1974, 2,248,335, October 14, 1974, and French Patent Application 77.11 370.12. April 1977, all in the name of the applicant.

The cathode according to the invention is also well suited for use in cells for the electrolysis of water and in cells for the production of alkali metal hypochlorite or 966772 chlorate, such as those described in French Patents 2,023,877, 19 November 1969 and 2,147,063, 17 July 1072, which are both in the name of applicant 5.

The nature of the invention will become more apparent from the following application examples, which are given for the purpose of illustrating the invention only.

In each of the following examples, electrolysis was performed on an aqueous saline solution containing 225 g of sodium chloride per kilogram in a laboratory cell equipped with vertical electrodes separable from the asbestos septum.

The cylindrical cell comprised an anode made of a ring-shaped perforated plate coated with an active crystal alloy material containing 50% by weight of ruthenium dioxide and 50% by weight of titanium dioxide.

The cathode consisted of a ring-shaped metal structure made of mesh, identical in shape in each example, but varying in composition from one example to another.

The total surface area 3 of each electrode of the cell was 113 cm, and the distance between the anode and the cathode was set to 5 mm.

The septum is attached to the anode side of the cathode using asbestos suspended in alkaline saline and heated to 90 ° C for 16 hours using the technique described in Luxembourg Patent Application 77.996.19, August 1977.

The resulting asbestos film had a basis weight of 1.3 kg per 35 square meters of cathode.

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In each example, the above brine was electrolysed in a cell at 85 ° C with a current density of 2 kA per square meter of anode and the brine flow rate was introduced into the anode chamber so that the basic brine leaving the cathode chamber contained about 100 g of sodium hydroxide and 140 g of sodium hydroxide. The cathode potential was periodically measured by the Luggin capillary measurement method using 10 saturated electromalium electrodes (ECS) as reference electrodes (Modern Electrochemistry, Bockris and Reddy, Pleunum Press, 1970, Vol. 2, pages 890 and 891).

Example 1

The cathode consisted of a mesh made of mild carbon steel coated with an active hematite layer according to the invention. To prepare the cathode, the mesh was first cleaned with formaldehyde-passivated hydrochloric acid. Then, an aqueous solution containing 200 g of ammonium ferric trioxalate of the general formula (NH 4) ^ Fe 2 C 2 O 4 • 3 3 H 2 O · m is sprayed on its surface. it was then heated in an oven to 225 ° C for 15 minutes in the presence of air to decompose ammonium ferric trioxalate and crystallize the first layer of hematite to the surface of the network.

Spraying with an ammonium ferric trioxalate solution and heating in an oxidizing atmosphere was repeated ten times to obtain ten overlapping layers of hematite crystals on the surface of the network. After forming the 30th hematite layer, the cathode was kept at 225 ° C for 1 hour in the presence of air and then allowed to cool. ° C.

The cathode of the present invention thus obtained was then subjected to the electrolysis test described above.

The results of the experiment are shown in Table I below.

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Table I

Electrolysis duration Cathode potential (days) (V) 10 10 -1.27 60 -1.28 90 -1.32

Example 2

The experiment of Example 1 was repeated using an aqueous bath containing 214 g of ammonium ferric trioxalate and 29 g of sodium chloride per liter to form the active region.

The mesh was cleaned as described in Example 1 and then heated to 250 ° C and immediately covered with a first layer of bath solution by immersion in the bath. The net was then heated for 15 minutes in an oven to 250 ° C in the presence of air to decompose the ammonium ferrite trioxalate and crystallize the first layer of hematite on its surface.

Immersion in a bath and heating to 250 ° C in an oxidizing atmosphere was repeated ten times to obtain ten overlapping layers of hematite on the surface of the mesh. After the formation of the tenth layer, the cathode was kept at 250 ° C for two hours in the presence of air, after which it was allowed to cool in air to about 20 ° C.

The results of the electrolysis test performed with the cathode thus prepared are shown in Table II.

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Table II

Electrolysis duration Cathode potential (days) (V) 5 10 -1.27 60 -1.27 90 -1.27 115 -1.27

Example 3 A cathode was prepared using the procedure described in Example 2, but this time using a bath containing 200 g of ammonium ferric trioxalate and 100 g of sodium chloride per liter.

After 2 months of electrolysis under the conditions described above, the cathode potential was equal to -1.26 V compared to the saturated calomel electrode.

Example 4

The experiment of Example 1 was repeated 20 more times, using as a coating bath a solution containing 120 g of ferrin nitrate in 1 liter of butanol.

A pre-cleaned mild carbon steel mesh as described in Example 1 was heated to 250 ° C and then immersed in a top bath to form a solution layer on its surface.

The heat treatment was identical to the treatment of Example 2, except that the heating step of each layer in an oven at 250 ° C lasted 10 minutes.

In addition, after the tenth iron oxide layer was formed on the cathode surface, this was kept at 350 ° C for 16 hours and then allowed to cool in air to about 20 ° C.

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After the electrolysis was continued for one month under the experimental conditions described above, the cathode potential was equal to -1.28 V compared to the saturated Kalo sense electrode.

Example 5 In this experiment, an active layer of cobalt oxide was formed on the surface of a matt carbon-steel mesh.

For this purpose, a mild carbon steel mesh purified as described in Example 1 was immersed in a water bath containing 291 g per liter of cobalt nitrate hexahydrate of the general formula Co (NO 2) 2 · 6 * ^ 0 '3a 255 g sodium nitrate.

After the first immersion in the bath, the net was heated in an oven for 15 minutes at 250 ° C in the presence of air to decompose the cobalt nitrate and form a cobalt oxide layer.

The immersion treatment and heating to 250 ° C was repeated five times to obtain five overlapping layers of cobalt oxide on the surface of the mesh. After forming the fifth layer, the cathode was kept at 250 ° C for one hour and then allowed to cool in air to room temperature.

After electrolysis was continued for 30 days under the conditions described above, the cathode potential was equal to -1.31 V compared to the saturated calomel electrode.

Example 6 For comparison, a cathode known before this invention was used, which consisted of a mild carbon steel mesh identical to that used in Examples 1-5, which was only purified by treatment with formaldehyde-passivated i4 67729 hydrochloric acid and then placed as such in the electrolytic cell used in the experiment.

At the beginning of the electrolysis, the cathode potential increased to -1.41 V as measured by comparison with the saturated Kalo-5 melode electrode; After 70 days of experiment, the cathode potential was equal to -1.45 V compared to the saturated calomel electrode.

Comparing the results of the electrolysis of Examples 1-5 according to the invention with the result of the electrolysis of Example 6 according to the pre-invention, the progress of the invention in terms of the magnitude of the cathode potential and consequently the energy efficiency of the electrolysis cells is immediately apparent.

Claims (9)

67729 Patented suction cups A cathode for the electrolytic production of hydrogen, comprising an active layer of metal oxide and optionally impurities obtained by thermally decomposing a thermally decomposable compound, characterized in that the metal oxide of the active layer is hematite obtained by thermally decomposing a thermally decomposable iron compound. 2. A cathode according to claim 1, which comprises a genomic thermal treatment of a thermally stable reaction, including an organic nitrate, a sulphate, a phosphate or a carboxylic acid. Cathode according to Claim 1, characterized in that the hematite is obtained by thermally decomposing a thermally decomposable compound of iron which is a nitrate, sulphate, phosphate or salt of a carboxylic acid. 3. A cathode according to claims 1 or 2, which comprises 15 parts of the invention, is shown in the genome as a thermal transfer agent under an oxidizing method. Cathode according to Claim 1 or 2, characterized in that the hematite is obtained by thermally decomposing a thermally decomposing compound of iron under oxidizing conditions. 4. A cathode according to claim 3, which can be used to heat the genome to a thermal condensing temperature of 20 to 100 ° C. Cathode according to Claim 3, characterized in that the hematite is obtained by decomposing the thermally decomposable compound at 100 to 300 ° C. 5. Cathodes according to claims 1-4, which are incorporated herein by reference, have shown that the genome is thermally bonded to a thermally bonded form, that is to say, in the form of an alkali metal and that the alkali metal is present. Cathode according to any one of claims 1 to 4, characterized in that the hematite is obtained by thermally decomposing a thermally decomposable compound in solution form in a bath containing a soluble salt of an alkali metal. 6. A cathode according to claim 5, which comprises an alkali metal salt having an alkali metal halide. Cathode according to Claim 5, characterized in that the soluble alkali metal salt is an alkali metal halide. 7. A cathode according to claim 6, which is labeled 30 times, with an alkali metal salt containing sodium chloride. Cathode according to Claim 6, characterized in that the soluble alkali metal salt is sodium chloride. 8. The cathodes are preferably made of a patent of 5-7, with a minimum of 0.5 mol per liter of alkali metal salt. Cathode according to one of Claims 5 to 7, characterized in that the bath contains at least 0.5 moles of soluble alkali metal salt per liter. Cathode 35 according to any one of claims 1 to 8, characterized in that under the hematite layer there is a support made of an electrically conductive material which is cobalt, chromium, iron, manganese, nickel or a mixture of these metals. 67729 Cathodes for electrolytic framing with the presence and effect of active metals, light-emitting metal and 5-oxide and, possibly, for the treatment and excitation of genomic thermal sounding systems vilken erhällits genom termisk sönderdelning av en termiskt sönderdelbar järnförening. 9. Cathodes according to claims 1-8, having a hematic coating on the surface of the material, free of electric lead material, chromium, manganese, nickel or bleached metal.