FI65284C - ELEKTROD FOER EN ELEKTROLYSCELL SAMT FOERFARANDE OCH AEMNESKOMPOSITION FOER OEVERDRAGNING AV DENSAMMA - Google Patents

Description

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Patent fs? Ddc? Lat V '(51) Kv.ik.3 / int.a.3 C 25 B 11/10 FINLAND —FINLAND (M) 77θ8θ6 (22) Hikundspaiv · —AmNailngwlig lU.03.T7 (23) AlkupUvi — GHtiftMtadag lU.03.77 (41) Announced - BIMt offmcllg 16.09.77

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Patent and Register of Publications A — ekan utlagd och utijkriften publicarad (32) (33) (31) fyr4 * «y • tueik · ——Bugam pnort — t 15.03.76 United States 667202 (71) Diamond Shamrock Corporation, 1100 Superior Avenue, Cleveland,

Ohio, USA (US) (72) David Lynn Lewis, Mentor, Ohio, Charles Richard Franks, USA (US) (7U) Berggren Oy Ab (5U) Electrode for electrolytic cell, and method and composition for coating it - Electrode for electrolytic cell This invention relates generally to a method of applying coating materials to an electrode substrate, which is generally a valve metal, in which the coating compositions contain the more complete utilization of the tin metal and the reduced atmospheric pollution caused by the evaporation of tin compounds during the coating process. More specifically, the present invention relates to a greatly improved method of manufacturing an electrode having a valve-metal substrate, such as titanium, having an electrode coating composition having a tin component on it, using a sulfate form of a tin compound in the coating composition.

Electrochemical manufacturing methods are becoming increasingly important for the chemical industry due to their better ecological acceptability, potential for energy conservation, and potential cost savings as a result. Therefore, a large amount of research and development efforts have been directed to electrochemical processes and equipment for these processes. One of the main elements of the devices is the electrode itself. The object has been to provide: an electrode which can withstand the corrosive environment in the electrolytic cell; an electrode having the lowest possible overvoltage for the desired electrochemical reaction; and an electrode that can be manufactured with a high level of quality control at a cost that is within commercial feasibility. Only a few materials can effectively form an electrode specifically for use as an anode due to the sensitivity of most other materials to the highly corrosive conditions prevailing in the anode compartment of the electrolytic cell. These materials include: graphite, nickel, lead, lead alloys, platinum or platinum-plated titanium. Electrodes of this type have limited uses due to various disadvantages, such as: lack of dimensional stability; high price; high wear speed; electrolyte contamination; cathodic deposition contamination; sensitivity to contaminants; or large overvoltages for the desired reaction. Overvoltage refers to the extra electrical potential relative to the theoretical potential at which the desired reaction occurs at a given current density.

The history of the electrodes is replete with examples of attempts and proposals to overcome some of the problems associated with the electrode in the electrolytic cell, none of which has achieved the desired optimization of properties with the electrode used in the electrolytic cell. The problem is to find an electrode that overcomes many of the undesirable properties listed above and, in addition, has low overvoltages at higher current densities to save energy. For example, it is known that platinum is an excellent material for use in electrodes to be used as anodes in the electrolytic separation process and satisfies many of the above criteria. However, platinum is expensive and therefore has not been considered suitable for industrial use in modern times. Carbon and lead alloy electrodes have been used commercially, but the carbon anode wears out rapidly, greatly contaminating the electrolyte, increasing the electrical resistance, and increasing the half-cell potential. This higher half-cell potential causes the electrolytic cell to consume more electrical energy than is desirable. The disadvantage of the lead alloy anode is that the lead dissolves in the electrolyte, forming a lead precipitate on the surface of the cathode, which contaminates the desired precipitate obtained. PbOj also converts to Pb ^ O ^ r, which is a bad conductor. Oxygen can penetrate under this layer and cause the film to crack, resulting in particles trapped in the cathode-deposited copper. This causes contamination of the copper coating, which is very undesirable.

It has been proposed that platinum or other precious metals be applied to titanium substrates in order to maintain their attractive electrical properties and further reduce manufacturing costs. However, even this limited use of precious metals such as platinum, which may cost about FIM 1,200 per square meter of electrode surface area, is expensive and therefore not desirable for industrial use. It has also been proposed that the surfaces of titanium be electrolytically coated with platinum, on which a second electrical layer of either lead dioxide or manganese dioxide is applied. Electrodes with a lead dioxide coating have the disadvantage of having relatively high oxygen overvoltages and both types of coatings have high internal stresses when deposited electrolytically and tend to come off the surface during commercial use, contaminating the electrolyte and cathode product. Thus, the current density of such anodes is limited and the handling of these anodes must be performed with great care. Another improvement tested has been to place a layer of manganese dioxide on the surface of a titanium substrate that is relatively porous in nature and grows multiple layers of manganese dioxide to provide a uniform coating. This results in relatively small overvoltages 2 as long as the current density remains below 77.5 mA / cm, but when the current density 2 is raised close to 155 mA / cm, the required overvoltage rises quite rapidly, leading to considerable disadvantage at higher current densities.

More recently, titanium, ruthenium and tin oxides or tin and antimony oxides, on which a top layer, either manganese or lead oxide, is galvanized have been used in various coatings. These coatings have proven to be remarkably promising for achieving overvoltage reduction and long life under corrosive conditions in an electrolytic cell. The main disadvantage of these materials is that, in particular, the methods of applying tin oxide materials have led to the evaporation of considerable amounts of tin when the coating is burned into tin oxides. This is because tin compounds such as stannous hydroxide pentahydrate are converted upon combustion to stannous hydroxide and then to stannous oxides, which are useful in a given electrode coating. During this process, much of the tin itself evaporates into the outside air instead of remaining in the coating. This is due, at least in part, to the boiling points of stannous chlorides in the order of 114 ° C and the conversion of tin compounds to their corresponding oxides at much higher temperatures, most of these materials are lost to the outside air, resulting in less than 50% utilization of tin material in the actual coating. This poses serious problems in the quality control of large electrode size and volume fabrication methods. The reproducibility of the coating composition is almost impossible with the evaporation caused by the current process for applying substrate coatings. Therefore, only theoretical tin calculations can be performed, which causes problems in calculating the possible lifetimes of a given electrode. Today, the use of tin in coating compositions has not achieved the commercial success it should have achieved, as evaporation of tin causes reproducibility problems, increases contamination that is subject to stringent regulations today, and increases the manufacturing cost of a given electrode due to tin losses.

It is therefore an object of the present invention to provide a manufacturing method for electrode coating compositions having the desired quality control properties and manufacturing costs within the scope of commercial implementation.

Another object of the present invention is to provide a method of manufacturing electrode coating compositions that significantly reduces the evaporation of tin into the ambient air, thereby reducing the atmospheric pollution problems associated with this manufacturing process.

These and other objects of the present invention, together with its advantages over existing and prior art embodiments, which will become apparent to those skilled in the art from the detailed description of the present invention below, will be realized by the improvements described and claimed herein.

It has been found that coating compositions can be prepared by a method comprising: selecting a substrate; applying to at least a portion of the surface of the substrate a coating material containing a tin sulfate compound; drying the coating material; and firing the coating material in an oxidizing atmosphere 65284 to convert the tin compound to its oxide form.

It has also been found that an improved electrode for use in an electrolysis cell can be made of a metal substrate having a coating material containing a tin sulfate compound applied to its surface and converted to an oxide form, and a top layer selected from the group consisting of manganese dioxide and lead dioxide.

The improved method of this invention for preparing electrode coating compositions can be used in the manufacture of any coating having a tin compound as a substituent that currently has a tin evaporation problem. In the past, electrode decomposable compounds in the form of chloride have been used in electrode coating compositions utilizing tin, which have a relatively low boiling point and therefore evaporation problems. The present invention utilizes the sulfate form of tin or chloride compounds in combination with sulfuric acid to provide the sulfate form, which has a simple decomposition mechanism to form the oxide form in the final electrode coating composition and therefore greatly reduces the evaporation of tin upon combustion. The substrate material used for such an electrode coating composition can be any electrically conductive metal having sufficient mechanical strength to support the coatings, and typical metals include: aluminum, molybdenum, niobium, tantalum, titanium, tungsten, zirconium, nickel, steel, and stainless steels thereof. The recommended valve metal based on price, availability, electrical and chemical properties is titanium. There are numerous forms that a titanium substrate can take in the manufacture of an electrode, such as: a solid sheet material; stretched metal mesh material with a high percentage open area; and porous titanium having a density of 30-70% of pure titanium, which can be produced by cold pressing titanium powder.

One type of coating in which the present invention can be used is where a substrate material as described above is coated with a semiconducting interlayer of tin and antimony oxides. These mixtures are generally mixtures of tin dioxide with minor amounts of antimony "filler", the latter being present in an amount of 0.1 to 30% by weight, based on the total weight percent of SnO 2 and Sb 2 O 2. The recommended amount of antimony trioxide in most of these 6 65284 cases is between 3 and 15% by weight. In the past, stannous chloride pentahydrate has generally been used in these interlayers as one of the materials of the mixture to be painted or in some way applied to the substrate material. The present invention uses a tin sulfate material or stannous chloride pentahydrate plus sulfuric acid to obtain the sulfate form of tin. The sulfate form has a simple decomposition mechanism at a temperature of approximately 320 ° C so that the combustion temperatures to convert these materials to their corresponding oxides result in very little evaporation of tin to the outside air. This makes it possible to apply the semiconducting intermediate layer in very few applications compared to previous methods which used multiple layered material applications to obtain the final tin weight in the desired area. On top of this semiconducting intermediate layer, a top layer of either manganese or lead dioxide can be applied to form electrodes with good current efficiency and good lifetimes.

There are many other examples of electrode coating compositions using tin compounds to provide useful electrode coating compositions. Those skilled in the art may wish to pre-coat the substrate material with a variety of other compositions prior to applying the tin sulfate-containing coating composition.

Another example is a single layer coating composition using titanium, ruthenium and tin oxides by a method similar to that described above. A coating composition of this type is described in more detail in U.S. Patent No. 3,855,092.

A third type of coating alloy utilizing tin-containing compounds is a mixture of tin, antimony, platinum group metal, and valve metal oxides. These coating compositions are described in more detail in U.S. Patent No. 3,875,043.

In order that those skilled in the art may more readily understand the present invention and certain preferred embodiments by which it may be utilized, the following typical examples are set forth.

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6 5 2 8 4

Example 1

A series of electrodes were prepared by coating a support metal, in this case titanium, with a solution of antimony trichloride, ruthenium trichloride and various tin-containing compounds in all amounts such that it was possible to calculate the initial tin / ruthenium ratio and compare it to the final tin / ruthenium ratio analysis. This indicates the amount of tin evaporation that occurred in each case. The initial tin / ruthenium ratio was determined from the weights of the starting materials in the coating solution. Since the ruthenium compound actually absorbs water to some extent to form hydrates, there is some inaccuracy in calculating the initial amount of proportionate ruthenium up to approximately 5%.

After application of these different materials to the substrate material, they were fired in an oxidizing atmosphere at 475-625 ° C for 5-10 minutes to convert the compounds to their corresponding oxides. This process was repeated several times to achieve a layer with the desired weight gain. The amount of coating material had no detectable effect on the resulting tin / ru ratios. Therefore, any suitable weight of coating material could be used. Once this was done, the final tin / ruthenium ratio was determined by stripping the catalyst layer off the titanium support with molten salts, which were later dissolved in water to precipitate metals and analyze the resulting solution by atomic absorption to determine the final tin / ruthenium ratio in the coating material. These ratios, together with the tin compounds used, are reported in Table I below.

65284 8

Table I

Used Sn compound Sn / Ru at the beginning Sn / Ru at the end

SnCl 4 * 5H 2 O 21.8 3.3 "10.9 1.7" 10.9 1.98 "4.3 0.5" 4.36 1.2 "4.36 1.8" 4.36 1 7

Sn (C4H9) 4 4.3 0.6

SnCl4-5H2O / H2SO4 5.7 6.4 "7.6 6.7" 7.6 7.5 "7.6 7.7" 7.6 7.8 "7.6 7.7

It can be seen that the evaporation loss of tin is of the order of 10-1 when stannous chloride pentahydrate was used, while the loss of tin is negligible when the tin compound used is stannous chloride pentahydrate reacted with sulfuric acid. In some cases, the final ratio is even higher than the initial ratio when using the sulfate form. It is thought that this is due to a test error caused by water absorption of the ruthenium compound and perhaps a small material loss during the stripping process.

Example 2

A second experiment was performed to show a significant increase in the amount of tin remaining in the coating. In this case, a known amount of the solution mixture of Example 1 using various tin-containing compounds was annealed in a crucible and the residue was analyzed by atomic absorption. The annealing temperatures and cycles were similar to those used in Example 1. The results of this experiment in terms of the percentage of element left in the coating material after this annealing are reported in Table II below.

li 9 65284

Table II

Used Sn compound% Sn% Ru% Sb preserved preserved preserved

SnCl 4 * 5H 2 O / H 2 SO 4 81 90 43

SnSO 4 94 95 61

SnCl4'5H2O 9 97 23

SnCl4 at reflux in amyl alcohol 19 94 15

It can be seen from Table II that the use of the sulfate form of tin results in significantly higher percentages of tin retention than the use of the chloride forms used so far.

Example 3

A series of electrodes were prepared to evaluate the half-cell potentials and lifetimes of these electrodes compared to electrodes using higher amounts of the chloride form compounds to result in an amount of tin remaining in the coating equal to that of the sulfate form compounds. It was found that 25.1 g of stannous chloride pentahydrate resulted in approximately the same amount of tin as the mixture containing 5.48 g of stannous chloride pentahydrate reacted with sulfuric acid. It can be seen that in this case approximately five times as much tin compounds are required when no sulfate form is used in the coatings. It was also found that when these two materials were applied in equal amounts in grams of ruthenium per surface area of the titanium sample, the resulting electrodes gave approximately the same half-cell potentials and had the lifetimes reported in Table III below.

Table III

Grams Ru square- Chloride Form Lifetime Sulfate Form Per Meter Hours Lifetime Hours 1.10 17 14 2.15 50 68 3.25 79 108 Thus, it can be seen that approximately five times as much tin chloride form as tin sulfate form is required from the electrodes obtained to achieve the same lifetimes. This means that a significantly smaller amount of tin compounds in sulphate form can be used for this purpose, which results in total manufacturing cost savings over a given electrode life. As can be seen from Table I, the reproducibility of the tin compounds in the sulfate form is significantly better than that of the compounds in the chloride form, making them much easier to increase the electrode fabrication process. The use of the sulfate form also results in significantly less evaporation of tin to the outside air, which eliminates one of the contamination problems of prior art manufacturing methods.

Thus, it should be apparent from the foregoing description of the preferred embodiment that the composition described herein accomplishes the purposes of the invention and solves the problems associated with the manufacture of electrode coating compositions and electrodes for use in electrolytic cells for electrochemical production.

Claims (4)

11 65284 A process for the preparation of electrodes for electrolysis by applying a tin compound coating to an electrode substrate, optionally in admixture with a catalytically active metal such as ruthenium, rhodium, iridium and / or antimony, and converting the metal compounds to the corresponding oxides by heating to that tin sulfate is used as the tin compound or that it is formed in situ on a substrate. Process according to Claim 1, characterized in that tin chloride and sulfuric acid are used to form tin sulphate in situ on the electrode substrate. Process according to Claim 1 or 2, characterized in that tin sulphate is applied together with anti-monitrichloride and optionally luthenium trichloride or iridium trichloride. Process according to Claim 1 or 2, characterized in that tin sulphate is applied together with a ruthenium and rhodium compound.