GB1573173A - Electrode manufacture - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRODE MANUFACTURE (71) We, DIAMOND SHAMROCK CORPORATION, of 1100 Superior Avenue, Cleveland, Ohio 44114, United States of America, a Corporation organised and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to performed, to be particularly described in and by the following statement::- This invention relates generally to the manufacture of electrodes and is particularly concerned with a method for applying a coating material to an electrode substrate, so as to produce an electrode which, in contrast to known electrodes, has significantly increased reproducibility in manufacture, provides savings in manufacturing costs due to more complete utilization of the components of the coating material and also provides reduced atmospheric pollution during the coating process.

Electrochemical methods of manufacture are becoming ever-increasingly important to the chemical industry, due to their greater ecological acceptibility, potential for energy conservation and the resultant cost reductions possible.

Therefore, a great deal of research and development efforts have been applied to electrochemical methods and the hardware for these methods. On major element of the hardware aspect is the electrode itself. The object has been to provide an electrode which (1) will withstand the corrosive environment within an electrolytic cell (2) has a minimum over-potential for the desired electrochemical reaction and (3) can be manufactured with high quality control at a cost within the range of commercial feasibility. Only a few materials may effectively constitute an electrode, especially one to be used as an anode, because of the susceptibility of most other substances to the intense corrosive conditions present within the anode compartment of an electrolytic cell. Among these materials are graphite, nickel, lead, lead alloy, platinum and platinized titanium.Electrodes of this type have limited applications because of various disadvantages, including a lack of dimensional stability, high cost, high wear rate, contamination of the electrolyte, contamination of a cathode deposit, sensitivity to impurities and high overpotentials for the desired reaction. Overpotential refers to the excess electrical potential over the theoretical potential at which the desired reaction occurs at a given current density.

The history of electrodes is replete with examples of attempts and proposals to overcome some of the problems associated with the use of electrodes in electrolytic cells, none of which have provided to the optimum extent all the characteristics desirable for electrodes used in electrolytic cells. The basic problem is to find an electrode which will overcome many of the undesirable characteristics listed above and additionally have low overpotentials at higher current densities so as to conserve energy. It is known, for instance, that platinum is an excellent material for use in electrodes to be used as anodes in electrowinning processes and satisfies many of the above-mentioned criteria. However, platinum is expensive and thus has not been found suitable for industrial. use to date. Carbon and lead alloy electrodes have been used commercially.However, carbon anodes wear quickly, which greatly pollutes the electrolytes, increases electrical resistance and increases the half cell potential. This higher half cell potential causes the consumption of more electrical energy than is desirable. A disadvantage of lead alloy anodes is that the lead dissolves in the electrolyte, producing a lead deposit on the cathode which contaminates the desired deposit obtained. Also, PbO2 changes to Pb3O4, which is a poor conductor. Oxygen may penetrate below the lead deposit layer, causing it to flake off and resulting, in copper plating, for instance, in particles becoming trapped in the copper deposited on the cathode. This causes a very undesirable degradation of the copper plating.

It has been proposed to apply platinum or other precious metals to a titanium substrate, so as to retain their attractive electrical characteristics and further reduce manufacturing costs. However, even this limited use of precious metals such as platinum, which can cost in the range of $30.00 per square foot ($323.00 per square metre) of electrode surface, is expensive and therefore not desirable for industrial use. It has also been proposed to plate the surfaces of titanium electrically with platinum and then apply another electrical deposit, either of lead dioxide or magnanese dioxide.Electrodes with lead dioxide coatings have the disadvantage of comparatively high oxygen overpotentials and, when electrolytically deposited, both types of coatings have high internal stresses, which make them liable to detach from the surface during commercial usage, contaminating the electrolyte and the product being deposited on the cathode surface. Thus, the current density of such anodes is limited and handling of such anodes must be done with extreme care. Another attempted improvement has been to put a layer of manganese dioxide on the surface of a titanium substrate which is relatively porous in nature, then building up a number of layers of the manganese dioxide so as to produce an integral coating.This yields electrodes having relatively low overpotentials, so long as the current density remains below 0.5 ampere per square inch (77.5 milliamperes per square centimetre), but as current density increases to near 1 ampere per square inch (155 milliamperes per square centimetre), the overpotential required rises rather rapidly, resulting in a considerable disadvantage at higher current densities.

More recently, a number of developments have made use of titanium, ruthenium and tin dioxides, or tin and antimony oxides, over which a top coating of either manganese or lead oxide is plated. These coatings have shown substantial promise in lowering overpotential and yielding good life-times in the corrosive conditions within an electrolytic cell. The major drawback of these methods, in which the desired oxides are provided on the electrodes by applying and converting other materials, is that the methods of applying the materials, especially those yielding tin oxide, have resulted in volatilization of substantial amounts of the tin upon baking the initial coating to produce tin oxide. This is because tin compounds, e.g. stannic chloride pentahydrate, when baked convert to a form of stannic hydroxide and then to the tin oxides which are desired in the resultant electrode coating.During this process, much of the tin itself is volatilized into the atmosphere, instead of remaining in the coating. This occurs at least partly because stannic chlorides have boiling points in the region of 114"C and, since the transformation of tin compounds to their respective oxides occurs at much higher temperatures, most of these materials are lost into the atmosphere, resulting in less than 50 /n utilization of the tin material in the actual coating. This causes a severe problem in the quality control of methods of manufacture for electrodes of large sizes and large quantities. It is nearly impossible to achieve the desired reproducibility of coating compositions with the level of volatilization of tin caused during the current methods for applying coatings to the substrate materials.

Therefore, only theoretical tin calculations can be made, causing problems with regard to calculating possible lifetimes of a given electrode. To date, the use of tin in coating compositions has not met with the commercial success it should have, because volatilization of the tin causes a reproducibility problem, increases pollution which is under stringent standards currently, and increases the cost of production of a given electrode due to the loss of the tin.

It has now been found that satisfactory coated electrodes which include tin oxide in their coatings can be made in ways which significantly reduce the loss of tin during manufacture.

According to one aspect of this invention, a method of manufacture of an electrode comprises applying a coating material containing an antimony compound and tin sulphate to at least a portion of the surface of a valve metal substrate selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof, drying the coating material and baking it in an oxidizing atmosphere to convert the antimony and tin compounds to their oxides.

The invention also consists in electrodes made by carrying out the method of manufacture of the invention.

According to a preferred embodiment of the invention, an electrode for use in an electrolytic cell is provided, comprising a valve metal substrate selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof, having on its surface an intermediate coating comprising antimony and tin oxides the tin oxide having been applied as a tin sulphate and converted to the respective oxide, and a top coating selected from manganese dioxide and lead dioxide.

According to another preferred embodiment of the invention, an electrode for use in an electrolytic cell is provided, comprising a valve metal substrate selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof, having on its surface a coating formed by applying ruthenium trichloride, antimony trichloride and tin sulphate and drying and baking them in an oxidizing atmosphere to convert them to their respective oxides.

In carrying out the method of the invention, the antimony compound used preferably is antimony trichloride. Most preferably, the amount of the antimony compound in the coating material is such that the resultant coating contains antimony trioxide in an amount in the range of 0.1% to 30% by weight of the total of antimony and tin oxides.

A preferred feature of the method of the invention is the inclusion in the coating material of a ruthenium or iridium compound, most preferably ruthenium trichloride or iridium trichloride. Another preferred feature is the inclusion of a rhodium compound. All these optional components of the coating material convert to the corresponding oxides in the baking step of the method.

A further preferred feature of the method of the invention is producing the tin sulphate of the coating material by reacting a tin chloride compound with sulphuric acid.

According to another preferred feature of the invention, a top coating comprising a manganese oxide or a lead oxide is applied to the surface of the coated substrate.

Improved electrode coating compositions suitable for use in carrying out the present invention are disclosed and claimed in our copending Application No.

23725/79 (Serial No. 1,573,297).

The improved method of manufacture of coated electrodes of the present invention may use, in the preparation of the tin sulphate component of the coating material to be employed, any tin compound which gives rise to the volatilization problem at present. In the past, electrode coating compositions including tin compounds have utilized thermally-decomposable compounds of the chloride form, which have a lower boiling point and therefore suffer from a volatilization problem. The present invention employs the tin in the sulphate form or as chloride compounds along with sulphuric acid, to result in a sulphate form of the tin, which has a simple decomposition mechanism for formation of the oxide form in the ultimate electrode coating and therefore drastically reduces volatilization of the tin upon baking.The substrate material for use in such an electrode can be any of the valve metals, as these all have sufficient mechanical strength to serve as a support for the coatings, such substrate metals being selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof.

The preferred valve metal based on cost, availability and electrical and chemical properties is titanium. There are a number of forms which the titanium substrate may take, when manufacturing electrodes, including solid sheet material, expanded metal mesh material with a large percentage of open area and porous material having a density of 30% to 70% pure titanium, which can be produced by coldcompacting titanium powder.

Preferably, the present invention is utilized in such a way that the substrate material is coated with a semi-conductive intermediate coating of the tin and antimony oxides. These compositions desirably are mixtures of tin dioxide with minor amounts of antimony "dopant", the latter being present in an amount from 0.1% to 30% by weight based on the total weight of SnO2 and Sb2O3 present in the coating. The preferred amount of antimony trioxide in most of these cases is from 3% to 15% by weight. In the past, these intermediate coatings have generally been made by using stannic chloride pentahydrate as one of the materials of the mixture to be painted on or otherwise applied to the substrate material. The present invention utilizes tin sulphate as such or e.g. stannic chloride pentahydrate plus sulphuric acid, to obtain the sulphate form of the tin compound.The sulphate form has a simple decomposition mechanism at a temperature in the region of 320"C, so that baking to transform these materials into their respective oxides takes place at temperatures which results in very little volatilization of the tin into the atmosphere. This allows the semi-conductive intermediate coating to be applied in a very few applications as compared with the past methods of using several applications of the material to obtain layers with a resultant tin weight in the desired range. Over the top of this semi-conductive intermediate coating may be applied a top coating of either manganese or lead dioxides, as this assists in producing electrodes having good current efficiencies and good lifetimes.

There are many other examples of electrode coating compositions utilizing tin compounds in their makeup to produce usable electrode coating compositions.

Those skilled in the art may desire to precoat the substrate material with numerous other compositions before applying the tin sulphate-containing coating composition.

An example of a known type of single layer coating is one having titanium, ruthenium and tin dioxides applied by a method similar to that described above.

Coatings of this type, which do not include antimony in oxide form, therefore, are described in U.S. Patent Specification No. 3,855,092, where use is made of anhydrous tin chloride to provide the tin oxide component.

Another known type of coating, which is described in U.S. Patent Specification No. 3,875,043, includes a mixture of the oxides of tin, antimony, a platinum group metal and a valve metal. This specification discloses the use of thermally-decomposable salts of tin, including the chlorides and nitrates. Neither of the aforesaid US Specifications discloses the use of tin sulphate.

In order that those skilled in the art may more readily understand the present invention and certain preferred aspects of it, the following specific examples are offered.

EXAMPLE 1 A series of electrodes were prepared by coating the substrate metal, in this case titanium, with a solution containing antimony trichloride, ruthenium trichloride and various compounds containing tin all in such amounts as to allow an initial tin/ruthenium ratio to be calculated and compared to an analysis of the final tin/ruthenium ratio. This shows the amount of volatilization of tin that occurred in each instance. The initial tin/ruthenium ratio was determined from the weights of the starting materials in the initial coating solution. Since ruthenium trichloride absorbs water to some extent to form hydrates, there is inaccuracy to the extent of approximately 5 percent on the calculation of the initial amount of ruthenium in the ratio.After the solution of these various materials had been applied to the substrate material, the coated substrate was dried and then baked in an oxidizing atmosphere at a temperature of 475" to 6250C for periods of 5 to 10 minutes to transform the compounds into their respective oxides. This process was repeated several times to achieve a layer of desired weight. The amount of coating material applied had no observable effect upon the final tin/ruthenium ratios achieved.

Therefore any convenient weight of coating material could be used. Once this step was accomplished, the final tin/ruthenium ratio was determined by stripping the catalytic oxide layer off the titanium substrate by means of molten salts, dissolving in water to precipitate the metals and analyzing the resulting solution by atomic absorption to ascertain the final ratio of tin/ruthenium in the oxide coating. These ratios along with the tin compounds used are reported in Table I below.

TABLE I Sn Compound Used Initial Sn/Ru Final Sn/Ru SnCl4 .5H2O 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(C4Hg)4 , 4.3 0.6 SnCI4.5H2O/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 there is a tin volatilization loss of the order of 10 to 1 when the stannic chloride pentahydrate was used alone, as compared with a negligible loss of tin when the tin compound used was stannic chloride pentahydrate reacted with sulphuric acid. In some cases, the final ratio is even higher than the initial ratio where the sulphate form is used. It is felt that this is due to experimental error caused by the ruthenium compound absorbing water and perhaps some material loss during the stripping process.

EXAMPLE 2 A second experiment to show the substantial increase in the amount of tin retained in the coating as compared with methods not using tin sulphate, was conducted. In this case, a known amount of the solutions according to Example 1 containing various tin compounds was fired in a crucible and the residues analyzed by atomic absorption. The firing temperatures and cycles were similar to those employed in Example 1. The results of this experiment in terms of percentage of the given element remaining in the coating material after such firing are reported in Table II below.

TABLE II Sn Compound % Sn % Ru % Sb Used Retained Retained Retained SnCl4. 5H2O/H2SO4 81 90 43 SnSO4 94 95 61 SnCl4. 5H2O 9 97 23 SnCl4 refluxed in amyl alcohol 19 94 15 From Table II, it can be seen that the use of a sulphate form of the tin yields significantly higher percentages of tin retention as compared with the chloride forms used heretofore.

EXAMPLE 3 A series of electrodes were prepared and tested to evaluate their half cell potentials and lifetimes, in comparison with electrodes made utilizing the chloride form of compounds in such larger amounts as to yield resultant amounts of tin in the coating equal to those given by the sulphate form of compounds. It was found that 25.1 grams of stannic chloride pentahydrate yielded approximately the same amount of tin as a mixture containing 5.48 grams of stannic chloride pentahydrate reacted with sulphuric acid. It can be seen in this case that approximately five times as much of the tin compound is necessary when the sulphate form is not used in the coatings.It was also found that when these two materials were applied in equal amounts in terms of grams per square foot of ruthenium on the titanium sample, the resultant electrodes gave approximately the same half cell potentials and had lifetimes as reported in Table III below.

TABLE III Grams per Lifetime of Lifetime of Square Foot Chloride form Sulphate form Ru in hours in hours 0.1 17 14 0.2 50 68 0.3 79 108 Thus it can be seen that approximately five times as much of the chloride form of the tin as compared with the sulphate form of tin is required to yield equal lifetimes from the resultant electrodes. This means that a significantly lesser amount of the sulphate form tin compounds can be used, therefore resulting in net manufacturing savings for a given electrode lifetime. As can be seen from Table I, the reproducibility of the sulphate form tin compounds is significantly higher than that for the chloride form compounds, thereby lending itself much more readily to scaling up of the manufacturing process for the electrode. Use of the sulphate form also results in significantly less tin being volatilized into the atmosphere, thus eliminating one pollution concern of the prior art processing methods.

WHAT WE CLAIM IS: 1. A method of manufacture of an electrode, which comprises applying a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. It can be seen that there is a tin volatilization loss of the order of 10 to 1 when the stannic chloride pentahydrate was used alone, as compared with a negligible loss of tin when the tin compound used was stannic chloride pentahydrate reacted with sulphuric acid. In some cases, the final ratio is even higher than the initial ratio where the sulphate form is used. It is felt that this is due to experimental error caused by the ruthenium compound absorbing water and perhaps some material loss during the stripping process. EXAMPLE 2 A second experiment to show the substantial increase in the amount of tin retained in the coating as compared with methods not using tin sulphate, was conducted. In this case, a known amount of the solutions according to Example 1 containing various tin compounds was fired in a crucible and the residues analyzed by atomic absorption. The firing temperatures and cycles were similar to those employed in Example 1. The results of this experiment in terms of percentage of the given element remaining in the coating material after such firing are reported in Table II below. TABLE II Sn Compound % Sn % Ru % Sb Used Retained Retained Retained SnCl4. 5H2O/H2SO4 81 90 43 SnSO4 94 95 61 SnCl4. 5H2O 9 97 23 SnCl4 refluxed in amyl alcohol 19 94 15 From Table II, it can be seen that the use of a sulphate form of the tin yields significantly higher percentages of tin retention as compared with the chloride forms used heretofore. EXAMPLE 3 A series of electrodes were prepared and tested to evaluate their half cell potentials and lifetimes, in comparison with electrodes made utilizing the chloride form of compounds in such larger amounts as to yield resultant amounts of tin in the coating equal to those given by the sulphate form of compounds. It was found that 25.1 grams of stannic chloride pentahydrate yielded approximately the same amount of tin as a mixture containing 5.48 grams of stannic chloride pentahydrate reacted with sulphuric acid. It can be seen in this case that approximately five times as much of the tin compound is necessary when the sulphate form is not used in the coatings.It was also found that when these two materials were applied in equal amounts in terms of grams per square foot of ruthenium on the titanium sample, the resultant electrodes gave approximately the same half cell potentials and had lifetimes as reported in Table III below. TABLE III Grams per Lifetime of Lifetime of Square Foot Chloride form Sulphate form Ru in hours in hours 0.1 17 14 0.2 50 68 0.3 79 108 Thus it can be seen that approximately five times as much of the chloride form of the tin as compared with the sulphate form of tin is required to yield equal lifetimes from the resultant electrodes. This means that a significantly lesser amount of the sulphate form tin compounds can be used, therefore resulting in net manufacturing savings for a given electrode lifetime. As can be seen from Table I, the reproducibility of the sulphate form tin compounds is significantly higher than that for the chloride form compounds, thereby lending itself much more readily to scaling up of the manufacturing process for the electrode.Use of the sulphate form also results in significantly less tin being volatilized into the atmosphere, thus eliminating one pollution concern of the prior art processing methods. WHAT WE CLAIM IS:

1. A method of manufacture of an electrode, which comprises applying a

coating material containing an antimony compound and tin sulphate to at least a portion of the surface of a valve metal substrate selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof, drying the coating material and baking it in an oxidizing atmosphere to convert the antimony and tin compounds to their oxides.

2. A method according to claim 1, wherein the antimony compound is antimony trichloride.

3. A method according to claim 1 or 2, wherein the amount of the antimony compound in the coating material is such that the resultant coating contains antimony trioxide in an amount in the range from 0.1% to 30% by weight of the total of antimony and tin oxides.

4. A method according to claim 1, 2 or 3, wherein the coating material also contains a ruthenium or iridium compound.

5. A method according to claim 4, wherein the coating material contains ruthenium trichloride or iridium trichloride.

6. A method according to claim 4 or 5, wherein the coating material also contains a rhodium compound.

7. A method according to any preceding claim, which comprises reacting a tin chloride compound with sulphuric acid to obtain the tin sulphate of the coating material.

8. A method according to any preceding claim, wherein a top coating comprising a manganese oxide or a lead oxide is applied to the surface of the coated substrate.

9. A method according to claim 1, substantially as herein described.

10. A coated electrode when made by a method according to any preceding claim.

11. An electrode for use in an electrolytic cell, comprising a valve metal substrate selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof, having on its surface an intermediate coating comprising antimony and tin oxides the tin oxide having been applied as a tin sulphate and converted to the respective oxide1 and a top coating selected from manganese dioxide and lead dioxide.

12. An electrode for use in an electrolytic cell, comprising a valve metal substrate selected from aluminium, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof, having on its surface a coating formed by applying ruthenium trichloride, antimony trichloride and tin sulphate and drying and baking them in an oxidising atmosphere to convert them to their respective oxides.