AN ELECTRODE, PROCESSES FOR THE MANUFACTURE THEREOF AND USE THEREOF

Abstract: Electrolytic electrode which has a hydrogen overvoltage-reducing coating essentially consisting of an alloy from iron, cobalt and/or nickel with tin, the percentage of tin of the alloy being between 0.01 and 10 per cent by weight. The electrode can advantageously be manufactured by accomplishing said coating galvanically or by a thermal spraying method. The electrode is especially well suited for use as cathode for hydrogen evolution in chlorate, chlorine-alkali or water electrolysis or as an anode in an hydrogen-oxygen or hydrogen-air fuel cell.

Description: TITLE An electrode, processes for the manufacture thereof and use thereof. 5 TECHNICAL FIELD The present invention relates to the field of electrolytic electrodes, the term electrolytic only meaning that the electrode is intended to be contacted with an electrolyte. Thus, the electrode is useful in • •*• • current-consuming as well as current-producing processes. More specifically the electrode according to the invention is an energy-saving electrode in that it shows a low hydro¬ gen overvoltage. The invention also relates to processes for the manufacture of the electrode and to especially in¬ teresting uses thereof, at which its overvoltage-reducing properties are especially advantageous. BACKGROUND OF THE INVENTION Large amounts of electrical energy are consumed with¬ in the electrolytic industry. Among the most energy-con¬ suming processes reference can be made to the manufacture of sodium chlorate and the chlorine-alkali electrolyzis. In cells for such processes it is common to utilize cathod¬ es of steel or titanium. At normal operational conditions the overvoltage for the hydrogen evolution can be between 0.2 and 0:6 V. As the total electrolyzis voltage is just above 3 V, the hydrogen overvoltage accordingly represents a considerable part of the total voltage and thereby the manufacturing costs. Several proposals have been made as to how the hydro¬ gen overvoltage could be reduced. Most of them are based on the idea of coating the cathodes with any suitable material. However, despite the fact that a very larg.e number of proposals have been made the technique has not to any great extent come to any industrial exploita- jUKifA OMPI tion. The reason therefor is that the coatings are not sufficiently good in relation to the costs involved there¬ by. For instance electrolytical coating methods have been proposed, but these give a slow deposition rate, a low current efficiency and brittle depositions while at the same time containing expensive ingredients. As examp¬ les of prior art in this respect reference is made to U.S. Patent specifications 1,818,579, 3,272,728, 4,033,837, 4,086,149, 4,170,536, 4,177,129 and 4,224,133. All plat¬ ing methods disclosed therein give high manufacturing costs. Also different methods for thermal spraying have been disclosed, but also in these cases the costs for the coating are high, especially as the thermally sprayed layers are intended to be subjected to so-called Raney processes in order to enhance the areas of the coatings. As examples of the prior art reference can be made to Canadian Patent specification 1,068,645 and U.S. Patent specifications 4,116,804 and 4,240,895. As concerns the so-called Raney processes there are several methods disclosed in the technical literature for the manufacture of electrodes by means of said tech¬ nique. The basic principle is the use of a noble basis metal and a base or so-called sacrificial metal. Espe¬ cially the technique of electrolytical deposition of an alloy from nickel and zinc and then leaching away the ignoble zinc is a popular method. Russian experiments with such, nickel-zinc coatings which are leached and utilized as electrodes in electrolyzis, were disclosed already in 1949 in Chemical Abstracts, vol 43, 505 g-i. Furthermore, in the above-mentioned U.S. Patent speci¬ fication 3,272,728 nickel-zinc alloys are disclosed which are deposited electrolytically and are leached and uti- lized as cathodes in electrolyzis of water. In DE OS - 2,527,386 there is also disclosed a Raney method, where¬ in the preferred embodiment is a nickel-zinc alloy to be subjected to a leaching operation. Nor has last-mentioned German specification anything to do with the present in- vention, since it works as in all prior art with just the combination of a noble basis metal and an ignoble base or sacrificing metal. It is true that there are enumerations of different basis metals in said specification, where iron is mentioned as one example thereof, and enumera- tions of different sacrificing metals, where tin is men¬ tioned as one example thereof, but it is completely ' obvious to a person skilled in the art that the combination of iron-tin is not useful at the technique according to the German specification. Thus, it is specifically stated that the sacrificing metals must be combined with a proper ba¬ sis, metal, and nowhere in the specification is there men¬ tioned any method by means of which tin could be leached out from the iron metals. To a person skilled in the art it is obvious that this could not be done, as tin is the nobler metal in this case. Rather, what is disclosed in DE OS 2,527.386 is a method of drastically enhancing the surface area, 50 % of the volume preferably being leached away. However, a prob¬ lem in connection with coatings of the Raney-type is their pyroforicity. When the layers have been leached, they are pyroforic and must be kept wet. Thus, when drying an oxidation of the layers takes place due to the large hydrogen covered reaction-active surface and the temperature may rise drastically. Furthermore, chlor- ine-alkali cells, must be dismounted for maintenance now and then, and for diaphragm cells this takes place as often as once every twelfth or fifteenth month. When the new diaphragm has been applied to the cathode it is dried and baked. At said drying operation the larger part of the effect of the Raney layer is lost. DISCLOSURE OF THE INVENTION The present invention relates to something complete¬ ly novel within the stated technical field. More specific¬ ally the invention makes it possible to manufacture in one single operational step a hydrogen overvoltage-reducing coating that does not need any further post or finishing treatment and that can be transported in a dry condition. The coating layer can be made completely dense, which has been confirmed in connection with the invention by means of microscopy, and a good corrosion protection can be ob¬ tained concurrently with the coating, which is not the case with porous coatings of the Raney type. The purpose of the invention is obtained by pro¬ viding an electrically conducting, metallic substrate with a hydrogen overvoltage-reducing coating which essen¬ tially consists of an alloy from any of the iron metals, i.e. iron, cobalt or nickel, or any combination thereof, with tin, the percentage of tin of the alloy being bet¬ ween 0.01 and 10 percent by weight, based on said alloy. In this way there is obtained an electrode having the above-mentioned advantages, primarily a strong re¬ duction of the hydrogen overvoltage, and it has also turn¬ ed out that the alloy is very easily applied to the me¬ tallic substrate. Of course the hydrogen overvoltage-reducing effect will be most pronounced if essentially the total area of the active part of the metallic substrate is coated, but no doubt some improved effect is obtained already when a minor part of said surface is coated with the alloy accord- ing to the invention, which means that the invention is not delimited to the coating of any special percentage of the surface. However, for economical purposes it is gene¬ rally preferable to coat at least 10 percent of the sur¬ face area. Within the wide range as to the amount of tin in the alloy it has been shown that especially preferable results are obtained if said amount is xvithin the range of 0.1 - 5 percent by weight, especially 0.5 - 4 percent by weight. The invention also relates to especially preferable processes for the manufacture of the electrolytic electrode according to the invention. According to a first process aspect the electrode is then prepared by coating the sub¬ strate galvanically. Thus, the coating containing a small amount of tin can be electro-plated with high efficiency and high current density from a plating bath that can be prepared by cheap chemicals. According to a second process aspect the electrode according to the invention is manufactured by coating the substrate by thermal spraying, preferably by means of an arc spraying device. Thus, the thermal spraying is per- . formed by means of a wire having the ' desired composition, and the thermally sprayed layer obtained does not either require any post-treatment. Other methods of preparing the electrode by means of an alloy having the desired composition are possible and within the field of what can be accomplished by a person skilled in the art with reference to the inven¬ tive idea now disclosed, but the above-mentioned methods are those methods which are to-day most preferable from an economical point of view. Finally, the invention relates to especially in¬ teresting uses of the electrode, a first alternative being the use of the electrode as a cathode for hydro- gen evolution in chlorate, chlorine-alkali or water electrolyzises. According to a second alternative the electrode is utilized as an anode in a hydrogen-oxygen or hydrogen-air fuel cell. At both these uses especially the hydrogen overvoltage-reducing effect is valuable for economical reasons. OMPl Especially in connection with the use of the elec¬ trode in a hydrogen-oxygen fuel cell or in a water electro- lyzer, but also at the other uses mentioned, it may also be valuable to utilize the alloy in the form of a powder by means of which the desired coating of the electrode has been accomplished, as it is possible to obtain a very high specific surface area by means of a powder. The manu¬ facture of the alloy in the form of a powder and the coat¬ ing of the electrode therewith can be accomplished in any way that is well known to a person skilled in the art. EXAMPLES The invention will now be described more in detail by the following examples concerning some embodiments of the invention. Example.1 A plating bath having the following composition was prepared: 250 g of Ee S0 4 .7 H £ 42 g of Fe Cl 2 -4 H 2 0 20 g of NH 4 C1 The above-mentioned ingredients were dissolved in distilled water up to a volume of 1 litre. In order to remove the trivalent iron the solution xvas stirred with 1 g (5 ml. of H 2 S0 4 and 5 g of Fe-powder during one night. The solution was filtered, and to the same there was added a solution prepared from: 1 g of Sn Cl 2 H 2 0 30 ml of 50 - gluconic acid 10 ml of distilled water. The pH-value was adjusted to 2.95. Platings were performed on four well cleaned copper plates, an iron plate being utilized as the anode of the plating bath . . The plating operation was performed at room temperature OMPI and at varying plating current densities. Coatings with a layer thickness of about 30 ,um were deposited. The electrodes were weighed after the plating operation and the current efficiency was calculated to more than 90 -. The electrodes xvrere analyzed by dissolving the layers in an acid (HC1) and determining the metal con¬ tents by means of atomic absorption. The tin percentages obtained at different plating current -densities are shown in table 1 below, and the connection between the tin per¬ centage and the plating current density is shown graphical¬ ly in Figure 1. Table 1 Tin percentage obtained versus plating current density Plating current density Ci) Tin percentage (Sn.) mA/cm percent by weight .. 7 6.7 20 2.9 47 1.7 80 1.2 The electrodes were then tested as to evolution of hydrogen gas in 5M KOH, the potential being determined relative to a reference electrode in the form of Hg/HgO. The potential at a current density of 1 kA/rn * " as a function of the previously measured tin percentage is shown in Table 2 below and shown graphically in Figure 2. table 2 Potential at hydrogen evolution in 5M KOH, 1 kA/m as a function of the tin percentage Tin percentage (Sn.) Potential (U/Hg-HgO) percent by weight CmV) 6.7 -1.250 2.9 -1192 1.7 -1182 1.2 -1180 OA'-PI As is clear from last-mentioned table and figure the most active layer shows such a good potential as -1180 mV relative to Hg/HgO, which is to be compared with -1380 mV for a very rough iron surface or -1500 mV for a polished iron surface. However, also the least good value of this test series, i.e. -1250 V, is very much lower than said comparative value. The mechanical properties of the obtained coating layers were good, since the electrodes ' could withstand bending back and forth without any exfoliation being caused. Example 2 A plating bath which had previously been used for the plating of electrodes had been depleted on bivalent tin so that the"contents of bivalent tin had been reduced to 0.3 g per litre of solution. A steel electrode was coat- ed with a layer by plating in this bath at 80 mA/cm". This layer had a hydrogen evolution potential of -1230 mV 2 relative to Hg/HgO at one kA/m . The contents of tin in the deposited layer was estimated to 0.4 percent by weight. The estimation was made by comparing the percentages of tin in the plating bath before and after the plating opera- • tion, respectively In this connection it can be mentioned that it is only the bivalent tin that is active at the plating. A plating bath prepared from stannic chloride does not give any active layers. The amount of bivalent tin is simply monitored by iodometric titration. When the plat- ing bath is not utilized it should be protected from air to prevent " any unnecessary oxidation of the tin to in¬ active fourvalent tin. In order to further illustrate the influence of the percentage of tin upon the potential at hydrogen evolution the result obtained in example 2 has been marked in Figure -U 2 as a fifth point. From said point it can be seen that the effect is a little less good than at the maximum re¬ sults, but the graph also shows that already very low amounts of tin give a very considerable improvement re- lative to a pure iron surface. Example 3 A nickel bath which was equivalent to the iron bath of Example 1 was prepared by dissolution of: 250 g of NiS0 4 .7H 2 0 50 g of NiCl 2 .6H 2 0 20 g of NH 4 C1 and 10 ml of 50 % gluconic acid and 1.4 of SnCl 2 .2H 2 0 in distilled water up to a volume of 1 litre. The pH-value was adjusted to 3.0, and a number of electrodes were plat¬ ed from said bath. Also in this case the most active layers were obtain¬ ed at a high plating current density. More specifically, the hvdrogen evolution potential xvas -1170 mV relative to 2 Hg/HgO for a layer deposited at 100 mA/cm . It is true that the tin percentages were not analyzed in each single case, but they should be approximately similar to the per¬ centages at the corresponding current densities in the iron bath. Thus, the tin is the nobler element in the iron bath as well as in the nickel bath. If the percentage of tin were the same as that of the iron, a plating operation would lead to the deposition of almost only tin. What is keeping the tin percentage down in the coating is the low percentage of tin of the solution. At the higher plating current densities of the iron bath the metal composition of the coating tends to be similar to that of the solution. 2 At 80 mA/cm the percentage of -tin in the coating is 1.2 % while the tin represents 0.8 % of the total metals of the solution. The same should be applicable to the nickel, bath. Example 4 By means of an arc spraying device a number of steel plate specimens were sprayed from a steel wire containing approximately 0.7 percent by x.eight of tin. All specimens had a hydrogen evolution potential of about -1150 V at 1 kA/m 2 . Consequently, in all coatings described above very large voltage gains have been accomplished. This is true also when comparing the results with very coarse iron surfaces. Sand blasted steel or steel that has been corrod- 2 ed in operation has a value of about -1380 mV at 1 kA/m , which means a difference of about 0.2 V. At elevated tem¬ peratures the differences will be somex*hat lower while even greater voltage gains can be expected at higher current densities. -BU E OλϊPI

Claims: CLAIMS 1. An electrolytic electrode comprising an elec¬ trically conducting, metallic substrate having a hydrogen overvoltage-reducing coating on at least part of its sur- face, c h a r a c t e r i z e d in that the coating con¬ sists essentially of an alloy from iron, cobalt and/or nickel with tin, the percentage of tin of the alloy being between 0.01 and 10 percent by weight. 2. An electrode according to claim 1, c h a r a c t e r i z e d in that the percentage of tin of the alloy is between 0.1 and 5 percent by xveight. 3. An electrode according to claim 2, c h a r a c t e r i z e d in that the percentage of tin of the alloy is between 0.5 and 4 percent by weight. 4. An electrode according to any one of the pre¬ ceding claims, . c h a r a c t e r i z e d in that the coating has been obtained by a thermal spraying operation. 5. An electrode according to any one of claims 1-4, c h a r a c t e r i z e d in that the coating has been ob- tained galvanically. 6. A process for the manufacture of an electrolytic electrode according to any one of claims 1-4, c h a r a c t e r i z e d by coating the substrate by a thermal spraying operation. 7. A process according to claim 6, c h a r a c t e r i z e d by performing the thermal spraying operation by means of an arc spraying device. 8. A process for the manufacture of an electro¬ lytic electrode according to any one of claims 1-3 and 5, c h a r a c t e r i z e d by coating the substrate gal¬ vanically. 9. Use of an electrolytic electrode according to any one of claims 1-5 as a cathode for hydrogen evolution in the electrolyzis of chlorate, chlorine-alkali or water, or as an anode in a hydrogen-oxygen or hydrogen-air fuel cell. 10. Use according to claim 9 of the electrode with said coating in the form of a powder of said alloy. "BU EA

Inventors: KARLSSON GUNDER, KARLSSON, GUNDER
Applicants: ELECTROCELL AB
Application year: 1984
Main classification: C25B11
IPC codes: C25B11, H01M4