NO791960L - ELECTRODE FOR USE IN AN ELECTROLYTICAL PROCESS - Google Patents

Description

This invention relates to electrodes for electrolytic processes, processes in which such electrodes are used and electrolytic cells in which such electrodes are used.

It is known in electrolytic processes to use electrodes comprising a thin film of an oxide of a metal of the platinum group placed on a substrate comprising a "film-forming

nende metal", see US. Patents 3,632,498 and 3,611,385. It is likewise known to apply a coating to a valve metal,

which, in addition to an oxide of a metal of the platinum group, includes carbides of boron, hafnium, chromium, molybdenum, tantalum,

titanium, tungsten and the like and likewise refractory oxides such as silicon dioxide, titanium dioxide, etc., see US. Patents No. 3,616,329, 3,654,121, 3,657,102, 3,677,812, 3,687,724 and 3,755,107 and the papers presented at a meeting held by the Electrochemical Society in Seattle in May 1978.

It has now been shown that electrodes for use as

anodes in electrolytic processes of the type described in the above-mentioned patents, for the production of chlorine, hypochlorites, chlorates and perchlorates, for the production of organic compounds, for the electrolysis of water and for cathodic protection systems, can advantageously be produced by that at least one part of the valve metal surface of a substrate is coated with at least one coating of a thin, electrically conductive, electrocatalytic coating material of a thickness effective for performing electrolysis, which coating, disregarding any binders or modifying agents that may be incorporated therein, is substantially amorphous and substantially free of crystallinity detectable by X-ray diffraction analysis. The electrode is advantageous in that it has a long service life in an electrolysis environment and low overvoltage.

The electrode according to the invention comprises a substrate which has a surface consisting mainly of a valve metal. As used here, the term "valve metal" has the same meaning as in the rest of the subject, including the representatives of the state of the art referred to above, and the term includes, for example, titanium, tantalum, tungsten, aluminium, hafnium, niobium and zircon as well as alloys of these. The substrate can be a homogeneous body with a surface of such a valve metal or it can comprise an electrically conductive base metal, such as copper, which has high-quality properties as an electrical conductor, but which is exposed to corrosion in an electrolytic environment, which base metal is then provided with a separate, adhesive coating of a valve metal.

The cover may comprise one or more layers of several classes, which will be explained in more detail below,

of substantially amorphous electronically conductive electrocatalytic materials substantially free of crystallinity detectable by X-ray diffraction analysis.

The invention also includes electrolytic cells and electrolytic processes where use is made of the electrodes described here.

A first class of amorphous coating materials are phases (the term should be understood in the usual way in the art) comprising

predominantly at least one element from group Illa and at least one element from group V a of the periodic table, as this system is presented in the Handbook of Chemistry & Physics (55th edition, CRC Press, 1974) This first class of coating materials can also contain a second and possibly also a third element from one of the groups Illa and Va or from both groups. Examples of compositions of covering materials

in this class and containing only one element from each of these groups, are systems comprising aluminum-antimony, gallium-antimony, indium-antimony, gallium-arsenic, indium-arsenic, gallium-phosphorus, indium-phosphorus, aluminum-phosphorus, aluminum-arsenic and especially boron-phosphorus, such as boron phosphide, BP. (With the expression "predominantly present", as used here in the description of the elements that make up the different classes of amorphous coating materials, it is meant that the elements from

given groups make up more than 50 atomic % of the amorphous coating material, and this implies the possibility of incorporating a smaller amount, namely less than 50 atomic %, of other elements, provided that the coating material exclusive of any binders

agents, which will be described later, are electronically conductive, electrocatalytic and free of crystallinity which is detectable

similar to X-ray diffraction analysis.)

Examples of the composition of bases of the first class

which include several elements from either group Illa or group Va, are indium-gallium-antimony, gallium-arsenic-phosphorus and indium-arsenic-antimony.

Examples of semiconducting materials corresponding in composition to the above, and likewise further examples of other compositions, will be found in "Semiconductors", by R.A..Smith, Cambridge University Press, 1959, on pages 392 and 409 and likewise

referred to The Proceedings of the International symposium on Chemical Bonds in Semiconducting Crystals, Vol. 4, entitled "Semiconductor Crystals, Glasses and Liquids", pages 49, 95 and 155

(English translation by the Consul tants Bureau, New York-London■ r 1972). The symposium was held in Minsk, Soviet Union, in 1967. The publication will be referred to below as "Semiconductor Crystals, Glasses and Liquids". The information in the above re-

said publications and in the publications that will be discussed below, with regard to materials with electronic wiring

ability, is incorporated by reference in the present description.

Another class of amorphous coating materials is constituted

of phases which predominantly comprise at least one element from group II and at least one element from group Via of the periodic table, which exist in a solid state under the standard conditions. This second class of coating materials can

also contain a second and possibly a third element from group II or Via or from both groups. Examples of composite materials, in this class are predominantly phases as in

quantity consists of zinc-selenium, beryllium-selenium, beryllium-sulphur, beryllium-tellurium, zinc-sulphur, cadmium-sulphur, cadmium-tellurium or cadmium-selenium. Further examples of semiconducting materials of composition corresponding in type to the second class of phases are described in "Semiconductors" by Smith, pages 413-34,

and in "Bands and Bonds in Semiconductors" by J.C. Phillips (Academic Press, New York and London, 1973, page 196.

A third class of amorphous coating materials is constituted

of phases which predominantly consist of at least one element from group Ila and at least one element with an atomic weight of

at least 28 from group IVa of the periodic table. The third class of coating materials can likewise contain a second element and possibly a third element from one or the other of these groups or from both groups. Examples of compounds in this class are magnesium-tin, calcium-germanium, calcium-tin, calcium-silicon, calcium-lead, magnesium-silicon and magnesium-germanium, where there are nominally two atoms of the element from group Ila where atom of the element from group IVa. Information on semiconducting materials of composition similar to the above will be found on pages 411-412 of "Semiconductors" by Smith.

A fourth class of amorphous coating materials are ternary phases consisting predominantly of at least one element from each of groups II, IV and 'Va in the periodic table. These are sometimes referred to as A 11 B IV ^v compounds, which are nitrides, phosphides, arsenides and antimonides. Examples of composite materials in the fourth class, in which the atomic ratio between the elements from groups II, IV and Va is nominally 1:1:2, are nitrides such as zinc-tin-nitride, zinc-germanium-nitride and calcium-silicon-nitride, phosphides such as beryllium tin phosphides, cadmium germanium phosphides, antimonides such as zinc tin antimonide, zinc germanium antimonide and cadmium germanium antimonide, and arsenides such as zinc tin arsenide, magnesium silicon arsenide , cadmium-tin-arsenide and magnesium-tin-arsenide. Examples of ternary phases in the fourth class in which the atomic ratio between the elements is different from that stated above are

Ca^SiN^and Ca^Si2Ng. Further information regarding semiconducting materials with a ternary composition corresponding in type to the ternary compositions discussed above will be found on pages 31-38, 55-59 and 88 of "Semiconductors Crystals, Glasses and Liquids".

A fifth class of amorphous coating materials are ternary phases which predominantly consist of at least one element,

which normally exists in a solid state at standard conditions

tendons, from each of groups II, V and VI in the periodic table

tem. This fifth class of coating materials can also contain additional elements of one of the groups II and V or from both of these groups. Phases in this fifth class are sometimes designated as A^B^-A^C^ phases. Examples of compositions in the fifth class of amorphous coating materials are Zn^As,, - 2 ZnTe; Zn3As2-2CdTe; Cd3As2-2CdTe; Sd^ - 2 CdTe; Cd3As22 CdSe and (Zn,Cd)3(P,As) - 2 (Zn,Cd)(S,Se,Te). Additional

ternary phases in the fifth class are Hg3PS4, Hg3PS3, Hg4P2S^and HgPS3. Information on semiconducting materials with ternary composition corresponding to the ternary compositions given above can be found on pages 69-72 and 97-103 in "Semiconductor Crystals, Glasses and Liquids" .

A sixth class of amorphous coating materials are ternary phases which predominantly consist of at least one element from group Ib and/or IIb together with at least one element from each of groups Illa and Via, exclusive of oxygen. This sixth class of coating materials may also contain additional elements from any of these groups. Phases belonging to the sixth class, and which contain elements from group lb, are sometimes designated A<I>B<III>C2<I->phases. Examples of compositions of this type are AgInTe2, AgGaTe2, CulnTe , AglnTe , AgGaS2, CuInS2, CuAlS2, AgAlSe2, CuAlSe2, CulnSe 0g Aglnåe Other amorphous materials belonging to the

sixth class is sometimes denoted A I BI,- II CVg I such as for example CuIn^Teg, AglnTeg and Agln^Seg. When an element from group II is used instead of an element from group Ib, the phases are sometimes designated as { A11^ 1) - (B*11^1) , for example ZnS-In2S3 and CdS-In2S3. Information on semiconducting materials with compositions similar to those mentioned above will be found on pages 31-38 and 73-77 of "Semiconductors Crystals, Glasses and Liquids" and on page 437 of "Semiconductors" by Smith.

A seventh class of amorphous coating materials are quaternary phases which predominantly consist of at least one element from each of groups II, III, v and Vi, exclusive of oxygen, in the periodic table; these can also contain several elements from which preferably single group. Examples of phases belonging to the seventh class are InAs-CdS; InAs-CdSe ; InAs-CdTe; InAs-ZnSe; InAs-ZnTe; and InAs-ZnS. Information on semiconducting materials of composition similar to those mentioned above will be found on pages 104-107 of "Semiconductor Crystals, Glasses and Liquids".

An eighth class of amorphous coating materials are fine-grained phases which predominantly consist of at least one basic substance from each of groups I, III, IV, V and Via in the periodic table. These are considered to be solid solutions of B^^^D^ compounds with A^C^E^ compounds of the general formula ^O.Sx+O.SyJ^S^-l.Sx-O.S^^Mo. S-y)^<1>-Examples of compositions of this phase are up to 40 mol% Cu2GeSe3 dissolved in 3 GaAs; about. 1% Cu2GeSe3 dissolved in either 3 InSb or 3 GaSb and approx. 1% Ag2GeSe3 dissolved in 3 GaSb. Further information regarding semiconducting materials of composition similar to those mentioned above can be found on pages 81-85 of "Semiconductor Crystals, Glasses and Liquids".

A ninth class of amorphous coating materials are phases that are predominantly made up of halide glasses. These are known in the art, see for example US Patent 3,271,591, which describes GeQ # 17Te0 >83 and ^eo . isTeo. 81^^0 .02^0 .02 As chalo^~ nider, US.Patent 3,876,985, which specifically states GeQ 075"

<Te>0.675<As>0.25<;><Ge>0.0675<Te>0.40<AS>0.35<Sl>0.18<In>0.002°g Ge0.155-Te0 28As0 34S0 22 and S"Marsand'INIS Atomic Index, Vol 8, (23)

(1977), to which reference is made in Chemical Abstracts, Vol. 88, at No. 114203, which publication describes TeQ ^qASq ^2 and

Asr>-,0Gen, ,Ten a^S*nc. The material of composition Si,,Ge,,-

- 0.38m 0.14 0.43 0.05 . _ ,. _ _a.. 11 11 35 3 40 er"es^revet 1 Proceedings of the Symposium on Semiconductor Effects in Amorphous Solids, page 172 (North Holland Publishing Co., Amsterdam, 1970. Compounds of sulfur with titanium, vanadium , chromium, manganese or zirconium is described in US Patent No. 3 571 669. Further halide glasses are described in "Semiconductor Crystals, Glasses and Liquids", pages 131-143. Halide glasses containing rare earth metals are discussed on the pages of the aforementioned publication 39-44, and by V. P. Zhuze, et al., Fiz. Tverd. Tela, Vol 6, pages 257 and 268 (1964). Still other halide glasses are described in the Proceedings of the Sym<p>osium on Semiconductor Effects in Amorphous Solids, p

372 (North-Holland Publishing Co., Amsterdam, 1970).

A tenth class of amorphous coating materials is constituted

of predominantly amorphous alloys of metallic elements from groups IV, Vd, VIb and the rare earth metals with each other or with a metal selected from group Ib or group

II or the base metals in group VIII. Examples of materials

in the tenth class is an alloy of 30-85 atom% nickel in niobium (see T.W. Barbee, et., Thin Solid Films, Vol. 45(3), page 591 (1977); alloys of silver and one of the rare earth elements -ments such as gadolinium, terbium, dysprosium, holmium or erbium (see B. Boucher, IEEE Trans. Magn., 1977 MAG-13(5), 1601);

alloys of cobalt and gadolinium (see O.S. Lutes, et al,, id. page 1615; alloys of the composition Ge^ Te where x is in the range from 0.1 to 0.9, preferably 0.5 or 0.6 (see S.K. Behal, et al., Thin Solid Films, Vol. 48(1), page 51 (1978); and alloys of magnesium with either bismuth or antimony.

An eleventh class jav amorphous coating materials consist

predominantly of borides, carbides, nitrides, silicides and phosphides, some of which are known as metallic glasses, such as Fe8o<B2>0og<F>e78M°2B20'Q$ composite materials corresponding to semiconductors described on pages 8 - 26 and 55 - 59 in "Semiconductor Crystals, Glasses and Liquids", such as Ca^N-^, Ca^SiN4, Ca5Si2Ng, CaSiN2, Si^N^, Mn^ 67-l' 75)Si and CrSi* Other suitable amorphous composite materials is formed from phosphorus with silicon, germanium, gallium, boron or aluminium, as specified in the US. Patent 3,571,673, based on boron with rare earth metals, as stated in the US. Patent 3,571,671 and based on boron with carbon, silicon, titanium, germanium, zirconium and hafnium, as indicated in the US. Patent document 3,571,670.

It should be noted that compositions of the various elements, which are included in the classes mentioned above but which are crystalline (and certain such composite materials can be produced in crystalline form) are not covered by the term "amorphous coating" or "amorphous coating materials", as these designations are used here.

Amorphous coatings produced from the above-mentioned classes can and do normally contain varying amounts of one or more modifiers. The incorporation of a modifier has a profound effect on the properties of the amorphous matrix material with regard to electrical and electronic properties and electrocatalytic activity. By incorporating a modifying agent, the material's electrical conductivity can be changed by a few to several orders of magnitude.

Incorporation of dopants ("dopants") in semi-conducting materials to achieve desired electronic states and electrical conductivity is well known in the art and is discussed, for example, in "Semi-Jf-tn. conductors" by Smith. In contrast to what is the case with crystalline semiconductors, for example transistors, where the chemical purity of the base material and the chemical purity and permissible variation in the concentration of the doping agent must satisfy strict requirements, however, the requirements for purity and concentration of a modifying agent are significant less critical for the performance of amorphous electrode coatings. Relatively wide ranges can be used for the concentration of a modifier to change the properties of the base material. Generally, effective overcoating can be achieved on valve metal electrodes when the concentration of the modifier in the amorphous matrix material varies from less than 1 atomic % to about 30 atom% of the amorphous matrix material.

As used here, the term "modifying agent" means an element that does not exist as a gas under the standard conditions, or a compound of such an element, since the element does not fall under the definition of the phase class that makes up the base material, and since the element, or a compound of this, is present in solid form in the base material and is mainly insoluble in water. Suitable modifiers include metals, and compounds of such metals, from group Ib, group II, group VI, the rare earth metals and the transition metals from groups I\b , Vb, vib , VITfc, and the base metals (iron, cobalt) from group VIII. Carbon and boron can also be used as modifiers.

The modifiers are distributed evenly or homogeneously in the base material to achieve the desired modification of the electrical conductivity and the electrocatalytic properties. The modifiers can be present either in the amorphous state or in the 1 h^ydipers microcrystalline form.

Optionally, a binder may be incorporated into the amorphous coating material to improve adhesion of the coating to the surface of the electrode's base material. If a binder is used, this should be chosen so as to avoid any adverse effect of this on the electronic conductivity and the electrocatalytic properties of the class of amorphous material with which it is used. Normally, the binder makes up a small proportion of the amorphous coating material.

Amorphous coating materials can be applied to the valve metal by any of several known methods. In the slurry method, the material to be applied is dispersed in an aqueous or organic liquid in the form of finely divided particles with an average particle size smaller than approx. 250^u , preferably in the range from approx. 10^,u and down to colloidal particle size.

The sludge is applied to the valve metal substrate by painting, brushing, spraying or dipping, after which the electrode is dried and heated in air at a temperature from approx. 200° to approx. 800°C and possibly up to 1200°C, or in an atmosphere that is inert to the substrate and the coating material, or in a vacuum. Optionally, suitable binders can be incorporated into the sludge, for example organometallic compounds such as resinates of. bismuth,

tin, titanium or uranium, as described in US . Patent document 3,687,724. The organometallic compounds decompose during the heating step, usually to the metal oxides and then act as diluting binders. Another application method involves the use of organometallic compounds such as recinates, mercaptides or carboxylates dissolved in an organic solvent. The solution is applied to the surface of the valve metal, after which the organometallic compound is split into metallic form by heating at temperatures in the range from approx. 200° to approx. 800°C in an atmosphere of hydrogen which is essentially free of oxidizing compounds such as gaseous oxide. This application method is preferred for carbides. A third application method is vacuum evaporation or sublimation with subsequent deposition of the coating material on the valve metal substrate. Other application methods are spraying, such as radio frequency spraying, and glow discharge of the material onto the valve metal surface. Generally, spraying is the preferred method of application.

The average thickness of the amorphous coating, including any binders or modifiers, should be sufficiently large to allow electrolysis for a longer time, and it is preferably in the range from approx. 100 Å to approx. 1 mm. The amorphous coating may* cover essentially the entire exposed anodic surface of the valve metal or as little as 20% of this surface, but can advantageously, depending partly on economic conditions, cover from approx. 80 to approx. 9 5% of said surface.

The electrodes described here can have one or more layers

of amorphous coating material. The outer layer must be electronically conductive and electrocatalytic and must also be resistant to the electrolysis environment in which it is to be used. If an inner layer is used, this must be electrically conductive, but it does not have to be electrocatalytic. The layers may comprise phases of the same or different or alternating classes of amorphous material. Each layer must adhere to the underlying layer or to the valve metal surface of the substrate.

The preferred embodiment of the invention is an electrode comprising a substrate with a titanium surface which is provided with an amorphous coating comprising predominantly a halide glass, or alternatively boron phosphide, in addition to a modifier.

Claims (10)

1 Electrode for use in an electrolysis reaction, which electrode comprises a substrate with a surface of a valve metal, characterized in that at least one part of this surface is provided with a thin, electronically conductive, electrocatalytic coating, of a thickness that is effective for execution of electrolysis, and that this coating, disregarding any binders and modifiers, is substantially amorphous and substantially free of crystallinity detectable by X-ray diffraction analysis. 2. Electrode for use in an electrolysis reaction, which electrode comprises a substrate with a surface of a valve metal, characterized in that at least part of this surface is provided with a thin, electronically conductive, electrocatalytic coating, of a thickness that is effective for performing electrolysis, which coating comprises a matrix material and a modifier and, disregarding any binders and the modifier, is substantially amorphous and substantially free of crystallinity detectable by X-ray diffraction analysis. 3. Electrode according to claim 2, characterized in that the overcoat consists predominantly of a phase of at least one element from group Illa and at least one element from group Va in the periodic table. 4. Electrode according to claim 3, characterized in that the phase is boron phosphide. 5. Electrode according to claim 2, characterized in that the overcoat consists predominantly of a halide glass. 6. Electrode according to claim 5, characterized in that the halide glass is germanium-phosphorus-tellurium. 7. Electrode according to claim 2, characterized in that the coating comprises an amorphous alloy. 8. Electrode according to claim 7, characterized in that the amorphous alloy is germanium-tellurium. 9. Process for the production of a chemical product by electrolysis in an electrolytic cell, in which an electrolyte containing all the necessary elements for the formation of the chemical product is used, and in which an electrode comprising a substrate with a surface of a valve metal is used, characterized in that at least part of the surface^ of the valve metal is provided with a thin, electronically conductive, electrocatalytic coating, of a thickness that is effective for carrying out electrolysis, and that this coating, when any binders and modifiers are disregarded, is essentially amorphous and essentially free of crystallinity which can <p> be detected by X-ray diffraction analysis. 10. Method according to claim 9, characterized in that the overcoat - in a predominant amount constitutes ac chalo- • genid glass.