CA1190184A - Coated film-forming metal electrode with surface oxide barrier layer incorporating rhodium and/or iridium - Google Patents
Coated film-forming metal electrode with surface oxide barrier layer incorporating rhodium and/or iridiumInfo
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- CA1190184A CA1190184A CA000361721A CA361721A CA1190184A CA 1190184 A CA1190184 A CA 1190184A CA 000361721 A CA000361721 A CA 000361721A CA 361721 A CA361721 A CA 361721A CA 1190184 A CA1190184 A CA 1190184A
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Abstract
COATED METAL ELECTRODE WITH IMPROVED
BARRIER LAYER
ABSTRACT
An electrode for use in electrolytic processes comprises a substrate of film-forming metal such as titanium having a porous electrocatalytic coating comprising at least one platinum-group metal and/or oxide thereof possibly mixed with other metal oxides, in an amount of at least about 2 g/m2 of the platinum-group metal(s) per projected surface area of the substrate. Below the coating is a preformed barrier layer constituted by a surface oxide film grown up from the substrate. This preformed barrier layer has rhodium and/or iridium as metal or compound incorporated in the surface oxide film during formation thereof in an amount of up to 1 g/m2 (as metal) per projected surface area of the substrate.
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BARRIER LAYER
ABSTRACT
An electrode for use in electrolytic processes comprises a substrate of film-forming metal such as titanium having a porous electrocatalytic coating comprising at least one platinum-group metal and/or oxide thereof possibly mixed with other metal oxides, in an amount of at least about 2 g/m2 of the platinum-group metal(s) per projected surface area of the substrate. Below the coating is a preformed barrier layer constituted by a surface oxide film grown up from the substrate. This preformed barrier layer has rhodium and/or iridium as metal or compound incorporated in the surface oxide film during formation thereof in an amount of up to 1 g/m2 (as metal) per projected surface area of the substrate.
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Description
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Our Reference: 2978C
COATED METAL ELECTRODE WITH IMPROVED BARRIER LAYER
Technical Field The invention relates to electrodes for use in electrolytic processes, of the type having a substrate of a film-forming metal such as titanium, tantalum, zirconium, niobium, tungsten, aluminium and alloys containing one or more of these metals as well as silicon-iron alloys, coated with an electrocatalytic coating containing one or more platinum-group metals or their oxides possibly mixed with other oxides.
By "film-Forming metal" is meant one which has the property that when connected as an anode in -the electrolyte in which the coated anode is subseguently to operate, there rapidly forms a passivating oxide film which protects the underlying metal from corrosion by the electrolyte.
These metals are also frequently referred to as "valve metals".
The invention is more particularly concerned with dimensionally-stable electrodes provided with an improved barrier or intermediate layer between the film-forming metal substrate and the electrocatalytic outer coating.
Background Art In early proposals (see for example U.K. Patent Specifications ~55 107 and 869 865), a titanium electrode with a coating of platinum group metal was provided with an inert barrier layer of titanium oxide in the porous places of the coating, this barrier layer preferably being formed or reinforced by a heat treatment. Later, in U.K. Patent Specification 92~ 080, the inert barrier layer of titanium oxide was preformed by electrolytically treating or heating the titanium substrate in an oxidizing atmosphere prior to application of the platinum group metal.
The preforming of such a barrier layer was also advocated in U.K. Patent SpeciFication 1 147 422 with a view to improving the anchorage of an active coating consisting of or containing platinum group metal oxides.
Later, the development of coatinys formed of mixed crystals or solid solutions of co-deposited oxides of film-Forming metals and platinum group metals (see U.S. Patent No. 3 632 49~) provided commercially viable electrodes which revolutionized the chlor-alkali industry and have become widely used in other applications. With these electrodes, excellent performance was achieved without the need for a reinforced or preformed inert barrier or anchorage layer on the substrate and today it is generally accepted that the preFormed or reinforced inert barrier layers are detrimental to performance. In retrospect, the early proposals for pre-formed or reinforced inert barrier layers appear to have been unsuccessful attempts to avoid defects which were inherent in the previous coatings rather than in the substrate.
_ 3 Nevertheless, some proposals attempting to improve inert barrier layers have still been made, for example by applying a titanium oxide barrier layer from a solution containing ~i4 ions. Again, this has been found to impair performance of the electrodes.
Another approach has been to provide a non-passivating barrier layer or intermediate layer underlying the active outer coating. Typical suggestions have been doped tin dioxide sub-layers; thin sub-layers of one or more platinum metals such as a platinum-iridium alloy; sub-layers of cobalt oxide or lead oxide, and so forth. Although various patents have claimed marginal improvements for these electrodes in specific applic-ations, in practice none of these suggestions has led to any significant improvement or any widespreatl commercial use~
Disclosure of the Invention The invention concerns an electrode with a film-forming metal sub-strate having a porous outer electrocatalytic coating containing at least about 2 g/m (as platinum group metal per projected surface area of the substrate) of at least one platinum ~roup metal and/or oxide thereof possibly mixetl with other metal oxides, and an improved non-passivating barrier layer between the substrate and coating.
According to the invention, this barrier layer is a preformed surface oxide film grown up from the ,ilm-forming base and having rhodium and/or iridium incorporated in the surface oxide film during formation thereof in~an amount of up to 1 9/m2 (as metal) per projected surface area of the substrate.
The surface oxide film of the barrier layer is rendered non-passivating by the incorporation of the rhodium and/or iridium as metal or as a compound, usually the oxide or a partially oxidized compound.
Another aspect of the invention is a method of manufacturing such an electrode in which the formation of the barrier layer involves the application of a very dilute acidic paint, i.e. one which contains a small quantity of a thermodecomposable iridium and/or rhodium compound that during decomposition and simultaneous formation of the surface film of film-forming metal oxide will be flIlly absorbed by this surface film, this dilute paint containing generally about 1-15 9/1 of iridium and/or rhodium (as metal).
The paint used will typically include an organic solvent sucIl as isopropyl alcohol, an acid (notably IICl, ~IBr or III) or another agent (e.~. NaF) which attacks tht! film-forIlling metal and encourages the formation of film-forming metal oxide durin~ the subseguent heat treatment, and one or more thermo-deconIposable salts of iri~Iium and/or rhodium. Usually this solution will be at least five times more dilute and preferably about 10 or more times cIilute (in terms of its precious metal content) than the paint solution whicilIllay be used for the production of the outer porous electrocatalytic oxide coatings; this means that the quantity of iridium, and/or rhodium, will be reduced, e.g. to 1/5 or 1/10 or even l/lOOth the amouIlt of the corresponding platinum-group meta1 in -the paint used for producing the outer coating for approximately the same quantity of solvent and acid.
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The action of the acid or other agent which attacks or corrodes the film-forming metal and promotes the formation of the oxide film during the subsequent heat -treatment is very important; without a suitable agent producing this effect, fo~-mation of the surface oxide film oF the ~ilm-forming metal would be substantially hindered or inhibited.
It has been observed that by applying one coat of a given solvent/
acid mixture to a film-forming metal base subjected previously to the usual cleaning and etching treatments and then heat;ng after drying to drive ofF
the solvent, a given quantity of film-forming metal oxide will be produced.
This procedure can be repeated a number of times (usually four or five times for 4 ml HCl in 60 ml isopropyl alcohol applied to a titaniunl base, dried and heated to 500C for ten minlltes) before the growth of film-forming metal oxide durillg successive treatments becomes inhibited. The first layer of the integraI surface oxide film formed will be relatively porous. This allows the subsecluently-applied coat of the acid paint to penetrate this porous first layer during the drying phase so that the acid attacks the underlying film-forming metal. Ions of the film-formirlg metal are thus provided by the base for conversion to oxide during the subsequent heating, this oxide being partly fonned within the pores of the first layer. The porosity of the resulting7 oxide film is thus reduced after each coating cycle until no more film forming metal -from the base can be converted to oxide. An extremely stable, relatively compact and impermeable film of film-forming metal oxide can thus be fom1ed by the application of a limited number oF coats of acid paint followed by drying and heating.
To prepare barrier layers according to the invention, each applied coat of paint includes such a small quantity of the iridium and/or rhodium compound that the electrocatalyst formed by thermodecomposition becomes fully incorporated in the integral surface film of film-forining metal oxide that is formed each time. Usually, each applied coat of the paint will contain at most about 0.2 9/m2 of iridium and/or rhodium per projected surface area of the base, usually far less. Additionally, application of further layers of the dilute paint is stopped after the number of coats beyond which growth oF the surface oxide film on the film-forming metal ceases or is inhibited. Thus, the optimum quantity ofelectrocatalytic agent in the dilute paint arld the optimulll number oF
coats to be applied to produce a satisfactory compact, impermeable barrier layer can be determined quite easily for any particular substrate, solvent/acid and electrocatalytic material. In many instances, two to ten layers of the very dilute paint will be applied, each followed by tlrying and heating from about ~00 to 600C for about 5 to 15 minutes~
with the possible exception of the final layer which may be heated for a longer period - possibly several hours or days at q50-600C in air or in a reducing atmosphere (e.9. amlnonia/hydrot3en).
l~hen viewcd by the naked eye or under a microscope, barrier layers produced in this manner on an etched or non-etched titanium base usually retain the same range of distinctive appearances as titanium oxide films prepared in the same manner ~Jhich do not contain -the iridiurn and/or rhodium electrocatalyst, typically a bright blue, yellow and/or red "interference" film colour.
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The dilute acidic paint solution used to prepare the barrier layer according to the invention preferably only includes a thermodecomposable iridium and/or rhodium compound, since the film-forming metal oxide component is provided by the base. However, the dilute paint may include S small amounts of other components such as other platinum-group metals (ruthen;um7 palladium, platinum, osmium, in particular ruthenium), gold, silver, tin, chromium, cobalt, antimony, molybdenum, iron, nickel, manganese, tungsten, vanadium, titanium, tantalum, zirconium, niobium, bismuth, lanthanum, tellurium, ph~sphorl~s , boron, beryllium, sodium, lithium, calcium, strontium, lead and copper compounds and mixtures thereof. Usually, if any small quantity of a film-forming metal compound is used it will be a different metal frc~ the .~ rming metal S~S
so as to contribute to doping of the surface film. Excellent results have been obtained with iridiuln/rutheniuln compounds in a weight ratio of about 2:1, as metal. ~hen such additives ar~ included in the dilute paint composition, they will of course be in an amount compatible with the small amount of the main electrocatalyst, i.e. an iridium and/or rhodillm compound, so that substantially all of the main electrocatalyst and additive is incorporatetl in the surface film of filln-fonming metal oxide. In any event, the total amollnt of iridium and/or rhodjum and other metals is below 1 g/rn2, and usually below 0.5 g/m2 and thg extra metal will be present in a lesser amount than the rhodium and/or iridium.
These iridium/rhodiuln compoun(ls and other metal compounds may be thenno-decomposable to form the metal or the oxide, but in neither casq is it necessary to proceed to fllll decomposition. For example, barrier layers containing partially decomposed iridium chloride containing up to about 3~
~ 8 ~
5% by ~eight of the original chlorine, have shown excellent properties.
Barrier layers containing as little as 0.1 to 0.3 ~/m2 (as metal) of iridium and/or rhodium o~ide/chloride in their surface films give excellent results. Tests have shown that a barrier layer containing 0.5 to 0.6 9/m2 (as metal) of iridium produces an optimum effect in terms of the increased lifetime of the coated electrodes. Increasing the quantity of ir;dium above these values does not further increase the lifetime.
When a titanium substrate is used, the s~lrface oxide film is found to be predominantly rutile titanium dioxide; presumably, the formation of rutile e.g. at about 400-500C is catalysed by the rhodiulll and/or iridiulll in the dilute coating solution.
After formdtion of the improved barrier layer which is imperllleable to electrolyte and to evolved oxygen, the porous outer electrocatalytic coating is applie(l using standard techniqlles, for example by applying over the pre~ormed barrier layer a p1ura1ity of coats of a re1ative1y concentrate(l solution containing a thennodeconlposab1e p1atinum-group nlet:al compollnd and heclting. Each appliecl outer coat wi11 contain at 1east 0.4 9/m2 of the platinllm-grollp metal per projected area of the subst~a-te, ancl the coating procedllre is repeated to build up an effective outer coating containing at least about 2 g/m2 of the platinum-group metal(s), usually in oxide form. The coating components may be chosen to provide a coating consisting predominarltly of a solid-solution of at least one film-forming metal oxide and at least one platinum-group metal oxide, as described in US Patent No. 3 632 498. Advantageously, the coating is a solid solution of ruthenium and titanium oxides having a ruthenium:titanium atomic ratio of from l:l to l:4. In this instance the coatin~ consists of several superinnposed layers typically having a micro-cracked appearance and is quite porous. Employing an improved barrier layer according to the invention with such a coating greatly improves the performance of the electrode in standard accelerated life-time tests in oxygen-evolution conditions. Predictably in the conclitions for normal commercial production of chlorine the improved electrode will have a substantially longer lifetime since it is known that one of the reasons for failure of these electrodes after extended use in chlorine pro(1uction is clue to the action of oxygen on tht? substrate. Also it will be possible to obtain the same lifetime with an appreciable reduction in the outer coating thicknt?ss enabling a saving in the quantity of coating material usec1 and in the lat)oùr and energy consumed for prodl1ction.
'15 'rhe outer coatin9 m(ly also be formetl of one or more platinum-group l))C tals for exa1))ple a platinu1n-iric1iu11) alloy useful for chlorate production a1)(1 lo a limitet1 extent in diaphrag1)) or 1))e11)l)rane cells for chlorine prot1l1ction. witil conventional Pt/lr coa1;ec1 electrodes the coatings must be rel(ltive'1y thick (at leclst at)o~1t 5 ~/m2) to avoid passivation problems.
With the in1provt!(1 bar1ier layer accor(1ing to the invention thinner ancl m()re porous '1ayers of the ~'1atinun)r11etals can be used without prob'lems arising dl1e to oxidation of the substrate or the drawbacks associa-ted with the previously known passive barrier layers of -titanium oxide.
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lo -It is also possible to apply the outer coating by plasma-spraying a solid solution o~ a film-forming metal oxide and a platinum-group metal oxide. For example a solid solution powder can be prepared by flame-spraying as described in US Patent No. 3 677 975 and this powder is then plasma-sprayed onto the base. Alternatively the coating is applied by plasma-spraying at least one film-forrning metal oxide over the preformed barrier layer and subseguently incorporating the platinum-group metal(s) and/or oxides -thereof in the plasma-sprayed film-forming metal oxide for example according to the procedure of US Patent No. 4 140 813.
Again the improved barrier layer increases lifetime and enables a reduction o-F the precious metal content of the coating.
In a preft?rre(I mt?thod of nnass-prodllcing the electrocles a set of t?lectro(Ice substralt?s are subjected together to a series of pre-treatnlents includinti etching an(I formatiorl of the barrier layer by diI)-coating the lS set of substrates in said dilute solution and heating the set of sub-strates and thereafter the outer electrocatalytic coating is applied to the s~lbstratt?s onc at a tirmt!. This proce(Illrt? obviates the drawback in comlllercitll electrutIt? coatin~J plants associateti with a bottleneck between the etctling bath anti the coatin9 line. In tht! llSl.lcll mdSS-pr()tIllction procetlllr(- a set of substrtltes is pretreated by sanclblasting follo\~ietl by etct~ing rinsing and drying and these substrates are therl individually coated at a coating/baking line. It has thus been necessary to synchronize the etching with the coating/baking because the etched substrates cannot be left for long periods (more than about t~o days) without detriment to -the electrode performance due to air oxidation of the substrate before coating, especially if dust or dirt becomes anchored in the thin oxide film. By pre-coating the sets of substrates with an improved barrier layer immediately after etching, this bottleneck effect is avoided and the surface-treated substrates can be stored without any risk of further oxidation. Any dl~st or dirt which may settle on the barrier layer can be easily blo~/n off prior to coating, since it cloes not get anchored in the film.
lurthermort?, the (lip-coating proce(lure of a set of` substrates piled a~lainst one another is satisfactory for the production of tht? improved barrier layer oxide film gro~n Up from tht! substrate. Similar halldling is not satisFactory for application of the conv(?lltional coa-tings where an a(itlecl thickness of eactl applie(l coating mllst; be built up over and on top of the film-Fornlirl(J nletal base and its very thin surface oxide film.
lt~t? t?leCtr`()(lt! t)aS(? may be a sht?t.~t of any film-forming metal, titanium being preft?rr(?d for cost reasons~ Ro(ls, tubt?s and expande(l meshes of titanillm or other film-forllling mettlls may likewise be surface -treated by -the method of the invention. Titanium or other film-form;ng rnetal clad on a condllcting core can also be used. For most applications, the base will be etched prior to the surface treatment to provide a rough surface giving good anchorage for the subsequently applied electrocatalytic coating. It is also possib'le to surface-treat porous sintered or plasma sprayed titanium with the dilute paint solutions in the same manner but preferably the porous titanium will be only a surface layer on a non-porous base.
The e'lectrodes with an improved barrier layer according to the S invention are excellently suited as anodes for chlor-a'lkali e'lectrolysis.
These electrodes have also shown outstanding performance when used for electrowinning in a mixed chloride-sulphate electrolyte giving mixed chlorine and oxygen evolution.
Best modes for carrying out the Invention This invention will be further illus-trated in the following examples.
Example I
Coupons medsuring 7.5 x 2 cm of titanium available under the trade name "Contimet 30" were degreased rinsed in water dried and etched for 1/2 hour in oxalic acid. A paint solution consisting of 6 ml n-propanol 0.~ ml HCl (concentrated) and 0.1 9 Or iridium and/or rhodium chloride was then applied by brush to both sides of -the coupons in four thin coats.
The coupons were dried to evaporate the solvent and then heated in air to 500C for ln minutes after each of the first three coats and for 30 mins.
after the final coat. This gives a content of about 0.2 to 0.3 g/rn2 of rhodium and/or iridium (calculated as metal) in the barrier layer depend-ing on the amount of solution in each applied coat as determined by weight measurement.
A titanium oxide-ruthenium oxide solid solution having a titaniurn:
ruthenium atomic ratio oF approximately 2:1 was then applied by brushing on a solution consisting of 6 ml n-propanol, 0.4 ml ilCl (concentrated), 3 ml butyl titanate and l 9 RuCl3 and heating in air at 400C for 5 mins.
(Note: this solution is 10 times more concentrated in terms of precious metal:propanol solvent than is the dilute solution used for producing the barrier layer). This procedure was repeated until the coating was present in thickness oF approximately lO g/m2 (i.e. approx. 4 g/m2 of Ru metal).
Electrodes so produced are being subjected to comparative electro-chemical tests'with similar electrodes (a~ having a TiO2 barrier layer produced by the same procedure but with a paint consisting solely of 6 ml n-propanol and 0.4 ml HCl (concen-trated) and (b) having no barrier layer rhe initial results indicate that the electrode according to the invention has a greatly superior lifetime in accelerated lifetime tests as anodes in oxygen evolving cond-itions and, in chlor-alkali electro-lysis, should have a lifetime many times longer than comparative anode (a) and considerab1y longer than comparative anocle (b).
Example Il ~ litaniun~ cout~on was degreased "~insed in water, dried, etched ant1 then surFace-treate~ as in Exa1llple I with a paint solution containing iridillm and rutheniu111 chlori(1es in the weight ratio of 2:1 (as metal).
The treatment was re1-eated four tin1es until the titanium dioxide film formed containec1 an amount of 0.2 g/m2 Ir anc1 O.l ~/m2 Ru, both calculated as metal. The heat treatme1-t was carrie(1 out at 400C For lO minutes after each applied coat. An outer coating of TiO2.Ru02 was then applied as in Example I. The same cornparative electrochemical tests have given the same initial promising results as for Exannple 1.
Example III
Titanium coupons were degreased, rinsed in water, dried and etched as in Example I and treated with an iridium chloride solution similar to that of Example I. The solution was applied in four thin coats and the coupons were dried to evaporate the solvent and then heated to 4gOC for 7 minutes at the end of each coat. The iridium concentration was varied to give a content of 0.3, 0.6 and 0.~ y/m2 of iridium (calculated as metal) in the barrier layer.
A titanium dioxide - ruthenium dioxide solid solutiorl coating was then applied as in Exanlple I, except that the coating thickness corresponded to 20 9/m2 (approx. ~3 g/m2 of Ru metal). These electrodes were subjected to accelerated lifetime tests in oxygen evolvintJ con(litions. The maximum lifetime~ was observed with the coupon having a barrier layer containing 0.6 9/m2 Ir. This represented an increase by a factor of 10.3 oF the 'life-tim~ oF a similar electrode without a barrier layer (or with a barrier layer of TiO2 containing no iridium). In comparison, a similar coated electrode with no barrier laye~r but with the a(ldition of 0.6 9 of iridium dispersed in the coatintJ shows only a mart~inal increase of lifetime.
Example l~
. _ , .. .. .
Electrodes were prepared in a sinlilar manner to Example I, but using a dilute paint containing chlorides of various platinum-group meta'ls, including palladium, platinum and ruthenium alone, as well as rhodium and iridium as previously described, for production of the barrier layer. These electrodes w~re subjected to comparative lifetirne tests as oxygen-evolution anodes. Only the electrodes having a barrier layer containing Rh and/or Ir showed a marked increase in lifetime in this test; combinations of Rh and/or Ir with smaller quantities of the other plati ~m-group metals or their compounds, in particular Ru and Pd also produced substantial improvements.
Example V
Titanium coupons were provided with barrier layers containing approx.
0.2 9/m2 of iridium and/or rhodium following the procedure of Example I.
They were then painted with a solution containing 0.5 9 of iridium chloride and 1 y of platinum chloride in 10 ml of isopropyl alcohol and 10 ml of linalool, and heated in an oven to 350C. An ammonia/hydrogen mixture was then passed for approximately 30 seconds to produce a coating containing 70%
Pt and 30% Ir. The coating procedure was repeated to build up a coating containing 4 g/m2 of the Pt/Ir alloy. ~or similar electrodes coated with less than 7 g/m2 of the Pt/lr alloy but without the impl^oved barrier layer, it has been rep(~rted that operation at elevated current density produces passivation and at least 7 y/m2 must be applied to obtain satisfactory operation over extended periods. This problelll is apparently overcomt! by the electrode accordin~] to the invention which opc~rates satisfactorily with a coating o~ 2 Example VI
.
Titaniuln coupons were provided with barrier layers containing approx.
0.2 g/m2 of iridium and/or rhodium fol10winy thc procedure of Example I.
A layer of approximately 400 9/",2 of titaniunl oxide was then plasma-sprayed onto the barrier layer, using standard techniques. The plasma-sprayed titanium oxide layer was then coated with coatings containing 2 y/m2 (as metal) of ruthenium oxide and/or iridium oxide in various ratios, by v~
painting with a solution of 6 ml propanol and 1 9 of RuC13 and/or Ir~13 and heating in air to 500C for 10 minutes after each coating. Preliminary electrochemical testing indicates that these electrodes should have an extremely long lifetime as anodes in mercury chlor-alkali cells operating at high current densities. From the data published in US Patent 4 140 813 it seems that the electrode of this invention will achieve the same excellent lifetime with as little as l/5th of the precious metal loading.
Example VII
Titanium coupons were provided with barrier layers containin3 approx.
0.3 9/m2 of iridium rhodium and iridium/ruthenilIm in a 2:1 weight ratio following the procedure of Example I (except that in some instances the final heatin(J was prolonged for severa'l hours).
An a~ueous solu-tion containing iridiulll chloride and tantalum chloride (with Ir an(l Ta metals in an equal weight ratio) was applied by brush over lS both sides of -the coupons in 5 10 and 15 coats. Each app'lied coat contained about 0.5 9/m2 of iridium. After each coat the coupons were dried and heated in air for 10 minutes at ~50C and for 1 hour after the final coat.
The resulting coating was a solid solution of iridiunI and tantalum oxides containing approx. 2.5 5 and 7.5 ~/m2 of iridiunI. The electrodes were teste(l as arlodes in 10~ sulfuric acid at 60C at a current density oF 1.2 kA/m2 the current bein~ stopped f'or 15 min~ltes in each 24-hour period without the electrodes being removed from the acid bath. The initial results indicate a superior performance over sinIilar electrodes on a plain titaniurlI substrate and on a substrate of a titanium-palladium alloy containing 0.15% palladium. The titanium substrate with a barrier layer according to the invention is of course far less expensive than this titanium-palladium alloy and provides a greatly improved resistance to cell shutdown and to the passivating action of oxygen evolution. From the preliminary indications, the electrodes according to the invention with a low iridium loading (2.5 9/m2 + 0.3 9/m2 in the barrier layer) should have an outstanding lifetime compared to similar electrodes without the barrier layer.
Our Reference: 2978C
COATED METAL ELECTRODE WITH IMPROVED BARRIER LAYER
Technical Field The invention relates to electrodes for use in electrolytic processes, of the type having a substrate of a film-forming metal such as titanium, tantalum, zirconium, niobium, tungsten, aluminium and alloys containing one or more of these metals as well as silicon-iron alloys, coated with an electrocatalytic coating containing one or more platinum-group metals or their oxides possibly mixed with other oxides.
By "film-Forming metal" is meant one which has the property that when connected as an anode in -the electrolyte in which the coated anode is subseguently to operate, there rapidly forms a passivating oxide film which protects the underlying metal from corrosion by the electrolyte.
These metals are also frequently referred to as "valve metals".
The invention is more particularly concerned with dimensionally-stable electrodes provided with an improved barrier or intermediate layer between the film-forming metal substrate and the electrocatalytic outer coating.
Background Art In early proposals (see for example U.K. Patent Specifications ~55 107 and 869 865), a titanium electrode with a coating of platinum group metal was provided with an inert barrier layer of titanium oxide in the porous places of the coating, this barrier layer preferably being formed or reinforced by a heat treatment. Later, in U.K. Patent Specification 92~ 080, the inert barrier layer of titanium oxide was preformed by electrolytically treating or heating the titanium substrate in an oxidizing atmosphere prior to application of the platinum group metal.
The preforming of such a barrier layer was also advocated in U.K. Patent SpeciFication 1 147 422 with a view to improving the anchorage of an active coating consisting of or containing platinum group metal oxides.
Later, the development of coatinys formed of mixed crystals or solid solutions of co-deposited oxides of film-Forming metals and platinum group metals (see U.S. Patent No. 3 632 49~) provided commercially viable electrodes which revolutionized the chlor-alkali industry and have become widely used in other applications. With these electrodes, excellent performance was achieved without the need for a reinforced or preformed inert barrier or anchorage layer on the substrate and today it is generally accepted that the preFormed or reinforced inert barrier layers are detrimental to performance. In retrospect, the early proposals for pre-formed or reinforced inert barrier layers appear to have been unsuccessful attempts to avoid defects which were inherent in the previous coatings rather than in the substrate.
_ 3 Nevertheless, some proposals attempting to improve inert barrier layers have still been made, for example by applying a titanium oxide barrier layer from a solution containing ~i4 ions. Again, this has been found to impair performance of the electrodes.
Another approach has been to provide a non-passivating barrier layer or intermediate layer underlying the active outer coating. Typical suggestions have been doped tin dioxide sub-layers; thin sub-layers of one or more platinum metals such as a platinum-iridium alloy; sub-layers of cobalt oxide or lead oxide, and so forth. Although various patents have claimed marginal improvements for these electrodes in specific applic-ations, in practice none of these suggestions has led to any significant improvement or any widespreatl commercial use~
Disclosure of the Invention The invention concerns an electrode with a film-forming metal sub-strate having a porous outer electrocatalytic coating containing at least about 2 g/m (as platinum group metal per projected surface area of the substrate) of at least one platinum ~roup metal and/or oxide thereof possibly mixetl with other metal oxides, and an improved non-passivating barrier layer between the substrate and coating.
According to the invention, this barrier layer is a preformed surface oxide film grown up from the ,ilm-forming base and having rhodium and/or iridium incorporated in the surface oxide film during formation thereof in~an amount of up to 1 9/m2 (as metal) per projected surface area of the substrate.
The surface oxide film of the barrier layer is rendered non-passivating by the incorporation of the rhodium and/or iridium as metal or as a compound, usually the oxide or a partially oxidized compound.
Another aspect of the invention is a method of manufacturing such an electrode in which the formation of the barrier layer involves the application of a very dilute acidic paint, i.e. one which contains a small quantity of a thermodecomposable iridium and/or rhodium compound that during decomposition and simultaneous formation of the surface film of film-forming metal oxide will be flIlly absorbed by this surface film, this dilute paint containing generally about 1-15 9/1 of iridium and/or rhodium (as metal).
The paint used will typically include an organic solvent sucIl as isopropyl alcohol, an acid (notably IICl, ~IBr or III) or another agent (e.~. NaF) which attacks tht! film-forIlling metal and encourages the formation of film-forming metal oxide durin~ the subseguent heat treatment, and one or more thermo-deconIposable salts of iri~Iium and/or rhodium. Usually this solution will be at least five times more dilute and preferably about 10 or more times cIilute (in terms of its precious metal content) than the paint solution whicilIllay be used for the production of the outer porous electrocatalytic oxide coatings; this means that the quantity of iridium, and/or rhodium, will be reduced, e.g. to 1/5 or 1/10 or even l/lOOth the amouIlt of the corresponding platinum-group meta1 in -the paint used for producing the outer coating for approximately the same quantity of solvent and acid.
~3~
The action of the acid or other agent which attacks or corrodes the film-forming metal and promotes the formation of the oxide film during the subsequent heat -treatment is very important; without a suitable agent producing this effect, fo~-mation of the surface oxide film oF the ~ilm-forming metal would be substantially hindered or inhibited.
It has been observed that by applying one coat of a given solvent/
acid mixture to a film-forming metal base subjected previously to the usual cleaning and etching treatments and then heat;ng after drying to drive ofF
the solvent, a given quantity of film-forming metal oxide will be produced.
This procedure can be repeated a number of times (usually four or five times for 4 ml HCl in 60 ml isopropyl alcohol applied to a titaniunl base, dried and heated to 500C for ten minlltes) before the growth of film-forming metal oxide durillg successive treatments becomes inhibited. The first layer of the integraI surface oxide film formed will be relatively porous. This allows the subsecluently-applied coat of the acid paint to penetrate this porous first layer during the drying phase so that the acid attacks the underlying film-forming metal. Ions of the film-formirlg metal are thus provided by the base for conversion to oxide during the subsequent heating, this oxide being partly fonned within the pores of the first layer. The porosity of the resulting7 oxide film is thus reduced after each coating cycle until no more film forming metal -from the base can be converted to oxide. An extremely stable, relatively compact and impermeable film of film-forming metal oxide can thus be fom1ed by the application of a limited number oF coats of acid paint followed by drying and heating.
To prepare barrier layers according to the invention, each applied coat of paint includes such a small quantity of the iridium and/or rhodium compound that the electrocatalyst formed by thermodecomposition becomes fully incorporated in the integral surface film of film-forining metal oxide that is formed each time. Usually, each applied coat of the paint will contain at most about 0.2 9/m2 of iridium and/or rhodium per projected surface area of the base, usually far less. Additionally, application of further layers of the dilute paint is stopped after the number of coats beyond which growth oF the surface oxide film on the film-forming metal ceases or is inhibited. Thus, the optimum quantity ofelectrocatalytic agent in the dilute paint arld the optimulll number oF
coats to be applied to produce a satisfactory compact, impermeable barrier layer can be determined quite easily for any particular substrate, solvent/acid and electrocatalytic material. In many instances, two to ten layers of the very dilute paint will be applied, each followed by tlrying and heating from about ~00 to 600C for about 5 to 15 minutes~
with the possible exception of the final layer which may be heated for a longer period - possibly several hours or days at q50-600C in air or in a reducing atmosphere (e.9. amlnonia/hydrot3en).
l~hen viewcd by the naked eye or under a microscope, barrier layers produced in this manner on an etched or non-etched titanium base usually retain the same range of distinctive appearances as titanium oxide films prepared in the same manner ~Jhich do not contain -the iridiurn and/or rhodium electrocatalyst, typically a bright blue, yellow and/or red "interference" film colour.
3V~
The dilute acidic paint solution used to prepare the barrier layer according to the invention preferably only includes a thermodecomposable iridium and/or rhodium compound, since the film-forming metal oxide component is provided by the base. However, the dilute paint may include S small amounts of other components such as other platinum-group metals (ruthen;um7 palladium, platinum, osmium, in particular ruthenium), gold, silver, tin, chromium, cobalt, antimony, molybdenum, iron, nickel, manganese, tungsten, vanadium, titanium, tantalum, zirconium, niobium, bismuth, lanthanum, tellurium, ph~sphorl~s , boron, beryllium, sodium, lithium, calcium, strontium, lead and copper compounds and mixtures thereof. Usually, if any small quantity of a film-forming metal compound is used it will be a different metal frc~ the .~ rming metal S~S
so as to contribute to doping of the surface film. Excellent results have been obtained with iridiuln/rutheniuln compounds in a weight ratio of about 2:1, as metal. ~hen such additives ar~ included in the dilute paint composition, they will of course be in an amount compatible with the small amount of the main electrocatalyst, i.e. an iridium and/or rhodillm compound, so that substantially all of the main electrocatalyst and additive is incorporatetl in the surface film of filln-fonming metal oxide. In any event, the total amollnt of iridium and/or rhodjum and other metals is below 1 g/rn2, and usually below 0.5 g/m2 and thg extra metal will be present in a lesser amount than the rhodium and/or iridium.
These iridium/rhodiuln compoun(ls and other metal compounds may be thenno-decomposable to form the metal or the oxide, but in neither casq is it necessary to proceed to fllll decomposition. For example, barrier layers containing partially decomposed iridium chloride containing up to about 3~
~ 8 ~
5% by ~eight of the original chlorine, have shown excellent properties.
Barrier layers containing as little as 0.1 to 0.3 ~/m2 (as metal) of iridium and/or rhodium o~ide/chloride in their surface films give excellent results. Tests have shown that a barrier layer containing 0.5 to 0.6 9/m2 (as metal) of iridium produces an optimum effect in terms of the increased lifetime of the coated electrodes. Increasing the quantity of ir;dium above these values does not further increase the lifetime.
When a titanium substrate is used, the s~lrface oxide film is found to be predominantly rutile titanium dioxide; presumably, the formation of rutile e.g. at about 400-500C is catalysed by the rhodiulll and/or iridiulll in the dilute coating solution.
After formdtion of the improved barrier layer which is imperllleable to electrolyte and to evolved oxygen, the porous outer electrocatalytic coating is applie(l using standard techniqlles, for example by applying over the pre~ormed barrier layer a p1ura1ity of coats of a re1ative1y concentrate(l solution containing a thennodeconlposab1e p1atinum-group nlet:al compollnd and heclting. Each appliecl outer coat wi11 contain at 1east 0.4 9/m2 of the platinllm-grollp metal per projected area of the subst~a-te, ancl the coating procedllre is repeated to build up an effective outer coating containing at least about 2 g/m2 of the platinum-group metal(s), usually in oxide form. The coating components may be chosen to provide a coating consisting predominarltly of a solid-solution of at least one film-forming metal oxide and at least one platinum-group metal oxide, as described in US Patent No. 3 632 498. Advantageously, the coating is a solid solution of ruthenium and titanium oxides having a ruthenium:titanium atomic ratio of from l:l to l:4. In this instance the coatin~ consists of several superinnposed layers typically having a micro-cracked appearance and is quite porous. Employing an improved barrier layer according to the invention with such a coating greatly improves the performance of the electrode in standard accelerated life-time tests in oxygen-evolution conditions. Predictably in the conclitions for normal commercial production of chlorine the improved electrode will have a substantially longer lifetime since it is known that one of the reasons for failure of these electrodes after extended use in chlorine pro(1uction is clue to the action of oxygen on tht? substrate. Also it will be possible to obtain the same lifetime with an appreciable reduction in the outer coating thicknt?ss enabling a saving in the quantity of coating material usec1 and in the lat)oùr and energy consumed for prodl1ction.
'15 'rhe outer coatin9 m(ly also be formetl of one or more platinum-group l))C tals for exa1))ple a platinu1n-iric1iu11) alloy useful for chlorate production a1)(1 lo a limitet1 extent in diaphrag1)) or 1))e11)l)rane cells for chlorine prot1l1ction. witil conventional Pt/lr coa1;ec1 electrodes the coatings must be rel(ltive'1y thick (at leclst at)o~1t 5 ~/m2) to avoid passivation problems.
With the in1provt!(1 bar1ier layer accor(1ing to the invention thinner ancl m()re porous '1ayers of the ~'1atinun)r11etals can be used without prob'lems arising dl1e to oxidation of the substrate or the drawbacks associa-ted with the previously known passive barrier layers of -titanium oxide.
o~
lo -It is also possible to apply the outer coating by plasma-spraying a solid solution o~ a film-forming metal oxide and a platinum-group metal oxide. For example a solid solution powder can be prepared by flame-spraying as described in US Patent No. 3 677 975 and this powder is then plasma-sprayed onto the base. Alternatively the coating is applied by plasma-spraying at least one film-forrning metal oxide over the preformed barrier layer and subseguently incorporating the platinum-group metal(s) and/or oxides -thereof in the plasma-sprayed film-forming metal oxide for example according to the procedure of US Patent No. 4 140 813.
Again the improved barrier layer increases lifetime and enables a reduction o-F the precious metal content of the coating.
In a preft?rre(I mt?thod of nnass-prodllcing the electrocles a set of t?lectro(Ice substralt?s are subjected together to a series of pre-treatnlents includinti etching an(I formatiorl of the barrier layer by diI)-coating the lS set of substrates in said dilute solution and heating the set of sub-strates and thereafter the outer electrocatalytic coating is applied to the s~lbstratt?s onc at a tirmt!. This proce(Illrt? obviates the drawback in comlllercitll electrutIt? coatin~J plants associateti with a bottleneck between the etctling bath anti the coatin9 line. In tht! llSl.lcll mdSS-pr()tIllction procetlllr(- a set of substrtltes is pretreated by sanclblasting follo\~ietl by etct~ing rinsing and drying and these substrates are therl individually coated at a coating/baking line. It has thus been necessary to synchronize the etching with the coating/baking because the etched substrates cannot be left for long periods (more than about t~o days) without detriment to -the electrode performance due to air oxidation of the substrate before coating, especially if dust or dirt becomes anchored in the thin oxide film. By pre-coating the sets of substrates with an improved barrier layer immediately after etching, this bottleneck effect is avoided and the surface-treated substrates can be stored without any risk of further oxidation. Any dl~st or dirt which may settle on the barrier layer can be easily blo~/n off prior to coating, since it cloes not get anchored in the film.
lurthermort?, the (lip-coating proce(lure of a set of` substrates piled a~lainst one another is satisfactory for the production of tht? improved barrier layer oxide film gro~n Up from tht! substrate. Similar halldling is not satisFactory for application of the conv(?lltional coa-tings where an a(itlecl thickness of eactl applie(l coating mllst; be built up over and on top of the film-Fornlirl(J nletal base and its very thin surface oxide film.
lt~t? t?leCtr`()(lt! t)aS(? may be a sht?t.~t of any film-forming metal, titanium being preft?rr(?d for cost reasons~ Ro(ls, tubt?s and expande(l meshes of titanillm or other film-forllling mettlls may likewise be surface -treated by -the method of the invention. Titanium or other film-form;ng rnetal clad on a condllcting core can also be used. For most applications, the base will be etched prior to the surface treatment to provide a rough surface giving good anchorage for the subsequently applied electrocatalytic coating. It is also possib'le to surface-treat porous sintered or plasma sprayed titanium with the dilute paint solutions in the same manner but preferably the porous titanium will be only a surface layer on a non-porous base.
The e'lectrodes with an improved barrier layer according to the S invention are excellently suited as anodes for chlor-a'lkali e'lectrolysis.
These electrodes have also shown outstanding performance when used for electrowinning in a mixed chloride-sulphate electrolyte giving mixed chlorine and oxygen evolution.
Best modes for carrying out the Invention This invention will be further illus-trated in the following examples.
Example I
Coupons medsuring 7.5 x 2 cm of titanium available under the trade name "Contimet 30" were degreased rinsed in water dried and etched for 1/2 hour in oxalic acid. A paint solution consisting of 6 ml n-propanol 0.~ ml HCl (concentrated) and 0.1 9 Or iridium and/or rhodium chloride was then applied by brush to both sides of -the coupons in four thin coats.
The coupons were dried to evaporate the solvent and then heated in air to 500C for ln minutes after each of the first three coats and for 30 mins.
after the final coat. This gives a content of about 0.2 to 0.3 g/rn2 of rhodium and/or iridium (calculated as metal) in the barrier layer depend-ing on the amount of solution in each applied coat as determined by weight measurement.
A titanium oxide-ruthenium oxide solid solution having a titaniurn:
ruthenium atomic ratio oF approximately 2:1 was then applied by brushing on a solution consisting of 6 ml n-propanol, 0.4 ml ilCl (concentrated), 3 ml butyl titanate and l 9 RuCl3 and heating in air at 400C for 5 mins.
(Note: this solution is 10 times more concentrated in terms of precious metal:propanol solvent than is the dilute solution used for producing the barrier layer). This procedure was repeated until the coating was present in thickness oF approximately lO g/m2 (i.e. approx. 4 g/m2 of Ru metal).
Electrodes so produced are being subjected to comparative electro-chemical tests'with similar electrodes (a~ having a TiO2 barrier layer produced by the same procedure but with a paint consisting solely of 6 ml n-propanol and 0.4 ml HCl (concen-trated) and (b) having no barrier layer rhe initial results indicate that the electrode according to the invention has a greatly superior lifetime in accelerated lifetime tests as anodes in oxygen evolving cond-itions and, in chlor-alkali electro-lysis, should have a lifetime many times longer than comparative anode (a) and considerab1y longer than comparative anocle (b).
Example Il ~ litaniun~ cout~on was degreased "~insed in water, dried, etched ant1 then surFace-treate~ as in Exa1llple I with a paint solution containing iridillm and rutheniu111 chlori(1es in the weight ratio of 2:1 (as metal).
The treatment was re1-eated four tin1es until the titanium dioxide film formed containec1 an amount of 0.2 g/m2 Ir anc1 O.l ~/m2 Ru, both calculated as metal. The heat treatme1-t was carrie(1 out at 400C For lO minutes after each applied coat. An outer coating of TiO2.Ru02 was then applied as in Example I. The same cornparative electrochemical tests have given the same initial promising results as for Exannple 1.
Example III
Titanium coupons were degreased, rinsed in water, dried and etched as in Example I and treated with an iridium chloride solution similar to that of Example I. The solution was applied in four thin coats and the coupons were dried to evaporate the solvent and then heated to 4gOC for 7 minutes at the end of each coat. The iridium concentration was varied to give a content of 0.3, 0.6 and 0.~ y/m2 of iridium (calculated as metal) in the barrier layer.
A titanium dioxide - ruthenium dioxide solid solutiorl coating was then applied as in Exanlple I, except that the coating thickness corresponded to 20 9/m2 (approx. ~3 g/m2 of Ru metal). These electrodes were subjected to accelerated lifetime tests in oxygen evolvintJ con(litions. The maximum lifetime~ was observed with the coupon having a barrier layer containing 0.6 9/m2 Ir. This represented an increase by a factor of 10.3 oF the 'life-tim~ oF a similar electrode without a barrier layer (or with a barrier layer of TiO2 containing no iridium). In comparison, a similar coated electrode with no barrier laye~r but with the a(ldition of 0.6 9 of iridium dispersed in the coatintJ shows only a mart~inal increase of lifetime.
Example l~
. _ , .. .. .
Electrodes were prepared in a sinlilar manner to Example I, but using a dilute paint containing chlorides of various platinum-group meta'ls, including palladium, platinum and ruthenium alone, as well as rhodium and iridium as previously described, for production of the barrier layer. These electrodes w~re subjected to comparative lifetirne tests as oxygen-evolution anodes. Only the electrodes having a barrier layer containing Rh and/or Ir showed a marked increase in lifetime in this test; combinations of Rh and/or Ir with smaller quantities of the other plati ~m-group metals or their compounds, in particular Ru and Pd also produced substantial improvements.
Example V
Titanium coupons were provided with barrier layers containing approx.
0.2 9/m2 of iridium and/or rhodium following the procedure of Example I.
They were then painted with a solution containing 0.5 9 of iridium chloride and 1 y of platinum chloride in 10 ml of isopropyl alcohol and 10 ml of linalool, and heated in an oven to 350C. An ammonia/hydrogen mixture was then passed for approximately 30 seconds to produce a coating containing 70%
Pt and 30% Ir. The coating procedure was repeated to build up a coating containing 4 g/m2 of the Pt/Ir alloy. ~or similar electrodes coated with less than 7 g/m2 of the Pt/lr alloy but without the impl^oved barrier layer, it has been rep(~rted that operation at elevated current density produces passivation and at least 7 y/m2 must be applied to obtain satisfactory operation over extended periods. This problelll is apparently overcomt! by the electrode accordin~] to the invention which opc~rates satisfactorily with a coating o~ 2 Example VI
.
Titaniuln coupons were provided with barrier layers containing approx.
0.2 g/m2 of iridium and/or rhodium fol10winy thc procedure of Example I.
A layer of approximately 400 9/",2 of titaniunl oxide was then plasma-sprayed onto the barrier layer, using standard techniques. The plasma-sprayed titanium oxide layer was then coated with coatings containing 2 y/m2 (as metal) of ruthenium oxide and/or iridium oxide in various ratios, by v~
painting with a solution of 6 ml propanol and 1 9 of RuC13 and/or Ir~13 and heating in air to 500C for 10 minutes after each coating. Preliminary electrochemical testing indicates that these electrodes should have an extremely long lifetime as anodes in mercury chlor-alkali cells operating at high current densities. From the data published in US Patent 4 140 813 it seems that the electrode of this invention will achieve the same excellent lifetime with as little as l/5th of the precious metal loading.
Example VII
Titanium coupons were provided with barrier layers containin3 approx.
0.3 9/m2 of iridium rhodium and iridium/ruthenilIm in a 2:1 weight ratio following the procedure of Example I (except that in some instances the final heatin(J was prolonged for severa'l hours).
An a~ueous solu-tion containing iridiulll chloride and tantalum chloride (with Ir an(l Ta metals in an equal weight ratio) was applied by brush over lS both sides of -the coupons in 5 10 and 15 coats. Each app'lied coat contained about 0.5 9/m2 of iridium. After each coat the coupons were dried and heated in air for 10 minutes at ~50C and for 1 hour after the final coat.
The resulting coating was a solid solution of iridiunI and tantalum oxides containing approx. 2.5 5 and 7.5 ~/m2 of iridiunI. The electrodes were teste(l as arlodes in 10~ sulfuric acid at 60C at a current density oF 1.2 kA/m2 the current bein~ stopped f'or 15 min~ltes in each 24-hour period without the electrodes being removed from the acid bath. The initial results indicate a superior performance over sinIilar electrodes on a plain titaniurlI substrate and on a substrate of a titanium-palladium alloy containing 0.15% palladium. The titanium substrate with a barrier layer according to the invention is of course far less expensive than this titanium-palladium alloy and provides a greatly improved resistance to cell shutdown and to the passivating action of oxygen evolution. From the preliminary indications, the electrodes according to the invention with a low iridium loading (2.5 9/m2 + 0.3 9/m2 in the barrier layer) should have an outstanding lifetime compared to similar electrodes without the barrier layer.
Claims (22)
1. An electrode for use in electrolytic processes comprising a substrate of film-forming metal having a porous electrocatalytic coating comprising at least one platinum-group metal as metal, oxide or metal-oxide mixtures in an amount of at least about 2g/m2 of the platinum-group metal(s) per projected surface area of the substrate, and the substrate having below the coating a preformed barrier layer constituted by a surface oxide film grown up from the substrate, characterized in that the preformed barrier layer has at least one of rhodium, iridium or rhodium-iridium mixture therein as metal, oxide or metal-oxide mixture in an amount of up to 1 g/m2 (as metal) per projected surface area of the substrate.
2. The electrode of claim 1, wherein the porous electrocatalytic coating consists of a plurality of superimposed layers of micro-cracked configuration.
3. The electrode of claim 2, wherein the porous electrocatalytic coating consists predominantly of a solid-solution of at least one film-forming metal oxide and at least one platinum-group metal oxide.
4. The electrode of claim 3, wherein the porous electrocatalytic coating is a solid solution of ruthenium and titanium oxides having a ruthenium:
titanium atomic ratio of from 1:1 to 1:4.
titanium atomic ratio of from 1:1 to 1:4.
5. The electrode of claim 1, wherein the porous electrocatalytic coating consists predominantly of one or more platinum-group metals.
6. The electrode of claim 5, wherein the porous electrocatalytic coating is a platinum-iridium alloy.
7. The electrode of claim 1, wherein the porous electrocatalytic coating is a plasma-sprayed layer of at least one film-forming metal oxide incorpor-ating the platinum-group metal(s) and/or oxides thereof.
8. The electrode of claim 1, wherein the surface oxide film of the barrier layer contains at least one extra added metal in addition to rhodium and/or iridium but in a lesser amount than the rhodium and/or iridium the total metal content of the barrier layer being up to 1 g/m2.
9. The electrode o-F claim 8 wherein said film contains up to 0.5 g/m2 of iridium and ruthenium in a weight ratio of about 2:1.
10. The electrode of claim 1, wherein the substrate is of titaniuln and the surface oxide film is predominantly rutile titanium dioxide.
11. A method of manufacturing an electrode for use in electrolytic processes, comprising forming a barrier layer on a substrate of film-forming metal and applying over the barrier layer a porous outer electro-catalvtic coating comprising at least one platinum-group metal as metal, oxide or metal-oxide mixture, in an amount of at least about 2 g/m2 of the platinum-group metal(s) per projected surface area of the substrate, characterized in that the barrier layer is formed by applying to the substrate one or more coats of a very dilute (as herein-after defined) acid solution containing a thermodecomposable compound selected from rhodium, iridium or rhodium-iridium mixture, drying and heating the or each coat to form on the sub-strate a surface film of film-forming metal oxide and simultaneously at least partially decompose said compounds, the or each applied coat of said very dilute solution containing a quantity of said compound that is substantially fully absorbed in the surface film formed during heating, and the number of applied coats being such that the barrier layer so formed contains rhodium, iridium or rhodium-iridium as metal, oxide or metal-oxide mixture in an amount of up to 1 g/m2 (as metal) per projected surface area of the substrate.
12. The method of claim 11, wherein the or each applied coat of the solution contains up to 0.2 g/m2 of metal selected from rhodium, iridium or rhodium-iridium mixture per projected surface area of the substrate.
13. A method of manufacturing an electrode for use in electrolytic processes comprising forming a barrier layer on a substrate of film-forming metal and applying over the barrier layer a porous outer electro-catalytic coating comprising at least one platinum group metal as metal, oxide or metal-oxide mixture, in an amount of at least about 2 g/m2 of the platinum group metal(s) per projected surface area of the sub-strate, characterized in that the barrier layer is formed by applying to the substrate several coats each containing up to 0.2 g/m2 (as metal per projected surface area of the substrate) of a thermodecomposable compound selected from rhodium, iridium or rhodium-iridium mixture in a solution which attacks the film-forming metal substrate, and heating after drying each coat to produce a barrier layer of oxide of the film-forming metal containing rhodium, iridium or rhodium-iridium as metal, oxide or metal-oxide mixture in an amount of up to 1.0 g/m2 (as metal) per projected surface area of the substrate.
14. The method of any one of claims 11 to 13, wherein from 2 to 5 coats of the dilute solution are applied each followed by heating to between about 300° and 600°C for about 5 to 15 minutes, the final coat possibly being heated for a longer period.
15. The method of any one of claims 11? 12 or 13 wherein the heating is carried out to incompletely decompose said compound.
16. The method of claim 13, wherein the porous outer electrocatalytic coating is formed by applying over the pre-formed barrier layer a plurality of coats of a relatively concentrated solution containing a thermodecomposable platinum group metal compound and heating.
17. The method of claim 16, wherein each applied outer coat contains at least 0.4 g/m2 of platinum group metal per projected area of the substrate base.
18. The method of any one of claims 11, 12 or 13 wherein the porous outer electrocatalytic coating is applied by plasma-spraying.
19. The method of any one of claims 11, 12 or 13 wherein the porous outer electrocatalytic coating is applied by plasma spraying at least one film-forming metal oxide over the pre-formed barrier layer and subse-quently incorporating the platinum group metal(s) as metal, oxide or metal oxide mixture in the plasma-sprayed film-forming metal oxide.
20. The method of any one of claims 11, 12 or 13 wherein a set of electrode substrates are subjected together to a series of pre-treatments including etching and formation of the barrier layer by dip-coating the set of substrates in said dilute solution and heating the set of substrates, and thereafter the outer electrocatalytic coating is applied to the substrates one at a time.
21. An electrode as in claim 1, 2 or 3, wherein the electrocatalytic coating comprising a metal and/or oxide thereof also includes other metal oxides.
22. A method as in claim 11, 12 or 13, wherein the electrocatalytic coating comprising a metal and/or oxide thereof also includes other metal oxides.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000361721A CA1190184A (en) | 1980-10-07 | 1980-10-07 | Coated film-forming metal electrode with surface oxide barrier layer incorporating rhodium and/or iridium |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000361721A CA1190184A (en) | 1980-10-07 | 1980-10-07 | Coated film-forming metal electrode with surface oxide barrier layer incorporating rhodium and/or iridium |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1190184A true CA1190184A (en) | 1985-07-09 |
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ID=4118082
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000361721A Expired CA1190184A (en) | 1980-10-07 | 1980-10-07 | Coated film-forming metal electrode with surface oxide barrier layer incorporating rhodium and/or iridium |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2447395A3 (en) * | 2010-10-28 | 2013-01-30 | Bayer MaterialScience AG | Electrode for producing chlorine through electrolysis |
-
1980
- 1980-10-07 CA CA000361721A patent/CA1190184A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2447395A3 (en) * | 2010-10-28 | 2013-01-30 | Bayer MaterialScience AG | Electrode for producing chlorine through electrolysis |
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