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(11) | EP 2 655 693 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
ELECTRODE FOR ELECTROLYTIC CELL ELEKTRODE FÜR ELEKTROLYSEZELLEN ÉLECTRODE POUR CELLULE ÉLECTROLYTIQUE |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
FIELD OF THE INVENTION
[0001] The invention relates to an electrode suitable for operating as anode in electrolysis cells, for instance as chlorine-evolving anode in chlor-alkali cells.
BACKGROUND OF THE INVENTION
[0002] The use of metal electrodes equipped with catalytic coatings in electrolytic applications is known in the art: electrodes consisting of a metal substrate equipped with a coating based on noble metals or oxides thereof are for instance utilised as cathodes for hydrogen evolution in water or alkali chloride electrolysis processes, as anodes for oxygen evolution in electrometallurgical processes of various kinds or for chlorine evolution in alkali chloride electrolysis. Electrodes of such kind can be produced via thermal route, i.e. by suitable thermal decomposition of solutions containing the precursors of metals to be deposited; by galvanic electrodeposition from suitable electrolytic baths; by direct metallisation via flame or plasma spraying processes or chemical or physical phase vapour deposition.
[0003] The electrolysis of sodium chloride brine directed to the production of chlorine and caustic soda, for instance, is often carried out with anodes consisting of a titanium or other valve metal substrate activated with a superficial layer or ruthenium dioxide (RuO2) in order to lower the overvoltage of the anodic chorine evolution reaction. For this type of electrolysis, catalyst formulations based on mixtures of oxides of ruthenium, iridium and titanium are also known, all capable of lowering the overvoltage of the anodic chorine evolution reaction.
[0005] Catalytic formulations can be deposited on the substrate by phase vapour deposition techniques, having the advantage of allowing an extremely accurate control of coating deposition parameters. However, these are fundamentally characterised by being batch-type processes, requiring the loading of the substrate in a suitable deposition chamber, which has to undergo a slow depressurisation process, lasting several hours, in order to be able to treat a single piece. Besides the remarkable duration of the process (several hours being usually necessary, depending on the required noble metal loading), the application of high amounts of catalytic coatings leads to coatings having a very limited lifetime.
[0006] From WO 95/05498 A1, an electrode for evolution of gaseous products in electrochemical cells is known, comprising an outer catalytic composition deposited by a plasma spray technique, wherein the amount of noble metal in said outer catalytic composition is comprised between 5 and 100 g/m2.
[0007] From US2005/0211553, an electrode for evolution of gaseous products in electrochemical cells is known, comprising an intermediate activating layer and an outer protection layer deposited by thermal decomposition, in which the weight ratio of valve metals to noble metals is higher in the outer layer than in the intermediate activating layer.
SUMMARY OF THE INVENTION
[0009] Under a first aspect, the present invention relates to an electrode for evolution of gaseous products in electrolysis cells consisting of a valve metal substrate coated with at least one first catalytic composition and with an outer catalytic composition, said at least one first catalytic composition comprising a mixture of oxides of a valve metal or of tin and of noble metals selected from the group of platinum metals (PM) or oxides thereof taken alone or in admixture, said at least one first catalytic composition obtained by thermal decomposition of precursors, said outer catalytic composition comprising noble metals selected from the group of platinum metals or oxides thereof taken alone or in admixture, said outer catalytic composition being deposited by means of a chemical or physical phase vapour deposition technique, the amount of noble metal on said first catalytic composition being higher than 5 g/m2 of surface and the amount of noble metal in said outer catalytic composition ranging between 0.1 and 3.0 g/m2 of surface.
[0010] The inventors have surprisingly found out that the deposition of one last catalytic layer, with the specified characteristics, through chemical or physical phase vapour allows obtaining an electrode with unexpected features both in terms of duration and of potential decrease.
[0011] In one embodiment, the first catalytic composition of the electrode according to the invention comprises titanium, iridium, ruthenium in form of metals or oxides.
[0012] In one embodiment, the outer catalytic composition comprises ruthenium and/or iridium in form of metals or oxides.
[0013] In one embodiment, the specific noble metal loading in the first catalytic composition ranges between 6 and 8 g/m2 and the specific metal loading in the outer catalytic composition ranges between 1.5 and 2.5 g/m2.
[0014] Under another aspect, the invention relates to a method of manufacturing an electrode comprising the deposition of an outer catalytic composition by chemical or physical phase vapour deposition, preferably by reactive sputtering of noble metals selected in the group of platinum metals.
[0015] A used electrode can be re-activated by chemical or physical phase vapour deposition of an outer catalytic composition including noble metals selected from the group of platinum metals or oxides thereof taken alone or in admixture.
[0016] Under a further aspect, the invention relates to an electrolysis cell of alkali chloride solutions, for instance a sodium chloride brine electrolysis cell directed to producing chlorine and caustic soda, which effects the anodic evolution of chlorine on an electrode as hereinbefore described.
[0017] The following examples are included to demonstrate particular embodiments of the invention, whose practicability has been largely verified in the claimed range of values. It should be appreciated by those of skill in the art that the compositions and techniques disclosed in the examples which follow represent compositions and techniques discovered by the inventors to function well in the practice of the invention; however, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the scope of the invention.
COUNTEREXAMPLE 1
[0018] A sample of titanium mesh of 10 cm x 10 cm size was blasted with corundum, cleaning the residues with a jet of compressed air. The sample was then degreased using acetone in a ultrasonic bath for about 10 minutes. After drying, the sample was dipped into an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO3 at about 100°C for 1 hour. After the alkaline treatment, the sample was rinsed in deionised water at 60°C for three times, changing the liquid every time. The last rinse was carried out adding a small quantity of HCl (about 1 ml per litre of solution). An air drying was effected, observing the formation of a brown hue due to the growth of a thin film of TiOx. 100 ml of a hydroalcoholic solution containing RuCl3*3H2O, H2IrCl6*6H2O, TiCl3 in a mixture of water and 2-propanol acidified with HCl were then prepared, having a molar composition of 36% Ru, 20% Ir, 44% Ti referred to the metals.
[0019] The solution was applied to the sample of titanium mesh by brushing in five coats; after each coat, a drying at 100-110°C for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450°C. The sample was cooled in air each time prior to applying the subsequent coat.
[0020] At the end of the whole procedure, a total noble metal loading of 9 g/m2, expressed as the sum of Ru and Ir referred to the metals, was obtained.
COUNTEREXAMPLE 2
[0022] A sample of titanium mesh of 10 cm x 10 cm size was blasted with corundum, cleaning the residues with a jet of compressed air. The sample was then degreased using acetone in a ultrasonic bath for about 10 minutes. After drying, the sample was dipped into an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO3 at about 100°C for 1 hour. After the alkaline treatment, the sample was rinsed in deionised water at 60°C for three times, changing the liquid every time. The last rinse was carried out adding a small quantity of HCl (about 1 ml per litre of solution). An air drying was effected, observing the formation of a brown hue due to the growth of a thin film of TiOx.
[0023] The mesh sample was then introduced into the vacuum chamber of the reactive sputtering equipment.
[0024] Upon establishing a dynamic vacuum of about 50 E-4 mbar feeding an oxygen mixture with 20% argon, the sputtering targets were polarised at the following powers: ruthenium 35 W, iridium 24 W, titanium 250 W. The target-electrode substrate gap was about 10 centimetres.
[0025] The process of deposition was carried out, at the same conditions, alternatively on the two sides of the titanium mesh for an overall duration of 220 minutes. The thus obtained electrode presented a catalytic coating of about 1 micron and a total noble metal loading of about 9 g/m2, expressed as the sum of Ru and Ir referred to the metals.
EXAMPLE 1
[0027] A sample of titanium mesh of 10 cm x 10 cm size was blasted with corundum, cleaning the residues with a jet of compressed air. The sample was then degreased using acetone in a ultrasonic bath for about 10 minutes. After drying, the sample was dipped into an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO3 at about 100°C for 1 hour. After the alkaline treatment, the sample was rinsed in deionised water at 60°C for three times, changing the liquid every time. The last rinse was carried out adding a small quantity of HCl (about 1 ml per litre of solution). An air drying was effected, observing the formation of a brown hue due to the growth of a thin film of TiOx.
[0028] 100 ml of a hydroalcoholic solution containing RuCl3*3H2O, H2IrCl6*6H2O, TiCl3 in a mixture of water and 2-propanol acidified with HCl were then prepared, having a molar composition of 36% Ru, 20% Ir, 44% Ti referred to the metals.
[0029] The solution was applied to the sample of titanium mesh by brushing in five coats; after each coat, a drying at 100-110°C for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450°C. The sample was cooled in air each time prior to applying the subsequent coat.
[0030] At the end of the whole procedure, a total noble metal loading of 7 g/m2, expressed as the sum of Ru and Ir referred to the metals, was obtained.
The semi-finished electrode was then introduced into the vacuum chamber of the reactive sputtering equipment.
[0031] Upon establishing a dynamic vacuum of about 100 E-4 mbar feeding an oxygen mixture with 20% argon, the sputtering targets were polarised at the following powers: ruthenium 30 W, iridium 35 W. The target-electrode substrate gap was about 10 centimetres. To confer optimal properties to the resulting coating, the substrate was also subjected to a residual polarisation of about 150 V.
[0032] The process of deposition was carried out, at the same conditions, alternatively on the two sides of the electrode for an overall duration of 40 minutes. The thus obtained electrode had an outer catalytic coating about 0.1 µm thick and a total noble metal loading of about 9 g/m2, expressed as the sum of Ru and Ir referred to the metals.
[0034] The samples of the previous examples were characterised as anodes for chlorine evolution in a lab cell fed with a sodium chloride brine at a concentration of 200 g/l, strictly controlling the pH at 3. Table 1 reports chlorine overvoltage measured at a current density of 4 kA/m2 and the volume percentage of oxygen in product chlorine.
TABLE 1
| Sample No. | ηCl2 (mV) |
| 1 | 73 |
| 2 | 63 |
| 3 | 44 |
[0035] The samples of the previous examples were also subjected to a duration test. Said duration test is the simulation in a separated cell of industrial electrolysis conditions as regards electrolyte concentration and temperature, but at a current density conveniently increased up to a value 2-3 times higher than the nominal one for the sake of accelerating the experimental response. Table 2 reports the precious metal lost per unit current.
TABLE 2
| Sample No. | Loss of precious metal per unit current (mgPM/kAh) |
| 1 | 0.016 |
| 2 | 0.5 |
| 3 | 0.005 |
[0036] The previous description is not intended to limit the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is univocally defined by the appended claims.
[0037] Throughout the description and claims of the present application, the term "comprise" and variations thereof such as "comprising" and "comprises" are not intended to exclude the presence of other elements or additives.
[0038] The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.
1. Electrode for evolution of gaseous products in electrochemical cells consisting of
a valve metal substrate coated with at least one first catalytic composition and an
outer catalytic composition, said at least one first catalytic composition comprising
a mixture of a valve metal or tin or an oxide thereof and noble metals chosen in the
platinum group metals or oxides thereof taken alone or in admixture, said at least
one first catalytic composition obtained by thermal decomposition of precursors, said
outer catalytic composition deposited by a chemical or physical vapor deposition technique,
the amount of noble metal in said at least one first catalytic composition being above
5 g/m2 and the amount of noble metal in said outer catalytic composition ranging between
0.1 and 3.0 g/m2.
2. The electrode according to claim 1, wherein said mixture of said at least one first
catalytic composition comprises titanium, iridium and ruthenium.
3. The electrode according to claim 1, wherein said outer catalytic composition comprises
ruthenium and/or iridium.
4. The electrode according to any one of claims 1 to 3, wherein the specific noble metal
loading of said at least one first catalytic composition is 6 to 8 g/m2 and the specific noble metal loading of said outer catalytic composition is 1.5 to
2.5 g/m2.
5. Method for manufacturing an electrode according to any one of claims 1 to 4 comprising
the deposition of said outer catalytic composition by a chemical or physical vapor
deposition technique.
6. Method for manufacturing an electrode according to claim 5 comprising the deposition
of said outer catalytic composition as a mixture of oxides by reactive sputtering
of noble metals selected from the group of platinum metals.
7. Electrolysis cell comprising a cathodic compartment containing a cathode and an anodic
compartment containing an anode separated by a membrane or diaphragm, said anodic
compartment being fed with an alkali chloride brine, wherein said anode of said anodic
compartment is an electrode according to any one of claims 1 to 4.
1. Elektrode für die Entwicklung gasförmiger Produkte in elektrochemischen Zellen, bestehend
aus einem Ventilmetallsubstrat, beschichtet mit zumindest einer ersten katalytischen
Zusammensetzung und einer äußeren katalytischen Zusammensetzung, die zumindest eine
erste katalytische Zusammensetzung umfassend eine Mischung eines Ventilmetalls oder
von Zinn oder eines Oxids davon und Edelmetallen, ausgewählt aus der Gruppe der Platinmetalle
oder Oxiden davon, verwendet allein oder als Beimischung, wobei die zumindest eine
erste katalytische Zusammensetzung durch thermische Zersetzung von Präkursoren erhalten
wird, wobei die äußere katalytische Zusammensetzung durch ein Verfahren der chemischen
oder physikalischen Dampfphasenabscheidung aufgebracht wird, wobei die Menge des Edelmetalls
in der zumindest einen ersten katalytischen Zusammensetzung mehr als 5 g/m2 beträgt und die Menge des Edelmetalls in der äußeren katalytischen Zusammensetzung
von 0,1 bis 3,0 g/m2 reicht.
2. Elektrode gemäß Anspruch 1, wobei die Mischung der zumindest einen ersten katalytischen
Zusammensetzung Titan, Iridium und Ruthenium umfasst.
3. Elektrode gemäß Anspruch 1, wobei die äußere katalytische Zusammensetzung Ruthenium
und/oder Iridium umfasst.
4. Elektrode gemäß einem der Ansprüche 1 bis 3, wobei die spezifische Edelmetallbeladung
der zumindest einen ersten katalytischen Zusammensetzung 6 bis 8 g/m2 beträgt und die spezifische Edelmetallbeladung der äußeren katalytischen Zusammensetzung
1,5 bis 2,5 g/m2 beträgt.
5. Verfahren zum Herstellen einer Elektrode gemäß einem der Ansprüche 1 bis 4, umfassend
die Abscheidung der äußeren katalytischen Zusammensetzung durch ein Verfahren der
chemischen oder physikalischen Dampfphasenabscheidung.
6. Verfahren zum Herstellen einer Elektrode gemäß Anspruch 5, umfassend die Abscheidung
der äußeren katalytischen Zusammensetzung als Mischung von Oxiden durch reaktives
Zerstäuben von Edelmetallen, ausgewählt aus der Gruppe der Platinmetalle.
7. Elektrolysezelle mit einer kathodischen Kammer, die eine Kathode enthält, und einer
anodischen Kammer, die eine an Mode enthält, getrennt durch eine Membran oder ein
Diaphragma, wobei der anodischen Kammer eine alkalische Chloridsole zugeführt wird,
wobei die Anode der anodischen Kammer eine Elektrode gemäß einem der Ansprüche 1 bis
4 ist.
1. Électrode de dégagement de produits gazeux dans des cellules électrochimiques, composée
d'un substrat métallique pour valve enrobé d'au moins une première composition catalytique
et d'une composition catalytique externe, ladite première composition catalytique,
au moins au nombre de une, comprenant un mélange de métal pour valve ou d'étain ou
d'un oxyde de celui-ci et de métaux nobles sélectionnés parmi les métaux du groupe
du platine et leurs oxydes, pris seuls ou en mélange, ladite première composition
catalytique, au moins au nombre de une, étant obtenue par décomposition thermique
de précurseurs, ladite composition catalytique externe étant déposée par une technique
de dépôt chimique ou dépôt physique en phase vapeur, la quantité de métal noble dans
ladite première composition catalytique, au moins au nombre de une, étant supérieure
à 5 g/m2, et la quantité de métal noble dans ladite composition catalytique externe allant
de 0,1 à 3,0 g/m2.
2. Électrode selon la revendication 1, dans laquelle ledit mélange de ladite première
composition catalytique, au moins au nombre de une, comprend du titane, de l'iridium
et du ruthérium.
3. Électrode selon la revendication 1, dans laquelle ladite composition catalytique externe
comprend du ruthénium et/ou de l'iridium.
4. Électrode selon l'une quelconque des revendications 1 à 3, dans laquelle la charge
spécifique en métal noble de ladite première composition catalytique, au moins au
nombre de une, est de 6 à 8 g/m2, et la charge spécifique en métal noble de ladite composition catalytique externe
est de 1,5 à 2,5 g/m2.
5. Procédé de fabrication d'une électrode selon l'une quelconque des revendications 1
à 4, comprenant le dépôt de ladite composition catalytique externe par une technique
de dépôt chimique ou physique en phase vapeur.
6. Procédé de fabrication d'une électrode selon la revendication 5, comprenant le dépôt
de ladite composition catalytique externe sous la forme d'un mélange d'oxydes par
pulvérisation réactive de métaux nobles sélectionnés dans le groupe des métaux du
platine.
7. Cellule d'électrolyse comprenant un compartiment cathodique qui contient une cathode
et un compartiment anodique qui contient une anode séparée par une membrane ou un
diaphragme, ledit compartiment anodique étant alimenté par une saumure de chlorure
alcalin, ladite anode dudit compartiment anodique étant une électrode selon l'une
quelconque des revendications 1 à 4.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description