HK1207892B - Electrode for oxygen evolution in industrial electrochemical processes - Google Patents

Description

Electrode for oxygen evolution in industrial electrochemical processes

Technical Field

The present invention relates to an electrode for electrolytic processes, in particular an anode suitable for oxygen evolution in industrial electrolytic processes and a method for its manufacture.

Background

The present invention relates to an electrode for electrolytic processes, in particular an anode suitable for oxygen evolution in industrial electrolytic processes. Anodes for oxygen evolution are widely used in various electrolytic applications, several of which fall within the field of cathodic metal electrodeposition (electrometallurgy) and cover a wide range of applied current densities, which can be very small (for example, several hundred a/m)²For example in metal electrowinning processes) or very high (for example in the case of some galvanic electrodeposition applications, it may be higher than 10kA/m in terms of the surface of the anode²Work); another field of application of anodes for oxygen evolution is cathodic protection with impressed current. In the field of electrometallurgy, in particular electrowinning of metals, lead-based anodes have traditionally been commonly used and are still valuable for some applications, but present a rather high oxygen evolution overpotential and cause well-known environmental and human health problems associated with the use of such materials. Recently, particularly for high current density applications (which benefit most from the energy savings associated with the more reduced oxygen evolution potential), electrodes for anodic oxygen evolution obtained from valve metal substrates, such as titanium and its alloys, coated with catalytic compositions based on noble metals or their oxides, have been introduced into the market.

It should also be considered that the working life of anodes based on valve metal substrates coated with metal or metal oxides is greatly reduced in the presence of particularly aggressive pollutants capable of establishing accelerated phenomena of corrosion or contamination of the anode surface. Anodes comprising a substrate coated with a catalytic composition and provided with an outer coating of valve metal oxide for improved durability are also known in fact. In the latter case, however, if the valve metal oxide outer layer is present too thick, the potential is increased to an unacceptable value.

Thus, a need has been identified for providing anodes for oxygen evolution characterized by a sufficient oxygen overpotential and duration to overcome the disadvantages of prior art electrodes under process conditions including the presence of additives, such as in decorative chromium plating with trivalent chromium.

Disclosure of Invention

Various aspects of the invention are set out in the appended claims.

In one aspect, the invention relates to an electrode suitable for oxygen evolution in electrolytic processes, comprising a valve metal substrate, a catalytic layer, a protective layer consisting of valve metal oxide between the substrate and the catalytic layer, and an outer layer of valve metal oxide, the catalytic layer comprising iridium oxide, tin oxide and an oxide of at least one doping element M selected from bismuth and tantalum, the molar ratio Ir (Ir + Sn) being in the range from 0.25 to 0.55, and the molar ratio M (Ir + Sn + M) being in the range from 0.02 to 0.15.

In one embodiment, the catalytic layer of the electrode according to the invention has a molar ratio M (Ir + Sn + M) in the range from 0.05 to 0.12.

In another embodiment, the molar concentration of iridium in the catalytic layer is between 40 and 50% with respect to the sum of iridium and tin; the inventors have found that in this composition range, elemental doping is particularly effective for allowing the formation of reduced-scale and highly catalytically active crystallites (e.g. having a size below 5 nm). The inventors have also observed that when the catalytic layer has the composition and crystallite size described, the deposition of an additional outer layer of valve metal with barrier function leads to a more regular and uniform overall morphology, thus greatly reducing the potential increase caused by the addition of such an outer layer on top of the catalytic layer.

In one embodiment, the protective layer interposed between the catalytic layer and the valve metal substrate comprises a valve metal oxide capable of forming a thin film impermeable to the electrolyte, for example selected from titanium oxide, tantalum oxide or a mixture of both. This has the advantage of further protecting the underlying substrate made of titanium or other valve metal from aggressive electrolytes, such as in those processes typical for metal plating.

In one embodiment, the electrode is obtained on a titanium substrate (optionally alloyed); titanium is characterized by reduced cost in combination with good corrosion resistance compared to other valve metals. Moreover, titanium has excellent processability, allowing it to be used to obtain substrates of various geometries, for example in the form of flat sheets, perforated sheets, expanded metal sheets or meshes, according to the requirements of different applications.

In another embodiment, the electrode has a specific loading of valve metal oxide in the outer layer of from 2 to 25g/m²Within the range of (1). The inventors have unexpectedly found that such a barrier layer pair as described hereinabove, applied by thermal decomposition on top of a catalytic layer, is useful for anodic oxygen evolution (in particular from 2 to 7 g/m), compared to what is observable when adding the same to catalytic layers of the prior art²In range) produces a beneficial increase in the duration of the electrode, and a lesser increase in the potential.

In another embodiment, the electrode of the invention has a specific loading of valve metal oxide in the outer layer of from 9 to 25g/m²Within the range of (1). The inventors have unexpectedly observed that even with these increased amounts of valve metal oxide in the outer layer, the anode potential is still higher than that typically added to the catalytic layers of the prior artIt is good, and it is also observed that this layer acts as an effective barrier against diffusion of compounds and ions present in the electrolyte to the catalytic layer. These combined features, i.e. lower anodic potential and significant reduction of diffusion, are for example very important for decorative chromium plating, since at 1000A/m²The potential reduction of even just 50mV combined with less diffusion of cr (iii) ions reduces the kinetics of the additional anodic reaction of oxidation of cr (iii) to cr (vi), which can seriously impair the quality of the cathodic deposit of chromium metal. In the prior art, cr (vi) production due to additional reactions is usually compensated by supplying additives, which require periodic cleaning of the baths and subsequent rejuvenation of them with fresh solution.

In one embodiment, the electrode of the invention is provided with a valve metal oxide outer layer made of one component selected from the group consisting of titanium oxide and tantalum oxide.

In another aspect, the invention relates to a method for manufacturing an electrode suitable for use as an oxygen evolution anode in an electrolytic process, comprising applying to a valve metal substrate one or more coatings of a solution comprising iridium, tin and precursors of said at least one doping element M, and subsequently decomposing said solution by heat treatment in air at a temperature of 480 to 530 ℃, forming said catalytic coating, and forming said outer layer by applying a solution comprising a precursor of titanium or tantalum followed by thermal decomposition.

Prior to the catalytic coating application step, the substrate may be provided with a valve metal oxide protective layer, which protective layer is applied by the following procedure: such as flame or plasma spraying, prolonged heat treatment in an air atmosphere, thermal decomposition of solutions containing compounds of valve metals such as titanium or tantalum, and the like.

In another aspect, the invention relates to a method for the cathodic electrodeposition of metals from aqueous solutions, wherein the anodic half-reaction is an oxygen evolution reaction carried out on the surface of an electrode as described above.

In another aspect, the invention relates to a method for cathodic electrodeposition of chromium from an aqueous solution comprising cr (iii).

The following examples are included to demonstrate particular embodiments of the invention, the usefulness of which has been greatly verified within the claimed numerical ranges. It should be understood 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.

Example 1

A primary titanium sheet of size 200mm × 200mm × 3mm was degreased in an ultrasonic bath for 10 minutes with acetone and grit blasted first with emery grit until the surface roughness Rz value was 40 to 45 μm, then annealed at 570 ℃ for 2 hours, then at a temperature of 85 ℃ at 27 wt% H₂SO₄Middle etch for 105 minutes, verify the resulting weight loss at 180 and 250g/m²In the meantime.

After drying, a protective layer based on titanium oxide and tantalum oxide in a molar ratio of 80:20 was applied to the sheet with a total loading of 0.6g/m metal²(corresponding to 0.87g/m in terms of oxide)²). The protective layer is applied by applying three coats of a precursor solution by TaCl acidified with HCl and then thermally decomposing at 515 deg.c₅Addition of the aqueous solution to TiCl₄In aqueous solution.

A 1.65M solution of Sn hydroxyacetyl chloride complex (hereinafter SnHAC) was prepared according to the procedure disclosed in WO 2005/014885.

A0.9M Ir hydroxyacetyl chloride complex (hereinafter IrHAC) was prepared by the following method: IrCl dissolved in 10 vol.% aqueous acetic acid₃Evaporating the solvent, adding 10% aqueous acetic acidAnd then the solvent was evaporated twice more and finally the product was dissolved again in 10% aqueous acetic acid to give the indicated concentration.

By cold dissolution of 7.54g of BiCl in a beaker containing 60ml of 10% by weight HCl with stirring₃A precursor solution containing 50g/l bismuth was prepared. Upon completion of dissolution, once a clear solution was obtained, the volume was adjusted to 100ml with 10 wt% HCl.

10.15ml of a 1.65M SnHAC solution, 10ml of a 0.9M IrHAC solution and 7.44ml of a 50g/l Bi solution are added to a second beaker, which is kept under stirring. The stirring was prolonged for more than 5 minutes, and then 10ml of 10 wt% acetic acid was added.

Part of the solution was applied to the previously treated titanium sheet by brushing 7 coatings, each followed by a drying step at 60 ℃ for 15 minutes and subsequent decomposition at high temperature for 15 minutes. The pyrolysis step is carried out at 480 ℃ after the first coating, at 500 ℃ after the second coating and at 520 ℃ after the subsequent coating.

In this way, a mixture having a molar ratio lr: Sn: Bi of 33:61:6 and about 10g/m is applied²Ir ratio of (a) to the supported catalytic layer.

Then, TaCl acidified with HCl by brushing₅Application of the outer layer was carried out with 8 coats of aqueous solution (12 g/m in terms of oxide)²Amount of). Three areas of 1cm were cut from the electrode thus obtained²And subjecting it to an accelerated duration test under anodic oxygen evolution: by measuring H at 150g/l₂SO₄Medium, at a temperature of 60 ℃ and at 30kA/m²Deactivation time at current density (defined as the working time required for observing a 1V increase in potential). The average deactivation time for the three samples was found to be 600 hours.

At 1000A/m²The anode potential was found to be 1.556V/NHE.

Example 2

As in the previous examples, a primary titanium sheet having a size of 200mm × 200mm × 3mm was pretreated and provided with a protective layer based on titanium oxide and tantalum oxide in a molar ratio of 80:20 by adding 10g of TaCl₅The whole was boiled under stirring for 15 minutes in a beaker containing 60ml of 37% by weight HCl to prepare a precursor solution containing 50g/l of tantalum. Then 50ml of deionised H were added₂O and keeping the solution heated for about 2 hours until the volume returns to 50 ± 3 ml. Then 60ml of 37 wt% HCl was added to give a clear solution, which was boiled again until the volume returned to 50. + -.3 ml. Then using deionized H₂O adjusted the volume to 100 ml. 10.15ml of the 1.65M SnHAC solution of the previous example, 10ml of the 0.9MIrHAC solution of the previous example and 7.44ml of the 50g/l Ta solution were added to a second beaker, which was kept under stirring. Stirring was extended for 5 minutes. Then 10ml of 10 wt.% acetic acid was added. Part of the solution was applied to the previously treated titanium sheet by brushing 8 coatings, each followed by a drying step at 60 ℃ for 15 minutes and then decomposition at high temperature for 15 minutes. The pyrolysis step is carried out at 480 ℃ after the first coating, at 500 ℃ after the second coating and at 520 ℃ after the subsequent coating.

In this way, a mixture having a molar ratio lr Sn to Ta of 32.5:60:7.5 and about 10g/m was applied²Ir ratio of (a) to the supported catalytic layer.

Then, TaCl acidified with HCl by brushing₅Application of the outer layer was carried out with 10 coatings of aqueous solution (15 g/m in terms of oxide)²Amount of). Three areas of 1cm were cut from the electrode thus obtained²And subjecting it to an accelerated duration test under anodic oxygen evolution: by measuring H at 150g/l₂SO₄Medium, at a temperature of 60 ℃ and at 30kA/m²Deactivation time at current density (defined as the working time required to observe a 1V increase in potential). The average deactivation time for the three samples was found to be 520 hours.

At 1000A/m²The anode potential was found to be 1.579V/NHE.

Comparative example 1

Primary titanium sheets of dimensions 200mm x 3mm were degreased and first grit blasted with silicon carbide grit until the surface roughness Rz value was 70 to 100 μm, and then etched in 20 wt% HCl at a temperature of 90-100 ℃ for 20 minutes.

After drying, a protective layer based on titanium oxide and tantalum oxide in a molar ratio of 80:20 was applied to the sheet with a total loading of 0.6g/m metal²(corresponding to 0.87g/m in terms of oxide)²). The application of the protective layer is carried out by applying three coats of a precursor solution by TaCl to be acidified with HCl and then thermally decomposing at 500 deg.c₅Addition of the aqueous solution to TiCl₄In aqueous solution.

A catalytic coating based on oxides of iridium and tantalum in a weight ratio of 65:35 (corresponding to a molar ratio of about 66.3:36.7) was then applied to the protective layer, the total iridium loading being 10g/m². The electrode was heat treated at 515 ℃ for 2 hours and then acidified with HCl by brushing TaCl₅Application of the outer layer was carried out with 10 coatings of aqueous solution (15 g/m in terms of oxide)²Amount of). Three areas of 1cm were cut from the electrode thus obtained²And subjecting it to an accelerated duration test under anodic oxygen evolution: by measuring H at 150g/l₂SO₄Medium, at a temperature of 60 ℃ and at 30kA/m²Deactivation time at current density (defined as the working time required to observe a 1V increase in potential). The average deactivation time for the three samples was found to be 525 hours.

At 1000A/m²The anode potential was found to be 1.601V/NHE.

Comparative example 2

First-grade titanium sheets of dimensions 200mm x 3mm were degreased and first grit-blasted with silicon carbide grit until the surface roughness Rz-value was 70 to 100 μm, and then etched in 20 wt% HCl at a temperature of 90-100 ℃ for 20 minutes.

After drying, a protective layer based on titanium oxide and tantalum oxide in a molar ratio of 80:20 was applied to the sheet with a total loading of 0.6g/m metal²(corresponding to 0.87g/m in terms of oxide)²). The application of the protective layer is carried out by applying three coats of a precursor solution by TaCl to be acidified with HCl and then thermally decomposing at 500 deg.c₅Addition of the aqueous solution to TiCl₄In aqueous solution.

A catalytic coating consisting of two different layers is then applied on the protective layer: based on a first (inner) layer of iridium and tantalum oxide in a weight ratio of 65:35 (corresponding to a molar ratio of about 66.3:36.7), the total iridium loading was 2g/m²And a second (inner) layer of oxides of iridium, tantalum and titanium in a weight ratio of 78:20:2 (corresponding to a molar ratio of about 80.1:19.4:0.5) with a total iridium loading of 10g/m²。

Then by brushing TaCl acidified with HCl₅Application of the outer layer was carried out with 10 coats of aqueous solution (15 g/m in terms of oxide)²Amount of). Three areas of 1cm were cut from the electrode thus obtained²And subjecting it to an accelerated duration test under anodic oxygen evolution: by measuring H at 150g/l₂SO₄Medium, at a temperature of 60 ℃ and at 30kA/m²Deactivation time at current density (defined as the working time required to observe a 1V increase in potential). The average deactivation time for the three samples was found to be 580 hours.

At 1000A/m²The anode potential was found to be 1.602V/NHE.

The above description should not be taken as limiting the invention, which may be used according to different embodiments without departing from the scope thereof, and whose extent is solely defined by the appended claims.

Throughout the description and claims of this application, the term "comprise" and variations thereof such as "comprises" and "comprising" are not intended to exclude the presence of other elements, components or additional process steps.

The discussion of documents, acts, materials, devices, articles and the like in this specification is included 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.

Claims (10)

1. An electrode suitable for oxygen evolution in electrolytic processes, comprising a valve metal substrate, a catalytic layer, a protective layer consisting of valve metal oxide between said substrate and said catalytic layer, and an outer layer of valve metal oxide, said catalytic layer comprising mixed oxides of iridium, tin and at least one doping element M selected from bismuth and tantalum, the molar ratio Ir (Ir + Sn) being in the range from 0.25 to 0.55, and the molar ratio M (Ir + Sn + M) being in the range from 0.02 to 0.15.

2. The electrode according to claim 1, wherein the molar ratio M (Ir + Sn + M) is in the range from 0.05 to 0.12.

3. The electrode according to claim 1 or 2, wherein the molar ratio Ir (Ir + Sn) is in the range from 0.40 to 0.50.

4. The electrode according to claim 1 or 2, wherein the oxides of iridium, tin and at least one doping element M in the catalytic layer consist of crystallites having an average size of less than 5 nm.

5. The electrode according to claim 1 or 2, wherein the valve metal oxide outer layer is made of one component selected from titanium oxide and tantalum oxide.

6. The electrode of claim 1 or 2, wherein the specific loading of the valve metal oxide in the outer layer is from 2 to 25g/m²Within the range of (1).

7. The electrode of claim 1 or 2, wherein the specific loading of the valve metal oxide in the outer layer is from 9 to 25g/m²Within the range of (1).

8. Method for manufacturing an electrode according to any one of claims 1 to 7, comprising applying a solution comprising precursors of iridium, tin and the at least one doping element M to a valve metal substrate provided with a protective layer consisting of a valve metal oxide, and subsequently decomposing the solution by heat treatment in air at a temperature of 480 to 530 ℃, the outer layer being formed by applying a solution comprising precursors of tantalum or titanium and subsequently thermally decomposing.

9. A process for the cathodic electrodeposition of a metal from an aqueous solution comprising anodically evolving oxygen on the surface of an electrode according to any one of claims 1 to 7.

10. The method of claim 9, wherein the cathodic electrodeposition is electrodeposition of chromium from an aqueous solution comprising cr (iii).