HK1245358B - Electrode for electrolytic processes - Google Patents

Description

Electrodes for electrolysis processes

Technical Field

The present invention relates to an electrode for electrochemical applications, in particular to an electrode for oxygen evolution in a metal electrowinning process.

Background Art

The present invention relates to an electrode for electrolytic processes, and in particular to an anode suitable for oxygen evolution in industrial electrolytic processes. Anodes for oxygen evolution are widely used in various electrolytic applications, many of which are related to the field of cathodic electrodeposition of metals (electrometallurgy). They operate over a wide range of applied current densities, from very low (hundreds of A/ ^m² , such as in metal electrowinning processes) to very high (for example, in some electrodeposition and electroplating applications, which can operate in excess of 10 kA/ ^m² relative to the anode surface). Another field of application for anodes for oxygen evolution is cathodic protection by impressed current. In the field of electrometallurgy, particularly with respect to metal electrowinning, lead-based anodes have traditionally been used, which remain effective for certain applications, although they present a relatively high oxygen evolution overpotential and also carry well-known environmental and human health risks. More recently, electrodes for anodic oxygen evolution obtained from substrates of valve metals (e.g., titanium and its alloys) coated with catalyst compositions based on metals or their oxides have been introduced on the market, particularly for high current density applications, which benefit largely from the energy savings associated with the reduced oxygen evolution potential. A typical composition suitable for catalyzing the anodic oxygen evolution reaction consists of, for example, a mixture of oxides of iridium and tantalum, wherein the iridium is the catalytically active species and the tantalum helps form a tight coating that protects the valve metal substrate from corrosion, particularly for operation in corrosive electrolytes. Another very effective formulation for catalyzing the anodic oxygen evolution reaction consists of a mixture of oxides of iridium and tin with small amounts of doping elements such as bismuth, antimony, tantalum, or niobium (which can be used to make the tin oxide phase more conductive).

Electrodes having the above-described composition are able to meet the needs of many industrial applications at low currents and high current densities, with a sufficiently reduced operating voltage and a reasonable duration. However, the economics of certain manufacturing processes, particularly in the metallurgical field (e.g., electrowinning of copper or tin), require even higher electrode durations than those of the above-described compositions. To achieve this, protective interlayers based on valve metal oxides, such as mixtures of tantalum and titanium oxides, are known to further protect the valve metal substrate from corrosion. However, the interlayers thus formulated are characterized by a rather low electrical conductivity and can only be used with a very reduced thickness, not exceeding 0.5 μm, so as to contain the resulting increase in operating voltage within acceptable limits. In other words, a compromise must be found between a suitable operating life supported by a higher thickness and an overpotential supported by a lower thickness.

Another problem observed with the above-mentioned catalyst formulations is the tendency of the iridium-containing catalytic coating to leach significant amounts of iridium into the electrolyte during the startup phase and the first few hours of operation. This appears to indicate that a portion of the iridium oxide of the coating, while electrochemically active, is present in a phase that is not easily corroded by the electrolyte. This phenomenon, which also occurs to a certain extent with other noble metal catalysts such as ruthenium, can be mitigated by covering the catalytic coating with a porous protective layer, for example, based on tantalum or tin oxide. However, such an external protective layer has limited effectiveness and leads to an increase in the operating voltage of the electrode.

It therefore proves necessary to provide an anode for oxygen evolution which is characterized by an increased operating duration and a reduced release of noble metals during the first hours of operation, while at the same time having a very high catalytic activity for the oxygen evolution reaction.

Summary of the Invention

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

In one aspect, the present invention relates to an electrode suitable for oxygen evolution in an electrolytic process, comprising a valve metal substrate (e.g., titanium or a titanium alloy) provided with a coating comprising at least one protective layer composed of a mixture of oxides, expressed by weight of the metals, comprising 89-97% tin, a total of 2-10% of one or more doping elements selected from bismuth, antimony, and tantalum, and 1-9% ruthenium. Experiments conducted by the inventors have shown that bismuth provides the best results compared to other doping elements, but the invention can also be successfully implemented with antimony and tantalum. The protective layer described above has no significant catalytic activity but is suitable for combination with a catalytic layer containing a noble metal oxide, the latter constituting an active component for reducing the overpotential of the oxygen evolution reaction. In one embodiment, the coating may comprise a protective layer interposed between the substrate and the catalytic layer, which is particularly effective in preventing corrosion of the substrate. In one embodiment, the coating may comprise a protective layer external to the catalytic layer, which is particularly effective in preventing the release of noble metals from the catalytic layer during the startup phase or the early hours of operation of the electrode. In another embodiment, both a protective layer interposed between the substrate and the catalytic layer and a protective layer external to the catalytic layer may be present. In one embodiment, each protective coating layer has a thickness of 1 to 5 micrometers. In practice, it has been possible to verify how the typical properties of the protective layers described above in terms of conductivity and porosity allow operation at such high thicknesses without adversely affecting the electrode potential and with significant benefits in terms of operating life.

In one embodiment, the catalytic layer of the coating has a composition, expressed by weight of metal, comprising 40-46% of a platinum group metal, 7-13% of one or more doping elements selected from bismuth, tantalum, niobium, or antimony, and 47-53% of tin, with a thickness ranging from 2.5 to 5 micrometers. It has been observed that this formulation of the catalytic layer allows the advantages of the aforementioned protective layer to be utilized to a greater extent, particularly when the platinum group metal is selected from iridium and a mixture of iridium and ruthenium, and the selected doping element is bismuth. In one embodiment, the selected platinum group metal is a mixture of iridium and ruthenium in a weight ratio of Ir:Ru of 60:40 to 40:60.

In one aspect, the present invention relates to a process for cathodic electrodeposition of metals from aqueous solutions, such as copper electrowinning, wherein the corresponding anodic reaction is oxygen evolution at the surface of an electrode as described above.

The following examples are included to demonstrate specific embodiments of the invention, the utility of which has been demonstrated to a large extent within the numerical ranges claimed. It should be understood by those skilled in the art that the compositions and techniques disclosed in the following examples represent compositions and techniques discovered by the inventors that function well in the practice of the invention; however, in light of the present disclosure, those skilled in the art should appreciate that many changes can be made in the specific embodiments disclosed and still obtain the same or similar results without departing from the scope of the invention.

All the samples cited in the following examples were manufactured starting from a grade 1 titanium mesh of dimensions 200 mm × 200 mm × 1 mm: degreased with acetone in an ultrasonic bath for 10 minutes, then first sandblasted with corundum until a surface roughness value Rz of 25-35 μm was obtained, then annealed at 570° C. for 2 hours and finally etched in 22% by weight HCl at boiling temperature for 30 minutes, the resulting weight loss being checked to be 180-250 g/m ² .

All layers of coating were applied by brushing.

Example 1

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

Two different 0.9 M solutions of Ir and Ru hydroxyacetyl chloride complexes (IrHAC and RuHAC) were prepared according to the procedure described in WO2010055065. A solution containing 50 g/ _l bismuth was prepared by dissolving 7.54 g of BiCl3 in 60 ml of 10 wt% HCl in a beaker at room temperature with stirring, and then adjusting the volume to 100 ml with 10 wt% HCl when a clear solution was observed (indicating complete dissolution).

While stirring, 5.11 ml of 1.65 M SnHAC solution, 0.23 ml of 9 M RuHAC solution, and 0.85 ml of 50 g/l Bi solution were added to the beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added.

The solution was applied to a pretreated titanium mesh sample by brush coating in 6 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an inner protective layer was obtained with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 μm and a specific Sn loading of about 9 g/m ² .

While stirring, add 10.15 ml of 1.65 M SnHAC solution, 10 ml of 0.9 M IrHAC solution, and 7.44 ml of 50 g/l Bi solution to the second beaker. Stirring is continued for 5 minutes. Then, 20 ml of 10 wt% acetic acid is added.

The solution was applied over the previously obtained inner protective layer by brushing in 13 coats, each followed by a drying step at 60° C. for 10 minutes and a thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained having an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 μm and an Ir specific loading of about 10 g/m ² .

Label this electrode "EX1".

Comparative Example 1

A protective layer based on titanium and tantalum oxides (in a molar ratio of 80:20) with a total loading of 1.3-1.6 g/m ² expressed as metal (corresponding to 1.88-2.32 g/m ² expressed as oxide) was applied to a titanium mesh sample. The protective layer was applied by brushing in four passes with a precursor solution obtained by adding an aqueous solution of TaCl ₅ acidified with HCl to an aqueous solution of TiCl ₄ , followed by thermal decomposition at 515°C.

While stirring, 10.15 ml of 1.65 M SnHAC solution, 10 ml of 0.9 M IrHAC solution, and 7.44 ml of 50 g/l Bi solution were added to the beaker. Stirring was continued for 5 minutes. Then, 20 ml of 10 wt% acetic acid was added.

The solution was applied to the protective layer previously obtained by brushing in 14 passes, each followed by a drying step at 60° C. for 10 minutes and a thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained having an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 μm and an Ir specific loading of about 10 g/m ² .

Label this electrode "CE1".

Comparative Example 2

A protective layer based on titanium and tantalum oxides (in a molar ratio of 80:20) with a total loading of 7 g/m ² expressed as metal (10.15 g/m ² expressed as oxide) was applied to a titanium mesh sample. The protective layer was applied in four passes using a precursor solution obtained by adding an aqueous TaCl ₅ solution acidified with HCl to an aqueous TiCl ₄ solution, followed by thermal decomposition at 515°C.

While stirring, 10.15 ml of 1.65 M SnHAC solution, 10 ml of 0.9 M IrHAC solution, and 7.44 ml of 50 g/l Bi solution were added to the beaker. Stirring was continued for 5 minutes. Then, 20 ml of 10 wt% acetic acid was added.

The solution was applied over the previously obtained protective layer by brushing in 14 passes, each followed by a drying step at 60° C. for 10 minutes and a thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained having an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 μm and an Ir specific loading of about 10 g/m ² .

Label this electrode "CE2".

Example 2

Several 20 mm x 50 mm specimens were cut from the electrodes of the above-described Examples and Comparative Examples, and their anodic potentials were measured in a 150 g/l _{H₂SO₄ aqueous} solution at 50°C under oxygen evolution, using a Luggin capillary and a platinum probe as is known in the art. The data reported in Table 1 (CISEP) represent the potential values measured at a current density of 500 A/ ^m₂ . Table 1 also shows the lifetimes demonstrated in an accelerated life test (ALT) in a ₁₅₀ g/l _H₂SO₄ aqueous solution at a current density of 30 kA/ ^m₂ and a temperature of 60°C.

The results of these tests show how providing an internal protective layer according to the invention allows obtaining a significant increase in duration, accompanied by an improvement in the oxygen evolution potential (compared to an internal protective layer consisting of a mixture of titanium and tantalum oxides according to the prior art).

Similar results are obtained by varying the concentrations of the components of the protective layer and the nature of the doping element as described in the appended claims.

Table 1

Example 3

While stirring, 5.11 ml of a 1.65 M SnHAC solution, 0.23 ml of a 9 M RuHAC solution, and 0.85 ml of a 50 g/l Bi solution were added to a beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added. The solution was applied to the pretreated titanium mesh sample by brush coating in six passes, followed by a drying step at 60°C for 10 minutes and a subsequent thermal decomposition step at 520°C for 10 minutes.

In this way, an inner protective layer was obtained having a weight ratio of Sn:Bi:Ru of 94:4:2, a thickness of 4 μm and a specific Sn loading of about 9 g/m ² .

While stirring, add 10.15 ml of 1.65 M SnHAC solution, 10 ml of 0.9 M IrHAC solution, and 7.44 ml of 50 g/l Bi solution to the second beaker. Stir for 5 minutes. Then, add 20 ml of 10 wt% acetic acid.

The solution was applied to the previously obtained inner protective layer by brushing in 13 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained having an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 μm and an Ir specific loading of about 10 g/m ² .

While stirring, 5.11 ml of 1.65 M SnHAC solution, 0.23 ml of 9 M RuHAC solution, and 0.85 ml of 50 g/l Bi solution were added to the third beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added.

The solution was applied over the previously obtained layer by brushing in 4 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an outer protective layer was obtained with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 3 μm and a specific Sn loading of about 6 g/m ² .

Label this electrode "EX3".

Example 4

While stirring, 5.11 ml of 1.65 M SnHAC solution, 0.23 ml of 9 M RuHAC solution, and 0.85 ml of 50 g/l Bi solution were added to the beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added.

The solution was applied to a pretreated titanium mesh sample by brush coating in 6 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an inner protective layer was obtained with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 μm and a specific Sn loading of about 9 g/m ² .

While stirring, add 10.15 ml of 1.65 M SnHAC solution, 10 ml of 0.9 M IrHAC solution, and 7.44 ml of 50 g/l Bi solution to the second beaker. Stir for 5 minutes. Then add 20 ml of 10 wt% acetic acid.

The solution was applied over the previously obtained inner protective layer by brushing in 13 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained with an Ir:Sn:Bi weight ratio of 42:49:9 and an Ir specific loading of about 10 g/m ² .

Then, 5 ml of 1.65 M SnHAC solution and 15 ml of 10 wt % acetic acid were added to the third beaker while maintaining stirring.

The solution was applied over the previously obtained layer by brushing in 6 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an outer protective layer was obtained, which had a specific Sn loading of about 9 g/m ² .

Label this electrode "EX4".

Example 5

A protective layer based on titanium and tantalum oxides (in a molar ratio of 80:20) with a total loading of 1.3-1.6 g/m ² expressed as metal (corresponding to 1.88-2.32 g/m ² expressed as oxide) was applied to a titanium mesh sample. The protective layer was applied by brushing in four passes with a precursor solution obtained by adding an aqueous solution of TaCl ₅ acidified with HCl to an aqueous solution of TiCl ₄ , followed by thermal decomposition at 515°C.

While stirring, 10.15 ml of 1.65 M SnHAC solution, 10 ml of 0.9 M IrHAC solution, and 7.44 ml of 50 g/l Bi solution were added to the beaker. Stirring was continued for 5 minutes. Then, 20 ml of 10 wt% acetic acid was added.

The solution was applied over the previously obtained protective layer by brushing in 14 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained with an Ir:Sn:Bi weight ratio of 42:49:9 and an Ir specific loading of about 10 g/m ² .

While stirring, 5.11 ml of 1.65 M SnHAC solution, 0.23 ml of 9 M RuHAC solution, and 0.85 ml of 50 g/l Bi solution were added to the second beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added.

The solution was applied to the previously obtained catalytic layer by brushing in 6 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an outer protective layer was obtained with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 μm and a specific Sn loading of about 9 g/m ² .

Label this electrode "EX5".

Example 6

While stirring, 5.11 ml of 1.65 M SnHAC solution, 0.23 ml of 9 M RuHAC solution, and 0.85 ml of 50 g/l Bi solution were added to the beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added.

The solution was applied to a pretreated titanium mesh sample by brush coating in 6 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an inner protective layer was obtained with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 μm and a specific Sn loading of about 9 g/m ² .

While maintaining stirring, add 5.15 ml of 1.65 M SnHAC solution, 2.5 ml of 0.9 M IrHAC solution, 4.75 ml of 0.9 M RuHAC solution, and 3.71 ml of 50 g/l Bi solution to the second beaker. Stirring is continued for 5 minutes. Then, 21.7 ml of 10 wt% acetic acid is added.

The solution was applied over the previously obtained inner protective layer by brushing in 9 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, a catalytic layer was obtained having an Ir:Ru:Sn:Bi weight ratio of 21:21:49:9, a thickness of 3.5 μm and a specific loading of Ir+Ru of about 7 g/m ² .

While stirring, 5.11 ml of 1.65 M SnHAC solution, 0.23 ml of 9 M RuHAC solution, and 0.85 ml of 50 g/l Bi solution were added to the third beaker. Stirring was continued for 5 minutes. Then, 18.57 ml of 10 wt% acetic acid was added.

The solution was applied to the previously obtained layer by brushing in 4 passes, each followed by a drying step at 60° C. for 10 minutes and a subsequent thermal decomposition step at 520° C. for 10 minutes.

In this way, an outer layer was obtained having a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 3 μm and a specific Sn loading of about 6 g/m ² .

Label this electrode "EX6".

Example 7

Several 20 mm x 50 mm specimens were cut from the electrodes of the above examples and their anodic potentials were measured in a 150 g/ _{l H₂SO₄} aqueous solution at 50°C under oxygen evolution, using a Luggin capillary and a platinum probe as is known in the art. The data reported in Table 2 (CISEP) represent the potential values measured at a current density of 500 A/ ^m₂ . Table 2 also shows the lifetimes demonstrated in an accelerated life test (ALT) in a 150 g/l _{H₂SO₄ aqueous} solution at a current density of 30 kA/ ^m₂ and a temperature of 60°C.

Table 2

The results show how an outer protective layer containing tin oxide allows the operating life of the electrode to be increased at the expense of an increase in its anodic overpotential. However, if the protective outer layer containing tin oxide is a protective layer according to the invention, the increase in operating life can be further enhanced, possibly due to the stabilization of iridium at start-up and during the first hours of operation, while the anodic potential remains low.

Similar results are obtained by varying the concentrations of the components of the protective layer and the nature of the doping element as described in the appended claims.

The foregoing description is not intended to limit the present invention, which can be employed according to different embodiments without departing from its scope, and the scope of which is defined solely by the appended claims.

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

The discussion of documents, works, materials, devices, articles of manufacture and the like in this specification is included solely for the purpose of providing a background to the present invention. It is not intended to suggest or represent 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 any claim of this application.

Claims (9)

1. An electrode suitable for oxygen evolution in an electrolytic process, comprising a valve metal substrate with a coating, the coating comprising a catalytic layer and at least one protective layer outside the catalytic layer, the protective layer being composed of a mixture of oxides, the mixture of oxides having, in metallic form, a weight composition of 89-97% tin, 2-10% of at least one dopant element selected from bismuth, antimony and tantalum and 1-9% ruthenium. 2. The electrode according to claim 1, wherein the at least one protective layer is composed of a mixture of oxides, the mixture of oxides having a weight composition of 89-97% tin, 2-10% bismuth and 1-9% ruthenium, expressed in metal form. 3. The electrode according to any one of the preceding claims, wherein the at least one protective layer has a thickness of 1 to 5 micrometers. 4. The electrode according to claim 1 or 2, wherein the catalyst layer is in contact with the protective layer, the catalyst layer comprising a mixture of oxides, the mixture of oxides having, in metal terms, a weight composition of 40-46% platinum group metals, 7-13% of at least one element selected from bismuth, antimony, niobium and tantalum and 47-53% tin, and the catalyst layer having a thickness of 2.5 to 5 micrometers. 5. The electrode of claim 4, wherein the catalyst layer comprises a mixture of oxides, the mixture of oxides having a weight composition of 40-46% iridium, 7-13% bismuth and 47-53% tin, and the catalyst layer having a thickness of 2.5 to 5 micrometers. 6. The electrode according to claim 4, wherein the catalyst layer is composed of a mixture of oxides, the mixture of oxides having a weight composition, expressed in metals, containing 47-53% tin, 7-13% bismuth, 40-46% ruthenium and iridium, and the catalyst layer having a thickness of 2.5 to 5 μm. 7. The electrode of claim 6, wherein the weight ratio of iridium to ruthenium in the sum of iridium and ruthenium, expressed as metals, is between 60:40 and 40:60. 8. The electrode according to any one of claims 5 to 7, comprising at least two of the protective layers, wherein the catalytic layer is located between the at least two protective layers. 9. A method for cathode electrodeposition of metal from an aqueous solution, comprising anodic oxygen evolution on the surface of an electrode according to any one of claims 1 to 8.