Patent Application
20240328010
The invention relates to an electrode for electrolytic processes, in particular to an anode suitable for oxygen evolution in an industrial electrolytic process and to a method of manufacturing thereof.
Anodes for oxygen evolution are widely used in different electrolysis applications, several of which pertain to the field of electrometallurgy, covering a wide range in terms of applied current density, which can be very reduced (for instance a few hundred A/m2, such as the case of electrowinning processes) or also very high (for instance in high speed electroplating, which can operate in excess of 10 kA/m2, referred to the anodic surface).
Electrodes suitable for anodic evolution of oxygen can be obtained starting from substrates of valve metals, for instance titanium and alloys thereof, coated with catalytic compositions based on transition metals or oxides thereof, characterized by their capability of lowering the overvoltage of the oxygen anodic discharge reaction, which is much too high to allow the execution of industrial processes in the absence of catalytic systems.
Mixed metal oxide (MMO) electrodes are characterized by low consumption rate, long service life (comparing with the sacrificial anode), dimensional stability almost during the whole operating life, and lightweight. Use of mixed metal oxide (MMO) catalytic coated titanium anodes have been growing over decades.
MMO coated titanium anode for electrochemical reaction is used for oxygen evolution reaction (OER) in various applications such as electrowinning, electroplating, electrogalvanizing and electrolytic copper-foil production.
Generally, in MMO coated titanium anode for OER, an iridium-based oxide (generally mixed with oxide of valve metal such as tantalum) coating is applied as catalyst. MMO coating is applied on titanium substrate as a liquid form of metals salt and then is thermally decomposed to form an adherent layer of mixed oxides. The MMO coatings are applied in many layers and heat treated after each coat.
Iridium is one of the rarest metals. As it is one of the least abundant noble metals in the earth's crust resulting not only in a high price but also in difficulties purchasing bulk quantities for industrial-scale manufacturing processes. Furthermore, due to stronger demand for organic light-emitting diode applications and for “green hydrogen”, the price of iridium has increased significantly over the last years.
Electrodes with iridium-based catalytic coatings are produced by thermal decomposition which is a well-established technology providing less hazards and is a cost effective technique compared to other processes, such as PVD (Physical Vapor Deposition) or plasma spraying. However, iridium-based coatings are typically multilayer coatings resulting in costly manufacturing processes.
Ruthenium based MMO coatings are generally used to produce chlorine or hypochlorite for the chemical industry. Ruthenium based MMO coatings are practically not applicable to electrolytic processes involving oxygen evolution reaction with acidic environments, such as electro galvanizing, anodizing of aluminum, recovery of metals, metal plating, etc. due to the instability of RuO2 which eventually affects their long-term performance.
It is an object of the present invention to provide an electrode for oxygen evolution utilizing ruthenium as part of the catalytic coating element with improved durability, while maintaining costs low by keeping the usage of iridium in check. The present invention provides an improved anode for industrial electrolytic processes such as electrowinning of base metals such as copper, nickel and cobalt, electroplating, by exploiting ruthenium as part of the catalyst element.
The present invention provides for an electrode for electrolytic processes comprising a valve metal substrate comprising a coating, wherein the coating is composed of a catalytic layer and a barrier layer interposed between the catalytic layer and the substrate, wherein the coating is characterized by a diffusion area between the barrier layer and the catalytic layer comprising oxides of metals that make up the barrier layer and the catalytic layer.
More specifically, the electrode for electrolytic processes according to the invention comprises:
The present invention also provides for a method for manufacturing an electrode for electrolytic processes comprising the steps of:
Furthermore, the present invention also provides for an electrode obtained by the method discussed above.
The present invention provides an electrode for electrolytic processes comprising:
In another embodiment, the electrode's coating has:
In yet, another embodiment, the catalytic layer can have a composition of metal oxides of about 2.75-5.24% Ta, about 8.81-17.63% Ru, about 3.73-8.24% Ir, and about 0.24-0.53% Sn in terms of the atomic % of the metal present in the catalytic layer, and/or corresponding to a percent ratio of 17-21% Ta, 50-57% of Ru, 22-26% Ir and 0-7% of Sn relative to the sum of Ta+Ru+Ir+Sn present in the catalytic layer,
In one embodiment, the metal oxides in the catalytic layer can have the following compositions as measured at different points across the coating thickness:
The ratio is relative to the sum of Ta+Ru+Ir+Sn present in the catalytic layer.
In another embodiment, the metal oxides in the barrier layer can have the following compositions as measured at different points across the coating thickness:
The ratio is relative to the sum of Ta+Ru+Ir+Sn present in the barrier layer.
In a further embodiment, the metal oxides in the diffusion area have the following compositions:
The ratio is relative to the sum of Ta+Ru+Ir+Sn present in the diffusion area.
The above are a few examples of the metal oxide composition in the coating according to the invention. Additional metal oxide compositions and data such as charts and graphs can be found in
The cross-section line scans in the figures were acquired through an Energy-dispersive X-ray spectroscopy (EDS) measurement performed on the sample (e.g. the examples and counter-examples) coupled with milling with a gallium ion beam. The operator chose a suitable defect-free spot of the coating, as a skilled person can recognize, and measured the relative amounts of the elements Ru, Ta, Sn and Ir across the coating thickness. On each data point, the amounts were automatically normalized with respect to the sum of the Ga, C, O, Ta, Ru, Ir, Cl, Sn, and Ti signals. The figures show the evolution of the relative amounts of Ru, Ta, Sn and Ir across the coating thickness. The other elements, including the Ti signal from the substrate, are not shown.
Here and throughout the present application, it is understood that the relative percentage of Ru, Ta, Sn and Ir describing the observed electrode composition is obtained via the EDS measurement (as seen in the figures), after further normalization with respect to the sum of the Ru, Ta, Sn and Ir signals alone.
Depending on the thickness of the coating, the amount of the metal oxides present in each of the coating including the diffusion area, which is calculated based on the distance from the substrate, will slightly vary. However, the relative amount of each metal oxide will be within the same range.
For purposes of the invention, diffusion area does not mean that a separate “layer” is in existence or created between layers that make up the coating. Rather, the diffusion area refers to a convergence or junction area where elements from the barrier layer and the catalytic layer overlap and diffuse on each other thereby creating this diffusion area within the coating. Indeed, it is known in the art that with a mono or multilayer coating composition, where each layer may be obtained by applying multiple hands of precursor solutions, with each hand of coating being thermally treated after drying, certain elements may eventually diffuse along the coating thickness. This effect is often attributed to the different volatility of the elements, their mutual interaction and the temperatures and baking times used. However, in most cases, the diffusion patterns cannot be easily and precisely anticipated upfront, even if the end result is reproducible based on the specific technique and parameters used for the coating preparation. The ways the elements diffuse through the overall mono or multilayer coating composition may impart unexpected properties to the coating.
In the case of the present invention, the diffusion of the elements across the barrier and catalytic layers alters their initial composition, i.e. the amounts and materials employed as precursors, and furthermore creates an area where the elements of the catalytic and barrier layer overlap. The seeping of Sn and Ir along greater portions of the coating than expected is likely related to the surprising robustness and durability of the present coating, as well as its activity.
Additionally, the inventors have surprisingly observed that adding Ta to the Ir—Ru coating the active elements of Ir and Ru are stabilized, and the catalytic coating becomes more compatible with the Sn—Ta barrier layer thereby improving the catalytic coating adhesion to the Sn—Ta barrier layer. This effectively delays delamination of the catalytic coating from the Sn—Ta barrier layer, which in turn results in improved durability.
Furthermore, Ta in the catalytic coating plays as a binder element for Ir and Ru and stabilizing them. In addition, presence of Ta in the catalytic layer leads to improved compatibility with the Sn—Ta barrier layer. The coating of the invention, allows for a broader area of diffusion which in turn provides for a longer lifetime of the electrode.
The combination of Ta and Sn of the barrier layer, and their diffusion pattern throughout the overall coating, protects the substrate from corrosion or passivation under oxidative corrosive environment.
Thus, Ta must always be present in the barrier layer. The barrier layer must be conductive and stable under oxidative and corrosive environment. To be conductive for the barrier layer, Ta must be mixed with another element, which has different valence and is stable under oxidative and corrosive environment. Since SnO2 is more conductive and stable under oxidative and corrosive environment, Sn—Ta is a superior choice as barrier layer of the present invention.
Under one aspect of the invention, the above electrode for electrolytic processes according to the invention is obtained by applying, to a valve metal substrate, a catalytic coating solution comprising 18-21 mol % iridium, 52-64 mol % ruthenium and 15-30 mol % tantalum; a barrier layer solution comprising valve metal oxides tin and tantalum.
More specifically, the method of manufacturing of an electrode with the above described characteristics, comprises the steps of:
With the method described above, it is possible to obtain an electrode for electrolytic processes, in particular for oxygen evolution reactions in electrowinning or similar applications such as electroplating characterized by operating current density in the range of 200 to 2,000 A/m2. The resulting electrode exhibits comparable or improved durability compared to Ir-rich coatings, while reducing the total Ir amount. This is obtained by partially substituting Ir with Ru and the stability issues of the latter in electrowinning applications are overcome by using the specific combination of elements and quantities in the catalytic and barrier layer coating solutions recited above.
The result is an electrode with a composite coating structure which, when inspected via SEM analysis, exhibits a dual layer fingerprint, but where the distribution of the elements departs from the well-defined quantities employed in the catalytic and barrier layer coating solutions.
Indeed as EDS (Energy Dispersive Spectroscopy) line scans performed on a plurality of samples obtained via the aforementioned method in the claimed ranges of elements show, the elements of the resulting electrode diffuse across the overall coating thickness and, without limiting ourselves to a specific theory, their distribution pattern is likely to impart the advantageous properties of the electrode, in terms of both durability and activity, even if other factors may come into play.
It would be impossible to capture the observed line scan patterns without unduly restricting the scope of the claims, as the electrode obtainable with the method described above may exhibit a large variation in structure, while still performing according to the desired specifications.
Advantageously, the above described method allows to obtain an electrode for electrolytic processes comprising a valve metal substrate comprising a coating, wherein the coating is composed of a catalytic layer and a barrier layer interposed between the catalytic layer and the substrate, wherein the coating is characterized by a diffusion area between the barrier layer and the catalytic layer comprising oxides of metals that make up the barrier layer and the catalytic layer. This electrode has been noted to perform particularly well in the execution of the invention.
In one embodiment, the method uses a catalytic coating solution comprising 18-21 mol % iridium, 52-64 mol % ruthenium and 15-30 mol % tantalum.
In another embodiment, the method uses a barrier layer solution comprising oxides of tin and tantalum in a ratio of 70-90:30-10 mol % Sn:Ta.
In a further embodiment, in steps d) and f) of the method described above, the drying temperature can be from 25° C. to 60° C. In yet a further embodiment in steps d) and f) of the method described above, the thermal decomposition treatment is carried out in an electric furnace, at a temperature range of 480-530° C. for 10 to 20 minutes.
If the decomposition treatment time is too short, the coated metal precursors cannot fully convert to their oxides. However, if the decomposition treatment time is too long, it will give thermal profile more than enough to coated titanium resulting in reduced durability. Thus, in the present invention the thermal decomposition treatment is carried out in an electric furnace for 10 to 20 minutes.
Additionally, the temperature range used during thermal decomposition in the invention is 480-530° C. If the temperature is below this range, the coated metal precursors are not converted to their oxides well and they does not exhibit a proper functionality as barrier layer.
In another aspect of the invention, in step a) of the method described above the pre-treating step (which is optionally carried out) is carried out first by sandblasting by alumina grit, followed by blasting with steel grit and then etching in aqueous solution of 20 wt % hydrochloric acid at boiling temperature for 20 minutes; after the etching, rinsing the substrate with deionized water and drying.
In a further aspect of the invention, in step b) of the method described above the Sn—Ta barrier layer solution is obtained by mixing 1.65 M of stannic hydroxyacetochloride (SnHAC) complex solution, 120 g/L of tantalum solution and aqueous solution of 10 wt % acetic acid.
In yet a further aspect of the invention, in step c) of the method described above the Ru precursor for the catalytic coating solution is RuCl3 or RuHAC. Ir precursor used in the catalytic coating solution of the invention is IrCl3, H2IrCl6 or IrHAC, and Ta precursor fused in the present invention is TaCl5.
In one embodiment, in step c) the precursor solution of Ru, Ir and Ta is obtained by mixing H2IrCl6 solution, 20 wt % RuCl3 solution, 120 g/L of tantalum solution and 10 wt % HCl.
The valve metal substrate can be niobium, zirconium, titanium or alloyed titanium.
In another aspect, the invention relates to an electrode that is obtainable by a method comprising applying a barrier layer solution comprising oxides of tin and tantalum in a ratio of 70-90:30-10 mol % Sn:Ta to the substrate, followed by drying, thermal decomposition treatment and cooling down; thereafter applying a catalytic coating solution comprising 18-21 mol % iridium, 52-64 mol % ruthenium and 15-30 mol % tantalum to the barrier layer, followed by drying, thermal decomposition treatment and cooling down.
Under a preferred embodiment, the electrode obtained by the method described above comprises a substrate with a coating, the coating comprising a catalytic layer and a barrier layer interposed between the catalytic layer and the substrate,
In one embodiment the specific loading of the catalytic layer according to the invention ranges from 2.0 to 20.0 g/m2 in terms of precious metals (Ru+Ir), preferably from 6.0 to 12.0 g/m2 terms of precious metals (Ru+Ir), more preferably 9.0 g/m2 terms of precious metals (Ru+Ir).
In another embodiment the specific loading of the barrier layer ranges from 1.0 to 13.0 g/m2 in terms of Sn and Ta metals, preferably from 3.0 to 9.0 g/m2 in terms of Sn and Ta metals, more preferably 6.0 g/m2 in terms of Sn and Ta metals.
Some of the most significant results obtained by the inventors are presented in the following examples, which are not intended as a limitation of the domain of the invention.
A titanium sheet was subjected first to sandblasting by alumina grit, followed by blasting with steel grit and then etching in aqueous solution of 20 wt % hydrochloric acid at boiling temperature at about 106° C. for 20 minutes. After the etching step, the titanium sheet substrate was rinsed with deionized water and then dried. The surface roughness of the substrate is 3.8 μm.
1.65 M of stannic hydroxyacetochloride (SnHAC) complex solution was prepared following the procedure described in WO2005014885.
120 g/L of tantalum solution was prepared by dissolving Ta(V) chloride salt in concentrated hydrochloric acid.
A barrier layer solution of Sn:Ta=70:30 mol % was prepared by mixing the 1.65 M of SnHAC solution, the 120 g/L of tantalum solution and an aqueous solution of 10 wt % acetic acid.
A catalytic coating solution of Ir:Ru:Ta=18:52:30 mol % was prepared by mixing 20 wt % H2IrCl6 solution, 20 wt % RuCl3 solution, 120 g/L of tantalum solution and 10 wt % HCl.
To the pretreated titanium sheet substrate, the prepared Sn—Ta barrier layer solution was applied by brushing the solution to the substrate, followed by drying at 60° Celsius for 10 minutes. After drying, thermal decomposition was conducted for 10 minutes in an electric furnace of air circulation type at 520° Celsius, followed by cooling down at room temperature.
The above cycle of brushing, drying, thermal decomposition and cooling was repeated until reaching an overall loading of 6.0 g/m2 in terms of Sn and Ta metals.
To the barrier layer coated titanium sheet substrate, the prepared Ir—Ru—Ta catalytic coating solution was applied by brushing the solution to the substrate, followed by drying at 60° Celsius for 10 minutes. After drying, thermal decomposition was conducted for 10 minutes in an electric furnace of air circulation type at 475° Celsius, followed by cooling down at room temperature.
This cycle of brushing, drying, thermal decomposition and cooling was repeated until reaching an overall loading of 9.0 g/m2 in terms of total precious metals Ir and Ru.
A titanium sheet was subjected first to sandblasting by alumina grit, followed by blasting with steel grit and then etching in aqueous solution of 20 wt % hydrochloric acid at boiling temperature (about 106° C.) for 20 minutes. After the etching, the titanium sheet substrate was rinsed with deionized water and then dried.
1.65 M of stannic hydroxyacetochloride (SnHAC) complex solution was prepared following the procedure described in WO2005014885.
120 g/L of tantalum solution was prepared by dissolving Ta(V) chloride salt in concentrated hydrochloric acid.
A barrier layer solution of Sn:Ta=70:30 mol % was prepared by mixing 1.65 M of SnHAC solution, the 120 g/L of tantalum solution and an aqueous solution of 10 wt % acetic acid.
A catalytic coating solution of Ir:Ru:Ta=21:64:15 mol % was prepared by mixing 20 wt % H2IrCl6 solution, 20 wt % RuCl3 solution, 120 g/L of tantalum solution and 10 wt % HCl.
To the pretreated titanium sheet substrate, the prepared Sn—Ta solution was applied by brushing the solution to the substrate, followed by drying at 60° Celsius for 10 minutes. After drying, thermal decomposition was conducted for 10 minutes in an electric furnace of air circulation type at 520° Celsius, followed by cooling down in air at room temperature.
The above cycle of brushing, drying, thermal decomposition and cooling was repeated until reach an overall loading of 6.0 g/m2 in terms of Sn and Ta metals.
To the barrier layer coated titanium sheet substrate, the prepared Ir—Ru—Ta solution was applied by brushing the solution to the substrate, followed by drying at 60° Celsius for 10 minutes. After drying, thermal decomposition was conducted for 10 minutes in an electric furnace of air circulation type at 475° Celsius, followed by cooling down in air at room temperature.
The cycle of brushing, drying, thermal decomposition and cooling was repeated until reach an overall loading of 9.0 g/m2 in terms of total precious metals Ir and Ru.
Examples 3-9 are prepared similarly by following the steps in Examples 1-2, except that the molar ratios of the barrier layer solution and the catalytic layer solution are as follows:
A titanium sheet was subjected first to sandblasting by alumina grit, followed by blasting with steel grit and then etching in aqueous solution of 20 wt % hydrochloric acid at boiling temperature for 20 minutes. After the etching, the titanium sheet substrate was rinsed with deionized water and then dried.
1.65 M of stannic hydroxyacetochloride (SnHAC) complex solution was prepared following the procedure described in WO2005014885.
120 g/L of tantalum solution was prepared by dissolving Ta(V) chloride salt in concentrated hydrochloric acid.
A barrier layer solution of Sn:Ta=70:30 mol % was prepared by mixing the 1.65 M of SnHAC solution, the 120 g/L of tantalum solution and an aqueous solution of 10 wt % acetic acid.
A catalytic coating solution of Ir:Ru:Ta=25:75:0 mol % was prepared by mixing 20 wt % H2IrCl6 solution, 20 wt % RuCl3 solution, n-butanol and acetic acid.
To the pretreated titanium sheet substrate, the prepared Sn—Ta barrier layer solution was applied by brushing the solution to the substrate, followed by drying at 60° Celsius for 10 minutes. After drying, thermal decomposition was conducted for 10 minutes in an electric furnace of air circulation type at 520° Celsius, followed by cooling down in air.
The above cycle was repeated until reach an overall loading of 6.0 g/m2 in terms of Sn and Ta metals.
To the coated titanium sheet substrate, the prepared Ir—Ru solution was applied by brushing the solution to the substrate, followed by drying at 60° Celsius for 10 minutes. After drying, thermal decomposition was conducted for 10 minutes in an electric furnace of air circulation type at 475° Celsius, followed by cooling down in air.
The above cycle of brushing, drying, thermal decomposition and cooling was repeated until reaching an overall loading of 9.0 g/m2 in terms of total precious metals Ir and Ru.
Counter-Examples 2-3 are prepared similarly by following the steps in Counter-Example 1, except that the molar ratios of the barrier layer solution and the catalytic layer solution are as follows:
Lifetime test of the coatings of the examples were carried out under the following conditions:
The lifetime (hours) was defined as duration for cell voltage to increase by 2 volts from the initial value. Lifetime was normalized by Ir loading amount. Two tests were conducted for each sample. The results are summarized in Table 2.
As can be seen from the results in Table 2, having Ta in the outer catalytic coating layer enables to disperse the noble metal active elements. Ta acts as a binder between the catalytic coating and the barrier layer which provides for higher durability. The results shown in Table 2 were obtained by dividing “Lifetime (hours)” by “precious metal loading (g/m2)”, which is our “normalized” lifetime.
Passivation of the titanium substrate (formation of poorly conductive TiO2), corrosion of the titanium substrate and resultant coating delamination from the titanium substrate, delamination of the catalytic coating from the barrier layer and consumption of active elements (Ir and Ru) are possible phenomenon around the end of the life test (voltage increase by 2 volts).
While Example 1 and 2 have Ta in the catalytic coating, Counter-Example 1 doesn't have Ta in the catalytic coating. Table 2 shows life test result representing longevity of each coating formulation. Improved longevity in Examples 1 and 2 are related to stabilization of Ir and Ru in the catalytic coating by adding the binder Ta and improved adhesion of the catalytic coating to the Sn—Ta barrier layer. The two tests are established procedure and representative for each sample.
Similarly, Examples 3-9, which have Ta in the catalytic coating vs. the Counter-Example 1 which does not have Ta in the catalytic coating shows life test results with improved longevity. This improved longevity in Examples 3-9 are also related to stabilization of Ir and Ru in the catalytic coating by adding the binder Ta and improved adhesion of the catalytic coating to the Sn—Ta barrier layer.
The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.
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, components or additional process steps.
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.
This application claims the benefit of U.S. Provisional Application No. 63/493,623 filed on Mar. 31, 2023, the contents of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63493623 | Mar 2023 | US |
