Extended life anode coatings

Abstract

The present invention provides an anode comprising a core substrate comprising a multi-layer coating having a base layer comprising palladium, the base layer directly coating the substrate.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of priority of U.S. Provisional Application No. 63 / 400,668, filed on Aug. 24, 2022, which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present disclosure generally relates to electrode coatings on substrates intended to operate as anodes in electrochemical processes, herein referred to as anode coatings. Background Art

[0004] Many commercial manufacturing processes employ electrochemical techniques. For example, the chlor-alkali process electrolyzes aqueous solutions of sodium chloride or potassium chloride to form valuable commodity materials such as chlorine gas, sodium hydroxide (caustic soda) or potassium hydroxide, and hydrogen gas. Water is electrolyzed to produce hydrogen and oxygen gases. Other electrochemical processes are used to prepare various commodity chemicals and intermediates for the chemical and pharmaceutical industries. Currently, commercial electrochemical processes are making efforts to reduce energy consumption, lower manufacturing costs, and improve electrode efficiency and durability.

[0005] Specific electrochemical processes in the field of the present invention described herein are those in which a chloride salt is present in solution and in which chlorine gas or hypochlorite is the main product. Such processes include chlor-alkali membrane cell units and diaphragm cell unit processes, chlorate production, and the generation of hypochlorite in brine from fresh to concentrated for disinfection purposes. The use and composition of gas evolution electrodes give rise to different problems and results compared to other electrode uses such as in batteries.

[0006] Most conductive materials can be used as electrodes. Preferably, the materials used to prepare the electrodes resist corrosion by the electrolyte and / or the products generated. Many other suitable electrode materials lack the ability to effectively catalyze electron transfer to the electrolyte, which requires the use of additional power. And the greater the additional power used, the higher the cost of performing the electrochemical process. Coatings can be applied to the electrodes to facilitate electron transfer and reduce the overpotential required in the electrolysis process. Thus, the coatings help to reduce the overall operating voltage and power consumption of the electrolysis process. Further details regarding electrode coatings are described in International Application No. PCT / US2020 / 037426, filed on Jun. 12, 2020, which is hereby incorporated by reference in its entirety.

[0007] Anode coatings known in the art have a limited lifespan and contain precious metals. The function of the anode coating is to reduce the voltage required for oxidation by acting as an electrocatalyst and protecting the substrate, thus giving the anode geometric stability. Anode coatings for the oxidation of chloride ions provide a lower overpotential for the oxidation of chloride to chlorine gas.

[0008] Anode coatings fail when the coating itself wears over time, loses conductivity, loses adhesion to the substrate, or the substrate oxidizes beneath the coating to form a passivation layer. Once the coating fails, the voltage rises rapidly and continued operation may cause heating or damage to process equipment. In general, the useful life of the coating is proportional to the precious metal loading of the coating and inversely proportional to the square of the current density in the battery cell. In prior art inventions, iridium and ruthenium are the primary precious metals used. Iridium is typically used for current densities greater than about 3 kA / m 2 Anodes that can operate for more than 4 years are iridium, but iridium is much more expensive than ruthenium.

[0009] It would be desirable to develop anodes having improved durability, reduced overpotential, and / or extended operating life. Summary of the invention

[0010] In one embodiment of the invention, the preferred anode substrate is a valve metal, specifically titanium or an alloy thereof.An advantage of embodiments of the invention is that the coating provided on the substrate exhibits a lower overpotential compared to prior art coatings, thereby reducing the power requirements of the process.

[0011] Compared to expectations, embodiments of the present invention achieve longer lifetimes at a given precious metal loading of the coating. Embodiments of the present invention are characterized in that current densities greater than 3 kA / m can be produced without the use of iridium. 2 Long-life anode coatings are provided for chlorate cells. When this exemplary anode coating is applied to a chlorate cell, chlorine produced at the anode immediately forms hypochlorite ions in solution, while hydrogen produced at the cathode leaves the cell. A particular problem with chlorate cells is that oxygen is produced at the anode by oxygen on the electrocatalytic coating or by catalyzing the decomposition of hypochlorite in solution. The coatings of the embodiments of the invention described herein achieve exceptionally low oxygen content in the hydrogen from the chlorate cell. This is achieved by both reducing electrochemical oxygen evolution, specifically oxygen evolution on newly activated electrodes, and avoiding contamination of the hypochlorite-containing solution in the electrolyzer with impurities that can catalyze the decomposition of hypochlorite to oxygen.

[0012] In the anode coating of an embodiment of the present invention, a coating formulation containing titanium chloride or titanium oxychloride is manufactured in an aqueous / alcoholic solution containing hydrochloric acid. In certain embodiments, titanium alkoxide can be used in combination with pre-baking, secondary / tertiary alcohols and / or oxidants to achieve the same effect as titanium oxychloride or titanium chloride. Ruthenium, palladium and optionally salts of platinum and iridium are also dissolved in the solution, preferably in the form of chloride salts. Optionally, chloride salts of transition metal elements can be added.

[0013] In a preferred embodiment of the present invention, when preparing the coatings of the present invention comprising palladium, various factors, individually and in combination with two or more factors, surprisingly contribute to extending the life of anodes having the anodic coatings of the present invention:

[0014] 1. Avoid primary alcohols when preparing the coating solution.

[0015] 2. Pre-oxidize or pre-bake the titanium surface of the titanium or titanium alloy substrate.

[0016] 3. In preparing the coating solution, the presence of peroxide places both titanium and ruthenium in the +4 oxidation state. In other embodiments, other oxidizing agents may be used in place of or in addition to peroxide when preparing the coating solution to produce the +4 oxidation state of titanium and ruthenium, including nitric acid, chromates, halogens, chlorine dioxide, chloric acid, and / or ozone, etc. Thus, where peroxide is mentioned as being used herein, it is to be understood that other possible oxidizing agents are also covered.

[0017] 4. Preferably use titanium as the titanium oxychloride solution. However, in embodiments, titanium alkoxides may be used in combination with pre-baking, secondary / tertiary alcohols, and / or oxidizing agents to achieve the same effect as titanium oxychloride or titanium chloride.

[0018] 5. Avoid using tin together with palladium to prevent the formation of PdSn2 which behaves like a metal. In other embodiments, it is desirable to avoid the formation of PdSn4 compounds which behave like metal alloys and form a phase separated from the rutile phase of the coating.

[0019] In an embodiment considered to be unique, hydrogen peroxide is optionally added to the anodic coating solution of the mixed salts in an amount that increases the oxidation potential of the coating and prevents palladium from being reduced to the metallic state during the coating process. Another optional component in the coating solution is secondary and / or tertiary alcohols, preferably isopropyl alcohol (2-propanol). In other embodiments, 2-butanol and / or tert-butanol (tert-butyl alcohol) are another optional component of the coating solution. A remarkable characteristic of the coatings of the embodiments of the present invention is that the molar ratio of titanium to noble metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and the molar ratio of palladium to the sum of the other noble metals is between about 0.04 and 0.3.

[0020] Importantly, in embodiments of the present invention, the anodic coating comprises titanium, ruthenium, and palladium, where palladium is distributed in a fine scale, where all three metals have the same crystal structure, and the separation of palladium into a single phase as in prior art coatings comprising palladium is avoided.

[0021] When preparing an anode coating for high current density applications, if the amount of iridium is reduced, the thickness of the coating must be increased and the wear rate must be reduced. The coating is applied in multiple layers with drying and baking steps between each layer. An advantage of embodiments of the present invention is that a coating with a low wear rate mainly containing ruthenium as a noble metal can be achieved using fewer layers of the coating.

[0022] Coating adhesion is typically measured by a tape test, in which a piece of transparent tape is applied to the coated anode surface and then quickly peeled off, and the removed coating is observed. The tape test is usually evaluated by appearance, but can also be quantitatively evaluated using X-ray fluorescence measurement of the tape. The most useful quantitative tape test results are quantified based on the percentage of the total removed coating. An object of embodiments of the present invention is to achieve a coating with quantitative tape test results, where the coating removed by the tape is less than 5%, preferably less than 2%, and more preferably less than 1%.

[0023] In other anode coating inventions of the prior art, it has been found that coatings containing palladium or platinum reduce the voltage required for the oxidation of chloride ions to chlorine or hypochlorite, thereby reducing the required power and also reducing the undesired generation of oxygen. However, in such prior art, the ratio of palladium or platinum to the low-cost ruthenium component in the coating is greater than 3:10, and palladium and platinum are lost from the coating faster than ruthenium. When Pd or Pt in the coating is lost, the voltage increases. An advantage of the present invention is that a smaller amount of Pd or Pt can be used as an effective additive to the coating and maintain a lower voltage for a longer time.

[0024] The anode substrate in embodiments of the present invention is prepared by methods known in the art to roughen and etch the surface to remove oxides and embedded grit. Preferably, a light oxide film is then restored by baking at a temperature of 400 to 550 °C for a sufficient time to form an orange to dark blue or light gray color on the titanium surface, thereby making the substrate surface more hydrophilic.

[0025] A coating is then formed on the substrate by dip coating, roll coating, brush coating, spray coating, or electrostatic spraying the substrate with a coating solution. The coating is completely dried, preferably at a temperature below about 110 °C and preferably at about 50 °C, and then baked at a temperature of 400° to 550 °C for 10 to 20 minutes. Then, additional layers are applied by repeating the process of application, drying, and baking.

[0026] Optionally, after the final coating, a longer baking step for an extended period of time, known as post-baking, can be used.

[0027] The noble metals used in the coating of the embodiments of the present invention are ruthenium, palladium, and optionally platinum and iridium. In the anode of the embodiments of the present invention, ruthenium is the main noble metal used in the coating, and the molar ratio of Pd to the total noble metal is preferably about 0.04 to 0.3, preferably about 0.12. Palladium in the optimal range shows a voltage that reduces the evolution of chlorine, while reducing the evolution of oxygen in the coating, and at the same time prolonging the life of the anode. In the case of higher levels of palladium, especially in the case of higher levels of palladium proposed in the palladium coating examples of the prior art, the formation of rutile is reduced, and the anatase phase is favorable, thereby shortening the life of the coating. Therefore, in the coating of the prior art, the extended life is not attributed to the presence of palladium in the coating, because the higher level of coating promotes the formation of anatase.

[0028] About palladium salt and platinum salt, the problem that is not solved in the prior art is that the palladium salt and platinum salt are more easily reduced to metal form than ruthenium salt or iridium salt. When palladium and platinum are in close contact with titanium metal substrate, they can be reduced to metal form. Another aspect of an embodiment of the present invention is that the coating is applied after the titanium surface is prepared by oxidation (pre-baking) in the air at an elevated temperature and avoiding the use of primary alcohols. When the titanium oxide film is exposed to an acidic coating solution, it may be dissolved, and if dried at an elevated temperature, bare titanium will be exposed to the coating solution. By first pre-baking (pre-oxidation) the substrate surface, the coating can effectively wet the surface and cause the coating to produce a favorable adhesion to the substrate surface.

[0029] In an embodiment of the present invention, when the pre-baking conditions cause the titanium substrate to appear blue, and the coating is dried at a temperature below 110°C, and preferably at about 50°C, a palladium-containing coating can be formed without forming a metallic palladium phase. Alternatively, the preferred coating solution of an embodiment of the present invention contains some peroxide. The peroxide forms a stable complex with titanium and oxidizes ruthenium to the +4 state in the solution. The reduced palladium or platinum metal in the coating is easily oxidized to a soluble chloride salt in the process to which the embodiment of the present invention is applicable, so these metal phases in the coating shorten their life. Therefore, the coating containing palladium or platinum in the prior art cannot achieve an extended life when a metallic phase is produced.

[0030] In another embodiment of the present invention, the anode coating may include iridium to promote rutile formation and extend the coating life. In coatings having a molar ratio of iridium to ruthenium greater than about 0.1, the prior art has determined that the coating life is primarily a function of the iridium loading of the coating. Unexpectedly, even in coatings having a molar ratio of iridium to ruthenium greater than 0.1, the presence of palladium in a molar ratio of 0.04 to 0.3 relative to the total amount of ruthenium and iridium significantly increases the anode life, and the coatings of the embodiments of the present invention described herein can achieve more than twice the coating life of the prior art coatings with a similar total loading of iridium.

[0031] In embodiments of the present invention, additional dopants such as transition metals selected from Fe, Ni or Co etc. may be added to the coating, as these metals are known to promote rutile formation and are known in the art to increase the conductivity of the coating. However, the anode coatings of embodiments of the present invention may be produced in the absence of dopants. While increasing conductivity, it has not been found that adding dopants can increase the coating life. In addition, in applications for chlorate production, nickel and cobalt are known to catalyze the decomposition of hypochlorite into oxygen, so these dopants are not present in the coatings used for chlorate production.

[0032] In embodiments of the disclosed invention, it has been found that using titanium in the form of titanium oxychloride in combination with hydrogen peroxide increases rutile formation, particularly when a portion of the solvent of the coating is an alcohol and the coating is dried completely at an air temperature below 110 °C. It has been found that the optimal titanium oxychloride content of the coating solution is 0.25% to 5% by mass percentage of titanium, while the optimal peroxide content is a molar ratio of peroxide to titanium of 0.1 to 2.0, and the optimal alcohol content is 5% to 75% by mass percentage of the coating solution. The alcohol is preferably a secondary alcohol and / or a tertiary alcohol, preferably isopropyl alcohol (2-propanol), to avoid reacting with the peroxide before the solvent evaporates. In other embodiments, 2-butanol and / or tert-butanol (tert-butyl alcohol) is another optional component of the coating solution. In embodiments, if compatible with the oxidative nature of the salt, another water-soluble and volatile organic solvent may be used to replace the above-mentioned alcohol.

[0033] The present invention has the potential to achieve performance breakthroughs and reduce the manufacturing cost of the coatings used in all chlor-alkali processes and chlorate production. Another broad application area is hypochlorite generators for swimming pool disinfection, municipal water treatment, wastewater treatment or bilge water disinfection. The potential uses are in the global market.

[0034] Other features and iterations of the present invention are described in more detail below. Detailed Description

[0035] When introducing the elements of the embodiments described herein, the articles "a", "an", "the" and "said" are intended to mean that there is one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the recited elements.

[0036] One aspect of the present disclosure encompasses an anode comprising: (a) a core substrate, which is preferably a valve metal such as titanium and its alloys, and (b) a coating, which is prepared, applied to and adhered to the core substrate, the core substrate comprising titanium (Ti), ruthenium (Ru) and palladium (Pd), and optionally platinum and / or iridium in embodiments. The coating is prepared to avoid palladium segregation and is single-phase, as in prior art coatings, but rather palladium is well distributed at a fine scale with Ti-Ru-Pd having the same crystal structure, i.e., palladium is well dispersed throughout the coating.

[0037] Embodiments of the present invention relate to an electrode coating on a substrate intended to operate as an anode in an electrochemical process, herein referred to as an anode coating. Specific electrochemical processes in the field of embodiments of the present invention are those in which a chloride salt is present in solution and in which chlorine gas or hypochlorite is the main product (however, other fields, and indeed, the global market can benefit from the present invention). Such processes include chlor-alkali membrane cell units and diaphragm cell unit processes, chlorate production, and the production of hypochlorite from brine to concentrated brine for disinfection purposes. Preferred substrates are valve metals, specifically titanium or its alloys. Anode coatings known in the art have a limited lifespan and contain noble metals. The function of the anode coating is to reduce the voltage required for oxidation by acting as an electrocatalyst and protecting the substrate, thus giving the anode geometric stability. The anode coating for the oxidation of chloride ions provides a lower overpotential for the oxidation of chloride to chlorine gas. An advantage of embodiments of the present invention is that, compared to prior art coatings, it provides a lower overpotential, thereby reducing the power requirements of the process. The anode coating fails when the coating itself wears over time, loses conductivity, loses adhesion to the substrate, or the substrate oxidizes beneath the coating to form a passivation layer. Once the coating fails, the voltage rapidly increases, and continued operation may cause heating or damage to the process equipment. Generally, the lifespan of the coating is proportional to the noble metal loading of the coating and inversely proportional to the square of the current density in the cell unit. Compared to expectations, embodiments of the present invention achieve a longer lifespan for a given noble metal loading of the coating. In prior art inventions, iridium and ruthenium are the mainly used noble metals. Iridium is typically used for anodes that operate for more than 4 years at a current density greater than about 3 kA / m2, but iridium is much more expensive than ruthenium because iridium is rarer and less abundant in the earth's crust. Embodiments of the present invention are characterized in that an anode coating with a current density greater than 3 kA / m can be prepared without using iridium. 2Long-life anode coating. When such an anode coating is applied to a chlorate cell unit, chlorine generated at the anode immediately forms hypochlorite ions in the solution, while hydrogen gas generated at the cathode exits the cell unit. A particular problem with chlorate cell units is the generation of oxygen at the anode through the decomposition of hypochlorite in the catalytic solution or oxygen on the electrocatalytic coating. The coatings of the embodiments of the present invention achieve an unusually low oxygen content in the hydrogen gas from the chlorate cell unit. To achieve this, both the electrochemical oxygen evolution, specifically the oxygen evolution on newly activated electrodes, needs to be reduced, and the solution containing hypochlorite in the electrolytic cell needs to be protected from contamination by impurities, as impurities can catalyze the decomposition of hypochlorite into oxygen.

[0038] In the anode coating of the present invention, the coating formulation contains titanium chloride or titanium oxychloride in a water / alcohol solution containing hydrochloric acid. Salts of ruthenium, palladium, and optionally platinum and iridium are also dissolved in this solution, preferably in the form of chloride salts. Optionally, chloride salts of transition metal elements can be added. What is unique about the embodiments of the present invention is that hydrogen peroxide is optionally added to this solution of mixed salts, and the addition amount increases the oxidation potential of the coating and prevents palladium from being reduced to the metallic state during the coating process. Another optional component in the coating solution is secondary alcohol and / or tertiary alcohol, preferably isopropyl alcohol (2-propanol). In other embodiments, 2-butanol and / or tert-butanol (tert-butyl alcohol) are another optional component of the coating solution. A significant characteristic of the coatings of the embodiments of the present invention is that the molar ratio of titanium to noble metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and the molar ratio of palladium to the sum of other noble metals is between about 0.04 and 0.3. The substrate is prepared by methods known in the art to roughen and etch the surface to remove oxides and embedded grit. Preferably, a light oxide film is then restored by baking at a temperature of 400 to 550 °C for a sufficient time to form an orange to dark blue or light gray color on the titanium surface, thereby making the surface more hydrophilic. The coating is then formed by dip coating, roll coating, brush coating, spray coating, or electrostatic spray coating the substrate with the coating solution. The coating is completely dried, preferably at a temperature below about 110 °C, and more preferably at about 50 °C, and then baked at a temperature of 400 to 550 °C for 10 to 20 minutes. Then, additional layers are applied by repeating the processes of application, drying, and baking. Optionally, after the final coating, a longer baking step with an extended duration, known as post-baking, can be used.

[0039] The noble metals used in the coating are ruthenium, palladium, and optionally platinum and iridium. In the anodes of embodiments of the present invention, ruthenium is the main noble metal used in the coating, and the molar ratio of Pd to the total noble metals is preferably about 0.04 to 0.3, preferably about 0.12. Palladium within the optimal range shows a reduced voltage for chlorine evolution, while reducing oxygen evolution in the coating and simultaneously extending the anode life. At higher levels of palladium, especially in the case of the higher levels of palladium proposed in prior art examples of palladium coatings, the formation of rutile is reduced, and the anatase phase is favorable, thus shortening the coating life. Therefore, in prior art coatings, the extended life is not attributed to the presence of palladium in the coating, as higher levels of the coating promote the formation of anatase.

[0040] Regarding palladium salts and platinum salts, the problem not solved in the prior art is that the palladium salts and platinum salts are more easily reduced to the metallic form than ruthenium salts or iridium salts. Palladium and platinum can be reduced to the metallic form when in close contact with a titanium metal substrate. Another aspect of embodiments of the present invention is to apply the coating after preparing the titanium surface by oxidation (pre-baking) in air at an elevated temperature. When the titanium oxide film is exposed to an acidic coating solution, it may be dissolved, and if dried at an elevated temperature, the bare titanium is exposed to the coating solution. In embodiments of the present invention, when the pre-baking conditions render the titanium substrate blue and the coating is dried at a temperature below 110 °C, a palladium-containing coating can be formed without forming a metallic palladium phase. Alternatively, the preferred coating solution contains some peroxide. The peroxide forms a stable complex with titanium and oxidizes ruthenium to the +4 state in the solution. The reduced palladium or platinum metal in the coating is easily oxidized to soluble chloride salts in the process applicable to embodiments of the present invention, so these metal phases in the coating shorten its life. Therefore, prior art coatings containing palladium or platinum cannot achieve an extended life when a metallic phase is produced.

[0041] In the coatings of embodiments of the present invention, iridium can be used to promote rutile formation and extend the coating life. In coatings where the molar ratio of iridium to ruthenium is greater than about 0.1, it has been established in the prior art that the coating life is primarily a function of the iridium loading of the coating. Unexpectedly, even in coatings where the molar ratio of iridium to ruthenium is greater than 0.1, the presence of palladium in a molar ratio of 0.04 to 0.3 relative to the total amount of ruthenium and iridium significantly increases the anode life, and the coatings of embodiments of the present invention can achieve a coating life more than twice that of prior art coatings with a similar total iridium loading. Additional dopants such as transition metals selected from Fe, Ni, or Co can be added to the coating, as these are known to promote rutile formation and are known in the art to increase the coating conductivity. However, the anode coatings of embodiments of the present invention can be produced in the absence of dopants. It has been found that these dopants do not increase the coating life. Additionally, in applications where chlorate is produced, nickel and cobalt are known to catalyze the decomposition of hypochlorite into oxygen, and thus these dopants are not present in coatings used for chlorate production. In embodiments of the present invention, it has been found that using titanium in the form of titanium oxychloride in combination with hydrogen peroxide increases rutile formation, particularly when a portion of the solvent of the coating is an alcohol and the coating is dried at an air temperature below 110 °C. It has been found that the optimal titanium oxychloride content of the coating solution is 0.25% to 5% titanium, while the optimal peroxide content is a molar ratio of peroxide to titanium of 0.1 to 2.0, and the optimal alcohol content is 5% to 75% of the coating solution. The alcohol is preferably a secondary and / or tertiary alcohol such as 2-butanol and / or tert-butanol to avoid reaction with the peroxide prior to solvent evaporation. It is apparent that these alcohols can be replaced with another water-soluble and volatile organic solvent if compatible with the oxidative nature of the salts.

[0042] The anode coatings of the present invention achieve longer life in accelerated wear tests by a previously unexpected mechanism. One explanation for this behavior is that the complex of peroxide and titanium has surprising stability, survives the drying process, and affects the phase behavior of the oxide coating formed during baking. These oxides transform into the rutile structure at a lower baking temperature compared to coatings without peroxide. It has also been demonstrated that the coating life is significantly extended in proportion to the palladium content of the coating.

[0043] The lifespan of the oxide coating formed with a solution containing hydrogen peroxide appears to be 2 to 8 times that of a similar coating without hydrogen peroxide or palladium added. One theory that can explain this longer lifespan is that the oxygen-to-metal ratio of the coatings in the prior art is close to 2:1, rather than at least 2.5:1 in the present invention. During electrolysis, especially under conditions where oxygen may volatilize, some oxygen may enter the surface of the coating, forming a crystal structure where the excess oxygen is larger in volume than the underlying coating. Thus, in the anode coatings of the prior art, severe mechanical stress accumulates on the coating surface, and this stress can only be relieved by using an oxygen evolution catalyst such as iridium. Otherwise, over time, the surface will gradually lose the coating due to damage. In the present invention, the excess oxygen already exists in the coating, so no stress is generated due to oxygen evolution because the coating surface cannot absorb more oxygen.

[0044] The coatings of the embodiments of the present invention are characterized in that they contain a mixed solid solution of titanium oxide, ruthenium oxide, and palladium oxide, mainly in the form of rutile crystals. The coatings of the prior art contain anatase crystals, and some also contain a large amount of noble metals, which are in the form of separate oxides or metal phases and are not in solid solution with rutile. It has been determined in the study that if these alternating phases exist, their wear rate is faster and they disappear before the rutile phase of the coating.

[0045] In the embodiments of the present invention, it has been found that when the crystal form of the coating contains more than about 80% rutile, the coating wear rate is unexpectedly greatly reduced, and even lower when the coating contains more than about 85% rutile. This is achieved in coatings with specific ratios of titanium to ruthenium and palladium to ruthenium, and the formation of rutile is enhanced when peroxides (and / or potentially other oxidants) are present in the coating. In the coatings of the prior art, it has been widely accepted that the optimal molar ratio of titanium to noble metals is between 1.5 and 2.5. However, experiments have shown that at these molar ratios, titanium is not sufficient to form a solid solution of noble metals in the form of separate phases of rutile and RuO2 or RuO2+IrO2 - single electrode potential (SEP) tests have shown that such separate phases will reduce the single electrode potential of oxygen evolution, thereby increasing the Faradaic inefficiency of oxygen evolution of the coating - which is not desirable in applications where products such as chlorine, hypochlorite, and chlorate are desired. In addition, in the coating formulations containing palladium, when the molar ratio of titanium to noble metals exceeds about 5, a separate titanium anatase phase is formed. The noble metal oxides with less titanium are lost faster than rutile, resulting in faster coating wear. The anatase phase of titanium is also lost faster than rutile, resulting in faster coating wear.

[0046] Palladium Coating Formulation Examples

[0047] Example 1

[0048] A multivariable test (MVT) was designed to evaluate and optimize some of the uncertain aspects of the chlorate coating formulation. The variables considered in this MVT include the ratio of titanium to precious metals (ruthenium, palladium) (Ti ratio), the ratio of palladium to ruthenium (Pd ratio), the pre-bake temperature, and the post-bake temperature. A customized experimental design was created using software, which took into account all the expected non-linear and two-way interactions of the above variables. The runs were carried out by depositing aqueous solutions of precious metal chlorides and titanium oxychloride on titanium metal plates. As shown in Table 1, the following 12 experimental designs were carried out, in which most of the potential interactions and first-order effects are orthogonal but not completely balanced:

[0049] Table 1: MVT test runs

[0050]

[0051] In the experimental design, there are seven unique coating formulations in which the molar ratio of palladium to ruthenium varies among three options: 0.02, 0.06, and 0.1; the molar ratio of titanium to precious metals (the combination of palladium and ruthenium) varies among three options: 2.6, 3.8, and 5; the pre-bake temperature varies between 420 and 490 °C; and the post-bake temperature varies between 490 and 525 °C.

[0052] Results: Formulations B, C, F, and G are examples of embodiments of the present invention, showing favorable wear properties, and the remaining formulations A, D, and E are counterexamples.

[0053] All coating formulations were made with the same concentration of ruthenium metal in solution, 34 g / L, and the dopant concentrations (Ni, Fe, and Co) were all set at 1 g / L. The palladium and titanium contents vary according to the formulation, so the molar ratios of the coating components in the formulation are as follows. Subsequent experiments will reveal that the molar ratio of hydrogen peroxide is important for the success of the coating, so these ratios are included in Table 2 below.

[0054] Table 2. Coating metal composition (in mole percentages) of each chlorate MVT formulation.

[0055] Formulation Ru Ti Pd Fe Ni Co <![CDATA[H 2 O 2 :Ru]]> <![CDATA[H 2 O 2 :Ti]]> A 26.15％ 69.36％ 0.52％ 1.32％ 1.32％ 1.32％ 1.01 0.38 B 19.81％ 76.79％ 0.40％ 1.00％ 1.00％ 1.00％ 1.01 0.26 C 15.95％ 81.32％ 0.32％ 0.80％ 0.81％ 0.80％ 1.01 0.20 D 25.20％ 69.46％ 1.51％ 1.27％ 1.28％ 1.27％ 1.01 0.36 E 24.32％ 69.56％ 2.43％ 1.23％ 1.23％ 1.23％ 1.01 0.35 F 18.41％ 76.96％ 1.84％ 0.93％ 0.93％ 0.93％ 1.01 0.24 G 14.81％ 81.46％ 1.48％ 0.75％ 0.75％ 0.75％ 1.01 0.18

[0056] Each coating formulation has an equal weight concentration of ruthenium, hydrogen peroxide, and the dopant metal salts of Fe, Ni, and Co. Finally, hydrochloric acid is used as a stabilizer in the titanium solution, so the HCl content in each coating solution varies with the change of the Ti ratio, although each formulation contains 4.32 wt% of aqueous HCl added independently.

[0057] According to XRF, different coating formulations were applied to give approximately the same total ruthenium metal loading, and 8 - 13 impregnations were required. Coating solution formulations with higher titanium concentrations had higher viscosities and thus required fewer layers to achieve the desired minimum loading of 500 μg / cm². It was found that the average weight gain per impregnation varied with different coating formulations, mainly varying with the titanium concentration.

[0058] It is understood that the coating composition is determined by methods known in the art, including non - destructive determination by XRF (X - ray fluorescence spectroscopy) or using an electron microscope with EDS (energy - dispersive X - ray spectroscopy).

[0059] A representative preparation and application procedure for plating formulation A is outlined below.

[0060] Surface Preparation

[0061] Preliminary surface treatment experiments on titanium plates determined the optimal preparation, including a combination of light sandblasting with fine sand and oxalic acid etching. For chlorate MVT, replicated samples were coated for each of the 12 design runs, resulting in a total of 24 flat - plate chlorate anode samples. A representative description of the preparation of one of them is detailed. A 4" x 4" x 0.025" titanium plate (grade 2) was sandblasted with 220 alumina sandblasting media. The nozzle size was 6 mm, and the sandblasting pressure was set at 25 psi. To ensure surface uniformity, the spray gun was sprayed six times on each side at an angle of 60 to 90 degrees. The sandblasted plate was then rinsed with DI water and dried, after which it was etched with 20 gpl oxalic acid dihydrate at 80 °C for 1 - 1.5 hours. After etching, the plate was rinsed again with DI water, where it was observed that the gray oxide had been removed, and then pre - baked at 420 °C or 490 °C for 20 minutes, producing a yellow or dark - blue surface, respectively. The samples were then set aside for dip - coating. After pre - baking, a thin TiO₂ protective layer was formed, and under ideal storage conditions, the shelf stability of the titanium substrate could be up to at least one month or even indefinitely.

[0062] Coating Solution Preparation

[0063] Coating solution A specifies a molar ratio of titanium metal to palladium and ruthenium metals of 2.6, and a molar ratio of palladium metal to ruthenium metal of 0.02. Formulation A is used to coat 4 anodes, so the target is 300 g of coating solution. Distilled water (133.61 g) is mixed with aqueous HCl solution (36% assay, 36.01 g), and then cobalt(II) chloride hexahydrate (24.6% Co assay, 1.22 g), iron(III) chloride hexahydrate (20.29% Fe assay, 1.48 g) are added and then nickel(II) chloride hexahydrate (37.18% Ni assay, 0.814 g). Then ruthenium(III) chloride hydrate (40.88% Ru assay, Johnson Matthey, 24.91 g) is added, followed by titanyl chloride solution (14.08% Ti assay, 34.5% HCl, Kronos, 91.01 g). Note that the titanyl chloride solution contains HCl for stabilization, so for Formulation A, the total HCl content is approximately 14.8 wt% (4.32% from the aqueous HCl solution, 10.47% from TiOCl2). The resulting solution is stirred until all solids are visibly dissolved, about 1 hour, but in some cases, the solution is stirred overnight. Then hydrogen peroxide (32% assay, 10.73 g) is added. After adding the peroxide, note to vent the solution bottle regularly. The solution is stirred for at least one hour before adding palladium(II) chloride (59.7% Pd assay, 1.81 g) to ensure the peroxide reacts. As good practice, after adding all ingredients, the coating solution should be stirred for at least one hour before dipping and the coating solution should be stirred between dips.

[0064] Dip Coating Procedure

[0065] To dip coat the anode substrate, select a Pyrex glass container or similar container large enough to lay the sample flat. Ideally, the container is airtight, with a rubber gasket to prevent evaporation. Add a sufficient amount of coating solution so that the substrate can be fully immersed in the solution during dipping, which typically requires a solution depth of at least 0.25 - 0.5".

[0066] Before coating the flat substrate, drill two holes within a 0.5" border along the top and bottom edges. Ideally, these holes should be centered to vertically suspend the sample in a balanced manner. Note that titanium wires are used to suspend the anode sample during impregnation to prevent corrosion and contamination. Apply the first layer by suspending the sample above the impregnation container and then gently dipping the bottom edge of the plate into the solution, after which the plate is laid flat and fully immersed. It is important to avoid contacting the sample; it is best to handle the sample by the titanium wires. Additionally, it is crucial to move slowly and smoothly to avoid generating bubbles. After the sample is immersed, the above actions are reversed and the sample is slowly lifted to the vertical position, after which the bottom edge of the sample is lifted out of the solution. Place the sample above the impregnation container while draining the excess solution and then suspend it on a titanium rack to air dry in a well-ventilated designated area for 20 minutes, although subsequent experiments have shown that 40 - 60 minutes is ideal. Then dry the sample in an oven at 110 °C for 20 minutes, after which the temperature is raised to 490 °C and baked for another 20 minutes. It is advisable to carefully remove the sample with pliers and heat-resistant gloves and cool it to room temperature. Samples coated in formulations with a higher titanium ratio were found to have stripy loose oxide powder along the bottom edge of the titanium plate; this powder was removed by brushing. Then the sample was rotated 180 degrees and suspended by the opposite edge. Repeat the above impregnation, drying, and baking steps until the minimum ruthenium load measured by XRF is 500 μg / cm2. After achieving the target load, dry the sample and then bake it at the specified post-baking temperature of 490 °C or 525 °C for 2 hours.

[0067] The presence of palladium in the base layer prolongs the anode life

[0068] In addition to finding improved anode physical properties by including palladium in the anode coating as described above, the embodiments of the present invention described in subsequent examples surprisingly show that including palladium in the coating, specifically in the base layer adjacent to the anode substrate, provides increased anode life (see the accelerated life test results in Table 3). Conventional wisdom holds that including a life-prolonging material such as palladium in the outermost layer of the coating is most beneficial. However, the coatings and tests of the present invention have determined that including palladium in the innermost or innermost few layers adjacent to the anode substrate is more beneficial than including palladium in the outermost layer, including even cases where there is no palladium in the outermost layer. Thus, the key discovery of the present invention is that even without palladium in the outermost layer, palladium included in the base layer of a multi-layer coating provides improved anode life. Palladium in the innermost layer of the anode coating may interact with the anode substrate in a synergistic effect, causing the substrate to last longer (i.e., have a longer life). For example, when a titanium anode substrate is present together with palladium in the innermost (base layer) coating of a multi-layer coating, a titanium / palladium alloy can gradually form, which alters the properties of the individual titanium substrate to increase the anode service life, including preferably an anode used for gas generation.

[0069] Example 2 (Counterexample)

[0070] Titanium mesh samples were prepared by sandblasting, etching, and pre-baking. Coating solution "Z" was prepared using isopropanol containing 0.24% titanium, 0.22% ruthenium, and 0.24% iridium with 5.1% hydrochloric acid. The materials used to prepare this solution were tetrapropyl titanate (trade name Tyzor TPT), an alcohol solution containing 16.8% titanium; ruthenium(III) chloride hydrate crystals containing 40.9% ruthenium; iridium(IV) chloride dihydrate, an alcohol solution containing 5.1% iridium; anhydrous HCl in isopropanol containing 22.6% HCl; and dry isopropanol for diluting the solution to the desired final concentration. The coating was applied by the following steps:

[0071] The mesh was immersed in the coating solution and then vertically suspended to allow the excess coating to flow over the surface.

[0072] The coating was allowed to dry completely at 50 °C for typically 20 minutes and then baked at 490 °C for 20 minutes.

[0073] Steps 1 - 3 were repeated 9 times with the mesh being periodically flipped vertically. A final bake of 40 minutes was performed after the 9th dip. The sample was labeled ID 13.

[0074] Example 3 (Counterexample)

[0075] Following the steps of Counterexample 2, except that the titanium mesh was prepared by sandblasting and washing. Coating solution "Z*" containing iridium(IV) chloride dihydrate crystals with 52.0% iridium was applied by the same steps for a total of 6 cycles. The sample was labeled ID 6.

[0076] Example 4 (Counterexample)

[0077] Titanium mesh samples with a low palladium content in the base layer were prepared by sandblasting, etching, and pre-baking.

[0078] Base layer coating solution "X" was prepared using isopropanol containing 0.40% titanium, 0.14% ruthenium, 0.16% iridium, and 0.027% palladium with 5.6% hydrochloric acid. The materials used to prepare this solution were tetrapropyl titanate (trade name Tyzor TPT), an alcohol solution containing 16.8% titanium; ruthenium(III) chloride hydrate crystals containing 39.9% ruthenium; hydrogen hexachloroiridate(IV) hydride containing 39.2% iridium; anhydrous HCl in isopropanol containing 22.6% HCl; and dry isopropanol for diluting the solution to the desired final concentration. The base layer coating was applied following the steps outlined in Counterexample 1 for a total of 4 cycles (ID 1) and 6 cycles (ID 22).

[0079] The topcoat solution "Y" was prepared using the same materials, isopropanol containing 0.40% titanium, 0.13% ruthenium, 0.15% iridium, and 0.048% palladium, and 5.6% hydrochloric acid. The topcoat was applied in a total of 3 cycles (ID 1) and 5 cycles (ID 22), followed by a final bake for 2 hours.

[0080] Example 5

[0081] Following the procedure of Comparative Example 4, except that only the topcoat solution "Y" was applied in a total of 7 cycles (ID 5) and 11 cycles (ID 14).

[0082] Example 6

[0083] Following the procedure of Comparative Example 2, except that the Pd-containing coating solution "Y" was applied as the base layer in 5 cycles, and the Pd-free coating solution "Z" was applied as the top layer in 4 cycles, followed by a final bake at 490 °C for 40 minutes. The sample was labeled as ID 34.

[0084] The coated anodes of Examples 1 - 5 were subjected to an accelerated life test; XRF measurements were also performed to determine the average ruthenium loading of each sample. A comparison of the results from this test is summarized in Table 3 below. Generally, in groups of samples with the same coating formulation, the AC life hours increase with an increase in the total noble metal loading (ID 6 vs. ID 13, ID 1 vs. ID 22, ID 5 vs. ID 14). It is noteworthy that even if the top layer does not contain palladium, any amount of palladium in the base layer will still extend the accelerated corrosion life, as in the case of ID 34. The coating life is extended substantially in proportion to the palladium content, specifically in the coating closer to the anode substrate.

[0085] Table 3. Comparative test of the effect of palladium on AC life in anode coatings.

[0086]

[0087]

[0088] Anode life effect resulting from the use of palladium with peroxide for coating preparation

[0089] In additional embodiments, it has been found that in industries where oxygen generation is harmful, formulations with palladium benefit from the presence of both peroxide and pre-baking, which can prevent unwanted oxygen generation while extending the life of the anode. The results of Examples 7 - 11 subsequently demonstrated the advantages of adding palladium to the coatings of gas-generating anodes.

[0090] Example 7 (Comparative)

[0091] Titanium flat samples were prepared by sandblasting, etching, and pre-baking. A coating solution was developed using 4.6 wt% titanium in solution, with a molar ratio of titanium metal to noble metals of 3.6. Distilled water (71 g) was combined with an aqueous HCl solution (23 g, 36% assay), followed by the addition of a titanium oxychloride solution (76 g, 12.06% Ti assay, 16% HCl). Then ruthenium(III) chloride hydrate (13 g, 40.78% Ru assay) was added. The resulting solution was stirred until all solids were visibly dissolved, approximately 1 hour. Then hydrogen peroxide (6 g, 30% assay) was added, followed by isopropyl alcohol (10 g, 99.5% assay).

[0092] The coating was applied to the prepared flats by the techniques previously described herein until a ruthenium target thickness of 500 μg / cm2 was measured. A final bake was performed for 2 hours. The sample was labeled ID 2-1.

[0093] Example 8 (counterexample)

[0094] Following the steps of counterexample 7, except that peroxide was not added. The sample was labeled 2-3.

[0095] Example 9 (counterexample)

[0096] Following the steps of counterexample 7, with the following exceptions. The sample was not pre-baked. After adding ruthenium, the solution was stirred for at least one hour before adding palladium(II) chloride (0.7 g, 59.71% Pd assay) and isopropyl alcohol (10 g, 99.5% assay). The sample was labeled 2-2-2.

[0097] Example 10

[0098] Titanium flat samples were prepared by sandblasting, etching, and pre-baking. A coating solution was developed using 4.6 wt% titanium in solution, with a molar ratio of titanium metal to noble metals (Pd, Ru) of 3.6, and a molar ratio of palladium metal to ruthenium metal of 0.08. Distilled water (71 g) was combined with an aqueous HCl solution (23 g, 36% assay), followed by the addition of a titanium oxychloride solution (76 g, 12.06% Ti assay, 16% HCl). Then ruthenium(III) chloride hydrate (13 g, 40.78% Ru assay) was added. The resulting solution was stirred until all solids were visibly dissolved, approximately 1 hour. Then hydrogen peroxide (6 g, 30% assay) was added. The solution was stirred for at least one hour before adding palladium(II) chloride (0.7 g, 59.71% Pd assay) and isopropyl alcohol (10 g, 99.5% assay) to ensure that the peroxide reacted.

[0099] The coating was applied to the prepared flat plate by the techniques described previously herein until a ruthenium target thickness of 500 μg / cm2 was measured. A final bake was carried out for 2 hours. The sample was labeled 2-0.

[0100] Example 11

[0101] Following the steps of Example 10, except that no peroxide was added. The sample was labeled 2-2.

[0102] Single electrode potential evaluations of chlorine and oxygen overvoltage and accelerated corrosion tests were performed on Examples 7-11. A comparison of the results from these evaluations is summarized in Table 4 below. Notably, the addition of palladium reduced the production of oxygen (by increasing the oxygen overvoltage), extended the lifetime, and decreased the chlorine overvoltage (2-0 vs 2-3). The use of peroxide in the absence of palladium was detrimental to the anode lifetime; in fact, in the absence of palladium, peroxide increased the likelihood of oxygen production (2-1). When palladium was present, the absence of both peroxide and pre-bake was extremely detrimental to the lifetime (2-2-2); in fact, in the absence of peroxide, pre-bake prevented the reduction of palladium, and thus a significant lifetime advantage was still observed (2-2).

[0103] Table 4.

[0104]

[0105] The present invention has been described in detail, and it will be apparent that modifications and variations can be made without departing from the scope of the invention as described herein.

Claims

1. An anode, comprising: a core substrate including titanium or a titanium alloy; and a coating having a molar ratio of titanium to noble metal of 3 to 5, wherein the noble metal includes at least ruthenium and palladium, and the molar ratio of palladium to the sum of other noble metals is 0.02 to 0.

3.

2. The anode according to claim 1, wherein the noble metal of the coating includes iridium.

3. The anode according to claim 2, wherein titanium and the noble metal in the coating are in a crystal structure, and wherein palladium is not in a single phase and is sufficiently dispersed throughout the coating.

4. The anode according to claim 1, wherein titanium and the noble metal in the coating are in a crystal structure, and wherein palladium is not in a single phase and is sufficiently dispersed throughout the coating.

5. The anode according to claim 1, wherein the coating is the innermost layer of a multi-layer coating, and the innermost layer directly contacts the surface of the core substrate.

6. The anode according to claim 5, wherein the multi-layer coating includes an outermost layer without palladium, and the outermost layer does not directly contact the surface of the core substrate.

7. A method for preparing an anode, the method comprising: mixing titanium, ruthenium, and palladium in the coating solution without using primary alcohol; and applying the coating solution to the surface of an anode substrate including titanium or a titanium alloy.

8. The method according to claim 7, further comprising pre-baking the surface of the anode substrate of titanium or a titanium alloy before applying the coating solution.

9. The method according to claim 7, further comprising mixing an oxidant in the coating solution to bring both titanium and ruthenium to a +4 oxidation state.

10. The method according to claim 8, further comprising mixing an oxidant in the coating solution to bring both titanium and ruthenium to a +4 oxidation state.

11. The method according to claim 10, wherein there is no tin in the coating solution.

12. The method according to claim 9, wherein there is no tin in the coating solution.

13. The method according to claim 8, wherein there is no tin in the coating solution.

14. The method according to claim 7, wherein there is no tin in the coating solution.

15. A method for preparing an anode, the method comprising: mixing titanium, ruthenium, and palladium in a coating solution; pre-baking the surface of an anode substrate of titanium or a titanium alloy before applying the coating solution; and applying the coating solution to the anode substrate surface including titanium or a titanium alloy.

16. A method for preparing an anode, the method comprising: mixing titanium, ruthenium, palladium, and a peroxide sufficient to bring both titanium and ruthenium to a +4 oxidation state in a coating solution; and applying the coating solution to the surface of an anode substrate including titanium or a titanium alloy.

17. A method for preparing an anode, the method comprising: mixing titanium, ruthenium, and palladium in a coating solution, wherein titanium is provided in the coating solution in the form of an oxide; and applying the coating solution to the surface of an anode substrate including titanium or a titanium alloy.

18. An anode, comprising: a core substrate including valve metal; and A multi-layer coating, the multi-layer coating including a base layer containing palladium, the base layer directly coating the surface of the core substrate.

19. The anode according to claim 18, wherein the valve metal is titanium or a titanium alloy.

20. The anode according to claim 18, wherein the base layer includes ruthenium and titanium.

21. The anode according to claim 20, wherein the base layer includes iridium.

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