Long-life anode coating

Abstract

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

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 400,668, filed August 24, 2022, which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates generally to electrode coatings on substrates intended to function as anodes in electrochemical processes (hereinafter referred to as anode coatings). [Background technology]

[0003] Many commercial manufacturing processes utilize electrochemistry. For example, the chlor-alkali process involves electrolyzing aqueous sodium chloride or potassium chloride solutions to form valuable commodity raw materials such as chlorine gas, sodium hydroxide (caustic soda) or potassium hydroxide, and hydrogen gas. Electrolysis of water produces hydrogen and oxygen gases. Other electrochemical processes are used to produce a variety of commodity chemicals and intermediates for the chemical and pharmaceutical industries. Current efforts in commercial electrochemical processes relate to reducing energy consumption, lowering manufacturing costs, and improving electrode efficiency and durability.

[0004] Specific electrochemical processes of interest to the field of the invention described herein are those in which chloride salts are present in solution and chlorine gas or hypochlorite is the primary product. Such processes include chlor-alkali membrane cell and diaphragm cell processes, chlorate production, and hypochlorite production in weakly to strongly brackish waters for disinfection purposes. The use and construction of gas-generating electrodes is recognized as presenting different challenges and consequences than the use of other electrodes, such as in batteries.

[0005] Most conductive materials can function as electrodes. Preferably, the materials used to manufacture the electrodes are resistant to corrosion by the electrolyte and / or the products produced. Many other suitable electrode materials lack the ability to efficiently catalyze electron transfer to the electrolyte, necessitating the use of additional power. The greater the amount of additional power consumed, the greater the cost of running the electrochemical process. Coatings can be applied to the electrodes to facilitate electron transfer and reduce the overpotential required for the electrolysis process. The coatings thus help reduce the overall operating voltage and power consumption of the electrolysis process. Details regarding electrode coatings are described in International Application No. PCT / US2020 / 037426, filed June 12, 2020, which is incorporated herein by reference in its entirety.

[0006] Anode coatings known in the art have limited life spans and contain precious metals. The function of the anode coating is to act as an electrocatalyst, protect the substrate, thereby reducing the voltage required for oxidation, and provide geometric stability to the anode. Anode coatings used for the oxidation of chloride ions lower the overpotential for the oxidation of chloride to chlorine.

[0007] Anode coatings fail over time when the coating itself wears away, loses conductivity, loses adhesion to the substrate, or the substrate oxidizes beneath the coating, forming a passivation layer. Failure of the coating results in a sudden increase in voltage, which can overheat and damage process equipment if operation continues. Generally, the life of a coating is proportional to the precious metal content of the coating and inversely proportional to the square of the current density in the cell. Prior art inventions have used iridium and ruthenium as the primary precious metals. Iridium typically provides a current density of about 3 kA / m. 2 Although iridium is used in anodes operating for more than four years at current densities above 1000 kJ / cm², it is significantly more expensive than ruthenium.

[0008] It is desirable to develop anodes with improved durability, reduced overvoltage, and extended operating life. Summary of the Invention

[0009] The preferred substrate for the anode in one embodiment of the present invention is a valve metal, particularly titanium or its alloys. One advantage of embodiments of the present invention is that they provide a coating on the substrate that exhibits a lower overpotential than prior art coatings, reducing the power required for the process.

[0010] Embodiments of the present invention achieve longer than expected lifetimes for a given precious metal content of the coating. A feature of embodiments of the present invention is that they achieve a 3 kA / m 2 The advantage of this exemplary anode coating is that it can produce long-life anode coatings at current densities exceeding 100 kJ / s. When this exemplary anode coating is applied to a chlorate cell, the anode produces chlorine, which immediately forms hypochlorite ions in solution, while the cathode produces hydrogen gas, which is exhausted from the cell. A particular problem with chlorate cells is the generation of oxygen at the anode through electrocatalysis of oxygen on the coating or catalytic decomposition of hypochlorite in solution. The coatings of the invention embodiments described herein achieve very low oxygen content in the hydrogen from chlorate cells. This is achieved by reducing electrochemical oxygen evolution, especially at newly activated electrodes, and by avoiding contamination of the hypochlorite-containing solution in the electrolytic cell with impurities that catalyze the decomposition of hypochlorite to oxygen.

[0011] In the anode coating of embodiments of the present invention, a coating formulation containing titanium chloride or titanium oxychloride is prepared in an aqueous / alcoholic solution containing hydrochloric acid. In certain embodiments, titanium alkoxides can be used in combination with pre-baking, secondary / tertiary alcohols, and / or oxidizing agents to achieve the same results as titanium oxychloride or titanium chloride. Salts of ruthenium, palladium, and optionally platinum and iridium are also dissolved in this solution, preferably as chloride salts. Optionally, chloride salts of transition metal elements can be added.

[0012] In preferred embodiments of the present invention, various factors, including palladium in the preparation of the coating of the present invention, individually or in combination of two or more factors, surprisingly contribute to the extended life of an anode having the anode coating of the present invention.

[0013] 1. Avoid the use of primary alcohols when preparing coating solutions.

[0014] 2. Pre-oxidation or pre-baking of the titanium surface of titanium or titanium alloy substrate.

[0015] 3. The presence of peroxide to produce both titanium and ruthenium in the +4 oxidation state when preparing the coating solution. In other embodiments, other oxidizing agents, including nitric acid, chromates, halogens, chlorine dioxide, chloric acid, and / or ozone, can 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. Thus, although reference is made herein to the use of peroxide, it will be understood that other oxidizing agents can also be used.

[0016] 4. Preferably, titanium is used as a titanium oxychloride solution. However, in embodiments, titanium alkoxides can be used in combination with pre-calcination, secondary / tertiary alcohols, and / or oxidizing agents to achieve the same results as titanium oxychloride or titanium chloride.

[0017] 5. Palladium and tin are not used together to avoid 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 a metal alloy and form a phase separate from the rutile phase of the coating.

[0018] In a believed unique embodiment, hydrogen peroxide is optionally added to the mixed salt anode coating solution in an amount to increase the oxidation potential of the coating and prevent the reduction of palladium to the metallic state during the coating procedure. Another optional component in the coating solution is a secondary alcohol and / or a tertiary alcohol, preferably isopropanol (2-propanol). In another embodiment, 2-butanol and / or tertiary butanol (tert-butyl alcohol) is another optional component of the coating solution. A distinguishing feature of the coating of embodiments of the present invention is that the molar ratio of titanium to precious metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and the molar ratio of palladium to the sum of the other precious metals is between about 0.04 and 0.3.

[0019] Importantly, in embodiments of the present invention, the anode coating comprises titanium, ruthenium and palladium, with the palladium being finely dispersed and all three metals being in the same crystal structure, avoiding the single phase segregated palladium of prior art coatings containing palladium.

[0020] To produce anode coatings for high current density applications with reduced iridium loading, it is necessary to increase the coating thickness and reduce the wear rate. The coating is applied in multiple layers with drying and firing steps between each layer. An advantage of embodiments of the present invention is that a low wear rate coating containing primarily ruthenium as the precious metal can be achieved with fewer coating layers.

[0021] Coating adhesion is typically measured by a tape test, in which transparent tape is applied to the coated anode surface and quickly removed to observe whether the coating has been removed. Tape tests are typically evaluated visually, but can also be quantitatively evaluated using X-ray fluorescence measurements of the tape. Quantitative tape test results are best quantified based on the percentage of the total coating removed. An objective of embodiments of the present invention is to achieve coatings with quantitative tape test results in which the tape removes less than 5%, preferably less than 2%, and more preferably less than 1% of the coating.

[0022] Other prior art anode coating inventions have shown that coatings containing palladium or platinum reduce the voltage required to oxidize chloride ions to chlorine gas or hypochlorite, reducing power requirements and reducing undesirable oxygen generation. However, in these prior art coatings, the ratio of palladium or platinum to low-cost ruthenium components is greater than 3:10, resulting in the loss of palladium and platinum from the coating more rapidly than ruthenium. Loss of Pd or Pt from the coating results in an increase in voltage. An advantage of the present invention is that very small amounts of Pd or Pt are effective additives to the coating, maintaining a lower voltage for a longer period of time.

[0023] Anode substrates in embodiments of the present invention are prepared by methods known in the art, the surface roughened, etched to remove oxides and embedded grit, and then preferably baked at temperatures between 400 and 550°C for a time sufficient to form an orange to dark blue or light gray color on the titanium surface, thereby restoring the light oxide layer and making the substrate surface more hydrophilic.

[0024] The coating solution is then applied to the substrate by dip coating, roller coating, brushing, spray coating, or electrostatic spray coating to form a coating on the substrate. The coating is thoroughly dried, preferably at a temperature below about 110°C, preferably about 50°C, and then baked at a temperature of 400°C to 550°C for 10 to 20 minutes. Additional coats are then applied by repeating the application, drying, and baking process.

[0025] Optionally, a longer baking step, called a post-bake, can be performed after the final coat.

[0026] The precious metals used in the coatings of embodiments of the present invention are ruthenium, palladium, and optionally platinum and iridium. In anodes of embodiments of the present invention, ruthenium is the primary precious metal used in the coating, with a molar ratio of Pd to total precious metals preferably ranging from about 0.04 to 0.3, preferably about 0.12. Palladium in the optimal range has been shown to reduce oxygen evolution from the coating while lowering the chlorine evolution voltage and simultaneously extending anode life. Higher palladium content, especially at higher levels as suggested by examples of prior art palladium coatings, reduces rutile formation and favors the anatase phase, shortening the coating's life. Therefore, in prior art coatings, the extended life was not due to the presence of palladium in the coating, but rather to the promotion of anatase formation at higher concentrations.

[0027] An unresolved problem in the prior art regarding palladium and platinum salts is that they are more easily reduced to their metallic form than ruthenium or iridium salts. Palladium and platinum are reduced to their metallic form upon intimate contact with a titanium metal substrate. Another aspect of an embodiment of the present invention is to prepare a titanium surface by oxidizing it in air at high temperatures (pre-baking), avoiding the use of primary alcohols before applying the coating. Titanium oxide films can dissolve when exposed to acidic coating solutions, and drying at high temperatures can expose bare titanium to the coating solution. Pre-baking (pre-oxidizing) the substrate surface first allows the coating to effectively wet the surface, resulting in good adhesion between the coating and the substrate surface.

[0028] In embodiments of the present invention, pre-baking conditions impart a blue appearance to the titanium substrate, and when the coating is dried at temperatures below 110°C, preferably about 50°C, a palladium-containing coating can be formed without forming a metallic palladium phase. Alternatively, preferred coating solutions of embodiments of the present invention contain a portion of peroxide. The peroxide forms a stable complex with titanium and oxidizes ruthenium to the +4 state in solution. Because reduced palladium or platinum metal in the coating is easily oxidized to soluble chloride salts in the process in which embodiments of the present invention are applied, these metallic phases in the coating shorten the coating's lifespan. Therefore, the formation of metallic phases in prior art coatings containing palladium or platinum did not extend the lifespan of the coating.

[0029] In further embodiments of the present invention, the anode coating may include iridium to promote rutile formation and extend the coating's lifespan. It has been established in the prior art that for coatings with an iridium to ruthenium molar ratio greater than about 0.1, coating lifespan is primarily a function of the iridium content in the coating. Surprisingly, even for coatings with an iridium to ruthenium molar ratio greater than 0.1, the presence of palladium in a molar ratio of 0.04 to 0.3 relative to the sum of ruthenium and iridium significantly extends anode lifespan, allowing coatings according to embodiments of the present invention described herein to achieve coating lifespans that are more than twice that of prior art coatings with similar total iridium content.

[0030] In embodiments of the present invention, an additional dopant selected from transition metals such as Fe, Ni, and Co can be added to the coating, as these metals are known to promote rutile formation and are known in the art to increase the electrical conductivity of the coating. However, the anode coating of embodiments of the present invention can be produced without the presence of a dopant. It has been found that adding a dopant improves electrical conductivity but does not extend the coating's lifespan. Furthermore, in chlorate production applications, nickel and cobalt are known to catalyze the decomposition of hypochlorite to oxygen, so these dopants are not present in coatings used for chlorate production.

[0031] In embodiments of the disclosed invention, the use of titanium in the form of titanium oxychloride in combination with hydrogen peroxide has been found to increase rutile formation, particularly when part of the coating solvent is alcohol and the coating is thoroughly dried at air temperatures below 110°C. The optimum titanium oxide content of the coating solution has been found to be between 0.25% and 5% by weight of titanium, with the optimum peroxide content being a peroxide to titanium molar ratio of 0.1 to 2.0, and the optimum alcohol content being between 5% and 75% by weight of the coating solution. To avoid reaction with the peroxide before solvent evaporation, the alcohol is preferably secondary and / or tertiary, preferably isopropanol (2-propanol). In other embodiments, 2-butanol and / or tertiary butanol (tert-butyl alcohol) are additional optional components of the coating solution. In embodiments, other water-soluble, volatile organic solvents may be used in place of the alcohols, provided they are compatible with the oxidizing properties of the salt.

[0032] This invention has the potential to achieve breakthrough performance and reduce the manufacturing costs of coatings used in all chlor-alkali processes and chlorate production. Another major application area is hypochlorite generators used to disinfect swimming pools, municipal water treatment, wastewater treatment, or bilge water. Potential applications are in the global market.

[0033] Other features and iterations of the present invention are described in more detail below. DETAILED DESCRIPTION OF THE INVENTION

[0034] When introducing elements of the embodiments described herein, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "including," "comprising," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0035] One aspect of the present disclosure encompasses an anode comprising (a) a core substrate preferably comprised of a valve metal such as titanium and its alloys, and (b) a coating fabricated, applied, and adhered to the core substrate, the coating comprising titanium (Ti), ruthenium (Ru), and palladium (Pd), and in some embodiments, optionally platinum and / or iridium. The coating is fabricated to avoid the palladium separating into a single phase as in prior art coatings, and instead the palladium is well dispersed on a fine scale with the same crystalline structure as Ti-Ru-Pd, i.e., the palladium is well dispersed throughout the coating.

[0036] Embodiments of the present invention relate to electrode coatings (hereinafter referred to as anode coatings) on substrates intended to function as anodes in electrochemical processes. Specific electrochemical processes contemplated for embodiments of the present invention are those in which chloride salts are present in solution and chlorine gas or hypochlorite is the primary product (although other fields, and indeed global markets, can benefit from the present invention). Such processes include chlor-alkali membrane cell and diaphragm cell processes, chlorate production, and hypochlorite production in weakly to strongly brackish water for disinfection purposes. Preferred substrates are valve metals, particularly titanium or its alloys. Anode coatings known in the art have limited life spans and contain precious metals. The function of the anode coating is to act as an electrocatalyst, protect the substrate, thereby reducing the voltage required for oxidation, and geometrically stabilize the anode. Anode coatings used for the oxidation of chloride ions lower the overpotential for the oxidation of chloride to chlorine. One advantage of embodiments of the present invention is that they provide a lower overpotential than prior art coatings, reducing the power required for the process. Anode coatings fail over time when the coating itself wears away, loses conductivity, loses adhesion to the substrate, or the substrate oxidizes and forms a passivation layer beneath the coating. Coating failure can result in a sudden increase in voltage, potentially causing overheating and damage to process equipment during continued operation. Generally, coating life is proportional to the precious metal content of the coating and inversely proportional to the square of the current density in the cell. Embodiments of the present invention achieve longer-than-expected life for a given precious metal content of the coating. Prior art inventions use iridium and ruthenium as the primary precious metals. Iridium is typically used in anodes operating for more than four years at current densities above approximately 3 kA / m². However, iridium is rarer and less abundant in the Earth's crust than ruthenium, making it significantly more expensive. A feature of embodiments of the present invention is the ability to achieve a current density of 3 kA / m² without the use of iridium. 2The advantage of this invention is that it is possible to produce a long-life anode coating at current densities exceeding 1000 kJ / s. When this anode coating is applied to a chlorate cell, the anode produces chlorine, which immediately forms hypochlorite ions in solution, while the cathode produces hydrogen gas, which is exhausted from the cell. A particular problem with chlorate cells is the generation of oxygen at the anode through the electrocatalytic action of oxygen on the coating or the catalytic decomposition of hypochlorite in solution. The coating of embodiments of the present invention achieves very low oxygen content in the hydrogen from chlorate cells. This is achieved by reducing electrochemical oxygen evolution, especially at newly activated electrodes, and by avoiding contamination of the hypochlorite-containing solution in the electrolytic cell with impurities that catalyze the decomposition of hypochlorite to oxygen.

[0037] In the anode coating of the present invention, a coating formulation containing titanium chloride or titanium oxychloride is prepared in an aqueous / alcoholic solution containing hydrochloric acid. Salts of ruthenium, palladium, and optionally platinum and iridium are also dissolved in this solution, preferably as chloride salts. Optionally, chloride salts of transition metal elements can be added. In a specific embodiment of the present invention, hydrogen peroxide is optionally added to this solution of mixed salts in an amount to increase the oxidation potential of the coating and prevent the reduction of palladium to its metallic state during the coating procedure. Another optional component in the coating solution is a secondary alcohol and / or a tertiary alcohol, preferably isopropanol (2-propanol). In another embodiment, 2-butanol and / or tertiary butanol (tert-butyl alcohol) are another optional component of the coating solution. A distinctive feature of the coating of embodiments of the present invention is that the molar ratio of titanium to precious metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and the molar ratio of palladium to the sum of the other precious metals is between about 0.04 and 0.3. The substrate is prepared by methods known in the art, and the surface is roughened and etched to remove oxides and embedded grit. Preferably, the titanium surface is then baked at 400-550°C for a time sufficient to form an orange to dark blue or light gray color on the surface, restoring the light oxide layer and making the surface more hydrophilic. The coating solution is then applied to the substrate by dip coating, roller coating, brushing, spray coating, or electrostatic spray coating. The coating is thoroughly dried, preferably at temperatures below about 110°C, more preferably about 50°C, and then baked at 400-550°C for 10-20 minutes. Additional coats are then applied by repeating the application, drying, and baking process. Optionally, a longer baking step, called a post-bake, can be performed after the final coat.

[0038] The precious 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 primary precious metal used in the coating, with the molar ratio of Pd to total precious metals preferably being about 0.04-0.3, preferably about 0.12. Palladium in the optimal range has been shown to reduce oxygen evolution from the coating while lowering the chlorine evolution voltage and simultaneously extending anode life. Higher palladium content, especially at higher levels as suggested by examples of prior art palladium coatings, reduces rutile formation and favors the anatase phase, shortening the coating's life. Therefore, in prior art coatings, the extended life was not due to the presence of palladium in the coating, but rather to the promotion of anatase formation at higher concentrations.

[0039] An unresolved problem in the prior art regarding palladium and platinum salts is that they are more easily reduced to their metallic form than ruthenium or iridium salts. Palladium and platinum are reduced to their metallic form upon intimate contact with a titanium metal substrate. Another aspect of embodiments of the present invention is that the coating is applied after the titanium surface has been prepared by oxidizing it in air at high temperatures (pre-baking). The titanium oxide film can dissolve when exposed to an acidic coating solution, and drying at high temperatures can expose bare titanium to the coating solution. In embodiments of the present invention, the pre-baking conditions impart a blue appearance to the titanium substrate, and when the coating is dried at temperatures below 110°C, a coating containing palladium can be formed without forming a metallic palladium phase. Alternatively, preferred coating solutions contain some peroxide. The peroxide forms a stable complex with titanium and oxidizes ruthenium to the +4 state in solution. Because reduced palladium or platinum metal in the coating is easily oxidized to soluble chloride salts in processes where embodiments of the present invention are applied, these metallic phases in the coating shorten the coating's lifespan. Thus, prior art coatings containing palladium or platinum did not provide extended life once the metallic phase was formed.

[0040] In coatings according to embodiments of the present invention, iridium can be used to promote rutile formation and extend the coating's lifespan. It has been established in the prior art that for coatings with an iridium-to-ruthenium molar ratio greater than about 0.1, coating lifespan is primarily a function of the iridium content in the coating. Surprisingly, even for coatings with an iridium-to-ruthenium molar ratio greater than 0.1, the presence of palladium at a molar ratio of 0.04 to 0.3 relative to the sum of ruthenium and iridium significantly extends anode lifespan, allowing coatings according to embodiments of the present invention to achieve coating lifespans more than double those of prior art coatings with similar total iridium content. Additional dopants selected from transition metals such as Fe, Ni, and Co can be added to the coating, as these are known to promote rutile formation and are known in the art to increase the electrical conductivity of the coating. However, anode coatings according to embodiments of the present invention can be fabricated without the presence of dopants. These dopants have not been found to extend the coating's lifespan. Furthermore, in chlorate production applications, nickel and cobalt are known to catalyze the decomposition of hypochlorite to oxygen, so these dopants are absent from the coating used for chlorate production. In embodiments of the present invention, the use of titanium in the form of titanium oxychloride in combination with hydrogen peroxide has been found to increase rutile formation, particularly when part of the coating solvent is alcohol and the coating is thoroughly dried at air temperatures below 110°C. The optimum titanium oxide content of the coating solution has been found to be between 0.25% and 5% titanium, the optimum peroxide content is a peroxide to titanium molar ratio of 0.1 to 2.0, and the optimum alcohol content is between 5% and 75% of the coating solution. The alcohol is preferably a secondary and / or tertiary alcohol, such as 2-butanol and / or tertiary butanol, to avoid reaction with the peroxide prior to solvent evaporation. It will be apparent that other water-soluble and volatile organic solvents can be used in place of these alcohols, provided they are compatible with the oxidizing properties of the salt.

[0041] The anode coatings of the present invention achieve longer lifetimes in accelerated wear tests through a previously unsuspected mechanism. One explanation for this behavior is that the peroxide complexes with titanium are surprisingly stable, survive the drying process, and affect the phase behavior of the oxide coatings formed during calcination. These oxides are converted to the rutile structure at lower calcination temperatures than peroxide-free coatings. It has also been demonstrated that the lifetime of the coatings increases significantly in proportion to the palladium content in the coating.

[0042] Oxide coatings formed from solutions containing hydrogen peroxide appear to achieve a lifespan 2 to 8 times longer than similar coatings without added hydrogen peroxide or palladium. One theory that could explain this long lifespan is that the oxygen-to-metal ratio in prior art coatings is approximately 2:1, rather than at least 2.5:1 as in the present invention. During electrolysis, particularly under conditions where oxygen generation is possible, some oxygen can penetrate the surface of the coating, forming crystalline structures of excess oxygen that are larger in volume than the underlying coating. Therefore, prior art anode coatings build up severe mechanical stresses at the coating's surface, which can only be relieved by the use of oxygen-evolving catalysts such as iridium, or the surface deteriorates over time, resulting in gradual loss of the coating. In the present invention, excess oxygen is already present within the coating, preventing further absorption of oxygen at the coating's surface, so stress due to oxygen generation does not occur.

[0043] Coatings according to embodiments of the present invention are characterized by the inclusion of titanium oxide, ruthenium oxide, and palladium oxide in a mixed solid solution, primarily in the form of rutile crystals. Prior art coatings contain anatase crystals and also contain significant amounts of precious metal in other oxide or metal phases, some of which are not in solid solution with the rutile. Research has shown that when these alternating phases are present, they wear faster and are lost before the rutile phase of the coating.

[0044] In embodiments of the present invention, we have found that the wear rate of coatings is unexpectedly significantly reduced when the coating's crystalline morphology contains approximately 80% or more rutile, and even more so when the coating contains approximately 85% or more rutile. This is achieved with coatings containing specific ratios of titanium to ruthenium and palladium to ruthenium, where the presence of peroxides (and / or potentially other oxidizers) within the coating enhances rutile formation. While it is widely accepted that the optimum titanium to noble metal molar ratio for prior art coatings is between 1.5 and 2.5, experiments have shown that at these molar ratios, there is insufficient titanium to dissolve the noble metal in the rutile phase, resulting in the formation of a separate phase of RuO2 or RuO2 + IrO2. Single electrode potential (SEP) testing has shown that this separate phase reduces the single electrode potential for oxygen evolution and increases the faradaic inefficiency of the coating's oxygen evolution. This is an undesirable characteristic in applications where chlorine, hypochlorite, or chlorate are desired products. Also, in coating formulations containing palladium, when the titanium to precious metal molar ratio exceeds about 5, a separate anatase phase of titanium is formed. This precious metal oxide phase, which contains little titanium, is lost more rapidly than rutile, causing the coating to wear more quickly. The anatase phase of titanium is lost more rapidly than the rutile phase, causing the coating to wear more quickly. [Example]

[0045] Example of a palladium coating recipe Example 1 A multivariate test (MVT) was designed to evaluate and optimize some of the uncertain aspects of the chlorate coating recipe. The variables considered in this MVT included the titanium to precious metal (ruthenium, palladium) ratio (Ti ratio), the palladium to ruthenium ratio (Pd ratio), pre-bake temperature, and post-bake temperature. A custom experimental design was created using software that accounted for all expected nonlinear and two-way interactions among the above variables. Experiments were performed by depositing aqueous solutions of precious metal chlorides and titanium oxychloride onto titanium metal flat plates. The following experimental design, consisting of 12 runs, was implemented. As shown in Table 1, most of the potential interactions and first-order effects are orthogonal, although not perfectly balanced.

[0046] [Table 1]

[0047] In the experimental design, there are seven unique coating recipes, where the molar ratio of palladium to ruthenium varies in three options: 0.02, 0.06, and 0.1, the molar ratio of titanium to precious metal (combination of palladium and ruthenium) varies in three options: 2.6, 3.8, and 5, the pre-bake temperature varies from 420 to 490 °C, and the post-bake temperature varies from 490 to 525 °C.

[0048] Results: Recipes B, C, F and G are examples of embodiments of the present invention that exhibit advantageous wear characteristics, while the remaining recipes A, D and E are counterexamples.

[0049] All coating recipes were made with the same concentration of ruthenium metal in solution (34 grams per liter), and the dopant concentrations (Ni, Fe, Co) were all set at 1 gram / liter. The palladium and titanium content varied between recipes, resulting in the following molar ratios of the coating components for the recipes: Subsequent experiments revealed that the hydrogen peroxide molar ratio was critical to the success of the coating, so these ratios are included in Table 2 below.

[0050] [Table 2]

[0051] Each coating recipe had equal weight concentrations of ruthenium, hydrogen peroxide, and dopant metal salts of Fe, Ni, and Co. Finally, because hydrochloric acid is used as a stabilizer for the titanium solution, each recipe contained 4.32 wt.% aqueous HCl added independently, but the HCl content per coating solution varied depending on the titanium ratio.

[0052] Different coating recipes were applied to give approximately the same total ruthenium metal content according to XRF, requiring between 8 and 13 dips. Coating solution recipes with higher titanium concentrations have higher viscosities, which means fewer coatings are required to achieve the required minimum content of 500 μg / cm². The average weight gain per dip was found to vary between coating recipes, primarily due to differences in titanium concentration.

[0053] It will be appreciated that the coating composition may be determined by methods known in the art, such as non-destructive testing by XRF (X-ray fluorescence spectroscopy) or electron microscopy using EDS (energy dispersive X-ray spectroscopy).

[0054] A typical procedure for preparing and applying Coating Recipe A is outlined below.

[0055] surface manufacturing Initial surface preparation experiments on titanium plates identified an optimal preparation involving a combination of light grit blasting with fine grit and oxalic acid etching. For the chlorate MVT, duplicate samples were coated for each of the 12 planned runs, resulting in a total of 24 flat-plate chlorate anode samples. A representative procedure for fabricating one of these is described in detail below. A 4" x 4" x 0.025" titanium plate (Grade 2) was grit blasted with 220 aluminum oxide blasting media. The blast nozzle diameter was 6 mm, and the blast pressure was set at 25 psi. To ensure a uniform surface, the blast gun was held at a 60-90 degree angle, and six passes were completed per side. The blasted plate was then rinsed with deionized water, dried, and then etched with 20 gpl of oxalic acid dihydrate at 80°C for 1-1.5 hours. After etching, the plate was rinsed again with DI water to ensure the gray oxide had been removed. It was then pre-baked at 420°C or 490°C for 20 minutes, resulting in a yellow or deep blue surface, respectively. The sample was then set aside for dip coating. Once pre-baked to form a thin protective layer of TiO, the titanium substrate remains shelf-stable for at least one month, and can remain shelf-stable indefinitely under ideal storage conditions.

[0056] Preparation of coating solution Coating solution A was specified with a combined molar ratio of titanium metal to palladium and ruthenium metals of 2.6 and a combined molar ratio of palladium metal to ruthenium metal of 0.02. Recipe A was used to coat four anodes, so 300 g of coating solution was targeted. Distilled water (133.61 g) was combined with aqueous HCl (36% content, 36.01 g), followed by the addition of cobalt(II) chloride hexahydrate (24.6% Co content, 1.22 g), iron(III) chloride hexahydrate (20.29% Fe content, 1.48 g), and then nickel(II) chloride hexahydrate (37.18% Ni content, 0.814 g). Next, ruthenium(III) chloride hydrate (40.88% Ru content, Johnson Matthey, 24.91 g) was added, followed by titanium oxychloride solution (14.08% Ti content, 34.5% HCl, Cronos, 91.01 g). Because the titanium oxychloride solution contains HCl for stabilization, the total HCl content for Recipe A is approximately 14.8 wt% (4.32% from aqueous HCl, 10.47% from TiOCl2). The resulting solution was stirred for approximately 1 hour until all solids were visibly dissolved; in some cases, the solution was stirred overnight. Next, hydrogen peroxide (32% content, 10.73 g) was added. Care was taken to periodically vent the solution bottle after adding the peroxide. The solution was stirred for at least 1 hour before adding palladium(II) chloride (59.7% Pd content, 1.81 g) to ensure the peroxide reacted. As a best practice, the coating solution should be stirred after all ingredients have been added for at least one hour before dipping, and also between dippings.

[0057] Dip Coating Procedure To dip-coat the anode substrate, select a Pyrex glass container or similar large enough to lay the sample flat. Ideally, this container will be airtight with a rubber gasket to prevent evaporation. Sufficient coating solution will be added to allow the substrate to be completely submerged in the solution upon immersion; typically, a solution depth of at least 0.25-0.5 inches is required.

[0058] Before coating the flat substrate, two holes were drilled within a 0.5-inch margin along the top and bottom edges. Ideally, these holes would be centered to allow the sample to be balanced and suspended vertically. Care was taken to use a titanium wire to suspend the anode sample during immersion to prevent corrosion and contamination. The first coating was applied by suspending the sample over the immersion container and gently dipping the bottom edge of the plate into the solution, then laying the plate flat and completely immersed. It is important not to touch the sample; it is best manipulated via the titanium wire. Furthermore, slow, smooth movements are essential to avoid the creation of air bubbles. After immersion, the above action was reversed, slowly lifting the sample to a vertical position and lifting its bottom edge out of the solution. The sample was then placed over the immersion container while excess solution drained. It was then hung on a titanium rack and air-dried in a designated, well-ventilated area for 20 minutes; however, subsequent experiments indicated that 40 to 60 minutes was ideal. The samples were then dried in an oven at 110°C for 20 minutes, then the temperature was increased to 490°C and fired for another 20 minutes. The samples were carefully removed, preferably using tongs and heat-resistant gloves, and allowed to cool to room temperature. Samples coated with the higher titanium recipes exhibited a band of oxide powder along the bottom edge of the titanium plate; this powder was removed by brushing. The samples were then rotated 180 degrees and suspended on the opposite edge. The above soaking, drying, and firing steps were repeated until a minimum ruthenium content of 500 μg / cm2 was achieved, as measured by XRF. Once the target content was achieved, the samples were dried and fired at the specified post-bake temperature (490°C or 525°C) for two hours. The presence of palladium in the base layer(s) extends anode life

[0059] In addition to the discovery that the inclusion of palladium in the improved anode coating described above improves the physical properties of the anode, embodiments of the present invention described in the subsequent examples surprisingly demonstrate that the inclusion of palladium in the base layer of the coating, particularly the layer adjacent to the anode substrate, extends anode life (see accelerated life test results in Table 3). Conventional thinking has held that including a life-enhancing material, such as palladium, in the outermost layer of the coating is most effective. However, the coatings and testing of the present invention have determined that including palladium in the innermost layer or in several layers adjacent to the anode substrate is more beneficial than including palladium in the outermost layer, even when the outermost layer does not contain palladium. Thus, a key discovery of the present invention is that the inclusion of palladium in the base layer of a multilayer coating extends anode life even when palladium is not present in the outermost layer. Palladium in the innermost layer of the anode coating may interact with the anode substrate, providing a synergistic effect that extends (i.e., extends) substrate life. For example, when a titanium anode substrate is present with palladium in the innermost (base) coating layer of a multi-layer coating, a titanium / palladium alloy is produced that alters the properties of the titanium substrate alone, extending the anode's useful life and making it a preferred choice for use in gas generation anodes.

[0060] Example 2 (Counterexample) Titanium mesh samples were prepared by grit blasting, etching, and pre-baking. Coating solution "Z" was prepared using 0.24% titanium, 0.22% ruthenium, and 0.24% iridium in isopropanol containing 5.1% hydrochloric acid. The materials used to prepare this solution were tetrapropyl titanate (brand name: Tyzor TPT), an alcoholic solution containing 16.8% titanium; ruthenium(III) chloride hydrate crystals containing 40.9% ruthenium; iridium(IV) chloride dihydrate, an alcoholic solution containing 5.1% iridium; anhydrous HCl in isopropanol containing 22.6% HCl; and dry isopropanol to dilute the solution to the desired final concentration. The coating was applied in the following steps:

[0061] The mesh was immersed in the coating solution and then suspended vertically to allow excess coating to run to the surface.

[0062] The coating was typically allowed to dry completely at 50°C for 20 minutes and then baked at 490°C for 20 minutes.

[0063] Steps 1–3 were repeated for nine cycles, with the mesh periodically flipped vertically. A final firing after the ninth dip was performed for 40 minutes. The sample was labeled ID 13.

[0064] Example 3 (Counterexample) The steps of Counter Example 2 were followed, except that the titanium mesh was prepared by grit blasting and cleaning. The coating solution "Z" was prepared using iridium (IV) chloride dihydrate crystals containing 52.0% iridium. * A total of six cycles of " were applied in the same step. The sample was labeled ID 6.

[0065] Example 4 (Counterexample) Titanium mesh samples with low palladium in the base layer were prepared by grit blasting, etching and pre-firing.

[0066] Base layer coating solution "X" was prepared using 0.40% titanium, 0.14% ruthenium, 0.16% iridium, and 0.027% palladium in isopropanol with 5.6% hydrochloric acid. The materials used to prepare this solution were tetrapropyl titanate (brand name: Tyzor TPT), an alcoholic solution containing 16.8% titanium, ruthenium(III) chloride hydrate crystals containing 39.9% ruthenium, iridium(IV) hexachloride hydride containing 39.2% iridium, anhydrous HCl in isopropanol containing 22.6% HCl, and dry isopropanol to dilute the solution to the desired final concentration. The base layer coating was applied using the steps outlined in Counterexample 1 for a total of four cycles (ID 1) and six cycles (ID 22).

[0067] Using the same materials, top layer coating solution "Y" was prepared using 0.40% titanium, 0.13% ruthenium, 0.15% iridium, and 0.048% palladium in isopropanol with 5.6% hydrochloric acid. The top layer coating was applied for a total of 3 cycles (ID 1) and 5 cycles (ID 22), followed by a final bake of 2 hours.

[0068] Example 5 The steps of Counter Example 4 were followed, except that only the top coating solution "Y" was applied for a total of 7 cycles (ID 5) and 11 cycles (ID 14).

[0069] Example 6 The steps of counter example 2 were followed, except that Pd-containing coating solution "Y" was applied as the base layer in five cycles, and Pd-free coating solution "Z" was applied as the top layer in four cycles, followed by a final bake at 490°C for 40 minutes. The sample was labeled ID 34.

[0070] Accelerated life tests were performed on the coated anodes of Examples 1-5, and XRF measurements were also performed to determine the average ruthenium content per sample. A comparison of the results of this test is summarized in Table 3 below. Generally, within a sample set of the same coating formulation, increasing the total precious metal content increases the AC life time (ID 6 vs. ID 13, ID 1 vs. ID 22, ID 5 vs. ID 14). In particular, even when the top layer does not contain palladium, as in the case of ID 34, any amount of palladium in the base layer extends the accelerated corrosion life. Coating life is significantly extended in proportion to the palladium content, especially in the coating layers closest to the anode substrate.

[0071] [Table 3]

[0072] The effect of palladium in combination with peroxides in the preparation of coatings on anode life. In a further embodiment, in industries where oxygen generation is detrimental, formulations with palladium have been found to benefit from the presence of both peroxide and pre-baking, extending anode life while preventing unwanted oxygen generation. The results of Examples 7-11 demonstrate this benefit of adding palladium to the coating of gas generating anodes.

[0073] Example 7 (Counterexample) Titanium plate samples were prepared by grit blasting, etching, and pre-baking. A coating solution was developed using 4.6% titanium by weight in solution and a molar ratio of 3.6 noble metal to titanium metal. Distilled water (71 g) was combined with aqueous HCl (23 g, 36% content), followed by titanium oxychloride solution (76 g, 12.06% Ti content, 16% HCl). Ruthenium(III) chloride hydrate (13 g, Ru content 40.78%) was then added. The resulting solution was allowed to stir for approximately 1 hour until all solids were visibly dissolved. Hydrogen peroxide (6 g, 30% content) was then added, followed by isopropyl alcohol (10 g, 99.5% content).

[0074] The coating was applied to the prepared plaques by the techniques previously described herein to a target thickness of 500 μg / cm2 of ruthenium. A final bake was performed for 2 hours. The sample was labeled ID 2-1.

[0075] Example 8 (Counterexample) The steps of counterexample 7 were followed except that the addition of peroxide was omitted. The sample was labeled 2-3.

[0076] Example 9 (Counterexample) Counterexample 7 steps were followed, with the following exception: the sample was not pre-baked. After the ruthenium was added, the solution was stirred for at least 1 hour, after which palladium(II) chloride (0.7 g, 59.71% Pd content) and isopropyl alcohol (10 g, 99.5% Pd content) were added. The sample was labeled 2-2-2.

[0077] Example 10 Titanium plate samples were prepared by grit blasting, etching, and pre-baking. A coating solution was developed using 4.6 wt% titanium in the solution, a molar ratio of 3.6 between precious metals (Pd, Ru) and titanium metal, and a molar ratio of 0.08 between ruthenium and palladium metal. Distilled water (71 g) was combined with aqueous HCl (23 g, 36% content), followed by the addition of titanium oxychloride solution (76 g, 12.06% Ti content, 16% HCl). Ruthenium(III) chloride hydrate (13 g, 40.78% Ru content) was then added. The resulting solution was allowed to stir for approximately 1 hour until all solids were visibly dissolved. Hydrogen peroxide (6 g, 30% content) was then added. The solution was stirred for at least 1 hour to ensure the peroxide reacted, after which palladium(II) chloride (0.7 g, 59.71% Pd content) and isopropyl alcohol (10 g, 99.5% content) were added.

[0078] The coating was applied to the prepared plaques by the techniques previously described herein to a target thickness of 500 μg / cm2 of ruthenium. A final bake was performed for 2 hours. The sample was labeled 2-0.

[0079] Example 11 The steps of Example 10 were followed except that the addition of peroxide was omitted. The sample was labeled 2-2.

[0080] For Examples 7-11, single-electrode potential evaluations of chlorine and oxygen overvoltages, as well as accelerated corrosion tests, were performed. A comparison of these evaluation results is summarized in Table 4 below. Notably, the addition of palladium reduces oxygen generation (increasing oxygen overvoltage), extends life, and lowers chlorine overvoltage (2-0 vs. 2-3). Utilizing peroxide in the absence of palladium provides no benefit to anode life; in fact, in the absence of palladium, peroxide increases the likelihood of oxygen generation (2-1). In the presence of palladium, the absence of both peroxide and pre-baking is highly detrimental to life (2-2-2); indeed, in the absence of peroxide, a significant life benefit is still observed because pre-baking prevents the reduction of palladium (2-2).

[0081] [Table 4]

[0082] Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention as described herein.

Claims

1. an anode, a core substrate comprising titanium or a titanium alloy; a coating layer having a molar ratio of titanium to a noble metal of 3 to 5, wherein the noble metal comprises at least ruthenium and palladium, and the total molar ratio of palladium to the other noble metal is 0.02 to 0.

3.

2. 10. The anode of claim 1, wherein the noble metal of the coating layer comprises iridium.

3. 3. The anode of claim 2, wherein the titanium and the noble metal in the coating layer are in a crystalline structure and the palladium is not in a single phase but is well dispersed throughout the coating.

4. 2. The anode of claim 1, wherein the titanium and the noble metal in the coating layer are in a crystalline structure and the palladium is not in a single phase but is well dispersed throughout the coating.

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

6. 6. The anode of claim 5, wherein the multi-layer coating includes a palladium-free outermost layer that is not in direct contact with the surface of the core substrate.

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

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

9. 8. The method of claim 7, further comprising mixing an oxidizing agent into the coating solution to bring both titanium and ruthenium to a +4 oxidation state.

10. 9. The method of claim 8, further comprising mixing an oxidizing agent into the coating solution to bring both titanium and ruthenium to a +4 oxidation state.

11. The method of claim 10, wherein the coating solution is tin-free.

12. 10. The method of claim 9, wherein the coating solution is tin-free.

13. The method of claim 8 , wherein the coating solution is tin-free.

14. 8. The method of claim 7, wherein the coating solution is tin-free.

15. 1. A method for manufacturing an anode, comprising: combining titanium, ruthenium, and palladium in a coating solution; pre-baking a titanium or titanium alloy anode substrate surface prior to applying the coating solution; and applying the coating solution to the titanium or titanium alloy anode substrate surface.

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

17. 1. A method for manufacturing an anode, comprising: combining titanium, ruthenium, and palladium in a coating solution, wherein titanium is provided in the coating solution as an oxide; and applying the coating solution to a surface of the anode substrate comprising titanium or a titanium alloy.

18. an anode, a core substrate including a valve metal; a multi-layer coating including a base layer comprising palladium directly coating the surface of the core substrate.

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

20. 20. The anode of claim 18, wherein the base layer comprises ruthenium and titanium.

21. 21. The anode of claim 20, wherein the base layer comprises iridium.

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