WO2024044701A2 - Extended life anode coatings - Google Patents

Abstract

The present invention provides an anode comprising a core substrate including a multi-layer coating with a base layer including palladium that directly coats the substrate.

Description

EXTENDED LIFE ANODE COATINGS

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application claims the benefit of priority of U.S. provisional application no. 63/400,668 filed August 24, 2022, incorporated herein by reference in its entirety.

FIELD

[002] The present disclosure generally relates to electrode coatings on a substrate that are intended to operate as anodes in electrochemical processes, hereto referred to as anode coatings.

BACKGROUND

[003] Many commercial manufacturing processes utilize electrochemistry. For example, the chlor-alkali process electrolyzes aqueous sodium chloride or potassium chloride to form valuable commodity materials, such as chlorine gas, sodium hydroxide (caustic) or potassium hydroxide, and hydrogen gas. Water is electrolyzed to produce hydrogen gas and oxygen gas. Other electrochemical processes are used to prepare a variety of commodity chemicals and intermediates for the chemical and pharmaceutical industries. Current endeavors in commercial electrochemical processes are related to reducing energy consumption, reducing manufacturing costs, and improving the efficiency and durability of the electrodes.

[004] The particular electrochemical processes intended for the field of inventions described herein are those where chloride salts are present in solution and where chlorine gas or hypochlorite salts are the principal products. Such processes include the chlor-alkali membrane-cell and diaphragm-cell processes, the production of chlorate salts, and hypochlorite generation in weak to strong brines for the purposes of disinfection. The use and composition of gas-evolving electrodes will be appreciated as presenting different problems and results than other electrode uses, such as in batteries.

[005] Most electrically conductive materials can serve as electrodes. Preferably, the materials used to make the electrodes resist corrosion by the electrolyte and/or the products produced. Many otherwise suitable electrode materials lack the ability to efficiently catalyze electron transfer to an electrolyte, which requires the use of additional power. And the greater the amount of additional power used, the greater the cost of performing the electrochemical process. Coatings can be applied to the electrodes to facilitate electron-transfer, and to reduce the overpotential needed in the electrolytic process. Thus, coatings help to reduce the overall operating voltage and power consumption of an electrolytic process. Further details regarding electrode coatings are described in international application number PCT/US2020/037426; filed June 12, 2020, which is incorporated herein by reference in its entirety.

[006] Anode coatings known in the art have a limited lifetime and contain precious metals. The function of an anode coating is to lower the voltage required for oxidation by serving as an electrocatalyst and protecting the substrate, thus making the anode geometrically stable. Anode coatings used for oxidation of chloride ions provide lower overpotential for chloride oxidation to chlorine.

[007] An anode coating fails when the coating itself wears away over time, when it loses conductivity, when it loses adhesion to the substrate, or when the substrate oxidizes underneath the coating creating a passivating layer. Once a coating has failed, voltage increases rapidly, and continuing operation can result in heating or damage to the process equipment. In general, the useful life of a coating is proportional to the precious metal loading of the coating and inversely proportional to the square of current density in the cell. In inventions of the prior art, iridium and ruthenium are the principal precious metals used. Iridium is typically employed in anodes that operate more than 4 years at current densities greater than about 3 kA/m², but iridium is much more expensive than ruthenium.

[008] It would be desirable to develop an anode with improved durability, reduced overpotential, and/or extended operating lifetime.

SUMMARY OF THE INVENTION

[009] The preferred substrate for an anode in an embodiment of the invention is a valve-metal, in particular titanium or its alloys. One advantage of embodiments of the invention is that it provides a coating on the substrate exhibiting lower overpotential than coatings of the prior art, reducing power requirements for the process. [0010] Embodiments of the inventions achieve longer life for a given precious metal loading of the coating than expected. It is a feature of embodiments of the invention that anode coatings with long life at current densities greater than 3 kA/m² can be prepared without the use of iridium. In the application of this exemplary anode coating to chlorate cells, the anode generates chlorine that immediately forms hypochlorite ions in solution while a cathode generates hydrogen gas that leaves the cell. A particular problem for chlorate cells is the production of oxygen at the anode by electrocatalysis of oxygen on the coating or by the catalyzed decomposition of hypochlorite in solution. The coatings of embodiments of inventions described herein achieve exceptionally low oxygen content in the hydrogen from a chlorate cell. This is achieved both by reduced electrochemical oxygen evolution, especially on a newly activated electrode, and also by avoiding the contamination of the hypochlorite containing solution in the electrolyzer with impurities that can catalyze decomposition of hypochlorite to oxygen.

[0011 ] In anode coatings of embodiments of the present invention, a coating formula is created containing titanium chloride or titanium oxychloride in an aqueous/alcoholic solution that contains hydrochloric acid. In certain embodiments titanium alkoxides could be used in conjunction with pre-baking, secondary/tertiary alcohols, and/or oxidizer to achieve the same result as titanium oxychloride or titanium chlorides. Salts of ruthenium, palladium, and optionally platinum, and iridium are also dissolved into this solution, preferably as chloride salts. Optionally, chloride salts of transition metal elements can be added.

[0012] In preferable embodiments of the inventions, various factors, individually and in combination of two or more factors in preparation of inventive coatings, including palladium, surprisingly contribute to extended life of the anode with an inventive anode coating:

[0013] 1 . Avoiding primary alcohols in preparing coating solution.

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

[0015] 3. The presence of peroxide to put titanium and ruthenium both in +4 oxidation state in preparing the coating solution In other embodiments, other oxidizers in place of or in addition to peroxide may be used in preparing the coating solution to produce +4 oxidation states of titanium and ruthenium, including nitric acid, chromate, halogens, chloride dioxide, chloric acid and/or ozone and the like. Accordingly, where references to peroxide use are disclosed herein, it will be appreciated that other oxidizers are encompassed for possible use.

[0016] 4. Preferably using titanium as a titanium oxychloride solution. However, in embodiments titanium alkoxides could be used in conjunction with pre-baking, secondary/tertiary alcohols, and/or oxidizer to achieve the same result as titanium oxychloride or titanium chlorides.

[0017] 5. Avoid use of tin with palladium to avoid forming PdSn2 that performs like a metal. In other embodiments, it is desirable to avoid forming PdSn4 compounds that behave similarly to a metallic alloy and form a phase separate from the rutile phase of the coating.

[0018] In embodiments considered to be unique, hydrogen peroxide is optionally added to an anode coating solution of mixed salts in an amount that raises the oxidation potential of the coating and keeps palladium from being reduced to metallic state as the coating procedure is carried out. Another optional ingredient in the coating solution is secondary and/or tertiary alcohol, preferably isopropanol (2-propanol). In other embodiments 2-butanol and/or tertiary butanol (tert-butyl alcohol) is another optional ingredient of the coating solution. A distinguishing characteristic of the coatings of embodiments of the invention is that the mole ratio of titanium to precious metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and that the mole ratio of palladium to the sum of other precious metals is between about 0.04 and 0.3.

[0019] Importantly, in embodiments of the invention, an anode coating includes titanium, ruthenium and palladium with palladium distributed on a fine scale in which all three metals are in the same crystal structure and avoiding palladium in a single phase and separated as in prior art coatings that include palladium.

[0020] When anode coatings for high current-density applications are made with use of reduced amounts of iridium, the thickness of the coating must increase, and the wear rate must be reduced. Coatings are applied in multiple layers, with drying and baking steps between each coat. It is an advantage of embodiments of the invention that a coating with low wear rate containing principally ruthenium as the precious metal can be achieved using fewer coats of coating.

[0021 ] Coating adhesion is typically measured by a tape test, where a piece of clear tape is applied to the coated anode surface and then quickly stripped off, observing the coating removed. The tape test is typically evaluated by appearance, but it can also be evaluated quantitatively, using X-ray fluorescence measurement of the tape. The quantitative tape test result is most usefully quantified based on the percentage of the total coating removed. It is an object of embodiments of the invention to achieve a coating with a quantitative tape test result with less than 5%, preferably less than 2%, and more preferable less than 1 %, of the coating removed by the tape.

[0022] In other inventions of the prior art for anode coatings, coatings containing palladium or platinum have been found to reduce the voltage required for oxidation of chloride ion to chlorine gas or hypochlorite, reducing power required and also decreasing the undesirable generation of oxygen. However in this prior art, the ratio of palladium or platinum to lower cost ruthenium component in the coating is greater than 3:10, and the palladium and platinum is more rapidly lost from the coating than ruthenium. When Pd or Pt is lost from the coating, voltage rises. It is an advantage of the present invention that much smaller amounts of Pd or Pt can be effective additives to the coating and maintain lower voltages for much longer.

[0023] The substrate of anode in embodiments of the invention is prepared by methods known in the art to roughen and etch the surface to remove oxides and embedded grit. Preferably the surface of the substrate is then made more hydrophilic by restoring a light oxide film by baking it at a temperature of from 400 to 550 C for a duration long enough to form an orange to deep blue or light gray color to the surface of the titanium.

[0024] The coating is then formed on the substrate by dip-coating, roller-coating, brushing, spray coating, or electrostatic spray coating of the substrate with the 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 from 400° to 550° C for 10 to 20 minutes. Additional coats are then applied by repeating the process of application, drying, and baking. [0025] Optionally, after the final coat, a longer baking step can be used for an extended time, referred to as a post-bake.

[0026] The precious metals employed in the coating of embodiments of the invention are ruthenium, palladium, and optionally platinum and iridium. In anodes of embodiments of the invention, ruthenium is the principal precious metal used in the coating, while the Pd to total precious metals mole ratio is preferably about 0.04 to 0.3, preferably about 0.12. Palladium in the optimal range is shown to decrease voltage for chlorine evolution while decreasing oxygen evolution from the coating, and simultaneously extends anode life. At higher levels of palladium, especially those suggested in examples of palladium coating of the prior art, the formation of rutile is decreased, and the anatase phase is favored, decreasing coating life. Therefore, in the coatings of prior art, extended life was not attributed to the presence of palladium in the coating because the higher levels of coating promoted anatase formation.

[0027] A problem not solved in the prior art concerning palladium and platinum salts is that they are more easily reduced to metallic form than either ruthenium or iridium salts. Palladium and platinum can be reduced to metallic form when in intimate contact with the titanium metal substrate. Another aspect of embodiments of the invention is that coatings are applied after preparing the titanium surface by oxidizing in air at elevated temperature (pre-baking) and avoiding use of primary alcohols. When the titanium oxide film is exposed to acidic coating solutions, it may be dissolved, and bare titanium exposed to the coating solution if drying is carried out at elevated temperature. By first pre-baking (pre-oxidizing) the substrate surface, the coating can provide an effective wetting to the surface and get favorable adhesion between the coating and substrate surface.

[0028] In embodiments of the invention, when pre-baking conditions create a blue colored appearance of the titanium substrate, and the coating is dried at a temperature of less than 110°C, and preferably at about 50°C, coatings containing palladium can be formed without formation of a metallic palladium phase. Alternatively, the preferred coating solutions of embodiments the invention contain some peroxide. Peroxide forms stable complexes with titanium and oxidizes ruthenium to the +4 state while in solution. Reduced palladium or platinum metal in the coating are easily oxidized to soluble chloride salts in the processes to which embodiments of the invention applies, so these metallic phases in the coating reduce its life. For this reason, coatings of the prior art containing palladium or platinum did not achieve extended life when metallic phases were created.

[0029] In further embodiments of the invention, the coatings of an anode can include iridium to promote rutile formation and extend the coating life. In coatings with a mole ratio of iridium to ruthenium of greater than about 0.1 , prior art has established that coating life is primarily a function of the iridium loading of the coating. Unexpectedly, even in coatings with iridium to ruthenium mole ratios greater than 0.1 , the presence of palladium in a mole ratio of 0.04 to 0.3 to total of ruthenium and iridium substantially increases anode life, and coatings of embodiments of inventions described herein can achieve more than twice the coating life of coatings of the prior art with similar total loadings of iridium.

[0030] Additional dopants selected from transition metals such as Fe, Ni, or Co can be added to the coating in embodiments of the invention as these metals are known to promote rutile formation and are known in the art to increase coating conductivity. However, anode coatings of embodiments of the invention can be produced with no dopants present. While increasing conductivity, the addition of dopants has not been found to increase coating life. Furthermore, in the application of chlorate production, nickel and cobalt are known to catalyze decomposition of hypochlorite to oxygen, so these dopants are absent from coatings used for chlorate production.

[0031 ] In embodiments of disclosed inventions, it has been found that using titanium in the form of titanium oxychloride in combination with hydrogen peroxide increases rutile formation, especially when a portion of the solvent for the coating is an alcohol, and the coating is dried completely at an air temperature of below 110°C. The optimum titanium oxychloride content of the coating solution has been found to be 0.25% to 5% mass percent titanium, while the optimum peroxide content is a peroxide to titanium mole ratio of from 0.1 to 2.0, and the optimum alcohol content is from 5% to 75% mass percent of the coating solution. The alcohol is preferably a secondary and/or tertiary, preferably isopropanol (2-propanol), to avoid reaction with peroxide prior to the evaporation of the solvent. In other embodiments, 2-butanol and/or tertiary butanol (tertbutyl alcohol) is another optional ingredient of the coating solution. In embodiments, another water-miscible and volatile organic solvent can be substituted for the noted alcohols if compatible with the oxidizing nature of the salts.

[0032] The invention can potentially achieve breakthrough performance and lower the manufacturing cost of coatings used in all chlor-alkali processes, chlorate production. Another large field of application is in hypochlorite generators used for disinfection of swimming-pool, municipal water treatment, wastewater treatment, or bilge-water. The potential uses are a world-wide market.

[0033] Other features and iterations of the invention are described in more detail below.

DETAILED DESCRIPTION

[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 "comprising", "including" 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 of preferably a valve metal such as titanium and its alloys and (b) a coating prepared, applied to and adhering to the core substrate including titanium (Ti), ruthenium (Ru), and palladium (Pd), and optionally in embodiments platinum and/or iridium. The coating is prepared to avoid palladium separated and in a single phase, as in prior art coatings, with palladium instead well-distributed on a fine scale with Ti-Ru-Pd in the same crystal structure, i.e. the palladium is well-dispersed throughout the coating.

[0036] Embodiments of the invention pertains to electrode coatings on a substrate that are intended to operate as anodes in electrochemical processes, hereto referred to as anode coatings. The particular electrochemical processes intended for the field of embodiments of the invention are those where chloride salts are present in solution and where chlorine gas or hypochlorite salts are the principal products (however, other fields and, in fact, worldwide markets can benefit from the invention). Such processes include the chlor-alkali membrane-cell and diaphragm-cell processes, the production of chlorate salts, and hypochlorite generation in weak to strong brines for the purposes of disinfection. The preferred substrate is a valve-metal, in particular titanium or its alloys. Anode coatings known in the art have a limited lifetime and contain precious metals. The function of an anode coating is to lower the voltage required for oxidation by serving as an electrocatalyst and protecting the substrate, thus making the anode geometrically stable. Anode coatings used for oxidation of chloride ions provide lower overpotential for chloride oxidation to chlorine. One advantage of embodiments of the invention is that it provides lower overpotential than coatings of the prior art, reducing power requirements for the process. Anode coatings fail when the coating itself wears away over time, when it loses conductivity, when it loses adhesion to the substrate, or when the substrate oxidizes underneath the coating creating a passivating layer. Once a coating has failed, voltage increases rapidly, and continuing operation can result in heating or damage to the process equipment. In general, the useful life of a coating is proportional to the precious metal loading of the coating and inversely proportional to the square of current density in the cell. Embodiments of the invention achieve longer life for a given precious metal loading of the coating than expected. In inventions of the prior art, iridium and ruthenium are the principal precious metals used. Iridium is typically employed in anodes that operate more than 4 years at current densities greater than about 3 kA/m2, but iridium is much more expensive than ruthenium since indium is rarer and less abundant in the earth’s crust. It is a feature of embodiments of the invention that anode coatings with long life at current densities greater than 3 kA/m² can be prepared without the use of iridium. In the application of this anode coating to chlorate cells, the anode generates chlorine that immediately forms hypochlorite ions in solution while a cathode generates hydrogen gas that leaves the cell. A particular problem for chlorate cells is the production of oxygen at the anode by electrocatalysis of oxygen on the coating or by the catalyzed decomposition of hypochlorite in solution. The coatings of embodiments of the invention achieve exceptionally low oxygen content in the hydrogen from a chlorate cell. This is achieved both by reduced electrochemical oxygen evolution, especially on a newly activated electrode, and also by avoiding the contamination of the hypochlorite containing solution in the electrolyzer with impurities that can catalyze decomposition of hypochlorite to oxygen.

[0037] In anode coatings of the present invention, a coating formula is created containing titanium chloride or titanium oxychloride in an aqueous/alcoholic solution that contains hydrochloric acid. Salts of ruthenium, palladium, and optionally platinum, and iridium are also dissolved into this solution, preferably as chloride salts. Optionally, chloride salts of transition metal elements can be added. Unique to embodiments of the invention, hydrogen peroxide is optionally added to this solution of mixed salts in an amount that raises the oxidation potential of the coating and keeps palladium from being reduced to metallic state as the coating procedure is carried out. Another optional ingredient in the coating solution is secondary and/or tertiary alcohol, preferably isopropanol (2-propanol). In other embodiments 2-butanol and/or tertiary butanol (tertbutyl alcohol) is another optional ingredient of the coating solution. A distinguishing characteristic of the coatings of embodiments of the invention is that the mole ratio of titanium to precious metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and that the mole ratio of palladium to the sum of other precious 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 the surface is then made more hydrophilic by restoring a light oxide film by baking it at a temperature of from 400 to 550°C for a duration long enough to form an orange to deep blue or light gray color to the surface of the titanium. The coating is then formed by dip-coating, rollercoating, brushing, spray coating, or electrostatic spray coating of 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 from 400 to 550°C for 10 to 20 minutes. Additional coats are then applied by repeating the process of application, drying, and baking. Optionally, after the final coat, a longer baking step can be used for an extended time, referred to as a post-bake.

[0038] The precious metals employed in the coating are ruthenium, palladium, and optionally platinum and iridium. In the anodes of embodiments of the invention, ruthenium is the principal precious metal used in the coating, while the Pd to total precious metal moles ratio is preferably about 0.04 to 0.3, preferably about 0.12. Palladium in the optimal range is shown to decrease voltage for chlorine evolution while decreasing oxygen evolution from the coating, and simultaneously extends anode life. At higher levels of palladium, especially those suggested in examples of palladium coating of the prior art, the formation of rutile is decreased, and the anatase phase is favored, decreasing coating life. Therefore, in the coatings of prior art, extended life was not attributed to the presence of palladium in the coating because the higher levels of coating promoted anatase formation.

[0039] A problem not solved in the prior art concerning palladium and platinum salts is that they are more easily reduced to metallic form than either ruthenium or iridium salts. Palladium and platinum can be reduced to metallic form when in intimate contact with the titanium metal substrate. Another aspect of embodiments of the invention is that coatings are applied after preparing the titanium surface by oxidizing in air at elevated temperature (pre-baking). When the titanium oxide film is exposed to acidic coating solutions, it may be dissolved, and bare titanium exposed to the coating solution if drying is carried out at elevated temperature. In embodiments of the invention, when pre-baking conditions create a blue colored appearance of the titanium substrate, and the coating is dried at a temperature of less than 110°C, coatings containing palladium can be formed without formation of a metallic palladium phase. Alternatively, the preferred coating solutions contain some peroxide. Peroxide forms stable complexes with titanium and oxidizes ruthenium to the +4 state while in solution. Reduced palladium or platinum metal in the coating are easily oxidized to soluble chloride salts in the processes to which embodiments of the invention applies, so these metallic phases in the coating reduce its life. For this reason, coatings of the prior art containing palladium or platinum did not achieve extended life when metallic phases were created.

[0040] In the coatings of embodiments of the invention iridium can be used to promote rutile formation and extend the coating life. In coatings with a mole ratio of iridium to ruthenium of greater than about 0.1 , prior art has established that coating life is primarily a function of the iridium loading of the coating. Unexpectedly, even in coatings with iridium to ruthenium mole ratios greater than 0.1 , the presence of palladium in a mole ratio of 0.04 to 0.3 to total of ruthenium and iridium substantially increases anode life, and coatings of embodiments of the invention can achieve more than twice the coating life of coatings of the prior art with similar total loadings of iridium. Additional dopants selected from transition metals such as 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 coating conductivity. However, anode coatings of embodiments of the invention can be produced with no dopants present. These dopants have been found not to increase coating life. Furthermore, in the application of chlorate production, nickel and cobalt are known to catalyze decomposition of hypochlorite to oxygen, so these dopants are absent from coatings used for chlorate production. In embodiments of the invention, it has been found that using titanium in the form of titanium oxychloride in combination with hydrogen peroxide increases rutile formation, especially when a portion of the solvent for the coating is an alcohol, and the coating is dried completely at an air temperature of below 110°C. The optimum titanium oxychloride content of the coating solution has been found to be 0.25% to 5% titanium, while the optimum peroxide content is a peroxide to titanium mole ratio of from 0.1 to 2.0, and the optimum alcohol content is from 5% to 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 peroxide prior to the evaporation of the solvent. It is to be considered obvious that another water-miscible and volatile organic solvent can be substituted for these alcohols if it is compatible with the oxidizing nature of the salts.

[0041 ] Anode coatings of the present invention achieve longer life in accelerated wear testing by a previously unexpected mechanism. One explanation for this behavior is that the peroxide complexes with titanium are surprisingly stable, surviving the drying process and influencing the phase behavior of the oxides coatings that form during baking. These oxides are converted into a rutile structure at lower baking temperatures than coatings without peroxide. It has also been demonstrated that coating life is substantially extended proportionally to the palladium content of the coating.

[0042] The oxide coatings formed from solutions containing hydrogen peroxide appear to achieve a life of 2 to 8 times longer than similar coatings made without addition of hydrogen peroxide or palladium. One theory that can explain this longer life is that coatings of the prior art have an oxygen to metal ratio of near 2:1 instead of the ratio of at least 2.5:1 as in the present invention. During electrolysis, especially under conditions where oxygen can be evolved, some oxygen can enter the surface of the coating, forming a crystal structure with excess oxygen that has greater volume than the underlying coating. Thus, in anode coatings of the prior art, severe mechanical stress accumulates in the surface of the coatings which can only be relieved with the use of an oxygen evolution catalyst such as iridium, or gradual coating loss by spoiling of the surface occurs over time. In the present invention, excess oxygen is already present in the coating which is therefore not stressed by oxygen evolution because no more oxygen can be taken up by the surface of the coating.

[0043] A feature of the coatings of embodiments of the invention is that they contain titanium oxide, ruthenium oxide, and palladium oxide in a mixed solid solution that is predominantly in the rutile crystal form. Coatings of the prior art contain anatase crystals and some also contain substantial amounts of precious metal in a separate oxide or metallic phase that is not in solid solution with rutile. It has been established in investigation that when these alternate phases exist, they wear more quickly and are lost before the rutile phases of the coating.

[0044] In embodiments of the invention, it has been found that coating wear rate is unexpectedly greatly reduced when the crystal form of the coating contains greater than about 80% rutile, and even more when the coating contains greater than about 85% rutile. This is achieved in coatings with a specific ratio of titanium to ruthenium, and palladium to ruthenium, and when peroxide (and/or potentially other oxidizers) is present in the coating, the formation of rutile is enhanced. In coatings of the prior art, it has been widely accepted that the optimal mole ratio of titanium to precious metals is between 1.5 and 2.5, but experiments show that at these mole ratios, there is insufficient titanium to form a solid solution of the precious metals in the rutile phase, and a separate phase of RuO2, or RuO2+lrO2 forms - single electrode potential (SEP) testing shows that such separate phases lower single electrode potential for oxygen evolution which increases the faradaic inefficiency for oxygen evolution of the coating - an undesirable feature in applications where chlorine, hypochlorite, and chlorate are desired products. Also, at titanium to precious metal mole ratios above about 5, in coating formulas containing palladium, a separate anatase phase of titanium forms. The phases of precious metal oxides that contain little titanium are more rapidly lost than rutile, causing coatings to wear more rapidly. The anatase phases of titanium are also more rapidly lost than rutile, causing coatings to wear more rapidly. Palladium Coating Recipe Examples

Example 1

[0045] A multi-variable test (MVT) was designed to evaluate and optimize some of the uncertain aspects of chlorate coating recipes. 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), pre-bake temperature, and post-bake temperature. A custom experimental design was created using software which accounted for all the anticipated non-linear and two-way interactions of the above variables. The runs were performed by depositing an aqueous solution of precious metal chlorides and titanium oxychloride on flat plates of titanium metal. The following experimental design of 12 runs was implemented, in which most of the potential interactions and first-order effects are orthogonal, though not perfectly balanced, as shown in Table 1 :

Table 1 : MVT Test Runs

Run Pd Ti

Order Recipe Ratio Ratio Postbake Prebake

1 C 0.02 5 490 420

2 G 0.1 5 490 490

3 F 0.1 3.8 490 420

4 C 0.02 5 525 420

5 B 0.02 3.8 490 490

6 D 0.06 2.6 525 420

7 C 0.02 5 525 490

8 E 0.1 2.6 525 490

9 A 0.02 2.6 490 420

10 G 0.1 5 525 420

11 D 0.06 2.6 490 490

12 A 0.02 2.6 525 490

[0046] In the experimental design, there are seven unique coating recipes in which the mole ratio of palladium to ruthenium is varied with three options, 0.02, 0.06, and 0.1 ; the mole ratio of titanium to precious metals (palladium and ruthenium combined) is varied with three options, 2.6, 3.8, and 5; the pre-bake temperature is varied between 420 and 490°C; and the post-bake temperature is varied between 490 and 525°C.

[0047] Results: Recipes B, C, F and G are examples of embodiments of the invention exhibiting advantageous wear properties, and the remaining recipes A, D and E are counterexamples.

[0048] The coating recipes were all created with the same concentration of ruthenium metal in solution, 34 grams per liter, and the dopant concentrations (Ni, Fe, and Co) were all set at 1 gram per liter. Palladium and titanium content varied according to the recipes, so that the recipes had the following molar ratios of coating components. Subsequent experimentation will reveal that the molar ratio of hydrogen peroxide is important to the success of the coating, so these ratios are included in Table 2 below.

[0049] Table 2. The coating metal compositions (in mole percent) of each chlorate MVT recipe.

[0050] Each coating recipe had equal weight concentrations 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 HCI content per coating solution changes with the titanium ratio, though each recipe contains 4.32% by weight aqueous HCI added independently.

[0051 ] The different coating recipes were applied to give approximately the same total loading of ruthenium metal according to XRF, requiring between 8-13 dips. Coating solution recipes with higher titanium concentration were more viscous and therefore required fewer coats to achieve the desired minimum loading of 500 ug/cm2. Average weight gain per dip was found to vary by coating recipe, mostly following the different concentrations of titanium.

[0052] It will be appreciated that coating composition is determined with methods known in the art, including non-destructively by XRF (X-ray fluorescence spectroscopy) or by use of electron microscopy with EDS (energy dispersive X-ray spectroscopy).

[0053] A representative procedure for the preparation and application of coating recipe A is outlined as follows.

Surface Preparation

[0054] An initial surface treatment experiment on titanium flat plates identified the optimal preparation to include 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 designed runs, yielding a total of 24 flat plate chlorate anode samples. A representative description of the preparation of one of these is detailed. A 4” x 4” x 0.025” titanium flat plate (grade 2) was grit blasted with 220 aluminum oxide blast media. The blast nozzle measured 6 mm, and the blast pressure was set at 25 psi. To ensure a uniform surface, six passes were completed per side, with the blast gun held at a 60-to-90-degree angle. The blasted plate was then rinsed with DI water and dried before etching in 20 gpl oxalic acid dihydrate at 80 °C for 1-1.5 hours. After etching, the plate was rinsed once more in DI water, in which grey oxide is observed to be removed, and then pre-baked for 20 minutes at either 420 °C or 490 °C, yielding either a yellow or dark blue surface, respectively. The sample was then set aside for dip coating. Once pre-baked, which establishes a thin protective layer of TiO2, titanium substrates are shelf stable for up to at least one month, perhaps indefinitely in ideal storage conditions.

Coating Solution Preparation

[0055] Coating solution A specified a 2.6 mole ratio of titanium metal to palladium and ruthenium metals combined and a 0.02 mole ratio of palladium metal to ruthenium metal. Recipe A was used to coat 4 anodes, so 300 g of coating solution was targeted. Distilled water (133.61 g) was combined with aqueous HCI (36% assay, 36.01 g) prior to adding cobalt(ll) chloride hexahydrate (24.6% Co assay, 1.22 g), iron(lll) chloride hexahydrate (20.29% Fe assay, 1 .48 g), and then nickel(ll) chloride hexahydrate (37.18% Ni assay, 0.814 g). Ruthenium(lll) chloride hydrate (40.88% Ru assay, Johnson Matthey, 24.91 g) was then added, followed by titanium oxychloride solution (14.08% Ti assay, 34.5% HCI, Kronos, 91.01 g). Note, titanium oxychloride solutions contain HCI for stabilization, so for recipe A, the total HCI content is approximately 14.8% by weight (4.32% from aqueous HCI, 10.47% from TiOCI2). The resulting solution was allowed to stir until all solids were visually dissolved, about 1 hour, though in some cases, the solution was stirred overnight. Hydrogen peroxide was then added (32% assay, 10.73 g). Care was taken to periodically vent the solution bottle following the peroxide addition. The solution was allowed to stir for at least one hour prior to the addition of palladium(ll) chloride (59.7% Pd assay, 1.81 g) to ensure the peroxide reacts. As good practices, the coating solution should be stirred at least one hour before dipping after all ingredients are added, and the coating solution should be stirred between dips.

Dip Coating Procedure

[0056] To dip coat anode substrates, a Pyrex glass container, or something similar, is selected with dimensions large enough to lay the sample flat. Ideally, this container is air-tight with a rubber gasket to prevent evaporation. A sufficient amount of coating solution is added so that the substrate can be fully immersed in solution when dipped, which typically requires at least 0.25-0.5” of solution depth.

[0057] Prior to coating a flat plate substrate, two holes were drilled within a 0.5” margin along the top and bottom edges. Ideally, these holes are centered, so that a sample can be vertically hung in a balanced manner. Note, titanium wire is used to hang anode samples while dipping to prevent corrosion and contamination. The first coat was applied by hanging the sample over the dipping vessel and then gently immersing the plate’s bottom edge in the solution before proceeding to laying the plate flat and fully immersed. It is important to avoid touching the sample; the sample is best manipulated via titanium wire. Furthermore, it is critical to move slowly and smoothly to avoid bubble generation. Following sample immersion, the above motion was reversed, and the sample was slowly lifted into a vertical position before lifting the sample’s bottom edge out of solution. The sample was held over the dipping vessel while excess solution drained and then hung suspended from a titanium rack to air-dry in a well-ventilated, designated area for 20 minutes, though subsequent experiments have shown 40-60 minutes is ideal. The sample was then dried in the oven at 110°C for 20 minutes before elevating the temperature to 490°C and baking an additional 20 minutes. The sample was carefully removed, ideally with tongs and heat-resistant gloves, and cooled to room temperature. Samples coated in recipes with higher titanium ratios were found to have a strip a loose oxide powder along the bottom edge of the titanium plate; this powder was removed by brushing. The sample was then rotated 180 degrees and hung by the opposite edge. The above steps of dipping, drying, and baking were repeated until a minimum loading of 500 pg/cm2 of ruthenium was measured by XRF. Once the target loading was achieved, the sample was dried and then baked at the specified post-bake temperature, either 490 °C or 525 °C, for 2 hours.

Palladium Prescence in Base Layer(s) Extending Anode Life

[0058] In addition to discovering improved physical anode characteristics with inclusion of palladium in improved anode coatings described above, embodiments of the invention described in the subsequent examples surprisingly showed that including palladium in base layers of coating, particularly adjacent the anode substrate, provided increased anode life (see accelerated life test results in Table 3). Conventional thought would assume that inclusion of life-extending materials, such as palladium, is most beneficial in an outermost layer of coating. However, coatings and testing of the invention determined that including palladium in the innermost layer or innermost few layers adjacent the anode substrate was more beneficial than palladium in the outermost layer, including where even no palladium was in the outermost layer. Accordingly, a key discovery of the invention is that palladium included in a base layer of a multi-layer coating provides improved anode life even with no palladium in the outermost layer. It is possible that palladium in an innermost layer of anode coating interacts with the anode substrate with synergistic effect that causes the substrate to last longer (i.e. longer life). For example, when a titanium anode substrate is present with palladium in an innermost (base layer) coating layer of a multi-layer coating, a titanium/palladium alloy may develop that changes the characteristics of the titanium substrate alone to increase the longevity for anode use, including preferably for gas-producing anodes. Example 2 (Counterexample)

[0059] A sample of titanium mesh was prepared by grit blasting, etching, and prebaking. A coating solution “Z” was prepared using 0.24% titanium, 0.22% ruthenium, and 0.24% iridium in isopropanol with 5.1 % hydrochloric acid. The materials used to prepare this solution are tetrapropyl titanate (brand name Tyzor TPT), an alcoholic solution containing 16.8% titanium; ruthenium(lll) chloride hydrate crystals containing 40.9% ruthenium; iridium(IV) chloride dihydrate, an alcoholic solution containing 5.1 % iridium; anhydrous HCI in isopropanol containing 22.6% HCI; and dry isopropanol to dilute the solution to the desired final strength. The coating was applied by the following steps:

[0060] The mesh was dipped in the coating solution and then hung vertically to allow excess coating to flow over the surface.

[0061 ] The coating was allowed to dry completely at 50 °C, typically for 20 minutes, and then baked at 490 °C for 20 minutes.

[0062] Steps 1 -3 were repeated for 9 cycles with the mesh flipped periodically flipped vertically. A final bake after the 9th dip was performed for 40 minutes. Sample was labeled ID 13.

Example 3 (Counterexample)

[0063] The steps of Counterexample 2 were followed except the titanium mesh was prepared by grit blasting and washing. Coating solution “Z*”, using iridium(IV) chloride dihydrate crystals containing 52.0% iridium, was applied by the same steps for a total of 6 cycles. Sample was labeled ID 6.

Example 4 (Counterexample)

[0064] Samples of titanium mesh with low palladium in the base layer were prepared by grit blasting, etching, and pre-baking.

[0065] A 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 are tetrapropyl titanate (brand name Tyzor TPT), an alcoholic solution containing 16.8% titanium; ruthenium(lll) chloride hydrate crystals containing 39.9% ruthenium; hydrogen hexachloroiridate(IV) hydride containing 39.2% iridium; anhydrous HCI in isopropanol containing 22.6% HCI; and dry isopropanol to dilute the solution to the desired final strength. The base layer coating was applied in the steps outlined in counterexample 1 for a total of 4 cycles (ID 1 ) and 6 cycles (ID 22).

[0066] A 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 using the same materials. The top layer coating was applied for a total of 3 cycles (ID 1 ) and 5 cycles (ID 22), which was followed by a final bake performed for 2 hours.

Example 5

[0067] The steps of counterexample 4 were followed except the top layer coating solution “Y” was solely applied for a total of 7 cycles (ID 5) and 11 cycles (ID 14).

Example 6

[0068] The steps of counterexample 2 were followed except the Pd-containing coating solution “Y” was applied for 5 cycles as base layers and the no Pd coating solution “Z” was applied for 4 cycles as top layers, followed by a final bake of 40 minutes at 490 °C. Sample was labeled ID 34.

[0069] An accelerated life test was performed on the coated anodes of examples 1 -5; XRF measurements were also conducted to determine the average ruthenium loading per sample. A comparison of the results from this testing is summarized in Table 3 below. In general, within sample sets with the same coating formulations, AC life hours increase with increasing precious metal total loadings (ID 6 vs. ID 13, ID 1 vs ID 22, ID 5 vs ID 14). Notably, any amount of palladium in the base layer will extend accelerated corrosion life, even if the top layers do not contain palladium, as in the case of ID 34. Coating life is substantially extended proportionately to palladium content, particularly in coating layers close to the anode substrate.

[0070] Table 3. Comparison testing of effects of palladium on AC Life in anode coatings.

6 0 0 2 126 84 0.67

1 0.11 4 114 102 0.89 0.22 3

5 0.22 108 182 1.69

22 0.11 _{6 0 22} 5 206 266 1.29

14 0.22 181 295 1.63

13 ₀ 241 246 1.02

5 0 4

34 0.22 200 274 1.37

Anode Life Effects from Palladium Utilized with Peroxide in Making Coating

[0071 ] In further embodiments, it has been discovered that in industries were oxygen production is detrimental, formulations with palladium benefit from the presence of both peroxide and pre-baking, which prevents unwanted oxygen production while extending the life of the anode. The results of Examples 7-11 subsequently demonstrate such advantages in adding palladium to coatings of gas-producing anodes.

Example 7 (Counterexample)

[0072] A sample of titanium flat plate was prepared by grit blasting, etching, and pre-baking. A coating solution was developed utilizing 4.6% by weight titanium in solution, 3.6 mole ratio of titanium metal to precious metals. Distilled water (71 g) was combined with aqueous HCI (23 g, 36% assay) prior to adding titanium oxychloride solution (76 g, 12.06% Ti assay, 16% HCI). Ruthenium(lll) chloride hydrate (13 g, 40.78% Ru assay) was then added. The resulting solution was allowed to stir until all solids were visually dissolved, about 1 hour. Hydrogen peroxide was then added (6 g, 30% assay), followed by isopropyl alcohol (10 g, 99.5% assay).

[0073] The coating was applied to the prepared flat plate by techniques previously described herein until a target thickness of 500 ug/cm2 ruthenium was measured. A final bake was performed for 2 hours. Sample was labeled ID 2-1 .

Example 8 (Counterexample)

[0074] The steps of counterexample 7 were followed except the peroxide addition was excluded. Sample was labeled 2-3. Example 9 (Counterexample)

[0075] The steps of counterexample 7 with the following exceptions. Sample was not pre-baked. After the ruthenium addition, the solution was allowed to stir for at least one hour prior to the addition of palladium(ll) chloride (0.7 g, 59.71 % Pd assay) and isopropyl alcohol (10 g, 99.5% assay). Sample was labeled 2-2-2.

Example 10

[0076] A sample of titanium flat plate was prepared by grit blasting, etching, and pre-baking. A coating solution was developed utilizing 4.6% by weight titanium in solution, 3.6 mole ratio of titanium metal to precious metals (Pd, Ru), and a 0.08 mole ratio of palladium metal to ruthenium metal. Distilled water (71 g) was combined with aqueous HCI (23 g, 36% assay) prior to adding titanium oxychloride solution (76 g, 12.06% Ti assay, 16% HCI). Ruthenium(lll) chloride hydrate (13 g, 40.78% Ru assay) was then added. The resulting solution was allowed to stir until all solids were visually dissolved, about 1 hour. Hydrogen peroxide was then added (6 g, 30% assay). The solution was allowed to stir for at least one hour prior to the addition of palladium^ I) chloride (0.7 g, 59.71 % Pd assay) and isopropyl alcohol (10 g, 99.5% assay) to ensure the peroxide reacts.

[0077] The coating was applied to the prepared flat plate by techniques previously described herein until a target thickness of 500 ug/cm2 ruthenium was measured. A final bake was performed for 2 hours. Sample was labeled 2-0.

Example 11

[0078] The steps of example 10 were followed except the peroxide addition was excluded. Sample was labeled 2-2.

[0079] Single electrode potential evaluations of chlorine and oxygen overvoltage, as well as accelerated corrosion tests, were performed on examples 7-11 . A comparison on the results from these evaluations are summarized in Table 4 below. Notably, the addition of palladium reduces oxygen production (by increasing oxygen overvoltage), extends life, and lowers chlorine overvoltage (2-0 vs 2-3). The utilization of peroxide when palladium is absent does not benefit life of the anode; in fact, in the absence of palladium, peroxide increases the likelihood of oxygen production (2-1 ). When palladium is present, lack of both peroxide and pre-baking is extremely detrimental to life (2-2-2); in fact, in the absence of peroxide, pre-baking prevents the reduction of palladium so that a significant life advantage is still observed (2-2).

[0080] Table 4.

[0081 ] Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention described herein.

Claims

CLAIMS What is claimed is:

1 . An anode comprising: a core substrate including titanium or titanium alloy; and a coating layer having a mole ratio of titanium to precious metals from 3 to 5, wherein the precious metals at least include ruthenium and palladium, and the mole ratio of palladium to the sum of other precious metals is from 0.02 to 0.3.

2. The anode of claim 1 , wherein the precious metals of the coating layer include iridium.

3. The anode of claim 2, wherein titanium and the precious metals in the coating layer are in a crystal structure, and wherein palladium is not in a single phase and is well- dispersed throughout the coating.

4. The anode of claim 1 , wherein titanium and the precious metals in the coating layer are in a crystal structure, and wherein palladium is not in a single phase and is well- dispersed throughout the coating.

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

6. The anode of claim 5, wherein the multi-layer coating includes an outermost layer free of palladium that does not directly contact the surface of the core substrate.

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

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

9. The method of claims 7, further comprising mixing in an oxidizer of the coating solution to put titanium and ruthenium both in +4 oxidation state.

10. The method of claims 8, further comprising mixing in an oxidizer of the coating solution to put titanium and ruthenium both in +4 oxidation state.

11 . The method of claim 10, wherein tin is absent from the coating solution.

12. The method of claim 9, wherein tin is absent from the coating solution.

13. The method of claim 8, wherein tin is absent from the coating solution.

14. The method of claim 7, wherein tin is absent from the coating solution.

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

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

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

18. An anode comprising: a core substrate including a valve metal; and a multi-layer coating including a base layer that includes palladium directly coating the core substrate surface.