Patent Application
20250154672
The present disclosure generally relates to electrode coatings on a substrate that are intended to operate as electrodes in electrochemical processes, such as gas-producing electrodes, hereto referred to as electrode coatings.
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.
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.
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 Jun. 12, 2020, which is incorporated herein by reference in its entirety. Additionally, the published article Kariman, A. & Marshall, A. (2019), Improving the Stability of DSA Electrodes by the Addition of TiO2 Nanoparticles, Journal of The Electrochemical Society, 166 (8) E248-E251, is incorporated herein by reference in its entirety.
Embodiments of the invention pertain to coatings on electrodes used in electrochemical applications such as electrosynthesis. In an electrochemical process, oxidation takes place on an anode, or reduction takes place on a cathode. The substrate is an electrically conductive material, and in this invention, the substrate for the electrode is not oxidized or reduced during operation, whereas the electroactive coatings act as electrocatalysts, reducing or oxidizing substances contained in an electrolyte.
Examples of such coatings on titanium metal substrates are coatings containing chloride-oxidizing anode electrocatalysts made of precious metal oxides of ruthenium, iridium, osmium, rhodium, platinum, or palladium and mixtures thereof. These precious metals are optionally mixed with oxides of elements such as titanium, tantalum, zirconium, niobium, hafnium, lanthanides, actinides, or tin and mixtures thereof. These coatings are typically applied by forming aqueous or alcoholic solutions of soluble salts by any suitable means of uniform application followed by drying and baking at temperature sufficient to substantially decompose the salts into oxides.
In embodiments of the invention, an insoluble nanomaterial is added to the coating solution in an amount less than about 50% and preferably less than about 10% of the dry solids content of the coating solution by volume. Electrode coatings wear over time from mechanical or chemical means at a rate generally proportional to the square of current passed through the electrode. As the coating wears, voltage needed to drive the process increases, and in some cases, the faradaic efficiency of the electrochemical process decreases.
One problem solved by embodiments of the invention is the creation of electrode coatings with increased life relative to the amounts of precious metals used.
Another problem solved by embodiments of the is the reduction in power required by electrolysis.
A further problem solved by embodiments of the invention is that when coating electrocatalysts on a substrate, as the coating dries and bakes, mud-cracks can form in the coating. Mud-cracks slightly increase the surface area of the coating but also greatly decrease its mechanical integrity and durability. In one practice of this invention, the loading of electrocatalysts provided on each coat can be increased independently of the method of application while avoiding the formation of mud-cracking as the coating dries and bakes.
Yet another advantage of embodiments of the invention is that coatings containing nanomaterial can better be used for dip-coating of a substrate of an expanded mesh or woven wire cloth. When coatings of the prior art are used in this application, especially when the coating solutions contain water and surface-active materials, the openings of the substrate can become covered with a film which can dry and close off the mesh opening. The undesired coating over the mesh openings must then be brushed off. But when coatings of this invention are created, even when the volume % of the coating is less than 3%, the presence of fibers makes these films unstable as they dry, so that the films no longer cover mesh openings after drying and baking.
Other features and iterations of the invention are described in more detail below.
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.
The invention is electrode coatings containing either non-catalytic micromaterial or nanomaterial, or a combination thereof, which forms a reinforcing network in conjunction with active electrocatalysts comprised of mixed oxides optionally mixed with other oxides such as titanium, tantalum, etc. In one aspect, nanomaterial is defined as inorganic materials possessing at minimum one external dimension less than about 1000 nm, preferentially between about 1-100 nm. The term nanomaterial is meant to be broad and includes inorganic nanofibers (with a diameter of from about 10-1000 nm and a length at least ten times the diameter) and nanoparticles (a nanomaterial with a mean particle size of less than about 100 nm). In addition, microfibers with a diameter of less than about 10 microns, more preferably less than about 5 microns, and even more preferably under 1 micron, may also be used in coatings of the invention to achieve similar advantages. In some embodiments glass microfibers with at least one submicron external dimension, and nanoparticles, ideally with a low bulk density of less than about 700 g/L, and preferably less than about 200 g/L, or preferentially less than about 100 g/L, which is characterized by a filamentous structure that acts similar to fibrous nanomaterial to suppress and stabilize mud-cracking may be particularly advantageous. The coating is formed by a step of suspending nanomaterial in a solution containing the electrocatalysts, then a step of applying this solution to a substrate, and drying it to a thin film, followed by a final baking step in which electrocatalyst oxides are formed. An optional step may be needed of dispersing the nanomaterial in a first solution containing dispersants, which may or may not be acidified, prior to mixing it with a second solution containing only electrocatalyst components.
The nanomaterials to be used are formed from materials compatible with the electrolyte of the electrode application and must remain insoluble in the solution of electrocatalyst used for coating, typically forming a colloidal dispersion that may require agitation to remain dispersed. Nanofiber material must retain their fibrous form at the baking temperature of the coating process. The preferred shape of these fibers is a fiber or filament with a thickness of less than about 1 micron, and a length of at least 1 to 10 times its thickness. The filament may be electrically conductive or not, and is typically chosen from metal, ceramic, glass, or carbon. The term nanomaterial is intended to be very broad and includes nanofibers and nanoparticles that assemble in aggregates having high surface area and a bulk density that is less than about 20% of the inherent density of the solid material. For example, fumed silica, with typical bulk density of about 5% of the density of silicon dioxide and having a specific surface area of 50-500 m2/g has been found to be an effective additive to the coatings of this invention.
The unique additives of nanofiber used in coatings of this invention are preferably used in an amount defined by the volume ratio of the solid fiber to the dry volume of the coating. The benefit in increased life, reduced mud-cracking, and lower overvoltage are achieved using a volume ratio of from 0.3:100 up to 15:100. At the low end of this range, fibers with a high length to diameter ratio (for example 100:1), are effective at reducing mud-cracking in coatings or reinforcing the coating despite mud-cracking while simultaneously providing increased surface area. In the middle of the range, in one example, at 5% with a nanofiber having a 10:1 L/D, the coatings exhibit the largest decrease in voltage.
At the high-end of the range, pores are created in the coating as it dries and bakes that can increase the surface area of the coating without compromising its structural stability. Nanofibers used in coatings of this invention are wetted by the coating solution so that as the solution containing dispersed fibers dries, surface tension compresses the fibers and orients the fibers in random directions parallel with the substrate. This self-orienting behavior enables the fibers to reinforce the coating as it dries, preventing mud-cracks from forming and stabilizing those that do. As the coating bakes, the fibers retain their shape and reinforce the final coating, preventing it from cracking under the mechanical stress of operation. By using a maximum of 50% fibers of the dry volume of the coating, the coating retains electrical conductivity even when the fibers themselves are non-conductive. In coatings applied to valve metals such as titanium, niobium, or tantalum for use as anodes, the coating must protect the metal from oxidation. In this case, it has been demonstrated that a coating containing too large an amount of microfibers can become too porous and has reduced life.
A sample of titanium mesh was prepared by grit blasting and washing. A coating solution was prepared using 0.967% titanium and 0.88% ruthenium in isopropanol with 5.6% hydrochloric acid. The materials used to prepare this solution are tetra propyl titanate (brand name Tyzor TPT), an alcoholic solution containing 16.8% Titanium; ruthenium (III) chloride hydrate crystals containing 59.7% ruthenium, and a stock solution of anhydrous HCl in isopropanol containing 27.8% HCl; dry isopropanol was used to dilute the solution to the desired final strength. The coating was applied by:
The steps of counterexample 1 were followed except that an amount of glass microfibers was added to achieve 2% of the dry weight of the coating in the oxide form. The weight of titanium and ruthenium in solution were converted to the equivalent weight of RuO2 (MW 133.07) and TiO2 (MW 79.87). The calculated total dry weight of coating (RuO2, TiO2) in example 1 is calculated to be 2.766% of the total weight of coating solution. A suspension of glass microfiber B-X9-F purchased from Unifrax Corporation (Tonawanda, New York), and having an average diameter of about 0.2 to 0.3 microns, was made by adding 5.2 grams of dry microfiber to 194.8 grams of DI water to which was added acetic acid to adjust the pH to 2.8. This mixture was suspended using an 800-watt kitchen blender for approximately 5 minutes, after which it was observed that the fiber remained in suspension without settling and had formed a thick slurry. This slurry (3.24 grams) was added to 147.6 grams of the coating solution of example 1. The coating containing glass microfibers was used to dip coat a titanium mesh sample prepared and coated exactly as the samples of example 1, except that only 5 dip-and-bake cycles were performed. Four coated mesh samples were prepared using this procedure labeled IDs 41 through 44.
An accelerated life test was performed on the coated anodes of Example 1 and 2, XRF measurements were conducted to determine the ruthenium loading, and one sample from each type was operated in a diaphragm test cell where voltage was measured over a 60-day period. A comparison of the results from this testing is summarized in the following Table 1:
A sample of titanium mesh was prepared by methods well known in the art to ensure good adhesion. A coating solution was prepared using 0.55% titanium, 0.18% ruthenium, 0.21% iridium, and 0.07% palladium in isopropanol with 5.6% hydrochloric acid and 0.3% hydrogen peroxide. 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; hydrogen hexachloroiridate (IV) hydride crystals containing 39.2% iridium; palladium (II) chloride crystals containing 59.9% palladium; anhydrous HCl in isopropanol containing at least 22% HCl; and dry isopropanol to dilute the solution to the desired final strength. The coating was applied by the following steps:
The mesh was dipped in the coating solution and then hung vertically to allow excess coating to flow over the surface.
The coating was allowed to dry completely prior to baking at 490° C. for 20 minutes.
Steps 1-2 were repeated for 7 cycles with the mesh periodically flipped back to front with the same vertical orientation. A final bake after the 7th dip was performed for 2 hours at 490° C. Samples were labeled ID 32 and ID 34 as detailed in Table 2.
The steps of Example 3 (Counterexample) were followed with the following exceptions. To the coating solution was also added N82 titanium oxide nanopowder obtained from Magneli Materials, LLC, in a quantity equal to 4% by weight of the total coating solids. Note, the nanopowder was pre-dispersed in isopropanol by shearing prior to its addition to the coating solution. The coating was applied in the same manner as previously described for a total of 9 dip-and-bake cycles, followed by a 2-hour final bake. Before each dip, the coating solution was recirculated/agitated by stirring to suspend the nanopowder additive. Samples were labeled ID 41 and 44 as detailed in Table 2 below.
Voltage evaluations were performed on Examples 3 (Counterexample) and 4. A comparison on the results from these evaluations are summarized in Table 2 below. Notably, IDs 41 and 44 gained more uptake of coating solution, as demonstrated by the increase in iridium loading per dip, versus the control samples with no additive.
Voltage evaluation was conducted in lab cells electrolyzers that are operated at current densities around 7 kA/m2. Cell temperature is maintained at 85° C.; brine strength and anolyte strength are maintained around 18-23%. Anode voltages were monitored using a Luggin probe connected to a reservoir backfilled with anolyte with an Ag|AgCl reference electrode. Voltage was collected around four weeks of operation, with ID 44 showing a 30-mV advantage over the control ID 32.
Imaging was conducted on both the control sample with no additive, as well as the sample containing Magneli nanopowder. Note, even though ID 44 has a thicker coating with higher iridium loading, mud-cracking is visually reduced, providing a coating surface resistant to mechanical wear with increased durability.
Referring to
The steps of Example 3 (Counterexample) were followed with the following exceptions. A diluted coating solution was prepared using the targets 0.40% titanium, 0.13% ruthenium, 0.15% iridium, and 0.05% palladium in isopropanol with 5.6% hydrochloric acid and 0.3% hydrogen peroxide. The coating was applied in the same manner as previously described for a total of 9 dips, followed by a 2-hour final bake at 490° C. Sample set (quantity 9) was coated at production-scale; results reported in Table 3 are an average.
Similar steps to example 4 were followed with key exceptions. To the coating solution was added titanium (IV) oxide, Aeroxide® P25, with specific surface area measuring 35 to 65 m2/g and particle sizing <100 nm, in a quantity equal to 3%, 5%, and 7% of the dry solids content of the coating solution by volume. Note, the titanium dioxide nanopowder was pre-dispersed in isopropanol by shearing prior to its addition to the coating solution. The coating was applied via 9 dip-and-bake cycles, with a final bake following the last dip. Samples were labeled IDs 16, 17, and 18 in Table 3.
Recognizing the advantage of thicker coatings while avoiding detrimental mud-cracking, a coating solution was prepared starting with 0.65% titanium, maintaining the same metal mole ratios. The materials used to prepare this solution were titanium oxychloride, an acidic aqueous solution containing 12.1% titanium; ruthenium (III) chloride hydrate crystals containing 40.9% ruthenium; iridium (IV) chloride crystals containing 52.0% iridium; palladium (II) chloride crystals containing 59.9% palladium; anhydrous HCl in isopropanol containing at least 22% HCl; and dry isopropanol and water to dilute the solution to maintain an overall concentration of 50% isopropanol by weight. To the coating solution was CAB-O-SIL® EH-5, a high surface fumed silica, in a quantity equal to 4% of the dry solids content of the coating solution by volume. The fumed silica was pre-dispersed in water by shearing prior to its addition to the coating solution. The coating was applied via 8 dip-and-bake cycles, with a final bake following the last dip. Sample was labeled ID 19 in Table 3.
Accelerated corrosion tests and voltage evaluations were performed on Examples 5 (Counterexample) and 6-9. A comparison of the results from these evaluations are summarized in Table 3 below. Accelerated life tests were conducted in 1.5 M sodium sulfate (pH 2 adjusted with sulfuric acid) at 60° C. The addition of nanomaterial significantly extended anode coating life relative to the amount of iridium.
In view of improvements provided by the invention, global applications are expected to include uses in fuel cells, water electrolysis, batteries, and electrosynthesis of chlorine and caustic, hypochlorite, and chlorate.
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.
This application claims the benefit of priority of U.S. provisional patent application No. 63/597,659 filed Nov. 9, 2023, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63597659 | Nov 2023 | US |
