Patent Application
20230151503
The present invention relates to a method for preparing a metal substrate suitable for use as electrode support in industrial electrochemical applications, and to an electrode prepared using the metal substrate thus obtained. Said method allows to obtain a metal substrate characterized by a surface with controlled roughness.
Generally electrochemical processes involve the evolution of different gases which subject the catalytic coating, which is generally deposited on a metal substrate, to a continuous chemical-physical stress; the adhesion of this coating to the metal substrate therefore plays a fundamental role in obtaining an electrode with an industrially acceptable service life.
It is known to those skilled in the art that the adhesion of the catalytic layer to the metal substrate is closely related to the surface roughness profile of the metal substrate itself, a roughness that guarantees a suitable anchoring base for the catalytic coating.
In literature several types of surface treatment suitable for imparting roughness to metal substrates are described. One procedure, for example, consists in dry sandblasting in which the surface of the metal substrate is abraded by a high pressure air impacting jet with sand or grit, or alternatively in wet sandblasting, in which the surface of the metal substrate is abraded by a high pressure water jet with sand or grit.
However, these treatments produce on the metal structure a considerable amount of residual compressive stress. When thin meshes or sheets are used, for example with a thickness of less than 1 mm, it is possible that the aforementioned residual stresses produce deformations on the metal substrate with consequent loss of planarity. For this reason, dry or wet blasting can only be applied to relatively thick metal substrates.
In addition, these mechanical impact treatments show disadvantages even when applied to thick metal substrates. The action of sandblasting, in fact, can cause a substantial increase in the hardness of the metal substrate which will result in cracks during the application of the catalytic coating and subsequent heat treatment.
A further drawback of these methods of the prior art is represented by the poor homogeneity of the roughness profile obtained, due to the complexity in controlling it as it depends on a combination of several treatment parameters such as sand or grit granulometry, the air or water pressure, the size of the nozzles and the angle of the jet with respect to the surface. Furthermore, once this type of surface treatment has been completed, the metal substrate may be polluted by residues of grit or sand which negatively affect the adhesion of the catalytic coating.
In addition, this method has the disadvantage due to the need to frequently change the sand or grit used since the particle size varies and decreases during the treatment, with a consequent reduction in the abrasion efficiency. Finally, the disposal of sand, or grit, polluted by the particles abraded by the treated metal substrates, is complex and expensive.
Other surface treatments known in the art include coupling sandblasting, with sand or grit, with a chemical etching in acid solutions, or with heat treatment followed by chemical etching in acid solutions and spraying by melting metals or ceramic oxides, thus favoring the growth of a rough layer. Each of these techniques, however, has drawbacks mainly related to the difficulty in imparting a homogeneous roughness profile to the entire treated surface.
From other fields of endeavor, surface treatment methods using electrochemical processes are known. For instance, Chinese patent application CN 110760862 A describes a surface treatment of very thin copper foils used in high-density PCB circuit boards where, within a complex multi-step procedure, an electrochemical treatment with low-concentration sulfuric acid at high current densities is employed. Chinese patent application CN 106521587 A relates to surface preparation of stainless steel strips used as structural components in nuclear plants for holding the fuel rods which employs an electrochemical treatment using concentrated sulfuric acid. The substrates used in these Chinese patent applications are not suitable as substrates for electrodes in industrial electrochemical applications.
Therefore, it would be desirable to provide a method of treating metal substrates suitable for electrodes in industrial electrochemical applications that overcomes both the drawbacks due to insufficient cleaning of the treated surface, which can negatively affect the adhesion of the catalytic coating, and the difficulty of obtaining homogeneous, reproducible roughness profile of the entire surface of a single metal substrate and on the different samples of a batch of an industrial production, as well as the costs of disposal of the spent sand or grit.
In the electrochemical process industry, competitiveness is linked to various factors, the main one being the reduction of energy consumption, directly linked to the current voltage of the global process.
The reduction of said current voltage can be achieved by using anodes and cathodes with catalytic coatings suitable to facilitate the required electrochemical processes, such as for example the evolution of hydrogen, chlorine or oxygen. The present invention is therefore directed, among other things, to a method for the treatment of metal meshes or sheets used as metal substrates of electrodes to be installed as anodes or cathodes in electrochemical cells where the minimization of electrical energy consumption is of utmost importance.
In the method according to the invention, the metal substrate can be any metal suitable to be used as electrodic support for electrochemical processes. In particular, as metal substrate for a cathode to be used in chlor-alkali electrolysis and water electrolysis processes. In this case, the most commonly used metal substrates can be chosen from nickel, nickel alloys, copper and steel.
When used as an electrode, a catalytic coating is typically applied onto the metal substrate. The catalytic coating of the metal substrate has the purpose of being electrochemically active for the reactions of interest and, by way of example, can comprise noble metals, their alloys or their oxides. The most commonly used noble metals can be chosen from ruthenium, platinum, palladium, rhodium or their alloys.
In addition to the claimed noble metals, the catalytic coating may comprise metals belonging to the rare earth group, or their oxides, without departing from the scope of the invention. The most frequently used metals belonging to the rare earth group can be chosen from praseodymium, cerium and lanthanum.
Said catalytic coating can be applied to the metal substrate by methods known in the art, which include, among others, galvanic methods or thermal decomposition methods, comprising several steps, of solutions containing suitable precursors of said metals conducted at temperatures ranging from 300° C. at 600° C. For each step of the catalytic layer deposition, the heat treatment has a typical duration ranging from a few minutes to a few tens, with an optional final heat treatment.
The performance of these types of electrodes depends, among other features, on the adhesion of the catalytic coating to the metal substrate, which is a function of a series of surface properties of said metal substrate and in particular of the cleanliness and degree of surface roughness.
The cleaning of said metal substrates can be obtained with any treatment known in the art to obtain clean metal substrates, including, among others, the use of particular solvents or mechanical cleaning treatments.
The degree of roughness, dependent on the roughness profile of the surface, is one of the parameters that affects the adhesion of the catalytic coating to the metal substrate. It is expressed through a numerical parameter Ra which is defined as the average value (in μm) of the absolute deviations of the roughness profile with respect to its average line.
In one embodiment, the invention is directed to a method for surface treatment of at least one face of a metal substrate of an electrode for electrochemical applications, comprising acid etching in the presence of electrical polarization at controlled current density. Current density is the amount of current applied to a surface, expressed in A/dm2.
Such as manufacturing process can have the advantage of being easily applicable to substrates of several geometries such as solid, perforated, stretched or woven sheets and/or meshes, possibly of very reduced thickness, without causing substantial changes to the surface treatment process according to the various geometries and dimensions, as would happen in the case of sandblasting treatments.
According to an embodiment, the surface treatment comprises a method for surface roughening of a metal substrate having a thickness below 1.2 mm. Said method comprises a step (a) of immersion of said metal substrate, of which it has to be provided the surface roughening, in an acid solution in the proximity of at least one further conductive element as counter-electrode. Said acid solution acts as the electrolyte in the electrochemical system thus formed. In the next step (b) of the present method, an electric potential difference is applied between the metal substrate and at least one counter-electrode such that an anodic current density is applied to the metal substrate to obtain the desired roughness on the surface of said metal substrate. Said metal substrate can comprise nickel, copper, nickel metal alloys. Under various embodiments, the metal substrate can be in any form able to accomplish its purpose and can be in the form of a mesh, for instance an expanded mesh or a woven mesh, or a sheet, for instance a punched or expanded sheet, with thicknesses below 1.2 mm.
In one embodiment, the metal substrate has a thickness of 0.5 mm or less.
In certain embodiments, the metal substrate has a thickness in the range between 0.01 to 1.2 mm, preferably in the range from 0.02 to 1 mm and. In certain embodiment, the metal substrate has a thickness in the range from 0.05 to 0.5 mm.
In one embodiment said acid solution acting as electrolyte in which the metal substrate is immersed is selected from the group of mineral acids comprising hydrochloric acid, nitric acid, sulfuric acid and boric acid at a concentration by weight of between 10-40%, preferably between 15-30%. In certain embodiments, the electrolyte comprises hydrochloric acid at a concentration by weight of between 10-40%, preferably between 15-30%.
Accordingly, the method according to the present invention for surface treatment of a metal substrate suitable for use as electrode support in electrochemical processes comprising the following steps:
Preferably steps (a) and (b) are only applied once with no intervening steps such as curing steps.
In certain embodiments, the electrolyte does not comprise any additional metals or metal salts, such as copper ions or chopper chloride, except for typical residual amounts in water, and in any case only in concentrations by weight of below 1%.
In an embodiment, said at least one counter-electrode can be in any form suitable for achieving its purpose; the choice, number, distance and dimensioning of said at least one counter-electrode depends on various factors, such as for example the degree of roughening to be provided to said metal substrate or the dimensions and thickness of said metal substrate.
The person skilled in the art will be able to determine the features of said at least one counter-electrode and the optimal distance of said at least one counter-electrode to said metal substrate according to the degree of roughening to be imparted to said metal substrate.
Said at least one counter-electrode can be advantageously sized and positioned in such a way as to be able to obtain different degrees of roughening on the same surface of said metal substrate.
Said at least one counter-electrode can be advantageously sized and positioned in such a way as to be able to provide a different degree of roughening on the two faces of said metal substrate.
Said at least one counter-electrode can be advantageously sized and positioned in such a way as to be able to provide a degree of roughening to a single face of said metal substrate.
It has been surprisingly observed that the appropriate regulation of said current density allows to operate making the most of the advantages of the invention such as in situations wherein said metal substrate has a thickness equal to or less than 0.1 mm, producing a homogeneous roughness profile sufficient to ensure optimal anchoring of the catalytic coating.
In the illustrated embodiment, the metal substrate and at least one counter-electrode are coupled to a power source. Said power source is configured to apply an anodic current density of between 0.1 and 30 A/dm2 to said metal substrate to roughen its surface. In one embodiment, the anodic current density is in the range between 5 and 10 A/dm2.
The current density, its application time and the temperature of the electrolyte are parameters that can be varied to obtain the desired roughening degree.
The inventors have surprisingly observed that said anodic current density can be applied for a time equal to or less than 120 minutes allowing to obtain a typical weight loss, less than 10%, corresponding to the roughening of the surface of a metal mesh with thickness equal or less than 1.2 mm. In one embodiment, the application time of said anodic current density is equal to or less than 60 minutes, preferably equal to or less than 30 minutes. In one embodiment, the application time of said anodic current density is between 2 and 10 minutes, for instance, an anodic current density in the range between 5 and 10 A/dm2 can be applied of a time between 2 and 10 minutes.
This result is particularly important when the metal substrate to be roughened is a thin metal surface, for example with a thickness of 0.5 mm or less. As is known to those skilled in the art, this type of metal substrate cannot be subjected to the surface treatment, necessary to ensure the best adhesion of the catalytic layer, by sandblasting, as the energy of the pressurized sand jet could cause severe deformation of said metal substrate which would also increase in the event of a heat treatment associated with the application of the catalytic coating.
When a voltage is applied between the metal substrate and at least one counter-electrode, the current flows between the metal substrate and at least one counter-electrode through the electrolyte. The positive and negative ions of the electrolyte are separated and are attracted to the metal substrate and by at least one counter-electrode having the opposite polarity to the ions. Positive ions are attracted to at least one counter-electrode which acts as a cathode and negative ions are attracted to the metal substrate which acts as an anode in the electrochemical system here described, oxidizing with consequent corrosion of its surface. As a result, a homogeneous roughness layer is formed on the surface of the metal substrate. The process requires a few minutes to have a weight loss corresponding to the roughening of the surface. The weight loss, expressed as a percentage with respect to the initial weight value, is usually used as an index of surface roughening due to the surface removal of material from the metal substrate.
In one embodiment the applied anodic current density is between 5 and 10 A/dm2; an anodic current density between these values has the advantage of allowing to obtain in a limited time of a few minutes, and in any case less than 10 minutes, a typical weight loss, between 3 and 6%, corresponding to the roughening of the surface of a metal mesh with a thickness of less than 1 mm. Thanks to the high speed of similar surface treatments, fast and continuous processes can be developed that can lead to greater efficiency of the production process.
In another embodiment, the invention is directed to an electrode for industrial electrochemical applications comprising a metal substrate and a catalytic coating, said metal substrate being a mesh or sheet having a controlled roughness profile provided by an acid etching treatment in the presence of polarization according to the method described above and having a catalytic coating comprising one or more noble metals, their alloy or their oxides and/or one or more metals belonging to the rare earth group or their oxides.
In some embodiments, the temperature of the electrolyte can be varied between 15 and 40° C.
In a further embodiment, the present invention is directed to a method for manufacturing an electrode for gas evolution in electrochemical processes, comprising the steps of treating a metal substrate with the method described above and applying a catalytic coating comprising one or more noble or alloy metals or their oxides and/or one or more metals belonging to the rare earth group or their oxides, to said treated metal substrate.
In a further embodiment, the invention is directed to an electrode for industrial electrochemical applications comprising a metal substrate and a catalytic coating, said metal substrate being a mesh or sheet having a controlled roughness profile provided by an acid etching treatment in the presence of polarization according to the method described above and having a catalytic coating comprising one or more noble metals selected from ruthenium, platinum, palladium, rhodium or their alloys or their oxides and/or one or more metals belonging to the selected rare earth group between praseodymium, cerium, lanthanum or their oxides. The method for treating the metal substrate according to the present invention results in the metal substrate having roughened surface with a high degree of homogeneity in the roughness profile. Typically, a degree of homogeneity of the roughness profile expressed as the mean square deviation of the Ra values of less than 25% (σ<25%), preferably less than 20% is achieved. The surface treatment typically results in the substrate experiencing a weight loss between 3 and 6% (calculated as the weight difference of the substrate before and after treatment divided by the weight of the substrate before treatment times 100%).
Under a further aspect, the invention is directed to an electrode for industrial electrochemical applications comprising a metal substrate and a catalytic coating, said metal substrate being a mesh or sheet provided with a controlled roughness profile, different in the two faces of said substrate, provided by an acid etching treatment in the presence of polarization according to the method described above.
Under a further aspect, the invention is directed to an electrode for industrial electrochemical applications comprising a metal substrate and a catalytic coating, said metal substrate being a mesh or sheet with a thickness equal to or less than 1.2 mm and provided with degrees of roughening different on the same surface of said substrate, provided by an acid etching treatment in the presence of polarization according to the method described above.
Under a further aspect, the invention is directed to an electrode for industrial electrochemical applications comprising a metal substrate and a catalytic coating, said metal substrate being a mesh or sheet with a thickness equal to or less than 0.1 mm and provided with a profile of controlled roughness provided by an acid etching treatment in the presence of polarization according to the method described above and having a catalytic coating comprising one or more noble metals or their alloy or their oxides and/or one or more metals belonging to the rare earth group or their oxides.
According to an embodiment, the method of preparation of said metal substrate can comprise a further step (c), subsequent to step (b), represented by the electrodeposition of nickel directly on the metal substrate through cathodic polarization. Said embodiment comprises the addition to said electrolyte of a nickel salt in a weight concentration of between 100 and 300 g/l and subsequent application of a cathodic current density to said metal substrate of between 0.1 and 3 A/dm2 for a time between 0.5 and 120 minutes at a temperature between 15 and 70° C.
Said further step (c) can contribute to increasing the surface area of said metal substrate through nickel electrodeposition which can serve to provide specific properties, for example, to further improve the conductivity and/or the catalytic activity of the coating, to inhibit undesirable side reactions or to improve the physical or chemical stability of the coating.
Under a further aspect, the invention relates to a cell for water electrolysis or for the electrolysis of alkaline chloride solutions comprising an anodic compartment and a cathodic compartment, separated by an ion exchange membrane or a diaphragm where the cathodic compartment is equipped with a cathode for hydrogen evolution having a metal substrate obtained with the method of the invention.
Under a further aspect, the invention relates to an electrolyser for the production of chlorine and alkali starting from alkaline brine, comprising a modular arrangement of electrolytic cells with the anodic and cathodic compartments separated by ion exchange membranes or diaphragms, wherein the cathodic compartment comprises a cathode having a metal substrate obtained with the method of the invention.
Under a further aspect, the invention relates to an electrolyzer for the production of hydrogen by electrolysis of water comprising an anodic compartment and a cathodic compartment separated by a diaphragm where the cathodic compartment is equipped with a cathode for the evolution of hydrogen having a metal substrate obtained with the method of the invention.
The following examples are included to demonstrate particular embodiments of the invention, the practicability of which has been extensively verified in the range of values claimed. It will remain clear to those skilled in the art that the compositions and techniques described in the following examples represent compositions and techniques of which the inventors have found a good functioning in the practice of the invention; however, the person skilled in the art will also appreciate that in light of the present description, various changes can be made to the various embodiments described, still giving rise to identical or similar results without departing from the scope of the invention.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to standard procedure, then subjected to a surface treatment by immersion in a 20% HCl solution at room temperature close to a nickel counter-electrode. An anodic current density equivalent to 10 A/m2 was applied to the nickel mesh for 1.7 minutes.
At the end of the treatment, the weight loss was verified and the degree of roughness was measured in different points of the mesh
The mesh thus obtained was identified as sample E1
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to standard procedure, then subjected to a surface treatment by immersion in a 20% HCl solution at room temperature close to a nickel counter-electrode. An anodic current density equivalent to 5 A/m2 was applied to the nickel mesh for 3.3 minutes.
At the end of the treatment, the weight loss was verified and the degree of roughness was measured in different points of the mesh.
The mesh thus obtained was identified as sample E2.
A nickel mesh of dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to the standard procedure, then subjected to a surface treatment by immersion in a 10% HCl solution at room temperature close to a nickel counter-electrode. An anodic current density equivalent to 12 A/m2 was applied to the nickel mesh for 1.6 minutes.
At the end of the treatment, the weight loss was verified and the degree of roughness was measured in different points of the mesh.
The mesh thus obtained was identified as sample E3.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to standard procedure, then subjected to a surface treatment by immersion in a 20% HCl solution at room temperature close to a nickel counter-electrode. An anodic current density equivalent to 1.5 A/m2 was applied to the nickel mesh for 6 minutes.
It was then coated with 5 coats of an aqueous solution containing Pt, Pr and Pd with a 15-minute heat treatment at 450° C. after each coat until obtaining a coating of 1.90 g/m2 of Pt, 1.24 g/m2 of Pd and 3.17 g/m2 of Pr.
On the catalytic layer thus obtained 4 coats of a second solution containing Pt, Pr and Pd were applied in a different ratio compared to the first solution with a heat treatment of 15 minutes at 450° C. after each coat until obtaining a coating of 1.77 g/m2 of Pt, 1.18 g/m2 of Pd and 1.59 g/m2 of Pr.
The electrode thus obtained was identified as sample E4.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to standard procedure, then subjected to a surface treatment by immersion in a 20% HCl solution at room temperature close to a nickel counter-electrode. An anodic current density equivalent to 1.5 A/m2 was applied to the nickel mesh for 3 minutes.
The nickel mesh was subsequently subjected to a nickel electro-deposition treatment after the addition of NiCl2, at a concentration of 190 g/l, to the HCl solution 20%. A cathodic current density equivalent to 1.5 A/m2 was applied to the nickel mesh for 13 minutes.
The nickel mesh thus obtained was coated with 5 coats of an aqueous solution containing Pt, Pr and Pd with a heat treatment of 15 minutes at 450° C. after each coat until obtaining a coating of 1.90 g/m2 of Pt, 1.24 g/m2 of Pd and 3.17 g/m2 of Pr.
On the catalytic layer thus obtained 4 coats of a second solution containing Pt, Pr and Pd were applied in a different ratio compared to the first solution with a heat treatment of 15 minutes at 450° C. after each coat until obtaining a coating of 1.77 g/m2 of Pt, 1.18 g/m2 of Pd and 1.59 g/m2 of Pr.
The electrode thus obtained was identified as sample E5.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to a standard procedure, then subjected to a process of sandblasting with corundum and etching in 20% HCl at room temperature for a time of 1100 minutes.
At the end of the treatment, the weight loss was verified and the degree of roughness was measured in different points of the mesh.
The mesh thus obtained was identified as sample CE1 sample.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to standard procedure, then subjected to a process of sandblasting with corundum and etching in 20% HCl at a temperature of 60° C. for a period of 40 minutes.
At the end of the treatment, the weight loss was verified and the degree of roughness was measured in different points of the mesh.
The mesh thus obtained was identified as sample CE2.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was washed and degreased with acetone according to standard procedure, then subjected to a process of sandblasting with corundum and etching in HNO3 21% at room temperature for a time of 15 minutes.
At the end of the treatment, the weight loss was verified and the degree of roughness was measured in different points of the mesh.
The mesh thus obtained was identified as sample CE3.
A nickel mesh with dimensions 100 mm×100 mm×0.89 mm was subjected to an etching process in 20% HCl at a temperature of 60° C. for 5 minutes.
The mesh was then coated with 5 coats of an aqueous solution containing Pt, Pr and Pd with a 15-minute heat treatment at 450° C. after each coat until obtaining a coating of 1.90 g/m2 of Pt, 1.24 g/m2 of Pd and 3.17 g/m2 of Pr.
On the catalytic layer thus obtained 4 coats of a second solution containing Pt, Pr and Pd were applied in a different ratio compared to the first solution with a heat treatment of 15 minutes at 450° C. after each coat until obtaining a coating of 1.77 g/m2 of Pt, 1.18 g/m2 of Pd and 1.59 g/m2 of Pr.
The electrode thus obtained was identified as sample CE4.
A nickel mesh with dimensions of 100 mm×100 mm×0.89 mm was subjected to a process of sandblasting with corundum, etching in 20% HCl at room temperature and stress relieving through heat treatment according to the procedure known in the art. The mesh was then coated with 5 coats of an aqueous solution containing Pt, Pr and Pd with a 15-minute heat treatment at 450° C. after each coat until obtaining a coating of 1.90 g/m2 of Pt, 1.24 g/m2 of Pd and 3.17 g/m2 of Pr.
On the catalytic layer thus obtained 4 coats of a second solution containing Pt, Pr and Pd were applied in a different ratio compared to the first solution with a heat treatment of 15 minutes at 450° C. after each coat until obtaining a coating of 1.77 g/m2 of Pt, 1.18 g/m2 of Pd and 1.59 g/m2 of Pr.
The electrode thus obtained was identified as sample CE5.
Table 1 reports the results of the tests carried out to evaluate the time necessary to achieve a weight loss of between 3 and 6% of a metal substrate; the degree of homogeneity of the roughness profile was also measured, expressed as the mean square deviation in % (% a) of the Ra values measured in different points of the nickel mesh.
The samples of example E5 and counterexample CE5 described above were subjected to performance tests, under hydrogen evolution, in a laboratory cell fed with 32% NaOH at a temperature of 90° C., furthermore they were subsequently subjected to cyclic voltammetry tests in the voltage range from −1 to +0.5 V/NHE with a scan rate of 10 mV/s.
Table 2 reports the initial cathodic voltage and the one after 25 cycles of cyclic voltammetry (25 CV), index of resistance to inversions and therefore of robustness, measured at a current density of 3 kA/m2.
The previous description does not intend to limit the invention, which can be used according to different embodiments without thereby deviating from the purposes and whose scope is uniquely defined by the attached claims.
In the description and claims of the present application, the term “comprises” and “contains” and their variants such as “comprising” and “containing” are not intended to exclude the presence of other elements, components or additional process steps.
Discussion of documents, records, materials, apparatuses, articles and the like is included in the text for the sole purpose of providing context to the present invention; However, it is not to be understood that this matter or part of it constituted general knowledge in the field relating to the invention before the priority date of each of the claims attached to this application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102020000006187 | Mar 2020 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/057498 | 3/23/2021 | WO |
