Electrode for electrochemical cells

Abstract

The electrode for electrochemical cells which are designed to carry out electrolysis is a carrier which is formed with an open-pore metal structure and which is provided, on the outward-pointing surface facing a separator of an electrochemical cell, with a coating which increases the specific surface area or with a coating which increases the specific surface area and has increased catalytic activity. The coating is formed with a metal, of which the particles are integrally bonded to one another and to the surface of the open-pore metal structure via sintering bridges and / or an organic binder. The coating is formed with a plurality of regions, between each of which uncoated joints are arranged.

Description

[0001] Electrode for electrochemical cells

[0002] The invention relates to an electrode for electrochemical cells designed to conduct electrolysis. The invention is particularly designed for the electrochemical splitting of water into gaseous hydrogen and oxygen.

[0003] Electrodes in these electrochemical cells must allow good permeability for the gas released during electrolysis. Furthermore, energy consumption during electrolysis should be as low as possible. For this purpose, catalysts can be used, which can also be applied in the form of a coating on an open-pore metal structure and can be used to great advantage. Sinterable powders, which can be suitable metal powders, alloy powders, metal oxide powders, or mixtures thereof, can and are used for this purpose. The particles are fixed to the respective surface by sintering. In addition to their catalytic functionality, the application of particles is also used to increase the specific surface area when suitable metal powders are used. Such coatings are typically applied to the surface of an electrode facing the separator of the electrochemical cell.An increased specific surface area improves the release and detachment of gas bubbles from the electrode surface. The most relevant manufacturing routes for depositing coatings, particularly a catalyst or catalyst precursor, or for increasing the specific surface area of ​​an electrode are electrochemical deposition, plasma spraying, knife coating of a dry powder bed or a wet powder suspension or powder paste, screen printing of powder pastes, wet powder spraying, and scattering powder particles using a polymer onto a substrate previously coated with a binder. It is also possible to apply a coating to form an electrode. A catalyst can then be applied to this coating.

[0004] The coating and / or, if applicable, a catalytically active chemical element can be applied in powder form and bonded to a substrate or coating surface using a polymer. A thermal treatment can then be performed, if necessary, to improve the bond.

[0005] It is also known and feasible in the invention to first apply a metallic powder to a surface, with which a porous layer of appropriate thickness for producing an electrode can be formed. A catalyst can then be applied to this porous metal layer in a second step. For example, nickel can first be applied and sintered, and then, in a second step, the catalyst can be applied to the surface of this sintered layer. The catalyst can either be a metal powder (which is then sintered again to form the active catalytic phase), a powder / polymer (no sintering, the powder is applied with a polymer (e.g. an ionomer), which is then fixed to the porous surface of the sintered metal layer), or it is also possible to apply the catalyst using a wet-chemical variant (i.e., deposition of metal salts followed by heat treatment).The manufacturing options described here can also be combined with each other or at least individual process steps can be carried out multiple times.

[0006] The best-known and most active catalysts are activated nickel catalysts (Raney-Ni alloys) made from Ni-Zn or Ni-Al-X (X = transition metal) precursors. Precious metal electrodes are also available, which are also described in WO 2021 / 083862 A1.

[0007] Among the activated nickel catalysts, Ni-Al-X alloys in particular show a special catalytic activity.

[0008] Two production routes are usually followed here:

[0009] - Plasma spraying of Ni-Al-X alloys and subsequent leaching of the Al species.

[0010] - The application of Al-X alloys, followed by heat treatment and subsequent leaching of the Al species.

[0011] Here, the route with additional heat treatment is particularly attractive because the heat treatment step creates a material and form-fitting bond between the developing Ni-Al-X coating and the substrate. During the heat treatment, an exothermic reaction occurs between an open-pore Ni substrate and the Al-X layer, or between a Ni-X substrate and an Al layer, resulting in the formation of Ni-Al-X phases. This results in, firstly, mechanical stresses in the Ni-Al-X layer and between the Ni-Al-X layer and the substrate. Secondly, a volume change occurs in the region of the forming aluminide phases, and thirdly, the intermetallic Ni-Al-X phases are generally very brittle. There are two major problems in the production of active electrodes that can be used for water electrolysis.

[0012] These are:

[0013] Distortion. The production of large (0.3 m to 1.6 m) electrodes is problematic because the coating of porous structures often leads to substrate distortion. This is usually associated with heat input, which leads to mechanical stresses and, due to the formation of new intermetallic phases, also to a change in volume. With a one-sided substrate coating, it is obvious that an increase in volume on the coated side leads to a convex curvature of the substrate, while a volume shrinkage results in a concave curvature. Examples of this include plasma coating processes, electroplating, and powder metallurgy production routes. Examples of these will be discussed later.

[0014] - Mechanical stability due to brittle phases. Raney Ni coatings have proven to be the most promising electrodes for both the hydrogen and oxygen evolution reactions. However, the resulting Ni-Al or Ni-Zn phases are brittle. Particularly when using Ni-Al-X, the Ni structures of the substrate, which is formed with an open-pore metal structure, sometimes transform into brittle intermetallic Ni-Al phases. This poses the risk of significantly deteriorating the stability and thus the structural integrity of the substrate, making the electrode unsafe to handle. The Ni structure has largely transformed into Ni-Al phases. As a result, the original ductile properties of the open-pore metal structure are lost in these areas.Previous electrode manufacturing routes circumvented these two problems by simply making it impossible to produce suitable electrodes without distortion, so other catalyst systems were used instead. To counteract embrittlement, very thick substrates (such as perforated sheets) were used, creating a very thick structure that provides stability to the electrode. However, this approach is not future-oriented in terms of resource conservation.

[0015] It is therefore an object of the invention to provide electrodes of electrochemical cells which are suitable for electrolysis, in particular the separate release of oxygen and hydrogen from water by means of electrolysis, and which have a relatively low intrinsic mass with sufficient strength, even to withstand complicated assembly, and in which only small internal mechanical stresses act.

[0016] According to the invention, this object is achieved with electrodes having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features defined in dependent claims.

[0017] In the electrodes according to the invention, a support formed with an open-pore metal structure, preferably a metal foam, is provided on the outwardly facing surface facing a separator of an electrochemical cell with a coating that increases the specific surface area or with a coating that increases the specific surface area and has an increased catalytic activity.

[0018] The coating is formed from a metal whose particles are bonded to each other and to the surface of the open-pore metal structure via sintered bridges. The coating is formed with several regions, each of which is separated by uncoated joints. Joints can also be at least partially filled with another material that has different properties than the electrode-forming material of a coating. These properties can include, for example, greater strength, elasticity, hydrophobicity, conductivity, or other catalytic activity (cascade reactivity).

[0019] The entire outward-facing surface of the electrode is therefore not completely coated. Uncoated areas remain between coated areas and form the gaps. In principle, the coating can be formed with all common metals used to date or metals suitable for electrolysis. The coating can be formed on the open-pore metal structure, which acts as a carrier, using all known and previously recognized methods. In addition to the use of a pure metal powder, a coating forming the electrode can also be formed with a metal oxide powder. In this case, at least partial chemical reduction, in which the respective metal is released, is required during the thermal treatment or a subsequent thermal treatment in a reducing atmosphere.Pure metal oxide powder can be used, or a mixture of the metal or another metal with which the respective oxide is formed, and a pure metal can be used. It is also possible to first apply metal powder to a surface and then metal oxide powder. During chemical reduction, the respective oxide is converted into the corresponding metal.

[0020] The material of the coating with which the electrode is formed should contain at least 90 mass% metal.

[0021] The use of metal oxide powder over pure metal powder has the particular advantage that the metal particles obtained during chemical reduction have smaller particle sizes than is cost-effectively possible with pure metal powder. With the smaller particle sizes of the metal used to form an electrode, the specific surface area of ​​an electrode can be significantly increased, which can be advantageously utilized in the respective electrochemical reaction. The chemical reduction can be carried out during one or more subsequent thermal treatments in a reducing atmosphere. Sintering can also be performed to ensure a cohesive bond between the coating on the respective surface and between the particles.

[0022] Areas that are not to be coated and where joints are to remain can be temporarily covered during the coating process. Suitable masks or screens can also be used.

[0023] The coating can be formed using known metals, including Raney alloys, which are known for their increased catalytic efficiency in electrolysis. The electrodes can be manufactured with or without heat treatment.

[0024] Options for the formation of the coating can also be found in DE 10 2017 216 566 A1 and DE 10 2017 126 569 A1.

[0025] The individual coated areas separated by joints should each have a maximum area of ​​1500 mm 2 , preferably maximum 1200 mm 2At least 80% of the surface of the substrate on which the coated areas are formed should be coated. The arrangement of the coated areas can be chosen in a variety of ways. They can be arranged in a row or column arrangement, but can also be formed, for example, in a spiral or meandering pattern.

[0026] The width of the joints should be at least 0.1 mm, preferably at least 0.5 mm, particularly preferably at least 1 mm, in order to ensure their desired effect.

[0027] The coated areas should be homogeneous, for example, in terms of the distribution of the metal powder over the surface, and have a thickness of at least 0.002 mm while still being gas-permeable. In addition to a uniform thickness, the porosity and electrochemical activity should also be uniform across the entire surface of the respective coated areas. On the outward-facing surface facing the separator, at least four coated areas should be formed.

[0028] Both respective electrodes of an electrochemical cell can be designed accordingly and arranged opposite one another on one side of a separator.

[0029] The coated areas can have a polygonal, circular, or elliptical outer contour. Square, hexagonal, or rectangular geometries are preferred. Coated areas can also be meandering or spiral-shaped.

[0030] The open-pore metal structure forming the support, in particular a metal foam, can consist of, for example, Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, an alloy containing at least one of these chemical elements, or an intermetallic compound, or be coated with it. A metal powder selected from Ni, Fe, Mo, Co, Cu, and Ag, or alloys of these metals, can be used for the coating.

[0031] In order to make Raney Ni electrodes in particular manageable for application, both distortion and brittleness should be minimized.

[0032] According to the invention, therefore, not the entire surface of the open-pore metal structure is coated. This creates a grid-like electrode that is both mechanically stable and warp-free. The uncoated areas, in the form of a grid-like lattice, create a kind of ductile hinge point, allowing the open-pore coated metal structure to compensate for warping in a ductile manner. This concept leads to easily manufacturable large electrodes that are mechanically stable while simultaneously offering high electrocatalytic activity and an increased surface area for water electrolysis.

[0033] The advantage of the electrode produced according to the invention over conventional electrodes, which are also made of porous, brittle materials, is that it remains ductile in two spatial directions, each along the uncoated linear surface areas, deviating by an angle of at least 10° from the horizontal plane, without breaking. This represents a decisive advantage when installed in an electrolyzer cell, where the contact pressure when using brittle electrodes can easily cause cracks or fractures, which can be avoided with the invention.

[0034] In surface areas where gaps are present, another material may also be applied that has different properties than the actual electrode material in the coated areas. Such a material may, for example, have greater ductility, altered wetting behavior, electrical conductivity, or catalytic activity than the electrode material.

[0035] The coating with joints between coated areas significantly increases the mechanical stability of the electrode, which in particular makes further production steps and installation much easier.

[0036] The invention also makes it possible to facilitate the removal of gas bubbles produced during electrolysis and to increase the energy efficiency of electrolysis.

[0037] It is also possible to weld the electrodes according to the invention to a bipolar plate. This also allows the ohmic losses at the anode of an electrolyzer to be minimized.

[0038] With the electrodes according to the invention, mechanical stresses or defects that could occur particularly during cooling after a thermal treatment can be avoided, at least to a large extent.

[0039] The invention will be explained below using examples. Example 1

[0040] The semi-finished product used was an open-pore shaped body made of nickel, average pore size of at least 400 pm, preferably at least 450 pm, with a porosity greater than 90%, preferably at least 95%, the dimensions 100 mm x 100 mm or, for example, 70 mm x 70 mm and a thickness of less than 2 mm, for example 1.6 mm.

[0041] Subsequently, using a 2 mm wide commercially available adhesive film, narrow areas of the surface to be coated can be covered on one side, both transversely and lengthwise, and protected from the coating material. This creates a pattern of 16 open-pore squares, each with an edge length of approximately 15 mm, separated from each other by the 2 mm wide adhesive strips. This allows for uncoated joints between coated areas with a width of at least 1 mm to be created.

[0042] For one-sided coating of the taped porous molded body, which had a mass of 4.9 g, a metallic aluminum powder with a particle size < 63 pm and a powder quantity of 1.0 g can be used.

[0043] For the one-sided coating of the surface of the open-pore nickel preform as a semi-finished product, steramide wax and an aqueous solution of polyvinylpyrrolidone were used as a binder. The surface of the semi-finished product was sprayed with the binder solution on the taped side before the aluminum powder was applied to the binder-coated surface and evenly distributed over the surface to be coated. The aluminum powder and the steramide wax were mixed for 10 minutes using a Turbula mixer and then sprinkled as a mixture onto the binder-sprayed surface of the nickel preform. The coated sample was then dried for 12 hours at room temperature, after which the adhesive strips were removed by hand. The masking and removal of the adhesive tape can also be performed mechanically.

[0044] Subsequently, a thermal treatment for debinding, in which the organic components were removed, and sintering were carried out in a nitrogen atmosphere. For this purpose, a tube furnace was heated at a heating rate of 9 K / min. The debinding, in which the organic components were removed, took place at 500 °C and a holding time of 30 minutes. After further heating, the aluminum reacted with the webs of the open-pore nickel molding and formed various nickel aluminide phases on the surface of the coated squares. These intermetallic phases exhibited high brittleness.

[0045] The one-sided segmented coated and sintered molded body was then leached. The aluminum-rich phases were leached in an aqueous solution of potassium hydroxide (1.8 M) and potassium / sodium tartrate (1.5 M) until gas evolution ceased. The samples were thoroughly washed with deionized water and stored in deionized water until use as cathodes in an alkaline laboratory electrolyzer setup.

[0046] The advantage of the electrode produced according to the invention over conventional electrodes, which are also made of porous, brittle materials, is that it remained ductile in two spatial directions, each along the uncoated lines, at an angle of at least 10° from the horizontal plane, without breaking. This represented a decisive advantage when installed in an electrolyzer cell, where cracks or fractures can easily occur due to the contact pressure when using brittle electrodes.

[0047] The coated areas on the surface had increased catalytic efficiency and an increased specific surface area due to the aluminide phases.

[0048] Example 2

[0049] The semi-finished product used was an open-pore shaped body made of nickel, average pore size 450 pm, with a porosity of approximately 95%, dimensions 190 mm x 70 mm and a thickness of 0.5 mm.

[0050] For the one-sided coating of the porous molded body as an open-pore metal structure, which has a mass of 5.6 g, a metallic nickel powder with an average particle size < 10 pm and a powder quantity of 4.0 g was used.

[0051] For the one-sided coating of the surface of the open-pore nickel preform as a semi-finished product, a suspension of nickel powder with an aqueous solution of polyvinylpyrrolidone was used. This suspension was homogenized in a speed mixer for 1 minute. The suspension was then passed through a fine-mesh sieve in a screen printing device, the mesh areas of which were open in areas measuring 80 mm x 15 mm, while the intermediate areas with a width of 3 mm were closed. This coated the rectangular areas of the semi-finished product with the nickel suspension, while the areas between the rectangles remained uncoated. However, other metals or alloys can also be screen printed. The coated sample was then dried for 24 hours at room temperature. This resulted in uncoated joints with a width of at least 2 mm.

[0052] Subsequently, a thermal treatment for debinding and sintering was carried out in a hydrogen atmosphere. For this purpose, a tube furnace was heated at a heating rate of 5 K / min. Debinding took place at 500 °C and a holding time of 60 min. After further heating at 3 K / min, the sintering process then took place at 850 °C and a holding time of 120 min. During this process, the nickel particles from the former suspension sintered to form a finely porous nickel powder bed on the surface of the coated rectangles. These powder bed regions are more brittle than the porous substrate. However, an increased specific surface area in the coated regions after sintering was also advantageously achieved.

[0053] Example 3

[0054] The semi-finished product used was an open-pore shaped body made of nickel, average pore size 580 pm, with a porosity of approximately 95%, dimensions 75 mm x 75 mm and a thickness of 1.0 mm.

[0055] Subsequently, narrow areas of the surface to be coated were covered on one side, both transversely and lengthwise, using a 2 mm wide, commercially available adhesive film to protect them from the coating material. The plan view resulted in a pattern of 16 open-pore squares, each with an edge length of just over 15 mm, separated from each other by the 2 mm wide film strips. This made it possible to create uncoated joints between the coated areas, which ultimately had a width of at least 1 mm. For the one-sided coating of the porous molded body as an open-pore metal structure, which had a mass of 4.6 g, a nickel oxide powder with an average particle size of < 1 pm and a powder quantity of 1.8 g was used.

[0056] For the one-sided coating of the surface of the open-pore nickel preform as a semi-finished product, a suspension of nickel oxide powder with an aqueous solution of polyvinylpyrrolidone with a volume of 15 ml was used. This suspension was homogenized in a speed mixer for 2 x 0.5 min. The suspension was then sprayed onto one side of the nickel preform, which was partially covered with foil, using a paint spray gun.

[0057] The coated sample was then dried at room temperature for 24 hours and the film strips were then removed manually.

[0058] Subsequently, a thermal treatment for calcination was carried out at 500°C for 30 minutes under air atmosphere in a muffle furnace. This created a surface structure with nickel oxide-coated squares on the open-pore nickel molded body, separated from each other by the former foil strips, thus enabling the entire molded body to maintain crack-free flexibility during further use.

Claims

Claims 1. An electrode for electrochemical cells designed to carry out electrolysis, in which a support formed with an open-pore metal structure is provided on the outwardly facing surface facing a separator of an electrochemical cell with a coating which increases the specific surface area or with a coating which increases the specific surface area and has increased catalytic activity, the coating being formed with a metal whose particles are bonded to one another and to the surface of the open-pore metal structure via sinter bridges and / or an organic binder, characterized in that the coating is formed with a plurality of regions, between which uncoated joints are arranged.

2. Electrode according to claim 1, characterized in that the coated areas each have a maximum area of ​​1500 mm2 , preferably maximum 1200 mm 2 and / or the width of the joints is at least 0.1 mm, preferably at least 0.5 mm, particularly preferably at least 1 mm.

3. Electrode according to one of the preceding claims, characterized in that at least four coated regions are formed on the outwardly facing surface.

4. Electrode according to one of the preceding claims, characterized in that the coated regions have a polygonal, circular, elliptical or winding outer contour or the coated regions are meander-shaped or spiral-shaped.

5. Electrode according to one of the preceding claims, characterized in that the open-pore metal structure, in particular a metal foam, consists of Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, an alloy of at least one of these chemical elements or an intermetallic compound and the coating is formed with metal powder selected from Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, Pt, Ru, Ir and Ag or alloys of these metals.

6. Electrode according to one of the preceding claims, characterized in that at least 80% of the surface of the carrier on which the coated regions are formed is coated.

7. Electrode according to one of the preceding claims, characterized in that the electrode is formed with a catalytically active chemical element or the surface is at least partially coated therewith.