Nickel-based anodes for oxygen evolution.

Abstract

The present invention relates to an anode for oxygen evolution in an electrolysis process comprising a nickel-based planar substrate having a first side and a second side, and a porous catalytic coating formed on at least one side of said substrate, said porous catalytic coating exhibiting a lamellar morphology consisting of metal patches and void patches, said metal patches being made of a material selected from nickel, nickel oxide, nickel-aluminum alloy, or combinations thereof. The present invention also relates to a method for the manufacture of such an anode using powder plasma spraying or electric wire arc spraying.

Description

[Technical field]

[0001] The present invention relates to nickel-based anodes for oxygen evolution in electrolysis processes, in particular oxygen evolution anodes for use in electrochemical cells for alkaline water electrolysis, and methods for their manufacture. [Background technology]

[0002] Alkaline water electrolysis is typically carried out in an electrochemical cell in which an anode compartment and a cathode compartment are separated by a suitable separator such as a diaphragm or membrane. An alkaline aqueous solution with a pH higher than 7, for example an aqueous KOH solution, is fed into the cell and a current flow is established between the electrodes of the respective compartments, i.e., the cathode and the anode, with a potential difference (cell voltage) typically in the range of 1.8-2.4 V. Under these conditions, water is decomposed into its components, gaseous hydrogen is evolved at the cathode and gaseous oxygen is evolved at the anode. The gaseous products are removed from the cell so that the cell can be operated continuously. The anodic oxygen evolution reaction in alkaline medium can be summarized as follows: 4OH - →O 2 +2H 2 O+4e -

[0003] Alkaline water electrolysis is typically carried out in the temperature range of 40-90 °C. Alkaline water electrolysis is a promising technology in the field of energy storage, especially for the storage of energy from fluctuating renewable energy sources such as solar and wind energy.

[0004] In this regard, it is particularly important to reduce the cost of the technology in terms of cheaper equipment, such as cheaper electrodes, but also in terms of the efficiency of the overall process. One important aspect of cell efficiency concerns the cell voltage required to effectively perform the electrolysis of water. The overall cell voltage is essentially governed by the reversible voltages, i.e. the thermodynamic contributions to the overall reaction, the voltage losses due to ohmic resistance in the system, the hydrogen overpotential, which is related to the rate of the hydrogen evolution reaction at the cathode, and the oxygen overpotential, which is related to the rate of the oxygen evolution reaction at the anode.

[0005] The kinetics of the oxygen evolution reaction is slow, which causes high anode overpotentials, resulting in high operating cell voltages and complicating large-scale commercialization of the technology.

[0006] Moreover, electrodes for water electrolysis must exhibit a certain resistance to unprotected shutdowns. Indeed, during a typical operation of an electrolysis plant consisting of a stack of single electrochemical cells, it is often required to shut down the power supply for maintenance of technical problems, causing and inversion of polarity harmful to the electrodes. Such inversions are usually avoided using an external polarization system (or polarizer), which maintains the current flow in the desired direction. This auxiliary component avoids potential electrode degradation due to metal dissolution or electrode corrosion, but increases the investment costs of the system.

[0007] In the prior art, preferred anodes / anode catalysts for alkaline water electrolysis include bare nickel (Ni) electrodes and electrodes having a precious metal-based catalytic coating, for example an iridium oxide (Ir)-based catalytic coating.

[0008] Bare nickel anodes are formed only from a nickel substrate such as a Ni mesh and can be easily manufactured at low cost, but they have a high oxygen overvoltage and a slow reaction rate.

[0009] Anodes with iridium-based catalytic coatings are manufactured by pyrolysis, a well-established technology. However, the iridium used in these electrodes is one of the least abundant precious metals in the earth's crust, making it not only expensive but also difficult to purchase in large quantities for industrial-scale manufacturing processes (e.g., gold is 40 times more abundant than iridium, and platinum is 10 times more abundant). Furthermore, iridium-based coatings are typically multi-layer coatings, which makes the manufacturing process costly. Prior art multi-layer catalytic coatings are, for example, LiNiO applied directly onto a Ni substrate. x Intermediate layer, NiCoO coated on the intermediate layer x The multi-layer composition may include an active layer, an iridium oxide outer layer, and a low resistance to ungraceful shutdown because the Co and Ir typically dissolve in the electrolyte solution upon polarity reversal.

[0010] Raney nickel electrodes are produced by thin film deposition of catalytic powder of Ni+Al by plasma spraying technique. Such Raney nickel electrodes are described, for example, in Henne R. et al., "Low pressure plasma spraying-properties and potential for manufacturing improved electrolysers", THIN SOLID FILMS, Vol. 119, No. 2, pp. 141-152 (1984-09-01). At industrial level, plasma spraying technique is not used much for catalytic coatings due to the high manufacturing costs and the health and safety hazards associated with this technique, such as noise, explosiveness, intense flames at temperatures above 3000 °C, and fumes. Furthermore, in the Raney nickel production process, there is an activation process achieved by leaching of aluminum from the catalytic coating, leaving almost pure nickel at the surface and greatly increasing the surface area. Another technical problem of Raney nickel deposited by plasma spraying is the highly pitted morphology of the resulting coating. In zero gap cells where the electrode is in contact with the membrane, the sharp pitted surfaces can damage the membrane. Furthermore, Raney nickel electrodes are typically used as hydrogen evolution cathodes in water electrolysis applications due to their high activity and, as a result, low hydrogen overvoltage. Under cathodic polarization, the pores of Raney nickel electrodes are stable. However, Raney nickel electrodes are not commonly used as oxygen evolution anodes because the pore structure tends to collapse under anodic polarization due to the formation of nickel hydroxide.

[0011] To overcome some of the shortcomings of Raney nickel cathodes, A. Kellenberger et al., "Roughness factor evaluation of thermal arc sprayed skeleton nickel electrodes", JOURNAL OF SOLID STATE ELECTROCHEMISTRY;CURRENT RESEARCH AND DEVELOPMENT IN SCIENCE AND TECHNOLOGY, Vol. 11, No. 1, pp. 84-89 (2005-11-04) describe a porous nickel cathode for hydrogen generation obtained by depositing layers of nickel and aluminum on a carbon steel plate by arc spraying technique using nickel wire (97% Ni, 3% Ti) and aluminum wire (99.5% Al). To increase the specific surface area of ​​the electrode, the samples were alkali leached.

[0012] It is therefore an object of the present invention to provide an anode for oxygen evolution in an electrochemical cell that avoids the electrochemical instability problems associated with Raney nickel electrodes used as anodes, while avoiding the use of expensive precious metals such as iridium and ruthenium by relying on high surface nickel-based anodes. The present invention also relates to a method for producing such an anode. Summary of the Invention

[0013] The present invention relates to an anode for oxygen evolution in an electrolysis process comprising a nickel-based planar substrate having a first side and a second side, and a porous catalytic coating formed on at least one side of said substrate, said porous catalytic coating exhibiting a lamellar morphology consisting of metal patches and void patches, said metal patches being made from a material selected from nickel, nickel oxide, nickel-aluminum alloy, or combinations thereof.

[0014] A "nickel-based" substrate in the sense of the present invention is a substrate that is made essentially from nickel or a nickel alloy.

[0015] A "planar" substrate is a substrate that is substantially flat and essentially two-dimensional, i.e., has dimensions perpendicular to a plane defined by its length and width, i.e., that are much greater than its height or thickness.

[0016] The present invention is based on the observation that the low electrochemical stability of electrocatalytic coatings based on Raney nickel under anodic polarization is due to the highly porous structure with voids in the submicrometer range, obtained by leaching aluminum from Raney nickel alloys, i.e. nickel / aluminum alloys. Although a large surface area of ​​the catalytic coating with an associated increase in catalytic activity is generally desirable, a balance must be established between the electrochemical stability of the anode and its electrochemical efficiency in industrial applications. The inventors have surprisingly found that an increase in the electrochemical and mechanical stability of the anode can be obtained by providing the electrocatalytic coating with a specific morphology, i.e. a lamellar morphology consisting of metal patches and void patches, where the metal patches are made of a material selected from nickel, nickel oxide, nickel-aluminum alloys or combinations thereof.

[0017] "Lamellar morphology" in the sense of the present invention refers to a coating that includes metal and void patches whose dimensions parallel to the planar substrate are primarily greater than those perpendicular to the planar substrate. In cross-sectional SEM (scanning electron microscope) images, the dimension parallel to the planar substrate can be considered as the diameter or length of the patch, and the dimension perpendicular to the planar substrate can be considered as the height of the patch. In the majority of metal and void patches, the height of the patch is less than 30%, more preferably less than 20%, of the diameter / length of the patch. According to the present invention, the metal patches are primarily made of nickel, i.e., they are distinguished from Raney nickel alloys and do not exhibit the microporous structure associated with Raney nickel after leaching of the aluminum component.

[0018] In one embodiment, the metal patches of the porous coating have a size distribution in which the diameter or length measured parallel to the planar substrate is predominantly at least 25 μm, i.e. at least 50% of the metal patches have a length of at least 25 μm. The height of the individual patches measured perpendicular to the planar substrate is typically less than 50% of their length, i.e. the majority of the metal patches extend parallel to the substrate. However, this does not exclude that the individual patches may have a circular cross-sectional shape or even extend perpendicular to the substrate.

[0019] In one embodiment, the porous catalyst coating has a porosity in the range of 5-50%, preferably 10-25%. "Porosity" refers to the ratio of void patches to the total area of ​​a frame of reference of a cross-sectional image of the coating, specifically an SEM image or an optical microscope image. The method for measuring porosity follows the scheme described by Ullah et al. in the paper "A framework for image processing, analysis and visualization of materials microstructures using ImageJ package" (Chinese Journal of Stereology and Image Analysis, Vol. 17, No. 4, December 2012, pp. 301-312).

[0020] The porous catalytic coating applied to one side of the substrate has a preferred thickness in the range of 50 μm to 200 μm, more preferably in the range of 100 μm to 180 μm. In one embodiment, the porous catalytic coating is formed on only one side of the metal substrate, but preferably, the porous catalytic coating is formed on both sides of the metal substrate. When combining a fairly thin substrate with a relatively thick coating, i.e., a coating in the range of up to 180 or 200 μm, mechanical stresses may cause the substrate to bend and the resulting anode to be non-planar. It is therefore generally preferred to apply the coating on both sides of the substrate. In this case, the preferred thickness of the coating of 50 to 200 μm or 100 μm to 180 μm refers to the coating on each side individually.

[0021] In one embodiment, the metal patches of the porous catalytic coating comprise 80-98 wt. % nickel or nickel oxide and 20-2 wt. % aluminum, with respect to metallic elements as determined by energy dispersive X-ray spectroscopy (EDX) scans. In one embodiment, the nickel component of the catalytic coating is primarily metallic nickel, preferably comprising 0.1-2 vol. % nickel oxide, as determined by image analysis of SEM or optical microscope images of cross sections of sample anodes. The volume percent of nickel oxide therefore refers to the frame of reference of these images that includes the metal patches and void patches.

[0022] The void patches are 3-20 m2 determined according to BET (Brunauer, Emmett, Teller) analysis. 2 / g, preferably 5 to 10m 2 / g. Thus, the surface area of ​​the catalyst coating of the present invention is typically 30-40 m due to the high proportion of sub-micrometer pores. 2 / g range, but provides improved mechanical and electrochemical stability, especially when functioning as an anode in alkaline water electrolysis.

[0023] The nickel-based substrate is preferably a porous substrate allowing electrolyte and gas to permeate through the substrate. The nickel-based substrate may be, for example, an expanded metal sheet. Preferably, however, the nickel-based substrate is a nickel mesh having a thickness in the range of 0.2-1.5 mm. Preferred mesh thicknesses are in the range of 0.5-1.2 mm, preferably about 0.9 mm. Typical mesh openings are diamond-shaped openings with a long width in the range of 2-10 mm and a short width in the range of 1-5 mm.

[0024] In order to obtain the lamellar morphology of the porous catalytic coating defined above, a suitable manufacturing method must be used, which avoids the submicron porosity structure associated with Raney nickel electrodes. The invention is based on the observation that, instead of using the nickel-aluminium alloy powder used in the manufacture of Raney nickel electrodes, the desired lamellar structure can be obtained by manufacturing an initial coating with at least two different starting components, i.e. a first component that is mainly a nickel component and a second component that is mainly an aluminium component, and then using a thermal spraying technique to leach the aluminium from the initial coating. In one embodiment, the first component is an essentially pure or low-alloyed nickel component, and the second component is an essentially pure or low-alloyed aluminium component. By "low-alloyed component" it is meant that the main metal species (i.e. nickel in the first component and aluminium in the second component) accounts for at least 80% by weight of the respective component. For example, it has been observed that using essentially pure nickel or low-alloyed nickel, e.g. up to Ni80Al20, as the first component results in a coating with similar properties. The preferred thermal spraying technique used to obtain the electrodes of the invention can be, for example, powder plasma spraying or electric wire arc spraying. Thus, in one embodiment, the invention relates to an anode for oxygen evolution, in which a porous catalytic coating is obtained by powder plasma spraying or electric wire arc spraying, on at least one side of a substrate, an initial coating comprising a mixture of a first component mainly comprising nickel and a second component mainly comprising aluminum, followed by leaching of the aluminum from the initial coating. When using a starting material comprising a first component mainly composed of nickel and a second component mainly composed of aluminum, the initial coating obtained will have much larger nickel and aluminum patches than electrodes obtained with Raney nickel. Subsequent leaching of the aluminum component will then result in the desired lamellar structure.

[0025] Therefore, the present invention provides the following a) providing a planar nickel-based metal substrate having a first surface and a second surface; b) providing a first coating material comprising at least nickel and providing a second coating material comprising at least aluminum; c) depositing molten droplets from the first coating material and the configured second coating material onto at least one side of the substrate to form an initial coating; d) leaching aluminum from said initial coating in an alkaline bath to form a porous catalytic coating. The present invention also relates to a method for producing an anode for oxygen evolution in an electrolytic process comprising:

[0026] In one embodiment, in step b), the first coating material is nickel powder and the second coating material is aluminum powder, and in step c), molten droplets of said nickel powder and molten droplets of said aluminum powder are deposited on the substrate by plasma spraying, preferably atmospheric plasma spraying.

[0027] In another embodiment, in step b), the first coating material is a first consumable metal wire comprising at least nickel and the second coating material is a second consumable wire comprising at least aluminum, and in step c), molten droplets from the first and second said metal wires are deposited on the substrate by electric wire arc spraying, preferably using air as carrier gas.

[0028] In addition to being advantageous in terms of cost, safety, and simplified manufacturing process, the use of air as a carrier has the additional advantage of partially pre-oxidizing the nickel particles, which imparts a beneficial effect on the coating catalytic properties for oxygen evolution.

[0029] In all the above embodiments, the initial coating may be applied on both sides of the substrate in step c).

[0030] The leaching step d) is preferably carried out in an aqueous alkaline hydroxide solution at a temperature in the range of 10 to 100° C. for 10 minutes to 36 hours, preferably 2 to 24 hours.

[0031] The invention will now be described in more detail with reference to certain preferred embodiments and the corresponding figures. [Brief description of the drawings]

[0032] [Figure 1] 1 is a schematic representation of a nickel mesh substrate of the present invention and an anode of the present invention having a lamellar coating on both sides of the substrate. [Diagram 2] 1 is a scheme for preparing an anode of the present invention using plasma spraying. [Diagram 3] 1 is a scheme for preparing an anode of the present invention using twin-wire arc spraying. [Figure 4] 1 is a SEM photograph of a cross-section an anode coated with an initial coating before Al leaching. [Diagram 5] FIG. 1 is a SEM photograph of a cross-section an anode coated with the final porous catalytic coating after Al leaching. [Figure 6] FIG. 2 shows the oxygen overvoltage of anodes of two embodiments of the present invention compared to anodes known in the art. [Figure 7] FIG. 2 shows a stability test of an anode according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The invention will now be described in more detail with reference to two embodiments in which the porous catalytic coating was prepared by electric wire arc spraying and plasma powder spraying, respectively. The anode of the invention will be compared with two counterexamples, namely a bare nickel anode and an anode comprising a precious metal catalytic coating.

[0034] A. Substrate In all examples and counter examples herein, a rhomboidal nickel mesh obtained by spreading and flattening a nickel sheet having a thickness of 0.5 mm has been used as the anode substrate. Figure 1a) shows a schematic perspective view of an anode 10 according to the invention. The anode 10 comprises a substrate 11 obtained by spreading and flattening a nickel sheet having a thickness of 0.5 mm, thereby obtaining a nickel mesh with rhomboidal openings with a long width of 0.5 mm and a short width of 2.8 mm. After spreading and flattening the mesh, the substrate 11 was sandblasted. Similar substrates were used in examples 1 and 2 and counter examples 1 and 2, respectively.

[0035] FIG. 1b) shows an enlarged cross-sectional view of the coated substrate in an embodiment in which both sides of the substrate are coated with a porous catalytic coating 12. As can be seen therein, the porous coating exhibits a lamellar morphology indicated by the white patches 13 representing metal patches and the black patches 14 representing void patches. In the schematic illustration, the metal patches 13 and the void patches 14 are shown to extend essentially parallel to the upper surface 15 and the lower surface 16 of the substrate 11. According to the invention, the desired lamellar morphology is obtained by producing molten drops of nickel and molten drops of aluminum which are sprayed onto the substrate. Upon impacting the substrate surface, the drops flatten and solidify, forming metallic nickel and metallic aluminum patches which extend essentially parallel to the substrate surface. As a result, the initial coating obtained already exhibits the lamellar morphology of the nickel and aluminum patches. By leaching the aluminum from the aluminum patches, the aluminum patches are transformed into void patches of the final porous catalytic coating.

[0036] To produce the molten droplets of metallic nickel and metallic aluminium, the invention proposes to use electric wire arc spraying or powder plasma spraying. EXAMPLES

[0037] B. Example 1 (EX1): Preparation of a nickel mesh electrode with a porous catalytic coating obtained by electric wire arc spraying In Example 1, a porous catalytic coating was obtained by applying an initial coating of nickel and aluminum on a substrate using a twin-wire arc spraying system as shown diagrammatically in Figure 2. The twin-wire arc spraying system is generally indicated by reference number 20. A first metal wire 21 made of nickel (or a nickel alloy such as Ni95Al5 or Ni80Al20) and having a diameter of 1.6 mm is fed by a first feed roller 22 through a first contact tube 23 that establishes an electrical contact between the first metal wire 21 and the anode 24 of a power source. The contact tube 23 guides the first wire towards a contact zone 25 in front of a nozzle 26 through which a jet of atomizing gas, for example compressed air, indicated by arrow 27, is sprayed towards the contact zone 25. A second wire 28 made of pure aluminum, also having a diameter of 1.6 mm, is fed by a second feed roller 29 through a second contact tube 30 that establishes an electrical connection of the wire with the cathode 31 of the power source. The power supply provides a variable current of 120A to 450A at a variable voltage of 20V to 35V. In the contact zone 25, the first and second wires 21, 28 are close enough together to form an electric arc between the tip of the first wire 21 and the tip of the second wire 28, without actually coming into physical contact that would result in a short circuit of the power supply. The electric arc melts the tip portions of the first and second wires 21, 28, and the atomizing gas 27, supplied at a pressure between 2 bar (30 psi) and 5.5 bar (80 psi), produces a spray jet 31 of molten droplets of material of the first wire 21 and the second wire 23, respectively. The molten droplets impact and solidify on the nickel mesh substrate 11, which is disposed in the path of the molten droplets of the spray jet 31, forming a coating 12 that includes metallic nickel and metallic aluminum patches. The coating 12 exhibits an essentially lamellar structure of individual nickel patches and individual aluminum patches. Before leaching, the lamellar structures exhibited a volumetric aluminum content between 3% and 15% by volume, as determined by image analysis of optical microscope or SEM images.The volume ratio of nickel to aluminum can be controlled, for example, by selecting appropriate feed rates for the first and second feed rollers 22, 29, respectively, and / or by selecting different diameters for the first and second wires 21, 28, respectively. However, even with similar wire diameters and similar feed rates, as in this embodiment, the volumetric aluminum content will generally be significantly lower than the volumetric nickel content, since a significant portion of the aluminum will vaporize in the contact zone 25 and never form any molten droplets.

[0038] After forming an initial coating with a thickness ranging from 50 μm to 200 μm, aluminum was leached from the coating by immersing the coated electrode in a 30 wt % KOH aqueous solution with a temperature of 80 °C for 5 h. After leaching, the final porosity of the porous coating was in the range of 10-25%. The residual aluminum content could no longer be determined by image analysis because the initial aluminum patches had leached out, resulting in void patches in the final porous coating. Thus, the residual aluminum content after leaching was determined by EDX analysis to be in the range of 2-20 wt %. Residual aluminum may result from aluminum present in the first component when using low-alloy nickel materials. Even if essentially pure nickel is used as the first component, some of the aluminum of the second component cannot be effectively leached out because it may diffuse into the nickel patches during the formation of the initial coating. The oxide content (nickel oxide) was determined to be in the range of 0.5-1.2 vol % by visual inspection and image analysis. The surface roughness R of the coating before leaching a After leaching, the diameter was 10-17 μm, and after leaching, the diameter was 350-550 μm.

[0039] C. Example 2 (EX2): Preparation of a nickel mesh electrode with a porous catalytic coating obtained by powder plasma spraying In Example 2, a porous catalytic coating was obtained by applying an initial coating of nickel and aluminum on a substrate using a powder plasma spray system as shown diagrammatically in FIG. 3. In Example 2, an atmospheric plasma spray system, generally designated by reference numeral 40 in FIG. 3, is used. The plasma spray system 40 includes a plasma torch 41 in which a plasma zone 42 is established between a cathode 43 connected to a negative pole 44 of a power supply and an anode 45 connected to an anode 46 of the power supply. The plasma torch 41 was operated at 20-60 kW, corresponding to a voltage of 40-75 V with a current of 400-750 A. The plasma zone 42 was formed by applying electrical power to a mixture of plasma gases, indicated by arrows 47 and 48. Typically, argon or helium was used as the plasma primary gas, and nitrogen or hydrogen was used as the plasma secondary gas. Powder particles were injected into the plasma zone 42 through a nozzle 49 using argon as a carrier gas, indicated by arrow 50. A preferred powder composition included 10 wt. % aluminum powder having a particle size in the range of 45-75 μm and 90 wt. % nickel powder having a particle size of 45-90 μm. The purity of the aluminum powder was at least 99% and the purity of the nickel powder was at least 90%. The powder particles melted in the plasma zone 42 to form molten droplets that were sprayed towards the substrate 11 via a spray cone, represented diagrammatically by reference numeral 51 in FIG. 3. Upon impact with the substrate, the molten droplets solidified to form a coating 12 comprising metallic nickel and metallic aluminum patches.

[0040] As in Example 1, after forming an initial coating with a thickness ranging from 50 μm to 200 μm, the aluminum was leached from the coating by immersing the coated electrode in a 30 wt % KOH solution having a temperature of 80° C. for typically 2 to 5 hours depending on the thickness of the coating. After leaching, the porous coating had similar properties to the coating obtained by wire arc spraying in Example 1.

[0041] D. Counterexample 1 (CEx1) Counterexample 1 is an anode with a precious metal-based catalytic coating. On a nickel mesh similar to the mesh used in Examples 1 and 2, a LiNiO base layer, NiCoO x Intermediate layer, and IrO x A coating consisting of a top layer and a three layer coating was obtained by sequentially applying each corresponding precursor solution onto the mesh substrate (or the respective underlayer) by brushing and pyrolysis.

[0042] E. Counterexample 2 (CEx2) Counterexample 2 is a bare nickel mesh anode with no additional coating. This anode corresponds to the substrate used in Examples 1, 2 and 3.

[0043] F. Morphology Scanning electron microscopy (SEM) was used to evaluate the morphology of the coating of the anode according to Example 1. Figure 4 shows a cross-sectional SEM image of the substrate coated with the initial coating, i.e. before leaching, and Figure 5 shows an SEM image of the final porous coating, i.e. after leaching. In Figures 4 and 5, reference 11 denotes the nickel substrate, reference 12 denotes the catalytic coating, and reference 13 denotes the nickel patches in the coating. In Figure 4, reference 14a denotes the aluminum patches, and in Figure 5, reference 14 denotes the voids obtained by leaching of aluminum from the aluminum patches 14a. As can be seen from Figure 5, the porous coating maintains its structural integrity after leaching, despite the fact that most of the aluminum has been leached out. As can be seen from the SEM images, the porous coating of the invention exhibits a lamellar structure of metal patches extending substantially parallel to the surface of the substrate and void patches also extending substantially parallel to the surface of the substrate.

[0044] The surface area of ​​the coating, measured by krypton adsorption, is between 1 and 10 m 2 / g range.

[0045] G. Oxygen overvoltage Corrected impedance single electrode potential (CISEP) testing was used to characterize the electrochemical performance of the anode of the present invention compared to prior art anodes used in alkaline water electrolysis. To measure the oxygen overpotential of the anode of the present invention, it was tested as an anode in a three-electrode beaker cell. The test conditions are summarized in Table 1. TIFF2025501727000002.tif63170

[0046] First, each sample was charged at 10 kA / m 2 The specimen is then subjected to a 2-hour pre-electrolysis (conditioning) at 10 kA / m. Afterwards, several chronopotentiometric steps are applied to the specimen. The final output of the CISEP test is 10 kA / m 2 The average value of three steps performed at 100° C. is corrected for the resistance of the electrolyte.

[0047] Table 2 and FIG. 6 summarize a comparison of the wire arc sprayed anode of Example 1 (Ex1), the powder plasma sprayed anode of Example 2 (Ex2), the iridium-based anode of Counterexample 1 (CEx1), and the bare nickel anode of Counterexample 2 (CEx2).

[0048] As can be seen from the figure, the noble metal anode of Comparative Example 1 (column 3 in FIG. 7) had a current of 10 kA / m 2 The wire arc sprayed electrode of Example 1 exhibits an oxygen overvoltage of 265 mV, which represents a typical value obtained from various samples determined individually in the range of 260-270 mV. The plasma sprayed anode of Example 2 exhibits an oxygen overvoltage of 290 mV, which represents an individual sample value in the range of 285-295 mV. TIFF2025501727000003.tif63170

[0049] The oxygen overvoltage obtained with the anode of the present invention is well below that of bare nickel anodes. The oxygen overvoltage of the wire arc spray anode of Example 1 is even in the range of the expensive precious metal anode of Counter Example 1.

[0050] H. Stability test (shutdown resistance) The anode of Example 1 (wire arc sprayed) was subjected to a stability test in which the anode was placed in an electrolytic cell with a nickel cathode acting as the counter electrode. To evaluate the anode's resistance to polarity reversal and to estimate the anode's resistance to simulated plant shutdowns, a shutdown test was performed under the operating conditions summarized in Table 3 below. TIFF2025501727000004.tif33170

[0051] The following test protocol was performed: After a 48 hour break-in period, a 6 hour shutdown was simulated by shortening the electrolysis cell and cooling to room temperature with the pump still running. After each shutdown period, electrolysis was continued for 6 hours at the operating conditions in Table 2 and the oxygen overvoltage was recorded.

[0052] In FIG. 7, the X-axis shows the number of shutdowns (#sd) and the Y-axis shows the change in cell voltage. As can be seen, the cell voltage difference was only 0.05V during 55 shutdown cycles. The arrow in FIG. 7 indicates the adjustment of the electrolyte back to 30 wt. % KOH after 25 shutdown cycles, resulting in a small step change in the cell voltage. These results demonstrate that the anode of the present invention provides exceptional mechanical and electrochemical stability even under the harsh conditions of intermittent operation typical of alkaline water electrolysis using renewable energy sources.

[0053] Compared with plasma sprayed Raney nickel electrodes, the anode of the present invention has the advantage of lower material costs since highly alloyed nickel-aluminum powder is not required. In particular, in the case of electric arc wire spraying, there is no need to generate high-energy plasma from a suitable gas mixture, resulting in lower process costs. According to the present invention, only compressed air is required as atomizing gas.

[0054] The method of the invention improves robustness due to the low content of nickel aluminides. Especially with electric are-wire spraying, coatings of several hundred microns can be prepared, which results in high mechanical robustness. On the other hand, the thickness of nickel Raney coatings must be limited to a few tens of micrometers, due to their poor mechanical properties, especially after leaching. Finally, a low crystal defect content is achieved by using only pure nickel and aluminum materials (wires or powders, respectively), or moderately alloyed nickel wires and powders. It is known that crystal defects increase the activity of the electrode, but the time stability is poor, since the defect-related activity decreases with time.

[0055] The foregoing description is not intended to limit the invention, which can be used according to various embodiments without departing from its scope, the scope of which is uniquely defined by the appended claims.

[0056] In the present description and claims, the terms "comprising," "including," and "containing" are not intended to exclude the presence of additional elements, components, or process steps.

[0057] The discussion of documents, items, materials, devices, articles and the like is included in this specification solely for the purpose of providing a background to the invention. No suggestion or representation is intended that any or all of these topics formed part of the prior art or common general knowledge in the art relevant to the invention prior to the priority date of each claim in this application.

Claims

1. 1. An anode for oxygen generation in an electrolytic process comprising: a nickel-based planar substrate having a first side and a second side; and a porous catalytic coating formed on at least one side of the substrate, wherein the porous catalytic coating exhibits a lamellar morphology consisting of metal patches and void patches, and the metal patches are made from a material selected from nickel, nickel oxide, a low-alloy nickel component containing at least 80% nickel by weight, or a combination thereof; An anode, wherein the metal patches of the porous coating have a size distribution such that their length measured parallel to a planar substrate is predominantly at least 25 μm.

2. 2. The anode of claim 1, wherein the porous catalytic coating has a porosity in the range of 5 to 50%, preferably 10 to 25%.

3. 10. The anode of claim 1, wherein the porous catalytic coating has a thickness in the range of 50 μm to 200 μm.

4. 4. The anode of claim 3, wherein the porous catalytic coating formed on at least one surface of the planar substrate has a thickness in the range of 100 μm to 180 μm.

5. 5. The anode of claim 4, wherein the porous catalytic coating is formed on both sides of the metal substrate.

6. 2. The anode of claim 1, wherein the metal patches of the porous catalytic coating comprise, in terms of metallic elements, 80 to 98 wt. % nickel or nickel oxide and 20 to 2 wt. % aluminum.

7. 7. The anode of claim 6, wherein the porous catalytic coating comprises 0.1 to 2 volume percent nickel oxide.

8. The porous catalyst coating has a thickness of 5 to 20 m ２ 10. The anode of claim 1 having a surface area in the range of 0.1 wt.

9. 2. The anode of claim 1, wherein the nickel-based substrate is a nickel mesh having a thickness in the range of 0.2 to 1.5 mm.

10. 2. The anode of claim 1, wherein the porous catalytic coating is obtained by powder plasma spraying or electric wire arc spraying onto at least one side of the substrate an initial coating comprising a mixture of a first component comprising primarily nickel and a second component comprising primarily aluminum, followed by leaching of aluminum from the initial coating.

11. 1. A method for producing an anode for oxygen evolution in an electrolysis process, comprising: a) providing a planar nickel-based metal substrate having a first side and a second side; b) providing a first coating material comprising at least 80% by weight nickel and a second coating material comprising at least 80% by weight aluminum; c) depositing molten droplets from the first coating material and molten droplets from the second coating material onto at least one side of the substrate to form an initial coating; d) leaching aluminum from the initial coating in an alkaline bath to form a porous catalytic coating.

1. A method for producing an anode for oxygen evolution in an electrolysis process, comprising:

12. 12. The method of claim 11, wherein in step b), the first coating material is nickel powder and the second coating material is aluminum powder, and in step c), molten droplets of the nickel powder and molten droplets of the aluminum powder are deposited on the substrate by plasma spraying.

13. 12. The method of claim 11, wherein in step b), the first coating material is a first consumable metal wire comprising at least nickel, and the second coating material is a second consumable wire comprising at least aluminum, and in step c), molten droplets from the first and second metal wires are deposited on the substrate by electric wire arc spraying.

14. The method of claim 11 , wherein in step c) an initial coating is applied on both sides of the substrate.

15. 12. The process according to claim 11, wherein step d) is carried out in an aqueous alkaline hydroxide solution at a temperature in the range of 10 to 100°C for a period of 10 minutes to 36 hours, preferably 2 to 24 hours.

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