Electrode for gas evolution in electrolytic processes

Abstract

The invention relates to an electrode for gas evolution in an electrolytic process, comprising a metal substrate and a coating formed on the substrate, the coating comprising at least a catalytic porous outer layer comprising porous nickel oxide regions dispersed within a solid nickel oxide binder, and a method for producing such an electrode from preformed nickel vanadium oxide particles.

Description

Technical Field

[0001] This invention relates to an electrode for gas evolution during electrolysis, comprising a nickel substrate and a nickel-based catalytic coating. Such an electrode can be particularly used as an anode in an electrochemical cell, for example as an oxygen evolution anode in alkaline water electrolysis. Background Technology

[0002] Alkaline water electrolysis is typically carried out in an electrochemical cell where the anode and cathode compartments are separated by a suitable separator, such as a diaphragm or membrane. An alkaline aqueous solution with a pH higher than 7, such as KOH, is supplied to the cell, and a current flows between the electrodes in the cathode and anode compartments, i.e., between the cathode and anode, with a potential difference (cell voltage) typically ranging from 1.8 to 2.4 V. Under these conditions, water is decomposed into its components, resulting in the evolution of gaseous hydrogen at the cathode and gaseous oxygen at the anode. Removal of the gaseous products from the cell allows for continuous operation. The oxygen evolution reaction at the anode in an alkaline medium can be summarized as follows:

[0003] 4OH - →O2 + 2H2O + 4e -

[0004] Alkaline water electrolysis is typically carried out in a temperature range of 40–90°C. Alkaline water electrolysis is a promising technology in the field of energy storage, especially for storing energy from fluctuating renewable energy sources such as solar and wind power.

[0005] In this regard, it is particularly important to reduce technology costs in terms of less expensive equipment, such as less expensive electrodes, and in terms of overall process efficiency. An important aspect of cell efficiency involves the required cell voltage for efficient water electrolysis. The overall cell voltage is essentially controlled by the following: the reversible voltage, i.e., its thermodynamic contribution to the overall reaction; the voltage loss due to ohmic resistance in the system; the hydrogen overpotential related to the kinetics of the hydrogen evolution reaction at the cathode; and the oxygen overpotential related to the kinetics of the oxygen evolution reaction at the anode.

[0006] The oxygen evolution reaction has slow kinetics, which is the reason for the high overpotential at the anode. This results in increased operating cell voltage and difficulties in the large-scale commercialization of this technology.

[0007] Furthermore, the electrodes in water electrolysis should exhibit some tolerance to unprotected shutdowns. In fact, during typical operation of an electrolysis unit consisting of a cluster of individual electrochemical cells, maintenance due to technical issues frequently necessitates power outages, causing harmful polarity reversals of the electrodes. Such reversals are typically avoided using an external polarization system (or polarizer) that maintains current flow in the desired direction. This auxiliary component mitigates potential electrode degradation caused by metal dissolution or electrode corrosion but increases the system's initial investment cost.

[0008] In the prior art, preferred anode / anode catalysts for alkaline water electrolysis include bare nickel (Ni) electrodes, Raney nickel (Ni+Al) electrodes, and electrodes with an iridium oxide (Ir)-based catalytic coating.

[0009] Bare nickel electrodes are formed solely from nickel substrates such as Ni mesh, which can be easily manufactured at low cost but exhibit high oxygen overpotentials, resulting in slow kinetics.

[0010] Lanny nickel electrodes are manufactured via plasma spraying, which involves the deposition of a thin film of Ni+Al catalytic powder. At the industrial level, plasma spraying is not commonly used for catalytic coatings due to its high production costs and associated health and safety hazards such as noise, explosions, intense flames exceeding 3000°C, and fumes. Furthermore, Lanny nickel manufacturing methods include activation processes, which involve leaching aluminum from the catalytic coating, leaving nearly pure nickel on the surface, and significantly increasing the surface area. During the Al dissolution reaction, H2 is generated, which poses a problem during the manufacturing process due to the sudden exothermic reaction. Another technical problem with Lanny nickel deposited via plasma spraying is the rather jagged morphology of the coating. Sharp jagged surfaces in the zero-gap grooves where the electrode contacts the film can cause damage to the film.

[0011] Electrodes with iridium-based catalytic coatings are produced via thermal decomposition, a well-established technique offering fewer hazards. However, iridium, used in these electrodes, is one of the least abundant precious metals in the Earth's crust, leading not only to high prices but also to difficulties in procuring its bulk for industrial-scale manufacturing (e.g., gold is 40 times more abundant than iridium and platinum is 10 times more abundant). Furthermore, iridium-based coatings are typically multilayers, resulting in expensive manufacturing processes. Existing multilayer catalytic coatings may comprise, for example, LiNiO applied directly to a Ni substrate. x Intermediate layer, NiCoO applied to the intermediate layer x The active layer and the outer iridium oxide layer. This multilayer composition exhibits low tolerance to unprotected shutdown because Co and Ir typically dissolve into the electrolyte solution during polarity reversal.

[0012] Other techniques for increasing the surface area of ​​nickel-based electrode substrates are also known: CN110394180A describes a method for preparing nickel oxide with photocatalytic properties, wherein a nickel substrate is subjected to direct anodic oxidation surface treatment to obtain a nickel hydroxide film, and then the film is annealed to obtain a nickel oxide film. CN 110863211 A, CN 109972158 A, CN110438528B, and CN 110952111 A describe foamed nickel electrodes having an outer surface layer comprising nickel hydroxide and nickel oxide. The foamed nickel substrate acts as a scaffold, providing a higher surface area than the initial bulk nickel material, and these prior art documents describe different surface treatment methods for further increasing the effective catalytic area of ​​the electrode.

[0013] In the applicant's Italian patent application IT 2020000020575 (corresponding to international patent application PCT / EP2021 / 073783), an improved electrode, particularly for alkaline water electrolysis, is described, which avoids reliance on expensive precious metal-based catalytic coatings by employing a porous nickel oxide catalytic coating free of precious metals. The porous coating is obtained by leaching vanadium from a nickel oxide / vanadium oxide coating. However, it has been found that leaching vanadium from the coating reduces its mechanical stability.

[0014] Therefore, the object of the present invention is to provide a cost-effective electrode with a porous nickel oxide coating that exhibits improved mechanical stability while maintaining the low oxygen overvoltage found in alkaline water electrolysis applications as described in IT 2020000020575. Summary of the Invention

[0015] This invention is based on the concept of an electrochemically active thin film for oxygen evolution exhibiting a very high surface area. The high surface area of ​​the coating allows a large number of electrolytes to contact the catalyst and its active sites, thereby enhancing electrochemical performance, for example, for the production of gaseous oxygen (O2). By employing a coating comprising porous nickel oxide regions dispersed within a uniform nickel oxide binder, a stable, highly porous nickel oxide coating particularly suitable for the oxygen evolution reaction can be produced.

[0016] The invention is described in the appended claims.

[0017] This invention relates to an electrode for gas evolution during electrolysis, comprising a metal substrate and a coating formed on said substrate, wherein the coating comprises at least a catalytic porous outer layer containing porous nickel oxide regions dispersed within a nickel oxide binder. The coating of this invention exhibits high porosity while maintaining high mechanical stability.

[0018] For the purposes of this invention, the "nickel oxide binder" is a substantially continuous and substantially homogeneous nickel oxide phase. Such a continuous phase can be produced by applying a homogeneous precursor solution containing nickel salts to a substrate, for example by spraying or brushing, followed by heat treatment. As is known in the art, such heat treatment will result in a substantially homogeneous coating, however, which acts as a binder in the context of this invention. Such a substantially homogeneous nickel oxide coating is mechanically very stable but exhibits a high oxygen evolution overpotential and is therefore unsuitable for alkaline water electrolysis.

[0019] For the purposes of this invention, a "porous nickel oxide region" is a region with increased porosity compared to the surrounding nickel oxide binder. The porous nickel oxide region is therefore a substantially concentrated area within the nickel oxide binder, where the porosity is significantly higher than any remaining porosity of the nickel oxide binder, typically at least twice as high. Therefore, this invention proposes to reduce the overpotential of oxygen evolution while maintaining the mechanical stability of the coating by providing porous nickel oxide regions dispersed within the nickel oxide binder.

[0020] Porous nickel oxide regions within a nickel oxide binder can be obtained in various ways. According to one embodiment of the invention, the catalytic coating substantially exhibits the characteristics of a solid / solid dispersion, wherein solid porous nickel oxide particles are embedded in a solid nickel oxide binder. The solid porous nickel oxide particles thus constitute the aforementioned defined regions of porous nickel oxide. Therefore, the present invention proposes a novel application technique for applying catalyst particles in powder form to an electrode substrate.

[0021] For example, in a preferred embodiment of the invention, the porous nickel oxide region is based on pre-fabricated nickel vanadium oxide particles (Ni(V)O). x The porosity of these particles is obtained by employing the concept of removing sacrificial metals through metallurgical selective leaching, i.e., removing vanadium oxide from the particles through alkaline leaching of the vanadium oxide component.

[0022] In one embodiment, the catalytic porous outer layer can be obtained by heat treatment of a suitable precursor solution applied to a metallic substrate of the electrode. The precursor solution may contain suitable pre-formed solid particles dispersed in a solution of a nickel salt. Such pre-formed solid particles can be obtained, for example, by a chemical synthesis method described in more detail below. During heat treatment, the nickel salt defines a substantially homogeneous binder phase for the catalytic coating, while the pre-formed solid particles define porous nickel oxide regions. For this purpose, the pre-formed solid particles may be pre-formed porous nickel oxide particles, obtained by preparing nickel vanadium oxide particles and subsequently leaching the vanadium component from these particles. Preferably, the pre-formed solid particles of the precursor solution are nickel oxide and vanadium oxide particles, and the porous nickel oxide regions are obtained by leaching the vanadium oxide component from the formed catalytic layer after heat treatment of the applied precursor solution. Heat treatment is preferably performed at a temperature far below the melting temperature of nickel, typically at a temperature of up to 500°C, at which the pre-formed particles remain stable and intact during the application of the precursor solution. Therefore, after vanadium leaching, the particles constitute a well-defined region of porous nickel oxide within the solid nickel oxide binder, and any sintering of the particles is avoided.

[0023] Therefore, nickel oxide coatings are produced via thermal decomposition, a well-developed method easily adaptable to large-scale production. Furthermore, the thermal decomposition technology can be readily adapted to a wide variety of nickel substrates, regardless of the substrate's geometry or size. Additionally, the coating is obtained solely from nickel and vanadium (i.e., metals highly abundant in the Earth's crust and significantly less expensive than precious metals such as iridium). Due to their high abundance, large-scale purchases required for industrial-scale production are readily achievable. Moreover, the leaching step required to remove vanadium oxide from the coating is less challenging than the leaching step in Rani nickel production because vanadium leaching does not generate hydrogen gas during its dissolution, thus avoiding associated health and safety hazards. Finally, the coating produced according to the method of the invention has a flat morphology, thereby avoiding damage to the film in a zero-gap electrolytic cell.

[0024] Preferably, the diameter of the porous nickel oxide region is in the range of 50 nm to 10 μm, more preferably in the range of 100 to 400 nm. In the case of pre-formed nickel-vanadium oxide particles, the pre-formed particles maintain their structural stability during application and leaching. Therefore, the size of the desired region of the porous nickel oxide substantially corresponds to the size of the initial particles. After leaching, the porous nickel oxide region exhibits a substructure of nickel oxide from the initial nickel oxide component and voids from the initial vanadium component, while the binder is substantially non-porous nickel oxide. If the porous nickel oxide / pre-formed particle region has an irregular shape, then for the purposes of this invention, "diameter" corresponds to the diameter of a sphere in which the particle can be inscribed.

[0025] The metallic substrate of the electrode of the present invention is preferably selected from the following substrates: nickel-based substrates, titanium-based substrates, and iron-based substrates. Nickel-based substrates include nickel substrates, nickel alloy substrates (particularly NiFe alloys and NiCo alloys and combinations thereof), and nickel oxide substrates. Iron-based substrates include iron alloys such as stainless steel. In the context of the present invention, metallic nickel substrates are particularly preferred. Like a bare nickel electrode, the electrode of the present invention benefits from the catalytic properties of nickel but does not exhibit the slow kinetics of a bare nickel electrode and does not require additional precious metals or other metals to improve reaction kinetics.

[0026] Therefore, in a preferred embodiment, the coating of the present invention is substantially free of precious metals such as iridium. "Substantially free" means that the corresponding metal is typically outside any detectable range when using, for example, typical laboratory X-ray diffraction (XRD) techniques.

[0027] When vanadium is leached to achieve the desired porosity, some residual vanadium will typically remain in the resulting solid porous nickel oxide particles. Therefore, the term "porous nickel oxide" is not intended to exclude the presence of any residual vanadium, as long as nickel remains the dominant metal by weight. Furthermore, due to the characteristic of forming porous nickel oxide by leaching vanadium in an alkaline medium, two distinct nickel oxide phases (i.e., different oxidation states of nickel) will typically be present in the outer layer: nickel oxide (NiO) and nickel hydroxide (Ni(OH)₂).

[0028] Therefore, in one embodiment, the outer catalytic layer consists only of nickel oxide (NiO) and nickel hydroxide (Ni(OH)2) with a possible remaining amount of vanadium. Thus, the catalyst does not contain any rare and expensive metals.

[0029] In one embodiment, the porous nickel oxide region in the porous outer layer has a diameter of at least 20m. 2 The surface area per g was determined according to BET (Brunauer, Emmett, Tel ler) measurements. In contrast, the nickel oxide binder region has a surface area of ​​no more than 10 m². 2 / g, preferably less than 5m 2 / g surface area. Preferably, the total surface area of ​​the porous outer layer (i.e., the region including the nickel oxide binder and the porous nickel oxide) is at least 30, more preferably at least 40 m². 2 / g(BET). In some embodiments, the surface area of ​​the porous outer layer is between 20 and 80, and between 30 and 60 m². 2 Between / g (BET). Therefore, the electrode of the present invention has a catalytic layer having a highly porous nickel-based catalytic outer layer, which, for example, is significantly higher than typically less than 10m. 2 Surface area of ​​iridium-based catalytic coatings in the range of / g.

[0030] In a preferred embodiment, the coating comprises a nickel-based interlayer deposited between a nickel substrate and a porous catalytic outer layer. Preferably, the nickel-based interlayer is a LiNiOx interlayer applied directly to the metal substrate. Such interlayers and their fabrication methods are known from noble metal-based catalytic coatings. It has been found that the presence of the interlayer improves electrode lifetime. It has also been found that while lifetime increases with interlayer thickness, the interlayer also increases the signal of the oxygen evolution overpotential. Therefore, depending on the intended application, the electrode can be optimized with respect to lifetime or oxygen evolution overpotential.

[0031] In one embodiment, the porous outer layer has a thickness in the range of 5 to 40 micrometers (μm), preferably in the range of 10 to 20 μm. The porous outer layer has a thickness of 5-50 g / m³ based on metal elements. 2 The preferred nickel loading within the range. When applied directly to a nickel substrate, the catalytic coating is particularly suitable for low current density applications (e.g., at 1 kA / m²). 2 Or up to several kA / m 2 (within the range). For these applications, the preferred nickel loading is typically 10 g / m³. 2 Within this range. If a porous outer layer is applied to the nickel intermediate layer, these implementations can be used for high current density applications (e.g., at 10 kA / m²). 2 (above), making it typically preferred at 20 g / m 2 Higher nickel loadings within the above range.

[0032] The coating consisting of a porous outer layer and an intermediate layer preferably has a thickness in the range of 30-300 μm, preferably about 50 μm.

[0033] Preferably, the niobium substrate is a nickel mesh, which can be used in various configurations with respect to mesh thickness and geometry. A preferred mesh thickness is in the range of 0.2 to 1 mm, more preferably about 0.5 mm. A typical mesh opening is a diamond-shaped opening with a long width in the range of 2 to 10 mm and a short width in the range of 1 to 5 mm.

[0034] Due to its low oxygen overvoltage value, the electrode of the present invention is preferably used as an anode for oxygen evolution, and particularly as an anode in an electrolyzer for alkaline water electrolysis.

[0035] This invention also relates to a method for producing electrodes, comprising the following steps:

[0036] a) The pre-formulated nickel-vanadium oxide (Ni(V)O) x The particles are dispersed in a solution containing nickel salt to obtain a precursor suspension;

[0037] b) Applying a precursor suspension to a metal substrate to obtain an applied coating;

[0038] c) The applied coating is dried at a temperature in the range of 80-150°C, typically at 90°C, preferably for 5-30 minutes, typically for 10 minutes.

[0039] d) The coating applied by calcination at a temperature in the range of 300-500°C, typically at 400°C, preferably for 5 to 15 minutes, typically for 10 minutes, is used to oxidize the metal salt into a metal oxide.

[0040] e) Repeat steps b) to d) until a coating with the desired nickel specific loading is obtained;

[0041] f) Heat-treat the coating at a temperature in the range of 300-500°C, typically at 400°C, preferably for 0.5 to 2 hours, typically for 1 hour;

[0042] g) Vanadium is leached from the coating in an alkaline bath.

[0043] The preparation of the pre-fabricated solid nickel-vanadium oxide particles used in step a) can be based on, as in Yokoshima, K. et al., Electrochemical supercapacitor behavior of nanoparticulate rutile-type Ru 1- x V x The method for synthesizing Ru-V binary oxide particles via a polymerizable complexation approach is described in *O2. Journal of Power Sources*, 2006, 160.2:1480-1486. ​​The described method is applicable to the synthesis of nickel-vanadium oxide particles. In the article by Yokoshima et al., the prepared ruthenium-vanadium oxide particles were used as supercapacitors. Yokoshima et al. did not anticipate the use of selectively leaching vanadium compounds or such particles as porous catalyst particles in catalytic coatings for electrodes.

[0044] In a preferred embodiment, the pre-formed solid nickel vanadium oxide particles used in step a) are obtained from a mixture of a suitable metal precursor, such as a salt or nitrate of the relevant metal (e.g., Ni(NO3)2·6H2O and VCl3), an organic solvent (e.g., ethanol and ethylene glycol), and citric acid. After a first heating step to evaporate the ethanol, the mixture is heated to obtain a hard resin. The resin is then pyrolyzed in air to obtain the final pre-formed synthetic nickel vanadium oxide particles (Ni(V)O). x ).

[0045] This can be achieved by dissolving nickel salt in a suspension of the pre-prepared Ni(V)O xA solution containing a nickel salt is formed in an aqueous alcoholic solution (e.g., in water and isopropanol) of the particles, which forms a nickel oxide binder in the coating of the present invention. The nickel salt is preferably a nickel halide, such as nickel chloride. Besides nickel, other suitable metal oxide binders are envisioned for different electrochemical applications. Furthermore, process conditions can be adapted to each type of metal oxide binder.

[0046] In step b), the precursor suspension from step a) is applied to the metal substrate and is achieved using various techniques known in the art, such as brushing or spraying.

[0047] The drying and calcination steps c) and d) yield to the formation of the coating layer, wherein nickel oxide acts as an inorganic binder for the pre-synthesized nickel oxide catalyst particles.

[0048] According to step e), a nickel oxide outer catalytic layer can be generated in a series of layers to precisely adjust the desired nickel loading. Because only one coating composition is used, the fabrication of the coated electrode is faster, simpler, and therefore cheaper than existing methods. Furthermore, the oxide coating is produced via thermal decomposition, which is a well-developed method for large-scale coating production.

[0049] The final heat treatment according to step f) enhances the lifespan of the electrode and can also help reduce oxygen overpotential.

[0050] The concept of selectively leaching vanadium from the mixed oxide metal particles in step g) can be adapted from studies found in the literature in which vanadium was recovered from fly ash by leaching, such as those described in Navarro, R. et al., Vanadium recovery from oil fly ash by leaching, precipitation and solvent extraction processes. Waste Management, 2007, 27.3: 425-438 and Tsai, S. and M. Tsai. A study of the extraction of vanadium and nickel in oil-firedfly ash. Resources, Conservation and Recycling, 1998, 22.3-4: 163-176. In the method of the present invention, after step f), the region / particles of the coating containing the nickel-vanadium oxide mixture comprise two separate crystalline phases, namely nickel oxide (NiO) and vanadium oxide (VO). In step g), vanadium oxide is removed by leaching in an alkaline solution (e.g., 6M KOH at 80°C) to obtain an activated microporous Ni oxide structure within the particles (specifically a mixed phase of NiO and Ni(OH)2). Therefore, step f) is preferably carried out in an aqueous alkaline hydroxide solution, such as in 6M NaOH or 6M KOH solution, at a temperature between 60 and 100°C, typically at 80°C, for a duration in the range of 12-36 hours, typically for a duration of 24 hours.

[0051] In a preferred embodiment, an initial step a0) is performed prior to step a), wherein a nickel-based intermediate layer is applied to a nickel substrate.

[0052] This invention relates to the following particular advantages: It has been surprisingly found that although no noble metals are used in the catalytic coating of the electrode of this invention, the increased porosity and similarly increased active surface area of ​​the coating significantly improve the electrochemical activity of the material, for example, by reducing the oxygen evolution overpotential in alkaline water electrolysis. The increased porosity is achieved by leaching vanadium from pre-formed catalyst particles, leaving voids / pores in the coating, which increases the oxygen evolution reaction sites and thus reduces the overall oxygen overpotential. Furthermore, by limiting the leaching method to the area within the coating defined by the pre-formed particles, the remaining portion of the coating, i.e., the nickel oxide binder, remains unchanged, thus defining improved mechanical stability of the overall structure. Additionally, the leaching step according to this invention does not release hydrogen, compared to known leaching processes in NiAl and NiZn coatings, where hydrogen evolution can lead to structural damage. Finally, according to this invention, an inorganic metal binder, such as nickel oxide, is used. Nickel oxide binder is particularly preferred when the electrode is used for alkaline water electrolysis. However, for other applications, different metal binders may be used depending on the desired mechanical stability of the coating and the intended electrochemical application. Attached Figure Description

[0053] The invention will now be described in more detail, together with some preferred embodiments and corresponding drawings.

[0054] In the attached diagram:

[0055] Figure 1 A schematic diagram illustrating the electrodes according to the present invention;

[0056] Figure 2 SEM images of the electrodes according to the present invention are shown, overlaid with the results of EDX scans;

[0057] Figure 3 The XRD patterns of the electrode according to the present invention before and after vanadium leaching are shown.

[0058] Figure 4 SEM images of the electrode of the present invention and the comparative electrode before and after vanadium leaching are shown.

[0059] Figure 5 The results of oxygen overpotential determined by CISEP testing are shown; and

[0060] Figure 6 This displays the results of the accelerated life test.

[0061] Detailed description of the preferred implementation scheme

[0062] Figure 1 Figure a) shows a schematic representation of the electrode 10 according to the invention. The electrode comprises a metal substrate, in this case a nickel mesh 11 coated with a thickness typically ranging from 0.1 to 5 mm. Figure 1Figures b) and c) show enlarged cross-sectional views of the coating lines 12 of mesh 1 according to two alternatives to the coating according to the invention. According to alternative b), the cross-sectional view depicts a nickel substrate, i.e., nickel wire 12, and a porous outer layer 13 comprising solid porous nickel vanadium oxide particles 14 dispersed in a nickel oxide binder 15. The coating according to alternative c) corresponds to alternative b), but differs in that a LiNi intermediate layer 16 is applied directly to the nickel wire substrate 12, and a porous outer layer 13 comprising solid porous nickel vanadium oxide particles 14 dispersed in a nickel oxide binder 15 is disposed on top of the intermediate layer 16.

[0063] Example 1: Preparation of Coating Suspension

[0064] a) Preparation of pre-fabricated solid catalyst particles via chemical synthesis

[0065] The synthesis process described above by K. Yokoshima et al. is considered as an example and modified for the synthesis of Ni(V)O. x Catalyst: A mixture of metal precursors (Ni(NO3)2·6H2O and VCl3), organic solvents (ethanol and ethylene glycol), and citric acid was thoroughly mixed at 25°C for 2 h, followed by mixing at 60°C for 12 h. After evaporating the ethanol by heating the mixture at 90°C for 4 h, the solution was heated at 130°C for 6 h to obtain a hard resin. The resin was then pyrolyzed in air at 400°C for 1 h to obtain the final pre-synthesized Ni(V)O with a diameter in the range of 100 to 200 nm. x Particles.

[0066] b) Preparation of coating solution

[0067] Nickel chloride was dissolved in water and isopropanol (1:1 volume ratio). Nafion (a fluoropolymer-polymer based on sulfonated tetrafluoroethylene commercialized by DuPont de Nemours, Inc.) was added as an ionomer.

[0068] c) Disperse the pre-prepared solid catalyst particles and coating solution

[0069] The pre-formed solid particles obtained in a) are added to the prepared solution b) and any agglomerates dispersed therein by ultrasonic and magnetic stirring to obtain the final coating suspension. Ni(V)O in the coating suspension x The particles maintain their initial size in the range of 100 to 200 nm.

[0070] Example 2 (Ex2): Contains Ni(V)O x -Particle / NiOx-binder coating with nickel mesh electrode without intermediate layer preparation

[0071] In order to prepare 1m 2The coated mesh was made by sandblasting and etching a 0.5 mm thick woven nickel mesh with diamond-shaped openings of 5 mm long width and 2.8 mm short width in a hydrochloric acid solution. The coating suspension of Example 1 was deposited by brushing onto each side of the mesh, dried at 90°C for 10 minutes, and calcined at 400°C for 10 minutes. The deposition, drying, and calcination steps were repeated until a coating density of 10 g / m³ was achieved. 2 The final nickel loading of the projected area (in the binder and particle areas). The coated electrode was then post-baked at 400°C for 1 hour. Finally, the electrode was immersed in an alkaline 6MKOH bath for vanadium removal at 80°C for a total time of 24 hours.

[0072] Example 3 (Ex3): Fabrication of a nickel mesh electrode with Ni(V)Ox particles / NiOx binder coating and interlayer

[0073] Similar to Example 2, the nickel mesh is provided with Li 0.5 Ni 1.5 An intermediate layer composed of O2. Nickel mesh was coated on both sides with a solution containing nickel acetate and lithium acetate until a density of 8 g / m² was achieved through repeated cycles of drying (10 minutes at 80°C) and baking (15 minutes at 500°C). 2 Nickel loading of projected area and 0.3 g / m² 2 The lithium loading of the projected area is used to obtain the intermediate layer.

[0074] The coating suspension of Example 1 was deposited on the intermediate layer, and vanadium was leached as described in Example 2 to obtain the final electrode.

[0075] Comparative Example 4 (CEx4)

[0076] Comparative Example 4 corresponds to an electrode with a noble metal-based catalytic coating commercialized by the applicant. A nickel wire mesh with 0.17 mm diameter wires comprising a three-layer coating was obtained by sequentially applying each corresponding precursor solution by brushing onto a mesh substrate (or its respective underlying layer) and thermally decomposing the coating. The three-layer coating consists of a LiNiO base layer, NiCoO... x Intermediate layer and IrO x The top layer is made.

[0077] Comparative Example 5 (CEx5)

[0078] Comparative Example 5 corresponds to another electrode with a noble metal-based catalytic coating commercially available from the applicant. On a nickel grid similar to that of Example 2, an electrode containing LiNiIrO is applied. x A single-layer coating made from a mixture of IrO2 and IrO2.

[0079] Comparative Example 6 (CEx6)

[0080] For comparative purposes, the bare nickel mesh electrode of Example 2 without any coating was used as another comparative example.

[0081] Comparative Example 7 (CEx7)

[0082] The bare nickel mesh electrode of Example 2, with a coating consisting only of nickel binder, was used as another comparative example. For this purpose, the coating solution of Example 1b) was applied in a similar manner to that described for the coating suspension of Example 2 until it reached 10 g / m³. 2 Nickel loading in the adhesive coating.

[0083] The electrodes of Embodiments 2 and 3 according to the present invention were characterized using different techniques and compared with Comparative Examples 4 to 7.

[0084] Comparative Example 8 (CEx8)

[0085] The bare nickel mesh electrode of Example 2 is provided with a coating according to the applicant's Italian patent application IT2020000020575, i.e., provided with only impregnated Ni(V)O. x Coating without pre-formulated (pre-synthesized) particles.

[0086] The electrodes of Embodiments 2 and 3 according to the present invention were characterized using different techniques and compared with Comparative Examples 4 to 8.

[0087] A. Mechanical and chemical coating properties

[0088] A.1 Uniformity

[0089] The electrode (with an intermediate layer) prepared according to Example 3 was characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Figure 2 SEM image 20 shows the electrode cross-sections overlaid with the results of EDX scanning. SEM image 20 shows the exposed nickel substrate 21 and Li. 0.5 Ni 1.5 O2 intermediate layer 22 and Ni(V)O x Catalytic outer layer 23. The darker area 24 on the right side of the image is derived from the carbon resin used in sample preparation. Overlaid on image 20 is the result of an EDX scan along scan line 25, showing the weight percentages (wt%) of nickel (line 26), vanadium (line 27), and oxygen (line 28). The sample was obtained after the final leaching step, but as can be seen from... Figure 2It was observed that the remaining vanadium remained in the outer catalytic layer 23. Therefore, vanadium was not completely leached from the coating. Using X-ray fluorescence (XRF) technology, the amount of remaining vanadium after leaching was determined to depend on the thickness of the intermediate layer, and typically found to be between 40% and 60% of the initial vanadium content before leaching. It can be concluded that even after the leaching step, the intermediate layer plays a role in stabilizing the vanadium content within the coating.

[0090] A.2 Chemical Composition

[0091] The chemical composition of the electrode was further analyzed using X-ray diffraction (XRD) technology. Figure 3 The typical results for the electrode prepared according to Example 3 are shown in the figure. The x-axis represents the diffraction angle 2θ and the y-axis represents the diffraction intensity in arbitrary units (e.g., in count / scan). Line 30 shows the diffraction pattern before leaching and line 31 shows the diffraction pattern after leaching. Before leaching, the spectra show that the crystalline substances present on the sample are: Ni (substrate) and Li. x Ni y O z And LiNiVO4 / LiV2O4. Although the same substances exist after leaching, the difference is the appearance of crystalline Ni(OH)2. It has less Li 0.5 Ni 1.5 Other samples with O2 cycle numbers (5 or 10) have reduced Li x Ni y O z The peak intensity is different, but the same crystalline substance exists before and after leaching.

[0092] When comparing the XRD results of the electrode of Example 2 with those of the electrode prepared according to Comparative Example 8, i.e., according to the applicant's Italian patent application IT 2020000020575, it was determined that although the electrode of Comparative Example 8 was used, the VO peak essentially disappeared. In contrast, with the electrode according to the present invention, a reduced VO peak could still be observed, which means that vanadium (not shown in the figure) was not completely leached from the coating.

[0093] A.3 Mechanical Stability

[0094] The stability of the electrode (without an intermediate layer) according to Example 2 was compared with that of the electrode according to Comparative Example 8. Figure 4 The SEM images show the effect of the leaching step on the stability of the catalytic outer layer. Figure 4 In a) and b), cross-sectional views of the electrode according to Example 2 are shown before (a) and after (b) vanadium leaching. Figure 4 c) and d show the electrodes (i.e., those with porous Ni(V)O) before (c) and after (d) vanadium leaching, according to Comparative Example 8. xA similar cross-sectional view of an electrode with a catalytic coating but without pre-made / pre-synthesized particles. Figure 4 In the figures, reference numeral 41 denotes the nickel substrate and reference numeral 42 denotes the carbon resin required for sample preparation. The porous catalytic outer layer according to the present invention is indicated by reference numerals 43 (showing the layer before vanadium leaching) and 44 (showing the layer after vanadium leaching), respectively. The porous catalytic outer layer according to Comparative Example 8 is indicated by reference numerals 45 (showing the layer before vanadium leaching) and 46 (showing the layer after vanadium leaching), respectively. As can be seen from the images, compared to Comparative Example 8, where vanadium leaching occurs throughout the outer layer, resulting in significant shrinkage during leaching, the porous catalytic outer layer of the present invention, comprising preformed particles and a nickel oxide binder, is more stable during vanadium leaching and shows no significant shrinkage. Figure 4 The average layer thicknesses in a) and 4b) are 16+ / -2μm and 15+ / -2μm, respectively. (As shown in the original text...) Figure 4 Compared to 18+ / -4μm and 5+ / -1μm in c) and 4d).

[0095] B. Electrochemical Coating Characteristics

[0096] B.1 Oxygen Overvoltage

[0097] Compared to existing anodes used in alkaline water electrolysis, the electrochemical performance of the electrode of this invention was characterized using a modified impedance single electrode potential (CISEP) test. To determine the oxygen overvoltage of the electrode of this invention, it was tested as the anode in a three-electrode beaker. The test conditions are summarized in Table 1.

[0098] Table 1:

[0099] electrolytes <![CDATA[25 wt% KOH (1.5 l) in deionized H2O]]> temperature 80℃ cathode <![CDATA[Nickel mesh (projected area 12 cm 2 )]]> Working anode electrolysis area <![CDATA[Projected area 1 cm 2 > Reference electrode Saturated calomel electrode (SCE)

[0100] Initially, the sample underwent 2 hours of 10 kA / m 2 Pre-electrolysis (conditioning) was performed to stabilize the oxygen overvoltage (OOV). Then, several chronopotential steps were applied to the sample. The final output of the CISEP test was 10 kA / m. 2 The average of the three steps performed is corrected for by the electrolyte impedance.

[0101] Figure 5 A comparison is summarized between the bare nickel anode at 340 mV in Comparative Example 4 (represented by “baseline” 51), the iridium-based anodes (CEx4, CEx5) of Comparative Examples 4 and 5, and the electrodes (Ex2, Ex3) of Examples 2 and 3 of the present invention.

[0102] The energy savings achieved by the anode of this invention (with an oxygen overvoltage greater than 120 mV lower than that of a bare nickel electrode, OOV) solve the problem of high operating costs caused by the slow kinetics of the anode reaction in uncoated nickel mesh, without involving expensive precious metals or harmful manufacturing methods.

[0103] B.2 Lifetime Test

[0104] Accelerated life testing (ALT) was used to evaluate the lifetime of the catalytic coating. The test consisted of long-term electrolysis in a beaker bath with a two-electrode setup and a continuous electrolytic current applied directly to them. The applied conditions were more stringent than those of the CISEP test and exceeded typical operating conditions to accelerate the wear process. The conditions used in the accelerated life test are summarized in Table 2 below:

[0105] Table 2

[0106] electrolytes <![CDATA[30 wt% KOH in deionized H2O]]> Current density <![CDATA[20-40kA / m 2 ]]> temperature 88℃ counter electrode Nickel mesh Working electrode electrolysis area <![CDATA[1cm 2 -pjt]]>

[0107] exist Figure 6 The ALT data is displayed. The x-axis represents the duration of the test in days, and the y-axis represents the cell voltage in volts. Data point 61 shows the results for the uncoated bare nickel electrode according to Comparative Example 6, indicating that the cell voltage increased from 2.5V to 2.7V after only a few hours of operation. The cell voltage remained stable at 2.7V, indicating that no further degradation occurred. Data point 62 shows the electrode according to Comparative Example 8, coated with a nickel oxide binder layer, exhibiting behavior substantially similar to the bare nickel electrode. Data point 63 corresponds to the electrode according to Example 2, which maintained a lower cell voltage between 2.55 and 2.6V, with only a slight increase for more than 60 days. This indicates that the electrode of Example 2, with its highly porous external catalytic nickel oxide layer (without an intermediate layer), has superior performance in terms of cell voltage compared to the bare nickel electrode. Data point 64 shows the noble metal-based electrode according to Comparative Example 4, which exhibited even better performance under the harsh conditions of the ALT test. Nevertheless, the electrode of the present invention, which can be manufactured at low cost and exhibits high mechanical stability and significantly improved electrical efficiency, is well-suited as an anode for alkaline water electrolysis.

[0108] The preceding description is not intended to limit the invention. The invention can be used according to various embodiments without departing from its purpose, and the scope of the invention is uniquely defined by the appended claims. In the specification and claims of this application, the terms "comprising," "including," and "containing" are not intended to exclude the presence of additional elements, components, or method steps. Discussions of documents, terminology, materials, devices, articles, etc., are included in this specification only to provide background information on the invention. It is not suggested or implied that any or all of these subjects constitute part of the prior art or general knowledge in the field related to this invention prior to the priority date of each claim.

[0109] Acknowledgments:

[0110] This project has been approved by the EU's Horizon 2020 research and innovation program in Marie Funding was obtained under license agreement number 722614—ELCOREL—H2020-MSCA-ITN-2016 / H2020-MSCA-ITN-2016.

Claims

1. An electrode for gas evolution during electrolysis, comprising a metal substrate and a coating formed on said substrate, said coating comprising at least a catalytic porous outer layer containing porous nickel oxide regions dispersed within a solid nickel oxide binder, said catalytic porous outer layer being obtained by heat-treating a precursor solution comprising pre-formed nickel vanadium oxide particles dispersed in a precursor solution containing a nickel salt and subsequently leaching vanadium oxide from said heat-treated layer.

2. The electrode according to claim 1, wherein the catalytic porous outer layer is a solid / solid dispersion in which solid porous nickel oxide particles are dispersed within the solid nickel oxide binder.

3. The electrode according to claim 1 or 2, wherein the diameter of the porous nickel oxide region is in the range of 50 nm to 10 µm.

4. The electrode according to any one of claims 1 to 2, wherein the metal substrate is selected from the following substrates: nickel-based substrates, titanium-based substrates, and iron-based substrates.

5. The electrode according to any one of claims 1 to 2, wherein the porous outer layer is composed of nickel oxide and nickel hydroxide.

6. The electrode according to any one of claims 1 to 2, wherein the porous outer layer is composed of nickel oxide, nickel hydroxide and the remaining vanadium.

7. The electrode according to any one of claims 1 to 2, wherein the porous nickel oxide region in the porous outer layer has a depth of at least 20 μm. 2 / g surface area (BET).

8. The electrode according to claim 7, wherein the porous nickel oxide region in the porous outer layer has 20 and 80 μm. 2 Surface area between / g (BET).

9. The electrode of claim 4, wherein the coating comprises a nickel-based intermediate layer deposited between the nickel-based substrate and the catalytic porous outer layer.

10. The electrode of claim 9, wherein the nickel-based intermediate layer is LiNiO directly applied to the metal substrate. x Intermediate layer.

11. The electrode according to any one of claims 1 to 2 and 8 to 10, wherein the substrate is a nickel mesh.

12. Use of the electrode as defined in any one of claims 1 to 11 as an oxygen evolution anode.

13. A method for producing an electrode as defined in any one of claims 1 to 11, comprising the following steps: a) The pre-made nickel-vanadium oxide (Ni(V)O) x The particles are dispersed in a solution containing nickel salt to obtain a precursor suspension; b) Applying a precursor suspension to a metal substrate to obtain an applied coating; c) Dry the applied coating at a temperature ranging from 80 to 150°C; d) Coatings applied by calcination at temperatures ranging from 300 to 500°C; e) Repeat steps b) to d) until a coating with the desired nickel loading is obtained; f) Heat-treat the coating at a temperature in the range of 300-500°C; g) Vanadium is leached from the coating in an alkaline bath.

14. The method of claim 13, wherein the pre-formed solid nickel-vanadium oxide particles of step a) are obtained by pyrolysis of a resin based on nickel and vanadium precursors.

15. The method according to any one of claims 13 or 14, wherein the solution in step a) comprises water and alcohol.

16. The method according to any one of claims 13 to 14, wherein the nickel salt in step a) is a nickel halide.

17. The method according to any one of claims 13 to 14, wherein step g) is carried out in an alkaline hydroxide aqueous solution at a temperature in the range of 60-100°C for a period of time between 12 and 36 hours.

18. The method according to any one of claims 13 to 14, comprising step a0) performed prior to step a), wherein a nickel-based intermediate layer is applied directly to the metal substrate.

19. The method of claim 15, wherein the alcohol is isopropanol.

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