Electrodes for gas generation in electrolytic processes

Abstract

The present invention relates to an electrode for gas generation in an electrolysis process and a method for the manufacture of such an electrode, the electrode comprising a metal substrate and a coating formed on the substrate, the coating comprising at least a highly porous catalytic outer layer containing nickel oxide and nickel hydroxide, the porous outer layer having a thickness of at least 40 mm. 2 / g(BET). The catalyst layer is prepared from an initial coating of Ni oxide / V oxide followed by V leaching.

Description

[Technical Field]

[0001] The present invention relates to an electrode for gas evolution in electrolysis processes, comprising a nickel substrate and a nickel-based catalytic coating. Such an electrode can be used in particular 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, in which the anode and cathode compartments are separated by a suitable separator, such as a diaphragm or membrane. An alkaline aqueous solution, e.g., KOH solution, with a pH greater than 7 is supplied to the cell, and a current flow is established between the electrodes of the cathode and anode compartments, i.e., the cathode and anode, resulting in a potential difference (cell voltage) typically in the range of 1.8 to 2.4 V. Under these conditions, water is decomposed into its components, thus generating hydrogen gas at the cathode and oxygen gas at the anode. The gaseous products are removed from the cell so that the cell can operate continuously. The anodic oxygen evolution reaction can be summarized as follows: 4OH - →O2+2H2O+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 storing energy from fluctuating renewable energy sources such as solar and wind energy.

[0004] In this regard, it is particularly important to reduce technical costs, not only 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 relates to the cell voltage required for water electrolysis to occur efficiently. The overall cell voltage is essentially determined by the reversible voltage, i.e., the thermodynamic contribution to the overall reaction, the voltage loss 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 oxygen evolution reaction is slow, which causes a high anode overpotential, resulting in increased operating cell voltage and making large-scale commercialization of this technology difficult.

[0006] Another important feature of the electrodes is their resistance to unprotected shutdowns. Indeed, during typical operation of an electrolysis plant composed of a stack of single electrochemical cells, maintenance due to technical problems often requires shutting down the power supply, resulting in polarity reversals that are harmful to the electrodes. Such reversals are usually avoided using external polarization systems (or polarizers) that maintain the current flow in the desired direction. This auxiliary component avoids potential electrode degradation caused by 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, Raney nickel (Ni+Al) electrodes, and electrodes with iridium oxide (Ir)-based catalytic coatings.

[0008] Bare nickel electrodes are formed solely from a nickel substrate such as Ni mesh, which can be easily manufactured at low cost, but exhibit a high oxygen overpotential and are slow in kinetics.

[0009] Raney nickel electrodes are manufactured by the thin-film deposition of Ni+Al catalytic powder using plasma spraying. At the industrial level, plasma spraying is not frequently used for catalytic coatings due to the high production costs and associated health and safety hazards, such as noise, explosiveness, intense flames at temperatures exceeding 3000°C, and fumes. Furthermore, the Raney nickel manufacturing process involves an activation process, which involves leaching aluminum from the catalytic coating, leaving nearly pure nickel on the surface and substantially increasing the surface area. During the Al dissolution reaction, H2 is produced, which causes problems during the manufacturing process due to a spontaneous exothermic reaction. Another technical issue with Raney nickel deposited by plasma spraying is the resulting coating, which can have a somewhat uneven morphology. In zero-gap cells where the electrode contacts the membrane, sharp, uneven surfaces can damage the membrane.

[0010] Electrodes with iridium-based catalytic coatings are manufactured by pyrolysis, a well-established technique that involves fewer risks. However, the iridium used in these electrodes is one of the rarest precious metals in the Earth's crust, making it expensive and difficult to purchase in bulk 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 multilayer coatings, resulting in expensive manufacturing processes. Multilayer catalytic coatings include, for example, an intermediate layer applied directly onto a Ni substrate, an active layer applied to the intermediate layer, and an iridium oxide outer layer. These multilayer compositions typically exhibit low resistance to unprotected shutdown because Ir and other non-Ni metals, such as Co, present in their formulations can dissolve in the electrolyte solution during polarity reversal.

[0011] CN110394180A describes an electrode having a nickel substrate and a surface comprising nickel hydroxide and nickel oxide, which can be used as an anode in alkaline water electrolysis.CN110863211A, CN109972158A, CN110438528A, and CN110952111A describe nickel foam electrodes having an outer surface layer comprising nickel hydroxide and nickel oxide. Summary of the Invention

[0012] It is therefore an object of the present invention to provide an improved electrode that exhibits low oxygen overvoltage in alkaline water electrolysis applications and that can be manufactured more safely and cost-effectively than prior art electrodes. Furthermore, it is desirable that the new electrode exhibit improved resistance to unprotected shutdown.

[0013] The present invention is based on the concept of electrochemically active thin films for oxygen evolution that exhibit very high surface areas. The high surface area of ​​the coating allows more electrolyte to come into contact with the catalyst and its active sites, increasing the electrochemical performance, for example, for the production of oxygen gas (O). By combining, coordinating, and designing techniques from different fields, such as sol-gel synthesis and metallurgy, it has become possible to form stable, highly porous nickel oxide coatings that are particularly suitable for the oxygen evolution reaction.

[0014] Various aspects of the invention are set forth in the following claims.

[0015] The present invention provides an electrode for gas generation in an electrolytic process, comprising a metal substrate and a coating formed on said substrate, said coating comprising at least a catalytic porous nickel oxide outer layer exhibiting a high degree of porosity, said porous outer layer having a porosity of at least 40 mm as determined by BET (Brunauer, Emmett, Teller) measurements. 2The present invention relates to an electrode having a surface area of ​​0.1 μm / g. Due to the characteristics of the formation of the highly porous nickel oxide outer layer of the electrode of the present invention, which will be described in more detail below, two different phases of nickel oxide (i.e., nickel in different oxidation states), namely, nickel oxide (NiO) and nickel hydroxide (Ni(OH)2), are present in the outer layer. The inventors have surprisingly found that highly porous nickel oxide / nickel hydroxide catalyst layers on metal substrates exhibit low values ​​of oxygen overpotential, and therefore, highly efficient electrolysis cells for alkaline water electrolysis can be fabricated using such electrodes. Of course, the electrodes of the present invention can also be advantageously used in any other applications that benefit from a low oxygen overpotential.

[0016] The metal substrate of the electrode of the present invention is preferably a substrate selected from the group consisting of 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, as well as 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 bare nickel electrodes, the electrodes of the present invention benefit from the catalytic properties of nickel but do not exhibit the slow reaction rate of bare nickel electrodes and do not require additional precious or other metals to improve reaction rate. As a result, the coatings of the present invention are essentially free of precious metals, such as iridium, or other transition metals, such as cobalt. "Essentially free" means that the corresponding metals are typically outside any detectable range, for example, using typical laboratory X-ray diffraction (XRD) techniques. However, the coatings may contain trace amounts of vanadium (V) resulting from the preferred manufacturing techniques described below, but in preferred embodiments, the electrodes are also essentially free of vanadium.

[0017] In one embodiment, the catalyst outer layer consists solely of nickel oxide (NiO) and nickel hydroxide (Ni(OH)2). Thus, the catalyst does not contain any rare and expensive metals.

[0018] Preferably, the surface area of ​​the porous outer layer is at least 60, more preferably at least 80 m 2 / g(BET). In certain embodiments, the surface area of ​​the porous outer layer is between 40 and 120, between 60 and 110, or between 80 and 100 m 2 / g (BET). Thus, the electrodes of the present invention have a catalyst layer with a highly porous nickel-based catalytic outer layer, which is typically 10 m 2 This results in a significantly higher surface area than that of commercially available iridium-based catalyst coatings, in the range of less than 1 / g.

[0019] According to a preferred embodiment of the present invention, the porous outer layer is obtained by leaching vanadium oxide from a heat-treated gel-like precursor coating containing nickel and vanadium salts. Therefore, the present invention combines two techniques for obtaining porous nickel oxide catalyst coatings: sol-gel synthesis with the thermal formation of nickel oxide (NiO) and vanadium oxide (VO). Furthermore, using the concept of sacrificial metal removal by selective leaching from metallurgy, vanadium oxide is removed, resulting in a further increase in surface area. Therefore, the oxide coating is produced by pyrolysis, a well-developed process that readily lends itself to large-scale manufacturing. Furthermore, the pyrolysis technique is easily adaptable to a wide variety of nickel substrates, regardless of the substrate's shape or size. Furthermore, highly porous nickel oxide coatings are obtained from only nickel and vanadium, metals that are highly abundant in the Earth's crust and are significantly cheaper than precious metals such as iridium. Due to their high abundance, the bulk purchase required for industrial-scale manufacturing is easily achieved. Furthermore, the leaching process required to remove vanadium oxide from the coating is less difficult than that of Raney nickel production because vanadium leaching does not produce hydrogen gas during its dissolution, thus avoiding the associated health and safety hazards. Finally, the morphology of the coating produced according to the method of the present invention is substantially flat, thus avoiding membrane damage in zero-gap electrolytic cells.

[0020] In a preferred embodiment, the coating comprises a nickel-based intermediate layer deposited between the nickel substrate and the catalytic porous outer layer. Preferably, the nickel-based intermediate layer comprises metallic nickel or a combination of metallic nickel and nickel oxide. The nickel / nickel oxide intermediate layer is preferably about 1 μm thick. 2 Surprisingly, it has been found that the catalytic coating, when applied over the nickel / nickel oxide interlayer described above, is able to withstand unprotected shutdowns required by the operation and maintenance of electrolysis plants without the need for costly additional polarization units.

[0021] The nickel intermediate layer is referred to as 100 to 3000 g / m 2 , and even more preferably 200 to 800 g / m 2 The preferred nickel loading is in the range of

[0022] The middle layer is typically denser than the outer catalyst layers.

[0023] In one embodiment, the intermediate layer has an electric double layer capacitance in the range of about 1.0 to about 10.0 mF / g.

[0024] The intermediate layer can be obtained using various techniques, such as thermal spraying techniques, laser cladding or electroplating. In a preferred embodiment, the thermal spraying technique is selected from the group consisting of wire arc spraying and plasma spraying.

[0025] 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 density of 5 to 50 g / m2 in terms of metal elements. 2 When applied directly to a nickel substrate, the catalytic coating is suitable for low current density applications (e.g., 1 kA / m 2 or a maximum of several kA / m 2In these applications, the preferred nickel loading is typically 6-15 g / m 2 When a porous outer layer is applied over a nickel intermediate layer, these embodiments are suitable for high current density applications (e.g., 10 kA / m 2 and above), so typically 15-25 g / m 2 Higher nickel dosages within these ranges are preferred.

[0026] The coating consisting of the porous outer layer and the intermediate layer has a thickness in the range of 30 to 300 μm, preferably about 50 μm.

[0027] The coating consisting of the porous outer layer and optionally the intermediate layer may be applied on one or both sides of the metal substrate of the electrode, as is conventional in the art and depending on the cell configuration and electrode placement within the cell.

[0028] Preferably, the metal substrate is nickel-based, and even more preferably, it is nickel mesh, which can be used in a variety of configurations with respect to mesh thickness and mesh shape. Preferred mesh thicknesses are in the range of 0.2-1 mm, preferably about 0.5 mm. Typical mesh openings are diamond-shaped, with a major width in the range of 2-10 mm and a minor width in the range of 1-5 mm.

[0029] Due to its low value of oxygen overvoltage, the electrode of the present invention is preferably used as an anode for oxygen evolution, in particular as an anode in an electrolysis cell for alkaline water electrolysis. The present invention therefore also relates to an electrolysis cell for electrochemical processes, in particular for alkaline water electrolysis, comprising an oxygen evolution anode and a cathode, wherein the anode is an electrode as defined above.

[0030] The present invention also relates to a method for the manufacture of an electrode as defined above, comprising the steps of: a) applying a coating solution comprising a nickel salt, a vanadium salt and a gelling agent to a metal substrate; b) thereafter drying at a temperature in the range of 80 to 150°C for preferably 20 to 40 minutes, typically 30 minutes; c) subsequent calcination at a temperature in the range of 300-500°C, typically 400°C, for preferably 5-15 minutes, typically 10 minutes, for oxidation of the metal salt to a metal oxide; d) repeating steps a) through c) until a coating having a specific desired loading of nickel is obtained (it being understood that if the desired loading is achieved in a single run of steps a) through c), then repeating steps a) through c) is not necessary); e) a final heat treatment (second firing) at a temperature in the range of 300 to 500°C, typically 400°C, for preferably 1 to 4 hours, typically 2 hours; f) leaching vanadium from the coating in an alkaline bath to form a highly porous catalytic outer layer comprising nickel oxide and nickel hydroxide; The present invention relates to a method, comprising:

[0031] According to the present invention, the nickel oxide / nickel hydroxide outer catalyst layer can be formed as a series of layers to precisely adjust the desired nickel loading. Because only one coating composition is used, the production of coated electrodes is faster, leaner, and therefore less expensive than prior art methods. Furthermore, the oxide coating is produced by pyrolysis, a process well-developed in industrial-scale coating production.

[0032] The application of the coating solution to the substrate in step a) is preferably achieved by brushing or spraying techniques, and the coating solution is preferably water-based.

[0033] The combination of organic and inorganic chemical precursors in the coating solution forms a macroporous gel structure with embedded metal salts. During the drying process, the solvent evaporates. During subsequent heat treatment at temperatures capable of calcining the precursor metal salts, the dissolved metals become oxides, while other components evaporate or burn off, leaving behind a metal oxide porous structure. The coating solution preferably contains a solvent formed from water and / or an alcohol, such as ethanol, and an acid, such as hydrochloric acid. Suitable additives that act as gelling agents include ethylene glycol and citric acid. In one embodiment, the solvent and gelling agent for the sol-gel approach include ethanol or water or an ethanol / water mixture as the solvent, and hydrochloric acid, ethylene glycol, and citric acid in molar ratios of 14:4 and 5:1 (i.e., solvent:ethylene glycol:citric acid). In addition to its function in sol-gel synthesis, ethylene glycol forms a "dry mud" after evaporation during heat treatment. Ethylene glycol is heated above its decomposition temperature and combusted as CO2, leaving a particularly open structure compared to conventional purely inorganic coating solutions for the production of dimensionally stable anodes.

[0034] The nickel salt is preferably a nickel halide, such as nickel chloride, and the vanadium salt is preferably a vanadium halide, such as vanadium chloride.

[0035] After application onto the metal substrate, the coating consists of two separate crystalline phases, namely nickel oxide (NiO) and vanadium oxide (VO), the vanadium oxide being removed by leaching in an alkaline solution (e.g. 6M KOH at 80°C) to obtain an activated microporous Ni oxide structure (a mixed phase of NiO and Ni(OH)2). Step f) is therefore preferably carried out in an aqueous alkaline hydroxide solution, e.g. 6M NaOH or 6M KOH solution, at a temperature between 60 and 100°C, typically 80°C, for a period in the range of 12 to 36 hours, typically 24 hours.

[0036] It has been found that the ratio of nickel oxide to nickel hydroxide can be adjusted by selecting a suitable ratio of nickel to vanadium in the coating solution. Preferably, the atomic ratio of Ni / V in the coating solution is about 100 / 100, which results in an atomic percentage of about 25-15 atomic % NiO and about 75-85 atomic % Ni(OH)2 in the final outer catalyst layer. Generally, the atomic percentage of Ni(OH)2 in the catalyst coating decreases with decreasing V content in the coating solution.

[0037] In the context of the present invention, a catalytic highly porous (HP) nickel oxide outer layer obtained from the pyrolysis of a dried gel-like coating containing nickel and vanadium salts followed by leaching of vanadium oxide is referred to as HP-NiO. x is shown as:

[0038] In a preferred embodiment, an intermediate step a0) is carried out before step a), in which a nickel or nickel / nickel oxide intermediate layer is applied onto the metal substrate before step a), preferably by thermal spraying, laser cladding or electroplating, so that the intermediate layer has a thickness of about 1 m. 2 / g(BET), which results in an electrode that is more resistant to unprotected shutdown, especially at high current densities.

[0039] Preferably, step a0) comprises plasma spraying the nickel powder onto the metal substrate in ambient air. In one embodiment, the nickel powder plasma sprayed onto the substrate has an average particle size of about 10 μm to about 150 μm, preferably about 45 μm to about 90 μm.

[0040] The present invention will now be described in more detail with reference to certain preferred embodiments and the accompanying drawings. [Brief explanation of the drawings]

[0041] [Figure 1] 1 is a SEM photograph of the surface and cross-sectional image of the catalytic outer layer of the electrode of Example 2 having no nickel intermediate layer. [Figure 2]1 is a graph showing the results of BET surface area measurements of the outer surface of the electrode of Example 2. [Figure 3] FIG. 10 is a diagram showing the diffraction pattern of the electrode of Example 2. [Figure 4] 1 is a graph showing the results of accelerated life testing of the electrodes of Example 2 compared to prior art electrodes. [Figure 5] 10 is an SEM photograph of the surface and cross-sectional images of the catalytic outer layer of the electrode of Example 3 having a nickel intermediate layer. [Figure 6] 1 is a graph showing the results of shutdown testing of the electrodes of Example 3 compared to a prior art bare nickel electrode. [Figure 7] 1 is a graph showing the results of shutdown testing of the electrode of Example 3 compared to a prior art iridium-based electrode. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0042] Example 1: Preparation of coating solution To prepare 1 liter (l) of coating solution, 0.4 l of demineralized water, 0.4 l of ethylene glycol, and 0.2 l of 37% hydrochloric acid were mixed in a flask and stirred for 10 minutes. 300 g of VCl3 was added to the solution and dissolved under stirring for 30 minutes. 450 g of NiCl2 6H2O was then added to the solution and dissolved under stirring for 30 minutes. 300 g of citric acid was added to the solution and dissolved under continuous stirring for 45 minutes.

[0043] Example 2: HP-NiO without an intermediate layer x Preparation of coated nickel mesh electrodes 1m 2 To prepare a coated mesh of 1 g / m, a 0.5 mm thick nickel diamond mesh was sandblasted and etched in a hydrochloric acid solution. 4 ml of the coating solution of Example 1 was deposited on each side of the mesh by brushing, dried at 130°C for 30 minutes, and baked at 400°C for 10 minutes to obtain a coating of 1 g / m. 2The nickel loading of one cycle of the projected area was obtained. The deposition, drying and firing processes were repeated a total of 10 times to obtain a nickel loading of 10 g / m 2 The final nickel loading of the projected area was obtained. The coated electrode was then post-baked at 400 °C for 2 hours. Finally, the electrode was leached in an alkaline NaOH bath at 80 °C for a total of 24 hours to remove vanadium.

[0044] Example 3: HP-NiO with a nickel interlayer x Preparation of coated nickel mesh electrodes A 0.5 mm thick nickel diamond-shaped mesh was plasma sprayed with 99.9% pure nickel powder with a particle size of 45 ± 10 μm (Fe < 0.5, O < 0.4, C < 0.02, S < 0.01, 4.8 ± 0.5 g / dm on both sides in ambient air). 2 The plasma sprayed wire mesh was then heated in a furnace in air at 350°C for 15 minutes. The plasma sprayed woven mesh was then cooled and then coated with the precursor composition by brush application in a series of coating, heating, and cooling steps. 2 To prepare a coated mesh of 3 g / m, 14 ml of the coating solution of Example 1 was deposited by brushing on each side of the mesh, dried at 130°C for 30 minutes, and baked at 400°C for 10 minutes to obtain a coating of 3 g / m 2 The nickel loading of one cycle of the projected area was obtained. The deposition, drying and firing processes were repeated a total of seven times, resulting in a nickel loading of 21 g / m 2 The final nickel loading of the projected area was obtained. The coated electrode was then post-baked at 400 °C for 2 hours. Finally, the electrode was leached in an alkaline NaOH bath at 80 °C for a total of 24 hours to remove vanadium.

[0045] Counterexample 4: LiNiO base layer, NiCoO x Intermediate layer and IrO xA 0.5 mm thick nickel diamond mesh containing a triple-layer coating formed on the top layer was obtained by sequentially applying the corresponding precursor solutions to the mesh substrate (or each underlayer) by brushing and pyrolyzing them.

[0046] Counterexample 5: LiNiO base layer, LiNiIrO x A 0.5 mm thick nickel diamond-shaped mesh containing a two-layer coating formed on the top layer was obtained by sequentially applying the corresponding precursor solutions to the mesh substrate (or each previous layer) by brushing and pyrolyzing them.

[0047] The electrodes of Examples 2 and 3 according to the invention were characterized using various techniques and compared with Counter-Examples 4 and 5.

[0048] A. Electrode of Example 2 (HP-NiO x Characterization of electrodes with catalytic layers but without nickel interlayers A.1 Scanning electron microscopy (SEM) was used to evaluate the morphology of each coating, both on the surface and in cross section. Analysis was performed on fresh and used samples to qualitatively estimate the coating's stability, adhesion, and wear properties. Figure 1 shows SEM images of the surface view (a) and cross section (b) of an electrode of the present invention prepared according to Example 2. Morphological surface analysis revealed that the HPNiO x The flat "dried mud" morphology of the coating is shown, while the cross-section reveals the porosity of the coating. Furthermore, the phase homogeneity of the coating can be observed in the cross-section. The images, particularly cross-section (b), show that the bulk nickel substrate 10 exhibits some roughness after sandblasting and etching, which is beneficial for adhesion / anchoring of the catalytic porous outer layer 11 to the substrate. However, the outer surface of the catalytic outer layer 11 applied according to the method of the present invention is smooth, thus preventing damage to the delicate membrane when assembled into an electrolysis cell.

[0049] A.2 Corrected Impedance Single Electrode Potential (CISEP) testing was used to characterize the electrochemical performance of the electrodes of the present invention compared to prior art anodes used in alkaline water electrolysis. To determine the oxygen overpotential of the electrodes of the present invention, the electrodes of the present invention were tested as anodes in a three-electrode beaker cell. The test conditions are summarized in Table 1. TIFF0007762710000001.tif58170

[0050] First, the sample was subjected to 10 kA / m 2 The sample is subjected to a 2-hour pre-electrolysis (conditioning) at 100 kJ / s for 1 hour to stabilize the oxygen overpotential (OOV). Then, several chronopotentiometric steps are applied to the sample. The final power of the CISEP test is 10 kA / m, corrected for the resistance of the electrolyte. 2 This is the average of three steps performed.

[0051] Table 2 shows the results for the bare nickel anode (base Ni), the iridium-based anode of counter example 4 (CEx4), the Raney nickel anode (Ni Raney), and the electrode of Example 2 (HP-NiO x ) is summarized. TIFF0007762710000002.tif57170

[0052] The energy savings obtainable with the anode of the present invention (OOV 140 mV lower than bare Ni) solves the problem of high operating costs caused by the slow kinetics of the anodic reaction of uncoated nickel mesh, without involving costly precious metals or hazardous manufacturing processes.

[0053] A.3 BET measurements were performed to determine the surface area of ​​the electrode of Example 2 compared to that of Counter Example 5 (CEx5), which is also suitable for alkaline water electrolysis. The results, shown in Figure 2, indicate that the electrode of Example 2 has a significantly higher surface area than prior art electrodes.

[0054] A.4 X-ray diffraction (XRD) techniques were used to characterize the type of oxide formed and its crystalline structure. A typical diffraction pattern obtained from the electrode according to Example 2 is shown in Figure 3. The x-axis represents the diffraction angle 2θ, and the y-axis represents the diffraction intensity in arbitrary units (e.g., counts per scan). The strong peaks (1), (2), and (3) correspond to the Ni substrate with crystal planes (111), (200), and (220), respectively. The weaker peaks (4), (5), and (6) correspond to the NiO phase of the highly porous outer catalyst layer with crystal planes (111), (200), and (220), respectively. The even weaker peaks (7), (8), (9), and (10) correspond to the Ni(OH)2 phase of the highly porous outer catalyst coating, corresponding to crystal planes (001), (100), (101), and (110), respectively. Therefore, the catalyst coating was determined to be composed of nickel oxide (NiO) and nickel hydroxide (Ni(OH)2). Furthermore, as can be clearly seen from the diffraction pattern in Figure 3, the highly porous catalytic coating of the present invention does not contain any iridium or other rare / expensive metals. Therefore, the cost and supply issues associated with prior art electrodes can be avoided with the electrodes of the present invention.

[0055] A.5 Accelerated Life Tests (ALT) were used to estimate the life of catalytic coatings. The tests consisted of long-term electrolysis in a beaker cell with a two-electrode configuration to which a continuous electrolysis current was applied directly. The applied conditions were more severe compared to those of the CISEP test in order to accelerate the wear process and exceeded typical operating conditions. The conditions implied in the accelerated life test are summarized in Table 3 below. TIFF0007762710000003.tif57170

[0056] The ALT data are shown in Figure 4. The x-axis represents the test duration in hours, and the y-axis represents the cell voltage in volts. Data point (1) shows the results for the uncoated Ni substrate, showing an increase in cell voltage from 2.5 V to 2.7 V after only about 2 hours of operation. The cell voltage remains stable at 2.7 V, indicating no further degradation. Data point (2) shows the electrode of Example 2, which maintains a lower cell voltage of 2.5 V for approximately 250 hours following the increase in cell voltage before electrode failure occurs. This indicates that the electrode of Example 2, with its highly porous outer catalytic nickel oxide layer (without an intermediate layer), has superior performance in terms of cell voltage compared to the bare nickel substrate, but is not suitable for long-term operation under the harsh conditions of the ALT. As noted above, the electrode of Example 2 is particularly suitable for operation under lower current densities. Data points (3) and (4) are discussed in more detail in connection with the characterization of the electrode of Example 3 below.

[0057] B) Characterization of the electrode of Example 3 (electrode with HPNiOx catalyst layer and nickel interlayer) B.1 Again, scanning electron microscopy (SEM) was used to evaluate the morphology of each coating, both on the surface and in cross section. Analysis was also performed on fresh and used samples to qualitatively estimate the coating's stability, adhesion, and wear properties. Figure 5 shows SEM images of the surface (a) and cross section (b) of an electrode of the present invention fabricated according to Example 3 (note that the image in Figure 5 was obtained at a lower resolution / magnification than the image in Figure 1). Again, cross section (b), in particular, shows that the bulk nickel substrate 10 exhibits some roughness after sandblasting and etching, while the application of the plasma-sprayed nickel intermediate layer 12 and catalytic outer layer 11 using the method of the present invention results in a smooth surface.

[0058] B.2 Accelerated life testing (ALT) as described in Section A.5 above was also performed on the electrodes of Example 3. The corresponding results are also shown in Figure 4. Data point (3) represents the plasma sprayed NiO x With an intermediate layer and additional HP-NiO xElectrode 3 shows the nickel substrate without a catalytic outer layer. The interlayer-electrode alone exhibits a cell voltage that is lower than the bare nickel substrate but still at least 100 mV higher than the electrode of Example 2, with a further continuous increase throughout the electrode life. Data point (4) shows the electrode of Example 3, i.e., the nickel substrate with the plasma-sprayed nickel interlayer and highly porous catalytic outer layer. Electrode 3 performed best in the accelerated life test, with a similarly low initial cell voltage of 2.5 V and a very slow continuous increase over an operating life of approximately 1,500 hours.

[0059] B.3 To evaluate the resistance of the electrode of Example 3 to polarity reversal and to estimate its resistance to simulated plant shutdown, a shutdown test was performed under operating conditions as summarized in Table 4 below. TIFF0007762710000004.tif39170

[0060] The following test protocol was carried out: After a 48-hour ramp-in period, a 6-hour shutdown was simulated by shortening the electrolysis cell with the pump on and allowing the temperature to drop to room temperature. After shutdown, electrolysis was continued for 6 hours under the operating conditions in Table 4. The shutdown cycle was repeated until the electrode failed.

[0061] FIG. 6 shows the results for the electrode of Example 3 (data point (1)) and the bare nickel electrode (data point (2)). The number of shutdowns is shown on the x-axis, while the y-axis shows the cell voltage. The results show that the bare nickel electrode could only withstand 40 shutdowns while operating at a higher cell voltage, while the electrode of Example 3 maintained its low cell voltage for up to 55 shutdowns.

[0062] FIG. 7 shows a comparison between the electrode of Example 3 (data point (1)) and the electrode of Counterexample 4 (data point (2)). The number of shutdowns is shown on the x-axis, while the y-axis shows the deviation from the normalized cell voltage for excluding the cathode and separator configuration. As can be seen from FIG. 7, the highly porous nickel oxide outer catalyst layer on the plasma-sprayed nickel intermediate layer can withstand more than 50 shutdowns without an increase in cell voltage. In contrast, the cell voltage of the electrode of Counterexample 4 already begins to increase after 20 shutdowns.

[0063] The above description is not intended to limit the present invention, which can be used according to various embodiments without departing from its scope in any manner, the scope of which is uniquely defined by the appended claims.

[0064] In the description and claims of this application, the terms "comprising," "including," and "containing" are not intended to exclude the presence of other additional elements, components, or process steps.

[0065] The discussion of documents, items, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present 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 fields relevant to the present invention prior to the priority date of each claim in this application.

Claims

1. 1. An electrode for gas generation in an electrolytic process comprising a metal substrate and a coating formed on the substrate, the coating including a catalytic porous outer layer containing at least nickel oxide and nickel hydroxide, the porous outer layer having a thickness of at least 40 mm. ２ / g BET surface area, said porous outer layer being obtained by leaching vanadium oxide from a heat-treated gel-like precursor coating containing nickel salts and vanadium salts.

2. 10. The electrode of claim 1, wherein the metal substrate is a substrate selected from the group consisting of nickel-based substrates, titanium-based substrates, and iron-based substrates.

3. 3. The electrode of claim 1, wherein the porous outer layer comprises nickel oxide and nickel hydroxide.

4. The porous outer layer is 40 and 120 mm ２ 4. The electrode of claim 1, having a BET surface area between 0.1 and 0.2g.

5. 5. The electrode of claim 1, wherein the coating comprises an intermediate layer deposited between the metal substrate and the catalytic porous outer layer, the intermediate layer comprising nickel and / or nickel oxide.

6. 6. The electrode according to claim 1, wherein the porous outer layer has a thickness in the range of 5 to 40 μm.

7. The porous outer layer has a metal element content of 5 to 50 g / m ２ 7. The electrode of claim 1, having a nickel loading in the range of:

8. The intermediate layer has a metal element content of 100 to 3000 g / m ２ 8. The electrode of claim 5, having a nickel loading in the range of:

9. The intermediate layer is about 1 m ２ 9. The electrode of claim 5, having a porosity of less than 1 / g BET.

10. 10. The electrode of claim 5, wherein the intermediate layer has an electric double layer capacitance, normalized by metal loading, in the range of about 1.0 to about 10.0 mF / g.

11. 11. An electrode according to any one of claims 5 to 10, wherein the coating consisting of a porous outer layer and an intermediate layer has a total thickness in the range of 30 to 300 μm.

12. 12. The electrode according to any one of claims 5 to 11, wherein the nickel intermediate layer is obtained by thermal spraying, laser cladding or electroplating.

13. 13. The electrode according to claim 12, wherein the nickel intermediate layer is obtained by thermal spraying, in particular by wire arc spraying or plasma spraying.

14. 14. The electrode of claim 1, wherein the substrate is a nickel mesh.

15. 15. An electrochemical cell for electrolysis processes comprising an oxygen evolution anode and a cathode, wherein the anode is an electrode according to any one of claims 1 to 14.

16. A method for manufacturing an electrode according to any one of claims 1 to 14, comprising the steps of: a) applying a coating solution comprising a nickel salt, a vanadium salt and a gelling agent to a metal substrate; b) drying at a temperature in the range of 80 to 150°C; c) firing at a temperature in the range of 300 to 500°C; d) repeating steps a) through c) until a coating having a specific desired loading of nickel is obtained; e) performing a final heat treatment at a temperature in the range of 300 to 500°C; f) leaching vanadium from the coating in an alkaline bath; A method comprising:

17. 17. The method of claim 16, wherein the coating solution comprises a solvent comprising water and / or an alcohol, preferably ethanol, and an acid, preferably hydrochloric acid.

18. 18. The method of claim 16 or 17, wherein the gelling agent comprises ethylene glycol and citric acid.

19. 19. The method of any one of claims 16 to 18, wherein the nickel salt is a nickel halide and the vanadium salt is a vanadium halide.

20. 20. The method of any one of claims 16 to 19, wherein step f) is carried out in an aqueous alkali hydroxide solution at a temperature in the range between 60 and 100°C for a period of between 12 and 36 hours.

21. The method includes an intermediate step a0) before step a), wherein step a0) includes forming an intermediate layer of nickel and nickel oxide on the metal substrate by thermal spraying, laser cladding or electroplating, and the intermediate layer has a thickness of about 1 m. ２ 21. The method of any one of claims 16 to 20, wherein the porous membrane has a porosity of less than 1 / g BET.

22. 22. The method according to claim 21, wherein the intermediate layer in step a0) is formed by wire spraying or by plasma spraying nickel powder onto the metal substrate in ambient air.

23. 23. The method of claim 22, wherein the nickel powder is plasma sprayed onto a metal substrate and has an average particle size of about 10 μm to about 150 μm, preferably about 45 μm to about 90 μm.

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