Electrode with improved shutdown tolerance

Abstract

The present invention relates to an electrode for use in an alkaline electrolytic process, Metal substrate; A catalyst layer disposed on the above metal substrate, comprising nickel and nickel oxide, and having a porosity of less than about 1 m² / g, and The present invention relates to an electrode comprising an active composition disposed on both the catalyst layer and within the catalyst layer, the composition comprising one or more metal compounds selected from the group consisting of cobalt compounds, iridium compounds, rhodium compounds, iron compounds, platinum compounds, lithium compounds, and manganese compounds. The present invention also relates to an alkaline water electrolysis unit comprising the electrode and a method for forming the electrode.

Description

Technology Field

[0001] The present invention relates to an electrode, and more particularly to an electrode for use in an electrolytic process. Background Technology

[0002] Hydrogen is emerging as an important part of the clean energy paradigm. Not only can hydrogen be used as a high-efficiency, low-emission fuel, but it can also serve as an energy carrier for moving, storing, and delivering energy from renewable sources. Furthermore, hydrogen can be produced from water, an abundant and relatively inexpensive feedstock.

[0003] A number of different processes can be used to produce hydrogen from water. These processes include water electrolysis, photoelectrolysis, photobiological production, and high-temperature decomposition. Among these processes, water electrolysis is the most practical method for producing hydrogen using renewable energy sources, such as power from wind turbines and / or solar panels.

[0004] There are various types of water electrolysis processes used for hydrogen generation, namely alkaline electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolytic cell (SOEC) electrolysis. Alkaline electrolysis and PEM electrolysis are more commonly used, and alkaline electrolysis is the most advanced and best suited for large-scale implementation.

[0005] In water electrolysis, a DC power source is connected to two electrodes placed in the water. Theoretically, a potential difference of 1.23 volts between the electrodes separates water into hydrogen and oxygen. Hydrogen is generated at the cathode, and oxygen is generated at the anode. More specifically, there is a hydrogen evolution reaction ("HER") at the cathode and an oxygen evolution reaction ("OER") at the anode. HER involves electrons (e) from the cathode - While OER is a reduction reaction that gives electrons to hydrogen cations to form hydrogen gas, it is an oxidation reaction that gives electrons to the anode to generate oxygen.

[0006] Theoretically, 1.23 volts is sufficient to separate water, but in practice, a higher voltage is required. Any amount exceeding 1.23 volts is called overpotential or overvoltage and represents lost energy or inefficiency. In the water electrolysis process, the greatest overpotential is the reaction overpotential for the oxidation of water to oxygen at the anode. Therefore, significant efforts have been made to reduce the overpotential at the anode.

[0007] In alkaline electrolysis, water contains an alkaline electrolyte, for example, potassium hydroxide (KOH) or sodium hydroxide (NaOH), and the electrodes are separated by a diaphragm. In PEM electrolysis, water is deionized, and the electrodes are separated by a solid polymer electrolyte that electrically insulates the electrodes while allowing protons to pass from the anode to the cathode.

[0008] Recent advancements in water electrolysis have enabled the process to operate at higher current densities. Additionally, renewable energy sources are frequently used to provide power for water electrolysis. The combined use of higher current densities and the intermittent nature of renewable energy increases stress on the electrodes, causing conventional electrodes to degrade more rapidly. Consequently, there is a need for more robust electrodes that provide superior performance while being less sensitive to degradation under high current densities and intermittent power conditions, i.e., numerous power interruptions. In particular, there is a need for anodes with reduced oxygen overpotential that are not sensitive to degradation caused by high current densities and shutdowns.

[0009] EP 3296431 A1 discloses an oxidized nickel foam electrode having a coating layer comprising crystalline nanoparticles comprising nickel, nickel oxide, and ionic oxide. WO 2019 / 172160 A1 discloses an alkaline water electrolysis anode comprising a conductive substrate having an intermediate layer and a catalyst layer made of nickel oxide. EP 3064614 A1 discloses an anode for alkaline water electrolysis having a nickel substrate and a lithium-containing nickel oxide catalyst layer formed on said substrate. EP 3375906 A1 discloses an anode for an electrochemical reaction comprising a nickel substrate and an oxidation catalyst layer disposed on a nickel oxide layer and an oxide layer on a conductive substrate.

[0010] According to the present invention, an electrode for use in an alkaline electrolytic process is provided. The electrode comprises a metal substrate and a catalyst layer disposed on the metal substrate. The catalyst layer comprises nickel and nickel oxide and has a porosity of less than about 1 m² / g as measured by BET. An active composition is disposed on both the catalyst layer and within the catalyst layer. The active composition comprises one or more metal compounds selected from the group consisting of cobalt compounds, iridium compounds, rhodium compounds, iron compounds, platinum compounds, lithium compounds, and manganese compounds.

[0011] Additionally, according to the present invention, an alkaline water electrolysis unit comprising an electrode functioning as an anode is provided. The unit also comprises a cathode and an electrolyte substantially free of chlorine.

[0012] A method for forming an electrode is also disclosed herein. According to the method, a metal substrate is provided, and a catalyst layer is formed on the metal substrate. The catalyst layer comprises nickel and nickel oxide formed by thermal spraying, cold spraying, or other surface treatment processes, such as laser cladding and electroplating, and has a porosity of less than about 1 m² / g (BET). An active composition is applied to the catalyst layer and thermally decomposed. The active composition comprises one or more metal compounds selected from the group consisting of cobalt compounds, iridium compounds, rhodium compounds, iron compounds, platinum compounds, lithium compounds, and manganese compounds. Brief explanation of the drawing

[0013] The characteristic configuration, aspects, and advantages of the present invention will be better understood in conjunction with the following description, the appended claims, and the accompanying drawings. Figure 1 shows a schematic diagram of a water electrolysis unit. Figure 2 shows a schematic diagram of the electrodes of a water electrolysis unit. Figure 3 shows a graph of the average capacitance for a plurality of test electrodes having different catalyst layers. Figure 4 shows a graph of the emitted charge for the test electrode. Figure 5 shows a graph of capacitance versus emitted charge for a test electrode. Figure 6 shows an SEM image of the terminal surface of an electrode according to the present invention. Figure 7 shows an SEM / EDAX image of the nickel and iridium distribution in the same region of the electrode of Figure 6. Figure 8 shows an SEM / EDAX image of the oxygen distribution of the same region of the electrode in Figure 6, overlaid on the SEM image of Figure 6. Specific details for implementing the invention

[0014] It should be noted that in the following detailed description, identical elements have the same reference numerals, regardless of whether they appear in other embodiments of the invention. It should also be noted that for clarity and brevity, the drawings may not necessarily be depicted in a fixed proportion, and specific characteristic features of the invention may be depicted in a somewhat schematic form.

[0015] The present invention relates to an electrode (10) configured for use in an electrolytic process. The electrode (10) is particularly suitable for use in an alkaline electrolytic process, and more particularly suitable for use as an anode in said process. A unit (12) that can be used to perform alkaline water electrolysis is illustrated in FIG. 1. The unit (12) may include an electrode (10) and a cathode (14) that function as an anode. The electrode (anode) (10) and the cathode (14) are separated by a diaphragm (16) and placed in a container (18) containing an electrolyte. It is preferable that the electrolyte used in the electrolytic process does not contain chlorine or other halides. The electrolyte may include water and potassium hydroxide (KOH) or water and sodium hydroxide (NaOH). The diaphragm (16) is composed of a microporous material with an average pore size of less than 1 μm, so that ions can move between the electrode (anode) (10) and the cathode (14) but gases cannot pass through, thereby preventing the mixing of hydrogen and oxygen gases generated. The diaphragm (16) may be composed of a porous polymer, for example, polytetrafluoroethylene (PTFE) or a composite material comprising zirconium dioxide (ZrO2) and polysulfone, which is commercially sold under the trademark Zirfon® at the time of writing.

[0016] The reactions occurring at the electrode (anode) (10) and cathode (14) are as follows:

[0017] Cathode: 2H2O + 2e - ------> H2 + 2OH -

[0018] Anode: 2OH - ------> ½O2 + H2O + 2e -

[0019] The cathode (14) is composed of a conductive metal that may be nickel, low-carbon steel, or stainless steel. The cathode (14) may have a structure as described below for the electrode (10) and may be coated with nickel or a nickel-iron alloy.

[0020] Referring now to FIG. 2, the electrode (10) comprises a substrate (22) on which a catalyst layer (24) is disposed. The catalyst layer (24) has pores formed within it, but is only slightly porous. An active composition (26) is disposed within the catalyst layer (24) and / or on the catalyst layer (24). The active composition (26) may form a layer and / or its components may be interspersed within the pores of the catalyst layer (24). In some embodiments, the active composition (26) does not form a layer, and its components are distributed throughout the porosity of the catalyst layer (24). In other embodiments, the active composition (26) forms a layer on the catalyst layer (24), and its components are distributed throughout the porosity of the catalyst layer (24). The catalyst layer (24) may be placed over all outer surfaces of the substrate (22), and similarly, the active composition (26) may be placed within and / or on all catalyst layers (24). In FIG. 2, the electrode (10) is shown having a rectangular configuration with opposing main surfaces covered with the catalyst layer (24) and the active composition (26). Although not shown, the terminal surface of the electrode (10) may also be covered with the catalyst layer (24) and the active composition (26). The catalyst layer (24) and the active composition (26) are applied to the substrate (22) so that the thickness and composition are substantially uniform across the outer surface of the substrate (22). Preferably, the variation in thickness is mostly within the range of + / -5 μm.

[0021] Structurally, the substrate (22) may be a punched plate, woven wire, wire mesh, metal sponge, expanded metal, perforated or unperforated metal sheet, flat or corrugated lattice work, spaced rods or strips, or other structural forms. Wire mesh has been found to work particularly well in the practice of the present invention. The thickness of the substrate (22) is preferably about 0.15 to about 3.0 mm, more preferably about 0.6 to about 2.0 mm. Preferably, the substrate (22) has openings to reduce current density. The substrate (22) is composed of a conductive metal, for example, iron, iron alloy, nickel, nickel alloy, or stainless steel. Iron alloys that may be used include iron-nickel alloys, iron-chromium alloys, and iron-nickel-chromium alloys. Nickel alloys that may be used include nickel-copper alloys and nickel-chromium alloys. Preferably, the substrate (22) is composed of nickel, nickel alloy, or iron alloy. It was confirmed that a nickel plain weave wire mesh is particularly suitable for use as a substrate (22).

[0022] The catalyst layer (24) is formed from metal using a suitable thermal spraying process, for example, flame spraying, wire-arc spraying, plasma spraying, high-velocity oxygen fuel (HVOF) spraying, high-velocity air fuel (HVAF) spraying, detonation gun, and combustion wire spraying. Wire-arc spraying and plasma spraying have been observed to work particularly well in the practice of the present invention. Alternatively, the catalyst layer (24) may be formed from metal using low-temperature spraying or other types of surface finishing processes, for example, laser cladding and electroplating.

[0023] The metal forming the catalyst layer (24) may be nickel or a nickel alloy and / or nickel oxide (NiO). However, nickel is preferred. In this regard, it has been found that using a nickel-aluminum alloy that is dealloyed after deposition with a caustic solution results in a porous nickel layer that is very brittle and tends to degrade too quickly. Preferably, nickel is deposited on the substrate (22) using nickel powder in an atmospheric plasma spraying (APS) process, a low-temperature gas spraying process, or a wire-arc spraying process. More preferably, nickel is deposited on the substrate (22) using nickel powder in an APS process. The average particle size of the nickel powder used to form the catalyst layer (24) is about 10 to about 150 µm, more preferably about 45 to about 90 µm.

[0024] Before metal (e.g., nickel) powder is sprayed onto the substrate (22), the substrate (22) is treated to remove contaminants and its surface is conditioned to accommodate the metal powder. The treatment may include removing oil and grease through a degreasing process using a solvent. After any degreasing process is performed, preferably, a grit blasting process is performed on the substrate (22). In the grit blasting process, an abrasive medium, e.g., cooled iron or aluminum oxide, is pressurized with compressed air and directed toward the surface of the substrate (22). Grit blasting further removes any contaminants and roughens the surface to form pits and crevices that facilitate the mechanical bonding of the metal powder sprayed onto the surface. Fragments from the grit blasting can be removed by vacuuming, brushing, and / or air blowing.

[0025] The APS process can be performed with a plasma torch, where a strong electric arc is generated between a positively charged electrode (anode) and a negatively charged electrode (cathode). The arc ionizes a flowing process gas (e.g., a mixture of argon, hydrogen, nitrogen, and helium) into a plasma state, forming a plasma jet that may have a temperature in the range of about 10,000 to about 30,000°C. Generally, the anode is circular and helps to form a nozzle through which the plasma jet is discharged. The anode may be composed of copper, while the cathode may be composed of tungsten thoriumide. Metal powder (e.g., nickel) is fluidized in a carrier gas (which may contain hydrogen) and fed into the plasma jet, where the powder particles are melted and propelled onto a substrate (22) at a high speed (e.g., 300 to 550 m / s). Some of the molten metal particles are oxidized while moving to the substrate (22) to form an oxide, for example, nickel oxide (NiO). When the catalyst layer (24) is formed using APS in this way, the catalyst layer (24) contains both nickel and nickel oxide.

[0026] Molten metal particles sprayed from a plasma torch collide with a substrate (22) to flatten into a "splat," shrink and solidify to form micro-features, for example, stacked lamellae with voids. As the splat shrinks and solidifies, the molten metal particles are mechanically bonded to the substrate (22) through mechanical hooking. Thus, the adhesion of the sprayed metal particles is primarily due to mechanical bonding.

[0027] In a low-temperature spraying process, metal (e.g., nickel) powder is fluidized in a high-pressure stream of carrier gas, which is preheated to a temperature of up to 1,000°C and may consist of nitrogen, helium, or air. The stream of carrier gas and metal particles passes through a convergent-divergent DeLaval nozzle, where the stream is cooled and accelerated to a supersonic speed exceeding 1,000 m / s. The supersonic stream of metal particles and carrier gas is directed onto a substrate (22), where the metal particles undergo plastic deformation upon impact. The accelerated metal particles collide with the substrate (22) with sufficient kinetic energy to induce a mechanical and / or metallurgical bond between the metal particles and the substrate (22). Unlike a thermal spraying process, e.g., APS, the metal particles are not melted during the low-temperature spraying process.

[0028] In some embodiments, for example, after the catalyst layer (24) is formed by APS or low-temperature spraying, the substrate (22) coated with the catalyst layer (24) is heated in an oven at a temperature of about 300 to about 500°C for about 10 to about 20 minutes. In some embodiments, the substrate (22) coated with the catalyst layer (24) may be heated in an atmosphere containing hydrogen.

[0029] The catalyst layer (24) is formed on one side of the substrate (22) to have a thickness of about 10 to about 300 μm. Surprisingly, it was confirmed that the electrode (10), with a thickness of 50 μm or less on each side of the substrate (22), exhibits surprisingly excellent properties in that it can reduce the reverse current generated during shutdown. Accordingly, the thickness of the catalyst layer (24) may be 50 μm or less, more specifically in the range of about 10 to about 50 μm, and more specifically in the range of about 30 to about 50 μm.

[0030] As mentioned above, the catalyst layer (24) is moderately porous. Porosity can be measured by different methods, for example, the widely known Brunauer-Emmett-Teller (BET) method or the mercury (Hg) porosity measurement method. The BET method measures the total specific external surface area and internal surface area (per unit mass) of a porous solid from the measurement of physical adsorption of gas molecules (e.g., nitrogen) by the porous solid. The measured BET surface area (m² / g) provides a measure of porosity. Using the BET method, the catalyst layer (24) has a porosity of less than 1 m² / g.

[0031] Porosity may be correlated with electrochemical properties. As is known, the double-layer capacitance of an electrode is directly related to the total surface area of ​​the electrode. In relation to BET, measuring the double-layer capacitance allows for a more specific measurement of the electrode's "active porosity," that is, the surface area that can actually access the electrolyte for electrochemical reactions. Thus, unlike or in addition to the measured BET surface area (m² / g), the porosity of the catalyst layer (24) may be characterized by the double-layer capacitance before or after coating with the active composition (26) normalized with respect to the loading of the catalyst layer (24). The loading of the catalyst layer (24) is measured as the amount of metal (g) per cm². The normalized double-layer capacitance of the catalyst layer (24) is about 1.0 to about 10.0 mF / g, preferably about 3.0 to about 7.0 mF / g. The double-layer capacitance prior to normalization is measured in units of mF / cm² according to the procedure described below. The loading of a typical catalyst layer (24) is 50 to 1,200 g / m², more preferably 100 to 600 g / m². A lower loading of the catalyst layer (24) is more cost-effective because it requires less material in the catalyst layer and requires less of the generally expensive active composition (26), or it can be excessively diluted. In some applications, for example, but not limited to, applications involving the leaching of the active composition (26), the typical loading may be higher to 100 to 3,000 g / m² for the metal.

[0032] The double-layer capacitance of catalyst layers (24) having different compositions was studied using test electrodes (11) under relevant conditions, namely 25% KOH and T = 80°C. Each test electrode (11) included a substrate (22) of nickel expanded mesh. In the first test electrode (11a), the substrate (22) was plasma coated with nickel; in the second test electrode (11b), the substrate (22) was plasma sprayed with a nickel-aluminum alloy; in the third test electrode (11c), the substrate (22) was wire-arc sprayed using two nickel wires; and in the fourth test electrode (11d), the substrate was wire-arc sprayed with two wires each composed of a nickel-aluminum alloy. The test electrodes (11) were initially conditioned by performing polarization at 2 kA / m² for 10 minutes. Continuously, voltage-current scans were performed at different scan rates in the potential range of 0.25 to 0 V vs Hg / HgO (generally, it is understood by those skilled in the art that an appropriate potential range compatible with the active composition (26) is selected to avoid interference caused by a possible Faraday process). The electrochemical double layer capacitance is given by the standard equation dE / dt = C dl ·dQ / dt was used for estimation. Test electrode (11) exhibited a specific capacitance (normalized with respect to the catalyst layer (24) load) in the range of about 0.5 to about 30 mF / g as shown in FIG. 3, plotting the average capacitance versus electrode types (11a to 11d). In particular, test electrodes (11a, 11c and 11d) are preferably in the range of about 1.0 to about 10.0 mF / g. Al-containing electrodes (11b and 11d) were prophylactically leached in a KOH (30%) solution at 80°C for 2 hours.

[0033] The test electrode (11) is characterized by the amount of charge it can emit when a reverse current is applied under the relevant conditions, namely KOH 25% and T = 80°C. The test electrode (11) was conditioned by performing anode polarization at 10 kA / m² for 2 hours initially. Subsequently, a time potential measurement step was performed by applying a cathode current of 120 A / m² until the electrode potential was recorded as -0.8 V for the SHE (standard hydrogen electrode). The total amount of charge emitted from the test electrode (11) was calculated by integrating the current over time. The test electrode (11) was able to emit a controlled charge (normalized for the coating load) in the range of about 1,000 to about 70,000 mC / g as shown in FIG. 4, and FIG. 4 shows the normalized average charge for electrode types (11a to 11d). In particular, the test electrodes (11a, 11c and 11d) were preferably in the range of about 2,000 to about 10,000 mC / g.

[0034] In a specific embodiment, more than 90 wt% of the nickel in the catalyst layer is in a metallic form, while up to 10 wt% of the nickel is in an oxide form, mainly NiO, i.e., in the case of the oxide form, the weight percentage represents NiO. Preferably, the metallic nickel content in the catalyst layer is in the range of 90 to 99.5 wt%, preferably in the range of 95 to 99 wt%. The nickel oxide content in the catalyst layer may be in the range of at least 0.5 to 10 wt%, preferably in the range of 1 to 5 wt%.

[0035] The electrode configuration with optimal performance is thought to be due to controlled charge release from the porous nickel layer. Additionally, a certain amount of porosity in the nickel / nickel oxide layer is desirable to improve the number of accessible catalyst sites. It can be expected that the relationship between porosity and charge release depends on the characteristics of the layer. Control of porosity / capacitance is used to control electrode robustness (charge). As shown in FIG. 5, which plots average capacitance (y-axis) versus average charge (x-axis) for electrodes (11a to 11d), there is a linear relationship between capacitance and released charge for electrode (11) having a catalyst layer (24). Thus, it has been demonstrated that the investigated electrode configuration features the same and appropriate relationship between porosity / capacitance and charge release.

[0036] The active composition (26) comprises one or more metals selected from the group consisting of iridium (Ir), cobalt (Co), rhodium (Rh), iron (Fe), platinum (Pt), lithium (Li), and manganese (Mn). The iridium in the active composition (26) may be an iridium compound, for example, iridium oxide (IrO2). The cobalt in the active composition (26) may be a cobalt compound, for example, cobalt oxide, i.e., cobalt(II) oxide (CoO) or cobalt(II, III) oxide (Co3O4) or nickel cobaltite (NiCo2O4). Preferably, the cobalt in the active composition (26) is nickel cobaltite. More preferably, the nickel cobalt in the active composition (26) is in the form of spinel. The rhodium in the active composition (26) may be a rhodium compound, for example, rhodium(III) oxide (Rh2O3); Manganese in the active composition (26) may be a manganese compound, for example, manganese oxide (MnO2); iron in the active composition (26) may be an iron compound, for example, iron oxide (Fe2O3 or Fe3O4); platinum in the active composition (26) may be a platinum compound, for example, platinum oxide (PtO2); and lithium in the active composition (26) may be a mixed compound, for example, lithium nickel oxide (LiNiO2).

[0037] In some embodiments, the active composition (26) may essentially consist of a cobalt compound, a nickel compound, an iridium compound, a lithium compound (e.g., lithium nickel oxide), or an iron compound (e.g., iron oxide). Alternatively, the active composition (26) may essentially consist of a cobalt compound and an iridium compound, or the active composition (26) may include a cobalt compound or an iridium compound, or both a cobalt compound and an iridium compound. Preferably, the active composition (26) includes a cobalt compound or an iridium compound, more preferably both a cobalt compound and an iridium compound, and even more preferably, the active composition (26) includes nickel cobaltite and iridium oxide. Accordingly, the active composition (26) may comprise 0 to 100 mol% of an iridium compound, 0 to 100 mol% of a cobalt compound, and 0 to 40 mol% of one or more transition metal compounds selected from the group consisting of a rhodium compound, a manganese compound, an iron compound, a platinum compound, and a lithium compound. More preferably, the active composition (26) may comprise about 10 to 50 mol% of an iridium compound, about 40 to 90 mol% of a cobalt compound, and 0 to 20 mol% of one or more transition metal compounds selected from the group consisting of a rhodium compound, a manganese compound, an iron compound, a platinum compound, and a lithium compound. More preferably, the active composition (26) may comprise about 10 to 30 mol% of an iridium compound, more than 70 mol% of a cobalt compound, and 0 to 10 mol% of one or more transition metal compounds selected from the group consisting of a rhodium compound, a manganese compound, an iron compound, a platinum compound, and a lithium compound.More preferably, the active composition (26) may comprise 10 to 30 mol% of an iridium compound, 70 to 90 mol% of a cobalt compound, and 0 to 10 mol% of one or more transition metal compounds selected from the group consisting of a rhodium compound, a manganese compound, an iron compound, a platinum compound, and a lithium compound.

[0038] The catalyst layer (24) and the active layer (26) together comprise, in combination, about 80 to about 99.9 mol% nickel, 0 to about 2 mol% nickel oxide, about 0 to about 0.8 mol% iridium compound, about 0 to about 2 mol% cobalt compound, and about 0 to about 2 mol% rhodium compound, manganese compound, iron compound, platinum compound and lithium compound, one or more transition metal compounds selected from the group consisting of rhodium compound, manganese compound, iron compound, platinum compound and lithium compound.

[0039] The active composition (26) may be formed from a single precursor composition applied to the catalyst layer (24) in one or more coating processes, or the active composition (26) may be formed from a plurality of different precursor compositions applied to the catalyst layer (24) in a plurality of coating processes. As described above, the active composition (26) may form a layer on top of the catalyst layer (24) as schematically illustrated in FIG. 2, and / or the active composition (26) may be absorbed into the catalyst layer (24) and distributed throughout the porosity of the catalyst layer. In this way, the thickness of the layer of the active composition (26) may be about 0 to about 20 μm, more preferably about 0 to about 10 μm.

[0040] Generally, the precursor composition(s) comprises a precursor compound (e.g., an organic or inorganic metal salt) that can be thermally decomposed to form a metal compound in the active composition (26). The metal salt may be a chloride and may be soluble in a solvent comprising an acid (e.g., hydrochloric acid and nitric acid) and an alcohol, e.g., isopropyl alcohol, n-propyl alcohol, n-butyl alcohol, and ethyl alcohol. Some examples of the precursor compound(s) are given below.

[0041] In the case of iridium oxide, the precursor compound(s) may comprise a chloride, sulfate, or nitrate salt of iridium dissolved in a solvent, for example, an acid or an alcohol. More specifically, the precursor compound(s) may comprise a solution of iridium(III) chloride (IrCl3) in the trihydrate form (IrCl3(H2O)3) together with hydrochloric acid and isopropyl alcohol and / or ethyl alcohol.

[0042] In the case of cobalt oxide (II), the precursor compound(s) may include cobalt chloride (CoCl2) and water. In the case of cobalt oxide (II, III), the precursor compound(s) may include cobalt (II) acetate tetrahydrate (C4H6CoO4_4H2O), ethyl alcohol and oxalic acid (C2H2O4).

[0043] In the case of nickel cobaltite, the precursor compound(s) may comprise nickel(II) acetate tetrahydrate (C4H6NiO4_4H2O) and cobalt(II) acetate tetrahydrate (C4H6CoO4_4H2O) mixed with urea (CO(NH)2)2), deionized water, ethyl alcohol, glycerol, and tetraethylene glycol (TEG), said nickel(II) acetate and cobalt(II) acetate are co-precipitated. Alternatively, the precursor compound(s) may comprise nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O) and cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), which may be soluble in NH4OH.

[0044] The precursor composition(s) can be applied to the catalyst layer (24) using a brush or by electrospraying, roller coating, or immersion coating.

[0045] After the precursor composition(s) are applied to the catalyst layer (24), the catalyst layer (24) and the substrate (22) coated with the precursor composition(s) are heated so that the precursor compound(s) are thermally decomposed to form the active composition (26). Heating may be performed for about 10 to about 20 minutes in an oven at a temperature of about 300 to about 500°C.

[0046] Example 1

[0047] A 100x100mm piece of nickel wire woven mesh with a diameter of 0.17mm was plasma-sprayed on both sides in ambient air with a target thickness of 50㎛ (on each side) using 99.9% purity nickel powder (Fe<0.5, O<0.4, C<0.02, S<0.01) with a particle size of -45 / +10㎛ at a rate of 4.8±0.5g / dm². Subsequently, the sprayed wire mesh was heated in an oven at 350℃ in air for 15 minutes. After cooling the plasma-sprayed woven mesh, it was coated with a precursor composition using a brush in a series of coating, heating, and cooling steps. The precursor composition consisted of 19.77g of nickel(II) nitrate hexahydrate and 39.56g of cobalt(II) / nitrate hexahydrate, and was made to a volume of 100mL with deionized water. Initially, a plasma-sprayed wire mesh was coated with a precursor composition and then heated in an oven at 350°C in air for 15 minutes. After cooling, the wire mesh was coated again with the precursor composition and heated again in an oven at 350°C for 15 minutes. This coating, heating, and cooling process was repeated to obtain 7.52g METAL Achieved a load of / ㎡.

[0048] The above process produced an electrode (E1) having a catalyst layer and an active composition on each side. The catalyst layer was mostly nickel, containing a small amount of NiO. The active composition contained 0.0427 mol / m² of nickel cobaltite. When viewed under a scanning electron microscope (SEM), the catalyst layer was a nickel matrix formed by a thin crust of nickel particles and NiO, and the nickel cobaltite was distributed within the porosity of the catalyst layer.

[0049] The combination of the catalyst layer and the active composition had a thickness (one side) of 51±6㎛ and a porosity of 0.3 m² / g (BET). The combination consisted of 97.6 mol% nickel, 0.9 mol% nickel oxide, and 1.5 mol% nickel cobaltite.

[0050] Example 2

[0051] A 100x100mm nickel expanded mesh piece was plasma sprayed on both sides in ambient air with a target thickness of 45㎛ (on each side) using 99.9% purity nickel powder (Fe<0.5, O<0.4, C<0.02, S<0.01) with a particle size of -45 / +10㎛ at a rate of 4.5±0.5g / dm². Subsequently, the sprayed wire mesh was heated in an oven at 350℃ in air for 15 minutes. After cooling the plasma-sprayed expanded mesh, it was coated with a first precursor composition and a second precursor composition as described below.

[0052] The first precursor composition was identical to the precursor composition applied in Example 1, namely, a solution of nickel(II) nitrate hexahydrate and cobalt(II) nitrate hexahydrate. The first precursor composition was also applied to the mesh in the same manner as in Example 1, and 7.52 g METAL Achieved a load of / ㎡.

[0053] The second precursor composition contained 4.27 g of hexa-ammineiridium(III) hydroxide ([Ir(NH3)6](OH)3) and was made to a volume of 100 mL with deionized water. After applying the first precursor composition, the second precursor composition was applied with a brush in a series of coating, heating, and cooling steps. Initially, a plasma-sprayed and coated mesh was coated with the second precursor composition and then heated in an oven at 350°C in air for 15 minutes. After cooling, the mesh was coated again with the second precursor composition and heated again in an oven at 350°C for 15 minutes. This coating, heating, and cooling process was repeated to obtain 1.88 g METAL Achieved a load of / ㎡.

[0054] The above process produced an electrode (E2) having a catalyst layer and an active composition on each side. The catalyst layer was mostly nickel, containing a small amount of NiO. The active composition was 9.78 mol / m² iridium oxide and 42.7 mmol / m² nickel cobaltite, which corresponds to 18.6 mol% iridium oxide and 81.4 mol% nickel cobaltite. When viewed under a scanning electron microscope (SEM), the catalyst layer was a nickel matrix formed by a thin crust of nickel particles and NiO, and the iridium oxide and nickel cobaltite were distributed within the porosity of the catalyst layer. The combination of the catalyst layer and the active composition had a thickness (on one side) of less than 50 μm and a porosity of 0.4 m² / g (BET). The combination consisted of 98.8 mol% nickel, 0.12 mol% iridium oxide, and 0.53 mol% cobaltite.

[0055] Example 3

[0056] A 100x100mm nickel expanded mesh piece was plasma sprayed on both sides in ambient air with a target thickness of 45㎛ (on each side) using 99.9% purity nickel powder (Fe<0.5, O<0.4, C<0.02, S<0.01) with a particle size of -45 / +10㎛ at a rate of 4.5±0.5g / dm². Subsequently, the sprayed expanded mesh was heated in an oven at 350℃ in air for 15 minutes. After cooling the plasma-sprayed expanded mesh, it was coated with a precursor composition using a brush in a series of coating, heating, and cooling steps. The precursor composition was identical to the second precursor composition of Example 2, namely containing hexa-ammineiridium(III) hydroxide ([Ir(NH3)6](OH)3), and was made to a volume of 100mL with deionized water. Initially, a plasma-sprayed expanded mesh was coated with a precursor composition and then heated in an oven at 350°C in air for 15 minutes. After cooling, the mesh was coated again with the precursor composition and heated again in an oven at 350°C for 15 minutes. By repeating this coating, heating, and cooling process, 1.88g METAL Achieved a load of / ㎡.

[0057] The above process produced an electrode (E3) having a catalyst layer and an active composition on each side. The catalyst layer was mostly nickel, containing a small amount of NiO. The active composition was 9.78 mol / m² of iridium oxide. When viewed under a scanning electron microscope (SEM), the catalyst layer was a nickel matrix formed by a thin crust of nickel particles and NiO, and iridium oxide was distributed within the porosity of the catalyst layer (see FIGS. 6 to 8). The combination of the catalyst layer and the active composition had a thickness (on one side) of less than 50 μm and a porosity of 0.4 m² / g (BET). The combination consisted of 98.7 mol% nickel, 0.9 mol% iridium oxide, and 0.4 mol% iridium oxide.

[0058] Comparative Example 1

[0059] A 100x100mm nickel expanded mesh piece was plasma sprayed in ambient air on both sides with a target thickness of 300㎛ and 100㎛ using 99.9% purity nickel powder (Fe<0.5, O<0.4, C<0.02, S<0.01) with a particle size of -45 / +10㎛ at a rate of 13.0±0.9 g / dm². Subsequently, the sprayed mesh was heated in an oven at 350℃ in air for 15 minutes. Afterward, the sprayed expanded mesh was cooled to form electrode CE1, which had only a catalyst layer containing nickel and nickel oxide. The catalyst layer had a thickness (on each side) of less than 200㎛ and a porosity of 0.4 m² / g (BET).

[0060] Comparative Example 2

[0061] A 100x100mm nickel expanded mesh piece was plasma sprayed onto both sides in ambient air with a target thickness of 45㎛ using 99.9% purity nickel powder (Fe<0.5, O<0.4, C<0.02, S<0.01) with a particle size of -45 / +10㎛ at a rate of 4.5±0.5g / dm². Subsequently, the sprayed expanded mesh was heated in an oven at 350℃ in air for 15 minutes. Afterward, the sprayed wire mesh was cooled to form an electrode CE2 having only a catalyst layer containing nickel and nickel oxide. The catalyst layer had a thickness (on one side) of less than 50㎛ and a porosity of 0.4m² / g (BET).

[0062] [Comparative Example 3]

[0063] Except as mentioned below, Example 2 was repeated on a different 100x100mm piece of nickel expanded mesh. The mesh was not plasma-sprayed, or otherwise coated with a nickel / nickel oxide or other metal layer. The active composition was formed directly on the expanded mesh. The same first precursor composition and second precursor composition were applied to the expanded mesh in the same manner as in Example 2, except that there was no plasma-sprayed layer.

[0064] The process produced an electrode (CE3) having an active composition on each side. The active composition was 9.78 mmol / m² iridium oxide and 42.7 mmol / m² nickel cobaltite, which corresponds to 18.6 mol% iridium oxide and 81.4 mol% nickel cobaltite.

[0065] test

[0066] Electrodes (E1, E2, E3, CE1, CE2, and CE3) were used as anodes in the electrolytic cells, and the oxygen overpotential at the anodes was measured using a VMP3 Biologic static spectroscopy equipped with an SCE reference electrode and a 20A booster at 80°C in 25% KOH with a three-electrode setup. The iR drop was measured by electrochemical impedance spectroscopy (EIS). The generated electrode potential measured at 10 kA / m² was subtracted from the thermodynamic oxygen evolution potential at pH 14. These measurement results are shown in Table 1 below. Additionally, electrolytic cells with anodes E2, E3, CE1, CE2, and CE3 were connected to receive power with a current of 10 kA / m², followed by a series of 50 power shutdowns with an on / off cycle of 6 hours. The voltage of the cells at 10 kA / m² was measured after each shutdown. These measurement results are also shown in Table 1 below.

[0067]

[0068] The test results show that the catalyst layer of electrode CE2 is substantially thinner than the catalyst layer of electrode CE1, but ( 50㎛ range (200 μm), showing that the oxygen overpotential of electrode CE2 is not much greater than that of electrode CE1 (380 mV vs. 350 mV). Electrodes E1 to E3 had substantially better oxygen overpotentials than both electrodes CE1 and CE2. Electrode E2 had a much better oxygen overpotential than CE1 and CE2, and had better shutdown resistance than electrodes E3 and CE3. Electrode CE3 had a good oxygen overpotential but did not have very good shutdown resistance. The results show that an electrode having only the active composition (26) without the catalyst layer (24) has a good oxygen overpotential but has poor shutdown resistance. Conversely, an electrode having only the catalyst layer (24) without the active composition (26) has a good shutdown resistance but has poor overpotential. The synergistic combination of the catalyst layer (24) and the active composition (26) produces an electrode having improved properties, namely excellent shutdown resistance and excellent overpotential, in particular an active layer containing both a cobalt compound and an iridium compound. Such an electrode (e.g., electrode E2) also has an improved oxygen overpotential.

[0069] FIGS. 6 to 8 illustrate SEM / EDAX images of cross-sections of an electrode according to Example 3, comprising a nickel substrate (30), a catalyst layer (31) made of nickel, and an iridium-based active composition heat-applied to a small amount of nickel oxide and catalyst coating. The SEM image of FIG. 6 shows the morphology of the catalyst layer obtained by depositing nickel onto a substrate made of a rough nickel mesh through thermal spraying. As can be seen from this, the catalyst layer (31), which exhibits some porosity, is clearly distinguished from the dense nickel substrate (30), which essentially does not exhibit porosity, by a well-defined interface (32). The porosity of the catalyst layer (31) is essentially caused by cracks (33) formed within the catalyst layer (31). Mainly, these cracks (33) are essentially oriented parallel to the interface (32) between the substrate (30) and the catalyst layer. The black area (34) corresponds to the resin required for sample preparation. The active composition (not shown in FIG. 6) can penetrate into the catalyst layer through cracks (33), as can be obtained from the SEM / EDAX image of FIG. 7, which illustrates the same electrode cross-sectional area as FIG. 6. The gray areas correspond to the nickel of the catalyst layer, while the white areas / dots correspond to the iridium of the active composition. Although the iridium EDAX map a is considerably noisy, distinct patches with increased iridium content, shown as black circles in FIG. 7, clearly and visibly indicate that the active composition has penetrated into the catalyst layer and accumulated in the area corresponding to the porous region of FIG. 6. FIG. 8 is a SEM / EDAX image illustrating the oxygen distribution in the same area of ​​the electrode of FIG. 6. The oxygen EDAX image (white) is superimposed on the SEM image of FIG. 6 (gray). Ignoring noise, from FIG. 8 it can be seen that the substrate (30) is composed of pure nickel, while the catalyst layer (31) contains a patch (black circle) containing oxygen corresponding to the nickel oxide component of the catalyst layer.

[0070] It should be understood that the description of the exemplary embodiment(s) above is intended to be illustrative only and not exhaustive. Those skilled in the art may make specific additions, deletions, and / or modifications to the embodiment(s) disclosed without departing from the spirit or scope of the invention.

Claims

Claim 1 An electrode for use in an alkaline electrolytic process, comprising: a metal substrate; a catalyst layer disposed on the metal substrate and comprising nickel and nickel oxide, and having a porosity of less than 1 m² / g as measured by BET; and an active composition disposed both on the catalyst layer and within the catalyst layer and comprising one or more metal compounds selected from the group consisting of cobalt compounds, iridium compounds, rhodium compounds, iron compounds, platinum compounds, lithium compounds, and manganese compounds. Claim 2 The electrode of claim 1, wherein the electrode has a double-layer capacitance in the range of 1.0 to 10.0 mF / g, normalized by the loading of the catalyst layer on the metal. Claim 3 An electrode according to claim 1 or 2, wherein the active composition comprises more than 30 mol% of one of the cobalt compound and the iridium compound. Claim 4 An electrode according to paragraph 3, wherein the active composition comprises more than 60 mol% of one of the cobalt compound and the iridium compound, the cobalt compound comprises nickel cobaltite, and the iridium compound comprises iridium oxide. Claim 5 In paragraph 4, the electrode wherein the active composition is essentially composed of nickel cobaltite. Claim 6 In paragraph 4, the electrode wherein the active composition is essentially composed of iridium oxide. Claim 7 An electrode according to claim 1, wherein the active composition comprises 40 to 90 mol% of the cobalt compound, 0 to 50 mol% of the iridium compound, and 0 to 20 mol% of one or more of the rhodium compound, the iron compound, the platinum compound, the lithium compound, and the manganese compound. Claim 8 An electrode according to claim 7, wherein the active composition comprises 70 to 90 mol% of the cobalt compound, 10 to 30 mol% of the iridium compound, and 0 to 10 mol% of one or more of the rhodium compound, the iron compound, the platinum compound, the lithium compound, and the manganese compound. Claim 9 An electrode according to claim 8, wherein the cobalt compound comprises nickel cobaltite and the iridium compound comprises iridium oxide. Claim 10 An electrode according to claim 1, wherein the active composition comprises one or more metal compounds selected from the group consisting of nickel cobaltite, iridium oxide, iron oxide, and lithium nickel oxide. Claim 11 An electrode according to any one of claims 1, 2, and 7 to 10, wherein the thickness of the catalyst layer is in the range of 10 to 50 μm. Claim 12 An electrode according to any one of claims 1, 2, and 7 to 10, wherein the metal substrate of the electrode comprises one or more metals selected from the group consisting of nickel, nickel alloys, and iron alloys. Claim 13 In claim 12, the catalyst layer is a first catalyst layer, and the metal substrate comprises a nickel substrate; the metal substrate has opposing first and second sides, and the first catalyst layer is disposed and attached on the first side of the metal substrate; the electrode further comprises a second catalyst layer, the second catalyst layer is disposed and attached on the second side of the metal substrate, and the second catalyst layer has substantially the same composition as the first catalyst layer; and the active composition is disposed on both the second catalyst layer and within the second catalyst layer, the electrode. Claim 14 An alkaline water electrolysis unit comprising an electrode described in any one of claims 1, 2, and 7 to 10, wherein the electrode is an anode, and the alkaline water electrolysis unit further comprises a cathode and an electrolyte substantially free of chlorine. Claim 15 A method for forming an electrode, comprising: a step of providing a metal substrate; a step of forming a catalyst layer on the metal substrate through thermal spraying, laser cladding, or electroplating, wherein the catalyst layer comprises nickel and nickel oxide and has a porosity of less than 1 m² / g as measured by BET; and a step of applying an active composition to the catalyst layer, wherein, after applying one or more precursor compositions to the catalyst layer, the one or more precursor compositions and the catalyst layer are heated to form the active composition. Claim 16 A method for forming an electrode according to claim 15, wherein the step of forming the catalyst layer is performed by thermal spraying, and the double layer capacitance of the catalyst layer is in the range of 1.0 to 10.0 mF / g. Claim 17 A method for forming an electrode according to claim 15 or 16, wherein the step of forming the catalyst layer is performed by thermal spraying, and the step of forming the catalyst layer comprises an electric wire or plasma that sprays nickel powder onto the metal substrate in ambient air. Claim 18 A method for forming an electrode according to claim 15, wherein the active composition comprises one or more metal compounds selected from the group consisting of cobalt compounds, iridium compounds, rhodium compounds, iron compounds, platinum compounds, lithium compounds, and manganese compounds, and the precursor composition comprises a precursor of the one or more metal compounds. Claim 19 A method for forming an electrode according to claim 18, wherein the active composition comprises 40 to 90 mol% of the cobalt compound and 10 to 50 mol% of the iridium compound. Claim 20 A method for forming an electrode according to claim 19, wherein the cobalt compound comprises nickel cobaltite and the iridium compound comprises iridium oxide.

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