Perovskite electrodes for electrolysis in alkaline media

Abstract

An electrode for use in the electrolysis of water under alkaline conditions comprises a nickel metal substrate, a ceramic material having a perovskite structure, and metal nanoparticles embedded in the ceramic material, the ceramic material comprising at least one metal oxide selected from the lanthanides, including lanthanum, cerium, and praseodymium, and the ceramic material forms a coating on the nickel metal substrate. The metal nanoparticles facing the alkaline aqueous solution are electrochemically active, while the metal nanoparticles facing the metal substrate form anchor points between the metal substrate and the ceramic material.

Description

[Technical Field]

[0001] The present invention relates to an electrode for electrolysis, particularly an electrode for the electrolysis of water under alkaline conditions. More specifically, the present invention relates to an electrode comprising a nickel metal substrate, a ceramic material having a perovskite structure comprising an oxide of at least one lanthanide metal including lanthanum, cerium, and praseodymium, and metal nanoparticles, wherein the ceramic material forms a coating on the metal substrate and the metal nanoparticles are embedded in the ceramic material. The present invention further relates to a method for manufacturing the electrode. The present invention further relates to an alkaline electrolytic stack comprising at least one of the electrodes, and a method for the electrolysis of water under alkaline conditions using the alkaline electrolytic stack. [Background technology]

[0002] In alkaline water electrolysis, water is electrochemically converted into hydrogen and oxygen under alkaline conditions (H2O = H2 + 0.5O2). The electrolytic cell contains two electrodes, an anode and a cathode. During operation, a potential is applied between the two electrodes, causing an electrolytic current to flow through the electrolytic cell. During operation, hydrogen is produced at the cathode by the hydrogen production reaction (HER) (2H2O + 2e⁻¹). - =H2+2OH - ), on the other hand, oxygen is produced at the anode by the oxygen production reaction (OER) (2OH - = 0.5O2 + H2O + 2e - The alkaline electrolytic cell contains an electrolyte consisting of a liquid alkaline medium such as an aqueous solution of hydroxide and / or carbonate. The electrolytic cell further includes a porous separator and / or an ion exchange membrane capable of conducting hydroxide ions.

[0003] In alkaline water electrolysis, it is desirable to minimize both the investment cost and energy consumption of the electrolytic cell. All other things being equal, the lower the investment cost and energy consumption of the cell, the lower the production cost of the hydrogen produced by electrolysis. The investment cost of the electrolytic cell is minimized by using low-cost materials and low-cost manufacturing methods. The energy consumption of the electrolytic cell is minimized by minimizing the required applied potential (voltage) at a given operating current or current density.

[0004] The required applied potential is the sum of multiple contributions, including the following: 1) The theoretical potential required to carry out an electrochemical reaction, 2) Potential due to porous separator and / or ion exchange membrane, 3) Overvoltage due to the anode, 4) Overvoltage due to cathode, 5) Potential that depends on the distance between the anode and cathode in the electrolytic cell.

[0005] Here, "overpotential" refers to the difference between the theoretical potential and the actual potential applied to each electrode, including, for example, potential contributions due to bubble formation. Those skilled in the art know that the overpotential of the anode, i.e., OER, is generally higher than that of the cathode, i.e., HER. Therefore, the development of improved electrodes for oxygen generation is extremely important. In particular, such electrodes should be composed of low-cost materials, manufactured using low-cost methods, and have a lower overpotential to the OER than state-of-the-art electrodes.

[0006] In industrial alkaline electrolytic cells, electrodes are typically nickel or precious metal-based. Nickel-based electrodes are relatively inexpensive but are characterized by high overpotential, leading to high energy consumption in the cell and consequently higher hydrogen production costs. On the other hand, electrodes based on precious metals or their compounds (e.g., IrO2, RuO2, Pt) have low overpotential but very high investment costs, resulting in high hydrogen production costs. A third type of alkaline electrolysis electrode material has been discovered that exhibits low overpotential and low cost. Specifically, ceramic materials with certain crystal structures, such as perovskite structures, have been shown to be particularly attractive alternatives to both nickel-based and precious metal-based electrodes. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] US 4497698 [Patent Document 2] EP3351659A1 [Non-patent literature]

[0008] [Non-Patent Document 1] Yarong Wang et al., “Enhanced overall water electrolysis on a bifunctional perovskite oxide through interfacial engineering”, Electrochimica Acta 318 (2019) 120-129

[0009] US4497698 (Texas A&M University [US], filed on November 8, 1983) discloses an improved oxide for an oxygen generation electrode, including a perovskite-type oxide composed of lanthanum nickelate. The electrode is manufactured from press-molded and sintered lanthanum nickelate powder. This manufacturing method is not suitable for industrial applications, and subsequently, a number of more active perovskite-type oxides have been identified.

[0010] EP3351659A1 (Asahi Kasei Corporation [JP] and Yokohama National University [JP], filed on September 16, 2016) discloses an anode for water electrolysis, including a porous nickel substrate and a perovskite thin film coating with a composition of LnNi x Co 1-x O3 (Ln is a rare earth element, 0 < x ≤ 1). This anode has a low overvoltage for the oxygen generation reaction, but (as well-known to those skilled in the art) the adhesion between the perovskite coating and the nickel substrate has been a problem. More specifically, in EP3351659A1, a plurality of additional processing steps were required to improve the adhesion between the perovskite coating and the substrate. First, the substrate was surface-treated before coating to form an irregular surface. For such irregularity formation, a combination of, for example, sandblasting and chemical etching using hydrochloric acid is generally used. Next, a nickel oxide layer (so-called "intermediate layer") can be deposited between the nickel substrate and the perovskite film. The nickel oxide film should be as thick as possible, preferably 50 nm or more, more preferably 100 nm or more, to improve adhesion. Importantly, since nickel oxide has a very low electrical conductivity, if the nickel oxide layer is too thick, the overvoltage of the anode will increase rapidly. According to EP3351659A1, the thickness of the nickel oxide layer is preferably 3000 nm or less, more preferably 5000 nm or less. Assuming the electrical conductivity of nickel oxide is 1.3×10 -5 S / cm and the thickness of the nickel oxide layer is 5000 nm, the resulting resistance penalty is 38.5 Ω·cm 2 and this is at a current density of 5 kA / m 2This corresponds to an additional anode overvoltage of 19.23V. Since the overvoltage of high-performance anodes is typically less than 0.5V, the approach using the 5000nm nickel oxide intermediate layer described above is not commercially viable. Even a 200nm nickel oxide layer increases the anode overvoltage by 0.77V, which adds an unacceptably large overvoltage contribution to the electrolytic cell.

[0011] Yarong Wang et al., “Enhanced overall water electrolysis on a bifunctional perovskite oxide through interfacial engineering”, Electrochimica Acta 318 (2019) 120-129, La 0.8 Sr 0.2 Cr 0.69 Ni 0.31 O 3-δ The catalytic activity of (LSCN) was improved by partial reduction and subsequent phosphating, which induced the exsolution of discrete Ni2P nanoparticles. LSCN was first partially reduced in a gas mixture of Ar and H2, and metallic Ni was added to the surface of the partially reduced LSCN (denoted as r-LSCN). 0 This promotes the exsolution of nanoparticles. Subsequently, r-LSCN is exposed to a reducing atmosphere containing a phosphorus source, and Ni 0The nanoparticles were converted to Ni2P nanoparticles (denoted as r-LSCN-P). The OER and HER activities of the perovskite oxides increased in the order of r-LSCN < LSCN < r-LSCN-P. That is, it was observed that the activity of the partially reduced LSCN was inferior to that of the initial LSCN in any case. According to Wang et al., both the reduction and phosphidation steps are essential to realize highly active perovskite oxides. The additional step of the phosphidation treatment was carried out by placing a porcelain boat filled with NaH2PO2 upstream of a porcelain boat filled with r-LSCN in a tubular furnace at 600 °C under an Ar / H2 flow. Such a processing step is very difficult to carry out on an industrial scale, and the control of the degree of phosphidation is also an issue. Therefore, it is desirable to obtain highly active perovskite oxides without an additional phosphidation step.

[0012] Therefore, there still remains a need to identify a new technical solution to improve the adhesion of perovskite oxides on a nickel substrate without performance degradation and preferably without additional processing steps. SUMMARY OF THE INVENTION

[0013] The inventor has discovered that there are still significant drawbacks in the methods employed for the anodes described in the prior art for the purpose of producing low-cost hydrogen by alkaline electrolysis using noble-metal-free electrodes. More specifically, an anode that coats a nickel substrate with a perovskite oxide and does not dispose an intermediate layer between the coating and the substrate lacks mechanical stability. In particular, adhesion problems become apparent during a temperature cycle in which the temperature of the electrolytic cell is raised from room temperature to an operating temperature of about 80 °C and then returned to room temperature. Poor adhesion between layers can also become apparent when operating the electrolytic cell at a high current density (e.g., 5 kA / m 2 or higher). This is because the adhesion between layers weakens due to the intensity of oxygen bubble formation on the anode surface. Furthermore, the anode should not contain noble metals and should be easy to manufacture.

[0014] The inventors have surprisingly discovered that it is possible to improve the adhesion between a perovskite coating and a nickel substrate while simultaneously achieving high electrochemical activity for oxygen production reactions, without providing a resistive intermediate layer between the coating and the substrate. More specifically, it was found that a desirable combination of strong adhesion and high electrochemical activity can be achieved by coating a ceramic material with nanoparticles embedded within the ceramic material. This embedding can be achieved, for example, by matching the chemical composition of the perovskite and then forming the nanoparticles by an exsolution process. The embedded nanoparticles provide additional electrochemical activity for oxygen production reactions and function as anchoring points that improve the adhesion between the perovskite and the nickel substrate.

[0015] Here, "perovskite" refers to a class of ceramic materials having the general crystal structure formula ABO3, where A and B are metal ions and O is an oxide ion, and these ions are located at energetically different positions within the crystal lattice. The sites where A ions are located in the lattice are called "A sites," and the sites where B ions are located are called "B sites." The oxygen content of perovskites varies depending on the selection of metal ions located at the A and B sites, and also depends on factors such as temperature and oxygen partial pressure. Therefore, ABO 3±δ is a more precise formula representing the class of perovskites, where δ is called the "oxygen unstoichiometric ratio" and is in the range of 0 ≤ δ ≤ 1. In the formulas and equations of this invention, the "oxygen unstoichiometric ratio" refers to the number of oxygen vacancies relative to the number of B-site cations in the ceramic material. For example, La 0.3 Sr 0.7 TiO 3.15 (La 0.3 Sr 0.7 TiO 3+0.15 In the case of (equivalent to), δ = 0.15. Note that for any double perovskite structure A2B2O 5+δ* is, δ * Assuming = 1-2δ, ABO 3-δIt can be expressed as follows. The oxygen unstoichiometry depends, for example, on temperature, oxygen partial pressure, and applied voltage. Non-limiting examples of ceramic materials having a perovskite structure that include at least one oxide of lanthanides, including lanthanum, cerium, and praseodymium, are La 0.5 Sr 0.5 FeO 3-δ La 0.9 Ba 0.1 NiO 3-δ Ce 0.6 Sr 0.4 OO 3-δ and Pr 0.95 OO 3-δ Includes.

[0016] In the context of this invention, "A is embedded in B" refers to a state in which particles of A are partially embedded in the surface of B. "A is embedded in B" does not include situations where particles of A are simply deposited or coated onto the surface of B. Experimentally, it is possible to distinguish between embedded particles and deposited / coated particles by chemically etching the particles of A using an etching agent such as HNO3. 。 In areas where particles A were embedded in B, etching the material of B forms pits, and the size and number of these pits are similar to the size and number of particles A. On the other hand, if particles A are coated onto the surface of B, no pits are formed when A is removed.

[0017] In the context of this invention, "adhesion" refers to the ability of two materials to bond to each other. Therefore, the expressions "strong adhesion" or "good adhesion" refer to a state in which two materials are strongly bonded and one material does not peel or separate from the other. At the macroscale, adhesive strength can be quantitatively evaluated using a dolly test as described in ASTM D4541-22. Qualitatively, it can be evaluated using a tape test as described in ASTM D3359. At the microscale, adhesive strength can be qualitatively evaluated by observing the polished cross-section of the interface between the two materials using an optical microscope or scanning electron microscope. The detection of areas of peeling or separation is a sign of weak adhesion. Finally, in applications to electrolysis, adhesion between two electrode materials is considered good or sufficient if neither material separates from the electrode during operation.

[0018] In one aspect of the present invention, an electrode is provided for electrolyzing water from an alkaline aqueous solution, the electrode comprising a nickel metal substrate, a ceramic material having a perovskite structure containing a lanthanide oxide, wherein the ceramic material forms a coating on the metal substrate, and metal nanoparticles, the metal nanoparticles being embedded in the ceramic material. In the context of the present invention, "A forms a coating on B" or "a coating of A on B" means a situation in which at least one surface of element B is substantially covered with material A. Importantly, materials A and B do not necessarily have to be in physical contact; for example, materials A and B may be separated by a third material, which may be, for example, nanoparticles embedded in material A and sandwiched between materials A and B.

[0019] The embodiments of the invention described above have several important advantages compared to state-of-the-art technology. First, the electrodes of the present invention have high electrochemical activity, enabling low overpotential and high efficiency, thereby enabling low hydrogen production costs. Second, the electrodes of the present invention are mechanically stable, meaning the perovskite coating adheres strongly to the nickel substrate. This prevents the perovskite coating from being lost over time during the operation of the electrolytic cell, otherwise it would cause a decrease in activity and even clogging of the filter and / or separator. Third, since the electrodes of the present invention do not contain precious metals such as iridium, platinum, and ruthenium, not only is the cost of electrode raw materials significantly reduced, but supply problems for electrode materials are also alleviated. Fourth, the electrodes of the present invention have a lower overpotential for the oxygen production reaction compared to conventional nickel-based electrodes. Since electricity costs are a major factor determining the cost of hydrogen production by electrolysis, the electrodes of the present invention enable hydrogen production at a lower cost (OPEX) than conventional nickel-based electrodes. Fifth, electrochemical activity and adhesion strength can be optimized by adjusting the composition of the metal embedded in the ceramic material.

[0020] Embedded nanoparticles can be realized in numerous different ways, but are generally achieved by a method called "exsolution." "Exsolution" refers to a property of a subset of perovskite materials, specifically the property of changing the solubility of at least one metal ion at the B-site of the perovskite lattice by changing external conditions. When the solubility of these metal ions decreases, at least some of the B-site metals can no longer dissolve in the perovskite and are extruded from the perovskite structure, forming an additional phase on the perovskite surface. Chemically, exsolution can be expressed as follows:

[0021] A 1-y BO 3-δ’ →(1-y)ABO 3-δ +yB (Equation 1) Here, A 1-y BO 3-δ’ The precursor ceramic material is ABO 3-δ is the desired ceramic material, and B is the exsoluble nanoparticles.

[0022] Here, A 1-y BO 3-δ Some metal ions at the B site of the structure cannot remain dissolved in the perovskite lattice and are exsoluble as a separate phase denoted as B. 1-y BO 3-δ ∫A refers to an A-site defective perovskite, that is, a perovskite in which the total number of A-site metals is less than the total number of B-site metals. The parameter "y" is called the "A-site defect rate". For example, in the case of a perovskite with an A-site defect rate of 0.05, Equation 1 can be rewritten as follows.

[0023] A 0.95 BO 3-δ’ →0.95ABO 3-δ +0.05B ​​(Equation 2)

[0024] Therefore, the A-site defect rate characterizes not only the relative number of metals at the A-site and B-site, but also the amount of B exsoluble from the perovskite lattice. ABO 3-δ This refers to a stoichiometric perovskite where the ratio of metals at site A to site B is 1. In other words, the oxygen unstoichiometric ratio δ in the stoichiometric perovskite phase may differ from the oxygen unstoichiometric ratio δ' in the original A-site-deficient perovskite phase.

[0025] Exsolution can be induced by changes in external conditions. These external conditions may refer to changes in temperature and / or changes in the partial pressure of oxygen in the atmosphere. For example, Neagu et al. “In situ growth of nanoparticles through control of non-stoichiometry”, Nature Chemistry, doi: 10.1038 / nchem.1773, published October 6, 2013, showed that species such as Fe, Ni, Mn, and Cu can be exsoluble from a lattice of A-site deficient doped lanthanum titanate exposed to a reducing atmosphere and high temperature. For example, Ni is exsoluble in La 0.4 Sr 0.4 Ni 0.06 Ti 0.94 O 3-δBased on its structure, it was excreted by exposure to a 5% H2 argon atmosphere at 930°C for 20 hours.

[0026] Exsolution can also be induced by other methods. For example, Myung et al. "Switching on electrochemical activity in solid oxide cells", Nature, doi: 10.1038 / nature19090, published August 22, 2016, shows that exsolution can be induced by applying an electric potential. For example, by applying a potential of 2 volts in a 50:50 mixed phase of H2O and N2 at 900°C, La 0.43 Ca 0.37 Ni 0.06 Ti 0.94 O 3-δ The structure demonstrated that Ni could be exsoluble. These materials were used as electrodes in solid oxide cells.

[0027] Furthermore, exsolution can also be induced with the help of plasma. For example, Kyriakou et al. “Plasma Driven Exsolution for Nanoscale Functionalization of Perovskite Oxides”, Small Methods, doi: 10.1002 / smtd.20210868, published October 22, 2021, describes exsolution by exposure to a high-frequency plasma of nitrogen or argon at a temperature of 650°C. 0.43 Ca 0.37 Ni 0.06 Ti 0.94 O 3-δ This demonstrated that Ni can be exsoluble.

[0028] Perovskite coated with exsolution nanoparticles was used as an electrode in alkaline electrolysis. For example, Ma et al. “Exsolution manipulated local surface cobalt / iron alloying and dealloying conversion in La0.95Fe0.8Co0.2O3 perovskite for oxygen evolution reaction”, Journal of Alloys and Compounds, doi: 10.1016 / j.jallcom.2020.157154, published September 11, 2020, describes La 0.95 Fe 0.8 Co 0.2 This study demonstrates that an active catalyst for OER can be obtained by exsolution of Co or a Co alloy from O3. However, the electrochemical measurements were performed using a model electrode consisting of perovskite, conductive carbon filler, and Nafion binder deposited on a polished 5 mm diameter substrate. While such a model electrode is suitable for material characterization experiments, it is not suitable for industrial applications.

[0029] In one aspect of the present invention, embedded metal nanoparticles (hereinafter referred to as metal nanoparticles) cover at least two surfaces of the ceramic material, with the metal nanoparticles facing the alkaline aqueous solution imparting electrochemical activity, while the metal nanoparticles facing the metal substrate form fixed points between the metal substrate and the ceramic material. Thus, the embedded nanoparticles play two roles: on the one hand, they improve the electrochemical activity of the electrodes by providing an additional surface area for the catalytically active metal, and on the other hand, they improve the adhesion between the metal substrate and the ceramic material having a perovskite structure.

[0030] In the context of this invention, "fixed point" means 10 × 10 nm 2 From 100 x 100 nm 2This refers to a localized area of ​​a certain size where the embedded nanoparticles are in physical contact with or form an alloy with the nickel metal substrate. Importantly, these fixed points improve the adhesion between the nickel metal substrate and the ceramic material compared to when nanoparticles are deposited on the ceramic material by other known methods, such as impregnation, spraying, chemical vapor deposition, or other similar methods. This improved adhesion is due to the partial embedding of the embedded nanoparticles in the perovskite surface, which promotes interdiffusion between the perovskite and the nanoparticle phase, significantly improving the adhesion between the metal and oxide phases.

[0031] For example, Neagu et al. “Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution”, Nature Communications, doi: 10.1038 / ncomms9120, published September 11, 2015, showed that, unlike nickel particles deposited on perovskite oxides, embedded nickel particles exhibited improved stability and a significant tendency to avoid coking, suggesting strengthening of the metal-oxide interface. Neagu et al. limited their research to gas-phase applications such as steam methane reforming and solid oxide cells.

[0032] In the context of this invention, "electrochemical activity" refers to the activity of a material or electrode that catalyzes an OER or HER reaction under alkaline conditions. Electrochemical activity is characterized by measuring the overpotential or polarization resistance per unit area of ​​the material or electrode. Electrochemical activity can be measured using methods such as cyclic voltammetry or impedance spectroscopy.

[0033] In one embodiment of the present invention, the ceramic material comprises a doped lanthanide titanate represented by the following formula: [Ln x A' (1-x) ] 1-y B z Ti (1-z) O3±δ Here, Ln is a lanthanide, A' is a lanthanide or alkaline earth metal, x is 0.1 ≤ x < 1, y is the A-site defect rate in the range of 0.05 ≤ y ≤ 0.05, B is a transition metal, Ti is titanium, z is in the range of 0.01 ≤ z ≤ 0.1, O is oxygen, δ is the oxygen unstoichiometry, 0 ≤ δ ≤ 1. A-site defect-doped lanthanide titanates are attracting attention as useful materials in alkaline electrolysis applications due to their excellent properties such as stability against etching and structural changes under alkaline conditions, relatively high electrical conductivity, and a wide selectability of transition metals that can be doped at the B site. Furthermore, A-site defects are desirable because they ensure that the B-site ions are strongly bound to the perovskite structure in the initial stage (i.e., before exsolution) and promote exsolution according to Equation 1. In the context of the present invention, a chemical element (e.g., Ti) and its corresponding ion (e.g., Ti 4+ Those two terms should be understood to be interchangeable. A person skilled in the art will be able to easily distinguish between the two in the appropriate context.

[0034] In one aspect of the present invention, Ln is La (lanthanum), Ce (cerium), or Pr (praseodymium), A' is Ce (cerium), Sr (strontium), Ca (calcium), or Ba (barium), and B is Ni (nickel), Fe (iron), Co (cobalt), Cr (chromium), Mn (manganese), or a combination thereof. The advantage of using La, Ce, or Pr as Ln is that La 3+、 Ce 3+ , and Pr 3+ This relates to the appropriate cation size in the perovskite lattice and the availability (price) of these elements. For example, Dy 3+ and Eu 3+Although it is suitable from the viewpoint of ionic radius, it is too expensive for large-scale use. The advantage of using Ce, Sr, Ca, or Ba as A' is that it generally increases not only the oxygen defect concentration of the ceramic material but also the electrical conductivity. There are two advantages of using Ni, Fe, Co, Cr, or Mn as B. First, the use of transition metals improves the electronic conductivity of the material due to the possibility of many oxidation states of the metal, enabling, for example, a polaron hopping conduction mechanism. Second, using Ni, Fe, Co, Cr, or Mn as B instead of using Ru or Ir as B or B' has the advantage that the cost of Ni, Fe, Co, Cr, or Mn is lower than that of Ru and Ir.

[0035] EP3444383B1, issued by Denora Permelec Co., Ltd. [JP], on February 20, 2019, discloses an anode for electrolysis having a conductive substrate having a surface made of at least nickel or a nickel-based alloy and an electrode catalyst layer formed on the surface of the conductive substrate. The catalyst component constituting the electrode catalyst layer is a nickel-cobalt spinel oxide represented by the structural formula NiCo2O4, or a structural formula XNi a Co 1-a O3 (where X represents at least one metal selected from lanthanoids including lanthanum, cerium, and praseodymium, and 0 < a < 1), a first catalyst component containing a lanthanoid-nickel-cobalt perovskite oxide, and a second catalyst component containing at least iridium oxide or ruthenium oxide. The amount of the second catalyst component calculated as the amount of the component metal is at least 0.2 g / m 2The electrode described in EP3444383B1 contains a substantial amount of iridium and ruthenium oxides to achieve the required activity level. However, iridium and ruthenium are expensive and rare, and it is desirable to achieve high electrochemical activity without using precious metals as catalysts. During manufacturing, the electrode is heat-treated at a temperature of 350°C to 550°C in an oxygen-containing atmosphere to form the desired oxides and improve the coating strength to the electrode substrate and the adhesion strength between catalyst layers. The heat treatment in the oxygen-containing atmosphere is important to prevent the separation of catalyst components from the catalyst layer, improve corrosion resistance, and ensure the achievement of low overpotential. Furthermore, neither the spinel-type nor the perovskite-type first catalyst reported in this specification is excipient.

[0036] In one embodiment of the present invention, the nanoparticles include nickel or a nickel alloy. The use of nickel or nickel alloys such as Ni-Co, Ni-Fe, Ni-Mn, Ni-Cr, and Ni-Co-Fe is advantageous for two reasons. First, nickel nanoparticles and nickel alloy nanoparticles are widely known in the industry to exhibit excellent electrochemical activity against HER and OER under alkaline conditions. Second, nickel nanoparticles and nickel alloy nanoparticles strongly interact with the nickel metal substrate, and upon physical contact, the nanoparticles attempt to partially dissolve into the nickel metal substrate by lowering the surface energy. The embedding of the nanoparticles improves the contact between the nanoparticles and the substrate, resulting in the formation of fixed points. The use of nickel alloys is particularly advantageous because the electrochemical activity and adhesion can be optimized simply by adjusting the alloy composition and the ratio of nickel to other metals in the alloy.

[0037] In one aspect of the present invention, the chemical composition of the nanoparticles facing the alkaline aqueous solution is not the same as the composition of the nanoparticles facing the metal substrate. Using different nanoparticles on each surface of a coating made of a ceramic material having a perovskite structure is advantageous because it is possible to optimize the composition of the nanoparticles facing the alkaline aqueous solution to maximize electrochemical activity, and at the same time optimize the composition of the nanoparticles facing the metal substrate for best adhesion. The chemical composition of the nanoparticles can be determined by measurement combining energy dispersive X-ray spectroscopy and an electron microscope. The difference in the composition of the nanoparticles can be achieved, for example, by changing the composition of the ceramic material in the ceramic coating by layering, gradation, gradual change in composition, or other methods well-known to those skilled in the art. For example, the nanoparticles facing the metal substrate can contain Ni, and the nanoparticles facing the alkaline aqueous solution can contain Ni-Fe.

[0038] In one embodiment of the present invention, the thickness of the coating of the ceramic material formed on the surface of the substrate is in the range of 1 to 5 microns, and the number density of the nanoparticles on the surface of the ceramic material is 2 at least 50 nanoparticles per μm 2 and less than 250 nanoparticles per μm.

[0039] In the context of this invention, "coating thickness" refers to the average thickness of the coating, which can be estimated, for example, by electron microscopy measurements of the polished or fractured surface of the coating. If the thickness of the ceramic material coating is too thin (e.g., less than 1 micron), the surface area of ​​the ceramic material coating exposed to the electrolyte becomes small, and the rate of the electrochemical reaction at the electrode is limited by the amount of available surface area of ​​the ceramic material catalyzing the reaction. On the other hand, if the ceramic material coating is too thick (e.g., more than 5 microns), the rate of the electrochemical reaction on the electrode is limited by the slow diffusion rate of reactants and reaction products through the coating layer. Furthermore, since the electrical conductivity of ceramic coatings is generally significantly lower than that of nickel metal substrates, very thick ceramic coatings cause undesirable additional ohmic resistance in the electrolytic cell.

[0040] In the context of this invention, "number density of nanoparticles" refers to the average number of nanoparticles detectable per unit area of ​​another material, for example, "number of particles / μm". 2 It is characterized in units of "". The number density of nanoparticles can be determined, for example, by scanning electron microscopy. Since the catalytic activity of the electrode is directly related to the exposed surface area of ​​the nanoparticles, the number density of nanoparticles on the surface of the ceramic material must be high, and to ensure sufficient catalytic activity of the electrode, for example, at least 50 nanoparticles / μm 2 The above is necessary. On the other hand, if the number density of nanoparticles is too high (for example, 250 nanoparticles / μm 2 If the value exceeds a certain limit, the particles get too close to each other and begin to aggregate excessively, for example, via the Ostwald maturation mechanism.

[0041] In one embodiment of the present invention, the electrode is an anode. Ceramic materials having a perovskite structure have been shown to exhibit particularly high electrochemical activity for OER. However, some perovskite materials are also suitable as cathodes (catalysts for the HER reaction).

[0042] In another embodiment of the present invention, a method for manufacturing an electrode is described, the method comprising a desolving step. In the desolving step, a species (e.g., Ni) that was originally dissolved in the perovskite structure is desolving to form nanoparticles embedded on the surface of the ceramic material.

[0043] In one aspect of the present invention, the exsolution of metal nanoparticles in the exsolution process is achieved by one or more of the following methods. a) Exposure to a temperature range of 700°C to 1100°C and a reducing atmosphere. b) Exposure to temperatures in the 700°C to 1100°C range and high-frequency plasma, or c) Exposure to a temperature range of 500 to 900°C and to electrical potentials.

[0044] The high electrochemical activity and strong adhesion of the ceramic coating to the metal substrate depend on the successful exsolution of nanoparticles from the ceramic material. Exsolution can be induced by many different methods, for example, by exposing the ceramic material of the present invention to a combination of high temperature and one of the following: a reducing atmosphere, high-frequency plasma, or a strong potential. The temperature needs to be high enough to accelerate the exsolution rate, but it needs to be kept low enough to prevent the ceramic coating from starting to sinter or coarseen, and to avoid structural changes in the nickel substrate. For exsolution to occur between the metal substrate and the ceramic material, the surface of the ceramic material in contact with the metal substrate must be able to reach a reducing gas atmosphere, plasma, or potential.

[0045] In yet another embodiment of the present invention, the method includes the following steps. 1) To provide a nickel metal substrate, 2) An impregnation process in which a ceramic material substrate is deposited onto the surface of a nickel metal substrate by impregnation, 3) After that, a drying process at a drying temperature of 50-150°C, 4) Subsequently, a firing process is carried out at a firing temperature in the range of 450 to 550°C, thereby forming a precursor ceramic material. 5) Dissolution process Here, the precursor ceramic material contains an A-site deficient doped lanthanoid titanate, and y is in the range of 0.05 ≦ y ≦ 0.2.

[0046] In the context of the present invention, "impregnation" refers to the process of impregnating, i.e., wet coating, a ceramic material substrate (i.e., a soluble, molten or liquid species containing metal ions in the same atomic ratio as the desired ceramic material) into the pores and on the surface of another material. By firing, the substrate decomposes into the precursor ceramic material. For example, for the production of x Sr (1-x) 1-y TiO 3±δ , an aqueous solution of La(NO3)3·6H2O, Sr(NO3)2 and [CH3CH(O-)CO2·NH4]2·Ti(OH)2 (dihydroxybisammonium, lactate, titanium(IV)) can be used, and the La 3+ , Sr 2+ , and Ti 4+ ​The atomic ratio of these ions must match the atomic ratio of these ions in a ceramic material having a desirable perovskite structure. The substrate solution typically contains citric acid, glycine, sucrose, or similar complexing agents. The advantage of impregnation is that the ceramic coating can uniformly cover the substrate surface, including the internal surfaces of any pores present in the substrate. The impregnation step is followed by a drying step, which must be carried out under controlled conditions to ensure uniform distribution of the solution across the entire substrate surface. When impregnating a porous substrate, the drying step must be carried out in a way that prevents the formation of a film or crust on the surface of the porous substrate, otherwise it will hinder the penetration of the solution into the pores of the substrate. The role of the subsequent firing step is to decompose the material substrate into the corresponding oxides while maintaining the underlying nickel metal substrate in an at least partially reduced state. The upper limit temperature of the firing step is determined by the oxidation rate of the nickel metal substrate, if carried out in air or another oxidizing atmosphere. In a preferred embodiment of the present invention, the nickel metal substrate does not oxidize during the firing step. The desolvation step described above is typically carried out after the firing step. The presence of A-site deficient perovskite in the fired ceramic material is particularly advantageous because it tends to promote exsolution according to Equation 1. After exsolution, the resulting ceramic material has a lower degree of A-site deficiency than the fired ceramic material and, under certain conditions, may have a stoichiometric composition (no A-site deficiency).

[0047] In one embodiment of the present invention, the method includes the following steps. 1) To provide a nickel metal substrate, 2) A-site defective perovskite structure (A 1-y BO 3-δ’ A synthesis process to obtain a precursor ceramic material having (y is in the range of 0.05 ≤ y ≤ 0.2), 3) A coating process in which a precursor ceramic material is deposited on the surface of a nickel metal substrate. 4) Subsequently, any drying process at a drying temperature in the range of 50 to 150°C, and 5) Subsequently, an arbitrary firing process at a firing temperature in the range of 450-550°C, and 6) Ceramic material ABO 3-δ and a dissolution step to form nanoparticles B.

[0048] Unlike the impregnation process, in this particular embodiment, the "coating process" is formed in a separate process before deposition on the substrate, rather than after deposition on the substrate as in impregnation. The precursor ceramic material is obtained in a synthesis process, and suitable synthesis methods include, but are not limited to, sol-gel synthesis, solid-phase synthesis, coprecipitation, spray pyrolysis, and hydrothermal synthesis. In some cases, the synthesis process may include additional processes such as heat treatment, particle size adjustment, and filtering. The ceramic material is coated onto a nickel metal substrate using methods including, but not limited to, tape casting, dip coating, electrophoresis, wet spray coating, plasma spray, cold spray, coating, and electrolytic deposition. Following the coating process, an optional drying process is performed to remove any solvent (if present) from the coating, followed by an optional firing process. The role of the firing process is to burn off any organic additives contained in the coating immediately after deposition, produced by spraying, tape casting, dip coating, etc., while maintaining the underlying nickel metal substrate in a state that is at least partially reduced. When using coating methods such as plasma spraying, the firing process is unnecessary. The desoldering process described above is usually performed as the final step in the procedure. After desoldering, the resulting ceramic material ABO 3-δ This is fired ceramic material A 1-y BO 3-δ’ This results in less A-site loss. 。 The resulting ceramic material may have a stoichiometric composition (no A-site defects) depending on the conditions.

[0049] In one aspect of the present invention, an alkaline electrolytic stack includes at least one electrode, the electrode comprising a nickel metal substrate and a ceramic material having a perovskite structure containing a lanthanide oxide, wherein the ceramic material forms a coating on the metal substrate, and metal nanoparticles, the metal nanoparticles being embedded in the ceramic material. An "alkaline electrolytic stack" refers to a device consisting of multiple alkaline electrolytic cells electrically connected in series. The more cells in the stack, the greater the amount of hydrogen and oxygen produced per unit time in a stack operating at a constant current density. Similarly, the larger the surface area (footprint) of each cell in the stack, the greater the amount of hydrogen and oxygen produced in a stack operating at a constant current density.

[0050] In yet another embodiment of the present invention, the stack is used for the electrolysis of water under alkaline conditions. [Brief explanation of the drawing]

[0051] [Figure 1] Figure 1 shows a schematic diagram of the electrodes in one embodiment of the present invention. [Figure 2] Figure 2 is a schematic diagram of an electrode in another embodiment of the present invention. [Figure 3] Figure 3 is a schematic diagram of an electrode in yet another embodiment of the present invention. [Figure 4] Figure 4 illustrates the method for manufacturing an electrode according to an embodiment of the present invention. [Figure 5] Figure 5 illustrates a description of the method for manufacturing electrodes in another embodiment of the present invention.

[0052] Detailed description of the drawing Figure 1 shows an electrode 100 according to the present invention. The electrode includes a nickel metal substrate 101, a coating 102 of a ceramic material having a perovskite structure, and metal nanoparticles 103 embedded in the ceramic material 102.

[0053] Figure 2 shows an electrode 200 according to another embodiment of the present invention. This electrode comprises a nickel metal substrate 101, a coating 102 of a ceramic material having a perovskite structure, and nanoparticles 103 facing an alkaline aqueous solution, and nanoparticles 201 facing the metal substrate 101. The metal nanoparticles 103 facing the alkaline aqueous solution provide electrochemical activity, and the nanoparticles 201 provide adhesion between the metal substrate 101 and the ceramic coating 102. The nanoparticles 103 and 201 are embedded in the ceramic material 102. The chemical composition of the nanoparticles 103 facing the alkaline aqueous solution and the chemical composition of the nanoparticles 201 facing the metal substrate may be the same or different.

[0054] Figure 3 shows an electrode 300 according to yet another embodiment of the present invention. The double-sided electrode includes a nickel metal substrate 101, a coating 102 of a ceramic material having a perovskite structure, and nanoparticles 103 facing an alkaline aqueous solution and nanoparticles 201 facing the metal substrate 101. The nanoparticles 103 and 201 are embedded in the ceramic material 102.

[0055] Figure 4 shows an electrode manufacturing method 400 according to an embodiment of the present invention. A liquid material substrate 401 is deposited on the surface of a nickel metal substrate 101 by impregnation 40. Subsequently, the liquid material substrate 401 is converted into a dried material substrate 402 by a drying step 41. Subsequently, the dried material substrate 402 is converted into a precursor ceramic material 403 by a firing step 42. The fired precursor ceramic material has a perovskite structure and is A-site defective, i.e., formula A 1-y BO 3-δ’ It can be represented as 。 Subsequently, the precursor ceramic material 403 is converted into ceramic material 102 coated with nanoparticles 103 and 201 as a result of the exsolution process 43. The resulting ceramic material 102 has fewer A-site defects than the fired precursor ceramic material 403.

[0056] Figure 5 shows an electrode manufacturing method 500 according to another embodiment of the present invention. In the coating step 50, a synthetic precursor ceramic material 501 is deposited on the surface of a nickel metal substrate 101. This synthetic ceramic material has a perovskite structure and is A-site defective, i.e., formula A 1-y BO 3-δ’ This can be expressed as follows. Subsequently, the coating of the synthetic precursor ceramic material 501 is converted into a dry coating of the precursor ceramic material 502 as a result of the drying step 51. Subsequently, the dry coating of the precursor ceramic material 502 is converted into a fired precursor ceramic material 503 as a result of the firing step 52. Subsequently, the fired precursor ceramic material 403 is converted into a ceramic material 102 having embedded nanoparticles 103 and 201 as a result of the exsolution step 43. The resulting ceramic material 102 has fewer A-site defects than the fired precursor ceramic material 503. [Modes for carrying out the invention]

[0057] This method will be described in more detail in the following non-limiting embodiments. [Examples]

[0058] (Example 1, Comparative Example) Table 1 shows the selection of lanthanum strontium titanate-based ceramic materials. The listed materials do not have A-site defects, and therefore y=0, and thus exsolution according to Equation 1 does not proceed. Exposure of such materials to exsolution conditions such as a combination of high temperature and a reducing atmosphere, a combination of high temperature and a potential, or a combination of high temperature and a high-frequency argon or nitrogen (N2) plasma does not achieve exsolution of nanoparticles.

[0059] For example, (La 0.3 Sr 0.7 ) 1.0 Ti 0.94 Ni 0.06 O 3.09 It is a perovskite-type material, where Ln=La, A'=Sr, x=0.3, y=0, B=Ni, z=0.06, and δ'=(x / 2+(mn)) ×z / 2 = 0.09, where Ti m+ (Ti 4+ ,m=4) and Ni n+ (Ni 2+ (n=2) 0.3 Sr 0.7 ) 1.0 Ti 0.94 Ni 0.06 O 3.09 The exsolution of Ni nanoparticles is inhibited by the A-site stoichiometric composition, i.e., the absence of A-site defects.

[0060] (La 0.5 Sr 0.5 ) 1.0 Ti 0.94 Ni 0.06 O 3.19 In another perovskite-type material with Ln=La, A'=Sr, x=0.5, y=0, B=Ni, z=0.06, and δ'=0.19, the exsolution of Ni nanoparticles is similarly inhibited by the A-site stoichiometric composition. Furthermore, a high δ' value tends to facilitate the mutual growth of the rutile phase in the nominal perovskite structure, which is undesirable.

[0061] (La 0.3 Sr 0.7 ) 1.0 Ti 0.88 Ni 0.12 O 3.03 In another perovskite-type material, Ln=La, A'=Sr, x=0.3, y=0, B=Ni, z=0.12, and δ'=0.03, the exsolution of Ni nanoparticles is similarly inhibited by the A-site stoichiometric composition. Furthermore, the high value of z (z≧0.1) makes it difficult to dissolve all nickel species within the perovskite lattice during synthesis. Instead of being incorporated into the perovskite structure during calcination, some nickel may form an additional nickel oxide phase. When exposed to exsolution conditions, this additional nickel oxide phase does not form embedded nanoparticles, which is undesirable.

[0062] (La 0.3 Sr 0.7) 1.1 Ti 0.94 Ni 0.06 O 3.105 In another perovskite-type material with Ln=La, A'=Sr, x=0.3, y=-0.1, B=Ni, z=0.06, and δ'=0.105, the exsolution of Ni nanoparticles is inhibited by an excess of A-sites, which may form additional co-growth phases such as the rutile phase. Exsolution according to Equation 1 cannot proceed.

[0063] [Table 1]

[0064] (Example 2) Table 2 shows the selection of lanthanide strontium titanate-based ceramic materials. The listed materials have A-site defects and satisfy the condition 0.05 ≤ y ≤ 0.2. Therefore, they are materials that can be exmelted according to Equation 1.

[0065] For example, (La 0.5 Sr 0.5 ) 0.8 Ti 0.97 Ni 0.03 O 2.97 Ln=La, A'=Sr, x=0.5, y=0.8, B=Ni, z=0.03, and δ'=-(mn)×z / 2=-0.03 are perovskite-type materials, where Ti m+ (Ti 4+ , m=4) and Ni n+ (Ni 2+ (n=2) 0.5 Sr 0.5 ) 0.8 Ti 0.97 Ni 0.03 O 2.97 The exsolution of Ni nanoparticles can be achieved by exposing the initial ceramic material to an argon atmosphere containing 5% H2 for 20 hours at, for example, 930°C. After complete exsolution, the chemical composition of the ceramic material is (La 0.5 Sr 0.5 ) 0.83 TiO 3-δ This is the result.

[0066] (La 0.5 Sr 0.5 ) 0.8 Ti 0.94 Ni 0.06 O 2.94 ,(Pr 0.5 Sr 0.5 ) 0.8 Ti 0.94 Ni 0.06 O 2.94 , and (Ce 0.5 Sr 0.5 ) 0.8 Ti 0.94 Ni 0.06 O 2.94 This is a perovskite-type material, where Ln is La, Pr, or Ce, respectively. Furthermore, A'=Sr, x=0.5, y=0.8, B=Ni, z=0.06, and δ'=-(mn)×z / 2=-0.06. (La 0.5 Sr 0.5 ) 0.8 Ti 0.94 Ni 0.06 O 2.94 The exsolution of Ni nanoparticles can be achieved by exposing the initial ceramic material to an argon atmosphere containing 5% H2 for 20 hours at, for example, 930°C. After complete exsolution, the chemical composition of the ceramic material is (La 0.5 Sr 0.5 ) 0.86 TiO 3-δ This is the result. [Table 2]

[0067] (Example 3) Table 3 shows the selection of lanthanum titanate strontium-based ceramic materials doped with multiple transition metals at the B site. The listed materials have A site defects and satisfy the condition 0.05 ≤ y ≤ 0.2. Therefore, they are materials that can be exmelted according to Equation 1.

[0068] For example, (La 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.075 Fe0.025 O 2.902 This is a perovskite-type material, where Ln=La, A'=Sr, x=0.5, y=0.8, B=Ni, z=0.075, B'=Fe, z'=0.025, and δ' is -0.0 9 It is 8. (La 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.075 Fe 0.025 O 2.902 The exsolution of Ni-Fe alloy nanoparticles can be achieved by exposing the initial ceramic material to an argon atmosphere containing 10% H2 for 10 hours at, for example, 900°C. After complete exsolution, the chemical composition of the ceramic material is (La 0.5 Sr 0.5 ) 0.9 TiO 3-δ This is the result.

[0069] For example, (La 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.05 Co 0.05 O 2.925 (La is a perovskite-type material, with Ln=La, A'=Sr, x=0.5, y=0.8, B=Ni, z=0.05, B'=Co, z'=0.05, and δ' is -0.075. 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.05 Co 0.05 O 2.925 The exsolution of Ni-Co alloy nanoparticles can be achieved by exposing the initial ceramic material to Ar containing 10% H2 at, for example, 900°C for 10 hours. After complete exsolution, the chemical composition of the ceramic material is (La 0.5 Sr 0.55 ) 0.9 TiO 3-δ This will be 。

[0070] [Table 3]

Claims

1. An electrode (100) for electrolyzing water from an alkaline aqueous solution, wherein the electrode is a. Nickel metal substrate (101) and b. A ceramic material (102) having a perovskite structure, comprising at least one oxide of a lanthanide including lanthanum, cerium, and praseodymium, wherein the ceramic material (102) forms a coating on the metal substrate (101), c comprises metal nanoparticles (103), The metal nanoparticles (103) are embedded in the ceramic material (102) and are formed by a desolving process. The metal nanoparticles (103) facing the alkaline aqueous solution have electrochemical activity, while the metal nanoparticles (103) facing the metal substrate (101) form a fixed point between the metal substrate (101) and the ceramic material (102), forming an electrode (100).

2. The ceramic material (102) is obtained by exsolution from a precursor ceramic material, and the precursor ceramic material is an A-site defective perovskite (A １－ｙ BO ３－δ’ The electrode according to claim 1, wherein y is in the range of 0.05 ≤ y ≤ 0.2, and δ' is the oxygen unstoichiometry of the precursor ceramic material, wherein 0 ≤ δ' ≤ 1, and the precursor ceramic material exhibits a change in the solubility of at least one metal ion at the B site during the exsolution process.

3. The electrode according to claim 1, wherein the ceramic material comprises a doped lanthanide titanate represented by the following formula. [Ln] ｘ A' （１－ｘ） ] １－ｙ B ｚ Till （１－ｚ） Oh ３±δ Here, Ln is a lanthanide, A' is a lanthanide or alkaline earth metal, x is in the range of 0.1 ≤ x < 1, y is the A-site defect in the range of 0 ≤ y ≤ 0.05, B is a transition metal, Ti is titanium, z is in the range of 0.01 ≤ z ≤ 0.1, O is oxygen, and δ is the oxygen unstoichiometry of the ceramic material in the range of 0 ≤ δ ≤ 1.

4. The electrode according to claim 3, wherein Ln is La (lanthanum), Ce (cerium), or Pr (praseodymium), A' is Ce (cerium), Sr (strontium), Ca (calcium), or Ba (barium), and B is Ni (nickel), Fe (iron), Co (cobalt), Cr (chromium), Mn (manganese), or a combination thereof.

5. The electrode according to claim 1 or 2, comprising nickel or a nickel alloy.

6. The electrode according to claim 5, wherein the chemical composition of the nanoparticles facing the alkaline aqueous solution is not the same as the composition of the nanoparticles in contact with the metal substrate.

7. The thickness of the ceramic material coated on the surface of the substrate is in the range of 1 to 5 microns, and the number density of the nanoparticles on the surface of the ceramic material is at least 50 nanoparticles per ２ μm and less than 250 nanoparticles per ２ μm. The electrode according to claim 1.

8. The electrode according to claim 1, wherein the electrode is an anode.

9. A method for manufacturing an electrode according to claim 1, wherein the method includes a desolvation step.

10. The method according to claim 9, wherein the exsolution of metal nanoparticles from the precursor ceramic material in the exsolution step is achieved by at least one of the following methods. a. Exposure to a temperature range of 700 to 1100°C and a reducing atmosphere. b. Exposure to temperatures in the range of 700 to 1100°C and to high-frequency plasma. c. Exposure to temperatures and potentials in the range of 500 to 900°C.

11. The method according to claim 10, comprising the following steps. 1) To provide a nickel metal substrate, 2) An impregnation step in which the base material is deposited on the surface of the nickel metal base material by impregnation, 3) Subsequently, a drying process at a drying temperature in the range of 50 to 150°C. 4) A firing process thereafter, in which a firing temperature in the range of 450 to 550°C is used to form the precursor ceramic material. 5) dissolution step, Here, the precursor ceramic material contains a lanthanide titanate doped with A-site defects, and y is in the range of 0.05 ≤ y ≤ 0.

2.

12. The method according to claim 10, comprising the following steps. 1) To provide a nickel metal substrate, 2) A-site defective perovskite structure (A １－ｙ BO ３－δ’ A synthesis step to obtain a precursor ceramic material having (y is in the range of 0.05 ≤ y ≤ 0.2), 3) A coating step of depositing the precursor ceramic material onto the surface of the nickel metal substrate. 4) Subsequently, any drying process at a drying temperature in the range of 50 to 150°C. 5) Subsequently, any firing process at a firing temperature in the range of 450 to 550°C, 6) Dissolution step.

13. An alkaline electrolytic stack comprising at least one electrode as described in claim 1.

14. A method for performing electrolysis of water under alkaline conditions using the alkaline electrolysis stack described in claim 13.

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