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 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 metal of the lanthanides, including lanthanum, cerium, and praseodymium, and metal nanoparticles, the ceramic material forming a coating on the metal substrate and the metal nanoparticles embedded in the ceramic material. The present invention also relates to a method for producing this electrode. The present invention also relates to an alkaline electrolysis stack comprising at least one such electrode, and a method for electrolyzing water under alkaline conditions using the alkaline electrolysis stack. [Background technology]

[0002] In alkaline water electrolysis, water is electrochemically converted to hydrogen and oxygen under alkaline conditions (H2O = H2 + 0.5O2). The electrolysis cell contains two electrodes, an anode and a cathode. During operation, an electrical potential is applied between the two electrodes, which causes an electrolysis current to flow through the electrolysis cell. During operation, hydrogen is produced at the cathode via the hydrogen evolution reaction (HER) (2H2O + 2e - =H2+2OH - ), while oxygen is produced at the anode by the oxygen evolution reaction (OER) (2OH - =0.5O2+H2O+2e - Alkaline electrolysis cells contain an electrolyte consisting of a liquid alkaline medium, such as an aqueous solution of hydroxides and / or carbonates. The electrolysis cell further contains a porous separator and / or an ion exchange membrane capable of conducting hydroxide ions.

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

[0004] The required applied potential is the sum of several contributions, including: 1) The theoretical potential required for an electrochemical reaction to proceed, 2) potential due to porous separators and / or ion exchange membranes; 3) Anode overvoltage, 4) Cathode overvoltage, 5) The potential depends on the distance between the anode and cathode in an electrolytic cell.

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

[0006] In industrial alkaline electrolysis cells, electrodes are based on nickel or precious metals. Nickel-based electrodes are relatively low cost but are characterized by high overpotentials, which leads to high cell energy consumption and, consequently, high hydrogen production costs. On the other hand, electrodes based on precious metals or their compounds (e.g., IrO2, RuO2, Pt) have low overpotentials but have very high investment costs, which leads to high hydrogen production costs. A third alkaline electrolysis electrode material has been discovered and shown to have low overpotentials and low cost at the same time. In particular, ceramic materials with specific crystalline 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 that includes a perovskite-type oxide made 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, which includes 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 and 0 < x ≦ 1). This anode has a low overvoltage for the oxygen generation reaction, but (as is well known to those skilled in the art) the adhesion between the perovskite coating and the nickel substrate was 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, in order 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 desirably 3000 nm or less, 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 anodic overvoltage of 19.23 V. Because the overvoltage of a high-performance anode is typically less than 0.5 V, the approach using the 5000 nm nickel oxide interlayer described above is not commercially viable. Even a 200 nm nickel oxide layer would increase the anodic overvoltage by 0.77 V, thereby adding an unacceptably large overvoltage contribution to the electrolysis 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 exsolution of discrete NiP nanoparticles induced by partial reduction and subsequent phosphorization. LSCN was first partially reduced in a gas mixture of Ar and H, and metallic NiP was deposited on the surface of the partially reduced LSCN (denoted as r-LSCN). 0 The r-LSCN was then exposed to a reducing atmosphere containing a phosphorus source to promote the exsolution of the nanoparticles. 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 both cases. 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 nickel substrates 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 techniques employed in 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 the 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 the 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 present inventors have surprisingly discovered that it is possible to achieve high electrochemical activity for the oxygen evolution reaction while simultaneously improving adhesion between a perovskite coating and a nickel substrate without providing a resistive intermediate layer between the coating and the substrate. More specifically, it has been found that the desirable combination of strong adhesion and high electrochemical activity can be achieved by coating a ceramic material with nanoparticles embedded in the ceramic material. This embedding can be achieved, for example, by matching the chemical composition of the perovskite and then forming the nanoparticles through an exsolution process. The embedded nanoparticles provide additional electrochemical activity for the oxygen evolution reaction and also serve as anchoring points that improve adhesion between the perovskite and the nickel substrate.

[0015] "Perovskite" as used herein refers to a class of ceramic materials with the general crystal structure ABO3, where A and B are metal ions and O is an oxide ion, and these ions are located at energetically distinct positions within the crystal lattice. The sites within the lattice where A ions are located are called "A-sites," and the sites where B ions are located are called "B-sites." The oxygen content of perovskites varies with the choice of metal ions located at the A-sites and B-sites, and also depends on factors such as temperature and oxygen partial pressure. Thus, ABO 3±δ is a more accurate formula for the class of perovskites, where δ is called the "oxygen nonstoichiometry" and is in the range of 0≦δ≦1. In the formulas and equations of the present invention, the "oxygen nonstoichiometry" 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 any double perovskite structure A2B2O 5+δ* is δ * = 1-2δ, ABO 3-δThe oxygen non-stoichiometry depends on, for example, temperature, oxygen partial pressure, and applied voltage. A non-limiting example of a ceramic material having a perovskite structure containing at least one oxide of a lanthanide, including lanthanum, cerium, and praseodymium, is La 0.5 Sr 0.5 FeO 3-δ , La 0.9 Ba 0.1 NiO 3-δ , Ce 0.6 Sr 0.4 CoO 3-δ and Pr 0.95 CoO 3-δ Includes:

[0016] In the context of this invention, "A is embedded in B" refers to the situation where particles of A are partially sunk into the surface of B. "A is embedded in B" does not include the situation where particles of A are simply deposited or coated on the surface of B. Experimentally, it is possible to distinguish between embedded and deposited / coated particles by chemically etching the particles of A using an etchant such as HNO3. 。 Where particles of A were embedded in B, etching the B material will result in pits, the size and number of which will be similar to the size and number of particles of A. On the other hand, if particles of A are coated on the surface of B, removing A will not result in pits.

[0017] In the context of this invention, "adhesion" refers to the ability of two materials to bond together. Therefore, the terms "strong adhesion" or "good adhesion" refer to a state in which two materials are strongly bonded together and one material does not peel or separate from the other. At the macroscale, adhesion strength can be quantitatively assessed using the Dolly test, as described in ASTM D4541-22. Qualitatively, it can be assessed using the tape test, as described in ASTM D3359. At the microscale, adhesion strength can be qualitatively assessed by examining polished cross-sections of the interface between two materials with an optical microscope or a scanning electron microscope. Detection of peeling or separated areas is a sign of poor adhesion. Finally, in electrolysis applications, 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 for electrolyzing water from an alkaline aqueous solution is provided, the electrode comprising a nickel metal substrate, a ceramic material having a perovskite structure including a lanthanide oxide, the ceramic material forming a coating on the metal substrate, and metal nanoparticles embedded in the ceramic material. In the context of the present invention, "A forming a coating on B" or "a coating of A on B" refers to 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 need to be in physical contact; for example, materials A and B may be separated by a third material. The third material may be, for example, nanoparticles embedded in material A, sandwiched between materials A and B.

[0019] The above-described embodiments of the invention have several important advantages over the state-of-the-art. First, the electrodes of the invention have high electrochemical activity, low overpotential, and high efficiency, thereby enabling low hydrogen production costs. Second, the electrodes of the invention are mechanically stable, i.e., the perovskite coating is strongly adhered to the nickel substrate. This prevents the perovskite coating from being lost over time during operation of the electrolysis cell, which would otherwise cause a decrease in activity and even filter and / or separator clogging. Third, the electrodes of the invention do not contain precious metals such as iridium, platinum, or ruthenium, significantly reducing electrode raw material costs and mitigating electrode material supply issues. Fourth, the electrodes of the invention have a lower overpotential for the oxygen evolution reaction compared to conventional nickel-based electrodes. Because electricity costs are a major factor determining the cost of producing hydrogen through electrolysis, the electrodes of the invention enable hydrogen production at a lower cost (OPEX) than conventional nickel-based electrodes. Fifth, the composition of the metal embedded in the ceramic material can be adjusted to optimize electrochemical activity and adhesion strength.

[0020] Embedded nanoparticles can be achieved by a number of different methods, but are commonly achieved by a process known as "exsolution." "Exsolution" refers to a property of a subset of perovskite materials where changing external conditions alter the solubility of at least one metal ion in the B-site of the perovskite lattice. As the solubility of these metal ions decreases, at least some of the B-site metal can no longer dissolve in the perovskite and is extruded from the perovskite structure, forming additional phases at the perovskite surface. Chemically, exsolution can be expressed as:

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

[0022] where A 1-y BO 3-δ Some metal ions in the B site of the structure cannot remain dissolved in the perovskite lattice and exsolve as a separate phase, denoted B. 1-y BO 3-δ refers to an A-site deficient perovskite, i.e., 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 deficiency ratio." For example, for a perovskite with an A-site deficiency ratio 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 vacancy rate characterizes not only the relative number of metals in the A and B sites, but also the amount of B exsolving from the perovskite lattice. 3-δ refers to a stoichiometric perovskite where the ratio of A-site to B-site metals is 1. That is, the oxygen nonstoichiometry δ in the stoichiometric perovskite phase can be different from the oxygen nonstoichiometry δ' in the original A-site deficient perovskite phase.

[0025] Exsolution can be caused by changes in external conditions. These external conditions may refer to changes in temperature and / or atmospheric oxygen partial pressure. 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 exsolved from the lattice of A-site deficiency doped lanthanum titanate exposed to a reducing atmosphere and high temperature. For example, Ni can be exsolved by exsolving La. 0.4 Sr 0.4 Ni 0.06 Ti 0.94 O 3-δBased on the structure, it was exsolved by exposure to a 5% H2 argon atmosphere at 930°C for 20 hours.

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

[0027] Furthermore, exsolution can also be induced with the aid 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, reported that La was extruded 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 It was shown that Ni can be exsolved.

[0028] Perovskites coated with exsolution nanoparticles have been used as electrodes 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, reported that La 0.95 Fe 0.8 Co 0.2 We demonstrate that exsolution of Co or Co alloys from O3 provides an active catalyst for OER. However, 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. Such a model electrode is suitable for material characterization experiments but 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 sides of the ceramic material, with the metal nanoparticles facing the alkaline aqueous solution providing electrochemical activity, while the metal nanoparticles facing the metal substrate form anchoring points between the metal substrate and the ceramic material. Thus, the embedded nanoparticles serve two purposes: on the one hand, they improve the electrochemical activity of the electrode by providing additional surface area for catalytically active metals, and on the other hand, they improve adhesion between the metal substrate and the ceramic material having a perovskite structure.

[0030] In the context of the present invention, a "fixed point" is defined as a 10×10 nm 2 to 100×100nm 2" refers to a localized region having a size of about 1 / 2" where the embedded nanoparticles are in physical contact with or alloy with the nickel metal substrate. Importantly, the anchoring points thus formed improve adhesion between the nickel metal substrate and the ceramic material compared to other known methods of depositing nanoparticles onto the ceramic material, such as impregnation, spraying, chemical vapor deposition, or other similar methods. This improved adhesion is due to the embedded nanoparticles being partially buried in the perovskite surface, which promotes interdiffusion between the perovskite and nanoparticle phases, significantly improving 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, implying a strengthened metal-oxide interface. Neagu et al. limited their work to gas-phase applications such as steam methane reforming and solid oxide cells.

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

[0033] In one embodiment of the present invention, the ceramic material comprises a doped lanthanide titanate represented by the formula: [Ln x A' (1-x) ] 1-y B z Ti (1-z) O3±δ where Ln is a lanthanide, A' is a lanthanide or alkaline earth metal, x is 0.1≦x<1, y is the A-site vacancy in the range of 0.05≦y≦0.05, B is a transition metal, Ti is titanium, z is 0.01≦z≦0.1, O is oxygen, and δ is the oxygen non-stoichiometry in the range of 0≦δ≦1. A-site vacancy-doped lanthanide titanates have attracted attention as useful materials for 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 selection of transition metals that can be doped at the B-site. Furthermore, A-site vacancies are desirable because they ensure that B-site ions are strongly bound to the perovskite structure in the early stage (i.e., before exsolution), facilitating exsolution according to Equation 1. In the context of the present invention, the relationship between a chemical element (e.g., Ti) and a corresponding ion (e.g., Ti 4+ ) should be understood to be used interchangeably. Those skilled in the art can 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+ The availability (price) of these elements is related to the appropriate cation size in the perovskite lattice. 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. The advantage of using Ni, Fe, Co, Cr, or Mn as B has two aspects. 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, the 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 nickel-cobalt spinel oxide represented by the structural formula NiCo₂O₄, or the 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 EP 3444383 B1 contains significant amounts of iridium and ruthenium oxides to achieve the required activity level. However, iridium and ruthenium are expensive and rare, and it would be desirable to achieve high electrochemical activity without using precious metals as catalysts. During fabrication, the electrode is heat-treated in an oxygen-containing atmosphere at temperatures between 350°C and 550°C to form the desired oxides and improve the coating strength to the electrode substrate and the adhesion strength between the catalyst layers. This heat treatment in an oxygen-containing atmosphere is important for preventing separation of the catalytic components from the catalyst layer, improving corrosion resistance, and ensuring low overvoltage. Furthermore, neither the spinel-type nor perovskite-type first catalyst reported in this specification undergoes exsolution.

[0036] In one embodiment of the present invention, the nanoparticles comprise nickel or a nickel alloy. The use of nickel or a nickel alloy, such as Ni-Co, Ni-Fe, Ni-Mn, Ni-Cr, or Ni-Co-Fe, is advantageous for two reasons. First, nickel and nickel alloy nanoparticles are widely known in the industry to exhibit excellent electrochemical activity for HER and OER under alkaline conditions. Second, nickel and nickel alloy nanoparticles strongly interact with nickel metal substrates, and upon physical contact, the nanoparticles lower their surface energy and tend to partially dissolve in the nickel metal substrate. The embedded nanoparticles improve contact between the nanoparticles and the substrate, resulting in the formation of anchoring points. The use of nickel alloys is particularly advantageous because 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 side of a coating made of a ceramic material with a perovskite structure is advantageous because it allows the composition of the nanoparticles facing the alkaline aqueous solution to be optimized to maximize electrochemical activity, while the composition of the nanoparticles facing the metal substrate can be optimized for best adhesion. The chemical composition of the nanoparticles can be determined by combining energy-dispersive X-ray spectroscopy and electron microscopy. The difference in nanoparticle composition can be achieved, for example, by varying the composition of the ceramic material within the ceramic coating through layering, gradation, a gradual change in composition, or other methods known to those skilled in the art. For example, the nanoparticles facing the metal substrate can contain Ni, while 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 in the range of 1 to 5 microns. 2 At least 50 nanoparticles per μm 2 There are less than 250 nanoparticles per

[0039] In the context of the present invention, "coating thickness" refers to the average thickness of the coating, which can be estimated, for example, by electron microscopy of polished or fractured cross sections of the coating. If 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 is reduced, and the rate of the electrochemical reaction at the electrode is limited by the available surface area of ​​the ceramic material that catalyzes the reaction. On the other hand, if the ceramic material coating is too thick (e.g., greater 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, because the electrical conductivity of ceramic coatings is generally significantly lower than that of the nickel metal substrate, very thick ceramic coatings introduce additional, undesirable ohmic resistance into the electrolytic cell.

[0040] In the context of the present invention, "number density of nanoparticles" refers to the average number of detectable nanoparticles per unit area of ​​another material, e.g., "number of particles / μm 2 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 at the surface of the ceramic material must be high, for example, at least 50 nanoparticles / μm, to ensure sufficient catalytic activity of the electrode. 2 On the other hand, if the number density of nanoparticles is too high (for example, 250 nanoparticles / μm 2 ), the particles get too close together and begin to agglomerate excessively, for example via the Ostwald ripening mechanism.

[0041] In one embodiment of the present invention, the electrode is an anode. Ceramic materials with 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 an exsolution step in which species (e.g., Ni) originally dissolved in the perovskite structure are exsolved to form nanoparticles embedded in the surface of the ceramic material.

[0043] In one aspect of the present invention, exsolution of the metal nanoparticles in the exsolution step is achieved by one or more of the following methods: a) exposure to temperatures ranging from 700°C to 1100°C and a reducing atmosphere; b) exposure to a radio frequency plasma at temperatures between 700°C and 1100°C; or c) Exposure to temperatures ranging from 500 to 900°C and electrical potentials.

[0044] High electrochemical activity and strong adhesion of the ceramic coating to the metal substrate depend on successful exsolution of the nanoparticles from the ceramic material. Exsolution can be induced in a number of different ways, for example, by exposing the ceramic material of the present invention to a combination of high temperature and one of a reducing atmosphere, a radio frequency plasma, or a strong electrical potential. The temperature must be high enough to accelerate the rate of exsolution, yet low enough so that the ceramic coating does not begin to sinter or coarsen, and to avoid structural changes to 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 accessible to a reducing gas atmosphere, plasma, or electrical potential.

[0045] In yet another embodiment of the present invention, the method comprises the following steps. 1) providing a nickel metal substrate; 2) an impregnation step in which a ceramic material substrate is deposited on the surface of the nickel metal substrate by impregnation; 3) Then, a drying process at a drying temperature of 50 to 150°C. 4) thereafter, a firing step at a firing temperature in the range of 450-550°C, thereby forming a precursor ceramic material; 5) Dissolution process wherein the precursor ceramic material comprises an A-site deficient doped lanthanide 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, or 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 surface of another material. Upon firing, the substrate decomposes into precursor ceramic materials. For example, [La x Sr (1-x) ] 1-y TiO 3±δ Aqueous solutions of La(NO3)3·6H2O, Sr(NO3)2, and [CH3CH(O-)CO2·NH4]2·Ti(OH)2 (dihydroxybisammonium, lactate, titanium(IV)) can be used to prepare 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 with the desired perovskite structure. The substrate solution typically contains citric acid, glycine, sucrose, or a similar complexing agent. The advantage of impregnation is that the ceramic coating can uniformly cover the substrate surface, including the interior surfaces of any open pores that may be present in the substrate. The impregnation process is followed by a drying process, 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 process must avoid the formation of a film or crust on the surface of the porous substrate, which would otherwise prevent the solution from penetrating into the pores of the substrate. The role of the subsequent calcination process is to decompose the material substrate into its corresponding oxide while maintaining the underlying nickel metal substrate in an at least partially reduced state. The upper temperature limit for the calcination process is determined by the oxidation rate of the nickel metal substrate when performed in air or other oxidizing atmosphere. In preferred embodiments of the present invention, the nickel metal substrate does not oxidize during the calcination process. The exsolution process described above is typically carried out after the calcination process. It is particularly advantageous for the fired ceramic material to contain an A-site deficient perovskite, as this tends to promote exsolution according to Equation 1. After exsolution, the resulting ceramic material will have a lower A-site deficiency than the fired ceramic material, and may, under certain conditions, be stoichiometric (no A-site deficiency).

[0047] In one embodiment of the present invention, the method includes the following steps. 1) providing a nickel metal substrate; 2) A-site deficient perovskite structure (A 1-y BO 3-δ’ a synthesis step of obtaining a precursor ceramic material having the formula (y) where y is in the range of 0.05≦y≦0.2; 3) a coating step of depositing a precursor ceramic material onto the surface of a nickel metal substrate; 4) followed by an optional drying step at a drying temperature in the range of 50-150°C; and 5) followed by an optional firing step at a firing temperature in the range of 450-550°C; and 6) Ceramic material ABO 3-δ and an exsolution step to form nanoparticles B.

[0048] The "coating process" differs from the "impregnation process" in that, in this particular embodiment, the precursor ceramic material is formed in a separate process prior to deposition onto the substrate, rather than after deposition onto the substrate, as in impregnation. The precursor ceramic material is obtained through a synthesis process, and suitable synthesis methods include, but are not limited to, sol-gel synthesis, solid-state synthesis, co-precipitation, spray pyrolysis, and hydrothermal synthesis. In some cases, the synthesis process may include additional processing such as heat treatment, particle size adjustment, and filtering. The ceramic material is coated onto the nickel metal substrate using methods that may include, but are not limited to, tape casting, dip coating, electrophoresis, wet spray coating, plasma spraying, cold spraying, painting, and electrolytic deposition. The coating process is followed by an optional drying process to remove solvent (if present) from the coating, and an optional firing process. The purpose of the firing process is to burn off any organic additives contained in the as-deposited coating produced by spraying, tape casting, dip coating, and the like, while maintaining the underlying nickel metal substrate in an at least partially reduced state. When using a coating method such as plasma spraying, a firing process is not required. The exsolution step described above is usually carried out as the final step in the procedure. After exsolution, the resulting ceramic material ABO 3-δ is a fired ceramic material A 1-y BO 3-δ’ There is less A-site loss than 。 Depending on the conditions, the resulting ceramic material may have a stoichiometric composition (no A-site deficiency).

[0049] In one aspect of the present invention, an alkaline electrolysis stack includes at least one electrode, the electrode comprising a nickel metal substrate and a ceramic material having a perovskite structure including a lanthanide oxide, the ceramic material forming a coating on the metal substrate, and metal nanoparticles embedded in the ceramic material. An "alkaline electrolysis stack" refers to a device consisting of multiple alkaline electrolysis cells electrically connected in series. Increasing the number of cells in a stack increases the amount of hydrogen and oxygen produced per unit time in the stack operating at a constant current density. Similarly, increasing the surface area (footprint) of each cell in the stack increases the amount of hydrogen and oxygen produced in the 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 drawings]

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

[0052] Detailed Description of the Drawings 1 shows an electrode 100 according to the present invention. The electrode comprises 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] 2 shows an electrode 200 according to another embodiment of the present invention. The electrode includes a nickel metal substrate 101, a coating 102 of a ceramic material having a perovskite structure, and nanoparticles 103 facing the alkaline aqueous solution, as well as 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, 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] 3 shows an electrode 300 according to yet another embodiment of the present invention. The double-sided electrode comprises a nickel metal substrate 101, a coating 102 of a ceramic material having a perovskite structure, and nanoparticles 103 facing the alkaline aqueous solution and nanoparticles 201 facing the metal substrate 101. The nanoparticles 103, 201 are embedded in the ceramic material 102.

[0055] 4 illustrates a method 400 for manufacturing an electrode 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. The liquid material substrate 401 is then converted into a dry material substrate 402 by a drying step 41. The dry material substrate 402 is then converted into a precursor ceramic material 403 by a calcination step 42. The calcined precursor ceramic material has a perovskite structure and is A-site deficient, i.e., has the formula A 1-y BO 3-δ’ can be expressed as 。 The precursor ceramic material 403 is then converted to a ceramic material 102 coated with nanoparticles 103, 201 as a result of the exsolution step 43. The resulting ceramic material 102 has fewer A-site deficiencies than the calcined precursor ceramic material 403.

[0056] 5 illustrates a method 500 for manufacturing an electrode according to another embodiment of the present invention. A coating process 50 deposits a synthetic precursor ceramic material 501 on the surface of a nickel metal substrate 101. The synthetic ceramic material has a perovskite structure and is A-site deficient, i.e., has the formula A 1-y BO 3-δ’ The coating of synthetic precursor ceramic material 501 is then converted into a dry coating of precursor ceramic material 502 as a result of a drying step 51. The dried coating of precursor ceramic material 502 is then converted into a calcined precursor ceramic material 503 as a result of a calcination step 52. The calcined precursor ceramic material 403 is then converted into a ceramic material 102 having embedded nanoparticles 103, 201 as a result of an exsolution step 43. The resulting ceramic material 102 has fewer A-site deficiencies than the calcined precursor ceramic material 503. DETAILED DESCRIPTION OF THE INVENTION

[0057] This method is described in further detail in the non-limiting examples below. [Example]

[0058] (Example 1, Comparative Example) Table 1 shows a selection of ceramic materials based on lanthanum strontium titanate. The listed materials are not A-site deficient, y=0, and therefore do not undergo exsolution according to Equation 1. Exposing such materials to exsolution conditions, such as high temperature combined with a reducing atmosphere, high temperature combined with an electric potential, or high temperature combined with a radio frequency argon or nitrogen (N2) plasma, does not result in exsolution of the nanoparticles.

[0059] For example, (La 0.3 Sr 0.7 ) 1.0 Ti 0.94 Ni 0.06 O 3.09is 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). (La 0.3 Sr 0.7 ) 1.0 Ti 0.94 Ni 0.06 O 3.09 The exsolution of Ni nanoparticles from the solution is inhibited by the A-site stoichiometry, i.e., the absence of A-site deficiencies.

[0060] (La 0.5 Sr 0.5 ) 1.0 Ti 0.94 Ni 0.06 O 3.19 In another perovskite material with Ln=La, A'=Sr, x=0.5, y=0, B=Ni, z=0.06, and δ'=0.19, exsolution of Ni nanoparticles is similarly hindered by the A-site stoichiometry. Furthermore, high δ' values ​​tend to favor rutile intergrowth 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 material, where Ln = La, A' = Sr, x = 0.3, y = 0, B = Ni, z = 0.12, and δ' = 0.03, exsolution of Ni nanoparticles is similarly hindered by the A-site stoichiometry. Furthermore, the high value of z (z ≥ 0.1) makes it difficult to dissolve all nickel species within the perovskite lattice during synthesis. Upon calcination, rather than being incorporated into the perovskite structure, 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 material with Ln = La, A' = Sr, x = 0.3, y = -0.1, B = Ni, z = 0.06, and δ' = 0.105, exsolution of Ni nanoparticles is inhibited by the excess A sites, which can lead to the formation of additional intergrown phases such as rutile. Exsolution according to Eq. 1 cannot proceed.

[0063] [Table 1]

[0064] Example 2 Table 2 shows a selection of ceramic materials in the strontium lanthanide titanate system. The listed materials are A-site deficient and satisfy the condition 0.05 ≤ y ≤ 0.2. Therefore, they are capable of exsolution according to Eq. 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, is a perovskite-type material, where Ti m+ (Ti 4+ , m=4) and Ni n+ (Ni 2+ , n=2). (La 0.5 Sr 0.5 ) 0.8 Ti 0.97 Ni 0.03 O 2.97 Exsolution of Ni nanoparticles from the ceramic can be achieved by exposing the initial ceramic material to an argon atmosphere containing 5% H2 at, for example, 930 °C for 20 hours. After complete exsolution, the chemical composition of the ceramic material becomes (La 0.5 Sr 0.5 ) 0.83TiO 3-δ This becomes:

[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 is a perovskite-type material, and 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 Exsolution of Ni nanoparticles from the pristine ceramic material can be achieved by exposing the material to an argon atmosphere containing 5% H2 at 930 °C for 20 hours. After complete exsolution, the chemical composition of the ceramic material is (La 0.5 Sr 0.5 ) 0.86 TiO 3-δ This becomes: [Table 2]

[0067] Example 3 Table 3 shows a selection of lanthanum strontium titanate-based ceramic materials doped with multiple transition metals at the B site. The listed materials are A-site deficient and satisfy the condition 0.05 ≤ y ≤ 0.2. Therefore, they are capable of exsolution according to Eq. 1.

[0068] For example, (La 0.5 Sr 0.5 ) 0.8Ti 0.9 Ni 0.075 Fe 0.025 O 2.902 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 -0.088. (La 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.075 Fe 0.025 O 2.902 Exsolution of Ni-Fe alloy nanoparticles from the ceramic can be achieved by exposing the initial ceramic material to an argon atmosphere containing 10% H2 at, for example, 900 °C for 10 hours. After complete exsolution, the chemical composition of the ceramic material becomes (La 0.5 Sr 0.5 ) 0.9 TiO 3-δ becomes 。

[0069] For example, (La 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.05 Co 0.05 O 2.925 is a perovskite-type material, where 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. (La 0.5 Sr 0.5 ) 0.8 Ti 0.9 Ni 0.05 Co 0.05 O 2.925 Exsolution of Ni-Co alloy nanoparticles from the ceramic can be achieved by exposing the initial ceramic material to, for example, 10% H2 in Ar at 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 becomes 。

[0070] [Table 3]

Claims

1. An electrode (100) suitable for performing the electrolysis of water from an alkaline aqueous solution, said electrode comprising a nickel metal substrate (101); b) a ceramic material (102) having a perovskite structure, comprising at least one oxide of a lanthanoid, including lanthanum, cerium, and praseodymium, said ceramic material (102) forming a coating on said metal substrate (101); c metal nanoparticles (103), The metal nanoparticles (103) are embedded in the ceramic material (102) and the electrode (100) is formed by an exsolution process.

2. 2. The electrode of claim 1, wherein the metal nanoparticles (103) facing the alkaline aqueous solution are electrochemically active, while the metal nanoparticles (103) facing the metal substrate (101) form anchoring points between the metal substrate (101) and the ceramic material (102).

3. The ceramic material (102) is obtained by exsolution from a precursor ceramic material, and the precursor ceramic material is an A-site deficient perovskite (A １－ｙ BO ３－δ’ , y in the range of 0.05≦y≦0.2, and δ′ is the oxygen non-stoichiometry of the precursor ceramic material, 0≦δ′≦1, wherein the precursor ceramic material exhibits a change in solubility of at least one metal ion at the B site during an exsolution process.

4. 4. The electrode of claim 1, 2 or 3, wherein the ceramic material comprises a doped lanthanide titanate having the formula: [Ln] ｘ A' （１－ｘ） ] １－ｙ B ｚ Till （１－ｚ） Oh ３±δ Here, Ln is a lanthanoid, A' is a lanthanoid or alkaline earth metal, x is in the range of 0.1≦x<1, y is an A-site deficiency 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 non-stoichiometry of the ceramic material in the range of 0≦δ≦1.

5. 5. The electrode of claim 4, 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.

6. 6. The electrode of claim 1, wherein the nanoparticles comprise nickel or a nickel alloy.

7. The electrode according to claim 6 , 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.

8. The ceramic material coated on the surface of the substrate has a thickness 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 8. The electrode of claim 1, wherein the number of nanoparticles per electrode is less than 250.

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

10. 10. A method for manufacturing an electrode according to any one of claims 1 to 9, said method comprising an exsolution process step.

11. 11. The method of claim 10, wherein exsolution of the metal nanoparticles from the precursor ceramic material in the exsolution step is accomplished 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 a temperature range of 700-1100°C and radio frequency plasma; c Exposure to a temperature range of 500-900°C and potential.

12. 12. The method of claim 11, comprising the steps of: 1) providing a nickel metal substrate; 2) an impregnation step of depositing a substrate on the surface of the nickel metal substrate by impregnation; 3) followed by a drying step at a drying temperature in the range of 50 to 150°C; 4) thereafter, firing at a firing temperature in the range of 450-550°C, thereby forming the precursor ceramic material; 5) dissolution step, wherein the precursor ceramic material comprises an A-site deficiency doped lanthanoid titanate, and y is in the range of 0.05≦y≦0.

2.

13. 13. The method of claim 12, comprising the steps of: 1) providing a nickel metal substrate; 2) A-site deficient perovskite structure (A １－ｙ BO ３－δ’ a synthesis step of obtaining a precursor ceramic material having a y in the range of 0.05≦y≦0.2; 3) a coating step of depositing the precursor ceramic material on the surface of the nickel metal substrate; 4) followed by an optional drying step at a drying temperature ranging from 50 to 150°C; 5) then an optional firing step at a firing temperature in the range of 450-550°C; 6) Dissolution step.

14. An alkaline electrolysis stack comprising at least one electrode according to any one of claims 1 to 9.

15. A method for electrolyzing water under alkaline conditions using the alkaline electrolysis stack of claim 14.

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