Nickel complex slurry, method for producing nickel complex slurry, nickel film, and method for producing nickel film

Abstract

The present invention provides a nickel complex slurry, a method for producing a nickel complex slurry, a nickel film, and a method for producing a nickel film, all of which are capable of forming a catalyst layer with a large specific surface area using an inexpensive metal. [Solution] A nickel complex slurry comprising a nickel complex represented by (HCOO)2Ni(2MAE)2, a nickel complex represented by (HCOO)2Ni(2MAE)4, and a glycol, wherein 2MAE is 2-methylaminoethanol, (HCOO)2Ni(2MAE)2 and (HCOO)2Ni(2MAE)4 are micelles, and the micelles are dispersed in the glycol.

Description

[Technical Field]

[0001] The present invention relates to a nickel complex slurry, a method for producing a nickel complex slurry, a nickel film, and a method for producing a nickel film. [Background technology]

[0002] In recent years, electrode catalysts used in fuel cells such as solid oxide fuel cells (SOFCs) have attracted attention, and for effective industrial utilization, it is important to improve and maintain the catalytic activity as well as maintain the conductivity of the electrode. For example, Patent Document 1 discloses an electrolytic electrode having a catalyst layer having a platinum-containing layer containing platinum and an iridium oxide-containing layer containing iridium oxide, and an electrode substrate containing a conductive metal. Patent Document 2 also discloses an electrolytic electrode comprising a conductive substrate, an intermediate layer, and a catalyst layer, and exemplifies that the catalyst layer contains at least one catalyst selected from the group consisting of nickel cobalt spinel oxide, lanthanide nickel cobalt perovskite oxide, iridium oxide, ruthenium oxide, and lithium nickel cobalt oxide.

[0003] Other examples of electrode layers include Ir oxide, Ru oxide and Ta oxide, Pt and B oxide, Ru oxide, Ti oxide and Sn oxide, and composite oxides of Mn and Mo. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6817080 [Patent Document 2] International Publication No. 2019 / 172160 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] To increase the catalytic activity of these catalyst layers, it is necessary to increase the specific surface area of ​​the surface. Such catalyst layers with a large specific surface area have been realized by using expensive metals such as Pt, as described above.

[0006] Furthermore, the formation of catalyst layers with such a large specific surface area has traditionally been carried out using physical vapor deposition methods such as pulsed laser deposition, or spray pyrolysis methods in which an aqueous solution of dissolved metal elements is sprayed using ultrasonic vibration elements, transported to a heating furnace by a carrier gas, and dehydration, decomposition, reaction, and sintering are performed in a continuous manner, which has been time-consuming.

[0007] Therefore, the present invention aims to provide a nickel complex slurry, a method for producing a nickel complex slurry, a nickel film, and a method for producing a nickel film, which can form a catalyst layer with a large specific surface area using an inexpensive metal. [Means for solving the problem]

[0008] To solve the above problems, the nickel complex slurry of the present invention is a nickel complex slurry comprising a nickel complex represented by (HCOO)2Ni(2MAE)2, a nickel complex represented by (HCOO)2Ni(2MAE)4, and a glycol, wherein the 2MAE is 2-methylaminoethanol, and the (HCOO)2Ni(2MAE)2 and (HCOO)2Ni(2MAE)4 are micelles, and the micelles are dispersed in the glycol.

[0009] The molar ratio of Ni to 2MAE may be Ni:2MAE = 1:2 to 5.

[0010] The glycol may also be ethylene glycol.

[0011] The molar ratio of Ni to glycol may be 1:0.5 to 1.5.

[0012] Furthermore, in order to solve the above problems, the present invention provides a method for producing a nickel complex slurry, which includes a nickel complex formation step of mixing nickel formate dihydrate, glycol, and 2-methylaminoethanol to form a nickel complex represented by (HCOO)2Ni(2MAE)2 and a nickel complex represented by (HCOO)2Ni(2MAE)4, wherein 2MAE is 2-methylaminoethanol.

[0013] In the nickel complex formation step, the molar ratio of Ni to 2MAE may be Ni:2MAE = 1:2 to 5.

[0014] The glycol may also be ethylene glycol.

[0015] The molar ratio of Ni to glycol may be 1:0.5 to 1.5.

[0016] The method for producing a nickel complex slurry of the present invention may include a first mixing step of mixing the nickel formate dihydrate with ethanol before the nickel complex formation step, and a first dehydration step of subjecting the nickel complex-containing mixture obtained in the nickel complex formation step to reduced pressure and dehydrating by azeotropic dihydration of water and ethanol under reduced pressure after the nickel complex formation step.

[0017] The present invention's method for producing a nickel complex slurry may include, after the nickel complex formation step, a second mixing step of mixing the nickel complex-containing mixture obtained in the nickel complex formation step with ethanol, and after the second mixing step, a second dehydration step of subjecting the mixture of the nickel complex-containing mixture and ethanol to a reduced pressure treatment, thereby dehydrating the mixture by azeotropic distillation of water and ethanol under reduced pressure.

[0018] Furthermore, in order to solve the above problems, the method for producing a nickel film of the present invention includes a nickel film formation step of heating the nickel complex slurry of the present invention in an inert gas atmosphere to form a nickel film.

[0019] Also, in order to solve the above problems, the nickel film of the present invention includes a plurality of polygonal plate-like bodies, and a joint portion that is between the plate-like bodies and connects the plurality of plate-like bodies. The joint portion is a porous body, and the joint portion includes a continuous film that connects the plate-like bodies.

[0020] The volume resistivity of the nickel film is 1×10 -4 Ω·cm to 9×10 -4 Ω·cm may be sufficient.

[0021] In the nickel film of the present invention, the ratio of the total area of the plurality of plate-like bodies to the total area of the joint portion may be 1:0.3 to 0.8.

Effect of the Invention

[0022] According to the present invention, it is possible to provide a nickel complex slurry, a method for producing a nickel complex slurry, a nickel film, and a method for producing a nickel film, which can form a catalyst layer having a large specific surface area with an inexpensive metal.

Brief Description of the Drawings

[0023] [Figure 1] Image of the sintered catalyst electrode of Example 1 taken with an optical microscope. [Figure 2] Image of the sintered catalyst electrode of Comparative Example 1 taken with an optical microscope. [Figure 3] Image of the sintered catalyst electrode of Comparative Example 2 taken with an optical microscope.

Mode for Carrying Out the Invention

[0024] Hereinafter, an embodiment of the nickel complex slurry, the method for producing the nickel complex slurry, the nickel film, and the method for producing the nickel film of the present invention will be described.

[0025] [Nickel Complex Slurry] The nickel complex slurry of the present invention comprises a nickel complex represented as (HCOO)2Ni(2MAE)2, a nickel complex represented as (HCOO)2Ni(2MAE)4, and a glycol. 2MAE is 2-methylaminoethanol.

[0026] <Nickel complex> (HCOO)2Ni(2MAE)2 and (HCOO)2Ni(2MAE)4 are micelles (i.e., colloidal particles), and for example, multiple micelles aggregate to form a colloid, which is then dispersed in glycol. Some of these micelles are also connected by water molecules, and these are dispersed in ethylene glycol.

[0027] 2MAE is stable as a ligand for Ni(HCOO)2 (nickel formate) in either 2-coordinate or 4-coordinate states, but both coordination states are present, and it is difficult to determine the ratio of 2-coordinate to 4-coordinate ligands. However, the molar ratio of Ni to 2MAE can be determined, and it is preferable that the molar ratio is Ni:2MAE = 1:2 to 5. Having the molar ratio within this range facilitates the formation of the nickel film of the present invention, as described later. The molar ratio of Ni:2MAE may also be 1:2.5 to 4.5.

[0028] Furthermore, it is important that the ligand is 2MAE, as this enables the formation of the nickel film of the present invention, described later, i.e., a continuous film having plate-like bodies and bonding portions (Figure 1). If other ligands are used instead of 2MAE, nickel is deposited in fragments, and such a continuous film cannot be formed (Figures 2 and 3).

[0029] <Glycol> Glycols are dispersion media used to disperse these nickel complexes. For example, glycols are selected as dispersion media because they can disperse nickel complexes while possessing sufficient solubility to avoid dissolving the substrate to which the nickel complex slurry is to be applied.

[0030] Examples of glycols include ethylene glycol, propylene glycol, and diethylene glycol. For example, ethylene glycol can be used because it can disperse the nickel complex without any problems and has good solubility in the substrate.

[0031] The ratio of Ni to glycol in a nickel complex slurry varies depending on the viscosity and Ni concentration of the nickel complex slurry, which are set considering the desired nickel film thickness, printing characteristics, and handling properties. For example, setting the molar ratio of Ni to glycol to 1:0.5 to 1.5 can satisfy the desired nickel film thickness, printing characteristics, and handling properties. More preferably, the molar ratio of Ni to glycol is 1:0.8 to 1.2.

[0032] (moisture content) The water content of the nickel complex slurry is preferably 2% by mass or less. Water is easily trapped within colloids formed by the aggregation of multiple micelles of the nickel complex, forming a Water-in-Oil (W / O) structure. Therefore, the size of the colloid varies depending on the amount of water trapped within it. Furthermore, not only does the amount of trapped water differ from colloid to colloid, but the water trapped within the colloid dissolves from the colloid to the dispersion medium over time after the nickel complex slurry is prepared, causing the size of the colloid to decrease over time. Moreover, the behavior of the colloid decreasing in size varies depending on the environment, such as the temperature in which the nickel complex slurry is stored.

[0033] In other words, if the nickel complex slurry contains a large amount of water, the size of the colloid will not be stable, which may result in variations in the particle size and shape of the nickel nanoparticles that form the nickel film, and consequently, variations in the nickel particle density of the formed electrode. Therefore, by lowering the water content of the nickel complex slurry, the properties of the nickel complex can be stabilized, and variations in the particle size and shape of the nickel nanoparticles can be controlled.

[0034] Specifically, if the water content of the nickel complex slurry is 2% by mass or less, the particle size is stable, and it is preferable that the lower limit of the water content is 0% by mass. However, it is acceptable to use the detection limit of analytical instruments, for example, and no specific lower limit is set.

[0035] [Method for producing nickel complex slurry] Next, a method for producing a nickel complex slurry will be described. This method includes the following nickel complex formation steps and is capable of producing the above-mentioned nickel complex slurry. It may also include a first mixing step, a first dehydration step, a second mixing step, and a second dehydration step.

[0036] <Nickel complex formation process> This process involves mixing nickel formate dihydrate, represented as (HCOO)2Ni·2H2O, a glycol, and 2-methylaminoethanol to form nickel complexes represented as (HCOO)2Ni(2MAE)2 and (HCOO)2Ni(2MAE)4.

[0037] This process aims to form the (HCOO)2Ni(2MAE)2 complex and the (HCOO)2Ni(2MAE)4 complex by coordinating the amino group of 2-methylaminoethanol to the nickel ion of nickel formate dihydrate.

[0038] Glycol is a dispersion medium that disperses colloids formed by aggregated micelles of nickelamine formate (hydrated) complex. Since 2-methylaminoethanol is poorly soluble in glycol, the nickelamine formate (hydrated) complex, to which 2-methylaminoethanol is coordinated, also does not dissolve in ethylene glycol, and the micelles of the nickelamine formate complex are dispersed colloidally in ethylene glycol.

[0039] The specific method for mixing nickel formate hydrate, glycol, and 2-methylaminoethanol is not particularly limited. For example, these raw materials can be placed in a container and mixed. One example is to place ethylene glycol and nickel formate hydrate in a beaker, stir with a stirrer, and then add 2-methylaminoethanol. Alternatively, these raw materials can be mixed in a container using a rotation-revolution type mixer.

[0040] In the nickel complex formation process, the molar ratio of Ni to 2MAE may be Ni:2MAE = 1:2 to 5. While 2MAE is stable as a ligand for Ni(HCOO)2 (nickel formate) in either 2-coordinate or 4-coordinate states, both types of 2MAE are present, and it is difficult to determine the ratio of 2-coordinate to 4-coordinate 2MAE. However, the molar ratio of Ni to 2MAE can be determined, and it is preferable that the molar ratio be Ni:2MAE = 1:2 to 5. Having the molar ratio within this range facilitates the formation of the nickel film described later in the present invention. The molar ratio of Ni:2MAE may also be 1:2.5 to 4.5.

[0041] Examples of glycols include ethylene glycol, propylene glycol, and diethylene glycol. For example, ethylene glycol can be used because it can disperse the nickel complex without any problems and has good solubility in the substrate.

[0042] The ratio of Ni to glycol in a nickel complex slurry varies depending on the viscosity and Ni concentration of the nickel complex slurry, which are set considering the desired nickel film thickness, printing characteristics, and handling properties. For example, setting the molar ratio of Ni to glycol to 1:0.5 to 1.5 can satisfy the desired nickel film thickness, printing characteristics, and handling properties. More preferably, the molar ratio of Ni to glycol is 1:0.8 to 1.2.

[0043] (1st mixing step) This step involves mixing nickel formate dihydrate and ethanol before the nickel complex formation step.

[0044] The actual removal of water occurs in the first dehydration step, which will be described later. However, by pre-mixing nickel formate hydrate with ethanol, which is compatible with water, the water can be made compatible with ethanol and the hydrate can be removed. This first mixing step is performed before the nickel complex formation step because if a colloid of nickel complexes is formed by the nickel complex formation step, the water derived from the hydrate will be trapped within the colloid as a hydrated complex, making it difficult to remove the water. By bringing the water derived from nickel formate hydrate into contact with ethanol before the colloid is formed, dehydration becomes easier.

[0045] The amount of ethanol mixed is not particularly limited, but considering that ethanol will be removed along with dehydration in a subsequent process, the molar ratio of ethanol to nickel formate dihydrate should theoretically be 19 or higher. There is no particular upper limit to the molar ratio of ethanol, but it should be 30 or lower, as making it unnecessarily large will not contribute to dehydration.

[0046] (1st dehydration step) This process involves a first mixing step and a nickel complex formation step, followed by a vacuum treatment of the nickel complex-containing mixture obtained in the nickel complex formation step, in which water and ethanol are azeotropically dissolved under reduced pressure to dehydrate the mixture.

[0047] The nickel complex-containing mixture consists specifically of colloidal nickel complex, glycol in which the colloid is dispersed, and water. Dehydration is possible under reduced pressure even without adding ethanol, but adding ethanol allows for azeotropic removal of water at lower temperatures and in a shorter time, thus facilitating dehydration.

[0048] The size of a colloid varies depending on the amount of water trapped within it. Furthermore, the amount of trapped water differs from one colloid to another. Additionally, the trapped water dissolves from the colloid into the ethylene glycol side over time after the nickel complex slurry is prepared, causing the colloid to decrease in size over time. This behavior is also affected by environmental factors such as the temperature at which the nickel complex slurry is stored.

[0049] Therefore, if the first mixing step and the first dehydration step are not performed, the size of the colloid will vary and be unstable immediately after the nickel complex formation step due to the amount of trapped water. However, by performing the first mixing step and the first dehydration step, the water trapped in the colloid can be sufficiently removed, thereby stabilizing the colloid size uniformly and without variation over a long period of time.

[0050] The method of dehydration is not particularly limited, but a specific method is to heat, stir, and reduce the pressure of the mixture in an evaporator. In addition to water, ethanol is also removed by dehydration.

[0051] (Second mixing process) This step involves mixing the nickel complex-containing mixture obtained from the nickel complex formation step with ethanol, following the nickel complex formation step. In the first mixing step, ethanol is mixed before the nickel complex formation step, but in this step, ethanol may be mixed after the nickel complex formation step instead of in the first mixing step. This step also allows for sufficient removal of water trapped in the colloid through the second dehydration step described later, thereby stabilizing the colloid size uniformly and without variation over a long period of time.

[0052] The amount of ethanol mixed is not particularly limited, but considering that ethanol will be removed along with dehydration in a subsequent process, the molar ratio of ethanol to nickel formate dihydrate should theoretically be 19 or higher. There is no particular upper limit to the molar ratio of ethanol, but it should be 30 or lower, as making it unnecessarily large will not contribute to dehydration.

[0053] (Second dehydration process) This process involves, after the second mixing step, treating the mixture of nickel complex-containing mixture and ethanol under reduced pressure to dehydrate it by azeotropic distillation of water and ethanol. The specific details of the second dehydration step are the same as those of the first dehydration step.

[0054] [Method for manufacturing nickel films] Next, the method for producing a nickel film according to the present invention will be described. This method includes the following nickel film formation steps.

[0055] <Nickel film formation process> This process involves heating the nickel complex slurry of the present invention in an inert gas atmosphere to form a nickel film. For example, a nickel complex slurry applied to a film-forming object such as a carrier can be heated in an atmosphere filled with nitrogen gas or argon gas (for example, oxygen concentration of 50 ppm or less) to remove formic acid, 2MAE, glycol, ethanol, water, etc., in order to prevent oxidation of nickel, thereby depositing nickel and forming a nickel film.

[0056] In addition to the nickel film formation process, a coating process may be included in which a nickel complex slurry is applied to the object to be coated. Furthermore, this coating process may be a process of printing or impregnating the object with the nickel complex slurry.

[0057] Furthermore, before heating the nickel complex slurry under an inert gas atmosphere, a drying step may be included in which the coating film is dried at a temperature of approximately 20°C to 200°C to remove dispersion media and solvents such as glycol and ethanol from the coating film. Also, the heating conditions for the coating film are not particularly limited; for example, it may be heated from room temperature to the set temperature at a heating rate of 1°C to 10°C / minute. Alternatively, the drying chamber may be heated to the set temperature in advance before placing the drying film into the drying chamber for drying.

[0058] Furthermore, the heating temperature in the nickel film formation process can be any temperature at which nickel precipitates, for example, 200°C to 270°C. Also, the heating conditions for the coated film after drying are not particularly limited, and for example, the temperature may be raised from the drying temperature to the set temperature at a heating rate of 1°C to 10°C / minute. Alternatively, the drying process may be omitted, and the heating furnace may be heated to the set temperature in advance before placing the coated film into the furnace for drying. Furthermore, the drying and heating of the coated film may be performed continuously.

[0059] Conventional methods for forming catalyst layers have involved physical vapor deposition or spray pyrolysis, which require considerable time. However, with the nickel film manufacturing method of the present invention, a film usable as a catalyst layer can be formed simply by applying and heating a nickel complex slurry, as described above, thus requiring less effort than conventional methods. Furthermore, it allows for the use of less expensive nickel than conventional methods. As a result, a nickel film useful as a catalyst layer can be formed inexpensively and easily.

[0060] The following describes the manufacturing method for nickel films in more detail. As specific examples, we will explain the printing process on a flat plate, the impregnation process on a porous material, and the heating process common to both processes.

[0061] <Printing process> The printing process involves printing the nickel complex slurry of the present invention in the form of a thin film on a flat plate. Examples of printing targets include ceramic or metal carriers of any thickness. During printing, a metal mask is placed over the printing target, the nickel complex slurry is filled into the openings of the metal mask, and the film is printed using a squeegee to achieve a consistent film thickness. Afterward, the metal mask is removed to form a thin film with the same thickness as the metal mask.

[0062] <Impregnation process> The impregnation process involves soaking a porous material with the nickel complex slurry, removing excess solvent and dispersion medium by volatilization or physical means, and forming a uniform film on the surface of the porous material in order to coat and dry the nickel complex slurry of the present invention in a thin film on the surface of the porous material. The impregnation process can be carried out by optimizing known methods. For example, the porous material is weighed into a round-bottom flask of an evaporator, and the pressure is reduced to a certain level while heating in an oil bath or water bath, allowing the nickel complex slurry to be absorbed. After performing appropriate homogenization methods such as stirring, heating, reduced pressure, and ultrasonic vibration while the nickel complex slurry is absorbed, the porous material that has absorbed the nickel complex slurry is removed and dried under reduced pressure at approximately 150°C. Depending on the pore size and surface area of ​​the carrier, the viscosity of the nickel complex slurry can be adjusted by changing the amount of nickel complex slurry and the ratio of glycol and ethanol content.

[0063] <Heating process> The heating process involves sintering a thin film obtained by the printing process or a porous body with a uniform film formed on its surface to obtain a metal sintered film. Since a small amount of oxide is acceptable, the heating process may be carried out in air, but it is preferable to heat in an inert gas atmosphere such as nitrogen gas or argon gas to prevent the metal from being completely oxidized before sintering. The heating conditions are those that cause the metal to sinter, but for example, if a thin film printed with the nickel complex slurry of the present invention or the porous body described above is used, a nickel film that can be used as a catalytic metal sintered film can be obtained by gradually increasing the temperature to 250°C at a rate of 1°C to 10°C per minute and holding it at that temperature for 10 minutes.

[0064] [Nickel film] Next, the nickel film of the present invention will be described. As shown in Figure 1, the nickel film is a continuous film comprising multiple polygonal plate-like bodies and joints between the plate-like bodies that connect the multiple plate-like bodies, with the joints forming a porous body connecting the plate-like bodies.

[0065] The nickel film of the present invention is characterized by having a surface with cracks and fissures, similar to the surface of dry soil, due to the bonding portion being formed by connecting plate-like bodies. As a result of these cracks and fissures, the specific surface area of ​​the nickel film is larger compared to a smooth surface without cracks, thus increasing its catalytic activity. Therefore, the nickel film of the present invention is useful as a catalyst layer.

[0066] Such a continuous nickel film can be formed by using the nickel complex slurry of the present invention and carrying out the nickel film manufacturing method of the present invention. For example, if a ligand other than 2MAE is used in the nickel complex, such a continuous film cannot be formed (Figures 2 and 3).

[0067] The volume resistivity of a nickel film is 1 × 10⁻⁶. -4 Ω·cm~9×10 -4 It is preferable that the volume resistivity is Ω·cm. Having the volume resistivity within this range ensures that the electrode catalyst can satisfy sufficient electrical properties. The volume resistivity is 5 × 10⁻⁶. -4 Ω·cm~9×10 -4 It is more preferable if it is Ω·cm.

[0068] Furthermore, it is preferable that the ratio of the total area of ​​the multiple plate-like bodies to the total area of ​​the bonding portions is 1:0.3 to 0.8. Since the plate-like portions enhance adhesion to the carrier and the bonding portions contribute to the electrical properties, the ratio of these areas greatly affects the performance of the nickel film as an electrode catalyst. If the above ratio is within the above range, it is possible to obtain an electrode catalyst with excellent performance. It is more preferable that the above ratio is 1:0.4 to 0.7.

[0069] [Regarding the behavior of the process by which nickel complex slurry transforms into a nickel film] The nickelamine formate complex, obtained by adding 4 moles of 2-methylaminoethanol to 1 mole of nickel ions in the nickel formate complex, takes on two complex structures: A) a four-coordinate structure with two coordination of formic acid and two coordination of amine per nickel ion, and B) a six-coordinate structure with two coordination of formic acid and four coordination of amine per nickel ion. Nickel complex slurry, mainly composed of these complex structures, undergoes reduction deposition of nickel nanoparticles of a size sinterable at that temperature from the two-coordinate nickelamine formate complex at around 200°C in a nitrogen gas atmosphere (oxygen concentration 50 ppm or less). By 220°C in a nitrogen gas atmosphere (oxygen concentration 50 ppm or less), a tortoise-shell-like semi-dried film consisting of individual sintered and shrunk plate-like nickel flakes is formed on the printed support surface.

[0070] Furthermore, when heated to approximately 230°C in a nitrogen gas atmosphere (oxygen concentration of 50 ppm or less), nickel particles generated by the reduction of the tetra-coordinate amine nickelamine formate (hexa-coordinate) complex remaining between the plate-like nickel flakes form a porous layer at that temperature, with the flakes linked together in a mesh-like structure (necking). This creates a sintered film that is electrically conductive, consisting of a continuous film of porous, low-density junctions and dense plate-like bodies.

[0071] To form an electrode that has both "sparse" and "dense" areas, a certain two-dimensional pattern is created on the electrode surface, and the rate of metal lamination in that area is increased to form "sparse" areas, while the rate of lamination in other patterned areas is decreased to form "dense" areas, thereby forming an electrode film that is both "sparse" and "dense".

[0072] On the other hand, nickel formate amine complexes reduce the central nickel ion through electron transfer during the elimination of the coordinated formic acid and amine, thereby precipitating nickel particles. Due to properties dependent on the structure of the ligand of this complex, the sintered films obtained by the reduction of the complex can be broadly classified into the following two forms.

[0073] One possibility is that a continuous, dense electrode film can be formed if the evaporation temperatures of formic acid and amine individually are lower than the elimination temperatures of formic acid and amine from the complex, and that temperature is above the melting point of the precipitated (melting point depressed) nickel nanoparticles.

[0074] The other case is when the evaporation temperatures of formic acid and amine individually are approximately the same as or higher than the desorption temperature of formic acid and amine from the complex, and nickel nanoparticles are generated in the formic acid and amine medium. Some of the nickel nanoparticles grow to a size that does not sinter at that temperature due to sintering between adjacent particles, and the nanoparticles partially connect the grown sintered masses to form an electrode film with a "sparse" density.

[0075] Furthermore, if the ligand's evaporation temperature is 300°C or higher, or even if it is below that temperature but polymerizes before evaporation and remains in the film as sludge, it remains as an aggregate of non-conductive metal particles in the film. Unless heated to the sintering temperature of normal nickel powder (approximately 1000°C or higher) in a hydrogen-formic acid reducing atmosphere, an electrically conductive electrode film cannot be formed.

[0076] As described above, in electrode film formation by reduction of metal formate salt amine complexes, the density of the electrode film ("dense" or "sparse") depends on the relationship between the evaporation rate of the ligand at the ligand's desorption temperature and the melting point of the precipitated metal nanoparticles. While this ligand desorption temperature may fluctuate due to azeotropes with residual water, it is largely determined by the coordination structure to the metal ion and the type of ligand.

[0077] In the nickel film formed by the above process, the specific surface area is increased due to the presence of junctions where the density is "sparse," and the adhesion between the nickel film and the support is improved because gases generated during the heating process are more easily discharged. Furthermore, electrode catalyst films used in SOFCs and the like repeatedly expand and contract due to temperature changes caused by operations such as starting and stopping, which reduces the adhesion to the support. However, in the nickel film of the present invention, the sparsely connected portions act as buffers against expansion and contraction, thereby suppressing the decrease in adhesion to the support.

[0078] Here, nickel formate can coordinate to nickel ions in two-coordinate formic acid form, plus two or four additional ligands, resulting in two types of structures: four-coordinate and six-coordinate relative to nickel ions. As described above, nickel formate amine complexes contain a mixture of two structures: a total of four-coordinate and a total of six-coordinate. In most cases, the six-coordinate ligands have stronger adsorption power than the four-coordinate ligands. Therefore, upon heating, the four-coordinate complex is reduced at lower temperatures, and the six-coordinate complex is reduced at higher temperatures. Consequently, when the nickel formate amine complex is heated, nanoparticles are generated in two stages, and after final heating, a sintered film or aggregated metal powder film containing metal particles of two different particle sizes is formed.

[0079] In developing this invention, various amine compounds were investigated as ligands, and the formation of nickel films by amine complexation of nickel formate and heat reduction was diligently studied. As a result, it was found that the nickel formate 2MAE (mixed 4-coordinate and 6-coordinate) complex, using 2MAE as a ligand, begins to generate nanoparticles at around 200°C due to the detachment of the 4-coordinate ligand. However, since this detachment temperature is higher than the boiling point of 2MAE (156°C), the detached 2MAE rapidly evaporates, forming a semi-dry film, which brings the distance between the generated nickel nanoparticles closer and promotes necking. At around 220°C during the continuous heating process, the semi-dry film cracks in a tortoise-shell pattern due to the shrinkage of the generated nickel nanoparticle sintered body, and the remaining nickel formate 2MAE (6-coordinate) complex is discharged into the cracks caused by the volume shrinkage due to nanoparticle sintering.

[0080] When the temperature reaches 230°C, the reduction of the nickel formate 2MAE (hexa-coordinate) complex confined at the cracks begins. However, because the temperature is significantly higher than the boiling point of 2MAE (156°C), ligand detachment and evaporation are severe, resulting in reduction in a bumpy boiling state. We found that this leads to the formation of a porous nickel film inside the cracks. The volume resistivity of the nickel film after sintering is 5 × 10⁻⁶. -4The density was Ωcm. Therefore, by optimizing the ligand by selecting 2MAE as the ligand for the nickel formate complex, it is possible to form a nickel film suitable for catalytic electrodes, which has a periodic structure of "sparse" and "dense" densities, and a "porous" structure in the "sparse" parts, using the simple method of continuous heating in a nitrogen atmosphere. [Examples]

[0081] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited in any way by the examples. Nickel complex slurries were prepared below, and nickel films were made using these slurries and their physical properties were evaluated.

[0082] [Manufacturing of nickel complex slurry] [Example 1] (Raw materials used) Copper formate tetrahydrate ((HCOO)2Ni·2H2O) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the amine compound, and 2-methylaminoethanol (manufactured by Tokyo Chemical Industry Co., Ltd.) was used. Ethanol and ethylene glycol (both manufactured by Nacalai Tesque Co., Ltd.) were used as the solvent and dispersion medium.

[0083] (Nickel complex formation process) 5.00 g of copper tetrahydrate hexahydrate and 1.00 g of ethylene glycol were placed in a container and mixed. The mixing was performed at room temperature (25°C) using a stirrer (Sinky Co., Ltd. ARE-310 self-rotating mixer).

[0084] 8.12 g of 2-methylaminoethanol was added to the above container, and these were further mixed to synthesize (HCOO)2Ni(2MAE)2 and (HCOO)2Ni(2MAE)4, obtaining a dispersion in which micelles of nickelamine formate (Aco) complex were dispersed in ethylene glycol. Mixing was performed at room temperature (25°C) using a stirrer (Sinky Co., Ltd., self-rotating mixer ARE-310).

[0085] (Second mixing process) 2.00 g of ethanol was added to the above container to obtain a nickel complex slurry before dehydration. Mixing was performed at room temperature (25°C) using a stirrer (ARE-310, manufactured by Thinky Co., Ltd.).

[0086] (Second dehydration process) The above dispersion was transferred from the container to a round-bottom flask, and water and ethanol were azeotropically dissolved using an evaporator. The mixture was thoroughly dehydrated until no further evaporation was observed, yielding the nickel complex slurry of Example 1.

[0087] [Manufacturing of nickel films] <Printing and firing on alumina substrates> The nickel complex slurry of Example 1 was printed onto an alumina substrate (2.54 mm × 2.54 mm) using a metal mask with an opening of 0.54 mm × 1.54 mm and a thickness of 100 μm. The printed alumina substrate was pre-dried in nitrogen gas (oxygen concentration of 50 ppm or less) by heating at 200°C for 10 minutes, 230°C for 10 minutes, and a maximum temperature of 250°C for 10 minutes to create a sintered nickel film.

[0088] [Observation and property evaluation of nickel films] The resistance of the nickel film after sintering was measured to confirm whether or not it conducted electricity. The nickel film was found to conduct electricity, and its volume resistivity was 5 × 10⁻⁶. -4 It showed a relatively low resistance value of Ωcm for a nickel electrode.

[0089] Figure 1 shows an optical microscope image of the nickel film surface. It can be seen that a continuous film is formed by the periodic formation of densely sintered tortoise-shell-like plate-like regions and porous, loosely sintered joint regions at the cracks. The area of ​​the tortoise-shell-like plate-like regions and the area of ​​the loosely sintered joint regions were measured as follows. Five locations on the nickel film surface were photographed using an optical microscope, and the images were analyzed using image analysis software (software used: MIPAR, manufactured by Lightstone Corporation; analysis template: Globular Alpha in Titanium). The ratio of the total area of ​​the flat regions to the total area of ​​the grid regions was 1:0.49 to 0.56.

[0090] Thereafter, a cellophane tape peeling test of the nickel film (conforming to JIS K 5600-5-6, but without cross-cutting) was carried out to evaluate the adhesion of the nickel film to the alumina substrate. The nickel film did not peel off.

[0091] [Comparative Example 1] 2-Methylaminoethanol (molecular weight 75.11, boiling point 156°C) used in Example 1 was changed to 2-amino-2-methyl-1-propanol (manufactured by Tokyo Chemical Industry Co., Ltd.) (molecular weight 89.14, boiling point 165°C), and the addition amount was changed to 9.65 g. Otherwise, in the same manner as in Example 1, a nickel formate·2-amino-2-methyl-1-propanol complex slurry and a nickel film were produced.

[0092] [Observation and Physical Property Evaluation of Nickel Film] The resistance value of the nickel film after sintering was measured to confirm the presence or absence of energization. The volume resistivity was 5×10 -2 Ωcm and there was energization, but the nickel electrode showed a relatively high resistance value.

[0093] The optical microscope image of the nickel film surface is shown in Figure 2. It can be seen that due to foaming during the formation process of the nickel film, a "porous" and "sparse" scaly joint is formed. However, the scaly joints are only connected and the formation of a continuous film was not observed.

[0094] Thereafter, a cellophane tape peeling test of the nickel film was carried out in the same manner as in Example 1 to evaluate the adhesion. The nickel film of Comparative Example 1 peeled off completely and had no adhesion.

[0095] [Comparative Example 2] 2-Methylaminoethanol (molecular weight 75.11, boiling point 156°C) used in Example 1 was changed to 2-ethylhexylamine (manufactured by Tokyo Chemical Industry Co., Ltd.) (molecular weight 129.24, boiling point 169°C), and the addition amount was changed to 13.99 g. Otherwise, in the same manner as in Example 1, a nickel formate·ethylhexylamine complex slurry and a nickel film were produced.

[0096] [Observation and property evaluation of nickel films] The resistance of the nickel film after sintering was measured to check for current flow, and it was found that it did not conduct electricity and was therefore unsuitable for use as an electrode.

[0097] Figure 3 shows an optical microscope image of the nickel film surface. It can be seen that "sparse" flakes of powder are scattered. However, the formation of a continuous film was not observed.

[0098] Furthermore, a cellophane tape peel test of the nickel film was performed in the same manner as in Example 1, and the adhesion strength was evaluated. The nickel film of Comparative Example 2 peeled off completely and showed no adhesion. [Industrial applicability]

[0099] The present invention makes it possible to form a nickel film having a periodic structure of "sparse" and "dense" areas, and a "porous" structure in the "sparse" areas, by a simple method of heating a nickel complex slurry in an inert gas atmosphere. The resulting nickel film is ideal for forming components that require being "sparse" and "porous," such as electrode catalysts for solid oxide fuel cells (SOFCs), and is industrially useful, as it is expected to expand applications through further miniaturization and cost reduction of solid oxide fuel cells (SOFCs) according to the present invention.

Claims

1. (HCOO) ２ Ni(2MAE) ２ Nickel complex shown, (HCOO) ２ Ni(2MAE) ４ Nickel complex shown, A nickel complex slurry containing glycol, The aforementioned 2MAE is 2-methylaminoethanol, Said (HCOO) ２ Ni(2MAE) ２ and the above (HCOO) ２ Ni(2MAE) ４ is a micelle, The micelles are dispersed in the glycol in a nickel complex slurry.

2. The nickel complex slurry according to claim 1, wherein the molar ratio of Ni to 2MAE is Ni:2MAE = 1:2 to 5.

3. The nickel complex slurry according to claim 1, wherein the glycol is ethylene glycol.

4. The nickel complex slurry according to claim 1, wherein the molar ratio of Ni to glycol is 1:0.5 to 1.

5.

5. Mix nickel formate dihydrate, glycol, and 2-methylaminoethanol to form a nickel complex represented by (HCOO) ２ Ni(2MAE) ２ and a nickel complex represented by (HCOO) ２ Ni(2MAE) ４ A method for producing a nickel complex slurry, comprising a nickel complex formation step of forming a nickel complex, wherein the 2MAE is 2-methylaminoethanol.

6. The method for producing a nickel complex slurry according to claim 5, wherein in the nickel complex formation step, the molar ratio of Ni to 2MAE is Ni:2MAE = 1:2 to 5.

7. The method for producing a nickel complex slurry according to claim 5, wherein the glycol is ethylene glycol.

8. The method for producing a nickel complex slurry according to claim 5, wherein the molar ratio of Ni to glycol is 1:0.5 to 1.

5.

9. Prior to the nickel complex formation step, a first mixing step is performed in which the nickel formate dihydrate and ethanol are mixed. A method for producing a nickel complex slurry according to claim 5, further comprising, after the nickel complex formation step, a first dehydration step of subjecting the nickel complex-containing mixture obtained by the nickel complex formation step to a reduced pressure treatment and dehydrating it by azeotropic dilution of water and ethanol under reduced pressure.

10. Following the nickel complex formation step, a second mixing step is performed in which the nickel complex-containing mixture obtained in the nickel complex formation step is mixed with ethanol. A method for producing a nickel complex slurry according to claim 5, further comprising a second dehydration step after the second mixing step, in which the mixture of the nickel complex-containing mixture and ethanol is subjected to reduced pressure treatment to dehydrate the mixture by azeotropic dilution of water and ethanol under reduced pressure.

11. A method for producing a nickel film, comprising a nickel film forming step of heating the nickel complex slurry described in claim 1 in an inert gas atmosphere to form a nickel film.

12. A nickel film, Multiple polygonal plate-like bodies, A joint located between the plate-like bodies, connecting a plurality of the plate-like bodies, Equipped with, The aforementioned joint is a porous material, A nickel film comprising a continuous film connecting the plate-like bodies at the aforementioned joint.

13. The volume resistivity of the nickel film is 1 × 10 －４ Ω・cm~9×10 －４ The nickel film according to claim 12, wherein the density is Ω·cm.

14. The nickel film according to claim 12, wherein the ratio of the total area of ​​the multiple plate-like bodies to the total area of ​​the joint is 1:0.3 to 0.8.

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