KR101210816B1 - Method for Preparing anode support for solid oxide fuel cells by high-frequency induction-heated sintering - Google Patents

Abstract

The present invention relates to a method for producing a porous composite material having a porous structure using high frequency induction heating sintering and to a cathode support for a solid oxide fuel cell thereby. According to the present invention, there is provided a method capable of sintering in water at a temperature of about 200 to 300 ° C. lower than the conventional sintering method. As a result, compared with the prior art, it is possible to suppress or limit grain growth of the composite and to prepare a porous composite material having a uniform particle size and pore distribution.

Description

Cathode support for fuel cell using high frequency induction heating sintering {Method for Preparing anode support for solid oxide fuel cells by high-frequency induction-heated sintering}

The present invention relates to a method for manufacturing a porous composite material, and more particularly, to a method for manufacturing a porous composite material having a porous structure using high frequency induction heating sintering and to a cathode support for a solid oxide fuel cell thereby.

The operating temperature of solid oxide fuel cell (SOFC) is 600 ~ 1000 ℃ high temperature, the highest power conversion efficiency among the existing fuel cells, has the advantage of the variety of fuel selection and the use of waste heat, 1- It can be applied to 5KW household fuel cell, medium and large power generation over 200KW and cogeneration with gas turbine.

A solid oxide fuel cell generally consists of an oxygen ion conductive electrolyte and a cathode and an anode positioned on both sides thereof. When air and fuel are supplied to each electrode of the unit cell, oxygen reduction reaction occurs in the cathode to produce oxygen ions, and oxygen ions moved to the anode through the electrolyte react with hydrogen supplied from the anode to generate water. do. In this case, electrons are generated at the anode and electrons are consumed at the cathode, so that electricity flows when the two electrodes are connected to each other.

As a unit cell of a conventional solid oxide fuel cell, yttria stabilized zirconia (YSZ) is used as an electrolyte, and strontium-doped lanthanum manganite (LSM) (for example, La 0.8 Sr 0.2 MnO 3) is used as an air electrode. ) Is used, and cermet (NiO / YSZ) in which nickel oxide (NiO) and yttria stabilized zirconia are mixed is used.

In general, a unit cell of a solid oxide fuel cell is manufactured by sintering a fuel electrode, an electrolyte, and an air electrode, or first sintering a fuel electrode serving as a support, then coating and sintering an electrolyte on the fuel electrode, and finally applying an air electrode to sinter it. do.

However, when the anode is sintered using the conventional sintering process (for NiO / YSZ, at 1400 ° C. or more for 12 hours or more), there is a problem that control of the sintered body is not easy due to grain growth due to long time and high sintering temperature.

The present invention is to provide a method for producing a porous composite that can be used for a cathode support for fuel cells by sintering at a short time and low temperature.

The present invention is to provide a method for controlling the microstructure and pores of the porous composite.

One aspect of the invention is a) mixing the ion conductive oxide, transition metal oxide and pore-forming agent, b) press forming and sintering by applying heat generated by current to the mixture of step a), and and c) cooling the press-molded and sintered products.

One embodiment of the present invention relates to a method for producing a porous composite material which is sintered at 1000 to 1200 ° C within 5 minutes.

Another aspect of the invention relates to a negative electrode support using the porous composite material.

According to the present invention, there is provided a method capable of sintering in water at a temperature of about 200 to 300 ° C. lower than the conventional sintering method. As a result, compared with the prior art, it is possible to suppress or limit grain growth of the composite and to prepare a porous composite material having a uniform particle size and pore distribution.

1 is a diagram showing a schematic configuration of an induction current heating / pressurizing sintering apparatus usable for forming a porous composite material according to a preferred embodiment of the present invention.
FIG. 2 is a schematic illustration of a die assembly of a pulsed current heating / pressurizing sintering apparatus usable for forming a porous composite material in accordance with a preferred embodiment of the present invention.
3 is a graph measuring changes in temperature and shrinkage length in the process of heating / pressurizing high frequency induction current of Examples 1 to 3;
4 is a Scanning Electron Microscope (SEM) photograph of the composite material (sintered body) obtained in Example 1 and Comparative Example 1. FIG.
Figure 5 is a graph showing the pore size distribution of the composite material (sintered body) obtained in Example 1 and Comparative Example 1.
6 is a graph showing the three-point bending strength of the composite material (sintered body) obtained in Example 1 and Comparative Example 1. FIG.

Method for producing a porous composite material according to an embodiment of the present invention is characterized in that the press molding and sintering by applying heat generated by the current to the mixed powder.

The manufacturing method of the present invention will be described in detail for each step.

The porous composite material may be applied to a ceramic filter, a porous ceramic separator, a catalyst carrier, a negative electrode support, and the like, but for convenience, the porous composite material will be described in detail below based on the negative electrode support for the fuel cell.

Mixing stage

The mixing step is a step of mixing the ion conductive oxide, transition metal oxide and pore forming agent.

The ion conductive oxide may be any one selected from doped zirconia, ceria, perovskite-based oxide, and a complex thereof.

The transition metal oxide may be used Ni, Cu or Ru.

In order to activate the oxidation reaction of the fuel, the cathode (fuel anode) must include a high activity catalyst component, as well as the concentration of the active reaction point must be maintained in the cathode. In addition, since the cathode support smoothly supplies the fuel to the reaction point at which the electrochemical reaction of the fuel occurs and also smoothly discharges the water vapor generated during the oxidation reaction of the fuel, the cathode support includes pores as a passage for easy movement of the reactants or products. It must have a porous structure.

In order to satisfy these properties at the same time, the cathode may use a complex of an ion conductive oxide and a transition metal oxide having electrochemical activity and excellent electron conductivity.

The most representative cathode support is Ni-YSZ (Yttria Stabilized Zirconia). In this case, Ni has a catalytic activity and electron conductivity with respect to fuel as a metal, and YSZ has ion conductivity as an oxide.

More specifically, in order to form a cathode support, NiO in an oxide form and YSZ, which is an ion conductive oxide, are mixed and sintered to form a composite. The Ni-YSZ composite, which is an actual anode form, is formed under a reducing atmosphere during unit cell operation.

In the present invention, graphite powder, carbon black powder or stainless powder may be used as the pore forming agent.

The ion conductive oxide 30 to 40% by volume based on 100% by volume of the total oxide may include 60 to 70% by volume of the transition metal oxide, the pore-forming agent may include 10 to 20% by volume based on 100% by volume of the total oxide. Can be.

If the content of the transition metal oxide is less than the above-mentioned amount, the conductivity of the transition metal is insufficient after reduction, so that the electrical conductivity is reduced, and if too much, the connectivity of the ion-conducting oxide is destroyed, resulting in grain growth and coarsening of the transition metal. This happens. In addition, when the content of the pore-forming agent exceeds the above amount, it is difficult to maintain the appropriate strength, when too small, the gas diffusion is reduced due to the low porosity.

Particles of the ion conductive oxide powder is 0.05 ~ 0.1㎛, particles of the transition metal oxide powder is 0.05 ~ 2㎛, preferably 0.1 ~ 0.5㎛, the pore-forming agent has a particle diameter of 1 ~ 20㎛ preferably 5 ˜10 μm. When the particle sizes of the powders used as starting materials are large, the mechanical and electrochemical properties of the entire composite are deteriorated due to grain growth and coarsening after sintering. Therefore, the smaller the particle size of the starting material is, the smaller the particle size of the sub-micron unit is appropriate because the filling rate is reduced by agglomeration if it is too small to go to several nanoscales. In particular, the ion conductive oxide must have a particle size smaller than that of the transition metal oxide because it must form a network structure between the transition metals to impart strength of the support and inhibit grain growth of the transition metal.

The mixture is preferably mixed evenly using a method known in the art without particular limitation. For this purpose, for example, a universal milling machine or the like can be used.

Press forming and sintering step

The step is press molding and sintering by applying heat generated by the current to the mixture.

The present invention utilizes heat generated by current (external current). Specifically, for example, heat and pressure generated by induced current or pulse current are applied, and the cathode support, which is a porous composite material, is formed through the press molding and sintering processes.

A pressure of about 10 to 1000 MPa, preferably about 10 to 200 MPa, and preferably about 40 to 80 MPa may be applied for pressure molding, but in some cases, atmospheric pressure is also possible. If the pressure is too low, it may be difficult to progress the sintering due to difficult current flow through the sample, whereas if the pressure is too high, there may be a problem of density difference due to the temperature difference between the surface portion under pressure and the sample. It is preferable to pressurize to one pressure range.

The sintering step may be performed within 5 minutes, preferably within 2 minutes at 1000 ~ 1200 ℃.

As described above, the mixed powder is sintered, and during the sintering process, the ion conductive oxide and the transition metal oxide powder form the joints as well as the respective particles. In particular, the ion conductive oxide forms a network structure. Pore formers are oxidized to form additional pores of porosity. When the cemented carbide is manufactured, densification occurs between the powders because sintering is performed at a high temperature of 1600 ° C. or higher, whereas in the present invention, only pores are formed between the powders due to the sintering temperature of 1000 ° C. to 1200 ° C. The oxidation of the pore former results in the presence of additional pores. Therefore, by adjusting the shape and quantity of the pore-forming material it is possible to control the distribution and fraction of the final pore. In addition, since graphite powder and carbon black powder used as pore-forming agents are excellent electrical conductors, the sintering temperature can be lowered even with a small output and the temperature gradient in the sintered body can be reduced. Therefore, there is an advantage that the thermal stress is smaller than the conventional cemented carbide sintered body.

According to a preferred aspect of the present invention, the heat generated by the current in the sintering process may be used, and heat generated by the induced current or the pulsed current may be used.

In the case of using an induction current, Joule heat generated by the induction current is applied by applying a high frequency induction current to an outer coil, for example, a conductive metal coil such as a copper coil, which surrounds the outer surface of the mixed powder without contacting the outer surface thereof. The mixed powder is indirectly heated through. The frequency of the high frequency induction current applied to the external coil is typically in the range of about 1 to 100 Hz. In the case of using high frequency induction heating, the frequency range of the induced current can be appropriately adjusted according to the size of the sintered object or the specimen because the penetration depth of the high frequency current depends on the frequency. For example, when the sintered object is large, it is preferable to lower the frequency because the penetration depth of the induced current must be increased.

On the other hand, in the case of using a pulse current, a pulse current is applied to the die member in which the mixed powder is accommodated, and heat required for sintering is supplied by Joule heat generated thereby. The period of the pulse current is preferably in the range of about 1 mA to 1 mA, since the shorter the pulse period, the easier the trapped gas is released and the easier the sintering is.

On the other hand, the heating rate in the sintering process is preferably set to about 100 to 5000 ℃ / min, more preferably about 100 to 1000 ℃ / min. If the heating rate is too low, the grain growth may occur as the time required for sintering increases, while in the case where the heating rate is too high, there may be a problem that thermal stress occurs in the sintering object. Therefore, the above-described heating rate range is preferable.

As the press forming and sintering proceeds while heating the mixed powder by the induction current heating / pressurizing sintering method or the pulse current heating / pressurizing sintering method as described above, the obtained negative electrode support shrinks due to the continuously applied pressure, and further shrinks. If the length does not change substantially, it is possible to cut off the induced current or the pulse current and to relieve the pressure.

When the pressure and the induction current or the pulse current is applied to the mixed powder as described above, when the shrinkage length of the sintering object reaches a state that no longer changes, the point of blocking the induction current or the pulse current and removing the pressure applied to the sintering object The journey can typically take a short time, within about 5 minutes, even within about 2 minutes.

As such, in the embodiment of the present invention, particle growth of the powder can be prevented by sintering the ion conductive oxide and the transition metal oxide in a short time. In addition, as the pore-forming agent in the mixture is oxidized and removed at a high temperature, a porous composite material or a negative electrode support for a fuel cell may be manufactured.

Cooling stage

The step may be a cooling step for the press-molded and sintered products, and may use a conventional method known in the art, for example, an air cooled cooling method and the like. At this time, it is preferably cooled to room temperature, the cooling rate is not particularly limited, it is suitable if it is about 100 to 500 ℃ / min.

According to the above process, a porous composite material or a negative electrode support for a fuel cell thereby can be obtained, it can be produced simply by a single process because no separate post-treatment process is required.

The grain size of the porous composite material produced by the present invention, thereby the negative electrode support, is preferably in the range of about 100 to 500 nm, more preferably in the range of about 50 to 100 nm and the grain size is uniform. In addition, the porosity and pore size may be adjusted according to the particle size of the initial powder used, the content to volume% of the pore-forming agent, the pressurization time, temperature pressure. The porous composite material suitable for use as a negative electrode support for a fuel cell has a porosity of 30 to 40%, a pore size of 250 to 500, preferably 290 to 400 nm, and most preferably 290 to 310 nm.

In another aspect, the present invention relates to a fuel cell comprising the negative electrode support prepared above. The fuel cell can be manufactured by a known method. That is, for example, a fuel cell according to the present invention includes a fuel cell (cathode support), a solid electrolyte deposited on one surface of the fuel electrode by an electron beam physical vapor deposition method, a pulsed laser deposition method, an aerogel deposition method, and the other of the solid electrolyte. It may be a solid oxide fuel cell composed of a cathode formed on one surface.

As described above, the negative electrode support for the fuel cell can preferably be manufactured using an induction current heating / pressurizing sintering apparatus or a pulse current heating / pressurizing sintering apparatus. Preferably, the apparatus which has a structure mentioned later can be used.

1 is a view showing a schematic configuration of an induction current heating / pressure sintering apparatus that can be used to manufacture a cathode support for a fuel cell according to a preferred embodiment of the present invention.

According to the drawing, the induction current heating / pressurizing sintering apparatus 100 includes a die member 110, a pressing member 120, and an induction current generating member 130.

The die member 110 is for accommodating a mixed powder (a mixture of an ion conductive oxide, a transition metal oxide, and a pore forming agent powder), and preferably may be made of graphite material. In addition, a through hole is formed in the die member 110, and the mixed powder is accommodated in a central portion of the inner space of the through hole. At this time, the vacuum degree of the space inside the through hole filled with the mixed powder is preferably maintained in the range of about 0.01 to 1 Torr.

The pressing member 120 serves to apply pressure transmitted from an external pressure generating device to the mixed powder located inside the through hole, and are respectively inserted into the upper and lower portions of the through hole to apply uniaxial pressure. do. As such, the sintered object may be densified due to the pressure applied from the pressing member 120. In addition, in order to measure the change in shrinkage length of the sintered object corresponding to the index of the degree of densification, a linear displacement differential transformer (LVDT) may be attached to the movable portion where the through hole and the pressing member 120 are connected. At this time, it is preferable to apply a pressure of about 10 to 1000 MPa through the pressing member 120.

On the other hand, the induction current generating member 130 is configured to be spaced apart around the die member 110, and serves to generate an induction current. The induction current generating member 130 is made of a high frequency current coil, and indirect heat is applied to the die member 110 and the sintering object (mixed powder) by the induction current generated from the applied current, thereby sintering under pressure. Lose. At this time, it is preferable to flow an induction current having a frequency in the range of about 1 to 100 kHz to an external (induction) coil, and the heating rate by the induction current is preferably about 100 to 5000 ° C / min.

2 is a schematic illustration of a die assembly of a pulsed current heating / pressurizing sintering apparatus usable for producing a cathode support for a fuel cell according to a preferred embodiment of the present invention.

In general, the pulse current heating / pressurizing sintering apparatus includes a water-cooled vacuum chamber, a die assembly, a pulse current supply member, a pressing member, a vacuum member, a cooling member, and various control and measurement members. The water-cooled vacuum chamber is a container (for example, stainless steel) for controlling the atmosphere of the press molding and sintering process, and may be provided with a double container having a viewing window for internal monitoring and a door for mounting the die assembly. It is configured to flow the coolant therein.

Referring to the drawings, the die assembly 200 of the pulse current heating / pressure sintering apparatus is, for example, a vertical punch 210, a cylindrical die 220 and a vertical pressure block of insulating material such as alumina (high purity graphite material) 230). At this time, the mixed powder is filled in the inner space formed by the upper and lower punches 210 and the cylindrical die 220, the vacuum of the inner space can be maintained at about 0.01 to 1 Torr, even at atmospheric pressure depending on the material used It is possible. The pulse current supply member 300 supplies a pulse current to the sintered object by the operation of the control switch 310, the pressing device (not shown) is the upper and lower punch of the die assembly 200 through the pressing block 230 ( A uniaxial pressure is applied to 210, and a linear displacement differential transformer LVDT is attached to the movable portion of the hydraulic cylinder to measure the change in length of the sintered object.

The pressure applied through the upper and lower punches 210 may be determined experimentally to the extent that the sintered object can be sufficiently densified, preferably in the range of about 10 to 1000 MPa. The pulse current is applied and maintained until the densification of the mixed powder is made during sintering after press molding, where the pulse period is suitably in the range of about 1 ms to 1 ms. In addition, the heating rate by the pulse current is preferably adjusted to about 100 to 5000 ℃ / min. In addition, conventional rotary pumps and cooling water pumps may be used as the vacuum member and the cooling member, respectively, and the control and measurement member controls process factors such as pressure and current, and measures various data during the process.

The following examples are provided to aid the understanding of the present invention and are not intended to limit the present invention to the following examples.

Example  One

YSZ powder (Tosoh, Japan, average particle diameter: 0.1㎛) 6.2g, NiO powder (Japan FP, average particle diameter: 0.5㎛) 18g, graphite powder (Grand CN, average particle diameter: 5㎛) 0.6g g were mixed, 4.5 g of these mixed powders were extracted and filled into a graphite die (outer diameter 45 mm; inner diameter 20 mm; height 40 mm) of the die member 110 shown in Fig. 1, followed by a uniaxial pressure of 40 MPa. Were added and a vacuum atmosphere of 0.04 Torr was made.

A high frequency induction current heating / pressurization sintering was started by applying a current of 14.4 mA to the external coil, that is, the induction current generating member 130 shown in FIG. At this time, the heating rate by Joule heat generated by the induction current heating was to be 500 ℃ / min. At this time, the temperature of the graphite die surface was measured by an optical thermometer (pyrometer).

The change in shrinkage length of the specimen during press molding and sintering was observed with a linear displacement differential transformer (LVDT) to remove the induced current and pressure at the point of stabilization without changing the length (at this time, the temperature was 1100 ℃ and the time was about 80 seconds), and then cooled to room temperature to obtain a porous composite material.

Example  2

In Example 1, it carried out similarly except applying each uniaxial pressure of 60 Mpa.

Example  3

In Example 1, it implemented similarly except applying each uniaxial pressure of 80 Mpa.

Comparative example  One

YSZ powder (Tosoh, Japan, average particle diameter: 0.1㎛) 6.2g, NiO powder (Japan FP, average particle diameter: 0.5㎛) 18g, graphite powder (Grand CN, average particle diameter: 5㎛) 0.6g g was mixed, and 4.5 g of the mixed powder was extracted, uniaxially formed, and then placed in a sintering furnace and maintained at 1400 ° C. for 12 hours. Then, cooled to room temperature to obtain a composite material.

Temperature and shrinkage length changes during the high frequency induction current heating / pressurizing sintering of Examples 1 to 3 were measured, and the results are shown in FIG. 3.

In addition, scanning electron microscopy (SEM) photographs of the composite material (sintered body) obtained in Example 1 and Comparative Example 1 are shown in FIG. 4.

5 and 6 show the pore distribution and the three-fold strength of the composite material (sintered body) obtained in Example 1 and Comparative Example 1, respectively.

3 illustrates a change in temperature and shrinkage displacement according to heating time during high frequency induction current heating / pressurizing sintering. From the figure, it can be seen that by using the high frequency induction current heating / pressure sintering method, the shrinkage is almost completed 80 seconds after the sintering is started. At this time, the sintering temperature was 1100 ° C, 300 ° C lower than that of Comparative Example 1, and the sintering time was 80 seconds, which is considerably shorter than that of 12 hours of Comparative Example 1.

In Fig. 4, (a), (b) and (c) show the microstructures of the NiO-YSZ sintered body, the Ni-YSZ reducing body, and the YSZ from which Ni was removed, respectively, in Example 1, (d) and (e ) and (f) respectively represent the microstructures of the NiO-YSZ sintered compact, the Ni-YSZ reduced body, and the YSZ from which Ni was removed. 4, the particle size of the sintered compact and the reduced body of Example 1 is smaller than that of Comparative Example 1, and its distribution is uniformly formed. In addition, the pore distribution is also uniformly formed, it can be confirmed that the porosity is large. This suppresses the grain growth of the crystal grains by heating / pressurizing for a short time, thereby obtaining a composite material (100-500 nm) which is almost similar to the particle size of the initial starting powder and significantly lower than that of Comparative Example 1, and is evenly uniform. The pores resulting from the oxidation of the mixed graphite powder are maintained as it is to show that the porous composite material can be produced.

Referring to FIG. 5, Comparative Example 1 shows a pore size of 650 nm while Example 1 shows a pore size of 350 nm, which is half the level.

Referring to FIG. 6, Comparative Example 1 exhibited an average strength of 170 MPa while Example 1 exhibited an average strength of 190 MPa. It is because the network of the ion conductive oxide which functions as a support body in an anode in the case of manufacture by Example 1 is formed well.

The method for producing a metal carbide-metal carbide material having a nanostructure according to the present invention described above is not limited to the above-described embodiment, and the present invention belongs without departing from the gist of the present invention as claimed in the following claims. It will be appreciated that anyone with ordinary knowledge in the art can include various changes and implementations.

100: induction current heating / pressure sintering apparatus 110: die member
120: pressing member 130: induction current generating member
200: die assembly 210: punch
220: cylindrical die 230: pressure block
300: pulse current supply member 310: control switch

Claims (16)

a) preparing a mixed powder by mixing an ion conductive oxide, a transition metal oxide and a pore forming agent;
b) press molding and sintering by applying heat generated by an induced current or a pulse current to the mixed powder of step a); And
c) cooling the press-formed and sintered product,
The mixed powder includes 30-40% by volume of the ion conductive oxide and 60-70% by volume of the transition metal oxide, based on 100% by volume of the total oxide, and the pore-forming agent is 10-20 based on 100% by volume of the total oxide. Volume%, the sintering step is a method for producing a negative electrode support for a fuel cell, characterized in that performed within 5 minutes at 1000 ~ 1200 ℃. delete The method of claim 1, wherein the pressing is performed in the range of 10 to 80 MPa. delete The method of claim 1, wherein the induced current is a method of manufacturing a negative electrode support for a fuel cell, characterized in that the induced current having a frequency of 1 to 100 kHz. delete The method of manufacturing a negative electrode support for a fuel cell according to claim 1, wherein the pulse current is a pulse current having a period of 1 mA to 1 mA. The method of claim 1, wherein the heating rate of the heat generated by the current is 100 to 5000 ℃ / min. delete The method of claim 1, wherein the ion conductive oxide is any one selected from doped zirconia, ceria, perovskite-based oxide and a composite thereof. 2. The method of claim 1, wherein the transition metal oxide is Ni, Cu, or Ru. The method of claim 1, wherein the pore-forming agent is graphite powder, carbon black powder or stainless powder, and has a size of 5 to 10 µm. The method of claim 1, wherein the particle size of the transition metal oxide is 0.1 ~ 0.5㎛, the particle size of the ion conductive oxide is 0.05 ~ 0.1㎛ manufacturing method of a negative electrode support for a fuel cell. The method of claim 1, wherein the step c) is to cool the press-molded and sintered product to room temperature. 15. A cathode support for a fuel cell prepared by any one of claims 1, 3, 5, 7, 8, and 10-14, wherein the cathode support has a grain size of 100. A cathode support for a fuel cell, characterized in that in the range of ~ 500nm.

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