JP4406355B2 - Fuel electrode material for solid electrolyte fuel cells - Google Patents

Description

  The present invention relates to a fuel electrode material for a solid electrolyte fuel cell. More specifically, the present invention relates to an improvement in the microstructure of a fuel electrode that can extend the life of a solid electrolyte fuel cell.

  Solid electrolyte fuel cells are roughly classified into two types: a cylindrical type and a flat plate type. For example, as an example of a cylindrical solid electrolyte fuel cell, a vertically striped cylindrical type is shown in FIG. In this vertically striped cylindrical electrolyte fuel cell, an air electrode 21, a solid electrolyte 22, and a fuel electrode 23 are formed concentrically around a cylindrical support 20, and the air is formed so as to divide the solid electrolyte 22 and the fuel electrode 23. A current on the air electrode 21 side is taken out by an interconnector 24 formed on the electrode 21. A groove 25 is provided between the interconnector 24 and the fuel electrode 23 for electrical insulation between the fuel electrode 23 and the interconnector 24. In this vertically striped cylindrical solid electrolyte fuel cell, air passes through the inside of the support 20 and is supplied to the air electrode 21. Further, the fuel gas is supplied to the fuel electrode 23 through the outside of the support 20. This vertical-striped cylindrical solid electrolyte fuel cell has relatively high mechanical strength, so it has been developed ahead of the flat plate type, and has already succeeded in generating power of 5 kW class, and has started production of 25 kW class. At this stage.

  Further, as a flat type solid electrolyte fuel cell, for example, as shown in an exploded perspective view in FIG. 13, flat unit cells 1 and separators 4 are alternately stacked via spacers 2 and 3. Fuel gas and air are supplied to an air supply space 5 and a fuel gas supply space 6 formed by the separator 4 via a fuel cell gas supply pipe 7 and an air supply pipe 8, respectively. An air electrode 10 and a fuel electrode 11 are formed on the surface side of the solid electrolyte 9. This flat plate type is currently successfully generating 1kW power.

By the way, as a fuel electrode material of these solid electrolyte fuel cells, nickel-zirconia cermet obtained by mixing fine particles of nickel oxide (NiO, but nickel nickel during fuel cell operation) and zirconia (ZrO ₂ ) is a high catalyst. It was considered suitable because of its high activity (reduction ability of hydrogen) and high conductivity (reciprocal of electrical resistance) even at high temperatures from room temperature to 1000 ° C. However, if the content of nickel in the fuel electrode material is large, when this is used as the fuel electrode, thermal stress may occur due to the difference in thermal expansion coefficient, which may lead to cell destruction. On the other hand, when the amount of nickel is small, the electrode characteristics are not very good, it is difficult to take out current, and there are problems such as high sinterability and easy densification. Therefore, conventionally, zirconia using zirconia (hereinafter referred to as 8YSZ) whose crystal structure is stabilized with 8 mol% of yttria has been adopted.

Thus, the conventional anode material for a solid electrolyte fuel cell was obtained by mixing fine powder NiO and 8YSZ. However, although this fuel electrode material has excellent initial characteristics, it deteriorates within several tens of hours after the start of power generation, making it impossible to generate power. As a result of elucidating the cause, it was found that the fuel electrode material was densified and volumetric shrinkage and Ni particles were agglomerated under battery operating conditions (Report of Electric Power Research Laboratory W93019, May 1994).

  There are other reports on the aggregation and densification of Ni (Abstracts of the 60th Annual Meeting of the Electrochemical Society, page 269, April 1-3, 1993).

  Furthermore, there is a report that attempts to increase the dispersion of Ni particles and extend the life of the fuel electrode by using Ni (Mg) O-8YSZ (Abstracts of the 33rd Battery Review Meeting Lecture, pages 35-36) , September 16-18, 1992; Electrochemical Society 59th Conference Abstracts, 197 pages, April 2-4, 1992; Electrochemical Society 60th Conference Abstracts, 270 pages , April 1-3, 1993).

  There is also a report that attempts to improve performance by coating Ni with a large particle size with YSZ with a small particle size (Abstracts of the 59th Annual Meeting of the Electrochemical Society, page 198, April 1992-2). 4th).

  Furthermore, YSZ is deposited on metal ruthenium by electrochemical vapor deposition (Abstracts of 18th Solid State Ionics Discussion Meeting, 5-8 pages, October 12-13, 1992) and YSZ is deposited on metal Ni by vapor phase method. The deposited materials (Abstracts of the 59th Annual Meeting of the Electrochemical Society, page 199, April 2-4, 1992) are also being studied.

  However, the various reports as described above are mainly intended to improve the power generation performance, and the operation data for a long period of time is poor. Furthermore, it is thought that the manufacturing method in which YSZ is chemically vapor deposited or deposited by a vapor phase method is costly (Power Central Research Institute report W92028 March 1993).

Japanese Patent Laid-Open No. 4-192261 JP-A-4-56070

As described above, the above-described prior art is particularly aimed at improving the performance of the anode material , and has not been sufficiently examined for deterioration during long-lasting operation, and considerations regarding the manufacturing process that directly affects the manufacturing cost. However, the manufacturing cost is high due to the complexity of the manufacturing process.

  Accordingly, the present invention has developed a manufacturing method capable of mass-producing a fuel electrode material for a solid electrolyte fuel cell having an optimum microstructure at a low cost using a simple manufacturing technique, and further, a fuel electrode for a solid electrolyte fuel cell. When used in the above, an object of the present invention is to provide a fuel electrode material that can stably maintain performance equal to or higher than that of a conventional material for a long period of time.

In order to achieve such an object, the present invention is a fuel electrode material for a nickel-zirconia-based solid electrolyte fuel cell, comprising a mixture of zirconia coarse particles, zirconia fine particles, and nickel oxide or nickel particles . The particle size of each particle group satisfies the relationship of zirconia coarse particles> nickel or nickel oxide particles> zirconia fine particles, and the weight ratio of zirconia coarse particles to nickel or nickel oxide particles to zirconia fine particles is 7 to 4: 3 to 6: 1, and, by sintering, skeleton is formed by zirconia grit及beauty di zirconia particles, nickel or nickel oxide particles in the gap skeletal are as have been dispersed.
Here, as in the fuel electrode material for a solid electrolyte fuel cell according to claim 2, the weight ratio of zirconia coarse particles to nickel or nickel oxide particles and zirconia fine particles is 7: 3: 1 and 6: 4: 1. 4: 6: it is good preferable is any one of 1.

  The zirconia is preferably stabilized zirconia, particularly zirconia (8YSZ) stabilized with 8 mol% yttria.

  The specific particle size of each of the particles is 20 to 75 [mu] m of zirconia coarse particles, 0.1 to 1 [mu] m of zirconia fine particles, and 5 to 20 [mu] m of nickel or nickel oxide particles. It is desirable.

Further, in the method for producing a fuel electrode material for a solid electrolyte fuel cell, the zirconia coarse particles and the nickel or nickel oxide particles and the zirconia fine particles have a relationship of zirconia coarse particles> nickel particles or nickel oxide particles> zirconia fine particles. a step of pre-grain diameter control, using a ball mill, in a dry condition, or by mixing the Zuji zirconia coarse particles and nickel or nickel oxide particle group, then further mixed by addition of di-zirconia particulate group in this mixture And a process of performing.

  Thus, in the present invention, the particle size of each raw material used in the fuel electrode material for a solid electrolyte fuel cell is changed, and the fine structure is improved.

  Conventional nickel-zirconia cermet-based solid electrolyte fuel cell anode material was a mixed powder of fine NiO and 8YSZ, but in the present invention, it is compared with a zirconia coarse particle group having a relatively large particle size. A mixture of a group of zirconia fine particles having a small particle size and a group of nickel oxide particles.

Each particle constituting the fuel electrode material is considered to have a function corresponding to the particle size as described below.
(1) Form a framework of zirconia coarse particles and fuel electrode material, and eliminate the difference in thermal expansion from the electrolyte.
• Create and maintain pores in the gaps between the particles (intergranular gaps).
・ Prevents aggregation of nickel particles during electrode operation and maintains an electron conduction path (hereinafter referred to as a current path).
(2) Nickel oxide particles (nickel particles when the electrode is activated)
-Disperse in the gaps between the zirconia coarse particles so as to cover the surface of the zirconia coarse particles and cope with the aggregation of nickel.
-Form a current path.
-Increase the interface with zirconia particles and increase the electrode reaction field.
(3) Improve adhesion between zirconia fine particles, zirconia coarse particles, and zirconia electrolyte plates.
・ Improve nickel particles and prevent current paths from being interrupted.

Therefore, the fuel electrode material according to the present invention in which the raw materials having different particle diameters are combined has a change in porosity under a high temperature / reducing atmosphere (an atmosphere close to battery operating conditions) as compared with the conventional material. The shrinkage of the volume is extremely small, and the current path is not blocked. Thereby, even in the long-lasting power generation, the deterioration of the fuel electrode material hardly occurs, and the performance of the fuel cell is not deteriorated. In addition, the performance of the fuel electrode material itself can be improved by the effect of increasing the electrode reaction field.

  On the other hand, the invention as a method for producing such a fuel electrode material is a fuel electrode material having a desired performance as described above, wherein dry mixing and agitation is performed by a ball mill to maintain the desired particle size of each raw material as described above. Since a general and simple device called a ball mill is used, it is excellent in terms of productivity and manufacturing cost.

  The fuel electrode material of the present invention is less susceptible to densification such as volume shrinkage and porosity reduction even in the air and in the battery operating atmosphere, and the present invention is more effective than a fuel electrode manufactured using conventional materials. The fuel electrode made of this material has excellent power generation performance, enabling stable power generation for a long time. Therefore, the use of the fuel electrode material of the present invention makes it possible to improve the performance and life of the solid electrolyte fuel cell. In addition, the method for producing a fuel electrode material can produce a fuel electrode material exhibiting excellent characteristics as described above using a conventional simple apparatus, which is advantageous from the viewpoint of productivity and production cost. It is.

  Hereinafter, the present invention will be described in more detail based on embodiments.

The fuel electrode material of the present invention comprises a mixture of zirconia coarse particles, zirconia fine particles, and nickel oxide or nickel particles. Figure 1 is a conceptual diagram illustrating the microstructure of the fuel electrode material charge according to the present invention, reference numeral 31 is zirconia grit, reference numeral 32 is a nickel oxide particle, reference numeral 33 denotes a zirconia fine.

As shown in FIG. 1, the zirconia coarse particles 31 form a skeleton in the fuel electrode material and form pores 34 by gaps (intergranular slits) formed between the particles. By these, thermal compatibility with the electrolyte (made of stabilized zirconia) is achieved, and when the fuel electrode material is used as the fuel electrode, the fuel electrode is prevented from contracting and the pores being closed due to the progress of sintering. Further, the zirconia fine particles 33 more firmly adhere the zirconia coarse particles 31 having a large particle diameter, or improve the adhesion between the electrolyte and the fuel electrode when the fuel electrode material is used as the fuel electrode . Then the control is Tsu by the zirconia particles 31 and 33 sinterability of the overall electrode size, it is achieved an increase in aggregation preventing the electrode reaction field of nickel. Further, the nickel oxide particles 32 are dispersed around the zirconia coarse particles having a large particle diameter, and change to nickel during battery operation. As a result, when the fuel electrode material is used as the fuel electrode, a current path of the fuel electrode is formed, and an electrode reaction occurs at the interface between the zirconia particles 31 and 33 and the pores.

  In order to provide such functionality, in the present invention, the particle size of each particle group is set to have a relationship of zirconia coarse particles> nickel or nickel oxide particles> zirconia fine particles. More specifically, for example, the particle diameter of zirconia coarse particles is 20 to 75 μm, more preferably 45 to 75 μm, and the particle diameter of zirconia fine particles is 0.1 to 1 μm, more preferably 0.1 to 0.5 μm. These particles are combined in such a way that the particle diameter of nickel oxide particles is 5 to 20 μm, more preferably 1/10 or less of the particle diameter of coarse zirconia particles.

As zirconia, stabilized zirconia, particularly 8YSZ is preferable. The reason for this is that, as described above, if the nickel content in the fuel electrode material is large, thermal stress may occur due to the difference in the thermal expansion coefficient, leading to cell destruction. On the other hand, if the amount of nickel is small, the electrode characteristics are not very good and the sinterability is high, so that the optimum nickel content can be obtained by using stabilized zirconia or 8YSZ. .

  The fuel electrode material of the present invention can be suitably used for production of the fuel electrode of various forms of solid electrolyte fuel cells as exemplified in FIGS. 12 and 13, and the fuel cell or the shape of the fuel electrode can be used. Without being limited in any way, in any case, excellent performance as described later can be exhibited.

  The method for producing the fuel electrode material of the present invention is not particularly limited, but the predetermined particle size of each particle as described above, in particular, the particle size of zirconia coarse particles is maintained and mixed stably. Therefore, it is desirable to use a ball mill and stir and mix under dry conditions. The ball mill is preferably a device having a relatively soft surface, such as a combination of a poly ointment bottle and a nylon ball.

  Stirring and mixing is performed by first mixing 8YSZ coarse particles and NiO particles, for example, for about 48 to 60 hours, and then adding 8YSZ fine particle groups to the mixture and further mixing for about 48 hours.

  Hereinafter, the present invention will be described more specifically based on examples.

(1) The fuel electrode material used in the experiment is made of powder having a mixing ratio as shown in Table 1, and hereinafter, each powder is indicated by a sample number in Table 1. The material was produced based on the flow shown in FIG. That is, as a first step, the particle size of the powder to be used is adjusted. The coarse particles of 8YSZ were obtained by pre-baking at 1400 ° C. for 20 hours and then classifying with a sieve. On the other hand, 8YSZ fine particles and NiO particles were obtained by pulverization under appropriate conditions in a wet ball mill composed of a special nylon resin container and partially stabilized zirconia balls. Next, as a second step, each powder is dry-mixed by a ball mill composed of a poly ointment bottle and nylon balls. First, 8YSZ coarse particles and NiO particles were mixed, and then the 8YSZ fine particle group was added to this mixture and further mixed.

  The conventional material is obtained by mixing NiO having a particle size of several μm and 8YSZ so that Ni after reduction is 40% by volume.

(2) The microstructure of the fuel electrode material thus obtained was observed with an electron microscope (EPMA) before and after power generation. The conventional material (FEM000) is composed of Ni and 8YSZ having a fine particle size (FIG. 3), but the FEM 461 according to the present invention has a microstructure as shown in the conceptual diagram as shown in FIG. It was observed.

(3) The shrinkage after each material powder was baked at 1400 ° C. for 10 hours in air for use in the experiment was examined. The results are shown in Table 2. From this result, it can be seen that the material according to the present invention is less contracted than the conventional material.

(4) Change in volume shrinkage and porosity after holding the sintered body of each material obtained in (3) above at 1000 ° C. in a hydrogen atmosphere using an electric furnace apparatus for reduction test as shown in FIG. The change in rate was examined. FIG. 7 shows the obtained volume shrinkage change result, and FIG. 8 shows the porosity change result. As is apparent from the results shown in FIG. 7, the shrinkage of NiO (sample FEM010) is extremely large, and the conventional material (sample FEM000) also shrinks by 17% after 300 hours. In contrast, the fuel electrode material according to the present invention has small shrinkage. Further, as shown in FIG. 8, the change in the porosity was similarly small in the fuel electrode material according to the present invention. Further, in the fuel electrode material according to the present invention, as the amount of 8YSZ fine particles mixed with 8YSZ coarse particles increases, shrinkage tends to increase when fired in air, and the volume when held at 1000 ° C. in hydrogen is increased. Since the shrinkage and the change in porosity tend to be small, it can be seen that the large and small 8YSZ controls the densification of the material. That is, the 8YSZ fine particles have an effect of strengthening the skeleton structure in the material, and when fired in air to obtain a sintered body, densification occurs and slightly shrinks the volume of the material. However, since the skeleton of the material is completed at this point, 8YSZ densification does not proceed in hydrogen, so the volume of the material hardly changes and the porosity does not change.

(5) Among the above materials, a solid electrolyte fuel cell was manufactured using FEM461 having the smallest change, and the power generation test and the performance evaluation of the fuel electrode were performed. For the evaluation, a current interpolation method was used using a measuring apparatus having a configuration as shown in FIG. In addition, after applying a slurry of a fuel electrode material on an electrolyte plate, the evaluation unit cell was baked at 1400 ° C. for 10 hours, and a slurry of lanthanum manganite added with strontium was applied to the air electrode. After that, it was baked at 1150 ° C. for 4 hours, and further a platinum paste was baked as a reference electrode.

(6) From the results shown in FIG. 10 obtained in the power generation test for FEM461, power generation for 3000 hours became possible. Note that the decrease in the cell voltage after 2500 hours in FIG. 10 is due to the separation of the air electrode and the cell destruction caused by the earthquake that occurred during the experiment, and the performance of the fuel electrode did not decrease. On the other hand, the lifetime of a single cell using a conventional fuel electrode material is at most a dozen hours (see Central Research Institute report W93019, May 1994), which shows a remarkable stabilization of performance. It was.

(7) FIG. 11A shows the result of the performance evaluation of the fuel electrode performed by the current interpolation method. The overvoltage after 500 hours is remarkably increased with respect to the initial performance, but this is because an overcurrent was passed, and no major deterioration has occurred after 500 hours. In order to obtain reproducibility, when 2500 hours of continuous power generation was performed without changing the current, no significant change was observed in the overvoltage (FIG. 11 (b)).

It is a conceptual diagram which shows the fine structure of a fuel electrode at the time of using the fuel electrode material of this invention for the fuel electrode of a solid electrolyte fuel cell. It is a figure which shows the measurement result of the particle size distribution of each particle | grain used in the Example of this invention, (a) relates to 8YSZ coarse particle, (b) relates to NiO particle, (c) relates to 8YSZ fine particle. It is an electron micrograph showing the fine structure before power generation and after power generation of a conventional fuel electrode, (a) is a secondary electron image before power generation, (b) is a Ni distribution image before power generation, (c) is after power generation. (D) is a Ni distribution image after power generation. It is an electron micrograph which shows the fine structure after the electric power generation of the fuel electrode concerning one example of the present invention, (a) is a secondary electron image, (b) is a Ni distribution image, (c) is a Zr distribution image, (D) is a Zr distribution image (enlarged portion A in (a)). It is a figure which shows the manufacturing process in the manufacturing method which concerns on this invention. It is a conceptual diagram of the apparatus used when hold | maintaining in a hydrogen reduction atmosphere in the Example of this invention. It is a graph which shows the time-dependent change of volume shrinkage in the reducing atmosphere of each material produced in the Example of this invention. It is a graph which shows the time-dependent change of the porosity in the reducing atmosphere of each material produced in the Example of this invention. It is a conceptual diagram of the apparatus used for the electric power generation and electrode evaluation in the Example of this invention, (a) shows a measurement apparatus structure, (b) shows a measurement circuit, respectively. It is a graph which shows the electric power generation condition of the single cell using the fuel electrode material of one Example of this invention. It is a graph which shows the time-dependent change of the performance of the fuel electrode material of one Example of this invention, (a) shows the case where an overcurrent is sent, (b) shows the case where a constant current is sent, respectively. It is a perspective view which shows the structure of an example of a vertical stripe cylindrical solid electrolyte fuel cell. It is a disassembled perspective view of a flat plate difficulty solid electrolyte fuel cell.

Explanation of symbols

1 Cell 11, 23 Fuel Electrode 31 Zirconia Coarse Particle 32 Nickel Oxide Particle 33 Zirconia Fine Particle

Claims (2)

A fuel electrode material for a nickel-zirconia-based solid electrolyte fuel cell, comprising a mixture of a zirconia coarse particle group, a zirconia fine particle group, and a nickel or nickel oxide particle group, wherein the particle size of each particle group is a zirconia coarse particle > satisfy the relationship of the nickel or nickel oxide particles> zirconia fine, the weight ratio of the zirconia coarse particles and the nickel or nickel oxide particles and the zirconia particles, 7-4-3-6: 1, and, baked As a result , a skeleton is formed by the zirconia coarse particles and the zirconia fine particles, and the nickel or nickel oxide particles are dispersed in the gaps of the skeleton. The weight ratio of the zirconia coarse particles, the nickel or nickel oxide particles, and the zirconia fine particles is any one of 7: 3: 1, 6: 4: 1, and 4: 6: 1. The fuel electrode material for a solid electrolyte fuel cell according to claim 1.