JP4603084B2 - Electrode for solid electrolyte - Google Patents

Description

The present invention relates to an oxygen ion conductive electrode for solid electrolyte applied to a fuel cell, an oxygen sensor and the like.

  As an oxygen ion conductive solid electrolyte, a material in which a stabilizer such as calcium oxide or yttrium oxide is dissolved in zirconium oxide is generally used as stabilized zirconia.

  Since the operating temperature of these solid electrolytes is high, the electrode materials for solid electrolyte oxygen sensors are required to have high conductivity, high corrosion resistance, and catalytic activity for the dissociation reaction of oxygen molecules. Currently, noble metals such as platinum are used. Is used.

On the other hand, ruthenium oxide (RuO ₂ ) is a metal oxide having electronic conductivity, and has high conductivity (−10 ⁵ S / cm) and catalytic activity, and is expected as a new electrode for oxygen sensor. . Here, an example in which ruthenium oxide is used as an electrode of a solid electrolyte type oxygen sensor is disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 8-122297). In this publication, ruthenium oxide is used as the inner electrode. Then, ruthenium oxide is used as a paste and heat-baked at a high temperature to form an electrode.

  However, in the method of forming a ruthenium oxide electrode by paste baking, the size of ruthenium oxide metal particles in the paste is as large as submicron to tens of microns, and it is influenced by solvent vaporization, residual carbides, etc. during baking. It is difficult to demonstrate the required properties such as the original high conductivity, high corrosion resistance, and high catalytic activity for the dissociation reaction of oxygen molecules, and platinum electrodes are actually used as electrode materials for solid electrolyte oxygen sensors .

JP-A-8-122297

  Stabilized zirconia conventionally used as a solid electrolyte needs to operate at about 1000 ° C. in order to obtain high oxygen ion conduction. Therefore, when the stabilized zirconia is used for a fuel cell or an oxygen sensor, heat resistance is required for the electrode and the constituent material, and there are problems in reliability and economy.

  In the present invention, attention is focused on ruthenium oxide as an electrode material for solid electrolytes, and in order to maintain the required physical properties such as high conductivity, high corrosion resistance, and high catalytic activity even at high operating temperatures, the original physical properties of ruthenium oxide are maximized. It is another object of the present invention to provide a method for producing an electrode for solid electrolyte that forms an electrode having a fine structure that can be maintained. That is, an object is to produce a ruthenium oxide electrode that does not change its microstructure even when used for a long time at a high operating temperature. Furthermore, it aims at providing the manufacturing method of the ruthenium oxide electrode which suppressed the interface resistance increase of a solid electrolyte substrate-electrode, and the electrode interface reaction advanced very easily.

  In addition, by using the prescribed high-purity dipivaloylmethanate ruthenium as a raw material, it is possible to suppress degradation of raw material usage due to side reactions during raw material transport, etc., and to uniformly decompose the raw material on the solid electrolyte substrate It is to provide a method for producing a ruthenium oxide electrode having a fine structure composed of particles having a uniform particle diameter.

The electrode for solid electrolyte of the present invention is formed by forming a ruthenium oxide electrode formed by MOCVD on an oxygen ion conductive solid electrolyte substrate made of zirconium oxide containing a stabilizer. The particle diameter of the ruthenium oxide particles constituting the ruthenium oxide electrode observed by (SEM) surface observation is 2 μm or less, and satisfies the microstructure evaluation criteria of (Condition 1) .
(Condition 1) The ruthenium oxide electrode formed by the MOCVD method is heat-treated at 850 ° C. for 8 hours. After the treatment, the particle diameter of the ruthenium oxide particles observed by surface observation with a scanning electron microscope (SEM) is 2 μm. The following.

The electrode for solid electrolyte of the present invention includes satisfying the microstructure evaluation criteria of (Condition 2) .
(Condition 2) The ruthenium oxide electrode formed by the MOCVD method is heat-treated at 850 ° C. for 8 hours, and before and after the treatment, the ruthenium oxide particles observed by a scanning electron microscope (SEM) are coarsened crystal grains. There is no phenomenon.

The electrode for solid electrolyte of the present invention includes that the ruthenium oxide electrode has a thickness of 2 to 3 μm.

  The method for producing an electrode for solid electrolyte of the present invention is an electrode film forming method belonging to a so-called organometallic CVD method (MOCVD method). The organic metal CVD method is a CVD method using an organic metal as a raw material. Here, the CVD (chemical vapor deposition) method means that a gas of a reaction system molecule or a mixed gas of this and an inert carrier is flowed on a heated substrate, and then hydrolysis, self-decomposition, photolysis, redox, substitution The method of vapor-depositing the product by reaction etc. on a board | substrate is said.

  According to the present invention, in order to maintain the required physical properties such as high conductivity, high corrosion resistance and high catalytic activity even at high operating temperature, an electrode having a fine structure capable of maximizing and maintaining the original physical properties of ruthenium oxide is formed. It became possible to squeeze. That is, this electrode is a ruthenium oxide electrode in which the electrode interface reaction has proceeded very easily, and there is no change in the microstructure even if it is used for a long time at a high operating temperature. Furthermore, an increase in the interface resistance between the solid electrolyte substrate and the electrode is suppressed.

  In addition, by using the prescribed high-purity dipivaloylmethanate ruthenium as a raw material, it is possible to suppress degradation of raw material usage due to side reactions during raw material transport, etc., and to uniformly decompose the raw material on the solid electrolyte substrate It is possible to provide a ruthenium oxide electrode having a fine structure composed of particles having a uniform particle diameter. Furthermore, there are few vaporization residues.

  The electrode manufactured according to the present invention has excellent compatibility with the stabilized zirconia when the stabilized zirconia is used in a fuel cell or an oxygen sensor, and even when operated at a high temperature to obtain high oxygen ion conduction, It has excellent heat resistance, reliability and economy.

It is the conceptual diagram which showed the manufacturing apparatus used for formation of the electrode for solid electrolytes by the Example of this invention. It is a figure which shows the thermogravimetric curve of a raw material, and shows two examples of Ru (DPM) ₃ and Ru (AcAc) ₃ . It is a figure which shows the electron micrograph of the electrode of Example 1, (a) is after the film-forming, (b) is the microstructure after annealing at 850 degreeC for 8 hours. 3 is an X-ray diffraction chart of an electrode of Example 1. FIG. The substrate is yttria stabilized zirconium oxide. It is a figure which shows the electron micrograph of an electrode, The thing after the film-forming of Example 1 and the thing quoted of the reference example 1 are shown. It is a figure which shows the electron micrograph of an electrode, and the thing after the film-forming of Example 1 and the thing quoted of the reference example 2 are shown. It is an output characteristic figure (IV characteristic figure) of Example 1 and Comparative Examples 1-6. It is the figure which showed the result of having performed the alternating current impedance analysis of the electrode of Example 1. FIG. It is a figure which shows a temperature-interface conductivity characteristic about the electrode interface resistance component of an electrode, and shows Example 1 and Comparative Example 1, respectively.

  Hereinafter, although an embodiment and an example are shown and the present invention is explained in detail, the present invention is limited to these descriptions and is not interpreted.

  Hereinafter, a procedure for forming a ruthenium oxide electrode on a solid electrolyte substrate using the CVD film forming apparatus shown in FIG. 1 will be described. The manufacturing apparatus of FIG. 1 is an example. This CVD film forming apparatus includes a carrier gas supply means 3 comprising an inert gas generation source 1 which is a carrier gas and a carrier gas flow rate controller 2, and an oxygen gas supply comprising an oxygen gas generation source 4 and an oxygen gas flow rate controller 5. In order to introduce the means 6 and the carrier gas into the reaction chamber 7, the carrier gas introduction pipe 8 inserted into the reaction pipe 7, and the gas disposed inside the reaction pipe 7 and in the carrier gas introduction pipe 8 are provided. A raw material tray 9 for placing the pivaloylmethanate ruthenium powder, a raw material heating heater 10 provided around the side of the reaction tube to heat the dipivaloylmethanate ruthenium powder placed on the raw material plate 9, Oxygen gas introduced so as to be mixed with the vaporized dipivaloylmethanate ruthenium conveyed together with the inert gas from the carrier gas introduction pipe 8 is introduced into the reaction pipe 7. An oxygen gas introduction pipe 11 connected to an oxygen gas supply means 6 for supplying, a mixer section 12 for uniformly mixing oxygen gas and vaporized dipivaloylmethanate ruthenium, and a reaction pipe 7 are installed. A substrate holder 14 for holding the substrate 13, a substrate heating heater 15 provided around the side of the reaction tube 7 to heat the substrate 13, a pressure gauge 16 for adjusting the inside of the reaction tube 7 to a predetermined pressure, a vacuum pump 17, And an exhaust means 19 including an exhaust duct 18.

  First, dipivaloylmethanate ruthenium is placed on the raw material tray 9 and installed in the reaction tube 7 and in the vicinity of the end of the carrier gas introduction tube 8 as shown in FIG.

The raw material is preferably dipivaloylmethanate ruthenium. Dipivaloylmethanate ruthenium (Ru (DPM) ₃ ) belongs to the β-diketone (R ₁ —CO—CH ₂ —CO—R 2) complex. For example, acetylacetonate ruthenium (Ru (AcAc) ₃ ) requires a high heating temperature in order to obtain the required vapor pressure. On the other hand, dipivaloylmethanate ruthenium can set the heating temperature for vaporization lower than acetylacetonate ruthenium. Moreover, there is almost no residue after vaporization. Therefore, the use efficiency of the raw material is increased.

  In the present invention, when synthesizing ruthenium dipivaloylmethanate by reacting ruthenium trichloride with dipivaloylmethane in the presence of an alkaline reaction accelerator, a crude material is obtained by refluxing in a nitrogen atmosphere, It is more preferable to use dipivaloylmethanate ruthenium obtained by purifying the crude material by column chromatography and further purifying by sublimation. The dipivaloylmethanate ruthenium obtained by the above production method does not undergo oxidative decomposition during the synthesis due to reflux under a nitrogen atmosphere, and contains a small amount of by-products. Therefore, the vaporization efficiency is high and the residue after vaporization is small. Moreover, since there is little content of a by-product, decomposition | disassembly before a board | substrate arrives will reduce, and dipivaloylmethanate ruthenium will be supplied with high purity to the substrate surface. Furthermore, because there are few foreign gases other than dipivaloylmethanate ruthenium, such as raw material semi-decomposition by-products, carbon compounds, etc., the reaction on the substrate surface proceeds homogeneously, and the oxidation consists of particles of uniform particle size. A ruthenium thin film can be deposited. It should be noted that if there is a large amount of foreign gas other than dipivaloylmethanate ruthenium, it may be a factor that inhibits the precipitation of a ruthenium oxide thin film composed of particles having a uniform particle diameter.

  The reflux conditions are, for example, 100 to 230 ° C., preferably 120 to 210 ° C., and 15 to 25 hours, preferably 18 to 22 hours, under a nitrogen atmosphere. Examples of the alkaline reaction accelerator include sodium hydrogen carbonate and potassium hydrogen carbonate.

  As the raw material dish, a dish made of a material inert to dipivaloylmethanate ruthenium is selected, for example, a quartz boat.

  Although FIG. 1 describes the method for sublimating dipivaloylmethanate ruthenium, dipivaloylmethanate ruthenium is dissolved in an organic solvent such as ethanol and stored in a vaporization vessel, and the raw material vapor is bubbled. You may introduce | transduce into the inside of the reaction tube 7, for example, a quartz tube.

  Next, the inside of the reaction tube 7 is depressurized by the exhaust means 19. The pressure is 13 to 4000 Pa, preferably 13 to 70 Pa.

  The substrate heating heater 15 is operated to heat the substrate 13 to a predetermined temperature. The substrate temperature is 400 to 800 ° C.

  The substrate is an oxygen ion conductive solid electrolyte substrate made of zirconium oxide containing a stabilizer. The stabilizer is preferably a metal oxide such as magnesium oxide, calcium oxide, yttrium oxide or scandium oxide and cerium oxide. Zirconium oxide containing a stabilizer is stabilized zirconium oxide or partially stabilized zirconium oxide.

  Next, the raw material heater 15 is operated to heat the raw material 26. Raw material heating temperature shall be 140-270 degreeC. In order to obtain a desired vaporization rate, the raw material heating temperature is appropriately adjusted.

  Next, the carrier gas introduction means 3 is operated to send an inert gas, for example, argon gas, to the raw material tray 9. Nitrogen gas may be used. At the same time, the oxygen gas introduction means 6 is operated to introduce oxygen gas into the reaction tube. In FIG. 1, the carrier gas introduction pipe 8 and the oxygen gas introduction pipe 11 are inserted in the reaction tube in parallel. However, in order to more reliably mix the carrier gas and the oxygen gas, the carrier gas introduction pipe 8 The oxygen gas introduction pipe 11 having an inner diameter larger than the outer diameter may be arranged in such a manner that the carrier gas introduction pipe 8 is inserted into the oxygen gas introduction pipe 11. The flow rates of the inert gas and the oxygen gas are appropriately adjusted according to the size of the reaction tube 7 and the size of the substrate.

  The carrier gas containing the raw material and the oxygen gas are sent into the reaction tube 7, stirred by the mixer 12, and introduced to the heated substrate 13 surface. Since the substrate heater 15 is set at a temperature higher than that of the raw material heater 10, the raw material does not aggregate on the inner wall surface of the reaction tube 7.

  The raw material that has reached the substrate surface is thermally decomposed, and ruthenium oxide is deposited on the substrate in the presence of oxygen. In addition, since dipivaloylmethanate ruthenium which is a raw material contains six oxygen atoms with respect to one ruthenium atom, in principle, ruthenium oxide can be deposited without introducing oxygen. However, since carbon atoms are included, oxygen is reduced when consumed for carbon combustion, and oxygen-deficient ruthenium oxide is likely to be deposited, and impurities such as carbon may be mixed in the ruthenium oxide electrode. . Therefore, it is preferable to introduce oxygen gas within a range that prevents these.

  In addition, when dipivaloylmethanate ruthenium is dissolved in an organic solvent and the raw material vapor is introduced by bubbling, the inside of the reaction tube 7 is brought into a reducing atmosphere by the organic solvent. Therefore, it is necessary to introduce a large amount of oxygen. Arise. Therefore, ruthenium oxide having an oxygen deficiency is likely to be precipitated, and impurities such as carbon are likely to be mixed into the ruthenium oxide electrode. Therefore, it is more preferable to vaporize dipivaloylmethanate ruthenium by sublimation.

  The ruthenium oxide thus obtained has a fine structure in which the electrode interface reaction proceeds very easily, and the physical properties inherent to ruthenium oxide can be exhibited and maintained to the maximum. Furthermore, by using the specified high-purity dipivaloylmethanate ruthenium as a raw material, it is possible to suppress a decrease in the efficiency of raw material use due to the progress of side reactions during the raw material transport, etc., and to uniformly decompose the raw material on the solid electrolyte substrate To have a fine structure composed of particles having a uniform particle diameter. The ruthenium oxide electrode having such a fine structure has no change in the fine structure even if it is used for a long time at a high operating temperature, and there is no characteristic deterioration. Further, an increase in the interface resistance between the solid electrolyte substrate and the electrode is suppressed. The particle diameter can range from a cluster of several atoms to a particle of several tens of microns, and depends on the film thickness, but is about 1 nanometer to 100 microns.

  The ruthenium oxide electrode has an optimum film thickness depending on the shape and size of the solid electrolyte oxygen sensor. The film thickness can be controlled by conditions such as the amount of raw material supplied and the film formation time.

(Raw material vaporization test)
When synthesizing ruthenium dipivaloylmethanate by reacting ruthenium trichloride with dipivaloylmethane in the presence of an alkaline reaction accelerator, the mixture is refluxed for 20 hours in the range of 155 to 200 ° C. in a nitrogen atmosphere. A crude raw material was obtained, and the crude raw material was purified by column chromatography and further subjected to thermogravimetric analysis using dipivaloylmethanate ruthenium purified by sublimation as an example. The results are shown in FIG. As is apparent from the thermogravimetric curve, sublimation started at around 140 ° C. and complete sublimation at around 230 ° C. On the other hand, when a similar sublimation test was performed using acetylacetonate ruthenium (Ru (AcAc) ₃ ) as a comparative example, sublimation started from around 200 ° C. and sublimation ended at around 270 ° C. Thus, dipivaloylmethanate ruthenium has higher vapor pressure properties than acetylacetonate ruthenium at low temperatures. Moreover, there is little residue because of good vaporization stability. This suggests that dipivaloylmethanate ruthenium stably reaches the substrate.

(Formation of ruthenium oxide electrode)
Using the apparatus of FIG. 1, a ruthenium oxide electrode was formed on a yttria-stabilized zirconium oxide substrate. Dipivaloylmethanate ruthenium was introduced into the reaction tube together with oxygen gas using argon as a carrier gas. The carrier gas flow rate was 8.0 × 10 ⁻⁷ m ³ / s, the oxygen gas flow rate was 3.2 × 10 ⁻⁷ m ³ / s, and the film was formed at a substrate temperature of 650 ° C. for 60 minutes.

(Microstructure evaluation)
Using the obtained film as Example 1, observation of the fine structure by an electron microscope (SEM) and phase identification by X-ray diffraction (XRD) were performed. FIG. 3A shows the result of SEM observation of Example 1. It was found that spherical particles having a diameter of about 0.3 μm were produced. Moreover, as shown in FIG. 4, it was confirmed by XRD that the film was ruthenium oxide (RuO ₂ ). When this film was heat-treated in air at 850 ° C. for 8 hours, the microstructure was hardly changed as shown in FIG. This is probably because the ruthenium oxide electrode was formed from uniform particles having a uniform particle size, and there were few particles smaller than this, so that the coarsening phenomenon of crystal particles due to melting of the small particles did not occur. The reason why the generation of small particles was suppressed is thought to be that there were few by-product formation reactions that occurred outside the substrate using dipivaloylmethanate ruthenium as a raw material. Further, the fact that the microstructure is not changed is also due to the very good combination of RuO ₂ and the stabilized zirconium oxide substrate. The film thickness was 2-3 μm.

  A platinum electrode formed by sputtering was used as Comparative Example 1. The microstructure changed greatly by the heat treatment, and an agglomerated structure was observed. In contrast, Example 1 maintained an excellent microstructure and was found to be an electrode having excellent heat resistance.

Kim et al. (Electrochemical and Solid-State Letters, 4 (5) A62-A64 (2001), FIG. 2) sprayed a ruthenium chloride ethanol solution onto a heated substrate to produce a RuO ₂ film. The film obtained by this method is composed of fine particles as shown in Reference Example 1 in FIG. 5, but the film is very sparse.

(In Journal of The Electrochemical Society, 148 ( 3) A275-A278 (2001), FIG. 2) lim colleagues were deposited RuO ₂ film on the substrate by sputtering RuO _2. As shown in Reference Example 2 in FIG. 6, this film is composed of particles of about several tens of nm and is dense, but the size of the particles is not uniform.

  When Reference Example 1 in FIG. 5 and Reference Example 2 in FIG. 6 are compared with Example 1, Example 1 is characterized in that it is a thin film composed of uniform particles having a uniform particle diameter. Since it is comprised by the particle | grains of a uniform size, it is thought that it is the outstanding electrode which shows the stable electrode performance.

(Electrical characteristics evaluation)
A gold electrode and a lead wire were attached to the surface of the ruthenium oxide electrode of Example 1 by a gold paste method, and output characteristics (IV characteristics) were evaluated by a direct current method, and complex impedances were evaluated by an alternating current impedance method. Similarly, the output characteristics (IV characteristics) were evaluated by the direct current method on the platinum electrode surface formed by sputtering in Comparative Example 1 in the same manner. FIG. 7 shows the evaluation results of the output characteristics, and FIG. 8 shows the evaluation results of the complex impedance by the AC impedance method of Example 1.

(Comparative Example 2)
A paste containing ultrafine platinum powder was applied on a yttria-stabilized zirconium oxide substrate, and heat treatment was performed to obtain a platinum electrode.

(Comparative Example 3)
A paste containing iridium ultrafine powder was applied onto a yttria-stabilized zirconium oxide substrate, and heat treatment was performed to obtain an iridium electrode.

(Comparative Example 4)
A paste containing ultrafine palladium powder was applied on a yttria-stabilized zirconium oxide substrate, and heat treatment was performed to obtain a palladium electrode.

(Comparative Example 5)
A paste containing ultrafine rhodium powder was applied onto a yttria-stabilized zirconium oxide substrate, and heat treatment was performed to obtain a rhodium electrode.

(Comparative Example 6)
A paste containing ultrafine gold powder was applied on a yttria-stabilized zirconium oxide substrate, and heat treatment was performed to obtain a gold electrode.

  For Comparative Examples 2 to 6, the output characteristics were evaluated by the direct current method. FIG. 7 also shows the evaluation results of the output characteristics.

According to FIG. 7, in Example 1, the output characteristics changed linearly, and it was confirmed that good characteristics as an electrode were obtained. Furthermore, an output current of 52 A / m ² was shown at 500 ° C. and an applied voltage of 5.0 kV / m. This value is a very high value of 2 to 3 times that of a platinum electrode (Comparative Example 1) produced by sputtering.

  In addition, about Example 1, when the same output characteristic evaluation was performed about the sample heat-processed at 850 degreeC for 8 hours, a change was not seen by conductivity. This coincided with the result that the microstructure did not change before and after the heat treatment by SEM observation.

  FIG. 9 shows the results of evaluating the temperature dependence of the electrode interface resistance component of Example 1 and Comparative Example 1. It was found that the ruthenium oxide electrode of Example 1 had higher electrical conductivity than Comparative Example 1 within the range of 800 to 1250 ° C., and had good temperature-current characteristics. It can be seen that Example 1 is suitable as an electrode for a solid oxide oxygen sensor that requires a high operating temperature.

FIG. 8 shows the results when AC impedance analysis was performed using the electrode of Example 1. This figure shows that there are almost three semicircles. A small semicircle (C-10 ⁻¹² F) near the origin is independent of the type of electrode and is the bulk conductivity of the stabilized zirconium oxide substrate. The following right semicircle (C = 2.1 × 10 ⁻⁸ F) shows the conductivity of the grain boundary component of the substrate. The subsequent right quarter circle (C = 1.5 × 10 ⁻⁶ F) indicates the conductivity of the interface component between the substrate and the electrode. In Example 1, the quarter circle of (C = 1.5 × 10 ⁻⁶ F) due to electrode interface resistance greatly depends on the type of electrode, and the conductivity of the interface component between the substrate and the electrode is small. Since it is large, it can be seen that the electrode interface reaction proceeds very easily. This is also attributed to the compatibility between the ruthenium oxide electrode and the stabilized zirconium oxide, as described in the section on microstructure observation.

  From the above, it became clear that the ruthenium oxide film produced by the MOCVD method is an excellent electrode.

1, inert gas generation source 2, carrier gas flow rate controller 3, carrier gas supply means 4, oxygen gas generation source 5, oxygen gas flow rate controller 6, oxygen gas supply means 7, reactor 8, carrier gas introduction pipe 9, raw material Dish 10, heater for raw material heating 11, oxygen gas introduction pipe 12, mixer section 13, substrate 14, substrate holder 15, heater for substrate heating 16, pressure gauge 17, vacuum pump 18, exhaust duct 19, exhaust means 20, thermocouple 21, Needle valves 23, 24, Sealed flange 25, Ribbon heater 26, Raw material powder

Claims (3)

A ruthenium oxide electrode formed by MOCVD is formed on an oxygen ion conductive solid electrolyte substrate made of zirconium oxide containing a stabilizer, and is observed by surface observation with a scanning electron microscope (SEM). The solid electrolyte electrode is characterized in that the ruthenium oxide particles constituting the ruthenium oxide electrode have a particle diameter of 2 μm or less and satisfy the fine structure evaluation criteria of (Condition 1) .
(Condition 1) The ruthenium oxide electrode formed by the MOCVD method is heat-treated at 850 ° C. for 8 hours. After the treatment, the particle diameter of the ruthenium oxide particles observed by surface observation with a scanning electron microscope (SEM) is 2 μm. The following. 2. The electrode for solid electrolyte according to claim 1, wherein the microstructural evaluation criteria of (Condition 2) are satisfied .
(Condition 2) The ruthenium oxide electrode formed by the MOCVD method is heat-treated at 850 ° C. for 8 hours, and before and after the treatment, the ruthenium oxide particles observed by a scanning electron microscope (SEM) are coarsened crystal grains. There is no phenomenon.   The electrode for solid electrolyte according to claim 1 or 2, wherein the ruthenium oxide electrode has a thickness of 2 to 3 µm.