JP5717067B2 - Composite cathode material for solid oxide fuel cell operating at medium and low temperature, composite cathode for solid oxide fuel cell, and method for producing electrolyte-composite cathode structure for solid oxide fuel cell - Google Patents

Description

  The present invention relates to a composite cathode material for a solid oxide fuel cell (hereinafter referred to as SOFC). More specifically, the present invention relates to a composite material of two kinds of conductive powder, which is 400 ° C to 700 ° C, preferably 450 ° C. The present invention relates to a composite cathode material for SOFC fuel cells and a method for producing the same, which exhibits good conduction characteristics at medium to low temperatures in the range of ˜600 ° C. (hereinafter simply referred to as “medium temperature”).

  New power generation fuel cells have been viewed as promising for a continuous increase in electrical energy demand. Among them, the development of SOFC is rapid, and it is operated at 800 to 1000 ° C., and part of it has been put into practical use. These are used for stationary generators that can easily realize high-temperature environments, but have not yet been realized for use in movable generators such as automobiles. If the operating temperature can be set in the mid-low temperature range, generators that can be used in a wide range including movable types will be produced, and the economic, global environmental and political impacts will be great, and a huge market is expected to be opened. Is done.

  There are two types of electrolytes used for SOFC: protons, ions, and electron conduction. Batteries that operate at medium and low temperatures have low activation energy and need to use proton conduction activated at lower temperatures, and development of proton conduction SOFCs is urgently needed. In proton-conducting SOFC, protons react with air at the cathode, and therefore the cathode material preferably has proton conductivity and excellent electrical conductivity. Moreover, since it is necessary to evaporate the water of the reaction product at the cathode, a larger surface area is better. Accordingly, a highly electrically conductive metal oxide solid (ceramics) that is porous and has proton conductivity is required as a cathode material for medium-low temperature operation SOFCs. Conventional electron-conducting SOFCs have the disadvantage that the reaction takes place at the anode and the product water dilutes the fuel, but the proton-conducting SOFC is a cathodic reaction that can overcome the disadvantages.

An oxide conductive material is zirconium oxide (zirconia, ZrO ₂ ), which exhibits electrical conduction at high temperatures. Yttria-stabilized zirconia (YSZ) in which yttrium oxide (yttria, Y ₂ O ₃ ) is dissolved is also well known as an electron conductor. BaZr _1-xy Pr _x Y _y O _{3-δ in} which other elements are dissolved in this (0.1 ≦ x ≦ 0.4, 0 <y ≦ 0.2, δ is oxygen deficiency, hereinafter BZPY Is a proton conductor, and it has been found that adjusting the compositions X and Y has both proton conductivity and electron conductivity (hereinafter referred to as proton-electron conductivity) and contributes to electrical conduction. Yes. On the other hand, La _1-x Sr _x Co _y Fe _1-y O _{3-δ of} the La—Sr—Co—Fe—O-based compound (0.2 ≦ x ≦ 0.6, 0 ≦ y ≦ 0.4, δ Is oxygen deficiency) (hereinafter referred to as LSCF) is a conductor having both oxygen ion conductivity and electron conductivity (hereinafter referred to as oxygen ion-electron both conductivity), and exhibits excellent electrical conductivity, It is used as a material for SOFC and also used as a cathode. Furthermore, it is also known that when a plurality of solid electrically conductive materials are combined, an excellent cathode material for SOFC can be obtained and the efficiency of SOFC can be improved.

  The present inventor studied what kind of oxide is useful for improving the electrical conductivity in the combination of a plurality of solid electrically conductive materials, and reached the present invention. It is an object of the present invention to provide a composite cathode material for SOFC that operates at medium and low temperatures and has excellent electrical conductivity.

According to one aspect of the present invention, a solid oxide fuel cell comprising a first solid oxide having proton conductivity or proton-electron conductivity and a second solid oxide having oxygen ion-electron conductivity. A composite cathode material is provided.
Here, the composition of the first solid oxide is BaZr _1-xy Pr _x Y _y O _3-δ (0.1 ≦ x ≦ 0.4, 0 <y ≦ 0.2), The composition of the second solid oxide is La _1-x Sr _x Co _y Fe _1-y O _3-δ (0.2 ≦ x ≦ 0.6, 0 ≦ y ≦ 0.4, where δ is a variation due to oxygen deficiency. And 0 ≦ δ ≦ 0.3). The composite cathode material for a solid oxide fuel cell according to claim 1.
Further, at least one of the first oxide and the second oxide may be a powder obtained by a combustion synthesis method.
The first oxide may have both proton-electron conductivity.
The first oxide may have a composition of BaZr _0.5 Pr _0.3 Y _0.2 O _3-δ .
According to another aspect of the present invention, there is provided a composite cathode for a solid oxide fuel cell, which is a sintered body of any one of the above composite cathode materials for a solid oxide fuel cell.
Here, the particle diameter of the composite cathode for a solid oxide fuel cell may be 50 nm to 1 μm, and the porosity may be 0.3 to 0.7.
According to still another aspect of the present invention, an electrolyte-composite cathode structure for a solid oxide fuel cell, which co-fires a proton conductive solid electrolyte and any one of the above composite cathode materials for a solid oxide fuel cell. The manufacturing method is given.

  The composite cathode material for SOFC of the present invention has high air conduction at medium and low temperatures, and is suitable for an SOFC electrolyte-composite cathode structure. Furthermore, according to the production method of the present invention, an excellent electrolyte-composite cathode structure made of fine powder and having an appropriate porosity can be obtained.

_{(A) LSCF-BaZr 0.7 Pr} 0.1 Y 0.2 O 3-δ ( hereinafter, referred to as BZPY10) illustrates a possible reaction pathways for the composite cathode. _{(B) LSCF-BaZr 0.5 Pr} 0.3 Y 0.2 O 3-δ ( hereinafter, referred to as BZPY30) illustrates a possible reaction pathways for the composite cathode. In the figure, the large sphere on the left represents LSCF particles, and the small sphere on the right represents BZPY10 / PZPY30 particles. Subscripts g and ad indicate a gas state and an adsorption state, respectively. The crescent-shaped part of the color different from the surrounding shown between the electrolyte and the LSCF particles, between the LSCF particles and the BZPY10 / BZPY30 particles, and in the upper right part of the BZPY30 particles is an active region in which the cathode reaction can occur. It is. (A) is (i) LSCF powder calcined at 1100 ° C. for 3 hours, (ii) BZPY10 powder calcined at 1100 ° C. for 6 hours, and (iii) PZPY30 powder calcined at 100 ° C. for 6 hours, and 1000 (Iv) XRD pattern of (LS) LSCF-BZPY10 mixed powder and (v) LSCF-BZPY10 mixed powder, (b) SEM photograph of LSCF powder, (c) SEM photograph of BZPY10 powder, (d) BPY30 SEM photograph of powder. (A) and (d) are SEM photographs of cross sections of LSCF-BZPY10 composite cathode co-fired at 1000 ° C. for 3 hours on dense BZPY10 electrolyte pellets, and (b) and (c) are dense BZPY10. The cross-sectional SEM photograph of LSCF-BZPY30 baked simultaneously at 1000 degreeC on the electrolyte pellet for 3 hours. ASR graph versus reciprocal temperature for LSCF-BZPY10 and LSCF-BZPY30 composite cathodes in humidified O ₂ . For the LSCF-BZPY30 composite cathode in the examples of the present invention, measured values of unit area electric conductivity (ASR) measured at 500 ° C. to 700 ° C. (circles), and other combinations according to the prior art The figure which shows the ASR measured value (solid line 1, 2, 3) of the composite cathode material for SOFC. The figure shows that at medium and low temperatures, the material of the present invention has a lower electrical resistance than the prior art material.

  In SOFC, BZPY, which is a proton conductor or both proton and electron conductors (hereinafter referred to as proton-electron both conductors), and both oxygen ion and electron conductors (hereinafter referred to as both oxygen ion-electron conduction). When the cathode is formed by combining LSCF, which is called a body, the cathode reaction is accelerated and a cathode material with high electrical conductivity is obtained.

Proton H ⁺ generated in the solid electrolyte travels to the cathode, quickly moves in the proton conductor such as BZPY or the proton-electron conductor in the cathode, and both oxygen ion-electron conduction such as LSCF as described in detail below. Water is generated by causing a reduction reaction with electrons and oxygen ions O- ^{2 on the} surface of the body (even when the proton-electron both conductors are used). For this reason, the electron resistance is lowered in the cathode formed using the composite cathode material formed from the two kinds of conductors such as the combination of BZPY and LSCF. Furthermore, the cathode is preferably made of fine powder, has an appropriate porosity, and promotes the reaction. According to the method of the present invention described below, a cathode material having such a structure can be realized.

  The composite cathode material for SOFC is synthesized from two kinds of fine powders, a solid oxide fine powder which is a conductor or a proton-electron both conductor and a solid oxide fine powder which is an oxygen ion-electron both conductor. Both fine powders are preferred as the particles are finer. Of course, the synthesis of these fine powders is not intended to be limited to this. However, for example, it is preferable to synthesize a chelated solution by adding an organic substance and a solvent to each metal compound and synthesize it using a combustion synthesis method. These fine powders are mixed and compounded to obtain an oxide solid electrolyte composite cathode material.

BZPY which is a proton conductor or a proton-electron both conductor is represented by BaZr _1-xy Pr _x Y _y O _3-δ (0.1 ≦ x ≦ 0.4, 0 <y ≦ 0.2). Here, the range of δ is 0 ≦ δ ≦ 0.3. Whether BZPY exhibits proton conductivity or electron conductivity depends on the compositions x and y. When x = 0.1 (BZPY10), the proton conductor is almost pure in humidified air. When the value of x is larger than this (for example, 0.3; BZPY30), a proton-electron conductor is obtained.

LSCF which is an oxygen ion-electron both conductor is La _1-x Sr _0x Co _y Fe _1-y O _3-δ (0.2 ≦ x ≦ 0.6, 0 ≦ y ≦ 0.4, δ is oxygen deficiency Here, the range of δ is 0 ≦ δ ≦ 0.3. An excellent SOFC cathode can be obtained when a cathode is made of a composite material obtained by combining both powders.

  Here, FIG. 1 shows a chemical reaction system that occurs at a composite cathode of a proton-electron conductor (or proton conductor) such as BZPY and an oxygen ion-electron conductor such as LSCF of the present invention. FIG. 1A shows a reaction path that can occur when a proton conductor (for example, BZPY10) is used, and FIG. 1B shows a reaction path that can occur when both a proton-electron conductor (for example, BZPY30) is used. As can be seen from FIG. 1, when a particle having proton-electron conductivity is used as one material of the composite cathode, both electrons and protons can flow through the particle. Reduction and subsequent reaction with protons can occur directly (see the active region in the upper right of the right particle in FIG. 1 (b)). This is a characteristic that cannot be obtained when a pure proton conductor is used (FIG. 1A). Thus, in the case of FIG. 1 (b), water generation can occur at the surface of the proton-electron conductor in addition to the electrolyte-cathode interface and the proton-electron conductor contact in FIG. 1 (a). Therefore, the area where the cathode reaction can occur is larger.

[Description of Examples]
As the composite cathode material for SOFC of the present invention, BaZr _1-xy Pr _x Y _y O _2-δ (0.1 ≦ x ≦ 0.4, 0 <y ≦ 0.2) and La _0.6 Sr are described below. A composite of two materials of _0.4 Co _0.2 Fe _0.8 O _3-δ will be described as an example. The uniform powder of each composition is synthesized by calcining a metal oxide powder of each element. Although not limited to this, the combustion synthesis method is most effective for producing a uniform fine powder.

  In combustion synthesis, a compound of each constituent metal element, for example, an oxide or a nitric acid compound is chelated with a solvent and an organic component, and is converted into a gel or a solid by a treatment such as pH adjustment or heating. It is burned with a flame to form a uniform fine powder, and further calcined at a high temperature for synthesis.

  The BZPY and LSCF powders are dispersed in an appropriate liquid by a conventional method, and mixed thoroughly with a mortar or mechanical mixer to form a mixed powder, which is then molded. In this way, a molded product composed of two kinds of particles at about 1 μm to 50 nm was obtained.

As a starting material for composite cathode materials for SOFC, Ba (NO ₃ ) ₂ , Zr (NO ₃ ) ₂ .2H ₂ O, Pr ₆ O ₁₁ and Y (NO ₃ ) ₃ · 6H ₂ O powder are used for BZPY. Using. For LSCF, La (NO ₃ ) ₃ · 6H ₂ O, Sr (NO ₃ ) ₂ , Co (NO ₃ ) ₂ · 6H ₂ O and Fe (NO ₃ ) ₃ · 9H ₂ O powder were used. These were dissolved in an aqueous nitric acid solution, and chelated with citric acid at a ratio of 2: 1 to the total metal elements to form a solution. When the pH was adjusted to 6 to 8 for LSCF and 3 to 4 for BZPY with ammonia water and stirred at 80 ° C., gelation occurred. The gel was burned with a flame to form a powder and then calcined at 1100 ° C. for 6 hours for BZPY and at 1000 ° C. for 3 hours for LSCF. By adjusting the mixing ratio of the raw materials, the composition is BaZr _0.7 Pr ₀₁ Y _0.2 O _3-δ (denoted as BZPY10), BaZr _0.5 Pr _0.3 Y _0.2 O _3-δ (with BZPY30) And a fine powder of LSCF (La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O _3-δ ). BZPY10 is an electrolyte used when evaluating the composite cathode material.

  Further, BZPY10 powder and LSCF powder mixed at a weight ratio of 1: 1 and BZPY30 powder and LSCF powder mixed at a weight ratio of 1: 1 were fired at 1000 ° C. for 3 hours, respectively. The products LSCF-BZPY10 and LSCF-BZPY30 were obtained.

  FIG. 2 shows XRD patterns of LSCF, BZPY10, BZPY30, LSCF-BZPY10 and LSCF-BZPY30 and SEM photographs of LSCF, BZPY10 and BZPY30 obtained as described above. In the diffraction pattern of FIG. 2A, since only diffraction lines related to BZPY10 / PZPY30 and LSCF are observed in the XRD patterns of LSCF-BZPY10 and LSCF-BZPY30, two types of oxidation constituting these composite cathodes are observed. It was suggested that no chemical reaction occurred between the objects.

  Powders obtained by weighing 1: 1 the synthesized BZPY30 and LSCF powder were mixed in a mortar, 10% by weight of terpineol or ethyl cellulose was added, and mechanically mixed to form.

  In order to evaluate the composite material as described above, the LSCF-BZPY30 composite cathode was baked at 1600 ° C. for 8 hours in the air on both sides of the sintered BZPY10 electrolyte pellets, and the LSCF-BZPY30 composite was obtained. An electrolyte-composite cathode structure in which a cathode-BZPY10 electrolyte-LSCF-BZPY30 composite cathode was laminated in this order was produced. The composite cathode in the structure pair thus prepared was an aggregate of particles having an average particle diameter of about 100 nm as measured from an SEM structural photograph, and its porosity was 50%. In addition, by using BZPY10 instead of BZPY30, a laminate having the same structure was prepared and co-fired under the same conditions, whereby an electrolyte-composite cathode structure having the same structure except that BZPY10 was used was also prepared. FIG. 3 shows cross-sectional SEM photographs of the two electrolyte-composite cathode structures thus prepared. All of these electrolyte-composite cathode structures have a fine microstructure and contain relatively small particles on average. Thereby, good adhesion between the large surface area and the electrolyte was obtained. In the present invention, the average particle diameter and the porosity generally have values in the range of 50 nm to 1 μm and 0.3 to 0.7, respectively.

  Electrodes were baked at both ends of these electrolyte-composite cathode structures at 1000 ° C. for 3 hours, and measurements for evaluation were performed using a multi-channel potentiometer and an electrochemical impedance meter. The evaluation was performed by unit area electrical conductivity (ASR (cathode area specific resistance), where ASR = RxA / 2, R: electrical resistance, A: area).

  FIG. 4 shows the relationship between the ASR in the humidified oxygen atmosphere and the reciprocal of the temperature. Both samples gave good results, but in particular the ASR value of the material using BZPY30 is 4-6 times higher than the similar composite electrode-electrolyte material synthesized so far (low electrical conductivity). Resistance).

[Comparison with comparative example]
FIG. 5 is a graph showing the relationship between the reciprocal of ASR vs. temperature in humid air of LSCF-BZPY30 among the examples of the present invention, and a composite that has been reported so far as a comparative example. It is the graph which added the value described in the said literature about the ASR of the cathode electrode material of the system as solid lines 1-3. ASR of LSCF-BZPY30 was 0.15Omucm ² in 1 .OMEGA.cm ^2, 600 ° C. at 500 ° C.. The solid line 1 is the measurement result of the PrBaCo ₂ O _{5 + δ} -BaZr _0.1 Ce _0.7 Y _0.2 O _3-δ composite system reported in Non-Patent Document 16, and the ASR is 1.669 Ωcm ² at 600 ° C. there were. The solid line 2 is the measurement result of the LSCF-BaCe _0.9 Yb _0.1 O _3-δ (BCYb) composite system reported in Non-Patent Document 15, and the electric resistance is about 6 times higher at 600 ° C. than the present invention. The solid line 3 is the measurement result of the Sm _0.5 Sr _0.5 CoO _3-δ -BaCe _0.8 Sm _0.2 O _3-δ composite system reported in Non-Patent Document 17, and the ASR is 600 ° C. The electric resistance is about 4 times higher than that of LSCF-BZPY30 which is an embodiment of the invention.

  As can be seen from the above examples and comparative examples, when the LSCF-BZPY-based solid electrolyte oxide composite cathode material of the present invention is used, a low electrical resistivity can be realized.

  The LSCF-BZPY composite material is an excellent composite cathode material for SOFC, and can be synthesized from metal powder without applying a special method. The raw material powder is synthesized by a combustion synthesis method because a fine powder can be obtained and is a process that can be industrially applied. By using this material, the performance of a fuel cell operating at a low temperature is greatly improved, and a portable fuel cell is also possible. Therefore, the industrial application potential of the present invention is great.

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Claims (7)

In a composite cathode material for a solid oxide fuel cell comprising a first solid oxide having proton conductivity or proton-electron conductivity and a second solid oxide having oxygen ion-electron conductivity,
The composition of the first solid oxide is BaZr _1-xy Pr _x Y _y O _3-δ (0.1 ≦ x ≦ 0.4, 0 <y ≦ 0.2), and the second solid oxide The composition of the solid oxide is La _1-x Sr _x Co _y Fe _1-y O _3-δ (0.2 ≦ x ≦ 0.6, 0 ≦ y ≦ 0.4, δ is a variation due to oxygen deficiency, 0 range of ≦ δ ≦ 0.3), solid bodies oxide fuel cell composite cathode material.   The composite cathode material for a solid oxide fuel cell according to claim 1, wherein at least one of the first solid oxide and the second solid oxide is a powder obtained by a combustion synthesis method.   The composite cathode material for a solid oxide fuel cell according to claim 1 or 2, wherein the first solid oxide has proton-electron conductivity. The first solid oxide has a composition _{BaZr 0.5 Pr 0.3 Y 0.2 O 3} -δ, composite cathode material for a solid oxide fuel cell according to claim 3.   A composite cathode for a solid oxide fuel cell, which is a sintered body of the composite cathode material for a solid oxide fuel cell according to any one of claims 1 to 4.   The composite cathode for a solid oxide fuel cell according to claim 5, wherein the particle diameter is 50 nm to 1 μm and the porosity is 0.3 to 0.7. A method for producing an electrolyte-composite cathode structure for a solid oxide fuel cell, comprising simultaneously firing a proton conductive solid electrolyte and the composite cathode material for a solid oxide fuel cell according to any one of claims 1 to 4. .