JP4608506B2 - Mixed ionic conductor and device using the same - Google Patents

Description

  The present invention relates to a mixed ionic conductor and an electrochemical device such as a fuel cell and a gas sensor using the same.

The present applicant has continued to develop a mixed ionic conductor of protons and oxygen ions (Patent Document 1, Patent Document 2, etc.). This mixed ionic conductor is basically a perovskite oxide based on barium and cerium, and a high ionic conductor is generated by substituting a part of cerium with a substitution element M ( general _{formula: BaCe 1-p M p O} 3-q). In particular, when the substitution amount p of the substitution element M is set to 0.16 to 0.23, it has high conductivity and is higher than the zirconia-based oxide (YSZ: yttria-stabilized zirconia) conventionally used as an oxygen ion conductor. High ionic conductivity is obtained. As the substitution element M, a rare earth element is appropriate, and in particular, a heavy rare earth element is optimal from the viewpoint of the atomic radius and charge balance.

  Electrochemical devices such as new fuel cells and sensors using this material as a solid electrolyte have also been developed. The discharge characteristics and sensor characteristics of the fuel cell using this material have been demonstrated to be superior and industrially superior. As these related patent applications, there are Patent Literature 3, Patent Literature 4, Patent Literature 5, Patent Literature 6, Patent Literature 7, Patent Literature 8, Patent Literature 9, Patent Literature 10, Patent Literature 11, and the like.

However, the above materials are not completely free from chemical stability. For example, barium may precipitate in carbon dioxide gas. In order to solve this problem, the present applicant presented a countermeasure in Patent Document 12.
Japanese Patent Laid-Open No. 5-28820 JP-A-6-231611 JP-A-5-234604 JP-A-5-290860 JP-A-6-223857 JP-A-6-290802 JP-A-7-65839 Japanese Patent Laid-Open No. 7-136455 JP-A-8-29390 JP-A-8-162121 Japanese Patent Laid-Open No. 8-22060 Japanese Patent Application Laid-Open No. 9-295866 (Japanese Patent Application No. 8-107918)

  However, the above measures are not perfect. For example, precipitation of barium is observed in a standing test at a low temperature of 85 ° C. in a humidity of 85% or a boiling test in water. Further, barium segregation is observed in the vicinity of the platinum electrode under a high water vapor pressure during discharge in a fuel cell. On the other hand, in gas sensors and the like, long-term retention of high ion conductivity at low temperatures and improvement in acid resistance of the oxide itself have been problems.

  Accordingly, an object of the present invention is to further improve the chemical stability of the mixed ionic conductor proposed by the present applicant.

  The main cause of corrosion due to humidity of the perovskite oxide is considered to be that segregated barium in the oxide once becomes barium hydroxide and then reacts with carbon dioxide to form stable barium carbonate. Therefore, in order to increase moisture resistance, the present invention uses a mixed ionic conductor made of the following perovskite oxide.

Mixed ionic conductor of the present invention has the formula _{Ba d Zr 1-xy Ce x} M 3 y O 3-t
(M ³ is at least one element selected from Bi, Ga, Sn, Sb, In, Nd, Sm, Eu, Gd, Tb, Yb, Y and Sc,
d is 0.98 or more and 1 or less,
x is 0.01 or more and 0.5 or less,
y is 0.01 or more and 0.3 or less,
t is (2 + y-2d) / 2)
It consists of the sintered compact of the perovskite type oxide represented by these.

In the mixed ionic conductor, M ³ is preferably at least one element selected from Nd, Sm, Eu, Gd, Tb, Yb, Y, Sc and In, particularly from Gd, In, Y and Yb. At least one element selected is preferred.

  The mixed ion conductor of the present invention has excellent moisture resistance while having conductivity necessary for electrochemical devices such as fuel cells.

  In the present specification, the rare earth element refers to Sc, Y, and lanthanoid (atomic number 57La to same number 71Lu). In the above formula, t is determined by the amount of oxygen vacancies with an indefinite ratio.

  The present invention also provides a device using the above mixed ionic conductor. That is, the fuel cell of the present invention is characterized by including the mixed ionic conductor as a solid electrolyte. The gas sensor of the present invention is characterized in that the mixed ion conductor is included as a solid electrolyte.

  According to the present invention, it is possible to provide an electric device such as a fuel cell and a gas sensor having high moisture resistance, high performance and long life.

  Hereinafter, preferred embodiments of the present invention will be described. The high conductivity of the mixed ionic conductor of the present invention is derived from the mixed ionic conductivity of oxygen ions and protons, as described in the above publication by the applicant. In order to improve the moisture resistance of the mixed ionic conductor, in the first mixed ionic conductor, an appropriate substitution element is introduced with the amount of barium in the perovskite oxide being less than the stoichiometric ratio. did. Hereinafter, this mixed ionic conductor is referred to as an “additive system” conductor.

  Moreover, according to this invention, the said 2nd mixed ion conductor and the said 3rd mixed ion conductor are provided as a mixed ion conductor with high moisture resistance. Hereinafter, these mixed ionic conductors are referred to as “barium zirconium-based” conductors and “barium zirconium cerium-based” conductors, respectively. In these systems, high moisture resistance is obtained while being a mixed ionic conductor exhibiting proton conductivity.

  The mixed ionic conductors of the above systems can be obtained by applying conventionally used raw materials and manufacturing methods. An example of the manufacturing method will be described later as an example.

  Hereinafter, examples of devices using the mixed ionic conductor of the present invention will be described.

  FIG. 1 is a perspective view of one embodiment of the fuel cell of the present invention. In this flat type fuel cell, an anode (air electrode) 1 and a cathode (fuel electrode) 3 are stacked with a solid electrolyte 2 interposed therebetween. And it has the structure where the separator 4 intervened between this lamination | stacking unit 7. FIG.

  During power generation, the anode 1 is supplied with an oxidizing gas 6 (for example, air), and the cathode 3 is supplied with a fuel gas 5 (for example, a reducing gas such as hydrogen or natural gas). Electrons generated along with the oxidation-reduction reaction at each electrode are taken out.

  FIG. 2 is a cross-sectional view of one embodiment of the gas sensor of the present invention. In this HC sensor (hydrocarbon sensor), an anode 15 and a cathode 16 are laminated via a solid electrolyte 14. This laminate is fixed on the substrate with an inorganic adhesive 18 so that a space is maintained between the laminate (ceramic substrate) 17. The internal space 20 is electrically connected to the outside through the diffusion rate limiting hole 13.

  In this sensor, if a state in which a predetermined voltage (for example, 1.2 V) is applied between the two electrodes 15 and 16 is maintained, a current value corresponding to the concentration of hydrocarbon existing in the space in contact with the anode 15 is obtained as an output. During measurement, the sensor is held at a predetermined temperature by a heater 19 attached to the substrate. In order to limit the inflow amount of the measurement species (hydrocarbon) flowing into the internal space 20, it is preferable to provide the diffusion rate controlling hole 13.

  In the above description, the HC sensor is described. However, in the configuration shown in the figure, an oxygen sensor can be formed by replacing the anode and the cathode. The mixed ionic conductor of the present invention is not limited to the above and can be applied to various electrochemical devices.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not restrict | limited by a following example.

In this example, perovskite oxides shown in (Table 1) to (Table 3 ) were synthesized. Each oxide was synthesized using a solid phase reaction method. Oxidized powders of each element such as barium, cerium, zirconium, rare earth elements were weighed so as to have the composition ratios in the table, and pulverized and mixed in an agate mortar using an ethanol solvent. After sufficiently mixing, the solvent was scattered, degreased using a burner, and pulverization and mixing were repeated in an agate mortar again. Thereafter, it was press-molded into a cylindrical shape and baked at 1300 ° C. for 10 hours. After firing, the mixture was coarsely pulverized and further granulated to about 3 μm by planetary ball mill pulverization in a benzene solvent. The obtained powder was vacuum-dried at 150 ° C. and then formed into a cylinder by isostatic pressing at 2 ton / cm ² and immediately fired at 1650 ° C. for 10 hours to synthesize a sintered body. For most samples, a sufficiently dense single phase perovskite oxide was obtained. The following items were evaluated for each sample thus obtained.

Boiling test As an accelerated test of the moisture resistance test, a sample was put into boiling water at 100 ° C, and the degree of Ba precipitation was evaluated by the pH value after 10 hours. This is an evaluation method using the fact that the pH value in the aqueous solution increases with the precipitation of barium. Humidity resistance is excellent when the pH change is 2 or less (A), better than 2 and more than 3.5 (B), better than 3.5 and less than 4 (C), more than 4 The case was determined to be defective (D).

Conductivity Next, the sample after the boiling test is processed from a cylindrical sintered body into a disk shape having a thickness of 0.5 mm and a diameter of 13 mm, and platinum is formed so that each of the surfaces of the disk has an area of 0.5 cm ^2. A paste was applied, baked, and used as a sample for measuring ionic conductivity. The conductivity of this sample was calculated from the resistance value by the AC impedance method in air. The measurement temperature is 500 ° C. The lead resistance component in the measuring device was completely corrected. The electrical conductivity (S / cm) was determined to be 0.007 or more as A, 0.001 or more and less than 0.007 as B, and less than 0.001 as C.

  In addition, in FIG. 3, an example of the electrical conductivity of this invention material is shown with an Arrhenius plot.

Crystallinity After sintering, it was determined that A was a single phase, B was a multiphase, and C was a case where sintering was not possible. Moreover, each table | surface is shown together with the electrical conductivity in each 500 degreeC, and the pH evaluation result of a boiling test.

  As is apparent from the evaluation results, the mixed ionic conductor of the present invention has significantly improved moisture resistance and also has a practical level of ionic conductivity.

  In this embodiment, the synthesis was performed using the solid-phase sintering method, but the present invention is not limited to this. For example, the oxide may be synthesized using a method such as a coprecipitation method, a nitrate method, or a spray granulation method. Alternatively, a film formation method such as a CVD method or a sputtering method may be applied. Moreover, you may produce by thermal spraying. The shape of the oxide is not limited, and may be any shape including a bulk and a film.

1 is a cross-sectional cutaway view showing one embodiment of a fuel cell using a mixed ion conductor of the present invention. It is sectional drawing which shows one form of the gas sensor using the mixed ion conductor of this invention. It is a graph which shows the example of the electrical conductivity of the mixed ion conductor of this invention.

1,15 Anode (air electrode)
2,14 Solid electrolyte 3,16 Cathode (fuel electrode)
4 Separator 5 Fuel gas (hydrogen, natural gas)
6 Oxidizing gas (air)
7 Laminate unit 13 Diffusion-controlled hole 17 Substrate 18 Inorganic adhesive 19 Heater

Claims (3)

Formula _{Ba d Zr 1-xy Ce x} M 3 y O 3-t
(M ³ is at least one element selected from Bi, Ga, Sn, Sb, In, Nd, Sm, Eu, Gd, Tb, Yb, Y and Sc,
d is 0.98 or more and 1 or less,
x is 0.01 or more and 0.5 or less,
y is 0.01 or more and 0.3 or less,
t is (2 + y-2d) / 2)
A mixed ionic conductor comprising a sintered body of a perovskite oxide represented by the formula :   A fuel cell comprising the mixed ionic conductor according to claim 1 as a solid electrolyte.   A gas sensor comprising the mixed ionic conductor according to claim 1 as a solid electrolyte.

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