Mixed ionic conductor and device using the same

Abstract: A mixed ionic conductor with an ion conductive oxide has a perovskite structure, e.g. of the formula Ba a (Ce 1-b M 1 b )L c O 3-α , wherein M 1 is at least one trivalent rare earth element other than Ce; L is at least one element selected from the group consisting of Zr, Ti, V, Nb, Cr, Mo, W, Fe, Co, Ni, Cu, Ag, Au, Pd, Pt, Bi, Sb, Sn, Pb and Ga; with 0.9 ≤ a ≤ 1; 0.16 ≤ b ≤ 0.26; 0.01 ≤ c ≤ 0.1; and (2+b-2a)/2 ≤ α ≤ < 1.5. Such a mixed ionic conductor has not only the necessary conductivity for electrochemical devices such as fuel cells, but also superior moisture resistance.

Description: The present invention relates to a mixed ionic conductor and an electrochemical device, such as a fuel cell or a gas sensor, using the same. The applicant has long been actively developing mixed conductors of protons and oxide ions (see for example Publication of Unexamined Japanese Patent Application (Tokkai) No. H5-28820 or H6-236114). These mixed ionic conductors are basically perovskite oxides containing barium and cerium wherein a portion of the cerium has been substituted by the substitute element M, so as to achieve a high ionic conductivity (chemical formula: BaCe 1-p M p O 3-α ). Especially, when the substitution amount p of the substitution element M is 0.16 to 0.23, the mixed ionic conductor has a high conductivity, higher even than zirconia-based oxides (YSZ: yttrium-stabilized zirconia), which conventionally have been used as oxide ionic conductors. As the substitution element M, rare earth elements are suitable, in particular heavy rare earth elements, because of their atomic radius and charge balance. New fuel cells, sensors and other electrochemical devices using such materials as a solid electrolyte have been developed. The sensor characteristics and the discharge characteristics of fuel cells using such materials have been shown to be superior to prior devices. Other patent applications related to these materials are Tokkai H5-234604, Tokkai H5-290860, Tokkai H6-223857, Tokkai H6-290802, Tokkai H7-65839, Tokkai H7-136455, Tokkai H8-29390, Tokkai H8-162121, and Tokkai H8-220060. However, these materials show some problems with regard to their chemical stability. For example, barium tends to precipitate in CO 2 gas. To solve these problems, the applicant has proposed a counter-strategy in Tokkai H09-295866 (JP A H09 (1997)-295866). However, even this counter-strategy is not perfect, and for example at low temperatures of 85°C and 85% humidity, precipitation can be observed in shelf tests and boiling tests in water. Moreover, under high water vapor pressures as during discharge of the fuel cells, barium can be seen to precipitate near the platinum electrodes. Furthermore, with gas sensors, there is the problem of maintaining high ion conductivity at lower temperatures over a long time and the problem of raising the acid resistance of the oxide itself. To solve these problems, it is desired to improve the chemical stability of the mixed ionic conductors. The main cause for decomposition of the oxides due to humidity is believed to be the fact that the segregated barium turning into barium hydroxide reacts with the carbon dioxide, and forms stable barium carbonate. To increase the moisture resistance, the present invention uses a mixed ionic conductor including the following perovskite structure oxide. A mixed ionic conductor according to the present invention comprises an ion conductive oxide having a perovskite structure of the formula Ba a (Ce 1-b M 1 b )L c O 3-α , wherein M 1 is at least one trivalent rare earth element other than Ce; L is at least one element selected from the group consisting of Zr, Ti, V, Nb, Cr, Mo, W, Fe, Co, Ni, Cu, Ag, Au, Pd, Pt, Bi, Sb, Sn, Pb and Ga; with 0.9 ≤ a ≤ 1; 0.16 ≤ b ≤0.26; 0.01 ≤ c ≤ 0.1; and (2+b-2a)/2 ≤ α < 1.5. In this mixed ionic conductor it is preferable that M 1 is at least one element selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y and Sc. More preferably, M 1 is Gd and/or Y. It is also preferable that L is at least one element selected from the group consisting of Zr, Ti, Fe, Co, Ni, Cu, Bi, Sn, Pb and Ga. More preferably, L is at least one element selected from the group consisting of Zr, Ti, Bi, Pb and Ga. The preferred mixed ionic conductor not only has the necessary conductivity for electrochemical devices such as fuel cells, but also superior moisture resistance. Throughout this specification, "rare earth element" means Sc, Y, and the lanthanides (elements 57La through 71Lu). In the above formula, α, is determined by the absent amount of disproportionate oxygen. The present invention also provides devices using such a mixed ionic conductor. A fuel cell in accordance with the present invention includes as a solid-state electrolyte a mixed ionic conductor as described above. A gas sensor in accordance with the present invention includes as a solid-state electrolyte a mixed ionic conductor as described above. Using the mixed ionic conductors of the present invention provides electric devices, such as fuel cells and gas sensors, with high moisture resistance, high performance, and long lifetimes. Various embodiments of the present invention will now be described, by may of example only, and with reference to the accompanying drawings in which: Fig. 1 is an exploded perspective cross-sectional view of an embodiment of a fuel cell using a mixed ionic conductor; Fig. 2 is a cross-sectional view of an embodiment of a gas sensor using a mixed ionic conductor; and Fig. 3 is a graph showing the conductivity of mixed ionic conductors. The following is an explanation of the preferred embodiments of the present invention. As the applicant has pointed out in the above-noted publications the high conductivity of mixed ionic conductors in accordance with the preferred embodiment stems from the mixed ion conductivity of oxygen ions and protons. In order to improve the moisture resistance of such mixed ionic conductors, a suitable substitute element is introduced into the above-mentioned first ionic conductor so as to reduce the amount of barium in the perovskite oxide to less than the stochiometric ratio. In the following, such a mixed ionic conductor is referred to as an "additive system" conductor. Mixed ionic conductors can be obtained with conventional raw materials and manufacturing methods. Specific examples of manufacturing methods are explained along with the examples further below. The following is an explanation of a device using a mixed ionic conductor in accordance with the preferred embodiment. Fig. 1 is a cross-sectional perspective view of an embodiment of a fuel cell in accordance with a preferred embodiment. This planar fuel cell has several layered units 7, which include a cathode (fuel electrode) 3, a solid electrolyte 2 layered on the cathode 3, and an anode (air electrode) 1 on the solid electrolyte 2. Separators 4 are arranged between the layered units 7. When generating power, an oxidation gas 6 (such as air) is supplied to the anodes 1, and a fuel gas 5 (a reduction gas such as hydrogen or natural gas) is supplied to the cathodes 3. The oxidation-reduction reaction at the electrodes generates electrons, so that the fuel cell serves as an electric power source. Fig. 2 is a cross-sectional view of an embodiment of a gas sensor in accordance with another embodiment. This HC sensor (hydrocarbon sensor) includes an anode 15, a solid electrolyte 14 on the anode 15, and a cathode 16 on the solid electrolyte 14. This layered structure is attached with an inorganic adhesive 18 to a (ceramic) substrate 17, providing a space 20 between the substrate and the layered structure. This space 20 is in communication with the outside via a diffusion limiting hole 13. When a certain voltage (for example 1.2V) is applied steadily between the two electrodes 15 and 16, a current that is proportional to the concentration of hydrocarbons in the space adjacent to the anode 15 is attained as output. During the measurement, the sensor is kept at a certain temperature with a heater 19 attached to the substrate. To provide the diffusion limiting hole 13 is advantageous to limit the inflow of the material to be measured (here, hydrocarbons) into the space 20. This embodiment has been explained for a HC sensor, but an oxygen sensor is also possible by exchanging anode and cathode in the structure shown in Fig. 2. Furthermore, the preferred mixed ionic conductor is not limited to the above, but also can be applied to all kinds of other electrochemical devices. Examples The following is a more detailed description of specific examples of the present invention. It should be noted that the present invention is in no way limited to these examples. As examples of the present invention, oxides as shown in Tables 1 to 4 have been synthesized. These oxides were synthesized by solid state sintering. An oxide powder of barium, cerium, zirconium, and rare earth elements was weighed to the composition ratio listed in the tables, and crushed and mixed with ethanol in an agate mortar. After sufficient mixing, the solvent was removed, defatted with a burner, and crushing and mixing were- repeated in the agate mortar. Then, the samples were pressed into columnar shape and fired for 10 hours at 1300°C. After the firing, granules of about 3µm were produced by coarse crushing, with further crushing in a benzene solution with a planetary ball mill. The resulting powder was vacuum-dried at 150°C, and columns were formed with a hydrostatic press at 2tons/cm 2 , which were immediately fired for 10 hours at 1650°C to synthesize a sintered product. For almost all samples, a very compact single-phase perovskite oxide was attained. The resulting samples were then evaluated as follows: · Boiling test As an accelerated test of moisture resistance, the samples were introduced into boiling water of 100°C, and the level of Ba precipitation was evaluated after 10 hours by measuring the pH value. This evaluation utilizes the fact that the pH value in the aqueous solution rises proportionally with the precipitation of barium. For a pH change of not more than 2, the moisture resistance was taken to be excellent (A), for more than 2 and not more than 3.5, it was taken to be good (B), for more than 3.5 and not more than 4, it was taken to be adequate (C), and for more than 4, it was taken to be poor (D). · Conductivity After the above-mentioned boiling test, disks of 0.5mm thickness and 13mm diameter were made of the columnar sintered product samples, both sides of the disks were coated with a platinum paste on an area of 0.5cm 2 each, which was baked onto the samples, and the ion conductivity was measured. In this experiment, the conductivity was calculated from the resistance with the alternating current impedance method in air. The measurement temperature was 500°C. The resistance of the leads of the measurement device was subtracted. When the conductivity (in S/cm) was at least 0.007, it was taken as A, for at least 0.001 and less than 0.007 it was taken as B, and for less than 0.001 it was taken as C. Fig. 3 is an arrhenius plot showing the conductivity of materials. · Crystallinity When the sintered product was single-phase it was taken as A, when it was multi-phase, it was taken as B, and sintering failures were taken as C. The tables show the conductivity at 500°C and the result of the pH evaluation in the boiling test. Material Boiling Test Crystallinity Conductivity BaCe 0.8 Gd 0.2 O 3-α D A A Ba 0.99 Ce 0.8 Gd 0.2 O 3-α D A A Ba 0.98 Ce 0.8 Gd 0.2 O 3-α D A A Ba 0.94 Ce 0.8 Gd 0.2 O 3-α D A B Ba 0.90 Ce 0.8 Gd 0.2 O 3-α D A B Additive System Material Boiling Test Crystallinity Conductivity BaCe 0.8 Gd 0.2 Zr 0.01 O 3-α B A A BaCe 0.8 Gd 0.2 Zr 0.04 O 3-α B A A BaCe 0.8 Gd 0.2 Zr 0.06 O 3-α B A A BaCe 0. 8 Gd 0. 2 Zr 0. 1 O 3-α B A A BaCe 0.8 Gd 0.2 Zr 0.11 O 3-α D B not measured BaCe 0.8 Gd 0.2 Zr 0.15 O 3-α D C not measured Ba 0.99 Ce 0.8 Gd 0.2 Zr 0.01 O 3-α B A A Ba 0.99 Ce 0.8 Gd 0.2 Zr 0.04 O 3-α B A A Ba 0.99 Ce 0.8 Gd 0.2 Zr 0.06 O 3-α B A B Ba 0.99 Ce 0.8 Gd 0.2 Zr 0.1 O 3-α B A B Ba 0.99 Ce 0.8 Gd 0.2 Zr 0.11 O 3-α B B C Ba 0.98 Ce 0.8 Gd 0.2 Zr 0.01 O 3-α B A A Ba 0.98 Ce 0.8 Gd 0.2 Zr 0.04 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.2 Zr 0.06 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.2 Zr 0.1 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.2 Zr 0.11 O 3-α B B C Ba 0.98 Ce 0.8 Gd 0.16 Zr 0.04 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.23 Zr 0.04 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.26 Zr 0.04 O 3-α B A A Ba 0.9 Ce 0.8 Gd 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 Gd 0.2 Zr 0.04 O 3-α B A C Ba 0.9 Ce 0.8 Gd 0.2 Zr 0.06 O 3-α B A C Ba 0.9 Ce 0.8 Gd 0.2 Zr 0.1 O 3-α A A C Ba 0. 9 Ce 0.8 Gd 0.2 Zr 0.11 O 3-α A B D Ba 0.89 Ce 0.8 Gd 0.2 Zr 0.01 O 3-α B A C Ba 0.85 Ce 0.8 Gd 0.2 Zr 0. 04 O 3-α B A D Additive System Material Boiling Test Crystallinity Conductivity Ba 0.98 Ce 0.8 Y 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Y 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Y 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 La 0.2 Zr 0.04 O 3-α B A C Ba 0.99 Ce 0.8 La 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 La 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Pr 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Pr 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 Pr 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Nd 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Nd 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 Nd 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Pm 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Pm 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 Pm 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Sm 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Sm 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Sm 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Eu 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Eu 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Eu 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Tb 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Tb 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Tb 0.2 Zr 0.1 O 3-α B A C Ba 0. 98 Ce 0.8 Dy 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Dy 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Dy 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Ho 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Ho 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 Ho 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Er 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Er 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Er 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Tm 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Tm 0.2 Zr 0.01 O 3-α B A B Ba 0.9 Ce 0.8 Tm 0.2 Zr 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Yb 0.2 Zr 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Yb 0.2 Zr 0.01 O 3-α B A A Ba 0.9 Ce 0.8 Yb 0.2 Zr 0.1 O 3-α B A C Additive System Material Boiling Test Crystallinity Conductivity Ba 0.99 Ce 0.8 Gd 0. Ti 0.01 O 3-α B A C Ba 0.99 Ce 0.8 Gd 0.2 Ti 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Ti 0.04 O 3-α B A C Ba 0.9 Ce 0.8 Gd 0.2 Ti 0.1 O 3-α A A C Ba 0.98 Ce 0.8 Gd 0.16 Ti 0.04 O 3-α B A C Ba 0.99 Ce 0.8 Gd 0.2 Bi 0.01 O 3-α B A A Ba 0.99 Ce 0.8 Gd 0.2 Bi 0.1 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.2 Bi 0.04 O 3-α B A B Ba 0.9 Ce 0.8 Gd 0.2 Bi 0.1 O 3-α A A C Ba 0.98 Ce 0.8 Gd 0.16 Bi 0.04 O 3-α B A B Ba 0.99 Ce 0.8 Gd 0.2 Pb 0.01 O 3-α B A B Ba 0.99 Ce 0.8 Gd 0.2 Pb 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Pb 0.04 O 3-α B A C Ba 0.9 Ce 0.8 Gd 0.2 Pb 0.1 O 3-α A A C Ba 0.98 Ce 0.8 Gd 0.16 Pb 0.04 O 3-α B A C Ba 0.99 Ce 0.8 Gd 0.2 Ga 0.01 O 3-α B A A Ba 0.99 Ce 0.8 Gd 0.2 Ga 0.1 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Ga 0.04 O 3-α B A B Ba 0.9 Ce 0.8 Gd 0.2 Ga 0.1 O 3-α A A C Ba 0.98 Ce 0.8 Gd 0.16 Ga 0.04 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.2 V 0.04 O 3-α C A C Ba 0.98 Ce 0.8 Gd 0.2 Nb 0.04 O 3-α C A C Ba 0.98 Ce 0.8 Gd 0.2 Cr 0.04 O 3-α C A C Ba 0.98 Ce 0.8 Gd 0.2 Mo 0.04 O 3-α C A C Ba 0.98 Ce 0.8 Gd 0.2 W 0.04 O 3-α C A C Ba 0.98 Ce 0.8 Gd 0.2 Fe 0.04 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Co 0.04 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Ni 0.04 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Cu 0.04 O 3-α B A B Ba 0.98 Ce 0.8 Gd 0.2 Ag 0.04 O 3-α C A B Ba 0.98 Ce 0.8 Gd 0.2 Au 0.04 O 3-α C A B Ba 0.98 Ce 0.8 Gd 0.2 Pd 0.04 O 3-α C A B Ba 0.98 Ce 0.8 Gd 0.2 Pt 0.04 O 3-α C A B Ba 0.98 Ce 0.8 Gd 0.2 Sb 0.04 O 3-α B A C Ba 0.98 Ce 0.8 Gd 0.2 Sn 0.04 O 3-α B A C As becomes clear from this evaluation, mixed ionic conductors in accordance with the preferred embodiment have considerably better moisture resistance, while the ion conductivity can be held at a practical level. The above examples have been synthesized by solid state sintering, but there is no limitation to this method, and the oxide also can be synthesized by coprecipitation, nitration, spray granulation or any other suitable method. It is also possible to use film forming methods such as CVD or sputtering methods. It is also possible to use thermal spraying. There is no limitation to the shape of the oxide, and it can be of any shape, including bulk shapes and films.

Claims: A mixed ionic conductor comprising an ion conductive oxide having a perovskite structure of the formula Ba a (Ce 1-b M 1 b )L c O 3-α , wherein M 1 is at least one trivalent rare earth element other than Ce; L is at least one element selected from the group consisting of Zr, Ti, V, Nb, Cr, Mo, W, Fe, Co, Ni, Cu, Ag, Au, Pd, Pt, Bi, Sb, Sn, Pb and Ga; with 0.9 ≤ a ≤ 1; 0.16 ≤ b ≤ 0.26; 0.01 ≤ c ≤ 0.1; and (2+b-2a)/2 ≤ α < 1.5. The mixed ionic conductor of Claim 1, characterized in that M 1 is at least one element selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y and Sc. The mixed ionic conductor of Claim 2, characterized in that M 1 is at least one element selected from the group consisting of Gd and Y. The mixed ionic conductor of Claim 1, 2 or 3, characterized in that L is at least one element selected from the group consisting of Zr, Ti, Fe, Co, Ni, Cu, Bi, Sn, Pb and Ga. A fuel cell comprising as a solid-state electrolyte a mixed ionic conductor of any of Claims 1 to 4. A gas sensor comprising as a solid-state electrolyte a mixed ionic conductor of any of Claims 1 to 4.

Inventors: TANIGUCHI NOBORU, TANIGUCHI, NOBORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 2000
Main classification: C04B35
IPC codes: C04B35, G01N27, H01M8