KR100777685B1 - Perovskite type solid electrolytes for solid oxide fuel cells and fuel cells containing the same - Google Patents

Abstract

A perovskite structured solid oxide electrolyte and a solid oxide fuel cell containing the electrolyte are provided to obtain pure ion conductivity free from electron conductivity under specific condition. A perovskite structured solid oxide electrolyte is an oxygen ion conductor represented by La_(1-x) Sr_x Ga_(1-y-z) Mg_y Fe_z O-(3-delta) and has an ion conductivity of 0.05 S/cm or more at a temperature of 800 deg.C, wherein 0.2<x<=0.3, 0.2<y<=0.4, 0<z<=0.08, and 0<=delta<=1.

Description

Perovskite structure solid electrolyte for solid oxide fuel cell and fuel cell comprising same {Perovskite type Solid Electrolytes for Solid Oxide Fuel Cells and Fuel Cells containing the same}

FIG. 1 shows X-ray diffraction results of a solid electrolyte prepared by partially substituting Fe for Mg in LaGaO ₃ having Sr and Mg substitution amounts of 30 mol% and 40 mol%, respectively. (●: Perovskite, La _{1 - x} Sr _x Ga _{1 - y} Mg _y O ₃ , ▼: LaSrGaO ₄ , ■: MgO) The marking principle of each specimen is indicated by the first letter of the alphabet, Mol% of (Sr, Mg and Fe) is shown in two digits.

FIG. 2 shows X-ray diffraction results of a solid electrolyte prepared by partially substituting Fe for Mg in LaGaO ₃ having Sr and Mg substitution amounts of 30 mol% and 20 mol%, respectively. (●: perovskite, La _{1 - x} Sr _x Ga _{1 - y} Mg _y O ₃ , ▼: LaSrGa ₃ O ₇ )

FIG. 3 shows X-ray diffraction results of a solid electrolyte prepared by partially substituting Fe for Mg in LaGaO ₃ having Sr and Mg substitution amounts of 25 mol% and 20 mol%, respectively. (●: perovskite, La _{1 - x} Sr _x Ga _{1 - y} Mg _y O ₃ , ▼: LaSrGa ₃ O ₇ )

Figure 4 is a change in the electrical conductivity according to the total charge and oxygen partial pressure in a common conductive oxide.

5 is a change in electrical conductivity according to the oxygen partial pressure of LaGaO ₃ of Sr and Mg substitution amounts of 25 mol% and 20 mol%, respectively.

6 is a change in electrical conductivity according to oxygen partial pressure of a solid electrolyte in which 2 mol% of Fe is substituted for Mg in LaGaO ₃ having Sr and Mg substitution amounts of 25 mol% and 20 mol%, respectively.

7 is a change in electrical conductivity according to the oxygen partial pressure of a solid electrolyte in which 4 mol% Fe is substituted for Mg in LaGaO ₃ having Sr and Mg substitution amounts of 25 mol% and 20 mol%, respectively.

8 is a change in electrical conductivity according to the oxygen partial pressure of a solid electrolyte in which 8 mol% Fe is substituted for Mg in LaGaO ₃ having Smol and Mg substitution amounts of 25 mol% and 20 mol%, respectively.

The present invention relates to a solid electrolyte used in a solid oxide fuel cell (SOFC), and more particularly, to a multi-component solid oxide electrolyte having a perovskite structure and a solid oxide fuel cell constituted using the same.

Yttria stabilized zirconia (YSZ) of fluorite structure has been widely used as a solid electrolyte for solid oxide fuel cells. Yttria stabilized zirconia has properties suitable for use as solid oxides such as high ionic conductivity, extremely low electron conductivity, and long term stability in a reducing atmosphere. However, in order to operate the solid oxide fuel cell using the same, there is a disadvantage in that a temperature of about 1000 ° C. is required to increase the ion conductivity of the solid oxide.

When the operating temperature of the solid oxide fuel cell is lowered to 800 ° C. or lower, the manufacturing cost of the fuel cell can be significantly lowered. As a method of lowering the operating temperature, there is a method of reducing the thickness of the electrolyte to lower the resistance, configuring the electrode as a support, and replacing the solid electrolyte with high ion conductivity even at low and low temperatures.

As materials having high ionic conductivity at low and low temperatures, fluorite oxides such as Scandia substituted zirconia, gadolinium substituted ceria, and samarium substituted ceria, and cation substituted lanthanum gallate having a perovskite structure are proposed.

T. Ishihara et al., J. Am. Chem. Soc., Vol. 116, pp. 3801-3803 (1994)] reported high ionic conductivity in multi-component oxides of perovskite structure based on LaGaO ₃ and partially substituted with cations such as Sr and Mg. There is a bar. However, the composition range in which Sr and Mg can be solvated is not wide. Especially, in case of 20 mol% or more, the solution exceeds the solubility limit, so that the conductive perovskite single phase structure is deformed. Impurity phases are known to form [K. Huang et al., J. Am. Ceram. Soc., Vol. 81, pp. 2565 (1998).

On the other hand, in LaGaO ₃ substituted with Sr and Mg, it has been reported that the ionic conductivity is increased by a small amount of transition metal such as Fe, Co, or Ni, but also applied in the range of Sr and Mg within 20 mol% Result [T. Ishihara et al., Solid State Ionics, Vol. 135, pp. 631 (2000). In the case of exceeding the high solubility limit, impurity phases such as LaSrGaO ₄ and LaSrGa ₃ O ₇ are generated. These have very high electrical resistance, which lowers the ionic conductivity of the entire electrolyte.

An object of the present invention is a solid electrolyte, which has a perovskite single phase and improved ion conductivity in a LaGaO ₃ composition in which Sr and Mg are substituted at least 20 mol%, in contrast to a composition range known in the art. To provide a solid oxide electrolyte having an ideal property as.

The present invention relates to a solid oxide electrolyte for a fuel cell of Formula 1 below.

(Where 0.2≤ x ≤0.3

0.2≤ y ≤0.4

0 <z ≤0.08
0 ≦ δ ≦ 1.)
In the formula (1), δ is an oxygen lattice defect, that is, a numerical value representing the number of moles of oxygen vacancy, which is dependent on the concentration of the substituted cation, temperature, or oxygen partial pressure.

The electrolyte is an oxygen ion conductor, and exhibits an ion conductivity of 0.05 S / cm or more at 800 ° C., and has a perovskite structure.

The perovskite structure can be represented by ABO ₃ . La and Sr correspond to A-position atom, and Ga, Mg, and Fe correspond to B-position atom.

The present invention also provides a solid oxide fuel cell comprising the solid oxide electrolyte and comprising an air electrode and a fuel electrode having both electron conductivity and ion conductivity.

The principle that the oxide electrolyte of the present invention has oxygen ion conductivity is as follows. Partial substitution of Sr having a divalent valence at the position of La having a trivalent valence produces an oxygen vacancy, as shown in the defect equation of Equation 1 below.

Since oxygen ions move through the oxygen vacancies, when the concentration of the oxygen vacancies increases, the oxygen ion conductivity increases, so that it can be used as a solid electrolyte.

The cation substituted at the A site in the perovskite structure ABO ₃ includes Ca, Sr, or Ba, and Sr, which is close to La and the ion radius, is most effective in improving conductivity. Mg is used for the B-site atom.

LaGaO ₃ substituted with Sr and Mg, which corresponds to the electrolyte composition of the present invention, exhibits sufficiently high ionic conductivity to be used as a solid electrolyte even at temperatures of 800 ° C. or lower, and has a low electron conductivity at all oxygen partial pressures. It can be used as a solid electrolyte of a battery.

On the other hand, it is known that the composition range (employment limit) in which Sr and Mg can be dissolved in LaGaO ₃ is 20 mol%. If the substitution amount is increased beyond the solid solution limit, the conductive perovskite single-phase structure is deformed to form an impurity phase of a high resistance component and lower the ionic conductivity of the solid electrolyte and thus cannot be applied to a solid oxide fuel cell [K. Huang et al., J. Am. Ceram. Soc., Vol. 81, pp. 2565 (1998).

The present invention relates to LaGaO ₃ having Sr or Mg of 20 mol% or more, unlike the conventional high solubility limit of Sr and Mg with respect to LaGaO ₃ , having a perovskite single phase, and having improved electrical conductivity and electron conductivity under a wide oxygen partial pressure. This extremely limited, characterized in that a slight substitution of the transition metal Fe to produce a solid oxide electrolyte having ideal characteristics as a solid electrolyte.

Representative manufacturing process for producing a solid oxide electrolyte of the present invention is as follows. The raw material in the form of an oxide or salt is weighed in an amount corresponding to the final composition and then wet mixed using a ball mill. The mixed powder is maintained at a temperature of 1400-1500 ° C. for at least 2 hours to form a perovskite single phase. The heat-treated powder is sintered in the air for at least 2 hours at a temperature of 1400 ~ 1500 ℃ after pressing.

The invention will be described in detail through the following examples.

Example 1

In LaGaO ₃ having Smol and Mg substitution amounts of 30 mol% and 40 mol%, respectively, 2, 4, 6 and 8 mol% of Fe was substituted for Mg to prepare a solid electrolyte.

1 is a phase analysis result using X-ray diffraction. As the Fe content increases, the impurities (high resistance component) LaSrGaO ₄ (marked with ▼) continue to decrease and approach the perovskite single phase structure.

Table 1 is an electrical conductivity for the composition of the Fe content of 4 mol% and 8 mol%, respectively, showing a relatively high value when the Fe content is high. On the other hand, a trace amount of MgO (denoted by ■) which is one of the high resistance components is detected in all compositions.

Example 2

In LaGaO ₃ having Smol and Mg substitution amounts of 30 mol% and 20 mol%, respectively, 2, 4, 6 and 8 mol% of Fe was substituted for Mg to prepare a solid electrolyte.

2 is a phase analysis result using X-ray diffraction. La _0.7 Sr _0.3 Ga _0.8 Mg _0.2 O _3-δ without Fe is composed of perovskite single phase without high resistivity, while LaSrGa ₃ O ₇ (▼) A trace amount of the high resistance component of the composition is detected.

Table 2 is an electrical conductivity for the composition of the Fe content of 0, 2 mol% and 8 mol%, respectively, showing a relatively high value when the Fe content is high.

Example 3

In LaGaO ₃ having Smol and Mg substitution amounts of 25 mol% and 20 mol%, respectively, 2, 4, 6 and 8 mol% of Fe was substituted for Mg to prepare a solid electrolyte.

3 shows the results of phase analysis using X-ray diffraction. A single phase of the perovskite structure was formed over all composition ranges.

Table 3 is an electrical conductivity for the composition of the Fe content of 0, 2 mol%, 4 mol% and 8 mol%, respectively, showing a relatively high value when the Fe content is high.

In LaGaO ₃ having 25 mol% and 20 mol% substitutions of Sr and Mg, in the case of specimens prepared by substituting 4 mol% of Fe instead of Mg, 0.161 S / cm at 800 ° C. and 0.395 S / cm at 1000 ° C. Indicated. In addition, when the content of Fe is added 8 mol%, it exhibits high electrical conductivity as 0.17 S / cm at 800 ° C and 0.465 S / cm at 1000 ° C.

In order to examine the applicability of the prepared composition as a solid electrolyte, the oxygen partial pressure dependence of electrical conductivity must be evaluated. When lanthanum gallate is placed in a reducing atmosphere having a low oxygen partial pressure, as shown in Equation 2 below, oxygen is released from the crystal lattice, and Ga is reduced to meet charge neutrality conditions (Stevenson et al. Solid State Ionics, Vol. 113, pp. 571-583 (1998). The equilibrium constant at this time is as shown in Equation 3 below.

That is, Ga ^{3 +} is reduced to Ga ^{2 +} to provide electrons. In LaGaO ₃ substituted with Sr and Mg, many oxygen vacancies are generated, and thus the concentration of oxygen vacancies is almost constant. The number t _e is proportional to -1/4 squared of the partial pressure of oxygen.

The correlation between the electrical conductivity and the oxygen partial pressure as described above may be expressed as shown in FIG. 4. As the temperature increases, the conductivity generally increases. First, if the logarithmic value of conductivity in the high oxygen partial pressure region is proportional to the slope of the logarithmic function of oxygen partial pressure, the conductivity is governed by the electron-hole, and the conductivity in the low oxygen partial pressure region. If the logarithmic value of is proportional to the slope -1/4 of the logarithmic value of the partial pressure of oxygen, the electrical conductivity is determined by the electron. In addition, if the electrical conductivity does not change with respect to the oxygen partial pressure, the number of oxygen ion conductance (transference number) is close to 1 and can be used as a solid electrolyte.

The three charge carriers may vary in concentration according to each temperature, so that the so-called electrolyte domain, whose conductivity does not change with oxygen partial pressure, may be used as an electrolyte.

Example 4

5, 6, 7 and 8 are LaGaO ₃ in which the substitution amounts of Sr and Mg are 25 mol% and 20 mol%, respectively, in which 0, 2 mol%, 4 mol% and 8 mol% of Fe are substituted for Mg, respectively. It is the change of electrical conductivity according to the oxygen partial pressure of the solid electrolyte prepared by substitution.

In the compositions having 0, 2 mol% and 4 mol% of Fe, respectively, there is little change in electrical conductivity in the oxygen partial pressure range of 10 ^-24 atm <P _O2 <10 ⁰ atm at the measurement temperature of 700 ° C or higher. Therefore, the measured electrical conductivity is due to the conduction of oxygen ions, and has suitable characteristics as a solid electrolyte of a solid oxide fuel cell.

On the other hand, in the composition in which 8 mol% Fe is substituted, the electrical conductivity increases in the low oxygen partial pressure region (10 ^-24 atm <P _O2 <10 ^-12 atm) as the oxygen partial pressure decreases, and Δ logσ / Δ logP _O2 Matches a straight line with a slope of -1/4. Therefore, it can be seen that the measured electrical conductivity is mainly due to electrons, and when used in a solid oxide fuel cell, the characteristics of a unit cell are greatly reduced, and thus, the electrolyte is not suitable as a solid electrolyte.

The solid electrolyte according to the present invention exhibited an electrical conductivity of 0.16 S / cm or more at 800 ° C., and exhibited pure ion conductivity with little electron conductivity in the oxygen partial pressure range of 10 ⁻²⁴ atm <P _{O 2} <10 ⁰ atm. It has properties suitable for solid electrolytes in fuel cells.

In particular, the measured electrical conductivity is LaGaO ₃ substituted with 20 mol%, 11.5 mol%, and 8.5 mol%, respectively, of Sr, Mg, and Co, which exhibit the highest conductivity among the oxides applicable to the solid electrolyte of solid oxide fuel cells. [T. Ishihara et al., Journal of the European Ceramic Society, Vol. 24, pp. 1329-1335 (2004)] are approximated to the electrical conductivity (~ 0.16 S / cm, 800 ℃).

Therefore, the oxide prepared according to the present invention can be used as the solid electrolyte of the solid oxide fuel cell in the mid-low temperature region below 800 ° C.

Claims (3)

An oxygen ion conductor having a composition of Formula 1 below, a solid oxide electrolyte having a perovskite structure having an ion conductivity of at least 0.05 S / cm at 800 ° C. [Formula 1] La _.1-x Sr _x Ga _1-yz Mg _y Fe _z O _3-δ 0.2 <x ≤0.3 0.2 <y ≤0.4 0 <z ≤0.08 0≤ δ ≤1 An oxygen ion conductor having a composition of Formula 2 below, a solid oxide electrolyte having a perovskite structure having an ion conductivity of 0.1 S / cm or more at 800 ° C. [Formula 2] La _.1-x Sr _x Ga _1-yz Mg _y Fe _z O _3-δ 0.25≤ x ≤0.3 0.2 <y ≤0.4 0 <z ≤0.08 0≤ δ ≤1 A solid oxide fuel cell comprising the air electrode and the fuel electrode, comprising the solid oxide electrolyte of claim 1 or 2, and having both electron conductivity and ion conductivity.

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