JP2009067643A - Proton conductor and electrochemical cell prepared by using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a proton conductor which has a composition represented by La<SB>(1-x)</SB>M<SB>x</SB>Sc<SB>(1-y)</SB>N<SB>y</SB>O<SB>3-α</SB>, and exhibits good sinterability and good proton conductivity. <P>SOLUTION: The proton conductor has a composition represented by La<SB>(1-x)</SB>M<SB>x</SB>Sc<SB>(1-y)</SB>N<SB>y</SB>O<SB>3-α</SB>(wherein M is strontium, calcium, or barium; N is zinc; x is a value satisfying 0<x<0.3; and y is a value satisfying 0.02≤y≤0.1), good sinterability, and good proton conductivity. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a proton conductor and an electrochemical cell using the proton conductor.

  Proton conductors are used in electrochemical cells such as batteries, sensors, and fuel cells. Examples of the proton conductor include a solid oxide electrolyte. This solid oxide electrolyte is widely used because it has good proton conductivity.

As this solid oxide electrolyte, a proton conductor having a composition of R _(1-x) M _x ScO _3-α , in which R is lanthanum and M is strontium, is known. Although the proton conductivity of this proton conductor is good, the sinterability is not so good. For example, a fine density of 90% or more necessary for a practical electrolyte cannot be obtained unless baking is performed at a high temperature of 1700 ° C. or higher or a special sintering method such as a spark plasma sintering method is used. Therefore, a proton conductor in which a part of the B site is substituted with magnesium or zinc is disclosed (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 10-67558

However, when magnesium is introduced into the B site of a proton conductor having a composition of La _(1-x) M _x ScO _3-α , the sinterability is improved, but the proton conductivity is greatly reduced. It was only recognized that zinc had the same effect as magnesium.

An object of the present invention is to provide a proton conductor having a composition of La _(1-x) M _x ScO _3-α and having good sinterability and good proton conductivity.

The proton conductor according to the present invention is a proton conductor having a composition of La _(1-x) M _x Sc _(1-y) N _y O _3-α , where M is Sr (strontium), Ca ( Calcium) or Ba (barium), N is Zn (zinc), x is a value satisfying 0 <x <0.3, and y satisfies 0.02 ≦ y ≦ 0.1. It is characterized by being a value. In the proton conductor according to the present invention, Sr, Ba or Ca is added to the A site, so that the oxygen deficiency concentration increases. Since the proton concentration is proportional to the oxygen deficiency concentration, the conductivity increases as the oxygen deficiency concentration increases. Moreover, since Zn is added to the B site and y satisfies 0.02 ≦ y ≦ 0.1, the sinterability is improved while suppressing a decrease in conductivity. Thereby, a high-density proton conductor can be obtained without using a special manufacturing method. Thus, by limiting the metal added to the A site and the B site and the composition thereof as described above, the proton conductor can have both good sinterability and good proton conductivity.

  Further, x is preferably a value satisfying 0.08 <x <0.3. In this case, good proton conductivity can be obtained.

Another proton conductor according to the present invention is a proton conductor having a composition of La _(1-x) M _x Sc _(1-y) N _y O _3-α , where M is strontium, calcium or barium. N is Cu (copper). In the other proton conductor according to the present invention, since Sr, Ba or Ca is added to the A site, the oxygen deficiency concentration increases. Since the proton concentration is proportional to the oxygen deficiency concentration, the conductivity increases as the oxygen deficiency concentration increases. Moreover, since Cu is added to the B site, the sinterability is improved while suppressing a decrease in conductivity. Thereby, a high-density proton conductor can be obtained without using a special manufacturing method. Thus, by limiting the metal added to the A site and the B site and the composition thereof as described above, the proton conductor can have both good sinterability and good proton conductivity.

  According to the present invention, good sinterability and good proton conductivity can be realized.

  Hereinafter, the best mode for carrying out the present invention will be described.

(First embodiment)
The proton conductor according to the first embodiment has a composition of La _(1-x) M _x Sc _(1-y) N _y O _3-α . In the present embodiment, M is Sr (strontium), Ca (calcium) or Ba (barium), and N is Zn (zinc). X is a value satisfying 0 <x <0.3.

  In this embodiment, since Sr, Ca, or Ba is added to the A site, the oxygen deficiency concentration increases. Since the proton concentration is proportional to the oxygen deficiency concentration, the proton conductivity increases as the oxygen deficiency concentration increases. Further, since x satisfies 0 <x <0.3, the proton conductor has a good proton conductivity while maintaining a single-phase perovskite structure. Note that x preferably satisfies 0.08 <x <0.3. In this case, good proton conductivity can be obtained. In addition, since Zn is added to the B site, the sinterability is improved while suppressing a decrease in proton conductivity of the proton conductor. Thereby, a high-density proton conductor can be obtained without using a special manufacturing method.

Here, the principle that the decrease in proton conductivity is suppressed and the sinterability is improved by adding Zn will be described. The stability of the perovskite crystal represented by ABX ₃ is represented by a tolerance factor t as shown in the following equation.
t = (R _A + R _X ) / √2 (R _B + R _X )
R _A = ion radius of A R _B = ion radius of _B R _X = ion radius of _X

In the case of t = 1, the perovskite is the most stable ideal crystal (a cubic crystal having no strain) in which ions of A, B, and X are closely packed. In the case of t> 1, the ionic radius of the B site is smaller than that of the ideal perovskite crystal. In the case of t <1, the ionic radius of the B site is larger than that of the ideal perovskite crystal. In pure LaScO ₃ to which no dopant is added, t = 0.910, and the ionic radius of Sc ³⁺ (hereinafter referred to as R _Sc ) is larger than that of an ideal crystal. Here, Table 1 shows the radius of each ion. In Table 1, X ^* is a virtual element satisfying t = 1 in the LaX ^* O ₃ structure.

Generally, the closer to t = 1, the higher the crystal stability and the higher the sinterability. On the other hand, the proton conductivity is said to be higher when the crystal has some strain. Therefore, the ionic radius of the dopant N of B site (hereinafter, referred to as _{R N)} is, ^{X *} ionic radius (hereinafter, referred to as _{R X *)} <preferably satisfies R N _{<R Sc.} This is because, in the case of _R N _<R X _* is sinterability is improved but greatly reduced proton conductivity, _R X _{* <R} N _<in the case of R _Sc sinterability and proton conductivity are both improved and, in the case of R _{Sc <R N} is because the proton conductivity is improved but sinterability decreases. As shown in Table 1, it can be seen that Mg satisfies R _N <R _{X *} , but Zn satisfies R _{X *} <R _N <R _Sc .

  Further, y preferably satisfies 0.02 ≦ y ≦ 0.1. In this case, the proton conductor can have both good sinterability and good proton conductivity. From the above, the proton conductor can have both good sinterability and good proton conductivity by limiting the metal added to the A site and the B site and the composition thereof as described above.

(Second Embodiment)
The proton conductor according to the second embodiment has a composition of La _(1-x) M _x Sc _(1-y) N _y O _3-α . In the present embodiment, M is Sr, Ca or Ba, and N is Cu (copper). In this case, since a part of La is replaced by Sr, Ca, or Ba, the oxygen deficiency concentration increases. Since the proton concentration is proportional to the oxygen deficiency concentration, the conductivity increases as the oxygen deficiency concentration increases. Moreover, as shown in Table 2, since Cu satisfies R _{X *} <R _N <R _Sc , the sinterability is improved while suppressing a decrease in the conductivity of the proton conductor. Thereby, a high-density proton conductor can be obtained without using a special manufacturing method. From the above, the proton conductor can have both good sinterability and good proton conductivity by limiting the metal added to the A site and the B site and the composition thereof as described above.

(Third embodiment)
In the third embodiment, a fuel cell including a proton conductive electrolyte that is an example of an electrochemical cell will be described. FIG. 1 is a schematic cross-sectional view of a fuel cell 100 according to a third embodiment of the present invention. As shown in FIG. 1, the fuel cell 100 has a structure in which an anode 10, an electrolyte membrane 20, and a cathode 30 are sequentially laminated. The electrolyte membrane 20 is made of the proton conductor according to the first embodiment or the second embodiment.

  A fuel gas containing hydrogen is supplied to the anode 10. Hydrogen contained in the fuel gas is dissociated into protons and electrons at the anode 10. Protons pass through the electrolyte membrane 20 and reach the cathode 30. An oxidant gas containing oxygen is supplied to the cathode 30. Water is generated and electric power is generated from oxygen in the oxidant gas and protons reaching the cathode 30. With the above operation, power generation by the fuel cell 100 is performed. In the present embodiment, since the electrolyte membrane 20 has good proton conductivity, good power generation performance can be obtained. In addition, since the electrolyte membrane 20 has good sinterability, gas leakage can be suppressed.

(Fourth embodiment)
In the fourth embodiment, a hydrogen separation membrane battery 200 which is an example of an electrochemical cell will be described. Here, the hydrogen separation membrane cell is a kind of fuel cell and is a fuel cell provided with a dense hydrogen separation membrane. The dense hydrogen separation membrane is a layer formed of a metal having hydrogen permeability and also functions as an anode. The hydrogen separation membrane battery has a structure in which an electrolyte having proton conductivity is laminated on the hydrogen separation membrane. Hydrogen supplied to the hydrogen separation membrane is converted into protons, moves through the proton conductive electrolyte, and combines with oxygen at the cathode to generate power. Hereinafter, details of the hydrogen separation membrane battery 200 will be described.

  FIG. 2 is a schematic cross-sectional view of the hydrogen separation membrane battery 200. As shown in FIG. 2, the hydrogen separation membrane battery 200 has a structure in which a power generation unit in which an electrolyte membrane 120 and a cathode 130 are sequentially stacked on a hydrogen separation membrane 110 is sandwiched between a separator 140 and a separator 150. In the present embodiment, the operating temperature of the hydrogen separation membrane battery 200 is about 300 ° C. or higher and 600 ° C. or lower.

  The separators 140 and 150 are made of a conductive material such as stainless steel. The separator 140 is formed with a gas flow path for flowing a fuel gas containing hydrogen. The separator 150 is formed with a gas flow path through which an oxidant gas containing oxygen flows.

  The hydrogen separation membrane 110 is made of a dense hydrogen permeable metal. In the present embodiment, the hydrogen separation membrane 110 has a structure so dense that hydrogen permeates in the form of hydrogen atoms and / or protons. The hydrogen separation membrane 110 functions as an anode to which fuel gas is supplied, and also functions as a support that supports and reinforces the electrolyte membrane 120. The metal constituting the hydrogen separation membrane 110 is, for example, a metal such as palladium, vanadium, titanium, tantalum, niobium, or an alloy thereof. Alternatively, a hydrogen separation membrane 110 may be used in which films of palladium, palladium alloy, etc. having hydrogen dissociation ability are formed on both surfaces of these hydrogen permeable metal layers. The hydrogen separation membrane 110 may be a self-supporting membrane or may be supported by a porous base metal plate.

The cathode 130 is made of a conductive material such as La _0.6 Sr _0.4 CoO ₃ or Sm _0.5 Sr _0.5 CoO ₃ . A catalyst such as platinum may be supported on the material constituting the cathode 130. The electrolyte membrane 120 is made of the proton conductor according to the first embodiment or the second embodiment. In the present embodiment, since the electrolyte membrane 120 has good proton conductivity, good power generation performance can be obtained. In addition, since the electrolyte membrane 120 has good sinterability, gas leakage can be suppressed.

  Here, in order to maintain good power generation efficiency in the hydrogen separation membrane battery 200, it is necessary that the adhesion between the hydrogen separation membrane 110 and the electrolyte membrane 120 be high. Since the electrolyte membrane 120 is not a mixed ion conductor but a proton conductive electrolyte, the generation of water on the anode side is suppressed. Therefore, the separation of the hydrogen separation membrane 110 and the electrolyte membrane 120 can be prevented by using the electrolyte membrane 120. From the above, the electrolyte having the configuration of the present invention is particularly effective for the hydrogen separation membrane battery.

(Fifth embodiment)
In the fifth embodiment, a hydrogen pump 300 that is an example of an electrochemical cell will be described. FIG. 3 is a schematic diagram of the hydrogen pump 300. As shown in FIG. 3, the hydrogen pump 300 includes an anode 210, an electrolyte membrane 220, a cathode 230, and a power source 240. The anode 210, the electrolyte membrane 220, and the cathode 230 are laminated in order. The anode 210 is connected to the plus terminal of the power source 240. On the other hand, the cathode 230 is connected to the negative terminal of the power supply 240. The electrolyte membrane 220 is made of the proton conductor according to the first embodiment or the second embodiment.

  When voltage is applied from the power source 240 to the anode 210 and the cathode 230, hydrogen dissociates into electrons and protons at the anode 210. The electrons move to the power source 240. Protons are conducted through the electrolyte membrane 220 and reach the cathode 230. At the cathode 230, hydrogen is generated from electrons and protons supplied from the power supply 240. From the above, hydrogen can be separated from the gas supplied to the anode side and moved to the cathode side using the hydrogen pump 300. Thereby, hydrogen gas with high purity can be generated.

  Since the electrolyte membrane 220 is made of the proton conductor according to the first embodiment or the second embodiment, good proton conductivity can be obtained. Therefore, good hydrogen separation efficiency can be obtained. Moreover, since the electrolyte membrane 220 has good sinterability, gas leakage can be suppressed.

  Hereinafter, the proton conductor according to the above embodiment was produced and the characteristics thereof were examined.

(Example 1 to Example 3)
In Examples 1 to 3, proton conductors according to the first embodiment were produced. Table 3 shows the composition of the proton conductor according to each example. Proton conductors according to the respective examples were prepared using La ₂ O ₃ , SrCO ₃ , Sc ₂ O ₃ and ZnO as raw materials and using a solid phase reaction method.

Example 4
In Example 4, the proton conductor according to the second embodiment was produced. The composition of the proton conductor according to Example 4 is shown in Table 3. A proton conductor according to Example 4 was prepared using La ₂ O ₃ , SrCO ₃ , Sc ₂ O ₃ and CuO as raw materials and using a solid phase reaction method.

(Comparative Example 1)
In Comparative Example 1, a proton conductor in which no element was added to the B site was produced. Table 3 shows the composition of the proton conductor according to Comparative Example 1. A proton conductor according to Comparative Example 1 was prepared using La ₂ O ₃ , SrCO ₃ and Sc ₂ O ₃ as raw materials and using a solid phase reaction method.

(Comparative Example 2)
In Comparative Example 2, one having the same composition as in Examples 1 to 3 and x = 0.3 was produced. The composition of the proton conductor according to Comparative Example 2 is shown in Table 3. A proton conductor according to Comparative Example 2 was produced using La ₂ O ₃ , SrCO ₃ , Sc ₂ O ₃ and ZnO as raw materials and using a solid phase reaction method.

(Comparative Example 3)
In Comparative Example 3, a proton conductor in which no element was added to the A site was produced. The composition of the proton conductor according to Comparative Example 3 is shown in Table 3. A proton conductor according to Comparative Example 3 was produced using La ₂ O ₃ , Sc ₂ O ₃ and ZnO as raw materials and using a solid phase reaction method.

(Comparative Example 4)
The proton conductor according to Comparative Example 4 is a proton conductor in which magnesium is added to the B site, and Solid State Ionics 125 (1999).
339-344. Regarding the proton conductor according to Comparative Example 4, the results of Analysis 1 to Analysis 3 shown in Table 4 are also extracted from the above literature.

(Analysis 1)
In Analysis 1, XRD measurement was performed on the proton conductors according to each Example and Comparative Examples 1 to 3. Table 4 shows the results. As shown in Table 4, it was found that proton conductors other than Comparative Example 2 consisted of a perovskite single phase. It was found that the proton conductor according to Comparative Example 2 contained peaks of layered perovskite SrO (LaScO ₃ ) _n , n = 1,2. Therefore, to obtain a single-phase proton conductor, x is less than 0.3.

(Analysis 2)
In Analysis 2, the relative density of the proton conductor according to each Example and Comparative Examples 1 to 3 was obtained. The relative density was measured after each proton conductor was fired at 1600 ° C. for 10 hours. Here, the relative density is measured density / theoretical density of each proton conductor. The theoretical density is the density of a single crystal having no voids calculated from the crystal structure and lattice constant. Therefore, the density increases as the relative density increases. The results are shown in Table 4 and FIG. In FIG. 4, the horizontal axis indicates the Zn doping amount (molar ratio) to the B site, and the vertical axis indicates the relative density. As shown in Table 4 and FIG. 4, the relative density improved as the amount of Zn added to the B site increased.

  Here, voids may occur in the crystal. This void includes an open pore and a closed pore. An open pore is a void into which water enters when immersed in water, and a closed pore is a void into which water does not enter when immersed in water. When a proton conductor is used as an electrolyte for a fuel cell, closed pores may be present, but open pores are preferably absent because they may cause gas leakage. In a normal ceramic sintered body, it is said that closed pores disappear when the relative density is 90% or more. Therefore, from FIG. 4, the Zn doping amount is preferably 0.02 or more. Moreover, as shown in Table 4, it was found that the relative density was significantly improved by adding Cu to the B site.

(Analysis 3)
In Analysis 3, the conductivity of the proton conductor according to each Example and Comparative Examples 1 to 3 was obtained. The conductivity was measured by a direct current four-terminal method in a humidified hydrogen atmosphere at 800 ° C. The results are shown in Table 4 and FIG. FIG. 5 shows the conductivity of a proton conductor obtained by adding Zn to the B site. In FIG. 5, the horizontal axis indicates the amount of Sr doping (molar ratio) to the A site, and the vertical axis indicates the specific conductivity. Here, the specific conductivity means the conductivity of each proton conductor when the conductivity of the proton conductor according to Comparative Example 1 is “1”.

  As shown in Table 4 and FIG. 5, the electrical conductivity improved as the doping amount to the A site increased. However, since the unstable layered perovskite phase begins to appear when the doping amount to the A site becomes 0.3 or more, the doping amount to the A site is preferably less than 0.3. Further, from FIG. 5, in order to obtain good proton conductivity, the doping amount to the A site is preferably more than 0.08. On the other hand, as shown in Table 4, when magnesium was added to the B site, the conductivity was low.

  From the above, it was found that good proton conductivity and good sinterability can be obtained by setting 0 <x <0.3 and 0.02 ≦ y ≦ 0.1. It was also found that a better proton conductivity can be obtained by setting 0.08 <x <0.3. In Examples 1 to 4, the conductivity was higher than in the case where an element was added to either the A site or the B site. Therefore, it was found that a synergistic effect was obtained by adding elements to the A site and the B site.

It is typical sectional drawing of the fuel cell which concerns on the 3rd Embodiment of this invention. It is a typical sectional view of the hydrogen separation membrane battery concerning a 4th embodiment of the present invention. It is a schematic diagram of the hydrogen pump which concerns on the 5th Embodiment of this invention. It is a figure which shows the relative density of each proton conductor. It is a figure which shows the electrical conductivity of each proton conductor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Anode 20,120,220 Electrolyte membrane 30,130 Cathode 100 Fuel cell 110 Hydrogen separation membrane 200 Hydrogen separation membrane cell 300 Hydrogen pump

Claims (3)

A proton conductor having a composition of La _(1-x) M _x Sc _(1-y) N _y O _3-α ,
M is strontium, calcium or barium;
N is zinc;
x is a value satisfying 0 <x <0.3;
y is a proton conductor having a value satisfying 0.02 ≦ y ≦ 0.1.   2. The proton conductor according to claim 1, wherein x is a value satisfying 0.08 <x <0.3. A proton conductor having a composition of La _(1-x) M _x Sc _(1-y) N _y O _3-α ,
M is strontium, calcium or barium;
A proton conductor, wherein N is copper.

Images