JP4794239B2 - Solid electrolyte body and fuel cell - Google Patents

Description

  The present invention relates to a solid electrolyte body composed of ceria-based crystal particles in which a rare earth element (excluding Ce) is dissolved, and a fuel battery cell.

  In recent years, ceria-based oxides have attracted attention as electrolyte materials for solid electrolyte fuel cells because they have excellent oxide ion conductivity. In particular, the above oxides can contribute to lowering the operating temperature, and are therefore indispensable materials for practical use of low-temperature operating fuel cells.

  A conventional method for producing a solid electrolyte composed of ceria-based crystal particles is a method of synthesizing a composite oxalate from a water-soluble salt of a rare earth element and a water-soluble salt of cerium by a coprecipitation method, and sintering this at 1500 ° C. It has been known. The ceria-based crystal particles synthesized by this method are composed of a uniform structure and do not contain foreign phases other than ceria in the grains and grain boundaries (see Patent Document 1).

Furthermore, conventionally, a method for producing a dense sintered body made of a single phase having a fluorite structure by sintering a raw material powder of about 30 nm synthesized using a bicarbonate at 1000 ° C. is known. (See Patent Document 2).
JP 2004-143023 A JP 2004-107186 A

However, in the solid electrolyte bodies described in Patent Documents 1 and 2, the valence change of cerium from tetravalent to trivalent occurs in a reducing atmosphere (Ce ⁴⁺ + e ⁻ → Ce ³⁺ ). Therefore, there has been a problem that the fuel electrode side of the solid electrolyte body extends and warps. That is, the ceria-based sintered body undergoes a reduction reaction from the surface of the ceria-based crystal particles in a reducing atmosphere, and finally the whole is reduced, resulting in large volume expansion of the crystal particles. As a result, when a fuel cell was produced using the solid electrolyte body, there was a problem that it was difficult to produce a crack due to cell deformation or a stack.

  An object of this invention is to provide the solid electrolyte body which can suppress the volume expansion in a reducing atmosphere, and the fuel battery cell which can prevent a curvature and a crack.

Solid-solid electrolyte of the present _{invention, (CeO 2) 1-x} (REO 3/2) x (x is 0.1 ≦ x ≦ 0.4, RE is a rare earth element other than Ce) ceria represented by It is made of a polycrystalline body in which crystal grains are aggregated, and the valence of Ce in the ceria-based crystal grains is mainly tetravalent at the central portion, and trivalent Ce is mainly at the surface.

The composite oxide of trivalent Ce and rare earth element RE (excluding Ce) present in the surface layer of the ceria-based crystal particles constituting the solid electrolyte body is in the central part of the ceria-based crystal particles in a reducing atmosphere. It functions as a barrier layer for the reduction reaction of Ce (Ce ⁴⁺ + e ⁻ → Ce ³⁺ ), and the presence of the composite oxide prevents the reduction gas from diffusing into the central part of the ceria crystal particles. The reduction of the volume is suppressed, and the volume expansion caused by the reduction can be suppressed.

  The effect of improving the reduction resistance due to the composite oxide is noticeable when the average particle diameter of the ceria-based crystal particles is 100 nm or less. By setting the average particle diameter to 100 nm or less, the sintered body can be densified while maintaining the surface layer generated by the heat treatment under the high oxygen partial pressure.

  The rare earth element RE (excluding Ce) is preferably at least one of Nd, Sm, Gd, Dy, Ho, Y, and Yb. Thereby, the ionic conductivity of the ceria-based crystal particles is sufficiently high, and the reliability of the solid electrolyte body can be further improved by the effect of suppressing the volume expansion under the reducing atmosphere due to the presence of the surface layer.

  The fuel battery cell of the present invention is characterized in that an oxygen electrode and a fuel electrode are formed on both sides of a solid electrolyte body. In such a fuel cell, since volume expansion of the solid electrolyte body on the fuel electrode side is suppressed, deformation and warpage of the fuel cell during power generation can be suppressed, and reliability can be improved.

The solid electrolyte body of the present invention is a ceria-based crystal represented by (CeO ₂ ) _1-x (REO _3/2 ) _x (x is 0.1 ≦ x ≦ 0.4, RE is a rare earth element excluding Ce). with particles comprises a central portion and the surface layer thereof is formed around the surface layer of the ceria-based crystal particles, since the trivalent Ce is the main, surface layer, under a reducing atmosphere, Ce-based It functions as a barrier layer against Ce reduction reaction (Ce ⁴⁺ + e ⁻ → Ce ³⁺ ) at the center of the crystal grain, and this composite oxide prevents the reduction gas from diffusing into the center of the ceria based crystal grain. Volume expansion of the solid electrolyte body caused by the reduction can be suppressed. As a result, in the fuel cell using the solid electrolyte body of the present invention, during power generation, deformation and cracking due to expansion of the solid electrolyte body on the fuel electrode side are suppressed, the reliability of the solid electrolyte body is improved, and the stack Is easy to manufacture.

  The present invention relates to a solid electrolyte body applied to various types of solid electrolyte fuel cells such as a cylindrical type, a flat plate type, and a hollow flat plate type, and FIG. 1 illustrates a cross-sectional view of the fuel cell.

  The fuel battery cell is configured by sandwiching the solid electrolyte body 3 between the fuel electrode 5 and the oxygen electrode 7 from both sides.

  The fuel electrode 5 is composed of a metal (for example, Ni) and / or a metal oxide (for example, NiO), or a porous conductive material composed of this and a ceria-based composite oxide constituting the solid electrolyte body 3. It is considered as a material.

The oxygen electrode 7 is made of particles having oxygen ion conductivity and electron conductivity, and is made of, for example, (La, Sr) (Co, Fe) O ₃ or the like.

Solid-solid electrolyte of the present invention, the solid electrolyte member 3 is, in _{(CeO 2) 1-x (} REO 3/2) x (x is 0.1 ≦ x ≦ 0.4, RE is a rare earth element other than Ce) As shown in FIG. 2, the valence of Ce in the ceria crystal particles 9 is mainly tetravalent in the central portion 9a and trivalent on the surface. It is important that Ce is the main. That is, it is important that the ceria-based crystal particles 9 have a two-layer structure in which the valence of Ce is different between the central portion 9a and the surface.

  Here, the surface in the present invention means a surface layer up to a depth at which trivalent Ce is 1.2 times the amount at the center of the ceria-based crystal particle 9.

Thus, by forming the surface layer of the upper SL, (excluding Ce) rare earth element RE in the reduction reaction can function effectively as a barrier layer of the reducing gas.

  Further, the thickness of the surface layer 9b of the ceria-based crystal particle 9 is preferably 1/10 or less, particularly 1/12 or less, more preferably 1/15 or less with respect to the radius of the ceria-based crystal particle 9. Thereby, it becomes easy to sufficiently exhibit the characteristics excellent in ion conductivity or electron conductivity due to tetravalent Ce.

  According to the present invention, the thickness of the surface layer 9b of the ceria-based crystal particle 9 is preferably 0.4 nm or more, particularly 1 nm or more, and more preferably 1.5 nm or more. Thereby, as will be described later, the effect of suppressing the reduction by the surface layer of the ceria-based crystal particles can be maximized, and the volume expansion in the reducing atmosphere of the solid electrolyte body can be further suppressed.

  The ceria-based crystal particle 9 has an average particle size of 100 nm or less, and as shown in FIG. 2, includes a central portion 9a and a surface layer 9b formed around the central portion 9a. The rare earth element RE is present more than the central portion 9a, and is composed of a complex oxide of trivalent Ce and rare earth element RE (excluding Ce).

  In this way, a large amount of rare earth element RE is present on the surface of the ceria-based crystal particles, whereby a composite oxide of trivalent Ce and rare earth elements (excluding Ce) is formed on the surface layer 9b.

  In the present invention, the ceria-based crystal particles 9 of the solid electrolyte body 3 are spherical particles having an average particle size of 100 nm or less. The reason that the Ce-based crystal particles 9 have an average particle size of 100 nm or less is that This is because a large number of different phases containing Ce and rare earth elements can be present in the surface layer in which a large amount exists. On the other hand, when the average particle size is larger than 100 nm, the distribution of the rare earth element RE in the ceria-based crystal particles 9 proceeds in a uniform direction, and the heterogeneous phase existing on the surface of the ceria-based crystal particles 9 is reduced. This is because the effect of suppressing the reduction by the surface layer 9b is reduced. In particular, the average particle size of the ceria-based crystal particles 9 is preferably 10 to 80 nm from the viewpoint of improving the reduction suppression effect.

In such a fuel cell, the surface layer 9b is composed of a composite oxide of trivalent Ce and rare earth element RE (except for Ce), and the surface layer 9b is formed in the center of the ceria-based crystal particle in a reducing atmosphere. It functions as a barrier layer against the Ce ⁴⁺ reduction reaction (Ce ⁴⁺ + e ⁻ → Ce ³⁺ ) of the part 9a and can suppress the volume expansion of the solid electrolyte body 3 caused by the reduction. In other words, the presence of the surface layer 9b made of the composite oxide prevents the reduction gas from diffusing into the ceria crystal particle central portion 9a, thereby suppressing the reduction of the ceria crystal particles 9.

The surface layer 9b itself is composed of a complex oxide (hereinafter sometimes referred to as a different phase) of trivalent Ce and a rare earth element RE (excluding Ce). In such a different phase, a large amount of trivalent Ce is present, and thus no reaction with the reducing gas occurs. On the other hand, as described above, such a different phase functions as a barrier layer for the Ce ⁴⁺ reduction reaction of the ceria-based crystal particle central portion 9a.

  Therefore, according to the present invention, the reduction of the ceria crystal particle central portion 9a is prevented by the surface layer 9b, and the reduction resistance of the solid electrolyte body made of ceria crystal particles is improved. As a result, the fuel cell produced using the ceria-based solid electrolyte body 3 of the present invention is prevented from being deformed or cracked due to reductive expansion of the solid electrolyte body 3, improving the reliability of the fuel cell, Fabrication is also easy.

  In particular, in order to suppress the reduction reaction more effectively, the average content of the rare earth element RE in the surface layer 9b is 1.2 times or more, particularly 1.4 times the average content of the rare earth element RE in the central portion 9a. The above is desirable.

The ceria-based crystal particle 9 is a composite represented by the general formula (CeO ₂ ) _1-x (REO _3/2 ) _x (x is 0.1 ≦ x ≦ 0.4, RE is a rare earth element excluding Ce). An oxide, that is, one in which REO _3/2 is dissolved in xO mol% in CeO ₂ .

  The rare earth element RE is preferably at least one of Nd, Sm, Gd, Dy, Ho, Y, and Yb. In particular, Sm and Gd are preferable because they have high ionic conductivity.

  The amount of rare earth element RE in the ceria-based crystal particles 9 is preferably 0.1 to 0.4 mol%, particularly preferably 0.1 to 0.2 mol%. By setting the content within this range, the conductivity can be improved and the crystal structure can be stabilized.

  The manufacturing method of the solid electrolyte body of this invention is demonstrated.

First, a CeO ₂ oxide powder in which a rare earth element RE is dissolved is synthesized by a coprecipitation method or an oxalate method. For example, a precipitating agent such as ammonium carbonate is added to a solution in which Ce ions and rare earth ions are dissolved, and the produced Ce and RE carbonate is calcined at 600 ° C. or lower to obtain a rare earth solid solution ceria powder (ceria raw material). Powder). The reason why the calcination temperature is set to 600 ° C. or less is that when calcination is performed at a temperature higher than this, grain growth and fusion between particles occur, and the sintering temperature becomes high.

Such a CeO ₂ raw material powder in which the rare earth element RE is dissolved is heat-treated at a high oxygen partial pressure, pulverized with a ball mill or the like, and fired at 1300 ° C. or lower. The heat treatment under a high oxygen partial pressure will be specifically described.

The CeO ₂ raw powder by heat treatment at a high oxygen partial pressure, deposition of powder out of the rare earth element is dissolved in CeO ₂ raw material powder (Gd, Sm, ^{_{etc.) (2Gd Ce + 3O O x}} + V o 2+ + 1 / 2O ₂ → Gd ₂ O ₃ + 2h ^· ) occurs. In this case, when oxidation treatment was performed under appropriate conditions, precipitation of rare earth elements to the outside of the powder did not occur, and there were many rare earth elements (heterogeneous phases) on the surface of the ceria-based crystal particles as shown in the present invention. It is possible to form a structure.

As described above, the ceria-based oxide powder to which the rare earth element RE is added is heat-treated (oxidized) under a high oxygen partial pressure, so that oxygen ions enter the oxygen vacancies in the ceria-based crystal particles. As a result, in the ceria-based crystal particles, precipitation of rare earth elements RE such as Gd ₂ O _{3 at} the grain boundaries occurs simultaneously with the generation of holes. At this time, when the oxidation treatment is performed under appropriate conditions, the rare earth element RE does not precipitate at the grain boundaries, and as shown in the present invention, a large amount of rare earth element RE exists in the surface layer of the ceria-based crystal particles. In addition, the surface layer can be formed of a composite oxide of trivalent Ce and rare earth element RE (excluding Ce) (hereinafter also referred to as a different phase).

The heat treatment conditions are as follows: the rare earth solid solution CeO ₂ raw material powder has an oxygen partial pressure higher than atmospheric pressure (oxygen partial pressure 2 × 10 ⁴ Pa), the heat treatment temperature is 600 to 1000 ° C., and the heat treatment time is 1 to 10 hours. . In particular, 600 to 800 ° C. and an oxygen partial pressure of 5 × 10 ⁴ Pa or more are desirable, and further, 600 to 700 ° C. and an oxygen partial pressure of 1 × 10 ⁵ Pa or more are desirable.

The reason why the heat treatment temperature is set to 600 to 1000 ° C. is that when the temperature is lower than 600 ° C., the oxidation reaction of CeO ₂ (1 / 2O ₂ + [Vo ^·· → Oo _x + 2h ^· ) does not proceed sufficiently. Insufficient generation of complex oxides of valence Ce and rare earth elements (excluding Ce), and when higher than 1000 ° C., the sintering of particles proceeds, grain growth occurs, and sintering becomes difficult Because. In addition, the oxygen partial pressure was made higher than 2 × 10 ⁴ Pa. Below this, the oxidation reaction of CeO ₂ did not proceed sufficiently, and a complex oxide of trivalent Ce and rare earth elements (excluding Ce) was not formed. This is because it is not generated.

By performing such a heat treatment condition, the rare earth element RE is present more in the raw material powder surface layer than in the center portion of the CeO ₂ raw material powder, and the surface layer has a different phase containing Ce ³⁺ and the rare earth element RE. Will exist.

A solid electrolyte body of the present invention can be obtained by adding a binder or the like to such rare earth solid-solved CeO ₂ raw material powder, mixing it, forming it into a predetermined shape, and firing it in air at 1300 ° C. or lower.

  The reason for setting the firing temperature to 1300 ° C. or lower is that when firing at a higher temperature, diffusion of rare earth elements excessively present in the surface layer to the ceria-based crystal particle central portion 9a proceeds, and the ceria-based crystal particles 9 This is because the reduction suppression effect by the surface layer 9b is reduced.

By producing the rare earth solid solution CeO ₂ raw material powder under the heat treatment conditions as described above, the surface layer contains more rare earth elements than the central part of the raw material powder, and the surface layer in which the rare earth elements are solid solution contains Ce. There will be a heterogeneous phase containing ³⁺ and rare earth elements.

<Measurement method of rare earth element content>
It can be confirmed by X-ray photoelectron spectroscopy (EDS) that the surface layer 9b contains more rare earth elements than the central portion 9a of the ceria-based crystal particles 9. The quantification at that time can be performed by using a NORAN VOYAGER quantitative total system.

<Method for measuring the valence of Ce>
In addition, in the particle surface layer 9b in which a large amount of rare earth elements are present, the presence of trivalent and tetravalent Ce can be confirmed by energy loss spectroscopy (EELS), which is mainly present. In other words, it can be determined which one is present more.

<Method for analyzing composition of GDC>
The composition of GDC can be measured using a fluorescent X-ray analyzer for a sintered body of 25 mmΦ.

<Measurement method of solid solution amount>
The lattice constant can be measured by an X-ray diffractometer, and the solid solution amount can be calculated from the shift amount of the lattice constant.

<Method for measuring Ce content>
Boron acid and sodium carbonate are added to a pulverized sintered body and melted in a hydrochloric acid solution. The rare earth element and Ce in the obtained solution can be quantitatively analyzed with an ICP emission spectroscopic analyzer. .

<Method for determining spherical particles>
It is possible to determine that the particles having a grain boundary that can be clearly confirmed by performing structure observation and having an aspect ratio (= major axis particle diameter / minor axis particle diameter) of 1.5 or less are spherical particles.

<Measuring method of average particle diameter>
The average particle size can be measured by the intercept method after polishing the cross section of the sintered body, performing thermal etching.

<Method for determining composition of composite oxide>
The composition of the composite oxide can be estimated based on the results of EDS and EELS.

First, a 0.05M cerium nitrate aqueous solution and a 0.05M RE nitrate aqueous solution were prepared, and the two aqueous solutions were mixed. Next, 1M ammonium carbonate aqueous solution as a precipitating agent is added dropwise with stirring, and the produced Ce and RE carbonate is calcined at 600 ° C., thereby producing RE _0.2 Ce _0.8 O having an average particle size of 80 nm. _1.9 powder (GDC) was produced.

  This powder was heat-treated at the oxygen partial pressure, temperature and time shown in Table 1, then paraffin wax was added to the raw material powder, mixed and pulverized by a ball mill, press-molded and fired at 1250 ° C. for 2 hours. .

  The lattice constant was measured with an X-ray diffractometer, and the solid solution amount was calculated from the shift amount of the lattice constant. At that time, it goes without saying that peak correction is performed in advance with a standard sample such as Si, and a calibration curve of the solid solution amount and the lattice constant is prepared.

  Observation of the structure was carried out, and particles having a grain boundary clearly confirmed and having an aspect ratio (= major axis particle diameter / minor axis particle diameter) of 1.5 or less were determined as spherical particles.

  The average particle size was measured by the intercept method after polishing the cross section of the sintered body, performing thermal etching.

  In X-ray photoelectron spectroscopy, a sample is machined and ion-etched using TEM-EDX under conditions of an acceleration voltage of 200 kV, a sample absorption current of 10-9A, a measurement time of 70 to 100 seconds, and a beam diameter of 1 nm. It was measured.

The surface layer of the crystal particles was measured by electron energy loss spectroscopy (EELS) under the conditions of an acceleration voltage of 200 kV, a sample absorption current of 10-9A, a measurement time of 5 seconds, and a beam diameter of 1 nm. In the EELS spectrum, the case where the peak was at 130 eV was Ce ⁴⁺ , and the case where the peak was at 125 eV was Ce ³⁺ .

Next, a sintered body having a length of 1 mm, a width of 4 mm, and a length of 15 mm was heat-treated at 850 ° C. for 16 hours in a reducing atmosphere (oxygen partial pressure: 10 ⁻²⁰ MPa), and the length change due to the reduction treatment (ΔL) Table 1 shows the value (ΔL / L) obtained by dividing by the initial length.

The results are shown in Table 1.

According to Table 1, sample no. 1 to 6 and 9 to 14 are formed of a two-layer structure having different valences of RE, so that ΔL / L before and after the reduction treatment is as small as 0.54% or less, and a solid electrolyte body excellent in reliability It turns out that it is.

  On the other hand, a sample No. 2 outside the scope of the present invention in which a two-layer structure having different valences of RE is not formed. In 7 and 8, ΔL / L before and after the reduction treatment was as large as 0.74% and 0.88%, respectively.

It is sectional drawing which shows the fuel battery cell of this invention. It is a schematic diagram which shows the ceria type crystal particle which comprises the solid electrolyte body of this invention. It is a scanning electron micrograph (50,000 times) which shows the solid electrolyte body of this invention. It is a spectrum figure by electron energy loss spectroscopy (EELS).

Explanation of symbols

DESCRIPTION OF SYMBOLS 3 ... Solid electrolyte body 5 ... Fuel electrode 7 ... Oxygen electrode 9 ... Ce-type crystal particle 9a ... Central part 9b ... Surface layer

Claims (4)

(CeO ₂ ) _1-x (REO _3/2 ) _x (where x is 0.1 ≦ x ≦ 0.4, RE is a rare earth element excluding Ce) The solid electrolyte body is characterized in that the valence of Ce in the ceria-based crystal particles is mainly tetravalent at the central portion and trivalent Ce at the surface. The ceria-based crystal grains, claim 1 Symbol placement of the solid electrolyte body, characterized in that it comprises the following spherical particles having an average particle size of 100 nm. 3. The solid electrolyte according to claim 1, wherein the rare earth element RE (excluding Ce) is at least one of Nd, Sm, Gd, Dy, Ho, Y, and Yb. On both sides of the solid electrolyte body according to any one of claims 1 to 3 fuel cell, characterized by comprising forming the oxygen electrode and the fuel electrode, respectively.

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