JP2023022333A - Solid electrolyte, laminate, and fuel cell - Google Patents

Abstract

To provide a solid electrolyte capable of exhibiting excellent proton conductivity and chemical stability in an intermediate-temperature range.SOLUTION: One aspect of the present disclosure provides a solid electrolyte that contains barium zirconate with scandium added, represented by a general formula: BaZr1-xScxO3-δHy, where x is greater than 0.20 and less than 0.65, y is 0.7x to x, and δ is 0 to 0.15x.SELECTED DRAWING: None

Description

The present disclosure relates to solid electrolytes, laminates and fuel cells.

Alkaline electrolyte fuel cells, phosphate fuel cells, solid polymer fuel cells, solid oxide fuel cells, and the like are known as fuel cells, depending on the type of electrolyte. Polymer electrolyte fuel cells are suitable for home use due to their ease of handling and low operating temperature, but they require the use of expensive materials such as platinum and nickel superalloys as catalysts. . Solid oxide fuel cells are attracting particular attention because they do not use a catalyst such as platinum, can be manufactured at low cost, and have high power generation efficiency.

Solid oxide fuel cells operate at a high temperature of 700 to 1000° C., and thus have problems such as the need for preheating for operation and the requirement for durability of constituent materials of the fuel cell. Therefore, for the purpose of lowering the operating temperature, studies have been made on using an oxide having a perovskite structure as an electrolyte as a proton-conducting oxide. In recent years, barium zirconate has been studied as a material that can exhibit proton conductivity in a relatively low temperature range and has excellent chemical stability.

Patent Literature 1 discloses a solid electrolyte laminate formed from yttrium-doped barium zirconate. Further, in Patent Document 2, at least one compound selected from the group consisting of lanthanum strontium cobalt composite oxide, lanthanum strontium cobalt iron composite oxide, and lanthanum strontium iron composite oxide, or a composite of the compound and an electrolyte material and a first solid electrolyte membrane having a composition formula of BaZr _1-x Lu _x O _3-δ (0<x<1), wherein the electrode and the first solid electrolyte A membrane-electrode assembly in contact with a membrane is disclosed. In the literature, the composition formula is BaZr _1-x1 M ¹ _x1 O _3-δ (M ¹ is La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, a second solid electrolyte membrane represented by at least one element selected from the group consisting of Yb, Y, Sc, Mn, Fe, Co, Ni, Al, Ga, and In, 0<x1<1); A form of provision is also disclosed.

JP 2013-206703 A JP 2018-085329 A

An object of the present disclosure is to provide a solid electrolyte that can exhibit excellent proton conductivity and chemical stability in a medium temperature range. Another object of the present disclosure is to provide a laminate comprising the solid electrolyte as described above. Another object of the present disclosure is to provide a fuel cell that is excellent in operation in a medium temperature range.

One aspect of the present disclosure is represented by the general formula: BaZr _1-x Sc _x O _3-δ H _y , where x is greater than 0.20 and less than 0.65, y is between 0.7x and x, and δ A solid electrolyte is provided comprising scandium-doped barium zirconate having a .

The solid electrolyte has a high substitution rate of zirconium (Zr) with scandium (Sc) and contains a predetermined amount of protons, so that it can exhibit excellent proton conductivity and chemical stability in a medium temperature range.

Oxides obtained by replacing zirconium in barium zirconate with trivalent elements (hereinafter also referred to as dopants) such as scandium, yttrium (Y), and lanthanides are known. The substitution rate of zirconium in these oxides is about 20 at % (where x in the above general formula is 0.20) in order to evenly disperse the dopant. Proton conductivity is represented by the product of proton concentration and proton mobility, and generally, improvement of both is necessary for improvement of proton conductivity. It is known that the proton concentration increases in proportion to the concentration of the trivalent dopant substituted for zirconium in barium zirconate. Therefore, it is considered that the proton concentration can be increased by increasing the substitution rate of zirconium with scandium. However, increasing the substitution rate of zirconium with a dopant increases the number of sites where protons form associations (associations of dopant elements and protons), so in general proton diffusibility is significantly reduced. It is said that this leads to a significant decrease in proton mobility. For the reasons described above, even in yttrium-doped barium zirconate, which has the highest proton conductivity, it is said that the substitution ratio of zirconium with a dopant of around 0.2 is excellent in proton conductivity. Also, compared to yttrium and lanthanides, scandium has a higher electronegativity and a higher electrostatic energy that attracts protons. Therefore, when scandium is used as a dopant, it is considered unsuitable as a dopant for obtaining a proton-conducting solid oxide because it lowers the proton conductivity.

Through investigations, the present inventors intentionally used scandium as described above as a dopant, and further increased the ratio of scandium to replace zirconium and the ratio of protons to the amount of scandium substituted more than before. It was found that the conductivity can be improved. Although the reason why the above effects are obtained is not clear, the present inventors presume as follows. First, scandium generally has a high electronegativity and a small ionic radius compared to yttrium and lanthanides. Therefore, an aggregate of protons and scandium is formed at a relatively short distance from the dopant scandium, and the protons are bound near the scandium site. Therefore, proton conductivity is low when scandium is used as a dopant. However, in the presence of multiple protons around scandium and a high concentration of scandium, the repulsive electrostatic interaction between the protons bound to the scandium sites causes the electrostatic Since the attractive force is canceled, the electrostatic attraction felt by protons is reduced. A high concentration of protons is introduced in advance into the solid electrolyte. With such a configuration, protons are supplied in advance to the vicinity of the site where the electrostatic attraction for protons is locally increased due to the proximity of scandium to each other, and the electrostatic attraction felt by the protons trying to pass through the solid electrolyte from the outside is increased. Homogenization can be achieved, and proton conductivity can be enhanced as a whole.

In the above general formula, y may range from 0.8x to x.

In the above general formula, x may be 0.30 or more and less than 0.65, and y may be 0.8x to x.

The solid electrolyte may have an association energy of −30 kJ/mol or more with respect to protons. When the association energy for protons is within the above range, the protons can move more easily, and the proton conductivity can be further improved.

The solid electrolyte may have crystal grains with an average grain size of 3 μm or more. When the average grain size of the crystal grains is within the above range, an increase in the grain boundary density in the solid electrolyte can be suppressed, and the proton conductivity can be further improved.

The solid electrolyte may have an average grain size of 6 to 7 μm. When the average grain size of the crystal grains is within the above range, an increase in grain boundary density in the solid electrolyte can be suppressed, and the proton conductivity can be further improved.

One aspect of the present disclosure includes a first electrode, an electrolyte membrane provided on the first electrode, and a second electrode provided on the side of the electrolyte membrane opposite to the first electrode side. , wherein the electrolyte membrane includes the solid electrolyte described above.

Since the laminate contains the above solid electrolyte as the electrolyte membrane, the laminate has excellent proton conductivity in the intermediate temperature range.

The electrolyte membrane may have a polycrystalline structure.

One aspect of the present disclosure provides a fuel cell comprising the laminate described above.

Since the fuel cell includes an electrolyte membrane containing the solid electrolyte described above, it is excellent in operation in a medium temperature range.

According to the present disclosure, it is possible to provide a solid electrolyte that can exhibit excellent proton conductivity and chemical stability in a medium temperature range. According to the present disclosure, it is also possible to provide a laminate comprising the solid electrolyte as described above. According to the present disclosure, it is also possible to provide a fuel cell that is excellent in operation in a medium temperature range.

FIG. 1 is a graph showing the results of proton conductivity measurement performed on the solid electrolytes of Example 1 and Comparative Example 1. FIG. FIG. 2 is a graph showing the results of proton concentration measurement performed on the solid electrolytes of Example 1 and Comparative Example 1. FIG. 3 is a graph showing the measurement results of the proton diffusion coefficients of the solid electrolytes of Example 1 and Comparative Example 1. FIG. FIG. 2 is a graph showing the evaluation results of chemical stability of the solid electrolyte of Example 1. FIG. FIG. 5 is a scanning electron micrograph showing a part of the fracture surface of the sintered body. FIG. 6 is a graph showing the frequency distribution regarding the grain size of crystal grains. FIG. 7 is a graph showing measurement results of proton diffusion coefficients at different proton introduction rates in BaZr _0.4 Sc _0.6 O _{3-δH y} . FIG. 8 is a graph showing the relationship between the association energy for protons and the introduction rate of protons with respect to the dopant concentration in the solid electrolyte.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings as the case may be. However, the following embodiments are examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents. Positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. The dimensional ratio of each element is not limited to the ratio shown in the drawings.

The materials exemplified in this specification can be used singly or in combination of two or more unless otherwise specified. The content of each component in the composition means the total amount of the multiple substances present in the composition unless otherwise specified when there are multiple substances corresponding to each component in the composition. . Further, in this specification, a numerical range indicated using "to" indicates a range including the numerical values described before and after "to" as the minimum and maximum values, respectively.

One embodiment of the solid electrolyte comprises scandium-doped barium zirconate represented by the general formula: BaZr _1-x Sc _x O _3-δ H _y . The scandium-doped barium zirconate is a compound having a perovskite structure in which a part of zirconium in barium zirconate is replaced with scandium and protons are introduced. The scandium-doped barium zirconate is chemically stable even in a medium temperature range of about 250 to 650.degree. The solid electrolyte is mainly used in the form of a film, sheet or plate, but may be powder. From this point of view, the solid electrolyte can be said to be a powder for a solid electrolyte.

In the scandium-doped barium zirconate, x is more than 0.20 and less than 0.65 in the above general formula, but for example, 0.30 or more and less than 0.65, 0.30 to 0.60, or 0.50 to It may be 0.60. In the above general formula, y ranges from 0.7x to x, but preferably ranges from 0.8x to x from the viewpoint of further improving proton conductivity. In the general formula above, y may be, for example, 0.40 to 0.60, or 0.50 to 0.60. The above x and y are the raw material composition when preparing scandium-doped barium zirconate, the relative humidity, heat treatment temperature, and holding time during heat treatment in a steam atmosphere for the purpose of introducing protons, and the cooling rate of the sample. can be controlled by adjusting In the above general formula, δ ranges from 0 to 0.15x, but may be represented by (xy)/2.

The composition of the solid electrolyte or scandium-doped barium zirconate herein can be determined by energy dispersive X-ray analysis (EDS analysis). A sample obtained by polishing the surface of a dense body of scandium-added barium zirconate sintered in dry air to a thickness of 100 μm or more with silicon carbide abrasive paper to level the surface is used as a measurement target. The surface of the sample is irradiated with an electron beam accelerated at 15 kV, and the characteristic X-ray intensity generated by this is observed using a scanning electron microscope (manufactured by Hitachi High-Tech Technologies Co., Ltd., product name: SU3500). Thus, the composition of the solid electrolyte and scandium-doped barium zirconate can be determined. The ZAF method is used for the quantitative method.

The solid electrolyte may contain other proton-conducting oxides in addition to the scandium-doped barium zirconate described above, and may consist of the above-described scandium-doped barium zirconate alone. When used as an electrolyte membrane for a fuel cell or the like, it is desirable to manufacture the electrolyte membrane more homogeneously. From the viewpoint of forming a more uniform film, the solid electrolyte may consist of only the above-described scandium-doped barium zirconate.

Since the solid electrolyte contains the above-described scandium-doped barium zirconate, the association energy with protons is relatively low. The lower limit of the association energy for protons of the solid electrolyte is, for example, -30 kJ/mol or more, -28 kJ/mol or more, -25 kJ/mol or more, -22 kJ/mol or more, -20 kJ/mol or more, -18 kJ/mol or more. , or −17 kJ/mol or more. When the lower limit of association energy for protons of the solid electrolyte is within the above range, the solid electrolyte can exhibit more excellent proton conductivity. The upper limit of the association energy of the solid electrolyte with protons is not particularly limited, but may be -10 kJ/mol or less, or -15 kJ/mol or less, for example. The association energy for protons of the solid electrolyte can be adjusted within the above range, and may be -30 to -10 kJ/mol, or -30 to -15 kJ/mol, for example.

The association energy for protons herein is determined by examining the temperature dependence of the proton diffusion coefficient. First, the proton conductivity is obtained from the proton concentration measured by thermogravimetric analysis and the AC impedance method, and the proton diffusion coefficient is calculated from the proton conductivity. An Arrhenius plot of the proton diffusion coefficient is generated for the obtained proton diffusion coefficient. It means a value calculated as the difference between the activation energy obtained from the temperature range of 600 to 400° C. in the Arrhenius plot and the activation energy obtained from the temperature range of 100° C. or lower.

The solid electrolyte may be an aggregate of multiple crystal grains, and the aggregate may have a polycrystalline structure. The solid electrolyte contains crystal grains having a relatively large grain size. The lower limit of the average grain size of crystal grains may be, for example, 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, or 6 μm or more. When the lower limit of the average grain size of the crystal grains is within the above range, an increase in grain boundary density in the solid electrolyte can be suppressed, and the proton conductivity can be further improved. The upper limit of the average grain size of the crystal grains is not particularly limited, but may be 10 μm or less, 9 μm or less, 8 μm or less, or 7 μm or less. The average grain size of the crystal grains can be adjusted within the range described above, and can be, for example, 0.5-10 μm, 3-10 μm, or 6-7 μm.

The average grain size of crystal grains in this specification can be determined by image analysis of a scanning electron microscope for a solid electrolyte. Specifically, the target solid electrolyte is mechanically fractured, and 300 or more crystal grains are arbitrarily selected in one field of view at a magnification of 540 times observed by a scanning electron microscope on an unpolished fractured surface. , to measure the grain size (major axis) of each. A frequency distribution curve is created from the measured results, the frequency distribution curve is fitted to the normal distribution curve, and the particle size corresponding to the median value (peak top) of the resulting normal distribution is defined as the average particle size.

Solid electrolytes have excellent proton conductivity in the medium temperature range. Therefore, by using the solid electrolyte, it is not always necessary to make the electrolyte membrane thinner in order to increase the proton conductivity, and the manufacturing cost of devices such as fuel cells can be reduced. The proton conductivity of the solid electrolyte can be, for example, 0.001 Scm ⁻¹ or more, 0.005 Scm ⁻¹ or more, or 0.01 Scm ⁻¹ or more.

The proton conductivity in the middle temperature range in this specification can be determined by AC impedance measurement. Specifically, it is determined according to the method described in the Examples of the present specification. The above proton conductivity means proton conductivity in crystal grains, that is, bulk proton conductivity (σ _H.bulk ), and is represented by the minimum value in the measurement temperature range.

The solid electrolyte described above can be prepared by either synthesis method of solution synthesis and solid phase synthesis. The solid electrolyte described above can be produced, for example, by the following method. One embodiment of a method for producing a solid electrolyte includes, for example, a raw material mixture containing a compound having barium as a constituent element, a compound having zirconium as a constituent element, and a compound having scandium as a constituent element, ethylenediaminetetraacetic acid, and citric acid. and a solvent, and a step of dissolving by adjusting the pH to 9.0 to 10.0 to prepare a reaction solution (reaction solution preparation step); obtaining a solid by reducing the solvent content in the gel (first heat treatment step); and obtaining a heat-treated product by heat-treating the solid at a temperature of 900° C. or higher ( a second heat treatment step), and a step of heat-treating (for example, 1600 ° C.) the pulverized product obtained by pulverizing the heat-treated product to obtain a sintered body containing scandium-added barium zirconate (third heat treatment step), and a step of heat-treating the sintered body under water vapor at a temperature lower than the temperature of the third heat treatment step (for example, 450 ° C.) to introduce protons into scandium-doped barium zirconate (proton introduction step). A third heat treatment step may include, for example, pressing the pulverized material. Pressing may include, for example, uniaxial pressing and cold isostatic pressing.

As a compound containing barium as a constituent element, for example, barium nitrate (Ba(NO ₃ ) ₂ ), barium carbonate (BaCO ₃ ), and the like can be used. Examples of compounds having zirconium as a constituent element include zirconium oxynitrate n-hydrate (ZrO(NO ₃ ) ₂ ·nH ₂ O) and zirconium oxide (ZrO ₂ ). Examples of compounds having scandium as a constituent element include scandium nitrate n-hydrate (Sc(NO ₃ ) _{3.nH 2} O) and scandium oxide (Sc ₂ O ₃ ).

The content ratio of the compound having barium as a constituent element, the compound having zirconium as a constituent element, and the compound having scandium as a constituent element in the raw material mixture is adjusted according to the target composition of the obtained scandium-doped barium zirconate. can do.

The raw material mixture may further contain other compounds in addition to the compound having barium as a constituent element, the compound having zirconium as a constituent element, and the compound having scandium as a constituent element. Other compounds include, for example, compounds containing cerium as a constituent element, sintering aids, and the like. When the raw material mixture further contains a compound containing cerium as a constituent element, the proton conductivity can be further improved. Examples of compounds containing cerium as a constituent element include cerium nitrate hexahydrate and cerium oxide. When the raw material mixture further contains a compound having cerium as a constituent element, the content thereof can be appropriately adjusted according to the zirconium content in the raw material mixture. When the raw material mixture further contains a sintering aid, it is possible to promote the growth of crystal grains. Examples of sintering aids include nickel oxide, zinc oxide, and cobalt oxide. When the raw material mixture contains a sintering aid, the content of the sintering aid is, for example, 0.1 to 10% by mass, or 0.1 to 1% by mass, based on the total amount of the raw material mixture. good.

The solid electrolyte described above is useful, for example, as a constituent member of a fuel cell or the like. A specific example of the constituent member is a laminate in which the above-described solid electrolyte is arranged between electrodes. One embodiment of the laminate includes a first electrode, an electrolyte membrane provided on the first electrode, and a second electrode provided on the side of the electrolyte membrane opposite to the first electrode side. And prepare. The electrolyte membrane contains the solid electrolyte described above.

The first electrode and the second electrode may be made of the same material, or may be made of different materials. When the laminate is a constituent member of a fuel cell, for example, the first electrode may be an anode electrode (fuel electrode) and the second electrode may be a cathode electrode (air electrode).

Materials for the anode electrode include, for example, silver, platinum, palladium, nickel, mixtures of nickel and zirconia oxides, nickel and ceria oxides, and mixtures of nickel and barium zirconate oxides. good.

Materials for the cathode electrode include, for example, silver, platinum, and perovskite-type oxides containing rare earth elements and 3d transition metals. As such a perovskite-type oxide, for example, an oxide represented by the general formula: A1 _1-a A2 _a B1 _1-b B2 _b O ₃ (0≦a≦1, 0≦b≦1) is used. be able to. Here, A1 and A2 are lanthanides (Ln) such as lanthanum (La), cerium (Ce), praseodymium (Pr), and samarium (Sm), or calcium (Ca), strontium (Sr), barium (Ba ) and other alkaline earth metals (AE). B1 and B2 may be manganese (Mn), iron (Fe), cobalt (Co), and the like. Specific examples of materials for the cathode electrode include Ln _1-a AE _a Mn _1-b Fe _b O ₃ , Ln _1-a AE _a Mn _1-b Co _b O ₃ , and Ln _1-a AE _a Co _1-b Fe _b O ₃ , AE _1-a AE _a Co _1-b Fe _b O ₃ and the like.

The thicknesses of the first electrode and the second electrode can be adjusted according to the use of the laminate.

The thickness of the electrolyte membrane can be adjusted according to the performance required of the laminate, the use of the laminate, and the like. The upper limit of the thickness of the electrolyte membrane may be, for example, 500 μm or less, 300 μm or less, 150 μm or less, 50 μm or less, 25 μm or less, or 15 μm or less. By setting the upper limit of the thickness of the electrolyte membrane within the above range, the proton conductivity can be further improved. The lower limit of the thickness of the electrolyte membrane may be, for example, 0.1 μm or more, 0.5 μm or more, 1.0 μm or more, 3 μm or more, 5 μm or more, or 10 μm or more. By setting the lower limit of the thickness of the electrolyte membrane within the above range, it is possible to suppress the deterioration of the mechanical strength of the electrolyte membrane itself and to more sufficiently suppress the dielectric breakdown between the electrodes. The thickness of the electrolyte membrane can be adjusted within the ranges described above, and can be, for example, 0.1-500 μm, 1.0-50 μm, or 10-15 μm.

The laminate may include other layers in addition to the first electrode, the electrolyte membrane, and the second electrode. Other layers include, for example, a layer inserted between the first electrode and the electrolyte and a layer inserted between the second electrode and the electrolyte. Examples of the layer inserted between the first electrode and the electrolyte include a nickel and barium zirconate-based mixture layer, a nickel and zirconia-based oxide mixed layer, and a nickel and cerium oxide-based oxide mixed layer. Examples of the layer inserted between the second electrode and the electrolyte include cerium oxide-based oxides, barium zirconate-based oxides, cerium-added barium zirconate-based oxides, and barium cerate-based oxides. .

Since the solid oxide fuel cell having the above laminate contains the above solid oxide as an electrolyte, the operating temperature can be set low. The upper limit of the operating temperature of the solid oxide fuel cell can be, for example, 600° C. or less, 550° C. or less, 540° C. or less, 530° C. or less, 500° C. or less, or 450° C. or less. The lower limit of the operating temperature of the solid oxide fuel cell can be, for example, 370° C. or higher, 380° C. or higher, or 390° C. or higher. The operating temperature of the solid oxide fuel cell can be adjusted within the ranges described above, and can be, for example, 370-550°C, 390-540°C. Since the solid oxide fuel cell having the above-described laminate contains the above-described solid oxide as an electrolyte, it can be stably operated for a long period of time (for example, 200 hours or more).

Although several embodiments have been described above, the present disclosure is not limited to the above embodiments. Also, the descriptions of the above-described embodiments can be applied to each other.

Hereinafter, the contents of the present disclosure will be described in more detail with reference to examples and comparative examples. However, the present disclosure is not limited to the following examples.

(Example 1)
[Preparation of scandium-doped barium zirconate represented by BaZr _0.4 Sc _0.6 O _{3-δH y} ]
Barium nitrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Ba(NO ₃ ) ₂ , purity: 99.9% by mass) and zirconium oxynitrate dihydrate (manufactured by Kanto Chemical Co., Ltd., ZrO(NO ₃ ) _{2.2H 2} O) and scandium nitrate n-hydrate (Kojundo Chemical Laboratory Co., Ltd., Sc(NO ₃ ) _{3.nH 2} O, purity: 3N) are combined into the desired scandium-added barium zirconate (a composition formula in which x is 0.6 in the general formula), and dissolved in about 800 mL of distilled water to prepare an aqueous solution. As for the hydrates, the blending amount was adjusted after confirming the amount of hydration by thermogravimetric analysis.

Ethylenediaminetetraacetic acid (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., EDTA, purity: 99.0% by mass) and citric acid (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., purity: 99.0% by mass) are added to the above aqueous solution. , based on the total number of moles of metal ions in the aqueous solution, is further added so that each amount is 0.5 moles, and the pH of the reaction solution is adjusted to 9.0 or more and 10 or less with an aqueous ammonia solution. The reagent was dissolved to obtain a reaction solution. Next, the reaction solution was heated with stirring on a hot stirrer for about 300 minutes to react the contents and remove the solvent out of the system. At this time, the surface temperature of the hot stirrer was adjusted to 180°C. The obtained gel was heated in a microwave heating furnace to remove the solvent contained in the gel to obtain a solid.

The solid material obtained as described above was placed in a heating furnace, heated from room temperature at a temperature increase rate of 5° C./min, and fired for 10 hours after reaching 900° C. to obtain a precursor. . The obtained precursor is placed in a container, a predetermined amount of ethanol and zirconia balls are added, and a planetary ball mill (manufactured by Fritsch, product name: planetary ball mill Pulverisette 7) is used for 2 hours at a rotational speed of 300 rpm. , wet pulverized to obtain a pulverized product. The grind was dried. At the end of the pulverization step, it was confirmed by X-ray diffraction that a mixture of scandium-added barium zirconate, scandium-added zirconium oxide, and barium carbonate was formed.

The pulverized mixture was molded into pellets having a diameter of 20 mm and a thickness of about 2 mm using a uniaxial pressure molding machine, and further pressed at 300 MPa by a cold isostatic pressing method. The pellets were heat-treated at 1600° C. for 24 hours while flowing dry air at a flow rate of 100 mL/min to obtain a sintered body. The sintered body was shrunk to a diameter of 15 mm by sintering, and the density of the sintered body was 5.3 g/cm ³ . The composition of the obtained sintered body was analyzed by the X-ray diffraction method (XRD method) and confirmed to have a single perovskite structure. Also, the composition of the obtained sintered body was analyzed by energy dispersive X-ray spectroscopy (EDS method), and it was confirmed to have a cation composition of BaZr _0.4 Sc _0.6 O _3-δ .

[Measurement of bulk proton conductivity (σ _{H. bulk} ) of scandium-doped barium zirconate (Zirconium substitution rate by scandium is 60 at%)]
For each of the scandium-doped barium zirconate (solid electrolyte) prepared in Example 1, the proton conductivity (σ _{H. bulk} ) in the bulk under an intermediate temperature range was measured by AC impedance measurement.

Specifically, first, both surfaces of the sintered body were polished to adjust the thickness to 458 μm. Next, on both sides of the sintered body after polishing, a silver layer with a thickness of 650 nm was formed using a DC sputtering device (manufactured by Sanyu Electronics Co., Ltd., product name: SC-701HMC II), and this was measured. was taken as a sample.

Prepare a current collector consisting of a silver mesh (manufactured by Nilaco Corporation) and a gold wire with a diameter of 0.1 mm, and apply silver paste (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., product name: TR-3025) to the current collector. Then, the paste was dried in a drier at a temperature of 55° C. with the current collector adhered to both polished surfaces of the measurement sample. A measurement sample was obtained by heat-treating the obtained laminate at about 800° C. for 1 hour in a dry argon atmosphere. The measurement sample was connected to an atmosphere-controllable electrochemical measurement cell via a gold wire. A measurement sample was heated to about 800° C. in a dry argon atmosphere, and upon reaching 800° C., the atmosphere was switched to an argon atmosphere with a water vapor pressure of 0.02 atm. After that, the proton conductivity was measured by sweeping the frequency from 0.1 to 10 ⁶ Hz by the AC impedance method in the temperature range of 623 to 48° C. while decreasing the temperature. Measurement at each temperature was performed by continuously measuring AC impedance and sufficiently stabilizing it until the resistance value did not change. The results are shown in FIG. In FIG. 1, the solid electrolyte of Example 1 is indicated by "60Sc".

(Comparative example 1)
Scandium-added zirconate was prepared in the same manner as in Example 1, except that the substitution rate of zirconium with scandium was changed from 60 at% to 20 at%, and the target composition was changed to BaZr _0.8 Sc _0.2 O _3-δ. A sintered body of barium was prepared.

[Measurement of proton conductivity (σ _{H. bulk} ) in bulk of scandium-doped barium zirconate (zirconium substitution rate by scandium is 20 at%)]
The proton conductivity of the measurement sample of Comparative Example 1 was measured. Specifically, first, both surfaces of the sintered body were polished to adjust the thickness to 491 μm. On both sides of the sintered body after polishing, a silver layer with a thickness of 1300 nm was formed using a DC sputtering device (manufactured by Sanyu Electronics Co., Ltd., product name: SC-701HMC II), and this was used as a measurement sample. .

Prepare a current collector consisting of a silver mesh (manufactured by Nilaco Corporation) and a gold wire with a diameter of 0.1 mm, and apply silver paste (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., product name: TR-3025) to the current collector. Then, the paste was dried in a drier at a temperature of 55° C. with the current collector adhered to both polished surfaces of the measurement sample. A measurement sample was obtained by heat-treating the obtained laminate at about 800° C. for 1 hour in the atmosphere. The measurement sample was connected to an atmosphere-controllable electrochemical measurement cell via a gold wire. The measurement sample was heated to about 800° C. in a dry argon atmosphere and held for 90 minutes to dehydrate the introduced protons. After that, the temperature was lowered to 700° C., and upon reaching 700° C., the atmosphere was switched to an argon atmosphere with a water vapor pressure of 0.02 atm. Protons were introduced by holding for 15 to 30 minutes, and proton conduction was then swept at a frequency of 4 to 8×10 ⁶ Hz by the AC impedance method in a temperature range of 389 to 88° C. while decreasing the temperature. degree was measured. The proton introduction time at each temperature was recorded for proton concentration measurement during this measurement. The results are shown in FIG. In FIG. 1, the solid electrolyte of Comparative Example 1 is indicated by "20Sc".

As shown in FIG. 1, it was confirmed that the solid electrolyte prepared in Example 1 as described above had an extremely excellent proton conductivity of 0.01 S/cm or more in a medium temperature range of 396 to 534°C. .

[Measurement of Proton Concentration in Bulk of Scandium-Added Barium Zirconate (Zirconium Substitution Rate by Scandium is 60 at %)]
The proton concentration in the scandium-doped barium zirconate (the substitution rate of zirconium by scandium was 60 at %) was measured by thermogravimetric analysis under the proton conductivity measurement conditions.

Specifically, the sintered body used in the measurement of proton conductivity was pulverized in an agate mortar, and a planetary ball mill (manufactured by Fritsch, product name: planetary ball mill Pulverisette 7) was used at a rotational speed of 300 rpm. A ground product was obtained by wet grinding for 2 hours. The obtained powder was subjected to heat treatment at 1000° C. for 1 hour in a dry argon atmosphere using a thermogravimetric analyzer (product name: STA449F3 Jupiter manufactured by Netsch). Protons were introduced into the scandium-doped barium zirconate by humidifying the atmosphere around the pulverized material with water vapor and adjusting the water vapor partial pressure ( _pH2O ) to 0.02 atm. The atmosphere was switched to a humidified atmosphere and the temperature was maintained until equilibrium was reached with no mass change. Assuming that oxygen vacancies in scandium-doped barium zirconate are hydrated by water molecules, the proton introduction rate was calculated from the mass increase observed in the above operation.

Next, the temperature of the sintered body was lowered stepwise to 200° C. to obtain the temperature dependence of the proton introduction rate. As the temperature was lowered stepwise, the proton introduction rate increased, and the proton concentration was introduced up to 0.55 mol per unit perovskite at around 400°C. Thus, y in BaZr _0.4 Sc _0.6 O _{3-δH y} was determined to be 0.55. The results are shown in FIG. In FIG. 2, the solid electrolyte of Example 1 is indicated by "60Sc". FIG. 2 also shows the results of a similar experiment performed on the sintered body obtained in Comparative Example 1, which will be described later.

[Measurement of Proton Concentration in Bulk of Scandium-Added Barium Zirconate (Zirconium Substitution Rate by Scandium is 20 at %)]
The proton concentration of the scandium-doped barium zirconate prepared in Comparative Example 1 was measured by thermogravimetric analysis under the proton conductivity measurement conditions.

Specifically, the proton concentration of the sintered body used in the proton conductivity measurement was measured using a thermogravimetric analyzer (product name: STA449F3 Jupiter manufactured by Netsch). Using the same temperature and atmosphere histories as the proton conductivity measurements, the proton concentration in the scandium-doped barium zirconate was determined during the proton conductivity measurements. After heat treatment at 800° C. for 90 minutes in a dry argon atmosphere. Assuming that the proton concentration in the dense body before treatment is zero and that the oxygen vacancies in the scandium-doped barium zirconate are hydrated by water molecules, the proton introduction rate was calculated.

Then, the proton introduction rate was increased by lowering the temperature stepwise, and the proton concentration was introduced up to 0.05 mol per unit perovskite. Thus, y in BaZr _0.8 Sc _0.2 O _{3-δH y} used for proton conductivity measurement was determined to be 0.05. The results are shown in FIG. In FIG. 2, the solid electrolyte of Comparative Example 1 is indicated by "20Sc".

[Calculation of Diffusion Coefficient (D _{H. bulk} ) and Association Energy in Scandium-Added Barium Zirconate Bulk]
Using the proton conductivity in scandium-doped barium zirconate measured by the AC impedance method and the proton concentration measured by thermogravimetric analysis, the temperature dependence of the proton diffusion coefficient was calculated.

Specifically, _D.H. It was calculated based on the Nernst-Einstein relational expression of _bulk = (σ _{H. bulk} RT)/(F ² C _H ). where R is the gas constant, T is the temperature, F is the Faraday constant, and _CH is the proton concentration. The proton diffusion coefficient was calculated by substituting the proton conductivity and proton concentration at each temperature. Here, the temperature dependence of the proton concentration employs the values indicated by the solid line and white symbols in FIG. An Arrhenius plot of the proton diffusion coefficient obtained is shown in FIG. In FIG. 3, BaZr _0.4 Sc _0.6 O _{3-δH 0.55} of Example 1 is indicated by “60Sc”, and BaZr _0.8 Sc _0.2 O _{3-δH 0.5 of Comparative Example 1 is indicated by “60Sc”. 05} was indicated by "20Sc".

The Arrhenius plot of the proton diffusion coefficient in BaZr _0.4 Sc _0.6 O _3-δ H _0.55 of Example 1 increased in slope as the temperature decreased. Activation energy in proton diffusion of the solid electrolyte of Example 1 was calculated from the slope of the Arrhenius plot in two regions of the temperature range of 623 to 400° C. and the temperature range of 100 to 48° C. . The activation energies in the high temperature range (temperature range of 623 to 400 ° C.) and low temperature range (temperature range of 100 to 48 ° C.) are 21 kJ / mol and 44 kJ / mol, respectively. It was confirmed that the energy increased in the low temperature range. The difference between these values, −23 kJ/mol, is the energy of proton aggregate formation. As described above, the association energy was calculated as the difference in activation energy for proton diffusion between the high temperature range and the low temperature range.

As shown in FIG. 3, the diffusion coefficient in BaZr _0.8 Sc _0.2 O _{3-δH 0.05} of Comparative Example 1 is compared with scandium-doped barium zirconate (the substitution rate of zirconium by scandium is 60 at%). It showed a low value. In the temperature range of 200° C. or less of the Arrhenius plot shown in FIG. 3, the activation energy for proton diffusion in the solid electrolyte of Comparative Example 1 was calculated from the slope of the Arrhenius plot, and was estimated to be 54 kJ/mol. In the case of scandium-doped barium zirconate (the substitution rate of zirconium by scandium is 20 at%), the activation energy in the high temperature range is equal to the value obtained with scandium-doped barium zirconate (the substitution rate of zirconium by scandium is 60 at%). Assuming that, the association energy was calculated to be −32 kJ/mol.

[Evaluation of chemical stability]
Chemical stability was evaluated using the measurement sample of Example 1 prepared during proton conductivity measurement. Specifically, the proton conductivity was measured continuously for 200 hours while replacing the water vapor supplied to the measurement sample with heavy water (D ₂ O) and light water (H ₂ O). The measurement conditions were a water vapor pressure of 0.02 atm and a temperature of 400°C. The proton conductivity when heavy water was supplied was 0.078 S/cm, and the proton conductivity when light water was supplied was 0.013 S/cm. The results are shown in FIG.

As shown in FIG. 4, the solid electrolyte prepared in Example 1 maintained proton conductivity for at least 200 hours, confirming that the obtained solid electrolyte has excellent chemical stability. did it.

[Measurement of average grain size of crystal grains]
The average grain size of the scandium-doped barium zirconate crystal grains prepared in Example 1 was measured. Specifically, first, the pulverized product is molded into pellets using a uniaxial pressure molding machine, and further pressurized at 300 MPa by a cold isostatic pressing method to obtain pellets having a thickness of 2 mm and a diameter of 20 mm. was prepared. The pellets were heat-treated at 1600° C. for 24 hours while flowing dry air at a flow rate of 100 mL/min to obtain a sintered body. The density of the sintered body was 5.3 g/cm ³ . Next, both sides of the sintered body were polished to adjust the thickness to 458 μm. The polished sintered body thus obtained was bent and mechanically fractured, and the resulting fractured surface was observed with a scanning electron microscope. In one field obtained at a magnification of 540 times, 300 crystal grains were arbitrarily selected and the grain size (major axis) of each was measured. A frequency distribution curve was created from the measured results, the frequency distribution curve was fitted to a normal distribution curve, and the average particle size was determined from the median value (peak top) of the normal distribution. The grain size obtained was 6.7 μm. The results are shown in FIGS. 5 and 6. FIG. FIG. 5 is a scanning electron micrograph showing a part of the fracture surface of the sintered body. FIG. 6 is a graph showing the frequency distribution of grain sizes of crystal grains in the polycrystalline structure shown in FIG.

(Example 2)
Scandium-doped barium zirconate having a composition represented by BaZr _0.4 Sc _0.6 O _3-δ H _0.51 (y=0.51) was prepared by changing the proton introduction rate. The proton introduction rate was adjusted using a thermogravimetric analyzer. After deprotonation for 1 hour at 800°C in a dry argon atmosphere, the atmosphere was switched to a humidified argon atmosphere with a water vapor partial pressure of 0.02 atm and the temperature was lowered in steps of 50°C to 450°C. After holding at 450° C. for 12 hours, it was rapidly cooled to room temperature, and the proton concentration was determined. The proton conductivity measurement was performed in a dry argon atmosphere in a temperature range of 130° C. or less where the proton concentration does not change kinetically.

(Comparative Examples 2 to 6)
By changing the proton introduction treatment temperature and retention time by the thermogravimetric analyzer, y (proton introduction rate) in the composition formula was changed from 0.37 (Comparative Example 2), 0.24 (Comparative Example 3), 0.37 (Comparative Example 2), 0.24 (Comparative Example 3), Scandium-doped barium zirconate was prepared in the same manner as in Example 2, except that the values were 16 (Comparative Example 4), 0.10 (Comparative Example 5), and 0.02 (Comparative Example 6).

[Evaluation of the influence of proton introduction rate 1]
For each of the scandium-doped barium zirconate (solid electrolytes) prepared in Example 2 and Comparative Examples 2 to 7, the proton diffusion coefficient (D _{H. bulk} ) in the bulk under an intermediate temperature range was measured by AC impedance measurement, and the proton The impact of adoption rate was evaluated. The method of calculating the proton diffusion coefficient is the same as the measurement of proton conductivity in Example 1. The results are shown in FIG. For reference, FIG. 7 also shows the results of using the solid electrolyte of Example 1 (results when the proton concentration was 0.55 mol per unit perovskite).

As shown in FIG. 7, when a solid electrolyte with a low proton introduction rate is used, the proton diffusion coefficient is low, and when the solid electrolytes prepared in Examples 1 and 2 with a high proton introduction rate are used, It was confirmed that an excellent proton diffusion coefficient was exhibited. As a result, it became clear that the high proton conductivity in the medium temperature range shown in Example 1 could be achieved. In addition, as shown in FIG. 7, in scandium-doped barium zirconate, the proton diffusion coefficient fluctuates greatly depending on the proton introduction rate, which is a behavior that cannot be seen when dopants other than scandium are used. It was confirmed that

[Evaluation of the influence of proton introduction rate 2]
Based on the proton diffusion coefficient measured in Example 2, the association energy with respect to protons was measured, and the influence of the proton introduction rate was evaluated. Specifically, from the activation energy (E _a.high ) in the high temperature range of BaZr _0.4 Sc _0.6 O _{3-δH 0.55} in the proton diffusion coefficient of 21 kJ/mol, in FIG. The association energy was calculated by subtracting the activation energy (E _a.low ) calculated in the low temperature range of 100° C. or less of the proton diffusion coefficient shown. The results are shown in FIG.

FIG. 8 is a graph showing the relationship between the association energy for protons and the introduction rate of protons into the solid electrolyte. As shown in FIG. 8, it was confirmed that as the proton introduction rate in scandium-doped barium zirconate approaches the scandium addition rate, the association energy with protons can be suppressed to a low level. In BaZr _0.4 Sc _0.6 O _{3-δH y} , proton introduction of 0.5 mol or more per unit perovskite with a value of y corresponding to about 80% of the substitution rate of zirconium by scandium (60 at%) It was confirmed that at a proton concentration that provides a ratio, the association energy is −30 kJ/mol or more, which enables excellent proton conductivity to be exhibited.

According to the present disclosure, it is possible to provide a solid electrolyte that can exhibit excellent proton conductivity and chemical stability in a medium temperature range. According to the present disclosure, it is also possible to provide a laminate comprising the solid electrolyte as described above. According to the present disclosure, it is also possible to provide a fuel cell that is excellent in operation in a medium temperature range.

Claims (9)

General formula: represented by BaZr _1-x Sc _x O _3-δ H _y , where x is more than 0.20 and less than 0.65, y is 0.7x to x, and δ is 0 to 0.15x A solid electrolyte comprising a scandium-doped barium zirconate. 2. The solid electrolyte according to claim 1, wherein y is 0.8x to x in the general formula. 3. The solid electrolyte according to claim 1, wherein in the general formula, x is 0.30 or more and less than 0.65, and y is 0.8x to x. 4. The solid electrolyte according to any one of claims 1 to 3, which has an association energy with respect to protons of -30 kJ/mol or more. The solid electrolyte according to any one of claims 1 to 4, wherein the average grain size of crystal grains is 3 µm or more. The solid electrolyte according to any one of claims 1 to 5, wherein the average grain size of crystal grains is 6 to 7 µm. a first electrode, an electrolyte membrane provided on the first electrode, and a second electrode provided on the opposite side of the electrolyte membrane to the first electrode side,
A laminate, wherein the electrolyte membrane comprises the solid electrolyte according to any one of claims 1 to 6. 8. The laminate according to claim 7, wherein said electrolyte membrane has a polycrystalline structure. A fuel cell comprising the laminate according to claim 7 or 8.

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