JP2009023883A - Proton conductor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a proton conductor having a reduced overall resistivity by enlarging crystal diameters of the proton conductor. <P>SOLUTION: In the proton conductor, which is the oxide having a perovskite structure represented by the composition formula of AZrO<SB>3</SB>, wherein A is an alkaline earth metal, Zr is substituted by Ma and Mb elements, wherein Ma element represents at least one element of an element group consisting of Y, Ho, Er, Tm, Yb and Lu, and Mb element represents at least one element of Sc and In. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a proton conductor, in particular, a proton conductor that can be used as an electrolyte or a hydrogen permselective membrane of a fuel cell and a fuel cell including the same.

  A fuel cell is a power generation device with high power generation efficiency that can directly convert chemical energy generated by an electrochemical reaction between hydrogen and oxygen into electric energy without passing through thermal energy. A wide range of applications is expected from portable electronic devices such as notebook computers and mobile phones to transportation facilities such as cars and trains, home generators, and power plants, and development is underway.

Fuel cells include solid polymer types, alkali types, phosphoric acid types, solid oxide types, and the like. Among them, the solid oxide type has a high power generation efficiency of 55 to 60%. Solid oxide fuel cells use an oxide having a perovskite structure as an electrolyte, and the development of electrolytes with high ion conductivity is being promoted along with the development of fuel cells. For example, Patent Document 1 proposes an oxide having a perovskite structure, which is a mixed ionic conductor of protons and oxygen ions.
JP 2000-302550 A

The solid electrolyte used in a fuel cell or the like is preferably as low as possible in order to reduce the loss due to Joule heat and improve the efficiency of the fuel cell or the like. Of the oxides having a perovskite structure proposed in Patent Document 1, those in which BaCeO ₃ is doped with Gd and part of Ce is substituted have a high electrical conductivity of about 10 ⁻² Scm ⁻¹ at 600 ° C. However, there is a drawback that it decomposes in an atmosphere containing CO ₂ and H ₂ O which are reaction products of the fuel cell. On the other hand, BaZrO ₃ doped with Y and partially substituted with Zr is stable even in an atmosphere containing CO ₂ and H ₂ O, but its electrical conductivity is 10 ⁻³ Scm ⁻¹ at 600 ° C. The degree is lower than that of BaCeO ₃ doped with Gd.

In BaZrO ₃ , the crystal grain boundary has a higher resistance than that in the crystal grain. Therefore, it is preferable that the crystal grain size is large in order to reduce the crystal grain boundary density and reduce the overall resistance. However, when BaZrO ₃ is doped with Y, crystal grains do not grow during sintering, and the crystal grains are small, so that the grain boundary density is high and the overall resistance is high.

  Accordingly, an object of the present invention is to provide a proton conductor made of an oxide having a perovskite structure with large crystal grains and low overall resistance.

To achieve the above object, the proton conductor of the present invention, one element of the composition formula _{AZr 1-xy Ma x Mb y} O 3 (A is an alkaline earth metal, Ma is Y, Ho, Er, At least one element of the element group consisting of Tm, Yb, and Lu, Mb is at least one element of Sc and In, and x and y are 0 <x <1, 0 <y <1, 0 <. x + y <1), and an oxide having a perovskite structure.

  Further, the present invention is characterized in that in the proton conductor configured as described above, A is Ba, Ma is Y, and Mb is Sc.

  In the proton conductor having the above-described configuration, x / y ≧ 1 may be satisfied, or x = 0.1 and y = 0.05 may be satisfied.

  In addition, the fuel cell of the present invention includes the proton conductor having the above-described configuration.

  According to the present invention, a proton conductor having a low resistance can be obtained because the crystal grain size is large and the proportion of the entire crystal grain boundary having a higher resistance than that in the crystal grains is small. Further, by using a proton conductor having a low resistance as the electrolyte, generation of Joule heat and loss due thereto can be suppressed, and the efficiency of the fuel cell can be increased.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a unit cell showing a structure of arrangement of atoms of a proton conductor according to the present invention.

In the unit cell shown in FIG. 1, the oxide having a perovskite structure represented by the composition formula of AZrO ₃ has an A atom at each vertex, a Zr (zirconium) atom at the center (body center), and an O (oxygen) atom at each Located at the center (face center) of the surface.

The proton conductor of the present invention is an oxide having a perovskite structure represented by the composition formula of AZrO ₃ , and a part of Zr is substituted with two kinds of elements Ma and Mb. In other words, the proton conductor of the present invention can be expressed as _{AZr 1-xy Ma x Mb y} O 3. Here, the element A is at least one element among alkaline earth metals. The element Ma is at least one element of the element group consisting of Y (yttrium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium), and the element Mb is , Sc (scandium) and In (indium). Y is preferable as the element Ma, and Sc is preferable as the element Mb. x and y preferably satisfy x / y ≧ 2, more preferably satisfy 0.05 ≦ x ≦ 0.3, more preferably 0.05 ≦ y ≦ 0.3, and satisfy x / y ≧ 1. More preferably.

The proton conductor of the present invention is obtained by replacing Zr, which is a cation having a valence of 4, with elements Ma and Mb, which are cations having a valence of 3. The element group composing the element Ma is composed of elements of rare earth elements having a stable cation valence of 3 and an ionic radius close to or less than Zr. Further, Sc constituting the element Mb has an ion radius of rare earth element and the smallest stable cation has a valence of 3, and In has an ion radius close to that of Sc, and the stable cation has a valence of 3. By adding the element Ma and further adding the element Mb, crystal growth at the time of sintering can be promoted, so that the crystal grain boundary density can be reduced. A solid electrolyte containing a BaZrO ₃ proton conductor has lower electrical conductivity at the grain boundary than inside the crystal grain, and therefore the overall electrical conductivity can be improved by reducing the grain boundary density. it can.

  Here, the alkaline earth metal refers to Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), and Ra (radium), and the rare earth elements include Sc, Y, La ( Lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho, It refers to Er, Tm, Yb, and Lu.

Further, since the element Ma and the element Mb have valences of 3, the proton conductor of the present invention has oxygen vacancies, and the oxygen number is stoichiometric if the valence of oxygen is −2 and stable. for less than the ratio, the composition formula is precisely should be denoted as _{AZr 1-xy Ma x Mb y} O 3- δ. However, in the present embodiment will be denoted as _{AZr 1-xy Ma x Mb y} O 3 even in such a case.

(Fuel cell using proton conductor of the present invention)
FIG. 2 shows a schematic diagram of a fuel cell using the proton conductor of the present invention. The fuel cell 10 includes an anode 11 that decomposes hydrogen molecules into protons, a solid electrolyte 12 that includes the proton conductor of the present invention that conducts protons supplied from the anode 11, and a proton supplied from the solid electrolyte 12 is oxidized. The cathode 13 is laminated in order. The anode 11 and the cathode 13 are connected to the load 20 by wiring. The anode 11 is also called a hydrogen electrode, and the cathode 13 is also called an air electrode.

Hydrogen gas (H ₂ ) is supplied to the anode 11, and oxygen gas (O ₂ ) or air containing oxygen gas is supplied to the cathode 13 from the outside. At the anode 11, hydrogen gas is oxidized to generate protons and electrons. Protons generated at the anode 11 are supplied to the cathode 13 via the solid electrolyte 12, and electrons are supplied to the cathode 13 via the load 20. At the cathode 13, oxygen gas is reduced by protons supplied from the solid electrolyte 12 and electrons supplied from the anode 11 to generate water (H ₂ O). The load 20 is driven by the potential difference generated by such a redox reaction. By using the proton conductor of the present invention having high electrical conductivity as the solid electrolyte 12, generation of Joule heat and loss due thereto can be suppressed, and the efficiency of the fuel cell 10 can be increased.

(Sample preparation method)
The powders of ACO ₃ , ZrO ₂ , Ma ₂ O ₃ and Mb ₂ O ₃ are used as starting materials to prepare by a solid phase reaction method. Each powder weighed in a predetermined distribution is put in a plastic container together with zirconia (ZrO ₂ ) balls, and 2-propanol is added to perform a ball mill for 24 hours. The ball-milled powder mixture is transferred to an eggplant flask and dried under reduced pressure at 50 ° C. using an evaporator, then transferred to a beaker and 2-propanol is evaporated at 120 ° C. using a hot stirrer.

  After calcination of the powder mixture in which 2-propanol was evaporated at 1000 ° C., 2-propanol was added, ball milled for 8 hours, transferred to an eggplant flask and dried under reduced pressure at 50 ° C. using an evaporator. Transfer and evaporate 2-propanol at 120 ° C. using a hot stirrer.

  The powder mixture obtained by evaporating 2-propanol is formed into pellets at a pressure of 9.8 MPa using a die having a diameter of 19 mm and held at 1300 ° C. for 10 hours. After that, the pellets were pulverized in an agate mortar at room temperature, 2-propanol was added, ball milled for 8 hours, transferred to an eggplant flask and dried under reduced pressure at 50 ° C. using an evaporator, then transferred to a beaker and used with a hot stirrer. Evaporate 2-propanol at 120 ° C.

  The powder mixture from which 2-propanol has been evaporated is again formed into pellets under the same conditions as described above, and held at 1300 ° C. for 10 hours. Thereafter, the pellet is pulverized in an agate mortar, and the obtained powder is subjected to phase identification using an X-ray diffractometer.

The powder in which the single phase of the AZrO ₃ structure was confirmed by the phase identification by the X-ray diffractometer was added with 2-propanol, ball milled for 100 hours, dried under reduced pressure using an evaporator, and then using a hot stirrer. Evaporate 2-propanol.

The powder in which the phase other than the AZrO ₃ structure was confirmed by the phase identification by the X-ray diffractometer was added with 2-propanol, ball milled for 8 hours, dried under reduced pressure using an evaporator, and then using a hot stirrer. Evaporate 2-propanol. The powder mixture obtained by evaporating 2-propanol is again formed into pellets under the same conditions as described above, held at 1300 ° C. for 10 hours, and then pulverized in an agate mortar. The obtained powder is identified by using an X-ray diffractometer, and the above process is repeated.

The powder in which the single phase of the AZrO ₃ structure is confirmed and the binder liquid having a mass 1/7 of the mass of the powder are mixed for 15 minutes in an agate mortar and passed through a stainless steel sieve having an opening of 150 μm. Here, the binder liquid is a liquid obtained by mixing 10 ml of ethanol, 2 g of polyvinyl alcohol (PVA), 1 ml of glycerin, and 200 ml of deionized water. The sieved sample is formed into pellets by holding for 10 minutes at a pressure of 392 MPa using a die having a diameter of 11 mm, and held at 600 ° C. for 8 hours to remove the binder liquid. The pellet is sintered at 1600 ° C. for 24 hours in an oxygen atmosphere. The sintered pellets are uniformly polished on the surface.

When sintering, the A component may be evaporated and dissipated from the pellets. Then, it heats in the state which embedded the pellet in the protective powder containing A component, and produces a sintered compact. For the protective powder, the sample powder (mixture of each powder weighed in the above-mentioned predetermined distribution) and ACO ₃ were weighed to a mass ratio of 9: 1, mixed in an agate mortar for 10 minutes, and sieved. Is.

The sintered pellets thus produced are measured for the diameter and thickness with a micrometer, and the volume is measured with the Archimedes method using an ionic liquid. As the ionic liquid, an aliphatic quaternary ammonium type TMHA-Tf2N (trimethyl-n-hexylammonium bis [(trifluoromethyl) -sulfonyl] amide) having a density of 1.33 g · cm ⁻³ at 25 ° C. can be used. From these measurement results, the dimensional factor and density used in the measurement of electrical conductivity can be determined.

(AC impedance method)
The AC impedance method can be used for measuring the electrical conductivity of the proton conductor of the present invention. For example, silver paste is applied as an electrode on the surface of the proton conductor when measuring the electrical conductivity, so that the measured resistance of the proton conductor is composed of an electrolyte resistance component and a resistance component at the electrolyte / electrode interface. Therefore, when the resistance of the proton conductor is measured with a direct current, only the resistance of the entire system including the electrolyte and the electrolyte / electrode interface can be measured, and only the components of the electrolyte cannot be extracted. On the other hand, since the impedance measured by the AC impedance method depends not only on the amplitude but also the frequency of the AC, these resistance components can be separated by the relaxation time (response time) according to the AC impedance method.

  In general, an ionic conductor including a proton conductor has a larger capacity component at the electrode / electrolyte interface than in the electrolyte because charge carriers are different in the electrode and in the electrolyte. This means that the resistance response seen when a voltage is applied is slower (longer relaxation time) at the electrode / electrolyte interface than at the electrolyte portion. When the impedance is measured with an alternating current having a frequency shorter than the relaxation time, a resistance component derived from the electrode / electrolyte interface does not appear.

Assuming that the electrolyte impedance of the proton conductor and the impedance of the electrode / electrolyte interface behave in the same way as the parallel circuit of the resistor and the capacitor (RC parallel circuit), the equivalent circuit of the entire measurement system is RC as shown in FIG. Two parallel circuits are connected in series. In FIG. 3, R1 and C1 represent the resistance and capacity of the electrolyte, and R2 and C2 represent the resistance and capacity of the electrode / electrolyte interface, respectively. Here, the impedance of each RC parallel circuit can be expressed as follows using the resistance R, the capacitance C, and the AC frequencies ω.
Z = Z ′ + iZ ″
= (1 / R) / {(1 / R) ² + (ωC) ² }
−i [ωC / {(1 / R) ² + (ωC) ² }]

Further, there is a relationship of f = ω / 2π between each frequency ω and frequency f. Therefore, when the impedance of the RC parallel circuit is measured by sweeping the AC frequency and plotted on the complex plane (Nyquist Plot), one semicircle is drawn as shown in FIG. Here, the diameter of the semicircle is R, and the apex frequency (characteristic frequency) f ₀ is f ₀ = ½πRC. Further, the higher the frequency f, the closer the impedance is to the origin on the complex plane.

On the other hand, in a circuit in which two RC parallel circuits, which are equivalent circuits of the entire measurement system composed of proton conductors and electrodes, are connected in series, if the f ₀ of the two RC parallel circuits are sufficiently different, the Nyquist Plot As shown in FIG. 5, two semicircles appear. Of the two semicircles, the one closer to the origin reflects the RC parallel circuit having the larger f ₀ . The RC parallel circuit having the larger f ₀ generally reflects the electrolyte in the solid electrolyte system.

  In actual measurement, the resistance of the electrolyte and the resistance of the electrode / electrolyte interface are separated and evaluated by comparing the shape of the complex impedance appearing on the Nyquist Plot with an equivalent circuit. In the actual measurement result, two semicircles reflecting the electrolyte appear on the Nyquist Plot as shown in FIG. Of the two semicircles, the one closer to the origin reflects the intragranular component R11 of the electrolyte resistance, and the other reflects the grain boundary component R12 of the electrolyte resistance.

(Measurement of electrical conductivity)
Impedance is measured using an impedance analyzer in a humidified Ar atmosphere with a partial pressure of water vapor of 0.05 atm for an electrode formed by applying silver paste to the upper and lower surfaces of the sintered pellets. From the Nyquist Plot obtained by the measurement, the intragranular resistance R11 and the grain boundary resistance R12 are obtained. When the thickness of the pellet is L and the area of the upper surface or the lower surface is A, the electric conductivity σ11 in the crystal grains and the electric conductivity σ12 in the crystal grain boundaries are σ11 = L / (A · R11) and σ12 = L, respectively. / (A ・ R12)
It can be expressed as. A may be the average of the areas of the upper and lower surfaces. The electrical conductivity obtained from the resistance value of the electrolyte represents the conductivity of ions such as oxygen ions or protons.

Examples of the present invention will be described. The proton conductor of this example is BaZr _0.85 Y _0.1 Sc _0.05 O ₃ in which the element A is Ba, the element Ma is Y, the element Mb is Sc, x = 0.1, and y = 0.05.

In the proton conductor of this example, BaCO ₃ , ZrO ₂ , Y ₂ O ₃ and Sc ₂ O ₃ powders were weighed as starting materials so that the molar ratio was 1: 0.85: 0.1: 0.05. Use what you did. And a sintered pellet is produced with the above-mentioned production method.

About this sintered pellet, the electrical conductivity in a crystal grain and the electrical conductivity of a crystal grain boundary were measured with the alternating current impedance method about several temperature from normal temperature to 200 degreeC vicinity. The result is shown in FIG. FIG. 7 shows the common logarithm of the horizontal axis obtained by multiplying the reciprocal of absolute temperature by 1000 and the vertical axis obtained by multiplying the electrical conductivity by absolute temperature. For reference, the upper horizontal axis shows the Celsius temperature, and the right vertical axis shows the common logarithm of electrical conductivity. In the graph, a line having a constant electrical conductivity is also shown. FIG. 7 also shows BaZr _0.85 Y _0.15 O _{3 in} which the element Ma is Y and x = 0.15 and the element Mb is not added, that is, y = 0 as a comparative example.

  FIG. 7 shows that the electrical conductivity of the proton conductor of this example and the comparative example is higher in the crystal grains (white symbols) than in the crystal grain boundaries (black symbols). In addition, the electrical conductivity of the grain boundaries is higher in this example (black triangle) than in the comparative example (black circle), and the electrical conductivity in the crystal grain is higher in this example (white triangle). Is lower than the comparative example (black circle).

  In addition, when the structure was observed by polishing the proton conductor in both of this example and the comparative example, the average crystal grain size was about 0.1 μm in the comparative example, and about 1-2 μm on average in the present example. It was. That is, it was found that the grain boundary density is reduced by adding the element Mb.

  Therefore, in the present example and the comparative example, the electrical conductivity of the sintered pellets, that is, the total electrical conductivity in the crystal grains and the grain boundaries were obtained by experiments for a plurality of temperatures from around room temperature to around 700 ° C. When extrapolated based on the obtained activation energy or the like, the result shown in FIG. 8 was obtained. As a result, it can be seen that the overall electrical conductivity of this example is higher than that of the comparative example. That is, in the sintered pellet of the proton conductor of this example, the increase in the electrical conductivity in the crystal grains was compensated by the low grain boundary density, and the overall electrical conductivity was higher than that in the comparative example. Conceivable.

  The proton conductor of the present invention can be used for a solid electrolyte of a fuel cell, a solid electrolyte of a hydrogen generator using steam electrolysis, a solid electrolyte of a hydrogen selective permeation device for purifying hydrogen gas, and the like.

Schematic diagram of unit cell of perovskite structure which is the structure of proton conductor according to the present invention Schematic diagram of a fuel cell according to the present invention Equivalent circuit of proton conductor impedance measurement system Nyquist Plot for RC parallel equivalent circuit Nyquist Plot for the equivalent circuit of proton conductor impedance measurement system Nyquist Plot for proton conductor The electric conductivity in the crystal grain of the proton conductor according to the embodiment of the present invention and the electric conductivity of the grain boundary FIG. 4 is a diagram showing the overall electric conductivity of a proton conductor according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fuel cell 11 Anode 12 Solid electrolyte 13 Cathode 20 Load

Claims (5)

One element of formula _{AZr 1-xy Ma x Mb y} O 3 (A is an alkaline earth metal, Ma is at least one element selected from the element group consisting of Y, Ho, Er, Tm, Yb and Lu , Mb is at least one element of Sc and In, and x and y have a perovskite structure represented by 0 <x <1, 0 <y <1, 0 <x + y <1) A proton conductor comprising an oxide.   The proton conductor according to claim 1, wherein A is Ba, Ma is Y, and Mb is Sc.   The proton conductor according to claim 1, wherein x / y ≧ 1.   The proton conductor according to claim 1, wherein x = 0.1 and y = 0.05.   A fuel cell comprising the proton conductor according to claim 1 as an electrolyte.

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