JP4933400B2 - Proton conductor and method for producing the same - Google Patents

Description

  The present invention relates to a proton conductor and a method for producing the same.

SnP ₂ O ₇ and Sn 3 P ₂ O ₇ doped with In ³⁺ are known to exhibit proton conductivity of about 0.2 S / cm in a temperature range of about 200 ° C. (Non-patent Documents 1 and 2). reference). It is also known that SnP ₂ O ₇ doped with Al ³⁺ or Ti ⁴⁺ instead of In ³⁺ exhibits proton conductivity (see Non-Patent Document 3 and Patent Document 1). These materials are expected to be used as solid electrolytes in fuel cells in place of sulfonated tetrafluoroethylene copolymers such as Nafion (registered trademark).

  The above-mentioned substances are difficult to obtain a sintered body, and in order to use as a solid electrolyte, it is necessary to produce a powder pressure-formed body. However, if the substance is produced according to the method described in each of the above-mentioned documents, it is in an extremely hard solid state and is in close contact with a reaction vessel such as a crucible. Is not good. Also, due to this, the inconvenience that impurities are likely to be mixed in, and the phosphorus concentration tends to be insufficient from the stoichiometric composition due to the evaporation of phosphoric acid (in this case, 2.0 or less), and the yield of the product There is also the inconvenience of lowering. Furthermore, since the extracted product is very hard, it is not easy to finely pulverize the product, and when pulverized strongly, the crystal structure of the product may be destroyed.

Nagao, Y. et al., “Abstracts of the 47th Battery Discussion Meeting”, Electrochemical Society, November 20, 2006, p. 186 Masahiro Nagao et al., "Proton Conduction in In3 + -Doped SnP2O7 at Intermediate Temperatures", J. Electrochem. Soc., 153 (8), A1604-A1609 (2006) Riichi Hiyama et al., “Abstracts of the 74th Annual Meeting of the Electrochemical Society”, Electrochemical Society, 2007, p. 67 JP 2005-294245 A

  Accordingly, it is an object of the present invention to provide a proton conductor that can eliminate the various drawbacks of the above-described prior art.

The present invention provides a proton conductor comprising TiP ₂ O ₇ doped with Al ³⁺ and / or Ca ²⁺ .

The present invention is also a preferred method for producing the proton conductor,
Crushing the titanium compound and the aluminum compound and / or calcium compound so that the average particle size is 3 μm or less,
Mixing the pulverized titanium compound and the aluminum compound and / or calcium compound with an aqueous phosphoric acid solution,
The mixture is heated with stirring to make the mixture gel.
The obtained gel is heated to 400 to 800 ° C. to provide a method for producing a proton conductor.

  According to the present invention, proton conductivity that is comparable to or higher than that of a conventionally known phosphate-type proton conductor, and operations such as taking out from the reaction vessel and grinding after being taken out Proton conductors that are easy to handle and excellent in handleability are provided.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The proton conductor of the present invention is mainly composed of TiP ₂ O ₇ and doped with Al ³⁺ or Ca ²⁺ . Either Al ³⁺ or Ca ²⁺ may be doped, or both may be doped. This proton conductor is characterized by high proton conductivity in an intermediate temperature range (for example, about 150 to 300 ° C.). Specifically, the proton conductivity at 200 ° C. shows a high value of 0.1 to 0.2 S / cm, for example. Therefore, the proton conductor of the present invention is a material suitable for applications such as a solid electrolyte of a fuel cell, a NOx reduction ceramic reactor, and a NOx · H ₂ sensor.

In particular, the doping amount of Al ³⁺ and / or Ca ²⁺ doped in the proton conductor is 0.05 to 0.30, particularly 0.10 to 0 in terms of a molar ratio of (Al + Ca) / (Ti + Al + Ca). .20 is preferable because proton conductivity is further increased.

The proton conductor of the present invention is generally obtained in the form of a powder. In this case, the average particle diameter is ₅₀ μm or less, particularly 0.1 to 10 μm, expressed as D ₅₀ , and the high proton conductivity of the solid electrolyte produced using the powder and the production of the solid electrolyte It is preferable from the point of easiness. The value of D ₅₀ is measured using, for example, a particle size distribution measuring apparatus using a laser diffraction scattering method such as a microtrack particle size distribution measuring apparatus.

  As will be described later, the proton conductor of the present invention is preferably produced using a titanium compound, an aluminum compound and / or a calcium compound, and phosphoric acid. The proton conductor contains phosphoric acid. It is preferable from the viewpoint of obtaining high conductivity. From this point of view, as will be described later, the amount of the titanium compound and the aluminum compound and / or calcium compound with respect to the phosphoric acid is expressed in terms of a molar ratio of P / (Ti + Al + Ca) of 2.6 to 3.6, particularly 2. As 8-3.4, it is preferable to make the preparation amount of phosphoric acid excessive.

Although depending on the detailed conditions of the production method described later, when the proton conductor of the present invention is doped with aluminum, the proton conductor contains aluminum metaphosphate. Aluminum metaphosphate is a substance corresponding to impurities as viewed from TiP ₂ O ₇ doped with Al ³⁺ , and it is generally considered that its content should be as small as possible. However, as a result of the study by the present inventors, the proton conductor of the present invention has high proton conductivity as described above despite containing aluminum metaphosphate, which is a substance generally regarded as an impurity. It becomes. There are still many unclear points about this reason, and further studies are expected.

As a result of the examination by the present inventors, the proportion of aluminum metaphosphate contained in the proton conductor of the present invention increases in proportion to the amount of Al ³⁺ doped. When the proton conductor contains aluminum metaphosphate, it is preferable because the proton conductor can be easily pulverized without lowering the proton conductivity. Whether or not the proton conductor of the present invention contains aluminum metaphosphate can be confirmed by XRD measurement of the proton conductor. The proportion of aluminum metaphosphate contained in the proton conductor can be estimated from, for example, the intensity ratio of the main peak of TiP ₂ O ₇ and aluminum metaphosphate. This strength ratio is preferably aluminum metaphosphate / TiP ₂ O ₇ = 1/10 to 1/20. When CaP2 ⁺ is doped into TiP ₂ O ₇ , calcium protonate may be contained in the obtained proton conductor, but this calcium metaphosphate does not significantly affect the conductivity of the proton conductor. .

  The proton conductor of the present invention can be made brittle to the extent that it can be crushed by hand by producing it by the method described below. Further, since the degree of close contact with the reaction vessel is low, it can be easily taken out from the reaction vessel. Therefore, the workability is good and the possibility of impurities being mixed during the work is extremely low. Furthermore, the powder can be easily pulverized after being taken out from the reaction vessel, and it is easy to obtain a powder having a desired particle size.

  The proton conductor of the present invention is preferably produced by the method described below. First, a titanium compound as a raw material, and an aluminum compound and / or calcium compound powder are prepared. Titanium oxide is preferably used as the titanium compound. As the aluminum compound, aluminum hydroxide, aluminum oxide, aluminum nitrate, aluminum chloride, or the like can be used. Of these aluminum compounds, it is preferable to use aluminum hydroxide because it is brittle and an object having good handleability can be easily obtained. As the calcium compound, calcium carbonate, calcium hydroxide, calcium oxide, calcium chloride and the like can be used. Of these calcium compounds, calcium carbonate is preferably used from the viewpoint of reactivity.

As a result of the examination by the present inventors, it is important to pulverize the titanium compound and the aluminum compound and / or calcium compound as raw materials so that the average particle diameter D ₅₀ is 3 μm or less. . This makes it possible to easily obtain an object that is brittle and has good handleability. The lower limit of the average particle diameter of the titanium compound and the aluminum compound and / or calcium compound is not particularly limited, but according to the study by the present inventors, it is satisfactory if the average particle diameter D ₅₀ is as small as about 0.1 μm. It is possible to easily obtain the desired object.

  The titanium compound, the aluminum compound and / or the calcium compound may be pulverized in a mixed state, or may be separately pulverized and mixed after pulverization. In any case, it is preferable to use a ball mill as a pulverizing means because it is brittle and an object with good handleability can be easily obtained. It is particularly preferable to use a planetary ball mill as the ball mill. The planetary ball mill is a device that revolves a pulverization container containing a pulverization object and a pulverization ball while rotating at high speed to generate strong pulverization energy and pulverize the pulverization object. Means other than the ball mill can be used as the grinding means. For example, an attritor or a bead mill can be used.

  The titanium compound and aluminum compound and / or calcium compound pulverized to a desired particle size are mixed with phosphoric acid and water using a stirrer or the like while heating in a reaction vessel such as a beaker. In this case, for the purpose of successfully dispersing the titanium compound and the aluminum compound and / or calcium compound in phosphoric acid, a commercially available 85% by weight phosphoric acid is used as phosphoric acid, and water (distilled water) is used. It is preferable to use an appropriately diluted one. The amount of water is preferably about 3 to 6 with respect to phosphoric acid 1 by weight. When commercially available phosphoric acid is not diluted, the viscosity of phosphoric acid is high, so that a homogeneous mixed solution can be obtained by directly mixing titanium compound and aluminum compound and / or calcium compound powder with phosphoric acid. Is not easy. Therefore, a homogeneous mixed solution can be easily obtained by diluting phosphoric acid with the above-mentioned amount of water and mixing it with a titanium compound and an aluminum compound and / or a calcium compound in a state where the viscosity is lowered. Obtaining a uniform mixed solution is important from the viewpoint of uniformly causing phosphoric acid to adhere to the surfaces of the titanium compound and the aluminum compound and / or calcium compound, thereby causing a uniform reaction.

  In mixing, the feed amount of the titanium compound and the aluminum compound and / or calcium compound with respect to phosphoric acid is expressed as a molar ratio of P / (Ti + Al + Ca) in the range of 2.6 to 3.6, particularly 2.8 to 3.4. As described above, it is preferable to make the amount of phosphoric acid charged excessively. When the charged amount is within this range, the obtained proton conductor has deliquescence, the density of the green compact is improved, the particles are strongly bonded, and high conductivity is obtained. On the other hand, when the molar ratio is less than 2.6, the conductivity of the proton conductor is likely to decrease to 0.001 S / cm or less, and when the molar ratio exceeds 3.6, the obtained proton conductor is liable to be in a liquid state. Cannot be manufactured. Furthermore, it is preferable that the amount of the aluminum compound and / or calcium compound charged with respect to the titanium compound is expressed in terms of a molar ratio of Al / (Ti + Al + Ca) to be in the range described above.

  After a titanium compound, an aluminum compound and / or a calcium compound are mixed with phosphoric acid and water to obtain a mixed solution, the mixed solution is heated to remove moisture to some extent. Heating is performed until the mixed solution becomes a gel. The heating temperature is preferably 200 to 350 ° C, particularly preferably 250 to 300 ° C. It is preferable to maintain the temperature in this range for 0.5 to 3 hours, particularly 1 to 2 hours, and to make the mixed solution into a gel. In this case, it is preferable to heat the mixed solution while stirring from the viewpoint of easily obtaining a target product that is brittle and has good handleability.

  Once the mixture has gelled, the gel is once returned to room temperature (20-25 ° C). Next, the gel is heated again and baked to cause a dehydration reaction to obtain a target product. The heating temperature is set to be equal to or higher than the above-described gelling temperature, specifically 400 to 800 ° C., and particularly preferably 550 to 700 ° C. from the viewpoint of obtaining an object having good proton conductivity. By setting the heating temperature to 400 ° C. or higher, the target product can be successfully synthesized. Moreover, by making heating temperature 800 degrees C or less, the difficulty of control resulting from excessive volatilization of phosphoric acid can be avoided, and a target object can be manufactured stably. The heating rate is preferably from 2 to 4 hours from room temperature to the calcination temperature, and maintained at the calcination temperature for 0.5 to 3 hours, particularly 1 to 1.5 hours to obtain the desired product.

  The dehydration reaction by heating may be completed once, or may be repeated a plurality of times as necessary. However, if the dehydration reaction is repeated a plurality of times, the phosphoric acid may be decomposed, which may make it difficult to express desired proton conductivity.

  The target product thus obtained is brittle enough to be crushed by hand, and is weakly attached to the inner wall of the reaction vessel. Therefore, it is possible to easily perform the operation of taking out the target product from the reaction vessel. As a result, it is very difficult for impurities to be taken out during extraction. Further, according to the above reaction system and the above production method, the phosphorus concentration is uniform and appropriate throughout the obtained object, and the yield is very high.

  The obtained object is made into a powder having a desired particle diameter using a crushing device or a crushing device. In this case, since the target product is brittle enough to be crushed by hand, crushing and pulverization operations can be easily performed.

  The proton conductor obtained by the above method can be formed into a solid electrolyte membrane, particularly a non-humidified solid electrolyte membrane, by mixing it with, for example, a binder and forming it into a membrane shape. This solid electrolyte membrane is suitably used, for example, as a constituent member of a fuel cell. Moreover, it can also be set as the membrane electrode assembly (MEA) for fuel cells by adhere | attaching a catalyst and an electrode on each surface of the obtained film | membrane. A fuel cell equipped with these solid electrolyte or membrane electrode assemblies is preferably operated in an intermediate temperature range of, for example, about 150 to 300 ° C.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” means “% by weight”.

[Example 1]
Anatase-type titanium oxide powder was pulverized with a planetary ball mill, and the average particle diameter D ₅₀ was 0.5 μm. Separately, aluminum hydroxide powder was pulverized with a planetary ball mill, and the average particle diameter D ₅₀ was set to 0.2 μm. 3.59 g of titanium oxide and 0.39 g of aluminum hydroxide obtained by pulverization were mixed with 19.6 g of phosphoric acid having a concentration of 85% and 100 g of distilled water to obtain a mixed solution. The Al doping amount was 0.10 in terms of Al / (Ti + Al). The amount of phosphoric acid charged was 3.4 in terms of P / (Ti + Al).

  The mixed solution was placed in a beaker, heated to 300 ° C. in the air using a hot plate, and this temperature was maintained for 2 hours. The mixture was kept stirring during heating. By this heating, the mixed solution became a viscous gel.

The mixed liquid in the form of a gel was put in a crucible, covered, heated from room temperature to 650 ° C. in the air using an electric furnace in 2 hours, and held at 650 ° C. for 1 hour. Thus, a dehydration reaction was caused to obtain a target product. As a result of analysis by XRD, it was confirmed that this substance was TiP ₂ 0 ₇ doped with Al. The doping amount of Al was 0.10 in terms of Al / (Ti + Al). As a result of analysis by XRD, this substance contains aluminum metaphosphate, and it is estimated from the ratio of the height of the main peak that several percent of aluminum metaphosphate is contained.

[Example 2]
Anatase type titanium oxide (3.79 g), aluminum oxide (0.195 g) and distilled water (20 g) were ground and mixed in a planetary ball mill for 24 hours. The average particle diameter D ₅₀ of the mixed powder after pulverization was 0.4 μm. To this solution, 18.45 g of phosphoric acid having a concentration of 85% and 80 g of distilled water were added to obtain a mixed solution. The target product was obtained in the same manner as in Example 1 except that this mixed solution was used. The charge amount of Al was 0.05 expressed by Al / (Ti + Al), and the charge amount of phosphoric acid was 3.2 expressed by P / (Ti + Al). As a result of analysis by XRD, this substance contains aluminum metaphosphate, and the ratio is estimated to be several percent.

Example 3
Anatase type titanium oxide (3.59 g), calcium carbonate (0.50 g), and distilled water (20 g) were ground and mixed in a planetary ball mill for 24 hours. The average particle diameter D ₅₀ of the powder mixture after pulverization was 0.3 [mu] m. To this solution, 19.6 g of phosphoric acid having a concentration of 85% and 80 g of distilled water were added to obtain a mixed solution. The target product was obtained in the same manner as in Example 1 except that this mixed solution was used. The charge amount of Al was 0.10 expressed by Al / (Ti + Ca), and the charge amount of phosphoric acid was 3.4 expressed by P / (Ti + Ca).

[Comparative Example 1]
3.994 g of anatase-type titanium oxide, 19.6 g of phosphoric acid having a concentration of 85%, and 100 g of distilled water were mixed to obtain a mixed solution. The target product was obtained in the same manner as in Example 1 except that this mixed solution was used. Analysis by XRD confirmed that the material was TiP ₂ O ₇ .

[Comparative Example 2]
6.78 g of tin (IV) oxide, 0.694 g of indium (III) oxide, 19.6 g of phosphoric acid having a concentration of 85%, and 100 g of distilled water were mixed to obtain a mixed solution. The charged amount of In was 0.10 expressed by In / (Sn + In), and the charged amount of phosphoric acid was 3.4 expressed by P / (Sn + In). Tin (IV) oxide and indium (III) oxide were not pulverized by a planetary ball mill. This mixed solution was put in a crucible and heated to 650 ° C. under the atmosphere, and this temperature was maintained for 1 hour. Thus, a dehydration reaction was caused to obtain a target product. As a result of XRD analysis, it was confirmed that this material was SnP ₂ O ₇ doped with In. This substance was firmly fixed to the inner wall of the crucible in a very hard solid state, and it was very difficult to remove it from the crucible.

[Comparative Example 3]
7.119 g of tin (IV) oxide, 0.195 g of aluminum (III) hydroxide, 17.29 g of phosphoric acid having a concentration of 85%, and 100 g of distilled water were mixed to obtain a mixed solution. The charge amount of Al was 0.05 as expressed by Al / (Sn + Al), and the charge amount of phosphoric acid was 3.0 as expressed as P / (Sn + Al). Tin (IV) oxide and aluminum hydroxide were not pulverized by a planetary ball mill. This mixed solution was put in a crucible and heated to 650 ° C. under the atmosphere, and this temperature was maintained for 1 hour. Thus, a dehydration reaction was caused to obtain a target product. As a result of XRD analysis, it was confirmed that this material was SnP ₂ O ₇ doped with Al. This substance was firmly fixed to the inner wall of the crucible in a very hard solid state, and it was very difficult to remove it from the crucible.

[Evaluation]
About the substance obtained by the Example and the comparative example, the temperature dependence of proton conductivity was measured. The result is shown in FIG. The measurement pellets have a diameter of 13 to 15 mm and a thickness of 2 to 3 mm, and the pellets are obtained by molding the synthesized object at a pressure of ˜2 t / cm ² . The obtained molded body was sandwiched between 10 mm square platinum plates (area: 1 cm ² ), and proton conductivity was measured from 50 ° C. to 300 ° C. in the atmosphere using the AC impedance method.

  As is clear from the results shown in FIG. 1, the materials obtained in Examples 1, 2, and 3 have higher proton conductivity around 200 ° C. than the materials obtained in Comparative Examples 1, 2, and 3. I understand that. In particular, it can be seen that the materials obtained in Examples 1, 2, and 3 exhibit a proton conductivity of 0.1 S / cm or more, which is a necessary and sufficient proton conductivity for an electrolyte of a fuel cell.

It is a graph which shows the temperature dependence of the proton conductivity of the substance obtained by the Example and the comparative example.

Claims (5)

A proton conductor obtained by doping TiP ₂ O ₇ with Al ³⁺ and / or Ca ²⁺ . The proton conductor according to claim 1, wherein the doping amount of Al ³⁺ and / or Ca ²⁺ is 0.05 to 0.30 in terms of a molar ratio of (Al + Ca) / (Ti + Al + Ca). The proton conductor according to claim 1 or 2, which is doped with Al ³⁺ and further contains aluminum metaphosphate. A method for producing a proton conductor according to claim 1,
Crushing the titanium compound and the aluminum compound and / or calcium compound so that the average particle size is 3 μm or less,
Mixing the pulverized titanium compound and the aluminum compound and / or calcium compound with an aqueous phosphoric acid solution,
The mixture is heated with stirring to make the mixture gel.
A method for producing a proton conductor, comprising heating the obtained gel to 400 to 800 ° C.   The production of a proton conductor according to claim 4, wherein the charged amount of the titanium compound and the aluminum compound and / or calcium compound with respect to phosphoric acid is 2.6 to 3.6 in terms of a molar ratio of P / (Ti + Al + Ca). Method.

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