JP6000537B2 - Alkaline electrolyte and fuel cell provided with the electrolyte - Google Patents

Description

  The present invention relates to an alkaline electrolyte and a fuel cell including the alkaline electrolyte.

  An alkaline electrolyte fuel cell (AFC: Alkaline Fuel Cell; hereinafter may be simply referred to as “AFC”) is a fuel cell using a hydroxide ion conductor as an electrolyte, and has high power generation efficiency at 100 ° C. Since it can be used in the following (for example, 20 ° C. to 100 ° C.), it is used for space applications (for mounting on a space shuttle) and the like. In addition, since an expensive catalyst material such as platinum (Pt) is unnecessary, research is being conducted as a next-generation power generator.

The structure of AFC is such that the cathode (air electrode) and the anode (fuel electrode) face each other through a hydroxide ion conductor (typically, a form in which a separator is impregnated with an alkaline aqueous solution) as an electrolyte. Configured. When a current is applied to the AFC, oxygen in an oxygen-containing gas (typically air) reacts with water (H ₂ O) at the cathode to form hydroxide ions (OH ⁻ ). The hydroxide ions are supplied from the cathode through the electrolyte to the anode. At the anode, power is generated by reacting with a fuel gas (typically hydrogen) to produce water and releasing electrons. The power generation efficiency of AFC generally depends on the operating temperature, and the power generation efficiency tends to increase as the temperature increases.

  In recent years, inorganic solid electrolytes have attracted attention as electrolytes for AFC. Since the solid electrolyte is solid (typically in the form of a film), the operating temperature can be set to be equal to or higher than the boiling point of water (for example, 100 ° C. to 200 ° C.). In a fuel cell using the electrolyte, power generation efficiency Further improvement can be expected. Further, since such a solid electrolyte does not use a liquid, there is no fear of liquid leakage and is excellent in terms of safety. As a prior art regarding an inorganic solid electrolyte, there are Patent Document 1 and Non-Patent Documents 1 to 3. For example, Patent Document 1 discloses an alkaline electrolyte made of a clay mineral. That is, the electrolyte is formed by laminating a layer of cation (typically, metal ion) hydroxide (hereinafter sometimes simply referred to as a hydroxide layer), and an anion and a water hydrate between the layers. It consists of layered double hydroxide (also called hydrotalcite) containing molecules, and exhibits ionic conductivity by moving anions between layers.

International Publication No. 2010/109670

Solid State Ionics, Vol. 181 (2010) pp. 883-888 Advanced Materials, Vol. 22 (2010) pp. 4401-4404 Solid State Ionics, Vol. 192 (2011) pp. 185-187

  However, the layered double hydroxide as described in the above-mentioned patent document has a very high humidity dependency, and when water molecules contained between the layers are washed away as the humidity decreases, the ionic conductivity is remarkably reduced. For this reason, when the layered double hydroxide is used as an electrolyte for a fuel cell, it is necessary to always maintain a high humidity state (for example, a state of 80% RH or higher). This is not preferable because it leads to a complicated battery structure and an increase in manufacturing cost. Moreover, since the humidity dependency as described above may destabilize the power generation performance, an electrolyte having a lower humidity dependency is desired.

  This invention is made | formed in view of this subject, The objective is to provide the alkaline electrolyte with less humidity dependence compared with the former. Moreover, this invention provides the fuel cell provided with the said alkaline electrolyte as another side surface.

In order to achieve the above object, an alkaline electrolyte is provided by the present invention. Here, the alkaline electrolyte is composed of a composition that forms a layered double hydroxide, and the composition includes divalent and trivalent cations, an anion containing at least hydroxide ions, and amphoteric. And an organic compound that can be an ion.
The alkaline electrolyte disclosed herein includes a layer composed of a divalent and trivalent cation hydroxide (hydroxide layer), and a substance (that is, an anion) interposed between the hydroxide layers. It is composed of a layered double hydroxide composed of water molecules and organic compounds that can be zwitterions. Here, since the hydroxide layer is positively charged, an organic compound (an element portion having a negative charge) having an anion or polarization is bonded to the surface of the hydroxide layer. Then, an anion or a polarized water molecule (H ^{δ +} -OH ^δ- ) is bonded to the element part having a positive charge of the organic compound, thereby forming a strong ion conduction path (path). Further, since the organic compound is contained between the hydroxide layers, the distance between the hydroxide layers (hereinafter referred to as the interlayer distance) is widened, and more hydroxide molecules can be included in the structure. . Therefore, the alkaline electrolyte disclosed here is less dependent on humidity than conventional ones, and can exhibit high ionic conductivity even under low humidity conditions.

In a preferred embodiment of the material disclosed herein, the divalent cation is a magnesium ion, and the trivalent cation is an aluminum ion.
Since the above elements have a relatively large ionization tendency, the layered double hydroxide provided with such elements exhibits high ionic conductivity. Moreover, since the layered double hydroxide is excellent in chemical stability, it is suitable as an alkaline electrolyte.

In a preferred embodiment of the materials disclosed herein, the molar ratio of the divalent cation ^{(M II)} and the trivalent cation ^{(M III) (M II /} M III) is 2 to 4 It is.
When the ratio of cations in the hydroxide layer is in the above range, the crystal structure is excellent in stability and can exhibit excellent performance as an ionic conductor. For this reason, it is suitable as an alkaline electrolyte.

In a preferred embodiment of the materials disclosed herein, the molar ratio of the divalent cation ^{(M II)} and the trivalent cation ^{(M III) (M II /} M III) is 2.5 or more 3 .5 or less.
When the ratio of the cation in the hydroxide layer satisfies the above, the crystal structure is more stable and can exhibit more excellent performance as an ionic conductor.

In a preferred embodiment of the material disclosed herein, the organic compound includes at least one amino acid and / or amino acid derivative.
Since the amino acid (or amino acid derivative) contained as the organic compound has an amino group (—NH ₂ or the like) and a carboxyl group (—COOH) in the molecular structure, polarization easily occurs in the molecule. Therefore, an alkaline electrolyte having the organic compound can form a stronger ion conduction path.

In a preferred embodiment of the material disclosed herein, the organic compound includes at least a general formula: R ₁ R ₂ R ₃ N ⁺ R ₄ COO ⁻ (where R ₁ , R ₂ , R ₃ , R ₄ are An amino acid and / or an amino acid derivative represented by a hydrogen atom or a linear, branched, or cyclic alkyl group having 1 to 5 carbon atoms.
Since the organic compound has high affinity with water (that is, water solubility), an alkaline electrolyte having the organic compound can exhibit high ionic conductivity.

In a preferable embodiment of the material disclosed herein, the organic compound includes at least a general formula: R ₁ R ₂ R ₃ N ⁺ CH ₂ COO ⁻ (wherein R ₁ , R ₂ , and R ₃ each represent a hydrogen atom Or a linear, branched or cyclic alkyl group having 1 to 5 carbon atoms).
Alkyl glycine contained as the organic compound can be preferably used because it has a relatively short alkyl chain and therefore can easily enter the interlayer of the hydroxide layer and can maintain the interlayer at a suitable distance.

In a preferred embodiment of the material disclosed herein, the organic compound includes at least trimethylglycine ((CH ₃ ) ₃ N ⁺ CH ₂ COO ⁻ ; also referred to as glycine betaine).
Trimethylglycine is excellent in chemical stability among the above alkylglycines. Moreover, since it exists widely naturally, it is suitable also from an environmental viewpoint.

According to the present invention, there is also provided a method for producing an alkaline electrolyte comprising a layered double hydroxide, comprising preparing an aqueous solution containing a divalent cation source and a trivalent cation source. Adding an organic compound capable of forming an amphoteric ion; adjusting the aqueous solution to which the organic compound is added to alkalinity to obtain a precipitate; recovering the precipitate from the alkaline aqueous solution; and Forming the precipitate.
According to such a production method, the alkaline electrolyte having a small humidity dependency disclosed here can be suitably synthesized.

Furthermore, the present invention provides a fuel cell comprising any of the alkaline electrolytes disclosed herein.
The alkaline electrolyte disclosed here has low humidity dependence and exhibits high ionic conductivity even in a low humidity region (for example, 20% RH to 60% RH). Therefore, in the fuel cell using the electrolyte, the power generation performance in the low humidity region is improved as compared with the conventional one, and the power generation stability can be improved.

It is sectional drawing which shows typically the structure of the alkaline electrolyte fuel cell (single cell) which concerns on one Embodiment of this invention. It is sectional drawing which shows typically the structure of the layered double hydroxide which comprises the alkaline electrolyte which concerns on one Embodiment of this invention. It is the chart which measured the X-ray-diffraction pattern of the layered double hydroxide (Mg2 ⁺ / Al3 ⁺ = 3) which ^concerns on one Example of this invention. It is the chart which measured the X-ray-diffraction pattern of the layered double hydroxide (Mg2 ⁺ / Al3 ⁺ = 2) which ^concerns on one Example of this invention. It is Cole-Cole Plot which measured the alternating current impedance of the layered double hydroxide concerning one example of the present invention. It is the graph which compared the ionic conductivity of the layered double hydroxide (Mg2 ⁺ / Al3 ⁺ = 3) which concerns on one Example of this invention. It is the graph which compared the ion conductivity of the layered double hydroxide (Mg2 ⁺ / Al3 ⁺ = 2) which concerns on one Example of this invention.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

The alkaline electrolyte disclosed herein is characterized by being constituted by a composition that forms a layered double hydroxide. Therefore, the content, composition, and the like of other components can be determined in light of various standards without departing from the object of the present invention.
Unless otherwise specified in this specification, “humidity” refers to a partial pressure of water vapor pressure with respect to saturated water vapor pressure (that is, relative humidity (RH)), and for convenience, a unit of “% RH”. write.

  The alkaline electrolyte disclosed here is composed of a composition that forms a layered double hydroxide. In this specification, “double hydroxide” refers to a compound composed of a composite state (composite state) of two or more hydroxides, and “layered double hydroxide” is formed by the double hydroxide. A layered structure in which sheet-like double hydroxides are regularly stacked.

FIG. 2 is a schematic diagram showing a typical structure of the layered double hydroxide 20 disclosed herein. The layered double hydroxide 20 is roughly composed of a layer 22 made of double hydroxide and a material existing between the layers 24, and these are alternately stacked by the interaction of van der Waals forces. Have The hydroxide layer 22 has a regular octahedral structure in which divalent and trivalent cations present at random are surrounded by hydroxide ions (OH ⁻ ), and is positively charged as a whole. . The interlayer 24 includes an organic compound 23 that can be an amphoteric ion, an anion 25, and a water molecule (interlayer water) 26, and is electrically charged negatively as a whole to maintain electrical neutrality. Yes.
Here, on the surface 22a of the positively charged hydroxide layer 22, anions 25 and polarized organic compounds (namely, amphoteric ions) 23 are bonded by Coulomb force (electrostatic attraction). For this reason, the resistance between the hydroxide layer 22 and the interlayer 24 is reduced. Further, an element portion having a positive charge of zwitterion and a water molecule (typically in a polarized state) 26 can be combined to form a stronger ion conduction path (path). Furthermore, when the organic compound 23 enters between the hydroxide layers, the distance between the layers 24 increases, and more water molecules 26 can be included in the structure. For this reason, the alkaline electrolyte disclosed here is less dependent on humidity than conventional ones, and can exhibit high ionic conductivity even under low humidity conditions.

  The interlayer distance can be measured by, for example, a general X-ray diffraction apparatus (XRD: X-ray diffraction). Specifically, X-rays (CuKα rays) are irradiated while changing the incident angle with respect to the sample stepwise or continuously, and the X-ray diffracted by the sample is captured by an inspection device. Then, the angle difference between the X-ray diffraction direction and the incident direction (diffraction angle 2θ) and the diffraction X-ray intensity are measured. Such measurement can be performed, for example, using an X-ray diffractometer manufactured by Rigaku Corporation, model “RINT-TTRIII”, under conditions such as those in Examples described later. Note that the sample used for measurement may be a layered double hydroxide (typically powdered) packed in a sample board, and a pellet formed using the layered double hydroxide. (Plate shape) may be sufficient.

Here, the cation constituting the hydroxide layer 22 is not particularly limited, but those having a relatively large ionization tendency can be preferably used. More specifically, examples of the divalent cation include magnesium ion (Mg ²⁺ ), calcium ion (Ca ²⁺ ), barium ion (Ba ²⁺ ), manganese ion (Mn ²⁺ ), and iron ion (Fe ²⁺ ). , One or more elements selected from cobalt ions (Co ²⁺ ), nickel ions (Ni ²⁺ ), copper ions (Cu ²⁺ ), zinc ions (Zn ²⁺ ), etc. Large alkaline earth metal ions (Mg ²⁺ , Ca ²⁺ ) can be preferably used. In addition, as the trivalent cation, for example, one or two or more kinds selected from aluminum ions (Al ³⁺ ), chromium ions (Cr ³⁺ ), iron ions (Fe ³⁺ ), gallium (Ga ³⁺ ), and the like. Elements can be used, and among these, aluminum ions can be preferably used. For example, a hydroxide layer containing magnesium ions and aluminum ions has high ion conductivity and excellent chemical stability (for example, acid resistance and alkali resistance), and therefore can be suitably used as an alkaline electrolyte. . The divalent cation and the trivalent cation may be selected from the same kind of elements (for example, Fe ²⁺ and Fe ³⁺ ) as long as they have different valences.
Although not particularly limited, the molar ratio of the divalent cation ^{(M II)} and the trivalent cation ^{(M III) (M II /} M III) is 2 to 4 (more preferably 2.5 or more and 3.5 or less). When the ratio of the cation in the hydroxide layer is in the above range, the crystal structure is excellent in stability and can be further improved as an ionic conductor. For this reason, it can be suitably used as an alkaline electrolyte.

The anions used here are one or more elements including at least hydroxide ions (OH ⁻ ) constituting the hydroxide layer 22. Anions other than hydroxide ions are not particularly limited, but those having a relatively large ionization tendency can be preferably used. More specifically, carbonate ion (CO ₃ ²⁻ ), nitrate ion (NO ₃ ⁻ ), nitrite ion (NO ₂ ⁻ ), sulfate ion (SO ₄ ²⁻ ), fluoride ion (F ⁻ ), chloride A product ion (Cl ⁻ ), a bromide ion (Br ⁻ ), an iodide ion (I ⁻ ), and the like. The anions 25 other than that constituting the hydroxide layer 22 are typically present between the hydroxide layer layers 24 and maintain electrical neutrality. The anions 25 do not have to exist between all the layers, and may exist between every other layer, for example.

As used herein, the organic compound 23 that can be an amphoteric ion refers to an organic compound that can be polarized in the molecule and have a positive charge and a negative charge, and does not always need to be in a polarized state. Further, the number of positive charges and / or negative charges contained in the compound may be one or two or more.
Examples of the organic compound include a compound that easily causes polarization, typically a compound having acidic and basic functional groups. For example, amino acids and derivatives thereof (salts) having an amino group (—NH ₂ or the like) and a carboxyl group (—COOH) in the molecule are typical examples. Specific examples of amino acids include those having charged side chains such as arginine, histidine, lysine, aspartic acid and glutamic acid, those having uncharged side chains such as serine, threonine, asparagine and glutamine, and alanine. And those having a hydrophobic side chain such as valine, isoleucine, leucine, methionine, phenylalanine, tyrosine and tryptophan, glycine, cysteine, selenocysteine, proline and the like. For example, the general formula: R ₁ R ₂ R ₃ N ⁺ R ₄ COO ⁻ (where R ₁ , R ₂ , R ₃ and R ₄ are each a hydrogen atom or a straight chain, branched or cyclic group having 1 to 5 carbon atoms. Since the amino acid or its derivative (salt) organic compound represented by (1) is highly compatible with water (ie, water-soluble), an alkaline electrolyte having the organic compound exhibits high ionic conductivity. be able to. Of these, alkyl glycine and its derivatives represented by the general formula: R ₁ R ₂ R ₃ N ⁺ CH ₂ COO ^{— in} which R ₄ in the above general formula is CH ₂ have a relatively short alkyl chain, and thus are hydroxylated. It can be preferably used because it can easily enter between physical layers and can be kept at a suitable distance. Specific examples of the alkyl glycine include, for example, trimethyl glycine (also referred to as glycine betaine, sometimes simply referred to as betaine), dimethyl glycine, methyl glycine, ethyl glycine, propyl glycine, and the like. Trimethylglycine which is easily adsorbed and excellent in chemical stability can be preferably used. Trimethylglycine is also suitable from the environmental viewpoint because it is widely present in nature. In addition, various amphoteric surfactants that are derivatives of the above amino acids and are generally used in fields such as industry can also be used.

  Moreover, the method of manufacturing suitably the alkaline electrolyte disclosed here is provided. In the production method provided herein, (1) preparing an aqueous solution containing a divalent cation source and a trivalent cation source, and (2) an organic that can be zwitterions in the prepared aqueous solution. Adding a compound, (3) preparing an aqueous solution to which the organic compound is added to be alkaline to obtain a precipitate, (4) collecting the precipitate from the alkaline aqueous solution, and (5) the deposit. Molding an object. According to this manufacturing method, the alkaline electrolyte having a small humidity dependency disclosed here can be synthesized more easily, which is preferable. Hereinafter, this manufacturing method will be described in detail.

<(1) Preparation of aqueous solution>
In the production method disclosed herein, first, a compound as a starting material (that is, a “divalent cation” source, a “trivalent cation” source) and other additives as required are prepared. And these are weighed so that it may become a desired composition ratio, and are dissolved in an aqueous solution.
The divalent cation source used here is the divalent cation already described above (for example, Mg ²⁺ , Ca ²⁺ , Ba ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ^2+, etc.). ) Is not particularly limited as long as it is a compound containing at least one of them, and can be appropriately selected according to the composition of the layered double hydroxide constituting the alkaline electrolyte as the production objective. For example, nitrates, sulfates and chlorides of the metal ions can be used. More specifically, magnesium nitrate, magnesium oxalate, magnesium carbonate, magnesium sulfate, magnesium chloride, magnesium hydroxide, and the like can be used. Among them, nitrates having a high ionization tendency (for example, magnesium nitrate (Mg (NO ₃ ) ₂ The trivalent cation source used here is also the trivalent cation (Al ³⁺ , Cr ³⁺ , Fe ³⁺⁾ already described above. , Ga ^{3+ and the} like) as long as the compound contains at least one of them, and can be appropriately selected according to the composition of the layered double hydroxide constituting the alkaline electrolyte that is the production object. Metal ion nitrates, sulfates, chlorides, etc. can be used, more specifically, for example, aluminum ions (A When using ³⁺⁾ is aluminum nitrate, aluminum oxalate, can be used aluminum chloride, aluminum hydroxide, etc., among them aluminum nitrate (Al (NO _{3) 3:} may be in the form of hydrates). Can be preferably used.

The solvent constituting the aqueous solution used here is typically water (pure water), but may be a mixed solvent containing water as a main component. As the solvent other than water constituting the mixed solvent, an organic solvent (such as a lower alcohol or a lower ketone) that can be uniformly mixed with water can be suitably used. The kind of the solvent to be used can be appropriately selected depending on the properties of the compound used in the production (for example, an organic compound described later) and the like, for example, a compound having poor solubility in water (typically a long-chain organic compound). When using the compound, an organic solvent may be used in combination. Typically, an aqueous solvent in which 50% by mass or more (typically 60% by mass or more, for example, 80% by mass or more) of the aqueous solvent is water can be used. It is mentioned as a suitable example.
In preparing the aqueous solution, various conventionally known stirring and mixing operations can be employed. Examples of such operations include irradiation with ultrasonic waves and stirring with a magnetic stirrer. The order of dissolution is not particularly limited, and the divalent cation source and the trivalent cation source may be added simultaneously to the aqueous solution. First, either one is added to the aqueous solution and dissolved. Then, the other cation source may be added and dissolved. In addition, in the aqueous solution used here, various additives (for example, a dispersant, a pH adjuster, a preservative, a colorant, etc.) other than the cation source are provided as long as the effects of the present invention are not significantly impaired. You may add suitably.

  The amount of the aqueous solvent is not particularly limited, but is preferably an amount that can completely dissolve the cation source (that is, an amount that can be lower than the saturated solution). For example, it can be about 150 to 3000 parts by mass, preferably about 200 to 1600 parts by mass with respect to 100 parts by mass of the cation source. If it is the amount of the above-mentioned solvent, the above-mentioned cation source can be dissolved uniformly, and the target product can be produced more uniformly. Further, the total number of moles (total ion concentration) of the divalent cation, trivalent cation and hydroxide ion contained in the aqueous solution is, for example, about 0.2 to 2.4 mol / L. It is preferably about 0.6 to 2.0 mol / L. By setting the concentration within the above range, the layered double hydroxide as the target can be suitably precipitated in the pH adjustment described later. The concentration of the aqueous solution is within a range that can maintain a desired composition ratio (that is, the ratio of divalent cation, trivalent cation, and zwitterion constituting the layered double hydroxide). It can be appropriately adjusted by adding an ion source or the like.

<(2) Addition of organic compound>
In the production method disclosed herein, next, an organic compound capable of forming zwitterions is weighed and added to the prepared aqueous solution so as to have a desired composition ratio.
The organic compound which can be an amphoteric ion used here can be used without particularly limiting one or more of the organic compounds described above. The amount of the organic compound added is not particularly limited, but can be, for example, about 0.5 to 20.0 moles relative to the number of moles of the trivalent cation contained in the aqueous solution. It is preferable to set it as 8-10.0 mol. Moreover, the addition method is not particularly limited. For example, when the organic compound is in a powder form (including powder form, granular form, etc.), it may be added to the aqueous solvent as it is. It may be added to the aqueous solvent after being dissolved in a second solvent. In addition, as a 2nd solvent used here, what can be mixed with the said aqueous solvent uniformly is preferable, for example, the same kind of solvent as the aqueous solvent mentioned above can be used. Moreover, regarding the stirring / mixing operation, the above-described method can be appropriately used. Although the time required for stirring and mixing is not particularly limited, it can be, for example, several minutes to several tens of minutes (typically 1 minute to 60 minutes, for example, 5 minutes to 40 minutes).

<(3) pH adjustment of aqueous solution>
In the production method disclosed herein, the aqueous solution to which the organic compound is added is adjusted to be alkaline, and the layered double hydroxide particles are precipitated from the aqueous solution (also referred to as a coprecipitation method).
Here, the method for adjusting the alkalinity is not particularly limited. For example, an alkali agent (that is, a compound having an action of tilting the liquidity to alkalinity) may be added to the aqueous solution. As such an alkali agent, for example, sodium hydroxide (NaOH), potassium hydroxide (KOH) and the like can be typically used in the form of an aqueous solution. In a preferred embodiment, an aqueous sodium hydroxide solution is used.
The pH of the aqueous solution varies depending on the cation species used and the concentration of the solution, and is not particularly limited. However, at extremely high pH, the precipitated hydroxide may be redissolved. In addition, from the viewpoint that it is easy to obtain homogeneous particles (for example, a small number ratio of particles whose particle size or particle structure greatly deviates from the average), a large number of nuclei are precipitated from the aqueous solution in a short time. It is preferable that it can be made. In order to suitably realize such a precipitation mode, it is typically preferable to adjust the pH to 8 or more, for example, 8 to 12 and preferably 9 to 11. In addition, in this specification, the value of pH points out the value measured at room temperature (liquid temperature of 25 degreeC) using the commercially available pH meter.

In a preferred embodiment disclosed herein, in the process of precipitating the layered double hydroxide, a step (nucleation step) of depositing nuclei of layered double hydroxide particles from the aqueous solution, A stage of growth (particle growth stage). In the particle growth stage, it is preferable to hold for a certain period of time in an environment where the particles grow uniformly. As a preferred embodiment, for example, the temperature is kept (left) for a certain time while keeping the temperature below the boiling point of the aqueous solvent (for example, a predetermined temperature ± 1 ° C.). The temperature to be held is a temperature not higher than the boiling point of the solvent (typically 100 ° C. or lower), and can be, for example, 30 ° C. to 95 ° C., preferably 50 ° C. to 90 ° C. The heating method is not particularly limited, and a known method (for example, an electric furnace) can be appropriately used. The holding time can be changed according to the growth of the particles, and is typically 0.5 hours to 100 hours, although depending on the type of layered double hydroxide and environmental conditions (for example, temperature). For example, it can be 1 hour to 48 hours, preferably 2 hours to 36 hours. By holding in such a heated state, the growth of particles can be further promoted. Incidentally, CO ₂ in the air is absorbed not a little alkaline aqueous solution, it will be present in solution as carbonate ions. For this reason, when it is desired to minimize the contamination of the ions, it is preferable to maintain the atmosphere in the reaction solution and the reaction tank in an inert atmosphere (for example, a nitrogen atmosphere) through the nucleation stage and the particle growth stage.

<(4) Collection of precipitate>
In the production method disclosed herein, next, the precipitated layered double hydroxide particles (that is, a composite containing the divalent and trivalent cations, the amphoteric ions, and the anions containing at least hydroxide ions) are used. The hydroxide particles are separated from the aqueous solution, washed and dried. As the separation method, conventionally known methods (centrifugation, filtration, decantation, etc.) can be used singly or in combination of two or more. Then, the obtained particles are appropriately pulverized and / or classified. In the pulverization treatment, one or two or more kinds of conventionally used apparatuses can be used without any particular limitation. For example, a non-media type dispersing machine such as a jet mill or a planetary mixer, or a medium type dispersing machine such as a ball mill can be used. The conditions for the pulverization treatment (for example, pulverization speed and pulverization time) may be adjusted as appropriate so that a desired particle size can be obtained.
Thus, for example, layered double hydroxide particles having an average particle size of about 1000 nm or less (typically 1 nm to 1000 nm, preferably 1 nm to 500 nm, more preferably 1 nm to 250 nm) are obtained. Can do. In the present specification, the “average particle diameter” refers to an average particle diameter calculated from a cumulant analysis method by particle size distribution measurement based on a dynamic light scattering method. Specifically, for example, it can be measured by a particle size distribution measuring apparatus using a dynamic light scattering method such as a model “Zetasizer Nano ZS ZEN 3600” (manufactured by Malvern Instruments).

  The layered double hydroxide obtained here has a structure in which hydroxide layers are laminated, and anions are intercalated (incorporated) between the layers. When it is desired to exchange the anionic species with other anionic species, for example, it can be performed as follows. First, the layered double hydroxide obtained above is baked at a high temperature (for example, 300 ° C. to 1000 ° C.) to dehydrate water constituting the hydroxide layer to obtain a double oxide. Next, an aqueous solution in which an anionic species to be exchanged is dissolved in an aqueous solvent is prepared. And the layered double hydroxide in which the target anion was intercalated can be obtained by immersing the said oxide in the prepared aqueous solution. In the ion exchange, ions having a higher charge density (that is, ions having a higher valence and a smaller ion radius) are more easily intercalated.

<(5) Alkaline electrolyte molding>
And the obtained layered double hydroxide particles are formed into a desired shape. The method of molding is not particularly limited, and it is manufactured using a conventionally known molding technique (mold molding, cold isostatic pressing, extrusion molding, injection molding, cast molding, press molding, etc.), etc. Can do.
Specifically, for example, first, a powdery layered double hydroxide and a binder (for example, a cellulose polymer) are dispersed in a dispersion solvent (water or an organic solvent) to form a slurry (a paste, an ink). A dispersion is prepared. The slurry-like dispersion may contain additives such as a dispersant and a surfactant as necessary. Next, the slurry-like dispersion is formed into a desired shape using the above-described known forming technique. And an alkaline electrolyte can be manufactured by drying (for example, heating) this molded object.
The alkaline electrolyte obtained by the production method disclosed herein can take various shapes depending on the application. Examples of the shape include a film shape, a flat plate (thin plate) shape, and a columnar shape (pellet shape), but the shape and size are not particularly limited. The thickness (typically the film thickness) of the alkaline electrolyte is typically about 10 μm to 500 μm, and preferably about 20 μm to 300 μm, but is not limited to such a film thickness.

<Fuel cell>
The present invention also provides a fuel cell including any of the alkaline electrolytes disclosed herein. The alkaline electrolyte disclosed here has low humidity dependence and exhibits high ionic conductivity even in a low humidity region (for example, 20% RH to 60% RH). Therefore, in the fuel cell using the electrolyte, the power generation performance in the low humidity region is improved and the power generation stability can be improved as compared with the conventional fuel cell.
The alkaline electrolyte disclosed here can be used without particular limitation as long as it is a fuel cell using hydroxide ions as an ionic conductor. Examples of such a fuel cell include a polymer electrolyte fuel cell (PEFC), a direct methanol fuel cell (DMFC), and a direct ethanol fuel cell (DEFC). and a fuel cell generically called a direct fuel cell (DFC) such as a cell).
Also, the shape is not particularly limited. For example, a conventionally known flat plate type (Planar), vertical stripe cylindrical type (Tubular), flat cylindrical type (Flat tubular) in which the peripheral side surface of the cylinder is vertically crushed, an integral laminated type, and the like It can be used for a fuel cell. The material used for the fuel cell (for example, the material constituting the electrode, the member constituting the battery cell, etc.) and the construction method of the battery can be the same as those used in the prior art and require special operations. And not.

An example will be described with reference to FIG. FIG. 1 is a cross-sectional configuration diagram schematically showing an AFC 1 provided with an alkaline electrolyte disclosed herein. A typical AFC 1 shown here has a structure in which an anode (fuel electrode) 10 and a cathode (air electrode) 30 are opposed to each other via an alkaline electrolyte (typically a membrane) 20 disclosed herein. doing. A fuel chamber 11 is disposed on the side of the anode 10 not in contact with the electrolyte 20, and an oxidant gas chamber 31 is disposed on the side of the cathode 30 not in contact with the electrolyte 20. The fuel chamber 11 is joined to a pipe or the like for supplying fuel (typically in the form of gas (gas)) and sealed so that gas (fuel or air) does not flow out (inflow). The fuel used here is typically hydrogen gas (H ₂ ), but can be used without particular limitation as long as it burns. For example, a gas such as hydrocarbon (methane (CH ₄ )) may be used, and a liquid alcohol such as methanol or ethanol may be used. The oxidant gas chamber 31 has a structure that is exposed to a gas (typically air) containing oxygen (O ₂ ).
When a current is applied to the AFC, oxygen in the oxygen-containing gas reacts with water (H ₂ O) at the cathode 30 to generate hydroxide ions (OH ⁻ ). The hydroxide ions are supplied from the cathode 30 to the anode 10 via the alkaline electrolyte 20. In the anode 10, electricity is generated by reacting with fuel to generate water and releasing electrons.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not intended to be limited to those shown in the specific examples. In this example, a device in which the ratio of the elements constituting the layered double hydroxide was changed was produced, and its performance was evaluated.

<Example 1>
First, magnesium nitrate hexahydrate (Mg (NO ₃ ) ₂ .6H ₂ O) as a divalent cation source and aluminum nitrate nonahydrate (Al (NO ₃ ) ₃ .3 as a trivalent cation source. 9H ₂ O) was weighed so that the molar ratio of cations (ie, Mg ²⁺ to Al ³⁺ ) was 3: 1 and dissolved in purified water (75 ml) to prepare an aqueous solution. Next, sodium carbonate (Na ₂ CO ₃ ) was weighed and dissolved in purified water (300 ml) to prepare an aqueous solution. Then, the amphoteric ion source in a solution state was added to the aqueous solution in which the cation source was dissolved, and the mixture was stirred with a stirrer for 20 minutes.
To the aqueous solution prepared above, 2M NaOH solution was added to adjust the pH of the aqueous solution to 9.5. The aqueous solution was further stirred for 30 minutes with a stirrer, and then placed in an electric furnace and kept (left) at 80 ° C. for 4 hours to obtain a precipitate. The precipitate was centrifuged and then filtered, washed three times with purified water, and then dried (80 ° C., 24 hours) to obtain a layered double hydroxide (Example 1) having an average particle size of 120 nm.

<Example 2>
Other than using trimethylglycine ((CH ₃ ) ₃ N ⁺ CH ₂ COO ⁻ ) that can be zwitterions instead of sodium carbonate, and maintaining the retention time at 80 ° C. for 24 hours, the above < A layered double hydroxide (Example 2) was obtained in the same manner as in Example 1>.

<Example 3>
Except for adding sodium dodecyl sulfate (CH ₃ (CH ₂ ) ₁₁ OSO ₃ Na), which is an anionic surfactant, to the aqueous solution containing magnesium nitrate hexahydrate and aluminum nitrate nonahydrate, A layered double hydroxide (Example 3) was obtained in the same manner as in <Example 1> above.

<Example 4>
Sodium stearate (CH ₃ (CH ₂ ) ₁₆ COONa), which is an anionic surfactant, is added to the aqueous solution containing magnesium nitrate hexahydrate and aluminum nitrate nonahydrate, and purified water is used as a solvent. A layered double hydroxide (Example 4) was obtained in the same manner as in the above <Example 1> except that a mixed solvent of ethanol and ethanol (mixing ratio was 1: 1) was used.

<Example 5>
Except that the magnesium nitrate hexahydrate as the divalent cation source and the aluminum nitrate nonahydrate as the trivalent cation source were weighed so that the molar ratio of the cations was 2: 1, A layered double hydroxide (Example 5) was obtained in the same manner as in the above <Example 1>.

<Example 6>
Other than using trimethylglycine ((CH ₃ ) ₃ N ⁺ CH ₂ COO ⁻ ) that can be zwitterions instead of sodium carbonate, and maintaining the retention time at 80 ° C. for 24 hours, the above < A layered double hydroxide (Example 6) was obtained in the same manner as in Example 5.
The configuration of each sample described above is summarized in Table 1 below.

[X-ray diffraction measurement]
Here, about the obtained layered double hydroxide (Examples 1-6), the X-ray diffraction (XRD: X-Ray Diffraction) measurement was performed on condition of the following. FIG. 3 shows an XRD chart of the layered double hydroxide (Examples 1 to 4) in which the ratio of the divalent cation (Mg ²⁺ ) to the trivalent cation (Al ³⁺ ) is 3: 1. FIG. 4 shows XRD charts of layered double hydroxides (Examples 5 and 6) having an ion ratio of 2: 1. Table 2 shows the results of summarizing the peak position of the XRD chart and the interlayer distance d (nm) obtained from the Bragg equation (2 d sin θ = nλ).
Measuring device: RINT-TTRIII (manufactured by Rigaku Corporation)
Target: Cu (Kα ray)
Slit: Diverging slit = 1 °, Receiving slit = 0.1 mm, Scattering slit = 1 °
Measurement range: 3 ° ≦ 2θ ≦ 80 °
Measurement temperature: Room temperature (25 ° C)

As shown in FIG. 3 and Table 1, in the XRD chart of Example 1, remarkable peaks appeared in the vicinity of 2θ = 10 ° and 20 °, and it was confirmed from the XRD pattern that the target product could be synthesized. In the XRD charts of Examples 3 and 4, it was confirmed that the peak observed near 2θ = 10 ° in the XRD chart of Example 1 was shifted to the low angle side, that is, the interlayer distance was widened. This is probably because an anionic surfactant having a relatively bulky structure entered the hydroxide layer. On the other hand, no peak shift was observed in the XRD chart of Example 2, but it was confirmed that the peak width in the vicinity of 2θ = 10 ° was slightly wider than that of Example 1. For this reason, it is considered that some change occurs in the crystal structure.
Further, as shown in FIG. 4, it was confirmed from the XRD pattern of Example 5 that the target product was synthesized. In Example 6, two peaks were observed in the vicinity of 2θ = 10 °, and it was considered that trimethylglycine entered the interlayer and the interlayer distance increased.

[Measurement of ion conductivity]
About the obtained layered double hydroxide (Examples 1-6), ion conductivity (sigma) (S / cm) was measured. Specifically, first, a measurement sample (pellet) was prepared. That is, the layered double hydroxide (0.2 g) obtained above is placed in a mold for pellet molding (diameter 10 mm) and pressed under predetermined conditions (for example, after pressing at 6 kN for 30 seconds here) And pressurizing with a stronger pressure (12 kN) for 5 minutes). And the electrically conductive paste (Dotite D-500, Fujikura Chemical Co., Ltd. product) was apply | coated to the front and back of the obtained pellet, and it was made to dry at 80 degreeC for 10 minutes. An electrode metal plate (thickness: 0.1 mm) was placed on the pellet to obtain measurement samples (Examples 1 to 6).

Next, ion conductivity σ (S / cm) was measured using a frequency response analyzer under the following measurement conditions. In this measurement, the measurement was first performed at a relative humidity of 80%, and then the samples having high ion conductivity were also measured under low humidity conditions (60% RH and 20% RH). As an example, FIG. 5 shows an impedance spectrum chart (Cole-Cole plot) measured at 80 ° C. and 20% RH. Moreover, the value of bulk resistance was calculated | required from the obtained Cole-Cole plot, and the ionic conductivity was computed. The results are summarized in FIGS. 5 and 6 and Table 1. In addition, "-" in the following Table 2 represents that it has not been measured.
Measuring device: 1260 type (manufactured by Solartron)
Measurement voltage: 10mV
Measurement frequency: 10 MHz to 0.1 Hz
Measuring method: Two-terminal method Measuring temperature: 80 ° C

As shown in FIG. 5, the resistance in Example 2 containing trimethylglycine was significantly lower than that in Example 1 containing no trimethylglycine under high temperature and low humidity conditions of 80 ° C. and 20% RH. The reason for this is that an element site having a negative charge of trimethylglycine ((CH ₃ ) ₃ N ⁺ CH ₂ COO ⁻ ) that is polarized is bonded to the surface of the positively charged hydroxide layer, thereby allowing hydroxylation. It is conceivable that the interface resistance between the physical layer and the interlayer has been reduced. In addition to the above, an element part having a positive charge of trimethylglycine and a water molecule (typically in a polarized state) are combined to form a stronger ion conduction path (path) than before. It is possible.

As shown in FIGS. 6 and 7, the ionic conductivity of Examples 1 and 5 which are simple layered double hydroxides is very dependent on humidity between 80% RH and 20% RH, and the humidity is reduced. As a result, the ionic conductivity decreased significantly. On the other hand, in the layered double hydroxide to which amphoteric ions were added (Example 2 and Example 6), the humidity dependency (that is, the decrease in ionic conductivity accompanying the decrease in humidity) was significantly suppressed. In particular, when the ratio of divalent cation to trivalent cation is 3: 1, Example 2 containing trimethylglycine is 10 times as much as Example 1 containing no trimethylglycine in an environment of 20% RH. The above ionic conductivity was shown. The reason for this is that in addition to the formation of the above-mentioned strong conduction path, the distance between layers is increased by the addition of the amphoteric organic compound between the hydroxide layers, and more water can be included. Conceivable.
In Examples 3 and 4 to which an anionic surfactant was added, even at 80% RH, the ionic conductivity was small compared to the other examples. The reason for this is that the presence of a large amount of the surfactant having a large molecular weight between the hydroxide layers significantly reduced the mobility of ions (that is, the charge transfer resistance increased significantly). It is possible.

  From the above results, the significance of using a layered double hydroxide having an organic compound capable of forming an amphoteric ion in the crystal structure as an alkaline electrolyte material requiring high ionic conductivity was shown.

1 Fuel Cell 10 Anode (Fuel Electrode)
11 Fuel chamber 20 Alkaline electrolyte 22 Hydroxide layer 23 Zwitterion (polarized organic compound)
24 Interlayer 25 Anion 26 Water molecule (interlayer water)
30 Cathode (Air electrode)
31 Oxidant gas chamber

Claims (7)

Alkaline electrolyte:
The alkaline electrolyte is composed of a composition that forms a layered double hydroxide,
The layered double hydroxide comprises two or more hydroxide layers and a substance disposed between the two or more hydroxide layers,
The hydroxide layer encompasses a divalent cation, a trivalent cation, and water oxide ions,
The material between the layers includes an anion and an organic compound that can be polarized in the molecule to become a zwitterion,
The divalent cation is one or more of magnesium ion, calcium ion and barium ion;
The trivalent cation is an aluminum ion,
The anion is one or more of hydroxide ion, carbonate ion, nitrate ion, nitrite ion, sulfate ion, fluoride ion, chloride ion, bromide ion and iodide ion;
The organic compound is a derivative of amino acids and / or the amino acid,
The amino acids are arginine, histidine, lysine, aspartic acid, glutamic acid, serine, threonine, asparagine, glutamine, alanine, valine, isoleucine, leucine, methionine, phenylalanine, tyrosine, tryptophan, glycine, cysteine, selenocysteine, proline, trimethylglycine One or more of dimethyl glycine, methyl glycine, ethyl glycine and propyl glycine;
The alkaline electrolyte has a molar ratio of the organic compound to the trivalent cation of 0.5 to 20.0 . The divalent cation is magnesium ion, an alkaline electrolyte according to claim 1. The molar ratio of the divalent cation ^{(M II)} and the trivalent cation ^{(M III) (M II /} M III) is 2 to 4, an alkali according to claim 1 or 2 Electrolytes. The molar ratio of the divalent cation ^{(M II)} and the trivalent cation ^{(M III) (M II /} M III) is 2.5 to 3.5, claims 1 to 3 Alkaline electrolyte as described in any one of these. The alkaline electrolyte according to any one of claims 1 to 4 , comprising at least trimethylglycine as the organic compound. A method for producing an alkaline electrolyte comprising a layered double hydroxide comprising:
Providing an aqueous solution containing a divalent cation source and a trivalent cation source;
In aqueous solution the prepared, and child adding an organic compound which can be a zwitterion;
Adjusting the aqueous solution to which the organic compound has been added to alkalinity to obtain a precipitate;
Recovering precipitates from the alkaline aqueous solution; and shaping the recovered precipitates;
It encompasses,
The layered double hydroxide comprises two or more hydroxide layers and a substance disposed between the two or more hydroxide layers,
The hydroxide layer includes a divalent cation, a trivalent cation, and a hydroxide ion,
The material between the layers includes an anion and an organic compound that can polarize in the molecule and become zwitterion,
As the divalent cation, one or more of magnesium ion, calcium ion and barium ion are used,
As the trivalent cation, an aluminum ion is used,
As the anion, one or more of hydroxide ion, carbonate ion, nitrate ion, nitrite ion, sulfate ion, fluoride ion, chloride ion, bromide ion and iodide ion are used,
As the organic compound, an amino acid and / or a derivative of the amino acid is used. , Tyrosine, tryptophan, glycine, cysteine, selenocysteine, proline, trimethylglycine, dimethylglycine, methylglycine, ethylglycine and propylglycine,
The method for producing an alkaline electrolyte, wherein a molar ratio of the organic compound to the trivalent cation is 0.5 to 20.0 . A fuel cell comprising the alkaline electrolyte according to any one of claims 1 to 5 .

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