JP2015182995A - Azole phosphoric acid and production method thereof, as well as proton-conductive electrolyte and electrolyte membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel material that can be used as a proton-conductive electrolyte for high temperature of 100°C or higher.SOLUTION: The problem is solved by an azole phosphoric acid of chemical formula MHPO-1HO (M is azole group) in which azole and phosphoric acid are ionically bonded; azole phosphoric acid can be synthesized by mixing and stirring azole and phosphoric acid; benzimidazole phosphoric acid, 1,2,4-triazole phosphoric acid, pyrazole phosphoric acid and 1,2,3-triazole phosphoric acid are synthesized; these azole phosphoric acids can be isolated as single crystal by adding acetone to the reaction mixture, and a high proton-conductivity is confirmed at high temperature region.

Description

  The present invention relates to an azole phosphoric acid synthesized by reacting an azole and phosphoric acid, and a proton conductive electrolyte and an electrolyte membrane using the azole phosphoric acid. The present invention further relates to a method for producing such azole phosphoric acid.

Phosphoric acid is a mineral acid (inorganic acid) having the chemical formula H ₃ PO ₄ . The conjugate base of phosphoric acid is a dihydrogen phosphate ion (H ₂ PO ₄ ⁻ ), a hydrogen phosphate ion (HPO ₄ ²⁻ ), and a phosphate ion (PO ₄ ³⁻ ). An azole is a nitrogen-containing hetero five-membered ring compound, and additionally contains at least one non-carbon atom of nitrogen, sulfur or oxygen (Non-patent Document 1). Phosphate, which is an inorganic chemical substance, is a salt of phosphoric acid. Organophosphates are important in biochemistry, biogeochemistry, ecology and electrochemistry (Non-Patent Documents 2-5).

  Solid electrolytes (Non-Patent Documents 5 to 7) that conduct ions, particularly protons, over a wide temperature range are attractive materials. This is because such solid electrolytes have potential applications in fuel cells, electrolysis, hydrogen separation and electrochemical sensors. However, proton transport at low temperatures relies on the presence of liquid transport molecules to achieve high mobility. Therefore, only ion exchange membranes such as Nafion (registered trademark of EI DuPont de Nemours and Company) are practical options for many applications such as polymer electrolyte fuel cells (PEFC) (non-patented). Reference 6).

On the other hand, electrolytes that can withstand high temperatures (> 100 ° C.) have attracted attention due to the greatly improved resistance of platinum electrodes to CO at high temperatures and the reduction of both electrode overvoltage and electrolyte resistance. Solid electrolytes such as cesium hydrogen sulfate (CsHSO ₄ ), cesium dihydrogen phosphate (CsH ₂ PO ₄ ), and PBI / H ₃ PO ₄ are widely studied due to high proton conductivity and high temperature stability under non-humidified conditions. (Non-Patent Documents 5 to 8). However, these existing solid electrolytes still do not satisfy the requirements for many applications.

  An object of the present invention is to provide a novel material that can be used as, for example, a solid electrolyte.

According to one aspect of the present invention, there is provided azole phosphate represented by the chemical formula MH ₂ PO ₄ .1H ₂ O (M is an azole group) in which azole and phosphoric acid are ionically bonded.
Here, the azole may be selected from the group consisting of benztriazole, 1,2,4-triazole, pyrazole and 1,2,3-triazole.
According to another aspect of the present invention, a proton conductive electrolyte containing the azole phosphate is provided.
Here, the proton conductive electrolyte may be one in which azole phosphoric acid is embedded in a polymer.
According to still another aspect of the present invention, there is provided a proton conductive electrolyte membrane in which the proton conductive electrolyte is formed in a film shape.
According to still another aspect of the present invention, there is provided a method for producing the azole phosphoric acid, wherein the azole and phosphoric acid are mixed and stirred.
Here, after dissolution by stirring, it may be mixed with acetone to precipitate a single crystal of azole phosphoric acid in acetone.

  The novel substance of the present invention can exhibit high proton conductivity at a high temperature of about 100 ° C. to 180 ° C., for example.

The figure which shows a chemical structure. (A) benzimidazole, (b) 1,2,4-triazole, (c) pyrazole, (d) 1,2,3-triazole, (e) phosphoric acid. Photo of single crystal of azole phosphate. (A) benzimidazole phosphate, (b) 1,2,4-triazole phosphate, (c) pyrazole phosphate, (d) 1,2,3-triazole phosphate. The figure which shows the chemical structure of azole phosphate. (A) benzimidazole phosphate, (b) 1,2,4-triazole phosphate, (c) pyrazole phosphate, (d) 1,2,3-triazole phosphate. The figure which shows the structure of the unit cell of an azole phosphate single crystal. (A) benzimidazole phosphate, (b) 1,2,4-triazole phosphate, (c) pyrazole phosphate, (d) 1,2,3-triazole phosphate. (A) And (b) is a figure which shows the result of TG and DTA of an azole phosphate single crystal, respectively. (I) benzimidazole phosphate, (ii) 1,2,4-triazole phosphate, (iii) pyrazole phosphate, (iv) 1,2,3-triazole phosphate. The figure which shows the temperature dependence of electrical conductivity. (A) pure benzimidazole, (b) benzimidazole phosphoric acid single crystal, (c) phosphoric acid.

  In the present invention, azole phosphate, which is a novel substance having characteristics that can be used as a solid electrolyte, is provided. The azole phosphate single crystals are azoles (benzimidazole, bz, 1,2,4-triazole, pyrazole, 1,2,3-triazole in the following examples ( It is grown by a solution process using 1,2,3-triazole)) and phosphoric acid. The characteristics of this crystal were investigated by elemental analysis, single crystal analysis, thermal stability and conductivity.

  The present invention will be described in more detail with reference to the following examples. However, it should be noted that the present invention is not limited to the examples.

[Experiment]
<Material and crystal growth>
Benzimidazole (Bz; C ₇ H ₆ N ₂ ; M _w = 118.14; M _p = 170 ° C .; B _p ≧ 360 ° C.), 1,2,4-triazole (C ₂ H ₃ N ₃ ; M _w = _{69.06; M p = 120~121 ℃;} B p = 260 ℃), pyrazole _{(C 3 H 4 N 2;} M w = 68.08; M p = 66~70 ℃; B p = 186~188 ℃ ) And 1,2,3-triazole (C ₂ H ₃ N ₃ ; M _w = 69.06; M _p = 23-25 ° C .; B _p = 203 ° C. were purchased from Aldrich, Inc. Phosphoric acid (85%) Was purchased from Wako Pure Chemical Industries, Ltd. Fig. 1 shows these chemical structures.

  Bz (0.5 mmol), 1,2,4-triazole (5 mmol), pyrazole (5 mmol) and 1,2,3-triazole (5 mmol) were each added to phosphoric acid. Each mixture was dissolved and stirred until clear. Acetone was then added to these solutions. Single crystals of azolephosphate grew in acetone.

<Chemical structure and thermal stability>
Elemental analysis for C, H, N, O, and P is as follows: C, H, N are MT-3 / MT-5 (Anatech Yanaco), O is EMGA-920 (Horiba, Ltd.), and P was performed by flow injection analysis (FIA).

  In order to determine the crystal structure of azole phosphate single crystal, room temperature X-ray diffraction data was collected with a Bruker AXS Co., Ltd. SMART Apex CCD diffractometer using Mo Kα radiation (λ = 0.10773１０７). Processed using a Bruker software package containing SHELX97.

The thermal stability of azole phosphate single crystals was examined using thermogravimetric analysis with STA8000 (PerkinElmer, Inc). The sample was heated from room temperature to 400 ° C. at a rate of 5 ° C./min under N ₂ atmosphere. Al ₂ O ₃ powder was used as a reference material.

<Conductivity measurement>
In order to investigate the conductive properties of azole phosphate single crystals at room temperature (29 ° C.), each saturated solution in ultrapure water was prepared, and these were impregnated in a glass film having a thickness of 0.55 mm. The electrical conductivity in the state was measured. Furthermore, in order to obtain the temperature dependence of the conductivity, solid pellets were produced. The membrane was placed between two circular electrodes with an area of about 0.2 cm ² . A frequency range of 1 Hz to 1 MHz and a peak-to-peak voltage of 10 mV were used for impedance measurements on Reference600 ™ (Gamry instruments). All cells were allowed to equilibrate for 30 minutes at each temperature before conducting conductivity measurements. Ionic conductivity was measured by applying a cycle of heating and cooling to 180 ° C. under non-humidified conditions.

<Results and examination>
FIG. 2 shows the appearance of an azole phosphate single crystal. The crystals of benzimidazole phosphate were fibrous, the crystals of 1,2,4-triazole phosphate and triazole phosphate were rod-shaped, and the crystals of 1,2,3-triazole phosphate were plate-like. As shown in Table 1 below, azole phosphoric acid crystals were dissolved in water, methanol, DMSO, and NMP solvents, but not in acetone, 1-propanol, and isopropanol solvents.

Table 2 shows the elemental analysis results of the azole phosphate single crystal. From this result, the chemical formula of azole phosphate single crystal is calculated as follows: Benzimidazole phosphate is (C ₇ H ₆ N ₂ ) H ₂ PO ₄ .1H ₂ O, 1,2,4-triazole phosphate (C ₂ H ₃ N ₃ ) H ₂ PO ₄ .1H ₂ O, pyrazole phosphate is (C ₃ H ₄ N ₂ ) H ₂ PO ₄ .1H ₂ O and phosphate 1,2,3-triazole is ( _{C 2 H 3 N 3) H} 2 PO 4 · 1H 2 O. This can be expressed as a general formula MH ₂ PO ₄ .1H ₂ O (M: azole molecule). One proton in phosphoric acid forms an ionic bond with an azole group, which is azole phosphoric acid as a salt. FIG. 3 shows the structures of the four types of azole phosphates thus formed by ionic bonds.

Chemical structure benzimidazole phosphate: (C ₇ H ₆ N ₂ ) H ₂ PO ₄ .1H ₂ O
1,2,4-triazole phosphate: (C ₂ H ₃ N ₃ ) H ₂ PO ₄ .1H ₂ O
Pyrazole phosphate: (C ₃ H ₄ N ₂ ) H ₂ PO ₄ .1H ₂ O
1,2,3-triazole phosphate: (C ₂ H ₃ N ₃ ) H ₂ PO ₄ .1H ₂ O

  The structure of the azole phosphate single crystal was examined by X-ray diffraction. As a result, the crystal group of benzimidazole phosphate is P212121 (orthorhombic), the crystal group of 1,2,4-triazole phosphate is Pna21 (orthorhombic), and the crystal group of pyrazole phosphate is P21 / n. It was found that the crystal group of (monoclinic) and 1,2,3-triazole phosphate was P21 / c (monoclinic). FIG. 4 shows the unit cell of these azole phosphate single crystals. A disorder phase was found in benzimidazole phosphate single crystals. In this phase, 50% of oxygen can be activated. No other irregular phase was found in other azole phosphates.

  The thermal stability of the phosphate azole single crystal was examined by TG-DTA. The result is shown in FIG. The thermal stability of benzimidazole phosphate was better than the other azole phosphates, and conversely, the thermal stability of pyrazole phosphate was worse than the others. The benzimidazole phosphoric acid single crystal showed a thermal stability of about 200 ° C. In the DTA data, azole phosphate showed three endothermic peaks at different temperatures. The endothermic peak at 42.3 ° C. can be attributed to the dissolution temperature of phosphoric acid. Benzimidazole phosphate, 1,2,4-triazole phosphate, pyrazole phosphate and 1,2,3-triazole phosphate had endothermic peaks at 144 ° C., 119 ° C., 115 ° C. and 142 ° C., respectively. . The endothermic peak is due to dissolution of the azole group in the azole phosphate single crystal. The endothermic peak around 200 ° C. can be attributed to the decomposition of phosphoric acid.

The proton conduction properties of azole phosphate single crystals were investigated by impedance measurement. The conductivity was measured at room temperature (29 ° C.) using an ultrapure water saturated solution. Moreover, it measured by heating up using the pellet of the benzimidazole phosphoric acid single crystal. Benzimidazole phosphoric acid, 1,2,4-triazole phosphoric acid, pyrazole phosphoric acid and 1,2,3-triazole phosphoric acid single crystal ultrapure aqueous solutions are 24 mS / cm, 42 mS / cm, 36 mS / cm and 35 mS, respectively. The conductivity was / cm. In addition, the proton conductivity of benzimidazole phosphate was examined by raising the temperature under non-humidified conditions. The result is shown in FIG. For comparison, the same measurements were made for pure benzimidazole and phosphoric acid. The proton conductivity of solid phase benzimidazole phosphate was lower than that of phosphoric acid and pure benzimidazole. A proton conductivity of 5 × 10 ⁻⁵ S / cm was obtained at 120 ° C. On the other hand, the proton conductivity of benzimidazole phosphate increased slowly with increasing temperature, but suddenly increased above 120 ° C. A proton conductivity of 0.06 S / cm was obtained at 160 ° C. Since the melting point of benzimidazole phosphoric acid is 140 ° C., benzimidazole phosphoric acid becomes a liquid phase from 140 ° C. and is stable up to 200 ° C. (see FIG. 5). Therefore, a high conductivity between 180 ° C. and 100 ° C. can be used as an electrolyte for PEFC at high temperatures.

  In the above-mentioned examples, benzimidazole, 1,2,4-triazole, pyrazole and 1,2,3-triazole are exemplified as azoles, but azole phosphate can be prepared by other azoles. For example, imidazole may be used. However, imidazole phosphate could not be isolated as a single crystal by the above method.

  In addition, since these azole phosphoric acids themselves do not form a film, when used as an electrolyte, they are used as composite materials such as embedded in other materials such as polymers.

An azole phosphate single crystal was grown by a solution process using benzimidazole, 1,2,4-triazole, pyrazole, 1,2,3-triazole, and phosphoric acid. The azole phosphate single crystal has a structure of MH ₂ PO ₄ .1H ₂ O (M is an azole group) which is a phosphate. The crystal group was orthorhombic or monoclinic, and benzimidazole phosphate was irregular in the crystal structure and was stable up to 200 ° C. A high proton conductivity of 0.06 S / cm was obtained with benzimidazole phosphoric acid liquefied between 180 ° C and 100 ° C. Due to this high proton conductivity, the azole phosphoric acid of the present invention is promising as an electrolyte for high-temperature PEFC. In addition, since this substance is an organic phosphate, it is expected to be applied in fields such as biochemistry, biogeochemistry, ecology, and electrochemistry other than electrolytes.

Iwanami Rikagakuziten, 5th edition, 20. Iwanami Rikagakuziten, 5th edition, 1479. Japan patent, PCT / JP2012 / 064605. G. Matsunaga, H. Sato, "Industrial use of phosphorus", The Society for Biotechnology, Japan, 8 (2012) 477-480. D.A.Boysen, S.M.Haile, H. Liu, R.A.Secco, "High-temperature behavior of CsH2PO4 under both ambient and high pressure conditions", Chem. Mater. 15 (2003) 727-736. K. D. Kreuer, Chem. Mater., 8 (1996) 610. J. H. Park, Phys. Rev. B, 69 (2004) 054104. J.-T. Wang, R.F.Savinell, J. Wainright, M. Litt, and H. Yu, Electrochim. Acta 41 (1996) 193.

Claims (7)

An azole phosphate represented by the chemical formula MH ₂ PO ₄ .1H ₂ O (M is an azole group), in which azole and phosphoric acid are ionically bonded.   The azole phosphoric acid according to claim 1, wherein the azole is selected from the group consisting of benztriazole, 1,2,4-triazole, pyrazole and 1,2,3-triazole.   A proton conducting electrolyte comprising the azole phosphate of claim 1 or 2.   The proton conductive electrolyte according to claim 3, wherein azole phosphoric acid is embedded in the polymer.   5. A proton conductive electrolyte membrane in which the proton conductive electrolyte according to claim 3 or 4 is formed into a film shape.   The method for producing azole phosphoric acid according to claim 1 or 2, wherein azole and phosphoric acid are mixed and stirred. The method for producing azole phosphoric acid according to claim 6, wherein after the dissolution by stirring, the mixture is mixed with acetone to precipitate a single crystal of azole phosphoric acid in acetone.