Preyssler type polyacid crystalline material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of fuel cells, and discloses a Preyssler type polyacid crystalline material, the structural formula of the Preyssler type polyacid crystalline material is {[Cd5 (mu2-H2O) (H2O) 14 (H5P5W30O110) (btp) 4]. 8H2O} n, and the asymmetric unit of the Preyssler type polyacid crystalline material comprises 2.5 crystallographically independent Cd (II) ions, a half {P5W30} unit, 2 btp ligands, 8 coordinated water molecules and 4 free water molecules. The powder proton conductivity of the compound reaches 1.70 * 10 <-3 > S cm <-1 > under the conditions of 85 DEG C and 98% RH, the proton conductivity of a composite membrane formed by the compound and Nafion reaches 2.20 * 10 <-2 > S cm <-1 >, and large-scale commercial application of proton exchange membrane fuel cells can be achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and relates to a Preyssler-type polyoxometalate crystalline material, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous growth of global energy demand and the increasingly severe environmental problems, the development of efficient and clean energy conversion technologies has become the focus of current research. Proton exchange membrane fuel cells (PEMFCs) are regarded as an important part of the future energy system due to their high energy conversion efficiency and environmental friendliness. As the core component of PEMFCs, the performance of proton exchange membranes (PEMs) directly determines the durability and electrochemical performance of the battery. Currently, commercial Nafion membranes are widely used due to their high proton conductivity and excellent chemical stability. However, the amorphous structure of Nafion membranes makes it difficult to clarify the relationship between their structure and performance, which limits the further improvement of their performance. Therefore, the development of crystalline proton-conducting materials with a clear structure has become an urgent task in current research.

[0003] In recent years, polyoxometalates (POMs) have received extensive attention in the field of proton conduction due to their abundant surface oxygen atoms that can serve as active sites for proton hopping. Among them, Preyssler-type polyoxoanions exhibit excellent proton conduction potential due to their unique cyclic geometric structure and high negative charge characteristics. Preyssler-type POMs can not only adsorb a large number of protons, but also have abundant surface oxygen atoms and diverse coordination modes, and can maintain stability in a wide pH range and high temperature conditions. Therefore, they are considered ideal candidates for proton-conducting materials. However, the strong water solubility of polyoxometalates limits their application in the field of proton conduction. Introducing N-heterocyclic ligands and transition metal ions to construct POM-based coordination polymers can not only enhance the stability of the structure, but also further expand their application in proton conduction. To sum up, the development of novel proton-conducting materials based on Preyssler-type POMs and N-heterocyclic ligands can not only solve the problem of unclear relationship between the structure and performance of existing proton exchange membrane materials, but also provide a new technical path for the development of high-performance PEMFCs. Summary of the Invention

[0004] In view of the technical problem of poor structural stability existing in Preyssler-type polyoxometalate crystalline materials, the present invention provides a Preyssler-type polyoxometalate crystalline material with high proton conductivity. This compound has good thermal stability and water stability, and its powder proton conductivity reaches 1.70×10 -3 S cm -1 at 85 °C and 98% RH. The proton conductivity of the composite membrane composed of it and Nafion reaches 2.20×10 -2 S cm -1 , which is improved by an order of magnitude, contributing to the large-scale commercial application of proton exchange membrane fuel cells.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In the first aspect, the present invention provides a Preyssler-type polyoxometalate crystalline material, and the structural formula of the Preyssler-type polyoxometalate crystalline material is {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n . Its asymmetric unit contains 2.5 crystallographically independent Cd(II) ions, half a {P5W 30} unit, 2 btp ligands, 8 coordinated water molecules and 4 free water molecules; in this compound, the btp ligand adopts two coordination modes, and two pairs of μ2-btp and μ3-btp ligands connect two Cd1, two Cd2 and two Cd3 to form a hexanuclear Cd6(btp)4 unit. The hexanuclear Cd6(btp)4 unit extends into a one-dimensional chain through μ2-H2O, and the {P5W 30} unit is connected to 8 Cd(II) ions from four hexanuclear Cd6(btp)4 units between the one-dimensional chains to expand and form a three-dimensional framework structure.

[0007] Furthermore, the Preyssler-type polyoxometalate crystalline material belongs to the monoclinic system, space group C2 / c, and the unit cell parameters are: α = 90°, β = 98.160(7)°, γ = 90°.

[0008] In the second aspect, the present invention provides a preparation method of the above-mentioned Preyssler-type polyoxometalate crystalline material, including the following steps: including the following steps: Mix CdCl2·2.5H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine (abbreviated as btp), K 12.5 Na 1.5 [NaP5W 30 O 110Mix CdCl₂·2.5H₂O, 2,6-bis(1,2,4-triazol-1-yl)pyridine, KNa[NaP₅W₁₂O₄₀]·15H₂O and water in a reaction kettle and stir for 30 min. Adjust the pH to 1.8, then react the mixture at 160 °C for 96 h. After cooling to room temperature, yellow block crystals are obtained.

[0009] Further, the molar ratio of CdCl₂·2.5H₂O, 2,6-bis(1,2,4-triazol-1-yl)pyridine and KNa[NaP₅W₁₂O₄₀]·15H₂O is 6:2:1. 12.5 Na 1.5 [NaP₅W 30 O 110 ·15H₂O is 6:2:1.

[0010] In a third aspect, the present invention provides the application of the Preyssler-type polyoxometalate crystalline material in the preparation of a fuel cell proton exchange membrane, and the proton exchange membrane is a composite membrane formed by dispersing the Preyssler-type polyoxometalate crystalline material in a Nafion solution.

[0011] Further, the dosage ratio of the Preyssler-type polyoxometalate crystalline material to the Nafion solution is 2.5 mg:1 ml.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] The present invention uses the Preyssler-type {P₅W₁₂} as a building block, and uses 2,6-bis(1,2,4-triazol-1-yl)pyridine and Cd to jointly modify the polyoxometalate, and hydrothermally synthesizes a compound: {[Cd₅(μ₂-H₂O)(H₂O)(H₅P₅W₁₂O₄₀)(btp)₄]·8H₂O}. 30} as a building block, and uses 2,6-bis(1,2,4-triazol-1-yl)pyridine and Cd 2+ to jointly modify the polyoxometalate, and hydrothermally synthesizes a compound: {[Cd₅(μ₂-H₂O)(H₂O) 14 (H₅P₅W 30 O 110 )(btp)₄]·8H₂O} n . This compound has good thermal stability and water stability, and the powder proton conductivity reaches 1.70×10 –3 S cm -1 ⁻¹ at 85 °C and 98% RH. The proton conductivity of the composite membrane composed of it and Nafion reaches 2.20×10 –2 S cm -1 ⁻¹, which can be comparable to commercial Nafion, and helps to realize the large-scale commercial application of proton exchange membrane fuel cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The structure of the compound {[Cd₅(μ₂-H₂O)(H₂O) 14 (H₅P₅W 30 O 110 )(btp)₄]·8H₂O} n of the present invention, wherein,Figure 1 a is the coordination environment of metal ions in the compound; Figure 1 b is the one-dimensional chain of the compound; Figure 1 c is the coordination environment of the {P5W 30} unit in the compound, Figure 1 d is the three-dimensional framework structure of the compound.

[0015] Figure 2 For the compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n is the coordination mode of bpt.

[0016] Figure 3 For the compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n is the TGA curve.

[0017] Figure 4 For the compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n are the PXRD patterns under different conditions.

[0018] Figure 5 For the compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n is the powder proton conductivity test structure, where Figure 5 a is the Nyquist plot of the compound at 25 °C, 55%–98% RH; Figure 5 b is the Nyquist plot of the compound at 98% RH and different temperatures; Figure 5 c is the Arrhenius plot of the compound at 98% RH and different temperatures.

[0019] Figure 6 For the characterization results of the proton exchange composite membrane prepared using the compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n where Figure 6a is the IR spectrum of the composite membrane; Figure 6 b is the PXRD pattern of the composite membrane; Figure 6 c is the physical photo of the composite membrane; Figure 6 d is the SEM image of the composite membrane.

[0020] Figure 7 is the proton conductivity test result of the proton exchange composite membrane prepared by using the compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O}, n wherein, Figure 7 a is the Nyquist plot of the composite membrane at 25 °C and 55%–98% RH; Figure 7 b is the Nyquist plot of the composite membrane at different temperatures under 98% RH; Figure 7 c is the Arrhenius plot of the composite membrane at different temperatures under 98% RH. Detailed implementation mode

[0021] The following examples are used to illustrate the present invention, but are not used to limit the protection scope of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. The test methods in the following examples are all conventional methods unless otherwise specified. The test reagents in the following examples are all commercially available unless otherwise specified.

[0022] Example 1 Compound {[Cd5(μ2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n Synthesis and characterization

[0023] K 12.5 Na 1.5 [NaP5W 30 O 110 ·15H2O (hereinafter abbreviated as: {P5W 30}) was synthesized according to the literature (Inorg. Chem. 1993, 32, 1573-1578).

[0024] CdCl2·2.5H2O (0.12 mmol, 0.0274 g), 2,6-bis(1,2,4-triazol-1-yl)pyridine (hereinafter referred to as btp, Jinan Henghua Technology Co., Ltd.) (0.04 mmol, 0.0085 g), K 12.5 Na 1.5 [NaP5W 30 O 110·15H2O (0.02 mmol, 0.1649 g) was placed in a reaction kettle, 6 mL of distilled water was added and stirred, and the pH value of the mixture was adjusted to about 1.8 with 1 mol / L HCl. The reaction kettle was placed in an oven at 160 °C and heated for 4 days. After cooling to room temperature, yellow block crystals were obtained with a yield of 30.20% (calculated based on the btp ligand).

[0025] 1) Elemental analysis (%) : Theoretical values: C 4.67, N 4.23, H 0.86; Calculated values: C 4.71, N 4.51, H 1.04.

[0026] 2) Infrared spectroscopy data (KBr, cm -1 ) : 3564 (s), 3119 (w), 1611 (m), 1518 (m), 1473 (m), 1166 (s), 1080 (s), 922 (s), 780 (s).

[0027] 3) Crystal structure characterization

[0028] A single crystal of appropriate size was selected, and the crystal data of the compound was collected using a Bruker APEX-II CCD single crystal diffractometer (Mo Kα, ) at a test temperature of 167 K. The structure was determined by the direct method and refined using SHELXS - 2008 and SHELXL - 2015. The crystallographic data is shown in Table 1.

[0029] Table 1 {[Cd5(μ2 - H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n Crystallographic data of the compound

[0030]

[0031] Single crystal X - ray diffraction (SCXRD) analysis showed that the compound crystallized in the monoclinic system, space group C2 / c. Its asymmetric unit contains 2.5 crystallographically independent Cd(II) ions, half a {P5W 30} unit, 2 btp ligands, 8 coordinated water molecules and 4 free water molecules.

[0032] As Figure 1 shown in a, Cd1 forms a six - coordination with one N atom in the btp ligand, one O atom in the {P5W 30} unit, one μ2 - H2O and three water molecules. Cd2 forms a coordination with four N atoms in two btp ligands and two {P5W 30The two O atoms in the {unit} and two water molecules form an octahedral coordination. Compared with the Cd2+ ion, Cd3+ exhibits a pentagonal bipyramidal structure with seven coordination, reducing one O atom from the {P5W 30} unit. The Cd-O and Cd-N bond lengths are 2.180 - 2.5 and

[0033] In the compound, the btp ligand adopts two coordination modes, as Figure 2 shown. Two pairs of μ2-btp and μ3-btp ligands connect two Cd1, two Cd2 and two Cd3 to form a hexanuclear Cd6(btp)4 unit, where the Cd…Cd distance is The hexanuclear Cd6(btp)4 unit extends into a one-dimensional chain through μ2-H2O (as Figure 1 shown in b). The {P5W 30} unit is connected to eight Cd(II) ions from four hexanuclear Cd6(btp)4 units in the one-dimensional chain (as Figure 1 shown in c), thus expanding to form a three-dimensional framework structure (as Figure 1 shown in d). Meanwhile, there is also a hydrogen bond interaction between the free water molecules and the polyoxometalate, increasing the stability of the structure.

[0034] 4) Thermal stability and water stability tests of the compound

[0035] Thermogravimetric analysis (TGA) was used to study the thermal stability of the compound. As Figure 3 shown, in the temperature range from room temperature to 160 °C, the first weight loss of the compound occurs, which can be attributed to the loss of lattice water and coordinated water molecules (experimental value 4.30%, calculated value 4.47%); subsequently, a plateau is maintained between 160 °C and 300 °C; as the temperature further increases, the organic ligand begins to decompose. This indicates that the compound has good thermal stability, and the framework decomposition temperature is about 300 °C.

[0036] As Figure 4 shown, in the X-ray diffraction (PXRD) pattern, the experimental peaks are basically consistent with the simulated peaks, indicating that the compound has good purity. After the compound was immersed in water for three days, the PXRD curve did not change significantly, which indicates that the compound has good water stability.

[0037] 5) Powder proton conductivity test of the compound

[0038] The crystal of the compound was ground into powder. The powder sample was placed in a mold with a diameter of 0.3 cm and kept under a pressure of 0.5 MPa for 30 seconds to obtain a disc with a thickness of about 1.0 mm. Subsequently, conductive silver paste was evenly coated on the upper and lower cross-sections of the disc, and it was fixed on the sample stage with a gold wire for testing.

[0039] The instrument used for the alternating current (AC) impedance test of the compound is a high-precision impedance gain phase analyzer (Solartron 1260 / 1296). The measurement frequency range is 10 MHz to 0.01 Hz, the input voltage is 100 mV, the measurement temperature range is from 25 °C to 85 °C, and the humidity range is from 55% RH to 98% RH. The conductivity σ (S cm -1 ) is calculated by the formula σ = L / (RS), where L (cm) and S (cm 2 ) represent the thickness and cross-sectional area of the wafer respectively, and R (Ω) is the resistance value of the sample, which is obtained by simulating the impedance data in the Nyquist plot through the Zeview equivalent circuit method. The activation energy (E a ) is obtained by the Arrhenius formula σT = σ o exp(-E a / kT), where k is the Boltzmann constant (eV / k) and T (K) is the temperature.

[0040] As Figure 5 shown in -8 a, at 25 °C, as the relative humidity increases from 55% to 98%, the conductivity of the compound continuously increases, from 3.20×10 -1 S cm -4 to 3.6×10 -1 S cm -3 , an increase of four orders of magnitude, indicating that humidity has a great influence on proton conductivity. When the relative humidity is 98% and the temperature continuously rises to 85 °C, the proton conductivity of the compound reaches 1.70×10 -1 S cm Figure 5 , as shown in

[0041] b. -1 To further explore the proton transport mechanism, the Arrhenius equation is used to fit the proton conductivity with temperature changes, and a plot of [ln(σT) vs 1000 / T Figure 5 is made (as shown in a c), and the activation energy E

[0042] of this compound is calculated to be 0.27 eV, following a hopping mechanism. Figure 4 In addition, we tested the sample after the proton conductivity test by X-ray powder diffraction (XRD). As shown in

[0043] Example 2 Preparation and Characterization of Proton Exchange Composite Membrane

[0044] The compound prepared in Example 1 was ground into powder. 10 mg was taken and evenly dispersed in a mixture of ethanol (1 mL) and 5% wt Nafion solution (Sigma-Aldrich, 4 mL), and ultrasonically dispersed for 1 h, then stirred at room temperature for 12 h to obtain a homogeneous solution. Subsequently, the obtained solution was poured into a petri dish and vacuum dried at 60 °C to prepare a composite membrane. The thickness of the composite membrane was measured with a vernier caliper, and the content of the compound in the composite membrane was 5.2%.

[0045] The infrared spectra of the compound and the composite membrane were tested in the range of 400 - 4000 cm -1 using a Bruker VERTEX-70 Fourier transform infrared spectrometer. Figure 6 a The results showed that characteristic peaks of -SO3H groups (1058–1053 cm -1 ) and characteristic peaks of W–O terminal vibrations in the {P5W 30} unit (930–904 cm -1 ) appeared in the composite membrane, indicating the successful preparation of the composite membrane. In the PXRD pattern, the diffraction peak of the Nafion membrane appeared at 2θ = 17°. With the addition of the sample, the characteristic peaks in the composite membrane curve shifted to lower angles (as shown in Figure 6 b), which can be attributed to the interaction between Nafion and the compound. It can be seen from the electron photos and SEM images that the composite membrane has a smooth surface, indicating that the compound powder is evenly distributed in Nafion (as shown in Figure 6 c, d).

[0046] The composite membrane was measured for AC impedance to evaluate its proton conduction performance. As shown in Figure 7 a, the proton conductivity of this composite was 1.84×10 –3 S cm -1 at 25 °C and 98% RH. As the temperature increased, the proton conductivity reached 2.20×10 –2 S cm -1 at 85 °C and 98% RH (as shown in Figure 7 b). Compared with the proton conductivity of the compound, the proton conductivity of the composite membrane increased by a factor of ten. The activation energy E a of the composite membrane was obtained by fitting with the Arrhenius formula to be 0.25 eV (as shown in Figure 7 c), following a hopping mechanism.

[0047] The above-described embodiments are only the preferred embodiments of the present invention, which are merely used to explain the present invention and do not limit the scope of implementation of the present invention. For those skilled in the art of this technology, of course, other implementation manners can be easily made by means of substitution or change according to the technical content disclosed in this specification. Therefore, all changes and improvements made on the principle of the present invention should be included within the scope of the patent application of the present invention.

Claims

1. A Preyssler-type polyoxometalate crystalline material, characterized in that, The structural formula of the Preyssler-type polyoxometalate crystalline material is {[Cd5( μ 2-H2O)(H2O) 14 (H5P5W 30 O 110 )(btp)4]·8H2O} n , and its asymmetric unit contains 2.5 crystallographically independent Cd(II) ions, half a {P5W 30} unit, 2 btp ligands, 8 coordinated water molecules and 4 free water molecules; in this compound, the btp ligand adopts two coordination modes, and two pairs of μ 2-btp and μ 3-btp ligands connect two Cd1, two Cd2 and two Cd3 to form a hexanuclear Cd6(btp)4 unit. The hexanuclear Cd6(btp)4 unit extends into a one-dimensional chain through μ 2-H2O. The {P5W 30} unit is connected to 8 Cd(II) ions from four hexanuclear Cd6(btp)4 units between the one-dimensional chains, thus expanding to form a three-dimensional framework structure.

2. The Preyssler-type polyoxoacid crystalline material according to claim 1, wherein The Preyssler-type polyoxometalate crystalline material belongs to the monoclinic system, C 2 / c space group, and the unit cell parameters are: a = 22.490(4) Å, b = 22.854(4) Å, c = 28.557(6) Å, α = 90°, β = 98.160(7)°, γ = 90°.

3. The preparation method of the Preyssler-type polyoxometalate crystalline material according to claim 1 or 2, characterized in that, comprising the following steps: Put CdCl2·2.5H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine, K 12.5 Na 1.5 [NaP5W 30 O 110 ·15H2O and water into a reaction kettle, stir for 30 min, adjust the pH to 1.8, then react the mixture at 160 °C for 96 h. After cooling to room temperature, yellow block crystals are obtained.

4. The preparation method according to claim 3, characterized in that, The molar ratio of the CdCl2·2.5H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine, and K 12.5 Na 1.5 [NaP5W 30 O 110 ·15H2O is 6:2:

1.

5. Use of the Preyssler-type polyoxoacid crystalline material according to claim 1 or 2 in the preparation of a proton exchange membrane for a fuel cell, characterized in that, The proton exchange membrane is a composite membrane formed by dispersing the Preyssler-type polyoxometalate crystalline material in a Nafion solution.

6. The application according to claim 5, wherein The dosage ratio of the Preyssler-type polyoxometalate crystalline material to the Nafion solution is 2.5 mg: 1 ml.

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