Preyssler type polyacid crystalline material with high proton conductivity as well as preparation method and application of Preyssler type polyacid crystalline material

Abstract

The invention belongs to the technical field of fuel cells, and discloses a Preyssler type polyacid crystalline material with high proton conductivity, the structural formula of the Preyssler type polyacid crystalline material is [Zn4 (H2O) 8 (H7P5W30O110) (btp) 4]. 6H2O, and the asymmetric unit of the Preyssler type polyacid crystalline material comprises an independent Zn (II) ion, a quarter {P5W30} unit, two half btp ligands, two coordinated water molecules and two free water molecules. The compound is a proton conduction material with great potential, the proton transmission mechanism of the compound and the proton transmission mechanism of the composite membrane follow a jumping mechanism, and the stable three-dimensional frame structure of the compound is beneficial to large-scale commercial application of proton exchange membrane fuel cells.

Description

Technical Field

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

[0002] Proton exchange membrane fuel cells (PEMFCs), as a type of fuel cell, exhibit significant advantages in many aspects: their power density performance indicators are particularly prominent, the system response speed is fast, the startup time is significantly shortened, and as a green and efficient energy conversion device, they are regarded as a highly potential alternative clean energy source. The commercial proton exchange membrane is the Nafion membrane, which has good stability. Under the conditions of 75 °C and 100% RH, the conductivity can reach 10 -1 S cm -1 . However, despite the many advantages of the Nafion membrane, its existing limitations cannot be ignored: high cost, which limits its large-scale use; dependence on water content, with a significant decline in proton conduction performance at low humidity and inability to be used under high-temperature conditions; relatively high fuel permeability; and environmental pollution caused by the degradation products of high-fluorine-content materials. Therefore, the work on modifying the Nafion membrane has become a current research hotspot.

[0003] Polyoxometalates (POMs) are metal-oxygen clusters with a rich oxygen atom surface and a well-defined structure. Their excellent charge transfer ability and relatively high proton capacity make them good proton conduction materials. In most cases, proton conduction is closely related to water molecules. The oxygen-rich surface of POMs can form a hydrogen bond network for proton transport with water molecules. However, the high solubility of POMs in water leads to dissolution under high-humidity conditions, thus limiting their application. Therefore, regulating the structure of POMs and enhancing their stability are problems that need to be solved for polyoxometalate-based materials.

[0004] Preyssler-type POMs generally refer to {P5W 30}, whose structure was measured and determined by the Pope research group in 1985. The characteristics of {P5W 30} are: (1) having a relatively large cavity that can capture metal cations of a suitable size; (2) high stability, still being able to remain stable in a wide pH range (1 - 10) and at high temperatures (up to 300 °C); (3) rich surface oxygen atoms, which are beneficial to the construction of a hydrogen bond network. In addition, introducing N-heterocyclic ligands and transition metal ions has been proven to be an effective strategy for constructing POM-based coordination polymers. Such materials can not only enhance the stability of the structure but also further expand their application in proton conduction. Based on this, combining Preyssler-type POMs with N-heterocyclic ligands to design and synthesize new proton conduction materials has important research value and application prospects. Summary of the Invention

[0005] In view of the technical problem of poor structural stability of Preyssler-type polyoxometalate crystalline materials, the present invention provides a Preyssler-type polyoxometalate crystalline material with high proton conductivity. The proton conductivity of the composite membrane composed of it and Nafion reaches 3.43×10 -2 S cm -1 , and its stable three-dimensional framework structure helps to realize the large-scale commercial application of proton exchange membrane fuel cells.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a Preyssler-type polyoxometalate crystalline material with high proton conductivity. The structural formula of the Preyssler-type polyoxometalate crystalline material is [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O. Its asymmetric unit contains an independent Zn(II) ion, a quarter of the {P5W 30} unit, two half btp ligands, two coordinated water molecules and two free water molecules; in this compound, the Zn(II) ion coordinates with two N atoms derived from the btp ligand, two O atoms derived from the {P5W 30} unit and two water molecules. Each btp ligand bridges two Zn(II) ions through the triazole nitrogen on both sides. Each {P5W 30} unit is connected to eight Zn(II) centers to form a two-dimensional layer, and the two-dimensional layer is further expanded into a three-dimensional framework structure through free water molecules and {P5W 30} units.

[0008] Furthermore, the Preyssler-type polyoxometalate crystalline material belongs to the monoclinic system, space group C2 / m, and the unit cell parameters are: α = 90°, β = 124.687(3)°, γ = 90°.

[0009] In the second aspect, the present invention provides a preparation method of the above-mentioned Preyssler-type polyoxometalate crystalline material with high proton conductivity, including the following steps: Mix Zn(NO3)2·6H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine (abbreviated as btp), K 12.5 Na 1.5 [NaP5W 30 O 110Mix Zn(NO3)2·6H2O, distilled water and ethanol in 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, black block crystals are obtained.

[0010] Furthermore, the molar ratio of Zn(NO3)2·6H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine and K 12.5 Na 1.5 [NaP5W 30 O 110 ·15H2O is 12:4:1.

[0011] In a third aspect, the present invention provides the application of a Preyssler-type polyoxometalate crystalline material with high proton conductivity 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.

[0012] Furthermore, the dosage ratio of the Preyssler-type polyoxometalate crystalline material to the Nafion solution is 2.5 mg:1 ml.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] The present invention uses Preyssler-type {P5W 30} as a building block, uses 2,6-bis(1,2,4-triazol-1-yl)pyridine and metal zinc ions to jointly modify polyoxometalate, and hydrothermally synthesizes a compound: [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O. The proton conductivity of the powder of this compound reaches 2.92×10 –3 S cm -1 under the conditions of 85 °C and 98% RH. The proton conductivity of the composite membrane composed of it and Nafion reaches 3.43×10 –2 S cm -1 , which can be comparable to commercial Nafion. Therefore, this compound is a very promising proton transfer material, and the proton transport mechanisms of it and the composite membrane both follow the hopping mechanism. Its stable three-dimensional framework structure helps to realize the large-scale commercial application of proton exchange membrane fuel cells. Description of the Drawings

[0015] Figure 1 This is the structure of the compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O of the present invention, wherein, Figure 1a is the coordination environment of Zn(II) in the compound; Figure 1 b is the environment around {P5W 30} in the compound; Figure 1 c is the 2D layer of the compound, Figure 1 d is the three-dimensional framework structure of the compound.

[0016] Figure 2 This is the PXRD pattern of the compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O after soaking in water in the present invention.

[0017] Figure 3 This is the powder proton conductivity test structure of the compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O in the present invention, wherein, Figure 3 a is the Nyquist plot of the compound at 25 °C and 55%–98% RH; Figure 3 b is the Nyquist plot of the compound at 98% RH and different temperatures; Figure 3 c is the Arrhenius plot of the compound at 98% RH and different temperatures; Figure 3 d is the PXRD pattern of the compound before and after the proton conductivity test.

[0018] Figure 4 This is the characterization result of the proton exchange composite membrane prepared by using the compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O in the present invention, wherein, Figure 4 a is the infrared spectrum of the composite membrane; Figure 4 b is the PXRD pattern of the composite membrane.

[0019] Figure 5 This is the proton conductivity test result of the proton exchange composite membrane prepared by using the compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O in the present invention, wherein, Figure 5 a is the Nyquist plot of the composite membrane at 25 °C and 55%–98% RH; Figure 5 b is the Nyquist plot of the composite membrane at 98% RH and different temperatures; Figure 5 c is the Arrhenius plot of the composite membrane at 98% RH and different temperatures. Specific Embodiments

[0020] The following examples are used to illustrate the present invention, but are not intended to limit the scope of protection 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.

[0021] Example 1 Synthesis and Characterization of the Compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O

[0022] 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).

[0023] Zn(NO3)2·6H2O (0.12 mmol, 0.0357 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), {P5W 30} (0.01 mmol, 0.0825 g), distilled water (4 mL) and ethanol (2 mL) were placed in a stainless - steel container with a polytetrafluoroethylene liner and stirred for 30 min. During the stirring process, the pH was adjusted to about 1.80 with 1 mol / L HCl, and then the mixture was maintained at 160 °C for 4 days. After cooling to room temperature, black block - shaped crystals were obtained. Based on the btp ligand calculation, the product yield was 28.7%.

[0024] 1) Elemental analysis (%): Theoretical values: C 4.91, N 4.45, H 0.72; Experimental values: C 4.92, N 4.64, H 1.12.

[0025] 2) Infrared spectrum data (KBr, cm -1 ) : 3455(s), 3140(m), 1613(m), 1527(m), 1479(s), 1165(s), 1073(s), 938(s), 910(s), 795(s).

[0026] 3) Crystal structure characterization

[0027] A single crystal of appropriate size was selected and used on a Bruker APEX - II CCD single - crystal diffractometer (Mo Kα, )The crystal data of the compound were collected at a test temperature of 271 K. The structure was determined by the direct method and refined using SHELXS-2008 and SHELXL-2015. The crystallographic data are shown in Table 1.

[0028] Table 1 Crystallographic data for the compound [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O

[0029]

[0030]

[0031] Single-crystal X-ray diffraction (SCXRD) analysis showed that the compound crystallized in the monoclinic system in the C2 / m space group. Its asymmetric unit contains one independent Zn(II) ion, a quarter of the {P5W 30} unit, two half btp ligands, two coordinated water molecules, and two free water molecules.

[0032] As Figure 1 shown in a, the Zn(II) ion in the compound adopts a six-coordinate mode, including two N atoms from the btp ligand, two O atoms from the {P5W 30} unit, and two water molecules. The bond lengths of Zn-O / N are in the range. Each btp ligand in the compound uses the triazole nitrogen on both sides to bridge two Zn(II) ions, thus expanding into a zigzag one-dimensional chain with a Zn…Zn distance of . The center of the {P5W 30} unit is located on the C2 axis, and each {P5W 30} is connected to eight Zn(II) centers. As Figure 1 shown in b, there are four Zn(II) ions on the upper part and four Zn(II) ions on the lower part, thus encapsulating the {P5W 30} unit in the cavity surrounded by four one-dimensional chains, finally forming a two-dimensional layer as Figure 1 shown in c. In addition, the two-dimensional layer is further extended into a three-dimensional framework structure through the O-H…O hydrogen bonds formed by free water molecules and the {P5W 30} unit ( Figure 1 d).

[0033] 4) Water stability test of the compound

[0034] Good water stability plays an important role in the application of proton conduction materials. After soaking the crystalline sample of the compound in distilled water for 3 days, powder PXRD test was carried out, as Figure 2As shown, the peak positions of the soaked samples are consistent with the diffraction peaks of the unsoaked samples, indicating that the compounds have not undergone structural changes after soaking treatment and have excellent water stability.

[0035] 5) Powder proton conductivity test of the compound

[0036] The crystals of the compound were 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 wafer with a thickness of about 1.0 mm. Subsequently, conductive silver paste was evenly coated on the upper and lower cross-sections of the wafer, and it was fixed on the sample stage with a gold wire for testing.

[0037] Under constant temperature and humidity conditions, a Solartron 1260 / 1296 impedance and gain-phase analyzer was used to measure the proton conductivity. The input voltage was 100 mV, and the frequency range was from 10 MHz to 0.01 Hz. The measurement temperature range was 25 °C to 85 °C, and the relative humidity (RH) range was 55% to 98%. The proton conductivity was calculated by the following formula: σ = L / RS, where σ is the conductivity (S cm -1 ), L is the thickness (cm), S is the cross-sectional area (cm2), and R is the bulk resistance (Ω) of the wafer. Impedance analysis was performed using Zview software to obtain the resistance value. The activation energy (E a ) corresponding to proton conduction at different temperatures was calculated by using the Arrhenius equation σT = σ0exp(-E a / kT) to discuss the proton conduction mechanism. Here, σ0 is the exponential factor, k is the Boltzmann constant (eV / K), and T is the temperature (K).

[0038] As Figure 3 shown in a, at room temperature, the proton conductivities of the compound at 55% relative humidity (RH) are 3.83×10 -7 S cm -1 , and when the humidity reaches 98% RH, the proton conductivity reaches 7.01×10 -4 S cm -1 . The proton conductivity increases by three orders of magnitude with the increase of humidity, indicating that the water molecule content has a crucial impact on proton conduction.

[0039] As Figure 3 shown in b, when the temperature increases from 25 °C to 85 °C, the highest proton conductivity of the compound at 98% RH can reach 2.92×10 -3 S cm -1 . This shows that the proton conductivity of the compound increases with the increase of temperature, which may be due to the fact that the proton movement speed increases with the increase of temperature.

[0040] To study the proton conduction mechanism of the compound, the activation energy (E a ) was calculated by the Arrhenius law. By fitting the linear curve of ln(σT) against 1000 / T, the E a value of the compound was 0.29 eV (see Figure 3 c), following the Grotthuss mechanism.

[0041] In addition, for the test of X-ray powder diffraction (XRD), as shown in Figure 3 d, the positions of the peaks in the XRD pattern were basically in agreement with the theoretical peaks of the XRD pattern simulated from single crystal data, indicating that it could maintain structural integrity before and after impedance testing.

[0042] Example 2 Preparation and Characterization of Proton Exchange Composite Membrane

[0043] The compound prepared in Example 1 was ground into powder. 10 mg was taken and uniformly 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 overnight at room temperature to obtain a uniform solution. Subsequently, the obtained solution was poured into a petri dish and vacuum dried at 60 °C to prepare the composite membrane. The thickness of the composite membrane was measured by a vernier caliper, and the content of the compound in the composite membrane was 5.2%.

[0044] The infrared spectra of the compound and the composite membrane were tested in the range of 400 - 4000 cm -1 by the potassium bromide tablet method, using a Bruker VERTEX-70 Fourier transform infrared spectrometer. It can be seen from the infrared spectra that the composite membrane was successfully prepared (see Figure 4 a). The characteristic peak of the -SO3H group appeared at 1058 cm -1 , the W–O terminal vibration of the {P5W 30} unit was located at 930 cm –1 , and the strong 723 cm –1 peak was attributed to v(W / TM-O b,c -W / TM, TM = Zn), where O b and O c are bridging oxygen atoms. In the PXRD pattern, the diffraction peak of Nafion appeared at 2θ = 17°, and with the addition of the compound, the characteristic peak of the composite membrane shifted to a lower angle in the curve (see Figure 4 b), which was attributed to the interaction between Nafion and Compound 1.

[0045] The composite membrane was measured by AC impedance to evaluate its proton conduction performance. At 25 °C and 98% relative humidity, the proton conductivities of the composite membrane were measured to be 3.23×10 –3S cm -1 (See Figure 5 a); as the temperature increases, the composite membrane exhibits the highest proton conductivity at 85 °C and 98% relative humidity, reaching 3.43×10 –2 S cm -1 (See Figure 5 b). Compared with the proton conductivity of the compound, the proton conductivity of the composite membrane is increased by one order of magnitude. According to the fitting of the Arrhenius law, the activation energy (Ea) of the composite membrane is 0.39 eV, indicating that proton conduction follows a hopping mechanism (see Figure 5 c).

[0046] The above-described embodiments are only preferred embodiments of the present invention, which are merely used to explain the present invention and do not limit the scope of the present invention. For those skilled in the art of the present technology, other embodiments can of course 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 with high proton conductivity performance, characterized in that, The structural formula of the Preyssler-type polyoxometalate crystalline material is [Zn4(H2O)8(H7P5W 30 O 110 )(btp)4]·6H2O. Its asymmetric unit contains an independent Zn(II) ion, a quarter of the {P5W 30} unit, two half btp ligands, two coordinated water molecules and two free water molecules; in this compound, the Zn(II) ion coordinates with two N atoms derived from the btp ligand, two O atoms derived from the {P5W 30} unit and two water molecules. Each btp ligand bridges two Zn(II) ions through the triazole nitrogen on both sides. Each {P5W 30} unit is connected to eight Zn(II) centers to form a two-dimensional layer, and the two-dimensional layer is further extended into a three-dimensional framework structure through free water molecules and {P5W 30} units.

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 / m Space group, and the unit cell parameters are: a = 28.100(2) Å, b = 18.2860(14) Å, c = 19.7598(14) Å. α = 90°, β = 124.687(3)°, γ = 90°.

3. The preparation method of the Preyssler-type polyoxoacid crystalline material according to claim 1 or 2, characterized in that, It includes the following steps: putting Zn(NO3)2·6H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine, K 12.5 Na 1.5 [NaP5W 30 O 110 ·15H2O, distilled water and ethanol into a reaction kettle, stirring for 30 min, adjusting the pH to 1.8, then reacting the mixture at 160 °C for 96 h, and after cooling to room temperature, obtaining black block crystals.

4. The preparation method according to claim 3, characterized in that, The molar ratio of Zn(NO3)2·6H2O, 2,6-bis(1,2,4-triazol-1-yl)pyridine and K 12.5 Na 1.5 [NaP5W 30 O 110 ·15H2O is 12:4:

1.

5. Use of the Preyssler-type polyoxoacid crystalline material with high proton conductivity 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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