Polyoxometallate material with high proton conductivity and pure inorganic three-dimensional frame structure as well as preparation method and application of polyoxometallate material

Abstract

According to the invention, a one-pot synthesis strategy is adopted, a polyoxometallate with high proton conductivity and a pure inorganic three-dimensional frame structure is successfully synthesized by accurately controlling the reaction temperature, the pH value and the proportion of raw materials, and the chemical formula of the polyoxometallate is [H2N (CH3) 2] 8K3Na2H4 [Se4W35Ce2O124 (H2O) 4] Cl. 22H2O; based on the structural characteristics, the proton conductivity is further systematically tested under different temperature and humidity conditions; then, Arrhenius fitting is performed on experimental data, and activation energy of proton migration is calculated, so that a proton conduction mechanism is deeply discussed; the polyoxometallate prepared by the invention is of a pure inorganic three-dimensional frame structure, is completely composed of metal-oxygen bonds, and has important structural innovation significance; in addition, the polyoxometallate has the advantages of high proton conductivity, simple preparation process, good chemical stability and the like, so that the polyoxometallate becomes a solid proton conduction material with great application potential.

Description

Technical Field

[0001] This invention addresses the growing depletion of traditional fossil fuels and the urgent need for environmental remediation by developing a polyoxometalate material with high proton conductivity and a purely inorganic three-dimensional framework structure, along with its preparation method and applications. This material utilizes rare earth ions as connecting units to construct trimer selenium-tungsten-oxygen clusters, and further extends the trimer structure into a stable three-dimensional framework structure using alkali metal ions as bridging units. This invention not only optimizes the structural design of this polyoxometalate semiconductor material but also conducts in-depth research on its proton conductivity performance. Test results show that the material exhibits significant proton conduction responsiveness under different temperature and humidity conditions, and demonstrates excellent proton conductivity in high-temperature and high-humidity environments. Furthermore, the material's preparation process is simple, non-toxic, and environmentally friendly, possessing good chemical stability and application prospects, providing a scientific basis and technical support for the future development of high-efficiency solid proton conduction materials. Background Technology

[0002] In recent years, with the dwindling reserves of fossil fuels, global attention and demand for renewable energy have continued to grow. Against this backdrop, proton exchange membrane fuel cells (PEMFCs) have become one of the most promising clean energy technologies due to their rapid start-up and high power density. However, the Nafion membrane, a key component of PEMFCs, exhibits a significant decrease in proton conduction efficiency under low humidity and high temperature environments, severely limiting its practical application and widespread adoption. Therefore, developing novel materials that maintain high proton conduction performance under harsh conditions has become a key research direction for improving PEMFC performance and advancing its commercialization.

[0003] Polyoxometalates (POMs) are a class of metal oxide clusters composed of high-valence transition metals (such as W, Mo, and V). They have attracted considerable attention in recent years due to their excellent proton conductivity, outstanding thermal stability, and multifunctional chemical properties. Based on their framework structures, POMs can be classified into several types, including Silverton-type [XMs]. 12 O 42 ] n- Wells-Dawson type [XM] 18 O 62 ] n- Keggin type [XM] 12 O 40 ] n- Lindqvist type [M6O] 19 ] n- , Preyssler type [X n+ P5W 30 O 110 ] (15-n)- And Anderson-Evans type [XM6O]24 ] n- Among them, Keggin-type and Dawson-type POMs have been extensively studied and explored in various fields such as catalysis, energy storage, and proton conduction due to their relatively simple synthesis conditions, stable structures, and excellent physicochemical properties. Especially in the development of proton conduction materials, these two types of POMs, relying on their unique tunable structures, efficient proton transport capabilities, and excellent thermal stability, are considered highly promising candidate materials and are expected to play an important role in future high-performance proton exchange membrane fuel cells and related technologies.

[0004] The advantages of Proton Molecular Oxide (POM) in proton conduction primarily stem from its unique structure and chemical properties. Firstly, the high density of oxygen atoms on the POM surface provides abundant hopping points for protons, thus facilitating proton conduction within the material. Proton migration depends not only on the overall material structure but also on the surface microstructure. The POM surface possesses nanoscale anion clusters, which exhibit low charge density due to the delocalization effect of negative charges, providing favorable conditions for dynamic dissociation and migration of protons. Furthermore, as an acidic material, POM's structure is composed of multiple oxygen anions and antications. The oxygen atoms in the multiple oxygen anions possess strong electron-withdrawing capabilities, enabling rapid exchange and transport of protons between oxygen atoms and hydrogen. This characteristic gives POM excellent stability and efficiency in proton conductivity, especially under high humidity and high temperature conditions, where it maintains good proton conductivity. Moreover, the structure of POM can be optimized by adjusting the metal center, oxygen atom arrangement, and anion cluster morphology to meet the needs of different applications. This structural tunability allows POM to provide customized proton conduction properties according to different working environments and application requirements. These advantages make POM a highly promising proton conduction material, especially in proton exchange membrane fuel cells (PEMFCs) and other energy conversion devices, where its application prospects are very broad. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies and explore the unique advantages of POM in proton conduction, this invention proposes a polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure. The POM molecular structure contains a large amount of water of crystallization, antications, and obvious proton transport channels, making it an excellent solid proton conducting material. Experimental results show that this material exhibits a significant temperature and humidity dependence in its proton conduction performance, demonstrating excellent proton conductivity under high temperature and high humidity conditions.

[0006] This invention also provides a method for preparing and applying the aforementioned polyoxometalate material with high proton conductivity and a purely inorganic three-dimensional framework structure. This method constructs trimer selenium-tungsten-oxygen clusters using rare earth ions as connecting units, and further extends the trimer structure into a stable three-dimensional framework structure using alkali metal ions as bridging units. Furthermore, the preparation process of this material is simple, non-toxic, and environmentally friendly, exhibits good chemical stability, and demonstrates broad application prospects.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure, its chemical formula is: [H2N(CH3)2]8K3Na2H4[Se4W 35 Ce2O 124 [(H₂O)₄]Cl·22H₂O, the polyoxometalate material has a cyclic trimeric anionic structure and belongs to the triclinic crystal system. P Space group -1, cell parameters are: a = 19.5903(12) Å, b = 20.2582(12) Å, c = 25.1571(16) Å, α = 94.171(2) °, β = 96.315(2) °, γ = 117.546(2) °.

[0008] This invention provides a method for preparing a polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure, comprising the following steps: 1) Dissolve Na2WO4·2H2O, [H2N(CH3)2]·Cl, KCl, NaAc and Na2SeO3 in deionized water to form a colorless and transparent solution; 2) While continuously stirring, adjust the pH of the solution to 4.0-4.5, and ensure that the solution is uniformly mixed; 3) While stirring, add Ce(NO3)3·6H2O to the solution and dissolve it completely. At this point, the solution turns yellow and transparent. Then adjust the pH of the solution to 4.0-4.5. 4) Heat at 70-90 ℃ for 1-3 hours. After heating, cool to room temperature; separate the solid and liquid to obtain a clear and transparent yellow solution. 5) Allow the solution to stand at room temperature to evaporate naturally, and orange-yellow crystals will precipitate out, which is the final product.

[0009] Specifically, in step 1), 1-3g Na2WO4·2H2O, 1-2g [H2N(CH3)2]·Cl, 0.1-0.4g KCl, 0.1-0.3g NaAc and 0.1-0.4g Na2SeO3 are dissolved in 15-30 mL of deionized water.

[0010] Specifically, in step 2), the pH is adjusted to 4.0-4.5 using 4-8 mol / L HCl.

[0011] Specifically, in step 3), add 0.1-0.3g of Ce(NO3)3·6H2O to the solution and ensure that it dissolves completely.

[0012] Furthermore, in step 3), the pH of the solution is subsequently readjusted to 4.0-4.5 using 4-8 mol / L HCl, and stirring is continued to ensure that the solution is homogeneous and stable.

[0013] This invention provides a polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure prepared by the above method.

[0014] This invention also provides the application of the above-mentioned polyoxometalate materials with high proton conductivity and pure inorganic three-dimensional framework structure in the preparation of proton exchange membrane fuel cells, etc.

[0015] The present invention also provides the application of the above-mentioned polyoxometalate materials with high proton conductivity and pure inorganic three-dimensional framework structure as solid proton conduction materials, sensors, or electrochemical devices.

[0016] The polyoxometalate material described in this invention is expected to meet the stringent requirements of proton conductor materials for high-performance energy devices such as proton exchange membrane fuel cells (PEMFC), and shows broad application prospects in the fields of fuel cells, sensors and other electrochemical devices.

[0017] This invention employs a one-pot synthesis strategy, and through precise control of reaction temperature, pH value, and the proportions of each raw material, successfully synthesized a polyoxometalate with high proton conductivity and a pure inorganic three-dimensional framework structure. Its chemical formula is: [H₂N(CH₃)₂]₈K₃Na₂H₄[Se₄W₂] ... 35 Ce2O 124 [(H2O)4]Cl·22H2O; Based on its structural characteristics, its proton conductivity was further systematically tested under different temperature and humidity conditions; Subsequently, by performing Arrhenius fitting on the experimental data, the activation energy of its proton migration was calculated, thereby exploring its proton conduction mechanism in depth; The polyoxometalate prepared in this invention has a pure inorganic three-dimensional framework structure, which is entirely composed of metal-oxygen bonds, and has important structural innovation significance; In addition, this polyoxometalate has advantages such as high proton conductivity, simple preparation process, and good chemical stability, which makes it a solid proton conduction material with great application potential.

[0018] The polyoxometalate prepared by the above method in this invention has the chemical formula: [H2N(CH3)2]8K3Na2H4[Se4W 35 Ce2O 124 [(H₂O)₄]Cl·22H₂O (referred to as Compound 1), as can be seen from the chemical composition of this polyoxometalate molecule, contains a large number of dimethylamine molecules and water of crystallization molecules in its crystal structure. These molecules help form a stable and continuous hydrogen bond network with oxygen atoms on the surface of the polyoxometalate, thereby significantly enhancing the proton transport efficiency. In addition, crystal structure analysis shows that there are obvious proton conduction channels in this crystal. The channels are rich in non-bridging oxygen atoms, which have a good affinity for protons, facilitating the migration of protonated water molecules and the efficient conduction of the proton "jumping" mechanism (Grotthuss mechanism) within the channels. The material's purely inorganic three-dimensional framework structure, constructed through bridging between metal and oxygen atoms, effectively avoids the decomposition or failure problems that may occur with organic ligands under extreme environments. This inorganic framework, composed entirely of metal and oxygen elements, endows the material with excellent structural stability, enabling it to maintain its integrity and functionality under harsh conditions such as high temperature and high humidity, providing a solid guarantee for its reliability in proton conduction-related applications.

[0019] Based on the aforementioned theoretical foundation and research findings in related fields, we have reason to speculate that this material possesses the potential to become an excellent solid-state proton conductor. Proton conductivity tests conducted on this material under different temperature and humidity conditions further validated this hypothesis, demonstrating that the material exhibits good proton conductivity and environmental adaptability. Specifically, the proton conductivity of the material was measured using a Solartron 1260 impedance / gain-phase analyzer equipped with a Solartron 1296 dielectric interface, employing a quasi-four-probe method.

[0020] The proton conductivity of the polyoxometalate material described herein was measured using the following steps: 1) Sample preparation: Weigh an appropriate amount of polyoxometalate material crystal powder and place it in a dry and clean self-made mold to press it into a circular sheet sample; 2) Electrode connection: Cut two gold wires of appropriate length and attach them to the upper and lower surfaces of the sample disc respectively using conductive silver paste, ensuring good contact, and then let them air dry naturally. 3) Sample installation: Assemble the sample with the fixed gold wire onto the test sample stage, ensuring stable electrode connection and centered sample position; 4) Conductivity test: AC impedance is tested under different temperature and humidity conditions using an impedance / gain-phase analyzer combined with the quasi-four-probe method.

[0021] This invention employs a one-pot synthesis strategy, successfully synthesizing a pure inorganic three-dimensional framework polyoxometalate material with high proton conductivity by precisely controlling the reaction temperature, pH value, and the proportions of each raw material. Based on its structural characteristics, its proton conductivity was further systematically tested under different temperature and humidity conditions. Simultaneously, by performing Arrhenius fitting on the experimental data, the activation energy for proton migration was calculated, thereby further exploring its proton conduction mechanism. Compared with existing polyoxometalate materials, this invention has the following innovative points and beneficial effects: 1) This invention adopts a "one-pot" synthesis process, which is simple to operate, mild under mild conditions, and does not require a complex multi-step reaction process, which is conducive to the large-scale preparation and industrial application of materials; 2) The polyoxometalates prepared in this invention are pure inorganic three-dimensional framework structures, entirely composed of metal-oxygen bonds, which differs from the common polyoxometalate-organic framework (POMOF) structures constructed through organic ligand bridging. This pure inorganic linkage mode is relatively rare in polyoxometalate systems and has significant structural innovation value; 3) The material prepared by this invention exhibits excellent proton conduction performance under high temperature and high humidity conditions, which is significantly better than most polyacid materials with similar structures, meeting the application requirements of high-performance devices such as proton exchange membrane fuel cells; 4) The polyacid crystals prepared by this invention have a relatively clear proton conduction mechanism. The clear conduction path not only helps to understand the conductivity behavior of this type of material, but also provides important theoretical support and research examples for the design and development of novel proton conductors in related fields. 5) The raw materials used in this invention are all inexpensive conventional chemicals that are easy to obtain, and no harmful byproducts are introduced during the synthesis process. The process is simple, energy consumption is low, and it conforms to the concept of green chemistry and sustainable development. Attached Figure Description

[0022] Figure 1 This is a simplified synthetic route diagram of compound 1 described in this invention; Figure 2 shows a schematic diagram of the construction of polyoxometalate trimers. (a) is a schematic diagram of the connection between the selenium tungsten oxide cluster and the central unit of the trimer; (b) is a schematic diagram of the connection between the hexahedral atoms of the central unit; (c) shows the dihedral angles of the faces formed by the three tungsten atoms and cerium atoms in the central unit; (d) is a diagram of the geometric coordination structure of tungsten and cerium atoms in the central unit. Figure 3 Alkali metal ions (K + Na +) Schematic diagram of bridging trimer clusters, (a) Quadrilateral structural unit diagram of adjacent trimers bridged by Na1 and K1; (b) One-dimensional chain structure diagram of trimers bridged by K2; (c) Irregular dodecagonal structural unit diagram of trimers bridged by Na2 and W10 and W20; (d) "Step-like" geometric structural unit diagram of K3 and K4 formed by oxygen bridges; Figure 4 Alkali metal ions (K + Na + Atomic coordination diagrams of K1 (nine-coordinate) and Na1 (six-coordinate), (a) coordination and bridging between K1 (nine-coordinate) and Na1 (six-coordinate); (b) eight-coordinate structure of K2 (including Cl⁻ participating in coordination); (c) six-coordinate structure of Na2; (d) "step-like" geometric units formed by K3 and K4 through oxygen bridges; (e) seven-coordinate structure of K3; (f) six-coordinate structure of K4; Color coding: K (purple), Na (gray), O (red), Cl (green); Figure 5 shows a schematic diagram of a polyhedron of a pure inorganic framework structure of polyoxometalates. (a) is a schematic diagram of a single-layer xy-plane structure obtained by viewing perpendicularly along the y-axis; (b) is a schematic diagram of a single-layer yz-plane structure obtained by viewing perpendicularly along the x-axis. Figure 6 The infrared spectrum of compound 1 is shown below. Figure 7 (a) shows the proton conductivity of compound 1 at 298 K under different relative humidities (RH), and (b) shows the temperature dependence test conducted at a relative humidity of 95%. Figure 8 The Arrhenius curve is used to investigate the temperature dependence of the material's electrical conductivity and derive its activation energy. Figure 9 Raman spectroscopy (a) and powder X-ray diffraction (PXRD) analyses were performed on the samples before and after the proton conductivity test (b). Figure 10 Raman spectroscopy results for compound 1 in different solvents; Figure 11 The thermogravimetric analysis (TGA) curve for compound 1 is shown. Detailed Implementation

[0023] The technical solution of the present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0024] In the following description, all raw materials used are common commercially available chemicals that can be purchased through conventional commercial channels, or can be conveniently prepared based on the general knowledge of those skilled in the art and existing synthetic methods. For example, sodium selenite (Na2SeO3) was purchased from Beijing Innocare Technology Co., Ltd., with a purity meeting analytical reagent grade requirements and an average molar mass of approximately 172.94 g / mol; the 6 mol / L hydrochloric acid solution was prepared by mixing concentrated hydrochloric acid (approximately 12 mol / L) with an equal volume of deionized water at a volume ratio of 1:1.

[0025] Room temperature refers to 25±5℃. Example 1

[0026] Synthesis and Structure of Polyoxometalate Materials

[0027] A polyoxometalate material (compound 1) with high proton conductivity and a pure inorganic three-dimensional framework structure has the chemical formula: [H2N(CH3)2]8K3Na2H4[Se4W 35 Ce2O 124 [(H₂O)₄]Cl·22H₂O, elemental analysis (%): Na, 0.46; K, 1.17; Se, 3.14; Ce, 2.79; W, 64.00; C, 1.91; N, 1.11; H, 1.20; O, 23.87; Cl, ​​0.35. This molecular crystal structure contains a large number of dimethylamine molecules and water of crystallization molecules. These molecules form multi-point hydrogen bonds with oxygen atoms in the polyoxometalate framework, constructing a continuous hydrogen bond network that runs through the entire three-dimensional framework. This network not only enhances the stability of the crystal structure but also provides a "jump channel" for efficient proton migration, thereby enabling rapid proton conduction.

[0028] The preparation method of the above-mentioned polyoxometalate materials with high proton conductivity and pure inorganic three-dimensional framework structure (synthetic route see below) Figure 1 Specifically, it includes the following steps: 1) Add 1.5g Na2WO4·2H2O, 1g [H2N(CH3)2]·Cl, 0.2g KCl, 0.1g NaAc and 0.2g Na2SeO3 to 20 mL of deionized water in sequence, and dissolve them completely with stirring until a colorless and transparent solution is formed; 2) While stirring continuously, slowly add 6 mol / L HCl dropwise to the above solution to adjust the pH to 4.0-4.5, and ensure that the solution is mixed evenly; 3) While stirring, slowly add 0.1 g Ce(NO3)3·6H2O (0.226 mmol) to the solution, ensuring complete dissolution. At this point, the solution will turn yellow and transparent. Then, readjust the pH of the solution to 4.0-4.5 using 6 mol / L HCl and continue stirring for 30 minutes to ensure the solution is homogeneous and stable. 4) Place the beaker containing the solution obtained in step 3) in a water bath at 80 °C and heat for 1.5 hours. After heating, cool to room temperature. Then filter out a large amount of precipitate using filter paper to obtain a clear and transparent yellow solution; 5) Place the beaker containing the solution obtained in step 4) in a clean environment and allow it to stand at room temperature to allow the solution to evaporate naturally. After about three weeks, large orange-yellow crystals will gradually precipitate at the bottom of the beaker, which is the polyoxometalate material.

[0029] In the above synthesis process, each component played a clear and synergistic role in the reaction system, ensuring the successful construction of the target polyoxometalate crystal: a) Tungsten ions, selenium ions, and cerium ions jointly participate in the construction of polyoxometalate trimer structures, forming stable metal-oxygen clusters. Tungsten ions mainly exist in polyhedral form (such as WO6 octahedrons), serving as the core building blocks of the polyoxometalate framework; selenium ions are usually in a lower oxidation state, embedded in the metal-oxygen cluster structure, regulating its geometry and charge distribution; while cerium ions are introduced in the form of rare-earth metal centers, playing a role in connecting, stabilizing, and regulating the degree of polymerization. b) [H2N(CH3)2] + and Ac - As a buffer component in solution, it not only adjusts the pH of the system, but also plays a structural assist role in the synthesis process by stabilizing the coordination environment and assisting conformational assembly. c) Maintaining a weakly acidic pH of 4.0–4.5 helps promote the formation of polyacid basic building blocks (Keggin and Wells-Dawson type POMs) and inhibits the formation of impurity phases, which is one of the key reaction conditions for obtaining high-quality crystals. d) Heating the reaction system in a water bath at 80 °C for 1.5 hours significantly accelerated the reaction kinetics between the components and promoted the ordered assembly of polyacid structures. In contrast, no crystal formation was observed in the control experiment without heating, further demonstrating the necessity of heating for the crystal formation process.

[0030] Figure 2 The diagram illustrates the detailed structural features of the trimer building blocks of this polyoxometalate molecule: Figure 2 a shows that the trimer structure consists of two triple-vacancy Keggin-type units {SeW9O 33} 8-A four-vacancy Wells–Dawson type unit {Se2W 14 O 52} 12- Composed of units, these units are connected by three {WO2}²⁺ units to form an interesting ring structure. Figure 2 a). In addition, three {WO2} 2+ The unit is connected to two Ce atoms through its oxygen atom, forming a stable hexahedral central structural unit. This unit plays an important bridging and supporting role in the formation of the cyclic trimer structure. Figure 2 (b) Figure 2c further reveals the geometric relationship between tungsten and cerium atoms in the central unit of the trimer from a spatial perspective. Specifically, the dihedral angle between the W1-Ce1-Ce2 and W6-Ce1-Ce2 faces is 75.5°, the dihedral angle between W1-Ce1-Ce2 and W4-Ce1-Ce2 is 76.0°, and the dihedral angle between W6-Ce1-Ce2 and W4-Ce1-Ce2 is 28.4°. (Combined with...) Figure 2 The bond length information shown in b indicates that the central connecting unit exhibits good central symmetry around the plane W1-Ce1-Ce2. Figure 2 d further illustrates the geometric coordination structure of tungsten (W) and cerium (Ce) atoms in the central unit, where the tungsten atoms exhibit a typical six-coordinate configuration (usually octahedral), while the cerium atoms are nine-coordinate, forming a complex but stable coordination environment.

[0031] Figure 3 The bridging mechanisms of alkali metal ions (K⁺, Na⁺) between polyacid trimers were demonstrated, revealing their crucial role in constructing three-dimensional framework structures. Figure 3 As shown in Figure a, adjacent trimer units are bridged by quadrilateral-like connecting units formed by two Na1 and K1 ions. This connection method exhibits good central symmetry, providing a basis for constructing ordered structural units among trimers. Figure 3 b indicates that the two trimers are bridged by K2 ions. This bridging method is repeated along a specific direction and eventually expands to form a one-dimensional chain structure, which reflects the structural guiding role of alkali metal ions in directional connection. Figure 3 c shows that two Na2 ions are connected to W10 and W20 atoms in two trimers through oxygen bridges, forming an irregular dodecagonal structure. The two trimers are centrally symmetrically distributed around the dodecagon, exhibiting high symmetry and spatial matching. Figure 3 d describes two K3 and K4 ions connected by oxygen atoms to form a geometric unit similar to a "ladder". This special connection structure not only gives the crystal framework more spatial hierarchy, but also makes the adjacent trimers exhibit a centrally symmetrical distribution on both sides of the "ladder" unit.

[0032] Figure 4 The geometric coordination ball-and-stick model of alkali metal ions (K⁺, Na⁺) in polyacid crystal structures is shown: (e.g.) Figure 4 As shown in a, K1 is nine-coordinated and Na1 is six-coordinated. The two are bridged by two shared oxygen atoms, O14 and O40, forming a quadrilateral-like connecting unit. Figure 4 In b, the K2 ion has an eight-coordinate structure. What is special is that it not only coordinates with multiple oxygen atoms, but also coordinates with an additional chlorine atom. Figure 4 In c, the Na2 ions exhibit a typical six-coordinate structure; Figure 4 In df, K3 is hexacoordinate and K4 is hexacoordinate. The two are bridged by shared O2 and O18, forming a geometric unit similar to a "step".

[0033] Figure 5 A polyhedral schematic diagram of a pure inorganic three-dimensional framework structure of polyacids is shown: Figure 5 a shows a schematic diagram of a single-layer xy-plane structure obtained by viewing it perpendicularly along the y-axis; Figure 5 Figure b shows a schematic diagram of the single-layer yz-plane structure obtained by viewing perpendicularly along the x-axis. As can be seen from the figure, both the xy and yz planes clearly contain large-sized proton conduction channels and a large number of non-bridging oxygen atoms. These oxygen atoms are distributed on the channel surface, providing dense acceptor sites for the proton's "jumping" behavior. Furthermore, the channel also accommodates a large number of dimethylamine and water of crystallization molecules (not shown in the figure for clarity of the relationships between atoms, obtained from single-crystal structure data). These molecules interact with the non-bridging oxygen atoms through hydrogen bonds, forming a stable hydrogen bond network. This not only lowers the energy barrier during proton migration but also enhances the proton's directional movement ability, thus providing crucial kinetic support for the efficient and rapid migration of protons within the channel.

[0034] Figure 6 The compound [H2N(CH3)2]8K3Na2H4[Se4W] was shown. 35 Ce2O 124 Infrared spectrum of [H₂O)₄]Cl·22H₂O. The potassium bromide pellet method was used, and the spectrum was obtained using a Bruker VERTEX-70 spectrometer in the mid-infrared region (400-4000 cm⁻¹). -1 The characteristic vibrations of the multi-anion framework can be measured at 500-1000 cm⁻¹. -1 In the low wavenumber range, the spectrum was observed at approximately 475, 646, 770, 857, and 960 cm⁻¹. -1 The characteristic vibrational peaks appearing at these locations are attributed to ν(Ce–O), ν(Ce–O–W), and ν(W–O), respectively. c ), ν(W–O b) and ν(W–O t The presence of characteristic peaks is consistent with the conclusion of "a polyanionic structure composed of Ce, W and O" obtained from single-crystal X-ray diffraction analysis, verifying the integrity of the framework.

[0035] Electrochemical impedance spectroscopy experiment.

[0036] First, 20 mg of the target sample was weighed and placed in a dry, clean mold (mold radius 0.15 cm), and pressed into a regular circular disc. The thickness of the sample was precisely measured using calipers for subsequent conductivity calculation. Next, two gold wires of appropriate length were cut and fixed to the upper and lower surfaces of the disc sample using conductive silver paste, ensuring good contact. After fixing, the sample was allowed to air dry naturally to ensure the silver paste fully cured. Subsequently, the prepared sample was mounted on the test stage and connected to the testing equipment. Proton conductivity was measured using a Solartron 1260 impedance / gain-phase analyzer (Ametek, UK) equipped with a Solartron 1296 dielectric interface, using a quasi-four-probe method. Electrochemical impedance spectroscopy (EIS) data were acquired in the frequency range of 0.1 Hz to 10 MHz and at an input voltage of 100 mV. The test temperature range was 298 K to 368 K (25, 35, 45, 55, 65, 75, 85, 95 °C), and the relative humidity range was 25%RH to 95%RH (25, 35, 45, 55, 65, 75, 85, 95 %RH). Under each temperature and humidity condition, the sample needed to equilibrate in the test environment for 2 hours to ensure that its structure and internal water content reached thermodynamic and kinetic stability before data acquisition, thereby ensuring the accuracy and repeatability of the obtained proton conductivity data.

[0037] The proton conductivity (σ) of the sample was calculated using the Nyquist plot and the following formula. Where L is the thickness of the sample pellet (cm), and A is the cross-sectional area of ​​the sample (cm²). 2 R is the sample volume resistance (Ω) obtained by fitting the Nyquist plot.

[0038]

[0039] The activation energy (Ea) is calculated using the Arrhenius equation, where σ is the proton conductivity (S·cm). - ¹), where T is the absolute temperature (K), σ0 is the pre-exponential factor, and k is the Boltzmann constant (8.6 × 10⁻⁶). -5 eV·K - ¹).

[0040]

[0041] This compound possesses a complete and abundant hydrogen bond network, water of crystallization, and antications, providing a solid foundation for studying its proton conductivity. To further investigate its conductivity properties, we measured the proton conductivity (σ) of the pressed sample under different humidity and temperature conditions using AC impedance spectroscopy. First, we tested the proton conductivity of this polyoxometalate at different relative humidities (RH) at 298 K to analyze the effect of humidity on its conductivity. The results are as follows... Figure 7 As shown in Figure a, the impedance spectrum of the material begins to change significantly with the gradual increase of relative humidity. When the humidity increases to 85%, a clear semi-circular feature appears in the impedance diagram. Further increasing the humidity to 95%, the proton conductivity increases significantly to 6.05 × 10⁻⁶. -4 S·cm - ¹ indicates that the proton conductivity of this material is sensitive to humidity, and the σ value increases with increasing humidity. This phenomenon may stem from the presence of a large number of water molecules in the material, which provide abundant active sites for proton transport, thus significantly enhancing its proton conductivity. Furthermore, under high humidity, this polyoxometalate can adsorb more water molecules, which form a dense hydrogen bond network with non-bridging oxygen atoms on the polyoxometalate surface, further accelerating the proton migration process. This trend is consistent with previous research results, verifying the crucial role of water molecules in promoting proton conduction. In addition, we further investigated the effect of temperature, a key factor, on the proton conductivity of this compound. Temperature dependence tests were conducted using the same compressed samples at a relative humidity of 95%. The results are as follows... Figure 7 As shown in b, as the temperature increases from 25 °C to 55 °C, the proton conductivity of compound 1 increases from 6.05 × 10⁻⁶. -4 Increased to 5.26 × 10 -3 S·cm⁻¹. When the temperature is further increased to 95℃, the proton conductivity reaches 1.07×10⁻¹. -2 The S·cm⁻¹ values ​​are approximately two orders of magnitude and one order of magnitude higher than those at 25 °C and 55 °C, respectively. The graph clearly shows that increasing the temperature enhances the proton conductivity of this polyoxometalate. This phenomenon may be attributed to the increased H₃O generated by the reaction of water molecules with protons at higher temperatures. + The increased temperature also lowers the energy barrier that protons need to overcome for migration, making it easier for protons to migrate within the hydrogen bond network and further enhancing conductivity.

[0042] The Arrhenius equation can be used to approximate the activation energy (Ea) of this compound during proton conduction. For example... Figure 8As shown, under conditions of 95% relative humidity and a temperature range of 328 K to 368 K, the activation energy obtained by Arrhenius fitting calculation based on the experimentally obtained conductivity data is 0.21 eV. This low activation energy value indicates that the proton conduction process is mainly driven by a Grotthuss-type mechanism. This mechanism relies on a continuous hydrogen bond network and abundant water molecules in the structure, with protons achieving efficient transport by hopping between hydrogen bonds. This mechanism is highly consistent with the stable hydrogen bond network constructed in this material and the presence of abundant water of crystallization in the proton transport channels, further verifying that its excellent proton conductivity stems from the organic combination of rational structural design and functional synergy.

[0043] To verify the structural stability of this compound under different testing conditions, we analyzed the samples before and after the proton conductivity test using powder X-ray diffraction (PXRD) and Raman spectroscopy. Figure 9 As shown in the figure. Experimental results show that the Raman and PXRD spectra of the sample before and after testing are highly consistent, with almost no significant differences. This result fully demonstrates that the compound can maintain the integrity of its crystal structure under different temperature and humidity environments, exhibiting good structural stability and providing strong support for its reliability in practical applications. Furthermore, we monitored the state of the compound in different solvents (H2O, MeOH, EtOH, EtCN, CH2Cl2) using Raman spectroscopy. The results are shown in the figure. Figure 10 The spectrum in the figure shows that its main characteristic peaks are retained, and no vibrational peaks belonging to new substances appear, indicating that the material does not undergo significant dissolution or chemical changes in the test solvent and has a certain degree of chemical stability.

[0044] To investigate the thermal stability and composition of compound 1, thermogravimetric analysis was performed on it at temperatures ranging from room temperature to 800 °C under a N2 atmosphere. The results are as follows: Figure 11 As shown, the TGA curve exhibits continuous weight loss behavior, with a total weight loss of approximately 11.5%. The weight loss process can be divided into three stages: the first stage occurs between 20 °C and 200 °C, with a weight loss rate of 3.7%, attributed to the loss of 22 lattice water molecules (calculated value: 3.9%); the second stage occurs between 200 °C and 400 °C, with a weight loss rate of 4.8%, attributed to the organic cation [H2N(CH3)2]. + The difference in experimental weight loss rate due to thermal decomposition and loss of coordinated water (calculated value: 4.4%) is attributed to mild deoxygenation of the POM cluster (weak bond breaking of WOW bridging oxygen); the weight loss in the third stage may correspond to the partial collapse of the polyoxometalate framework.

[0045] Single-crystal X-ray diffraction analysis showed that this polyoxometalate crystallized in a triclinic crystal system. PSpace group -1 (see Table 1 below). Its molecular structure contains a purely inorganic anion [Se4W]. 35 Ce2O 124 (H2O )4 ] 16- Eight [H2N(CH3)2] + Cations, three K + Ions, two Na + Ions, four H + Ion, one Cl - The compound contains ions and twenty-two lattice water molecules. It possesses a cyclic trimeric anionic structure. Structural analysis reveals that the trimeric anion consists of two triple-vacant Keggin-type subunits ({SeW9O...). 33} 8- ) and a four-missing Wells-Dawson type subunit ({Se2W) 14 O 52} 12- These subunits are composed of three {WO2} 2+ The bridging units are interconnected.

[0046] The bond valence and (BVS) calculation results are shown in Table 2 below, indicating that the oxidation states of all Se, W and Ce atoms are +4, +6 and +3, respectively, which is consistent with the expected structure.

[0047] Table 1. Crystallographic data and structural refinement parameters of compound 1

[0048] Table 2. Bond valences and (BVS) values ​​of Ce, W, Se, and O atoms in Compound 1

[0049]

[0050] In summary, this study successfully synthesized a novel rare-earth polyoxometalate compound [H2N(CH3)2]8K3Na2H4[Se4W 35 Ce2O 124 [(H₂O)₄]Cl·22H₂O was confirmed to possess a cyclic trimeric anionic structure. This structure contains two triple-vacancy Keggin-type units ({SeW₉O₂}). 33} 8- ) and a four-missing Wells-Dawson type subunit ({Se2W) 14 O 52} 12- Composed of three {WO2} 2+The components are bridged together. Notably, compound 1 exhibits excellent proton conductivity, reaching 1.07 × 10⁻² S·cm⁻¹ at 95 °C and 95% relative humidity. Furthermore, compound 1 demonstrates good structural stability under various conditions. This work provides new structural design strategies and experimental basis for the study of proton conduction in polyoxometalate semiconductors.

Claims

1. A polyoxometalate material with high proton conductivity and a purely inorganic three-dimensional framework structure, characterized in that, The chemical formula is: [H2N(CH3)2]8K3Na2H4[Se4W 35 Ce2O 124 [(H₂O)₄]Cl·22H₂O, the polyoxometalate material belongs to the triclinic crystal system. P Space group -1, cell parameters are: a = 19.5903(12) Å, b = 20.2582(12) Å, c = 25.1571(16) Å, α = 94.171(2) °, β = 96.315(2) °, γ = 117.546(2) °.

2. The method for preparing the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 1, characterized in that, Includes the following steps: 1) Dissolve Na2WO4·2H2O, [H2N(CH3)2]·Cl, KCl, NaAc and Na2SeO3 in deionized water to form a colorless and transparent solution; 2) Adjust the pH of the solution to 4.0-4.5 while continuously stirring; 3) While stirring, add Ce(NO3)3·6H2O to the solution and dissolve it completely. At this point, the solution turns yellow and transparent. Then adjust the pH of the solution to 4.0-4.

5. 4) Heat at 70-90 ℃ for 1-3 hours, then cool to room temperature; separate the solid and liquid phases to obtain a clear and transparent yellow solution; 5) Allow the solution to stand at room temperature to evaporate naturally, and orange-yellow crystals will precipitate out, which is the final product.

3. The method for preparing the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 1, characterized in that, In step 1), dissolve 1-3g Na2WO4·2H2O, 1-2g [H2N(CH3)2]·Cl, 0.1-0.4g KCl, 0.1-0.3g NaAc and 0.1-0.4g Na2SeO3 in 15-30 mL of deionized water.

4. The method for preparing the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 1, characterized in that, In step 2), the pH is adjusted to 4.0-4.5 using 4-8 mol / L HCl.

5. The method for preparing the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 3, characterized in that, In step 3), add 0.1-0.3g of Ce(NO3)3·6H2O to the solution and dissolve it completely.

6. The method for preparing the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 3, characterized in that, In step 3), the pH of the solution is adjusted to 4.0-4.5 using 4-8 mol / L HCl.

7. A polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure prepared by any of the methods described in claims 2 to 6.

8. The application of the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 1 or 7 in the preparation of proton exchange membrane fuel cells.

9. The application of the polyoxometalate material with high proton conductivity and a pure inorganic three-dimensional framework structure as described in claim 1 or 7 as a solid proton conducting material, sensor, or electrochemical device.

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