A polyoxometalate composite electrode material, a preparation method and application thereof

Abstract

The application belongs to the technical field of electrode materials, and particularly relates to a polyoxometalate, a composite electrode material and a preparation method and application thereof. The polyoxometalate is prepared by the following steps: adding Ln(NO3)3 into a H6PMo9V3O 40 orange-red transparent solution, and orange-red crystals are precipitated by standing the orange-red transparent solution. Orange-red crystals are collected by recrystallization of the mother liquor. The orange-red crystals are washed, dried and combined to obtain the polyoxometalate. Ln is a lanthanide metal. The polyoxometalate of the application forms a dimer structure to obtain a relatively stable crystal structure.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a polyoxometalate, a composite electrode material, its preparation method, and its application. Background Technology

[0002] Electrode materials are the core components of electrocatalytic water splitting. Obtaining highly active electrocatalysts requires research into the design and controllable preparation of electrode materials. Polyoxometalates (POMs) are nanoscale polynuclear metal-oxygen cluster anions composed of high-oxidation-state transition metals (such as molybdenum, tungsten, and vanadium) and oxygen atoms. Compared to transition metal oxides such as RuO2, MnO2, and V2O5, POMs exhibit significant advantages: they possess excellent molecular conductivity, enabling rapid and reversible multi-electron redox reactions ("electron sponge behavior") while maintaining a highly stable main framework structure; furthermore, their redox effects can be systematically modulated by precisely replacing their heteroatoms (central atoms) or coordinating atoms (framework metal atoms). These characteristics make POMs highly promising in the field of electrochemical catalysis.

[0003] In the field of electrocatalysis, polymorphic oxygen molecules (POMs) can form molecular-level complexes with almost all transition metals, making them ideal precursors for the precise design and construction of multi-metal electrocatalysts at the molecular level. Furthermore, POMs possess excellent electron storage capacity, high thermal stability, and inherent lattice oxygen reactivity (easily forming oxygen vacancies). Through precise structural control, structures rich in oxygen vacancies can be directionally generated, thereby optimizing the adsorption free energy of reaction intermediates (e.g., to improve the kinetic performance of the hydrogen evolution reaction HER). However, the direct application of unmodified POMs in electrochemical devices faces key bottlenecks: their inherently low specific surface area severely limits the number of effective active sites available for electrochemical reactions; although intramolecular charge transport is good, the large intermolecular charge transport resistance as macroscopic materials results in weak overall conductivity. Keggin-type heteropolyacids, as a classic structural type of POMs, possess excellent structural modifiability. By substituting their framework metal atoms or central heteroatoms, or by combining them with other functional materials such as carbon nanotubes, metal-organic frameworks, and ionic liquids, their electronic structure and catalytic performance can be precisely controlled.

[0004] However, existing research mainly focuses on structurally simple, intact Keggin-type POMs (such as phosphomolybdic acid H3PMo). 12 O 40 phosphotungstic acid H3PW 12 O 40Research on vacancy-deficient heteropolyacids (e.g.) is relatively scarce. This is mainly due to the fact that their structural stability is more difficult to guarantee than that of complete Keggin-type structures, making it difficult to systematically carry out the preparation and performance study of composite materials of vacancy-deficient heteropolyacids and metal conductive substrates. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a polyoxometalate, a composite electrode material, its preparation method, and its application.

[0006] A polyoxometalate is prepared by the following steps: [The process involves] reacting H6PMo9V3O... 40 · Add Ln(NO3)3 to the 14H2O heteropolyacid solution and mix well. Adjust the pH value to 3-5 and stir the reaction to obtain an orange-red transparent solution. Let the orange-red transparent solution stand to precipitate orange-red crystals. Collect the orange-red crystals by recrystallization of the mother liquor. Combine the orange-red crystals, wash and dry them to obtain the polyoxometalate. Wherein, Ln is a lanthanide metal, H6PMo9V3O 40 The concentration of the heteropolyacid solution was 0.05~0.12 g / mL, and the concentration of H6PMo9V3O was... 40 The ratio of 14H2O heteropolyacid solution to Ln(NO3)3 is 10~15mL:0.375~0.5g.

[0007] This invention achieves a La by using proportions within a certain range of proportional coefficients. 3+ It can coordinate with two missing clusters at the same time, thus forming a dimer structure, which can accurately realize the construction of metal oxoate clusters and obtain a relatively stable crystal structure.

[0008] A method for preparing a composite electrode material includes the following steps: The dispersion of the polyoxometalate and nickel foam with metal oxides grown on the surface of spinel structure were heated at 110℃~120℃ for 5~6 hours, washed and dried to obtain the composite electrode material. The mass of the polyoxometalate in the dispersion of the said polyoxometalate used per square centimeter of said nickel foam is 0.05~0.2g.

[0009] Preferably, the metal oxide with a spinel structure is Fe-doped Co3O4.

[0010] Preferably, the preparation steps of the nickel foam with a spinel structure metal oxide grown on its surface are as follows: Fe(NO3)3 . 9H2O, Co(NO3)2 .6H2O and CO(NH2)2 are uniformly dispersed in water to obtain a clear orange solution. Pretreated nickel foam is added and reacted at 150~160℃ for 5~6 hours. After the reaction is completed, the solution is washed and dried, and then calcined at 200℃~350℃ for 0.5~1 hours under nitrogen protection to obtain nickel foam with metal oxides with spinel structure grown on the surface. Among them, Fe(NO3)3 . 9H2O, Co(NO3)2 . The molar ratio of 6H2O to CO(NH2)2 is (0.15-0.2):(0.7-0.8):(0.015-0.020).

[0011] The composite electrode material prepared by the aforementioned preparation method.

[0012] Application of the aforementioned composite electrode material in electrocatalytic water splitting.

[0013] Preferably, in the electrocatalytic water splitting process, the composite electrode material is used as the working electrode, the mercury oxide electrode is used as the reference electrode, and the carbon rod electrode is used as the counter electrode to form a three-electrode system, which is connected to an electrochemical workstation to perform electrocatalytic water splitting in the electrolyte.

[0014] Preferably, during the electrocatalytic water splitting, the composite electrode materials are used as the cathode and anode, respectively, forming a two-electrode system connected to an electrochemical workstation, and the electrocatalytic water splitting is carried out in the electrolyte.

[0015] This invention provides a self-supporting electrode material of heterostructured nanoneedles anchored by polyoxometalates grown in situ in nickel foam, its preparation method, and its application. This invention employs a two-step hydrothermal strategy to grow the polyoxometalate precursor {La(PMo8V3O...}... 39 The LPMV is interfacially coupled with a nickel foam substrate with Fe-doped Co3O4 (FCO) nanoneedles. By controlling the hydrothermal reaction temperature and time, the directional growth of the FCO nanoneedle structure and the precise anchoring of the LPMV are achieved, which significantly improves the conductivity and stability of the electrode material and effectively solves the problems of insufficient exposure of active sites and low charge transport efficiency in the electrocatalysis process of traditional electrode materials.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention enables the precise construction of polyoxometalate clusters within a specified ratio range, resulting in relatively stable crystal structures and corresponding crystal chemical formulas. It achieves precise control over the structure of novel polyoxometalates (LPMVs) at the molecular level, creatively combining POMs with transition metal oxide substrates to effectively leverage their synergistic effect and broaden the application of POMs in the field of electrocatalysis.

[0017] This invention pioneered the use of an LPMV-FCO heterostructure to in-situ anchor LPMV onto the surface of a nickel foam electrode containing FCO nanoneedles via a hydrothermal method, overcoming the interfacial compatibility bottleneck of traditional catalysts. The "electron sponge" behavior of LPMV enhances charge storage capacity and optimizes the adsorption energy of reaction intermediates. The curvature of the FCO nanoneedle tips induces a localized electric field, significantly increasing the OH⁻ concentration, noticeably reducing bubble adhesion, and further enhancing La… 3+ Large-radius lattice distortion and oxygen vacancies jointly activate the intrinsic activity of Co3O4, accelerating charge transfer kinetics. Using it as an electrode material for electrocatalytic water splitting and studying its catalytic performance, thereby achieving molecular-level regulation of the electrocatalytic performance of POMs, will provide new ideas and experimental foundations for the development and application of high-performance electrocatalytic water splitting systems, and has significant scientific research value.

[0018] This invention combines molecular design concepts to successfully achieve a perfect combination of POMs and transition metal oxide substrates, and the constructed electrocatalytic water splitting system exhibits revolutionary electrocatalytic performance. In 1M KOH electrolyte, it achieves a significant leap in dual-function performance for complete water splitting, exhibiting ultra-low overpotentials in both hydrogen evolution and oxygen evolution reactions, and a high hydrogen evolution reaction (η... 10 =84mV), oxygen evolution reaction (η 10 =219mV). It also has high energy conversion, with an electrolyzer start-up voltage as low as 1.55V, hydrogen production energy consumption of 1.40 kWh / Nm³H2, and low current density decay rate after 120 hours of constant current testing (10mA / cm²), and the structural integrity is almost completely preserved. Attached Figure Description

[0019] Figure 1 This is a scanning electron microscope image of the polyoxometalate LPMV crystal material prepared in Example 1 of the present invention.

[0020] Figure 2 The images shown are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the LPMV-FCO electrode material prepared in Example 1 of this invention.

[0021] Figure 3 The figures show the electrochemical properties of LPMV-FCO prepared in Example 1 of this invention, FCO of Comparative Example 1, LPMV of Comparative Example 2, and 20% Pt / C of Comparative Example 3 in 1M KOH for hydrogen evolution; wherein, (a) is a comparison of polarization curves at a scan rate of 1mV / s; (b) is a comparison of Tafel slopes; and (c) is a comparison of EIS.

[0022] Figure 4The following are the electrochemical properties of oxygen evolution in 1M KOH for LPMV-FCO prepared in Example 1 of the present invention, FCO of Comparative Example 1, LPMV of Comparative Example 2, and RuO2 of Comparative Example 4; wherein, (a) is a comparison of polarization curves at a scan rate of 1mV / s; (b) is a comparison of Tafel slopes; and (c) is a comparison of EIS.

[0023] Figure 5 The figures show the electrochemical properties of the electrolytic cells formed by LPMV-FCO prepared in Example 1 of this invention as cathode and anode, and the electrolytic cells formed by Comparative Example 3 as cathode and Comparative Example 4 as anode, in 1M KOH. (a) The figure shows a comparison of the polarization curves of the electrolytic cells formed by LPMV-FCO prepared in Example 1 as cathode and anode, 20% Pt / C in Comparative Example 3 as cathode, and RuO2 in Comparative Example 4 as anode in 1M KOH. (b) The figure shows the water electrolysis (it) curve test of LPMV-FCO prepared in Example 1 as cathode and anode at a current density of 10 mA / cm². The inset in the figure shows the image of bubbles overflowing from the cathode and anode during the reaction. (c) The figure shows a comparison of the polarization curves of the electrodes before and after the it stability test in Figure (b).

[0024] Figure 6 Comparison diagrams of FCO electrodes prepared at different reaction temperatures, where (a) is 120℃ for 6 h, (b) is 180℃ for 6 h, and (c) is 160℃ for 10 h. Detailed Implementation

[0025] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0026] The polyoxometalate refers to an anionic structure having Z(XM) 11 O 39 )2-type polyoxometalates (Z is a lanthanide metal ion Ln) 3+ (where X is Si or P, and M is Mo, W, or V). The general molecular formula of the polyoxometalate in this invention is K. 14 H3[Ln(PMo8V3O 39 )2],K 14 H3[Ln(PMo8V3O 39 In [2], Ln represents the lanthanide metals La, Ce, and Eu.

[0027] In the preparation method of the LPMV-FCO composite electrode material of the present invention, when preparing the LPMV heteropolyacid, H6PMo9V3O is used. 40 The heteropolyacid precursor ·14H2O is dissolved in deionized water, and excess La(NO3)3 is added and stirred to ensure sufficient La content in the solution. 3+ Re-coordinated in place of its position, a La 3+ It can coordinate with two vacancy clusters simultaneously, thus forming a dimer structure, namely LPMV. The pH is adjusted to 4 using a buffer solution (KAc-H2O-HAc). This pH adjustment creates vacancy polyoxometalate clusters, i.e., allowing H6PMo9V3O 40 A vacancy is created at the position of Mo in the 14H2O structure, and this pH favors vacancy clusters and La. 3+ into a bond.

[0028] In the preparation of FCO, Fe(NO3)3·9H2O, Co(NO3)2·6H2O, and CO(NH2)2 were uniformly dispersed in deionized water at room temperature to obtain a clear orange solution. The resulting solution was then placed in an autoclave, and pretreated NF was added. The mixture was then subjected to a hydrothermal reaction at 160°C for 6 hours, allowing the metal ions in the solution to react with the nickel foam substrate, synthesizing in-situ grown hydroxides, thus forming preliminary nanoneedle structures on the surface of the nickel foam.

[0029] In preparing the LPMV-FCO composite electrode material, LPMV heteropolyacid was dispersed in deionized water. The resulting dispersion and FCO precursor were placed together in an autoclave and heated at 120°C for 6 hours to allow LPMV to undergo an ion exchange reaction with the FCO grown on the NF surface, thereby anchoring LPMV onto the FCO surface to form a composite structure. Anchoring LPMV onto the FCO surface firstly enhances the surface roughness of FCO, and secondly, LPMV itself has a strong proton transport capability. Therefore, the addition of LPMV effectively solves the problems of insufficient exposure of active sites and low charge transport efficiency.

[0030] H6PMo9V3O 40 Preparation of the 14H2O heteropolyacid precursor: Molybdenum trioxide (0.09 mol) and vanadium pentoxide (0.015 mol) were dispersed in 250 mL of deionized water (DI water) and stirred under reflux to 100 °C in a three-necked flask. Then, 85% (w / w) phosphoric acid solution was slowly added dropwise, and the mixture was kept under reflux at 100 °C for 24 hours to obtain an orange-red solution. The solution was then kept under reflux at 100 °C for 24 hours to obtain an orange-red transparent solution. Finally, the solution was dried in a vacuum oven at 54 °C and recrystallized three times at 4 °C to obtain H6PMo9V3O. 40 ·14H2O heteropolyacid precursor.

[0031] Example 1 A method for preparing an LPMV-FCO composite electrode material includes the following steps: Add 1.5g of H6PMo9V3O 40 The 14H2O heteropolyacid precursor was dissolved in 15 mL of deionized water, and 0.45 g of La(NO3)3 was added and stirred. The pH was adjusted to 4 with KAc-H2O-HAc buffer, and stirring was continued for 4 hours to obtain an orange-red transparent solution. After standing at 3°C ​​for 24 hours, orange-red crystals precipitated. The remaining liquid was used as the mother liquor. Finally, the orange-red crystals were recrystallized three times with the mother liquor. After filtration and collection, the orange-red crystals were washed and dried at room temperature to obtain a novel {Ln(PMo8V3O]3} precursor with abundant oxygen vacancies. 39 )2} series polyoxometalates K 14 H3[La(PMo8V3O 39 [2], denoted as LPMV heteropolyacid. The KAc-H2O-HAc buffer solution was prepared by mixing 1g:3mL:1mL of KAc, H2O, and HAc.

[0032] Fe(NO3)3·9H2O (0.15 mmol), Co(NO3)2·6H2O (0.78 mmol), and CO(NH2)2 (0.015 mmol) were uniformly dispersed in deionized water at room temperature to obtain a clear orange solution. The resulting solution was then placed in an 18 mL autoclave, and pretreated nickel foam (NF) was added. The mixture was hydrothermally reacted at 160 °C for 6 hours. After the reaction was complete, the product was repeatedly washed with deionized water and ethanol, then dried in an electrically heated forced-air drying oven at 60 °C for 12 hours. Finally, it was calcined at 250 °C for 0.5 hours under nitrogen protection to obtain FCO. The pretreated nickel foam was obtained by ultrasonically washing the nickel foam with 3M hydrochloric acid, deionized water, and anhydrous ethanol for 30 minutes each.

[0033] 0.1 g of LPMV heteropolyacid was dispersed in 10 mL of deionized water. The resulting dispersion was placed together with 1 cm * 1 cm FCO in a 15 mL autoclave. After heating at 120 °C for 6 hours, the product was repeatedly washed with deionized water and ethanol. Then, it was dried at 60 °C for 12 hours to obtain a composite electrode material with a unique tip effect nanoneedle structure, denoted as LPMV-FCO.

[0034] Example 2 The difference between Example 2 and Example 1 lies in the concentration of LPMV heteropolyacids, as detailed below: Take 0.05g of LPMV heteropolyacid and disperse it in 10mL of deionized water. Place the resulting dispersion and 1cm*1cm FCO in a 15mL autoclave. Heat at 120℃ for 6 hours and wash the product repeatedly with deionized water and ethanol. Then dry at 60℃ for 12 hours to obtain LPMV-FCO.

[0035] Example 3 The difference between Example 3 and Example 1 lies in the concentration of LPMV heteropolyacids, as detailed below: Take 0.2g of LPMV heteropolyacid and disperse it in 10mL of deionized water. Place the resulting dispersion and 1cm*1cm FCO in a 15mL autoclave. Heat at 120℃ for 6 hours and wash the product repeatedly with deionized water and ethanol. Then dry at 60℃ for 12 hours to obtain LPMV-FCO.

[0036] Comparative Example 1 A method for preparing an FCO electrode material is the same as the FCO preparation method in Example 1.

[0037] Comparative Example 2 A method for preparing an LPMV electrode material includes the following steps: The preparation method of LPMV heteropolyacid was the same as in Example 1. The pretreated NF was then calcined at 250°C for 0.5 hours under nitrogen protection to obtain the NF precursor. Next, LPMV heteropolyacid (0.01 g / mL) was dispersed in 10 mL of deionized water. The resulting dispersion and the NF precursor were placed together in a 15 mL autoclave and heated at 120°C for 6 hours. The product was then repeatedly washed with deionized water and ethanol, and finally dried at 60°C for 12 hours to obtain the LPMV electrode.

[0038] Comparative Example 3 Commercially available electrode material for hydrogen evolution: 20% Pt / C.

[0039] Preparation method of 20%Pt / C electrode: Mix 2mg (20%Pt / C) with 20μL deionized water, 80μL anhydrous ethanol and 15μL Nafion (5wt%), and sonicate for 3 hours to obtain a slurry. Then carefully add the slurry dropwise to a volume of 1*1cm. 2 The NF surface was naturally dried at room temperature to obtain a 20% Pt / C electrode.

[0040] Comparative Example 4 RuO2 is a commercially available electrode material for oxygen evolution.

[0041] Preparation method of RuO2 electrode: Mix 2 mg RuO2 with 20 μL deionized water, 80 μL anhydrous ethanol and 15 μL Nafion (5 wt%), and sonicate for 3 hours to obtain a slurry. Then carefully add the slurry dropwise to a volume of 1*1 cm. 2 The NF surface was naturally dried at room temperature to obtain the RuO2 electrode.

[0042] The LPMV-FCO electrode materials prepared in Examples 1-3 of this invention have the same structure and similar performance. The following comparative study is based solely on the samples from Example 1 and Comparative Examples 1-4. The composite electrode material prepared in Example 1 is designated as LPMV-FCO, the unmodified metal oxide compound of LPMV in Comparative Example 1 is designated as FCO, the compound electrode of Comparative Example 2 is designated as LPMV, and the commercial electrodes of Comparative Examples 3 and 4 are designated as 20%Pt / C and RuO2, respectively.

[0043] The above electrode materials were compared and studied separately, and the results are as follows: Figures 1-5 As shown: Test conditions and contents for electrochemical detection: All electrochemical tests were performed using an electrochemical workstation (CHI660E, Shanghai Chenhua Instruments Co., Ltd., China). In the electrocatalytic hydrogen evolution and oxygen evolution performance test, 1M KOH was used as the electrolyte, Hg / HgO as the reference electrode, and a carbon rod as the counter electrode. The electrodes obtained in Example 1, Comparative Examples 1-2, and Comparative Examples 3-4 were used as working electrodes. Linear sweep voltammetry (LSV) curves were scanned at a scan rate of 1 mV / s, with the electrode-to-electrode (HER) potential range of -1.5 to -0.9 V and the oxygen reduction electrode (OER) potential range of 0 to 0.9 V. Each electrode underwent 100 voltammetric cycles before the LSV test at a scan rate of 10 mV / s. According to the Nernst equation: E RHE =E Hg / HgO +0.098 +0.0592 × pH, all potential values ​​in this invention are calculated relative to the RHE potential. Charge transfer resistance (R ct Electrochemical impedance spectroscopy (EIS) was used to determine the results in the frequency range of 100 kHz to 0.01 Hz. All tests were repeated 2–4 times to ensure reproducibility. The overall water splitting performance in 1 M KOH was studied using a two-electrode system, with LSV polarization curves measured at a scan rate of 10 mV / s in the range of 1.20–2.00 V. The long-term stability of HER, OER, and all-water separated samples was determined using multi-step chronoamperometry and chronoamperometry (it) experiments. All data were further analyzed after 100% iR correction.

[0044] Figure 1The results show that LPMV crystals were successfully prepared, and the constituent elements were uniformly distributed within them, proving that La 3+ Successful doping.

[0045] Figure 2 The results show that: Figure 2 (a) The scanning electron microscope image shows that the nanoneedles grow uniformly and at a high density on the nickel foam matrix. Figure 2 (b) and (c) show that at different magnifications, a large number of nanoneedles with a diameter of about 30 nanometers are visible on the surface. Figure 2 Transmission electron microscopy (TEM) observations revealed significant LPMV anchoring on the surface of individual nanoneedles. The LPMV-FCO nanoneedle structure has a radius of only 30 nm, giving it a greater specific surface area advantage compared to other geometries. Figure 2 (e) High-resolution transmission electron microscopy imaging shows that its lattice fringes are consistent with the {311} and {400} crystal planes of spinel Co3O4 structure and the {311} crystal plane of Fe3O4 structure, confirming the lattice embedding of iron. Figure 2 (f) shows the ring-shaped dark-field scan pattern, which reveals the diffraction rings corresponding to these crystal planes, further confirming the high crystallinity of LPMV-FCO.

[0046] Figure 3 The results show that: Figure 3 (a) Shows the linear sweep voltammetry (LSV) polarization curves (100% iR compensation, scan rate: 1 mV / s). Compared with FCO (η 10 =163mV) and the original LPMV (η 10 Compared to 212mV, the LPMV-FCO exhibits a lower overpotential (η) when achieving a current density of 10mA / cm². 10 =84mV), its performance is close to that of Pt / C catalysts (Pt / C, η 10 =70mV). This significant improvement indicates that oxygen vacancy engineering effectively promotes the HER kinetics. Figure 3 (b) The kinetic properties of the catalyst were evaluated by Tafel analysis. The results showed that LPMV-FCO had the lowest Tafel slope (90 mV / dec), significantly lower than FCO (150 mV / dec), LPMV (126 mV / dec), and Pt / C (99 mV / dec), confirming the synergistic effect of Fe atom doping and LPMV anchoring in accelerating the rate-determining step of HER. Furthermore, electrochemical impedance spectroscopy (EIS) provided important insights into the charge transfer characteristics. Figure 3 (c) EIS shows that the LPMV-FCO has the lowest charge transfer resistance (Rc). ct=6.32Ω), lower than FCO (7.81Ω) and LPMV (16.55Ω), and comparable to Pt / C (6.07Ω), indicating that it has better conductivity and faster interfacial charge transfer kinetics.

[0047] Figure 4 The results show that: Figure 4 The LSV polarization curve in (a) shows that, compared to FCO (η) 10 =275mV), commercial RuO2 catalyst (η 10 =303mV) and the original LPMV (η 10 =353mV), LPMV-FCO requires only a lower overpotential (η). 10 =219mV) can achieve 10mA / cm 2 The current density. Figure 4 (b) Tafel slope analysis further confirms its excellent kinetic properties, with LPMV-FCO exhibiting the smallest slope (40 mV / dec). The enhanced oxygen reduction electrode activity validates the feasibility of the Fe doping strategy. The introduction of Fe may amplify the "tip effect" by increasing the tortuosity of the nanotip. This localization phenomenon concentrates OH⁻ ions, generates an interfacial electric field, accelerates surface charge transfer, and optimizes reaction kinetics, thereby synergistically promoting the performance of a highly efficient oxygen reduction electrode. Figure 4 (c) Electrochemical impedance spectroscopy (EIS) shows that LPMV-FCO has a small reaction rate constant (Rc). ct =0.83Ω), indicating that the interfacial electron transfer dynamics are significantly accelerated.

[0048] Figure 5 The results show that: Figure 5 As shown in (a), the LPMV-FCO-based cell exhibits an onset potential of 1.55V, significantly lower than the benchmark Pt / C||RuO2. Furthermore, Figure 5 The it curve test in (b) shows that the electrolysis unit assembled with LPMV-FCO can achieve stable operation for up to 120 hours at a voltage of 1.55V, and intense bubble generation was observed at both electrodes. Figure 5 (c) shows that the linear mild activation curve of the electrode remained basically unchanged before and after the stability test.

[0049] Figure 6The figures show a comparison of FCO electrodes prepared at different reaction temperatures. Figure a shows FCO prepared at 120℃ for 6 hours, exhibiting a morphology tending towards disordered nanosheets. Figure b shows FCO prepared at 180℃ for 6 hours, exhibiting a morphology tending towards a mixed structure of nanoneedles and microspheres. Figure c shows FCO prepared at 160℃ for 10 hours, exhibiting a morphology tending towards an aggregated nanoneedle structure. LPMV has high solubility in water, thus anchoring itself on the FCO substrate surface. The morphology of the LPMV-FCO composite sample is mainly determined by FCO. The precise anchoring of LPMV is due to the ion exchange reaction between LPMV and FCO under hydrothermal conditions.

[0050] In summary, this invention successfully developed a highly efficient bifunctional electrocatalyst (LPMV-FCO) by in-situ integrating oxygen-vacancy-rich POM materials with transition metal oxide nanoneedles. The La in Keggin-type POM materials... 3+ Substitution induces lattice distortion and oxygen vacancies, while Fe doping enhances the "tip effect" of the nanoneedles. These synergistic effects optimize the adsorption free energy and accelerate ion / electron transfer through a local electric field. The catalyst exhibits benchmark-level overpotentials (84 mV for HER, 219 mV for OER) and a small Tafel slope (40 mV / dec for OER), attributed to the synergistic effect of POM-derived vacancies and the conductive Co3O4 matrix. Symmetric electrolyzers (-)LPMV-FCO||LPMV-FCO(+) achieved industrial-grade current densities at 1.55 V while maintaining excellent energy efficiency (1.40 kWh / Nm³). 3 H2) and 120-hour durability. This study not only demonstrates high-performance materials, but also establishes a design principle: combining in-situ grown morphology-optimized substrates with multifunctional molecular clusters, opening up new pathways for advanced energy conversion technologies.

[0051] This invention, based on molecular design principles, combines polyoxymethylene (POMs) with transition metal oxides (TMOs) possessing tunable electronic structures, excellent reactivity, and high electron density through an in-situ hydrothermal reaction, effectively promoting the adsorption and activation of reactant molecules. This combination aims to enhance the overall activity and selectivity of the composite catalytic system by optimizing the chemical bonding state between the materials. Although noble metal oxides (such as RuO2, IrO2, and PtO2) exhibit high catalytic activity, their scarcity and high cost significantly limit their widespread application. Therefore, an effective strategy is to grow structurally diverse transition metal nanomaterials on a conductive substrate and modify TMOs using POMs through heterostructure engineering. This strategy fully utilizes the synergistic effect between the POMs and TMOs components, significantly improving the overall stability and catalytic activity of the composite material. Simultaneously, this composite structure helps increase the exposure of active sites, improves the conductivity of the material, and effectively reduces the solubility of the POMs component.

[0052] Among the diverse types of POMs, Keggin-type heteropolyacids, as a classic structural type, are nanoscale metal-oxygen clusters formed by bridging former transition metal atoms (such as molybdenum (Mo), tungsten (W), vanadium (V), niobium (Nb), etc.) with oxygen atoms. Their typical molecular formula is [XM]. 12 O 40 [ⁿ⁻ (where X is a heteroatom such as phosphorus (P), silicon (Si), germanium (Ge), etc.; M is a metal atom such as Mo or W)]. These compounds have a highly ordered cage-like structure with symmetry close to the Td point group. Their structural characteristic is that the heteroatom is located at the center of a tetrahedron, surrounded by twelve MO6 octahedral units. The Keggin structure contains four different bonding states of oxygen atoms (such as terminal oxygen, bridging oxygen, etc.). These oxygen atoms exhibit differentiated coordination abilities and reactivity. In particular, the terminal oxygen atom, due to its strong coordination ability, readily combines with transition metal ions or protons, thus forming transition metal-substituted heteropolyacid derivatives. In the field of electrocatalysis, Keggin-type heteropolyacids are considered ideal functional materials due to their unique redox properties and significant structural tunability. Their core advantage lies in their ability to reversibly accept or release multiple electrons (typically 4-6 electrons per molecule) while maintaining structural stability, exhibiting efficient "electron sponge" behavior and proton transport carrier function.

[0053] This invention develops a novel {Ln(PMo8V3O)} electrocatalytic active center by precisely controlling the metal ions coordinated at vacancy sites to directly serve as electrocatalytic active centers. 39 The 2} series of polyoxometalates are designed to fully leverage their intrinsic advantages of multi-active-site synergy and enhanced charge transfer, thereby overcoming the limitations of their application in harsh electrochemical environments.

[0054] It should be noted that when numerical ranges are mentioned in the claims of this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. To avoid redundancy, the present invention describes preferred embodiments.

[0055] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0056] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A polyoxometalate, characterized in that, Prepared by the following steps: Add H6PMo9V3O 40 · Add Ln(NO3)3 to the 14H2O heteropolyacid solution and mix well. Adjust the pH value to 3-5 and stir the reaction to obtain an orange-red transparent solution. Let the orange-red transparent solution stand to precipitate orange-red crystals. Collect the orange-red crystals by recrystallization of the mother liquor. Combine the orange-red crystals, wash and dry them to obtain the polyoxometalate. Wherein, Ln is a lanthanide metal, H6PMo9V3O 40 The concentration of the heteropolyacid solution was 0.05~0.12 g / mL, and the concentration of H6PMo9V3O was... 40 The ratio of 14H2O heteropolyacid solution to Ln(NO3)3 is 10~15mL:0.375~0.5g.

2. The polyoxometalate according to claim 1, characterized in that, The Ln is La, Ce, or Eu.

3. A method for preparing a composite electrode material, characterized in that, Includes the following steps: The dispersion of the polyoxometalate described in claim 1 and nickel foam with metal oxides grown on the surface of spinel structure were heated at 110°C to 120°C for 5 to 6 hours, then washed and dried to obtain the composite electrode material. The mass of the polyoxometalate in the dispersion of the said polyoxometalate used per square centimeter of said nickel foam is 0.05~0.2g.

4. The preparation method according to claim 3, characterized in that, The metal oxide with a spinel structure is Fe-doped Co3O4.

5. The preparation method according to claim 3, characterized in that, The preparation steps for nickel foam with a metal oxide of spinel structure grown on its surface are as follows: Fe(NO3)3 . 9H2O, Co(NO3)2 . 6H2O and CO(NH2)2 are uniformly dispersed in water to obtain a clear orange solution. Pretreated nickel foam is added and reacted at 150~160℃ for 5~6 hours. After the reaction is completed, the solution is washed and dried, and then calcined at 200℃~350℃ for 0.5~1 hours under nitrogen protection to obtain nickel foam with metal oxides with spinel structure grown on the surface. Among them, Fe(NO3)3 . 9H2O, Co(NO3)2 . The molar ratio of 6H2O to CO(NH2)2 is (0.15-0.2):(0.7-0.8):(0.015-0.020).

6. The composite electrode material prepared by the preparation method according to any one of claims 3-5.

7. The application of the composite electrode material according to claim 6 in the electrocatalytic splitting of water.

8. The application according to claim 7, characterized in that, In the electrocatalytic water splitting process, the composite electrode material is used as the working electrode, the mercury oxide electrode is used as the reference electrode, and the carbon rod electrode is used as the counter electrode to form a three-electrode system, which is connected to an electrochemical workstation to carry out the electrocatalytic water splitting in the electrolyte.

9. The application according to claim 7, characterized in that, In the electrocatalytic water splitting process, the composite electrode materials are used as the cathode and anode, respectively, forming a two-electrode system connected to an electrochemical workstation, where the electrocatalytic water splitting is carried out in the electrolyte.

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