A UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger, its preparation method and application

Abstract

The application discloses a kind of CePO4 radical scavenger of UiO-66 topological cerium-based MOF derivative and its preparation method and application, it is related to proton exchange membrane fuel cell oxidation-resistant functional filler, rare earth radical scavenging material, topological MOF derivative composite material technical field.Preparation method includes the following steps: synthesis of UiO-66 topological cerium-based MOF precursor;Low-concentration phosphoric acid mild impregnation load phosphorus;Polydopamine coating modification;High temperature topological pyrolysis conversion.CePO4 active phase prepared by the application has more excellent acid resistance and structural stability, can effectively inhibit the dissolution of cerium ions under acidic conditions;Surface carbon-coated shell can simultaneously improve the conductivity and interface adaptability of the material, solve the defects of traditional rare earth oxidation-resistant materials, such as easy agglomeration, poor stability and high impedance, and is suitable for long-life proton exchange membrane fuel cell modification application.

Description

Technical Field

[0001] This invention relates to the fields of antioxidant functional fillers for proton exchange membrane fuel cells, rare earth radical scavenging materials, and topological MOF-derived composite materials. Specifically, it relates to a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger, its preparation method, and its application. Background Technology

[0002] During the operation of a proton exchange membrane fuel cell, hydrogen-oxygen transmembrane permeation, incomplete oxygen reduction side reactions, and dynamic start-stop load fluctuations continuously generate Fenton reactions, producing highly oxidizing hydroxyl radicals and superoxide anion radicals. These reactive oxygen species indiscriminately attack the perfluorosulfonic acid proton exchange membrane, carbon support, and platinum-based catalyst particles, causing membrane pinhole defects, carbon support corrosion, and platinum particle detachment and aggregation. This is the core bottleneck leading to rapid performance degradation and insufficient service life of the fuel cell stack.

[0003] Currently, commercially available antioxidant fillers for fuel cells mainly use nano-CeO2, relying on Ce... 3+ / Ce 4+ While reversible valence state cycling can capture free radicals, it suffers from several inherent technical drawbacks: First, cerium dioxide has weak acid resistance, and cerium ions are easily dissolved and lost in the strongly acidic working environment of fuel cells, resulting in a significant decrease in antioxidant capacity and poor long-term stability after long-term cycling; Second, nano-CeO2 particles have high surface energy and are prone to spontaneous aggregation, burying a large number of active sites and limiting the efficiency of free radical quenching; Third, CeO2 has poor intrinsic conductivity, and direct doping will increase the interfacial impedance of the catalyst layer, sacrificing the initial power density and electrochemical kinetic performance of the battery.

[0004] Compared to cerium dioxide, CePO4 exhibits superior acid-base stability and a significantly reduced risk of cerium ion leaching, making it a promising next-generation high-performance antioxidant candidate to replace traditional CeO2. However, existing CePO4 materials are mostly prepared using traditional co-precipitation methods, resulting in randomly aggregated nanoparticles lacking an ordered hierarchical porous structure. These particles have low specific surface area, insufficient exposure of active sites, and lack a conductive protective layer, leading to poor compatibility with membrane electrode interfaces and hindering the full realization of long-lasting antioxidant advantages.

[0005] MOF materials, with their advantages of ordered topological channels, high specific surface area, and atomically uniform dispersion of metal sites, are ideal precursors for the preparation of derived rare-earth functional materials. UiO-66 possesses a rigid and stable topological framework, regular channels, and complete crystal morphology, making it the most widely used benchmark topological structure in the field. Existing studies mostly use zirconium-based UiO-66 as a support to load rare-earth components. However, the rare earth elements only adhere to the material surface, resulting in poor dispersion uniformity and unstable active sites, failing to achieve intrinsically uniform distribution of metal sites.

[0006] If a pure cerium-based MOF with the same topology as UiO-66 is synthesized, cerium atoms are uniformly distributed in the MOF metal nodes, achieving a high degree of dispersion of active sites from the source. However, conventional high-concentration phosphoric acid impregnation processes can damage the coordination framework of cerium-based MOFs due to high ionic strength, causing crystal collapse and destruction of the pore structure, completely losing the advantages of high specific surface area and ordered mass transfer channels. At the same time, the intrinsic conductivity of pure CePO4 is insufficient, and the lack of a conductive coating layer can easily deteriorate the mass transfer at the three-phase reaction interface of the catalyst layer.

[0007] Therefore, how to develop a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger, its preparation method, and its application are technical problems that urgently need to be solved by those skilled in the art. Summary of the Invention

[0008] To address the shortcomings of existing technologies, such as the poor acid resistance and easy dissolution and aggregation of commercial CeO2, the lack of ordered channels and poor dispersibility of traditional CePO4, and the easy destruction of the cerium-based MOF crystal structure by high-concentration phosphoric acid impregnation, this invention provides a UiO-66-like topological cerium-based MOF-derived CePO4 free radical scavenger, its preparation method, and its applications. Through a mild phosphoric acid loading process using a low-concentration dilute phosphoric acid solution, the MOF channels and crystal form are preserved. Combined with a polydopamine carbonized conductive coating shell, highly active free radical scavenging, improved acid resistance, stability, and conductivity are simultaneously achieved, significantly extending the service life of proton exchange membrane fuel cells.

[0009] A method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger includes the following steps:

[0010] (1) Synthesis of UiO-66 topological cerium-based MOF precursor: Soluble cerium salt and terephthalic acid were dissolved in deionized water, and the reaction was carried out at room temperature with magnetic stirring. After centrifugation, washing, and vacuum drying, cerium-based MOF powder with complete morphology and regular crystal form was obtained. (2) Mild impregnation of phosphorus-loaded powder with low-concentration phosphoric acid solution: The cerium-based MOF powder obtained in step (1) is dispersed in the impregnation solution and impregnated by low-temperature constant temperature stirring. After impregnation, the powder is centrifuged and dried to obtain phosphorus-loaded cerium-based MOF powder. The complete crystal skeleton of MOF is preserved throughout the process. (3) Polydopamine coating modification: The phosphorus-loaded cerium-based MOF powder obtained in step (2) is dispersed in Tris-HCl weakly alkaline buffer solution, and then dopamine hydrochloride is added and stirred at room temperature to complete uniform coating. After centrifugation, washing with water and drying, PDA@phosphorus-containing cerium-based MOF composite precursor is obtained. (4) High-temperature topological pyrolysis conversion: The PDA@phosphorus-containing cerium-based MOF composite precursor obtained in step (3) is placed in a tube furnace and heated and calcined under an inert atmosphere. Simultaneously, phosphate and Ce sites are generated in situ to form cerium phosphate, and polydopamine is carbonized to form a conductive carbon coating shell. After cooling, grinding, and sieving, the free radical scavenger is obtained.

[0011] Furthermore, in step (1), the reaction temperature is 15-40 ℃ and the reaction time is 24-48 h; The molar ratio of soluble cerium salt to terephthalic acid is 3:2. The soluble cerium salt and terephthalic acid are dissolved in deionized water, wherein the concentration of terephthalic acid is 0.1 mol / L and the concentration of soluble cerium salt is 0.15 mol / L. The soluble cerium salt is cerium ammonium nitrate. The cerium-based MOF powder has an ordered hierarchical porous structure, and its topological framework is consistent with that of UiO-66. It has a complete crystal form without collapse or particle agglomeration.

[0012] The beneficial effects of adopting the above-mentioned further technical solutions are as follows: Maintaining the complete crystal form and UiO-66 topology through reaction time control. This invention limits the use of deionized water as the sole solvent and introduces formic acid as a coordination competitor. Formic acid can gently slow down the coordination polymerization rate of cerium ions and terephthalic acid, avoiding lattice defects and amorphous byproducts caused by rapid nucleation. Extending the reaction to 24-48 h at room temperature ensures that the crystal fully self-repairs through the "dissolution-recrystallization" process. This topological integrity directly determines the permeability of the channels during subsequent phosphoric acid impregnation, which is the structural basis for avoiding stress concentration during phosphorus loading that leads to framework collapse.

[0013] Furthermore, in step (2), the impregnation temperature is 15-40 ℃, and the stirring impregnation time is 2-6 h; The concentration of the dilute aqueous solution of phosphoric acid is 0.1-0.2 mol / L, and the ratio of cerium-based MOF powder to impregnation solution is 1 g:(50-100) mL; In phosphorus-loaded cerium-based MOF powder, the phosphorus source is uniformly adsorbed inside the MOF channels without damaging the MOF crystal framework and morphology.

[0014] The beneficial effects of adopting the above-mentioned further technical solutions are as follows: 1) Impregnation temperature 15-40 ℃: This temperature range can provide the activation energy required for phosphate ions to diffuse into the MOF channels, accelerating mass transfer, and can also avoid the hydrolytic attack of the protonic acid-cerium-terephthalic acid coordination bond at high temperature (>60℃), thus protecting the integrity of the skeleton.

[0015] 2) Phosphoric acid concentration 0.1-0.5 mol / L: This extremely low concentration range ensures that phosphoric acid slowly dissociates into PO4 in a sustained-release manner. 3-This allows it to preferentially anchor to the surface of cerium metal nodes within the MOF channels via electrostatic adsorption and ligand exchange, rather than remaining free in the solution. When the concentration is below 0.1 mol / L, the phosphorus loading is insufficient for complete conversion, while when it is above 0.5 mol / L, the high ionic strength will disrupt the coordination equilibrium, leading to edge corrosion.

[0016] 3) Solid-liquid ratio 1 g: (50-100) mL: This ratio ensures that the powder is in a uniform suspended and dispersed state in the impregnation liquid, and the liquid volume is sufficient to fill all the pores, ensuring that the phosphorus source is enriched inside the pores rather than only on the outer surface of the particles, thus achieving uniform phosphorus loading "from the inside out".

[0017] Furthermore, in step (3), dopamine hydrochloride is added and stirred at room temperature for 4-12 hours; The mass ratio of dopamine hydrochloride to cerium phosphorus-loaded MOF powder is (0.05-0.3):1, and the ratio of cerium phosphorus-loaded MOF powder to Tris-HCl weakly alkaline buffer solution is 1 g : (50-100) mL.

[0018] The beneficial effects of adopting the above-mentioned further technical solutions are as follows: 1) Dopamine to MOF mass ratio (0.05-0.3):1: This ratio range can ensure that the conductive layer is continuous and effectively blocks acid penetration after the final carbonization of PDA.

[0019] 2) Solid-liquid ratio 1 g:(50-100) mL: This dilute solution system ensures that the dopamine monomer maintains a low concentration gradient in the solution, which is conducive to the occurrence of "heterogeneous nucleation-growth" mechanism on the MOF surface, rather than "homogeneous nucleation" which wastes raw materials, and ensures uniform coating and batch repeatability.

[0020] Furthermore, in step (4), the inert atmosphere is high-purity N2 or Ar, the heating rate is 2-5 ℃ / min, the calcination temperature is 600-800 ℃, and the calcination constant temperature holding time is 1-3 h.

[0021] The beneficial effects of adopting the above-mentioned further technical solutions are as follows: 1) Heating rate 2-5 ℃ / min: This slow heating condition ensures that the PDA layer undergoes pre-crosslinking stabilization in the early stage (<300℃), avoiding the violent decomposition of PDA and release of a large amount of gas during rapid heating, which would cause the shell to bubble and break; at the same time, slow heating gives phosphate ions enough time to migrate, allowing them to complete in-situ topological chemical reactions with the adjacent Ce nodes in the solid phase.

[0022] 2) Calcination temperature 600-800 ℃: This temperature range has a dual key role—the lower limit of 600 ℃ is the minimum threshold for PDA to be completely carbonized into a conductive carbon layer, and at the same time, it can drive PO4 3- With Ce 4+A solid-state reaction occurs to form crystalline CePO4; the upper limit of 800℃ avoids excessive growth and coarsening of CePO4 grains (leading to a sharp drop in specific surface area) and excessive graphitization and densification of the carbon shell (loss of mesoporous permeability). Within this temperature range, disordered D bands and ordered G bands coexist in the carbon shell, ensuring conductivity while retaining structural defects to facilitate interfacial charge transfer.

[0023] 3) Keep warm for 1-3 hours: This time is sufficient for the cerium phosphate crystal phase to be completely transformed and the carbon layer structure to become stable. If the time is too short, the reaction will not be complete and an intermediate phase will remain. If the time is too long, the energy consumption will increase and there will be no additional performance gain.

[0024] The present invention also provides a method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 free radical scavenger.

[0025] Furthermore, the free radical scavenger has an integrated core-shell structure, with the core being the CePO4 active phase and the outer shell being a multi-level porous conductive carbon coating layer formed by high-temperature carbonization of polydopamine.

[0026] The beneficial effects of adopting the above-mentioned further technical solutions are as follows: Compared with CeO2, the CePO4 active phase of this invention has a lower lattice oxygen mobility and higher phosphate structure rigidity. In a strongly acidic perfluorosulfonic acid membrane environment with pH < 1, the cerium ion dissolution rate is reduced by about one order of magnitude compared with CeO2. Its free radical scavenging follows a Fenton-like catalytic mechanism: Ce 3+ The site reduces ·OH to OH. - It oxidizes to Ce 4+ Ce 4+ Then O2· - It is oxidized to O2 and then reduced to Ce. 3+ This forms a closed redox cycle. This cycle does not depend on an externally applied potential and is entirely driven by the intrinsic redox potential of the material, thus possessing the characteristics of "activation-free, self-sustaining" long-lasting antioxidant properties.

[0027] The present invention also provides the application of the UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger in proton exchange membrane fuel cells.

[0028] Furthermore, the free radical scavenger is added to the catalyst layer slurry or proton exchange membrane casting solution at a doping ratio of 1wt%-10wt%.

[0029] The beneficial effects of adopting the above-mentioned further technical solutions are: to remove reactive oxygen free radicals generated during battery operation in situ, to inhibit the degradation of the proton exchange membrane backbone, the oxidation and corrosion of the carbon support, the migration and aggregation of platinum nanoparticles, and to reduce the rate of decay of the membrane electrode's long-term cycle performance.

[0030] The inventive mechanism of this invention: This invention first prepares a cerium-based MOF with the same topological structure as UiO-66 using terephthalic acid as a ligand, with cerium uniformly distributed in the MOF metal nodes. A low-concentration phosphoric acid solution is used for low-temperature, gentle impregnation to slowly introduce phosphate ions, avoiding the coordination framework collapse problem caused by high-concentration phosphorus sources and preserving the MOF's hierarchical channels and complete morphology. Subsequently, a polydopamine layer is uniformly coated onto the particle surface. During high-temperature calcination in an inert atmosphere, the phosphate adsorbed in the MOF channels reacts in situ with the cerium metal nodes to generate a cerium phosphate active phase. Simultaneously, the polydopamine undergoes high-temperature carbonization to form a continuous hierarchical porous conductive carbon coating shell, ultimately yielding an integrated core-shell structure composite material of a cerium phosphate active core and a hierarchical porous conductive carbon coating shell.

[0031] In this invention, the cerium site depends on Ce 3+ / Ce 4+ Reversible valence cycle continuously captures ·OH and O2 - Two types of destructive reactive oxygen species; the fully preserved MOF-derived hierarchical channels provide a large number of accessible active sites, improving the free radical quenching efficiency; the outer carbon-coated shell not only constructs a continuous conductive network and optimizes the mass transfer of electrochemical reactions, but also acts as a physical barrier layer to isolate acidic electrolytes, inhibit the dissolution of cerium ions from cerium phosphate, and ensure the long-term antioxidant stability of the material.

[0032] Compared with the prior art, the present invention has the following beneficial effects: 1. Based on a UiO-66-like topological cerium-based MOF, atomic-level uniform dispersion of active sites is achieved. Unlike traditional zirconium-based UiO-66 followed by rare earth loading, cerium in this invention is an intrinsic metal node in the MOF framework, enabling atomic-level uniform dispersion. The well-ordered hierarchical porous structure greatly improves the exposure of active sites and mass transfer efficiency, and the free radical quenching efficiency is far higher than that of co-precipitated random CePO4 powder and CeO2.

[0033] 2. Low-concentration phosphoric acid mild impregnation process, completely preserving the MOF crystal framework. This invention defines a low-concentration phosphoric acid mild impregnation system, which can uniformly introduce phosphate groups while completely preserving the original crystal form and ordered hierarchical channels of MOF, avoiding the framework collapse and structural damage defects caused by high-concentration phosphorus sources, and maximizing the high specific surface area and mass transfer advantages of the topological structure.

[0034] 3. Controllable coating with a single carbon source precisely addresses the inherent performance limitations of CePO4. This invention uses only polydopamine as a single carbon source to construct a multi-level porous conductive carbon coating shell, which has a clear structural logic and strong controllability. The carbon shell can both construct a continuous conductive path, improving the inherent poor conductivity of CePO4 and reducing the interfacial impedance of the membrane electrode; and form an acid-resistant physical barrier, significantly inhibiting the dissolution of cerium ions under acidic conditions and improving the long-term cycling stability of the material.

[0035] 4. The overall antioxidant performance is significantly better than that of traditional CeO2. The cerium phosphate active phase prepared by this invention has much better acid resistance and structural stability than CeO2. It can simultaneously and efficiently remove two types of core reactive oxygen free radicals. The doped and modified membrane electrode can significantly slow down the degradation of the proton exchange membrane, the corrosion of the carbon support and the agglomeration and shedding of the platinum catalyst, effectively reducing the long-term cycle degradation of the battery.

[0036] 5. The preparation process is mild and controllable, suitable for large-scale mass production. This invention features a simple one-step synthesis, low-temperature dilute solution impregnation, room-temperature coating, and high-temperature calcination, all under mild conditions. The raw materials are inexpensive and readily available, and the crystal morphology, phosphorus doping amount, and carbon shell thickness can all be precisely controlled. The process has high repeatability and is suitable for the large-scale preparation and industrialization of antioxidant fillers for fuel cells.

[0037] In summary, compared to traditional CeO2 materials in the fuel cell field, the CePO4 active phase prepared in this invention exhibits superior acid resistance and structural stability, effectively suppressing cerium ion dissolution under acidic conditions; the fully preserved UiO-66 regular hierarchical porous structure can significantly expose Ce. 3+ / Ce 4+ The reversible variable-valence active sites continuously and efficiently quench hydroxyl radicals and superoxide anion radicals; the surface carbon coating can simultaneously improve the material's conductivity and interfacial compatibility, solving the defects of traditional rare earth antioxidant materials such as easy agglomeration, poor stability, and high impedance. The process of this invention is mild, the structure is controllable, and the crystal integrity is high. It can significantly delay the free radical-induced decay of the membrane electrode of proton exchange membrane fuel cells, improve the long-term service durability of the stack, and is suitable for large-scale modification applications of long-life proton exchange membrane fuel cells. Attached Figure Description

[0038] Figure 1 The XRD patterns of the phosphorus-loaded cerium-based MOF powder after impregnation with phosphoric acid solutions of different concentrations (0.1 mol / L, 0.2 mol / L, 0.5 mol / L) in step (1) of Example 1 are shown.

[0039] Figure 2 The XRD diffraction pattern of CePO4@C obtained by high-temperature pyrolysis in step (4) of Example 1 and its comparison with the CePO4 standard PDF card are shown.

[0040] Figure 3 The image shows a comparison of the Raman spectra of the CePO4@C composite materials obtained in Example 1 after impregnation with 0.1 mol / L and 0.2 mol / L phosphoric acid and followed by high-temperature pyrolysis.

[0041] Figure 4 The electrochemical impedance spectroscopy (EIS) test curves of the conductivity of CePO4@C composite material in step (4) of Example 1 are shown.

[0042] Figure 5 The image shows the scanning electron microscope (SEM) morphology of the cerium-based MOF powder obtained in step (1) of Example 1.

[0043] Figure 6 The image shows the SEM morphology of the phosphorus-loaded cerium-based MOF powder obtained in step (2) of Example 1.

[0044] Figure 7 This is a transmission electron microscope (TEM) image of the phosphorus-loaded cerium-based MOF powder obtained in step (2) of Example 1.

[0045] Figure 8 The first comparative spectrum is shown in the UV-Vis spectrophotometric free radical scavenging experiment.

[0046] Figure 9 The second image shows a comparison of free radical scavenging experiments using ultraviolet-visible spectrophotometry.

[0047] Figure 10 The third image shows a comparison of free radical scavenging experiments using ultraviolet-visible spectrophotometry.

[0048] Figure 11 The fourth image shows a comparison of the free radical scavenging experiment using ultraviolet-visible spectrophotometry.

[0049] Figure 12 Fifthly, a comparative spectrum of the free radical colorimetric scavenging experiment using ultraviolet-visible spectrophotometry.

[0050] Figure 13 Comparison of linear sweep voltammetry (LSV) polarization curves of a CePO4@C composite modified rotating disk electrode in acidic medium: CePO4@C obtained in step (4) of Example 1 was used as an antioxidant additive; blank control group (Pt / C catalyst layer, no scavenger added); Pt / C catalyst layer with 5 wt% of target material added; Pt / C catalyst layer with 10 wt% of target material added.

[0051] Figure 14 Comparison of EIS spectra of the CePO4@C composite material modified rotating disk electrode obtained in step (4) of Example 1: The impedance response of pure cerium phosphate@carbon material (without Pt / C) is slightly greater than that of the blank control group; impedance spectra of blank control group (Pt / C, without scavenger), added 5 wt% target material and added 10 wt% target material.

[0052] Figure 15 A comparison of the number of electrons transferred in the oxygen reduction reaction (n) measured by the rotating ring electrode (RRDE) under different CePO4@C composite material addition amounts.

[0053] Figure 16This is a schematic diagram illustrating the antioxidant mechanism of cerium phosphate free radical scavenger in proton exchange membrane fuel cells.

[0054] Figure 17 The image shows the XRD spectrum of CePO4@C obtained in Example 3.

[0055] Figure 18 The image shows the Raman spectrum of CePO4@C obtained in Example 3.

[0056] Figure 19 The image shows the Raman spectrum of CePO4@C obtained in Comparative Example 3. Detailed Implementation

[0057] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] Example 1 A method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger includes the following steps: (1) Synthetic UiO-66 topological cerium-based MOF precursors: 1.644 g of cerium ammonium nitrate and 0.332 g of terephthalic acid were weighed and dissolved separately in 20 mL of deionized water. The mixture was stirred at 100 rpm at room temperature for 48 h until the solution turned pale yellow. After the reaction was completed, the solid product was separated by centrifugation and washed three times each with deionized water and anhydrous ethanol. The product was then vacuum dried at 60 ℃ for 12 h to obtain cerium-based MOF powder with complete morphology and regular crystal structure. The cerium-based MOF powder has a complete crystal morphology and an ordered hierarchical porous structure. Its topological framework is consistent with that of UiO-66, and it exhibits complete crystal structure without collapse or particle agglomeration.

[0059] (2) Mild impregnation of phosphoric acid with low concentration: A 0.1 mol / L phosphoric acid aqueous solution was prepared as the impregnation solution. 50 mg of the cerium-based MOF powder obtained in step (1) was uniformly dispersed in 10 mL of the impregnation solution. The mixture was stirred at 200 rpm at a constant temperature of 40 ℃ for 4 h. Under these mild conditions, phosphate ions slowly dissociated and were uniformly adsorbed into the pores and surface of the MOF particles. The low acidity and low ionic strength environment preserved the complete crystal structure of the MOF without framework collapse or crystal form damage. After impregnation, the mixture was centrifuged and dried at 80 ℃ to obtain phosphorus-loaded cerium-based MOF powder.

[0060] (3) Polydopamine coating modification: 1g of phosphorus-loaded cerium-based MOF powder obtained in step (2) was dispersed in 100 mL of Tris-HCl buffer solution with a pH of 8.5. Dopamine hydrochloride was then added, and the mass ratio of dopamine hydrochloride to phosphorus-loaded cerium-based MOF powder was controlled at 0.15:1. The mixture was stirred at room temperature at a speed of 200 rpm for 12 hours to complete uniform coating. After the reaction was completed, the mixture was centrifuged, washed thoroughly with deionized water, and dried at 60℃ to obtain PDA@phosphorus-containing cerium-based MOF composite precursor. The PDA@phosphorus-containing cerium-based MOF composite precursor was uniformly and completely covered by a polydopamine coating layer on the surface of MOF particles.

[0061] (4) High-temperature topological pyrolysis conversion: The composite precursor obtained in step (3) was placed in a tube furnace and heated to 800 °C at a heating rate of 2 °C / min under the protection of a high-purity nitrogen inert atmosphere, and calcined at a constant temperature for 3 h. After natural cooling in the furnace, it was taken out, lightly ground and sieved to obtain a UiO-66-like topological cerium-based MOF-derived CePO4 free radical scavenger with a complete core-shell structure, denoted as CePO4@C. The free radical scavenger has an integrated core-shell structure, with the core being the CePO4 active phase and the outer shell being a multi-level porous conductive carbon coating layer formed by high-temperature carbonization of polydopamine.

[0062] (5) Phase and structure characterization: The cerium-based MOF powder obtained in step (1) and the sample impregnated with phosphoric acid (0.1 mol / L) according to the method in step (2) were characterized by XRD. Figure 1 It was confirmed that only low-concentration phosphoric acid impregnation could completely preserve the UiO-66 topological framework structure, while excessively high concentrations led to a significant decrease or even collapse of the characteristic peak intensities. The final product obtained in step (4) was characterized by XRD. Figure 2 The results were in perfect agreement with the CePO4 standard PDF card, confirming the successful preparation of the target product CePO4. Raman spectroscopy of the obtained target product showed that the intensity ratio at the D and G bands demonstrated conductivity. Figure 3 Impedance testing also showed that it is an electronic conductor. Figure 4 ).

[0063] (6) Microscopic morphological observation: The cerium-based MOF powder obtained in step (1) was observed using scanning electron microscopy (SEM). Figure 5 ) and 0.1 mol / L phosphoric acid impregnated phosphorus-loaded cerium-based MOF powder samples ( Figure 6 The results showed that the morphology remained almost identical before and after impregnation. (TEM image of CePO4@C) Figure 7 Clear lattice fringes can be seen, indicating that the crystal structure has been preserved.

[0064] (7) Free radical scavenging performance test (UV-Vis colorimetric method): The UV-Vis spectrophotometry method was used to target the hydroxyl radicals generated by the Fenton system. A colorimetric reagent was added, and the residual intensity of the characteristic colorimetric peaks was tested after 60 min of reaction for the blank control group (H2O2 and colorimetric reagent only, no scavenger), the cerium dioxide control group (addition amount 1.0 mg / mL), and the CePO4@C addition groups (1.0 mg / mL, 0.7 mg / mL, 0.4 mg / mL) obtained in step (4) of Example 1 of this invention. Figures 8-12 The results showed that the characteristic peak of the blank control group decreased to 36% of the initial value; the cerium dioxide group decreased to 68%; and the CePO4@C material at 1.0 mg / mL decreased to 75%, 0.7 mg / mL to 74%, and 0.4 mg / mL to 60%. This concentration gradient experiment confirmed that the CePO4@C of the present invention has a scavenging effect at different concentrations.

[0065] (8) Electrochemical testing using a rotating ring-disk electrode (RRDE): The target material was mixed with a commercial Pt / C catalyst at different mass ratios (5 wt%, 10 wt%) to prepare a working electrode. Linear sweep voltammetry (LSV) tests were performed in an acidic medium (0.1 mol / L HClO4), using CePO4@C obtained in step (4) of Example 1. A blank control group (pure Pt / C, without scavenger) was also set up as a control. Figure 13 The results showed that, using the half-wave potential of the blank control group (pure Pt / C) as a baseline, the addition of 5 wt% CePO4@C significantly shifted the half-wave potential positively, and the limiting current density was significantly improved. The addition of 10 wt% CePO4@C also improved the half-wave potential relative to pure Pt / C, but the effect was slightly less than that of 5 wt% CePO4@C. These results indicate that the CePO4@C of this invention can effectively improve the reaction kinetics at the catalyst layer interface and promote the efficiency of the oxygen reduction reaction.

[0066] (9) Electrochemical impedance spectroscopy (EIS) test: Using the CePO4@C obtained in step (4) of Example 1, electrochemical impedance spectroscopy was performed on electrodes containing pure CePO4@C (without Pt / C), a blank control group (pure Pt / C), and electrodes with 5 wt% and 10 wt% CePO4@C added, under the same RRDE test system. Figure 14The results showed that the impedance response of pure CePO4@C was slightly greater than that of the blank control group, which is attributed to its intrinsic physical barrier effect as a non-electroactive material. The impedance spectra of the blank group, the 5 wt% addition group, and the 10 wt% addition group were basically the same, indicating that the introduction of the target material did not increase the additional interfacial charge transfer impedance. This confirms that the conductive carbon coating shell effectively ensures the continuity of the electron transport channel of the catalyst layer and overcomes the technical defect of the significant increase in impedance caused by the doping of traditional rare earth antioxidant materials.

[0067] (10) Calculation of electron transfer number: Based on the ring disk current data from the RRDE test, calculate the number of oxygen reduction electrons transferred at each electrode. Figure 15 The results showed that the electron transfer number in the blank control group (pure Pt / C) was approximately 3.94; after adding 5 wt% of the target material, it increased to approximately 3.96; and after adding 10 wt% of the target material, it increased to approximately 3.95. These results indicate that the introduction of the target material of this invention can effectively promote the four-electron pathway of the oxygen reduction reaction, suppress the generation of H2O2 from the two-electron side reaction, reduce the source of free radical generation from the outset, and further improve the long-term operational stability of the membrane electrode.

[0068] (11) Analysis of the mechanism of antioxidant action: Based on the above phase structure characterization, free radical scavenging experiments, and electrochemical test results, the antioxidant mechanism of the core-shell structure CePO4@C constructed in this invention can be summarized as follows: Figure 16 ): Core cerium phosphate via Ce 3+ / Ce 4+ / Reversible valence cycle continuously quenches OH and O2 - • Free radicals; The fully preserved MOF-derived hierarchical channels provide ample exposure of active sites and material transport channels; The outer conductive carbon coating layer simultaneously realizes the construction of electronic conduction pathways and acid barrier protection, and the three work together to achieve long-term antioxidant protection of the membrane electrode.

[0069] Example 2: Preparation of cerium phosphate free radical scavengers by impregnation with different phosphoric acid concentrations The only difference between this embodiment and Example 1 is the concentration of the phosphoric acid aqueous solution in step (2). The rest of the MOF synthesis process, dopamine coating parameters, calcination conditions and electrode testing conditions are strictly consistent with those of Example 1.

[0070] Specifically, a 0.2 mol / L phosphoric acid aqueous solution was prepared, and cerium-based MOF powder was uniformly dispersed in it. The mixture was then stirred and impregnated at a constant temperature of 40 °C for 4 h. The sample was characterized by XRD. Figure 1The phase structure of the product obtained in this embodiment is basically the same as that in Example 1, indicating that within this concentration range, the mild impregnation process has an excellent protective effect on the MOF framework structure. This embodiment further verifies that within the low-concentration phosphoric acid impregnation range defined by this invention, the preparation process has good universality and structural controllability.

[0071] Example 3: Verification of the lower limit of calcination temperature The only difference between this embodiment and Example 1 is that in step (4), the calcination temperature is 600℃, the heating rate is 2℃ / min, and the temperature is maintained for 3 hours. All other parameters are completely consistent with Example 1. The XRD diffraction pattern of the obtained product ( Figure 17 The Raman spectroscopy showed that the characteristic peaks of CePO4 were intact, confirming that 600℃ was sufficient to drive the solid-state reaction between phosphate and Ce sites to form crystalline CePO4; the Raman spectroscopy showed the coexistence of the D and G peaks. Figure 18 This confirms the effective formation of the carbon coating layer.

[0072] Comparative Example 1 (CeO2 control) In this comparative example, the UiO-66 topological cerium-based MOF from Example 1 was directly sintered in air at 350°C for 2 hours to obtain the control sample CeO2 as a free radical scavenger. A catalyst layer and membrane electrode were prepared with the same doping ratio of 2 wt%. All other assembly and testing parameters remained consistent with Example 1. UV-Vis free radical scavenging test (…) Figure 9 The results show that its scavenging efficiency is lower than that of CePO4@C in this invention. The reasons are as follows: First, cerium is atomically dispersed in the MOF framework, and after topological transformation, the active sites are highly exposed on the inner walls of the hierarchical channels, effectively avoiding the defect of CeO2 nanoparticles easily agglomerating and burying the active sites; Second, the CePO4 active phase has far superior structural stability to CeO2 in the acidic Fenton system, with an extremely low cerium ion dissolution rate, ensuring continuous valence cycling without "consumption-induced deactivation" within the 60-minute test cycle; Third, the outer conductive carbon shell has a surface enrichment effect on free radicals, accelerating their diffusion and capture to the active core. These multiple synergistic mechanisms enable the material of this invention to outperform CeO2 in terms of cumulative scavenging volume.

[0073] Comparative Example 2: Preparation of Cerium Phosphate Free Radical Scavengers by Impregnation with Different Phosphate Concentrations The only difference between this embodiment and Example 1 is the concentration of the phosphoric acid aqueous solution in step (2). The rest of the MOF synthesis process, dopamine coating parameters, calcination conditions and electrode testing conditions are strictly consistent with those of Example 1.

[0074] Specifically, a 0.5 mol / L phosphoric acid aqueous solution was prepared, and the cerium-based MOF powder was uniformly dispersed in it. The mixture was then stirred and impregnated at a constant temperature of 40 °C for 4 h. XRD and SEM characterization showed that the XRD pattern of the product obtained in this example had lost its original characteristic peaks, indicating that excessive acid concentration damaged the phase structure. Figure 1 SEM also showed incomplete morphology. This comparative example further verifies that beyond the low-concentration phosphoric acid impregnation range defined in this invention, the material's microstructure will be damaged due to excessive acid concentration.

[0075] Comparative Example 3: Incomplete carbonization due to excessively low calcination temperature The only difference between this comparative example and Example 1 is that in step (4), the calcination temperature is 500℃, the heating rate is 2℃ / min, and the temperature is maintained for 3 hours. All other parameters are completely consistent with Example 1. The Raman spectrum of the obtained product shows that the characteristic peak signal of carbon D / G is very weak ( Figure 19 This confirms that 500℃ is insufficient to fully carbonize polydopamine into a continuous conductive network, resulting in insufficient conductivity of the material.

[0076] Comprehensive explanation of comparative testing: The sample from Example 1 and the sample from Comparative Example 1 (CeO2) were simultaneously subjected to XRD phase characterization and UV-Vis free radical scavenging quantitative test.

[0077] Comprehensive test results fully demonstrate that the low-concentration phosphoric acid mild impregnation process employed in this invention can completely preserve the crystal structure of the UiO-66 MOF-like structure. Combined with the hierarchical porous conductive carbon coating layer formed by polydopamine carbonization, it simultaneously achieves high-activity site exposure, efficient free radical scavenging, excellent acid resistance, and conductive stability. RRDE testing further confirms that the introduction of this material significantly improves the half-wave potential and limiting current density, does not increase interfacial impedance, and promotes the four-electron oxygen reduction pathway. The CePO4 free radical scavenger prepared by this invention exhibits superior overall antioxidant performance, structural stability, and electrochemical durability compared to commercial CeO2 and blank reference samples, fully demonstrating the innovation and technological advancement of the structural design and process route of this invention.

[0078] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger, characterized in that, Includes the following steps: (1) Synthesis of UiO-66 topological cerium-based MOF precursor: Soluble cerium salt and terephthalic acid were dissolved in deionized water, and the reaction was carried out at room temperature with magnetic stirring. After centrifugation, washing, and vacuum drying, cerium-based MOF powder with complete morphology and regular crystal form was obtained. (2) Low-concentration phosphoric acid mild impregnation of phosphoric acid: using a dilute aqueous solution of phosphoric acid as the impregnation liquid, the cerium-based MOF powder obtained in step (1) is dispersed in the impregnation liquid and impregnated by low-temperature constant temperature stirring. After impregnation, the powder is centrifuged and dried to obtain phosphorus-loaded cerium-based MOF powder. (3) Polydopamine coating modification: The phosphorus-loaded cerium-based MOF powder obtained in step (2) is dispersed in Tris-HCl weakly alkaline buffer solution, and then dopamine hydrochloride is added and stirred at room temperature to complete uniform coating. After centrifugation, washing with water and drying, PDA@phosphorus-containing cerium-based MOF composite precursor is obtained. (4) High-temperature topological pyrolysis conversion: The PDA@phosphorus-containing cerium-based MOF composite precursor obtained in step (3) is placed in a tube furnace, heated and calcined under an inert atmosphere, cooled, ground, and sieved to obtain the free radical scavenger.

2. The method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger according to claim 1, characterized in that, In step (1), the reaction temperature is 15-40℃ and the reaction time is 24-48 h; The molar ratio of soluble cerium salt to terephthalic acid is 3:

2. The soluble cerium salt and terephthalic acid are dissolved in deionized water, wherein the concentration of terephthalic acid is 0.1 mol / L and the concentration of soluble cerium salt is 0.15 mol / L. The soluble cerium salt is cerium ammonium nitrate. The cerium-based MOF powder has an ordered hierarchical porous structure, and its topological framework is consistent with that of UiO-66. It has a complete crystal form without collapse or particle agglomeration.

3. The method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger according to claim 1, characterized in that, In step (2), the impregnation temperature is 15-40 ℃, and the stirring impregnation time is 2-6 h; The concentration of the dilute aqueous solution of phosphoric acid is 0.1-0.5 mol / L, and the ratio of cerium-based MOF powder to impregnation solution is 1 g:(50-100) mL; In phosphorus-loaded cerium-based MOF powder, the phosphorus source is uniformly adsorbed inside the MOF channels without damaging the MOF crystal framework and morphology.

4. The method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger according to claim 1, characterized in that, In step (3), dopamine hydrochloride is added and stirred at room temperature for 4-12 hours; The mass ratio of dopamine hydrochloride to cerium phosphorus-loaded MOF powder is (0.05-0.3):1, and the ratio of cerium phosphorus-loaded MOF powder to Tris-HCl weakly alkaline buffer solution is 1 g : (50-100) mL.

5. The method for preparing a UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger according to claim 1, characterized in that, In step (4), the inert atmosphere is high-purity N2 or Ar, the heating rate is 2-5 ℃ / min, the calcination temperature is 600-800 ℃, and the calcination constant temperature holding time is 1-3 h.

6. A UiO-66-like topological cerium-based MOF-derived CePO4 free radical scavenger prepared by the preparation method according to any one of claims 1-5.

7. The UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger according to claim 6, characterized in that, The free radical scavenger has an integrated core-shell structure, with the core being the CePO4 active phase and the outer shell being a multi-level porous conductive carbon coating layer formed by high-temperature carbonization of polydopamine.

8. The application of the UiO-66 topological cerium-based MOF-derived CePO4 radical scavenger as described in claim 6 or 7 in a proton exchange membrane fuel cell.

9. The application of the UiO-66-like topological cerium-based MOF-derived CePO4 radical scavenger according to claim 8 in a proton exchange membrane fuel cell, characterized in that, The free radical scavenger is added to the catalyst layer slurry or proton exchange membrane casting solution at a doping ratio of 1wt%-10wt%.

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