Rare earth doped iron oxyhydroxide / ferric oxide-three-dimensional graphene composite material and preparation method thereof

Abstract

The invention discloses a rare earth doped iron oxyhydroxide / ferric oxide-three-dimensional graphene composite material and a preparation method thereof, the material is x% RE-FeOOH / Fe2O3 (at) 3D Gr, x is the molar ratio of rare earth ions to iron ions, the value range of x is 1-8.5, RE is one of rare earth elements Ce, La, Sm, Nd or Y, and RE is one of rare earth elements Fe, La, Sm, Nd or Y; the preparation method comprises the following steps: preparing a mixed solution A from an iron salt, a rare earth salt and an alkaline solution, adding a complexing agent into a graphene oxide solution to prepare a mixed solution B, performing ultrasonic treatment on the two solutions, mixing, adding a reducing agent, performing ultrasonic treatment again, and finally performing hydrothermal treatment, washing, freeze drying and low-temperature sintering. Rare earth ions can induce oxygen vacancies, increase active sites and promote ion transmission, the maximum mass specific capacitance of the material under 0.5 A / g reaches 1218.7 F / g, and the maximum capacitance retention rate of a flexible supercapacitor assembled by the material is 97% after 5000 times of charging and discharging.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of inorganic composite materials, and relates to a rare earth doped hydroxyl ferric oxide / ferroferric oxide-three-dimensional graphene composite material and a preparation method. BACKGROUND

[0002] Among the numerous electrode materials of supercapacitors, iron-based oxides or hydroxides are considered as ideal electrode materials due to their advantages such as multiple valence states, wide potential window in negative potential region, low toxicity, environmental friendliness, low cost, and abundant natural reserves. Fe2O3, as an important iron-based material, has become a research hotspot due to its advantages such as high theoretical capacitance, non-toxicity, and low cost. However, the low conductivity of Fe2O3 to some extent affects its electrochemical performance. FeOOH, as another important iron-based material, exists in multiple crystal forms. The crystal structure of FeOOH contains a large number of hydroxyl (-OH) groups, which not only enhance the hydrophilicity of the material but also provide rich active sites for the material, promoting the electrochemical reaction. However, the poor conductivity and easy aggregation of FeOOH during the charging and discharging process limit the effective diffusion of charges / ions, restricting its further application and greatly limiting its practical application.

[0003] In order to solve the problems of poor intrinsic conductivity and poor cycle stability of iron-based materials, they can be compounded with high-conductivity materials (such as graphene, carbon nanotubes) and oxygen vacancies are introduced to improve their conductivity. At the same time, the good resilience of three-dimensional graphene can relieve the volume effect during the cycle process after the iron-based material is compounded with graphene, thereby improving the cycle life. In addition, it is found that metal doping engineering can also improve the conductivity and electrochemical reaction activity of the electrode material, and excellent electrochemical performance can be obtained. However, the materials prepared at present still have room for improvement in terms of electrochemical performance, such as specific capacitance or cycle stability, and the performance and practicality of flexible supercapacitors still need to be improved in terms of mechanical properties. For example, Chinese invention patent CN111725003 A discloses a cubic iron-based hydroxyl oxide / graphene composite material for supercapacitors and a preparation method thereof. Metal iron source, Na2SO4, and GO are used as raw materials, and a hydrothermal method is used to prepare the composite material. According to electrochemical tests, the highest specific capacitance obtained is only 688 F / g at 0.5 A·g -1 Chinese invention patent CN120527164 A discloses an iron-doped flexible supercapacitor electrode, a preparation method, and a supercapacitor. First-order or second-order FeCl3 intercalated graphite is used as the main conductive filler, and conductive polymers such as PVDF and PANI are used as the bonding resin. After being mixed with NMP or DMF and sand milling, a uniform electrode slurry is prepared and coated on a flexible substrate (PET). Fe 3+Ion catalysis decomposes H₂O₂ to produce O₂, thereby forming a porous structure in situ within the electrode film. Electrochemical tests showed that at 0.5 A·g -1 The highest capacitance obtained at the current density is only 400 F / g, and the capacitance retention rate obtained after 10,000 cycles is 85%.

[0004] Therefore, it is necessary to develop a novel electrode material preparation method that balances high performance, low cost, and environmental friendliness, ultimately achieving a synergistic improvement in electrochemical and mechanical properties on flexible devices. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention aims to provide a rare-earth-doped iron hydroxyl oxide / ferric oxide-three-dimensional graphene composite material and its preparation method. The material is x% RE-FeOOH / Fe2O3@3D Gr, where x is the molar ratio of rare-earth ions to iron ions, ranging from 1 to 8.5. RE is one of the rare-earth elements Ce, La, Sm, Nd, or Y. The preparation method involves: preparing a mixture A with iron salt, rare-earth salt, and an alkaline solution; adding a complexing agent to a graphene oxide solution to prepare a mixture B; mixing the two solutions after ultrasonication; adding a reducing agent and ultrasonicating again; and finally, hydrothermal treatment, washing, freeze-drying, and low-temperature sintering. Rare-earth ions can induce oxygen vacancies, increase active sites, and promote ion transport. This material exhibits a maximum specific capacitance of 1218.7 F / g at 0.5 A / g, and the flexible supercapacitor assembled with it retains up to 97% of its capacitance after 5000 charge-discharge cycles.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A rare earth-doped iron hydroxyl oxide / ferric oxide-three-dimensional graphene composite material, wherein the rare earth-doped iron hydroxyl oxide / ferric oxide-three-dimensional graphene composite material is x% RE-FeOOH / Fe2O3@3D Gr, wherein: x is the molar ratio of rare earth ions to iron ions, and the value of x ranges from 1 to 8.5, and RE is one of the rare earth elements Ce, La, Sm, Nd or Y.

[0007] This invention also provides a method for preparing rare earth-doped iron hydroxyl oxide / ferric oxide-three-dimensional graphene composite material, which is carried out in the following order: S1. Dissolve iron salt and rare earth salt together in deionized water and stir for 15-30 min. Then add alkaline solution and continue stirring for 15-30 min to obtain mixed solution A. S2. Add graphene oxide solution to a beaker, then add complexing agent to graphene oxide solution and stir for 15-20 min to obtain mixed solution B; S3. Mixed solution A and mixed solution B are ultrasonically dispersed for 20 min respectively, then the two are mixed evenly, a reducing agent is added to the mixture, and after thorough stirring, it is ultrasonically dispersed again. The resulting mixture is transferred to a reaction vessel for hydrothermal reaction. After the reaction is completed, the product is washed to remove impurities and then freeze-dried to obtain x% RE-FeOOH@3D Gr composite material. S4. Place the x% RE-FeOOH@3D Gr composite material in a muffle furnace and sinter at 280~370 ℃ for 2 h to obtain the x%RE-FeOOH / Fe2O3@3D Gr ternary composite material.

[0008] As a limitation of the preparation method of the present invention, in step S1, the iron salt is one of Fe(NO3)3·9H2O, FeCl3·6H2O or Fe2(SO4)3·9H2O; the rare earth salt is one of Ce(NO3)3·6H2O, La(NO3)3·6H2O, Sm(NO3)3·6H2O, Nd(NO3)3·6H2O or Y(NO3)3·6H2O.

[0009] As another limitation of the preparation method of the present invention, in step S1, the mass ratio of the iron salt to the rare earth salt is 1:(0.011~0.136).

[0010] In this invention, the proportion and type of rare earth salt doping are crucial. Rare earth ions have much larger radii than iron ions, and this size difference leads to localized lattice expansion. To maintain the charge balance of the system, the lattice self-regulates through two mechanisms: firstly, the strain energy generated by size mismatch lowers the formation energy of oxygen vacancies, resulting in a significant increase in the equilibrium concentration of oxygen vacancies at a given temperature; secondly, some Fe... 3+ It will be reduced to Fe by capturing electrons. 2 + To compensate for the charge, while Fe 2+ The coordination stability is better than Fe 3+ Worse still, this instability further promotes the detachment of neighboring oxygen ions, resulting in the formation of more oxygen vacancies. These oxygen vacancies provide more electrochemical reaction sites and conductive channels. Therefore, the proportion of rare earth salt doping affects the number of oxygen vacancies formed in x% RE-FeOOH / Fe2O3@3D Gr composites, which in turn affects the capacitance and cycling stability.

[0011] As a third limitation of the preparation method of the present invention, in step S1, the alkaline solution is a NaOH solution or KOH solution with a concentration of 2 mol / L, and the mass-volume ratio of the iron salt to the alkaline solution is 0.6:(2~8) g / mL.

[0012] As a fourth limitation of the preparation method of the present invention, in step S2, the graphene oxide solution is an aqueous solution formed by dispersing graphene oxide in deionized water, with a concentration of 10 mg / mL; the mass-to-volume ratio of the iron salt to the graphene oxide solution is 0.6:(6~20) g / mL.

[0013] As a fifth limitation of the preparation method of the present invention, in step S2, the complexing agent is one of polyethylene glycol-2000, polyethylene glycol-1000 or polyvinylpyrrolidone; the mass ratio of the iron salt to the complexing agent is 0.6:(0.147~0.6).

[0014] As a sixth limitation of the preparation method of the present invention, in step S3, the reducing agent is one of ascorbic acid, sodium ascorbate or hydrazine hydrate, and the mass ratio of the iron salt to the reducing agent is 0.6:(0.3~2.06).

[0015] As a seventh limitation of the preparation method of the present invention, in step S3, the temperature of the hydrothermal reaction is 150~200℃ and the time is 10~20 h.

[0016] As an eighth limitation of the preparation method of the present invention, in step S3, the freeze-drying process is carried out according to the following procedure: (a) First stage: Cooling from room temperature to -80 to -60 ℃ at a cooling rate of 0.5 ℃ / s, and holding frozen for 8 to 14 h; (b) Second stage: First, the temperature is increased from -80~-60 ℃ to 0 ℃ at a heating rate of 0.3 ℃ / s, and then increased from 0 ℃ to 8~20 ℃ at a heating rate of 0.1 ℃ / s, and held for 12~24 h.

[0017] In this invention, the freeze-drying process ensures the removal of water while preserving the integrity of the material's morphology. In the first stage, the temperature is lowered from room temperature to -80 to -60°C at a rate of 0.5°C / s, causing the liquid water in the sample to transform into solid ice. Holding this temperature for 8 to 14 hours ensures uniform freezing temperature and complete crystallization. In the second stage, the temperature is first raised from -80 to -60°C to 0°C at a rate of 0.3°C / s to allow the solid ice to sublimate under vacuum, directly transforming into water vapor while preserving the original 3D Gr structure. Then, the temperature is raised from 0°C to 8 to 20°C at a rate of 0.1°C / s to remove any remaining bound water. Holding this temperature for 12 to 24 hours ensures the sample is thoroughly dried at this temperature, facilitating subsequent storage and use.

[0018] As is well known, the morphology and structure of materials are closely related to their properties. The product prepared in this invention can achieve the construction and stability of three-dimensional porous graphene through hydrothermal and low-temperature sintering. During the morphology construction process, graphene oxide (GO) sheets self-assemble into a three-dimensional interconnected network, solving the problem of two-dimensional sheet stacking and providing abundant ion adsorption sites. In addition, the addition of a reducing agent before hydrothermal treatment deeply reduces GO to rGO, which can significantly improve conductivity; it can also reduce the stability of some Fe-O bonds in FeOOH and Fe2O3, making it easier to form oxygen vacancies. This provides more electrochemical reaction sites and conductive channels. Under the action of ultrasound, the iron / rare earth precursor solution is mixed with GO, so that metal ions are uniformly anchored on the oxygen-containing groups of GO, allowing FeOOH to grow between rGO sheets during hydrothermal treatment, effectively inhibiting nanoparticle aggregation. At the same time, the morphology regulation and dispersing effects of the complexing agent ensure that RE-FeOOH nanosheets are uniformly dispersed on the 3D rGO framework while preventing the graphene nanosheets from recombining. During subsequent low-temperature sintering, the removal of structural hydroxyl groups under thermal drive leads to the collapse of the metastable aqueous structure. Subsequently, some ferro-oxygen atoms rearrange into a more thermodynamically stable and denser Fe2O3 lattice. This ultimately forms a FeOOH / Fe2O3 heterostructure. The atomic structure at the heterojunction interface typically exhibits unsaturated coordination and lattice distortion. These sites are highly active catalytic sites and charge storage sites, which can significantly increase the effective electrochemical active surface area.

[0019] During the reaction, the strongly alkaline solution provides OH-. - Precisely regulate Fe 3+ The hydrolysis rate is optimized to prevent rapid precipitation and the formation of amorphous impurities. Furthermore, alkaline substances can etch graphene, creating more nanopores and thus increasing the specific surface area of ​​the composite material. Three-dimensional porous graphene facilitates ion transport in the composite material, primarily because the nanopores shorten the ion transport distance between graphene sheets. Ultimately, combining rare-earth-doped electronic structure modulation with the spatial structure design of 3D rGO breaks through the performance limits of single components.

[0020] The above-mentioned technical solution of the present invention is a whole in which each step is closely related and mutually influential, and together they determine the morphological characteristics and performance of the product.

[0021] The above technical solution has the following advantages or beneficial effects: 1. This invention uses graphene oxide and soluble iron salts as precursors to rapidly prepare x% RE-FeOOH / Fe2O3@3D Gr composite materials doped with rare earth elements through hydrothermal and low-temperature sintering methods. The process is simple, efficient, and low-cost. 2. The x% RE-FeOOH / Fe2O3@3D Gr composite material prepared by this invention has a stable structure, adjustable loading rate, and good conductivity. At a current density of 0.5 A / g, the mass specific capacitance can reach up to 1218.7 F / g, and it can be directly used as an electrode material for supercapacitors. 3. The supercapacitor assembled using the x% RE-FeOOH / Fe2O3@3D Gr composite material prepared by this invention can achieve a capacitance retention rate of up to 97% after 5000 charge-discharge cycles, and has broad application prospects.

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0023] Figure 1 The images shown are scanning electron microscope (SEM) images of the 5% Nd-FeOOH / Fe2O3@3D Gr composite material prepared in Example 1 of the present invention, wherein: (a), (b), and (c) are morphology images of the 5% Nd-FeOOH / Fe2O3@3D Gr composite material at different magnifications, respectively. Figure 2 The electrochemical test diagrams are as follows: (a) is a cyclic voltammetry curve, and (b) is a constant current charge-discharge curve. Figure 3 The EPR test image is shown for the 5% Nd-FeOOH / Fe2O3@3D Gr composite material prepared in Example 1 of this invention. Figure 4 The diagrams above show the flexible supercapacitor assembled from the 5% Nd-FeOOH / Fe2O3@3D Gr composite material prepared in Example 1 of this invention, placed under different weights and at different bending angles. (a) shows the original state of the assembled flexible capacitor; (b) to (e) show the state of the assembled flexible capacitor with 10 g, 20 g, 50 g, and 100 g weights respectively; and (f) to (i) show the state of the capacitor after bending at 0... ° 30 ° 45 ° 90 ° State diagram; Figure 5 Comparison of cyclic voltammetry curves of a flexible supercapacitor assembled from 5% Nd-FeOOH / Fe2O3@3D Gr composite material prepared in Example 1 of this invention, placed under different weights and at different bending angles at a scan rate of 100 mV / s. Figure 6This is a comparison chart of the constant current charge-discharge curves of the composite materials prepared in Examples 1, 2, 6 and Comparative Example 2 of the present invention. Detailed Implementation

[0024] The following embodiments are merely some, not all, of the embodiments of the present invention. Therefore, the detailed descriptions of the embodiments provided below are not intended to limit the scope of the claimed invention, but merely to illustrate selected 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.

[0025] In this invention, unless otherwise specified, all equipment and raw materials are commercially available or commonly used in the industry. The methods described in the following embodiments are conventional methods in the art, unless otherwise specified.

[0026] This embodiment prepares a 5% Nd-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g Fe(NO3)3·9H2O and 0.03239 g Nd(NO3)3·6H2O in deionized water and stir for 15 min. Then add 5 mL of 2 mol / L NaOH solution and continue stirring for 15 min to obtain mixed solution A. S2. Add 10 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.27 g of polyethylene glycol-2000 to the graphene oxide solution and stir for 15 min to obtain mixed solution B. S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 1 mL of hydrazine hydrate was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 200 ℃ for 12 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -60 ℃ at a cooling rate of 0.5 ℃ / s and held for 12 h. Then, the temperature was raised from -60 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 10 ℃ at a rate of 0.1 ℃ / s and held for 12 h to obtain 5% Nd-FeOOH@3D Gr composite material. S4. Place the 5% Nd-FeOOH@3D Gr composite material in a muffle furnace and sinter at 340 °C for 2 h to obtain the 5% Nd-FeOOH / Fe2O3@3D Gr ternary composite material.

[0027] The sintered product was ground in a mortar and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce a 5% Nd-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the 5% Nd-FeOOH / Fe2O3@3D Gr composite material was 1218.7 F / g.

[0028] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, Nd-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 5% Nd-FeOOH / Fe2O3@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10g PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 97%.

[0029] Figures 1-3 The images shown are the scanning electron microscope image, cyclic voltammetry curve, constant current charge-discharge curve, and EPR test image of this embodiment. Figure 1 Images (a) and (b) at low magnification show that FeOOH / Fe2O3, after low-temperature sintering, forms a unique flower-like heterostructure growing on the 3D rGO surface, achieving a large specific surface area. Figure 1 (c) clearly shows that these nanoflowers are composed of high-purity, well-crystallinity, and uniformly sized FeOOH / Fe2O3 nanocubes and short nanorods, obtained with the assistance of the surfactant polyethylene glycol-2000. This results in a large specific surface area and a relatively dense arrangement. This indicates that the three-dimensional graphene not only provides good support but also significantly reduces the aggregation of FeOOH / Fe2O3 nanoblocks and short nanorods, providing more active contact area. By observing the cyclic voltammetry scanned at rates in the range of 2–100 mV / s, as shown... Figure 2 In (a), it can be seen that the response current increases with increasing scan rate, indicating that the composite material is beneficial for rapid and reversible redox reactions. Furthermore, with increasing scan rate, the magnitude of the positive shift of the oxidation peak and the negative shift of the reduction peak in the CV curve decreases, indicating that the composite material has low internal resistance and less severe polarization. During constant current charge-discharge... Figure 2In (b), the constant current charge-discharge curves all have obvious plateaus, which means that the capacitance of the composite material is mainly controlled by the redox reaction. Moreover, the constant current charge-discharge curves also show symmetrical characteristics, which indicates that the composite material has good electrochemical capacitance behavior and good reversibility. Figure 3 This is the EPR test pattern of the 5% Nd-FeOOH / Fe2O3@3D Gr composite material prepared in this embodiment. The pattern clearly shows a symmetrical EPR signal peak at g=2.001, indicating the presence of oxygen vacancies in the crystal lattice.

[0030] Figure 4 This is a schematic diagram of the flexible supercapacitor assembled from the 5% Nd-FeOOH / Fe2O3@3D Gr composite material prepared in this embodiment under different weights and bending angles. Figure 5 This is a comparison chart of cv curve changes at a scan rate of 100 mV / s. Figure 4 Figure (a) shows the initial state of the assembled flexible capacitor. Figure 4 Images (b) through (e) show the placement of 10 g, 20 g, 50 g, and 100 g weights onto the assembled flexible capacitor, respectively. Figure 4 (f)~(i) represent the bending 0. ° 30 ° 45 ° 90 ° The image. From Figure 5 As can be seen from (a) and (b), the CV curve did not change significantly under different weights and different bending angles. The weights and bending angles showed similar capacitive behavior, indicating a good combination of flexibility and capacitive behavior.

[0031] Example 2 This embodiment prepares a 5% Sm-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g Fe(NO3)3·9H2O and 0.03255 g Sm(NO3)3·6H2O in deionized water and stir for 20 min. Then add 5 mL of 2 mol / L KOH solution and continue stirring for 15 min to obtain mixed solution A. S2. Add 10 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.27 g of polyethylene glycol-2000 to the graphene oxide solution and stir for 20 min to obtain mixed solution B. S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 1 mL of hydrazine hydrate was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 180 ℃ for 10 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -60 ℃ at a cooling rate of 0.5 ℃ / s and held for 12 h. Then, the temperature was raised from -60 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 10 ℃ at a rate of 0.1 ℃ / s and held for 12 h to obtain 5% Sm-FeOOH@3D Gr composite material. S4. Place the 5% Sm-FeOOH@3D Gr composite material in a muffle furnace and sinter at 360 ℃ for 2 h to obtain the 5% Sm-FeOOH / Fe2O3@3D Gr ternary composite material.

[0032] The sintered product was poured into a mortar and ground, and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce a 5% Sm-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the composite material was 1033.5 F / g.

[0033] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, Sm-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 5% Sm-FeOOH / Fe2O3@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10g PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 94%.

[0034] Example 3 This embodiment prepares a 4% La-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g Fe2(SO4)3·9H2O and 0.03736 g La(NO3)3·6H2O in deionized water and stir for 15 min. Then add 3 mL of 2 mol / L NaOH solution and continue stirring for 15 min to obtain mixed solution A. S2. Add 10 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.147 g of polyethylene glycol-1000 to the graphene oxide solution and stir for 15 min to obtain mixed solution B. S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 0.4 g of ascorbic acid was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 150 ℃ for 20 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -70 ℃ at a cooling rate of 0.5 ℃ / s and held for 14 h. Then, the temperature was raised from -70 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 15 ℃ at a rate of 0.1 ℃ / s and held for 17 h to obtain 4% La-FeOOH@3D Gr composite material. S4. Place the 4% La-FeOOH@3D Gr composite material in a muffle furnace and sinter at 280 °C for 2 h to obtain the 4% La-FeOOH / Fe2O3@3D Gr ternary composite material.

[0035] The sintered product was poured into a mortar and ground, and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce a 4% La-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the 4% La-FeOOH / Fe2O3@3D Gr composite material was 892.5 F / g.

[0036] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, La-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 4% La-FeOOH / Fe2O3@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10g PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 92%.

[0037] Example 4 This embodiment prepares an 8.5% La-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g FeCl3·6H2O and 0.08170 g La(NO3)3·6H2O in deionized water and stir for 30 min. Then add 8 mL of 2 mol / L NaOH solution and continue stirring for 30 min to obtain mixed solution A. S2. Add 20 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.6 g of polyvinylpyrrolidone to the graphene oxide solution and stir for 20 min to obtain mixed solution B. S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 2 mL of hydrazine hydrate was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 200 ℃ for 12 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -60 ℃ at a cooling rate of 0.5 ℃ / s and held for 10 h. Then, the temperature was raised from -60 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 20 ℃ at a rate of 0.1 ℃ / s and held for 24 h to obtain 8.5% La-FeOOH@3D Gr composite material. S4. Place the 8.5% La-FeOOH@3D Gr composite material in a muffle furnace and sinter at 330 °C for 2 h to obtain the 8.5% La-FeOOH / Fe2O3@3D Gr ternary composite material.

[0038] The sintered product was ground in a mortar and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce an 8.5% La-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the 8.5% La-FeOOH / Fe2O3@3D Gr composite material was 863.4 F / g.

[0039] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, La-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 8.5% La-FeOOH@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10 g of PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 92%.

[0040] Example 5 This embodiment prepares a 1% Sm-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g Fe(NO3)3·9H2O and 0.00651 g Sm(NO3)3·6H2O in deionized water and stir for 15 min. Then add 2 mL of 2 mol / L NaOH solution and continue stirring for 20 min to obtain mixed solution A. S2. Add 6 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.2 g of polyethylene glycol-2000 to the graphene oxide solution and stir for 18 min to obtain mixed solution B; S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 0.6 mL of hydrazine hydrate was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 180 ℃ for 12 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -80 ℃ at a cooling rate of 0.5 ℃ / s and held for 8 h. Then, the temperature was raised from -80 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 8 ℃ at a rate of 0.1 ℃ / s and held for 24 h to obtain 1% Sm-FeOOH@3D Gr composite material. S4. Place the 1% Sm-FeOOH@3D Gr composite material in a muffle furnace and sinter at 280 °C for 2 h to obtain the 1% Sm-FeOOH / Fe2O3@3D Gr ternary composite material.

[0041] The sintered product was ground in a mortar and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce a 1% Sm-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the 1% Sm-FeOOH / Fe2O3@3D Gr composite material was 891.4 F / g.

[0042] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, Sm-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 1% Sm-FeOOH / Fe2O3@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10g PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 93%.

[0043] Example 6 This embodiment prepares a 5% Ce-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g Fe(NO3)3·9H2O and 0.03192 g Ce(NO3)3·6H2O in deionized water and stir for 15 min. Then add 5 mL of 2 mol / L NaOH solution and continue stirring for 15 min to obtain mixed solution A. S2. Add 10 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.27 g of polyethylene glycol-2000 to the graphene oxide solution and stir for 15 min to obtain mixed solution B. S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 1 mL of hydrazine hydrate was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 150 ℃ for 12 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -60 ℃ at a cooling rate of 0.5 ℃ / s and held for 10 h. Then, the temperature was raised from -60 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 10 ℃ at a rate of 0.1 ℃ / s and held for 12 h to obtain 5% Ce-FeOOH@3D Gr composite material. S4. Place the 5% Ce-FeOOH@3D Gr composite material in a muffle furnace and sinter at 340 °C for 2 h to obtain the 5% Ce-FeOOH / Fe2O3@3D Gr ternary composite material.

[0044] The sintered product was poured into a mortar and ground, and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce a 5% Ce-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the 5% Ce-FeOOH / Fe2O3@3D Gr composite material was 907.2 F / g.

[0045] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, Ce-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 5% Ce-FeOOH / Fe2O3@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10g PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 94%.

[0046] Example 7 This embodiment prepares a 6% Y-FeOOH / Fe2O3@3D Gr ternary composite material, and the preparation process and steps are as follows: S1. Dissolve 0.6 g Fe(NO3)3·9H2O and 0.03379 g Y(NO3)3·6H2O in deionized water and stir for 15 min. Then add 5 mL of 2 mol / L KOH solution and continue stirring for 15 min to obtain mixed solution A. S2. Add 10 mL of graphene oxide solution with a concentration of 10 mg / mL to a beaker, then add 0.2 g of polyethylene glycol-2000 to the graphene oxide solution and stir for 15 min to obtain mixed solution B. S3. After sonicating mixed solution A and mixed solution B separately for 20 min, they were mixed together, and 0.3 g of sodium ascorbate was added. The mixture was stirred for 20 min and sonicated for 20 min. The mixture was then placed in a reaction vessel and reacted at 180 ℃ for 12 h. After the reaction was completed, the reaction vessel was cooled to room temperature and then washed with deionized water until neutral. Finally, the washed sample was placed in a freeze dryer for freeze drying. The freeze drying process was as follows: First, the temperature was lowered from room temperature to the target temperature of -80 ℃ at a cooling rate of 0.5 ℃ / s and held for 8 h. Then, the temperature was raised from -80 ℃ to 0 ℃ at a rate of 0.3 ℃ / s. Finally, the temperature was raised from 0 ℃ to 8 ℃ at a rate of 0.1 ℃ / s and held for 12 h to obtain 6% Y-FeOOH@3D Gr composite material. S4. Place the 6% Y-FeOOH@3D Gr composite material in a muffle furnace and sinter at 370 °C for 2 h to obtain the 6% Y-FeOOH / Fe2O3@3D Gr ternary composite material.

[0047] The sintered product was poured into a mortar and ground, and then placed in a vacuum dryer for drying. After drying, it was pressed onto a current collector to produce a 6% Y-FeOOH / Fe2O3@3D Gr composite electrode material. At a current density of 0.5 A / g, the specific capacitance of the 6% Y-FeOOH / Fe2O3@3D Gr composite material was 895.7 F / g.

[0048] Subsequently, an asymmetric flexible supercapacitor device with a sandwich structure, Y-FeOOH / Fe2O3@3D Gr‖3DGr, was assembled. 6% Y-FeOOH / Fe2O3@3D Gr ternary composite material was directly pressed onto a nickel sheet at 10 MPa using a tablet press without any pretreatment, serving as the negative electrode material. The positive electrode material was fabricated using 3D Gr in the same manner. PVA / KOH was used as the solid electrolyte (10g PVA powder was dissolved in 90 mL of deionized water, followed by the addition of 45 mL of 6 M KOH solution, and stirred at 90 °C). The resulting flexible supercapacitor was then tested, and after 5000 charge-discharge cycles, the capacitance retention was 93%.

[0049] Comparative Example To investigate the effects of different rare earth element contents and different synthesis processes on the performance of the product of this invention, the following comparative experiments were conducted. Different composite materials for flexible supercapacitors were prepared according to the following comparative examples: Comparative Example 1 This comparative example prepared an 8.5% Nd-FeOOH / Fe2O3@3D Gr ternary composite material. The preparation process was similar to that of Example 1, except that in step S1, 0.03239 g of Nd(NO3)3·6H2O was replaced with 0.05506 g of Nd(NO3)3·6H2O. All other steps and parameters were the same as in Example 1. At a current density of 0.5 A / g, the specific capacitance of the 8.5% Nd-FeOOH / Fe2O3@3D Gr composite material was 781.3 F / g. The asymmetric flexible supercapacitor assembled using the 8.5% Nd-FeOOH / Fe2O3@3D Gr ternary composite material (assembled using the same method as in Example 1) retained 89% of its capacitance after 5000 cycles.

[0050] The 8.5% Nd-FeOOH / Fe2O3@3D Gr composite electrode material prepared in this comparative example, compared with Example 1, increases the proportion of rare-earth neodymium doped to 8.5%. The aim is to explore the effect of higher neodymium dopant content on the structure and electrochemical performance of the electrode material. Results show that with increasing Nd doping... 3+ With the increase in Nd content, the conductivity of the electrode initially increased because of the Nd content. 3+ After doping, free electrons are generated in the FeOOH lattice. However, when the doping concentration exceeds 5%, the conductivity begins to decrease because the overdoped metal ions act as electron traps, increasing the internal resistance. Therefore, at a current density of 0.5 A / g, the specific capacitance of the 8.5% Nd-FeOOH@3D Gr composite material is only 861.3 F / g, which is lower than the specific capacitance when the doping concentration is 5%.

[0051] Comparative Example 2 This comparative example prepared a FeOOH / Fe2O3@3D Gr composite electrode material. The preparation process was similar to that of Example 1, except that Nd(NO3)3·6H2O was not added in step S1. The remaining steps and parameters were the same as in Example 1. At a current density of 0.5 A / g, the specific capacitance of the FeOOH@3D Gr composite material was 719.2 F / g. The asymmetric flexible supercapacitor assembled using the FeOOH / Fe2O3@3D Gr composite electrode material (assembly method was the same as in Example 1) retained 88% of its capacitance after 5000 cycles.

[0052] Compared to Example 1, the FeOOH / Fe2O3@3D Gr composite electrode material prepared in this comparative example was not doped with neodymium (NdO3), aiming to investigate the influence of NdO3 on the structure and electrochemical performance of the composite material. The results show that NdO3 doping can induce oxygen vacancy generation, thereby enhancing the electrochemical performance of FeOOH / Fe2O3@3D Gr. Furthermore, the unique electronic structure of rare earth ions can significantly improve the cycling stability of the supercapacitor.

[0053] Figure 6 The constant current charge-discharge curves of FeOOH / Fe2O3@3D Gr and FeOOH / Fe2O3@3D Gr with different rare earth element doping at a current density of 1 A / g show that the composite material has the largest capacitance when the doping amount is 5% Nd. This indicates that Nd doping generates more oxygen vacancies, which effectively enhances the conductivity of intrinsic electrons, thereby giving the electrode superior energy storage performance.

[0054] Comparative Example 3 This comparative example prepared a 5% Nd-Fe2O3@3D Gr composite electrode material. The preparation process was similar to that of Example 1, except that in step S4, the sintering temperature was increased to 400 °C. The remaining steps and parameters were the same as in Example 1. At a current density of 0.5 A / g, the specific capacitance of the 5% Nd-Fe2O3@3D Gr composite material was 815.6 F / g. The asymmetric flexible supercapacitor assembled using the 5% Nd-Fe2O3@3D Gr composite electrode material (assembly method was the same as in Example 1) retained 90% of its capacitance after 5000 cycles.

[0055] Compared to Example 1, the 5% Nd-Fe2O3@3D Gr composite electrode material prepared in this comparative example had its sintering temperature increased to 400 °C. The aim was to investigate the effect of sintering temperature on the structure and electrochemical performance of the composite material. The results showed that when the sintering temperature was increased to 400 °C, FeOOH was completely converted to Fe2O3. However, the electrochemical performance of Fe2O3 was weaker than that of FeOOH. Furthermore, due to the inability of a single component to construct a synergistic heterostructure, the electrochemical performance was significantly different from that of Example 1. This indicates that a suitable sintering temperature can improve the structure and electrochemical performance of the composite material.

[0056] Comparative Example 4 This comparative example prepared a 5% Nd-FeOOH@3D Gr composite electrode material. The preparation process was similar to that of Example 1, except that step S4 (sintering) was omitted. All other steps and parameters were the same as in Example 1. At a current density of 0.5 A / g, the specific capacitance of the 5% Nd-FeOOH@3D Gr composite material was 861.4 F / g. The asymmetric flexible supercapacitor assembled using the 5% Nd-FeOOH@3D Gr composite electrode material (assembly method same as in Example 1) retained 91% of its capacitance after 5000 cycles.

[0057] Compared to Example 1, the 5% Nd-FeOOH@3D Gr composite electrode material prepared in this comparative example was not sintered after freeze-drying. The purpose of this comparative example was to investigate the effect of sintering on the structure and electrochemical performance of the composite material. The results showed that when sintering was not performed, the product was FeOOH, and the capacitance of FeOOH was larger than that of Fe2O3. At the same time, due to the heterogeneous structure that cannot be constructed by a single component, the electrochemical performance was significantly different from that of Example 1. This indicates that low-temperature sintering can improve the structure and electrochemical performance of the composite material.

[0058] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A rare earth-doped ferric hydroxide / ferric oxide-three-dimensional graphene composite material, characterized in that, The rare earth-doped iron hydroxyl oxide / ferric oxide-three-dimensional graphene composite material is an x% RE-FeOOH / Fe2O3@3D Gr ternary composite material, wherein: x is the molar ratio of rare earth ions to iron ions, and the value of x ranges from 1 to 8.5, and RE is one of the rare earth elements Ce, La, Sm, Nd or Y.

2. The method for preparing a rare earth-doped ferric oxide / ferric oxide-three-dimensional graphene composite material according to claim 1, characterized in that, Follow these steps in sequence: S1. Dissolve iron salt and rare earth salt together in deionized water and stir for 15-30 min. Then add alkaline solution and continue stirring for 15-30 min to obtain mixed solution A. S2. Add graphene oxide solution to a beaker, then add complexing agent to graphene oxide solution and stir for 15-20 minutes to obtain mixed solution B; S3. Mixed solution A and mixed solution B are ultrasonically dispersed for 20 min respectively, then the two are mixed evenly, a reducing agent is added to the mixture, and after thorough stirring, it is ultrasonically dispersed again. The resulting mixture is transferred to a reaction vessel for hydrothermal reaction. After the reaction is completed, the product is washed to remove impurities and then freeze-dried to obtain x% RE-FeOOH@3D Gr composite material. S4. Place the x% RE-FeOOH@3D Gr composite material in a muffle furnace and sinter at 280~370 ℃ for 2 h to obtain the x% RE-FeOOH / Fe2O3@3D Gr ternary composite material.

3. The method for preparing a rare earth-doped hydroxyl iron oxide / ferric oxide-three-dimensional graphene-based flexible supercapacitor according to claim 2, characterized in that, In step S1, the iron salt is one of Fe(NO3)3·9H2O, FeCl3·6H2O, or Fe2(SO4)3·9H2O; the rare earth salt is one of Ce(NO3)3·6H2O, La(NO3)3·6H2O, Sm(NO3)3·6H2O, Nd(NO3)3·6H2O, or Y(NO3)3·6H2O.

4. The method for preparing a rare earth-doped ferric hydroxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S1, the mass ratio of the iron salt to the rare earth salt is 1:(0.011~0.136).

5. The method for preparing a rare earth-doped ferric oxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S1, the alkaline solution is a NaOH solution or KOH solution with a concentration of 2 mol / L, and the mass-to-volume ratio of the iron salt to the alkaline solution is 0.6:(2~8) g / mL.

6. The method for preparing a rare earth-doped ferric oxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S2, the graphene oxide solution is an aqueous solution formed by dispersing graphene oxide in deionized water, with a concentration of 10 mg / mL; the mass-to-volume ratio of the iron salt to the graphene oxide solution is 0.6:(6~20) g / mL.

7. The method for preparing a rare earth-doped ferric hydroxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S2, the complexing agent is one of polyethylene glycol-2000, polyethylene glycol-1000, or polyvinylpyrrolidone; the mass ratio of the iron salt to the complexing agent is 0.6:(0.147~0.6).

8. The method for preparing a rare earth-doped ferric oxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S3, the reducing agent is one of ascorbic acid, sodium ascorbate or hydrazine hydrate, and the mass ratio of the iron salt to the reducing agent is 0.6:(0.3~2.06).

9. The method for preparing a rare earth-doped ferric oxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S3, the temperature of the hydrothermal reaction is 150~200 ℃ and the time is 10~20h.

10. The method for preparing a rare earth-doped hydroxyl iron oxide / ferric oxide-three-dimensional graphene composite material according to claim 2, characterized in that, In step S3, the freeze-drying process is carried out according to the following procedure: (a) First stage: Cooling from room temperature to -80 to -60 ℃ at a cooling rate of 0.5 ℃ / s, and holding frozen for 8 to 14 h; (b) Second stage: First, the temperature is increased from -80~-60 ℃ to 0 ℃ at a rate of 0.3 ℃ / s, and then increased from 0 ℃ to 8~20 ℃ at a rate of 0.1 ℃ / s, and held for 12~24 h.

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