Three-dimensional porous nanocellulose-based supercapacitor electrode and integrated preparation method

Abstract

The invention provides a three-dimensional porous nanocellulose-based supercapacitor electrode and an integrated preparation method, and relates to the technical field of supercapacitor electrode preparation. Comprising a nanocellulose three-dimensional network skeleton; the conductive material is uniformly dispersed in the nanocellulose three-dimensional network framework and forms a chemical bonding interface with the cellulose fibers; the functional group is cross-linked to the three-dimensional network structure through a covalent bond; and the metal oxide nanoparticles are distributed on the surface of the nanocellulose three-dimensional network skeleton. In the structural design, a three-dimensional interpenetrating network with a compact surface layer, middle transition and internal penetration is constructed, and in the interface performance, a conductive material and a cellulose skeleton form a C-O-M chemical bonding interface; according to the preparation process, the steps of directional freezing, vacuum impregnation, microwave reduction and gas phase functionalization are integrated in the integrated process, and the production period of a traditional step-by-step process is shortened.

Description

Technical Field

[0001] The present invention relates to the technical field of supercapacitor electrode preparation, and specifically relates to a three-dimensional porous nanocellulose-based supercapacitor electrode and an integrated preparation method. Background Art

[0002] In recent years, three-dimensional porous nanocellulose-based supercapacitor electrodes have become a research hotspot in the field of energy storage materials due to their reproducibility, high specific surface area, and tunable pore structure. As supercapacitors evolve from activated carbon and metal oxides to composite and bio-based materials, nanocellulose, with its unique nanofiber network, rich surface active groups, and excellent mechanical properties, is considered an ideal substrate for building high-performance flexible electrodes. Current technologies mostly use freeze-drying to construct a three-dimensional porous skeleton, and achieve synergistic energy storage of double-layer and pseudocapacitance by compounding with conductive materials such as graphene and MXene. At the same time, a gradient pore size design is introduced to balance ion transport kinetics and active site density.

[0003] There are also some technical problems. The weak interfacial bonding caused by physical mixing severely limits the charge transfer efficiency. The disordered pores formed by the traditional freezing process lead to low electrolyte infiltration rate and mechanical strength, which leads to the collapse of the cycle structure. The step-by-step preparation process of impregnation-reduction-functionalization separation has problems such as high energy consumption, long cycle time and chemical pollution, which makes it difficult to meet the needs of large-scale production. These defects restrict the practical application of electrodes in high-energy-density, long-cycle-life supercapacitors. Summary of the Invention

[0004] The purpose of the present invention is to provide a three-dimensional porous nanocellulose-based supercapacitor electrode and an integrated preparation method to improve the charge transfer efficiency and mechanical strength of the supercapacitor electrode and shorten the preparation process cycle.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solutions: a three-dimensional porous nanocellulose-based supercapacitor electrode, comprising: a nanocellulose three-dimensional network skeleton with a porosity of 85% to 98% and a pore size distribution of 5nm to 500um; a conductive material uniformly dispersed in the nanocellulose three-dimensional network skeleton, with a mass proportion of 10% to 300% of the nanocellulose three-dimensional network skeleton, and forming a chemically bonded interface with the cellulose fibers; functional groups cross-linked to the three-dimensional network structure by covalent bonds, the functional groups having a density of 0.5 to 3.2 mmol / g, and being one or more of carboxyl groups, amino groups, and sulfonic acid groups; metal oxide nanoparticles distributed on the surface of the nanocellulose three-dimensional network skeleton, with a particle size of 2 to 50 nm and a mass proportion of 5% to 40% of the conductive material.

[0006] Preferably, the nanocellulose is one or more of wood pulp fiber, bacterial cellulose, and seaweed cellulose.

[0007] Preferably, the surface of the conductive material is plasma treated or chemically modified, and the conductive material is one or more of carbon nanotubes, graphene, MXene, polyaniline, and polypyrrole.

[0008] Preferably, the three-dimensional porous structure of the three-dimensional porous nanocellulose has a gradient pore size distribution, specifically:

[0009] Surface dense area: thickness 10-200 μm, pore diameter 5-50 nm, porosity 85-90%;

[0010] Transition gradient zone: thickness 300-800 μm, pore size 50-100 nm, porosity 90-95%;

[0011] Internal support area: thickness ≥1mm, pore diameter 100~500nm, porosity 95~98%.

[0012] Preferably, the metal oxide nanoparticles are one or more of transition metal oxides, rare earth oxides, and composite oxides.

[0013] Preferably, the invention further comprises additives, wherein the additives are one or more of a cross-linking agent, a toughening agent and a flame retardant.

[0014] Reference Figure 1 The present invention also provides an integrated preparation method for three-dimensional porous nanocellulose-based supercapacitor electrodes, which specifically includes:

[0015] S1. Nanocellulose slurry pretreatment: ultrasonically treat a nanocellulose dispersion with a mass ratio of 0.1-5% and adjust the pH to 3-11;

[0016] S2, directional freezing molding: freezing the nanocellulose dispersion obtained in step S1 at a temperature range of -20°C to -196°C at a rate of 5-50°C / min, and making the ice crystal growth direction form a controllable angle of 0-90° with the fiber axis to obtain a three-dimensional porous nanocellulose skeleton;

[0017] S3, vacuum impregnation compounding: under a vacuum degree of 10 to 100 kPa, the dispersion containing the conductive material is infiltrated into the three-dimensional porous nanocellulose skeleton obtained in step S2 for an impregnation time of 1 to 6 hours;

[0018] S4. Microwave-assisted reduction: In a microwave field of 300-1000W, a mixture of hydrogen and argon is introduced to promote interfacial bonding;

[0019] S5, vapor phase functionalization: introducing functional groups using chemical vapor deposition;

[0020] S6. Post-treatment to improve the mechanical strength, interface stability and electrochemical activity of the three-dimensional porous nanocellulose.

[0021] Preferably, a multi-stage freezing process is adopted in step S2, specifically comprising:

[0022] The first stage: -5℃~-20℃, rate 2~5℃ / min, forming micron-sized pores;

[0023] The second stage: -20℃~-80℃, speed 10~30℃ / min, forming submicron pores;

[0024] The third stage: -80℃~-196℃, rate>50℃ / min, fixing nano-scale pores.

[0025] Preferably, the microwave-assisted reduction in step S4 specifically comprises the following steps:

[0026] Pre-reduction stage: power 300-500W, time 5-10min, hydrogen concentration 10-30vol%;

[0027] Main reduction stage: power 800-1000W, time 10-20min, hydrogen concentration 50-80vol%;

[0028] Annealing stage: power 200-300W, time 3-5min, slow cooling under argon atmosphere.

[0029] Preferably, the post-processing in step S6 specifically includes:

[0030] Hot pressing: compress the capacitor electrodes to 20-50% of their original thickness at 80-150°C and 5-20 MPa pressure.

[0031] Surface coating: spraying polyvinylidene fluoride or polyurethane protective layer;

[0032] Activation treatment: Treat in CO2 supercritical fluid for 1 to 3 hours.

[0033] Compared with the existing technology, the beneficial effects of the present invention are: through the innovative integrated preparation method and structural design, the key technical difficulties existing in the current three-dimensional nanocellulose-based supercapacitor electrodes are effectively solved, and multi-dimensional performance improvement is achieved. Based on microwave-assisted chemical reduction and covalent bonding interface construction technology, the electron migration between the conductive material and the cellulose skeleton is accelerated, and the formation of COM (M = metal / carbon material) covalent bonds is induced to enhance the heterogeneous interface bonding strength. Through gradient freeze-drying and multi-level pore size control technology, a three-dimensional interpenetrating network with dense surface-middle transition-internal penetration is constructed, the ion diffusion coefficient is improved, while maintaining high porosity and tensile strength; in addition, the integrated preparation system shortens the production cycle of the traditional step-by-step process, providing a solution that is both environmentally friendly and economical for the development of high energy density, long life, and flexible supercapacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 The figure is a flow chart of the preparation method of the present invention. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," "the other end," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0037] The three-dimensional porous nanocellulose-based supercapacitor electrode includes: a three-dimensional nanocellulose network skeleton with a porosity of 85% to 98% and a pore size distribution of 5nm to 500um; a conductive material uniformly dispersed in the three-dimensional nanocellulose network skeleton, with a mass proportion of 10% to 300% of the three-dimensional nanocellulose network skeleton and forming a chemically bonded interface with the cellulose fibers; functional groups cross-linked to the three-dimensional network structure through covalent bonds, with a functional group density of 0.5 to 3.2 mmol / g, which are one or more of carboxyl, amino, and sulfonic acid groups; metal oxide nanoparticles distributed on the surface of the three-dimensional nanocellulose network skeleton, with a particle size of 2 to 50nm and a mass proportion of 5% to 40% of the conductive material.

[0038] Nanocellulose is one or more of wood pulp fiber, bacterial cellulose, and seaweed cellulose.

[0039] The surface of the conductive material is plasma treated or chemically modified, and the conductive material is one or more of carbon nanotubes, graphene, MXene, polyaniline, and polypyrrole.

[0040] The three-dimensional porous structure of three-dimensional porous nanocellulose has a gradient pore size distribution, specifically:

[0041] Surface dense area: thickness 10-200 μm, pore diameter 5-50 nm, porosity 85-90%;

[0042] Transition gradient zone: thickness 300-800 μm, pore size 50-100 nm, porosity 90-95%;

[0043] Internal support area: thickness ≥1mm, pore diameter 100~500nm, porosity 95~98%.

[0044] The metal oxide nanoparticles are one or more of transition metal oxides, rare earth oxides, and composite oxides.

[0045] The invention also includes additives, which are one or more of a cross-linking agent, a toughening agent, and a flame retardant.

[0046] The three-dimensional porous nanocellulose-based supercapacitor electrode provided by the present invention has a structural design that uses gradient freeze-drying and multi-level pore size control technology to construct a three-dimensional interpenetrating network with a dense surface layer, a transitional middle layer, and a continuous interior. This breaks through the ion transport bottleneck of the traditional disordered pore structure, increases the electrolyte infiltration rate from 70% to over 95%, and achieves an ion diffusion coefficient of 1.2×10 -6 cm 2 / s, while maintaining a high porosity of 98% and a mechanical strength of 25MPa, solving the difficult problem of compatibility between high specific surface area and structural stability;

[0047] In terms of interface performance, microwave-assisted chemical reduction and covalent bonding technology are used to form a COM chemical bonding interface between the conductive material and the cellulose skeleton, and the interface charge transfer resistance is reduced from >10Ω·cm in traditional physical mixing to 2 Down to 0.8Ω·cm 2 , the capacity retention rate at 10C rate is increased by 37%, and the cycle life is extended by more than 3 times;

[0048] The integrated process combines directional freezing, vacuum impregnation, microwave reduction, and gas-phase functionalization, shortening the traditional step-by-step process's 12-hour production cycle to 3.5 hours, reducing energy consumption by 40%, and avoiding the use of toxic reducing agents, meeting green manufacturing requirements.

[0049] In terms of comprehensive performance, the electrode specific surface area is increased to 2200m through the synergistic optimization of supercritical CO2 activation and surface coating. 2 / g, the pseudocapacitance contribution rate increased from 45% to 78%, the volume energy density reached 28Wh / L, and flexible adaptability with no performance degradation after 5000 bends was achieved.

[0050] The present invention also provides an integrated preparation method for three-dimensional porous nanocellulose-based supercapacitor electrodes, which specifically includes:

[0051] S1. Nanocellulose slurry pretreatment: ultrasonically treat a nanocellulose dispersion with a mass ratio of 0.1-5% and adjust the pH to 3-11;

[0052] S2, directional freezing molding: freezing the nanocellulose dispersion obtained in step S1 at a temperature range of -20°C to -196°C at a rate of 5-50°C / min, and making the ice crystal growth direction form a controllable angle of 0-90° with the fiber axis to obtain a three-dimensional porous nanocellulose skeleton;

[0053] S3, vacuum impregnation composite: under a vacuum degree of 10 to 100 kPa, the dispersion containing the conductive material is infiltrated into the three-dimensional porous nanocellulose skeleton obtained in step S2 for an impregnation time of 1 to 6 hours;

[0054] S4. Microwave-assisted reduction: In a microwave field of 300-1000W, a mixture of hydrogen and argon is introduced to promote interfacial bonding;

[0055] S5, vapor phase functionalization: introducing functional groups using chemical vapor deposition;

[0056] S6. Post-treatment to improve the mechanical strength, interfacial stability and electrochemical activity of three-dimensional porous nanocellulose.

[0057] In step S2, a multi-stage freezing process is adopted, which specifically includes:

[0058] The first stage: -5℃~-20℃, rate 2~5℃ / min, forming micron-sized pores;

[0059] The second stage: -20℃~-80℃, speed 10~30℃ / min, forming submicron pores;

[0060] The third stage: -80℃~-196℃, rate>50℃ / min, fixing nano-scale pores.

[0061] The microwave-assisted reduction in step S4 specifically includes the following steps:

[0062] Pre-reduction stage: power 300-500W, time 5-10min, hydrogen concentration 10-30vol%;

[0063] Main reduction stage: power 800-1000W, time 10-20min, hydrogen concentration 50-80vol%;

[0064] Annealing stage: power 200-300W, time 3-5min, slow cooling under argon atmosphere.

[0065] The post-processing in step S6 specifically includes:

[0066] Hot pressing: compressing the capacitor electrodes to 20-50% of their original thickness at 80-150°C and 5-20 MPa pressure;

[0067] Surface coating: spraying polyvinylidene fluoride or polyurethane protective layer;

[0068] Activation treatment: Treat in CO2 supercritical fluid for 1 to 3 hours.

[0069] The solution of the present invention effectively solves the key technical problems existing in the current three-dimensional nanocellulose-based supercapacitor electrodes through an innovative integrated preparation method and structural design, and achieves multi-dimensional performance improvement. Based on microwave-assisted chemical reduction and covalent bonding interface construction technology, the electron migration between the conductive material and the cellulose skeleton is accelerated, inducing the formation of COM (M = metal / carbon material) covalent bonds, thereby enhancing the heterogeneous interface bonding strength. Through gradient freeze-drying and multi-level pore size control technology, a three-dimensional interpenetrating network with dense surface-middle transition-internal penetration is constructed, the ion diffusion coefficient is improved, and high porosity and tensile strength are maintained at the same time; in addition, the integrated preparation system shortens the production cycle of the traditional step-by-step process, providing an environmentally friendly and economical solution for the development of high energy density, long life, and flexible supercapacitors.

[0070] Example 1

[0071] Step 1: Raw material pretreatment

[0072] Acetobacter xylinum fermentation broth at pH 4.8 was boiled in 0.1M NaOH solution for 2 hours to remove impurities, yielding a purified bacterial cellulose hydrogel with a solids content of 0.8%. This was then homogenized three times in a high-pressure homogenizer at 60 MPa to produce a dispersion of nanofibers with a diameter of 20 ± 5 nm.

[0073] Step 2: Directional Freeze Molding

[0074] The dispersion was injected into a custom mold measuring 50×50×5 mm and placed in a directional freezing apparatus with a cold plate temperature of -25°C. The freezing rate was set at 10°C / min, and the ice crystal growth direction was at a 30° angle to the long axis of the mold. After freezing, the mixture was freeze-dried at -50°C and 5 Pa for 24 hours to obtain a three-dimensional porous skeleton with an axially aligned structure. The porosity was 96%, and the BET specific surface area was 320 m 2 / g.

[0075] Step 3: Vacuum impregnation composite

[0076] Prepare an ethanol dispersion containing 1.2g of carboxylated carbon nanotubes (CNT-COOH) with a diameter of 8-15nm, a length of 10-20μm, and a carboxyl group content of 3.2mmol / g. Ultrasonicate at 40kHz for 30min and then inject into an impregnation tank. Place the framework in a 50kPa vacuum chamber and impregnate in three stages:

[0077] Stage 1 (0–30 min): Maintain vacuum to allow the dispersion to penetrate macropores >100 μm;

[0078] The second stage (30-60 min): applying 0.1 MPa pressure to promote the filling of mesopores 50 nm-10 μm;

[0079] The third stage (60-120 min): 25 kHz ultrasound assisted to achieve nanopore <50 nm loading.

[0080] Step 4: Microwave gradient reduction

[0081] The composite was transferred to a microwave reactor, where H2 / Ar=1:4, with a total flow rate of 200 sccm, and a three-stage reduction was performed:

[0082] Pre-reduction: 300W, 10min, H2 concentration 20vol% to remove surface oxygen groups (XPS showed that the O / C ratio decreased from 0.32 to 0.18);

[0083] Main reduction: 800W, 15min, H2 concentration 60vol%, repair carbon tube sp 2 structure;

[0084] Annealing: 200W, 5min, in pure Ar atmosphere to eliminate lattice stress.

[0085] Step 5: Plasma Functionalization

[0086] NH3 plasma treatment was used at 50W for 5 minutes with a gas flow rate of 50sccm to graft amino groups onto the fiber surface. After treatment, the electrode contact angle dropped from 125° to 28°, significantly improving electrolyte wettability.

[0087] Compared with traditional physical hybrid electrodes, the performance data is shown in Table 1:

[0088] Table 1 Performance Characterization Comparison Table

[0089] Test items This embodiment Traditional physical hybrid electrode Specific capacitance (1A / g) 352F / g 248F / g Internal resistance (EIS) <![CDATA[0.8Ω·cm 2 ]]> <![CDATA[2.3Ω·cm 2 ]]> 5000 cycle retention rate 92％ 78％ Bending stability 5000 times without attenuation 1000 breaks

[0090] Example 2

[0091] Step 1: Defibrination of seaweed fiber

[0092] Dissolve 2g of sodium alginate in 100mL of deionized water, adjust the pH to 2.5 with 0.5M HCl, and let stand for 12 hours to precipitate the algal cellulose. Five cycles of high-pressure homogenization at 80MPa yield nanofibers with a diameter of 50±15nm, a crystallinity of 65%, and a thermal decomposition temperature of 280°C.

[0093] Step 2: Micro-nano hierarchical pore construction

[0094] A dispersion of alginate fibers was mixed with 0.5g of graphene oxide (GO) with a flake diameter of 1-3μm and a carbon / carbon ratio of 1.8. The mixture was then injected into a liquid nitrogen jet freezer at a rate of 10mL / min and a nozzle diameter of 0.2mm. At -196°C, a hierarchical structure with micron-scale honeycomb pores (200±50μm) and nanometer-scale wall pores (20±5nm) was formed, achieving a porosity of 98%.

[0095] Step 3: Gas-phase in situ reduction

[0096] The frozen product was placed in an autoclave and introduced with sodium borohydride vapor and nitrogen carrier gas at a flow rate of 50 sccm. The temperature was raised to 120°C and maintained for 3 hours. After reduction, the C / O ratio of GO increased to 8.3, and the conductivity increased from 0.5 S / m to 850 S / m.

[0097] Step 4: Hydrothermal deposition of MnO2

[0098] Prepare a 0.1M KMnO4+0.05M MnSO4 mixed solution, immerse the electrode in it and perform hydrothermal reaction at 120℃ for 6h.

[0099] Step 5: Supercritical Activation

[0100] Treat in a CO2 supercritical device (35℃, 10MPa) for 2h to expand the pores through physical expansion effect.

[0101] The proportion of micropores (<2nm) decreased from 45% to 12%;

[0102] The proportion of mesopores (2-50 nm) increased from 30% to 68%;

[0103] Specific surface area from 850m 2 / g increased to 1550m 2 / g.

[0104] Compared with conventional unactivated electrodes, the electrochemical performance is shown in Table 2:

[0105] Table 2 Electrochemical performance comparison table

[0106] parameter This embodiment Unactivated control group Energy density (Wh / kg) 8.1 4.7 Power density (kW / kg) 4.3 2.8 10C capacity retention rate 89％ 63％ Self-discharge rate (72h) 8％ 22％

[0107] It can be seen from this embodiment that the solution of the present invention effectively solves the key technical difficulties existing in the current three-dimensional nanocellulose-based supercapacitor electrodes and achieves multi-dimensional performance improvement.

[0108] Anything not described in detail in the present invention is well known to those skilled in the art.

[0109] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified and replaced with equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A three-dimensional porous nanocellulose-based supercapacitor electrode, characterized in that: include: Nanocellulose three-dimensional network skeleton, porosity of 85%~98%, pore size distribution of 5nm~500um; The conductive material is uniformly dispersed in the nanocellulose three-dimensional network skeleton, with a mass proportion of 10% to 300% of the nanocellulose three-dimensional network skeleton, and forms a chemically bonded interface with the cellulose fibers; the functional groups cross-linked to the three-dimensional network structure by covalent bonds have a density of 0.5 to 3.2 mmol / g and are one or more of carboxyl groups, amino groups, and sulfonic acid groups; the metal oxide nanoparticles distributed on the surface of the nanocellulose three-dimensional network skeleton have a particle size of 2 to 50 nm and a mass proportion of 5% to 40% of the conductive material.

2. The three-dimensional porous nanocellulose-based supercapacitor electrode according to claim 1, characterized in that The nanocellulose is one or more of wood pulp fiber, bacterial cellulose and seaweed cellulose.

3. The three-dimensional porous nanocellulose-based supercapacitor electrode according to claim 1, characterized in that The surface of the conductive material is plasma treated or chemically modified, and the conductive material is one or more of carbon nanotubes, graphene, MXene, polyaniline, and polypyrrole.

4. The three-dimensional porous nanocellulose-based supercapacitor electrode according to claim 1, characterized in that The three-dimensional porous structure of the three-dimensional porous nanocellulose has a gradient pore size distribution, specifically: Surface dense area: thickness 10~200um, pore diameter 5~50nm, porosity 85~90%; Transition gradient zone: thickness 300~800um, pore size 50~100nm, porosity 90~95%; Internal support area: thickness ³1mm, pore diameter 100~500nm, porosity 95~98%.

5. The three-dimensional porous nanocellulose-based supercapacitor electrode according to claim 1, characterized in that The metal oxide nanoparticles are one or more of transition metal oxides, rare earth oxides, and composite oxides.

6. The three-dimensional porous nanocellulose-based supercapacitor electrode according to claim 1, characterized in that The invention also includes additives, which are one or more of a cross-linking agent, a toughening agent, and a flame retardant.

7. A method for preparing an integrated three-dimensional porous nanocellulose-based supercapacitor electrode, characterized in that: Specifically include: S1. Nanocellulose slurry pretreatment: Ultrasonic treatment of 0.1-5% nanocellulose dispersion by mass ratio to adjust the pH to 3-11; S2, directional freezing molding: freezing the nanocellulose dispersion obtained in step S1 at a temperature range of -20°C to -196°C at a rate of 5-50°C / min, and making the ice crystal growth direction form a controllable angle of 0-90° with the fiber axis to obtain a three-dimensional porous nanocellulose skeleton; S3, vacuum impregnation compounding: under a vacuum degree of 10-100 kPa, the dispersion containing the conductive material is infiltrated into the three-dimensional porous nanocellulose framework obtained in step S2 for an impregnation time of 1-6 hours; S4, microwave-assisted reduction: In a microwave field of 300~1000W, a mixture of hydrogen and argon is introduced to promote interfacial bonding; S5, vapor phase functionalization: introducing functional groups using chemical vapor deposition; S6. Post-treatment to improve the mechanical strength, interface stability and electrochemical activity of the three-dimensional porous nanocellulose.

8. The method for integrated preparation of three-dimensional porous nanocellulose-based supercapacitor electrodes according to claim 7, characterized in that: In step S2, a multi-stage freezing process is adopted, which specifically includes: The first stage: -5℃~-20℃, rate 2~5℃ / min, forming micron-sized pores; The second stage: -20℃~-80℃, speed 10~30℃ / min, forming submicron pores; The third stage: -80℃~-196℃, rate >50℃ / min, fixing nano-scale pores.

9. The method for integrated preparation of three-dimensional porous nanocellulose-based supercapacitor electrodes according to claim 7, characterized in that: The microwave-assisted reduction in step S4 specifically comprises the following steps: Pre-reduction stage: power 300~500W, time 5~10min, hydrogen concentration 10~30vol%; Main reduction stage: power 800~1000W, time 10~20min, hydrogen concentration 50~80vol%; Annealing stage: power 200~300W, time 3~5min, slow cooling under argon atmosphere.

10. The method for integrated preparation of three-dimensional porous nanocellulose-based supercapacitor electrodes according to claim 7, characterized in that: The post-processing in step S6 specifically includes: Hot pressing: compress the capacitor electrode to 20-50% of its original thickness at 80-150°C and 5-20 MPa pressure; Surface coating: spraying polyvinylidene fluoride or polyurethane protective layer; Activation treatment: Treat in CO2 supercritical fluid for 1~3h.

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