Graphene oxide / walnut shell biochar / MXene ternary composite electrode material and preparation method and application thereof

Abstract

This invention belongs to the field of supercapacitor electrode material technology, and relates to a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene, its preparation method and application. The ternary composite electrode material of graphene oxide / walnut shell biochar / MXene is composed of graphene oxide, walnut shell biochar and MXene. The walnut shell biochar is inserted into the interlayer of graphene oxide sheets as a layered spacer to inhibit the stacking and agglomeration of graphene oxide. The MXene and graphene oxide / walnut shell biochar form a three-dimensional conductive network structure to optimize the charge transport path. The invention systematically studies the inhibitory effect of WAC content on GO stacking behavior and the enhancing effect of MXene on the conductive network.

Description

Technical Field

[0001] This invention belongs to the field of supercapacitor electrode material technology, and relates to a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene, its preparation method and application. Background Technology

[0002] With the increasing scarcity of fossil fuels and the resulting environmental pollution, the urgency of developing renewable and efficient energy systems has become paramount. The utilization of renewable energy relies on efficient energy conversion and storage technologies; therefore, developing advanced energy storage devices is crucial for achieving a green and sustainable energy economy. Supercapacitors, as a novel energy storage device, demonstrate significant potential in renewable energy storage due to their high power density, rapid charge and discharge capabilities, long cycle life, and environmental friendliness, offering an effective solution to alleviate the energy crisis and environmental pollution problems.

[0003] Supercapacitors possess advantages such as high power density, long cycle life, fast charge / discharge speed, and good operational stability. Their working principle is based on the rapid adsorption / desorption of charges at the electrode-electrolyte interface or surface Faraday reactions. According to different energy storage mechanisms, supercapacitors are mainly classified into electric double-layer capacitors, pseudocapacitors, and hybrid supercapacitors. Among them, electric double-layer capacitors have become the most widely used type due to their high power characteristics, extremely long cycle life, and low cost. They mainly rely on the physical adsorption of carbon materials for energy storage, exhibiting stable structure and rapid response, making them suitable for high-power output and frequent charge / discharge scenarios. However, the fatal drawback of electric double-layer capacitors is their low energy density. To improve their energy density, synergistic optimization is needed in both electrode materials and electrolytes. This includes developing carbon materials with high specific surface area or hierarchical porous structures, widening the voltage window, and optimizing ion size and conductivity.

[0004] To improve the energy storage performance of electric double-layer supercapacitors, optimizing the structure and properties of electrode materials is a crucial approach. Common electrode materials for electric double-layer supercapacitors include graphene, biomass-derived char, carbon nanotubes (CNFs), and activated carbon. Graphene oxide (GO), as a derivative of graphene, possesses both a two-dimensional sheet structure and abundant oxygen-containing functional groups. It can achieve charge storage by constructing an electric double layer, and its surface functional groups can provide additional Faraday reaction sites, making it a research hotspot for supercapacitor electrode materials.

[0005] However, the strong van der Waals forces between GO sheets lead to their easy aggregation and stacking, which not only significantly reduces the effective specific surface area but also blocks pore channels, hindering electrolyte ion transport and severely limiting their electrochemical performance. To address this issue, researchers often modify GO through doping and composite methods: for example, introducing nanoparticles as "spacers" to expand the sheets, but poor particle dispersion easily leads to secondary aggregation; or combining with carbon materials to construct porous structures, but the interfacial bonding between traditional carbon materials and GO is weak, making it difficult to form a stable conductive network. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene, its preparation method, and its applications. Using graphene oxide (GO) as the core matrix, the composite electrode material is prepared through a two-step modification strategy: first, walnut shell biochar (WAC) is introduced as a "layer-spreading agent" to regulate the pore structure and specific surface area of ​​GO; then, MXene is composited to construct a three-dimensional conductive network, optimizing the charge transport path. Through systematic research on the inhibitory effect of WAC content on GO stacking behavior and the enhancing effect of MXene on the conductive network, the optimal ratio for synergistic modification of the three components is explored. Furthermore, the key properties of the composite material, such as specific surface area, pore structure, specific capacitance, and cycle stability, are investigated, providing new ideas for the design of graphene oxide-based high-performance supercapacitor electrode materials.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A ternary composite electrode material of graphene oxide / walnut shell biochar / MXene, wherein the ternary composite electrode material of graphene oxide / walnut shell biochar / MXene is composed of graphene oxide, walnut shell biochar and MXene.

[0008] The walnut shell biochar is inserted as a layered spacer between the graphene oxide sheets to inhibit the stacking and aggregation of graphene oxide.

[0009] The MXene sheets are interspersed within the graphene oxide / walnut shell biomass carbon sheets to form a three-dimensional conductive network structure.

[0010] In a preferred embodiment of the present invention, the mass ratio of graphene oxide, walnut shell biochar and MXene is 1-3:1-3:0.5-1.5.

[0011] A method for preparing a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene includes the following steps: Walnut shells are crushed, mixed with potassium hydroxide solution, impregnated, and then carbonized to obtain walnut shell biochar.

[0012] Graphene oxide, walnut shell biochar, and MXene were mixed in a certain proportion, and after stirring, dispersion, and hydrothermal reaction, C-Ti bonds were formed to obtain a GO / WAC / MXene ternary composite material.

[0013] In a preferred embodiment of the present invention, the mass ratio of graphene oxide, walnut shell biochar and MXene is 2:1:1.

[0014] In a preferred embodiment of the present invention, the reaction temperature in the hydrothermal reaction is 175℃-180℃, and the reaction time is 10h-11h.

[0015] In a preferred embodiment of the present invention, carbonization is carried out under nitrogen protection, the carbonization temperature is 700°C, the heating rate is 5°C / min, and the carbonization time is 2h.

[0016] In a preferred embodiment of the present invention, the mass ratio of walnut shell to potassium hydroxide solution is 1:3, the soaking temperature is 80°C, and the soaking time is 10 hours.

[0017] In a preferred embodiment of the present invention, the post-carbonization treatment involves soaking in a 1 mol / L hydrochloric acid solution for 4 hours to remove residual KOH and inorganic impurities, followed by repeated washing with deionized water until the pH of the filtrate is neutral, and finally drying for 12 hours.

[0018] In a preferred embodiment of the present invention, the particle size of the walnut shell after crushing is 80-100 mesh.

[0019] The nitrogen adsorption-desorption isotherm of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material of the present invention exhibits type IV characteristics, has an H3 type hysteresis loop, and the pore size is mainly distributed in the mesoporous range of 3.1nm-4.8nm.

[0020] The specific surface area of ​​the graphene oxide / walnut shell biochar / MXene ternary composite electrode material described in this invention is 694-1107 m². 2 / g, pore volume is 0.178-0.887 cm³ 3 / g, with an average pore size of 3.65-4.75 nm.

[0021] The graphene oxide / walnut shell biochar / MXene ternary composite electrode material of the present invention has a performance of 1 A·g -1 The specific capacitance at current density is 326 F·g -1 -559F·g -1 After 3000 charge-discharge cycles, the capacitance retention rate is no less than 88%.

[0022] The application of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material described in this invention in supercapacitor electrodes.

[0023] Compared with the prior art, the beneficial effects of the present invention are: 1. The ternary composite electrode material of graphene oxide / walnut shell biochar / MXene described in this invention is composed of graphene oxide, walnut shell biochar, and MXene. The walnut shell biochar acts as a layered spacer inserted between the graphene oxide sheets to inhibit the stacking and aggregation of graphene oxide. The MXene forms a three-dimensional conductive network structure with the graphene oxide / walnut shell biochar. Biochar (such as walnut shell char WAC) has the advantages of wide availability, low cost, rich pore structure, and abundant active sites on its surface. Its three-dimensional porous framework can act as a "physical support" to be inserted into the GO sheets, effectively inhibiting aggregation. At the same time, the microporous-mesoporous structure of WAC can synergize with the sheet structure of GO to optimize the electrolyte ion transport path. As a novel two-dimensional transition metal carbide, MXene has ultra-high conductivity and good interlayer regulation ability. It can form a "sheet-to-sheet" interwoven three-dimensional conductive network with GO, further reducing charge transfer resistance and improving electrode rate performance.

[0024] 2. This invention uses waste walnut shell biochar as a "three-dimensional physical spacer." Its porous framework structure effectively inserts into the interlayer of graphene oxide sheets, breaking van der Waals forces and significantly suppressing GO stacking. Compared to single GO materials, the specific surface area of ​​the composite material is increased to 694.5391-1107.1 m². 2 ·g -1 This effectively solved the technical problem of low GO utilization.

[0025] 3. Construction of a highly efficient three-dimensional conductive network: MXene nanosheets form a sheet-to-sheet interwoven three-dimensional conductive network in the GO / WAC system. Through strong coupling with the carbon matrix via C-Ti covalent bonds, the charge transfer resistance is as low as 0.47 Ω, which is more than 38% lower than that of pure biochar. This structure not only compensates for the brittleness of the GO conductive pathway but also avoids the stacking problem of MXene itself, achieving a performance balance of "high specific surface area - low charge transfer resistance".

[0026] 4. Optimized pore structure enhances ion transport: The composite material exhibits a typical Type IV nitrogen adsorption-desorption isotherm, with a significantly increased proportion of mesopores and a concentrated pore size distribution between 3.1 nm and 4.8 nm. This mesoporous structure provides a low-resistance transport channel for electrolyte ions, effectively shortening the ion diffusion path and enabling the electrode to maintain ideal double-layer capacitance behavior at high scan rates, resulting in excellent rate performance.

[0027] 5. Breakthrough in electrochemical performance: Under the optimal ratio of GO:WAC:MXene = 2:1:1, the electrode material achieves a high electrochemical performance at 1 A·g -1 The specific capacitance reaches 559.26 F·g at current density.-1 It showed a 71.2% improvement over binary GO / WAC composite materials and nearly 20 times the improvement over pure biochar. (At 10 A·g) -1 The capacitance retention rate reaches 22.51% under high current density, and 90.84% ​​after 3000 cycles. The average decay rate per cycle is only 0.0032%, and the cycle life is 3 to 5 times longer than that of traditional carbon materials.

[0028] 6. This invention uses waste walnut shells as biochar raw material, realizing the high-value utilization of agricultural and forestry waste resources. The raw material cost is low and the sources are wide. The preparation process adopts a two-step hydrothermal method, which does not require complex equipment or harsh conditions, making it suitable for large-scale production and showing good industrialization prospects. By adjusting the mass ratio of GO, WAC, and MXene, the pore structure and conductive network of the composite material can be precisely designed. When the WAC content is too high, its own aggregation will destroy the mesoporous continuity; when MXene is excessive, the lamellar stacking will block the pores and lead to a decrease in performance. This invention determines the optimal ratio range through systematic optimization, providing clear guidance for material design. Attached Figure Description

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] Figure 1 (a) is a technical roadmap for the preparation of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material of the present invention, (b) is a SEM of WAC10, (c) is a SEM of WAC / GO 1:2, and (d) is a SEM of a WAC / GO / MXene sample of 1:2:1.

[0031] Figure 2 (a) XRD patterns of WAC / GO at different ratios. (b) XRD patterns of WAC / GO / MXene samples at ratios of 1:2:0.5, 1:2:1, and 1:2:1.5. (c) Total XPS spectra of WAC / GO / MXene samples at ratios of 1:2:1, 1:2, and WAC10. (d) C1s of WAC10. (e) C1s of WAC / GO. (f) C1s of WAC / GO / MXene sample at ratio of 1:2:1. (g) Ti 2p of WAC / GO / MXene sample at ratio of 1:2:1.

[0032] Figure 3(a) shows the N2 adsorption / desorption isotherms for WAC / GO 1:1, WAC / GO 1:2, WAC / GO 1:3, WAC / GO 2:1, and WAC / GO 3:1; (b) shows the N2 adsorption / desorption isotherms for WAC / GO / MXene 1:2:0.5, WAC / GO / MXene 1:2:1, and WAC / GO / MXene 1:2:1.5; (c) shows the N2 adsorption / desorption isotherms for WAC10, WAC / GO 1:2, and WAC / GO / MXene 1:2:1; (d) shows the N2 adsorption / desorption isotherms for WAC / GO 1:1, WAC / GO 1:2, WAC / GO 1:3, WAC / GO 2:1, and WAC / GO 3:1. Pore ​​size distributions of 3:1: (e) WAC / GO / MXene1:2:0.5, WAC / GO / MXene1:2:1, WAC / GO / MXene1:2:1.5, (f) WAC10, WAC / GO1:2 and WAC / GO / MXene1:2:1.

[0033] Figure 4 (a) shows the CV curves for different ratios of WAC / GO, (b) shows the GCD for different ratios of WAC / GO, (c) shows the EIS for different ratios of WAC / GO, (d) shows the CV curves for WAC / GO 1:2 at different scan rates, (e) shows the GCD for WAC / GO 1:2 at different current densities, (f) shows the EIS for WAC10 and WAC / GO 1:2, (g) shows the capacitance retention rate of WAC / GO 1:2 and WAC10 after 3000 GCD cycles in 1 M KOH solution, and (h) shows the specific capacitance and rate performance of different ratios of WAC / GO at different current densities.

[0034] Figure 5 (a) is the CV curve of WAC / GO / MXene 1:2:1, (b) is the CV curve of WAC / GO / MXene with different ratios, (c) is the CV curve of WAC10, WAC / GO 1:2 and WAC / GO / MXene 1:2:1, (d) is the GCD of WAC / GO / MXene 1:2:1, (e) is the GCD of WAC / GO / MXene with different ratios, and (f) is the GCD of WAC10, WAC / GO 1:2 and WAC / GO / MXene 1:2:1.

[0035] Figure 6(a) EIS of WAC / GO / MXene, (b) EIS of WAC10, WAC / GO1:2 and WAC / GO / MXene1:2:1, (c) Specific capacitance and rate performance of WAC / GO / MXene in different ratios at different current densities, and (d) Capacitance retention of WAC10, WAC / GO1:2 and WAC / GO / MXene1:2:1 in 1 M KOH solution for 3000 GCD cycles. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0037] The following detailed description, in conjunction with embodiments of the present invention and accompanying drawings, provides a clear and complete illustration of the technical solutions in these embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] It should be noted that all technical terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0039] Figure 1 Figure (a) shows the preparation technology roadmap of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material of the present invention. Walnut shells are crushed and mixed with potassium hydroxide solution for impregnation, followed by carbonization to obtain walnut shell biochar. Graphene oxide is mixed with the walnut shell biochar and impregnated. After a first hydrothermal reaction, the interface between graphene oxide and walnut shell biochar bonds to obtain a graphene oxide / walnut shell biochar binary composite material. MXene is added to the binary composite material and a second hydrothermal reaction is carried out to form C-Ti bonds, thus obtaining the graphene oxide / walnut shell biochar / MXene ternary composite electrode material.

[0040] Example 1 (1) The walnut shells were washed to remove impurities, dried, and then pulverized into 80-mesh particles. The walnut shell particles were added to the KOH solution at a mass ratio of 1:3 and impregnated at 80°C under reflux for 10 hours to ensure that the KOH fully penetrated into the pores inside the walnut shells. The impregnated mixture was transferred to a tube furnace and heated to 700°C at a rate of 5°C / min under the protection of high-purity nitrogen (99.99% purity) to achieve carbonization at a constant temperature for 2 hours, thereby activating the carbon skeleton and constructing a porous structure. After the carbonized product was cooled to room temperature, it was soaked in 1 mol / L hydrochloric acid solution for 4 hours to remove residual KOH and inorganic impurities. Then, it was repeatedly washed with deionized water until the pH of the filtrate was neutral. Finally, it was dried in a vacuum drying oven at 80°C for 12 hours to obtain activated walnut shell biochar (WAC10, which is referred to as WAC in the subsequent preparation process for the sake of simplification of the composite material name).

[0041] (2) First, a suspension with a WAC to GO mass ratio of 1:2 was prepared as the base system. Then, MXene dispersion (concentration 1 mg·ml⁻¹) was mixed with WAC:GO:MXene at a mass ratio of 1:2:0.5. After adding deionized water, the mixture was placed on a magnetic stirrer and stirred continuously at 500 r / min for 6 h to ensure that MXene was uniformly dispersed in the GO / WAC system and formed a homogeneous suspension. Next, the suspension was transferred to a hydrothermal reactor and reacted at 180℃ for 10 h. After the reaction, the mixture was cooled to room temperature, and the product was transferred to a centrifuge tube. The precipitate was collected by centrifuging at 8000 r / min for 10 min. The precipitate was washed three times with deionized water and anhydrous ethanol to remove unrecombined free MXene. Finally, the mixture was dried in a vacuum drying oven at 60℃ for 8 h to obtain a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene.

[0042] Example 2 (1) The walnut shells were washed to remove impurities, dried, and then pulverized into 90-mesh particles. The walnut shell particles were added to the KOH solution at a mass ratio of 1:3, and continuously impregnated at 80℃ under reflux for 10 hours to ensure that the KOH fully penetrated into the pores inside the walnut shells. The impregnated mixture was transferred to a tube furnace and heated to 700℃ at a heating rate of 5℃ / min under the protection of high-purity nitrogen (purity 99.99%), and carbonized at a constant temperature for 2 hours to achieve the activation of the carbon skeleton and the construction of a porous structure. After the carbonized product was cooled to room temperature, it was soaked in 1mol / L hydrochloric acid solution for 4 hours to remove residual KOH and inorganic impurities. Then it was repeatedly washed with deionized water until the pH of the filtrate was neutral. Finally, it was dried in a vacuum drying oven at 80℃ for 12 hours to obtain activated walnut shell biochar (WAC10).

[0043] (2) First, a suspension of WAC and GO with a mass ratio of 1:2 was prepared as the base system. Then, MXene dispersion (concentration 1 mg·ml⁻¹) was mixed with WAC:GO:MXene at a mass ratio of 1:2:1. After adding deionized water, the mixture was placed on a magnetic stirrer and stirred continuously at 500 r / min for 6 h to ensure that MXene was uniformly dispersed in the GO / WAC system and formed a homogeneous suspension. Next, the suspension was transferred to a hydrothermal reactor and reacted at 180℃ for 10 h. After the reaction, the mixture was cooled to room temperature, and the product was transferred to a centrifuge tube. The precipitate was collected by centrifuging at 8000 r / min for 10 min. The precipitate was washed three times with deionized water and anhydrous ethanol to remove unrecombined free MXene. Finally, the mixture was dried in a vacuum drying oven at 60℃ for 8 h to obtain a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene.

[0044] Example 3 (1) The walnut shells were washed to remove impurities, dried, and then pulverized into 90-mesh particles. The walnut shell particles were added to the KOH solution at a mass ratio of 1:3, and continuously impregnated at 80℃ under reflux for 10 hours to ensure that the KOH fully penetrated into the pores inside the walnut shells. The impregnated mixture was transferred to a tube furnace and heated to 700℃ at a heating rate of 5℃ / min under the protection of high-purity nitrogen (purity 99.99%), and carbonized at a constant temperature for 2 hours to achieve the activation of the carbon skeleton and the construction of a porous structure. After the carbonized product was cooled to room temperature, it was soaked in 1mol / L hydrochloric acid solution for 4 hours to remove residual KOH and inorganic impurities. Then it was repeatedly washed with deionized water until the pH of the filtrate was neutral. Finally, it was dried in a vacuum drying oven at 80℃ for 12 hours to obtain activated walnut shell biochar (WAC10).

[0045] (2) First, a suspension of WAC and GO with a mass ratio of 1:2 was prepared as the base system. Then, MXene dispersion (concentration 1 mg·ml⁻¹) was mixed with WAC:GO:MXene at a mass ratio of 1:2:1.5. After adding deionized water, the mixture was placed on a magnetic stirrer and stirred continuously at 500 r / min for 6 h to ensure that MXene was uniformly dispersed in the GO / WAC system and formed a homogeneous suspension. Next, the suspension was transferred to a hydrothermal reactor and reacted at 180℃ for 10 h. After the reaction, the mixture was cooled to room temperature, and the product was transferred to a centrifuge tube. The precipitate was collected by centrifuging at 8000 r / min for 10 min. The precipitate was washed three times with deionized water and anhydrous ethanol to remove unrecombined free MXene. Finally, the mixture was dried in a vacuum drying oven at 60℃ for 8 h to obtain a ternary composite electrode material of graphene oxide / walnut shell biochar / MXene.

[0046] The microstructure of the prepared samples was observed using a field emission scanning electron microscope (SEM, OXFORD ULTIM Max65). For further analysis, the elemental composition and oxidation state were determined using X-ray photoelectron spectroscopy (XPS, Shimadzu / Krayos AXIS UITRA DLD). The crystal structure of the samples was characterized using X-ray diffraction (XRD) at an accelerating voltage of 40 kV and an emission current of 20 mA. The surface area and pore size distribution of the electrode material were measured after drying and degassing the samples at 200 °C for 2 h to remove moisture using a high-performance surface area and micropore analyzer (BSD-660M, model BSD-660).

[0047] The electrochemical performance of the prepared electrode material was tested on an electrochemical workstation. A three-electrode configuration was used, with the prepared electrode material coated on nickel foam as the working electrode, a platinum sheet (Pt) as the counter electrode, and Hg / HGO as the reference electrode. The mass of the material coated on the nickel foam was approximately 0.5 mg / cm³. 2 Cyclic voltammetry (CV) testing was performed at 5 mVs. -1 10 mVs -1 20 mVs -1 50 mVs -1 75 mVs -1 100 mVs -1 The scan rate was measured at a range of 0 to 1 V, with an operating potential window of 0–1 V. Furthermore, galvanostatic charge-discharge (GCD) tests were performed at 0.5–10 A·g. -1 The current density was measured within a certain range. Electrochemical impedance spectroscopy (EIS) was used to study the impedance of the material in the frequency range of 0.01–100,000 Hz. To demonstrate the electrochemical performance of the material, the specific capacitance was calculated based on the discharge curve, using the following formula:

[0048] (1) Where m(g) is the mass of the active material, ΔV(V) is the potential window, I(A) is the discharge current, and Δt(s) is the discharge time.

[0049] Comparative Example 1 Walnut shells were washed to remove impurities, dried, and then pulverized to 90 mesh. The walnut shell particles were added to a KOH solution at a mass ratio of 1:3, and impregnated at 80°C under reflux for 10 hours to ensure sufficient KOH penetration into the pores of the walnut shells. The impregnated mixture was transferred to a tube furnace and carbonized at 700°C at a rate of 5°C / min under high-purity nitrogen (99.99% purity) for 2 hours to activate the carbon framework and construct a porous structure. After cooling to room temperature, the carbonized product was soaked in 1 mol / L hydrochloric acid solution for 4 hours to remove residual KOH and inorganic impurities. It was then repeatedly washed with deionized water until the pH of the filtrate was neutral, and finally dried in a vacuum drying oven at 80°C for 12 hours to obtain activated walnut shell biochar (WAC10).

[0050] Comparative Example 2 (1) The walnut shells were washed to remove impurities, dried, and then pulverized into 90-mesh particles. The walnut shell particles were added to the KOH solution at a mass ratio of 1:3, and continuously impregnated at 80℃ under reflux for 10 hours to ensure that the KOH fully penetrated into the pores inside the walnut shells. The impregnated mixture was transferred to a tube furnace and heated to 700℃ at a heating rate of 5℃ / min under the protection of high-purity nitrogen (purity 99.99%), and carbonized at a constant temperature for 2 hours to achieve the activation of the carbon skeleton and the construction of a porous structure. After the carbonized product was cooled to room temperature, it was soaked in 1mol / L hydrochloric acid solution for 4 hours to remove residual KOH and inorganic impurities. Then it was repeatedly washed with deionized water until the pH of the filtrate was neutral. Finally, it was dried in a vacuum drying oven at 80℃ for 12 hours to obtain activated walnut shell biochar (WAC10).

[0051] (2) GO dispersion (concentration 1 mg·ml⁻¹) and WAC powder were added to deionized water at WAC to GO mass ratios of 1:1, 1:2, 1:3, 3:1, and 2:1, respectively. The mixture was ultrasonically dispersed at 300 W for 30 min under ice bath conditions to form a uniform and stable suspension. Hydrothermal composite was then performed. The suspension was transferred to a polytetrafluoroethylene-lined hydrothermal reactor (80% filling) and placed in an oven at 180℃ for 10 h to promote interfacial bonding and structural regulation of GO and WAC. After the reaction, the mixture was naturally cooled to room temperature. The product was collected by filtration and washed three times alternately with deionized water and anhydrous ethanol, soaking for 10 min each time to remove unreacted free GO and WAC. The product was then dried in a vacuum drying oven at 60℃ for 8 h to obtain five binary composite materials: WAC / GO 1:1, WAC / GO 1:2, WAC / GO 1:3, WAC / GO 2:1, and WAC / GO 3:1.

[0052] Results Analysis The microstructures of WAC10, WAC / GO, and WAC / GO / MXene were observed using scanning electron microscopy (SEM). Figure 1 (b) shows that the KOH-activated walnut shell char WAC10 exhibits a porous framework structure, indicating that the activated WAC10 has a complex porous structure, providing abundant active sites for ion transport in the material. Figure 1 (c) shows the microstructure of WAC / GO 1:2, where it can be observed that the sheet-like GO is inserted into the pores of WAC10 after hydrothermal treatment, which slows down the stacking of GO. Figure 1 (d) clearly shows that a large number of monolayer MXene nanosheets are inserted into GO. MXene supports GO to form abundant slit-like channels and is composite in the porous structure of WAC10, providing more active sites.

[0053] Figure 2 (a) and (b) are the XRD patterns of WAC10, GO, WAC / GO, and WAC / GO / MXene composites, respectively. WAC10 shows two broad diffraction peaks near 23° and 43°, corresponding to the (002) and (100) crystal planes of graphite, indicating that it has a typical amorphous carbon structure. GO shows a characteristic (001) diffraction peak at 11°. All WAC / GO composites exhibit an amorphous structure. With the increase of GO ratio, the intensity of the (002) peak increases and the full width at half maximum (FWHM) narrows, indicating an increase in the number of graphite crystallites. The WAC / GO / MXene composite shows diffraction peaks near 9°, 23°, and 43°. Compared to the (002) diffraction peak of the original MXene (approximately 5°), the peak in the composite shifts to 9°, and its intensity increases significantly with the increase of MXene addition. Meanwhile, the intensity of the (002) diffraction peak associated with MXene near 23° also increased with its proportion, indicating that the MXene component in the composite material has good crystal order. The (100) diffraction peak at 43° remained stable in all samples, indicating that the in-plane lattice structure of MXene did not change significantly during the composite process.

[0054] Figure 2 (c) shows the XPS full spectra of WAC10, WAC / GO 1:2, and WAC / GO / MXene 1:2:1, indicating that all three are mainly composed of C and O elements, with WAC / GO / MXene 1:2:1 also showing significant Ti element presence. The C 1s of WAC10 are as follows... Figure 2 (d) can be fitted with four peaks: 284.8 eV (CC / C=C), 286.1 eV (CO), 287.8 eV (C=O), and 289.4 eV (OC=O). The C 1s of WAC / GO 1:2 is shown below. Figure 2(e) also shows four functional groups: sp² carbon (284.8 eV), CO (286.2 eV), C=O (287.8 eV) and OC=O (289.1 eV). Figure 2 (f) shows the C 1s phase of WAC / GO / MXene in a 1:2:1 configuration, with a C-Ti bond appearing at 282.1 eV, confirming a strong interfacial coupling between MXene and the carbon matrix. Its Ti 2p spectrum ( Figure 2 g) can be fitted as Ti-C (455.5 eV), Ti²⁺ (456.7 eV), Ti 4 The material contains components such as ⁺ (458.5 eV). The presence of C-Ti bonds facilitates rapid electron transport and enhances electrode structural stability. The abundant oxygen-containing functional groups in the material improve electrode wettability, shorten ion diffusion time, and contribute to improved electrochemical performance.

[0055] To further investigate the effect of different GO ratios on the pore structure, the specific surface area and pore size distribution of WAC / GO ratios of 1:1, 1:2, 1:3, 2:1, and 3:1 were characterized. The N2 gas adsorption-desorption isotherm at 77 K is shown below. Figure 3 As shown in (a), all samples exhibit typical type IV adsorption curves and type H3 hysteresis loops, indicating the presence of mesoporous structures. The specific surface area of ​​WAC / GO1:2 is 694.5391 m². 2 ·g -1 Higher than WAC / GO 1:1 (390.4262 m2·g) -1 WAC / GO1:3 (355.4433 m) 2 ·g -1 WAC / GO2:1 (297.8896 m) 2 ·g -1 WAC / GO3:1 (604.0129 m) 2 ·g -1 As the proportion of GO increases, the hysteresis loop increases. However, when the ratio of WAC10 to GO is 1:2, the hysteresis loop is greater than that of 1:3. This is because the large pores in the WAC10 framework are blocked, leading to an increase in GO layers and a stronger ink bottle effect. Figure 3(d) shows a composite material structure with numerous mesopores. With increasing GO content, the peak pore size distribution at 3-5 nm exhibits a trend of first increasing and then decreasing, reaching its maximum at a WAC / GO ratio of 1:2. This phenomenon stems from the synergistic regulation of GO lamellar stacking behavior and the interaction with the carbon substrate. At low GO content, GO is sparsely dispersed, resulting in a discrete number of stacked layers and a broad pore size distribution; while at a ratio of 1:3, the lamellars are tightly stacked. The composite material with a WAC / GO ratio of 1:2 exhibits the highest proportion of 3.5 nm equally spaced slit pores, and its optimized pore structure significantly improves ion transport efficiency.

[0056] like Figure 3 (b) shows the N2 adsorption / desorption curves and pore size distribution diagrams for three different ratios of WAC / GO / MXene. All samples exhibited obvious type IV isotherm characteristics within a relative pressure (P / P0) range of 0.45-0.95, accompanied by a type H3 hysteresis loop, indicating the presence of a pore structure dominated by slit-like mesopores in the material. The adsorption capacity of the 1:2:1 ratio was significantly higher than the other two groups throughout the process, and its hysteresis loop area was also the most significant, indicating that this ratio has a more developed mesoporous structure and higher gas adsorption capacity. The 1:2:0.5 ratio was the second highest, while the 1:2:1.5 ratio had the lowest adsorption capacity and the smallest hysteresis loop. The desorption curves of all samples completely closed with the adsorption curves at P / P0≈0.45, indicating good pore connectivity and the absence of ink bottle pore effect. Figure 3 (e) The pore size distribution further reveals key differences: the 1:2:1 sample exhibits the strongest peak in the 3.3–4.3 nm range, the 1:2:0.5 sample shows a second strongest peak in the 3.6–4.5 nm range, while the 1:2:1.5 sample has the lowest peak intensity in the 3.6–4.3 nm range. Table 5 shows that the specific surface area of ​​the 1:2:1 sample is 1107.1 m²·g⁻¹, while that of the 1:2:0.5 sample is 248.2 m²·g⁻¹. -1 ) and 1:2:1.5 (233.2 m²·g -1 4.5 times and 4.7 times that of ), pore volume (0.8865 cm³·g) -1The MXene content was 1.4 times and 4.0 times that of the other two groups, respectively; the average pore size (4.75 nm) was also within the range of most active mesopore sizes. MXene content reached optimal balance at a ratio of 1:2:1, without excessive MXene causing lamellar stacking and pore blockage. The oxygen-containing functional groups of GO promoted the interfacial bonding between MXene and the carbon matrix, forming a stable hierarchical porous framework. The WAC / GO / MXene 1:2:1 sample showed the strongest pore size distribution peak in the 3.1-4.8 nm range, while the pore size decreased to 3.99 nm in the 1:2:1.5 ratio, leading to increased diffusion resistance. Although the 1:2:0.5 ratio had a higher pore volume, the specific surface area was not significantly improved due to the wide pore size distribution and insufficient peak intensity. An appropriate amount of MXene acts as a "sacrificial template" during the composite process. Its interlayer gaps and nanocavities formed by surface etching are transformed into new mesopores after heat treatment, directly contributing to the ultra-high specific surface area. However, excessive MXene inhibited this effect due to lamellar stacking.

[0057] Figure 3 (c) shows that the N2 adsorption-desorption isotherms indicate that WAC10 exhibits a typical Type I isotherm, confirming its micropore-dominated closed structure. The WAC / GO 1:2 isotherm transforms into a Type IV isotherm and an H3 hysteresis loop appears, suggesting mesoporous formation. The WAC / GO / MXene 1:2:1 has the highest adsorption capacity throughout the process and the hysteresis loop is significantly expanded, highlighting the synergistic effect of MXene. Figure 3 (f) The pore size distribution further quantifies the structural differences. WAC10 shows no significant peaks in the 2-8 nm range, WAC / GO 1:2 exhibits a weak peak in the 3.5-4.3 nm range, while WAC / GO / MXene 1:2:1 shows a sharp and strong peak in the 3.1-4.8 nm range, indicating that the mesopore concentration and size distribution of WAC / GO / MXene 1:2:1 are significantly optimized. Although WAC10 has an ultra-high specific surface area, its micropore size is too small and its pore volume is extremely low, resulting in severe obstruction of electrolyte ion diffusion. WAC / GO / MXene 1:2:1 solves the fundamental contradiction of "high specific surface area - low availability" of WAC10 by constructing a highly concentrated mesoporous network and using a pore volume expansion strategy. Moreover, the ultra-high pore volume provides a huge ion storage space for the double-layer capacitor, achieving a breakthrough in capacitor performance.

[0058] Table 1 is a summary table of specific surface area data for WAC10, WAC / GO 1:1, WAC / GO 1:2, WAC / GO 1:3, WAC / GO 2:1, WAC / GO 3:1, WAC / GO / MXene 1:2:0.5, WAC / GO / MXene 1:2:1, and WAC / GO / MXene 1:2:1.5. In a three-electrode testing system, using 1 M KOH solution as the electrolyte, the capacitive behavior of WAC / GO 1:1, WAC / GO 1:2, WAC / GO 1:3, WAC / GO 2:1, WAC / GO 3:1, and WAC10 was investigated within a voltage range of -1 V to 0 V. The capacitance values ​​were measured at 5 mV·s. -1 10 mV·s -1 20 mV·s -1 50 mV·s -1 75 mV·s -1 100 mV·s -1 Cyclic voltammetry was performed at a scan rate of 50 mV·s. WAC10 and different WAC / GO ratios were used. -1 The CV curve below is as follows Figure 4 As shown in (a), all samples exhibit rectangular curves, with the WAC / GO 1:2 electrode showing the largest enclosed rectangular area. This is mainly related to the excellent pore structure of the WAC / GO 1:2 electrode; the abundant microporous structure can store a large number of active sites, indicating its superior capacitance performance. Figure 4 (d) WAC / GO1:2 samples at all scan rates (5~100 mV s) -1 The CV curves under these conditions all maintain a nearly rectangular symmetrical shape, indicating ideal double-layer capacitance behavior. The area under the curve continuously increases with increasing scan speed, reaching a maximum when the scan rate increases to 50 mV / s. -1 At the specified time, the area under the curve is the largest, and the CV curve deformation is the smallest, indicating low charge transport impedance and excellent rate capability. The absence of significant curve deformation suggests that the WAC / GO1:2 material possesses rapid ion transport capabilities. Figure 4 (b) GCD curves for WAC / GO and WAC10 with different ratios. The GCD curves reveal the high reversibility of the WAC / GO1:2 sample, with no significant voltage drop observed, demonstrating the good conductivity of the WAC / GO1:2 electrode. To further analyze the performance of the WAC / GO1:2 electrode material, Figure 4 (e) shows the GCD curves of the WAC / GO1:2 composite material. It exhibits a highly symmetrical linear triangle within the current density range of 1–10 A g⁻¹, indicating its efficient reversible charge storage and stable double-layer dominance mechanism. As the current density increases, the discharge time gradually shortens; even with a 10-fold increase in current density, the GCD curves maintain symmetry, further verifying that WAC / GO1:2 represents the optimal doping ratio for conductivity and active sites.

[0059] pass Figure 4(c) Comparing the Nyquist plots of WAC / GO composites with different ratios, it was found that the 1:2 ratio exhibits the best charge transport characteristics. Fitting analysis revealed that the semicircular diameter of the 1:2 WAC / GO composite in the high-frequency region corresponds to the charge transfer resistance (R0). ct The lowest resistance (0.15 Ω) is found in the WAC / GO1:1 ratio, indicating the lowest resistance to interfacial electrochemical reactions. The WAC / GO1:1 ratio is 0.49 Ω, WAC / GO1:3 is 0.46 Ω, WAC / GO2:1 is 0.79 Ω, and WAC / GO3:1 is 1.48 Ω. This result is consistent with the optimal rate performance of WAC / GO1:2 in CV and GCD tests, confirming that the WAC / GO1:2 ratio achieves the best synergistic effect between WAC and GO. The WAC / GO1:2 ratio exhibits the closest Warburg impedance slope to vertical in the mid-frequency region, with a phase angle >80°, indicating that the composite material structure effectively shortens the ion diffusion path. This is corroborated by the low IR drop characteristic of WAC / GO1:2 in GCD tests. The Nyquist plots of WAC / GO1:2 and WAC10 are shown in Figure 4(f). Further comparison of the Rct values ​​of the WAC / GO1:2 composite material and pure walnut shell char (WAC10) shows that the Rct of WAC / GO1:2 is significantly lower than that of WAC10. This is attributed to the formation of a continuous electron transport network by GO in the composite material, reducing the interfacial resistance. The introduction of GO significantly increases the formation rate of the electric double layer at the electrode / electrolyte interface. The GO sheets provide long-range conductive pathways, reducing the Rct of the composite material; the micropores (<2 nm) of WAC10 ensure the high specific surface area of ​​the composite material; and the mesopores of WAC / GO1:2 optimize the ion diffusion path, thus improving its overall electrochemical performance.

[0060] Figure 4 (g) shows the capacitance retention changes of WAC / GO1:2 composite material and WAC10 after 3000 GCD cycles in 1 M KOH electrolyte. The results show that at a current density of 10 A g... -1After 3000 GCD cycles, the WAC / GO1:2 capacitance retention rate was 88.62%, demonstrating superior cycling stability compared to WAC10's 85.5%. Although the difference is relatively small, it still reflects the structural advantages of the composite material. WAC / GO1:2 maintained a capacitance retention rate of >99% in the first 1200 cycles, while pure WAC only maintained the same level in the first 600 cycles. This phenomenon is attributed to the local collapse of the rigid microporous framework of WAC10 during repeated ion insertion-extraction processes, while the GO sheets form a flexible support network in WAC / GO1:2, which can effectively disperse stress and suppress the volume deformation of WAC particles. The GO capping layer blocks the direct contact between WAC and the electrolyte, reduces side reactions, and delays structural degradation until after 1200 cycles. After more than 1200 cycles, even though GO inhibited the oxidation of the WAC surface, some uncovered areas still decayed slowly. The decay rate of WAC / GO 1:2 at 0.008% / cycle was still lower than that of WAC at 0.012% / cycle, and the final retention rate difference widened to 3.12%. Figure 4 (h) shows the variation of the specific capacitance of WAC / GO electrodes with different ratios as a function of current density, calculated from the GCD curves. As the current density increases, the specific capacitance of all electrodes gradually decreases. This is mainly attributed to the fact that at lower current densities, electrolyte ions have more time to migrate to the electrode surface and can more fully utilize the active sites of the electrode material, thus achieving a higher specific capacitance. At a current density of 1 A g⁻¹, the WAC / GO 1:2 electrode exhibits the highest specific capacitance, reaching 326.7 F·g⁻¹; when the current density increases to 10 A·g⁻¹, its capacitance drops to 67.99 F·g⁻¹, with a capacitance retention of approximately 20.8%. At this high current density, the specific capacitances of other WAC / GO electrode ratios are: WAC / GO 1:1 (54.6 F·g⁻¹), 1:3 (48.9 F·g⁻¹), 2:1 (45.1 F·g⁻¹), and 3:1 (41.6 F·g⁻¹), with corresponding capacitance retention rates of 20.1%, 19.8%, 19.6%, and 20.0%, respectively. The comparison reveals that the WAC / GO 1:2 electrode exhibits the best rate performance, mainly due to its low internal resistance. Other WAC / GO electrode ratios may have relatively poor rate performance due to insufficient ion diffusion.

[0061] Figure 5 (a) WAC / GO / MXene 1:2:1 at a scan rate of 5-100 mV·s -1The CV curves of all WAC / GO / MXene 1:2:1 curves are rectangular, indicating that the composite material has good double-layer capacitance characteristics. However, as the scan rate increases, the CV curves change from "wide and flat" rectangles to "narrow and tall" rectangles, and sharp points appear on both sides, indicating that the ion diffusion rate is limited at high magnification but still maintains capacitance behavior. Figure 5 (b) WAC / GO / MXene 1:2:0.5, WAC / GO / MXene 1:2:1, and WAC / GO / MXene 1:2:1.5 at a scan rate of 50 mV·s -1 Comparing the CV curves at different times, it was found that WAC / GO / MXene 1:2:1 has the largest CV curve area, which strictly corresponds to the maximum pore volume and optimal mesopore distribution of this ratio in the aforementioned pore structure analysis. Figure 5 (c) WAC10, WAC / GO 1:2 and WAC / GO / MXene 1:2:1 at 50 mV·s -1 The CV curves of WAC / GO / MXene 1:2:1 show that it has the largest CV curve area, further revealing the advantages of ternary composite materials. Although WAC10 has the highest specific surface area, it has the smallest CV area, while the 1:2:1 sample, although having a wider voltage window and a slightly weaker ordinate response, achieves superior capacitance performance due to its significantly expanded curve area. Figure 5 (d) is WAC / GO / MXene 1:2:1 in 1-10 A ·g -1 The GCD curves at current density are all approximately isosceles triangles. As the current density increases, the charge / discharge time also decreases. At 1 A·g -1 The longest discharge time was 583.4 s, confirming that the reaction was highly reversible. Figure 5 (e) WAC / GO / MXene 1:2:0.5, WAC / GO / MXene 1:2:1 and WAC / GO / MXene 1:2:1.5 in 1 A · g -1 At the given time, the discharge time was longest for the WAC / GO / MXene 1:2:1 ratio. Although all three curves approximate isosceles triangles, the curve for the WAC / GO / MXene 1:2:1.5 ratio was more distorted during discharge. This is because the excessive MXene caused stacking polarization, resulting in a more severe distortion of the 1:2:1.5 GCD curve. Insufficient MXene content in the WAC / GO / MXene 1:2:0.5 ratio led to a shortage of active sites, resulting in a lower specific capacitance of the composite material compared to the other two ratios. Figure 5 (f) represents WAC10, WAC / GO 1:2 and WAC / GO / MXene 1:2:1 in 1A·g -1At high GCD, the discharge time of WAC / GO / MXene 1:2:1 is much longer than that of the material. The discharge time of the WAC / GO binary is only 43% of that of WAC10. However, compared with WAC10, the bridging role of GO in the ternary material is also very important. It enhances the interfacial conductivity and inhibits MXene aggregation, ensuring the capacitance retention rate at high scan rates.

[0062] Figure 6 (a) shows the Nyquist plots of WAC / GO / MXene ratios of 1:2:0.5, 1:2:1, and 1:2:1.5. The 1:2:1 ratio of WAC / GO / MXene exhibits the optimal charge transport characteristics. Fitting analysis reveals that the charge transfer resistance (Rct) corresponding to the semicircular diameter in the high-frequency region is the smallest, only 0.47 Ω, while the Rct for WAC / GO / MXene 1:2:0.5 is 0.51 Ω, and for WAC / GO / MXene 1:2:1.5 it is 0.63 Ω. Furthermore, the composite materials of all three ratios show approximately linear patterns in the low-frequency region, with the 1:2:1 ratio exhibiting the steepest slope, indicating optimal ion diffusion efficiency. This phenomenon corresponds precisely to the aforementioned mesoporous structure, suggesting that this ratio concentrates the mesoporous peaks and constructs low-resistance ion channels. Figure 6 (b) shows the Nyquist plots for WAC10, WAC / GO1:2, and WAC / GO / MXene1:2:1. In the high-frequency region, the Rct for WAC / GO1:2 is 0.15 Ω, the Rct for WAC10 is 0.29 Ω, and the Rct for WAC / GO / MXene1:2:1 is... ct The resistance is 0.47 Ω. In WAC / GO1:2, the introduction of GO improves the conductivity of the material, but insufficient mesopore development restricts ion diffusion. In WAC / GO / MXene1:2:1, the introduction of MXene slightly increases the interfacial resistance, but the MXene sheets interspersed in the GO network form a "point-to-surface" contact conductivity mode, which offsets the effect of a single interfacial resistance. Figure 6 In (b), all three samples exhibit approximately linear slopes in the low-frequency region. Among them, WAC10 and WAC / GO1:2 show greater slopes in the low-frequency region than WAC / GO / MXene1:2:1. The slope in the low-frequency region reflects the ion diffusion control capability. The near-vertical slope of WAC10 is essentially a "pseudo-ideal capacitor," as its microporous structure forces ions to diffuse slowly, exhibiting a blockage-like electrode behavior. The slope of WAC / GO / MXene1:2:1 is approximately 75°, slightly slower than the other two groups, which precisely demonstrates its mesoporous permeability. Excessive MXene content, however, would clog the pores. Figure 6 Figure (c) shows the different WAC / GO / MXene composite electrodes calculated based on GCD curves at 1–10 A·g. -1The variation of specific capacitance under current density. All electrodes exhibit a typical capacitance decay trend: as the current density increases from 1 A·g... -1 Increased to 10 A·g -1 At that time, the specific capacitance of the WAC / GO / MXene 1:2:1 electrode increased from 559.26 F·g. -1 It decreased significantly to 125.88 F·g -1 The retention rate was 22.51%; while the 1:2:0.5 and 1:2:1.5 electrodes retained 535.72 F·g. -1 and 501.06 F·g -1 Decreased to 117.29 F·g -1 and 111.91 F·g -1 The retention rates were 21.86% and 22.34%, respectively. The 1:2:1 electrode was used at a low current density (1 A·g⁻¹). -1 The optimal capacitance performance is observed at this ratio, indicating that the synergistic effect of the three components is most significant at this ratio. This may be due to the hierarchical conductive network formed by MXene and WAC / GO, which improves charge storage efficiency. Although the WAC / GO / MXene 1:2:1 electrode exhibits outstanding performance at low rates, its absolute capacitance at high rates (125.88 F·g) is significantly lower. -1 The result is still higher than other ratios, indicating that optimizing the component ratio can partially alleviate capacitance loss under high current. Figure 6 Figure (d) shows three electrodes—WAC10, WAC / GO 1:2, and WAC / GO / MXene 1:2:1—in 1M KOH solution at a current density of 10 A·g. -1 The capacitance retention after 3000 GCD cycles under certain conditions was measured. The results showed that WAC10 retained 85.5% of its capacitance, WAC / GO1:2 88.62%, and WAC / GO / MXene1:2:1 90.84%. During cycling, WAC / GO / MXene1:2:1 maintained a capacitance retention exceeding 99.9% for the first 1450 cycles, WAC / GO1:2 maintained above 99% for the first 1200 cycles, while WAC10 only maintained similar stability for the first 600 cycles. This difference indicates that the introduction of MXene forms a protective layer at the electrode interface, effectively suppressing side reactions during ion insertion / extraction, thus exhibiting excellent stability in the early and mid-cycle stages. Simultaneously, the abundant mesoporous structure within the material provides excellent mechanical buffering and ion transport channels, contributing to maintaining electrode structural integrity and electrochemical reversibility. At 10 A·g -1After 3000 continuous cycles at high current density, WAC / GO / MXene 1:2:1 still retains 90.84% ​​of its capacity, with an average decay rate of only 0.0032% per cycle. Its cycle life is 3 to 5 times higher than that of traditional porous carbon materials, providing a new approach for the structural design of electrodes for high-power supercapacitors.

[0063] This study uses graphene oxide (GO) as the core matrix and innovatively employs a stepwise synergistic modification strategy with walnut shell biochar (WAC) and MXene to specifically address the key issues of GO sheet agglomeration, disordered pore structure, and discontinuous conductive network. It systematically elucidates a multi-mechanism synergistic modification pathway of "sheet opening-pore regulation-three-dimensional conductivity," providing a practical technical solution for performance breakthroughs in GO-based supercapacitor electrode materials. The main conclusions are as follows: 1. Suppression of GO sheet aggregation and directional regulation of pore structure by WAC: WAC, with its porous framework properties, can effectively insert itself into the interlayer of GO sheets as a "three-dimensional physical spacer," breaking the strong van der Waals forces between the sheets. Structural characterization shows that the WAC / GO 1:2 composite exhibits a typical Type IV nitrogen adsorption-desorption isotherm, with a significantly increased proportion of 3-5 nm mesopores, achieving efficient development of the mesoporous structure. Its specific surface area reaches 694.5391 m²·g. -1 Furthermore, the pore size distribution is concentrated. This not only provides a smooth channel for electrolyte ion transport but also prevents secondary aggregation of the lamellar sheets during cycling through interfacial hydrogen bonds and covalent bonds between WAC and GO, laying a structural foundation for improved electrochemical performance.

[0064] 2. Synergistic Enhancement of the Conductive Network in the WAC / GO System by MXene: The introduction of MXene further constructs a three-dimensional conductive network interwoven with WAC and GO through a "sheet-particle-sheet" structure, achieving strong interfacial coupling among the three through C-Ti covalent bonds. This structural design keeps the charge transfer resistance of the WAC / GO / MXene composite material at a low level of 0.47 Ω. Compared to the WAC / GO system, MXene not only compensates for the problem of conductive path breakage caused by GO sheet agglomeration, but also works synergistically with WAC to optimize pore distribution, achieving a performance balance of "high specific surface area - low charge transfer resistance," significantly improving the charge storage and transport efficiency of the electrode.

[0065] 3. Optimization of the GO / WAC / MXene ternary system ratio and breakthrough in electrochemical performance: Through screening multiple ratios, the GO / WAC / MXene ratio of 2:1:1 (originally 1:2:1) was determined to be the optimal system. This ratio achieves excellent electrochemical performance at 1 A·g⁻¹. -1 It exhibits 559.26 F·g at current density. -1 High specific capacitance, and at 10 A·g -1It maintains good capacitance retention even at high current densities; after 3000 GCD cycles, the capacitance retention reaches 90.84%, far superior to the 88.62% of WAC / GO 1:2, demonstrating excellent rate performance and cycle stability. Comparison with other formulations shows that excessive WAC leads to self-agglomeration, disrupting mesoporous continuity; excessive MXene clogs pores, reducing the specific surface area to 233.2482 m²·g. -1 This confirms that only by precisely controlling the ratio of the three components can their synergistic effect be fully realized.

[0066] In summary, this study, through stepwise modification of GO with WAC and MXene, not only provides a new approach to solving the aggregation problem of GO-based materials, but also establishes a multi-component synergistic design paradigm of "biochar-two-dimensional carbon materials-transition metal carbides," providing new ideas for promoting the development and industrial application of supercapacitor electrode materials towards high capacity, long lifespan, and low cost.

[0067] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A ternary composite electrode material of graphene oxide / walnut shell biochar / MXene, characterized in that, The graphene oxide / walnut shell biochar / MXene ternary composite electrode material is composed of graphene oxide, walnut shell biochar and MXene; The walnut shell biochar is inserted between the layers of graphene oxide as a layered spacer to inhibit the stacking and aggregation of graphene oxide. The MXene sheets are interspersed within the graphene oxide / walnut shell biomass carbon sheets to form a three-dimensional conductive network structure.

2. The graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 1, characterized in that, The mass ratio of graphene oxide, walnut shell biochar and MXene is 1-3:1-3:0.5-1.

5.

3. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 1, characterized in that, Includes the following steps: Walnut shells are crushed, mixed with potassium hydroxide solution, impregnated, and then carbonized to obtain walnut shell biochar. Graphene oxide, walnut shell biochar, and MXene were mixed in a certain proportion, and after stirring, dispersion, and hydrothermal reaction, C-Ti bonds were formed to obtain a GO / WAC / MXene ternary composite material.

4. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 3, characterized in that, The mass ratio of graphene oxide, walnut shell biochar, and MXene was 2:1:

1.

5. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 3, characterized in that, The hydrothermal reaction temperature is 175℃-180℃, and the reaction time is 10h-11h.

6. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 3, characterized in that, Carbonization is carried out under nitrogen protection at a temperature of 650℃-750℃, a heating rate of 3℃ / min-6℃ / min, and a carbonization time of 1.8h-2.5h.

7. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 3, characterized in that, The mass ratio of walnut shells to potassium hydroxide solution is 1:3-3.5, the soaking temperature is 80℃-85℃, and the soaking time is 10h-11h.

8. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 3, characterized in that, The post-carbonization treatment involves soaking the filtrate in 1 mol / L-1.2 mol / L hydrochloric acid solution for 4-6 hours to remove residual KOH and inorganic impurities. The filtrate is then repeatedly washed with deionized water until the pH of the filtrate is neutral, and finally dried for 12-14 hours.

9. The preparation method of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 3, characterized in that, The particle size of the crushed walnut shells is 80-100 mesh.

10. The application of the graphene oxide / walnut shell biochar / MXene ternary composite electrode material according to claim 1 in supercapacitor electrodes.

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