A nitrogen-doped yellow peel core-based porous carbon material, a preparation method and application thereof

Abstract

This invention relates to the field of activated carbon material preparation technology, specifically to a nitrogen-doped wampee kernel-based porous carbon material, its preparation method, and its application in supercapacitors. The preparation method includes: washing, drying, and pulverizing wampee kernels, followed by pre-carbonization under an inert atmosphere; then, high-temperature activation using potassium citrate as an activator, followed by acid washing and drying to obtain porous carbon; finally, mixing the porous carbon with urea in a specific ratio, and achieving nitrogen doping through a hydrothermal reaction to obtain the nitrogen-doped wampee kernel-based porous carbon material. This invention achieves a balance between high specific surface area, porous structure, and effective nitrogen doping under relatively mild conditions through the synergistic effect of potassium citrate activation and urea hydrothermal doping. The obtained material has a loose, sheet-like porous structure with a specific surface area of ​​up to 1358 m² / g. In 6 M KOH electrolyte, it has a specific capacitance of up to 305 F / g at a current density of 0.5 A / g and still maintains 242 F / g at 10 A / g. The symmetrical supercapacitor assembled with it has an energy density of 16.7 Wh / kg at a power density of 250 W / kg and a capacity retention of 82.9% after 10,000 cycles.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials technology, specifically relating to a nitrogen-doped yellow peel kernel-based porous carbon material, its preparation method, and its application as a supercapacitor electrode material. Background Technology

[0002] Supercapacitors, as a novel energy storage device, possess advantages such as high power density, rapid charge-discharge capability, and ultra-long cycle life, and have broad application prospects in fields such as electric vehicles, smart grids, and portable electronic devices. Electrode materials are a core factor determining the performance of supercapacitors, with porous carbon materials attracting significant attention due to their large specific surface area, good chemical stability, and excellent conductivity. However, traditional commercial activated carbon mainly relies on fossil fuels such as coal and petroleum coke, which are non-renewable and face resource depletion and carbon emission problems, failing to meet the requirements of green and sustainable development.

[0003] Biomass-based porous carbon, using renewable agricultural and forestry waste as raw materials, is inexpensive and environmentally friendly, and has become a research hotspot in recent years. However, raw biochar usually suffers from uneven pore structure distribution, low specific surface area, and insufficient conductivity, making it difficult to use directly as an electrode material for high-performance supercapacitors. To address this, researchers have developed various activation and modification methods to improve its electrochemical performance. Common chemical activation methods use strong bases / acids such as potassium hydroxide, sodium hydroxide, or zinc chloride as activators. While these methods can achieve high specific surface areas, they suffer from problems such as large activator dosages, strong corrosiveness, severe equipment wear and tear, and high subsequent wastewater treatment costs. For example, when using potassium hydroxide as an activator, the mass ratio of carbon precursor to KOH is usually as high as 1:3 to 1:5, making the alkaline wastewater generated after the reaction difficult to treat.

[0004] Patent CN117550602A discloses a petroleum-based activated carbon and its preparation method, using potassium hydroxide and potassium carbonate as a composite activator, achieving a specific surface area of ​​up to 2813 m² / g. However, it relies on fossil raw materials, requires a large amount of activator, resulting in high cost and heavy pollution. Patent CN111017925A discloses a method for preparing porous carbon using waste distiller's grains, achieving a specific capacitance of up to 463 F / g. However, it requires a high amount of alkaline activator (the mass ratio of carbon precursor to alkaline inorganic matter is 1:4-5), leading to equipment corrosion and increased wastewater treatment costs.

[0005] Besides activation processes, heteroatom doping (especially nitrogen doping) is another important strategy for improving the electrochemical performance of biomass carbon. Nitrogen doping can introduce active functional groups such as pyridine nitrogen and pyrrole nitrogen into the carbon framework, contributing additional pseudocapacitance and improving the polarity of the material surface and electrolyte wettability. Currently, there are two main nitrogen doping methods: one is to directly introduce nitrogen-containing gases (such as ammonia or acetonitrile vapor) during carbonization for high-temperature heat treatment; the other is to mix the biomass precursor with nitrogen-containing compounds (such as urea or melamine) and then calcine at high temperature. However, these high-temperature gas-phase or solid-phase doping methods have obvious drawbacks: the nitrogen doping efficiency is low, and a large amount of nitrogen source is lost in gaseous form at high temperatures; the doped nitrogen atoms are mostly concentrated on the material surface and are difficult to distribute uniformly deep in the pores; prolonged high-temperature treatment can also easily lead to the collapse of the pre-formed pore structure, resulting in a significant decrease in specific surface area.

[0006] Furthermore, existing technologies typically optimize activation and pore formation as two independent steps, lacking a systematic design that considers their synergistic effects. Process parameters such as the selection and dosage of activators, the type and doping method of nitrogen sources, and the pretreatment conditions of carbon precursors are interdependent; optimizing a single parameter alone often fails to yield materials with optimal overall performance. No research has been reported on the preparation of nitrogen-doped porous carbon materials using wampee kernels as precursors. Wampee kernels, a common agricultural processing waste in tropical and subtropical regions, are currently mostly discarded or treated at low value, and their unique advantages in biomass structure (such as high lignin content and abundant natural pores) have not been fully explored.

[0007] The existing technologies have the following main shortcomings: (1) The amount of strong alkali activator used is large, resulting in high cost, strong corrosiveness, and serious environmental pollution; (2) High-temperature gas-phase or solid-phase nitrogen doping methods are inefficient and easily damage the pore structure; (3) There is a lack of synergy between the activation and nitrogen doping steps, making it difficult to simultaneously achieve a unified high specific surface area, hierarchical porous structure, and effective nitrogen doping; (4) The kernels of yellow peel fruit have not yet been developed for the preparation of high-performance nitrogen-doped porous carbon materials. Therefore, developing a preparation method with reasonable activator dosage, mild process, and the ability to synergistically regulate pore structure and surface chemical properties is of great significance for improving the electrochemical performance of biomass-based carbon materials and realizing the high-value utilization of agricultural waste. Summary of the Invention

[0008] The main objective of this invention is to provide a nitrogen-doped wampee kernel-based porous carbon material and its preparation method, thereby solving the technical problems of large activator dosage, low nitrogen doping efficiency, and difficulty in synergistic control of pore structure and surface chemistry in existing technologies. Another objective of this invention is to provide the application of the aforementioned nitrogen-doped wampee kernel-based porous carbon material in supercapacitors.

[0009] To achieve the above objectives, the present invention provides a method for preparing nitrogen-doped wampee kernel-based porous carbon material, comprising the following steps:

[0010] (1) Pretreatment: The kernels of the yellow peel fruit are washed, dried and crushed to obtain kernel powder; (2) Pre-carbonization: The kernel powder is heated to 450-550 ℃ at 2-10 ℃ / min under an inert atmosphere and kept at the temperature for 1-3 h to obtain a pre-carbonized product; (3) Activation: The pre-carbonized product is mixed with potassium citrate at a mass ratio of 1:(2-4), heated to 650-750 ℃ ​​at 2-10 ℃ / min under an inert atmosphere and kept at the temperature for 1-3 h, and then acid washed, water washed and dried to obtain a porous carbon material; (4) Nitrogen doping: The porous carbon material is mixed with urea at a mass ratio of 1:(4-6), deionized water is added and stirred and dispersed, and then hydrothermally reacted at 140-180 ℃ for 8-16 h, and then washed and dried to obtain a nitrogen-doped yellow peel kernel-based porous carbon material.

[0011] The core inventive concept of this invention lies in using potassium citrate as an activator to construct a rich porous structure under relatively mild conditions (compared to strong bases such as KOH); subsequently, utilizing the hydrothermal reaction of urea at a specific temperature, uniform nitrogen doping of porous carbon is achieved in a liquid phase environment. The inventors discovered a significant synergistic effect between potassium citrate activation and urea hydrothermal doping: the loose, sheet-like porous framework formed by potassium citrate activation provides ample reaction sites and transport channels for subsequent nitrogen doping; while the decomposition products of urea under hydrothermal conditions (such as ammonia and isocyanate) can uniformly penetrate into the deep pores of the porous carbon in the liquid phase, reacting chemically with the carbon framework to achieve efficient nitrogen introduction. Compared to traditional high-temperature calcination nitrogen doping, the hydrothermal method is milder, effectively avoiding nitrogen loss and pore collapse caused by high temperatures, thus achieving effective nitrogen doping while maintaining a high specific surface area.

[0012] Preferably, the drying temperature in step (1) is 60-100 ℃, and the particle size of the powder after pulverization is 100-300 mesh.

[0013] Preferably, the inert atmosphere in step (2) is nitrogen or argon, the heating rate is 4-6 °C / min, the pre-carbonization temperature is 500 °C, and the holding time is 2 h.

[0014] Preferably, in step (3), the mass ratio of the pre-carbonized product to potassium citrate is 1:3, the heating rate is 4-6℃ / min, the activation temperature is 700℃, and the holding time is 2 h.

[0015] Preferably, the acid washing in step (3) is performed using hydrochloric acid, sulfuric acid or nitric acid with a concentration of 0.5 to 2 mol / L.

[0016] Preferably, the mass ratio of the porous carbon material to urea in step (4) is 1:5.

[0017] Preferably, the hydrothermal reaction temperature in step (4) is 160 °C and the reaction time is 12 h.

[0018] Preferably, the stirring time in step (4) is 0.5 to 2 h, and the drying temperature is 60 to 100 °C.

[0019] This invention also provides a nitrogen-doped wampee kernel-based porous carbon material prepared by the above-described method. The material exhibits a loose, sheet-like porous structure with a specific surface area of ​​1000–2500 m² / g. This invention further provides a supercapacitor electrode material comprising the above-described nitrogen-doped wampee kernel-based porous carbon material as an active material.

[0020] The present invention also provides a symmetrical supercapacitor, in which the above-mentioned nitrogen-doped yellow peel kernel-based porous carbon material is used as the positive electrode and negative electrode active material, respectively, and an alkaline aqueous solution is used as the electrolyte.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. This invention uses waste wampee seeds as biomass carbon precursors. The raw materials are widely available, inexpensive, and environmentally friendly, achieving high-value utilization of agricultural waste and facilitating large-scale production. Wampee seeds, a common agricultural processing waste in tropical and subtropical regions, are currently mostly discarded. This invention provides an effective way to utilize them for resource recovery.

[0023] 2. This invention uses potassium citrate as an activator, which significantly reduces corrosiveness compared to traditional strong alkaline activators such as KOH and NaOH, and the dosage is moderate (carbon-potassium mass ratio 1:2-4). While ensuring a high specific surface area, it reduces corrosion to equipment and subsequent wastewater treatment costs, making it more environmentally friendly.

[0024] 3. This invention innovatively combines potassium citrate activation with urea hydrothermal doping, resulting in a significant synergistic effect: the porous structure formed by potassium citrate activation (mainly micropores with a small amount of mesopores) provides abundant active sites and unobstructed diffusion channels for ion transport and charge storage; urea hydrothermal doping, under mild conditions, uniformly introduces nitrogen-containing functional groups such as pyridine-N and pyrrole-N into the carbon framework, providing not only additional pseudocapacitance but also increasing surface defects and active sites, thus improving electrolyte wettability. This synergistic effect enables the material to achieve an effective nitrogen doping of 3.64 at% while maintaining a high specific surface area (1358 m² / g).

[0025] 4. The nitrogen-doped porous carbon material prepared in this invention exhibits excellent electrochemical performance: in a three-electrode system with 6 MKOH electrolyte, the specific capacitance reaches 305 F / g at a current density of 0.5 A / g, and even at a high current density of 10 A / g, the specific capacitance remains at 242 F / g, with a capacitance retention rate of 79.3%, far exceeding that of the undoped sample (67.2%). The charge transfer resistance Rct is only 0.10 Ω, indicating that it possesses rapid charge transfer kinetics. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 SEM image of N-CKPC-5 (Example 1 of the present invention);

[0028] Figure 2 GCD curve of N-CKPC-5 (Example 1 of the present invention) at 1 A / g;

[0029] Figure 3 Cyclic voltammetry of N-CKPC-5 (Example 1 of the present invention) at different scan rates;

[0030] Figure 4 The graph shows the cyclic test results of N-CKPC-5 (Example 1 of this invention) at 10A / g.

[0031] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0033] Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0034] A method for preparing a nitrogen-doped wampee kernel-based porous carbon material includes the following steps:

[0035] (1) Pretreatment: Waste yellow peel fruit kernels were washed with deionized water and ethanol to remove surface impurities, then dried at 80℃ and ground into fine powder; (2) Pre-carbonization: Yellow peel fruit kernel powder was heated to 500℃ at a heating rate of 5℃ min⁻¹ under a nitrogen atmosphere and kept at a constant temperature for 2 h to obtain pre-carbonized product; (3) Activation: The pre-carbonized product was thoroughly mixed with potassium citrate at a mass ratio of 1:3, heated to 700℃ at a heating rate of 5℃ min⁻¹ under a nitrogen atmosphere and kept at a constant temperature for 2 h; the obtained product was repeatedly washed with 1 M hydrochloric acid and deionized water and then dried to obtain porous carbon material; (4) Nitrogen doping: Porous carbon material was mixed with urea at a mass ratio of 1:(4~6), deionized water was added and stirred continuously for 0.5 h, then transferred to a reaction vessel and hydrothermally reacted at 160℃ for 12 h; the reaction product was washed with deionized water and placed at 60℃ The material was dried in an oven at ℃ to obtain nitrogen-doped yellow peel kernel-based porous carbon material.

[0036] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0037] Example 1

[0038] (1) Pretreatment: Waste yellow peel fruit kernels were washed with deionized water and ethanol to remove surface impurities, then dried at 80℃ and ground into fine powder; (2) Pre-carbonization: Yellow peel fruit kernel powder was placed in a tube furnace and heated to 500℃ at a heating rate of 5℃ min⁻¹ under a nitrogen atmosphere, and kept at a constant temperature for 2 h to obtain pre-carbonized product; (3) Activation: The pre-carbonized product was thoroughly mixed with potassium citrate at a mass ratio of 1:3, and heated to 700℃ at a heating rate of 5℃ min⁻¹ under a nitrogen atmosphere, and kept at a constant temperature for 2 h; The resulting black product was repeatedly washed with 1 M hydrochloric acid and deionized water and then dried to obtain porous carbon material; (4) Nitrogen doping: The porous carbon material was mixed with urea at a mass ratio of 1:5, placed in a beaker, 60 mL of deionized water was added and stirred continuously for 0.5 h, then poured into a 100 mL reaction vessel inner liner, and dried in an oven at 5℃. The temperature was increased to 160 °C for hydrothermal reaction for 12 h; the reaction product was washed with deionized water and dried in an oven at 60 °C to finally obtain nitrogen-doped yellow peel kernel-derived carbon material.

[0039] Example 2

[0040] In this embodiment, all preparation steps and parameters are the same as in Example 1. The difference is that in step (4), the mass ratio of porous carbon material to urea is 1:4, and nitrogen-doped yellow peel kernel-derived carbon material is finally obtained.

[0041] Example 3

[0042] In this embodiment, all preparation steps and parameters are the same as in Example 1. The difference is that in step (4), the mass ratio of porous carbon material to urea is 1:6, and nitrogen-doped yellow peel kernel-derived carbon material is finally obtained.

[0043] Comparative Example 1

[0044] In this comparative example, all preparation steps and parameters are the same as in Example 1. The difference is that the urea hydrothermal doping process in step (4) is omitted, that is, the porous carbon obtained by activation is not treated by urea hydrothermal reaction and is directly used as a comparative sample.

[0045] Comparative Example 2

[0046] In this comparative example, all preparation steps and parameters are the same as in Example 1, except that: (1) in step (3), potassium hydroxide is used instead of potassium citrate as the activator, and the mass ratio of potassium hydroxide to the pre-carbonized product is 1:3. (2) The hydrothermal doping step of urea in step (4) is omitted, and the other conditions remain unchanged. After the reaction, obvious corrosion marks were observed on the inner wall of the furnace tube, and the pH value of the wastewater after washing was 12.5.

[0047] Comparative Example 3

[0048] In this comparative example, all preparation steps and parameters are the same as in Example 1. The difference is that the hydrothermal reaction in step (4) is cancelled, and the porous carbon material and urea are directly mixed and ground at a mass ratio of 1:5, and then calcined at 700°C for 2 h under a nitrogen atmosphere to carry out high-temperature nitrogen doping.

[0049] Comparative Example 4

[0050] In this comparative example, all preparation steps and parameters are the same as in Example 1. The difference is that the pre-carbonization temperature in step (2) is changed to 400 ℃ (Note: 400 ℃ is lower than the preferred range of 450~550 ℃ in this invention, which is used to illustrate that if the pre-carbonization temperature is too low, the carbon skeleton will not develop completely, which will affect the subsequent activation pore-forming efficiency and the final electrochemical performance). The other conditions remain unchanged.

[0051] Performance testing and characterization

[0052] The porous carbon materials prepared in Examples 1-3 and Examples 1-4 were characterized structurally and tested electrochemically. The specific results are as follows:

[0053] Table 1 Comparison of structural parameters of each sample

[0054]

[0055] Note: Specific surface area, pore size distribution, and pore volume were obtained through N2 adsorption-desorption tests; nitrogen content was obtained through XPS tests.

[0056] Table 1 shows that Comparative Example 1 (potassium citrate activation but no nitrogen doping) has a specific surface area of ​​941 m² / g and a nitrogen content of 0.56. Comparative Example 2 (potassium hydroxide activation), although having a higher specific surface area (1523 m² / g), is highly corrosive due to its strong alkali nature and lacks nitrogen doping. Comparative Example 3 (high-temperature calcination with nitrogen doping) has a specific surface area of ​​only 1186 m² / g and a nitrogen content of 2.12 at%, indicating that high-temperature calcination leads to partial collapse of the pores and significant nitrogen loss. Comparative Example 4 (pre-carbonization temperature 400 ℃) has a specific surface area of ​​1245 m² / g, lower than Example 1, indicating that a low pre-carbonization temperature is detrimental to subsequent activation and pore formation. Example 1, while maintaining a high specific surface area (1358 m² / g), achieved 3.64 at% nitrogen doping, with a moderate degree of defect, which is beneficial for improving capacitor activity.

[0057] Table 2. Comparison of electrochemical performance of each sample in a three-electrode system (6 M KOH)

[0058]

[0059] Note: Specific capacity is calculated by constant current charge-discharge (GCD) test; the rate retention rate is the ratio of specific capacity at 10 A / g to that at 0.5 A / g.

[0060] As shown in Table 2, Example 1 exhibits a high specific capacity of 305 F / g at 0.5 A / g and maintains 242 F / g at 10 A / g, achieving a rate retention of 79.3%. Comparative Example 2 (activated with potassium hydroxide), while having a higher specific surface area, only has a specific capacity of 301 F / g (0.5 A / g) and poor rate performance, indicating that simply increasing the specific surface area has limitations in improving capacitance performance. The pseudocapacitance and surface active sites introduced by nitrogen doping are crucial for performance enhancement. Comparative Example 3 (high-temperature calcination nitrogen doping) performs worse than Example 1, demonstrating that hydrothermal doping has significant advantages in maintaining pore structure and improving nitrogen doping efficiency.

[0061] Performance testing of symmetrical supercapacitor devices

[0062] The N-CKPC-5 prepared in Example 1 was assembled into a symmetrical supercapacitor (N-CKPC-5 / / N-CKPC-5), and electrochemical tests were performed in 6 MKOH electrolyte. The results are shown in the table below:

[0063] Table 3 Device performance of N-CKPC-5 symmetric supercapacitor

[0064]

[0065] Note: Energy density and power density are calculated based on the overall mass of the symmetrical supercapacitor device;

[0066] As shown in Table 3, the N-CKPC-5 symmetrical supercapacitor achieves a specific capacitance of 120.6 F / g at 0.5 A / g and an energy density of 16.75 Wh / kg at a power density of 250 W / kg. Even when the power density is increased to 5000 W / kg, the energy density remains at 12.5 Wh / kg, demonstrating excellent rate performance. After 10,000 charge-discharge cycles at 10 A / g, the capacity retention is 82.9%, and the coulombic efficiency remains essentially 100%, exhibiting outstanding cycle stability.

[0067] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing a nitrogen-doped wampee kernel-based porous carbon material, characterized in that, Includes the following steps: (1) Pretreatment: The kernels of the yellow peel fruit are washed, dried and crushed to obtain kernel powder; (2) Pre-carbonization: The fruit kernel powder is heated to 450-550 ℃ at 2-10 ℃ / min under an inert atmosphere and kept at the temperature for 1-3 h to obtain the pre-carbonized product; (3) Activation: The pre-carbonized product is mixed with potassium citrate at a mass ratio of 1:(2-4), and heated to 650-750 ℃ ​​at 2-10 ℃ / min under an inert atmosphere, and kept at the temperature for 1-3 h. After acid washing, water washing and drying, porous carbon material is obtained. (4) Nitrogen doping: The porous carbon material is mixed with urea at a mass ratio of 1:(3-6), deionized water is added and stirred to disperse, and then hydrothermally reacted at 140-180 °C for 8-16 h. After washing and drying, nitrogen-doped yellow peel kernel-based porous carbon material is obtained.

2. The preparation method according to claim 1, characterized in that, The drying temperature in step (1) is 60-100 ℃, and the particle size of the powder after pulverization is 100-300 mesh.

3. The preparation method according to claim 1, characterized in that, The acid washing in step (3) is carried out using hydrochloric acid, sulfuric acid or nitric acid with a concentration of 0.5 to 2 mol / L.

4. The preparation method according to claim 1, characterized in that, The stirring time in step (4) is 0.5 to 2 hours, and the drying temperature is 60 to 100 °C.

5. A nitrogen-doped wampee kernel-based porous carbon material, prepared by the method described in any one of claims 1 to 4, characterized in that, The material has a fluffy, sheet-like porous structure with a specific surface area of ​​1000–2500 m² / g.

6. A symmetrical supercapacitor, characterized in that, As described in claim 5, nitrogen-doped yellow peel kernel-based porous carbon materials are used as positive and negative electrode active materials, respectively.

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