A highly graphitized porous activated carbon material, its preparation method and application

Abstract

The present invention discloses a highly graphitized porous activated carbon material, its preparation method and application. A carbon precursor and water are mixed in a graphite crucible, and heat preservation treatment is carried out at 155 °C for 2 h under an inert atmosphere, and then the temperature is raised to 955 °C for heat preservation treatment for 2 h for pre-expansion carbonization treatment to obtain a pre-expanded intermediate; the pre-expanded intermediate and potassium hydroxide are weighed respectively according to a mass ratio of 1:2, mixed evenly in water, and after drying, it is placed again under a nitrogen atmosphere for heat preservation treatment at 755 °C for 2 h for activation treatment; it is cooled to room temperature with the furnace, taken out, washed with water until neutral, and dried to obtain the product. By regulating the water content of the carbon precursor, the present invention obtains a highly graphitized porous activated carbon material by using the water-bubble expansion carbonization-assisted KOH activation strategy, increases the number of reactive sites of the electrode material, improves the conductivity of the material, can improve the specific capacitance of the supercapacitor, and at the same time maintains good electrochemical performance parameters such as the cycle life of the electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of nano - material preparation, and particularly relates to a highly graphitized porous activated carbon material, a preparation method thereof, and an application thereof. Background Art

[0002] Porous activated carbon materials are the most widely used electrode active materials in commercial supercapacitors at present. Their rich micro / mesoporous channels increase the specific surface area of the materials and form abundant electrochemically active sites, which can provide unobstructed pathways for the penetration / diffusion of ions in the electrolyte, accelerate the kinetic process of ion migration, and thus exhibit excellent electrochemical energy storage characteristics. At present, the main method for constructing the pore structure of activated carbon materials and increasing their specific surface area is the activation method, including gas activation, salt activation, acid / alkali activation, etc. Among them, the KOH activation strategy is the most commonly used carbon material activation method. For example: Shang (Nano Energy, 75, (2525) 154531) et al. synthesized a carbon material with a specific surface area as high as 3577 m 2 / g based on the KOH activation method using walnut shell as the carbon source. The fabricated flexible supercapacitor exhibited an ultra - high energy density (125 Wh kg -1 ) and power density (155 kW kg -1 ).

[0003] However, the heat treatment activation temperature of KOH is generally low (<755 °C. Too high a heat treatment activation temperature will cause excessive corrosion of the carbon material, resulting in a low product yield). The low activation temperature leads to low crystallinity and poor conductivity of the carbon material, seriously affecting the electrochemical energy storage performance of activated carbon - based supercapacitors. In addition, from the perspective of the activation mechanism, KOH activation requires the carbon material to have a certain basic specific surface area to ensure sufficient contact between KOH and the carbon material and improve the activation efficiency.

[0004] At present, there has been no report on using water - bubble expansion carbonization pretreatment to obtain an initial carbon material with a certain basic specific surface area and high crystallinity, and then performing KOH activation to prepare a highly crystallized and large - specific - surface - area porous activated carbon material. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a preparation method of a highly graphitized porous activated carbon material to realize the preparation of a porous activated carbon material with high crystallinity and large specific surface area, and improve its electrochemical energy storage characteristics.

[0006] Another purpose of the present invention is to provide a highly graphitized porous activated carbon material prepared by the above - mentioned preparation method.

[0007] The third purpose of the present invention is to provide an application of the above - mentioned highly graphitized porous activated carbon material.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] In a first aspect, the present invention provides a method for preparing a highly graphitized porous activated carbon material, comprising the following steps:

[0010] (1) Mix a carbon precursor containing C element and water in a certain ratio in a graphite crucible, stir evenly, place it in a tube furnace, keep it at 155 °C for 2 h under an inert atmosphere, then raise the temperature to 955 °C and keep it for 2 h for pre-expansion carbonization treatment, and finally cool it to room temperature with the furnace to obtain a pre-expanded intermediate;

[0011] (2) Weigh the pre-expanded intermediate obtained in step (1) and potassium hydroxide respectively according to a mass ratio of 1:2, mix them evenly in water, dry them, and then place them in a tube furnace again. Carry out activation treatment at 755 °C for 2 h under a nitrogen atmosphere;

[0012] (3) Cool it to room temperature with the furnace, take it out, wash it with water until neutral, and dry it to obtain a porous activated carbon material.

[0013] Preferably, the carbon precursor containing C element in step (1) is maltose powder.

[0014] Preferably, the mixing ratio of the maltose powder and water in step (1) is 1 g: 5.8 mL. If the water volume is too low, it is not enough to dissolve maltose. If the water volume is too high, the viscosity of the mixture is likely to decrease, reducing the expansion pore formation rate.

[0015] Preferably, the heating rate in the first stage of step (1) is 5 °C / min, and the heating rate in the second stage is 2 °C / min.

[0016] Preferably, in step (2), the heating rate of the tube furnace is 3 °C / min.

[0017] In a second aspect, the present invention provides a highly graphitized porous activated carbon material prepared by the above preparation method.

[0018] The porous carbon nanomaterial obtained by pre-expansion treatment assisted by KOH activation has obvious pores and uniform pore size distribution, achieving the purpose of increasing the ion transport rate.

[0019] The porous carbon nanomaterial obtained by pre-expansion treatment assisted by KOH activation has high crystallinity, achieving the purpose of increasing the conductivity of the material.

[0020] In a third aspect, the present invention provides the application of the above highly graphitized porous activated carbon material as a supercapacitor electrode material.

[0021] The highly graphitized porous activated carbon material, binder polytetrafluoroethylene, and conductive carbon black are coated on nickel foam at a mass ratio of 85:15:5, dried in a vacuum drying oven at 155 °C for 12 h, and then pressed at 15 MPa to produce a working electrode with a loading of about 1.5 mg cm -2 .

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. By regulating the water content of the carbon precursor, the present invention uses the "pre-expansion strategy" to prepare an intermediate conducive to the entry of the activator, and then uses the KOH activation method to finally obtain a highly graphitized porous activated carbon material.

[0024] 2. The water-bubble expansion carbonization heat treatment process of the present invention has a high temperature, which is conducive to improving the graphitization degree of the activated carbon material.

[0025] 3. The preparation process of the present invention is simple and controllable, and has good repeatability.

[0026] 4. The highly graphitized porous activated carbon prepared by the present invention as an electrode material increases the number of reactive sites of the electrode material, improves the conductivity of the material, can increase the specific capacitance of the supercapacitor, and at the same time maintains good electrochemical performance parameters such as the cycle life of the electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 (a) is a digital photo of the initial activated carbon material obtained by pre-expansion carbonization in Example 1;

[0028] Figure 1 (b, c) are scanning electron microscope (SEM) images of the porous activated carbon material prepared in Example 1 at different magnifications;

[0029] Figure 1 (d) is a transmission electron microscope (TEM) image of the porous activated carbon material prepared in Example 1;

[0030] Figure 2 is an X-ray diffraction (XRD) pattern of the porous activated carbon material prepared in Example 1;

[0031] Figure 3 is a Raman spectrum of the porous activated carbon material prepared in Example 1;

[0032] Figure 4 (a) is an X-ray photoelectron spectroscopy (XPS) pattern of the porous activated carbon material prepared in Example 1;

[0033] Figure 4(b) High-resolution X-ray photoelectron spectroscopy (XPS) C1s spectrum of the porous activated carbon material prepared in Example 1;

[0034] Figure 4 (c) High-resolution XPS O1s spectrum of the porous activated carbon material prepared in Example 1;

[0035] Figure 4 (d) High-resolution XPS N1s spectrum of the porous activated carbon material prepared in Example 1;

[0036] Figure 5 (a) Comparison chart of nitrogen isothermal adsorption and desorption curves of the porous activated carbon materials prepared in the comparative example and Examples 1-4;

[0037] Figure 5 (b) Pore size distribution chart of the porous activated carbon materials prepared in the comparative example and Examples 1-4;

[0038] Figure 6 (a) Cyclic voltammogram of the electrodes of the porous activated carbon materials prepared in the comparative example and Examples 1-4 in a three-electrode system at a scan rate of 25 mV s -1 ;

[0039] Figure 6 (b) Galvanostatic charge-discharge curve of the electrodes of the porous activated carbon materials prepared in the comparative example and Examples 1-4 in a three-electrode system at a current density of 2 Ag -1 ;

[0040] Figure 7 Curve diagram of the relationship between the specific capacitance and the current density of the supercapacitor assembled with the electrode material prepared in Example 1. Detailed implementation manners

[0041] The technical solutions of the present invention will be further described and illustrated below through specific examples and accompanying drawings. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all common raw materials in the art, and the methods used in the embodiments are all conventional methods in the art.

[0042] Example 1

[0043] Weigh 2 g of maltose powder as the carbon precursor in a graphite crucible, add 1.6 mL of deionized water and stir evenly. Under N 2 atmosphere protection, heat it in a tube furnace at a rate of 5 °C / min to 155 °C and hold for 125 min, then heat it at a rate of 2 °C / min to 955 °C and hold for 125 min for pre-puffing treatment, and finally cool it to room temperature with the furnace to obtain a pre-puffed intermediate.

[0044] The pre-puffed intermediate and KOH were dissolved in 25 mL of deionized water at a mass ratio of 1:2. After stirring evenly, it was placed in a forced-air drying oven for drying. The dried mixture was transferred to a graphite crucible and heated in a tube furnace under N 2 atmosphere protection to 755 °C at a rate of 3 °C / min and held for 125 min for KOH etching activation treatment. Finally, it was cooled to room temperature with the furnace, and the mixture was taken out and rinsed with deionized water until the product was neutral to obtain a nitrogen-doped porous activated carbon material.

[0045] The temperature of the puffing carbonization heat treatment is preferably 955 °C. Too low a heat treatment temperature is not conducive to improving the graphitization degree of the initial carbon material prepared by bubble puffing carbonization.

[0046] The mass ratio of the pre-puffed intermediate to potassium hydroxide is preferably 1:2. If the mass ratio is too small, it is not sufficient to achieve the purpose of etching and forming pores. If the mass ratio is too large, it will cause excessive corrosion and reduce the product yield.

[0047] During activation, calcination in a nitrogen atmosphere not only prevents the loss of carbon materials but also performs nitrogen doping on the carbon materials. Through high-temperature calcination, a highly graphitized nitrogen-doped porous activated carbon material with a large specific surface area and uniform pores is obtained.

[0048] Example 2

[0049] The preparation steps of this example are basically the same as those of Example 1, except that: 5.8 mL of deionized water was added to dissolve the maltose powder.

[0050] Example 3

[0051] The preparation steps of this example are basically the same as those of Example 1, except that: 1.2 mL of deionized water was added to dissolve the maltose powder.

[0052] Example 4

[0053] The preparation steps of this example are basically the same as those of Example 1, except that: 2 mL of deionized water was added to dissolve the maltose powder.

[0054] Comparative Example

[0055] 2 g of maltose powder was weighed as a carbon precursor and placed in a graphite crucible. It was heated in a tube furnace under N 2 atmosphere protection to 155 °C at a rate of 5 °C / min and held for 125 min, then heated to 955 °C at a rate of 2 °C / min and held for 125 min for pre-puffing treatment. Finally, it was cooled to room temperature with the furnace to obtain a pre-puffed intermediate.

[0056] The pre-expanded intermediate and KOH were dissolved in 25 mL of deionized water at a mass ratio of 1:2, stirred evenly, and then dried in a forced-air drying oven. The dried mixture was placed in a graphite crucible and heated in a tube furnace under N 2 atmosphere protection at a rate of 3 °C / min to 755 °C and held for 125 min for KOH etching activation treatment. Finally, it was cooled to room temperature with the furnace, the mixture was taken out, and rinsed with deionized water until the product was neutral to obtain a nitrogen-doped porous activated carbon material.

[0057] Figure 1 (a) is an optical picture of the pre-expanded intermediate prepared in Example 1. After annealing treatments at 155 °C and 955 °C, a rich surface pore structure was generated due to the expansion of water molecules and the cleavage of maltose functional groups. Figure 1 (b-c) are SEM images of the nitrogen-doped porous activated carbon material prepared in Example 1 at different magnifications. It shows that the prepared nitrogen-doped porous activated carbon material is composed of nanoparticles and contains a large number of cracks inside, which provides channels for the penetration / diffusion of electrolytes and is conducive to the full utilization of active substances. Figure 1 (d) is a high-resolution transmission electron microscopy (HTEM) image of the nitrogen-doped porous activated carbon material prepared in Example 1, showing that the adjacent lattice spacing of the prepared material is about 5.341 nm, corresponding to the crystal plane spacing of graphite (552).

[0058] Figure 2 is the X-ray diffraction pattern (XRD) of the nitrogen-doped porous activated carbon material prepared in Example 1. Its two characteristic peaks near 26° and 43° indicate that they are related to the specific planes (552) and (155) of graphite.

[0059] Figure 3 is the Raman spectrum of the nitrogen-doped porous activated carbon material prepared in Example 1. The "disordered" D peak (~1355 cm -1 ) and the "crystalline" G peak (~1586 cm -1 ) appear in the figure, indicating that these peaks are related to the carbon material. The calculated I D / I G ratio is 5.93, indicating a high degree of graphitization transformation of the prepared sample.

[0060] Figure 4 (a) is the X-ray photoelectron spectroscopy (XPS) measurement scan of the nitrogen-doped porous activated carbon material prepared in Example 1, showing that the surface composition of the prepared nitrogen-doped porous activated carbon material is composed of three elements: C, N, and O, and nitrogen element has been successfully doped into the carbon material. Figure 4 (b)- Figure 4(d) High-resolution C1s, O1s, and N1s XPS spectra collected for the nitrogen-doped porous activated carbon materials prepared in Examples 2-4, indicating that the prepared nitrogen-doped porous activated carbon materials have rich functional groups, greatly promoting the wettability of the electrode and enabling effective mass transfer.

[0061] Figure 5 (a)- Figure 5 (b) Comparative diagrams of nitrogen isothermal adsorption-desorption curves and pore size distribution diagrams for the nitrogen-doped porous activated carbon materials prepared in the comparative example and Examples 1-4. The pictures show that as the water content increases, its specific surface area gradually increases, reaching the highest at a water addition of 1.6 mL. When the water content continues to increase to 2 mL, the specific surface area shows a downward trend. Due to the excessive water reducing the viscosity of maltose, during the pre-puffing carbonization process, it is difficult for low-viscosity maltose to establish a stable balance in the internal / external space of H 2 O bubbles, thus restricting the growth of pores and reducing the specific surface area;

[0062] The nitrogen-doped porous activated carbon materials prepared in the comparative example and Examples 1-4 were respectively coated on nickel foam with a binder polytetrafluoroethylene and conductive carbon black in a mass ratio of 85:15:5. After drying in a vacuum drying oven at 155 °C for 12 h, it was pressed at 15 MPa, and finally a working electrode with a loading of about 1.5 mg cm -2 was made. Using Hg / HgO as the reference electrode, a platinum sheet electrode as the counter electrode, and 6 mol / L KOH solution as the electrolyte, a three-electrode system was constructed to characterize the electrochemical behavior at room temperature.

[0063] Figure 6 (a) Cyclic voltammograms of the electrodes of the nitrogen-doped porous activated carbon materials prepared in the comparative example and Examples 1-4 in a three-electrode system at a scan rate of 25 mV s -1 show that at the same scan rate, the electrode material obtained in Example 1 has the largest areal specific capacitance;

[0064] Figure 6 (b) Galvanostatic charge-discharge curves of the electrodes of the nitrogen-doped porous activated carbon materials prepared in the comparative example and Examples 1-4 in a three-electrode system at a current density of 2 Ag -1 show that at the same current density, the electrode material obtained in Example 1 has the longest discharge time. Therefore, the sample C-1.6 obtained in Example 1 is the optimal electrode material.

[0065] A supercapacitor was constructed with the working electrode prepared in Example 1, 6 mol / L KOH solution as the electrolyte, and porous polypropylene as the separator, and its electrochemical performance was tested at room temperature.

[0066] Figure 7Graph of the relationship between specific capacitance and current density of the supercapacitor assembled with the electrode material prepared in Example 1. At a current density of 5.5 A / g, the specific capacitance obtained in Example 2 is 132.7 F / g, and when the current density increases to 25 A / g, 62.3% of the specific capacitance is still maintained. This indicates that the electrode material has good rate performance.

Claims

1. A preparation method of a highly graphitized porous activated carbon material, characterized in that, it comprises the following steps: (1) Mix a carbon precursor containing C element and water in a graphite crucible at a ratio of 1 g: 0.8 mL. After stirring evenly, place it in a tubular furnace. Under an inert atmosphere, keep it at 155 °C for 2 h, then heat it to 955 °C and keep it for 2 h for pre-expansion carbonization treatment. Finally, cool it to room temperature with the furnace to obtain a pre-expanded intermediate; (2) Weigh the pre-expanded intermediate obtained in step (1) and potassium hydroxide respectively according to a mass ratio of 1:2, mix them evenly in water, dry them, and then place them in a tubular furnace again. Under a nitrogen atmosphere, keep it at 755 °C for 2 h for activation treatment; (3) Cool it to room temperature with the furnace, take it out, wash it with water until neutral, and dry it to obtain a porous activated carbon material.

2. The preparation method of a highly graphitized porous activated carbon material according to claim 1, characterized in that, the carbon precursor containing C element in step (1) is maltose powder.

3. The preparation method of a highly graphitized porous activated carbon material according to claim 1, characterized in that, the heating rate in the first stage of step (1) is 5 °C / min, and the heating rate in the second stage is 2 °C / min.

4. The preparation method of a highly graphitized porous activated carbon material according to claim 1, characterized in that, in step (2), the heating rate of the tubular furnace is 3 °C / min.

5. A highly graphitized porous activated carbon material, characterized in that, it is prepared by the preparation method according to any one of claims 1 to 4.

6. Application of the highly graphitized porous activated carbon material according to claim 5 as a supercapacitor electrode material.

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