Nitrogen, phosphorus, sulfur co-doped coal-based carbon aerogel and preparation method and application thereof

Abstract

The application discloses a preparation method of nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel, and comprises the following steps: firstly, ball-milling and screening raw coal to obtain coal powder; pre-oxidizing the coal powder in air at 250-350 o C to obtain pre-oxidized coal powder; and preparing the pre-oxidized coal powder, K2CO3 and PVP into a coal-based carbon aerogel precursor, and co-doping the coal-based carbon aerogel precursor with nitrogen, phosphorus and sulfur heteroatoms to obtain the coal-based carbon aerogel. The method can not only improve the specific surface area, porosity and hydrophilicity of the material, but also improve the specific capacitance of the coal-based aerogel, and make the coal-based aerogel have better electric conductivity.

Description

TECHNICAL FIELD

[0001] The present application relates to a nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel and its preparation method and application, and belongs to the technical field of inorganic carbon material preparation. BACKGROUND

[0002] As a new type of energy storage device, supercapacitors have been widely used in energy, transportation, electronic equipment, industry and other fields due to their high power density, fast charge and discharge speed, excellent life cycle and other outstanding advantages. The electrode material determines the energy storage mode and performance of the supercapacitor. Common electrode materials include carbon materials, transition metal oxides / hydroxides, conductive polymers and their composite materials. Among them, carbon materials are widely used as supercapacitor electrode materials due to their advantages of wide source, high specific surface area, stable structure and controllable electrical conductivity.

[0003] Coal has attractive advantages such as low price, abundant reserves, and geographical distribution, and can be used as a high-quality carbon material precursor. However, it is a difficult problem to convert raw coal into a supercapacitor electrode material through a suitable method. At present, coal-based carbon materials are generally synthesized by activating raw coal through various technologies such as KOH, ZnCl2, MgCO3, K2CO3 activation, NaCl, Na2CO3, Mg(OH)2 template method for pore making, and ionic liquid activation for pore making.

[0004] Carbon aerogels have efficient ion transport and surface activity due to their hierarchical structure, three-dimensional interconnected channels and high specific surface area, which is beneficial to improve the energy storage capacity of the material. At present, aerogels formed by stacking nanosheets into a skeleton are mainly focused on graphene, MXene and biomass carbon, while aerogels synthesized from coal generally have a honeycomb structure. The main goal of constructing coal-based aerogel structure is to effectively adjust the pore structure, increase the specific surface area, promote electrolyte permeation and carrier transfer, and expect to exhibit good reversible capacity, rate performance and stability.

[0005] Therefore, it is still a challenge to find a novel and simple synthesis strategy to prepare hierarchical heteroatom-doped microporous / mesoporous carbon aerogels with excellent electrochemical performance. SUMMARY

[0006] The purpose of the present application is to provide a nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel and its preparation method, and to assemble an asymmetric supercapacitor device for high-performance supercapacitors.

[0007] The implementation process of the present application is as follows:

[0008] A preparation method of a nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel, comprising the following steps:

[0009] (1) Ball milling and sieving raw coal to obtain coal powder;

[0010] (2) pre-oxidizing the coal powder in air at 250-350 o C to obtain pre-oxidized coal powder;

[0011] (3) adding K2CO3 and PVP to the pre-oxidized coal powder, stirring to form a hydrogel, and freeze-drying to obtain a coal-based carbon aerogel precursor;

[0012] (4) using melamine, sodium dihydrogen phosphate and sulfur powder as nitrogen source, phosphorus source and sulfur source respectively, annealing the coal-based carbon aerogel precursor in an inert atmosphere at 600-900 o C to obtain a heteroatom-doped coal-based carbon aerogel;

[0013] (5) acid washing, water washing and drying the heteroatom-doped coal-based carbon aerogel to obtain a nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel.

[0014] In the above step (3), the mass ratio of the pre-oxidized coal powder, K2CO3 and PVP is 1:(0.5-2):(0.5-1), and the preferred mass ratio of the pre-oxidized coal powder, K2CO3 and PVP is 1:1:1.

[0015] In the above step (4), the mass ratio of melamine, sodium dihydrogen phosphate and sulfur powder is (1-2):(1-2):(1-2), and the preferred mass ratio of melamine, sodium dihydrogen phosphate and sulfur powder is 1:1:1.

[0016] In the above step (4), the annealing temperature is 700 o C / min, and the annealing temperature is 700 o C.

[0017] In the above step (5), the concentration of the dilute hydrochloric acid is 1-2 mol / L.

[0018] The coal-based carbon aerogel prepared by the above method can be assembled into an asymmetric supercapacitor device for manufacturing high-performance supercapacitors.

[0019] The present application uses K2CO3 as an activator and PVP as a crosslinking agent to obtain a coal-based hydrogel through a sol-gel reaction, and a precursor carbon aerogel is obtained after freeze-drying. At this time, K2CO3 is recrystallized and fixed in situ in the precursor carbon aerogel. When annealing at 700 o C under an argon atmosphere, a 3D hierarchical porous carbon aerogel with a honeycomb structure is formed. The specific process is as follows: K2CO3 is decomposed into K2O and CO2 at 700 C, and when carbon is present, K2O is reduced to K, and CO2 continuously etches the carbon skeleton to form micropores. On the other hand, K2CO3 directly reacts with carbon in the aerogel to form K, and the carbon skeleton is continuously etched by potassium vapor to form a large number of micropores.

[0020] When nitrogen, sulfur and phosphorus sources are present, the precursor is activated into a hierarchical porous structure supported by carbon nanosheets. This unique structure composed of two-dimensional nanosheets is due to the presence of one or more of melamine, disodium hydrogen phosphate and sulfur powder, which, together with K2CO3, plays a key role in the exfoliation of the carbon skeleton and the formation of layered carbon nanosheets. In combination with the literature, the formation process is as follows: melamine decomposes into g-C3N4 and NH3 upon heating, NH3 reacts with the carbon skeleton, and ultimately promotes the formation of a porous structure in the carbon matrix. While g-C3N4 further decomposes into inorganic gas due to its poor thermal stability, it eventually disappears. This process not only promotes the formation of pores in the carbon matrix, but also forms N-doping, i.e. pyridine-N, pyridine-N (N-5), graphite-N and oxidized pyridine-N.

[0021] 3Na2HPO4 + 3NH3 = 2Na3PO4 + (NH4)3PO4

[0022] (NH4)3PO4 generated by disodium hydrogen phosphate under the action of NH3 can be gasified at high temperature, uniformly distributed on the surface of carbon, and at the same time decomposed into NH3 and H3PO4, introducing P elements into the carbon skeleton. NH3, H3PO4, sulfur powder promote the formation of pore structure in the carbon matrix, and together with the micropores formed by the decomposition of K2CO3, finally form a 3D structure aerogel supported by two-dimensional nanosheets.

[0023] Compared with the prior art, the present application has the following beneficial effects: nitrogen, phosphorus and sulfur doped coal-based carbon aerogel material, nitrogen, phosphorus and sulfur doping not only improves the specific surface area and porosity of the material, but also improves the hydrophilicity of the carbon matrix, so that the porous carbon is better infiltrated in the electrolyte, thereby increasing the contact interface area. At the same time, the pyridine nitrogen produced during carbonization improves the pseudo-capacitance of the porous carbon, and the graphite nitrogen improves the conductivity of the porous carbon. The doped phosphorus and sulfur also provide more active reaction sites and additional pseudo-capacitance. Under the synergistic action of nitrogen, phosphorus and sulfur doping, not only the specific capacitance of the coal-based aerogel is improved, but also the coal-based aerogel has better conductivity. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 SEM image of coal-based aerogel prepared for Example 1;

[0025] Figure 2 SEM image of nitrogen-doped coal-based aerogel prepared with melamine as a nitrogen source;

[0026] Figure 3 SEM image of nitrogen and phosphorus co-doped coal-based aerogel;

[0027] Figure 4 SEM image of nitrogen and sulfur co-doped coal-based aerogel;

[0028] Figure 5 SEM images of nitrogen, phosphorus, sulfur co-doped coal-based carbon aerogels;

[0029] Figure 6 XRD images of N-C, N-P-C, N-S-C and N-P-S-C;

[0030] Figure 7 Raman images of N-C, N-P-C, N-S-C and N-P-S-C;

[0031] Figure 8 Specific capacity of aerogels prepared by changing the mass ratio of K2CO3 and PVP at different current densities;

[0032] Figure 9 Rate performance curves of C, N-C, P-C, S-C at a current density of 0.5-10 A / g;

[0033] Figure 10 Rate performance curves of C, N-C, N-S-C, N-P-S-C at a current density of 1-10 A / g;

[0034] Figure 11 Cycle performance curve of N-P-S-C at a current density of 10 A / g;

[0035] Figure 12 Energy density of C, N-C, N-S-C, N-P-S-C and commercial AC assembled into asymmetric devices. DETAILED DESCRIPTION

[0036] The technical solutions of the present application and their effects will be further described below in combination with specific embodiments / examples. The following embodiments / examples are only used to illustrate the content of the present application, and the present application is not limited to the following embodiments or examples.

[0037] Example 1 Preparation of undoped coal-based carbon aerogel

[0038] (1) The raw coal was ball milled and sieved to obtain 200 mesh coal powder; the rotation speed of the ball mill was 100 r / min, and the ball milling time was 24 h;

[0039] (2) The 200 mesh coal powder was subjected to a pre-oxidation treatment of 300 o C / 2h in an air atmosphere, and then naturally cooled to room temperature to obtain pre-oxidized coal powder;

[0040] (3) Preparation of aerogel precursor: 1 g of K2CO3 was dissolved in 10 mL of deionized water, 1 g of PVP and 1 g of pre-oxidized coal were added and stirred for 6 h to form a hydrogel, which was freeze-dried for 24 h to obtain a coal-based carbon aerogel precursor;

[0041] (4) The precursor obtained in step (3) was heated at a rate of 2 o C / min under an inert atmosphere, and annealed at 700 o C / 2h to obtain a hierarchical coal-based carbon aerogel;

[0042] (5) Preparation of coal-based carbon aerogel: after cleaning and drying, the coal-based carbon aerogel obtained in (4) was subjected to acid washing with 2 mol / L hydrochloric acid, water washing and drying treatment to obtain a coal-based carbon aerogel.

[0043] It can be seen from Figure 1 that the coal-based carbon aerogel is a honeycomb-like hierarchical structure.

[0044] Example 2 Preparation of nitrogen, phosphorus and sulfur single-doped coal-based carbon aerogel

[0045] The difference from Example 1 is that in step (4), 2 g of melamine is used as a nitrogen source, or 2 g of disodium hydrogen phosphate is used as a phosphorus source, or 2 g of sulfur powder is used as a sulfur source, and the aerogel precursor is heated at a rate of 2 o C / min under an inert atmosphere, and annealed at 700 o C / 2h to obtain nitrogen, phosphorus and sulfur single-doped coal-based carbon aerogel electrode materials (N-C, P-C, S-C).

[0046] Figure 2 The SEM image of the nitrogen-doped coal-based aerogel prepared by using melamine as a nitrogen source shows that the presence of melamine makes the skeleton thinner, and obvious mesopores appear on the skeleton, and part of the skeleton is converted into nanosheets.

[0047] Example 3 Preparation of nitrogen and phosphorus co-doped coal-based aerogel

[0048] The difference from Example 1 is that in step (4), 1 g of melamine and 1 g of disodium hydrogen phosphate are used as a nitrogen source and a phosphorus source, respectively, and the aerogel precursor is heated at a rate of 2 o C / min under an inert atmosphere, and annealed at 700 o C / 2h to obtain nitrogen and phosphorus co-doped coal-based carbon aerogel electrode materials (N-P-C).

[0049] Figure 3 The SEM image of the nitrogen and phosphorus co-doped coal-based aerogel prepared by using melamine and disodium hydrogen phosphate shows that the nitrogen and phosphorus co-doped coal-based carbon aerogel is supported by a multi-layer nanosheet structure.

[0050] Example 4 Preparation of nitrogen and sulfur co-doped coal-based aerogel

[0051] The difference from Example 1 is that 1 g of melamine and 1 g of sulfur powder are respectively used as nitrogen source and sulfur source in step (4), and the aerogel precursor is placed downstream of the tube furnace, and the temperature is raised at a rate of 2 o C / min in an Ar atmosphere, and the aerogel precursor is annealed at 700 o C / 2h to obtain a nitrogen and sulfur co-doped coal-based carbon aerogel electrode material (N-S-C).

[0052] Figure 4 The SEM image of the nitrogen and sulfur co-doped coal-based aerogel prepared from melamine and sulfur powder shows that the nitrogen and sulfur co-doped coal-based carbon aerogel is supported by a multi-layer nanosheet structure, and the nanosheet is relatively thin.

[0053] Example 5 Preparation of nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel

[0054] The difference from Example 1 is that melamine, disodium hydrogen phosphate and sulfur powder are respectively used as nitrogen source, phosphorus source and sulfur source in step (4), and the aerogel precursor is placed downstream of the tube furnace at a rate of 2 o C / min in an inert atmosphere, and the aerogel precursor is annealed at 700 o C / 2h to obtain a nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel electrode material (N-P-S-C).

[0055] Figure 5 The SEM image of the nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel shows that the presence of nitrogen source, phosphorus source and sulfur source makes the nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel present a multi-layer nanosheet stacking structure.

[0056] Figure 6 The XRD image of N-C, N-P-C, N-S-C and N-P-S-C shows that the broad peak of carbon corresponds to (002) and (100) crystal planes, indicating that the prepared carbon aerogel is amorphous carbon.

[0057] Figure 7 The Raman image of N-C, N-P-C, N-S-C and N-P-S-C shows that the defects of the sample increase due to the doping of more heteroatoms into the carbon skeleton, resulting in more disordered carbon structure.

[0058] Table 1 Specific surface area and pore volume of C, N-C, N-P-C, N-S-C and N-P-S-C

[0059] Sample Specific surface area (m 2 / g) Micropore volume (cm3 / g) 3 / g) Total volume (cm 3 / g) Micropore / total pore volume (%) C 15.38 0.0027 0.0115 23.48 N-C 86.73 0.0213 0.059 36.1 N-P-C 55.45 0.0113 0.028 40.4 N-S-C 181.80 0.0514 0.121 42.5 N-P-S-C 285.59 0.0777 0.178 43.6

[0060] From Table 1, the specific surface areas of C, N-C, N-P-C, N-S-C, N-P-S-C are 15.38 m 2 / g, 86.73 m 2 / g, 55.45 m 2 / g, 181.8 m 2 / g and 285.59 m 2 / g, respectively. And the micropore contents are 23.48%, 36.1%, 40.4%, 42.5% and 43.6%, respectively. It shows that heteroatom doping can improve the specific surface area and micropore content of aerogel. (C represents pure coal-based carbon aerogel, N-C represents nitrogen-doped coal-based carbon aerogel, N-P-C represents nitrogen and phosphorus double-doped coal-based carbon aerogel, N-S-C represents nitrogen and sulfur double-doped coal-based carbon aerogel, and N-P-S-C represents nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel).

[0061] Figure 8 The specific capacity of the aerogel at different current densities when the mass of K2CO3 and PVP is 0.5 g:0.5 g, 0.5 g:1 g, 1 g:1 g, 1.5 g:1 g and 2 g:1 g, respectively. It can be seen that the specific capacity is higher when the mass ratio of K2CO3 and PVP is 1:1, and the mass of 1 g:1 g is the best ratio.

[0062] Figure 9 The rate performance curves of C, N-C, P-C and S-C at a current density of 0.5-10 A / g. The results show that nitrogen doping can improve the specific capacity, and phosphorus and sulfur doping have poor effect.

[0063] Figure 10 The rate performance curves of C, N-C, N-S-C and N-P-S-C at a current density of 1-10 A / g. The results show that N-P-S-C has a specific capacity as high as 435 F / g at a current density of 1 A / g, and the capacity retention rate is 55.6% at a current density of 15 A / g.

[0064] Figure 11 The cycle performance curve of N-P-S-C at a current density of 10 A / g. The results show that N-P-S-C exhibits good cycle stability after 5000 cycles, reaching 88.2%.

[0065] Figure 12 The energy density of C, N-C, N-S-C, N-P-S-C and commercial AC assembled into asymmetric devices. The results show that nitrogen, phosphorus and sulfur co-doping makes the device N-S-P-C / / AC have the highest energy density of 14.5 Wh / kg (899 W / kg).

[0066] In summary, the coal-based carbon aerogel prepared by heteroatom doping solves the problems of low specific capacity and poor rate performance of pure carbon aerogel. The specific capacity of the coal-based aerogel electrode material (at 15 A / g) is 200-450 F / g. In the experiments of the present application, the specific capacities of C, N-C, N-P-C, N-S-C and N-P-S-C are 233 F / g, 272 F / g, 324 F / g, 433 F / g and 435 F / g, respectively (C represents pure coal-based carbon aerogel, N-C represents nitrogen-doped coal-based carbon aerogel, N-P-C represents nitrogen and phosphorus double-doped coal-based carbon aerogel, N-S-C represents nitrogen and sulfur double-doped coal-based carbon aerogel, and N-P-S-C represents nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel).

[0067] The rate performance of the coal-based aerogel electrode material (at 15 A / g) is 40-70%. In the experiments of the present application, the capacity retention rates of C, N-C, N-P-C, N-S-C and N-P-S-C at 15 A / g are 59.7%, 61.8%, 55.5%, 44.1% and 55.6%, respectively.

[0068] The asymmetric device composed of the nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel electrode material and activated carbon of the present application solves the problem of low energy density of carbon materials. In the experiments of the present application, the energy densities of N / / AC, N-C / / AC, N-S-C / / AC and N-S-P-C / / AC are 7.25 Wh / kg (905.3 W / kg), 8.55 (901 W / kg), 14.27 (899.4 W / kg) and 14.5 Wh / kg (899 W / kg), respectively (AC represents activated carbon).

[0069] It should be noted that the above examples do not limit the present application in any form, and any technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present application.

Claims

1. A method for the preparation of nitrogen, phosphorus, sulfur co-doped coal-based carbon aerogel, characterized by The method comprises the following steps: (1) ball-milling and screening raw coal to obtain coal powder; (2) pre-oxidizing the coal powder in air at 250-350 o C to obtain pre-oxidized coal powder; (3) adding water to the pre-oxidized coal powder, K2CO3 and PVP to form a hydrogel, and freeze-drying the hydrogel to obtain a coal-based carbon aerogel precursor; (4) Melamine, sodium dihydrogen phosphate and sulfur powder are respectively used as nitrogen source, phosphorus source and sulfur source, and the coal-based carbon aerogel precursor is annealed at 600-900 o C under inert atmosphere to obtain a heteroatom-doped coal-based carbon aerogel. (5) acid washing, water washing and drying the heteroatom-doped coal-based carbon aerogel to obtain a nitrogen, phosphorus and sulfur co-doped coal-based carbon aerogel.

2. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal-based carbon aerogel as claimed in claim 1, wherein: In step (3), the mass ratio of the pre-oxidized coal powder, K2CO3 and PVP is 1:(0.5-2):(0.5-1).

3. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal-based carbon aerogel according to claim 2, characterized by the fact that: The mass ratio of the pre-oxidized coal powder, K2CO3 and PVP is 1:1:

1.

4. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal based carbon aerogel as claimed in claim 1, wherein: In step (4), the mass ratio of the melamine, sodium dihydrogen phosphate and sulfur powder is (1-2):(1-2):(1-2).

5. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal-based carbon aerogel as claimed in claim 4, wherein: The mass ratio of the melamine, sodium dihydrogen phosphate and sulfur powder is 1:1:

1.

6. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal-based carbon aerogel as claimed in claim 1 wherein: In step (4), the annealing treatment is performed at a temperature increasing rate of 1 to 3 o C / min.

7. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal-based carbon aerogel as claimed in claim 1 wherein: In step (4), the annealing temperature is 700 o C.

8. The process for the preparation of nitrogen, phosphorus, sulphur co-doped coal-based carbon aerogel as claimed in claim 1 wherein: In step (5), the concentration of the dilute hydrochloric acid is 1-2 mol / L.

9. The coal-based carbon aerogel prepared by the method of claim 1.

10. The use of the coal-based carbon aerogel of claim 9 in supercapacitors for electrochemical energy storage.

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