Nitrogen and phosphorus co-doped carbon aerogel material and method for preparing the same

Abstract

The application relates to the technical field of supercapacitor electrode material preparation, and particularly discloses a nitrogen and phosphorus co-doped carbon aerogel material and a preparation method thereof. Nanocellulose is prepared by taking bleached coniferous wood pulp as a base, inorganic or organic matter containing N and P double elements is used as a dopant, and the N and P double-doped porous three-dimensional network carbon aerogel with excellent performance and stable structure is prepared through one-step doping by ultrasonic mixing, freeze drying and high-temperature carbonization. Compared with the prior art, the raw materials selected by the application have a wide source and low price, the carbon aerogel prepared by the application has good mesoporous structure, uniform distribution of nitrogen and phosphorus, and excellent electrochemical performance, and the application provides an effective way for developing supercapacitors with low price, sustainable utilization, good flexibility, excellent electrochemical performance and good cycle stability.

Description

Technical Field

[0001] This invention relates to the field of supercapacitor electrode material preparation technology, specifically to a nitrogen-phosphorus co-doped carbon aerogel material and its preparation method. Background Technology

[0002] Porous carbon materials, particularly carbon aerogels (CAs), have been extensively studied as electrode materials. The unique structure of CAs provides efficient diffusion / mass transfer channels for electrolyte ion / electron migration and multiple active sites for electrostatic attraction, resulting in electrodes with excellent electrochemical performance. Recent studies have revealed some drawbacks in the preparation of CAs as electrode materials, including expensive and hazardous precursors and complex preparation processes. Nanocellulose aerogels are among the most promising candidates for CAs due to their environmentally friendly and sustainable sources, excellent mechanical strength, large specific surface area, and impressive flexibility. Nanocellulose aerogels possess highly active -OH groups, allowing for easy surface modification and bonding with conductive fillers via covalent or hydrogen bonds to prepare high-performance electrodes. Y. Ma et al. reported the preparation of ultralight carbon aerogels using bacterial cellulose precursors and their application in supercapacitors, achieving a specific capacitance of 158 F / g. -1 (J. Mater. Chem. A, 2021, 9,900).

[0003] In summary, these advantages give carbon aerogels derived from nanocellulose through freeze-drying and carbonization excellent properties, including large specific surface area, high specific capacitance, good cycle stability, and good energy and power density. However, due to the energy storage mechanism of the double layer in nanocellulose carbon aerogels and the fact that most of the pores in the prepared carbon aerogels are microporous, they are difficult to be penetrated by electrolytes, resulting in relatively low specific capacitance and energy density. Heteroatom doping is an effective strategy to improve the physicochemical properties of carbon materials. For example, patent application CN107265438A discloses a method using bacterial cellulose as a precursor for carbon aerogels, which involves impregnating the aerogel with ammonium dihydrogen phosphate solution and then carbonizing it to obtain carbon aerogels. However, using bacterial cellulose as a precursor requires a long soaking and acid washing process, which is time-consuming and environmentally unfriendly. The impregnation doping method also suffers from the drawback of a long processing cycle. For example, in application number CN108238596A, carbon aerogels are made by synthesizing polymer gel materials using pyrrole, formaldehyde, glyoxal, and glutaraldehyde as monomers. The selected monomers are harmful and expensive; the high carbonization activation temperature also leads to energy consumption. Another example is in application number CN105692587A, where nitrogen is doped in an NH3 atmosphere. However, NH3 is highly toxic to humans, and its decomposition at high temperatures produces hydrogen gas, making the process complex and dangerous. Summary of the Invention

[0004] To address the shortcomings and deficiencies of existing technologies, and to further improve the current limitations of nanocellulose carbon aerogels, this invention provides a nitrogen-phosphorus co-doped carbon aerogel material and its preparation method. By selecting inexpensive inorganic or organic materials as dopants and employing a one-step doping method to dope N and P dual elements, a carbon aerogel with a multi-level microporous and mesoporous channel structure and excellent electrochemical performance is prepared. This provides an effective approach for developing inexpensive, sustainable, flexible, electrochemically superior, and cycle-stable supercapacitors.

[0005] This invention can be achieved through the following technical solutions:

[0006] A method for preparing a nitrogen-phosphorus co-doped carbon aerogel material, which is used as an electrode material for supercapacitors, includes the following steps:

[0007] (1) Preparation of oxidized nanocellulose: Bleached softwood pulp was dispersed in deionized water with pH 10-10.5 and stirred evenly. Oxidation was carried out using NaClO and NaBr as catalysts and TEMPO (2,2,6,6-tetramethylpiperidin-1-oxy) as oxidant. After the reaction was completed, the pulp was washed with deionized water until neutral. Finally, the pulp was filtered and freeze-dried to obtain oxidized nanocellulose (TOCNFs). The charge density of the obtained TOCNFs was 200-1800 μmol / g.

[0008] (2) Preparation of carbon aerogel: Inorganic or organic materials containing N and P elements and oxidized nanocellulose prepared in step (1) are ultrasonically mixed in deionized water. The ultrasonic power is 800-1800W (generally 20-60min). After solvent exchange, the mixture is rapidly frozen with liquid nitrogen and then freeze-dried for 24-48h to obtain aerogel. The aerogel is then carbonized in a nitrogen atmosphere to obtain a three-dimensional network carbon aerogel material with excellent performance and stable structure that is doped with N and P. The mass ratio of inorganic or organic materials containing N and P elements to freeze-dried oxidized nanocellulose is 1-4:5.

[0009] Furthermore, the inorganic compound containing both N and P elements is ammonium polyphosphate, and the mass ratio of ammonium polyphosphate to oxidized nanocellulose is 2:5.

[0010] Furthermore, the carbonization process in step (2) involves heating to 600-800°C at a heating rate of 5-10°C / min for 2-5 hours.

[0011] A nitrogen-phosphorus co-doped carbon aerogel material prepared by the method described above.

[0012] An application of the nitrogen-phosphorus co-doped carbon aerogel material as described above, used to prepare electrode materials for supercapacitors, is as follows: the nitrogen-phosphorus co-doped carbon aerogel is ground and mixed with carbon black and PVDF, then placed in an ultrasonic cleaner for ultrasonic mixing and drying, thus obtaining the nitrogen-phosphorus co-doped electrode material for supercapacitors.

[0013] Furthermore, the mass ratio of the nitrogen-phosphorus co-doped carbon aerogel, carbon black, and PVDF is 8:0.8-1.2:0.8-1.2.

[0014] Compared with existing technologies, this invention achieves the following beneficial effects: It employs a one-step doping method to dope N and P dual elements, preparing carbon aerogels with microporous and mesoporous hierarchical pore structures. The operation is simple and quick, and the prepared porous carbon material has a high specific surface area. The N and P elements are uniformly dispersed, improving the surface wettability and conductivity of the carbon material, providing more chemically active sites, enhancing the electrode material's energy storage capacity, improving electrochemical stability, and exhibiting extremely high specific capacitance (compared to currently published or disclosed literature (CN201710997775.4, CN20...)). (Data from CN201810279884.1 and CN201810841339.2 is several times higher); This invention changes the pyrolysis route of nanocellulose by a one-step doping method with N and P elements, thereby increasing the residual carbon content after pyrolysis of nanocellulose and improving the yield of nanofiber carbon aerogel; Using bleached softwood pulp as a base to prepare nanocellulose as a carbon precursor saves costs and is inexpensive and environmentally friendly; The selected inorganic or organic substances are non-toxic and odorless, do not produce corrosive gases, have low hygroscopicity and high thermal stability, and are excellent non-halogenated flame retardants, effectively improving material safety. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the accompanying drawings used in the description of the specific embodiments will be briefly introduced below.

[0016] Figure 1 The images show the infrared spectra of cellulose at different degrees of oxidation.

[0017] Figure 2 CV diagrams of carbon aerogel materials at different current densities in comparative examples and Examples 1-4.

[0018] Figure 3 GCD diagrams of carbon aerogel materials at different current densities in comparative examples and Examples 1-4.

[0019] Figure 4 Elemental distribution diagram of carbon aerogel in Example 1.

[0020] Figure 5 A schematic diagram of the contact angle of Comparative Example (A) and Example 1 (B). Detailed Implementation

[0021] The present invention will be further described below with reference to the embodiments. The embodiments provide detailed implementation methods and specific operating procedures, but the scope of protection of the present invention is not limited to the following embodiments.

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments (in the following embodiments and comparative examples, carbon black and PVDF are auxiliary reagents required for electrode preparation; in the actual working electrode of the test, these two reagents have been excluded).

[0023] Example 1

[0024] (1) Weigh 3g of bleached softwood pulp, disperse it in 500ml of deionized water, stir it evenly, add 20mL of 2mg / mL TEMPO solution, 400mg of NaBr as catalyst, and 1.5mmol of NaClO as oxidant for oxidation. During the oxidation process, add 0.5M NaOH dropwise to adjust the pH to 10-10.5. After the reaction is completed, wash with deionized water until neutral, and finally filter and dry.

[0025] (2) Preparation of APP-CA: 0.2g of ammonium polyphosphate (APP) and 0.5g of oxidized nanofibers were weighed and dissolved in 100ml of deionized water and ultrasonically mixed at 1200W. The mixture was subjected to tert-butanol solvent exchange and then rapidly frozen with liquid nitrogen. Finally, it was placed in a freeze dryer for 48h overnight to prepare aerogel. The aerogel was carbonized at 800℃ for 2h under nitrogen atmosphere to obtain carbon aerogel material 1 (APP-CA-1).

[0026] Electrochemical performance testing of carbon aerogel:

[0027] The electrochemical performance of the prepared nitrogen-phosphorus co-doped carbon electrode was tested using an electrochemical workstation in a three-electrode system. The working electrode was a nitrogen-phosphorus co-doped carbon aerogel, the counter electrode was a carbon rod electrode, and the reference electrode was an Ag / AgCl electrode. 6M KOH solution was used as the electrolyte, and CV and GCD curves were measured.

[0028] Example 2

[0029] Example 2 differs from Example 1 in that the mass ratio of ammonium polyphosphate to the lyophilized sample is 1:5, while other operations remain the same, resulting in carbon aerogel 2 (APP-CA-2).

[0030] Example 3

[0031] Example 3 differs from Example 1 in that the mass ratio of ammonium polyphosphate to the lyophilized sample is 3:5, while other operations remain the same, resulting in carbon aerogel 3 (APP-CA-3).

[0032] Example 4

[0033] Example 4 differs from Example 1 in that the mass ratio of ammonium polyphosphate to the lyophilized sample is 4:5, while other operations remain the same, resulting in carbon aerogel 4 (APP-CA-4).

[0034] Example 5

[0035] Example 5 differs from Example 1 in that the inorganic or organic compounds of the N and P dual elements are replaced by an equal mass of diamine hydrogen phosphate (DAP), while the other operations are the same, resulting in carbon aerogel 5.

[0036] Comparative Example

[0037] Compared with Example 1, the comparative example differs in that no N and P dual-element inorganic or organic substances are added, while other operations are the same, resulting in carbon aerogel.

[0038] Table 1. Performance changes of carbon aerogel before and after ammonium polyphosphate doping.

[0039]

[0040] Table 2 provides the characterization results of the carbon aerogels of the comparative examples and Examples 1-5 (current density 1 Ag). -1 )

[0041] project <![CDATA[Specific capacitance (F g -1 )]]> Comparative Example 161.2 Example 1 1518.8 Example 2 924.1 Example 3 1088.8 Example 4 713.2 Example 5 313.8

[0042] As can be seen from the above embodiments, the nitrogen-phosphorus co-doped carbon aerogel of the present invention is simple and inexpensive. The carbon composite aerogel obtained has a high specific surface area and a reasonably distributed three-dimensional porous structure. When used as an electrode material for supercapacitors, it has the characteristics of large specific capacitance and good cycle stability. It is suitable for supercapacitors with high energy density, high power density and long life, and has high practical value.

[0043] In all examples shown and described herein, unless otherwise specified, any specific value should be interpreted as merely exemplary and not as a limitation. Therefore, other examples of embodiments may have different values. The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solutions and concepts of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a nitrogen and phosphorus co-doped carbon aerogel material for use as an electrode material for supercapacitors, characterized in that: Includes the following steps: (1) Preparation of oxidized nanocellulose: bleached softwood pulp was dispersed in deionized water with pH 10~10.5 and stirred evenly. NaClO and NaBr were used as catalysts and TEMPO (2,2,6,6-tetramethylpiperidin-1-oxy) was used as oxidant for oxidation. After the reaction was completed, the mixture was washed with deionized water until neutral. Finally, the mixture was filtered and freeze-dried to obtain oxidized nanocellulose with a charge density of 200~1800 μmol / g. (2) Preparation of carbon aerogel: Inorganic or organic materials containing N and P elements and oxidized nanocellulose prepared in step (1) are ultrasonically mixed in deionized water with an ultrasonic power of 800~1800W. After solvent exchange with tert-butanol, the mixture is rapidly frozen with liquid nitrogen and finally freeze-dried for 24~48h to obtain aerogel. The aerogel is carbonized in a nitrogen atmosphere to obtain N and P doped carbon aerogel material. The mass ratio of inorganic or organic materials containing N and P elements to freeze-dried oxidized nanocellulose is 2:

5. In step (2), the carbonization process involves heating at a rate of 5-10℃ / min to 600-800℃ for 2-5 hours.

2. The method for preparing nitrogen-phosphorus co-doped carbon aerogel material according to claim 1, characterized in that: The ultrasound time in step (2) is 20~60 min.

3. The nitrogen-phosphorus co-doped carbon aerogel material prepared by the method described in claim 1 or 2.

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