Method for preparing activated carbon with high specific capacitance by activating bagasse pith with monopotassium phosphate

Abstract

The invention relates to a method for preparing high-specific-capacitance activated carbon by activating bagasse pith through monopotassium phosphate, and belongs to the technical field of activated carbon preparation, and the method comprises the following steps: S1, crushing and drying the bagasse pith; s2, mixing the product in S1, KH2PO4 and deionized water according to a mass ratio of 1: (1-3): (5-15), and performing freeze drying; and S3, putting the product of S2 into a muffle furnace, pre-carbonizing for 0.5-1.5 hours at 350-450 DEG C in a nitrogen atmosphere, calcining for 1-2 hours at 800-950 DEG C, cooling, and washing to be neutral, thereby obtaining the high-specific capacitance activated carbon. The biochar with excellent molecular structure and high specific capacitance is obtained by processing bagasse pith, a new thought is provided for recycling of bagasse pith and preparation of activated carbon with high specific capacitance, and the method has high social value and economic value.

Description

Technical Field

[0001] The invention relates to the technical field of activated carbon preparation, and in particular to a method for preparing high specific capacitance activated carbon by activating sugarcane pith with potassium dihydrogen phosphate. Background Art

[0002] Supercapacitor is a new type of energy storage device between conventional capacitors and secondary batteries, which has the advantages of high power density of conventional capacitors and high energy density of secondary batteries. It also has the characteristics of no pollution to the environment, high efficiency, long cycle life, wide operating temperature range, high safety, etc., so it is an ideal energy storage unit and is widely used in the field of new energy.

[0003] The electrode material of supercapacitors is the most critical factor affecting their performance and production cost. Super activated carbon has the advantages of high specific surface area, good chemical stability, high porosity and adjustable size, high conductivity, good electrolyte wettability, and wide potential window. It is currently a successfully commercialized electrode material. However, compared with traditional activated carbon, the manufacturing process and process control of super activated carbon are more complicated, the technology is difficult, and the cost is high, which limits the widespread promotion of supercapacitors.

[0004] Improving the raw materials and activation process is an effective way to prepare super activated carbon. First, it is to select the super activated carbon precursor with high carbon yield, large specific surface area, low cost and excellent adsorption performance. Second, it is to continue to innovate in the activation method and develop a preparation process with low activator dosage, simple process, easy operation and environmental friendliness.

[0005] Chinese patent CN 112520738A reports a highly efficient and low-cost preparation method for super activated carbon, in which sugar organic matter can be carbonized at low temperature to generate raw carbon through molecular polymerization under the action of oxidizing salt. If an activator is added to the reactants in advance, it can be evenly dispersed around the raw carbon particles, and a large number of pores can be formed in the subsequent temperature-raising activation to obtain super activated carbon. However, this technology not only requires the addition of oxidizing salts, but also the specific capacitance of the activated carbon obtained is not high.

[0006] Therefore, it is necessary to develop a new process to achieve the preparation of low-cost, high-quality, and high-specific capacitance activated carbon. Summary of the invention

[0007] The purpose of the present invention is to provide a method for preparing high specific capacitance activated carbon by activating sugarcane pith with potassium dihydrogen phosphate, which can obtain high specific capacitance activated carbon and has good effects of low cost and high quality.

[0008] The technical solution of the present invention is achieved in this way: The present invention provides a method for preparing high specific capacitance activated carbon by activating sugarcane pith with potassium dihydrogen phosphate, comprising the following steps: S1, crushing and drying the sugarcane pith; S2, mixing the product of S1, KH2PO4 and deionized water in a mass ratio of 1:1-3:5-15, and freeze-drying; S3. Place the product of S2 in a muffle furnace, pre-carbonize at 350-450°C under a nitrogen atmosphere, calcine at 800-950°C for 1-2h, cool and wash to neutrality to obtain high specific capacitance activated carbon.

[0009] As a further improvement of the present invention, the crushing is crushing to less than 200 meshes.

[0010] As a further improvement of the present invention, the freeze-drying temperature is -5~0°C.

[0011] As a further improvement of the present invention, the heating rate of the pre-carbonization is 4-6°C / min.

[0012] As a further improvement of the present invention, the pre-carbonization temperature is 400°C.

[0013] As a further improvement of the present invention, the pre-carbonization time is 0.5-1.5h.

[0014] As a further improvement of the present invention, the heating rate of the calcination is 6-8°C / min.

[0015] As a further improvement of the present invention, the specific capacitance of the high specific capacitance activated carbon is 220-270 F / g.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The method of the present invention uses sugarcane pith as a raw material, which is easy to obtain, low in cost, simple in steps, and has low requirements on equipment, thus providing a new idea for the high-value transformation of sugarcane pith. Potassium dihydrogen phosphate is used as an activator, which melts into a liquid at 400 degrees Celsius and decomposes to generate potassium metaphosphate; it further decomposes at high temperature to generate metal K, phosphoric acid and water vapor; the metal K atoms can effectively enrich the pore structure, and the P / O heteroatoms in the phosphoric acid dope the activated carbon, promote the formation of functional groups, enhance the surface wettability and conductivity of the carbon material, and greatly improve the specific capacitance of the carbon material. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0018] Figure 1 This is a test chart of the charge and discharge performance of the biochar prepared in Examples 1-3 of the present invention and Comparative Examples 1-4. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] Test conditions: The sample was mixed with PTFE and carbon black in a weight ratio of 8:1:1 to prepare an active material mixture (with anhydrous ethanol as a dispersant). Then, the mixture (about 1 mg) was pressed onto 1×2.5 cm nickel foam (collector) using a tablet press to prepare a working electrode. The measurement process adopted a three-electrode system with 6M KOH aqueous electrolyte. The specific capacitance of the activated carbon was calculated by constant current charge and discharge (GCD) test.

[0021] The specific surface area of ​​the samples was tested using a Micromeritics ASAP 2460 multi-station fully automatic specific surface area and pore size analyzer.

[0022] Example 1 40g of sugarcane pith, 80g of potassium dihydrogen phosphate and 280g of deionized water were mixed, stirred at room temperature for 4h, and then vacuum freeze-dried for 12h overnight, with a freezing temperature of -2°C; the dried product was placed in an ark, and a small amount of nitrogen was added to seal the lid; the product was heated to 400°C at a temperature rise of 5°C / min and kept warm for 1h, and then heated to 850°C at a temperature rise of 7°C / min and kept warm for 1h, with a nitrogen range of 150ml / min; after cooling to room temperature, the product was washed with water until neutral and then dried to obtain 13.3g of activated carbon, which had a specific capacitance of 224.3F / g and a specific surface area of ​​1811.27m in a three-electrode system supercapacitor. 2 / g.

[0023] Example 2 20g of sugarcane pith, 30g of potassium dihydrogen phosphate and 150g of deionized water were mixed, stirred at room temperature for 5h, and then vacuum freeze-dried for 12h overnight, with a freezing temperature of -5℃; the dried product was placed in an ark, and a small amount of nitrogen was added to seal the lid; the product was heated to 400℃ at a temperature rise of 5℃ / min and kept warm for 1h, and then heated to 900℃ at a temperature rise of 7℃ / min and kept warm for 1.5h, with a nitrogen range of 150ml / min; after cooling to room temperature, the product was washed with water until neutral and then dried to obtain 6.2g of activated carbon, which had a specific capacitance of 234.3F / g and a specific surface area of ​​2024.14m in a three-electrode system supercapacitor. 2 / g.

[0024] Example 3 10g sugarcane pith, 15g potassium dihydrogen phosphate and 75g deionized water were mixed, stirred at room temperature for 5h, and then vacuum freeze-dried for 12h overnight, with a freezing temperature of -5℃; the dried product was placed in an ark, and a small amount of nitrogen was added to seal the lid; the product was heated to 400℃ at 5℃ / min and kept warm for 1h, and then heated to 950℃ at 7℃ / min and kept warm for 1h, with a nitrogen range of 150ml / min; after cooling to room temperature, the product was washed with water until neutral and then dried to obtain 2.42g activated carbon, which had a specific capacitance of 268.3F / g and a specific surface area of ​​2931.31m in a three-electrode system supercapacitor. 2 / g.

[0025] Comparative Example 1 40g of sugarcane pith, 30g of ammonium sulfate and 40g of potassium dihydrogen phosphate were uniformly mixed and placed on the bottom of a crucible, and then kept at 200℃ in a muffle furnace for 4h for carbonization treatment; then the temperature was further raised to 750℃ and kept for 2h under the protection of inert gas for activation; after cooling, 7.6g of activated carbon was obtained after washing and drying with hydrochloric acid. The specific capacitance in the three-electrode system supercapacitor was 94.2F / g and the specific surface area was 646.44m 2 / g.

[0026] Comparative Example 2 30g of sugarcane pith, 20g of ammonium sulfate and 45g of potassium dihydrogen phosphate were uniformly mixed and placed at the bottom of a crucible. The mixture was kept at 200℃ for 4h in a muffle furnace for carbonization. The mixture was then heated to 700℃ and kept at this temperature for 3h under the protection of inert gas for activation. After cooling, the mixture was washed and dried with hydrochloric acid to obtain 7.6g of activated carbon. The specific capacitance in the three-electrode system supercapacitor was 106.9F / g and the specific surface area was 1203.4m 2 / g.

[0027] Comparative Example 3 40g of sugarcane pith, 80g of potassium phosphate and 280g of deionized water were mixed, stirred at room temperature for 4h, and then vacuum freeze-dried for 12h overnight, with a freezing temperature of -2°C; the dried product was placed in an ark, and a small amount of nitrogen was added to seal the lid; the product was heated to 400°C at a temperature rise of 5°C / min and kept warm for 1h, and then heated to 850°C at a temperature rise of 7°C / min and kept warm for 1h, with a nitrogen range of 150ml / min; after cooling to room temperature, the product was washed with water until neutral and then dried to obtain 11.6g of activated carbon, with a specific capacitance of 125.1F / g and a specific surface area of ​​993.21m in a three-electrode system supercapacitor. 2 / g.

[0028] Comparative Example 4 40g of sugarcane pith, 30g of ammonium sulfate, 80g of potassium dihydrogen phosphate and 280g of deionized water were mixed, stirred at room temperature for 4h, and then vacuum freeze-dried for 12h overnight, with a freezing temperature of -2°C; the dried product was placed in an ark, and a small amount of nitrogen was added to seal the lid; the product was heated to 400°C at 5°C / min and kept warm for 1h, and then heated to 850°C at 7°C / min and kept warm for 1h, with a nitrogen range of 150ml / min; after cooling to room temperature, the product was washed with water until neutral and then dried to obtain 10.3g of activated carbon, which had a specific capacitance of 123.4F / g and a specific surface area of ​​1102.1m in a three-electrode system supercapacitor. 2 / g.

[0029] Comparative Example 5 40g of sugarcane pith, 80g of potassium dihydrogen phosphate and 280g of deionized water were mixed, stirred at room temperature for 4h, and then dried at 60℃ for 12h overnight; the dried product was placed in an ark, and a small amount of nitrogen was added to seal the lid; the product was heated to 400℃ at 5℃ / min and kept warm for 1h, and then heated to 850℃ at 7℃ / min and kept warm for 1h, and the nitrogen range was 150ml / min; after cooling to room temperature, the product was washed with water until neutral and then dried to obtain 13.6g of activated carbon, which had a specific capacitance of 104.2F / g and a specific surface area of ​​992.21m in a three-electrode system supercapacitor. 2 / g.

[0030] Combination Figure 1 The specific capacitance of the activated carbon in Comparative Examples 1, 2, and 4 is lower than 120 F / g, and the specific surface area is not high either. This is because ammonium sulfate will undergo a replacement reaction with potassium dihydrogen phosphate, which affects the activation process of the activated carbon. In Comparative Example 3, the specific capacitance of the activated carbon was 125.1 F / g and the specific surface area was 993.21 m 2 / g of activated carbon, while the present invention uses potassium dihydrogen phosphate to activate sugarcane pith to obtain a specific capacitance greater than 200F / g and a specific surface area greater than 1800m 2 / g, because the K atom content in potassium phosphate is too high, which affects the heteroatom doping effect and has a side effect. On the other hand, potassium dihydrogen phosphate will dissolve at 400°C, greatly increasing the contact area with the carbon skeleton and enhancing the activation effect. In Comparative Example 5, the activated carbon specific capacitance obtained by drying the pretreated sample at 60°C for 12h overnight is only 104.2F / g. This is because the molecular motion under thermal drying is so intense that the effect left by the pretreatment cannot be retained, while freeze drying can well fix the embedded position of potassium dihydrogen phosphate in the sugarcane pith, greatly improving the activation effect.

[0031] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing high specific capacitance activated carbon by activating sugarcane pith with potassium dihydrogen phosphate, characterized in that: The steps include: S1, crushing and drying the sugarcane pith; S2, mixing the product of S1, KH2PO4 and deionized water in a mass ratio of 1:1-3:5-15, and freeze-drying; S3. Place the product of S2 in a muffle furnace, pre-carbonize at 350-450°C under a nitrogen atmosphere, calcine at 800-950°C for 1-2h, cool and wash to neutrality to obtain high specific capacitance activated carbon.

2. The method according to claim 1, characterized in that The crushing is to crush to less than 200 meshes.

3. The method according to claim 1, characterized in that The freeze-drying temperature is -5~0°C.

4. The method according to claim 1, characterized in that: The heating rate of the pre-carbonization is 4-6°C / min.

5. The method according to claim 1, characterized in that The pre-carbonization temperature is 400°C.

6. The method according to claim 1, characterized in that The pre-carbonization time is 0.5-1.5h.

7. The method according to claim 1, characterized in that The heating rate of the calcination is 6-8°C / min.

8. The method according to claim 1, characterized in that The specific capacitance of the high specific capacitance activated carbon is 220-270 F / g.

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