Method for preparing porous carbon material based on supercritical CO2 / ozone mixed fluid activation technology

Abstract

The invention discloses a method for preparing a porous carbon material based on a supercritical CO2 / ozone mixed fluid activation technology. The method comprises the following steps: loading a carbon precursor into a pressure-resistant reactor at a low temperature of 400-600 DEG C, injecting liquid CO2, adjusting the pressure to 8-15 MPa to make the liquid CO2 reach a supercritical state, and directly introducing ozone with a mass concentration of 10-30% at the same time. In the activation process, the ozone is partially decomposed at high temperature to generate high-activity free radicals, and the free radicals play roles in directional etching and pore regulation and control on the carbon precursor; meanwhile, the oxygen and the supercritical CO2 also synergistically promote the activation of the carbon material through oxidation and physical activation effects. The specific surface area of the finally prepared porous carbon material is larger than or equal to 1000 m / g to 2500 m / g, the pore volume distribution is concentrated between 0.6 cm < 3 > / g and 1.2 cm < 3 > / g, and the porous carbon material is rich in oxygen doped porous carbon materials. According to the invention, the energy consumption and the equipment load are greatly reduced, and meanwhile, the operation cost and the maintenance frequency are reduced.

Description

Technical Field

[0001] The present invention relates to a carbon material preparation technology, and in particular to a method for rapidly preparing porous carbon materials with a hierarchical pore structure, controllable pore size distribution, and abundant surface functional groups under low temperature (400-600°C) conditions by using a supercritical CO2 and ozone mixed fluid activation technology. The method is suitable for use in supercapacitors, CO2 adsorption, electrocatalysis, and flexible energy storage devices. Background Art

[0002] Traditional carbon material preparation technologies mainly include two categories: physical activation and chemical activation. Physical activation often uses water vapor or CO2 as an activating agent, and the activation reaction is carried out at a high temperature of 800-950°C. Although this method can produce a large number of micropores, due to the high temperature, some surface functional groups are easily destroyed. At the same time, the equipment consumes a lot of energy, the equipment wears seriously, and it is difficult to control the pore size distribution. On the other hand, the chemical activation method usually uses strong bases such as KOH and NaOH to react with carbon precursors. Although it can obtain a higher specific surface area and a richer pore structure at a lower temperature, this method often has problems such as impurity introduction, environmental pollution and complex post-processing steps. In addition, the type and density of functional groups introduced during the chemical activation process are difficult to accurately control, which limits the application of materials in high-end energy storage, catalysis and adsorption fields.

[0003] In recent years, with the widespread application of new carbon materials in fields such as energy storage, catalysis, and environmental governance, higher requirements have been placed on preparation methods: not only is it necessary to prepare porous carbon materials with high specific surface area and ideal pore size distribution, but the surface of the materials is also required to have controllable functional groups to meet specific functional requirements. In addition, more refined requirements are placed on the pore size distribution of porous carbon materials. In supercapacitors, micropores are required to provide high specific surface area (energy storage) and mesopores are required to ensure rapid ion transport (high power); in the field of gas adsorption, the size and distribution of micropores directly affect the selective adsorption capacity of gases such as CO2 and CH4; and in catalytic carrier applications, hierarchical pore structures (micropore-mesopore-macroporous synergy) can simultaneously achieve highly dispersed loading and rapid mass transfer. These requirements highlight the shortcomings of traditional activation methods in precisely controlling pore size.

[0004] Supercritical CO2 activation technology, due to its dual properties as both a gas and a liquid, offers advantages such as high diffusivity and high density, enabling rapid penetration into carbon precursors. Ozone, a strong oxidant, decomposes at relatively low temperatures to produce highly reactive free radicals. These free radicals not only react directly with the carbon precursor, promoting pore formation and expansion, but also introduce a rich supply of oxygen-containing functional groups. Combining supercritical CO2 with ozone leverages the advantages of both: the excellent permeability and relatively low-temperature physical activation effect of supercritical CO2, combined with the highly reactive free radical activation produced by the partial decomposition of ozone at high temperatures, creates a synergistic, temperature-controlled, and energy-efficient activation process. It is important to note that the free radicals generated by ozone decomposition are only one of the multiple mechanisms that promote activation. Oxygen and supercritical CO2 themselves can also activate the carbon precursor through oxidative and physical interactions, enabling precise control of the hierarchical pore structure and surface functional groups. Summary of the Invention

[0005] The present invention provides a method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology. The core of the method is to give full play to the respective advantages of supercritical CO2 and ozone to achieve low-temperature and high-efficiency activation, while introducing the activation effect of oxygen and CO2 itself during the reaction process, thereby precisely controlling the pore structure and surface functional groups of the carbon material at a lower temperature.

[0006] The technical solution adopted by the present invention to solve the technical problem includes the following steps: (1) Loading the carbon precursor into a rotatable reactor with a filling amount of 10-40% of the reactor volume; (2) using CO2 to replace the air in the reactor, heating the reactor to 400°C to 600°C after the replacement, and then injecting liquid CO2 into the reactor, adjusting the pressure in the reactor so that the CO2 is in a supercritical state, and rotating the reactor to mix the supercritical CO2 and the carbon precursor evenly; (3) introducing a mixed gas of ozone and oxygen into the reactor, wherein the volume of the mixed gas introduced at normal pressure (1 standard atmosphere) is 1% to 10% of the effective volume of the reactor; (4) Activate at 400-600°C for 0.5-2 hours, and reduce the pressure to normal pressure after the reaction to obtain a porous carbon material.

[0007] The core technical means of the present invention are introduced as follows: First, the present invention utilizes the high diffusivity and high density characteristics of supercritical CO2, making the activation process easier and the pores more likely to develop inward.

[0008] Secondly, the present invention directly introduces an ozone / oxygen mixed gas (ozone concentration of 10% to 30%, volume of 100 to 500 mL). At a reaction temperature of 400 to 600°C, the ozone partially decomposes to produce highly active free radicals. CO2, oxygen and these free radicals etch the carbon precursor, promoting the formation and expansion of pores.

[0009] In addition, oxygen and supercritical CO2 themselves produce a synergistic activation effect on the carbon precursor through oxidation and physical action, jointly achieving precise control of the pore structure and surface functional groups.

[0010] The beneficial effects of the present invention are as follows: Traditional steam activation typically requires high temperatures of 800-950°C, resulting in extremely high energy consumption and severe equipment wear. However, this method performs activation at low temperatures of 400-600°C, significantly reducing energy consumption and equipment load, while also lowering operating costs and maintenance frequency.

[0011] Compared to traditional activation processes, this method significantly shortens reaction time (typically only 1-3 hours), thanks to the synergistic effect of the high mass transfer efficiency of supercritical CO2 and the highly reactive ozone radicals. Traditional physical activation often requires 5-8 hours of high-temperature maintenance, while chemical activation, while shorter, is limited by time-consuming post-processing steps and environmental issues. This technology, leveraging in-situ activation and the rapid penetration of supercritical fluids, significantly improves activation efficiency at low temperatures, avoiding the equipment and energy consumption associated with prolonged, high-temperature processes.

[0012] The present invention achieves activation through the synergistic effect of supercritical CO2 and ozone. Supercritical CO2, with its high diffusivity, can quickly penetrate the interior of the carbon precursor, allowing the activation reaction to spread evenly from the inside out. Simultaneously, the active free radicals generated by the partial decomposition of ozone at high temperatures promote directional etching, accelerating pore formation and introducing abundant oxygen-containing functional groups onto the material surface, thereby optimizing the pore structure and surface chemical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is the FTIR spectrum of the product in Example 1.

[0014] Figure 2 This is the nitrogen adsorption-desorption curve of the product in Example 1.

[0015] Figure 3 This is the pore size distribution diagram of the product in Example 1.

[0016] Figure 4 These are the SEM image (a) and EDS element (O element) mapping image (b) of the porous carbon product of Example 1. DETAILED DESCRIPTION

[0017] The present invention will be further described below with reference to the accompanying drawings and examples.

[0018] like Figure 1-Figure 4 As shown, the present invention places a carbon precursor into a pressure-resistant reactor at a low temperature of 400-600°C, first injects liquid CO2 and adjusts it to 8-15 MPa to achieve a supercritical state, and simultaneously directly introduces ozone (ozone mass concentration is 10%-30%). During the activation process, ozone partially decomposes at high temperature to generate highly active free radicals, which play a role in directional etching and pore regulation of the carbon precursor; at the same time, oxygen and supercritical CO2 themselves also synergistically promote the activation of the carbon material through oxidation and physical activation effects. The final porous carbon material prepared has a specific surface area greater than 1000 m² / g, and the pore volume distribution is concentrated in the range of 0.6-1.2 cm 3 / g, and rich in oxygen-doped porous carbon materials.

[0019] Example 1 Step 1: Coconut shells were crushed and sieved to a particle size of 200 ± 20 μm. The coconut shell particles were soaked in a 5% HCl solution and stirred at 80°C for 6 hours to remove ash and metallic impurities. The particles were then repeatedly washed with deionized water until neutral and then dried at 105°C for 12 hours. The dried coconut shell particles were placed in a tube furnace and heated to 800°C at a rate of 5°C / min under nitrogen protection. Carbonization was performed for 2 hours to obtain a coconut shell carbon precursor (specific surface area 300 ± 30 m² / g, pore volume 0.3 ± 0.02 cm³ / g, average pore diameter 1.2 ± 0.1 nm).

[0020] Step 2: Evenly load the carbonized coconut shell carbon precursor into a 1 L rotary reactor, filling it with 30% of the reactor volume (approximately 130 g). Close the reactor, connect a CO2 gas source, and slowly introduce high-purity carbon dioxide gas to displace the air in the reactor, ensuring that no oxygen remains in the system. After air replacement is complete, raise the temperature in the reactor to the activation temperature of 500°C. Continue to introduce liquid CO2, and use a high-pressure plunger pump to increase the pressure in the reactor to 10 MPa, bringing the CO2 to a supercritical state. Maintain the pressure in the supercritical state for 10 minutes to confirm that there are no leaks in the system. Simultaneously, start the rotary reactor (10 rpm) and maintain it for 1 hour to ensure that the gas and material are fully and evenly distributed.

[0021] Step 3: Ozone gas was generated using an external ozone generator using oxygen as the raw material. A predetermined amount of ozone / oxygen mixture (120 mL at atmospheric pressure, 20 wt% ozone concentration) was introduced into the rotary reactor before sealing. The rotary reactor's air inlet was then closed to ensure that the internal gas was not disturbed by external factors. The rotary reactor was kept rotating continuously (10 rpm) for 1.5 hours at the activation temperature (500°C).

[0022] Step 4: After the reaction is completed, the reactor pressure is reduced to normal pressure at a rate of 2 MPa / min to avoid rapid pressure reduction resulting in pore structure collapse. The resulting porous carbon material is then dried in a vacuum drying oven at 60 °C for 12 hours to obtain the final product. Figure 1 Shown is the FTIR spectrum of the product of Example 1, Figure 2 Shown is the nitrogen adsorption-desorption curve of the product of Example 1. Figure 3 The pore size distribution diagram of the product of Example 1 is shown. Figure 4 Shown are the SEM image and EDS element mapping image of the porous carbon product of Example 1.

[0023] Example 2 (Temperature Gradient Optimization) The process and conditions are the same as those in Example 1, except that the parameters are adjusted: the activation temperature is set to 400° C. (other parameters are the same as those in Example 1).

[0024] Example 3 (Temperature Gradient Optimization) The process and conditions are the same as those in Example 1, except that the parameters are adjusted: the activation temperature is set to 600° C. (other parameters are the same as those in Example 1).

[0025] Example 4 (Ozone Concentration Control) The process and conditions were the same as in Example 1, except that the parameters were adjusted: the ozone mass concentration was 10% (the volume was fixed at 120 mL, and the other parameters were the same as in Example 1).

[0026] Example 5 (Ozone Concentration Control) The process and conditions were the same as those in Example 1, except that the parameters were adjusted: the ozone mass concentration was 30% (the volume was fixed at 120 mL, and the other parameters were the same as those in Example 1).

[0027] Example 6 (Supercritical CO2 Pressure Change) The process and conditions were the same as those in Example 1, except that the parameters were adjusted: the pressure was set to 8 MPa (activation temperature 500° C.).

[0028] Example 7 (Supercritical CO2 Pressure Change) The process and conditions were the same as those in Example 1, except that the parameters were adjusted: the pressure was set to 15 MPa (activation temperature 500° C.).

[0029] Example 8 (Control of Ozone / Oxygen Mixed Gas Concentration) The process and conditions are the same as those in Example 1, except that the parameters are adjusted: the volume of the mixed gas is 60 ml (other parameters are the same as those in Example 1).

[0030] Example 9 (Control of Ozone / Oxygen Mixed Gas Concentration) The process and conditions are the same as in Example 1, except that the parameters are adjusted: the volume of the mixed gas is 300 ml (other parameters are the same as in Example 1).

[0031] Example 10 (Phenolic resin carbonized material) Step 1: Carbon precursor preparation Phenolic resin was used as the raw material and pulverized and sieved to a particle size of 250 ± 20 μm. The particles were placed in a tube furnace and carbonized for 3 hours at 600°C (the resin has a low pyrolysis temperature) under nitrogen protection at a rate of 5°C / min. This yielded a phenolic resin carbon precursor (specific surface area 280 ± 20 m² / g, pore volume 0.25 ± 0.03 cm³ / g, and average pore diameter 1.0 ± 0.2 nm).

[0032] Steps 2 to 4: The process and conditions are the same as in Example 1.

[0033] Example 11 (Petroleum coke carbonized material) Step 1: Carbon precursor preparation Petroleum coke was used as the raw material, crushed and sieved to 160±20 μm particles. The particles were then soaked in 5% HCl (80°C / 8 hours) to remove ash. After washing and drying, the mixture was heated to 900°C at a rate of 10°C / min under nitrogen and carbonized for 4 hours to obtain a petroleum coke precursor (specific surface area 180±15 m² / g, pore volume 0.18±0.02 cm³ / g, average pore diameter 0.9±0.1 nm). Steps 2-4: The process and conditions were the same as in Example 1.

[0034] Comparative Example 1 (low temperature) The process and conditions are the same as those in Example 1, except that the parameters are adjusted: the activation temperature is set to 300° C. (other parameters are the same as those in Example 1).

[0035] Comparative Example 2 (Pure Supercritical CO2 Activation) The process and conditions were the same as those in Example 1, except that, in the process parameters, only 12 MPa supercritical CO2 (500°C) was used in the ozone removal step (i.e., step 3: closing the air inlet of the rotary reactor to ensure that the internal gas is not disturbed by the outside world, while the rotary reactor was kept rotating continuously (10 rpm) and maintained at this temperature (activation temperature 500°C) for 1.5 hours).

[0036] Comparative Example 3 (Low Concentration CO2 + Ozone Activation) The process and conditions were the same as those in Example 1, except that, based on Example 1, the process parameters were adjusted as follows: the pressure was set to 5 MPa (maintaining the gaseous state, not reaching the supercritical state), the ozone mass concentration was 20% (added volume 200 mL), and the activation temperature and time were maintained at 500°C and 1.5 hours.

[0037] Other steps, including carbonization of the carbon precursor, ozone injection, and gradient pressure reduction, are the same as those in Example 1. Experimental data analysis: Oxygen content was measured using an Elementar vario EL cube oxygen, nitrogen, and hydrogen elemental analyzer. Specific surface area and pore size distribution were calculated using a Micromeritics ASAP 2460 physical adsorption instrument using N adsorption / desorption isotherms (77 K) combined with BET theory and NLDFT modeling. Micromorphology was observed using a Zeiss Sigma 500 field-emission scanning electron microscope (SEM). Based on experimental data and comparative analysis, this study reveals the key mechanism of supercritical CO2 / ozone mixed fluid activation in the preparation of porous carbon materials. Comparison of the data from the examples with the control group reveals a significant synergistic effect between the high diffusivity of supercritical CO2 and the strong oxidizing property of ozone. Under conditions of 500°C and 10 MPa (Example 1), the material achieved a specific surface area of ​​2019 m² / g, an oxygen content of 8.1%, and a total pore volume of 1.02 cm³ / g. This represents a 3.7-fold improvement over the single-activator system (e.g., the specific surface area of ​​432 m² / g in Comparative Example 2), confirming that CO2 infiltration etches the pore structure through supercritical CO2. Furthermore, combined oxygen, nitrogen, and hydrogen elemental analyzers and infrared spectroscopy confirmed that ozone selectively oxidized the pore walls and introduced abundant oxygen-containing functional groups. Temperature manipulation revealed that low temperatures (400°C) inhibited activation efficiency (specific surface area 1321 m² / g), while high temperatures (600°C), while increasing the specific surface area to 2231 m² / g, accelerated functional group thermal decomposition (oxygen content only 8.5%), indicating that 500°C represents the optimal equilibrium point. Ozone concentration exhibited a nonlinear effect with mixed gas volume: a 20% ozone concentration (120 mL) achieved both pore expansion (pore volume 1.02 cm³ / g) and surface modification (oxygen content 8.1%). However, increasing the concentration to 30% or the volume to 300 mL (Example 9) further increased the oxygen content to 10.8%, but over-oxidation led to a broadening of the pore size distribution (pore volume 1.30 cm³ / g but increased the risk of micropore collapse). Supercritical CO2 pressure regulation verifies its physical-chemical synergy. Under a pressure of 10 MPa (Example 1), the ratio of micropores to mesopores is balanced, while high pressure (15 MPa) increases the specific surface area to 2114 m² / g.

[0038] At the same time, by regulating the activation temperature, ozone concentration (and mixed gas volume) and supercritical CO2 pressure, this study achieved fine control of the pore size distribution of porous carbon materials: at low temperatures (about 400°C), the reaction is mild, the ozone decomposition rate is low, and the microporous structure is mainly retained; at high temperatures (about 600°C), ozone decomposition and free radical generation are intensified, resulting in a significant increase in mesopores and macropores, but at the same time may cause thermal decomposition of functional groups; while at 500°C (as in Example 1), a relatively balanced distribution of micropores, mesopores and macropores is presented, while a high specific surface area (about 2019 m² / g) and suitable pore volume (about 1.02 cm³ / g) as well as abundant surface oxygen-containing functional groups (about 8.1%) are obtained. In addition, low-concentration ozone and a smaller volume of mixed gas mainly promote the formation of micropores, while high-concentration ozone or a larger volume enhances the oxidation etching effect, resulting in a decrease in the proportion of micropores (as low as 56.5%) and an increase in the proportion of mesopores and macropores, and an increase in the total pore volume (up to 1.30 cm³ / g), but excessive oxidation poses a risk of micropore collapse; at the same time, a supercritical CO2 pressure of 10 MPa facilitates uniform penetration and etching, while a higher pressure (15 MPa) will further increase the specific surface area but may increase the number of mesopores.

[0039] Table 1 Specific surface area, pore volume and oxygen content of porous carbon

[0040] In summary, the present invention can achieve continuous regulation from the dominance of micropores to the gradual increase of mesopores and macropores according to different application requirements through comprehensive regulation of parameters, as shown in Table 1.

Claims

1. A method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology, characterized in that: The method comprises the following steps: (1) loading the carbon precursor into a rotatable reactor in an amount of 10-40% of the reactor volume; (2) Using CO2 to replace the air in the reactor, after the replacement is completed, the reactor temperature is raised to 400 ℃ ~ 600 ℃, and then liquid CO2 is injected into the reactor. The pressure in the reactor is adjusted to make the CO2 in a supercritical state, and the reactor is rotated to mix the supercritical CO2 and the carbon precursor evenly; (3) introducing a mixed gas of ozone and oxygen into the reactor, wherein the volume of the mixed gas introduced at normal pressure is 1% to 10% of the effective volume of the reactor; (4) Activate at 400-600 °C for 0.5-2 hours. After the reaction is completed, reduce the pressure to normal pressure to obtain a porous carbon material.

2. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 1, characterized in that: The specifications of the carbon precursor are as follows: its specific surface area is between 250 and 350 m² / g, its pore volume is between 0.2 and 0.4 cm³ / g, and its average pore diameter is between 0.8 and 1.4 nm.

3. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 1, characterized in that: In step (3), a mixed fluid of supercritical CO2 and ozone / oxygen is used as an activator, wherein the pressure of supercritical CO2 is 8 MPa-15 MPa; the mass concentration of ozone in the mixed gas of ozone and oxygen is 10% to 30%, and this concentration has a regulating effect on the pore size distribution.

4. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 3, characterized in that: By adjusting the ozone concentration and volume of the mixed gas, the activation temperature and activation time are jointly controlled to achieve continuous regulation of the ratio of micropores, mesopores and macropores in the porous carbon material.

5. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 4, characterized in that: Continuous regulation of the ratio of micropores, mesopores, and macropores in porous carbon materials is achieved as follows: (a) When the activation temperature is 400-500 °C, the activation time is 1-2 h, the ozone mass concentration is 10%-20%, and the volume of the mixed gas accounts for 1%-3% of the effective volume of the reactor, the micropore volume accounts for 70%-85%; the rest are mesopores and macropores; Or (b) when the activation temperature is greater than 500-600 °C, the activation time is 2-3 h, the ozone mass concentration is increased to greater than 20%-30%, and the volume of the mixed gas is increased to greater than 3%-10%, the activity of the free radicals generated by the high-temperature decomposition of ozone is enhanced, the proportion of mesopores and macropores is increased, and the proportion of micropores is reduced to 55%-70%.

6. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 4, characterized in that: Continuous regulation of the ratio of micropores, mesopores, and macropores in porous carbon materials is achieved as follows: (a) When the activation temperature is 430-480 °C, the activation time is 1-2 h, the ozone mass concentration is 15%-18%, and the volume of the mixed gas accounts for 1%-2% of the effective volume of the reactor, the micropore volume accounts for 70%-85%; the rest are mesopores and macropores; Or (b) when the activation temperature is greater than 530-580 °C, the activation time is 2-3 h, the ozone mass concentration is increased to greater than 25%-28%, and the volume of the mixed gas is increased to greater than 5%-8%, the activity of the free radicals generated by the high-temperature decomposition of ozone is enhanced, the proportion of mesopores and macropores is increased, and the proportion of micropores is reduced to 55%-70%.

7. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 5 or 6, characterized in that: The carbon precursor is biomass with a particle size of 120-300 μm. Or petroleum coke particles are soaked in a 4-6% HCl solution at 20-90°C for 4-48 hours, washed with water, dried, and carbonized at 700-900°C for 1-12 hours; Or the phenolic resin particles with a particle size of 160-300 μm are obtained by carbonizing at 600-800°C for 1-12 hours.

8. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 5 or 6, characterized in that: The rotary reactor is a reactor that can rotate along a rotation axis; the rotation speed range of the rotary reactor is 5-20 rpm.

9. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 5 or 6, characterized in that: The ozone / oxygen mixed gas introduced into the reactor is produced by an ozone generator using oxygen as raw material.

10. A porous carbon material, characterized in that: The porous carbon material is prepared by the method according to any one of claims 1 to 6, and has a specific surface area greater than or equal to 1000 m² / g to 2500 m² / g, and a pore volume distribution of 0.6 to 1.2 cm 3 / g, and has a hierarchical pore structure and controllable pore size distribution, and is rich in oxygen-doped porous carbon materials.

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