Carbon material low temperature graphitization method based on dual transition metal salt synergistic catalysis and freeze-drying process

Abstract

This invention discloses a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process. This method requires no organic dispersants or anchoring agents. First, a specific Mn-containing transition metal salt solution is mixed with carbon black. Then, a freeze-drying process with precisely controlled temperature and pressure is used to fix the dispersion state of the metal precursor. Next, catalytic graphitization is carried out in an inert atmosphere at a temperature below 1100°C. Finally, the carbon skeleton is purified by acid washing and stabilized by secondary annealing, yielding carbon materials with high graphitization degree (Raman ID / IG value of 0.6–0.91), high specific surface area, and abundant pore structure. This invention overcomes the problems of reliance on organic additives, complex processes, and high energy consumption in traditional technologies by combining the binary metal synergistic effect with a unique physical drying process and secondary annealing process, providing a new approach for the low-cost and green preparation of high-performance fuel cell catalyst supports and other materials.

Description

Technical Field

[0001] This invention belongs to the field of carbon material preparation and fuel cell technology, and relates to a low-temperature graphitization method for carbon materials based on binary transition metal salt synergistic catalysis and freeze-drying process. Background Technology

[0002] For on-board fuel cells, the hydrogen / air interface generated during start-up and shutdown can raise the cathode potential to 1.5V, accelerating catalyst degradation and the oxidation of carbon materials. Common carbon black materials, due to their disordered crystalline structure, are more susceptible to thermodynamic corrosion under actual operating conditions, leading to decreased overall battery performance and lifespan. Converting ordinary carbon black into ordered graphitized carbon materials is one way to reduce carbon corrosion. However, common graphitization processes require temperatures above 2000℃, which presents challenges such as limited graphitization equipment, high energy consumption, and long processing cycles. Furthermore, directly graphitizing carbon supports at high temperatures results in smooth surfaces with few pores, making it difficult to further load functional metals. Therefore, developing low-temperature catalytic graphitization methods, while also developing graphitized materials with abundant pore structures, is a key aspect of solving the carbon durability problem in fuel cells.

[0003] A search of relevant patents revealed that invention patent CN118712402A provides a method for preparing conductive carbon black support, belonging to the field of fuel cell catalysts. This method uses commercial carbon black as raw material and employs a high-temperature heat treatment process at 800℃~1800℃ to obtain graphitized material with a D-peak to G-peak intensity ratio of 1.0~1.8. Furthermore, oxygen-containing functional groups are introduced to the surface of the carbon black support by adding an oxidizing solution to further improve the material's conductivity. However, the selected graphitization process, even at high temperatures below 1800℃, still cannot yield highly crystalline carbon materials with a Raman ID / IG ratio below 1. Invention patent CN117886314A proposes a method for ultrafast preparation of graphitized carbon support, mainly involving thorough mixing of a magnesium ion-containing solution and carbon material. After drying, the mixture is subjected to ultrafast Joule heating treatment under an inert atmosphere at a temperature of 1500-3000℃ to obtain the graphitized carbon support. The first two patents disclosed above both employ high-temperature graphitization processes, which limits the application scope and scale-up of graphitization. CN 113506885A proposes a method for preparing a graphitized carbon support for hydrogen fuel cells. Its core innovation lies in adding organic additives (such as ethylenediamine, aniline, and polyvinylpyrrolidone) to assist in the dispersion and anchoring of transition metal ions on the carbon support surface. The specific process includes: carbon support pretreatment → reflux of transition metal salts and additives in isopropanol → medium-low temperature drying (30-70℃) → catalytic graphitization (700-1000℃) → acid washing and purification. This patent improves graphitization uniformity by converting the additives into carbonaceous "barriers" during heat treatment, inhibiting metal particle agglomeration. However, this low-temperature graphitization patent uses organic additives (such as aniline) to anchor and disperse transition metals to achieve catalytic graphitization of the carbon support; however, this technical route suffers from problems such as potential additive decomposition residues and complex process steps. This invention aims to overcome these limitations by proposing a novel method that requires no organic additives and combines the synergistic effect of binary transition metal salts with impregnation mixing and precisely controlled freeze-drying processes. This method achieves high graphitization and uniform structure of carbon materials at temperatures below 1200°C. Compared to CN 113506885A, which uses organic matter to anchor metal ions, the uneven distribution of organic matter can lead to uneven distribution of metal ions. After anchoring with organic matter, metal ions may re-accumulate during drying, resulting in uneven distribution. Furthermore, during catalysis, metal ions may catalyze the graphitization of carbon decomposed from organic matter; whether this catalysis can effectively catalyze the graphitization of bulk carbon is not addressed in the patent. This invention uses liquid nitrogen rapid cooling, which instantly fixes metal ions in the carbon support material, forming an atomic-level distribution. This distribution remains unchanged after freeze-drying, resulting in higher catalytic graphitization efficiency. Solving the problem of uneven distribution of metal elements in the carbon support and achieving more uniform catalytic graphitization of the carbon support are also key technical issues addressed by this invention. Summary of the Invention

[0004] To address the shortcomings of the prior art, the present invention aims to provide a method for the synergistic promotion of low-temperature graphitization of carbon materials by binary transition metal salts, which features simple process, abundant raw materials, and high crystallinity of the obtained carbon materials.

[0005] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0006] This invention provides a method for low-temperature graphitization of carbon materials, the method comprising the following steps: S1. Mix two transition metal salts with deionized water at a mass ratio of (0.02-1):1, so that the two metal salts are fully dissolved in the deionized water; one of the two transition metal salts is a manganese salt. S2. The carbon black and the two transition metal salts in the S1 solution are mixed uniformly at a mass ratio of (0.5~8):1. After stirring at room temperature, the suspension is first rapidly cooled in liquid nitrogen (-196℃) for 5-30 minutes, and then dried under vacuum conditions of -45℃ to -70℃ and vacuum degree ≤10 Pa for 20-28 hours. The dried solid is a black powder, which is the mixed material A. S3. Transfer the mixed material A obtained in S2 to a heat treatment apparatus and heat-treat it for 10 to 60 minutes at a temperature of 10 to 15 °C / min to 1000 °C to 1200 °C under a protective atmosphere to obtain mixture B. S4. The mixture B obtained in S3 is washed with an acid solution, heated to 60~90℃, and the acid washing time is 12~72h. S5. The graphitized carbon material after S4 pickling is subjected to a second annealing treatment under a protective atmosphere at a temperature of 600-800℃ to obtain highly graphitized porous carbon material.

[0007] As one implementation, in S1, the first transition metal salt in the combination of the two transition metal salts must be a manganese salt, and the combination with the second transition metal salt (iron, cobalt, and nickel salt) is one of the following combinations: manganese salt and iron salt, manganese salt and cobalt salt, or manganese salt and nickel salt. If the system in this invention uses a combination of nickel salt and iron or cobalt salt, or a combination of iron and cobalt salt, although it can catalyze graphitization, excessive graphitization will lead to a significant decrease in the specific surface area of ​​the support, which is detrimental to the dispersion and loading of the fuel cell metal catalyst. Specifically, the degree of graphitization of the support required in this invention needs to be controlled within a suitable range (ID / IG ratio between 0.6 and 0.91). If the above-mentioned metal combinations with excessively strong catalytic activity are used, the ID / IG ratio will be lower than 0.6 (as in Comparative Example 6), that is, the degree of graphitization is too high. At this time, the specific surface area of ​​the support is greatly reduced, seriously affecting the uniform loading and dispersion performance of the metal particles.

[0008] In one implementation scheme, the molar ratio of the manganese salt to the metal element of the other transition metal salt in the combination of two transition metal salts is (0.05-10):1.

[0009] As one implementation scheme, the carbon black in S2 is one or more of the following: biomass carbon black, acetylene black, Ketjen black, Cabot black, mesoporous carbon, carbon fiber, etc.

[0010] In one implementation scheme, S2 involves rapid cooling in liquid nitrogen for 5-30 minutes. The freeze-drying temperature is -50℃±2℃, and the drying time is 24-26 hours. In this invention, the liquid nitrogen rapid cooling + freeze-drying process treats a mixed suspension of metal ion solution and carbon powder (in ionic form, the metal exists in the solution in an ionic state at this stage and is adsorbed into the porous carbon support). It innovatively transforms the entire freeze-drying process into an integrated "drying-anchoring" step—the rapid freezing with liquid nitrogen instantly solidifies the weak adsorption state of metal ions on the surface of the functionally rich carbon support, "freezing and fixing" the ion positions. Subsequently, ice crystal sublimation achieves migration-free drying, thus completing the in-situ uniform anchoring of metal ions during the drying stage, ensuring the uniform distribution of metal ions on the surface and inside the support. The "anchoring" in this invention is essentially the instantaneous suppression and spatial fixation of metal ion migration during the drying process (the ions and the support remain in a weakly bound state), rather than the physical reinforcement of the already formed metal-support composite.

[0011] As one implementation, the protective atmosphere in S3 is either nitrogen or argon.

[0012] As one implementation, the acid solution in S4 is one or more of hydrochloric acid, sulfuric acid, nitric acid, and glacial acetic acid.

[0013] In one embodiment, the concentration of the acid solution in S4 is 1 to 3 M.

[0014] In one implementation scheme, the annealing time in S5 is 1-3 hours.

[0015] As another specific implementation scheme, the annealing temperature in S5 is 600℃ and the annealing time is 2h.

[0016] As one embodiment, the highly graphitized porous carbon material in S5 has a Raman peak ratio (ID / IG) of 0.6 to 0.91. Preferably, the Raman peak ratio (ID / IG) is 0.65 to 0.87.

[0017] This invention uses two transition metal salts to synergistically catalyze the graphitization of carbon black, which can provide a better adjustment effect for the catalytic graphitization process. This invention achieves atomic-level uniform anchoring of metal active centers in the carbon matrix through co-impregnation of binary transition metal salts (mainly Mn, synergistic with Ni, Co or Fe (Mn has a relatively high catalytic temperature)), liquid nitrogen rapid cooling solidification and controllable freeze drying; then, the carbon skeleton is driven to be highly graphitized by heat treatment at ≤1200℃, and the degree of graphitization is improved by secondary annealing to obtain carbon materials with high graphitization degree, excellent structural uniformity and good electrical conductivity: (1) Two transition metal salts containing Mn are selected for synergistic catalysis, which significantly reduce the energy barrier of carbon atom rearrangement and promote the formation of graphite ordered structure through mechanisms such as "dissolution-precipitation" and "decomposition of carbide intermediates" at high temperature. The bimetallic system can not only enhance catalytic activity and graphitization uniformity through electronic synergy, but also mutually inhibit particle agglomeration at high temperature, thereby obtaining graphitized carbon materials with tunable structure and performance; (2) The freeze-drying technology is used to replace the traditional organic additives to achieve metal salt dispersion. In this process, the solvent is directly sublimated from the solid state, avoiding agglomeration caused by liquid phase surface tension, so that the metal salt is fixed in situ inside the carbon black, achieving atomic-level dispersion and forming a high specific surface area precursor structure with well-developed channels; (3) Secondary annealing, as a post-treatment step, can eliminate internal stress and unstable defects in the carbon skeleton, balance the activity and stability of the material, thereby improving its electrochemical durability.

[0018] This invention proposes a method using two transition metal salts as catalytic graphitization aids. Through process design methods such as impregnation and mixing with commercial carbon black, heat treatment, and acid washing, its advantages are: (1) efficient catalytic graphitization, low-temperature preparation of graphitized carbon materials with high crystallinity; (2) freeze-drying process anchors the metal salts, promoting the uniformity of graphitization, and secondary annealing stabilizes the carbon skeleton and improves electrochemical stability; (3) low process cost, no aids introduced; (4) simple process and easy operation; (5) abundant natural reserves of raw materials, which can effectively promote the industrialization of graphitization process. Attached Figure Description

[0019] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 The figure shows the CV curves obtained by accelerated aging (AST) testing of the prepared graphitic carbon material in Example 1. As can be seen from the figure, after graphitization treatment by binary manganese and cobalt salt catalysis, the capacitance region of the Ketjen carbon shrinks after 10,000 cycles of CV aging test, indicating that the number of quinone / hydroquinone groups in the material is reduced and the redox peak current density is reduced.

[0020] Figure 2The figure shows the peak current value change curve obtained by accelerated aging test (AST) of the carbon material prepared in Example 1. As can be seen from the figure, under the same number of AST cycles, the peak current growth rate of the oxidation of Ketjen carbon catalyzed by manganese and cobalt salts slows down and the peak current value decreases.

[0021] Figure 3 TEM images of the ungraphitized carbon material (EC-300J) and the graphitized carbon material (MnCo-EC-300J) prepared in Example 1; the graphitized carbon material (MnCo-EC-300J) exhibits a distinct thin-layer cage-like graphite structure.

[0022] Figure 4 The Raman spectrum and ID / IG ratio of the graphitized carbon material prepared in Example 1 are shown. The graphitized carbon after manganese-cobalt salt catalysis produced obvious 2D peaks, and the intensity ratio of the D peak to the G peak decreased. Detailed Implementation

[0023] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0024] Example 1 This embodiment relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process, comprising the following steps: S1: Weigh out 4g of manganese sulfate and cobalt sulfate, with a Mn:Co molar ratio of 3:1, and disperse them evenly in 75ml of deionized water; S2: Ketjon carbon and the two transition metal salts in the S1 solution are uniformly mixed at a mass ratio of 2.5:1. After stirring at room temperature for 3.5 hours, the mixture is first rapidly cooled in liquid nitrogen for 30 minutes and then freeze-dried in vacuum at -50°C for 24 hours. The solid is then ground and pulverized to obtain mixed material A. S3: Transfer the mixed material A obtained in S2 to a heat treatment device and heat treat it at 1000℃ for 30 min under an argon atmosphere, with the heating rate controlled at 15℃ / min, to obtain mixture B; S4: The mixture B obtained in S3 was acid-washed with a sulfuric acid solution with a molar concentration of 2M, heated to 90℃, and the acid washing time was 24h.

[0025] S5: The solid powder in S4 was subjected to a second annealing treatment at 600℃ in an inert atmosphere for 2 hours to obtain highly graphitized porous carbon material.

[0026] The carbon durability of the highly graphitized carbon material prepared in Example 1 was evaluated by AST and CV tests, and the results are as follows: Figure 1 and Figure 2 After 10,000 cycles at a high voltage of 1.0 to 1.5V, compared with untreated carbon black, the rate of increase of the carbon oxidation peak current in its CV curve slowed down, and the peak current value decreased by about 18%.

[0027] Figure 3 The image shows a TEM image of the graphitized carbon material (MnCo-EC-300J) prepared in Example 1. Compared with untreated carbon black (EC-300J), the carbon support after catalytic graphitization exhibits a cage-like graphene thin-layer structure, which is significantly different from carbon black.

[0028] Figure 4 The image shows the Raman spectrum of the graphitized carbon material prepared in Example 1. It can be seen that the highly graphitized carbon material exhibits a high Raman spectrum at 2670 cm⁻¹. - The graphite 2D peak is clearly visible at position ¹, and the ID / IG ratio is less than 1. This result confirms that the bimetallic salt catalytic graphitization process of the present invention significantly improves the order of the carbon black lattice.

[0029] Example 2 This embodiment relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process, comprising the following steps: S1: Weigh out 3g of ferrous sulfate and manganese sulfate, with a Mn:Fe molar ratio of 0.5:1, and disperse them evenly in 30ml of deionized water; S2: Cabot carbon and the two transition metal salts in the S1 solution are uniformly mixed at a mass ratio of 3:1. After stirring at room temperature for 5 hours, the mixture is first rapidly cooled in liquid nitrogen for 20 minutes, then freeze-dried in vacuum at -50℃ for 26 hours. The dried solid is then ground and pulverized to obtain mixed material A. S3: Transfer the mixed material A obtained in S2 to a heat treatment device and heat treat it at 1100℃ for 15 min under an argon atmosphere, with the heating rate controlled at 15℃ / min, to obtain mixture B; S4: The mixture B obtained in S3 was acid-washed with a sulfuric acid solution with a molar concentration of 1M, heated to 80℃, and the acid washing time was 72h.

[0030] S5: The solid powder in S4 was subjected to a second annealing treatment at 600℃ in an inert atmosphere for 2 hours to obtain highly graphitized porous carbon material.

[0031] The carbon durability of the highly graphitized carbon material prepared in Example 2 was evaluated by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5 V, compared with untreated carbon black, the carbon oxidation peak current growth in its CV curve slowed down, and the peak current value decreased by about 29%. The ID / IG ratio of this highly graphitized carbon material was less than 1 (Table 1).

[0032] Example 3 This embodiment relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process, comprising the following steps: S1: Weigh out 8g of manganese sulfate and nickel sulfate, with a Mn:Ni molar ratio of 0.1:1, and disperse them evenly in 100ml of deionized water; S2: Cabot carbon and the two transition metal salts in the S1 solution are uniformly mixed at a mass ratio of 5:1. After stirring at room temperature for 4 hours, the mixture is first rapidly cooled in liquid nitrogen for 30 minutes, then freeze-dried in vacuum at -50℃ for 24 hours. The dried solid is then ground and pulverized to obtain mixed material A. S3: Transfer the mixed material A obtained in S2 to a heat treatment device and heat treat it at 1200℃ for 60 min under an argon atmosphere, with the heating rate controlled at 10℃ / min, to obtain mixture B; S4: The mixture B obtained in S3 was acid-washed with a sulfuric acid solution with a molar concentration of 2.5M, heated to 85℃, and the acid washing time was 36h.

[0033] S5: The solid powder in S4 was subjected to a second annealing treatment at 600℃ in an inert atmosphere for 2 hours to obtain highly graphitized porous carbon material.

[0034] The highly graphitized carbon material prepared in Example 3 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5 V, compared with untreated carbon black, the carbon oxidation peak current growth in its CV curve slowed down, and the peak current value decreased by about 22%. The ID / IG ratio of this highly graphitized carbon material was less than 1 (Table 1).

[0035] Example 4 This embodiment relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process, comprising the following steps: S1: Weigh out 3.5g of manganese sulfate and ferrous sulfate, with a Mn:Fe molar ratio of 0.05:1, and disperse them evenly in 60ml of deionized water; S2: Cabot carbon and two transition metal salts in solution S1 are uniformly mixed at a mass ratio of 0.5:1. After stirring at room temperature for 2 hours, the mixture is first rapidly cooled in liquid nitrogen for 30 minutes and then freeze-dried in vacuum at -50°C for 24 hours. The dried solid is then ground and pulverized to obtain mixed material A. S3: Transfer the mixed material A obtained in S2 to a heat treatment device and heat treat it at 1000℃ for 60 min under an argon atmosphere, with the heating rate controlled at 10℃ / min, to obtain mixture B; S4: The mixture B obtained in S3 was acid-washed with a sulfuric acid solution with a molar concentration of 1.5M, heated to 85℃, and the acid washing time was 48h.

[0036] S5: The solid powder in S4 was subjected to a second annealing treatment at 600℃ in an inert atmosphere for 2 hours to obtain highly graphitized porous carbon material.

[0037] The highly graphitized carbon material prepared in Example 4 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5 V, compared with untreated carbon black, the carbon oxidation peak current growth in its CV curve slowed down, and the peak current value decreased by about 40%. Increasing the amount of transition metal significantly improved the graphitization degree of the support, and the ID / IG ratio of this highly graphitized carbon material was less than 1 (Table 1).

[0038] Example 5 This embodiment relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process, comprising the following steps: S1: Weigh out 3.5g of manganese sulfate and ferrous sulfate, with a Mn:Fe molar ratio of 0.05:1, and disperse them evenly in 60ml of deionized water; S2: Biochar and two transition metal salts in solution S1 are mixed uniformly at a mass ratio of 1.5:1. After stirring at room temperature for 2 hours, the mixture is first rapidly cooled in liquid nitrogen for 30 minutes and then freeze-dried in vacuum at -50℃ for 24 hours. The dried solid is then ground and pulverized to obtain mixed material A. S3: Transfer the mixed material A obtained in S2 to a heat treatment device and heat treat it at 1000℃ for 60 min under an argon atmosphere, with the heating rate controlled at 10℃ / min, to obtain mixture B; S4: The mixture B obtained in S3 was acid-washed with a sulfuric acid solution with a molar concentration of 1.5M, heated to 85℃, and the acid washing time was 48h.

[0039] S5: The solid powder in S4 was subjected to a second annealing treatment at 600℃ in an inert atmosphere for 2 hours to obtain highly graphitized porous carbon material.

[0040] The highly graphitized carbon material prepared in Example 5 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5 V, compared with untreated carbon black, the carbon oxidation peak current growth in its CV curve slowed down, and the peak current value decreased by about 25%. The ID / IG ratio of this highly graphitized carbon material was less than 1 (Table 1).

[0041] Comparative Example 1 This comparative example relates to a method for promoting the low-temperature graphitization of carbon materials using transition metal salts, comprising the following steps: S1: Weigh 4g of cobalt sulfate and disperse it evenly in 50ml of deionized water; S2: Ketjon carbon and cobalt salt in solution S1 are mixed uniformly at a mass ratio of 2.5:1. After stirring at room temperature for 3.5 hours, the mixture is first rapidly cooled in liquid nitrogen for 30 minutes and then freeze-dried in vacuum at -50°C for 24 hours. The dried solid is then ground and pulverized to obtain mixed material A. The remaining steps are the same as in Example 1.

[0042] Compared with Example 1, the carbon-supported cage-like graphene structure obtained in Comparative Example 1 was sparse, the 2D peak in the Raman spectrum was not obvious, the ratio of ID to IG was 0.95, the degree of graphitization was low, and the expected effect was not achieved.

[0043] The graphitized carbon material prepared in Comparative Example 1 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5V, compared with untreated carbon black, the carbon oxidation peak current growth rate in its CV curve did not change significantly, and the peak current value decreased by only 11%.

[0044] Comparative Example 2 This comparative example relates to a method for promoting the low-temperature graphitization of carbon materials using transition metal salts, comprising the following steps: S1: Weigh 4g of manganese sulfate and disperse it evenly in 50ml of deionized water; S2: Mix the Ketjen carbon and manganese salt in the S1 solution at a mass ratio of 2.5:1. Stir at room temperature for 3.5 h, then rapidly cool in liquid nitrogen for 30 min, freeze dry in vacuum at -50℃ for 24 h, and grind the dried solid to obtain mixed material A. The remaining steps are the same as in Example 1.

[0045] Compared with Example 1, the carbon-supported cage-like graphene structure obtained in Comparative Example 2 was sparse, the 2D peak in the Raman spectrum was not obvious, the ratio of ID to IG was 0.98, the degree of graphitization was low, and the expected effect was not achieved.

[0046] The graphitized carbon material prepared in Comparative Example 2 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5V, compared with untreated carbon black, the carbon oxidation peak current growth rate in its CV curve did not change significantly, and the peak current value decreased by only 9%.

[0047] Comparative Example 3 This comparative example relates to a method for the synergistic promotion of low-temperature graphitization of carbon materials by binary transition metal salts. The main difference between this method and Example 1 is the use of 70°C vacuum drying, without liquid nitrogen quenching and subsequent freeze-drying steps. The method specifically includes the following steps: S1: Weigh out 4g of manganese sulfate and cobalt sulfate, with a Mn:Co molar ratio of 3:1, and disperse them evenly in 75ml of deionized water; S2: Ketjon carbon and the two transition metal salts in the S1 solution are mixed uniformly at a mass ratio of 2.5:1, stirred at room temperature for 3.5 h, and then vacuum dried at 70 °C for 24 h. The dried solid is then ground and pulverized to obtain mixed material A. The remaining steps are the same as in Example 1.

[0048] Compared with Example 1, the carbon support obtained in Comparative Example 3 showed a significantly larger distribution of transition metal particles before acid treatment, which affected the uniformity of graphitization. The 2D peaks in the Raman spectrum were not obvious, and the ratio of ID to IG was greater than 0.91, indicating a less favorable performance compared to the examples.

[0049] The graphitized carbon material prepared in Comparative Example 3 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5V, compared with untreated carbon black, the carbon oxidation peak current growth rate in its CV curve did not change significantly, and the peak current value decreased by 12%.

[0050] Comparative Example 4 This comparative example relates to a method for synergistically promoting the low-temperature graphitization of carbon materials using binary transition metal salts. The main difference between this method and Example 5 is the elimination of the secondary annealing step. The method specifically includes the following steps: S1: Weigh out 3.5g of manganese sulfate and ferrous sulfate, with a Mn:Fe molar ratio of 0.05:1, and disperse them evenly in 60ml of deionized water; S2: Biochar and two transition metal salts in solution S1 are mixed uniformly at a mass ratio of 1.5:1. After stirring at room temperature for 2 hours, the mixture is first rapidly cooled in liquid nitrogen for 30 minutes and then freeze-dried in vacuum at -50℃ for 24 hours. The dried solid is then ground and pulverized to obtain mixed material A. The secondary annealing is cancelled, and the remaining steps are the same as in Example 5.

[0051] Compared with Example 5, the degree of graphitization of the carbon support obtained in Comparative Example 4 was comparable to that in Example 5. The ratio of ID to IG was 0.92, which was a decrease compared to Example 5.

[0052] The graphitized carbon material prepared in Comparative Example 4 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5 V, compared with untreated carbon black, the carbon oxidation peak current growth rate in its CV curve did not change significantly, and the peak current value decreased by 14%. This indicates that annealing plays an important role in the structural stability of the carbon support.

[0053] Comparative Example 5 This comparative example relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process. Its main difference from Example 1 is the absence of liquid nitrogen quenching. The method specifically includes the following steps: S1: Weigh out 4g of manganese sulfate and cobalt sulfate, with a Mn:Co molar ratio of 3:1, and disperse them evenly in 75ml of deionized water; S2: Ketjen carbon and the two transition metal salts in the S1 solution are mixed uniformly at a mass ratio of 2.5:1. After stirring at room temperature for 3.5 hours, the mixture is freeze-dried under vacuum at -50°C for 24 hours. The dried solid is then ground and pulverized to obtain mixed material A. The remaining steps are the same as in Example 1.

[0054] Compared with Example 1, the carbon support obtained in Comparative Example 5 was similar to that in Comparative Example 3. Before acid treatment, the distribution of transition metal particles was significantly larger and less uniform, which also affected the uniformity of graphitization. The 2D peaks in the Raman spectrum were not obvious, and the ratio of ID to IG was greater than 0.91, which was worse than the example.

[0055] The graphitized carbon material prepared in Comparative Example 5 was evaluated for carbon durability by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5V, compared with untreated carbon black, the carbon oxidation peak current growth rate in its CV curve did not change significantly, and the peak current value decreased by only 10%, which was close to that of Comparative Example 3.

[0056] Comparative Example 6 This comparative example relates to a low-temperature graphitization method for carbon materials based on a binary transition metal salt synergistic catalysis and freeze-drying process. The main difference between this method and Example 2 is the use of a combination of ferrous sulfate and cobalt sulfate. The method specifically includes the following steps: S1: Weigh out 3g of ferrous sulfate and cobalt sulfate, with a Co:Fe molar ratio of 0.5:1, and disperse them evenly in 30ml of deionized water; S2: Cabot carbon and the two transition metal salts in the S1 solution are uniformly mixed at a mass ratio of 3:1. After stirring at room temperature for 5 hours, the mixture is first rapidly cooled in liquid nitrogen for 20 minutes, then freeze-dried in vacuum at -50℃ for 26 hours. The dried solid is then ground and pulverized to obtain mixed material A. S3: Transfer the mixed material A obtained in S2 to a heat treatment device and heat treat it at 1100℃ for 15 min under an argon atmosphere, with the heating rate controlled at 15℃ / min, to obtain mixture B; S4: The mixture B obtained in S3 was acid-washed with a sulfuric acid solution with a molar concentration of 1M, heated to 80℃, and the acid washing time was 72h.

[0057] S5: The solid powder in S4 was subjected to a second annealing treatment at 600℃ in an inert atmosphere for 2 hours to obtain highly graphitized porous carbon material.

[0058] The carbon durability of the highly graphitized carbon material prepared in Comparative Example 6 was evaluated by AST and CV tests. After 10,000 cycles at a high voltage of 1.0 to 1.5 V, compared with untreated carbon black, the carbon oxidation peak current in its CV curve showed a significant increase that tended to level off, and the peak current value decreased by more than 50%. The ID / IG ratio of this highly graphitized carbon material was less than 0.6 (Table 1).

[0059] Table 1

[0060] The specific surface area of ​​the graphitized carbon materials used in Examples 2, 4, and Comparative Example 6 is recorded in Table 2. A Pt / C catalyst with a platinum loading of 40 wt% was prepared using an impregnation reduction method, and its particle size was characterized by transmission electron microscopy (TEM). The data are listed in Table 2. The results show that excessive graphitization significantly reduces the specific surface area of ​​the support. When the value is below 400 m² / g, the size of the supported Pt particles increases significantly, and the particle size distribution broadens. This phenomenon is generally accepted to reduce the electrochemical active area of ​​the catalyst, decrease platinum utilization, and thus adversely affect the overall battery performance, and may accelerate the Ostwald ripening process of Pt particles.

[0061] Table 2

[0062] In summary, the low-temperature graphitization method for carbon materials based on Mn salt-based binary transition metal salt synergistic catalysis and freeze-drying provides by this invention requires no addition of any organic dispersants or anchoring agents. First, two specific transition metal salt solutions are uniformly mixed with commercial carbon black and rapidly solidified by liquid nitrogen quenching. Then, a freeze-drying process with precise temperature and pressure control is used to fix the dispersion state of the metal precursor. Next, catalytic graphitization is performed in an inert atmosphere at a temperature below 1200°C. Finally, the carbon framework is purified by acid washing and stabilized by secondary annealing, yielding carbon materials with suitable high graphitization degree (Raman ID / IG value of 0.6–0.91), high specific surface area, and abundant pore structure. This invention overcomes the problems of reliance on organic additives, complex processes, and high energy consumption in traditional technologies by combining the synergistic effect of binary metals with a unique physical drying process and secondary annealing process, providing a new approach for the low-cost and green preparation of high-performance fuel cell catalyst supports and other materials.

[0063] Furthermore, the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for low-temperature graphitization of carbon materials, characterized in that, The method includes the following steps: S1. Mix two transition metal salts with deionized water at a mass ratio of (0.02-1):1, so that the two transition metal salts are fully dissolved in the deionized water; one of the two transition metal salts is a manganese salt. S2. Mix carbon black and the two transition metal salts in the S1 solution at a mass ratio of (0.5~8):

1. After stirring at room temperature, the suspension is first rapidly cooled and pre-frozen in liquid nitrogen for 5-30 minutes, and then dried under vacuum conditions of -45℃ to -70℃ and vacuum degree ≤10 Pa for 20-28 hours. The dried solid is a black powder, which is the mixed material A. S3. Transfer the mixed material A obtained in S2 to a heat treatment apparatus and heat-treat it for 10 to 60 minutes at a temperature of 10 to 15 °C / min to 1000 °C to 1200 °C under a protective atmosphere to obtain mixture B. S4. The mixture B obtained in S3 is washed with an acid solution, heated to 60~90℃, and the acid washing time is 12~72h. S5. The graphitized carbon material after S4 pickling is subjected to a second annealing treatment under a protective atmosphere at a temperature of 600-800℃ to obtain highly graphitized porous carbon material.

2. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S1, the other transition metal salt is selected from iron salt, cobalt salt, and nickel salt.

3. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In the combination of the two transition metal salts, the molar ratio of the manganese salt to the metal element of the other transition metal salt is (0.05-10):

1.

4. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S2, the carbon black is one or more of the following: biomass carbon black, acetylene black, Ketjen black, Cabot black, mesoporous carbon, and carbon fiber.

5. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S2, the stirring time at room temperature is 0.5~6 h; and the liquid nitrogen is rapidly cooled for 5-30 min.

6. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S3, the protective atmosphere is either nitrogen or argon.

7. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S4, the acid solution is one or more of hydrochloric acid, sulfuric acid, nitric acid, and glacial acetic acid.

8. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S4, the concentration of the acid solution is 1 ~ 3M.

9. The low-temperature graphitization method for carbon materials according to claim 1, characterized in that, In S5, the annealing time is 1-3 hours.

10. The method for low-temperature graphitization of carbon materials according to claim 1, characterized in that, In S5, the Raman peak ratio (ID / IG) of the highly graphitized porous carbon material is 0.6~0.91.

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