CN102569842A - Preparation method of hybrid ordered mesoporous carbon coat for protecting stainless steel bipolar plate of proton exchange membrane fuel cell - Google Patents

Abstract

A method for preparing a hybrid ordered mesoporous carbon coating for the protection of stainless steel bipolar plates in proton exchange membrane fuel cells, comprising the following steps: dissolving a surfactant in absolute ethanol, stirring, and then adding dropwise a phenolic resin ethanol solution, stir evenly, then add boric acid or phosphoric acid ethanol solution, stir to obtain a light yellow transparent sol-gel, drop by drop on the stainless steel sheet, and carry out dripping and homogenizing treatment; then the solution coated The solvent of the stainless steel sheet is evaporated for at least 8 hours, and then thermally polymerized at 70-120°C for at least 24 hours to form a light yellow coating on the surface of the stainless steel; finally carbonized in a nitrogen atmosphere to obtain a black ordered mesoporous carbon coating or carbon-boron coating or carbon - Phosphorus coating. In the present invention, heteroatoms are directly doped into the precursor of carbon to obtain uniformly distributed heteroatoms to maintain an ordered mesoporous structure, and at the same time improve the conductivity and hydrophobicity of the mesoporous carbon coating on the 304 type stainless steel substrate properties, and its corrosion resistance in sulfuric acid solution.

Description

A preparation method of hybrid ordered mesoporous carbon coating for proton exchange membrane fuel cell stainless steel bipolar plate protection

technical field

The invention relates to a method for preparing a hybrid ordered mesoporous carbon coating used for the protection of stainless steel bipolar plates of proton exchange membrane fuel cells, belonging to the technical field of preparation of protective coatings for stainless steel bipolar plates.

Background technique

The bipolar plate is an important part of the proton exchange membrane fuel cell, accounting for nearly 80% of the total weight and 50% of the total cost. Materials that can be used as bipolar plates must have the characteristics of high electrical conductivity, high corrosion resistance, good mechanical strength, low gas permeability and low price. In recent years, recent studies have shown that stainless steel has become the best choice for metallic bipolar plates due to its advantages in mechanical strength, corrosion resistance, hardness, surface finish, and cleanliness. However, there are still some problems with stainless steel as a bipolar plate. The main problem is that after working in a corrosive medium for a long time, a passivation film will be formed on the surface of the metal bipolar plate to prevent further corrosion of the metal. However, the surface contact resistance increases, and as the working time increases, the passivation layer will become thicker and thicker. Another problem is that there are SO ₄ ²⁻ , Cl ⁻ , F ⁻ plasmas under weakly acidic conditions, so that the stainless steel will be corroded and dissolved during use, so as to contaminate the membrane electrode. The passivation and dissolution of metals will undoubtedly convert part of the electrical energy into heat and dissipate it, which will affect the overall efficiency of the fuel cell. Various coatings are used to protect metallic bipolar plates, such as carbon coatings and metallic coatings. Carbon-film-coated stainless steel is considered a good material for bipolar plates in proton membrane fuel cells due to its low cost and small size. Many studies have shown that when a carbon coating is applied to a metal bipolar plate from which the passivation film has been completely removed, the interfacial contact resistance is significantly reduced [Tomokazu Fukutsuka, Takayuki Yamaguchi, Shin-Ichi Miyano, Yoshiaki Matsuo, Yosohiro Sugie, Zempachi Ogumi, Journal of Power Sources 174 (2007) 199–205].

Therefore, various carbon materials, such as graphite, conductive polymers, diamond-like carbon, and organic self-assembled materials, have been extensively studied as carbon coatings for bipolar plates. Fukutsuka indicated that by the plasma-assisted vapor deposition method [T. Fukutsuka, T. Yamaguchi, S.I. Miyano, Y. Matsuo, Y. Sugie, Z. Ogumi, J. Power Sources 1 (2007) 199-205.] A carbon film is formed on the surface of stainless steel (304SS), which has high conductivity compared with before deposition, and there is no carbon film between the carbon layer and 304SS A passivation film is formed, but under the working conditions of a proton exchange membrane fuel cell, the 304SS deposited with a carbon film shows a strong polarization phenomenon. Chung [C.Y. Chung, S.K. Chen, P.J. Chiu, M.H. Chang, T.T. Hung, T.H. Ko, J. Power Sources 1 (2008) 276-281.] and others systematically studied the corrosion behavior of carbon-coated 304SS as a proton membrane fuel cell bipolar plate, and found that the carbon film exhibited similar The chemical stability of 304SS can be protected in the corrosive environment of the proton membrane fuel cell. Feng et al[Kai Feng, Xun Cai, Hailin Sun, Zhuguo Li, Paul K. Chu, Diamond & Related Materials 19 (2010) 1354-1361. Kai Feng, Yao Shen, Hailin Sun, Dongan Liu, Quanzhang An, Xun Cai, , Paul K. Chu, i n t e r n a t i o n a l j o u rna l o f hydrogen energy 3 4 ( 2 0 0 9 ) 6771 – 6777] A dense, continuous, amorphous carbon film was coated on the surface of a 316L stainless steel sample by unbalanced magnetron sputtering, and its performance is superior to that of a traditional graphite bipolar plate. The hydrophobicity and interfacial contact resistance of this carbon film are superior to those of graphite, and because of its good chemical inertness, it significantly enhances the corrosion resistance of the coated 316L stainless steel.

A new method is to form a mesoporous carbon film on a stainless steel substrate by spin coating. The carbon-based composite film exhibits the best protection performance, and its corresponding potential polarization process is very stable in a simulated fuel cell environment. [T. Wang, J.P. He, D. Sun, J.H. Zhou, Y.X. Guo, X.C. Ding, S.C. Wu, J.Q. Zhao, J. Tang, Corrosion Science 53 (2011) 1498-1504.] Although mesoporous carbon films have many advantages, such as good protection performance, due to the presence of a large amount of silicon dioxide, the electrical conductivity needs to be further improved. Many studies have shown that doping electron-donating or electron-withdrawing elements in carbon materials, such as N, P, and B, can change the electrical properties of carbon films and form additional functional groups on the surface of carbon films. Most ordered mesoporous carbon materials are doped with other atoms through post-treatment. This method is very simple, but it is not easy to control the content and distribution of dopants, so that the ordered mesoporous structure is partially or completely destroyed. It is a very serious deficiency.

Contents of the invention

Technical problem: In order to solve the above problems, the object of the present invention is to propose a method for preparing a hybrid ordered mesoporous carbon coating with simple process and excellent corrosion resistance.

Technical solution: 1. A method for preparing a hybrid ordered mesoporous carbon coating for protection of stainless steel bipolar plates in proton exchange membrane fuel cells, characterized in that it comprises the following steps:

(1) Dissolve the surfactant in absolute ethanol, stir to form a transparent solution; then add the ethanol solution of phenolic resin dropwise, stir, and set aside;

(2), set 0.6 Add the mol/L HCl solution into absolute ethanol, adjust the pH to 3~5, then add boric acid or phosphoric acid, and stir to obtain the solution; set aside;

(3) Add the solution obtained in step (2) dropwise to the solution obtained in step (1). The amount of boric acid or phosphoric acid added per 1 g of surfactant ranges from 0.02 to 0.16 g, and stir to make it evenly mixed , to obtain light yellow transparent sol-gel;

(4), add the sol-gel prepared in step (1) or (3) drop by drop on the stainless steel sheet, and perform glue dropping and uniform glue treatment;

(5) Evaporate the solution-coated stainless steel sheet obtained in step (4) at 25°C for at least 8 hours, and then thermally polymerize at 70-120°C for at least 24 hours to form a light yellow coating on the surface of the stainless steel; finally Carbonization in nitrogen atmosphere to obtain black ordered mesoporous carbon-boron coating or carbon-phosphorus coating or carbon coating.

The surfactant in the step (1) is polyoxyethylene/polyoxypropylene/polyoxyethylene amphiphilic block copolymer F127 with a molecular formula of PEO ₁₀₆ -PPO ₇₀ -PEO ₁₀₆ .

In the step (1), every 1 g of surfactant is dissolved in 15 mL of absolute ethanol, and the mass fraction of the ethanol solution of phenolic resin is 20%.

In the step (4), the dispensing speed is 400~800 rpm, the glue dispensing time is 10-20 s, the speed of dispensing glue is selected as 1500-3000 rpm, the dispensing time is 45-60 s, and the dispensing and dispensing process is repeated 5 times.

The carbonization in the step (5) is carried out in an atmosphere tube furnace with a nitrogen gas flow, at 350°C for 3-5 hours, at the target temperature of 400-700°C for 2 hours, and the heating rate is strictly controlled at 1°C /min

The ethanol solution of phenolic resin in step (1) (phenolic resin mass fraction 20%), the specific preparation method is: first, weigh 6.1 g of phenol in a beaker, slowly heat up to 40~42 ℃ to make it melt; then slowly add 1.3 g of NaOH (mass fraction 20%) aqueous solution into the beaker dropwise, and magnetically stir for 10 min at the same time to make the solution uniform; then, add 10.5 g formaldehyde solution (mass fraction 37%), and heated to 70~75 ℃, stirred vigorously for 60 min to polymerize phenol and formaldehyde; after the reaction, the solution was naturally cooled to room temperature, and then 0.6 mol/L HCl solution to adjust the pH value of the solution in the beaker to neutral (7.0); put the adjusted pH value solution in a vacuum drying oven, and evaporate the water at 45 ℃, during which white NaCl can be seen Crystals are precipitated; finally, part of the phenolic resin in the beaker is dissolved in absolute ethanol to prepare a phenolic resin ethanol solution with a mass fraction of 20% for later use.

Beneficial effects: the present invention directly dopes heteroatoms into carbon precursors to obtain evenly distributed heteroatoms to maintain an ordered mesoporous structure, and at the same time improve the electrical conductivity and Hydrophobicity, and its corrosion resistance in sulfuric acid solution.

Description of drawings

Figure 1 is the XRD pattern of the ordered mesoporous CB-x-500 coating,

Figure 1a is the small-angle XRD pattern of the ordered mesoporous CB-x-500 coating. It can be seen from the pattern that as the amount of boric acid increases, the small-angle XRD diffraction peak at the position of 0.8 ^o gradually decreases, which means that the CB-x-500 coating has The order degree decreases, indicating that the addition of boric acid in the range of 0.2-0.8 is more conducive to the maintenance of the ordered mesoporous structure;

Figure 1b is the large-angle XRD pattern of CB-x-500 coating;

Figure 2 is the XRD pattern of the ordered mesoporous CB-0.08-y coating,

Among them: Figure 2a is the small-angle XRD spectrum of the ordered mesoporous CB-0.08-y coating, and Figure 2b is the large-angle XRD spectrum of the ordered mesoporous CB-0.08-y coating; it can be seen from Figure 2a that the coating Order is maintained intact;

Figure 3 is the XRD spectrum of the ordered mesoporous CP-0.06-500 coating,

Among them: Figure 3a is the small-angle XRD spectrum of the ordered mesoporous CP-0.06-500 coating, and Figure 3b is the large-angle XRD spectrum of the ordered mesoporous CP-0.06-500 coating;

Figure 4 is the TEM image of the ordered mesoporous CB-x-500 coating: (a,b) CB-0-500; (c,d) CB-0.02-500; (e,f) CB-0.04-500 ; (g,h) CB-0.08-500; (i,j) CB-0.16-500; where the inset in Figure 4h is the CB-0.08-500 electron diffraction pattern, indicating that the coating is mainly amorphous carbon;

Figure 5 is the TEM image of the ordered mesoporous CB-0.08-y coating: (a,b) CB-0.08-400; (c,d) CB-0.08-500; (e,f) CB-0.08-600; (g, h) CB-0.08-700;

Fig. 6 is the _N2 adsorption-desorption isotherm curve and pore size distribution curve of the ordered CB-x-500 coating,

Among them: Figure 6a is the _N adsorption-desorption isotherm curve of CB-x-500 coating, and Figure 6b is the pore size distribution curve of CB-x-500 coating;

Figure 7 is the _N2 adsorption-desorption isotherms and pore size distribution curves of the ordered CB-0.08-y coating,

Among them: Figure 7a is the N adsorption _- desorption isotherm curve of CB-0.08-y coating, and Figure 7b is the pore size distribution curve of CB-0.08-y coating;

Figure 8 is the Tafel curve of the ordered CB-x-500 coating in the 0.5 MH ₂ SO ₄ system; the electrochemical data calculated from the figure are listed in Table 2, and it can be seen from the table that the amount of boric acid added is The corrosion protection of the coating between 0.02 and 0.08 g is better;

Fig. 9 is the Tafel curve of ordered CB-0.08-y coating under 0.5 MH ₂ SO ₄ system. The electrochemical data calculated from the figure are listed in Table 2. It can be seen from the table that the corrosion protection of the coating is better when the heat treatment temperature is between 400 and 600 °C.

Detailed ways

The naming method of the coating obtained in the following examples is: the boron-doped carbon coating product is marked in the form of CB-xy, where x indicates the mass (g) of boric acid added, and y indicates the final heat treatment temperature, in the form of CP-xy Mark the phosphorus-doped carbon coating product, where x represents the mass (g) of phosphoric acid added, and y represents the final heat treatment temperature, and the samples in each embodiment are named in the following table:

Example 1:

(1) Dissolve 1 g of polyoxyethylene/polyoxypropylene/polyoxyethylene amphiphilic block copolymer F127 (purchased from Sigma-Aldrich) in 15.0 ml of absolute ethanol, and stir for 1 h to form a transparent solution A; At the same time, 1.8 ml of HCl solution (0.6 mol/L) into 5 ml of absolute ethanol, adjust the pH in the range of 3 to 5, then slowly add 0.02 g of boric acid, and magnetically stir to obtain solution B; slowly add 5 g of ethanol solution of phenolic resin dropwise (mass fraction of phenolic resin 20%) in solution A, stir for 10 min; then add solution B to solution A drop by drop, stir for 1 h to mix evenly, so as to obtain light yellow transparent carbon-boron solution;

(2) Add the previously prepared carbon-boron sol-gel drop by drop on the stainless steel sheet on the desktop glue homogenizer. rpm/s, glue homogenization time 60 s, repeat the process of dispensing glue and glue homogenization 5 times;

(3) The stainless steel sheet coated with the solution was transferred to an oven, the solvent was evaporated at 25 °C for 8 h, and then thermally polymerized at 70-120 °C for 24 h to form a light yellow coating on the surface of the stainless steel;

(4) Put the coated stainless steel sheet into the porcelain boat and prepare for carbonization. The carbonization was carried out in an atmosphere tube furnace with a nitrogen gas flow. The temperature was kept at 350 °C for 3 h, and at the target temperature of 500 °C for 2 h. The heating rate was strictly controlled at 1 °C/min;

The CB-0.02-500 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 4(c,d)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 500 mV, and the corrosion current density decreased by an order of magnitude.

Example 2:

In this embodiment, except that the addition of boric acid is 0.04 g, all the other contents are the same as in Example 1.

The CB-0.04-500 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 4(e,f)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 500 mV, and the corrosion current density decreased by an order of magnitude.

Example 3:

In this embodiment, except that the addition of boric acid is 0.08 g, all the other contents are the same as in Example 1.

The CB-0.08-500 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 4(g,h)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 500 mV, and the corrosion current density decreased by an order of magnitude.

Example 4:

In this embodiment, except that the addition of boric acid is 0.16 g, all the other contents are the same as in Example 1.

The CB-0.16-500 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 4(i,j)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 500 mV, and the corrosion current density doubled.

Example 5:

In this example, except that the target temperature is kept at 400°C for 2 hours, the rest of the content is the same as in Example 3.

The CB-0.08-400 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 5(a,b)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion Positive shift of about 500 mV corrosion current density decreased by an order of magnitude.

Embodiment 6:

In this example, except that the target temperature is 600 °C for 2 hours, the rest of the content is the same as that of Example 3.

The CB-0.08-600 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 5(e,f)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 500 mV, and the corrosion current density decreased by an order of magnitude.

Embodiment 7:

In this example, except that the target temperature is 700 °C for 2 hours, the rest of the content is the same as that of Example 3.

The CB-0.08-700 coating is obtained, and the B element is highly dispersed in the carbon matrix during the carbonization process, which ensures that the ordered mesoporous structure of the coating is not destroyed (see Figure 1 and Figure 5(g,h)) , see Table 1 for specific structural data. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 300 mV, and the corrosion current density doubled.

Embodiment 8:

(1) Dissolve 1 g of surfactant F127 in 20.0 ml of absolute ethanol, stir for 1 h to form a transparent solution A; slowly add 5 g of ethanol solution of phenolic resin (mass fraction of phenolic resin 20%) in solution A , stirred to obtain light yellow transparent carbon solution;

(2) Add the previously prepared carbon sol-gel drop by drop on the stainless steel sheet on the desktop glue homogenizer, the glue dropping speed is selected as 800 rpm/s, the glue dropping time is 20 s, and the glue uniform speed is selected as 3000 rpm/s, glue homogenization time 45s, repeat the glue dropping and glue homogenization process 5 times;

(3) The stainless steel sheet coated with the solution was transferred to an oven, the solvent was evaporated at 25 °C for 8 h, and then thermally polymerized at 70-120 °C for 24 h to form a light yellow coating on the surface of the stainless steel;

(4) Put the coated stainless steel sheet into the porcelain boat and prepare for carbonization. The carbonization was carried out in an atmosphere tube furnace with a nitrogen gas flow, held at 350 °C for 5 h, and at the target temperature of 500 °C for 2 h, and the heating rate was strictly controlled at 1 °C/min;

The obtained CB-0-500 coating has a highly ordered mesoporous structure (see Figure 1 and Figure 4(a,b)), and the specific structural data are shown in Table 1. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential shifted positively by about 300 mV, and the corrosion current density decreased by an order of magnitude.

Embodiment 9:

In this embodiment, except that the additive in step (1) replaces boric acid by phosphoric acid, and the addition amount is 0.06g, all the other contents are the same as in Example 1.

The obtained CP-0.06-500 coating has a highly ordered mesoporous structure (see Figure 3), and the specific structural data are shown in Table 1. The data of the Tafel test in 0.5 _MH2SO4 are shown in Table _2. From Table 2, it can be seen that compared with the uncoated Type 304 stainless steel, the self-corrosion The potential is shifted positively by about 550 mV, and the corrosion current density is doubled.

Table 1 Pore structure parameters of ordered mesoporous CB and CP coatings calculated from _N2 adsorption-desorption isotherms

From Table 1, it can be seen that the ordered mesoporous carbon coating after adding B or P has a large specific surface area, between 400 and 700 m ² /g, and the mesopore ratio is relatively high, both at 78 % or more, and combined with Figure 6b and Figure 7b, it can be seen that the pore size distribution of the CB coating is very concentrated.

Table 2 is the electrical conductivity, contact angle, and self-corrosion potential and self-corrosion current density of the Tafel test in 0.5 MH ₂ SO ₄

From Table 2, it can be seen that the conductivity and contact angle of the ordered mesoporous carbon coatings increase correspondingly after adding B or P, and the greater the addition amount, the more the conductivity and contact angle increase. However, through the characterization of corrosion protection performance, it can be deduced that the amount of boric acid added should be 0.02~0.08 g, the heat treatment temperature is preferably between 400 and 600 °C.

Claims (6)

1. A preparation method for a hybrid ordered mesoporous carbon coating for proton exchange membrane fuel cell stainless steel bipolar plate protection, characterized in that comprising the following steps: (1) Dissolve the surfactant in absolute ethanol, stir to form a transparent solution; then add dropwise the ethanol solution of phenolic resin, the mass ratio of surfactant to phenolic resin is 1:5, stir evenly, and set aside; (2) Adjust the pH of absolute ethanol to 3~5 with HCl solution, then add boric acid or phosphoric acid, and stir to obtain a solution; set aside; (3) Add the solution obtained in step (2) dropwise to the solution obtained in step (1). The amount of boric acid or phosphoric acid added per 1 g of surfactant ranges from 0.02 to 0.16 g, and stir to make it evenly mixed , to obtain light yellow transparent sol-gel; (4), add the sol-gel prepared in step (1) or (3) drop by drop on the stainless steel sheet, and perform glue dropping and uniform glue treatment; (5) Evaporate the solution-coated stainless steel sheet obtained in step (4) at 25°C for at least 8 hours, and then thermally polymerize at 70-120°C for at least 24 hours to form a light yellow coating on the surface of the stainless steel; finally Carbonization in nitrogen atmosphere to obtain black ordered mesoporous carbon coating or carbon-boron coating or carbon-phosphorus coating. 2. A method for preparing a hybrid ordered mesoporous carbon coating for protection of stainless steel bipolar plates in proton exchange membrane fuel cells according to claim 1, characterized in that: in the step (1), the surface activity The agent is polyoxyethylene/polyoxypropylene/polyoxyethylene amphiphilic block copolymer F127, and its molecular formula is PEO ₁₀₆ -PPO ₇₀ -PEO ₁₀₆ . 3. A method for preparing a hybrid ordered mesoporous carbon coating for protection of stainless steel bipolar plates in proton exchange membrane fuel cells according to claim 1, characterized in that: each step in the step (1) 1 g of surfactant was dissolved in 15 mL of absolute ethanol, and the mass fraction of the ethanol solution of phenolic resin was 20%. 4. A method for preparing a hybrid ordered mesoporous carbon coating for protection of stainless steel bipolar plates in proton exchange membrane fuel cells according to claim 1, characterized in that: in the step (2), HCl The solution concentration is 0.6 mol/L. 5. A method for preparing a hybrid ordered mesoporous carbon coating for protection of stainless steel bipolar plates of proton exchange membrane fuel cells according to claim 1, characterized in that: in the step (4), Glue dispensing speed is 400~800 rpm, the glue dispensing time is 10-20 s, the speed of dispensing glue is selected as 1500-3000 rpm, the dispensing time is 45-60 s, and the dispensing and dispensing process is repeated 5 times. 6. A method for preparing a hybrid ordered mesoporous carbon coating for protection of stainless steel bipolar plates in proton exchange membrane fuel cells according to claim 1, characterized in that: carbonization in the step (5) It was carried out in an atmosphere tube furnace with a nitrogen gas flow, at 350 °C for 3-5 h, at the target temperature of 400-700 °C for 2 h, and the heating rate was strictly controlled at 1 °C/min.

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