Iron monatomic modified carbon nanotube array, preparation method and application thereof in aqueous zinc-iodine battery

Abstract

The present application relates to the technical field of energy storage, in particular to a kind of iron single atom modified carbon nanotube array, preparation method and its application in aqueous zinc-iodine battery, simple solid phase reaction and high temperature carbonization strategy are used, carbon cloth is used as substrate, acetylacetone iron, boric acid, polyethylene glycol and urea are used as iron source, boron source, carbon source and nitrogen source respectively, after high temperature calcination, iron single atom modified carbon nanotube array is in situ grown on carbon cloth.Iron single atom not only enhances the chemical capture ability of carbon nanotube to polyiodide, but also promotes the redox conversion of iodine;Carbon nanotube array provides physical space for the confined adsorption of iodine species, and effectively promotes ion diffusion.Therefore, the obtained aqueous zinc-iodine battery shows high specific capacity and good cycle stability.The whole process is simple, easy to operate, and has industrial application prospect, which provides a new preparation method for carbon-based iodine positive electrode host material of aqueous zinc-iodine battery.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, specifically to an iron single-atom modified carbon nanotube array, its preparation method, and its application in aqueous zinc-iodine batteries. Background Technology

[0002] With increasing concerns about lithium resource reserves, battery safety, and environmental friendliness, researchers are accelerating their exploration of alternatives to lithium batteries. Aqueous rechargeable zinc batteries are considered a powerful supplement to the electrochemical energy storage of lithium-ion batteries due to their high safety and low cost. Among them, aqueous zinc-iodine batteries are competitive because they possess abundant iodine reserves, a high theoretical capacity (211 mAh / g), and a reasonable redox potential (Ig). - / I0 (0.54V relative to the standard hydrogen electrode) avoids the oxygen evolution reaction in aqueous batteries. Compared to lithium-ion batteries, aqueous zinc-iodine batteries show advantages such as ease of assembly, low installation cost, and high energy density due to the elimination of supporting equipment such as storage tanks, pumps, and pipelines. However, aqueous zinc-iodine batteries still suffer from problems such as low utilization rate of positive electrode active material, rapid capacity decay, and low coulombic efficiency in applications. These shortcomings are mainly due to the high solubility of I... 3- This is caused by problems such as the shuttling between the positive and negative electrodes, the slow conversion reaction between the positive electrode active material iodine and zinc iodide, and the inherent poor conductivity of iodine.

[0003] To avoid these problems, porous carbon-based materials (such as carbon fibers, activated carbon, and carbon nanotubes) are considered effective conductive hosts, confining iodine species through physical adsorption and improving the conductivity of iodine. Furthermore, hierarchical porous structures facilitate the construction of rapid mass transfer pathways at the solid-liquid interface and accelerate reaction kinetics. However, the weak physical adsorption of the nonpolar carbon framework makes it difficult to limit the shuttle effect of polyiodides, resulting in poor cycle performance. Therefore, finding novel multifunctional catalysts to improve the performance of aqueous zinc-iodine batteries is an effective measure to promote their commercial application.

[0004] In addition to having the highest atomic utilization efficiency, carbon-based metal single atoms also have advantages such as high intrinsic activity, flexible selectivity, and low cost. At the same time, by utilizing the synergistic effect of metal single atoms and carbon matrix, composite materials can be endowed with unique physicochemical properties, which is expected to solve the problems faced by iodine cathodes. Specifically, carbon-based metal single atoms have the following advantages in aqueous zinc-iodine batteries: (1) The excellent conductivity of carbon support can improve the utilization rate of iodine and its discharge products; (2) Reasonable pore structure and specific surface area can physically adsorb and limit iodine species; (3) Metal-nitrogen sites regulate the surface charge environment of adjacent carbon atoms, generating polar surfaces and active sites to strongly anchor polyiodine, thereby enhancing chemical adsorption and catalytic conversion of iodine species. However, the activity of single-atom catalysts mainly depends on the reasonable design and optimization of their coordination structure. In addition, constructing hollow structures is another effective way to improve catalytic performance. Therefore, by making reasonable use of material preparation methods, loading metal single-atom catalysts onto hollow carbon material matrices and combining them with iodine, the conversion reaction kinetics of iodine cathodes can be greatly improved, and the electrochemical performance of aqueous zinc-iodine batteries can be enhanced.

[0005] In view of the above-mentioned defects, the inventors of this invention have finally obtained this invention after a long period of research and practice. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of low utilization rate of positive electrode active material, rapid capacity decay and low coulombic efficiency in aqueous zinc-iodine batteries. It provides an iron single-atom modified carbon nanotube array, its preparation method and its application in aqueous zinc-iodine batteries.

[0007] To achieve the above objectives, this invention discloses a method for preparing iron single-atom modified carbon nanotube arrays, comprising the following steps:

[0008] S1, improve the hydrophilicity of carbon cloth by oxidizing it through gas-phase heat treatment;

[0009] S2, the carbon cloth obtained in step S1 is immersed in an aqueous solution of polyethylene glycol, urea, boric acid and acetylacetone iron and kept for 24 hours, and then heated in a vacuum drying oven at 80°C for 12 hours to remove the solvent.

[0010] S3. The material obtained in step S2 is placed in a tube furnace and carbonized at high temperature in an inert gas atmosphere to obtain an iron single-atom supported carbon nanotube array.

[0011] In step S1, the conditions for gas phase heat treatment are as follows: after calcining the carbon cloth in air at 450°C for 2 hours, the hydrophobic surface of the carbon cloth has been adjusted to a superhydrophilic surface.

[0012] In step S2, the mass ratio of polyethylene glycol, urea, boric acid, and acetylacetone iron is 1:10:0~0.3:0.02~0.2.

[0013] In step S3, the inert gas is argon or nitrogen, the gas flow rate is 450 mL / min, the heating rate is 2℃ / min, the high-temperature carbonization temperature is 800℃, and the time is 6 h.

[0014] The present invention also discloses an iron single-atom modified carbon nanotube array prepared by the above preparation method.

[0015] The present invention also discloses the application of the above-mentioned iron single-atom modified carbon nanotube array in aqueous zinc-iodine batteries.

[0016] (1) Preparation of iodine-supported cathode material:

[0017] Iodine solution was added dropwise to an iron single-atom modified carbon nanotube array and dried at 40℃ for 1 h. Then, it was transferred to a tube furnace and kept at 120℃ for 6 h under an argon atmosphere. After the temperature dropped to room temperature, it was cut into 1 cm × 1 cm square positive electrodes for later use.

[0018] (2) Assembly of aqueous zinc-iodine batteries:

[0019] The iodine self-supporting positive electrode, zinc foil negative electrode and glass fiber separator prepared in step (1) are assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator are 30 μL on the positive electrode side and 30 μL on the negative electrode side.

[0020] In step (1), the amount of iodine solution added is 20-40 mL.

[0021] In step (1), the amount of iodine solution added is 20 mL, and the surface loading of iodine is 3 mg / cm³. 2 .

[0022] In step (1), the charging and discharging mechanism of the aqueous zinc-iodine battery is as follows:

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. This invention provides a method for preparing iron-modified carbon nanotube array materials. It employs a simple solid-state reaction and high-temperature carbonization strategy, using carbon cloth as a substrate. Iron acetylacetone, boric acid, polyethylene glycol, and urea are used as iron, boron, carbon, and nitrogen sources, respectively. After high-temperature calcination, iron-modified carbon nanotube arrays are grown in situ on the carbon cloth. The method is simple, involving only a few steps such as dissolution, drying, and high-temperature carbonization. It is environmentally friendly and easily scalable for large-scale production.

[0025] 2. The material obtained by the preparation method of the present invention has high conductivity, large porosity, and hollow carbon nanotube structure, which can achieve effective adsorption of polyiodides. Furthermore, the captured and adsorbed polyiodides are catalyzed by iron single atoms to accelerate the reaction kinetics of iodine.

[0026] 3. The iron single-atom modified carbon nanotube array material prepared by this invention has a uniform distribution of single-atom iron on the surface of carbon nanotubes, which effectively improves the exposure of active sites and enhances the synergistic catalytic efficiency of the material; it also effectively improves the cycle and rate performance of aqueous zinc-iodine batteries.

[0027] 4. The iron single-atom modified carbon nanotube array material obtained in this invention has high capacity, excellent cycle performance and rate performance when used in aqueous zinc-ion batteries. Attached Figure Description

[0028] Figure 1 The XRD patterns of the iron single-atom modified carbon nanotube array obtained in Example 1 and the materials obtained in Example 2 and Comparative Example 1 are shown.

[0029] Figure 2 SEM images of the iron-modified carbon nanotube array (a) obtained in Example 1, and the materials obtained in Comparative Example (b) and Example 2 (c);

[0030] Figure 3 TEM images of the iron-modified carbon nanotube array (a) obtained in Example 1 and the materials obtained in Comparative Example 1 (b) and Example 2 (c);

[0031] Figure 4 This is a spherical aberration electron microscope image of the iron single-atom modified carbon nanotube array obtained in Example 1;

[0032] Figure 5 The nitrogen adsorption / desorption curves are for the iron single-atom modified carbon nanotube array obtained in Example 1.

[0033] Figure 6 The rate performance of the materials obtained in Examples 1 and 2 and Comparative Example 1 when applied to aqueous zinc-iodine batteries;

[0034] Figure 7 The cycling performance of the materials obtained in Examples 1, 2 and Comparative Example 1 in an aqueous zinc-iodine battery at 1 A / g is shown. Detailed Implementation

[0035] The above-mentioned and other technical features and advantages of the present invention will be described in more detail below with reference to the accompanying drawings.

[0036] Example 1

[0037] Step 1: The method for preparing iron single-atom modified carbon nanotube arrays is as follows:

[0038] 0.5 g polyethylene glycol, 5 g urea, 0.15 g boric acid, and 0.02 g ferric acetylacetone were dissolved in 50 mL of deionized water. After stirring at room temperature for 30 min, a piece of vapor-phase heat-treated carbon cloth (2 cm × 2 cm) was immersed in the solution and kept for 24 h. The solvent was then removed by heating in a vacuum furnace at 80 °C for 12 h. After removal, the sample was placed in a tube furnace and calcined at 800 °C for 6 h in an argon atmosphere at a heating rate of 2 °C / min to obtain an iron single-atom modified carbon nanotube array material.

[0039] Step 2: Preparation of iodine-supported cathode material:

[0040] 20 mL of a 5 mg / mL iodine aqueous solution was added dropwise to an iron single-atom modified carbon nanotube array with a size of 2 cm × 2 cm. The array was dried at 40 °C for 1 h, then transferred to a tube furnace and maintained at 120 °C for 6 h under an argon atmosphere. After the temperature cooled to room temperature, the array was cut into 1 cm × 1 cm square positive electrodes for later use.

[0041] Step 3: Preparation of iodine-supported cathode material:

[0042] The iodine self-supporting positive electrode, commercial zinc foil negative electrode, and Whatman glass fiber (GF / C) separator prepared above were assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator were 30 μL on the positive electrode side and 30 μL on the negative electrode side, respectively. After the assembled battery was left to stand for 8 hours, electrochemical tests were performed.

[0043] Example 2

[0044] Step 1: The method for preparing iron single-atom modified carbon nanotube arrays is as follows:

[0045] 0.5 g polyethylene glycol, 5 g urea, 0.15 g boric acid, and 0.2 g ferric acetylacetone were dissolved in 50 mL of deionized water. After stirring at room temperature for 30 min, a piece of vapor-phase heat-treated carbon cloth (2 cm × 2 cm) was immersed in the solution and kept for 24 h. The solvent was then removed by heating in a vacuum furnace at 80 °C for 12 h. After removal, the sample was placed in a tube furnace and calcined at 800 °C for 6 h in an argon atmosphere at a heating rate of 2 °C / min to obtain an iron single-atom modified carbon nanotube array material.

[0046] Step 2: Preparation of iodine-supported cathode material:

[0047] 20 mL of a 5 mg / mL iodine aqueous solution was added dropwise to an iron single-atom modified carbon nanotube array with a size of 2 cm × 2 cm. The array was dried at 40 °C for 1 h, then transferred to a tube furnace and maintained at 120 °C for 6 h under an argon atmosphere. After the temperature cooled to room temperature, the array was cut into 1 cm × 1 cm square positive electrodes for later use.

[0048] Step 3: Preparation of iodine-supported cathode material:

[0049] The iodine self-supporting positive electrode, commercial zinc foil negative electrode, and Whatman glass fiber (GF / C) separator prepared above were assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator were 30 μL on the positive electrode side and 30 μL on the negative electrode side, respectively. After the assembled battery was left to stand for 8 hours, electrochemical tests were performed.

[0050] Example 3

[0051] Step 1: The method for preparing iron single-atom modified carbon nanotube arrays is as follows:

[0052] 0.5 g polyethylene glycol, 5 g urea, 0.15 g boric acid, and 0.1 g ferric acetylacetone were dissolved in 50 mL of deionized water. After stirring at room temperature for 30 min, a piece of vapor-phase heat-treated carbon cloth (2 cm × 2 cm) was immersed in the solution and kept for 24 h. The solvent was then removed by heating in a vacuum furnace at 80 °C for 12 h. After removal, the sample was placed in a tube furnace and calcined at 800 °C for 6 h in an argon atmosphere at a heating rate of 2 °C / min to obtain an iron single-atom modified carbon nanotube array material.

[0053] Step 2: Preparation of iodine-supported cathode material:

[0054] 20 mL of a 5 mg / mL iodine aqueous solution was added dropwise to an iron single-atom modified carbon nanotube array with a size of 2 cm × 2 cm. The array was dried at 40 °C for 1 h, then transferred to a tube furnace and maintained at 120 °C for 6 h under an argon atmosphere. After the temperature cooled to room temperature, the array was cut into 1 cm × 1 cm square positive electrodes for later use.

[0055] Step 3: Preparation of iodine-supported cathode material:

[0056] The iodine self-supporting positive electrode, commercial zinc foil negative electrode, and Whatman glass fiber (GF / C) separator prepared above were assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator were 30 μL on the positive electrode side and 30 μL on the negative electrode side, respectively. After the assembled battery was left to stand for 8 hours, electrochemical tests were performed.

[0057] Example 4

[0058] Step 1: The method for preparing iron single-atom modified carbon nanotube arrays is as follows:

[0059] 0.5 g polyethylene glycol, 5 g urea, and 0.02 g ferric acetylacetone were dissolved in 50 mL of deionized water. After stirring at room temperature for 30 min, a piece of vapor-phase heat-treated carbon cloth (2 cm × 2 cm) was immersed in the solution and kept for 24 h. The solvent was then removed by heating in a vacuum furnace at 80 °C for 12 h. After removal, the sample was placed in a tube furnace and calcined at 800 °C for 6 h in an argon atmosphere at a heating rate of 2 °C / min to obtain an iron single-atom modified carbon nanotube array material.

[0060] Step 2: Preparation of iodine-supported cathode material:

[0061] 20 mL of a 5 mg / mL iodine aqueous solution was added dropwise to an iron single-atom modified carbon nanotube array with a size of 2 cm × 2 cm. The array was dried at 40 °C for 1 h, then transferred to a tube furnace and maintained at 120 °C for 6 h under an argon atmosphere. After the temperature cooled to room temperature, the array was cut into 1 cm × 1 cm square positive electrodes for later use.

[0062] Step 3: Preparation of iodine-supported cathode material:

[0063] The iodine self-supporting positive electrode, commercial zinc foil negative electrode, and Whatman glass fiber (GF / C) separator prepared above were assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator were 30 μL on the positive electrode side and 30 μL on the negative electrode side, respectively. After the assembled battery was left to stand for 8 hours, electrochemical tests were performed.

[0064] Example 5

[0065] Step 1: The method for preparing iron single-atom modified carbon nanotube arrays is as follows:

[0066] 0.5 g polyethylene glycol, 5 g urea, 0.15 g boric acid, and 0.02 g ferric acetylacetone were dissolved in 50 mL of deionized water. After stirring at room temperature for 30 min, a piece of vapor-phase heat-treated carbon cloth (2 cm × 2 cm) was immersed in the solution and kept for 24 h. The solvent was then removed by heating in a vacuum furnace at 80 °C for 12 h. After removal, the sample was placed in a tube furnace and calcined at 800 °C for 6 h in an argon atmosphere at a heating rate of 2 °C / min to obtain an iron single-atom modified carbon nanotube array material.

[0067] Step 2: Preparation of iodine-supported cathode material:

[0068] 30 mL of a 5 mg / mL iodine aqueous solution was added dropwise to an iron single-atom modified carbon nanotube array with a size of 2 cm × 2 cm. The array was dried at 40 °C for 1 h, then transferred to a tube furnace and maintained at 120 °C for 6 h under an argon atmosphere. After the temperature cooled to room temperature, the array was cut into 1 cm × 1 cm square positive electrodes for later use.

[0069] Step 3: Preparation of iodine-supported cathode material:

[0070] The iodine self-supporting positive electrode, commercial zinc foil negative electrode, and Whatman glass fiber (GF / C) separator prepared above were assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator were 30 μL on the positive electrode side and 30 μL on the negative electrode side, respectively. After the assembled battery was left to stand for 8 hours, electrochemical tests were performed.

[0071] Example 6

[0072] Step 1: The method for preparing iron single-atom modified carbon nanotube arrays is as follows:

[0073] 0.5 g polyethylene glycol, 5 g urea, 0.15 g boric acid, and 0.02 g ferric acetylacetone were dissolved in 50 mL of deionized water. After stirring at room temperature for 30 min, a piece of vapor-phase heat-treated carbon cloth (2 cm × 2 cm) was immersed in the solution and kept for 24 h. The solvent was then removed by heating in a vacuum furnace at 80 °C for 12 h. After removal, the sample was placed in a tube furnace and calcined at 800 °C for 6 h in an argon atmosphere at a heating rate of 2 °C / min to obtain an iron single-atom modified carbon nanotube array material.

[0074] Step 2: Preparation of iodine-supported cathode material:

[0075] 40 mL of a 5 mg / mL iodine aqueous solution was added dropwise to an iron single-atom modified carbon nanotube array with a size of 2 cm × 2 cm. The array was dried at 40 °C for 1 h, then transferred to a tube furnace and kept at 120 °C for 6 h under an argon atmosphere. After the temperature cooled to room temperature, the array was cut into 1 cm × 1 cm square positive electrodes for later use.

[0076] Step 3: Preparation of iodine-supported cathode material:

[0077] The iodine self-supporting positive electrode, commercial zinc foil negative electrode, and Whatman glass fiber (GF / C) separator prepared above were assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator were 30 μL on the positive electrode side and 30 μL on the negative electrode side, respectively. After the assembled battery was left to stand for 8 hours, electrochemical tests were performed.

[0078] Comparative Example 1

[0079] The difference between Comparative Example 1 and Example 1 is that iron acetylacetone is not added in the first step, while the other conditions are the same as in Example 1.

[0080] The XRD pattern of the prepared material is as follows: Figure 1As shown, the test results indicate that the material in Example 1 is carbon material, and there are no characteristic peaks of iron-related compounds, indicating that iron exists in the form of single atoms. The SEM images show that the structures of Example 1, Example 2, and Comparative Example 1 are all carbon fibers uniformly covered with vertically grown carbon nanotube arrays. Figure 2 From TEM image ( Figure 3 It can be seen more clearly that, similar to Comparative Sample 1, Example 1 consists of hollow carbon nanotubes, and no iron-related compound particles are dispersed on the surface. In contrast, Example 2 shows multiple iron carbide nanoparticles randomly dispersed on the inner surface of the carbon layer and inside the shell of the hollow carbon nanotubes. High-angle annular dark-field scanning transmission electron microscopy with spherical aberration correction was used. Figure 4 The images show that the iron is atomically dispersed, indicating that individual iron atoms are stably loaded on the surface of the carbon nanotubes. Figure 5 It can be seen that the material in Example 1 has a typical type IV adsorption-desorption curve, indicating that mesopores and micropores coexist in the material.

[0081] The above examples and comparative examples were used to assemble aqueous zinc-iodine batteries to test their electrochemical performance. Figure 6 It can be seen that Example 1 has good rate capability. When the current density is 0.2A / g, its specific capacity can reach 210mAh / g, and when the current density is increased to 5A / g, its specific capacity is 148mAh / g. Figure 7 It can be seen that the material obtained in Example 1 exhibits excellent cycle stability, maintaining 88% capacity after 30,000 cycles at 3 A / g. Figure 6 , Figure 7 It can be seen that the rate capability and cycle performance of Example 1 are better than those of Example 2 and Comparative Example 1.

[0082] The material parameters and electrochemical properties of Examples 1-5 and Comparative Example 1 are shown in Table 1 below:

[0083] Table 1 Summary of material parameters and electrochemical performance of each embodiment and comparative example.

[0084]

[0085] As can be seen from Table 1, the present invention can control the catalyst morphology, single-atom configuration and content, and iodine loading in the cathode by controlling the raw material ratio and iodine content, and finally obtain a carbon-based iodine cathode host material suitable for aqueous zinc-iodine batteries. The aqueous zinc-iodine battery exhibits high specific capacity and good cycle stability.

[0086] The above description is merely a preferred embodiment of the present invention and is illustrative rather than restrictive. Those skilled in the art will understand that many changes, modifications, and even equivalents can be made within the spirit and scope defined by the claims of the present invention, all of which will fall within the protection scope of the present invention.

Claims

1. A method for preparing an iron single-atom modified carbon nanotube array, characterized in that, Includes the following steps: S1, improve the hydrophilicity of carbon cloth by oxidizing it through gas-phase heat treatment; S2, the carbon cloth obtained in step S1 is immersed in an aqueous solution of polyethylene glycol, urea, boric acid and acetylacetone iron and kept for 24 hours, and then heated in a vacuum drying oven at 80°C for 12 hours to remove the solvent. S3, the material obtained in step S2 is placed in a tube furnace and carbonized at high temperature in an inert gas atmosphere to obtain an iron single-atom supported carbon nanotube array with high catalytic activity. In step S3, the inert gas is argon or nitrogen, the gas flow rate is 450 mL / min, the heating rate is 2℃ / min, the high-temperature carbonization temperature is 800℃, and the time is 6 h.

2. The method for preparing an iron single-atom modified carbon nanotube array as described in claim 1, characterized in that, In step S1, the conditions for gas phase heat treatment are as follows: after calcining the carbon cloth in air at 450°C for 2 hours, the hydrophobic surface of the carbon cloth has been adjusted to a superhydrophilic surface.

3. The method for preparing an iron single-atom modified carbon nanotube array as described in claim 1, characterized in that, In step S2, the mass ratio of polyethylene glycol, urea, boric acid, and acetylacetone iron is 1:10:0~0.3:0.02~0.

2.

4. An iron-modified carbon nanotube array prepared by the preparation method according to any one of claims 1 to 3.

5. The application of the iron single-atom modified carbon nanotube array as described in claim 4 in an aqueous zinc-iodine battery.

6. The application of an iron-modified carbon nanotube array as described in claim 5 in an aqueous zinc-iodine battery, characterized in that, The preparation of the aqueous zinc-iodine battery includes the following steps: (1) Preparation of iodine-supported cathode material: Iodine solution was added dropwise to an iron single-atom modified carbon nanotube array and dried at 40℃ for 1 h. Then, it was transferred to a tube furnace and kept at 120℃ for 6 h under an argon atmosphere. After the temperature dropped to room temperature, it was cut into 1 cm × 1 cm square positive electrodes for later use. (2) Assembly of aqueous zinc-iodine batteries: The iodine self-supporting positive electrode, zinc foil negative electrode and glass fiber separator prepared in step (1) are assembled into an aqueous zinc-iodine battery. The electrolyte droplets added to both sides of the separator are 30 μL on the positive electrode side and 30 μL on the negative electrode side.

7. The application of an iron-modified carbon nanotube array as described in claim 6 in an aqueous zinc-iodine battery, characterized in that, In step (1), the amount of iodine solution added is 20-40 mL.

8. The application of an iron-modified carbon nanotube array as described in claim 6 in an aqueous zinc-iodine battery, characterized in that, In step (1), the amount of iodine solution added is 20 mL, and the surface loading of iodine is 3 mg / cm³. 2 .

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