Graphite is divided into artificial graphite and natural graphite, the world’s proven reserves of natural graphite in about 2 billion tons.
Artificial graphite is obtained by the decomposition and heat treatment of carbon-containing materials under normal pressure. This transformation requires high enough temperature and energy as the driving force, and the disordered structure will be transformed into an ordered graphite crystal structure.
Graphitization is in the broadest sense of the carbonaceous material through above 2000 ℃ high temperature heat treatment carbon atoms rearrangement, however some carbon materials in the high temperature above 3000 ℃ graphitization, this kind of carbon materials was known as the “hard charcoal”, for easy graphitized carbon materials, the traditional graphitization method include high temperature and high pressure method, catalytic graphitization, chemical vapor deposition method, etc.
Graphitization is an effective means of high added value utilization of carbonaceous materials. After extensive and in-depth research by scholars, it is basically mature now. However, some unfavorable factors limit the application of traditional graphitization in industry, so it is an inevitable trend to explore new graphitization methods.
Molten salt electrolysis method since the 19th century was more than a century of development, its basic theory and new methods are constantly innovation and development, now is no longer limited to the traditional metallurgical industry, at the beginning of the 21st century, the metal in the molten salt system solid oxide electrolytic reduction preparation of elemental metals have become the focus in the more active,
Recently, a new method for preparing graphite materials by molten salt electrolysis has attracted much attention.
By means of cathodic polarization and electrodeposition, the two different forms of carbon raw materials are transformed into nano-graphite materials with high added value. Compared with the traditional graphitization technology, the new graphitization method has the advantages of lower graphitization temperature and controllable morphology.
This paper reviews the progress of graphitization by electrochemical method, introduces this new technology, analyzes its advantages and disadvantages, and prospects its future development trend.
First, molten salt electrolytic cathode polarization method
1.1 the raw material
At present, the main raw material of artificial graphite is needle coke and pitch coke of high graphitization degree, namely by the oil residue and coal tar as raw material to produce a high-quality carbon materials, with low porosity, low sulfur, low ash content and advantages of graphitization, after its preparation into graphite has good resistance to impact, high mechanical strength, low resistivity,
However, limited oil reserves and fluctuating oil prices have restricted its development, so seeking new raw materials has become an urgent problem to be solved.
Traditional graphitization methods have limitations, and different graphitization methods use different raw materials. For non-graphitized carbon, traditional methods can hardly graphitize it, while the electrochemical formula of molten salt electrolysis breaks through the limitation of raw materials, and is suitable for almost all traditional carbon materials.
Traditional carbon materials include carbon black, activated carbon, coal, etc., among which coal is the most promising one. The coal-based ink takes coal as the precursor and is prepared into graphite products at high temperature after pre-treatment.
Recently, this paper proposes a new electrochemical methods, such as Peng, by molten salt electrolysis is unlikely to graphitized carbon black into the high crystallinity of graphite, the electrolysis of graphite samples containing the petal shape graphite nanometer chips, has high specific surface area, when used for lithium battery cathode showed excellent electrochemical performance more than natural graphite.
Zhu et al. put the deashing treated low-quality coal into CaCl2 molten salt system for electrolysis at 950 ℃, and successfully transformed the low-quality coal into graphite with high crystallinity, which showed good rate performance and long cycle life when used as anode of lithium ion battery.
The experiment shows that it is feasible to convert different types of traditional carbon materials into graphite by means of molten salt electrolysis, which opens up a new way for future synthetic graphite.
1.2 the mechanism of
Molten salt electrolysis method uses carbon material as cathode and converts it into graphite with high crystallinity by means of cathodic polarization. At present, existing literature mentions the removal of oxygen and long-distance rearrangement of carbon atoms in the potential conversion process of cathodic polarization.
The presence of oxygen in carbon materials will hinder graphitization to some extent. In the traditional graphitization process, oxygen will be slowly removed when the temperature is higher than 1600K. However, it is extremely convenient to deoxidize through cathodic polarization.
Peng, etc in the experiments for the first time put forward the molten salt electrolysis cathodic polarization potential mechanism, namely the graphitization most the place to start is to be located in solid carbon microspheres/electrolyte interface, first carbon microsphere form around a basic same diameter graphite shell, and then never stable anhydrous carbon carbon atoms spread to more stable outer graphite flake, until completely graphitized,
The graphitization process is accompanied by the removal of oxygen, which is also confirmed by experiments.
Jin et al. also proved this point of view through experiments. After carbonization of glucose, graphitization (17% oxygen content) was carried out. After graphitization, the original solid carbon spheres (Fig. 1a and 1c) formed a porous shell composed of graphite nanosheets (Fig. 1b and 1d).
By electrolysis of carbon fibers (16% oxygen), the carbon fibers may be converted into graphite tubes after graphitization according to the conversion mechanism speculated in the literature
Believed that, the long distance movement is under cathodic polarization of carbon atoms the high crystal graphite to amorphous carbon rearrange must process, synthetic graphite unique petals shape nanostructures benefited from oxygen atoms from, but the specific how to influence graphite nanometer structure is not clear, such as oxygen from carbon skeleton after how at the cathode reaction, etc.,
At present, the research on the mechanism is still in the initial stage, and further research is needed.
1.3 Morphological characterization of synthetic graphite
SEM is used to observe the microscopic surface morphology of graphite, TEM is used to observe the structural morphology of less than 0.2 μm, XRD and Raman spectroscopy are the most commonly used means to characterize the microstructure of graphite, XRD is used to characterize the crystal information of graphite, and Raman spectroscopy is used to characterize the defects and order degree of graphite.
There are many pores in the graphite prepared by cathode polarization of molten salt electrolysis. For different raw materials, such as carbon black electrolysis, petal-like porous nanostructures are obtained. XRD and Raman spectrum analysis are carried out on the carbon black after electrolysis.
At 827 ℃, after being treated with 2.6V voltage for 1h, the Raman spectral image of carbon black is almost the same as that of commercial graphite. After the carbon black is treated with different temperatures, the sharp graphite characteristic peak (002) is measured. The diffraction peak (002) represents the degree of orientation of the aromatic carbon layer in graphite.
The sharper the carbon layer is, the more oriented it is.
Zhu used the purified inferior coal as the cathode in the experiment, and the microstructure of the graphitized product was transformed from granular to large graphite structure, and the tight graphite layer was also observed under the high rate transmission electron microscope.
In Raman spectra, with the change of experimental conditions, the ID/ Ig value also changed. When the electrolytic temperature was 950 ℃, the electrolytic time was 6h, and the electrolytic voltage was 2.6V, the lowest ID/ Ig value was 0.3, and the D peak was much lower than the G peak. At the same time, the appearance of 2D peak also represented the formation of highly ordered graphite structure.
The sharp (002) diffraction peak in the XRD image also confirms the successful conversion of inferior coal into graphite with high crystallinity.
In the graphitization process, the increase of temperature and voltage will play a promoting role, but too high voltage will reduce the yield of graphite, and too high temperature or too long graphitization time will lead to the waste of resources, so for different carbon materials, it is particularly important to explore the most appropriate electrolytic conditions, is also the focus and difficulty.
This petal-like flake nanostructure has excellent electrochemical properties. A large number of pores allow ions to be quickly inserted/deembedded, providing high-quality cathode materials for batteries, etc. Therefore, the electrochemical method graphitization is a very potential graphitization method.
Molten salt electrodeposition method
2.1 Electrodeposition of carbon dioxide
As the most important greenhouse gas, CO2 is also a non-toxic, harmless, cheap and easily available renewable resource. However, carbon in CO2 is in the highest oxidation state, so CO2 has high thermodynamic stability, which makes it difficult to reuse.
The earliest research on CO2 electrodeposition can be traced back to the 1960s. Ingram et al. successfully prepared carbon on gold electrode in the molten salt system of Li2CO3-Na2CO3-K2CO3.
Van et al. pointed out that the carbon powders obtained at different reduction potentials had different structures, including graphite, amorphous carbon and carbon nanofibers.
By molten salt to capture CO2 and preparation method of carbon material success, after a long period of research scholars have focused on carbon deposition formation mechanism and the effect of electrolysis conditions on the final product, which include electrolytic temperature, electrolytic voltage and the composition of molten salt and electrodes, etc., the preparation of high performance of graphite materials for electrodeposition of CO2 has laid a solid foundation.
By changing the electrolyte and using CaCl2-based molten salt system with higher CO2 capture efficiency, Hu et al. successfully prepared graphene with higher graphitization degree and carbon nanotubes and other nanographite structures by studying electrolytic conditions such as electrolysis temperature, electrode composition and molten salt composition.
Compared with carbonate system, CaCl2 has the advantages of cheap and easy to obtain, high conductivity, easy to dissolve in water, and higher solubility of oxygen ions, which provide theoretical conditions for the conversion of CO2 into graphite products with high added value.
2.2 Transformation Mechanism
The preparation of high value-added carbon materials by electrodeposition of CO2 from molten salt mainly includes CO2 capture and indirect reduction. The capture of CO2 is completed by free O2- in molten salt, as shown in Equation (1) :
CO2+O2-→CO3 2- (1)
At present, three indirect reduction reaction mechanisms have been proposed: one-step reaction, two-step reaction and metal reduction reaction mechanism.
The one-step reaction mechanism was first proposed by Ingram, as shown in Equation (2) :
CO3 2-+ 4E – →C+3O2- (2)
The two-step reaction mechanism was proposed by Borucka et al., as shown in Equation (3-4) :
CO3 2-+ 2E – →CO2 2-+O2- (3)
CO2 2-+ 2E – →C+2O2- (4)
The mechanism of metal reduction reaction was proposed by Deanhardt et al. They believed that metal ions were firstly reduced to metal in cathode, and then the metal was reduced to carbonate ions, as shown in Equation (5~6) :
M- + E – →M (5)
4 m + M2CO3 – > C + 3 m2o (6)
At present, the one-step reaction mechanism is generally accepted in the existing literature.
Yin et al. studied the Li-Na-K carbonate system with nickel as cathode, tin dioxide as anode and silver wire as reference electrode, and obtained the cyclic voltammetry test figure in Figure 2 (scanning rate of 100 mV/s) at nickel cathode, and found that there was only one reduction peak (at -2.0V) in the negative scanning.
Therefore, it can be concluded that only one reaction occurred during the reduction of carbonate.
Gao et al. obtained the same cyclic voltammetry in the same carbonate system.
Ge et al. used inert anode and tungsten cathode to capture CO2 in the LiCl-Li2CO3 system and obtained similar images, and only a reduction peak of carbon deposition appeared in the negative scanning.
In the alkaline metal molten salt system, alkali metals and CO will be generated while carbon is deposited by the cathode. However, because the thermodynamic conditions of carbon deposition reaction are lower at a lower temperature, only the reduction of carbonate to carbon can be detected in the experiment.
2.3 CO2 capture by molten salt to prepare graphite products
High-value-added graphite nanomaterials such as graphene and carbon nanotubes can be prepared by electrodeposition of CO2 from molten salt by controlling experimental conditions. Hu et al. used stainless steel as cathode in the CaCl2-NaCl-CaO molten salt system and electrolyzed for 4h under the condition of 2.6V constant voltage at different temperatures.
Thanks to the catalysis of iron and the explosive effect of CO between graphite layers, graphene was found on the surface of cathode. The preparation process of graphene is shown in Fig. 3.
The picture
Later studies added Li2SO4 on the basis of CaCl2-NaClCaO molten salt system, electrolysis temperature was 625 ℃, after 4h of electrolysis, at the same time in the cathodic deposition of carbon found graphene and carbon nanotubes, the study found that Li+ and SO4 2- to bring a positive effect on graphitization.
Sulfur is also successfully integrated into the carbon body, and ultra-thin graphite sheets and filamentous carbon can be obtained by controlling the electrolytic conditions.
Material such as electrolytic temperature of high and low for the formation of graphene is critical, when the temperature higher than 800 ℃ is easier to generate CO instead of carbon, almost no carbon deposition when higher than 950 ℃, so the temperature control is extremely important to produce graphene and carbon nanotubes, and restore the need carbon deposition reaction CO reaction synergy to ensure that the cathode to generate stable graphene.
These works provide a new method for the preparation of nano-graphite products by CO2, which is of great significance for the solution of greenhouse gases and preparation of graphene.
3. Summary and Outlook
With the rapid development of new energy industry, natural graphite has been unable to meet the current demand, and artificial graphite has better physical and chemical properties than natural graphite, so cheap, efficient and environmentally friendly graphitization is a long-term goal.
Electrochemical methods graphitization in solid and gaseous raw materials with the method of cathodic polarization and electrochemical deposition was successfully out of the graphite materials with high added value, compared with the traditional way of graphitization, the electrochemical method is of higher efficiency, lower energy consumption, green environmental protection, for small limited by selective materials at the same time, according to the different electrolysis conditions can be prepared at different morphology of graphite structure,
It provides an effective way for all kinds of amorphous carbon and greenhouse gases to be converted into valuable nano-structured graphite materials and has a good application prospect.
At present, this technology is in its infancy. There are few studies on graphitization by electrochemical method, and there are still many unknowable processes. Therefore, it is necessary to start from raw materials and conduct a comprehensive and systematic study on various amorphous carbons, and at the same time explore the thermodynamics and dynamics of graphite conversion in a deeper level.
These have far-reaching significance for the future development of graphite industry.
Post time: May-10-2021