Recent Progress in the Electrochemical Exfoliation of Colloidal Graphene: A Review

Graphene is a wonder nanomaterial which is used in a wide variety of electronics applications because of its excellent electrical, optical, chemical and mechanical properties. For the efficient use of graphene in the preparation of modern electronics devices it is imperative to first prepare a colloidal solution of graphene. Although various techniques are being used for the synthesis of colloidal form of graphene, the synthesis of colloidal graphene via electrochemical exfoliation is time saving and easy, facile method which can be easily performed in the laboratory without any expensive and sophisticated equipment as required in other techniques. Through electrochemical exfoliation of colloidal graphene, high quality graphene can be obtained within short time. Further, after the electrochemical exfoliation of colloidal graphene, the colloidal solution is stable in the organic solvent for few weeks. The conducting electrodes prepared by this colloidal solution of graphene have wide application in the areas of flexible energy storage devices and sensors fabrication.


Introduction
Graphene is the wonder nanomaterial discovered in 2004, most widely investigated because of its excellent electrical, mechanical, optical, chemical properties [1,2]. The main properties are it is 97.7% transparent and is used for making transparent conducting electrodes. Its high carrier mobility (200,000 cm 2 v −1 s −1 ), Young's modulus of 1.0 TPa is another important properties of graphene. It is considered 200 times more conductive than copper and 100 times stronger than steel [3][4][5][6][7]. In addition to this it is very flexible in nature as it can be stretched to 20% of its original length. These exceptional properties of graphene are highly suitable for the fabrication of various modern electronics device applications such as energy storage devices and sensors etc. However, for the efficient use of graphene in these modern devices, the colloidal dispersion of graphene has to be prepared for using in solution phase. The colloidal solution of graphene has some advantages in comparison to the other forms of graphene to be utilized for the formation of various devices.
Nowadays the modern electronics devices are being fabricated using printable electronics process. For the fabrication of the electronics devices such as sensors, energy storage devices using printable electronics the graphene has to be available in the colloidal solution form so that it will be easy to fabricate these graphene based flexible electronics devices using spray coating, brush coating, screen printing techniques [8]. Therefore, it is absolutely necessary to produce the colloidal solution of graphene.

Limitations of graphene synthesis methods
Various methods have been used for the preparation of graphene each having its own limitations as compared to others. Some of the prominent methods are mechanical exfoliation, Hummer's method, liquid phase exfoliation, epitaxial growth, chemical vapor deposition etc. First, the mechanical exfoliation method by which the graphene was first exfoliated from solid graphite source is a very time consuming technique of producing graphene from graphite [1]. It is a hit and trial method in which the researcher is not sure whether the graphene exfoliated on the scotch tape is single layered or multi layered graphene sheet. So, it is not a controllable and leads to a lot of wastage of time. Similarly the hummer's method uses very harmful acids and is also very time consuming method. The quality of graphene obtained from this method is not of high quality as indicated from the TEM results of graphene sheets.
Chemical vapor deposition requires very expensive and sophisticated equipment, hence the synthesis cannot be performed in all the laboratories [9,10]. Liquid phase exfoliation is a technique where long hours of sonication process is required which is very time consuming process [11]. Further the long hours of sonication deteriorates the quality and size of graphene nanosheets in the dispersion. Therefore, among other methods of graphene synthesis it is found that the colloidal dispersion of graphene prepared by electrochemical exfoliation is time saving method, economical and easy to use in any laboratory without any sophisticated and expensive equipment. The relative advantages of electrochemical exfoliation method in comparison to others methods is shown in Table 1.

Colloidal graphene
A colloidal solution of graphene (Figure 1) has graphene nanoparticles evenly distributed throughout the solution. In the colloidal solution of graphene the graphene nanoparticles remain dispersed in the solution without settling to the bottom, for quite a long time. Further, The colloidal solution of graphene has very large surface area and exhibits high electrochemical behavior. Further, the advantage of using solution phase to form various graphene devices such as sensors, electrodes, energy storage devices makes it more prominent method as compared

Electrochemical exfoliation
The colloidal graphene has high specific surface area and does not exhibit aggregation. The colloidal graphene was produced by the intercalation of the sulfate ions in between the individual graphene nanosheets present in the graphite rod. This intercalation process separates the individual graphene nanosheets which accumulate in the electrolyte solution at the end of electrochemical exfoliation process to form the colloidal solution of graphene [14]. Various steps involved in the formation of colloidal solution of graphene through electrochemical exfoliation process are shown in Figure 2.
The quality of colloidal graphene produced by the electrochemical exfoliation depends upon the type of the aqueous electrolyte used. Therefore, to improve the quality of colloidal graphene various electrolytes have been studied by researchers [8]. Some of the electrolytes used to prepare colloidal solution of graphene are ammonium sulfate, phosphoric acid, potassium sulfate, sodium sulfate, sulfuric acid electrolytes. In addition of these aqueous electrolytes lithium sulfate has also been observed to produce colloidal solution of graphene via electrochemical route [16].
The electrochemical exfoliation of graphene is performed by using one graphite rod and one platinum wire immersed in any aqueous solution containing sulfate ions. A DC voltage source is used for the exfoliation process [8]. Usually, a DC voltage of 10 V is applied for 1 to 1.5 hour for the exfoliation of graphene to complete. After 1.5 hour, the colloidal solution of graphene is obtained in the aqueous electrolyte solution (Figure 3). When the exfoliation process is completed, the graphite rod has been completely converted into graphene colloids in the solution. Later, the colloidal solution of graphene is used for the preparation of the graphene based electrodes.

Recent works on electrochemical exfoliation of graphene
Due to the various advantages of the electrochemical exfoliation technique in comparison to others, it has been extensively utilized recently for the preparation of graphene nanosheets for various applications. Recently industrial scale synthesis of few-layer graphene nanosheets have been synthesized where the electrochemical experiment shows that rate of graphene exfoliation increases with the higher concentration of intercalates and accordingly the colloidal conductivity changes [17]. In comparison to graphene, graphene oxide is more environment friendly because of the presence of oxygen-containing functional groups. In Comparison to standard Hummers' method for synthesis of GO, electrochemical exfoliation of graphite is considered facile and green with better crystallinity and higher oxidation degree [18].  The role of electrolytes is very important in the quality of the graphene produced. The graphene synthesis mechanism in acidic (0.5 M H 2 SO 4 ), neutral (0.5 M Li 2 SO 4 ) is compared to non-destructive intercalation in organic electrolyte (1 M NaClO 4 in acetonitrile) [19]. Another important advantage of electrochemical exfoliation is that it can be utilized for the synthesis of graphene from electronic waste because it is a great threat to the environment due to difficulty in recycling, difficulty in conversion of waste to useful materials. So, researchers have reported a facile and fast production method of electrochemical exfoliation of graphene from graphite of used Zn-C batteries [20]. The quality of graphene produced by the electrochemical exfoliation can be enhanced by taking mixtures of various electrolytes with pencil rods as electrodes. For the synthesis of graphene sheets, different mixtures of H 2 SO 4 and HNO 3 solution were investigated with different volume ratios of H 2 SO 4 : HNO 3 , with maximum oxidation was achieved at 1:1 ratio of both mixtures [21]. In another study, electrochemical exfoliation method was used for the preparation of high-quality water-dispersible graphene using molybdate aqueous solutions as the electrolyte [22]. Further, graphene is very effective against the cancer cells for treatment of cancer patients. Some researchers have demonstrated the cytotoxic effect of graphene with high content of nitrogen on colon cancer cells and antioxidant and protective properties on human endothelial cells [23]. Further, the researchers have investigated the role of Transition Metal Salts During Electrochemical Exfoliation of Graphite for Energy Storage Applications [24]. Recently, colloidal graphene produced by the electrochemical exfoliation of graphite in potassium sulphate and sulfuric acid electrolytes has been successfully utilized for the supercapacitor applications [25,26]. Table 2 shows various electrolytes used recently for the electrochemical exfoliation of graphene. Colloids -Types, Preparation and Applications 6

Application of colloidal graphene
The colloidal dispersion of graphene produced by the electrochemical exfoliation method is ideal for the preparation of the graphene based flexible paper electrodes. For this, colloidal graphene is coated using brush coating method on A4 paper and dried. These graphene based flexible paper electrodes are used for the preparation of the flexible energy storage devices [8]. First a gel electrolyte is prepared by using PVA (polyvinyl alcohol) and water [34], then two graphene based paper electrodes are joined using this gel electrolyte in between electrodes. Later, two aluminum foils are joined to form the contacts with these electrodes for taking various measurements of these flexible energy storage devices (Figure 4). The actual fabricated flexible energy storage device using colloidal graphene is shown in Figure 5.
The performance of the flexible energy storage devices is measured by the CV Curves [8]. Further, the type of the gel electrolyte changes the shape of the CV curve measured and hence the performance of the energy storage device.

Conclusion
This review article discusses electrochemical synthesis of colloidal graphene. Colloidal form of graphene is better than other forms of graphene available because of its solution route is easy to be used for the formation of graphene based electrodes. It is observed that electrochemical exfoliation of graphite is the simple, time saving and economical method for the production of colloidal form of graphene. Various aqueous electrolytes can be used for the electrochemical exfoliation of graphene from graphite. After the colloidal solution is obtained it is used for the preparation of the flexible paper electrodes which are suitable for the formation of flexible energy storage devices.

Conflict of interests
The authors declare no conflict of interests. © 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.