**4. 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.

#### **Figure 2.**

*Various steps involved in the formation of colloidal solution of graphene through electrochemical exfoliation process.*

**Figure 3.** *Electrochemical exfoliation of graphite for the formation of colloidal graphene.*
