**3. Aqueous electrolytes**

Usually, regarding the energy density study, the aqueous electrolytes really are a poor selection for technical supercapacitor products because of their small voltage window. This could be the reason for why nearly all the commercial supercapacitors use organic electrolyte rather than aqueous electrolytes. Though aqueous electrolytes possess an insufficient cell voltage, they were considerably employed in the literature from 1997 onward because of that aqueous electrolytes seem to be cheaper and can be quite easily taken care of in the laboratory without having a special circumstance. However, ionic liquid and organic electrolytes generally call for complex purification treatments under a rigidly controlled environment to stay clear of moisture. All of these features of aqueous electrolytes tremendously explain the design and assembling of supercapacitors. Generally, aqueous electrolytes exhibit extremely high conductivity (**Table 1**) which is at least one magnitude larger than that of organic or IL electrolytes.


**Table 1.** Electrolytic conductivity and operating voltage of various electrolytes at room temperature.

The high conductivity of the aqueous electrolyte is propitious for reducing equivalent series resistance (ESR) which leads to significantly high power density supercapacitors.

To evaluate the overall performance of aqueous electrolytes, some typical criteria should be taken into consideration such as the dimensions of hydrated and bare ions (**Table 2**), the flow of ions which alters the ionic conductivity, as well as the specific capacitance.

The aqueous electrolytes can be categorized into three groups such as alkaline, acid, and neutral solutions. The most commonly used aqueous electrolytes are KOH, H<sup>2</sup> SO4 , and Na<sup>2</sup> SO4 , respectively.
