**2.2 Separators and electrolyte**

Separators are filters through which ions migrate in and out, and they can reject connections between two electrodes. Resistance and stability are the key parameters for separators. Many types of commercial separators are available, which consist of fiber separators, high polymer separators, electrospinning separators, and biological separators. The poly(vinyl alcohol) (PVA), which is a high polymer separator, has shown high conductivity, biodegradability, and high alkaline stability. PVA-based gels in aqueous electrolytes have been researched for solid-state supercapacitor applications and fuel cells. However, PVA-based gels have some limitations in aqueous solutions, i.e. the voltage window should be under or equal to 1 V. Researchers have now focused on finding inorganic ceramic solid-state electrolytes. Polyacrylonitrile (PAN) is a type of electrospinning separator that has shown great shrinkage and porosity, with many applications. In addition, studies have been conducted on biological separators, such as egg shell membranes [38], fish bubbles [39], and other biological separators. Furthermore, separators should be stretchable, stable, and in a staining state in flexible and stretchable supercapacitors, which has shown to be a challenging property in separators. Electrolytes consist of a conductive liquid mixture containing an aqueous or organic solvent, based on active materials and ions that include OH<sup>−</sup> , H+ , Li+ , Na+ , K+ , SO4 2−, and Cl− [40]. The concentration of ions and the PH value have shown the impact of power intensity in supercapacitors, and the less volume ions performed better in the transfers with less impedance.
