**4. Summary and future research directions**

In summary, this chapter contributes comprehensive review of the development and modern trends concerning aqueous electrolyte for ESs. The effect of aqueous electrolyte properties including ionic conductivity, ion size, ESPW, etc. on the ES performance like specific capacitance, cyclic stability, energy density, and temperature performance have been reviewed in details. In particular, the effect of interaction among the electrodes and electrolytes has been discussed. This argument shows that the current explored ESs relying on neutral aqueous electrolytes provide numerous benefits such as low cost, environmentally friendly, and safe, over the other electrolyte systems. Despite of considerable accomplishments in this field, still, some difficulties exist such as lower energy density and operating voltage in aqueous electrolytes. To overcome these problems, several research directions can be considered in the future as follows:

