The development of hydrogel electrolytes for electronic devices has garnered significant attention due to their remarkable flexibility, operation safety, and electrochemical stability, making them a highly competitive material for wearable and flexible electronic devices, such as flexible supercapacitors. However, hydrogel electrolytes also have certain drawbacks that limit their utility in flexible supercapacitors. Specifically, hydrogel electrolytes face two primary challenges: 1) insufficient cross-linking leading to unfavourable mechanical properties, and 2) delamination between electrode and electrolyte under deformation due to weak adhesion at the interface. Consequently, it is crucial to develop mechanically robust hydrogel electrolytes with promising adhesion to the electrodes for their widespread application in future flexible electronic devices. This thesis provides a design and synthesis of a self-repairable, adhesive, strong, and stretchable hydrogel electrolyte for supercapacitors. A highly stretchable and tough hydrogel was firstly synthesized by incorporating green nanomaterial, cellulose nanocrystal (CNC) as nano-reinforcement, and physical (hydrophobic, electrostatic, and hydrogen bonding) interactions to reinforce the hydrogel matrix. The synthesized hydrogel demonstrated outstanding mechanical performance with the best tensile stress of 1085 ± 14 kPa and elongation of 4106 ± 311%. Then the prepared hydrogels were loaded with 1 M KOH by soaking to make hydrogel electrolytes. The results of this work demonstrate that CNC-incorporated hydrogel electrolytes are promising and competitive materials for flexible supercapacitors. The best capacitance was obtained as 67.31 F/g at 0.05 A/g by using the hydrophobized CNC hydrogel with 6-hr soak-loading of KOH. And almost 100% capacitance retention was obtained at 0.1A/g after 2200 cycles. This thesis also provides a fundamental understanding of how the CNC and different interactions will affect the mechanical and electrochemical performances of the hydrogel as a supercapacitor with thoughtful evaluations in various aspects.