A flexible energy storage device with a redox-active polymer hydrogel electrolyte is provided. The flexible energy storage device can include a pair of electrodes separated by the redox-active polymer hydrogel electrolyte. The redox-active polymer hydrogel electrolyte can include a polymer hydrogel, charge balancing anions and redox-active transition metal cations at least one selected from the group consisting of vanadium, chromium, manganese, cobalt, and copper. The flexible energy storage device may retain greater than 75% of an unbent specific capacitance when bent at an angle of 10° to 170°.