A poly(vinylphosphonic acid) (PVPA)-(NH4)2MoO4), gel polymer electrolyte can be prepared by incorporating redox-mediated Mo, or similar metal, into a PVPA, or similar polymer, matrix. Gel polymer electrolytes including PVPA/MoX, x representing the percent fraction Mo in PVPA, can be used to make supercapacitors including active carbon electrodes. The electrolytes can be in gel form, bendable and stretchable in a device. Devices including this gel electrolyte can have a specific capacitance (Cs) of 1276 F/g, i.e., a more than 50-fold increase relative to a PVPA system without Mo. A PVPA/Mo10 supercapacitor can have an energy density of 180.2 Wh/kg at power density of 500 W/kg, and devices with this hydrogel structure may maintain 85+% of their initial capacitance performance after 2300 charge-discharge cycles.