We report a successful design and synthesis method for developing a graphene/mesoporous carbon (G@PE40-MC700) electrode materials from upcycled waste polyethylene (PE) plastic combined with graphene oxide (GO) and flame retardant by low-temperature carbonization at 700 degrees C. The G@PE40-MC700 exhibits a high surface area (1175 m(2) g(-1)) and a considerable amount of mesopores (2.30 cm(3)g(-1)), thus improved electrochemical performance in both symmetric and hybrid supercapacitors with wide voltage windows. The hybrid supercapacitor assembled from G@PE40-MC700 as anode and LiMn2O4 as cathode operating at 2.0 V in 0.5 M Li2SO4 was investigated. The hybrid supercapacitor delivers an energy density of 47.8 Wh kg(-1) at a power density of 250 W kg(-1), as well as high cycling stability of 83.8 % after 5000 cycles. Furthermore, the much higher energy density of 63.3 Wh kg(-1) by using G@PE40-MC700 as electrode material was achieved in high-voltage (4.0 V) symmetric supercapacitors using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) as the electrolyte with enhanced cycling stability of 89.3% after 5000 cycles. The high capacitance and rate capability of G@PE40-MC700 can be attributed to the synergistic effect of graphene and the mesoporous carbon composites. Our work not only offers a sustainable approach to turn waste plastic into valuable carbon materials but also an opportunity for its applications in "gold capacitors." (C) 2019 Published by Elsevier Inc.