An integrated pyrolysis and chemical vapor deposition using CaCrO4 as a precursor were applied to convert mixed plastics into a nanocomposite comprising CaCr2O4 and carbon nanoplatelets (CaCr2O4/CNPs). During pyrolysis, plastics were decomposed into oil and non-condensable gas. While reacting with the precursor, non-condensable pyrolysis gas containing hydrocarbons and H2 played a dual role: (1) provided carbon for nanoplatelets growth and (2) facilitated reduction of Cr(VI) to Cr(III). Cyclic voltammetry demonstrated the synergistic effect of CaCr2O4 and CNPs on the heterogeneous electron transfer rate. The nanocomposite showed good performance as an electrocatalyst for oxygen reduction reaction increasing current density and reducing overpotential to −0.27 V vs Ag/Ag/KCl (sat.) at 0.06 mA cm−2. Furthermore, the nanocomposite exhibited a specific electrochemical capacitance as high as 60 F g−1 at 2 A g−1 and 67% stability over 3000 cycles, demonstrating the capability of electrode for fast charge/discharge, long cycling life and high reversibility.