Abstract: Electrochemical supercapacitors are regarded as a promising electrochemical energy storage device, because of its fast chargeability, long cycle life, and high power density. The connection of porous carbon and extensive electroactive substances have significantly improved their performance. However, the unsuitable constructing strategy and unclear electrochemical mechanism lead to a mismatch between the fast double-layer effect and the sluggish Faradaic behavior at the electrode, degrading the energy density and capacitance performances. Herein, the construction strategy based on the double-wall carbon foam, carbon nanotubes and copper nanocubes provides enhanced electronic structure and ion transport path, resulting in accelerated proton transport and redox insertion/deinsertion processes, as well as surprising electrochemical behaviors of Cu-Co oxide on both positive and negative electrodes (927.9 and 427.0 F g^-1 at 1 A g^-1). A flexible symmetric supercapacitor assembled with the SCF/CNT@Cu/Cu-Co-O delivers a extremely excellent performance. At 1 A g^-1, the specific capacitance is 337.8 F g^-1, the energy density hits 60 Wh kg^-1 at the power density of 17.8 kW kg^-1. At 10 A g^-1, the energy density is up to 120.1 Wh kg^-1, representing the top energy density of supercapacitors. Combining systematical structural optimization and mechanism study, this work broadens the fabricating strategy of hierarchical nanostructures and high-performance electrode materials for energy storage.

Key words: Carbon foam, Cu-Co oxides, Symmetric supercapacitors, Energy density