Ultra‐Stable Asymmetric Supercapacitors Constructed by In‐Situ Electro‐Oxidation Activated Ni@CNTs Composites

Abstract Up to now, it is still an enormous challenge to build high‐performance asymmetric supercapacitors showing long cycling life and superior rate capability. In this work, unique three‐dimensional nickel deposited onto carbon nanotubes (Ni@CNTs) composites are synthesized by a reduction and catalytic growth process with ethanol as both, reductive agent and green carbon source. Asymmetric supercapacitors are assembled by only using the Ni@CNTs composites as positive and negative electrodes with our concept “one for two”, in rational combination with electro‐oxidation activation. Here, the three‐dimensional CNTs are responsible for the electric double layer capacitance and the encapsulated NiOOH nanoparticles derived from electro‐oxidation reaction contribute to pseudocapacitance. The asymmetric devices exhibit remarkable cycling stability with 100 % capacitance retention even up to 60,000 cycles at a current density of 10 mA cm −2 and excellent rate capability with 72.9 % capacitance retention with the current density increasing from 5 to 50 mA cm −2 . To better understand the electrochemical behavior, the relative contributions from electric double layer capacitance and pseudocapacitance of the positive electrode as well as the electrochemical kinetics of the devices are quantified.