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

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.