Nickel oxide nanoparticles supported onto oriented multi-walled carbon nanotube as electrodes for electrochemical capacitors

We report an electrode material for supercapacitors composed of nickel oxide (NiO) nanoparticles supported onto radially oriented multi-walled carbon nanotubes (CNTs) using a stainless-steel fine-mesh as the support (AISI: CNT-NiO). CNT scaffolds showed a turbostratic multi-walled structure with an interplanar spacing of 0.32 +/- 0.02 nm and a diameter of similar to 20-100 nm. NiO nanoparticles exhibited a diameter of similar to 2-7 nm. X-ray data confirmed the presence of NiO in the scaffold. A large pseudocapacitive voltage range of 2.0 V was obtained in a 1.0 M Li2SO4 aqueous solution. The main contribution to the overall pseudocapacitance is due to the presence of reversible solid-state surface Faradaic reactions involving the Ni(II)/Ni(III) redox couple. High specific capacitance values of similar to 1200 F g(-1) at 5 A g(-1) for the AISI: CNT-NiO electrode were extracted from galvanostatic discharge curves. Considering the contribution of negative voltages, the specific power and energy determined using cyclic voltammetry exhibited values of similar to 140 Wh kg(-1) and similar to 9 W kg(-1), respectively, at 0.02 V s(-1). A specific capacitance of similar to 1028 F g(-1) was obtained at this scan rate. Even after 40,000 cycles carried out under galvanostatic conditions, the symmetric coin cell remained stable with a very high coulombic efficiency of similar to 99%, which is a remarkable result. Also, we attributed to carbon nanotubes an extraordinary stability as electron drain on the current collector. The morphology factor analysis revealed that 19% of the electrochemically active surface area is confined to the inner surface regions of the porous nanostructured active layer. A low value of 0.15 m Omega g was extracted for the equivalent series resistance. New insights are presented concerning the true meaning of negative voltages for coin cells. Interesting findings regarding the porous nature of electrodes were elucidated using the impedance technique. (C) 2018 Published by Elsevier Ltd.