Controllable nitrogen-doped carbon layers coated on NiCoO2 as electrodes for high-performance hybrid supercapacitors

Controllable nitrogen-doped carbon layers coated on NiCoO2 (NiCoO2@N-C) as anode materials are successfully synthesised by facile hydrothermal method and post-annealing treatment. Benefiting from the synergistic merits of carbon and nitrogen, the as-synthesised NiCoO2@N-C delivered an excellent capacitance of 1000 F g-1 at 1 A g-1 and a high specific capacitance of 491 F g-1 at 50 A g-1. After 5000 cycles at 10 A g-1, the cyclic stability remains 91% of the initial capacitance. Moreover, the assembled asymmetric supercapacitor exhibits a high energy density of 45.9 Wh kg-1 at a power density of 375 W kg-1. The NiCoO2@N-C hybrid nanostructure was investigated by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) techniques. In situ X-ray absorption near edge structure (XANES) spectroscopy was used to track the changes of NiCoO2@N-C electrode under electrochemical working conditions and confirmed that Ni had more significant electrochemical activity than Co. These findings provide new insights into the role of heteroatom doping for high-performance supercapacitors in future applications.