Co9S8 nanotube arrays supported on nickel foam for high-performance supercapacitors.

Uniform Co9S8 nanotube arrays on conductive nickel foam were successfully synthesized through a facile two-step hydrothermal method and were directly applied as the electrode for high-performance electrochemical capacitors. The formation of the tubular structure of Co9S8 can be attributed to the nanoscale Kirkendall effect. SEM and TEM images show that most of the Co9S8 nanotubes have hexagonal sections. The diameter of the Co9S8 nanotubes is about 120–200 nm and the wall thickness is of 40–60 nm. Cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance measurements are applied to investigate the electrochemical performance of the Co9S8 nanotubes. The specific capacitance of the Co9S8 nanotubes is 1775 F g−1 at 4 A g−1, and 1483 F g−1 at 24 A g−1, indicating the high rate capability. Also, the Co9S8 nanotubes exhibit stable cycling performance.