Multi-scale biomass-based carbon microtubes decorated with Ni-Co sulphides nanoparticles for supercapacitors with high rate performance

Micron-grade carbon microtubes (CMTs) decorated with layered Ni-Co sulphides of several hundred nanometers and accompanied by Ni-Co sulphides nanoparticles composite structures have been fabricated by facile hydrothermal synthesis method, using willow catkins derived CMTs as carbon scaffolds and ethylene glycol mixture solution containing nickel acetate, cobalt acetate and thiourea as precursors for Ni-Co sulphides. The controllable Ni/Co molar ratios of Ni-Co sulphides can be altered by adjusting the ratio of nickel acetate and cobalt acetate in mixture solution. The electrochemical performance of CMTs/Ni-Co sulphides can be greatly affected by CMTs conductivity, Ni-Co sulphides deposition amounts and crystalline structures. Nevertheless, the hollow tubular structure of CMTs/Ni-Co sulphides composites can provide fast transport channels for ions diffusion while the great conductivity of CMTs scaffolds may facilitate the access of electrons transportation. Owing to this unique structure, the CMTs-1000/Ni2CoS4 composite exhibits an optimal pseudocapacitance (1210 F g−1 at 0.5 A g−1), good rate capability of 83.8% (1014 F g−1 at 10 A g−1) and good cycling durability of 95% retention of the initial value over 3000 charge-discharge cycles, well above the value of pure Ni2CoS4 that is merely 66.1% (1210 F g−1 at 0.5 A g−1 and 800 F g−1 at 10 A g−1) with inferior cycling durability of 78%. Besides, a maximum energy density of 28.1 Wh kg−1 for the CMTs-1000/Ni2CoS4//AC asymmetric supercapacitor (ASC) device can be achieved at power density of 753 W kg−1, and the energy density can still maintain 17.7 Wh kg−1 even at high power density of 17.2 kW kg−1.