N-doped carbon coated hollow NixCo9−xS8 urchins for a high performance supercapacitor†
Abstract
N-doped carbon coated NixCo9−xS8 (NixCo9−xS8@C) hollow urchins have been synthesized via a two-step solvothermal synthesis and an in situ polymerization in dopamine together with a post-annealing process. The characterization indicated that NixCo9−xS8@C hollow urchins have urchin-like morphology and a uniform size distribution. Furthermore, there is a complete phrase transformation from the as obtained NiCo2S4/NixCo9−xS8 hybrid to NixCo9−xS8 during the thermal annealing process. More importantly, as electrochemical materials, NixCo9−xS8@C has a high specific capacitance (1404.0 F g−1 at 2.0 A g−1) and excellent cycling performance (95.8% capacitance retention of the highest value after 2000 cycles). These results can be attributed to the coating of N-doped carbon, which gives the composite good conductivity. Additionally, the phase transformation from NiCo2S4/NixCo9−xS8 to NixCo9−xS8 during the thermal annealing greatly enhanced the redox reaction of the Co and Ni species.