Novel NiCo2S4/graphene composites synthesized via a one-step in-situ hydrothermal route for energy storage

A uniform composite with NiCo2S4 nanoparticles anchored on graphene sheets is fabricated through an innovative one-step hydrothermal method. The XRD, Raman, XPS, SEM, HRTEM and EDS analyses are performed for characterizing the morphology and microstructure of the as-prepared NiCo2S4/graphene composite. It can be found that NiCo2S4 nanoparticles with a length of 20-50 nm were densely grown on the highly conductive graphene sheets, forming a 3D conductive network for rapid and efficient charge transport. The electrochemical performance is evaluated by CV, GCD and EIS, demonstrating that the NiCo2S4/graphene composite exhibits a very high specific capacitance (1040.6 F g−1) and greatly improved cycling stability (89.3% capacitance retention after 2000 cycles), which are due to the synergistic effect of graphene and NiCo2S4 nanoparticles. An asymmetric device has been assembled with NiCo2S4/graphene as positive electrode and graphene as negative electrode in PVA-KOH gel electrolyte solution. The as-prepared device demonstrates a energy density of 2.9 W h kg−1 with a power density of 872 W kg−1, indicating that the NiCo2S4/graphene composite can serve as a promising electrode material for asymmetric capacitor.