Vital effect of sufficient vulcanization on the properties of Ni-Co-S/graphene composites for supercapacitor

Ni-Co-S/graphene-based composites have a broad prospect in the application of supercapacitors. Whether the vulcanization process is sufficiently performed seriously affect the performance of the composites. Here, a series of transition metal sulfide/graphene composites is synthesized through a two-step hydrothermal method by introducing different sulfur sources selected from organic (thiourea and thioacetamide) to inorganic (sodium sulfide and sodium thiosulfate). Both thioacetamide and sodium sulfide can vulcanize Ni-Co hydroxy-carbonate precursor to form the Ni-Co sulfide, but the other two cannot. And the Ni-Co-S/graphene composites synthesized by using sodium sulfide as sulfur source possess the best performance in asymmetric supercapacitor with an initial specific capacitance of 178.1 F g−1, and a capacitance retention of 89.47% after 10,000 cycles at 10 A g−1. The excellent stability is related to the ideal hollow structure with rich electrochemical active sites, and dispersed constituents of the Ni-Co-S on the surface of rGO.