Formation of CoNi2S4 nanofibers with 3D hierarchical pompom-like structure for high-rate electrochemical capacitors

Constructing a 3D hierarchical structure of electrode materials is a fast-developing technology to meet the increasing demand of high-rate electrochemical capacitors. Here, hierarchical CoNi2S4 nanofibers with pompom-like morphology are successfully synthesized via a two-step anion exchange reaction. The obtained CoNi2S4 nanofibers interweave densely with each other from inner core to external surface, constructing a stable 3D nanostructure. The pompom-like CoNi2S4 nanofibers have a high specific surface area of 169.7 m2 g−1 and a uniform nanopore size distribution in the range of 1–10 nm. Benefiting from these structural features, the pompom-like CoNi2S4 nanofibers provide more effective charge transport during electrochemical reactions compared with other 3D hierarchical structured CoNi2S4, such as flower-like nanosheets and urchin-like nanoneedles prepared by conventional methods. When evaluated as electrode materials in electrochemical tests, CoNi2S4 nanofibers with pompom-like hierarchical structure deliver a high specific capacitance of 2078.9 F g−1 at 0.5 A g−1 and an excellent rate capability of 844.4 F g−1 at 50 A g−1. The assembled CoNi2S4 nanofibers//activated carbon asymmetric electrochemical capacitor exhibits a good energy density of 48.2 W h kg−1 at a power density of 850 W kg−1, and a good cycling stability with 92.6% of its capacitance retained after 1000 cycles, suggesting that the pompom-like CoNi2S4 nanofibers can be applied as promising electrode materials for high-rate electrochemical capacitors.