Ionic Liquid‐Controlled Growth of NiCo2S4 3D Hierarchical Hollow Nanoarrow Arrays on Ni Foam for Superior Performance Binder Free Hybrid Supercapacitors

Abstract A significant development in the design of a NiCo 2 S 4 3D hierarchical hollow nanoarrow arrays (HNA)‐based supercapacitor binder free electrode assembled by 1D hollow nanoneedles and 2D nanosheets on a Ni foam collector through controlling ionic liquid 1‐octyl‐3‐methylimidazolium chloride ([OMIm]Cl) concentration is reported. The unique NiCo 2 S 4 ‐HNA electrode acquires high specific capacity (1297 C g −1 at 1 A g −1 , 2.59 C cm −2 at 2 mA cm −2 ), excellent rate capability (maintaining 73.0% at 20 A g −1 ), and long operational life (maintaining 92.4% after 10 000 cycles at 5 A g −1 ), which are superior to those for 1D hollow nanoneedle arrays (HNN) and 2D porous nanoflake arrays (PNF). The outstanding electrochemical performance is attributed to the novel 3D structure with large specific surface, hollow cores, high porosity as well as stable architecture. In addition, a hybrid supercapacitor applying 3D NiCo 2 S 4 ‐HNA as the positive electrode and active carbon as the negative electrode exhibits a high energy density of 42.5 Wh kg −1 at a power density of 2684.2 W kg −1 in an operating voltage of 1.6 V. Robust cycling stability is also expressed with 84.9% retention after repeating 10 000 cycles at 5 A g −1 , implying their great potential in superior‐performance supercapacitors.