A metal–organic framework derived hierarchical nickel–cobalt sulfide nanosheet array on Ni foam with enhanced electrochemical performance for supercapacitors

Metal–organic frameworks (MOFs) have emerged as a new platform for the construction of various functional materials for energy related applications. Here, a facile MOF templating method is developed to fabricate a hierarchical nickel–cobalt sulfide nanosheet array on conductive Ni foam (Ni–Co–S/NF) as a binder-free electrode for supercapacitors. A uniform 2D Co-MOF nanowall array is first grown in situ on Ni foam in aqueous solution at room temperature, and then the Co-MOF nanowalls are converted into hierarchical Ni–Co–S nanoarchitectures via an etching and ion-exchange reaction with Ni(NO3)2, and a subsequent solvothermal sulfurization. Taking advantage of the compositional and structural merits of the hierarchical Ni–Co–S nanosheet array and conductive Ni foam, such as fast electron transportation, short ion diffusion path, abundant active sites and rich redox reactions, the obtained Ni–Co–S/NF electrode exhibits excellent electrochemical capacitive performance (1406.9 F g−1 at 0.5 A g−1, 53.9% retention at 10 A g−1 and 88.6% retention over 1000 cycles), which is superior to control CoS/NF. An asymmetric supercapacitor (ASC) assembled by using the as-fabricated Ni–Co–S/NF as the positive electrode and activated carbon (AC) as the negative electrode delivers a high energy density of 24.8 W h kg−1 at a high power density of 849.5 W kg−1. Even when the power density is as high as 8.5 kW kg−1, the ASC still exhibits a high energy density of 12.5 W h kg−1. This facile synthetic strategy can also be extended to fabricate other hierarchical integrated electrodes for high-efficiency electrochemical energy conversion and storage devices.