CoNi2S4 Nanosheet Arrays Supported on Nickel Foams with Ultrahigh Capacitance for Aqueous Asymmetric Supercapacitor Applications

We report that CoNi2S4 nanosheet arrays exhibit ultrahigh specific capacitance of 2906 F g(-1) and areal capacitance of 6.39 F cm(-2) at a current density of 5 mA cm(-2), as well as good rate capability and cycling stability, and superior electrochemical performances with an energy density of 33.9 Wh kg(-1) at a power density of 409 W kg(-1) have been achieved in an assembled aqueous asymmetric supercapacitor. The CoNi2S4 nanosheet arrays were in situ grown on nickel foams by a facile two-step hydrothermal method. The formation mechanism of the CoNi2S4 nanosheet arrays was based on an anion-exchange reaction involving the pseudo Kirkendall effect. The two aqueous asymmetric supercapacitors in series using the CoNi2S4 nanosheet arrays as the positive electrodes can power four 3-mm-diameter red-light-emitting diodes. The outstanding supercapacitive performance of CoNi2S4 nanosheet arrays can be attributed to ravine-like nanosheet architectures with good mechanical and electrical contact, low crystallinity and good wettability without an annealing process, rich redox reactions, as well as high conductivity and transport rate for both electrolyte ions and electrons. Our results demonstrate that CoNi2S4 nanosheet arrays are promising electrode materials for supercapacitor applications.