Strategically Tuned Ultrathin Nickel Phosphate Nanosheet Thin-Film Electrode as Cathode for High-Power Hybrid Supercapacitor Device

Improved capacitive performance of the electrochemical energy storage devices can be achieved by manipulating the morphology of electroactive electrode material. Therefore, in the present work, the size and shape of microsheets of nickel phosphate electrode material are manipulated by varying hydrolyzing agent (urea) concentration in the chemical bath deposition method. Formation of hydrous nickel phosphate (Ni3(PO4)2·8H2O) in thin-film form over stainless steel substrate is confirmed by structural analysis, and change in microstructure from microsheets to nanosheets with hydrolyzing agent variation is observed in field emission scanning electron microscopy (FE-SEM) analysis. Strategically tuned ultrathin nanosheets of nickel phosphate deliver a high specific capacity of 471 C g–1 (specific capacitance, 1177 F g–1) at a 0.5 mA cm–2 current density. Moreover, a hybrid asymmetric supercapacitor device made up of nickel phosphate and reduced graphene oxide as cathode and anode, respectively, exhibits a specific capacitance of ∼108 F g–1 at a 4 A g–1 current density. Also, the hybrid supercapacitor device exhibits a maximum energy density of 38.2 Wh kg–1 at a high power density of 3.2 kW kg–1 with excellent cyclic stability (99%) over 5000 cycles. The excellent cyclic stability and actual practical demonstration [lightning 12 white light-emitting diode (LEDs)] suggest that a morphologically tuned nickel phosphate thin film is the best cathode for hybrid energy storage devices.