Trisodium citrate assisted synthesis of hierarchical NiO nanospheres with improved supercapacitor performance

Abstract Hierarchical NiO nanospheres composed of porous nanosheets are prepared by a facile trisodium citrate assisted precipitation route followed by a calcination process. Effects of the trisodium citrate on the microstructure and electrochemical performances of NiO nanospheres are systematically investigated. The XRD, SEM, TEM, BET, and TG analyses show that the key point of the successful realization is that the citrate positioned in the precursor α-Ni(OH) 2 layer, which can prevent the restacking of α-Ni(OH) 2 sheets, yielding better crystallinity, high surface area (182 m 2  g −1 ) as well as pore volume (0.15 cm 3  g −1 ) and hierarchical porous ball-like morphology of NiO nanospheres by the calcination of the precursor. Electrochemical results show that the hierarchically porous NiO obtained with trisodium citrate assisted route exhibits high rate charge–discharge performance (463 F g −1 at 4.5 A g −1 ), longer cyclic stability (95% capacitance remained after 1000 charge–discharge cycles at 0.5 A g −1 ) as compared to the NiO prepared in the absence of sodium citrate (182 F g −1 at 4.5 A g −1 ; 70% capacitance retention after 1000 charge–discharge cycles at 0.5 A g −1 ). Further, due to facile mass transfer in the perfectly porous nanosheet, the citrate-assisted NiO show lower equivalent series resistance as revealed from the impedance studies.