LAUSR

We report a surfactant-free, facile, and scalable synthesis of porous nickel oxalate (NiOX) and its derived cabbage-like nickel oxide (NiO) nanostructures. These cabbage-like NiO nanostructures are derived from NiOX by simple open atmosphere annealing for a prolong time at different temperatures. Structural data revealed the NiOX as NiC 2 O 4 ·2H 2 O with defective layered carbon and NiO phase formation before and after annealing, respectively. Morphological changes have been observed using a scanning electron microscope, which revealed the effect of annealing in inducing the surface area, by surface modified phase formation. Further, surface area analysis corroborated the above finding confirming an increase in surface area after annealing. As a proof-of-concept application, these as-synthesized and annealed powders are used as electrode materials for super capacitor and electrochemically tested in a three-electrode configuration using KOH as electrolyte. It is estimated that the electrodes fabricated using as-synthesized NiOX, annealed at exhibit specific capacity (Cs) of 428 Cg −1 (@ 2 Ag − 1 ) in a potential range of − 0.3 to + 0.4 V following the electrical double layer charge storage mechanism. After annealing, the Cs value of this material increased by twofold having the highest estimated value of 888 Cg − 1 (0 to + 0.46 V) for 450 ° C in comparison to 835 and 547 Cg − 1 for 350 and 550 ° C annealed samples at 3 Ag − 1 , respectively, following pseudo-capacitive mechanism. Further, a symmetric device was fabricated using 450 ° C annealed sample which exhibit a maximum energy density of 26.1 Wh kg −1 and power density of 302.5 W kg −1 at a current density of 1 Ag −1 . Moreover, a systematic study on the facile synthesis of NiOX powder and its further modification to tune the structure, morphology, electrochemical charge storage capacity, and working potential window for super capacitor application has been demonstrated. Graphical abstract