Facile Synthesis\nof a Pyrovanadate Ni<sub>2</sub>V<sub>2</sub>O<sub>7</sub> Hollow\nSphere/Reduced Graphene Oxide Nanocomposite\nas a Solid-State Hybrid Supercapacitor

Nickel pyrovanadate (NVO) and compositing rGO in different\nconcentrations\nwith NVO are synthesized via the solvothermal process. XRD patterns\nreveal the formation of crystalline NVO and amorphous rGO in the nanocomposite.\nThe morphology of the material resembles the formation of an NVO hollow\nnanosphere through a template-free synthesis route with the effect\nof ethylene glycol. From the CV oxidation and reduction curve, the\nbattery-type faradic reaction is observed. The specific surface area\nincrement via the rGO concentration increment in nanocomposites is\ndue to the partially encapsulated hollow sphere on the 2D active surface\narea of rGO owing to better specific capacitance and electrochemical\nstability. In addition, the maximum specific capacitance of 3807 F\ng<sup>–1</sup> at 1 A g<sup>–1</sup> for NVO@rGO 20\nis obtained via a three-electrode system. The solid-state device shows\nthe specific capacitance retention of ∼70% even after 10,000\ncycles for a scan rate of 10 A g<sup>–1</sup>. The liquid electrolyte\ndevice shows the specific capacitance retention of ∼90% from\nits initial value and the successive charge–discharge process\nseen over the 10,000 cycles for the scan rate 10 A g<sup>–1</sup>. The suitable device is identified from this work in terms of high\nstability, high specific capacitance, and excellent reversibility\nfor electrochemical performance.