Solvent-free solid-state synthesis of NiFe₂O₄ nanoparticles: calcination temperature-dependent structure-property...

This work presents the synthesis of NiFe₂O₄ (NFO) nanoparticles by the solid-state reaction method for supercapacitor applications and investigates the effect of calcination temperature. A cubic inverse spinel phase of nickel ferrite is confirmed by X-ray diffraction (XRD), and Ostwald ripening causes crystallite size to increase with calcination temperature. Field-Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HRTEM) confirm particle agglomeration with an average particle size of approximately 27.60 nm. Infrared spectra collected through Fourier Transform Infrared Spectroscopy (FTIR) provided the evidence for the associated functional groups. Optical properties and absorption behavior are investigated using UV-vis spectroscopy. The oxidation states of Ni, Fe and O atoms on the surface of NFO are confirmed by X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray Spectroscopy (EDX). Vibrating Sample Magnetometry (VSM) magnetic studies reveal the ferromagnetic nature of NFO. Porosity and the specific surface area of the samples were characterized using Brunauer-Emmett-Teller (BET) measurements. The thermal degradation profile was studied via Thermogravimetric Analysis (TGA). Redox activity and ion transport are evaluated by electrochemical techniques such as Cyclic Voltammetry (CV), Galvanostatic Charge Discharge (GCD) and Electrochemical Impedance Spectroscopy (EIS). The specific capacitance of the optimized NFO sample calcined at 700 °C is 1525.54 F g^-1 at 1 A g^-1 in 6 M KOH. An asymmetric supercapacitor device fabricated using NFO 700 °C and activated carbon delivered a specific capacitance of 70.34 F g^-1 at 1 A g^-1 and 87.5% capacitance retention after 4500 cycles. The enhanced electrochemical performance demonstrates that NFO is a viable electrode material for emerging energy storage and supercapacitor applications.