Effect of Co2+ content on supercapacitance properties of hydrothermally synthesized Ni1-xCoxFe2O4 nanoparticles

Rich physical properties of ferrite nanoparticles, which exhibit a spinel structure, are sensitive to the distribution of cations in tetrahedral and octahedral sites as well as the crystal field effect, both of which can modify the electronic structure. Here, using a hydrothermal method, we incorporated Co+2 ions into nickel ferrite (NiFe2O4) nanoparticles to synthesize Ni1-xCoxFe2O4 (x = 0.0, 0.5, 1) nanoparticles and studied how their structural, electronic, and electrochemical properties are influenced by the Co+2 content. We found that the employed surfactant agent (here, CTAB) is critically important and affects the size and the specific surface area of the nanoparticles. X-ray diffractometry indicated that the obtained nanoparticles have an inverse spinel structure and field emission scanning electron microscopy showed that the nanoparticles' size is in the range 15-20 nm. Fourier-transform infrared spectroscopy and energy dispersive X-ray spectroscopy confirmed the chemical compositions of the samples. UV-visible spectrophotometry indicated that the optical bandgap of Ni1-xCoxFe2O4 decreases from 2.2 to 1.95 eV as the Co+2 content increases. Electrochemical properties of the samples were studied using cycling voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques. The specific capacitances achieved by the supercapacitors constructed from the prepared NiFe2O4, Ni0.5Co0.5Fe2O4, and CoFe2O4 nanoparticles were calculated from cycling voltammetry curves at the scan rate of 5 mV/s as 534.5, 630, and 991 F/g, respectively, and from galvanostatic charge-discharge curves at the current density of 0.25 A/g as 467.43, 522.80 and 655.60 F/g, respectively. We discussed that the increase in the specific capacitances as the Co+2 content increases is because the Co+2 content increases the available ways for the electrode to react with the electrolyte. Finally, we performed a density functional theory study on the structures to provide more insight into the increased specific capacitance.