Ni-CeO₂ spherical nanostructures for magnetic and electrochemical supercapacitor applications

The synthesis of nanoparticles has great control over the structural and functional characteristics of materials. In this study, CeO₂ and Ni-CeO₂ spherical nanoparticles were prepared using a microwave-assisted method. The prepared nanoparticles were characterized via thermogravimetry, X-ray diffraction (XRD), Raman, FTIR, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM) and cyclic voltammetry (CV). The pure CeO₂ sample exhibited a flake-like morphology, whereas Ni-doped CeO₂ showed spherical morphology with uniform shapes. Spherical morphologies for the Ni-doped samples were further confirmed via TEM micrographs. Thermogravimetric analyses revealed that decomposition varies with Ni-doping in CeO₂. XRD revealed that the peak shifts towards lower angles for the Ni-doped samples. Furthermore, a diamagnetic to ferromagnetic transition was observed in Ni-doped CeO₂. The ferromagnetic property was attributed to the introduction of oxygen vacancies in the CeO₂ lattice upon doping with Ni, which were confirmed by Raman and XPS. The pseudo-capacitive properties of pure and Ni-doped CeO₂ samples were evaluated via cyclic voltammetry and galvanostatic charge-discharge studies, wherein 1 M KOH was used as the electrolyte. The specific capacitances were 235, 351, 382, 577 and 417 F g^-1 corresponding to the pure 1%, 3%, 5% and 7% of Ni doped samples at the current density of 2 A g^-1, respectively. The 5% Ni-doped sample showed an excellent cyclic stability and maintained 94% of its maximum specific capacitance after 1000 cycles.