Hydrothermal synthesis of spherical NiCO₂O₄ nanoparticles as a positive electrode for pseudocapacitor applications

Hydrothermal method was adapted to synthesis NiCo₂O₄ nanoparticles by varying nickel and cobalt precursor concentration as 1:1, 1:2, and 1:3 ratios. X-ray diffraction (XRD) results revealed the spinel NiCo₂O₄ structure belongs to \({\rm{Fd}}\overline {\rm{3}} {\rm{m}}\) space group system with face-centered cubic crystal structure. Raman characteristic peaks observed at 495 and 654 cm⁻¹ explored E_g and A_2g modes of spinel NiCo₂O₄ product. Photoluminescence (PL) results revealed the hole recombination of Ni²⁺/Co²⁺ ions from 3d-E_g and 3d-T_g electronic state of spinel NiCo₂O₄ material. The characteristic Fourier transform infrared spectroscopy (FTIR) metal–oxygen bands appeared at 658 and 558 cm⁻¹ revealed the spinel-type crystal structure. SEM image revealed the NiCo₂O₄ spherical nanoparticles formation with an average particle size of around 500 nm. The cyclic voltammetry studies revealed the estimated average specific capacitance value of NC3 (NiCo₂O₄ spherical nanoparticles) as 542 F g⁻¹ relatively higher than NC1 and NC2. The electro impendence spectroscopy results explored the small arc formation in high frequency range and very low charge transfer resistance (R _ct), which resulted high conductive active materials. The estimated specific capacitance for NC3 exhibited superior galvanstatic charging and discharging (GCD) characteristics with high specific capacitance of 294 F g⁻¹ at high current density of 1 A g⁻¹ and revealed that the obtained electrode is suitable for supercapacitor applications.

Hydrothermal synthesis using an excess of Co source leads to smaller and more uniform particle size. This particle size and the slightly larger crystallite size formed in the materials leads to the improved electrochemical performance of the particles.