Physical Properties and Theoretical Study of NiCo_3-O₄ (0 ≤ x ≤ 1.5) Nanostructures as High-Performance Electrode...

Study evaluates the electrochemical performance of off-stoichiometric NiCo_3-O₄ compounds. The off-stoichiometric samples were prepared via hydrothermal technique by systematically varying Co/Ni molar ratio. Physical and electrochemical properties of NiCo_3-O₄ were observed to be stoichiometry dependent. The increase in Ni/Co ratio in NiCo_3-O₄ leads to the morphological transformation from fibrous bundles to urchin like nanospheres with a concomitant increase in the surface area reaching up to 132 m2/g. The optimal specific capacitance of 225 F/g at a current density of 1 A/g and 524 F/g at 10 mV/s scan rate was observed of x 1.0 sample, with an increased retention capacity ∼120% measured at 2 A/g current density. The hybrid density functional theory (DFT) calculations of the electronic density of states identified Ni_1.0Co₂O₄ with optimal band-gap of 2.38 eV with an expectation of displaying higher electrocapacitive performance. Experimentally, Ni_0.92Co_2.08O₄ displayed superior electrocapacitive performance among all Ni/Co ratio in NiCo_3-O₄. The DFT study also predicted Ni preference to the octahedral site, which is in-line with the observed increase in ferromagnetic nature, decreased lattice parameter, and increased structural disorder with increasing Ni/Co ratio. The improved electrochemical performance of NiCo_3-O₄ (x > 0) is attributed to the mesoporous hierarchical structure, with a high electroactive surface which can effectively improve structural stability, and reduce the ionic and electron diffusion length. Compared to the pure Co₃O₄, the reduction of Co content in NiCo_3-O₄ is desired due to the high cost and toxicity of Co element.