Physical Properties and Theoretical Study of NixCo3-xO4 ( 0≤ x ≤ 1.5) Nanostructures as High-Performance Electrode Materials for Supercapacitors

Study evaluates the electrochemical performance of off-stoichiometric NixCo3-xO4 compounds. The off-stoichiometric samples were prepared via hydrothermal technique by systematically varying Co/Ni molar ratio. Physical and electrochemical properties of NixCo3-xO4 were observed to be stoichiometry dependent. The increase in Ni/Co ratio in NixCo3-xO4 leads to the morphological transformation from fibrous bundles to urchin like nanospheres with a concomitant increase in the surface area reaching up to 132 m(2)/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 similar to 120% measured at 2 A/g current density. The hybrid density functional theory (DFT) calculations of the electronic density of states identified Ni(1.0)Co(2)O(4 )with optimal band-gap of 2.38 eV with an expectation of displaying higher electrocapacitive performance. Experimentally, Ni 0.92 Co 2.08 04 displayed superior electrocapacitive performance among all Ni/Co ratio in NixCo3-xO4. 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 NixCo3-xO4 (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 NixCo3-xO4 is desired due to the high cost and toxicity of Co element.