Tuning structural and microstrain features in Ni–Co substituted spinel ferrite nanoparticles for high-rate and stable pseudocapacitive energy storage

This study investigates Ni²⁺/Co²⁺ substitution effects on microstructure of spinel ferrite [Ni_1-yCo_yFe₂O₄ (0 ≤ y ≤ 1)] composite nanoparticles, prepared by sol-gel driven hydrolysis. Electron microscopy studies revealed the formations of uniformly distributed nanostructures with agglomerated and strongly interconnected tetragonal grains. X-ray diffraction (XRD) study confirmed the formation of single-phase cubic spinel structured nanocrystals in all compositions, with inconsistent variations of the lattice parameter and crystallite size as the Ni²⁺/Co²⁺ ratio changes. The Raman spectra revealed additional vibrational modes which confirmed the localized symmetry distortions, that could have occurred as a result of ionic radius mismatch of Ni²⁺/Co²⁺/Fe³⁺ cations at the octahedral sites. The vibrational stretching of metal-oxygen bond within the tetrahedral and octahedral sites were validated from infrared (FTIR) study. Charge storage studies also indicates that although, substituting Ni²⁺ with Co²⁺ in ferrite compounds can enhance electron hopping by occupying the octahedra B-sites, however, excessive Co content can create severe structural distortion by increasing the interplanar d-spacing, which reduces active sites and lowers electrical conductivity response. Thus, with its high specific capacity/capacitance (70.28 mAh g⁻¹/632.4 Fg^-1) at 0.5A/g, outstanding rate capability, exceptional cycling stability, and low charge transfer resistance, Ni_0.5Co_0.5Fe₂O₄ showed superior electrochemical performance among the Ni_1-yCo_yFe₂O₄ series, making it a suitable material for advanced energy storage applications.