Ni-driven modulation of magneto-electrochemical properties in CoFeCrO₄ spinels: integrated experimental and DFT...

In this work, Co_(1-x)Ni_xFeCrO₄ (x = 0-1) spinel ferrites were synthesized via a solution combustion method and systematically investigated to understand the effect of Ni²⁺ substitution on their structural, magnetic, electronic, and electrochemical properties. X-ray diffraction confirmed the formation of a single-phase cubic spinel structure, with progressive lattice contraction induced by the smaller ionic radius of Ni²⁺, leading to modified superexchange interactions and lattice strain. Optical and spectroscopic analyses revealed band gap modulation governed by enhanced 3d-3d electronic interactions and p-d hybridization. Magnetic measurements indicated tunable ferrimagnetic behavior arising from cation redistribution and altered A-B site coupling. Electrochemical studies demonstrated improved hydrogen evolution reaction (HER) activity with increasing Ni content, with the x = 0.75 composition exhibiting optimal performance due to a synergistic balance between electronic conductivity, active site availability, and charge transfer kinetics. The enhanced catalytic activity is attributed to Ni-induced electronic structure modulation, facilitating efficient electron transport and adsorption of reaction intermediates. Density functional theory (DFT) calculations further support these findings by revealing charge density redistribution and band structure evolution upon Ni substitution. This study establishes a clear structure-property-performance relationship and highlights the effectiveness of cation engineering in designing multifunctional spinel ferrites for energy conversion applications.