Composition-Controlled of High Electrocatalytic Activity of CoNi Thin Films in Ethanol Electrooxidation Reaction

Abstract This study aims to identify the optimal Co–Ni composition for ethanol electrooxidation reaction by investigating the structural, morphological, and electrochemical properties of electrodeposited CoNi electrocatalyst. CoNi electrocatalysts with varying cobalt-to-nickel ratios were synthesized via electrodeposition and characterized using field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. Their electrocatalytic performance was evaluated using cyclic voltammetry and electrochemical impedance spectroscopy. field-emission scanning electron microscopy observations revealed a clear morphological evolution from spherical particles to a compact mixed spherical–pebbly structure with increasing cobalt content. X-ray diffraction analysis indicated a phase transition from a face-centered cubic structure to a dual-phase hexagonal close-packed/face-centered cubic structure, while Raman spectroscopy confirmed the formation of CoNi phases. Among the investigated compositions, the Co-rich catalyst exhibited the highest electrocatalytic activity toward ethanol oxidation, delivering a peak current density of 12.72 mA cm−2, the lowest onset potential, a low charge-transfer resistance of 5.70 Ω, and a reduced Tafel slope of 146 mV dec−1. The superior performance is attributed to the higher cobalt content, smaller crystallite size, increased defect density, and enhanced lattice strain, which collectively improve the density of active sites, optimize the electronic structure, and accelerate reaction kinetics. These findings demonstrate that precise control of CoNi composition is an effective strategy for developing efficient and stable non-noble metal electrocatalysts for ethanol electrooxidation and fuel cell applications.