Electrochemical anomalous codeposition of Co-Ni alloys from sulfamate electrolytes and its mechanism

The anomalous codeposition behavior of Co-Ni alloys from sulfamate electrolytes containing various Co²⁺/Ni²⁺ ratios was investigated by steady polarization curves and electrochemistry impedance spectroscopy (EIS) measurements. It was demonstrated that the electrochemical behavior of Co-Ni alloy deposition from sulfamate electrolytes was different significantly from that of from other electrolytes such as chloride or sulfate solutions. It is believed that the codeposition of CO-Ni alloys is inhibited by the adsorption of heteronuclear complexes on the electrode surface and the complexes are formed by the sulfamate anion as the bidentate ligand. From the standpoint of crystal-field theory, high-spin cobalt complexes with higher energy are less stable than that of nickel. Therefore the inhibition of sulfamate anion to Ni²⁺ is stronger than that to Co²⁺ and it cause the preferential reduction of Co²⁺ from the sulfamate solution. Moreover, according to the Nyquist plot, a electrical equivalent circuits for simulating the EIS of Co-Ni codeposition was composed and the reaction kinetic mechanism of the Co-Ni alloy electrochemical codeposition was studied.