Cathodic process of nickel electrodeposition from ammonia–ammonium chloride solutions

Mechanism of cathodic process on nickel electrodeposition from ammonia–ammonium chloride solutions was studied by Tafel polarization curves, linear sweep voltammetry (LSV), cyclic voltammograms (CV) and chronopotentiometry (CE) technologies. In the electrolyte with 1.0 mol·L −1 NiCl 2 ·6H 2 O, 4.0 mol·L −1 NH 3 ·H 2 O and 2.5 mol·L −1 NH 4 Cl, nickel ion mainly complexed with ammonia and almost no free nickel ion could be found. It is confirmed that the dominant form of nickel ammonia complex ion in the system is Ni(NH 3 ) 4 2+ by measuring the equilibrium potential of the electrode at various concentrations of ammonia. The species of Ni(NH 3 ) 4 2+ turns into Ni(NH 3 ) 2 2+ during pre-chemical reaction step before discharging reduction reaction. Then, Ni(NH 3 ) 2 2+ discharges at the cathodic interface to form metal nickel deposit directly. Nucleation process is involved during nickel electrodeposition according to CV results. The reduction reaction of Ni(NH 3 ) 2 2+ is irreversible which is controlled by diffusion step. Other processes are also accompanied during the electrodeposition process, except for the diffusion-controlled process.