Mechanism of Nanocrystalline Nickel Electrodeposition from Novel Citrate Bath

The electrochemical behavior of nanocrystalline nickel deposition in citrate bath was investigated by electrochemical impedance spectroscopy (EIS). The two-step discharge of Ni (II) cations was confirmed, and an intermediate absorbate Ni(OH)ads originated from the reversible first step. The analysis of the impedance of nickel deposition showed that apart from the capacitive loop at the high frequency due to the double layer, the inductive loop at low-frequency was observed in a citrate bath which is ascribed to the relaxation of the electrode coverage by Ni(OH)ads. The electrochemical behavior observed in the experiment processes and results could be accounted for by the reaction mechanism implying two successive transfer reactions in which Ni(OH)ads acts as an intermediate compound consumed at the electrode. By quantitative analysis and simulation of the results of the cathodic impedance, the impedance parameters of nickel deposition in a bath with or without sodium citrate were obtained. The presence of sodium citrate in the bath increases the charge transfer resistance of nickel deposition and reduces the capacitance of the electrochemical double-layer.