Mechanism for nucleation and growth of electrochemical deposition of palladium(II) on a platinum electrode in hydrochloric acid solution

Palladium(II) and chloride ions tend to form complexes in aqueous solution. Both theoretical and experimental (by UV spectrum) results indicate that there are four complexes formed in aqueous solution containing 3 mol/L hydrochloric acid and 20 mmol/L PdCl2. This work evaluates the kinetics of electrochemical deposition of palladium on a Platinum electrode. For this purpose, palladium electrodeposition was investigated by means of cyclic voltammetry (CV), potentiostatic current-time transients (CTTs) and Tafel curve. By CTTs curves, the regions corresponding to the charge transfer control, mixed control and diffusion control were identified. In the diffusion control region, palladium electrodeposition mechanism was characterized as progressive nucleation with three-dimensional (3D) growth under diffusion control; as for the mixed control region, an adsorption (I (Ads)), ion transfer (I (IT)), and nucleation and growth (I (NG)) model were proposed to analyze the current-time transients quantitatively, which could separate the I (Ads), I (IT) and I (NG) perfectly.