The Effect of pH on the Electrodeposition of Pd Clusters onto Highly Ordered Pyrolytic Graphite-A Kinetic and Morphological Study

In this study, we carried out an electrochemical investigation of the palladium electrodeposition process at pH 5 and 8, evaluating the kinetic parameters related to its nucleation and growth processes on a Highly Oriented Pyrolytic Graphite (HOPG) electrode from a plating bath containing 1 mM of Pd and 1 M NH4Cl. The voltammetric study allowed us to identify the potential values at which palladium can be electrodeposited, along with the adsorption and desorption processes of hydrogen absorbed on the deposited Pd. Analysis of the peak currents of the deposited Pd indicated diffusional control at both pH values. The evaluation of kinetic parameters, such as the number of active nucleation sites (N0), the nucleation rate (A), and the rate constant of the proton reduction process (kPR), was determined via potentiostatic studies, revealing their dependence on the applied potential to the electrode. The number of active nucleation sites predicted by the nucleation model correlated well with the number of nuclei observed via Scanning Electron Microscopy (SEM). SEM images revealed that at pH 5, the Pd clusters had an average diameter of 27 nm and a height of 39 nm, while at pH 8, the clusters had an average diameter of 12.8 nm and a height of 16.6 nm. At pH 5, homogeneous and dispersed Pd clusters were obtained, while at pH 8, agglomeration of Pd clusters was observed.