Electrochemical nucleation of palladium on graphene: A kinetic study with an emphasis on hydrogen co-reduction

Abstract

The nucleation mechanisms of palladium nanoparticles from 1mM [PdCl4]2−+1M NH4Cl electrolyte (pH=3.4) on graphene surface were studied using cyclic voltammetry, chronoamperometry and field emission scanning electron microscopy (FESEM). It was found that due to high catalytic properties of Pd nanoparticles/graphene surface, the hydrogen co-reduction during Pd electrodeposition occurs. The kinetics of electrochemical nucleation and growth of Pd nanoparticles was also evaluated using Mirkin–Nilov–Heerman–Tarallo (MNHT) model. Regarding the simultaneous reduction of hydrogen ion on the surface of the growing Pd nuclei, a modified model developed by Palomar-Pardavé et al. [M. Palomar-Pardavé, B.R. Scharifker, E.M. Arce, M. Romero-Romo, Electrochimica Acta 50 (2005) 4736] was employed to estimate the kinetic parameters of the process. It was shown that the steady state nucleation rate (Ist) and the rate constant of hydrogen reduction (kH) increase exponentially with a potential shift to more cathodic values. Using the relation between Gibbs free energy of formation for a critical nucleus (ΔGc) and Ist, it is concluded that there is no thermodynamic barrier against nucleation of Pd on graphene in the electrode potential range studied in this work (E=−0.3 to −0.5V vs. Ag/AgCl).