Galvanic replacement of Ni thin film in Pt(IV) + Pd(II) solutions and evaluation of structure and electrocatalytic features of the synthesized nanostructures

Abstract

Pt, Pd and Pt–Pd nanostructures were grown by simple galvanic replacement of electrodeposited Ni thin film in [PtCl62−]x:[PdCl42−]5-x (x = 0–5 mM) solutions. The replacement processes were monitored by chronopotentiometry, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS). The effects of solution composition on growth mechanisms, morphology and electrocatalytic features of nanostructures were studied. The results indicated that the catalyst growth mechanisms changes from surface limited 3D-islands growth to high-surface-area dendritic growth by increasing the concentration of Pd(II) in the solution due to the predominance of surface active features of Pd. Cyclic voltammetry in H2SO4 solution with and without HCOOH demonstrated that the absorption of hydrogen in Pd lattice reduces considerably by introduction of low amounts of Pt in the nanostructures. This inhibiting effect of Pt on Pd beside the mutual ensemble effect of Pd on Pt, change the onset potential and Tafel constant of formic acid oxidation; i.e. variation of oxidation mechanisms. The hydrodynamic polarization measurements in O2 saturated H2SO4 solution revealed that the dendritic Pt–Pd nanostructures synthesized in 2.5 mM PtCl62−: 2.5 mM PdCl42− solution exhibits the highest activity for oxygen reduction reaction (ORR) due to the high-surface-area morphology as well as the ligand effect of Pd atoms on surface Pt atoms.