Palladium Nanoparticles Supported on Highly Oriented Pyrolytic Graphite: Preparation, Reactivity and Stability

Palladium nanoparticles (Pd NPs) were deposited electrochemically on highly oriented pyrolytic graphite (HOPG) substrates by using a potentiostatic double-pulse technique. The particle densities were in the order of 10⁹cm^-2; the radius of the deposited Pd NPs (2-20nm) was proportional to the 1/3 power of the growth pulse duration (t_g^1/3). The open-circuit potential measured during potentiodynamic potential scans of hydrogen evolution/oxidation (HER/HOR) at Pd/HOPG electrodes (average radii of Pd NPs larger than 8nm) was more negative than at a bulk Pd electrode; this was caused by the different phases of palladium hydride (PdH_x) formed in the latter case. Pd/HOPG samples showed an increased specific current density with decreasing particle size in HER. The activity of Pd NPs for HER is primarily affected by the absorbed-hydrogen-atom content in the Pd lattice. The electrochemical activity of Pd NPs for the oxygen reduction reaction (ORR) in acidic media decreased for smaller particle sizes. The stability of the Pd NPs was significantly influenced by the pH of the electrolyte. Pd NPs had a higher dissolution rate in solution with lower pH. The degeneration mechanisms seen for Pd NPs on HOPG were the dissolution of metal atoms, detachment of particles, and particle agglomeration. Corrosion of the graphite substrate after the potential cycles was also observed in AFM images.