Oxygen Reduction Kinetics on PtxNi100-x Thin Films Prepared by Pulsed Laser Deposition

Mixed Pt-Ni thin films were prepared by crossed beam pulsed laser deposition. The depositions were performed at high kinetic energy conditions in the presence of 0.1 Torr He. X-ray diffraction patterns and X-ray reflectometry measurements showed that all films are made of a single face-centered cubic phase compound whose lattice parameter and density vary linearly with the bulk Pt composition. Likewise, X-ray photoelectron spectroscopy revealed that the Pt surface composition of PtxNi100-x thin films closely follows the bulk Pt concentration. The electrochemical active surface area (EASA) was evaluated by estimating Q(Hupd), the hydrogen desorption charge in the potential region of ca. 0.05-0.35 V (where UPD stands for underpotential deposited). Q(Hupd) is almost constant for PtxNi100-x with x >= 60 but increases steadily for lower values of x. This is thought to reflect the fact that dissolution of Ni atoms occurs, leading to an increase in the EASA and Pt enrichment at the surface of the films, as determined by XPS measurements. A positive potential shift (60 mV) of the half-wave current potential for oxygen reduction is observed as the Pt content is reduced from x = 100 to x = 27. This effect is mostly related to an increase in the EASA as the intrinsic ORR activity of PtxNi100-x thin films, as determined from normalized kinetic current density values, is constant and does not vary with the bulk Pt content of the film. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428363] All rights reserved.