New insights into the halide-related reactions on platinum surfaces : an electrochemical investigation

Abstract

The results of an electrochemical investigation of polycrystalline platinum are presented. The aim of the study was the elucidation of aspects of the electrochemistry of polycrystalline platinum in 0.5 M sulphuric acid solution and differing concentrations of halides (chloride, bromide and iodide). Conventional cyclic voltammetry (CV) (both single cycle and multicycle techniques) formed the cornerstone of the study, while a variety of supplementary experimental techniques were also employed, namely the electrochemical quartz crystal microbalance (EQCM), ICP-analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Both glassy carbon and platinum metal electrodes were employed. By studying eight linked potential peaks in the CVs it was ascertained that the reduction of [PtCl6]2− to [PtCl4]2− occurred between 0.15 and 0.03 V (SHE), followed by the incomplete reduction of [PtCl4]2− to Pt. Furthermore, the simultaneous adsorption and desorption reactions of (H2+) and (H3O+) could be identified in the CVs and correlated with published results. A notable observation is the occurrence, under certain experimental conditions, of an isopotential point in the CVs. The interplay between the reduction of [PtCl6]2−/[PtCl4]2− and the reduction/oxidation of hydrogen-containing species perfectly fit the H2+/H3O+ model, and supports the mechanism for the HER to proceed via the adsorbed molecular hydrogen ion (H2+)ads as intermediate. The formation of oxide films from oxygen species was viewed from a metal passivation perspective and led to a new model for the oxidation process which, inter alia suggests that the frequently reported “place exchange” process involve oxygen species entering the platinum subsurface lattice by occupation of tetrahedral and octahedral interstices, rendering them electrochemically inert, thereby explaining the phenomenon of hysteresis associated with the reduction of platinum anodic oxide films. By interrupting the positive-going CV potential scans for a specified time (100 s) at specific holding potentials, followed by the reduction cycle, the electrochemical reactions occurring at those potentials could be amplified, leading to a better understanding of the processes involved. Another innovation was to graphically represent Pt mass loss at different potentials together with mass gains as determined by EQCM. Valuable information on the adsorption/desorption and reactions of species at the different holding potentials was obtained, especially when halide ions were present. The influence of the halide ions (Cl–, Br– and I–) on the Pt oxidation was studied in electrolytes containing 6, 60 and 600 μM ions. Regarding hydrogen evolution a clear tendency was observed in going from Cl–, to Br–, to I–, in that hydrogen adsorption/desorption diminishes with two peak pairs being evident for Cl–, one peak pair for Br–, and no peaks for I–.