An electrochemical study of well-defined nafion coatedplatinum and platinum-bimetallic electrodes

In this investigation, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and rotating disc electrode (RDE) measurements of the oxygen reduction reaction (ORR) have been used to explore the complex three-phase Nafion-platinum-electrolyte interface. This interface is at the heart of the functioning membrane electrode assembly (MEA) of a fuel cell. CV was primarily used to analyse ultra-thin Nafion films, deposited (without contamination) onto various flat and stepped platinum and platinum bimetallic single crystal electrodes. For Pt{111}, XPS measurements were also used to determine Nafion surface layer thickness and to obtain surface chemical composition. CV results have shown that Nafion is a probe of adsorbed OH on platinum electrodes and for stepped surfaces, unusual structural sensitivity of Nafion-induced voltammetric peaks, ascribable to Nafion interactions with step sites, is observed as a function of average terrace width. Voltammetric results for palladium adlayers (up to two monolayers) adsorbed on Nafion coated Pt{111} and {100} in aqueous 0.1M HClO4, show the first layer palladium hydrogen underpotential deposition (HUPD) peak being much sharper and intense as compared to Nafion free surfaces. A similar phenomenon was found for platinum-palladium surface alloys in that Nafion adsorption would produce sharper, palladium HUPD peaks. This behaviour is ascribed to stronger specific adsorption of the Nafion sulphonate groups with palladium compared to platinum. It was interesting to note that for bismuth adlayers adsorbed onto Nafion coated Pt{111} and {100}, attenuation of HUPD features was identical whether or not Nafion was adsorbed but the Bi-OH redox features for Nafion coated surfaces exhibited marked differences, again ascribable to competitive adsorption of sulphonate and OH. Using RDE, it was found that the ORR for various Nafion coated Pt{hkl} electrodes was inhibited compared to Nafion free electrodes. The electrooxidation of formic acid on palladium modified, Nafion coated Pt{111}, in aqueous 0.1M HClO4, was found not to be affected by the presence of Nafion. However methanol electrooxidation was inhibited on palladium modified, Nafion coated Pt{111}. Finally a number of actual fuel cell electrocatalysts, provided by Johnson Matthey were characterised using CV. The {111} and {100} surface site densities were quantified using bismuth and germanium as surface probes. Comparisons between Nafion coated electrocatalysts and Nafion free electrocatalysts are also reported. It was found that only very marginal differences between the CV responses of both types of catalyst are recorded (in contrast to the single crystal data).