Impurity Effects on Oxygen Reduction Electrocatalysis at Platinum Ultramicroelectrodes: A Critical Assessment

Impurity effects associated with an increase in the rate of mass transport induced by a decrease in the size of electrodes have been examined with emphasis on species capable of undergoing irreversible adsorption. Calculations show that for hemispherical ultramicroelectrodes 50 nm in radius, the flux of a solution-phase contaminant, such as a small organic molecule, present at ppb levels, is large enough to reach saturation coverage over times on the order of seconds. In the case of dioxygen reduction on Pt in aqueous electrolytes, such adsorbed species are believed to be responsible for marked changes in the mechanism, promoting a two-electron over a four-electrode reduction pathway. Such aspects must be considered in the analysis of results obtained with Pt ultramicroelectrodes, either as single particles or in dilute arrays. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3264089] All rights reserved.