N. Vasiljevic

Diffusion-Limited Current Density of Oxygen Reduction on Copper

There are many important processes in corrosion for which the diffusion-limited current density of oxygen reduction, i L , plays a dominant role in terms of kinetic control. The conventionally accepted value of i L (for the four-electron reduction mechanism) which can be found in many corrosion textbooks is in the range of 50-100 μA cm -2 , but the origins of this range of values are a bit mysterious. Previous research in our group aimed at ascertaining i L (under stagnant conditions) on a planar Cu electrode and a Cu microelectrode array in a naturally aerated 0.1 M Na 2 SO 4 electrolyte found that i L was in the range of 20-30 μA cm -2 . In situ scanning tunneling microscopy was used to characterize the Cu surface at relevant potentials. Rotating disk electrode studies were used to measure i L in a naturally aerated 0.1 M Na 2 SO 4 electrolyte as a function of pH for both Pt and Cu electrodes. By comparing results for Pt and Cu we conclude that oxygen reduction occurs on a Cu surface via the four-electron mechanism. The oxygen diffusion-limited current density was found to be independent of pH (in the range 1.5-14). Finally, we conclude that our previous determination of i L in a stagnant electrolyte reflects an accurate range of values of the oxygen diffusion-limited current density.

A Kinetic Model for Redox Replacement of UPD Layers

The redox replacement at open-circuit potential OCP of the underpotentially deposited UPD metal layer in the presence ofoxygen and/or more noble metal ions is discussed and an analytical model describing this process is proposed. The modelestablishes a functional relationship between time and potential during the replacement of the UPD layer featuring eitherLangmuir- or Frumkin-type adsorption behavior. A model validation is performed by fitting experimental OCP transients foroxidative Pb stripping from Cu 111 and Ag 111 substrates. The excellent agreement between theory and experiment enablesquantitative prediction and control of the replacement reaction kinetics in a wide variety of systems.© 2007 The Electrochemical Society. DOI: 10.1149/1.2735817 All rights reserved.Manuscript submitted February 7, 2007; revised manuscript received March 8, 2007. Available electronically May 9, 2007.

Oxidation of the Cu ( 100 ) Surface Induced by Local Alkalization

In this work we present an electrochemical and in situ scanning tunneling microscopy (STM) study of locally induced oxidation of the Cu(100) surface in naturally aerated 0.1 M NaClO 4 , pH≤ 4 acidic solutions. The oxygen reduction reaction induces a near-electrode alkalization by the steady generation of OH - , producing a decaying pH gradient from the surface toward the bulk of the solution. The pH gradient profile and the concomitant surface processes are dependent upon the bulk solution pH, polarization potential, and polarization time. The local alkalization occurring in the presence of dissolved oxygen in the solution influences substantially the surface properties. These are manifested by distinctive changes in electrochemical behavior and illustrated by in situ STM images in solutions with different bulk pH values. A semiquantitative estimate suggests almost 6 units of pH increase near the Cu(100) electrode in 0.1 M NaClO 4 solution with bulk pH 2 after 25 min of polarization at anodic potentials. The similarity between electrochemical and STM results obtained in naturally aerated solution with bulk pH 4 and those registered in alkaline solutions with pH 9 confirms the local alkalization effect.