(681a) Towards an Understanding of Surface Electrochemical Reaction Kinetics

In its early development electrochemistry was concerned primarily with potential dependent processes occurring in bulk solution or in the so-called double layer region near the electrode surface. While the nature of an electrode (i.e., platinum vs. mercury) was recognized to be of importance, the actual state of the surface, so long as it was clean, was thought to be of secondary importance. Beginning in the 1960s more attention began to be paid to electrode surfaces. Attention increased steadily in the following years and accelerated greatly in the 1980s with the advent of surface sensitive techniques like potential and polarization modulated infrared spectroscopy, extended x-ray absorption fine structure, and ex-situ vacuum/electrochemical methods.

It is now known that an electrochemical response is extremely sensitive to the state of the surface, both in terms of structure and adsorbates. Indeed, electrochemical measurements such as cyclic voltammetry and other potential-current measurements can provide sensitive, quantitative measures of surface properties, so long as the response can be linked to a known condition. Nonetheless, our understanding of chemical reactions at electrochemical interfaces remains in its infancy. Developing accurate measurements of reaction kinetics that satisfy the demands of both electrochemist and surface scientist is a significant undertaking. An electrochemical reaction is actually a network of reactions both with and without charge transfer. There is, as yet, no widely established framework for interpreting surface electrochemical kinetic rate constants apart from current measurements.

This presentation will review work from the author's research group that addresses the need for a better understanding of surface electrochemical kinetics. Examples will be given in the following areas: (1) double layer modeling, in which prototypical double layers are constructed at well defined surfaces and studied by ultrahigh vacuum techniques; (2) ex-situ electrochemical studies, in which a charged electrochemical interface survives removal from the electrolyte for analysis in vacuum; (3) field induced surface chemistry, in which high surface electric fields of up to 100 MV/cm enable the study of water ionization and the basic process for generating electricity in fuel cells; (4) a Langmuir-Hinshelwood electrochemical surface reaction mechanism with kinetic rate constants that successfully explains the poisoning problem in direct methanol fuel cells; and (5) a surface reaction study of direct oxidation and internal reforming in the oxidation of methane and higher hydrocarbons on ceria-based anodes in solid oxide fuel cells.

The author gratefully acknowledges funding of this work by the Office of Naval Research, the National Science Foundation, and the Petroleum Research Fund of the American Chemical Society and the inspiration to pursue this line of research by Prof. Robert Madix.