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Materials

Electrochemical Insights Gained

Method elucidates elementary steps of water activation at metal electrodes

by Mitch Jacoby
January 9, 2006 | A version of this story appeared in Volume 84, Issue 2

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Credit: Courtesy of Jean-Sébastien Filhol
Electrooxidation of water on palladium forms surface hydroxyl species (spheres at center) and H+ ions, which migrate away from the anode by means of H5O2+ intermediates. H is white; O, red; and Pd, blue.
Credit: Courtesy of Jean-Sébastien Filhol
Electrooxidation of water on palladium forms surface hydroxyl species (spheres at center) and H+ ions, which migrate away from the anode by means of H5O2+ intermediates. H is white; O, red; and Pd, blue.

Basic interactions between water molecules and metal electrodes lie at the heart of a variety of industrially relevant processes. Yet details of molecular-scale events in electrochemistry, such as fundamental steps in solution-phase reaction mechanisms, generally have eluded scientists.

Now, Matthew Neurock, a professor of chemical engineering and chemistry, and postdoc Jean-Sébastien Filhol at the University of Virginia have developed a quantum-mechanical modeling technique that simulates specific changes in molecular structures and reactivity at the interface between an aqueous solution and an electrode during electrochemical reactions as a function of applied potential (Angew. Chem. Int. Ed. 2006, 45, 402).

The new method sheds light on molecular-scale events that govern electrodeposition, electrocatalysis in fuel cells, and other types of electrochemical reactions. The method was applied to a test case: electrochemical activation of water at a palladium electrode.

Among various findings, the model predicts, as expected, that as the potential applied to the metal grows increasingly negative, water molecules become oriented with their hydrogen sides toward the electrode and react to form surface hydrides and solution-phase hydroxyl ions.

Conversely, as the potential becomes increasingly positive, water bonds to the electrode through the oxygen side and dissociates, forming surface hydroxides and liberating H+ ions into solution and electrons to the electrode. Neurock points out that the ions are transported away from the electrode via a concerted proton-transfer process, which proceeds by way of a hydrogen-bonded water network. The process delivers the protons to water molecules in the solution layer above the electrode.

Many of the study's results are summarized in the form of an interfacial phase diagram that maps the electrooxidation and electroreduction of water on palladium as a function of applied potential.

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