Electrochemical NanoScience Group
‹— Research topics

Electrochemical double layer

The interface between the electrode and the electrolyte is the “heart” of electrochemistry. It is the place where charge transfer takes place, where gradients in electrical and chemical potentials constitute the driving force for electrochemical reactions. How does this interface look like? The classical approach describes the electric double layer (EDL) of a metal electrolyte interface by a plate condenser of molecular dimensions. One plate is the metal surface with its excess charge, the other is formed by the solvated ions at closest approach. The solvated ions that form the outer Helmholtz plane (OHP) and that are held in position by purely electrostatic forces are termed “nonspecifically adsorbed”. These are mainly solvated cations. Most anions however give away part of that solvation shell when entering the double layer to form a chemical bond with the electrode surface. These ions are termed “specifically adsorbed” and their centers form the inner Helmholtz plane (IHP). In addition, the electrolyte may also contain specifically or non-specifically adsorbed organic molecules as well as redox-active species.
A characteristic feature of metal/aqueous electrolyte interfaces is their remarkably high capacity, which ranges between 20 and 50 μF·cm-2. By applying a potential drop across the electrochemical interface of up to 1 V (which for noble metal electrodes is indeed possible) high surface charges of up to about 10–50 μC·cm-2 can be achieved (corresponding to a charge of about 0.1–0.2 electrons per surface atom), and extremely high electric fields of about 3·107 Vcm-1 can be obtained, assuming a “plate distance” of 0.3 nm. These properties make the electrochemical double layer (EDL) unique, and it is important to understand them. The combination of macroscopic measurements of current vs. potential characteristics with the power of structure-sensitive in situ techniques, such as scanning probe microscopy (SPM) and surface X-ray scattering provide opportunities to explore, at least step by step, how an interfacial system and its functional behaviour evolve from the macroscopic through the microscopic to the molecular/atomic level.
Revised: 07.12.2007     ©: 2005-2007