Structures of water at electrified interfaces: Microscopic understanding of electrode potential in electric double layers on electrode surfaces

Water adsorption on M(111) (M = Pt, Cu, Ni, Ru(001)) surfaces and a new double layer structure of water at a Cu(111) electrode surface were investigated by surface X-ray diffraction, scanning tunneling microscopy and infrared reflection absorption spectroscopy methods. Model electrochemical double layer structures - fabricated outside an electrochemical cell under vacuum conditions without electrode potential - were extensively elucidated with a view to simulating the electrochemical interfaces under electrode potential control. There exists a clear relationship between a double layer structure on an electrode under an electrochemical potential control and a simulated electric double layer structure in UHV. The origin of an "immersed gap" (the difference between the UHV and electrochemical situation) is attributed to charge transfer from water molecules to metal electrodes, and the potential drop near the electrified interface depends on the orientation of water dipoles of cationic or anionic water at inner and outer Helmholtz layer. Water molecules in an electric double layer exhibit an ordered and a disordered structure at negative and positive electrode potentials, respectively. Therefore, potential polarization (negative or positive electrode potential application) from an equilibrium potential operates the electrified interface to cause increased or decreased ordering, orientation and charge transfer of water molecules as well as water dissociation on the electrode surfaces. (C) 2008 Elsevier B.V. All rights reserved.