Theory of electron transfer at electrified interfaces

Some recent achievements in condensed phase molecular charge transfer theory are overviewed, with focus on interfacial electrochemical electron transfer (ET). Elements of available and new formalism are addressed in Sections 2 and 3. New elements considered are, firstly, a new convenient parametric scheme for calculation of the rate constant and electrochemical current. The scheme goes beyond the commonly used quadratic free energy relations and extends straightforwardly to vibrational frequency changes, anharmonic nuclear motion, and nuclear tunnelling in local and vibrationally dispersive environmental modes. Other new elements addressed are adiabatic electrochemical processes, and self-consistent calculations of the electronic-vibrational interaction in long-range ET. Self-consistency leads to non-linear electronic vibrational features, These enhance charge transfer both by increasing the electronic tunnel factor and by decreasing the activation Gibbs free energy. Elements of charge transfer formalism are followed in Section 4 by a discussion of the theoretical basis of several electrochemical ET systems. Attention is given to electrochemical ET across well-characterized thin films, hot electron electrochemistry, electrochemistry at superconducting electrodes, and ET near a temperature or field induced high order phase transition. Particular attention is given to in situ electrochemical scanning tunnelling microscopy where a dielectric continuum or configurationally fluctuating water molecules in the tunnel gap, molecular adsorbates, and bias voltage and overvoltage spectroscopy for different tunnelling mechanisms, are considered. Section 5 gives a short discussion of some new views on proton conduction in strongly hydrogen bonded systems, and of the electrochemical dihydrogen evolution reaction, with focus on low-temperature features. This part is concluded by some perspectives in areas of molecular scale science (nanotechnology, molecular electronics) which can be expected to hold new cases for pure and applied molecular charge transfer theory. (C) 2000 Elsevier Science Ltd. All rights reserved.