1.1 Introduction
This chapter is devoted to single outer‐sphere electron transfers taking place between an electrode and a molecule. The electrode is an electronic conductor, often a metal, although graphite and glassy carbon are also frequently used. The molecule may be attached (or adsorbed) onto the electrode surface or free to move in the bathing solution. The latter contains a strong electrolyte – the supporting electrolyte – which ensures the conduction of electricity through this section of the circuit. The presence of the supporting electrolyte also minimizes migration of charged reactants in solution. Diffusion (and in special cases, forced convection) is thus the sole mode of transport to and from the electrode. The supporting electrolyte is usually chosen so as to offer a maximally extended electroactivity window. To this end, it contains a hard‐to‐oxidize anion and a hard‐to‐reduce cation, allowing the investigation of the largest possible number of electron donor and electron acceptor reactants.
Single‐electron transfers may be categorized as outersphere and innersphere processes according to whether they are not or whether they are accompanied in a concerted manner with breaking or formation of bonds. The classification originates from the electron transfer chemistry of metallic complexes [1], making a distinction between the reactions where one electron but no ligand are transferred, and those where a ligand is transferred concertedly with one electron, which amounts to the transfer of an atom (or of a group of atoms). The notion was extended afterward [2] according to the definition above so as to include all kinds of molecules, including organic molecules.
What follows consists of a depiction of the main characteristics of single outersphere electron transfer reactions and of the way in which they have been gathered. A strong emphasis will be put on cyclic voltammetry, together with, in places, a comparison with other existing electrochemical techniques, such as steady state, potential‐step, and impedance methods. The reason for this choice is not only that cyclic voltammetry has progressively become the most popular of the electrochemical techniques [3] but also the conviction that, although not always recognized, the performances of these techniques are all essentially equivalent if compared on an equal footing. One main reason for the popularity of cyclic voltammetry is most probably the pictorial nature of the current–potential response and of its variation with the scan rate. At a glance, a coarse idea of the reaction mechanism emerges, even though it has next to be carefully checked and analyzed to obtain quantitative characterization. The price to pay is that this quantitative analysis requires a somewhat heavier algebra than with the abovementioned techniques. Algebra is, however, nothing but a compliant servant...
