Redox reactions ionic liquid electrochemistry

Corrosion is an electrochemical process. Therefore, some understanding of the fundamentals of electrochemistry is necessary [11-13]. Electrochemistry is the study of reactions that occur at the interface of an electrode, which is a metallic or semiconducting solid or liquid, and an electrolyte, which is a liquid or solid ionic conductor. These reactions typically involve the transfer of charge across the interface. There are two types of charge transfer reactions. Ion transfer reactions involve the transfer of ions from the electrode to the electrolyte, or vice versa. Electron transfer reactions involve the transfer of charge between ions in the electrolyte (or adsorbed on the surface), and typically occur heterogeneously at an electrode surface. Redox reactions are pure electron transfer reactions that occur at inert electrode surfaces. A more detailed discussion of electrochemical concepts can be found in the other volumes of this encyclopedia. A simplified view of certain aspects relevant to corrosion will be presented in this section. 

Electrochemistry is dominated by the study of species dissolved in solution. The use of a solvent as the reaction medium helps electrochemists to control important reaction conditions such as pH, rate of mass transfer, concentration of reactant, solubility, solvation, etc. Water and organic solvents are the most popular media. However, by using appropriate ionic liquids, reactants and products that are unstable in those media remain stable, and redox reactions that are impossible in water and organic solvents become possible. The reaction environments are markedly wider in some ionic liquids than in other solvent systems. In spite of this, some fundamental electrochemical concepts generally used in conventional solvent systems are not always valid in ionic liquids. 

The mechanism depicted in Scheme 18.3 which involves the disproportionation of HO2 radicals was the more accepted one at that time [3]. Posteriorly, the role of a proton source in the oxygen reduction reaction was evaluated in a similar ionic liquid, [C2mim][BF4] (Scheme 18.2), in the presence of2.1 mM and 2.64Mof water [11]. The increase in water concentration modified the electrochemistry of the oxygen reduction reaction from a reversible reduction process corresponding to the 02/02 redox couple to an irreversible cathodic process. In summary, the main features observed upon addition of water were (1) an increase of the current density due to more favourable mass transport condition (increased fluidity and conductivity in the medium), (2) shift in potential for the reduction process to more positive values caused by changes to the protonation equilibria and the solvation of the electrogenerated species [13], and (3) loss of reversibility for the reduction process. 

Kavan [28] and Kijima et al. [29] have used the electrochemical method to synthesize carbyne. This technique may be realized by classical electrochemistry whereby the charge transfer reaction occurs at interface of a metal electrode and liquid electrolyte solution. Electrons in reaction were supplied either through redox active molecules or through an electrode, which contacts an ionically conducting solid or liquid phase and the precursor. In general, the structure and properties of electrochemical carbon may differ considerably from those of usual pyrolytic carbons. The advantage of this technique is the synthesis of carbyne at low (room) temperature. It was shown that the best product was prepared by cathodic defluorination of poly(tetrafluoroethylene) and some other perhalo-//-alkanes. The carbyne... 

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