Solvent and Electrode Kinetic Effects on Cathodic Reduction of I3−+2e→3I− in Some Pure and Mixed Dipolar Aprotic Solvents

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Abstract
Exchange current densities (i0) of the redox reaction I3−+2e−=3I− have been determined at 25 °C on platinum (Pt) and graphite (C) electrodes in acetonitrile (ACN), propylene carbonate (PC), N,N-dimethylformamide (DMF), and their binary mixtures by potentiostatic polarization studies under near equilibrium conditions. The studies in pure solvents have also been extended to 15 and 35 °C and also on platinized graphite (C)Pt electrode. The kinetic solvent effect data at 25 °C have been analyzed in the light of relative solvation of I2 and transition state (TS) as reflected from the previously reported transfer free energies (ΔGt°) of I2 and that of TS derived therein for the respective solvent systems. The electrocatalytic activities of (C)Pt electrode in different solvent systems are found to be of the same order as that of Pt electrode possibly because of its large roughness factor. But the observed energies of activation (Ea) being of the order C>>C(Pt) ≈Pt suggest that graphite is intrinsically a weaker electrocatalyst than both the (C)Pt and Pt electrodes, which are however equivalent to each other so far as electrocatalytic activity is concerned. Moreover, while the electrocatalytic activity of Pt and (C)Pt electrodes is found to be in the order ACN>PC>DMF, that of C is ACN>DMF>PC, which have been ascribed to result from the differential adsorption propensities of the solvent molecules with respect to Pt and C surfaces respectively.
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