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

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.

Electrochemical and Spectroscopic Studies on the Oxidation of the cis-(Et2-dcbpy)2RuX2 Series of Photovoltaic Sensitizer Precursor Complexes (Et2-dcbpy = 2,2‘-Bipyridine-4,4‘-diethoxydicarboxylic Acid, X = Cl-, I-, NCS-, CN-)

“…For other complexes containing easily oxidizable ligands, , electrochemically generated Ru III complexes have been shown to undergo an internal reaction to produce Ru II and the oxidized ligand . In the particular case of L 2 RuI 2 the oxidized form of the ligand is I 2 , which itself is electroactive in the potential range of interest. ,− …”

“…43 In the particular case of L 2 RuI 2 the oxidized form of the ligand is I 2 , which itself is electroactive in the potential range of interest. 41,[44][45][46] The following kind of reaction scheme therefore may be postulated in order to rationalize the electrochemical oxidation of L 2 RuI 2 :…”

Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I− Oxidation and I3− Photodissociation

“…26,27 The second explanation is that the TiO 2 (e -)s are good one electron reductants, perhaps best thought of as Ti(III) trap states, that are unable to facilitate the 2e -reduction of I 3 -, which is in fact the only reaction observed for I 3 -at metallic electrodes. [28][29][30] In addition, quinone/hydroquinone redox mediators that achieved some success in the older dye sensitized literature also involve two-electron transfer. 31 Significantly, it has been known for some time that the reaction of aqueous Ti(III) ions with I 3 -is also kinetically sluggish with an activation energy of 80 kcal/ mol, Reaction 3.…”

Anomalous Charge Transfer Behavior of an Iodide/Triiodide Redox Couple at an Ionic Liquid/Pt-Electrode Interface

“…Meanwhile, though several mechanisms have been suggested for reaction , ,− this reaction is assumed to be composed of the following multistep processes proposed by Dané et al: normalI 3 − ⇄ normalI 2 + normalI − normalI 2 + normale − ⇄ normalI ads + normalI − normalI ads + normale − ⇄ normalI − where I ads means the iodine molecule adsorbed on the electrode surface, and step is considered to be the rate-determining step. This is one of the most widely accepted mechanisms and has often been cited to discuss the exchange reaction rates of the I – /I 3 – couple. − On the basis of the above reaction mechanism, the exchange current density can be expressed as the following equation (cf. Supporting Information): i 0 , overall = 2 F k app 0 [ I 3 − ] 1 / 2 ( 1 + α ( c ) ) ...…”

“…It was reported previously, on the basis of results on i 0 values on platinum or graphite electrode in AN, propylene carbonate (PC), or N,N -dimethylformamide (DMF), that the Δ G * values for the interfacial I – /I 3 – redox reaction depend not only on the electrode material but also on solvent. The reported dependence of Δ G * on the electrode material, in which the Δ G * value on platinum in AN is almost 4 times lower than that on graphite, is strongly reflected by the fact that the rate-determining step of the I – /I 3 – redox reaction (eq ) includes the adsorption/desorption of the I – /I 3 – couple, whereas the solvent dependence of Δ G * indicates that the reorganization of the surrounding molecules during the adsorption/desorption relates also with the rate-determining step. It can also be recognized in literature that the solvent dependence of Δ G * found for the interfacial process of the I – /I 3 – couple is almost similar to that for heterogeneous outer-sphere electron transfers of several redox couples, , i.e., Δ G * in AN < Δ G * in DMF < Δ G * in PC.…”