Electrode Reaction and Mass Transport Mechanisms Associated with the Iodide/triiodide Couple in the Ionic Liquid 1-ethyl-3-methylimidazolium Bis(Trifluoromethanesulfonyl)Imide
By Bentley, Cameron Luke; Bond, Alan M; Hollenkamp, Anthony F.; Mahon, Peter John & Zhang, Jie
Published in The Journal of Physical Chemistry C
2014
Abstract
The oxidation and reduction processes associated with the iodide/triiodide (I-/I3-) couple have been investigated in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide using cyclic voltammetry, convolution voltammetry and chronoamperometry. Analogous to the case in acetonitrile, two processes with a relative electron stoichiometry of 2:1 are observed at a platinum electrode, and are assigned to the I-/I3- and I3-/I2 processes at lower and higher potentials respectively. The electro-oxidation of I- has been simulated using a termolecular charge transfer mechanism: I2 + 2e- ⇌ I- + I- (E0, ks, α), I2 + I- ⇌ I3- (Keq, kf), where E0, ks, α, Keq and kf are the standard potential, standard heterogeneous electron-transfer rate constant, transfer coefficient, equilibrium (stability) constant and bimolecular (forward) rate constant respectively. The stability constant of I3- in this ionic liquid (Keq = 2.3 × 106, estimated by simulation) is comparable to that reported in acetonitrile (Keq ≈ 107). The reduction of I3- to I- has also been modelled in the ionic liquid using a kinetically controlled CE mechanism. For this process, the delicate balance between kinetic and diffusion control is sensitive to the voltammetric scan rate, electrode geometry and the total concentration/ratio of I-/I2, thereby making it difficult to estimate the diffusion coefficient of I3-. In contrast, the oxidation of I- to I3- proceeds via a diffusion-controlled EC mechanism and the diffusivity of I- is directly proportional to the bulk concentration of I3-, showing an enhancement of ≈50% when the bulk concentration of I3- is increased from 0 to 500 mM. This enhancement cannot be explained by a decrease in solution viscosity and has been attributed to electron hopping and/or a Grotthuss-type bond-exchange reaction between I- and I3-, in agreement with previous reports.