The role of the electrical double layer and ion pairing on the electrochemical oxidation of hexachloroiridate(III) at Pt electrodes of nanometer dimensions

The steady-state voltammetric oxidation of hexachloroiridate(III), IrCl6(3-) (1-5 mM), in the presence and absence of an excess supporting electrolyte was investigated at disk- and hemispherical-shaped Pt electrodes with radii ranging from 48 nm to 12.5 microm. Thermodynamic, kinetic, and transport parameters that define the shape and magnitude of the voltammetric wave exhibit a complex dependence on whether a supporting electrolyte is present in the solution. First, the half-wave potential, E1/2, for oxidation of IrCl6(3-) shifts to more positive potentials in the presence of a supporting electrolyte, a consequence of the relative difference in the strength of ion pairing of IrCl6(3-) and IrCl6(2-) by the supporting electrolyte cation. E1/2 increases in the order no electrolyte < n-tetrabutylammonium < Na+ approximately K+ approximately Ca2+, but is independent of the supporting electrolyte anion (Cl-, NO3-, PF6-). Second, the heterogeneous electron-transfer rate constant for oxidation of IrCl6(3-) increases by approximately an order of magnitude in the presence of a supporting electrolyte. Third, in the absence of electrolyte, mass transport limited currents deviate significantly from predicted values based on the Nernst-Planck equation, but only when the electrode radius is smaller than ca. 1 microm. The latter two effects (Frumkin and dynamic diffuse layer effects) result from the dependence of interfacial electrical fields and, thus, the rates of electron-transfer and ion migration, on the supporting electrolyte concentration. We also demonstrate that the theoretical shape of the voltammetric response for oxidation or reduction of a highly charged redox species (e.g., IrCl6(3-)) is essentially independent of whether a supporting electrolyte is present in the solution. This finding can greatly simplify the analysis of heterogeneous electron-transfer rates using steady-state voltammetry in low ionic strength solutions.