Electrode Material Dependence, Ion Pairing, and Progressive Increase in Complexity of the α-[S₂W₁₈O₆₂]^{4-/5-/6-/7-/8-/9-/10-}Reduction Processes in Acetonitrile Containing [ n-Bu₄N][PF₆] as the Supporting Electrolyte

Large-amplitude Fourier transform alternating current (FTAC) voltammetry has been used to parameterize the electrode kinetics associated with the reduction of α-[S2W18O62]4- in acetonitrile containing [n-Bu4N][PF6] as the supporting electrolyte at glassy carbon (GC), gold (Au), and platinum (Pt) electrodes by experimenter-based heuristic and computer-assisted automated approaches. The electron-transfer kinetics described by the Butler-Volmer relationship are faster at GC than at the metal electrodes. Progressively increasing departures from ideality in the experimental versus simulated data comparisons were found with reduction processes that occur at more negative potentials and with higher electrolyte concentrations. Ion pairing between α-[S2W18O62]4- or its reduced forms and the electrolyte cation may contribute to nonconformance between theory and experiment. Electrochemical quartz crystal microbalance experiments along with other experiments reveal that adsorption of more extensively reduced species may modify the electrode surface and contribute to the asymmetry found in the reduction and oxidation components of the FTAC voltammetric data. Enhanced double-layer effect at negative potentials could also explain why the level of nonideality increases with reduction processes that occur at more negative potentials. The findings in this study are expected to apply to the voltammetric reduction of other negatively charged polyoxometalates.