Modelling limitations encountered in the thermodynamic and electrode kinetic parameterization of the α-[S2W18O62]4−/5−/6− processes at glassy carbon and metal electrodes

Abstract

Fourier transformed large amplitude alternating current voltammetry has been used to examine the electrode material and electrolyte dependence of the thermodynamics (E0 value) and heterogeneous electron-transfer kinetics (k0 and α values) for the α-[S2W18O62]4−/5−/6− polyoxometalate reduction processes in acetonitrile (0.50 or 0.10 M [n-Bu4N][PF6]). Initial estimates of the E0, k0 and α values were made heuristically, and used to allow a computationally based data optimization method of data analysis to be performed more efficiently and to ensure the computer derived parameters are physically significant. At glassy carbon (GC), gold (Au) and platinum (Pt) electrodes, the first α-[S2W18O62]4−/5− electron transfer process is always significantly faster than the second α-[S2W18O62]5−/6− process, with k0 values following the order Pt < Au < GC, using the Butler-Volmer model for electron transfer. However, at the Pt electrode, agreement between simulation and experiment for the α-[S2W18O62]5−/6− process was poor. Since E0 significantly depends on the electrolyte concentration, implying the presence of ion-pairing reactions, the implication of neglecting ion-pairing in data analysis was investigated.