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

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.