Electrolyte cation dependence of the electron transfer kinetics associated with the [SVW11O40]3–/4– (VV/IV) and [SVW11O40]4–/5– (WVI/V) processes in propylene carbonate

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Changing the supporting electrolyte cation from tetrabutylammonium to 1-butyl-3-methylimidazolium is known to significantly increase the apparent heterogeneous electron transfer rate constants (k0 value at the formal reversible potential, (EF0)) associated with the [SVW11O40]3−/4− (VV/IV) and [SVW11O40]4−/5− (WVI/V) processes in aprotic organic media. In this study, supporting electrolytes containing 7 different cations, namely 1-ethyl-3-methylimidazolium ([EMIM]+), 1-butyl-3-methylimidazolium ([BMIM]+), 1-butyl-1-methylpyrrolidinium ([Py14]+), tetraethylammonium ([TEA]+), tetrapropylammonium ([TPA]+), tetrabutylammonium ([TBA]+) and tetrahexylammonium ([THA]+), have been investigated in order to provide a systematic account of the influence of the electrolyte cations on the rate of polyoxometalate (POM) electron transfer at a platinum disk electrode. Fourier transformed alternating current (FTAC) voltammetry has been used for the measurement of fast kinetics and DC cyclic voltammetry for slow processes. The new data reveal the formal reversible potentials and electron-transfer rate constants associated with the VV/IV (kV0) and WVI/V (kW0) processes correlate with the size of the supporting electrolyte cation. kV0 and kW0 values decrease in the order, [EMIM]+>[BMIM]+>[Py14]+≈[TEA]+>[TPA]+>[TBA]+>[THA]+ for both processes. However, while kV0 decreases gently with increasing cation size (k0=0.1 and 0.002cms−1 with [EMIM]+ and [THA]+ electrolyte cations, respectively), the decrease in kW0 is much more drastic (k0=0.1 and 2×10−6cms−1 for [EMIM]+ and [THA]+, respectively). Possible explanations for the observed trends are discussed (e.g., ion-pairing, viscosity, adsorption and the double-layer effect), with inhibition of electron-transfer by a blocking “film” of electrolyte cations considered likely to be the dominant factor, supported by a linear plot of ln(k0) vs. ln(d) (where d is the estimated thickness of the adsorbed layer on the electrode surface) for both the VV/IV and WVI/V processes.