A Systematic Study of the Mass Transport, Kinetic and Thermodynamic Properties of the FeIII/II Process at Glassy Carbon and Boron-Doped Diamond Electrodes

Abstract

The heterogeneous electron transfer kinetics (k0 values), mass transport and thermodynamic properties (E0 values) associated with the FeIII/II process have been determined in aqueous solutions containing 0.1M HCl, HClO4, bis(trifluoromethanesulfonyl)imide (HNTf2) or 0.05M silicotungstic acid (H4[α-SiW12O40]) supporting electrolytes. The diffusion coefficient (D) values for FeIII and FeII are dependent on the radius of the electrolyte anion and follow the order D(SiW12O404−)<D(NTf2−)<D(ClO4−)<D(Cl−). The k0 values are found to be smaller at boron doped diamond (BDD) electrodes than glassy carbon (GC) electrodes, and influenced by the ion-pairing between electrolyte anions and FeIII and FeII. Based on the formal potential value, HNTf2 is found to be a more innocent electrolyte than commonly used HClO4 with regard to determine the true k0 values of the outer-sphere [Fe(H2O)6]3+/2+ process. Fourier transformed large amplitude alternating current voltammetry, which provides favorable signal to background ratio, was used to determine k0 values associated with the FeIII/II process in H4[α-SiW12O40] and HNTf2 electrolyte media at GC electrodes and probe the implications of the heterogeneity of the BDD electrode surface on the electrode kinetic determination in H4[α-SiW12O40] electrolyte media.