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

The heterogeneous electron transfer kinetics (k⁰ values), mass transport and thermodynamic properties (E⁰ values) associated with the Fe^III/II process have been determined in aqueous solutions containing 0.1 M HCl, HClO₄, bis(trifluoromethanesulfonyl)imide (HNTf₂) or 0.05 M silicotungstic acid (H₄[α-SiW₁₂O₄₀]) supporting electrolytes. The diffusion coefficient (D) values for Fe^III and Fe^II are dependent on the radius of the electrolyte anion and follow the order D(SiW₁₂O₄₀ ⁴⁻) < 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 Fe^III and Fe^II. 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 Fe^III/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.