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 (k(0) values), mass transport and thermodynamic properties (E-0 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 (H-4[alpha-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(SiW12O404-) < D(NTf2-) < D (ClO4-) < D(Cl-). The k(0) 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 k(0) 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 k(0) values associated with the Fe-III/II process in H-4[alpha-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 H-4[alpha-SiW12O40] electrolyte media. (C) 2017 Elsevier Ltd. All rights reserved