RESUMO
The kinetics of the V5+/V4+ redox reaction is investigated in a three-electrode configuration on a Vulcan XC-72 modified glassy carbon rotating disk electrode at four different temperatures (25 to 40 °C, with 5 °C interval). The values of enthalpy of activation (ΔH#) and pre-exponential factor (Af) estimated using the Eyring equation are in the range of 0.25-0.53 eV (24-51 kJ mol-1) and -1.3 to 5, respectively. The Eyring plots tend to diverge with overpotential, causing an increase in the values of the estimated ΔH# and Af. This is perhaps due to the retarding effect of the precipitates/adsorbates on the electrode surface. The investigation of the kinetics suggests that the V5+/V4+ redox reaction is electrocatalysed through an increase in the entropy of activation (ΔS#).
RESUMO
Transport of redox species through porous thin-film electrodes is investigated using electrochemical impedance spectroscopy (EIS). Redox species of small size and fast electron-transfer kinetics show two arcs in the EIS pattern: a high frequency arc corresponding to the charge-transfer process (electron-transfer) and a low frequency arc corresponding to the mass-transport process (transport of the redox species from the bulk of the solution to the electrode interface). Often, the features of the Nyquist plot corresponding to the transport of the redox species through the porous electrode and that through the bulk of the electrolyte are not resolved. It is shown that the resolution of such features depends on the (1) composition of the porous thin-film, (2) electron-transfer kinetics, (3) interaction of the redox species with the electrode components, and (4) bulkiness of the redox species and (5) its concentration.
