The impact of overpotential on the enthalpy of activation and pre-exponential factor of electrochemical redox reactions.

The kinetics of the V 5+ /V 4+ 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 (Δ H # ) and pre-exponential factor ( A f ) 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 Δ H # and A f . This is perhaps due to the retarding effect of the precipitates/adsorbates on the electrode surface. The investigation of the kinetics suggests that the V 5+ /V 4+ redox reaction is electrocatalysed through an increase in the entropy of activation (Δ S # ).