Kinetics of the V5+/V4+ redox reaction on Vulcan XC-72 modified glassy carbon disk electrode is investigated in a three-electrode configuration. Cyclic voltammograms of V5+/V4+ redox couple suggest that the overpotential range for the kinetic analysis is limited to ±300 mV, after excluding V4+/V3+ redox reaction at the negative overpotential and the oxygen evolution reaction at the positive overpotential. The linear sweep-voltammograms (LSVs) are corrected for potential drop due to solution resistance (iR
s
), mass-transfer resistance, and most importantly, for the back reaction current. These corrections are imperative to estimate the Tafel slope in the limited range of overpotential for V5+/V4+ redox reaction. The charge-transfer coefficient (α) estimated from the Tafel slope deviates significantly from the expected value of 0.5 for the single electron-transfer reaction. Moreover, the instantaneous slope of the Tafel plot suggests that the α is overpotential dependent. Therefore, Marcus theory of electrochemical kinetics is applied to estimate the α. The reorganization energy (λ) calculated from the Arrhenius plots is in the range of values reported in the literature for the other redox couples.
Redox reactions of organic redox couples are investigated on Nafion-free and Nafion-containing carbon (Vulcan XC-72)-modified glassy carbon disk (GCD) electrodes. The model system chosen is the p-benzoquinone (BQ)/hydroquinone (BQH 2 ) couple because BQ derivatives are common to most of the organic redox couples. The electrochemical impedance spectroscopy patterns show an unexpected rise in real resistance with time and with rotation speed (rpm). This increase in resistance is remarkable with Nafion-containing GCDs. It seems that the sulfonate group on the polymer chain attacks the redox couple to bind the species to the electrode surface. Moreover, the internal and external mass transfer features are better resolved than their nonbinding inorganic counterparts (V 4+ /V 5+ couple). These findings have enormous implications to the development of organic redox flow batteries, organic synthesis, supercapacitors, sensors, and so on.
Improvement in catalytic activity of electrochemically treated carbon (relative to the untreated carbon) toward various redox reactions is widely reported in the literature. In this work, the origin of such activity enhancement due to electrochemical treatment in a 1 M H 2 SO 4 electrolyte in a potential range of 1−2.5 V is investigated using physical, electrical, and electrochemical methods. The physical characterizations suggest intercalation of anions (bisulfate) between the graphite layers from the H 2 SO 4 electrolyte. Electrical characterizations (both Hall measurement and Mott−Schottky analysis) show that the samples switch from n-type to p-type behavior upon electrochemical treatment. The improvement in the catalytic activity on electrochemical treatment of carbon is explained on the basis of the change in surface characteristics, carrier concentration (N D ), and active site density. The same is validated with oxygen reduction reaction in alkaline media.
n-Type
and p-type Si samples of various doping concentrations (1014–1019 cm–3) are investigated
in hydrogen fluoride electrolyte using voltammetry and electrochemical
impedance spectroscopy (EIS) at negative overpotentials. In addition
to the features of solution resistance, space charge, and double layer
common to all the doped-Si samples, up to six different types of low-frequency
(lf) features are observed in the EIS spectra, depending on the dopant
and its concentration. These lf features corresponding to the sequential
or simultaneous hydride formation and evolution processes are well
resolved on Si of appropriate dopant concentration, unlike in the
case of the widely investigated Pt and Pd. The results reported here
with Si, a semiconducting material, lend experimental credence to
the proposed theoretical models of hydrogen evolution reaction (HER),
and the spectra are similar to the simulated spectra reported in the
literature.
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