Hydrogen evolution reaction (HER) are investigated on Pt, Pd, and MoS 2 in a 0.5 M H 2 SO 4 electrolyte in a rotating disk electrode (RDE) configuration in the temperature range of 285−335 K. The reaction is temperature-sensitive on all of the three catalyst surfaces at their respective overpotential ranges. The kinetic parameters (activation enthalpy (ΔH # ), free energy of activation (ΔG # ), and preexponential factor (A f )) toward HER are obtained from the Arrhenius and Eyring relations, and the overall kinetics on the catalyst surfaces is analyzed. ΔH # for HER is a strong function of the overpotential in the case of both Pt and Pd. On the other hand, the trend in A f suggests that the electrocatalysis of HER on MoS 2 originates from an increase in entropy factor, perhaps due to the solvent−dipole interaction at the interface. Such analysis is pivotal to the investigation of electrocatalysis of HER, especially on surfaces for which determination of active-site density is not established.
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.
The rate of electron-transfer reactions, irrespective of electrochemical or electrocatalytic, is universally explained on the basis of the Butler-Volmer (B-V) theory. The charge-transfer coefficient (α) obtained is typically in the...
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.
The oxygen reduction reaction (ORR)
is investigated on
metal-free
carbon (Vulcan XC-72) and nitrogen-doped (∼≤1%) carbon
(N/C-900) in 0.1 M KOH. The product distribution (O2 to
OH– and HO2
–) as a function of overpotential (η)
in the temperature range of 293–323 K is analyzed using a rotating
ring-disk electrode (RRDE) assembly. The kinetic current due to reduction
of O2 to HO2
– is estimated and used in the Eyring
analysis to determine the change in enthalpy of activation (ΔH
#). It is shown that doping of carbon with nitrogen
(even with ≤1 wt %) causes substantial increase in the number
of active sites (almost 2-fold) and reduction in ΔH
# at any η. Moreover, ΔH
# is a stronger function of η on N/C-900 as compared
to that on the carbon surface.
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