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.

Effect of redox species concentration across the electrode/electrolyte interface on the EIS features.

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.

Estimated ESA of nitrogen-doped carbon and carbon using AQS-adsorption method.