Transport of redox species (VO2+/VO2 +, V2+/V3+, Ti3+/Ti4+, and Fe2+/Fe3+) across the electrode/electrolyte interface is investigated in a thin-film rotating disk electrode configuration using electrochemical impedance spectroscopy (EIS). The transport features depend on the constituents of the thin-film catalyst layer and on the rate constant of the redox reaction. On Nafion-free porous electrodes, semi-infinite linear and finite transport features are observed under static and hydrodynamic conditions of the electrode, respectively. Depending on the rate constant of the electrochemical reaction, an equivalent circuit consisting of either resistance (R) and constant phase element (Q) or the Warburg short (W s) element is proposed to explain the finite transport features. Addition of Nafion (binder) in the electrode offers extra resistance to the transport of redox species, which helps resolve EIS features of the transport of redox species through the porous thin-film electrode and that through the bulk of the electrolyte. The features of the transport of redox species through the porous electrode media are independent of the hydrodynamic conditions.

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.

The energetics of the electrochemical, electro-inhibiting and electrocatalytic reactions as a function of overpotential.