Heyrovsky Step Research Articles

The maximum hydrogen (deuterium) loading ratio in the Pd|H2O (D2O) electrochemical system

Abstract Based on the Volmer-Heyrovsky-Tafel mechanism, Frumkin adsorption and thermodynamic data of hydrogen (deuterium) adsorption in Pd, effects of surface parameters on the hydrogen (deuterium) loading ratio into Pd in the hydrogen (deuterium) evolution reaction are discussed. There is a change of mechanism from the Volmer-Tafel route to the Volmer-Heyrovsky route when the current density rises, and there exists the maximum loading ratio at a certain current density when the symmetry factor of the Heyrovsky step is less than that of the Volmer reaction. The theoretical results fit the experimental data presented before very well; other factors that affect the loading ratio are discussed as well.

The polarisation resistance, exchange current density and stoichiometric number for the hydrogen evolution reaction: theoretical aspects

A relationship between the polarisation resistance and the equilibrium reaction rate of the Volmer, Heyrovsky and Tafel steps has been developed for the hydrogen evolution reaction. Furthermore, the exchange current density for the simultaneous occurrence of both Volmer-Heyrovsky and Volmer-Tafel routes has been analysed on the basis of an extension of the Temkin development for a single reaction route. In this context, a relationship between the true exchange current density and that obtained by extrapolation of the Tafel lines has been obtained, as well as their relation to the mean stoichiometric number. The application of these expressions to some selected experimental results is also analysed. It is concluded that the determination of the dependence of the current density on overpotential should be carried out in a potential range as wide as possible, in order to evaluate the polarisation resistance at low overpotentials and to determine whether a Tafel region with a slope equal to RT/(1 α) F is present at high overpotentials.

The hydrogen evolution reaction on amorphous nickel and cobalt alloys

The mechanism of the hydrogen evolution reaction (HER) on Ni 70P 30, Co 72P 28, Ni 40Co 23P 37 and Ni 72Fe 1P 27 was studied by impedance spectroscopy and steady-state polarization measurements. Only the Ni-Fe-P presents a good electrocatalytic activity with an overpotential ≈200 mV lower than that for mild steel or Ni (the traditional materials used as cathodes in water electrolysers). Impedance studies of the HER were carried out on these amorphous alloys and it was concluded that Ni-Fe-P is the only material to present a Volmer-Heyrovsky mechanism with the Heyrovsky step being the r.d.s. In contrast, on Co-P, Ni-P and Ni-Co-P the mechanism is Volmer-Tafel with the Volmer step being the r.d.s. From the rate constants the following catalytic sequence was determined: Ni-Fe-P > Co-P > Ni-Co-P > Ni-P.

The Mechanism of Hydrogen Evolution Reaction on a Modified Raney Nickel Composite‐Coated Electrode by AC Impedance

The hydrogen evolution reaction (HER) on a modified Raney nickel electrode, consisting of Ni‐Al‐Cu‐Cr alloy, was investigated with ac impedance measurements and by dc polarization current behavior in 5.36M solution at 70°C. Comparative measurements also were carried out on commercial Raney nickel (Ni‐Al). The rate constants of the forward and backward reactions of Volmer, Heyrovsky, and Tafel steps were estimated by a nonlinear fitting method. The modified Raney nickel composite‐coated electrode provides a lower overpotential for the HER than the commercial Raney nickel‐coated electrode. The main pathway for the HER is Volmer‐Tafel with Volmer as the rate determining step.

Investigation of hydrogen evolution on Raney-Nickel composite-coated electrodes

The hydrogen discharge on the Raney nickel composite coated electrode is investigated in 30% wt NaOH aqueous solution at 70°C by ac impedance technique. The easiest pathway for the hydrogen evolution reaction (HER) is the Volmer-Heyrovsky mechanism. Both Volmer and Heyrovsky steps control the reaction. The increase in the electrocatalytic activity from 30% wt NaOH to 1.04 M NaOH at 70°C is related to a larger exchange current density of the HER.