Abel C. Chialvo

Oxygen evolution reaction on thick hydrous nickel oxide electrodes

Abstract The electrocatalytic characteristics of thick hydrous oxide films obtained by repetitive square-wave potential perturbations for the oxygen evolution reaction in alkaline solutions are studied. Two kinds of films, fresh and aged, which can be related to the α and β nickel oxide structures respectively were used. The results show a marked difference between their Tafel slopes. Besides, a linear dependence of the apparent electrocatalytic activity upon the amount of oxide present in the film is obtained, which is explained on the basis of an increase in the roughness factor.

Kinetics of hydrogen evolution reaction with Frumkin adsorption: re-examination of the Volmer–Heyrovsky and Volmer–Tafel routes

Abstract A re-examination of the basic kinetic derivations of the Volmer–Heyrovsky and Volmer–Tafel routes for the hydrogen evolution reaction with a Frumkin adsorption of the intermediate was carried out. Expressions for the dependence of the surface coverage and current density on overpotential were derived for both routes without kinetic approximations. On the basis of these dependencies, the kinetic behavior was simulated for different values of the parameters involved at 298.16 K. Conditions for the existence of Tafelian domains were discussed and the independence of the Tafel slopes on the type of adsorption of the reaction intermediate was demonstrated. The results obtained were critically compared with those derived from approximated expressions customarily used and the differences with them were pointed out.

Hydrogen diffusion effects on the kinetics of the hydrogen electrode reaction. Part I. Theoretical aspects

The effect of the hydrogen diffusion on the hydrogen electrode reaction has been studied and rigorous kinetic expressions for the Tafel–Heyrovsky–Volmer mechanism has been derived. The analysis of the dependences of the current density (j) on overpotential (η), particularly oriented to the hydrogen oxidation reaction (hor), leads to the following main results: (i) in the Tafel–Volmer (TV) route, the current density reaches a maximum value (jmax) less or equal to the limiting diffusion current density (jL); (ii) jmax is always equal to jL for the Heyrovsky–Volmer (HV) route; (iii) in the simultaneous occurrence of both routes (THV), the current density always reaches the jL value, although in the range of overpotentials of applied interest (0 ≤ η/V ≤ 0.6) the jmax value, characteristic of the TV route, can also be obtained. The Levich–Koutecky plots have also been analysed and it has been demonstrated that the j(η) dependence for the hor under activated control cannot always be obtained from these plots.

Hydrogen diffusion effects on the kinetics of the hydrogen electrode reaction

The present work proposes a methodology for the evaluation of the kinetic parameters of the hydrogen electrode reaction for the Tafel–Heyrovsky–Volmer mechanism from experimental results corresponding to the study of the hydrogen oxidation on a rotating disc electrode. It is based on the correlation of the experimental dependence j(η,ω) with a theoretical expression together with some constraints that involve the use of the equilibrium polarisation resistance, which reduces the number of adjustable parameters. This methodology was applied to the hydrogen oxidation reaction on a platinum electrode, which was studied in the range 900 ≤ ω/rpm ≤ 8100 and −0.05 ≤ η/V ≤ 0.40 in 0.5 M H2SO4. The results obtained have been discussed in the light of the theoretical treatment described in Part I of this work.

Oxygen evolution reaction on NixCo(3—xO4 electrodes with spinel structure

Abstract The oxygen evolution reaction on Ni x Co (3— x )O 4 oxide electrodes with x values between 0.6 and 1.3 in alkaline solution was studied. The characterization of these oxides was carried out by X-ray diffraction, ir , XPS and SEM. The results obtained demonstrate the presence of the spinel structure on all types of electrodes studied and that the oxide, with a cobalt—nickel atomic surface ratio equal to two, has the maximum electrocatalytic activity.

Hydrogen evolution reaction: Analysis of the Volmer-Heyrovsky-Tafel mechanism with a generalized adsorption model

Abstract The Volmer-Heyrovsky-Tafel mechanism for the hydrogen evolution reaction in alkaline solutions, using a generalized adsorption model for the reaction intermediate, was solved under steady state conditions without any kinetic approximation. The electrode surface was modeled through a distribution function of the standard Gibbs energy of the adsorbed atomic hydrogen. This is characterized by its property of describing a generalized Freundlich-type adsorption. The dependences of the current density and the surface coverage on the overpotential were evaluated for the Volmer-Heyrovsky and Volmer-Tafel routes, and for the parallel course of these two routes, with different values of the model parameters. The results obtained show that the present model can correlate a great variety of kinetic behavior, and demonstrate the importance of the evaluation of the equilibrium surface coverage and the distribution function of the adsorbed species through alternative methods of kinetic study.

Existence of Two Sets of Kinetic Parameters in the Correlation of the Hydrogen Electrode Reaction

It has been demonstrated that the dependence of the current density on overpotential for the hydrogen electrode reaction described through the Volmer-Heyrovsky-Tafel mechanism can be correlated by two different sets of the kinetic parameters. Such sets lead to two different dependencies of the surface coverage on overpotential, while a unique dependence of the corresponding pseudocapacitance of adsorption is obtained. The expressions that relate both sets of kinetic parameters have been established and analyzed for the simultaneous occurrence of the three steps and for the different kinetic routes.

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

Abstract 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.

Preparation of gold nanoparticles supported on glassy carbon by direct spray pyrolysis

A method for the preparation of gold nanoparticles supported on glassy carbon by direct spray pyrolysis in air at low temperature is presented. The procedure is based on the property of some gold salts which, unlike many other metal salts, decompose by heating at low temperature to produce metallic gold and gaseous species which depend on the salt nature. In the case of the present work, HAuCl4.nH2O was used, which decomposed to solid Au and gaseous H2O and chlorine containing compounds such as HCl and Cl2. This behaviour was verified by thermogravimetric analysis and X-ray diffraction. The gold nanoparticles supported on a glassy carbon electrode were obtained by the impact of HAuCl4 solution droplets generated by an ultrasonic atomizer. These droplets are directed through a nozzle towards the glassy carbon substrate located on a stainless steel block heated by electrical resistance. The supported gold nanoparticles were electrochemically characterized by cyclic voltammetry in acid solution and the surface morphology was characterized by SEM and AFM operated in the tapping mode.

Hydrogen evolution reaction on a smooth iron electrode in alkaline solution at different temperatures

The hydrogen evolution reaction (HER) has been studied on a rotating polycrystalline smooth iron disc electrode in the temperature range 20⩽T/°C⩽60 in 2 M NaOH. It has been demonstrated that only for ∣η∣0.1 V can the polarisation curve be considered as that corresponding to the HER, due to the corrosion process that takes place at low overpotentials. The experimental results were adequately correlated by the Volmer–Heyrovsky–Tafel mechanism with a Langmuirian adsorption, considering αV≠αH. On this basis, two sets of kinetic parameters were found for each of the three elementary steps and the corresponding apparent activation energies were also evaluated. These results allowed the calculation of the experimental kinetic parameters, Tafel slope and extrapolated exchange current density, for all the temperatures studied.