María R. Gennero de Chialvo

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.

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.

Isopiestic determination of osmotic coefficients and evaluation of activity coefficients of aqueous mixtures of sodium and potassium chloride at 45°C

Abstract The osmotic coefficients of aqueous mixtures of potassium chloride and sodium chloride were evaluated at 45°C by the isopiestic method, varying the total molal concentration between 1 and 5 moles kg−1 in the whole range of reduced molar fraction. The results obtained were correlated by the Pitzer equation and by that derived by Pan and Han, for the application of the McKay-Perring method. From these correlations, the activity coefficients of both salts in the mixture were calculated. A comparative analysis of the results obtained were also made.

Evaluation of the kinetic parameters of the hydrogen electrode reaction from the analysis of the equilibrium polarisation resistance

The theoretical expression of the equilibrium polarisation resistance as a function of the partial hydrogen pressure and the activity of protons has been established for the hydrogen electrode reaction operating through the Volmer–Heyrovsky–Tafel mechanism. The corresponding elementary kinetic parameters have been evaluated using experimental results reported in the literature, obtained on smooth platinum electrodes. The advantage of the proposed method for the calculation of these parameters with respect to the conventional determination of the polarisation curves at high overpotential values has been also demonstrated.

Analysis of the Volmer–Krishtalic mechanism for the chlorine electrode reaction

Abstract The dependencies of the current density and surface coverage of the adsorbed intermediates on overpotential were established without kinetic approximations for the chlorine electrode reaction under the Volmer–Krishtalic mechanism. Tafelian regions were obtained which slope values cannot be derived from the use of the rate determining step criteria, such as 2.3026(2RT/3F) and 2.3026(RT/F), as well as two or three Tafel regions with different slopes in the same anodic or cathodic curve. The existence of limiting kinetic current densities was also demonstrated. Finally, the results obtained were analysed and discussed, comparing them with those obtained by the usual methods.

Resolution of the mechanism of CO electrooxidation on steady state and evaluation of the kinetic parameters for Pt and Ru electrodes

The carbon monoxide oxidation reaction (COOR) was studied on steady-state conditions by chronoamperometry on polycrystalline smooth platinum and ruthenium rotating disc electrodes in CO-saturated acid solution. The chronoamperometric response did not show current oscillations and therefore the current density (j) vs. overpotential (η) curves on steady state could be obtained. In order to interpret these results, kinetic expressions were derived starting from the mechanism proposed by S. Gilman, which considers two adsorbed reaction intermediates, carbon monoxide (COad) and hydroxyl (OHad). Analytical expressions as a function of overpotential for the current density, the surface coverage of the adsorbed species (θCO and θOH) and the CO and CO2 pressures at the electrode surface on steady state were obtained. This set of equations was used for the correlation of the experimental polarization curves and the evaluation of the corresponding kinetic parameters. From these values, the dependences of the surface coverage of the adsorbed intermediates on overpotential were simulated, as well as those of the partial pressure of CO and CO2. Thus, it was demonstrated that the Gilman’s mechanism accurately describes the experimental results on steady state of the COOR on these metals.

Intermetallic junction contribution to the CO electrooxidation on a Pt/Au electrode: the excess voltammetric current

The contribution of the intermetallic junction to the electrocatalytic activity of a Pt/Au electrode for the carbon monoxide oxidation reaction (COOR) in alkaline solution is evaluated on the basis of the excess voltammetric current. This contribution is measured on a new type of bimetallic electrode characterized by a large ratio between the total intermetallic perimeter and the electrode area. The results obtained demonstrate that Pt/Au heterojunction enhances the reaction rate in two potential regions. This improvement is explained on the basis of the spillover of the CO adatoms from one side of the intermetallic junction to the other.

A Simple Method to Establish the Relationship Between the Equilibrium Polarization Resistance and the Elementary Kinetic Parameters of an Electrocatalysed Reaction

AbstractA simple and rigorous methodology to establish the relationship between the faradaic equilibrium polarization resistance of an electrocatalytic reaction with the elementary kinetic parameters involved in the reaction mechanism is proposed. It was derived through an alternative method, which avoided the differentiation of the corresponding current-overpotential dependence. This formalism includes the cases where both, reactants and products, exhibit diffusion contributions. It is demonstrated that the equilibrium polarization resistance is the sum of both, faradaic and diffusion, contributions. Each diffusion term has a linear variation with the inverse of the limiting diffusion current density of the species involved. This behavior was exemplified with two different experimental data sets for the hydrogen electrode reaction, obtained on a rotating disc and microelectrodes, respectively. Graphical AbstractExperimental equilibrium polarization resistance is the sum of faradaic (Rpo