C.F. Zinola

Kinetics and mechanism of the electrochemical reduction of molecular oxygen on platinum in KOH: influence of preferred crystallographic orientation

The oxygen electroreduction reaction has been studied at both preferred oriented and conventional polycrystalline platinum rotating disc electrodes in xm KOH (0.05 ⩽ x ⩽ 3.0) aqueous solutions under oxygen saturation at 25°C. At low current densities, Tafel lines with slope -0.060 V decade−1 have been obtained at all platinum electrodes. At high current densities, higher Tafel slopes ranging from -0.18 to -0.40 V decade−1 have been observed, depending on the type of preferred oriented Pt and KOH concentration. Rotating ring-disc electrode data have shown that a higher amount of H2O2 is produced on one type of preferred oriented surface at all KOH concentrations. A complex reaction scheme has been used to evaluate the electrochemical rate constants of the reaction steps at three platinum electrodes.

Electroreduction of molecular oxygen on preferentially oriented platinum electrodes in acid solution

The oxygen electroreduction reaction was studied on two different preferentially oriented ((111)-type and (100)-type and on a conventional polycrystalline (PC) platinum rotating disc electrodes in acid solutions at 30 °C. At low overpotentials, Tafel lines of −0.060 V decade−1 were obtained on the three electrodes in oxygen-saturated 1.0m H2SO4 and 1.0m H2SO4 + ym K2SO4 (0 ⩽y ⩽ 1). At high over-potentials the usual Tafel slope of -0.120V decade−1 was observed on both (111)-type and PC platinum electrodes in 1.0m H2SO4, whereas a slope of −0.165V decade−1 was found on (100)-type platinum. In oxygen-saturated 1.0m H2SO4 the surface coverage by O-containing adsorbates on (100)-type platinum was greater than on both (111)-type and PC platinum. Rotating ring-disc electrode data showed that a higher amount of H2O2 was produced on (100)-type platinum than on the other platinum surfaces. The overpotential against current density plots are influenced by the anion concentration depending on the type of preferentially oriented platinum.

Temperature dependence of kinetic parameters related to oxygen electroreduction in acid solutions on platinum electrodes

The oxygen electroreduction reaction (OERR) kinetics at (111)- and (100)-type Pt electrodes has been studied in aqueous 1.0 M H2SO4 solutions in the 8–62°C range. On both Pt electrodes and at all temperatures, two Tafel regions have been observed. The Tafel slope in the low current density range is −0.053 ± 0.005 V decade−1, irrespective of temperature and the surface texture of the electrode. For (111)-type Pt, the Tafel slope in the high current density range increases with temperature, approaching the −2.303(2RT/F) ratio, whereas for (100)-type Pt, it reaches a temperature-independent value equal to −0.165 ± 0.003 V decade−1. Temperature dependences of OERR Tafel slopes on both types of Pt electrodes offer the possibility of further insights into the mechanism of the reaction, through the interpretation of both the adsorption enthalpy and entropy of intermediates produced along the OERR. Compensation effects, involving activation enthalpy and entropy variations with the applied potential, are also considered to account for experimental data.

New effects in the electrochemistry of carbon dioxide on platinum by the application of potential perturbations

The electroformation and electrooxidation of carbon dioxide adsorbates were studied on polycrystalline and preferentially oriented platinum surfaces by the application of medium frequency potential perturbations in saturated carbon dioxide aqueous sulphuric acid solutions. Preferentially oriented platinum surfaces were obtained by the application of fast symmetric square wave potential programmes to polycrystalline electrodes based on well-known methodology. Two new electrochemical characteristics for the electrochemistry of carbon dioxide were observed after the application of the medium frequency routines, that is, an extended adsorption potential domain for the formation of carbon dioxide adsorbates, and novel voltammetric features for the two anodic stripping peaks. These effects upon carbon dioxide electrochemistry are dependent on the morphology of the platinum electrode surface and adsorption potentials.

The influence of surface facetting upon molecular oxygen electroreduction on platinum in aqueous solutions

Abstract The O 2 -electroreduction reaction was studied at polycrystalline and [(111)-type and (100)-type] facetted Pt rotating disc electrodes in O 2 -saturated acid and alkaline solutions at 25°C. From the Tafel plots, two linear regions, one at a low ( lcd ) and another at a high ( hcd ) current density range, with Tafel slopes, ( b T ) lcd = −0.06 V decade −1 and ( b T ) hcd ⩾ −120 V decade −1 were found. In contrast to ( b T ) lcd , the value of ( b T ) hcd showed a dependence on the type of Pt electrode, the solution pH and the O 2 partial pressure. Otherwise, rotating ring-disc electrode ( rrde ) data on both acid and alkaline solutions showed that the amount of H 2 O 2 produced on (100)-type facetted Pt was greater than that formed on the other Pt surfaces. The analysis of these data has provided new evidence that the O 2 -electroreduction to H 2 O for the three Pt electrodes proceeds principally via the 4-electron pathway, but for (100)-type facetted Pt a significant contribution of the 2-electron pathway has also been found.

Electrochemical and optical study of rhenium layers formed on gold electrodes

Films obtained after cathodic polarization at Ec, −0.2 V≤Ec≤0.1 V (vs. RHE), in aqueous acid perrhenate have been investigated using cyclic voltammetry and ellipsometry. The thickness and optical indices corresponding to the electrodeposited rhenium layer were calculated. The results indicate the formation of a composite layer, with a volumetric fraction of about 30% of metallic rhenium and hydrogen occlusion.

Kinetics and mechanism of the oxygen electroreduction reaction on faceted platinum electrodes in trifluoromethanesulfonic acid solutions

The kinetics of the oxygen electroreduction reaction (OERR) were investigated on (1 1 1)-type and (1 0 0)-type faceted, and polycrystalline platinum electrodes in aqueous (0.05–1.0)m trifluoromethanesulfonic acid (TFMSA) using the rotating disc and ring-disc electrode techniques at 25°C. Reaction orders with respect to oxygen close to either 1/2 or 1 were found, depending on the TFMSA concentration and platinum surface morphology. At all TFMSA concentrations the formation of H2O2 was enhanced at (1 0 0)-type platinum surfaces. The difference in the electrocatalytic activity of platinum surfaces can be explained through data derived from the OERR formalism proposed by Damjanovicet al. The rate of the direct O2 to H2O electroreduction reaction increased steadily with the cathodic overvoltage irrespective of the platinum surface morphology, whereas a maximum H2O2 formation rate was found at about 0.5 V, depending on the TFMSA concentration. The H2O2 decomposition rate on (1 0 0)-type platinum electrode yielding H2O approached zero within a certain potential range.

A comparative study of electrochemical and optical properties of rhenium deposited on gold and platinum

Rhenium-containing films were grown on gold and platinum after different potentiostatic and potentiodynamic polarizations in the - 0.20 V to 0.70 V range (vs rhe) in aqueous acid perrhenate. Experimental data were obtained using cyclic voltammetry and ellipsometry, from which the thickness and optical indices of the electrodeposited rhenium layer were calculated. Metallic rhenium deposition on gold takes place at potentials within the hydrogen evolution reaction. Rhenium oxide on platinum is formed in the hydrogen adatom potential domain, whereas metallic rhenium is deposited concurrently with the hydrogen adsorption and evolution reactions on the same metal.

Adsorption configurations of ethylene and acetylene on gold

Abstract An improved extended Huckel molecular orbital study was conducted to predict the adsorbate configuration of ethylene and acetylene on Au(100) and Au(111) single crystal cluster surfaces at different simulated electric potentials. Three adsorbed configurations on both cluster surfaces were found for ethylene and acetylene, i.e. π , di- σ and μ -bridging species, whose relative contributions to the total organic surface coverage depend on the electric potential. Acetylene is chemisorbed on gold in a π -bonded mode when a positively charged surface is simulated, whereas di- σ and μ -bridging state are the most likely adsorption modes at negative electric potentials, the binding energies on Au(100) being larger than those of Au(111) clusters. Ethylene, on the other hand, is physisorbed on gold at equilibrium and positive electric potentials, however the chemisorption of the molecule is achieved when a negatively charged surface is simulated. Adsorption of ethylene at potentials below that of zero charge of the gold solution interface was observed. The anodic stripping of adsorbed ethylene at potentials lower than 0.20 V in 1 M H 2 SO 4 (0.25 V in 1 M HClO 4 ) exhibits two residues, in the double layer and in the gold oxide region. On the other hand, the adsorption of acetylene took place in two potential regions, one coinciding with the ethylene adsorption domain, and the other involving potential values positive to that of zero charge.

The Electrochemical Development of Pt(111) Stepped Surfaces and Its Influence on Methanol Electrooxidation

The progress in the preparation of new electrode surfaces by electrochemical treatments exhibiting high faradaic efficiencies towards industrial electrocatalytic processes has gained more attention in todays scientific community. Most of the papers report new catalysts dispersed on different substrates, but some fundamental studies required for electrochemical and physical characterizations are sometimes forgotten. In this paper, we make a full staging of two electrochemical treatments that can be conducted to enhance the electrocatalytic activity of platinum surfaces, such as, electrofacetting through square wave potential programs and constant cathodic polarizations in the net hydrogen evolution region. The cathodic treatment applied at −2 V clearly develops (111) stepped planes similarly to the electrofacetting performed after applying the square wave program between 1.40 V and 0.70 V at 2.5 kHz. The X-ray diffraction patterns of both surfaces as well as on other electrofacetted platinum electrodes are obtained for comparison purposes. Moreover, the electrocatalytic activity towards methanol electrooxidation also exhibits equivalent coulombic efficiencies and 200% higher than on polycrystalline platinum as demonstrated by linear sweep voltammetry and potentiostatic current decays.