Modification of platinum surfaces by spontaneous deposition: Methanol oxidation electrocatalysis

The presence of a second metal on platinum surfaces affects the performance of methanol oxidation. However, most of the electrocatalytic reactions are studied by using electrochemically deposited platinum alloys, but in the case of spontaneous deposition the situation is not so clear since the surface distribution, stability and morphology are usually not well documented. The formation of surface decorated samples on mono- and poly-crystalline platinum is followed by electrochemical and spectroscopic techniques and analysis of their performance towards methanol adsorption and oxidation compared with that on pure platinum. Pt/Sn and Pt/Ru are of special interest because of their well-known performance in methanol fuel cells. Methanol oxidation on Pt(111)/Ru, Pt(111)/Sn and Pt(111) shows that ruthenium is the only one able to promote the reaction since the simultaneous dissolution of tin occurs and competes with the process of interest. The in situ infrared spectroscopy is used to compare methanol oxidation on Pt(111)/Ru and Pt(111) in acid media using p-polarized light. The formation of bridge bound carbon monoxide is inhibited in the presence of ruthenium ad-species, whereas on Pt(111) the three adsorption configurations are observed. Linear sweep polarization curves and Tafel slopes (calculated from steady state potentiostatic plots) for methanol oxidation are compared on polycrystalline surfaces modified by tin or ruthenium at different coverages. There is almost no change in the Tafel slopes due to the presence of the foreign metal except for Pt/Ru, where a 0.09 V decade⁻¹ slope was calculated below 0.55 V due to hydroxyl adsorbates on ruthenium islands. The anodic stripping of methanol residues on the three surfaces indicates a lower amount of carbon monoxide-type adsorbates on Pt/Ru, and the simultaneous tin dissolution process leading to residues oxidation on Pt/Sn electrodes.     

Heterogeneously assisted oxidation of adsorbates from carbonmonoxide, methanol and ethanol by hydrogen peroxide solutions on platinum electrodes in sulphuric acid

The influence of hydrogen peroxide on the adsorption and oxidation of carbon monoxide, methanol and ethanol adlayers on porous Pt electrodes were studied in 2 M sulphuric acid solution by means of cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). The oxidation of adsorbed species is observed at electrode potentials far less negative than those required for electrochemical adsorbate oxidation. The oxidation by H₂O₂ is dependent on its concentration in solution, as well as on the adsorbates and their coverages. In all cases the isolated adlayers are oxidised by dissolved H₂O₂. However, the presence of H₂O₂ during adsorption partially inhibits adlayer formation from CH₃OH and C₂H₅OH, but avoids almost completely the adsorption of carbon monoxide. The removal of the residues from the surface by dissolved hydrogen peroxide probably occurs through O_ad species formed during the heterogeneous decomposition reaction of H₂O₂ on Pt.     

New experimental evidence on the formation of platinum superactive sites in an electrochemical environment

New electrochemical results are found after the application of a constant cathodic polarization to the platinum/aqueous acid solution interface within the hydrogen evolution region. Voltammograms reveal highly stable anodic peaks emerging in the double-layer region assigned to the so-called “superactive” states. The severe cathodization produces the disruption of the platinum/aqueous solution interface leading to an open outer layer composed of metallic species highly coordinated to the solvent. These active sites are related with electrocatalytic features since the onset potential for water discharge is diminished. Voltammetry and modulated voltammetry explain the fact that platinum species highly coordinated to the solvent can be more oxidized since the reduction potential shifts toward more negative values. The alkalinization of the interface and the formation of new surface oxides in a different potential range are proposed. The latter produces electrocatalytic effects on methanol electrooxidation.     

A novel Pt/Cr/Ru/C cathode catalyst for direct methanol fuel cells (DMFC) with simultaneous methanol tolerance and oxygen promotion

New carbon supported electrocatalysts Pt/Cr/Ru with distinct compositions and preparation methods were studied with the help of different electrochemical and spectroscopic techniques. The purposes of obtaining these catalysts lie on their possibilities towards methanol/oxygen fuel cells. In this sense, the oxygen reduction reaction and methanol oxidation reaction were analyzed using stationary and fluid dynamic methodologies. Pt_7.8/Ru_1.3/Cr_0.5 and Pt_8.0/Ru_2.0/Cr_0.1 were the most interesting prepared substrates, on which the first one shows the best catalytic properties towards methanol oxidation and the second the finest performance towards oxygen reduction reaction. Reaction orders with respect to oxygen for the oxygen reduction reaction were obtained being equal to ½ at potentials lower than 0.80 V for both catalysts. Polarization curves run for this reaction depicted two Tafel slopes, i.e. 0.09 V dec⁻¹ above 0.8 V and 0.20 V dec⁻¹ below 0.8 V for both catalysts. An analysis of the most likely mechanism for the oxygen reduction was proposed on the base of those reaction orders and Tafel slopes.     

Kinetics of methanol electrooxidation on Pt/C and PtRu/C catalysts

This paper analyzes the performance of platinum and platinum:ruthenium carbon-supported catalysts modified by the application of in-situ cathodic polarizations towards the methanol oxidation reaction. These new electrodes are characterized by electrochemical techniques together with transmission electron microscopy images to envisage the dispersion of the catalyst. We measure methanol electrooxidation current transients, fitting the results with a general kinetic equation for a mixed mass and charge transfer processes for adsorbed reactant species. The kinetic equation also helps to predict the exponent of the chronoamperometric decay as directly related to the fractal dimension of the catalyst surface and to discuss the possible processes involved in the electrocatalytic reaction.     

Film formation and surface growth on tin electrodes in bicarbonate solutions: an impedance spectroscopy study

The electrochemical behaviour of potentiodynamically formed thin anodic films of polycrystalline tin in aqueous sodium bicarbonate solutions (pH ≈ 8.3) were studied using cyclic voltammetry and electrochemical impedance spectroscopy. Different equivalent circuits corresponding to various potential regions were employed to account for the electrochemical processes taking place under each condition.     

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.     

Gas diffusion electrodes for methanol electrooxidation studied by a new DEMS configuration: Influence of the diffusion layer

A novel differential electrochemical mass spectrometry (DEMS) cell has been developed to study gas diffusion electrodes (GDEs) used in fuel cells under operating conditions. In this way, catalytic and diffusion properties of the electrodes can be evaluated at the same time. Moreover, DEMS, allows the detection of volatile and gaseous products and intermediates generated in the electrochemical reactions with good sensitivity. In this way, CO₂ conversion efficiencies are evaluated during alcohol oxidation reaction. In the present communication, the electrochemical behaviour towards hydrogen evolution and CO and alcohol electrooxidation at different platinum-based catalysts has been studied using the new DEMS cell configuration. The relative yields of CO₂ and by-side products during methanol oxidation have been evaluated to determine the CO₂ conversion efficiency. In addition, the diffusional properties of diverse GDEs have been considered.     

The inhibition of Ni corrosion in H2SO4 solutions containing simple non-saturated substances

The inhibition of Ni electrodissolution in H₂SO₄ solutions containing carbon monoxide, ethylene or acetylene was investigated after adsorption equilibrium at room temperature. The interaction of these molecules with Ni was followed by the anodic stripping of adsorbed residues. The Ni electrodissolution inhibitory effect caused by the different adsorbates increases in the following order: ethylene < carbon monoxide < acetylene. Anodic stripping curves were simulated by applying a numerical method, such as Runge-Kutta's, and the different voltammetric contributions calculated by numeric deconvolution. These analyses provide a further insight into the mechanism of the Ni electrodissolution in acid containing carbon monoxide, ethylene or acetylene.     

The anode efficiency in methanol direct fuel cells—A chronoamperometric approach

The methanol oxidation reaction has been studied on polycrystalline platinum surfaces, cathodically treated platinum and platinum surfaces modified by a second (or a third) metal deposited spontaneously in 0.50 M methanol + 0.5 M sulphuric acid using chronoamperometry. The electrochemical behaviour of methanol on the three types of electrodes shows that the initial oxidation rate is higher on the cathodically treated electrodes, whereas it depends on the nature of the second spontaneously deposited metal; silver shows inhibition and ruthenium and osmium exhibit a catalytic behaviour for the same 20% amount of surface metal ad-atoms.