C.E. Mayer

An EIS study of aluminium barrier-type oxide films formed in different media

Abstract Impedance measurements were performed on aluminium in different aggressive (0.5 M NaCl) and non-aggressive (0.5 M Na 2 SO 4 , 0.16 M NH 4 -tartrate) electrolytes. Similar impedance responses of the formed passive film were found. The capacitive behaviour observed at high frequencies is related to the thickness and dielectric properties of the barrier oxide film. The impedance spectra obtained at low frequencies were interpreted in terms of a diffusion-controlled process within the oxide film. The Faradaic impedance given by the metal dissolution at the oxide film/electrolyte interphase is strongly affected by the anion type. This fact and the local pH developed at the interphase passive film/electrolyte determine the chemical stability of the film. The increased impedance observed in chloride solution under rotation conditions is explained by a local change of pH at the film/solution interphase. EIS can also be used to study pitting phenomena. In the stabilized active pitted area an ohmic control given by the presence of a salt film is responsible for the capacitive and low frequency inductive behaviour obtained.

Structural changes of surface oxide layers on palladium

Abstract The anodic formation of oxide layers on Pd and their cathodic reduction have been investigated in perchloric acid solutions using cyclic voltammetry and linear sweep polarization. The structure of the voltammograms depends strongly on the anodic polarization and the polarization routine, but less on the sweep rate. From the measured cathodic charge amounts of the reduction process as functions of these parameters, it is assumed that slow structural changes of the oxide layer occur.

AC-Impedance measurements on aluminium in chloride containing solutions and below the pitting potential

Impedance measurements were performed on aluminium in 0.5M NaCl in the frequency range 5×10−4-104 Hz and before the onset of pitting corrosion. The behaviour of the system was characterized by a high frequency capacitive loop related to the thickness of the oxide film, an inductive loop at medium frequency, which was interpreted on the basis of the dielectric relaxation model proposed by Dignam, and a second capacitive loop obtained at low frequencies which was ascribed to the film dissolution through the formation of a soluble chloride containing aluminium salt.

Ag UPD on Au(100) and Au(111)

The underpotential deposition (UPD) of Ag in the systems Au(hkl)/Ag + , ClO - 4 and Au(hkl)/ Ag + , SO 2- 4 with (hkl) = (100), (111) was studied by in situ STM and different electrochemical techniques including charge-coverage measurements with the twin-electrode thin-layer (TTL) technique. Ag UPD was found to occur stepwise and similar in both systems. At relatively high underpotentials expanded and commensurate Ag adlayers Au(100)-c(√2x5√2)R45°Ag and Au(111)-(4x4)Ag are formed. At low underpotentials, condensed commensurate overlayers with Au(hkl)-(1 x 1)Ag structures are formed via a first order phase transition. On Au(111), the condensed Ag phase is preferentially created on monatomic steps, whereas on Au(100) a simultaneous 2D nucleation of Ag on flat terraces is observed. Electrosorption valency measurements in the system Au(hkl)/Ag + , SO 2- 4 show γ = z = 1, indicating that coadsorption or competitive adsorption of anions can be excluded. Electrochemical and in situ STM results indicate similar charge-coverage behaviour in the system Au(hkl)/Ag + , ClO - 4 . Ag UPD on Au(hkl) occurs at positive potentials with respect to the potentials of zero charge of Au(hkl) and Ag(hkl). Therefore, a nearly constant anion coverage in the entire Ag UPD range cannot be excluded.

Electrochemical nucleation and growth of copper on polycrystalline palladium

Abstract The electrochemical nucleation of copper on palladium has been investigated by applying the potential step technique. The nucleation process was found to be progressive and interpretation of current-time transients over a wide overpotential interval made it possible to determine the overpotential dependence of the steady state nucleation rate, the size of the critical nucleus and the work of nucleation.

Mixed electrosorption processes of silver and oxygen underpotential deposition on platinum surfaces

Abstract The UPD of silver on platinum was studied as a function of the initial oxidation state of the surface. Twin-electrode thin-layer and semi-infinite voltammetric measurements show a strong interaction between silver and oxygenated species. The suerimposed processes are described in terms of the mixed electrosorption valency. However, the irreversibility of both processes restricts exact thermodynamic analysis.

Structural investigation of the initial stages of copper electrodeposition on polycrystalline and single crystal palladium electrodes

Abstract The electrodeposition of Cu on polycrystalline and single crystal Pd electrodes has been studied by combining electrochemical techniques with optical ones. On Pd(111) the deposition process begins with the formation of quasi-two-dimensional Cu crystallites which transform into three-dimensional truncated pyramids with increasing deposition time. On Pd(100) a multilayer process is observed together with a pyramidal outgrowth on the top of the underlying layers. The morphology of deposits on the polycrystalline substrate, depends strongly on the surface pretreatment. On mechanically polished electrodes Cu crystallites, some of them with well defined geometric shapes, are observed all over the surface. On the contrary, on electrochemically polished substrates different grains appear on the surface. Depending on the crystallographic orientation of each grain, Cu deposits as those described for Pd(111) and Pd(100) are observed. This indicates that the polycrystalline material is composed mainly of long range order regions possessing a (111) and a (100) orientation, in agreement with the results obtained by comparing the behaviour of each substrate within the underpotential deposition range.

Electro‐Oxidation of Methanol on PtRu Nanostructured Catalysts Electrodeposited onto Electroactivated Carbon Fiber Materials

The surface of different carbon substrates, such as glassy carbon (GC), graphite cloth (GC‐10), graphite felt (GF‐S2), and carbon fiber paper (CFP) was modified by electrochemical treatment to generate high concentrations of oxygenated functional groups. These activated carbons were used as substrates for the simultaneous electrodeposition of Pt and Ru by a double potentiostatic pulse program. The different catalyst/carbon systems were evaluated as electrodes for methanol oxidation in acid solution. Comparing the results for the oxidized and nonoxidized substrates, the oxidation of the different carbon materials prior to the catalyst deposition was found to lead to an increase in the electrode activity for methanol oxidation. This enhancement could be associated with a remarkable improvement of metal dispersion, reduction of particle size, and a higher active surface area of the catalyst. The electrodes prepared with oxidized graphite felt exhibited the greatest catalytic activity.

STM tip-induced local electrochemical dissolution of silver

Local dissolution/deposition processes under in situ scanning tunneling microscopy (STM) imaging conditions are studied in the systems Ag(111)/Ag+, ClO4− and Ag(111)/Ag+, SO42−. The results show that in both systems the local kinetics of these processes strongly depend on the polarization conditions. At STM-tip potentials more positive than the Ag/Ag+ equilibrium potential, a local dissolution of the Ag(111) substrate is observed even at cathodic substrate overpotentials at which the overall substrate current density is cathodic. This tip-induced Ag dissolution is in agreement with results obtained recently in the system Cu(111)/Cu2+. The enhanced local Ag dissolution is explained by a reduced Ag+ concentration underneath the STM tip promoted by both an electrostatic repulsion of Ag+ and a reduction of the mass transport due to the shielding effect of the tip. The possibility for a preparation of negative Ag nanostructures by STM tip-induced electrochemical dissolution is demonstrated.

Electrooxidation of Mn(II) to MnO2 on graphite fibre electrodes

Electrooxidation of Mn2+ to MnO2 on carbon felt and carbon cloth electrodes (C-materials) in a weak sulfuric acid electrolyte was investigated using stationary and rotating electrodes. These materials exhibit better performance than more conventional materials such as lead. Scanning electron micrographs of the deposits show a uniform cylindrical growth of MnO2 encapsulating the carbon fibres. The deposits spontaneously fracture following straight-lines indicating a fibrous microstructure. X-ray diffraction reveals a γ-MnO2 structure for all deposits.