K. Jüttner

AC-impedance measurements on aluminium barrier type oxide films

Abstract AC -impedance measurements in the frequency range 5 × 10 −3 Hz ≤ f ≤ 10 4 Hz were performed in the system polycrystalline pure aluminium/0.16 M NH 4 -tartrate at pH 5–7, in the potential range −900 mV ≤ E H ≤ 400 mV. The impedance of the system is characterized by a high-frequency capacitive and a low-frequency inductive behaviour. The capacity is unequivocally related to the thickness and dielectric properties of the barrier oxide film. The faraday impedance corresponds to a metal dissolution process at the oxide film/electrolyte interphase and can be interpreted by a model previously proposed by Armstrong.

Frequency response analysis of the Ag/Ag+ system: a partially active electrode approach☆

The dc and ac polarization behaviour of the Ag/Ag+ system was studied theoretically and experimentally. The model proposed is based on bulk diffusion towards a partially active electrode surface. The experimental data obtained on stationary and rotated disc electrodes of polycrystalline and monocrystalline silver in 0.1 M AgNo3 + 1.0 M HClO4 solution are interpreted in terms of different model parameters characterizing the dc and ac behaviour of the system quantitatively. It is demonstrated that surface relaxation may give rise to an inductive part in the ac-impedance diagram.

Lead adsorption on silver single crystal surfaces

Abstract Twin-electrode thin layer experiments have been carried out in the system Ag(hkl)/Pb 2+ , ClO 4 − in order to determine independently the charge and Pb 2+ coverage fluxes of the lead underpotential adsorption. The electrosorption valency was found to be equal to the charge number of the lead ion. Therefore, the sorption behaviour of the system can be described by the metal monolayer model. The peak potentials of the cyclic voltammograms as well as the saturation coverages are only slightly dependent on the orientation of the silver single crystal surfaces used. Hypothetical superlattice structures for the monolayer formation are discussed. The Kolb-Gerischer linearity rule is considered critically.

Voltammetric investigation of thallium adsorption on silver single crystal electrodes

The adsorption behaviour of Tl+ on Ag(100) and Ag(111) in 0.5 M NaClO4 and Na2SO4 has been studied by voltammetric methods including flow-through thin layer voltammetry (FTTL) and single electrode thin layer voltammetry (STL). The Tl-deposits formed on both electrode substrates exhibit the ideal charge stoichiometry of metal monolayer adsorbates, indicating the absence of anion adsorption. The adsorption on Ag(100) can be characterized by an equilibrium Γ(E)-isotherm compatible with a stepwise formation of three stable sorption layers. The Tl-deposit on Ag(111) features the successive build up of two adsorbate layers. In case of incomplete formation of the first layer, however, a slow structural transformation occurs, involving a strong interaction with the Ag(111) substrate and accompanied by partial Tl-desorption. The complex behaviour of this system is interpreted in terms of internal strain within the Tl-superlattice structures.

Electrocatalysis of oxygen and hydrogen peroxide reduction by UPD of bismuth on poly- and mono-crystalline gold electrodes in acid solutions

The electrocatalytic effect of underpotential deposition (UPD) of bismuth on the cathodic reduction of oxygen and hydrogen peroxide has been investigated on poly- and mono-crystalline (111) and (100) gold electrodes in 0.5 M HClO4 solution. On the bare gold substrates an incomplete 2-electron reduction of O2 to H2O2 predominates, which in the presence of Bi3+ in the solution is positively catalysed indicated by a decrease of the overvoltage and an increase of the rate of H2O2 reduction. The catalytic effects can be correlated with the degree of Bi adsorbate coverage and the structural arrangement of Bi adatoms depending on the crystallographic orientation of the substrate. The effect of a mixed anion (Cl−) and cation (Bi3+) adsorption on the reduction process was also studied.

Oxygen reduction electrocatalysis by underpotential deposited metal atoms at different single crystal faces of gold and silver

Electrocatalytic effects of underpotential lead and bismuth adsorbates on the cathodic reduction of O2 and H2O2 in perchloric acid solution obtained on rotating disc Au and Ag single crystal surfaces of (111), (100) and (110) crystallographic orientation are discussed. On the clean Ag substrates the complete 4-electron reduction of O2 to H2O takes place whereas on Au the incomplete 2-electron reduction to H2O2 predominantly occurs, exhibiting pronounced structural effects. The underpotential deposition of Pb on Ag surfaces leads to a partial inhibition of the O2 reduction whereas on Au (111) and (100) surfaces positive catalytic effects are observed. Measurements of H2O2 reduction indicate that an important part of the catalytisis on Ag and Au can be attributed to a decrease or increase of the rate of further reduction of H2O2 to H2O. The results are discussed in terms of different adsorption models for the O2 molecule.

Electrocatalysis of oxygen reduction by UPD of lead on gold single-crystal surfaces

Abstract The electrocatalytic effect of underpotential lead adsorbates on the cathodic reduction of O2 and H2O2 has been investigated in 0.5 M HClO4 solutions on rotating disc gold (111), (100) and (110) single-crystal surfaces. On the bare gold substrates a two-electron reduction of O2 to H2O2 predominantly takes place. In the presence of underpotential lead adsorbates a positive catalytic effect is observed, dependent on the degree of lead adsorbate coverage and on the crystallographic orientation of the substrate. Measurements of H2O2 reduction in solution free of O2 indicate that the positive catalysis can be mainly attributed to an increase of the rate of H2O2 reduction to H2O. The results are discussed in terms of different O2 adsorption models and are compared with those obtained previously on silver single-crystal surfaces.

Characterization of the electrocatalytic properties of amorphous metals for oxygen and hydrogen evolution by impedance measurements

The electrocatalytic properties of amorphous metals containing Fe, Co, and Ni as main components have been studied for the oxygen and hydrogen evolution in 1 M KOH at four different temperatures, T = 298, 323, 348, and 363 K. The methods employed were ac-impedance, cyclic voltammetry and steady state polarization techniques. For comparison, measurements were carried out on polycrystalline Fe, Co, Ni, and Pt. It was found that the amorphous metal Co50Ni25Si15B10 is the best catalyst for the oxygen evolution and Fe60Co20Si10B10 is a good catalyst for both oxygen and hydrogen evolution reactions comparable with Pt.

Underpotential alloy formation in the system Ag(hkl)/Cd2+

The underpotential deposition (UPD) of Cd on “real” and “quasi-ideal” silver single crystal surfaces of (111) and (100) crystallographic orientation as well as on polycrystalline silver substrate has been investigated in 0.5 M Na2SO4 supporting electrolyte solutions. At high underpotentials 100 < ΔE < 350 mV, the UPD is characterized by a quasi-reversible adsorption/desorption of Cd whereas at low underpotentials. ΔE < 50 mV, an increase of the anodic stripping charge with the polarization time is found due to the formation of an AgCd alloy at the substrate surface. The time dependence of this process can be described by a parabolic rate law, the rate constant of which is a function of ΔE and temperature T. Relatively low activation energies of about 70 kJmol−1 were determined from measurements at 293 ⩽ T ⩽ 338 K. The results are discussed in terms of a semi-infinite-linear diffusion model. The alloy formation process is assumed to be initiated by a place exchange between Ag substrate atoms and vacancy sites within a mobile Cd adsorbate layer thus forming the initial stage of a highly distorted AgCd alloy. The further growth will take place by the movement of Ag atoms through the vacancy-rich surface alloy and the simultaneous deposition of Cd at the interface AgCd/Cd2+.

The electrocatalytical influence of underpotential lead and thallium adsorbates on the cathodic reduction of oxygen on (111), (100) and (110) silver single-crystal surfaces

Abstract The influence of underpotential Pb and Tl adsorbates on the electrochemical reduction of oxygen on rotating-disc Ag(111), (100), and (110) single-crystal surfaces has been studied in aerated 0.5 M HClO 4 solutions at various concentrations of Cl − . On the bare silver substrates oxygen is reduced completely to H 2 O. Depending on the degree of coverage and the structural arrangements of Pb and Tl adsorbates on the different crystal planes, a partial inhibition of the oxygen reduction is obtained predominatly leading to the formation of the stable intermediate, H 2 O 2 . In the presence of Cl − ions in solution, the overvoltage for charge-transfer controlled oxygen reduction increases according to (∂ E /∂ log c Cl − ) i_ =−60 mV, due to a specific adsorption of chloride on the silver substrate. In 0.5 M HCl solutions a stimulating effect on the oxygen reduction induced by the underpotential deposition of Pb has been found, which can be interpreted in terms of a competitive adsorption-desorption mechanism involving a replacement of chloride by lead.