M. Lübke

Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentiostatic control. I. Experimental

The experimental characterization of the current/outer potential (I/U) behavior during the electrochemical CO oxidation on Pt(100), Pt(110) and Pt(111) is used as the first step towards a thorough investigation of the processes occurring during the electrochemical formic acid oxidation. The CO study is followed by new cyclovoltammetric results during the electrochemical formic acid oxidation on the corresponding Pt single crystals. At high concentrations of formic acid, the cyclovoltammograms revealed a splitting of the large current peak observed on the cathodic sweep into two peaks whose dependence on scan rate and reverse potential was investigated. It turned out that the presence of a sufficiently large ohmic resistance R was crucial for oscillatory instabilities. Given an appropriate resistance, all three Pt surfaces were found to exhibit current oscillations at both low and high formic acid concentrations. On Pt(100) stable mixed-mode oscillations were observed. In addition, the sensitivity of the o...

Experimental and theoretical description of potentiostatic current oscillations during H2 oxidation

A recently proposed model to describe the galvanostatic oscillating oxidation of hydrogen on Pt in the presence of copper and chloride is also able to explain the potentiostatic behaviour of the experimental system. For sufficiently high series resistance it exhibits current oscillations on a branch where the stationary current-voltage curve has a positive polarization slope. This is in contrast to most of the potentiostatic oscillators examined and described previously, where the oscillations are caused by an interplay between the IR drop and a region of negative polarization slope. The mechanism is discussed and compared with other models which describe potentiostatic oscillators. The main difference is shown to lie in the existence of a hidden negative impedance, which is detected experimentally by impedance spectroscopy.

A hierarchy of transitions to mixed mode oscillation in an electrochemical system

Abstract The dynamic behavior of the electrocatalytic oxidation of hydrogen on a platinum anode in the presence of Cu2+ and Cl− ions under galvanostatic conditions has been studied experimentally in two control parameters: current density and copper ion concentration. This parameter plane can roughly be divided into three broad regions: A steady state is found at low values of the current density, at medium values the system shows small amplitude oscillations, whereas at high current densities typical mixed mode oscillations (MMOs) are found. Depending on the Cu2+ concentration, the transition from the small amplitude to the mixed mode regime takes place from a simple periodic, period doubled, or chaotic attractor arising from a Feigenbaum route. In this paper we focus on the characteristics of these different transitions to MMOs and try to construct a bifurcation diagram. The bifurcation from the small chaotic attractor to MMOs very likely constitutes an interior crisis (a bifurcation at which the small chaotic attractor is destroyed by colliding with a stable manifold of a saddle type limit set). At lower copper ion concentrations, the transition to MMOs moved through the period-double cascade. We compile different properties of the bifurcation diagram and discuss a scenario which fits the observations in a consistent way.

Modelling oscillations in galvanostatic H2 oxidation at Pt in the presence of metal ions

Abstract A simple model of the galvanostatic oscillations that occur during the oxidation of hydrogen at platinum electrodes in the presence of electrosorbing metal ions and specifically adsorbing anions is presented. It consists of three ordinary differential equations describing the degree of coverage of the electrode with underpotential-deposited metal and with specifically adsorbed anions; the third variable is the potential drop across the double layer. This model is able to reproduce the potentiostatic experiments of the system. The galvanostatic behavior is examined by integration of the equations and by bifurcation analysis. The experimental dependence on the control parameters is reproduced in all main features. The parameters were altered to check the model for robustness and to determine the essential processes leading to oscillations. Mixed-mode and period-doubled oscillations were also found. Inclusion of a fourth variable, the concentration of metal ions in front of the electrode, leads to a better quantitative description of oscillations at low metal ion concentrations.

Modeling galvanostatic potential oscillations in the electrocatalytic iodate reduction system

Abstract An experimental and theoretical description of the oscillatory electrocatalytic iodate reduction system in alkaline solution is presented. Experimental measurements suggested the crucial role of a negative differential resistance as well as the presence of hydrogen evolution in the oscillatory instability. A simple kinetic model with one chemical species was developed which involved: (i) an iodate reduction current with N-shaped potential dependence due to a Frumkin repulsion effect; and (ii) an additional ‘iodate-independent’ current providing process. The dynamical behavior of the model compares favorably with experiments as far as voltammetric profiles, stirring effects and bifurcations between stable and oscillatory states are concerned. It is further shown that the model can account for oscillatory states in a number of related electrocatalytic systems.

Photoelectrochemical Studies of Nearly Intrinsic Semiconductors Made Possible by Using Photoconductivity

Donnees sur la generation du courant photoelectrique par des electrodes en semiconducteurs presque intrinseques. Cas des plaquettes de GaS et de CdS

Structural effects on the dynamics of an electrocatalytic oscillator

Abstract The electrocatalytic oxidation of hydrogen in the presence of Cu 2+ and Cl − ions under galvanostatic conditions has been studied on the three low-index plane surfaces of platinum. Over the parameter ranges investigated and with the electrode pretreatment used, no oscillations were observed on Pt(110), whereas only simple periodic oscillations were seen with Pt(111). The Pt(100) surface, on the other hand, exhibited a variety of oscillation types, including simple periodic oscillations, period-doubling, chaos and mixed-mode behavior as two parameters, copper ion concentration and current density, were varied. The dynamics on Pt(100) turned out qualitatively very similar to that observed previously on polycrystalline platinum electrodes.

The role of triplet states in dye sensitization of ZnO electrodes

Abstract The quantum efficiency of photooxidation of a number of xanthene dyes at ZnO single crystal electrodes has been found to depend on the solution concentration of a triplet quenching agent, FeCN 4− 6 , providing evidence for triplet state participation in the oxidation reaction.

Quantitative Modeling of the Oscillatory Electrooxidation of Hydrogen on Pt in the Presence of Poisons

Abstract A quantitative model of oscillations observed during hydrogen oxidation on platinum in the presence of electrosorbing metal ions and specifically adsorbing anions is presented and the model predictions are compared with experiments. Mass and charge balances of all reactants lead in a first step to a seven variable model which is governed by reaction steps that have been widely studied. We demonstrate that attractive interactions between metal and halide ions on the electrode surface, which were recently reported, are crucial for the observed dynamics. The model parameters were almost exclusively taken out of the literature. The model is then reduced to its minimal form without losing dynamic features arriving at a four variable system. Experimental time series of three of the four variables of the model and measured bifurcation diagrams are presented. It is shown that the integrated time evolution and the calculated bifurcation diagrams of the model agree almost quantitatively with the experiment.

Electrochemistry of n-Type MoSe2: A Comparison of Photocorrosion and Dissolution under High Anodic Bias

Electrochemistry / Semiconductor / Photocorrosion / Surface states / MoSe2 It has been found that «-type MoSe2 is under illumination anodically oxidized to Mo(VI) and Se(IV) while under sufficient anodic bias in the dark, Mo(VI) and Se(VI) is generated and the corrosion occurs much more locally. This is explained by the different ways ofcharge generation. Photocorrosion is induced by holes which are homogeneously generated underneath the surface by light absorption. Limited localization is caused by the differences in the reactivity of steps and structural defects on the surface. Corrosion in the dark however is extremely localized because the initial process is tunnelling ofelectrons from surface states into the conduction band, which is particularly favoured at a few defective areas on the surface. At such a high local current density, oxidation to a higher valency is enhanced.