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Katharina Krischer

Publications and source records attributed to Katharina Krischer.

8 recordsLinked to original sources
PubMed2006

Two-dimensional electrochemical turbulence during the electrodissolution of metal disk electrodes: Model calculations.

We present numerical studies of the spatio-temporal dynamics of disk electrodes with local limit cycle oscillations. The simulations are done with a realistic 3-D geometry of the electrochemical cell and disk-shaped working electrodes (WE). Spatio-temporal chaos is shown to exist from a critical electrode size onwards. It is analyzed by Karhunen-Loève decomposition and Hilbert transform. The former shows that the chaos becomes more complex with increasing system size, the latter allows features that generate the spatio-temporal complexity to be identified, namely, spatially extended 1-D phase defects and topological defects.

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PubMed2005

A hierarchy of global coupling induced cluster patterns during the oscillatory H2-electrooxidation reaction on a Pt ring-electrode.

We report experimental results on spatiotemporal pattern formation during the oscillatory hydrogen electrooxidation reaction on a Pt ring-electrode under negative (desynchronizing) global coupling (GC). Spatially one-dimensional profiles of the interfacial potential drop along the angular direction of the ring electrode are recorded by means of a potential probe. The dynamics is investigated as a function of two control parameters, the applied voltage U and the strength of the global coupling. The latter is adjusted either by varying the distance between the working electrode (WE) and the reference electrode (RE) or by inserting a negative impedance device in series with the WE. In the absence of global coupling, uniform oscillations were destabilized by migration coupling, and electrochemical turbulence developed at large values of U (H. Varela, C. Beta, A. Bonnefont and K. Krischer, Phys. Rev. Lett., 2005, 94, 174104; ). Already low global coupling strengths sufficed to suppress turbulence. Instead, regular two-phase clusters formed. At higher coupling strength, a second type of two-phase cluster was observed as well as two types of irregular cluster patterns, which were connected with an irregular motion of the cluster boundaries and the emergence and disappearance of clusters through splitting and merging of the boundaries, respectively. Upon increasing the coupling strength even further, five-phase clusters were stabilized and at the highest coupling strength applied the cluster patterns transformed into strongly modulated pulses. The two types of two-phase clusters and the five-phase clusters are analyzed employing several signal processing techniques.

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PubMed2005

Transitions to electrochemical turbulence.

We report experimental evidence of transitions from limit cycle oscillations through a phase turbulent regime to space-time defect turbulence in a spatially (quasi-)one-dimensional electrochemical system with nonlocal coupling. The transitions are characterized in terms of the defect density, the Karhunen-Loève decomposition dimension, and a measure of the degree of spatial correlation in the data. Furthermore, these quantities give the first experimental confirmation that the spatial coupling range in electrochemical systems indeed depends on the distance between the working and the counterelectrode.

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PubMed2005

Stationary spatial patterns during bulk CO electrooxidation on platinum.

We present experimental studies and mathematical modeling on pattern formation during bulk CO electrooxidation on Pt ring electrodes. Profiles of the interfacial potential drop in front of the working electrode were recorded with a potential probe. Stationary self-organized potential patterns were observed under potentiostatic conditions in dilute acidic and basic supporting electrolytes. The amplitude and shape of these potential patterns can be modified by an appropriate local perturbation of the interfacial potential drop. A mathematical model of the formation of these patterns reveals that the homogeneous state becomes unstable through a subcritical Turing-like bifurcation and that several patterned electrode states coexist in wide parameter ranges.

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PubMed2002

Organic compound adsorption on Au(111): simultaneous SHG/electrochemical studies.

Camphor has attracted considerable attention in electrochemical research because it adsorbs strongly on metal surfaces. Due to its surface activity it is able to inhibit surface reactions. Recently, camphor has been used in investigations of nonlinear surface dynamics and pattern formation. Details regarding its influence on the morphology of the metal surface, the significance of surface reconstruction, structural changes in the camphor adlayer and oxide formation remain unclear. We employ second harmonic generation (SHG) to elucidate the structural and electronic behaviour of Au(111) surfaces during camphor adsorption and desorption processes. Our technique allows measurement of the anisotropy of the SHG intensity while simultaneously performing cyclic voltammetry (CV) using the hanging meniscus configuration. The anisotropy data can be refined, yielding the symmetry components of the second order susceptibility tensor chi2 which are analysed as a function of external potential and related to the system's electrochemical behaviour. Results of SHG measurements are presented together with corresponding CV data and discussed with respect to the open questions mentioned above.

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PubMed2001

Fronts, Waves, and Stationary Patterns in Electrochemical Systems In the appendix we list the minimal equations (prototype equations) that describe the phenomena of self-organization discussed in the individual sections.

Oscillatory behavior has been observed for almost all electrochemical reactions in a certain, although sometimes small, range of external parameters. Only in the past ten years has it been possible, however, to find a common explanation for the occurrence of these temporal self-organization phenomena of chemically completely different electrochemical reactions. The breakthrough was achieved because new methods and concepts, which had been developed in nonlinear dynamics to describe the spontaneous formation of order in various disciplines, could be applied. This development in turn was only possible because the underlying laws are universal at a certain abstract level. Oscillations are only one possible manifestation of nonlinear behavior. Examples of other features that are often closely associated with temporal instabilities are spatial structures and waves. Initiated by the theoretical progress and the development of new experimental techniques, spatial pattern formation in electrochemical systems has been targeted for investigations in the past few years. Based on these investigations, it can be predicted under which conditions temporal or spatial pattern formation can be expected. Furthermore, the possibility of predicting the occurrence of instabilities indicates that it might be feasible to exploit nonlinear effects to increase, for example, the yield of electrocatalytic reactions. Here we discuss physicochemical mechanisms that lead to pattern formation in electrochemical systems. At the same time, we stress the generic principles that are responsible for self-structuring processes in many chemical and biological systems.

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