Adrian Birzu

Modelling of spatiotemporal patterns during metal electrodissolution in a cell with a point reference electrode

A model for the simulation of experimentally observed spatiotemporal patterns in the bistable and oscillatory regions during the electrodissolution of metals under potentiostatic control is presented. A two-dimensional geometry was used with a point-like reference electrode located in the electrolyte between the working and the counter electrodes. The dynamical variables of the model are the potential drop across the electric double layer and the proton concentration near the working electrode surface. The effects of changing parameter values such as cell geometry, electrolyte conductivity, and double layer capacitance on time series and spatial patterns are presented. In a bistable system the reference electrode position influences both the front behavior and also the structure (uniform or nonuniform) of the steady state. In the oscillatory region the direction of the passivation wave can be changed by changes in the location of the reference electrode. An increase in electrode length can change a periodic oscillation with almost uniform spatial distribution to a chaotic behavior with nonuniform pattern.

Resonance tongues in a system of globally coupled FitzHugh–Nagumo oscillators with time-periodic coupling strength

We investigate the dynamics of a population of globally coupled FitzHugh-Nagumo oscillators with a time-periodic coupling strength. While for synchronizing global coupling, the in-phase state is always stable, the oscillators split into several cluster states for desynchronizing global coupling, most commonly in two, irrespective of the coupling strength. This confines the ability of the system to form n:m locked states considerably. The prevalence of two and four cluster states leads to large 2:1 and 4:1 subharmonic resonance regions, while at low coupling strength for a harmonic 1:1 or a superharmonic 1:m time-periodic coupling coefficient, any resonances are absent and the system exhibits nonresonant phase drifting cluster states. Furthermore, in the unforced, globally coupled system the frequency of the oscillators in a cluster state is in general lower than that of the uncoupled oscillator and strongly depends on the coupling strength. Periodic variation of the coupling strength at twice the natural frequency causes each oscillator to keep oscillating with its autonomous oscillation period.

Spatiotemporal patterns on a disk electrode: Effects of cell geometry and electrolyte properties

Spatiotemporal patterns on a disk electrode are simulated with a model for the electrodissolution of a metal that exhibits passivity; a 3-dimensional geometry with a point reference electrode is used. The two variables of the model are the potential drop across the electric double layer and the proton concentration near the surface of the working electrode. Non-local coupling among reaction sites is through the electric field. Several patterns seen previously in experiments are reproduced in the simulations and the dependence of transitions among them on system geometry, electrolyte conductivity and double layer capacitance are explored. For example, a state with an inhomogeneous oscillation is changed to one with a rotating wave as the reference electrode is moved closer to the working electrode and negative coupling develops; under other conditions the negative coupling produces anti-phase oscillations.

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.

Confined Spatio-Temporal Chaos During Metal Electrodissolution: Simulations

The electrodissolution of metal electrodes exhibits often sustained oscillations of the current density. We study the spatio-temporal dynamics of a metal disk electrode embedded in an insulator in a cylindrical electrochemical cell with a ring-shaped CE in the oscillatory region. The chosen cell geometry introduces a strong radial parameter dependence into the system. For low conductivity the dynamics is spatio-temporally chaotic. A small aspect ratio of the cell supports 2-dimensional space-time chaos on the entire disk electrode. For larger aspect ratios and thus larger parameter gradients, however, we observe a ‘self-confinement’ of the turbulent dynamics to the central part of the disk electrode, the outer ring of the electrode exhibiting angularly uniform oscillations. It can be expected that the state of confined chaos is not restricted to electrochemical systems but can be established also in other dynamical systems by introducing an appropriate parameter gradient.