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Hydrogen electrooxidation; nonlinear dynamics; electrochemistry; pattern formation; global coupling
Abstract:
This experimental thesis deals with self-organization phenomena in electrochemical systems. Two aspects are considered. On the one hand, the impact of a periodic variation of the applied voltage on a bistable electrochemical system is investigated. On the other hand, the interaction of an oscillatory reaction kinetics with a spatial coupling via the electric field of the electrolyte (migration coupling) and with a global coupling through the electric control is studied. The first type of studies was carried out with the Pt|H2SO4|H2 system. The instabilities occurring in this system are linked to the presence of a negative differential resistance (NDR) in an N-shaped current/potential curve, and this system exhibits bistability at low electrolyte conductivity. In the second type of studies a prototype electrochemical oscillator, namely the Pt|H2SO4,Cl-,Cu2+|H2 system, was employed. The addition of Cl- and Cu2+ to sulfuric acid shifts the NDR of the first system towards less positive values and partially hides it, inducing the occurrence of oscillations in a wide parameter range. Complex voltammetric responses including high-order periodic and aperiodic cyclic voltammograms were observed in the bistable Pt|H2SO4|H2 system. The behavior was rationalized in terms of the interplay between the presence of the NDR and the processes of roughening of the platinum electrode accompanying the oxide reduction and its subsequent healing during the cycling. A theoretical model was developed and numerical simulations corroborated the key premises underlying the complex responses. Studies on the spatiotemporal pattern formation in the oscillatory Pt|H2SO4,Cl-,Cu2+|H2 system covered most of the present work. When subject only to migration coupling a transition from periodic spatiotemporal structures to turbulence was observed upon increasing the applied voltage. This observation seems to be the first one exhibiting a transition to chemical turbulence in oscillatory media. The turbulent regime is characterized by the existence of several modes excited in a random manner. Moreover, it was demonstrated that when decreasing the range of the migration coupling the transition towards turbulence occurs through a different route, and in the developed turbulent state the number of excited modes is larger. As for the influence of a desynchronizing global coupling upon spatiotemporal pattern formation, in a first step it was experimentally demonstrated that a global coupling is induced in electrochemical experiments whenever some portion of the cell resistance is compensated, irrespective of the method used to achieve the compensation. Then the impact of the strength of the global coupling on the dynamics of the system was studied. The results are divided into two main classes, corresponding to different copper concentrations. At lower copper concentration the addition of a weak global coupling was already enough to suppress the turbulence observed in its absence. Instead, two-phase clusters formed at high applied voltages. Increasing the strength of the global coupling, a plethora of patterns was found including different types of two-phase clusters, five-phase clusters, irregular clusters and traveling pulses. At higher copper concentrations modulated oscillations, target patterns (as well as its asymmetric version) and pulses dominated the dynamics. Spatiotemporal patterns trapped between oxide covered domains were also observed under strong global coupling as a result of the interplay between two different regions exhibiting an NDR.
