Georg Flätgen

Autocatalytic mechanism of H2O2 reduction on Ag electrodes in acidic electrolyte: experiments and simulations

It is shown that the cathodic reduction of hydrogen peroxide (H2O2) on silver electrodes in acidic electrolyte can proceed by two parallel mechanisms: first, by the ‘normal’ mechanism that has been discussed in the literature, second, by a novel mechanism proceeding at a significantly more positive potential. It is proposed that the second mechanism involves the activating adsorbate (OH)ad, that forms in the course of the H2O2 reduction reaction as an unstable intermediate. The coverage of the electrode with (OH)ad increases with the rate of H2O2 reduction, i.e., the process is autocatalytic. At more negative potentials the coverage decreases as the rate of adsorbate reduction/desorption rises. This leads to a potential region of negative differential charge-transfer resistance and thus to complex dynamic phenomena, in particular to electrochemical oscillations. Model calculations based on these considerations yield the potential dependent OH-adsorption, the N-shaped current/voltage curves and current oscillations that agree well with the experimental findings.

Two-Dimensional Imaging of Potential Waves in Electrochemical Systems by Surface Plasmon Microscopy

The potential dependence of resonance conditions for the excitation of surface plasmons was exploited to obtain two-dimensional images of the potential distribution of an electrode with high temporal resolution. This method allows the study of spatiotemporal patterns in electrochemical systems. Potential waves traveling across the electrode with a speed on the order of meters per second were observed in the bistable regime of an oscillatory electrochemical reaction. This velocity is close to that of excitation waves in nerve fibers and is far greater than the velocity of reaction-diffusion waves observed in other chemical systems.

A GENERAL MODEL FOR PATTERN FORMATION IN ELECTRODE REACTIONS

Experiments on pattern formation in electrochemical systems show qualitative differences in comparison with phenomena observed with other chemical systems. In this paper we derive a general model from the basic transport equations which takes into account the special aspects of electrochemical systems. Stepwise introduction of three approximations considerably simplifies the equations and demonstrates the dominant role the electric potential plays for pattern formation. The simplest form of the model contains migration as the only transport mechanism, and the importance of intrinsic global coupling for the dynamic behavior of electrochemical systems becomes apparent. This simple model already reproduces the experimentally observed front behavior we recently reported [G. Flatgen and K. Krischer, Phys. Rev. E 51, 3997 (1995)].

The Impact of the Operation Mode on Pattern Formation in Electrode Reactions From Potentiostatic to Galvanostatic Control

Applying a constant voltage across an external resistor in series with an electrochemical cell allows the continuous variation of the experimental control mode from potentiostatic to galvanostatic. The model for electrochemical pattern formation presented for potentiostatic conditions in Ref. 1 is extended to account for this general case. The resistor introduces a global coupling into the model, interacting in a nontrivial manner with the nonlocal coupling through migration currents in the electrolyte which occur independently of the control mode. The complex interplay of these two coupling terms is investigated as a function of operating parameters, and is illustrated by simulations in the bistable regime of an electrochemical reaction.

Spatio-temporal pattern formation during the reduction of peroxodisulfate in the bistable and oscillatory regime: a surface plasmon microscopy study

Abstract Using surface plasmon microscopy we demonstrate that oscillations as well as transitions in the bistable regime during the reduction of S 2 O 8 2− at Ag electrodes are accompanied by wave phenomena. Typical velocities of the waves range from some centimeters per second to meters per second, depending on the conductivity of the electrolyte. The characteristic length of the patterns is determined by the temporal dynamics of the potential drop across the double layer at high mass transport and by the dynamics of the concentration of S 2 O 8 2− at the electrode at low mass transport. The first gives rise to potential fronts with front widths between 0.1 and 0.5 mm; the second leads to smooth structures on the order of centimeters. Further, the influence of the electrode boundary on the pattern formation in electrochemical systems is discussed, as well as the generality of the above-mentioned characteristic times and lengths.

Adaptive detection of instabilities: an experimental feasibility study

Abstract We implement a practical protocol for the active, on-line detection of bifurcations in experimental systems, based on real-time identification and feedback control ideas. Current experimental practice for the detection of bifurcations typically requires long observation times in the vicinity of marginally stable solutions, as well as frequent re-settings of the experiment for the detection of turning point or subcritical bifurcations. The approach exemplified here addresses these issues drawing from numerical bifurcation detection procedures. The main idea is to create an augmented experiment, using the experimental bifurcation parameter(s) as additional state variables. We implement deterministic laws for the evolution of these new variables by coupling the experiment with an on-line, computer-assisted identification/feedback protocol. The “augmented” experiment (the closed-loop system) thus actively converges to what, for the original experiment (the open-loop system), is a bifurcation point. We apply this method to the real-time, computer-assisted detection of period-doubling bifurcations in an electronic circuit. The method succeeds in actively driving the circuit to the bifurcation points, even in the presence of modest experimental uncertainties, noise, and limited resolution. The active experimental tracing of a codimension-1 bifurcation boundary in two-parameter space is also demonstrated.

Investigation of Front Propagation in an Electrochemical System: Experiments and Numerical Simulations

Abstract We examined the spatio-temporal behavior of an electrochemical system in the bistable regime, in which the system might take on either a high current density state (active) or a low current density state (passive) at one value of the externally applied voltage. The transition from the passive to the active state is accompanied by accelerating fronts with sharp interfaces, whereas the reverse transition from the active to the passive state exhibits much smoother spatial variations. Also the evolution of the total current density displays qualitative differences in the two cases. Both, the differences of the spatial patterns as well as those of the total current densities are reproduced with a mathematical model, which also reveals the origin of the asymmetry of the transitions: a global coupling, intrinsic in all electrochemical systems, in combination with the specific dependence of the reaction current on the electrode potential.