Accelerating fronts in an electrochemical system due to global coupling

We examine spatiotemporal patterns during the reduction of peroxodisulfate to sulfate at silver ring electrodes in an electrochemical cell. The reaction displays bistable behavior in the current-voltage characteristics under potentiostatic conditions. The two stable states are characterized by different voltage drops across the double layer. We investigate transitions in the bistable region between the different states. Spatiotemporally resolved measurements of the voltage drop across the electrode-electrolyte interface show that the transition occurs via fronts. We are able to trigger these fronts by local perturbations of the electric field and thus to examine their dependence on operating parameters. It is shown that the movement of the fronts results from migration currents (movement of ions due to the electric field), and hence the system is described by reaction-migration-diffusion equations. The velocity of the fronts increases with time; hence they are accelerated. This contrasts with usual front solutions of reaction-diffusion systems, which possess a constant velocity in one-dimensional geometries. This acceleration is shown to be a consequence of a global, or more precisely nonlocal, coupling mediated through the electric field.