Spatiotemporal patterns on electrode arrays

Experiments were carried out with arrays of iron electrodes in sulfuric acid solution under conditions in which slow active-passive relaxation oscillations occur. The arrays consisted of a number of small disks which were made by exposing the ends of wires embedded in an insulator. Three array geometries were used: (a) 2 x 8 which approximates a one-dimensional geometry, (b) 4 x 4 square array, and (c) 61 electrodes in a hexagonal pattern. The experiments were done potentiostatically, and the current in each electrode was measured independently; therefore, the spatiotemporal patterns which occurred were directly determined. For the oscillatory conditions, which occur at potentials above the Flade potential, a wave moves from the center of the electrode to the edge during the activation phase and another moves in the opposite direction, from edge to center, during the passivation. The velocities of these waves depend on array size and applied potential; the activation velocities are much faster than those of passivation. Both the activation and passivation wave fronts accelerate as they propagate along the array. As the potential is lowered, a spatiotemporal period doubling occurs. Long-range coupling plays an important role in the dynamics of such electrochemical reactions; the arrays of electrodes behave qualitatively similar to single electrodes of the same total surface area. The arrays can thus be used to gather information on the rate of reaction at various sites on a reacting surface.