Modulated electrochemical waves

Experimental studies have been carried out with ribbon cobalt electrodes in buffered phosphate solutions of different concentration and pH under potentiostatic control. Two conditions are investigated. Under oscillatory conditions the system undergoes slow relaxation oscillations between active and passive states, where passivation is due to the presence of a passivating surface film. Oscillations occur in both the total and local currents, although no uniform, spatially independent oscillations are observed. During the activation phase of the oscillations a wave propagates from each end of the electrode toward the center. In the second type of behavior the system has a single, stable passive steady state. Activation past a threshold value of a small region of the ribbon leads to the growth and propagation of the active region. The system is excitable in the sense that a small disturbance, corresponding to a low current or integrated rate of reaction, produces a large excursion before the system returns to its original stable state. Under both types of behavior the leading edge of each wave is visible. The visible leading edge accelerates as it travels along the ribbon; long-range coupling plays a major role in producing this acceleration. When the applied voltage is very close to the Flade potential, the trailing edge of the active region propagates intermittently in the reverse direction, and thus the width of the active region alternately increases and decreases. The velocity of the reverse propagation of the trailing edge is significantly higher than the primary forward velocity of the leading edge. This modulation can be identified as an activation process; it leads to an additional (intermittent) acceleration of the leading edge and to an increase in the total current.