Effect of the ohmic potential drop on the onset of current oscillations at the transition between active and passive states of a cobalt electrode in phosphoric acid solutions

Abstract

Oscillations of the relaxation type, jump phenomena and hysteresis have been observed experimentally in the electrochemical system of a cobalt electrode polarized in phosphoric acid solutions. Sharp transitions between stable branches representing the active and passive state of the Co electrode occur as the applied potential increases or decreases gradually within a certain potential range. This behaviour results in the appearance of a hysteresis loop since the unstable state of the system is not accessible. The width of the hysteresis loop increases by increasing the potential sweep rate. The delay of the jump transition satisfies a 2 3 power law. The shape of the polarization curve, which is characteristic of the activation—passivation processes of Co/H 3PO 4, depends also on the IR potential drop in the electrochemical cell. This was verified by examining the effect of an externally controlled resistance which was added between the ground and the working electrode as well as by varying the electrolyte concentration. For small deviations of the applied potential from the bifurcation point, defined as the potential for the onset of current oscillations, harmonic current oscillations of small amplitude are observed. As the deviations of the applied potential increase, current oscillations of the relaxation type appear. The oscillatory potential range is within the hysteresis loop formed between the active and passive state of the Co/H 3PO 4 system. The experimentally observed current oscillations of the relaxation type can be reproduced by a system of differential equations having two independent dynamic variables with widely different timescales. These equations describe the physicochemical processes which contribute to the activation—passivation of the Co. An equivalent RC electric circuit of the electrochemical cell, with an element having dynamic negative resistance, demonstrates clearly the role of the IR potential drop for the appearance of the current relaxation oscillations.