Mechanistic aspects of oscillations during CO electrooxidation on Pt in the presence of anions: Experiments and simulations

The electrooxidation of carbon monoxide on platinum in the presence of anions is one of the very few strictly potentiostatic oscillators, in which the electrode potential takes the role of a parameter rather than an essential variable. In this paper we investigate the oscillatory behavior as a function of five parameters: applied voltage, rotation rate of the electrode, anion concentration, electrolyte resistance and external resistor. The experiments exhibit regular oscillations also in parameter regions in which the hitherto assumed mechanism predicts only the existence of stable fixed points, and they show irregular oscillations in wide parameter ranges. Model considerations reveal that an asymmetric inhibition of the adsorption of CO and OH by adsorbed anions introduces a positive feedback loop. This so far undetected source of oscillations could be responsible for the wide oscillatory range found in the experiments. These oscillations are solely driven by the reaction kinetics and do not involve transport processes or, as the overwhelming number of electrochemical oscillators, an IR drop in the electric circuit. Furthermore, observations are compiled which suggest that the irregular oscillations are a manifestation of spatial pattern formation.