Transient chaotic behavior during simultaneous occurrence of two electrochemical oscillations

We have found that a chaotic oscillation transiently appears when two kinds of current oscillations, named oscillation A and oscillation alpha, occur simultaneously. Oscillations A and alpha appear during the reduction of H2O2 and S2O8 (2-) respectively on the Pt electrode. When the electrode potential is stepped from the rest potential to a potential where both the oscillations appear, a period-1 oscillation first appears, and then a period-doubling bifurcation cascade occurs. After that, the transient chaos appears, which is followed by a sequence of mixed-mode oscillations (MMOs). The appearance of the chaotic behavior and the MMOs can be explained on the basis of the reported mechanisms for oscillations A and alpha. Both the oscillations are caused by an N-shaped negative differential resistance (N-NDR) due to the formation of under-potential deposited H (upd-H), indicating that a positive feedback mechanism works during the simultaneous occurrence of oscillations A and alpha. On the other hand, negative feedback mechanisms for oscillations A and alpha include the surface concentration of H2O2 and S2O8 (2-), respectively. We can, then, conclude that the combination of the positive feedback mechanism and the two negative feedback mechanisms of different time scales gives rise to the chaotic behavior and the MMOs, which has been verified by numerical simulations.