Bifurcation Behavior in Oscillations during H2O2 Reduction at Pt Electrodes

Abstract

Electrochemical oscillations are of interest from the viewpoint of dynamic self-organization of molecular systems. Extensive studies have revealed that N-shaped negative differential resistance (N-NDR) plays an essential role in the majority of the oscillations because an N-NDR induces a positive feedback mechanism. Simple periodic oscillations are explained by the combination of a positive feedback loop and a negative one, latter of which involves a slow process such as the surface concentration of an electroactive species and the surface coverage of an adsorbed species [1]. Among the electrochemical oscillations, the reduction of H2O2 at Pt electrodes in H2SO4 solutions (H2O2 + 2H+ + 2e- → 2H2O) has an especially unique feature because it exhibits ten kinds of oscillations, as we have reported before [2]. One of the oscillations, named oscillation A, appeared in the potential region where under-potential deposited H (upd-H), which suppressed the H2O2 reduction, was formed on Pt surface (see Figure 1a). Another oscillation, named oscillation D, appeared when a small amount of Br- ions, which adsorbed on Pt surface, was added to the solutions. The reduction of S2O8 2- at Pt electrodes in acidic solutions (S2O8 2- + 2e- → 2 SO4 2-) was also unique because it showed several kinds of oscillations. The formation of upd-H also suppressed the S2O8 2- reduction; hence one of the oscillations, named oscillation a, appeared in the potential region of upd-H [3]. The electrochemical oscillations are also of interest from the viewpoint of nonlinear dynamics because they often exhibit transitions among non-oscillatory state, periodic oscillations, quasi-periodic ones, mixed-mode ones, and chaotic ones via period-adding bifurcation, period-doubling bifurcation, and so on. We have been studying bifurcation behavior in the oscillations during the H2O2 and S2O8 2- reductions. The studies showed [3,4] that the complex oscillations such as period-n, mixed-mode, and chaotic oscillations were observed when oscillation A appeared simultaneously with oscillation a (or D) though each was period-1 oscillation (see Figure 1b for the chaotic oscillation induced by the simultaneous occurrence of oscillations A and a). Furthermore, the studies led us to the conclusion that the complex oscillations appeared when two kinds of simple periodic oscillations occurred at the same time, and also that they were caused by the interplay among a positive feedback loop and two negative feedback loops. Recently, we found [5] that oscillation A changed into the complex oscillations when inorganic salts such as Na2SO4 and K2SO4 are added to the solutions (see Figure 1c for the chaotic oscillation). The solutions originally contained (bi)sulfate ions dissociated from H2SO4, and hence we explained that alkali metal ions, i.e., Na+ and K+ ions, caused a negative feedback loop that induced the complex oscillations. This presentation will show the bifurcation behavior in detail and also provide a definitive factor for the appearance of the complex oscillations. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin, 2012. [2] Y. Mukouyama, H. Kawasaki, D. Hara, M. Kikuchi, Y. Yamada, S, Nakanishi, ECS Trans., 69 (39) 37-45, 2015. [3] Y. Mukouyama, H. Kawasaki, D. Hara, S. Nakanishi, J. Solid State Electrochem., 19, 3253-3263, 2015. [4] Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Solid State Electrochem., 9, 290-295, 2005. [5] Y. Mukouyama, D. Hara, H. Kawasaki, M. Kikuchi, Y. Yamada, S, Nakanishi, ECS Trans., 69 (39) 47-57, 2015. FIGURE CAPTION Figure 1. The I – E curves for a Pt-disc electrode in 0.1 M H2SO4 + 0.2 M H2O2 (a) without Na2S2O8 nor Na2SO4, (b) with 0.1 M Na2S2O8, and (c) with 0.2 M Na2SO4, measured under potential-controlled conditions. The insets in panels a, b, and c are the waveforms of oscillations observed at −0.065, −0.070, and −0.085 V, respectively. Figure 1