Electrochemical Oscillation during H2O2 Reduction in Basic Solutions
Authors: Hasegawa Mitsuhisa , Nakanishi Shuji , Mukouyama Yoshiharu 
Published: February 2020

INTRODUCTION
Chemical and electrochemical oscillations have been studied extensively because they are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems.
The H2O2 reduction at Pt electrodes in acidic solutions (H2O2 + 2H+ + 2e- → 2H2O) has a unique feature where various kinds of oscillations, named oscillations A, B, C, D, E, F and G, are observed, as we reported more than a decade ago. Oscillation A is observed in the potential region of formation of under-potential deposited H (upd-H), whereas oscillation B is observed in the region of hydrogen evolution reaction (HER) (see. Fig. 1b). Oscillations C and D are observed in the presence of a small amount of Cl- or Br-ions which adsorb on Pt surface. Oscillations E, F and G are observed when a single-crystal Pt(111) or Pt(100) electrodes are used.
We have also reported that another oscillation, named oscillation H, appears under current controlled conditions in the presence of Na2SO4 or K2SO4 in the H2O2-H2SO4-Pt electrochemical system [1]. In the presence of the Na+ or K+ ions originating from the salts, the transport rate of H+ to the electrode surface by the electromigration decreases [2] and consequently the local pH at the electrode surface becomes basic during the reduction of H2O2. When the solution is basic, H2O2 dissociates to form HO2
- ion (H2O2 ⇌ HO2
- + H+) because H2O2 is a weak acid. The presence of HO2
-ion probably attributes to the appearance of oscillation H [1].
Recently, we have found another current oscillation appears during the reduction of H2O2when the solution is basic. In this work, we studied the conditions for the appearance of the oscillation and the mechanism of the oscillation itself.
RESULTS and DISCUSSION
Figure 1 shows the current (I) – potential (E) curves for a Pt-disc electrode in 0.14 M NaOH + 0.25 M H2O2 and 0.05 M H2SO4 + 0.25 M H2O2 with or without Na2SO4, measured under current controlled conditions. In the basic solution (Fig. 1a), the H2O2-reduction current started to flow at ca. 0.1 V vs. SHE. A current oscillation appeared at around -0.85 V. The HER occurred in the potential region below ca. -0.95 V.
In the acidic solution without Na2SO4 (Fig. 1b), oscillations A and B appeared. However, in the presence of Na2SO4 (Fig. 1c), both oscillations A and B did not appear. The current oscillation did at around -0.85 V and the HER occurred in the potential region below ca. -0.95 V, similarly to the basic solution. This was because the local pH at the electrode surface was basic in the presence of Na+ ions. The waveform of the oscillation was periodic, as shown in Figure 2. The oscillation period was around 0.1 s and it increased as the potential decreased. Very similar behavior was observed by the addition of K2SO4.
These results show that the current oscillation appeared during the H2O2 reduction when the solution was basic. That is, the current oscillation appeared during the electrochemical reduction of HO2
- ion (HO2
- + H2O + 2e- → 3OH-). It appeared in the potential region where a negative differential resistance (NDR) was formed. Therefore, the NDR played an important role for the appearance of the oscillation.
This work is partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grants Q12E-02.
REFERENCES
[1] Y. Mukouyama, M. Hasegawa, S. Yamamoto, S, Nakanishi and H. Okamoto, ECS Trans., 58 (2013) in press.
[2] Y. Mukouyama, M. Kikuchi and H. Okamoto, J. Electroanal. Chem.,
617, 179 (2008).