Appearance of New Oscillation (Named Oscillation K) Induced By Na2so4 and K2so4 in Electroreduction of H2O2 on Platinum
Authors: Mizuochi Ryusuke , Kikuchi Mitsunobu , Yamada Yuri , Mukouyama Yoshiharu , Nakanishi Shuji 
Published: February 2020

INTRODUCTION

Electrochemical oscillations, which are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems, have been reported in a variety of electrochemical reactions [1]. Among them, the reduction of H2O2 at Pt electrodes in H2SO4 solutions (H2O2 + 2H+ + 2e- → 2H2O) has an especially unique feature: the H2O2 reduction shows ten kinds of oscillations, named sequentially from A to J, because various types of N-shaped negative differential resistances (N-NDRs) are involved in the reduction [2-4]. The first seven oscillations (from A to G) were studied more than a decade ago, and their mechanisms were clarified except for oscillation G [2].
Oscillations H, I, and J, which were found recently [3, 4], appeared when the H2SO4 solution contained the salt such as Na2SO4 and K2SO4 (see Figures 1c and 1d). In the presence of the salt, the transport rate of H+ to the electrode surface decreased because the electromigration of H+ was suppressed by the alkali cation (Na+ and K+). Thus, the local pH at the electrode surface became basic during the H2O2 reduction though the solution was strongly acidic. In the vicinity of the electrode surface, H2O2 dissociated to form HO2
- ion (H2O2 ⇌ HO2
- + H+). Note that H2O2 is a weak acid. The ion was electrochemically reduced to OH- or oxidized to O2, resulting in the appearance of oscillations H, I, and J.
The salts are often used as supporting electrolytes for electrochemical reactions in aqueous solution because they usually do not react in the solution. Thus, it is of interest that the salts cause oscillations H, I, and J as well as the pH increase. In this present work, a further study is conducted on the effect of the salts on the “H2O2-H2SO4-Pt” electrochemical system. The presence of the salts has been found to induce a new oscillation, named oscillation K. The present work studies the conditions for the appearance of oscillation K and also discusses its mechanism.

RESULTS and DISCUSSION

Figure 1 shows the current (I) – potential (E) curves for a Pt-wire electrode. When the solution was 0.1 M H2SO4 + 0.25 M H2O2, the H2O2 reduction occurred in the potential region below ca. 0.8 V under potential controlled conditions. It was suppressed, that is, an N-NDR appeared, at around ca. –0.15 V due to the formation of upd-H (Figure 1a). Oscillation A appeared as a current oscillation at the potentials of the N-NDR and the HER due to the H+ reduction occurred in the potential region below ca. –0.2 V. Oscillation B appeared in the potential region of the HER both under potential controlled conditions and under current controlled ones (Figures 1a and 1b).
When 0.1 M Na2SO4 was added to the solution, the I – E curves were strongly affected because the local pH increased during the H2O2 reduction. Oscillations A and B disappeared whereas oscillation I appeared at around –0.8 V under potential controlled conditions (Figure 1c). The HER due to the water reduction occurred in the potential region below ca. –1.0 V. Oscillation H appeared in the potential range between 0.5 and 0.1 V under current controlled conditions (Figure 1d). The new oscillation, oscillation K, appeared in the potential range between -0.2 and -0.5 V under current controlled conditions. In this potential range, not only the H2O2 reduction but also the HO2
- reduction occurred. Thus, these reductions play major roles for the appearance of oscillation K, which will be discussed in the presentation.

REFERENCES

[1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012).
[2] S. Nakanishi, S.-i. Sakai, M. Hatou, K. Fukami, Y. Nakato, J
. Electrochem
. Soc
.,
150 (2003) E47.
[3] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc.,
164 (2017) H1.
[4] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc.,
164 (2017) H675.

FIGURE CAPTION

Figure 1. I – E curves for 0.1 M H2SO4 + 0.25 M H2O2 + x M Na2SO4 where x = (top) 0 and (bottom) 0.1, measured (left) under potential controlled conditions and (right) under current controlled ones.

Figure 1