Hydrogen evolution reaction (HER) has been extensively studied using various kinds of electrodes because it is a fundamental electrochemical reaction and also because it is an important reaction for the production of H₂. The HER can occur more or less at a cathode electrode in aqueous solution under a variety of conditions. When the overpotential of the cathode is high, i.e., when the rate of the HER is high, hydrogen bubbles are formed vigorously at electrode surface. This often causes a disturbance in electrochemical reactions, leading to difficulties in electrochemical measurements, controls, and analysis. Thus, the HER has been mostly studied in a low overpotential region where no bubbles are formed or in an intermediate region where bubbles do not disturb electrochemical measurements. On the other hand, the HER in a high overpotential region attracts recent interest from the viewpoint of application such as production of metal nanoparticles [1].

We have reported [2] that the HER on Pt and Au in 0.10 M (M = mol/dm³) H₂SO₄ solution is anomalously affected in a high overpotential region, e.g., more negative than -0.5 V vs. SHE, as shown in Figure 1a. The reduction current due to the HER became low (in absolute value) when salt such as Na₂SO₄ and K₂SO₄ was added to the solution. Interestingly, a potential oscillation, named HER oscillation, appeared under current controlled conditions (Figures 1b and 1c). We recently reported that the potential oscillation appeared during the HER not only on Pt and Au electrodes but also on Rh, Ag, Cu, Fe, Ni, W, Zn, Sn, and In electrodes in a variety of electrolytes such as H₂SO₄, HNO₃, and HClO₄ [3].

As discussed in the earlier paper [2], the decrease in the HER current by adding salts is caused by the presence of cations, Na⁺ or K⁺, which changes the transport velocity of H⁺ to the electrode surface. The transport is caused not only by diffusion and solution-stirring but also by electromigration, when no salt is added to the solution. When the salt is added to the solution, the cation reduces the electromigration transport of H⁺, resulting in the decrease in the HER current. On the other hand, the potential oscillation appears in the absence of the salts when the concentration of H₂SO₄ is as low as 0.03 M [3], and thus the appearance of the oscillation is not attributed to the presence of cations but to the hydrogen bubbles formed on the electrode surface. The behavior of hydrogen bubbles oscillate synchronously with the potential oscillation. The bubbles evolve more vigorously in the low potential region where the reduction of water occurs than in the high potential region where the reduction of H⁺ occurs. Therefore, we have proposed that the bubble evolution is involved in both the positive and negative feedback mechanisms, the combination of which causes an oscillatory instability.

This proposed mechanism, however, cannot explain a negative differential resistance (NDR), which is essential generally for the appearance of electrochemical oscillations. In the present study, to clarify how the bubble evolution is related to an NDR, the bubble behaviour is carefully observed by using a high-speed camera. Furthermore, electrochemical impedance spectra are measured repeatedly during the HER on a Pt electrode because the Nyquist plots obtained by the impedance measurements can show the presence of the NDR. In this presentation, the major factor that induces the NDR will be discussed and the oscillatory instability, or the positive and negative feedback mechanisms, will be reconsidered.

REFERENCES

[1] T. Nishimura, T. Nakade, T. Morikawa, H. Inoue, Electrochimi. Acta, 129, (2014) 152.

[2] Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Electroanal. Chem., 617 (2008) 179.

[3] Y. Mukouyama, R. Nakazato, T. Shiono, S. Nakanishi, H. Okamoto, J. Electroanal. Chem., 713 (2014) 39.

FIGURE CAPTION

Figure 1. Current (I) - potential (E) curves for a Pt electrode in 0.10 M H₂SO₄ solution with or without 0.05 M K₂SO₄, measured (a) under potential controlled conditions and (b) under current controlled conditions. (c) Time course of E measured at I = -16 mA.