Potential Oscillation during Hydrogen Evolution Reaction: Hidden N-Shaped Negative Differential Resistance Due to Hydrogen Bubbles

Abstract

Electrochemical oscillations are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems. In general, an N-shaped negative differential resistance (N-NDR) plays a crucial role in the appearance of oscillations because it gives rise to an oscillatory instability [1]. Most of the oscillations can be classified into an N-NDR type or “hidden” N-NDR (HN-NDR) type oscillator. The former shows current oscillations under potential controlled conditions and hysteresis loops under current controlled conditions, whereas the latter shows not only current oscillations but also potential oscillations. Thus, the electrochemical system that shows potential oscillations falls into an HN-NDR type oscillator. However, it is not always true because there are different types of oscillators that show potential oscillations, namely, strictly potentiostatic type, S-shaped NDR (S-NDR) type, coupled N-NDR (CN-NDR) type, and electrochemical reactions and diffusion–convection (ERDC) type oscillators. Hydrogen evolution reaction (HER), which is the cathode reaction in water electrolysis, 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. When the electrode potential is much negative than the onset potential of HER, a large number of hydrogen bubbles are formed on 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 affect electrochemical measurements. We have reported [2] that the HER on Pt and Au in 0.10 M (M = mol/dm3) H2SO4 solution is anomalously affected by salts such as Na2SO4 and K2SO4 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 Na2SO4 or K2SO4 was added to the solution. Interestingly, a potential oscillation, named HER oscillation, appeared under current controlled conditions (Figures 1b and 1c). As reported recently [3], the oscillation also appeared in the absence of the salts when the concentration of H2SO4 was as low as 0.03 M. The amount of hydrogen bubbles oscillated synchronously with the potential oscillation: the bubbles evolved more vigorously in the low potential region where the reduction of water occurred than in the high potential region where the reduction of H+ occurred. Therefore, we proposed that the bubble evolution played role in both the positive and negative feedback mechanisms, the combination of which caused an oscillatory instability. The proposed mechanism, however, cannot explain N-NDR characteristics though the oscillation seems to be an HN-NDR type oscillator because it appears as potential oscillations. In the present study, to clarify how the bubble evolution is related to N-NDR characteristics and an NDR-hiding factor, the bubble evolution is studied using Pt electrodes under atmospheric pressure (0.1 M Pa) conditions and also under high-pressure (up to 1 M Pa) conditions. Furthermore, the bubble behaviour is carefully observed by using a high-speed camera. In this presentation, we will show how hydrogen bubbles concurrently induce and hide the N-NDR characteristics. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012). [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 Na2SO4, measured (a) under potential controlled conditions and (b) under current controlled conditions. (c) Time course of E measured at I = -16 mA. Figure 1