Influence of Alkali Metal Ions on Electrochemical Oscillations Via an Increase in the Electrode Surface pH
Authors: Mukouyama Yoshiharu 
Published: July 2021

The local pH of an electrochemical interface (pHs) determines both the kinetics and thermodynamics of various electrochemical reactions in aqueous solutions, and might, therefore, have a strong impact on the activity and selectivity of such reactions. pHs is different from the bulk pH of the electrolyte solution (pHb) in electrochemical reactions that consume H+ or OH- ions. This difference is particularly pronounced when the electrolyte solution contains a salt like Na2SO4 or K2SO4, because the transport of H+ in the solution is suppressed, owing to the alkali metal ion (Na+ or K+) that originates from the salt. For example, pHs significantly increases during the hydrogen evolution reaction (HER) on various metal electrodes in 0.1 M H2SO4 solutions, when an appropriate amount of Na2SO4 or K2SO4 is added to these solutions, as reported by our previous studies [1, 2]. In this case, even though pHb is ~1, pHs increases up to 13 at high overpotentials [2]. pHs also increases significantly during the hydrogen peroxide reduction reaction (HPRR) on Pt electrodes in H2SO4 solutions containing Na2SO4 or K2SO4 [3].
Salts like Na2SO4 and K2SO4 are often chosen as supporting electrolytes in electrochemical reactions because the ions constituting the salts, such as Na+ and SO4
2-, are nonreactive in aqueous solutions. Typically, such supporting electrolytes are used to reduce the solution resistance, which presumably increases the reaction efficiency. However, as mentioned above, salts with alkali metal ions have a strong impact especially on electrochemical reactions that consume H+. In other words, alkali metal ions suppress such electrochemical reactions.
Interestingly, as will be shown in this presentation, alkali metal ions can lead to oscillatory instability [1, 3]. For example, HER shows a potential oscillation under current-controlled conditions (HER oscillation). Detailed studies using a high-pressure apparatus, which facilitates the study of the HER in a high overpotential region where hydrogen bubbles evolve vigorously, have revealed that the HER oscillation can be explained in a framework based on the electrochemical reactions and diffusion–convection (ERDC) mechanism [4]. HPRR exhibits four types of oscillations that are associated with the increase in pHs because the perhydroxyl ion formed at the electrode surface (H2O2 ⇌ HO2
- + H+, pK
a = 11.7) electrochemically reduces to a hydroxyl ion or oxidizes to oxygen. Various studies including electrochemical impedance measurements have revealed that the four types of oscillations in HPRR can be explained by considering the N-shaped negative differential resistance (N-NDR) characteristics.

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