A Systematic Study on Three Kinds of Oscillations during Simultaneous Occurrence of Nitrate Reduction and Hydrogen Evolution Reaction on Cu and Ag Electrodes

Abstract

The reduction of nitrate ions on Cu and Ag electrodes shows three kinds of oscillations in the potential region of the hydrogen evolution reaction (HER). The oscillations, which appear both under potential-controlled and current-controlled conditions, are sequentially named oscillations I, II, and III as the rate of the HER increases. Oscillation I appears when the electrolyte is acidic, while the other oscillations appear irrespective of the electrolyte pH. To reveal how N-shaped negative differential resistances (N-NDRs) are associated with the HER, a high-speed camera, electrochemical impedance analyzer, and high-gravity electrode are used, which leads to the following conclusions: oscillations I and III are hidden N-NDR (HN-NDR) types, with the former caused by a reaction intermediate of the HER, while the latter is attributed to the inherent nature of bubbles. This paper discusses how these N-NDR characteristics are induced and hidden by the occurrence of the HER. On the other hand, oscillation II is explained by the diffusion limitation of nitrate ions, enhanced convection due to the hydrogen bubble evolution, and change in the pH at the electrode surface.