We have previously reported that the hydrogen evolution reaction (HER) in acidic water electrolysis shows a potential oscillation with amplitude of about 1 V. The oscillation, named HER oscillation, is accompanied with a periodic change in the evolution rate of hydrogen bubbles, i.e., hydrogen bubbles evolve more vigorously at low potentials than at high potentials, which has led us to propose a mechanism for HER oscillation (J. Electroanal. Chem., 713, 39 (2014)). In order to obtain a deeper insight into the mechanism of HER oscillation, this present work studies the effect of high pressure (e.g. 0.7 MPa) on the oscillation and current-potential curves. It reveals that any N-shaped negative differential resistance characteristics are not involved in HER oscillation unlike the majority of electrochemical oscillations. It also shows that a solution-stirring effect due to the hydrogen bubble evolution, which causes an enhancement of convection near the electrode surface, plays an essential role in HER oscillation. We thus conclude that the appearance of HER oscillation can be explained by considering that the enhancement occurs only at low potentials at which hydrogen bubbles evolve vigorously.

A nitrobenzene (NB) droplet moves like amoeba on the surface of an Au electrode during the Sn electrodeposition in HNO3 solution. As explained in our previous paper (J. Electrochem. Soc., 165, H473 (2018)), the motion is due to a Marangoni effect (i.e. a hydrodynamic flow induced by an interfacial tension gradient). The mechanism of the Marangoni motion, which has been proposed in the previous paper, is complicated because some processes such as reduction of nitrate ions, that of NB, and transfer of aniline occurs at the same time. In this report, the proposed mechanism is verified by simple experiments, and the factors inducing the Marangoni motion are discussed.

Photoelectrochemical (PEC) and photocatalytic (PC) processes are promising candidates to address environmental and energy issues. Previous studies have shown that Nb-related perovskite oxides such as KNbO3 and NaNbO3 have excellent PC activities. However, as for (K, Na)NbO3 (KNN), nothing has been studied on its PEC and PC behaviour. This work studies PEC properties of a KNN thin film prepared by a radio-frequency magnetron sputtering method. The properties, namely, the PEC activity, flat-band potential, and band gap of the KNN film, are examined by measuring cyclic voltammograms with and without UV-vis light irradiation, Mott-Schottky method, and Tauc method. This work also studies PEC properties of a KNN thin film prepared by a chemical solution deposition method. The KNN films exhibit a PEC activity similar to a n-type semiconductor photoanode. We thus conclude that KNN is an active material for PEC water splitting.