Operando observation of bubble formation and mass transfer overpotential in alkaline water electrolysis

Alkaline water electrolysis (AWE) has attracted significant attention for producing hydrogen. One of the problems with AWE is the increase in overpotential, which is attributed to bubbles covering the electrode surface at high current densities. Therefore, understanding the relationship between the bubble formation behavior and reactant transfer mechanism is important for reducing the overpotential. In this study, an operando observation system for the oxygen and hydrogen bubble formation behavior on nickel wire electrodes was developed using a microscope-type high-speed video camera. The relationship between the current density-electrode potential curve and bubble generation behavior was then investigated. The results showed that the generation of oxygen/hydrogen bubbles was activated as current density increased. At a high current density, bubbles covered the electrode surface, and the overpotential increased. Electrochemical impedance spectroscopy (EIS) measurements were performed to analyze the resistance components of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). The EIS results showed that the diffusion rate of hydroxide ions through the bubble-covering layer on the anode can be modeled by the Warburg impedance with a finite diffusion boundary. The migration rate of ions driven by the potential gradient in OER and HER was modeled by a tortuosity model with void fraction and bubble-covering layer thickness. A method for promoting bubble detachment via the addition of tert-butyl alcohol and external magnetic fields was introduced. Finally, a computational fluid dynamics (CFD) simulation of the gas-liquid two-phase flow and a deep-learning analysis method for the size distribution of crowded bubbles in an electrolysis cell were introduced.