Operando Optical and Electrochemical Study of Dynamic Hydrogen Bubble Formation in Confined Systems

The efficiency of the electrocatalytic hydrogen evolution reaction (HER) is critically influenced by gas bubble nucleation, growth, and detachment dynamics at the electrode–electrolyte interface. However, correlating individual bubble events with electrochemical responses remains challenging on the macroscale due to overlapping signals from concurrent surface processes. In this study, we use scanning electrochemical cell microscopy (SECCM), integrated with optical imaging, to investigate hydrogen bubble dynamics under confined electrochemical conditions at the microscale, enabling a direct correlation between visual observations and electrochemical responses. We first demonstrate that correlating individual H2 bubble formation with electrochemical responses is difficult at the bulk scale when visualizing the electrode surface during HER on platinum (Pt). SECCM, however, provides a confined environment at the microscale, allowing for systematic variation of H2 mass transport. By adjusting the probe–substrate distance (h), we reveal a strong relationship of gas bubble formation with local confinement by the glass probe and mass transport through the three-phase boundary (electrode–electrolyte–air) at the meniscus. Current–time (i–t) responses directly reflect bubble formation, as the gas bubble blocks the probe orifice, a finding supported by COMSOL simulations. Aerophobic and aerophilic surface modifications of SECCM probes offer further control over interfacial wettability between the gas bubble and Pt surface. Aerophilic probes promote repeated rapid gas film formation and removal inside the glass wall of the probe that is reflected in periodic i–t transients (at ∼1 Hz), while aerophobic probes induce broader droplet contact areas and variable nucleation profiles. Amperometric measurements conducted at 20 spatially distinct locations on a polycrystalline Pt surface generated a comprehensive dataset for statistical analysis, revealing consistent nucleation and detachment times. These findings establish SECCM as a powerful tool for studying liquid–gas phase transitions and interfacial gas management for the development of electrocatalysts.