Dynamic Interfacial Processes in Single Hydrogen Microbubbles Probed by Operando Opto-Electrochemistry

Understanding how individual gas bubbles nucleate, grow, and interact with electrode surfaces is critical for improving the efficiency of gas-evolving electrochemical reactions. Here, we combine pseudo-dark-field optical microscopy with electrochemical measurements to visualize the growth and dynamics of single H₂ microbubbles during the H₂ evolution reaction at a 10 μm diameter Pt ultra-microelectrode. The electrochemical response exhibits a rapid rise in current during H₂ evolution, followed by an abrupt decrease to a residual value upon bubble formation and electrode blocking, consistent with single-bubble nucleation behavior. From the peak current, a critical nucleus diameter of ∼0.3 μm, a critical dissolved H₂ concentration of 0.007 M, and an internal pressure of ∼9.6 atm are determined. Simultaneous optical measurements reveal a sharp increase in scattering intensity during bubble growth. Notably, the bubble expands to ∼54 μm without inducing current shutdown and subsequently relaxes to ∼44 μm while undergoing an abrupt ∼10 μm lateral displacement within 200 ms, coincident with faradaic current shutdown. We propose that this motion arises from asymmetric Marangoni convection driven by surface-tension gradients generated by local variations in dissolved H₂ concentration and temperature. Together, these results highlight the importance of interfacial hydrodynamics in governing bubble behavior at three-phase boundaries.