Coalescence driven dynamics of hydrogen bubbles during water electrolysis

The evolution of electrogenerated gas bubbles during water electrolysis significantly hampers the overall process efficiency. It is therefore beneficial to promote their detachment. For a single bubble, a departure from the electrode surface occurs when buoyancy wins over the downward-acting forces (e.g. contact, Marangoni, and electric forces). In this work, the dynamics of a pair of H₂ bubbles produced during water electrolysis in 0.5 mol/L H₂SO₄ at dual microelectrode is systematically studied by varying the cathodic potential. By combining high-speed imaging and electrochemical methods, we demonstrate the importance of bubble-bubble interactions for the detachment process. We show that bubble-bubble coalescence on the one hand may lead to a substantially earlier departure than that one defined by buoyancy, resulting in significantly higher reaction rates at constant potential. On the other hand, intensive coalescence events may reverse the direction of a departed bubble, driving it back toward the surface. The latter leads to the resumption of bubble growth near the electrode surface followed by buoyancy-driven detachment. We experimentally explore the phase diagram for these different behaviors as a function of the electrode distance and the applied potential and provide a simple model explaining the observed trends.