In the quest to make green hydrogen production more efficient, a new study published in eScience suggests that larger bubbles—long considered detrimental—may actually enhance the hydrogen evolution reaction (HER) under high-current conditions. The research, conducted by teams from East China University of Science and Technology and Southern University of Science and Technology, reveals that bubble coalescence can act as a self-cleaning mechanism that improves mass transfer and frees active sites on electrodes.
Water electrolysis is a key technology for producing green hydrogen, which is expected to play a crucial role in decarbonizing industries such as chemical manufacturing, transportation, and steelmaking. However, the process is hampered by bubbles that form on electrode surfaces, blocking catalytic sites and slowing ion transport. Traditional approaches have focused on making bubbles detach earlier and at smaller sizes through surface engineering or external fields. Yet at high current densities, bubble-bubble interactions become dominant, and the new research indicates that promoting coalescence rather than suppressing it can lead to significant efficiency gains.
The team used a platinum disk electrode in a three-electrode cell, combining electrochemical measurements, high-speed imaging, and numerical simulations. In sulfuric acid, bubbles coalesced readily, but when perchloric acid or sodium sulfate was added, coalescence was suppressed and bubbles departed at smaller sizes. Surprisingly, smaller bubbles did not improve performance. At -40 mA, adding perchloric acid reduced bubble size but caused about a 20% drop in HER efficiency; at -60 mA, the performance gap reached about 30%.
Mechanistic analysis showed that a just-detached bubble can linger above the electrode and merge with surface-anchored microbubbles, pulling them away at sizes below 10 μm before they block active sites. This coalescence also generates local flows exceeding 1 m/s, disrupting the stagnant interfacial layer and enhancing heat and mass transfer. In alkaline media, where coalescence is naturally suppressed, adding hydrophobic polystyrene microparticles promoted coalescence and improved efficiency by 2–6%.
The authors emphasize that future electrolysis design should consider bubble interactions after formation, not just bubble size. “Bubble coalescence can act like a self-driven cleaning and mixing process,” they note, re-opening reaction sites and bringing fresh electrolyte to regions where transport is usually slow. This explains why larger departing bubbles can indicate better performance under high-current conditions.
These findings suggest a new design principle for gas-evolving electrochemical systems. In acidic systems, electrodes or flow fields could be designed to increase beneficial bubble collisions. In alkaline water electrolysis, seawater electrolysis, and chlor-alkali processes, where coalescence is often inhibited, electrolyte additives or particle-assisted strategies may restore beneficial merging. The study points to broader applications in industrial electrolysis, where surface bubble removal and interfacial transport remain major limits. By treating coalescence as a controllable tool, future devices may reduce energy loss without relying solely on catalyst or electrode-surface improvements.
The research was funded by the National Natural Science Foundation of China, the Shanghai Pilot Program for Basic Research, the Special Project for Peak Carbon Dioxide Emissions-Carbon Neutrality from the Shanghai Municipal Science and Technology Commission, and the Guangdong Basic and Applied Basic Research Foundation. The full article is available at https://doi.org/10.1016/j.esci.2025.100472.
