For decades, water electrolysis research has assumed that small, quickly departing bubbles are optimal for efficiency. However, a new study published in eScience challenges this notion, revealing that under high-current conditions, larger bubbles formed by coalescence can actually enhance the hydrogen evolution reaction (HER). The findings, reported by researchers from East China University of Science and Technology and Southern University of Science and Technology, suggest that bubble coalescence acts as a self-cleaning and mixing mechanism at the electrode surface, improving performance significantly.
The study, available online in July 2026 with DOI: 10.1016/j.esci.2025.100472, investigated how electrolyte composition influences bubble behavior and HER efficiency in both acidic and alkaline water electrolysis. Using a three-electrode electrolytic cell with a platinum disk electrode, the team employed electrochemical measurements, high-speed imaging, and numerical simulations. In sulfuric acid, bubbles coalesced readily, but adding perchloric acid or sodium sulfate suppressed coalescence, leading to smaller bubble departure sizes. Surprisingly, smaller bubbles did not improve performance. At −40 mA, adding perchloric acid reduced bubble size but caused approximately a 20% drop in HER efficiency; at −60 mA, the gap reached about 30%.
The mechanistic analysis revealed that a just-detached bubble can linger above the electrode and continuously merge with surface-anchored microbubbles. This late departure pulls microbubbles away at sizes below 10 μm, freeing active sites before they become blocked. Additionally, coalescence 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 (PS) microparticles promoted coalescence and improved efficiency by 2–6%.
The authors emphasize that the key question in bubble management should shift from how to make bubbles smaller to how bubbles interact after formation. "Bubble coalescence can act like a self-driven cleaning and mixing process at the electrode surface," they noted, explaining that it removes microbubbles early, reopens reaction sites, and brings fresh electrolyte into regions where transport is usually slow. This explains why larger departing bubbles can signal better performance under high-current conditions.
These findings offer a new design principle for gas-evolving electrochemical systems. In acidic systems, where bubbles already merge easily, 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 help restore beneficial merging. The study also points to broader applications in industrial electrolysis, where surface bubble removal and interfacial transport remain major limitations. 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.
