A recent study challenges a long-held assumption in water electrolysis research: that smaller, faster-departing bubbles are always better. Instead, under high-current conditions, bubbles that merge and leave the electrode later can enhance the hydrogen evolution reaction (HER). The key is not their size but their coalescence, which clears tiny bubbles from the electrode and stirs the surrounding liquid. By tuning the electrolyte composition, the research team achieved up to 30% higher HER efficiency in systems that promoted coalescence compared to those that inhibited it, offering a fresh perspective on reducing energy losses in green hydrogen production.
Green hydrogen is poised to play a critical role in decarbonizing industries like chemical manufacturing, transportation, and steelmaking. However, electrolysis efficiency is hampered by bubbles that form on electrode surfaces, covering catalytic sites, slowing ion transport, and impeding heat and mass transfer. Traditional strategies have focused on making bubbles detach earlier and at smaller sizes through surface design, wettability control, or external fields. But at high current densities, bubble collisions become frequent, and bubble–bubble interactions dominate. This has created a need to understand how bubble coalescence affects interfacial transport and overall electrolysis efficiency.
Researchers from East China University of Science and Technology and Southern University of Science and Technology reported their findings in eScience (DOI:10.1016/j.esci.2025.100472), with the article available online in July 2026. The study explored how electrolyte composition controls bubble coalescence, bubble departure, and HER performance in both acidic and alkaline water electrolysis. Using a three-electrode electrolytic cell with a platinum disk electrode, electrochemical measurements, high-speed imaging, and numerical simulations, they found that promoting coalescence can actually improve efficiency by reshaping how bubbles leave the electrode surface.
In sulfuric acid, bubbles naturally coalesced, but adding perchloric acid or sodium sulfate suppressed coalescence and reduced bubble departure size. 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 roughly 30%. The mechanism became clear: 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, breaking up 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 this work shifts the focus from merely making bubbles smaller to understanding how bubbles interact after they form. Bubble coalescence acts like a self-driven cleaning and mixing process: it removes microbubbles early, reopens reaction sites, and brings fresh electrolyte into a region where transport is usually slow. This explains why larger departing bubbles can signal better performance under high-current conditions.
These findings suggest 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 useful 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 has broader applications in industrial electrolysis, where surface bubble removal and interfacial transport remain major limits. By treating coalescence as a controllable tool, future devices could reduce energy loss without relying solely on catalyst or electrode-surface improvements.
