In a surprising twist that could reshape green hydrogen production, a new study published in the journal eScience finds that larger departing bubbles—long considered a drawback—can actually improve the efficiency of water electrolysis under high-current conditions. The research, conducted by teams from East China University of Science and Technology and Southern University of Science and Technology, challenges decades of conventional wisdom that focused on making bubbles smaller and faster to detach.
The study, available online at DOI:10.1016/j.esci.2025.100472, demonstrates that bubble coalescence—the merging of bubbles on the electrode surface—acts as a self-driven cleaning mechanism. When bubbles merge, they sweep away tiny microbubbles that block catalytic sites and stir the nearby liquid, enhancing heat and mass transfer. This process, the researchers found, can lead to up to 30% higher hydrogen evolution reaction (HER) efficiency compared to systems where coalescence is suppressed.
Green hydrogen is critical for decarbonizing hard-to-electrify sectors like chemical manufacturing, steelmaking, and transportation. However, electrolysis efficiency is hampered by bubbles that accumulate on electrodes, covering active sites and slowing ion transport. Traditional strategies have focused on surface design, wettability control, and external fields to encourage early bubble detachment at smaller sizes. But at high current densities, bubble-bubble interactions dominate, making coalescence an inevitable and potentially beneficial factor.
The team used a platinum disk electrode in a three-electrode cell, employing electrochemical measurements, high-speed imaging, and numerical simulations. In sulfuric acid, bubbles readily coalesced, but adding perchloric acid or sodium sulfate suppressed coalescence, resulting in smaller departing bubbles. Surprisingly, the smaller bubbles did not improve performance. At −40 mA, suppressing coalescence caused about a 20% drop in HER efficiency, and at −60 mA, the gap widened to 30%.
The mechanism behind this improvement lies in the behavior of just-detached bubbles. These large bubbles linger near the electrode and continuously merge with surface-anchored microbubbles, pulling them away at sizes below 10 micrometers. This clears active sites before they become blocked. Additionally, coalescence generates local fluid flows exceeding 1 meter per second, breaking up the stagnant interfacial layer and improving mass transport. In alkaline media, where coalescence is naturally suppressed, adding hydrophobic polystyrene microparticles promoted coalescence and boosted efficiency by 2–6%.
The authors argue that the key question in bubble management should shift from "how to make bubbles smaller" to "how bubbles interact after they form." Bubble coalescence, they say, acts like a self-driven cleaning and mixing process at the electrode surface, which explains why larger departing bubbles can signal better performance under high-current conditions.
These findings open new design avenues for gas-evolving electrochemical systems. In acidic electrolyzers, where bubbles already merge easily, electrodes and flow fields could be engineered to encourage beneficial collisions. In alkaline water electrolysis, seawater electrolysis, and chlor-alkali processes—where coalescence is often inhibited—electrolyte additives or particle-assisted strategies could restore beneficial merging. By treating coalescence as a controllable tool, future devices may reduce energy losses without relying solely on catalyst or 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 study is published in eScience, a golden open-access journal cooperated with KeAi, which is indexed in major databases and boasts an impact factor of 52.9, ranking first in electrochemistry.
