A new study challenges decades of assumptions in water electrolysis research, suggesting that larger bubbles that merge and depart later from electrodes can actually enhance hydrogen production efficiency under high-current conditions. The findings, published in the journal eScience, reveal that bubble coalescence acts as a self-driven cleaning and mixing mechanism that removes micro-bubbles and stirs the electrolyte, leading to significant performance gains.
Green hydrogen is poised to play a crucial role in decarbonizing industries such as chemical manufacturing, transportation, and steelmaking. However, the efficiency of water electrolysis—the process of splitting water into hydrogen and oxygen—has been hampered by the formation of gas bubbles on electrode surfaces. These bubbles can cover catalytic active sites, impede ion transport, and reduce heat and mass transfer near the electrode. Traditionally, strategies have focused on making bubbles detach earlier and at smaller sizes through surface engineering, wettability control, or external fields. But at high current densities, bubble-bubble interactions become dominant, and the conventional wisdom may not hold.
Researchers from East China University of Science and Technology and Southern University of Science and Technology investigated how electrolyte composition influences bubble coalescence and its impact on the hydrogen evolution reaction (HER). Using a three-electrode electrolytic cell with a platinum disk electrode, they combined electrochemical measurements, high-speed imaging, and numerical simulations. In sulfuric acid, bubbles readily coalesced, while adding perchloric acid or sodium sulfate suppressed coalescence and led to smaller departing bubbles. Surprisingly, smaller bubbles did not translate to better performance. At −40 mA, suppressing coalescence caused about a 20% drop in HER efficiency; at −60 mA, the gap widened to 30%.
The mechanistic analysis revealed that a just-detached bubble can linger above the electrode and merge with surface-anchored micro-bubbles. This late departure pulls micro-bubbles away at sizes below 10 μm, freeing active sites before they become blocked. Additionally, coalescence generates local fluid velocities 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 argue that future electrolysis design should focus not only on making bubbles smaller but on controlling how bubbles interact after formation. Bubble coalescence can be viewed as a beneficial, self-driven process that cleans the electrode and brings fresh electrolyte to the reaction zone. This insight suggests new design principles for gas-evolving electrochemical systems. In acidic systems, where coalescence is already favorable, electrodes or flow fields could be engineered 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, published with DOI 10.1016/j.esci.2025.100472, was funded by the National Natural Science Foundation of China, among others. It highlights a potential pathway to reduce energy losses in industrial electrolysis without relying solely on catalyst or electrode-surface improvements.
