Oxygen sticks, hydrogen glides — but current density decides!: Bubble dynamics in alkaline water electrolysers

Gas bubble behaviour in water electrolysis has been extensively studied, but research under industrially relevant conditions in flow cells remains limited. In this work, high-resolution, micron scale visualization was achieved by imaging the bottom region of the electrodes through a microscopic lens. A clear difference in size distribution and bubble dynamics between hydrogen and oxygen bubbles was observed, especially at high current densities. Oxygen bubbles exhibited “sticky” behavior and significant coalescence, resulting in a considerable increase in mean bubble diameter with current density. In contrast, hydrogen bubbles remained misty and glided along the electrode surface with limited coalescence. Polarity-switch experiments revealed that adhesion and repulsion are independent of the nature of the evolved gas. Furthermore, current step experiments showed that bubble dynamics correlate with current density rather than electrode potential. These results are supported by an order-of-magnitude force analysis, which indicates negligible electrostatic contributions and highlights the roles of surface tension and solutal Marangoni forces. Overall, the findings suggest that current density dependent effects, such as solutal Marangoni forces, may play a dominant role in governing bubble behaviour at higher current densities, providing insights for minimizing bubble-induced ohmic resistance in alkaline water electrolyzers.