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 orderof-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.