Solutal Marangoni convection at growing oxygen bubbles during water electrolysis

Electrochemical water splitting leads to variations in the electrolyte concentration which might impact the surface tension at both hydrogen and oxygen bubbles. Although the potentially resulting solutal Marangoni convection has been predicted, it has not yet been observed in experiments. To discover solutal Marangoni convection, the potential applied during the oxygen evolution reaction at microelectrodes, as well as the electrolyte concentration, have been systematically varied. To capture the bubble dynamics and local variations of the refractive index, a combination of high-speed shadowgraphy, particle tracking velocimetry, and schlieren imaging was employed. The analysis has identified an exotic regime of oxygen bubbles at high potentials, characterized by slow growth that is interrupted by intermittent current spikes, causing fast growth. While the fast growth is expectedly dominated by thermocapillary convection, the slow growth displays a vortical flow that circulates in the reverse direction at a much slower velocity. This flow pattern is attributed to an increased sulfuric acid concentration at the bubble foot, induced by proton generation during oxygen evolution. This locally elevates the surface tension and causes the solutal Marangoni flow. These findings confirm the theoretical prediction of solutal effects in the bubble dynamics during water electrolysis and reveal the fascinating physics behind.