Solutal Marangoni effects on growing hydrogen and oxygen bubbles in alkaline water electrolysis

We investigate solutal Marangoni flow around a single growing bubble at a macro electrode in a small-scale model of alkaline water electrolysis at 30 wt% KOH. The hydrogen and the oxygen side are simulated separately, using an immersed boundary method. The Marangoni flow is caused by surface tension variations due to the gradient in the KOH concentration. At a current density of 1600 A/m², the Marangoni flow is found to exert a large force on the bubble, larger than the buoyancy force, for a bubble radius up to 25 μm. Lowering the current density decreases the force on the bubble. The force on the hydrogen bubble is directed away from the electrode, while the force on the oxygen bubble is directed towards the electrode. This could explain why hydrogen bubbles detaching from a macro electrode are often smaller than oxygen bubbles detaching from a macro electrode. Moreover, in this specific small-scale model, the Marangoni flow reduces the hydrogen mass flow rate into the hydrogen bubble, while it increases the oxygen mass flow rate into the oxygen bubble.