Coalescence induced late departure of bubbles improves water electrolysis efficiency

In this study, we examine the effects of bubble collisions and coalescence on electrolysis performance using micro-electrodes in a 0.5 M H2SO4. We observe that the addition of electrolytes such as HClO4, despite increasing conductivity, significantly reduces energy conversion efficiency by inhibiting bubble coalescence. A phase diagram illustrates the trade-off between improved conductivity and inhibition of coalescence with varying concentrations of supporting electrolytes (HClO4 or Na2SO4) at different electrolysis current. Contrary to conventional understanding, we find that larger bubbles resulting from coalescence enhance efficiency. This can be attributed to the early release of smaller bubbles, approximately 10 micron in size, facilitated by the coalescence of the surface microbubble and the just-detached large bubble. Additionally, the merge of two bubbles, at a critical velocity of ~2 m/s, enhances agitation at the electrode surface that significantly improves electrolyte transport within the stagnant layer. These findings challenge conventional perspectives on bubble detachment size and suggest innovative approaches to optimize energy consumption in electrolysis, especially in systems where the bubble coalescence is inhibited, such as alkaline water electrolysis and the chlor-alkali industry.