Coalescence induced late departure of bubbles improves water electrolysis efficiency

In water electrolysis, bubbles form on the electrode and interact through processes such as collision and coalescence. However, the impact of bubble coalescence a fundamental process governing electrolytic bubble behaviour-on electrolysis efficiency remains unclear. Here, we show that enhancing bubble coalescence improves electrolysis efficiency by more than 30% compared to systems where coalescence is inhibited. One key feature is the continuous coalescence of a newly detached bubble with microbubbles on the electrode, which delays the former from departing. Experimental observations and numerical simulations reveal two key benefits of bubble coalescence for electrolysis efficiency: (1) it liberates surface bubbles from the electrode at much smaller sizes, reducing their diameter from approximately 60-80 um to less than 10 um, thus freeing the active sites of the electrode from bubble coverage; (2) it induces strong agitation, with velocities reaching 1m/s in a small region near the electrode (at a depth of 10-5 m), thereby significantly improving the heat/mass transfer locally. Importantly, the chaotic agitation effect lasts for approximately 10 ms, two orders of magnitude longer than the coalescence process, which occurs in around 0.2 ms. This work provides valuable insight into bubble management in water electrolysis and other gas-evolution electrochemical reactions.