How dual hydrogen bubble evolution inhibits electrolytic performance

Controlling bubble evolution on the electrodes of water-splitting cells or chemical reactors is crucial for achieving higher energy efficiencies in these devices. However, the evolution of dual bubbles on electrodes and their impact on electrolysis efficiency remain largely unexplored. This study combines high-speed imaging and electrochemical analysis to investigate the growth dynamics of two bubbles generated by dual platinum microelectrodes with different reaction kinetics, using an interelectrode distance of 800 μm, while varying the electrolyte concentration and the cathodic potential. The evolution of dual bubbles reveals three distinct modes: Mode I (coalescence-driven detachment), Mode II (alternating coalescence and buoyancy-driven detachment), and Mode III (buoyancy-driven detachment), each associated with unique electrochemical signals. Although bubble coalescence in Mode I results in earlier detachment, the electrolytic efficiency at constant potential decreases by up to 31 % compared to that of a single electrode, at -6V and 0.6 M electrolyte concentration. This reduction is attributed to coalescence-induced premature detachment, which shortens the time from plateau to peak current, thereby lowering efficiency. In Mode III, the reaction rate is slower than with a single electrode, attributed to greater Marangoni forces in the two-electrode setup. Mode II, however, is influenced by both of the above factors.