Bubble evolution during hydrogen production under normal gravity and microgravity: A review

Hydrogen production by water electrolysis is a promising energy source for space missions, yet the lack of buoyancy in space reduces the efficiency of electrochemical reactions because bubbles of hydrogen accumulate on electrode surfaces. In this review, we systematically compare and discuss the differences in bubble evolution behaviors and key factors under normal gravity and microgravity environments, focusing on nucleation, growth, detachment, coalescence, and adhesion. We discover higher bubble nucleation probability and faster growth direction are provided in hydrophobic electrode, and this effect being further amplified in microgravity. Moreover, Marangoni forces are accentuated in microgravity, further inhibiting bubble detachment. Then, effective strategies under normal gravity, like applying ultrasonic fields, magnetic fields, flow fields, and electrode structure modifications, are proposed to mitigate bubble coverage, offering valuable references for space applications. Lastly, future perspectives are proposed to address technical challenges and advance the development of reliable water electrolysis systems for space missions.