High Speed Video Observation of Gas Bubble Evolution Phenomena Accompanying Water Electrolysis on Transparent Electrode under Quasi-Microgravity

Galvanostatic water electrolysis was conducted in 2 wt. % KOH solution for 5 s under terrestrial and quasi-mu-G condition. These conditions were realized by using two types of cell arrangement: (1) the vertical arrangement and (2) the downward facing horizontal electrode arrangement over the counter electrode, respectively. The latter arrangement can be regarded as a quasi-mu-G condition because the natural convection induced by generating gas bubbles is macroscopically excluded. The gas bubble formation of O-2 or H-2 was in-situ observed by using high speed camera. Sputter-deposited Pt film on transparent FTO glass was used as the working electrode to record backside images of evolving bubbles. O-2 gas bubbles were less hydrophilic than H-2 and tended to coalesce with each other to grow in larger size. They stayed much longer on electrode surface than H-2. The attachment of bubbles decreased the effective reaction area to result in energy loss for O-2 gas evolution. Quasi-mu-G experiment made it possible to measure the larger contact angle of O-2 than H-2, but difficult to realize H-2 froth layer formation. Thus, the measured results in the downward facing electrode arrangement were partially coincident with those in microgravity experiment conducted in a drop tower. It suggests that careful and appropriate experimental design with downward facing electrode may sometimes reduce the expenses and time necessary for the scientific research in the international space station.