Correlation between the Forming Sites of Hydrogen Bubbles and Micro/Nanostructures on the Electrode Surface

We investigate the correlation between the forming sites of hydrogen bubbles and micro/nanostructures on the electrode surface. For this purpose, we optically monitor the formation of hydrogen bubbles over a wide area on a vertical Ni disk cathode during water electrolysis from the front of the cathode surface to identify the bubble forming sites, and label them on the surface profile of the same area with ∼μm accuracy. It turns out that these sites have very shallow microholes with a few tens of μm width and a few hundred nm depth. However, neither the width nor the depth of such holes has a clear correlation with the growth rates or formation periods of bubbles. Instead, irrespective of the local surface morphologies of the bubble forming sites, the growth rates and formation periods of the bubbles exhibit a clear correlation, and these findings strongly imply that the other factor, probably the local convection, plays a more important role to determine the fates of bubbles.