The electrodes in modern electrolysers for the production of green hydrogen have holes to let bubbles escape, but what size should these holes be? Despite more than half a century since its first usage, there is no clear guidance on this question available in the open literature.
Therefore, we performed an extensive experimental campaign under industrially relevant conditions with 17 electrodes, with varying hole sizes and shapes. We found that by far the most important feature determining the electrode performance was the hole size, not its shape, or the electrode thickness or other features like the presence of pillars.
We found that for most relevant current densities and atmospheric pressure a hole size of several millimeter is optimal. Much larger holes lead to larger cell voltages as the current becomes more inhomogeneously distributed, increasing the resistance. But particularly much smaller holes lead to excessive overpotentials.

To study why this is, we looked at the behavior of gas bubbles, as they leave the holes from the back, but also from the front, through a transparent membrane. It turned out that if the holes become so small that many times per second bubbles are generated that can fill these holes, the chance of them clogging these holes becomes large. Holes substantially smaller than a millimetre tend to clog very frequently. This clogging leads to large gas films forming between the electrode and the diaphragm, inactivating large parts of the electrode surface, forcing the reaction to the backside, increasing the resistance.
Millimeter-sized holes are largely immune to this clogging and perform much better, which explains why commercial expanded metal electrodes have holes in this size range.