The optimal electrode hole size in zero gap alkaline water electrolysis: A combined electrochemical, theoretical, and bubble imaging approach

The ohmic resistance of an alkaline water electrolyser for green hydrogen production can be reduced by minimising the distance between the electrodes and the diaphragm. A zero-gap configuration requires holes in the electrode to transport the produced gases to the backside of the electrode. Industry typically uses expanded metals and perforated plates with hole sizes of one or a few millimetres, but the optimal hole size is not known. In this study, we experimentally assess the overpotentials as a function of hole size, shape, and open area fraction using a wide variety of electroformed nickel electrodes of 30 cm 2 up to 1 0 4 A/m 2 in 80 °C 30 w % KOH. We find that for sub-millimetre holes, the overpotentials strongly increase as hole size decreases. The reason is that small hole sizes make it difficult for the gas bubbles to leave effectively, leading to coalescence and clogging. Consequently, a gas film can arise between the electrode and the diaphragm, as shown by through-the-membrane images. Therefore, the increased surface area associated with these small holes is not effectively used. We show that performance can be improved by taking away surface area through additional larger holes in a small hole-size electrode, which allow bubbles to effectively evacuate from the electrodes. • Holes of several millimetres are preferred for zero gap alkaline water electrolysers. • Smaller holes get clogged by bubbles, especially at high current densities. • Larger holes increase ohmic losses due to inhomogeneous current distribution. • A semi-empirical simple expression for optimal hole size is derived. • Using small and large holes combines high surface area with good bubble evacuation.