Membraneless flow-through water electrolysis

Abstract

Membraneless flow-through water electrolysers keep the generated hydrogen away from oxygen bubbles by forced convection, removing membrane costs while improving energy efficiency. Previously, micrometre holes were used to ensure a homogeneous flow distribution. Here, we strongly reduce flow resistance using electrodes with large holes, while a discharge channel that widens in the direction of the main flow improves flow distribution. At too low liquid velocities, we provide direct in-situ measurements showing that the bubble size between the electrodes increases over time and with height due to coalescence. Counterintuitively, we find that a straight discharge channel requires less liquid flow. Using a combination of multiphase simulations and imaging experiments, we find that the higher velocity through the final downstream holes in a straight discharge channel prevents gas bubbles in the gap. The superficial velocity at which the electrolyte enters the feed channel must be an order of magnitude greater than that of the hydrogen produced at the electrode. The electrolyte velocity through the last holes should be even two orders of magnitude greater, so the gas fraction stays well below 1 percent there. Interestingly, at lower liquid flow rates, computational fluid dynamics predicts that natural electrolyte convection diverts the flow towards the hydrogen-producing electrode. By identifying the local flow requirements, this work helps to balance low pressure drop with effective gas bubble removal in membraneless electrolysers.