Reducing the gap between the electrodes and diaphragm to zero is an often adopted strategy to reduce the ohmic drop in alkaline water electrolyzers for hydrogen production. We provide a thorough account of the current–voltage relationship in such a zero-gap configuration over a wide range of electrolyte concentrations and current densities. Included are voltage components that are not often experimentally quantified like those due to bubbles, hydroxide depletion, and dissolved hydrogen and oxygen. As is commonly found for zero-gap configurations, the ohmic resistance was substantially larger than that of the separator. We find that this is because the relatively flat electrode area facing the diaphragm was not active, likely due to separator pore blockage by gas, the electrode itself, and or solid deposits. Over an e-folding time-scale of ten seconds, an additional ohmic drop was found to arise, likely due to gas bubbles in the electrode holes. For electrolyte concentrations below 0.5 M, an overpotential was observed, associated with local depletion of hydroxide at the anode. Finally, a high supersaturation of hydrogen and oxygen was found to significantly increase the equilibrium potential at elevated current densities. Most of these voltage losses are shown to be easily avoidable by introducing a small 0.2 mm gap, greatly improving the performance compared to zero-gap.

Flow-through electrolyzers, with flow parallel to the current, are used in a wide range of industrial applications. The presence of flow avoids concentration gradients but can also be used to separate evolved gases, allowing membrane-less operation. In this work, we propose a simple multiphase flow-through electrode model. We derive and experimentally validate an analytical expression for the minimum velocity required to ensure effective gas separation. From this relation, we analytically derive design parameters that show that significant energy savings can be made using flow, compared to a physical separator.

A new compact electrode architecture with hollow pillar-shaped anodes and cathodes arranged in a ‘checkerboard’ pattern is analysed and shown to be equivalent to a particular arrangement of corrugated plate electrodes. Because all four sides of the flow channels are electrodes, this design takes up at least 1.5 to two times less volume compared to conventional ‘sandwich’-type configurations. The assumption underlying this theoretical scaling is illustrated with a 3D-printed metal prototype for alkaline water electrolysis using natural convection. For mass-transfer-limited electrolysers, fuel cells, electrowinning cells, and flow-batteries, the expected volume savings easily increase to a factor three or more.

Under alkaline conditions, hydroxide ions can deplete at the anode of a water electrolyser for hydrogen production, resulting in a limiting current density. We found experimentally that in a micro-porous separator, an electro-osmotic flow from anode to cathode lowers this limiting current density. Using the Nernst-Planck equation, a useful expression for the potential drop in the presence of diffusion, migration, and advection is derived. A quasi-stationary, one-dimensional model is used to successfully describe the transient dynamics. Electro-osmotic flow-driven cross-over of dissolved oxygen is argued to impact the hydrogen purity.