context: The increased environmental hazards from the use of carbon-emitting fuels and the rapid depletion of conventional fossil fuels have triggered the research community to find alternatives to fossil-fuel-based technologies. In many innovative approaches to address these challenges, electrocatalytic water splitting driven by a renewable energy input to produce clean H2 has been widely viewed as a promising strategy as hydrogen is produced from renewable sources (such as solar and wind energy), which can be stored, transported and consumed without generating any carbon-based byproducts. Electrolysers are a crucial factor for water electrolysis. Currently, great efforts are being made to develop new electrocatalysts with high efficiency for overall water splitting. In addition to the advancement of electrocatalysts, another equally important aspect of water electrolysis is the development of novel electrolyzer design. The conventional cell design of a alkaline water electrolyser is illustrated in Scheme 1a. Both electrodes are dipped into an aqueous alkaline electrolyte with a separator (ion exchange membrane or porous diaphragm) between them. H2 and O2 are produced on the electrodes via two half-cell reactions: the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). In the lab, these electrolysers are used directly, while in industry, these electrolysers are used in stacks to scale-up H2 production. For electrolysers, major costs are due to the fabrication and material for the three main components: anodes, cathodes and separators. The existence of a separator not only increases the cost of the electrolyser, but also induces extra overpotentials for water splitting. A new electrolyser with a simple, condensed, membrane-free design is highly desired since it not only requires lower overpotentials, but also needs smaller areas and fewer materials, which reduces the overall cost of the electrolyser cells and promotes the electrochemical water splitting.