A Zero-Gap Electrolyzer Enables Supporting Electrolyte-Free Seawater Splitting for Energy-Saving Hydrogen Production

Membrane-assisted direct seawater splitting (DSS) technologies are actively studied as a promising route to produce green hydrogen (H₂), whereas the indispensable use of supporting electrolytes that help to extract water and provide electrochemically-accelerated reaction media results in a severe energy penalty, consuming up to 12.5 % of energy input when using a typical KOH electrolyte. We bypass this issue by designing a zero-gap electrolyzer configuration based on the integration of cation exchange membrane and bipolar membrane assemblies, which protects stable DSS operation against the precipitates and corrosion in the absence of additional supporting electrolytes. The heterolytic water dissociation function of the bipolar membrane in situ creates an asymmetric acidic-alkaline environment, kinetically facilitating H₂ and O₂ evolution reactions. When working in natural seawater without any chemical inputs, this zero-gap electrolyzer sustains nearly 100 % Faradaic efficiency toward H₂ for 120 h at a current density of 100 mA cm^-2. With the high-integrity merit, our electrolyzer can be facilely scaled up into practical cell stacks with significantly increased active area and promising prospects for volume/space-sensitive application scenarios. This electrolyzer concept opens an underexplored design space for energy-saving H₂ production from low-grade saline water sources, being complementary to, and potentially competitive with the pre-purification Schemes.