Pretreatment-Free Direct Seawater Electrolysis Using Chloride-Blocking Bipolar Membranes

Direct seawater electrolysis (DSE) is an attractive route to sustainable hydrogen production but suffers from competing chlorine evolution reaction (CER) and hydroxide scaling, typically requiring prealkalization or desalination. Here, we report a pretreatment-free DSE system enabled by a bipolar membrane (BPM) comprising an ultrathin cation-exchange layer (CEL) and a catalytic MXene@FeOOH nanosheet interlayer. This architecture creates a cooperative ion-sieving environment, where electrostatic exclusion from the cation-exchange layer and overlapping electric double layers within the nanosheets synergistically suppress chloride crossover, thereby mitigating parasitic CER at the anode. Simultaneously, protons generated at the BPM junction maintain a mildly acidic catholyte, preventing Mg(OH)₂ and Ca(OH)₂ deposition. In an asymmetric configuration with real seawater at the cathode and air at the anode, the electrolyzer sustains nearly 100% hydrogen Faradaic efficiency for 80 h at 100 mA cm^-2, with negligible chlorine evolution and hydroxide precipitation. This work establishes a practical membrane strategy for efficient, pretreatment-free seawater-to-hydrogen conversion.