Hydrogen produced from neutral seawater electrolysis faces many challenges including high energy consumption, the corrosion/side reactions caused by Cl-, and the blockage of active sites by Ca2+/Mg2+precipitates. Herein, we design a pH-asymmetric electrolyzer with a Na+exchange membrane for direct seawater electrolysis, which can simultaneously prevent Cl-corrosion and Ca2+/Mg2+precipitation and harvest the chemical potentials between the different electrolytes to reduce the required voltage. In-situ Raman spectroscopy and density functional theory calculations reveal that water dissociation can be promoted with a catalyst based on atomically dispersed Pt anchored to Ni-Fe-P nanowires with a reduced energy barrier (by 0.26 eV), thus accelerating the hydrogen evolution kinetics in seawater. Consequently, the asymmetric electrolyzer exhibits current densities of 10 mA cm−2and 100 mA cm−2at voltages of 1.31 V and 1.46 V, respectively. It can also reach 400 mA cm−2at a low voltage of 1.66 V at 80 °C, corresponding to the electricity cost of US$1.36 per kg of H2($0.031/kW h for the electricity bill), lower than the United States Department of Energy 2025 target (US$1.4 per kg of H2).