 Seawater electrolysis is promising for grid-scale H2 production without freshwater reliance, but high energy costs and detrimental Cl chemistry reduce its practical potential. Here, authors developed an energy-saving hybrid seawater electrolyzer for chlorine-free H2 production, an N2H4 degradation. This article was authored by Fu Sun, Jingxian Qing, Jiyu Wang, and others.