Modulating built-in electric field <i>via</i> Br induced partial phase transition for robust alkaline freshwater and seawater electrolysis

Repulsing Cl^- to reduce its negative effects during seawater electrolysis is a promising strategy to guard against the corrosion of high-valence metal sites. Herein, we synthesized Fe₂P/Ni₂P by a facile Br-induced partial <i>in situ</i> phase transition strategy. This Fe₂P/Ni₂P possessed intensified built-in electric field (BEF) due to large work function difference (Δ<i>Φ</i>), demonstrating outstanding OER and HER activity in alkaline freshwater/seawater solution and exhibiting a low cell voltage for an anion exchange membrane water electrolyzer (AEMWE) system. Both experiments and theoretical results verify that the interfacial charge redistribution induced by the enhanced BEF optimizes the adsorption strength for the intermediates. Moreover, the appropriate phosphorus-oxygen anion self-transformation can protect the NiOOH active species from corrosion by repulsing Cl^- in alkaline seawater. This work not only proposes a fresh perception of the water/seawater splitting mechanism but also provides new design principles to defend active sites in seawater-to-H₂ conversion systems.