Fig. 1: Schematic illustrations of different surface properties of Pt-based cathodes during HER in natural seawater.

a Conventional Pt/Ti cathode which will be gradually covered by Mg(OH)2 precipitates. b Halogen-modified Pt-X/Ti (X: F, Cl, Br, I) cathode which can be stably operated due to the like-charge repulsion towards OH-.