Hexafluorophosphate additive enables durable seawater oxidation at ampere-level current density.

Direct seawater electrolysis at ampere-level current densities, powered by coastal/offshore renewables, is an attractive avenue for sustainable hydrogen production but is undermined by chloride-induced anode degradation. Here we demonstrate the use of hexafluorophosphate (PF₆⁻) as an electrolyte additive to overcome this limitation, achieving prolonged operation for over 5,000 hours at 1 A cm and 2300 hours at 2 A cm using NiFe layered double hydroxide (LDH) as anode. Together with the experimental findings, PF₆⁻ can intercalate into LDH interlayers and adsorb onto the electrode surface under an applied electric field, blocking Cl⁻ and stabilizing Fe to prevent segregation. The constant-potential molecular dynamics simulations further reveal the accumulation of high surface concentrations of PF on the electrode surface that can effectively exclude Cl, mitigating corrosion. Our work showcases synchronous interlayer and surface engineering by single non-oxygen anion species to enable Cl rejection and marks a crucial step forward in seawater electrolysis.