Defective Antifluorite MnO₂-Layered RuO₂ for Direct Seawater Electrolysis at Circum-Neutral pH

The direct electrolysis of unbuffered seawater at pH 8.2 remains challenging. This study synthesizes RuO₂ electrodes with electrodeposited polymorphs of MnO₂ (δ-, ε-, γ-, and defective antifluorite [DA]-MnO₂) as a semipermeable overlayer that selectively allows water transport while blocking chloride diffusion. Whereas the oxygen evolution reaction (OER) and chlorine evolution reaction occur simultaneously with the bare RuO₂ electrode, only the former is observed with the MnO₂-coated RuO₂ electrodes. Compared to the crystalline δ-, ε-, and γ-MnO₂ polymorphs, the DA-MnO₂/RuO₂ electrode drives the OER at a Faradaic efficiency (FE) of ≈100% for over 100 h in unbuffered seawater, with an OER-FE of ≈98.7% and 90.0% in 1 M and 5 M NaCl, respectively. Based on theoretical calculations, the excellent electrocatalytic behavior of DA-MnO₂ is attributed to the steric structure of the staggered, narrower Mn-O polyhedron channels, inhibiting the diffusion of Cl^- through the MnO₂ overlayer.