Defective Antifluorite MnO 2 ‐Layered RuO 2 for Direct Seawater Electrolysis at Circum‐Neutral pH

Abstract The direct electrolysis of unbuffered seawater at pH 8.2 remains challenging. This study synthesizes RuO 2 electrodes with electrodeposited polymorphs of MnO 2 (δ‐, ε‐, γ‐, and defective antifluorite [DA]‐MnO 2 ) 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 RuO 2 electrode, only the former is observed with the MnO 2 ‐coated RuO 2 electrodes. Compared to the crystalline δ‐, ε‐, and γ‐MnO 2 polymorphs, the DA‐MnO 2 /RuO 2 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‐MnO 2 is attributed to the steric structure of the staggered, narrower Mn‐O polyhedron channels, inhibiting the diffusion of Cl − through the MnO 2 overlayer.