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.