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.