A Bilayer Structure Composed of Mg|Co-MnO Deposited on a Co(OH) Film to Realize Selective Oxygen Evolution from Chloride-Containing Water.

A fluorine-doped tin oxide-coated glass electrode modified with a bilayer film of underlying α-Co(OH) and overlying Mg-intercalated and Co-doped δ-type (layered) MnO (Mg|Co-MnO) preferentially yielded oxygen with a Faradaic efficiency as high as 79% in the presence of chloride ions at high concentration (0.5 M). This noble metal-free electrode was fabricated by cathodic electrolysis of aqueous Co(NO) followed by anodic electrolysis of a mixture of Mn, Co, and cetyltrimethylammonium (CTA) ions in water. The CTA ions accommodated in the interlayer spaces of Co-doped δ-MnO were replaced with Mg by ion exchange. The upper Mg|Co-MnO could effectively block the permeation of Cl ions and allow only HO and O, while the under α-Co(OH) acted as an oxidation catalyst for the HO penetrated through the upper coating. Thus, the oxygen evolution reaction (OER) was preferred to the chlorine evolution reaction (CER). In artificial seawater (pH 8.3), the blocking effect against Cl decreased because of ion exchange of the intercalated Mg ions with Na in solution, but the OER efficiency still remained at 57%, much higher than that (28%) without the upper Mg|Co-MnO. This demonstrates that the interlayer spaces between MnO layers acted as pathways for HO molecules to reach the active sites of the underlying Co(OH). Density functional theory (DFT) calculations revealed that the most stable structure of hydrated Mg ion, in which a part of coordinated HO molecules is hydrolyzed, has less affinity toward Cl ion than that of hydrated Na ion.