A Bilayer Structure Composed of Mg|Co-MnO₂ Deposited on a Co(OH)₂ Film to Realize Selective Oxygen Evolution from...

A fluorine-doped tin oxide-coated glass electrode modified with a bilayer film of underlying α-Co(OH)₂ and overlying Mg-intercalated and Co-doped δ-type (layered) MnO₂ (Mg|Co-MnO₂) 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(NO₃)₂ followed by anodic electrolysis of a mixture of Mn²⁺, Co²⁺, and cetyltrimethylammonium (CTA⁺) ions in water. The CTA⁺ ions accommodated in the interlayer spaces of Co-doped δ-MnO₂ were replaced with Mg²⁺ by ion exchange. The upper Mg|Co-MnO₂ could effectively block the permeation of Cl^- ions and allow only H₂O and O₂, while the under α-Co(OH)₂ acted as an oxidation catalyst for the H₂O 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 Mg²⁺ ions with Na⁺ in solution, but the OER efficiency still remained at 57%, much higher than that (28%) without the upper Mg|Co-MnO₂. This demonstrates that the interlayer spaces between MnO₂ layers acted as pathways for H₂O molecules to reach the active sites of the underlying Co(OH)₂. Density functional theory (DFT) calculations revealed that the most stable structure of hydrated Mg²⁺ ion, in which a part of coordinated H₂O molecules is hydrolyzed, has less affinity toward Cl^- ion than that of hydrated Na⁺ ion.