Ni- and Cu-co-Intercalated Layered Manganese Oxide for Highly Efficient Electro-Oxidation of Ammonia Selective to...

We fabricated a thin film of layered MnO₂ whose interlayer space was occupied by hydrated Ni²⁺ and Cu²⁺ ions. The process consisted of electrodeposition of layered MnO₂ intercalated with tetrabutylammonium cations (TBA⁺) by anodic oxidation of aqueous Mn²⁺ ions in the presence of TBA⁺, followed by ion exchange of the initially incorporated bulkier TBA⁺ with the denser transition metals in solution. The resulting layered MnO₂ co-intercalated with Ni²⁺ and Cu²⁺ ions (NiCu/MnO₂) catalyzed the ammonia oxidation reaction (AOR) in an alkaline electrolyte with a much lower overpotential than its Ni²⁺- and Cu²⁺-intercalated single-cation counterparts. Surprisingly, the NiCu/MnO₂ electrode achieved a faradic efficiency as high as nearly 100% (97.4%) for nitrogen evolution at a constant potential of +0.6 V vs Hg/HgO. This can be ascribed to the occurrence of the AOR in the potential region where water is stable and dimerization of the partially dehydrogenated ammonia species is preferred, thereby forming an N-N bond, rather than to be further oxidized into NO species.