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

摘要

We fabricated a thin film of layered MnO_(2) whose interlayer space was occupied by hydrated Ni~(2+) and Cu~(2+) ions. The process consisted of electrodeposition of layered MnO_(2) intercalated with tetrabutylammonium cations (TBA~(+)) by anodic oxidation of aqueous Mn~(2+) 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 MnO_(2) co-intercalated with Ni~(2+) and Cu~(2+) ions (NiCu/MnO_(2)) catalyzed the ammonia oxidation reaction (AOR) in an alkaline electrolyte with a much lower overpotential than its Ni~(2+)- and Cu~(2+)-intercalated single-cation counterparts. Surprisingly, the NiCu/MnO_(2) 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_(x) species.