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

We fabricated a thin film of layered MnO2 whose interlayer space was occupied by hydrated Ni-2(+) and Cu-2(+) 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 Ni-2(+)- 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 NOx species.