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
x
species.
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