The electrochemical conversion of ammonia to dinitrogen in a direct ammonia fuel cell (DAFC) is a necessary technology for the realization of a Nitrogen Economy. Previous efforts to catalyze this reaction with molecular complexes require the addition of exogenous oxidizing reagents or application of potentials that lie significantly above the thermodynamic potential for the oxygen reduction reaction (ORR) under alkaline conditions. These high operating potentials prevent such complexes from being viable mediators for the ammonia oxidation reaction (AOR) at the anode of DAFCs that utilize ORR at the cathode. We report a stable metal-metal bonded diruthenium complex Ru₂^II,III(chp)₄OTf (chp⁻ = 6-chloro-2-hydroxypyridinate) that spontaneously produces dinitrogen from ammonia under ambient conditions. The resulting reduced Ru₂^II,II diruthenium material can be fully re-oxidized to the Ru₂^II,III redox state via bulk electrolysis at applied potentials as negative as −255 mV vs Fc^0/+ and by direct reaction with molecular oxygen. In both cases the re-oxidized diruthenium product can perform subsequent reactions with ammonia to again release dinitrogen. These findings demonstrate the most negative operating potential reported for electrocatalytic oxidation of ammonia by a molecular complex and the unprecedented regeneration of a catalytically active species using molecular oxygen as the terminal oxidant, marking Ru₂^II,III(chp)₄OTf as the first homogeneous electrocatalyst capable of performing the fundamental chemistry necessary for a DAFC.

Michael J Trenerry

Graduate Student, UW Madison Department of Chemistry

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