Rapid Aqueous Ammonia Oxidation to N₂ Using a Molecular Ru Electrocatalyst

The storage of renewable energy in ammonia (NH3) isa promising alternative to hydrogen (H-2) for our transitionfrom fossil fuels. Solid oxide fuel cells (SOFCs) are the leadingtechnology for direct ammonia fuel cells (DAFCs); however, their hightemperature of operation makes them unsuitable for light-duty vehicles.As such, there has been growing interest in aqueous-fed DAFCs. Suchtechnologies face challenges with respect to reaction selectivity,precious metal catalyst loading, and stability; however, there isa dearth of reported molecular catalysts to address such issues. Rutheniumbipyridinedicarboxylate complexes are known for catalyzing water oxidationat rapid rates and were recently reported to catalyze the oxidationof ammonia, i.e., nitrogen evolution reaction (NER) in acetonitrile,albeit at sluggish rates. Herein, we present our use of the knowncomplex [Ru(bipyridinedicarboxylate)(4-methylpyridine)(2)] (RuBda, 1) to electrocatalyze ammonia oxidation ofaqueous NH3 to N-2 at high faradaic efficiencies(>80%), unprecedented rates (turnover frequency & AP; 3757 s(-1)), and high turnover. Our kinetic analyses suggestthat the catalyst operates via a unimolecular mechanism, which ishighly applicable for commercially viable fuel cells.