Rapid Aqueous Ammonia Oxidation to N₂ Using a Molecular Ru Electrocatalyst

Abstract

The storage of renewable energy in ammonia (NH3) is a promising alternative to hydrogen (H2) for our transition from fossil fuels. Solid oxide fuel cells (SOFCs) are the leading technology for direct ammonia fuel cells (DAFCs); however, their high temperature of operation makes them unsuitable for light-duty vehicles. As such, there has been growing interest in aqueous-fed DAFCs. Such technologies face challenges with respect to reaction selectivity, precious metal catalyst loading, and stability; however, there is a dearth of reported molecular catalysts to address such issues. Ruthenium bipyridinedicarboxylate complexes are known for catalyzing water oxidation at rapid rates and were recently reported to catalyze the oxidation of ammonia, i.e., nitrogen evolution reaction (NER) in acetonitrile, albeit at sluggish rates. Herein, we present our use of the known complex [Ru(bipyridinedicarboxylate)(4-methylpyridine)2] (RuBda, 1) to electrocatalyze ammonia oxidation of aqueous NH3 to N2 at high faradaic efficiencies (>80%), unprecedented rates (turnover frequency ≈ 3757 s–1), and high turnover. Our kinetic analyses suggest that the catalyst operates via a unimolecular mechanism, which is highly applicable for commercially viable fuel cells.