Critical Assessment of the Electrocatalytic Activity of Vanadium and Niobium Nitrides toward Dinitrogen Reduction to Ammonia

Previous theoretical work has predicted vanadium and niobium nitrides to be catalytically active toward the electrochemical reduction of dinitrogen to ammonia and inactive for the hydrogen evolution reaction. The present experimental study investigates the electrocatalytic activity of vanadium(III) nitride, niobium(III) nitride, and Nb4N5 for the nitrogen reduction reaction in aqueous electrolyte solutions of different pH under ambient conditions using a robust testing protocol and thoroughly controlled experimental conditions to exclude any contamination with adventitious sources of ammonia and nitrogen oxides. VN and Nb4N5 (supported on carbon cloth) were synthesized by annealing of hydrothermally produced hydroxide precursors in an NH3 atmosphere at 600-1100 degrees C; NbN was obtained by a solid-state reaction between niobium(V) chloride and urea at 1000 degrees C. Comprehensive testing of the materials under a wide range of conditions unambiguously demonstrates their inability to catalyze the electrosynthesis of ammonia from dinitrogen, as well as the propensity of VN (synthesized at 600 degrees C) and Nb4N5 to release lattice nitride in a noncatalytic process, which leads to the formation of ammonia under reductive conditions. Thus, polycrystalline nitrides of vanadium and niobium are concluded to be catalytically inactive toward the ammonia electrosynthesis from N-2 dissolved in water. The present work additionally emphasizes the compulsory requirement for the implementation of reliable testing and analysis procedures for the assessment of the catalytic properties of materials for the nitrogen reduction reaction.