Comprehensive Assessment of the Electrocatalytic Activity of Vanadium, Niobium Nitrides and Molybdenum-Based Materials Towards Dinitrogen Reduction to Ammonia

Electrochemical nitrogen reduction reaction (NRR) that can be powered by renewable energy is currently broadly investigated as a sustainable alternative method for the industrial ammonia synthesis to replace, at least partially, the Haber-Bosch technology. Inspired by recent theoretical reports advocating several types of materials as active NRR catalysts, our experimental work aimed to assess the validity of some of these predictions. Vanadium and niobium nitrides were suggested by the previous computational work to be catalytically active towards the electrochemical reduction of dinitrogen to ammonia occurring via the Mars-van Krevenlen (MvK) mechanism [1]. The present experimental study thoroughly investigates the electrocatalytic activity of cubic vanadium(III) nitride, niobium(III) nitride and tetragonal Nb 4 N 5 for the nitrogen reduction reaction in aqueous electrolyte solutions of different pH under ambient conditions [2]. VN and Nb 4 N 5 (supported on carbon cloth) were synthesised by annealing of hydrothermally produced hydroxide precursors in NH 3 atmosphere at 600-1100 °C; NbN was obtained by solid state reaction between niobium(V) chloride and urea at 1000 °C. Comprehensive testing of the materials under a wide range of aqueous conditions unambiguously demonstrates their inability to catalyse the electrosynthesis of ammonia from dinitrogen, as well as the propensity of VN synthesised at 600 °C and Nb 4 N 5 to release lattice nitride in a non-catalytic process, which produces ammonia under reductive conditions. Thus, polycrystalline nitrides of vanadium and niobium are concluded to be catalytically inactive towards the ammonia electrosynthesis from N 2 dissolved in water. When tested in non-aqueous aprotic electrolyte, Nb 4 N 5 sustained slow ammonia electrosynthesis (4 pmol s -1 cm -2 ) at low faradaic efficiency (20 %). The present work additionally emphasises 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. Another type of materials suggested by DFT to be catalytically active for the NRR at low overpotentials is molybdenum disulphide promoted with Mo metal particles (Mo/MoS 2 ), which was suggested to provide energetically favourable binding with N 2 molecule [3]. However, MoS 2 is a well-known hydrogen evolution catalyst rendering any reasonable selectivity for the N 2 reduction with Mo/MoS 2 in aqueo