Investigation into the mechanism of electrochemical nitrogen reduction reaction to ammonia using niobium oxynitride thin-film catalysts

Highlights: • Growth and optimization of Nbo_xN_y thin films with controlled N/O stoichiometries. • Ammonia synthesis in a micro-reactor flow-cell following strict protocols. • Reactor connected to ammonia quantification system for accurate ammonia measurement. • Multiple electrochemical techniques and surface analysis. • Ammonia synthesis mechanism on Nbo_xN_y proposed based on experimental results. -- Abstract: Niobium oxynitride (NbO_xN_y) thin films with varying combined non-metal vs. metal stoichiometries (x + y) and N/O stoichiometric ratios (y/x) are investigated for their ability to catalyze the nitrogen reduction reaction and ammonia synthesis at ambient conditions. Electrochemical impedance spectroscopy and ammonia measurements show stark differences both in nitrogen vs. argon media on each surface and on the surfaces in the series when the combined stoichiometry of N+O vs. Nb increases. Surface stability checks at fixed intervals during the experiments and surface characterization after the experiments using X-ray diffraction reveal the least changes occurred to the surface with the highest N+O stoichiometry. Based on these observations, an ammonia synthesis mechanism is proposed. Isotope labeling experiments on the most promising surface of the series, however, show no sign of catalytically produced ammonia, possibly due to the lack of stability of the surface to endure through the ammonia production cycle.