Crystal structures of molybdenum borides dictate electrocatalytic ammonia synthesis efficiency.

Rather than multivalence molybdenum (Mo), the low valence Mo has rarely been reported as active site for nitrogen reduction. Herein, molybdenum borides with various Mo-B stoichiometry ratios (Mo 2 B, α-MoB, and Mo 2 B 4) in which Mo shows low valence (<1) are synthesized as electrochemical nitrogen reduction reaction (eNRR) catalysts. Mo 2 B 4 demonstrates the highest NH 3 yield of 7.65 µg h− 1/mg at − 0.15 V with Faradaic efficiency (FE) of 12.47 %, while α-MoB exhibits the fastest intrinsic eNRR reaction rate with a higher FE of 17.17 % after considering electrochemically active surface area. DFT calculations reveal that both enzymatic and consecutive mechanisms via side-on configuration can proceed on α-MoB. Additionally, α-MoB exhibits suppressed HER activity due to an optimal surface B occupancy. The eNRR of molybdenum borides were verified qualitatively and quantitatively by 15N 2 isotope experiments. This study demonstrated a synergistic design of eNRR and HER activity to achieve efficient electrocatalytic ammonia production with high eNRR selectivity. [Display omitted] • Phase-pure molybdenum borides with tunable composition are synthesized without high pressure. • eNRR performance depends strongly on the crystallographic structure of molybdenum borides. • A synergistic design of eNRR and HER to achieve efficient electrocatalytic ammonia production with high eNRR selectivity was demonstrated. [ABSTRACT FROM AUTHOR]