Coadsorption of NRR and HER Intermediates Determines the Performance of Ru-N₄ toward Electrocatalytic N₂ Reduction

Electrochemical N2 reduction (NRR) to ammonia is seriously limited by the competing hydrogen evolution reaction (HER), but atomic-scale factors controlling HER/NRR competition are unknown. Herein we unveil mechanism, thermodynamics, and kinetics determining efficiency on state-of-the-art NRR electrocatalyst, Ru-N4, using grand canonical ensemble density functional theory (GCE-DFT). We show that NRR/HER intermediates coadsorb catalyst where suppress HER selectivity determined initial step forming *NNH or *H. Our results provide crucial insight into complex competition, necessity of GCE-DFT calculations, suggest ways improve selectivity.