Coadsorption of NRR and HER intermediates determines the performance of Ru-N4 towards electrocatalytic N2 reduction

Efficiency of the electrochemical N2 reduction reaction (NRR) to ammonia is seriously limited by the competing hydrogen evolution reaction (HER) but our current atomic-scale insight on the factors controlling HER/NRR competition are unknown. Herein we unveil the elementary mechanism, thermodynamics, and kinetics determining the HER/NRR selectivity on the state-of-the-art NRR electrocatalyst, Ru-N4 using constant potential density functional theory calculations (DFT). The calculations show that NRR and HER intermediates coadsorb on the catalyst where HER is greatly suppressed by the NRR intermediates. The first reaction step leading to either *NNH or *H determines the selectivity towards NRR or HER. Our results also demonstrate that an explicitly potential-dependent treatment of reaction kinetics is needed to understand NRR selectivity. We provide crucial insight into the complex NRR/HER competition and the role of non-innocent adsorbates, show the necessity of constant potential DFT calculations, and suggest that interfacial proton donors will improve NRR selectivity.