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. We have employed grand canonical ensemble density functional theory (GCE-DFT) calculations together with the hybrid solvation model to address the reaction mechanism, thermodynamics, and kinetics determining the HER/NRR efficiency on the state-of-the-art NRR electrocatalyst, a Ru single atom anchored on nitrogen doped graphene (Ru-N₄). Our calculations show that the catalyst studied exhibits stronger affinity towards N₂ than H adsorption, suggesting that NRR could be preferred. The N₂ adsorption hinders hydrogen deposition through the Volmer step compared to the empty Ru-N₄ site. However, the first Volmer step after N₂ adsorption is still more favoured over N₂ hydrogenation. Once *N₂ and *H are coadsorbed, H₂ is easily generated through the Heyrovsky step whereas the formation of the first NRR intermediate, *NNH, is kinetically and thermodynamically more difficult limiting the overall NRR activity and selectivity. If one can overcome the difficult formation of *NNH, HER is significantly suppressed until NH₃ is released. The presence of *N₂ and NRR species significantly suppresses HER activity and thus we suggest that the experimentally observed 30% selectivity towards NRR on Ru-N₄ results from the hindered Volmer step in the presence of adsorbed NRR intermediates and a “non-innocent” *N₂ ligand. The obtained computational results provide crucial insight into the complex NRR/HER competition, show the necessity of using GCE-DFT calculations, and suggest ways to improve NRR selectivity.

Karoliina Honkala

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