Ruthenium single-atom catalysis for electrocatalytic nitrogen reduction unveiled by grand canonical density functional theory

To substitute the intensive energy-consuming Haber-Bosch process for the industrial production of ammonia, the theoretical exploration of catalysts for electrocatalytic nitrogen reduction reaction (eNRR) into ammonia is critical to obtain an insightful mechanism and principles for designing electrocatalysts. However, most studies adopted the constant charge model (CCM), although an electrochemical system corresponds to the constant potential model (CPM) in reality. Herein, considering the benchmark of carbon-supported Ru single atom catalysts, we systematically examined the adsorption of eNRR intermediates under the condition of constant electrode potential to obtain the eNRR mechanism and optimal active site based on the grand canonical density functional theory (GC-DFT). The comparison study demonstrated that, during the electrochemical processes, the inherent electrons exchanged, ignored by the CCM, play an important role in the quantitative determination of Gibbs free energy change, particularly at the potential-determining step, leading to the conclusion that the RuN4 motif has a lower limiting potential (-0.66 V) than the RuN3 motif (-1.22 V) in the CPM. Importantly, Ru, a single atom bonded with two carbon atoms and two nitrogen atoms, is identified as the optimal eNRR reaction site with the lowest limiting potential of -0.43 V. This work both accurately describes the practical eNRR process for Ru-based single-atom catalysts from the prospective of electrocatalysis and improves the identification protocol for the theoretical exploration and design of electrocatalysts.