Tuning the Coordination Environment to Effect the Electrocatalytic Behavior of a Single-Atom Catalyst toward the Nitrogen Reduction Reaction

Continual development of more advanced catalysts to support the more efficient electrocatalytic nitrogen reduction reaction (NRR) as a qualified substitute of the industrial Haber–Bosch reaction holds great significance but still remains largely underexplored. Many recent works have focused on research on the active central atom of single-atom catalysts (SACs) for electrochemical NRR; yet, a comprehensive investigation on the coordinating environment of the central atom of SACs to enhance the performance of electrochemical NRR has seldom been done and is thus imperative to be developed. Herein, from a DFT perspective, we propose a systematic research on modifiying the coordinating environment of Mn-SACs via diversification of two-dimensional (2D) monolayer supports, usage of the unsaturated form of N-doping, introduction of extra heteroatoms (F, P, and S), and external strain engineering to tune the performance of the electrocatalytic NRR. According to our thermodynamics, kinetics, and selectivity analysis, the N-doped graphene used as the support to anchor Mn-SAC (Mn@g-N4) exhibits excellent behavior for electrocatalytic NRR. In addition, usage of the unsaturated form of N-doping, Mn@g-N3C1 and Mn@g-N1C3 in particular, can help improve the catalytic performance. Furthermore, introduction of extra O-heteroatoms to form a dual-heteroatom coordination structure (Mn@g-N3C1O1) would significantly decrease the free energy requirement of fixation and activation of N2 (ΔGN2 = −0.61 eV; ΔGNNH = 0.29 eV), thereby facilitating the electrocatalytic NRR efficiently. Finally, an appropriate external compressive strain engineering can be accepted as the effective strategy to enhance the electrocatalytic performance of NRR. All of the improvement of the NRR performance is mainly due to the energy decrease of the d-band center of the central atom of Mn-SCAs. Importantly, our efforts toward understanding the influence of the coordinating environment of the central atom of SACs on electrochemical NRR and strengthening the performance via decreasing the energy of the d-band center of the central atom would provide guidelines for further research including theory and experiment.