Structural Design of π-d Conjugated TM_x B₃ N₃ S₆ (x=2, 3) Monolayer Toward Electrocatalytic Ammonia Synthesis

Single-atom catalysts (SACs) have attracted wide attention to be acted as potential electrocatalysts for nitrogen reduction reaction (NRR). However, the coordination environment of the single transition metal (TM) atoms is essential to the catalytic activity for NRR. Herein, we proposed four types of 3-, 4-coordinated and π-d conjugated TM_x B₃ N₃ S₆ (x=2, 3, TM=Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re and Os) monolayers for SACs. Based on density functional theory (DFT) calculations, I-TM₂ B₃ N₃ S₆ and III-TM₃ B₃ N₃ S₆ are the reasonable 3-coordinated and 4-coordinated structures screening by structure stable optimizations, respectively. Next, the structural configurations, electronic properties and catalytic performances of 30 kinds of the 3-coordinated I-TM₂ B₃ N₃ S₆ and 4-coordinated III-TM₃ B₃ N₃ S₆ monolayers with different single transition metal atoms were systematically investigated. The results reveal that B₃ N₃ S₆ ligand is an ideal support for TM atoms due to existence of strong TM-S bonds. The 3-coordinated I-V₂ B₃ N₃ S₆ is the best SAC with the low limiting potential (U_L ) of -0.01 V, excellent stability (E_f =-0.32 eV, U_diss =0.02 V) and remarkable selectivity characteristics. This work not only provides novel π-d conjugated SACs, but also gives theoretical insights into their catalytic activities and offers reference for experimental synthesis.