Structural Design of -d Conjugated TM_xB₃N₃S₆ (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 pi-d conjugated TMxB3N3S6 (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-TM2B3N3S6 and III-TM3B3N3S6 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-TM2B3N3S6 and 4-coordinated III-TM3B3N3S6 monolayers with different single transition metal atoms were systematically investigated. The results reveal that B3N3S6 ligand is an ideal support for TM atoms due to existence of strong TM-S bonds. The 3-coordinated I-V2B3N3S6 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 pi-d conjugated SACs, but also gives theoretical insights into their catalytic activities and offers reference for experimental synthesis.