N,S coordination in Ni single-atom catalyst promoting CO 2 RR towards HCOOH.

Carbon-based single atom catalysts (SACs) are attracting extensive attention in the CO 2 reduction reaction (CO 2 RR) due to their maximal atomic utilization, easily regulated active center and high catalytic activity, in which the coordination environment plays a crucial role in the intrinsic catalytic activity. Taking NiN 4 as an example, this study reveals that the introduction of different numbers of S atoms into N coordination (Ni-N x S 4- x ( x = 1-4)) results in outstanding structural stability and catalytic activity. Owing to the additional orbitals around -1.60 eV and abundant Ni d xz , d yz , d x 2 , and d z 2 orbital occupation after S substitution, N,S coordination can effectively facilitate the protonation of adsorbed intermediates and thus accelerate the overall CO 2 RR. The CO 2 RR mechanisms for CO and HCOOH generation via two-electron pathways are systematically elucidated on NiN 4 , NiN 3 S 1 and NiN 2 S 2 . NiN 2 S 2 yields HCOOH as the most favorable product with a limiting potential of -0.24 V, surpassing NiN 4 (-1.14 V) and NiN 3 S 1 (-0.50 V), which indicates that the different S-atom substitution of NiN 4 has considerable influence on the CO 2 RR performance. This work highlights NiN 2 S 2 as a high-performance CO 2 RR catalyst to produce HCOOH, and demonstrates that N,S coordination is an effective strategy to regulate the performance of atomically dispersed electrocatalysts.