N,S coordination in Ni single-atom catalyst promoting CO₂RR towards HCOOH

Carbon-based single atom catalysts (SACs) are attracting extensive attention in the CO₂ reduction reaction (CO₂RR) 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 NiN₄ as an example, this study reveals that the introduction of different numbers of S atoms into N coordination (Ni-NS_4- (x = 1-4)) results in outstanding structural stability and catalytic activity. Owing to the additional orbitals around -1.60 eV and abundant Ni d, d, d, and d orbital occupation after S substitution, N,S coordination can effectively facilitate the protonation of adsorbed intermediates and thus accelerate the overall CO₂RR. The CO₂RR mechanisms for CO and HCOOH generation via two-electron pathways are systematically elucidated on NiN₄, NiN₃S₁ and NiN₂S₂. NiN₂S₂ yields HCOOH as the most favorable product with a limiting potential of -0.24 V, surpassing NiN₄ (-1.14 V) and NiN₃S₁ (-0.50 V), which indicates that the different S-atom substitution of NiN₄ has considerable influence on the CO₂RR performance. This work highlights NiN₂S₂ as a high-performance CO₂RR catalyst to produce HCOOH, and demonstrates that N,S coordination is an effective strategy to regulate the performance of atomically dispersed electrocatalysts.