Structural rule of N-coordinated single-atom catalysts for electrochemical CO2 reduction

Metal single-atom catalysts (SACs) on nitrogen-doped carbons exhibit an attractive prospect in catalysis. However, how to quickly collocate various metal centers with diverse N-coordination topological structures to maximize the catalytic performance is still vague and always challenging in practice, and cannot be easily realized using the traditional descriptors. Herein, a new descriptor, i.e., the cohesive energy (E-c) of a metal that corresponds to the intrinsic property of the metal center, is put forward, which facilitates assessing the catalytic performance of SACs expediently. Taking the electrochemical CO2 reduction to CO (CO2RR) on transition metal (TM) SACs anchored on different N-doped graphenes (GN(x), local N-coordination x = 2, 3 and 4) as an example, we reveal the general rule of "TM-N-x" combinations in SACs for the CO2RR from the obtained activity trends in terms of E-c, which interestingly shows that GN(2) and GN(4) generally provide a superior structural platform for a majority of metal centers and exhibit better catalytic activities than GN(3). In particular, SACs anchored by the least N-coordinated GN(2) can hold an outstanding catalytic activity for the most common metal centers and simultaneously keep a high selectivity of the CO2RR. Accordingly, we proposed that Co (or Ni, Fe, Cr, Mn, Cu, Pd) anchored on GN(2) could be potential SACs for the CO2RR through high-throughput calculations, where the high catalytic efficiencies of some candidates such as GN(2)-anchored Ni (or Co, Fe and Cu) SACs for the CO2RR have been demonstrated in the experiments. This work provides a significant insight into the structural origin of different N-x-coordinations modulating the catalytic performances of single-atom metal centers, and we believe that the obtained design rule could pave the way to rapidly designing efficient TM SACs for the CO2RR and other reactions.