Regulating the Coordination Environment of MOF-Templated Single-Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single-atom catalysts (SACs) remains a great challenge. Herein, a general host-guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal-organic framework. As an example, the introduction of Mg2+ in MgNi-MOF-74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single-atom Ni catalysts (named Ni-SA-N-x-C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni-SA-N-2-C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h(-1)), far superior to those of Ni-SA-N-3-C and Ni-SA-N-4-C, in electrocatalytic CO2 reduction. Theoretical calculations reveal that the low N coordination number of single-atom Ni sites in Ni-SA-N-2-C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.