Metal-organic framework derived single-atom catalysts for electrochemical CO₂ reduction

With maximum atomic utilization, transition metal single atom catalysts (SACs) show great potential in electrochemical reduction of CO₂ to CO. Herein, by a facile pyrolysis of zeolitic imidazolate frameworks (ZIFs) assembled with tiny amounts of metal ions, a series of metal-nitrogen-carbon (M-N-C) based SACs (M = Fe, Ni, Mn, Co and Cu), with metal single atoms decorated on a nitrogen-doped carbon support, have been precisely constructed. X-ray photoelectron spectroscopy (XPS) for M-N-C showed that the N 1s spectrum was deconvoluted into five peaks for pyridinic (∼398.3 eV), M-N coordination (∼399.6 eV), pyrrolic (∼400.4 eV), quaternary (∼401.2 eV) and oxidized (∼402.9 eV) N species, demonstrating the existence of M-N bonding. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) indicates homogeneous distribution of metal species throughout the N-doped carbon matrix. Among the catalysts examined, the Fe-N-C catalyst exhibits the best catalytic performance in electrocatalytic CO₂ reduction reaction (CO₂RR) with nearly 100% faradaic efficiency for CO (FE_CO) at -0.9 V vs. the reversible hydrogen electrode (RHE). Ni-N-C is the second most active catalyst towards CO₂RR performance, then followed by Mn-N-C, Co-N-C and Cu-N-C. Considering the optimum activity of Fe-N-C catalyst for the CO₂RR, we then further investigate the effect of pyrolysis temperature on CO₂RR of the Fe-N-C catalyst. We find the Fe-N-C catalyst pyrolyzed at 1000 °C exhibits the best catalytic activity in CO₂RR with excellent CO selectivity.