Transient Dangling Active Sites of Fe(III)-N-C Single-Atom Catalyst for Efficient Electrochemical CO₂ Reduction...

The Fe single-atom catalyst (SAC) with an oxidation state of III anchored on the N-doped carbon substrate (Fe(III)-N-C) delivers superior activity for catalyzing the electrochemical CO₂ reduction reaction (eCO₂RR) to produce CO, but its mechanism remains contentious and the commonly adopted FeN₄-C model is not a conformant model for Fe(III)-N-C but for Fe(II)-N-C. Herein, employing the grand-canonical ensemble modeling with the density functional theory method benchmarked against the high-level wavefunction theory method, we first identify the conformant model for Fe(III)-N-C to be FeN₁C₃-C, and we then unveil that the Fe(III)N₁C₃-C SAC generates a novel type of dangling active site transiently under working conditions, in which the Fe single-atom leaves from the anchoring site by breaking all the Fe-C bonds but retains a stable binding to the substrate by the Fe-N bond. Thus, we further elucidate that this flexible dangling active site of Fe(III)-N-C renders a convoluted reaction network with facile CO₂ activation, which delivers superior activity for eCO₂RR. Our findings provide a novel understanding of the structure-activity relationship for Fe-N-C and concrete insights into the design of highly active SACs.