Peripheral nitrogen microenvironment engineering of Cu–N4 single-atom catalysts enables selective electrochemical CO2 reduction to formate

Rational design of metal–nitrogen–carbon (M–N–C) single-atom catalysts (SACs) for selective CO 2 electro-reduction is still largely guided by first-shell coordination engineering, while the catalytic impact of the surrounding non-coordinated “second-shell” microenvironment remains underexplored. Here, we show that tailoring the peripheral nitrogen microenvironment can decisively switch product selectivity in Cu-based SACs, even with an identical Cu–N 4 first-shell motif. Using a polymer coordination strategy, pyrrolic-N–regulated Cu–N pr –C and pyridinic-N–regulated Cu–N py –C exhibit strikingly divergent behaviors: Cu–N pr –C selectively produces formate with 72.6% Faradaic efficiency (FE) at −0.7 V vs. RHE and sustains >67% FE over 10 h, whereas Cu–N py –C predominantly drives H 2 evolution (up to 69% FE). In situ attenuated total reflection surface enhanced infrared spectroscopy captures an earlier emergence of the key HCOO* intermediate on Cu–N pr –C, evidencing accelerated formate-pathway kinetics enabled by the pyrrolic microenvironment. Density functional theory calculations further support that pyrrolic second-shell regulation promotes the O-bound formate route while disfavoring hydrogen adsorption, whereas the pyridinic microenvironment renders H adsorption more competitive and biases the reaction toward hydrogen evolution reaction. Extending this second-shell strategy to Ni SACs confirms its generality: Ni–N py –C achieves 96.8% CO selectivity, while Ni–N pr –C shows mixed CO/H 2 production. This work establishes second-shell microenvironment regulation as a general and actionable design principle for steering selectivity in M–N–C SACs toward targeted CO 2 reduction products.