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

• • Pyrrolic-N–regulated Cu–Npr–C achieves 72.6% Faradaic efficiency (FE) for formate at −0.7 V vs. RHE, sustaining >67% FE over 10 h, demonstrating industrially relevant stability for continuous CO2-to-formate conversion. • • Pyridinic-N–regulated Cu–Npy–C, with identical Cu–N4 first-shell, predominantly drives hydrogen evolution (up to 69% FE), underscoring that second-shell nitrogen chemistry dictates product selectivity, a critical factor for catalyst design. • • In situ ATR-SEIRAS detects earlier emergence of HCOO* intermediate on Cu–Npr–C, providing direct spectroscopic evidence of accelerated formate-pathway kinetics, which is essential for mechanistic understanding and process optimization. • • The second-shell strategy is generalizable: Ni–Npy–C achieves 96.8% CO selectivity, while Ni–Npr–C yields mixed CO/H2, demonstrating tunable selectivity across different metal centers for tailored CO2 reduction products.