NH4Cl-assisted preparation of single Ni sites anchored carbon nanosheet catalysts for highly efficient carbon dioxide electroreduction

• Single-atomic electrocatalyst with abundant Ni-N 4 active sites is developed via NH 4 Cl-assited pyrolysis method. • High catalytic performance with CO Faradaic efficiency of 98% is observed at a small overpotential of 510 mV. • Optimized mesopore size, increased concentrations of Ni-N 4 active sites and pyridinic N species are achieved by NH 4 Cl addition. • Unveiling the synergistic catalytic effect of Ni-N 4 active sites and pyridinic N species in catalyzing CO 2 reduction. Single-atomic transition metal-nitrogen codoped carbon (M-N-C) are efficient substitute catalysts for noble metals to catalyze the electrochemical CO 2 reduction reaction (CO 2 RR). However, the uncontrolled aggregations of metal and serious loss of nitrogen species constituting the M-N x active sites are frequently observed in the commonly used pyrolysis procedure. Herein, single-atomic nickel (Ni)-based sheet-like electrocatalysts with abundant Ni-N 4 active sites were created by using a novel ammonium chloride (NH 4 Cl)-assited pyrolysis method. Spherical aberration correction electron microscopy and X-ray absorption fine structure analysis clearly revealed that Ni species are atomically dispersed and anchored by N in Ni-N 4 structure. The addition of NH 4 Cl optimized the mesopore size to 7–10 nm and increased the concentrations of pyridinic N (3.54 wt%) and Ni-N 4 (3.33 wt%) species. The synergistic catalytic effect derived from Ni-N 4 active sites and pyridinic N species achieved an outstanding CO 2 RR performance, presenting a high CO Faradaic efficiency (FE CO ) up to 98% and a large CO partial current density of 8.5 mA cm −2 at a low potential of –0.62 V vs. RHE. Particularly, the FE CO maintains above 80% within a large potential range from –0.43 to –0.73 V vs. RHE. This work provides a practical and feasible approach to building highly active single-atomic catalysts for CO 2 conversion systems.