Layer-by-layer enriching active Ni-N_(3)C sites in nickel-nitrogen-carbon electrocatalysts for enhanced CO_(2)-to-CO reduction

Transition metal and nitrogen co-doped carbons(M-N-C)have proven to be promising catalysts for CO_(2)electroreduction into co because of the high activity and selectivity.Effective enrichment of the active transition metal coordinated nitrogen sites is desirable but is challenging for a practical volumetric productivity.Herein,we report four kinds of model electrocatalysts to unveil this issue,which include the NC structures with surface N-functionalities,Ni-N-CI with one layer of surface Ni-NsC sites,NC@Ni-N-CI with surface N-functionalities and underneath Ni-N3C sites as well as Ni-N-C_II with doubled surface Ni-NsC sites.The X-ray absorption spectroscopy indicates the coordination configuration of Ni-NsC.For NC catalysts,when N-doping level increased from 3.5 at%to 8.4 at%,the CO partial current density increased from below 0.1 mA/cm^(2)to 3 mA/cm^(2).Introducing one layer of Ni-N_(3)C onto the NC structures leads to a 54 times higher CO partial current density than that of NC,in the meantime the FE_(CO) is 66 times higher.Furthermore,doubling the density of surface Ni-N_(3)C sites by a layer-by-layer method doubles the CO partial current density Gco,indicating its potential to achieve a high density of active coordinated sites and current densities.