Revealing the synergistic effect of Ni single atoms and adjacent 3d metal doped Ni nanoparticles in electrocatalytic CO reduction.

Herein, we report the successful fabrication of a series of transition metal doped Ni nanoparticles (NPs) coordinated with Ni single atoms in nitrogen-doped carbon nanotubes (denoted as NiM-NCNTs, M = Mn, Fe, Co, Cu and Zn; Ni = Ni single atom). X-ray absorption fine structure reveals the coexistence of Ni single atoms with Ni-N coordination and NiM NPs. When applied for electrocatalytic CORR, the NiM-NCNT compounds show the Faradaic efficiency of CO (FE) with a volcano-like tendency of Mn < Fe ≈ Co < Zn < Cu, in which the NiCu-NCNT exhibits the highest FE of 96.92%, a current density of 171.25 mA cm and a sustainable stability over 24 hours at a current density of 100 mA cm, outperforming most reported examples in the literature. Detailed experiments and theoretical calculations reveal that for NiCu-NCNTs, the electron transfer from NiCu NPs to Ni single atoms strengthens the adsorption of *COOH intermediates. Moreover, the d-band center of Ni-N in NiCu-NCNT is upshifted, providing stronger binding with the reaction intermediates of *COOH, whereas the NiCu NPs increase the Gibbs free energy change of the Volmer step, suppressing the competitive HER.