Carbon-encapsulated nickel nanoparticles spatial confinement *H enhance electrocatalytic CO₂ to CO

Atomically dispersed Ni sites on N-doped carbon catalyst exhibits outstanding catalytic performance for the electrochemical CO₂ reduction reaction (CO₂RR). However, the roles of Ni single atom (Ni_SA) and Ni nanoparticles (Ni NP) were not clear. Herein, a series of feasible nitrogen doped carbon encapsulated Ni catalysts with different Ni species composites were synthesized by a facile pyrolysis method for electrocatalytic CO₂ to CO. The self-tandem catalytic mechanism between Ni NP and graphite carbon with Ni-N_x sites (Ni/Ni_SA@GCN), mainly originates from their unique spatially confined structure. Specifically, Ni-N_x obtained by controllable carbothermic reaction is deemed to be the active center to capture CO₂ and stabilize *CO₂ intermediate. Meanwhile, H₂O is activated and dissociated to *H by Ni NP at the defect site of the carbon layer, which accelerates the formation of the key reaction intermediate *COOH of CO₂RR. Due to the molecule-level confined space of carbon layer defects, the multiple H₂O dissociation was blocked and the *H+*H reaction was inhabited. It results in nearly all the *H was captured by *CO₂ rather than forming H₂. The Faraday efficiency of CO(FE_CO) can reach 98.4 % at -0.88 V vs. RHE and maintained 94 % at -200 mA cm^-2. This work revealed a self-tandem mechanism and clarified the synergistic mechanism of Ni/Ni_SA@GCN for CO₂ electroreduction to CO, which provides valuable insights for designing and identifying the active sites on Ni-N_x catalysts.