Mechanistic insight into the synergy between nickel single atoms and nanoparticles on N-doped carbon for electroreduction of CO2

The synergy of single atoms (SAs) and nanoparticles (NPs) has demonstrated great potential in promoting the electrocatalytic carbon dioxide reduction reaction (CO₂RR); however, the rationalization of the SAs/NPs proportion remains one challenge for the catalyst design. Herein, a Ni²⁺-loaded porous poly(ionic liquids) (PIL) precursor synthesized through the free radical self-polymerization of the ionic liquid monomer, 1-allyl-3-vinylimidazolium chloride, was pyrolyzed to prepare the Ni, N co-doped carbon materials, in which the proportion of Ni SAs and NPs could be facilely modulated by controlling the annealing temperature. The catalyst Ni-NC-1000 with a moderate proportion of Ni SAs and NPs exhibited high efficiency in the electrocatalytic conversion of CO₂ into CO. Operando Ni K-edge X-ray absorption near-edge structure (XANES) spectra and theoretical calculations were conducted to gain insight into the synergy of Ni SAs and NPs. The charge transfer from Ni NPs to the surrounding carbon layer and then to the Ni SAs resulted in the electron-enriched Ni SAs active sites. In the electroreduction of CO₂, the co-existence of Ni SAs and NPs strengthened the CO₂ activation and the affinity towards the key intermediate of *COOH, lowering the free energy for the potential-determining *CO₂ → *COOH step, and therefore promoted the catalysis efficiency.