Size-Dependent Effect of Ni Nanoparticles Confined in Ni-Modified Flower-Like Carbon Toward CO2 Electroreduction Reaction.

The Ni anchored nitrogen-doped carbon (Ni-N-C) are considered a promising catalysts toward electrocatalytic CO2 reduction reaction (CO2RR). During preparation process, the inevitable formation of Ni nanoparticles is generally considered unfavorable. However, emerging evidence suggests that the impact of Ni nanoparticles on CO2RR is governed by confinement and structural evolution, which remains poorly understood and motivates systematic investigation. In this work, a flower-like carbon framework was designed to confine Ni species, enabling the coexistence of Ni-N-C sites and different sizes Ni nanoparticles. The results reveal that moderate Ni introduction (0.2-1 mmol) stabilizes small Ni nanoparticles and enhances micropore. Consequently, the Ni-N-C/Ni-0.2 and Ni-N-C/Ni-1 samples achieved superior CO2RR activity, delivering a CO faradaic efficiency (FECO) of ∼90% at -1.0 V versus. RHE. Importantly, the confinement of flower-like carbon ensured that FECO could be retained above 85% even after 30 h continuous operation. Time-resolved structural characterizations combined with density functional theory calculations reveal that catalyst deactivation originates from the gradual transformation of confined Ni species into exposed metallic nanoparticles, which promote hydrogen evolution and hinder *COOH formation. This work establishes a direct correlation between Ni nanoparticle size evolution and CO2RR performance, highlighting confinement engineering as an effective strategy for designing durable catalysts.