Thermal Transformation of Molecular Ni²⁺–N₄ Sites for Enhanced CO₂ Electroreduction Activity

Atomically dispersed nickel sites complexed on nitrogen-doped carbon (Ni–N/C) have demonstrated considerable activity for the selective electrochemical carbon dioxide reduction reaction (CO₂RR) to CO. However, the high-temperature treatment typically involved during the activation of Ni–N/C catalysts makes the origin of the high activity elusive. In this work, Ni­(II) phthalocyanine molecules grafted on carbon nanotube (NiPc/CNT) and heat-treated NiPc/CNT (H-NiPc/CNT) are exploited as model catalysts to investigate the impact of thermal activation on the structure of active sites and CO₂RR activity. H-NiPc/CNT exhibits a ∼4.7-fold higher turnover frequency for CO₂RR to CO in comparison to NiPc/CNT. Extended X-ray absorption fine structure analysis and density functional theory (DFT) calculations reveal that the heat treatment transforms the molecular Ni²⁺–N₄ sites of NiPc into Ni⁺–N₃V (V: vacancy) and Ni⁺–N₃ sites incorporated in the graphene lattice that concomitantly involves breakage of Ni–N bonding, shrinkage in the Ni–N–C local structure, and decrease in the oxidation state of the Ni center from +2 to +1. DFT calculations combined with microkinetic modeling suggest that the Ni–N₃V site appears to be responsible for the high CO₂RR activity because of its lower barrier for the formation of *COOH intermediate and optimum *CO binding energy. In situ/operando X-ray absorption spectroscopy analyses further corroborate the importance of reduced Ni⁺ species in boosting the CO₂RR activity.