Regulating Local Coordination Environment Of Metal Phthalocyanines Via Facile Temperature For Enhancing CO₂ Electrochemical Reduction Reaction

The electrochemical carbon dioxide reduction（ECO₂RR）to valuable chemicals and fuels is one of the promising approaches for reducing excess CO₂ concentration in the atmosphere.However,it faces the great challenge of high overpotential since CO₂ is a thermodynamically stable molecule.Thus,an efficient catalyst and high-energy input are required to drive the transformation.Among many strategies for improving CO₂RR,Metal phthalocyanines（MPcs）have been identified as significant catalysts due to their central metal ions,which act as a Lewis acid catalyst for providing the stable and confined environment of the CO₂ molecules absorption.MPcs are thermally stable complex catalysts made of metal and organic molecules that have clearly defined local coordination configurations（i.e.,M-N₄,M=Transition metal）with a precisely tuned ability that makes them prominent for CO₂RR to carbon monoxide（CO）product.However,high symmetric electronic density around M-N₄ moiety in MPcs makes it unconducive for CO₂ adsorption and activation,leading to low selectivity,instability,and low conductivity for CO₂RR.So,regulating local environmental coordination of MPcs is an efficient strategy to achieve precise electronic properties for enhancing their catalytic properties.This dissertation,therefore,presents a facile molecular engineering strategy for regulating the electronic structure of MPcs,specifically cobalt phthalocyanines（Co Pc）,iron phthalocyanines（Fe Pc）,and nickel phthalocyanines（Ni Pc）,by coordinating their local catalytic sites with the functional nitrogen-doped carbon substrate for CO₂RR electrocatalysis.It also seeks to explore the coordination effect between M-N₄ catalytic site and substrates.The first chapter of this dissertation lays out the technological motivation in the context of the urgent need to develop an effective electrocatalyst to mitigate climate change and meet global energy demand.The second chapter describes the chemicals and tools used to synthesize and characterize electrocatalysts,respectively.Primarily gives the details of equipment employed,such as Fourier transform infrared（FTIR）and Ultraviolet-visible spectrophotometry（UV-vis）for exploring the ligation environment of the absorber,while coordination number and bond lengths of the absorber are unravelled by X-ray absorption spectroscopy（XAS）.The computation method is introduced;it provides the details about density functional theory calculations applied to study the coordination effect between metal centre and substrate as well as elucidating intrinsic electrocatalytic mechanism during the CO₂RR of synthetic electrocatalysts.The third chapter reports nitrogen-rich carbon nitride-induced electron delocalization on Co-N₄site to enhance CO₂RR.Here,nitrogen-rich carbon nitride（MPF）is coordinated with Co Pc through axial coordination under a mild temperature of 400 ^oC,thereby forming an atomically dispersed Co-N₅catalytic site.The synthetic Co-N₅/MPF catalyst exhibits high activity and selectivity(FE_CO>99%)for CO₂RR and remarkable stability for 40 h.Theoretical calculations reveal that MPF helps to modulate the intrinsic electronic structure of Co Pc,thereby d-band centre by inducing electronic delocalization on Co-N₄ site,which enhances the conductivity and optimizes the interaction between the active sites and intermediates.The fourth chapter reports Fe Pc with axial nitrogen coordination-induced electronic localization to enhance CO₂RR.The Fe-N₄ of Fe Pc is coordinated with nitrogen-doped hierarchically mesoporous carbon support via a facile pyrolysis-free approach for the Fe-N₅ active site formation.The synthetic catalyst demonstrates excellent CO₂RR performance with a turnover frequency（TOF）value of 5283 h^-1 and remarkable selectivity of 96%,and stability for 20 h at-0.7 V vs.RHE.Theoretical calculations show that the axial N coordination with Fe Pc induces electronic localization on Fe-N₅site,which simultaneously breaks the electron density symmetry around the Fe and prompts its electron transfer to CO₂ for improved adsorption and activation.Chapter five reports constraining Ni-N₄ and Co-N₄ catalytic sites to single atom dimer site（Co Ni Pc-SAD）formation for boosting CO₂RR kinetics.Here,a highly dispersed and homogeneous heterostructure Co Ni Pc-SAD catalyst supported on nitrogen-doped porous carbon（PNC）by a facile pyrolysis-free strategy is fabricated.The resultant Co Ni Pc-SAD catalyst exhibits excellent activity and selectivity(J_CO=6.5～9 m A cm^-2,FE_CO>99%at-0.74～-0.84 V vs.RHE)for CO₂RR and remarkable stability for 30 h with a TOF value of 7836.75 h^-1,outperforming individual Co and Ni in PNC.More conspicuously,theoretical calculations reveal that the synergistic interaction in Co Ni Pc-SAD can effectively reform the electronic structure of dimer sites for facilitating electron transfer ability and regulating the adsorption energy of intermediates for further improving CO₂RR kinetics.These findings provide a new point of view to regulate the local environment of the catalytic site of MPcs via mild temperatures or pyrolysis-free pathways for a better understanding reaction mechanism of CO₂ on the catalyst surface.Furthermore,provide a deeper understanding of the bimetal synergistic effect for future energy-related applications.