Design And Electrocatalytic Mechanism Of Fe/Ni Single Atom Catalysts For Carbon Dioxide Reduction

To solve a series of environmental problems caused by excessive carbon dioxide emissions,China has pronounced to achieve carbon emission peak in 2030 and carbon neutrality in 2060.Electrocatalytic CO₂ reduction has been regarded as a promising approach to realize recycling of CO₂.However,most of reported catalysts for CO₂reduction suffered from low selectivity and poor activity,which is difficult to meet the practical needs.Trasition metal-nitrogen-carbon single-atom catalysts are promising catalysts for CO₂ reduction owning to high activity and low-cost.Unfortunately,it still hindered by sluggish kinetic process,competing hydrogen evolution reaction and fast activity decay.In this paper,CO₂ reduction performance of Fe-N-C and Ni-N-C single-atom catalysts was imporved by regulating the active sites and modifying the carbon support.Furthermore,density functional theory was adopted to clarify the structure-activity relationship.The research contents are listed as follows:To facilitate the desorption of CO on single Fe atoms,we constructed atomically dispersed Fe N₄Cl active sites with axial Fe-Cl bond.After introducing high-electronegative Cl atom,electron transfers from the axial Cl atom to the central Fe atom,making Fe N₄Cl more favorable for CO desorption than Fe N₄.At-0.6 V vs.RHE,the as-prepared Fe N₄Cl single atom catalysts exhibited an ultrahigh current density of 12.41m A·cm^-2,a selectivity of 90.5%and excellent stability for working continuously for 18h.To improve the desorption of CO on single Fe atoms and suppress the hydrogen evolution reaction,this work reported a electrospinning and multistep thermal treatment strategy to prepare atomically dispersed Fe N₅ active sites supported on defective carbon substrate.Density functional theory reveal that the synergistic effect between the Fe N₅center and carbon defects could induce electronic localization,thus improve the CO desorption and inhibit the hydrogen evolution reaction.The prepared single atom catalysts exhibited a high selectivity of 93.1%and a large current density of 9.4 m A·cm^-2.The assembled rechargeable Zn-CO₂ batteries delivered a high powder density of 1.3m W·cm^-2 and good stability of 25 h,demonstrating excellent application.To enhance the activation of CO₂ and improve the desorption of CO on catalyst surface,this work reported an electrospinning and an in-situ gas-phase selenization strategy to prepare Fe-Se dual active sites catalyst.During this process,Se could be doped into the carbon framework and forming Se-C bond.The optimized catalysts exhibited an excellent selectivity of 95.6%at a low potential of-0.45 V vs.RHE.Density functional theory reveal that the construction of Fe-Se dual active sites could break linear relationship between different intermediates,enhancing the adsorption of COOH*and faciliting the desorption of CO*,giving rise to excellent CO₂ reduction performance.Nickel-nitrogen-carbon single-atom catalysts have attracted widespread interest for CO₂ electroreduction but they suffer from poor stability owning to continuous metal dissolution.This work prepared Cl-and N-doped porous carbon nanosheets supported atomically dispersed Ni N₄Cl active sites with high Ni loading of 1.9 wt%through a molten-salt-assisted pyrolysis.The optimized catalysts exhibited a high selectivity of 98.7%at-0.7 V vs.RHE.Moreover,this catalyst also displayed extraordinary stability,maintaining a superior selectivity（95%）as well as activity(12 m A·cm^-2)for 220 h of long-term electrolysis.Density functional theory results reflected that coupling Ni N₄Cl sites on a Cl-doped carbon support synergetically induced electron delocalization of the Ni center,promoting the activation of CO₂ and suppressing demetalation,thus enabling a superior activity and stability.This work showed that molten salt-assisted pyrolysis is a promising strategy to synthesis single atom catalysts.