Design Of Transition Metal-nitrogen Doped Carbon Catalyst For Electrocatalytic CO₂ Reduction

The electro-catalytic CO₂ reduction reaction（CO₂RR）with renewable energy as a driving force is an achievable technology that can selectively produce high-value chemicals and fuels while achieving carbon peaking and carbon neutrality goals.Among different products of CO₂RR,CO has been widely investigated as a basic product.How to design low cost,highly efficient and long stability catalyst is a key issue in realizing the commercialization of CO₂RR to CO.Among transition metal nitrogen-doping carbon catalysts,transition metal nanoparticles encased by nitrogen-doped carbon（M@NC）composites,which have special morphology,structure and electronic properties,have attracted many attentions due to their outstanding catalytic performance of CO₂RR to CO.However,the real active sites in CO₂RR are still controversial,especially the effects between M and N species is not clear.Therefore,identifying the active sites of M@NC catalysts in CO₂RR and determining their interactions between metal and carbon shell are essential for guiding designing efficient encapsulated catalysts.Currently,there continue to be some gaps for industrialization in the requirement of activity,selectivity and stability.On the basis of identifying active sites,it is necessary to explore the factors affecting the catalytic activity and stability of M@NC to designate the orientation for the design of M@NC catalyst with excellent selectivity,activity and stability.The specific research contents and results are as follows:（1）To elucidate active sites of M@NC catalysts in CO₂RR and determining their interactions between metal and carbon shell,four kinds of metal nanoparticles encapsulated by carbon shells（Co@C,Co@NC,Ni@C,and Ni@NC）composites are prepared through hydrothermal method.Electrochemical performance tests,control experiments and density functional theory（DFT）results verified that the adsorption and activation of CO₂ molecules mainly occurred on the pyrrorole N species in the shell of the catalyst.The encased metal nanoparticles inject electrons into theπ*orbital of nitrogen-doped carbon by the feedbackπbond,which improved the electronic structure of the N species,thereby decreasing their adsorption energy for*COOH,*CO and*H intermediates and enhancing the catalytic performance in CO₂RR.The free energy of generating*COOH and*CO intermediates for Ni@NC is lower than that for Co@NC.Therefore,Ni@NC exhibits superior performance compared to Co@NC.This work can provide theoretical guidance for designing high efficiency M@NC catalyst for CO₂RR to CO.（2）Currently,most reported catalysts only possess high FE_CO%under a narrow potential range,and this shortage largely limits its practical viability.Herein,on the basis of determining the N@NC active site,we develop a facile method to synthesize nickel encased by nitrogen-doped carbon nanotubes on carbon support（Ni@NCNT-C）catalyst by controlling the content of Ni and pyrolysis temperature to regulate the electronic properties of NC and the crystal structure of catalyst.The results indicated that 0.1Ni@NCNT-C-800catalyst had excellent activity and stability in CO₂RR to CO owing to its high surface area,special physical structure and surface electronic properties,as well as the limited domain effect of NCNT.The 0.1Ni@NCNT-C-800 catalyst provides a low overpotential（-0.27 V vs.RHE）and high FE_CO%（～100%）.The partial current density of CO can attain 230 m A cm^-2 and the activity can be kept over 40 h.More importantly,a wide potential range of 900m V can be obtained when FE_CO%is continuously maintained about 95%.Therefore,it has important practical applications,and may provide a route to seek a robust catalyst for high efficiency CO₂RR over a wide potential range.（3）The long-term durability of catalyst under reduction condition remains a key issue.The Ni@NC catalysts with different coating thicknesses and Ni-NC single-atom catalysts were designed and synthesized.In addition,the stability of the catalysts during the CO₂RR process was detailedly studied.The characterization of electrochemical in-situ Raman and the structure of catalyst before and after the CO₂RR show that Ni@NC-0.25 catalyst has a moderate thicknesses of NC layer（<5 nm）,and the strong electronic regulation ability of Ni to a few NC coating layers enables Ni@NC-0.25 catalyst to have the optimal adsorption strength for intermediates,which is conducive to the regeneration of active sites.Ni@NC-0.25 exhibits superior selectivity and activity,with FE_CO%approaching 100%and current density reaching 340 m A cm^-2 at-1.27 V vs.RHE.The excellent performance can be maintained over 110 h at-1.07 V vs.RHE.In addition,the strong hydrophobic property of Ni@NC-0.25 material effectively prevents the carbonate or bicarbonate generated by the reaction of CO₂ with the cathode KOH electrolyte in the void of the gas diffusion electrode（GDE）,ensuring the smooth progress of the reaction,thus maintaining the stability of the performance.This work can provide a novel idea for designing efficient and stable coated catalyst.