Research On Construction Of Transition Metal （Fe,Co,and Ni） Composite Catalyst For Electrocatalytic Reduction Of CO₂ To CO

The electrocatalytic CO₂ reduction reaction（CO₂RR）is a technology that drives sustainable carbon cycle and stores intermittent renewable energy by converting CO₂ into valuable chemicals and fuels.CO₂ electrocatalytic reduction to CO is one of the most potential commercialization technologys.However,there is still a certain gap between the technology and industrial application.The design and development of electrocatalyst with exceptional activity,selectivity,and stability is a key issue to be solved urgently for industrial production.Recently,transition metal-based catalyst has been recognized as attractive electrocatalyst due to the low price and high activity,which has great potential for CO production.Therefore,this work focuses on the development of transition metal composite catalyst and its study on electrocatalytic reduction of CO₂ to CO.Phthalocyanine and porphyrin molecular catalysts with well-defined M-N_X center have been widely used in CO₂RR on account of its easy regulation of active centers.In view of coordination chemistry,the local coordination environment of catalyst is highly sensitive to its catalytic performance.However,most studies are concentrated on the regulation of the metal center and the first coordination layer,while the influence of heteroatoms in the second coordination layer on the CO₂RR performance has less attention.In this work,a reasonable crystal model system was constructed to verify the effect of N atom in the second coordination layer on the CO₂RR activity of porphyrin molecular catalysts.Specifically,5,10,15,20-tetraphenyl-21H,23H-porphene nickel（Ⅱ）（NiTPP）with a saturated symmetrical coordination Ni-N₄ structure was selected as the main research catalyst,and anchored on carbon black or N-doped carbon black to study the active site and mechanism of the catalyst.The results demonstrated that NiTPP catalyst anchored on N-doped carbon black showed remarkable catalytic performance compared with NiTPP catalyst loaded on pure carbon black.This is because the doped N atom on the carbon support can activate Ni-N₄ in NiTPP,so that NiTPP without CO₂ reduction ability showed certain catalytic activity.In conclusion,the coordination structure can be adjusted through the design of carbon support and the synergistic action of catalyst and carbon support,thereby improving the CO₂ reduction performance of the catalyst.Here,C₃N₄ was selected as an excellent C/N source,and the catalyst was prepared by pyrolysis strategy.The synergistic effect of NiTPP molecular catalyst and N-doped carbon support was further studied,clarifying the structure-performance relationship of catalyst,so as to rationally develop transition metal-based catalyst.NiTPP-C₃N₄-900 delivered the highest intrinsic CO₂reduction activity,maintaining a CO Faraday efficiency(FE_CO)of more than 90%over the wide potential range of-0.57 V to-0.97 V vs.RHE.More importantly,at-0.87 V vs.RHFE _CO reached 96.3%,and CO current density(j _CO)was 16.0 mA cm^-2.This work provides a clear idea for the regulation of the coordination environment of catalyst and the design of transition metal-based catalyst with extraordinary performance.At present,most of the reported catalysts are subject to complex preparation processes,which cannot be large-scale production of catalyst.From the practical application,the strategy of large-scale catalyst preparation has a wide application prospect.In addition,the first work elucidated that metal species,heteroatomic-doped carbon support（such as C₃N₄）and high-temperature pyrolysis play an important role in the preparation of catalyst.Thus,in the second work,a general and easy-to-scale one-step synthesis strategy was adopted to construct a group of N-doped carbon-encapsulated metal catalysts,named M@N-C（M=Fe,Co,and Ni）,for CO₂ electrocatalytic reduction of CO,and the effect of transition metals on the structure and properties of the catalysts were studied in detail.Compared with Fe@N-C and Co@N-C,Ni@N-C exhibited high catalytic activity and selectivity due to its complex three-dimensional carbon network structure and moderate degree of graphitization.Specifically,in the H-type cell,Ni@N-C catalyst achieved an excellent FE_CO（>90%）over a wide reachable potential window of-0.67 V to-0.97 V vs.RHE.In the flow cell,Ni@N-C maintained a FE_CO of 98%at-0.57 V vs.RHE and an industrial-level current density of 430 mA cm^-2 at-1.07 V vs.RHE.Based on further characterization and experiments,it was found that the active pyridine N atom and Ni metal center were indispensable factors for the enhancement of CO₂RR electrocatalytic performance.As transition metals exhibit different electron transfer properties,the activity and selectivity can be significantly affected by adjusting the type of transition metals.This work provides a simple and feasible method for large-scale preparation of industrial CO₂electroreduction catalyst with excellent performance.