Study On The Fabrication And Performance Of Carbon-based Catalysts With High-dispersive Single Transition Metal Atoms

Nowadays,due to the increasingly serious energy crisis and environmental issues,the development of high-efficient and low-cost clean energy conversion devices is critical to the transformation of energy structure.Among them,electrolyzed water devices,hydrogen fuel cells and metal-air batteries have been widely attached attention.However,the sluggish cathodic reaction kinetic restricts their development severely.The precious-metal-based catalysts,such as Pt-based catalysts,have been proposed to settle this issue;however,their high-cost hampers the applications of the above devices.Therefore,it is urgent to develop high-activity,high-stability and low-cost non-precious metal catalysts to improve the reaction kinetic of the cathode reactions.At present,metal-nitrogen doped carbon catalysts?M-N/C?with abundantly accessible M-Nx sites,especially metal single atom M-N/C?SAM-N/C?,have been developed to substitute high cost Pt-based catalysts for sluggish oxygen reduction reaction?ORR?and hydrogen evolution reaction?HER?.However,the agglomerated metal nanoparticles are usually easy to form but very difficult to remove due to the protection of the surface-coating carbon layers,significantly hampering SAM-N/C fabrication.Therefore,it is of vital important to explore the simple and low-cost method of synthesis for single-atom carbon-based catalysts.Based on this,this paper mainly carried out the following two aspects of work:?1?Aimed to solve the easy formation of Co-rich particles@carbon shell structure?Co@C?,we used Co²⁺-SCN^-coordination compound as the metal precursor to inhibit the formation of complete Co-rich particles@carbon shell structure?Co@C?.Thanks to the decomposition of Co²⁺-SCN^-compound,the Co-rich particles would form at the lower temperature compared to that of carbon deposition,resulting in large Co-rich agglomerates generation at high temperature.These formed large Co-rich agglomerates play the crucial roles in SACo-N/C generation via effectively delaying the generation temperature of Co@C.Therefore,the single atom Co-N/C catalyst?SACo-N/C?was fabricated successfully at the proper temperature.The SEM,TEM and Raman results proved the graphene structure of SACo-N/C.And HADDF-STEM results demonstrated that there are cobalt atoms rather than cobalt nanoparticles presenting in SACo-N/C.And thus,the as-prepared SACo-N/C^-possessed superior ORR and HER activity.Moreover,the Zinc-O₂ battery assembled by SACo-N/C shown high power density,indicating that SACo-N/C has the potential to replace Pt-based catalysts.Thus,this study might open up new opportunities for metal single atom nitrogen-doped carbon catalyst fabrication.?2?Based on the results of the previous chapter,a series of transition metal-based catalysts were successfully synthesized using a transition metal salt and SCN^-as a metal precursor for HER and ORR.And TEM,XRD and XPS results confirmed that there are no metal particles in SAM-N/C,indicating the atomic dispersion metal in SAM-N/C,which proves the universality of this ligand regulation strategy in preparing transition metal single-atom carbon-based catalysts.The results of electrochemical tests indicated that the order of oxygen reduction activity of SAM-N/C catalyst was as follows:SACo-N/C?SAFe-N/C>SAMn-N/C>SANi-N/C;the order of hydrogen evolution activity of SAM-N/C catalyst was as follows:SACo-N/C>SANi-N/C>SAMn-N/C>SAFe-N/C;and the order of the corresponding power density of the zinc-air battery assembled by the catalysts above is as follows:SACo-N/C>SAFe-N/C>SANi-N/C>SAMn-N/C.The above results showed that the Co-N_x and Fe-N_x structures have the best performance in oxygen reduction reaction;nevertheless,the Co-N_x structure has superior catalytic activity in Zinc-O₂ battery.For hydrogen evolution reaction,Co-N_x structure still possessed the best performance.This chapter demonstrated SCN^-could anchor metal ions to assist the formation of transition metal single-atom carbon-based catalysts.This method is simple and cost-controllable,showing certain methodological significance in the preparation of transition metal single-atom carbon-based catalysts.