Design,Construction,Electrochemical Performance And Mechanism Of Functional Carbon-based Materials

Functional carbon-based materials have attracted extensive attention in electrochemical energy storage and conversion due to their low cost,abundant resources,high conductivity and relatively high reactivity.Herein,in this study,starting from metal-organic coordination polymers,a series of general and efficient synthesis strategies are developed to obtain functional carbon-based materials including carbon nanotube assemblies,carbon-coated composite structures,nitrogen-doped graphene hollow framework,graphene-supported single-atom catalysts,carbon-confined niobium pentoxide nanoparticles and graphene-supported carbon-confined metal oxide nanodots.Based on experimental analysis and theoretical calculation,the electrocatalytic and energy storage mechanisms of functional carbon-based materials are systematically revealed,and the correlations between structure and performance are further established.These in-depth understandings provide new ideas and scientific foundations to develop low-cost,long-life and high-performance electrode materials for energy conversion and storage.The following interesting results were achieved:(1)A facile,general and high-yield strategy is developed to realize the oriented formation of CNTs from metal-organic frameworks(MOFs)through a low-temperature(as low as 430°C)pyrolysis process.The selected MOF crystals act as a single precursor for both nanocatalysts and carbon sources.The key to the formation of CNTs is obtaining small nanocatalysts with high activity during the pyrolysis process.This method is successfully extended to obtain various oriented CNT-assembled architectures by modulating the corresponding MOFs.Specifically,the resulting nitrogen-doped CNT-assembled hollow structures exhibit excellent oxygen reduction activity and stability.On basis of experimental analyses and density functional theory simulations,these superior performances are attributed to synergistic effects between ideal components and multilevel structures.Additionally,the appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function,hence efficiently enhancing its oxygen reduction activity.(2)A facile,efficient,and general method for the oriented synthesis of precise carbon-confined nanostructures is developed by low-pressure vapor superassembly of a thin MOF shell and subsequent controlled pyrolysis.The selected nanostructured metal oxide precursors not only act as metal ion sources but also orient the superassembly of gaseous organic ligands through the coordination reactions under the low-pressure condition,resulting in the formation of a tunable MOF shell on their surfaces.This strategy is further successfully extended to obtain various precise carbon-confined nanostructures with diverse compositions and delicate morphologies.Notably,these as-prepared carbon-confined architectures exhibit outstanding electrochemical performances in water splitting.The remarkable performances are mainly attributed to the synergistic effect from appropriate chemical compositions and stable carbon-confined structures.(3)A facile and scalable template method is developed to construct unique nitrogen-doped carbon hollow frameworks(NC),exhibiting excellent ORR activity.The formation mechanism is clearly revealed,including low-pressure vapor superassembly,in situ carbonization and template removal.The resulting NC possesses relatively high nitrogen concentration,high surface area,high conductivity and robust structure,thus increasing the number of exposed active sites and facilitating mass transport.The obtained NC-800 as an ORR catalyst displayed more positive half-wave potential and higher current density compared with other NC-700and NC-900 samples.On basis of experimental analysis and theoretical calculations,the superior ORR activity of NC-800 is mainly attributed to the synergistic effect between proper nitrogen doping concentration and robust hollow framework.(4)A general template approach is developed to synthesize a series of high-content metal atoms(over 1.2 at%)anchored into hollow nitrogen-doped graphene frameworks(M-N-Grs;M represents Fe,Co,Ni,Cu,etc.)at gram-scale.The highly compatible doped Zn O solid solutions can react with the incoming gaseous organic ligands to form doped metal-organic framework thin shells,whose composition determines the heteroatom species and contents in M-N-Grs.In alkaline media,the Fe-N-Gr catalyst displayed higher oxygen reduction activity and better stability than other obtained M-N-Grs,even surpassing most of the reported nonprecious metal catalysts.Both experimental analyses and theoretical calculations suggest that the superior performance in Fe-N-Gr is mainly attributed to its unique electronic structure,rich exposed active sites and robust hollow framework.(5)A facile and efficient method is developed to construct three typical carbon-confined Nb₂O₅(TT-Nb₂O₅@C,T-Nb₂O₅@C,and H-Nb₂O₅@C)nanoparticles via a mismatched coordination reaction during the solvothermal process and subsequent controlled heat treatment.Different phase effects are investigated on their lithium storage properties on the basis of both experimental and computational approaches.The thin carbon coating and nanoscale size can endow Nb₂O₅ with a high surface area,high conductivity,and short diffusion length.When employed as LIB anode materials,the resulting T-Nb₂O₅@C nanoparticles display higher rate capability and better cycling stability compared with TT-Nb₂O₅@C and H-Nb₂O₅@C nanoparticles.Furthermore,systematic crystal structure analysis,in situ X-ray diffraction analysis,and density functional theoretical calculations demonstrate a synergistic effect between fast diffusion pathways and stable hosts in T-Nb₂O₅ on its ultrafast and stable lithium storage.(6)A facile and general method to synthisize uniform carbon-confined metal oxide nanodots on graphene was developed via a well-designed process including surfactant-induced assembly,mismatched coordination reaction and subsequent in situ carbonization.On the basis of experimental analyses and density functional theory calculations,the key mismatched coordination reaction mechanism is clearly revealed,resulting in the formation of small amorphous metal-ligand complexes.This versatile oriented assembly strategy is then generally applied to obtain various carbon-confined metal oxide(SnO₂,Cr₂O₃,Fe₃O₄ and Al₂O₃)nanodots on graphene.Notably,the as-prepared C@SnO₂@Gr electrode as a LIB anode material possesses a high reversible discharge capacity of 702 m Ah g^-1 and an excellent capacity retention of over 100%at 2 A g^-1 after 1200 cycles.