Mixed-Ligand-MOF Derived Heteroatom-doped Carbon Nanocomposite: Preparation And Application In Energy Conversion And Storage

With the continuous increase of energy consumption in modern society,and the consequent problems of depletion for traditional fossil energy and environmental pollution,the demand for clean energy is gradually increased.Renewable clean energy sources such as wind energy,solar energy,and water energy have shortcomings such as intermittent and regional differences,which are not conducive to the effective use of clean energy sources.Therefore,it is very necessary to develop efficient catalyst to convert or store surplus clean energy.In this process,the catalyst involved in the energy conversion reaction is very important,which can ensure the efficient energy conversion process,improving the efficient and sustainable use of clean energy.Therefore,the research on energy conversion and storage（ECS）catalytic materials has been continuously paid attention to by researchers in all world.Hydrogen is an ideal energy medium with high energy density,which stores energy in the form of chemical energy.Hydrogen Evolution Reaction（HER）is a process of converting electrical energy into chemical energy,in which HER catalyst plays a crucial role.In fuel cells,hydrogen is oxidized to break the chemical bond between two hydrogen atoms,thereby converting chemical energy into electrical energy.The key response to limiting this energy conversion efficiency is the Oxygen Reduction Reaction（ORR）on the cathode.An efficient ORR catalyst is needed to accelerate the reaction rate and improve energy conversion efficiency.The carbon-cycle is also a strategy for energy conversion.By using CO₂ as a reactant to form chemical intermediates or high-value fuel molecules through energy conversion reactions,which can alleviate global warming caused by carbon emissions and achieve clean energy.In addition,the energy storage device is an effective means of directly and quickly using clean energy,especially lithium-sulfur batteries（Lithium-Sulfur Battery:Li-S battery）with higher energy density than traditional lithium-ion batteries,which has attracted the research enthusiasm of many scientists.Carbon-based nanocomposites with heteroatom doping（nitrogen,phosphorus,sulfur,boron,fluorine,etc.）have important catalytic effects in the above ECS reactions,especially multi-heteroatom doped carbon-based composite materials.The preparation of heteroatom-doped carbon-based nanocomposites with metal-organic framework（MOF）as precursor has unique advantages.The spatial periodic intervals of metal nodes and organic ligands in MOF materials not only prevents the aggregation of metal elements during calcination,but also ensures the uniform distribution of various chemical elements in the product.And the diverse components and structures also provide more for the preparation of carbon-based nanocomposites.However,in most reported literatures,the MOF precursors are mainly concentrated in a few typical MOF structures（such as ZIF-67/ZIF-8,MIL series）.Furthermore,in the synthesis of multiple heteroatom-doped carbon-based nanocomposites with MOF materials as precursors,additional additives containing other heteroatoms are usually required.The preparation of multi-heteroatom co-doped carbon-based nanomaterials has rarely been reported by using a mixed-ligand-MOF composed of two or more organic ligands containing different heteroatoms as a single source precursor.Based on the above research background,this thesis focuses on the structural design of mixed-ligand-MOF as single-source precursor,and explores the characteristics of MOF structure for effective preparation of heteroatom-doped carbon-based nanocomposites and studies the potential of mixed-ligand-MOF as single-precursor in ECS applications,the relationship between the structural characteristics of heteroatom-doped carbon-based nanocomposites and their catalytic ECS reactions.The main research work is divided into the following three aspects:1.A mixed-ligand-MOF was prepared by self-assembly of nitrogen-containing ligand（pyrazine）andphosphorus-containingligand（1-hydroxyethylidene-1,1-diphosphonic acid:HEDP）and Cu²⁺.The as-synthesized Cu-NPMOF-1 was used as single-source precursor to prepare successfully a composite electrocatalyst of nitrogen and phosphorus co-doped porous carbon matrix encapsulating Cu3P nanoparticles（Cu3P@NPPC）after calcination and subsequent phosphidation.The performance of electrocatalytic HER and ORR were studied in detail.The optimized Cu₃P@NPPC-650 composite showed the smallest overpotential of 89 mV at a current density of 10 mA cm^-2.Meanwhile,it exhibited the best ORR catalytic activity with a half-wave potential of 0.78 V,which is only 29 mV less than that of Pt/C.And Cu₃P@NPPC-650 also showed the excellent catalytic stability for HER and ORR.In addition,the performance of zinc-air battery with the cathode material of Cu₃P@NPPC was studied.2.On the basis of part 1,the ionic host-guest framework NiMeP-MOF was prepared through the rapid synthesis of nitrogen-containing ligand（sodium dicyandiamide）and Ni²⁺induced by cationic template（phosphorus-containing ligand:methyltriphenylphosphonium bromide）.The nitrogen and phosphorus co-doped porous carbon embedding nickel/nickel phosphide composite（Ni-P@NPPC）was obtained by one-step calcination of Ni-P@NPPC as single-source precursor.The Ni-P@NPPC-900 composite（pyrolysis of NiMeP-MOF at 900°C）showed the largest specific surface area（1468.4 m² g^?1）.The performance of photocatalytic CO₂reduction for Ni-P@NPPC catalyst in ternary system containing[Ru（bpy）₃]²⁺of photosensitizer and triethanolamine（TEOA）of sacrificial agent was studied.In the mixed solvent of acetonitrile and H₂O（3:2）,the CO generation rate could reach up to2673μmol h^?1 g^?1with the high CO selectivity of 92.6%.In addition,the recycling results showed the outstanding cycle stability of Ni-P@NPPC-900 hybrid.3.We selected 4,4’-bipyridyl（bpy）as a nitrogen-containing ligand to replace the previously used pyrazine ligand,to prepare the Cu-NPMOF-2 with HEDP ligand and Cu²⁺.The nitrogen and phosphorus co-doped porous carbon framework（NPPCF）with ultra-high specific surface area(1961 m² g^?1)was obtained by the one-step calcinaton of Cu-NPMOF-2 as a single-source precursor.The performance of CO₂-adsorption of NPPCF and the performance of lithium-sulfur batteries constructed by the NPPCF as cathode were studied.The optimal NPPCF-700 showed a high CO₂ sorption capacity of 99.4 mL g^-1 at 273 K with the significant CO₂adsorption selectivity.As the cathode material of lithium-sulfur battery,the sulfur content of S@NPPCF-700 was as high as 85.3%,and S@NPPCF-700 exhibited good cycling stability with a high specific capacity of 840 mA h g^-1 after 200 cycles at a current rate of 0.5 C.Furthermore,the interaction of polar polysulfide molecules with heteroatom-sites was verified by XPS.