Heteroatoms-doped Porous Carbon Materials Derived from Phosphazenes and Their Electrochemical Properties

Author:Liu Wei

Supervisor:wu zhan peng


Degree Year:2018





Phosphazene is a type of organic-inorganic polymer with alternating nitrogen-phosphorus atoms as the main chain structure and side groups can be regulated by nucleophilic substitution reactions.The high nitrogen and phosphorus content is favorable for the in-situ preparation of heteroatoms-doped carbon materials and the in-situ formation of pores during the removal of the pyrolysis gases to prepare porous carbon materials with interconnected channels.Heteroatoms-doped micro/mesoporous carbon materials have been widely used in supercapacitors and electrocatalysis owing to their excellent electrochemical properties.However,during the carbonization of macromolecular precursors,molecular chain will move freely under thermal treatment.As a result of the aggregation of the molecular chain,the final product is mostly a carbon block with fewer pores.In this thesis,different chemical mechanisms were applied to prepare phosphazene-based porous carbon materials.The unique chemical activity of polyphosphazene compounds containing different functional substituents can be prepared according to the requirement.Several phosphazene precursors with different functional groups have been selected,such as poly(diphenolphosphazene)(PDPP)with high phenyl content,hexa(eugenol)cyclotriphosphazene(6ECP)with ENE self-polymerization of allyl,poly(diaryloxyphosphazene)(PDAP)with chemical crosslinking sites of p-ethylphenoxy.Porous carbon materials have been prepared with different structures and properties by different chemical mechanisms.Their applications in electrochemistry were investigated.1.A novel heteroatoms-doped porous carbon material was prepared from PDPP by the principle of direct carbonization and activation.The surface morphology and elemental comparison of the carbon materials prepared at 800℃ with/without following activation at 800℃ were investigated as an example.The activation process produced a large number of micropores of the bulk carbon material.Its specific surface area was greatly increased to 1798 m2 g-1 after activation.Heteroatoms-doped porous carbon material was prepared by in-situ carbonization.Though N1s(1.22%)and P2p3(3.42%)remained in 800AC,most heteroatoms were greatly reduced during the activation process.Because low temperature is conducive to the doping of heteroatoms and the high temperature facilitates the graphitization of carbon materials,800AC electrode material seized the best chemical properties owing to the synergy of electric double layer capacitance and faradaic pseudocapacitance.In 6 M KOH aqueous electrolyte,it showed relatively high specific capacitance,good rate performance and cycling stability.The specific capacitance can reach 105 F g-1 at the current density of 0.3 A g-1.2.A Heteroatoms-doped mesoporous carbon material was prepared from 6ECP by the principle of self-polymerization reaction mechanism of the double bonds on side groups.The influences of hydrogen,heating rate,carbonization temperatures and template content in the carbonization system were studied on the performance of aqueous capacitors and oxygen reduction reaction.The performances of mesoporous carbon electrode materials in ionic liquid electrolyte were also investigated.The carbonization mechanisms were studied through SEM,XRD,Raman,XPS and BET analyses methods in detail.During the thermal treatment,6ECP monomer took self-polymerization reaction to produce P6ECP with a hypercrosslinked structure,which can effectively prevent the material from melting,shrinking,and collapsing into a dense and pore-free morphology during the carbonization process.The BET specific surface area of the mesoporous material MC-m-1000H was 717.3 m2 g-1,which was prepared at 1000 ℃ for 2 h in an Ar/H2 mixed atmosphere with a high heating rate.In 6 M KOH aqueous electrolyte,the fast charge-discharge process of electric double-layer capacitor and characteristics of pseudocapacitor attributing to heteroatoms were demonstrated.The interconnected channels,which were distributed between the mesopores,can also provide rapid ion diffusion and transportation for larger species in ionic liquid electrolytes.When the power density was 738 W kg-1,the energy density of a symmetrical supercapacitor in the[BMIM]BF4 electrolyte can reach up to 26.6 Wh kg-1.3.Hierarchical porous carbon materials were prepared from PDAP precursor by chemical crosslinking theory.PDAP elastomer precursor took chemical crosslinking reactions with peroxides under thermal treatment.The crosslinking sites can prevent the movement of polymer chains in a high temperature environment to a certain extent and effectively prevent them to form dense carbon blocks during carbonization.The carbon materials prepared from crosslinked PDAP filled with Si02 with a specific surface area of only 404.1 m2 g-1,provided channels for the rapid transfer and diffusion of the ions in electrolyte owing to the hierarchical porous structure.As electrode materials in supercapacitors,its symmetric cyclic voltammetry and galvanostatic charge and discharge curves indicate the perfect electric double-layer capacitor characteristics.H2O2 was prepared by oxygen reduction reaction of porous carbon materials through two-electron pathway in alkaline.The yield of H2O2 reached 159 mg L-1 at 120 min(-0.7V vs SCE)and the current efficiency can reach 78%at 30 min(-0.5 V vs SCE).4.PDPP microspheres were prepared by emulsion method and their carbonation properties were preliminary explored.In order to maintain its morphology,the PDPP@Ag composite microspheres were fabricated to employ metal shell as a mechanical support for carbon@noble metal composites.The structure and morphology of PDPP@Ag microspheres were compared with polyimide@silver(PI@Ag)microspherees.The silver particles were dispersed onto the surface of PDPP microspheres as the their movements were restricted by the highly flexible polyphosphazene polymer chains.So the silver failed to provide the desired mechanical support.However,a continuous silver layer formed on the surface of PI microspheres owing to the rigid chains of PI.Since PI is also a polymer rich in nitrogen,its composites with other metals,metal oxides and sulfides were expanded,which may be used in the energy field.