Preparation, Functionalization And Energy Storage Of Polyphosphazene Derived Carbon Materials

A major issue on the development of energy storage systems for storing electricity from sustainable and renewable energy has emerged in the past decades,owing to the ever-increasing global energy demand,severe environmental pollutions,and decreasing availability of fossil fuels.Electrochemical capacitors,which are also called supercapacitors,have been recognized as a new class of energy-storage device due to their unique advantages on high power density,long cycling life,rapid charge-discharge process,and favorable safety considerations.However,the low energy density restricts the wide application of supercapacitors,and the structural design towards electrode materials is the key to solve the problem.Nanostructured carbon materials with multiple heteroatoms doping are considered as a promising new carbon electrode material for supercapacitors,which own great superiority in the aspects of improving the specific capacitance of electrode material,electrolyte ion penetrability and transmission efficiency.Therefore,Nanostructured carbon materials with multiple heteroatoms doping have received extensive attention in recent years.However,the development of novel multiple heteroatom doping carbon materials is still a challenge.Polyphosphazene is a kind of polymer with multiple heteroatoms and abundant carbon,which is a good precursor for carbon materials.In addition,Because of its low cost,natural enrichment and high theoretical capacity,manganese dioxide has become a research hotspot in recent years.In this work,based on polyphosphazene and manganese dioxide,three nanostructured carbon materials and their composites were designed,that is,N,P,S-codoped hollow carbon shell（NPS-HCS）,N,P,O-codoped hollow carbon spheres（NPO-HCSs）,and NPO-HCS@MnO₂ composites.And their electrochemical performance was characterized systematically when used as supercapacitor electrode materials.The main contents are summarized as follows:（1）Using hexachlorocyclotriphosphazene and 4,4’-dihydroxydiphenylsulfone as comonomers,and ZIF-8 as template,core@shell structured ZIF-8@PZS composites were prepared by template induced assembly mechanism.Then,N,P,S-codoped hollow carbon shells（NPS-HCS）were obtained by one-step pyrolysis of ZIF-8@PZS under nitrogen atmosphere.Results show that the NPS-HCS has a high specific surface area(865 m² g^-1),high heteroatom content（6.01%）and rich porous structure.Also,NPS-HCS exhibits high specific capacitance(302 F g^-11 of specific capacitance at a current density of 1 A g^-1),good rate performance(206 F g^-1of specific capacitance at a density of 20 A g^-1),and excellent cycle stability(97.1%retention of the initial specific capacitance after 10000 charge-discharge cycles at a current density of 20 A g^-1).The assembled symmetrical supercapacitor has an energy density of 26.5 Wh kg^-11 at a power density of 900 W kg^-11 in a wide voltage range of 0-1.8 V.（2）Ternary-doped（nitrogen,phosphorus and oxygen）hollow carbon microspheres（NPO-HCSs）are fabricated by one-step pyrolysis of single poly（cyclotriphosphazene-co-phloroglucinol）（PCPP）microsphere,whichis generated through a facile polymerization between hexachlorocyclotriphosphazene and phloroglucinol at mild conditions.The whole preparation process is not used any additional template or activating agent.The obtained NPO-HCS-950 with average diameter of 580 nm and shell thickness of about 80 nm have a high specific surface area(2390 m²g^-1),a large pore volume(1.35 cm³g^-1),hierarchically interconnected pore texture,and uniform ternary heteroatom doping（O:3.04 at%;N:1.33 at%and P:0.67 at%）.As an electrode material for supercapacitors,the specific capacitance of the NPO-HCS-950 reaches 253 F g^-1of 1 A g^-1and 176 F g^-1at 20 A g^-1,revealing superior rate performance.The capacity retention after 10000 consecutive charge-discharge cycles at 20 A g^-11 is up to 98.9%,demonstrating excellent cycling stability.（3）The NPO-HCS@MnO₂ composites with core-shell structure were prepared by one-step hydrothermal method for growing MnO₂ nanofibers on N,P,O-codoped hollow carbon spheres（NPO-HCSs）.As the supercapacitor anode material,the NPO-HCS@MnO₂ composites have excellent structure advantages.Its conductive carbon core and distributed evenly MnO₂ nanofibers can provide good mechanical support and provide rapid transmission path for ions and electrons.Importantly,NPO-HCS@MnO₂ exhibits a specific capacitance of up to 222 F g^-11 in 1 M Na₂SO₄electrolyte.Inaddition,theasymmetricsupercapacitorsassembledwith NPO-HCS@MnO₂ as positive electrodes material and NPO-HCS as negative electrodes material exhibit high energy densities up to 32.21 Wh kg^-1at a power density of 449.8 W kg^-1.It is suggested that NPO-HCS@MnO₂ composites have a promising application as a supercapacitor electrode material.In a word,three types of nanostructured carbon materials and their composite materials were constructed by structure design.And they can be used as positive and negative electrodes materials for high-performance supercapacitors respectively.The assembled symmetric supercapacitor and asymmetric supercapacitor show good application prospect.At the same time,we also established the relationship between the microstructure of nanostructured carbon materials and their electrochemical properties.These studies will lay a good theoretical basis and experimental basis for the future structural design of supercapacitor electrode materials.