Design of Carbon Micro/Nano Materials Based on Polyphosphazene,and Their Electrochemical Applications

Author:Chen Kui Yong

Supervisor:liu hong


Degree Year:2016





Climate change and the over-consumption of fossil fuels have negative impacts on world economy and ecology.Development of novel electrochemical energy storage and conversion devices is inevitable for human society.Because of the characteristics including cleaning and high efficiency,supercapacitor and fuel cell are considered of great potential in the future society.However,there are some obstacles that restricting the commercialization of those devices.For example,the low energy density of supercapacitor can not meet the basic requirements in some fields(for example new energy vehicles).On the other hand,the high cost,low activity and poor durability restrict the application of fuel cells.The main problems could be attributed to the electrode materials,which are of poor electrochemical properties.Dvelopment of novel electrode materials with high electrochemical performance is of key importance to solve the problems.Because of the large specific surface area,well conductivity and stable physical and chemcial properties,carbon materials are considered to be the ideal candidates.However,there are still some problems in the controllable preparation and structural optimization of carbon materials.Polyphosphazene materials,which have the features of simple preparation,controlable micro/nano-structures,highly crosslinked structure and high char yield,are ideal precursors of carbon micro/nano materials.This study is mainly focused on the design and preparation of functional carbon nano/micro materials based on the above-mentioned features of polyphosphazene,and further investigates their performance in the fields of supercapacitor and oxygen reduction reaction(ORR).The details are as follows:Based on the remarkable molecular structure designability,polyphosphazene nanotubes are simply synthesized via the polycondensation between hexchlorophosphazene(HCCP)and 4,4’-Sulfonyldiphenol(BPS).Heteroatoms doped porous carbon nanotubes(HMCNTs)are produced via facile carbonization of highly cross-linked polyphosphazene nanotubes under an inert atmosphere,without the use of any template or activating agent.Tubular structure of the polymeric nanotubes can be easily maintained through carbonization due to the highly cross-linked structure.High content of heteroatoms and uniform mesopores(with diameter4 nm)are incorporated into the carbon nanotubes.Electrochemical tests manifest high supercapacitor performance and oxygen reduction catalytic performance of the carbon nanotubes.On the one hand,electrodes prepared by using HMCNTs show a specific capacitance of 214.9 F/g at the current density of 0.1 A/g in 6 M KOH electrolyte,because of the large specific surface area and the high content of heteroatoms.On the other hand,heteroatoms in the surface chemical structure of the carbon nanotubes ensure effective ORR active sites of the materials,and HMCNTs show good ORR catalytic performance(low over potentiality,4e-process,excellent methanol tolerance and durability).Based on the remarkable surface building performance of polyphosphazene,hollow polyphosphazene microspheres(HPMSs)are simply synthesized via the in situ polycondensation between HCCP and p-phenylenediamine(p-PDA)on the surface of vesicle.The microstructure of HPMSs could be facilely controlled by changing the formation condition of the vesicle.Furthermore,heteroatoms doped porous hollow carbon microspheres(HCMSs)are prepared by carbonization of HPMSs in an inert atmosphere,followed by a KOH activating process.Electrodes prepared by using HCMSs showed a specific capacitance of 314.6 F/g at the current density of 0.2 A/g in6 M KOH electrolyte.As the applied current density increased from 0.2 A/g to 10 A/g,the specific capacitance retention ratio of HCMSs could reach 76.1%.When the current density reached 30 A/g,the specific capacitance could reach as high as 180.0F/g.The excellent supercapacitor performance benefits from the high surface area and the high content of heteroatoms of HCMSs.In addition,hollow structure of HCMSs ensures the satisfactory supercapacitor performance by increasing utilization efficiency of the surface area as well as accelerating the electrolyte ion transportation.Efficient oxygen reduction catalysts based on heteroatom-doped mesoporous carbon nanotubes loaded with Co2P nanoparticles are fabricated via facile carbonization the composite of highly cross-linked polyphosphazene nanotubes and cobaltous acetate.The Co2P nanoparticles are embedded in graphitic carbon layers,and uniformly scattered on the surface of the carbon nanotubes.Electrochemical tests show good electrocatalytic activity toward oxygen reduction reaction of the catalysts in the aspects of typical 4e-process,low over potentiality,excellent methanol tolerance and good durability.The effect of the Co2P nanoparticles to the ORR catalytic performance is mainly based on the synergistic effect between Co2P nanoparticles and the doped carbon materials via electronic interaction.Co2P nanoparticles can improve the highest occupied molecular orbital(HOMO)of heteroatoms doped carbon via electron donating property,and results in enhanced ORR catalytic property.This synergistic effect between metal compounds and doped graphitic carbon shell would be useful to further improve the catalytic performance of heteroatoms doped carbon materials toward oxygen reduction,and provide a new way for development of high efficient ORR catalysts.Various transition metal phosphides(Co2P,Mn2P,Ni2P)decorated heteroatom-doped carbon nanotubes(HCNTs-Co2P,HCNTs-Mn2P,HCNTs-Ni2P)are prepared by direct carbonization the composites of polyphosphazene modified carbon nanotubes and transition metal acetates.Transition metal phosphide nanoparticles are uniformly scattered on the surface of the carbon nanotubes,and embedded in graphitic carbon layers,indicating that they can’t play the active sites in the catalytic process,and the active sites are heteroatom-doped carbon.Electrochemical tests show good electrocatalytic activity of HCNTs-Co2P and HCNTs-Mn2P toward oxygen reduction reaction in the aspects of more inclined 4e-process,low over potentiality,excellent methanol tolerance and durability.It is mainly due to the intense electron donating ability of Co2P and Mn2P nanoparticles that can enhance the catalytic activity of heteroatoms doped carbon materials.Because of the poor electron donating ability of Ni2P,HCNTs-Ni2P shows weak impact on the catalytic activity of the heteroatom-doped carbon materials.In consideration of electron donating ability of transition metal compounds,electronegativity and binding energy shift of the elements(for example transition metals)need to be considered at the same time.This work is important for the further research of the active sites of ORR catalysts as well as design of high performance ORR catalysts.