Preparation And Supercapacitor Property Research Of Poly(Diaryloxyphosphazene)-based Carbon Nanomaterials

Electrical double layer capacitance(EDLC)also termed as super capacitance has gained immense importance in advanced energy storage and management devices owing to its excellent energy storage properties including high power density,long lifecycle,low maintenance cost,wide working temperature and short charging time.Porous carbon materials are one of the most commonly used materials as electrodes in supercapacitor because its high surface area,excellent chemical stability and environment friendly in nature.Nanoporous carbon materials doped with heteroatoms such as oxygen,nitrogen and phosphorous,have gained much importance in supercapacitors,as electron donor/acceptor properties of their electrochemical active sites would have resulted in pseudo capacitative effects.Oxygen doped functional groups are the easiest to be prepared by various activation methods but now the attention of the researcher have tilted towards the doping of nitrogen and phosphorous groups in to activated carbon-based EDLC super capacitors.It is because of the fact that the nitrogen-containing functional groups improve the wettability to electrolyte solution of carbon materials to increase the electronic conductivity of them.It is reported that phosphorus doping affects the degree of graphitization and surface areas of carbon materials,the doped phosphorus can modify the electron transport properties and the affinity towards acceptor molecules(such as 02),which makes phosphorus-doped carbons more efficient electrocatalysts.Some polymers(such as polystyrene,polyimide,polyacrylonitrile,etc.)have been used as carbon precursors to prepare carbon materials.The advantage of this method is that different precursors can be designed to produce the carbon materials with specific structures or containing specific elements.Take nitrogen and phosphorus containing materials as carbon precursors or add nitrogen and phosphorus source during the carbonization process is a common element doping method.Polyphosphazenes are an important class of inorganic polymers comprising of alternate single-double bonds between phosphorus and nitrogen atoms.Polyphosphazene is a kind of rubber elastomer,which had excellent mechanical properties and flame retardant properties.However,polyphosphazenes have not been used as a source of carbon materials.Therefore we predict that polyphosphazene may become a new type of hetero-doped carbon material precursor.Then we use aryloxy groups which have high carbon content as the side groups to synthesis poly(diaryloxyphosphazene),trying to prepare a new carbon precursor to prepare hetero-doped carbon materials.The results of this thesis:(1)The application of polyphosphazenes precursors in supercapacitance has been exploited as a novel idea in this research work.Doping of these precursors with nitrogen(N),phosphorus(P)and oxygen(O)have played a vital role in further development of super capacitance.N,P and O-doped nanoporous carbon materials are prepared by using poly(diaryloxyphosphazene)(PDPP)as carbon precursor and this polymer carbon precursor was synthesized by the reaction of linear polydichlorophosphazene(PDCP)with sodium phenolate.These carbon materials possess a high percentage of O,N and P as functional groups.An additional chemical activation process with potassium hydroxide(KOH)has been performed which has considerably enhanced the microporous network.After activation,the surface area of these carbon materials has reached up to 1798 m2 g-1,pore volume up to 0.91 cm3 g-1 with the 20%O,5%N and 9%P contents.Moreover,it is revealed that different carbonization temperature has a strong influence on textual properties of the resulting carbon materials(including surface area,pore volume,pore size and pore size distribution).As super capacitor electrodes,these hetero atom-doped carbon material exhibits a high specific capacitance(up to 329 F g-1 at 0.5 A g-1 in 3-electrode cell)and excellent rate performance(with 63.8%maintenance from 0.5 to 50 A g-1).(2)In carbon-based materials,graphene has been widely used as a supercapacitor electrode.Its excellent performance,especially the high theoretical specific capacitance is about 550 F g-1,high conductivity(ca.2000 S cm-1)and high theoretical specific surface area(about 2630 m2 g-1)have large contributions to electrochemical performance.In the third chapter,the composite materials of polyphosphazene and reduced grapheme oxide(rGO)were prepared by solvating method because the poly(diaryloxyphosphazene)can be dissolved in tetrahydrofuran(THF).It is desirable to add rGO to improve the electrochemical properties of carbon materials.In the experiment,the graphene oxide were chemically modified by using octadecylamine and reduced to dispersed in THF.The precursors were prepared by complexing PDPP and rGO in the THF.Change the mass ratio of PDPP and rGO to see whether the rGO can improve the electrochemical properties of the carbon materials.The experimental data showed that when the mass of rGO is 2%of the mass of PDPP,the specific surface area of the obtained carbon material is 2471 m2 g-1,and the pore volume is 1.2 cm3 g-1.The prepared carbon material has a high specific surface area and a high pore volume.As super capacitor electrodes,these carbon materials exhibited a high specific capacitance(up to 428 F g-1 at 0.5 A g-1 in 3-electrode cell)and excellent rate performance(with 69%maintenance from 0.5 to 10 A g-1).It is shown that the method of preparing composite by adding rGO has successfully improved the electrochemical performance.Through the above experimental study,we can draw the preliminary conclusion that polyphosphazene is a new type of carbon material precursors with very broad research prospects.