Polymers-derived Hollow Nitrogen-doped Carbon Spheres For Supercapacitors And Oxygen Reduction Reaction

Development of sustainable power/energy devices including as supercapacitors(SCs),lithium-ion batteries,solar cells and fuel cells(FCs)has gained serious attention in recent years to meet the rising global energy demands.Amongst the energy storage devices,electrochemical SCs have gained serious consideration owing to their high power density,charge-discharge rates,and cycle efficiency.However,the drawback of low energy density limits its large-scale applications.In addition to energy storage,energy conversion is also very crucial to meet the increasing demand for energy.The oxygen reduction reaction(ORR)is believed to be one of the most significant reactions in sustainable energy conversion devices such as FCs and metal-air batteries.At present,widespread application of FCs is hindered by the high-cost of cathode electrocatalyst(such as platinum)for ORR.Therefore,to evade the drawbacks,it demands to synthesize efficient electrode materials with high energy density,low-cost,and eco-friendliness for the potential applications of energy storage and conversion devices(ESCDs).Heteroatom-doping provides an important option to improve the performance of carbon electrode materials in support of a wide range of potential applications for renewable ESCDs.Additionally,hollow structures can provide high porosity in the carbon framework which facilitates the electroactive properties.This study focuses on the production of metal-free hollow nitrogen-doped carbon spheres(HNCSs).The resulting HNCSs were used as electrode materials to investigate the electrochemical properties for SCs and electrocatalytic ORR performances for alkaline FCs.Three N-containing polymers such as polyaniline(PANI),polypyrrole(PPY),and polydopamine(PDA)were used as nitrogen(N)and carbon(C)source while polystyrene(PS)was used as a template to prepare HNCSs.Summary of the main results of this dissertation is as follows:PS@PANI core-shell spheres were produced by coating PANI layers on polyvinylpyrrolidone(PVP)modified polystyrene(PS)spheres.The core was removed by carbonizing the PS@PANI under the N2 atmosphere at 500 ? for 2 h to obtain the HNCSs.The contribution of a highly intact hollow structure,high surface area,and pyridinic N-contents can progress the mass transportation by facilitating the diffusion pathways which consequences the improvement of electrochemical and electrocatalytic performances.As an electrode material,the maximum specific capacitance of 266.7 F g-1 was obtained in 1 M KOH at a scan rate of 5 mV s-1.Additionally,it had good catalytic activity and stability via the more favorable nearly 4e-ORR pathway in an alkaline solution.Coating of both PPY and PANI on modified PS was further applied to examine the supercapacitive and ORR performances.The HNCSs derived from PS@PPY-PANI exhibited a maximum specific capacitance of 294.9 F g-1 at a 5 mVs-1 with good cycling stability of 95%after 5000 cycles at 0.5 A g-1.It also showed good ORR activity with a high onset potential of 0.83 V vs.RHE at 1600 rpm via more favorable near to the 4e-pathway in an alkaline medium.Pyrolysis of PS@PPY-PANI commences the incorporation of N-units along with the high degree of graphitization which contributes exceptionally to improve the ion/charge transfers.The improved electrochemical and electrocatalytic performances could arise from the hollow porous spherical structure,high surface area,and considerable pyrrolic-pyridinic N-contents.Dopamine(inexpensive/sustainable raw material)was also used as a N and C source to coat on modified PS for the preparation and performance evaluation of HNCSs.The resulting hollow spheres show a high surface area of 31 1.07 m2 g-1,the large pore volume of 0.32 cm3 g-1,and N-contents of 8.17 wt.%.Its low resistance value in electrochemical impedance spectroscopic(EIS)measurements also proved its enhanced electro-conducting properties.It showed a specific capacitance of 261.6 F g-1 at a scan rate of 2 mV s-1 in 1 M KOH aqueous electrolyte and moderate electrocatalytic activity via the almost '4e-' process(with the more positive onset potential of 0.919 V vs.RHE at 1600 rpm)towards ORR.Charge transfers,redox transformations(caused by N-H/catechol groups of the PDA)and pyrolysis(introduce the N-atoms along with the high degree of graphitization)may attribute to increasing the electrochemical and electrocatalytic performances of HNCSs.To prepare HNCSs,polymers such as polyaniline,polypyrrole,and polydopamine were used as nitrogen and carbon source while polystyrene as a template.As cost-effective,eco-friendly,and easily obtained N-containing hollow structure with high surface area,HNCSs are indicated to be efficient electrode materials for the application of high-performance SCs and ORR towards alkaline FCs.Therefore,this work provided an efficient way to obtain high-performance electrode materials.