The Preparation Of Polypyrrole Nanotubes-Based Composites For The Performance Of Supercapacitors

Supercapacitors,as a fast-rising class of energy storage devices,have attracted extensively research interest because of their high power density,fast charge/discharge process,environmentally friendliness and long lifespan.With the increasing energy demand in hybrid electric vehicles and mobile electronic devices,there is an urgent need to explore new materials with high specific capacitance,outstanding conductivity and electrochemical stability to meet the requirement of energy storage.In recent years,Transition metal oxides,hydroxides,and sulfides such as NiO,Co₃O₄,Ni（OH）₂,NiCo₂S₄ and etc.have been paid increasing attention to the application of pseudocapacitors owing to their low cost,low toxicity,large capacitance and excellent thermal stability.However,the poor conductivity of pseudocapacitive materials hinders the rate of electron transfer,leading to a huge loss of capacitance and restricting their practical applications.Polypyrrole（PPy）is an important conductive polymer,which is considered as one of the most valuable functional materials for supercapacitors due to its high degree of flexibility,high conductivity and excellent energy storage ability.In this study,we used the self-degrading template method to synthesize polypyrrole nanotubes（PNTs）,and then loaded Ni（OH）₂ nanoparticles and NiCo₂S₄ nanosheets on the surface to systematically study the electrochemical performance of the supercapacitors.The main contents and results are as follows:（1）Preparation of PNTs/Ni（OH）2 electrode materials and study on the performance of supercapacitorsA facile and novel method of growing nickel hydroxide nanoparticles on polypyrrole nanotubes（Ni（OH）₂/PNTs）is presented using methyl orange and ferric chloride formed fibrillar complex as self-degrading tubular template coupled with hydrothermal synthesis.The Ni（OH）₂/PNTs samples were characterized by SEM,TEM,FTIR,XRD and XPS.The results showed that Ni（OH）₂/PNTs composites were successfully prepared and Ni（OH）₂ nanoparticles were evenly distributed on the surface of PNTs.Electrochemical tests show that the obtained Ni（OH）₂/PNTs in three-electrode configuration display a significantly enhanced specific capacitance（864 F/g at 1 A/g）,better rate performance,lower charge-transfer resistance and higher cycling performance（91.1%of the initial capacitance retention at 5 A/g over2000 cycles）compared to the individual Ni（OH）₂ and PNTs,and previously reported composite electrodes based on Ni（OH）₂ or PNTs.Besides,the symmetric supercapacitors of Ni（OH）₂/PNTs in two-electrode configuration show a maximum energy density of 18.8 Wh/kg at the power density of 414.6 W/kg and a maximum power density of 3.4 kW/kg at the energy density of 8.4 Wh/kg.The high electrochemical performance of Ni（OH）₂/PNTs can be attributed to the synergistic effect of both components and the unique nanostructure.These results reveal that the Ni（OH）₂/PNTs can be used as promising electrode materials for supercapacitors in energy storage.In addition,the method described in this paper provides a new route for the construction of transition metal oxides（hydroxides）/PNTs-based composite nanostructures with improved electrochemical performance of supercapacitors.（2）Preparation of PNTs@NiCo2S4 nanosheets core-shell composites and study the performance of supercapacitorsWe coated NiCo₂S₄ nanosheets on Pre-synthesised conductive polypyrrole nanotubes to form a hierarchical core-shell PNTs@NiCo2S4 by an oil bath and hydrothermal method,aiming to enhance the electron transport and electrochemical stability of NiCo₂S₄.The PNTs@NiCo₂S₄ samples were characterized by SEM,TEM,FTIR,XRD,EDX and XPS.The results showed that PNTs@NiCo2S4 is successfully synthesized and NiCo2S4 nanosheets are uniformly coated on PNTs.Electrochemical tests show that the PNTs@NiCo₂S₄ exhibits a low charge-transfer resistance（0.55?）and a high specific capacitance（911 F/g at 1 A/g）,which is higher than that of pure NiCo₂S₄（470 F/g）and other NiCo₂S₄-based materials.Furthermore,93.2%of the initial specific capacitance is remained after 4000 cycles at 5 A/g,indicating the excellent stability of PNTs@NiCo2S4.Besides,an symmetric supercapacitor built with PNTs@NiCo2S4 shows high energy density of 21.3 Wh/kg at 417 W/kg,high power density of 8.6 kW/kg at 11.3 Wh/kg,and good cycling stability（91.6%capacitance retention after 4000 cycles at 5 A/g）.The remarkable performance of PNTs@NiCo2S4 is mainly attributed to the good conductivity of PNTs,novel core-shell structure,and synergetic effect of NiCo2S4 and PNTs.The results indicate that PNTs@NiCo₂S₄ is potential electrode material for supercapacitors.