Preparation And Supercapacitive Performance Of Graphene-based Nanomaterials

As the cumulative problems of energy consumption and environmental destruction,developing the renewable energy resources has become the urgency.Among the various energy storage devices,supercapacitor has attracted wide attention owing to its high power density,long cycle life,fast charge/discharge capabilities and low maintenance cost.Graphene is a promising electrode material for supercapacitor due to its unique thermal,mechanical,electrical properties and large surface area.However,the aggregation of graphene during processing leads to significant decrease of real surface area and therefore becomes a serious obstacle to take the full advantage of the structural characteristics of graphene.Recent researches have focused on fabricating modified graphene-based materials to solve this problem.This study focuses on preparing a series of graphene-based materials by various methods.The morphologies and structures of as-synthesized graphene-based materials are characterized.Then the supercapacitive behaviors of these materials are also investigated by electrochemical tests.The specific contents of this study are as follows:(1)A novel scroll-sheet structured graphene(SSG)was fabricated by a simple hydrogen inner-detonation method.The morphology and structure were characterized by a range of techniques.The results exhibited that the SSG material with obvious tube structure has an enhanced specific surface area and more ion-accessible pores compared to reduced graphene oxide(rGO).The electrochemical performance of synthesized SSG was investigated and it offered an outstanding capacitive behavior.In the two-electrode system,the SSG//SSG symmetrical supercapacitor exhibited a high specific capacitance of 57.6 F g-1 at 1 A g-1,equaling to 230.4 F g"1 for a single SSG electrode.The SSG//SSG capacitor also showed excellent cycling stability,the specific capacitance remained 96.8%of the initial value after 5000 cycles.(2)3D porous N-doped graphene(PNG)was facilely prepared via a quasi-gas template method by pyrolysis of GO at the presence of urea.The 3D PNG with large BET surface and high N-doping content was obtained by adjusting the pyrolysis process and the mass ratio of urea to GO.The first stage of pyrolysis process was set at 300 ? for 3 h,and then urea and GO could release large amounts of gas(such as NH3,HCNO and H2O)gradually.The released gases were functioned as pore generators to tear and strike GO nanosheets,which are beneficial to forming the 3D nanostructured graphene.The second stage of pyrolysis process was set at 800 ? for 2 h,the N-doping was achieved.The results showed that the PNG-10 with an optimal N-doping content of 6.9 at%displays a high surface area of 490.2 m2 g-1 when the mass ratio of urea to GO was 10:1.Remarkably,the PNG-10 exhibited a specific capacitance of 337.0 F g-1 at 1 A g-1 and a low capacity shrinkage of 2.4%after 5000 cycles compared with its initial capacitance.Moreover,a symmetric supercapacitor was fabricated and displayed a cell voltage of 1.3 V,and it showed both high energy density and high power density of 19.9 W h kg-1 and 650 W kg-1,severally.(3)Nitrogen-doped unzipped carbon nanotubes-graphene(NUCNT-rGO)composite was prepared by a self-assemble method for supercapacitor electrode material.The morphologies and structures of as-prepared samples were characterized by various techniques.As a result,the carbon nanotubes were cut breadthwise and slit lengthwise successfully,forming analogous graphene nanoribbon structure.The nitrogen was also doped to the unzipped carbon nanotubes successfully with nitrogen content of 7.5 at%by hydrothermal treatment.The NUCNT-rGO composite,with more defects and larger interlayer spacing,showed outstanding supercapacitive performance.In the three-electrode system,the NUCNT-rGO electrode delivers a specific capacitance of 296.0 F g-1 at a current density of 1 A g-1.A NUCNT-rGO//NUCNT-rGO symmetric supercapacitor was also fabricated,and it displayed a specific capacitance of 65.5 F g-1 at 1 A g-1.After 5000 cycles,the specific capacitance of NUCNT-rGO//NUCNT-rGO supercapacitor could still remain 92.7%of the initial value,implying good long-term cycle stability of the composite.(4)Hierarchical porous carbon spheres-graphene(CSG)composite was prepared by a simple self-assembled method for supercapacitor electrode material.Carbon spheres(CS)was chosen as the spacer for graphene due to its submicron size.As a result,the aggregation of graphene was reduced by CS effectively,which leads to an enhanced specific surface area and more ion-accessible pores.This unique structure is conducive to the penetration/mobility of ions,especially for ionic liquid with large ionic sizes.The electrochemical performance of synthesized CSG was investigated and the composite offered an outstanding capacitive behavior.In the two-electrode system,CSG electrode displayed a specific capacitance of 303.8 F g-1 in KOH and 280.0 F g-1 in[BMIM]BF4.Remarkably,the CSG//CSG supercapacitor exhibited a high energy density of 87.5 W h kg-1 in[BMIM]BF4 electrolyte.After 5000 cycles,the specific capacitance of CSG could still remain 95.3%in KOH and 90.1%in[BMIM]BF4 of the initial value,implying good long-term cycle stability of the composite.(5)Unzipped carbon nanotubes(UCNT)were prepared successfully by using modified Hummers method.Carbon spheres(CS),with uniform morphology and size,was prepared by hydrothermal treatment.And then,the unzipped carbon nanotubes-carbon spheres-graphene(UCNT-CS-rGO)composite was prepared by a simple self-assembled method for supercapacitor electrode material.As a result,the UCNT and CS were intercalated between rGO sheets and the composite showed increased defects and enhanced interlayer spacing.This structure is conducive to the penetration/mobility of ions,especially for ionic liquid with large ionic size.Hence,the composite offered an outstanding capacitive behavior in both KOH electrolyte and[BMIM]BF4 electrolyte.In the two-electrode system,UCNT-CS-rGO//UCNT-CS-rGO supercapacitor displayed a specific capacitance of 74.8 F g-1 in KOH and 71.7 F g-1 in[BMIM]BF4.Remarkably,the supercapacitor also exhibited a high energy density of 89.6 W h kg-1 in[BMIM]BF4 electrolyte due to the wide working voltage of 3 V for[BMIM]BF4.After 5000 cycles,the specific capacitance of the supercapacitor could still remain 94.8%in KOH and 90.2%in[BMIM]BF4 of the initial value,implying good long-term cycle stability of the composite.(6)The pompon-like MnO2/graphene oxide composite was prepared by in-suit growth method.And the composite was reduced to MnO2/graphene hydrogel(GH)composite via a hydrothermal process.Micro-nanostructured MnO2/GH composites with different mass ratios were also prepared for supercapacitor electrodes.The morphology,structure and electrochemical performance were characterized and the effect of MnO2 content on capacitance of the composites were studied.It could be observed that graphene nanosheets constituted the 3D structure of porous hydrogels and most of the pompon-like MnO2 embedded in these pores.The electrochemical performance of the prepared electrode was investigated,and the MnO2/graphene hydrogel composite electrode with a MnO2 content of 50 wt%delivered a specific capacitance of 445.7 F g-1 at a current density of 0.5 A g-1,and the specific capacitance remained 82.4%of the initial value after 5000 cycles.The high performance was ascribed to the 3D pompon-like micro-nanostructure of MnO2/GH that not only provided a good support to stabilize MnO2 but also facilitated the fast ion mobility and electron transfer.