| Supercapacitors,as an energy storage device,have attracted extensive attention due to their high power density,long cycle life and safety.They are highly needed in fast charging-discharging situation and energy self-sufficient devices.Unfortunately,the low energy density of commercially available supercapacitors(normally using carbon materials,with energy density of less than 10 Wh kg-1)is remarkably lower than those of batteries and fuel cells.In general,the charge storage properties of supercapacitors are strongly dependent on the electrode materials used.Nitrogen doped carbon material can can improve the energy storage capacity of carbon materials and maintain good electrochemical stability.At present,the specific capacitance of these materials has been reported to be as high as 855 F g-1.However,nitrogen-doped powdered porous materials need to be prepared into electrodes with the aid of conductive agents and binders.The introduction of these non-active materials reduces the overall performance of the electrodes.While the performance of self-supporting membrane electrodes needs to be further improved.The specific capacity of the free-standing membrane electrode is generally no more than 340 F g-1,and the rate performance is poor.This is mainly due to the difficulties in the preparation of interconnected porous nitrogen-doped carbon materials,with the routine methods such as chemical vapor deposition,chemical activation or pyrolysis method.Hence,to control the composition of electrode material,design and construct a proper structure,in order to obtain free-standing electrode with high specific capacitance,high energy density and good cycle stability,is still a challenge.Gelation is widely used in the preparation of nitrogen doped porous bulk materials.However,limited by the stability of the material,it is seldom used in the preparation of membrane electrode materials.In this paper,a series of flexible self supporting nitrogen doped carbon material membrane electrodes with interconnected macroporous structure were obtained by the combination of gel strategy and flexible substrate.For one thing,the interconnected macro-porous structure is in favor of the transport and adsorption-desorption of ions,for another,the uniform nitrogen doping and graphene network can provide conductive pathways for quick charge transfer.Therefore,high mass specific capacity and energy density were obtained.1.The preparation of nitrogen-doped graphene flexible filmsIn this section,we first fabricated graphene oxide?GO?gel with the aid of pyrrole as nitrogen-containing cross-linking agent.The nitrogen-doped graphene?NG?flexible films were prepared via vacuum filtration process.During filtration,the hydrogel filter is supported by a thin GO substrate as the flexible carrier of the gel.Free-standing NG film is obtained after subsequent freeze-drying and pyrolysis process.The gelation is benificial to the formation of interconnected macroporous structure.In this process,pyrrole is used as nitrogen-containing linking agent.Thus,uniform nitrogen-doped structure can be formed in the interconnected macroporous network.A series of self-supporting nitrogen-doped graphene films?NG-x?were prepared by adjusting the amount of crosslinking agent containing nitrogen.They were characterized by morphology,structure and electrochemistry.The results show that NG-3 films exhibit abundant interconnected macroporous structures and excellent electrochemical properties.Nitrogen atoms were effectively incorporated into the sp2 carbon structure,and the nitrogen content was 3.27%.The specific capacity of 455.4 F g-1 is obtained at current density of 1 A g-1.Besides,at 5 A g-1,it shows good cyclic stability.2.Acquisition of High Energy Density Based on Nitrogen Doped Graphene Flexible FilmsOn the basis of the above results,we prepared a series of polypyrrole/graphene?RGO/PPy?composite aerogel films by using aerogel got after the same preparation process as NG-3 but without the pyrolysis procedure as skeleton.The polymerization reaction is conducted under different pyrrole concentration respectively.These RGO/PPy composite membranes have double-layer macroporous structure similar to NG-3.Furthermore,we use NG-3 as the negative electrode and the polypyrrole composite aerogel membrane as the cathode to assemble an asymmetric supercapacitor.Its working voltage window can reach1.7 V,and a high energy density of 34.5 Wh kg-1 is achieved at 849.8 W kg-1 power density.Besides,it shows good cycling stability.3.Fabrication of macro-porous nitrogen doped carbon flexible films with high nitrogen content and their electrochemical propertiesIn this section,a similar gel strategy is used to explore large-area and low-cost preparation of nitrogen doped flexible membranes.GO/CNT hydrogel was prepared and used as starting material.It was drop casted onto a flexible thin layer of PAN?polyacrylonitrile?film substrate and further polymerized and pyrolyzed to obtain a free-standing flexible film.Then a series of characterization show that the flexible membrane has a two-layer interconnected macroporous structureand a nitrogen content of 9.1%.The symmetric supercapacitor based on this nitrogen doped films show a specific capacitance of 351.6 F g-1 at 1A g-1.At current density of 20 A g-1,there is still 55.7%capacitance retention.An asymmetric supercapacitor assembled with two film electrodes got after pyrolysis and before pyrolysis?experienced acid reduction?process can achieve a 1.7 V working window.A high energy density of 60.6 Wh kg-1 is achieved at 850.2 W kg-1 power density and shows good cycling stability. |
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