Preparation And Electrochemical Performance Of Nitrogen-doped Graphene-based Supercapacitor Electrode Materials

Energy storage is one of the key technologies that promise to solve serious problems such as increasing energy consumption and global warming.In a variety of energy storage devices,supercapacitors have attracted much attention due to their high power density,good rate performance and excellent cyclic stability.However,the low energy density(4-10wh kg^-1)of supercapacitors limits their further application in the field of energy storage.Therefore,it is necessary to design or use electrode materials and electrolytes that improve the energy and power density.Among various electrode materials,due to the unique properties of graphene,such as excellent conductivity,corrosion resistance in electrolytes,highly modifiable nanostructures,long cycle life and large theoretical specific surface area,so it as an electrode material for supercapacitor has become the focus of scientific research in the field of energy storage device.Unfortunately,due to the strong interlayer van der Waals forces and theπ-πstacking interaction,graphene generally suffers from irreversible agglomeration and significant reduction of surface area,resulting in a low specific capacitance.Doping heteroatoms（N,S,B,P,etc.）into graphene can reduce the restacking of graphene and improve the electrochemical performance of graphene.Compared with aqueous electrolytes and organic electrolytes,ionic liquids have the advantages of non-volatile,non-inflammable,good thermal stability and wide electrochemical window（>3 V）,and are considered as electrolytes that are expected to improve the energy density of supercapacitors.In this paper,a series of nitrogen-doped graphene aerogel materials were prepared by hydrothermal reduction of graphene oxide（GO）,in which using GO as the precursor and different nitrogen sources as dopants,and studied for their application in the ionic liquid based supercapacitors.The main contents include the following aspects:1.Diammonium hydrogen phosphate（（NH₄）₂·HPO₄）has relatively green and mild characteristics,avoiding the use of irritating dopants（hydrazine hydrate,pyridine,acetonitrile,etc）.Therefore,three-dimensional N-doped graphene aerogel（3D-HPNGA）with hierarchical porous structure was synthesized by hydrothermal treatment with GO dispersion as precursor and（NH₄）₂·HPO₄ as dopant.After optimization,the as-prepared nitrogen-doped graphene aerogel has high BET specific surface area and nitrogen content（504.1 m2 g-1 and 6.52 at.%,respectively）.It was assembled into the symmetrical supercapacitor,and the electrochemical test was carried out in the EMIMBF₄ ionic liquid electrolyte.When the current density was 1 A g^-1,the specific capacitance of the sample was 184.7 F g^-1.2.The mixture of ammonium fluoroborate（H₄BF₄N）,ammonia（NH₃·H₂O）and graphene oxide was hydrothermal treatment to synthesize 3D covalent cross-linked N-doped porous graphene material（NPG）.Ammonium fluoroborate reacts with the GO dispersion to form nitrogen-doped graphene materials with relatively small amount of pyridine nitrogen,while ammonia water and GO hydrothermal to generate materials with high pyridine nitrogen content.The pyridine nitrogen has higher activity than other types of N,which can induce enough defects and more active sites.The ratio of ammonium fluoroborate to ammonia was adjusted to obtain materials with different nitrogen contents,among which NHG-NH₃0.5 has the specific surface area of 320.1 m² g^-1,rich hierarchical pore structure,and higher nitrogen doping amount of 7.65 at%.Due to the unique internal pore structure of NHG-NH30.5,it has low internal resistance and excellent rate performance.The NHG-NH₃0.5 was used as electrode material to assemble the symmetrical supercapacitor.At the power density of 0.9 k W kg^-1,the capacitor exhibited the energy density of 91.3 Wh kg^-1.3.Using hydrogen peroxide as an etchant and ammonium oxalate（（NH₄）₂·C₂O₄）as a dopant,a 3D nitrogen-doped holey reduced graphene oxide aerogel（NHr GA）was prepared by two-step hydrothermal method.Hole defects were introduced on the surface of graphene by etching of hydrogen peroxide,while（NH₄）₂·C₂O₄ was used as a dopant to effectively regulate the electrochemical behavior of NHr GAs.The effect of different nitrogen concentrations in doping on the electrochemical performance of NHr GA was studied.At a current density of 1 A g^-1,the specific capacitance of NHr GA-2 is 193.1 F g^-1,and the symmetric supercapacitor based on NHr GA-2 has the energy density of 95.8 Wh kg^-1 at the power density of 0.9 k W kg^-1.