Efficient PH-universal Aqueous Supercapacitors Based On Azine Dye Functionalized N-doped Graphene Aerogels

As a green energy storage technology,supercapacitors（SCs）show wonderful promise in many portable and industrial applications.Current aqueous SCs have relatively low energy densities,so there is widespread interest in the cost-effective fabrication of SC electrode materials with higher specific capacitance and wider voltage windows.Graphene has been of great interest in SC applications due to its extremely high specific surface area and high theoretical specific capacitance.However,its actual capacitive performance is always unsatisfactory,and thus the development of effective methods is needed to improve its capacitive performance.Among them,one effective way is to modulate the composition and structure of graphene.Heteroatom doping can provide additional specific capacitance to graphene due to it stronger electronic conductivity,higher specific surface area and more redox active sites.The atomic radius of N is similar to that of carbon,but N has a more electronegative value than the latter.N has been a commonly used and ideal graphene dopant.N-doped graphene aerogel（NGA）not only has the inherent advantages of two-dimensional N-doped graphene（NG）,but also has a highly porous three-dimensional（3D）structure.This pore structure of NGA can effectively avoid 2D NG stacking or aggregation,and accelerate ion transfer,thus further improving the capacitive performance of 2D NG.However,the specific capacitance of pure NGA is still insufficient for practical applications.Recent studies have shown that electroactiveπ-conjugated fused-ring organic small molecules can form extendedπ-conjugated materials through non-covalentπ-πinteractions with graphene aerogels（GA）,and thus the resalting composites possess enhanced capacitive properties.However,there are few studies on electroactiveπ-conjugated organic small molecule non-covalently functionalized NGA for SCs.Taking into account that the specific capacitve property of NGA is better than that of GA,in this thesis,azure C,neutral red and aniline tetramer are introduced into NGAs through strong non-covalentπ-πinteractions to prepare two electrode materials with high electrochemical performance and cycling stability.The main studies are as follows:（1）A redox-active azure C decorated N-doped graphene aerogel（AC-NGA）was fabricated by a simple hydrothermal method.The resultant AC-NGA exhibits excellent charge storage performance at full p H values(specific capacitance of 591.1 F g^-1at a current density of 1.0 A g^-1and a high voltage window of 1.2 V).Its capacitive contribution of charge storage at a scan rate of 2 m V s^-1is 91.70%.In addition,a symmetrical AC-NGA//AC-NGA device achieves high energy and high power densities(15.2-60.2 Wh kg^-1,650-23,000 W kg^-1),as well as excellent cycling stability in acidic,neutral and alkaline aqueous solutions.（2）An electroactive neutral red functionalized N-doped graphene aerogel（NR-NGA）was fabricated by a simple hydrothermal method.The resulting NR-NGA exhibits excellent charge storage performance at all p H values(specific capacitance of 784.9 F g^-1at a current density of 1.0 A g^-1and a high voltage window of 1.2 V).Its capacitive contribution of charge storage is 91.74%at a scan rate of 3 m V s^-1.In addition,an asymmetric NR-NGA//1.0 mol L^-1KOH//LDH device achieves an energy density of up to 215.1 Wh kg^-1at900 W kg^-1and 77.4 Wh kg^-1at a high power density of 18 k W kg^-1,the energy density of NR-NGA//1.0 mol L^-1H₂SO₄//GA device is up to 41.7 Wh kg^-1at a power density of 950W kg^-1,NR-NGA//1.0 mol L^-1Na₂SO₄//Na Mn O_2-xdevices have an energy density of 91.4Wh kg^-1at a power density of 1.55 k W kg^-1,and excellent cycling stability in acidic,neutral and alkaline aqueous solutions.（3）Aniline tetramer was formed on the surface of graphene oxid through chemical polymerization and then an aniline tetramer-modified NGA（AT-NGA）was prepared by a two-step hydrothermalmethod.The resultant AT-NGA exhibits excellent charge storage properties over a full p H range(specific capacitance of 699.1 F g^-1at a current density of1.0 A g^-1and a wide voltage window of 1.1 V).Moreover,the capacitance contribution was94.1%at a low scan rate of 1 m V s^-1.The energy density of the AT-NGA//1.0 mol L^-1H₂SO₄//GA asymmetric supercapacitor was 35.8 Wh kg^-1at 900 W kg^-1;the AT-NGA//GA/1.0 mol L^-1Na₂SO₄//Na Mn O_2-xasymmetric supercapacitor has an energy density of 71.8Wh kg^-1at 1.65 k W kg^-1;at 900 W kg^-1,AT-NGA//1.0 mol L^-1KOH//LDH asymmetric supercapacitor has an energy density of 48.2 Wh kg^-1.This work provides a cost-effective strategy for combining redox dye basedπ-conjugated fused-ring organic small molecules orπ-conjugated aniline tetramer with heteroatom-doped graphene aerogels to construct green and efficient p H-universal water-based supercapacitors.