Regulation Of Oxygen-containing Functional Groups Of Graphene By Heat Treaement And Its Supercapacitive Performance

Since its discovery in 2004,graphene has been very promising candidate for supercapacitors（SCs）due to its excellent electrical conductivity(carrier mobility 2×10⁵cm² V^-1 s^-1),large theoretical specific surface area(2 630 m² g^-1)and high theoretical double layer capacitance(550 F g^-1).However,the inevitable agglomeration of graphene during the preparation process results in the actual double layer capacitance of graphene generally not higher than 200 F g^-1.Although agglomeration can be avoided by preparing three-dimensional porous graphene,it leads to a very low volumetric capacitance,which is unfavorable for practical applications.In this paper,we proposed three methods to regulate the oxygen-containing functional groups（OFGs）on graphene,such as two-step atmosphere heat treatment,phosphoric acid heat treatment and metal cation-assisted heat treatment.These strategies can increase the content of electrochemically active quinone-type C=O and aromatic C-OH groups on the graphene surface to enhance the redox pseudocapacitance in graphene;the packing density of graphene materials can be enhanced through the self-assembly process of graphene.Thus we obtained graphene supercapacitor electrode and device with high volumetric capacitance.In this paper,we proposed an efficient and environmentally friendly two-step atmosphere heat treatment of graphene oxide（GO）.The first step is argon thermal treatment in which interlayer water and some OFGs were removed,leading to the expansion of graphene,increase of its specific surface area and formation of three-dimensional structure.The sencond step is air heat treatment,by controlling the temperature of the second air oxidation step,quinone-type C=O groups were selectively introduced in graphene to enhance the pseudocapacitance in graphene electrode materials.The experimental results showed that the highest content of C=O groups（5.3 at.%）was achieved after the air oxidation at 400°C for 10 min;the specific gravimetric capacitance（Cg-m）and the specific volumetric capacitance（CV-m）of RGO-Ar-O400 were 303 F g^-1and 82 F cm^-3@0.2 A g^-1 with a mass loading of～1.0 mg cm^-2,respectively.The value of Cg-m of RGO-Ar-O400 is 3.4 times that of GO.The gravimetric energy density of the symmetric supercapacitor assembled by RGO-Ar-O400 electrode with a mass loading of2.0 mg cm^-2 reached 8.1 Wh kg^-1@59.5 W kg^-1.The volumetric energy density of the electrode acitve material was 2.2 Wh L^-1 and the capacitance retention of the device was88%after 5 000 cycles.Graphene powder materials with well self-assembled structure and high packing density were prepared by low-temperature phosphoric acid heat treatment（PAT）strategy.The PAT selectively improved the relative content of aromatic C-OH groups by the formation of C-O-P bond between C-OH and H3PO4.In the PAT process,the self-assembly process between the reduced graphene oxide（RGO）was achieved due to the van der Waals forces between the graphene sheets and the hydrophobicity of the graphene after reduction.As a result,the packing density of the obtained graphene powder was as high as 1.55 g cm^-3.Thanks to the high content of electrochemically active C-OH groups on the graphene surface and the high packing density,the Cg-m and CV-m of RGO-MP40graphene powder reached 312 F g^-1 and 484 F cm^-3@0.1 A g^-1 with a mass loading of2.5 mg cm^-2,respectively.As the mass loading incresed to 19.7 mg cm^-2,the Cg-m and CV-m slightly decresed to 248 F g^-1 and 384 F cm^-3.The gravimetric energy density of the symmetric supercapacitor assembled by RGO-MP40 electrode with a mass loading of20.0 mg cm^-2 can reach 8.9 Wh kg^-1@14.9 W kg^-1.The volumetric energy density of the electrode active material was 13.8 Wh L^-1,and the cycling stability of the device was 89%after 5 000 cycles.In this paper,we proposed a method to prepare RGO by metal cation-assisted heat treatment of GO.The supercapacitor assembled by this RGO exhibited excellent electrochemical performance.The assembly process of graphene sheets was achieved through the interactions of cation-πand cation-OFGs,and the metal cation-assisted heat treatment favours the transformation of OFGs in graphene to C-OH and C=O groups,leading to the high stacking density and high pseudocapacitance of graphene electrode materials.Graphene powder materials with a packing density of 1.67 g cm^-3 were obtained by Li⁺(3.5 mol L^-1)assisted heat treatment at 400°C.Due to the high content of C-OH and C=O groups and high pacing density,the Cg-m and CV-m of of RGO-3.5 M Li Cl-400C were 307 F g^-1 and 512 F cm^-3@0.1 A g^-1 with a mass loading of～2.5 mg cm^-2,respectively.The gravimetric energy density of the symmetric supercapacitor assembled by RGO-3.5 M Li Cl-400C electrode with a loading of 2.0 mg cm^-2,the gravimetric energy density can reach 9.5 Wh kg^-1@60 W kg^-1,while the volumetric energy density of the active material can reach 15.9 Wh L^-1,and the capacitance retention of the device is 98%after 5 400 cycles.