High Performance Carbon Nanosheets And Their Composites: Controllable Synthesis And Application In Energy Storage

As a carbon matrix material,2D carbon nanosheets（CNS）are one of the most promising materials in the application of supercapacitors and other energy storage devices because of their small thickness,abundant carbon source,good electrical conductivity and controllable heteroatom doping.However,they still have the inherent disadvantages of poor specific capacitance and energy density.To solve these problems,the composite system can be built between CNS and transition metal oxides to make full use of their complementary advantages and synergistic effects.In this paper,functional CNS generated by activation is taken as the main research object.The MnO₂ nanosheets,MoO₂ nanoparticles and CuO nanoparticles were combined with functional CNS to make full use of complementary advantages of carbon materials and transition metal oxides.The results significantly improve the performance in supercapacitor devices.（1）2D surface functional CNS was prepared by carbonization and activation method.The surface of functional CNS is rough and wrinkled in margin because of a series of oxygen-containing functional groups.These functional groups could be used as the nucleation sites of growing materials,which greatly improved the success of composite experiments.（2）The MnO₂/CNS composite was prepared.The MnO₂ nanosheets with high capacitance were grown on the functional CNS.This micro-nano structure design has more efficient use of the contact area between electrolyte and active materials with the increase of the active site.The MnO₂/CNS electrode was carried out in three-electrode system using alkaline electrolyte（KOH）.The results show the typical pseudocapacitance.The specific capacitance is up to 395 F g^-1 at 1 A g^-1,and the capacitance retention rate is maintained at 72%after 2000 cycles.（3）The MoO₂/CNS composite was synthesized.The self-assembled interconnected MoO₂ nanoparticles were loaded onto the functional CNS.This structure could reduce the transport resistance and provide an efficient way for charge transfer.The electrode was tested in a neutral electrolyte（Na₂SO₄）.The MC-0.12shows the best specific capacitance(190.9 F g^-1)and superior rate ability.The capacitance retention rate of 2000 cycles is 79%.The symmetric coin cell supercapacitor was successfully assembled,and the result of 2000 cycles of the assembled device is maintained at 92%.The device achieves a maximum power density of 1682 W kg^-1 at 7.9 Wh kg^-1,and still maintains 378 W kg^-1 at 10.3 Wh kg^-1.（4）CuO/CNS composite with excellent properties were prepared.The integration strategy was made full use of the high conductivity of functional CNS and the rich chemical activity of CuO.It combined the stable electrical double-layer of functional CNS and the pseudocapacitance effect of CuO.The electrochemical characteristics of the prepared samples were obtained in neutral electrolyte and alkaline electrolyte.Different charge storage mechanisms were analyzed.In Na₂SO₄ and KOH electrolyte,the specific capacitance of CuO/CNS-20 electrode at 1 A g^-1 reach to 183.9 and 371.1F g^-11 in different elecytolytes.After 2000 cycles,the results are maintained at 87%and51.1%,respectively.Na₂SO₄ and KOH were used as electrolytes to assemble two kinds of symmetric coin cell supercapacitor devices.In two devices,the voltage window reaches to 1.2 and 1.4 V,respectively.The results are maintained at 94.4%and 52%after 2000 cycles.As for the devices with neutral electrolyte,the maximum energy density generated can reach 12.46 Wh kg^-1 at 300.3 W kg^-1.Even at the high power density of 1504.3 W kg^-1,the energy density still reaches 8.9 Wh kg^-1.In devices with alkaline electrolyte,the energy density is up to 19.36 Wh kg^-1 at 355.6 W kg^-1and the maximum power density is 1750.7 W kg^-1 at 12.06 Wh kg^-1.