Preparation And Assembly Of Graphene Nanosieve And The Effect Of Surface Functional Groups On Capacitance

Supercapacitor is a kind of energy storage devices which bridge the gaps between traditional capacitor and rechargeable battery because of its fast charge-discharge rate, high power density and ultralong cycling stability. Among all the componemts which constitute a supercapacitor, the property of electrode materials is the key component which determines the performance of supercapacitor. With the rapid development of graphene materials over the past decades, graphene has show promising application as supercapacitor electrode. However, the ?-? interaction and strong van der Waals interactions lead graphene sheets to form serious agglomeration and stacking, which decreases its specific surface area and increases the ion diffusion resistance. To address this problem, in this work, we prepared two kinds of graphene nanomesh by solvothermal method and chemical vapor deposition, and studied the influence of the holey structure and the surface oxygen functional groups on its capacitive performance. The following results have been achieved:A green yet efficient solvothermal method has been developed for preparing holey graphene aerogel ?HGA? in a mixed solvents consisting of water and EG. The high temperature and pressure caused by solvothermal reaction enable the dissolved O2 molecules to serve as reactive oxidant to initiate the oxidative-etching reaction at the sp3 carbon-rich regions of GO sheets. The removal of oxygenated carbon atoms accompanying with the release of CO or CO2 leaves behind vacancies which progressively extend into nanopores in graphene basal plane. The holey graphene sheets can facilely assemble into compressible graphene aerogel by increasing the starting GO dispersion concentration. The synthetic condition for graphene aerogel including the composition of mixed solvent, different gases ?air, oxygen and argon? and solvothermal time were systematically investigated. With nanopores of 1.5-5.0 nm on graphene sheets, graphene aerogel composed of holey graphene exhibited a specific surface area of 528 m² g"1. The electrochemical behaviors of HGA was measured in a two-electrode system. Its capacitance retention remains 73% as the current density varied from 0.5 A g^-1 to 20 A g^-1, and its relaxation time constant was found to be 3.03 s. These results demonstrate holey graphene sheets with numerous nanopores on graphene sheets could promote the ion diffusion across the 2D graphene sheets, thus improving the rate capability of graphene aerogel.The CVD derived graphene nanomesh ?GNM? was reduced by hydrogen under different temperature ?200 ?,400 ?,600 ?, and 800 ??, and the influence of the surface oxygen functional groups on the capacitive performance was investigated by Raman, XPS, CV, charge-discharge and electrochemical impedance spectroscopy. The amount of surface oxygen functional groups was found to gradually decreases with the increase of the reduction temperature. An abnormal results is HGNM treated in H2 at 400 ?, which showed dramatic increase in specific capacitance and good capacitance retention (250 F g^-1 at the current density of 0.2 A g^-1, and 83% capacitance retention as current density ranging from 1.0 to 30 Ag^-1).