Rearch On Improving The Capacitive Energy Storage Performance Of Activated Carbon By Graphene Quantum Dots

As a kind of green electrochemical energy storage device,supercapacitor has acttracted great attention of researchers because of their high power density,long cycle life,low cost,and simple configuration.Activated carbons are the most widely used electrode materials for supercapacitors because of their low cost and large specific surface areas.But the activated carbon have poor capacitive performance and low working voltage,leading to low energy density of the devices.Generally,enlarging specific surface area of activated carbon will improve their specific capacitance.But the well-developed porous structure reduces the conductivity of activated carbons through breaking the conductive networks.The deep and branches micropores will restrict the fast transport and storage of electrolyte ions,thus resulting in very low utilization of specific surface and poor electrochemical performance.To solve the above critical problems,herein,we prepared several graphene quantum dots?GQDs?embedded activated carbon using GQDs and glucosamine as precursors through hydrothermal,carbonization,and activation process.The influence of GQD addition ration and KOH dosage on the morphology and capacitive performance of activated carbons were detailedly investigated.The improvement mechanism of the electrochemical performance was proposed.1.The activated carbons were prepared with different GQDs to glucosamine ratios and further activated under different KOH dosages.The morphology of the samples gradually transformed from coralline nanoparticles to micro-spheres with the increasing of GQDs.The specific capacitance and rate performance also gradually improved.With the increase of KOH dosage,the specific surface area of activated carbons increased gradually.Under the optimal condition,the specific surface area of GQD embedded activated carbon is 2829 m² g^-1,the total pore volume is 1.265 cm³g^-1,and the pore dimater is mainly distributed at 0.48-3.2 nm.In three-electrode system using 6 M KOH as electrolyte,it shows high specific capacitances of 354 F g^-1@1 A g^-1 and 220 F g^-1@100A g^-1.The capacitance retention at 100 A g^-1 is up to62%.In two-electrode system,it also performs superior specific capacitance of 388 F g^-1@1 A g^-1 as well as excellent rate performance with 60%capacitance retention at100 A g^-1.The specific surface area of the unactivated sample is only 357 m² g^-1 with the capacity of 56 F g^-1@1 A g^-11 and 12 F g^-1@20 A g^-1.The energy storage performance is far lower than the optimized sample.Chemical activation can increase the specific surface area of carbon materials,leading to improved energy storage performance.2.The improving mechanism of the energy storage performance by GQDs was also investigated.The electrochemical performance of four samples including GQD embedded activated carbon,activated carbon without GQDs,graphene nanosheet embedded activated carbon,and commercial activated carbon were detailedly investigated.We found that the GQD embedded activated carbon showed the best capacitive and rate performance.We propose that the embedded GQDs can improve the electric conductivity and charge-transfer kinetics of the activated carbon through constructing entire conductive networks,leading to effective utilization of the specific surface area especially in deep and branched pores.As a result,the symmetric supercapacitor achieves an ultrahigh energy of 13.47 Wh kg^-1 in alkaline electrolyte,which is much better than most of the reported activated carbons.Moreover,the capacitance reaches to the highest value of 4.17 and 2.44 F cm^-2 at 1 and 5 A g^-1 under a high mass loading of 15 mg cm^-2(higher than commercial level of 10 mg cm^-2),respectively,revealing great potential of the GQD embedded activated carbon for practical applications.In this work,we have developed a novel GQD embedding strategy to improve the electrochemical performance of activated carbon.Embedding highly crystallized GQDs results in the formation of entire conductive networks inside activated carbon skeleton owing to their small size and excellent compatibility,which significantly improves the charge-transfer kinetics and ion migration rate in deep and branched micropores.This strategy provides a new thought for designing advanced porous carbon materials for high performance energy storage.