2.1.4 Cellulose/graphene quantum dot paper based composite electrode
Graphene quantum dots refer to a quasi zero dimensional material with graphene sheet size less than 100 nm and sheet size less than 10[74]. Since its discovery in 2008[75], GQD has attracted extensive attention due to its excellent optical and electrical properties. And it has been applied to energy storage, sensing, catalysis, environment and other fields. Among them, GQD has ultra-fine size and ultra-high specific surface area because its dimensions in the three dimensions are at the nanometer level[76]. These superior properties make it a new rising star of electrode materials for supercapacitors. The composite of GQD and cellulose can overcome the shortcomings of limited specific surface area, poor conductivity and low utilization rate of cellulose. For example, Xiong et al.[77]obtained CNF/GQD paper-based composites by vacuum filtration for the first time. First, GQD was prepared by electrolysis and dialysis purification with a stone mill (Figure 4D). Then, CNF/GQD paper-based electrode is obtained by mixing GQD and CNF uniformly and vacuum filtering (Figure 4E). Figure 4F shows SEM and TEM images of CNF/GQD. It can be seen from the figure that GQD is evenly distributed on the CNF surface. Moreover, CNF crosslinked with each other to form a porous structure. This provides a fast transport path for electrolyte ions in the electrochemical energy storage process. The test results show that the paper based material can maintain an area capacitance of 118mF cm-2 even at a high scanning rate of 1000mV s-1, indicating that it has excellent magnification performance. This is mainly due to the excellent energy storage characteristics of GQD itself and the porous structure of CNF providing sufficient load sites for GQD.