2.1.3 Cellulose/carbon nanotube paper based composite electrode
Carbon nanotubes (CNTs) have been widely concerned by researchers because their unique 1D structure endows them with excellent mechanical strength, electronic conduction efficiency, heat transfer efficiency and stable chemical properties[69]. And CNT has a large aspect ratio and easy surface modification[70], which has great advantages for its application in supercapacitors. Among them, the large aspect ratio makes it possible to form a good conductive network without using adhesives. However, CNT and graphene have the same defect, that is, due to the strong interaction between CNTs, it is a fatal defect for electrode materials[71]. Therefore, it is particularly important to find a material that can disperse CNT evenly. It is a best of both worlds method to prepare paper based composite materials by mixing with cellulose materials. For example, Fang et al.[72]prepared BC/CNT paper-based electrode materials by vacuum filtration. Figure 4A shows the preparation process of the paper-based material. It is worth mentioning that the author used juglone to chemically modify the surface of CNT, because it is difficult for unmodified CNT to show high energy density. Some studies have shown that the organic quinones rich in redox carbonyls have high theoretical capacity, so the chemically modified CNT surface has a large number of juglone molecules. The test results showed that the conductivity of the paper matrix composite prepared by juglone modified CNT was 1674S m-1 (the conductivity of the unmodified composite was 1111S m-1). At the same time, the influence of the content of juglone molecules on the conductivity of paper matrix composites was also discussed. As shown in Figure 4B, when the content of juglone molecules is low, electrons can only have discrete jumps between the molecules. When the content is high, due to the crowding between molecules, electrons jump randomly in disorder during conduction. Therefore, when juglone molecules are uniformly distributed on the surface of CNT, the conductivity and energy storage performance of the composite are greatly improved thanks to the coordination between the uniformly arranged quinone molecules and CNT. The electrochemical test results of the paper based electrode showed that the modified BC/CNT paper based electrode showed a specific capacitance of 461.8F g-1 at 0.5 A g-1 (this value was five times that of the non chemically modified electrode). The paper based electrode materials prepared in this work have better performance than most reported CNT and paper based electrode materials prepared by vacuum filtration. In conclusion, this work provides an excellent strategy for preparing high-performance paper based electrode materials by vacuum filtration.
As shown in Figure 4C, Jyothibasu et al. [73] first treated the CNT surface with nitric acid to obtain CNT (f-CNT) loaded with a large number of hydroxyl groups. The prepared cellulose is then uniformly dispersed with the cellulose and then subjected to vacuum filtration. Finally, the cellulose/f-CNT paper based film was obtained after drying. In order to enhance its electrochemical performance, the author soaked the paper based film in the mixed solution of potassium permanganate and sulfuric acid to load manganese dioxide. The test results showed that after 120 min of loading reaction, the solid paper based device assembled by cellulose/CNT/MnO2 paper based electrode showed an excellent specific capacitance of 1812mF cm-2. This work shows that the cellulose/CNT paper based material obtained by vacuum filtration is also a good conductive substrate, and its electrochemical performance can be improved by introducing the third component. This provides a perfect solution for customizing high-performance flexible paper based supercapacitors.