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.