2.1.1 Cellulose/activated carbon paper based composite
electrode
In the field of supercapacitors, typical activated carbon materials with
double electric layers have been widely used due to their low cost,
extensive sources and mature preparation process. However, when
traditional activated carbon based electrode materials are assembled
into devices, it is inevitable to combine them with inactive binders
(such as polytetrafluoroethylene), conductive substances (such as carbon
black) and collectors (such as foam nickel). The preparation process is
not only complicated, but also the prepared electrode materials lack a
certain flexibility, which limits its application in flexible electronic
devices. To solve this problem, Chen et al. [59] prepared porous
carbon (PC)/MXene/nano cellulose (CNF) paper based composite electrode
materials through simple vacuum filtration. As shown in Figure 2A, the
preparation process of CNF/PC/Mxene paper-based material is shown. The
paper-based electrode is first prepared by simple mixing and dispersion,
and then by vacuum assisted filtration process. As shown in Fig. 2B, the
author points out that the densified structure formed by vacuum
filtration is the key to the excellent performance of the material. This
provides a strong interface interaction between the oxygen-containing
groups on the surfaces of CNF, PC and MXene, which makes paper based
materials have obvious mechanical advantages. It can be seen from the
microscopic image of the composite (Figure2C) that it is inside the
material. Mxene provides excellent conductivity (83.1S
cm-1), ensuring fast electron transfer. On the other
hand, PC provides excellent double layer capacitors for the composite.
And the PCs are evenly distributed between the network structures built
by MXene and CNF. This also further avoids the stacking effect between
two-dimensional MXene and CNF, which makes the paper based material form
an intercalation structure extremely conducive to the rapid movement of
electrolyte ions, and shortens the diffusion distance of ions. The test
results show that the excellent structure of the paper-based material
gives it excellent electrochemical performance, such as the area
specific capacitance of 143mF cm-2 (current density
0.1mA cm-2, thickness 0.2mm) and 2.4μWh
cm-2 area energy density.
As mentioned above, the adhesive similar to PTFE will coat the surface
of some activated carbon, resulting in insufficient activation of
electrochemical performance. In the cellulose/activated carbon paper
based composite electrode materials prepared by vacuum filtration
similar to the above research work, cellulose molecules are only used as
the adhesive and structural support of an active substance in the
device. In the electrochemical reaction process, it can not provide a
transfer path for electron transfer. Therefore, in paper based electrode
materials, it is particularly important to build cellulose with
excellent electrical conductivity and mechanical properties. Luo et al.
[60] constructed BC/AB/AC paper based composite electrode materials
by vacuum filtration of bacterial cellulose (BC), acetylene black (AB)
and activated carbon (AC). Figure 2D shows the TEM image of the
paper-based material. It can be seen that due to the existence of a
large number of hydroxyl and carboxyl groups with electronegativity on
the molecular face of BC, the electrically neutral AB particles can be
adsorbed on its surface during the ultrasonic dispersion process, thus
building conductive cellulose on the microstructure. This provides an
excellent path for electron transfer in the electrochemical reaction
process (Figure 2E), which greatly enhances the energy storage
performance of paper based materials. As shown in Figure 2F, the author
also discussed the performance difference between PVDF as the binder and
BC as the binder. It can be seen from the figure that compared with
PVDF, the paper based electrode material constructed by BC has excellent
porous structure and AC has more active sites for electrochemical
reaction. The paper based electrode shows excellent energy storage
performance (the specific capacitance is 275F g-1 at
current density of 1A g-1). This work provides a
simple and efficient method for the construction of paper based
electrode materials with excellent mechanical properties by non
carbonization method, and provides an idea for the wide application of
paper based electrode materials.