2.1.5 Cellulose/MXene paper based composite electrode
As a 2D material with high specific surface area and high conductivity,
MXene has shown great prospects in the field of supercapacitor energy
storage[78]. In recent years, the composite paper based electrode
materials prepared by vacuum filtration of cellulose and Mxene have been
fully studied. This section summarizes cellulose/MXene paper based
electrode materials. When 2DMxene chips are used as supercapacitors and
electrode materials of supercapacitors, stacking between layers is easy
to occur. In order to solve this problem, scientists also tried to use
1D carbon nanotubes and other carbon materials as spacers to prevent
their stacking. However, the preparation process of these carbon
materials is complex and costly. Cellulose, as a green and sustainable
renewable resource, is the first choice as a 2D active material spacer.
For example, Jiao et al. [79] first prepared BC/MXene composite
paper with excellent new mechanical properties and electrochemical
properties through vacuum pumping power. In addition, stretchable and
patterned paper based electrode materials are prepared by laser cutting
process to adapt to various use environments (Figure5A). Thanks to the
close stacking structure between BC fiber and Mxene lamellae (Figure5B).
The prepared paper based electrode was assembled into an all solid state
paper based supercapacitor, showing an area capacitance of 115mF
cm-2 and flexible mechanical properties. This work
provides a promising method for designing and manufacturing flexible
electronic equipment used in different environments through the
patterned scheme of laser cutting. Similarly, Tian et al. [80]
prepared CNF/Mexene paper-based electrodes by vacuum filtration, which
also showed excellent electrochemical performance (298F
g-1). The above two works show that the
cellulose/MXene paper-based electrode prepared by simple vacuum
filtration process has a very broad application prospect.
Although the paper based electrode obtained by direct filtration of
cellulose and Mxene has good performance, it can not meet people’s needs
under certain conditions. Therefore, it is very necessary to improve the
energy storage performance of paper-based materials through certain
methods. For example, the introduction of the third component or the
surface modification of MXene can improve its energy storage
performance. In order to improve the electron transfer rate of MXene in
paper based materials, as shown in Figure 5C, it is inspired by the
structure of plant leaves. Tang et al. [81] prepared a paper based
composite material with leaf like structure by vacuum filtration of
cellulose, Mxene and silver nanowires (AgNWs). As shown in Figure5D, the
structure diagram of natural leaf veins and prepared
cellulose/Mxene/AgNWs paper based electrode (PMxAg) is similar to leaf
veins. Among them, cellulose as the main pulse provides excellent
mechanical properties for paper based materials and provides a transport
channel for electrolyte ions. Secondly, AgNWs as the secondary pulse
provides a fast channel for electron transfer. Finally, as the
”mesophyll”, Mxene provides sufficient energy storage capacity for
composite materials. Thanks to the ”blade like structure”, the paper
based electrode shows a specific capacitance of up to 505F g-1. In
addition, the author also successfully prepared large area composite
paper by using paper machine. The batch preparation of this flexible
supercapacitor provides a solution. In addition, surface treatment of
MXene is also an effective method to improve the electrochemical
performance. Previous research shows that the energy storage performance
of MXene mainly comes from the=O group, while the -F and -OH groups in
it have no contribution to the energy storage performance, and even more
content will affect the energy storage performance. For this method,
Chen et al. [82] modified the surface of MXene by KOH and high
temperature treatment to reduce the -F and -OH groups on its surface.
Subsequently, NFC/Mxene paper based composite electrode was prepared by
combining nano fibrillar cellulose (NFC) prepared from soybean straw
with modified Mxene through vacuum filtration process. The preparation
process is shown in Figure 5E. The test results show that the paper
electrode based supercapacitor has a specific capacitance of 303.1F
g-1 at 1mA cm-2 and a capacitance
retention of 92.84% after 10000 cycles. As shown in Figure 5F, Cai et
al. [83] prepared SnS2 on the surface of MXene by hydrothermal
method Mxene@SnS2 Nanosheets, then CNF was prepared by
vacuum filtration/ Mxene@SnS2 Paper based composite materials. To
illustrate Mxene@SnS2 The contribution of nano sheets to
the electrochemical performance of composite materials is shown in
Figure5G. The author compares CNF/MXene, CNF/MXene/SnS2and CNF/ Mxene@SnS2 Energy storage performance among the
three. CNF/ Mxene@SnS2 The specific capacitance of
171.6F g-1 is significantly higher than that of CNF/Mxene (163.3F g-1)
and CNF/Mxene/SnS2 (130F g-1). At the same time, because
of the light heat conversion function of SnS2, the
assembled supercapacitor has the function of solar energy driving. As
shown in Figure 5H, the discharge time of the paper-based supercapacitor
increases with the increase of light intensity (under the light
intensity of 1kW m-2, the energy storage performance of the device is
improved by 60%).